The characteristics of adsorption-desorption of Cd in yellow brown earth with pH

Adsorption and desorption of Cd in yellow brown earth at pH range from 2 to 7 under different Cd concentration were measured. The results showed that amount of adsorption increased with increasing concentration, the relationship could be described by the Langmuir isotherms. It is noteworthy that the isotherm parameters change with pH, thus, the parameters were calculated from the relative with pH. The isotherm was therefore transformed into pH-dependent form. For example, the Langmuir isotherm for adsorption of Cd~(2+) was as below. C/X=69.334e~(-1.7183pH) + (5.5793pH~(-3.0383) )·C. Where X(mg/kg)=the amount of Cd adsorbed and C(mg/L)= the concentration of Cd~(2+) in the equilibrium solution.The amount of desorption of Cd increased with increasing pH and amount of Cd adsorption. However, the proportion of Cd desorption to adsorption decreased with pH. At pH 3-7, the ratio of desorption to adsorption of Cd in yellow brown earth was 42%～95% with average 75%, which indicated that Cd was mainly nonspecifically adsorbed by yellow brown earth and the availability of Cd adsorbed was high.