Quartz Sand Columns Research Articles

Biosurfactant-mediated transport of tetracycline antibiotics in saturated porous media: Combined effects of the chemical properties of contaminants and solution chemistry conditions

The mobility of tetracycline antibiotics (TCs) in saturated aquifers is possibly affected by the presence of biosurfactants, which are widespread in the aquatic/soil environments. This study investigated the mobility characteristics of various tetracyclines—specifically tetracycline (TC), oxytetracycline (OTC), and chlortetracycline (CTC)—within quartz sand columns in the presence of rhamnolipid, a common biosurfactant. Exogenous rhamnolipid significantly inhibited the transport of the three TCs over the pH range of 5.0–9.0 (e.g., the mass of retained TC, OTC, and CTC increased from 32.6 %, 26.9 %, and 39.2 % (in the absence rhamnolipid) to 39.4 %, 38.9 %, and 51.7 % (in the presence of rhamnolipid), respectively). This observation could be attributed to the bridging effects of this biosurfactant. Specifically, the hydrophilic head of rhamnolipid molecules is likely associated with the surfaces of sand grains through surface complexation and/or hydrogen bonding interactions. Accordingly, the hydrophobic moieties of the deposited rhamnolipid molecules (i.e., the aliphatic chains) interact with the hydrophobic groups of TCs molecules via hydrophobic interactions. Interestingly, the extent of the inhibitory effect on CTC mobility was greater than that on OTC and TC, which was related to the different hydrophobic characteristics of the three antibiotics. Furthermore, the inhibitory effect of rhamnolipid on the transport of TCs diminished as the pH of the background solution increased. This observation was attributed to the weakened bridging effects, resulting from the reduced deposition of the biosurfactant on the sand surfaces. Additionally, the cation-bridging mechanism involved in the retention of TCs in the addition of rhamnolipid when the background electrolyte was Ca2+ (i.e., Ca2+ ions served as bridging agents between the deposited rhamnolipid molecules and TCs). The insightful findings enhance our understanding of the critical roles of biosurfactants in influencing the environmental dynamics and ultimate fate of conventional antibiotic pollutants within groundwater systems.

Molecular characteristics of dissolved organic matter regulate the binding and migration of tungsten in porous media

Tungsten (W) is an emerging contaminant that poses potential risks to both the environment and human health. While dissolved organic matter (DOM) can significantly influence the W's environmental behavior in natural aquifers, the mechanisms by which DOM's molecular structure and functional group diversity impact W binding and migration remain unclear. Using molecular weight-fractionated soil and sediment DOM (<1 kDa, 1–100 kDa, and 100 kDa–0.45 μm), this study systematically investigated the relationship between DOM molecular characteristics and tungstate (WO42−) binding properties using multiple spectroscopic methods, including FTIR, fluorescence spectroscopy and XPS. The migration behavior of WO42− in porous media was also investigated through quartz sand column experiments. Results revealed that approximately 75 % of W was controlled by DOM, with over 50 % binding to low molecular weight DOM (<1 kDa). Tungsten bound to medium-high molecular weight DOM (1–100 kDa, >100 kDa) showed a greater propensity for retention, with the >100 kDa fractions demonstrating stronger selective binding to W, exhibiting distribution coefficients (Kmd) of 6.11 L/g and 10.69 L/g, respectively. Further analysis indicated that W primarily binds with aromatic rings, phenolic hydroxyls, polysaccharides, and carboxyl groups in DOM, potentially affecting DOM structural stability and consequently influencing W migration characteristics. Free W migration in quartz sand was primarily controlled by Langmuir monolayer adsorption, leading to local enrichment (Da = 6.83, Rd = 86.98). When bound to DOM, W's migration ability significantly increased (Rd = 8–10), with adsorption shifting to a Freundlich multilayer model, primarily controlled by convective transport (Npe = 27–62> > 1.96), while adsorption effects weakened (Da ≈ 1). This study, for the first time, systematically reveals the regulatory mechanisms of DOM molecular characteristics on tungsten's environmental behavior. It offers crucial parameter support for constructing tungsten migration models and provides important guidance for tungsten pollution risk assessment and remediation strategies.

