Abstract
The triazine-based herbicides removal from aqueous solution on specific polymeric adsorbent was studied. Poly(divinylbenzene) modified with maleic anhydride in Diels-Alder reaction was selected for the sorption experiments because molecular structures of triazine derived herbicides exhibit complementarity to the arrangement of functional groups in the polymer. The presence of carboxyl groups in adsorbent structure resulted in specific directional interactions, such as hydrogen bonds, which can intensify adsorption ability towards triazines. In the case of both atrazine and terbuthylazine the effect is more intensive, whereas in sorption of simazine and propazine the non-specific interactions have higher importance than hydrogen bonds. Specific interactions in investigated systems are between the hydrogen atom of the amino group of triazine and the carbonyl oxygen atom of the carboxyl group of the modified poly(divinylbenzene) (O…H–N). Only in the case of terbuthylazine the creation of hydrogen bonds between hydroxyl hydrogen atom of carboxylic group and nitrogen atom containing free electron pair from triazine (O–H…N) was observed. The sorption of simazine, atrazine and propazine does not depend on pH in the acidic region, whereas in the case of terbuthylazine an increase in sorption efficiency is observed while pH decreases.
Introduction
In past decades problem of contamination with triazine-based herbicides has been investigated in many research groups [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. Triazines are one group of herbicides which are used in agriculture to protect crops such as wheat, corn, potatoes and other against weeds, but at the same time display high toxicity and are highly stable in soil [19]. One of the strongest member of these herbicides group is atrazine, which has been used since 1950. Exposure to atrazine causes reproductive dysfunction in animals [20]. Additionally, it affects the hormones and immunological system in man and animals. The results of investigations done in USA indicate that atrazine has an effect on the weight of newborns and may cause serious birth defects [21]. A small amount of atrazine in drinking water may cause infertility in men, prostate cancer and breast and ovarian cancer in females or other types of cancer [22], [23]. Despite these atrazine is still in use and present in the drinking and surface water in USA. In European Union atrazine has been banned since 2004 because the concentration of this herbicide was frequently above the allowed limits [24]. Despite the ban and due to atrazine stability in the environment this herbicide is still present in the bottom sediments of ponds, lakes and other bodies of water. Also, in the post-industrial grounds there are large areas contaminated with atrazine. Atrazine may be present in these places in the unchanged form as well as in the form of various products of partial degradation. It should be also pointed out that ban on atrazine resulted in the syntheses of new compounds, which are also based on triazine. For example, terbuthylazine replaced atrazine and has somewhat different biological activity but the same long-term stability in the environment [2]. Currently several herbicides in which the active substance is terbuthylazine are used 250 g L−1 of terbuthylazine is used in commercial herbicides such as Cornmax 340 SE, Defender 340 SE, Korn 340 SE, Kukugran 340 SE, Maizgard 340 SE, Successor T550 SE, Zeagran 340 SE, and 187.5 gL−1 in herbicide Lumax 537.5 SE. Therefore, the methods for the selective sorption of triazines and selective materials for this process are still needed. The removal of terbuthylazine from aqueous solution by various treatment methods including adsorption is described in [2].
Selective adsorption is one of the most popular and most effective process of the removal of toxic substances from aqueous solutions. Polymeric materials, because of relatively high specific surface area, easy regeneration and simplicity of synthesis and modification, are widely used in removal of herbicides [25], [26], [27], [28], [29], [30], [31]. It can be seen that the presence of highly polar groups in adsorbents increases the sorption of herbicides. Those groups can be introduced into the structure of adsorbents either by choice of the suitable monomer or by the modification of adsorbents surface. The aim of such modification is to increase the number of specific interactions between adsorbents surface and sorbate molecule and thus to increase the sorption capacity and selectivity [32]. One example of such modification may be the introduction of the carboxyl groups to the polystyrene matrix [33]. Also, in the studies of sorption of triazine-based herbicides, the materials containing carboxyl groups, most often obtained from methacrylic acid, are used. They are mainly based on the molecular imprinting technique, in which the imprint ‘left’ on the surface of polymer is highly complementary in terms of its shape and functionality to the molecules resembling an imprinted molecule [34], [35], [36], [37], [38], [39]. Selectivity of such obtained adsorbents is extremely high, but unfortunately, their capacities are very low, so they can be effectively used only to detect herbicides.
