All possible minerals should be known and investigated. Mixed minerals or, in other words, coprecipitation cannot be considered. Mixed solid formation is common Table 3. For example, when minerals such as montmorillonite and Fe or Mn oxides precipitate from soil solutions, they are characterized by a wide range of isomorphous substitutions in which metal cations in their usual structures can be partly replaced by trace metal ions of the. There is a lack of equilibrium solubility data see point 1 of many minerals of trace metals.
This problem can be circumvented to some degree by using empirical methods to estimate Ksp values as yet unavailable see references in Mattigod et al. The computation of ion activities can be inaccurate, especially for high ionic strength. Direct measurements are difficult. All complexes especially organics and ion pairs must be considered.
Major and minor cations and anions must all be analyzed to permit these calculations. It is an equilibrium method. For example, the composition of soil solutions is often controlled by fast-forming metastable solid phases. Consequently, all solids that are likely to occur, stable or metastable, must be considered. Chemical speciation can be defined as the distribution of a given element or compound into the various chemical forms which together make up the total concentration of that element or compound in a sample.
It is now well recognized that it is not the total amount of a metal pollutant which is most significant for the understanding of geochemical fluxes. One of the keys to the description of the mobility of a metal is its speciation. This is especially important for trace metals because they are largely influenced by the major ion chemistry of their aqueous environment: KM. Let us note that one advantage of the surface complexation model described earlier is that surfaces can be considered as classical polymer-type ligands.
The activity of any com pie xed species can be written as: 10 Two types of effects can thus modify the speciation of M: 1. Indirect effect: an increase in the concentration of a competitor of M for complexation causes a decrease in U- and thus M is less complexed.
The chemical forms of trace metals in soils and aquatic systems have been reviewed by Florence and Batley , Sposito , and Salomons and Forstner Analytical in situ determination ofthe dissolved speciation can provide at best very limited information. A significant effort has been made in the last few years to determine the complexing capacity of natural waters Kramer and Duinker , but while everyone agrees that the dissolved organic carbon is a major complexing agent, there is no consensus yet as to the complexing intensity or the density of complexing sites.
The conditional constants for the complexation of trace metals with organic matter vary largely with the pH of the medium in which they were determined. In a first approximation, the simple linear relationship between the logarithmic values of the constant and the pH, observed by Buffie , for eu can be extended to other metals Bourg and Vedy The effect on the speciation of trace metals of some ofthe most significant types of parameters pH for hydrolysis and carbonate complexes, chloride for inorganic complexation, NTA for organic complexation, and adsorbing surface for adsorp- tion is illustrated in Fig.
The consequences for the mobility of heavy metal will be discussed later. Since the appearance of the first paper on chemical speciation of a natural water Garrels and Thompson , the use of computers to solve chemical equilibrium speciation calculations has expanded enormously Nordstrom et al. Zn so,. ZnCO ;i!.. Speciation of trace metals as a function of various parameters. NTA diagram from Salomons et al. In the absence of a dynamic geochemical model, equilibrium calculations can provide useful qualitative information on potential tluxes between several com- partments of a given ecosystem.
For example, an equilibrium speciation computer code with the weakest reported binding constants between trace metals and dissolved organic matter was used to investigate expected forms of dissolved trace metals in the gravitational water of two acid-soil proilles Bourg and Vedy In spite of the weak constants used, organic complexes were predicted to be significant species of trace metals in an acid brown earth and a humo-ferrugineous podzol. It was concluded that the fate of trace metals associated with the dissolved organics in these soils should follow the dynamics and evolution of their organic vectors.
The acid brown soil Al horizon organic trap is effective, as shown, for example, by the strong accumulation ofPb in the organic surface layer of an acid brown soil from the SoIling area, FRG Heinrichs and Mayer On the other hand, even though they are known to retain organics, podzol Bt. Other uses of equilibrium speciation calculations are numerous Zirino and Yamamoto ; Sibley and Morgan ; Long and Angino ; Mantoura et al.