Effects of Mono- and Multicomponent Nonaqueous-Phase Liquid on the Migration and Retention of Pollutant-degrading Bacteria in Porous Media

The successful implementation of in-situ bioremediation of nonaqueous-phase liquid (NAPL) contamination in soil-groundwater systems is greatly influenced by the migration performance of NAPL-degrading bacteria. However, the impact and mechanisms of NAPL on the migration/retention of pollutant-degrading bacteria remain unclear. This study investigated the migration/retention performance of A. lwoffii U1091, a strain capable of degrading diesel while producing surfactants, in porous media without and with the presence of mono- and multicomponent NAPL (n-dodecane and diesel) under environmentally relevant conditions. The results showed that under all examined conditions (5 and 50 mM NaCl solution at flow rates of 4 and 8 m/d), the presence of n-dodecane/diesel in porous media could reduce the migration and enhance retention of A. lwoffii in quartz sand columns. Moreover, comparing with mutlicomponent NAPLs of n-dodecane, the monocomponent NAPLs (diesel) exhibited a greater reduction effect on the retention of A. lwoffii in porous media. Through systemically investigating the potential mechanisms via tracer experiment, visible chamber experiment, and theoretical calculation, we found that the reduction in porosity, repulsive forces and movement speeds, the presence of stagnant flow zones in porous media, particularly the biosurfactants generated by A. lwoffii contributed to the enhanced retention of bacteria in NAPL-contaminated porous media. Moreover, owing to presence of the greater amount of hydrophilic components in diesel than in n-dodecane, the available binding sites for the adsorption of bacteria were lower in diesel, resulting in the slightly decreased retention of A. lwoffii in porous media containing diesel than n-dodecane. This study demonstrated that comparing with porous media without NAPL contamination, the retention of strain capable of degrading NAPL in porous media with NAPL contamination was enhanced, beneficial for the subsequent biodegradation of NAPL.

Transport and retention of COVID-19-related antiviral drugs in saturated porous media under various hydrochemical conditions

Antiviral drugs have garnered considerable attention, particularly in the global battle against the COVID-19 pandemic, amid heightened concerns regarding environmentally acquired antiviral resistance. A comprehensive understanding of their transport in subsurface environments is imperative for accurately predicting their environmental fate and risks. This study investigated the mobility and retention characteristics of six COVID-19 antiviral drugs in saturated quartz sand columns. Results showed that the mobility of the drugs was primarily contingent on their hydrophobicity, with ribavirin and favipiravir exhibiting the highest transportability, while arbidol displaying the greatest retention. The transport characteristics of ribavirin and favipiravir remained largely unaffected by pH, whereas the retention of the other four antivirals remained consistently minimal under alkaline conditions. Elevating ionic strength marginally facilitated the transport of these antivirals, while the presence of Ca2+ notably enhanced their retention in quartz sand compared to Na+. Ribavirin and remdesivir warrant particular attention due to their relatively high transportability and propensity for environmentally acquired antiviral resistance. These findings contribute to an enhanced understanding of the leachate potential and transport of COVID-19-related antivirals in sandy porous media, furnishing fundamental data for predicting their environmental fate and associated risks.

