Adsorption of Hg (II) ions from water by activated carbons

The adsorption isotherms of Hg (II) ions from aqueous solutions have been studied on three granulated activated carbons and a sample of activated carbon cloth. The adsorption data obeys both the Langmuir and Freundlich isotherm equations. The x(m) value obtained from the linear Langmuir plots has been used to determine the fraction of the carbon surface occupied by the Hg (II) ions while the linear Freundlich plot constant n has been used to look at the heterogeneity of the carbon surface. The adsorption takes place on certain specific sites on the carbon surface and only a fraction of the BET surface area is occupied by the Hg (II) ions. The adsorption is determined by the amount of acidic carbon - oxygen surface groups present on the carbon surface. It increases with increase in the amount of these surface groups on oxidation and decreases when these surface groups are eliminated on degassing of the oxidized carbon. The adsorption involves attractive electrostatic interactions between the negative sites created by the ionization of the acidic carbon oxygen surface groups and the positively charged Hg (II) cations.