Facilitated transport of heavy metals by bacterial colloids in sand columns

Abstract
The aim of this work is to evaluate the ability of biotic collois (e.g. bacterial cells) to facilitate the transport of heavy metals in soils. and to identify the main factors influencing colloid transport in order to detelmine the geo-chemical conditions where this secondary transport process may become dominant. The model colloids studied here are living cells of Escherichia coli and Ralstonia metallidurans . We studied the transport of mercury zinc, and cadmium in columns of Fontainebleau sand. The properties (i.e. optical and morphological properties, charge (zeta potential, ) and hydrophobia (water/hexadecane distribution parameter, )) of the bacterial cells surface were characterised, as well as their potential for heavy metals sorption (kinetic and isotherm). Both surface charge ( and -14 mV) and hydrophobia ( and 0.05) differ strongly for the two bacteria. Column studies were conducted with bacteria and heavy metals separately or simultaneously. The cell surface differences led to different transport behaviour of the two bacteria, although the retardation factor is close to 1 for both. We observed that colloid mobility increases when increasing bacterial cells concentration and when decreasing the ionic strength. We also observed that bacterial colloids appeared as excellent vectors for Hg, Zn and Cd. Indeed, heavy metals adsorbed on the Fontainebleau sand when injected alone in columns (retardation factors of 1.4 ; 2.9 and 3.8 for Hg, Zn and Cd, respectively); whereas no retardation ( ) is observed when injected in the presence of both bacteria. Moreover, transport of bio-sorbed metal appears to be 4 to 6 times higher than dissolved heavy-metal.