Effects of phosphate on the transport of Escherichia coli O157:H7 in saturated quartz sand

Facilitated transport of ferrihydrite with phosphate under saturated flow conditions

With increasing phosphate (P) entering the environment during agricultural application, the subsurface flow of particular P has been recently discussed as a vital P transport pathway. Iron (oxyhydr)oxide colloid-facilitated P transport is critical for iron and P biogeochemical processes in the subsurface. This study investigated the ferrihydrite colloid-facilitated P transport through adsorption and column experiments under different P concentrations and three pH conditions. Increased P loading on ferrihydrite colloids decreased the transport of ferrihydrite colloids (< 8.0%) under acid conditions through pore straining and irreversible attachment. Under neutral and alkaline conditions, ferrihydrite colloids exhibited more negative surfaces and smaller diameters with increasing P, which further enhanced ferrihydrite colloid transport (maximum to 95.6%). Ferrihydrite colloid-facilitated P transport was limited under acid conditions, and it was 10% - 57% enhancement under neutral and alkaline conditions with increasing P adsorption. Under neutral conditions, ferrihydrite colloid-facilitated P transport was strongest (maximum to 68.84%) because of its stronger ferrihydrite colloid transport than under acid conditions and larger P adsorption capacity than under alkaline conditions. Our findings indicate that the facilitated transport of ferrihydrite colloids in the presence of P may be appreciable in iron and phosphate-rich soil and subsurface systems, which is essential for evaluating the fate of iron and iron-facilitated P and potential environmental risks of P transport in the subsurface.

Mechanistic insights into aggregation process of graphene oxide and bacterial cells in microbial reduction of ferrihydrite

Microbial reduction of ferrihydrite is prevalent in natural environments and plays an important role in reductive dissolution of Fe(III) minerals. With consistent release of anthropogenic graphene oxide (GO) into water bodies, new changes in the Fe(III)-reducing microorganisms/ferrihydrite binary system demand attention. Herein, we focused on the interaction of GO and bacterial cells in view of colloidal stability and interfacial forces, and on the consequences for microbial ferrihydrite reduction. The results showed that the addition of GO decreased the bioreduction efficiency of ferrihydrite down to 1/15 of the control. Meanwhile, the GO nanosheets were found not depositing on ferrihydrite but spontaneously aggregating with Shewanella spp., the representative dissimilatory Fe(III) reduction bacterial species. Using the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory and atomic force microscopy (AFM), the aggregation process can be interpreted in three steps according to the interaction energy calculation, namely, colloidal instability, reversible aggregation and irreversible aggregation. The motility of living cells seems the reason inducing the colloidal instability between GO and bacteria. While, the aggregation remains reversible even the secondary minimum achieved at the separation distance of 8.74-9.24 nm from XDLVO. When the separation distance <5.74-6.01 nm, the adhesion work predominates and causes irreversible aggregation, validated by AFM. Additionally, the probable ecological risks raised by this aggregation behavior for the imbalance of iron biogeochemical cycle were demonstrated.

Escherichia coli and phosphate interplay mediates transport of nanoscale zero-valent iron synthesized by green tea in water-saturated porous media

Green synthesized nano-zero-valent iron (GT-nZVI) has been considered an excellent material for in-situ soil remediation due to its high stability and environmental benignity. However, sufficient transportability of GT-nZVI downstream towards the contaminated sites, likely affected by the physicochemical properties of soil-groundwater, is required for improved in-situ remediation. Thus, the effect of soil components (i.e., bacteria and phosphate) on GT-nZVI transportability is significant. Hence, we studied the transport of GT-nZVI (Fe0 core wrapped by green tea polyphenols) with the existence of E. coli and phosphate in water-saturated porous sand media in NaCl and CaCl2 solutions at pHs 6.0 and 8.0. Also studied were the stability, surface characteristics, and two-site kinetics attachment modeling (TKAM) with Escherichia coli or/and phosphate. The results showed that phosphate could further enhance GT-nZVI co-transport with E. coli by increasing the negative charge on GT-nZVI at pH 6.0. However, E. coli reduced GT-nZVI mobility at pH 8.0 because the cell-cell interactions could mask the negative charges of pre-deposited GT-nZVI on E. coli, forming the large clusters between GT-nZVI and E. coli. Then, phosphate occurrence diminished E. coli inhibition by detaching GT-nZVI from nZVI-E. coli-phosphate polymers due to the stronger phosphate adsorption on E. coli than GT-nZVI at pH 8.0. Overall, TKAM describes the transport and retention of GT-nZVI adequately under various conditions, indicating the deposition order with k2str value as follows: GT-nZVI alone > with (w.) E. coli > w. phosphate > w. combined E. coli & phosphate at pH 6.0. By contrast, w. phosphate > w. E. coli > w. combined E. coli & phosphate > GT-nZVI alone ensued at pH 8.0. This investigation highlights the transport behavior of GT-nZVI associated with surface property changes in complex environments for effective in-situ remediation.

Physiological characteristics, geochemical properties and hydrological variables influencing pathogen migration in subsurface system: What we know or not?

The global outbreak of coronavirus infectious disease-2019 (COVID-19) draws attentions in the transport and spread of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) in aerosols, wastewater, surface water and solid wastes. As pathogens eventually enter the subsurface system, e.g., soils in the vadose zone and groundwater in the aquifers, they might survive for a prolonged period of time owing to the uniqueness of subsurface environment. In addition, pathogens can transport in groundwater and contaminate surrounding drinking water sources, possessing long-term and concealed risks to human society. This work critically reviews the influential factors of pathogen migration, unravelling the impacts of pathogenic characteristics, vadose zone physiochemical properties and hydrological variables on the migration of typical pathogens in subsurface system. An assessment algorithm and two rating/weighting schemes are proposed to evaluate the migration abilities and risks of pathogens in subsurface environment. As there is still no evidence about the presence and distribution of SARS-CoV-2 in the vadose zones and aquifers, this study also discusses the migration potential and behavior of SARS-CoV-2 viruses in subsurface environment, offering prospective clues and suggestions for its potential risks in drinking water and effective prevention and control from hydrogeological points of view.

Insight into the effect of phosphate on ferrihydrite colloid-mediated transport of tetracycline in saturated porous media

Colloid-mediated contaminant mobility is absolutely critical for the environmental behavior of contaminants such as antibiotics in water resources. In this study, the influences of phosphate (a commonly inorganic ligand in the environment) on the ferrihydrite colloid-mediated transport of tetracycline (TC, a typical antibiotic) in porous media were investigated. In the absence of colloids, phosphate promoted TC mobility due to the competitive deposition of phosphate and TC on the sand surface as well as the electrostatic repulsion. Interestingly, ferrihydrite colloids could inhibit TC transport; however, the inhibitory effect of the colloids was weakened by the addition of phosphate. This phenomenon stemmed from colloid-associated TC mobility, the increased electrostatic repulsion induced by adsorbed phosphate, and deposition site competition effect. Another interesting finding was that the impacts of phosphate on the colloid-mediated mobility of TC were pH-dependent. That is, phosphate exhibited a weaker effect on the inhibitory role of ferrihydrite colloids in TC mobility at pH 5.0 than that at pH 7.0; specially, ferrihydrite colloids acted as possible carriers of TC and facilitated antibiotic transport at pH 9.0. The observations were ascribed to different influences of phosphate on the binding affinity of ferrihydrite toward TC and the mobility of free TC under different pH conditions. Therefore, the findings of this study provide useful information about the fate and co-transport of antibiotics and natural mineral colloids in the presence of inorganic ligands in the aquatic environment.

