Study On The Immobilization Effect And Stability Of Silicon-modified Biochar On Heavy Metals In Contaminated Soil

With the increasing frequency of industrial activities and mining,the problem of heavy metal pollution has become increasingly serious in recent years.The adsorption amount of raw biochar is limited,and it can be improved by chemical modification.Silicon（Si）is the second most abundant element in the earth’s crust after oxygen.In addition,the silicon-rich biochar is more conducive to the removal of heavy metal pollutants.Increasing the silicon content of biochar artificially can also promote the adsorption of heavy metal ions by biochar.However,few studies have reported the adsorption performance of heavy metals by inorganic silicate-modified biochar.Moreover,for a large amount of agricultural and forestry wastes in China,there are few studies on the applicability of silicon modification methods.Therefore,in order to prove the adsorption capacity of silicon-modified biochar on heavy metal,5 common agricultural and forestry wastes were selected in this study:rice straw,corn straw,tobacco straw,grapefruit peel and sycamore leaves.They were mixed with sodium silicate by dry treatment,followed by pyrolysis to synthesize silicon-supported biochars（Si-RSBC,Si-CSBC,Si-GBC,Si-YBC,and Si-WBC）.We performed an isothermic adsorption study on Pb.The pyrolysis product was applied to the heavy metal composite polluted soil,and the passivation culture and simulated acid rain leaching experiments were carried out.The effects of silicon-modified biochars on the mobility and buffering behavior of lead,cadmium,copper and zinc under acid rain exposure were investigated.The underlying mechanism of silicon-modified biochar adsorption and immobilization of heavy metals in aqueous solution and soil is discussed.We evaluate the mobility of heavy metals in contaminated soil affected by simulated acid rain over a range of p H values.The effect and passivation stability of silicon-modified biochar on heavy metal immobilization in soil affected by acid rain were explored.And we provide scientific basis for waste utilization and improvement of heavy metal removal efficiency.The main results are as follows:（1）The silicon content of corn stalks,tobacco stalks,sycamore leaves and grapefruit peel biochar can be increased by mixing with sodium silicate and co-heating.The silicon content increased by 14.31,18.18,44.42 and 71.01 mg/g,respectively.But it has little effect on rice straw biochar.At the same time,co-pyrolysis with silicon can improve the aromaticity of biochar,thereby enhancing the stability of biochar.Moreover,the p H of the biochar was increased,the Zeta potential was reduced,and the oxygen-containing functional groups of the biochar were increased.Through cation exchange,precipitation and surface complexation with functional groups,its adsorption capacity for heavy metals is improved.The adsorption capacity of each biochar after modification increased.The most notable of these are Si-WBC and Si-CSBC.Among them,when the Pb concentration in the equilibrium solution is 108.24 mg/L,the adsorption capacity of Si-WBC reaches141.76 mg/kg,and the adsorption capacity is still on an upward trend;when the equilibrium Pb concentration is 100.31 mg/L,the adsorption capacity of Si-CSBC reaches 212.77 mg/L.mg/kg and tend to be stable.（2）Original biochar（BC）,mixed application of original biochar and sodium silicate（BC+Na₂SiO₃）and silicon-modified biochar（Si-BC）were applied in the soil contaminated with heavy metals.All three passivating agents transformed exchangeable lead,cadmium,copper and zinc to more stable forms in soil.Moreover,its TCLP extract state content is reduced,and the ecological risk index of polluted soil is reduced.Among them,the effect of Si-BC is better than that of BC+Na₂SiO₃,which is better than that of BC.Sodium metasilicate can reduce the mobility of heavy metals by increasing soil p H,leading to enhanced complexation reactions of heavy metals,and co-precipitation with heavy metals.After co-pyrolysis of sodium metasilicate and biochar,the content of effective silicon,DOC and oxygen-containing functional groups and specific surface area of silicon-modified biochar increased.Combination and other methods improve the fixation of heavy metals in soil.In conclusion,silicon modification can effectively improve the remediation ability of biochar on composite polluted soil.（3）The heavy metal composite contaminated soil was treated by BC,BC+Na₂SiO₃and Si-BC.Under different p H（4.0,5.6 and 7.0）simulated acid rain environment,the biochar was aged due to acid rain leaching.Changes in its properties can affect soil p H,DOC and available silicon content.This in turn affects the migration and transformation of heavy metals in soil.After each treatment was washed with acid rain,the mobility of heavy metals was Si-BC<BC+Na₂SiO₃<BC.And the lower the p H of acid rain,the higher the mobility of heavy metals.However,after leaching by acid rain,the loss of DOC and available silicon appeared,and the leaching amount decreased with the decrease of p H of acid rain.Therefore,silicon modification can effectively improve the stability of biochar passivation of heavy metals in soil and reduce the desorption and release of heavy metals in soil.In conclusion,exogenous addition of silicon can improve the adsorption capacity of biochar to heavy metals.Reduce soil desorption and release of heavy metals.Reduce potential ecological risks of contaminated soil.Compared with the original biochar and the mixed application of silicon and biochar,the silicon-modified biochar has better ecological value.