Removal of Selected Pharmaceutical Residues from Wastewater using Activated Carbon from Rice Husks and their Biodegradation using Microorganisms

Abstract

Presence of pharmaceutical residues in wastewater discharges into rivers disrupts aquatic ecosystems and human health. The risk of exposure to these pharmaceutical residues becomes greater since most of them are not degraded during sewage treatment. The aim of this study was to develop a method of removing paracetamol, trimethoprim and nevirapine residues from wastewater using activated carbon made from rice husks and subject them to biodegradation by use of selected microorganisms. Analysis of pharmaceutical residues was done using liquid chromatography tandem mass spectrometry. Powdered rice husks were carbonated at different temperatures in the range of 300℃ - 600 ℃ then activated using phosphoric acid. Biochar carbonated at 500 ℃ was found to have the best adsorption properties. Characterization of the biochar was done using scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction and their efficacy as adsorbents for the three pharmaceutical drugs evaluated. Adsorption of the three pharmaceutical drugs was found to increase with increase in contact time and adsorbent dosage, decrease with increase in initial drug concentration while pH did not have a significant effect on the adsorption rate. The optimum contact time for the three pharmaceutical drugs was 30 minutes while the optimum adsorbent dosage found to be 0.10 grams. Adsorption isotherms for the three pharmaceutical drugs fitted well in both Langmuir and Freundlich models. The Langmuir isotherms R2 values were 0.9996, 0.9994 and 0.9831 while the Freundlich isotherm R2 values were 0.977, 0.994 and 0.977 for paracetamol, trimethoprim and nevirapine, respectively. Kinetic studies were found to fit well in the pseudo-second order model with R2 values approaching unity. This indicated that chemisorption was favoured over physisorption. Biodegradation of the three pharmaceutical drugs by Pseudomonas aeruginosa, Escherichia coli, Bacillus subtilis and Staphylococcus aureas bacteria was also studied. Biodegradation was found to decrease with an increase in concentration from 0.5 ppm to 1.5 ppm for paracetamol and nevirapine but increased with an increase in pharmaceutical concentration for P. aeroginosa. Biodegradation by the bacteria was highest for trimethoprim (82%) followed by paracetamol (75%) and nevirapine the least (51%). The study gave a reliable assessment of the risks associated with the pharmaceuticals to living organisms and the remedial measures through possible biodegradation. According to the investigations, chemically activated rice husk biochar was shown to be an effective, low-cost adsorbent for use in removal of pharmaceutical residues from wastewater. Biodegradation of the three pharmaceuticals drugs using the selected microorganisms has been shown to be an effective method for their removal hence it should be tested in the field. Key words: Pharmaceutical residues; Biodegradation; Wastewater; Activated carbon