Development, characterization and the application of hybrid materials in coagulation/flocculation of wastewater: A review

Abstract

Hybrid materials are products which are composed of two or more different types of components in one polymeric matrix. Hybrid materials have been receiving increased attention by researchers in recent years due to their unique properties and superior performance compared to that of conventional inorganic coagulants and organic polymeric flocculants. This paper attempts to discuss various scientific aspects of hybrid materials that are used in coagulation–flocculation process. This review covers the fundamental aspects of preparation, characterization as well as the application of hybrid materials in coagulation/flocculation of wastewater. The properties of hybrid materials are reviewed in terms of chemical, physical, thermal, morphological as well as structural characteristics. Emphasis is given on the application of hybrid materials in treating wastewater under different conditions, through coagulation and flocculation method. It covers various research works efforts which are being carried out over the past decade with respect to hybrid materials used in treating wastewater.

Introduction

Solid–liquid separation through coagulation–flocculation is an important unit operation in wastewater treatment and for sludge dewatering for industries such as pulp and paper processing, pharmaceutical, cosmetics, food, mineral processing, metal working, textile and so forth [1], [2], [3]. Coagulation of dissolved and colloidal substances in wastewater is essentially understood in light of Derjaguin–Landau–Verwey–Overbeek theory (DLVO theory) whereby coagulation refers to the process of overcoming the interparticle repulsive energy barrier by simply increasing its ionic strength. The introduction of common inorganic coagulant such as aluminum and iron based substances plays a major role in neutralizing the surface charge of suspended particles or colloidal systems and facilitating particle aggregation and settling under gravity as a result of electrical double layer compression [4]. Flocculation process involves the addition of floc-forming chemical reagent usually after coagulation to agglomerate non-settable and slow-settling colloidal solids and it plays a major role in the fate and transport of contaminants in aquatic environments by bridging the aggregated flocs to form larger agglomerates in the presence of polymeric materials [5], [6].

Various materials have been developed in recent years for the coagulation and flocculation purposes. Among them are inorganic-based coagulants, organic-based flocculants as well as hybrid materials [7]. Although many materials have been developed and successfully used in removing pollutants from wastewater, there is still a need to improve their performance. The continuous increase of market needs for efficient and effective materials in wastewater treatment has induced the development of hybrid materials for coagulation–flocculation of wastewater. Hybrid materials thus have emerged as new materials that pose tremendous potential in treating wastewater due to their better performance compared to that of conventional inorganic-based coagulants, and its lower cost than that of organic-based flocculants [8].

Hybrid materials used in coagulation/flocculation of wastewater are materials obtained from the addition of effective components into the original material to enhance the aggregating power. It is logical to introduce functional chemical groups or components into the initial chemical which can strengthen the aggregating power [9]. Due to the synergetic effect of hybrid components in one material, hybrid materials hence pose a superior performance than that of individual component [10], [11]. Compared to individual coagulant/flocculant, hybrid materials, which have combined functional components into one prescription, would be a convenient alternative material for the operation of wastewater treatment facilities since the whole wastewater treatment can be conducted with the addition of one chemical and in one tank instead of two unit operations in the conventional coagulation–flocculation system. Reduction of operation time as a result of the application of these hybrid materials in a single operation is favorable to the industries that are discharging large volumes of wastewater [2].

The present review article attempts to classify the hybrid materials and provides an overview of recent information concerning the development and characterization of hybrid materials, in view of hybrid materials which have been receiving increased attention by researchers in recent years in treating wastewater. Their preparation methods and relatively unique properties of hybrid materials have become an essential area to be reviewed in this paper considering there is no systematic compilation available for hybrid materials for coagulation/flocculation of wastewater. A particular emphasis is given to the application of hybrid materials in treating wastewater, under different conditions, through coagulation or flocculation method.