Nano-size plastics inhibited Cr(VI) species transformation during facilitated transport of green synthesized nano-iron in the presence of oxyanions

Escherichia coli and phosphate mediated the distinct retention of small-sized nano-plastic particles in seawater-saturated porous sands

Small nano-plastics (NPs, < 30 nm) with a high accumulation in biological organisms in coastal areas might react with widely presented bacteria and phosphate, which remains unclear. Therefore, the mechanisms governing the transport of two-sized NPs with Escherichia coli (E. coli) and phosphate were investigated in hyper-saline water-saturated sand porous media. The results showed that 20 nm NPs exhibited more hetero-aggregation with E. coli than 80 nm NPs, associated with lower k1d/k1 values (0.268 vs. 0.412) and more substantially suppressed depth of φmax (17.83 KBT vs. 23.44 KBT), based on two-site kinetic attachment retention model fitting and extended-Derjaguin-Landau-Verwey-Overbeek theory. Accordingly, even though the mass recovery percentage of both sized NPs alone was similar, the irreversible deposition of 20 nm NPs doubled by E. coli, increasing the coastal environmental risks. In contrast, 80 nm NPs reversibly attached to the sands with less effect by E. coli, causing secondary pollution. The copresence of phosphate pronouncedly enhanced the transportability of two-sized NPs with E. coli, especially increasing 20 nm NP mobility from 17.7 % to 39.2 % in 200 mM NaCl by preferentially adsorbing onto E. coli to avoid agglomeration with NPs. This study highlights the potential risk of small NPs in complicated coastal ecosystems.

Residual mulch-film characteristics affect heavy metal migration of different soil layers in the subtropical croplands of China

In recent years, as the abundance of residual mulch film (RMF) in agricultural soil continues to increase, whether the adsorption capacity of its surface affects the migration of heavy metals is a topic of current interest for scholars. Herein, this study investigated the distribution of RMF abundance and metal concentration in different soil layers of 75 plastic-mulching croplands in subtropical China; meanwhile, we also explored the associations of RMF characteristics with metal concentration. The results showed that land type, film mulching amount, and film mulching time were the main factors affecting RMF abundance, distribution, and particle size composition. The highest abundance of RMF was found in the garden soils (910 n·kg-1) with more than 15 years mulching period and more than 19.5 kg hm-2 of annual mulch amount. The lowest abundance of RMF was occurred in the group of field and conservation agricultural land (237 n·kg-1). Moreover, the concentrations of metals in soil, especially Cd, Cr, Cu, and Pb, were closely related to the extent of RMF contamination in the soil environment. In the 0-10 cm and 10-20 cm soil layers, microplastic abundance exhibited a negative correlation with Cr and Cu concentrations and a positive correlation with Pb concentration. Based on the above findings, it is demonstrated that RMF significantly influences the mobility of metals in soil via adsorption processes, with potential synergistic effects between RMF and heavy metals posing a heightened risk to the soil environment.

Cr(VI) removal during cotransport of nano-iron-particles combined with iron sulfides in groundwater: Effects of D. vulgaris and S. putrefaciens

