ABSTRACT
The objective of this work was to prepare modified adsorbents from fuller's earth (FE) by acid and alkali treatment for enhancement cationic dye adsorption. Toluidine blue (TB) was selected as adsorbate for evaluating the adsorption performance of fuller's earth samples, which was affected significantly by acid and alkali modification. The adsorption of TB was studied by visible spectra. The absorption band of the monomer at low loading of TB in FE suspension with respect to its maximum in aqueous solution is red-shifted, which is related to accessibility of dye interlamellar space in the presence of positively charged surface sites. Since all surfaces are negatively charged under experimental conditions, this effect has not been observed in acid- and alkali-treated FE suspensions. It was seen that the adsorption capacity of alkali-treated surface (FEAl) for TB was higher than these of acid-treated adsorbent (FEAc) and FE. Scanning electron micrographs (SEM) and X-ray diffraction (XRD) and fluorescence (XRF) spectra were applied to analyze the structure of the raw and modified FE samples. Absence of any identifiable amount of a crystalline compound in the solid reaction products after acid treatment was confirmed by XRD and SEM, whereas the crystalline form of FEAl was preserved. Experimental data for high-concentration regions were well described by Freundlich and Langmuir adsorption equations. The thermodynamic parameters were estimated for FE, FEAc, and FEAl by using temperature dependence of adsorption equilibrium constants.
ABSTRACT
The adsorption behavior of methylene blue (MB) on four fuller's earth (FE) samples of varying compositions was investigated using a spectrophotometric technique to obtain information on the color removal. The distribution coefficient (K(D)) increased with an increase in the initial concentration (C(0)) of the dye, attained a maximum value, and decreased again at higher initial concentrations. Dye solutions became colorless for a C(0) value corresponding to maximum K(D). A progressively increased flocculation behavior in the clay suspensions was observed and the maximum value of K(D) corresponds to optimum flocculation of the clay. The K(D) values were found to decrease exponentially after of the solution again became colored while the amount adsorbed increased with an increase in the initial concentration of MB. Only adsorption data obtained for this region could be defined by adsorption isotherm equations. The shifts of the C(0) values corresponding to K(D)(max) toward higher concentrations were correlated with the composition of FE samples by using XRF, XRD patterns, and SEM images. The influence of temperature on MB adsorption was also studied and thermodynamic parameters were calculated.
ABSTRACT
A bauxite waste of alumina manifacture, i.e., red mud (RM), is an oxide-like adsorbent capable of removing radiocesium and strontium. The adsorption behavior of these radionuclides is dominated by the surface charge of the adsorbent and the number of available adsorption sites. In this study, the surface charge densities (sigma), microscopic acidity constants (pK(s)), and site distributions of the RM in 10(-3)-1 M concentrations of NaCl, CsCl, and SrCl(2) solutions were evaluated from potentiometric titration data. The reciprocal slopes of the pH-sigma curves are higher than those predicted by double-layer theory. This suggests that surface charge and the counter charge are located in a region inside the surface because the porous and/or gel surface layer is permeable to these ions. Ionic strength dependency of sigma in CsCl solutions is similar to those found for other oxides. In SrCl(2) and NaCl solutions, at any pH the surface charge decreases as the electrolyte concentration increases. This behavior of the RM may be attributed to the existence of differently charged oxide surface sites of variable affinity for electrolyte ions. Uptake of protons on these sites could be interpreted in terms of H(+) adsorption and well described by the Freundlich equation. The empirical Freundlich parameters were used to characterize a site distribution function which provides information about the affinity ratio of the adsorption sites to H(+) and supporting electrolyte cations. Copyright 2000 Academic Press.
