A study on the reductive dechlorination of chloroform with nano fe/cu bimetallic particles in aqueous solution - Phan Kim Nguyen

ISSN 1859-1531 - THE UNIVERSITY OF DANANG, JOURNAL OF SCIENCE AND TECHNOLOGY, NO. 6(127).2018 13 A STUDY ON THE REDUCTIVE DECHLORINATION OF CHLOROFORM WITH NANO Fe/Cu BIMETALLIC PARTICLES IN AQUEOUS SOLUTION Phan Kim Nguyen, Bui Xuan Vung University of Education – The University of Danang; vungxuanbui@gmail.com Abstract - In this work, nano-Fe/Cu bimetallic particles are synthesized and used to reduce chloroform to methane in aqueous solution. The synthesized particles are characterized by X ray diffraction (XRD) pattern, Transmission electron microscopy (TEM) images and energy dispersive X ray (EDX) analysis. Such key parameters on the reduction of chloroform as pH, nano-Fe/Cu dosage, treatment time have been investigated. Closed batch experiments have been conducted for this investigation. Experimental results show that the de-chlorination of 50 mL of 20 ppm chloroform aqueous solution has the highest degradation efficiency of 88.93% under the experimental conditions such as pH = 3, reaction time of 30 minutes and nano-Fe/Cu dosage of 0.05 gram. GC-MS analysis for a 20 ppm chloroform aqueous solution before and after treatment has shown that there is no formation of such products containing chlorine as CH2Cl2 and CH3Cl. Key words - Nano-Fe/Cu; bimetallic particles; chloroform; de-chlorination; degradation; aqueous solution. 1. Introduction Trihalomethanes including mainly chloroform (CHCl3) are disinfection by-products formed when using chlorine for disinfecting drinking water [1] and treated wastewater before it is conveyed into water distribution systems [2]. Chlorine is by far the most widely used chemical disinfectant in water and wastewater treatment. These by- products are linked to a direct health risk such as liver and kidney cancer, nervous system and reproductive effects. The recommended concentration value by WHO for chloroform in drinking water is 0.3 mg/L [1]. Many technologies such as advanced oxidation, air stripping, and physical adsorption have been applied to the removal of chloroform in water [3-6]. A reductive system with zero- valence iron and the reductive process coupled with Fenton’s reagent were also used for such a purpose. However, the destruction of chloroform requires additional treatment [7]. Another efficient approach for degrading a variety of contaminants is that using nano-Fe0 coated with another metal such as Ag, Pd, Pt, Ni or Cu because the rate of reduction by bimetallic particles is significantly faster than those observed for Fe0 alone [8]. An investigation shows that nano-Fe/Cu particles increase the rate of reduction 1,1,1-trichloroethane related to Fe/Ni combination and the bimetals show a dramatically faster rate than Fe0 alone [9]. In this regards, Fe/Cu combination was chosen to degrade chloroform in aqueous solution. In this study, nano Fe/Cu particles are firstly synthesized and characterized and then used for the investigation of effects on the removal of chloroform from aqueous solution. 2. Experimental 2.1. Synthesizing nano-Feo and nano-Fe/Cu particles To synthesize nano-Fe0 particles, two solutions of A and B were respectively prepared by dissolving 4 gr of FeSO4.7H2O (99%, China) into 50 mL of distilled water and 0.4 gr of NaBH4 (99%, Merck) into 10 mL of distilled water to form the solution that was then added with 10 mL of 1% w/v starch solution. Solution B was added slowly in the rate of 3-4 mL.min−1 to solution A at ambient temperature and vigorous stirring. All aqueous solutions removed dissolved oxygen by bubbling argon gas for 20 min. During this reaction, ferrous ion (Fe2+) was reduced into black particles by sodium borohydride reductant in the following reaction: 4Fe2+ + 2BH4- + 3H2O → 4Fe0 + 2H2BO3- + 8H+ + 2H2 The black precipitates were filtered by vacuum filtration and then, washed with distilled water and ethanol at least three times. The prepared Fe0 particles were mixed with 10 mL of 1% w/v starch solution, and then distilled water was added to obtain 50 mL of solution C. Bimetallic nano-Fe/Cu particles were prepared by adding drop by drop 10 mL of aqueous solution D containing 0.500 gr CuSO4.7H2O to solution C in vigorous stirrer and ambient temperature. After a few minutes, a redox reaction occurred between Cu2+ and nano-Fe0 as follows: Fe0 + Cu2+ → Fe2+ + Cu0 The resulting nano-Fe/Cu particles were washed with distilled water, and stored in ethanol. The whole process above was carried out under the condition of bubbling the solutions with clean argon gas [10,11]. 