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Treatment of chloramphenicol-contaminated soil by microwave radiation
Chemosphere 78 (2010) 66–71
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Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Technical Note
Treatment of chloramphenicol-contaminated soil by microwave radiation Li Lin, Songhu Yuan, Jing Chen, Linling Wang, Jinzhong Wan, Xiaohua Lu * Environmental Science Research Institute, Huazhong University of Science and Technology, Wuhan 430074, PR China
a r t i c l e
i n f o
Article history: Received 6 June 2009 Received in revised form 22 September 2009 Accepted 23 September 2009 Available online 28 October 2009 Keywords: Microwave radiation Soil treatment Antibiotics Activated carbon
a b s t r a c t This paper describes the microwave (MW) treatment of soil contaminated by chloramphenicol (CAP), using granular activated carbon (GAC) as MW absorbent. It was found that the addition of GAC effectively increased the temperature of soil. Large MW power and GAC dosage were beneficial for a completed decomposition of CAP. The effect of initial CAP concentration on decomposition was minute and a small scale of soil/GAC was disadvantageous. The degradation mechanism by MW radiation was also explored. The decomposition product of 4-nitrobenzoic acid after MW radiation was confirmed by LC–MS. The analysis by GC–MS and FTIR proved that parts of the decomposed fragment of CAP reacted with soil organic matters and formed compounds with larger molecular weight than CAP, but the concentration of each product was extremely low. It was suggested that MW radiation was an alternative technology for the treatment of antibiotics-contaminated soils. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction In both human and veterinary medicine, a large number of antibiotics have been used extensively to treat disease. After excretion, antibiotics and their metabolites can enter the environment through municipal effluents, sewage sludge, solid waste, and manure applications. Recently, a number of antibiotics have been detected worldwide in soil, wastewater, surface water and even drinking water (Sarmah et al., 2006). Concentrations of antibiotics from ng L 1 up to lg L 1 have been measured in surface waters and drinking waters (Ternes et al., 2002). Certain point sources such as effluents generated by hospitals, pharmaceutical production facilities and aquaculture have much higher antibiotic concentrations, in the level of several mg L 1 (Kummerer, 2001). Chloramphenicol (CAP) with the concentration of 37 lg kg 1 was detected in the sediments in Guiyang, China (Liu et al., 2007). Tetracycline and sulfadimidine were measured in liquid manure at concentrations up to 20 and 40 mg L 1, respectively (Hamscher et al., 2002). The residues of antibiotics in environment may pose potential risk to public health. To date, studies on antibiotics in environment have been focused on the environmental monitoring, transport behavior and systematic fate of antibiotics in soil and water bodies (Tolls, 2001; Loke et al., 2002), whereas only a few papers have been published dealing with the remediation. Ternes et al. (2002) investigated the removal of pharmaceuticals during drinking water * Corresponding author. Tel./fax: +86 27 87792159. E-mail address: [email protected] (X. Lu). 0045-6535/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2009.09.054
treatment, ozonation and filtration with granular activated carbon (GAC) were both effective. Photodegradation of tetracycline in wastewater was studied by Chen et al. (2008), and direct photolysis was found to be the predominant process. According to our knowledge, few investigations dealt with the remediation of antibiotics-contaminated soil. Considering the environmental risk of antibiotics and public security, it is necessary to explore effective methods for the treatment of antibiotics-contaminated soils. In recent years, microwave (MW) radiation has attracted great attention in environmental field. Interesting reports have appeared on the application of MW heating technology for regenerating activated carbon (Liu et al., 2004) and removal of ammonia from wastewater (Lin et al., 2009a,b). MW radiation was also used to remediate soils contaminated by persistent organic pollutants (Abramovitch et al., 1998, 1999a,b; Yuan et al., 2006) and heavy metals (Tai and Jou, 1999), and promising results were achieved. In this study, MW radiation was used to remediate soils contaminated by CAP. A large amount of CAP had already existed in environment due to its wide application (Bjorklund et al., 1991). Although it has been banned as a veterinary pharmaceutical in many countries due to the highly toxic effects to humans (Impens et al., 2003), it is still illegally used in animal farming because of its easy access and low cost (Huang et al., 2006) and led to the pollution of CAP in waters and soils. The objectives of present work are to evaluate the feasibility of MW treatment of CAP-contaminated soil using GAC as MW absorbent, and to investigate the degradation mechanism of CAP in soil by MW radiation.
L. Lin et al. / Chemosphere 78 (2010) 66–71
67
2. Experimental
2.2. Procedures
2.1. Chemicals and materials
CAP-contaminated soil was placed in a quartz column reactor (22 mm id) and certain amount of GAC was added and mixed with the soil thoroughly. The soil was radiated in a domestic MW oven (700 W, 2.45 GHz) for a preset time. At least triplicate runs were repeated for each case. A sheltered type-K thermocouple was used to record the temperature profiles during MW radiation (Liu and Yu, 2006).
