Adsorption of pharmaceuticals by nano-molecularly imprinted polymers (nano-MIPs) from wastewater: kinetics, isotherms, and thermodynamics studies.

Abstract

It has been reported that pharmaceuticals are not entirely removed or broken down during the wastewater treatment process, allowing them to escape into effluent water. This stems from the pharmaceuticals widespread use and the inefficient wastewater treatment methods. Therefore, the objective of this study was to develop more effective methods for removing pharmaceuticals from wastewater systems. Adsorption-based pharmaceutical removal is one of the most promising approaches because it is easily incorporated into current water treatment systems. The first part of this work reports on literature studies for recent advancements in the adsorption process involving the incorporation of an artificial molecularly imprinted polymer (MIP), that is an effective molecular receptor that can selectively recognize and remove pollutants. In magnetic solid-phase extraction, dispersive solid-phase microextraction, and solid-phase extraction, MIPs can be used as a selective adsorbent for analyte cleanup and preconcentration. Moreover, MIPs can be produced by combining nanoparticles to develop composite nanomaterials (nanoMIPs). In comparison to conventional bulk adsorbents, the enhanced selective adsorption capacity and kinetics are attributed to the large surface area per unit volume and specific functionality of nanomaterials. Nonetheless, some significant barriers to the application of nanomaterials are their dispersive qualities, difficulty in cycling, and secondary pollution from the loss of adsorbent during treatment. Another way to use nanoparticles for detectability enhancement is to modify the molecularly imprinted polymers chemical or physical characteristics. The nanoparticles' embedding in the MIP enhances the material's surface area or gives the adsorbent new features. This study describes a method for creating reusable, economical, and effective polymer-based silver nanoparticles-adsorbents. Notably, silver nanoparticles have a wide range of applications due to their unique properties which include their large surface area, shape and size. Plant-mediated synthesis plays a significant role in their synthesis. Remarkably, the synthesis of silver nanoparticles from plant extracts is inexpensive, easily scalable, and harmless for the environment. Plant extracts can be used to produce nanoparticles with controlled sizes and shapes. The molecular imprinting technique was used to create species-specific functionalities like carboxylic acid (-COOH) on a polymer surface. MIPs offer several advantages, including large surface area, targeted functionalities for high reactivity, and the ability to minimise nanoparticles from leaking into the surrounding environment when MIP-based adsorbents are being handled. To further comprehend the behaviour of adsorbents and the adsorption process, kinetics, thermodynamics, and isotherm models were explored. The second part of the work involved synthesizing the MIPs for efficient and selective removal of pharmaceuticals from specific groups. All target compounds were employed as multiple templates in a bulk polymerization process carried out at 70 °C to synthesize MIPs. Additional reagents utilized in the synthesis included toluene as a porogenic solvent, ethylene glycol dimethacrylate as cross-linker, 1,1'-azobis-(cyclohexane carbonitrile) as an initiator and 2- vinyl pyridine as functional monomer, respectively. The synthesis of a non-imprinted polymer (NIP) was conducted without templates, using reaction conditions similar to those of MIP. Furthermore, following the synthesis, the polymers were characterized using X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, Fourier transform infrared spectroscopy, and scanning electron microscopy. Liquid chromatography-mass spectrometry (LC-MS) was successfully used to develop an analytical method for detection and quantification of the target pharmaceuticals. The method yielded quantification limits of 0.42 to 0.75 mg L-1 and detection limits of 0.14 to 0.25 mg L-1 for the target pharmaceuticals. The synthesized polymer attained maximum matrix-matched adsorption capacities of 3.89, 4.97 and 3.40 mg g-1 for sulfamethoxazole, nevirapine and ibuprofen, respectively within 10 minutes. Competitive adsorption of the target pharmaceuticals demonstrated a link between adsorption and the pharmaceuticals pKa, log Kow, and molecular size. Studies on batch adsorption and kinetics revealed that the binding of pharmaceuticals to the MIP particles suited the pseudo-second order kinetics, leading to various interactions through chemisorption. The data also fitted well in Langmuir isotherm which meant that the target pharmaceuticals adsorption occurred on the homogeneous binding sites of the MIP. Furthermore, the thermodynamic data demonstrated the adsorption process's endothermic and spontaneous nature. Notably, the synthesized MIP was highly selective and its application in environmental studies led to the development of a less expensive analytical method. Moreover, the MIP particles that had been generated were recovered to be reusable up to five cycles with removal efficiency >90%. The third part involved incorporating silver nanoparticles (AgNPs) into MIPs