Some of these will be described in the next section on control of dissolved trace metals. The dispersion of metal pollutants from mine tailings and dredged materials is influenced by redox reactions at two stages during their release from the deposition area to the surrounding environment: 1. The disposal on land or in oxic aquatic environments of dredged material, the resuspension of contaminated sediments, the disposal of mining wastes, and the operation of mines all provoke drastic changes from reducing to oxidizing conditions. These changes are not due to the introduction in the global envi- ronment, but rather to the physical transfer of pollutants concentrated by given solids mineral deposits, sediments through natural processes and suddenly exposed to new chemical conditions.
Metal pollutants released from the deposition area because of the transition from anoxic to oxic conditions can later, in specific areas of the receiving surroundings, be exposed to further redox varia tions daily, seasonal, or spatial capable of retarding or even blocking their migration. The weathering of mine tailings can be exemplified by the oxidation of pyrite, in which the side effect of oxidation is the release of acidity Fig. After the oxidizing reaction is initiated, the ferric ion is capable of taking over the role of oxygen to produce additional Fe II and acidity.
For dredged material, organic matter is a significant component of the oxidation process Fig. The organic compounds formed by the microbial decay of organic matter can inhibit the oxidation of Fe II. Moreover, the solubility of Fe III can be drastically enhanced by the formation of dissolved organic complexes.
A natural analog of the oxidation of dredged material is the fate oftrace metals in anoxic areas of aquatic systems. Upon entering the more oxic estuarine water, trace metals are released from the suspended matter Salomons and Eysink A similar behavior was observed in an experiment simulating the dumping of dredged material in the North Sea. A control experiment carried out in fresh water produced only a little release. An adsorption was even observed for some metals. The transition from anoxic to oxic conditions can be explained by the following consecutive reactions: 1.
An oxidation of metal sulfides; 2. The formation under oxic conditions of new species, such as dissolved chloro- complexes in marine and estuarine environments or adsorbed compounds in fresh water. The latter depends on the chemistry of the immediate environment of the metal remobilized during the former.
As mentioned earlier, pH and complexing agents have a significant influence on the dissolved metal speciation Fig.
Moreover, these two parameters as well as the availability of solid surfaces will affect the metal adsorption Fig. The solubility and mobility of trace metals are influenced by the redox status of their environment, even though they are usually not directly involved in oxidation-reduction reactions. The oxidation ofsulfide into sulfate will increase the solubility of heavy metals by lowering the ionic activity product of solid metal sulfides. The acidity produced during the reaction will also affect the solubility of trace metals see Sect.
The redox chemistry of iron and manganese, ubiquitously present as coatings on natural solids Jenne , will also tremendously affect the mobility of trace metals Fig. Oxidizing conditions provoke the formation of coprecipitating and later adsorbing amorphous oxyhydroxides of Fe and Mn. Adsorption of cations J [ Desorption of sorbed cations. The solubilization of iron and manganese in reducing environments is well documented Lyons et al. Khalid et al. Lg S g-l solids indicating cadmium sulfide precipitation. In the mercury investigation the addition of 2. Lg ofHg g-l sediment resulted in recoveries always lower than 0.
The addition of 60 J. Lg of Hg g-l. Under reduced and moderately oxidized conditions, essentially all the mercury added was sorbed by the sediment. A greater decomposition of organic matter under oxidized condi- tions may partially explain the lower uptake ofHg.
A more realistic explanation for the increase in solubility with decrease in pH is the use of HCl to maintain pH at 6. Complexation with chloride could also explain the high Cd recovery for low pH and high Eh. The retention of trace metals in aquatic and terrestrial systems is due to hetero- geneous geochemical processes and to physical deposition or filtering of natural solids. Which of the sorption processes, adsorption or precipitation, controls the dissolved concentration and thus the mobility of trace metals? The equilibrium solubility approach see Sect. Under some conditions, dissolved trace metals are regulated by precipitation phenomena Table 4.
Under others, adsorption is Mouvet and Bourg ; Bourg or is suggested to be the control mechanism Table 5. Equilibrium calculations performed with available solubility products and adsorption constants on Meuse River sediments Mouvet and Bourg suggest that depending on the chemical characteristics of the water phase, both adsorption and precipitation are regulating mechanisms of the dissolved heavy metal content of natural waters Fig. For pH values lower than 8, adsorption is likely to control the dissolved Cu of rivers with low Cu load.
