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.
Copper has been found to play a key role in the physiology of methanotrophic micro-organisms, and methane oxidation may critically depend on the availability of Cu. In natural environments, such as soils, sediments, peat bogs, and surface waters, the presence of natural organic matter (NOM) can control the bioavailability of Cu by forming strong metal complexes. To promote Cu acquisition, methanotrophs exude methanobactin, a ligand known to have a high affinity for Cu. In this study, the capability of methanobactin for Cu acquisition from NOM was investigated using humic acid (HA) as a model substance. The kinetics of ligand exchange between Cu-HA and methanobactin was observed by UV-vis spectroscopy, and the speciation of Cu bound to methanobactin was determined by size-exclusion chromatography coupled to an ICP-MS. The results showed that Cu was mobilized from HA by a fast ligand exchange reaction following a second-order rate law with first-order kinetics for both methanobactin and Cu-HA complexes. The reaction rates decreased with decreasing temperature. Equilibrium experiments indicated that methanobactin was not sorbed to HA and proved that methanobactin is competitive with HA for Cu binding by forming strong 1:1 Cu-methanobactin complexes. Consequently, our results demonstrate that methanobactin can efficiently acquire Cu in organic-rich environments.
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