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The synthesis of humic acids graft copolymer and its adsorption for organic pesticides
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JIEC-1440; No. of Pages 7 Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx
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The synthesis of humic acids graft copolymer and its adsorption for organic pesticides Chengjian Yang a,b, Qingru Zeng a,*, Yang Yang a, Ruiyang Xiao a, Yunzhong Wang b, Hui Shi b a b
Department of Environment and Science, Hunan Agricultural University, Changsha 410000, PR China College of Environment & Municipal Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, PR China
A R T I C L E I N F O
Article history: Received 8 December 2012 Accepted 1 July 2013 Available online xxx Keywords: Humic acid Modification Adsorption Organic pesticide
A B S T R A C T
humic acid graft copolymer (PSt-g-HA) was prepared by graft copolymerization of Humic acid (HA) with styrene and the sorption of three selected organic pesticides, parathion-methyl, carbaryl and carbofuran by PSt-g-HA acid and untreated humic were also examined, respectively. The PSt-g-HA had relatively high aromatic carbons content because the grafted copolymerization of polystyrene entered into condensed domains in HA, whereas the removal of the polar function groups, such as carboxyl and phenolic hydroxyl group, was performed. For above reasons, the sorption capacity of three organic pesticides (parathion-methyl, carbaryl and carbofuran) on PSt-g-HA increased by 64.1% to 95.2%. ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Organic pesticides such as parathion-methyl, carbaryl and carbofuran are highly toxic organic pollutants, and are widely used in cotton, sugar cane, rice, citrus and other crops in China. Due to wide applications of these pesticides, they are often detected in fresh water exceeding permissible levels and impact on human health. In order to minimize the possible damages to human, various kinds of treatments technologies such as adsorption, chemical flocculation, chemical oxidation, ultra filtration and biological treatment technologies have been employed. Among these methods, adsorption is generally considered to be a simple, relatively low-cost and effective method in removing organic pesticides from water and wastewater. Meanwhile, it is an important task to research on selection in absorbent of these pesticides. Humic acid (HA) is a kind of natural mixture of organic macromolecule which contains both hydrophilic and hydrophobic molecules as well as phenolic, carboxyl and hydroxyl groups connected to the skeleton of aliphatic or aromatic units and has loose sponge texture, large surface area and surface energy [1], thereby, HA has highly potential for treatment of heavy metals or organic contaminant from wastewater with adsorption method.
* Corresponding author at: College of Resources and Environment, Hunan Agricultural University, Changsha, Hunan 410128, PR China. Tel.: +86 0731 84673620; mobile: +86 1397 5819100. E-mail addresses: [email protected], [email protected] (Q. Zeng).
Recently, more attention has been paid to interaction between HAs and organic contaminant. The studies suggested that the adsorption activity of organic contaminant were significantly affected by the chemical structure of HA [2]. Thus, there is a need to identify the overall chemical structure of HA in adsorption processes so that a significant theoretical basis could be provided to treatment techniques of organic contaminant from wastewater with HA adsorption. Many studies reported that the adsorption process of hydrophobic organic chemicals (HOCs) could be related to different content and chemical structure of HA. For instance, Gauthier et al. [3] reported that the binding of pyrene to humic and fulvic acids was modified to a significant extent by the degree of aromaticity in the humic material. Xing et al. [4] observed that isotherm nonlinearity increased with the increase of aromatic carbon in adsorption of two HOCs, naphthalene and phenanthrene, by six pedogenetically related HAs. Besides, another group of authors investigated that aromatic carbon content influenced the adsorption of contaminant in HA [5–8]. Aromatic components also were recognized as condensed domain in soil organic matter (SOM) dominating the adsorption of HOCs in soils and sediments [9,10]. However, several contradictory arguments about the positive correlation between Kf (or Koc) and aliphatic content were reported. Some investigations revealed that the aliphatic carbon was the most important factor which could lead to changes in adsorption coefficient, instead of the aromatic carbon [12,13]. With pyrene adsorption to natural organic matter varying in chemical composition (e.g., high aliphaticity or aromaticity),
1226-086X/$ – see front matter ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.07.001
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Chefetz observed a positive trend between the Koc and the aliphaticity of sorbent [11]. Since structure and contents of chemical groups in humic materials, especially those of polar functional group, were recognized as the most important factors that affected the isotherm nonlinearity and adsorption capacity, further process has been conducted by various methods of sorbent modification to determine the change of structure in HA and its consequent results on adsorbing soil contaminant. Sachs et al. identified the changes in organic matter chemistry due to structural modification by combining the Nuclear Magnetic Resonance (NMR) spectra [12]. Witte et al. studied binding behavior of the metabolites 2aminobenzothiazole (ABT) and 2-(methylamino) benzothiazole (MABT) toward to humic acids in oxidative atmosphere and argon atmosphere [1]. However, modifying chemical structure of the humic acid with polystyrene has not been investigated before. To the best of our knowledge, this will be the first study that identifies the chemical and structural heterogeneity of polystyrened-HA and unmodified HA by element analysis, potentiometric titration, Fourier-Transform Infrared (FT-IR) spectroscopy, and cross polarization magic angle spinning carbon-13 nuclear magnetic resonance (CPMAS 13C NMR) spectroscopy. In addition, no previous reports have examined the adsorption behavior of three selected organic pesticides, parathion-methyl, carbaryl, and carbofuran, using structurally modified HA. The results will help us determine the chemical groups of the adsorption domains and their contributions to adsorption of organic pesticides. 2. Materials and methods 2.1. Structural modification The humic acid used in the study was purchased from JingKe Precise Chemical Academy in Tianjin, China. Structural modification was performed by mixing 5 g of HA with 200 ml of dimethyl sulfoxide (DMOS) in a 500 ml beaker and the mixture was washed with 50 ml of dimethyl sulfoxide three times. The solution was heated in a temperature controlled water bath shaker at temperatures varying between 70 and 80 8C. 20 min later, 30 ml of fluid polystyrene, 5 mmol FeCl2 and 15 mmol H2O2 (30%) were added to the suspension by shaken 30 min. After cooling to room temperature, this solution was precipitated with methanol and concentrated hydrochloric acid, and then centrifugated at 4000 g for 20 min. After removal supernatant fluid of the mixture, the precipitate was washed with dilute hydrochloric acid and deionized distilled water to neutral, freeze-dried, ground, and stored for their characterization and adsorption work. 2.2. Chemical characterization of HAs The C, H, and N contents of HAs were determined on an Elemental Analyzer (Elementar Vario EL, Germany). The content of Oxygen was calculated according to the mass difference and the atomic ratios of H to C, O to C were calculated consequently. 2.3. Potentimetric titration of HAs 0.20 g of each of HA sample was added to 75 ml of 0.10 M NaCl solution in auto-controlled temperature cabinet at a temperature of 25 0.2 8C. After reaching equilibrium by bubbling N2 for 20 min, the solution was titrated with 0.05 mol/L HCl until the pH was 2.5, and then was titrated with 0.10 M NaOH solution until the pH increased to 9.0. Details were reported in elsewhere [14]. The carboxyl and phenolic hydroxyl group were neutralized by alkaline solution.
