Sulfide immobilization

Availability of reactive iron in sediments also has been postulated to control S retention. Reactive iron may limit fluxes of S recycled from sediments by rendering sulfide immobile and less amenable to oxidation by bacteria or chemical agents. Availability of iron strongly influences total S content and isotopic signature of marine sediments (198). Canfield (94) ob-... 

The samples were stored for 3 weeks for curing. Each sample was then crushed and was subjected to the TCLP test. The TCLP test results on both the waste stream and the treated CBPC waste form are given in Table 16.6. The results on the untreated waste streams show that the leaching levels far exceed the regulatory limits. The results for the waste forms, on the other hand, are an order of magnitude below the EPA limit. These results indicate superior stabilization of Hg in the phosphate ceramic waste forms coupled with sulfide immobilization. 

Arsenic is another element with different bioavailabiUty in its different redox states. Arsenic is not known to be an essential nutrient for eukaryotes, but arsenate (As(V)) and arsenite (As(III)) are toxic, with the latter being rather more so, at least to mammals. Nevertheless, some microorganisms grow at the expense of reducing arsenate to arsenite (81), while others are able to reduce these species to more reduced forms. In this case it is known that the element can be immobilized as an insoluble polymetallic sulfide by sulfate reducing bacteria, presumably adventitiously due to the production of hydrogen sulfide (82). Indeed many contaminant metal and metalloid ions can be immobilized as metal sulfides by sulfate reducing bacteria. 

Ion exchange, in which cation and/or anion resins are used to replace undesirable anionic species in liquid solutions with nonhazardous ions. For example, cation-exchange resins may contain nonhazardous, mobile, positive ions (e g., sodium, hydrogen) which are attached to immobile acid groups (e.g., sulfonic or carboxylic). Similarly, anion-exchange resins may include nonhazardous, mobile, negative ions (e.g., hydroxyl or chloride) attached to immobile basic ions (e.g., amine). These resins can be used to eliminate various species from wastewater, such as dissolved metals, sulfides, cyanides, amines, phenols, and halides. 

Biological activity can be used in two ways for the bioremediation of metal-contaminated soils to immobilize the contaminants in situ or to remove them permanently from the soil matrix, depending on the properties of the reduced elements. Chromium and uranium are typical candidates for in situ immobilization processes. The bioreduction of Cr(VI) and Ur(VI) transforms highly soluble ions such as CrO and UO + to insoluble solid compounds, such as Cr(OH)3 and U02. The selenate anions SeO are also reduced to insoluble elemental selenium Se°. Bioprecipitation of heavy metals, such as Pb, Cd, and Zn, in the form of sulfides, is another in situ immobilization option that exploits the metabolic activity of sulfate-reducing bacteria without altering the valence state of metals. The removal of contaminants from the soil matrix is the most appropriate remediation strategy when bioreduction results in species that are more soluble compared to the initial oxidized element. This is the case for As(V) and Pu(IV), which are transformed to the more soluble As(III) and Pu(III) forms. This treatment option presupposes an installation for the efficient recovery and treatment of the aqueous phase containing the solubilized contaminants. 

Precipitation is the most promising method for immobilizing dissolvable metals such as lead, cadmium, zinc, and iron.15 Some forms of arsenic, chromium, mercury, and some fatty acids can also be treated by precipitation.47 The common precipitating chemicals for metal cations are sulfide, phosphate, hydroxide, or carbonate. Among them, sulfide is the most promising, because sulfides have low solubility over a broad pH range. Precipitation is most applicable to sites with sand or coarse silt strata. 

Redox reactions may cause mobile toxic ions to become either immobile or less toxic. Hexavalent chromium is mobile and highly toxic. It can be reduced to be rendered less toxic in the form of trivalent chromium sulfide by the addition of ferrous sulfate. Similarly, pentavalent (V) or trivalent (III) arsenic, arsenate or arsenite are more toxic and soluble forms. Arsenite (III) can be oxidized to As(IV). Arsenate (V) can be transformed to highly insoluble FeAs04 by the addition of ferrous sulfate. 

