The impact of capping agents and environmental conditions (pH, ionic strength, and background electrolytes) on surface charge and aggregation potential of silver nanoparticles (AgNPs) suspensions were investigated. Capping agents are chemicals used in the synthesis of nanoparticles to prevent aggregation. The AgNPs examined in the study were as follows: (a) uncoated AgNPs (H(2)-AgNPs), (b) electrostatically stabilized (citrate and NaBH(4)-AgNPs), (c) sterically stabilized (polyvinylpyrrolidone (PVP)-AgNPs), and (d) electrosterically stabilized (branched polyethyleneimine (BPEI)-AgNPs)). The uncoated (H(2)-AgNPs), the citrate, and NaBH(4)-coated AgNPs aggregated at higher ionic strengths (100 mM NaNO(3)) and/or acidic pH (3.0). For these three nanomaterials, chloride (Cl(-), 10 mM), as a background electrolyte, resulted in a minimal change in the hydrodynamic diameter even at low pH (3.0). This was limited by the presence of residual silver ions, which resulted in the formation of stable negatively charged AgCl colloids. Furthermore, the presence of Ca(2+) (10 mM) resulted in aggregation of the three previously identified AgNPs regardless of the pH. As for PVP coated AgNPs, the ionic strength, pH and electrolyte type had no impact on the aggregation of the sterically stabilized AgNPs. The surface charge and aggregation of the BPEI coated AgNPs varied according to the solution pH.

As a result of the extensive number of applications of silver nanoparticles (AgNPs), their potential impacts, once released into the environment, are of concern. The toxicity of AgNPs was reported to be dependent on various factors such as particle size, shape and capping agent. Although these factors may play a role in AgNPs toxicity, the results presented herein suggest that surface charge is one of the most important factors that govern the toxicity of AgNPs. In the current study, the toxicity of four AgNPs representing various surface charging scenarios ranging from highly negative to highly positive was investigated. These AgNPs were (1) uncoated H(2)-AgNPs, (2) citrate coated AgNPs (Citrate-AgNPs), (3) polyvinylpyrrolidone coated AgNPs (PVP-AgNPs), and (4) branched polyethyleneimine coated AgNPs (BPEI-AgNPs). Our results clearly demonstrate that the AgNPs exhibited surface charge-dependent toxicity on the bacillus species investigated. Furthermore, ultrafiltration membranes were utilized to purify the AgNPs suspensions from residual impurities prior to the introduction to the microbes. This step was crucial in determining the true AgNPs toxicity and is either missing or not explicitly mentioned in most of the reported toxicity studies.

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It is expected that the antibacterial property of bulk silver is carried over and perhaps enhanced, to silver nanoparticles. Therefore, when one examines the environmental issues associated with the manufacture and use of silver nanoparticle-based products, the antibacterial effects should always be taken into account particularly at the different stages of the product lifecycle. Currently, there are two arguments in the scientific literature about the mechanisms of antimicrobial properties of silver nanoparticles as they relate to colloidal silver particles and inonic silver. Methodologies of risk assessment and control have to account for both arguments.

Synchrotron-based X-ray fluorescence (S-XRF) was utilized to locate arsenic (As) in polished (white) and unpolished (brown) rice grains from the United States, China, and Bangladesh. In white rice As was generally dispersed throughout the grain, the bulk of which constitutes the endosperm. In brown rice As was found to be preferentially localized at the surface, in the region corresponding to the pericarp and aleurone layer. Copper, iron, manganese, and zinc localization followed that of arsenic in brown rice, while the location for cadmium and nickel was distinctly different, showing relatively even distribution throughout the endosperm. The localization of As in the outer grain of brown rice was confirmed by laser ablation ICP-MS. Arsenic speciation of all grains using spatially resolved X-ray absorption near edge structure (µ-XANES) and bulk extraction followed by anion exchange HPLC-ICP-MS revealed the presence of mainly inorganic As and dimethylarsinic acid (DMA). However, the two techniques indicated different proportions of inorganic:organic As species. A wider survey of whole grain speciation of white (n ) 39) and brown (n ) 45) rice samples from numerous sources (field collected, supermarket survey, and pot trials) showed that brown rice had a higher proportion of inorganic arsenic present than white rice. Furthermore, the percentage of DMA present in the grain increased along with total grain arsenic.

