The effect of micro- and nano-?-Al2O3 on the synthetic hydrotalcite and Congo red adsorption: Site energy distribution

It is significant to reveal the structure and properties of micro-nano reactants and their relationships in preparing high-performance materials. In this study, the effect on the structures and Congo red (CR) removal properties of synthetic hydrotalcite (HT) by micro & gamma;-Al2O3 (MA) and nano & gamma;-Al2O3 (NA) (named HT-MA and HT-NA, respectively) were investigated. FTIR, XRD, TGA-DTG, SEM-EDS, and BET, amongst others, were used to analyze the morphological and structural differences between HT-MA and HT-NA. The effects of synthetic HT addition, initial pH, CR initial concentration, contact time, and other factors on CR adsorption were also investigated. The results showed that the CR adsorption efficiency of HT-NA was higher than that of HT-MA under the same conditions and was not affected by pH in the range of 4-12. Kinetic analysis indicated that the adsorption process fits well with pseudo-second-order and intraparticle diffusion models, and the isotherm curves fit better with the Freundlich and Dubinin-Ashtakhov models, suggesting a heterogeneous porous multilayer adsorption mechanism. The site energy distribution analysis results further elucidated that nano & gamma;-Al2O3 was more favorable to increasing the number of high-energy sites and a stronger heterogeneous surface interaction of synthetic HT, thus improving CR adsorption efficiency. HT-NA and HT-MA showed good regeneration performances after being recycled four times and are expected to be used in wastewater treatment.