Timothy A. Kramer

Arsenic stabilization on water treatment residuals by calcium addition.

A common method of removing arsenic from contaminated water is the co-precipitation or sorption of arsenic onto oxy-hydroxides formed by the addition of metal salts. Arsenic co-precipitation produces solids containing high concentrations of arsenic. The elevated arsenic content poses leaching problems requiring expensive disposal in certified hazardous impoundments. The objective of this research is to determine the effect of calcium addition as a stabilization agent, on arsenic desorption from ferric water treatment residuals. Due to the treatment residuals buffer capacity, desorption experiments in this study did not follow the standard Toxicity Characteristic Leaching procedure (TCLP) test. Arsenate desorption was induced in two ways: controlling solution pH in de-ionized water, and controlling solution pH in a 1.33 mM phosphate solution where phosphate is a competing anion. Desorption from laboratory treatment residuals did not generate any arsenic when calcium was present in solution, especially when excess calcium that did not join the surface of the treatment residual was present. Similarly, arsenic leaching decreased when field treatment residuals were treated with lime as stabilizing agent. Ordinary Portland cement (OPC) was also tested as a stabilizing agent in conjunction with lime since long term lime stabilization can be slowly consumed when directly exposed to atmospheric CO(2). The solidification and stabilization (S/S) technique with lime and OPC was shown to be successfully applied to the immobilization of arsenic tainted water treatment residuals.

Perchlorate reduction during electrochemically induced pitting corrosion of zero-valent titanium (ZVT).

Zero-valent metals and ionic metal species are a popular reagent for the abatement of contaminants in drinking water and groundwater and perchlorate is a contaminant of increasing concern. However, perchlorate degradation using commonly used reductants such as zero-valent metals and soluble reduced metal species is kinetically limited. Titanium in the zero-valent and soluble states has a high thermodynamic potential to reduce perchlorate. Here we show that perchlorate is effectively reduced to chloride by soluble titanium species in a system where the surface oxide film is removed from ZVT and ZVT is oxidized during electrochemically induced pitting corrosion to produce reactive soluble species. The pitting potential of ZVT was measured as 12.77±0.04 V (SHE) for a 100 mM solution of perchlorate. The rate of perchlorate reduction was independent of the imposed potential as long as the potential was maintained above the pitting potential, but it was proportional to the applied current. Solution pH and surface area of ZVT electrodes showed negligible effects on rates of perchlorate reduction. Although perchlorate is effectively reduced during electrochemically induced corrosion of ZVT, this process may not be immediately applicable to perchlorate treatment due to the high potentials needed to produce active reductants, the amount of titanium consumed, the inhibition of perchlorate removal by chloride, and oxidation of chloride to chlorine.