Sensitivity of the Transport of Plastic Nanoparticles to Typical Phosphates Associated with Ionic Strength and Solution pH

The influence of phosphates on the transport of plastic particles in porous media is environmentally relevant due to their ubiquitous coexistence in the subsurface environment. This study investigated the transport of plastic nanoparticles (PNPs) via column experiments, paired with Derjaguin–Landau–Verwey–Overbeek calculations and numerical simulations. The trends of PNP transport vary with increasing concentrations of NaH₂PO₄ and Na₂HPO₄ due to the coupled effects of increased electrostatic repulsion, the competition for retention sites, and the compression of the double layer. Higher pH tends to increase PNP transport due to the enhanced deprotonation of surfaces. The release of retained PNPs under reduced IS and increased pH is limited because most of the PNPs were irreversibly captured in deep primary minima. The presence of physicochemical heterogeneities on solid surfaces can reduce PNP transport and increase the sensitivity of the transport to IS. Furthermore, variations in the hydrogen bonding when the two phosphates act as proton donors will result in different influences on PNP transport at the same IS. This study highlights the sensitivity of PNP transport to phosphates associated with the solution chemistries (e.g., IS and pH) and is helpful for better understanding the fate of PNPs and other colloidal contaminants in the subsurface environment.

Freeze-thaw cycles induce diverse bacteria release behaviors from quartz sand columns with different water saturations

Bacteria present in natural environment especially those in cold regions would experience freeze-thaw (FT) process during day-night and season turns. However, knowledge about the influence of FT on bacteria release behaviors in porous media was limited. In present study, the bacteria release behaviors from quartz sand columns without and with 1 and 3 FT treatment cycles under three water saturations (θ=100%, 90%, and 60%) were investigated. We found that for all three water saturated columns without FT treatment, negligible bacteria released from columns via background salt solution elution, while the subsequent release of bacteria from sand columns via low ionic strength (IS) solution elution decreased with decreasing column water saturations. More importantly, we found unlike the negligible bacteria release in columns without FT treatment, for columns with high saturations (θ=100% and 90%), FT treatment could promote bacteria release with background salt solution elution. Moreover, for high saturated columns, FT treatment would decrease subsequent bacteria release with low IS solution elution. This phenomenon was more obvious with increasing FT treatment cycles. In contrast, FT treatment had negligible influence on bacteria release from columns with lower saturation (θ=60%). The decreased bacterial sizes, the loss of bacterial flagella, as well as the change of local configuration of porous media (via changing water into ice and ice back into water) during the FT processes contributed to increased bacteria release via background salt solution elution from high saturated sand columns. While, the reduced amount of bacteria being retained at secondary energy minima drove to the subsequently decreased bacteria release via low IS solution elution. The results of this study clearly showed that for porous media with high saturations, FT cycles would increase the risk of bacteria detaching from porous media with flushing by the background solution.

Transport of tetracycline in saturated porous media: combined functions of inorganic ligands and solution pH

To date, there is still very little knowledge about the combined effects of typical inorganic ligands and solution pH values on mobility characteristics of tetracycline (TC) through saturated aquifer media. In this work, three typical inorganic ligands (i.e., phosphate, silicate, and iodate) were employed in the transport experiments. Generally, all the ligands promoted TC mobility over the pH range of 5.0-9.0 owing to the enhanced electrostatic repulsion between sand grains and TC anionic forms (i.e., TC^- and TC^2-) as well as the competitive deposition between ligands and antibiotic molecules for attachment sites. Furthermore, the transport-enhancement effects of ligands on TC intensively depended on ligand type and followed the sequence of phosphate > silicate > iodate. This phenomenon was ascribed to their different molecular sizes and binding abilities to sand grains. Interestingly, the differences in extents of enhanced effects of various inorganic ligands on TC transport varied with background solution pH due to pH-induced different extents of deposition site competition effects. Moreover, the two-site nonequilibrium model (which accounts for an equilibrium site and a kinetic site) as well as adsorption and kinetic studies were performed to help interpret the controlling mechanisms for the synergistic effects of inorganic ligands and solution pH on TC transport in saturated quartz sand. The findings of our study clearly demonstrate that inorganic ligands may be critical factors in assessing the fate and transport of antibiotics in groundwater systems.

Effect of phytic acid and morphology on Fe (oxyhydr)oxide transport under saturated flow condition

Phytic acid (myo-inositol hexaphosphate, IHP) is a dominant form of organic phosphate (OP) in organic carbon-rich surface soil. The IHP impact on Fe (oxyhydr)oxide transport is critical for iron and phosphorus (bio)geochemical processes in iron and phosphorus rich soil and subsurface systems. Three typical Fe (oxyhydr)oxides (ferrihydrite, hematite, and goethite) were studied in this research. The effects of IHP and morphology on Fe (oxyhydr)oxide transport and IHP cotransport had been investigated using saturated sand columns. The results showed that IHP significantly enhanced the mobility of Fe (oxyhydr)oxide by 30-90% due to the stronger electrostatic repulsion. At low IHP concentration (< 50 µM IHP), the rod-like goethite and goethite-facilitated IHP showed high mobility due to their orientation and motion along the water flow, which is 70% faster than ferrihydrite and hematite at pH 5 and 90% faster at pH 10. The mobility of amorphous ferrihydrite was slowest among three selected iron oxides (< 37% at pH 5 and < 72% at pH 10). At high IHP concentration (> 50 μM IHP), the surface precipitation might have occurred on ferrihydrite because of its poorly ordered crystallinity, contributing to its less negatively charged surface and weak transport. The new insight provided in this study is essential for evaluating the fate and transport behavior of iron and iron-facilitate OP in soil rich in iron and OP.

Effects of phosphate on the transport of graphene oxide nanoparticles in saturated clean and iron oxide-coated sand columns

In this study, transport behaviors of graphene oxide (GO) in saturated uncoated (i.e., clean sand) and goethite-coated sand porous media were examined as a function of the phosphate. We found that phosphate enhanced the transport of GO over a wide range of solution chemistry (i.e., pH 5.0-9.0 and the presence of 10 mmol/L Na⁺ or 0.5 mmol/L Ca²⁺). The results were mainly ascribed to the increase of electrostatic repulsion between nanoparticles and porous media. Meanwhile, deposition site competition induced by the retained phosphate was another important mechanism leading to promote GO transport. Interestingly, when the phosphate concentration increased from 0.1 to 1.0 mmol/L, the transport-enhancement effect of phosphate in goethite-coated sand was to a much larger extent than that in clean sand. The observations were primarily related to the difference in the total mass of retained phosphate between the iron oxide-coated sand and clean sand columns, which resulted in different degrees of the electrostatic repulsion and competitive effect of phosphate. When the background solution contained 0.5 mmol/L Ca²⁺, phosphate could be bind to sand/ goethite-coated sand surface by cation bridging; and consequently, promoted competition between phosphate and nanoparticles for deposition sites, which was an important mechanism for the enhanced effect of phosphate. Moreover, the DLVO theory was applicable to describe GO transport behaviors in porous media in the absence or presence of phosphate. Taken together, these findings highlight the important status and role of phosphate on the transport and fate of colloidal graphene oxide in the subsurface environment.

