Development of low-carbon and porous biochar-slag composites for efficient phosphorus removal

A CO₂-sequestering porous biochar-slag composite was fabricated via carbonation coupled with biochar modification for phosphorus (P) removal. Carbonation created meso-/macropores and increased surface area from 0.32 m²/g to 5.13 m²/g, biochar addition further raised surface area to 7.24 m²/g and enhanced CO₂ uptake from 82.07 mg/g to 141.29 mg/g, likely by promoting CaCO₃ nucleation and improving CO₂ diffusion. In batch tests, carbonation improved effluent quality (pH 11.51 to 9.93; electrical conductivity 1074.90 to 116.90 μS/cm) but lowered P uptake. An optimised 10 wt% biochar-carbonated composite (BC₁₀_C) balanced capacity and leaching, achieving 3.16 mg/g at P₀ = 10 mg/L. P removal was sensitive to solution chemistry: humic acid and added Ca²⁺ enhanced P removal, whereas CO₃^2- most strongly inhibited it. Column study showed that BC₁₀_C maintained ∼100% P removal over 930 bed volumes without surface ponding observed for BC₁₀_UC, while exhibiting more moderated effluent chemistry, including lower and more stable electrical conductivity (∼600 μS/cm), lower TOC release, and post-operational P immobilisation of 38.91 mg/g. SEM-EDS mapping and fractionation revealed Ca-P precipitation was the dominant removal mechanism. This study proposed a scalable, low-carbon route for sustainable P remediation.