Effect of auxiliary substances on the adsorption of anionic dyes on low-moor peat

The effect of auxiliary substances (NaCl, Na2CO3, and CH3COOH) used in real dyeing processes on the adsorption capacity of low-moor peat for reactive dyes (Reactive Blue 19, Reactive Blue 81, Reactive Black 5) and acid dyes (Acid Black 1, Acid Blue 9) was investigated. The adsorption of the dyes was studied as a function of dye concentration (1-1,000 mg L-1) and the properties of peat, using a batch method. The experimental data were analyzed using the Langmuir, Freundlich, Dubinin-Radushkevich, and Sips isotherm models. The effect of auxiliary substances depended on the dye-adsorbent system - the lower the binding ability of the dye on the peat, the greater the effect of the substances. The maximum dyes adsorption capacity and the affinity of the binding sites increased in the presence of auxiliaries in the solution. The presence of CH3COOH was an important factor since H+ in the solution affected the peat surface charge by protonation reaction and the possibility of binding anionic dyes via electrostatic interaction. Only CH3COOH at low pH affected the peat surface charge because of the very good buffering capacity of peat. NaCl and Na2CO3 increased the intermolecular forces as well as the dimerization process of reactive dyes in the solution. The Sips equation fitted the experimental results best. This model combines the Langmuir and Freundlich equations, suggesting that the mechanism of dyes binding is more complex and does not follow an ideal monolayer adsorption.