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Phytoremediation of abandoned mining areas using native plant species: a Sardinian case study
... Some plant species that have adapted to grow in highly contaminated environments have been suggested as some of the most feasible and effective tools in phytostabilization through bio-mineralization processes [6,7]. This can eventually play an important role in the immobilization of elements around the root systems and the recovery of the polluted mine sites [8][9][10]. In mine waste environments, soil elements can be associated with different geochemical forms, such as soluble exchangeable, carbonates, iron-manganese oxides, residues and organic materials. ...
... Toxics 2022, 10, 728 4 of 19 In this study, the term soil refers to the collected mine waste samples. In the previous research study, between the years 2008 and 2010, a successful phytoremediation study on some plant species was carried out in an experimental plot amended with compost produced from the organic fraction of Municipal Solid Waste (MSW) in -CP tailing site [10,37]. The contaminated sampling points of this study are located in the same experimental site and its surrounding area. ...
... Six Pinus halepensis specimens with a nearly similar height (2 m) and age (10-12 years old) were selected from both non-contaminated and contaminated sites in the south-west of Sardinia (November 2020) where they grow spontaneously (Figure 1a,b). Six bulk soil samples were collected from the soils around the roots of P. halepensis together with its root samples: (i) CP1: located inside the contaminated Campo Pisano (CP) site in the compost-amended plot aged 10 years old [10,37], (ii) CP2: outside of the CP experimental plot distancing 3-4 m from it aged 10 years old, and (iii) CP3: located outside of the CP experimental plot distanced 6-7 m from it and aged 12 years old (Figure 1c,d) (see more in [32]), (iv) B1: Blank sample in the non-contaminated site (Santa Margherita, Pula) at approximately 80 km far away from CP mine site, (v) B2: Blank sample in the noncontaminated site (Calamosca, Cagliari) at about 60 km far away from the CP mine site, (vi) B3: Blank sample from the less-contaminated site (Fontanamare, Gonnesa) at about 10 km distance from the CP mine site (Figure 1b). The contaminated soils around the root and root samples were exploited the previous study [32]. ...
The study aimed at evaluating the geochemical fractions of Zn, Pb, Cd and their bioavailability in soil in-depth and around the root of Pinus halepensis grown on heavily contaminated mine tailing in south-western Sardinia, Italy. The contaminated substrates were partly investigated in a previous study and are composed of pyrite, dolomite, calcite, quartz, gypsum, barite, iron-sulfate and
iron-oxide. The geochemical fractions and bioavailability of Zn, Pb and Cd were measured through the BCR extractions method. Cadmium in the superficial contaminated substrates was mainly found in the exchangeable BCR fraction. Zinc and lead were often found in the residual BCR fraction. PCA confirmed that the uppermost alkaline-calcareous layers of mine waste were different with respect to
the deeper acidic layers. We demonstrated that Pb and Zn were less present in the exchangeable form around the roots of P. halepensis and in soil depth. This can be due to uptake or other beneficial effect of rhizospheres interaction processes. Further studies will shed light to confirm if P. halepensis is a good candidate to apply phytostabilization in mine tailing.
The plant species Pinus halepensis grows spontaneously on heavily polluted mine tailings dumps of Campo Pisano (Sardinia, southwestern Italy). The area is characterized mainly by Zn, Pb, and Cd. Sampling campaign was done, related to soils and plant materials (roots, barks, wood, and needles), aimed at evaluating the main mineralogical characteristics, metal content, plant accumulation, and translocation behavior. The polluted substrates were composed of pyrite, dolomite, calcite, quartz, gypsum, and barite with iron sulfate, and iron oxide. Zn ore minerals (smithsonite) and muscovite detected mostly in the deeper soil layers. Zn was the most abundant metal in the substrate as well as plant tissues. Roots accumulated high metal concentrations (664.65–2710 Zn, 58.39–735.88 Pb, and 4.86–11.02 mg kg⁻¹ Cd) reflecting high metal contamination in soil. The biological accumulation and translocation values were reported below one for all plant tissues. Pb, Zn and Cd Translocation Factor (TF) in needles ranged 0.03–0.32, 0.03–0.19, 0.04–0.14. Biological Concentration Factor (BCF) estimated up to 0.17, 0.18, and 0.19, respectively. The results indicate that P. halepensis is an excluder, tolerates high Zn, Pb, and Cd concentrations, restricts their accumulation and translocation to the aerial parts and may be applied for long-term phytostabilization and revegetation processes in abandoned mine tailing sites.
Phytoremediation is an efficient, economical, solar-driven innovative technology employed in plant species to remediate contaminated soil, sediment, water, and air using energy from sunlight. The chapter illustrates the potential of 14 native mangrove species for absorption, accumulation and transport processes of nine trace metal(loid)s in plant organs inhabiting in Indian Sundarban mangrove wetland. The mangroves exhibited a wide range of variations between the species and between sampling sites experiencing diverse environmental stresses. Avicennia officinalis (Acanthaceae) proved to be the highly promising plant species acting as a hyperaccumulator by assimilating and tolerating high trace metal (TM) concentrations without showing any sign of injury or stunted growth. These results suggest that mangrove plants may be used in a synergistic way to remediate and restore the TM contaminated estuary.
The mitigation of metals contamination is currently a crucial issue for the reclamation of mine sites. Indeed, mine wastes are often disposed in open dumps and consequently pollutants are subjected to dispersion in the surrounding areas. In this study, the potential use of Helichrysum microphyllum subsp. tyrrhenicum for phytostabilization was evaluated in ex situ conditions. Ninety specimens were randomly selected and were planted in three substrates (reference substrate, mine waste materials, and mine wastes with compost). Mineralogical compositions of substrates, rhizosphere, and roots were assessed through X-ray diffraction (XRD). Zn, Pb, and Cd concentrations of substrates, rhizosphere, soil pore waters, and plant tissues were determined. The phytostabilization potential was determined through the application of biological accumulation coefficient (BAC), biological concentration factor (BCF), and translocation factor (TF). Moreover, survival and biometric parameters were assessed on plant specimens. The polluted substrates and related rhizosphere materials were mainly composed of dolomite, quartz, pyrite, and phyllosilicate. Zn was the most abundant metal in substrates, rhizosphere, and soil pore waters. XRD analysis on roots showed the presence of amorphous cellulose and quartz and Zn was the most abundant metal in plant tissues. H. microphyllum subsp. tyrrhenicum restricts the accumulation of the metals into roots limiting their translocation in aereal parts, indicating its potential use as phytostabilizer (BCF, BAC, TF < 1). Survival and growth data showed a great adaptability to different substrates, with an evident positive effect of the implementation of compost which increased the plant survival and decreased the metals uptake into roots.
Sardinia was known as an important mine pole in Europe during his history. Still after decades from mine closure, 75.000.000 m3 of mine waste, rich in heavy metals, were left abandoned causing a huge environmental legacy on the mine district area. Consequently, cost effective remediation is required. In this frame, phytoremediation is considered a feasible candidate. This research was focused on Helichrysum microphyllum subsp. tyrrhenicum, which is pioneer in xeric soils with low-functions, like mine tailings. The aim of this study was to evaluate its ability to extract heavy metals from mine soils and accumulate them in plant tissues and its suitability for phytostabilization. Sundry samples of soil, roots and epigean organ were collected through field sampling and analysed in order to obtain metals concentration and mineralogical characteristics. Our results indicate that this species tolerates high concentration of zinc, lead and cadmium, behaving as a species suitable for phytostabilization.
