Transport, retention, and long-term release behavior of polymer-coated silver nanoparticles in saturated quartz sand: The impact of natural organic matters and electrolyte

Co-transport of citrate-modified biochar nanoparticles and released plant-available silicon in saturated porous media: Effect of LMWOAs and solution chemistry

Citrate-modified biochar nanoparticles (CBCNPs) represent a promising amendment with plant-available silicon (PASi) releasing capacity. However, the co-transport behavior with released PASi remain poorly understood. This study investigated their co-transport in saturated porous media under various solution chemistry and low molecular weight organic acids (LMWOAs). Experimental and two-site kinetic model results revealed that higher ionic strength caused favorable aggregation and size-selective, hindering CBCNPs transport. Divalent Ca2+ ions retained CBCNPs more effectively than K+ due to stronger charge screening and cation bridging. The pH buffering capacity of CBCNPs resulted in consistent transport behavior across a broad pH range (4-8). XDLVO calculation clarified the impact mechanisms of IS, ion types and pH on CBCNPs transport. Furthermore, LMWOAs exhibited a time-dependent blocking effect on CBCNPs transport. Oxalic acid (OA) and citric acid (CA) facilitated CBCNPs transport though mechanisms beyond XDLVO, including steric hindrance, competitive adsorption, and surface hydrophilicity. The presence of LMWOAs significantly hindered PASi co-transport, with the inhibitory effect ranked as acetic acid (AA) ≈ CA > OA > absence of organic acids. The inhibition is attributed to the blocking effect and formation of Si-organic acid complexes, as evidenced by breakthrough curves and density functional theory calculations. This study provides novel insights into the co-transport of CBCNPs with released PASi through mutual mechanisms, indicating both potential environmental benefits and risks.

The long-term release and particle fracture behaviors of nanoplastics retained in porous media: Effects of surfactants, natural organic matters, antibiotics, and bacteria

The transport of nanoplastics (NPs) in porous media has received a lot of attention, but the studies on the long-term release of NPs retained in porous media and the particle fracture during this process are seriously lacking. For filling this deficiency, we examined the individual or synergistic effects of surfactants, natural organic matters (NOMs), antibiotics, and bacteria on the desorption, long-term release, and particle fracture behaviors of polystyrene NPs (PS-NPs) retained in porous media. It was found that the change in hydrophilicity of PS-NPs dominated the long-term release of PS-NPs retained in porous media when surfactants were present. In the single system of surfactants and the dual system of surfactants and NOMs, the release of PS-NPs were improved owing to the increasing hydrophilicity of PS-NPs, although cationic surfactants also reduced the electrostatic repulsion between PS-NPs and porous media. Increasing antibiotic concentration reduced the electrostatic repulsion between PS-NPs and porous media to inhibit the release of PS-NPs. When bacteria were present whether containing antibiotics or not, the effects on roughness of PS-NPs dominated the release of PS-NPs. The effects of surfactants and NOMs on the PS-NP desorption were similar with the long-term release, with changes in hydrophilicity dominating the process. Whereas the effects of antibiotics and bacteria on the PS-NP desorption were different with the long-term release. Surfactants and NOMs in the presence of surfactants inhibited the fracture of PS-NPs by increasing the hydrophilicity of PS-NPs brought about the coating of water molecules on PS-NPs for protection. Antibiotics had no significant effects on the fracture of PS-NPs due to unaltered vertical forces on PS-NPs and no protective effect. Bacteria in the presence or absence of antibiotics inhibited the fracture of PS-NPs by coating PS-NPs retained in porous media to protect PS-NPs from fracture.

Co-impacts of cation type and humic acid on migration of polystyrene microplastics in saturated porous media

The aging process of microplastics (MPs) could significantly change their physical and chemical characteristics and impact their migration behavior in soil. However, the complex effects of different cations and humic acids (HA) on the migration of aged MPs through saturated media are not clear. In this research, the migration and retention of pristine/aged PSMPs (polystyrene microplastics) under combined effects of cations (Na+, Ca2+) (ionic strength = 10 mM) and HA (0, 5, 15 mg/L) were investigated and analyzed in conjunction with the two-site kinetic retention model and DLVO theory. The findings showed that the aging process accelerated PSMPs migration under all tested conditions. Aged PSMPs were less susceptible to Ca2+ than pristine PSMPs. Under Ca2+ conditions, pristine/aged PSMPs showed higher retention than under Na+ conditions in the absence of HA. Furthermore, under Na+ conditions, the migration of aged PSMPs significantly increased at higher concentrations of HA. However, under Ca2+ conditions, the migration of aged PSMPs decreased significantly at higher concentrations of HA. In higher HA conditions, HA, Ca2+, and PSMPs interact to cause larger aggregations, resulting in the sedimentation of aged PSMPs. The DLVO calculations and two-site kinetic retention models' results showed the detention of PSMPs was irreversible under higher HA conditions (15 mg/L) with Ca2+, and aged PSMPs were more susceptible to clogging. These findings may help to understand the potential risk of migration behavior of PSMPs in the soil-groundwater environment.

