Effects of organic acids, o-phenylenediamine and pyrocatechol on cadmium adsorption and desorption in soil

This article studied the interaction of cadmium (Cd) and low molecular weight organics in Indicotic black (IB) soil. Cadmium adsorption isotherm in this soil was satisfactorily simulated using Freundlich equation as Q = 905.6C(0.49) with high correlation (r(2) = 0.984), and its adsorption quantity increased with increasing pH. The presence of citric acid and EDTA significantly reduced Cd adsorption in soil, which was due to the formation of soluble Cd-organic complex. Concentration of cadmium ions in equilibrium solution was determined and percentages of [CdH(2)Cit(+)]/[Cd-complex(Total)], [CdHCit]/[Cd-complex(Total)] and [CdCit(-)]/[Cd-complex(Total)] vs. pH were successfully calculated. Desorption percentage of Cd, adsorbed in the presence of citric acid and EDTA decreased, compared with that adsorbed in NaNO3 media. It suggests that free sites for Cd adsorption in soil increased in the presence of organic acid. When o-phenylenediamine, pyrocatechol and aminoethonic acid appeared in Cd equilibrium media, Cd adsorption quantity increased by increasing cation exchange of positively charged cadmium complex with soil at low pH. Compared with that performed in the absence of these organics, the exchangeable Cd, desorbed by 0.1 mol L-1 NaNO3, reduced obviously, which suggests that Cd-complex was more difficult to be desorbed than Cd2+. Moreover, % Cd desorbed was linearly correlated with the reverse of the total Cd adsorption under unsaturated adsorption.