Adsorption, immobilization mechanisms and potential risks of Cd in soil-biochar-microplastics system

The problem of cadmium (Cd) and microplastic compound contamination in agricultural soils is becoming more and more prominent. However, research on biochar remediation for heavy metal-contaminated soils often overlooks the impact of microplastics. This investigation explored Cd adsorption and immobilization of biochar in soils containing varying percentages (0 %, 2 %, 4 %, 6 %, 8 %, and 10 %) of 500-mesh polyethylene microplastics (MP500). Additionally, it examined the quantitative Cd adsorption mechanisms of biochar in the soil. The findings revealed that the Cd adsorption capacity of biochar continuously increased 1.5 %–33.1 % as the percentage of MP500 in the soil rose from 0 % to 10 %. However, higher MP500 concentrations also led to greater Cd leaching into the environment (continuously increased, 17.5 %–58.6 %) and a reduction in the proportion of relatively stable Cd (continuously decreased, 7.9 %–15.4 %) in soil-biochar samples. This suggested that microplastics interfered with biochar's effectiveness in remediating Cd contamination in soils. For adsorption mechanisms, the larger the percentage of MP500 in the soil, the greater the percentage of mineral mechanism of biochar. This study highlighted that Cd adsorbed on microplastics was a greater threat to the agricultural soil environment compared to Cd in soils; microplastics should be paid special attention to as an influencing factor in the practical application of biochar for the management of Cd pollution.