A commercially available activated carbon (AC) was oxidized using 1 M H 2 SO 4 solution containing 2 M (NH 4 ) 2 S 2 O 8 oxidant with the solution to carbon ratio of 50 cm 3 g ¹1 by changing oxidation period ranging 1 to 10 days at ambient temperatures of 20 or 30°C to introduce carboxy groups onto the carbon surface to capture Pb(II) ions. Adsorptive removal of Pb(II) ions from aqueous solution was also examined with the oxidized ACs. The Pb(II) ion adsorption progressed via ion exchange with protons of carboxy groups bound to graphite and 2.2 mmol g ¹1 of Pb(II) could be accommodated at equilibrium pH above 4.0 for ACs oxidized for at least 10 days at 20°C and 4 days at 30°C. Decrease in specific surface area and yield and increase in oxygen and hydrogen content in ACs also observed during the (NH 4 ) 2 S 2 O 8 treatment implied that the oxidation could convert graphite sheets to smaller size by hydrolysis while many carboxy groups would be introduced to the peripherals of the graphite sheets to scavenge Pb(II) ions.Activated carbons (ACs) have long been widely applied for water purification to principally remove organic pollutants such as phenol and nitrobenzene utilizing their hydrophobic nature on the carbon surface. 13 Currently ACs have been extensively utilized and modified by numerous methods to change their structure and surface nature to remove such other contaminants as bulky and polar dye molecules, 4 heavy metal ions, 5,6 and anionic nitrate ions.7 For the removal of cationic heavy metal ions, a negatively charged surface is required; one is a basic site generated by introducing basic nitrogen and oxygen groups on the carbon surface and π-electrons in hydrophobic graphite layers, on the contrary the other is an acidic site such as carboxy groups in which cationic heavy metals are effectively adsorbed via ion-exchange with protons of carboxy groups on the hydrophilic carbon surface, and the ion-exchange mechanism has been supported in the literature.811 Hydroxy, lactone, and other oxygen functional groups, however, did not contribute to heavy metals adsorption very much particularly in acidic and neutral aqueous solution in our previous study. 9 Comparing to basic sites such as amino-groups and π-electrons, acidic carboxy sites adsorb heavy metal cations over a wide pH range from 4 to above 7 although basic sites work in basic solutions above pH 8, thereby increasing carboxy groups is one strategy to enhance the uptake of heavy metal cations.10,12 Kasnejad et al. found that Cu(II) adsorption was increased as much as 3.1 mmol g ¹1 at equilibrium aqueous solution pH about 6 for a commercial activated carbon modified by NH 3 after preoxidation with HNO 3 .13 Rangel-Mendez and Streat examined the effect of electrochemically oxidized activated carbon cloth made from polyacrylonitrile on Cd(II) adsorption and adsorption of 1.2 mmol g ¹1 could be achieved in solution pH above 5 at the expense of the reduction of specific surface area of AC by 35%.14 In this study, ammonium persulfate was used as an oxidant...

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