A two-year study has been conducted in an abandoned Pb/Zn mining site, with the aim of investigating the feasibility of phytoremediation using two native Mediterranean plants (Pistacia lentiscus and Scrophularia bicolor) and of assessing the performance of amendments able to reduce the toxic effects of heavy metals. The amendments used were compost, chemical fertilizer, and zeolites, used singly or in combination. Depending on the amendments applied, the two species showed different mortality rates in the different plots, but all produced an increase in P. lentiscus survival, while S. bicolor survival improved only when amended with zeolite or zeolite + fertilizer. Scrophularia bicolor proved to be a more efficient accumulator than P. lentiscus, especially for Pb uptake. Pistacia lentiscus accumulated metals mostly in the roots. The effect of amendments was to generally reduce the bioavailable metal fraction, especially lead, in the plots amended with compost. Pistacia lentiscus proved to be the most suitable species for phytostabilization and environmental restoration, both for its resistance to metals and high phytomass production. The experiments demonstrate that the use of compost not only encourages this kind of revegetation in degraded areas, but is also an economical option that uses a by-product of solid municipal waste treatment.
Trace elements in soils exist as components of several different fractions. We have analyzed the correlation between total and extractable (EDTA, calcium chloride and deionized water) Zn, Pb and Cu concentrations in soils and the concentration of these elements in plant leaves. Soil and plant samples have been taken from Sulcis-Iglesiente (Sardinia), an area rich in mining tailings. This has made that the concentrations of the trace element under study in soils were varied. Three plants have been studied: Dittrichia viscosa, Cistus salviifolius, and Euphorbia pithyusa subsp. cupanii. Soil samples beneath each of them at depths of 0–30 and 30–60 cm have been considered. The highest concentration of trace elements in the leaves of the studied species has been found for Zn. The calcium carbonate content and the crystalline and amorphous forms of iron in the soil have determined the concentration of metal in plant leaves. The soil concentrations that have been found with the extraction methods are uncorrelated with Pb and Cu concentrations in plants, but Zn is correlated with the fraction extracted with EDTA and calcium chloride. The concentrations of trace metals in plants are most closely related to the soil contents of CaCO3, electrical conductivity, Feox, and Fedc.
A stream sediment survey carried out in southern Iglesiente, southwestern Sardinia, comprised 249 samples which were analysed for 35 elements. The data from 83 samples derived from a previous survey by Ente Minerario Sardo were also used. Maps showing element distribution were created using the GIS program Arc View 3.1. For the compilation of potential risk maps, the regional distribution of the elements has been reclassified using as thresholds the Italian intervention criteria for residential/recreational and commercial/industrial land use. The main concentrations of heavy elements mainly follow the carbonate lithologies, hosting most of the ore deposits of Mississippi Valley and Sedimentary Exhalative types. Fairly high contents of Pb, Zn, Cd, and As are reflected in the stream sediments sampled along the carbonate ridges bordering the mining district along the Iglesias syncline. Treatment plants, tailings and dumps are also located in this area. Another area with high metal values is the swampy river mouth of Rio Sa Masa, near Gonnesa, draining not only the main mining sites of the Iglesias syncline, but also the small industrial areas around the town of Iglesias and a few villages. Potential risk maps have been compiled for four elements: Zn, Pb, Cd and As. Among those, the state of geochemical pollution is very serious for Cd, Pb, and Zn, which are the main elements contained in the stratabound orebodies, but less so for As. If even only partly transferred to the soils, the concentration in these elements should limit any further development in this area, unless remediation is carried out.
The mobilization of heavy metals by man through extraction from ores and processing for different applications has led to the release of these elements into the environment. Since heavy metals are nonbiodegradable, they accumulate in the environment and subsequently contaminate the food chain. This contamination poses a risk to environmental and human health. Some heavy metals are carcinogenic, mutagenic, teratogenic and endocrine disruptors while others cause neurological and behavioral changes especially in children. Thus remediation of heavy metal pollution deserves due attention. Different physical and chemical methods used for this purpose suffer from serious limitations like high cost, intensive labor, alteration of soil properties and disturbance of soil native microflora. In contrast, phytoremediation is a better solution to the problem. Phytoremediation is the use of plants and associated soil microbes to reduce the concentrations or toxic effects of contaminants in the environments. It is a relatively recent technology and is perceived as cost-effective, efficient, novel, eco-friendly, and solar-driven technology with good public acceptance. Phytoremediation is an area of active current research. New efficient metal hyperaccumulators are being explored for applications in phytoremediation and phytomining. Molecular tools are being used to better understand the mechanisms of metal uptake, translocation, sequestration and tolerance in plants. This review article comprehensively discusses the background, concepts and future trends in phytoremediation of heavy metals.
Mining activities generate spoils and effluents with extremely high metal concentrations of heavy metals that might have adverse
effects on ecosystems and human health. Therefore, information on soil and plant metal concentrations is needed to assess
the severity of the pollution and develop a strategy for soil reclamation such as phytoremediation. Here, we studied soils
and vegetation in three heavily contaminated sites with potential toxic metals and metalloids (Zn, Pb, Cd, As, TI) in the
mining district of Les Malines in the Languedoc region (southern France). Extremely high concentrations were found at different
places such as the Les Aviniéres tailing basins (up to 160,000mgkg–1 Zn, 90,000mgkg–1 Pb, 9,700mgkg–1 of As and 245mgkg–1 of Tl) near a former furnace. Metal contamination extended several kilometres away from the mine sites probably because of
the transport of toxic mining residues by wind and water. Spontaneous vegetation growing on the three mine sites was highly
diversified and included 116 plant species. The vegetation cover consisted of species also found in non-contaminated soils,
some of which have been shown to be metal-tolerant ecotypes (Festuca arvernensis, Koeleria vallesiana and Armeria arenaria) and several Zn, Cd and Tl hyperaccumulators such as Anthyllis vulneraria, Thlaspi caerulescens, Iberis intermedia and Silene latifolia. This latter species was highlighted as a new thallium hyperaccumulator, accumulating nearly 1,500mgkg–1. These species represent a patrimonial interest for their potential use for the phytoremediation of toxic metal-polluted
areas.
KeywordsSoil pollution–Hyperaccumulation–Phytostabilisation–Mine tailings–Metal-tolerant species
Work carried out at the abandoned copper (Cu) and sulphur (S) mine at Avoca (south east Ireland) has shown acid mine drainage
(AMD) to be a multi-factor pollutant. It affects aquatic ecosystems by a number of direct and indirect pathways. Major impact
areas are rivers, lakes, estuaries and coastal waters, although AMD affects different aquatic ecosystems in different ways.
Due to its complexity, the impact of AMD is difficult to quantify and predict, especially in riverine systems. Pollutional
effects of AMD are complex but can be categorized as (a) metal toxicity, (b) sedimentation processes, (c) acidity, and (d)
salinization. Remediation of such impacts requires a systems management approach which is outlined. A number of working procedures
which have been developed to characterise AMD sites, to produce surface water quality management plans, and to remediate mine
sites and AMD are all discussed.
This study assessed the distribution and availability of plant uptake of Zn, Pb, and Cd present in an abandoned mine at Ingurtosu, Sardinia (Italy). Geological matrix samples (sediments, tailings, and soil from a nearby pasture site) and samples of the predominant plant species growing on sediments and tailings were collected. Mean values of total Zn, Pb and Cd were respectively (mg kg(-1)) 7400, 1800, and 56 in tailings, 31000, 2900, and 100 in sediments, and 400, 200, and 8 in the pasture soil. The metal concentration values were high even in the mobile fractions evaluated by simplified sequential extraction (Zn 7485-103, Pb 1015-101, Cd 47-4 mg kg(-1)). Predominant native species were identified and analyzed for heavy metal content in various tissues. Among the plant species investigated Inula viscosa, Euphorbia dendroides, and Poa annua showed the highest metal concentration in aboveground biomass (mean average of Zn: 1680, 1020, 1400; Pb: 420, 240, 80; Cd: 28, 7, 19 mg kg(-1), respectively). The above mentioned species and A. donax could be good candidates for a phytoextraction procedure. Cistus salvifolius and Helichrysum italicus generally showed behavior more suitable for a phytostabilizer.
