Reactivity characterization of SiO2-coated nano zero-valent iron for iodoacetamide degradation: The effects of SiO2 thickness, and the roles of dehalogenation, hydrolysis and adsorption.

The effect of SiO2-layer thickness in SiO2-coated nano zero-valent iron (nZVI) particles on the reactivity characteristics of iodoacetamide (IAcAm) degradation was evaluated. SiO2-layer thicknesses ranging from 3.6 to 27.3 nm were obtained through varying tetraethyl orthosilicate dosages of 0.001-1 M. The crystallinity, surface chemical composition, and physicochemical properties were evaluated for their effects on synergetic degradation mechanisms, dehalogenation, hydrolysis, and adsorption. At a thickness of 3.6 nm, the SiO2 layer offered the highest observed pseudo-first-order rate (kobs) and higher rates of IAcAm degradation were maintained under pH fluctuations (pH 5-7) and aerobic conditions compared to pristine nZVI. At this SiO2-layer thickness (3.6 nm), the rate of iron oxide-layer formation was reduced and the migration of reactive iron species (Fe0 and Fe2+) for the dehalogenation and hydrolysis reactions was enabled. In a single-solute solution, IAcAm elimination was greater than bromoacetamide and chloroacetamide elimination due to the weak ionic I-C bond. In mixed solute conditions, the hydrophobicity of chloroacetamide played a more significant role in competitive degradation through greater adsorption. The proportion of dehalogenation relative to hydrolysis during IAcAm degradation by pristine nZVI and SiO2-coated nZVI was approximately 0.6:0.4. Iodoacetic acid and acetic acid were detected as intermediates in the degradation pathway of IAcAm by pristine nZVI. In contrast, the SiO2 layer on nZVI can accelerate the transformation of IAcAm to acetamide and iodoacetic acid. The electrolyte background of tap water exhibited a slight inhibitory effect on the degradation of IAcAm for both nZVI and SiO2-coated nZVI.

Role of oxidative stress in inactivation of Escherichia coli BW25113 by nanoscale zero-valent iron.

An Escherichia coli BW25113 wildtype strain and mutant strains lacking genes that protect against oxidative stress were examined at different growth phases for susceptibility to zero-valent iron (nZVI). Viability of cells was determined by the plate count method. All mutant strains were more susceptible than the wild type strain to nZVI; however, susceptibility differed among the mutant strains. Consistent with the role of rpoS as a global stress regulator, an rpoS gene knockout mutant exhibited the greatest susceptibility to nZVI under the majority of conditions tested (except exponential and declining phases at longer exposure time). Mutants lacking genes encoding the inducible and constitutively expressed cytosolic superoxide dismutases, sodA and sodB, respectively, were more susceptible to nZVI than a mutant lacking the gene encoding sodC, a periplasmic superoxide dismutase. This suggests that nZVI induces oxidative stress inside the cells via superoxide generation. Quantitative polymerase chain reaction was used to examine the expression of katG, a gene encoding the catalase-peroxidase enzyme, in nZVI-treated E. coli at different growth phases. Results showed that nZVI repressed the expression of katG in all but lag phases.

Impact of nanoscale zero valent iron on bacteria is growth phase dependent.

The toxic effect of nanoscale zero valent iron (nZVI) particles on bacteria from different growth phases was studied. Four bacterial strains namely Escherichia coli strains JM109 and BW25113, and Pseudomonas putida strains KT2440 and F1 were experimented. The growth curves of these strains were determined. Bacterial cells were harvested based on the predetermined time points, and exposed to nZVI. Cell viability was determined by the plate count method. Bacterial cells in lag and stationary phases showed higher resistance to nZVI for all four bacterial strains, whereas cells in exponential and decline phases were less resistant to nZVI and were rapidly inactivated when exposed to nZVI. Bacterial inactivation increased with the concentration of nZVI. Furthermore, less than 14% bacterial inactivation was observed when bacterial cells were exposed to the filtrate of nZVI suspension suggesting that the physical interaction between nZVI and cell is necessary for bacterial inactivation.

Iron nanoparticles coated with amphiphilic polysiloxane graft copolymers: dispersibility and contaminant treatability.

Amphiphilic polysiloxane graft copolymers (APGCs) were used as a delivery vehicle for nanoscale zerovalent iron (NZVI). The APGCs were designed to enable adsorption onto NZVI surfaces via carboxylic acid anchoring groups and polyethylene glycol (PEG) grafts were used to provide dispersibility in water. Degradation studies were conducted with trichloroethylene (TCE) as the model contaminant. TCE degradation rate with APGC-coated NZVI (CNZVI) was determined to be higher as compared to bare NZVI. The surface normalized degradation rate constants, k(SA) (Lm(2-) h(-1)), for TCE removal by CNZVI and bare NZVI ranged from 0.008 to 0.0760 to 007-0.016, respectively. Shelf life studies conducted over 12 months to access colloidal stability and 6 months to access TCE degradation indicated that colloidal stability and chemical reactivity of CNZVI remained more or less unchanged. The sedimentation characteristics of CNZVI under different ionic strength conditions (0-10 mM) did not change significantly. The steric nature of particle stabilization is expected to improve aquifer injection efficiency of the coated NZVI for groundwater remediation.

Entrapment of iron nanoparticles in calcium alginate beads for groundwater remediation applications.

Zero-valent iron nanoparticles (nZVI) have been successfully entrapped in biopolymer, calcium (Ca)-alginate beads. The study has demonstrated the potential use of this technique in environmental remediation using nitrate as a model contaminant. Ca-alginate beads show promise as an entrapment medium for nZVI for possible use in groundwater remediation. Based on scanning electron microscopy images it can be inferred that the alginate gel cluster acts as a bridge that binds the nZVI particles together. Kinetic experiments with 100, 60, and 20mg NO(3)(-)-NL(-1) indicate that 50-73% nitrate-N removal was achieved with entrapped nZVI as compared to 55-73% with bare nZVI over a 2-h period. The controls ran simultaneously show little NO(3)(-)-N removal. Statistical analysis indicates that there was no significant difference between the reaction rates of bare and entrapped nZVI. The authors have shown for the first time that nZVI can be effectively entrapped in Ca-alginate beads and no significant decrease in the reactivity of nZVI toward the model contaminant (nitrate here) was observed after the entrapment.

Nitrate removal by entrapped zero-valent iron nanoparticles in calcium alginate.

Zero-valent iron nanoparticles (nZVI) were successfully entrapped in calcium alginate beads. The potential use of this technique in environmental remediation using nitrate as a model contaminant was investigated. Kinetics of nitrate degradation using bare nZVI (approximately 35 nm dia) and entrapped nZVI were compared. Calcium alginate beads show promise as the entrapment medium for nZVI for possible use in permeable reactive barriers for groundwater remediation. Based on scanning electron microscopy images it can be inferred that the alginate gel cluster acts as a bridge that binds the nZVI particles together. Kinetic experiments with 100, 60, and 20 mg NO3--N L(-1) indicate that 50-73% nitrate-N removal was achieved with entrapped nZVI as compared to 55-73% with bare nZVI over a 2 h period. The controls ran simultaneously show little or no NO3--N removal. Statistical analysis indicates that there was no significant difference between the reaction rates of bare and entrapped nZVI. The authors have shown for the first time that nZVI can be effectively entrapped in Ca-alginate beads and no significant decrease in the reactivity of nZVI toward the model contaminant (nitrate here) was observed after the entrapment.