Statuses, shortcomings, and outlooks in studying the fate of nanoplastics and engineered nanoparticles in porous media respectively and borrowable sections from engineered nanoparticles for nanoplastics

This review discussed the research statuses, shortcomings, and outlooks for the fate of nanoplastics (NPs) and engineered nanoparticles (ENPs) in porous media and borrowable sections from ENPs for NPs. Firstly, the most important section was that we reviewed the research statuses on the fate of NPs in porous media and the main influencing factors, and explained the influencing mechanisms. Secondly, in order to give NPs a reference of research ideas and influence mechanisms, we also reviewed the research statuses on the fate of ENPs in porous media and the factors and mechanisms influencing the fate. The main mechanisms affecting the transport of ENPs were summarized (Retention or transport modes: advection, diffusion, dispersion, deposition, adsorption, blocking, ripening, and straining; Main forces and actions: Brownian motion, gravity, electrostatic forces, van der Waals forces, hydration, filtration, bridging; Affecting elements of the forces and actions: the ENP and media grain surface functional groups, size, shape, zeta potential, density, hydrophobicity, and roughness). Instead of using the findings of ENPs, thorough study on NPs was required because NPs and ENPs differed greatly. Based on the limited existing studies on the NP transport in porous media, we found that although the conclusions of ENPs could not be applied to NPs, most of the influencing mechanisms summarized from ENPs were applicable to NPs. Combining the research thoughts of ENPs, the research statuses of NPs, and some of our experiences and reflections, we reviewed the shortcomings of the current studies on the NP fate in porous media as well as the outlooks of future research. This review is very meaningful for clarifying the research statuses and influence mechanisms for the NP fate in porous media, as well as providing a great deal of inspiration for future research directions about the NP fate in porous media.

Combined effects of bacteria and antibiotics on surface properties and transport of nanoplastics in porous media

Currently, research on the individual effects of bacteria and antibiotics on the transport of nanoplastics (NPs) in porous media is in its infancy, while research on their combined effect is absent. It is well known that bacteria and antibiotics also interact with each other, so this synergistic transport of bacteria, antibiotics, and NPs in porous media must be very interesting. For exploring this aspect, we investigated the individual and combined effects of bacteria and antibiotics on the transport of polystyrene NPs (PS-NPs) in saturated porous media. Hydrophobicity, roughness, and the Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energy were measured and calculated. The PS-NPs' transport in porous media was fitted using a mathematical model. Enhanced roughness and size of PS-NPs with increased bacterial concentration dominated and inhibited the PS-NPs' transport in porous media, although the hydrophilicity of PS-NPs and the energy barrier between PS-NPs and porous media were also increased. An increase in antibiotic concentration reduced the energy barrier between PS-NPs and porous media, thereby decreasing the PS-NPs' transport. Combined effects of bacteria and antibiotics on the PS-NPs' transport were complex and distinct from individual effects, but the mechanisms were clear. Roughness and hydrophilicity of PS-NPs and the DLVO interaction energy between PS-NPs and porous media together influenced this process. In the presence of bacteria, antibiotics could alter the bacterial surface roughness by altering bacterial extracellular polymeric substances, and thus alter the PS-NPs' surface roughness, thereby affecting the PS-NPs' transport in porous media. When antibiotics were present, enhanced bacterial concentration increased the PS-NPs' hydrophilicity and the energy barrier between PS-NPs and porous media, thus promoting the PS-NPs' transport. The findings of this study provided a theoretical basis for clarifying the transport of NPs in porous media under complex environments, facilitating a reduction in environmental pollution of NPs.

Investigating the transport and colloidal behavior of Fe3O4 nanoparticles in aqueous and porous media under varying solution chemistry parameters

The possible adverse effects of engineered iron oxide nanoparticles, especially magnetite (Fe3O4 NP), on human health and the environment, have raised concerns about their transport and behavior in soil and water systems. Accumulating these NPs in the environment can substantially affect soil and water quality and the well-being of aquatic and terrestrial organisms. Therefore, it is essential to examine the factors that affect Fe3O4 NP transportation and behavior in soil and water systems to determine their possible environmental fate. In this work, experiments were conducted in aqueous and porous media using an environmentally relevant range of pH (5, 7, 9), ionic strength (IS) (10, 50, 100 mM), and humic acid (HA) (0.1, 1, 10 mg L-1) concentrations. Fe3O4 NPs exhibited severe colloidal instability at pH 7 (⁓ = pHPZC) and showed an improvement in apparent colloidal stability at pH 5 and 9 in aquatic and terrestrial environments. HA in the background solutions promoted the overall transport of Fe3O4 NPs by enhancing the colloidal stability. The increased ionic strength in aqueous media hindered the transport by electron double-layer compression and electrostatic repulsion; however, in porous media, the transport was hindered by ionic compression. Furthermore, the transport behavior of Fe3O4 NPs was investigated in different natural waters such as rivers, lakes, taps, and groundwater. The interaction energy pattern in aquatic systems was estimated using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. This study showed the effects of various physical-chemical conditions on Fe3O4 NP transport in aqueous and porous (sand) media.

