Removal of mercury from water by fixed bed activated carbon columns

The breakthrough curves for Hg(II) ions on a sample of granulated activated carbon (GAC) and a sample of activated carbon cloth (ACC) are generally S-shaped. The breakthrough time increases with increase in the bed depth but decreases on increasing the hydraulic loading rate (HLR) and the feed concentration. The adsorption of Hg(II) ions increases with HLR and attains a maximum value at HLR around 7 m(3)/h/m(2). At low HLR, laminar flow conditions prevail so that the mass transfer takes place across a nearly stationary film of the liquid covering the carbon particles. This high resistance leads to low mass transfer and results in smaller adsorption. On increasing HLR, the interface resistance decreases resulting in an increase in adsorption. Beyond a certain HLR, the rate of adsorption decreases due to decrease in the residence time of the solution within the carbon bed and a lower time available for mass transfer. The adsorption zone parameters of the carbon column have been determined using the carbon bed column data and invoking the mathematical treatment suggested by Michaels. Bed Depth Service Time (BDST) theoretical model has been used to calculate the critical bed depth and the depth of the mass transfer zone. These have been found to be in agreement with the experimental values. (C) 2009 Elsevier B.V. All rights reserved.