Although forest biomass estimates are vital for estimating carbon fluxes, the database for estimating the biomass or carbon pool in Mediterranean shrub ecosystems is lacking. This paper reports the above-ground phytomass estimates obtained for shrub ecosystems in Southern Spain. The phytomass values obtained were 1143 g m –2 for heath, 447 g m –2 for rock rose thicket-heath, 788 g m –2 for rock rose thicket and 1404 g m –2 for gorse thicket formations; the corresponding values for bushes were 695 g m –2 for mancha phytomass and 1966 g m –2 for Pistacia lentiscus formations. The photosynthetic-to-total phytomass ratio ranged from 0.156 for mancha highbush to 0.213 in rock rose thickets, with a mean value of 0.187 for the study ecosystems as a whole. The estimates of sequestered carbon differed between the shrub-type ecosystems in the interval between 2.23 t ha –1 for Cistus ladanifer to 9.83 t ha –1 for Pistacia lentiscus. A better understanding of biomass in Mediterranean shrub communities will provide useful information on the growth pattern of these species, biomass mapping, remote sensing and regional estimations of primary productivity in these areas.
Abandoned metalliferous mine wastes can result in severe pollution and have aesthetic impacts on the local environment. Use of a vegetation cover gives a cost-effective and environmentally sustainable method of stabilising and reclaiming wastes such as mine-spoils and tailings. Many characteristics of metalliferous wastes are often inimical to successful vegetation establishment, most notably phytotoxic levels of residual heavy metals, low nutrient status and poor physical structure of the substratum. Current approaches to revegetation and reclamation involve both ameliorative and adaptive strategies to allow plant establishment and encourage subsequent vegetation development. Different techniques of revegetation are available for temperate and arid, subtropical regions depending on the characteristics of the waste. These include direct seeding with commercially available plants, use of cover and barrier systems and the enhancement of natural revegetation processes.
In this paper the results of a recent characterization of Rio Piscinas (SW of Sardinia, Italy) hydrological basin are reported. In such area (about 50 km2), previous mining activities caused a serious heavy metal contamination of surface waters, groundwater, soils and biota. Acid mine drainage phenomena were observed in the area. The main sources of contamination are the tailings stored in mine tunnels and abandoned along fluvial banks. A methodological approach was adopted in order to identify relations between tailings and water contamination. Representative samples of tailings and stream sediments samples were collected. XRD analyses were performed for mineralogical characterization, while acid digestion was carried out for determining metal contents. Batch sequential leaching tests were performed in order to assess metal mobility. Also groundwater and stream water were sampled in specific locations and suitably characterized. All information collected allowed the understanding of the effect of tailings on water contamination, thus contributing to the qualitative prediction of pollution evolution on the basis of metal mobility. Finally, a potential remediation strategy of stream water is proposed.
This paper reviews the ecological aspects of mined soil restoration, with special emphasis on maintaining a long-term sustainable vegetation on toxic metal mine sites. The metal mined soils are man-made habitats which are very unstable and will become sources of air and water pollution. Establishment of a vegetation cover is essential to stabilize the bare area and to minimize the pollution problem. In addition to remediate the adverse physical and chemical properties of the sites, the choice of appropriate vegetation will be important. Phytostabilization and phytoextraction are two common phytoremediation techniques in treating metal-contaminated soils, for stabilizing toxic mine spoils, and the removal of toxic metals from the spoils respectively. Soil amendments should be added to aid stabilizing mine spoils, and to enhance metal uptake accordingly.
Unreclaimed mine tailings sites are a worldwide problem, with thousands of unvegetated, exposed tailings piles presenting a source of contamination for nearby communities. Tailings disposal sites in arid and semiarid environments are especially subject to eolian dispersion and water erosion. Phytostabilization, the use of plants for in situ stabilization of tailings and metal contaminants, is a feasible alternative to costly remediation practices. In this review we emphasize considerations for phytostabilization of mine tailings in arid and semiarid environments, as well as issues impeding its long-term success.
We reviewed literature addressing mine closures and revegetation of mine tailings, along with publications evaluating plant ecology, microbial ecology, and soil properties of mine tailings.
Data were extracted from peer-reviewed articles and books identified in Web of Science and Agricola databases, and publications available through the U.S. Department of Agriculture, U.S. Environmental Protection Agency, and the United Nations Environment Programme.
Harsh climatic conditions in arid and semiarid environments along with the innate properties of mine tailings require specific considerations. Plants suitable for phytostabilization must be native, be drought-, salt-, and metal-tolerant, and should limit shoot metal accumulation. Factors for evaluating metal accumulation and toxicity issues are presented. Also reviewed are aspects of implementing phytostabilization, including plant growth stage, amendments, irrigation, and evaluation.
Phytostabilization of mine tailings is a promising remedial technology but requires further research to identify factors affecting its long-term success by expanding knowledge of suitable plant species and mine tailings chemistry in ongoing field trials.
This study is part of research work aimed atidentifying zeolitic occurrences of economic interest in Sardinia. The aim of the present work is to define genetic models for zeolite occurring in Tertiary pyroclastic flows outcropping in northern Sardinia (Logudoro region). The sample area, covering about 1000 km2, includes the villages of Ozieri, Ploaghe, Ittireddu, Chilivani, Mores and Bonorva. The most important volcanic formations, outcropping here, include theoldest manifestations (24-15 Ma) ofoligo-Miocene orogenic magmatism in Sardinia. Tertiary volcanics include poorly welded (ILP and IC) and welded (IS) ash and pumice flows; epiclastic (EV and EN) and base surge (BS) deposits are locally interbedded. Field research, devoted to lithostratigraphic reconstruction of outcropping suites, also showed numerous degassing pipes inside ignimbritic rocks. Laboratory researchidentified various rock components (matrix, phenocrysts, pumice and xenoliths) and modal analyses were also carried out near Bonorva. The distributive and textural features of secondary minerals (zeolite, smectite, silica minerals and glauconite) were also examined. Microscopic (optical and SEM) data defined the following crystallization order: smectite → clinoptilolite → opal-CT → mordenite. The most common zeolitic variety is clinoptilolite, which: 1) fills vugs and degassing pipe structures; 2) lines both tubular and sub-spherical vesicles in pumice; 3) grows on cuspate glassy fragments; 4) intergrows with silica minerals in patches randomly distributed in the matrix; 5) constitutes the felt-form intergrowths of well-developed crystals growing on the matrix; 6) lines veins which, in the central part, are filled with anhedral calcitic aggregates. The overall data set suggests the following three genetic models for this zeolitic material: a) crystallization from fluids entrapped inside sub-spherical vesicles hosted in pumiceous fragments; b) crystallization linked to polyphase tranformation of glassy components by juvenile fluids and c) crystallization linked to interaction processes on glassy components by external fluids.
Phytoremediation is a group of technologies that use plants to reduce, degrade, or immobilize environmental toxins, primarily those of anthropogenic origin, with the aim to clean-up contaminated areas. In this review paper, different types of phytoremediation processes, typical plants used and their application for clean-up of metal contaminated sites were reviewed. Plant responses to heavy metals and mechanisms of metal uptake and transport were also discussed. Phytoremediation of Pb, Zn, Cu and Fe tailings and mine spoils were carried out by grasses, herbs and shrubs, which could be categorised as As accumulator (Paspalum, Eriochloa, Holcus, Pennisetum Juncus, Scirpus and Thymus), Pb accumulator (Brassica juncea, Vetiveria, Sesbania, Minuartia, Juncus, Scirpus and Thymus), Cu accumulator (Ammania baccifera, Scleranthus) Zn and Cd accumulator (Vetiveria, Sesbania, Viola, Sedum, Rumex). The research work showed that, bioavailability and metal uptake by plants could be accomplished by ameliorating pH, addition of organic amendment, fertiliser and chelating agents. Further research is required to develop fast growing high biomass plants with improved metal uptake ability, increased translocation and tolerance of metals through genetic engineering for effective phytoremediation of metal mine wastes.
