- Research Article
- Open Access
Catalytic oxidation of arsenite and reaction pathways on the surface of CuO nanoparticles at a wide range of pHs
- Lingqun Zeng†1,
- Biao Wan†1,
- Rixiang Huang2,
- Yupeng Yan1,
- Xiaoming Wang1,
- Wenfeng Tan1,
- Fan Liu1 and
- Xionghan Feng1Email author
© The Author(s) 2018
Received: 17 January 2018
Accepted: 11 June 2018
Published: 22 June 2018
Recently, the wide application of CuO nanoparticles (NPs) in engineering field inevitably leads to its release into various geologic settings, which has aroused great concern about the geochemical behaviors of CuO NPs due to its high surface reactivity and impact on the fate of co-existing contaminants. However, the redox transformation of pollutants mediated by CuO NPs and the underlying mechanism still remain poorly understood. Here, we studied the interaction of CuO NPs with As(III), and explored the reaction pathways using batch experiments and multiple spectroscopic techniques. The results of in situ quick scanning X-ray absorption spectroscopy (Q-XAS) analysis verified that CuO NPs is capable of catalytically oxidize As(III) under dark conditions efficiently at a wide range of pHs. As(III) was firstly adsorbed on CuO NPs surface and then gradually oxidized to As(V) with dissolved O2 as the terminal electron acceptor. As(III) adsorption increased to the maximum at a pH close to PZC of CuO NPs (~ pH 9.2), and then sharply decreased with increasing pH, while the oxidation capacity monotonically increased with pH. X-ray photoelectron spectroscopy and electron paramagnetic resonance characterization of samples from batch experiments indicated that two pathways may be involved in As(III) catalytic oxidation: (1) direct electron transfer from As(III) to Cu(II), followed by concomitant re-oxidation of the produced Cu(I) by dissolved O2 back to Cu(II) on CuO NPs surface, and (2) As(III) oxidation by reactive oxygen species (ROS) produced from the above Cu(I) oxygenation process. These observations facilitate a better understanding of the surface catalytic property of CuO NPs and its interaction with As(III) and other elements with variable valence in geochemical environments.
Compared with its micro or bulk counterparts, CuO nanoparticles (NPs) possesses better optical, semiconductive and surface reactive properties, and is thus widely used for the production of ceramics, glass and pigments, catalysts, battery anodes, and antimicrobial agents [1–5]. Due to the increasing application of CuO NPs in industrial fields, its release and geochemical behaviors in the environment have aroused great concern [6–9]. Its potential toxicity to the organisms has been studied in detail [10–14]. Apart from the direct biotic effects, CuO NPs might also impact the mobility, transformation and toxicity of the co-existing contaminants through (de)sorption, redox and catalytic reactions [15–17]. However, little is known about the interaction of CuO NPs with redox-sensitive contaminants and the underlying reaction mechanism.
Arsenic (As) is the most common heavy metal in natural waters primarily in the forms of inorganic arsenate [As(V)] and arsenite [As(III)] . Redox processes on the surface of oxide minerals largely determine the speciation of As [19–21]. Compared with As(V), As(III) has higher toxicity, solubility and mobility .
CuO NPs is an excellent adsorbent to remove As(III) from water due to its large specific surface area and high point of zero charge (PZC) [17, 23–25]. In addition, X-ray photoelectron spectroscopy (XPS) analysis suggested that As(III) could be oxidized on CuO NPs surface, which remarkably enhances the immobility of As in the form of As(V) [17, 24]. A previous study has proposed the direct electron transfer from CuO NPs to As(III), which leads to As(III) oxidation . Given that XPS measurement is performed under a high vacuum condition, more solid evidences are needed to verify As(III) oxidation on the surface of CuO NPs.
Furthermore, it has been reported that reactive oxygen species (ROS) is involved in the oxidation pathway of organic matter [26–28]. For example, remarkable amounts of Cu+/Cu2+ and H2O2 were formed in zero-valent copper (ZVC) acidic system due to the corrosive dissolution of ZVC and the concurrent reduction of oxygen, which lead to highly efficient oxidation of diethyl phthalate under aerobic atmosphere condition . In addition, a synergistic effect of Fe(II) and copper oxide (CuO) was observed on the degradation of acetaminophen in the presence of O2, and the resulting Cu(I) significantly accelerated the destruction of acetaminophen by serving as an electron-mediator between the adsorbed Fe(II) and O2 to produce ROS . Therefore, it is possible that ROS might be produced through the activation of O2 on the surface of CuO NPs, which leads to the catalytic oxidation of As(III).
