Chih-ping Tso

Stability of metal oxide nanoparticles in aqueous solutions

The application of nanoparticles in the processes of making commercial products has increased in recent years due to their unique physical and chemical properties. With increasing amount of commercial nanoparticles released into nature, their fate and effects on the ecosystem and human health are of growing concern. This study investigated the stability and morphology of three metal oxide nanoparticles in aqueous solutions. The commercially available nanoparticles, TiO(2), ZnO, SiO(2), aggregated quickly into micrometer-size particles in aqueous solutions, which may not threaten human health. Their changes in morphology and characteristics were further examined by dynamic light scattering (DLS) method and transmission electron microscopy (TEM). Among the several dispersion approaches, ultrasonication was found to be the most effective for disaggregating nanoparticles in water. For these three selected nanoparticles, ZnO could not remain stable in suspensions, presumably due to the dissolution of particles to form high concentration of ions, resulting in enhanced aggregation of particles. In addition, the existence of dissolved organic matters stabilized nanoparticles in lake water and wastewater for several hours in spite of the high concentration of cations in these real-water samples. The fate of metal oxide nanoparticles in natural water bodies would be determined by the type and concentration of cations and organic matters. Results obtained in this study revealed that the stability of nanoparticles changed under different aqueous conditions and so did their fate in the environment.

The effect of electrolytes on the aggregation kinetics of three different ZnO nanoparticles in water.

Nanoscale ZnO particles are receiving increasing attention because they are widely used in commercial products, but they do have potentially hazardous effects. The aggregation behavior of ZnO nanoparticles (NPs) in the environment contributes to the real risk assessment of nano-toxicity, and the real size of the nano-aggregates should be investigated. In this study, the influences of electrolytes on the stabilities of three ZnO NPs were compared: the commercial powder (NP1), the lab synthesized suspension (NP2) and the commercial suspension (NP3). The initial particle size of NP2 and NP3 in water was at a nanoscale whilst NP1 tended to form microscale aggregates. The capping reagents helped to retain their suspension. The stability of ZnO NPs depends on their zeta potential under specific pH value, ionic types and ionic strength. In general, neutralization plays a major role in aggregation. The effect of divalent counter-ions on ZnO NP aggregation was more than that of monovalent ones. The stabilities of NP2 and NP3 were confirmed by the large critical coagulation concentration (CCC) values of these particles. The experimental results also fit the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The aggregation of different ZnO NPs is relevant to their basic properties and is influenced by electrolytes, which decreases the possibility of the penetration of NPs into cells to cause toxicity in the environment. An understanding of the basic properties of NPs is crucial for assessing their fate in the environment as well as for setting up usage regulation and treatment strategy.

The reactivity of well-dispersed zerovalent iron nanoparticles toward pentachlorophenol in water

In order to prevent the aggregation of nanoparticles (NPs), surface modification or the addition of a stabilizer are used for stabilization. However, the real reactivity of NPs is still unclear because of the surface coating. For different physical dispersion methods, the particle stabilization for nanoscale zerovalent iron (NZVI) particles and their reactivity are studied. The particle properties of different preparations and their reactivity toward one polychlorinated aromatic compound, pentachlorophenol (PCP), with different electrolytes are also evaluated. Ultrasonication (US) with magnetic stirring disperses NZVI and Pd/Fe NPs well in water and does not affect the surface redox property a lot under the operating conditions in this study. The well-suspended NZVI cannot dechlorinate PCP but adsorption removal is observed. Compared to shaking, which gives limited removal of PCP (about 43%), Pd/Fe NPs remove 81% and 93% of PCP from water in the US and the US/stirring systems, respectively, which demonstrates that a greater surface area is exposed because of effective dispersion of Pd/Fe NPs. As the Pd doping increases, the dechlorination kinetics of PCP is improved, which shows that a catalyst is needed. With US/stirring, chloride ions do not significantly affect the removal kinetics of PCP, but the removal efficiency increases in the presence of nitrate ions because PCP anions were adsorbed and coagulated by the greater amount of iron (hydro)oxides that are generated from the reduction of nitrate on Pd/Fe. However, bicarbonate ions significantly block the adsorption and reaction sites on the Pd/Fe NP surface with US/stirring. The US/stirring method can be used to evaluate the actual activity of NPs near the nanoscale. The use of Pd/Fe NPs with US/stirring removes PCP from water effectively, even in the presence of common anions expect a high concentration of bicarbonate.