Ifthe total Cu were to be higher, the adsorption curves of Fig. The malachite solubility would then be controlling the dissolved Cu at lower pH values. Further evidence of the importance of adsorption as a control mechanism is provided 1 by the equilibrium model of Hunter for the scavenging of reactive metals from the deep oceans by suspended particles covered with an organic film and 2 by the simple black box adsorption model ofSalomons for the regulation of dissolved Zn in the IJsselmeer, Holland.
The most important characteristics of suspended or deposited sediments, soils, and subsoils for the retention of metal pollutants are: 1. Content of hydrous oxides of Fe and Mn; 3. Organic matter content; 4. Surface properties cation-exchange capacity ; 6. Nature and concentrations of constituents of the aqueous phase; 7. Quantity and flow ra te of solution which moves through the aq uatic or terrestrial system.
Acid mine drainage Chapman et al. OH '2 Acid mine drainage Chapman et al. O PbO. CI Mining drainage Jenne et al. Geochemical control of dissolved eu in pH average river water Bourg and Mouvet Let us simply point out that the influence of clays on the total adsorbing strength of a natural solid is negligible compared to the contribution of organic matter and oxides of Fe and Mn MacLaren et al.
A number of ions or compounds and processes can be involved in keeping heavy metals in solution or in remobilizing them from temporary retention substrates. It is quite clear that complexation with chloride ions is a mobilizing factor. The perchlorate experiments were used as references since significant complexing ofCIO:; with Cd, Cu, and Ni does not occur. Chloride complexation increases the metal mobility Fig. Mercury migrated through a sand column much faster in the presence of chloride Behra Moreover, mercury retained by the sand column when introduced as the nitrate salt was completely remobilized when the supporting electrolyte of the percolating fluid was shifted to sodium chloride Behra et al.
The presence of chloride also decreased the adsorption of Hg on sediments Reimers and Krenkel and on silica MacNaughton and James , of Cd on soils Christensen b , on alumina Bourg b , and on montmorillonite Garcia-Miragaya and Page ; Egozy and of Cd, Cu, and Zn on sediments Bourg b. A large fraction of the metal added as inorganic salts was removed by leaching during the first week of the experiment, while metal added as a humic compound was strongly immobilized with the exception ofNi. Under acid leaching pH 3 by dilute solution 0. These experimental conditions are close to that expected from acid-mining waste leachates.
Even though they are not represen- tative of natural conditions soil pH values between 4. However, only small fractions of metals like Cu and Pb are solubilized because either they were associated with the soil organic matter or, ifnot, after leaching, they can complex with solid organic matter or adsorb on other surfaces. If inorganic comp1exing agents tend to remobilize heavy metals or maintain them in solution, the behavior of organic che1ates is harder to ascertain. Natural organic matter is, however, subject to precipitation-coagulation phenomena and it is therefore susceptible to inhibit the mobility of some heavy metals such as Cu and Pb Bourg and Schindler Natural municipal solid waste 1eachates individually enriched with metals were percolated through soil columns under anaerobic saturated conditions at flow rates ranging from 1 to 15 ml h- I Alesii et al.
This need to investate the inclusion of sorption dynamics for the adequate description of practical situations, particularly those involving intermittent flow is further supported by the experiments of Murali and Aylmore The breakthrough curves in Fig. S mM phosphate, 1. The soil used was in a finely ground state and the depressions in outflow concentrations for phosphate and selenite following the no-flow period cannot be attributed to intra-aggregate diffusion.
This is verified by the absence of a measurable depression for sulfate, despite the measurable adsorption of this ion, and for tritium. This brief introductory chapter of a volume on the environmental impact and management of mine tailings and dredged materials was intended to present briefly the general fundamental aspects of the processes involved in the retention and mobility of trace metals in aquatic and terrestrial environments.