Flagella and Their Properties Affect the Transport and Deposition Behaviors of Escherichia coli in Quartz Sand

The effects of flagella and their properties on bacterial transport and deposition behaviors were examined by using four types of Escherichia coli (E. coli) with or without flagella, as well as with normal or sticky flagella. Packed column, quartz crystal microbalance with dissipation, visible parallel-plate flow chamber system, and visible flow chamber packed with porous media system were employed to investigate the deposition mechanisms of bacteria with different properties of flagella. We found that the presence of flagella favored E. coli deposition onto quartz sand/silica surfaces. Moreover, by changing the porous media porosity and directly observing the bacterial deposition process, local sites with high roughness, narrow flow channels, and grain-to-grain contacts were found to be the major sites for bacterial deposition. Particularly, flagella could help bacteria swim near and then deposit at these sites. In addition, we found that due to the stronger adhesive forces, sticky flagella could further enhance bacterial deposition onto quartz sand/silica surfaces. Elution experiments indicated that flagella could help bacteria attach onto sand surfaces more irreversibly. Clearly, flagella and their properties would have obvious impacts on the transport/deposition behaviors of bacteria in porous media.

Protein corona-mediated transport of nanoplastics in seawater-saturated porous media

The offshore aquaculture environment is a potential water area with high concentrations of tiny plastics and feeding proteins. In this study, the negatively charged bovine serum albumin (BSA) and the positively charged lysozyme (LSZ) were used to explore the effects of protein corona on the aggregation, transport, and retention of polystyrene nanoplastics (NPs; 200, 500, and 1000 nm) in sea sand saturated with seawater of 35 practical salinity units (PSU). The BSA corona, which was formed by the adsorption of BSA on the surface of NPs, drove the dispersion of NPs (200 and 500 nm) due dominantly to the induced colloidal steric hindrance. For example, the aggregate sizes of 500 nm NP decreased from 1740 ± 87 nm to 765 ± 8 nm at 40 min, which resulted in the enhanced transportation of NP. The calculated interaction energies indicated the BSA corona-induced high energy barriers (>10⁴ K_BT) between 1000 nm NPs and sand surface, demonstrating the BSA-enhanced transport of 1000 nm NPs. The mass percentage recovered from the effluent (M_eff) increased from 33.4% to 61.7%. Inversely, the LSZ corona triggered the aggregation of 200 nm NPs into the large aggregates via electrostatic adsorption and bridging effect, thereby inhibiting the transport of 200 nm NPs. Nevertheless, no LSZ corona was formed on the surface of 500 and 1000 nm NPs due to extremely low protein adsorption. Accordingly, LSZ cannot affect the stability and transport of these NPs. In the diluted seawater (3.5 PSU), the strong electrostatic attraction between positively charged LSZ and 500 nm NPs significantly increased and the LSZ corona formed, which induced the aggregation of 500 nm NPs. The M_eff of NPs therefore decreased from 53.1% to 11.2%. Overall, the protein corona-mediated transport of NPs in seawater-saturated porous media depends on protein type, NP size, and seawater salinity.

Facilitated transport of nTiO2-kaolin aggregates by bacteria and phosphate in water-saturated quartz sand

The soil major component of clay plays an important role in governing the fate and transport of engineered nanomaterials (e.g., the most commonly used titanium dioxide nanoparticles; nTiO₂) in the subsurface environments via forming nTiO₂-clay aggregates. This research is designed to unravel the interplay of naturally-occurring bacteria (Escherichia coli) and phosphate on the transport and retention of nTiO₂-kaolin aggregates in water-saturated porous media. Our results showed that nTiO₂-nTiO₂ homoaggregates and nTiO₂-kaolin heteroaggregates dominated in the nTiO₂-kaolin nanoaggregate suspension. Transport of nTiO₂-kaolin aggregates was enhanced with the copresence of E. coli and phosphate, particularly at the low pH of 6.0. This effect is due to the greater adsorption of phosphate and thus the greater enhancement in repulsive interaction energies between aggregates and sand grains at pH 6.0 (vs. pH 9.0). The charged "soft layer" of E. coli cell surfaces changed the aggregation state and the heterogeneous distribution of nTiO₂-kaolin aggregates, and subsequently stabilized the nTiO₂-nTiO₂ homoaggregates and nTiO₂-kaolin heteroaggregates via TEM-EDX measurements and promoted the physical segregation between the aggregates (separation distance = 0.486 vs. 0.614 μm without vs. with the presence of E. coli) via 2D/3D AFM identifications, both of which caused greater mobility of nTiO₂-kaolin aggregates with the presence of E. coli. Nonetheless, transport of nTiO₂-kaolin aggregates was lower with the copresence of E. coli and phosphate vs. the singular presence of phosphate due to the competitive adsorption of less negatively charged E. coli (vs. phosphate) onto the aggregates. Taken altogether, our findings furnish new insights into better understanding the fate, transport, and potential risks of nTiO₂ in real environmental settings (soil and sediment aquifer) where clay, bacteria, and phosphate ubiquitously cooccur.

Synergistic oxidation - filtration process analysis of catalytic CuFe2O4 - Tailored ceramic membrane filtration via peroxymonosulfate activation for humic acid treatment

This work synthesized catalytic CuFe₂O₄ tailored ceramic membrane (CuFeCM), and systematically investigated the intercorrelated oxidation - filtration mechanism of peroxymonosulfate (PMS)/CuFeCM catalytic filtration for treating humic acid (HA). PMS/CuFeCM filtration exhibited enhanced HA removal efficiency while reduced the irreversible fouling resistance as compared with the conventional CM filtration. Results from HA characterizations showed that PMS/CuFeCM catalytic filtration oxidized HA into conjugated structures of smaller molecular weight. The unsaturated bonds further caused the re-agglomeration of HA, hence enhancing the size exclusion of CuFeCM. Meanwhile, oxidized HA particles with changing physicochemical properties reduced the total attractive interaction energy between CuFeCM and HA, mainly attributed to the reduced acid-base interaction energy according to the Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) analysis. The changing of HA properties and HA-CuFeCM physicochemical interactions rendered more re-agglomerated HA particles retained above membrane with less attachment, which induced decreasing irreversible fouling resistance and facilitated easier external fouling removal by hydraulic cleaning. Overall, the PMS/CuFeCM configuration demonstrated in this study could provide a new insight into the synergistic oxidation - filtration interaction mechanism of hybrid catalytic ceramic membrane filtration process.

Advanced redox zonation of the San Pedro Sula alluvial aquifer (Honduras) using data fusion and multivariate geostatistics

The incorrect wastewater management and the land use distribution lead to severe environmental problems, creating heavy eutrophication condition in surface-water; when surface-water/groundwater relationships exist, the organic matter transferred to the aquifer oxidizes and triggers redox processes (i.e. Terminal Electron Accepting Processes, TEAPs), that provoke severe groundwater quality modifications and complicate its exploitation and management. For this reason, the definition of the redox zonation within an aquifer is an effective tool for the identification of the contamination sources and for the conceptual model refinement, when remediation strategies need to be planned. Although the redox processes are dynamic reactions, the aquifer redox zonation is generally aimed to identify homogenous zones, characterized by a predominant TEAP. To overcome this methodological approach, the Multi-Collocated Factorial Kriging (MCFK) was applied to redox-related physico-chemical parameters, which allowed identifying their spatial relationships at different scales, transferring this method from precision agriculture and soil science to hydrogeochemistry. The selected study area is the San Pedro Sula aquifer (Honduras), a multi-layer alluvial aquifer characterized by well-known surface-water/groundwater interactions and heavy eutrophicated streams. Here, high concentrations of Mn and Fe were found in the aquifer. The MCFK results identified a short-range (2300 m) factor, highlighting a strong relation between Mn concentrations and anoxic conditions, due to the organic matter transfer from eutrophicated surface-water into the aquifer. Simultaneously, the relationship between Fe and turbidity is related to a fine Fe(III) oxi-hydroxide colloidal phase, developed when different redox conditions of groundwater mix up in the wells. The long-range (6000 m) factor points out that Fe is related to redox processes at a wider scale, especially in the northern San Pedro Sula alluvial plain. These results are supported by both the Principal Component Analysis and the hydrogeochemical numerical modeling. As a result, different TEAPs occur simultaneously in contaminated areas, acting at multiple scales.

