3.1 Reaction kinetics of humic substances undergoing persulfate oxidation
3.1.1 Degradation rates at different UV wavelengths
We delivered 185 nm and 254 nm UV irradiation in the presence or absence of persulfate, as shown in Figure 1. Under direct photolysis at 254 and 185 nm, the non-purgeable organic carbon (NPOC), UV254 absorbance, and SUVA254 decreased continually; the reductions mediated by 254 nm UV were lower. Under 185 nm UV, the UV254 absorbance and SUVA254 decreased more rapidly than did the NPOC, especially in the initial stages. UV at 185 nm was more effective. However, UV at 254 nm was chosen for further work, given its limited efficiency in terms of humic acid treatment. Similar results were obtained by Gao et al., (2019) and Corin et al, (1996). Gao et al., (2019) found that the UV wavelength acompromise treatment, as followsffected the molar absorption coefficient of humic acid. UV at 185 nm better treated humic acid, because irradiation simultaneously created active intermediates including hydrogen atom, hydroxyl radical and other oxidative species such as hydrogen peroxide probably from water (Liu et al., 2016, Han et al., 2004); these efficiently decomposed humic acid:
When persulfate was added to 1 mM, the NPOC, UV254 absorbance, and SUVA254 fell dramatically. After 30 min, the NPOC, UV254 absorbance, and SUVA254 reductions were 82.4%, 99.0%, and 94.1% for 254 nm UV and 92.7%, 98.9%, and 84.9% for 185 nm UV, respectively. However, under UV irradiation only, the corresponding values were only 1.2%, 2.06%, and 0.86% (254 nm UV) and 28.1%, 58.1%, and 41.8% (185 nm UV), respectively. Therefore, the sulfate radical was taking the main role to effectively remove humic substances. After persulfate addition, UV at 185 nm was more effective than UV at 254 nm because the shorter wavelength delivered more energy, creating more sulfate radicals. However, when the water is irradiated by 185 nm lamp, hydroxyl radicals were created (eq.1) which makes it difficult to directly investigate sulfate radicals’ treatment efficiency and mechanism. On the other hand, generally, 254 nm UV lamp was utilized in the water treatment plant. Hence, the following experiments were utilized 254 nm UV lamp instead of 185 nm lamp
3.1.2 Dose-dependency of humic substances degradation by the UV/persulfate process
188.8.131.52 Dose-dependent degradation
Figure 2a and 2b presents the reductions in humic acid (Sigma) levels at persulfate concentrations of 0, 0.1, 1, and 10 mM. The removal rate was initially rapid and then slower. Figure 2c and 2d presents the fulvic acid data. With regard to humic acid, the reduction rate was greater at higher persulfate concentrations. At the end of the reaction, the reductions in UV254 absorbance and NPOC were 99.0%, 82.4% for humic acid and 98.5% and 83.1% for fulvic acid (1 mM persulfate), respectively. Although the final reductions were similar, the fulvic acid reaction was more rapid than the humic acid reaction; fulvic acid was more susceptive to persulfate treatment.
When humic and fulvic acids were treated with 10 mM persulfate, the NOM degradation rate increased. Higher sulfate radical concentrations increased degradation efficiency.
However, this does not mean that the higher the persulfate concentration, the better the treatment. On the contrary, a high persulfate concentration reduces efficiency because the sulfate radical can react with the persulfate anion or self-decay in the absence of an appropriate solute (Matzek and Carter., 2016). Hence, there is an optimum concentration of persulfate:
Given this reaction, even if persulfate is abundant, the sulfate radical will transform into non-toxic sulfate. Thus, sulfate radical-based advanced oxidation is environmentally friendly.
184.108.40.206 Mechanism of elimination of humic and fulvic acids
Persulfate at 1 mM reduced humic and fulvic acid levels (Fig. 2); the reductions in UV254 absorbance and NPOC were similar. During humic acid degradation, the UV254 absorbance decreased more rapidly from 0–5 min (24.3%); the NPOC fell more slowly (2.6%). This is because humic acid removal is a multi-step process. Humic acid and persulfate initially form an unstable adduct; aromatic portions are then degraded to smaller intermediate products via irradiation, reducing UV254 absorbance, but reducing the NPOC level only minimally (Pedro et al., 2008). Next, the sulfate radical oxidizes the intermediate products which are main low molecular weight products into end products, improving NPOC removal (Ahn et al., 2017, Gao et al., 2019). In other words, as aromatic organic matter is transformed, the UV254 absorbance decrease rapidly from 0–5 min but the fall in NPOC level is limited during this interval.
For fulvic acid, the UV254 absorbance also decreased more rapidly (74.1%) than did the NPOC level (30.4%) from 0-5 min. However, compared to humic acid, initial NPOC removal was more rapid can be considered that fulvic acid contains fewer aromatic portions and has small size than does humic acid. Sulfate radicals directly oxidize low molecular weight portion into end products, increasing initial NPOC removal.
