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Bioavailability of Metals in Estuarine Sediments and Their Possible Impacts on the Environment

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Mangroves and mudflats, important sub environments within the estuaries, are rich in nutrients and are potential sources for flora and fauna and thus provide shelter to thousands of animal and plant species. The sediments in these sub-environments composed of different geochemical phases that act as potential binding sites for metals. The metals are present in various forms and metal bioavailability includes metal species that are bio-accessible. A modified sequential extraction procedure, screening quick reference table, sediment quality guidelines as standard approaches are available to quantify different forms of metals. Our investigation revealed that Mn values are above Apparent effects Threshold (AET), indicating Mn is bioavailable and toxic to biota in mudflats of many estuaries in India. Among the trace metals Co and Zn show higher bioavailability. Distribution pattern of bioavailable metals can also be used to understand anthropogenic input to the estuaries and their mobilization.
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Environmental and Social
Sciences
Open Science Publications
01
Abstract
Mangroves and mudats, important sub environments within the estuaries, are rich in nutrients and are potential sources for ora and fauna and thus
provide shelter to thousands of animal and plant species. The sediments in these sub-environments composed of different geochemical phases that act as
potential binding sites for metals. The metals are present in various forms and metal bioavailability includes metal species that are bio-accessible. A modied
sequential extraction procedure, screening quick reference table, sediment quality guidelines as standard approaches are available to quantify different
forms of metals. Our investigation revealed that Mn values are above Apparent effects Threshold (AET), indicating Mn is bioavailable and toxic to biota in
mudats of many estuaries in India. Among the trace metals Co and Zn show higher bioavailability. Distribution pattern of bioavailable metals can also be
used to understand anthropogenic input to the estuaries and their mobilization.
estuaries which are formed wherever the tidal range is between 2 to
4 m. ese estuaries have meandering characteristics. Macrotidal
estuaries which are formed wherever there are strong tidal currents
(tidal range is more than 4 m). ese are trumpet in shape.
e estuaries are complex systems which receive inputs from
dierent sources like land derived material through river, banks,
marine, atmosphere etc. e river waters that mix with sea water
in estuaries varies with the rate of freshwater discharge from the
drainage basin and with the geological and geochemical characters
of each drainage basin. Estuaries are the favourable environments
of deposition of sediments derived from both the catchment area
(terrestrial) and the marine sources. So, the sedimentation in estuaries
is within three distinguishable regimes viz. estuarine uvial, estuarine
brackish and estuarine marine. Fine sedimentary deposits or mud are
a characteristic feature of estuaries. e most signicant sorting is the
coarse (gravel and sand), which are found in the more energetic areas
and ne (silt and clay) sediments, which accumulate in low energy
conditions or quite waters. It is also necessary to mention here that
the sediments are composed of dierent geochemical phases such
as clay, silt, sand, organic material, oxides of iron and manganese,
carbonate and sulphide complexes that act as potential binding sites
for metals entering an estuarine system. In the sediments, metals can
Introduction
e coastal zone is characterized by variety of landforms like
estuaries, lagoons, beaches, islands. Estuaries are one of the important
sub environments of the coastal zone. Estuaries are rich in nutrients
and are potential sources for ora and fauna and thus provide shelter
to thousands of animal and plant species. Estuaries are coastal bodies
of water, occupying an existing river valley and their characters are
typied by the discharge of rivers and therefore they are regarded
as complementary extensions of rivers. Estuarine regimes are
governed by several factors such as river inow, tides, waves, wind
and meteorological forces making the system more complicated and
dynamic thus temperature, salinity and turbidity uctuate on daily,
fortnightly and seasonal basis and reach more extremes in estuarine
waters than they do at sea or in rivers.
Pritchard [1] dened an estuary as ‘’An estuary is a semi enclosed
coastal body of water, which has a free connection with the open sea,
and within which sea water is measurably diluted with fresh water
derived from land drainage’’. Estuaries are classied based on tides,
as Microtidal estuaries which are formed wherever the tidal range is
less than 2 m and are dominated by freshwater discharge, which leads
to ‘’salt wedge’’ type estuary. ese are highly stratied. Mesotidal
Volume 2, Issue 1 - 2015
© G. N. Nayak 2015
www.opensciencepublications.com
Bioavailability of Metals in Estuarine Sediments
and eir Possible Impacts on the Environment
Research Article
G. N. Nayak*
Department of Marine Sciences, Goa University, Goa - 403 206, India
*Corresponding author: Dr. G. N. Nayak, Department of Marine Sciences, Goa University, Goa - 403 206, India, E-mail:
gnnayak@unigoa.ac.in
Article Information: Submission: 10/02/2015; Accepted: 14/03/2015; Published: 19/03/2015
JOURNAL OF ENVIRONMENTAL AND SOCIAL SCIENCES G. N. Nayak
Citation: Nayak GN. Bioavailability of Metals in Estuarine Sediments and Their Possible Impacts on the Environment . J Environ Soc Sci. 2015;2(1): 105.
