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Selection of sorbent for removing pesticides during water treatment
Journal of Hazardous Materials 169 (2009) 953–957
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Selection of sorbent for removing pesticides during water treatment Katarzyna Ignatowicz ∗ Department of Engineering and Environment Protection Technology, Technical University Białystok, ul. Wiejska 45a, 15-351 Białystok, Poland
a r t i c l e
i n f o
Article history: Received 6 August 2008 Received in revised form 9 April 2009 Accepted 13 April 2009 Available online 22 April 2009 Keywords: Sorption Activated carbon Herbicides Pesticides Freundlich isotherm
a b s t r a c t This paper presents research on phenoxyacid pesticides removal using sorption methods on activated carbons. It was noted, that physico-chemical properties of adsorbent and adsorbate as well as parameters of the process have influence on adsorption of pesticides, derivatives of phenoxyacetic acid on carbon. The experimental data were analyzed by the Freundlich isotherm. The best for remove from water on carbon NP-5 was 2,4-D. Equilibrium data fitted well with the Freundlich model with maximum adsorption capacity of NP-5 carbon. The exemplary sorption capacity at equilibrium concentration 10 mg L−1 were: 2,4-D 70 mg g−1 , MCPA 2 mg g−1 , MCPP 0.5 mg g−1 . The results indicated that coconut shell-based NP-5 carbon is most effective for the adsorption of phenoxyacetic acid from aqueous solutions. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The presence of pesticides in natural waters of such countries as: USA, Canada, Hungary, India, Russia, Germany, Greece, France, Switzerland, Poland, and also in the Baltic Sea and the North Sea has been proved many times [1–9]. For the last few years the concentration of compounds belonging to phenoxyacid group has been observed to increase. An alarming fact is that these compounds are more and more often detected in shallow or even deep underground waters. Concentration of marked pesticides in Polish surface waters reaches a level as high as 290 g L−1 . The following compounds are detected most often and in the large quantities: lindan, DDT, which has not been used for many years now, simazina and atrazina, chlorofenwinfos and fenitrotion, 2,4-D, MCPA, MCPP. The presence ˙ of pesticides in surface waters in the Zuławy Wi´slane region has ˙ been described many times by Zelechowska and Makowski [9]. Additionally, my own research confirmed the presence of phenoxyacid herbicides (2,4-D, MCPA, MCPP) in the surface waters in the northern-eastern region—in the Supra´sl, Białka, Narew and Biebrza rivers [6]. We have comparatively little reliable information on the presence of pesticides in underground waters in Poland. A reconnaissance research carried out by the Institute of Protection of Plants has proved the presence of 27 compounds. In shallow underground waters (depth of water less than 4 m) the most often present
∗ Tel.: +48 085 7469644. E-mail address: [email protected]. 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.04.061
compound was atrazina in quantities up to half a dozen g L−1 . The examination of wells carried out during the period of intensive pesticides usage showed their presence in 305 country-wells in Lublin Region. Examination of ground waters in Poznan´ Region also confirmed the presence of pesticide compounds [4]. There is an inseparable connection between the presence of pesticides in natural waters and their presence in drinking water. Even relatively small quantities of pesticides cause changes of organoleptic characteristics of water. A feature disqualifying polluted water is its very specific smell caused by different substances and deteriorated taste. Pesticides can make water smell of earth, mould, chlorine, onion, etc. Some compounds cause white turbidity, yellow tinge, foaming, etc. Acceptable concentration of pesticides in drinkable water was tightened following the introduction (March, 2007) of new standards for drinkable water. The amount of a single pesticide compound cannot exceed 0.1 g L−1 while all of the pesticides—0.5 g L−1 . It should be mentioned at this point that those strict standards follow the directives of the European Union. There is consequently the need to intensify the potable water treatment. Additionally, special attention should be given to subsequent stages of the removal of organic compounds residue, including pesticide residue. Several methods are available for pesticides removal such as photocatalytic degradation [10,11], combined photo-Fenton and biological oxidation [12], advanced oxidation processes [13], aerobic degradation [14], nanofiltration membranes [15], ozonation [16] and adsorption [17–19]. Adsorption on activated carbon is the most widespread technology used to deal with purification of water contaminated by pesticides.
