Copper and cadmium sorption onto kraft and organosolv lignins Hengky Harmita, K.G. Karthikeyan * , XueJun Pan Department of Biological Systems Engineering, University of Wisconsin-Madison, 460 Henry Mall, Madison, WI 53706, USA article info Article history: Received 26 August 2008 Received in revised form 23 June 2009 Accepted 24 June 2009 Available online 29 July 2009 Keywords: Kraft lignin Organosolv lignin Cadmium Copper Ion exchange abstract The relative metal sorption ability of kraft and organosolv lignins was examined as a function of solution chemistry (pH, ionic strength (I), sorbate-to-sorbent ratio) and reaction time. The surface charge charac- teristics and functional group composition of these lignins, especially kraft lignin, are favorable for metal sorption. Sorption of Cu and Cd increased with increasing pH and decreasing I. Description of sorption isotherms required the more complex Sips equation, as compared to the simpler Langmuir and Freund- lich formulations, indicative of the presence of binding sites with varying affinities on these lignin biosor- bents. Sorption capacity varied in the following order: softwood organosolv lignin < hardwood organosolv lignin < hardwood kraft lignin < softwood kraft lignin with sorption maximum of 21.5, 40, 66.7, and 80.6 lmol/g, respectively, for Cu, and 8.2, 18.3, 25.2, and 28.7 lmol/g for Cd. Both Cu and Cd sorption kinetics were rapid with equilibrium levels attained within 80 min and faster uptake was noticed for Cu. Strong competitive effects exhibited by H + and Na + in limiting Cu and Cd sorption are sug- gestive of the occurrence of weak ion-exchange type interactions involving the carboxylic and phenolic functional groups. Additional pretreatment and surface modifications of these biosorbents might be required to increase metal sorption capacity and enhance their use in waster/wastewater treatment. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Copper (Cu) and cadmium (Cd) are heavy metals with broad range applications such as catalysts, pigments, pesticides, fertiliz- ers, and stabilizers for PVC (Bradl, 2005; Krishnani and Ayyappan, 2006). However, their toxicity and potential to bioaccumulate have led the United States Environmental Protection Agency (USEPA) to specify a threshold concentration limit, defined as the maximum contaminant level (MCL) for Cu and Cd. The MCL for Cu and Cd is 1.3 and 0.005 mg/L, respectively (USEPA, 2001). Due to their environmental persistence and intense utilization in several applications, development of cost-effective remediation strategies is required for treating metal-containing water/waste- water (Förstner and Wittmann, 1981). A variety of well-known treatment methods exist for metals including lime precipitation, coagulation and co-precipitation as metal hydroxides, electro- chemical treatment, ion exchange, and membrane separation (Sawyer et al., 2003; Tchobanoglous et al., 2003). However, high capital costs for the above processes have led researchers to ex- plore more cost-effective options including the use of sorption media developed from natural materials (i.e., biosorbents). Biosorbents due to their natural abundance and inherent ability to react with metals have been investigated for their potential to serve as sorbents (Gérente et al., 2000; Chen and Wu, 2004). Bio- sorbents contain several surface functional groups (e.g., amino, car- boxylic, phenolic) capable of serving as ion-exchange sites (Chubar et al., 2004). Metal removal occurs via ion exchange and complex- ation (McBride, 1994) and the extent is influenced by physico- chemical characteristics of the biosorbents and solution chemistry (van Riemsdijk et al., 1987). Many low-cost biosorbents such as chitosan, clay, saw dust, peat moss, lignin, pectin, seaweed, zeolite, bark materials, iron- oxide-coated sand have been previously investigated for metal re- moval (e.g., Bailey et al., 1999; Basso et al., 2002; Suhas et al., 2007). Lignin, the by-product of paper industry and emerging cel- lulose ethanol industry, is a potential metal sorbent with abundant supply. With the rapid growth of bioethanol industries across the United States, the production of the next generation (cellulose) ethanol could potentially generate several million tons of residues, mainly consisting of lignin, that should be appropriately utilized (USDA, 2005). In addition, the current paper industry produces million tons of lignin, mainly kraft lignin (Lora and Glasser, 2002). Currently, lignin is typically utilized as a fuel to power the paper and ethanol production facility to further increase process efficiency. However, burning lignin to generate steam, electricity, and heat may exacerbate global warming due to high CO 2 emis- sions (Demirba, 2001). The alternative utilization of lignin as potential low-cost metal sorbents has been explored by other researchers (Srivastava et al., 1994; Dizhbite et al., 1999). This option appears attractive due to the natural abundance of lignin and the availability of appropriate 0960-8524/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.06.093 * Corresponding author. Tel.: +1 608 262 9367; fax: +1 608 262 1228. E-mail address: kkarthikeyan@wisc.edu (K.G. Karthikeyan). Bioresource Technology 100 (2009) 6183–6191 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech