The pH and soil/solution ratio are two parameters influence the sorption of heavy metals in the soil. Selectivity sequences of cadmium (Cd), copper (Cu), nickel (Ni), and zinc (Zn) in a single and competitive system under different soil/solution ratios (1:2, 1:5, 1:10, 1:25, 1:50, and 1:100) and initial pH (3, 4, 5, 6, 7, and 8) was tested in a calcareous soil. The results showed that heavy metals sorption decreased with increasing soil/solution ratio, while it increased with increasing pH, in all soil/solution ratios and under both single and competitive systems. The most common selectivity sequence calculated on the basis of the Freundlich distribution coefficient at different soil solution ratios and pHs was Cd > Ni > Zn > Cu in a single system, while in a competitive system, the selectivity sequence was Ni > Zn > Cu > Cd. It seems at high heavy metals addition, both sorption and precipitation of Cd, Cu, Ni, and Zn (only at low soil/solution ratio) control their concentration in soil under all pHs and both at low and high soil/ solution ratios in single and competitive systems. It was revealed that competition between studied heavy metals for exchange sites reduced Cd sorption and thus the mobility of Cd in a multiple system is higher than a single system, posing a serious health risk. The results of this study may be used to evaluate the risks and develop tools to minimize the risks of heavy metals in soils polluted with multiple heavy metals.