Thus, the objectives of this study are to (i) in situ confirm whether As(III) oxidation occurs on the surface of CuO NPs under dark condition; (ii) determine the effects of geochemical parameters [such as pH, As(III) concentration and O2] on As(III) adsorption and oxidation by CuO NPs; (iii) elucidate the adsorption and oxidation pathways of As(III) on the surface of CuO NPs at a wide range of pH values under dark condition. To achieve these objectives, batch experiments and spectroscopic analysis were performed. Time-resolved quick scanning X-ray absorption spectroscopy (Q-XAS) and electron paramagnetic resonance (EPR) spectroscopy were used to in situ measure the species of As and ROS, respectively. XPS spectroscopy was also used to determine the changes in oxidation state of Cu and As on the surface of CuO NPs after the reaction.
Preparation and characterization of CuO NPs
CuO NPs was prepared via a previously reported method [5, 29]. The synthesized CuO NPs contained no impurity phases as examined by powder X-ray diffraction (PXRD), Fourier-transformed infrared (FTIR) spectroscopy and transmission electron microscopy (TEM) analyses. The specific surface area was determined by Brunauer–Emmett–Teller (BET) N2 adsorption method. Additional details of analytical procedures and characterization results are provided in Additional file 1: Figures S1, S2, S5.
As(III) adsorption and oxidation kinetics
The adsorption-oxidation experiments were conducted by the reaction of 0.2 g CuO NPs with 200 mL 10 mg L−1 As(III) at pH 6, 9, and 11, respectively. The background electrolyte was 0.01 M NaCl solution. The CuO NPs suspensions were agitated by magnetic stirring at 10 Hz. Solution pH was measured using a pH meter (FE20, Mettler-Toledo) and manually adjusted to desired pH values ± 0.1 using 0.1 M HCl and 0.1 M NaOH. To examine the effect of dissolved oxygen (O2) on the oxidation of As(III), the reaction solution was purged by N2 before and during the reaction. At the selected reaction time, 5 mL suspension was filtered through 0.22 μm Millipore membrane to analyze the concentrations of As(III) and As(V) in the supernatant. The wet solids on the membrane were dissolved by 1 mL 1 M HCl to analyze the amount of As(III) and As(V) adsorbed onto CuO NPs surface. Another 5 mL suspension was directly dissolved by 1 M HCl (1 mL) to analyze the total amount of As(III) and As(V) in the suspension. All experiments were performed in triplicates. The As(V) concentration was measured by the molybdene blue method . The total As was determined by hydrate generation atomic fluorescence spectrometry (HG-AFS) (AFS-230E) . Additionally, the volume of 0.1 M NaOH consumed in open system and N2 system at pH 11 was recorded by automatic titrator (Metrohm 907 Titrando). All As(III) adsorption and oxidation experiments were carried out in reactors covered with aluminum foil to avoid the effect of light.
Effect of pH and initial As(III) concentration on As(III) adsorption and oxidation
To investigate the effect of pH values on the species distributions of As(III) and As(V) in solution and on CuO NPs surface, As(III) adsorption and oxidation were evaluated at pH 5, 6, 7, 8, 9, 10 and 11 by adjusting to the desired pH values ± 0.1 using 0.1 M HCl and 0.1 M NaOH. The experiments were carried out in 50-mL polyethylene centrifuge tubes by mixing 0.025 g of CuO NPs with 25 mL of 0.01 M NaCl containing fresh 10 mg L−1 As(III). To investigate the effect of initial As(III) concentration on As(III) adsorption and oxidation, the experiments were performed at initial As(III) concentrations ranging from 0.5 to 80 mg L−1 at pH 8 and 11. The reaction suspensions were equilibrated by shaking at 200 rpm and at 25 °C for 48 h. During the reaction, the pH of each batch sample was adjusted to the designed pH ± 0.05 at 1, 6, 12, 24, 36 and 48 h, respectively. After 48 h of reaction, the species distributions of As(III) and As(V) in solution and on CuO NPs surface were analyzed with the same procedures as described in “As(III) adsorption and oxidation kinetics”.
Quick Scanning K-edge X-ray absorption spectroscopy (Q-XAS) of As
Q-XAS was used to in situ investigate the changes in oxidation state of As with increasing reaction time. The Q-XAS spectra were measured at room temperature on the 1W2Bbeamline at the Beijing Synchrotron Radiation Facility(BSRF). Considering the detect limitations (≥ 100 mg L−1 for As) of Lytle prober, higher concentrations of AS(III) (150 mg L−1) and CuO NPs (5 g L−1) were used for the in situ Q-XAS measurement. The reaction was performed in 50-mL polypropylene reaction vessels (see Additional file 1: Scheme S1), into which a 1 × 3 cm slit was cut and sealed with Kapton tape, backed with Kapton film to prevent the interaction between the tape and suspension. The As K-edge XAS spectra was collected immediately when As(III) solution was added into the suspension. The measurement time for each XAS spectrum is 1 min and the total time for the in situ XAS experiment is 8 h. Additional experimental details are described in Additional file 1: S3.
