Effects of electrolyte composition on chromium desorption in soils contaminated by tannery waste

We conducted batch studies to investigate the effect of phosphate and domin ant cations present in tannery waste on desorption of chromium (Cr) from su rface and subsurface soil horizons from a contaminated tannery waste site a t Mount Barker, South Australia. The surface horizon (0-15 cm) of the soil profile was alkaline (pH 7.9) and the subsurface horizon (50-90 cm) was aci dic (pH 3.9). Aqua-regia extractable Cr concentration ranged from 62 g/kg i n the surface to 0.26 g/kg in the subsurface soils. X-ray diffraction and s canning electron microscopic studies revealed the predominance of carbonate and kaolin minerals in the surface soils, and highly weathered framboidal particles, with morphology similar to that of pyrites, in the subsurface so ils. The amount of Cr desorbed from the contaminated soils varied considera bly with both the electrolyte's cationic charge and soil properties. The ef fect of cations, Ca2+ and Na+ (CaCl2, NaCl), and phosphate on desorption of Cr was investigated. While the ionic strength of the solutions was standar dised at approximately 0.03 mol/L, the concentration of phosphate (as KH2PO 4) was varied from 0 to 3.2 mmol/L. On the basis of studies on solution to soil ratio and desorption kinetics we chose a 20:1 solution to soil ratio a nd 2 h equilibration time. In the surface alkaline soil, the amount of Cr r eleased decreased in the order Na+ (286 mug/L) water (256 mug/L) > Ca2+ (15 6 mug/L). In contrast subsurface acidic soil showed a reverse trend, i.e. C a2+ (52 mug/L) > Na+ (29 mug/L) > water (20 mug/L). Speciation of Cr in the extracts showed predominantly Cr(VI) in the alkaline surface soil and Cr(I II) in the subsurface acidic soil. A strong effect of phosphate on the cumu lative amount of chromate desorbed was observed. The study shows that the p resence of high concentrations of Na+ (0.03 mol/L) and phosphate (3.2 mmol/ L) in soil solution enhances Cr(VI) mobility. In contrast, Ca2+ may induce retention of Cr(VI) in Cr-contaminated alkaline soils.