The aim of this research was to identify wild plant species applicable for remediation of mine tailings in arid soils. Plants growing on two mine tailings were identified and evaluated for their potential use in phytoremediation based on the concentration of potentially toxic elements (PTEs) in roots and shoots, bioconcentration (BCF) and translocation factors (TF). Total, water-soluble and DTPA-extractable concentrations of Pb, Cd, Zn, Cu, Co and Ni in rhizospheric and bulk soil were determined. Twelve species can grow on mine tailings, accumulate PTEs concentrations above the commonly accepted phytotoxicity levels, and are suitable for establishing a vegetation cover on barren mine tailings in the Zimapan region. Pteridium sp. is suitable for Zn and Cd phytostabilization. Aster gymnocephalus is a potential phytoextractor for Zn, Cd, Pb and Cu; Gnaphalium sp. for Cu and Crotalaria pumila for Zn. The species play different roles according to the specific conditions where they are growing at one site behaving as a PTEs accumulator and at another as a stabilizer. For this reason and due to the lack of a unified approach for calculation and interpretation of bioaccumulation factors, only considering BCF and TF may be not practical in all cases.
The aim of this study was to identify metal-tolerant plants with potential application in phytoremediation strategies. For this we evaluated the distribution and chemical fractionation of heavy metals in soils and their accumulation or exclusion by native plant species growing in an abandoned Pb/Zn mine in NW Spain. Mine-soils (I = 0–20 cm, II = 20–40 cm) and shoots of the dominant plants were sampled at 39 sampling points in a (100 m × 100 m) regular grid at two sites (mine zone (MZ) and tailing ponds (TP)).
A finite element code was used for investigating the effect of some relevant characteristics of a phytoremediation project (crop type and density, presence of an irrigation system, soil capping and root depth). The evolution of the plume of contamination of Cd2+, Pb2+ and Zn2+ was simulated taking into account reactive transport and root processes. The plant contaminant uptake model was previously calibrated using data from greenhouse experiments. The simulations adopted pedological and climatological data representative of a sub-tropical environment. Although the results obtained were specific for the proposed scenario, it was observed that, for more mobile contaminants, poor water conditions favour stabilization but inhibit plant extraction. Otherwise an irrigation system that decreases crop water stress had an opposite effect. For less mobile contaminants, the remediation process did not have appreciable advantages. Despite its simplifying assumptions, particularly about contaminant sorption in the soil and plant system, the numerical analysis provided useful insight for the phytoextraction process important in view of field experiments.
Extensions of land in which the degree of pollution is so high that they are defined contaminated sites are very vast in all the industrialized countries. The need to mantein unaltered the indispensable protective action that the soil exerts has become one of the central points of much environmental legislation which has been introduced in the latest years. The presence of heavy metals in contaminated soils represents one of the most important environmental hazards. A knowledge of the total concentration of heavy metals in a contaminated soil could be considered a starting point for evaluating the degree of pollution. Chemical tests modified from sludge treated soil studies, are able to quantify the mobile fractions in a highly polluted soil, and, if inserted into a proper sampling strategy can supply a kind of "finger print" useful for a good choice of clean-up procedures. After remediation treatments, by the same test it is possible to determine their effect on soil quality and vulnerability. In this paper we present a procedural approach for characterizing pollution in sites contaminated by heavy metal . Trigger and intervention limits are defined. These are based on a knowledge of the concentrations both of the total and of the mobile species typical of Italian soils.
In the last two centuries, several Pb–Zn mines were active in the Rio Mannu basin near Narcao (SW Sardinia, Italy), but are now abandoned. These abandoned mines pose a serious risk to the population and required an assessment of the hazard sources and the contaminant pathways in the area. The characterization plan of the Rio Mannu basin was carried out according to Italian protocols. Specifically, samples of soil, stream sediment, fine-grained ore-processing waste (from washery and flotation processes), surface water and groundwater have been analyzed in order to assess the levels of contamination in each mine area and the risks in the plains surrounding the site. Several potential chemical contaminants were considered both in solid materials and water samples. The Rosas mine was recognized as the most contaminated area in the basin, due to the presence of a large tailing pond, some fine-grained waste piles and two mine adits with concentrations of the toxic elements As, Cd, Cu, Pb, Sb and Zn exceeding Italian regulatory standards. The dispersion of contaminants occurs downstream from the small stream draining the area. In case of heavy rain the runoff into the streamlet transports the contaminated material far into the plain. The results of this study show that the characterization plan is a relatively cheap tool for establishing mitigation actions, prior to the realization of a complete, and usually expensive, remediation project at abandoned mine sites. Urgent recommended actions in the Rio Mannu basin include the treatment of the adit water prior to its discharge into the Rio Barisonis; the construction of drainage barriers on waste piles to reduce runoff and solid transport into the Rio Barisonis; the consolidation of the earthen dam containing the tailing pond of Rosas to avoid the deepening of erosion furrows that may threaten its stability.
Plants from Cistus genus show a great plasticity and are able to grow both in contaminated and non-contaminated soils. Cistus salviifolius L. was identified in several mines from Portuguese Iberian Pyrite Belt (PIPB) while Cistus populifolius L. was collected in 2009 in Chança and Caveira mines for the first time. In these mine areas, a hybrid of both species Cistus × hybridus Pourr. was also collected being this its first report from those areas. This study aimed to compare the biogeochemical behavior of the three Cistus species growing in the polymetallic mine areas of Caveira, Chança and São Domingos (PIPB). Their possible use in phytoremediation programs of multielemental contaminated soils was evaluated.
Metal mining in Caveira and São Domingos (Portugal) generated large volumes of tailings with high total concentrations of trace elements, low pH and fertility. Autochthones plants growing spontaneously in mine areas are usually tolerant to unfavorable characteristics of tailings. The aim of this investigation was to analyze the behavior (trace elements uptake, accumulation and translocation from roots to shoots) of different populations of Cistus salviifolius plants growing spontaneously in contaminated and non-contaminated sites in order to evaluate the potential of this plant species for soils and mine wastes phytostabilization. Cistus salviifolius plants (roots and aerial parts) and soils were collected, in two mine areas (São Domingos and Caveira) and a reference area (Grândola), and were analyzed for arsenic, antimony, copper, lead, manganese and zinc concentrations. Cistus salviifolius showed tolerance for moderate acid soils (5.34
Metaphors, such as those used in the title of this article, are often useful for the comprehension of specialised topics in plant biology. A brief attempt is made to elucidate one of these metaphors, plant “intelligence”, as it relates to the plastic responses of roots and root systems to their environment. Tropisms and nastic movements of root apices are two expressions of an inherent plasticity of form exhibited by roots. In soil, roots are exposed to multiple stimuli, many of which can potentially elicit such movements. Hence, a key question is how roots respond to and process the different stimuli which simultaneously reach their surfaces. Assuming that roots always use the same site along their length to express their movement responses, and that they also use an auxin‐based information‐transduction pathway, the most evident choices for the processing of stimuli are that roots either prioritise the various incoming stimuli and respond only to the strongest or they amalgamate stimuli and mount an averaged compromise response to all of them. The proposal that plants may be “intelligent”, especially in respect to their plastic growth responses, is one that draws upon knowledge of this faculty from animal biology. Also implied is that plants and animals are sufficiently similar to share usage of this term “intelligence”. But an alternative view is that plants and animals are sufficiently different and so intelligence is an unfitting term. Following the line of enquiry into creative evolution initiated by Henri Bergson, plants can be viewed differently to animals. The tendency of plants is towards instinctive behaviour rather than intelligent behaviour.
