New insights into physicochemical properties of different particulate size-fractions and dissolved organic matter derived from biochars and their sorption capacity for phenanthrene.

To improve the knowledge of the heterogeneity and sorption behavior of biochars on hydrophobic organic contaminants (HOCs), pristine biochars (PBCs, 400 and 700 °C) were fractionated into four particulate fractions (SfBCs) and dissolved organic matter derived from biochars (DBC), then the sorption capacities of them towards phenanthrene were examined. Results showed that the OC-normalized sorption distribution coefficients (Koc) of PBCs were generally at intermediate levels among that of SfBCs and DBCs. The logKoc values of SfBCs increased as particle sizes decreased. By virtue of the higher micropore volume, specific surface area, aromaticity and hydrophobicity, the lowest SfBCs (0.45-10 µm, BC0.45-10) exhibited remarkably higher logKoc. Besides, although SfBCs from 700 °C generally showed larger logKoc than counterparts from 400 °C, almost no difference was observed for logKoc values of BC0.45-10 fractions from 400 and 700 °C. We thus speculated that particle size might have stronger effect on their sorption capacity than pyrolysis temperature. Although DBCs exhibited dramatically lower logKoc values than nano-scale SfBCs, they were interestingly comparable to large-sized SfBCs. Our findings thus suggested the importance of small particulate biochar species and DBCs on HOCs transport should be both highlighted since these fractions were highly dynamic in the environment.

Identifying dust as the dominant source of exposure to heavy metals for residents around battery factories in the Battery Industrial Capital of China.

Heavy metals (HMs) are constantly released into the environment during the production and use of batteries. Battery manufacturing has been ongoing for over six decades in the "Battery Industrial Capital" (located in Xinxiang City) of China, but the potential exposure pathways of residents in this region to HMs remain unclear. To clarify the exposure pathways and health risk of human exposure to HMs, hand wipe samples (n=82) and fingernail samples (n=36) were collected from residents (including young children (0-6 years old), children (7-12 years old) and adults (30-60 years old)) living around battery factories. The total concentrations of the target HMs (Zn, Mn, Cu, Pb, Ni, Cr, Cd, Co) in hand wipes ranged from 133 to 8040 µg/m2, and those in fingernails ranged from 9.7-566 µg/g. HM levels in the hand wipe and fingernail samples both decreased with age, and higher HM levels were observed for males than females. The HM composition profiles in these two matrices represented a high degree of similarity, with Zn as the predominant element, and thus, oral ingestion and dermal exposure via dust were expected to be the most important HM exposure pathways for residents in this region. The non-carcinogenic risks (HQs) from dermal and oral ingestion exposure to Cd, Cr, and Pb were higher than those of the other five elements for all three populations, and the HQderm of Cd for young children was 2.1 (HQoral=0.6). Moreover, the hazard index (HI) values of ∑8HMs for young children (HItotal=5.2, HIoral=2.0, HIdermal=3.2) and children (HItotal=1.6, HIoral=1.3, HIdermal=0.3) exceeded the safe threshold (1.0). Therefore, young children and children should be prioritized for protection from HM pollution, and more attention should be paid to young children's dermal exposure to Cd in this region.

Aggregation kinetics of different surface-modified polystyrene nanoparticles in monovalent and divalent electrolytes.