More specifically, the invention involves the use of Thiobacillus denitrificans under anaerobic conditions to oxidize sulfur compounds such as hydrogen sulfide to sulfate. The process may be carried out in various ways such as in a batch or a continuous bioreactor system using a suspended or an immobilized biocatalyst. The method is particularly applicable to treating natural gas containing hydrogen sulfide and producing a biomass byproduct. 

Gruzdkov, Yu.A., Savinov, E.N., and Parmon, V.N., Photocatalytic decomposition of hydrogen sulfide in the presence of polymer immobilized cadmium sulfide. Promotion by I and VIII group metals, Int.. Hydrogen Energ., 12, 393, 1987. 

Metal contaminants can in some cases be immobilized in situ by oxidation or reduction, or precipitated by reaction with sulfide. Sulfate reducing bacteria are sometimes stimulated to produce sulfide, or a sulfur-bearing compound such as calcium polysulfide can be injected into the subsurface as a reductant and sulfide source. In certain cases where the contamination poses little immediate threat, it can safely be left to attenuate naturally (e.g., Brady et al., 1998), a procedure known as monitored natural attenuation. 

Alkali-immobile dye-releasing quinone compounds, 19 293-294 Alkali lignins, 15 19-20 Alkali manganate(VI) salts, 15 596 Alkali manganates(V), 15 592 Alkali-metal alkoxide catalysts, 10 491 Alkali-metal alkoxides, effects of, 14 252 Alkali-metal alkylstannonates, 24 824 Alkali-metal fluoroxenates, 17 329-330 Alkali-metal hydrides, 13 608 Alkali-metal hydroxides, carbonyl sulfide reaction with, 23 622 Alkali-metal metatungstates, 25 383 Alkali-metal perchlorates, 18 211 Alkali-metal peroxides, 16 393... 

Divinyl sulfide, 25 630 Divinylsulfone method, for covalent ligand immobilization, 6 3961 Division of Chemical Nomenclature and Structure Representation (IUPAC),... 

Rao, Jayalakshmi and coworkers have prepared NPs of SnO, SnS and ZnS in order to study their capacitance behavior [88-90]. The metal oxide and metal sulfide NPs were prepared using hydrothermal methods. After immobilization in PIGE electrodes, their electrochemical properties were examined. Capacitance values in the 4—15 Fg were reported for SnS. Comparable values were reported for ZnS. 

There may be a cycling of S compounds of different oxidation state between anaerobic and aerobic zones in the soil, such as at the soil—floodwater interface. In reduced lake and marine sediments this leads to accumulation of insoluble sulfides as S04 carried into the sediment from the water above is immobilized. Such deposits function as sinks for heavy metals. Plants absorb S through their roots as S04 H2S is toxic to them. Therefore HS must be oxidized to S04 in the rhizosphere before it is absorbed. 

Like Zn, Cd is a Group IIB element and occurs in soils exclusively in the +2 oxidation state as the Cd + cation. Cadmium and zinc are often co-precipitated with each other in sulfide minerals in rocks (p/fCdS = 27.0). Hence Cd tends to be highly immobile under anaerobic sulfate-reducing conditions, but under acid, oxidizing conditions it is released in soluble and mobile forms. Hence soils... 

Mercury occurs in soils predominantly in the +2 oxidation state. Elemental Hg in the atmosphere is oxidized to Hg + and deposited in rainfall. It is a strong chalcophile and under anaerobic conditions forms the extremely insoluble sulfide cinnabar (HgS, pK = 52.7). Nonetheless it is not entirely immobilized under anaerobic conditions because it is reduced to volatile Hg° or methylated to volatile methyl mercury compounds by microbial action, and so returned to the atmosphere. The methylation is mediated by various bacteria, especially methanogens, through the reactions ... 

Immobile Element Lithogeochemistry of felsic volcanic rocks hosting the Restigouche Volcanogenic Massive Sulfide Deposit, Bathurst Mining... 

Bein, A. Lentz, D.R. 2009. Immobile Element Lithogeochemistry of felsic volcanic rocks hosting the Restigouche Volcanogenic Massive Sulfide Deposit, Bathurst Mining Camp, New Brunswick Canada. Proceedings of lAGS 2009. 

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