Oral bioaccessibility of silver nanoparticles and ions in natural soils: Importance of soil properties

Trophic transfer of nanomaterials and their effects on high-trophic-level predators

Nanotechnology offers great opportunities for numerous sectors in society. One important challenge in sustainable nanotechnology is the potential of trophic transfer of nanomaterials (NMs), which may lead to unintentional impacts on environmental and human health. Here, we highlight the key advances that have been made in recent 15 years with respect to trophic transfer of heterogeneous NMs, including metal-based NMs, carbon-based NMs and nanoplastics, across various aquatic and terrestrial food chains. Particle number-based trophic transfer factors (TTFs), rather than the variable mass-based TTFs, capture the particle-specific transfer, for which NMs exhibit dynamic and complex biotransformation (e.g., dissolution, sulfidation, reduction, and corona formation). Trophic transfer of NMs has toxicological significance to predators at molecular (e.g., increased oxidative stress and modified metabolites), physiological (e.g., feeding inhibition) and population (e.g., reproduction inhibition) levels. However, linking NM exposure and toxicity remains a challenge, partly due to the dynamic biotransformation along the food chain. Although NMs have been used to increase crop yield in agriculture, they can exert detrimental impacts on crop yield and modify crop quality, depending on NMs type, exposure dose, and crop species, with unknown consequences to human health via crop consumption. Given this information, we describe the challenges and opportunities in understanding the significance of NMs trophic transfer to develop more sustainable, effective and safer nanotechnology.

Bioaccessibility and reliable human health risk assessment of heavy metals in typical abandoned industrial sites of southeastern China

Heavy metal pollution caused by a large number of abandoned industrial sites cannot be underestimated, but its human health risks have not been accurately assessed. This study investigated the pollution of heavy metals in soils of the typical abandoned industrial sites in southeastern China. Based on the bioaccessibility of different heavy metals (Pb, Ni, Cu, Zn, Cd, Cr) in the industrial soils, the human health risks were accurately evaluated, and the controlling factors were quantitatively assessed. The results showed that the heavy metals in each typical abandoned industrial sites had a high degree of spatial heterogeneity. Among them, Cd was the most susceptible to relevant discrete input from external factors such as human activities, followed by Zn, Pb, Cr, Ni and Cu. The bioaccessible concentration of heavy metals by the physiological-based extraction test (PBET) had a good correlation (R² = 0.58 ∼ 0.86) with its bioavailable concentration by diethylenetriaminepentaacetic acid (DTPA) extraction. The regression model based on soil parameters had great potential to predict the bioaccessibility of heavy metals in abandoned industrial sites (R² = 0.49 ∼ 0.95). The total concentration of heavy metals, Fe, soil texture and pH were the controlling factors of the metal bioaccessibility. Compared with the total concentration, the hazard index (HI) and carcinogenic risk (CR) values calculated based on gastrointestinal bioaccessibility were decreased by 39.0∼77.9% and 68.2∼79.9% in adults, and 45.3∼88.0% and 73.9∼83.5% in children, respectively. This work provides a feasible theoretical basis for reliable assessment of the human health risks of heavy metals in the abandoned industrial sites in the future.

Ambivalent effects of dissolved organic matter on silver nanoparticles/silver ions transformation: A review

The numerous applications of silver nanoparticles (AgNPs) lead to their spread in aquatic systems and the release of silver ions (Ag⁺), which brings potential risks to environment and human health. Owing to the different toxicity, the mutual transformations between AgNPs and Ag⁺ has been a hot topic of research. Dissolved organic matter (DOM) is ubiquitous on the earth and almost participates in all the reactions in the nature. The previous studies have reported the roles of DOM played in the transformation between AgNPs and Ag⁺. However, different experiment conditions commonly caused contradictory results, leading to the difficulty to predict the fate of AgNPs in specific reactions. Here we summarized mechanisms of DOM-mediated AgNPs oxidation and Ag⁺ reduction, and analyzed the effects of environmental parameters. Moreover, the knowledge gaps, challenges, and new opportunities for research in this field are discussed. This review will promote the understanding of the fate and risk assessments of AgNPs in natural water systems.

