Improved holding and releasing capacities of coal gangue toward phosphate through alkali-activation

Activation is an effective method to improve adsorption capacity of coal gangue, however, most activated gangues reported have limited adsorption capacity for phosphate because of the electronegativity, affecting their use in agricultural production and environmental protection. In order to enhance the phosphate absorption capacity of coal gangue through alkali-activation, three alkali-activated silicate coal gangue according to fine-size (AS-FCG¹), medium-size (AS-MCG²), and large-size (AS-LCG³) were prepared through grinding, calcining (800 °C) and stimulating with Ca(OH)₂ solution (0.1 M), and raw coal gangue (RCG⁴) was used as control. The results revealed that AS-FCG has the best modified effects, its maximum phosphate adsorption capacity, according to Langmuir model, was 11.796 mg g^-1, which was 4.41, 8.16 and 73.73 times higher than that of AS-MCG, AS-LCG and RCG, respectively. The adsorption penetration time of AS-FCG packed column was over 30 h, which was longer than that of other samples. Besides, when the AS-FCG packed column loaded with phosphate was desorbed with distilled water, the phosphate concentration of filtrate was low and rapidly tended to 0 mg L^-1, while when it was desorbed with NaHCO₃ solution, the concentration remained high sustainably. The best adsorption performance of AS-FCG were associated with generation of calcium silicate hydrate gel and ettringite, which provided more Ca²⁺, Al³⁺, hydroxyl and larger specific surface area. It is suggested that alkali-activation is able to truly realize the recycling of solid waste, AS-FCG is an inexpensive, durable and eco-friendly material, which could not only be used to hold phosphate, but also release it slowly as fertilizer.