Elsevier

Journal of Hazardous Materials

Volume 377, 5 September 2019, Pages 259-266
Journal of Hazardous Materials

2,4-Dichlorophenol removal from water using an electrochemical method improved by a composite molecularly imprinted membrane/bipolar membrane

https://doi.org/10.1016/j.jhazmat.2019.05.064Get rights and content

Highlights

  • A molecularly imprinted/bipolar membrane enhanced 24DCP removal by a Pd/Ti electrode.

  • 24DCP hydrodechlorination increased with current density and electrolyte concentration.

  • Complete 24DCP removal was achieved by Fenton treatment after hydrodechlorination.

  • The sequential treatment decreased COD and TOC more than Fenton's reagent alone.

  • 24DCP was removed and degraded via adsorption, reduction by Hads, and Fenton oxidation.

Abstract

Low efficiency is often a problem in electrochemical reductive hydrodechlorination (ERHD) to remove chlorinated compounds such as 2,4-dichlorophenol (24DCP) from water. In this study, a composite molecularly imprinted membrane (MIM)/bipolar membrane (BPM) was introduced onto a palladium-coated titanium mesh electrode (BPM/MIM@Pd/Ti) to increase the concentration of 24DCP on the surface of electrode and ERHD efficiency. The efficiency of ERHD of 24DCP increased from 70 to 88% by introduction of the two membranes, from 71 to 89% by increasing current density from 5.0 to 30 mA/cm2, and from 80 to 94% by increasing the electrolyte concentration from 0.25 to 1.00 mol/L. Treatment with Fenton’s reagent after ERHD achieved 100% 24DCP removal, with chemical oxygen demand and total organic carbon reductions of 91 and 87%, respectively. Notably, these reductions were greater than obtained from the direct oxidation of the 24DCP solution by Fenton’s reagent alone (i.e., 98, 84, and 72%, respectively). No products were detected in solution by GC–MS after treatment with the proposed combination technology. The mechanism of 24DCP removal and degradation involved adsorption, electrochemical hydrodechlorination via Hads, and Fenton oxidation. Results show the process has high potential for removing 24DCP from aqueous solution.

Introduction

Chlorophenols (CPs) are a group of chlorinated organic compounds often employed in the production of wood preservatives, biocides, and pesticides. However, chlorophenols such as 2,4-dichlorophenol (24DCP) pose a serious threat to human and animal health due to their high toxicities, as well as their recalcitrant, mutagenic, and possibly carcinogenic properties [1]. Indeed, 24DCP is listed as a priority pollutant by the United States Environmental Protection Agency [2]. Moreover, the World Health Organization (WHO) has established a concentration limit for CPs in drinking water of less than 1.0 mg/L [3]. Therefore, the development of suitable methods for the removal of 24DCP from water bodies is essential.

To date, a number of methods, including adsorption [4], microbiological degradation [5], zero valent iron reduction [6], advanced oxidation processes [[6], [7], [8], [9]], and dechlorination [10], have been employed for 24DCP removal. However, these processes have a number of disadvantages. Adsorption simply transfers 24DCP from one phase to another, while biological methods often yield unsatisfactory results due to its the toxicity and resistance to biodegradation. Advanced oxidation technologies may generate more harmful and complex degradation products, and reduction treatments, as with zerovalent iron, can also generate toxic/harmful products, which are often difficult to remove in subsequent treatment processes. As such, the development of a more effective method for the degradation of 24DCP is of particular importance.

