| Textile dye wastewaters contain high concentrations of pollutants such as colors, organics and heavy metals, which require extensive treatment prior to discharge. Removal of color and organic pollutants from the wastewaters by conventional techniques is difficult and costly.; In this study, a new and efficient technology was developed for simultaneous removal of color, organic substances and heavy metal pollutants from textile dye wastewaters. The process consists of three basic steps: (1) hydrophobic precipitation of ionic dyes by cationic amine surfactants; (2) self-association or flocculation using polymers to form large size aggregates; and (3) separation of the dye aggregates from solution. It was shown that for all the dye solutions studied, more than 99% color, 90-95% total organic carbon, and 95-99% metal ions were removed using the newly developed process. Results obtained on actual dyeing wastewater showed that the new process efficiently removed more than 99% of color and 90-95% of organic carbon and heavy metals.; Several different types of cationic surfactants were evaluated to remove dyes, including monoamine, diamine, triamine and tetramine. Several synthetic dyes were selected for the study including acid, direct, reactive, and mordant dyes. The extensive fundamental studies on the new process showed that chemical reaction, charge neutralization, electrostatic interaction, and hydrophobic coagulation mechanisms are involved in the process. A stoichiometric amount of cationic surfactant to dye is required to initiate the hydrophobic precipitation process and to obtain the optimal removal of colors. The surface charge plays an extremely important role in the formation of large and rigid dye aggregates. The zeta potential range for efficient precipitation of dye was found to be from {dollar}-{dollar}30 mV to +60 mV. Using the principles of surface and colloid chemistry, it was found that the stability of dye flocs was mainly determined by the hydrophobic attraction and the electrostatic repulsion. The hydrophobic/hydrophilic characteristic of the surfactants was an important parameter in controlling the effectiveness of the process. With the addition of small quantity of polymers, the size of final dye-surfactant flocs/aggregates can be significantly increased from submicrons to several hundred microns, thereby making it easy to separate the dye flocs from the solution by simple physical methods such as filtration or flotation. |
