Durable Composite Alginate Hydrogels for the Remediation of High-Concentration Cr(VI) in Industrial Effluents

The transition of adsorbent materials from laboratory synthesis to industrial application requires a rigorous assessment of their durability under high concentrations. This study evaluates composite alginate hydrogels incorporating mill scale-derived magnetite (Fe3O4), activated carbon, and L-ascorbic acid (Fe3O4/AC/VitC alginate hydrogels) for the detoxification of highly concentrated chromium effluents. We investigated the operational limits of these hydrogel bubbles through three distinct scenarios: (1) Single-Batch Adsorption–Regeneration Cycling in Fresh Cr(VI) Solutions, (2) Sequential Reuse of Single-Batch Hydrogel Bubbles in the Same Effluent for Complete Cr(VI) Removal, and (3) the Multi-Batch Treatment Capacity Test for highly concentrated Cr(VI) loads. The composite demonstrated exceptional robustness, maintaining >80% removal efficiency over five cycles for 100 mg/L Cr(VI) without chemical treatment. For higher loads (150–400 mg/L), a simple regeneration protocol effectively restored performance, reducing Cr(VI) levels to below stringent discharge limits (<0.25 mg/L). Furthermore, in “worst-case” scenarios involving synthetic solutions (2500 mg/L) and real electroplating wastewater (~2000 mg/L), a multi-batch approach achieved complete abatement within 8–9 cycles. Notably, the treatment simultaneously neutralized the effluent pH from ~1.0 to 7.0, confirming the potential of this waste-derived material as a resilient, multi-functional solution for complex industrial wastewater management. Beyond performance, a techno-economic analysis was conducted to evaluate the economic feasibility of industrial-scale production. The results indicate that by utilizing industrial-grade reagents and waste-derived precursors, the production cost of the dry adsorbent can be estimated at ~45 EUR/kg. These findings confirm the potential of this material as a cost-effective, multi-functional solution for SMEs struggling with high operational costs and stringent environmental regulations.