Adsorption of Emerging Ionizable Contaminants on Carbon Nanotubes: Advancements and Challenges

The superior adsorption capacity of carbon nanotubes has been well recognized and there is a wealth of information in the literature concerning the adsorption of unionized organic pollutants on carbon nanotubes. Recently, the adsorption of emerging environmental pollutants, most of which are ionizable, has attracted increasing attention due to the heightened concerns about the accumulation of these emerging contaminants in the environment. These recent studies suggest that the adsorption of emerging ionizable contaminants on carbon nanotubes exhibit different characteristics than unionized ones. For example, a new charge-assisted intermolecular force has been proposed for ionizable compounds because some adsorption phenomenon cannot be easily explained by the conventional force theory. The adsorption of ionizable compounds also displayed much stronger dependence on solution pH and ionic strength than unionized compounds. This article aims to present a brief review on the current understanding of the adsorption of emerging ionizable contaminants to carbon nanotubes and discuss further research needs required to advance the mechanistic understanding of the interactions between ionizable contaminants and carbon nanotubes

Accumulation of zinc, copper, or cerium in carrot (Daucus carota) exposed to metal oxide nanoparticles and metal ions

The release of engineered nanoparticles (ENPs) into the environment has raised concerns about the potential risks to food safety and human health. There is a particular need to determine the extent of ENP uptake into plant foods. Belowground vegetables growing in direct contact with the growth substrate are likely accumulate the highest concentration of ENPs. Carrot (Daucus carota) was grown in sand amended with ZnO, CuO, or CeO2 NPs or the same concentrations of Zn2+, Cu2+, or Ce4+. Treatment with ZnO or Zn2+ produced a concentration-dependent decrease in root and total biomass. Ionic Cu2+ and Ce4+ caused a greater reduction in shoot biomass as compared to the corresponding ENP treatments. Accumulation of Zn, Cu, or Ce in the taproot was restricted to the taproot periderm. Metal concentrations in the taproot periderm were higher for the ionic treatments than for the ENP treatments. Radial penetration of the metals into the taproot and subsequent translocation to shoots was also generally greater for plants receiving the ionic treatment than the ENP treatment. The distribution of the metals from the ENP treatments across the periderm, taproot, and shoots differed from that observed for the ionic treatments. Overall, the ENPs were no more toxic than the ionic treatments and showed reduced accumulation in the edible tissues of carrot. The results demonstrate that the understanding of ionic metal transport in plants may not accurately predict ENP transport and that additional comparative study is needed for this and other crop plants

Uptake and accumulation of bulk and nanosized cerium oxide particles and ionic cerium by radish (Raphanus sativus L.).

The potential toxicity and accumulation of engineered nanomaterials (ENMs) in agricultural crops has become an area of great concern and intense investigation. Interestingly, although below-ground vegetables are most likely to accumulate the highest concentrations of ENMs, little work has been done investigating the potential uptake and accumulation of ENMs for this plant group. The overall objective of this study was to evaluate how different forms of cerium (bulk cerium oxide, cerium oxide nanoparticles, and the cerium ion) affected the growth of radish (Raphanus sativus L.) and accumulation of cerium in radish tissues. Ionic cerium (Ce(3+)) had a negative effect on radish growth at 10 mg CeCl3/L, whereas bulk cerium oxide (CeO2) enhanced plant biomass at the same concentration. Treatment with 10 mg/L cerium oxide nanoparticles (CeO2 NPs) had no significant effect on radish growth. Exposure to all forms of cerium resulted in the accumulation of this element in radish tissues, including the edible storage root. However, the accumulation patterns and their effect on plant growth and physiological processes varied with the characteristics of cerium. This study provides a critical frame of reference on the effects of CeO2 NPs versus their bulk and ionic counterparts on radish growth

Bioavailability of cerium oxide nanoparticles to Raphanus sativus L. in two soils.

