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Article

Electrochemical Oxidation of Reverse Osmosis Concentrates from Landfill Leachate Treatment Using Ti4O7-Nanotube Reactive Electrochemical Membrane

by
Qiujie Qian
,
Pingzhong Xiao
,
Tinghui Du
,
Demin Xu
,
Chuanshuo Guo
,
Yue Guan
,
Minxuan Yan
,
Feihong Wang
,
Yonghao Zhang
* and
Yan Li
*
School of Environmental Science and Engineering, Yancheng Institute of Technology, Yancheng 224051, China
*
Authors to whom correspondence should be addressed.
Water 2024, 16(24), 3579; https://doi.org/10.3390/w16243579
Submission received: 13 November 2024 / Revised: 7 December 2024 / Accepted: 9 December 2024 / Published: 12 December 2024
(This article belongs to the Special Issue Advancement of Advanced Oxidation Processes (AOPs) for Water and Wastewater Treatment and Water Reuse)
Browse Figures
Versions Notes

Abstract

:
With the growing demand for high-quality discharge, reverse osmosis (RO) technology has been widely applied in landfill leachate treatment; however, the problem of reverse osmosis concentrates (ROCs) has worsened. In this work, a Ti4O7-nanotube reactive electrochemical membrane (Ti4O7-NT-REM) was employed to treat ROCs from landfill leachate treatment. The effects of current density, flow rate and pH on COD removal were evaluated, and the appropriate conditions were a current density of 20 mA cm−2, a flow rate of 10 mL s−1 and a pH of 7. Under these conditions, COD, TOC, NH4+-N and NO3−-N were removed by 82%, 68%, 100% and 73%, respectively. In addition, the 3D-EEM fluorescence spectra and GC–MS results revealed that the organics significantly decreased after 120 min of treatment, and aliphatic compounds were the major organic compounds. The stable performance of the REM was illustrated by cyclic treatment (20 cycles) with the assistance of cathodic polarization. In addition, a long service lifetime of 267.3 h and a low energy consumption of 7.6 kWh·kg COD−1 were obtained by related testing and evaluation. The excellent and stable performance confirmed that the Ti4O7-NT-REM has broad application prospects in the treatment of ROCs.