02
e fractions can be digested in Teon beakers using the
combination of HF, HClO4 and HNO3. e digested samples can be
analyzed for various metals on Varian 240FS model Atomic
Discussion
In recent years, there has been increasing concerns about
pollutants entering the aquatic environment. A need to discover
simple and reliable pathways to monitor levels of particular metals
or other pollutants in the aquatic environment has resulted in a
proliferation of studies. Jonathan et al. [4] studied recent sediments
o Gulf of Mannar along the southeast coast of India and their studies
revealed the enrichment of Cr, Pb, Cd, Cu, Ni, and Zn and indicated
that the area has been contaminated by riverine sources and industries
nearby. Alagarsamy [5] had studied the seasonal distribution of trace
metals such as Fe, Mn, Co, Cu, Zn and Pb in the Mandovi Estuary,
west coast of India. His results revealed that the surface sediments of
Mandovi Estuary are moderately or strongly contaminated to some
extent by Fe and Mn. Cu and Zn showed the inuence of organic
wastes from municipal sewage entering into the estuary, in the river
mouth region. Janaki - Raman et al. [6] have reported that the trace
metals in sediments of Muthpet mangroves, South - East Coast
of India are diagenetically modied and anthropogenic processes
control Pb and to some extent Ni, Zn and Fe. Nayak et al. [7] have
studied the abundance and distribution of total suspended matter
(TSM) from Mandovi and Zuari estuaries in three dierent seasons
over the last seventeen years. ey have reported that in Zuari estuary,
TSM concentration increased in both surface and bottom waters from
year 1991 to 2004. Recently, Department of Marine Sciences of Goa
University under the research project “Reading pollution history,
paleoclimate and sea level changes from the study of mudats, central
west coast of India’ has carried out a detailed study on mudats along
central west coast of India [8-11].
Mud deposition is characteristic of protected low energy
environments such as estuaries and lagoons. is occurs in the
intertidal zone where regular and increased depth of ooding prevents
salt tolerant plants growing. Intertidal mudats are a prominent
geomorphological component of estuaries and the development of
an estuarine mudat is both complex and dicult to predict because
of the multiple relationship between the physical, chemical and
biological properties of the sediment. Further, due to their potential
role as contaminant storage areas, mudats tend to release heavy
metals into the estuarine waters by various chemical, physical and
biological processes which may facilitate mixing and remobilization
[10].
Speciation of metals is the identication of metals that are bound
to dierent components of the sediment [12] and the phases have
been dened [2,13]; as exchangeable ions, adsorbed ions / carbonates,
Fe and Mn oxides, sulphides / organics and metals bound to
lithogenic minerals and residual. Bioavailable metal fractions include
Exchangeable fraction which consist of metals bound to colloidal or
particulate material. Generally, clay and organic matter controls ionic
exchange thus responsible for availability of metal in exchangeable
metal form. Further it is understood that pH plays an important role
in governing concentrations of soluble metals. Mn solubility is low at
high pH and with high organic matter content, while in acid soils with
be present in various forms and generally exhibit dierent physical
and chemical behaviour in terms of chemical interactions, mobility,
biological availability and potential toxicity. erefore, the fate
of various metals, in the natural environment is of great concern.
Metals may be partitioned into six fractions: dissolved, exchangeable,
carbonate, iron-manganese oxide, organic, and crystalline. e
metal bioavailability includes metal species that are bio-accessible
and are absorbed or adsorbed by an organism with the potential for
distribution, metabolism, elimination, and bioaccumulation.
Methods
e sediment samples may be surface and/or subsurface can be
collected from sub-environments like mudats, mangroves etc. within
estuaries using a hand driven PVC coring tube. Representative of
each sub-sample can be powdered by using an agate mortar and pestle
for geochemical analysis. A modied sequential extraction procedure
[2] can be adopted to quantify the metals in dierent operationally
dened geochemical phases. e steps involved in separating the
phases are detailed below.
