Microbial degradation of urea was investigated as a potential geochemical catalyst for Ca carbonate precipitation and associated solid phase capture of common groundwater contaminants (Sr, UO2, Cu) in laboratory batch experiments. Bacterial degradation of urea increased pH and promoted Ca carbonate precipitation in both bacterial control and contaminant treatments. Associated solid phase capture of Sr was highly effective, capturing 95% of the 1 mM Sr added within 24 h. The results for Sr are consistent with solid solution formation rather than discrete Sr carbonate phase precipitation. In contrast, UO2 capture was not as effective, reaching only 30% of the initial 1 mM UO2 added, and also reversible, dropping to 7% by 24 h. These results likely reflect differing sites of incorporation of these two elements-Ca lattice sites for Sr versus crystal defect sites for UO2. Cu sequestration was poor, resulting from toxicity of the metal to the bacteria, which arrested urea degradation and concomitant Ca carbonate precipitation. Scanning electron microscopy (SEM) indicated a variety of morphologies reminiscent of those observed in the marine stromatolite literature. In bacterial control treatments, X-ray diffraction (XRD) analyses indicated only calcite; while in the presence of either Sr or UO2, both calcite and vaterite, a metastable polymorph of Ca carbonate, were identified. Tapping mode atomic force microscopy (AFM) indicated differences in surface microtopography among abiotic, bacterial control, and bacterial contaminant systems. These results indicate that Ca carbonate precipitation induced by passive biomineralization processes is highly effective and may provide a useful bioremediation strategy for Ca carbonate-rich aquifers where Sr contamination issues exist.
As a first step towards understanding microbial dissolution processes, our research focuses on characterizing attachment features that form between a Pseudomonas sp. bacteria and the Fe(III)‐(hydr)oxide minerals hematite and goethite. Microbial growth curves in Fe‐limited growth media indicated that the bacteria were able to obtain Fe from the Fe(III)‐(hydr)oxidesfor use in metabolic processes. A combination of scanning electron microscopy, epifluorescence, and Tapping Mode™ atomic‐force microscopy showed that the bacteria colonized some fraction of mineralogical aggregates. These aggregates were covered by bacteria and were linked together by relatively open biofilms consisting of networks of fiber‐like attachment features intertwined through thin films of amorphous‐looking organic material. The biofilm material encompassed numerous individual bacteria, as well as minéralogie particles. We hypothesize that the bacteria first attached to mineral aggregates, perhaps via their flagella, forming colonies. Following initial attachment, the bacteria exuded additional attachment features in the form of fine, branching fibrils intertwined through thin films. The detailed structures of these attachment features were highlighted by Phase Imaging atomic‐force microscopy, which served as a real‐time contrast enhancement technique and showed some poorly defined sensitivity to different surface materials, most probably related to differences in stiffness or viscoelasticity. Although the mechanism of the microbially enhanced dissolution remains unknown, we hypothesize that the bacteria may have produced micro environments conducive to dissolution through the use of observed extracellular materials.
Dissolution processes of Pb uranyl-hydroxy-hydrate phases with framework and sheet structural units of polymerized uranyl polyhedra were studied in order to understand the role of the interstitial Pb cations and the degree of polymerization of the structural unit during those processes. Batch-dissolution experiments on single crystals of fourmarierite, Pb 1– x (H 2 O) 4 [(UO 2 ) 6 O 3–2 x (OH) 4+2 x ] (sheet structural-unit) and synthetic Pb 2 (H 2 O)[(UO 2 ) 10 UO 12 (OH) 6 (H 2 O) 2 ] ( PbUOH , framework structural-unit) were done in HCl solution of pH 2, in distilled water, in 0.1 mol L −1 Na 2 CO 3 solution of pH 10.5, in 1.0 mol L −1 M Cl solutions of pH 2 ( M = Na, K), and in 0.5 mol L −1 M Cl 2 solutions of pH 2 ( M = Ba, Ca, Sr, Mg). Dissolution features on the basal surface of these phases were examined with atomic force microscopy, scanning electron microscopy and optical microscopy. Hillocks on the basal surface of fourmarierite form in distilled water, in HCl, KCl and SrCl 2 solutions of pH 2 and in a Na 2 CO 3 solution. Etch pits form only on the basal surface in solutions of pH 2, indicating that their formation is promoted by a higher activity of protons in solution. The symmetry and elongation of etch pits formed in electrolyte solutions of pH 2 can vary with the type of cation in solution. The formation of hillocks was not observed on the (001) face of PbUOH , which is in contrast to observations on the basal surfaces of curite, becquerelite, billietite and fourmarierite. The outline of etch pits on the basal surface of PbUOH formed in distilled water and in Na 2 CO 3 solution can be described as a regular parallelogram, whereas etch pits formed in aqueous SrCl 2 , MgCl 2 , CaCl 2 and NaCl solutions of pH 2 have the shape of a distorted parallelogram. The cations in solution have a different effect on the lateral dimensions and depth of etch pits formed on the prominent surfaces of fourmarierite and synthetic PbUOH . The relief of dissolution features on the basal surface of fourmarierite increases in electrolyte solutions in the sequence BaCl 2 2 2 2 , and on the prominent (001) surface of synthetic PbUOH , in the sequence KCl 2 2 2 2 . The relief of dissolution features on the basal faces of both phases is inversely correlated with the size of the cation in solution. This correlation is explained by the higher affinity of large alkali and alkaline-earth cations (Ba, K) to oxide and silicate surfaces, which may result in a stronger adsorption of these cations to specific surface-sites. A stronger adsorption of cations on surface sites lowers the number of protonated anion-terminations (activated sites) and may result in a lower rate of dissolution perpendicular to the basal surface and edges.
Apatites provide a mechanism for in-situ remediation of Pb-contaminated sites, by supplying phosphate which combines with dissolved Pb to form highly insoluble pyromorphite. This study expanded upon previous research using hydroxylapatite (HAP), to focus on how the surface properties of natural chlorapatite (CAP) and fluorapatite (FAP) affect pyromorphite nucleation and growth, when aqueous Pb2+ ([Pb] < 50 mg/L as PbCl2) is reacted with apatite at pH = 4.2, 22 °C. A combination of atomic force microscopy (AFM), scanning electron microscopy (SEM), optical microscopy, energy dispersive X-ray fluorescence spectroscopy (EDS), infrared spectroscopy (FTIR), and X-ray diffraction (XRD) was used for in-situ and ex-situ examination of the interface and the reaction products.
