Influence of choline chloride based natural deep eutectic solvent on the separation and rheological behavior of stable bentonite suspension

Colloids in wastewater are considered a serious problem due to their stability and separation difficulty. Coagulation/ flocculation processes are the most common for the separation of colloids because of their highperformance efficiency, simplicity, and economical characteristic. Natural deep eutectic solvents (NADES) are green solvents that proved to be highly effective in the destabilization of colloidal suspensions. In the present study, the influence of choline chloride (ChCl) and lactic acid (LA) based NADES on the rheological behavior of bentonite suspension was investigated. Furthermore, it examines the difference in the rheological behavior of NADES treated suspension with others treated with ChCl-LA mixture (LA and ChCl added one after the other). The rheological behavior are correlated to the destabilization degree of each coagulants through turbidity reduction, zeta potential, particle size distribution, and capillary suction time. The study revealed that while untreated bentonite suspension follows a Newtonian behavior, treated suspensions are non-Newtonian fluids with shear thinning behavior. Furthermore, all treated suspensions showed elastic behavior under moderate to low oscillatory frequencies. The changes in the studied rheological parameters (i.e., initial viscosity, elastic modulus, and Bingham yield stress) were influenced by the concentration in addition to the structure of the selected coagulant. The addition of higher coagulant concentration enhances the elastic properties from two to more than ten times depending on the concentration and coagulant type. At concentration of 6.76 x 10-2M, suspension treated with NADES achieved an initial viscosity of 194, 110mPa.s and a yield stress of 2959.3mPa which indicates the formation of stronger and stiffer flocs. On the other hand, ChCl-LA treated suspension had viscosity and yield stress of 9 and 15 magnitudes lower, respectively. Enhancing the rheological behavior of suspensions is usually attributed to higher destabilization degree, which is desirable for the further treatment processes. By increasing the concentration from 5.9 x 10-3M to 6.76 x 10-2M, turbidity reduction increases from 12% to 99.9% with an increase in the floc size from 3.3 mu m to 27.3 mu m for NADES treated suspension. The increase in the destabilization degree was associated with an enhancement in the suspension viscosity by more than two folds.