Humic acid enhances the co-transport of colloids and phosphorus in saturated porous media

Phosphorus (P) has been widely recognized as a substance that is difficult to transport due to its tendency to become easily fixed in the soil. However, many reports demonstrate that groundwater P pollution is rising in humus-rich areas. Research is urgently needed to confirm (or reject) the hypothesis that increased P pollution is related to humus, as there is currently limited quantitative research on this topic. In this study, we conducted a series of batch equilibrium adsorption-desorption experiments and column experiments to quantify the effects of montmorillonite colloids (MCs) and humic acids (HCs, the main components of humus) on the P transport behavior. The results indicate that P's adsorption and desorption behavior on MCs can be well simulated using the Langmuir and Temkin models (R2 > 0.91). Compared to the non-HC treatments, HCs significantly increased MCs' P adsorption and desorption capacity 5.18 and 7.21 times, respectively. Moreover, HCs facilitated the transport ability of the MC-P mixture through the saturated quartz sand column. In a 0.1 M NaCl solution, the MC-P mixture is nearly completely adsorbed on the surface of quartz sand, with a penetration rate of only 0.5%. In contrast, the HC-MC-P mixture can evidently penetrate further at a rate of 26.1%. The transport parameters fitted using HYDRUS-1D further indicated that the presence of humic acids significantly decreased the deposition coefficients of colloids, thereby enhancing the co-transport of colloids and P through the quartz sand porous medium. The potential mechanism of P pollution in humus-rich areas is likely enhanced by the formation of an HC-colloid-P mixture, which greatly increases the adsorption amount of P on colloids and enhances the electrostatic and spatial repulsion between colloids as well as between colloids and quartz sand. It reduces the aggregation and adsorption of colloids, ultimately transferring P into groundwater through colloid-facilitated co-transport. The findings of this study clarified the relationship between the transport of P, colloids, and HCs, which provides a theoretical basis for explaining the P pollution mechanism in humus-rich areas.

Combination of magnesium modified biochar and iron oxides down-regulates phosphates transport in porous media

The practical effects of biochar on soil phosphate (P) transport are significantly affected by P adsorption capacity of biochar itself and its interaction process with iron oxides. Here, the ability of pyrolysis to shape P adsorption capacity of biochar was investigated, while the interaction of biochar and iron oxides on P transport was explored with quartz sand columns and numerical modeling. Adsorption experiments showed that pyrolysis increased P adsorption capacity of Mg-modified biochar (∼248 mg g−1) through enriching surface morphology and pore structures, optimizing composition of functional groups, and forming MgO crystals. It also caused that P adsorption at MgBC-700°C was mainly controlled by surface precipitation, electrostatic attraction and pore interception. Simultaneously, the desorption of P was sensitive to pH changes and with low-stability. Transport experiments showed that iron oxides reduced P transport, shown to be goethite > hematite (P leaching: 69.59 % vs 98.29 %). Further, MgBC-700°C continued to decrease P transport from 75.56 % to 31.50 %, and the effect of goethite was more effective, with ligand reaction playing an important role. This process was influenced by cation types (Ca2+ and K+), ionic strength (1 mM and 50 mM), and pH (5, 7 and 9) through changing available retention sites of columns. Furthermore, since P adsorption mechanisms were different in the presence of MgBC-700°C and goethite than MgBC-700°C only, the changes of pH could no longer cause the release of P, but surprisingly, citric acid and oxalic acid could activate and slowly release the retained P via chelation, ligand exchange, and solubilization, which suggested that MgBC-700°C, as an environmentally-friendly P adsorption and release material, has great potential in reducing P loss and facilitating P utilization.

Characteristic study of biological CaCO3-supported nanoscale zero-valent iron: stability and migration performance

