Abstract
Binding of nanoparticles (NPs) to mineral surfaces influences their transport through the environment. The potential, however, for growing minerals to immobilize NPs via occlusion (the process of trapping particles inside the growing mineral) has yet to be explored in environmentally relevant systems. In this study, the ureolytic bacteria Sporosarcina pasteurii was used to induce calcium carbonate precipitation in the presence of organo-metallic manufactured nanoparticles. As calcite crystals grew the nanoparticles in the solution became trapped inside these crystals. Capture of NPs within the calcite via occlusion was verified by transmission electron microscopy of thin foils. Nanoparticles with a negative surface charge were captured with greater efficiency than those with a positive surface charge, resulting from stronger attachment of negative nanoparticles to the positively charged calcite surfaces, which in turn facilitated occlusion. Thermodynamic and kinetic analysis, however, did not reveal a significant difference in kp (calcite precipitation rate constant) or the critical saturation at which precipitation initiates (Scrit), indicating the presence of different charged nanoparticles did not influence calcite precipitation at the concentrations used here. Overall, these findings demonstrate that microbially driven mineral precipitation has potential to immobilize nanoparticles in the environment via occlusion.
Highlights
Microorganisms have the ability to drive the precipitation of a wide range of minerals
While this study has focused upon bacterially driven calcite precipitation, there are numerous other microbially driven mineral precipitation systems which have the potential for occlusion of NPs, such as the oxidation of Mn(II) (Sujith and Bharathi, 2011), Fe oxidation and reduction (Gault et al, 2012), or enzymatic precipitation of phosphate minerals (Schulz and Schulz, 2005)
The results presented here demonstrate that microbially mediated calcite precipitation captured negatively charged NPs while positively charged NPs were captured much less successfully which facilitated occlusion
Summary
Microorganisms have the ability to drive the precipitation of a wide range of minerals. The capacity to hydrolyse urea (ureolysis) is common in soil and aquifer microorganisms and has the ability to drive calcite precipitation (Fujita et al, 2010) This process can be manipulated for solid phase capture of heavy metals and radionuclides, where the foreign ion gets incorporated into the calcium carbonate crystal structure as it forms (e.g., 90Sr replacing Ca in the crystal lattice), preventing their mobility in the subsurface (Warren et al, 2001). During ureolysis-driven calcium carbonate precipitation, urea is hydrolysed by the microbial enzyme urease, producing ammonia and carbonic acid (Eq (1)), which equilibrates in water to form bicarbonate, ammonium and hydroxide ions. This leads to a pH rise and if soluble calcium is present, an increase in CaCO3 saturation state. Once CaCO3 becomes supersaturated, CaCO3 minerals such as calcite precipitate (Tobler et al, 2011) (Eq (2))
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