Microbial bioprocess performance in nanoparticle-mediated composting

Microbial composting is one of the most cost-effective techniques for degradation, remediation, nutrition, etc. Currently, there is faster growth and development in nanotechnology in different sectors. This development leads nanoparticles (NPs) to enter into the composts in different ways. First, unintentional entry of NPs into the composts via: waste discharge, buried solid waste, surface runoff, direct disposal into wastes (consumer goods, food, pharmaceuticals, and personal care products). Second, intentional mediation of the NPs in the composting process is a novel approach developed to enhance the degradation rate of wastes and as a nutrient for plants. The presence of NPs in the composts can cause nanotoxicity. Conversely, their presence might also be beneficial, such as soil reclamations, degradation, etc. Alternatively, metal NPs are also helpful for all living organisms, including microorganisms, in various biological processes, such as DNA replication, precursor biosynthesis, respiration, oxidative stress responses, and transcription. NPs show exemplary performance in multiple fields, whereas their role in composting process is worth studying. Consequently, this article aids the understanding of the role of NPs in the composting process and how far their presence can be beneficial. This article reviews the significance of NPs in: the composting process, microbial bioprocess performance during nano composting, basic life cycle assessment (LCA) of NP-mediated composting, and mode of action of the NPs in the soil matrix. This article also sheds insight on the notion of nanozymes and highlights their biocatalytic characterization, which will be helpful in future composting research.

Transport of polystyrene nanoplastics in porous media: Combined effects of two co-existing substances

Both surfactants and natural organic matters (NOMs) are substances commonly found in aqueous environments, and their effects on the transport of nanoplastics that is gradually gaining widespread attention in porous media are currently in their infancy, while their combined effects are absent. We investigated innovatively the combined effect of surfactants and NOMs on the transport of polystyrene nanoplastics (PS-NPs) in saturated porous media. Adsorption tests of surfactants and NOMs onto PS-NPs, adsorption tests of PS-NPs onto quartz sand, and transport tests of PS-NPs in saturated quartz sand were conducted. Hydrophobicity and Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energy were measured and calculated. A mathematical model was employed to fit the transport of PS-NPs in porous media. It was found that the effects and action mechanisms of cationic cetyl trimethylammonium bromide (CTAB) and anionic sodium dodecylbenzene sulfonate (SDBS) on the transport of PS-NPs in porous media were distinct. In the presence of CTAB, 1 mg/L humic acid (HA) and 10 mg/L sodium alginate (SA) could promote aggregation of PS-NPs by decreasing the absolute zeta potential of PS-NPs, and reducing the energy barrier between PS-NPs and porous media and increasing the blocking and straining, thus inhibiting the transport of PS-NPs. In the presence of SDBS, SA and HA could improve the adsorption of SDBS onto PS-NPs by bridging and increasing adsorption sites, thus increasing the hydrophilicity of PS-NPs and improving the transport of PS-NPs. Whether or not NOMs were present, the transport of PS-NPs in porous media was mainly governed by the DLVO interaction energy in the presence of cationic surfactants and by hydrophobicity in the presence of anionic surfactants. This innovative observation has led to an understanding on the environmental behaviour of nanoplastics in porous media under complex environments.

Aquatic Ecosystem Risk Assessment Generated by Accidental Silver Nanoparticle Spills in Groundwater

This paper aims to create a new model for assessing the ecosystem risk in rivers and wetlands that are linked to accidental spills of silver nanoparticles (AgNPs) in soil/groundwater. Due to the uncertainty of the modeling inputs, a combination of two well-known risk assessment methodologies (Monte Carlo and fuzzy logic) were used. To test the new model, two hypothetical, accidental AgNP soil spill case studies were evaluated; both of which were located at the end of the Llobregat River basin within the metropolitan area of Barcelona (NE Spain). In both cases, the soil spill reached groundwater. In the first case, it was discharged into a river, and in the second case, it recharged a wetland. Concerning the results, in the first case study, a medium-risk assessment was achieved for most cases (83%), with just 10% of them falling below the future legal threshold concentration value. In the second case study, a high-risk assessment was obtained for most cases (84%), and none of the cases complied with the threshold value. A sensitivity analysis was conducted for the concentration and risk. The developed tool was proven capable of assessing risk in aquatic ecosystems when dealing with uncertain and variable data, which is an improvement compared to other risk assessment methodologies.