It is well known that the C-Cl bond in CPs plays an important role in determining their toxicity and persistence in the environment [11]. Selective removal of chlorine atoms can decrease the toxicity of CPs, thereby rendering their subsequent treatment both convenient and economical. In this context, electrochemical reductive hydrodechlorination (ERHD) is an efficient method due to its rapid reaction rate, low cost, and lack of secondary pollutant formation [12,13]. In this process, it is essential that the chemisorbed hydrogen atoms (Hads) generated on the electrode surface through the electrolysis of H2O or H3O+ exchange with chlorine atoms in the CP structure [14,15]. This detailed reaction process can be represented as shown in Eqs. (1), (2), (3), (4), (5), (6) (M represents the electrode surface):M + H2O(H3O+) + e → (H)adsM + OH (H2O)M + R-Cl ⇋ (R-Cl)adsM2(H)adsM + (R-Cl)adsM → (R-H)adsM + HCl(R-H)adsM ⇋ R-H + MHadsM + HadsM ⇋ H2 + MHadsM + H2O + e → H2 + M + OH

More specifically, in the initial stage of this process, Hads is generated on the cathode surface through the reduction of H2O or H3O+ (Eq. (1)). This is followed by the reaction of Hads with the adsorbed R-Cl on the cathode surface to produce (R-H)ads (Eq. (3), R-H adsorption shown as Eq. (2)). The reduction product is then desorbed into the solution (Eq. (4)). However, in the case where lower quantities of (R-H)ads and greater quantities of Hads are present on the cathode surface, H2 can be produced via the thermal and electrochemical desorption of Hads (Eqs. (5) and (6)). Since competition for Hads exists in this system, an increase in the efficiency of reaction 3 in addition to a reduction in the efficiencies of reactions 5 and 6 are essential to optimise the R-Cl dechlorination efficiency.

As indicated above, both the production of Hads and the ERHD reaction itself take place at the electrode surface [13], thereby indicating that the hydrogen storage capacity and the quantities of (R-H)ads present on the cathode surface have a direct influence on the R-Cl ERHD efficiency. To increase the reaction efficiency of Eq. (1), noble metals such as silver, platinum, rhodium, and palladium have been employed as working electrodes or modified electrode materials [[16], [17], [18], [19], [20]]. Among these, Pd is a popular choice due to its high rates of hydrogen production, atom efficiency, and ability to insert hydrogen atoms into its lattice [21,22]. However, due to the high cost, a reduction in the required quantity of Pd is necessary. This could be achieved through the electrodeposition of Pd on a low cost substrate to form highly dispersed and large specific surface area Pd particles, which exhibit comparable efficiencies in the ERHD of CPs [12]. In addition, titanium is often employed as a cathode substrate due to its high penetrability and excellent mass transfer during the ERHD process. Thus, to enhance the ERHD efficiency, Pd/Ti electrodes are commonly employed in the electrochemical reduction of CPs [12,23,24].

As indicated in Eq. (3), the quantity of R-Cl adsorbed to the electrode is also an important factor in the ERHD process. However, to our knowledge, no direct method for increasing the concentration of R-Cl on the cathode surface has been reported to date. The use of molecularly imprinted membranes (MIMs) has received significant attention in the field of water purification due to their selectivity towards specific organic pollutants [25]. MIMs are generally prepared via the copolymerisation of functional monomers and cross-linking agents to form complexes with the template molecules [26]. Subsequent removal of the template molecules provides MIMs exhibiting recognition properties for these template molecules. Thus, the combination of a MIM exhibiting selection adsorption towards 24DCP with a Pd/Ti electrode (MIM@Pd/Ti) is expected to enhance the R-Cl removal upon increasing the concentration of 24DCP on the electrode surface. In addition, bipolar membranes (BPMs) are composed of a cation/anion-exchange layer, which permits the dissociation of water into H+ and OH ions in the presence of an electric field [27,28]. Therefore, the concentration of H+ on the cathode surface can also be increased through the combination of a BPM with a MIM@Pd/Ti electrode (BPM/MIM@Pd/Ti). According to Eq. (1), greater quantities of Hads would be produced, thereby further increasing the CP removal efficiency.