The influence of different charged poly (amido amine) dendrimer on the transport and deposition of bacteria in porous media

The influence of dendrimer on the bacterial transport and deposition behaviors in saturated porous media (quartz sand) was investigated in both NaCl (10 and 25 mM) and CaCl₂ solutions (1.2 and 5 mM). 3.5G and 4G poly (amido amine) (PAMAM) dendrimer was employed as negatively and positively charged dendrimer, respectively. Three dendrimer concentrations (10 μg/L, 1 and 10 mg/L) were considered in present study. We found that regardless of the solution chemistry (ionic strength and ion types) and dendrimer concentrations, the presence of negatively charged PAMAM 3.5G in suspensions enhanced bacterial transport and inhibited their deposition in quartz sand; while the presence of positive charged PAMAM 4G yet induced the opposite effects (decreased bacterial transport and increased their deposition in quartz sand). The increased repulsive force between cell and quartz sand due to the adsorption of PAMAM 3.5G onto both cell and sand surfaces, the competition deposition sites as well as the steric repulsion via the suspended PAMAM 3.5G drove to the increased bacterial transport with PAMAM 3.5G copresent in suspensions in quartz sand. While the reduced repulsive force between cell and quartz sand induced by the chemical heterogeneity on both cell and sand surfaces (due to the adsorption of positive charged PAMAM 4G) increased bacterial retention in quartz sand with copresence of PAMAM 4G (10 μg/L and 1 mg/L) in suspensions. Steric repulsion due to the presence of great amount of suspended PAMAM 4G yet lead to the enhanced bacterial transport with furthering increasing PAMAM 4G to 10 mg/L relative to the lower PAMAM 4G concentration.

Adhesion mediated transport of bacterial pathogens in saturated sands coated by phyllosilicates and Al-oxides

The current knowledge of bacterial migration is mainly derived from work using bare or Fe-coated quartz sands as porous media. However, mineral coatings on quartz by phyllosilicates and Al-oxides prevail in natural soils, and their effect on bacterial transport remains unknown. Herein, we systematically explored the transport of two bacterial pathogens (Escherichia coli and Staphylococcus aureus) through saturated bare quartz and those coated by kaolinite (KaoQuartz), montmorillonite (MontQuartz) or Al-oxides (AlQuartz) under various solution ionic strength (IS) and pH levels. Elevating IS or decreasing pH discouraged bacterial mobility in all cases, with one exception for the migration of S. aureus through AlQuartz at various IS levels. E. coli showed a higher mobility than S. aureus in all cases. All the three coatings, especially the Al-oxides inhibited bacterial transport through quartz. Overall, the two phyllosilicates-coated sands showed transport behaviors (mobility trends with IS, pH, and cell type) similar to those for the bare quartz which could be explained by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Nevertheless, for transport within AlQuartz, there were deviations between the observations and the DLVO predictions, probably because of the existence of non-DLVO forces such as hydrophobic and chemical interactions. More importantly, the bacterial retention was found to correlate well with the adhesion regardless of the solution condition and the bacteria and media type, thereby revealing a central role of adhesion in mediating migration through mineral-coated sands. These findings highlight the significance of mineral coating and adhesion in pathogen dissemination in natural soils.

A review of bacteriophage therapy for pathogenic bacteria inactivation in the soil environment

The emerging contamination of pathogenic bacteria in the soil has caused a serious threat to public health and environmental security. Therefore, effective methods to inactivate pathogenic bacteria and decrease the environmental risks are urgently required. As a century-old technique, bacteriophage (phage) therapy has a high efficiency in targeting and inactivating pathogenic bacteria in different environmental systems. This review provides an update on the status of bacteriophage therapy for the inactivation of pathogenic bacteria in the soil environment. Specifically, the applications of phage therapy in soil-plant and soil-groundwater systems are summarized. In addition, the impact of phage therapy on soil functioning is described, including soil function gene transmission, soil microbial community stability, and soil nutrient cycling. Soil factors, such as soil temperature, pH, clay mineral, water content, and nutrient components, influence the survival and activity of phages in the soil. Finally, the future research prospects of phage therapy in soil environments are described.

Effects of divalent metal cations and inorganic anions on the transport of tetracycline in saturated porous media: column experiments and numerical simulations

Tetracycline is one of the most commonly used antibiotics in the world. Eventually, large amounts of this contaminant will enter into the subsurface environment, where a variety of ions exist. In this study, the effects of divalent metal cations (Mg2+, Ca2+, Pb2+ and Cu2+) and inorganic anions (Cl-, NO3-, SO42- and H2PO4-) on the transport of tetracycline in saturated porous media were investigated. Both batch and column experiments were conducted to determine the interactions between tetracycline and sand. Batch sorption experimental results showed that the presence of divalent metal cations could increase the sorption of tetracycline onto sand due to the cation-bridging mechanism. When Na+ was the counterion in the background solution, anions caused a significant decrease in tetracycline sorption owing to the occupation of some adsorption sites by anions and the decrease of electrostatic attraction. Column experiments indicated that the inhibition effects of divalent cations followed the order of Cu2+ > Pb2+ > Ca2+ ≈ Mg2+; the regular pattern might be related to their different complexing strengths. The presence of inorganic anions enhanced the mobility of tetracycline following the order of H2PO4- > SO42- > NO3- > Cl-. Transport-enhancement effects of anions were ascribed to competition between inorganic anions and tetracycline for deposition sites on sand surfaces. However, when Ca2+ was the counterion, the differences in the breakthrough curve of tetracycline among three inorganic anions (i.e., SO42-, NO3- and Cl-) were very small. In this case, the transport-inhibiting effects of anions could be counterbalanced by the transport-enhancement effects of the cation-bridging effect. Also, the two-site nonequilibrium transport model was applied to analyze the transport data. Findings from this study improve our understanding of the transport of tetracycline in saturated aquifer materials.

