Adsorption of Pharmaceutical Compounds in Wastewater Using Ash Derived From Agri-Residue and It’s Solidification

Abstract

The improper discharge of drugs from hospitals and pharmaceutical industries is the leading cause of negative impacts on the natural environment's physical, chemical and biotic features, especially water. In addition, the production of several categories of pharmaceutical compounds due to the rise in geometric population and falling health status of the general public has enhanced the problem. These contaminants, usually stated as “emerging pollutants” are the real threat to human health. Various types of pharmaceutical compounds have been extensively examined in aqueous environments in concentration ranges of μg/L to mg/L. The high rate of bioaccumulation, persistence and non-biodegradability causes great resistance in eliminating these contaminants by conventional treatment processes. Among the different categories of pharmaceutical compounds, antibiotics are an important class of pharmaceuticals that have their unique properties because they kill pathogens without disturbing the metabolic system of human beings. The term “antibiotic” is typically used to characterize several classes of organic molecules which could successfully prevent bacterial growth. Based on chemical structures, antibiotics have been classified as fluoroquinolones, tetracyclines, sulfonamides and chloramphenicol. The estimated worldwide average consumption of antibiotics is more than 100 x 106 kg per year. Such great consumption of antibiotics for agriculture and veterinary purposes results to its excessive emission, which may lead to several environmental issues. Recently, the residues of the antibiotic pollutants have been detected in various aqueous systems, particularly in industrial effluent, domestic influent and effluent, drinking water, groundwater and surface water. According to the present status of COVID-19 coronavirus, Various antibiotics have sparked interest as potential COVID-19 treatment options in the year 2020. The huge demand and consumption of OFL and DOX, its incomplete metabolism and complex behavior in the atmosphere are causing a great ecological issue, which needs to be solved. Antibioticresistant bacteria (ARB) and antibiotic-resistant genes (ARG) in aquatic systems are a critical concern due to their ecotoxicity. In this regard, researchers have widely preferred the adsorption process in the past two decades due to its various advantages like simple operation, cost-effective, no by-products, no sludge generation and environmental friendliness. Therefore, tremendous endeavors have been done for exploring the different types of adsorbents with excellent performance. As a result, numerous natural, modified and functionalized materials have been evolved to adsorb IV pharmaceuticals from aqueous systems. However, the molecular structure, specific surface area, surface charge and affinity towards adsorbate are the most important features for the effective performance of a particular adsorbent. Thus, Adsorption is a significantly important process from a technological point of view due to the micro and macroporous nature of the materials. In the present study, Doxycycline hydrochloride (DOX) and Ofloxacin (OFL), which belongs to the tetracyclines and fluoroquinolones category, respectively were chosen as model pollutants. Moreover, for appropriate management of exhausted adsorbents, its disposability studies were carried out by stabilization technique, using Portland cement as a solidifying agent. Further, the toxicity study of leachate from solidified adsorbents using different microbes confirmed almost complete encapsulation of OFL and DOX and ensured that concentration of antibiotics in leachates were insufficient to affect the microbial growth. Thus, Cytotoxicity assessment indicated that the solidified matrix of OFL and DOX did not exhibit any toxic effect after adsorption and stabilisation processes. Adsorptive removal of antibiotics such as OFL and DOX using several types of natural and modified adsorbents has been reported by various authors. In the present study, RHA, PJAC and PSSAC adsorbents were prepared using rice husk ash, Prosopis juliflora and pumpkin seed shell respectively. Further, RHA and PSSAC were modified using deep eutectic solvent (DES) as functionalization agent to give DES-RHA and DES-PSSAC adsorbents. The studied adsorbents were characterized to determine their chemical and morphological characteristics employing FTIR, XRD, 1HNMR, TGA, SEM-EDX, FESEM, HR-TEM and zeta potential. Surface area and pore size distribution were evaluated with the help of BET and BJH characterization techniques, respectively. Batch adsorption experiments were performed for OFL and DOX removal. The influence of various adsorption parameters suggested by the central composite design (CCD) model was evaluated by Response surface methodology (RSM). A set of 30 experiments with 6 replicates were performed for each antibiotic. The interactive effects of initial adsorbate concentration (C0), adsorbent amount (m), pH and removal time (t) were optimized. The 3D response surface graphs were also obtained for capacity (mg/g) and removal (%) of both responses. Furthermore, as suggested by ANOVA, the polynomial quadric model was significant for both antibiotics with higher coefficient values of R2. At optimized experimental conditions, thermodynamic and kinetic studies were also performed. The experimental data were examined by applying various kinetic and isotherm models. Pseudo-first order and pseudo-second order kinetic models were used to study the V adsorption kinetics. Further, the adsorption rate controlling mechanism was investigated using an intra-particle diffusion model. Interactions of OFL and DOX molecules with the surfaces of RHA, PJAC, PSSAC, DES-RHA and DES-PSSAC adsorbents were examined and the adsorption process controlling mechanism was explored. The effect of temperature on the adsorption of OFL and DOX was studied at the different ranges of temperature from 288 to 318 K. The well-suited isotherms for fitting experimental data were Langmuir, Freundlich, Redlich-Peterson (R-P) and Temkin equilibrium models. The thermodynamics parameters were obtained from the linear plot of ln KD versus 1/T. The negative value of ΔG° at each reaction temperature directed that the adsorption of antibiotics on the adsorbents was instantaneous and thermodynamically favorable. Moreover, the positive value of parameter △H° established the endothermic nature of antibiotics onto adsorbents. Furthermore, the increase in the randomness of the adsorbates molecules at the liquidsolid interface was approved by the positive ΔS° values. Thus, from the thermodynamic study, the adsorption of antibiotics on adsorbents was spontaneous and endothermic. Finally, adsorbents' good regeneration capability and high adsorption capacity represent their excellent potential to alleviate pharmaceutical wastes from industrial effluents.