Adsorption of tungstate on kaolinite: adsorption models and kinetics

The adsorption characteristics of tungstate onto kaolinite have been studied using batch experiments under ambient temperature. The effects of various operating variables, viz., contact time, initial concentration, solution pH, ionic strength and competitive anions, have been investigated. The optimum contact time needed to reach equilibrium was found to be 48 h. Tungstate adsorption isotherms fitted well with both Langmuir and Freundlich models and the maximal adsorption capacity of the kaolinite sample is 35.54 mmol kg(-1). Tungstate adsorption increases in the pH range of 3-5 and reaches a maximum at an average equilibrium pH of 4.24, but decreases from 83.7% to 10.6% as the pH increases from 5.35 to 11.03. Tungstate adsorption increases from 71.9% to 89.4% as ionic strength increases from 0.001 M to 0.1 M NaCl. As the P/W molar ratio increases from 0 to 10, tungstate adsorption decreases from 24.38 mmol kg(-1) to 21.61 mmol kg(-1), while phosphate adsorption increases from -2.09 mmol kg(-1) to 27.45 mmol kg(-1). These results demonstrate that tungstate might be adsorbed onto tungstate-specific adsorption sites of the kaolinite minerals, mainly via inner-sphere complexation.