Effects of pH and low molecular weight organic acids on competitive adsorption and desorption of cadmium and lead in paddy soils

The bioavailability and ultimate fate of heavy metals in the environment are controlled by adsorption-desorption process. Batch equilibrium experiments were performed to assess the effects of pH and low molecular weight organic acids (LMWOAs) on competitive adsorption and desorption of cadmium and lead in paddy soils from China. The results indicated that both soils exhibited greater sorption capacity for lead (Pb) (1.37-1.61-fold) than cadmium (Cd) as estimated by the maximum sorption parameter (Q) from the Langmuir equation. The Langmuir bonding energy coefficient (b) and distribution coefficient (K (d)) were greater for Pb than for Cd, furthermore, b (binary) and K (d) (single) were greater than b (single) and K (d binary), indicating that competition for sorption sites promote the retention of both metals on more specific sorption sites. Both Cd and Pb desorption as a function of solution pH was characteristic of S pattern. The presence of LMWOAs inhibited Cd or Pb desorption at the low concentrations (a parts per thousand currency sign0.1 mmol L-1) but promoted Cd and Pb desorption at higher concentrations (a parts per thousand yen0.5 mmol L-1 for citric acid and a parts per thousand yen1 mmol L-1 for malic and oxalic acid). The two paddy soils had a greater d (*) (Cd) than d (*) (Pb) in the presence of LMWOAs, indicating that Cd desorption was more affected by the presence of LMWOAs in binary metal system.