using ibuprofen, nevirapine, and sulfamethoxazole as templates. In this part, starch (St) and macadamia nutshells (MCD) were employed in the synthesis of AgNPs as reducing and stabilizing agents. Following that, each of these AgNPs was incorporated with MIP, and the most effective combination was identified through comparison. The synthesized adsorbents were further optimized for the adsorptive removal of selected target pharmaceuticals. The % removal efficiencies were greater than 70%, indicating that the adsorbents are suitable for use in water treatment processes. The material's adsorption mechanisms and performance were examined through the application of various kinetics and isotherm models. Both the St and MCD-AgNPs experimental data fit to Freundlich and Langmuir adsorption isotherms. However, based on the somewhat higher correlation coefficients, the Langmuir isotherm model provided a better fit. The St/MCD nanoMIPs best suited the Freundlich model, indicating that the adsorption occurred on the multilayer heterogeneous surface. Further, both the St/MCD nanoMIP adsorbents underwent spontaneous, endothermic adsorption, as demonstrated by the thermodynamic data, whereas the behaviour of the kinetics was effectively anticipated by pseudo-second order model, which suggested adsorption through chemisorption. Accordingly, large internal surface area, greater loading capacity, thermal stability, and reusability were among the advantageous properties of the nanoMIPs adsorbent materials. Moreover, both adsorbents showed improved qualities and were highly selective and effective in removing the selected pharmaceuticals in wastewater. As a result, during the course of five adsorption/desorption cycles, the St/MCD nanoMIPs show a removal efficiency of more than 90%. As a result, they demonstrated proficiency in efficient application. The fourth part involved the incorporation of MIP with Platanus acerifolia and Moringa oleifera silver nanoparticles. Using plants to synthesize AgNPs is a more cost-effective and lowmaintenance method; in contrast, using other organisms requires a particular medium and a specific amount of time. Therefore, the leaves of both the platanus acerifolia (PL) and moringa oleifera (MO) served as stabilizing and reducing agents during the synthesis of AgNPs. Each optimized parameter that could influence the adsorption potential, such as temperature, adsorbate concentration, pH, adsorbent dose, and contact time, was examined in relation to the removal effectiveness of the MO/PL nanoMIP adsorbents. These evaluated parameters were optimum at pH 7, concentration of 0.2 mg/L and contact time of 10 minutes for both MO and PL-nanoMIPs, mass dosage of 30 mg and 20 mg, and temperature of 40 and 30 °C for MO and PL-nanoMIP, respectively. Further, the maximum removal efficiencies obtained at these optimum conditions were >97% for both MO-nanoMIP and PL-nanoMIP. The adsorption experimental data for both MO/PL-AgNPs and MO/PL-nanoMIPs nano-adsorbents fitted with the linear Langmuir model which suggests that the binding took place on the homogenous monolayer surface. Additionally, compared to MO/PL-AgNPs, the MO/PL-nanoMIPs adsorption capacities for the target pharmaceuticals were higher, suggesting that the nanoMIPs larger surface areas contribute to their enhanced adsorption capacity. The linear pseudo-second order kinetic model best fitted on MO/PL-nanoMIPs which implied adsorption through chemisorption, whereas the thermodynamic data demonstrated that the adsorption process was endothermic and spontaneous. Moreover, the values of ΔH° for the MO/AgNPs were less than 40 kJ/mol and more than 40 kJ/mol for the MO/PL-nanoMIPs. This therefore confirmed that the MO/AgNPs was dominated by physical adsorption whereas the MP/PL-nanoMIPs was dominated by chemical adsorption. The MO/PL-nanoMIPs confirmed the high efficiency for the removal of target pharmaceuticals in wastewater. Upon recycling the adsorbents for five cycles, it was noted that the MO-nanoMIP adsorbent was effective continued to remove 86.7- 88.8% and 97-98% for PL-nanoMIP even in the fifth cycle. Indeed, the removal of sulfamethoxazole, nevirapine, and ibuprofen by nanoMIP adsorbents has demonstrated the importance of the surface area, structural stability, pore size and the electrostatic interactions brought about by the charges on the nanoMIPs surface. Consequently, among the investigated nanoMIP adsorbents, PL-nanoMIP demonstrated strong adsorption capacities for the targeted pharmaceuticals due to it large surface area and narrow size distribution as compared to the other nanoMIP adsorbents. The usability of plant leaves as a reducing and capping agent for nanoparticles as well as the recycling of nanoMIPs has the potential to transform waste that is no longer useful into valuable pollutants adsorbents. This would solve the problem of waste disposal and have beneficial impacts on the environment pollution and the economy. Notably, the nanoMIPs synthesized in this study are highly selective, reusable adsorbents that are cost effective and environmentally friendly. In contrast, as a substitute for more costly synthetic materials, these nanoMIPs are a promising material for the removal of different classes of pharmaceuticals in wastewater treatment plants and they can possibly be applied on a large scale.