CAP (98%) was obtained from Acros organics, Belgium. GAC (analytical grade) was purchased from Shanghai Experimental Reagent Corporation. The GAC was immerged in 10% HCl for 24 h and heated in boiling water for 30 min, and washed with deionized water to a neutral pH, then dried at 105 °C for 5 h and stored for usage. The particle size of GAC was 0.35–1.2 mm, with BET surface area 324 m2 g 1, average pore width 2.2 nm. Deionized water (18.2 MX cm) was obtained from a Millipore Milli-Q system. All the other reagents were above analytical grade. Actual soils were generally used in MW remediation researches (Abramovitch et al., 1999b; Liu and Yu, 2006). Nevertheless, since the composition of actual soils was complex, it was hard to explore the remediation mechanism. Diatomite (chemical purity, Tianjin Kermel Chemical Reagent Development Center, China), which has a simple composition, was used in present work as simulated soil. The adsorption of CAP on diatomite was experimentally found to be low. Infrared spectra with an Equinox 55 FTIR spectrophotometer in the region 4000–400 cm 1 were used to analyze the samples. FTIR spectrum of diatomite showed that the main features of the spectra were characteristic bands for SiO2 at 1092, 793 and 473 cm 1. The organic content of diatomite was measured as 0.39% by potassium dichromate digestion (Ko et al., 1998). The FTIR spectrum of the extract of diatomite by 1:1 acetone–hexane solvent is shown in Fig. 1. It indicated the characteristic bands for Si–O–Si at 1110–1000 cm 1, and the characteristic bands for Si– (CH3)3 at around 1260 cm 1, the characteristic bands for –CH3 at around 2961 cm 1. These results displayed that the organic matters in the soil contained the function groups of Si–O–Si and Si– (CH3)3. A total of 250 g dry diatomite was spiked with 250 mL CAP methanol solution (100 mg L 1) to achieve the load of 100 mg CAP kg 1 soil. The CAP-contaminated soil was dried in air for 24 h to evaporate the methanol, and then it was stored and used as needed. The water content of the resultant soil was 0.65%.
2.3. Analysis For the determination of the antibiotic in soil, methanol extraction was used (Rabolle and Spliid, 2000). When the soil was cooled to room temperature, methanol was added into the soil and the extraction process was assisted by ultrasonication (20 kHz) for 1 h. The mixture was further centrifuged for 10 min at 4000 rpm and the supernatant was filtered through 0.45 lm filter. The recoveries of CAP in soil were verified to be above 80%. CAP in the filtrate was measured by a high performance liquid chromatography (HPLC) system (Hitachi Company, Japan) equipped with an ultraviolet–visible detector L-7420 and a reverse-phase Akasil C18 column (250 mm 4.6 mm id, 5 lm, Agela Technologies, China). The mobile phase was set as a mixture of methanol and deionized water with the ratio of 55:45 (v/v). The wavelength was 278 nm and the flow rate of mobile phase was 1.0 mL min 1. The decomposition products in the soil were determined by LC– MS and GC–MS. In order to acquire more information about intermediate products, 3 g soil and 0.3 g GAC were used and the mixture was radiated by 20 min at 700 W MW power. The soil was extracted with 1:1 acetone–hexane solvent (30 mL) assisted by ultrasonic. Then the mixture was centrifuged and the supernatant was evaporated on a rotary evaporator until dry, and 3 mL ethyl acetate was used to dissolve residue. The extracts before and after MW radiation were then analyzed by LC–MS and GC–MS. HPLC system employed was Agilent 1100 series and mass spectrometry was carried out using a XCT ion trap with electrospray ionization
Fig. 1. The FTIR spectrum of the extract of diatomite by acetone–hexane solvent.
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source. The GC–MS condition was the same as our previous research (Yuan et al., 2006).
3. Results and discussion 3.1. Effect of MW power Since MW power is an important parameter in MW process, the effect of MW power was investigated firstly. The investigated power series were 230, 385, 540 and 700 W, and the results are displayed in Fig. 2a. It can be seen that higher MW power led to higher remediation efficiency. It is noted that the remediation effi-
ciencies at 0 min were resulted from the recoveries of CAP in the pretreatment process. For a given material, the temperature can in principle be modified by adjusting the input power (Menendez et al., 1999). The temperature profiles of soil during MW radiation at different treatment conditions are shown in Fig. 2e. When the MW power was as high as 700 W, the temperature rose up to 310 °C in 10 min, but the temperature was only 170 °C at 385 W. At the MW power of 700 W, a degradation efficiency of above 80% was reached in 20 min, whereas the degradation efficiency was only 60% at 385 W. For a higher MW power, a higher temperature can be reached in the soil/GAC system, which benefits the degradation of CAP in the soil.
Fig. 2. Degradation efficiencies of CAP in soil by MW with different MW power (a) (GAC dosage 0.1 g, soil mass 0.5 g, initial CAP concentration 100 mg kg 1), GAC dosage (b) (MW power 700 W, soil mass 0.5 g, initial CAP concentration 100 mg kg 1), soil mass (c) (MW power 700 W, the ratio of GAC to soil 20%, initial CAP concentration 100 mg kg 1), initial CAP concentration (d) (MW power 700 W, GAC dosage 0.1 g, soil mass 0.5 g) and temperature profiles of soil during MW radiation at different conditions (e) (1–700 W, 0.5 g soil, 0.0 g AC, 100 mg kg 1 CAP; 2–700 W, 0.5 g soil, 0.1 g AC, 100 mg kg 1 CAP; 3–700 W, 1.5 g soil, 0.3 g AC, 100 mg kg 1 CAP; 4–700 W, 0.5 g soil, 0.1 g AC, 1000 mg kg 1 CAP; 5–385 W, 0.5 g soil, 0.1 g AC, 100 mg kg 1 CAP).
