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Hydrogen peroxide (H2O2) is a green oxidant widely used in water treatment and sustainable chemistry. Although many advanced materials exist for photo- and electrocatalytic production, H2O2 output and stability depend on reactor design and water quality. This study explores a scalable photochemical system employing bismuth vanadate-coated polymeric optical fibers (POF-BVO) illuminated by 440 nm LEDs. A single 20 cm, 3 mm diameter fiber generates H2O2 at 4.3 mg H2O2 h−1 (430 mg H2O2 gcat −1 h−1), with enhanced rates achieved using bundled fibers. The bundled configuration increases fiber packing density in the reactor to >120 m2 m−3, tripling that of flat-plate photocatalytic reactors. High H2O2 production is achieved using oxygen-permeable hollowfiber membranes to deliver pure O2 or air. The system performs consistently across pH 4−9 and in tap water, wastewater, or seawater. Phosphate ions improve H2O2 stability, resulting in higher concentrations. Over 21 days of continuous operation, the system produces >6 g L−1 of H2O2 with minimal performance degradation. Energy analysis reveals a 2−30x reduction in energy use compared to traditional slurry-based photocatalytic systems, with a three-fiber bundle reaching 27 kWh kg−1 comparable to electrochemical processes. These results demonstrate the potential of the POF-BVO platform as an energy-efficient and modular solution for decentralized H2O2 production. 

Zero liquid discharge (ZLD) and minimal liquid discharge (MLD) are brine management approaches that aim to reduce the environmental impacts of brine discharge and recover water for reuse. ZLD maximizes water recovery and avoids the needs for brine disposal, but is expensive and energy-intensive. MLD (which reduces the brine volume and recovers some water) has been proposed as a practical and cost-effective alternative to ZLD, but brine disposal is needed. In this Review, we examine the concepts, technologies and industrial applications of ZLD and MLD. These brine management strategies have current and potential applications in the desalination, energy, mining and semiconductor industries, all of which produce large volumes of brine. Brine concentration and crystallization in ZLD and MLD often rely on mechanical vapour compression and thermal crystallizers, which are effective but energy-intensive. Novel engineered systems for brine volume reduction and crystallization are under active development to achieve MLD and/or ZLD. These emerging systems, such as membrane distillation, electrodialytic crystallization and solvent extraction desalination, still face challenges to outcompete mechanical vapour compression and thermal crystallizers, underscoring the critical need to maximize the full potential of reverse osmosis to attain ultrahigh water recovery. Brine valorization has potential to partially offset the cost of ZLD and MLD, provided that resource recovery can be integrated into treatment trains economically and in accordance with regulations. 

Carbon dioxide (CO2) can be converted into valuable organic chemicals using light irradiation and photocatalysis. Today, light-energy loss, poor conversion efficiency, and low quantum efficiency (QE) hamper application of photocatalytic CO2 reduction. To overcome these drawbacks, we developed an efficient photocatalytic reactor platform for producing formic acid (HCOOH) by coating iron-based metal-organic framework (Fe-MOF) onto side-emitting polymeric optical fiber (POFs) and using hollow-fiber membranes (HFMs) to deliver bubble-free CO2. The photocatalyst, Fe-MoF with amine-group (−NH2) decoration, provided exceptional dissolved inorganic carbon (DIC) absorption. The dual-fiber system gave a CO2-to-HCOOH conversion rate of 116 ± 1.2 mM h-1 g-1, which is ≥18-fold higher than rates in photocatalytic slurry systems. The 12% QE obtained using the POF was 18-fold greater than the QE obtained by a photocatalytic slurry. The conversion efficiency and product selectivity of CO2-to-HCOOH were up to 22% and 99%, respectively. Due to the dual efficiencies of bubble-free CO2 delivery and the high QE achieved using the POF platform, the dual-fiber system had energy consumption of only 0.60 ± 0.05 kWh mole-1, 3000-fold better than photocatalysis using slurry-based systems. This innovative dual-fiber design enables efficient CO2 valorization without use of platinum group metals or rare earth elements.