Adsorptive Removal of Pb(II) Ions from Aqueous Solution by (NH4)2S2O8 Oxidized Activated Carbons

Abstract

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...

Adsorption of Pb(II) from Aqueous Solution on Oxidized Activated Carbon Fibers

“…The recovery could result from the estimation that the effect of pore entrance obstruction by surface functional groups was lessened because surface functional groups were eliminated during the outgassing, and the surface area in the inner part of the outgassed ACFs could be measured again by BET measurements using nitrogen gas. However, the pore diameter was still kept approximately 1.80 nm for all ACFs, which was different from our previous research [26], indicating that the decrease in surface area and total pore volume was not caused by the erosion of carbon during the oxidation procedure. This assumption was also supported by the calculated result that the decrease in carbon content by liberated CO and CO2 from the surface carboxy and lactone was only 0.83% in total.…”

Adsorptive Removal of Nitrate from Aqueous Solution Using Nitrogen Doped Activated Carbon

“…The pH pzc value of 7.1 for non-treated AC was decreased to 4.0 for Ox-9.5AG implying that positively charged N-Q species could be involved in the species on the carbon surface. Generally speaking, pH pzc of activated carbons is always decreased by oxidation 19,22) and it is increased by outgassing 23) and reduction. 24) In oxidation, carboxy groups play a decisive role to reduce pH pzc since carboxy groups easily discharge protons to aqueous phase generating negative charge of carbon surface, whereas in outgassing and reduction, acidic functional groups as carboxy and lactone are removed and/or transformed to basic functional groups such as carbonyl and quinone receiving protons resulting in positively charged carbon surface.…”

“…Elemental composition was measured by Perkin-Elmer PE2400 CHN analyzer. Since sulfur content has been already confirmed to be 0.02% or less for both AC and Ox, 19) oxygen content was determined by balance assuming all other elements except C, H and N would be O (oxygen). Adsorption experiments of nitrate were carried out using 15 mL NaNO 3 solution dosing 30 mg adsorbents of Ox-9.5AG and AC as a control.…”