0957–5820/02/$10.00+0.00 # Institution of Chemical Engineers Trans IChemE, Vol 80, Part B, May 2002 MERCURY AND CADMIUM SORPTION PERFORMANCE OF A FIBROUS ION EXCHANGER AND GRANULAR ACTIVATED CARBON J. R. RANGEL-MENDEZ and M. STREAT Department of Chemical Engineering, Loughborough University, Loughborough, UK A commercially available wood-based granular activated carbon (WHK) and a weak acid brous ion exchanger (K-4) were evaluated for the removal of mercury and cadmium from aqueous solution. As-received granular WHK was modied electrochemically to enhance cation sorption capacity for comparison with K-4. Granular carbon samples were characterized by direct titration, X-ray photoelectron spectroscopy (XPS), elemental analysis, surface area, electrophoretic mobility measurements, Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The sodium sorption capacity of K-4 was 4.28 mmol g ¡1 compared to 1.27 mmol g ¡1 for as-received WHK. Electrochemical oxidation for 3 h increased the ion exchange capacity of WHK to 2.57 mmol g ¡1 . Elemental analysis and XPS showed a noticeable increase in the oxygen content of electrochemically modied WHK, which was reected in a higher content of acidic oxygen-containing groups and a shift of the isoelectric point to lower pH values compared to as-received WHK. Batch sorption experiments showed that K-4 is more effective for the removal of cadmium than mercury when compared to modied WHK. The effect of pH on sorption isotherms indicated that metal uptake increased with increasing pH. Small-scale studies indicated that breakthrough occurred at 20 and 75 bed volumes for mercury and cadmium respectively when a 1 mM mixed cadmium and mercury feed solution at pH 6 was passed through a column packed with modied WHK. This indicated that modied WHK is more selective for cadmium than for mercury. Keywords: sorption; mercury; cadmium; oxidation; granular activated carbon; ion exchange bre. INTRODUCTION The importance of environmental pollution control has in- creased signicantly in recent years. Environmentalists are primarily concerned with the presence of heavy metals, radio- nuclides and synthetic chemicals in groundwater, surface water, drinking water and aqueous efuents due to their high toxicity and impact on human and aquatic life. Wastewaters that contain a high concentration of heavy metal ions usually originate from industrial sites and=or domestic residences. The Environmental Protection Agency (EPA) and the European Community (Directive 98=78=EC 1 ) have highlighted the most common heavy metals arising in residual water, namely arsenic, cadmium, chromium, copper, lead, mercury, nickel and zinc. Cadmium and mercury are two of the most toxic metals present in the aqueous environment; hence, the maximum permissible concentration in drinking water has been set at 5 and 1 mgl ¡1 (ppb), respectively. Many chronic diseases have been identied with cadmium and mercury, e.g. brain damage, nasopharynx, change in vision and hearing 2,3 . Moreover, the EPA has established that cadmium and mercury are possible human carcinogens. Several techniques have been developed and used to re- move and=or recover a wide range of micro-pollutants from a variety of industrial efuents. Adsorption is well estab- lished for the removal of organic molecules from aqueous solution but to a much lesser extent for the removal of toxic heavy metals. Several adsorbents have been used, for example chitosan 4 , biosorbents 5 , iron oxide 6 , magnesium oxide 7 and silica gel 8 . However, activated carbon is the most widely used. Alternatively, polymeric ion exchange resins can be applied under extreme conditions, ranging from highly acidic to highly alkaline. The high sorption capacity of these materials, usually greater than carbonaceous adsorbents, and good selectivity towards metal ions, make them very attractive candidates for application in wastewater treatment. However, the application of ion exchange resins involves signicant capital and operating costs. The metal ion capacity of conventional granular activated carbons is rather limited, but it can be signicantly enhanced by chemical modication of the surface. The surface of car- bonaceous materials can be modied by oxidation, e.g. reac- tion with hydrogen peroxide 9 , nitric acid 10 , air and ozone at different temperatures 11 , or by electrochemical treatment 12 150