The intentional production and degradation of plastic debris may result in the formation of nanoplastics. Currently, the scarce information on the environmental behaviors of nanoplastics hinders accurate assessment of their potential risks. Herein, the aggregation kinetics of different surface-modified polystyrene nanoparticles in monovalent and divalent electrolytes was investigated to shed some light on the fate of nanoplastics in the aquatic environment. Three monodisperse nanoparticles including unmodified nanoparticles (PS-Bare), carboxylated nanoparticles (PS-COOH) and amino modified nanoparticles (PS-NH2), as well as one polydisperse nanoparticles that formed by laser ablation of polystyrene films (PS-Laser) were used as models to understand the effects of surface groups and morphology. Results showed that aggregation kinetics of negatively charged PS-Bare and PS-COOH obeyed the DLVO theory in NaCl and CaCl2 solutions. The presence of Suwannee river natural organic matters (SRNOM) suppressed the aggregation of PS-Bare and PS-COOH in monovalent electrolytes by steric hindrance. However, in divalent electrolytes, their stability was enhanced at low concentrations of SRNOM (below 5 mg C L-1), while became worse at high concentrations of SRNOM (above 5 mg C L-1) due to the interparticle bridging effect caused by Ca2+ and carboxyl groups of SRNOM. The cation bridging effect was also observed for PS-laser in the presence of high concentrations of divalent electrolytes and SRNOM. The adsorption of SRNOM could neutralize or even reverse surface charges of positively charged PS-NH2 at high concentrations, thus enhanced or inhibited the aggregation of PS-NH2. No synergistic effect of Ca2+ and SRNOM was observed on the aggregation of PS-NH2, probably due to the steric repulsion imparted by the surface modification. Our results highlight that surface charge and surface modification significantly influence aggregation behaviors of nanoplastics in aquatic systems.

Influence of Air Pollution on Inhalation and Dermal Exposure of Human to Organophosphate Flame Retardants: A Case Study During a Prolonged Haze Episode.

The health impact of haze is of great concern, but few studies have explored its influence on human inhalation and dermal exposure to trace pollutants. Size-segregated atmospheric particles ( n = 72) and forehead wipe samples ( n = 80) from undergraduates were collected in Xinxiang, China, during a prolonged haze episode and analyzed for 10 organophosphate flame retardants (OPFRs). ∑TCPP and TCEP were the most abundant OPFR substances in all samples. The arithmetic mean particle-bound and forehead OPFR concentrations under a heavy pollution condition (air quality index (AQI), 350-550) were 41.9 ng/m3 (∑8OPFRs) and 7.4 µg/m2 (∑6OPFRs), respectively, apparently greater than the values observed under a light pollution condition (AQI, 60-90) (19.5 ng/m3 and 3.9 µg/m2, respectively). Meteorological conditions played distinctive roles in affecting the OPFR occurrence in atmospheric particles (statistically significant for TCEP and ∑TCPP) and forehead wipes (excluding TPHP), implying that OPFR exposure through inhalation and dermal absorption was synchronously influenced by air quality, and OPFRs on the forehead may be mainly absorbed from the air. Inhalation contributed dominantly to the total OPFR exposure dose for humans when using the relative absorption method to assess dermal exposure, while according to the permeability coefficient method, dermal exposure was much more significant than inhalation. The results of this study indicate that OPFR exposure should attract particular concern in regions with heavy air pollution.

Spatial, seasonal and particle size dependent variations of PAH contamination in indoor dust and the corresponding human health risk.

To investigate the particle size distribution, spatial variation, and corresponding health risks of polycyclic aromatic hydrocarbons (PAHs) in indoor environments, composite settled dust samples were collected from four types of microenvironments (offices, hotels, dormitories and kindergartens) in Beijing, and each pooled dust sample was homogenized and fractionated into 9 fractions (F1 (900-2000 µm), F2 (500-900 µm), F3 (400-500 µm), F4 (300-400 µm), F5 (200-300 µm), F6 (100-200 µm), F7 (74-100 µm), F8 (50-74 µm), and F9 (<50 µm)). The total concentrations of 15 PAHs varied from 388 ng g-1 (kindergarten dust, F1) to 8140 ng g-1 (hotel dust, F7) in the 31 size-segregated samples. Particle size distribution patterns of PAHs were found to vary for the different types of dust samples. The seasonality of PAH contamination in indoor dust was discussed within 36 samples collected weekly and biweekly from two offices of one building in Beijing. Generally, the seasonal trends of PAHs in dust from these two offices were consistent, showing that PAH levels in cold seasons were higher than those in warm seasons. Diagnostic ratios and principal component analysis (PCA) indicated the important contribution of fuel combustion to PAHs in the indoor dust samples. The estimated incremental lifetime cancer risk (ILCR) values ranged from 10-6 to 10-5 for all relevant populations corresponding to the four types of microenvironments.

Insights into aggregation and transport of graphene oxide in aqueous and saturated porous media: Complex effects of cations with different molecular weight fractionated natural organic matter.

