Microplastics alter Cr accumulation and fruit quality in Cr(VI) contaminated soil-cucumber system during the lifecycle: Insight from rhizosphere bacteria and root metabolism.

Both microplastics and Cr(VI) potentially threaten soil and crops, but little is known about their interaction in the soil-plant system. This study investigated the effect and mechanism of polyethylene (PE), polyamide (PA), and polylactic acid (PLA) microplastics on Cr bioaccumulation and toxicity in a Cr(VI) contaminated soil-cucumber system during the lifecycle. The results show that microplastics had a greater effect on Cr accumulation in cucumber roots, stems, and leaves than in fruits. PE microplastics increased, but PA and PLA microplastics decreased the Cr accumulation in cucumber. Microplastics, especially high-dose, small, and aged microplastics, exacerbated the effects of accumulated Cr in cucumber on fresh weight and fruit yield. The nutrient contents in fruits except soluble sugars were reduced by microplastics. The random forest regression model shows that the microplastic type was the most important factor causing changes in the soil-cucumber system except for Cr(VI) addition. Under Cr(VI) and microplastic co-exposure, bacteria that could simultaneously tolerate Cr(VI) stress and degrade microplastics were enriched in the rhizosphere soil. The partial least squares path model shows that microplastics reduced the beneficial effect of the bacterial community on cucumber growth. Microplastics, especially PLA microplastics, alleviated the adverse effects of Cr(VI) stress on root metabolism.

Organic matters adsorbed on goethite inhibited the heterogeneous aggregation and adsorption of CdSe quantum dots: Experiments and extended DLVO theory.

The widespread use of Cd-based quantum dots (Cd-QDs) has led to their inevitable release into the environment, and the prevalent iron oxides and natural organic matter (NOM) are the key factors affecting the environmental behavior and fate of Cd-QDs. However, the impact of NOM adsorbed on iron oxides on the behavior of Cd-QDs with iron oxides and the mechanism of its interaction are not clear. In this study, two kinds of water-soluble QDs (CdSe QDs and core-shell CdSe/ZnS QDs) were selected to study the aggregation and adsorption behavior on goethite (Goe) and goethite-humic acid/fulvic acid composites (Goe-HA/FA). Aggregation kinetics and adsorption experiments between QDs and Goe(-HA/FA), characterization, and extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory calculations indicated that electrostatic interaction was the dominant force for QDs adsorption on Goe(-HA/FA). HA/FA changed the surface charge of Goe and increased the electrostatic repulsion and steric hindrance between the particles, which in turn inhibited the adsorption of QDs on Goe. Besides, unsubstituted aromatic carbons, carboxy carbons, and carbonyl carbons played an important role in the adsorption process, and chemisorption occurred between QDs and Goe(-HA/FA). Our findings are important for better assessing the transport, fate, and potential environmental impacts and risks of Cd-QDs in iron-rich environments.

Water-dependent effects of biodegradable microplastics on arsenic fractionation in soil: Insights from enzyme degradation and synchrotron-based X-ray analysis.

The abundance of biodegradable microplastics (BMPs) is increasing in soil due to the widespread use of biodegradable plastics. However, the influence of BMPs on soil metal biogeochemistry, especially arsenic (As), under different water regimes is still unclear. In this study, we investigated the effects of two types of BMPs (PLA-MPs and PBAT-MPs) on As fractionation in two types of soils (black soil and fluvo-aquic soil) under three water regimes including drying (Dry), flooding (FL), and alternate wetting and drying (AWD). The results show that BMPs had limited indirect effects on As fractionation by altering soil properties, but had direct effects by adsorbing and releasing As during their degradation. Enzyme degradation experiments show that the degradation of PLA-MPs led to an increased desorption of 4.76 % for As(III) and 15.74 % for As(V). Synchrotron-based X-ray fluorescence (µ-XRF) combined with micro-X-ray absorption near edge structure (µ-XANES) analysis show that under Dry and AWD conditions, As on the BMPs primarily bind with Fe hydrated oxides in the form of As(V). Conversely, 71.57 % of As on PBAT-MP under FL conditions is in the form of As(III) and is primarily directly adsorbed onto its surface. This study highlights the role of BMPs in soil metal biogeochemistry.

