Electrochemical Hydrogen Looping to Enable Efficient Reverse Osmosis Concentrate Valorization via pH-Mediated Mineral Precipitation

Abstract

Electrochemical technologies enable a chemical-free, pH-induced precipitation of scaling ions present in reverse osmosis concentrate (ROC) or brine, thereby advancing both water and resource recovery. Here, we pioneer a proof-of-concept electrochemical hydrogen-looping process that significantly reduces the energy consumption required for precipitation without generating byproducts, thereby opening a new pathway for brine valorization. The key innovation involves the reuse of hydrogen produced by the hydrogen evolution reaction (HER) for the hydrogen oxidation reaction (HOR). The OH^– generated via the HER (5 mA cm^–2) enabled >70% hardness reduction using a simulated brackish water ROC sample. The required cell voltage (0.89 V) was 66% lower than that coupled with the conventional oxygen evolution reaction (2.64 V). This reduction was achieved by the thermodynamically/kinetically favored HOR, which also suppressed the undesired chlorine evolution reaction. The energy consumption for precipitation (0.45 kWh kg^–1 as CaCO₃) was reduced by approximately 90% relative to the previous study reporting the highest precipitation rate, while simultaneously achieving the highest productivity (2.43 kg CaCO₃ h^–1 m^–2) reported to date. We also validated the process by recovering both CaCO₃ and Mg(OH)₂ from actual ROC samples, demonstrating its potential for brine mining. Overall, these findings establish electrochemical hydrogen looping as a promising strategy for brine valorization.