Comparative studies on catalytic mechanisms for natural chalcopyrite-induced Fenton oxidation: Effect of chalcopyrite type

Journal of Hazardous Materials 381 (2020) 120998

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Comparative studies on catalytic mechanisms for natural chalcopyriteinduced Fenton oxidation: Effect of chalcopyrite type Xiaotao Huanga,b, Tonghe Zhub, Weijian Duanb, Sheng Liangb, Ge Lib, Wei Xiaoa,

T

⁎

a

School of Resources Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, PR China The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of Education, School of Environment and Energy, South China University of Technology, Guangzhou 510006, PR China

b

A R T I C LE I N FO

A B S T R A C T

Editor: Xiaohong Guan

The type of chalcopyrite plays a key role in determining its physicochemical properties. In this study, we present a systematic comparative study on the use of p- and n-type chalcopyrite (Cpy A and Cpy B, respectively) as Fenton catalysts for wastewater treatment. Experimental results showed that 60% of AO7 removal could be achieved in 30 min at a natural pH when H2O2 was activated by Cpy A. The removal rate could be further enhanced by up to 100% within 5 min using Cpy B as the catalyst. This is because Cpy B released far more Cu+ and Fe2+ ions, and less Cu2+ after being washed, and then activated H2O2 to produce more ·OH radicals (main active species). On the other hand, the excess copper ions released from Cpy A could react with AO7 to generate an intermediate product that would negatively affect the degradation process. Finally, the relative contribution of the homogeneous vs. heterogeneous process was calculated. Although only about 20% of the contribution for AO7 degradation was provided by heterogeneous processes in both systems, the time for full removal could be obviously reduced to 5 min from 20 min (homogeneous process) in the Cpy B/H2O2 system.

Keywords: N- and p- type chalcopyrite Fenton oxidation Hydroxyl radical Adsorptive action

1. Introduction Fe-bearing solids such as iron-oxides species (ferrihydrite, hematite, goethite, magnetite, etc.) have been widely used as catalysts in advanced oxidation processes (AOPs) for wastewater treatment owing to their good H2O2 catalyzing ability, which generates strong oxidative characteristics of free radicals •OH (Barreiro et al., 2007; Zhao et al., 2018; Pouran et al., 2014; Expósito et al., 2007; Huang et al., 2016). They bear significant advantages over the corresponding ions-homogenous catalysts including an applicability to a wider pH range (from 3.0 to 9.0), low iron sludge production, and ease of regeneration and recycling (Munoz et al., 2015; Garrido-Ramírez et al., 2010; Bokare and Choi, 2014; Ganiyu et al., 2018). Apart from studying iron-oxides catalysts, sulfide materials like pyrite have been widely reported as an effective Fenton catalyst to activate H2O2 in recent years (Ye et al., 2018; Farshchi et al., 2018a, b). This is because pyrites offer unique benefits of continuous generation of aqueous Fe2+ and self-regulation of pH in acidic solutions, which results from rapid dissolution of Fe (II) from solid pyrite into the solution in the presence of oxygen and H2O2 according to Eqs (1)–(4) (Ye et al., 2018; Zhang et al., 2018; Li et al., 2018a; Schoonen et al., 2010). These characteristics should facilitate faster Fenton oxidation towards the degradation of organic pollutants,

⁎

as the traditional Fenton process is limited by the slow kinetics of Fe (II) regeneration and by the replenishment of acids (Dong et al., 2018; Xing et al., 2018). Additionally, the availability of Cu in the Fe-bearing solids plays a significant role in promoting Fenton-like reactions, owing to the fact that the cycle between Cu2+ and Cu+ can stimulate H2O2 decomposition (Eqs. (5) and (6)), accompanied by the production of •OH (Dai et al., 2018; Ding et al., 2016; Sun et al., 2019). It is thus expected that chalcopyrite (CuFeS2), a copper-iron sulfide mineral found in the earth’s crust (easy to obtain and to allow self-regulation of solution pH), represents a promising alternative Fenton catalyst attributed to the immediate release of Fe2+, Cu2+, and protons (Eqs. (7)–(12)) (Panda et al., 2015; Zhao et al., 2017; Barhoumi et al., 2017; Li et al., 2013). FeS2 + 7 O2 + 2 H2O → 2 Fe2+ + 4 SO42− + 4 H+

(1)

FeS2 + 15 H2O2 → 2 Fe

(2)

3+

+ 4 SO4

2−

+

+2H

+ 14 H2O

FeS2 + 14 Fe3+ + 8 H2O → 15 Fe2+ + 2 SO42− + 16 H+ 2+

+ H2O2 → Fe

Cu

2+

+ H2O2 → Cu

Cu

+

Fe

+ H2O2 → Cu

Corresponding author. E-mail address: [email protected] (W. Xiao).

https://doi.org/10.1016/j.jhazmat.2019.120998 Received 9 May 2019; Received in revised form 8 August 2019; Accepted 11 August 2019 Available online 12 August 2019 0304-3894/ © 2019 Published by Elsevier B.V.

