Journal of Hazardous Materials 177 (2010) 899–907 Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat The role of pectin in Cd binding by orange peel biosorbents: A comparison of peels, depectinated peels and pectic acid Silke Schiewer ∗ , Muhammad Iqbal 1 Department of Civil & Environmental Engineering, University of Alaska Fairbanks, PO Box 755900, Fairbanks, AK 99775, USA article info Article history: Received 16 December 2008 Received in revised form 15 December 2009 Accepted 4 January 2010 Available online 11 January 2010 Keywords: Biosorption Cadmium Mechanism Titration FTIR Pectin abstract Biosorption by cheaply and abundantly available materials such as citrus peels can be a cost efficient method for removing heavy metals from wastewater. To investigate the role pectin plays in metal binding by citrus peels, native orange peels, protonated peels, depectinated peels, and extracted pectic acid were compared. Kinetic experiments showed that equilibrium was achieved in 1 h. The 1st-order model was more effective in describing the kinetics than the 2nd-order model. Titrations showed two acidic sites with pK a values around 4 (carboxyl) and 10.5 (hydroxyl), respectively. The pH dependent surface charge was described well by a two-site model. Sorption isotherms were best modeled assuming a 1:2 binding stoichiometry, followed by the Langmuir and the Freundlich model. The binding capacity was highest for pectic acid (2.9 mequiv./g) followed by protonated peels and native peels, being lowest for depectinated peels (1.7 mequiv./g). This showed the importance of pectin in metal binding by citrus peels. However, even depectinated peels were still good sorbents which still provided carboxyl groups that were involved in metal binding. FTIR spectra confirmed the presence of carboxyl and hydroxyl groups in all materials and their involvement in metal binding. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The occurrence of heavy metals such as cadmium in surface waters is of large environmental concern [1]. Beyond traditional methods such as precipitation or ion exchange for removing heavy metals such from wastewater, biosorption, the passive accumula- tion of metals by biomass, presents an efficient low cost alternative, especially when byproducts from other industries are used as biosorbents [2,3]. One such group of materials is citrus peels, which are generated in large quantities by the juice producing industry. A number of recent studies have explored the use of several types of citrus peels as biosorbents for the removal of different contam- inants from wastewater [4–14]. There has been some indication that the carboxyl groups of pectic acid, a biopolymer known to be present in citrus peels, are responsible for the good metal uptake by citrus peels [15]. Pectin is a polysaccharide mainly based on par- ∗ Corresponding author at: Department of Civil & Environmental Engineering, University of Alaska, Institute of Northern Engineering, 459 Duckering Building, Fair- banks, PO Box 755900, Fairbanks, AK 99775, USA. Tel.: +1 907 474 2620; fax: +1 907 474 6087. E-mail addresses: sschiewer@alaska.edu (S. Schiewer), iqbalmdr@brain.net.pk (M. Iqbal). 1 Current address: Environmental Biotechnology Group, Centre for Environment Protection Studies, PCSIR Laboratories Complex, Ferozepur Road, Lahore 54600, Pakistan. Tel.: +92 42 9230688; fax: +92 42 9230705. tially methoxylated galacturonic acid [16], with some other sugars such as rhamnose, arabinose, galactose present in smaller amounts [17]. Chemical modification such as saponification at pH around 11 can improve sorption performance of peels or pectin [16,14]. The role of pectin structure and functional groups in metal binding has been the focus of several studies [17,18]. If pectin is indeed respon- sible for the metal uptake by citrus peels, then the pectin content can serve as an easy indicator for the biosorption capacity of such materials. Many developing nations, where heavy metals are not always sufficiently eliminated from industrial wastewater, are situated in the (sub)tropics where citrus fruits are cultivated and citrus peel waste is generated in large quantities. Biosorption using citrus peel could make metal removal economically feasible in situ- ations where pollution control is otherwise insufficient due to prohibitively high cost, e.g. of ion exchange resins. The metal laden citrus peel biosorbent can be regenerated by acidic desorption and reused in multiple adsorption/desorption cycles while maintaining its sorption capacity [15]. Therefore, the goal of the present study was to compare the biosorption potential for peels themselves, pectic acid extracted from those peels, and the remaining depectinated material. The comparison also included protonated peels since protonated biosorbents are typically more effective than native material, and since acidic desorption generates protonated peels. A com- prehensive evaluation of this family of peel-derived biosorbents 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.01.001