Synthesis and characterization of municipal waste incinerator fly ash based Geopolymers for the removal of Endosulfan from water: kinetics, isotherms, and thermodynamics

Abstract

Endosulfan is a broad spectrum organochlorine insecticide which acts as a contact poison for controlling a wide variety of insects. However, extensive and uncontrolled use of insecticides leads to water pollution. Conventionally, adsorption onto activated carbon is used to remove water pollutants from influent wastewater. A suitable adsorbent must be cheap, possess high adsorption capacity, fast adsorption kinetics and recyclable among other properties. Geopolymers are emerging low-cost and efficient adsorbents for removal of various contaminants from water. The performance of the geopolymers varies with both sorption conditions and adsorbent structural and textural characteristics, which are controlled by several factors such as; the precursor material and the preparation conditions. While silicate to NaOH ratios affect mechanical properties of geopolymers for construction purposes, the impact of silicate to NaOH ratios on the adsorptive performance of geopolymers is unknown. Furthermore, although research has been done on remediation of pesticides from water using various adsorbents, no article has reported the use of solid waste incinerator fly ash MWFA-based geopolymers especially on their synthesis and/or application as adsorbent for endosulfan removal from water. The present work investigated the adsorption of endosulfan on alkaline activated geopolymers, synthesized from MWFA. Since there is no reported theory for predicting the geopolymer sorption capacity and adsorption rate based on precursor materials and silicate to NaOH ratios conditions, this leaves a clear gap in our understanding of the role of these factors. The objective of this study was to synthesize MWFA-based geopolymers and evaluate the effect of sodium silicate to NaOH ratio of the activator solution on morphology, chemical composition, and adsorptive performance (adsorption capacity and mechanism) in batch-mode. Alkali-activated MWFA-based geopolymers, GPA, GPB, and GPC, were synthesized using 0.17, 0.21 and 0.24 sodium silicate to sodium hydroxide mole ratios, respectively. The geopolymers were applied in the removal of endosulfan from water. The adsorbents were characterized by XRD, SEM-EDX, and FTIR. Variation of sodium silicate to sodium hydroxide mole ratios resulted in morphologically distinguishable geopolymers with different compositions. Adsorption experiments were done at different parameters such as initial endosulfan concentration, contact time, pH, geopolymer dosage and temperature. The adsorption equilibrium data were best described by Langmuir isotherm. The maximum adsorption capacities increased with an increase in sodium silicate to sodium hydroxide mole ratios in the order 1.872, 15.899, 16.970, and 20.010 mg/g for MWFA, GPA, GPB, and GPC, respectively. The kinetic data were best described by the pseudo-first-order model wherein the adsorption rate (k1) was independent of the sodium silicate to sodium hydroxide mole ratio and the geopolymer composition. The thermodynamic parameters, that is enthalpy (∆H > 0), Gibbs free energy (∆G < 0), entropy (∆S > 0), and activation energy (Ea > 0) show that the processes were endothermic, spontaneous, physical (Ea and ∆H < 40 kJ/mol) and entropy-driven. Alkalination was beneficial since the geopolymers had a higher adsorption capacity (~8-10 times) and affinity for endosulfan (~30 times) than the precursor material (MWFA). The adsorption mechanism entailed electrostatic interactions and hydrogen bonding. Increase in sodium silicate to NaOH ratio increases the adsorption capacity of the geopolymers for endosulfan. The MWFA-based geopolymers prepared with high silicate to NaOH ratios are recommended as alternative high adsorbing materials for wastewater treatment and a strategy for the valorization of MWFA.