3+ +

2+

+ OH + O2

−

−·

+ OH

−

(4)

+

(5)

·

(6)

+ OH +2H

(3)

·

+ OH

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X. Huang, et al.

CuFeS2(s) + 4 O2 → Fe2+ + Cu2+ + 2 SO42− CuFeS2(s) + 4 O2 + 4 H CuFeS2(s) + 16 Fe H+

3+

+

→ Fe

2+

+ Cu

+ 8 H2O → 17 Fe

2+

2+

+2

+ Cu

2+

S0(s) + 2 H2O + 3 O2 → 2 SO42− + 4 H+ CuFeS2(s) → Fe

2+

+ Cu

+

CuFeS2(s) + 8 H2O2 → Fe

+2 3+

S0(s)

+ Cu

+ 2 SO4

+ 2 H2O

+2

SO42−

(8) + 16 (9) (10)

+3e 2+

nitrate, further rinsed with distilled water, then with 95% ethanol twice and finally dried at 30 °C. Table S1 presents the element analysis results of chalcopyrite samples. Fe (27.32%), Cu (35.82%), and S (29.41%) were the major elements for Cpy A, and Fe (36.73%), Cu (30.11%), and S (27.62%) were the major elements for Cpy B.

(7) S0(s)

2.3. Fenton reaction

(11) 2−

A total of 200 mg (0, 50, and 100 mg) of Cpy A or Cpy B powders were added to 100 mL of 100 mg/L AO7 containing 0.05 M Na2SO4 aqueous solution. The solution was poured into a 500 mL Bunsen breaker equipped with a magnetic rod. The stirring speed was fixed at a predetermined value. Then, 0–40 mM H2O2 (30 wt%) was added into the above solution. At designed sampling intervals, samples were taken out using a 5.0 mL syringe, and then filtered via a 0.22 mm Suporfone film to obtain a clear supernatant, then finally quenched with 1 mL of methanol if necessary. All the above experiments were repeated in triplicate unless particularly stated.

+ 8 H2O + 2 H+ (12)

Despite of the advantages of a chalcopyrite-aided Fenton system, few reports in the literature concerning the use of natural chalcopyrite for catalyzing Fenton-like reactions are available (Barhoumi et al., 2017; Ltaïef et al., 2018; Xu et al., 2019). The complex metal oxidation, including Fe2+, Fe3+, Cu+, and Cu2+, surface structure, and crystal defects of chalcopyrite varies extensively depending on its natural source (Zhao et al., 2017), and greatly influences its behavior as a Fenton catalyst. Chalcopyrite is indeed an antiferromagnetic semiconductor categorized into n-type and p-type depending on the kind and level of doping (Wen et al., 2018; Carr and Morelli, 2015). To date, no previous study has been devoted to investigating the mechanism of how the natural chalcopyrite type affects the dissolution of the metal ions and their roles in decomposing H2O2, which in turn determine the overall Fenton efficiencies. The random selection of chalcopyrite makes it difficult to determine appropriate natural Fenton-like catalysts with good performance. Herein, for the first time, we report a systematic comparative study of n- and p-type chalcopyrite as Fenton catalysts for contaminant degradation in water. In addition to elucidating the differences in the dissolution behavior of Fe and Cu ions, and in the pH fluctuation between two chalcopyrite types, efforts are made to unravel the catalytic mechanisms for natural chalcopyrite-induced Fenton oxidation. These include (i) the catalyzing effects of reactive Fe and Cu species for breaking H2O2 bonds and the spontaneous formation of radicals, (ii) the relative contribution of homogeneous vs. heterogeneous processes (differentiated as the H2O2 decomposition catalyzed by the Fe/Cu ions in the solution or the surface-bound Fe/Cu species on the solid), and (iii) the interactions between the released ions, the solid surface, and the aqueous species (particularly pollutants). In this study, representative n- and p-type chalcopyrite samples were mined from the Chambishi Copper mine of Zambia and obtained from the geological museum of Guangxi, China, respectively. Azo-dye orange II (AO7) was selected as a model organic pollutant because it is a stable compound and widely used in pulp and textile industries.

