Removal of ammonia from aqueous solutions by catalytic oxidation

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of copper-based rare earth composite metal materials,aqueoussolutions containing 400 mg/L of ammonia were oxidized ina batch-bed reac- tor with a catalyst ...

JOURNAL OF RARE EARTHS, Vol. 29, No. 7, Jul. 2011, P. 632

Removal of ammonia from aqueous solutions by catalytic oxidation with copper-based rare earth composite metal materials: catalytic performance, characterization, and cytotoxicity evaluation Chang-Mao Hung (⋾ᕄស) (Department of Vehicle Engineering, Yung-Ta Institute of Technology and Commerce, 316 Chung-Shan Road, Linlo, Pingtung 909, Taiwan, China) Received 28 December 2010; revised 25 February 2011

Abstract: Ammonia (NH3) has an important use in the chemical industry and is widely found in industrial wastewater. For this investigation of copper-based rare earth composite metal materials, aqueous solutions containing 400 mg/L of ammonia were oxidized in a batch-bed reactor with a catalyst prepared by the co-precipitation of copper nitrate, lanthanum nitrate and cerium nitrate. Barely any of the dissolved ammonia was removed by wet oxidation without a catalyst, but about 88% of the ammonia was reduced during wet oxidation over the catalysts at 423 K with an oxygen partial pressure of 4.0 MPa. The catalytic redox behavior was determined by cyclic voltammetry (CV). Furthermore, the catalysts were characterized using thermogravimetric analyzer (TGA) and scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX), which showed that the catalytic behavior was related to the metal oxide properties of the catalyst. In addition, the copper-lanthanum-cerium composite-induced cytotoxicity in the human lung MRC-5 cell line was tested, and the percentage cell survival was determined by 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetra-zolium (MTS) analysis in vitro. No apparent cytotoxicity was observed when the human lung cells were exposed to the copper-lanthanum-cerium composite. Keywords: wet oxidation; ammonia; copper-lanthanum-cerium composite metal catalyst; cytotoxicity; rare earths

Nitrogen compounds (including ammonia and organic nitrogen compounds) are common in industrial wastewaters and have significant utility in the chemical industry in Taiwan[1,2]. Direct biological treatment is unfeasible because it is either toxic to microorganisms or must be employed at concentrations and temperatures so high as to make them toxic[3]. Moreover, typical biological and physicochemical treatments, such as anaerobic biological oxidation, activated carbon adsorption and ion exchange resins, induce a phase transformation and may generate contaminated sludge and an adsorbent, both of which must be further treated. Thus, eradicating ammonia from air and waste streams is important. Wet oxidation technology was originally developed to oxidize organic substances in solution into intermediate products with low molecular weights at temperatures between 398 and 623 K under pressures ranging from 0.5 to 20.0 MPa. However, ammonia is normally an end product of the wet oxidation process and thus is difficult to oxidize. Previously, Mishra et al.[4] explored the wet oxidation processes and made proposals for future work. Wet oxidation is a promising method of pretreating wastewater that contains ammonia at concentrations up to 600 mg/L[5]. However, the efficient elimination of ammonia by non-catalytic wet oxidation requires pressures up to 4.0 MPa and a high temperature (513 K) and thus seriously affects the economic workability of this technology. To meet the wastewater permitted limits, catalytic wet

oxidation can increase the applicability of wet oxidation technology by using dedicated catalysts, which may promote oxidation in a shorter reaction period under milder operating conditions. Furthermore, ammonia pollution can be eliminated by the selective catalytic oxidation (SCO) of ammonia-containing water to produce molecular nitrogen and water[6]. For example, Levec and Pintar[7] showed that the wet oxidation of organic aqueous solutions from wastewaters at low temperatures and pressures was easier with heterogeneous catalysts than without them. Moreover, Inazu et al.[8], who developed several heterogeneous catalysts for wet oxidation, found a 2 wt.% Pt/AC catalyst to be more active than a Pt/ MgO catalyst in the wet oxidation of ammonia. The 2 wt.% Pt/AC catalysts were active in the reaction at temperatures above 453 K and a pressure of 0.5 MPa. Additionally, Lee et al.[9] investigated selective ammonia photocatalytic oxidation (photo-SCO) on Pt/TiO2 in water. Li and Liu[10] observed that ammonia conversion reached 88% by electrochemical oxidation with a RuO2/Ti catalyst. Recently, Hung[11] described the catalytic oxidation of NH3 in an effluent stream using a nanoscale, calcined Cu-La-Ce ternary catalyst at 773 K with an oxygen partial pressure of 4.0 MPa. The catalytic behavior is related to the copper (II) oxide, which has the highest NH3 reduction activity, while the lanthanum (III) and cerium (IV) oxides may only serve to provide active sites for the reaction during catalyzed wet oxida-