The transformation of hexabromocyclododecane using zerovalent iron nanoparticle aggregates

Hexabromocyclododecane (HBCD), an emerging contaminant, is a brominated flame retardant that has been widely detected in the environment. In this study, nanoscale zerovalent iron (NZVI) aggregates are firstly used to treat HBCD and its removal under different geochemical conditions is evaluated. HBCD is almost removed from solutions by NZVI, with a kSA of 4.22×10(-3)Lm(-2)min(-1). An increase in the iron dosage and temperature increases the removal rate. The activation energy for the removal of HBCD by NZVI is 30.2kJmol(-1), which suggests that a surface-chemical reaction occurs on NZVI. HBCD is adsorbed on the NZVI surface, where electrons were transferred to HBCD, and consequently forms byproducts with less bromide. Three common groundwater anions decrease the reaction kinetics and efficiency of NZVI. The kobs of HBCD in the presence of anions is in the order: pure water >Cl(-)>NO3(-)≒HCO3(-). The inhibitory effect of these anions may be a result of the possible complexation of anions with the oxidized iron surface. The oxidized sites on NZVI and oxidized species of iron also contribute to the removal of HBCD by adsorption on NZVI from solutions.

Concurrent oxidation and reduction of pentachlorophenol by bimetallic zerovalent Pd/Fe nanoparticles in an oxic water.

Under the oxic condition, the most effective removal of pentachlorophenol (PCP) with Pd/Fe nanoparticles (NPs) is demonstrated as compared to the anoxic condition. Concurrent oxidation and reduction of polychlorinated compounds such as PCP by zerovalent Pd/Fe were first observed. The optimal Pd content of the bimetallic NPs is only around 0.54 mg g(-1) Fe. Increases in both dosage of Pd/Fe NPs and temperature enhance degradation rates and efficiency. The activation energy of 29 kJ/mol indicates that the degradation is a surface-mediated mechanism. The removal mechanism also includes adsorption, which explains that the dechlorination of Cl on PCP molecules at ortho and meta positions is easier than that at para position. Overall, Pd/Fe NPs can apply directly to degrade polyhalogenated compounds in water without deaeration.

The influence of carboxymethylcellulose (CMC) on the reactivity of Fe NPs toward decabrominated diphenyl ether: The Ni doping, temperature, pH, and anion effects.

Polybrominated diphenyl ethers (PBDEs) are commonly used brominated flame retardants in many products. They have accumulated in the environment and become widely dispersed. In this study, carboxymethylcellulose (CMC) was applied to modify nanoscale zerovalent iron (NZVI) and bimetallic Ni/Fe nanoparticles (NPs) to prevent NP aggregation. In this study the removal kinetics of the decabrominated diphenyl ethers (DBDE) with CMC-stabilized Fe NPs were evaluated. CMC-stabilized Ni/Fe NPs with an average size of 86.7nm contained metallic Fe0 and reduced Ni. The colloidal stability decreased with a decrease in pH, which was further accompanied by a change in the removal rate of DBDE. Our results showed that anions do not change the removal rates of DBDE, with the exception of 10mM NO3-, which induced the formation of Fe (hydro)oxides on the Fe NP surface, which could further coagulate with DBDE. This study provides important information for our understanding of the influence of CMC coatings on the reactivity of Fe NPs. Because CMC coatings prevent the passivation of Fe in the presence of anions, CMC-coated Fe NPs show potential for the in-situ remediation of PBDEs in the environment.