Phototrophic microalgae-driven comprehensive remediation of arsenic and cadmium: A novel iron mineralization pathway enabling arsenic immobilization

Abstract

Phototrophic microalgae have demonstrated remarkable potential in remediating heavy metals like cadmium (Cd), yet their efficacy for metalloids like arsenic (As) remains limited due to the lack of efficient immobilization pathways. While iron (Fe) minerals are known to strongly bind As, the potential for microalgae to drive Fe mineralization for As immobilization has not been systematically investigated. To bridge this knowledge gap, a microcosm experiment was conducted, focusing on the role of Fe mineralization and comprehensive remediation of As and Cd, which is strongly linked to both biotic and abiotic processes driven by microalgae and their associated bacterial consortia. Results revealed a dual mechanism for metal(loid) immobilization. First, microalgal photosynthesis-induced pH elevation (from 3.46-3.54 to 5.45–5.50) triggered Cd hydrolysis and precipitation (28%–30% removal within 20 days), while sustained alkalization enhanced Fe(II) oxidation rates by 45% compared to abiotic controls during the 90-d cultivation, achieving >99% As removal through Fe mineral coprecipitation. Second, the microalgal-bacterial consortia established synergistic interactions that simultaneously mitigated metal(loid) toxicity and enhanced remediation efficiency. Specifically, Fe-oxidizing bacteria generated biogenic Fe minerals with higher As adsorption capacity (As/Fe ratio 0.28 vs. 0.20 in controls), resulting in faster As removal, while sulfate-reducing bacteria facilitated CdS precipitation, collectively enabling >97% Cd removal. These findings highlight microalgae-induced Fe mineralization as a previously overlooked mechanism enabling comprehensive remediation of both heavy metals and metalloids, expanding the scope of phycoremediation applications.