Hydrogeochemical surveys were carried out in SW Sardinia (Italy) to investigate the impact of past mining activities on the quality of groundwater. The chemistry of waters from flooded galleries, adits and dumps has been compared with that of springs and wells in the same area at sites relatively far from any mine legacy. A feature, common to all waters, is the circumneutral pH, since the carbonate formations in the area neutralise the acidity produced by the oxidation of Fe-bearing sulphide minerals in the mine impacted water. However, groundwater interacting with mine workings is degraded in quality; it shows high dissolved SO4, Zn, Cd and Pb contents. In some cases groundwater exceeds the limit established by the guidelines of the World Health Organization for Pb content in drinking water, so that groundwater is mixed before entering the local aqueducts. Results from this study suggest that more attention needs to be paid to the impact on the streams from contaminated water flowing out from some mine areas because during the dry season these streams are only fed by mine groundwater. We recommend focusing efforts to reduce the chemical contamination prior to discharge.
Remediation of sites contaminated with toxic metals is particularly challenging. Unlike organic compounds, metals cannot be degraded, and the cleanup usually requires their removal. However, this energy-intensive approach can be prohibitively expensive. In addition, the metal removing process often employs stringent physicochemical agents which can dramatically inhibit soil fertility with subsequent negative impacts on the ecosystem. Phytoremediation has been proposed as a cost-effective, environmental-friendly alternative technology. A great deal of research indicates that plants have the genetic potential to remove many toxic metals from the soil. Despite this potential, phytoremediation is yet to become a commercially available technology. Progress in the field is hindered by a lack of understanding of complex interactions in the rhizosphere and plant-based mechanisms which allow metal translocation and accumulation in plants. In this paper, four research areas relevant to metal phytoextraction from contaminated soil are reviewed. The review concludes with an assessment of the current status of technology deployment and suggestions for future phytoremediation research.
Please view the pdf by using the Full Text (PDF) link under 'View' to the left. Copyright © 2002. American Society of Agronomy, Crop Science Society of America, Soil Science Society . Published in J. Environ. Qual.31:109–120.
At the Monteponi Pb–Zn mine located in south-western Sardinia intensive dewatering has been carried out over about 100 a. A marked increase in the salinity and Hg concentration of deep groundwater occurred as the water table level was lowered from +15 to −160 m a.s.l. over this period. Closure of the mine implied the cessation of the expensive pumping system, and prompted the assessment of the contamination risk for the shallow groundwater system supplying Iglesias town. This study shows that deep water was mixed into the shallow aquifer as the water table level rose. An increase of salinity, mainly due to Na and Cl−, has been observed in shallow groundwater. The input of Cl− facilitates the dissolution of Hg. Moreover, the progressive mine flooding is causing an increase of other dissolved metals, mainly due to the weathering of primary sulphides and secondary minerals present in the ore, and remobilization of metals in the mine waste left in the galleries. A stratification process will allow the saline water at depth to settle, and this is expected to occur in a relatively short time (few years). The leaching of metals represents a greater concern for the quality of shallow groundwater, and the time necessary to clean up will be much longer (probably several decades) than that expected for the stratification of the water body.
Several native plants, able to grow in an unconfined mining impacted area that is now in close vicinity with urban areas, were evaluated for their ability to accumulate heavy metals. The main soil contaminants were As, Pb, Cu, and Zn. Sampling of the rhizospheric metal polluted soil showed that Euphorbia prostrata Aiton, Parthenium incanum Kunth, and Zinnia acerosa (DC.) A. Gray were able to grow in the presence of high amounts of mixtures of these elements. The plants accumulated the metals in the above ground parts and increased the synthesis of thiol molecules. E. prostrata showed the highest capacity for accumulation of the mixture of elements (588 μg g DW(-1)). Analysis of the thiol-molecules profile showed that these plants synthesized high amounts of long-chain phytochelatins, accompanied by low amounts of monothiol molecules, which may be related to their higher resistance to As and heavy metals. The three plants showed translocation factors from roots to leaves >1 for As, Pb, Cu, and Zn. Thus, by periodically removing aerial parts, these plants could be useful for the phytoremediation of semi-arid and arid mining impacted areas, in which metal hyper-accumulator plants are not able to grow.
The vegetation of mining dumps of SW‐Sardinia includes several endemic or rare species, that have been confined to such artificial habitats by a millennial mining activity. The extreme edaphic specialization of these species reflects with unusual fidelity the heterogeneity of substrata, therefore creating a system of plant communities that largely contributes to the environmental diversity of this part of the island. The phytosociological study presented in this paper recognized eight associations, distinguished by the cluster analysis in two main groups, the first including hemicrypto‐chamaephytic vegetation, the second including nanophanerophytic garigues.
Six associations are belonging to the first group, five of which newly described: Helichryso tyrrhenici‐Dianthetum sardoi ass. nova occurring on gross‐grained, hard‐sloping dumps deriving from the geologic unit of Gonnesa; Coincyo recurvatae‐Helichrysetum tyrrhenici ass. nova colonizing dumps made of metamorphitic flakes and finer particles; Resedo luteolae‐Limonietum merxmuelleri ass. nova colonizing small‐grained mining dumps with high concentrations in sulphides and heavy metals ; Ptilostemono casabonae‐Iberidetum integerrimae ass. nova replacing the previous one on older and more consolidated dumps, where the concentration of phytotoxic elements was reduced by the meteoric agents; Epipactidetum tremolsii ass. nova colonizing muddy deposits originating from ferrous clay; Euphorbio cupanii‐Santolinetum insularis Angiolini & Bacchetta 2003, occurring on very old, stabilized dumps. All these associations can be ascribed to the class Scrophulario‐Helichrysetea Brullo, Scelsi & Spampinato 1998. Within this class, a new Sardo‐Corsican alliance, Ptilostemono casabonae‐Euphorbion cupanii all. nov. is here proposed, differentiated by a pool of rare or endemic species. In addition to the above‐mentioned associations, it is proposed, as well, to include into the new alliance the Polygono scoparii‐Helichrysetum tyrrhenici Biondi, Vagge, Fogu & Mossa 1996 corr., that was described for the gravelly riverbeds of Central‐Southern Sardinia.
As concerns the second group, two new association have been recognized: Dorycnio suffruticosi‐Genistetum corsicae ass. nova, found on abandoned, well consolidated mining dumps, with an upper layer altered by pedogenetic processes; Polygalo sardoae‐Linetum muelleri ass. nova, occurring in the vicinities of the mines on steep rocky slopes made of Paleozoic metalliferous limestones. Both of them can be ascribed to the alliance Teucrion mari Gamisans & Muracciole 1985, grouping the Sardo‐Corsican aspects of Cisto‐Lavanduletea Br.‐Bl . in Br.‐Bl., Molinier & Wagner 1940.
Two taxa are new for science: Dianthus sardous Bacchetta, Brullo, Casti & Giusso , sp. nova and Helichrysum microphyllum ( Willd.) Camb . subsp. tyrrhenicum Bacchetta, Brullo & Giusso , subsp. nova. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
We have analyzed the relationship between total Zn, Pb and Cu concentrations in the soil and the capacity
of three plant species to accumulate these elements in their leaves. The study was carried out in a highly
contaminated area at Sulcis-Iglesiente (SW-Sardinia, Italy). We took samples of the leaves of Dittrichia
viscosa, Cistus salviifolius and Euphorbia pithyusa subsp. cupanii and samples of the soil beneath each of
them at depths of 0–30 and 30–60 cm, both in contaminated mine tailings and surrounding areas. Due to
the anthropic origin of the soil materials the results varied considerably. Bioavailability of trace elements
was mainly related to the calcium-carbonate content and the crystalline and amorphous forms of iron in
the soil. The concentration of Zn in the leaves of the three plant species studied was highest, followed by
Pb and finally Cu. The leaves of Dittrichia viscosa contained the highest concentrations of trace elements
and this species may be considered as being a “phytoextractor” in soils where the trace-element concen-
trations are not too high. Euphorbia pithyusa subsp. cupanii had low trace-element concentrations in its
leaves despite growing in highly contaminated soils, and so might be used as a “phytostabilizer”. Although
Cistus salviifolius does not grow in the most contaminated soils, could be considered as a contamination
indicator up to a given level.
