Colloids In Saturated Porous Media Research Articles

Soil colloids can significantly enhance spreading of polybromodiphenyl ethers in groundwater by serving as an effective carrier

Polybromodiphenyl ethers (PBDEs), the widely used flame retardants, are common contaminants in surface soils at e-waste recycling sites. The association of PBDEs with soil colloids has been observed, indicating the potential risk to groundwater due to colloid-facilitated transport. However, the extent to which soil colloids may enhance the spreading of PBDEs in groundwater is largely unknown. Herein, we report the co-transport of decabromodiphenyl ester (BDE-209) and soil colloids in saturated porous media. The colloids released from a soil sample collected at an e-waste recycling site in Tianjin, China, contain high concentration of PBDEs, with BDE-209 being the most abundant conger (320 ± 30 mg/kg). The colloids exhibit relatively high mobility in saturated sand columns, under conditions commonly observed in groundwater environments. Notably, under all the tested conditions (i.e., varying flow velocity, pH, ionic species and ionic strength), the mass of eluted BDE-209 correlates linearly with that of eluted soil colloids, even though the mobility of the colloids varies markedly depending on the specific hydrodynamic and solution chemistry conditions involved. Additionally, the mass of BDE-209 retained in the columns also correlates strongly with the mass of retained colloids. Apparently, the PBDEs remain bound to soil colloids during transport in porous media. Findings in this study indicate that soil colloids may significantly promote the transport of PBDEs in groundwater by serving as an effective carrier. This might be the reason why the highly insoluble and adsorptive PBDEs are found in groundwater at some PBDE-contaminated sites.

Cover Image, Volume 22, Issue 1

Vadose Zone JournalVolume 22, Issue 1 COVER IMAGEFree Access Cover Image, Volume 22, Issue 1 First published: 20 January 2023 https://doi.org/10.1002/vzj2.20248AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract On the cover: Scanning electron microscope image of a clay colloid. See Shi et al., “Impact of heavy metal cations on deposition and release of clay colloids in saturated porous media,” https://doi.org/10.1002/vzj2.20179. Volume22, Issue1January/February 2023 RelatedInformation

Experimental and numerical investigation of the effect of temporal variation in ionic strength on colloid retention and remobilization in saturated porous media

Temporal variations in the chemistry of infiltrating water into the subsurface are known to cause remobilization of colloids from the grain surfaces, thereby increasing the travel distance of the colloidal contaminants. Hence, it is essential to thoroughly understand the transport, deposition, and release mechanisms of colloids in the subsurface, through laboratory experiments and modeling. There are only a few experiments in which the chemistry of inflow water is changed rapidly during colloid transport. Also, although some models have been presented for simulating the effect of transient chemistry on the fate of colloids, there is no consensus in this regard, as the proposed models suffer from shortcomings. In this study, we systematically investigated the effect of temporal variations in ionic strength on the remobilization of deposited colloids in saturated porous media through laboratory column experiments and numerical modeling. Four sets of column experiments were performed, in which we injected carboxylate-modified latex colloids at a given ionic strength for a specified period. After breakthrough of colloids, the ionic strength of inflowing water was decreased in a stepwise manner to 0 mM (DI water). The initial ionic strength values of the four experiments were 100, 50, 25, and 10 mM. We observed partial release of deposited colloids after several steps of ionic strength decrease with significant release observed only when the ionic strength was reduced to below 10 mM. We also found that the fraction of released colloids decreased with increasing value of initial ionic strength of inflow water. We have developed a mathematical model incorporating a novel formulation for ionic strength-dependent deposition and release. The model is found to capture the colloid breakthrough curves reasonably well for all experiments with the same set of parameter values, except the one at the initial ionic strength of 25 mM.