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K. Ignatowicz / Journal of Hazardous Materials 169 (2009) 953–957
Therefore, the purpose of this work was to evaluate the adsorption potential of activated carbon for phenoxyacetic acids. The equilibrium of the adsorption was then studied to understand the adsorption mechanism of 2,4-D, MCPA and MCPP molecules using activated carbon made with different materials. 2. Materials and methods 2.1. Sorbates Technical grade phenoxyacetic acid herbicide (2,4-D, MCPA, MCPP) of 99% purity obtained from Institute of Industrial Organic Chemistry in Poland were used as adsorbates. The concentrations of prepared solutions were applied: 130 mg 2,4-D per litre, 100 mg MCPA per litre and 150 mg MCPP per litre. 2.2. Sorbents Commercial coal-based activated carbon WD-extra, WD-1, DTO and coconut shells-based NP-5 were used in this study. Prior to use, carbons were repeatedly washed with distilled water in order to remove pollutants and subsequently dried using hot air oven at 150 ◦ C for at least 3 h [17,20,21]. Table 1 presents physical properties of the activated carbons. 2.3. Analytical procedure The phenoxyacetic acids and their metabolites were also determined by means of chromatographic technique HPLC in Institute of Plant Protection (IPP). IPP has used DiCorcia and Marchetti techniques determined for phenoxyacetic acids derivatives [22–25]. The initial and equilibrium concentrations of adsorbate solutions were determined by HPLC by means of Waters Alliance 2695 XC with UV–vis spectrophotometric detector. Liquid–liquid extraction methods were employed for the extraction of phenoxy herbicides from water. Thus, a 20 mL water sample was acidified with 100 mL of 35% HCl, and extracted with three times with 2 mL of n-hexane ethyl acetate (20:80). The extracts were evaporated to ca. 0.5 mL. The residue was mixed with 10 mL of 0.25 mmol 9-anthryl-diazomethane (ADAM) in acetone and allowed to react for 4 h at 40 ◦ C in the dark. After derivatization, the solvents were evaporated at 40 ◦ C and the fluorescent derivatives were purified on a silica column with n-hexane ethyl acetate (95:5, v/v) [22]. The efficiencies of liquid–liquid extraction and SPE for the extraction of chlorophenoxy acid herbicides from water were compared [22]. Water samples (20 mL each) were treated with Na2 SO3 , to remove free chlorine, and then acidified with HCl. Samples were extracted with 3 mL of benzene followed by 4 mL of ethyl acetate–n-hexane (8 + 2 mL). The organic phases were evaporated and derivatized with ADAM, as described above. RP-18 cartridges were conditioned with 3 mL of dichloromethane, methanol and 0.1 M HCl. Samples were passed through the cartridge at 4 mL min−1 and were removed with 0.5 mL of methanol and 3 mL Table 1 Physical properties of activated carbon. Parameter
Surface area (m2 g−1 ) Total pore volume (cm3 g−1 ) Granulation (mm) Dechloration (cm) Methylene blue (cm3 ) Iodine number (mg g−1 ) Hardness (%) Grindability (%)
Carbon WD-extra
WD-1
NP-5
DTO
950–1050 0.85–0.95 1.0–1.5 4–5 22 900–1000 90 3.0
900–1000 0.8–0.9 1.2–2.0 5–7 min 18 850–950 90 3.0
1300–1500 min 0.7 0.75–1.2 5–8 min 40 1390 95–97 0.3
– – 0.3–0.75 <3 min 50 750 – –
Fig. 1. HPLC of 2.5 mg of each phenoxyacid extracted from 300 mL of tap water using an optimized method on Separon SGX C18 with a mixture of methanol and 0.01 mol L acetic acid (61:39, v/v) as the mobile phase. Phenoxyacids were eluted in the following order: (1) 2,4-D (7.10 min), (2) MCPA (8.80 min), (3) 2,4,5-T (10.23 min) and (4) MCPP (13.88 min). (Reprinted with permission from Ref. [22].)
of dichloromethane–methanol (8:2, v/v). The organic phase was evaporated and the residue was derivatized with ADAM. MCPA, MCPP and 2,4-D were successfully separated on a C18 column using an isocratic eluent (methanol–0.01 M acetic acid, 61:39, v/v), as illustrated in Fig. 1. The flow-rate was 0.35 mL min−1 and the phenoxyalkyl acid herbicides were detected at 235 nm. No interference from the coextracted compounds was observed and the detection limit was 0.03 g L−1 . The method was proposed for the analysis of drinking water. 2.4. Adsorption procedure Adsorption experiments were carried out by adding a known mass of active carbon (0.01; 0.02; 0.05; 0.1; 0.25; 0.5 and 1.0 g on every 100 mL of the solution) to glass flasks filled phenoxyacetic acids aqueous solution. The flasks were shaken in an isothermal water bath shaker at 130 rpm and 20 ◦ C for 24 h and then were left for 24 h in order to reach sorption equilibrium [18–21]. The initial and equilibrium herbicide concentrations were determined by HPLC by means of Waters Alliance chromatograph. 3. Results and discussion Freundlich’s isotherms (A = kc1/n ) were plotted on the base of achieved results applying Statistica version 6.0 and Sorp-Lab software in order to analyze the processes. Constants k and 1/n were estimated by means of the Marquardt-Levenberg optimization procedure. In this optimization procedure, two criteria (sum of squares and parameter convergence) are met to obtain parameter values. Values of k and 1/n parameters as well as correlation coefficients R for particular adsorbates are presented in Table 2. Figs. 2–7 present adsorption isotherms for studied pesticides on applied adsorbents as a function of adsorbate amount adsorbed by adsorbent weight unit (x/m) vs. adsorbate’s equilibrium concentration (c0 ). According to the Giles classification two groups of isotherms (Fig. 6) were obtained: S and L [26,27]. Group S includes isotherms for systems in which the solvent undergoes
K. Ignatowicz / Journal of Hazardous Materials 169 (2009) 953–957
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Table 2 Isotherm parameters for removal pesticide. Coefficient
Phenoxyacetic compound 2,4-D
MCPA
MCPP
NP-5 k 1/n R
0.695 1.942 0.984
WD-1 k 1/n R
0.0026 2.907 0.900
20.420 0.141 0.655
0.003 2.857 0.849
DTO k 1/n R
0.684 1.600 0.975
19.166 0.221 0.943
0.001 2.469 0.833
WD-extra k 1/n R
0.0097 2.321 0.888
9.117 0.366 0.938
0.026 1.761 0.768
0.0001 3.690 0.951
0.0004 3.145 0.967
Fig. 4. The Freundlich adsorption isotherm of phenoxyacetic acid derivative on carbon WD-1: y = 22.46x0.5 , R = 0.52.
