Application of a novel heterogeneous sulfite activation with copper(i) sulfide (Cu2S) for efficient iohexol abatement

Transition metal ion-activated sulfite autoxidation processes for the production of sulfate radicals (SO4˙−) have been widely investigated to achieve efficient abatement of recalcitrant organic pollutants. However, these homogeneous processes suffered from narrow effective pH range and metal release, thus restricting their practical application. In order to address this problem, we report a simple and efficient approach to iohexol abatement by a combined Cu2S and sulfite process (simplified as Cu2S/sulfite process) based on the superior activation performance of copper and the excellent electron donating capacity of the low-valent sulfur species. Compared with typical copper oxides, Cu2S can significantly accelerate the sulfite autoxidation to generate radicals, leading to 100% iohexol abatement in the Cu2S/sulfite process. The influence of solution pH and dissolved oxygen on iohexol abatement is also investigated. Qualitative and quantitative analysis of reactive radicals is performed by electron paramagnetic resonance (EPR) and radical quenching experiments. Generation of SO4˙− from sulfite activation with Cu2S mainly contributes to the iohexol abatement. X-ray photoelectron spectroscopy (XPS) suggests that copper is the main activation site and the reductive sulfur species can achieve the continuous regeneration of copper. Application potential of the Cu2S/sulfite process is also assessed. This study provides a new method for the treatment of water and wastewater containing organic micropollutants.


Introduction
Advanced oxidation processes (AOPs) based on sulfate radicals (SO 4 c À ) have attracted increasing research interest owing to SO 4 c À possesses having high standard redox potential (2.5-3.1 V), excellent selectivity and favorably long life (s ¼ 30-40 ms). 1,2 The generation of SO 4 c À and other reactive radicals including SO 3 c À , SO 5 c À and HOc from sulte autoxidation has been recently investigated for the abatement of organic contaminants. [3][4][5] In comparison with other traditional SO 4 c À precursors including persulfate (PS) and peroxymonosulfate (PMS), sulte has great potential for application due to its cost-effectiveness and nontoxicity. 6 However, the single sulte process is not applicable due to its negligible autoxidation rate. Therefore, activation of sulte with addition of the appropriate activators is necessary in the water treatment process.
Homogeneous processes of sulte activation with transition metal ions including Fe(II), Cu(II), Co(II) and Mn(II) have been demonstrated to be effective for the abatement of organic contaminants. 6 In the sulte activation process, transition metal ions (Me(II)) react with sulte to form complexes (Me(II)-SO 3 2À ) via a rapid complexation equilibrium process. This complex is then transformed to Me(III)-SO 3 2À complexes in the presence of dissolved oxygen, which can be decomposed to Me(II) and SO 3 c À , initiating the following oxysulfur radicals chain reactions. In comparison with Fe(II), Co(II) and Mn(II), Cu(II) exhibits great capability in promoting sulte autoxidation to generate radicals under circumneutral conditions, in which Cu(II)-SO 3 2À complexes can directly generate Cu(I) and SO 3 c À .
For example, Zhao et al. reported that iohexol abatement was mainly attributed to SO 4 c À produced in the Cu(II)/sulte process under aerobic conditions. 7 Yang et al. demonstrated that Cu(I) was mainly responsible for diatrizoate abatement for the Cu(II)/ sulte process under anaerobic conditions. 8 However, the application of Cu(II) can be compromised by the potential toxicity and difficult operation of recycling. To address these limitations, heterogeneous activators can be an important substitute for the sulte activation.