ABSTRACT nZVI has attracted much attention in the remediation of contaminated soil and groundwater, but the application is limited due to its aggregation, poor stability, and weak migration performance. The biological CaCO3 was used as the carrier material to support nZVI and solved the nZVI agglomeration, which had the advantages of biological carbon fixation and green environmental protection. Meanwhile, the distribution of nZVI was characterised by SEM-EDS and TEM carefully. Subsequently, the dispersion stability of bare nZVI and CaCO3@nZVI composite was studied by the settlement experiment and Zeta potential. Sand column and elution experiments were conducted to study the migration performance of different materials in porous media, and the adhesion coefficient and maximum migration distances of different materials in sand columns were explored. SEM-EDS and TEM results showed that nZVI could be uniformly distributed on the surface of biological CaCO3. Compared with bare nZVI, CaCO3@nZVI composite suspension had better stability and higher absolute value of Zeta potential. The migration performance of nZVI was poor, while CaCO3@nZVI composite could penetrate the sand column and have good migration performance. What’s more, the elution rates of bare nZVI and CaCO3@nZVI composite in quartz sand columns were 5.8% and 51.6%, and the maximum migration distances were 0.193 and 0.885 m, respectively. In summary, this paper studies the stability and migration performance of bare nZVI and CaCO3@nZVI composite, providing the experimental and theoretical support for the application of CaCO3@nZVI composite, which is conducive to promoting the development of green remediation functional materials.

Reporting nanoparticle tracers: Validation of performance in flow-through experiments simulating reservoir conditions

Characterization and monitoring of reservoir properties and conditions are key problems in the exploitation of subsurface aquifers and reservoirs, with tracer tests being an important tool that provides valuable insights into groundwater dynamics. The Reporting Nanoparticle Tracers (RNTs) approach was recently presented with the aim of expanding the scope of measureable parameters and the potential benefits of tracer tests in comparison to tests performed with traditional tracers. However, successful implementation of the concept depends on the ability of the nanoparticle tracers to exhibit a stable dispersion that facilitates sufficiently high recovery rates to allow for meaningful analysis. As a step forward toward the implementation of the approach, we used flow‑through studies with packed-bed quartz sand columns to compare the transport and retention properties of the RNTs with those of prevalent conservative molecular tracers and to show the feasibility of temperature detection. In the main experiments, the RNTs showed recovery rates around 80%, outperformed only by uranine (95 %) and surpassing eosin and sulforhodamine G (15 % and 50 % respectively). Thermally-activated RNTs could also be clearly differentiated from non-activated ones by a signal ratio difference of 50 %. These results validate the feasibility of application, especially considering the modularity of the nanoparticles, which enables adjustment of the RNTs to the hydrochemical conditions prevalent in the tested aquifer.

Influence of Concentration, Surface Charge, and Natural Water Components on the Transport and Adsorption of Polystyrene Nanoplastics in Sand Columns.

Information about the influence of surface charges on nanoplastics (NPLs) transport in porous media, the influence of NPL concentrations on porous media retention capacities, and changes in porous media adsorption capacities in the presence of natural water components are still scarce. In this study, laboratory column experiments are conducted to investigate the transport behavior of positively charged amidine polystyrene (PS) latex NPLs and negatively charged sulfate PS latex NPLs in quartz sand columns saturated with ultrapure water and Geneva Lake water, respectively. Results obtained for ultrapure water show that amidine PS latex NPLs have more affinity for negatively charged sand surfaces than sulfate PS latex NPLs because of the presence of attractive electrical forces. As for the Geneva Lake water, under natural conditions, both NPL types and sand are negatively charged. Therefore, the presence of repulsion forces reduces NPL's affinity for sand surfaces. The calculated adsorption capacities of sand grains for the removal of both types of NPLs from both types of water are oscillating around 0.008 and 0.004 mg g-1 for NPL concentrations of 100 and 500 mg L-1, respectively. SEM micrography shows individual NPLs or aggregates attached to the sand and confirms the limited role of the adsorption process in NPL retention. The important NPL retention, especially in the case of negatively charged NPLs, in Geneva Lake water-saturated columns is related to heteroaggregate formation and their further straining inside narrow pores. The presence of DOM and metal cations is then crucial to trigger the aggregation process and NPL retention.

Cu(II) inhibited the transport of tetracycline in porous media: role of complexation.