1. Introduction

Water shortages have long been an important factor restricting the development of human society, and China is designated a water-deficient country with low water resources per capita [1,2]. Wastewater recycling is a promising strategy and necessary approach to address this critical problem. For many recycling strategies, reverse osmosis (RO) technology emerged as a promising method in wastewater treatment, one of which is landfill leachate treatment [3,4,5]. However, the RO system achieves high-quality effluents but also produces reverse osmosis concentrates (ROCs) at percentages between 30% and 40% of the incoming wastewater [6]. This generated ROC usually involves increased salinity and high concentrations of inorganic ions and refractory organic pollutants [7,8]. Moreover, some chemicals (e.g., disinfectants, reducing agents and anti-scaling agents) that maintain RO system performance often unavoidably exist in ROCs [9]. With the continuous expansion of the scope and quantity of the RO process in landfill leachate treatment worldwide, the amount of ROC is also increasing. Direct discharge of this type of wastewater severely threatens the water environment. Therefore, there is an urgent need to find an effective method to address ROCs before discharge. Recently, various methods, including evaporation, coagulation, adsorption, biological treatment and advanced oxidation processes (AOPs), have been investigated to treat ROC, aiming to remove refractory organics and reduce biological toxicity [10].
To address the refractory and toxic organic pollutants of ROC, AOPs are considered effective because they can generate strong oxidants of OH· either by photocatalysis, electrocatalysis or reactions between chemicals, such as ultraviolet oxidation, electrochemical oxidation (EO), Fenton oxidation or ozone-based oxidation [11,12,13,14]. Among them, EO is more suitable for ROC treatment because it adapts to high-salinity circumstances and also can take advantage of salt to generate various new oxidants, such as reactive chlorine species (RCS) and sulfate radicals [5,15]. Currently, EO is extensively used in the treatment of ROC, which can not only remove refractory pollutants but also contribute to the removal of conventional indexes, such as COD, BOD5, ammonium nitrogen (NH4+-N) and total nitrogen (TN). However, the traditional EO process involving plate electrodes suffered from poor mass transfer, which results in high energy consumption. To solve this critical problem, reactive electrochemical membranes (REMs) have been developed and subsequently intensively studied in recent years [16,17,18]. The REMs have been tested in “flow-through” mode instead of “flow-by” mode, which significantly improves the mass transfer efficiency via the convection flow of the solution toward the electrode. Accordingly, REMs have shown great potential in the treatment of ROC.
In addition to the operation mode, the catalytic coating of REMs also plays an important role in EO, and different materials exhibit large differences when removing pollutants. Related studies have reported various catalytic coatings in ROC treatment, including IrO2, RuO2-IrO2, Pd, SnO2-Sb, PbO2 and BDD, and some of them referred to REMs [12,19,20,21,22,23,24]. Among these coatings, nonactive electrodes (e.g., SnO2, PbO2, BDD and Ti4O7) have a stronger ability to destroy organic pollutants because they weakly interact with the generated OH· [25]. Nevertheless, the SnO2 electrode suffers from low stability and service life, the PbO2 electrode faces the challenge of leaching toxic Pb2+ and the BDD electrode has a high material and fabrication cost [26,27]. In contrast, Ti4O7 has shown great potential in terms of applications because of its great electroconductibility, chemical resistance and relatively low price [28,29]. These properties indicate that Ti4O7 is a suitable material for REMs in ROC treatment. In a previous study, we fabricated a Ti4O7-nanotube REM (Ti4O7-NT-REM), which showed excellent performance in the treatment of ceftriaxone sodium-containing antibiotic wastewater [30]. After treatment, the biodegradability and toxicity of the wastewater significantly increased and decreased, respectively. However, ROCs are more complex in terms of organics and high salinity than the above-mentioned antibiotic wastewater. The performance, durability and cost of Ti4O7-NT-REM for the treatment of actual ROC are still unclear. Thus, it is necessary to study the treatment of actual ROCs by Ti4O7-NT-REM before large-scale application.
In this study, Ti4O7-NT-REM was employed to treat actual ROCs from landfill leachate treatment. The effects of the operation parameters (current density, flow rate and pH) on COD removal were investigated. The performance of REM on the removal of total organic carbon (TOC), NH4+-N and NO3−-N was also investigated. Furthermore, variations in dissolved organic matter (DOM) were studied in detail via 3D-EEM fluorescence spectroscopy and GC–MS. Finally, the long-term stability and operation cost were also investigated.

2. Materials and Methods

2.1. Reactant and Material

The chemicals were analytical grade or higher. Sodium chloride, sulfuric acid, sodium hydroxide, hydrofluoric acid and potassium fluoride dihydrate were purchased from Sigma-Aldrich (Shanghai, China). Dichloromethane for GC–MS was chromatographic grade and obtained from Aladdin Industrial Co., Ltd. (Shanghai, China). The porous Ti substrate was bought from Baoji Yinggao Metal Materials Co., Ltd. (Baoji, China). Ultrapure water was obtained from a Milli-Q® EQ 7000 system (Merck Chemical Technology Co., Ltd., Shanghai, China). The reverse osmosis concentrate sample was obtained by a landfill leachate treatment plant located in Yancheng, China. The RO system was placed after two-stage AO and MBR. The detailed characteristics are shown in Table S1.

2.2. Fabrication of Ti4O7-NT-REM

The Ti4O7-NT-REM employed in this study was fabricated through our previous study [31]. Briefly, a porous titanium substrate (diameter of 30 mm, thickness of 1 mm, pore size of ~20 μm) was first anodically oxidized in a solution consisting of hydrofluoric acid and potassium fluoride dihydrate to generate TiO2-NT and then reduced TiO2-NT into Ti4O7-NT by hydrogen at a high temperature to obtain the Ti4O7-NT-REM.