L. Lin et al. / Chemosphere 78 (2010) 66–71
3.2. Effect of GAC dosage Four levels of GAC amount (0–0.1 g) were investigated, and the corresponding degradation efficiencies are presented in Fig. 2b. It is clearly seen that the degradation efficiencies rose as GAC dosage increased. The increased amount of GAC in soil enhanced the ability to absorb MW and led to much higher degradation efficiencies. The degradation efficiencies were insignificant in the absence of GAC, which was less than 20% within 20 min. Whereas, the degradation efficiency increased quickly with MW radiation time when 0.10 g GAC was added, and the concentration of CAP decreased from 100 to 19.35 mg kg 1 after 20 min radiation. From Fig. 2e, it can be seen that the temperature of the soil could only reach 150 °C when no GAC was used. Whereas, when 0.1 g GAC was added, the mixture temperature stabilized at 300–315 °C after 5 min MW radiation. A larger GAC dosage led to a higher temperature of the soil/GAC system, which subsequently led to a higher degradation efficiency of CAP in the soil. 3.3. Effect of the soil mass It is widely accepted that the volume and shape of a sample are important parameters that determine the MW energy absorption (Diprose, 2001). Fig. 2c shows the degradation efficiencies at different soil masses under MW radiation. It is obvious that the degradation efficiency increased with the soil mass in the range from 0.5 to 3.0 g, which was consistent with the results reported by Liu and Yu (2006). For 3 g soil, 93% of CAP degradation efficiency was reached in 5 min, whereas for 0.5 g soil, it was only 80% even in 20 min MW radiation. The soil mass is an important parameter which affects the capability of soil in absorbing MW energy. Soil is a weaker microwave absorbent, without GAC the temperature of the soil only reached 150 °C and with no more increase in 700 W MW power, and the output MW energy has not yet been fully absorbed by the soil. When GAC was added into soil, the MW absorbed capability of the soil/GAC mixture increased rapidly. From Fig. 2e it can be clearly seen that the increase of the soil mass effectively raised its ability to absorb MW and led to much higher temperature. Since the ratio of GAC and soil was fixed at 1:5, the increase in soil mass meant the increasing dosage of GAC. The MW absorbing capability of the soil increased continuously with the increase of soil mass and GAC dosage in this range. The temperatures for 0.5 g soil mixed with 0.1 g GAC and 3 g soil mixed with 0.6 g GAC rose up to 315 and 800 °C by 20 min MW radiation, respectively. Since more MW energy was absorbed and a higher temperature of the soil was reached, the degradation efficiency of CAP increased with soil mass in this range. Liu and Yu (2006) found that when the soil mass increased from 10 to 20 g, the degradation efficiencies of PCB increased; whereas, a comparable difference of the degradation kinetics between 20 and 30 g soil was not found. 3.4. Effect of initial CAP concentration In order to examine the treatment capacity of MW radiation for CAP-contaminated soil, the effect of initial CAP concentration was studied. The degradation efficiencies by MW radiation with different initial CAP concentration are illustrated in Fig. 2d. The results demonstrated that lower initial concentration led to higher degradation efficiency. When the initial concentration was 10 mg kg 1, CAP in the soil was facile to be treated, and the degradation efficiency was nearly 100% by 20 min MW radiation. When the initial concentration increased to 10 000 mg kg 1, the degradation efficiencies were less than 50% after 20 min MW radiation. For a high initial concentration, larger GAC dosage and longer treatment time were supposed to be needed to get higher degradation efficiency.
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Summarily, trial results suggested a significant effect of MW power and GAC dosage on the degradation efficiencies by MW radiation. The degradation efficiencies increased with the soil mass in the range from 0.5 to 3.0 g, which was beneficial for the treatment of practical contaminated-soil in engineering application in some extent. The effect of initial CAP concentration was minute because the CAP concentration in actual soil was from lg kg 1 to mg kg 1. 3.5. CAP degradation mechanism To reveal the degradation mechanisms of CAP in soil by MW radiation, the extracts before and after MW radiation were directly analyzed by LC–MS. No intermediates were detected, which might be due to the extremely low concentration of the products. In order to acquire more information about intermediate products, the concentrated extracts before and after MW were both analyzed by LC– MS and GC–MS, as described in Section 2.3. The extracts were firstly analyzed by LC–MS in both negative and positive mode. In the negative mode, the deprotonated molecules [M–H] of CAP were seen in abundance with a mass of m/z 321 in the total ion chromatogram (TIC) of the extract before MW radiation. CAP with the parent m/z = 321 ionized well producing daughters of m/z = 257, 194, 176 and 152 in MS/MS spectra, which was consistent with the results reported by literature (Sheridan et al., 2008). Before MW radiation, the CAP (m/z = 321) concentration in the extract was high. After 20 min MW radiation, the CAP concentration decreased rapidly, the degradation efficiency was about 90%, which was analyzed by HPLC. The products with m/z = 249, 452, 385 and 166 were detected on the TIC after MW radiation, but the concentration of each product was very low. In positive mode of LC–MS, products with m/z = 217, 261, 453 and 475 were presented on TIC with low content. Fig. 3a shows the MS/MS fragment of m/z 166 and the proposed molecule structure of the product. By comparing the fragment of the product m/z 166 with 4-nitrobenzoic acid (C7H5NO4) (Fig. 3a), the fragment information of the product with m/z 166 corresponded well with the structure of 4-nitrobenzoic acid. Thus, the product was confirmed to be 4-nitrobenzoic acid. This result proved that CAP experienced decomposition process in MW radiation, and the proposed decomposition pathway of CAP to 4-nitrobenzoic acid is shown in Fig. 3b. CAP experienced carbon–carbon bond rupture reaction in MW radiation and the fragment containing phenyl was converted to 4-nitrobenzoic acid by oxidation. Hong et al. (2002) reported that when CAP powder was irradiated by c-radiation, the same degradation product of 4-nitrobenzoic acid was found in their study, and CH2Cl2 and CHCl3 were also detected as degradation products by headspace-GC–MS analysis. We proposed that the degradation mechanisms between c-radiation and MW radiation have somewhat similar, with the same kind of reactions of bond rupture and oxidation. The concentrations of chloride ion in soil before and after MW radiation were also measured by ion chromatogram in our study, and no increase of chloride ion concentration was found. Thus, it was proposed that no dechlorination process occurred during MW radiation. We inferred that the rest part of CAP may be degraded to CH2Cl2 and/or CHCl3 by MW radiation, but the two products can not be detected by LC– MS or GC–MS due to rotary evaporation process in the pretreatment procedure. The products in negative mode with m/z = 249, 385 and 452 and in positive mode with m/z = 217, 261, 453 and 474 were also detected, but the structures of these compounds could not be confirmed from their fragment information by LC–MS. GC–MS was also performed to analyze the products after MW radiation, many peaks of high boiling-point alkyl hydrocarbons appeared on the TIC, which were produced from GAC by MW radiation (Liu and Yu, 2006). Besides the alkyl hydrocarbons, some
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L. Lin et al. / Chemosphere 78 (2010) 66–71
Fig. 3. Fragmentation of m/z 166 in LC–MS (a) and scheme of decomposition pathway of CAP to C7H5NO4 (b) and the typical products detected in GC–MS (c).