Impact of heavy metal cations on deposition and release of clay colloids in saturated porous media

AbstractWhile the facilitated transport of heavy metals by colloids such as clay particles has been widely recognized, the influence of heavy metals on transport of the colloids has received much less attention. This study conducted saturated column experiments to systematically examine influence of multivalent heavy metal cations on transport of natural clay colloids and fullerene nC60 nanoparticles in glass bead porous media. Results showed that the presence of Cd2+ in the solution can cause more deposition of clay colloids in glass beads compared to Ca2+ at a given ionic strength (IS), and the subsequent release by IS reduction was less, demonstrating that the presence of Cd2+ increased the irreversibility of attachment. When the glass beads were initially adsorbed with Ca2+ and Cd2+, the release of clay colloids was significantly reduced due to strong cation bridge between the clay colloids and collector surfaces via the heavy metal cations. The release of clay colloids was significantly increased upon reduction of solution IS if the Na+ was used to exchange for the Ca2+ or Cd2+ before the IS reduction. Additional experiments showed that the nC60 nanoparticles that were deposited in the presence of Fe3+ and Cu2+ cannot be released by reduction of solution IS. However, the nanoparticle release occurred when the Ca2+ was used to exchange for the Fe3+ and Cu2+. Our work was the first to reveal the influence of heavy metal cations on the irreversibility of colloid attachment, and the findings have important implication to fabrication of functional nanomaterials for stabilization of heavy metals in soil and development of mathematic models for accurate prediction of cotransport of heavy metals and colloids in subsurface environments.

PAHs sorption to biochar colloids changes their mobility over time

Biochar is an efficient sorbent for polycyclic aromatic hydrocarbons (PAHs). However, little is known about PAHs sorption kinetics and the effect on the mobility of biochar colloids in saturated porous media. In this study, naphthalene (NAP) and phenanthrene (PHE) were chosen as typical PAHs. In different stages, different sorption sites of PAHs on biochar played a distinct role in affecting the transport of biochar colloids in saturated porous media. The biochar colloids showed less negative surface charge as the contact time between biochar colloids and PAHs increased from 0.017 h to 96 h, which led to the mobility of biochar colloids decreasing over time. But after 168 h contact time, the surface charge of biochar colloids became more negative again, and the inhibition effect of PAHs on biochar colloid transport was weakened. This was related to the sorption kinetics of PAHs on biochar colloids: (1) PAHs sorption onto outer biochar surface; and (2) adsorbed PAHs diffusion into internal biochar pores. PAHs sorption onto the outer biochar surface shielded the negative surface charge and then decreased the mobility of biochar colloids, and then the adsorbed PAHs diffusion into internal biochar pores increased their mobility again. Our results suggested that porous colloids-facilitated PAHs transport in porous media might be highly related to interaction time between PAHs and porous colloids.

Cotransport of uranyl carbonate loaded on amorphous colloidal silica and strip-shaped humic acid in saturated porous media: Behavior and mechanism

Uranyl carbonate (UC(VI)) is a stable form of uranyl (U(VI)) that widely coexists with amorphous colloidal silica (ACSi) and humic acid (HA) in carbonate-rich U-contaminated areas. In this context, the cotransport behavior and mechanism of UC(VI) with ACSi (100 mg L−1) and HA colloids in saturated porous media were systematically investigated. It was found that the ACSi and strip-shaped HA have a strong adsorption capacity for UC(VI), and their adsorption distribution coefficient (Kd) is 4–5 orders of magnitude higher than that of quartz sand (QS). In the ternary system, UC(VI) was mainly existing in the colloid-associated form at low UC(VI) concentration (4.2 × 10−6 M). Compared with the individual transport of UC(VI), the presence of ACSi and strip-shaped HA in the binary system promotes the transport of low-concentration UC(VI) (4.2 × 10−6 M) but shows a hindering effect when UC(VI) = 2.1 × 10−5 M. When ionic strength (IS) increased from 0 to 100 mM, the individual transport of UC(VI) and ACSi was weakened owing to the masking effect and the compression of the electrical double layer, respectively; this weakening effect is more pronounced in the binary (UC(VI)–ACSi) system. Notably, the transport of UC(VI) and ACSi in the ternary system is independent of the changes in IS due to the surface charge homogeneity strengthening the electrostatic repulsion between HA and QS. The Derjaguin–Landau–Verwey–Overbeek theory and retention profiles reveal the co-deposition mechanism of ACSi and UC(VI) in the column under different hydrochemical conditions. The nonequilibrium two-site model and the mathematical colloidal model successfully described the breakthrough data of UC(VI) and ACSi, respectively. These results are helpful for evaluating the pollution caused by UC(VI) migration in an environment rich in HA and formulating corresponding effective control strategies.

