The highly cross-linked polymers have many useful features, such as their well-developed porous structure, high thermal resistance, and low ability to swelling. This makes them very useful materials as adsorbents for purification and separation techniques [1
]. In order to make their porous structure more suitable to adsorb a specific group of compounds, methods like surface sensitization or imprinting are used [6
]. Additionally, functionalization reactions can be carried out to modify the chemical nature of their surface [7
]. Other methods allow for the design of surface properties of the polymer (e.g., polarity, acidity) while already at the stage of synthesis [9
]. These methods rely on the usage of monomers with functional groups, which give the polymer a specific character and are able to interact with an analyte. Their presence also helps to improve the wettability of the polymer, which is advantageous when uptake is carried out from the aqueous solutions.
The chemistry of polymeric sorbent is particularly important in the case of analysis of polar analytes like phenols or pharmaceuticals that exhibit biological activity. Although their concentration in natural waters or sewage is on a low level, they pose a serious threat to the environment. All living organisms are exposed to the harmful effects of these substances and products of their decomposition. A lot of such compounds are not biodegradable, and remain in the environment for a very long time. This type of pollution may lead to contamination of drinking water. For this reason, governments around the world introduced directives on maximum limit values for the most toxic compounds and the requirement for constant monitoring of their presence [11
Moreover, if pollutants are very diluted, it is an additional difficulty from an analytical point of view, as a stage of quantitative isolation and concentration should be included in the determination procedure [1
]. For many years analytical methods for extraction of phenolic compounds based on liquid-liquid extraction (LLE) and Soxhlet extraction, for liquid and solid samples, respectively [16
]. These methods are expensive and time consuming therefore SPE (solid phase extraction) as more convenient, cheap, fast and environmentally friendly method was proposed [1
]. Due to differentiated chemical character of pollutants there is no one universal adsorbent suitable “for all purposes”. Depending on the properties and molecular size of an analyte, the right adsorbent should be selected. The most important features of the adsorbent are: Porosity, surface chemistry and stability, both chemical and thermal. Porosity affects sorption capacity. Chemical structure determines its usefulness for a particular type of analytes. Chemical and thermal stability influences the regime of working conditions, especially in the stage of recyclability.
Many studies were performed to evaluate the usefulness of commercially available sorbents like Isolute ENV, PLRP-S, PGC, LiChrolut EN, Bond Elut Env, OASIS HLB [17
] also newly prepared materials are examined [20
]. Researchers used stationary and dynamic methods, different experimental conditions and sample matrices. In addition, the way of presentation of the data is not unified. For this reason, it is difficult to compare results obtained by various groups.
In order to model the sorption process usually one-component solutions are used, but from the practical applications point of view, studies performed for multicomponent mixtures are more useful.
In this work we present the series of highly cross-linked porous polydivinylbenzenes (pDVB). The aim of our study was the evaluation if the addition of a potential adsorbate to pore forming agents in the stage of polymerization will have an influence on a porous structure and sorption properties of the synthesized polymer towards the added adsorbate. Moreover, the influence of the sulphonation reaction on the porous structure of the chemically modified polymer, as well as its sorption properties towards phenols and pharmaceuticals was evaluated. All the polymers were synthesized via a suspension method using toluene as a basic pore-forming diluent. To prepare polymers with modified porous structure, 1 mL of toluene was replaced with an appropriate amount of phenol (P), 2,4,6-trichlorophenol (T) or their mixture (M). Moreover, the chemical modification of pDVB was performed in order to introduce sulfonic groups onto the polymer surface. The prepared materials were tested in the sorption processes of phenolic compounds (a multicomponent mixture consisted of phenol, 2-chlorophenol, 2,4-dichlorophenol and 2,4,6-trichlorophenol), salicylic acid and ibuprofen using dynamic method—solid phase extraction (SPE). The elemental and ATR-FTIR analyses confirmed the chemical structure of the tested materials and the correct course of the modification process. The polymers possessed well developed porous structure with specific surface areas of 440 to 560m2/g and bimodal pore size distribution. It was shown that the type of the porogen, as well as the sulphonation reaction influenced the porosity of pDVB polymers. SPE experiments revealed that changes in the porous structure of the polymers had an impact on their sorption properties. In the case of 2,4-dichlorophenol and 2,4,6-trichlorophenol, pDVB-T and pDVB-M had higher sorption capacity than pDVB, pointing out the positive role of 2,4,6-trichlorophenol in the formation of pores suitable for the sorption of these compounds. The similar effect was observed for ibuprofen uptake. The opposite effect was obtained for salicylic acid.
