The IR spectra of the 2,4-dinitrophenol imprinted polymer, non-imprinted polymer (NIP) and monomer materials were recorded using the KBr pellet method. The spectral differences between MIP and monomer acrylamide are shown in Figure 1. Both spectra indicated the presence of CO carbonyl groups. However, the spectrum of acrylamide in the 3,356–3,188 cm⁻¹ range showed the characteristic NH₂ stretch of the amide group. Fortunately, the interaction between template and monomer gave changeable peaks in the MIP spectrum, which showed a board OH stretching peak at 3,452 cm⁻¹ and CH stretching at 2,960 cm⁻¹, while the CH stretching in the spectrum of acrylamide was at 2,802 cm⁻¹. For the NIP, the N-H stretching vibration occurs in the 3,460 cm⁻¹ range. The C-N stretching absorption occurs in the 1390 cm⁻¹ region. C=O stretching vibrations occur in the 1,730 and 1,726 cm⁻¹ range, which shows a very strong ketone group band. The amide shows a very strong band for the C=O group that appears in the 1,674–1,612 cm⁻¹ range. N-H bending bands, in solid primary amides give strong bending vibrational bands in the 1,674–1,612 cm⁻¹ range. They often nearly overlap with the C=O stretching bands. Primary amides give other bending bands and a very broad band around 704 cm⁻¹. A C-N stretching band appears at about 1,430 cm⁻¹ for arylamide. The broadening at 3,400 cm⁻¹ indicates that a hydrogen bonding interaction takes place between the phenolic hydroxyl group and the carbonyl from the amido group (as shown in Figure 2). The size of the MIP particles is in the range of 130.02 nm, as shown in Figure 3 (data from the particle size analyzer).

The pH of the solution is one of the most important variables affecting the adsorption process. Adsorption of 2,4-dinitrophenol was measured at different pH values (1–10). At every pH range, the 2,4-dinitrophenol capacity on MIP was higher than on NIP. Figure 4 shows that the maximum adsorption of 2,4-dinitrophenol was found to be 2.88 mg/g under acidic conditions at pH 6.0. The capacity decreased when the pH values increased.

The rate of the adsorption of 2,4-dinitrophenol by the MIP was measured as a function of time. The result of this kinetics study is shown in Figure 5. The 2,4-dinitrophenol sorption process was rapid in the initial stage and rather slow when approaching equilibrium. The maximum adsorption occurred after 1 hour at a capacity around 3.50 mg/g.

The sorption kinetic data of 2,4-dinitrophenol was analyzed using the Langergen first order rate model. The equations involved are as below: (1) d q t d t = k 1 ( q e − q )where k₁ (min⁻¹) is the rate constant of pseudo-first-order sorption, q_t denotes the amount of 2,4-dinitrophenol sorption (mg/g) at time, t (min) and q_e denotes the amount of 2,4-dinitrophenol adsorption (mg/g) at equilibrium. After definite integration and applying the initial conditions q_t = 0 at t = 0 and q_t = q_t at t = t, Equation (1) becomes: (2) ln ( q e − q t ) = ln ( q e ) − k 1 t

In addition, a pseudo-second-order equation based on adsorption equilibrium capacity may be expressed in the form: (3) d q t d t = k 2 ( q e − q t ) 2where k₂ is the rate constant of pseudo-second-order sorption. Integrating Equation 3 and applying the initial conditions, (4) 1 ( q e − q t ) = 1 q e + k 2 tor equivalently: (5) t q t = 1 k 2 q e 2 + t q e

The plot t/q versus t should give a straight line if second order kinetics are applicable and the values q_e and k₂ can be calculated from the slope and the intercept of the plot, respectively.

The equilibrium removal of 2,4-dinitrophenol was mathematically expressed in terms of the adsorption kinetics. The data does not fall on a straight line and had a low correlation coefficient, indicating the first order kinetic model is less appropriate. On the other hand, the plot of t/q against t results in a high correlation coefficient (0.999) (Figure 6). This indicates that 2,4-dinitrophenol adsorption by using MIP follows a second order kinetics reaction.

The amount of materials adsorbed is determined as a function of the concentration at constant temperature that could be explained in adsorption isotherms. Adsorption isotherm was measured for 2,4-dinitrophenol with MIP using the optimized conditions and the results are shown in Figure 8. The maximum adsorption capacity was observed to be 2.87 mg/g. Equations that are often used to describe the experimental isotherm data were developed by Langmuir and Freundlich. These equations are commonly used for describing sorption equilibrium for water and wastewater treatment applications.

The Langmuir model is probably the best known and most widely applied adsorption isotherm. It has produced good agreement with a wide variety of experiment data and may be represented as follows: (6) q e = q m b C e 1 + b C e

The above equation can be rearranged to the following linear form: (7) C e q e = C e q m + 1 b q mwhere C_e is the equilibrium concentration (mg/L), q_e the amount of 2,4-dinitrophenol adsorbed at equilibrium (mg/g), q_m is amount of 2,4-dinitrophenol sorbed for a complete monolayer (mg/g), b is a constant related to the energy or net enthalpy of sorption (L/mg). The adsorption data were analyzed using the linear form, Equation (7) of the Langmuir isotherm. The plots of specific sorption, C_e/q_e, against the equilibrium concentration, C_e, for MIP are shown in Figure 8.

The Freundlich expression is an exponential equation and therefore assumes that as the sorbate concentration increases, the concentration of sorbate on the adsorbent surface will increase also: (8) q e = K f C e 1 / n

The equation is frequently used in the linear form by taking the logarithm of both sides as: (9) log   q e   =   log   K f   +   1 / n   log   C ewhere K_f and n are isotherm constant, respectively.