Foundation item: Project supported by the National Science Council of Taiwan (NSC 98-2221-E-132-003-MY3) Corresponding author: Chang-Mao Hung (E-mail: [email protected]; Tel.: +886-8-7233733 ext 508) DOI: 10.1016/S1002-0721(10)60512-1

Chang-Mao Hung, Removal of ammonia from aqueous solutions by catalytic oxidation with copper-based rare earth …

tion. Information about cytotoxicity for most of the metal composites is not available from the manufacturers[12]. Additionally, cumulating evidence demonstrates that copper-based rare earth composite metal materials are important in the generation of reactive oxygen species (ROS) that induce oxidative stress and DNA damage in human lung epithelial cells and potentially negatively affect the health of living organisms and the environmental [13,14]. Information on the cytotoxicity of copper-based rare earth composite metal materials, which have been used for decades, has been published, but very little is known about the toxicological effects of the most newly developed materials and their interaction with cellular structures. So far, few investigations have been performed on the applicability of copper-based rare earth composite metal materials in catalytic wet oxidation. Hence, the activity of a copper-lanthanum-cerium composite metal catalyst in the wet oxidation of NH3 solutions under various conditions and the effect of this catalyst on the elimination of NH3 from the effluent stream in the processes were investigated. The catalysts were characterized by TGA, CV, and SEM-EDX. Also, the human lung epithelia MRC-5 cell line was used to perform in vitro cytotoxicity assays to determine the biological effect of the copper-lanthanum-cerium composite metal catalyst.

1 Experimental 1.1 Material and chemicals The copper, lanthanum and cerium composite metal catalysts used in this work were prepared by the co-precipitation of copper (II) nitrate, lanthanum (II) nitrate, and cerium (III) nitrate with a molar ratio of 7:2:1. The catalysts were then calcined at 773 K in an airstream for 4 h. The resulting powder was made into tablets using acetic acid as a binder. The tablets were reheated at 573 K to burn off the binder and then crushed and sieved into various particle sizes ranging from 0.25 to 0.15 mm for later application. Thermogravimetric and differential thermal analysis (TGA-DTA) experiments were conducted in a TGA-DTA unit (Seiko SSC-5000, Japan) at a heating rate of 283 K/min. Cyclic voltammetric (CV) experiments were conducted at room temperature with an electrochemical analyzer (CHI 6081D, USA) using a three-electrode electrochemical cell to investigate the oxidation/reduction of the powder samples. The working electrode (WE) was a glassy carbon electrode at a scan rate of 20 mV/s with the potential cycled between –0.2 and 1.2 V. The counter electrode (CE) used was platinum wire, and a saturated hydrogen electrode (SHE) was employed as the reference electrode (RE). H2SO4 (0.5 mol/L) was used as the electrolyte solution. The composition of the catalyst’s surface was determined using an energy dispersive X-ray spectrometer (SEM/EDX, JEOL, JSM-6400, Kevex, DeltaII). 1.2 Experimental methods All feed solutions were made using Millipore (Bedford,