As summa- rized in Fig. Moreover, there is some, even if limited, evidence that the participation of metals in these processes may be affected by the flow rates of the vectors of transport water, solid particles. Further theoretical work, in tight connection with field investigations, should be conducted on the characterization of adsorption by natural solids, of com- plexation with natural organic matter as well as on the kinetics of the various processes mentioned and especially for the redox reactions involving the solubilization and precipitation of Fe and Mn.
Geochim Cosmochim Acta Behra P Migration or retention of mercury II salts when percolating through a porous medium constituted of a natural quartz sand. Document du B. In: Trace Elements in Natural Waters. Internat Union Pure and Applied Chem. I Complex formation in the system silica-Cu II -ethylenediamine. Chimia Bowers AR Adsorption characteristics of various heavy metals at the oxide-solution interface: effect of complex formation. Geoderma Buffle J A critical comparison of studies of complex formation between copper II and fulvic substances of natural waters.
J Soil Sci Calvet R Etude du comportement physico-chimique de metaux lourds et des pesticides et de leur transport dans les sols. Report Institu! Nat Rech Agronomique, Paris, grant Effect of time, cadmium load, pH and calcium. Reversibility, effect of changes in solute composition and effect of soil aging. Prediction and observation of mobility. Science Davis JA la personal communication Davis JA b Complexation of trace metals by adsorbed natural organic matter. Geochim Cosmochim Acta Davis JA, Leckie JO Surface ionization and complexation at the oxide-water interface: 2 Surface properties of amorphous iron oxyhydroxide and adsorption of metal ions.
Elliott HA Adsorption behavior of cadmium in response to soil surface charge. Am J Sci Gomez A, Juste C Mobilite du cadmium, du mercure, du nickel et du plomb associes ades acides humiques ou ades anions mineraux dans des colonnes de sable. Li YH Ultimate removal mechanisms of elements from the ocean. Observed results compared to values calculated with a chemical equilibrium computer program.
In: Jenne EA ed Chemical modelling in aqueous systems. Geochim Cosmochim Acta Parks GA Aqueous surface chemistry of oxides and complex oxide minerals; isoelectric point and zero point of charge. Salomons W Dredged material and mine tailings: Similarities in behavior and impact. Int Conf. Holland is plated by the Rhine environmental problems associated with contaminated sediments. Effects of waste disposal on groundwater. Proc Exeter Symp! Geoderma, Sholkovitz ER, Copland 0 The chemistry of suspended matter in Esthwaite Water, a biologically productive lake with seasonally anoxic hypolimnion.
Proc Internat Conf. Heavy metals in the environment, Oct , Toronto, Vol Sigg L Surface chemical aspects ofthe distribution and fate of metal ions in lakes. Colloids and Surfaces 2: Sigg L, Stumm W, Zinder B Chemical processes at the particle-water interface; implications concerning the form of occurrence of solute and adsorbed species. Water Res Sposito G The chemical forms of trace metals in soils. The composition of associated carbonates. Vuceta J, Morgan JJ Chemical modeling of trace metals in fresh waters: role of complexation and adsorption. Applied Geochemistry 1: Zirino A, Yamamoto S A pH-dependent model for the chemical speciation of copper, zinc, cadmium and lead in seawater.
KELLEyl and o. Industrially significant metal sulfides are relatively stable in their natural crys- talline form. As a result of mining activities, these minerals are exposed and interact with water, oxygen, carbon dioxide, and soluble chemical species, factors which collectively enhance the mineral dissolution. The very nature of mining practices such as milling and grinding, which are designed to maximize metal recovery, also ensure maximum surface area exposure of mine tailings and waste materials to subsequent oxidative processes.
An important prerequisite prior to oxidative dissolution is the chemical dissociation of the mineral sulfide Sato Following mineral sulfide dissociation, the chemical oxidation ofthe reduced valence state ionic species is greatly enhanced by the catalytic activity of the ubiquitous mixed communities of microorganisms associated with sulfide ore wastes and tailings.
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A major contributor to the microbial consortia is the chemolithoautotrophic bacterium Thiobacillus ferrooxidans. For a more com- prehensive listing of the sulfur- and iron-oxidizing bacteria, the reader is directed to the excellent reviews by Ralph and Norris and Kelly Under field conditions, the degradation of these mineral materials is a dynamic process involving a succession of microbial populations Ralph ; Lundgren and Malouf which develop according to the prevailing environmental condi- tions.
