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Experimental Study on the Formation of Heavy Metal Sulphides Using Layer Silicates: Environmental Aspects
Abstract
Clay minerals saturated with metal ions (Cu, Pb, Cd, Zn, Tl, Ag, Hg) were treated by aqueous H2S solutions under controlled Eh and pH conditions. The heavy metal ions were released and precipitated at 80 °C as sulphides and in most cases appeared to be crystalline. They often formed coatings or crystals overgrown on clay minerals or formed tight intergrowths with clay particles. After 5 days, well crystallised sulphides were formed; covellite (CuS), galena (PbS) and sphalerite (ZnS) were identified by X-ray. The microscopic observations revealed some other Cu-Fe-minerals, like chalcopyrite (CuFeS2), idaite, and bornite. Native copper and pyrite were also observed. Cadmium formed hawleyite (CdS), whereas the greenockite crystals were rarely found in the reaction products. Mercury reacted very quickly with S2 ions and in the heavy fraction of the reaction products the metacinnabar (HgS) was determined. Several thallium sulphides were obtained: T1S, T12S (carlinite), T12S5, T14S3 and TlFeS2 (raquinite), Cu3Tl2S5, Cu2TlS3, AgTlS, Ag4Tl2S3 and AgTlS2.
... However, different results can be obtained as a function of the system applied and/or temperature. Rybicka et al. [54] obtained crystalline CuS and ZnS from the precipitation in clay columns at 80 @BULLET C, whereas the precipitates were amorphous at room temperature. ...
Copper was continuously and selectively precipitated with Na(2)S to concentrations below 0.3 ppb from water containing around 600 ppm of both Cu and Zn in a Continuously Stirred Tank Reactor. The pH was controlled at 3 and the pS at 25 (pS=-log(S(2-))) by means of an Ag(2)S sulfide selective electrode. Copper's recovery and purity were about 100%, whereas the total soluble sulfide concentration was below 0.02 ppm. X-ray diffraction (XRD) analysis showed that copper precipitated as hexagonal CuS (covellite). The mode of the particle size distribution (PSD) of the CuS precipitates was around 36 microm. The PSD increased by high pS values and by the presence of Zn. Depending on the turbulence, the CuS precipitates can grow up to 200 microm or fragment in particles smaller than 3 microm in a few seconds. Zn precipitation with Na(2)S at pH 3 and 4, in batch, always lead to Zn concentrations above 1 ppm. Zn precipitated as cubic ZnS (spharelite).
The sulfides characteristic of Kupferschiefer ores show partly very low temperature stability. Basing on this fact attempts have been made to synthesize sulfide minerals in water medium through the solid phase solution interaction by means of thermostat.
In the sulfide formation the layer silicates act as "acseptor" and "donor" of heavy metals ions through their specific features such as the ion exchange capacity and larg surface area. The folowing clay were used: Kaolinite, Illite and Smectite.
Chemical extractdion experiments for estimating charactereistic particulate binding forms of heavy metals in anoxic marine and freshwater sediments were carried out under both the presence and absenceof atmospheric oxygen during the analytical procedure. Storage methods including aeration of anoxic sediments do not preserve the initial fractionation patterns of heavy metals and may both increase (Ni, Pb, Cu, Zn, Cd) and decrease (Fe, Mn) their mobility. Elutriate tests of methanogenic freshwater sedimenten from Hamburg harbour basins show that metals with increased carbonatic fractions (Fe, Mn) and those with predominantly sulphidic/organic fractions (Zn, Cu, Cd) are particularly sensitive to sample aeration. Transformations of oxidizable Ni (11%), zinc (62%) and Cd (67%) predominantly lead to easily reducible associations. Drying of the sediments decreases the proportion of the sulphidic/organic fraction of the metals to a stronger degree. Oxidized Zn and Cd is then found even in the least stable exchangeable fraction. If pretreatment of sediment samples is done adequately the chemical leaching approach may provide information on diagenetic effects and reaction kinetics, which influence the mobility of contaminants in highly complex matrices.
The adsorption /desorption behaviour of Cd, Cu, Pb, Zn and Ni on illite, beidellite and montmorillonite was investigated using a multichamber device. This method allows one to study the adsorption/desorption rates and metal transfer reactions simultaneously on a few different solids. Sorption experiments show the highest enrichment of metal ions on smectites, but for Pb ions also on illite. The adsorbed amounts of Pb and Cu ions were very high: 80% and 70% of the total concentration of the metal ions, whereas the Zn, Ni and Cd ions were adsorbed in smaller (40–50%) amounts. A desorption experiment of metals adsorbed on clay minerals shows the following sequence of metal mobility from investigated minerals: Pb > Cd ⪢ Cu > Ni > Zn for beidellite, and Pb > Cd ≌ Cu > Ni > Zn for illite and montmorillonite. The lower metal accumulation on clay minerals after reaction with ferrous sulphate is supposed due to the premature iron hydroxide precipitation in the system, which has caused beside metals sorption their precipitation as well. Data from the sorption/desorption studies provides information regarding the different type of metal surface complexes formed with selected clay minerals and will help to predict the fate of metals in soils and sediments.
