Bromate oxidized from bromide during sonolytic ozonation

Underestimated role of hydroxyl radicals for bromate formation in persulfate-based advanced oxidation processes

In persulfate-based advanced oxidation processes (PS-AOPs), sulfate radicals (SO4•-) have been recognized to play more important roles in inducing bromate (BrO3-) formation rather than hydroxyl radicals (HO•) because of the stronger oxidation capacity of the former. However, this study reported an opposite result that HO• indeed dominated the formation of bromate instead of SO4•-. Quenching experiments were coupled with electron paramagnetic resonance (EPR) detection and chemical probe identification to elucidate the contributions of each radical species. The comparison of different thermal activated persulfates (PDS and PMS) demonstrated that the significant higher bromate formation in HEAT/PMS ([BrO3-]/[Br-]0 = 0.8), as compared to HEAT/PDS ([BrO3-]/[Br-]0 = 0.2), was attributable to the higher concentration of HO• radicals in HEAT/PMS. Similarly, the bromate formation in UV/PDS ([BrO3-]/[Br-]0 = 1.0), with a high concentration of HO•, further underscored the dominant role of HO•. As a result, we quantified that HO• and SO4•- radicals accounted 66.7% and 33.3% for bromate formation. This controversial result can be reconciled by considering the critical intermediate, hypobromic acid/hypobromate (HOBr/BrO-), involved in the transformation of Br- to BrO3-. HO• radicals have the chemical preference to induce the formation of HOBr/BrO- intermediates (contributing ∼ 60%) relative to SO4•- radicals (contributing ∼ 40%). This study highlighted the dominant role of HO• in the formation of bromate rather than SO4•- in PS-AOPs and potentially offered novel insights for reducing disinfection byproduct formation by controlling the radical species in AOPs.

Effective removal of organics from Bayer liquor through combined sonolysis and ozonation: Kinetics and mechanism

The presence of organic compounds in the waste liquor is of serious environmental concern that has plagued the development of alumina industry (Bayer Process). The present work attempts to develop a green and efficient process for removal of organics utilizing combined effect of sonolysis and ozonation (US/O₃). The effects of reaction duration, ozone concentration and ultrasonic power are assessed for sonolysis (US), ozonation (O₃) and combination of sonolysis and ozonation (US/O₃). The optimal conditions for US/O₃ treatment system is identified to be a reaction duration of 7 h, ozone concentration of 7.65 g/h, and ultrasonic power of 600 W. The total organic carbon (TOC) removal and decolorization are 60.13% and 87.1%, respectively. The process can be scaled-up to industrial scale, which could potentially serve to be a convenient, safe and sustainable alternative to the exisiting treatment technologies. Additionally, the treated waste water can be reused contributing to an improvement in the overall economics.

Effects of organic matter, ammonia, bromide, and hydrogen peroxide on bromate formation during water ozonation

Ozone is widely applied for disinfection in drinking water treatment and the disinfection by-product bromate would be produced during the ozonation of bromide-bearing water. Hydrogen peroxide (H₂O₂) addition could effectively control the formation of bromate. However, the bromate depression performance would be impacted by water qualities. In this study, typical source water containing bromide in eastern China was selected to investigate bromate depression effect under different organic matter, ammonia and bromide concentrations during the H₂O₂-O₃ process. The results display that organic matter, ammonia and bromide concentration could influence the formation of bromate significantly. As tyrosine was applied to increase the dissolved organic carbon (DOC) concentration of source water by 2.0 and 3.0 mg/L, the total concentration of bromate produced decreased gradually as the H₂O₂/O₃ (g/g) doses increased from 0 to 1.0 and bromate concentration could be controlled below 10 μg/L as H₂O₂/O₃ (g/g) was 0.5 and 1.0. As ammonia concentration increased by 0.1 and 0.5 mg/L, lower H₂O₂/O₃ (g/g) doses would lead to an increase in bromate generation. As more H₂O₂ was added in water, the bromate formation would be suppressed. The increase of bromide concentration induced higher bromate formation. When the bromide concentration increased by 50 and 200 μg/L, bromate concentration was 10.7 μg/L and 41.2 μg/L respectively at the H₂O₂/O₃ (g/g) of 1.0, higher than the standard level. As 200 μg/L of bromide was added to the water, bromate concentration increased significantly and then decreased as H₂O₂/O₃ (g/g) increased and more H₂O₂ would be needed for bromate control.