These, in turn, are controlled by a complex array of physicochemical conditions which are generally site-specific. The oxidative dissolution of a sulfide mineral is commonly associated with an increasing acidification of the surrounding medium. This alone provides a heavy selection pressure on the development of the microbial succession since a range of pH conditions are encountered. Variations in pH also affect the development and viability of microbial populations through the availability of electron donors such as ferrous iron, the oxidation of which is sensitive to pH. Kelley and O.
In a field environment, however, mineral sulfide degradation is affected by a far more complex interaction of physicochemical parameters. These have been previously discussed Ralph , ; Lundgren and Malouf and include the availability of oxygen, carbon dioxide, water, and suitable nutrients at the reaction site. Climatic conditions also play an important role, particularly tem- perature and rainfall. The mineralogical composition of the tailings or waste material influences both mineral sulfide dissociation and bacterial establishment. Highly siliceous or carbonaceous gangue associations consume acid, thereby displacing conditions beyond the pH range suitable to many leaching organisms.
The reduction in proton levels can also reduce the rate of chemical dissociation of many metal sulfides thus reducing their availability as bacterial substrates fributsch and Bennett a,b. Gangue minerals may also contain metal species such as arsenic, silver, and molybdenum, which on being rendered soluble, may be toxic to the chemoautotrophic bacteria Tuovinen et al. Other aspects of gangue mineralogy and its associated effects have been discussed by Lundgren and Malouf and Ralph The type and relative abundance of sulfide minerals will vary according to the nature of the mineral deposit and the efficiency of the primary metal extraction process.
Since the various mineral sulfides differ in their free energies of formation, their abilities to act as substrates for bacterial oxidative processes vary quite considerably CL Brierley It is apparent that tailings dams and waste deposits represent extremely complex and variable environments. All mineral deposits differ in composition and extensive mineralogical and grade variations can exist within anyone deposit. In mining practice, attempts are often made to blend feed ores in an effort to maintain uniform grades.
Despite this, variations in the nature of the tailings occur between different sections of the same dam as a result of changes in the grade of the run-of-mine feed, fluctuations in metallurgical extraction efficiency, and physical characteristics of the dam including water availability and depth. The residence time of the deposited material must also be considered. Such complexity does not permit uniform modeling of sulfide mineral dissolution and bacterial oxidation of mine tailings.
Nevertheless, significant progress has been made in our under- standing of the fundamental chemical and biological processes associated with these materials. This must contribute to our attempts to industrially exploit the sizeable metal values contained within these low-grade deposits and to minimizt' the possibilities of pollution. The dissolution of a large number of metal sulfides has been shown to be biologically assisted. Very few natural metal sulfides have been studied as purified mineral fractions and, although the stoichiometry of bacterial oxidation can be predicted, incomplete oxidations, side reactions, by-products, and impurities complicate the understanding of the microbiology of mineral leaching.
Iron, both in its divalent and trivalent state, plays a central role in sulfide mineral degradation. Attempts to demonstrate direct bacterial oxidation of these sulfides must necessarily exclude iron. This is extremely difficult to accomplish as most naturally occurring mineral sulfides are associated with various iron minerals. Our understanding of the detailed mechanisms of bacterial attack remains ob- scure. Ralph , has considered the mechanisms oftheiroxidative process in some detail.
Electrochemical processes may be im portant in providing substrates for bacterial metabolism. The cathodic reaction, the reduction of oxygen on the mineral sulfide surface, is the rate-limiting step owing to the formation of a passive sulfur layer over the surface of the mineral. The presence of this layer could be oxidized to sulfuric acid by bacterial action. In an elegant series of experiments, Vanselow was further able to demonstrate that certain strains of Thiobacilli could oxidize the mineral while physically separated from it.
He concluded that while physical attachment of the bacterium to the mineral sulfide surface is not an essential component of the oxidative mechanism, it would be of importance in reducing diffusion distances at low substrate concentrations. These findings were supported by Torma and Bosecker , who suggested that substrate dis- sociation from the mineral surface rather than bacterial attachment is the pre- requisite for bacterial oxidation of the insoluble metal sulfides.
