Adsorption/desorption Kinetics Behavior Of PCP On Wetland Soil

As a typical organic pesticide, pentachlorophenol （PCP） once be used forpesticides universally, and a large number of residues were still in the environmentnow. Therefore, the study of adsorption and desorption behavior characteristics ofPCP on estuarine wetland soils was necessary for understanding the environmentalbehavior, ecological risk assessment and water pollution control. This paper selected20stations of surface soils from Liaohe estuarine wetlands as the object of study. Theenvironmental factors（pH,temprature and salinity）,dissolved organic matter（oxalicacid, tartaric acid, citric acid, glycine and glucose）and heavy metal ion(sPb²⁺,Cd²⁺andZn²⁺)were investigated in the adsorption and desorption process of PCP. Theexperiment results and conclusions of dynamic diffusion mechanism were studied andanalyzed.With increasing temperature, the adsorption of PCP on the estuarine wetland soildecreased, but desorption increased. In the temperature range from0℃-25℃,chemical diffusion and intraparticle diffusion were the main rate-determiningdiffusion steps for both adsorption and desorption of PCP according to the fittingresults of kinetics equations.The impact of salinity on adsorption and desorption process of PCP was complex.The adsorption amount of PCP increased as the increasing salinity in theconcentration range from0.02mol/L-0.20mol/L. But in the salinity range from0.10mol/L-0.50mol/L, desorption process of PCP was inhibitted by salinity significantly.The maximum amount of desorption of PCP appear in appropriate salinity conditionsmerely.The fitting results of kinetic equations showed that both the adsorption anddesorption diffusion process of PCP were combined results of several diffusionprocesses. Chemi-sorption was the main controlling step of adsorption rate, butdesorption process was affected by intraparticle diffusion process and physicaldiffusion.The adsorption of PCP on wetland soil was promoted by acidic condition butinhibited by alkaline condition. The largest amount of desorption of PCP appeared inpH=8of Liaohe estuarine wetland soils. Chemi-sorption was the main step of the adsorption of PCP, but the desorption rate of PCP was controlled by the intraparticlediffusion process mainly, accompanied by heterogeneous diffusion and physicaldiffusion.The adsorption of PCP on wetland soils was promoted by oxalic acid, tartaricacid and citric acid, but desorption was inhibitted. But when the concentration ofoxalic acid was2mmol/L and5mmol/L, the desorption process of PCP was inhibitedby oxalic acid as the strong reduction.Due to the difference between numbers andtypes of active functional groups in three organic acids, the chemical reaction wasdifferent. The strength order of three kind organic acids was: citric acid>tartaricacid>oxalic acid. Because of the dual role, the adsorption and desorption of PCP wereboth inhibitted by glucose and glycine.The experimental results showed that the adsorption and desorption of PCP onsoil was inhibitted by metal ions. In the complex pollution system of single metal ionand PCP, chemi-sorption was the main rate-determining diffusion step for adsorptionof PCP, but desorption of PCP was controlled by intraparticle diffusion, accompaniedby the effect of heterogeneous diffusion and physical diffusion. Because of thedifferent physicochemical properties of Pb²⁺,Cd²⁺and Zn²⁺, the impact on theadsorption and desorption of PCP was different. The impact strength order was:Pb²⁺≥Cd²⁺>Zn²⁺.But in the complex pollution system of two heavy metal ions andPCP, because of the competition of heavy metal ions, different compound effect wasgenerated by different combination of metal ions.The combination of Zn²⁺and Cd²⁺or Cd²⁺and Pb²⁺was antagonistic effect, but the combination of Zn²⁺and Cd²⁺wassynergistic effect. In addition, the concentration of metal ions in composite system hasan important impact on adsorption and desorption of PCP diffusion process in the soil.