Effects of input concentration, media particle size, and flow rate on fate of polystyrene nanoplastics in saturated porous media

Nanoplastics have gradually attracted widespread attention, but the studies in this area are still very scarce. In this research, the adsorption, transport, long-term release, and particle fracture of polystyrene nanoplastics (PS-NPs) in saturated porous media were investigated at different media particle sizes, input concentrations, and flow rates. The increased PS-NPs concentration and sand grain size promoted the adsorption of PS-NPs onto quartz sand. In transport tests, the peak breakthrough amounts of PS-NPs ranged from 0.5761 to 0.8497, demonstrating their high mobility in saturated quartz sand. Transport of PS-NPs in saturated porous media increased with decreasing input concentration and increasing media particle sizes. The effect of input concentration could be predicted by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, in which adsorption played a dominant role. The effect of media particle size was mainly dominated by filtration rather than adsorption. As a result of higher shear force, increasing flow rate might boost transport of PS-NPs. With increasing media particle size and flow rate, more retained PS-NPs were released, which was in line with the findings of the transport tests on the mobility of PS-NPs. Notably, PS-NPs could be broken down into smaller PS-NPs during long-term release and the percentage of released PS-NPs (<100 nm) was gradually increased from 1st to 3rd PV effluent in all media particle sizes and flow rates. The fracture of released PS-NPs from medium quartz sand was the most in relation to fine and coarse and showed a decreased trend with increasing flow rate, which was likely to be governed by the force perpendicular to the contact surface with the media particle. This study showed that PS-NPs have strong mobility in porous media and are easily broken into smaller particles during long-term release. The findings of this research provided fundamental information for clarifying transport laws of nanoplastics in porous media.

Influences of input concentration, media particle size, metal cation valence, and ionic concentration on the transport, long-term release, and particle breakage of polyvinyl chloride nanoplastics in saturated porous media

The environmental impact of nanoplastics has gradually attracted widespread attention; however, nanoplastics of polyvinyl chloride, one of the most commonly used plastics, have not yet been studied. In this study, we investigated the transport, long-term release behavior, and particle fracture of polyvinyl chloride nanoplastics (PVC NPs) in saturated quartz sand with different metal cations, ionic concentrations, input concentrations, and sand grain sizes by determining breakthrough, long-term release, and particle size distribution curves. The breakthrough curves and retention profiles were simulated by a mathematical model. The transport of PVC NPs increased with increased input concentration and sand grain size, which could be predicted by the Derjaguin-Landau-Verwey-Overbeek (DLVO) and colloid filtration theories. Increased ionic concentration and metal cation valence could restrain the transport of PVC NPs in saturated quartz sand owing to the decreased energy barrier between PVC NPs and sand grains. The total released amount of PVC NPs in the long-term release tests with different experimental conditions ranged from 3.91 to 21.95%. Increased sand grain size and decreased metal cation valence and ionic concentration resulted in an increased released amount of retained PVC NPs in saturated quartz sand, indicating increased release ability and mobility. The particle fracture results indicated that the PVC NPs were not broken down during long-term release under the experimental conditions of this research. This opens up a completely new and meaningful study of whether nanoplastics are broken down into smaller nanoplastics during their long-term release under various conditions.

The transport and retention of CQDs-doped TiO2 in packed columns and 3D printed micromodels

CQDs-doped TiO₂ (C-TiO₂) has drawn increased attention in recent because of its excellent catalytic performance. Understanding the transport of C-TiO₂ in porous media is necessary for evaluating the environmental process of this new nanomaterial. Column experiments were used in this study to investigate ionic strength (IS), dissolved organic matter (DOM) and sand grain size on the transport of C-TiO₂. The mobility of C-TiO₂ was inhibited by the increased IS and decreased sand grain size, but was promoted by the increased DOM concentration. The promotion efficiency of DOM ranked as humic acid (HA) > alginate (Alg) > bovine serum albumin (BSA), which was in the same order as their ability to change surface charges. The micromodels of pore network were prepared via 3D printing to further reveal the deposition mechanisms and spatial/temporal distribution of C-TiO₂ in porous space. C-TiO₂ mainly attached to the upstream region of collectors because of interception. The collector ripening was observed after long-time deposition. The existence of DOM caused visible decrease of C-TiO₂ deposition in the pore network. HA caused the most remarkable reduce of deposition in the three types of DOM, which was consistent with the column experiment results. This research is helpful to predict the transport of C-TiO₂ in natural porous media.

Uptake, translocation, and transformation of silver nanoparticles in plants

This article reviews the plant uptake of silver nanoparticles (AgNPs) that occurred in soil systems and the in planta fate of Ag.