Nickel bioaccessibility in soils with high geochemical background and anthropogenic contamination

Abnormally high concentrations of metals including nickel (Ni) in soils result from high geochemical background (HB) or anthropogenic contamination (AC). Metal bioaccessibility in AC-soils has been extensively explored, but studies in HB-soils are limited. This study examined the Ni bioaccessibility in basalt and black shale derived HB-soils, with AC-soils and soils without contamination (CT) being used for comparison. Although HB- and AC-soils had similar Ni levels (123 ± 43.0 vs 155 ± 84.7 mg kg^-1), their Ni bioaccessibility based on the gastric phase of the Solubility Bioaccessibility Research Consortium (SBRC) in vitro assay was different. Nickel bioaccessibility in HB-soils was 6.42 ± 3.78%, 2-times lower than the CT-soils (12.0 ± 9.71%) and 6-times lower than that in AC-soils (42.6 ± 16.3%). Based on the sequential extraction, a much higher residual Ni fractionation in HB-soils than that in CT- and AC-soils was observed (81.9 ± 9.52% vs 68.6 ± 9.46% and 38.7 ± 16.0%). Further, correlation analysis indicate that the available Ni (exchangeable + carbonate-bound + Fe/Mn hydroxide-bound) was highly correlated with Ni bioaccessibility, which was also related to the organic carbon content in soils. The difference in co-localization between Ni and other elements (Fe, Mn and Ca) from high-resolution NanoSIMS analysis provided additional explanation for Ni bioaccessibility. In short, based on the large difference in Ni bioaccessibility in geochemical background and anthropogenic contaminated soils, it is important to base contamination sources for proper risk assessment of Ni-contaminated soils.

Effect of metal cations on antimicrobial activity and compartmentalization of silver in Shewanella oneidensis MR-1 upon exposure to silver ions

Silver is an antimicrobial agent that is used extensively in consumer products, such as fabrics and humidifiers. Silver ion (Ag⁺) uptake in bacteria represents a crucial phase of antimicrobial activity. However, the uptake mechanism of Ag⁺ in bacteria remains largely unknown. The genus Shewanella drives many geochemical processes of nutrients and pollutants in soils. In the present study, Ag⁺ uptake by Shewanella oneidensis MR-1 was first investigated in a laboratory in defined anaerobic, oligotrophic, and inorganic media with or without cations (potassium ions [K⁺], magnesium ions [Mg²⁺], and zinc ions [Zn²⁺]). Our results revealed variations in antimicrobial activity of Ag⁺ in the presence of Mg²⁺ and Zn²⁺. First, Mg²⁺ significantly decreased antimicrobial activity of Ag⁺ in S. oneidensis MR-1 by inhibiting cellular Ag⁺ uptake when compared with K⁺. The results were consistent with that of Co²⁺ (Mg²⁺ channel blocker) decreased Ag⁺ uptake by S. oneidensis MR-1. Moreover, Mg²⁺ promoted riboflavin secretion and facilitated the formation of metallic Ag nanoparticles on bacterial surfaces, which was beneficial for extracellular electron transfer and consequently reduced antibacterial activity of Ag⁺. Second, Zn²⁺ increased the antimicrobial activity of Ag⁺ in S. oneidensis MR-1, although the effect on Ag⁺ uptake was minimal. A synergistic interaction between Zn²⁺ and Ag⁺ led to an increase in dead cells and decreased ferrihydrite reduction capacity. The findings suggest that Mg²⁺ could reduce the environmental risk of Ag⁺ to soil bacteria, while Zn²⁺ should be of particular concern due to its synergistic antimicrobial effect on bacteria.

Greater Bioaccessibility of Silver Nanoparticles in Earthworm than in Soils

The buildup of silver nanoparticles (AgNPs) in soil has raised mounting concerns on their impact on human health. Human are exposed to AgNPs in soils via hand-to-mouth activities (direct exposure) and food consumption (indirect exposure). However, the bioaccessibility of AgNPs under these exposure scenarios remains largely unknown. We used a physiologically based extraction test (PBET) to assess Ag bioaccessibility in AgNP-containing soils and in earthworms (Pheretima guillemi) cultured in these soils. Silver bioaccessibility was 1.2 - 8.4% and 8.1 - 78.7% upon direct exposure and indirect exposure, respectively. These results indicated greater Ag bioaccessibility in earthworms than in soils. Moreover, particle size decreased upon direct exposure, but remained constant upon indirect exposure in wetland soil, as revealed by single particle inductively coupled plasma-mass spectrometry (spICP-MS) analysis. Our results highlight the importance of indirect exposure to NPs.