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Massachusetts) water (18 Mȍ), and the pH value of the aqueous ammonia solution was adjusted to 11.5±0.2 using 1 mol/L sodium hydroxide. The experiments were performed in a 400-ml autoclave made of stainless steel (SS-316), which can perform batch experiments at pressures up to 10 MPa and temperatures up to 573 K. The autoclave resists corrosion in the pH range of 5–12.5. A laboratory DC power supply (Rex-C100, HIPOINT, Taiwan) and an electric heating belt (BRANSTEAD, Iowa, USA) were used to convert electric power into heat and to control the temperature to within ±2 °C throughout the reaction. Takes of oxygen and helium were used as the oxygen supply and the idle gas, respectively, in the wet oxidation reaction. A regulator and barometer (Model 101, Harris, Ohio, USA) were used to control and read the total pressure in the reactor, respectively. Additionally, the temperature of the sample was cooled to 308 K before being withdrawn from the sampling port using a 12-L cooling water bath. In each WO experiment, a fixed bed was formed by measuring 10–50 g of the copper-lanthanum-cerium composite metal catalyst and installing it in the reactor to create a bed volume of about 15.6 ml. A hydrophilic inert material of Ȗ-Al2O3 spheres was used to increase the interfacial area between the liquid and gas phase to increase the mass transfer efficiency of oxygen to water. Fig. 1 schematically depicts the catalytic wet oxidation system used in this study. In the experiments, 250 ml of ammonia solution was added to the reactor, and then nitrogen gas at a flow rate of 1 L/min was introduced to strip the residual air from the reactor for 5 min. The reactor was quickly heated to the desired temperature, and a 20 ml portion of the solution was taken out of the reactor as the first (0 min) sample. At the same time, oxygen was introduced into the reactor to begin the oxidation reaction. Samples (10 ml) were taken at 5, 10, 15, 20, 40, 60, 90, 120, 150 and 180 min for analysis. The

Fig. 1 Schematic diagram of the wet oxidation process 1-Helium cylinder; 2-Oxygen cylinder; 3-Barometer; 4-Check valve; 5-Liquid pump; 6-Fixed bed reactor; 7-Heater; 8-Pressure gauge; 9-Thermal sensor; 10-Thermal controller; 11-Ball valve; 12-Buffing bottle; 13-Sample reservoir; 14-Heat exchanger; 15-Needle valve

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oxygen partial pressure in the reactor was maintained at a constant 4.0 MPa. The pH of the samples was measured using a pH meter (SP701, Suntex, Kaohsiung Taiwan). The samples were appropriately diluted, and a Merck kit (Merck, Spectroquant Pharo 300, Darmstadt, Germany) was used to measure the ammonia concentration within the range of 0.03 to 3.00 mg/L (Spectroquant 14752). The NO3– concentrations (Spectroquant 14773) could be measured between 0.2 and 20.0 mg/L, and the NO2– concentration (Spectroquant 14776) could be measured between 0.005 and 1.00 mg/L also using a Merck kit, as described above. The amounts of NO and NO2 in the gas samples were quantified using a gas analyzer unit (IMR3000, IMR, Germany). The amounts of O2, N2, and N2O were determined using a Shimadzu GC-14A equipped with a TCD (Shimadzu, Kyoto, Japan). A stainlesssteel column (Porapak Q 80/100 mesh) was used to isothermally separate them along with a refinery analyzer. 1.3 Cytotoxicity assay procedure A 7-ml sample of the copper-lanthanum-cerium composite metal catalyst in water was dried under a stream of nitrogen, dissolved in 1 ml of dimethyl sulfoxide (DMSO), and passed through a sterile filter with a pore size of 0.2 ȝm. The resulting solution was then diluted with DMSO at various ratios. The mixture was incubated for 24 h at 310 K. All cytotoxicity assays were conducted on MRC-5 (male lung epithelial) cell lines that were purchased from the Institute of Food Industrial Development and Research in Taiwan and had code number CCRC 60023 (ATCC CCL-171). These cells were derived from the normal lung tissue of a 14-week-old male fetus and were capable of 42–46 population doublings before the onset of senescence. The cells were grown in a minimum essential medium (MEM), which was supplemented with 10% fetal bovine serum (FBS) and maintained within an atmosphere of 5% CO2 and 95% ambient air in a humidified incubator at 310 K. The composite material-induced cytotoxicity was evaluated by performing a 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2 H-tetra-zolium (MTS) cytotoxicity assay[15], for which 10,000 cells of MRC-5 MEM were seeded into each well of a 96-well plate overnight. On the following day, the cells were fed with the test samples and incubated for one day. Then, 20 ȝl of a mixture of MTS and phenazine methosulfate (PMS; MTS/PMS 20:1) was added to the wells, and they were incubated for 4 h at 310 K (in a 5% CO2/95% ambient air mixture). The reaction mixture in each well was measured using a Fluostar reader (Fluostar, Germany) at 492 nm. Each sample was tested in four different wells. A cumene hydroperoxide (CH) solution with a concentration of 12.5 ȝmol/L was used as a positive control, and the MEM was used as a negative control. The cytotoxicity of CH to lymphocytes was induced in vitro by treating the lymphocytes with different doses of CH for 48 h. In the MTS assay, the absorbance at 492 nm of each sample was compared to that of the same sample exposed to the 12.5 ȝmol/L CH to obtain the relative effect potential percentage (REP%) of the sample.