2.4. Spectroscopic characterization of HAs Fourier-Transform Infrared (FT-IR) spectra were recorded on an FT-IR spectrophotometer (NEXUS 670X, Perkin Elmer). Samples were prepared in the form of KBr pellets by mixing 1 mg of HA and 400 mg of KBr at 10,000 kg/cm2 pressure for 30 min. Details of this procedure were described elsewhere [15]. Two HAs samples were subjected to 13C NMR analysis to acquire their chemical group distribution. Spectra were run on a Bruker DSX-300 operating at a 75 MHz for 13C and 300 MHz for 1H observation frequency, MAS spinning rate of 5 KHz, contact time of 1 ms, 4 s recycle delay, approximately 10,000 scans per sample, using a ramp cross-polarization pulse program with magic angle spinning. Structural carbons determined were alky carbon (0– 50 ppm), oxygen-alky carbon (50–108 ppm), aromatic carbon (108–168 ppm), carboxyl carbon (168–192 ppm), and carbonyl carbon (192–220 ppm). 2.5. Adsorption experiments Parathion-methyl, carbaryl and carbofuran (>99.5% purity), were purchased from Drug Control Institution of the Ministry of Agriculture of China and used without further purification, and their solubility in water at 20 8C were 55 mg/L, 120 mg/L and 250 mg/L, respectively. These pesticides were chosen because they are common organic pollutants in soil and sediment and have often been used in environmental research. All adsorption isotherms were obtained by adding 0.1 g of each HA sample in three flasks, respectively, at room temperature based on a batch of equilibration technique [12]. Preliminary experiments with UV-visible spectroscopy showed little difference (if any) between 0.01 M CaCl2-NaNO3 solution and the same solution mixed with HAs after 7-d equilibration and centrifugation, due to the effective flocculation by the CaCl2 and low pH [16]. 20 ml of varying concentration of these three pesticides and 5 ml of 0.025 mol/L CaCl2 solution were added into the 50 ml flasks which contained 0.1 g of HA sample, respectively. Isotherms consisted of ten varying concentration points; each point, including the blank, was run in duplicate. After 24 h of vibration and equilibrium, partial of each solution was distracted to a 10 ml centrifugal tube and centrifuged at 3000 r/min for 20 min, and then the pesticides tested in supernatant solutions were separated and purified by using solid phase extraction (SPE) technique and analyzed by gas chromatography [17,18]. Because of little adsorption by flasks and no biodegradation, adsorbed pesticides by the sorbents were calculated by mass difference. The other experimental details were reported elsewhere [19]. All adsorption data were fitted to the logarithmic form of the Freundlich equation:
log S ¼ log K f þ N log C e where S is the solid-phase concentration (mg/g) and Ce is the liquid-phase equilibrium concentration (mg/ml), Kf for adsorption capacity coefficient [(mg/g)(mg/ml)N] and N are constants with N < 1. The parameters, Kf and N (dimensionless) indicating isotherm nonlinearity were determined by linear regression of log-transformed data. Linear fitting of log-transformed data was justified over direct nonlinear curve fitting in this paper for two reasons: (i) concentrations spread evenly over the log scale, thus, nonlinear curve fitting would underestimate the importance of the low concentration data, and (ii) the relative uncertainty in the measurement was not greatly dependent on concentration [9].
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2.6. Pesticide analysis The final configuration used for the analysis of supernatant for organic pesticides was as follows. 500 mg C18 plus cartridges (500 mg/3 mL, Supelco, USA) were used for purification and concentration during SPE analysis. SPE cartridges were preconditioned by successive washing with 5 ml of methanol and 10 ml of ultrapure water. The supernatant was passed through the cartridges. The cartridges were dried by blowing nitrogen over for 20 min. The pesticides retained on the surface of solid phase were eluted with 10 ml ethyl acetate. The final extract was concentrated to less than 1 ml with the aid of a stream of nitrogen, internal standard was added, and the volume made up to 1 ml with ethyl acetate. The purified solution was stored at 4 8C until analyzed by GC. A gas chromatograph, Agilent 6890N, equipped with a capillary column (HP-5, 30 m 0.32 mm ID, 0.25 mm thickness film) and a NP detector was employed. Helium was used carrier gas at a flow rate of 1.0 ml min1. The injector temperature was set to 250 8C and NP detector was 300 8C. The temperature program used for the analysis was: from 110 8C (hold 1 min) to 300 8C (hold 4 min) at 15 8C min1. The injection volume was 2 ml, operating in the splitless mode. Blank experiments without soil were performed for the tested compound; the recoveries ranged from 85–95%. Measured equilibrium concentrations were not adjusted for the recoveries. All analytical determinations utilized standard external calibration curves over their linear response regions and were made well above the instrumental and method detection limits. 3. Results and discussion 3.1. The possible mechanism of graft-copolymerization Modification of HA by graft copolymerization with styrene was the first report at present and, therefore, it is a challenge to establish the mechanism of graft-copolymerization of HA. But there was systematic research about polystyrene (PS) chains were grown from carbon black (CB) surface by surface-initiated atom transfer radical polymerization (SI-ATRP), obtaining carbon black grafted with polystyrene (CB-g-PS) [20]. CB is a kind of HA analogs which can be extracted from soil organic matter [21], and its chemical groups is similar to HA, the mechanism of carbon black grafted with polystyrene can be used for reference deducing the mechanism of HA grafted with polystyrene. Using the synthetic mechanism of CB-g-PS for reference, the possible process about graft copolymerization of styrene onto HA, using H2O2–FeCl2 redox system as initiator in dimethylsulphoxide (DMOS) medium is shown as Scheme 1. In this redox systems, the primary hydroxyl radical (OH) is generated from the reaction of Fe2+ and H2O2, at the same time, Fe2+ can be reduced by HA to Fe2+ (see Scheme 1a). Since OH is highly reactive and apt to attack and initiate the organic matter, when OH attack styrene, it will add to the double bonds (C5 5C) of styrene to form 2-phenylethanol radical (see Scheme 1b), then the polystyrene radical containing hydroxylterminated (PSt) is formed by the further polymerization reaction of 2-phenylethanol radical with styrene (see Scheme 1e) [22]. In the meantime, carboxyl groups (COOH) and phenol hydroxyl in HA, as main functional groups, will be given first attacked by OH and lead to the formation of HAs with stable free radical (these humic acids are represented by HA)(see Scheme 1c and d). In this solution systems, the chain termination reactions of stable free radical PSt and HA include three parts: The first, the coupling reaction of PSt with HA to form graft copolymerization (PSt-gHA) of styrene onto HA (see Scheme 1f and g), the second, the coupling reaction between PSt to form polystyrene containing the hydroxyl group (see Scheme 1h), and the last one is the coupling