The stability of nanomaterials in aquatic environment is a critical factor that governs their fate and ecotoxicity. Meanwhile, the interaction between nanomaterials and ubiquitous natural organic matter (NOM) is a vital process that influences the transport and biological effects of nanomaterials in the environment. However, impacts of NOM on the aggregation and transport of two-dimensional nanomaterials, especially for the increasingly used graphene oxide (GO), are not well understood. Particularly, there is lack of exploration on potential impacts of the heterogeneous properties of NOM on GO behaviour, especially that induced by the wide molecular weight (MW) span of NOM. In this study, effects of several kinds of well-characterized MW fractionated Suwannee River NOM (Mf-SRNOMs) on the aggregation and transport of GO in aqueous media and saturated porous media were investigated. Our results suggest that the stability and migration capacity of GO under most investigated electrolyte conditions are promoted by all Mf-SRNOMs, and efficiencies of different Mf-SRNOMs are generally positively correlated with their MW. Primarily, mechanisms including MW-dependent steric hindrance and sorption of Mf-SRNOMs onto GO are critical in stabilizing GO, and thus facilitating its transport. However, the stronger sorption of higher Mf-SRNOMs onto the GO basal plane through π-π interaction further facilitated the cation bridging between both ends of Mf-SRNOM and GO, and resulted in heteroaggregation of NOM-GO. Moreover, the weight analysis indicated that despite the fact that high Mf-SRNOMs only occupied a small percentage of pristine-SRNOM, they showed a stronger contribution towards pristine-SRNOM's capacity in stabilizing GO, when compared with that of lower MW counterpart. These findings pointed out that complex effects of the heterogeneities of NOM and cations should be highly relevant when the aggregation and transport behaviour of two-dimensional nanomaterials is investigated, and NOM fractions that are highly aromatic and of a higher MW should receive greater attention.

Amplification effect of haze on human exposure to halogenated flame retardants in atmospheric particulate matter and the corresponding mechanism.

The health impact of haze is of great concern; however, few air quality studies have investigated trace pollutant contamination in the air. Size-segregated atmospheric particles (nine size fractions derived from PM10) were collected in dwelling (indoor) and traffic (outdoor) environments in Xinxiang, China, during light pollution conditions (air quality index (AQI), 60-90) and heavy pollution conditions (AQI, 350-550), and they were analysed for halogenated flame retardants (HFRs), including polybrominated diphenyl ethers (PBDEs), novel brominated flame retardants (NBFRs) and Dechlorane Plus (DP) isomers. HFR occurrence levels generally decreased in the order of PBDEs > NBFRs > DPs. The total mean abundance ratios of heavy pollution/light pollution were 4.0, 2.9, 4.4 and 3.6 for PBDEs, NBFRs, DPs and HFRs, respectively. Meteorological conditions played distinctive roles in the HFR distribution in the air. Apparent differences were found for the particle size distribution of HFRs under light and heavy pollution conditions. In general, for adults, the estimated hazard quotient (HQ) and incremental lifetime cancer risk (ILCRBDE-209) values were approximately 1.7 × 10-2 and 9.3 × 10-9 in heavy pollution conditions, respectively, which were significantly higher than those in light pollution conditions (1.8 × 10-3 and 2.1 × 10-9, respectively).

Contamination characteristics of trace metals in dust from different levels of roads of a heavily air-polluted city in north China.

Concentrations of eight trace metals (TMs) in road dust (RD) (particles < 25 µm) from urban areas of Xinxiang, China, were determined by inductively coupled plasma mass spectrometry. The geometric mean concentrations of Zn, Mn, Pb, As, Cu, Cr, Ni and Cd were 489, 350, 114, 101, 60.0, 39.7, 31.6, and 5.1 mg kg-1, respectively. When compared with TM levels in background soil, the samples generally display elevated TM concentrations, except for Cr and Mn, and for Cd the enrichment value was 69.6. Spatial variations indicated TMs in RD from park path would have similar sources with main roads, collector streets and bypasses. Average daily exposure doses of the studied TMs were about three orders of magnitude higher for hand-to-mouth ingestion than dermal contact, and the exposure doses for children were 9.33 times higher than that for adults. The decreasing trend of calculated hazard indexes (HI) for the eight elements was As > Pb > Cr > Mn > Cd > Zn > Ni > Cu for both children and adults.