JOURNAL OF RARE EARTHS, Vol. 29, No. 7, Jul. 2011

2 Results and discussion Fig. 2 plots the results of the TGA thermal decomposition of the copper-lanthanum-cerium composite metal catalyst precursor to determine the calcination temperature. The catalyst did not lose mass until the temperature from 773 to 1100 K was reached. Above this temperature, a significant loss of mass may contribute to the decomposition and desorption of metal salts, oxygen and water, and this temperature limit indicated that the amount of converted ammonia increased with a calcination temperature up to 773 K[11]. Moreover, the TGA curve revealed whether the reaction that involved the copper-lanthanum-cerium composite metal catalyst was exothermic or endothermic. Fig. 3 plots the effect in terms of removal, of the copper-lanthanum-cerium composite metal catalysts with various metal content on the conversion of ammonia in catalytic wet oxidation. The amount of ammonia that decomposed during catalytic wet oxidation was determined by the metal loading mass of the catalyst. Although the catalyst with a copper-lanthanum-cerium composite metal mass of 50 g was found to be optimal in terms of its capacity to eliminate ammonia, the difference between the catalysts with mass of 50 and 10 g was significant. The wet oxidation of a 400 mg/L ammonia solution to nitrogen was conducted

Fig. 2 Thermal gravimetric analysis (TGA) diagram of the copperlanthanum-cerium composite metal catalyst

Fig. 3 Effect of various catalytic content on ammonia removal in the catalytic wet oxidation process with the copper-lanthanum- cerium composite metal catalyst in a batch-bed reactor (temperature= 423 K, partial pressure of O2=4.0 MPa, initial concentration of ammonia=400 mg/L)

Chang-Mao Hung, Removal of ammonia from aqueous solutions by catalytic oxidation with copper-based rare earth …

without a catalyst in the fixed-bed reactor at 423 K and 4.0 MPa. The conversions were raised to 88% and 59% when the wet oxidation was performed under the same operating conditions over the catalysts with mass of 50 and 10 g, respectively. A low pH promoted the removal of the by-product NO2– but not the removal of ammonia. The results in Fig. 4 indicate that NO2– selectivity was minimized and ammonia removal maximized when the reaction solution pH approached 9.8 for a given ammonia solution feed. Imamura et al.[16] demonstrated that the reaction did not proceed at all in an acidic solution with Co/Bi catalysts and only proceeded at a pH over 9.0. Thus, he concluded that ammonia was more reactive than ammonium when Co/Bi catalysts were used. The reactions must occur in the liquid phase when catalysts are used. The data presented here indicated that a low pH promoted nitrite reduction. However, at a pH under 9.8, substantially more ammonia was converted at 423 K, and NO2– formation was minimized. Although some parts of the primary pathways are poorly understood, this study clarified the possible mechanisms of ammonia oxidation and provided significantly new information about this process. The CV procedure was used for the redox profiles during the oxidation and reduction reaction study of the catalysts. Fig. 5 displays the profile information of the fresh and aged