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reaction between HA, but this reaction is very weak since the HA molecular too large to stable polymerize together [23]. Hence, in the graft-copolymerization experiment of HAs, the main synthetic products are PSt-g-HA, polystyrene and untreated HA, but the polystyrene suspended in solution and untreated HA dissolved by DMOS could be removed by precipitation and centrifugal separation in the last experimental procedures [22]. 3.2. Implications of structural modification on the HA Elements analysis data of untreated HA and treated HA (PSt-gHA) were showed in Table 1. The percentages of untreated HA and PSt-g-HA were 4.2%, 5.1% for H and 50.2%, 68.7% for C, whereas the percentages of N and O dropped from 1.4% to 0.8% and from 44.1% to 25.0%, espectively, after chemical treatment. Further more, the ratio of H to C decreased from 0.0842 to 0.0747, which was consistent with the aromaticity increased with PSt-g-HA. Meanwhile, the ratio of O to C also decreased from 0.8737 to 0.3600, because the carboxyl of HA had been replaced by PSt (see Scheme 1). and this similar result also can be obtained in the potentiometric titration(Table 1 and Fig. 1). Table 1 showed that the decrease of content of carboxyl and phenolic hydroxyl group was obvious which was evident from the potentiometric titration in Fig. 1. The FT-IR spectra illustrate that chemical treatment had specific structural modifications on the HA (Fig. 2). The OH stretching at 3200–3600 cm1 and C5 5O stretching at 1720 cm1 of COOH in these two HAs are in the order of untreated HA cPSt-g-HA, implied that the carboxyl group of HA decreased during synthesized process, It can be concluded that the PSt-g-HA has lower polarity, compared with untreated HA. Moreover, the PSt-g-HA showed relatively high intensity of aromatic C-H stretching at 3000– 3100 cm1 and had a peak of aromatic C5 5C stretching at 1510 cm1, indicating increased aromaticity of HA modified by graft copolymerization with styrene [24]. The CPMAS 13C NMR spectra of HAs are showed in Fig. 3. They reveal clear signals due to O-substituted aromatic C or phenolic C (140–160 ppm), ary C (105–140 ppm) and alkyl C (0–50 ppm). Compared with untreated HA, the wide band from 105 to 160 ppm became sharp and strong after chemical treatment, it can be due to grafting copolymerization of HA with polystyrene and the coupling reaction of phenol hydroxyl in HA with PSt. The band from 41 to 42 ppm became sharp and strong after chemical treatment, which is assigned to CH2 in long polystyrene chains. In adtion, the 165– 185 ppm signal of PSt-g-HA is weaker than untreated HA, indicating decreased carboxyl group content, it is corresponded to the result of FT-IR spectra. 3.3. Adsorption of parathion-methyl, carbaryl, and carbofuran The adsorption of three selected pesticides is well described by Freundlich equation and the isotherm parameters are shown in Table 2. For the same HA, the adsorption coefficient (Kf) of three pesticides decreased in following order: parathionmethyl > carbaryl > carbofuran. This may be attributed to the hydrophobicity difference among these three compounds. Parathion-methyl, carbaryl and carbofuran are highly hydrophobic organic pesticide. Their hydrophobicity are at the order of parathion-methyl > carbaryl > carbofuran. According to the previous data in this study, the adsorption capacity of these three organic pesticides on treated HA is corresponded to the hydrophobicity i.e., the adsorption capacity order of parathionmethyl > carbaryl > carbofuran. This is because the stronger the hydrophobic organic compounds, the greater the ability to give electron, the higher the adsorption capacity [25]. Additionally, for different HAs, the Kf increased from 1814.9 to 44130 from 223.75 to 3348.6 and from 1.1212 102 to 703.75,
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Scheme 1. Possible process about graft-copolymerization mechanism of HA by styrene.
Table 1 Elemental contents (%), H/C, O/C ratios and potentiometric titration of HAs. Sample
untreated HA PSt-g-HA
Elemental analysis
Potentiometric titrations (mmol/g)
O
C
N
H
O/C
H/C
Phenolic
Carboxylic
Total acid
44.1 25.0
50.2 68.7
1.4 0.8
4.2 5.1
0.8737 0.3600
0.0842 0.0747
1.50 0.10
1.10 0.10
2.60 0.20
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Fig. 1. HA potentiometric titration curves.
for parathion-methyl, carbaryl, and carbofuran, respectively. The Kf of two HAs decreased in this order: PSt-g-HA > untreated HA. The adsorption capacity of parathion-methyl, carbaryl and carbofuran on PSt-g-HA increased by 95.2%, 90.4% and 64.1%, compared to that on original HA. Xing and Chen [16] proposed that precise comparison cannot be made between the Kf values from different isotherms because of their different units as a result of nonlinearity. However, it is obvious that Kf values of PSt-g-HA were much larger than that of untreated HA. The sorption isotherms of three pesticides are plotted in Fig. 4. The sorption nonlinearity, as determined by the Freundlich parameter n, varies from 0.56 to 0.91, with the exception of curbofuran. Compared with the untreated HA, the PSt-g-HA produced higher nonlinearity with n values of 0.91, 0.56, and 0.77, respectively. Whether parathion-methyl, carbofura or curbaryl, n values all increased in the order of PSt-g-HA > ununtreated HA, which is consistent with Kf values, confirming that the isotherm nonlinearity increased after HA is grafted copolymerization by polystyrene. In order to explain the nonlinear isotherms, HA was thought to be a kind of rubbery and glassy synthetic polymers, and the rubbery domain was expanded and the glassy domain was condensed, simultaneously and respectively. This is referred as dual-mode sorption [26]. Although the sorption of expanded domain appears to be linear because of the partition while nonlinear isotherms are found in the condensed
Fig. 2. The Fourier transforms infrared (FTIR) spectra.
