Steam- and carbon dioxide-gasification of coal combustion ash for liquid phase cadmium removal by adsorption

This paper is collocated after two other works in which sieved or demineralized coal combustion ashes were used as cadmium (Cd2+) sorbent for water treatments, and the changes in raw ash properties were followed upon steam- and CO2-gasification. In the present paper, ash samples activated by steam- or CO2-gasification were used to adsorb Cd2+ in model aqueous solutions, to elucidate the relationships among ash properties, gasification treatments, properties of the gasified ash and its sorption behavior. To this end, equilibrium and kinetic cadmium adsorption tests were carried out using the gasified ash (treated with either steam or CO2) as sorbent and under fixed operating conditions (for example, the contact time was 7 days and the initial Cd2+ concentration in the aqueous solution to be treated varied between 5 and 50 mg L−1 for equilibrium tests, while kinetic tests were carried out for times ranging from 10 min to 7 days using an initial Cd2+ concentration equal to 50 mg L−1). Results were expressed in terms of adsorption isotherms, removal efficiency and kinetic curves. Post-processing of experimental data allowed to provide estimations of relevant parameters such as the equilibrium and the kinetic constants, the separation factor, the initial adsorption rate, the characteristic adsorption time. Besides a direct confrontation between the two gasification treatments, results were also compared with former experiments on the same raw material, and with the main outcomes emerging from the pertinent literature, confirming that the gasified materials resulted as very attractive sorbents for Cd2+ capture from wastewaters.

• Coal ash beneficiation by gasification was investigated by tests on Cd adsorption.
• The CO2-gasified ash appeared as a better Cd sorbent than the steam-treated ash.
• Both treatments were able to improve the sorbent characteristics.
• Gasified ashes showed higher removal efficiencies at low pollutant/sorbent ratios.
• An analysis concerning the involved processes characteristic times was developed.