new peaks of products containing siloxanes appeared on the TIC, the structures of typical products and their molecular weights are shown in Fig. 3c. But the concentration of each product was very low. The FTIR spectrum results displayed that the soil organic matter contained the function groups of Si–O–Si and Si–(CH3)3, which was discussed in the preceding section. These new compounds (Fig. 3c) were the reaction products of CAP fragment and the soil organic matter. Therefore, it was proposed that a part of CAP decomposed and the fragment reacted with the soil organic matter and formed new compounds with larger molecular weight than CAP. Diatomite which was not contaminated by CAP was also treated in the same way as the CAP-contaminated diatomite, alkyl hydrocarbons were detected and no products containing siloxanes were found on the TIC of GC–MS. Summarily, CAP experienced carbon–carbon bond rupture and oxidation reactions in MW radiation, and a part of CAP fragment reacted with the soil organic matter and formed new compounds, but the concentration of each product was extremely low, no dechlorination process occurred during this process. The toxicity of the degradation products was also estimated. Hong et al. (2002) reported that healthy hazard data showed that 4-nitrobenzoic acid was safe to human health at trace level. Since the concentration of 4-nitrobenzoic acid was extremely low in soil, it was considered to be safe to human health and ecological
environment at such trace level. On the other hand, CAP contains a nitrobenzene ring, an amide bond and an alcohol function. Nitrobenzene ring is the main functional group of CAP and it is relevant because it leads to the formation of aromatic amines that may be carcinogenic (Hong et al., 2002). The molecular structures of these degradation products (Fig. 3c) contained no extra toxic functional groups than CAP. Thus, it could be expected that the toxicities of those identified products were similar or less than that of CAP. Overall, the toxicity of the organic compounds in the soil decreased greatly after MW radiation. References Abramovitch, R.A., Huang, B.Z., Mark, D., Luke, P., 1998. Decomposition of PCBs and other polychlorinated aromatics in soil using microwave energy. Chemosphere 37, 1427–1436. Abramovitch, R.A., Huang, B.Z., Abramovitch, D.A., Song, J.G., 1999a. In situ decomposition of PCBs in soil using microwave energy. Chemosphere 38, 2227–2236. Abramovitch, R.A., Huang, B.Z., Abramovitch, D.A., Song, J.G., 1999b. In situ decomposition of PAHs in soil and desorption of organic solvents using microwave energy. Chemosphere 39, 81–87. Bjorklund, H., Rabergh, C.M.I., Bylund, G., 1991. Residues of oxolinic acid and oxytetracycline in fish and sediments from fish farms. Aquaculture 97, 85–96. Chen, Y., Hu, C., Qu, J.H., Yang, M., 2008. Photodegradation of tetracycline and formation of reactive oxygen species in aqueous tetracycline solution under simulated sunlight irradiation. J. Photochem. Photobiol. A 197, 81–87.
L. Lin et al. / Chemosphere 78 (2010) 66–71 Diprose, M.F., 2001. Some considerations when using a microwave oven as a laboratory research tool. Plant Soil 229, 271–280. Hamscher, G., Sczesny, S., Hoper, H., Nau, H., 2002. Determination of persistent tetracycline residues in soil fertilized with liquid manure by high-performance liquid chromatography with electrospray ionization tandem mass spectrometry. Anal. Chem. 74, 1509–1518. Hong, L., Horni, A., Hesse, M., Altorfer, H., 2002. Identification and evaluation of radiolysis products of irradiated chloramphenicol by HPLC–MS and HPLC–DAD. Chromatographia 55, 13–18. Huang, J.F., Zhang, H.J., Feng, Y.Q., 2006. Chloramphenicol extraction from honey, milk, and eggs using polymer monolith microextraction followed by liquid chromatography–mass spectrometry determination. J. Agric. Food Chem. 54, 9279–9286. Impens, S., Reybroeck, W., Vercammen, J., Courtheyn, D., Ooghe, S., Wasch, K.D., Smedts, W., Brabander, H.D., 2003. Screening and confirmation of chloramphenicol in shrimp tissue using ELISA in combination with GC–MS2 and LC–MS2. Anal. Chim. Acta 483, 153–163. Ko, S.O., Schlautman, M.A., Carraway, E.R., 1998. Effects of solution chemistry on the partitioning of phenanthrene to sorbed surfactants. Environ. Sci. Technol. 32, 3542–3548. Kummerer, K., 2001. Drugs in the environment: emission of drugs, diagnostic aids and disinfectants into wastewater by hospitals in relation to other sources – a review. Chemosphere 45, 957–969. Lin, L., Chen, J., Xu, Z.Q., Yuan, S.H., Cao, M.H., Liu, H.C., Lu, X.H., 2009a. Removal of ammonia nitrogen in wastewater by microwave radiation: a pilot-scale study. J. Hazard. Mater. 168, 862–867. Lin, L., Yuan, S.H., Chen, J., Xu, Z.Q., Lu, X.H., 2009b. Removal of ammonia nitrogen in wastewater by microwave radiation. J. Hazard. Mater. 161, 1063–1068. Liu, X.T., Yu, G., 2006. Combined effect of microwave and activated carbon on the remediation of polychlorinated biphenyl-contaminated soil. Chemosphere 63, 228–235.