Transport and retention of polymeric and other engineered nanoparticles in porous media

Increasing applications of nanoparticles (NPs) in agriculture have raised potential risks to soil and aquatic ecosystems. A comparative study examining the transport of commonly used NPs in porous media is of critical significance for their application and regulation in agroecosystems. In this study, laboratory column leaching experiments were conducted to investigate the transport and retention of polysuccinimide NPs (PSI-NPs) in two saturated porous media with different grain sizes, as compared with multi-walled carbon nanotubes (MWCNTs), nano-Ag and nano-TiO₂. Zeta potential of the NPs was negative at pH_6.3 and decreased in an order of PSI-NPs > nano-TiO₂ > MWCNTs > nano-Ag. The coarse and fine sands used in this study had negative charges with similar zeta potentials. The movement of NPs was affected by grain size, with larger sizes facilitating mobility while finer sizes favoring retention of NPs in the porous matrix. The retention profile significantly varied between the two sand columns, with more NPs transported to deeper layers in the coarse sand than the fine sand. The relative percentage of NPs detected in leachate was found to be positively correlated with the zeta potential of NPs (r = 0.931). Among the NPs, nano-Ag had the most negative zeta potential, and therefore was the most mobile, followed by MWCNTs and nano-TiO₂. Having the least negative zeta potential, PSI-NPs had the lowest mobility, as compared with other NPs regardless of matrix grain size. This work reveals grain size and zeta potential of NPs are major factors that influence transport of NPs along the vertical porous profile, as well as demonstrating the relative unimportance of NP composition, which could serve as important guideline in nanomaterials application, risk assessment, and waste management in agroecosystems.

Low mobility of CuO and TiO2 nanoparticles in agricultural soils of contrasting texture and organic matter content

The fate of nanoparticles (NPs) in soil under relevant environmental conditions is still poorly understood. In this study, the mobility of two metal-oxide nanoparticles (CuO and TiO₂) in contrasting agricultural soils was investigated in water-saturated soil columns. The transport of TiO₂ and CuO-NPs were assessed in six soils with three different textures (from sand to clay) and two contrasted organic matter (OM) contents for each texture. TiO₂ mobility was very low in all soils, regardless of texture and OM content. Mass recoveries were always less than 5%, probably in relation with the strong homo-aggregation of TiO₂-NPs observed in all soil solutions, with apparent sizes 3-6 times larger than their nominal size. This low mobility suggests that TiO₂-NPs present a low risk of direct groundwater contamination in contrasted surface soils. Although their retention was also generally high (more than 86%), CuO nanoparticles were found to be mobile in all soils. This is probably related to their smaller apparent size and low capacity of homo-aggregation of CuO-NPs in all soil solutions. No clear influence of neither soil texture or soil total organic matter content could be observed on CuO transport. However, this study shows that in contrasted agricultural soils, CuO-NPs transport is mainly controlled by the solutes dissolved in soil solution (DOC and PO₄ species), rather than by the properties of the soil solid phase.

Transport of nanoparticles in porous media and its effects on the co-existing pollutants

Nanomaterials are widely used in daily life owing to their superior characteristics. The release and transport of nanoparticles (NPs) in the environment is inevitable during their entire life cycle, posing a risk to the aquatic environment. Thus, considerable attention has been focused on the fate and behavior of NPs in porous media, as well as the co-transport of NPs with other pollutants. In this review, current knowledge about the retention and transport behavior of NPs in porous media is summarized. NP transport in porous media is dominated by various internal and external factors, including the characteristics of NPs, porous media, and water flow. Generally, NPs with high density, small particle size, and surface coating are easily transported in porous media with the characteristics of large size, smooth surface, and low water saturation. Meanwhile, high pH and velocity, low temperature, and natural organic matter-containing fluids are also conducive to NP transport. Aggregation, adsorption, straining, and blocking are the primary mechanisms by which NPs affect the transport of co-existing pollutants in porous media. Current research on NP transport has been performed predominantly using modal porous media (e.g., sand and glass beads); however, there is a large gap between simulated and natural porous media. Further studies should focus on the transport, fate, and interaction of NPs and coexistent pollutants in natural porous media, as well as the coupling mechanisms under actual environmental conditions.

The impacts of metal-based engineered nanomaterial mixtures on microbial systems: A review

The last decade has witnessed tremendous growth in the commercial use of metal-based engineered nanomaterials (ENMs) for a wide range of products and processes. Consequently, direct and indirect release into environmental systems may no longer be considered negligible or insignificant. Yet, there is an active debate as to whether there are real risks to human or ecological health with environmental exposure to ENMs. Previous research has focused primarily on the acute effects of individual ENMs using pure cultures under controlled laboratory environments, which may not accurately reveal the ecological impacts of ENMs under real environmental conditions. The goal of this review is to assess our current understanding of ENM effects as we move from exposure of single to multiple ENMs or microbial species. For instance, are ENMs' impacts on microbial communities predicted by their intrinsic physical or chemical characteristics or their effects on single microbial populations; how do chronic ENM interactions compare to acute toxicity; does behavior under simplified laboratory conditions reflect that in environmental media; finally, is biological stress modified by interactions in ENM mixtures relative to that of individual ENM? This review summarizes key findings and our evolving understanding of the ecological effects of ENMs under complex environmental conditions on microbial systems, identifies the gaps in our current knowledge, and indicates the direction of future research.

