Aggregation, Sedimentation and Dissolution of Cu(OH)2-Nanorods-Based Nanopesticide in Soil Solutions.

Along with the development of nanotechnology, nanomaterials have been gradually applied to agriculture in recent years, such as Cu(OH)2-nanorods-based nanopesticide, an antibacterial agrochemical with a high efficacy. Nevertheless, knowledge about physical stability of Cu(OH)2 nanopesticide in soil solutions is currently scarce, restricting comprehensive understanding of the fate and risk of Cu(OH)2 nanopesticide in the soil environment. Herein we investigated aggregation, sedimentation and dissolution of Cu(OH)2 nanopesticide in soil solutions extracted from three different soil samples, wherein commercial Cu(OH)2 nanopesticide formulation (NPF), as well as its active ingredient (AI) and laboratory-prepared Cu(OH)2 nanorods (NR) with similar morphology as AI, were used as model Cu(OH)2 nanopesticides. We found that NPF compared to AI showed less extents of aggregation in ultrapure water due to the presence of dispersing agent in NPF. Yet, moderated aggregation and sedimentation were observed for Cu(OH)2 nanopesticide irrespective of NPF, AI or NR when soil solutions were used instead of ultrapure water. The sedimentation rate constants of AI and NPF were 0.023 min-1 and 0.010 min-1 in the ultrapure water, whereas the rate constants of 0.003-0.021 min-1 and 0.002-0.007 min-1 were observed for AI and NPF in soil solutions, respectively. Besides aggregation and sedimentation, dissolution of Cu(OH)2 nanopesticide in soil solutions was highly dependent on soil type, wherein pH and organic matter played important roles in dissolution. Although the final concentrations of dissolved copper (1.08-1.37 mg/L) were comparable among different soil solutions incubating 48 mg/L of AI, NPF or NR for 96 h, a gradual increase followed by an equilibrium was only observed in the soil solution from acidic soil (pH 5.16) with the low content of organic matter (1.20 g/kg). This work would shed light on the fate of Cu(OH)2 nanopesticide in the soil environment, which is necessary for risk assessment of the nanomaterials-based agrochemical.

Response of soil enzyme activity and bacterial community to copper hydroxide nanofertilizer and its ionic analogue under single versus repeated applications.

Nanosized agrochemicals like nanofertilizers are being applied to soils. Adverse impacts of nanofertilizers on soil microflora were reported in past studies, but only considering a single application. Repeated applications are however more likely to occur in agriculture. We investigated effects of single versus repeated applications of a copper hydroxide nanofertilizer formulation (NFF) on soil enzyme activity and bacterial community. One or three applications were performed within 21 days to achieve same final level of Cu in soil (48 mg(Cu)/kg: the recommended dose of NFF). Besides, the active ingredient (i.e., copper hydroxide nanotubes (NT)) and dispersing agent (DA) of NFF, and an ionic fertilizer (i.e., CuSO4) were examined. Fluorescein diacetate hydrolase (FDAse), N-acetylglucosaminidase (NAG), leucine aminopeptidase (LAP), and urease (URE) showed negligible changes in the activities between the control and DA treatment. Bacterial community abundance, composition and diversity exhibited similar phenomena. Exposures to copper hydroxide NFF and NT or CuSO4 enhanced the activities of FDAse and NAG, weakened the activity of URE, and showed negligible changes in the LAP activity irrespective of single and repeated applications. Concentrations of NO3--N and NH4+-N in soil were also affected by the application mode of NFF. More importantly, responses of soil bacterial community to copper hydroxide NFF were highly dependent on its application mode, whereas similar responses were observed in the CuSO4 treatment regardless of single or repeated applications. This study provided new insights into environmental risk of copper hydroxide NFF that were ignored in previous studies using a single exposure.

Identification and Speciation of Nanoscale Silver in Complex Solid Matrices by Sequential Extraction Coupled with Inductively Coupled Plasma Optical Emission Spectrometry.