Cerium oxide nanoparticles (CeO2 NP) are a common component of many commercial products. Due to the general concerns over the potential toxicity of engineered nanoparticles (ENPs), the phytotoxicity and in planta accumulation of CeO2 NPs have been broadly investigated. However, most previous studies were conducted in hydroponic systems and with grain crops. For a few studies performed with soil grown plants, the impact of soil properties on the fate and transport of CeO2 NPs was generally ignored even though numerous previous studies indicate that soil properties play a critical role in the fate and transport of environmental pollutants. The objectives of this study were to evaluate the soil fractionation and bioavailability of CeO2 NPs to Raphanus sativus L (radish) in two soil types. Our results showed that the silty loam contained slightly higher exchangeable fraction (F1) of cerium element than did loamy sand soil, but significantly lower reducible (F2) and oxidizable (F3) fractions as CeO2 NPs concentration increased. CeO2 NPs associated with silicate minerals or the residue fraction (F4) dominated in both soils. The cerium concentration in radish storage root showed linear correlation with the sum of the first three fractions (r(2) = 0.98 and 0.78 for loamy sand and silty loam respectively). However, the cerium content in radish shoots only exhibited strong correlations with F1 (r(2) = 0.97 and 0.89 for loamy sand and silty loam respectively). Overall, the results demonstrated that soil properties are important factors governing the distribution of CeO2 NPs in soil and subsequent bioavailability to plants

Reproductive Toxicity and Life History Study of Silver Nanoparticle Effect, Uptake and Transport in Arabidopsis thaliana

Concerns about nanotechnology have prompted studies on how the release of these engineered nanoparticles impact our environment. Herein, the impact of 20 nm silver nanoparticles (AgNPs) on the life history traits of Arabidopsis thaliana was studied in both above- and below-ground parts, at macroscopic and microscopic scales. Both gross phenotypes (in contrast to microscopic phenotypes) and routes of transport and accumulation were investigated from roots to shoots. Wild type Arabidopsis growing in soil, regularly irrigated with 75 μg/L of AgNPs, did not show any obvious morphological change. However, their vegetative development was prolonged by two to three days and their reproductive growth shortened by three to four days. In addition, the germination rates of offspring decreased drastically over three generations. These findings confirmed that AgNPs induce abiotic stress and cause reproductive toxicity in Arabidopsis. To trace transport of AgNPs, this study also included an Arabidopsis reporter line genetically transformed with a green fluorescent protein and grown in an optical transparent medium with 75 μg/L AgNPs. AgNPs followed three routes: (1) At seven days after planting (DAP) at S1.0 (stages defined by Boyes et al. 2001 [41]), AgNPs attached to the surface of primary roots and then entered their root tips; (2) At 14 DAP at S1.04, as primary roots grew longer, AgNPs gradually moved into roots and entered new lateral root primordia and root hairs; (3) At 17 DAP at S1.06 when the Arabidopsis root system had developed multiple lateral roots, AgNPs were present in vascular tissue and throughout the whole plant from root to shoot. In some cases, if cotyledons of the Arabidopsis seedlings were immersed in melted transparent medium, then AgNPs were taken up by and accumulated in stomatal guard cells. These findings in Arabidopsis are the first to document specific routes and rates of AgNP uptake in vivo and in situ

Projected Dietary Intake of Zinc, Copper, and Cerium from Consumption of Carrot (Daucus carota) Exposed to Metal Oxide Nanoparticles or Metal Ions

The expanding production and use of engineered nanomaterials have raised concerns about the potential risk of those materials to food safety and human health. In a prior study, the accumulation of Zn and Cu from ZnO, CuO, or CeO2, respectively, was examined in carrot (Daucus carota L.) grown in sand culture in comparison to accumulation from exposure to equivalent concentrations of ionic Zn2+, Cu2+, or Ce4+. The fresh weight concentration data for peeled and unpeeled carrots were used to project dietary intake of each metal by seven age-mass classes from child to adult based on consumption of a single serving of carrot. Dietary intake was compared to the oral reference dose (oral Rfd) for chronic toxicity for Zn or Cu and estimated mean and median oral RfD values for Ce based on nine other rare earth elements. Reverse dietary intake calculations were also conducted to estimate the number of servings of carrot, the mass of carrot consumed, or the tissue concentration of Zn or Cu that would cause the oral RfD to be exceeded upon consumption. The projections indicated for Zn and Cu, the oral RfD would be exceeded in only a few highly unrealistic scenarios of exceedingly high Zn or Cu concentrations in the substrate from ZnO or CuO or consumption of excessive amounts of unpeeled carrot. The implications associated with the presence of Ce in the carrot tissues depended upon whether the mean or median oral RfD value from the rare earth elements was used as a basis for comparison. The calculations further indicated that peeling carrots reduced the projected dietary intake by one to two orders of magnitude for both ENM- and ionic-treated carrots. Overall in terms of total metal concentration, the results suggested no specific impact of the ENM form on dietary intake. The effort here provided a conservative view of the potential dietary intake of these three metals that might result from consumption of carrots exposed to nanomaterials and how peeling mitigated that dietary intake. The results also demonstrate the utility of dietary intake projections for examining potential risks of nanomaterial exposure from agricultural foods