Interactions between engineered nanoparticles and dissolved organic matter: A review on mechanisms and environmental effects.

Dissolved organic matter (DOM) is ubiquitous in the environment and has high reactivity. Once engineered nanoparticles (ENPs) are released into natural systems, interactions of DOM with ENPs may significantly affect the fate and transport of ENPs, as well as the bioavailability and toxicity of ENPs to organisms. However, because of the complexity of DOM and the shortage of useful characterization methods, large knowledge gaps exist in our understanding of the interactions between DOM and ENPs. In this article, we systematically reviewed the interactions between DOM and ENPs, discussed the effects of DOM on the environmental behavior of ENPs, and described the changes in bioavailability and toxicity of ENPs caused by DOM. Critical evaluations of published references suggest further need for assessing and predicting the influences of DOM on the transport, transformation, bioavailability, and toxicity of ENPs in the environment.

Evaluation of methods to determine adsorption of polycyclic aromatic hydrocarbons to dispersed carbon nanotubes.

A number of methods have been reported for determining hydrophobic organic compound adsorption to dispersed carbon nanotubes (CNTs), but their accuracy and reliability remain uncertain. We have evaluated three methods to investigate the adsorption of phenanthrene (a model polycyclic aromatic hydrocarbon, PAH) to CNTs with different physicochemical properties: dialysis tube (DT) protected negligible depletion solid phase microextraction (DT-nd-SPME), ultracentrifugation, and filtration using various types of filters. Dispersed CNTs adhered to the unprotected polydimethylsiloxane (PDMS)-coated fibers used in nd-SPME. Protection of the fibers from CNT adherence was investigated with hydrophilic DT, but high PAH sorption to the DT was observed. The efficiency of ultracentrifugation and filtration to separate CNTs from the water phase depended on CNT physicochemical properties. While non-functionalized CNTs were efficiently separated from the water phase using ultracentrifugation, incomplete separation of carboxyl functionalized CNTs was observed. Filtration efficiency varied with different filter types (composition and pore size), and non-functionalized CNTs were more easily separated from the water phase than functionalized CNTs. Sorption of phenanthrene was high (< 70%) for three of the filters tested, making them unsuitable for the assessment of phenanthrene adsorption to CNTs. Filtration using a hydrophilic polytetrafluoroethylene (PTFE) filter membrane (0.1 µm) was found to be a simple and precise technique for the determination of phenanthrene adsorption to a range of CNTs, efficiently separating all types of CNTs and exhibiting a good and highly reproducible recovery of phenanthrene (82%) over the concentration range tested (70-735 µg/L).

Speciation Analysis of Labile and Total Silver(I) in Nanosilver Dispersions and Environmental Waters by Hollow Fiber Supported Liquid Membrane Extraction.

Hollow fiber supported liquid membrane (HFSLM) extraction was coupled with ICP-MS for speciation analysis of labile Ag(I) and total Ag(I) in dispersions of silver nanoparticles (AgNPs) and environmental waters. Ag(I) in aqueous samples was extracted into the HFSLM of 5%(m/v) tri-n-octylphosphine oxide in n-undecane, and stripped in the acceptor of 10 mM Na2S2O3 and 1 mM Cu(NO3)2 prepared in 5 mM NaH2PO4-Na2HPO4 buffer (pH 7.5). Negligible depletion and exhaustive extraction were conducted under static and 250 rpm shaking to extract the labile Ag(I) and total Ag(I), respectively. The extraction equilibration was reached in 8 h for both extraction modes. The extraction efficiency and detection limit were (2.97 ± 0.25)% and 0.1 µg/L for labile Ag(I), and (82.3 ± 2.0)% and 0.5 µg/L for total Ag(I) detection, respectively. The proposed method was applied to determine labile Ag(I) and total Ag(I) in different sized AgNP dispersions and real environmental waters, with spiked recoveries of total Ag(I) in the range of 74.0-98.1%. With the capability of distinguishing labile and total Ag(I), our method offers a new approach for evaluating the bioavailability and understanding the fate and toxicity of AgNPs in aquatic systems.