Tetracycline (TC) and Cu(II) coexist commonly in various waters, which may infiltrate into the subterranean environment through runoff and leaching, resulting in substantial ecological risks. However, the underlying mechanisms why Cu(II) affects the transport of TC in porous media remain to be further explored and supported by more evidence, especially the role of complexation. In this study, the transport of TC with coexisting Cu(II) was comprehensively explored with column experiments and density functional theory (DFT) calculation. At natural environmental concentrations, Cu(II) significantly inhibited the transport of TC in the quartz sand column. Cu(II) augmented the retention of TC in the column mainly via electrostatic force and complexation. The interaction between TC and TC-Cu complexes on the surface of SiO2 was investigated with first-principles calculations for the first time. There were strong van der Waals forces and coordination bonds on the surface of complexes and SiO2, leading to higher adsorption energy than that of TC and inhibiting its penetration. This study offers novel insights and theoretical framework for the transport of antibiotics in the presence of metal ions to better understand the fate of antibiotics in nature.

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

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

Cotransport of Tl(I) and kaolinite colloids in water-saturated porous media: Two systems with different components

Media components and colloid particles may affect Tl(I) transport in water-saturated porous media. It is significant to study the cotransport of Tl(I) and kaolinite in two systems with different components under different ISs, pH and kaolinite concentrations. The results of experiments conducted in quartz sand columns indicated that Tl(I) transport was promoted under low pH and high IS, due to competition between Tl(I) and cations (Na+ and H+) for adsorption sites. The effect of kaolinite on Tl(I) transport changed from inhibition to promotion with increasing kaolinite concentration and decreasing pH, because colloidal particles and H+ occupy more adsorption sites and competed with Tl(I). In the mica-quartz sand column, Tl(I) transport was promoted at low pH, which was consistent with the results of the quartz sand column experiments. However, Tl(I) transport was inhibited under high IS, due to the adsorption of heavy metals caused by the unique structure of mica. Tl(I) transport was promoted when kaolinite was present, which occurred mainly because of the adsorption of Tl(I) by kaolinite colloids and competitive adsorption between Tl(I) and colloids. The transport curves were fitted by two-site nonequilibrium sorption model, two-site kinetic attachment/detachment model, and colloid-facilitated solute transport model, respectively. In this study, the transport process of Tl(I) and kaolinite in porous media with different components was explored in depth, and the role of media in Tl(I) transport was investigated using novel approaches. The results facilitate the predicting of Tl(I) transport in underground environments and support remediation efforts for Tl contaminated soil and groundwater.

A model to quantify permeability in solute mixing precipitation porous media

The effects of the concentration and flow velocity of the solution on calcite precipitation process in porous media and its permeability were investigated with a combined experimental and modeling approach. In the column precipitation experiments, calcite precipitation was mainly distributed at the inlet of experimental column and quickly decreases along the flow direction, and precipitates completely wrapped the quartz sands at the inlet, and the gaps between the quartz sands were filled. The permeability coefficient of experimental column decreases quickly in the early stage and then decreases slowly. The calcite precipitates in the experimental column increased with the saturation index and gradually stretched along the flow direction with the flow velocity increase. The precipitates were distributed within 1.0 cm of the experimental column inlet. In the model, based on the classical Kozeny-Carman equation and the principle of series superposition, a coupled model of mineral precipitation and permeability coefficient in porous media was developed, the model was verified, and the parameter sensitivity was analyzed. It revealed that the mineral precipitation at the inlet interface of experimental column was the dominant factor determining the permeability coefficient of porous media. When considering solution conditions, the saturation indices SI and flow velocity V were the main factors affecting calcite precipitation and permeability coefficient in porous media. When the solution conditions were not considered, the quartz sand diameter ds, the length of quartz sand column L and porosityφ aiming at mineral precipitation had higher sensitivity, and the porosity ϕ aiming at permeability coefficient had higher sensitivity.