The role of natural organic matter in the silver release from sludge generated from coagulation of wastewater spiked with silver nanoparticles

Sludge is an integral part in the migration pathway of silver nanoparticles (AgNPs) from manufacture to the terrestrial environment. However, the detailed information on the role of natural organic matters (NOMs) remains limited. In this study, the sludge generated from coagulation of wastewater spiked with AgNPs (denoted as sludge_C-AgNPs) was taken as the model. Effects of humic acid (HA), alginate (AA) and bovine serum albumin (BSA) on the release amount, dynamics and speciation of silver from the sludge_C-AgNPs were investigated by a series of leaching experiments. The results showed that HA, AA and BSA in the leaching solution could enhance the silver release from the sludge_C-AgNPs. The concentrations of the dissolved and colloidal silver in the BSA solution were the highest at the initial stage of dynamic leaching. The controlling step of the silver release was internal diffusion in the HA and AA solution, while the release of dissolved silver was controlled by both chemical reaction and internal diffusion in the BSA solution. In addition, the released colloidal silver fractions in the BSA solution contained more particles with size >50 nm compared with the HA and AA solutions. The results suggested that the properties of NOMs may be the key factor affecting the transfer of AgNPs from the sludge to the terrestrial environment.

High pyrolysis temperature biochar reduced the transport of petroleum degradation bacteria Corynebacterium variabile HRJ4 in porous media

Biochar has been widely applied for the remediation of petroleum-contaminated soil. However, the effect of biochar on the transport of petroleum degradation bacteria has not been studied. A typical Gram-positive petroleum degradation bacteria-Corynebacterium variabile HRJ4 was used to study the effect of different biochars on bacterial transport and retention. Results indicated that the addition of biochar in sand was effective for reducing the transport of bacteria and poplar sawdust biochar (PSBC) had a stronger hinder effect than corn straw biochar (CSBC). The hindrance was more evident with pyrolysis temperature of biochar raised from 300°C to 600°C, which was attributed to the increase of specific surface area (309 times). The hindrance effect also enhanced with higher application rate of biochar. Furthermore, the reduction of HRJ4 transport was more obvious in higher (25 mmol/L) concentration of NaCl solution owing to electrostatic attraction enhancement. The adsorption of biochar to HRJ4 was defined to contribute to the hindrance of HRJ4 transport mainly. Combining the influence of feedstocks and pyrolysis temperature on HRJ4 transport, it suggested that specific surface area had the greatest effect on HRJ4 transport, and pore-filling, electrostatic force also contributed to HRJ4 retained in quartz sand column. At last, phenol transportation experiment indicated that the restriction of biochar on HRJ4 enhanced the phenol removal rate in the column. This study provides a theoretical basis for the interaction of biochar and bacteria, which is vital for the remediation of oil-contaminated soil and groundwater in the field.

Factors affecting pluronic-coated iron oxide nanoparticle binding to petroleum hydrocarbon-impacted sediments

Effective targeted delivery of nanoparticle agents may enhance the remediation of soils and site characterization efforts. Nanoparticles coated with Pluronic, an amphiphilic block co-polymer, demonstrated targeted binding behaviour toward light non-aqueous phase liquids such as heavy crude oil. Various factors including coating concentration, oil concentration, oil type, temperature, and pH were assessed to determine their effect on nanoparticle binding to heavy crude oil-impacted sandy aquifer material. Nanoparticle binding was increased by decreasing the coating concentration, increasing oil concentration, using heavier oil types, and increasing temperature, while pH over the range of 5-9 was found to have no effect. Nanoparticle transport and binding in columns packed with clean and oily porous media demonstrated the ability for efficient nanoparticle targeted binding. For the conditions explored, the attachment rate coefficient in columns packed with clean sand was 2.10 ± 0.66 × 10^-4 s^-1; however, for columns packed with oil-impacted sand a minimum attachment rate coefficient of 8.86 ± 0.43 × 10^-4 s^-1 was estimated. The higher attachment rate for the oil-impacted sand system indicates that nanoparticles may preferentially accumulate to oil-impacted zones present at heterogeneous impacted sites. Simulations were used to demonstrate this hypothesis using the set of parameters generated in this effort. This work contributes to our understanding of the application conditions that are required for efficient targeted binding of nanoparticles to crude-oil impacted porous media.