Recently, activation of sulte with the heterogeneous copperbearing nanoparticles has been investigated extensively. Zhao et al. reported that copper ferrite (CuFe 2 O 4 ) signicantly accelerated the sulte autoxidation rate to generate SO 4 c À for the effective removal of iohexol (80%) within 2 min at pH 8.0. 3 Xiao et al. demonstrated that copper oxides with dominantly exposed (0 0 1) reactive facets exhibited an excellent sulte activation performance with remarkable degradation of tetracycline (87%) within 5 min for the pH range of 4.5-9.0. 9 Ding et al. prepared a low-leaching silica-supported copper catalyst, which exhibited excellent activation efficacy of sulte with 90% of As(III) oxidation in neutral wastewater. 10 In the sulte activation process with copper-bearing nanoparticles, it is proposed that the decomposition of the surface >Cu(II)-SO 3 2À complexes to Cu(I) and SO 3 c À is the key step and the >Cu(II)/>Cu(I) redox cycling is critical to the acceleration of sulte autoxidation. However, the efficiency of this >Cu(II)/>Cu(I) redox cycling is limited due to the absence of electron-donating species. Recently, sulfur, as an efficient electron donor, was reported to be favorable for the radical generation and contaminant abatement in AOPs. 11,12 Especially, He et al. demonstrated that copper suldes (Cu 2 S and CuS) showed good performance in the heterogeneous Fenton reactions due to the synergy between copper and S(ÀII). 13 Therefore, it can be proposed that Cu 2 S and CuS could be employed as the activator to accelerate the sulte autoxidation. Especially, Cu 2 S is expected to be more advantageous than CuS in maintaining stable surface functional groups via the enhanced >Cu(I)/>Cu(II) cycling by sulfur and its application in sulte activation has never been studied previously and needs to be explored. Herein, the Cu 2 S/sulte process is studied for abatement of iohexol, which is a widely-used iodinated pharmaceutical and frequently detected in the aquatic environment. The objectives of this study are (i) to validate the feasibility of Cu 2 S in the sulte activation and examine the corresponding iohexol abatement performance; (ii) to explore the dominant radicals in the iohexol abatement by EPR, uorescence detection and radical scavenging experiments; (iii) to investigate the sulte activation mechanism by Cu 2 S via XPS and (iv) to assess the application potential of the Cu 2 S/sulte process in the real water matrixes.
2 Results and discussion 2.1 Characterization of the activator Fig. 1(a) illustrates the phase information of Cu 2 S via X-ray diffraction (XRD) measurement. It can be seen that Cu 2 S exhibits the characteristic diffraction peak of the standard Cu 2 S spectra (JCPDS card no. 84-0208). The diffraction peaks at 2q ¼ 37.1 , 45.6 and 48.0 are attributable to the (1 0 2), (1 1 0) and (1 0 3) crystal planes of Cu 2 S. Fig. 1(b) shows the microstructure of Cu 2 S via transmission electron microscopy (TEM) characterization. Irregular shape of the Cu 2 S particles is observed with the lattice spacings of 0.243 nm corresponding to the (1 0 2) crystalline plane.
2.2 Feasibility of sulte activation by Cu 2 S for iohexol abatement Fig. 2 presents a comparison of iohexol abatement efficiency by sulte activation with the copper based activators including CuO, Cu 2 O, CuS and Cu 2 S. It can be seen that the iohexol abatement efficiency by sulte activation with Cu 2 S, CuS, CuO and Cu 2 O is 95.6%, 88.6%, 81.0% and 15.0%, respectively. Apparently, CuS and Cu 2 S exhibit better iohexol abatement performance than the corresponding copper oxides. Especially, iohexol can be completely degraded within 30 s during sulte activation with Cu 2 S, which is signicantly higher than that of Cu 2 O. It is worthy to point out that the adsorption of iohexol by Cu 2 S is negligible. The above results suggest that the reductive sulfur might play an important role in the sulte activation process. The rate of sulte decay in the presence of the copper based activators follows an order of Cu 2 S > CuS > CuO > Cu 2 O ( Fig. 2(b)), which also reects that copper suldes show superior activation performance in accelerating sulte autoxidation.
