Water pollution causes many problems for individual organisms, populations, and communities. The relationship between water quality and human activity is very complicated. Water used in domestic, industrial, and agricultural sectors is usually returned to rivers, lakes, estuaries, or oceans; however, industrial effluent is usually polluted by organic compounds, phenolic compounds and their derivatives being the most common. Nitrophenols are considered priority toxic pollutants by the U.S. Environmental Protection Agency and are therefore of particular interest [1
Various methods are available to remove nitrophenols from water, including adsorption [2
], microbial degradation [7
], chemical oxidation [8
], membrane separation [9
], catalytic oxidation [10
], and electrochemical treatment [11
]. Some of these methods, such as adsorption on activated carbon, have high costs, as well as the potential to cause secondary pollution, and low-cost, highly selective removal methods continue to be sought [1
Molecularly imprinted polymers (MIPs) are being utilized in an increasing number of applications, as “tailor-made” separation materials, antibody–receptor binding site mimics in recognition and assay systems, enzyme mimics in catalytic applications, recognition elements in sensors, and in facilitated chemical synthesis [13
]. To date, their most extensively investigated application has been as separation materials for the analysis of various compounds, including drugs [16
], pesticides [18
], and amino acids [20
]. A highly specific detection technology, MIPs have been used for the separation of isomers and enantiomers [21
], solid extraction [22
], in biochemical sensors [23
] and chemosensors [24
], in simulating enzyme-catalysed pharmaceutical analysis [25
], in sorbents, and in membrane separation technologies [26
]. They have been prepared in various configurations—including polymer beads, monoliths, and membranes—and have numerous advantages, such as physical robustness, high strength, resistance to elevated temperatures and pressures, and inertness towards organic solvents, acids, and bases [27
]. Further, MIPs are stable, easy to prepare, and inexpensive.
Investigations of MIP membranes have been increasing in the last few years. Several articles have been published on the preparation of MIP membranes, showing specific permeability and separation for template/ligands such as cholesterol [28
], nucleotides, and various drugs [29
]. The transport properties and applications of MIP membranes in sensor technology have also been investigated.
The present study used a MIP membrane as a sorbent for the removal of 2,4-dinitrophenol (2,4-DNP) from aqueous solution. The aim of this study is to investigate the adsorption of 2,4-DNP by the MIP membrane, specifically using cellulose acetate (CA) and polysulfone (PSf). To the best of our knowledge, this work is the first to report on the development of an MIP membrane for the removal of 2,4-nitrophenol. The parameters studied include pH, sorption kinetics, and sorption isotherms. A selectivity study is also conducted. The prepared MIP membrane can also be used for the separation, enrichment, and trace analysis of targeted phenolic compounds in aqueous samples.
3. Experimental Section
Acrylamide and 2,4-DNP were supplied by Fisher Chemical, and Sigma-Aldrich, respectively. Benzoyl peroxide, acetic acid, and tetrahydrofuran were supplied by R & M Chemical Technologies. Acetonitrile and methanol were supplied by HmbG Chemical. Ethylene glycol dimethacrylate (EDGMA) was supplied by Fluka Chemical. The CA and polystyrene (PS) were supplied by Acros Organic. Distilled water was used throughout the experiment.
The MIP membrane was characterized by FTIR with a Perkin Elmer 1600 Spectrophotometer and SEM JEOL JSM 6700 and JEOL JSM 6400. The sorption experiments were carried out using Varian Cary WinUV spectrophotometer.
3.3. MIP Synthesis
The MIP was prepared using a non-covalent approach. The template, 2,4-DNP (DNP, 1.0 mmol), was dissolved in a beaker of acetonitrile (30 mL). The functional monomer acrylamide (5.0 mmol), the cross-linker EDGMA (15 mmol), and the initiator benzoyl peroxide (1.6 mmol) were then added to the flask. After 10 min of degassing and nitrogen purging, the flask was sealed and the contents allowed to polymerize in a water bath at 70 °C for 24 h. The bulk polymers obtained were crushed, ground, and sieved into regularly sized particles between 80 μm and 100 μm. Finally, the MIP particles were extensively washed with distilled water to remove any unreacted monomer or diluents. The polymer was then washed with 1:2 (v/v) methanol–acetic acid until the template was completely removed.
3.4. Preparation of the MIP Membrane
The MIP particles were hybridized in porous membranes of CA and P by a phase inversion process. Tetrahydrofuran was used as a solvent. A total of 500 mg of each polymer, CA and PS, were dissolved in 30 mL of tetrahydrofuran. The MIP particles (1000 mg) were mixed into each polymer solution by stirring within 4 h at 50 °C. The resultant viscous solution was spread on a glass plate and dried overnight at room temperature.
3.5. Sorption of 2,4-DNP by the MIP Membrane
Sorption experiments for 2,4-DNP by the MIP membrane underwent characterization by pH, sorption isotherms, and sorption kinetics and a selectivity study. A 2 × 2 cm2 piece of MIP membrane was cut and applied to the sorption of 10 mg/L of 2,4-DNP and stirred for 24 h. The concentration of 2,4-DNP in the aqueous solution after the desired treatment periods was analysed using an ultraviolet–visible spectrometer.
3.5.1. Effect of pH
The MIP membrane was stirred for 24 h with 20 mL of 10 mg/L 2,4-DNP at pH values of 1–12. The pH was adjusted with hydrochloric acid or sodium hydroxide. The MIP was then filtered and the final pH was measured.
3.5.2. Sorption Kinetics
For sorption kinetics, 20 mL of 10 mg/L 2,4-DNP solution, at the optimum pH obtained from the previous experiment were stirred with the MIP membrane for various time periods (5 min, 10 min, 20 min, 30 min, 60 min, 120 min, 240 min, 480 min, 960 min, and 1440 min). The MIP membrane was then filtered and the filtrate analysed for 2,4-DNP concentrations with the ultraviolet–visible spectrometer.
3.5.3. Selectivity Study
The selectivity of the fabricated MIP towards phenol, 3-CP, and 2,4-DCP with respect to 2,4-nitrophenol was studied. A solution (25 mL) containing 10 mg/L (of each compound) was mixed together and treated with MIP membrane at room temperature. The concentrations of the phenolic compounds, after treatment, were measured by the ultraviolet–visible spectrometer. The binding capacity was then calculated.
A MIP membrane was successfully prepared by the hybridization of MIP particles with CA and PS. The MIP was characterized by FTIR and SEM. The maximum sorption of 2,4-DNP by the fabricated CA-MIP and PS-MIP was found at pH 7.0 and pH 5.0, respectively. Kinetics found that the rate of sorption of 2,4-DNP increased rapidly in the initial stage, and then slowed until it reached equilibrium. A pseudo–second-order kinetic model is more suitable to describe the sorption process onto CA-MIP and PS-MIP, based on the correlation coefficient values. The selectivity experiments showed that the MIP is selective towards 2,4-DNP in the presence of 2,4-DCP, 3-CP, and phenol interference. It was confirmed that the shape and size of the template, as well as the strength of interaction between the target molecule and binding sites, determine MIP selectivity.