Biochar and zero-valent iron sand filtration simultaneously removes contaminants of emerging concern and Escherichia coli from wastewater effluent

Advanced treated municipal wastewater is an important alternative water source for agricultural irrigation. However, the possible persistence of chemical and microbiological contaminants in these waters raise potential safety concerns with regard to reusing treated wastewater for food crop irrigation. Two low-cost and environmentally-friendly filter media, biochar (BC) and zero-valent iron (ZVI), have attracted great interest in terms of treating reused water. Here, we evaluated the efficacy of BC-, nanosilver-amended biochar- (Ag-BC) and ZVI-sand filters, in reducing contaminants of emerging concern (CECs), Escherichia coli (E. coli) and total bacterial diversity from wastewater effluent. Six experiments were conducted with control quartz sand and sand columns containing BC, Ag-BC, ZVI, BC with ZVI, or Ag-BC with ZVI. After filtration, Ag-BC, ZVI, BC with ZVI and Ag-BC with ZVI demonstrated more than 90% (> 1 log) removal of E. coli from wastewater samples, while BC, Ag-BC, BC with ZVI and Ag-BC with ZVI also demonstrated efficient removal of tested CECs. Lower bacterial diversity was also observed after filtration; however, differences were marginally significant. In addition, significantly (p < 0.05) higher bacterial diversity was observed in wastewater samples collected during warmer versus colder months. Leaching of silver ions occurred from Ag-BC columns; however, this was prevented through the addition of ZVI. In conclusion, our data suggest that the BC with ZVI and Ag-BC with ZVI sand filters, which demonstrated more than 99% removal of both CECs and E. coli without silver ion release, may be effective, low-cost options for decentralized treatment of reused wastewater.Graphical

Cotransport of different electrically charged microplastics with PFOA in saturated porous media

The fate and transport behavior of microplastics (MPs), emerging colloidal contaminant ubiquitous in natural environments, would be greatly affected by other copresent pollutants. PFOA (emerging surfactant pollutant) would interact with MPs after encounter with them in natural environments, which could alter the transport behavior of both pollutants. Relevant knowledge is still lacking, affecting accurate prediction the fate and distribution of these two emerging contaminants in natural porous media. The cotransport behavior of different surface charged MPs (negatively/positively charged, CMPs/AMPs) with PFOA (three concentrations ranging from 0.1 to 10 mg/L) in porous media in both 10 and 50 mM NaCl solutions thus was investigated in the present study. We found PFOA inhibited CMPs transport in porous media, while enhanced AMPs transport. The mechanisms leading to the altered transport of CMPs/AMPs caused by PFOA were found to be different. The decreased electrostatic repulsion between CMPs-sand induced by the decreased CMPs negative zeta potentials via the adsorption of PFOA led to the inhibited transport of CMPs in CMPs-PFOA suspension. The enhanced electrostatic repulsion between AMPs-sand due to the decreased positive charge of AMPs via the adsorption of PFOA together with steric repulsion induced by suspended PFOA resulted in the increased transport of AMPs in AMPs-PFOA suspension. Meanwhile, we found that the adsorption onto MPs surfaces also impacted the transport of PFOA. Due to the lower mobility of MPs than PFOA, the presence of MPs despite their surface charge decreased the transport of PFOA of all examined concentrations in quartz sand columns. This study demonstrates that when MPs and PFOA are co-existing in environments, their interaction with each other will alter the fate and transport behavior of both pollutants in porous media and the alteration is highly correlated with the amount of PFOA adsorbed onto MPs and original surface properties of MPs.

