High-performance nanofluidic membranes for salinity gradient energy conversion coupled with Cr (VI) reduction

In the face of rising energy demands for high-salinity wastewater treatment, the abundant salt content presents a novel opportunity for salinity gradient energy harvesting, which can drive metal recovery processes. A “waste-controls-waste” strategy was developed by exploiting the salinity gradient to drive chromium reduction, achieving synergistic energy recovery and heavy metal removal. Based on this, an asymmetric nanochannel was designed to couple high-salinity and Cr-containing wastewater, enabling a green, self-powered treatment process. A cation-selective heterogeneous membrane was fabricated via a facile vacuum filtration method, wherein one-dimensional aramid nanofibers (ANFs) were intercalated between two-dimensional graphene oxide (GO) sheets to form an ordered lamellar structure. The structure was subsequently assembled onto an anodized aluminum oxide (AAO) substrate. The ANF-GO/AAO membrane, with a “line-plane” hybrid architecture, exhibited high cation selectivity and pronounced ion rectification due to its bipolar charge configuration. By mixing high-salinity wastewater and Cr-laden wastewater through the ANF-GO/AAO membrane, substantially high-power density of up to 39.22 W m⁻² was achieved, along with an energy conversion efficiency of 25.60% under extreme salinity gradients. Nernst-Planck simulations revealed that ion selectivity and energy conversion in the heterogeneous membranes are dictated by their asymmetric architecture and the negatively charged selective layer. The harvested electrical energy directly drove the electrochemical reduction of Cr (VI), achieving a removal amount of 18.77 mol cm⁻² under acidic conditions. A “waste-controls-waste” strategy harnesses salinity gradients to drive Cr(VI) reduction, achieving simultaneous energy recovery and heavy metal removal. An asymmetric nanochannel couples high-salinity and Cr-laden wastewater, enabling a green, self-powered treatment system.