Destabilization of stable bentonite colloidal suspension using choline chloride based deep eutectic solvent: Optimization study

Abstract

Choline chloride (ChCl) based natural deep eutectic solvents (NADES) were synthesized and tested as coagulant for stable bentonite colloidal suspensions. In this work, three ChCl based natural deep eutectic solvent NADES were synthesized by mixing choline chloride as the hydrogen bond acceptor (HBA) with lactic acid (LA), malic acid (MA), and citric acid (CA) as the hydrogen bond donors (HBD) with a molar ratio of 1:1. The analysis showed that there is no significant difference in the result, however, ChCl:LA was the easiest to synthesize and showed slightly higher efficiency at low dosage. For the determination of the optimum conditions, response surface methodology with central composite design was employed to generate an experimental design with two controlled variables: the ChCl:LA dosage and the bentonite solid content. The turbidity removal percentage, the reduction in zeta potential, the variation in the floc size of the sediments, and pH of the system were the design responses. However, due to the high turbidity removal percentage even at lower dosage of the coagulant and the constant removal percentage of almost 99 % led to excluding the turbidity as a response as it proves to be insignificant. By performing a multiple response optimization on the designed experiment, the optimum condition for the destabilization and separation process that results in a zeta potential of -5.8 mV, a pH of 2.2, D50 of 31.5 μm and a D90 63.8 μm was found to be 77 mM and 3.48 g L−1 for the ChCl:LA dosage and the bentonite solid content, respectively.