REMOVAL OF CADMIUM USING ELECTROCHEMICALLY OXIDIZED ACTIVATED CARBON J. R. RANGEL-MENDEZ, M. H. TAI and M. STREAT (FELLOW) Department of Chemical Engineering, Loughborough University, Loughborough, UK A wood-based activated carbon, AUG WHK, was oxidized electrochemically to enhance its metal binding capacity and subsequently studied for the removal of cadmium ions from aqueous solution. Treated adsorbents were characterized by N 2 adsorption at 77K before and after oxidation, and a quantitative determination of weak-acid surface groups was carried out by direct titration. The BET surface area decreased considerably after oxidation; however, the total amount of oxygen-containing surface groups was 3.36 times higher compared to the untreated adsorbent. Batch equilibrium tests indicated that the oxidized material has 16.5 times higher sorption capacity for cadmium than conventional activated carbon. Equilibrium isotherms were determined at pH 4, 5 and 6 and showed that there was a slight increase in cadmium uptake with increase in pH. The experimental data were ®tted by Langmuir and Freundlich isotherms and it was found that the Freundlich isotherm ®tted better in all the cases. Overall, the results indicated a rapid adsorption rate with over 96% fractional uptake of metal occurring in the ®rst 12 minutes. Small-scale mini-column experiments were performed and indicated that breakthrough occurred after about 140 bed volumes (BV) of feed solution, containing 1 mM Cd at pH 6, was passed at 10 BV h ±1 for oxidized WHK, whereas breakthrough was instantaneous for the unoxidized sample. Keywords: sorption; cadmium; adsorption; breakthrough; pH; kinetics. INTRODUCTION The presence of heavy metals in ef¯uents is a matter of primary environmental concern due to toxicity. Cadmium is one of the most toxic elements, which can seriously affect human health. Dispersion of cadmium into the environ- ment results from industrial processes, domestic activities, agricultural practices (use of fertilizers), use of cadmium- containing goods, and disposal of wastes. Adsorption has been widely applied for the removal of trace contaminants from potable water, domestic water and industrial ef¯uents. Sorption of heavy metals on activated carbon is not a simple process because it depends on several factors such as metal speciation, water chemistry and chemistry of the carbon surface. Activated carbons are extensively used in wastewater treatment for the removal of a wide range of contaminants. They offer extensive surface area for sorption of metal ions from aqueous solu- tions and have been investigated for treatment of solutions bearing inorganic species. By far the most extensively developed large-scale application of activated carbon is the recovery of gold from dilute cyanide leach solutions. A detailed study of this process is reported by Bailey 1 . The metal binding ability of conventional activated carbon is very limited. However, this can be signi®cantly enhanced by chemical modi®cation. These modi®cations can be carried out by addition of oxygen, nitrogen, sulphur and phosphorus onto the surface of the original precursor. Oxidation is commonly used to increase the number of weakly acidic groups on the carbon surface, e.g. carboxylic, carbonic, phenolic and lactonic. Hence, metal ions can interact directly with these mainly oxygen-containing groups and are thereby removed from solution. The groups added during the oxidation affect acidity, porosity and the adsorption performance of modi®ed carbons 2 . Oxidation has been carried out mostly by using air at high temperature or nitric acid solutions 3 . Other oxidants such as ozone and hydrogen peroxide have also been used 4 . Electrochemical oxidation of carbon ®bres has been carried out mainly to increase tensile strength as well as improving metal binding capacity 5 . However, only a limited amount of work has been carried out on electrochemical oxidation of granular activated carbons. Some preliminary studies of the adsorption and kinetics of cadmium uptake on conventional and modi®ed granular activated carbon are reported in this work. EXPERIMENTAL Materials Granular activated carbon, designated WHK, was sup- plied by AUG, Germany. This material was sieved to a particle size fraction of 170±210 mm, washed carefully with distilled water and then dried in an oven at 378 K until no change in weight was observed. Cadmium solution was prepared using CdCl 2 ´H 2 O laboratory grade purchased from May & Baker Ltd, Dagenham, England. Sodium hydroxide, nitric acid, hydrochloric acid and potassium chloride were prepared from analytical reagents supplied 143 0957±5820/00/$10.00+0.00 q Institution of Chemical Engineers Trans IChemE, Vol 78, Part B, March 2000