Sorption Of Microcystins In Agricultural Soils

Water eutrophication occurred frequently in China with the rapid economic development and the increase in water consumption. Consequently, cyanobacterial blooms have become more serious and almost all major freshwater resources were affected. Microcystins(MCs) are biotoxins generated by cyanobacteria in eutrophic fresh water bodies. However, MCs in water may be transferred to agricultural soil via irrigation using toxin-contaminated water and fertilization using harvested toxic cyanobacterial paste, which may lead to the soil pollution and endanger the crops and the safety of human finally. Sorption procedure, which related to the mobility, bio-availability of pollutant in soil, plays an important role in the environmental behavior of contaminants. In this study, sorption characters and mechanism of microcystin(MC-RR, MC-LR, MC-YR) in agricultural soils and clay minerals were investigated using batch sorption experiments. The results will provide essential data for the assessment of ecological risk and environmental pollution control of MCs. The main conclusions were as follows:(1) The sorption of MC-RR in the soils reached equilibrium within 4 h. The sorption kinetics of MC-RR followed the pseudo-second-order kinetic models with R2 > 0.994 and they were controlled by the liquid film diffusions. The MC-RR sorption isotherms followed the Langmuir models well with R2 > 0.827 under different temperature(15 ℃, 25 ℃, 35 ℃). The MC-RR sorption in paddy soil was a spontaneous, exothermic, and physical sorption process, and the sorption decreased when temperature rises. However, the MC-RR sorption in latosolic red soil and salted paddy soil were spontaneous, endothermic, and chemical sorption processes, and the sorption increased with temperature. Soil clay contents and organic matters acted as predominant role of MCRR sorption process, with the promotion effect and inhibition effect, respectively. Therefore, MCRR was easily sorbed by latosolic red soil(with lower organic matter) and salted paddy soil(with higher clay content), while it was hardly sorbed by paddy soil(with higher organic matter). MC-RR could easily be adsorbed in paddy soil and salted paddy soil under acid condition. The sorption of MC-RR in latosolic red soil was not affect by p H significantly.(2) The sorption of MC-RR in the chlorite, halloysite and kaolinite reached equilibrium within 4 h, 4 h and 6 h respectively. The sorption kinetics of MC-RR followed the pseudo-second-order kinetic models with R2 > 0.999. The sorption procedure were controlled by the liquid film diffusions. The MC-RR sorption isotherm followed the Langmuir models and D-R models well with R2 > 0.971 and R2 > 0.863 respectively under 25 ℃. MC-RR could easily be adsorbed by the tested clay minerals, among which the sorption of MC-RR is strongest in halloysite. The MC-RR sorption in chlorite and kaolinite were spontaneous and physical sorption process. However, the MC-RR sorption in halloysite was a spontaneous and chemical sorption processe. MC-RR could be easily adsorbed in halloysite and kaolinite under acid and alkaline condition, or in chlorite under alkaline condition.(3) The sorption of MCs in the paddy soil reached equilibrium within 6 h. The sorption kinetics of MCs followed the pseudo-second-order kinetic models with R2 > 0.895, and they were controlled by the liquid film diffusions. The MCs sorption isotherms followed the Langmuir model and D-R model well with R2 > 0.815 and R2 > 0.826 respectively under 25 ℃. MCs in algal extract could hardly be adsorbed in paddy soil and MC-YR is the most hardly one to be adsorbed in paddy soil. The MC-RR and MC-LR sorption in paddy soil were spontaneous and physical sorption process and the MC-YR was a spontaneous and chemical sorption processes. The maximum MCs sorption in paddy soil was observed at pH 3.