Competition adsorption of NO3- with Cl- on constant charge soils and variable charge soils

Adsorption of chloride and nitrate ions by three typical variable charge soils and four typical constant charge soils of China was studied under different pH, concentration and coexisting cations and anions When identical concentration of Cl- or NO3- was added to soil, the adsorption of Cl- by variable charge soils was higher than that of NO3- The selective coefficient was from 0.51 to 0.78 although the adsorption both Cl- and NO3- increased with decreasing pH and with increasing Cl- or NO3- concentration The concentration ratio Cl-/NO3- in soil suspension was less than 1.0 under different pH and coexisting cations such as Na+, K+, Ca2+, Mg2+, Fe3+ These results indicated that the variable charge soils had a higher preference for chloride ions relative to nitrate ions, and this tendency could not be changed by factors such as the pH, the concentration of the anions, the accompanying cations and etc Which suggested that this difference in affinity is caused by the difference in the nature of the interactions between these two ions and variable charge soils. However, the ratio Cl-/NO3- in equilibrium soil suspension of constant charge soils was almost near 1.0 under various pH, concentrations and species of coexisting anions and cations, which indicated that the adsorption mechanism and affinity of Cl- and NO3- by constant charges soils were not different significantly. Moreover, it was observed that a negative adsorption of Cl- and NO3- by constant charge soils in a mixed system containing equal amount of chloride and nitrate ions at low concentration.Which do a competitive adsorption of chloride and nitrate ions and repulsion of negative charge on the surface of soil particles for anion cause.