Interaction Analysis between Gravity-Driven Ceramic Membrane and Smaller Organic Matter: Implications for Retention and Fouling Mechanism in Ultralow Pressure-Driven Filtration System

Gravity-driven membranes (GDM) generally achieve high retention performance in filtration of organic matter with a smaller size than the membrane pore, yet the in-depth mechanism remains unclear. Thorough analysis of the retention mechanism is crucial for optimizing GDM properties and improving GDM filtration performance. The performance and interaction mechanism of gravity-driven ceramic membrane (GDCM) filtrating smaller organic matter (SOM) were systematically studied. Rejection rate grew noticeably for like-charged foulant, whereas it only grew slightly for opposite-charged foulant as operation height decreased. Flux declined more seriously at lower operation height, probably due to heavier cake fouling caused by the rejected foulant. Interactions of ceramic membrane-SOM were analyzed through extended Derjaguin-Landau-Verwey-Overbeek theory (XDLVO) and hydrodynamic permeation drag (PD). Among van der Waals (LW), acid-base (AB), and electrostatic (EL) forces in XDLVO, EL played a significant role on GDCM filtrating SOM, and altering membrane electrostatic property could greatly influence SOM filtration. Furthermore, the rising PD force largely weakened the EL dominant zone with operation height increasing, while barely influencing the LW and AB dominant zones. Therefore, the weakened EL-dominant repulsive zone caused less rejection of like-charged foulant with operation height increasing. Fe₂O₃- and MnO₂-modified membranes further validated the comprehensive influence of LW, AB, EL, and PD interactions on GDCM filtration. The possible "trade-off" of pore blocking-cake fouling with operation height decreasing demonstrated potential enhancement for both rejection and antifouling performance by electrically modified membrane under ultralow pressure. This study provides insight on membrane selection/preparation/modification and performance control of ultralow pressure-driven filtration.

Effects of Escherichia coli and phosphate on the transport of titanium dioxide nanoparticles in heterogeneous porous media

Transport behaviors of titanium dioxide nanoparticles (nTiO₂) were examined in the individual- and co-presence Escherichia (E.) coli and phosphate in heterogeneous sand (uncoated and iron oxyhydroxide-coated sand) columns. The results showed that for the individual presence of phosphate, the degree of nTiO₂ deposition was less in uncoated than in iron oxide-coated sands. In contrast, an opposite trend that greater deposition of nTiO₂ in uncoated than in coated sands occurred in the individual presence of E. coli. These observations are due to the phosphate adsorption changing the charge of NPs and iron oxyhydroxide-coated sand, or the preferential adhesion of bacterial to coated sand. In the copresence of E. coli and phosphate, interestingly, the phosphate level plays an important role in influencing nTiO₂ transport. At a high phosphate concentration (>1.0 mM), the deposition of nTiO₂ with the individual presence of E. coli was stronger than nTiO₂ in the copresence of both E. coli and phosphate, regardless of sand type. The potential mechanism was that phosphate adsorption led to the formation of more negatively charged NPs-bacteria complexes that have higher mobility in sand columns. At a low phosphate level (≤0.1 mM), a similar observation occurred in uncoated sand. Nevertheless, the deposition of nTiO₂ with copresence of E. coli and phosphate was greater than nTiO₂ with E. coli in oxyhydroxide-coated sand. It was attributed to the formation of large NPs-bacteria-phosphate clusters (less mobile) and the preferential adhesion of E. coli cells to iron oxyhydroxide coating simultaneously. Taken together, our findings provide crucial knowledge for better understanding the fate, transport, and potential risks of engineered nanoparticles in complicated environmental settings where bacteria and phosphate are ubiquitous.

Synergistic effects of phosphorus and humic acid on the transport of anatase titanium dioxide nanoparticles in water-saturated porous media

The (un)intentional release of titanium dioxide nanoparticles (TiO₂ NPs) poses potential risks to the environment and human health. Phosphorus (P) and humic acid (HA) usually coexist in the natural environments. This study aims at investigating the transport and retention behaviors of TiO₂ NPs in the single and binary systems of P and HA in water-saturated porous media. The experimental results showed that HA alone favored the transport of TiO₂ NPs in sand columns to a greater extent than that of P alone at pH 6.0. Interestingly, the co-presence of P and HA acting in a synergistic fashion enhanced the transport of TiO₂ NPs in sand-packed columns more significantly compared to that in the single-presence of P or HA. Particularly, P plays a dominant role in the synergistic effect. This is largely due to the competitive effect between P and HA for the same adsorption sites on the sand surfaces favorable for TiO₂ NPs retention. A two-site kinetic attachment model that considers Langmuirian blocking of particles at one site provided a good approximation of TiO₂ NPs transport. Modeled first-order attachment coefficient (k₂) and the maximum solid-phase retention capacity on site 2 (S_max2) for P or HA alone were larger than those in the co-presence of P and HA, suggesting a less retention degree of TiO₂ NPs in the binary system of P and HA. Our findings indicate that the mobility of TiO₂ NPs is expected to be appreciable in soil and water environments, where P and HA are rich and always co-present at low pH conditions.

Different electrically charged proteins result in diverse bacterial transport behaviors in porous media

The influence of proteins on bacterial transport and deposition behaviors in quartz sand was examined in both NaCl (10 and 25 mM) and CaCl₂ solutions (1.2 and 5 mM). Bovine Serum Albumin (BSA) and bovine trypsin were used to represent negatively and positively charged proteins in natural aquatic systems, respectively. The presence of negatively charged BSA in suspensions increased the transport and decreased bacterial deposition in quartz sand, regardless of the ionic strength and ion types. Whereas, positively charged trypsin inhibited the transport and enhanced bacterial deposition under all experimental conditions. The potential mechanisms controlling the changes of bacterial transport behaviors varied for different charged proteins. The steric repulsion resulting from BSA adsorption onto both bacteria and quartz sand was found to play a dominant role in the transport and deposition of bacteria in porous media with BSA copresent in suspension. BSA adsorption onto bacterial surfaces and competition for deposition sites onto sand surfaces (adsorption of quartz sand surfaces) contributed to the increased cell transport with BSA in suspension. In contrast, the attractive patch-charged interaction induced by the adsorption of trypsin onto both bacteria and quartz sand had great contribution to the decreased bacterial transport in porous media with trypsin copresent in suspension. The increase in bacteria size, and the adsorption of trypsin onto cell surfaces (resulting in less negative cell surface charge) and quartz sand surfaces (providing extra deposition sites) were found to be the main contributors to the decreased transport and increased deposition of bacteria in quartz sand with trypsin in suspension.

Enhanced transport of ferrihydrite colloid by chain-shaped humic acid colloid in saturated porous media

Both humic acid and colloid particle size effectively regulate colloid transport. However, little is known about effect of particle size and configuration of humic acid colloid (HA_colloid) on enhanced-transport of ferrihydrite colloid (FH_colloid) in porous media. Co-transport of HA_colloid and FH_colloid at different pH was systematically investigated by monitoring breakthrough curves (BTCs) in saturated sand columns. The colloid transport model and the (X)DLVO theory were used to reveal the mechanism of HA_colloid-enhanced FH_colloid transport in the columns. Results showed that HA_colloid enhanced FH_colloid transport in neutral and alkaline conditions. In neutral conditions, small HA_colloid (F-HA_colloid) with chain-shaped structure enhanced FH_colloid transport more prominently than pristine granular HA_colloid. The chain-shaped F-HA_colloid caused osmotic repulsion and elastic-steric repulsion between colloids and sand, leading to enhanced transport. However, the granular HA_colloid readily occurred as deposition due to attachment and straining, which decreased the enhanced transport of FH_colloid. In alkaline conditions, both HA_colloid and F-HA_colloid were chain-shaped, with longer chains of HA_colloid than F-HA_colloid. Ferrihydrite colloid transport was enhanced by HA_colloid more significantly than F-HA_colloid due to stronger repulsion between mixed HA_colloid-FH_colloid and sand. It suggested that regulation of particle size and morphology of HA_colloid would enhance FH_colloid transport and further help in understanding FH_colloid-facilitated contaminants transport in porous media.

