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Triazines removal by selective polymeric adsorbent

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Published/Copyright: November 1, 2014
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Pure and Applied Chemistry
From the journal Pure and Applied Chemistry Volume 86 Issue 11

Abstract

The objective of the study was to investigate sorption of simazine, atrazine, propazine and terbuthylazine on specific polymeric adsorbent and thereby evaluate the possibility of triazine-based herbicide removal from the aqueous solution. In order to obtain polymer adsorbent for triazines removal, the poly(divinylbenzene) was synthesized in radical polymerization using bead polymerization, and modified with maleic anhydride in Diels–Alder reaction with subsequent base hydrolysis. The porous material containing carboxyl groups was obtained. Experiments have been performed in single and multi-component mixtures of herbicide in the ppm concentration range. Introduction of carboxyl groups into polymer structure resulted in obtaining specific interactions, such as hydrogen bonds between modified poly(divinylbenzene) and triazines, therefore intensification of adsorption was observed. Calculated distribution coefficients of triazines (K = 2600–35 100) testify to their effective removal from aqueous solutions on the studied adsorbent. Selective sorption of triazines is observed and explained in relation to the binding mechanism which involve hydrophobic interactions and hydrogen bonding. The effect of the adsorbate structure on the ability to form specific interactions with the tested adsorbent was investigated. The kinetic of sorption and the parameters of Langmuir and Freundlich isotherms for the studied systems were determined.

Keywords: adsorption; herbicides; POC-2014; polymers

Introduction

Triazines are commonly used herbicides for the control of weeds in many agricultural crops like corn, wheat, and sorghum productions. Unfortunately, these substances, as well as products of their degradation are toxic, highly resistant and survive many years in the soil [1], water, animals [2] and plants. Furthermore, relatively low absorption in soil provides easy migration through soil into the surface and ground water [3, 4]. The widespread applications of triazines herbicides over the past few decades have resulted in rising concern about effects of their usage on the environment and human health and resulted in prohibition of triazines usage in many countries. However, even though described herbicides are still observed in groundwater, due to the fact that in the past it has been heavily applied for preemergence control in the agriculture. Therefore in past decades problem of contamination with pesticides, including triazine-based herbicides has been investigated in many research groups [5–10]. Triazines removal from water is widely studied using various materials including modified zeolites [11], nanotubes [12] and molecularly imprinted polymers [13, 14]. The examples of triazines adsorption on different materials are shown in Table 1. As presented, there is huge variety of materials applied for triazines adsorption, however, the most efficient methods of water purification is adsorption using polymer materials. They are characterized by highly developed surface area, therefore sorption process is highly efficient. Furthermore, easy modification and regeneration provides possibility of applying these materials as the efficient adsorbents [21–25]. Moreover, numerous possibilities of modification and implementing various functional groups into polymer structure [26–28] can be used in synthesis of selective adsorbents, hence specific interactions between adsorbate and adsorbent are developed and the adsorption performance towards specific adsorbent is improved.

Table 1

Sorption of triazines using various materials.

AdsorbateAdsorbentSorption capacity (mg g-1)References
SimazinePoly(ε-Caprolactone)27.54[15]
Hypercrosslinked polymer: Hypersol Macronet™15.70[16]
Greenwaste biochar1.00[17]
AtrazinePoly(ε-Caprolactone)10.98[15]
Hypercrosslinked polymer: Hypersol Macronet™39.70[16]
Atrazine imprinted polymer1.40[18]
Greenwaste biochar0.45[17]
PropazineAluminum pillared clay0.90[19]
TerbuthylazineActivated carbon10.00[20]

Our previous study described in [29] 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 atrazine removal from water. Implementation of carboxyl groups into polymer structure resulted in obtaining specific interactions between modified poly(divinylbenzene) and atrazine and therefore intensification of adsorption was observed. Adsorption capacities were highly dependent on porous structure of polymer, designed by modification of solvents mixture used during polymerization step. The highest adsorption capacity (32 mg g–1) was obtained for porous adsorbent, where atrazine removal was an effect of both trapping the atrazine molecules in polymer pore structure and forming specific interactions between adsorbent and adsorbate. It has been proven that intensification of atrazine sorption using synthesized sorbent is caused by forming hydrogen bond between herbicide and poly(divinylbenzene) containing carboxyl groups. Due to the presence of amino groups in the structure of triazine herbicides and carboxyl groups on the polymer surface the possibility of specific interactions related to hydrogen bonds formation can be predicted not only in the case of atrazine, but also other triazines. Therefore, in the presented paper we expand the previous research on the sorption of propazine, simazine and terbuthylazine using the most effective adsorbent. Moreover the aim of the study is to examine the mechanism of triazines adsorption onto porous poly(divinylbenzene) adsorbent containing carboxyl groups.

