[Adsorption Properties of Fluorine onto Fulvic Acid-Bentonite Complex].

Fulvic Acid-Bentonite (FA-BENT) complex was prepared using coprecipitation method, and basic properties of the complex and sorption properties of fluorine at different environmental conditions were studied. XRD results showed that the d001 spacing of FA- BENT complex had no obvious change compared with the raw bentonite, although the diffraction peak intensity of smectite in FA-BENT complex reduced, and indicated that FA mainly existed as a coating on the external surface of bentonite. Some functional groups (such as C==O, −OH, etc. ) of FA were observed in FA-BENT FTIR spectra, thus suggesting ligand exchange-surface complexation between FA and bentonite. Higher initial pH values of the reaction system were in favor of the adsorption of fluorine onto FA-BENT, while the equilibrium capacity decreased with the increase of pH at initial pH ≥ 4.50. The adsorption of fluorine onto FA-BENT was also affected by ionic strength, and the main reason might be the "polarity" effect. The adsorption of fluorine onto FA-BENT followed pseudo-second-order kinetic model and was controlled by chemical process ( R² = 0.999 2). Compared with the Freundlich model, Langmuir model was apparently of a higher goodness of fit (R² > 0.994 9) for absorption of fluorine onto FA-BENT. Thermodynamic parameters indicated that the adsorption process of fluorine was an spontaneously endothermic reaction, and was an entropy-driven process (ΔH 32.57 kJ · mol⁻¹, ΔS 112.31 J · (mol · K)⁻¹, ΔG −0.65- −1.76 kJ · mol⁻¹).

[Adsorption Characteristics for Humic Acid by Binary Systems Containing Kaolinite and Goethite].

In this study, the binary systems of kaolinite-goethite mixture (KGM) and kaolinite-goethite complex (KGC) were prepared by different methods, and the surface properties and humic acid adsorption of the samples were investigated. Results showed that the specific surface area (SSA) of the samples followed the order of goethite> KGC> KGM> kaolinite, and the SSAs increased significantly for KGC while slightly for KGM when compared to the average value of kaolinite and goethite. The isoelectric point (IEP) of kaolinite, goethite, KGM and KGC appeared around 3.2, 7.9, 6.1 and 6.7, and the Zeta potential at pH 5.0 was -13.9, 38.2, 14.3 and 19.7 mV, respectively. The adsorption kinetic data for humic acid were well fitted using the pseudo-second-order kinetic models, suggesting that chemisorption was important in the adsorption process. Both one-site and two-site Langmuir models were suitable to describe the isotherm adsorption data (R² 0.962-0.993), and the correlation coefficients of two-site model for the binary systems were relatively higher (R2>0.989). The R2 values of Freundlich model fiting the adsorption data were low for the four samples, especially for the two pure samples. This indicated that the adsorption with various sites and mono-layer model was important in adsorbing humic acid onto the binary systems. At the initial pH of 5.0, the adsorption capacity (qmax) of kaolinite, goethite, KGM and KGC was 6.02, 61.83, 35.13 and 42.10 mg·g^-1, respectively. The qmax values of KGC and KGM increased to different extents when compared to the average of kaolinite and goethite. Thermodynamic parameters indicated that the adsorption of humic acid were endothermic for the four samples and non-spontaneous for kaolinite while spontaneous for the other samples.