The adsorption behavior of four anionic dyes and one disperse dye in single solution and binary solutions on fly ash was investigated in order to elucidate the effect of competitive adsorption on the kinetics. The experimental findings showed that adsorption equilibriums of four anionic dyes were reached within 50 min either in the single solution or in binary mixtures. Competitive adsorption increased the time of attaining equilibrium of disperse dye. Desorption of dyes suggested the predominant adsorption mechanisms, that is, chemisorption for anionic dyes and physisorption for disperse dye. For the binary mixtures, the anionic dyes could be adsorbed preferentially on fly ash at the first stage. Second-order kinetic models fitted better to the equilibrium data of all dyes in the single solution as well as in the binary mixtures. The maximum rate constant of intraparticle diffusion and the minimum external mass transfer coefficient was found for disperse dye both in single and in binary solutions. The intraparticle diffusion constants and external mass transfer coefficients of the four anionic dyes in binary solution are similar to those obtained in single solution. The Biot number confirmed that the intraparticle diffusion was the rate-limiting step in the dye sorption process.

Raw bentonite (RB), a known low-cost versatile clay was used as an adsorbent. RB was treated thermally and chemically to increase its adsorption capacity. For thermal treatment (TTB), the bentonite was heated at 400 ° C for 60 min, and for the chemical modification, its surface was treated by cetyltrimethylammonium bromide (CTAB) to prepare organo-modified bentonite (CTAB-B). The removal of Congo red dye (CR) from aqueous solution was investigated in the batch mode. The prepared adsorbents were characterized by SEM, BET, and FTIR analyses. The effects of various experimental parameters such as contact time, pH, adsorbent dosage, dye concentration and temperature were investigated. The obtained results were in good agreement with the Langmuir isotherm model, and the maximum adsorption capacity of RB, TTB and CTAB-B was 43. 1, 55. 86 and 116. 28 mg/g, respectively. The adsorption kinetic was better described by the pseudo-second order kinetic model. The results showed that thermally or chemically modified bentonite could be proposed as a low-cost adsorbent for the removal of CR from water.

The interaction of Congo Red (CR) with a series of Alkyl trimethyl ammonium bromide (CnTAB), N-hexadecyl pyridinium bromide (CPB) and N-hexadecyl pyridinium chloride (CPC) were investigated using conductometry and UV-Vis spectroscopy technique. Job’s method was used to determine the stoichiometric ratio of dye and surfactant in ion pairs. The equilibrium constant and other thermodynamic parameters for the ion pair formation were calculated on the basis of a theoretical model using the data obtained by conductometry. It was found that an increase in temperature reduces the tendency for ion pair formation as equilibrium constants decrease with increasing temperature. The results showed that as the chain length of the CnTAB molecule in Dye/CnTAB systems was increased, the equilibrium constants were significantly increased. Therefore, the short range as well as long range interactions is responsible for the formation of the ion pair. The importance of long range electrical forces is basically to bring the dye anion and the surfactant cation close enough to enable the action of short range interactions whose contribution represents the major part of the standard free energy change for the formation of the anionic dye-cationic surfactant ion pair. The reaction between the dye and the surfactants was revealed to be an enthalpy-driven reaction which is highly dependent on the temperature and the structures of both dye and the surfactant.

The present work describes the adsorption of the anionic dye Congo Red (CR) by materials based on Activated Montmorillonite (AM), graphene oxide (GO), and gelatin (G). The materials were prepared and characterized by X-Ray Diffraction (XRD), infrared spectroscopy (FT-IR), and thermal analysis (ATG/DTG) in the previous study. Adsorption experiments of CR dye on GO/AM at different ratios of GO were performed to evaluate the adsorption efficiencies. The maximum adsorption capacity of anionic dye (CR) onto (GO / AM10%) composite was insufficient. To improve the adsorption capacity, a cationic-charged component such as gelatin was added to obtain a new composite (GO/AM/G). Therefore, the effects of several factors on the adsorption capacity of (GO/AM/G) composite, such as the pH of dye solution, adsorbent dosage, contact time, initial dye concentrations, temperature, and regeneration, were investigated. In addition, the kinetics of dye adsorption followed the pseudo-second-order model, and the adsorption isotherm was very well described by the Freundlich model. Afterward, the study of the temperature’s effect on the adsorption rate indicated that the reaction was exothermic with the medium disorder. The values of the free energy showed that the nature of the adsorption was physisorption. The reusability of the composites using 0.1N HCl for over six cycles indicated the economic significance of these materials as adsorbents. The fast removal rate in a wider range of pH and the easy reusability and regeneration make the composite (GO/AM/G) a prospective material for dye adsorption from aquatic environments.

Polypyrrole (PPy) and polyaniline (PAni) conducting polymers were synthesized chemically on the surface of wood sawdust (SD) to remove Eosin Y (EY), an anionic dye, from aqueous solution. The important parameters such as pH, initial concentration, sorbent dosage and contact time on uptake of EY solution were also investigated. The experiments were carried out using both batch and column systems at room temperature. It was found that both polymers can be used to remove anionic dyes such as EY from aqueous solutions very efficiently. The sorption process of EY dye using both polypyrrole and polyaniline coated onto sawdust (respectively termed as PPy/SD and PAni/SD) was less dependent on the pH of the solutions at least at pH values from 2-10. Considerable decrease in sorption performance occurred only at high pH values (pH ³ 12). According to our "breakthrough analysis", it was found that PAni/SD adsorbent is more efficient for EY removal in flow or column system. Adsorption of EY on the PPy/SD and PAni/SD under different conditions was examined by pseudo-first-order and pseudo-second-order models, and their respective rate constants of first-order adsorption (k1) and second-order adsorption (k2) were estimated. The experimental data fitted very well into pseudo-second-order kinetic model. The experimental data were also analyzed by the Langmuir and Freundlich models of adsorption. Based on the correlation coefficient values obtained (R2), it was found that equilibrium data fitted well with both models. In order to find out the possibility of desorption for frequent use, the chemical regeneration of the employed adsorbents was also investigated. It was found that more than 96% of the EY dye can be recovered from PPy/SD column using a dilute NaOH solution (0.05 M).