2. Materials and Methods
2.1. Chemicals and Reagents
Divinylbenzene (DVB), α,α’-Azoiso-bis-butyronitrile (AIBN), poly(vinyl alcohol) (PVA), methanol (MeOH), 2-chlorophenol (ChP), 2,4-dichlorophenol (DChP) and 2,4,6-trichlorophenol (TChP), 1,4-dioxane and (S)-(+)-2-(4-Isobutylphenyl)propionic acid (ibuprofen), 2-hydroxybenzoic acid (salicylic acid) were purchased from Merck (Darmstadt, Germany). 98% H2SO4, 36% HCl, NaOH, hexane, acetone, toluene and phenol (P) were from POCh (Gliwice, Poland).
2.2. Synthesis of Polymeric Microspheres
In a three-necked flask equipped with a stirrer, a thermometer and a water condenser 5 g of PVA and 175 of distilled water was dissolved. Next, the mixture of the monomer (DVB), pore forming agent (toluene, P, T) and the initiator (AIBN) was prepared and poured into the dispersion medium, all details are collected in Table 1
. The reaction mixture was stirred at 250 rpm at 80 °C for 18 h. After the reaction the obtained polymers were washed in an ultrasonic bath successively with distilled water (150 mL), methanol (150 mL), hexane (150 mL) and again methanol (150 mL) (each solvent for half an hour). After the washing the solvent was filtered off and the polymers were dried in the open air for 10 days.
The sulphonation process was carried out in a two-neck flask equipped with a thermometer and a water condenser. A mixture of 13 g pDVB and 120 mL H2SO4 (98%) was gradually heated from 35 to 100 °C for 4 h. After the reaction was completed the mixture was poured into distilled water, the polymer was filtered off, washed with water to remove the acid (neutral pH) and dried in the open air for 10 days.
Elemental analyses (CHN) were performed on a Perkin Elmer CHN 2400 analyzer (Palo Alto, CA, USA).
The acid number was determined by standard titration method.
Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra were collected using a Bruker FTIR spectrophotometer TENSOR 27 in the frequency range of 4000 to 600 cm−1, the resolution of the apparatus was 4 cm−1.
Differential scanning calorimetry (DSC) analyses were carried out on a Netzsch DSC 204 calorimeter (Netzsch, Germany). Samples of about 7 mg weight were placed in an aluminum pan with a pierced lid and heated to 500 °C at a heating rate of 10 °C·min−1. The measurements were conducted under argon atmosphere. An empty pan was used as a reference. The samples were analyzed as received. Additionally, pDVB-SO3H was dried at 100 °C for 30 min. to remove adsorbed moisture.
The porous structure parameters of the studied sorbents were determined by nitrogen sorption experiments, the isotherms were measured at −196 °C with adsorption analyzer ASAP 2405 (Micrometrics Inc., USA). Before analyses, the samples were outgassed at 140 °C for 1 h. The specific surface area was calculated using the standard BET method while the total pore volume was calculated as the volume of liquid adsorbate at a relative pressure of 0.99.