Source: Tabulated on the basis of this study.
Fig. 5. The Freundlich adsorption isotherm of phenoxyacetic acid derivative on carbon DTO: y = 54.51x0.314 , R = 0.51.
Fig. 2. The Freundlich adsorption isotherm of phenoxyacetic acid derivative on carbon WD-extra: y = 4x0.76 , R = 0.66.
strong sorption, that is to say it competes with the adsorbed dissolved substance. The isotherm’s shape also testifies to the fact that the adsorbate particle may be placed vertically or at an angle in the active centre of the adsorbent. Group L is includes isotherms for systems in which solvent is not strongly adsorbed and is not competitive for dissolved substance being adsorbed [28]. Isotherm’s
Fig. 3. The Freundlich adsorption isotherm of phenoxyacetic acid derivative on carbon NP-5: y = 45,958x0.495 , R = 0.60.
shape also proves the flat arrangement of adsorbate’s molecule at adsorbent’s active centres. Knowledge on 1/n parameter value in Freundlich’s formula allows for assessing the adsorption intensity of a given substance from water phase on adsorbent; value of k constant determines the sorption capacity of a adsorbent at equilibrium concentration in a
Fig. 6. The comparison of the Freundlich isotherms of phenoxyacetic pesticide on chosen sorbents.
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K. Ignatowicz / Journal of Hazardous Materials 169 (2009) 953–957
sorption. Dissolubility of the compounds examined can be ordered as follows: MCPP > MCPA > 2, 4-D The exemplary values of sorption capacity of carbon NP5 at equilibrium concentration 10 mg L−1 are: 2,4-D—70 mg g−1 ; MCPA—2 mg g−1 ; MCPP—0.5 mg g−1 . Similarly, substances with high molecular weight have tendency to be adsorbed more strongly than chemical compounds with low molecular weight. This rule was confirmed in our experiments. The phenoxyacid compounds examined can be ordered according to the increasing molecular weight as follows: 2, 4-D > MCPP > MCPA
Fig. 7. The comparison of the Freundlich isotherms of phenoxyacetic pesticide on chosen sorbents.
solution (Table 2). Higher k value corresponds to higher sorption capacity. In own studies, higher value of k coefficient was achieved for MCPA on WD-1 carbon. Constants 1/n in Freundlich’s formula are directional coefficients of isotherms equal to the tangent of line inclination angle in logarithmic coordinates. Therefore, the higher 1/n value, the more intensive adsorption process. Also 1/n coefficient for the WD-extra carbon is two times higher, which proves higher intensity of the phenoxyacetic pesticides retained. Sorption of phenoxyacetic acid pesticides is similar for the whole group. No substantial differences were noticed in detoxication of the substances previously discussed. This fact could be described by means of Freundlich isotherm (Figs. 6–7) for the whole group of phenoxyacid pesticides on each type of sorbent: WD-extra A = 4.0c 0.76
R = 0.66
DTO A = 54.51c 0.314
R = 0.51
WD-1 A = 22.46c 0.5
R = 0.52
NP-5 A = 45.96c 0.49
R = 0.60
It is an important because in future it will be possible, judging from the behaviour of just one phenoxyacid acid, to draw preliminary conclusions about the rest of the phenoxyacid group subjected to sorption during the water treatment process. In this way the study of the process of sorption will be more cost-effective and less timeconsuming. The easiest to remove from water on carbon NP-5 was acid 2,4-D, the worst to remove was MCPP. This can be explained by the presence of additional group—CH3 in MCPP acid, its chiral properties as well as by the fact that it is a strong enantiomer R. On WD-extra carbon the acid 2,4-D was the most effectively sorbed, and MCPA was the least effectively sorbed. Numerous experiments point out that the effectiveness of sorption depends on dissolubility of the compound, the adsorbent and the adsorbate. The behaviour of a given system can be predicted assuming that adsorption involves a distribution of the adsorbate into two phases: liquid and solid. Generally speaking there is an inversely proportional dependence between the adsorption of a given substance and its solubility in the solvent. Namely, the less soluble the substance, the more likely it is to be adsorbed. The above remarks fully reflect the results of the experiment: the least soluble compound, i.e. acid 2,4-D had the highest capacity of