Our previous study described in [5], [6] revealed that poly(divinylbenzene) beads modified in Diels–Alder reaction with maleic anhydride and subsequent base hydrolysis can be effectively used for preparation of material for triazines removal from water. Selectivity of the tested adsorbent towards herbicides from the group of triazines has been confirmed in previous publication [5]. Implementation of carboxyl groups into polymer structure resulted in obtaining specific interactions between modified poly(divinylbenzene) and triazines. It has been proven that intensification of triazines sorption using synthesized adsorbent is caused by formation of hydrogen bonds between amino groups in the structure of triazine herbicides and carboxyl groups on the polymer surface. The affinity of proposed adsorbent to herbicide from the group of triazines is presented in [5], where the effect of the chemical structure of triazine homologues on the efficiency of sorption was examined. Changing the number of methyl substituents around the amine nitrogen in the triazine determines the type of interaction between the sorbate and sorbent. Participation of specific interactions was observed only during the sorption of atrazine and terbuthylazine. This is an unexpected result, because the participation of hydrogen bonds in the simazine sorption processes has been described in the literature [40], [41]. However, even when the H-bonding between simazine and adsorbent can potentially occur, the authors investigating sorption on biochars have observed only non-specific interactions like hydrophobic effect, charge transfer (π-π*) interaction and pore-filling mechanism [11]. In numerous studies, it was hypothesized that the sorption mechanism of triazines depends on both hydrogen bonding and hydrophobic interactions [2], [25], [30], [41], [42], [43]. In this work, the experiments have been done in order to examine the impact of various factors such as pH, temperature or ionic form of adsorbent functional groups on the efficiency of triazines sorption on poly(divinylbenzene) modified with maleic anhydride. Moreover, the aim of the study is to examine the mechanism of triazines adsorption onto proposed adsorbent.
Materials and methods
Materials
The following chemicals were obtained from commercial sources: divinylbenzene (80%) (DVB), benzoyl peroxide (BPO), sodium chloride, polyvinyl alcohol 88% hydrolyzed (PVA), toluene, n-heptane purchased from Sigma-Aldrich. Monomer was purified by distillation before use. Analyzed herbicides: simazine, atrazine, propazine and terbuthylazine were obtained from Warsaw Institute of Industrial Organic Chemistry. All purities were ≥99% unless stated otherwise.
Preparation of polymer adsorbent
Polymer beads of poly(divinylbenzene) were synthesized in radical suspension polymerization. The continuous water phase comprised 2% w/w sodium chloride and 1% w/w PVA (calculated for organic phase). The dispersed organic phase contained monomer (DVB), initiator – BPO (0.5% w/w calculated for organic phase) and solvents: toluene and n-heptane (1:7 w/w). Prepared polymer beads were modified with maleic anhydride in Diels-Alder reaction as described in [44]. First, maleic anhydrate was dissolved in toluene. Then, previously prepared poly(divinylbenzene) beads were added. Modification was carried out at 110°C for 48 h. After that, base hydrolysis was performed using 3 M sodium hydroxide. Later, products were placed in ion-exchange columns and washed with water (for having carboxyl groups in sodium form on the polymer surface), hydrochloric acid and again with water (for having carboxyl groups in acidic form). Detailed synthesis and modification were described in [6].
Materials characterization
Water regain
Water regain, W (g g−1) of the adsorbent was determined using the centrifugation method and was calculated using Equation (1):
where mw (g) is the weight of wet polymer after centrifugation in a small column with fritted-glass bottom and md (g) is the weight of polymer after drying at 100°C overnight.