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 using. Deionized water (Millipore) was used for making the aqueous phase and solutions during sorption studies. Analyzed herbicides: simazine, atrazine, propazine, terbuthylazine and metamitron 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 polymerization using 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 [30]. First maleic anhydrate was dissolved in toluene. Then previously prepared poly(divinylbenzene) beads were added. Modification was held at <softenter;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 was described in [29].

Materials characterization

Water and ethanol/water solution regain

Water and ethanol/water solution regain, W (g g–1) of the adsorbent was measured using the centrifugation method and was calculated using Eq. (1):

(1)W = (mw − md)/md (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 measured by reversed hydrochloric acid titration. First polymer beads were presoaking 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.

Scanning Electron Microscopy

Scanning electron micrographs (SEM) were done to obtain more direct insight into the porous polymer structure. Polymer beads were coated with gold using Edwards Scancoat Six, Pirani 501 apparatus, time of gold-coating 600 s. Micrographs were taken on a Zeiss EVO LS 15.

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 Brauner–Emmet–Teller (BET) treatment.

Sorption studies

In sorption experiments 10 ppm ethanol/water solutions (1/9, v/v) of herbicides were used. Due to the fact that the triazines are poorly soluble in water, they were previously dissolved in ethanol and then diluted with water. A batch method was used in which herbicide solution was contacted in 50 mL Erlenmeyer flask with an appropriate amount of polymer adsorbent. After shaking at room temperature for 48 h, the adsorbent was separated by filtration and the concentrations of triazines and metamitron were measured using UV/VIS spectroscopy, Jasco V-630 apparatus. Wavelength was set at 222.0 nm for simazine, atrazine and propazine, 225.0 nm for terbuthylazine and 307.0 nm for metamitron, respectively. The physical and chemical properties of the herbicides are given in Table 2. Sorption kinetics was examined by mixing adsorbent with 10 ppm herbicide solution, collecting samples at fixed times and analyzing the residual herbicide concentration. The sorption capacity as amount of herbicide adsorbed at equilibrium, qeq (mg g–1) was calculated using Eq. (2):

Table 2

The physical and chemical properties of the herbicides.

HerbicideStructureMolecular weightSolubility in water (in ethanol) 20 °C (mg L–1) [31, 32]
Simazine
201.76.2 (570)
Atrazine
215.733.0 (15 000)
Propazine
229.75.0 (N/A)
Terbuthylazine
229.78.5 (15 000)
Metamitron
202.21700 (1100)
(2)qeq = (C0 − Ceq)V / m (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 Eq. (3):

(3)R = (C0 − Ceq) ⋅ 100 % / C0 (3)

where C0 and Ceq (mg dm–3) are the liquid-phase concentrations of herbicide at initial and at equilibrium.

The static sorptions for multicomponent solutions of herbicides containing equal proportions by weight and a total concentration 10 ppm (for 2-component solutions) and 12 ppm (for 4-component solution) were also done. Due to the fact that the maximum absorption signal of the tested triazines occurs in the same wavelength (λ = 222.0 nm) the concentrations of adsorbates in multicomponent solutions were determined by high performance liquid chromatography (HPLC). HPLC studies was performed using a Dionex chromatograf ISC-5000+. Conditions for use of HPLC were as follows: column – C-18, 250 mm × 4.6 mm, injection – 100 μL, flow rate – 1 mL/min, eluents: A: water, B: 90 % acetonitrile (ACN) + 10 % water. To the eluent was added 1 % of trifluoroacetic acid (TFA). Analysis of sorption isotherms for the multicomponent triazines solutions allow 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 Eq. (4):

(4)K = qeq ⋅ ρ / Ceq (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). Sorption selectivity was determined by selectivity coefficient, α (–) using Eq. (5), which is the ratio of the distribution coefficients of the two compounds (i,j):

(5)α = Ki / Kj (5)

where Ki(j) is distribution coefficient [i,j = (S,A,P,T) – indexes that corresponds to the herbicide: S- simazine, A- atrazine, P- propazine, T- terbuthylazine].

