Performance Investigation of Electrochemical Assisted HClO/Fe2+ Process For The Treatment of Land ll Leachate


 The feasibility of removal of COD and ammonia nitrogen (NH4+-N) from landfill leachate by electrochemical assisted HClO/Fe2+ process is demonstrated for the first time. The performance of active chlorine generation at the anode was evaluated in Na2SO4/NaCl media, and a higher amount of active chlorine was produced at greater chloride concentration and higher current density. The probe experiments confirmed the coexistence of hydroxyl radical (·OH) and Fe(IV)-oxo complex (FeIVO2+) in the HClO/Fe2+ system. The influence of initial pH, Fe2+ concentration and applied current density on COD and NH4+-N abatement was elaborately investigated. The optimum pH was found to be 3.0, and the proper increase in Fe2+ dosage and current density resulted in higher COD removal due to the accelerated accumulation of ·OH and FeIVO2+ in the bulk liquid phase. Whereas, the NH4+-N oxidation was significantly affected by the applied current density because of the effective active chlorine generation at high current, but was nearly independent of Fe2+ concentration. The reaction mechanism of electrochemical assisted HClO/Fe2+ treatment of landfill leachate was finally proposed. The powerful ·OH and FeIVO2+, in concomitance with active chlorine and M(·OH) were responsible for COD abatement and active chlorine played a key role in NH4+-N oxidation. The proposed electrochemical assisted HClO/Fe2+ process is a promising alternative for the treatment of refractory landfill leachate.


Introduction 43
Sanitary landfill disposal is the most widely used method for municipal solid waste treatment in 44 the world due to its economic advantages (Wu et al. 2018). However, this disposal method leads 45 to the production of complex liquids, namely landfill leachate, containing large amounts of 46 organic pollutants, NH4 + -N, inorganic salts and heavy metals (Fu et al. 2021 (7), and also acts as a strong oxidizer to 83 destroy some organic pollutants (Cabeza et al. 2007). 84 2Cl   Cl2(aq) + 2e  (5) 85 Cl2(aq) + H2O  HClO + Cl  + H + (6) 86 2NH4 + + HClO  N2 + 2H2O + 6H + + 2Cl  (7) 87 Recently, Kishimoto et al. (2015) proposed a new electrochemical assisted Fenton-like process 88 to form  OH using Fe 2+ and HClO via reaction (8) for the decontamination of wastewater 89 containing Cl  , while Fe 2+ can be regenerated upon cathodic reduction of Fe 3+ through reaction 90 (9). This electro-Fenton-like process shows numerous advantages over the conventional electro-91 Fenton. Firstly, HClO is generated by the chlorine-based reaction (5) and (6) HClO + H2O2  Cl  + O2(g) + H2O + H + (10) 106 The objective of this study was to investigate, for the first time, the performance of 108 electrochemical assisted HClO/Fe 2+ process regarding the treatment of old landfill leachate. The 109 ability to generate HClO of the anode was firstly evaluated by conducting the electrolysis in NaCl 110 or mixed Na2SO4 + NaCl media at pH 3.0 with different chloride concentrations and current 111 densities in the absence of Fe 2+ . The formation of  OH and Fe IV O 2+ in the HClO/Fe 2+ system was 112 further confirmed by using dimethyl sulfoxide (DMSO) and methyl phenyl sulfoxide (PMSO) as 113 the radical probes, respectively (Shao et al. 2018). Then, the effect of initial pH, Fe 2+ 114 concentration and current density on COD and NH4 + -N decay was examined during the electrochemical assisted HClO/Fe 2+ treatment of landfill leachate. At last, a specific reaction 116 mechanism for COD and NH4 + -N removal was proposed. 117