2.4. Apparatus and analytical procedures AO7 concentration was determined at a wavelength of 484 nm (λ = 484 nm) by a UV–vis spectrophotometer (SP-754PC, China). The total organic carbon (TOC) concentration of the degradation product was analyzed using a Shimadzu TOC (1020A, Japan). The H2O2 concentration was determined using potassium titanium (IV) oxalate as a color indicator by a UV–vis spectrophotometer at λ = 400 nm. Concentrations of metal ions, such as iron and copper, were measured using a UV–vis spectrophotometer unless noted otherwise. Total dissolved Fe and aqueous Fe (II) concentration were detected by means of a 1,10-phenantroline spectrophotometric method at λ = 510 nm with the addition of ascorbic acid or not. Dissolved Cu (II) and dissolved Cu (I) were determined using the bis(cyclohexanone) oxaldihydrazone spectrophotometric method at λ = 600 nm, and by 2,9-Dimethyl-1,10phenanthroline at λ = 457 nm, respectively. An N2 adsorption-desorption test at 77 K (NOVA 4200e, Quantachrome, USA) was carried out to measure the specific surface area and pore distribution of chalcopyrite samples. Their composition and morphologies were characterized before and after reaction by scanning electron microscopy (SEM, FESEM; Merlin, Zeiss, Germany) and X-ray diffraction (XRD, Empyrean, PANalytical B.V., Holland), respectively. X-ray photoelectron spectroscopy (XPS, Escalab, Thermo, USA) was used to determine the identification of functional groups and element chemical states. Spectra were analyzed using XPS Peak software (Version 4.1) with a standardized C1 s peak (284.4 eV). Fourier transform infrared spectroscopy (FT-IR, CCR-1, Thermo-Nicolet, USA) was performed to identify the surface groups of the catalysts. The adsorption pattern of catalysts was determined before and after AO7 addition using zeta-potential measurements (Zetasizer Nano ZS, Malvern, U.K.). Hydroxyl radical ·OH was detected using DMPO as the spin trap agents by EPR spectroscopy (Bruker EMX-8/2.7, Germany).

2. Materials and methods 2.1. Chemicals Distilled water, orange II, methanol, sodium sulfate, sulfuric acid, sodium hydroxide, hydrogen peroxide (30 wt%), ferrous sulfate, and anhydrous cupric sulfate were used. All these chemicals were of analytical grade and used directly without any pretreatment (Xiao et al., 2019).

3. Results and discussion 3.1. Catalytic effects of natural chalcopyrite-induced Fenton oxidation for AO7 degradation Hall effect analysis (Table S2) was employed to investigate the semiconductor properties of two chalcopyrite samples. The results indicate that Cpy A is p-type chalcopyrite whose charge carrier concentration (7.01 E +19 cm−3) is positive, and Cpy B is n-type chalcopyrite whose charge carrier concentration (-5.25 E +18 cm−3) is negative. To our knowledge, different types of chalcopyrite would result in varying dissolution behavior (H+, Fe2+, and Cu2+) after being washed or leached (Zhao et al., 2017). In addition, the amount of released metal ions, such as copper and iron, would have an influence on

2.2. Materials Chalcopyrite samples (Cpy A and Cpy B) used in this work were mined from the Chambishi Copper mine of Zambia and obtained from the geological museum of Guangxi province of China, respectively. They were crushed and sieved to have particle diameters of less than 74 μm. Before use, the surface impurities of obtained powder should be removed by a series of processes. Briefly, powder was added into 95% ethanol and then ultrasonicated for 5 min, washed with 1 M hydrogen 2

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Fig. 1. a) AO7 degradation in different systems; b) TOC concentration (Conditions: AO7 = 100 mg/L; H2O2 =40 mM; Cpy A =2.0 g/L, Cpy B =2.0 g/L, natural pH = 6.26).

cases, the final aqueous solutions of samples were acidic, indicating that the proton (H+) release reaction occurred after chalcopyrite entered into the solution (Eqs. (9) and (10)). Furthermore, the solution pH dramatically decreased within 60 s; and then remained constant in all samples, which shows that the equilibrium had been achieved between the reactions that generated and consumed H+ (Eqs. (8) and (9)-(10)). This result agrees with that of Oturan (2017), who used chalcopyrite as a catalyst to remove TC (Barhoumi et al., 2017). Furthermore, the final solution pH in the Cpy A/AO7 system (5.42 for 0.5 g/L, 5.16 for 1.0 g/L, 4.98 for 2.0 g/L) was much higher than that in Cpy B/AO7 system (4.93, 4.70, 4.51), indicating the higher H+ generation capacity of Cpy B. After the addition of H2O2 (Fig. 2(c and d)), a lower pH value was observed in the Cpy B/H2O2 process, although the solution rapidly pH decreased further within 1 min in both systems. This result indicates that protons could be further released by decomposition of chalcopyrite by H2O2 (Eqs. (12)). Therefore, chalcopyrite could be viewed as an excellent potential catalyst for H2O2 activation to degrade organic containments. Furthermore, it does not require pH adjustment, which eliminates the necessity to purchase expensive acidic or alkaline reagents. Based on the above analysis, it is clear that metal ions such as Cu+, 2+ Cu , and Fe2+, are also released into the aqueous solution through the reactions described in Eqs. (7)–(9) and (11). All of these ions could catalyze the decomposition of H2O2 to form ·OH (Eqs. (4)–(6)), which in turn would facilitate the degradation of aqueous organic pollutants. Fig. 3 shows the amount of metal ions released (TFe, Fe2+, Cu+, and Cu2+) in the chalcopyrite/AO7 system with and without H2O2. Note that Cu+ was measured in pure water from 1 min to 30 min, as shown in Fig.3b, and the Fig. 3d did not show the line of Cu+ in Cpy A, because the color of the color AO7 will cover the color of detecting method for Cu+. Unsurprisingly, the TFe, Fe2+, Cu+, and Cu2+ ions were all detected in both Cpy A and Cpy B systems, regardless of the presence or absence of H2O2. As shown in Fig. 3(a and b), the TFe, Fe2+, and Cu+ concentrations from Cpy B were much higher than those from Cpy A. Furthermore, over the same duration, the concentrations of TFe, Fe2+, and Cu+ with Cpy A were nearly 2.5 times, 2.5 times, and 3 times more than those with Cpy B, respectively. However, the Cu2+ concentration with Cpy A, which slightly increased from about 37.2 mg/L in 5 min to 38.3 mg/L in 30 min, was much higher (nearly 3.2 times) than that with Cpy B which remained nearly constant at 12 mg/L. This result indicates that Cpy A primarily released Cu2+ ions and smaller amounts of Fe2+ and Cu+ ions, whereas the opposite was true for Cpy B, which primarily released Fe2+ and Cu+ ions and smaller amounts of Cu2+ ions. The variation of metal ions from Cpy A and Cpy B is displayed in Fig. 3(c and d) when H2O2 was added to the solution. First, it is evident that the amount of TFe and Cu2+ ions increased dramatically, and Fe2+ and Cu+ concentrations decreased in both two processes. This is because H2O2 would inevitably react with chalcopyrite to release more metal ions via Eq. (12), and it was easy for Fe2+ and Cu+ ions to donate

