Adsorption of anionic dyes onto natural, thermally and chemically modified smectite clays

The aim of this study was to determine the adsorption capacity of the smectite clays (from the overburden of the lignite deposit in Belchatow) for two anionic dyes, i.e. Reactive Blue 81 (RB-81) and Direct Blue 74 (DB-74). Additionally, the infl uence of the thermal and chemical (acid and alkali) clay modifi cations on the amount of bonded dyes was investigated. The adsorption capacity of the clay (natural and modifi ed) was different for studiem dyes and depended on the initial concentration and modifi cation type. All the modifi ed clays adsorbed the dyes AT pH>pHPZC as the negatively charged surfaces of their particles (in accordance with the formula: AOH ↔ AO– +H+) prevented the formation of electrostatic bonds between the anionic dyes and the clay surface. The dyes were mainly bound with the hydrogen bonds forming between the donor groups in the dyes and the acceptor groups (–SiO and –Al2OH) in the clays. The coeffi cients in the adsorption isotherms were estimated with the linear and non-linear regression. The linear regression method was found that the Freundlich and Dubinin-Radushkevich isotherms described the dye sorption much better than the Langmuir model. On the other hand, all three models described well the experimental data in the non-linear regression method. Furthermore, the 1/n value (<1) obtained from the Freundlich equation for all the dye-sorbent systems indicated the favorable sorption.