Removal of manganese (II) by edge site adsorption on raw and milled vermiculites

Abstract

The adsorption mechanism of manganese nitrate on vermiculite was studied by analysis of the isotherms of the cations exchanged from the vermiculite (K+ and Ca2+) and the ions (NO3− and Mn2+) adsorbed on the vermiculite. The Langmuir-type manganese adsorption isotherm of the raw vermiculite shows a maximum uptake of 28.32mg·g−1. The adsorption temperature dependence between 25°C and 45°C has demonstrated an endothermic and spontaneous adsorption. The amount of exchanged Mn2+ ions in equivalent per gram represents only 17% of the total adsorbed uptake, of which 14% have been exchanged against Ca2+ ions and 3% against K+ ions. 83% of the Mn2+ and NO3− ions have been co-adsorbed by complexation at the edges of the layers. The grinding of the vermiculite has involved a decrease in the grains size from 2–3mm to 100μm and thus an increase in the edge layer adsorption site content allowing an increase of 19% of the maximum adsorption uptake compared to raw vermiculite. The ultrasonic milling of vermiculite in aqueous dispersion at 20kHz allowed to decrease the size of the grains further (8μm) and led to increase the maximum uptake by 31%. The sonication of the vermiculite dispersion in 35% hydrogen peroxide has allowed to obtain a maximum adsorption uptake of 36.77mg·g−1 (30% of increase compared to raw vermiculite). The linear variation of the Mn2+ adsorption capacity with respect to the sum of the estimated geometric perimeter of the sonicated particles (assuming square shaped particles) confirms an adsorption process at the edge of layers.