the effect of sewage sludge and incubation time on the availability and speciation of nickel and cadmium in the calcareous soil solutions

abstract in order to study the chemical behavior of nickel and cadmium in the presence of organic matter, an experiment with two different soils (4 and 30 percent caco3), two different texture (8 and 40 percent clay), two levels of nickel (0 and 100 mg.kg-1), two levels of cadmium (0 and 25 mg.kg-1) and organic matter in the form of sewage sludge in two levels (0 and 100 t. ha-1), at three times (0, 30, 60 days) in a completely randomized design with three replications was carried out. concentration of cations, anions, dissolved organic carbon and ph were measured in soil extracts. soil solutions were speciated for chemical forms of nickel and cadmium by minteqa2 model. the results of this study showed that nickel, cadmium and dissolved organic carbon in sewage sludge treated soils increased significantly. the amount of available nickel and cadmium decreased significantly with time. nickel and cadmium in sandy loam soil was significantly higher than clay soil. sewage sludge application decreased the amount of free hydrated cadmium significantly. in solution phase the proportion of free hydrated cadmium and nickel was higher in soil with 4 percent lime. in contrast, in soil with high lime, the percentage of nickel and cadmium in the form of carbonate was higher. in general, it seems that in the study soils calcium carbonate plays a key role in chemical behavior of nickel and cadmium in solution phase. therefore, it can be expected that the recommended threshold by environmental protection agencies should be higher in calcareous soils.