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Fig. 3. CP/MAS13C-NMR spectra of HA.
domain, the sorption of the integrity of these two domains are observed as nonlinear isotherm. It reflected that there might be more condensed substance in PSt-g-HA than that in untreated HA, according to the increase of nonlinearity of three isotherms. Using X-ray diffraction and solid-state NMR, Xing and Chen [9] have observed rigid, condensed aromatic domains in HA. Moreover, there is other evidence in literature to supported that the condensed domains is tightly packed by aromatic regions in HA [27]. Therefore the grafted copolymerization of polystyrene enters into condensed domains in HA. These data (Table 2 and Fig. 4) demonstrate that the PSt-g-HA has the capability to sorb more organic pesticides (parathionmethyl, carbaryl, and curbofuran) than the untreated HA due to the structural and chemical difference between these two HAs. Chiou et al. [8] proposed that the increasing of aromatic content leaded to the partitioning of nonionic compounds to HA getting larger, in accordance with its high hydrophobicity in aromatic solvent (benzene and n-hexane, respectively. According to the theory that similarities can be solvable easily, the sorption abilities of parathion-methyl, carbaryl and carbofuran containing hydrophobic aromatic rings on PSt-g-HA with more phenyl rings than untreated HA will naturally increase. Furthermore, Kukkonen et al. [25] also reported that hydrophobic compound as electron-donors could be more easily adsorbed on PSt-g-HA with aromatic structure as electron-acceptors than untreated HA. Meanwhile, the existence of carboxyl and phenolic hydroxyl group and could decrease the adsorption capacity of the hydrophobic organic pesticide based on the high polarity of carboxyl and phenolic hydroxyl group [5,28]. It can be concluded that polystyrenemodification method causes aromatic structure and content changes of carboxyl and phenolic hydroxyl group, which results in polarity of HA decreased, and that the lowest polarity showed the highest sorption capacity and nonlinearity as compared with the two HAs. Basically the sorption of hydrophobic organic pesticides on HAs is a consequence of competitive adsorption between hydrophobic organic pesticide molecular and water molecular [4]. As shown in Fig. 5, for untreated HA, hydrophobic organic pesticides cannot effectively compete with polar water molecules for adsorption on the HA surface since water molecules can be bonded to polar carboxyl and phenolic hydroxyl group and form water film inhibited the partitioning of organic pesticide molecular to untreated HA. However, for PSt-g-HA, the remove of polar carboxyl and phenolic hydroxyl group and the grafting of hydrophobic polythene chains can restrain the form of water film on PSt-g-HA
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Table 2 Linear isotherm equation parameters for sorption of organic pesticides on HAs. pesticide
Parathion-methyl Curbaryl Carbofuran
HA
Linear equation
Untreated HA PSt-g-HA Untreated HA PSt-g-HA Untreated HA PSt-g-HA
Freundlich equation 2
Kd
R
Kf
n
R2
186.91 50866 106.84 1479.7 18.323 369.47
0.9865 0.9852 0.9445 0.8208 0.7534 0.9583
1814.9 44130 223.75 3348.6 1.1212 102 703.75
1.02 0.91 0.78 0.56 3.51 0.77
0.9867 0.9897 0.9723 0.9950 0.9547 0.9887
Fig. 4. Parathion-methyl, carbaryl and carbofuran sorption to HAs.
Fig. 5. The schematic plot about the sorption of organic pesticide molecular on HAs.
surface, thus, as in the case of the Kd and Kf values reported in Table 2, the PSt-g-HA appeared to have a higher adsorption capacity for hydrophobic organic pesticides, such as parathionmethyl, carbaryl and carbofuran, than untreated HA.
4. Conclusions In this study, modification of HA by graft copolymerization with styrene was achieved and the treated humic (PSt-g-HA) acid had
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relatively high aromatic carbon content and less polar function groups, such as carboxyl and phenolic hydroxyl group, compared to that of untreated humic acid. Meanwhile, PSt-g-HA was found to be a good adsorbent for organic pesticides, which was attributed to the decrease of the polar substances contents and the increasing of aromatic carbons content in rigid (condensed) domain after HA is grafted copolymerization by polystyrene. Therefore, this new HA have great potentials in water treatment for organic pesticide removal. Acknowledgements The authors are thankful to department of environment science, university of Hunan agriculture for providing laboratory facilities. References [1] H.R. Schulten, The three-dimensional structure of humic substances and soil organic matter study by computational analytical chemistry, Fresenius’ Journal of Analytical Chemistry 351 (1995) 62–73. [2] S. Kang, D. Amarasiriwardena, P. Veneman, B.S. Xing, Characterization of ten sequentially extracted humic acids and a humin from a soil in Western Wassachusetts, Soil Science 168 (2003) 880–887. [3] T.D. Gauthier, W.R. Seitz, C.L. Grant, Effects of structure and compositional variation of dissolved humic materials on pyrene Koc values, Environmental Science & Technology 21 (1987) 243–248. [4] B. Xing, Sorption of naphthalene and phenanthrene by soil humic acids, Environmental Pollution 111 (2001) 303–309. [5] P. Grathwohl, Influence of organic matter from soils and sediments from various origins on the sorption of some chlorinated aliphatic hydrocarbons: Implications on Koc correlations, Environmental Science & Technology 24 (1990) 1687–1693. [6] R. Ahmad, R.S. Kookana, A.M. Alston, J.O. Skjemstad, The nature of soil organic matter affects sorption of pesticides. 1. Relationships with carbon chemistry as determined by 13C CPMAS NMR spectroscopy, Environmental Science & Technology 35 (2001) 874–884. [7] I.V. Perminova, N.Y. Grechishcheva, V.S. Peterosyan, Relationships between structure and binding affinity of humic substances fro polycyclic aromatic hydrocarbons: relevance of molecular descriptors, Environmental Science & Technology 33 (1999) 3781–3787. [8] C.T. Chiou, S.E. McGroddy, D.E. Kile, Partition characteristics of polycyclic aromatic hydrocarbons on soils and sediments, Environmental Science & Technology 32 (1998) 264–269. [9] B. Xing, Z. Chen, Spectroscopic evidence for condensed domains in soil organic matter, Soil Science 164 (1999) 40–47. [10] M.D. Johnson, M. Huang, W.J. Weber, A distributed reactivity model for sorption of soils and sediments: 13. Simulated diagenesis of natural sediment organic matter and its impact on sorption/desorption equilibrium, Environmental Science & Technology 35 (2001) 1680–1687. [11] B. Chefetz, A.P. Deshmukh, P.G. Hatcher, E.A. Guthrie, Pyrene sorption by natural organic matter, Environmental Science & Technology 34 (2000) 2925–2930.