Accumulation of sulfamethazine and ciprofloxacin on grain surface decreases the transport of biochar colloids in saturated porous media

The increasing amount of antibiotics entering the environment through manure usage and sludge application from wastewater treatment plants (WWTP) attracts much concern due to their potential threat to ecological security and human health. When biochar, a soil and water amendment, is introduced into the soil for remediation, the antibiotics are usually co-present with the biochar colloids (BC) or pre-accumulated in soils. However, little is known about the effect of antibiotics on the behavior of BC. Column experiments were conducted at three different pH values to study the effect of sulfamethazine (SMT) or ciprofloxacin (CIP) on BC transport. Under certain conditions (co-present in the influent and pre-sorbed on quartz sand), large numbers of cation and zwitterion forms of the less mobile CIP at pH 5 and 7 led to less negatively-charged surface of BC and quartz sand, resulted in higher BC retention compared to the highly mobile SMT. The decrease in BC transport became more significant with a higher amount of SMT or CIP pre-sorption. Therefore, when biochar is applied into soils polluted by antibiotics, the pH-dependency and the loading amount of antibiotics in soil matrix should be paid attention to as they might affect the transport of BC and the related facilitated-contaminants transport.

Release of colloids in saturated porous media under transient hydro-chemical conditions: A pore-scale study

The deposition and consequent release of colloids pose a significant challenge to the environment, groundwater quality and human health. Subsurface soil contains numerous types of colloids that exhibit a diverse range of interaction favorability with the soil grains and their impact on release behavior remains unclear. The objective of this study was to investigate, at the pore-scale, the impact of colloid interaction favorability on colloid deposition and subsequent release in response to perturbations of flow rate and solution chemistry in saturated porous media. Pore-scale experiments were conducted using a micromodel that is geometrically representative of a real sand-stone rock. Favorable, medium-favorable and unfavorable colloids (i.e., repulsion absent, repulsion at long-range of separation distances and attraction absent with the micromodel surface, respectively) were deposited in the micromodel and then a series of colloid release experiments were conducted at different conditions including increasing the flow rate, decreasing the ionic strength and increasing the solution pH. Favorable colloids exhibited extensive deposition on the collector center where the flow streamlines are parallel to the collector surface, as adhesion forces overcome hydrodynamic forces. However, at medium and high ionic strength, deposition in Forward Flow Stagnation Zone (FFSZ) was dominant for unfavorable colloids as the hydrodynamic forces are negligible. Pore-scale images showed that, upon perturbations in flow rate and solution chemistry, colloids that were initially deposited on collector centers were more susceptible to release as compared to colloids that were initially deposited in FFSZs. The negligible hydrodynamic drag forces in FFSZ and deep primary minimum interaction at short separation distances were the major factors that hindered the release of colloids in FFSZ under transient hydro-chemical conditions. The intensity of colloid deposition and release decreases as the favorability of colloids decreases and as the ionic strength decrease for unfavorable colloids. This study provides a clear insight to the pore-scale colloid deposition and release mechanisms during transient hydro-chemical conditions that help in the modeling of environmental and engineering applications including managed aquifer recharge, groundwater contamination and wastewater treatment processes.

Enhanced role of humic acid on the transport of iron oxide colloids in saturated porous media under various solution chemistry conditions

Humic acid (HA), a naturally occurring and widely distributed natural organic matter in the environment, is one of the most important factors controlling the transport and fate of mineral colloids. This study examined the influence of HA (0–10 mg/L) on the transport of goethite and hematite colloids in saturated quartz sand-packed columns under various solution chemistry conditions (including ionic strength and pH). We found that the transport-enhancement effect of HA on goethite colloids was greater than that on hematite colloids because the steric hindrance and electrostatic repulsion between HA-bound goethite colloids and sand stronger than those between HA-bound hematite colloids and sand. Besides, Ca2+ could be a bridging agent between HA and sand; and consequently, enhanced the steric hindrance between iron oxide colloids and sand, which was an important mechanism for the transport-enhancement effect of HA in the presence of Ca2+. Interestingly, with increasing HA concentration from 3 to 10 mg/L, the transport-enhancement effect of HA at higher ionic strength (i.e., 20 mM NaCl) was to a larger extent than that at lower ionic strength (i.e., 5 mM NaCl); meanwhile, the enhancement effect of HA at lower pH (i.e., 5.0) was to a larger extent than that at higher pH (i.e., 9.0). The observations were attributable to the surface charge and aggregation properties of HA-bound iron oxide colloids affected by solution chemistry coupled with HA concentration. Furthermore, DLVO theory extended by steric repulsion (XDLVO) could explain the observed trend of colloid transport under the test conditions.

