The influence of Mn(II) on transformation of Cr-absorbed Schwertmannite: Mineral phase transition and elemental fate.

Schwertmannite (Sch) is considered as an effective remover of Chromium (Cr) due to its strong affinity for toxic Cr species. Since the instability of Sch, the environmental fate of Cr deserves attention during the transformation of Sch into a more stable crystalline phase. The ubiquitous manganese(II) (Mn(II)) probably affects the transformation of Sch and thus the environmental fate of Cr. Therefore, this study investigated the impact of Mn(II) on the transformation of Cr-absorbed Sch (Cr-Sch) and the associated behavior of SO42- and Cr. We revealed that the transformation products of Cr-Sch at pH 3.0 and 7.0 were goethite and Sch, respectively. The presence of Mn(II) weakened the crystallinity of the transformation products, and the trend was positively correlated with the concentration of Mn(II). However, Mn(II) changed the transformation products of Cr-Sch from hematite to goethite at pH 10.0. Mn(II) replaced Fe(III) in the mineral structures or formed Mn-O complexes with surface hydroxyl groups (-OH), thereby affecting the transformation pathways of Sch. The presence of Mn(II) enhanced the immobilization of Cr on minerals at pH 3.0 and 7.0. Sch is likely to provide an channel for electron transfer between Mn(II) and Cr(VI), which promotes the reduction of Cr(VI). Meanwhile, Mn(Ⅱ) induced more -OH production on the surface of secondary minerals, which played an important role in increasing the Cr fixation. In addition, part of the Mn(Ⅱ) was oxidized to Mn(Ⅲ)/Mn(Ⅳ) at pH 3.0 and pH 7.0. This study helps to predict the role of Mn(II) in the transformations of Cr-Sch in environments and design remediation strategies for Cr contamination.

Influence of dissolved organic matter with different molecular weight from chicken manure on ferrihydrite adsorption and re-release of antimony(V).

Applying organic fertilizer is the main way to enhance soil fertility through the interfacial reaction between mineral and dissolved organic matter (DOM). However, the interfacial reaction between minerals and DOM may influence antimony(V) (Sb(V)) mobility in agricultural soils around antimony mines. In our study the ferrihydrite (Fh) was chosen as a representative mineral, to reveal the effect of its interaction with chicken manure organic fertilizer (CM-DOM) with Fh on Sb(V) migration. In this study, we investigated different organic matter molecular weights and C/Fe molar ratios. Our findings indicated that the addition of CM-DOM decreased the adsorption of Sb(V) by Fh and promoted the re-release of Sb(V) adsorbed on Fh. This effect was enhanced by increasing the C/Fe molar ratio. Fh mainly affects its interaction with Sb(V) through electrostatic gravitational interaction and ligand exchange, but the presence of CM-DOM weakens the electrostatic interaction between Fh and Sb(V) as well as competes with Sb(V) for the hydroxyl reactive site on Fh surface. In addition, the smaller molecular weight fraction (<10 kDa) of CM-DOM has higher aromaticity and hydrophobicity, which potentially leads to more intense competition with Sb(V) for the reaction sites on Fh. Therefore, the application of organic fertilizer may promote Sb(V) migration, posing significant risks to soil ecosystems and human health, which should be a concern in field soil cultivation.

An environmentally friendly strategy for reducing the environmental risks of heavy metals adsorbed by kaolinite.

Plenty of heavy metals (HMs) that are adsorbed on clay minerals (such as kaolinite), in addition to low molecular-weight organic acids (such as oxalic acid (OA)) with high activities, are widespread in the natural environment. In the present study, the effects of OA on the environmental behaviors of Pb2+/Cd2+ adsorbed by kaolinite have been investigated. The effectiveness and mechanisms of calcium silicate (CS) and magnesium silicate (MS) in reducing the environmental risks of the HMs have also been studied. The results showed that the releases of Pb2+/Cd2+ increased with an increasing concentration of OA. When different dosages of CS/MS were added to the aging system, a redistribution of HMs took place and the free form of Pb2+/Cd2+ decreased to very low levels. Also, the unextractable Pb2+/Cd2+ increased to high levels. Furthermore, a series of characterizations showed that the released HMs were re-captured by the CS/MS. In addition, the CS immobilized the OA in the solution during the aging process, which also facilitated an immobilization of the carbon element in the environment. In general, the present study has contributed to a further understanding of the transport behaviors of the HMs in natural environments, and of the interactions between CS (or MS), the environmental media, and the heavy metal contaminants. In addition, this study has also provided an eco-friendly strategy for an effective remediation of heavy metal pollution.