the Fenton process for the degradation of organic pollutants. Therefore, we hypothesized that these two chalcopyrite samples (Cpy A and Cpy B) would show different efficiencies in the decomposition of organic contaminants. To directly prove the above hypothesis, organic matter removal experiments were designed and carried out using CuFeS2 as a catalyst to activate H2O2 for the degradation of AO7. Fig. 1 shows the reduction in AO7 concentration with time, and the TOC concentration after reacting for 30 min in Cpy A/H2O2, Cpy B/H2O2, Cpy A alone, Cpy B alone, and H2O2 systems. H2O2 alone was almost unreactive toward AO7. Complete removal of AO7 was achieved in 5 min with the Cpy B/H2O2 coupled process. In contrast, 60 mg/L of AO7 was removed in 30 min when H2O2 was activated by Cpy A, and less than 20 mg/L was removed in Cpy A/B alone, indicating that AO7 was decolored in Cpy A/ H2O2 and Cpy B/H2O2 but not significantly adsorbed on those particles. Owing to the fact that AO7 is easy to decolor, but difficult to completely mineralize, TOC concentration was determined to assess the mineralization effect. As shown in Fig. 1(b), only 6.985 mg/L of TOC remained after the Cpy B/H2O2 process, indicating that AO7 was effectively reduced. However, 31.86 mg/L persisted after the Cpy A/H2O2 process, and more than 35.0 mg/L remained after treatment using only Cpy A/B. It is generally accepted that the catalyst dosage and H2O2 concentration are important factors influencing the decomposition of organic pollutants. The variation in AO7 degradation with different H2O2 concentrations (0–40 mM) and chalcopyrite dosages (0.0–2.0 g/L) is discussed. Fig. S1 and Fig. S2 show that AO7 and TOC concentrations decreased with increasing chalcopyrite dosage (both Cpy A and Cpy B) and H2O2 concentration. In other words, 2.0 g/L of chalcopyrite and 40 mM of H2O2 was yielded the best AO7 decolorization in the two systems. Notably, in all cases, the removal efficiency in the Cpy B/H2O2 system was much higher than that in the Cpy A/H2O2 system under the same conditions. The results clearly show that H2O2 activation by Cpy B is better than that by Cpy A, although the surface area of Cpy B (2.811 m2/g) is lower than that of Cpy A (4.404 m2/g) (Fig. S3). Cu+ and Fe2+ were reported to provide one electron to H2O2, thus inducing H2O2 decomposition into a free radical ·OH and a hydroxyl ion OH− (Wang et al., 2017; Chen et al., 2014). In addition, the above process is rapid, which facilitates the effective degradation of aquatic pollutants. Therefore, the next section addresses the differences in AO7 degradation efficiency between the two chalcopyrite samples in terms of the amounts of H+, Fe2+, Cu+, and Cu2+ released by the samples. 3.2. Release of H+, TFe (Fe2+ & Fe3+), Fe2+, Cu+, and Cu2+ in the aqueous solution To ascertain the properties of the two types of chalcopyrite in suspension, preliminary experiments were carried out before using the CuFeS2/H2O2 process to degrade AO7. Fig. 2(a and b) shows the timeseries of solution pH starting from the natural pH value of AO7 solution (nearly 6.3) with chalcopyrite concentrations from 0.5 to 2.0 g/L. In all 3

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Fig. 2. Change of solution pH during CuFeS2/AO7 system without H2O2 (a and b) and with H2O2 (c and d) (Conditions: AO7 = 100 mg/L; Cpy A = 0.5–2.0 g/L; Cpy B = 0.5–2.0 g/L; H2O2 =40 mM; natural pH = 6.26).