An overview of bromate formation in chemical oxidation processes: Occurrence, mechanism, influencing factors, risk assessment, and control strategies

Chemical oxidation processes have been extensively utilized in disinfection and removal of emerging organic contaminants in recent decades. Some undesired byproducts, however, are produced in these processes. Of them, bromate has attracted the most intensive attention. It was previously regarded as a byproduct that typically occurred in ozone-based oxidation processes. However, for the past decade, bromate formation has been detected in other oxidation processes such as CuO-catalyzed chlorination, SO₄^--based oxidation, and ferrate oxidation processes. This review summarizes the occurrences, mechanisms, influencing factors, risk assessment, and control strategies of bromate formation in the four oxidation processes, i.e., ozone-based oxidation, chlorine-based oxidation, SO₄^--based oxidation, and ferrate oxidation. Besides, some unresolved issues for future studies are provided: (1) Clarification of the relative contributions of SO₄^- and Br to the oxidation of bromine for bromate formation in SO₄^--based oxidation processes; (2) evaluation of the role of different reactive species in the bromate formation in the process of UV/HOCl; (3) quantification of the dual role of alkalinity in bromate formation during ozonation; (4) assessment of the risks of bromate formation in SO₄^--based oxidation processes for practical applications; and (5) exploration of strategies for inhibiting bromate formation in SO₄^--based oxidation, UV/chlorine, and metal oxide-catalyzed chlorination processes.

Oxidation byproducts from the degradation of dissolved organic matter by advanced oxidation processes - A critical review

Advanced oxidation processes (AOPs) have been increasingly used for the treatment of source waters and wastewaters. AOPs characteristically produce oxidation byproducts (OBPs) from the partial degradation of dissolved organic matter (DOM) and/or the transformation of inorganic ions (especially, halides) into highly toxic substances including bromate and halogenated organic OBPs (X-OBPs). However, despite the enormous health and environmental risks posed by X-OBPs, an integral understanding of the complex OBP formation mechanisms during AOPs is lacking, which limits the development of safe and effective AOP-based water treatment schemes. The present critical and comprehensive review was intended to fill in this important knowledge gap. The study shows, contrary to the hitherto prevailing opinion, that the direct incorporation of halide atoms (X^•) into DOM makes an insignificant contribution to the formation of organic X-OBPs. The principal halogenating agent is hypohalous acid/hypohalite (HOX/XO^-), whose control is, therefore, critical to the reduction of both organic and inorganic X-OBPs. Significant generation of X-OBPs has been observed during sulfate radical AOPs (SR-AOPs), which arises principally from the oxidizing effects of the unactivated oxidant and/or the applied catalytic activator rather than the sulfate radical as is commonly held. A high organic carbon/X^- molar ratio (>5), an effective non-catalytic activator such as UV or Fe²⁺, a low oxidant concentration, and short treatment time are suggested to limit the accumulation of HOX/XO^- and, thus, the generation of X-OBPs during SR-AOPs. At present, there are no established techniques to prevent the formation of X-OBPs during UV/chlor(am)ine AOPs because the maintenance of substantial amounts of active halogen is essential to these processes. The findings and conclusions reached in this review would advance the research and application of AOPs.

Quantitatively assessing the role played by carbonate radicals in bromate formation by ozonation

Bicarbonate scavenges OH to form CO₃^- that enhances the bromate formation by ozonation. However, the role of CO₃^- in the bromate formation during ozonation has never been quantitatively investigated. Herein, we establish a quantitative approach for evaluating the role played by CO₃^- based on the detection of CO₃^--involved bromate and CO₃^- exposure. Experiments demonstrated that the CO₃^--involved bromate was responsible for 33.7-69.9% of the total bromate formed with bicarbonate concentrations from 0.5 mM to 4 mM. The CO₃^- exposure was two orders of magnitude higher than the corresponding OH exposure during ozonation. These results demonstrate that CO₃^- plays a comparable or even more pronounced role in the oxidation of bromine during bromate formation than OH. A model was developed based on the ratio of bromine oxidized by CO₃^-, which could predict the CO₃^--involved bromate formation well. Modeled and experimental results illustrated that the contribution of the CO₃^--involved bromate to the total bromate decreased with increasing pH or initial bromide, but almost remained unchanged at different ozone dosages. Moreover, the presence of humic acid led to an increase in this contribution during ozonation. The results of this study provide a more in-depth understanding of the mechanism of bromate formation during ozonation.

Enhancement of bromate formation by pH depression during ozonation of bromide-containing water in the presence of hydroxylamine

This work investigated the fate of bromate formation during ozonation in the presence of hydroxylamine (HA). Results indicated that pH depression, as a commonly feasible control strategy for bromate formation during ozonation, unexpectedly enhanced the bromate formation during ozonation in the presence of HA. A dramatically high level of bromate was observed at acidic pH in the ozone/HA process. The scavenging experiments demonstrated the essential role of OH produced in the reaction of ozone with HA in bromate formation. In the process, OH mainly oxidizes bromide to Br, which is further oxidized by ozone and eventually converts to bromate. Further investigations suggested that the unexpected enhancement on bromate formation by pH depression can be mainly ascribed to the pH-dependent ozone decay, OH exposures and formation rate of Br. As pH decreased from 7 to 5, the reduced OH scavenging capacity of HA led to higher OH exposures, which contributed to the enhancement of bromate formation. As pH decreased from 5 to 3, the enhanced formation rate of Br largely augmented the formation of bromate. In addition, the ozone decay slowed down by pH depression provided more available ozone for the oxidation of the formed Br to bromate. The enhanced effect of pH depression on bromate formation was still observed in the real water samples in the ozone/HA process. Accordingly, pH depression might be avoided to control the bromate formation during ozonation in the presence of HA.