118
Landfill leachate characteristics 119 The old landfill leachate was collected from a municipal sanitary landfill located in Wuhan, China. 120 The samples were stored in a refrigerator at 4 °C to maintain the characteristics unaltered, and 121 they were directly used in electrochemical systems without any pre-treatment (Ye et al. 2016). 122 The main characteristics of the leachate were summarized in Table 1. was 12 cm×10 cm×20 cm. The anodic material was titanium coated by iridium dioxide, 138 ruthenium dioxide and titanium dioxide (Ti/IrO2-RuO2-TiO2, 75 cm 2 ) and the cathodic material 139 was titanium. The gap between the anode and the cathode was adjusted parallel at a distance of 140 2 cm. All trials were conducted under constant current conditions provided by a direct current 141 (DC) power supply (LW-3030KD) under room temperature at 20 ± 3 C, and the solution was 142 vigorously stirred with a magnetic bar at 700 rpm. The assessment of the ability to generate HClO 143 at the anode was firstly carried out with 400 mL NaCl or mixed Na2SO4 + NaCl solutions at pH in Cl  concentration can accelerate the accessibility between Cl  and active sites on the anode 178 surface, and promote active chlorine production. 179 The effect of current density on active chlorine generation was further investigated at 4.0 g L 1 180 of Cl  ion. As shown in Fig. 1(b), a larger accumulation of active chlorine was achieved as the 181 current density became higher. Only 550 mg L 1 active chlorine was yielded at the lowest current 182 stage. This is consistent with the result achieved on Fe 2+ evolution in Fig. 2(b), Fe 2+ concentration 198 underwent a very quick decay once the reaction initiated, and reached 96% disappearance at 30 199 min. Therefore, it is believed that the ability of the cathode to reduce Fe 3+ by reaction (9)  However, most recently, some researchers suggested that Fe IV O 2+ could also make a significant 206 contribution to wastewater decontamination (Liang et al. 2020). Therefore, to fully identify the 207 potential reactive species involved in the HClO/Fe 2+ process, DMSO was used as a capturing 208 agent for  OH, and PMSO was selected to determine Fe IV O 2+ by measuring the conversion rate 209 of PMSO to PMSO2 in this work. 210 DMSO has been widely employed in the detection of  OH in advanced oxidation processes due 211 to its high reactivity with  OH (k = 4.5~7.1 × 10 9 M 1 s 1 ) forming methanesulfinic acid and 212 methyl radicals via reaction (12), the generated methyl radicals were further converted to HCHO 213 thorough reaction (13)   The old landfill leachate was firstly treated by EO-HClO in the absence of Fe 2+ at different 238 current density (7 and 14 mA cm 2 , respectively). As shown in Fig. S1, only 8.1% COD abatement was obtained after 2 h treatment at low current density 7 mA cm -2 , and further increase 240 in the current density to 14 mA cm 2 led to slight enhancement on COD removal (13.7%). The 241 old landfill leachate is usually characterized by complex refractory organic compounds, such as 242 humic and fulvic acids, which are highly resistant to the oxidation. The active anode, Ti/IrO2-243 RuO2-TiO2, used in this study presents low oxygen evolution potential and allows the generation  Similar results were achieved for NH4 + -N treatment in EO-HClO system, i.e., 7.3% and 9.1% 250 removal efficiency at current density of 7 and 14 mA cm 2 , respectively. As reported, the 251 contribution of  OH on NH4 + -N oxidation is assumed as negligible, active chlorine thus became 252 the dominant active species for the elimination of NH4 + -N, which was also competitively 253 consumed by high amounts of organics in landfill leachate (Mandal et al. 2020). Summarily, 254 single EO-HClO process failed to achieve powerful performance on landfill leachate treatment. 255

Effect of initial pH 256
To investigate the effect of initial pH on COD and NH4 + -N removal, experiments were carried 257 out with a current density of 14 mA cm 2 and Fe 2+ dosage of 4.0 mM at different initial pH (2.0, 258 3.0 and 9.0). The decay of COD and NH4 + -N as a function of time is displayed in Fig. 4. Similar and 3.0, whereas the value was dropped to 28% when the pH increased to 9.0. Worth noting, the 261 initial 30 min treatment already led to 45% COD abatement at pH 3.0, which was much higher 262 than that obtained in EO-HClO process (3%), and the following 7.5 h treatment only contributed 263 10.8% more COD removal. This highly agrees with the results achieved in Section 3.1, where it 264 has been demonstrated that the production of  OH and Fe IV O 2+ from reactions (8)  In addition, the aforementioned rate limited factor after the disappearance of added Fe 2+ is the 296 Fe 3+ reduction ability of the cathode, rather than the total concentration of iron species. The 297 generated excess Fe 3+ from reaction (8) and (11) tended to precipitate on the cathode surface due 298 to the formation of OH  by water splitting reaction, which retarded the regeneration of Fe 2+ via 299 reaction (9). 300 The profiles of NH4 + -N removal with different Fe 2+ dosage possessed similar trends during 8 h 301 treatment, as depicted in Fig. 5. This is because NH4 + -N elimination is mainly attributed to the indirect oxidation by active chlorine generated via chloride oxidation at the anode, rather than 303  OH and Fe IV O 2+ species, whose production highly relied on Fe 2+ concentration. 304 Effect of current density 305 HClO +  OH   ClO + H2O (18) 320 2 H2O  4H + + O2 + 4e  (19) 321 On the contrary, more rapid NH4 + -N oxidation was observed at higher current density due to the 323 enhanced generation of active chlorine, which proved again that NH4 + -N elimination dominantly 324 arose from the active chlorine oxidation, but barely affected by M(  OH),  OH and Fe IV O 2+ species. 325 Proposed reaction mechanism 326 Based on the results summarized in this work, the abatement mechanism of COD and NH4 + -N 327 during the electrochemical assisted HClO/Fe 2+ treatment of old landfill leachate was proposed in 328 showed superior ability to generate active chlorine, which was more rapid in the presence of a 340 greater Cl  concentration or at a higher current density. The production of both  OH and Fe IV O 2+ 341 species was verified in the HClO/Fe 2+ system by employing DMSO and PMSO as the probes, 342 respectively, despite the lower oxidizing potential of Fe IV O 2+ compared with  OH, it is advantageous due to the high selectivity and activity for the oxidation of pollutants. The addition 344 of Fe 2+ to construct electrochemical assisted HClO/Fe 2+ system led to more rapidly abatement of 345 COD, especially at the initial stage, than that in EO-HClO process. Acidic pH was found to favor 346 better COD and NH4 + -N removal due to the fact that hypochlorous acid was the dominant active     In uence of Fe2+ concentration on the time course of COD and NH4+-N removal during the electrochemical assisted HClO/Fe2+ treatment of 1 L land ll leachate at initial pH 3.0 and current density 14 mA cm-2 (black and solid: COD; red and hollow: NH4+-N).

Figure 6
Effect of current density on the removal of COD and NH4+-N during the electrochemical assisted HClO/Fe2+ treatment of 1 L land ll leachate with 4.0 mM Fe2+ at initial pH 3.0 (black and solid: COD; red and hollow: NH4+-N).

Figure 7
Proposed mechanism for COD and NH4+-N removal during electrochemical assisted HClO/Fe2+ treatment of land ll leachate.

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