Rice (Oryza sativa) is the staple food for over half the world's population yet may represent a significant dietary source of inorganic arsenic (As), a nonthreshold, class 1 human carcinogen. Rice grain As is dominated by the inorganic species, and the organic species dimethylarsinic acid (DMA). To investigate how As species are unloaded into grain rice, panicles were excised during grain filling and hydroponically pulsed with arsenite, arsenate, glutathione-complexed As, or DMA. Total As concentrations in flag leaf, grain, and husk, were quantified by inductively coupled plasma mass spectroscopy and As speciation in the fresh grain was determined by x-ray absorption near-edge spectroscopy. The roles of phloem and xylem transport were investigated by applying a 6 stem-girdling treatment to a second set of panicles, limiting phloem transport to the grain in panicles pulsed with arsenite or DMA. The results demonstrate that DMA is translocated to the rice grain with over an order magnitude greater efficiency than inorganic species and is more mobile than arsenite in both the phloem and the xylem. Phloem transport accounted for 90% of arsenite, and 55% of DMA, transport to the grain. Synchrotron x-ray fluorescence mapping and fluorescence microtomography revealed marked differences in the pattern of As unloading into the grain between DMA and arsenite-challenged grain. Arsenite was retained in the ovular vascular trace and DMA dispersed throughout the external grain parts and into the endosperm. This study also demonstrates that DMA speciation is altered in planta, potentially through complexation with thiols.

Summary• Arsenic (As) contamination of rice grains and the generally low concentration of micronutrients in rice have been recognized as a major concern for human health. Here, we investigated the speciation and localization of As and the distribution of (micro)nutrients in rice grains because these are key factors controlling bioavailability of nutrients and contaminants.• Bulk total and speciation analyses using high-pressure liquid chromatography (HPLC)-inductively coupled plasma mass spectrometry (ICP-MS) and X-ray absorption near-edge spectroscopy (XANES) was complemented by spatially resolved microspectroscopic techniques (µ-XANES, µ-X-ray fluorescence (µ-XRF) and particle induced X-ray emission (PIXE)) to investigate both speciation and distribution of As and localization of nutrients in situ.• The distribution of As and micronutrients varied between the various parts of the grains (husk, bran and endosperm) and was characterized by element-specific distribution patterns. The speciation of As in bran and endosperm was dominated by As(III)-thiol complexes.• The results indicate that the translocation from the maternal to filial tissues may be a bottleneck for As accumulation in the grain. Strong similarities between the distribution of iron (Fe), manganese (Mn) and phosphorus (P) and between zinc (Zn) and sulphur (S) may be indicative of complexation mechanisms in rice grains.

The rapid development and commercialization of nanomaterials will inevitably result in the release of nanoparticles (NPs) to the environment. As NPs often exhibit physical and chemical properties significantly different from those of their molecular or macrosize analogs, concern has been growing regarding their fate and toxicity in environmental compartments. The wastewater-sewage sludge pathway has been identified as a key release pathway leading to environmental exposure to NPs. In this study, we investigated the chemical transformation of two ZnO-NPs and one hydrophobic ZnO-NP commercial formulation (used in personal care products), during anaerobic digestion of wastewater. Changes in Zn speciation as a result of postprocessing of the sewage sludge, mimicking composting/stockpiling, were also assessed. The results indicated that "native" Zn and Zn added either as a soluble salt or as NPs was rapidly converted to sulfides in all treatments. The hydrophobicity of the commercial formulation retarded the conversion of ZnO-NP. However, at the end of the anaerobic digestion process and after postprocessing of the sewage sludge (which caused a significant change in Zn speciation), the speciation of Zn was similar across all treatments. This indicates that, at least for the material tested, the risk assessment of ZnO-NP through this exposure pathway can rely on the significant knowledge already available in regard to other "conventional" forms of Zn present in sewage sludge.