Hybrid treatment technologies for chromium-contaminated wastewater: Mechanisms, challenges, and future perspectives

Chromium (Cr) water resource pollution is a persistent worldwide environmental and health threat mainly caused by tanning, electroplating, and textile manufacturing. Conventional treatment technologies, such as chemical precipitation, adsorption, ion exchange, and membrane filtration, have performed variably; however, they are often limited by high processing cost, the generation of secondary wastes, and low efficiency under site conditions. Hybrid processes that involve two or more processes have emerged as potential substitutes for individual processes with synergistic removal efficiency, selectivity, and sustainability advantages. This review critically evaluates the past and present use of hybrid systems for chromium removal, emphasizing membrane-adsorption combinations, bio-electrochemical systems, and algal-based technologies. New case studies illustrate their potential to convert toxic Cr(VI) to less toxic Cr(III), facilitate the recovery of resources, and support circular economy approaches. The review also addresses scalability issues, energy demands, and operational stability, with scope for integrating hybrid processes with renewable energy sources and low-cost biomaterials. Future research must involve the design of pilot-scale demonstrations, techno-economic analyses, and multidisciplinary strategies combining environmental engineering, microbiology, and materials science. Combining laboratory knowledge with practical realities, hybrid technologies hold significant promise for offering efficient, cost-effective, and environmentally benign solutions to chromium-polluted wastewater.