Dissolving metals sulfides

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. 

GC/FPD has been used to measure hydrogen sulfide, free disulfide, and dissolved metal sulfide complexes in water (Radford-Knoery and Cutter 1993). Hydrogen sulfide was measured in the headspace of the sample (100 mL) with a detection limit of 0.6 pmol/L. A detection limit of 0.2 pmol/L was obtained for total dissolved sulfide. This method allows for the determination of the concentration of free sulfide that is in equilibrium with hydrogen sulfide. Complexed sulfide can be estimated from the difference between total dissolved sulfide and free sulfide. 

Sulfide (S ) is a bivalent monoanion produced from the decomposition of metal sulfide salts. It occurs in groundwaters, hot springs, and wastewaters. It is also formed from the bacterial reduction of sulfate. Sulfide salts in solid wastes in contact with an acid can produce hydrogen sulfide. H2S, which is highly toxic. In an aqueous sample, sulfide may be present as dissolved H2S and HS , dissolved metallic sulfide, and acid-soluble metallic sulfide contained in suspended particles. All these soluble and insoluble sulfides and dissolved H2S and HS together are termed as total sulfide. The sulfide remaining after the removal of suspended solids is termed the dissolved sulfide. Copper and silver sulfides are insoluble even under acidic conditions. Therefore, these two sulfides are not determined in the following tests. 

The standard free energy, G, of various dissolved metallic, sulfide, oxide and other species in aqueous solution are listed in the Appendix. 

Technology Description To achieve precipitation, acid or base is added to a solution to adjust the pH to a point where the constituents to be removed have their lowest solubility. Chemical precipitation facilitates the removal of dissolved metals from aqueous wastes. Metals may be precipitated from solutions as hydroxides, sulfides, carbonates, or other soluble salts. A comparison of precipitation reagents is presented in Table 7. Solid separation is effected by standard flocculation/ coagulation techniques. 

Reactions of the various sulfides of arsenic call for little further comment. AS2S3 bums when heated in air to give AS2O3 and SO2. Chlorine converts it to ASCI3 and S2CI2. It is insoluble in water but dissolves readily in aqueous alkali or alkali-metal sulfide solutions to give thioarsenites ... 

An alternative procedure for removing an ion from solution is to change its identity by changing its oxidation state. The metal ions in very insoluble heavy metal sulfide precipitates can be dissolved by oxidizing the sulfide ion to elemental sulfur. For example, copper(II) sulfide, CuS, takes part in the equilibrium... 

The solubility of a solid can be increased by removing one of its ions from solution acid can be used to dissolve a hydroxide, sulfide, sulfite, or carbonate precipitate and nitric acid can be used to oxidize metal sulfides to sulfur and a soluble salt. 

To overcome some of the problems associated with aqueous media, non-aqueous systems with cadmium salt and elemental sulfur dissolved in solvents such as DMSO, DMF, and ethylene glycol have been used, following the method of Baranski and Fawcett [48-50], The study of CdS electrodeposition on Hg and Pt electrodes in DMSO solutions using cyclic voltammetry (at stationary electrodes) and pulse polarography (at dropping Hg electrodes) provided evidence that during deposition sulfur is chemisorbed at these electrodes and that formation of at least a monolayer of metal sulfide is probable. Formation of the initial layer of CdS involved reaction of Cd(II) ions with the chemisorbed sulfur or with a pre-existing layer of metal sulfide. 

The corrosion of metal surfaces and the precipitation of a metal sulfide by an aqueous acid solution can be prevented by an aldol-amine adduct. Aldol (from acetaldehyde) CH3CH(OH)CH2CHO has been utilized as a H2S scavenger that prevents the precipitation of metal sulfides from aqueous acid solutions. However, when the aldol or an aqueous solution of the aldol is stored, the solution separates quickly into two layers, with all of the aldol concentrated in the bottom layer. The bottom layer is not redispersible in the top layer or in water or acid. In addition, the aldol in the bottom layer has very little activity as a sulfide scavenger. Thus the use of aldol as a H2S scavenger in aqueous acid solutions can result in unsatisfactory results [245,247]. However, the aldol can be reacted with an amine, such as monoethanoleamine (=aminoethanol), to form an aldol-amine adduct to overcome these difficulties. The amine utilized to prepare the aldol-amine adduct must be a primary amine. The aldol-amine adduct preferentially reacts with sulfide ions when they are dissolved in the... 