Interaction of polystyrene nanoplastics with impurity-bearing ferrihydrite and implication on complex particle sedimentation.

Nanoplastics (NPs) usually coexist with impurity-bearing ferrihydrite (ImFh), and their interaction is related to their environmental fate. In this study, the aggregation between ImFh (impurities: Al, Mn and Si) and polystyrene nanoplastics (PSNPs), as well as the sedimentation of ImFh-PSNP complex particles in the aqueous phase were investigated systematically with particle concentrations of 100 mg/L ImFh and 10 mg/L PSNPs. Our results revealed that the PSNP suspension was dispersive and stable under various pH values and low ion strength. After coexisting with ImFh, PSNPs aggregated with the positively charged ImFh to form ImFh-PSNP complex particles, which destroyed the stability of PSNPs. The increase in pH and Na+ concentration could inhibit their aggregation, but high Na+ concentration (>20 mM) caused the homoaggregation of PSNPs. The aggregation capacity of PSNPs with ImFh was in the order of Al-bearing Fh > Fh > Mn-bearing Fh > Si-bearing Fh. Zeta potential and Derjaguin-Landau-Verwey-Overbeek (DLVO) calculations indicated that Al-bearing Fh showed higher positive potential than pure Fh, which caused stronger electrostatic interactions with PSNPs. However, Mn and Si in ImFh decreased the positive potential and inhibited the electrostatic interaction with PSNPs, and the effect of Si was greater than that of Mn. The aggregation between ImFh and PSNPs inhibited the sedimentation of their complex particles, and the higher aggregation capacity appeared to have a greater inhibition degree. Due to the "electrostatic patches" effect of PSNPs, the energy barrier of the ImFh-PSNPs particles was higher than that of the ImFh particles. Our findings clarified the influence of impurities on the interaction between ImFh and PSNPs and provided insight regarding their fate in the environment.

Impact of persistent rain on microplastics distribution and plastisphere community: A field study in the Pearl River, China.

Microplastic contamination is a global problem which has been threatening human health and the environment. There is still a knowledge gap about the effect of persistent rain on microplastics distribution and plastisphere community in fluvial environments. In this study, the abundance and composition of microplastics in the sediment and surface water from the Pearl River was investigated. Thirty polymers (10-500 µm) were identified from thirty-eight samples collected at ten sites using the newly developed laser direct infrared (LDIR) technique. The average concentrations of microplastics in the sediment and surface water were 1974 particles kg-1 and 290 particles L-1, respectively. Abnormally high concentrations of polyurethanes (PU) were possibly due to particulate pollution from ship antifouling. The persistent rain increased the abundance and diversity of microplastics in the surface water, whereas an opposite trend was observed in the sediment. Sediments could temporarily switch from microplastics sinks to potential sources under the effect of violent hydrodynamic disturbances. Additionally, plastisphere communities and predicted functional profiles indicated significant differences before and after the rain. Our study highlights the important impact of persistent rain on microplastic contamination in the environment.

The heteroaggregation behavior of nanoplastics on goethite: Effects of surface functionalization and solution chemistry.

Nanoplastics have attracted extensive attention in recent years. However, little is known about the heteroaggregation behavior of nanoplastics on goethite (FeOOH), especially the contribution of surface functional groups. In this study, the heteroaggregation behavior between polystyrene nanoplastics (PSNPs) and FeOOH was systematically investigated under different reaction conditions. Moreover, the effect of different functional groups (-NH2, -COOH, and bare) of PSNPs and solution chemistry was evaluated. The results showed that PSNPs could heteroaggregate with FeOOH, and the heteroaggregation rate of PSNPs with surface functionalization was significantly faster. The removal of suspended PSNPs was enhanced with increasing NaCl or CaCl2 concentration. However, heteroaggregation was significantly inhibited with the increase of solution pH. The zeta potentials analysis, time-resolved dynamic light scattering (DLS) and heteroaggregation experiments suggested that the electrostatic force affected the heteroaggregation process significantly. Fourier transform infrared (FTIR) spectra proved that the adsorption affinity between PSNPs and FeOOH was stronger after surface functionalization, especially for CH, O-C=O, and -CH2- groups, indicating that chemical bonding also made a contribution during the heteroaggregation process. This work is expected to provide a theoretical basis for predicting the environmental behavior between PSNPs and FeOOH.