Biotic Process Dominated the Uptake and Transformation of Ag+ by Shewanella oneidensis MR-1

Silver ions (Ag⁺) directly emitted from industrial sources or released from manufactured Ag nanoparticles (AgNPs) in biosolid-amended soils have raised concern about the risk to ecosystems. However, our knowledge of Ag⁺ toxicity, internalization, and transformation mechanisms to bacteria is still insufficient. Here, we combine the advanced technologies of hyperspectral imaging (HSI) and single-particle inductively coupled plasma mass spectrometry to visualize the potential formed AgNPs inside the bacteria and evaluate the contributions of biological and non-biological processes in the uptake and transformation of Ag⁺ by Shewanella oneidensis MR-1. The results showed a dose-dependent toxicity of Ag⁺ to S. oneidensis MR-1 in the ferrihydrite bioreduction process, which was primarily induced by the actively internalized Ag. Moreover, both HSI and cross-section high-resolution transmission electron microscopy results confirmed that Ag inside the bacteria existed in the form of particulate. The Ag mass distribution in and around live and inactivated cells demonstrated that the uptake and transformation of Ag⁺ by S. oneidensis MR-1 were mainly via biological process. The bioaccumulation of Ag⁺ may be lethal to bacteria. A better understanding of the uptake and transformation of Ag⁺ in bacteria is central to predict and monitor the key factors that control Ag partitioning dynamics at the biointerface, which is critical to develop practical risk assessment and mitigation strategies.

Effect of Soil Properties and Aging Time on Oral and Inhalation Bioaccessibility of Copper Oxide Nanoparticles in Soils

In this study, soils spiked with copper oxide nanoparticles (CuO NPs) or Cu(NO₃)₂ and aged as long as 90 days were utilized to investigate effect of soil properties and aging on oral and inhalation bioaccessibility of CuO NPs. Results showed that oral bioaccessibility of CuO NPs in gastric phase (GP) ranged from 70% to 84%, it significantly decreased to 50%-70% in intestinal phase (IP). The inhalation bioaccessibility of CuO NPs in artificial lysosomal fluid (ALF) ranged from 66% to 85%, and much higher than that in Gamble's solution (GS, 3.3%-23%). By comparing CuO NPs to Cu(NO₃)₂ bioaccessibility, insignificant difference was found. The aging time (D15 and D90) had limited effect on their oral and inhalation bioaccessibility. CEC and free Al were positively and clay content was negatively correlated with CuO NPs inhalation bioaccessibility, while Cu(NO₃)₂ inhalation bioaccessibility decreased with increasing soil clay content and pH. Our findings provide an essential basis to evaluate the human health risks of CuO NPs.

In vitro bioaccessibility, phase partitioning, and health risk of potentially toxic elements in dust of an iron mining and industrial complex

Dust emitted from mining, ore processing, and tailing dumps have direct effects on miners who work close to these operations. The Gol-E-Gohar (GEG) mining and industrial company is one of the most important iron concentrate producers in the Middle East. The objective of the present study was to estimate the distribution, fractionation, and oral bioaccessibility of potentially toxic elements (PTEs) in dust generated by the GEG mining and industrial company. Total PTE content including Al, As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Mo, Ni, Pb, V, and Zn was quantified for suspended particulate matter (PM) in PM_2.5, PM₁₀, and total suspended particulate matter (TSP). As, Cd, Co, Cu, Fe, Ni, and Pb were quantified in fallout dust samples for oral bioaccessibility using in vitro Unified BARGE (UBM) Method and modified BCR fractionation analysis. Enrichment factors (EF) were calculated for the studied elements in PM; Cu, Fe, and As were found to be extremely enriched. Oral bioaccessibility of selected PTEs in fallout dust samples ranged from 0.35% to 41.55% and 0.06-37.58% in the gastric and intestinal phases, respectively. Regression modeling revealed that the bioaccessibilities of the PTEs could mostly be explained by total concentrations in dust particles. Average daily intake (ADI) calculations revealed that the intake of PTEs did not exceed the tolerable daily intake (TDI) values and as such was not considered a significant risk to workers. Additionally, the hazard quotients (HQ) and carcinogenic risk (CR) values were lower than the acceptable level. This study can provide further risk assessment and management of PTE pollution in occupational environments.

Soil particle size fraction and potentially toxic elements bioaccessibility: A review

In the last decade, extensive studies have been conducted to quantify the influence of different factors on potentially toxic elements (PTE) bioaccessibility in soil; one of the most important is soil size fraction. However, there is no agreement about the size fraction and the methods to investigate bioaccessibility, as very few review articles are available on soil PTE bioaccessibility and none addressed the influence of particle size on PTE bioaccessibility. This study provides a review of the relations between PTE bioaccessibility and soil particle size fractions. The available research indicates that PTE bioaccessibility distribution across different size fractions varies widely in soil, but a general trend of higher bioaccessibility in finer size fraction was found. The different elements may exhibit different relationships between bioaccessibility and soil size fraction and, in some cases, their bioaccessibility seems to be more related to the source and to the chemico-physical form of PTE in soil. Often, soil pollution and related health risk are assessed based on PTE total concentration rather than their bioaccessible fraction, but from the available studies it appears that consensus must be pursued on the methods to determine PTE bioaccessibility in the fine soil size fractions to achieve a more accurate human health risk assessment.

