Effect Of Field Aging And Sulfur Modified Biochar On Remediation Of Vegetable Heavy Metal Contaminated Soil

Agricultural soil pollution has become a global problem,especially heavy metal pollution of agricultural soils in developing countries.It has been reported that biochar has the advantages of abundant raw material sources,simple preparation method,developed pore structure,large specific surface area and good adsorption performance,which can effectively fix heavy metals in soil,reduce the bioavailability of heavy metals and reuse the soil contaminated by heavy metals for agricultural production.However,field aging and biochar chemical stability may affect heavy metal mobility,bioavailability,environmental toxicity,and heavy metal immobilization stability.Therefore,this study investigated the effects of field aging and sulfur modification on biochar physicochemical properties,soil physical,chemical and biological properties,and heavy metal enrichment by plants.The main study contents and results are as follows:（1）Biochar after 7 years of aging in the field was characterized by using a series of characterization methods.The results showed that the specific surface area,pore volume and pore size increased by 2.8,10.76 and 3.15 times,respectively,the relative content of oxygen-containing functional groups on aged biochar surface increased by14.41%,the relative content of element C decreased by 42.12%and the relative content of element O increased by 72.49%.compared with the unaged biochar.The changes in physicochemical properties caused by field aging not only increased the adsorption of cadmium（Cd）and lead（Pb）in single-or binary-metal systems by biochar,but also changed the adsorption mechanism of biochar for Cd and Pb.The main adsorption mechanisms of field-aged biochar for Cd and Pb were surface complexation and ion exchange,while the unaged biochar was dominated by precipitation and cation-πelectron interaction.（2）The sulfur-modified biochar was prepared by Na OH and CS₂ modification method,and its properties and adsorption mechanism were investigated by a series of characterization methods.The results showed that the relative content of S element in the sulfur modified biochar increased from 0.047%to 0.68%,sulfhydryl functional groups were detected on the surface of sulfur modified biochar.Compared with the unmodified biochar,the adsorption of Cd and Pb by sulfur modified biochar in the single metal system increased by 46.81%and 21.95%,respectively,and the bioaccumulation of Cd and Pb in plant tissues decreased by 12.40%and 23.47%,respectively,and improved the stability of biochar in fixing heavy metals.It shows that sulfur-modified biochar has some potential application in the remediation of heavy metal contaminated soil.（3）The effect of long-term,short-term and repeated applications of biochar on the remediation of heavy metal-contaminated soil was investigated through pot experiments.The results showed that long-term application of biochar significantly increased soil p H,EC,organic matter,total pore space,available P,available K,the relative abundance of Proteobacteria phylum with high tolerance to heavy metals and the number of genes of Sphingomonas with degradation of aromatic compounds,while the relative abundance of Acidobacteria phylum was significantly decreased compared with the soil not amended by biochar.The long-term application of biochar reduced the accumulation of Cd and Pb in leaf lettuce tissues by 36.03%and 23.58%,respectively,the mobility and bioavailability of Cd and Pb in soil,but had no significant effects on leaf lettuce biomass,quality and photosynthesis.The short-term,long-term and repeated applications of biochar significantly reduced the accumulation of Cd and Pb in leaf lettuce tissues,which the greatest reduction was observed with the repeated application,followed by the long-term application and the least with the short-term application.