Degradation of 4-chlorophenol by mixed Fe⁰/Fe₃O₄ nanoparticles: from the perspective of mechanisms

Chlorophenols are important chemical materials with wide application including biocides, disinfectants, dyes, preservatives and pesticides. They occur as pollutants and industrial waste in the environment, and are widely detected in water and soil (Gao et al. 2008). Most chlorophenols are listed as priority pollutants by the United States Environmental Protection Agency due to their toxicity and carcinogenicity (Hwang et al. 2015). Moreover, they can persist in the environment for a long time, and resist chemical and biological degradation (Wang & Qian 1999). As a result, the exposure to chlorophenols in the environment would do harm to human health by bioaccumulation and pose a series of environmental problems. Therefore, necessary measures must be taken to remove chlorophenols from the environment.

Zero-valent iron (Fe⁰) has been applied for recalcitrant chemicals treatment including pesticides, nitro-aromatic compounds, chlorinated solvents, azo dyes and chloric organic pollutants (Dror et al. 2005; Liu et al. 2015). However, surface passivation of iron would seriously decrease the activity of iron and further decrease pollutant removal efficiency (Carniato et al. 2012). Due to large specific surface area and high surface reactivity, Fe⁰ nanoparticles have been studied increasingly for pollution abatement. In previous studies, degradation performance (Cheng et al. 2007; Kim et al. 2010), kinetics (Yang & Lee 2005), mechanisms (Shih et al. 2011; Chen et al. 2012; Kaifas et al. 2014), and reaction conditions (Tian et al. 2009) for Fe⁰ nanoparticles removing chlorinated organics were studied. However, regarding the mechanisms for pollutant degradation, studies have mainly focused on the reductive dechlorination by Fe⁰, and the dynamic process has not been clarified completely. It has been reported that some organics could be degraded during the oxidation process of Fe⁰ by O₂, and hydrogen peroxide was supposed to be produced (Joo et al. 2005; Keenan & Sedlak 2008). Moreover, Fe₃O₄, the main oxidation product of Fe⁰ nanoparticles (Kumar et al. 2014a; Cheng et al. 2015), could catalyze the oxidation of 4-chlorophenol (4-CP) with hydrogen peroxide (Cheng et al. 2015). However, the study of the role of Fe₃O₄ in the nanosized Fe⁰/pollutants system is limited to date.

In this study, Fe₃O₄, the main oxidation product of Fe⁰ nanoparticles, was used to treat the pollutant combining with Fe⁰ nanoparticles; and 4-CP, a representative chlorophenol which is highly toxic and recalcitrant towards chemical and biological degradation in the environment, was used as the model compound. The removal of 4-CP by Fe⁰ nanoparticles and mixed Fe⁰/Fe₃O₄ nanoparticles was studied, and the transformation of nanoparticles was also studied. Moreover, the mechanisms of 4-CP removal were analyzed. The results would analyze the role of Fe₃O₄ and provide a complement for the current pollutant degradation mechanism with Fe⁰ nanoparticles.

Chemicals used in the experiments were of reagent grade. 4-CP was supplied by Tianjin Jinke Fine Chemical Industry Research Institute. Ferrous sulfate (FeSO₄·7H₂O) was provided by Shenyang Reagent Factory. Ferric sulfate (Fe₂(SO₄)₃) was provided by Nankai Fine Chemical Factory. Hydrogen peroxide, sodium borohydride, sodium hydroxide, sulfuric acid, methanol and ethanol were purchased from Beijing Chemical Factory. Argon gas was purchased from Beijing Aolin Gas Company.

The synthesized particles were washed three times with degassed ultrapure water, and then dried in a vacuum dryer.

Morphology of the synthesized Fe⁰ nanoparticles and Fe₃O₄ nanoparticles was observed using a transmission electron microscope (TEM, Hitachi H-7650B) and a scanning electron microscope (SEM, JSM-6301F). The crystal structure and transformation products of nanoparticles were characterized using X-ray powder diffraction (XRD, D8-advance) on a Rigaku D/max-RB X-ray diffractometer with Cu Kα radiation (λ = 0.1542 nm).

The TEM and SEM results showed that the two samples were relatively uniform. The average particle size of Fe⁰ nanoparticles was 50 nm, and the average particle size of Fe₃O₄ nanoparticles was 30 nm. Peaks identified on the two XRD patterns of synthesized particles were Fe⁰ and Fe₃O₄, respectively. No other peaks were detected in the two XRD patterns, indicating the high purity of the samples.