Coupled effect of flow velocity and structural heterogeneity on transport and release of kaolinite colloids in saturated porous media.

Understanding the behavior and fate of clay colloids in water-saturated porous media is critical to assess its environmental impact and potential risk since clay is commonly a carrier of many contaminants. Column experiments with four-packing configurations were designed to understand the coupled effects of column structural heterogeneity and the flow velocity on the transport and fate of kaolinite colloids in the saturated porous media. The results showed that the structural heterogeneity could have facilitated the transport of kaolinite colloids in saturated porous media. For the columns with strong heterogeneity, the preferential flow paths led to an early breakthrough of kaolinite. Only few kaolinite colloids were released with slow flow rate; however, the released peak concentration and release percentage of kaolinite colloids had further increased with the high flow velocity. In the layered column, there was significant kaolinite's retention at the interface where water passed from fine to coarse quartz sand. All results indicated that both flow rates and media characteristics played an important role in controlling kaolinite's fate and transport in porous media. A thorough understanding of these processes had an important significance for pollution control in subsurface natural environment where heterogeneous soil and variation in flow pattern are usually common.

Influence of chromate adsorption and reduction on transport and retention of biochar colloids in saturated porous media

Biochar (BC) has been proposed as an effective and promising adsorbent for the remediation of contaminated soils. While growing numbers of studies have reported the adsorption and immobilization of Cr(VI) on BC materials, very little is known about the mobility and retention of Cr-sorbed BC (Cr-BC) colloids, which may act as a carrier of the adsorbed Cr species, migrate along the soil profile and pose risk to deep subsurface environments. In this study, wheat straw BC samples prepared through pyrolysis at 450 °C and 600 °C (BC450 and BC600) were applied in Cr(VI) adsorption, and the resultant Cr-BC450 and Cr-BC600 colloids were further used to examine the effects of Cr(VI) adsorption amount, ionic strength and pyrolysis temperature on Cr-BC transport and retention in saturated porous media. Our results indicated that the reductive adsorption of Cr(VI) resulted in less negative zeta potential and enlarged hydrodynamic diameter of the BC colloids. For BC produced at a specific temperature, the mobility of Cr-BC colloids decreased with increasing Cr(VI) adsorption amount. For BC450 incubated with Cr(VI) (pH 2.0, 20 h), increasing ionic strength (1-5 mM NaCl) decreased the colloid mobility, and a transition from blocking to ripening occurred. In comparison to BC600, BC450 not only possessed higher Cr(VI) adsorption capacity, but also demonstrated higher mobility after Cr(VI) adsorption and reduction. The Derjaguin-Landau-Verwey-Overbeek theory and the two-site kinetic retention model were utilized to interpret the transport behaviors. Our findings suggested that the mobility of BC colloid was retarded after Cr(VI) adsorption and reduction, which could provide lasting benefits for Cr(VI) stabilization. However, the transport of Cr-BC cannot be neglected and should be considered during the application of BC in remediation of contaminated soil. This study for the first time revealed the importance of reductive adsorption in affecting the mobility of BC colloids. The transport of Cr species that associated with BC carriers should be considered during remediation of Cr(VI)-contaminated sites.

