A Novel Au-Decorated Ni-MOF for the Photo and Electrochemical Treatment of MC-LR

Microcystins (MCs) are toxins produced by cyanobacteria commonly found in harmful algal blooms (HABs) threatening many surface waters. Conventional removal methods, including membrane filtration and activated carbon, are costly and only achieve phase separation. This study introduces a novel Au-decorated Ni-metal-organic framework (Au/Ni-MOF) as a platform for the multi-modal treatment of MC-Leucine Arginine (MC-LR), utilizing photodegradation (PD), electrochemical oxidation (EO), and photoelectrochemical oxidation (PEO) by evaluating and comparing their effectiveness. A 4 cm2 Au/Ni-MOF achieved complete photodegradation of 20 ppb of MC-LR in 7 mL of DI water with a degradation rate constant of 0.0599 min-1 under 35,000 lux UV-Vis illumination. In comparison, the EO process, conducted with a 2 cm2 electrode at a current density of 5 mA cm-2 in 25 mL of 0.1M phosphate buffer (PB) solution, achieved 71% removal with a rate of 0.0053 min-1 and an energy consumption of 0.41 kWh m-3. The PEO process demonstrated superior performance, reaching 84% removal at 0.0074 min-1 and 0.43 kWh m-3 using a 2 cm2 electrode at 5 mA cm-2 under 26,000 lux illumination. In all cases, degradation was primarily driven by reactive oxygen species (ROS) production, particularly hydroxyl radicals. In lake water spiked with 20 ppb of MC-LR, the PD rate decreased 10-fold to 0.006 min-1, due to the presence of competing ROS scavengers. In contrast, the EO and PEO exhibited enhanced performance with rate constants of 0.0192 min-1 and 0.0202 min-1, respectively. The application of external electrical potential in EO and PEO facilitated more stable energy input, enabled diverse ROS generation pathways, and reduced environmental sensitivity. PEO showed a synergistic improvement over EO, following the overall trend: PEO > EO > PD. Overall, the Au/Ni-MOF photoanode shows strong potential as a low-cost, energy-efficient platform for the multi-modal treatment of MC-LR in both natural and engineered water systems.