Carboxyl groups content
Content of carboxyl groups was determined by reversed hydrochloric acid titration. First, polymer beads were presoaked in water for 24 h. Later they were centrifuged for 5 min at 3000 rpm, and after that analyzed material (~1.2 g) and 50 mL of 0.1 mol dm−3 sodium chloride were placed in the shaker for 24 h. Finally, 20 cm3 of solution were sampled and titrated with 0.1 mol dm−3 hydrochloric acid using phenolphthalein as an indicator.
Surface area measurement and pore size estimation
Pore size and surface area were obtained by examining nitrogen adsorption at the liquid nitrogen temperature using Micromeritics ASAP 2020 analyzer. Resultant data were subjected to Brunauer-Emmett-Teller (BET) analysis. The total pore volume was estimated from a single point adsorption at the relative pressure of 0.988.
Sorption studies
The physical and chemical properties of investigated herbicides are given in Table 1. In sorption experiments a batch method was used in which 5 mg dm−3 triazine solution (ethanol:water 1:9 v/v) was contacted in 50 mL Erlenmeyer flask with an appropriate amount of polymer adsorbent. After shaking at room temperature (23°C) for 48 h, the adsorbent was separated by filtration and the concentrations of triazines were measured using UV/VIS spectroscopy, Jasco V-630 apparatus. Wavelength was set at 225.0 nm for terbuthylazine and 222.0 nm for simazine, atrazine and propazine. The same experiment was carried out at 16 and 50°C. The impact of pH and ionic form of adsorbent functional groups on the efficiency of triazines sorption was checked using 5 mg dm−3herbicide solutions at 23°C. The effect of pH of adsorptive solutions on the adsorption process was studied with the use of pesticide solutions at fixed concentration (5 mg dm−3) and different pH values from 3 to 7. The pH was controlled by the addition of a 0.1 M solution of HCl. Sodium hydroxide (0.1 M) was used to prepare the adsorbent having the functional groups in the form of its sodium salt. After ion-exchange, the polymer was washed using distilled water to neutral pH.
The physical and chemical properties of the herbicides.
| Herbicide | Structure | Molecular weight | Solubility in water (in ethanol) 20°C (mg L−1) [45], [46] | pKaa [47] | logKowa [47] |
|---|---|---|---|---|---|
| Simazine |  | 201.7 | 6.2 (570) | 1.6 | 2.3 |
| Atrazine |  | 215.7 | 33.0 (15 000) | 1.7 | 2.7 |
| Propazine |  | 229.7 | 5.0 (N/A) | 1.7 | 3.9 |
| Terbuthylazine |  | 229.7 | 8.5 (15 000) | 1.9 | 3.4 |
aKa, acid dissociation constant; Kow, octanol/water partition coefficient.
The sorption capacity as amount of herbicide adsorbed at equilibrium, qeq (mg g−1) was calculated using Equation (2):
where C0 and Ceq (mg dm−3) are the liquid-phase concentrations of pesticide at initial and at equilibrium, V (dm3) is the volume of solution and m (g) is the mass of dry adsorbent used. Retention rate of the herbicide, R (%) was calculated using Equation (3):
where C0 and Ceq (mg dm−3) are the liquid-phase concentrations of herbicide at initial and at equilibrium. Analysis of sorption isotherms for triazines solutions allows to determine the distribution coefficients. Distribution coefficient is determined as the amount of substance adsorbed by defined unit of weight of the adsorbent to the amount of the substance in the same volume of the solution after sorption. Distribution coefficient, K (–) was calculated using Equation (4):
where qeq (mg g−1) is the amount of herbicide adsorbed at equilibrium, Ceq (mg dm−3) is the liquid-phase concentrations of herbicide at equilibrium, ρ− solvent density (g dm−3).