The role of particle size and molar ratio of Ca2+/Na+ in the retention and remobilization of colloids in saturated porous media

The paper examined the effects of colloid size and molar ratio of Ca2+/Na+ on the retention and release of colloids under transients in ionic strength (IS). Column experiments were conducted to elucidate the transport behavior of three-sized polystyrene latex microspheres (300 nm, 500 nm, and 1000 nm) by varied molar ratios of Ca2+/Na+. Colloid concentration breakthrough curves showed the retention of colloids enhanced with the increase of molar ratio and the decrease of colloid size, which can be explained by Derjaguin–Landau–Verwey–Overbeek theory and the existence of nanoscale surface roughness. When the IS of the eluting solution was reduced from 10 to 1 mM, and when the eluting solution changed to ultrapure water, peak concentrations of colloid release were observed. Colloid recovery rates during the injection of ultrapure water were low, and substantial proportion of colloid still retained in the column even after the injection of ultrapure water, which suggested a stronger influence caused by the primary minimum compared with the secondary minimum on the retention of colloid. A modified numerical model considering the processes of Langmuirian dynamics blocking associated with a conservative tracer equation was successfully developed to simulate the transport process of all the experiments.

Co-transport behavior of ammonium and colloids in saturated porous media under different hydrochemical conditions.

To investigate co-transport behavior of ammonium and colloids in saturated porous media under different hydrochemical conditions, NH4+ was selected as the target contaminant, and silicon and humic acid (HA) were selected as typical organic and inorganic colloids in groundwater. Column experiments were then conducted to investigate the transport of NH4+ colloids under various hydrochemical conditions. The results showed that because of the different properties of colloidal silicon and HA after combining with NH4+, the co-transport mechanism became significantly different. During transport by the NH4+-colloid system, colloidal silicon occupied the adsorption sites on the medium surface to promote the transport of NH4+, while humic acid (HA) increased the number of adsorption sites of the medium to hinder the transport of NH4+. The co-transport of NH4+ and colloids is closely related to hydrochemical conditions. In the presence of HA, competitive adsorption and morphological changes of HA caused NH4+ to be more likely to be transported at a higher ionic strength (IS = 0.05m, CaCl2) and alkalinity (pH = 9.3). In the presence of colloidal silicon, blocking action caused the facilitated transport to be dependent on higher ionic strength and acidity (pH = 4.5), causing the recovery of NH4+ to improve by 7.99%, 222.25% (stage 1), and 8.63%, respectively. Moreover, transport increases with the colloidal silicon concentrations of 20mg/L then declines at 40mg/L, demonstrating that increased concentrations will lead to blocking and particle aggregation, resulting in delayed release in the leaching stage. Graphical abstract.

Factors affecting the transport of petroleum colloids in saturated porous media

Batch and column experiments were performed to evaluate the responses of attachment and transport of petroleum colloids to the changes in solution chemistry conditions. We found that transport of petroleum colloids was inhibited with increasing the ionic strength or decreasing the pH. This observation can be explained by the increase in particle size of petroleum colloids and the reduction in electrostatic repulsion between colloids and grain surfaces. Meanwhile, the extent of transport inhibition by ionic strength depended on the type of porous media, that is, greater mobility of petroleum colloids could be found in sand rather than in soil at the same ionic strength. This can probably be ascribed to surface chemistry of soil minerals. In the case of divalent ion (e.g., Ca2+), the retention of petroleum colloids was more obvious in soil than in sand resulting from the complexing between the oxygen functional groups from both petroleum colloids and grains, i.e., petroleum colloids could bind to the sand/soil grains through Ca2+ bridging. Additionally, the presence of sodium dodecyl sulfate (SDS, selected as a model surfactant) significantly enhanced the mobility of petroleum colloids as a result of enhanced steric hindrance that remarkably inhibited aggregation of petroleum colloids. Also, a two-site transport model was used to fit the transport data to gain further insights on deposition mechanisms. These findings from this research are helpful to understand the transport and fate of petroleum colloids in subsurface systems.

Transport of biochar colloids in saturated porous media in the presence of humic substances or proteins

Application of biochar in the field has received considerable attention in recent years, but there is still little known about the fate and transport of biochar colloids (BCs) in the subsurface. Natural organic matter (NOM), which mainly consists of humic substance (HS) and proteins, is ubiquitous in the natural environment and its dissolved fraction is active and mobile. In this study, the transport of BCs in saturated porous media has been examined in the presence of two HS (humic and fulvic acids) and two proteins. Bull serum albumin (BSA) and Cytochrome c (Cyt) were selected to present the negatively and positively charged protein, respectively. At low and high salt concentration and different pH conditions, the transport of BCs was strongly promoted by HS. HS significantly increased the mobility of BCs in porous media under both low and high salt conditions due to the enhanced electrostatic repulsion and modification of surface roughness and charge heterogeneity. While BC mobility in porous media was suppressed by both BSA and Cyt in the low salt solution, the presence of BSA largely promoted and Cyt slightly enhanced the transport of BCs in high salt solutions. BSA and Cyt adsorption onto BC surface decreased the negative charge of BC and resulted in a less repulsive interaction in low salt solutions. In high salt solutions, the adsorbed BSA layers disaggregated BCs and reduced the strength of the interaction between BC and the sand. Adsorbed Cyt on BCs caused more attractive patches between BC and sand surface, and greater retention than BSA.