The applicability of the Freundlich sorption is also analyzed by plotting log q_e versus log C_e. To determine the constant K_f and n, the linear form of equation, Eq (10) can be used. Compared to Freudlich, the Langmuir plots have a higher correlation coefficient of 0.985. It can be confirmed that 2,4-dinitrophenol adsorption by MIP follow the Langmuir Model with a maximum adsorption capacity of 2.87 mg/g at room temperature (Figure 9).

Competitive adsorptions of phenol/2,4-dinitrophenol, 3-chlorophenol/2,4-dinitrophenol and 2,4-dichlorophenol/2,4-dinitrophenol from their mixtures were conducted using imprinted and non-imprinted polymer. The effect of imprinting on selectivity was defined as: (10) K d = [ C i − C f ] × ν Mwhere K_d is distribution coefficient, Ci and Cf the initial and final solution concentration, respectively. v (mL) is the volume of the solution and M (g) is the weight of MIP.

The selectivity coefficient for the binding of phenol compound in the presence of competitor species can be obtained from equilibrium binding data according to: (11) K d   =   K d ( 2 , 4 − d i n i t r o p h e n o l ) K d ( p h e n o l i c )where k is the selectivity coefficient and (phenolic) represents the compounds phenol, 3-chlorophenol and 2,4-dichlorophenol. A comparison of k values of the imprinted polymers with those phenolic compounds allows an estimate of the effect of imprinting on selectivity. A relative selectivity coefficient k’ can be defined as: (12) k ′ = k i m p r   int   e d k c o n t r o l

The binding capacity adsorption of MIP for 2,4-dinitrophenol is higher than NIP. Table 1 summarizes K_d, k and k’ values of phenol, 3-chlorophenol and 2,4-dichlorophenol with respect to 2,4-dinitrophenol. The k’ values for MIP of phenol/2,4-dinitrophenol, 3-chlorophenol/2,4-dinitrophenol and 2,4-dichlorophenol/2,4-dinitrophenol were 2.88, 2.46 and 1.95, respectively, indicating a good selectivity towards 2,4-dinitrophenol. The selectivity of the MIP and NIP towards 2,4-dinitrophenol in the presence of phenol, 3-chlorophenol and 2,4-dichlorophenol were also evaluated and the results (Figure 11) showed that the MIP has a greater selectivity towards the target 2,4-dinitrophenol compared to NIP.

Acrylamide, 2,4-dinitrophenol, ethylene glycol dimethacrylate, and benzoyl peroxide were obtained from Fluka (Switzerland). All other chemicals were of reagent grade and purchased from Merck (Germany).

Molecular imprinted polymer was prepared using the non-covalent approach. The template, 2,4-dinitrophenol (1.0 mmol) was dissolved in acetonitrile (30 mL) in a flask. The functional monomer acrylamide (5.0 mmol), the cross-linker ethylene glycol dimethacrylate (EDGMA) (15 mmol) and the initiator benzoyl peroxide (BPO, 1.6 mmol) were then added to the flask. After degassing and nitrogen purging for 10 minutes, the flask was sealed and the contents allowed to polymerize in a water bath at 70 °C for 24 hours. The obtained bulk polymers were crushed and ground to obtain regularly sized particles between 80–100 μm. The template molecules were leached out using methanol-acetic acid (1%) with a 2:1 ratio three times. The particles were then extensively washed with water until no more 2,4-dinitrophenol was released. Non-imprinted polymer (NIP) was prepared and treated with the same method, but in the absence of 2,4-dinitrophenol.

Adsorption of 2,4-dinitrophenol from aqueous solutions was investigated in batch experiments. Effects of pH, adsorption isotherm, kinetics and selectivity of the fabricated MIP were examined. MIP (0.05 g) was shaken for 1 hour in 10 ppm 2,4-dinitrophenol (20 mL). The pH was adjusted with hydrochloric acid (HCl) or sodium hydroxide (NaOH). The concentration of 2,4-dinitrophenol in the aqueous after desired treatment periods was analyzed using UV-Vis Spectroscopy. The adsorption capacity was calculated as below: (13) q = ( C o − C e ) × V Mwhere q (mg/g) is the amount of total adsorption of 2,4-dinitrophenol, C_o and C_e are initial and equilibrium concentration of 2,4-dinitrophenol in solution (mg/L), respectively. V (L) is the volume of the solution and M (g) is the weight of MIP.

The adsorption kinetics were studied by stirring MIP (0.05 g) in 10 ppm 2,4-dinitrophenol (20 mL) for different times (5, 10, 20, 30, 60, 120, 240, 480, 960 and 1440 min). The adsorption isotherms were studied by stirring MIP (0.05 g) in 2,4-dinitrophenol solution (20 mL) at different concentration (1, 2, 4, 8, 10, 15, 20 and 30 mg/L).

Selectivity of the fabricated MIP towards phenol, 3-chlorophenol and 2,4-dichlorophenol with respect to 2,4-nitrophenol was studied. A solution (25 mL) containing 10 mg/L (for each compound) was mixed together and treated with MIP (0.05 g) at room temperature. The concentration of the phenolic compounds after treatment was measured by UV-Vis Spectroscopy. The binding capacity and the distribution coefficient were calculated.

Characterizations of the imprinted polymer were carried out using Fourier Transformed Infrared (FTIR) spectroscopy. The imprinted polymer particles (~0.01g) were thoroughly mixed with KBr and pressed into a pellet and the FTIR spectrum was recorded. A Nanophox particle size analyzer was used to determine the size of the particles. The MIP (~0.01g) in deionized water (10 mL) was sonicated for 10 minutes before being analyzed with the Nanophox particle size analyzer.