Utilization of alkaline silicate wastes for removal of cadmium ions from aqueous solution: Comparative performances and removal mechanisms

In this work, alkaline silicate wastes including blast furnace slag (BFS), coal fly ash (CFA), straw ash (SA) and lithium slag (LS) were used to remove Cd2+ from aqueous solution. The removal performances of Cd2+ by silicate wastes were compared in batch experiments, and the removal mechanisms by SA were proposed. All of four silicate wastes showed high acid neutralization capacity with solution pH increasing from 2.0 at initial to 6.27–9.70 at equilibrium. The removal of Cd2+ by four wastes followed the pseudo‒second‒order kinetic model and the equilibrium data were well described by the Langmuir isotherm. The maximum removal capacity of Cd2+ for four wastes had the sequence of SA > BFS > CFA > LS. The SA with the maximum removal capacity of 88.75 mg/g was the best adsorbent for Cd2+ removal. The XRD tests showed that Cd2+ could be chemically precipitated as Cd(OH)2 on the SA surface. The FTIR and XPS analysis exhibited that oxygen functional groups of SA could undergo surface complexation with Cd2+. Hydration products such as calcium silicate hydrate derived from SA could immobilize Cd2+ ions through ion exchange. In summary, the removal of Cd2+ by straw ash had a complicate involvement of chemical precipitation, ion exchange and surface complexation with oxygen functional groups.