Carbon 03-S urface modification of a granular activated carbon by citric

a
Department of Chemical and Environmental Engineering National University of Singapore, 10 Kent Ridge Crescent, Singapore 129791, Singapore b Institute of High Performance Computing, 1 Science Park Road, [01 -01, The Capricorn, Singapore 117528, Singapore Received 19 February 2003; accepted 5 May 2003
* Corresponding author. Tel.: 1 65-6874-8092; fax: 1 656779-1936. E-mail address: checjp@.sg (J.P. Chen).
carbon had much higher adsorption of cadmium, lead and zinc. Lakov et al. [3] modified activated carbons with 3-methyl-1-phenyl-pyrazolone-5 (HMPP). The copper adsorption capacity was enhanced by 50% even though the specific surface area of the carbon decreased by 80%. More recently, Monser and Adhoum [4] reported that, when tetrabutyl ammonium iodide (TBA) or sodium diethyl dithiocarbamate (SDDC) were applied onto an activated carbon, a substantial improvement in Cu, Zn and Cr removal capacity was achieved. It has been reported that citric acid has a higher binding affinity to copper ions [5]. It is biodegradable and widely used in the food and pharmaceutical industries. As a result, it was selected for the modification of a commercially available granular activated carbon in this study. The main objective of this paper was to compare the adsorption performance of citric acid-modified activated carbons with the unmodified activated carbons. The copper adsorption equilibrium and kinetics were determined. Langmuir and Freundlich isotherm equations were employed to quantify the adsorption equilibrium. The effect of solution pH on
0008-6223 / 03 / $ – see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016 / S0008-6223(03)00197-0
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J.P. Chen et al. / Carbon 41 (2003) 1979–1986
copper ion adsorption behaviour was examined. An intraparticle diffusion model was used to investigate the adsorption mechanisms.
2. Experimental
2.1. Materials
pure nitrogen gas overnight at a temperature of 150 o C to remove any contaminants as well as moisture that might be present on the activated carbon surfaces. The surface morphology of the activated carbon samples was visualized by scanning electron microscopy (JEOL, JSM-5600V, Japan). SEM enables the direct observation of any surface microstructure changes in the activated carbons that would have occurred due to the citric acid modification. Potentiometric titration experiments were carried out with an automatic titrator (Metrohm 716 DMS Titrino, Switzerland). A 0.25-g amount of activated carbon was added to a conical flask containing 50 ml of 0.1 M sodium nitrate as background ionic strength. The solution was shaken for 48 h on a shaker. The titration was carried out using 0.1 M NaOH and 0.1 M HNO 3 solutions.
Carbon 41 (2003) 1979–1986
Surface modification of a granular activated carbon by citric acid for enhancement of copper adsorption
J. Paul Chen a , *, Shunnian Wu b , Kai-Hau Chong a
Abstract In this study, citric acid was used to modify a commercially available activated carbon to improve copper ion adsorption from aqueous solutions. The carbon was modified with 1.0 M citric acid, followed by an optional step of reaction with 1.0 M sodium hydroxide. It was found that the surface modification reduced the specific surface area by 34% and point of zero charge (pH pzc ) of the carbon by 0.5 units. Equilibrium results showed that citric acid modification increased the adsorption capacity to 14.92 mg Cu / g, which was 140% higher than the unmodified carbon. Higher initial solution pH resulted in higher copper adsorption. The chemical surface modification adversely affected the copper adsorption rate. Adsorption kinetic mechanisms were investigated with an intraparticle diffusion model. It was found that the modification did not change both external diffusion and intraparticle diffusion. 2003 Elsevier Ltd. All rights reserved.
In this study, Fitrasorb 200 (Calgon, Pittsburgh, PA, USA) was used. Copper chloride dihydrate from J.T. Baker (Phillipsburg, USA), and sodium nitrate, lead nitrate, sodium hydroxide, sodium acetate, acetic acid, hydrochloric acid, nitric acid and citric acid monohydrate from Merck (Germany) were used. All chemicals were of reagent grade. Solution pH was adjusted by 0.1 M HCl or 0.1 M NaOH. The citric acid has a chemical formula of HOOC–CH 2 –COH(COOH)–CH 2 –COOH, indicating that it is a small organic compound rich in carboxylic groups. The concentration of metal ions was measured using inductively coupled plasma emission spectroscopy (ICPES) (Perkin-Elmer Optima 3000, USA). Samples were acidified with concentrated nitric acid, and filtered with a 0.45-mm Whatman Autovial filter (USA) before analysis. Solution pH was measured using an Accumet basic pH meter (Fisher Scientific, USA).