In the previous studies, Liu et al. demonstrated that basic copper(II) carbonate showed efficient activation of sulte with 94% of As(III) removal at 20 min (pH 8.0). 14 An et al. reported a hollow-structured self-oating copper-loaded catalyst for sulte activation, which could achieve the efficient abatement of As(III) (93.7%) within 20 min at pH 7.5. 15 Dong et al. prepared a zero-valent Fe-Cu bimetallic nanoparticles to facilitate sulte activation with 87% abatement of sulfamethazine at 60 min (pH 6.0). 16 Compared to the aforementioned copper-based activated sulte processes, Cu 2 S exhibits excellent iohexol abatement efficiency of 95.6% within 0.5 min at pH 8.0. The rapid iohexol abatement is likely due to the electron transfer promoted by the reducing sulfur species, which is further conrmed by electrochemical characterization. As shown in Fig. 2 plots conrm the lower electrical impedance of the copper suldes, which is favorable for the advanced oxidation processes. The i-t response curve in the Cu 2 S/sulte process demonstrates the electron transfer between Cu 2 S and sulte based on the occurrence of current response aer addition of sulte ( Fig. 2(d)).    3 shows the effect of solution pH on iohexol abatement and sulte decay in the Cu 2 S/sulte process. As illustrated in Fig. 3(a), iohexol abatement is dependent on solution pH. Lower iohexol abatement efficiencies (<30%) are observed in the pH range of 4.0-6.0. With the increase of solution pH, iohexol abatement is elevated with the efficiency being 72.0% at pH 7.0. For further increase of pH from 7.0 to 10.0, complete iohexol abatement ($100%) can be achieved. The trend of sulte decay, as shown in Fig. 3(b), is consistent with the iohexol abatement. This suggests that the formation of reactive radicals is corresponding to the sulte autoxidation process, which is inuenced by solution pH.

(c), EIS Nyquist
The effect of solution pH on iohexol abatement and sulte decay can be interpreted as the following reasons. On the one hand, the distribution of HSO 3 À species (HSO 3 À /SO 3 2À , pK a ¼ 7. 2) is dependent on pH. HSO 3 À is the main species for the acidic conditions while SO 3 2À is the dominating species for the alkaline conditions. According to a previous study, SO 3 2À is easily activated to produce SO 3 c À compared with HSO 3 À owing to the lower redox potential of SO 3 c À /SO 3 2À (0.63 V) than that of SO 3 c À /HSO 3 À (0.84 V). 17 On the other hand, the presence of the deprotonated copper complexes (>Cu(I)-OH and >Cu(II)-OH) under the alkaline conditions is favorable for the reaction with SO 3 2À with the corresponding formation of the surface >Cu(I)-SO 3 2À and >Cu(II)-SO 3 2À complexes. This formation of complexes has been conrmed to be important for accelerating sulte autoxidation to generate reactive radicals in the presence of the heterogeneous transition metal based activators. 10 Regarding the combined effect of distribution of HSO 3 À species and the formation of the above surface complexes, iohexol abatement is more favorable in the Cu 2 S/sulte process for the near-neutral and alkaline conditions than the acidic conditions.

Mechanistic insights
2.3.1 Identication of reactive radicals. In the typical sulte activation with transition metals, SO 3 c À , SO 4 c À and SO 5 c À can be produced in the presence of dissolved oxygen. 18 Fig. 4 represents the production of reactive radicals in the Cu 2 S/sulte process from the qualitative and quantitative perspective. EPR and uorescence method were adopted for the qualitative identication of reactive radicals. As shown in Fig. 4(a), a signicant uorescence intensity of HOc appears for sulte activation with the copper based activators, which follows an order of Cu 2 S > CuS > CuO > Cu 2 O. This trend of HOc generation also accords with the iohexol abatement and sulte decay. Fig. 4(b) presents the EPR signals using DMPO as the spin trapping agent to capture the reactive radicals produced in the Cu 2 S/sulte process. Compared with the single Cu 2 S and single sulte process, the Cu 2 S/sulte process shows the characteristic signal of SO 3 c À with the distinct hyperne splitting constants of DMPO-SO 3 c À (a N ¼ 14.5 G, a H ¼ 16.0 G). This indicates that the formation of SO 3 c À is signicantly enhanced during sulte activation in the presence of Cu 2 S. Apparently, the signals originating from SO 4 c À , SO 5 c À and HOc are not observed because of the high efficiency of SO 3 c À captured by DMPO. Although the apparent second order rate constant for the reaction of DMPO with SO 3 c À (k DMPO,SO3 c À ¼ 1.2 Â 10 7 M À1 s À1 ) is much lower than that of dissolved oxygen ðk O2 ; SO3 À ¼ 1:5 Â 10 9 M À1 s À1 Þ, 19 the higher dosage of DMPO (100 mM) than dissolved oxygen (0.25 mM) leads to the interruption of the subsequent chain reactions of the oxysulfur radicals. The formation of SO 3 c À reects the potential generation of the other oxysulfur radicals.
