Abstract
Over the past two decades, the alkaline persulfate oxidation (PO) with thermal and/or ultraviolet (UV) assisted digestion method has been widely used for digestion of nitrogen containing compounds (N-compounds) in water quality routine analysis in laboratory or on-line analysis, due to its simple principle, high conversion rate, high percent recovery, low-cost. However, this digestion method still has some inevitable problems such as complex operations, high contamination potential, batch N blanks, higher reaction temperature (120–124 °C) and time-consuming (30–60 min). In this study, ozone (O3) was selected as the oxidant for digestion of N-compounds through analysis and comparison firstly. Secondly, we proposed and compared the UV and/or ultrasound (US) combined with ozone (UV/O3, US/O3 and UV/US/O3) synergistic digestion methods based on O3 with sole O3 oxidation method on digestion efficiency (digestion time and conversion rate) of standard N-compounds. Simultaneously, the influence of reaction temperature, pH of water sample, concentration of O3 and mass flow rate, UV intensity, US frequency and power on digestion efficiency were investigated, and then the optimum parameters for digestion system were obtained. Experimental results indicated that UV radiation can effectively induce and promote the decomposition and photolysis of O3 in water to generate hydroxyl radicals (•OH), while US can promote the diffusion and dissolution of O3 in water and intensify the gas-liquid mass transfer process for the reaction system. Meanwhile, results showed that the UV/US/O3 synergistic digestion method had the best digestion efficiency under the optimum conditions: water sample volume, 10 mL; pH of water sample, 11; O3 mass flow rate, 3200 mg/h; reaction temperature, 30 °C; digestion time, 25 min; UV lamp power, 18 W; distance between UV lamp and reactor, 2 cm; US frequency, 20 kHz; US power, 75 W. The conversion rate (CR) of synthetic wastewater samples varied from 99.6% to 101.4% for total dissolved nitrogen (TDN) in the range of 1.0~4.0 mg/L. The UV/US/O3 synergistic digestion method would be an effective and potential alternative for digestion of N-compounds in water quality routine analysis in laboratory or on-line analysis.
Highlights
Total nitrogen (TN) is an important indicator for water quality determination and on-line monitoring [1]
Results showed that the UV/US/O3 synergistic digestion method had the best digestion efficiency under the optimum conditions: water sample volume, 10 mL; pH of water sample, 11; O3 mass flow rate, 3200 mg/h; reaction temperature, 30 ◦ C; digestion time, 25 min; UV lamp power, 18 W; distance between UV lamp and reactor, 2 cm; US frequency, 20 kHz; US power, 75 W
The main objectives of this study were: (a) to compare the UV/O3, US/O3 and UV/US/O3 synergistic digestion methods based on O3 with sole O3 oxidation method on digestion efficiency of standard N-compounds; (b) to investigate the influence of reaction temperature, pH of water sample, concentration of O3 and mass flow rate, UV intensity, US frequency and power on digestion efficiency, and to obtain the optimum parameters for digestion system; (c) to seek and develop a novel and reliable digestion method for safe, environmentally-friendly, convenient and efficient digestion of nitrogen containing organic and inorganic compounds for improving the precision and stability of determination of total dissolved nitrogen in natural waters and waste waters
Summary
Total nitrogen (TN) is an important indicator for water quality determination and on-line monitoring [1]. In order to determine and quantify TDN in waters, all N-compounds should be converted to a single species, such as ammonia, ammonium, nitrite and nitrate. Ammonia and ammonium can be distilled off in the form of ammonia and determined by titration with standardized mineral acid, nitrate and nitrite (need reduction reaction, diazo-coupling reaction and Griess reaction) can be measured by various techniques [5], but the most widely used and convenient methods are spectroscopic methods [6], such as the ultraviolet (UV)/Vis spectrophotometric method. The first step for the determination of TDN is the digestion of the water sample in order to convert DON into an inorganic form of nitrogen, it is the most tedious and time-consuming step and the greatest source of errors in the analytical determination of TDN
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