Dye removal from aqueous solution using coal fly ash for continuous flow adsorption

Coal fly ash was characterized at first in batch adsorption with acid and basic dyes. Good results of removal were obtained, although strongly dependent on initial dye concentration, ash dosage, and above all on pH of the solution. The maximum adsorption capacities per g of fly ash were 410 mg of Acid Blue 25 and 142 mg of Basic Blue 9, both with an ash dosage of 2 g/L at pH 11 after a contact time of 1 h. However, increasing the ash dosage to 20 g/L and contact time to 24 h, the maximum adsorption capacity of Basic Blue 9 raised 187 mg/g fly ash at pH 11, but this value was lowered to 8.4 mg/g at pH 7. Afterwards, flow experiments were carried out in semi-batch mode with a fixed amount of ash and continuous flow of dye solution in continuous stirred tank reactor, to determine the exhaustion curves of fly ash. The best fit of the results was obtained by a sigmoidal function referable to the breakthrough curve model of Yoon-Nelson. Successively, to reduce the alkali leaching by solution flow, in a semi-pilot scale plant, a continuous stream of dye solution was mixed in stirred tank reactor with a continuous ash slurry supply, and the resulting slurry was decanted in a settling tank to obtain a clarified supernatant stream outlet. In this last equipment, the optimum conditions were investigated to obtain the highest yields of dye removal from solutions at low concentrations (5–20 mg/L) such as those occur in rinsing waters arising from dyeing processes.