X-ray photoelectron spectroscopic (XPS) analysis
To determine the oxidation state of As and Cu on the surface of CuO NPs, the samples prepared from the reaction of 0.05 g CuO with 50 mL of 10 mg L−1 As(III) for 12 h were measured with XPS using a monochromatic Al Kα X-ray source (VG Multilab 2000 X-ray photoelectron spectrometer). The scans were carried out in an energy range of 1100–5 eV to obtain XPS spectra for C1 s, Cu 2p, and As 3d. The position of binding energy was corrected by fixing the C1 s peak at 284.6 eV using the Advantage v6.5 software.
Electron paramagnetic resonance (EPR) spectroscopy
For the EPR experiment, 50 mL reaction suspension was prepared to contain 1 g L−1 CuO NPs and 10 mg L−1 As(III) at pH 11 over 2 h under stirring. At the selected time, 3 aliquots of 2 mL suspension were sampled for the detection of ROS speciation. The detection methods and procedures for different ROS species are described in Additional file 1: S4.
Results and discussion
As(III) adsorption and oxidation kinetics
Apparent, first-order rate constants determined from batch and Q-XAS experiments
Time period (h)
No. of data points
Effects of pH and initial As(III) concentration on As(III) adsorption and oxidation
The pH-dependent distribution of As species is most likely related to the species transformation of aqueous As(III) and As(V) and the surface charge of CuO NPs. At pH below 9, the main species of aqueous As(III) is H3AsO3, but As(V) mainly exists in the forms of H2AsO4−and HAsO42−. At pH above 9, As(III) exists in the forms of H2AsO3− and HAsO32−, and aqueous As(V) as HAsO42− and AsO43− . As(III) (in form of H3AsO3) is adsorbed on CuO NPs via Van der Waals force at pH 6–9 . The adsorption percentage of As(V) was observed to be obviously higher than that of As(III) under the same initial As concentration at pHs ranging from 6 to 11 . The aqueous As(V) in forms of H2AsO3− and HAsO32− may have a higher affinity than As(III) to the positively charged surface of CuO NPs and tend to form inner-sphere complexes. In this study, a large number of surface reactive sites were occupied by the formed As(V), resulting in the low oxidation rate of As(III) by CuO NPs at low pHs. Due to the strongly negative surface charge of CuO NPs at high pHs, the formed As(V) (mainly in the forms of HAsO42− and AsO43−) is electrostatically repulsed from CuO NPs surface. Furthermore, redox potential of As(V)/As(III) would increase with decreasing pH, indicating that As(III) is more readily oxidized at higher pH. For instance, in acid medium, the standard potential for the half reaction (H3AsO4+H+)/H3AsO3 is 0.56 V; but in alkaline medium, that of AsO43−/(AsO2−+OH−) is − 0.71 V . Therefore, CuO NPs surface remains highly reactive and can continuously oxidize As(III) at pH above the PZC.
Q-XAS test results
EPR analysis for ROS
As(III) oxidation mechanisms
Based on the first sharp increase and the subsequent slow decrease of As(III) on CuO NPs surface (Fig. 1), we propose that As(III) might be firstly adsorbed on the surface of CuO NPs and then slowly oxidized to As(V). Therefore, at pH below PZC, As(III) and the produced As(V) are mainly adsorbed on the surface of CuO NPs; at pH above PZC, the adsorbed As(III) can be rapidly oxidized to As(V) on the surface and then the produced As(V) is abruptly desorbed into the solution from CuO NPs surface, which ensures that an active surface is available for further oxidation of newly adsorbed As(III).
In addition, the Cu(I) in CuO NPs could be responsible for the production of ROS . O2·− could be produced on the reactive site with electron transfer from Cu(I) to O2. However, O2·− was not detected using EPR in our study. It is possible that O2·− was not a stable species under our experimental conditions, and its concentration was too low to be detected. In fact, O2·− is readily transformed to 1O2 via disproportionation reaction , which was detected in CuO NPs suspension at the absence or presence of As(III) (Fig. 6). The standard redox potential of 1O2/H2O is 2.204 eV , indicating that 1O2 is a strong oxidant similar to ·OH, which has a standard redox potential of 2.538 eV (·OH/H2O). Therefore, we suggest the second reaction pathway: As(III) is oxidized by ROS produced via the activation of O2 on Cu(I) sites of the CuO NPs surface (Fig. 7). CuO will be re-generated from Cu(I) after reaction with O2, and participates in the further oxidation of As(III) via the above-mentioned first reaction pathway. Figure 1d shows that dissolved O2 is an essential factor for the high rate of As(III) oxidation by CuO NPs. Formation of 1O2 from O2 in the presence of CuO NPs might also contribute to As(III) oxidation by CuO NPs (Eqs. 3 and 4). The observation of continuous 1O2 production in the experiments (Fig. 6) indicates a close coupling of these two pathways, which can explain the cycling of CuO NPs catalyst and efficient As(III) oxidation. That is to say, the oxidation of As(III) by the re-generated Cu(II) active sites via the first pathway yields Cu(I) sites again, which could trigger the continuous production of 1O2.