BACKGROUND: The application of phytostabilization and assisted phytoextraction to the remediation of abandoned mining areas can be a valuable method to reclaim these areas without modifying soil and landscape characteristics. An in situ application of a continuous phytoextraction technique was carried out in the area of Campo Pisano (Sardinia, Italy), followed by a laboratory assisted phytoextraction test using the biodegradable chelating agents methylglycine diacetic acid (MGDA) and iminodissuccinic acid (IDSA). The plants used were Scrophularia canina subsp. bicolor, Cistus salviifolius and Teucrium flavum subsp. glaucum.RESULTS: The plant that accumulated more Pb was T. glaucum (353 mg kg−1) while C. salviifolius demonstrated better ability to accumulate Zn (1560 mg kg−1). S. bicolor showed a better tolerance to metals but accumulated 119 mg kg−1 of Pb. Accumulation of metals immediately after chelant application was up to 300 mg kg−1 of Pb and 3000 mg kg−1 of Zn which did not further increase during the assisted phytoextraction experiment.CONCLUSION: The plant that demonstrated to be most suitable for phytoremediation application was S. bicolor due to its higher biomass production and tolerance to metals. The low cation exchange capacity and the high concentration of Ca and Mg in soil determined a low chelant effectiveness. Copyright © 2009 Society of Chemical Industry
In consideration of their origin the adaptive strategies of the evergreen species of the Mediterranean maquis were analysed. Rosmarinus officinalis L., Erica arborea L., and Erica multiflora L. had the lowest net photosynthetic rate (P
N) in the favourable period [7.8±0.6 μmol(CO2) m−2s−1, mean value], the highest P
N decrease (on an average 86 % of the maximum) but the highest recovery capacity (>70 % of the maximum) at the first rainfall in September. Cistus incanus L. and Arbutus unedo L. had the highest P
N during the favourable period [15.5±5.2 μmol(CO2) m−2s−1, mean value], 79 % decrease during drought, and a lower recovery capacity (on an average 54 %). Quercus ilex L., Phillyrea latifolia L., and Pistacia lentiscus L. had an intermediate P
N in the favourable period [9.2±1.3 μmol(CO2) m−2s−1, mean value], a lower reduction during drought (on an average 63 %), and a range from 62 % (Q. ilex and P. latifolia) to 39 % (P. lentiscus) of recovery capacity. The Mediterranean species had higher decrease in P
N and stomatal conductance during drought and a higher recovery capacity than the pre-Mediterranean species. Among the pre-Mediterranean species, P. latifoliahad the best adaptation to long drought periods also by its higher leaf mass per area (LMA) which lowered leaf temperature thus decreasing transpiration rate during drought. Moreover, its leaf longevity determined a more stable leaf biomass during the year. Among the Mediteranean species, R. officinalis was the best adapted species to short drought periods by its ability to rapidly recover. Nevertheless, R. officinalis had the lowest tolerance to high temperatures by its P
N dropping below half its maximum value when leaf temperature was over 33.6°C. R. officinalismay be used as a bioindicator species of global change.
Different plant communities have established spontaneously on Sanmen Pb/Zn mine tailing. The site was inspected and four different
plant communities were identified according to their species composition. To understand the effects of different communities
on mine tailing physico-chemical properties, a community survey was carried out in Sanmen Pb/Zn mine tailing, and the physico-chemical
properties and heavy metal (Cu, Pb, Cd and Zn) distribution of mine tailings were determined. Results showed that there were
four types of communities (I, II, III and IV) in Sanmen Pb/Zn mine tailing. From community I to IV, the number of plant species
and community characteristics (aboveground biomass, underground biomass, coverage and height) consistently increased. Moreover,
the nutrient pool and physico-chemical properties of mine tailing consistently reestablished from community I to IV, while
the total heavy metal content consistently decreased. The contents of residual fractions, Fe–Mn oxide fractions for Pb, Zn,
Cu and Cd and exchangeable fractions for Pb and Zn also consistently decreased. However, the contents of organically bound
fraction had no obvious change from community I to IV. Moreover, the contents of Cu organically bound fraction reversely increased.
Results demonstrate that communities I, II, III and IV should be a progressive community succession. Moreover, along with
the progressive community succession, phytostabilization and phytoextraction of mine tailings are more and more effective.
KeywordsHeavy metal fraction–Mine tailing–Physico-chemical properties–Plant communities
This study aims to assess the extent of metal accumulation by plants found in a mining area in Hamedan Province in the central west part of Iran. It also investigates to find suitable plants for phytoextraction and phytostabilization as two phytoremediation strategies. Plants with a high bioconcentration factor (BCF) and low translocation factor (TF) have the potential for phytostabilization while plants with both BCFs and TFs greater than one have the potential to be used for phytoextraction. In this study, shoots and roots of the 12 plant species and the associated soil samples were collected. The collected samples were then analyzed by measurement of total concentrations of trace elements (Pb, Zn, Mn and Fe) using atomic absorption spectrophotometer. Simultaneously, BCF and TF parameters were calculated for each element. Results showed that although samples suitable for phytoextraction of Pb, Zn, Mn and Fe and phytostabilization of Fe were not detected, Scrophularia scoparia was the most suitable for phytostabilization of Pb, Centaurea virgata,
Echinophora platyloba and Scariola orientalis had the potential for phytostabilization of Zn and Centaurea virgata and Cirsium congestum were the most efficient in phytostabilization of Mn. Present study showed that native plant species growing on contaminated sites may have the potential for phytoremediation.
Phytoremediation is an emerging technology for the remediation of mine tailings, a global problem for which conventional remediation
technologies are costly. There are two approaches to phytoremediation of mine tailings, phytoextraction and phytostabilization.
Phytoextraction involves translocation of heavy metals from mine tailings to the plant shoot biomass followed by plant harvest,
while phytostabilization focuses on establishing a vegetative cap that does not shoot accumulate metals but rather immobilizes
metals within the tailings. Phytoextraction is currently limited by low rates of metal removal which is a combination of low
biomass production and insufficiently high metal uptake into plant tissue. Phytostabilization is currently limited by a lack
of knowledge of the minimum amendments required (e.g., compost, irrigation) to support long-term plant establishment. This
review addresses both strategies within the context of two specific climate types: temperate and arid. In temperate environments,
mine tailings are a source of metal leachates and acid mine drainage that contaminate nearby waterways. Mine tailings in arid
regions are subject to eolian dispersion and water erosion. Examples of phytoremediation within each of these environments
are discussed. Current research suggests that phytoextraction, due to high implementation costs and long time frames, will
be limited to sites that have high land values and for which metal removal is required. Phytostabilization, due to lower costs
and easier implementation, will be a more commonly used approach. Complete restoration of mining sites is an unlikely outcome
for either approach.