The electrochemical process differs from the chemical process by the fact the solid to be dissolved has to be electrically conducting (as for example, a metal), or a semiconductor (as for example, certain oxides and metal sulfides). As some specific examples of dissolution occurring electrochemically, mention may be made of (i) metals in oxygenated water,... 

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. 

Precipitation may be significant for heavy metals and other inorganic constituents in injected wastes. For example, sulfide ions have a strong affinity for metal ions, precipitating as metal sulfides. The dissolved constituents in injected wastes and reservoir fluids would not be in equilibrium with the in situ brines because of the fluids different temperature, pH, and Eh. When the fluids are mixed, precipitation reactions can lead to injection-well plugging. 

Oxidation by chemolithotropic microorganisms of the sulfur entities of metal sulfides to obtain energy is an example of direct dissolving action under aerobic condition (Kurek 2002). Compounds of many other... 

Acid drainage is a persistent environmental problem in many mineralized areas. The problem is especially pronounced in areas that host or have hosted mining activity (e.g., Lind and Hem, 1993), but it also occurs naturally in unmined areas. The acid drainage results from weathering of sulfide minerals that oxidize to produce hydrogen ions and contribute dissolved metals to solution (e.g., Blowes et al., 2005). 

Geochemical results show that the degree of sulfide oxidation is dependent upon the temperature of the piles and the rock sulfide content. In the basal drain effluent of the Type III pile, a decrease in pH was observed throughout the summer as the pile warmed. Sulfate concentrations also increase with the warming of the pile and decrease as the pile cools. The decrease in pH is correlated with an increase in the concentrations of dissolved metals. 

Reductive dissolution of Fe oxyhydroxides holding sorbed As appears to explain the very large concentrations of As in water from wells drilled into alluvial sediments of the Brahmaputra and Ganges Rivers in Bangladesh and West Begal (Nickson et al 1998, 2000). Dissolved As has accumulated from the reduction of As-rich Fe oxyhydroxides formed upstream of the contaminated areas by weathering of As-rich base metal sulfides. The reduction is driven by sedimentary organic matter in the deposits. Release of As from oxidation of pyrite in shallow wells contributes little to the water contamination because any As(IV) released would be re-sorbed on Fe oxides formed in pyrite oxidation. 

A special case represents acid sulfate waters released from mines where metal sulfide ores and lignite have been exploited. S- and 0-isotope data may define the conditions and processes of pyrite oxidation, such as the presence or absence of dissolved oxygen and the role of sulfur-oxidizing bacteria (i.e. Taylor and Wheeler 1994). 

Pure S is unsuitable because the operating temperature of these cells is so high (about 400 ) that S dissolves in the LiCl-KCl electrolyte and reacts with Li at the anode, leading to self-discharge. However, in metal-sulfide cells these problems have been circumvented. Attempts have been made to replace S by... 

Yellow-brown glassy amorphous sohd sublimes on heating decomposes around 500°C insoluble in cold water ( 1.4 mg/L at 0°C) dissolves in alkahes and solutions of alkali metal sulfides, and in nitric acid. 

Mercury(ll) sulfide dissolves in concentrated solutions of alkali or alkaline-earth metal sulfides forming thiosalts, such as Na2[HgS2] XFI2O. Such thio-salts are stable in solution only when alkaline hydroxides are present in excess. These salts also are obtained as bright and deliquescent needles when HgS is heated with sulfur and alkaline hydroxides. 

Sulfur can be analyzed by x-ray, GC and GC/MS techniques. Alpha-octacy-closulfur is dissolved in benzene, toluene, or chloroform and analyzed for sulfur by GC using a flame photometric detector or by GC/MS. The characteristic mass ions for its identification are multiples of 32 (i.e. 32, 64, 128, and 256). Sulfur may be identified by mixing a little powder with copper, silver, or mercury at room temperature and identifying the metal sulfide from color change and various instrumental methods. 

In solid state physics, it is well known that many inorganic solids, e.g., the oxides and sulfides, can dissolve metals and nonmetals in excess, and that by this process electron and ion defects in the lattice will be formed. Wagner and co-workers (1) have developed the basic theory of... 

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