Harnessing the rhizosphere sponge to smooth pH fluctuations and stabilize contaminant retention in biofiltration system.

Fluctuating pH conditions can affect heavy metal mobility, thereby limiting the efficiency of biofiltration systems (BS). To address this, we developed an innovative rhizosphere sponge, biochar-based bioreactor (RBB), designed to stabilize Cd²⁺ removal across a pH range of 5 to 9. RBB consistently outperformed the control, achieving a notable 91.3 % Cd²⁺ removal at pH 5. By creating optimized oxygen and redox zoning, the rhizosphere sponge enhanced both biochar surface reactions and microbial activity. Under acidic conditions, biochar facilitated Fe²⁺/Mn²⁺ precipitation into stable (oxy)hydroxides, a process further driven by microbial oxidation. Consequently, RBB accumulated 1.54 times more Fe-Mn oxide-bound Cd than the control, effectively reducing Cd²⁺ mobility. Additionally, loosely bound extracellular polymeric substances claimed preferential Cd²⁺ sequestration after acidification. The stabilized microecology and increased ecological niches, allowing RBB to better buffer against pH fluctuations, presenting it as a robust solution for sustainable heavy metal remediation in variable environments.