Chlorinated phenols are a kind of environmental priority pollutant that attract much attention. Nanosized Fe and Fe/Ni materials are considered as promising options for chlorinated phenol removal. The effect of Ni morphology on the removal of pentachlorophenol (PCP) with Fe/Ni nanomaterials was investigated in this study. Iron nanoparticles and nickel nanomaterials with different shapes were synthesized using a chemical reduction method and wet chemical techniques, respectively. The concentrations of PCP and chloride in solutions were measured with and without Ni present. The intermediates of PCP were also analyzed. The results showed that the dechlorination of PCP was promoted by Ni nanomaterials, among which the tubular porous Ni nanomaterials expressed the most promotion, then those with net shape and nanochains. However, the tubular porous Ni nanomaterials inhibited the removal of PCP, and the other two expressed a certain promotion. In the Fe/Ni system, Fe nanoparticles transformed into magnetite (Fe3O4) and/or maghemite (Fe2O3), and Ni nanomaterials were still pure Ni after reaction. The introduction of Ni nanomaterials would improve dechlorination of PCP, but the removal of PCP might be inhibited or improved as the morphology of Ni changed.
Chlorophenols, which are bioaccumulative, refractory and carcinogenic, are recognized as environmental priority control pollutants (Squillace et al. 1999). Because of extensive bactericidal and insecticidal effects, chlorophenols have been widely used as insecticides, disinfectants and preservatives. In addition, chlorophenols are a class of aromatic compounds with high water solubility, and the adsorption and fixation of chlorophenols in soil is weak, so chlorophenols can spread widely into the environment with water as the carrier (Droste et al. 1998). The effective treatment of chlorophenols is an important issue to be solved.
Because of the special interface, physical and chemical properties, zero-valent iron nanoparticles (ZVI NPs) can transform a variety of environmental pollutants effectively. As a result, ZVI NPs can be used for in situ or ex situ remediation of groundwater pollution. In recent years, ZVI NPs have been extensively studied and applied in the field of environmental remediation and pollution control. Based on the current study, various pollutants including chlorinated organic compounds, heavy metals, nitrate, dyes, radionuclides, phosphate, and sulphion can be removed by ZVI NPs (Cheng et al. 2007; Zhang & Fang 2010; Suzuki et al. 2012; Huang et al. 2013; Nakatsuji et al. 2015). In addition, ZVI NPs can also remove the disinfection byproducts of drinking water including bromate effectively (Lim et al. 2007).
However, ZVI NPs still have some disadvantages. For example, ZVI NPs can be easily oxidized, which will decrease the reactivity of ZVI NPs. The dechlorination process of some chlorinated organic compounds is slow, and other toxic chlorinated byproducts will be produced. Studies have shown that the degradation rate of pollutants can be improved when a second metal (usually an inert metal) is introduced into the ZVI NP system, and some organic pollutants which are refractory in the ZVI NP system can be degraded effectively (Smuleac et al. 2011). Subsequently, bimetal systems have been widely studied to achieve rapid and complete dechlorination including Fe/Ni, Fe/Pd, Fe/Cu, etc. (Zhang et al. 2010; Han & Yan 2014; Lai et al. 2014).
As a widely used catalyst, Ni nanomaterials show high activity and selectivity in reactions such as catalytic hydrogenation, oxidation and cracking (Park et al. 2005; Busca et al. 2014; Zhou et al. 2014). However, few studies have focused on the effect of the morphology of the catalyst (Mahjoub et al. 2012). In this research, pentachlorophenol (PCP) was selected as the model pollutant, and Ni nanomaterials with different morphology were introduced into the ZVI NP system. The effect of Ni morphology on the removal of PCP in the nano-Fe/Ni system was investigated, and the role of nano-Ni in the system was also analyzed.
Preparation and characterization of nanomaterials
Ni nanomaterials were synthesized using polyvinyl pyrrolidone (PVP, K30, Sigma-Aldrich Chemical Co. Ltd) as the template and wet chemical techniques were adopted (Liu et al. 2004). The specific preparation method was as follows: 15 mL NiSO4 (Shanghai Hengxin Chemical Reagent Co. Ltd, AR) solutions of 0.025 mol·L−1 and 30 mL PVP of 0.3 mol·L−1 were dissolved in 60 mL ethylene glycol (EG, Shanghai Chemical Reagent Company, AR), and ultrasonic treatment was then used for 15 min in order to make sure the Ni2+ was evenly dispersed in the solution. Then 0.75 mL of hydrazine hydrate (50%, Tianjin Chemical Reagent Company) was added into the solution until the color of the solution became light purple. The purple solution was heated to the boiling point of EG (197°C), then refluxing for 3 h under intense magnetic stirring. Finally the black precipitate was separated, and washed with distilled water and anhydrous ethanol. The obtained sample was the tubular porous Ni. As for the Ni nanochains, UV radiation was applied for 1 h in the crystal growth process. For Ni with net shape, the refluxing time was 12 h.