Attenuation of bacterial cytotoxicity of carbon nanotubes by riverine suspended solids in water

The impact of solid particles on ecotoxicity of nanomaterials in water environments is poorly understood. This study investigated the effect of natural riverine suspended solids (SPS) on the cytotoxicity of single-walled carbon nanotubes (SWCNTs) towards a bacterium, Ochrobactrum sp. in water. Compared with SWCNT suspension without SPS, the presence of SPS at different concentrations ranging from 20 to 400 mg L^-1 markedly increased the survival rates of bacteria exposed to 50 mg L^-1 SWCNTs and bacterial survival rates increased with SPS concentrations by a power law. Sedimentation experiments and field emission scanning electron microscopy revealed the occurrence of heteroaggregation between SWCNTs and SPS, probably responsible for the reduced SWCNT toxicity. Furthermore, the extended Derjaguin-Landau-Verwey-Overbeek (ExDLVO) calculation showed the mitigated toxicity might also result from the decreased SWCNT-bacterium interaction energy with the increased SPS concentrations and the stronger SPS-SWCNT interaction than the SWCNT-bacterium interaction. This work provides new insights into our understanding of environmental hazards of engineered nanomaterials in aquatic systems.

Influence of Bisphenol A on the transport and deposition behaviors of bacteria in quartz sand

The influence of Bisphenol A (BPA) on the transport and deposition behaviors of bacteria in quartz sand was examined in both NaCl (10 and 25 mM) and CaCl₂ solutions (1.2 and 5 mM) by comparing the breakthrough curves and retained profiles of cell with BPA in suspensions versus those without BPA. Gram-negative Escherichia coli and Gram-positive Bacillus subtilis were employed as model cells in the present study. The extended Derjaguin-Landau-Verwey-Overbeek interaction energy calculation revealed that the presence of BPA in cell suspensions led to a lower repulsive interaction between the cells and the quartz sand. This suggests that, theoretically, increased cell deposition on quartz sand would be expected in the presence of BPA. However, under all examined solution conditions, the presence of BPA in cell suspensions increased transport and decreased deposition of bacteria in porous media regardless of cell type, ionic strength, ion valence, the presence or absence of extracellular polymeric substances. We found that competition by BPA through hydrophobicity for deposition sites on the quartz sand surfaces was the sole contributor to the enhanced transport and decreased deposition of bacteria in the presence of BPA.

Quantitative Effects of Biochar Oxidation and Pyrolysis Temperature on the Transport of Pathogenic and Nonpathogenic Escherichia coli in Biochar-Amended Sand Columns

The present study quantifies the transport of Escherichia coli pathogenic O157:H7 and nonpathogenic K12 strains in water-saturated Quincy sand (QS) columns amended with oxidized (OX) or unoxidized (UO) pine wood (PW) or pine bark (PB) biochar produced at either 350 or 600 °C. Our results showed that (1) the addition of oxidized biochar into QS columns enhanced the transport of E. coli O157:H7 by 3.1 fold compared to the unoxidized counterparts, likely because of an increase in the repulsive forces due to their higher negative charge densities. (2) The retention of E. coli O157:H7 was 3.3 fold higher than that of E. coli K12 in all biochar-amended sand columns. (3) Increased application rates of unoxidized PW600 biochar from 0 to 20 wt % led to a reduction in the transport of E. coli O157:H7 and K12 from 98 to 10% and from 95 to 70%, respectively. Our data showed that mixing sand with PW350-UO at a 20 wt % application rate almost completely retained the pathogenic E. coli in the subsurface, suggesting that utilizing sand mixed with biochar can act as a promising biofilter capable of protecting natural aquafers from pathogens.

Effect of low-concentration rhamnolipid biosurfactant on Pseudomonas aeruginosa transport in natural porous media

The effect of low-concentrations of monorhamnolipid biosurfactant on transport of Pseudomonas aeruginosa ATCC 9027 in natural porous media (silica sand and a sandy soil) was studied with miscible-displacement experiments using artificial groundwater as the background solution. Transport of two types of cells was investigated, glucose- and hexadecane-grown cells with lower and higher cell surface hydrophobicity (CSH), respectively. The effect of hexadecane presence as a residual non-aqueous phase liquid (NAPLs) on transport was also examined. A clean-bed colloid deposition model was used to calculate deposition rate coefficients (k) for quantitative assessment. Significant cell retention was observed in the sand (81% and 82% for glucose- and hexadecane-grown cells, respectively). Addition of a low-concentration rhamnolipid solution enhanced cell transport, with 40 mg/L of rhamnolipid reducing retention to 50% and 60% for glucose- and hexadecane-grown cells, respectively. The k values for both glucose- and hexadecane-grown cells correlate linearly with rhamnolipid-dependent CSH represented as bacterial-adhesion-to-hydrocarbon rate of cells. Retention of cells by the soil was nearly complete (>99%). Addition of 40 mg/L rhamnolipid solution reduced retention to 95%. The presence of NAPLs in the sand increased the retention of hexadecane-grown cells with higher CSH. Transport of cells in the presence of the NAPL was enhanced by rhamnolipid at all concentrations tested, and the relative enhancement was greater than in was in the absence of NAPL. This study shows the importance of hydrophobic interaction on bacterial transport in natural porous media and the potential of using low-concentration rhamnolipid for facilitating the transport in subsurface for bioaugmentation efforts.

Blocking effect of colloids on arsenate adsorption during co-transport through saturated sand columns

Transport of environmental pollutants through porous media is influenced by colloids. Co-transport of As(V) and soil colloids at different pH were systematically investigated by monitoring breakthrough curves (BTCs) in saturated sand columns. A solute transport model was applied to characterize transport and retention sites of As(V) in saturated sand in the presence of soil colloids. A colloid transport model and the DLVO theory were used to reveal the mechanism and hypothesis of soil colloid-promoted As(V) transport in the columns. Results showed that rapid transport of soil colloids, regulated by pH and ionic strength, promoted As(V) transport by blocking As(V) adsorption onto sand, although soil colloids had low adsorption for As(V). The promoted transport was more significant at higher concentrations of soil colloids (between 25 mg L(-1) and 150 mg L(-1)) due to greater blocking effect on As(V) adsorption onto the sand surfaces. The blocking effect of colloids was explained by the decreases in both instantaneous (equilibrium) As adsorption and first-order kinetic As adsorption on the sand surface sites. The discovery of this blocking effect improves our understanding of colloid-promoted As transport in saturated porous media, which provides new insights into role of colloids, especially colloids with low As adsorption capacity, in As transport and mobilization in soil-groundwater systems.

Effect of bacteria on the transport and deposition of multi-walled carbon nanotubes in saturated porous media

The influence of bacteria on the transport and deposition behaviors of carbon nanotubes (CNTs) in quartz sand was examined in both NaCl (5 and 25 mM ionic strength) and CaCl2 (0.3 and 1.2 mM ionic strength) solutions at unadjusted pH (5.6-5.8) by direct comparison of both breakthrough curves and retained profiles in both the presence and absence of bacteria. Two types of widely utilized CNTs, i.e., carboxyl- and hydroxyl-functionalized multi-walled carbon nanotubes (MWCNT-COOH and MWCNT-OH, respectively), were employed as model CNTs and Escherichia coli was utilized as the model bacterium. The results showed that, for both types of MWCNTs under all examined conditions, the breakthrough curves were higher in the presence of bacteria, while the retained profiles were lower, indicating that the co-presence of bacteria in suspension increased the transport and decreased the deposition of MWCNTs in porous media, regardless of ionic strength or ion valence. Complementary characterizations and extra column tests demonstrated that competition by bacteria for deposition sites on the quartz sand surfaces was a major (and possibly the sole) contributor to the enhanced MWCNTs transport in porous media.