2.3. Electrolysis Process

The ROC was electrochemically treated in a reactor assembled with Ti4O7-NT-REM, nickel foam and an O-ring (Figure 1). During the process, the reactor was immersed in a container containing 500 mL of ROC. The current was inputted and controlled by a DC power supply (Hanshengpuyuan, Beijing, China). Herein, Ti4O7-NT-REM was used as the anode, and nickel foam was used as the cathode. The electrodes were isolated by an O-ring with a distance of 1 cm. During the treatment, the ROC was continually circulated between the reactor and beaker via a peristaltic pump (Longer, Baoding, China), which also can control flow rate. The pH value was adjusted with 1 M H2SO4 and NaOH. For cyclic experiments, cathodic polarization was undertaken before each cycle at a current density of 2.5 mA cm−2 for 10 min to ensure that the performance achieved was in line with previous methods [32]. Cathodic polarization was achieved by simply reversing the anode and cathode.

2.4. Analytical Methods

The concentrations of COD, NH4+-N, NO3− and NO2− were measured via an LH-5B-3B multiparameter water quality analyzer (Lianhua, Beijing, China) with the corresponding reagents purchased from Beijing Lianhua Co., Ltd. TOC was measured via a TOC analyzer (TOC-LCP H, Shimadzu, Japan). The pH value was measured with a pH meter (PHS-3B, Shanghai Precision & Scientific Instrument Co. Ltd., Shanghai, China). The excitation–emission matrix fluorescence spectroscopy analysis was performed with a spectrophotometer (Horiba, Japan), and the excitation and emission slits were set to 3 nm. The organic composition of ROC was detected via gas chromatographic mass spectrometry (TRACE ISQ, Thermo, Waltham, MA, USA) after extraction with CH2Cl2 via a previously reported method (Text S1). The acute toxicity tests were conducted with five D. magna neonates (less than 24 h old) in 50 mL test solution at five test concentration gradients with four replicates [33]. Accelerated service life tests were performed at current densities up to 1 A cm−2 in ROC and 0.5 M NaCl, respectively. The calculations of removal and energy consumption are presented in Text S2.

3. Results

3.1. ROC Treatment by Ti4O7-NT-REM

3.1.1. Contaminant Removal

The removal of COD, TOC, NH4+-N and NO3−-N by Ti4O7-NT-REM was evaluated, as shown in Figure 2. Figure 2a–c present the effects of the current density, flow rate and pH on COD removal. The current density is a crucial factor in EO because the degree of molecular transformation and the reaction rate are highly related to the current density [34]. It can be found in Figure 2a that the COD removal increased with increasing current density. The COD concentration was reduced to 82 mg L−1 after treatment when the current density was 20 mA cm−2, whereas it was 307, 234 and 102 mg L−1 for 5, 10 and 15 mA cm−2, respectively. The higher current density contributes to a high yield of reactive chlorine species (RCS) and ·OH, leading to this high performance. Nevertheless, the improvement in COD removal was not significant (20%) when the current density increased from 15 to 20 mA cm−2. This is attributed to the serious side reactions of O2 evolution induced by the high current density. Moreover, a high current density also caused the conversion of some electric energy into thermal energy, which resulted in invalid consumption of the given charge [35]. Figure 2b shows the COD removal under different flow rates. Like the current density, the flow rate also has a positive influence on COD removal. This is because of the increased mass transfer from the increasing flow rate. As expected, the COD removal increased when the flow rate increased [36]. The COD concentrations after 120 min of treatment for 4, 6, 8 and 10 mL s−1 were 173, 125, 95 and 82 mg L−1, respectively. Furthermore, the improvement in COD removal was almost negligible when the flow rate increased from 8 to mL s−1, indicating that improving the flow rate was inefficient up to 8 mL s−1. To evaluate the effect of pH, the ROC was adjusted to pH 3, 5 and 9 before starting the treatment. Figure 2c shows that acidic conditions are favorable for removing COD. The COD concentration decreased to 42 and 44 mg L−1 under acidic conditions (pH 3 and 5), which was lower than that under neutral and alkaline conditions (82 mg L−1 for pH 7 and 184 mg L−1 for pH 9). This can be explained by the fact that the side oxygen evolution reaction can be alleviated because of the higher oxygen evolution potential in acidic solutions [37]. Moreover, bicarbonate ions (HCO3−), which can quench radicals, are eliminated when the pH value was 3 [38]. As presented above, although the conditions of 20 mA cm−2 and 10 mL s−1 can remove more COD in each comparison, the improvement was not significant when compared with the nearby conditions. Considering that a higher current density and higher flow rate cause considerable electric energy wasting in terms of electrolytic heating and driving forces, the appropriate current density and flow rate for ROC treatment should be 15 mA cm−2 and 8 mL s−1, respectively. On the other hand, the COD was efficiently removed at pH 7, and adjusting the pH requires many chemicals (H2SO4), which is not economical or environmentally friendly. Thus, no pH adjustment was conducted in later experiments.
In addition, the removal performance of Ti4O7-NT-REM for TOC, NH4+-N and NO3−N was further evaluated. As shown in Figure 2d, 68% of the total organic carbon (TOC) was removed after treatment, indicating that the REM is more suitable for degrading persistent organic matter. Furthermore, NH4+-N removal reached approximately 100% after treatment. This is attributed to the generated RCS, which has a high reaction rate with NH4+-N. The high removal of NO3−-N (73%) might be due to cathode reduction by nickel foam, which can generate massive amounts of atomic H* when serving as a cathode [39].

