Oxidation of As(III) and adsorption of As(V) using bioactive Mn- and Fe-sludge waste in a continuous dual-column system

This study investigated the use of waste Mn- and Fe-sludges from abandoned mines in a continuous two-column system for the oxidative removal of As(III) from contaminated water. Batch tests revealed distinct functional roles of the sludges: Mn-sludge primarily catalyzed As(III) oxidation with limited As(V) adsorption capacity (3.1 mg/g), while Fe-sludge acted as the main adsorbent with a significantly higher As(V) adsorption capacity (58.8 mg/g). These results underline the complementary roles of the two sludges in the dual-column system. Over a period of 243 days, the system achieved over 99 % removal of As(III) from an influent concentration of 15 mg/L, consistently meeting the effluent standard of 0.1 mg/L at a hydraulic retention time (HRT) of ≥6 h. In the upper Mn column, microbial Mn oxidation prevented surface passivation and dynamically regenerated Mn oxide (birnessite), as reflected by an increase in the average Mn oxidation state (3.58 to 3.67). In the lower Fe-column, adsorbed As(V) remained stable, with toxicity characteristic leaching procedure (TCLP) results showing minimal As leachability (<0.96 μg/L) over 218 days, ensuring safe disposal. Notably, the system did not require additional carbon feed as the microbial community in the Mn-column provided sufficient bioactivity, while unexpected microbial activity in the Fe-column did not affect the overall results. This study highlights the potential of using waste Mn- and Fe-sludges, exploiting their microbial and mineralogical properties, as a cost-effective and sustainable method for As(III) remediation. • Achieved 99 % As(III) removal over 243 days using waste Mn- and Fe-sludge. • Sustainable alternative to conventional media, using waste for effective As(III) remediation. • Bioactive and self-sustaining Mn sludge prevented passivation, extending column life. • Fe-sludge adsorbs As(V) with minimal leachability as confirmed by TCLP tests. • Operates without additional carbon feed, utilizing natural microbial activity.