Properties of novel adsorbent produced by hydrothermal treatment of waste fly ash in alkaline solution and its capability for adsorption of tungsten from aqueous solution

• FA48, FA60, and FA72 consisted of phillipsite, zeolite X, and zeolite A, respectively.
• The amount of tungsten adsorbed onto FA48 was greater than that onto another FA.
• Adsorption mechanism was related to not only chemical but also physical interaction.

Zeolites were produced by hydrothermal treatment of fly ash for 48, 60, and 72 h (referred to as FA48, FA60, and FA72) in alkaline solution. Physical and chemical analyses were performed on the FA samples. Moreover, the amount of tungsten adsorbed on the FA samples, derived from the adsorption isotherms under different pH conditions, and the effect of the contact time on the adsorption were evaluated. Parent FA consisted primarily of mullite crystals, whereas the hydrothermally treated FA48, FA60, and FA72 samples consisted of phillipsite, zeolite X, and zeolite A, respectively. The specific surface areas and pore volumes of parent FA were smaller than those of FA48, FA60, and FA72. FA48 generated the largest specific surface area and pore volume. The saturated amount of tungsten adsorbed on FA48 was greater than that of the other FA samples. Tungsten adsorption was more effective (larger amount) at pH 2.0–3.0 than at pH 6.1–6.5 or pH 11.0–12.0. These results suggest that the tungsten was adsorbed on the surface of FA48 through interactions between the electrons of the positively charged FA48 surface and the tungsten anions in solution. Analysis of the equilibrium adsorption data using the Langmuir and Freundlich equations showed that the correlation coefficient of the Freundlich isotherm was higher than that of the Langmuir model. The data obtained in this study fit more adequately to the pseudo-second-order model than the pseudo-first-order model. Collectively, these results suggest that FA48 is prospectively useful for the adsorption of tungsten from aqueous solutions.

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