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

Antagonistic Effects of Humic Acid and Iron Oxyhydroxide Grain-Coating on Biochar Nanoparticle Transport in Saturated Sand

Biochar land application may result in multiple agronomic and environmental benefits (e.g., carbon sequestration, improving soil quality, and immobilizing environmental contaminants). However, our understanding of biochar particle transport is largely unknown in natural environments with significant heterogeneity in solid (e.g., patches of iron oxyhydroxide coating) and solution chemistry (e.g., the presence of natural organic matter), which represents a critical knowledge gap in assessing the environmental impact of biochar land application. Transport and retention kinetics of nanoparticles (NPs) from wheat straw biochars produced at two pyrolysis temperatures (i.e., 350 and 550 °C) were investigated in water-saturated sand columns at environmentally relevant concentrations of dissolved humic acid (HA, 0, 1, 5, and 10 mg L^–1) and fractional surface coverage of iron oxyhydroxide coatings on sand grains (ω, 0.16, 0.28, and 0.40). Transport of biochar NPs increased with increasing HA concentration, largely because of enhanced repulsive interaction energy between biochar NPs and sand grains. Conversely, transport of biochar NPs decreased significantly with increasing ω due to enhanced electrostatic attraction between negatively charged biochar NPs and positively charged iron oxyhydroxides. At a given ω of 0.28, biochar NPs were less retained with increasing HA concentration due to increased electrosteric repulsion between biochar NPs and sand grains. Experimental breakthrough curves and retention profiles were well described using a two-site kinetic retention model that accounted for Langmuirian blocking or random sequential adsorption at one site. Consistent with the blocking effect, the often observed flat retention profiles stemmed from decreased retention rate and/or maximum retention capacity at a higher HA concentration or smaller ω. The antagonistic effects of HA and iron oxyhydroxide grain-coating imparted on the mobility of biochar NPs suggest that biochar colloid transport potential will be dependent on competitive influences exerted by a number of environmental factors (e.g., natural organic matter and metal oxides)

Transport of Biochar Particles in Saturated Granular Media: Effects of Pyrolysis Temperature and Particle Size

Land application of biochar is increasingly being considered for potential agronomic and environmental benefits, e.g., enhancing carbon sequestration, nutrient retention, water holding capacity, and crop productivity; and reducing greenhouse gas emissions and bioavailability of environmental contaminants. However, little is known about the transport of biochar particles in the aqueous environment, which represents a critical knowledge gap because biochar particles can facilitate the transport of adsorbed contaminants. In this study, column experiments were conducted to investigate biochar particle transport and retention in water-saturated quartz sand. Specific factors considered included biochar feedstocks (wheat straw and pine needle), pyrolysis temperature (350 and 550 °C), and particle size (micrometer-particle (MP) and nanoparticle (NP)). Greater mobility was observed for the biochars of lower pyrolysis temperatures and smaller particle sizes. Extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) calculations that considered measured zeta potentials and Lewis acid–base interactions were used to better understand the influence of pyrolysis temperature on biochars particle transport. Most biochars exhibited attractive acid–base interactions that impeded their transport, whereas the biochar with the greatest mobility had repulsive acid–base interaction. Nonetheless, greater retention of the MPs than that of the NPs was in contrast with the XDLVO predictions. Straining and biochar surface charge heterogeneity were found to enhance the retention of biochar MPs, but played an insignificant role in the biochar NP retention. Experimental breakthrough curves and retention profiles were well-described using a two-site kinetic retention model that accounted for depth-dependent retention at one site. Modeled first-order retention coefficients on both sites 1 and 2 increased with increasing pyrolysis temperature and particle size

Effect of Size-Selective Retention on the Cotransport of Hydroxyapatite and Goethite Nanoparticles in Saturated Porous Media

Attributable to their nanoscale size and slow phosphorus (P) release kinetics, hydroxyapatite nanoparticles (HANPs) are increasingly advocated as a promising P nanofertilizer. Additionally, HANPs have been extensively used to remediate soils, groundwater, and nuclear wastewaters contaminated with metals and radionuclides. Increasing application of HANPs for agronomic and environmental advantages will expedite their dissemination in subsurface environments. Because the biogeochemical cycling of P is intimately coupled with iron, it is anticipated that HANPs and released P from HANPs interact with iron oxides, particularly naturally occurring goethite nanoparticles (GNPs) because of their nanoscale size and high reactivity toward P. Here, we investigated the cotransport and retention of HANPs and GNPs in water-saturated sand columns under environmentally relevant transport conditions (pH and natural organic matter type and concentration). Our results indicated that the “size-selective retention”, i.e., preferential retention of larger particles near the column inlet and elution of smaller particles occurred during cotransport of HANPs and GNPs, and the cotransport of both NPs is highly sensitive to solution chemistry that determines NPs dissolution, homo- and heteroaggregation, and co- and competitive-retention. These findings have important insights into application of HANPs as a promising P nanofertilizer and an in situ amendment for contaminated site remediation