From the quantitative perspective, radical quenching experiments and direct quantication of SO 4 c À and HOc were performed. Methanol (MeOH) and tert-butanol (TBA) are used as the radical scavengers, 20,21 and the contribution of SO 4 c À and HOc to iohexol abatement in the Cu 2 S/sulte process can be differentiated, as shown in Fig. 4(c). It can be seen that TBA exerts an unnoticeable inhibitory effect on iohexol abatement, which rules out the contribution of HOc. In comparison, iohexol abatement in the presence of MeOH is considerably suppressed with a reduction of abatement efficiency as high as 80%. Therefore, SO 4 c À is the dominant radical being contributable to iohexol abatement. Then, MeOH and dimethyl sulfoxide (DMSO) are employed to quantify the production of SO 4 c À and HOc in the Cu 2 S/sulte process. 22,23 Formaldehyde is quantitatively produced via the respective reactions of MeOH and DMSO with SO 4 c À and HOc. Then the formed formaldehyde can react with 2,4-dinitrophenylhydrazine to form the corresponding hydrazone, as illustrated in Fig. 4(d). It can be seen that the Cu 2 S/sulte process under the studied conditions can produce 9.8 mM of SO 4 c À and 1.2 mM of HOc. This result further conrms that SO 4 c À is the dominating radical contributing to iohexol abatement.
2.3.2 Role of dissolved oxygen on iohexol abatement. Fig. 5(a) displays the effect of dissolved oxygen on iohexol abatement in the Cu 2 S/sulte process. As can be seen, iohexol abatement is signicantly inhibited under anaerobic conditions, suggesting that the generation of the reactive species is suppressed in the absence of the dissolved oxygen. According to the previous literature, dissolved oxygen plays signicant roles in the sulte activation with the heterogeneous transition metal based activators. 24 With regard to the Cu 2 S/sulte process, sulte autoxidation with a concomitant generation of SO 3 c À is initiated by the transformation of >Cu(II) by sulte to >Cu(I), in which a pre-equilibrium process occurs to form the >Cu(I)-SO 3 2À complexes prior to the electron transfer. 25 On the other hand, the presence of dissolved oxygen is vital for the generation of SO 4 c À , SO 5 c À and HOc on the basis of EPR and uorescence results. Fig. 5(b) displayed the variation of dissolved oxygen concentration. The concentration of dissolved oxygen is signicantly declined in the initial 1.0 min and then increased rapidly from the lowest concentration of 5.4 mg L À1 to 8.0 mg L À1 , indicating the indispensable role of dissolved oxygen in the Cu 2 S/sulte process. Fig. 5(c) manifests the iodide release during the iohexol abatement in the Cu 2 S/sulte process with an iodide release. The deiodination of iohexol under aerobic conditions also means that iohexol undergoes oxidative deiodination by SO 4 c À .

Identication of active sites.
Electrochemical measurement and high-resolution X-ray photoelectron spectroscopy (XPS) were employed for analyzing the chemical composition of Cu 2 S being responsible for sulte activation, as displayed in Fig. 6. The corrosion current of 1.8 Â 10 À5 A in the Tafel curve tting (Fig. 6(a)) evidences the occurrence of electron transfer between Cu 2 S and sulte, which reects the higher performance of Cu 2 S in accelerating the sulte autoxidation process. Fig. 6(b) presents a comparison of the chemical valence of Cu 2 S before and aer the reaction. As shown in the Cu 2p spectra (Fig. 6(c)), two pairs of peaks at the binding energies of 932.3/951.1 eV and 934.0/954.1 eV, are attributable to >Cu(I) and >Cu(II) with proportions of 53.4% and 46.6%, respectively. Aer sulte activation, the relative intensity of >Cu(I) is reduced to 50.8% with a 2.6% decrease. The relative intensity of >Cu(II) is increased from 46.6% to 49.2%, suggesting the oxidation of >Cu(I) to >Cu(II) by dissolved oxygen. The S 2p spectra displayed in Fig. 6(d)  and guarantees the regeneration of the >Cu(I) species, which in turn explains the negligible variation of the chemical valence of the copper species. In conclusion, the sulfur species on the Cu 2 S particles mainly contributes to the enhanced performance of Cu 2 S in accelerating the sulte autoxidation and the corresponding radical generation. On the basis of the above results, the sulte activation mechanism by Cu 2 S for efficient iohexol abatement is illustrated in Fig. 7. The copper species on the Cu 2 S surface serves as the dominant active sites, which is critical for accelerating sulte autoxidation in the presence of dissolved oxygen. The regeneration of Cu(I) by S 2À is thermodynamically feasible because the standard reduction potential of sulfur (S 2À /S 0 , À0.48 V) is much lower than that of copper (Cu(II)/Cu(I), 0.167 V). 