In addition, a microscopic confocal Raman spectrometer (RM2000, Renishaw) was used to investigate the transformation of nanoparticles under anoxic conditions and aerobic conditions.

In the test of 4-CP removal by Fe⁰ nanoparticles and mixed Fe⁰/Fe₃O₄ nanoparticles, experiments were conducted in 20 mL flasks containing 15 mL solutions of 4-CP. The initial concentration of 4-CP was 20 mg L⁻¹. Fe⁰ nanoparticles (20 mg) and mixed Fe⁰ (20 mg)/Fe₃O₄ (20 mg) nanoparticles were added to the Fe⁰ nanoparticles system and mixed Fe⁰/Fe₃O₄ nanoparticles system, respectively. The initial pH value of the systems was not adjusted. In order to keep the reactions under anoxic condition, the flasks were sealed completely and placed on a rotary shaker (150 rpm, 30 °C). Samples were withdrawn from various test groups at predetermined time intervals and then filtered by 0.22 μm membranes before analysis.

In the test of 4-CP removal by mixed Fe⁰/reused Fe₃O₄ nanoparticles, experiments were conducted in 50 mL flasks containing 15 mL solutions of 4-CP (20 mg L⁻¹). The reused Fe₃O₄ nanoparticles were the solid residues from the reaction of the 4-CP/Fe₃O₄/H₂O₂ system. In the 4-CP/Fe₃O₄/H₂O₂ system, Fe₃O₄ nanoparticles (20 mg) and hydrogen peroxide (1.0 ‰) were added to the 50 mL flask with 15 mL solutions of 4-CP (20 mg L⁻¹), and the flask was sealed with the sealing film for sterile culture vessel and placed on a rotary shaker (150 rpm, 30 °C). After 30 h, the flask was taken out of the rotary shaker and the solutions were withdrawn completely. The solid residues were the reused Fe₃O₄ nanoparticles. After this, new 4-CP solutions (20 mg L⁻¹) and Fe⁰ nanoparticles (20 mg) were added into the flask containing the reused Fe₃O₄ nanoparticles. The initial pH value of the system was not adjusted. The flask with mixed Fe⁰/reused Fe₃O₄ nanoparticles was sealed with sealing film to create a sterile culture vessel and the systems were under aerobic conditions. The reaction conditions were the same as mentioned above (150 rpm, 30 °C). Samples were withdrawn from various test groups at predetermined time intervals and then filtered by 0.22 μm membranes before analysis.

4-CP and its byproducts were quantified using a high-performance liquid chromatograph (HPLC) (Agilent 1100, Shanghai Agilent Ltd, China). The HPLC was equipped with an L-4000 UV-visible detector and a C¹⁸ column. The mobile phase for 4-CP consisted of 60% methanol and 40% distilled water. The detection wavelength was 280 nm for 4-CP and the flow rate was 1 mL min⁻¹. Chlorine ions were quantified using a DX-100 ion chromatograph (DIONEX Company, Germany). Eluent was Na₂CO₃ (3.5 mM) and NaHCO₃ (1.0 mM), and eluent flow was 1.2 mL min⁻¹.

Comparing the removal efficiency (Figure 1) with the dechlorination efficiency of 4-CP (Figure 2) by Fe⁰ and mixed Fe⁰/Fe₃O₄ nanoparticles, clearly, the removal efficiency of 4-CP was higher than the dechlorination efficiency in the two systems. Moreover, the difference between the removal efficiency and the dechlorination efficiency in mixed Fe⁰/Fe₃O₄ nanoparticles was less than that of Fe⁰ nanoparticles within 6 h. It illustrated that the proportion of the dechlorination in the 4-CP removal was increased by the addition of Fe₃O₄ nanoparticles. In other words, the catalytic oxidation of 4-CP with Fe₃O₄ nanoparticles contributed to the dechlorination. The difference between the removal efficiency and the dechlorination efficiency of 4-CP gradually increased with the increase of reaction time. It illustrated that the catalytic oxidation of 4-CP with Fe₃O₄ nanoparticles was gradually weakened, and the dechlorination was mainly attributed to the reducing action of Fe⁰ nanoparticles in the later stage of the reaction.

