| With the rapid development of the economy,human activities such as electroplating,mining,and smelting have intensified the widespread contamination of water and soil with heavy metals in China.Heavy metal pollution is characterized by its hidden,persistent,and irreversible nature and has become a global critical issue threatening the ecological environment and human health.Among soil pollutants,typical heavy metals such as cadmium（Cd）,lead（Pb）,and arsenic（As）have the highest over-limit ratios.Heavy metals in soil can flow into surface water and even groundwater due to rainfall and irrigation,exacerbating waterborne heavy metal pollution,and causing long-term potential harm to the human body through enrichment in the food chain.Meanwhile,the interaction between new pollutants such as microplastics and soil heavy metals is also threatening the development of healthy agriculture.Although iron-modified biochar has a certain remediation effect on heavy metal complex pollution in the environment,traditional preparation methods have high energy consumption and certain environmental toxicity.Some green chemical preparation methods for iron-modified biochar have complex processes,poor remediation effects,and low environmental benefits,requiring further optimization.In addition,China has the world’s largest green tea production area,and a large amount of residual tea leaves rich in reducible tea polyphenols and cellulose are left after picking and pruning.Directly returning them to the field or burning them may exacerbate the environmental crisis caused by carbon emissions.Based on this,this study aims to achieve the comprehensive utilization of waste tea leaves by preparing green synthesized nanoscale zero-valent iron modified tea waste biochar（G-n ZVI/TB）for experiments.Firstly,single and combined Cd,Pb,and As absorption experiments were carried out to study the adsorption function and mechanism of the G-n ZVI/TB.Secondly,dry-wet alternation soil culture experiments were carried out to summarize the dynamic effects of different amounts of G-n ZVI/TB on soil physicochemical properties,immobilization effects of heavy metals,soil fertility in multi-contamination chao soil（CS）and blue purple clay（BPC）,as well as the interactive effects with polypropylene microplastics.Finally,the impact of different levels of G-n ZVI/TB,culture time,and polypropylene microplastics on the microbial community composition and diversity in multi-contamination soil was explored using microbial absolute quantitative sequencing technology.The research results provide new theoretical and scientific support for the potential application of low-cost,efficient,and environmentally friendly green nano-biochar-based materials in heavy metals multi-contaminated water and soil environments.The following are the main findings:（1）The results of the adsorption experiments on polluted water indicate that the G-n ZVI/TB successfully overcame the defect of tea waste biochar（TB）inability to absorb As.Under the optimal adsorption conditions of contact time of 24 hours and p H 6,the maximum fitting adsorption capacity of the G-n ZVI/TB for Cd（II）,Pb（II）,and As（III）reached 49.57 mg Cd（II）g-1,177.2 mg Pb（II）g-1,and 6.289 mg As（III）g-1,respectively,which was notably higher than the adsorption capacity of TB(22.52 mg Cd（II）g-1,64.90 mg Pb（II）g-1,and 0.6855 mg As（III）g-1).Scanning electron microscopy and Fourier transform infrared spectroscopy results demonstrated that the G-n ZVI/TB achieved good dispersion of nanoscale zero-valent iron particles and inhibited their oxidation.Additionally,functional groups such as O-H,C-H,and C=C appeared on the surface of the G-n ZVI/TB,which was beneficial for adsorption of heavy metals.X-ray diffraction and X-ray photoelectron spectroscopy results confirmed that surface complexation,coprecipitation,and redox reactions were the main mechanisms by which G-n ZVI/TB adsorbs Cd（Ⅱ）,Pb（Ⅱ）,and As（Ⅲ）.The competitive adsorption ability of the three heavy metals on the adsorption sites of the G-n ZVI/TB was ranked from high to low as Pb（II）>Cd（II）>As（III）.（2）Results from the dry-wet alternate soil cultivation experiment showed that treatment with 30 g kg-1G-n ZVI/TB significantly reduced the Eh and p H of CS and BPC,while significantly increasing soil EC and DOC（P<0.05）.Compared to treatment with 15 and 30 g kg-1TB,treatment with 30 g kg-1G-n ZVI/TB significantly reduced the content of available heavy metals in CS and BPC by 4.0%-67.8%and the concentration of heavy metals in pore water by 28.2%-68.5%（P<0.05）.At the same time,G-n ZVI/TB significantly increased the proportion of residual Cd,Pb,and As in CS and the proportion of residual Pb in BPC（P<0.05）,transforming heavy metals from acid-soluble to stable form.G-n ZVI/TB significantly increased the organic matter content by 27.3%-72.9%and the available potassium content by 9.4%-57.7%in CS and BPC（P<0.05）,and enhanced the ability of soil to retain nitrogen and phosphorus.The addition of polypropylene microplastics had no significant effect on soil physicochemical properties,but polypropylene microplastics can promote and inhibit the formation of residual Cd and Pb in CS and BPC,respectively.Within four dry-wet alternate cycles of 112 days,the G-n ZVI/TB had a stable effect on the immobilization of Cd,Pb,and As in the soil.Characterization results from G-n ZVI/TB by magnetic separation showed that co-precipitation and redox were the main mechanisms for the long-term simultaneous immobilization of the three heavy metals.（3）The results of 16S rRNA gene amplicon absolute sequencing showed that adding G-n ZVI/TB had no notable impact on the soil bacterial diversity index during the soil cultivation period（P>0.05）.However,compared to the blank treatment,the addition of 30 g kg-1G-n ZVI/TB obviously increased the absolute abundance of bacteria in the CS（P<0.05）,while maintaining the relative stability of the absolute abundance of bacteria in the BPC throughout the cultivation period,with the most significant increase observed in the absolute abundance of dominant genus such as Gp4 and Gp6.The impact of G-n ZVI/TB on the evolution of bacterial community structure in CS increased significantly over time,while the impact on the evolution of bacterial community structure in BPC was relatively small.The main environmental factors that impact the structure of the bacterial community were found to be soil p H,DOC,EC,and the concentration of Fe in the soil pore water.However,in the coexistence environment of heavy metals and polypropylene microplastics,the improvement effect of G-n ZVI/TB on soil bacterial communities was significantly inhibited. |
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