Batch adsorption experiments

Stock Co²⁺ solution was prepared by dissolving appropriate amount of CoCl₂·6H₂O in distilled water. Batch adsorption experiments were conducted under different conditions: neutral pH (6.8–7.5), room temperatures, initial Co²⁺ concentrations (10–300 mg·L^-1), and contact time (0.5–36 h). The pH of Co²⁺ containing solution was adjusted by HCl (aq) and NaOH (aq). 0.05 g of the adsorbent was added into a 50 mL flask containing 25 mL of Co²⁺ containing solution, and the flask was agitated in water bath for a period of time. After that the mixture was centrifuged and atomic adsorption spectrometry (AAS) (Japan, Z-2000) was used to determine the concentration of Co²⁺ in the supernatant. To prevent any risk of metal contamination, all the flasks and tubes were presoaked in HNO₃ for 24 h, washed strongly with distilled water and then dried in an oven. The desorption of Co²⁺ from Mt and APTES-Mt were determined. Mt and APTES-Mt after the adsorption experiment were mixed with 25.0 mL deionized water and agitated for 36 h to allow desorption of Co²⁺ to occur.

The adsorption capacity q_e (mg·g^-1) is calculated according to the following equation.

Where V (mL) is the volume of Co²⁺ solution, C₀ and C_e (mg·L^-1) are the initial and equilibrium concentrations, respectively. m (g) is the mass of the adsorbent. All experiments were conducted in duplicate.