Ultrasonic-enhanced nascent H<sub>2</sub> activation on Raney Ni for the degradation of triclosan

Abstract

<p indent=0mm>Triclosan (TCS) is a broad-spectrum antibacterial agent that has been widely used in medicines and personal care products, such as fungicide, disinfectants, soap, toothpaste, etc. Most of the products containing TCS after use enter the sewage treatment plant through domestic sewage. However, TCS entering the sewage treatment plant is only partially degraded due to its stable halogenated aromatic structure, and the remaining TCS and its by-products will enter the external environment. Studies have found that TCS in the aquatic environment exhibits significant biological toxicity to the growth and metabolism of algae, luminescent bacteria, zooplankton and fish. In addition, TCS will cause allergic dermatitis through direct contact with the human body through skin and oral cavity, and can also indirectly accumulate in the body through the food chain, thereby disrupting the body’s normal endocrine function. The development of efficient treatment technology for TCS in water has important theoretical significance and application value. Reductive dechlorination can effectively reduce the toxicity and stability of TCS (improve its biodegradability), and plays an important role in the restoration of TCS polluted water. The currently used reduction techniques include zero-valent iron reduction, electrochemical reduction, photochemical reduction and catalytic hydrogenation. Among them, catalytic hydrogenation has attracted wide attention due to its simple operation and high efficiency. However, its upscale application is being restricted by its excessive dependence on noble metal catalysts and the closed high-pressure environment constructed to improve hydrogenation efficiency. This work aims to improve the existing catalytic hydrogenation technology. Specifically, cheap and available Raney Ni (R-Ni) was selected as the hydrogenation catalyst, and tiny nascent H₂ (Nas-H₂) bubbles generated <italic>in situ</italic> by the cathode hydrogen evolution reaction were used as the hydrogen source. The effects of current density and catalyst dosage on TCS hydrodechlorination were studied. The experimental results showed that the reduction process of TCS in the R-Ni/Nas-H₂ system followed a pseudo-first order reaction kinetics. The conversion and dechlorination ratios of TCS (<italic>C</italic>₀=<sc>30 mg/L)</sc> were 96.3% and 68.8%, respectively, within <sc>2.0 h.</sc> Compared with the existing catalytic hydrogenation system, H₂ consumption was significantly reduced (from ≥<sc>10 mL/min</sc> to <sc>1.05 mL/min),</sc> which was attributed to that the refined Nas-H₂ bubbles were more likely to be activated by R-Ni due to their higher solubility and longer residence time in the reaction liquid. Furthermore, ultrasonic cavitation was introduced to achieve deep dechlorination of TCS. The conversion and dechlorination ratios of TCS increased to 99.0% and 86.5%, respectively, with <sc>0.58 W/cm³</sc> ultrasonic power density of the reaction solution, and the atom utilization ratio of Nas-H₂ reached 0.21%. TCS reduction conformed to the catalytic hydrogenation mechanism of adsorption→activation→hydrogenation, and the hydrogen adatoms (H_ads*) were reactive species. The ultrasound-enhanced hydrogenation performance was attributed to the cavitation which improved the catalytic activity of R-Ni and blasted Nas-H₂ into easily activatable nanoscale hydrogen (Nano-H₂), thereby promoting the production of H_ads* and increasing the effective number of collisions between reactive species (H_ads*, TCS_ads). In addition, through the determination and analysis of the reaction intermediate products, the stepwise hydrodechlorination removal mechanism of TCS was proposed, and the final product was 2-hydroxydiphenyl ether. This research is expected to be applied to the efficient dehalogenation of polyhalogenated organic pollutants in water.