skip to main content

Title: Effects of rhamnolipid biosurfactant on the dissolution and transport of silver nanoparticles in porous media
The effects of nanoscale silver (nAg) particles on subsurface microbial communities can be influenced by the presence of biosurfactants, which have been shown to alter nanoparticle surface properties. Batch and column studies were conducted to investigate the influence of rhamnolipid biosurfactant (1–50 mg L −1 ) on the stability and mobility of silver nanoparticles (16 ± 4 nm) in batch reactors and water-saturated columns with three solution chemistries: pH = 4 and dissolved oxygen concentration (DO) = 8.8 mg L −1 , pH = 7 and DO = 8.8 mg L −1 , pH = 7 and DO = 2.0 mg L −1 . In batch studies, the presence of rhamnolipid (2–50 mg L −1 ) reduced nAg dissolution by 83.3–99.1% under all pH and DO conditions. Improved nAg stability was observed when rhamnolipid was present in batch reactors at pH = 7 ± 0.2, where the hydrodynamic diameter remained constant (∼50 nm) relative to rhamnolipid-free controls (increased to >230 nm) in 48 hours. Column experiments conducted at pH 4.0 ± 0.2 demonstrated that co-injection of nAg with rhamnolipid (2, 5 and 50 mg L −1 ) decreased Ag + breakthrough from ∼22% of total applied mass in rhamnolipid-free columns more » to less than 8.1% in the presence of rhamnolipid and altered the shape of the nAg retention profile from a hyper-exponential to a uniform distribution. Column experiments performed at pH 7.0 ± 0.2 and DO levels of either ∼2.0 or ∼8.8 mg L −1 showed that co-injection of 5 mg L −1 and 50 mg L −1 rhamnolipid increased nAg mass breakthrough by 25–40% and ∼80%, respectively, enhancements in nAg stability and mobility were attributed to rhamnolipid adsorption on nAg surfaces, which effectively slowed the oxidation and thus release of Ag + , and adsorption of rhamnolipid on the porous medium, which competed for nAg attachment sites. These results indicate that the presence of rhamnolipid significantly influenced nAg dissolution and mobility under dynamic flow conditions. A mathematical model based on modified filtration theory (MFT) accurately reproduced nAg transport and retention behavior when aggregation and reaction processes were minimal and when rhamnolipid was present, providing a tool to predict the effects of biosurfactants on nAg transport in porous media. « less
 ;  ;  ;  ;  ;  ;  
Award ID(s):
Publication Date:
Journal Name:
Environmental Science: Nano
Sponsoring Org:
National Science Foundation
More Like this
  1. Although prior studies have investigated the effects of solution constituents, including dissolved organic matter and synthetic polymers, on nanoparticle mobility in porous media, far less attention has been directed toward evaluating the impacts of biosurfactants secreted by microorganisms on the transport and retention behavior of nanomaterials. The objective of this study was to explore the influence of rhamnolipid, a biosurfactant associated with biofilms, on the transport and retention of iron oxide nanoparticles (IONPs) in a water-saturated quartz sand. Column experiments were conducted using aerobic medium (ionic strength = 50.4 mM) or 10 mM NaCl as background electrolyte at a pore velocity of 0.43 m per day and pH 6.8 ± 0.2. In aerobic medium columns, nearly all introduced nanoparticles were retained when IONPs were injected alone, whereas the presence of 10 mg L −1 or 50 mg L −1 rhamnolipid resulted in ∼25% and ∼50% breakthrough of the injected IONP mass, respectively. Moreover, preflushing media with 50 mg L −1 rhamnolipid further increased IONP mass breakthrough by ∼30%. Similar enhancement of nanoparticle mobility by 50 mg L −1 rhamnolipid was also measured in lower ionic strength (10 mM NaCl) columns. Mathematical models that incorporated nanoparticle filter ripening and biosurfactant competitivemore »adsorption successfully reproduced experimental observations. Modeling results predicted an order-of-magnitude decrease in IONP filter ripening rate coefficient and a three-fold drop in average IONP retention capacity in the presence of rhamnolipid, consistent with a stabilizing effect and competition for surface sites. These findings demonstrate that rhamnolipid biosurfactant can potentially enhance nanomaterial stability and mobility in subsurface environments and that these effects should be considered when evaluating the impact of biological process on nanoparticle fate and transport in porous media.« less
  2. Silver nanoparticles (AgNPs) are widely used in commerce, however, the effect of their physicochemical properties on toxicity remains debatable because of the confounding presence of Ag+ ions. Thus, we designed a series of AgNPs that are stable to surface oxidation and Ag+ ion release. AgNPs were coated with a hybrid lipid membrane comprised of L-phosphatidylcholine (PC), sodium oleate (SOA), and a stoichiometric amount of hexanethiol (HT) to produce oxidant-resistant AgNPs, Ag–SOA–PC–HT. The stability of 7-month aged, 20–100 nm Ag–SOA–PC–HT NPs were assessed using UV–Vis, dynamic light scattering (DLS), and inductively coupled plasma mass spectrometry (ICP-MS), while the toxicity of the nanomaterials was assessed using a well-established, 5-day embryonic zebrafish assay at concentrations ranging from 0–12 mg/L. There was no change in the size of the AgNPs from freshly made samples or 7-month aged samples and minimal Ag+ ion release (<0.2%) in fishwater (FW) up to seven days. Toxicity studies revealed AgNP size- and concentration-dependent effects. Increased mortality and sublethal morphological abnormalities were observed at higher concentrations with smaller nanoparticle sizes. This study, for the first time, determined the effect of AgNP size on toxicity in the absence of Ag+ ions as a confounding variable.
