ELIMINATION OF EFFLUENT ARSENIC RESULTING FROM HYDROLYSIS OF LEWISITES

A detoxication process of aqueous solutions obtained by hydrolysis of Lewisites, where arsenic and chloride contents are respectively equal to 0.1 and 1 mol.L-1 has been studied. The elimination of arsenite ion s, obtained during the hydrolysis, can be carried out by their precipi tation under an insoluble compound form. A two-step chemical treatment is proposed in order to obtain both a good confining of arsenic in th e precipitates and a concentration in effluents in agreement with the maximum permissible level (100 ppb in France, 50 ppb in USA and Japan) . The first one will consist of eliminating in batch the major part of arsenic by precipitation of an insoluble compound. The second step wi ll be a chromatographic treatment with selective stationary phases. In this article, the arsenic elimination by precipitation (both calcium arsenite and arsenate), trapping or coprecipitation in presence of iro n(III) has been studied. The calcium arsenite precipitation entails an elimination rate of 80% at pH 11 and for an addition of calcium ions in a molar ratio Ca2+/As-III equal to 15. Unfortunately the arsenic re sidual content is too high (300 ppm As) in the perspective of column t reatment. The calcium arsenate precipitation is much more efficient. T he elimination rate is greater than 99% at pH 12.5 for five times stoi chiometric calcium addition. The arsenite and arsenate ions eliminatio n at optimal pH (pH 9 for the arsenite and 4.5 for the arsenate) durin g the hydroxide ferric precipitation gives identical or even better (> 99.9%) elimination rates than for calcium arsenate precipitation. The elimination rate of 99% is reached for a molar ratio Fe(III)/As(III o r V) equal to one. In such operating conditions, the mole ratio As/Fe in the precipitate is equal to one. The optimal pH of the arsenite eli mination is more compatible with the pH (approximate to 14) of the sol utions from the Lewisite hydrolysis than the optimal pH of the arsenat e elimination. 0