3.1.2. Variations in Dissolved Organic Matter

To evaluate the variation in dissolved organic matter (DOM), Figure 3 presents the 3D-EEM fluorescence spectra obtained during the treatment. According to previous studies, the spectra can be categorized into four regions: aromatic protein substances (I), fulvic acid-like substances (II), soluble microbial byproduct-like substances (III) and HA-like substances (IV) [40]. Apparently, soluble microbial byproduct-like substances and HA-like substances were the predominant components of DOM in ROC, as the majority of the fluorescence intensity is located in regions (III) and (IV). As we can see in Figure 3b, the fluorescence intensity area significantly decreased, and fulvic acid-like and HA-like substances almost disappeared, whereas only a few soluble microbial byproduct-like substances remained. This result illustrated that the strong degradability of Ti4O7-NT-REM decomposed HA-like substances into other soluble microbial byproducts after 60 min treatment [41]. Moreover, the fluorescence intensity area was almost completely eliminated from the spectra after 120 min of treatment, as shown in Figure 3c. Only some areas of scattered fluorescence intensity could be found in region (I), indicating that the organics either were mineralized into CO2 and H2O or transformed into smaller and non-fluorescent organics.
Considering that some DOM (e.g., aliphatic compounds) cannot be reflected directly via 3D-EEM fluorescence spectra, GC–MS was further employed to study the changes in organics. The types and proportions of DOM as well as its sequential degradation are presented in Tables S2 and S3 and Figure S1. As we can see, 43 organics were detected in the influent, while 55 organics were detected in the effluent, which can be classified as aliphatic compounds, aromatic compounds, alcohol compounds, esters, amides, silanes, phenols, ketones, etc. Interestingly, silanes, ketones and alcohols were removed from the ROC after 120 min of treatment, whereas chlorinated compounds were newly detected (Figure 4a). This phenomenon can be attributed to the anticipation of RCSs during treatment, which were generated by the chlorine evolution reaction at the surface of Ti4O7-NT-REM (Figure S2) [42]. Figure 4b shows that the range of molecular weight (M.W.) of the organics in the influent was 170~592 g mol−1, and most of the organics had M.W. values of 206, 212, 256, 276, 282, 296 and 478 g mol−1. After 120 min of treatment, the molecular weights of the organics in the effluent were in the range of 124~500 g mol−1, and most organics had M.W. values of 124, 140, 152, 216, 256 and 284 g mol−1. In particular, the organics with M.W. values of 124, 134, 140 and 152 corresponded to the newly generated chlorinated compounds (red circles). In addition, the change in the number of carbon atoms was also investigated (Figure 4c). Figure 4c shows the number of carbon atoms distributed at C10~C39 for the influent and C4~C40 for the effluent. Two significant changes can be observed after treatment: (i) the number of organics with C14~C23 carbon atoms significantly decreased, and (ii) several organics with less carbon atoms (C4~C6) were detected. Combined with the results of the evaluation of M.W. and carbon atoms, these findings indicate that the macromolecular organics in ROCs were efficiently degraded into smaller molecules through electrochemical treatment. In addition, acute toxicity tests further support the results of the GC–MS study as the IC50 values increased from 31.7% to 67.3% (Figure 4d) after treatment, indicating a reduced threat of the treated ROC to the environment.