  3. The controlled synthesis of stable silver nanoparticles (AgNPs), that do not undergo surface oxidation and Ag + ion dissolution, continues to be a major challenge. Here the synthesis of robust hybrid lipid-coated AgNPs, comprised of l -α-phosphatidylcholine (PC) membranes anchored by a stoichiometric amount of long-chained hydrophobic thiols and sodium oleate (SOA) as hydrophobic binding partners, that do not undergo surface oxidation and Ag + ion dissolution, is described. UV-Visible (UV-Vis) spectroscopy, transmission electron microscopy (TEM), and inductively coupled plasma mass spectrometry (ICP-MS) demonstrate that in the presence of strong oxidants, such as potassium cyanide (KCN), the hybrid lipid-coated AgNPs are stable and do not undergo surface oxidation even in the presence of membrane destabilizing surfactants. UV-Vis studies show that the stability of hybrid lipid-coated AgNPs of various sizes and shapes is dependent on the length of the thiol hydrocarbon chain and can be ranked in the order of increasing stability as follows: propanethiol (PT) < hexanethiol (HT) ≤ decanethiol (DT). UV-Vis and ICP-MS studies show that the hybrid lipid-coated AgNPs do not change in size or shape confirming that the AgNPs do not undergo surface oxidation and Ag + ion dissolution when placed in the presence of strong oxidants,more »chlorides, thiols, and low pH. Long-term stability studies, over 21 days, show that the hybrid lipid-coated AgNPs do not release Ag + ions and are more stable. Overall, these studies demonstrate hybrid membrane encapsulation of nanomaterials is a viable method for stabilizing AgNPs in a “shape-locked” form that is unable to undergo surface oxidation, Ag + ion release, aging, or shape conversion. More importantly, this design strategy is a simple approach to the synthesis and stabilization of AgNPs for a variety of biomedical and commercial applications where Ag + ion release and toxicity is a concern. With robust and shielded AgNPs, investigators can now evaluate and correlate how the physical features of AgNPs influence toxicity without the confounding factor of Ag + ions present in samples. This design strategy also provides an opportunity where the membrane composition can be tuned to control the release rate of Ag + ions for optimizing antimicrobial activity.« less
  4. After release into the aquatic environment, engineered nanomaterials (ENMs) undergo complex chemical and physical transformations that alter their environmental fate and toxicity to aquatic organisms. Hyalella azteca are sediment-dwelling amphipods predicted to have a high exposure level to ENMs and have previously shown to be highly sensitive to ZnO nanoparticles (NPs). To investigate the impacts of environmentally transformed ZnO NPs and determine the route of uptake for these particles, we exposed H. azteca to ZnSO 4 , ZnO NPs, and environmental aged ZnO NPs which resulted in three types of particles: 30 nm ZnO–Zn 3 (PO 4 ) 2 core–shell structures (p8-ZnO NPs), micron scale hopeite-like phase Zn 3 (PO 4 ) 2 ·4H 2 O (p6-ZnO NPs), and ZnS nano-clusters (s-ZnO NPs). Treatments included freshwater, saltwater (3 ppt), and the presence of sediment, with a final treatment where animals were contained within mesh baskets to prevent burrowing in the sediment. Dissolution was close to 100% for the pristine ZnO NPs and phosphate transformed NPs, while s-ZnO NPs resulted in only 20% dissolution in the water only exposures. In the freshwater exposure, the pristine and phosphate transformed ZnO NPs were more toxic (LC 50 values 0.11–0.18 mg L −1 )more »than ZnSO 4 (LC 50 = 0.26 mg L −1 ) and the s-ZnO NPs (LC 50 = 0.29 mg L −1 ). Saltwater treatments reduced the toxicity of ZnSO 4 and all the ZnO NPs. In the presence of sediment, water column concentrations of Zn were reduced to 10% nominal concentrations and toxicity in the sediment with basket treatment was similarly reduced by a factor of 10. Toxicity was further reduced in the sediment only treatments where the sediments appeared to provide a refuge for H. azteca . In addition, particle specific differences in toxicity were less apparent in the presence of sediment. Bioaccumulation was similar across the different Zn exposures, but decreased with reduced toxicity in the saltwater and sediment treatments. Overall, the results suggest that H. azteca is exposed to ZnO NPs through the water column and NP transformations in the presence of phosphate do not reduce their toxicity. Sulfidized ZnO NPs have reduced toxicity, but their similar level of bioaccumulation in H. azteca suggests that trophic transfer of these particles will occur.« less
  5. Abstract Highlights

    IHP promotes the dissolution of kaolinite mainly through the formation of aluminium phytate complex.

    IHP sorption presents a sharp maximum at pH 4.0.

    IHP forms inner‐sphere complexes at the surface of kaolinite.

    Formation of aluminium phytate surface precipitates is favourable at relatively low pH.

    « less