Effects of Environmental Factors on the Sorption of Per- and Polyfluoroalkyl Substances by Biochars

Biochar emerges as a cost-effective sorbent for removing perfluoroalkyl and polyfluoroalkyl substances (PFAS) in water, but our knowledge of how environmental factors affect PFAS sorption by biochar remains unclear. One in-house produced biochar from Douglas Fir feedstock at 900 °C and one commercial biochar were employed to investigate PFAS sorption effectiveness and efficiency, including sorption kinetics, isotherms, and effects of salt and humic acid concentrations. An artificial groundwater solution was also selected to assess biochar’s ability for PFAS removal under a real-world water treatment scenario. PFAS removal efficiency by both Douglas Fir 900 biochar and commercial biochar was negatively affected by humic acid, despite the negative effect being less for commercial biochar compared to Douglas Fir 900 biochar. Conversely, salt (1–10 mM NaCl and 0.5–2 mM CaCl2) increased PFAS sorption by biochars, likely due to their charge screening effect of biochar surface charge. PFAS removal efficiency by both Douglas Fir 900 biochar and commercial biochar in artificial groundwater solution was largely inhibited (versus that in relatively clean water matrices); however, the commercial biochar can still remove >70% of most PFAS in water. These findings support the feasibility of using cost-effective biochars, especially the commercially produced biochar, for removing PFAS in water

Modeling the Transport of the “New-Horizon” Reduced Graphene OxideMetal Oxide Nanohybrids in Water-Saturated Porous Media

Little is known about the fate and transport of the “new-horizon” multifunctional nanohybrids in the environment. Saturated sand-packed column experiments (n = 66) were therefore performed to investigate the transport and retention of reduced graphene oxide (RGO)metal oxide (Fe3O4, TiO2, and ZnO) nanohybrids under environmentally relevant conditions (mono- and divalent electrolytes and natural organic matter). Classical colloid science principles (Derjaguin–Landau–Verwey–Overbeek (DLVO) theory and colloid filtration theory (CFT)) and mathematical models based on the one-dimensional convection-dispersion equation were employed to describe and predict the mobility of RGO-Fe3O4, RGO-TiO2, and RGO-ZnO nanohybrids in porous media. Results indicate that the mobility of the three nanohybrids under varying experimental conditions is overall explainable by DLVO theory and CFT. Numerical simulations suggest that the one-site kinetic retention model (OSKRM) considering both time- and depth-dependent retention accurately approximated the breakthrough curves (BTCs) and retention profiles (RPs) of the nanohybrids concurrently; whereas, others (e.g., two-site retention model) failed to capture the BTCs and/or RPs. This is primarily because blocking BTCs and exponential/hyperexponential/uniform RPs occurred, which is within the framework of OSKRM featuring time- (for kinetic Langmuirian blocking) and depth-dependent (for exponential/hyperexponential/uniform) retention kinetics. Employing fitted parameters (maximum solid-phase retention capacity: Smax = 0.0406–3.06 cm3/g; and first-order attachment rate coefficient: ka = 0.133–20.6 min–1) extracted from the OSKRM and environmentally representative physical variables (flow velocity (0.00441–4.41 cm/min), porosity (0.24–0.54), and grain size (210–810 μm)) as initial input conditions, the long-distance transport scenarios (in 500 cm long sand columns) of the three nanohybrids were predicted via forward simulation. Our findings address the existing knowledge gap regarding the impact of physicochemical factors on the transport of the next-generation, multifunctional RGOmetal oxide nanohybrids in the subsurface

Humic Acid Facilitates the Transport of ARS-Labeled Hydroxyapatite Nanoparticles in Iron Oxyhydroxide-Coated Sand

