Effects of Organic Acids, o-Phenylenediamine and Pyrocatechol on Cadmium Adsorption and Desorption in Soil

This article studied the interaction of cadmium (Cd) and lowmolecular weight organics in Indicotic black (IB) soil.Cadmium adsorption isotherm in this soil was satisfactorilysimulated using Freundlich equation as Q = 905.6C0.49 with high correlation (r2 = 0.984), and its adsorption quantity increased with increasing pH. The presence of citricacid 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 [CdH2Cit+]/[Cd-complexTotal],[CdHCit]/[Cd-complexTotal] and [CdCit-]/[Cd-complexTotal] vs. pH were successfully calculated. Desorption percentage ofCd, adsorbed in the presence of citric acid and EDTA decreased, compared with that adsorbed in NaNO3 media. It suggeststhat free sites for Cd adsorption in soil increased in thepresence of organic acid. When o-phenylenediamine, pyrocatechol and aminoethonic acid appeared in Cd equilibrium media, Cd adsorption quantity increased by increasing cation exchange ofpositively 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 moredifficult to be desorbed than Cd2+. Moreover, % Cd desorbedwas linearly correlated with the reverse of the total Cd adsorptionunder unsaturated adsorption.