3.1.3. Long-Term Durability

The long-term durability of Ti4O7-NT-REM in the ROC treatment was evaluated via cyclic experiments (20 cycles). As shown in Figure 5a, the COD removal rate was maintained within 70~81% over 20 cycles, and the removal rate of the last cycle was 92.7% of that of the first cycle, suggesting the great stability of degrading organics. Figure 5b shows that the REM is highly stable for TOC removal. The fluctuation range of TOC removal was within 13%, and at least 64% of the total organic carbon (TOC) was removed. The removal percentage of the last circle was 90.8% of that of the first circle. For NH4+-N (Figure 5c), the REM not only shows stable removal but also exhibits great efficiency. The NH4+-N was barely detected in the effluent of any of the circles. Furthermore, the REM still steadily removed NO3−-N, as shown in Figure 5d. The largest difference in removal was 7%, and the removal decreased by only 4% between the last cycle and the first cycle. In conclusion, 70~81%, 64~78%, 99.1~100% and 68~75% of the COD, TOC, NH4+-N and NO3−-N, respectively, were removed during the cyclic experiments, confirming the high long-term durability of the Ti4O7-NT-REM. Moreover, the mean concentrations of COD, TOC, NH4+-N and NO3−-N in the effluent were 93, 29, 0.10 and 8.1 mg L−1, respectively. The treated ROC met the standard for pollution control at the landfill site of municipal solid waste in China (GB16889-2008) (Table S1) [43], indicating that the ROC can now be discharged. In addition, Figure S3 displays the SEM images of Ti4O7-NT-REM before and after 20 cycles, where the REM still possesses a compact nano-structure after long-term study and even some cracking and impurities could be observed. These results strongly support the durability of Ti4O7-NT-REM.

3.1.4. Service Lifetime and Energy Consumption Evaluation

The service lifetime of REMs is a core factor for actual application. In our previous study, we investigated the service lifetime of Ti4O7-NT-REM in a Na2SO4 solution and reported that the REM’s service lifetime was 288.2 h [31]. To gain further insight into the service lifetime of Ti4O7-NT-REM in an actual ROC treatment, accelerated service life tests were performed with actual ROCs (Figure 6). As shown in Figure 6, the service lifetime of Ti4O7-NT-REM in the actual ROC was 267.3 h, which was slightly shorter than that in the Na2SO4 solution. This was not surprising because the ROC was much more complex than the Na2SO4 solution, and many ions in the ROC had a strong impact on the service lifetime, such as scaling by Ca2+ and Mg2+ or deactivation by the generated RCS. Moreover, considering that chloride ions were the most common anion in ROCs (Table S1), the REM’s service lifetime was tested in a 0.5 M NaCl solution. The results show that the REM still has a relatively long service lifetime of 241.2 h, demonstrating strong resistance of Ti4O7-NT-REM to the chloride-induced corrosion.
Besides the service lifetime, operating cost is also a crucial parameter of REMs in actual application [44]. Energy consumption (EC) is a common indicator used to evaluate operating costs. As shown in Table 1, the EC of Ti4O7-NT-REM was only 7.6 kWh·kg COD−1, which was the lowest among all the listed electrodes or REMs. This is attributed mainly to the advantages of the flow-through mode together with the superior electrochemical characteristics of Ti4O7. In conclusion, the relatively long service life, excellent electrochemical stability (if continuous reverse polarity treatments are employed) as well as the low toxicity of titanium oxide indicated that Ti4O7-NT-REM technology is environmentally sustainable. Furthermore, the low EC together with the low fabrication cost of ~USD 0.36 per m2 (including this study) indicated that this technology also is economically sustainable [29]. Thus, Ti4O7-NT-REM has exhibited promising potential for application in the treatment of ROC.

