## 2 Principal Textile Colored WW Characteristics and Composition
### Coagulation-Flocculation Technologies and Its Performances in Colored Textile WWs Treatment
The critical concentration of the coagulant (commonly a metal-containing salt or a certain metal-containing hybrid material) used for the aggregation of colloidal particles depends on the co-ions rather than the counter ions. The Hofmeister series rule must be considered that relates to coagulation effectiveness as SO4\({}^{2}\) - \(>\) Cl- \(>\) NO3\({}^{-}\) \(>\) I\({}^{-}\). (_ii_) _charge neutralization and adsorption_: The retention of species on colloid surfaces can be produced via charge neutralization and specific binding. Adsorption phenomena (physical process) is based on electrostatics (coulombic electrostatic forces that are weak in comparison to covalent, coordinative, or hydrogen bonds) when the charge density on both the colloid and water/WW species determines the extent of adsorption. The colloid-coagulant, coagulant-WW, and colloid-WW interactions are important when compared with coulombic energy (i.e., colloids-surfactant-like molecules such as dodecyl ammonium chloride). When enough counterions are adsorbed, charge reversal takes place, and re-stabilization occurs. If long-chain counterions (polymers, such as polyelectrolytes, and  
Figure 4: Schematic representation of coagulation–flocculation mechanism. (**a**) Electric double layer of a colloidal particle; (**b**) interaction mechanism between colloids, colored species, and polymer-based coagulants; (**c**) flocculation mechanism with polymer-based hybrid materials [95; 96; 150]. certain polymeric hybrid materials) are attached to the colloid surface, the effective charge outside of the shear layer is reduced in contrast to double-layer repression (double-layer compression), which alters the charge distributions within the diffuse layer (Figure 4b). _(iii) entrapment in a precipitate (co-precipitation) and adsorption_: The addition of electrolytes as coagulants (e.g., Fe\({}^{3+}\) and Al\({}^{3+}\) salts) in colloid-containing WW leads to the formation of polynuclear hydrolysis products, such as M(OH)n\({}^{2+}\), which are adsorbed at solid-WW interfaces (e.g., hydrous metal oxide interfaces). It is possible that the formation of a surface complex is due to cation and anion adsorption at hydrous colloid interfaces and the establishment of ligand exchange equilibrium, e.g., the coordination/complexation of cations and anions onto amphoteric hydrous metal oxides. _(iv) interparticle bridging_: Polyelectrolytes (e.g., polymer/co-polymer/or macromolecular compounds with multiple ionizable functional groups on its chain, which are soluble in water and have macromolecular structures with flexible chains and charges) and certain polymeric hybrid materials can modify the surface of mineral solids, leading to floc formation. The use of excess polymer, prolonged agitation, or a lack of intramolecular adsorption onto solids can re-stabilize colloidal systems (Figure 4c). In practice, all four mechanisms can act on a system, but the first two mechanisms are implicated in solid particle neutralization and agglomeration, and the last two are responsible for the growth in aggregate size and/or aggregation as a flocculation process (Figure 4). In the case of using hybrid materials as coagulants/floculants, all mechanisms act together and depend on the type, composition (metal-containing species and charged functional groups of organic polymeric chains and metal salt content), WW characteristics, and imposed operating conditions. As a result, the coagulation-floculation process applied in colored WW treatment can act according to the following [71; 117; 118; 119; 120; 121; 122; 123]:  
* Attraction forces: these decrease the surface charge and enhance the aggregation of solids in distinct, separable aggregates or flocs (coagulation process). * The simple electrostatic adsorption of counterions: effectively neutralizes the solid surface charge and decreases the surface potential (dependent on ionic species or large, complex molecules, and ordinary adsorption) (coagulation-floculation process).