Mechanism of reactive co-transport of Fe2+ and antibiotics in hyporheic zone simulated by quartz sand column

Iron redox is often coupled with the degradation and conversion of co-existing organic pollutants, however, their co-transport behavior in hyporheic zone is not clear. In this study, the characteristics and mechanism of reactive co-transport of Fe2+ and antibiotics during groundwater discharge were revealed by quartz sand column simulation. It was found that Fe2+ inhibited the transport of three antibiotics sulfamethoxazole (SMX), ofloxacin (OFL), and oxytetracycline (OTC) in the quartz column, and the average inhibitory intensity was OTC (∼80%) > OFL (∼20%) > SMX (∼2%). Meanwhile, the content of dissolved oxygen in the column decreased from more than 7 mg/L to 3 mg/L. Fe2+ transport in the column was promoted with the increase of OTC and OFL concentrations (2–50 mg/L), as Fe2+-OTC and Fe2+-OFL complexes inhibited the formation of lepidocrocite and induced Fe2+/ Fe3+ cycling during co-transport of Fe2+ and OTC. In contrast, the interaction between SMX and Fe2+ was not obvious due to the electrostatic repulsion. Due to the prevalence of Fe2+ and antibiotics in groundwater system (especially with high antibiotic contamination), this study provides a new perspective on the potential environmental impacts of Fe and pollutants co-transport during groundwater discharge.

Co-transport of polystyrene microplastics and kaolinite colloids in goethite-coated quartz sand: Joint effects of heteropolymerization and surface charge modification

This study investigated the transport behavior of polystyrene microplastics (MPs) in saturated quartz sand and goethite-coated sand in the presence of coexisting kaolinite colloids. Column experiments were conducted under a wide range of solution chemistry conditions, including pH levels of 6.0, 7.0, and 9.0, as well as background Na+ concentrations of 5 mM and 25 mM. We found that: (1) The individual transport of MPs in porous media diminished both with increasing background ion strength and decreasing pH, and its transport ability was significantly dominated by the interactions between MPs and porous media rather than the interplay between MPs, which has been further corroborated by the aggregation stability experiments of MPs particles. (2) MPs had a much lower ability to move through goethite-coated sand columns than quartz sand columns. This is because goethite coating reduces the repulsion energy barriers between porous media and MPs. The increased specific surface area and surface complexity of sand columns after goethite coating should also account for this difference. (3) MPs transport would be subjected to the differentiated impact of co-transported kaolinite colloids in the two types of porous media. The promotion effect of kaolinite colloid on MPs' transport capacity is not significantly affected by background ionic strength changes when quartz sand is served as the porous medium; however, the promotion effect is highly correlated with the background ionic strength when goethite-coated sand is served as the porous medium. In comparison with low background ionic strength conditions, kaolinite colloids under high background ionic strength conditions significantly facilitated MPs transport. This is mainly because under high background ionic conditions, kaolinite colloids are more likely to be deposited on the surface of goethite-covered sand, competing with MPs for the limited deposition sites. The extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory is applicable to describe the transport behavior of MPs.

Role of tailing colloid from vanadium-titanium magnetite in the adsorption and cotransport with vanadium.

The geochemical cycling of vanadium (V) in mining areas has attracted much attention. However, little knowledge was about the effects of tailing colloids on the fate and transport of vanadium in tailing reservoirs which was ignored before. This study investigated the interactions of tailing colloids from vanadium-titanium magnetite with vanadium. Colloid characterization, tailing leaching, adsorption, and column experiments of single and cotransport of tailing colloid with V were conducted. Results show that 98.08% V in the vanadium-titanium magnetite tailing was in the residual state with limited leachable V under various conditions. The adsorption of V to the tailing colloid was via electrostatic attraction and surface complexation on the heterogeneously distributed sorption sites on the colloid surface. The adsorption control step was the diffusion of V into the tailing colloid pores. The increase in pH and the decrease in ionic strength (IS) promoted the single transport of tailing colloid and V in quartz sand columns. In cotransport scenarios, V promoted the transport of tailing colloids via the surface coating effect. In contrast, the transport of V was retarded by the adsorbed tailing colloid on the quartz sand surface. The pre-adsorbed V in the column enhanced the subsequent transport of tailing colloids by electrical repulsion, while the pre-adsorbed tailing colloids facilitated the subsequent transport of V via cotransport of the released colloids with V. The high mobility of the tailing colloid and V and their cotransport in the porous media highly demonstrated the potential V pollution pathways that need to be taken into account.