Results
Adsorbent characterization
Proposed adsorbent was designed in such way that it will have sets of donor and acceptor atoms complementary to triazine-based herbicides. In order to achieve this the Diels-Alder reaction between free pendant vinyl groups of the polymer and maleic anhydride was done. The efficiency of the modification reaction of poly(divinylbenzene) microspheres with maleic anhydride was 54%. Resulted groups were subjected to hydrolysis to get carboxyl groups. Cycloaddition reaction scheme and the basic hydrolysis are shown in [6], and the fragment of the modified poly(divinylbenzene) surface is presented in Fig. 1. Carboxyl groups are able to form hydrogen bonds, their position and distance between them allow for the formation of complexes with adsorbate molecules such as triazines. Such specific, directional interactions are responsible for better sorption and selectivity when compared to the traditional polymeric adsorbents [5]. Selected polymer beads having porous structure and 3.58 mmol g−1 acidic groups were used in sorption experiments of herbicides based on triazine. Characteristics of this adsorbent are presented in Table 2. The study shows that the tested sorbent has a well-developed surface area, which has a direct influence on the adsorption capacity of the material. The water and ethanol/water regain of the tested polymer are characteristic for porous materials. The content of acidic groups indicates the presence of the carboxyl groups in the adsorbent structure that have been introduced by modification of the free vinyl groups in the poly(divinylbenzene). These groups can increase the sorption capacity by creating specific interactions between the sorbent and sorbate. Specific interactions are consisted of two hydrogen bonds: between hydroxyl hydrogen atom of carboxylic group from modified divinylbenzene and nitrogen atom containing free electron pair from triazine (O–H…N) and between hydrogen atom of triazine amino group and carbonyl oxygen atom from modified divinylbenzene (O…H–N).
Fragment of the modified poly(divinylbenzene) surface.
Characteristics of synthesized adsorbent.
| Properties of adsorbent | |
|---|---|
| Water regain (g g−1) | 2.54 |
| Ethanol/water solution (1/9 v/v) regain (g g−1) | 2.41 |
| Carboxyl group content (mmol g−1) | 3.58 |
| Surface area (m2 g−1) | 578 |
| Average pore size (nm) | 6.62 |
| Total pore volume (cm3 g−1) | 0.96 |
| Micropore size (nm) | 0.82 |
| Micropore volume (cm3 g−1) | 0.13 |
Sorption experiments
Sorption studies were performed for triazine-based herbicides such as simazine, atrazine, propazine and terbuthylazine. Changing the number of methyl substituents around the amine nitrogen in the triazine determines the type of interaction between herbicide and investigated sorbent. In the studied systems the triazines removal may be the result of both non-specific adsorption of the herbicide molecules in polymer pore structure and forming specific interactions between adsorbent and adsorbate. Previous studies have shown that participation of specific interactions occur only in adsorption of atrazine and terbuthylazine [5]. Therefore, there is a need for a wider analysis of the investigated adsorption systems. For this purpose, a series of sorption was carried out under various conditions for the aforementioned triazines. Based on the batch studies the sorption isotherms were plotted, which show the dependency of sorption on equilibrium concentration. Examining of the sorption isotherms gives the efficiency of the sorption of each herbicide. The analysis of the curves made it possible to determine the maximum of sorption (qeq) and retention rate for each herbicide (R). It also allows for determination of the distribution coefficients (K) of triazines between the adsorbent and the adsorbate solution. All these values were calculated for investigated triazines based on results obtained for 5 mg dm−3herbicide solutions at 23°C. The sorption isotherms are presented in Fig. 2 and results are shown in Table 3. Terbuthylazine shows the best sorption (28.6 mg g−1). It is also noted that with the reduction of the presence of methyl groups in the structure of triazines, the sorption capacity decreases and therefore the lowest value of the sorption is observed in the case of simazine (10.1 mg g−1). However, for the propazine and terbuthylazine which have the same chemical composition and molecular weight but different geometry arrangement of methyl groups various values of sorption were observed (28.6 and 24.9 mg g−1, respectively). Propazine has a higher value of octanol/water partition coefficient (Kow), therefore, we can expect a greater participation of hydrophobic interactions, however, terbuthylazine has lower solubility in water which also can increase non-specific sorption. The triazine-based herbicides solubility in water is 5 mg L−1, 33 mg L−1, 8.6 mg L−1 and 6.6 mg L−1 for simazine, atrazine, propazine and terbuthylazine, respectively. The inverse dependence of the sorption efficiency on the adsorbate solubility is not observed. The retention rate of herbicides from aqueous solutions was also examined. The studied herbicides were removed in the range from 77 to 95%. The greatest retention rate was observed for terbuthylazine. Calculated distribution coefficients for investigated triazine-based herbicides have high values, especially for terbuthylazine (11 700) and propazine (9300).