2.2. Effect on the CHCl3 de-chlorination To find out the de-chlorination capacity of the synthesized material, experiments were set up to investigate effects of pH, material dosage, and treatment time on the degradation of chloroform. These de-chlorination experiments were performed in a closed batch system. Determinations of pH were carried out by using the pH meter (Sension+ PH31, Hatch (UK)) that was daily calibrated at pH 4.00 and 7.00 using commercial buffers. In most cases, each bottle received 50 mL of 20 ppm CHCl3. 2.3. Analytical methods Chloroform degradation was analyzed by gas chromatography coupled with mass spectrometry (GC-MS Triple Quad 7098A-7001B Agilent, USA). The injection temperature and detector temperature of the GC were set at 110 and 230oC, respectively, and a gradient program was applied in the oven with an initial temperature of 50oC held for 1 min and then gradually increased to 230oC at a rate of 15oC min-1, and remained at 230oC for 1 min. Chloride ion concentration was determined by spectrometry (Lamda 650 UV-VIS spectrometer, USA) at a wavelength of 460 nm after reaction with mercury thiocyanate to form an 14 Phan Kim Nguyen, Bui Xuan Vung orange-red compound [12]. The remaining chloroform concentration after treatment was calculated based on the chloride formation, which was quantitatively analyzed by UV-VIS spectrometry. Finally, the efficiency of chloroform degradation was calculated using initial chloroform concentration and its remaining concentration after the reduction. 3. Results and Discussion 3.1. Characterization of synthesized Fe/Cu nano particles The synthesized nano-Fe/Cu particles were characterized by XRD, TEM and EDX. Figure 1A shows X- ray diffraction of the synthesized Fe/Cu nano particles were obtained by a D8 Advanced Bruker diffractometer. It can be seen from Figure 1A and 1B that there is a similarity of XRD spectra obtained from this study and from the work of Chien- Li Lee and Chih-Ju G Jou [13]. Figuge 2A presents TEM image of nano-Fe/Cu particles which have been recorded by a JEOL JEM-1010 transmission electron microscope. It is found that the diffraction patterns indicate the state of chemical combination of the bimetallic nanoparticles, and the TEM image shows the particles are well-combined and crystalline sizes are less than 1000Ao (or 100nm). Elemental analysis performed by energy dispersive spectrometry (EDX) with Horiba EMAX EDS detectors were presented in Figure 2B. The weight percentage of Fe and Cu in the synthesized nano-Fe/Cu particles obtained from the elemental analysis is 81.72% and 13.04% respectively. Figure 1. (A) Fe/Cu nano particle XRD patterns of this study and (B) of Chien-Li Lee & Chih-Ju G Jou Figure 2. (A) TEM image and (B) EDX spectrum of fresh Fe/Cu particles 3.2. Effect on the chloroform de-chlorination 3.2.1. Effect of pH Figure 3. Effect of initial pH on the CHCl3 degradation The effect of initial pH on the reduction reaction of chloroform by the synthesized nano-Fe/Cu is shown in Figure 3. In each experiment 0.025 gr of the material was added to 50 mL of 20 ppm CHCl3. As can be seen from Figure 3 in the pH range from 3 to 7 the more acidic medium, the faster rates of chloroform reduction are achieved. When the initial pH is 3.0, the degradation efficiency of chloroform at 10 min reaction reaches the maximum at 85.35%. When the initial pH is 7.0, the degradation efficiency of chloroform at 10 min reaction decreases to 44.49%, much smaller than that at pH 3.0. However, at the initial pH 2 the degradation efficiency is only 35.50 %, a much smaller value as compared with that at pH 3.0. This abnormal issue could be explained by the dehalogenation mechanism