Arsenic removal by self-regenerating Mn oxides and Fe-surface revitalization in a mixed biosludge column system

A self-regenerating arsenic (As) removal system was developed using a mixed Mn/Fe biosludge (MMFS) derived from mine drainage treatment waste. Unlike conventional two-step systems that separate As(III) oxidation and As(V) adsorption, the MMFS enables both within a single matrix. In a continuous two-column setup, the system achieved >99 % As removal over 314 days under variable hydraulic retention times (HRTs), consistently maintaining effluent As concentrations below both Japan's national effluent standard (0.1 mg/L) and the U.S. EPA freshwater chronic criterion (0.15 mg/L) at HRTs ≥4 h. Spectroscopic and surface analyses (SEM-EDS, XRD, XANES) revealed a self-sustaining Mn redox cycle, where biogenic Mn oxides oxidized As(III) and were continuously regenerated via microbial Mn(II) oxidation. Meanwhile, Fe oxides stably adsorbed As(V), with dynamic surface exposure preventing passivation. Microbial analysis confirmed the persistence of Mn-oxidizing bacteria under nutrient-limited and high-As conditions. Batch tests and XANES further clarified the mechanism: initial As(III) adsorption, subsequent oxidation to As(V), and long-term retention by Fe oxides. The lower column became active only after the upper column approached saturation, demonstrating spatial buffering. Toxicity characteristic leaching procedure (TCLP) tests, which assess the leaching potential of solid waste based on the U.S. EPA regulatory limit for As (5 mg/L), confirmed the solid-phase stability of the system throughout most of the operation, with leaching exceeding this limit only under prolonged extreme stress. These findings highlight MMFS as a robust, low-cost, and low-maintenance medium for As remediation, with long-term efficacy sustained by microbial Mn regeneration and Fe-surface revitalization. The integrated passive design is promising for decentralized water treatment applications. • >99 % arsenic removal sustained for 314 days using mixed Mn/Fe biosludge (MMFS). • Simultaneous As(III) oxidation and As(V) adsorption achieved in a single matrix. • Mn oxides were self-regenerated by native Mn(II)-oxidizing bacteria. • Fe oxide surfaces remained active via Mn cycling, avoiding surface passivation. • Demonstrates a low-input, scalable system for passive arsenic remediation.