Nanoscale silver (n-Ag) including silver nanoparticles (Ag-NPs), silver chloride nanoparticles (AgCl-NPs), and silver sulfide nanoparticles (Ag2S-NPs) and their corresponding ionic counterpart, namely, dissolved Ag, may coexist in soils. X-ray absorption near edge spectroscopy (XANES) is used to elucidate the speciation of n-Ag in soils, whereas it possesses drawbacks like high costs, rare availability of the instrument, and providing semiquantitative data. We developed a new method for the identification and speciation of n-Ag in soils and sediments based on a sequential extraction technique coupled with inductively coupled plasma optical emission spectrometry. Extraction conditions were first evaluated, establishing the optimal extraction procedure; Ag-NPs, AgCl-NPs, and dissolved Ag in soil were simultaneously extracted by using an aqueous solution of 10 mM tetrasodium pyrophosphate, followed by selective isolation and quantification via AgCl-NPs dissolution (4.45 M aqueous ammonia), centrifugation (Ag-NPs), and detection. The Ag2S-NPs remaining in the soil were then extracted with Na2S solution at pH 7.0 through selective complexation. Optimal recoveries of Ag-NPs, AgCl-NPs, Ag2S-NPs, and dissolved Ag were 99.1 ± 2.4%, 112.0 ± 3.4%, 96.4 ± 4.0%, and 112.2 ± 4.1%, respectively. The method was validated to investigate the speciation of n-Ag in soils and sediments, exhibiting the distribution of Ag-NPs, AgCl-NPs, Ag2S-NPs, and dissolved Ag in each sample, wherein Ag2S-NPs, the major species of n-Ag, accounted for 35.42-68.87% of the total Ag. The results of n-Ag speciation in soil are comparable to those obtained through the linear combination fitting of XANES. This method thus is a powerful, yet convenient, substitute for XANES to understand the speciation of n-Ag in complex solid matrices.

Chemical transformations of nanoscale zinc oxide in simulated sweat and its impact on the antibacterial efficacy.

Nanoscale zinc oxide (n-ZnO) is widely used in personal care products and textiles, thus, it would likely be released into human sweat. To better evaluate the potential human health risks of n-ZnO, it is essential to understand its chemical transformations in physiological solutions, such as human sweat, and the resulting changes in the n-ZnO bioavailability. Here, two types of n-ZnO, ZnO nanoparticles (ZnO-NPs) and nanorod-based ZnO nanospheres (ZnO-NSs) were synthesized and incubated in 3 types of simulated sweat with different pH values and phosphate concentrations. The content of Zn3(PO4)2 in the transformed n-ZnO was quantified by selective dissolution of Zn3(PO4)2 in 0.35 M ammonia solution where 100% and 5.5% of Zn3(PO4)2 and ZnO were dissolved, respectively. The kinetics analysis indicated that by 24-48 h the content of Zn3(PO4)2 reached the maximum, being 15-21% at pH 8.0 and 45-70% at pH 5.5 or 4.3. Interestingly, no correlation was observed between the rate constants of Zn3(PO4)2 formation and the specific surface areas of n-ZnO, implying that chemical transformations from n-ZnO to Zn3(PO4)2 in the simulated sweat might not be simply attributed to dissolution and precipitation. Using a variety of characterization techniques, we demonstrated the formation of a ZnO‒Zn3(PO4)2 core-shell structure with the shell consisting of amorphous Zn3(PO4)2 at pH 8.0 and additionally of crystalline Zn3(PO4)2 and Zn3(PO4)2•4H2O at pH 5.5 or 4.3. The phosphate-induced transformation of n-ZnO in the simulated sweat at pH 5.5 and 4.3 greatly reduced the antibacterial efficacy of n-ZnO through moderating the nanoparticle dissolution, indicating limited bioavailability of the NPs upon transformation. The results improve the understanding of the fate and hazards of n-ZnO.

Transformations of Ag2S nanoparticles in simulated human gastrointestinal tract: Impacts of the degree and origin of sulfidation.

