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Assessment of Arsenic Treatment Residuals

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Residuals were obtained by the removal of arsenite, As(III), or arsenate, As(V), in pure solutions or in the presence of kaolinite, montmorilinite, organic material and calcium carbonate. Production was achieved by co-precipitation/sorption using aluminum sulfate, ferric chloride or lime/soda addition. The extent of arsenite and arsenate removal for each sample composition was quantified and the residuals were analyzed for iron, aluminum and calcium content. Desorption studies were conducted consisting of TCLP, pH variance (4, 6, 8 and 10), and the presence of competing anions (chloride, sulfate and phosphate). Kinetic studies of desorption under the conditions cited were also conducted and the buffering capacity of the residuals documented. The extent of removal for both arsenite and arsenate in a variety of mixtures was conducted and quantified. The following can be concluded:

(1) Iron oxy-hydroxides are far superior at removing both arsenate and arsenite as opposed to lime/soda softening or aluminum sulfate.

(2) Desorption of arsenic from the iron oxy-hydroxides is minimal for both oxidation states of arsenic. However, the amount of arsenate desorbed was below detection limit at or below a pH of 8 for the iron residuals.

(3) Arsenite residuals formed using alum showed by far the most extensive amount of desorption when compared to calcium carbonate or iron oxy-hydroxides.

(4) While both aluminum and iron show increased desorption at higher pH for arsenate, calcium carbonate actually has lower desorption. Thus, a mixed calcium and iron system may be desirable.

(5) In general, high pH favors arsenate desorption while low pH favors arsenite release (except for the calcium system).

(6) All of the residuals are powerful buffers. The iron and aluminum residuals tend to maintain a neutral pH while the calcium residuals raise pH.

(7) Phosphate significantly influences desorption of iron based residuals; sulfate less so and chloride has a negligible effect. Calcium based arsenic residuals also show competing ion influence, while the aluminum residuals desorb to a very high extent regardless of the presence of competing ions.

From these results we can conclude the following at this stage of the research.

(1) The oxidation of arsenite to arsenate is highly desirable.

(2) The use of ferric chloride is preferred over alum in general. Overall coagulation studies that include particulate removal need to be conducted on each metal salt before a final conclusion can be made.

(3) Calcium addition with ferric chloride base coagulation may be the key to obtaining residuals that remain stable (resist leaching of arsenic) over a broad range of pH.

(4) The metal salt-based residuals have a strong tendency to buffer the water and thus resist pH extremes. Lime/soda residuals tend to raise pH.
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Document Type: Research Article

Publication date: 2003-01-01

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