Sunlight-driven reduction of silver ion to silver nanoparticle by organic matter mitigates the acute toxicity of silver to Daphnia magna.

Due to the unique antibacterial activities, silver nanoparticles (AgNPs) have been extensively used in commercial products. Anthropogenic activities have released considerable AgNPs as well as highly toxic silver ion (Ag(+)) into the aquatic environment. Our recent study revealed that ubiquitous natural organic matter (NOM) could reduce Ag(+) to AgNP under natural sunlight. However, the toxic effect of this process is not well understood. In this work, we prepared mixture solution of Ag(+) and AgNPs with varied Ag(+)% through the sunlight-driven reduction of Ag(+) by NOM and investigated the acute toxicity of the solutions on Daphnia magna. Formation of AgNPs was demonstrated and characterized by comprehensive techniques and the fraction of unconverted Ag(+) was determined by ultrafiltration-inductively coupled plasma mass spectrometry determination. The formation of AgNPs enhanced significantly with the increasing of solution pH and cumulative photosynthetically active radiation of sunlight. The toxicity of the resulting solution was further investigated by using freshwater crustacean D. magna as a model and an 8hr-median lethal concentration (LC50) demonstrated that the reduction of Ag(+) by NOM to AgNPs significantly mitigated the acute toxicity of silver. These results highlight the importance of sunlight and NOM in the fate, transformation and toxicity of Ag(+) and AgNPs, and further indicate that the acute toxicity of AgNPs should be mainly ascribed to the dissolved Ag(+) from AgNPs.

Seasonal and Particle Size-Dependent Variations of Hexabromocyclododecanes in Settled Dust: Implications for Sampling.

Particle size is a significant parameter which determines the environmental fate and the behavior of dust particles and, implicitly, the exposure risk of humans to particle-bound contaminants. Currently, the influence of dust particle size on the occurrence and seasonal variation of hexabromocyclododecanes (HBCDs) remains unclear. While HBCDs are now restricted by the Stockholm Convention, information regarding HBCD contamination in indoor dust in China is still limited. We analyzed composite dust samples from offices (n = 22), hotels (n = 3), kindergartens (n = 2), dormitories (n = 40), and main roads (n = 10). Each composite dust sample (one per type of microenvironment) was fractionated into 9 fractions (F1-F9: 2000-900, 900-500, 500-400, 400-300, 300-200, 200-100, 100-74, 74-50, and <50 µm). Total HBCD concentrations ranged from 5.3 (road dust, F4) to 2580 ng g(-1) (dormitory dust, F4) in the 45 size-segregated samples. The seasonality of HBCDs in indoor dust was investigated in 40 samples from two offices. A consistent seasonal trend of HBCD levels was evident with dust collected in the winter being more contaminated with HBCDs than dust from the summer. Particle size-selection strategy for dust analysis has been found to be influential on the HBCD concentrations, while overestimation or underestimation would occur with improper strategies.

Particle coating-dependent interaction of molecular weight fractionated natural organic matter: impacts on the aggregation of silver nanoparticles.

Ubiquitous natural organic matter (NOM) plays an important role in the aggregation state of engineered silver nanoparticles (AgNPs) in aquatic environment, which determines the transport, transformation, and toxicity of AgNPs. As various capping agents are used as coatings for nanoparticles and NOM are natural polymer mixture with wide molecular weight (MW) distribution, probing the particle coating-dependent interaction of MW fractionated natural organic matter (Mf-NOM) with various coatings is helpful for understanding the differential aggregation and transport behavior of engineered AgNPs as well as other metal nanoparticles. In this study, we investigated the role of pristine and Mf-NOM on the aggregation of AgNPs with Bare, citrate, and PVP coating (Bare-, Cit-, and PVP-AgNP) in mono- and divalent electrolyte solutions. We observed that the enhanced aggregation or dispersion of AgNPs in NOM solution highly depends on the coating of AgNPs. Pristine NOM inhibited the aggregation of Bare-AgNPs but enhanced the aggregation of PVP-AgNPs. In addition, Mf-NOM fractions have distinguishing roles on the aggregation and dispersion of AgNPs, which also highly depend on the AgNPs coating as well as the MW of Mf-NOM. Higher MW Mf-NOM (>100 kDa and 30-100 kDa) enhanced the aggregation of PVP-AgNPs in mono- and divalent electrolyte solutions, whereas lower MW Mf-NOM (10-30 kDa, 3-10 kDa and <3 kDa) inhibited the aggregation of PVP-AgNPs. However, all the Mf-NOM fractions inhibited the aggregation of Bare-AgNPs. For PVP- and Bare-AgNPs, the stability of AgNPs in electrolyte solution was significantly correlated to the MW of Mf-NOM. But for Cit-AgNPs, pristine NOM and Mf-NOM has minor influence on the stability of AgNPs. These findings about significantly different roles of Mf-NOM on aggregation of engineered AgNPs with various coating are important for better understanding of the transport and subsequent transformation of AgNPs in aquatic environment.

