Raina Margaret Maier

Abstract:

The efficiency of biosurfactant-facilitated removal of soil-bound metals is affected by biosurfactant sorption to soil. In this study, batch and column experiments were performed to minimize rhamnolipid biosurfactant sorption and to optimize rhamnolipid application for removal of cadmium from four soils. In batch studies, rhamnolipid sorption to a model coarse loamy soil was found to vary with applied rhamnolipid and K⁺ concentration of the rhamnolipid matrix. The presence of solution-phase biosurfactant was correlated to the release into solution of a soil-bound metal (cadmium). A series of column experiments was performed to evaluate whether rhamnolipid could remove cadmium from soil under saturated flow conditions. Four different soils were contaminated with cadmium and treated first with an KNO3 electrolyte solution (3.5 or 7 mM K⁺) and then with a rhamnolipid- containing solution (5 or 10 mM). Results showed that between 15 and 36% of the cadmium was removed by the initial electrolyte treatment and an additional 8-54% of the cadmium was removed by rhamnolipid treatment. Rhamnolipid treatment was very effective for three of the soils tested, but for the soil with the highest clay content, rhamnolipid application caused soil dispersion and column plugging.

Abstract:

Bacterial adhesion is the first step in biofilm formation which impacts numerous environmental, industrial and medical processes. Examples of undesirable consequences of biofilm formation include metal rust, sewage sludge and bacteria-related diseases. Desirable consequences are biofiltration and bioremediation. Bacteria are resilient and can survive in harsh environments. A severe stress is desiccation since dehydration can damage DNA and change the properties of proteins. Some bacteria protect against dehydration by accumulating sugars such as sucrose and trehalose while others undergo a transformation from an active to a dormant state. Evaporative deposition of bacteria on a surface shows that some bacteria aggregate to form two dimensional patterns which may be important for nutrient sharing and survival in dry conditions'. Since bacteria are increasingly being employed as components in biosensors and biofilm reactors, it is important to understand the material properties of bacteria in dry conditions for these applications. For a decade, Atomic Force Microscopy (AFM) has been the primary tool used to study the adhesion and elastic properties of individual bacteria. In this work we show it is possible to use a Surface Forces Apparatus (SFA) to measure elastic and adhesive properties of small collections of surface bound bacteria. The measurements are conducted with incomplete, patterned bacterial films and we have developed a protocol to image the contact area with AFM after the experiment. Using the SFA, we measured the force profile between a Pseudomonas aeruginosa PAO1 film and a bare mica surface. P. aeruginosa PAO1 is a . ubiquitous gram-negative soil bacterium and is also an opportunistic pathogen. We repeated the measurement in the same contact position for six days to determine the effect of desiccation on the film material properties. © 2006 Materials Research Society.

Abstract:

Complexation of cadmium, lead, and zinc (singly and in a mixture) by a monorhamnolipid biosurfactant produced by Pseudomonas aeruginosa ATCC 9027 was studied in batch solution and soil experiments. Conditional stability constants (log K(L)) for metal-rhamnolipid complexation in a buffered medium (0.1 M Pipes, pH 6.8) were determined in duplicate using an ion-exchange technique and averaged 6.5 (Cd²⁺), 6.6 (Pb²⁺), and 5.4 (Zn²⁺); these values are similar or slightly higher than literature values for Cd²⁺ and Pb²⁺ complexation with fulvic acid and activated sludge solids. To determine the ability of rhamnolipid to desorb soil-bound metals, rhamnolipid solutions (12.5, 25, 50, and 80 mM) were added to soil containing sorbed Cd²⁺ (1.46 mmol kg^-1), Pb²⁺ (1.96 mmol kg^-1), or a mixture of Pb²⁺- Cd²⁺-Zn²⁺ (3.4 mmol kg^-1). At 12.5 and 25 mM rhamnolipid, rhamnolipid sorption to soil exceeded 78%, and less than 11% of soil-bound Cd²⁺ and Zn²⁺ was desorbed. However, ion exchange of bound metals with K⁺ present in the rhamnolipid matrix could account for the removal of between 16 and 48% of the sorbed Cd²⁺ and Zn²⁺. At 50 and 80 mM rhamnolipid, rhamnolipid sorption to soil decreased to between 20 and 77%, and the removal of Cd²⁺ and Zn²⁺ could exceed the removal by ion exchange by as much as 3-fold. The behavior of Pb²⁺ was quite different. Less than 2% of soil-bound Pb²⁺ was desorbed due to ion exchange, although up to 43% was desorbed by 80 mM rhamnolipid.

