Dual role of water in the process of methyl mercaptan adsorption on activated carbons

Adsorption of methyl mercaptan in moist conditions was performed on numerous samples of activated carbons of various origins. To broaden the spectrum of surface pH, carbons were acidified, treated with bases, or modified with urea. Their structural parameters were determined using sorption of nitrogen. Methyl mercaptan adsorption was tested by a dynamic method. The amount of products of surface reaction was evaluated using thermal analysis. The results revealed that the main product of oxidation, dimethyl disulfide, is adsorbed in pores smaller than 50 A. There is apparent competition for adsorption sites between water (moist conditions) and dimethyl disulfide. The competition is won by the latter molecule due to its strong adsorption in the carbon pore system. Although dimethyl disulfide has to compete with water for the adsorption sites it cannot be formed in a significant quantity without water. Water facilitates dissociation of methyl mercaptan, provided that the pH of carbon is above the required threshold of about 7.5, and thus ensures the efficient removal process. Statistical-molecular calculations confirm that in a system activated carbon-water-DMDS in equilibrium conditions DMDS tends to substitute water in an adsorbed state and the highest capacity for DMDS adsorption is to be found for microporous carbon.