Preparation and conversion mechanism of different geopolymer-based zeolite microspheres and their adsorption properties for Pb2+

In this study, geopolymer zeolite microspheres (GZMs) were prepared from metakaolin by suspension dispersion solidification and in-situ conversion method. Different GZMs were prepared by adjusting n(Na2O)/n(Al2O3), n(H2O)/n(Na2O), curing time and temperature. The data showed that NaA structure was easily formed under low temperature and low alkalinity conditions, which was transformed into sodalite (SOD) with more stable structure under high temperature and high alkalinity conditions. When n(Na2O)/n(Al2O3) = 1.9 and n(H2O)/n(Na2O) = 12, the sphericity of GZMs was the best according to the sphericity and particle size distribution results. Furthermore, different ZMs were prepared by adjusting the curing temperature and time. These ZMs showed maximum adsorption capacities for Pb2+ of 529.67 mg/g (NaA), 345 mg/g (NaA + SOD: the mixture of NaA and sodalite) and 308.30 mg/g (SOD), following pseudo-second-order kinetic model and the Langmuir isotherm model. The fresh and post-adsorption GZMs were analyzed by SEM-EDX, XRD, FTIR and XPS, and the △c (Pb2+): △c (Na+) = 1: 1.92 (≈1:2) in the solition, which confirmed chemisorption phenomenon via ion exchange mechanism. Moreover, the adsorption rate of Pb2+ on NaA ZMs was higher than that of traditional zeolite materials. This study accredited to its simplicity, low cost and remarkable adsorption performance of the NaA ZMs for Pb2+, can be deemed of potential value in practical application of wastewater processing.