Role of surface chemical design in sorption specificity of functionalized silica gels

One of the most important tasks of environmental chemistry is the development of effective methods for the extraction and chemical analysis of highly toxic oxyanions, such as nitrate, phosphate and arsenate. They enter the environment, raw materials and commercial products with waste from chemical and metallurgical industries. This problem can be solved by synthesizing selective materials that absorb anions due to the complementarity of their active centers and ions. The aim of this work is a directed chemical design of silica surface for the construction of sorption active centers with a high affinity for nitrate, orthophosphate, and orthoarsenate anions. Chemical design of β-cyclodextrin-containing supramolecular structures on the surface of granular mesoporous silica gel to obtain sorbents of highly toxic oxyanions was carried out. The structure and surface chemistry of the initial and functionalized silica gels were characterized by means of IR spectroscopy, spectrophotometry, thermogravimetric and chemical analysis, pH metry, low-temperature adsorption-desorption of nitrogen and sorption techniques. The sorption of nitrate, orthophosphate, and orthoarsenate anions from aqueous one- and multicomponent salt solutions was studied as dependent on the time and ion concentration, as well as in the cyclic sorption-desorption mode. The results obtained are interpreted using kinetic models of pseudo-first and pseudo-second order processes and the equilibrium adsorption models of Langmuir and Freundlich. The main characteristics of the specificity and selectivity of the obtained β-cyclodextrin silica gel were calculated. Conclusions are drawn regarding the possibility of using functionalized silica gel for the sorption of oxyanions from water and aqueous solutions, their concentrating, chromatographic separation, and chemical analysis.