Results

Adsorbent characterization

In order to obtain polymer beads poly(divinylbenzene) was synthesized in radical polymerization using suspension polymerization. The polymerization conditions (e.g., time, temperature) were optimized in order to obtain on the surface of the polymer as much as possible of free pendant vinyl groups. Number of free pendant vinyl groups determines the degree of chemical modification of the polymer and at the same time the level of ability of the adsorbent to form specific interactions. Chemical modification of polymer was done by Diels–Alder reaction in which poly(divinylbenzene) reacts as diene and maleic anhydride as dienophile. Generation of the carboxyl groups capable for specific interactions followed by ring opening of the maleic anhydride with the use of base hydrolysis. Cycloaddition reaction scheme and the basic hydrolysis are shown in Fig. 1. Porous structure and carboxyl group content in obtained products were studied. Selected material having porous structure and 2.8 mmol g–1 acidic groups were used in sorption experiments of herbicides based on triazine. Characteristic of this adsorbent is presented in Table 3. Its SEM micrographs are shown in Fig. 2. The study shows that the tested adsorbent 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 and are the result of retention of water or ethanol/water solution in the pores of the material. 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.

Fig. 1 Modification of poly(divinylbenzene) with maleic anhydride [28].
Fig. 1

Modification of poly(divinylbenzene) with maleic anhydride [28].

Table 3

Characteristic of synthesized adsorbent.

Properties of adsorbent
Water regain (g g–1)3.24
Ethanol/water solution (1/9 v/v) regain (g g–1)3.04
Carboxyl group content (mmol g–1)2.80
Surface area (m2 g–1)713
Average pore size (nm)5.70
Total pore volume (cm3 g–1)1.00
Micropore size (nm)0.80
Micropore volume (cm3 g–1)0.17
Fig. 2 SEM images of modified poly(divinylbenzene) beads.
Fig. 2

SEM images of modified poly(divinylbenzene) beads.

Sorption experiments

Poly(divinylbenzene) modified with maleic acid anhydrate in Diels–Alder reaction was selected for the sorption experiments because molecular structures of triazines exhibit complementarity to the arrangement of functional groups present in its structure. The group of triazine-based herbicides, such as simazine, atrazine, propazine and terbuthylazine were tested. In the case of homologue of triazines the differences in sorption can be resulted from the change of the number of methyl substituents around the amine nitrogen. It is possible that the increased number of methyl groups around the amino nitrogen caused the spatial distribution, due to the fact that the methyl groups are much greater than the hydrogen atoms, and stronger interact with the surroundings. Moreover, the appearance of methyl groups changed the stabilization energy of the molecules due to the linkage, and also prevents the association of water molecules, which could destabilize formation of the complex. For comparison the sorption of metamitron was done.

Adsorption of herbicides from single-component solutions

Adsorption kinetics Batch adsorption experiments were conducted to determine the time required to reach the equilibrium in the adsorption process. Sorption kinetics study was performed by measuring the absorbance of the herbicide solution at a specified interval. The rate of sorption was determined by shaking the polymer sample with a solution containing an excess of the herbicide, exceeding the sorptive capacity of the test material. A plots of the equilibrium sorption (qeq) versus time (t) are presented in Fig. 3. On the basis of sorption kinetics it can be seen that the adsorption processes of the tested herbicides proceed quite long. The most intense sorption occurred within the first 10 h and after this time the sorption value is from 65 % for triazines to 100 % for metamitron. The longest time to reach adsorption equilibrium shows terbuthylazine. For other triazines sorption equilibrium was reached after about 30 h.

Fig. 3 Sorption kinetics for herbicides on modified poly(divinylbenzene) adsorbent.
Fig. 3

Sorption kinetics for herbicides on modified poly(divinylbenzene) adsorbent.