Single component adsorption isotherms for (a) simazine, (b) atrazine, (c) propazine and (d) terbuthylazine at various temperature:16°C, 23°C and 50°C.
Sorption capacity (qeq), retention rate (R) and distribution coefficients (K) for herbicides adsorbed from single component solutions.
| Herbicide | qeq (mg g−1) | R (%) | K (–) |
|---|---|---|---|
| Simazine | 10.1 | 77 | 2550 |
| Atrazine | 18.6 | 84 | 5300 |
| Propazine | 24.9 | 88 | 9300 |
| Terbuthylazine | 28.6 | 85 | 11 700 |
Figure 2 shows also graphs describing the sorption isotherms obtained at other temperatures (16 and 50°C). In all cases, the experiments conducted at elevated temperature gave the worst results. It can be seen that with increasing of sorption temperature the sorption efficiency decreased probably due to the weakening of hydrogen bonds created between amino groups in the structure of triazine herbicides and carboxyl groups on the polymer surface. Adsorption is an exothermic process, so the adsorption coefficient decreases with increasing temperature and, therefore, physical adsorption is more effective at lower temperatures. We can also see, that in the case of atrazine and terbuthylazine the effect is more intensive, so probably in sorption of simazine and propazine the non-specific interactions have higher importance than hydrogen bonds.
Next graphs, presented in Fig. 3, show the results of triazines sorption on sorbent having carboxyl groups in acidic (P–COOH) or sodium (P–COONa) form. In this study we can observe the impact of limiting possibilities of creating hydrogen bonds between the hydroxyl hydrogen atoms of the carboxyl groups in the adsorbent and the nitrogen atoms in the triazine molecules. Results clearly show that only in the case of terbuthylazine there is a significant effect. The difference in the adsorption effectivity between the adsorbent containing the acidic groups and the groups in the form of a salt for terbuthylazine is about 10 mg g−1. This may be a result of the elimination of hydrogen bonds between hydroxyl hydrogen atom of carboxylic group from modified poly(divinylbenzene) and nitrogen atom containing lone electron pair from triazine (O–H…N). In the case of other triazines such a result was not observed. This would suggest that in the investigated sorption systems (except terbuthylazine) the hydrogen bonds between hydroxyl hydrogen and nitrogen atom in triazine did not exist and the hydrogen bonds are formed only between the carbonyl oxygen atom of the carboxyl group and the hydrogen atom of the amino group of triazine.
Single component adsorption isotherms for (a) simazine, (b) atrazine, (c) propazine and (d) terbuthylazine on adsorbent having carboxyl groups in acidic (P–COOH) and sodium (P–COONa) form.
It would be necessary to pay attention to the fact that the presence of sodium ions in tested polymer results also in the higher polymer swelling (2.68 g g−1), which increases the availability of carboxyl groups and thereby increases formation of hydrogen bonds between the carbonyl oxygen atom of the carboxyl group and the hydrogen atom of the amino group of triazine. For this reason, in the case of atrazine and propazine, we can observe a small increase of sorption efficiency on adsorbent having sodium salt form. On the other hand, the greater polymer swelling in water decreases terbuthylazine sorption on the polymer in the sodium salt. The higher affinity of the investigated polymer for water decreased hydrophobic interaction between adsorbate and adsorbent what in the case of terbuthylazine, which is one of the most hydrophobic among investigated triazines, has a great impact on the sorption efficiency. This effect was not observed for propazine having the highest octanol/water partition coefficient (Kow) (the logarithm of octanol/water partition coefficient (logKow) is 2.3, 2.7, 3.9 and 3.4 for simazine, atrazine, propazine and terbuthylazine, respectively). Therefore, we can conclude that lower terbuthylazine sorption on the polymer in sodium salt form is mainly due to limitation in formation of hydrogen bonds between the hydroxyl hydrogen atoms of the carboxyl groups in the adsorbent and the nitrogen atoms in the triazine molecules.
