Ion exchange to immobilize Cd(II) at neutral pH into silicate matrix prepared by co-grinding kaolinite with calcium...

A novel silicate-based composite material was simply prepared by co-milling kaolinite and calcium compounds to endow the well studied clay minerals with active calcium for efficient removal of heavy metals. Batch experiments were carried out to investigate the main affecting factors such as raw material ratio, ball milling time, contact time, etc.. Even at a neutral solution pH, the silicate adsorbent exhibited excellent performance for the adsorption of Cd(II), reaching equilibrium in 30 min with a removal efficiency over 95%, and allowed a direct discharge of the treated solution without the need of acidic neutralization as usually used in the alkaline precipitation. A set of analytical methods including SEM/EDS and ²⁹Si MAS NMR etc. were used to analyze the adsorption mechanism of Cd(II), revealing that the adsorption process was mainly dominated by ion exchange to accommodate Cd ions inside silicate matrix, accompanied with partial hydroxide precipitation, rather than normally reported surface adsorption on pristine minerals. Furthermore, the as-prepared adsorption material exhibited similar excellent immobilization capacity for multiple heavy metals including Cu(II), Zn(II), Ni(II), Cd(II) and Mn(II). These findings provide a novel concept for the activation of the widely available cheap silicate minerals by the same widely available cheap calcium compounds and high contribution may be expected on its potentials to the environmental purification of heavy metal pollution in water and soil.