Enhanced Fe0-based autotrophic Cr(VI) biorduction with eggshell as an alternative inorganic carbon source: bioavailability mechanisms and microbial response.

Zerovalent iron-based autotrophic bioreduction of Cr(VI) [FAR-Cr(VI)] is a promising remediation strategy for Cr(VI) contamination in groundwater, but its performance depends on inorganic carbon (IC) delivery and microbial colonization. This study investigated waste derived eggshell as a multifunctional IC source and biostimulatory substrate for enhancing FAR-Cr(VI). Bioreactors were operated with NaHCO3 (LC), CaCO3 (SC), and eggshell (ES) as IC supplements during three consecutive 48 h cycles. The ES biosystem achieved complete Cr(VI) removal (100%) in all cycles, whereas the LC and SC biosystems reached final removal efficiencies of 88.9 ± 0.9% and 93.7 ± 0.6%, respectively. The pseudo-first-order rate constant in the ES biosystem (0.147 h-1) was 3.3- and 2.6-fold higher than those in the LC (0.045 h-1) and SC (0.057 h-1) biosystems. ES promoted denser biofilm formation and significantly increased electron transfer system activity, cell viability and counts. XPS analysis showed higher relative Fe(III) and Cr(III) signals in ES precipitates, indicating more efficient Fe0 oxidation and Cr(VI) reduction. Metagenomic analysis revealed higher abundances of Fe oxidation genes (feoA and fetA) and Wood-Ljungdahl pathway genes in the ES biosystem, suggesting an ATP-efficient carbon fixation strategy. Component-based experiments showed that calcined eggshell still improved the rate constant by 28.3% over SC, demembraned eggshell by 46.3-78.3%, and the inner membrane provided an additional 39.9-70.5% enhancement beyond demembraned eggshell. These findings indicate that eggshell enhances FAR-Cr(VI) through coupled carrier effects, and native organic, and trace element biostimulation, supporting a "waste-treating-waste" strategy for Cr(VI) contaminated groundwater remediation.