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X.T., Quan, X., Bo, L.L., Chen, S., Zhao, Y.Z., 2004. Simultaneous pentachlorophenol decomposition and granular activated carbon regeneration assisted by microwave irradiation. Carbon 42, 415–422. Liu, H., Zhang, G.P., Liu, C.Q., 2007. Determination of chloramphenicol and three tetracyclines by solid phase extraction and high performance liquid chromatography–ultraviolet detection. Chinese J. Anal. Chem. 35, 315– 319. Loke, M.L., Tjornelund, J., Halling-Sorensen, B., 2002. Determination of the distribution coefficient (log Kd) of oxytetracycline, tylosin A, olaquindox and metronidazole in manure. Chemosphere 48, 351–361. Menendez, J.A., Menendez, E.M., Pis, J.J., 1999. Thermal treatment of active carbons: a comparison between microwave and electrical heating. J. Microwave Power EE 34, 137–143. Rabolle, M., Spliid, N.H., 2000. Sorption and mobility of metronidazole, olaquindox, oxytetracycline and tylosin in soil. Chemosphere 40, 715–722. Sarmah, A.K., Meyer, M.T., Boxall, A.B.A., 2006. A global perspective on the use, sales, exposure pathways, occurrence, fate and effects of veterinary antibiotics (VAs) in the environment. Chemosphere 65, 725–759. Sheridan, R., Policastro, B., Thomas, S., Rice, D., 2008. Analysis and occurrence of 14 sulfonamide antibacterials and chloramphenicol in honey by solid-phase extraction followed by LC/MS/MS analysis. J. Agric. Food Chem. 56, 3509– 3516. Tai, H.S., Jou, C.J.G., 1999. Immobilization of chromium-contaminated soil by means of microwave energy. J. Hazard. Mater. B65, 267–275. Ternes, T.A., Meisenheimer, M., Mcdowell, D., Sacher, F., Brauch, H.J., Haist-Gulde, B., Preuss, G., Wilme, U., Zulei-Seibert, N., 2002. Removal of pharmaceuticals during drinking water treatment. Environ. Sci. Technol. 36, 3855–3863. Tolls, J., 2001. Sorption of veterinary pharmaceuticals in soils: a review. Environ. Sci. Technol. 35, 3397–3406. Yuan, S.H., Tian, M., Lu, X.H., 2006. Microwave remediation of soil contaminated with hexachlorobenzene. J. Hazard. Mater. B137, 878–885.
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Technical Note
Treatment of chloramphenicol-contaminated soil by microwave radiation Li Lin, Songhu Yuan, Jing Chen, Linling Wang, Jinzhong Wan, Xiaohua Lu * Environmental Science Research Institute, Huazhong University of Science and Technology, Wuhan 430074, PR China
a r t i c l e
i n f o
Article history: Received 6 June 2009 Received in revised form 22 September 2009 Accepted 23 September 2009 Available online 28 October 2009 Keywords: Microwave radiation Soil treatment Antibiotics Activated carbon
a b s t r a c t This paper describes the microwave (MW) treatment of soil contaminated by chloramphenicol (CAP), using granular activated carbon (GAC) as MW absorbent. It was found that the addition of GAC effectively increased the temperature of soil. Large MW power and GAC dosage were beneficial for a completed decomposition of CAP. The effect of initial CAP concentration on decomposition was minute and a small scale of soil/GAC was disadvantageous. The degradation mechanism by MW radiation was also explored. The decomposition product of 4-nitrobenzoic acid after MW radiation was confirmed by LC–MS. The analysis by GC–MS and FTIR proved that parts of the decomposed fragment of CAP reacted with soil organic matters and formed compounds with larger molecular weight than CAP, but the concentration of each product was extremely low. It was suggested that MW radiation was an alternative technology for the treatment of antibiotics-contaminated soils. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction In both human and veterinary medicine, a large number of antibiotics have been used extensively to treat disease. After excretion, antibiotics and their metabolites can enter the environment through municipal effluents, sewage sludge, solid waste, and manure applications. Recently, a number of antibiotics have been detected worldwide in soil, wastewater, surface water and even drinking water (Sarmah et al., 2006). Concentrations of antibiotics from ng L 1 up to lg L 1 have been measured in surface waters and drinking waters (Ternes et al., 2002). Certain point sources such as effluents generated by hospitals, pharmaceutical production facilities and aquaculture have much higher antibiotic concentrations, in the level of several mg L 1 (Kummerer, 2001). Chloramphenicol (CAP) with the concentration of 37 lg kg 1 was detected in the sediments in Guiyang, China (Liu et al., 2007). Tetracycline and sulfadimidine were measured in liquid manure at concentrations up to 20 and 40 mg L 1, respectively (Hamscher et al., 2002). The residues of antibiotics in environment may pose potential risk to public health. To date, studies on antibiotics in environment have been focused on the environmental monitoring, transport behavior and systematic fate of antibiotics in soil and water bodies (Tolls, 2001; Loke et al., 2002), whereas only a few papers have been published dealing with the remediation. Ternes et al. (2002) investigated the removal of pharmaceuticals during drinking water * Corresponding author. Tel./fax: +86 27 87792159. E-mail address: [email protected] (X. Lu). 0045-6535/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2009.09.054
