Influence of polyelectrolytes and other polymer complexes on the flocculation and rheological behaviors of clay minerals: A comprehensive review

Abstract

Separation of clay minerals from industrial wastewaters is of great academic and practical importance. Current treatment techniques are either not economically viable, not environmentally friendly, or both. Thus, researchers are actively trying to develop optimal and more environmentally friendly wastewater treatment processes. Clay minerals like montmorillonite, bentonite, kaolinite and illite have numerous applications in various industries including, mineral processing, cosmetics, pharmaceuticals, paint, dyes, cement, concrete, functional fillers, paper making, clarification of wines and oils, water treatment and improving drilling mud properties. Their wide applications increased the volume and treatment complexity of water contaminated with them since they form highly stable suspensions in water. Flocculating agents such as polyelectrolytes have the potential to separate the above-mentioned minerals from industrial wastewater effluents. Polyelectrolytes are more effective and environment-friendly flocculants, in contrast to inorganic metal salts and some non-biodegradable synthetic polymers that pose serious hazards to human health and the environment. The development of polyelectrolytes is considered to be among the most important breakthroughs in solid-liquid separation processes, which have resulted in an improved treatment of water polluted with minerals. In the characterization of clay mineral separation using polyelectrolytes, it has been common practice in the past to either emphasize on the flocculation behavior of colloidal clay-polyelectrolyte suspensions (i.e., settling behavior or floc sizes) or on the behavior of networked clay-polyelectrolyte suspensions (i.e., filterability or dewaterability of sediment). However, flocculation and rheological parameters, which are very important factors in optimizing the wastewater treatment process, have rarely been reported in the literature. The aim of this paper, therefore, is to offer a comprehensive review of the state-of-the-art contributions for polyelectrolyte systems, focusing on the development of different types of polyelectrolytes and their applications in flocculating and dewatering clay minerals. Electrokinetics and rheological behavior of different clay minerals using different polyelectrolytes are critically evaluated. The effect of several parameters related to the clay mineral (type, composition), the polyelectrolyte (e.g., synthetic/natural, molecular weight, charge type, charge density, linear vs branched) and the flocculation medium properties (e.g., pH, ionic strength, clay mineral and polyelectrolyte concentration and type) are reviewed. This paper provides up-to-date progress in the treatment of water contaminated with clay minerals using various polyelectrolytes. The gaps and potential parameters of investigation in flocculation optimization studies are identified and more economical and environmentally friendly reagents in wastewater treatment are highlighted.