The modeling of adsorption kinetics was investigated using two models, namely, the pseudo-first- and pseudo-second-order. Due to the higher correlation coefficient pseudo-second-order kinetics equation fits the experimental data better than pseudo-first order kinetics model. The parameters of the pseudo-second-order model were calculated using Eq. (6):

(6)t/qt=1/(k2⋅qeq2)+t/qeq (6)

where t – time (min), qeq – sorption capacity in equilibrium (mg g–1), qt – sorption capacity in time (mg g–1), qeq – maximum amount of adsorbed herbicide (mg g–1) corresponded to monolayer saturation, k2 – pseudo-second-order adsorption rate constant (g mg–1 min–1). The results are presented in Table 4. According to high correlation coefficients (R2 ≥ 0.99) pseudo-second order kinetics model can describe adsorption of triazines on analyzed adsorbent with success. Moreover better fitting the pseudo-second kinetics model than pseudo-first model confirms that triazines uptake using prepared adsorbents is a result of creation of the specific interaction between adsorbate and adsorbent and kinetics depends more on the availability of adsorption sides than the concentration of herbicide in solution.

Table 4

Parameters of pseudo-second adsorption kinetic model for modified poly(divinylbenzene) adsorbent and herbicides.

HerbicideCorrelation coefficient R2qeq (mg g–1)k2 (g mg–1 min–1)
Simazine0.99618.92.7·10–4
Atrazine0.99731.32.6·10–4
Propazine0.98636.47.1·10–5
Terbuthylazine0.99533.91.6·10–4
Metamitron0.9989.733.9·10–3

Adsorption isotherms Based on the static sorption studies the sorption isotherms are plotted, which show the dependency of sorption from equilibrium concentration, as shown in Fig. 4. Examination of the sorption isotherms allows to specify the efficiency of the sorption of each herbicide. The results show a substantially better sorption of triazine-based herbicides compared to triazinone, which confirms the impact of the structure complementarity on the sorption efficiency in the system under study. 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 determine the distribution coefficients (K) of herbicides between the adsorbent and the adsorbate solution. The calculated values are shown in Table 5. The best sorption shows terbuthylazine (64.4 mg g–1). Sorption of this triazine is more than 7-fold higher compared with the value obtained for metamitron. As is well known the adsorption of organic substances from aqueous solutions is influenced by the physical and chemical properties as well as molecular weight, size and geometrical shape, the type of functional groups, polarity and solubility of the adsorbate. It is noted that with the reduction of the presence of methyl groups in the structure of triazines, the sorption capacity decreases and therefore the smallest value of the sorption is observed in the case of simazine (17.5 mg g–1). However, an analysis should not be based on only one criterion which could lead to false conclusions. For the two triazines: propazine and terbuthylazine which have the same chemical composition and molecular weight but different geometry arrangement of methyl groups we observed various values of sorption. For terbuthylazine the sorption is almost twice higher than for propazine. Probably due to the presence of a tertbuthylamine group terbuthylazine is a more hydrophobic molecule what increase the contribution of non-specific sorption. From the same reason simazine and metamitron have lower adsorption capacity than other herbicide because they are far more hydrophilic. The inverse dependence of the sorption efficiency from the adsorbate solubility is not observed. The retention rate of herbicides particles from aqueous solutions was also examinated. The studied triazines were removed in the range from 85.0 to 99.7 %. The greatest retention rate was observed for terbuthylazine. It was noted a large discrepancy between the retention rate for triazines and for metamitron. Metamitron was removed only in 55.3 %. Generally triazines are eliminated about 30–40 % more effective than metamitron. Calculated distribution coefficients for investigated triazines, especially for terbuthylazine (35 100) and propazine (20 500) are high. The worst results (K = 950) were obtained for matamitron probably due to the lack of the ability to create specific directional interactions with the adsorbent.

Fig. 4 Single component adsorption isotherms for simazine, atrazine, propazine, terbuthylazine and metamitron.
Fig. 4

Single component adsorption isotherms for simazine, atrazine, propazine, terbuthylazine and metamitron.

Table 5

Sorption capacity, retention rate and distribution coefficients for herbicide adsorbed from single component solutions.

