Roles of physical and chemical properties of activated carbon in the adsorption of lead ions

Abstract

Elucidation of the roles of chemical and physical properties of activated carbons is an important basis for the systematic development of adsorbents with optimal properties specific for certain applications. Such an understanding has challenged most researchers and this has been attributed with the difficulty in decoupling the effect of chemical and physical properties that characterize activated carbons. This study proposed empirical modeling in resolving the effects of individual carbon properties in lead adsorption. A model based on lead adsorption and carbon properties including total surface area, mean pore size and heteroatom concentrations has been shown to adequately describe the lead adsorption onto activated carbons prepared from bagasse. To support this investigation a series of activated carbons were prepared from bagasse by physical and by chemical activation techniques. The surface chemical properties of the carbons were inferred from carbon pH and heteroatom concentrations. The physical characterizations of the carbons included total surface area by the BET technique and mean pore size measured using the Horvath–Kawazoe equation. Adsorption tests were conducted using a low concentration of lead (5 ppm) and the solution pH was maintained at 1.0 to maintain lead speciation to the un-complexed Pb²⁺ ion. The adequacy of the proposed empirical models was statistically assessed. This form of analysis was shown to provide valuable information in tailor making adsorbents and selecting appropriate adsorbents for lead adsorption.

Introduction

Lead is an element which holds significant importance industrially and has been used by man since ancient times. All lead compounds are considered cumulative poisons. Acute lead poisoning can affect the gastrointestinal track and nervous system. Treatments for lead removal from solution include precipitation, coagulation, and adsorption (Chu, 1999, Deshpande et al., 2001, Ho et al., 2002, Krishnan and Anirudhan, 2002, Matlock et al., 2002, Chiron et al., 2003, Deng et al., 2003). The maximum allowable lead in drinking water has been set at a concentration of 15 ppb by the US Environmental Protection Agency (EPA, 2002). More stringent requirements for treating lead effluents have led to the need for inexpensive methods that will provide low levels of lead. Lead removal, however, by chemical treatment such as precipitation is ineffective in removing low levels of lead. Current methods based on ion exchange are expensive. Lead removal by adsorption in a variety of inexpensive organic and inorganic adsorbents including by-product of the mineral industry, have thus been considered. Adsorbents which have been tested include kaolinitic clay (Orumwense, 1996), red mud (Apak et al., 1998) and activated carbon (Netzer and Hughes, 1984, Corapcioglu and Huang, 1987, Gajghate et al., 1990, Budinova et al., 1994, Akhtar and Qadeer, 1997, Strelko et al., 1998, Ho et al., 2002). Activated carbon, has by far shown the greatest efficiency in removing lead from solution. The majority of research in lead sorption in activated carbon has been focused in the effect of lead solution, speciation, concentration and carbon properties such as total surface area and particle size (Netzer and Hughes, 1984, Gajghate et al., 1990, Akhtar and Qadeer, 1997, Strelko et al., 1998). There has been little work in regard to the specific properties of the activated carbon that would optimize adsorption and there appears to be no attempt to separate the roles of the physical and chemical carbon properties in lead adsorption. In choosing or designing appropriate adsorbents for heavy metal removal require an extensive testing of various adsorbents. This has largely been associated with the difficulty in relating adsorption to carbon properties which all simultaneously changes with activation. As such, decoupling the individual effects of carbon properties on metal adsorption is complex.

This study aims to decouple the roles of the chemical and physical properties of activated carbon in lead adsorption and thus develop a basis for developing adsorbent for lead removal from solutions. Recent works reported by Syna and Valix, 2003a, Syna and Valix, 2003b established the modeling strategy in tailor making activated carbon for recovery of gold from thiourea and cyanide complexes. This study has followed this technique as a basis of developing activated carbons that would be suitable in removing lead complexes from waste water.

Adsorption tests were conducted in activated bagasse prepared by chemical and physical activation. Bagasse is the fibrous by-product resulting after the milling of sugar cane. Bagasse from the sugar milling process, is of particular interest because it is a renewable resource and because of its availability, quantity and low cost. Approximately 11 million tonnes of bagasse or greater and equivalent quantity of cane field trash are produced in Australia annually. Most of this fiber is used in co-generating both steam and power for the sugar mills. However, in most seasons considerable residual quantities are produced which incur storage cost and pose potential environmental safety hazards. Production of activated carbon from bagasse has the potential to alleviate these problems and represents a commercial opportunity to turn an underutilized by-product to income for sugar mills.