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The effectiveness of gibbsite (GB), an amorphous aluminum oxide, for the recovery of Mo(VI) from eluates of fly ash of two coal-fired thermal power stations and of roof tile waste was investigated. Upon the qualitative analysis of an eluate of fly ash, 16 elements were detected. Greater amounts of these elements were eluted under acidic conditions (pH 2) than from the neutral or basic eluate of fly ash. GB was used for the adsorption of Mo(VI). Equilibrium adsorption was reached within 1 min. Optimal solution acidity for the adsorption of Mo(VI) onto GB400 (calcined at 400 C) was pH 2. The main adsorption mechanism was ion exchange with a number of hydroxyl groups of GB400. For repeated ad-and desorption of Mo(VI), GB400 could be used at least four times and the recovery percentage of Mo(VI) with sodium hydroxide solution as eluent surpassed 90%. Our results showed that GB400 was very effective for the recovery of Mo(VI) from fly ash.

In the present study, calcined gibbsite (GB) was prepared by calcination treatment between 200 and 1000 • C (GB200-GB1000). The properties of GBs (i.e., the amount of hydroxyl groups, specific surface area, mean pore diameter, and pore volume) were evaluated. GB400 had the highest amount of hydroxyl groups (0.46 mmol/g), as well as the highest specific surface area (295.3 m 2 /g) and micropore volume (0.036 mL/g). It also provided the highest amount of the adsorbed Pt(IV) and Pd(II), which suggests that the adsorption of Pt(IV) and Pd(II) onto GB is related to the amount of hydroxyl groups, specific surface area, and micropore volume. The optimal pH values for the adsorption of Pt(IV) and Pd(II) onto GB400 were 4.5-5.0 and about 6.0, respectively. The equilibrium adsorption of both cations onto GB400 was reached within 30 min. Moreover, the experimental data of the adsorption of Pt(IV) and Pd(II) onto GB400 was fitted by pseudo-second-order model (r = 0.997-0.999) and the Langmuir equation (r = 0.934-0.953). The calculated thermodynamic parameters ∆G, ∆H, and, ∆S indicate that the adsorption of Pt(IV) onto GB400 is feasible, spontaneous, and exothermic. In addition, the amount of Pt(IV) and Pd(II) adsorbed onto GB400 decreased with increasing chloride ion concentration.

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