Adsorption experiments were carried out in 10 mL centrifuge tube containing appropriate volume of deionised water (18 MΩ cm), 30 mg of kaolinite or the composite of kaolinite (30 mg) and EPS. The suspensions of kaolinite and the composite of kaolinite + EPS were incubated on a shaker for 30 min with a speed of 120 rpm, and then appropriate mount of Cd(II) and supporting electrolyte (KNO3, final concentration 0.01 M) were added into centrifuge tube, and the total volume was brought to 6 mL using deionised water. The centrifuge tubes were agitated on a shaker at a speed of 120 rpm for 4 h (reached equilibrium). The pH value was set as 7 except pH experiments, and the temperature was set as 25 °C except temperature experiments. In order to study the effect of EPS concentration on the adsorption of Cd(II) by kaolinite, the final EPS concentration was set as 0.1, 0.3, 0.6, 1, 2, and 3 g/L, and the initial Cd(II) concentration was set as 5 mg/L. In pH experiments, the pH value was set as 5, 6, 7, and 8, respectively, and the initial Cd(II) concentration was also set as 5 mg/L. For determination of the effect of initial Cd(II) concentration on its adsorption by kaolinite and the composite of kaolinite + EPS, the initial Cd(II) concentration was set from 5 to 500 mg/L, and the final EPS concentration in the treatment of kaolinite + EPS was set as 0.6 g/L. In temperature experiments, the temperature was set as 20, 25, 30, 35, and 40 °C, respectively. After equilibrium, the suspensions were centrifuged at 12,000×g for 10 min, and the Cd (II) in the supernatant was determined using atomic absorption spectrometer (Varian Techtron Pty. Ltd., Victoria, Australia). The amount of adsorbed Cd(II) was calculated from the differences between the initial Cd(II) concentration and the residual concentration after sorption. In order to study the adsorption isotherm of Cd(II) by kaolinite and the composite of kaolinite + EPS, adsorption data were fitted using Langmuir model and Freundlich model in linear form (Eqs. 1 and 2), respectively.
$$\frac{1}{{q_{e} }} = \frac{1}{{q_{\text{max} } K_{L} C_{e} }} + \frac{1}{{q_{\text{max} } }}$$
(1)
$$\ln q_{e} = \ln K_{f} + \frac{1}{n}\ln C_{e}$$
(2)
where q
e
is the amount of adsorbate absorbed by adsorbent, C
e
, the equilibrium concentration, q
max, the maximum adsorption capacity upon monolayer saturation adsorbent, K
L
, the constant related to the adsorption energy, and the K
F
and n are Freundlich parameters involved in the relative adsorption capacity and the affinity between adsorbent and adsorbate, respectively.
X-ray diffraction and Fourier transform infrared spectroscopy analysis
The crystal structures of kaolinite and the composite of kaolinite + EPS were recorded using a XRD-6000 instrument (Shimadzu Seisakusho Ltd., Japan) employing graphite monochromatized Cu Kα radiation, with scanning rate of 4°/min and ranging from 5° to 75°. Fourier transform infrared (FT-IR) spectra of kaolinite and the composite of kaolinite + EPS were obtained on a spectrometer (PerkinElmer Spectrum 65, Perkin-Elmer Co., Norwalk, CT, USA).
Desorption of Cd(II)
Desorption of Cd(II) from the kaolinite and the composite of kaolinite + EPS was performed using deionised water or NH4NO3 or EDTA as desorbent according to the methods previously described by Fang et al. (2011).
Statistical analysis
Statistical analysis was done by one-way ANOVA using SPSS (version 13.0, SPSS Inc., Chicago, IL, USA). Difference was considered to be significant at P < 0.05 (LSD).