The mobilisation and transfer of copper and lead from polluted soil (sample taken from the vicinity of copper smelter) to five different — mainly clayey — competing sorbents have been studied using a multichamber device. The system consists of a central chamber which contained the soil suspension and six external chambers containing the different sorbents: zeolite, smectite, vermiculite, diatomite and halloysite. The solids in the central and external chambers are separated by 0.45 μm membranes which allow diffusion of the mobilised, dissolved metal constituents. In the central chamber the metals from the soil were mobilised by decrease of pH (< 5). The readsorbed amounts of Cu and Pb were obtained by taking samples after different time periods. The copper is mobilised to a higher degree than lead. The distribution of Cu is dominated by transfer and readsorption on smectite and zeolite, and also on halloysite at higher pH. Lead is mainly readsorbed on zeolite, and, at lower pH, on halloysite and smectite as well. Under the applied experimental conditions no significant transfer of both the Cu and Pb to the vermiculite and diatomite could be observed. These results indicate the higher affinity of lead on zeolite if compared with smectite; however, the latter remains the most effective sorbent of Cu. A mixture of smectite and zeolite is proposed for soil remediation procedures.
A multisite model was applied to Cd-sorption data across a range of pH and at two ionic strengths on specimen layer silicates and on untreated and dithionite-citrate-bicarbonate treated isolates from two Ultisols. The multisite model included the exchange of Cd2+ on fixed-charge sites on the layer silicates, complexation to hydroxyl sites on the layer silicates and Fe oxides, and electrostatic interactions . The combined modeling results indicated that: Cd2+ ion exchange to fixed charge sites was the dominant sorption reaction for pH<6.5 and ionic strength = 0.01; Cd complexation to hydroxyl sites produced strong pH dependency in Cd sorption; and Fe oxides decreased Cd sorption by blocking access of Cd to exchange sites. However, the results are unable to determine whether Fe oxides were important sorbents for Cd. -from Authors
A study of the ion-exchange properties of montmorillonite has been performed in order to facilitate computer predictions of the chemical properties of natural fluids and mineral assemblies. Clay/electrolyte interactions can be described using a technique based on the concept of hypothetical surface complex formation. Effects such as variable cation-exchange capacity and compositionally dependent exchange constants, normally indicating non-ideal behaviour, can be simulated using different combinations of ideal reactions involving charged surfaces and complexing groups representing clay edges. The techniques were applied to data on the ion-exchange characteristics of Wyoming bentonite to yield thermodynamic data for the reactivity of this clay with alkali metals, alkaline earths and a range of first-row transition metals at 25°C. -from Authors
The investigations reported are concerned with the synthesis of simple sulphides, Tl-bearing sulphosalts, and antimony-iron sulphides in aqueous environment using a combination of clay/dolomite or clay/quartz buffer at a temperature range of 85–90°C. Clay minerals as “catalyst” have been employed because their K+, Na+, Ca2+, etc. contents have to be substituted by cations of Fe, Cu, As, Sb, Tl, Bi, etc. After substitution, the saturated clay minerals were in turn treated by aqueous H2S solution under controlled Eh-pH condition, where the heavy elements were extracted and precipitated as sulphide minerals.
The data obtained show that the pH for simple sulphides varied between 6.46–9.70, and for sulphosalts from 4.14 up to 10.1. The antimony-copper and antimony-iron sulphides had an average pH value of 8.7. The combination clay/dolomite was characterized by high pH, that of clay/quartz by lower pH values. A linear relation between the Eh and pH is recognized for the sulphides, the sulphosalts, the antimony-copper and iron as well as the copper-arsenic sulphides in all environments.
In most cases, the mineral assemblages obtained depend on the initial materials and on the environment available. Using this procedure, we were able to synthesise thallium-sulphosalts at temperatures below 100°C.
A series of exploratory experiments on the concentration of base metals through ion exchange reactions on clay minerals show promising results. The extraction of Pb, Zn and Cd from bentonite, illite, kaolinite and montmorillonite was successful and megascopic quantities of PbS, CdS and ZnS could be obtained at specific loci, depending essentially on the flow velocity, and the other cations present. These or similar exchange reactions are tentatively proposed as possible mechanisms of economic metal concentration during diagenesis.Es werden experimentelle Resultate zur Anreicherung von Schwermetallen durch Tonminerale (Montmorillonite, Illite und Kaolinite) mitgeteilt. Eine erste Anreicherung kann ber den Kationenaustausch bewirkt werden. In dem darauffolgenden zweiten, sehr viel wirksameren Schritt werden die Schwermetallionen aus den Tonlagern extrahiert und in deren Nhe als schwerlsliche Sulfide oder Carbonate gefllt. Die Extraktion beruht darauf, da Tonschichten als Kationen-permeable Membranen wirken knnen. Sie wird von der Art der Tonminerale und der Kationen, den Lsungsgenossen, den Konzentrationsverhltnissen, dem pH-Wert, der Reaktionstemperatur, dem Porenvolumen und der Porenverteilung im Sediment beeinflut.