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[12] S. Sachs, M. Bubner, K. Schmeide, R. Choppin, H. Heise, Carbon-13 NMR spectroscopic studies on chemically modified and unmodified synthetic and natural humic acids, Talanta 57 (2002) 999–1009. [13] E.G. Witte, H. Philipp, H. Vereecken, Binding of 13C-labelled 2-aminobenzothiazoles to humic acid as derived from 13C NMR spectroscopy, Organic Geochemistry 29 (1998) 1829–1835. [14] C. Garnier, S. Mounier, J.Y. Benaı¨m, Influence of dissolved organic carbon content on modelling natural organic matter acid–base properties, Water Research 38 (2004) 3685–3692. [15] C. Luisella, S. Morris, N. Michele, Analysis of carboxyl groups in soil humic acids by a wet chemical method, Fourier-transform infrared spectrophotometry, and solution-state carbon-13 nuclear magnetic resonance: a comparative study, Soil Science 162 (1997) 189–197. [16] B. Xing, K.J. Wang, Chemical extraction affect the structure and phenanthrene sorption of soil humin, Environmental Science & Technology 39 (2005) 8333– 8340. [17] C.E. Lindley, J.T. Stewart, M.W. Sandstrom, Determination of low concentrations of acetochlor in water by automated solid-phase extraction and gas chromatography with mass-selective detection, Journal of AOAC International 79 (1996) 962–966. [18] C. Lo´pez-Blanco, S. Go´mez-A´lvarez, M. Rey-Garrote, B. Cancho-Grande, J. SimalGa´ndara., Determination of pesticides by solid phase extraction followed by gas chromatography with nitrogen–phosphorous detection in natural water and comparison with solvent drop microextraction, Analytical and Bioanalytical Chemistry 384 (2006) 1002–1006. [19] A.U. Baes, P.R. Bloom, Diffuse reflectance and transmission Fourier transform infrared (DRIFT) spectroscopy of humic and fulvic acids, Soil Science Society of America Journal 53 (1989) 695–700. [20] S. Yoshikawa, S. Machida, N. Tusbokawa, Polymer grafting onto carbon black by use of TEMPO-terminated polystyrene with controlled molecular weight, Journal of Polymer Science Part A: Polymer Chemistry 36 (1998) 3166–3172. [21] Z.S. Jan, A.P. Ping, L.H. Wei, Black carbon and kerogen in soils and sediments. 1. Quantification and characterization, Environmental Science & Technology 36 (2002) 3960–3967. [22] J. Shan, M. Nuopponen, H. Jiang, T. Viitala, E. Kauppinent, K. Kontturi, H. Tenhu, Amphiphilic gold nanoparticles grafted with poly (N-isopropylacrylamide) and polystyrene, Macromolecules 38 (2005) 2918–2926. [23] H. Kong, C. Gao, D.Y. Yan, Controlled functionalization of multiwalled carbon naotubes by in situ atom transfer radical polymerization, Journal of the American Chemical Society 126 (2004) 412–413. [24] D.P. Xu, Z.H. Xu, S.Q. Zhu, Y. Wang, X.M. Du, Q.B. Gu, F.S. Li, Adsorption behavior of herbicide on typical soils in China and humic acids from the soil samples, Journal of Colloid and Interface Science 285 (2005) 27–32. [25] J. Kukkonen, J.F. McCarthy, A. Oikari, Effects of XAD-8 fractions of dissolved organic carbon on the sorption and bioavailability of organic micropollutants, Archives of Environmental Contamination and Toxicology 19 (1990) 551–557. [26] B.S. Xing, J.P. Joseph, Dual-mode sorption of low-polarity compounds in glassy poly(vinyl chloride) and soil organic matter, Environmental Science & Technology 31 (1997) 792–799. [27] M. Schnitzer, H. Kodama, J.A. Ripmeester, Determination of the aromaticity of HA by X-ray diffraction analysis, Soil Science Society of America Journal 55 (1991) 745–750. [28] L. Li, Z.Q. Yu, G.Y. Sheng, The influence of molecular structure to the adsorption behavior of Philippines on humic acid, Environmental Chemistry 23 (4) (2004) 381–386.
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The synthesis of humic acids graft copolymer and its adsorption for organic pesticides Chengjian Yang a,b, Qingru Zeng a,*, Yang Yang a, Ruiyang Xiao a, Yunzhong Wang b, Hui Shi b a b
Department of Environment and Science, Hunan Agricultural University, Changsha 410000, PR China College of Environment & Municipal Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, PR China
A R T I C L E I N F O
Article history: Received 8 December 2012 Accepted 1 July 2013 Available online xxx Keywords: Humic acid Modification Adsorption Organic pesticide
A B S T R A C T
humic acid graft copolymer (PSt-g-HA) was prepared by graft copolymerization of Humic acid (HA) with styrene and the sorption of three selected organic pesticides, parathion-methyl, carbaryl and carbofuran by PSt-g-HA acid and untreated humic were also examined, respectively. The PSt-g-HA had relatively high aromatic carbons content because the grafted copolymerization of polystyrene entered into condensed domains in HA, whereas the removal of the polar function groups, such as carboxyl and phenolic hydroxyl group, was performed. For above reasons, the sorption capacity of three organic pesticides (parathion-methyl, carbaryl and carbofuran) on PSt-g-HA increased by 64.1% to 95.2%. ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Organic pesticides such as parathion-methyl, carbaryl and carbofuran are highly toxic organic pollutants, and are widely used in cotton, sugar cane, rice, citrus and other crops in China. Due to wide applications of these pesticides, they are often detected in fresh water exceeding permissible levels and impact on human health. In order to minimize the possible damages to human, various kinds of treatments technologies such as adsorption, chemical flocculation, chemical oxidation, ultra filtration and biological treatment technologies have been employed. Among these methods, adsorption is generally considered to be a simple, relatively low-cost and effective method in removing organic pesticides from water and wastewater. Meanwhile, it is an important task to research on selection in absorbent of these pesticides. Humic acid (HA) is a kind of natural mixture of organic macromolecule which contains both hydrophilic and hydrophobic molecules as well as phenolic, carboxyl and hydroxyl groups connected to the skeleton of aliphatic or aromatic units and has loose sponge texture, large surface area and surface energy [1], thereby, HA has highly potential for treatment of heavy metals or organic contaminant from wastewater with adsorption method.
* Corresponding author at: College of Resources and Environment, Hunan Agricultural University, Changsha, Hunan 410128, PR China. Tel.: +86 0731 84673620; mobile: +86 1397 5819100. E-mail addresses: [email protected], [email protected] (Q. Zeng).