Influence of Perfluorooctanoic Acid on the Transport and Deposition Behaviors of Bacteria in Quartz Sand

The significance of perfluorooctanoic acid (PFOA) on the transport and deposition behaviors of bacteria (Gram-negative Escherichia coli and Gram-positive Bacillus subtilis) in quartz sand is examined in both NaCl and CaCl2 solutions at pH 5.6 by comparing both breakthrough curves and retained profiles with PFOA in solutions versus those without PFOA. All test conditions are found to be highly unfavorable for cell deposition regardless of the presence of PFOA; however, 7%-46% cell deposition is observed depending on the conditions. The cell deposition may be attributed to micro- or nanoscale roughness and/or to chemical heterogeneity of the sand surface. The results show that, under all examined conditions, PFOA in suspensions increases cell transport and decreases cell deposition in porous media regardless of cell type, presence or absence of extracellular polymeric substances, ionic strength, and ion valence. We find that the additional repulsion between bacteria and quartz sand caused by both acid-base interaction and steric repulsion as well as the competition for deposition sites on quartz sand surfaces by PFOA are responsible for the enhanced transport and decreased deposition of bacteria with PFOA in solutions.

Stability of nTiO2 particles and their attachment to sand: Effects of humic acid at different pH

The fate and transport of nano-scale or micro-scale titanium dioxide particles (nTiO2) in subsurface environments are strongly influenced by the stability of nTiO2 and their attachment to sediment grains. nTiO2 may carry either positive or negative charges in natural water, therefore, environmental factors such as pH, humic substances, and Fe oxyhydroxide coatings on sediment grains, which are known to control the stability and transport of negatively-charged colloids, may influence nTiO2 in different manners. The objective of this study is to investigate the effects of pH and humic acid (HA) on the stability and attachment of nTiO2 to sand at HA concentrations that are relevant to typical groundwater conditions, so that mechanisms that control nTiO2 immobilization and transport in natural systems can be elucidated. Stability and attachment of nTiO2 to quartz sand and Fe oxyhydroxide coated quartz sand are experimentally measured under a range of HA concentrations at pH5 and 9. Results show that at pH5, negatively-charged HA strongly adsorbs to positively-charged nTiO2 and Fe oxyhydroxide, which, at low HA concentrations, partially neutralizes the positive charges on nTiO2 and Fe oxyhydroxide, and therefore decreases the repulsive electrostatic forces between the surfaces, resulting in nTiO2 aggregation and attachment. At high HA concentrations, adsorbed HA reverses the surface charges of nTiO2 and Fe oxyhydroxide, and makes nTiO2 and Fe oxyhydroxide strongly negatively charged, resulting in stable nTiO2 suspension and low nTiO2 attachment. At pH9, HA, nTiO2, and Fe oxyhydroxide are all negatively charged, and HA adsorption is low and does not have a strong impact on the stability and attachment of nTiO2. Overall, this study shows that changes in surface charges of nTiO2 and Fe oxyhydroxide coating caused by HA adsorption is a key factor that influences the stability and attachment of nTiO2.

Cotransport of bacteria with hematite in porous media: Effects of ion valence and humic acid

This study investigated the influence of multiple colloids (hematite and humic acid) on the transport and deposition of bacteria (Escherichia coli) in packed porous media in both NaCl (5 mM) and CaCl2 (1 mM) solutions at pH 6. Due to the alteration of cell physicochemical properties, the presence of hematite and humic acid in cell suspensions significantly affected bacterial transport and deposition in quartz sand. Specifically, the presence of hematite (5 mg/L) decreased cell transport (increased cell deposition) in quartz sand in both NaCl and CaCl2 solutions, which could be attributed to the less negative overall zeta potentials of bacteria induced by the adsorption of positively charged hematite onto cell surfaces. The presence of a low concentration (0.1 mg/L) of humic acid in bacteria and hematite mixed suspensions reduced the adsorption of hematite onto cell surfaces, leading to increased cell transport in quartz sand in NaCl solutions, whereas, in CaCl2 solutions, the presence of 0.1 mg/L humic acid increased the formation of hematite-cell aggregates and thus decreased cell transport in quartz sand. When the concentration of humic acid was increased to 1 mg/L, enhanced cell transport was observed in both NaCl and CaCl2 solutions. The decreased adsorption of hematite onto cell surfaces as well as the competition of deposition sites on quartz sand with bacteria by the suspended humic acid contributed to the increased cell transport.

Transport and viability of Escherichia coli cells in clean and iron oxide coated sand following coating with silver nanoparticles

A mechanistic understanding of processes controlling the transport and viability of bacteria in porous media is critical for designing in situ bioremediation and microbiological water decontamination programs. We investigated the combined influence of coating sand with iron oxide and silver nanoparticles on the transport and viability of Escherichia coli cells under saturated conditions. Results showed that iron oxide coatings increase cell deposition which was generally reversed by silver nanoparticle coatings in the early stages of injection. These observations are consistent with short-term, particle surface charge controls on bacteria transport, where a negatively charged surface induced by silver nanoparticles reverses the positive charge due to iron oxide coatings, but columns eventually recovered irreversible cell deposition. Silver nanoparticle coatings significantly increased cell inactivation during transit through the columns. However, when viability data is normalised to volume throughput, only a small improvement in cell inactivation is observed for silver nanoparticle coated sands relative to iron oxide coating alone. This counterintuitive result underscores the importance of net surface charge in controlling cell transport and inactivation and implies that the extra cost for implementing silver nanoparticle coatings on porous beds coated with iron oxides may not be justified in designing point of use water filters in low income countries.

Dual Roles of Capsular Extracellular Polymeric Substances in Photocatalytic Inactivation of Escherichia coli: Comparison of E. coli BW25113 and Isogenic Mutants

The dual roles of capsular extracellular polymeric substances (EPS) in the photocatalytic inactivation of bacteria were demonstrated in a TiO2-UVA system, by comparing wild-type Escherichia coli strain BW25113 and isogenic mutants with upregulated and downregulated production of capsular EPS. In a partition system in which direct contact between bacterial cells and TiO2 particles was inhibited, an increase in the amount of EPS was associated with increased bacterial resistance to photocatalytic inactivation. In contrast, when bacterial cells were in direct contact with TiO2 particles, an increase in the amount of capsular EPS decreased cell viability during photocatalytic treatment. Taken together, these results suggest that although capsular EPS can protect bacterial cells by consuming photogenerated reactive species, it also facilitates photocatalytic inactivation of bacteria by promoting the adhesion of TiO2 particles to the cell surface. Fluorescence microscopy and scanning electron microscopy analyses further confirmed that high capsular EPS density led to more TiO2 particles attaching to cells and forming bacterium-TiO2 aggregates. Calculations of interaction energy, represented by extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) potential, suggested that the presence of capsular EPS enhances the attachment of TiO2 particles to bacterial cells via acid-base interactions. Consideration of these mechanisms is critical for understanding bacterium-nanoparticle interactions and the photocatalytic inactivation of bacteria.

