Removal of Arsenical By-Products from Chemical Warfare Destruction Effluents

Abstract

Within the context of the chemical warfare destruction (April, 29(th) 1997 convention), the removal of arsenic, which is highly toxic, from the lewisite hydrolysis effluents has been studied. A process in two steps is presented, which is compatible with industrial and economical constraints. An efficient way to eliminate the major part of arsenic consists in precipitating it into an insoluble form (calcium arsenate, ferric arsenate or arsenite, etc...). Indeed, the elimination of arsenite and arsenate ions at optimal pH (pH 9 for As-III and 4.5 for As-V) during the precipitation of ferric hydroxide yields very good elimination rates (> 99.9 %). A molar ratio Fe(III)/As(III or V) as low as I is sufficient to get an elimination rate higher than 99%. In addition, this method allows to obtain a good confining of arsenic in the precipitate; for an addition of iron(III) in a molar ratio Fe(III)/As(III or V) equal to 1, the molar ratio As/Fe into the precipitate is also equal to 1. In this way the solid waste volume is minimized. However, the residual concentration of arsenic is unfortunately greater than the arsenic's maximum contaminant level (M.C.L.). For instance, this one reaches (10.8 +/- 0.2) ppm for As-III at pH 9 and (1.4 +/- 0.1) ppm for As-V at pH 4.5. In both cases, iron(III) is added so that the molar ratio Fe(III)/As(III or V) is equal to 1.5. A second step is therefore necessary. After the batch treatment, the arsenite or arsenate traces can be eliminated by selective adsorption on iron(III) hydroxide precipitated within the pores of a macroporous cation-exchange resin; with a 13 cm x 1 cm i.d column (bed volume equal to 10 mi), an effluent volume equal to 2.8 liters, where the arsenic content is lower than the maximal permissible concentration, is obtained. An extrapolation from these results shows a resin bed volume of 3.6 liters (for instance, a 72 cm x 8 cm i.d. column) would be required to decrease the residual arsenic content of 1 m(3) of effluent from 10 to 0.1 ppm.