Recently, more attention has been paid to interaction between HAs and organic contaminant. The studies suggested that the adsorption activity of organic contaminant were significantly affected by the chemical structure of HA [2]. Thus, there is a need to identify the overall chemical structure of HA in adsorption processes so that a significant theoretical basis could be provided to treatment techniques of organic contaminant from wastewater with HA adsorption. Many studies reported that the adsorption process of hydrophobic organic chemicals (HOCs) could be related to different content and chemical structure of HA. For instance, Gauthier et al. [3] reported that the binding of pyrene to humic and fulvic acids was modified to a significant extent by the degree of aromaticity in the humic material. Xing et al. [4] observed that isotherm nonlinearity increased with the increase of aromatic carbon in adsorption of two HOCs, naphthalene and phenanthrene, by six pedogenetically related HAs. Besides, another group of authors investigated that aromatic carbon content influenced the adsorption of contaminant in HA [5–8]. Aromatic components also were recognized as condensed domain in soil organic matter (SOM) dominating the adsorption of HOCs in soils and sediments [9,10]. However, several contradictory arguments about the positive correlation between Kf (or Koc) and aliphatic content were reported. Some investigations revealed that the aliphatic carbon was the most important factor which could lead to changes in adsorption coefficient, instead of the aromatic carbon [12,13]. With pyrene adsorption to natural organic matter varying in chemical composition (e.g., high aliphaticity or aromaticity),
1226-086X/$ – see front matter ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.07.001
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Chefetz observed a positive trend between the Koc and the aliphaticity of sorbent [11]. Since structure and contents of chemical groups in humic materials, especially those of polar functional group, were recognized as the most important factors that affected the isotherm nonlinearity and adsorption capacity, further process has been conducted by various methods of sorbent modification to determine the change of structure in HA and its consequent results on adsorbing soil contaminant. Sachs et al. identified the changes in organic matter chemistry due to structural modification by combining the Nuclear Magnetic Resonance (NMR) spectra [12]. Witte et al. studied binding behavior of the metabolites 2aminobenzothiazole (ABT) and 2-(methylamino) benzothiazole (MABT) toward to humic acids in oxidative atmosphere and argon atmosphere [1]. However, modifying chemical structure of the humic acid with polystyrene has not been investigated before. To the best of our knowledge, this will be the first study that identifies the chemical and structural heterogeneity of polystyrened-HA and unmodified HA by element analysis, potentiometric titration, Fourier-Transform Infrared (FT-IR) spectroscopy, and cross polarization magic angle spinning carbon-13 nuclear magnetic resonance (CPMAS 13C NMR) spectroscopy. In addition, no previous reports have examined the adsorption behavior of three selected organic pesticides, parathion-methyl, carbaryl, and carbofuran, using structurally modified HA. The results will help us determine the chemical groups of the adsorption domains and their contributions to adsorption of organic pesticides. 2. Materials and methods 2.1. Structural modification The humic acid used in the study was purchased from JingKe Precise Chemical Academy in Tianjin, China. Structural modification was performed by mixing 5 g of HA with 200 ml of dimethyl sulfoxide (DMOS) in a 500 ml beaker and the mixture was washed with 50 ml of dimethyl sulfoxide three times. The solution was heated in a temperature controlled water bath shaker at temperatures varying between 70 and 80 8C. 20 min later, 30 ml of fluid polystyrene, 5 mmol FeCl2 and 15 mmol H2O2 (30%) were added to the suspension by shaken 30 min. After cooling to room temperature, this solution was precipitated with methanol and concentrated hydrochloric acid, and then centrifugated at 4000 g for 20 min. After removal supernatant fluid of the mixture, the precipitate was washed with dilute hydrochloric acid and deionized distilled water to neutral, freeze-dried, ground, and stored for their characterization and adsorption work. 2.2. Chemical characterization of HAs The C, H, and N contents of HAs were determined on an Elemental Analyzer (Elementar Vario EL, Germany). The content of Oxygen was calculated according to the mass difference and the atomic ratios of H to C, O to C were calculated consequently. 2.3. Potentimetric titration of HAs 0.20 g of each of HA sample was added to 75 ml of 0.10 M NaCl solution in auto-controlled temperature cabinet at a temperature of 25 0.2 8C. After reaching equilibrium by bubbling N2 for 20 min, the solution was titrated with 0.05 mol/L HCl until the pH was 2.5, and then was titrated with 0.10 M NaOH solution until the pH increased to 9.0. Details were reported in elsewhere [14]. The carboxyl and phenolic hydroxyl group were neutralized by alkaline solution.
2.4. Spectroscopic characterization of HAs Fourier-Transform Infrared (FT-IR) spectra were recorded on an FT-IR spectrophotometer (NEXUS 670X, Perkin Elmer). Samples were prepared in the form of KBr pellets by mixing 1 mg of HA and 400 mg of KBr at 10,000 kg/cm2 pressure for 30 min. Details of this procedure were described elsewhere [15]. Two HAs samples were subjected to 13C NMR analysis to acquire their chemical group distribution. Spectra were run on a Bruker DSX-300 operating at a 75 MHz for 13C and 300 MHz for 1H observation frequency, MAS spinning rate of 5 KHz, contact time of 1 ms, 4 s recycle delay, approximately 10,000 scans per sample, using a ramp cross-polarization pulse program with magic angle spinning. Structural carbons determined were alky carbon (0– 50 ppm), oxygen-alky carbon (50–108 ppm), aromatic carbon (108–168 ppm), carboxyl carbon (168–192 ppm), and carbonyl carbon (192–220 ppm). 2.5. Adsorption experiments Parathion-methyl, carbaryl and carbofuran (>99.5% purity), were purchased from Drug Control Institution of the Ministry of Agriculture of China and used without further purification, and their solubility in water at 20 8C were 55 mg/L, 120 mg/L and 250 mg/L, respectively. These pesticides were chosen because they are common organic pollutants in soil and sediment and have often been used in environmental research. All adsorption isotherms were obtained by adding 0.1 g of each HA sample in three flasks, respectively, at room temperature based on a batch of equilibration technique [12]. Preliminary experiments with UV-visible spectroscopy showed little difference (if any) between 0.01 M CaCl2-NaNO3 solution and the same solution mixed with HAs after 7-d equilibration and centrifugation, due to the effective flocculation by the CaCl2 and low pH [16]. 20 ml of varying concentration of these three pesticides and 5 ml of 0.025 mol/L CaCl2 solution were added into the 50 ml flasks which contained 0.1 g of HA sample, respectively. Isotherms consisted of ten varying concentration points; each point, including the blank, was run in duplicate. After 24 h of vibration and equilibrium, partial of each solution was distracted to a 10 ml centrifugal tube and centrifuged at 3000 r/min for 20 min, and then the pesticides tested in supernatant solutions were separated and purified by using solid phase extraction (SPE) technique and analyzed by gas chromatography [17,18]. Because of little adsorption by flasks and no biodegradation, adsorbed pesticides by the sorbents were calculated by mass difference. The other experimental details were reported elsewhere [19]. All adsorption data were fitted to the logarithmic form of the Freundlich equation:
log S ¼ log K f þ N log C e where S is the solid-phase concentration (mg/g) and Ce is the liquid-phase equilibrium concentration (mg/ml), Kf for adsorption capacity coefficient [(mg/g)(mg/ml)N] and N are constants with N < 1. The parameters, Kf and N (dimensionless) indicating isotherm nonlinearity were determined by linear regression of log-transformed data. Linear fitting of log-transformed data was justified over direct nonlinear curve fitting in this paper for two reasons: (i) concentrations spread evenly over the log scale, thus, nonlinear curve fitting would underestimate the importance of the low concentration data, and (ii) the relative uncertainty in the measurement was not greatly dependent on concentration [9].