3.1.3 Evaluation of humic acids of different SUVA254
Due to the different structure of the humic acid and fulvic acid, it is hard to investigate the effect of the amount of aromatic compounds and molecular weight on treatment efficiency directly. Hence, we used two humic acids that differed in terms of the initial SUVA254 and UV254 absorbance (Fig. 3). From 0-10 min, 94.5% UV254 absorbance, 48.2% NPOC, 89.4% SUVA254 decreasing for HAs (IHSS) and 42.0% UV254 absorbance, 16.6% NPOC, 30.6% SUVA254 decreasing for HAs (Sigma) was investigated. Humic acid from Sigma had a higher UV254 absorbance and SUVA254 (0.179 cm-1, 4.8 L mg-1m-1) than humic acid from IHSS (0.104 cm-1, 2.8 L mg-1m-1). Thus, the former material contained more aromatic and chromophore portion than the latter, was degraded less efficiently. Aromatic and chromophore proportion are more susceptible to sulfate radicals. On the other hand, high SUVA254 value indicated high molecular weight for humic substances which showed low treatment efficiency. And at initial stage, high SUVA254 reduction was investigated for both two humic acids but low NPOC removal which indicated that most humic acid are not decomposed into final products. According to that, the oxidation mechanism can be considered that, aromatic and chromophore portion in humic acid was firstly decomposed into lower molecular intermediate products which lead to UV254 absorbance and SUVA254 decreasing but limit NPOC removing. Then lower molecular intermediate products were finally oxidized into final products like water and carbon dioxide, resulting in NPOC reducing.
3.1.4 Efficiencies of various oxidants
Figure 4 presents the decomposition efficiencies of various oxidants. All of the NPOC level, UV254 absorbance, and SUVA254 decreased continually. Hydrogen peroxide reduced the NPOC level, UV254 absorbance, and SUVA254 by 47.2%, 19.8%, and 34.2%, respectively; the corresponding figures for persulfate were 99.0%, 82.4%, and 94.1%, thus significantly better. Sarathy and Mohseni, 2010 and Bazri et al., (2012) reported that the SUVA254 serves as an indicator of organic matter biodegradability and the potential for THMs and HAAs formation. Compared to persulfate, hydrogen peroxide did not effectively reduce the SUVA254. In previous studies, organic compounds were not completely decomposed during oxidation, and were rather partially oxidized to toxic and assimilable transformation products (TPs) (Wang and Wang, 2018b). Hence, the UV/H2O2 process may create new pollutants because NPOC removal is limited; persulfate provides cleaner water. The better treatment efficacy of persulfate is attributable to the fact that the O-O bond dissociation energy is lower than that of hydrogen peroxide (Table 1). This increases the sulfate radical concentration; that radical has a higher standard redox potential than the hydroxyl radical. Hence, persulfate was more effective than hydrogen peroxide when used to treat humic acid. Although humic substances can be completely eliminated using the UV/H2O2 process, the oxidant concentration was greater and irradiation time was longer than those of the UV/persulfate system.
However, when hydrogen peroxide was added, the decreases in NPOC levels were similar to those afforded by persulfate, gradually increasing from 10 min; the reductions in UV254 absorbance and SUVA254 were already rapid from 0–10 min. This is explained by fractional conversion of the aromatic and chromophoric components of humic acid, as reflected in the UV254 data (Bazri et al., 2012). Aromatic and chromophoric components are initially transformed into lower molecular weight components, causing rapid reductions in UV254 absorbance and the SUVA254, but only limited NPOC removal. This has been previously described; Ahn et al., (2017) found that the levels of humic substances decreased, whereas the levels of low molecular weight acids increased, during the UV/H2O2 process.
3.2 Degradation efficiency of raw water
Figures 5a, b present raw water purification via the UV/persulfate process. The reductions were 92% for UV254 absorbance, 56% for the NPOC level, and 82% for the SUVA254; the sulfate radical effectively treated aquatic organic compounds. The limited NPOC removal was attributable to the high concentration of Br – (0.302 mg/L). Lou et al., (2016) reported that halide ions, especially Br –, compromise treatment, as follows:
However, in raw water, NPOC features both hydrophobic and hydrophilic NOM. The treatment efficiencies of various NOMs vary: longer reaction times or higher persulfate concentrations enhance NPOC removal. Chen et al., (2003) separated EEM data into five components containing specific NOMs. Figure 5b shows that the raw water contained humic acid-like compounds (region V), fulvic acid-like compounds (region III), and soluble microbial byproduct-like materials (region IV). All peaks eventually disappeared as reaction proceeded; all organic compounds were oxidized. The peaks of fulvic and humic acids were almost completely eliminated by 30 min.