Divalent metal adsorption by the thermophile Anoxybacillus flavithermus in single and multi-metal systems

Recent studies have applied surface complexation models (SCMs) to describe metal adsorption by mesophilic bacteria. However, only one SCM has been developed for metal biosorption by a thermophile and the study was limited to adsorption of Cd. In this study, we quantify adsorption of a variety of metals onto the thermophile Anoxybacillus flavithermus with surface complexation modeling. We conduct both single and multi-metal sorption studies using Cd, Cu, Mn, Ni, Pb, and/or Zn, in order to compare the relative affinities of the metals for the cell surface and investigate if these individual affinities change in the presence of other metals. We also use linear free energy relationships (LFERs) to compare the stability constants for the metal-bacteria surface complexes with a variety of aqueous metal–ligand stability constants, and we compare the metal binding capacity of Cd, Cu, Mn, Pb, Ni, and Zn of A. flavithermus to that of the commonly studied mesophile Bacillus subtilis. We find that the metals exhibit different preferences for the bacterial surface in the order Mn ≈ Ni < Zn < Cd < Pb ≈ Cu. Metal binding is described best by formation of a M–carboxyl complex and either a MOH–carboxyl, M–phosphoryl, or MOH–phosphoryl complex (where M is the metal cation). Stability constants determined from the single metal and multi-metal systems are comparable. It is therefore possible to obtain reasonable stability constants in multi-metal systems. This has the potential to greatly simplify the acquisition of metal-bacteria thermodynamic data. However, stability constants obtained from the single metal systems remain more accurate because the experimental conditions are more ideal. Under our experimental conditions, competition effects are observed for the low affinity metals in the presence of higher affinity metals. The LFER plot using the formation of M–carboxyl and MOH–carboxyl with M–acetate complexes yields linear correlation coefficients (r) of 0.82 and 0.73, respectively. We also observe that A. flavithermus generally adsorbs less metal than B. subtilis at similar metal-to-biomass concentration ratio over the pH range studied.