Coupled adsorption–reduction–immobilization of Cr(VI) and benzene in co-contaminated groundwater by a sodium alginate-stabilized nZVI–activated carbon permeable reactive barrier

Groundwater contaminated with toxic metals and petroleum hydrocarbons presents long-term risks due to the different transport and removal mechanisms of these contaminants. Co-occurrence of hexavalent chromium (Cr(VI)) and benzene (C 6 H 6 ) is particularly challenging, requiring combined redox transformation and hydrophobic adsorption for effective remediation. In this study, a porous and mechanically robust permeable reactive barrier (PRB) material, composed of sodium alginate, nanoscale zero-valent iron, and activated carbon (SA–nZVI–AC) was synthesized using a combined sol–gel and freeze-drying method. Unlike conventional nZVI systems that primarily focus on metal reduction, this composite integrates polymer stabilization, carbon adsorption, and iron-mediated redox for simultaneous removal of mixed metal–organic pollutants. Freeze-drying proved to be a key processing step, improving structural integrity, anti-clogging capacity, and thus long-term applicability of nZVI-based PRBs. Material characterization revealed a mesoporous framework, stabilized iron phases, and abundant oxygen-containing functional groups. Batch experiments demonstrated efficient simultaneous removal of Cr(VI) and C 6 H 6 , with adsorption kinetics following pseudo-second-order models ( k 2 values for total Cr and C 6 H 6 were 0.50 and 0.06 g mg −1  min −1 , respectively) and isotherms fitting the Langmuir model. Removal efficiencies exceeded 95.28% for Cr(VI) and 85.11% for C 6 H 6 at pH 7–9 and remained high (>78.15% and > 91.31%, respectively) under interfering ions. Column experiments confirmed sustained contaminant attenuation with breakthrough delayed to approximately 60 days. Cr(VI) was immobilized within the barrier through coupled adsorption and reduction, while C 6 H 6 was effectively retained via physical adsorption within the PRB matrix. Post-column microbial analyses suggested secondary biochemical interactions within the PRB. These results demonstrate that freeze-dried SA–nZVI–AC functions as a coupled adsorption–reduction–immobilization system, showing strong potential for long-term PRB applications in remediating groundwater co-contaminated with metals and organic compounds.