Herbicideqeq (mg g–1)R (%)K (–)
Simazine17.5852600
Atrazine30.0926750
Propazine38.79720 500
Terbuthylazine64.410035 100
Metamitron8.855950

Modification of poly(divinylbenzene) with maleic anhydride in Diels–Alder reaction and subsequent hydrolysis leads to the carboxyl groups being in a specific arrangements on the polymer surface and able to form specific directional interactions with triazine molecules what can increase sorption. The formation of hydrogen bonds between atrazine molecule and the surface of modified poly(divinylbenzene) was confirmed in the paper [29]. Complementarity of both molecules in tested system can be illustrated for the modified divinylbenzene and two molecules of atrazine (Fig. 5). This arrangement should also substantially increase the selectivity of sorption of triazines herbicides. The influence of specific interactions on triazines sorption was also investigated using sorption experiments on poly(divinylbenzene) containing carboxyl groups in sodium form. These studies were carried out in respect of the sorption studies on the adsorbent containing carboxyl groups in acidic form. Carboxyl groups on the polymer surface are able to form hydrogen bonds, their position and distance between them allow for the formation of complexes with adsorbate molecules. In the case of the adsorbent containing acidic groups, it is possible to form two 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) and between hydrogen atom of triazine amine group and carbonyl oxygen atom form modified poly(divinylbenzene) (O---H-N). Sorption on poly(divinylbenzene) containing carboxyl groups in sodium form excludes hydrogen bonds formation between hydroxyl hydrogen atom of carboxylic group from modified poly(divinylbenzene) and nitrogen atom containing free electron pair from triazine (O-H---N). The results of triazines sorption on adsorbent containing carboxyl groups in sodium form and for comparison on adsorbent having acidic groups are shown on the Fig. 6. The effect of specific interactions on the sorption efficiency is observed only in the case of sorption of terbuthylazine and atrazine. The difference in the adsorption effectivity between the adsorbent containing the acidic groups and the groups in the form of a salt for atrazine is 10 mg g–1 while for terbuthylazine is 25 mg g–1. In the case of propazine the difference between sorption on adsorbent containing acidic or sodium form of carboxyl groups was not observed. It can be ascribed to the presence of two isopropyl groups in the propazine structure and the steric hindrance that prevent the creation of the hydrogen bonds. Probably removal of propazine occurs due to non-specific interactions. This could also explain the much higher sorption of terbuthylazine compared to propazine where the contribution of the specific sorption is significant. In the case of simazine sorption intensification resulting from the formation of specific interactions is also not observed. This is an unexpected result, because the simazine molecule should easily form hydrogen bonds with the tested adsorbent.

Fig. 5 The structural formula of the complex of the modified divinylbenzene with two molecules of atrazine [29].
Fig. 5

The structural formula of the complex of the modified divinylbenzene with two molecules of atrazine [29].

Fig. 6 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.
Fig. 6

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.

In the present study, two isotherm models have been used to fit the equilibrium data, namely Langmuir and Freundlich models. The Langmuir model assumes the monolayer adsorption onto a surface containing number of adsorption sites of uniform energies. Langmuir isotherm is expressed as:

(7)qeq = qm ⋅ Kα ⋅ Ceq / (1 + Kα ⋅ Ceq) (7)

where qeq is the amount of solute adsorbed per mass of adsorbent and Ceq is the equilibrium concentration of solute in the bulk solution. The constant Kα and qm are characteristic of the Langmuir equation and can be evaluated from the linearised form represented by Eq. (8).

(8)1 / qeq = 1 / qm + 1/(qm ⋅ Kα) ⋅ (1 / Ceq) (8)

Second analyzed model was the Freundlich isotherm based on a assumption of adsorption on heterogeneous surfaces, given by Eq. (9)

(9)q = KF ⋅ Ceq1/n (9)

where: q – sorption capacity at monolayer saturation (mg g–1), Ceq – equilibrium concentration of herbicide (mg dm–3), KF, n – Freundlich constants. KF provides an indication of the adsorption capacity, while 1/n is a function of the strength of adsorption in the adsorption process [33]. If n = 1 then the partition between the two phases are independent of the concentration. If value of 1/n is below one it indicates a normal adsorption. On the other hand, 1/n being above one indicates cooperative adsorption [34]. Linear form of Freundlich model used to calculate parameters of isotherms was described by Eq. (10).

(10)logq = logKF + 1 / n ⋅ logCeq. (10)

Parameters of Langmuir and Freundlich equation are presented in Table 6. From fitting the experimental data it occurred that Langmuir isotherm is not applicable for metamitron on analyzed adsorbent, because maximum adsorption capacity obtained from linearization has a negative value. The very high correlation coefficient for propazine shows that Langmuir model is suitable for describing the equilibrium of their adsorption. The correlation coefficients for other triazines are also high, but the qeq values are too high when compared to the experimental sorption data (see Table 5). Freundlich isotherm model provided good correlation of equilibrium data for the adsorption of metamitron and triazines (except propazine) using prepared polymeric adsorbent. The good fit obtained with the Freundlich model suggests that the adsorption of triazines onto poly(divinylbenzene) containing carboxylic groups may involve interactions between the pesticide molecules and adsorbent surface.