Iron-based materials such as nanoscale zerovalent iron (nZVI) are effective candidates to in situ remediate hexachromium (Cr(VI))-contaminated groundwater. The anaerobic bacteria could influence the remediation efficiency of Cr(VI) during its cotransport with nZVI in porous media. To address this issue, the present study investigated the adsorption and reduction of Cr(VI) during its cotransport with green tea (GT) modified nZVI (nZVI@GT) and iron sulfides (FeS and FeS2) in the presence of D. vulgaris or S. putrefaciens in water-saturated sand columns. Experimental results showed that the nZVI@GT preferred to heteroaggregate with FeS2 rather than FeS, forming nZVI@GT-FeS2 heteroaggregates. Although the presence of D. vulgaris further induced nZVI@GT-FeS2 heteroaggregates to form larger clusters, it pronouncedly improved the dissolution of FeS and FeS2 for more Cr(VI) reduction associated with lower Cr(VI) flux through sand. In contrast, S. putrefaciens could promote the dispersion of the heteroaggregates of nZVI@GT-FeS2 and the homoaggregates of nZVI@GT or FeS by adsorption on the extracellular polymeric substances, leading to the improved transport of Fe-based materials for a much higher Cr(VI) immobilization in sand media. Overall, our study provides the essential perspectives into a chem-biological remediation technique through the synergistic removal of Cr(VI) by nZVI@GT and FeS in contaminated groundwater. ENVIRONMENTAL IMPLICATION: The green-synthesized nano-zero-valent iron particles (nZVI@GT) using plant extracts (or iron sulfides) have been used for in situ remediation of Cr(VI) contaminated groundwater. Nevertheless, the removal of Cr(VI) (including Cr(VI) adsorption and Cr(III) generation) could be influenced by the anaerobic bacteria governing the transport of engineered nanoparticles in groundwater. This study aims to reveal the inherent mechanisms of D. vulgaris and S. putrefaciens governing the cotransport of nZVI@GT combined with FeS (or FeS2) to further influence the Cr(VI) removal in simulated complex groundwater media. Our findings provides a chemical and biological synergistic remediation strategy for nZVI@GT application in Cr(VI)-contaminated groundwater.

Mechanistic insights into manganese oxide impacting the oxidation and transport of Cr(III) immobilized by nano-zero valent charged ion particles in water-saturated porous media

The presence of manganese oxide (MnO2) could influence the stability of green-synthesized nano-zero valent iron (nZVI@GT) associated with trivalent chromium (Cr(III)) after its excess application in the in situ remediation of hexachromium (Cr(VI)) contaminated soil. The research findings revealed that the co-transport of the remaining nZVI@GT with Cr(III) was substantially inhibited by high δ-MnO2 concentrations due to the formation of hetero-aggregates between nZVI@GT and δ-MnO2, resulting in an increased irreversible attachment parameter at second-site in a two-site kinetic attachment model. Simultaneously, the Cr(III) complex immobilized on nZVI@GT could be oxidized leading to high levels of Cr(VI) leaching at high δ-MnO2 concentrations. During this process, Mn(IV) was converted to Mn(III)/Mn(II). Subsequently, leachate containing a partial amount of Cr(VI) preferentially adsorbed onto the nZVI@GT surface, enhancing the dispersion of the nZVI@GT and δ-MnO2 agglomerates. Thereafter, nZVI@GT transportability was enhanced with a decreased second-site attachment parameter and the flow content of dissolved Cr(VI) was increased to double, also increasing the potential risk of Cr(VI) being carried by nZVI@GT to underground water systems. This study provides theoretical support for preserving the long-term stability of nZVI@GT after the in situ remediation of heavy metal-contaminated sites in the presence of δ-MnO2.

Mechanisms of increased small nanoplastic particle retention in water-saturated sand media with montmorillonite and diatomite: Particle sizes, water components, and modelling

The processes by which small nanoplastics (NPs) accumulate in soil are unclear. To clarify the different deposition processes that affect small NPs (< 30 nm) compared to larger NPs in the soil environment, due to their interaction with clays as major soil components, the transport behavior of two-sized NPs (20 and 80 nm) with two clays (diatomite (Diat) and montmorillonite (Mont)) in NaCl and CaCl2 solutions were investigated in water-saturated quartz sand columns. The experimental results showed that more 20 nm NPs could enter the lattice structure of Diat than Mont in NaCl solution. This contributed to the stronger deposition of 20 nm NPs by Diat on sand, which was associated with a lower k1d/k1 value (obtained from two-site kinetic attachment model). In contrast, 80 nm NPs had a stronger reversible retention than 20 nm NPs with Mont, even though both sizes of NPs-Mont displayed a similar transportability. In CaCl2 solution, the larger NPs-Mont hetero-aggregates formed with a stronger suppressed depth of φmax based on Derjaguin-Landau-Verwey-Overbeek theory. Thus, Mont had a stronger transport inhibition than Diat for both NPs sizes, with a lower k1d/k1. These findings could benefit in predicting the size-based deposition of NPs in a heterogenous soil environment.