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catalyst. The CV plots indicated that the fresh catalyst exhibited more reversible redox ability than the aged catalyst because they yielded reduction peaks at 0.2 and 0.1 V, respectively. This reversible redox ability may explain the high activity of the catalyst. An earlier investigation demonstrated that CeO2 is the most active phase in the aforementioned catalytic reaction because it is a strong promoter of an oxygen storage medium. Whenever cerium dioxide is in a copper catalyst, it can be assumed to promote the formation of the CuO active phase under the conditions of ammonia oxidation. Additionally, CeO2-based materials can act as oxygen buffers by storing and releasing O2 as demonstrated by the automotive three-way catalytic converter that contains the cerium (+3)-cerium (+4) redox couple[17]. Based on the experimental results, the catalytic activity of the copper-lanthanum-cerium composite metal system in oxidizing ammonia may be explained by the reversible redox behavior of copper-lanthanum-cerium couples in promoting the catalyst bifunctional mechanism. Fig. 6 shows the surface morphological changes of the catalyst elucidated by SEM to provide information on the fresh and aged catalyst surface structure. Fig. 6(a) shows that the surface of the catalyst was more aggregated and crystalline than that observed in Fig. 6(b). Fig. 6(b) indicates that the disaggregated and dispersed phases were formed when the surface of the catalyst was aged or when poisoning occurred because of plugging, implying that the porosity of the particles had changed. These crystal phases may be responsible for the high activity of the catalysts. These results also

Fig. 4 Plot of the effect of pH on NO2– and NO3– formation during the wet oxidation of the ammonia solution (temperature=423 K, partial pressure of O2=4.0 MPa, initial concentration of ammonia= 400 mg/L)

Fig. 5 Cyclic voltammograms of copper-lanthanum-cerium composite metal catalyst in a 0.5 mol/L H2SO4 electrolyte at a scan rate of 20 mV/s

Fig. 6 SEM-EDX photograph of fresh (a) and aged (b) copperlanthanum-cerium composite metal catalyst (temperature=423 K, partial pressure of O2=4.0 MPa, initial concentration of ammonia=400 mg/L)

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confirm that the dispersion phenomena of the catalyst increased the efficiency of the ammonia removal. Also, Fig. 6 indicates that copper, lanthanum and cerium leached from the catalyst. The elemental composition analysis of the test catalysts’ surfaces revealed that the leaching of metal ions from the copper-lanthanum-cerium composite metal catalyst varied slightly. Copper and lanthanum both exhibited elution from the catalyst probably making the active site of the cerium metal an exposed surface. Occupational exposure to copper-based composite materials by inhalation may detrimentally affect human pulmonary cells. Fig. 7 plots the REP (%) of the MEM solution of cells at dilution ratios of 1.0, 2.0, 3.0, and 4.0. Clearly, increasing the dilution ratio decreased the cytotoxicity. Therefore, the copper-lanthanum-cerium composite exhibited only minor cytotoxicity to human lung cells. However, previous epidemiological studies[18,19] have established that exposure to copper-based composite material has adverse respiratory effects and causes an observable exacerbation of lung disease. Accordingly, a cytotoxicity test was conducted to assess copper-based composite material-induced cytotoxicity. Furthermore, oxidative stress and DNA damage that are caused by copper-based composite material need to be explored in the future.

Fig. 7 Cytotoxicity assay of fresh (a) and aged (b) copper-lanthanumcerium composite metal catalyst with various dilution ratios (temperature=423 K, partial pressure of O2=4.0 MPa, initial concentration of ammonia=400 mg/L)

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3 Conclusions In this study, the copper-lanthanum-cerium composite metal catalyst effectively removed ammonia, and catalytic wet oxidation was more effective than a non-catalytic process at 423 K. The catalytic wet oxidation exhibited potential for treating highly concentrated ammonia solutions, which could help industrial plants adhere to discharge regulations. Also, changes in the redox state and the crystalline composition of the catalyst were identified. Finally, for the copper-lanthanum-cerium composite catalyst, no apparent cytotoxicity was found when human MRC-5 lung cells were exposed. These data provided important information about the application of eco-materials in the fields of environmental engineering, public safety and health. Acknowledgments: The author thanks Prof. M. F. Shue, in the Department of Environmental Engineering and Science at Tajen University of Science and Technology, for her support and discussions.

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