Removal of heavy metallic ions from the aqueous solution using raw and modified coal ash

The purpose of the present investigation is to explore the capability of Indian coal ash as a low-cost adsorbent for the adsorption of Cu2+, Cd2+, and Pb2+ metallic ions present in wastewater. It is observed that 5.14, 5.04 and 15.89% of Cu2+, Cd2+, and Pb2+ are removed using coal fly ash samples, which sharply increases to 99.8, 100 and 27.68%, when the same fly ash is treated with calcium. The removal of Cu2+, Cd2+, and Pb2+ ions increases from 41.3 - 99.9%, 79.1 - 100%, and 5.9 - 41%, respectively, when Ca-CFA dosage varies from 1000-50000 ppm. These metallic ions adsorb on the silanol sites of raw coal ash. When calcium-treated coal ash (Ca-coal ash) is used, some metallic ions get precipitated as metal hydroxides, and the rest is adsorbed on calcium silicate hydrate (CSH) in the form of metal silicate. Moreover, the metal removal rate also enhances from 0.99-5.14%, 1.31-5.04%, and 6.49-15.89% for Cu2+, Cd2+, and Pb2+ ions, respectively, when fine coal fly ash (d50 = 3.07 μm) is used instead of coarse bottom ash (d50 = 37.87 μm). The pseudo-first-order and pseudo-second-order kinetics models describe the experimental data reasonably well.