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Advanced Electrochemical Methods Enabled Hybrid Water Treatment and Desalination
Abstract
Wastewater, a significant byproduct of human activity, often contains a complex mixture of pollutants that are detrimental to both public health and the environment. Despite significant advancements in wastewater treatment technologies, efficiently and thoroughly removing these contaminants remains a daunting task. Electrochemical degradation, an advanced oxidation process (AOP), has emerged as a compelling method for pollutant removal. As a promising and environmentally-friendly wastewater treatment method, electrochemical degradation warrants comprehensive exploration and discussion. In this dissertation, several hybrid water treatment and desalination processes via advanced electrochemical methods have been proposed and developed to remove hazardous pollutants, including electrocatalysis methods combined with filtration, electrocoagulation, and solar evaporation.
First, an advanced flow-through process was developed using modified stainless steel (SS) mesh with high catalytic activity. The surface of SS mesh was decorated by catalytically-active FexCo3-xO4 nanoparticles and functionalized carbon nanotubes (CNTs). The synergistic effect between Fe and Co elements enhanced the electro-Fenton efficiency, and the optimal n(Fe): n(Co) ratio was determined at 1:2 from the degradation rate of pollutants and H2O2. The addition of FeCo2O4/CNT enhanced the first-order reaction rate k to 2.60 times on bisphenol A (BPA) removal, and 2.16 times on sulfamethoxazole (SMX) removal, compared to an undecorated mesh. Consequently, 94% of BPA was eliminated after 60 min, and 100% of SMX was eliminated after 120 min, respectively, under a low current density of 2.84 mA cm-1. The total concentration of leached Fe/Co ions into the electrolyte was only around 2.4 µmol L-1 after the treatment.
Also, we have coupled electrocatalysis with electrocoagulation and applied an atomic layer deposition (ALD) enabled TiO2 ultrathin overcoating at a nanometer scale on a stainless-steel cathode. The electrocatalytic overcoating increased the elimination efficiency of organics and microorganisms, likely due to the electro-generation of adequate reactive oxygen species (ROS). The thickness of TiO2 nanofilm was controlled by the number of ALD cycles, and it was found that nanofilms processed with 50 to 100 cycles led to the maximum benefit of pollutant removal. By using the novel electrocoagulation–electrocatalysis cell to treat synthetic wastewater, a remarkable removal of 99.92% of E. Coli, 92.1% of suspended solids, 98.3% of heavy metal ions, and 88.8% of methylene blue was observed. This hybrid electrochemical treatment process may have the potential to treat wastewater at a larger scale.
Moreover, we developed a unique water desalination process by integrating solar steam generation with electrochemical degradation to treat saline water containing organics, and strong synergistic effects have been experimentally demonstrated. The process used a dual-functional solar absorber that simultaneously served as a cathode of the electrochemical reactor, whose structural design was optimized by numerical simulation to balance heat transfer and mass transport. Degradation of three model organic pollutants, bisphenol A, phenol (VOC), and humic acid (natural organic matter) was evaluated, and the degradation rate constants were doubled under simulated sunlight compared to that without illumination, likely due to the high local temperature in the electrochemical reactor induced by the photothermal effect and preserved by the rational thermal insulation design. Furthermore, the concentration of VOCs in the condensate was reduced by 20 folds when electrochemical degradation of feed water was applied. In addition, the electrochemical degradation effectively mitigated humic acid fouling on the solar absorber, improving the steam generation rate by 20% after 12 h treatment, compared to the conventional solar evaporation process. Finally, the integrated solar desalination system achieved a thermal efficiency of 92.6% under real sunlight testing.
Last, A synergistic, adaptive, continuous-flow, and low-carbon solar evaporation and electrochemical treatment (SEET) system was proposed and researched for energy-efficient and sustainable decentralized water treatment. The hybrid system integrated anodic oxidation with solar evaporation to enhance organic degradation and optimize mass transport through the photo-thermal effect. A novel four-step numerical simulation method was proposed to design the system and examine the water evaporation process and mass transport of salts and organics. A case study was implemented, revealing that system parameters related to evaporation and organics degradation exhibited strong interdependence. The relationships between these parameters were well-established, and adaptive water flow rate ranges were also identified to prevent salt accumulation while ensuring efficient organic degradation. The adaptability demonstrated the system's potential for use in varying influent scenarios. A prototype of the system was constructed, and the experimental data matched well with the simulation results. In the experiments, the local water temperature reached 45-50 ℃ in the continuous-flow mode under one sun condition, resulting in a 2-5 times reduction in outlet organic concentrations compared to traditional electrochemical systems. Energy analysis confirmed that the system primarily relied on clean and sustainable solar energy, maintaining a low carbon footprint. In conclusion, this innovative approach offers significant potential for addressing the clean drinking water crisis and enhancing pollutant removal in future decentralized water treatment systems.
Citation
Fan, Tianzhu (2023). Advanced Electrochemical Methods Enabled Hybrid Water Treatment and Desalination. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /199981.