Modification of a commercial activated carbon for metal adsorption by several approaches

It has been widely recognized that metal removal by activated carbon adsorption is due to the surface complex formation between the metal ions and the acidic surface functional groups. Improvement of Filtrasorb 200 by using several modification approaches is investigated in this study. The carbon was first modified by nitric acid or by citric acid and subsequently by sodium hydroxide. It was found that modification slightly altered the surface area of the activated carbon. Kinetic study showed that the modifications greatly decreased the adsorption equilibrium time. In addition, the metal uptake was dramatically increased. Maximum adsorption capacities (qmax) determined by the Langmuir isotherm equation were 0.124, 0.208, and 0.244 mmol/gram for unmodified, modified by citric acid and nitric acid, respectively. Sodium hydroxide modification resulted in faster adsorption kinetics, but slightly decrease the maximum adsorption capacity. The effect of solution pH on metal ion adsorption was also discussed in this paper. Adsorption by activated carbon has been widely studied as an effective technique for removing heavy metal from wastewater. The removal efficiency is influenced by various factors, such as solution concentration, solution pH and ionic strength, and adsorbent modification procedure. Numerous attempts have been made to correlate the adsorption capacity of activated carbons with their surface areas and pore structures, but such a relationship seems to be limited to pure physical adsorption. In many applications, the adsorption properties of activated carbons may be correlated with the chemical nature of the carbon surface rather than with the surface area and the porosity of the carbon. At present, many investigations are directed towards modifying carbon surfaces to increase their acidic surface functional groups. Mostafa (1997) studied adsorption of 1 Research Scholar, Department of Chemical and Environmental Engineering, National University of Singapore, 10 Kent Ridge, Singapore 119260, [email protected] 2 Assistant Professor, Department of Chemical and Environmental Engineering, National University of Singapore, 10 Kent Ridge, Singapore 119260, [email protected] (corresponding author) mercury, lead and cadmium on activated carbon modified with sulphuric acid and observed a significant increase in metal ion adsorption. He proposed that sulphuric acid might introduce acidic surface oxides on the carbon surface. Toles et al. (1999) reported that air oxidation of phosphoric acid activated carbons yielded carbons with greater copper uptake. Copper adsorption showed good correlation with surface functional groups. Moreno-Castilla et al. (1997) claimed that modification of activated carbon with HCl, HF or HNO3, apart from removal of the mineral matter, introduced oxygen surface complexes that change the surface chemistry and can alter the surface area and porosity of the original samples. Modification of the surface chemistry of activated carbons might be a viable attractive route toward novel applications of these materials as adsorbent for heavy metal removal. In this paper, the commercial activated carbon Filtrasorb 200 was ground and sieved to obtain particles of size of 20-30 meshes. The carbon was washed with deionised water (DW-C) to remove impurity and potential powdered activated carbon. The yielded carbon was divided and chemically modified in different approaches: by reaction with 1M citric acid (CA-C) and subsequently with 1M sodium hydroxide (CA-SC), by oxidation with concentrated of nitric acid (NA-C) and subsequently by reaction with 1M sodium hydroxide (NA-SC). The porous structure characteristics and the pH values of aqueous suspensions of the carbon samples are summarized in Table 1. It shows that the surfaces of the modified carbon posses different chemical properties, but similar surface area (BET). It indicates that modification of activated carbon only slightly alters their porosity. Table 1. Surface characteristics of the carbons Carbon BET surface area (m/g) pH Adsorption capacity (mmol/g) DW-C 642 6.88 0.124 NA-C 668 3.74 0.244 NA-SC 646 8.49 0.227 CA-C 624 3.58 0.208 CA-SC 663 8.79 0.179 Kinetic study of copper ion adsorption on the activated carbon samples is shown in Figure 1a and 1b. Most of the copper ion adsorption occurred at the first 2 hours, followed by a relatively slow adsorption process. The modified activated carbons demonstrate larger adsorption rate compared with unmodified carbon. It is observed that modification with sodium hydroxide resulted in further faster adsorption kinetics. By plotting log (%adsorption) vs. log (time) of the results, it is found that the obtained straight line deviates from the origin, which indicates that intraparticle transport is not the only rate limiting step. Probably, the transport of the copper ions from the solution through the particle solution interface into the pores of the particle as well as the adsorption on the available surface the carbons are both responsible for the uptake process. time, hour 0 20 40 60 80 q/ q m ax