Transport behavior of micro polyethylene particles in saturated quartz sand: Impacts of input concentration and physicochemical factors

The long-term contamination of soil by microplastics may pose risks that are often still not well understood, and the ecological effects of microplastics are mainly dependent on their environmental behavior in environments. This study used saturated quartz sand as a solid porous medium to study the migration and influencing factors of 40-48 μm polyethylene (PE) particles in saturated porous media. The breakthrough curves at different injection concentrations (0.3, 0.4, 0.5 mg/L), flow rates (1.0, 1.5, 2.0, 2.5 ml/L), porous medium particle sizes (1-2, 2-4 mm), ionic strengths (0, 0.01, 0.05 mol/L) and concentrations of fulvic acid (FA) (0, 5, 10 mg/L) were compared and analyzed. The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was used to more accurately explain relevant transport behaviors. The results showed that the input concentration, flow rate, and particle size can affect the migration of PE particles individually or in combination. As ionic strength increased, the repulsion between microplastics and quartz sand gradually disappeared according to DLVO theory, and their attraction gradually strengthened. As a result, fewer microplastics could penetrate the sand column and reach the water body. With the continuous addition of FA, the repulsive energy between microplastics and quartz sand rose from DLVO theory, and the migration ability of microplastics initially increased before becoming stable because of the effect of straining. In all cases, the migration ability of PE was low (C/C₀ < 0.35), and most PE particles remained in the porous media during the whole experimental periods. This study provides new insights of understanding the migration of microplastics in environment.

Long-term fate of ZnO-FexOy mix-nanoparticles through the saturated porous media under constant head condition

The objective of this study is to evaluate the long-term fate of the nZnO-nFe_xO_y mix nanoparticles through the natural sediment in the presence of humic acid (HA) under varying pH and natural groundwater conditions. To achieve the objectives, a series of experiments were carried out where 50 mg/l of nZnO-nFe_xO_y mix suspensions were injected for 5 pore volumes (PVs) in that porous media in the presence of 10 mg/l HA under varying pH (6 and 8) followed by flushing with deionized and natural groundwater for another 95 PVs under constant head condition. The outcome of the study suggests that during the injection of the nZnO-nFe_xO_y mix suspension, more nZnO particles retain when the suspension is prepared at pH 6 (>90%). With an increase in pH of nZnO-nFe_xO_y mix suspension and background water, the long-term release of retained nZnO particles has increased significantly (from 29.97% at pH 6 to 95.89% at pH 8). The surface charge and the electrostatic repulsion are likely to govern the detachment and release of nZnO particles. Certain fraction (3.58-7.97%) of the Zn was also found to be dissolved and eluted at the outlet when the pH of background water is maintained at 6. In the case of nFe_xO_y, extensive retention of particles is observed during injection at both pHs (6 and 8). The release of the retained particles is limited (6.34%) specifically at lower pH (pH 6). There is an increase in the release of nFe_xO_y particles (24.76%) with an increase in the pH (pH 8) of both the suspension and background solution. When groundwater is used as the background water, a slight reduction in the release of Zn (22.04%) and Fe (2.06%) is observed at a pH of 6. However, at higher pH (pH 8), significantly less amount of retained particles (2.24% of Zn and 4.96% of Fe) are released. This is mainly due to the presence of co-ions in the groundwater which resulted in less negative charge of ENPs thus having less detachment and release of Zn and Fe particles. Overall, it could be concluded that there is a risk of release of Zn and Fe (especially at high pH) in the long run in the presence of organic matter when exposed in the porous media. The extent of release of Zn and Fe would be more at higher pH and might be less in the presence of other ions and under groundwater conditions.

Transport and retention of copper oxide nanoparticles under unfavorable deposition conditions caused by repulsive van der Waals force in saturated porous media

Currently, copper oxide nanoparticles (CuO NPs) have been widely used in industry, manufacturing and agriculture. The transport and retention of CuO NPs are vital to understanding the fate as well as the life cycle of CuO NPs in the environment. This study systematically investigates the transport and retention of CuO NPs in saturated porous media, and the experimental results were explained by the CFT and DLVO theory. The van der Waals force between CuO NPs and collector was repulsive, resulting in the unfavorable deposition condition. Column experiments were conducted with saturated quartz sand under environmentally relevant pH (6, 8, 9), ionic strength (IS, 1, 10, 50 mM), and humic acid (HA, 0.1-10 mg-C/mL). Experimental results show that the breakthrough curves (BCTs) were affected by different pH and IS. Under pH 6 and 9, the mobility of CuO NPs was enhanced by high IS while the mobility was inhibited by high IS under pH 8. The mobility of CuO NPs was enhanced by humic acid and the effect was best at 0.5 mg-C/mL HA. The experimental results were successfully explained by CFT and DLVO theory, the main mechanisms were aggregation of CuO NPs, interaction energy and collision between CuO NPs and collector. In general, these findings can improve our understanding of the transport and retention of CuO NPs in subsurface environments, and suggest pH, IS, HA may be key factors governing mobility and stability of CuO NPs in natural environment.