Moreover, the initial pH value was about 7.0, and pH values were decreased to about 5.0 and 6.0 after 20 h reaction in the Fe⁰ and mixed Fe⁰/Fe₃O₄ nanoparticles system, respectively. Opposite results were reported by other studies (Kumar et al. 2013, 2014b). It could be due to the different experimental systems and conditions. Formic acid and acetic acid were also detected in the Fe⁰ and mixed Fe⁰/Fe₃O₄ nanoparticles systems. It illustrated that 4-CP could convert into the acids with low molecular weight as the reaction proceeded, which resulted in the decrease of pH value. However, the reducing action of Fe⁰ nanoparticles did not produce carboxylic acids. Thus, the catalytic oxidation action of Fe₃O₄ nanoparticles to remove 4-CP did take place. In the Fe⁰ nanoparticles system, Fe⁰ nanoparticles could convert into Fe₃O₄ (Cheng et al. 2010). And acids with low molecular weight could also be produced by catalytic oxidation of 4-CP.

Comparing to the mixed Fe⁰/Fe₃O₄ nanoparticles systems, both the removal efficiency and the dechlorination efficiency of 4-CP were higher in the mixed Fe⁰/reused Fe₃O₄ nanoparticles system. On the one hand, hydrogen peroxide generated by Fe⁰ nanoparticles and oxygen in solution under aerobic conditions was higher than that in anoxic conditions. And then the catalytic oxidation of 4-CP with Fe₃O₄ nanoparticles could be improved. On the other hand, Fe₃O₄ nanoparticles were eroded and aggregated, and chain and flower structures were developed with the increase of reaction time, i.e. rough surfaces were formed. The erosion points were the sources of the surface active sites of Fe₃O₄ nanoparticles (Cheng et al. 2015). In addition, some surface defects of nanoscale iron could form, which could be used as active sites in the process of dechlorination (Gotpagar et al. 1999). Thus, surface active sites of Fe₃O₄ nanoparticles could increase after they were used. As a result, the removal efficiency and dechlorination efficiency of 4-CP were improved by the mixed Fe⁰/reused Fe₃O₄ nanoparticles.

In addition, the peak at 380 cm⁻¹ could be produced by the molecular vibration of 4-CP adsorbed on the surface of nanoparticles (Cheng et al. 2015). The peak at 250 cm⁻¹ indicated the bending vibration produced by the aliphatic chain. Some acids with low molecular weight were produced in the system, and then they were adsorbed on the surface of nanoparticles, which could produce the vibration peaks. The peak at 514 cm⁻¹ might be produced by the stretching vibration of C-Cl, and the peak at 1,294 cm⁻¹ might be produced by the stretching vibration of C-C.

4-CP could be catalytically oxidized by Fe₃O₄ nanoparticles in the presence of hydrogen peroxide (Cheng et al. 2015). In the mixed Fe⁰/Fe₃O₄ nanoparticles system under anoxic conditions, hydrogen peroxide was very limited due to the lack of oxygen. As a result, catalytic oxidation action of Fe₃O₄ nanoparticles only played a limited role for the 4-CP removal, and the removal of 4-CP was mainly attributed to the reducing action of Fe⁰. However, in the mixed Fe/Fe₃O₄ nanoparticles system under aerobic conditions, hydrogen peroxide was relatively high due to adequate oxygen. As a result, more 4-CP was catalytically oxidized by Fe₃O₄ nanoparticles. Meanwhile, the reducing action of Fe⁰ would be affected due to the oxidation of Fe⁰ nanoparticles.

Under anoxic conditions, the removal efficiency of 4-CP was lower in the system with mixed Fe⁰/Fe₃O₄ nanoparticles than that with Fe⁰ nanoparticles alone, which suggested that the addition of Fe₃O₄ nanoparticles hindered the removal of 4-CP by Fe⁰ nanoparticles. However, the dechlorination efficiency of 4-CP was improved in the initial 6 h when Fe₃O₄ nanoparticles were added, which indicated that the catalytic oxidation of 4-CP with Fe₃O₄ nanoparticles contributed to the dechlorination. Under aerobic conditions, the reused Fe₃O₄ nanoparticles would improve the removal and dechlorination of 4-CP, and the dechlorination efficiency of 4-CP was increased obviously. Fe⁰ nanoparticles were transformed to Fe₃O₄ and FeOOH during the process, and Raman spectra reflected that 4-CP, the acids with low molecular weight (products of 4-CP) and some other chlorinated byproducts could be adsorbed on the surface of Fe₃O₄ nanoparticles. It is proposed that the removal of 4-CP was due to the effects of both Fe⁰ and Fe₃O₄ nanoparticles – reducing action of Fe⁰ and catalytic oxidation action of Fe₃O₄. The reducing action of Fe⁰ was the major factor under anoxic conditions. And the catalytic oxidation action of Fe₃O₄ became an important reason that could not be ignored under aerobic conditions.

The research was supported by the National Natural Science Foundation (Grant No. 51478460, 51108454), which is greatly acknowledged.