Influence of Biochar on Deposition and Release of Clay Colloids in Saturated Porous Media.

Although the potential application of biochar in soil remediation has been recognized, the effect of biochar on the transport of clay colloids, and accordingly the fate of colloid-associated contaminants, is unclear to date. This study conducted saturated column experiments to systematically examine transport of clay colloids in biochar-amended sand porous media in different electrolytes at different ionic strengths. The obtained breakthrough curves were simulated by the convection-diffusion equation, which included a first-order deposition and release terms. The deposition mechanisms were interpreted by calculating Derjaguin-Landau-Verwey-Overbeek interaction energies. A linear relationship between the simulated deposition rate or the attachment efficiency and the fraction of biochar was observed ( ≥ 0.91), indicating more favorable deposition in biochar than in sand. The interaction energy calculations show that the greater deposition in biochar occurs because the half-tube-like cavities on the biochar surfaces favor deposition in secondary minima and the nanoscale physical and chemical heterogeneities on the biochar surfaces increase deposition in primary minima. The deposited clay colloids in NaCl can be released by reduction of ionic strength, whereas the presence of a bivalent cation (Ca) results in irreversible deposition due to the formation of cation bridging between the colloids and biochar surfaces. The deposition and release of clay colloids on or from biochar surfaces not only change their mobilizations in the soil but also influence the efficiency of the biochar for removal of pollutants. Therefore, the influence of biochar on clay colloid transport must be considered before application of the biochar in soil remediation.

Effect of naphthalene on transport and retention of biochar colloids through saturated porous media

In this study, we conducted batch and column experiments to study the effect of naphthalene on retention and transport of biochar colloids in saturated porous media under different solution chemistry. The results of the batch adsorption experiments showed that the ionic strength and pH had negligible effect on naphthalene sorption onto biochar colloids. The results from the column experiments presented that biochar colloid transport decreased with increasing naphthalene concentrations, the presence of naphthalene in suspensions reduced the mobility of biochar colloids in porous media under various ionic strength and pH conditions. The reason was that nonpolar naphthalene was strongly attached on biochar colloid surface, and further lowered the zeta potential of biochar colloids due to charge-shielding. The findings from this study have important implications for biochar application in naphthalene contaminated soils.

Influence of graphene oxide on the transport and deposition behaviors of colloids in saturated porous media

The effects of graphene oxide (GO) on the transport and deposition behaviors of colloids with different sizes in packed quartz sand were investigated in both NaCl (10 and 50 mM) and CaCl2 solutions (1 and 5 mM) at pH 6. Fluorescent carboxylate-modified polystyrene latex microspheres (CMLs) with size ranging from 0.2 to 2 μm were utilized as model colloids. Both breakthrough curves and retained profiles of colloids in the presence and absence of GO in suspensions under all examined solution conditions were analyzed. The breakthrough curves of all three different-sized CMLs with GO were higher yet the retained profiles were lower than those without GO at both examined ionic strengths in NaCl solutions. The observation showed that GO increased the transport and decreased the deposition of all three different-sized CMLs in NaCl solutions. However, in CaCl2 solutions, opposite observation was achieved at two different ionic strength conditions. Specifically, the presence of GO increased the transport and decreased the deposition of all three different-sized CMLs in 1 mM CaCl2 solutions, whereas, it decreased the transport and increased the deposition of all three different-sized CMLs in 5 mM CaCl2 solutions. Comparison the breakthrough curves and retained profiles of CMLs versus those of GO yielded that the overall transport and deposition behaviors of all three different-sized CMLs with GO copresent in suspensions agreed well with the transport and deposition behaviors of GO under all examined conditions. The transport and deposition behaviors of CMLs in packed porous media clearly were controlled by those of GO under the conditions investigated in present study due to the adsorption of CMLs onto GO surfaces. Our study showed that once released into natural environment, GO would adsorb (interact with) different types of colloids and thus have significant influence on the fate and transport of colloids in porous media.

