Removal of Cr(VI) from water using microalgae-based modified biochar: Adsorption performance, mechanisms, and life cycle assessment

Two types of biochar adsorbents, ZnCl₂-modified biochar (ZABC-5 and ZLBC) and H₃PO₄-modified biochar (HABC-5), were synthesized using algae powder and lake bloom algae. ZnCl 2 as activator can change the surface structure of algal biochar and increase the adsorption site. ZABC-5 exhibited superior adsorption efficiency, achieving a maximum removal rate of 98.68 % for Cr(VI), featuring a defined surface area of 34.58 m 2 ·g −1 . Furthermore, it demonstrated excellent regeneration performance, maintaining over 98 % removal rate for Cr(VI) after three cycles using HCl as the desorbing agent. The multifaceted adsorption mechanism of Cr(VI) removal by ZABC-5 encompassed pore filling, electrostatic interaction, redox reactions, surface complexation, and ion exchange. Response surface optimization determined the optimal adsorption conditions for ZLBC, yielding an actual removal rate of 76 ± 4 %. Life cycle assessment revealed that the production of 1kg ZLBC and its utilization significantly reduce environmental impact, and the global warming potential is 12.70 kg CO 2 -Eq, while remaining cost-effective. Overall, this work provides a precedent by using lake bloom algae to create biochar adsorbents, showcasing the effectiveness and sustainability of algal biochar for removing Cr(VI) from water bodies and aiding environmental remediation.