Adsorption performance of phosphate in water by mixed precursor base geopolymers.

Design of low-cost and high-removal efficiency phosphate adsorbents is of great significance for the control of eutrophication. In this study, fly ash and metakaolin were used as raw materials to evaluate the capability of phosphate adsorption and to investigate the mechanism of phosphate adsorption. The results of comparing the adsorption effect of geopolymers prepared with different modulus of alkali activator showed that the removal efficiency of phosphate in water at 0.8 M was on average 30.33% higher than 1.2 M. Therefore, FA + MK-0.8 had the maximum removal efficiency of 94.21% for phosphate in water with the maximum adsorption capacity of 36.02 mg/kg. In addition, the adsorption of phosphate could be well fitted by pseudo-second-order model and the process was mainly controlled by film diffusion. The alkali activation process can destroy the octahedral structure of the raw material, so the geopolymer are mainly tetrahedral in structure. Interestingly, new zeolite structures were formed in the mineral crystal phase of FA + MK-0.8, which may facilitate the adsorption of phosphate by geopolymers. Furthermore, the combined FTIR and XRD analysis results indicated that the underlying mechanisms of phosphate adsorption were electrostatic gravitation, ligand exchange, and surface complexation. This research not only synthesizes low-cost and high removal efficiency wastewater purification materials, but also provides a promising application for the elimination and resource utilization of industrial solid waste.

Influence of modulus of alkaline activator on the removal of Pb2+ by mesoporous geopolymer adsorbent.

In this study, we synthesised metakaolin-based mesoporous geopolymer adsorbent and investigated the effect of alkaline activator modulus (molar ratios of SiO2/Na2O) on Pb2+ adsorption. The geopolymer prepared using 1.2 M alkaline activator performs excellent Pb2+ removal with a maximum adsorption capacity of 172.71 mg g-1. The pseudo-second-order model fit the adsorption kinetics satisfactorily, indicating that the adsorption process is dominated by chemical adsorption. The adsorption data appropriately fit the Langmuir isotherm model. The contributions of adsorption methods corresponding to the total quantity adsorbed declined in the following order: EDTA extraction (formation of Pb aluminium oxide and Pb-containing amorphous materials) > residual fraction (Pb stabilisation in the tetrahedral aluminosilicate network) > ion exchange. Overall, the alkaline activator modulus significantly influenced the Pb2+ adsorption characteristics of the geopolymer adsorbent.