Table 6

Parameters of Langmuir and Freundlich isotherm model obtained for systems containing analyzed herbicides and polymeric adsorbent.

HerbicideLangmuir parametersFreundlich parameters
qm (mg g–1)Kα (dm3 mg–1)R21/nKF [(mg g–1)(L mg–1)1/n]R2
Simazine470.060.9800.832.810.972
Atrazine2040.030.9741.125.530.983
Propazine500.470.9930.5614.140.752
Terbuthylazine1180.300.9300.5628.000.986
Metamitron–4.89–0.070.8931.900.110.948

Adsorption of herbicides from multicomponent solutions

Adsorption isotherms Selectivity is a measure of sorbents utility in the process of extraction, which relies on the preferential sorption of triazine from multicomponent solutions containing herbicides from other groups, such as triazinone. In order to verify the selectivity of the sorption with respect to triazine herbicides the static sorption of two-component solutions was carried out: two ethanol-water herbicide mixtures of equal proportions and total concentration of 10 ppm. The composition of the first mixture of herbicides was: terbuthylazine and metamitron, and the second: atrazine and metamitron. The sorption isotherms for the studied mixtures were plotted (Figs. 7 and 8) and determination of sorption (qeq), distribution coefficients (K) for individual herbicides and selectivity coefficients (α) was done (Table 7). On the basis of the graphs it can be estimated that the tested sorbent is selective with the respect to herbicides from the group of triazines. Sorption of terbuthylazine relative to metamitron was about four times greater, and atrazine relative to metamitron nine times greater. Calculated distribution coefficients suggest that the tested adsorbent effectively removes triazines than triazinone from aqueous solutions. Selectivity coefficients clearly indicate that the tested specific adsorbent prefers sorption of herbicides from the triazine group. Atrazine and terbuthylazine in a mixture with metamitron are better adsorbed by the tested adsorbent, what confirm the importance of complementarity in the structure of the sorbent and the sorbate in sorption studies.

Fig. 7 Multicomponent adsorption isotherms for terbuthylazine and metamitron.
Fig. 7

Multicomponent adsorption isotherms for terbuthylazine and metamitron.

Fig. 8 Multicomponent adsorption isotherms for atrazine and metamitron.
Fig. 8

Multicomponent adsorption isotherms for atrazine and metamitron.

Table 7

Sorption capacity, distribution coefficients and selectivity coefficients for herbicide adsorbed from multicomponent solutions.

MixtureHerbicideqeq (mg g–1)K (–)α (–)
1Terbuthylazine19.711 90010.8
Metamitron4.811000.1
2Atrazine18.3500010.1
Metamitron2.15000.1

Previous studies have demonstrated the selectivity of sorption towards triazine herbicides, whereas study the sorption of a mixture of four triazines allows to specify the sorption selectivity towards herbicides belonging to the same group and the impact of triazines structure on the sorption efficiency. The sorption of herbicides from a mixture of four triazines: simazine, atrazine, propazine and terbuthylazine was studied in static conditions. The triazines are of equal weight proportions in the solution and their total concentration was 12 ppm. The concentration of triazines was determined using HPLC technique. The adsorbed mass was calculated based on the concentration of the solution after sorption. The sorption isotherms for the mixtures of herbicides are shown in Fig. 9. The analysis of this curves made it possible to determine the maximum of sorption (qeq) and retention rate (R) for each herbicide in the mixture. It also allows determine the distribution coefficients (K) of triazines between the adsorbent and the adsorbate solution. The calculated values are shown in Table 8. Maximum sorption values, qeq, for each compound in the mixture are decreased by 3–4 times in comparison to those obtained for the sorption of single herbicide. But the retention rate of the investigated compounds slightly changed for all tested triazines. Therefore, we can assume that the presence of competing compounds did not affect the efficiency of their removal from aqueous solutions. Calculated distribution coefficient increased in all cases compared with the values designated by the static sorption from single solutions. On the basis of the determined maximum value of the sorption and calculated distribution coefficients (Table 8) the selectivity coefficients (α) of competing compounds during the sorption on investigated sorbent was determined (Table 9). Calculated values of the selectivity coefficients indicate that the investigated adsorbent can remove triazines from aqueous solution selectively (α = 1.3–9.5). Effective separation occurs for pairs of herbicides with the greatest difference in the molecular weights. Superior separation is visible when the competing compounds are terbuthylazine and simazine (αT/S = 9.5). The decrease in the difference in the molecular weight decreases the selectivity coefficient for a given pair of herbicides. It was observed that most strongly adsorbed is terbuthylazine, what confirms the previous considerations on the influence of the spatial distribution of the methyl groups in the structure of herbicide molecule. Presumably, this arrangement of methyl groups in the terbuthylazine molecule increases its hydrophobicity (the presence of tertbuthylamine groups), so that its interact strongly with the hydrophobic molecule of the adsorbent and facilitates sorption intensification resulting from the possibility of formation of hydrogen bonds.