Influence of soil properties on the bioaccessibility of Cr and Ni in geologic serpentine and anthropogenically contaminated non-serpentine soils in Taiwan

Serpentine minerals with high levels of geologic chromium (Cr) and nickel (Ni) and non-serpentine farmlands polluted by irrigation water causing high anthropogenic Cr and Ni levels are both found in Taiwan. Elevated levels of Cr and Ni in these soils are a concern due to their potential to promote cancer mortality in humans. Bioaccessibility is a crucial factor determining the actual health risk via oral ingestion when children are exposed to metal-contaminated soils. Furthermore, the bioaccessibility of metals varies with the source, soil properties, and fractionation of metals in the soil. Therefore in this study, soil pH, total organic carbon (TOC), texture, and the total concentrations, fractionation, and bioaccessibility of Cr and Ni were analyzed and correlated for soils collected from serpentine mineral-containing deposits and contaminated non-serpentine farmlands. The low bioaccessibility and low mobility of Cr and Ni in serpentine soils suggested that incidental ingesting of soils posed a low health risk; however, the higher bioaccessibility and mobility of Ni in non-serpentine soils contaminated by electroplating wastewater could lead to potential risks for humans. Additionally, a significant difference in the bioaccessibility of Ni was observed between serpentine and non-serpentine soils, but this was not shown for Cr. Accordingly, a correlation analysis showed that Cr bioaccessibility was positively correlated with TOC, with no distinction between serpentine and non-serpentine soils. In contrast, TOC and the fractions of the sequential extraction procedure were significantly correlated with Ni bioaccessibility both in anthropogenically contaminated non-serpentine soils and in natural serpentine soils.

Assessing the Impacts of Cu(OH)2 Nanopesticide and Ionic Copper on the Soil Enzyme Activity and Bacterial Community

Nanopesticides are being introduced in agriculture, and the associated environmental risks and benefits must be carefully assessed before their widespread agricultural applications. We investigated the impacts of a commercial Cu(OH)₂ nanopesticide formulation (NPF) at different agricultural application doses (e.g., 0.5, 5, and 50 mg of Cu kg^-1) on enzyme activities and bacterial communities of loamy soil (organic matter content of 3.61%) over 21 days. Results were compared to its ionic analogue (i.e., CuSO₄) and nano-Cu(OH)₂, including both the commercial unformulated active ingredient of NPF (AI-NPF) and synthesized Cu(OH)₂ nanorods (NR). There were negligible changes in the activity of acid phosphatase, regardless of exposure dose, whereas significant (p < 0.05) variations in activities of invertase, urease, and catalase were observed at a dose of 5 mg kg^-1 or higher. Invertase activity decreased with an increasing bioavailable Cu concentration in soil under various treatments. In comparison to CuSO₄, both Cu(OH)₂ nanopesticide (i.e., NPF) and nano-Cu(OH)₂ (i.e., AI-NPF and NR) caused a significant (p < 0.05) inhibition of urease activity, wherein a significant (p < 0.05) increase in the activity of catalase was observed, representing serious oxidative stress. Accordingly, NPF, AI-NPF, and NR differently affected soil bacterial abundance, diversity, and community compared to CuSO₄, which could have resulted from the changes in the bioavailable Cu concentration as a result of the distinct nature of copper spiked (i.e., nano form versus salt). Moreover, minor differences in the soil enzyme activity and bacterial community were observed between NPF and AI-NPF, reflecting that the impact of the Cu(OH)₂ nanopesticide was primarily attributed to the presence of nano-Cu(OH)₂. In total, the impacts of nano-Cu(OH)₂ on the soil bacterial community and enzyme activity tested in this study differed from CuSO₄, shedding light on the environmental risks of the Cu(OH)₂ nanopesticide in the long run.

