Available online at www.sciencedirect.com Journal of Hazardous Materials 158 (2008) 401–409 Mechanisms of heavy metal sorption on alkaline clays from Tundulu in Malawi as determined by EXAFS S.M.I. Sajidu a,∗ , I. Persson b , W.R.L. Masamba c , E.M.T. Henry  a Chemistry Department, University of Malawi, P.O. Box 280, Zomba, Malawi b Department of Chemistry, Swedish University of Agricultural Sciences, Box 7015, 75007 Uppsala, Sweden c Harry Oppenheimer Okavango Research Centre, University of Botswana, P/Bag 285, Maun, Botswana Received 28 August 2007; received in revised form 24 January 2008; accepted 25 January 2008 Available online 6 February 2008 Abstract Chromium(III), copper(II), zinc(II), cadmium(II), mercury(II) and lead(II) cations are among the most common heavy metal pollutants in industrial waste waters. In our continued work on cost effective wastewater heavy metal removal agents and methods using local material, this study examines the interactions of chromium(III), copper(II), zinc(II), cadmium(II), mercury(II) and lead(II) cations with natural mixed clay minerals from Tundulu in Malawi using extended X-ray absorption fine structure (EXAFS) spectroscopy. The mixed clays were previously characterised and found to contain illite, low ordered kaolinite, mixed layer minerals and the non-clay mineral carbonate fluoroapatite with a mean pH PZC of 9.63. The EXAFS analyses provided qualitative evidence that oxygen atoms occupy the first coordination shells in all the studied central atoms. The metal species on the clay mineral surfaces seem to be adsorbates and/or precipitates of hydrolysis products. Chromium(III) forms a polynuclear hydrolysis complex on the mineral surface with Cr–O bond and Cr···Cr distances of 2.00 and 3.03 ˚ A, respectively, which is indicative of a chain structure with edge sharing CrO 6 octahedra. Copper(II) is bound to phosphate groups on the surface at low pH and has a first shell of coordinated oxygen atoms with Jahn–Teller distortion as revealed by different Cu–O bonds of 1.96 ˚ A for the equatorial ones, at 2.30 and 2.65 ˚ A for the axial oxygens, and a Cu–P distance at 3.29 ˚ A is distinguished as well. Upon treatment at neutral pH copper(I) oxide seems to be the main precipitation product on the clay surface. At neutral pH zinc(II) forms also polynuclear hydrolysis complexes with Zn–O bond and Zn···Zn distances of 2.01 and 3.11 ˚ A, respectively, which shows the presence of edge sharing ZnO 4 tetrahedra. Cadmium(II) is adsorbed to the clay surfaces as a six-coordinated CdO 6 complex in octahedral fashion, but it is not possible to distinguish if cadmium is hydrated or partly hydrolysed. Mercury(II) is present as linear O–Hg–O units but without any observable Hg···Hg distance at high pH showing that mercury(II) is hydrolysed but not present as mercury(II) oxide. At low pH, linear O–Hg–Hg–O units are present showing that mercury(II) is reduced to mercury(I). No precise chemical environment around the lead(II) could be obtained for the lead(II) treated clays due to formation of different hydrolysis structures with multiple coordination numbers by lead(II) salts on the mixed clays. © 2008 Elsevier B.V. All rights reserved. Keywords: Heavy metal; Tundulu mixed clay; pH PZC ; EXAFS; Polynuclear hydrolysis complex 1. Introduction There have been studies on the removal of heavy metals from water and wastewater including chemical precipitation, physical treatment such as ion exchange, solvent extraction and ∗ Corresponding author. Tel.: +265 1 524 222; fax: +265 1 524 046. E-mail addresses: sajidu@chanco.unima.mw, ssajidu@yahoo.co.uk (S.M.I. Sajidu), Ingmar.Persson@kemi.slu.se (I. Persson), wmasamba@orc.ub.bw (W.R.L. Masamba).  Deceased. adsorption. However, due to high maintenance costs and chemi- cal importation using scarce convertible foreign currency, these methods are unsustainable in developing countries. Recently, natural materials which are good sorbents and inexpensive have received much attention in sorption of heavy metals from con- taminated water. Clay minerals such as montmorillonite and vermiculite [1–3], allophane [4], kaolinite [5], bentonite [6,7] and illite [8] are among the natural materials which have been investigated as heavy metal sorbents owing to their high cationic exchange capacity and negative charges due to isomorphic sub- stitution in their structures. Our work on the removal of heavy metals from contaminated water using locally available cost 0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.01.087