Effect of polyethylene, polyamide, and polylactic acid microplastics on Cr accumulation and toxicity to cucumber (Cucumis sativus L.) in hydroponics.

Microplastics (MPs) in farmland soil may affect the environmental fate and toxicity of heavy metals; however, how non-biodegradable and biodegradable MPs change the accumulation and phytotoxicity of Cr(VI) to the plants is still unknown. In this study, we explored the impacts of Cr(VI) concentrations (0, 20, 50, 100, 200, and 500 µmol/L), MP types (polyethylene (PE), polyamide (PA), and polylactic acid (PLA)), sizes (13, 48, and 500 µm), and concentrations (40, 200, and 1000 mg/L) on the Cr accumulation and toxicity to cucumber (Cucumis sativus L.) under hydroponic conditions for 14 days. The results show that the presence of PE-MPs promoted the Cr accumulation in root by 8-39.8%. However, PA-MPs inhibited the Cr accumulation in the whole plant under less than 100 µmol/L Cr(VI). Notably, 1000 mg/L PA-MPs significantly reduced Cr accumulation in root and stem by 44.70% and 48.20%, respectively. Moreover, PE-MPs and PLA-MPs reduced the chlorophyll content and slowed down the growth of seedlings, while PA-MPs were beneficial to the growth of cucumber under 50-500 µmol/L Cr(VI) treatments, increasing the biomass by 20.99-189.99%. Furthermore, PE-MPs enhanced the content of MDA, especially under 500 µmol/L Cr(VI) concentration by 27.39%; however, the addition of PA-MPs and PLA-MPs slightly enhanced the enzyme activities including superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT). Significantly, 1000 mg/L PA-MPs promoted biomass and reduced MDA content compared the control due to their high Cr(VI) adsorption efficiency. Thus, MP type, especially PE-MPs, mainly determined the Cr accumulation and phytotoxicity, which was attributed to the various adsorption capacities of MPs to Cr(VI).

Polyamide microplastics as better environmental vectors of Cr(VI) in comparison to polyethylene and polypropylene microplastics.

The adsorption, desorption, and their influence factors of Cr(VI) by microplastics (MPs) in the solution was investigated in this study. The results demonstrated that UV aging promoted adsorption, while the increase of salinity and natural organic matter (NOM) inhibited adsorption. The particle size affected the total Cr(VI) active adsorption sites on MPs, while the pH changed the electrostatic force. The Cr(VI) adsorption by MPs conformed to the pseudo-first and pseudo-second order kinetic models, and was monolayer and inhomogeneous. SEM-EDS, XPS and ATR-FTIR analyses demonstrated that the physical adsorption dominated the adsorption process. Especially, the highest adsorption capacity of Cr(VI) by polyamide (PA) MPs was attributed to their hydrophilic amide groups. However, Cr(VI) was only released from PA MPs, of which desorption rate followed the sequence of seawater (79.5 %) > fresh water (66.6 %) > deionized water (34.8 %). Thus, PA MPs might be non-negligible environmental vectors for the transport of Cr(VI).

Response mechanism of lettuce (Lactuca sativa L.) under combined stress of Cd and DBDPE: An integrated physiological and metabolomics analysis.

