Prashant Srivastava1, Balwant Singh2, and Markus Gräfe2. (1) Faculty of Agriculture, Food and Natural Resources, The University of Sydney, Australia, Sydney, Australia, (2) The University of Sydney, Faculty of Agriculture, Food and Natural Resources, Sydney, NSW 2006, Sydney, Australia
Desorption experiments were conducted of four commonly occurring heavy metals with different hydrolysis behaviour, viz., Cd, Cu, Pb and Zn on kaolinite in single- and multi-element systems. The initial metal loadings were 66.67 and 333.33 ìM in single-element systems and 16.67 and 83.33 ìM in multi-element systems. The metal-adsorbed kaolinite was aged in moist condition under N2-environment for 0, 30 and 60 days at 20 and 40°C. After ageing for appropriate periods, the metal-adsorbed aged kaolinite slurry was dried at 40°C for 48 hours and ground to fine powder. The dried metal-adsorbed aged kaolinite powder was used for desorption of heavy metal(s) with 0.01 M NaNO3 (pH 6.0) as eluent. Desorption of all four metals from kaolinite was characterized by an initial rapid step followed by a slower desorption. The desorption kinetics of all the studied metals were best described by first- and second-order equations. Desorption of heavy metals from kaolinite decreased with ageing and increase in temperature in both the single- and multi-element systems. The decrease in the desorption of heavy metals due to ageing and increase in temperature could be attributed to the processes like movement of weakly bound metal ions to a more strongly bound phase (e.g., changes in the type of surface complex), diffusion into micropores or intraparticle spaces. The desorbability of heavy metals from kaolinite followed the sequence of Cd > Zn > Cu > Pb in both single- and multi-element systems. The results of EXAFS spectroscopy of the aged Cd- and Pb-adsorbed kaolinite samples suggested that average coordination number (CN) of Cd for the first oxygen shell was ~6.7 for most samples and the average distance between Cd and the first shell oxygen backscatterer was ~2.28 Ǻ. Whereas the average CN of Pb for the first ligand shell (O) was found to be ~1.8 and the average distance between Pb and O atoms was ~2.3 Ǻ. With ageing and temperature, the CN of Cd for the first shell oxygen atoms increased while the Cd-O distance remained the same which suggests more water molecules being attached to Cd, whereas CN of Pb for the first shell O atoms decreased and the CN for the second shell O atoms increased. The bond distance between the central absorber Pb atom and neighbouring O atoms decreased with ageing and temperature. The coordination environments of Cd and Pb suggested that Cd forms an outer sphere complex and Pb forms inner sphere complex.