A highly efficient advanced oxidation process for the degradation of benzoic acid (BA) during activation of peroxomonosulfate (PMS) by nanoscale zero-valent copper (nZVC) in acidic solution is reported. BA degradation was almost completely achieved after 10 min in the nZVC/PMS process at initial pH 3.0. PMS could accelerate the corrosion of nZVC in acidic to release Cu+ which can further activate PMS to produce reactive radicals. Both sulfate radical (SO4−•) and hydroxyl radical (•OH) were considered as the primary reactive oxidant in the nZVC/PMS process with the experiments of methyl (MA) and tert-butyl alcohol quenching. Acidic condition (initial pH ≤ 3.0) facilitated BA degradation and pH is a decisive factor to affect the oxidation capacity in the nZVC/PMS process. Moreover, BA degradation in the nZVC/PMS process followed the pseudo-first-order kinetics, and BA degradation efficiency increased with the increase of the nZVC dosage.
Increasing attention has been paid to advanced oxidation processes (AOPs) resulting from the generation of reactive radicals (e.g. SO4−• and •OH) (Duan et al. 2016; Peluffo et al. 2016) and its high efficiency on decomposing persistent organic pollutants. Among these AOPs, hydrogen peroxide (H2O2) (Chen et al. 2011), persulfate (PS) (Al-Shamsi & Thomson 2013), and peroxomonosulfate (PMS) (Zhou et al. 2015) are considered as inexpensive oxidants for the remediation of contaminated water resulting from these three common peroxides, precursors of SO4−• and •OH. Recently, PMS has gained more attention from investigators because it is easily activated by transition metals, heat, ultraviolet, and ultrasound (Anipsitakis & Dionysiou 2004; Guan et al. 2011).
The aim of this study is to investigate the degradation efficiency of BA in the nZVC/PMS process and to specifically focus on the mechanism of the nZVC/PMS process, identification of primary reactive oxidants, effect of initial pH, and effect of nZVC dosage.
MATERIALS AND METHODS
nZVC (size: 10–30 nm, >99.9%) and neocuproine hemihydrate (NCP, >98%) were of analytic purity and purchased from Aladdin Industrial Corporation. Oxone (KHSO5·0.5KHSO4·0.5K2SO4, PMS), copper sulfate pentahydrate (≥99.0%), and BA (≥99.5%) were of American Chemical Society (ACS) reagent grade and supplied by Sigma-Aldrich, Inc. Sodium sulfite, tert-butyl alcohol (TBA), sulfuric acid, and sodium hydroxide were of analytic purity and purchased from Sinopharm Chemical Reagent Co. Ltd, China. Methyl alcohol (MA) and ammonium acetate, which were purchased from Sigma-Aldrich, were of high performance liquid chromatography (HPLC) grade. Pure oxygen (O2, ≥99.2%) and pure nitrogen (N2, ≥99.99%) were stored in the special high-pressure gas cylinder.
Experiments were performed in 500 mL under constant stirring with a polytetrafluoroethylene (PTFE)-coated magnetic stirrer at 10 ± 0.5 °C. Each 500 mL reaction solution with desired concentration of BA was prepared with ultrapure water and adjusted to desired initial pH with sulfuric acid and sodium hydroxide, and each run was switched on by adding the desired dosage of PMS and nZVC. Most of the experiments were operated open to the air. However, in order to investigate the effect of O2 on the process, part of experiments were aerobic or anaerobic aqueous solutions through constant feeding of pure O2 or N2. Desired TBA and MA were added into the reaction solution before the addition of PMS to identify the primary reactive radicals. Moreover, NCP was used as Cu(I)-chelating to investigate the role of Cu+ in the process. Samples were withdrawn at set intervals and quenched by sodium sulfite after filtration with glass fiber membrane of 0.45 μm pore size.
The concentration of BA was analyzed on HPLC (Waters e2695), equipped with 2489λ absorbance detector (227 nm for BA) and reverse-phase C18 column (4.6 × 150 mm). The pH in aqueous solution was monitored by pH meter (Shanghai Leici Apparatus Fac., China). Total concentration of dissolved copper (TCu) were measured by a PerkinElmer® PinAAcle 900T flame atomic absorption spectrometer (Sheltom, CT, USA) equipped with two hollow multi-element cathode lamps. Moreover, nZVC was characterized before and after reaction with X-ray diffraction (XRD, X'Pert Pro MPD diffractometer (Philips, The Netherlands)) and scanning electron microscopy (SEM, JSM-7500F (JEOL, Japan)).
RESULTS AND DISCUSSION
Degradation of BA in the nZVC/PMS process
Effective copper species to activate PMS
Although Cu+ has not been reported as an activator for PMS, Cu+ can induce the generation of SO4−• and •OH via activating PS and H2O2 (Kolthoff & Woods 1966; Masarwa et al. 1988). Recently, Wen et al. (2014) reported that Cu+ is a key intermediate during the corrosion of ZVC to induce the generation of •OH in the ZVC acidic system. Therefore, Cu+ may be a potential intermediate to activate PMS during corrosion of nZVC in the nZVC/PMS process.
Identification of reactive oxidants
It has been reported that SO4−•, SO5−•, and •OH could be produced for activation of PMS catalyzed by transition metals (Anipsitakis & Dionysiou 2004; Liang & Su 2009). All three reactive radicals may be formed in the nZVC/PMS process as shown in Equations (4)–(7). Resulting from the high rate constants with SO4−• (k = 2.5 × 107 M−1s−1) (Neta et al. 1988) and •OH (k = 9.7 × 108 M−1s−1) (Buxton et al. 1988), MA is an effective quencher for both SO4−• and •OH. Owing to the high rate constant with •OH (k = 6.0 × 108 M−1s−1) (Buxton et al. 1988) and the much slower rate constant with SO4−• (k = 8.0 × 105 M−1s−1) (Neta et al. 1988), TBA is an effective quencher for •OH but not for SO4−•. Simultaneously, SO5−• is relatively inert toward MA and TBA with low rates (Hayon et al. 1972). On the basis of these properties, the quenching experiments with MA could allow us to differentiate the contribution between •OH/SO4−• and SO5−•, and the quenching experiments with TBA could allow us to differentiate the contribution between SO4−• and •OH on BA degradation.
Pathway of nZVC corrosion
Effect of initial pH
Effect of nZVC dosage
This study investigated the activation of PMS by nZVC and set up the nZVC/PMS process to effectively degrade BA in acidic aqueous solution. BA was almost completely degraded after 10 min, and BA degradation fit well with pseudo-first-order kinetics in the nZVC/PMS process at initial pH 3.0. PMS can accelerate the corrosion of nZVC to release Cu+ resulting in further activation of PMS via the Fenton-like reaction to produce reactive radicals, and Cu+ is the main active copper species to activate PMS at acidic solution. Due to the partial or almost complete inhibition of BA degradation with the addition of MA or TBA, both SO4−• and •OH were considered as the primary reactive oxidants in the nZVC/PMS process. Moreover, pH is a decisive factor to affect the oxidation capacity and acidic condition (initial pH ≤ 3.0) facilitated BA degradation in the nZVC/PMS process. BA degradation efficiency increased with the increase of nZVC dosage. In addition, the nZVC/PMS process overcame the drawback of instability of Cu+ and put forward an interesting idea to make full use of intermediate Cu+ to activate active oxidants during the corrosion of nZVC to degrade organic pollutants.
Appreciation and acknowledgment are given to the National Natural Science Foundation of China (No. 51508353) and the National Natural Science Foundation of China (No. 51408349).