Bioavailability and cytotoxicity of Cerium- (IV), Copper- (II), and Zinc oxide nanoparticles to human intestinal and liver cells through food

Anthropogenic nanoparticles (NPs) are emitted to the environment and may be present in vegetables for human consumption. However, the toxicity of NPs exposure through food lack systematical investigations. In order to propose a systematical study, lettuce grown in a Cerium- (IV), Copper- (II) and Zinc oxide NP contaminated environment were digested. This digestate was used to culture human intestine cells (i.e. epithelial colorectal adenocarcinoma cells, Caco-2). The basolateral juice produced by the intestinal cells was then used to culture normal human liver (HL-7702) cells. Bioavailability and biotoxicity of the NPs in the vitro models were assessed. NPs were found to be taken up from the environment by vegetables, and may thus be transferred to humans through oral exposure. Bioavailability and the effect of their concentration in the digestate medium differed in regards to NP materials. The levels of NPs found in the digestate were detrimental to intestine cells, while the liver cells exposed to lower concentrations of NP in the bodily fluid showed no statically significant change in cell necrosis. A closer assessment of the detrimental effect of the studied NPs to Caco-2 cells revealed that the damage was mainly related to the solubility of the NPs. This may partly be due to that the more soluble NP material (ZnO > CuO > CeO₂) render higher metal ion release and thus higher bioavailability. This appeared to cause more cell death, and even lead to local intestinal inflammation. Although no liver cells died, there was an increase of ROS level, causing ROS-related DNA damage prior to cell necrosis. The findings in this study enhances our understanding of the relative detrimental effect of different types of NPs, and the mechanisms causing their biotoxicity in human cells through food.

Effects of biodegradable chelator combination on potentially toxic metals leaching efficiency in agricultural soils

Soil washing with chelators, a viable method for treating soils contaminated with potentially toxic metals, has drawn increasing attentions. The objective of this study was to determine a new generation of mixed degradable chelating agents from N, N-bis (carboxymethyl) glutamic acid (GLDA), [S, S]-stereoisomer of ethyleneiaminedisucc--inic acid (EDDS), nitrilotriacetic acid (NTA), and citric acid (CA), and to evaluate its effectiveness and feasibility to reduce toxic metals contamination in two different agricultural soils. A comparative leaching test conducted on the four individual degradable chelating agents showed that the capacity of single chelator in mobilizing copper (Cu), zinc (Zn), cadmium (Cd), and lead (Pb) varied significantly. Using a combination of GLDA and NTA was more advantageous than using a single chelating agent in extracting potentially toxic metals. The removal efficiencies of Cu, Zn, Cd, and Pb reached 38.2, 9.8, 71.4, and 19.5% for soil 1, and 25.0, 5.2, 59.7, and 18.5% for soil 2, respectively, at mixed chelator (MC) concentrations of 3 mmol/L (GLDA) and 2 mmol/L (NTA), pH of 6.0, and a contact time of 4.0 h. The effects of washing conditions, chelator concentration, pH values, and contact time on the removal efficiencies of target toxic metals were investigated. The results showed that the combined chelating agent has a lower pH dependence, making it feasible for a wider range of applications. The effects of the chelating agents on the morphological distribution of potentially toxic metals and the soil enzyme activity before and after the treatments were also studied. After washing, the content of the water-soluble, acid-soluble, reducible, and oxidizable target metals showed a certain degree of decrease. Although the activities of catalase, urease, and invertase appeared to be inhibited during a short period of time, their activities were stimulated and later promoted with the degradation of the chelating agent. In general, the chelating agent combination has a great potential for toxic metals leaching.

Interaction of silver nanoparticles with mediterranean agricultural soils: Lab-controlled adsorption and desorption studies

The production of silver nanoparticles (AgNPs) has increased tremendously during recent years due to their antibacterial and physicochemical properties. As a consequence, these particles are released inevitably into the environment, with soil being the main sink of disposal. Soil interactions have an effect on AgNP mobility, transport and bioavailability. To understand AgNP adsorption processes, lab-controlled kinetic studies were performed. Batch tests performed with five different Mediterranean agricultural soils showed that cation exchange capacity and electrical conductivity are the main parameters controlling the adsorption processes. The adsorption kinetics of different sized (40, 75, 100 and 200 nm) and coated (citrate, polyvinylpyrrolidone and polyethyleneglycol (PEG)) AgNPs indicated that these nanoparticle properties have also an effect on the adsorption processes. To assess the mobility and bioavailability of AgNPs and to determine if their form is maintained during adsorption/desorption processes, loaded soils were submitted to leaching tests three weeks after batch adsorption studies. The DIN 38414-S4 extraction method indicated that AgNPs were strongly retained on soils, and single-particle inductively coupled plasma mass spectrometry confirmed that silver particles maintained their nanoform, except for 100 nm PEG-AgNPs and 40 nm citrate-coated AgNPs. The DTPA (diethylenetriaminepentaacetic acid) leaching test was more effective in extracting silver, but there was no presence of AgNPs in almost all of these leachates.