Electrocatalytic Oxidation of Hydrothermal Liquefaction- Derived Aqueous Phase for Onsite Wastewater Treatment and Simultaneous H2 Production

Abstract

Electrocatalytic oxidation (ECO) is garnering significant attention for the treatment of urban and industrial wastewater. There are many challenges to using traditional methods to treat the aqueous phase (AP) derived from hydrothermal liquefaction (HTL) due to high concentration of organic and inorganic compounds. In this study, we electrocatalytically oxidized HTL-AP derived from different feedstocks (e.g., algae, food waste, sludge, wood) and demonstrated chemical oxygen demand (COD) removal while simultaneously producing molecular hydrogen (H2) that can be used for hydrotreating HTL-derived bio-crude into biofuels. In this work, we performed batch electrolysis to understand the effect of HTL-AP composition on ECO performance using a modified Tafel analysis, the Boruah-Lopez-Ruiz-Strange (BLoRS) analysis. We then demonstrated the stability of the electrodes in a continuous flow electrolyzer and achieved 10% to 65% COD removal depending on the HTL-AP composition. We conducted electrocatalyst stability experiments at 2.5, 2.0, and 1.5 V vs a reversible hydrogen electrode (RHE) and demonstrated that the anode stability increased from 20 to ≥2,800 h. Furthermore, the energy required to produce H2 and remove COD decreased when the applied potential was decreased from 2.5 to 1.5 V vs. RHE; thus, the ECO reaction become more efficient [i.e., higher current efficiency (CE)] at lower applied potentials. We observed that the HTL-AP samples with higher NH4+ concentration appeared to deactivate at a faster rate. Furthermore, the presence of Cl- ions appears to enable indirect oxidation of organics and improve the anode stability and CE even when operating under higher NH4+ concentration. We performed a preliminary energy and H2 balance for an HTL of sewage sludge case, which revealed that the HTL-AP contains enough COD to generate all the H2 needed for bio-oil hydrotreating with a 47% excess that can be used for heat and power generation and sold for additional revenue from the HTL process. Thus, ECO of HTL-AP enables the production of endogenous H2 from HTL-AP with renewable electricity without using fossil fuels.

Keywords: Electrolysis, wastewater treatment, H2 production, hydrothermal liquefaction