Synthesis and characterization of stabilized oxygen-releasing CaO2 nanoparticles for bioremediation.

Bioremediation is one of the general methods to treat pollutants in soil, sediment, and groundwater. However, the low concentration and restricted dispersion of dissolved oxygen (DO) in these areas have limited the efficiency of remediation especially for microorganisms that require oxygen to grow. Calcium peroxide (CaO2) is one of the oxygen-releasing compounds and has been applied to magnify the remediation efficacy of polluting areas. In this study, CaO2 nanoparticles (NPs) were synthesized and evaluated by wet chemistry methods as well as dry and wet grinding processes. The characteristics of CaO2 particles and NPs were analyzed and compared by dynamic light scattering, transmission electron microscopy, scanning electron microscopy, and X-ray powder diffraction. Our results showed that wet-grinded CaO2 NPs had an average particle size of around 110 nm and were more stable compared to other particles from aggregation and sedimentation tests. In addition, we also observed that CaO2 NPs had better DO characteristics and patterns; these NPs generated higher DO levels than their non-grinded form. Accordingly, our results suggested that wet-grinding CaO2 particles to nanoscale could benefit their usage in bioremediation.

Effects of water chemistry on the destabilization and sedimentation of commercial TiO2 nanoparticles: Role of double-layer compression and charge neutralization.

Nanomaterials are considered to be emerging contaminants because their release into the environment could cause a threat to our ecosystem and human health. This study aims to evaluate the effects of pH, ions, and humic acid on the destabilization and sedimentation of commercial stabilized TiO2 nanoparticles (NPs) in aquatic environments. The average hydrodynamic size of TiO2 NPs was determined to be 52 ± 19 nm by dynamic light scattering. The zero point charge (ZPC) of the commercial TiO2 NPs was found to occur at pH 6. The stability of commercial TiO2 NPs is independent of its concentration in the range of 50-200 mg/L. In the absence of NaCl, the commercial TiO2 NPs rapidly settled down near pHzpc when the aggregated nanoparticle size surpassed 1 µm. However, when the commercial TiO2 NPs aggregated with the increase of NaCl concentrations, the large aggregates (>1 µm) were found to remain suspended. For example, even at the critical aggregation concentration of NaCl (100 meq/L), TiO2 NP aggregates suspended for 45 min and then slowly deposited. This implies an increase in the exposure risk of NPs. In the presence of Suwannee river humic acid (SRHA), the commercial TiO2 NPs did not settle down until the SRHA concentration increased to 20 mg/L, and were seen to restabilize at SRHA concentrations of 50 mg/L. The uncommon behaviors of the commercial TiO2 NPs we observed may be attributed to the different destabilization mechanisms caused by different species (i.e., NaCl and SRHA) in water.