Mechanism Of Silicon To Reduce Lead Bioavailability In Alkaline Soil

Lead is one of the heavy metal elements,which can enter the human body through the food chain,accumulating to a certain extent and causing multiple hazards.Remediation and utilization of lead-contaminated soil is an important measure to reduce the risk of human lead exposure and achieve intensive land use.Passivation repair has been proved to be an effective way to repair lead.The effect of using silicon-containing materials to repair lead-contaminated soil has been recognized,but its repair mechanism has not been fully defined.To this end,the research on the mechanism of silicon to reduce the bioavailability of lead in soil is of great significance for enriching the theory of soil environmental chemistry and guiding the practice of lead-contaminated soil remediation.To clarify the effectiveness of silicon on reducing the bioavailability of lead in alkaline soil,the chemical behavior of lead in liquid,solid and solid-liquid interface of alkaline soil,as well as the toxic effects of lead on rice were studied by bioassay and pot experiments.The experiments are based on neutralizing the alkalinity of sodium silicate and eliminating the influence of sodium ion,also along with the application of Donnan Membrane Technique（DMT）,X-ray diffraction technique and infrared spectroscopy technology.How silicon affects the chemical behavior of lead in soils and the effectiveness of silicon on reducing the bioavailability of lead were discussed in order to provide the basis for the restoration and utilization of lead-contaminated soil.The main conclusions are as follows:1.Silicon application can alleviate the toxic effect of lead on rice and reduce the lead content in rice.While compared with the control,when the lead content of the soil reached1000 mg kg^-1,rice yield decreased by 51.49%（P<0.01）,and the lead content of brown rice increased significantly（P<0.01）,reaching 0.36 mg kg^-1.Compared with lead stress treatment,silicon addition increased rice yield by 58.24%（P<0.01）,while the lead content in brown rice decreased significantly,to the extent that the content of lead in brown rice（P<0.01）met the safety requirements specified in the applicable national standards.2.Silicon changes the proportion of solid phase lead in soils and promotes the conversion of soil solid phase lead to an ineffective state.Compared with lead addition alone,the application of silicon reduces the proportion of acid extractable lead and reducible lead（P<0.01）,which is reduced by 10.10%-21.66%and 4.01%-9.61%,respectively.The proportion of lead in lead oxide and residual lead is increased（P<0.01）.It has increased by12.64%-27.78%and 4.13%-30.90%respectively.3.Silicon affects the chemical behavior of lead on the solid-liquid interface of the soil.The Langmuir and Freundlich isotherm adsorption models can well fit the adsorption and desorption of soil lead.The addition of silicon reduces the capacity and strength of lead adsorbed by alkaline soils and enhances the hysteresis effect during lead adsorption-desorption.4.Silicon addition increased the concentration of water-soluble lead in soil solution,but decreased the concentration of different forms of free lead.Silicon application does not change the trend of chemical properties of soil solutions,but increase the pH,Eh and water-soluble silicon concentrations of the soil solution.It was found that silicon reduced the concentration of free lead by using DMT.5.Lead and silicon undergo a coordination reaction in solution to form a soluble complex.X-ray diffraction spectroscopy was used to find silicon-lead coordination complexes such as Pb₂SiO₄ in silicon and lead.The infrared spectroscopy was used to characterize the reaction production of silicon and lead in solution.It was found that,with the addition of the silicon and lead,the Si-O bending vibration absorption peak and the Si-O-Si anti-symmetric stretching vibration characteristic peak were red-shifted respectively compared with the single-silicon treatment.The 1086 cm^-1 shifts to 1072 cm^-1 and the 466 cm^-1 shifts to 462 cm^-1,suggest that the coordination reactions of either silicic acid or polysilicic acid with lead resulted in the formation of water-soluble complexes.In conclusion,the experiments showed that silicon addition promoted the transformation of soil liquid,Converted solid lead to an ineffective state and inhibited desorption of solid lead,thus demonstrated it demonstrated the chemical mechanism of reducing the bioavailability of lead in alkaline soils.