one electron to H2O2, which would then be transferred to Fe3+ and Cu2+. Note that the amount of TFe ions in the Cpy A/H2O2 process did not increase, but did decrease in the first 20 min, which contributed to the formation of absorbed Fe onto the chalcopyrite (IEP = 3.52) surface and to the precipitation of Fe via Fe (OH)3 that is easy to form under the above pH range (Fig. 2). Additionally, the amount of remaining Fe2+ and Cu+ ions in the Cpy B/H2O2 system was much higher than that in the Cpy A system. More specifically, the Fe2+ and Cu+ concentrations in the Cpy B/H2O2 process decreased to 6 and 0.2 mg/L from 8.1 and 7.6 mg/L during the reaction, respectively. Only a small amount of Fe2+ was detected in the Cpy A/H2O2 process in the first 15 min, and then increased to 4 mg/L in the final period. Apart from this, no Cu+ was found in the entire range. The Cu2+ concentration of Cpy A was

about 3 times as large as that of Cpy B, although the amount of other metal ions from Cpy A was lower. Fe2+ and Cu+ rather than Cu2+ can effectively decompose H2O2 to form free radicals that help to quickly degrade organic pollutants. Owing to more Fe2+ and Cu+ ions released in Cpy B, it had greater performance in activating H2O2 and then degrading AO7. Chalcopyrite has been reported as a potential heterogeneous catalyst (Barhoumi et al., 2017; Ltaïef et al., 2018), but it would inevitably release metal ions that decompose H2O2 in solution. Thus, the main process for AO7 degradation, homogeneous or heterogeneous, is needed to be explained in detail. To the best of our knowledge, this work is the first of its kind to report on the contribution of homogeneous or heterogeneous processes in AO7 mineralization with

Fig. 3. Change of TFe, Fe2+, Cu2+ and Cu+ concentration during CuFeS2/AO7 system without H2O2 (a and b), and with H2O2 (c and d) (Conditions: AO7 = 100 mg/ L; Cpy A =2.0 g/L; Cpy B =2.0 g/L; H2O2 =40 mM; natural pH = 6.26). 4

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Fig. 4. a) Degradation of AO7 under different conditions; b) the corresponding first order kinetic results (Conditions: AO7 = 100 mg/L; Cpy A =2.0 g/L; Cpy B =2.0 g/L; H2O2 =40 mM; natural pH = 6.26).

chalcopyrite as a catalyst. Further experiments were carried out on the homogeneous vs. heterogeneous process to determine their contributions. 3.3. Relative contribution of the homogeneous vs. heterogeneous process To estimate the relative contribution of the homogeneous and heterogeneous processes, the supernatant of the samples must be obtained. In brief, the sample was added to pure water for a few minutes. Then, the solution was filtered by sequentially adding AO7 powder and H2O2 to start the AO7 degradation reaction to obtain the supernatant. As shown in Fig. 4a, AO7 was completely removed in 5 min and 20 min with Cpy B and the supernatant of Cpy B process, respectively. This indicates that the homogeneous reaction was the main process; however, the heterogenous process only contributed to approximately 17% (calculated by the difference between the remained AO7 concentration at 5 min) for AO7 degradation. In contrast, approximately 60% and 38% of AO7 was removed in the Cpy A and supernatant of Cpy A system, which shows that the homogenous reaction made the largest contribution to AO7 decolorization, and the heterogeneous only accounted for 22% of AO7 removal. Moreover, the kinetic study (Fig. 4b) was carried out, and the degradation rate of AO7 removal fitted well with a pseudo-first-order reaction model for each condition. The rate constants were 1.003, 0.1995, 0.0144, and 0.0049 min−1 for Cpy B, the supernatant of Cpy B, Cpy A, and the supernatant of Cpy A, respectively, indicating a much higher Fenton efficiency of Cpy B compared with that of Cpy A. Further, this shows that the heterogeneous process played a key role in accelerating the degradation of organic waste (approximately 15 min were saved). Therefore, it is necessary to study the heterogeneous process during the reaction, although the heterogenous process only contributes to approximately 20% of removal rate.

Fig. 5. XRD patterns of raw chalcopyrite and used chalcopyrite.