Phytoextraction is a green in situ technology which aims to diminish the concentration of the chemical element(s) (often synonymous with heavy metals) of contaminated soils to such a level that the soil can be used without danger for agriculture, horticulture, forestry or amenity.To achieve this goal within a reasonable time, growth of highly metal-accumulating plants, the so-called hyperaccumulators is often recommended, ignoring their inherent low biomass production, their low ability to extract more than one or two metals and their sensitivity to other elements inclusive metals in surplus. It has been demonstrated that the success of phytoextraction depends on the degree of soil contamination and on the number of metals in surplus at the site, the metal-resistance of the plant species and its association with mycorrhizal fungi and other rhizosphere organisms, and the desired degree of the final metal concentration. Up to now, transgenic higher plants have shown no advantage in the phytoextraction process, compared to the high potential of genetically not modified plant species. Phytoextraction demands that the metals are extracted from the harvested plant material by ashing, so that the ash can be deposed or even better recycled in metal-processing industries. Disposal of the contaminated plant material without further condensation of the biomass at storage sites is not a solution, but only a transfer of the problem from one contaminated site to another, often less contaminated site. Phytovolatilization after phytoextraction is nothing more than a pollution transfer to an unpolluted environment.As soon as a soil is highly contaminated with more than one metal (polymetallic soils), the phytoextraction procedure will last more than a centennium, thus this period is too long for an economically feasible technology. At such sites, phytostabilization by revegetation is the most relevant technology in order to ensure a stabilization of the metals in the soil, and to hamper the contamination of groundwater and/or the surrounding landscape by wind and/or water erosion of the mine waste.
A laboratory weathering study using a humidity cell procedure was conducted on two sulfide-bearing tailing samples from a metallurgical site in Ontario (Canada). The test was accompanied by microbiological studies to enumerate the major groups of sulfur-oxidizing bacteria and determine their potential role at different stages during the oxidation process. To evaluate the utility of this method, results were compared with those of previous laboratory and field studies on the same materials. The mineralogy of the laboratory samples differs only by the addition of a small amount of hydronium-bearing natrojarosite [(Na,H3O)Fe3(SO4)2(OH)6] to one sample. The progress of sulfide oxidation and the rates of solute release were determined to evaluate the extent of mineral dissolution. These processes were influenced strongly by the capacity of the material to generate acidity, which was enhanced by the presence of hydronium-bearing natrojarosite. Acid-neutralization processes occurring during the laboratory tests were affected by reaction kinetics, consistent with field observations. In particular, the extent of carbonate-mineral dissolution appears to be different in the laboratory than in the field, where more prolonged rock–water interaction allowed more complete chemical equilibration. As a consequence, the capacity of this test procedure to predict weathering reactions in mine tailings is limited by its inability to reproduce the weathering sequence observed in the field. The results of the microbiological study showed that distinct groups of sulfur-oxidizing bacteria operate at different stages of the oxidative process, as was observed in field studies where tailings oxidation occurred under natural conditions, suggesting that microbiological tests performed for laboratory studies are reflective of field conditions.
Metals including lead, chromium, arsenic, zinc, cadmium, copper and mercury can cause significant damage to the environment and human health as a result of their mobilities and solubilities. The selection of the most appropriate soil and sediment remediation method depends on the site characteristics, concentration, types of pollutants to be removed, and the end use of the contaminated medium. The approaches include isolation, immobilization, toxicity reduction, physical separation and extraction. Many of these technologies have been used full-scale. This paper will review both the full-scale and developing technologies that are available. Contaminants can be isolated and contained to minimize further movement, to reduce the permeability of the waste to less than 1×10−7 m/s (according to U.S. guidelines) and to increase the strength or bearing capacity of the waste. Physical barriers made of steel, cement, bentonite and grout walls can be used for isolation and minimization of metal mobility. Another method is solidification /stabilization, which contains the contaminants in an area by mixing or injecting agents. Solidification encapsulates contaminants in a solid matrix while stabilization involves formation of chemical bonds to reduce contaminant mobility. Another approach is size selection processes for removal of the larger, cleaner particles from the smaller more polluted ones. To accomplish this, several processes are used. They include: hydrocyclones, fluidized bed separation and flotation. Addition of special chemicals and aeration in the latter case causes these contaminated particles to float. Electrokinetic processes involve passing a low intensity electric current between a cathode and an anode imbedded in the contaminated soil. Ions and small charged particles, in addition to water, are transported between the electrodes. This technology have been demonstrated in the U.S. full-scale, in a limited manner but in Europe, it is used for copper, zinc, lead, arsenic, cadmium, chromium and nickel. The duration of time that the electrode remains in the soil, and spacing is site-specific. Techniques for the extraction of metals by biological means have been not extensively applied up to this point. The main methods include bioleaching and phytoremediation. Bioleaching involves Thiobacillus sp. bacteria which can reduce sulphur compounds under aerobic and acidic conditions (pH 4) at temperatures between 15 and 55°C. Plants such as Thlaspi, Urtica, Chenopodium, Polygonum sachalase and Alyssim have the capability to accumulate cadmium, copper, lead, nickel and zinc and can therefore be considered as an indirect method of treating contaminated soils. This method is limited to shallow depths of contamination. Soil washing and in situ flushing involve the addition of water with or without additives including organic and inorganic acids, sodium hydroxide which can dissolve organic soil matter, water soluble solvents such as methanol, nontoxic cations, complexing agents such as ethylenediaminetetraacetic acid (EDTA), acids in combination with complexation agents or oxidizing/reducing agents. Our research has indicated that biosurfactants, biologically produced surfactants, may also be promising agents for enhancing removal of metals from contaminated soils and sediments.In summary, the main techniques that have been used for metal removal are solidification/stabilization, electrokinetics, and in situ extraction. Site characteristics are of paramount importance in choosing the most appropriate remediation method. Phytoremediation and bioleaching can also be used but are not as well developed.
The term “hyperaccumulator” describes a number of plants that belong to distantly related families, but share the ability to grow on metalliferous soils and to accumulate extraordinarily high amounts of heavy metals in the aerial organs, far in excess of the levels found in the majority of species, without suffering phytotoxic effects. Three basic hallmarks distinguish hyperaccumulators from related non-hyperaccumulating taxa: a strongly enhanced rate of heavy metal uptake, a faster root-to-shoot translocation and a greater ability to detoxify and sequester heavy metals in leaves. An interesting breakthrough that has emerged from comparative physiological and molecular analyses of hyperaccumulators and related non-hyperaccumulators is that most key steps of hyperaccumulation rely on different regulation and expression of genes found in both kinds of plants. In particular, a determinant role in driving the uptake, translocation to leaves and, finally, sequestration in vacuoles or cell walls of great amounts of heavy metals, is played in hyperaccumulators by constitutive overexpression of genes encoding transmembrane transporters, such as members of ZIP, HMA, MATE, YSL and MTP families. Among the hypotheses proposed to explain the function of hyperaccumulation, most evidence has supported the “elemental defence” hypothesis, which states that plants hyperaccumulate heavy metals as a defence mechanism against natural enemies, such as herbivores. According to the more recent hypothesis of “joint effects”, heavy metals can operate in concert with organic defensive compounds leading to enhanced plant defence overall.
This study was conducted to evaluate the phytoremediation and phytomining potential of 10 plant species growing naturally at one of the largest lead-zinc mines in Northern Vietnam. Total concentrations of heavy metals and arsenic were determined in the plant and in associated soil and water in and outside of the mine area. The results indicate that hyperaccumulation levels (mg kg(-1) dry weight) were obtained in Houttuynia cordata Thunb. (1140) and Pteris vittata L. (3750) for arsenic, and in Ageratum houstonianum Mill. (1130), Potamogeton oxyphyllus Miq. (4210), and P. vittata (1020) for lead. To the best of our knowledge, the present paper is the first report on metal accumulation and hyperaccumulation by H. cordata, A. houstonianum, and P. oxyphyllus. Based on the obtained concentrations of metals, bioconcentration and translocation factors, as well as the biomass of these plants, the two latter species and P. vittata are good candidates for phytoremediation of sites contaminated with arsenic and multi-metals. None of the collected plants was suitable for phytomining, given their low concentrations of useful metals (e.g., silver, gallium, and indium).