In an iron-free environment, the rate of metal dissolution is proportional to the solubility product ofthe metal sulfide.
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Tributsch and Bennett la, b also demonstrated that metal sulfides with high solubility products are good substrates for bacterial leaching. However, this is not the only condition for bacterial leaching. The amenability of minerals to bacterial leaching is also critically dependent on the composition, structure, and impurities of metal sulfides Tributsch and Bennett a,b.
The pres- ence of lattice imperfections and impurities has been shown to effect the sus- ceptibility to bacterial degradation of a range ofzinc sulfide minerals Khalid and Ralph This will be discussed in more detail later in this section. This is based on the observation that electronically unfavorable systems e. There appears to be no direct correlation with the energy difference between the position of the electron transport system ofthe bacterium and the highest filled energy band ofthe sulfides.
Tributsch and Bennett Ia,b have, therefore, postulated that protons are important in chemically breaking surface bonds, including shifting electron levels of sulfides with large energy gaps into the energy range of the forbidden energy gap, where they can participate in electron transfer reactions. The proton catalyzed reaction produces weakly bonded "SHLl- groups which are in turn removed from the sulfide surface through complex formation with an unidentified molecular carrier used by the bacterium Tributsch and Bennett a,b.
This proposed mechanism is not applicable to sulfides in which the valence band of the semiconductor is derived from metal orbitals instead of from sulfur orbitals. Copper selenide has also been reported to be oxidized by T. Demonstrations of the coupling of these substrate oxidations with energy conservation in T. DiSpirito and Tuovinen have reviewed the current state of knowledge with respect to oxidation of these metals.
The coupling ofCu I and Sn lI oxidation to oxygen uptake and carbon dioxide fixation awaits confirmation. The oxidation of uranous compounds in the absence of iron has been linked to both oxygen uptake and carbon dioxide fixation DiSpirito and Tuovinen , a,b , although growth dependency on U IV compounds as sole sources of energy has yet to be demonstrated. The addition offerrous iron at concentrations of up to 0. As already indicated, in natural environments iron is nearly always associated with the leaching system. The kinetics of mixed substrates e. The direct biological oxidation of chalcopyrite is represented as follows: 5 However, chalcopyrite displays both chemical and microbiological recalci- trance and is typically oxidized at a slow rate in comparison to secondary copper sulfides such as chalcocite and covellite.
Its incomplete oxidation in leaching systems is a common problem and produces secondary copper sulfides and elemental sulfur. One of the most important naturally occurring growth substrates for T ferrooxidans is the iron sulfide mineral, pyrite FeS 2 , which is often associated with other, more valuable minerals. The direct biological oxidation of pyrite would require metal sulfide dissociation as a prerequisite as already discussed. The bacteria probably oxidize pyrite by the following reaction: 6 Other iron sulfide minerals differ in their degree of recalcitrance CL Brierley Pyrrhotite Fe1.
XS leaches more readily than pyrite Ahonen et al. Iron can be brought into solution by the aqueous oxidation of pyrite by molecular oxygen. In an excellent review, Lowson identified three reaction paths for pyrite oxidation, namely bacterial, chemical, and electrochemical, res- tricting discussion to the latter two. The chemical oxidation path is a three-step process: 1. The oxidation of pyrite by molecular oxygen; 7 2.
The oxidation of ferrous iron to ferric iron by molecular oxygen, which is thought to be the rate-limiting step; 3. The oxidation of pyrite by ferric iron. In a natural environment, the large number of physicochemical parameters involved would greatly influence the kinetics and direction of pyrite dissolution. Elemental sulfur has a passivation effect due to the formation of a diffusion barrier between the mineral surface and bulk solution, a problem particularly associated with oxidative dissolution of chalcopyrite.
