A mechanistic approach to arsenic adsorption and immobilization in aqueous solution, groundwater, and contaminated...

Arsenic (As) poisoning in groundwater and rice paddy soil has increased globally, impacting human health and food security. There is an urgent need to deal with As-contaminated groundwater and soil. Biochar can be a useful remedy for toxic contaminants. This study explains the synthesis of pinecone-magnetic biochar (PC-MBC) by engineering the pinecone-pristine biochar with iron salts (FeCl₃.6H₂O and FeSO₄.7H₂O) to investigate its effects on As(V) adsorption and immobilization in water and soil, respectively. The results indicated that PC-MBC can remediate As(V)-contaminated water, with an adsorption capacity of 12.14 mg g^-1 in water. Isotherm and kinetic modeling showed that the adsorption mechanism involved multilayer, monolayer, and diffusional processes, with chemisorption operating as the primary interface between As(V) and biochar. Post-adsorption analysis of PC-MBC, using FTIR and XRD, further revealed chemical fixing and outer-sphere complexation between As(V) and Fe, O, NH, and OH as the main reasons for As(V) adsorption onto PC-MBC. Recycling of PC-MBC also had excellent adsorption even after several regeneration cycles. Similarly, PC-MBC successfully immobilized As in paddy soil. Single and sequential extraction results showed the transformation of mobile forms of As to a more stable form, confirmed by non-destructive analysis using SEM, EDX, and elemental dot mapping. Thus, Fe-modified pine-cone biochar could be a suitable and cheap adsorbent for As-contaminated water and soil.