Studies were conducted to determine the best management practice for immobilisation of toxic Pb and Mn in soil and the interaction of these metal contaminants with the associated plants. The research protocol comprises addition of soil amendments to accelerate physico-chemically driven sorption processes and growth of appropriate plant species to reduce physiologically driven uptake of Pb and Mn. Lolium perenne L (perennial rye grass), Festuca rubra L (creeping red fescue) and Poa pratensis L (Kentucky blue grass) were tested in the presence of soil amendments (lime, phosphate and compost, both individually and in combination). The effectiveness of treatments in stabilizing metals was assessed on the basis of metal speciation in soil, partitioning of metals in plants, and metal uptake. Significant partitioning of Pb in immobile forms was noticed by the growth of P. pratensis and Mn by the growth of L. perenne. Lime application lowered plant Pb and Mn, while phosphate decreased plant Pb and increased plant Mn. Combined amendment addition resulted in a significant decrease in the exchangeable (mobile) metal fraction in soils growing Poa for Pb and in soils growing Lolium for Mn. EC(root) (ratio of root concentration to soil concentration) and EC(shoot) (ratio of shoot concentration to soil concentration) for Pb in Poa decreased by 72% and 60% with combined application of amendments, while the corresponding decreases for Mn in Lolium were 48% and 43%.
Phytoremediation is one of the cost-effective and environmental friendly technologies used to remove contaminants from contaminated soils, which has been intensively studied during the last decade. Presently, few economical and effective remediation methods are available for the remediation of Pb contaminated sites. This study was conducted to assess the potential of 19 plants growing on contaminated sites in Pb mine area. Plants and associated soil samples were collected and analyzed for total metal concentrations. While total soil Pb, Cu and Zn concentrations varied from 1,239 to 4,311, 36 to 1,020 and 240 to 2,380 mg/kg, those in the plant shoots ranged from 6.3 to 2,029, 20 to 570, and 36 to 690 mg/kg, respectively. Among the plants, we found that one cultivated crop (Ricinus communis L.) and two native species (Tephrosia candida and Debregeasia orientalis) have a great potential for phytoremediation of Pb contaminated soils, the Pb hyperaccumulation capacity of the 3 plants was found as the order: R. communis > D. orientalis > T. candida in the investigated area.
This paper presents an analysis of the site restoration techniques that may be employed in a variety of contaminated site cleanup programs. It is recognized that no single specific technology may be considered as a panacea for all contaminated site problems. An easy-to-use summary of the analysis of the important parameters that will help in the selection and implementation of one or more appropriate technologies in a defined set of site and contaminant characteristics is also included.
Contamination of heavy metals represents one of the most pressing threats to water and soil resources as well as human health. Phytoremediation can be potentially used to remediate metal-contaminated sites. This study evaluated the potential of 36plants (17species) growing on a contaminated site in North Florida. Plants and the associated soil samples were collected and analyzed for total metal concentrations. While total soil Pb, Cu, and Zn concentrations varied from 90 to 4100, 20 to 990, and 195 to 2200mg kg(-1), those in the plants ranged from 2.0 to 1183, 6.0 to 460, and 17 to 598mg kg(-1), respectively. None of the plants were suitable for phytoextraction because no hyperaccumulator was identified. However, plants with a high bioconcentration factor (BCF, metal concentration ratio of plant roots to soil) and low translocation factor (TF, metal concentration ratio of plant shoots to roots) have the potential for phytostabilization. Among the plants, Phyla nodiflora was the most efficient in accumulating Cu and Zn in its shoots (TF=12 and 6.3) while Gentiana pennelliana was most suitable for phytostabilization of sites contaminated with Pb, Cu and Zn (BCF=11, 22 and 2.6). Plant uptake of the three metals was highly correlated, whereas translocation of Pb was negatively correlated with Cu and Zn though translocation of Cu and Zn were correlated. Our study showed that native plant species growing on contaminated sites may have the potential for phytoremediation.
The restoration of heavy metal contaminated areas requires information on the response of native plant species to these contaminants. The sensitivity of most Mediterranean woody species to heavy metals has not been established, and little is known about phytotoxic thresholds and environmental risks. We have evaluated the response of four plant species commonly used in ecological restoration, Pinus halepensis, Pistacia lentiscus, Juniperus oxycedrus, and Rhamnus alaternus, grown in nutrient solutions containing a range of copper, nickel and zinc concentrations. Seedlings of these species were exposed to 0.048, 1 and 4 microM of Cu; 0, 25 and 50 microM of Ni; and 0.073, 25 and 100 microM of Zn in a hydroponic silica sand culture for 12 weeks. For all four species, the heavy metal concentration increased in plants as the solution concentration increased and was always higher in roots than in shoots. Pinus halepensis and P. lentiscus showed a higher capacity to accumulate metals in roots than J. oxycedrus and R. alaternus, while the allocation to shoots was considerably higher in the latter two. Intermediate heavy-metal doses enhanced biomass accumulation, whereas the highest doses resulted in reductions in biomass. Decreases in shoot biomass occurred at internal concentrations ranging from 25 to 128 microg g-1 of Zn, and 1.7 to 4.1 microg g( -1) of Cu. Nickel phytoxicity could not be established within the range of doses used. Rhamnus alaternus and J. oxycedrus showed higher sensitivity to Cu and Zn than P. halepensis and, especially, P. lentiscus. Contrasted responses to heavy metals must be taken into account when using Mediterranean woody species for the restoration of heavy metal contaminated sites.
Chelant-enhanced phytoextraction has received a lot of attention in the past decade. In theory, this technique could cleanse metal polluted soils by solubilizing contaminating metals, allowing them to be taken up by plants that would subsequently be removed from the site. We review the processes of metal solubilization, uptake by plants, and leaching during chelant-enhanced phytoextraction. A large excess of chelant is required to solubilize the target metal due to the co-solubilization of Ca and Fe. Chelated metals are taken up via the apoplastic pathway. Disruption of the Casparian Band is required to achieve the high shoot concentrations needed for phytoextraction. Therefore, adding chelants to a soil increases not only the total dissolved metal concentration but also changes the primary route of plant metal-uptake from the symplastic to the apoplastic pathway. Depending on metal, plant species, and chelant concentration, significant increases in metal uptake are likely. Soil solution chelate concentrations of at least several mM are required to induce appreciable shoot concentrations. A simple calculation reveals that at such soil solution concentrations plants will remove only a small fraction of the solubilized metals. Leaching, exacerbated by preferential flow processes, is unavoidable. Chelant-enhanced phytoextraction is therefore limited to areas where the connection with groundwater has been broken, orwhere leaching is unimportant. Chelant-enhanced phytoextraction may nonetheless have a role in enhancing the uptake of essential trace metals. Such a role warrants further investigations into the use of biodegradable chelants such as ethylenediaminedisuccinic acid (EDDS).
The removal of inorganic contaminants by plants is termed phytoextraction. Recent studies have looked at the feasibility of phytoextraction, and demonstrate that both good biomass yields and metal hyperaccumulation are required to make the process efficient. Adding chelating agents to soil to increase the bioavailability of contaminants can sometimes induce hyperaccumulation in normal plants, but may produce undesirable environmental risks. Thus, it is necessary to investigate the mechanisms responsible for hyperaccumulation, using natural hyperaccumulators as model plant species. Recent advances have been made in understanding the mechanisms responsible for hyperaccumulation of Zn, Cd, Ni and As by plants. Attempts to engineer metal tolerance and accumulation have so far been limited to Hg, As and Cd, and although promising results have been obtained they may be some way from practical application. More fundamental understanding of the traits and mechanisms involved in hyperaccumulation are needed so that phytoextraction can be optimised.
Rhizon soil solution samplers as artificial roots
- F Meijboom
- M Van Noordwijk
Meijboom, F. and van Noordwijk, M. (1992). Rhizon soil solution samplers as artificial
roots. In Hübl E. Root ecology and its practical application. Klagenfurt: Verein fur
Wurzelforschung. pp. 793-795.