Mechanism of Inversion of Montmorillonite Sorption Properties by Cationic Surfactant
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RIALS FOR ENSURING HUMAN VITAL ACTIVITY AND ENVIRONMENTAL PROTECTION
Mechanism of Inversion of Montmorillonite Sorption Properties by Cationic Surfactant T. V. Kon’kovaa, *, A. P. Ryseva, **, and E. V. Mishchenkoa, *** a
Mendeleev University of Chemical Technology of Russia, Moscow, 125480 Russia *e-mail: [email protected] **e-mail: [email protected] ***e-mail: [email protected] Received June 28, 2019; revised July 11, 2019; accepted July 12, 2019
Abstract—Organo-mineral derivatives of montmorillonite modified by salts of quaternary ammonium compounds are promising adsorbents for wastewater purification from heavy metal anions, as well as anionic dyes and pesticides. The understanding of the mechanism of anion adsorption by the example of montmorillonite modified by didecyldimethylammonium chloride cationic surfactant is expanded. The high rate of chromium anion adsorption, the appearance of adsorption capacity only during charge exchange of the surface, and the pH dependence of adsorption, including for pH-independent nitrate anions, indicate that a significant contribution to adsorption of anions is made by silanol and aluminol groups on the edge surface of the aluminosilicate layers of the mineral. The redistribution of access to various types of adsorption centers of the surface for adsorptive is the mechanism of inversion of the adsorption properties for montmorillonite. The inner negatively charged surface is screened by the double layer of the surfactant molecules that block the interlayer space. The surfactant molecules are not hydrated, which impedes delamination of the mineral in aqueous medium. Keywords: montmorillonite, anion-exchange adsorbent, cationic surfactant DOI: 10.1134/S2075113320050184
INTRODUCTION Montmorillonite is a layered mineral with an expanding unit cell, the most distributed amphoteric ion exchanger in nature. Its cation-exchange capacity is related to a permanent negative charge of alumosilicate layers determined by isomorphous substitutions in the O-grid of the mineral. The exchange interlayer cations that reduce electrical neutrality of the structure are weakly held by the surface and can be replaced by other cations. On the other hand, on the lateral edges of alumosilicate layers, there are so-called “terminal” aluminol and silanol groups that may carry both a negative and positive charge depending on the pH of the medium. Despite the fact that the surface of the lateral edges does not exceed 7–9% of the particle surface, they can be potential centers of anion adsorption [1, 2]. Since the interlayer cations are hydrated, the mineral particles are delaminated into elementary alumosilicate layers or their small aggregates in an aqueous medium, depending on the conditions and the type of interlayer cation. In fact, the inner negatively charged surface of montmorillonite becomes an outward negatively charged surface of its particles. As a result, montmorillonite in an aqueous medium has almost no adsorption capacity for anions because of their elec-
trostatic repulsion from the surface
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