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2.6. Pesticide analysis The final configuration used for the analysis of supernatant for organic pesticides was as follows. 500 mg C18 plus cartridges (500 mg/3 mL, Supelco, USA) were used for purification and concentration during SPE analysis. SPE cartridges were preconditioned by successive washing with 5 ml of methanol and 10 ml of ultrapure water. The supernatant was passed through the cartridges. The cartridges were dried by blowing nitrogen over for 20 min. The pesticides retained on the surface of solid phase were eluted with 10 ml ethyl acetate. The final extract was concentrated to less than 1 ml with the aid of a stream of nitrogen, internal standard was added, and the volume made up to 1 ml with ethyl acetate. The purified solution was stored at 4 8C until analyzed by GC. A gas chromatograph, Agilent 6890N, equipped with a capillary column (HP-5, 30 m 0.32 mm ID, 0.25 mm thickness film) and a NP detector was employed. Helium was used carrier gas at a flow rate of 1.0 ml min1. The injector temperature was set to 250 8C and NP detector was 300 8C. The temperature program used for the analysis was: from 110 8C (hold 1 min) to 300 8C (hold 4 min) at 15 8C min1. The injection volume was 2 ml, operating in the splitless mode. Blank experiments without soil were performed for the tested compound; the recoveries ranged from 85–95%. Measured equilibrium concentrations were not adjusted for the recoveries. All analytical determinations utilized standard external calibration curves over their linear response regions and were made well above the instrumental and method detection limits. 3. Results and discussion 3.1. The possible mechanism of graft-copolymerization Modification of HA by graft copolymerization with styrene was the first report at present and, therefore, it is a challenge to establish the mechanism of graft-copolymerization of HA. But there was systematic research about polystyrene (PS) chains were grown from carbon black (CB) surface by surface-initiated atom transfer radical polymerization (SI-ATRP), obtaining carbon black grafted with polystyrene (CB-g-PS) [20]. CB is a kind of HA analogs which can be extracted from soil organic matter [21], and its chemical groups is similar to HA, the mechanism of carbon black grafted with polystyrene can be used for reference deducing the mechanism of HA grafted with polystyrene. Using the synthetic mechanism of CB-g-PS for reference, the possible process about graft copolymerization of styrene onto HA, using H2O2–FeCl2 redox system as initiator in dimethylsulphoxide (DMOS) medium is shown as Scheme 1. In this redox systems, the primary hydroxyl radical (OH) is generated from the reaction of Fe2+ and H2O2, at the same time, Fe2+ can be reduced by HA to Fe2+ (see Scheme 1a). Since OH is highly reactive and apt to attack and initiate the organic matter, when OH attack styrene, it will add to the double bonds (C5 5C) of styrene to form 2-phenylethanol radical (see Scheme 1b), then the polystyrene radical containing hydroxylterminated (PSt) is formed by the further polymerization reaction of 2-phenylethanol radical with styrene (see Scheme 1e) [22]. In the meantime, carboxyl groups (COOH) and phenol hydroxyl in HA, as main functional groups, will be given first attacked by OH and lead to the formation of HAs with stable free radical (these humic acids are represented by HA)(see Scheme 1c and d). In this solution systems, the chain termination reactions of stable free radical PSt and HA include three parts: The first, the coupling reaction of PSt with HA to form graft copolymerization (PSt-gHA) of styrene onto HA (see Scheme 1f and g), the second, the coupling reaction between PSt to form polystyrene containing the hydroxyl group (see Scheme 1h), and the last one is the coupling
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reaction between HA, but this reaction is very weak since the HA molecular too large to stable polymerize together [23]. Hence, in the graft-copolymerization experiment of HAs, the main synthetic products are PSt-g-HA, polystyrene and untreated HA, but the polystyrene suspended in solution and untreated HA dissolved by DMOS could be removed by precipitation and centrifugal separation in the last experimental procedures [22]. 3.2. Implications of structural modification on the HA Elements analysis data of untreated HA and treated HA (PSt-gHA) were showed in Table 1. The percentages of untreated HA and PSt-g-HA were 4.2%, 5.1% for H and 50.2%, 68.7% for C, whereas the percentages of N and O dropped from 1.4% to 0.8% and from 44.1% to 25.0%, espectively, after chemical treatment. Further more, the ratio of H to C decreased from 0.0842 to 0.0747, which was consistent with the aromaticity increased with PSt-g-HA. Meanwhile, the ratio of O to C also decreased from 0.8737 to 0.3600, because the carboxyl of HA had been replaced by PSt (see Scheme 1). and this similar result also can be obtained in the potentiometric titration(Table 1 and Fig. 1). Table 1 showed that the decrease of content of carboxyl and phenolic hydroxyl group was obvious which was evident from the potentiometric titration in Fig. 1. The FT-IR spectra illustrate that chemical treatment had specific structural modifications on the HA (Fig. 2). The OH stretching at 3200–3600 cm1 and C5 5O stretching at 1720 cm1 of COOH in these two HAs are in the order of untreated HA cPSt-g-HA, implied that the carboxyl group of HA decreased during synthesized process, It can be concluded that the PSt-g-HA has lower polarity, compared with untreated HA. Moreover, the PSt-g-HA showed relatively high intensity of aromatic C-H stretching at 3000– 3100 cm1 and had a peak of aromatic C5 5C stretching at 1510 cm1, indicating increased aromaticity of HA modified by graft copolymerization with styrene [24]. The CPMAS 13C NMR spectra of HAs are showed in Fig. 3. They reveal clear signals due to O-substituted aromatic C or phenolic C (140–160 ppm), ary C (105–140 ppm) and alkyl C (0–50 ppm). Compared with untreated HA, the wide band from 105 to 160 ppm became sharp and strong after chemical treatment, it can be due to grafting copolymerization of HA with polystyrene and the coupling reaction of phenol hydroxyl in HA with PSt. The band from 41 to 42 ppm became sharp and strong after chemical treatment, which is assigned to CH2 in long polystyrene chains. In adtion, the 165– 185 ppm signal of PSt-g-HA is weaker than untreated HA, indicating decreased carboxyl group content, it is corresponded to the result of FT-IR spectra. 3.3. Adsorption of parathion-methyl, carbaryl, and carbofuran The adsorption of three selected pesticides is well described by Freundlich equation and the isotherm parameters are shown in Table 2. For the same HA, the adsorption coefficient (Kf) of three pesticides decreased in following order: parathionmethyl > carbaryl > carbofuran. This may be attributed to the hydrophobicity difference among these three compounds. Parathion-methyl, carbaryl and carbofuran are highly hydrophobic organic pesticide. Their hydrophobicity are at the order of parathion-methyl > carbaryl > carbofuran. According to the previous data in this study, the adsorption capacity of these three organic pesticides on treated HA is corresponded to the hydrophobicity i.e., the adsorption capacity order of parathionmethyl > carbaryl > carbofuran. This is because the stronger the hydrophobic organic compounds, the greater the ability to give electron, the higher the adsorption capacity [25]. Additionally, for different HAs, the Kf increased from 1814.9 to 44130 from 223.75 to 3348.6 and from 1.1212 102 to 703.75,
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Scheme 1. Possible process about graft-copolymerization mechanism of HA by styrene.