After the ERHD of 24DCP, the total organic carbon (TOC) content of the solution was previously found to be comparable to that of the original mixture [13], consistent with reaction such as 2-chlorophenol (2CP), 4-chlorophenol (4CP), phenol, and other dechlorinated organic compounds. Subsequent treatment of the obtained mixture is therefore required to further reduce TOC. For this purpose, the Fenton oxidation could be employed, as this procedure is commonly used to degrade toxic and recalcitrant organic compounds due to its high efficiency, environmentally friendly nature, and the ease of handling of the required reagents [29]. Therefore, we expect that this combined ERHD/Fenton oxidation system will result in essentially complete removal of 24DCP from aqueous solutions.

Herein we report preparation and application of a BPM/MIM@Pd/Ti electrode to enhance the ERHD efficiency towards 24DCP by increasing the concentrations of 24DCP and Hads on the electrode surface. Moreover, we examined the effects of current densities, solution pH values, and original 24DCP concentrations on 24DCP removal efficiency. Complete removal of 24DCP from the aqueous solution was then attempted by following ERHD treatment with the Fenton oxidation reaction. Degradation products were identified by gas chromatography–mass spectrometry (GC-MC) and the degradation mechanism proposed.

Section snippets

Pd/Ti electrode

Titanium mesh was cut into 2 × 2 cm squares and rinsed with dilute sulfuric acid (0.5 M) and acetone to clean the surface. A PdCl2 (250 mL, 4.0 g/L) was prepared by dissolving PdCl2 in dilute HCl [1]. The titanium mesh (cathode) and a carbon rod (anode) were immersed in the solution and a direct current of 0.17 A was applied. The solution was stirred at 200 rpm during the electrolytic deposition. After stirring for 10 min, the titanium mesh was removed from the solution, washed with ultrapure

Morphology of the MIM

Surfaces of the MIM and NIM (no 24DCP imprinted CMC-PVA cation exchange membrane) were examined using SEM (Fig. 3a and b). As expected, a particularly rough surface was observed for the MIM compared to that of the NIM due to the formation of holes and channels in the MIM structure following imprinting of 24DCP.

Adsorption of 24DCP on MIM

The isothermal adsorption and kinetic curves of 24DCP by the MIM are shown as Figs. S1 and S2. Lagergren first-order (Eq. (7)) and pseudo-second-order (Eq. (8)) kinetic models were

Analysis of the reaction products

Removal efficiencies of 24DCP with ERHD increased from 47 to 93% by 240 min (Fig. 9a). Treatment products, including phenol and 4CP, were found and the concentration of phenol increased with time during the ERHD treatment.

Theoretically, the 24DCP could be reduced to 2CP and 4CP, which would then be reduced to phenol, or directly reduced to phenol. However, 4CP and phenol were detected, but 2CP was not (Fig. 9a). This is attributed to stronger electron induction at C-2 than at C-4 by the

Mechanism

The postulated mechanism of 24DCP degradation is that the H+ generated by BPM or in the 24DCP solution were reduced to yield Hads, which reacted with 24DCP to give 2CP, 4CP, phenol, and other degradation products under the catalytic action of Pd (Fig. 2). The function of the MIM was to increase the amount of 24DCP adsorbed on the Pd/Ti electrode, which led to an enhanced ERHD efficiency, according to Eq. (3). In contrast, the function of the BPM was to increase the H+ concentration on the

Conclusions

A BPM/MIM@Pd/Ti electrode was used to remove 24DCP from water. Introduction of MIM and BPM onto the electrode increased the 24DCP ERHD efficiency. Removal of 24DCP to a non-detectable concentration was obtained after the Fenton’s reagent oxidation following the ERHD process. Moreover, no degradation products were detected by GC–MS after the two processes were completed. Therefore, the approach presented herein has a potential for application in the removal of halogen-containing organic

Acknowledgments

This work was supported by the National Nature Science Foundation of China (Grant No. 21507069), the Natural Science Funds (Grant No. 2017J01573, 2017J01407 and 2018J01672) of Fujian Province, and a Major Projects grant (Grant No. 2015YZ0001-1) from the Fujian Province Department of Science and Technology.

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