Fig. 9 Multicomponent adsorption isotherms for simazine, atrazine, propazine and terbuthylazine.
Fig. 9

Multicomponent adsorption isotherms for simazine, atrazine, propazine and terbuthylazine.

Table 8

Sorption capacity, retention rate and distribution coefficients for herbicide adsorbed from multicomponent solutions.

Herbicideqeq (mg g–1)R (%)K (–)
Simazine6.53774200
Atrazine7.92899500
Propazine14.519629 700
Terbuthylazine19.849639 800
Table 9

Selectivity coefficients for investigated triazines.

i/jaα (–)
S/A0.4
S/P0.1
S/T0.1
A/S2.3
A/P0.3
A/T0.2
P/S7.1
P/A3.1
P/T0.8
T/S9.5
T/A4.2
T/P1.3

ai, j = (S,A,P,T) – indexes that corresponds to the herbicide: S- simazine, A- atrazine, P- propazine, T- terbuthylazine.

In general, triazines removal using prepared adsorbent is more efficient, than in the case of other materials presented in the Table 1. Moreover, the process of adsorption is selective, whereas the triazines uptake described in literature is mostly based on trapping adsorbate in pore structure.

Conclusions

Triazines removal on modified poly(divinylbezene) from aqueous solution is an effective process. The kinetic data fit well by a pseudo-second-order kinetic model what suggests that the adsorption rate is dependent on the availability of adsorption sites more than on herbicide concentration. The equilibrium data followed the Freundlich isotherm, except propazine for which the Langmuir model is suitable. The affinity order of investigated polymer sorbent towards triazines is: terbuthylazine > propazine > atrazine > simazine. This order is in correlation with molecular weight and hydrophobicity of the triazines molecules. In order to study selectivity properties of obtained adsorbent sorption of triazines from two-component solution with metamitron was done. Selectivity coefficients clearly indicate that the tested specific adsorbent prefers sorption of triazine-based herbicides. Selective sorption can be explained in relation to the binding mechanism which involve hydrophobic interactions and hydrogen bonding. Intensification of sorption on the synthesized adsorbent resulting from the formation of specific interactions, such as system of complementary hydrogen bonds, only for terbuthylazine and atrazine has been observed. In propazine molecule there are two isopropyl groups, whose probably can cause the steric hindrance in the creation of hydrogen bonds, but for simazine this is an unexpected result, because the simazine molecule should easily form hydrogen bonds with the tested adsorbent. In order to explain mechanism of these effects further study are needed.

Article note

A collection of invited papers based on presentations at the 15th International Conference on Polymers and Organic Chemistry (POC-2014), Timisoara, Romania, 10–13 June 2014.

Corresponding author: Sylwia Ronka, Faculty of Chemistry, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland, e-mail:

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 Technology. Department of Polymer and Carbon Materials.