Table 1 Elemental contents (%), H/C, O/C ratios and potentiometric titration of HAs. Sample
untreated HA PSt-g-HA
Elemental analysis
Potentiometric titrations (mmol/g)
O
C
N
H
O/C
H/C
Phenolic
Carboxylic
Total acid
44.1 25.0
50.2 68.7
1.4 0.8
4.2 5.1
0.8737 0.3600
0.0842 0.0747
1.50 0.10
1.10 0.10
2.60 0.20
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Fig. 1. HA potentiometric titration curves.
for parathion-methyl, carbaryl, and carbofuran, respectively. The Kf of two HAs decreased in this order: PSt-g-HA > untreated HA. The adsorption capacity of parathion-methyl, carbaryl and carbofuran on PSt-g-HA increased by 95.2%, 90.4% and 64.1%, compared to that on original HA. Xing and Chen [16] proposed that precise comparison cannot be made between the Kf values from different isotherms because of their different units as a result of nonlinearity. However, it is obvious that Kf values of PSt-g-HA were much larger than that of untreated HA. The sorption isotherms of three pesticides are plotted in Fig. 4. The sorption nonlinearity, as determined by the Freundlich parameter n, varies from 0.56 to 0.91, with the exception of curbofuran. Compared with the untreated HA, the PSt-g-HA produced higher nonlinearity with n values of 0.91, 0.56, and 0.77, respectively. Whether parathion-methyl, carbofura or curbaryl, n values all increased in the order of PSt-g-HA > ununtreated HA, which is consistent with Kf values, confirming that the isotherm nonlinearity increased after HA is grafted copolymerization by polystyrene. In order to explain the nonlinear isotherms, HA was thought to be a kind of rubbery and glassy synthetic polymers, and the rubbery domain was expanded and the glassy domain was condensed, simultaneously and respectively. This is referred as dual-mode sorption [26]. Although the sorption of expanded domain appears to be linear because of the partition while nonlinear isotherms are found in the condensed
Fig. 2. The Fourier transforms infrared (FTIR) spectra.
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Fig. 3. CP/MAS13C-NMR spectra of HA.
domain, the sorption of the integrity of these two domains are observed as nonlinear isotherm. It reflected that there might be more condensed substance in PSt-g-HA than that in untreated HA, according to the increase of nonlinearity of three isotherms. Using X-ray diffraction and solid-state NMR, Xing and Chen [9] have observed rigid, condensed aromatic domains in HA. Moreover, there is other evidence in literature to supported that the condensed domains is tightly packed by aromatic regions in HA [27]. Therefore the grafted copolymerization of polystyrene enters into condensed domains in HA. These data (Table 2 and Fig. 4) demonstrate that the PSt-g-HA has the capability to sorb more organic pesticides (parathionmethyl, carbaryl, and curbofuran) than the untreated HA due to the structural and chemical difference between these two HAs. Chiou et al. [8] proposed that the increasing of aromatic content leaded to the partitioning of nonionic compounds to HA getting larger, in accordance with its high hydrophobicity in aromatic solvent (benzene and n-hexane, respectively. According to the theory that similarities can be solvable easily, the sorption abilities of parathion-methyl, carbaryl and carbofuran containing hydrophobic aromatic rings on PSt-g-HA with more phenyl rings than untreated HA will naturally increase. Furthermore, Kukkonen et al. [25] also reported that hydrophobic compound as electron-donors could be more easily adsorbed on PSt-g-HA with aromatic structure as electron-acceptors than untreated HA. Meanwhile, the existence of carboxyl and phenolic hydroxyl group and could decrease the adsorption capacity of the hydrophobic organic pesticide based on the high polarity of carboxyl and phenolic hydroxyl group [5,28]. It can be concluded that polystyrenemodification method causes aromatic structure and content changes of carboxyl and phenolic hydroxyl group, which results in polarity of HA decreased, and that the lowest polarity showed the highest sorption capacity and nonlinearity as compared with the two HAs. Basically the sorption of hydrophobic organic pesticides on HAs is a consequence of competitive adsorption between hydrophobic organic pesticide molecular and water molecular [4]. As shown in Fig. 5, for untreated HA, hydrophobic organic pesticides cannot effectively compete with polar water molecules for adsorption on the HA surface since water molecules can be bonded to polar carboxyl and phenolic hydroxyl group and form water film inhibited the partitioning of organic pesticide molecular to untreated HA. However, for PSt-g-HA, the remove of polar carboxyl and phenolic hydroxyl group and the grafting of hydrophobic polythene chains can restrain the form of water film on PSt-g-HA
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Table 2 Linear isotherm equation parameters for sorption of organic pesticides on HAs. pesticide
Parathion-methyl Curbaryl Carbofuran
HA
Linear equation
Untreated HA PSt-g-HA Untreated HA PSt-g-HA Untreated HA PSt-g-HA
Freundlich equation 2
Kd
R
Kf
n
R2
186.91 50866 106.84 1479.7 18.323 369.47
0.9865 0.9852 0.9445 0.8208 0.7534 0.9583
1814.9 44130 223.75 3348.6 1.1212 102 703.75
1.02 0.91 0.78 0.56 3.51 0.77
0.9867 0.9897 0.9723 0.9950 0.9547 0.9887
Fig. 4. Parathion-methyl, carbaryl and carbofuran sorption to HAs.
Fig. 5. The schematic plot about the sorption of organic pesticide molecular on HAs.
surface, thus, as in the case of the Kd and Kf values reported in Table 2, the PSt-g-HA appeared to have a higher adsorption capacity for hydrophobic organic pesticides, such as parathionmethyl, carbaryl and carbofuran, than untreated HA.
4. Conclusions In this study, modification of HA by graft copolymerization with styrene was achieved and the treated humic (PSt-g-HA) acid had
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