To investigate the sorption properties of the prepared materials, laboratory cartridges were filled with 100 mg of the studied polymer. Next, the aqueous solution of ibuprofen (100 mg·L−1
), salicylic acid (100 mg·L−1
) and a standard methanolic mixture of phenols (phenol (P), 2-chlorophenol (ChP), 2,4-dichlorophenol (DChP), and 2,4,6-trichlorophenol (TChP)—100 mg·L−1
of each compound) were prepared. The procedures of the sorption experiments and HPLC analyses for phenolic compounds were the same as in our previous works [7
]. The aqueous solution of the phenolic compounds (2 mg·L−1
) was prepared by dilution (1:50 v
) of the standard methanolic solution. Different volumes of the aqueous solution were sucked through the cartridge connected to the water aspirator. Next, the phenolic compounds were eluted from cartridge with adequate volume of methanol (2 mL of MeOH for each 100 mL of sucked aqueous solution). The obtained eluates were analyzed using HPLC system - Waters 2690 Alliance (Waters, USA) equipped with an automatic sampler, UV detector Waters 2487 and μ-BondapakTMC18 (3.9 × 300 mm) column. As a mobile phase methanol: Water (60:40, v
) at a flow rate of 1 mL/min−1
was applied. Detection was performed at λ = 210 nm.
The sorption of pharmaceuticals (ibuprofen and salicylic acid) was performed in the same way as phenols. The only difference regarded the parameters of HPLC analysis: A mobile phase methanol: Water (80:20, v/v), a flow rate of 1 mL·min−1, detection of ibuprofen was performed at 222 nm, and salicylic acid at 200.5 nm, respectively.
This work presents the way of synthesis and characterization of the porous polymers obtained by suspension polymerization of divinylbenzene. In order to modify porosity of this materials, in the procedure of synthesis, 1 mL of porogenic solvent was replaced with an adequate quantity of phenol, 2,4,6-trichlorophenol or their mixture (1:1 molar ratio). The chemical modification of pDVB was performed using the sulphonation reaction. The successful course of polymerization and modification reactions was confirmed by the elemental and FTIR analyses.
The DSC analysis proved that all the studied highly cross-linked polymers were thermally stable up to 350 °C (pDVB) and after sulphonation even to 450 °C. Moreover, the DSC data indicated that pDVB-SO3H polymer has hygroscopic properties.
The polymers possessed specific surface areas in the range of 440 to 560 m2/g and they were mainly mesoporous with bimodal pore size distribution in the range up to 12 nm. Mean pore widths were ca. 3.75 and 4.75–7.15 nm. It is worth noticing that, although the materials with porosity regulated by the addition of phenol, trichlorophenol or their mixture had specific surface areas 2–5% lower and pore volumes 5–18% lower than for pDVB obtained in the presence of toluene, their porous structures were more uniform. It was also proved that the process of chemical modification with sulphuric acid took place in wider mesopores resulting in a decrease of porous structure parameters.
The studies on phenols sorption with the use of the SPE method showed that the best sorption properties possessed pDVB (unmodified), pDVB-T and pDVB-M. In the case of the two latter polymers positive influence of the compounds modifying porosity was observed by a ca 5% increase in the recoveries in comparison to pDVB. The similar relationship was observed for SPE of ibuprofen. The highest recovery 37.7% was obtained in the case of pDVB-T. Salicylic acid had very low affinity to the surface of the tested polymers. The highest recoveries in SPE experiments were received for pDVB-T and pDVB-SO3H: 7.6 and 6.6%, respectively.
Studies of the sorption properties of the tested polymers towards phenols and ibuprofen revealed that the process of uptake is affected by both porosity and chemical character of the adsorbent, while sorption of strongly acidic molecules like salicylic acid only depends on the sorbent porosity.
The performed studies showed that it is possible to improve the sorption properties of the prepared polymeric materials by changing the chemical composition of the polymerization mixture. Even if the increase is not so significant, the method of the synthesis is really simple and worth considering. The proposed procedure can be optimized to get better results.
After the sulphonation, the sorption properties of pDVB-SO3H were worse, in comparison, to unmodified pDVB, in the case of adsorbates possessing acidic character. However, due to the presence of a sulphonic group in the structure of adsorbent it can be presumed that sorption of compounds with basic functionalities or those in ionic forms (e.g. paracetamol, tramadol or sodium ibuprofen) could be more effective.