Study of adsorptive materials obtained by wet fine milling and acid activation of vermiculite

Wet fine milling, as a pretreatment step to acid activation of vermiculite, was applied in order to decrease the environmental impact of the procedure commonly used to increase the mineral's adsorption capacity. Milling caused fragmentation of the material and several changes in its structure: edges of the flocks became frayed, the surface cracked, cation exchange capacity (CEC) increased, and most of the iron in oligonuclear and bulk form was removed. At the same time the specific surface area, crystallinity, chemical composition and adsorption capacity did not change significantly. Fine ground material was more susceptible to acid activation, which caused a decrease in the crystallinity and CEC, development of meso- and microporosity, an increase in the total volume of pores, in the specific surface and external surface areas. Micropores were developed faster in lower acid concentrations in the rough ground material, while the external surface area and total pores volume increased faster in the fine ground vermiculite. The latter material also had a higher CEC. Application of 0.5 mol L− 1 HNO3 to rough ground vermiculite did not change its adsorption capacity, however it changed from 55 ± 7 to 110 ± 8 mg g− 1 when the material was fine ground. The optimal treatment conditions for both materials were obtained for 1.0 mol L− 1 HNO3, however the adsorption capacity for the fine ground vermiculite increased more (i.e., from 55 ± 7 to 136 ± 7 mg g− 1) than for its rough ground counterpart (i.e., 52 ± 7 to 93 ± 7 mg g− 1). Concentrations higher than 1.0 mol L− 1 resulted in deterioration of the adsorption capacities in both cases. Considering all the experimental outcomes, it can be concluded that the environmental impact of acid activation of vermiculite may be diminished by application of fine grinding of the material before the chemical activation process. Such treatment resulted in higher adsorption capacity at a given acid concentration compared to the rough ground material.This work, within the scope of the project of Labóratorio Associado para Química Verde – Technologia e Processos Limpos – UID/QUI/50006, is financed by national founds of FCT/MEC and co-financed by Fundos FEDER (POCI-01-0145-FEDER-007265) within the scope of the partnership agreement PT2020. Part of the research was carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (contract no. POIG.02.01.00-12-023/08). To all financing sources the authors are greatly indebted.