Cotransport of biochar and Shewanella oneidensis MR-1 in saturated porous media: Impacts of electrostatic interaction, extracellular electron transfer and microbial taxis

Biochar widely applied to soil can influence microbial community composition and participate in extracellular electron transfer (EET). However, little is known about the cotransport behaviors of bacteria and biochar in aquifer and soil-water environments, which can affect the fate and application performance of biochar. In this study, we found that in comparison to their individual transport behaviors, the mobilities of cotransporting Shewanella oneidensis MR-1 and biochar colloid (BC) were significantly inhibited. The decreasing colloidal mobilities at higher ionic strengths signified the importance of electrostatic interaction between cell and BC in cotransport. Moreover, the less suppressed cotransport of BC and mutants defective of EET and the elevated inhibition effects on cotransport by adding exogenous electron donor suggested the importance of EET. Difference in cotransport behavior was also observed with BC having different redox states. Compared with oxidized BC, reduced BC with higher hydrophobicity led to easier aggregation with cell and higher retention in column. More importantly, MR-1 exhibited EET-dependent taxis towards biochar, which also contributed to the enhanced heteroaggregation and decreased mobilities of cell and biochar. Our results highlight that metabolic activities of microbes towards abiotic colloids cannot be neglected when assessing their transport behaviors, especially in subsurface environments abounded with redox-active inorganic particles and microbes performing extracellular respiration.

Transport and retention of surfactant- and polymer-stabilized engineered silver nanoparticles in silicate-dominated aquifer material

Packed column experiments were conducted to investigate the transport and blocking behavior of surfactant- and polymer-stabilized engineered silver nanoparticles (Ag-ENPs) in saturated natural aquifer media with varying content of material < 0.063 mm in diameter (silt and clay fraction), background solution chemistry, and flow velocity. Breakthrough curves for Ag-ENPs exhibited blocking behavior that frequently produced a delay in arrival time in comparison to a conservative tracer that was dependent on the physicochemical conditions, and then a rapid increase in the effluent concentration of Ag-ENPs. This breakthrough behavior was accurately described using one or two irreversible retention sites that accounted for Langmuirian blocking on one site. Simulated values for the total retention rate coefficient and the maximum solid phase concentration of Ag-ENPs increased with increasing solution ionic strength, cation valence, clay and silt content, decreasing flow velocity, and for polymer-instead of surfactant-stabilized Ag-ENPs. Increased Ag-ENP retention with ionic strength occurred because of compression of the double layer and lower magnitudes in the zeta potential, whereas lower velocities increased the residence time and decreased the hydrodynamics forces. Enhanced Ag-ENP interactions with cation valence and clay were attributed to the creation of cation bridging in the presence of Ca²⁺. The delay in breakthrough was always more pronounced for polymer-than surfactant-stabilized Ag-ENPs, because of differences in the properties of the stabilizing agents and the magnitude of their zeta-potential was lower. Our results clearly indicate that the long-term transport behavior of Ag-ENPs in natural, silicate dominated aquifer material will be strongly dependent on blocking behavior that changes with the physicochemical conditions and enhanced Ag-ENP transport may occur when retention sites are filled.

Transport and long-term release behavior of polymer-coated silver nanoparticles in saturated quartz sand: The impacts of input concentration, grain size and flow rate

This study investigated the transport and long-term release of stabilized poly vinylpyrrolidone-coated silver nanoparticles (PVP-AgNPs) in a quartz sand column with various sand grain sizes (0.3-0.5 μm, 0.5-1.0 μm, 1.0-2.0 μm), input concentrations of PVP-AgNP solution (1, 5, 15, 25 mg/L), and flow rates corresponding to a filter velocities (1.0, 1.5, 2.0, 2.5 mL/min-0.14, 0.21, 0.28, 0.35 cm/min) by determining breakthrough curves, retention profiles, and long-term release curves. Breakthrough curves and retention profiles were simulated by a mathematical model based on the advection dispersion equation coupled with second-order kinetics. The increased transport of PVP-AgNPs in quartz sand occurred with increased grain sizes and reduced input concentrations, and the transport can be predicted by the colloid filtration theory and DLVO theory. The long-term (one week) release amounts of retained PVP-AgNPs were 42.78%, 31.45%, and 10.95% in the fine, medium, and coarse sand columns, respectively, and were 34.70%, 40.79%, 47.24%, and 57.32% at flow rates of 0.0363, 0.0436, 0.0545, and 0.0726 mL/min, respectively. The released quantity of retained PVP-AgNPs decreased as the sand grain size increased. This phenomenon is opposite with the trend of increased transport of PVP-AgNPs with increased grain size in the transport test, which most likely because colloidal filtration regulates the transport process and adsorption (and desorption) dominates the release process. Increasing the flow rate increased the shear force on the particles, which improved the release of PVP-AgNPs. The results of the release tests further verified our previous published studies showing that the long-term release of retained PVP-AgNPs in the quartz sand was mostly in the form of released nanoparticles rather than ions. The results of this study indicated that sand grain size, input concentration, and flow rate have a prominent influence on the transport and long-term release behavior of PVP-AgNPs in saturated quartz sand.