Fraction 1: It is a soluble / exchangeable fraction in which the
contaminants are weakly adsorbed to the clays and organics in the
sediment via electrostatic attraction. From a biological availability
point of view, metals in this fraction are readily available [2].
Magnesium chloride is an eective reagent for desorbing adsorbed
trace metals.
Fraction 2: In this fraction the contaminants are bound to
carbonates. e metals are very susceptible to changes in temperature
and pH of the solution and therefore the next most readily available
[2]. Buered acetic acid and sodium acetate was applied to leach the
metals in this fraction.
Fraction 3: In this fraction the contaminants are bound to Fe and
Mn oxy- hydroxides. ey exist in sediment as cement, nodules and
concretions and tend to be thermodynamically unstable in anoxic
conditions. ey are most susceptible to changes in Eh (increased
availability at low Eh). ese contaminants are most biologically
available under reducing conditions [2].
Fraction 4: e contaminants here are bound to various forms
of organic matter with strong bonds. Metals in this fraction may be
associated with organic matters, such as organic coatings on inorganic
particles including biotic detritus. Under oxidising conditions, these
contaminants are released upon degrading the organic matter.
Contamination in the Organic matter fraction is the least biologically
available [2] among the rst four phases. Stability of the metals in this
fraction is high when compared with above three fractions and thus
incorporation in geochemical cycle is dicult. Hydrogen peroxide in
acid medium used to oxidise organic matter whereas, Ammonium
acetate is used to prevent adsorption of extracted metals on the
oxidized sediment.
Fraction 5: Residual fraction includes metals incorporated into
the crystal structure of the primary and secondary minerals. ese
contaminants are not biologically available and can only be released
with the use of a very strong acid such as HF [2,3]. Metals in this
fraction are inert and may not take part in the biochemical or chemical
reactions under normal environmental conditions.
JOURNAL OF ENVIRONMENTAL AND SOCIAL SCIENCES G. N. Nayak
Citation: Nayak GN. Bioavailability of Metals in Estuarine Sediments and Their Possible Impacts on the Environment . J Environ Soc Sci. 2015;2(1): 105.
03
low organic matter its solubility and availability is high. e solubility
of Mn is high at pH above 6 in anaerobic condition. Metals associated
with carbonate minerals constitute the carbonate fraction, which can
be newly precipitated in soil. e element in carbonate fraction would
be released more into the environment if the conditions became more
acidic. e co-precipitation with carbonate minerals is of importance
for a number of metals in addition to Mn which includes Zn and
Pb. e iron-manganese oxide fraction consists of metals adsorbed
to iron-manganese oxide particles or coatings. Fe-Mn oxide fraction
includes the metal oxides/hydroxides soluble under slightly acidic
pH, as well as the metals associated with reducible amorphous Fe-
Mn Oxy-hydroxides. Metals from this fraction are released into the
environment with a decrease in pH and if sediments change from oxic
to anoxic condition. e organic fraction consists of metals bound to
various forms of organic matter. Most metal hydroxide minerals have
very low solubility under pH conditions in natural water. Adsorption,
which occurs when dissolved metals are attached to surfaces of
particulate matter (notably iron, manganese, and aluminium oxide
minerals, clay, and organic matter), is also strongly dependent on
pH and, of course, the availability of particulate surfaces and total
dissolved metal content. erefore, organic matter/Sulphide fraction
represents the amount of metals bound to the organic matter and
sulphides that would be released into the environment if conditions
became oxidative. However, the intensity of complexation is found to
vary progressively with the metal concentration, with high-intensity
sites being lled rst, followed by sites of lower intensity. Residual
fraction named as inert phase, corresponds to that part of the metal,
which cannot be mobilized.
Dessai and Nayak [14] had studied speciation of metals in Zuari
estuary and reported that Mn and Co are higher in concentration in
exchangeable fraction. Li et al. [15] had studied the metal distribution
in the coastal wetland of the Pear River Estuary, China, and they
have reported higher content of Cd and Zn in the exchangeable
fraction indicating their ecological risk. Farkas et al. [16] had studied
chemical speciation by using sequential extraction procedure and
also evaluated geoaccumulation Index of the sediments to assess the
heavy metal pollution in surface sediments of the River Po.