4. Conclusions

In this work, Ti4O7-NT-REM was applied to treat actual ROCs. The results indicated that the REM performed well in terms of COD, TOC, NH4+-N and NO3−-N removal, and the appropriate conditions were a current density of 15 mA cm−2, a flow rate of 8 mL s−1 and a pH value of 7. The significant changes in DOM before and after treatment suggested that the organics in ROC were efficiently removed by the Ti4O7-NT-REM. The REM also exhibited stable performance in that 70~81% of the COD, 64~78% of the TOC, 99.1~100% of the NH4+-N and 68~75% of the NO3−-N were removed during the cyclic experiments (20 cycles). An accelerated service life test indicated that Ti4O7-NT-REM possesses a relatively long service lifetime during actual ROC treatment. Energy consumption was as low as 7.6 kWh⋅kg COD−1 after calculation, which was competitive with many other electrochemical technologies and acceptable for engineering applications. However, actual ROC treatment engineering is not yet mature enough at this time; the pilot or above scale must be investigated, and reactor or device design is also needed before implementation.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/w16243579/s1: Text S1: GC–MS analyses; Text S2: Calculation equations for removal and energy consumption; Table S1: Water quality of the collected actual reverse osmosis concentrates; Table S2: Organic composition of influent detected by GC–MS; Table S3: Organic composition of effluent detected by GC–MS. Figure S1: GC–MS analysis and proportions of DOM (a) before and (b) after treatment; Figure S2: Mechanism diagram of the generation of reactive species (·OH and RCS) and atomic H* as well as their attack on the organics; Figure S3. SEM images of Ti4O7-NT-REM (a) before and (b) after 20 cycles.

Author Contributions

Conceptualization, Q.Q. and Y.Z.; methodology, P.X., Y.G. and M.Y.; validation, F.W. and Y.L.; data curation, T.D., D.X. and C.G.; writing—original draft preparation, Q.Q.; writing—review and editing, Q.Q. and Y.Z.; supervision, F.W. and Y.L.; project administration, F.W. and Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research is a project funded by the Natural Science Foundation of Jiangsu Province (No. BK20220697), the Natural Science Foundation of China (No. 52400059) and the Funding for School-Level Research Projects of Yancheng Institute of Technology (No. xjr2022029).