Engineered silver sulfide nanoparticles (e-Ag2S-NPs) are used in industry and can be released into the environment. Besides e-Ag2S-NPs, transformed silver sulfide nanoparticles (t-Ag2S-NPs) from silver nanoparticles are more likely to be the form that is widely distributed in the environment. Both e-Ag2S-NPs and t-Ag2S-NPs may be ingested and get into human gastrointestinal tract (GIT) through trophic transfer, posing a potential threat to human health. Nevertheless, knowledge of chemical stability of t-Ag2S-NPs and e-Ag2S-NPs in the human GIT is very limited. Herein e-Ag2S-NPs and a series of t-Ag2S-NPs with different degrees of sulfidation were selected as models for exposure to the simulated human GIT including mouth, stomach and small intestine phases under fed and fasted conditions. Silver ions were detected in the simulated saliva, gastric and small intestine fluids when t-Ag2S-NPs or e-Ag2S-NPs were incubated in the simulated GIT, but the amount (e.g., < 20 µg) of silver ion in each phase accounted for < 0.2‰ (w/w) of the silver added (i.e., 100 mg). Silver species of the residual particulate from each phase of the simulated GIT with t-Ag2S-NPs or e-Ag2S-NPs were thus analyzed through a developed analytical method that could selectively, successively and efficiently dissolve and quantify AgCl, Ag(0), and Ag2S in particulates. Both e-Ag2S-NPs and fully sulfidized t-Ag2S-NPs were shown to be highly stable in the simulated human GIT. Conversely, partially sulfidized t-Ag2S-NPs primarily underwent transformations in the mouth phase relative to stomach and small intestine phases regardless of fed or fasted status, wherein AgCl and Ag2S were observed besides Ag(0). The amount of Ag2S in the mouth phase negatively (r = -0.99, p < 0.001) correlated with the sulfidation degree of initial t-Ag2S-NPs. This work improved our understanding of potential transformations of t-Ag2S-NPs in the simulated human GIT, providing valuable information for future researches on evaluating health risks of ingested Ag2S-NPs.

Sulfidation of sea urchin-like zinc oxide nanospheres: Kinetics, mechanisms, and impacts on growth of Escherichia coli.

Nanoscale zinc oxide (n-ZnO) with different morphology and sizes has been used in personal care products due to their antibacterial properties, resulting in discharge of n-ZnO into the environment with potential toxic effect to ecological systems. Sulfidation is one of pathways of transformation of n-ZnO, but a very limited information on the conversion of n-ZnO under sulfidic environment with special morphology such as sea urchin-like zinc oxide nanospheres (ZnO-NSs) is available to know the potential environmental risks of n-ZnO. Herein, sea urchin-like ZnO-NSs with an average size of 78 nm were synthesized and adopted as the model n-ZnO of special morphology. The ZnO-NPs at average sizes of 71 nm (ZnO-NPs-71), 48 nm (ZnO-NPs-48), and 17 nm (ZnO-NPs-17) nm were used to examine possible differences in the sulfidation between the sea urchin-like ZnO-NSs and ZnO-NPs. A new analytical method selectively dissolving ZnO over ZnS in partially sulfidized n-ZnO was developed and applied to understand the kinetics of n-ZnO sulfidation. The sulfidation rate constant (ks) of sea urchin-like ZnO-NSs was 2.9 × 10-3 h-1, comparable to that of ZnO-NPs-71 (4.1 × 10-3 h-1), but much lower than those of ZnO-NPs-48 (20.1 × 10-3 h-1) and ZnO-NPs-17 (67.8 × 10-3 h-1). This might be attributed to the differences in the specific surface area; ks positively correlated with the specific surface area (R2 = 0.97). Natural organic matter (NOM) decreased dissolution and sulfidation of the sea urchin-like ZnO-NSs. Aggregate ZnS nanocrystals instead of the original sea urchin-like ZnO-NSs were observed. We proposed that sea urchin-like ZnO-NSs were transformed to ZnS through a dissolution-precipitation pathway, consistent with the sulfidation pathway of ZnO-NPs. Sulfidation drastically reduced toxicity of sea urchin-like ZnO-NSs to Escherichia coli due to negligible dissolution of ZnS nanocrystals. These results greatly improved our understanding of the transformation and potential risks of n-ZnO with special morphology.