Impact of Redox Reactions on Colloid Transport in Saturated Porous Media: An Example of Ferrihydrite Colloids Transport in the Presence of Sulfide.

Transport of colloids in the subsurface is an important environmental process with most research interests centered on the transport in chemically stable conditions. While colloids can be formed under dynamic redox conditions, the impact of redox reactions on their transport is largely overlooked. Taking the redox reactions between ferrihydrite colloids and sulfide as an example, we investigated how and to what extent the redox reactions modulated the transport of ferrihydrite colloids in anoxic sand columns over a range of environmentally relevant conditions. Our results reveal that the presence of sulfide (7.8-46.9 μM) significantly decreased the breakthrough of ferrihydrite colloids in the sand column. The estimated travel distance of ferrihydrite colloids in the absence of sulfide was nearly 7-fold larger than that in the presence of 46.9 μM sulfide. The reduced breakthrough was primarily attributed to the reductive dissolution of ferrihydrite colloids by sulfide in parallel with formation of elemental sulfur (S(0)) particles from sulfide oxidation. Reductive dissolution decreased the total mass of ferrihydrite colloids, while the negatively charged S(0) decreased the overall zeta potential of ferrihydrite colloids by attaching onto their surfaces and thus enhanced their retention in the sand. Our findings provide novel insights into the critical role of redox reactions on the transport of redox-sensitive colloids in saturated porous media.

Cotransport of hydroxyapatite nanoparticles and hematite colloids in saturated porous media: Mechanistic insights from mathematical modeling and phosphate oxygen isotope fractionation

The fate and transport of individual type of engineered nanoparticles (ENPs) in porous media have been studied intensively and the corresponding mechanisms controlling ENPs transport and deposition are well-documented. However, investigations regarding the mobility of ENPs in the concurrent presence of another mobile colloidal phase such as naturally occurring colloids (colloid-mediated transport of ENPs) are largely lacking. Here, we investigated the cotransport and retention of engineered hydroxyapatite nanoparticles (HANPs) with naturally occurring hematite colloids in water-saturated sand columns under environmentally relevant transport conditions, i.e., pH, ionic strength (IS), and flow rate. Particularly, phosphate oxygen isotope fractionation of HANPs during cotransport was explored at various ISs and flow rates to examine the mechanisms controlling the isotope fractionation of HANPs in abiotic transport processes (physical transport).During cotransport, greater mobility of both HANPs and hematite occurred at higher pHs and flow rates, but at lower ISs. Intriguingly, the mobility of both HANPs and hematite was substantially lower during cotransport than the individual transport of either, attributed primarily to greater homo- and hetero-aggregation when both particles are copresent in the suspension. The shapes of breakthrough curves (BTCs) and retention profiles (RPs) during cotransport for both particles evolved from blocking to ripening with time and from flat to hyperexponential with depth, respectively, in response to decreases in pH and flow rate, and increases in IS. The blocking BTCs and RPs that are flat or hyperexponential can be well-approximated by a one-site kinetic attachment model. Conversely, a ripening model that incorporates attractive particle-particle interaction has to be employed to capture the ripening BTCs that are impacted by particle aggregation during cotransport.A small phosphate oxygen isotope fractionation (≤1.8‰) occurred among HANPs populations during cotransport responding to IS and flow rate changes. This fractionation is most likely a result of hetero-aggregation between hematite and HANPs that favors light phosphate isotopes (P16O4). This interpretation is further supported by the increase in isotope fractionation at higher ISs (i.e., greater aggregation). However, the fractionation was progressively erased by decreasing flow rate, ascribed to the reduced mass transfer of HANPs between the influent and effluent. Together our findings suggest that the cotransport and retention of HANPs and hematite colloids are highly sensitive to the considered physicochemical factors, and isotope tracing could serve as a promising tool to identify the sources and transport of phosphate-based NPs in complex subsurface environments due to insignificant transport-related isotope fractionation.