Comparative efficacies of iron oxide-modified biochar and pyrite-modified biochar for simultaneous passivation of cadmium and arsenic in aqueous solutions and lettuce (Lactuca sativa. L) cultivation

Abstract Effective simultaneous passivation of cationic (Cd) and anionic (As) heavy metal (metalloids) still is a critical environmental challenge. In this study, rice husk biochar was ball-milled with iron-based materials magnetite (Fe 3 O 4 ) and pyrite (FeS 2 ), then re-pyrolyzed at 600 o C to produce modified biochars Fe 3 O 4 -BC and FeS 2 -BC respectively. Short-term removal performance was evaluated after 24 h adsorption in dual-element aqueous systems where Fe 3 O 4 -BC displayed Cd (99.62%) and As (62.39%) removal, and FeS 2 -BC displayed Cd (81.73%) and As (55.54%) removal, and BC displayed Cd (99.04%) and As (54.31%) removal. Tessier and Wenzel sequential extraction of Cd and As sorbed biochar solids revealed both modifications led to enhanced immobilization mechanisms (precipitation and complexation) absent in unmodified BC. XRD and FTIR spectra identified heavy metal precipitates and surface complexation respectively. Sorbed Cd, As was visualized with SEM-EDS. Long-term passivation effects were studied in co-contaminated soil systems with 1% (w/w) treatment with biochars, quantifying> soil-to-plant heavy metal translocation by bioconcentration factors in lettuce plant, shoot, and root. Differing passivation superiority was observed for each individual metal, where Fe ,3, ,O, ,4, ,-BC treatment led to lowest plant Cd BCF (70.77%) while FeS, ,2, ,-BC treatment resulted in lowest plant As BCF (65.72%),. Interestingly, in comparison to the control, application of unmodified BC led to,increased plant As BCF (101.03%),, suggesting biochar modification with inorganic iron materials leads to increased long-term stability by decelerating DOC release. Overall, Fe 3 O 4 -BC treatment appeared most effective in countering simultaneous Cd and As accumulation in edible lettuce portions,displaying shoot Cd BCF (35.33%) and shoot As BCF (9.17%). Graphical abstract