Oxidation state arsenic removal

The washed slime is dried and melted to produce slag and metal. The slag is usually purified by selective reduction and smelted to produce antimonial lead. The metal is treated ia the molten state by selective oxidation for the removal of arsenic, antimony, and some of the lead. It is then transferred to a cupel furnace, where the oxidation is continued until only the silver—gold alloy (dorn) remains. The bismuth-rich cupel slags are cmshed, mixed with a small amount of sulfur, and reduced with carbon to a copper matte and impure bismuth metal the latter is transferred to the bismuth refining plant. 

Valence state Also called the oxidation number or oxidation state. An integer (positive, negative, or zero) that describes the number of electrons that must be added or removed from an atom to give it a neutral charge. Typical valence states for arsenic are -3, 0, +3, and +5. 

Arsenic is a frequent contaminant in metal ores (especially sulfides) and therefore appears in hydrometallurgical processes applied to these ores. Because of its toxicity, it must be efficiently removed from aqueous process streams before discharge. Traditionally this has been done with lime, but a large excess is required and the product (calcium arsenite and arsenate) can re-release some arsenic under the influence of atmospheric carbon dioxide. A more reliable process is co-precipitation with iron salts under near-neutral conditions. This requires both element ores to be in their higher oxidation states and hydrogen... 

Arsenic can exist in several oxidation states, as both inorganic and organometallic species, and in dissolved and gaseous phases (Table I). Dissolved arsenic species can adsorb to suspended solids and be carried down to the sediments in an aquatic system. Since gaseous arsenic compounds can form, arsenic can be removed from the sediments as dissolved gas or in gas bubbles (e.g. CH ). Thus, arsenic can cycle within aquatic ecosystems and this cyclic behavior has been reviewed by Ferguson and Gavis (1 ) and Woolson 2). In any given system, it is necessary to understand the behavior of a variety of different arsenic compounds as well as a variety of environmental compartments in order to totally characterize the cyclic behavior of this element. 

Pre-treatment to destroy organic matter. Organic selenium species are more widespread in the environment than comparable arsenic species. The determination of total selenium by most analytical methods requires samples to be pre-treated to remove organic matter, release selenium, and change its oxidation state. 

In contrast to inorganics such as mercury, copper, arsenic, lead, and cadmium, there have been fewer studies that focus only on zinc removal by carbonaceous adsorbents [199-203]. The presence of zinc in water appears to be due primarily to corrosion of galvanized metals [33]. The dominant oxidation state in aqueous solution is Zn(II), while the dominant species (see Fig. Al) are Zn at pH < 9 and Zn(OH), at pH > 9 [202]. 

Although the occurrence of arsenic in the source water will determine whether treatment would be required to meet a new standard, other water quality parameters will also influence the feasibility of various treatment technologies. Both the oxidation state of arsenic and the concentrations of co-occurring source water constituents will affect the efficiency of arsenic removal (see Chaps. 6 and 9). 

A cycle for arsenic in a stratified lake is shown in Fig. 4. The reactions include transfers from solution to solid phases, conversions from one oxidation state to another, and ligand exchanges. Some of the processes are chemical, some occur through microbial mediation, and some can occur either way. Fish and plants enter the cycle by concentrating arsenic, especially trimethylarsine. Upon death the organisms settle to the bottom where the arsenic is removed to the sediments or recycled depending on the physical and chemical conditions [6]. 

The niter and fresh caustic soda, required to maintain the fluidity of the salt bath in the reactor chamber, are added gradually. When the color of the saturated salts turns from a dark gray to white, the impurity metals are at their highest state of oxidation, and the lead content of the spent salts is very low. In a modification, the arsenic and tin are selectively removed as sodium arsenate and sodium stannate, followed by the removal of antimony as sodium antimonate. 

In general, the direct-oxidation processes employ a redox couple that has sufficient oxidation potential to convert H2S into elemental sulfur but insufficient potential to oxidize sulfur to higher states. Examples of materials that have this redox potential are vanadium compounds, arsenic compounds, iron compounds, and certain organic species. Typically, the redox materials, dissolved in a hot potassium carbonate solution with the species in its oxidized form, contacts the I S-laden gas and the H2S dissolves as the hydrosulfide. This sulfur reacts with the redox couple, forming elemental sulfur and the reduced state of the couple. Airblowing of the solution reoxidizes the couple and removes the elemental sulfur from solution as a product froth. 

SORB33 A process for removing arsenic from public water supplies by adsorption on granular ferric oxide. The adsorbent is Bayoxide 33, a proprietary product made by Bayer. Developed by Bayer and Severn Trent PLC and first demonstrated at the Burton Joyce waterworks in Nottingham, UK. In 2004, the process was in use at 15 plants in the UK and planned for use in 6 demonstration plants in the United States. 

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