Sorption behaviour of arsenate by non-crystalline aluminosilicate minerals: implications for arsenic immobilisation during the disposal of alkaline coal fly ash materials

Understanding the interaction of non-crystalline aluminosilicates with arsenate has environmental implications in controlling the mobility of dissolved anionic species in circum-neutral conditions during the utilisation and disposal of coal fly ash. This study focused on the determination of mineral phases in Si-Al system formed at circum-neutral pH condition and their interaction with arsenate in relation to the effect of temperature. The coprecipitation and surface adsorption behaviour of arsenate in the mineral phases formed was examined at 25, 50 and 75 degrees C. To determine arsenic association with the minerals, leaching test using phosphate bearing solution was also conducted. The results showed that allophane-like material was formed at relatively lower Si/Al ratio of 0.67 and amorphous aluminosilicates at higher Si/Al ratio of 1.5. Allophane showed higher arsenate removal efficiency of more than 90% during coprecipitation experiments compared to amorphous aluminosilicates. Moreover, allophane and amorphous aluminosilicates showed lower arsenate desorption efficiency of less than 20%. A significant irreversible fraction of sorbed arsenate was also observed which could be attributed to inner-sphere complex formation and the possible incorporation within the mineral structure. Hence, these non-crystalline minerals can act as chemical barrier for dissolved anionic species during the utilisation and disposal of coal fly ash.