Zhaobing Li,Zhi Chen,Yingying Xiang,Li Ling,Jingyun Fang,Chii Shang,Dionysios D. Dionysiou
Bromate formation in bromide-containing water through the cobalt (Co)-mediated activation of peroxymonosulfate (PMS) was investigated. Increasing the PMS dosage and the cobalt dosage increased the formation of bromate and bromate yields of up to 100% were recorded under the test conditions. The bromate yield increased to a maximum as the pH rose from 2.7 to 6 before decreasing by over 90% as the pH rose further from 6 to above 9. The bromate formation is a two-step process involving free bromine as a key intermediate and bromate as the final product. In the first step, apart from the known oxidation of bromide to free bromine and of free bromine to bromate by sulfate radicals (SO 4 − ), Co(III) produced from the oxidation of Co(II) by PMS and SO 4 − also oxidizes bromide to free bromine. The contribution of Co(III) to the bromate formation was verified with the addition of methanol and EDTA, a radical scavenger and a Co(III) ligand, respectively. In the presence of methanol, free bromine formation increased with increasing Co(II) dosage but no bromate was detected, indicating that Co(III) oxidized bromide to form free bromine but not bromate. In the presence of both EDTA and methanol, no free bromine or bromate was detected, as Co(III) was stabilized by EDTA to form the Co III EDTA – complex, which could not oxidize bromide. Mathematical simulation further suggested that Co(III) outweighed SO 4 − to oxidize bromide to free bromine. On the other hand, SO 4 − is essential for the oxidation of free bromine to bromate in the second step. In real water, the presence of NOM significantly decreased the bromate formation but caused the brominated organic DBP formation with high quantity. This is the first study to demonstrate the significant bromate formation in the Co/PMS system and the substantial contribution of Co(III) to the formation.
Wang Lu,Ji Yuefei,Lu Junhe,Kong Deyang,Yin Xiaoming,Zhou Quansuo
The objective of this research was to compare the transformation of Br − and formation of brominated byproducts in UV/persulfate (PS) and UV/H 2 O 2 processes. It was revealed that Br − was efficiently transformed to free bromine which reacted with humic acid (HA) or dihydroxybenzoic acid resulting in the formation of brominated byproducts such as bromoacetic acids (BAAs) in UV/PS system. In contrast, no free bromine and brominated byproducts could be detected in UV/H 2 O 2 system, although the oxidization of Br − was evident. We presumed that the oxidation of Br − by hydroxyl radicals led to the formation of bromine radicals. However, the bromine radical species could be immediately reduced back to Br − by H 2 O 2 before coupling to each other to form free bromine, which explains the undetection of free bromine and BAAs in UV/H 2 O 2 . In addition to free bromine, we found that the phenolic functionalities in HA molecules, which served as the principal reactive sites for free chlorine attack, could be in situ generated when HA was exposed to free radicals. This study demonstrates that UV/H 2 O 2 is more suitable than UV/PS for the treatment of environmental matrices containing Br − .
Graphical abstract
Formation of halogenated oxidation by-products (OBPs) in sulfate radical based advanced oxidation process (SR-AOPs) has raised growing concern. In the present study, we found that brominated OBPs (Br-OBPs) were generated in heat-activated peroxydisulfate (PDS) oxidation of humic acid (HA) in the presence of Br − . The yields of bromoform, dibromoacetic acid, and tribromoacetic acid reached 2.52, 2.01, and 0.11 μM, respectively, in 2 h at a working temperature of 60 °C with an initial Br − , PDS, and HA concentrations of 0.1 mM, 5.0 mM, and 2.0 mg/L, respectively. Similar results were found when HA was replaced by 3,5-dihydroxybenzoic acid (DHBA) as the substrate, suggesting that the phenolic moieties in HA molecules are the reactive sites for halogenation. Almost no Br-OBPs could be detected when 5.0 mM H 2 O 2 was added to the reaction solutions. The reduced Br-OBPs formation was due to the reduction of free bromine by H 2 O 2. Both experimental data and kinetic modeling indicated that the concentration of free bromine decreased significantly in the presence of H 2 O 2 . However, H 2 O 2 did not eliminate the formation of organic bromines completely. Certain brominated DHBA intermediates formation in the presence of H 2 O 2 was even higher than that with H 2 O 2 absent. Bromine radicals, such as Br 2 − and HBrO − , are presumed responsible for the organic bromines. Without free bromine, these organic bromines were relative stable and could not be further degraded to small molecule Br-OBPs. The findings of this study indicate that adding H 2 O 2 to SR-AOPs can significantly suppress the generation of halogenated-OBPs. However, the formation of organic bromines is still a concern.
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