Closed-loop chromium recovery from tannery wastewater: Mechanism of multiphase Cr(III) electrode position under low-concentration sulfate and chloride coordination.

Chromium recovery is pivotal for sustainable resource utilization, yet conventional approaches remain plagued by inefficiency and hazardous chromium sludge generation. While Cr(III) electrodeposition offers a promising alternative, its practical implementation under low-concentration conditions is fundamentally constrained by two underexplored barriers: anion-specific coordination effects and multiphase competitive inhibition. Here, we systematically decode the electrodeposition landscape by revealing critical phase disparity between sulfate and chloride systems. Three original findings emerge: (1) Sulfate systems enable superior metallic chromium (Cr(0)) deposition uniformity (83.68 % Cr purity) compared to chloride systems (57.40 %) due to anion-modulated coordination chemistry; (2) Multiphase self-inhibition arises through a stratified deposition architecture-base Cr(0) overlain by hydroxide/oxide passivation layers-intensified by hydrogen evolution reaction (HER) in sulfate electrolytes; (3) Anion-specific oxidation pathways dominate byproduct divergence: sulfate suppresses Cr(VI) generation via hydroxide-passivation dominance, whereas chloride promotes anode-driven Cr(VI) significant accumulation through Cl-mediated competitive oxidation. By strategically optimizing current density (50 mA cm) and duration (30 min), we maximize single-cycle Cr(0) yield (67.72 % purity) through selective suppression of competing byproducts. Crucially, field validation using tannery wastewater demonstrates industrial viability under realistic contaminant loads. Beyond these mechanistic advances, we propose a paradigm-shifting "single-cycle maximal enrichment with multi-cycle acid-assisted regeneration" strategy, achieving dual breakthroughs: closed-loop chromium recovery concurrent with near-zero sludge generation and secondary pollution. This work redefines electrochemical metal recovery by bridging molecular-scale coordination effects to macroscale process engineering, offering a template for sustainable heavy metal resource circularity. ENVIRONMENTAL IMPLICATIONS: This study offers significant environmental benefits by advancing the recovery of chromium from industrial wastewater through an electrochemical method. The proposed approach effectively prevents the formation of hazardous chromium sludge, which is a common issue in conventional treatment methods, and instead facilitates the recovery of high-purity Cr(0). The optimization of electrodeposition parameters, particularly in sulfate-based systems, ensures more efficient chromium recovery while minimizing the generation of Cr(VI), a highly toxic species. By improving the efficiency of Cr(0) deposition and enabling the reuse of chromium from wastewater, this approach aligns with sustainable waste management practices. Additionally, the reduction in hazardous by-products promotes safer industrial wastewater treatment processes, enhancing both environmental protection and resource conservation. The findings of this study provide a valuable framework for the development of scalable, environmentally friendly technologies for the recovery of chromium and other heavy metals from wastewater, contributing to more sustainable industrial practices.