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Water 16 03579 g001
Figure 1. Schematic diagram of Ti4O7-NT-REM reactor for the experiments.
Figure 1. Schematic diagram of Ti4O7-NT-REM reactor for the experiments.
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Figure 2. (a) The effect of current density on COD removal (conditions: flow rate 10 mL s−1, pH 7); (b) the effect of flow rate (conditions: current density 20 mA cm−2, pH 7) on COD removal; (c) the effect of pH (conditions: current density mA cm−2, flow rate 10 mL s−1) on COD removal; (d) TOC NH4+-N and NO3−-removal for treatment process (conditions: flow rate 8 mL s−1, pH 7 and current density at 15 mA cm−2).
Figure 2. (a) The effect of current density on COD removal (conditions: flow rate 10 mL s−1, pH 7); (b) the effect of flow rate (conditions: current density 20 mA cm−2, pH 7) on COD removal; (c) the effect of pH (conditions: current density mA cm−2, flow rate 10 mL s−1) on COD removal; (d) TOC NH4+-N and NO3−-removal for treatment process (conditions: flow rate 8 mL s−1, pH 7 and current density at 15 mA cm−2).
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Figure 3. Excitation–emission matrix (EEM) fluorescence spectra of ROC (conditions: flow rate 8 mL s−1, pH 7 and current density at 15 mA cm−2): (a) untreated, (b) after 60 min treatment and (c) after 120 min treatment.
Figure 3. Excitation–emission matrix (EEM) fluorescence spectra of ROC (conditions: flow rate 8 mL s−1, pH 7 and current density at 15 mA cm−2): (a) untreated, (b) after 60 min treatment and (c) after 120 min treatment.
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Figure 4. (a) Amounts of different organics in influent and effluent; (b) distribution of organics with different molecular weight (M.W.) values in influent and effluent; (c) carbon atom of organics in influent and effluent and (d) IC50 value of influent and effluent (conditions: flow rate 8 mL s−1, pH 7 and current density at 15 mA cm−2).
Figure 4. (a) Amounts of different organics in influent and effluent; (b) distribution of organics with different molecular weight (M.W.) values in influent and effluent; (c) carbon atom of organics in influent and effluent and (d) IC50 value of influent and effluent (conditions: flow rate 8 mL s−1, pH 7 and current density at 15 mA cm−2).
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Figure 5. (a) The COD concentration of the influent and effluent over 20 cycles; (b) the TOC concentration of the influent and effluent over 20 cycles; (c) the NH4+-N concentration of the influent and effluent over 20 cycles; (d) the NO3−-N concentration of the influent and effluent over 20 cycles (conditions: flow rate 8 mL s−1, pH 7 and current density at 15 mA cm−2).
Figure 5. (a) The COD concentration of the influent and effluent over 20 cycles; (b) the TOC concentration of the influent and effluent over 20 cycles; (c) the NH4+-N concentration of the influent and effluent over 20 cycles; (d) the NO3−-N concentration of the influent and effluent over 20 cycles (conditions: flow rate 8 mL s−1, pH 7 and current density at 15 mA cm−2).
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Figure 6. Accelerated service life test of Ti4O7-NT-REM in two solutions.
Figure 6. Accelerated service life test of Ti4O7-NT-REM in two solutions.
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Table 1. A comparison of Ti4O7-NT-REM with other anodes in ROC treatment.
Table 1. A comparison of Ti4O7-NT-REM with other anodes in ROC treatment.
AnodeOperating
Mode		Current
Density		COD
Removal		TOC
Removal		Energy
Consumption		Ref.
BDDFlow-by40 mA cm−291.9%76.63%1500 kWh·kg COD−1[23]
Ti/TiO2-NTs/PbO2Flow-by40 mA cm−247.59%/190 kWh·kg COD−1[45]
BDDBatch25 mA/cm−2100%/158 kWh·kg COD−1[46]
BDDBatch5 mA/cm−2100%/59 kWh·kg COD−1[47]
Ti/IrO2–Ta2O5Batch25 mA/cm−2<50%/50 kWh·kg COD−1[46]
Si/BDDFlow-by12.5 mA cm−2100%~70%34 kWh·kg COD−1[48]
Ti4O7-NT-REMFlow-through20 mA cm−274%68%7.6 kWh·kg COD−1This study
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Qian, Q.; Xiao, P.; Du, T.; Xu, D.; Guo, C.; Guan, Y.; Yan, M.; Wang, F.; Zhang, Y.; Li, Y. Electrochemical Oxidation of Reverse Osmosis Concentrates from Landfill Leachate Treatment Using Ti4O7-Nanotube Reactive Electrochemical Membrane. Water 2024, 16, 3579. https://doi.org/10.3390/w16243579

AMA Style

Qian Q, Xiao P, Du T, Xu D, Guo C, Guan Y, Yan M, Wang F, Zhang Y, Li Y. Electrochemical Oxidation of Reverse Osmosis Concentrates from Landfill Leachate Treatment Using Ti4O7-Nanotube Reactive Electrochemical Membrane. Water. 2024; 16(24):3579. https://doi.org/10.3390/w16243579

Chicago/Turabian Style

Qian, Qiujie, Pingzhong Xiao, Tinghui Du, Demin Xu, Chuanshuo Guo, Yue Guan, Minxuan Yan, Feihong Wang, Yonghao Zhang, and Yan Li. 2024. "Electrochemical Oxidation of Reverse Osmosis Concentrates from Landfill Leachate Treatment Using Ti4O7-Nanotube Reactive Electrochemical Membrane" Water 16, no. 24: 3579. https://doi.org/10.3390/w16243579

APA Style

Qian, Q., Xiao, P., Du, T., Xu, D., Guo, C., Guan, Y., Yan, M., Wang, F., Zhang, Y., & Li, Y. (2024). Electrochemical Oxidation of Reverse Osmosis Concentrates from Landfill Leachate Treatment Using Ti4O7-Nanotube Reactive Electrochemical Membrane. Water, 16(24), 3579. https://doi.org/10.3390/w16243579

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