The porous material containing carboxyl groups was investigated in fixed bed system for the triazine-based herbicides removal from aqueous solution. In order to obtain adsorbent capable of generating specific interactions with triazines, the poly(divinylbenzene) was synthesized in radical suspension polymerization and then was modified with maleic anhydride in Diels-Alder reaction with subsequent base hydrolysis. The introduction of carboxyl groups into polymer structure resulted in obtaining specific interactions, such as hydrogen bonds between modified poly(divinylbenzene) and triazines, therefore the selectivity and the high adsorption capacity towards terbuthylazine, propazine, atrazine and simazine was observed. The total and usable adsorptive capacities, the breakthrough and exhaustion times, the coefficients of sphericity of isoplanes, the heights of adsorption fronts and the mass exchange moving rates were calculated based on the analysis of the breakthrough curves. Results show that the best sorption parameters in dynamic conditions were achieved for terbuthylazine and propazine. For them the highest values of adsorptive capacities, the smallest heights of mass transfer fronts and their slow movement along the bed height were obtained. The use of ethanol for herbicides elution provided a high recovery degree of adsorbed substances. Reusability of investigated polymer bed was studied in three adsorption/desorption cycles.
Triazines are one group of herbicides which are used for decades to protect crops. We know that triazine-based herbicides display high toxicity and aptitude for the bioaccumulation , , ,  but at the same time are highly stable in soil . They are still present in the bottom sediments of ponds, lakes and other bodies of water. The highest concentrations of herbicides can be found in surface waters and the content of herbicides in the environment has a growing tendency. A significant increase of herbicides concentration is frequently seen during their applications in the fields, after heavy rains and floods. Because of their wide applicability, stability in the environment and long-term toxicity herbicides must be removed and this removal is an important issue in ecology. The importance of the above is reflected in a large number of research groups working in the area of triazine-based herbicides removal and numerous papers in scientific journals were published in recent years , , , , , , , , , , , , . We can find a large variety of materials applied for triazines sorption (activated carbons, carbon nanotubes, zeolites, aluminum pillared clay, green waste biochar), however, the most efficient method of water purification is adsorption using polymeric materials . Adsorption can be carried out in batch systems with granulated adsorbents or in the continuous flow processes in packed bed columns. In fixed bed column studies a solution of adsorbate(s) is passed through a bed of adsorbent granules where solution composition is changed due to sorption. The composition of the effluent and its change with time depend on the properties of the adsorbent, the composition of the feed, and the operating conditions (flow rate, temperature etc.). Sorption of herbicides under dynamic conditions is most commonly performed in mini-columns filled with adsorbent. Several techniques such as e.g. solid-phase extraction (SPE) or solid phase microextraction (SPME) are used. Due to the fact that triazine-based impurities are present in the environment at low concentrations, it is necessary to use adsorbents that exhibit high selectivity for the target compounds. As it turns out the numerous possibilities of modification and implementation of various functional groups into polymer structure allow the synthesis of selective adsorbents. For example, in SPE techniques molecularly imprinted polymers (MIPs) based on methacrylic acid are most commonly used, especially in detection of triazine-based herbicides , , , , . Polymeric adsorbents are also used in the method based on solid-phase dynamic extraction (SPDE), where methacrylic acid-based MIP is used as adsorbent coated on the internal surface of a needle . Single-hole hollow molecularly imprinted microspheres (h-MIMs) are another polymeric material which can be used in triazines detection. These microspheres exhibit bigger specific surface area, higher binding capacity and faster adsorption rate than the MIPs prepared by precipitation polymerization and surface polymerization . Not only MIPs were applied in detection and extraction of triazine-based herbicides using dynamic processes. Porous poly(octyl methacrylate-co-ethylene glycol dimethacrylate) is an example of polymeric filling in in-tube solid-phase microextraction technique (IT-SPME), where it is used as monolith in capillary column doped with magnetic nanoparticles .