The role of bacteria in the formation and oxidation of the sulfur layer surrounding the mineral surface is poorly understood and not well documented. As a phenomenon, sulfur accumulation indicates the predominance of ferric ion-mediated leaching. Essentially no information is available on soluble sulfur intermediates sulfo- oxyanions in mineral bioleaching systems. Compounds such as thiosulfate and polythionates are extremely metastable in acid, metal-containing solutions. Trithionate has been detected as an intermediate in bacterial pyrite oxidation Basaran and Tuovinen , but its pathway is not presently known.
Unlike the direct mechanism of bacterial attack, when ferric iron is present, there is less need to break chemical bonds in the mineral sulfide surface by proton interaction Tributsch and Bennett a,b , although the proton concentration is critical to maintain ferric iron solubility. This provides an efficient means of sulfide oxidation. In the natural leaching system, it is likely that all of these parameters including the solid state, chemical and electrochemical properties, and electronic structures of the mineral sulfide, the presence of iron, the proton concentration, and the physicochemical environment are all important in the relationship between T.
Microbiological Oxidations of Minerals in Mine Tailings The physical characteristics of the tailings dam or waste material are of prime importance in the microbial oxidation of sulfide minerals. These include factors such as galvanic effects associated with mixtures of sulfide minerals Mehta and Murr , ; Natarajan and Iwasaki and the particle size distribution.
In most tailings deposits, the gangue associated with the mineral sulfide represents the bulk of the material present and therefore largely contributes to the envi- ronment in which the sulfide minerals are degraded. The gangue may consist of silicates, clays, and limestone-type materials which are highly acid-consuming.
Response of Plants and Vegetation to Mine Tailings and Dredged Materials
Acid removal can bring about the precipitation offerric iron which in tum reduces the diffusion of water, nutrients, and dissolved gases to sites of microbial activity. The physical nature of the gangue may also bring about similar problems. The adsorption characteristics of the gangue must also be considered.
The large buffering capacity of the gangue minerals will affect factors such as heat dissipa- tion. Thus, the thermal conductivity and heat capacity of these materials is of importance. As pointed out by Ralph , the composition of the water associated with tailings deposits will also influence the microbial oxidation of sulfide minerals.
Factors such as nutrient status, pH, redox status, heavy metal ion concentrations, iron concentration, and the presence of potentially toxic materials will interact and affect sulfide mineral oxidation. These in tum are also influenced by prevailing climatic conditions including rainfall, the length of dry periods, and ambient temperatures.
Changes in mining practices are obviously also of importance. Acid mine drainage has prompted the development of some models to describe the problem particularly in coal strip mine areas Cathles and Apps ; Jaynes et al. Such models incorporate several interactive components including aspects of bacterial dynamics. Stabilization of Dredged. Biological and Geochemical Aspects.
Environmental management of solid waste: dredged material and mine tailings W. The Predictive Assessment of the Migration of Leachate in. Titration of Bottom Ash Samples. Heavy metal fluxes by leachate are expected to be compatible with the environment; however, additional laboratory and field studies are necessary to assess their behavior over longer time periods. With an experimental approach, which was first used by Patrick et al. Here, an ion-exchanger system is used for extracting and analyzing the released metals at an adequate frequency Figure 4.
In these experiments, special attention was given to the efficiency of individual components with respect to long-term behavior of critical trace elements in such mixed deposits. The kinetics of element release from "conditioned" waste material i. By treatment with pH 5 solutions, mobilization of As is essentially completed after the initial five weeks of the experiment.
Mobilization of Zn is strongly enhanced toward the end of the study period. Regarding the latter element, it can be expected that the cumulative percentage of release from the treated material would sig- nificantly be enhanced upon continuation of the experiment.
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Chemical Characterization of Dredged Material. With regard to the selection of disposal options for dredged materials, the study of the water phase only with not be fully satisfying. While most of the actual situation would be reflected in these data, the potentialities of future adverse effects as well as the possible measures for reducing such hazards cannot be predicted.
In this respect, extractability of pollutants with chemical agents of different strength will provide more reliable information of the potential release of these substances under typical environmental conditions. Elutriate Test. To estimate short-term chemical transformations, the interrelations between solid phases and water has been increasingly subjected to laboratory experimentation. The ad- vantage of such experiments is that especially important parameters can be directly observed and particularly unfavourable conditions simulated. The Army Corps of Engineers and the U.