dissolution result. Zhao et al. also (Zhao et al., 2017) indicated that ptype chalcopyrite was much easier to dissolve and to release more copper ions in solution. Because there is no obvious difference in the XRD results except the intensity, FESEM images were obtained to further discuss interactions at the interface. As shown in Fig.6, it can be obviously seen that the surface of Cpy A (a and c) became much rougher (like surface cracks, yellow color in picture) after use, indicating the serious corrosion occurred. For Cpy B (b and d), however, no distinct difference was founded in the overall structure of sample before and after action. Only partial structure of the sample was worn out and some raised particle was formed onto the surface after reaction. The result further verified the high stability of Cpy B, although metal ions were leached into the solution after being reacted in both systems. To determine the elements in the raised particle, an EDS analysis of two samples before and after use was conducted; the results are shown in Table 1. Levels of Cu in Cpy A decreased dramatically from 26.68 wt % to 12.84 wt% after use, indicating that a large amount of copper dissolved into the solution; this agreed with the metal leaching results. Levels of Fe and S in fresh Cpy A decreased from 21.98 wt% and 19.17 wt% to become 18.15 wt% and 12.66 wt% after use, respectively. Interestingly, the amounts of C and N were 16.07 wt% and 0.00 wt% before use; they then increased to 33.61 wt% and 0.09 wt% after use; this result demonstrated that a nitrogenous substance was formed on the surface of Cpy A. Cu and S in Cpy B, however, changed slightly from 29.03 wt% and 24.44 wt% to 24.45 wt% and 20.24 wt% after use, respectively, indicating that a lesser amount of Cu was leached. Because the Fe concentration in Cpy B was 2–4 times higher than that in Cpy A according to the metal leaching analysis, the decrease in Fe in Cpy B should have corresponded to the decrease in Fe in Cpy A. However, the decrease in Fe of Cpy B obtained from the EDS result was equal to that of Cpy A; this indirectly proved that Fe was absorbed onto the Cpy B surface, causing the formation of the raised particle. This result agrees well with the metal leaching analysis. The amounts of C and N in fresh Cpy B were 10.12 wt% and 0.00 wt% and those in the used sample were

3.4. Variations in the physicochemical properties of chalcopyrite before and after use To illustrate the role of the heterogenous process in the chalcopyrite/H2O2 system, a series of modern characterization techniques such as XRD patterns, FESEM images, XPS analysis, and FT-IR were used. The samples used here were of high purity (higher than 95%) without any obvious diffraction peaks of impurities (except the existence of less amount of SiO2, can be ignored). Fig. 5 shows the crystal structure of the chalcopyrite powder determined by XRD spectrum, which indicates a series of peaks indexed to (112), (204), and (312) planes of standard CuFeS2 structure (No. chalcopyrite #83-0983, space group I-42d). Although chalcopyrite was the main component of the residual solid sample before and after the reaction in all cases, the intensity of utilized Cpy A sample was clearly lower than that of the other samples. This result illustrates that Cpy A (p-type chalcopyrite) was considerably more unstable in solution, thus leading to a number of metal leaching (copper ions), which was in accordance with the 5

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Fig. 6. (a) FESEM images of (a: Cpy A; b: Cpy B) fresh CuFeS2; (c: Cpy A; d: Cpy B) used CuFeS2.

942.1, ˜935.0, and ˜932.0 eV, which can be assigned to the characteristic Cu2+ shake-up satellite peak, Cu2+ and Cu+ species, respectively, on the basis of the respective binding energies (Liu and Hensen, 2013; Fu et al., 2013; Zhang et al., 2012). Because the binding energy for reduced copper species like Cu0 and Cu+ are similar enough that it is very difficult to identify the two species simply by XPS alone, Auger Cu LMM spectra was used to confirm the presence of Cu+ (Platzman et al., 2008; Fierro, 1992). The auger parameters at ˜571.0 and ˜568.7 eV confirm the existence of Cu2+ and Cu+, respectively. The Cu2+ shakeup satellite peak appeared at a Cpy A raw sample, indicating the presence of Cu2+ peak on the surface, that is, higher Cu2+ and lower Cu+ concentration could be obtained after being washed. In the other samples, the Cu2+ shake-up satellite peak seemed to be disappeared. As we known, the disappearance of the Cu2+ shake-up satellite peak, which is viewed as a fingerprint of Cu2+, indicated the presence of either metallic Cu0 or Cu+ (Lee and Yong, 2012). However, the Cu2+ peak can be divided and fitted very well. The reason for this may be the peak of the Cu2+ shake-up satellite was so weak that it cannot be detected (Cu LMM confirmed). The pattern for the ratio of Cu+ and Cu2+ is consistent with that for Fe2+ and Fe3+, so there is no repetition here. Fig. 7(e and f) shows the N 1s spectrum, which could be deconvoluted into two peaks expect for the utilized Cpy B (one peak). The peak at around 400.0 eV that occurred in all cases was attributed to the pyrrolic-N (Li et al., 2019). The peaks at a binding energy of 401.7 and 401.8 eV in two raw chalcopyrite samples resulted from the oxidizing-N (Li et al., 2018b) that could be regarded as the background, so its role can be ignored. Interestingly, the peak at 400.8 eV only appeared in the utilized chalcopyrite, which was assigned to the C-N-Cu (Crist, 2000).

