Adsorption, separation and recovery properties of blocky zeolite-biochar composites for remediation of cadmium contaminated soil

Cadmium (Cd) contamination in soils is a global ecological threat. Conventional powdered biochar added to soil can temporarily immobilize Cd but is difficult to separate from soil, leading to secondary release of Cd and posing potential ecological and human health risks. The blocky biochar is also difficult to separate from the soil due to its fragile nature. One of the keys to overcome the difficulties in separating biochar from soil is to improve its mechanical strength. Blocky zeolite-biochar composites (ZBC) that have good mechanical strength were obtained after pyrolyzing the mixture of 50% feedstock and 50% zeolite powder at 400 °C. ZBC and NaOH-activated ZBC (ZBCa) were applied to remove Cd from soil. After sieving Cd-loaded ZBC and ZBCa from soil, the bioavailable Cd content in the soil decreased by 59.70% and 68.54%, respectively. Zeolite contributed to improving both adsorption performance and mechanical properties of the composites. After repeating the process of “remediation-sieving-desorption-regeneration” three times, the recoveries of ZBC and ZBCa were above 97.00%, and regeneration rates were 48.70–83.26%, respectively. Under simulated mechanical sieving conditions, ZBC and ZBCa lost only 4.06% and 5.40% of their mass and retained their integrity. Remediation of Cd-contaminated soil with blocky zeolite-biochar composite is sustainable and safe because the removal of bioavailable Cd from soil is permanent rather than a temporary decrease of bioavailability. This study provides a reference for the preparation of separable and recyclable adsorbents for the removal of contaminants from soil.