Abstract
Based on a synergistic digestion method of ultraviolet combined with ozone (UV/O3), this article investigates the reaction characteristics of nitrogen-containing compounds (N-compounds) in water and the influence of ions on digestion efficiency. In this respect, a novel and efficient AOPs-based dual-environmental digestion method for the determination of total dissolved nitrogen (TDN) in waters with complex components is proposed, in the hopes of improving the detection efficiency and accuracy of total nitrogen via online monitoring. The results show that inorganic and organic N-compounds have higher conversion rates in alkaline and acidic conditions, respectively. Meanwhile, the experimental results on the influence of Cl−, CO32−, and HCO3− on the digestion process indicate that Cl− can convert to radical reactive halogen species (RHS) in order to promote digestion efficiency, but CO32− and HCO3− cause a cyclic reaction consuming numerous •OH, weakening the digestion efficiency. Ultimately, to verify the effectiveness of this novel digestion method, total dissolved nitrogen samples containing ammonium chloride, urea, and glycine in different proportions were digested under the optimal conditions: flow rate, 0.6 L/min; reaction temperature, 40 °C; pH in acidic conditions, 2; digestion time in acidic condition, 10 min; pH in alkaline conditions, 11; digestion time in alkaline conditions, 10 min. The conversion rate (CR) of samples varied from 93.23% to 98.64%; the mean CR was greater than 95.30%. This novel and efficient digestion method represents a potential alternative for the digestion of N-compounds in the routine analysis or online monitoring of water quality.
Highlights
In natural waters, proper environmental nutrient balance is essential for maintaining a healthy ecosystem
Sun et al studied the effects of three types of ultraviolet lamp with different power (15, 18, and 23 W) on the reaction, and the results showed that the 18 W UV lamp could provide enough energy to induce the conversion of O3 into OH
The results show that different pH environments should be selected for the digestion of different dissolved N-compounds: for dissolved organic nitrogen (DON), acidic conditions can weaken the capture effect of inhibitors such as CO3 2−
Summary
Proper environmental nutrient balance is essential for maintaining a healthy ecosystem. When aiming to quantitatively determine TDN in waters, all dissolved N-compounds should be converted into a single species, such as nitrate, nitrite, ammonium, or ammonia [4]. TDN is determined by converting all dissolved N-compounds into detectable forms [5], which can generally be summarized into four categories: nitrate ions by adequate oxidation [6], nitrite ions by adequate oxidation and inadequate reduction [7], ammonium ions by reduction [8], and activated NO2 *(*-activated status) by high-temperature oxidation [9]. Spectrophotometry is an effective method to detect nitrate concentrations in solution, and is widely utilized in the field of online monitoring of water quality due to its low cost, simplicity, high efficiency, and lack of pollution byproducts [10]
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