X-ray photoelectron evidence for bacteria-enhanced dissolution of hornblende

An Arthrobacter species capable of extracting Fe from hornblende was isolat ed from a soil from the Adirondacks, NY (USA). This bacteria isolate, used in batch experiments with hornblende, accelerated the release of Fe from ho rnblende without measurably affecting Al release. The isolate produces both low molecular weight organic acids (LMWOA) and a catecholate siderophore. Polished hornblende (glass and crystal) discs were analyzed with X-ray phot oelectron spectroscopy (XPS) before and after incubation with growing Arthr obacter sp. to investigate whether the bacteria caused a distinguishable ch emical signature on the upper 100 Angstrom of mineral surface. After remova l of the arthrobacter grown on hornblende crystal or glass substrates using lysozyme, XPS revealed surface depletion of Fe for samples grown for sever al days in buffered (crystal) and unbuffered (crystal and glass) media. Fe/ Si ratios of hornblende surfaces dissolved under biotic conditions are sign ificantly lower than Fe/Si ratios on surfaces dissolved under abiotic condi tions for similar amounts of time. Enhanced Fe release and the formation of Fe-depleted surfaces is inferred to be caused by catechol complexation at the mineral surface. Because natural siderophore was not isolated in sufficient quantities to ru n bacteria-free leaching experiments, parallel investigations were run with a commercially available siderophore (desferrioxamine B). Desferrioxamine B was observed to enhance release of Fe, Si, and Al from hornblende both wi th and without added bacteria. Formation of desferrioxamine-Fe surface comp lexes were probed by studying the multiple splitting and shift in intensiti es of the N 1s line analyzed by XPS on siderophore +/- Fe on gold surfaces and siderophore + hornblende crystal surfaces. Based upon the observed form ation of an hydroxamate (desferrioxamine) surface complex on hornblende, we infer that catecholate siderophores, such as those produced by the arthrob acter, also complex on the hornblende surface. Surface complexation is favo red because of the extremely high association constants for siderophore + F e(III). X-ray photoelectron spectroscopic data is therefore consistent with a model wherein enhanced Fe release by these bacteria or desferrioxamine B is caused by Fe-siderophore complexation at the silicate surface. Such com plexation presumably weakens bonds between the Fe and the oxide lattice, ca using enhanced Fe leaching and an Fe-depleted surface. Some leaching may al so be due to LMWOA, although this is interpreted to be of secondary importa nce. Copyright (C) 2000 Elsevier Science Ltd.