Due to the high toxic effect of triazines and limitation of their permissible concentrations in waters (e.g. atrazine regulation limit in drinking water is 3 ppb and 0.1 ppb in the US and EU, respectively ), there is a need to search for sorptive materials having a large sorption capacity that will ensure effective removal of hazardous materials from the environment. Our previous study described in ,  revealed that poly(divinylbenzene) beads modified in Diels-Alder reaction with maleic anhydride and subsequent base hydrolysis can be effectively used as an efficient adsorbent material for triazines removal from water. 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 trazines sorption using synthesized sorbent is caused by forming hydrogen bond between amine groups in the structure of triazine-based herbicides and carboxyl groups on the polymer surface. The affinity of proposed adsorbent towards herbicides from the group of triazines is presented in , where the effect of the structure of triazine homologues on the efficiency of sorption was examined. In this work, we were interested in the study of triazines adsorption under dynamic conditions on modified poly(divinylbenzene) from an aqueous solution. Comparing with batch procedure, fixed bed is more effective for the cycle operation of adsorption/desorption, as it makes the best use of the concentration difference known to be a driving force for adsorption and allows more efficient usage of the sorbent capacity, what should result in a better efficiency in triazines removal. Additionally, fixed bed adsorption is simple to operate, and it can be relatively easily scaled up from a laboratory-scale study.
Materials and methods
The following chemicals were obtained from commercial sources: divinylbenzene (80%) (DVB), benzoyl peroxide (BPO), sodium chloride, polyvinyl alcohol 88% hydrolyzed (PVA), ethanol, toluene, n-heptane purchased from Sigma-Aldrich. Monomer was purified by distillation before use. 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 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 monomer) 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 . 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 the 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 .
Water regain, W (g g−1) of the adsorbent was determined using the centrifugation method and was calculated using Eq. (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 pre-soaked in water for 24 h. Later, they were centrifuged in a small column with fritted-glass bottom 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 performing 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.
Dynamic conditions were realized by the column filtration method. Fixed-bed sorption was done on 1.65 cm3 polymer bed (equivalent to 0.4 g of dry polymer) placed in a microcolumn (0.9 cm in diameter) by passing downwards a solution which was 10 ppm ethanol:water (1:9 v/v) herbicide solution. The feeding solution was pumped down-flow through the column by a peristaltic pump with flow rate 0.3 mL min−1. The effluent was collected at regular time intervals from the bottom of the column and was then analyzed by UV/VIS spectrophotometry using 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. Flow of solution lasted each time until the moment of bed exhaustion, i.e. at the point where the concentration in the discharge became equal to that of the initial solution.
In desorption process ethanol was used as the eluent. It was passed through the adsorbent bed with constant flow equal to (0.3 mL min−1) and the sampling time was 15 min. Desorption was carried out until maximum elution of the herbicides from the bed. Reusability of adsorbent was determined by its adsorption performance in three consecutive sorption/desorption cycles. All of the experiments were performed at room temperature (23±1°C).
Analysis of experimental data
The herbicide adsorption breakthrough curves were obtained by plotting the triazines concentration in the effluent (C) versus the effluent volume (V). The calculations were made based on the analysis included in the publication  and were carried out using the diagram presented in Fig. 1. Total adsorptive capacity Pt (mg g−1) of specific material was calculated from the following formula, Eq. (2):
were M is the mass of bed prior to sorption process (g). The total amount of triazine retained in the column, Ot, was calculated from the Eq. (3):
where PDFBA is the area representing the quantity of compounds introduced to sorption system (the point of bed exhaustion, C=Co=10 mg L-1), PDFB is the area representing the quantity of compounds not retained on the bed of adsorbent (the point of bed exhaustion, C=Co=10 mg L−1).
The usable adsorptive capacity Pu (mg g-1) was calculated as follows, Eq. (4):
The amount of triazine retained in the column to the breakthrough point Ou, was calculated from the Eq. (5):
where PDGHA is the area representing the quantity of compounds introduced to the sorption system (the breakthrough point, C=Cp=0 mg L-1), PDGI – the area representing the quantity of compounds not retained on the bed of adsorbent (the breakthrough point, C=Cp=0 g L−1).