Further, Buchman [17] introduced Screening Quick Reference
Table (SQUIRT) for metals in marine sediments, which can be used
as a standard to declare the given estuarine sediments have reached
the threshold with respect to pollution of metals. Screening Quick
Reference Table (SQUIRT) developed by (NOAA) is presented in
Table 1. Based on SQUIRT, the guideline values are categorized into
ve classes which are also presented in the Table 2.
TEL: reshold eect level; ERL: Eect range low; PEL: Probable
eects level;
ERM: Eect range median; AET: Apparent eects threshold
(Except for Fe, all values are in ug/g)
When the speciation data is compared with SQUIRT (Table 1),
Mn shows values above AET indicating Mn is bioavailable and toxic
to biota in mudats of many estuaries in India. Among the trace
metals Co and Zn show higher bioavailability.
e metals in acid soluble fractions are considered to be the
weakest bonded metals in sediments which may equilibrate with the
aqueous phase, and thus become more easily bioavailable [18]. us,
metal speciation is of critical importance to their potential toxicity
and mobility [19]. A criterion called ‘‘Risk Assessment Code (RAC)’’
has been used to assess the potential mobility and hazard of metal
based on the percentage of exchangeable and bound to carbonate
metal in the sediment [20,21]. e metals in dierent fractions are
bound with dierent strengths in the sediments. If a sediment sample
can release in these fractions less than 1% of the total metal, it will
be considered safe for the environment. On the contrary, sediment
releasing in the same fractions more than 50% of the total metal has
to be considered highly dangerous and can easily enter into the food
chain. e classes based on RAC is given in Table 3.
Our studies have shown class III to V RAC for Mn, Co and Pb
ELEMENTS TEL ERL PEL ERM AET
Fe - - - - 22% (Neanthes)
Mn - - - - 260 (Neanthes)
Cu 18.7 34 108 207 390 (Microtox & oyster larvey)
Zn 124 150 271 410 410 (Infaunal community)
Cr 52.3 81 160 370 62 (Neanthes)
Co - - - - 10 (Neanthes)
Table 1: Screening quick reference table for metals in marine sediments [17].
Sediment guidelines
Threshold Effect Level (TEL) Maximum concentration at which no toxic effects are observed
Effects Range Low (ERL) 10th percentile values in effects or toxicity may begin to be observed in sensitive species
Probable Effects Level (PEL) Lower limit of concentration at which toxic effects are observed
Effects Range Median (ERM) 50th percentile value in effects
Apparent effects Threshold (AET) Concentration above which adverse biological impacts are observed
Table 2: Sediment guidelines and terms used in SQUIRT.
JOURNAL OF ENVIRONMENTAL AND SOCIAL SCIENCES G. N. Nayak
Citation: Nayak GN. Bioavailability of Metals in Estuarine Sediments and Their Possible Impacts on the Environment . J Environ Soc Sci. 2015;2(1): 105.
04
in some of the estuaries along west coast of India. e bioavailability
of Mn is higher with organic fraction as a major phase in some
areas. Mn association with carbonate fraction shows its ability to
replace calcium in carbonate minerals as a result of their similar
ionic radii and charges. Mn is also higher near surface compared
to subsurface. is could be due to Mn remobilization from the
reducing deeper sediment sections and accumulation in oxidized
surface sections. Distribution pattern of bioavailable metals can also
be used to understand anthropogenic input to the estuaries and their
mobilization.
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... Calcium carbonate in tropical estuaries is mostly originated from the continental shelf and calcareous shell material from organisms. Whereas metal complexing with organic matter may vary from weakly to strongly bound, association with carbonates is generally a weak binding event that is susceptible to changes in temperature and pH (Costa et al. 2015;Nayak 2015). Therefore, mobility and bioavailability of metals in the estuarine environment strongly depends on metal binding to carbon substrates present in sediments. ...
... In addition to natural Mn source, anomalous values are probably linked to harbor activities involving manganese ore shipment rather than the binding capacity of SMB sediments, as observed by the negative correlation with fine-grained fraction. Higher Mn contents in sediments from ARE result from the association with clay-sized particles and the abundant organic matter in this estuary relative to SMB, as observed in other studies in organic-rich estuarine sediments (Nayak 2015;Williams and Block 2015). The anomalous values for Cr in SMB and ARE indicated lithogenic sources that were deposited in sites more upstream and diluted downstream of each estuary; these values were also similar to observed results in other tropical estuaries in Brazil (Amorim et al. 2007). ...
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