Bioavailability of Contaminants in Soil. In: Singh A, Kuhad RC, Ward OP, editors. Advances in Applied Bioremediation. Berlin: Springer Berlin Heidelberg; 2009. pp. 35-71. [DOI: 10.1007/978-3-540-89621-0_3]

Bioavailability of Soil-Sorbed Tetracycline to Escherichia coli under Unsaturated Conditions

Increasing concentrations of anthropogenic antibiotics in soils are partly responsible for the proliferation of bacterial antibiotic resistance. However, little is known about how soil-sorbed antibiotics exert selective pressure on bacteria in unsaturated soils. This study investigated the bioavailability of tetracycline sorbed on three soils (Webster clay loam, Capac sandy clay loam, and Oshtemo loamy sand) to a fluorescent Escherichia coli bioreporter under unsaturated conditions using agar diffusion assay, microscopic visualization, and model simulation. Tetracycline sorbed on the soils could be desorbed and become bioavailable to the E. coli cells at matric water potentials of -2.95 to -13.75 kPa. Bright fluorescent rings were formed around the tetracycline-loaded soils on the unsaturated agar surfaces, likely due to radial diffusion of tetracycline desorbed from the soils, tetracycline uptake by the E. coli cells, and its inhibition on E. coli growth, which was supported by the model simulation. The bioavailability of soil-sorbed tetracycline was much higher for the Oshtemo soil, probably due to faster diffusion of tetracycline in coarse-textured soils. Decreased bioavailability of soil-sorbed tetracycline at lower soil water potential likely resulted from reduced tetracycline diffusion in soil pore water at smaller matric potential and/or suppressed tetracycline uptake by E. coli at lower osmotic potential. Therefore, soil-sorbed tetracycline could still exert selective pressure on the exposed bacteria, which was influenced by soil physical processes controlled by soil texture and soil water potential.

Nanoparticle-supported lipid bilayers as an in situ remediation strategy for hydrophobic organic contaminants in soils

Polycyclic aromatic hydrocarbons (PAHs) are persistent environmental organic contaminants due to their low water solubility and strong sorption onto organic/mineral surfaces. Here, nanoparticle-supported lipid bilayers (NP-SLBs) made of 100-nm SiO2 nanoparticles and the zwitterionic lipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) are investigated as constructs for removing PAHs from contaminated sites, using benzo[a]pyrene (BaP) as an example. DMPC in the form of small unilamellar vesicles (SUVs) or DMPC-NP-SLBs with excess DMPC-SUVs to support colloidal stability, when added to saturated BaP solutions, sorb BaP in ratios of up to 10/1 to 5/1 lipid/BaP, over a 2-week period at 33 °C. This rate increases with temperature. The presence of humic acid (HA), as an analog of soil organic matter, does not affect the BaP uptake rate by DMPC-NP-SLBs and DMPC-SUVs, indicating preferential BaP sorption into the hydrophobic lipids. HA increases the zeta potential of these nanosystems, but does not disrupt their morphology, and enhances their colloidal stability. Studies with the common soil bacteria Pseudomonas aeruginosa demonstrate viability and growth using DMPC-NP-SLBs and DMPC-SUVs, with and without BaP, as their sole carbon source. Thus, NP-SLBs may be an effective method for remediation of PAHs, where the lipids provide both the method of extraction and stability for transport to the contaminant site.