The mass transfer zone was calculated using the equation of Michaels and Treybal, Eq. (6):
where: Ho – the adsorption front height, cm, H – the adsorbent bed height, cm, te – bed operation time until exhausted, min, tb – bed operation time until breakthrough, min, φ – coefficient of sphericity of exit curves was calculated by dividing the surface area of the rectangle dx by the surface area of the rectangle HBIJ.
Mass exchange zone moving rate, u (cm min−1), was calculated from the following formula, Eq. (7):
where: Ho – the adsorption front height, cm, te – bed operation time until exhausted, min, tb – bed operation time until breakthrough, min.
In order to obtain polymer beads the poly(divinylbenzene) was synthesized in radical suspension polymerization. Product in the form of regular microspheres with diameters in the range 50–400 μm was obtained. Chemical modification of polymer was done by Diels-Alder reaction in which poly(divinylbenzene) reacts as diene and maleic anhydride as dienophile. The efficiency of the modification reaction was 49%. Generation of the carboxyl groups (see Fig. 2) capable of 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 . Selected material having porous structure and acidic groups on the surface was used in sorption experiments of herbicides based on triazine. Characteristics of this adsorbent are presented in Table 1. The study showed that the tested adsorbent has a well-developed surface area, which has a direct influence on the adsorption capacity of the material. The content of acidic groups (2.80 mmol g−1) 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 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 .
|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|
Poly(divinylbenzene) modified with maleic anhydride 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. Sorption studies were performed in a dynamic mode for triazine-based herbicides such simazine, atrazine, propazine, terbuthylazine and, for comparison, for metamitron. The adsorbate solutions continuously pass through a column packed with adsorbent with flow rate 0.3 mL min−1. The pH values of the feed solutions were seven and the concentrations of herbicides were 10 ppm. The performance of fixed bed is usually described using the breakthrough curve. Plots of the ratio C/Co (outlet adsorbate concentration/adsorbate feed concentration) or adsorbate concentration in the effluent (C) versus effluent volume (V) or time (t) are denominated as breakthrough curves. On the basis of obtained results the breakthrough curves (isoplanes) were plotted and presented in Fig. 3. The shape of the breakthrough curve and the time for a breakthrough are very important characteristics for determining the operation and the dynamic response of the fixed bed column. The analysis of the breakthrough curves clearly indicates that the tested specific adsorbent prefers sorption of triazine-based herbicides. For metamitron the polymeric bed is rapidly saturated. It was found that the breakthrough time increases with increasing the molecular weight of triazine and is much higher for more hydrophobic triazines (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 ). In the case of terbuthylazine an extended breakthrough curve can be observed, indicating that a higher volume of solution could be treated. Calculating the parameters of sorption on the basis of the breakthrough curve is an important issue because it provides the basic information for the design of the column adsorption system and can be used to determine the rational scale of the column adsorption for practical application. The calculations were carried out in accordance with the diagram presented in Fig. 1. The parameters that characterize sorption of investigated herbicides in dynamic conditions were calculated using Eqs. (2–7). The obtained breakthrough curves can be described using mathematical equations presented in Table 2. The correlation coefficient R2 for breakthrough curves are around 0.98–0.99. The analysis of isoplanes allows for determination of the total adsorption capacity (Pt) – up to the point of exhaustion and useful adsorption capacity (Pu) – up to the point of breakthrough and also the bed operation time until breakthrough (tb) and exhaustion (te). Results are presented in Table 3. The adsorptive capacities obtained under dynamic conditions were comparable with those determined in batch sorption system for the same triazines concentration used, as shown in Table 3. The total adsorption capacity (Pt) obtained in the dynamic conditions were 15.3 mg g−1, 26.1 mg g−1, 43.5 mg g−1, 66.2 mg g−1 and 3.2 mg g−1 for simazine, atrazine, propazine, terbuthylazine and metamitron, respectively. The sorption capacity of metamitron is 5–20 times smaller than the sorption capacity of triazine-based herbicides, which confirms the presence of a specific interaction between the sorbent and investigated triazines that enhance sorption. Selectivity of the tested adsorbent towards herbicides from the group of triazines has been confirmed in the previous publication . The introduction of carboxyl groups into polymer structure resulted in creating hydrogen bonds with triazines. 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). The highest values of adsorptive capacity are observed for terbuthylazine and propazine. In the case of propazine, which has the same molecular weight as terbuthylazine, but is more hydrophobic, the sorption efficiency is smaller. This results can suggest that for propazine only non-specific interactions are possible, while sorption efficiency for terbuthylazine is intensified by additional specific interactions like hydrogen bonds. In the propazine molecule there are two bulky isopropyl groups, which probably can cause the steric hindrance in the creation of hydrogen bonds.