It is known that surface functional groups, behaving as volumetric compounds take part in the adsorbate–adsorbent interaction [29]. The chemical composition of the active carbon surface in an essential way influences its following properties: adsorption capacity, electrochemical, hydrophilic–hydrophobic, acidic–basic, oxido-reduction, catalytic, and others. In the case of ordinary active carbons the presence of oxygen on the surface has a decisive influence on its chemical properties. The carbons examined were activated with water vapour at over 850 ◦ C, which proves that groups of alkaline character were formed on their surface. These are the so called H type carbons, which adsorb acids from water solutions. According to previous studies it is quite difficult to assess the quality of basic/alkaline functional groups of H type carbons. It is difficult to predict the kind of interaction between the surface of H type carbons and phenoxyacetic acids. It is unquestionable, however, that these compounds are well adsorbed [28–30]. 4. Conclusions The present study shows that activated carbon prepared from coals and coconut shells can be used as an adsorbent for the removal of phenoxyacetic pesticides from aqueous solutions. The equilibrium data were fitted to the Freundlich isotherms models. The adsorption of phenoxyacetic acids herbicides on activated carbons is influenced by physico-chemical properties of the adsorbent (alkaline character of the BET surface area, total pore volume, methylene blue, iodine number) and the adsorbate (the acid character of phenoxyacid pesticides, the presence of aromatic ring, dissociation constant, dissolubility, molecular weight) as well as the parameters of the process conducted (active carbon mass). Carbon NP-5 proved to be the best sorbent for the process of removal of phenoxyacetic acids from water. Acknowledgments Financial support for this research was provided by Ministry of ´ Science and Higher Education within the project W/IIS/23/07 and N305 070 32/2535. References [1] B.T. Croll, J. Inst. Water Environ. Manage. 5 (1991) 389. [2] R. Seux, P. Chambon, M. Sebillote, J. Bontoux, J.P. Duguet, D. D’arras, D. Cleret, J.P. Godet, P. Schulhof, Tech. Sci. Methodes: Genie Urbain Genie Rural 2 (1999) 34. [3] P. Anderson, R. Jack, Ch. Burke, J. Cowles, B. Moran, Surface Water Monitoring Program for Pesticides in Salmonid-Bearing Streams April to December 2003, WSDA, Washington State, Department of Agriculture, Ecology Publication 0403-048, 2004. [4] J. Siepak, J. Zerbe, M. Kabacinski, Przyroda i Człowiek 4 (1993) 117. [5] J.K. Stamer, W.A. Battaglin, D.A. Goolsby, Herbicides in Midwestern Reservoir Outflows, 1992–93, U.S. Geological Survey Open-File Report 134-98, 1998. [6] K. Ignatowicz, Arch. Environ. Prot. 30 (2004) 51.
K. Ignatowicz / Journal of Hazardous Materials 169 (2009) 953–957 [7] D.A. Lambropoulou, V.A. Sakkas, D.G. Hela, T.A. Albanis, J. Chromatogr. A 963 (2002) 107. [8] M.B. Woudneh, M. Sekela, T. Tuominen, M. Gledhill, J. Chromatogr. A 1139 (2007) 121. ´ ˙ [9] A. Zelechowska, Z. Makowski, Ochrona Srodowiska 3 (1997) 7. [10] T. Aungpradit, P. Sutthivaiyakit, D. Martens, S. Sutthivaiyakit, A.A.F. Kettrup, J. Hazard. Mater. 146 (2007) 204. [11] M. Mahalakshmi, B. Arabindoo, M. Palanichamy, V. Murugesan, J. Hazard. Mater. 143 (2007) 240. [12] M.M. Ballesteros Martin, J.A. Sanchez Perez, J.L. Garcia Sanchez, L. Montes de Oca, J.L. Casas Lopez, I. Oller, S. Malato Rodriguez, J. Hazard. Mater. 155 (2008) 342. [13] P. Saritha, C. Aparna, V. Himabindu, Y. Anjaneyulu, J. Hazard. Mater. 149 (2007) 609. [14] H.M. Rajashekara Murthy, H.K. Manonmani, J. Hazard. Mater. 149 (2007) 18. [15] A.L. Ahmad, L.S. Tan, S.R.A. Shukor, J. Hazard. Mater. 151 (2008) 71. [16] M.I. Maldonado, S. Malato, L.A. Pérez-Estrada, W. Gernjak, I. Oller, X. Doménech, J. Peral, J. Hazard. Mater. 38 (2006) 363. [17] C.F. Chang, C.Y. Chang, K.E. Hsu, S.C. Lee, W. Höll, J. Hazard. Mater. 155 (2008) 295.
