The effect of different doses of sewage sludge and liming on total cobalt content and its speciation in soil

Abstract

The aim of this paper is to evaluate the effect of liming and various doses of municipal sewage sludge (5, 10, and 15% of the weight of the soil) on cobalt total content and its speciation. The incubation experiment lasted 420 days and was conducted in controlled laboratory conditions. Soil was sampled after 30, 60, 90, and 120 days and then, with a break of 180 days, after 360 and 420 days. In all samples, cobalt total content was determined by means of ICP-AES (emission spectrophotometer), and fractions of this metal were measured with the seven-step Zeien and Brümmer method, with seven of them separated: F1-easily soluble, F2-exchangeable, F3-bound to MnOx, F4-bound to organic matter, F5-bound to amorphous FeOx, F6-bound to crystalline FeOx, and F7-residual. Compared to the control, the average content of total cobalt in the soil increased more than two times in experimental units with higher doses of sewage sludge (10% and 15% of the weight of the soil). The metal was mainly bound to the residual fraction, where it constituted 40% of its total content, while in the organic fraction, its share was 20%. In the soil incubated with sludge, cobalt in mobile fractions constituted a small percentage of its total content. Liming limited the release of this metal.