Multi-targeted removal of coexisted antibiotics in water by the synergies of radical and non-radical pathways in PMS activation

Various antibiotics often coexist in contaminated water which can pose a threat to the ecological environment and human health. Conventional water treatment technologies generally have low efficiency to simultaneously remove multiple antibiotics. To address this issue, a bimetal composite corncob biochar catalyst (Fe-MOF-CC@MoS2) was explored for the peroxymonosulfate (PMS) activation to degrade various antibiotics simultaneously in this study. The Fe-MOF-CC@MoS2 was prepared by a simple green hydrothermal carbonization method and characterized. The degradation performance of Fe-MOF-CC@MoS2 for coexisted antibiotics from the aqueous phase was evaluated. The results indicated that the Fe-MOF-CC@MoS2/PMS system exhibited a superior degradation efficiency of antibiotics compared to Fe-MOF-CC/PMS system, due to the acceleration of the Fe2+/Fe3+ cycling by active Mo4+. Fe-MOF-CC@MoS2/PMS system could simultaneously remove four different types of antibiotics, and the removal efficiencies of tetracycline hydrochloride, ciprofloxacin, nitrofurantoin, and sulfamethoxazole were 96.51 %, 92.30 %, 88.96 %, and 80.76 %, respectively. Electron spin resonance and quenching experiments demonstrated that the cooperation of radical (SO4−, OH and O2−) and non-radical (1O2) in the Fe-MOF-CC@MoS2/PMS system led to 14 times higher degradation rate constant than that in Fe-MOF-CC/PMS system. In addition, the Fe-MOF-CC@MoS2 presented high removal efficiency (76.54 %) for the antibiotics after five cycles. Moreover, the toxicity of contaminated water after degradation was significantly reduced through the growth of Vigna radiata. Finally, the flowing experiment using Fe-MOF-CC@MoS2/quartz sand column proved that Fe-MOF-CC@MoS2 could effectively activate PMS and remediate water contaminated with four coexisted antibiotics. This study may provide a promising alternative for the multi-targeted removal of coexisted antibiotics from real water, meanwhile recovering resources.

Transport of montmorillonite colloid in unsaturated packed column: The combined effects of sand grain size, flow rate and colloid concentration

The transport of colloid in unsaturated porous media affects the migration of contaminants and thus is closely related to groundwater resources protection. To figure out the combined effects of grain size, colloid concentration and injection flow rate on montmorillonite colloid transport characteristics in unsaturated quartz sand, a total of 27 sets of column experiments were conducted with three kinds of quartz sand (20, 40, 60 mesh), three flow rates (1.98, 3.96, 5.94 cm3/min) and three colloid concentrations (300, 600, 900 mg/L), using three packed columns with the inner diameter of 11 cm and the height of 40 cm. The experimental results showed that the transport of Na-montmorillonite colloid particles in the unsaturated quartz sand columnoccurred with significant retention. In the 20 mesh quartz sand column, the average peak values of the penetration curves for low and high concentration colloidal solutions of 300 and 900 mg/L increased by 44% and 27%, respectively, as the flow rate increased from 1.98 to 5.94 cm3/min. The average peak value of the colloidal solution with concentration 300 mg/L increased 17% more than that of the colloidal solution with 900 mg/L for increasing flow rate. When the injection flow rate of the colloidal solution was increased from 1.98 to 5.94 cm3/min, the effect of flow rate on colloidal transport was the most obvious, followed by the effect of media particle size, while the effect of colloidal solution concentration was the least. The calculation based on the total potential energy of Derjaguin-Landau-Verwey-Overbeek (DLVO) and collision efficiency further explained the retention of colloids in unsaturated porous media.