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Diverse sources of wastewater organic carbon can be microbially funneled into biopolymers like polyhydroxybutyrate (PHB) that can be further valorized by conversion to hydrocarbon fuels and industrial chemicals. We report the vapor-phase dehydration and decarboxylation of PHB-derived monomer acids, 3-hydroxybutyric acid (3HB) and crotonic acid (CA), in water to propylene over solid acid catalysts using a packed-bed continuous-flow reactor. Propylene yields increase with increased Brønsted acidity of catalysts, with amorphous silica–alumina and niobium phosphate yielding 52 and 60 %C (percent feedstock carbon, max 75 %C) of feedstock 3HB and CA, respectively; additional products include CO 2 and retro-aldol products (acetaldehyde and acetic acid). Deactivation studies indicate progressive and permanent steam deactivation of amorphous silica–alumina, while re-calcination partially recovers niobium phosphate activity. Experiments demonstrating sustained reactor operation over niobium phosphate provide a promising technology pathway for increasing valorization of organic-rich wastewater. 

Ion exchange is widely used to treat nitrate-contaminated groundwater, but high salt usage for resin regeneration and management of waste brine residuals increase treatment costs and add environmental burdens. Development of palladium-based catalytic nitrate treatment systems for brine treatment and reuse has showed promising activity for nitrate reduction and selectivity towards the N2 over the alternative product ammonia, but this strategy overlooks the potential value of nitrogen resources. Here, we evaluated a hybrid catalytic hydrogenation/membrane distillation process for nitrogen resource recovery during treatment and reuse of nitrate-contaminated waste ion exchange brines. In the first step of the hybrid process, a Ru/C catalyst with high selectivity towards ammonia was found to be effective for nitrate hydrogenation under conditions representative of waste brines, including expected salt buildup that would occur with repeated brine reuse cycles. The apparent rate constants normalized to metal mass (0.30 ± 0.03 mM min−1 gRu−1 under baseline condition) were comparable to the state-of-the-art bimetallic Pd catalyst. In the second stage of the hybrid process, membrane distillation was applied to recover the ammonia product from the brine matrix, capturing nitrogen as ammonium sulfate, a commercial fertilizer product. Solution pH significantly influenced the rate of ammonia mass transfer through the gas-permeable membrane by controlling the fraction of free ammonia species (NH3) present in the solution. The rate of ammonia recovery was not affected by increasing salt levels in the brine, indicating the feasibility of membrane distillation for recovering ammonia over repeated reuse cycles. Finally, high rates of nitrate hydrogenation (apparent rate constant 1.80 ± 0.04 mM min−1 gRu−1) and ammonia recovery (overall mass transfer coefficient 0.20 m h−1) with the hybrid treatment process were demonstrated when treating a real waste ion exchange brine obtained from a drinking water utility. These findings introduce an innovative strategy for recycling waste ion exchange brine while simultaneously recovering potentially valuable nitrogen resources when treating contaminated groundwater. 

Conventional wastewater treatment processes can be tailored to recover organic carbon from wastewater as intracellular polyhydroxybutyrate (PHB) polymer granules while simultaneously meeting effluent discharge standards. Traditional applications of PHB as a bioplastic are hampered by its suboptimal properties (e.g., brittle), lack of efficient and sustainable approaches for recovering PHB from cells, and concerns about wastewater-derived impurities. In this study, we report on the conversion of PHB and its monomer acids – 3-hydroxybutyric acid (3HBA) and crotonic acid (CA) – under hydrothermal conditions (in condensed water at elevated temperature and pressure) to form propylene, a valuable chemical intermediate that self-separates from water. PHB depolymerization results in a mixture of 3HBA and CA, which can interconvert via (de)hydration reactions that vary with prevailing reaction conditions. Further hydrothermal conversion of the monomer acids yields propylene and CO2. Conversion of 3HBA occurs at lower temperatures than CA, and a new concerted dehydration-decarboxylation pathway is proposed, which differs from the sequential dehydration (3HBA to CA) and decarboxylation (CA to propylene and CO2) pathway reported for dry thermal conversion. A kinetics network model informed by experimental results reveals that CA conversion to propylene and CO2 proceeds predominantly via hydration to 3HBA followed by the concerted dehydration-decarboxylation pathway rather than by direct decarboxylation of CA. Demonstrative experiments using PHB-containing methanotrophic biomass show results consistent with the model, producing propylene at near-theoretical yields at lower temperatures than reported previously.