Effect of Added Silicon on the Adsorption and the Thermodynamics of Lead in Paddy Field Soil with Different pH

The effect of added silicon on the adsorption and the thermodynamics of lead(Pb)in paddy soils with two different pH was investi- gated by the adsorption isotherm experiment. The HNO3 was used to neutralize the alkalinity of Na2SiO3, and the different concentration of NO3- and Na+ between treatments were balanced with the addition of NaNO3, thus the effect of pH and accompanying ions by added silicate on Pb adsorption was eliminated. Results showed that Freundlich adsorption models could fit well to simulate the Pb adsorption characteristics at three different temperatures and in two types of soils. Added silicon enhanced Pb adsorption in acid soil, while added silicon decreased Pb adsorption in alkaline soil. The adsorption of Pb in the two types of soils was physically endothermic progress with increasing entropy as indi- cated by the results of thermodynamics function:△G0,△H0,△S0.After addition silicon, △G decreased,and △H and△S increased in acid soil, while in alkaline soil, △G increased, and △H and △S decreased, indicating that the spontaneity of Pb adsorption increased in acid soil but decreased in alkaline soil after addition of silicon.