Extractable Chromium as Related to Soil pH and Applied Chromium

Abstract

AbstractThe objective of this study was to evaluate the soil chemistry of added Cr(III), Cr(VI), and sludge Cr as affected by initial soil pH. Treatments of 0 and 500 ppm Cr as CrCl3 and CrO3, and sludge Cr at a rate of 1,400 ppm Cr were applied to Rubicon sand (Entic Haplorthod, pH 4.7), Morley clay loam (Typic Hapludalf, pH 6.0), and limed Morley clay loam (pH 7.5). Pots were sampled at 24 hours, 1, 2, 4, 8, and 16 weeks of incubation. Soil samples were extracted in succession with distilled water, 1M NH4Cl, 0.1M CuSO4, 0.3M (NH4)2C2O4, and citrate‐dithionite‐bicarbonate.Chromium(III) addition reduced soil pH while sludge raised soil pH. Chromium(VI) treatment initially lowered soil pH but then raised it above the pH of control soils after a short time period. Water‐soluble Cr(III) decreased with time, though more rapidly as soil pH increased. When added as Cr(VI), water‐soluble Cr also decreased with time but less rapidly as soil pH increased. Exchangeable (1M NH4Cl extractable) and organic‐bound Cr (0.1M CuSO4 extractable) were negligible. Oxalate and dithionite extractions removed large quantities of Cr from all treatments. Water‐soluble Cr compounds appeared to convert to insoluble compounds of Cr(III) in soils. Sludge Cr is probably a colloidal precipitate prior to incorporation.