Building high-capacity mesoporous adsorbents for fluoride removal through increased surface oxygen anions using organogel-assisted synthesis

Abstract

In this work, mesoporous calcium-based adsorbents are prepared with the assistance of an organogel, 1,3:24-Bis (3,4-dimethylobenzylideno) sorbitol (DMDBS), which can self-assemble into nanoscale fibrillar networks and act as soft template to create mesopores. Particularly, it is found that the specific surface area of adsorbent is increased about 25 times by adding an extremely low concentration of DMDBS template (0.4 wt%) during the adsorbent production, which is much more efficient than common template materials. Additionally, the physical property and adsorption capability of calcium-based adsorbents can be tuned by control over calcination temperatures and DMDBS loadings. Under optimal preparation conditions (700 °C and 0.4 wt% DMDBS), the synthesized mesoporous adsorbent can achieve a Langmuir adsorption capacity of 48.8 mmol g−1 at pH of 7 and a removal efficiency of 70% in a mimicking natural water with the existence of NaCl. Thermodynamic analysis of equilibriums indicates that the adsorption reaction of fluoride with synthesized adsorbents from water is endothermic and spontaneous in nature. Most significantly, the results demonstrate that adsorbents with surface mesopores possess enriched ionized surface hydroxyl groups, CaO-, which act as the dominant species for fluoride attraction via electrostatic force at a neutral environment. This finding can be supported by the zeta potential analysis and surface speciation calculation. Moreover, the fluoride adsorption capacities of DMDBS-synthesized adsorbents are stable over neutral pH range of 6–8, revealing a promising potential for practical applications in fluoride-contaminated groundwater.