treatment, ozonation and filtration with granular activated carbon (GAC) were both effective. Photodegradation of tetracycline in wastewater was studied by Chen et al. (2008), and direct photolysis was found to be the predominant process. According to our knowledge, few investigations dealt with the remediation of antibiotics-contaminated soil. Considering the environmental risk of antibiotics and public security, it is necessary to explore effective methods for the treatment of antibiotics-contaminated soils. In recent years, microwave (MW) radiation has attracted great attention in environmental field. Interesting reports have appeared on the application of MW heating technology for regenerating activated carbon (Liu et al., 2004) and removal of ammonia from wastewater (Lin et al., 2009a,b). MW radiation was also used to remediate soils contaminated by persistent organic pollutants (Abramovitch et al., 1998, 1999a,b; Yuan et al., 2006) and heavy metals (Tai and Jou, 1999), and promising results were achieved. In this study, MW radiation was used to remediate soils contaminated by CAP. A large amount of CAP had already existed in environment due to its wide application (Bjorklund et al., 1991). Although it has been banned as a veterinary pharmaceutical in many countries due to the highly toxic effects to humans (Impens et al., 2003), it is still illegally used in animal farming because of its easy access and low cost (Huang et al., 2006) and led to the pollution of CAP in waters and soils. The objectives of present work are to evaluate the feasibility of MW treatment of CAP-contaminated soil using GAC as MW absorbent, and to investigate the degradation mechanism of CAP in soil by MW radiation.
L. Lin et al. / Chemosphere 78 (2010) 66–71
67
2. Experimental
2.2. Procedures
2.1. Chemicals and materials
CAP-contaminated soil was placed in a quartz column reactor (22 mm id) and certain amount of GAC was added and mixed with the soil thoroughly. The soil was radiated in a domestic MW oven (700 W, 2.45 GHz) for a preset time. At least triplicate runs were repeated for each case. A sheltered type-K thermocouple was used to record the temperature profiles during MW radiation (Liu and Yu, 2006).
CAP (98%) was obtained from Acros organics, Belgium. GAC (analytical grade) was purchased from Shanghai Experimental Reagent Corporation. The GAC was immerged in 10% HCl for 24 h and heated in boiling water for 30 min, and washed with deionized water to a neutral pH, then dried at 105 °C for 5 h and stored for usage. The particle size of GAC was 0.35–1.2 mm, with BET surface area 324 m2 g 1, average pore width 2.2 nm. Deionized water (18.2 MX cm) was obtained from a Millipore Milli-Q system. All the other reagents were above analytical grade. Actual soils were generally used in MW remediation researches (Abramovitch et al., 1999b; Liu and Yu, 2006). Nevertheless, since the composition of actual soils was complex, it was hard to explore the remediation mechanism. Diatomite (chemical purity, Tianjin Kermel Chemical Reagent Development Center, China), which has a simple composition, was used in present work as simulated soil. The adsorption of CAP on diatomite was experimentally found to be low. Infrared spectra with an Equinox 55 FTIR spectrophotometer in the region 4000–400 cm 1 were used to analyze the samples. FTIR spectrum of diatomite showed that the main features of the spectra were characteristic bands for SiO2 at 1092, 793 and 473 cm 1. The organic content of diatomite was measured as 0.39% by potassium dichromate digestion (Ko et al., 1998). The FTIR spectrum of the extract of diatomite by 1:1 acetone–hexane solvent is shown in Fig. 1. It indicated the characteristic bands for Si–O–Si at 1110–1000 cm 1, and the characteristic bands for Si– (CH3)3 at around 1260 cm 1, the characteristic bands for –CH3 at around 2961 cm 1. These results displayed that the organic matters in the soil contained the function groups of Si–O–Si and Si– (CH3)3. A total of 250 g dry diatomite was spiked with 250 mL CAP methanol solution (100 mg L 1) to achieve the load of 100 mg CAP kg 1 soil. The CAP-contaminated soil was dried in air for 24 h to evaporate the methanol, and then it was stored and used as needed. The water content of the resultant soil was 0.65%.
2.3. Analysis For the determination of the antibiotic in soil, methanol extraction was used (Rabolle and Spliid, 2000). When the soil was cooled to room temperature, methanol was added into the soil and the extraction process was assisted by ultrasonication (20 kHz) for 1 h. The mixture was further centrifuged for 10 min at 4000 rpm and the supernatant was filtered through 0.45 lm filter. The recoveries of CAP in soil were verified to be above 80%. CAP in the filtrate was measured by a high performance liquid chromatography (HPLC) system (Hitachi Company, Japan) equipped with an ultraviolet–visible detector L-7420 and a reverse-phase Akasil C18 column (250 mm 4.6 mm id, 5 lm, Agela Technologies, China). The mobile phase was set as a mixture of methanol and deionized water with the ratio of 55:45 (v/v). The wavelength was 278 nm and the flow rate of mobile phase was 1.0 mL min 1. The decomposition products in the soil were determined by LC– MS and GC–MS. In order to acquire more information about intermediate products, 3 g soil and 0.3 g GAC were used and the mixture was radiated by 20 min at 700 W MW power. The soil was extracted with 1:1 acetone–hexane solvent (30 mL) assisted by ultrasonic. Then the mixture was centrifuged and the supernatant was evaporated on a rotary evaporator until dry, and 3 mL ethyl acetate was used to dissolve residue. The extracts before and after MW radiation were then analyzed by LC–MS and GC–MS. HPLC system employed was Agilent 1100 series and mass spectrometry was carried out using a XCT ion trap with electrospray ionization
Fig. 1. The FTIR spectrum of the extract of diatomite by acetone–hexane solvent.
