Adsorption of Polymeric Flocculants at Goethite Surfaces: Hydration Screening versus Salt Bridging

It is important to dispose of the red mud in the alumina production. Polymeric flocculants can effectively enhance the sedimentation of red mud, whose molecular mechanism remains to be explored and clarified. Herein, all-atom molecular dynamics simulations are employed to systematically investigate the adsorption behaviors and flocculation mechanism of polymeric flocculants at the surface of goethite, which is the main component in red mud. Seven types of polymeric flocculants are studied with consideration of the effects of amide groups, ether bonds, guaiacyl units, carboxyl groups, hydroxamic acid groups, and sulfonic acid groups. The compact hydration layer at the mineral surface is identified as the primary barrier to the polymer adsorption. In contrast, the salt ions can bridge the polymer chains and the surface, which can destroy the hydration layer and enhance the polymer adsorption. According to the analysis for roles of functional groups in destroying the hydration layer, amino nitrogen atoms can effectively enhance the polymer adsorption, whereas carbonyl oxygen atoms drive interchain bridging interactions. Moreover, by integrating the adsorption and bridging abilities, an evaluation model has been developed to give an overall flocculation performance. It is found that high performance of flocculants requires a balanced conformation for polymers at the surface, i.e., a high ratio of train conformations, long length of tail conformations, and a low ratio of loop conformations, which promote stable adsorption. These results provide fundamental molecular guidance for designing high-performance flocculants for the red mud treatment.