27 The consumption of sulfur species and the corresponding accumulation of SO 4 2À , as shown in the XPS results, lead to a reduction of the sulte activation efficiency of Cu 2 S, which is conrmed by the decrease of iohexol abatement efficiency during reusability tests. The reductive sulfur species of Cu 2 S is transformed to SO 4 2À via a series of reaction and the generated electron could be favorable for the transformation of the >Cu(II)-SO 3 2À complexes to the >Cu(I)-SO 3 2À complexes with a concomitant generation of SO 3 c À . This process can in turn achieve the redox cycling of >Cu(I)/>Cu(II) and maintain the stable surface copper species, which guarantees the higher overall iohexol abatement efficiency. In the presence of dissolved oxygen, the produced sulte radical (SO 3 c À ) initiates the oxysulfur radical chain reactions (SO 4 c À , SO 5 c À and HOc). The produced reactive radicals (SO 4 c À ) are the dominant species being responsible for efficient abatement of iohexol.
2.4 Application potential of the Cu 2 S/sulte process in the real water matrix Fig. 8(a) and (b) displays the effect of coexisting cations and anions on iohexol abatement in the Cu 2 S/sulte process. As can be seen, the presence of cations exerts slightly inhibitory effect on iohexol abatement. This phenomenon is probably due to that the increase of ionic strength can affect the interaction between the copper and sulte. In contrast, the anions exhibit inhibitory effect to different extent. Cl À and HPO 4 2À exhibit considerably inhibitory effect on iohexol abatement whereas the presence of HCO 3 À exerts relatively lower inhibition and NO 3 À shows minimal effect. The increase of Cl À concentration from 0 mM to 2 mM leads to a signicant reduction of iohexol abatement ($73%), which could be ascribed to the radical quenching effect of Cl À . 28 With regard to the effect of HPO 4 2À , the iohexol abatement efficiency is reduced by 79% when the concentration of HPO 4 2À is increased to 8 mM. This phenomenon may be due to the complexation of the active copper sites by HPO 4 2À , which hinders the complexation reaction between copper and sulte. 29 Regarding the effect of HCO 3 À , iohexol abatement efficiency is reduced from 77.6% to 55.7% when the concentration of HCO 3 À is in the range of 2 mM to 8 mM. This inhibitory effect is caused by the radical quenching effect of HCO 3 À . 30 The minor effect of NO 3 À on iohexol abatement is owing to negligible quenching effect of SO 4 c À . 10 Fig. 8(c) illustrates the effect of humic acid on iohexol abatement in the Cu 2 S/sulte process. An increase of humic acid concentrations from 0 to 4.0 mg L À1 results in a decrease of iohexol abatement efficiency from 91.5% to 35.3%. This indicates that humic acid shows a signicantly inhibitory effect due to its radical quenching effect with the apparent second order rate constant of 2.35 Â 10 7 M À1 s À1 and 3.0 Â 10 8 M À1 s À1 for SO 4 c À and HOc, respectively. 31 Fig. 8(d) presents the iohexol abatement in the tap water, surface water and secondary effluent from a municipal wastewater treatment plant. The iohexol abatement in the Cu 2 S/sulte process is inhibited in the real water matrixes. This can be ascribed to the coexistence of anions and humic matter. The reusable performance of Cu 2 S shown in Fig. 8(e) suggests that Cu 2 S exhibits good reusability with slight reduction of iohexol abatement efficiency aer recycled for 4 times. The decrease of the activation performance may be due to the consumption of the reductive sulfur species and the accumulation of SO 4 2À on the Cu 2 S surface. 32,33 Fig. 8(f) gives the efficacy of the Cu 2 S/sulte process towards different organic pollutants including tetracycline, cefalexin, atrazine, diatrizoate and nitrobenzene. The abatement efficiencies for the above pollutants are 95.9%, 90.9%, 78.9%, 80.3% and 62.7%, respectively. The above results demonstrate the selectivity of the Cu 2 S/sulte process for the abatement of organic pollutants.

