Low Concentrations of Surfactants Enhance Siderophore-Promoted Dissolution of Goethite

Abstract

Surface-active agents (surfactants) are released by many soil bacteria and plant roots and are also important as environmental contaminants. Their presence at interfaces could influence important biogeochemical processes in soils such as ligand-controlled dissolution, an important process in biological iron acquisition. To investigate their potential influence on ligand-controlled dissolution of iron oxides, we studied the dissolution kinetics of goethite (alpha FeOOH) at pH 6 in the presence of the bacterial siderophore desferrioxamine B (DFOB) and the anionic surfactant sodium dodecyl sulfate (SDS). The adsorption isotherm of SDS on goethite showed an increase in the slope at concentrations ranging between 300 and 400 microM SDS in solution. This increase in slope suggested the onset of admicelle formation. Adsorption of DFOB onto goethite increased strongly with increasing concentrations of adsorbed SDS. Small concentrations of SDS (5 microM) resulted in a 3-fold acceleration of DFOB-controlled goethite dissolution in the presence of 80 microM DFOB, compared to the suspensions without SDS. The effects of SDS on the goethite dissolution rates were less pronounced at higher SDS concentrations, and became negligible above 600 microM total SDS. The dissolution rates of goethite were not proportional to the adsorbed DFOB concentrations, as would be expected for ligand-controlled dissolution. We speculate that increasing concentrations of adsorbed SDS result in a change in DFOB surface speciation from inner-sphere to outer-sphere complexes and, consequently, the ligand-controlled dissolution rates are not linearly related to the adsorbed DFOB concentration. Our results provide the first evidence for an important role of biosurfactants in biological iron acquisition involving siderophores.

Metallophores and Trace Metal Biogeochemistry

“…Ligand-promoted mineral dissolution rates, and thus the relative availability of the metals contained in the surface layers of the mineral, are controlled by environmental factors including pH, temperature, element substitution, particle size, surfactant adsorption (Barton et al 2012;Carrasco et al 2007;Cervini-Silva and Sposito 2002;Cocozza et al 2002;Reichard et al 2007a), as well as the presence of other organic molecules and exudates. Dissolution rates of oxyhydroxide minerals in the presence of metallophores and low-molecular-mass organic acids (LMMOA) have been shown to exhibit a synergistic effect, i.e., the sum of the dissolution rates of the mineral in the presence of LMMOA and of the mineral in the presence of the metallophore is smaller than the dissolution rate when both are present at the same time (Cervini-Silva and Sposito 2002;Cheah et al 2003).…”

Mechanisms of goethite dissolution in the presence of desferrioxamine B and Suwannee River fulvic acid at pH 6.5

“…However, for our systems at pH 6.5, we 454 observe a surface excess of DFOB ranging from 14.4 to 26.5 μmol g -1 (Table 1). Much of this 455 DFOB will be adsorbed via inner-sphere surface complexes (Carrasco et al, 2007), however 456 electrostatic factors may be significant in increasing overall uptake. The predicted 457 electrostatic repulsion at pH 6.5 between DFOB (pK a ~ 8.6) and the positively charged 458 goethite surface (PZC = 9.2) can be minimised through orientation of the approaching 459 siderophore such that the hydroxamate group furthest from the protonated amine makes first 460 contact with the surface (Cocozza et al, 2002).…”

Coupled biogeochemical cycling of iron and manganese as mediated by microbial siderophores

“…Dissolution rates of (hydr)oxides are generally pH independent over the environmentally relevant range (i.e., pH 5-9) and dissolution proceeds exclusively by a ligand-promoted reaction, producing Fe(III)-complexes (Lloyd 1999). Hydroxamate siderophore-promoted dissolution rates, compiled from a number of sources (Hersman et al 1995;Lloyd 1999;Cocozza et al 2002;Neubauer et al 2002;Yoshida et al 2002;Cheah et al 2003;Carrasco et al 2007;Wolff-Boenisch and Traina 2007;Carrasco et al 2008) and spanning a large range of pH (3-9), concentration (1.3 9 10 -5 -10 -3 M), and solid phases [a-FeOOH, a-Fe 2 O 3 , and a poorly crystalline Fe(III)-hydroxide] vary by less than a factor of 20 (R = 10 -11.5 -10 -12.8 mol m -2 s -1 ), strongly suggesting commonalities among the mechanisms of hydroxamate siderophore-promoted dissolution. Desferrioxamine B promoted-dissolution of goethite appears to be surface-controlled (Cheah et al 2003) and possibly mediated by the formation of dissolutionactive bidentate mononuclear surface structures (Holmen et al 1997;Cocozza et al 2002).…”