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[18] B.H. Hameed, J.M. Salman, A.L. Ahmad, Adsorption isotherm and kinetic modeling of 2,4-D pesticide on activated carbon derived from date stones, J. Hazard. Mater., in press. [19] N.K. Hamadi, S. Swaminathan, X.D. Chen, J. Hazard. Mater. B112 (2004) 133. [20] A. Kumar, S. Kumar, S. Kumar, Carbon 41 (2003) 3015. [21] N. Witbowo, L. Setyadhi, D. Witbowo, J. Setiawan, S. Ismadji, J. Hazard. Mater. 146 (2007) 237. [22] T. Cserhati, E. Forgaes, J. Chromatogr. B 717 (1998) 157. [23] S. Moret, J.M. Sanchez, V. salvado, M. Hidalgo, J. Chromatogr. A 1099 (2005) 55. [24] A. Di Corcia, M. Marchetti, Anal. Chem. 63 (1991) 580. [25] A. Balinova, J. Chromatogr. A 754 (1996) 125. [26] C.H. Giles, T.H. MacEwan, S.M. Nakhwa, D. Smith, J. Chem. Soc. 3 (1960) 3973. [27] Yuh-Shan Ho, Pol. J. Environ. Stud. 15 (2006) 81. [28] T.E. Ochsner, B.M. Stephens, W.C. Koskinen, R.S. Kookana, Soil Sci. Soc. Am. 70 (2006) 1991. [29] R. Zbytniewski, B. Buszewski, Pol. J. Environ. Stud. 11 (2002) 179. [30] C.A. Spadotto, A.G. Hornsby, J. Environ. Qual. 32 (2003) 949.
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Selection of sorbent for removing pesticides during water treatment Katarzyna Ignatowicz ∗ Department of Engineering and Environment Protection Technology, Technical University Białystok, ul. Wiejska 45a, 15-351 Białystok, Poland
a r t i c l e
i n f o
Article history: Received 6 August 2008 Received in revised form 9 April 2009 Accepted 13 April 2009 Available online 22 April 2009 Keywords: Sorption Activated carbon Herbicides Pesticides Freundlich isotherm
a b s t r a c t This paper presents research on phenoxyacid pesticides removal using sorption methods on activated carbons. It was noted, that physico-chemical properties of adsorbent and adsorbate as well as parameters of the process have influence on adsorption of pesticides, derivatives of phenoxyacetic acid on carbon. The experimental data were analyzed by the Freundlich isotherm. The best for remove from water on carbon NP-5 was 2,4-D. Equilibrium data fitted well with the Freundlich model with maximum adsorption capacity of NP-5 carbon. The exemplary sorption capacity at equilibrium concentration 10 mg L−1 were: 2,4-D 70 mg g−1 , MCPA 2 mg g−1 , MCPP 0.5 mg g−1 . The results indicated that coconut shell-based NP-5 carbon is most effective for the adsorption of phenoxyacetic acid from aqueous solutions. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The presence of pesticides in natural waters of such countries as: USA, Canada, Hungary, India, Russia, Germany, Greece, France, Switzerland, Poland, and also in the Baltic Sea and the North Sea has been proved many times [1–9]. For the last few years the concentration of compounds belonging to phenoxyacid group has been observed to increase. An alarming fact is that these compounds are more and more often detected in shallow or even deep underground waters. Concentration of marked pesticides in Polish surface waters reaches a level as high as 290 g L−1 . The following compounds are detected most often and in the large quantities: lindan, DDT, which has not been used for many years now, simazina and atrazina, chlorofenwinfos and fenitrotion, 2,4-D, MCPA, MCPP. The presence ˙ of pesticides in surface waters in the Zuławy Wi´slane region has ˙ been described many times by Zelechowska and Makowski [9]. Additionally, my own research confirmed the presence of phenoxyacid herbicides (2,4-D, MCPA, MCPP) in the surface waters in the northern-eastern region—in the Supra´sl, Białka, Narew and Biebrza rivers [6]. We have comparatively little reliable information on the presence of pesticides in underground waters in Poland. A reconnaissance research carried out by the Institute of Protection of Plants has proved the presence of 27 compounds. In shallow underground waters (depth of water less than 4 m) the most often present
∗ Tel.: +48 085 7469644. E-mail address: [email protected]. 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.04.061
compound was atrazina in quantities up to half a dozen g L−1 . The examination of wells carried out during the period of intensive pesticides usage showed their presence in 305 country-wells in Lublin Region. Examination of ground waters in Poznan´ Region also confirmed the presence of pesticide compounds [4]. There is an inseparable connection between the presence of pesticides in natural waters and their presence in drinking water. Even relatively small quantities of pesticides cause changes of organoleptic characteristics of water. A feature disqualifying polluted water is its very specific smell caused by different substances and deteriorated taste. Pesticides can make water smell of earth, mould, chlorine, onion, etc. Some compounds cause white turbidity, yellow tinge, foaming, etc. Acceptable concentration of pesticides in drinkable water was tightened following the introduction (March, 2007) of new standards for drinkable water. The amount of a single pesticide compound cannot exceed 0.1 g L−1 while all of the pesticides—0.5 g L−1 . It should be mentioned at this point that those strict standards follow the directives of the European Union. There is consequently the need to intensify the potable water treatment. Additionally, special attention should be given to subsequent stages of the removal of organic compounds residue, including pesticide residue. Several methods are available for pesticides removal such as photocatalytic degradation [10,11], combined photo-Fenton and biological oxidation [12], advanced oxidation processes [13], aerobic degradation [14], nanofiltration membranes [15], ozonation [16] and adsorption [17–19]. Adsorption on activated carbon is the most widespread technology used to deal with purification of water contaminated by pesticides.