DBDPE and Cd are representative contaminants commonly found in electronic waste (e-waste), which tend to be gradually discharged and accumulated in the environment during e-waste dismantling, resulting in frequent outbreaks and detection of these pollutants. The toxicity of both chemicals to vegetables after combined exposure has not been determined. The accumulation and mechanisms of phytotoxicity of the two compounds, alone and in combination, were studied using lettuce. The results showed that the enrichment ability of Cd and DBDPE in root was significantly higher than that in aerial part. Exposure to 1 mg/L Cd + DBDPE reduced the toxicity of Cd to lettuce, while exposure to 5 mg/L Cd + DBDPE increased the toxicity of Cd to lettuce. The absorption of Cd in the underground part of lettuce of 5 mg/L Cd + DBDPE was significantly increased by 108.75 % compared to 5 mg/L Cd. The significant enhancement of antioxidant system activity in lettuce under 5 mg/L Cd + DBDPE exposure, and the root activity and total chlorophyll content were decreased by 19.62 % and 33.13 %, respectively, compared to the control. At the same time, the organelles and cell membranes of lettuce root and leaf were significantly damaged, which was significantly worse than that of single Cd and DBDPE treatment. Combined exposure significantly affected the pathways related to amino acid metabolism, carbon metabolism and ABC transport in lettuce. This study filled the safety gap of DBDPE and Cd combined exposure on vegetables and would provide a theoretical basis for the environmental behavior and toxicological study of DBDPE and Cd.

Simultaneous stabilization of Pb, Cd, and As in soil by rhamnolipid coated sulfidated nano zero-valent iron: Effects and mechanisms.

Sulfidation effectively improves the electron transfer efficiency of nanoscale zero-valent iron (nZVI), but decreases the specific surface area of nZVI. In this study, sulfidated nZVI (S-nZVI) coated with rhamnolipid (RL-S-nZVI) was synthesized and used to stabilize Pb, Cd, and As in combined polluted soil. The stabilization efficiency of 0.3% (wt) RL-S-nZVI to water soluble Pb, Cd, and As in soil reached 88.76%, 72%, and 63%, respectively. Rhamnolipid coating inhibited the reduction of specific surface area and successfully encapsulated nZVI, thus reducing the oxidation of Fe0. The types of iron oxides in RL-S-nZVI were reduced compared to S-nZVI, but the content and strength of Fe0 iron were obviously enhanced. Furthermore, rhamnolipid functional groups (-COOH and -COO-) were also involved in the stabilization process. In addition, the stabilization efficiency of RL-S-nZVI to the bioavailable Pb, Cd, and As in soil increased by 41%, 41%, and 50%, respectively, compared with nZVI. The presence of organic acids, especially citric acid, improved the stabilization efficiency of RL-S-nZVI to the three metals. The result of BCR sequential extraction indicated that RL-S-nZVI increased the residual state of Pb, Cd, and As and reduced the acid-soluble and reducible state after 28 days of soil incubation. XRD and XPS analyses showed that the stabilization mechanisms of RL-S-nZVI on heavy metals involved in ion exchange, surface complexation, adsorption, co-precipitation, chemisorption, and redox.

Exogenous iron alters uptake and translocation of CuO nanoparticles in soil-rice system: A life cycle study.

The abundant iron in farmland soil may affect the environmental fate of metal-based nanoparticles (MNPs). In this study, the effect of FeSO4 and nano-zero-valent iron (nZVI) as exogenous iron on the uptake and translocation of CuO nanoparticles (NPs) in soil-rice system was performed in a life cycle study. The results show that exogenous iron basically elevated the soil pH and electrical conductivity but lowered the redox potential. Moreover, the Cu bioavailability in soil was significantly increased by 86-269% with exogenous iron at the tillering stage, while was reduced by 15-45% with medium and high concentrations of Fe(II) at the maturation stage. Meanwhile, the addition of exogenous iron resolved the unfilling of grains caused by CuO NPs. Notably, except for highest Fe(II) treatment, both Fe(II) and nZVI reduced Cu accumulation from 31% to 84% in roots and leaves due to more iron plaque. Especially, medium Fe(II) level markedly decreased the Cu content in the brown rice. µ-XRF analysis suggests that high intensity of Cu was primarily located in the rice hull and embryo under Fe(II) treatment. The reduction of CuO NPs to Cu2O caused by Fe(II) can explain the positive effect of exogenous iron on controlling the environmental risk of MNPs.