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Published Online: 2014-11-1
Published in Print: 2014-11-1

©2014 IUPAC & De Gruyter

Articles in the same Issue

  1. Frontmatter
  2. Preface
  3. 15th International Conference on Polymers and Organic Chemistry (POC-2014)
  4. Conference papers
  5. Polymer engineering for drug/gene delivery: from simple towards complex architectures and hybrid materials
  6. Synthesis of biobased phosphate flame retardants
  7. Reengineering classic organic reactions using polymeric tools
  8. Naturally derived and synthetic polymers as biomimetic enhancers of silicic acid solubility in (bio)silicification processes
  9. Interfacial polycondensation method used in the synthesis of polymers containing phosphorus in the main chain
  10. Synthesis of cyclopolyolefins via ruthenium catalyzed ring-expansion metathesis polymerization
  11. Microwave-assisted modifications of polysaccharides
  12. Advances in interpenetrating polymer network hydrogels and their applications
  13. Microwave-assisted alcoholysis of dialkyl phosphites by ethylene glycol and ethanolamine
  14. Phosphonium grafted styrene–divinylbenzene resins impregnated with iron(III) and crown ethers for arsenic removal
  15. Use of styrene–divinylbenzene grafted with aminoethylaminomethyl groups and various ionic liquids in the removal process of thallium and strontium
  16. Triazines removal by selective polymeric adsorbent
  17. Decontamination of colored wastewater using synthetic sorbents
  18. Optimization of enzymatic ring-opening copolymerizations involving δ-gluconolactone as monomer by experimental design
  19. Effect of polymer support functionalization on enzyme immobilization and catalytic activity
  20. Advantages of polymer-supported multivalent organocatalysts for the Baylis-Hillman reaction over their soluble analogues
  21. Inherently chiral calix[4]arenes with planar chirality: two new entries to the family
  22. A facile method for generating Michael acceptor thin films via amine substituted poly(vinyl methyl ketone)
  23. Reactions of 2-cyano-3-ferrocenylacrylonitrile with malononitrile: formation of 4-ferrocenylpyridine-3,5-dicarbonitrile derivatives and sodium polymeric complexes containing carbanionic ligands
  24. Polyaniline composite designed for solid polymer electrolyte
  25. Gaseous mercury behaviour in the presence of functionalized styrene–divinylbenzene copolymers
  26. Ionic polymers based on quaternized polysulfones: hydrodynamic properties of polymer mixtures in solution
https://doi.org/10.1515/pac-2014-0722
Keywords for this article
adsorption; herbicides; POC-2014; polymers

Articles in the same Issue

  1. Frontmatter
  2. Preface
  3. 15th International Conference on Polymers and Organic Chemistry (POC-2014)
  4. Conference papers
  5. Polymer engineering for drug/gene delivery: from simple towards complex architectures and hybrid materials
  6. Synthesis of biobased phosphate flame retardants
  7. Reengineering classic organic reactions using polymeric tools
  8. Naturally derived and synthetic polymers as biomimetic enhancers of silicic acid solubility in (bio)silicification processes
  9. Interfacial polycondensation method used in the synthesis of polymers containing phosphorus in the main chain
  10. Synthesis of cyclopolyolefins via ruthenium catalyzed ring-expansion metathesis polymerization
  11. Microwave-assisted modifications of polysaccharides
  12. Advances in interpenetrating polymer network hydrogels and their applications
  13. Microwave-assisted alcoholysis of dialkyl phosphites by ethylene glycol and ethanolamine
  14. Phosphonium grafted styrene–divinylbenzene resins impregnated with iron(III) and crown ethers for arsenic removal
  15. Use of styrene–divinylbenzene grafted with aminoethylaminomethyl groups and various ionic liquids in the removal process of thallium and strontium
  16. Triazines removal by selective polymeric adsorbent
  17. Decontamination of colored wastewater using synthetic sorbents
  18. Optimization of enzymatic ring-opening copolymerizations involving δ-gluconolactone as monomer by experimental design
  19. Effect of polymer support functionalization on enzyme immobilization and catalytic activity
  20. Advantages of polymer-supported multivalent organocatalysts for the Baylis-Hillman reaction over their soluble analogues
  21. Inherently chiral calix[4]arenes with planar chirality: two new entries to the family
  22. A facile method for generating Michael acceptor thin films via amine substituted poly(vinyl methyl ketone)
  23. Reactions of 2-cyano-3-ferrocenylacrylonitrile with malononitrile: formation of 4-ferrocenylpyridine-3,5-dicarbonitrile derivatives and sodium polymeric complexes containing carbanionic ligands
  24. Polyaniline composite designed for solid polymer electrolyte
  25. Gaseous mercury behaviour in the presence of functionalized styrene–divinylbenzene copolymers
  26. Ionic polymers based on quaternized polysulfones: hydrodynamic properties of polymer mixtures in solution
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