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source. The GC–MS condition was the same as our previous research (Yuan et al., 2006).
3. Results and discussion 3.1. Effect of MW power Since MW power is an important parameter in MW process, the effect of MW power was investigated firstly. The investigated power series were 230, 385, 540 and 700 W, and the results are displayed in Fig. 2a. It can be seen that higher MW power led to higher remediation efficiency. It is noted that the remediation effi-
ciencies at 0 min were resulted from the recoveries of CAP in the pretreatment process. For a given material, the temperature can in principle be modified by adjusting the input power (Menendez et al., 1999). The temperature profiles of soil during MW radiation at different treatment conditions are shown in Fig. 2e. When the MW power was as high as 700 W, the temperature rose up to 310 °C in 10 min, but the temperature was only 170 °C at 385 W. At the MW power of 700 W, a degradation efficiency of above 80% was reached in 20 min, whereas the degradation efficiency was only 60% at 385 W. For a higher MW power, a higher temperature can be reached in the soil/GAC system, which benefits the degradation of CAP in the soil.
Fig. 2. Degradation efficiencies of CAP in soil by MW with different MW power (a) (GAC dosage 0.1 g, soil mass 0.5 g, initial CAP concentration 100 mg kg 1), GAC dosage (b) (MW power 700 W, soil mass 0.5 g, initial CAP concentration 100 mg kg 1), soil mass (c) (MW power 700 W, the ratio of GAC to soil 20%, initial CAP concentration 100 mg kg 1), initial CAP concentration (d) (MW power 700 W, GAC dosage 0.1 g, soil mass 0.5 g) and temperature profiles of soil during MW radiation at different conditions (e) (1–700 W, 0.5 g soil, 0.0 g AC, 100 mg kg 1 CAP; 2–700 W, 0.5 g soil, 0.1 g AC, 100 mg kg 1 CAP; 3–700 W, 1.5 g soil, 0.3 g AC, 100 mg kg 1 CAP; 4–700 W, 0.5 g soil, 0.1 g AC, 1000 mg kg 1 CAP; 5–385 W, 0.5 g soil, 0.1 g AC, 100 mg kg 1 CAP).
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3.2. Effect of GAC dosage Four levels of GAC amount (0–0.1 g) were investigated, and the corresponding degradation efficiencies are presented in Fig. 2b. It is clearly seen that the degradation efficiencies rose as GAC dosage increased. The increased amount of GAC in soil enhanced the ability to absorb MW and led to much higher degradation efficiencies. The degradation efficiencies were insignificant in the absence of GAC, which was less than 20% within 20 min. Whereas, the degradation efficiency increased quickly with MW radiation time when 0.10 g GAC was added, and the concentration of CAP decreased from 100 to 19.35 mg kg 1 after 20 min radiation. From Fig. 2e, it can be seen that the temperature of the soil could only reach 150 °C when no GAC was used. Whereas, when 0.1 g GAC was added, the mixture temperature stabilized at 300–315 °C after 5 min MW radiation. A larger GAC dosage led to a higher temperature of the soil/GAC system, which subsequently led to a higher degradation efficiency of CAP in the soil. 3.3. Effect of the soil mass It is widely accepted that the volume and shape of a sample are important parameters that determine the MW energy absorption (Diprose, 2001). Fig. 2c shows the degradation efficiencies at different soil masses under MW radiation. It is obvious that the degradation efficiency increased with the soil mass in the range from 0.5 to 3.0 g, which was consistent with the results reported by Liu and Yu (2006). For 3 g soil, 93% of CAP degradation efficiency was reached in 5 min, whereas for 0.5 g soil, it was only 80% even in 20 min MW radiation. The soil mass is an important parameter which affects the capability of soil in absorbing MW energy. Soil is a weaker microwave absorbent, without GAC the temperature of the soil only reached 150 °C and with no more increase in 700 W MW power, and the output MW energy has not yet been fully absorbed by the soil. When GAC was added into soil, the MW absorbed capability of the soil/GAC mixture increased rapidly. From Fig. 2e it can be clearly seen that the increase of the soil mass effectively raised its ability to absorb MW and led to much higher temperature. Since the ratio of GAC and soil was fixed at 1:5, the increase in soil mass meant the increasing dosage of GAC. The MW absorbing capability of the soil increased continuously with the increase of soil mass and GAC dosage in this range. The temperatures for 0.5 g soil mixed with 0.1 g GAC and 3 g soil mixed with 0.6 g GAC rose up to 315 and 800 °C by 20 min MW radiation, respectively. Since more MW energy was absorbed and a higher temperature of the soil was reached, the degradation efficiency of CAP increased with soil mass in this range. Liu and Yu (2006) found that when the soil mass increased from 10 to 20 g, the degradation efficiencies of PCB increased; whereas, a comparable difference of the degradation kinetics between 20 and 30 g soil was not found. 3.4. Effect of initial CAP concentration In order to examine the treatment capacity of MW radiation for CAP-contaminated soil, the effect of initial CAP concentration was studied. The degradation efficiencies by MW radiation with different initial CAP concentration are illustrated in Fig. 2d. The results demonstrated that lower initial concentration led to higher degradation efficiency. When the initial concentration was 10 mg kg 1, CAP in the soil was facile to be treated, and the degradation efficiency was nearly 100% by 20 min MW radiation. When the initial concentration increased to 10 000 mg kg 1, the degradation efficiencies were less than 50% after 20 min MW radiation. For a high initial concentration, larger GAC dosage and longer treatment time were supposed to be needed to get higher degradation efficiency.