954
K. Ignatowicz / Journal of Hazardous Materials 169 (2009) 953–957
Therefore, the purpose of this work was to evaluate the adsorption potential of activated carbon for phenoxyacetic acids. The equilibrium of the adsorption was then studied to understand the adsorption mechanism of 2,4-D, MCPA and MCPP molecules using activated carbon made with different materials. 2. Materials and methods 2.1. Sorbates Technical grade phenoxyacetic acid herbicide (2,4-D, MCPA, MCPP) of 99% purity obtained from Institute of Industrial Organic Chemistry in Poland were used as adsorbates. The concentrations of prepared solutions were applied: 130 mg 2,4-D per litre, 100 mg MCPA per litre and 150 mg MCPP per litre. 2.2. Sorbents Commercial coal-based activated carbon WD-extra, WD-1, DTO and coconut shells-based NP-5 were used in this study. Prior to use, carbons were repeatedly washed with distilled water in order to remove pollutants and subsequently dried using hot air oven at 150 ◦ C for at least 3 h [17,20,21]. Table 1 presents physical properties of the activated carbons. 2.3. Analytical procedure The phenoxyacetic acids and their metabolites were also determined by means of chromatographic technique HPLC in Institute of Plant Protection (IPP). IPP has used DiCorcia and Marchetti techniques determined for phenoxyacetic acids derivatives [22–25]. The initial and equilibrium concentrations of adsorbate solutions were determined by HPLC by means of Waters Alliance 2695 XC with UV–vis spectrophotometric detector. Liquid–liquid extraction methods were employed for the extraction of phenoxy herbicides from water. Thus, a 20 mL water sample was acidified with 100 mL of 35% HCl, and extracted with three times with 2 mL of n-hexane ethyl acetate (20:80). The extracts were evaporated to ca. 0.5 mL. The residue was mixed with 10 mL of 0.25 mmol 9-anthryl-diazomethane (ADAM) in acetone and allowed to react for 4 h at 40 ◦ C in the dark. After derivatization, the solvents were evaporated at 40 ◦ C and the fluorescent derivatives were purified on a silica column with n-hexane ethyl acetate (95:5, v/v) [22]. The efficiencies of liquid–liquid extraction and SPE for the extraction of chlorophenoxy acid herbicides from water were compared [22]. Water samples (20 mL each) were treated with Na2 SO3 , to remove free chlorine, and then acidified with HCl. Samples were extracted with 3 mL of benzene followed by 4 mL of ethyl acetate–n-hexane (8 + 2 mL). The organic phases were evaporated and derivatized with ADAM, as described above. RP-18 cartridges were conditioned with 3 mL of dichloromethane, methanol and 0.1 M HCl. Samples were passed through the cartridge at 4 mL min−1 and were removed with 0.5 mL of methanol and 3 mL Table 1 Physical properties of activated carbon. Parameter
Surface area (m2 g−1 ) Total pore volume (cm3 g−1 ) Granulation (mm) Dechloration (cm) Methylene blue (cm3 ) Iodine number (mg g−1 ) Hardness (%) Grindability (%)
Carbon WD-extra
WD-1
NP-5
DTO
950–1050 0.85–0.95 1.0–1.5 4–5 22 900–1000 90 3.0
900–1000 0.8–0.9 1.2–2.0 5–7 min 18 850–950 90 3.0
1300–1500 min 0.7 0.75–1.2 5–8 min 40 1390 95–97 0.3
– – 0.3–0.75 <3 min 50 750 – –
Fig. 1. HPLC of 2.5 mg of each phenoxyacid extracted from 300 mL of tap water using an optimized method on Separon SGX C18 with a mixture of methanol and 0.01 mol L acetic acid (61:39, v/v) as the mobile phase. Phenoxyacids were eluted in the following order: (1) 2,4-D (7.10 min), (2) MCPA (8.80 min), (3) 2,4,5-T (10.23 min) and (4) MCPP (13.88 min). (Reprinted with permission from Ref. [22].)
