Adsorption of copper ions by fly ash modified through microwave-assisted hydrothermal process

In this study, a microwave-assisted hydrothermal process (MAHP) is used to modify municipal solid waste incineration (MSWI) fly ash. The influences of the dosage, additives, liquid-to-solid ratio (L/S), temperature, and reaction time are investigated in detail, and it is found that the modified fly ash (MFA) exhibits the highest adsorption capacity of Cu²⁺ (32.05 mg/g) when modified under the conditions of 1 mol/L Na₂HPO₄, an L/S ratio of 3 mL/g, a reaction temperature of 200 °C and a reaction time of 30 min. The cation exchange capacity (CEC) of the fly ash remarkably increases from 0.022 to 0.498 meq/g after treatment, which is an increase of approximately 22 times. X-ray diffraction results reveal the formation of zeolitic crystals in the MFA. To study the adsorption mechanism, the Cu²⁺ adsorption isotherms and kinetics are measured. The adsorption behaviors are well described by the Freundlich isotherm equation with a correlation coefficient of 0.986 and by a pseudo-second-order kinetic equation with a correlation coefficient of 0.998. Overall, utilizing MSWI fly ash as adsorbents should receive more attention, and the MAHP is considered to be a promising technology.