SEQUESTERING AGENTS FOR ACTIVE CAPS - REMEDIATION OF METALS AND ORGANICS

This research evaluated organoclays, zeolites, phosphates, and a biopolymer as sequestering agents for inorganic and organic contaminants. Batch experiments were conducted to identify amendments and mixtures of amendments for metal and organic contaminants removal and retention. Contaminant removal was evaluated by calculating partitioning coefficients. Metal retention was evaluated by desorption studies in which residue from the removal studies was extracted with 1 M MgCl{sub 2} solution. The results indicated that phosphate amendments, some organoclays, and the biopolymer, chitosan, were very effective sequestering agents for metals in fresh and salt water. Organoclays were very effective sorbents for phenanthrene, pyrene, and benzo(a)pyrene. Partitioning coefficients for the organoclays were 3000-3500 ml g{sup -1} for benzo(a)pyrene, 400-450 ml g{sup -1} for pyrene, and 50-70 ml g{sup -1} for phenanthrene. Remediation of sites with a mixture of contaminants is more difficult than sites with a single contaminant because metals and organic contaminants have different fate and transport mechanisms in sediment and water. Mixtures of amendments (e.g., organoclay and rock phosphate) have high potential for remediating both organic and inorganic contaminants under a broad range of environmental conditions, and have promise as components in active caps for sediment remediation

Environmental release, fate and ecotoxicological effects of manufactured ceria nanomaterials

Recent interest in the environmental fate and effects of manufactured CeO2 nanomaterials (nanoceria) has stemmed from its expanded use for a variety of applications including fuel additives, catalytic converters, chemical and mechanical planarization media and other uses. This has led to a wave of publications on the toxicological effects of nanoceria in ecological receptor species, but only limited information is available on possible environmental releases, concentrations in environmental media, or environmental transformations. In this paper, we make initial estimates of likely environmental releases and exposure concentrations in soils and water and compare them to published toxicity values. Insufficient information was available to estimate aquatic exposures, but we estimated inputs to a hypothetical wastewater treatment plant that could result in effluent concentrations that would result in acute toxicity to the most sensitive aquatic organisms tested so far, cyanobacteria. The purpose of this exercise is to identify which areas are lacking in data to perform either regional or site specific ecological risk assessments. While estimates can be made for releases from use as a diesel fuel additive, and predicted toxicity is low in most terrestrial species tested to date, estimates for releases from other uses are difficult at this stage. We recommend that future studies focus on environmentally realistic exposures that take into account potential environmental transformations of the nanoceria surface as well as chronic toxicity studies in benthic aquatic organisms, soil invertebrates and microorgansims

Fate and transport of volatile organic compounds (VOCs) in phytoremediation systems

Phytoremediation is the technology of treating contaminated soil and groundwater with plants. Since its advent, phytoremediation has been applied to the treatment of various contaminants. In the phytoremediation of volatile organic compounds (VOCs), several mechanistic pathways have been suggested, but these pathways have not been fully researched and described. This research focused on the processes involved in uptake and volatilization of organic compounds by plants. --Abstract, page iii

Adsorption of Emerging Ionizable Contaminants on Carbon Nanotubes: Advancements and Challenges

The superior adsorption capacity of carbon nanotubes has been well recognized and there is a wealth of information in the literature concerning the adsorption of unionized organic pollutants on carbon nanotubes. Recently, the adsorption of emerging environmental pollutants, most of which are ionizable, has attracted increasing attention due to the heightened concerns about the accumulation of these emerging contaminants in the environment. These recent studies suggest that the adsorption of emerging ionizable contaminants on carbon nanotubes exhibit different characteristics than unionized ones. For example, a new charge-assisted intermolecular force has been proposed for ionizable compounds because some adsorption phenomenon cannot be easily explained by the conventional force theory. The adsorption of ionizable compounds also displayed much stronger dependence on solution pH and ionic strength than unionized compounds. This article aims to present a brief review on the current understanding of the adsorption of emerging ionizable contaminants to carbon nanotubes and discuss further research needs required to advance the mechanistic understanding of the interactions between ionizable contaminants and carbon nanotubes