|Herbicide||Mathematical equation of the breakthrough curve||Correlation coefficient, R2|
|Herbicide||Adsorption capacity (mg g−1)||Bed operation time until breakthrough tb (h)||Bed operation time until exhausted te (h)|
|Ps||Pt (total)||Pu (useful)|
aThese data were published in .
Analyzing the breakthrough curves one can notice that in spite of dosing a solution onto the column with the same initial concentration of the herbicide the breakthrough and exhaustion times were varied for the tested compounds. The longer time necessary to reach the breakthrough and exhaustion points (in this study observed for terbuthylazine and propazine) indicates much better filling of the polymer bed by triazines. Breakthrough time for both terbuthylazine and propazine is about 70 h, but exhaustion time is much higher for terbuthylazine (see Table 3). It means that the synthesized adsorbent has a much greater affinity for terbuthylazine. On the basis of the derived parameters the coefficients of sphericity of isoplanes (φ), the heights of adsorption fronts (Ho) and the mass exchange moving rates (u) were calculated (see Eq. 6–7). Results are presented in Table 4. In fixed bed experiments, the concentration of adsorbate in the mobile phase as well as in the solid phase changes with time and with the position in polymer bed. The mass transfer zone in a column moves from the top of the bed and proceed towards the bottom. In good sorption systems are small heights (Ho) of mass transfer fronts and low mass exchange moving rate (u). Small values of this parameters mean that the breakthrough curve is close to an ideal step with negligible mass-transfer resistance . Analysis of the results shows that the best sorption parameters in dynamic conditions have been obtained for terbuthylazine and propazine. A much higher height of adsorption front was obtained for metamitron (7.01 cm). Results also show that only in the case of propazine and terbuthylazine the heights of mass transfer fronts are smaller than adsorbent bed height, which was 2.7 cm in performed experiments. The highest mass exchange moving rate for metamitron results probably from the weak affinity of this herbicide to the investigated polymer. For the same reason the breakthrough and exhaustion times have the smallest values and consequently the saturation of the adsorbent occurs earlier.
|Herbicide||Coefficient of sphericity of isoplane φ (–)||Height of adsorption front Ho (cm)||Mass exchange moving rate u (cm min–1)|
Due to the good solubility of tested herbicides in ethanol it was decided to apply it in the desorption process as eluent. Based on the mass balance of herbicide adsorbed on the polymer bed and then desorbed the desorption process was investigated. The desorption efficiency, the desorption time and the volume of the eluent required to elute herbicides from the surface of the adsorbent were determined. The results are shown in Table 5. The use of ethanol as eluent assured the high recovery degree of adsorbed substances. Simazine can be completely desorbed. For other herbicides the desorption efficiency is in the range 88–97%. The desorption efficiency is related to the solubility of the herbicides in ethanol. For propazine and terbuthylazine the solubility in ethanol is 15 g L−1. It was observed that the better solubility of the compound in the eluent, the time required to eluting substances from the adsorbent bed was shorter. Due to the low solubility of simazine in ethanol (0.57 g L−1) its desorption time was the longest (1230 min). Despite only slightly higher solubility of metamitron in ethanol (1.1 g L−1), elution was significantly faster (135 min), which may be caused by low binding affinity of metamitron to investigated sorbent. Desorption time for other triazines was comparable and was about 350 min. For maximum desorption of triazines showing the greatest affinity for the tested adsorbent the amount of needed eluent was about 100 mL. Furthermore, during the study it was observed that for all herbicides after passage a small volume of ethanol (ca. 4.5 mL) through the column already about 90% of investigated herbicides were desorbed (see Fig. 4), but the process was continued until complete elution of the herbicides from the bed. Thus, the effective elution of herbicides from polymer bed demonstrates that in industrial conditions already small volume of eluent provides sufficient regeneration of the bed.