The morphologies of the iron and nickel nanomaterials were observed with scanning electron microscopy (SEM, FEI Quanta 200F), and the structure and composition of the samples were determined by X-ray diffraction (XRD, Rigaku Dmax 2200) analysis.
Taking reagent bottles (18 mL) with rubber plugs as reactors, 15 mL PCP solutions (Beijing Changping ShiYing Chemical Factory, CP) were added into each reaction bottle, 15 mg ZVI NPs and 5 mg Ni nanomaterials were put into the solution, then the reagent bottles were sealed with rubber plugs and were put into the table constant-temperature incubator shaker (TZ-2EH, Beijing Ward Company). The initial concentration of the PCP (C0) was 50 mg·L−1, the temperature was 30°C and the rotation speed was 150 r/min. Sampling at different times, the concentrations of residual PCP and released Cl− were analyzed. Based on the initial concentration of PCP, the removal of PCP and relative concentration of released Cl−C/C0 were calculated, then the removal rate of PCP and the dechlorination efficiency were obtained. PCP solutions without ZVI NPs or Ni nanomaterials were used as a control.
PCP was measured with a high performance liquid chromatograph (HPLC, Agilent 1200, Shanghai Agilent Company), equipped with a C18 reversed phase chromatographic column and 1,200 photodiode array detector, and manual sampling. The mobile phase was methanol and ultrapure water with volume ratio V (CH3OH)/V (H2O) = 85/15, the volume flow rate was 0.8 mL·min−1, and the detection wavelength was 320 nm. The intermediates of PCP were analyzed qualitatively by mass spectrometer (PE API 3000 MS), and then were quantitatively analyzed by HPLC. The corresponding detection wavelengths of 2-chlorophenol (2-CP), 3-chlorophenol (3-CP) and 4-chlorophenol (4-CP) were 275 nm, 279 nm and 280 nm, respectively.
Cl− was measured with ion chromatography (DX-100, German DIONEX Company). The eluent consisted of 3.5 mmol·L−1 Na2CO3 and 1.0 mmol·L−1 NaHCO3, the volume flow rate was 1.2 mL·min−1, and manual sampling; the sampling volume was 250 μL, the analysis time was 6 min. Deionized water was taken as blank.
All of the samples were filtered with 0.45 μm microporous membranes.
RESULTS AND DISCUSSION
The structure and morphology of the nanomaterials
All the diffraction peaks of the prepared Ni nanomaterials corresponded to the face-centered cubic structure of pure Ni (JCPDS 04-0850). The apparent morphology of the three kinds of Ni nanomaterials was displayed as Ni nanochains (Figure 1(b)), tubular porous Ni (Figure 1(c)) and Ni with net shape (Figure 1(d)), respectively.
The removal of PCP and Cl− release
The concentration fluctuation of these monochlorophenols in the ZVI NP system and nano-Fe/Ni system showed that these monochlorophenols were not the final products and would be degraded continually. However, in the nano-Fe/Ni system, the ‘wave peak’ of chlorophenol concentration was delayed, which resulted from the effect of Ni on the PCP degradation process. Although phenol showed a tendency of slow growth in the nano-Fe/Ni system, it was not the final product, which had been proved in our related study (Cheng et al. 2010). The concentration of three kinds of chlorophenols showed the pattern 2-CP > 4-CP > 3-CP in both systems, which indicated more accumulation of 2-CP in the systems.
Transformation of nanomaterials
The apparent morphology of Ni affected the PCP removal rate in the nano-Fe/Ni system: the tubular porous Ni nanomaterials inhibited the removal of PCP, and those with net shape and nanochains expressed a certain promotion. The dechlorination of PCP was promoted by the Ni nanomaterials, among which the tubular porous Ni nanomaterials expressed the most promotion, then those with net shape and nanochains. The accumulation of some intermediates, such as 2-chlorophenol (2-CP), 3-CP and 4-CP showed the pattern 2-CP > 4-CP > 3-CP. When Ni nanomaterials were present, the accumulation of chlorophenol was increased and the ‘wave peak’ of chlorophenol concentration was delayed. It is concluded that the introduction of Ni nanomaterials would improve dechlorination of PCP, but the removal of PCP might be inhibited or improved as the morphology of the Ni is changed.