Conclusions
In this study, a novel Cu 2 S/sulte process for production of reactive radicals is proposed to achieve effective abatement of iohexol. High abatement efficiency of iohexol is obtained for the pH range of 7.0-10.0, which is ascribed to the speciation of sulte. The radical scavenging tests and EPR analysis demonstrate that SO 4 c À and HOc are the dominant reactive radicals in the Cu 2 S/sulte process. , Cl À and humic acid exerts inhibitory inuence due to radical scavenging effect. Superior iohexol abatement efficiencies have been obtained for the four recycles, conrming the good reusability and satisfactory activation stability. Cu 2 S exhibits signicant advantages of applicability in wide pH range and the elevated sulte activation efficiency with lower dosages of the activator and thus is a promising alternative for the abatement of organic pollutants in the water and wastewater treatment.

Characterizations
The electrochemical analysis including electrochemical impedance spectroscopy Nyquist plots and i-t response curve were conducted by electrochemical workstation (CHI 760E, China). The morphological details of Cu 2 S were observed with transmission electron microscopy (FEI Tecnai G2 F20, USA). XRD patterns were analyzed by X-ray diffractometer (SmartLa) at room temperature with Cu-Ka radiation in the range of 10-80 . XPS of Cu 2 S before and aer activation were obtained on a Thermo Scientic Escalab 250Xi. The obtained spectra were then analyzed by XPSPEAK soware and calibrated by adventitious C 1s peak at 284.8 eV as the internal standard.

Experimental procedures
Iohexol abatement reactions were carried out in a 100 mL glass reactor with Teon-lined caps and stirred constantly with mechanical agitation (200 rpm). The desired amounts of iohexol, sulte and Cu 2 S were added in the aqueous solution, which was kept at 25 C with a thermostat. In a typical iohexol abatement experiment, the concentrations for the chemicals in the reaction solution were iohexol 10 mM, sulte 500 mM and Cu 2 S 0.025 g L À1 . During the reaction, 1 mL sample was withdrawn at predetermined time intervals, ltered with 0.22 mm syringe lters, and immediately terminated with methanol for HPLC analysis. Diluted NaOH and H 2 SO 4 were used to adjust solution pH. All the experiments were performed in duplicate to calculate the average values and the error bars for reproducibility.

Analytical methods
Iohexol was quantied by high performance liquid chromatography (HPLC, Agilent 1290 Innity) equipped with a UV-vis photodiode array detector (245 nm) and Waters Sunre C18 column (4.6 mm Â 150 mm, 5 mm particle size). The acetonitrile/water mixture (5% : 95%) was employed as the mobile phase at a ow rate of 1 mL min À1 . The quantication limit for iohexol was 0.15 mM with a lower standard deviation (<5%). The release of iodide during the iohexol degradation process was also quantied by HPLC at the wavelength of 226 nm. Mixture of acetonitrile and monobasic potassium phosphate (2% : 98%) was used as the mobile phase at a ow rate of 0.80 mL min À1 . The sulte concentrations were determined employing DTNB as chromogenic reagent. At a predetermined time interval, samples were directly added into a colorimetric tube containing 1 mM EDTA, 1 mM DTNB and phosphate buffer solution. UV-vis spectrophotometer (V-1600) was used to quantify the sulte decay during the reaction process at the wavelength of 412 nm. 34 In electrochemical analysis, activators were uniformly loaded onto the glassy carbon electrode, with platinum electrode as opposite electrode, Ag/AgCl electrode as reference electrode, and all experiments were carried out in 0.1 M sodium nitrate electrolyte. EPR signals with DMPO as the spin-trapping agent (100 mM) were recorded on Bruker EMX-10/12 spectrometer (Germany). With regard to the quantication of reactive radicals, 5 mM MeOH and dimethyl sulfoxide (DMSO) were employed to react with SO 4 c À and HOc to generate quantitatively formaldehyde, which 1 mol formaldehyde was produced from 1 mol SO 4 c À and 2.17 mol HOc. Then, 2,4-dinitrophenylhydrazine (240 mM) reacted with formaldehyde with a 1 : 1 ratio to form the corresponding hydrazone, which can be measured by HPLC. 22,23