PMID: 7621801;PMCID: PMC1519337;Abstract:

Bioremediation of metal-contaminated wastestreams has been successfully demonstrated. Normally, whole cells or microbial exopolymers are used to concentrate and/or precipitate metals in the wastestream to aid in metal removal. Analogous remediation of metal-contaminated soils is more complex because microbial cells or large exopolymers do not move freely through the soil. The use of microbially produced surfactants (biosurfactants) is an alternative with potential for remediation of metal-contaminated soils. The distinct advantage of biosurfactants over whole cells or exopolymers is their small size, generally biosurfactant molecular weights are less than 1500. A second advantage is that biosurfactants have a wide variety of chemical structures that may show different metal selectivities and thus, metal removal efficiencies. A review of the literature shows that complexation capacities of several bacterial exopolymers was similar to the complexation capacity of a rhamnolipid biosurfactant produced by Pseudomonas aeruginosa ATCC 9027.

Toxic metalliferous mine-tailings pose a significant health risk to ecosystems and neighboring communities from wind and water dispersion of particulates containing high concentrations of toxic metal(loid)s (e.g., Pb, As, Zn). Tailings are particularly vulnerable to erosion before vegetative cover can be reestablished, i.e., decades or longer in semi-arid environments without intervention. Metal(loid) speciation, linked directly to bioaccessibility and lability, is controlled by mineral weathering and is a key consideration when assessing human and environmental health risks associated with mine sites. At the semi-arid Iron King Mine and Humboldt Smelter Superfund site in central Arizona, the mineral assemblage of the top 2 m of tailings has been previously characterized. A distinct redox gradient was observed in the top 0.5 m of the tailings and the mineral assemblage indicates progressive transformation of ferrous iron sulfides to ferrihydrite and gypsum, which, in turn weather to form schwertmannite and then jarosite accompanied by a progressive decrease in pH (7.3 to 2.3). Within the geochemical context of this reaction front, we examined enriched toxic metal(loid)s As, Pb, and Zn with surficial concentrations 41.1, 10.7, 39.3 mM kg(-1) (3080, 2200, and 2570 mg kg(-1)), respectively. The highest bulk concentrations of As and Zn occur at the redox boundary representing a 1.7 and 4.2 fold enrichment relative to surficial concentrations, respectively, indicating the translocation of toxic elements from the gossan zone to either the underlying redox boundary or the surface crust. Metal speciation was also examined as a function of depth using X-ray absorption spectroscopy (XAS). The deepest sample (180 cm) contains sulfides (e.g., pyrite, arsenopyrite, galena, and sphalerite). Samples from the redox transition zone (25-54 cm) contain a mixture of sulfides, carbonates (siderite, ankerite, cerrusite, and smithsonite) and metal(loid)s sorbed to neoformed secondary Fe phases, principally ferrihydrite. In surface samples (0-35 cm), metal(loid)s are found as sorbed species or incorporated into secondary Fe hydroxysulfate phases, such as schwertmannite and jarosites. Metal-bearing efflorescent salts (e.g., ZnSO4·nH2O) were detected in the surficial sample. Taken together, these data suggest the bioaccessibility and lability of metal(loid)s are altered by mineral weathering, which results in both the downward migration of metal(loid)s to the redox boundary, as well as the precipitation of metal salts at the surface.