Methylene blue sorption equilibria on hydroxylated silica surfaces as wellas on carbon-silica adsorbents (Carbosils)

The adsorption properties of hydroxylated silica gel as well as carbon-sili ca adsorbents (Carbosils) containing 5 and 18 w/w% carbon, both treated hyd rothermally (with hydrogen peroxide solution) (HTT samples) and untreated, towards Methylene Blue (MB) were compared. The equilibrium adsorption of MB solutions of 6.5-7.0 x 10(-4) mol/l concentration and its dependence on pH were studied. At this concentration, ca. 30% of the adsorbent surfaces inv estigated were covered with MB molecules under optimal adsorption condition s. The presence of carbon deposits on the silica surface did not lead to a decrease in the adsorption capacity of Carbosils relative to that of hydrox ylated silica. However, such deposits increased the resistance of the adsor bent towards the action of alkalis. Oxidation of the carbon deposit in the HTT process caused a further increase in this resistance. Hence, the adsorp tion mechanism of MB at equilibrium in individual systems of the type inves tigated can differ significantly. On the one hand, this mechanism is associ ated with the differing behaviour of silica over a wide range of pH relativ e to treated and untreated Carbosils and on the other hand with the specifi c properties of MB.