Surface charge of mesoporous calcium silicate and its adsorption characteristics for heavy metal ions

Mesoporous calcium silicate (MCS) was synthesized using the ultrasonic-assisted template method by using calcium nitrate tetrahydrate and sodium metasilicate nonahydrate as raw materials and cetyltrimethylammonium bromide–methenamine as the template. X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, and Brunauer–Emmett–Teller method were used for the characterization of MCS. The surface charge characteristics of MCS and its adsorption characteristics for Ni2+, Cu2+, Zn2+, Pb2+, Mn2+, and Cd2+ were also investigated. MCS formed through the accumulation of thin flakes with various shapes and exhibited a slit-pore structure and uneven surfaces with a specific surface area of 239.41 m2 g−1 and a pore size of 11.74 nm. MCS possessed a distinct, wide, near-zero potential point pH range of 5–9, and the point of zero charge, pHPZC, reached 8.6 ± 0.1. The MCS surface was alkaline, and MCS exhibited excellent acid–base buffering capacity at pH 5–11. The high adsorption capacity of MCS for the six heavy metal ions at pH 5.0–7.5 followed the order Pb2+ > Cd2+ > Zn2+ > Cu2+ > Ni2+ > Mn2+ (in mgg−1). The adsorption for the six heavy metal ions occurred rapidly and attained equilibrium within 120 min. The order of adsorption activation energy was Mn2+ > Zn2+> Ni2+ > Pb2+ > Cu2+ > Cd2+. The adsorption processes of the six heavy metal ions were all endothermic, entropy increasing, and spontaneous. MCS remained stable during adsorption, but the adsorbed heavy metal ions underwent transformation, especially Cu2+, Pb2+, and Cd2+, which formed new phases. The adsorption mechanisms included physical adsorption, chemical adsorption (particularly surface complexing adsorption and surface precipitation), and ion exchange. MCS exhibits an excellent adsorption and recycling performance and the potential to become an adsorption material with excellent properties for heavy metal ions.