Escherichia coli removal in biochar-augmented biofilter: effect of infiltration rate, initial bacterial concentration, biochar particle size, and presence of compost

Bioretention systems and biofilters are used in low impact development to passively treat urban stormwater. However, these engineered natural systems are not efficient at removing fecal indicator bacteria, the contaminants responsible for a majority of surface water impairments. The present study investigates the efficacy of biochar-augmented model sand biofilters for Escherichia coli removal under a variety of stormwater bacterial concentrations and infiltration rates. Additionally, we test the role of biochar particle size and "presence of compost on model" biofilter performance. Our results show that E. coli removal in a biochar-augmented sand biofilter is ∼ 96% and is not greatly affected by increases in stormwater infiltration rates and influent bacterial concentrations, particularly within the ranges expected in field. Removal of fine (<125 μm) biochar particles from the biochar-sand biofilter decreased the removal capacity from 95% to 62%, indicating biochar size is important. Addition of compost to biochar-sand biofilters not only lowered E. coli removal capacity but also increased the mobilization of deposited bacteria during intermittent infiltration. This result is attributed to exhaustion of attachment sites on biochar by the dissolved organic carbon leached from compost. Overall, our study indicates that biochar has potential to remove bacteria from stormwater under a wide range of field conditions, but for biochar to be effective, the size should be small and biochar should be applied without compost. Although the results aid in the optimization of biofilter design, further studies are needed to examine biochar potential in the field over an entire rainy season.

Adhesion of bacterial pathogens to soil colloidal particles: influences of cell type, natural organic matter, and solution chemistry

Bacterial adhesion to granular soil particles is well studied; however, pathogen interactions with naturally occurring colloidal particles (<2 μm) in soil has not been investigated. This study was developed to identify the interaction mechanisms between model bacterial pathogens and soil colloids as a function of cell type, natural organic matter (NOM), and solution chemistry. Specifically, batch adhesion experiments were conducted using NOM-present, NOM-stripped soil colloids, Streptococcus suis SC05 and Escherichia coli WH09 over a wide range of solution pH (4.0-9.0) and ionic strength (IS, 1-100 mM KCl). Cell characterization techniques, Freundlich isotherm, and Derjaguin-Landau-Verwey-Overbeek (DLVO) theory (sphere-sphere model) were utilized to quantitatively determine the interactions between cells and colloids. The adhesion coefficients (Kf) of S. suis SC05 to NOM-present and NOM-stripped soil colloids were significantly higher than E. coli WH09, respectively. Similarly, Kf values of S. suis SC05 and E. coli WH09 adhesion to NOM-stripped soil colloids were greater than those colloids with NOM-present, respectively, suggesting NOM inhibits bacterial adhesion. Cell adhesion to soil colloids declined with increasing pH and enhanced with rising IS (1-50 mM). Interaction energy calculations indicate these adhesion trends can be explained by DLVO-type forces, with S. suis SC05 and E. coli WH09 being weakly adhered in shallow secondary energy minima via polymer bridging and charge heterogeneity. S. suis SC05 adhesion decreased at higher IS 100 mM, which is attributed to the change of hydrophobic effect and steric repulsion resulted from the greater presence of extracellular polymeric substances (EPS) on S. suis SC05 surface as compared to E. coli WH09. Hence, pathogen adhesion to the colloidal material is determined by a combination of DLVO, charge heterogeneity, hydrophobic and polymer interactions as a function of solution chemistry.

Efficient bacterial capture with amino acid modified magnetic nanoparticles

Traditional chemical disinfectants are becoming increasingly defective due to the generation of carcinogenic disinfection byproducts and the emergence of antibiotic-resistant bacterial strains. Functionalized magnetic nanoparticles yet have shown great application potentials in water treatment processes especially for bacterial removal. In this study, three types of amino acids (arginine, lysine, and poly-l-lysine) functionalized Fe3O4 nanoparticles (Fe3O4@Arg, Fe3O4@Lys, and Fe3O4@PLL) were prepared through a facile and inexpensive two-step process. The amino acid modified Fe3O4 nanoparticles (Fe3O4@AA) showed rapid and efficient capture and removal properties for both Gram-positive Bacillus subtilis (B. subtilis) and Gram-negative Escherichia coli 15597 (E. coli). For both strains, more than 97% of bacteria (initial concentration of 1.5 × 10(7) CFU mL(-1)) could be captured by all three types of magnetic nanoparticles within 20 min. With E. coli as a model strain, Fe3O4@AA could remove more than 94% of cells from solutions over a broad pH range (from 4 to 10). Solution ionic strength did not affect cell capture efficiency. The co-presence of sulfate and nitrate in solutions did not affect the capture efficiency, whereas, the presence of phosphate and silicate slightly decreased the removal rate. However, around 90% and 80% of cells could be captured by Fe3O4@AA even at 10 mM of silicate and phosphate, respectively. Bacterial capture efficiencies were over 90% and 82% even in the present of 10 mg L(-1) of humic acid and alginate, respectively. Moreover, Fe3O4@AA nanoparticles exhibited good reusability, and greater than 90% of E. coli cells could be captured even in the fifth regeneration cycle. The results showed Fe3O4@AA fabricated in this study have great application potential for bacteria removal from water.

Influence of phosphate and solution pH on the mobility of ZnO nanoparticles in saturated sand

The mobility of nanoparticles (NPs) strongly depends on the chemical characterization of the environmental medium. However, the influence of phosphate on NPs mobility was ignored by scientists despite the serious phosphate contamination in natural environments. Hence, the influence of phosphate and solution pH on the mobility of zinc oxide nanoparticles (ZnO-NPs) was investigated in water-saturated sand representative of groundwater aquifers, which encompassed a range of P/Zn molar ratios (P/Zn: 0-4) and pH (4.8-10.0). The transport of ZnO-NPs was dramatically enhanced in the presence of phosphate, even at a low P/Zn molar ratio namely 0.25, and the retention of ZnO-NPs in the saturated sand decreased with increasing P/Zn molar ratio. Moreover, attachment efficiencies (α) and deposition rates (kd) of ZnO-NPs rapidly decreased with increasing P/Zn molar ratio. In contrast, the solution pH had negligible effects on ZnO-NP transport behavior under phosphate-abundant condition (P/Zn: 4). The distinct effects may be explained by the energy interaction between ZnO-NPs and sand surface under different conditions. Interestingly, under phosphate-abundant condition (P/Zn: 4), solution pH could strongly affect the transport of Zn(2+) in the water-saturated sand. Overall, this study outlines the importance of taking account of phosphate into risk assessment of NPs in the environment.

Impact of the exopolysaccharides Pel and Psl on the initial adhesion of Pseudomonas aeruginosa to sand

In this study, the impact of the exopolysaccharides Pel and Psl on the cell surface electron donor-electron acceptor (acid-base) properties and adhesion to quartz sand was investigated by using Pseudomonas aeruginosa PAO1 and its isogenic EPS-mutant strains Δpel, Δpsl and Δpel/Δpsl. The microbial adhesion to hydrocarbon (MATH) test and titration results showed that both Pel and Psl contribute to the surface hydrophobicity of the cell. The results of contact angle measurement, however, showed no correlation with the cell surface hydrophobicity measured by the MATH test and the titration method. Packed-bed column experiments indicated that the exopolysaccharides Pel and Psl are involved in the initial cell attachment to the sand surface and the extent of their impact is dependent on the ionic strength (IS) of the solution. Overall, the Δpel/Δpsl double mutant had the lowest adhesion coefficient to sand compared with the wild-type PAO1, the Δpel mutant and the Δpsl mutant. It is hypothesized that in addition to bacterial surface hydrophobicity and DLVO forces, other factors, eg steric repulsion caused by extracellular macromolecules, and cell surface appendages (flagella and pili) also contribute significantly to the interaction between the cell surface and a sand grain.