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Summarily, trial results suggested a significant effect of MW power and GAC dosage on the degradation efficiencies by MW radiation. The degradation efficiencies increased with the soil mass in the range from 0.5 to 3.0 g, which was beneficial for the treatment of practical contaminated-soil in engineering application in some extent. The effect of initial CAP concentration was minute because the CAP concentration in actual soil was from lg kg 1 to mg kg 1. 3.5. CAP degradation mechanism To reveal the degradation mechanisms of CAP in soil by MW radiation, the extracts before and after MW radiation were directly analyzed by LC–MS. No intermediates were detected, which might be due to the extremely low concentration of the products. In order to acquire more information about intermediate products, the concentrated extracts before and after MW were both analyzed by LC– MS and GC–MS, as described in Section 2.3. The extracts were firstly analyzed by LC–MS in both negative and positive mode. In the negative mode, the deprotonated molecules [M–H] of CAP were seen in abundance with a mass of m/z 321 in the total ion chromatogram (TIC) of the extract before MW radiation. CAP with the parent m/z = 321 ionized well producing daughters of m/z = 257, 194, 176 and 152 in MS/MS spectra, which was consistent with the results reported by literature (Sheridan et al., 2008). Before MW radiation, the CAP (m/z = 321) concentration in the extract was high. After 20 min MW radiation, the CAP concentration decreased rapidly, the degradation efficiency was about 90%, which was analyzed by HPLC. The products with m/z = 249, 452, 385 and 166 were detected on the TIC after MW radiation, but the concentration of each product was very low. In positive mode of LC–MS, products with m/z = 217, 261, 453 and 475 were presented on TIC with low content. Fig. 3a shows the MS/MS fragment of m/z 166 and the proposed molecule structure of the product. By comparing the fragment of the product m/z 166 with 4-nitrobenzoic acid (C7H5NO4) (Fig. 3a), the fragment information of the product with m/z 166 corresponded well with the structure of 4-nitrobenzoic acid. Thus, the product was confirmed to be 4-nitrobenzoic acid. This result proved that CAP experienced decomposition process in MW radiation, and the proposed decomposition pathway of CAP to 4-nitrobenzoic acid is shown in Fig. 3b. CAP experienced carbon–carbon bond rupture reaction in MW radiation and the fragment containing phenyl was converted to 4-nitrobenzoic acid by oxidation. Hong et al. (2002) reported that when CAP powder was irradiated by c-radiation, the same degradation product of 4-nitrobenzoic acid was found in their study, and CH2Cl2 and CHCl3 were also detected as degradation products by headspace-GC–MS analysis. We proposed that the degradation mechanisms between c-radiation and MW radiation have somewhat similar, with the same kind of reactions of bond rupture and oxidation. The concentrations of chloride ion in soil before and after MW radiation were also measured by ion chromatogram in our study, and no increase of chloride ion concentration was found. Thus, it was proposed that no dechlorination process occurred during MW radiation. We inferred that the rest part of CAP may be degraded to CH2Cl2 and/or CHCl3 by MW radiation, but the two products can not be detected by LC– MS or GC–MS due to rotary evaporation process in the pretreatment procedure. The products in negative mode with m/z = 249, 385 and 452 and in positive mode with m/z = 217, 261, 453 and 474 were also detected, but the structures of these compounds could not be confirmed from their fragment information by LC–MS. GC–MS was also performed to analyze the products after MW radiation, many peaks of high boiling-point alkyl hydrocarbons appeared on the TIC, which were produced from GAC by MW radiation (Liu and Yu, 2006). Besides the alkyl hydrocarbons, some
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Fig. 3. Fragmentation of m/z 166 in LC–MS (a) and scheme of decomposition pathway of CAP to C7H5NO4 (b) and the typical products detected in GC–MS (c).
new peaks of products containing siloxanes appeared on the TIC, the structures of typical products and their molecular weights are shown in Fig. 3c. But the concentration of each product was very low. The FTIR spectrum results displayed that the soil organic matter contained the function groups of Si–O–Si and Si–(CH3)3, which was discussed in the preceding section. These new compounds (Fig. 3c) were the reaction products of CAP fragment and the soil organic matter. Therefore, it was proposed that a part of CAP decomposed and the fragment reacted with the soil organic matter and formed new compounds with larger molecular weight than CAP. Diatomite which was not contaminated by CAP was also treated in the same way as the CAP-contaminated diatomite, alkyl hydrocarbons were detected and no products containing siloxanes were found on the TIC of GC–MS. Summarily, CAP experienced carbon–carbon bond rupture and oxidation reactions in MW radiation, and a part of CAP fragment reacted with the soil organic matter and formed new compounds, but the concentration of each product was extremely low, no dechlorination process occurred during this process. The toxicity of the degradation products was also estimated. Hong et al. (2002) reported that healthy hazard data showed that 4-nitrobenzoic acid was safe to human health at trace level. Since the concentration of 4-nitrobenzoic acid was extremely low in soil, it was considered to be safe to human health and ecological
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