Exposure medium: key in identifying free Ag+ as the exclusive species of silver nanoparticles with acute toxicity to Daphnia magna.

It is still not very clear what roles the various Ag species play in the toxicity of silver nanoparticles (AgNPs). In this study, we found that traditional exposure media result in uncontrollable but consistent physicochemical transformation of AgNPs, causing artifacts in determination of median lethal concentration (LC50) and hindering the identification of Ag species responsible for the acute toxicity of AgNPs to Daphnia magna. This obstacle was overcome by using 8 h exposure in 0.1 mmol L(-1) NaNO3 medium, in which we measured the 8-h LC50 of seven AgNPs with different sizes and coatings, and determined the concentrations of various Ag species. The LC50 as free Ag(+) of the seven AgNPs (0.37-0.44 µg L(-1)) agreed very well with that of AgNO3 (0.40 µg L(-1)), and showed the lowest value compared to that as total Ag, total Ag(+), and dissolved Ag, demonstrating free Ag(+) is exclusively responsible for the acute toxicity of AgNPs to D. magna, while other Ag species in AgNPs have no contribution to the acute toxicity. Our results demonstrated the great importance of developing appropriate exposure media for evaluating risk of nanomaterials.

Fullerene-associated phenanthrene contributes to bioaccumulation but is not toxic to fish.

The present study investigated the effects of aqueous fullerene suspensions (nC60 ) on the bioavailability and toxicity of phenanthrene (Phe) to junior carp (Cyprinus carpio). Bioaccumulation factors (BAFs) were calculated based on total as well as free concentrations of Phe. Equal BAF values were obtained with and without nC60 based on the total concentrations, whereas greater BAFs were found in the presence of nC60 when free Phe concentrations were applied. The results demonstrated that nC60 could act as a contaminant carrier to facilitate Phe bioaccumulation. The concentration-response relationship of induced hepatic 7-ethoxysorufin-O-deethylase activity was established in regard to the total and free concentrations of aqueous Phe solutions as well as the body residues. The concentration-response curves were reliant on the nC60 concentration when the total concentration of Phe was employed as a variable but were independent of nC60 presence when free concentration or body residue was employed as a variable, implying that the latter 2 parameters were more accurate in evaluating biological effects. Particles of C60 were mostly distributed in fish liver and intestines, which indicated the primary routine of uptake was through ingestion. Approximately 22% to 100% of the Phe-nC60 complex contributed to the bioaccumulation, whereas the complex did not contribute to the toxicity.

Effects of molecular weight-dependent physicochemical heterogeneity of natural organic matter on the aggregation of fullerene nanoparticles in mono- and di-valent electrolyte solutions.