The pH of adsorbate solution usually has a remarkable influence on the adsorption process because it can affect the chemistry of both adsorbate and adsorbent in the solution. Triazines are weak bases, with the logarithmic acid dissociation constant (pKa) of 1.6, 1.7, 1.7 and 1.9 for simazine, atrazine, propazine and terbuthylazine, respectively. Therefore, at the studied pH (3–7), the investigated herbicides are almost exclusively present as neutral molecules and accordingly the weak interactions such as hydrophobic interactions and hydrogen bonds could be involved in their adsorption onto the modified poly(divinylbenzene). The sorption isotherms for tested triazines obtained at different pH conditions are shown in Fig. 4. It can be seen that the sorption of simazine, atrazine and propazine does not depend on the pH. The pKa value of the carboxylic groups on the investigated polymer is about 6±0.5. At pH lower than this value the acidic groups are undissociated. Only in the case of terbuthylazine the increase of the hydrogen ions concentration (lower pH) increases the efficiency of the sorption, which probably resulted from the more effective creation of the hydrogen bond due to the lower polymer hydration following the withdrawal of dissociation of acid groups. At the same time, the formation of hydrogen bonds between hydroxyl hydrogen atom of carboxylic group and nitrogen from terbuthylazine ring (O–H…N) can be confirmed. Additionally, no reduction in the sorption efficiency with decreasing the pH allows to use the polymers effectively in various environments, what is a great advantage of the tested sorbent. For example, removal of simazine by porous silica becomes ineffective at the pH lower than 5 and higher than 6 [7].
Single component adsorption isotherms for (a) simazine, (b) atrazine, (c) propazine and (d) terbuthylazine at various pH: 3, 5 and 7.
Conclusions
Sorption of investigated triazine-based herbicides on modified poly(divinylbenzene) can be explained in terms of the adsorption mechanism, which involves hydrophobic interactions and specific sorption. In the case of terbuthylazine and atrazine the creation of hydrogen bonds is more intensive, so probably in sorption of simazine and propazine the non-specific interactions have higher importance. The affinity order of investigated polymer adsorbent towards triazines is terbuthylazine>propazine>atrazine>simazine. This order is in correlation with molecular weight and is much higher for more hydrophobic triazines. However, comparing the sorption of terbuthylazine and propazine, the higher affinity of tested adsorbent for terbuthylazine is probably the result of greater participation of specific interaction. The specific sorption of triazines is based on hydrogen bonds formed between hydrogen atom of the amino groups of triazine and the carbonyl oxygen atom of the carboxyl group of the modified poly(divinylbenzene) (O…H–N). Only in the case of terbuthylazine the creation of hydrogen bonds between hydroxyl hydrogen atom of carboxylic group and nitrogen atom from triazine (O–H…N) was observed. The sorption of simazine, atrazine and propazine does not depend on the acidic pH, and in the case of terbuthylazine the increase in sorption efficiency is observed while pH decreases. Examining the impact of various factors such as pH, temperature or ionic form of adsorbent functional groups on the efficiency of triazines sorption it can be concluded that poly(divinylbenzene) modified with maleic anhydride has good sorption properties towards triazine-based herbicides in different media and therefore it may found potential application in water purification or solid phase extraction (SPE) technique.
Article note:
A collection of invited papers based on presentations at the 16th International Conference on Polymers and Organic Chemistry (POC-16), Hersonissos (near Heraklion), Crete, Greece, 13 – 16 June 2016.
Acknowledgments
The work was financed by a statutory activity subsidy from the Polish Ministry of Science and Higher Education for the Faculty of Chemistry of Wrocław University of Science and Technology.
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