Raina Margaret Maier

The rare earth elements (REE) are increasingly important in a variety of science and economic fields, including (bio)geosciences, paleoecology, astrobiology, and mining. However, REE distribution in early rock-microbe-plant systems has remained elusive. We tested the hypothesis that REE mass-partitioning during incipient weathering of basalt, rhyolite, granite and schist depends on the activity of microbes, vascular plants (Buffalo grass), and arbuscular mycorrhiza. Pore-water element abundances revealed a rapid transition from abiotic to biotic signatures of weathering, the latter associated with smaller aqueous loss and larger plant uptake. Abiotic dissolution was 39% of total denudation in plant-microbes-mycorrhiza treatment. Microbes incremented denudation, particularly in rhyolite, and this resulted in decreased bioavailable solid pools in this rock. Total mobilization (aqueous + uptake) was ten times greater in planted compared to abiotic treatments, REE masses in plant generally exceeding those in water. Larger plants increased bioavailable solid pools, consistent with enhanced soil genesis. Mycorrhiza generally had a positive effect on total mobilization. The main mechanism behind incipient REE weathering was carbonation enhanced by biotic respiration, the denudation patterns being largely dictated by mineralogy. A consistent biotic signature was observed in La:phosphate and mobilization: solid pool ratios, and in the pattern of denudation and uptake.

PMID: 11285908;Abstract:

The presence of toxic metals in natural environments presents a potential health hazard for humans. Metal contaminants in these environments are usually tightly bound to colloidal particles and organic matter. This represents a major constraint to their removal using currently available in situ remediation technologies. One technique that has shown potential for facilitated metal removal from soil is treatment with an anionic microbial surfactant, rhamnolipid. Successful application of rhamnolipid in metal removal requires knowledge of the rhamnolipid-metal complexation reaction. Therefore, our objective was to evaluate the biosurfactant complexation affinity for the most common natural soil and water cations and for various metal contaminants. The conditional stability constant (log K) for each of these metals was determined using an ion-exchange resin technique. Results show the measured stability constants follow the order (from strongest to weakest):Al³⁺ > Cu²⁺ > Pb²⁺ > Cd²⁺ > Zn²⁺ > Fe³⁺ > Hg²⁺ > Ca²⁺ > Co²⁺ > Ni²⁺ > Mn²⁺> Mg²⁺ > K⁺. These data indicate that rhamnolipid will preferentially complex metal contaminants such as lead, cadmium, and mercury in the presence of common soil or water cations. The measured rhamnolipid-metal stability constants were found in most cases to be similar or higher than conditional stability constants reported in the literature for metal complexation with acetic acid, oxalic acid, citric acid, and fulvic acids. These results help delineate the conditions under which rhamnolipid may be successfully applied as a remediation agent in the removal of metal contaminants from soil, as well as surface waters, ground water, and wastestreams.

PMID: 1444363;PMCID: PMC183091;Abstract:

A microbial surfactant (biosurfactant) was investigated for its potential to enhance bioavailability and, hence, the biodegradation of octadecane. The rhamnolipid biosurfactant used in this study was extracted from culture supernatants after growth of Pseudomonas aeruginosa ATCC 9027 in phosphate- limited proteose peptone-glucose-ammonium salts medium. Dispersion of octadecane in aqueous solutions was dramatically enhanced by 300 mg of the rhamnolipid biosurfactant per liter, increasing by a factor of more than 4 orders of magnitude, from 0.009 to >250 mg/liter. The relative enhancement of octadecane dispersion was much greater at low rhamnolipid concentrations than at high concentrations. Rhamnolipid-enhanced octadecane dispersion was found to be dependent on pH and shaking speed. Biodegradation experiments done with an initial octadecane concentration of 1,500 mg/liter showed that 20% of the octadecane was mineralized in 84 h in the presence of 300 mg of rhamnolipid per liter, compared with only 5% octadecane mineralization when no surfactant was present. These results indicate that rhamnolipids may have potential for facilitating the bioremediation of sites contaminated with hydrocarbons having limited water solubility.

Mine tailings can host elevated concentrations of toxic metal(loid)s that represent a significant hazard to surrounding communities and ecosystems. Eolian transport, capable of translocating small (micrometer-sized) particles, can be the dominant mechanism of toxic metal dispersion in arid or semiarid landscapes. Human exposure to metals can then occur via direct inhalation or ingestion of particulates. The fact that measured doses of total lead (Pb) in geomedia correlate poorly with blood Pb levels highlights a need to better resolve the precise distribution of molecularly speciated metal-bearing phases in the complex particle mixtures. Species distribution controls bioaccessibility, thereby directly impacting health risk. This study seeks to correlate Pb-containing particle size and mineral composition with lability and bioaccessibility in mine tailings subjected to weathering in a semiarid environment. We employed X-ray absorption spectroscopy (XAS) and X-ray fluorescence (XRF), coupled with sequential chemical extractions, to study Pb speciation in tailings from the semiarid Arizona Klondyke State Superfund Site. Representative samples ranging in pH from 2.6 to 5.4 were selected for in-depth study of Pb solid-phase speciation. The principle lead-bearing phase was plumbojarosite (PbFe(6)(SO(4))(4)(OH)(12)), but anglesite (PbSO(4)) and iron oxide-sorbed Pb were also observed. Anglesite, the most bioavailable mineral species of lead identified in this study, was enriched in surficial tailings samples, where Pb concentrations in the clay size fraction were 2-3 times higher by mass relative to bulk. A mobile and bioaccessible Pb phase accumulates in surficial tailings, with a corresponding increase in risk of human exposure to atmospheric particles.

PMID: 12371483;Abstract:

Lowering pH of a microbiological medium from 7 to 4 decreased cadmium toxicity during naphthalene biodegradation by a Burkholderia sp. Cadmium speciation and cadmium accumulation in the system were studied to explain this effect. Cadmium speciation was determined by direct measurement and by geochemical modeling. Previous studies have implicated the monovalent hydroxylated cadmium (CdOH⁺) species in the effect of pH on cadmium toxicity. Modeling analysis predicted CdOH⁺ formation only at very low concentrations (≤0.0128 μM), while the measured concentration of divalent ionic cadmium (Cd²⁺) was at least three orders of magnitude greater, suggesting that Cd²⁺ is the more significant metal form. With respect to cadmium accumulation, cells contained in media adjusted to pH 4 accumulated only 2.76 ± 0.76 mg Cd/g cells, whereas cells in media adjusted to pH 7 accumulated 8.52 ± 0.71 mg Cd/g cells. These data suggest that cadmium toxicity is correlated with increased cadmium accumulation rather than the formation of CdOH⁺ as pH is increased. At low pH, the decrease in cadmium accumulation may be caused by increased competition between hydrogen and cadmium ions for binding sites on the cell surface or by an increase in metal efflux pump activity due to an increase in the proton gradient that drives the efflux pump.