of dichloromethane–methanol (8:2, v/v). The organic phase was evaporated and the residue was derivatized with ADAM. MCPA, MCPP and 2,4-D were successfully separated on a C18 column using an isocratic eluent (methanol–0.01 M acetic acid, 61:39, v/v), as illustrated in Fig. 1. The flow-rate was 0.35 mL min−1 and the phenoxyalkyl acid herbicides were detected at 235 nm. No interference from the coextracted compounds was observed and the detection limit was 0.03 g L−1 . The method was proposed for the analysis of drinking water. 2.4. Adsorption procedure Adsorption experiments were carried out by adding a known mass of active carbon (0.01; 0.02; 0.05; 0.1; 0.25; 0.5 and 1.0 g on every 100 mL of the solution) to glass flasks filled phenoxyacetic acids aqueous solution. The flasks were shaken in an isothermal water bath shaker at 130 rpm and 20 ◦ C for 24 h and then were left for 24 h in order to reach sorption equilibrium [18–21]. The initial and equilibrium herbicide concentrations were determined by HPLC by means of Waters Alliance chromatograph. 3. Results and discussion Freundlich’s isotherms (A = kc1/n ) were plotted on the base of achieved results applying Statistica version 6.0 and Sorp-Lab software in order to analyze the processes. Constants k and 1/n were estimated by means of the Marquardt-Levenberg optimization procedure. In this optimization procedure, two criteria (sum of squares and parameter convergence) are met to obtain parameter values. Values of k and 1/n parameters as well as correlation coefficients R for particular adsorbates are presented in Table 2. Figs. 2–7 present adsorption isotherms for studied pesticides on applied adsorbents as a function of adsorbate amount adsorbed by adsorbent weight unit (x/m) vs. adsorbate’s equilibrium concentration (c0 ). According to the Giles classification two groups of isotherms (Fig. 6) were obtained: S and L [26,27]. Group S includes isotherms for systems in which the solvent undergoes
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Table 2 Isotherm parameters for removal pesticide. Coefficient
Phenoxyacetic compound 2,4-D
MCPA
MCPP
NP-5 k 1/n R
0.695 1.942 0.984
WD-1 k 1/n R
0.0026 2.907 0.900
20.420 0.141 0.655
0.003 2.857 0.849
DTO k 1/n R
0.684 1.600 0.975
19.166 0.221 0.943
0.001 2.469 0.833
WD-extra k 1/n R
0.0097 2.321 0.888
9.117 0.366 0.938
0.026 1.761 0.768
0.0001 3.690 0.951
0.0004 3.145 0.967
Fig. 4. The Freundlich adsorption isotherm of phenoxyacetic acid derivative on carbon WD-1: y = 22.46x0.5 , R = 0.52.
Source: Tabulated on the basis of this study.
Fig. 5. The Freundlich adsorption isotherm of phenoxyacetic acid derivative on carbon DTO: y = 54.51x0.314 , R = 0.51.
Fig. 2. The Freundlich adsorption isotherm of phenoxyacetic acid derivative on carbon WD-extra: y = 4x0.76 , R = 0.66.
strong sorption, that is to say it competes with the adsorbed dissolved substance. The isotherm’s shape also testifies to the fact that the adsorbate particle may be placed vertically or at an angle in the active centre of the adsorbent. Group L is includes isotherms for systems in which solvent is not strongly adsorbed and is not competitive for dissolved substance being adsorbed [28]. Isotherm’s
Fig. 3. The Freundlich adsorption isotherm of phenoxyacetic acid derivative on carbon NP-5: y = 45,958x0.495 , R = 0.60.
shape also proves the flat arrangement of adsorbate’s molecule at adsorbent’s active centres. Knowledge on 1/n parameter value in Freundlich’s formula allows for assessing the adsorption intensity of a given substance from water phase on adsorbent; value of k constant determines the sorption capacity of a adsorbent at equilibrium concentration in a
Fig. 6. The comparison of the Freundlich isotherms of phenoxyacetic pesticide on chosen sorbents.
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K. Ignatowicz / Journal of Hazardous Materials 169 (2009) 953–957
sorption. Dissolubility of the compounds examined can be ordered as follows: MCPP > MCPA > 2, 4-D The exemplary values of sorption capacity of carbon NP5 at equilibrium concentration 10 mg L−1 are: 2,4-D—70 mg g−1 ; MCPA—2 mg g−1 ; MCPP—0.5 mg g−1 . Similarly, substances with high molecular weight have tendency to be adsorbed more strongly than chemical compounds with low molecular weight. This rule was confirmed in our experiments. The phenoxyacid compounds examined can be ordered according to the increasing molecular weight as follows: 2, 4-D > MCPP > MCPA
Fig. 7. The comparison of the Freundlich isotherms of phenoxyacetic pesticide on chosen sorbents.