In addition, titration experiment was performed to compare the differences in the amount of consumed OH− between the N2 and open systems at pH 11. In the N2 system, only 0.022 mmol OH− was consumed at the very beginning, which might result from the surface hydrolysis of CuO NPs. However, in the open system, OH− consumption gradually increased from 0.022 mmol at 2 h to 0.080 mmol at 12 h as the reaction proceeded (Additional file 1: Fig. S8). These results suggest that H+ production or OH− consumption is associated with As(III) oxidation.
In general, ROS is mainly formed in acidic solution, and Cu2+ cations in solution play an important role in ROS production [26–28]. But the above XPS results indicate that the changes of Cu valence mainly occur at pH > 9, when Cu2+ could not be present in solution (Additional file 1: S4). Therefore, ROS is probably formed via electron transfer from Cu(I) to O2 on the surface of CuO NPs. Even when O2 is absent in the system, direct electron transfer from As(III) to Cu(II) can occur on the surface of CuO NPs. These findings can also facilitate a better understanding about the impact of CuO NPs on the mobility and transformation of some redox-sensitive substances in various geochemical settings.
Our results verify that CuO NPs is capable to catalytically oxidize As(III) efficiently with dissolved O2 as the terminal electron acceptor using in situ spectroscopic techniques (Fig. 4). Therefore, CuO NPs can be a potential catalyst and adsorbent to affect the geochemical behaviors of As, which can help researchers to predict the risk of CuO NPs before the application of it in some industrial and environmental fields. Also, this study provides a new perspective for investigation of As(III) oxidation process related to Cu-based NPs.
It can be indicated that the amount and rate of As(III) oxidation by CuO NPs are greatly enhanced by increasing pH to a high alkaline range (Fig. 3b). It should be noted that the adsorption of produced As(V) decreases to a certain degree in alkaline solution. Thus, other adsorbents with high As(V) retention ability at alkaline pHs could be applied simultaneously to enhance the removal of aqueous As to meet the environmental standard in As contaminated areas. Besides, it will be of great environmental significance to further improve the catalytic oxidation capacity of CuO NPs at around neutral pHs. Actually, our undergoing study has shown that the addition of aqueous Mn(II) could remarkably enhance the oxidation of As(III) and the adsorption of As(V) at near neutral pH (Additional file 1: Fig. S9).
Furthermore, two coupled reaction pathways, i.e. direction oxidation by CuO and oxidation by ROS produced via O2 activation on Cu(I) surface sites, are proposed for As(III) oxidation (Fig. 7). These findings further demonstrate the high catalytic activity of CuO NPs towards the oxidation reactions in water, implying the important role of CuO NPs to affect the fate and geochemical behaviors of some reducing pollutants and redox-sensitive organic substances in the environment. The stability and potential reusability of CuO NPs also make it an ideal candidate to be applied in permeable reactive barrier (PRB) in ground water purification. Actually, some developing countries (e.g., Bangladesh and West Bengal of India ) suffer from heavy As contamination in the groundwater, and usually there is a serious lack of water treatment facilities to purify the As-contaminated groundwater [47, 48]. Systematic studies of the adsorption–oxidation mechanisms of As(III) on CuO NPs surfaces are significant for a full understanding of the potential influence of CuO NPs on the reductive pollutants, and for the further development of reliable techniques to deal with As contamination efficiently.
XF lead project conceptualization and all authors contributed to the research design. LZ and BW collected data, all authors analyzed data and contributed to writing the manuscript. All authors read and approved the final manuscript.
The authors gratefully acknowledge National Natural Science Foundation of China (No. 41471194), the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB15020402) and the Fundamental Research Funds for the Central Universities (No. 2662017PY070) for financial support for this research. We also thank Aiwen Lei from Wuhan University and Jing Zhang and Lirong Zheng from Beijing Synchrotron Radiation Facility for experimental help and Songhu Yuan from China University of Geosciences (Wuhan) for helpful suggestion on paper corrections.
The authors declare that they have no competing interests.
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This work was funded by National Natural Science Foundation of China (Grant Nos 41471194), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant no XDB15020402).
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