|Herbicide||Desorption efficiency (%)||Amount of eluent (mL)||Desorption time (min)|
Efficient removal of loaded herbicides from the adsorbent was necessary to ensure their long-term use for repeated sorption-desorption cycles. The tested adsorbent showed the greatest affinity for terbuthylazine, therefore, it was decided to investigate the ability to remove this triazine in three subsequent cycles. Ethanol was used as eluent. Desorption efficiency of investigated herbicide from modified poly(divinylbenzene) was evaluated based on the amount of terbuthylazine released to ethanol. In each cycle, after passing 100 mL of eluent, the polymer bed was washed using distilled water. Based on the results obtained after three sorption/desorption cycles it can be concluded that the removal of terbuthylazine is still effective. It was observed that the sorption capacities of regenerated modified poly(divinylbenzene) adsorbent in the first and second cycles are comparable (67.1 and 65.6 mg g−1). In the third cycle the sorption capacity of the bed is reduced by 20% to 54.2 mg g−1, but it is still a good result for effective removal of investigated triazine.
This investigation showed that the poly(divinylbenzene) modified in Diels-Alder reaction is a promising material for removal of triazine-based herbicides from aqueous solution using fixed bed sorption method. Fixed bed column studies clearly indicate that the tested specific adsorbent prefers sorption of triazine-based herbicides. The adsorptive capacities obtained under dynamic conditions were comparable with those determined in batch sorption system for the same concentration of triazines. The affinity order of investigated polymeric adsorbent towards triazines in fixed bed sorption is terbuthylazine>propazine>atrazine>simazine. Bed operation time until breakthrough and until exhausted increases with increasing the molecular weight of triazines and is much higher for more hydrophobic herbicides. Analysis of the results shows that the best sorption parameters in dynamic conditions were achieved for terbuthylazine and propazine, for which the smallest heights of mass transfer fronts and their slow movement along the bed height were obtained. Furthermore, the adsorbed triazines can be quantitatively desorbed by ethanol, so the synthetized adsorbent can be used repeatedly without significant decrease of sorption capacity. Satisfactory results allow to conclude that the proposed adsorbent can be used not only in the detection of triazines, but also can be used as selective filling of fixed bed columns used in the processes of water and wastewater treatment.
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.
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.
 M. A. Marin-Morales, B. C.Ventura-Camargo, M. M. Hoshina. Toxicity of Herbicides: Impact on Aquatic and Soil Biota and Human Health, Herbicides - Current Research and Case Studies in Use, (A. Price and J. A. Kelton, ed.), pp. 399–443. InTech, Croatia (2013).Search in Google Scholar
 U.S. Department of Health and Human Services Public Health, Toxicological profile for atrazine, pp.119–129. Service Agency for Toxic Substances and Disease Registry (2003).Search in Google Scholar
 EPA National Primary Drinking Water Regulations. Technical Factsheet on: Atrazine (2003).Search in Google Scholar
 B. R. Stranix, G. D. Darling, Cycloaddition functional polymers from (vinyl)polystyrene, US Patent: US 6534611 B1. 2003.Search in Google Scholar
 PPDB: Pesticide Properties DataBase [WWW Document]. University of Hertfordshire (2007).Search in Google Scholar
©2016 IUPAC & De Gruyter. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. For more information, please visit: http://creativecommons.org/licenses/by-nc-nd/4.0/