25.26 wt% and 0.00 wt%, respectively. No N was detected. This result shows that the degradation of AO7 was complete; however, its mineralization was not. Based on above analysis, we inferred that AO7 could be absorbed onto the surface of chalcopyrite; metal ions such as Cu2+, Fe2+, and Cu+ could be adsorbed. Therefore, the zeta potential and FT-IR analysis (Fig. S4) of chalcopyrite samples before and after reaction with AO7 were carried out, and the results proved that AO7 adhered onto the surface of the catalyst through chemical absorption. Once catalysts come in contact with organic pollutants, the degradation efficiency increases rapidly; this is because the process can reduce resistance to mass transfer. Thus, it is reasonable to employ a heterogeneous process to reduce the time taken to remove AO7. XPS measurements were carried out to determine the elemental composition and chemical bonding configurations of the two chalcopyrite samples before and after use. In Fe 2p XPS spectra (Fig. 7(a and b)), according to the previous studies (Wu et al., 2015; Lin et al., 2014; Gupta et al., 2009), the peaks at 727.4 and 724.1 eV can be matched to the binding energies of the 2p1/2 orbitals of Fe3+ and Fe2+ species, respectively. For the 2p3/2 band, the peak at 715.0 eV is attributed to the binding energy of Fe3+, and the peak for Fe2+ is obtained at 711.3 eV. The ratio of Fe2+ and Fe3+ decreased from 1.25:1 to 1.19:1 when Cpy A was used as a catalyst to decolor AO7, indicating that Fe (II) of the sample was released into the solution, and only a small amount was adsorbed onto the surface. For Cpy B, however, it increased from 1.07:1 to 2.74:1 after utilization, which showed a large amount of Fe2+ from the solution could adhere to the surface of chalcopyrite. The Cu 2p3/2 spectrum (Fig. 7(c and d)) can be divided into 2–3 peaks at Table 1 EDS result of raw CuFeS2 and utilized CuFeS2. Element Cpy A Cpy B

Raw Used Raw Used

Cu (wt%)

Fe (wt%)

S (wt%)

C (wt%)

N (wt%)

O (wt%)

Total

26.68 12.84 29.03 24.55

21.98 18.15 26.66 22.83

19.17 12.66 24.44 20.24

16.07 33.61 10.12 25.26

0.00 0.09 0.00 0.00

16.09 22.64 9.75 7.13

100.0 100.0 100.0 100.0

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Fig. 7. XPS spectra of the Cpy A and Cpy B catalysts before and after use. (a), (c), (e): Fe 2p, Cu 2p3/2, and N 1s XPS spectra for Cpy A, respectively; (b), (d), (f): Fe 2p, Cu 2p3/2, and N 1s XPS spectra for Cpy B, respectively.

thereby favoring the formation of aqueous Fe (II) and Cu (I) via Eqs. (5), (9) and (12), and the reactions between H2O2, Fe2+ and Cu+ in solution generate large amounts of ·OH radicals. It should be noted that here, the signal intensity of Cpy B/H2O2 was obviously stronger than that of Cpy A/H2O2, indicating that massive quantities of ·OH radicals were generated in the former system. Therefore, we conclude that the main active species was the free radical ·OH. To further verify the effect of ·OH radicals, methanol (0.10 and 1.0 mol/L) was added into the above systems at different concentrations. As shown in Fig. 8b, the presence of 0.10 and 1.0 M methanol in both systems effectively inhibited the decolorization of AO7, although there was no significant difference in degradation effectiveness between them. Logically, the free radical ·OH participated in the process of AO7 degradation by chalcopyrite/H2O2. All these results demonstrated that ·OH radicals play a key role in the chalcopyrite/H2O2 system.

The result indicates that certain reactions occurred between AO7 and Cpy A and a new chemical key (C-N-Cu) formed during the Cpy A/H2O2 process, which was consistent with the result from the EDS analysis that found the N element on the surface. 3.5. Identification of radicals Generally, free radical ·OH is the main reactive species for the mineralization of organic pollutants in the Fenton reaction, although it remains controversial whether ferryl is involved (Yang et al., 2018). Here DMPO trapped ESR spectrum was used to identify the active species. The quenching experiments using methanol as a scavenger on the AO7 decolorization were designed to further verify the active species. The ESR spectrum and quenching experiments are depicted in Fig. 8. Fig. 8a shows the ESR spectrum of pure water (blank), hydrogen peroxide, Cpy A, Cpy B, Cpy A/H2O2 and Cpy B/H2O2. A weak free radical ·OH signal was observed in both Cpy A and Cpy B systems. This arises from the fact that the reactions between chalcopyrite (Cu+ and Fe2+), water, and dissolved oxygen would produce a small amount of hydrogen peroxide, and then form the free radical ·OH (Kaur and Schoonen, 2017). With the addition of H2O2, the signal intensity of the free radical ·OH increased promptly in the above systems. The reaction between chalcopyrite and H2O2 produces aqueous Fe (III) and Cu (II),

3.6. Proposed catalytic mechanism Based on the comparative study between two chalcopyrite samples, we found that the existence of copper ions might negatively affect the removal of AO7, especially at high Cu2+ concentrations (such as Cpy A). Therefore, we proposed that the large amounts of Cu2+ in solution would interact with AO7 molecules in processes containing Fe2+/H2O2, and then generate some intermediate products (Azo metal-complexed 7

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Fig. 8. (a) ESR spectra; (b) the effect of methanol as a scavenger on the degradation efficiency of AO7 (Conditions: AO7 = 100 mg/L; H2O2 =40 mM; Cpy A =2.0 g/ L, Cpy B =2.0 g/L, natural pH = 5.94; methanol = 0.10 and 1.0 M).

