Removal of dissolved glyphosate from waters by adsorption under the influence of metals

• Main Author: Herath, Mudiyanselage Gayana Anjali Dissanayake
• Other Authors: Ng Wun Jern
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Chemical engineering::Water in chemical industry; Engineering::Civil engineering::Water resources
• Online Access: https://hdl.handle.net/10356/137655
• Institution: Nanyang Technological University

Glyphosate, which is founded in 1974 is the most widely used weed control product in the world. The International Agency for Research on Cancer (IARC) declared glyphosate as “probably carcinogenic” to humans in 2015. Research on potential toxicity has gained momentum because of the new findings revealing higher toxicity of glyphosate than originally reported. As a result, public perception that this was a rather benign herbicide is beginning to erode. Due to its toxicity, detection of glyphosate and finding effective methods to control the glyphosate concentration levels in surface waters in agricultural areas where the herbicide is used extensively became an immediate concern. Chronic kidney disease associated with glyphosate has been reported in agricultural areas such as Central province in Sri Lanka, sub-regions of El Salvador, Nicaragua, Costa Rica, and Andra Pradesh of India. Researches have reported the detection of glyphosate and heavy metals cations in the drinking waters of these areas. According to previous studies, glyphosate tends to form soluble or insoluble complexes with the metal cations in waters containing heavy metal cations such as iron, zinc, copper and magnesium. This thesis work discusses the removal of dissolved glyphosate from surface waters by adsorption using activated carbon and biochar under the influence of metals, namely ferrous, copper and zinc. During the adsorption experiments, glyphosate is degraded when in contact with Fe2+ and Cu/ Cu+ ions. By-products such as amino-methyl-phosphonic acid (AMPA), which are more persistent and more toxic than glyphosate are formed by the degradation of glyphosate. Findings emphasize the importance of exploration of glyphosate heterogeneous degradation as glyphosate removal techniques such as adsorption can impair the water quality due to the formation of toxic by-products. The study is divided into two main parts. Firstly, commonly used adsorbent types are analysed focusing on the effect of surface area and pore distribution on the removal of glyphosate from surface waters. In this study, four coconut shell activated carbons, two wood activated carbons, and two porous polymers with different particle and pore sizes are tested. The finely powdered coconut shell based, and wood-based activated carbon types effectively remove the glyphosate. For coconut shell based activated carbon, the kinetics experiment data better fit with the pseudo-second-order model than the pseudo-first-order model; indicating chemisorption. Surface variations of activated carbon are analysed before and after glyphosate adoption to analyse the effects of surface charge and ligands on glyphosate adsorption. Hydroxyl and carbonyl groups favour the removal of glyphosate from aqueous solution. Finely powdered coconut shell activated carbon is then compared with iron loaded wood biochar adsorbent for glyphosate removal from aqueous solution. The adsorption data for the finely powdered coconut shell activated carbon and iron loaded wood biochar better fit the Freundlich isotherm model. The rate of glyphosate adsorption is found to follow the pseudo-second-order model. Box–Behnken design and percentage contribution with Pareto analysis techniques are used in surface response and efficiency calculations; modelling the process conditions and their effects. The pH of the solutions is regulated by buffering the solution during the adsorption process. Highest efficacy of glyphosate removal is obtained by optimizing parameters; operating pH, initial glyphosate concentration, temperature, adsorbent dose, and contact time. The conditions yielding the best removal for the aforementioned parameters are; pH 8.0, 0.2 mg/L, 50.0 °C, 11.4 g/L, 1.7 h for activated carbon, and pH 5.0, 0.7 mg/L, 50.0 °C, 12.3 g/L, 1.9 h for iron loaded wood biochar respectively. The maximum removal capacity and efficiency for the optimised conditions are; 0.0173 mg/g, 98.45% for activated carbon, and 0.0569 mg/g, 100.00% for biochar. During the adsorption process with iron loaded wood biochar, with the reduction of dissolved glyphosate concentration, an increase in the AMPA concentrations in the samples is detected. Iron attached to the surface of wood biochar degraded glyphosate to form AMPA. Evidently, glyphosate is strongly adsorbed onto iron oxides, yet few studies focus on the following degradation. Therefore, as the second part of the thesis, further studies are conducted to examine glyphosate decomposition in an aqueous medium when in contact with prevailing inorganic ions in agricultural areas, viz. ferrous, copper and zinc. Findings confirm; if there exist Fe2+ and Cu/ Cu+; it mediates the energy barriers opposing bond dissociation leaning towards C−N bond cleavage to form AMPA. Glyphosate degradation forming AMPA is a reduction reaction. Fe2+ and Cu/ Cu+ oxidise reducing glyphosate. Box–Behnken design and percentage contribution with Pareto analysis techniques are used respectively in surface response and efficiency calculations to determine the environmental conditions which induce glyphosate degradation with Fe2+ and Cu/ Cu+. Parameters such as operating pH, initial glyphosate concentration, temperature, iron loaded biochar/ porous carbon dose, and contact time are investigated for their effect on the degradation efficiency. The conditions yielding the highest AMPA concentration are; pH 5.0, 50.0 mg/L, 50.0°C, 11.4 g/L, 24.0 h for iron loaded wood biochar, and pH 5.0, 35.9 mg/L, 50.0°C, 34.7 g/L, 24.0 h for copper respectively. At these conditions, for copper; 18.61% of degraded glyphosate is converted to AMPA and for iron loaded wood biochar; 26.1% of degraded glyphosate is converted to AMPA. Glyphosate is degraded into non-detectable levels when in contact for >24 h at the aforenamed optimised environmental conditions. The abiotic degradation pathway for glyphosate is studied. Porous carbon materials such as activated carbon and biochar catalyse the degradation. C−N and C−P bond cleavage can happen at the surface of the adsorbent, where the preference for cleavage rest on glyphosate and porous carbon material ratios. Intermediate identification on glyphosate adsorption reveal further reactions; a polymerisation of intermediates forming complex phosphonic acids (i.e. Bis-2-aminoethyl-aminomethyl phosphonic acid, Bis-phosphonomethyl-amino-phosphonomethyl-aminomethyl phosphonic acid) in addition to AMPA. Existence of zinc ions does not impact the degradation of glyphosate.