Foam fractionation is a proven technique for separation of heavy metals. This technique was used for separation of mercury from aqueous solutions. It was found that knowledge of mercury-containing species is essential for this process. A rigorous method is presented for estimating the distribution of free and complex mercury-containing species in aqueous solutions. The chelates of Hg (2+) with ligands such as Cl- and OH- are quite stable leading to conclude that poor or no separation results when the pH is reduced by HCl or held alkaline. Experimental results indicated that the efficiency of mercury removal closely correlates with pH as well as the concentration of positively charged mercury-containing species. They also indicated that this efficiency is higher at lower Hg concentrations. A removal efficiency of similar to 80% was resulted for solutions containing 2.5 x 10(-5) M Hg in highly acidic media. It was noticed that this efficiency would drop almost to zero as pH was raised to around 5.5. The theoretical findings were in close agreement with the experimental results. (c) 2006 Elsevier B. V. All rights reserved.