Removal of Heavy Metals by Using Activated Carbon Produced from Cotton Stalks

Utilization of agricultural waste to produce valuable products has opened opportunities in Egypt to minimize serious public health risks from exposure to dangerous fumes that come from burning the agricultural wastes in open fields. In this study, activated carbon produced locally from cotton stalks was examined for the removal of target heavy metal contaminants from water and wastewater. Adsorption studies conducted in completely mixed batch reactors showed the ability of the produced activated carbon to remove heavy metals; namely lead, cadmium and copper; from aqueous solutions in a pH range below that of precipitation and with high uptake capacity after an equilibrium reaction time of 72 hours. The surface titration experiment indicated a negative surface charge of the produced activated carbon in solution at pH as low as 6, meaning that electrostatic attraction of the divalent heavy metals can occur below the pH required for precipitation. The highest adsorption capacity was for lead, followed by copper and then cadmium. Multicomponent metal adsorption experiments indicated a competition for the available surface sites. Adsorption capacities in the mixture were reduced from their single-solute values for all metals. Introduction Heavy metals are any element in the d-block of the periodic table, or transition metals. Many are also known to be toxic to both humans and other living forms, with their accumulation over time causing damage to the kidney, liver, and reproductive system in addition to cancer (Lentech, 1998). Heavy metal pollution derives from a number of sources, including lead in petrol, industrial effluents, and leaching of metal ions from the soil into lakes and rivers by acid rain. Moreover, they arise from the purification of metals, for instance smelting of ores, preparation of nuclear fuels, and electroplating (Lef, 1998). They precipitate into soil, underground water, and surface water. Unlike organic contaminants, heavy metals do not normally undergo biological decay and are thus considered a challenge for remediation. Many governments have enacted laws to hinder discharging heavy metals into water bodies and using toxic substances such as lead (Abdelshafi, 2007). However, heavy metals still find their way to water supplies. Accordingly, many studies have been done for removal of heavy metals. Ion exchange, reverse osmosis, and chemical precipitation have been investigated for the removal process, but they are too expensive or incapable of meeting treatment objectives. Adsorption has been proved to be a potentially feasible alternative. Adsorption by using activated carbon is the most common method, but this too may be expensive, particularly if proper raw materials are not available and therefore the carbon has to be imported. Motivated by cost considerations, locally generated agricultural wastes such as cotton stalks, rice straw, sugar cane bagasse, and others have been tested in the production of activated carbon in developing countries (Logan, 2002). The use of these raw materials in carbon production shows from the past studies that they are available at low cost, contain high carbon content, and may be effective in the removal of heavy metals and other toxic pollutants. This research investigated the effectiveness of activated carbon produced from cotton stalks in the adsorption of three target heavy metals – lead, cadmium, and copper – from aqueous solution in both single and multicomponent metal cases. Materials and Methods Activated Carbon The activated carbon used in this work was produced from cotton stalks, with the production procedure and carbon properties described in Louis (2006). The carbon had a surface area of approximately 850 m 2 /g. For this research, a sample of the produced carbon was sieved using U.S standard sieves. The pass 200 fraction was used for the surface titration experiment, and 35-50 mesh fraction was used in the adsorption experiments.