Fig. 9. The mechanism of n- and p-type CuFeS2/H2O2.

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Acknowledgements

dyes) that could make the color of solution change. To directly prove the above hypothesis, a series of control experiments was carried out, and their results are shown in Fig. S5. Note that the reaction time of all the experiments used here was 5 min, unless stated otherwise. All processes using Cu2+ alone, Fe2+ alone, Cu2+/ Fe2+ and Cu2+/H2O2 to decolor AO7 did not change the colors of their respective solution. As anticipated, in the Fe2+/H2O2 system, the solution became yellowish, nearly colorless. But with the addition of Cu (II), the color did change obviously. Interestingly, the color became much darker as the Cu2+ concentration increased (0–600 mg/L). These results directly prove our hypothesis that Cu2+ could react with some types of chemical bonds like -N = N- to form Azo metal-complexed dyes when the solution contains both Fe2+ and H2O2. This is why the N element was found only on the surface of Cpy A (EDS result) and formed new chemical key (N 1s XPS spectra) that may be induced by the azo metal-complexed dyes. Therefore, the plausible mechanism of n- and p-type CuFeS2/H2O2 for AO7 degradation can be summarized as shown in Fig. 9. First, chalcopyrite released metal ions after being washed. Second, the formed metal ions decompose the H2O2 to produce free radical ·OH. The released Cu (I), Cu (II) and Fe (II) formed n-type chalcopyrite (Cpy B), were absorbed onto the chalcopyrite surface, and then ran into the solution. Both of them would activate H2O2 to form large amounts of ·OH radicals, and then remove the AO7. For the another one (Cpy A), however, the released Cu (II) and Fe (II) adhered onto the surface of chalcopyrite, and then were carried into the solution. Only one of them could effectively activate H2O2 to form small amounts of ·OH radicals, and to react with Cu2+ ions to generate an intermediate product (azometal complex dyes) that would negatively affect the removal of AO7. Releasing copper ions into the water body would result in serious harm to the ecological environment and result in wastage of copper resources; fortunately however many methods to recycle and reuse the ions have been developed in recent years (Wang et al., 2018; Yue et al., 2017; Li et al., 2017). For example, Fang et al (Fang et al., 2019) reported that a green approach was obtained to recycle copper ions by preparing CuO nanosheets containing Cu (II) wastewater. Therefore, chalcopyrite is still a great potential catalyst to activate H2O2, although copper ions are leached into the solution.

This work was supported by the National Natural Science Foundation of China (Nos. 21577041 and 41673090). And thanks professor Hongbo Zhao (Central South University), professor Jun Wang (Central South University), professor Chunhua Feng (South China University of Technology), and Xiaotao Huang’s girlfriend Murphy Zheng (Hunan University). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.120998. References Barreiro, J.C., Capelato, M.D., Martin-Neto, L., Hansen, H.C.B., 2007. Oxidative decomposition of atrazine by a Fenton-like reaction in a H2O2/ferrihydrite system. Water Res. 41, 55–62. Zhao, L., Lin, Z.R., Ma, X., Dong, Y.H., 2018. Catalytic activity of different iron oxides: insight from pollutant degradation and hydroxyl radical formation in heterogeneous Fenton-like systems. Chem. Eng. J. 352, 343–351. Pouran, S.R., Raman, A.A.A., Daud, W.M.A.W., 2014. 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4. Conclusions In this study, we report a systematic comparative study of the use of n- and p-type chalcopyrite as Fenton catalysts for AO7 degradation. The results showed that Cpy A belongs to p-type chalcopyrite with a positive charge carrier concentration, and Cpy B is an n-type chalcopyrite whose charge carrier concentration is negative. The removal experiments indicated that 100% of AO7 was degraded by the Cpy B Fenton system within 5 min, whereas only 60% was degraded by the Cpy A Fenton system. The degradation of AO7 was significantly enhanced by the formation of greater quantities of ·OH free radicals in the Cpy B/H2O2 system, for less Cu2+ and a greater amount of Cu+ and Fe2+ released from Cpy B could easily activate H2O2. However, Cpy A was the main component that released Cu2+ ions and low amounts of Fe2+ and Cu+ ions after being washed. Interestingly, the excess copper ions could react with AO7 and H2O2 to generate an intermediate product (azometal complex dyes) that would negatively affect AO7 removal. Finally, the relative contribution of the homogeneous vs. heterogeneous process was determined. Although only about 20% of the contribution for AO7 degradation was provided by heterogeneous processes in both systems, the effectiveness for AO7 removal could be obviously improved (nearly 10 times), which was ascribed to the adsorptive action and proved by XRD, XPS, and SEM. Releasing copper ions would cause serious harm to the ecological environment; fortunately however, in recent years, many methods have been successfully developed to recycle and reuse them.

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