Given the wide presence of heterogeneous natural organic matter (NOM) and metal ions (Na(+)/Ca(2+)/Mg(2+)), as well as their significant role in governing nanoparticle stability in aqueous environments, it is of great importance to understand how the molecular weight (MW)-dependent physicochemical properties of NOM impact fundamental transportation processes like the aggregation of engineered nanoparticles (ENPs) in the presence of Na(+)/Ca(2+)/Mg(2+). Here, we report on the aggregation behavior of a model ENP, fullerene nanoparticles (nC60) in the presence of five MW fractions of Suwannee River NOM (Mf-SRNOMs, separated by ultrafiltration techniques) and three electrolytes (NaCl, CaCl2 and MgCl2). We found that in all NaCl treatments and low concentration CaCl2/MgCl2 treatments, the enhancement of nC60 stability positively correlated with the MW of Mf-SRNOMs. Whereas, the stability efficiency of identical Mf-SRNOM in different electrolytes followed an order of NaCl > MgCl2 > CaCl2, and the enhanced attachment of nC60-SRNOM associations was observed in high MW Mf-SRNOM (SRNOM>100 kD and SRNOM 30-100 kD) at high concentration CaCl2/MgCl2. Our results indicate that although the high MW NOM with large humic-like material is the key component for stabilizing nC60 in monovalent electrolyte, it could play a reversed role in promoting the attachment of nC60, especially in long term aggregations and at high concentrations of divalent cations. Therefore, a detailed understanding of the effects of heterogeneous NOM on the aggregation of ENPs should be highly valued, and properly assessed against different cation species and concentrations.

Photoreduction and stabilization capability of molecular weight fractionated natural organic matter in transformation of silver ion to metallic nanoparticle.

Photoinduced reduction of silver ion (Ag(+)) to silver nanoparticles (AgNPs) by dissolved organic matter (DOM) plays a crucial role in the transformation and transport of engineered AgNPs and Ag(+) in aquatic environments. DOM is a mixture of natural polymers with wide molecular weight (MW) distribution, and the roles of specific components of DOM in the photoreduction of Ag(+) to AgNPs are still not understood. In this study, MW fractionated natural organic matter (Mf-NOM) were investigated for their roles on the photoreduction process and stabilization of the formed AgNPs. This photoinduced reduction process depends highly on pH, concentration of Ag(+) and NOM, light quality, and the MW of Mf-NOM. Monochromatic radiation and light attenuation correction suggested that the difference of Mf-NOM on reduction was mainly ascribed to the differential light attenuation of Mf-NOM rather than the "real" reductive ability. More importantly, compared with low MW fractions, the high MW Mf-NOMs exhibit drastically higher capability in stabilizing the photosynthesized AgNPs against Ca(2+)-induced aggregation. This finding is important for a better understanding of the differential roles of Mf-NOM in the transformation and transport of Ag(+) and engineered AgNPs in DOM-rich surface water.

Highly dynamic PVP-coated silver nanoparticles in aquatic environments: chemical and morphology change induced by oxidation of Ag(0) and reduction of Ag(+).

The fast growing and abundant use of silver nanoparticles (AgNPs) in commercial products alerts us to be cautious of their unknown health and environmental risks. Because of the inherent redox instability of silver, AgNPs are highly dynamic in the aquatic system, and the cycle of chemical oxidation of AgNPs to release Ag(+) and reconstitution to form AgNPs is expected to occur in aquatic environments. This study investigated how inevitable environmentally relevant factors like sunlight, dissolved organic matter (DOM), pH, Ca(2+)/Mg(2+), Cl(-), and S(2-) individually or in combination affect the chemical transformation of AgNPs. It was demonstrated that simulated sunlight induced the aggregation of AgNPs, causing particle fusion or self-assembly to form larger structures and aggregates. Meanwhile, AgNPs were significantly stabilized by DOM, indicating that AgNPs may exist as single particles and be suspended in natural water for a long time or delivered far distances. Dissolution (ion release) kinetics of AgNPs in sunlit DOM-rich water showed that dissolved Ag concentration increased gradually first and then suddenly decreased with external light irradiation, along with the regeneration of new tiny AgNPs. pH variation and addition of Ca(2+) and Mg(2+) within environmental levels did not affect the tendency, showing that this phenomenon was general in real aquatic systems. Given that a great number of studies have proven the toxicity of dissolved Ag (commonly regarded as the source of AgNP toxicity) to many aquatic organisms, our finding that the effect of DOM and sunlight on AgNP dissolution can regulate AgNP toxicity under these conditions is important. The fact that the release of Ag(+) and regeneration of AgNPs could both happen in sunlit DOM-rich water implies that previous results of toxicity studies gained by focusing on the original nature of AgNPs should be reconsidered and highlights the necessity to monitor the fate and toxicity of AgNPs under more environmentally relevant conditions.