Environmental Protection Agency have developed an elutriate test that is designed to detect any significant release of chemical contaminants in dredged material. This test involves the mixing of one volume of the dredged sediment with four volumes of the disposal site water for a min shaking period. If the soluble chemical constituent in the water exceeds 1. Sequential Extraction. One of the widely applied extraction sequences of Tessier and co- workers has been modified by various authors.
Examples are presented in Table 3, e. It seems that thermodynamic models are still restricted because of various reasons: i adsorption characteristics are related not only to the system conditions i. One approach uses a multi-chambered device Calmano et al. A mass balance for the element copper in Figure 6b indicates that only 1. Only one third stays in solution, equivalent to approx.
Two thirds of the released copper is readsorbed at different affinities to the model substrates. The dominant role of organic substrates in the binding of metals such as Cd and Cu is of particular relevance for the transfer of these elements into biological systems. It can be expected that even at relatively small percentages of organic substrates these materials are primarily involved in metabolic processes and thus may constitute the major carriers by which metals are transferred within the food chain.
Metal Transfer Between Sedimentary Components.
New Developments in Environmental Geochemistry However, contaminants released either gradually from an imperfect impermeable barrier or catastrophically from failure of the barrier could produce substantial damage Kester et al. On the other hand, near-shore marine containment e.
In a review of various marine disposal options, Kester et al. Disposal in capped mound deposits above the prevailing sea-floor, disposal in sub-aqueous depressions, and capping deposits in depressions provide procedures for contaminated sediment Bokuniewicz, In some instances it may be worthwhile to excavate a depression for the disposal site of contaminated sediment which can be capped with clean sediment.
This type of waste deposition under stable anoxic conditions, where large masses of polluted materials are covered with inert sediment became known as "subsediment-depo- sit". The first example was planned for highly contaminated sludges from Stamford Harbour in the Central Long Island Sound following intensive discussions in the U.
Congress Morton, Under sub-sediment conditions there is a particular low solubility of metal sulphides, compared to the respective carbonate, phosphate, and oxide compounds. One major prerequisite is the microbial reduction of sulphate. Thus, this process is particularly important in the marine environment, whereas in anoxic freshwaters milieu there is a tendency for enhancing metal mobility due to the formation of stable complexes with ligands from decomposing organic matter.
Marine sulphidic conditions, in addition, seem to repress the formation of mono-methyl mercury, one of the most toxic substances in the aquatic environment, by a process of disproportionation into volatile dimethyl mercury and insoluble mercury sulphide Craig and Moreton, There are indications that degradation of highly toxic chlorinated hydrocarbons is enhanced in the sulphidic environment relative to oxic conditions Sahm et al. The aim of these techniques is a stronger fixation of contaminants to reduce the emission rate to the biosphere and to retard exchange processes.
Most of the stabilization techniques aimed for the immobilization of metal-containing wastes are based on additions of cement, water glass alkali silicate , coal fly ash, lime or gypsum Malone et al. Laboratory studies on the evaluation and efficiency of stabilization processes were performed by Calmano et al. Best results are attained with calcium carbonate, since the pH-conditions are not changed significantly upon addition of CaCO3.
Generally, maintainance of a pH of neutrality or slightly beyond favours ad- sorption or precipitation of soluble metals Gambrell et al. On the other hand it can be expected that both low and high pH-values will have unfavourable effects on the mobility of heavy metals. Experimental studies of the processes taking place with mixed residues from lignite coal incineration indicate favorable effects of incorporation of both chloride and heavy metals in newly formed minerals Figure 7.
The former may be incorporated at up to kg CaCl2 per m3 of the mineral mixture. Calcium-silicate-hydrate phases may be formed in a subsequent process, and by filling further pore space these minerals can significantly reduce permeability of the waste body for percolating solutions. Experimental studies of the leachability of salts and trace elements from samples of "stabilisates", with a pressure-filtration method, indicate relative high rates of release for sulphate ions, but not for zinc and cadmium in the eluate Bambauer, In general, microscale methods, e.