Analysis and study of the migration pattern of microplastic particles in saturated porous media pavement.

This work aims to establish an analytical and comparative model of pavement stormwater runoff and determine how to solve water pollution in saturated porous media pavements. Heavy metal element particles in the stormwater runoff due to the rainfall will cause inevitable environmental pollution. First, the pavement runoff and materials of saturated porous media are analyzed. Besides, particle migration laws and separation effects of different materials are compared. Based on this, microplastics are selected as the primary material for pavement filling. Then, the adsorption effect of microplastics and the parameters of rainwater infiltration rate and infiltration ratio are analyzed to propose a multi-level ecological integrated treatment system for pavement runoff. Specifically, the environmental resource pollution and saturated porous media materials are analyzed. In addition, the adsorption effect of microplastic particles is analyzed to establish a model to study the selection process of the optimal adsorption material. The main contribution of the research is to analyze the migration process of metal particles in the soil in combination with the internal particle migration rules of plastic granular materials. The research results demonstrate that the rain runoff coefficient gradually increases with the expansion of the permeable area of the pavement. The rain runoff coefficient reaches the maximum value under the pavement of 120 square meters. In addition, a comparative analysis of three street pavements is conducted on the residential street pavement (RSP), commercial street pavement (CSP), and active street pavement (ASP). When comparing the two sets of data, the overall average permeability of the RSP is better than CSP and ASP. The research materials are compared under isothermal conditions. The particle adsorption effect of the same material at 50 °C is significantly better than that at 30 °C. Therefore, it is feasible to resolve the pavement runoff water pollution through technical schemes.

The heteroaggregation and deposition behavior of nanoplastics on Al2O3 in aquatic environments.

The ubiquitous Al2O3 is anticipated to interact with nanoplastics, affecting their fate and transport in aquatic environments. In this study, the heteroaggregation and deposition behaviors of polystyrene nanoplastics (PSNPs) on Al2O3 were systematically investigated under different conditions (ionic strength, pH, and natural organic matter). The results showed that significant heteroaggregation occurred between PSNPs and Al2O3 particles under acidic and neutral conditions. When the NaCl concentration was increased from 50 to 500 mM, the heteroaggregation ratio gradually increased. However, poly (acrylic acid) (PAA) inhibited the heteroaggregation of PSNPs-Al2O3 due to steric repulsion. The deposition of PSNPs on Al2O3 surfaces was inhibited as the NaCl concentration or pH values increased. Due to charge reversal and steric repulsion, humic acid (HA) and fulvic acid (FA) prevented the deposition of PSNPs onto Al2O3 surfaces, and the former was more effective in reducing the deposition rate. The interaction mechanism between PSNPs and Al2O3 was revealed by using various characterization techniques and density function theory (DFT) calculation. The results demonstrated that in addition to the dominant electrostatic interaction, there were also weak hydrogen bonds and van der Waals interactions. Our research is of great significance for predicting the migration and fate of PSNPs in aquatic environments.

Abundance and distribution characteristics of microplastic in plateau cultivated land of Yunnan Province, China.

Microplastic pollution in cultivated soil has received increasing attention recently. There may be more serious microplastic abundance but little research has been done in cultivated soil in plateau areas. To survey the pollution characteristics of microplastics in inland cultivated soil, 100 soil samples collected from 10 counties of Yunnan Province were investigated through density separation and microscopic examination. The research results showed that microplastic abundance was in the range of 0.9 × 103 to 40.8 × 103 particles (kg Ds)-1 with average abundance of 9.8 × 103 particles (kg Ds)-1. Moreover, compared with other studies on sediments, it was found that microplastic abundance in inland soil was one order of magnitude higher than that in offshore sediments. The use of plastic mulch and its long-term residue in cultivated soil was an important reason for microplastic pollution. In this survey, various morphologies of microplastics existed, including fragment (78.3%), transparent/translucency (49.7%), and micro-size microplastics (< 500 µm) (89.3%). And the microplastic morphologies occurred in different degrees of aging phenomenon under the influence of the environment factors such as ultraviolet radiation. The findings provided the pollution status of microplastics in cultivated soil, and more attention should be paid to inland soil microplastic pollution. Grapical abstract.