solution (Table 2). Higher k value corresponds to higher sorption capacity. In own studies, higher value of k coefficient was achieved for MCPA on WD-1 carbon. Constants 1/n in Freundlich’s formula are directional coefficients of isotherms equal to the tangent of line inclination angle in logarithmic coordinates. Therefore, the higher 1/n value, the more intensive adsorption process. Also 1/n coefficient for the WD-extra carbon is two times higher, which proves higher intensity of the phenoxyacetic pesticides retained. Sorption of phenoxyacetic acid pesticides is similar for the whole group. No substantial differences were noticed in detoxication of the substances previously discussed. This fact could be described by means of Freundlich isotherm (Figs. 6–7) for the whole group of phenoxyacid pesticides on each type of sorbent: WD-extra A = 4.0c 0.76
R = 0.66
DTO A = 54.51c 0.314
R = 0.51
WD-1 A = 22.46c 0.5
R = 0.52
NP-5 A = 45.96c 0.49
R = 0.60
It is an important because in future it will be possible, judging from the behaviour of just one phenoxyacid acid, to draw preliminary conclusions about the rest of the phenoxyacid group subjected to sorption during the water treatment process. In this way the study of the process of sorption will be more cost-effective and less timeconsuming. The easiest to remove from water on carbon NP-5 was acid 2,4-D, the worst to remove was MCPP. This can be explained by the presence of additional group—CH3 in MCPP acid, its chiral properties as well as by the fact that it is a strong enantiomer R. On WD-extra carbon the acid 2,4-D was the most effectively sorbed, and MCPA was the least effectively sorbed. Numerous experiments point out that the effectiveness of sorption depends on dissolubility of the compound, the adsorbent and the adsorbate. The behaviour of a given system can be predicted assuming that adsorption involves a distribution of the adsorbate into two phases: liquid and solid. Generally speaking there is an inversely proportional dependence between the adsorption of a given substance and its solubility in the solvent. Namely, the less soluble the substance, the more likely it is to be adsorbed. The above remarks fully reflect the results of the experiment: the least soluble compound, i.e. acid 2,4-D had the highest capacity of
It is known that surface functional groups, behaving as volumetric compounds take part in the adsorbate–adsorbent interaction [29]. The chemical composition of the active carbon surface in an essential way influences its following properties: adsorption capacity, electrochemical, hydrophilic–hydrophobic, acidic–basic, oxido-reduction, catalytic, and others. In the case of ordinary active carbons the presence of oxygen on the surface has a decisive influence on its chemical properties. The carbons examined were activated with water vapour at over 850 ◦ C, which proves that groups of alkaline character were formed on their surface. These are the so called H type carbons, which adsorb acids from water solutions. According to previous studies it is quite difficult to assess the quality of basic/alkaline functional groups of H type carbons. It is difficult to predict the kind of interaction between the surface of H type carbons and phenoxyacetic acids. It is unquestionable, however, that these compounds are well adsorbed [28–30]. 4. Conclusions The present study shows that activated carbon prepared from coals and coconut shells can be used as an adsorbent for the removal of phenoxyacetic pesticides from aqueous solutions. The equilibrium data were fitted to the Freundlich isotherms models. The adsorption of phenoxyacetic acids herbicides on activated carbons is influenced by physico-chemical properties of the adsorbent (alkaline character of the BET surface area, total pore volume, methylene blue, iodine number) and the adsorbate (the acid character of phenoxyacid pesticides, the presence of aromatic ring, dissociation constant, dissolubility, molecular weight) as well as the parameters of the process conducted (active carbon mass). Carbon NP-5 proved to be the best sorbent for the process of removal of phenoxyacetic acids from water. Acknowledgments Financial support for this research was provided by Ministry of ´ Science and Higher Education within the project W/IIS/23/07 and N305 070 32/2535. References [1] B.T. Croll, J. Inst. Water Environ. Manage. 5 (1991) 389. [2] R. Seux, P. Chambon, M. Sebillote, J. Bontoux, J.P. Duguet, D. D’arras, D. Cleret, J.P. Godet, P. Schulhof, Tech. Sci. Methodes: Genie Urbain Genie Rural 2 (1999) 34. [3] P. Anderson, R. Jack, Ch. Burke, J. Cowles, B. Moran, Surface Water Monitoring Program for Pesticides in Salmonid-Bearing Streams April to December 2003, WSDA, Washington State, Department of Agriculture, Ecology Publication 0403-048, 2004. [4] J. Siepak, J. Zerbe, M. Kabacinski, Przyroda i Człowiek 4 (1993) 117. [5] J.K. Stamer, W.A. Battaglin, D.A. Goolsby, Herbicides in Midwestern Reservoir Outflows, 1992–93, U.S. Geological Survey Open-File Report 134-98, 1998. [6] K. Ignatowicz, Arch. Environ. Prot. 30 (2004) 51.
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