suggested by Leah J. Matheson and Paul G. Tratnyek [14]. Alkyl halides, RX, ISSN 1859-1531 - THE UNIVERSITY OF DANANG, JOURNAL OF SCIENCE AND TECHNOLOGY, NO. 6(127).2018 15 approach to the surface of the reductive material and then can be reduced by iron according to the following reaction: Fe0 + RX + H+ → Fe2+ + RH + X- The increase in pH favors the formation of iron hydroxide precipitates that may eventually form a surface layer on the metal, which leads to inhibiting further dissolution of the metal. Otherwise, at a more acidic pH, there is an additional reaction between Fe0 and H+ to form H2. In the absence of an effective catalyst such as Pd or Pt, H2 is not a facile reductant, and this reaction will not contribute directly to dehalogenation. In fact, excessive H2 accumulation at the metal surface inhibits the continuation of reduction reactions in organic synthesis. 3.2.2. Effect of treatment time For each experiment to investigate the effect of treatment time on the chloroform de-chlorination, 0.025 gr of the material was added to 50 mL of 20 ppm CHCl3 at pH 3 in the reaction time intervals of 5, 10, 30, 60 minutes. Figure 4 shows that the degradation efficiency rises up from 45.15% for 5 min treatment to 87.88% for 30 min treatment. The degradation efficiency of 88.76% for 60 min treatment implies that there is an insignificant change in the degradation efficiency after 30 min treatment. Figure 4. Effect of treatment time on the CHCl3 degradation 3.2.3. Effect of nano-Fe/Cu dosage In order to investigate the effect of synthesized nano Fe/Cu dosage on the chloroform de-chlorination, the dosage of 0.01, 0.025, 0.05 and 0.1 gr was respectively added to 50 mL of 20 ppm CHCl3 at pH 3 with the treatment time of 30 min. Figure 5. Effect of nano Fe/Cu particle dosage on the CHCl3 degradation From Figure 5 we can see when the material dosage increases from 0.01 to 0.05 gr, the degradation efficiency of chloroform increases from 79.33% to 86.84% and then when adding 0.1 gr of the material, the degradation efficiency is almost unchanged any more. So the material dosage of 0.05 gr per 50 mL of 20 ppm CHCl3 can be optimum for the investigation. 3.3. GC/MS analysis of chloroform degradation In order to investigate whether such fewer chlorine intermediate products as CH2Cl2, CH3Cl were formed from chloroform de-chlorination by the synthesized nano Fe/Cu particles, GC-MS analysis was performed for 20 ppm chloroform solution before and after the treatment with the reaction conditions of pH 3, treatment time of 30 min, nano-Fe/Cu dosage of 0.05 gr, and illustrated by GC-MS chromatograms in Figure 6E and 6F respectively. In addition to this purpose, based on GC-MS analysis, the degradation efficiency was also evaluated to compare with that calculated by UV-VIS method as mentioned above. A comparison between Figure 6A and 6B shows that the peak of CH2Cl2 impurity at the retention time of 6.623 min almost disappeared after the treatment. Meanwhile, the area of the CHCl3 peak at the retention time of 8.037 min is decreased from 4344995 to 482577, corresponding to the degradation efficiency of 88.93%. This efficiency shows a resemblance to the one calculated by using UV-VIS method. From the comparison of Figure 6A and Figure 6B, there is no evidence for the formation of fewer chlorine intermidiate products from the treatment. Figure 6. (A) GC-MS chromatograms of the sample before treatment and (B) the sample after the treatment 16 Phan Kim Nguyen, Bui Xuan Vung 4. Conclusion Under the conditions of our experiments, chloroform undergoes rapid reductive dehalogenation in the presence of the synthesized nano-Fe/Cu particles with the degradation efficiency is nearly up to 90%. Dehalogenation efficient of chloroform is higher in a more acidic medium for the pH range from 3 to 7, but this trend will be reversed at a pH of 2. This indicates that a strongly acidic medium is not in favor of the chloroform de-chlorination. Results of GC-MS analysis show that chloroform is completely transformed into methane without forming products containing chlorine such as CH2Cl2 or CH3Cl. 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