Eco-utilization of hierarchically structured calcium silicate hydrate for efficient removal of cadmium from synthetic wastewater

Developing a low-cost and high-performance adsorbent to eliminate cadmium (Cd) pollution in farmland irrigation water is highly desired. Herein, a hierarchically structured calcium silicate hydrate (CSH) material was prepared from the alumina-extracted residue (AR) of coal fly ash to remove low concentration of cadmium [Cd(II)] (0.1 mg/L) from simulated farmland irrigation water. The results indicated that CSH with different Si/Ca ratios exhibited excellent adsorption performance with a removal efficiency of over 90 %. Among them, CSH with Si/Ca ratios of 0.83 (CSH-0.83) had the highest removal rate, reaching 99.38 %. Under the optimum operation conditions of adsorbent dosage of 0.04 g/L, pH value of 4.0, contact time of 120 min, and initial concentration of 0.06 mg/L, the maximum adsorption rate of Cd(II) was 99.42 %. The adsorption isotherms and kinetics results showed that the adsorption and desorption process of Cd(II) by CSH-0.83 could be well fitted by pseudo-second-order (R2 = 99 %) and its adsorption isotherms accorded with the Langmuir model (R2 = 98 %). The adsorption mechanism was mainly chemosorption. The released calcium (Ca2+) and hydroxyl (OH−) from the CSH-0.83 reacted with Cd(II) via complexation, or Cd(II) underwent ion exchange with Ca2+ adsorbed on the surface or edge of the particle. Furthermore, CSH-0.83 exhibited good stability and regeneration capacity, and its adsorption efficiency remained at 86.45 % in the sixth cycle. This study provides new ideas for high-value utilization of similar solid waste of aluminosilicates.