Degradation of refractory contaminants in water by chemical-free radicals generated by ultrasound and UV irradiation

Abstract

The contamination of water bodies by endocrine disrupting compounds (EDCs) has drawn increasing attention in recent decades because of their potential adverse effects on aquatic lives and human health. The inefficiency of biological treatment methods in degrading EDCs promoted the development of various kinds of advanced oxidation technologies (AOTs). In this study, a chemical-free AOT by combining high frequency ultrasound and ultraviolet irradiation (i.e. US/UV) was proposed and intensively investigated to degrade different kinds of EDCs, including dimethyl phthalate (DMP), di-n-butyl phthalate (DBP), diethyl phthalate (DEP), monomethyl phthalate (MMP), atrazine (ATZ), and nonylphenol (NP). Initially, a prototype US/UV unit consisting of a 400 kHz ultrasonic system and a photolytic system at 253.7 nm was set up by using DMP as a probe compound. Several important parameters were evaluated, including ultrasonic frequency, ultrasonic power, initial concentration of the probe compound, initial solution pH, UV light intensity, and the effect of H₂O₂, etc. Both the ultrasonic frequency and power density played important roles in ultrasonic process. The optimal performance in this study was obtained by a 400 kHz frequency and the maximum input power of 120 W. In addition, the ultrasonic degradation rate increased directly with power density, and decreased moderately with increasing pH in the range typically found in the environment (pH 5 - 9). Higher initial concentration resulted in a smaller rate constant. The addition of hydrogen peroxide can increase the radical generation to some extent. It was also found that higher UV light intensity was beneficial for both the photolysis and sonophotolysis of DMP. Furthermore, the synergistic effect and mechanisms of the hybrid process (US/UV) was investigated. The role of ultrasonically generated hydrogen peroxide was examined qualitatively and quantitatively, and its generation and photo-decomposition were found to be the principal reason for the process synergy. The sonolysis and photolysis of the investigated compounds all followed pseudo first-order kinetics, but the sonophotolysis of different compounds exhibited different kinetics. A novel inverted S-curve model was developed and found to successfully describe the sonophotolytic process and the degradation of different compounds. Different compounds also demonstrated different magnitudes of synergistic effect due to their different physicochemical properties (e.g. the hydrophobicity and the photochemical properties). In addition, the distinct feature of ultrasonic process, i.e. the heterogeneous micro-environment that exists between cavitation bubbles and bulk solution, was investigated by comparing the degradation performance of ATZ in two different ultrasonic processes (20 kHz and 400 kHz). The better heterogeneous distribution of solutes in the 400 kHz ultrasound allowed a faster degradation of hydrophobic compounds than that in the 20 kHz ultrasound. Moreover, the sonophotolytic degradation of a group of phthalate acid esters (PAEs) with different physicochemical properties was compared. It was found that although stronger hydrophobicity was beneficial for the sonochemical degradation of PAEs, it was adverse to obtaining remarkable synergistic effect in the sonophotolytic process due to the less accumulation of H₂O₂. Moreover, the influence of some background species (e.g. radical scavengers, ions in wastewater) was also evaluated. The mechanism of sonochemical degradation of the involved compounds in this study was found all via radical oxidation near the bubble-liquid interfaces and in the bulk solution rather than pyrolysis inside the cavitation bubbles by employing different kinds of radical scavengers. The presence of ferrous ions (Fe²⁺) in solution gave rise to a homogeneous sono-photo-Fenton (US/UV/Fe²⁺) process, and clear synergistic effects were observed in the US/UV/Fe²⁺ process which led to it having the greatest impact on DBP degradation. The presence of NaCl demonstrated different effects on the 400 and 20 kHz ultrasonic processes, mainly because of the competition between "salting out" effect and surface tension, and they were also influenced by the different heterogeneous environments in different ultrasonic processes. The background total organic carbon (TOC) coming from wastewater effluent hindered the sonophotolysis of PAEs, and PAEs with stronger hydrophobicity experienced less inhibition by background TOC. Nitrate ions (NO₃⁻), the most commonly seen constituent in the secondary effluent and the main component of total nitrogen, had a beneficial effect on the sonophotolytic degradation of NP and the main mechanism was found to be acting as a photosensitizer. Finally, the degradation mechanisms of the investigated compounds and mineralization efficiencies in the US, UV, and US/UV processes were investigated. Results showed that the ultrasonic process was effective in degrading more hydrophobic target compounds, but less efficient in degrading their more hydrophilic intermediates. However, the involvement of UV process could promote the degradation of these less hydrophobic intermediates via direct photolysis or producing additional hydroxyl radicals in the bulk solution by photo-dissociation of hydrogen peroxide. The combined US/UV process also had the best mineralization performance compared to individual US or UV process.