Hydroxyapatite nanoparticles (nHAP) have been widely used to remediate soil and wastewater contaminated with metals and radionuclides. However, our understanding of nHAP transport and fate is limited in natural environments that exhibit significant variability in solid and solution chemistry. The transport and retention kinetics of Alizarin red S (ARS)-labeled nHAP were investigated in water-saturated packed columns that encompassed a range of humic acid concentrations (HA, 0–10 mg L^–1), fractional surface coverage of iron oxyhydroxide coatings on sand grains (λ, 0–0.75), and pH (6.0–10.5). HA was found to have a marked effect on the electrokinetic properties of ARS-nHAP, and on the transport and retention of ARS-nHAP in granular media. The transport of ARS-nHAP was found to increase with increasing HA concentration because of enhanced colloidal stability and the reduced aggregate size. When HA = 10 mg L^–1, greater ARS-nHAP attachment occurred with increasing λ because of increased electrostatic attraction between negatively charged nanoparticles and positively charged iron oxyhydroxides, although alkaline conditions (pH 8.0 and 10.5) reversed the surface charge of the iron oxyhydroxides and therefore decreased deposition. The retention profiles of ARS-nHAP exhibited a hyperexponential shape for all test conditions, suggesting some unfavorable attachment conditions. Retarded breakthrough curves occurred in sands with iron oxyhydroxide coatings because of time-dependent occupation of favorable deposition sites. Consideration of the above effects is necessary to improve remediation efficiency of nHAP for metals and actinides in soils and subsurface environments

Activated Carbon Application Simultaneously Alleviates Paddy Soil Arsenic Mobilization and Carbon Emission by Decreasing Porewater Dissolved Organic Matter

Flooding of paddy fields during the rice growing season enhances arsenic (As) mobilization and greenhouse gas (e.g., methane) emissions. In this study, an adsorbent for dissolved organic matter (DOM), namely, activated carbon (AC), was applied to an arsenic-contaminated paddy soil. The capacity for simultaneously alleviating soil carbon emissions and As accumulation in rice grains was explored. Soil microcosm incubations and 2-year pot experimental results indicated that AC amendment significantly decreased porewater DOM, Fe(III) reduction/Fe2+ release, and As release. More importantly, soil carbon dioxide and methane emissions were mitigated in anoxic microcosm incubations. Porewater DOM of pot experiments mainly consisted of humic-like fluorophores with a molecular structure of lignins and tannins, which could mediate microbial reduction of Fe(III) (oxyhydr)oxides. Soil microcosm incubation experiments cospiking with a carbon source and AC further consolidated that DOM electron shuttling and microbial carbon source functions were crucial for soil Fe(III) reduction, thus driving paddy soil As release and carbon emission. Additionally, the application of AC alleviated rice grain dimethylarsenate accumulation over 2 years. Our results highlight the importance of microbial extracellular electron transfer in driving paddy soil anaerobic respiration and decreasing porewater DOM in simultaneously remediating As contamination and mitigating methane emission in paddy fields

Ionic Strength-Dependent Attachment of Pseudomonas aeruginosa PAO1 on Graphene Oxide Surfaces

Graphene oxide (GO) is a widely used antimicrobial and antibiofouling material in surface modification. Although the antibacterial mechanisms of GO have been thoroughly elucidated, the dynamics of bacterial attachment on GO surfaces under environmentally relevant conditions remain largely unknown. In this study, quartz crystal microbalance with dissipation monitoring (QCM-D) was used to examine the dynamic attachment processes of a model organism Pseudomonas aeruginosa PAO1 onto GO surface under different ionic strengths (1–600 mM NaCl). Our results show the highest bacterial attachment at moderate ionic strengths (200–400 mM). The quantitative model of QCM-D reveals that the enhanced bacterial attachment is attributed to the higher contact area between bacterial cells and GO surface. The extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory and atomic force microscopy (AFM) analysis were employed to reveal the mechanisms of the bacteria–GO interactions under different ionic strengths. The strong electrostatic and steric repulsion at low ionic strengths (1–100 mM) was found to hinder the bacteria–GO interaction, while the limited polymer bridging caused by the collapse of biopolymer layers reduced cell attachment at a high ionic strength (600 mM). These findings advance our understanding of the ionic strength-dependent bacteria–GO interaction and provide implications to further improve the antibiofouling performance of GO-modified surfaces

Fate of As(III) and As(V) during Microbial Reduction of Arsenic-Bearing Ferrihydrite Facilitated by Activated Carbon

Microbial reduction of arsenic (As)-bearing Fe­(III)-(oxyhydr)­oxides is one of the major processes for the release of As in various environmental settings such as acid mine drainage, groundwater, and flooded paddy soil. Pyrogenic carbon has recently been reported to facilitate microbial extracellular reduction of Fe­(III)-(oxyhydr)­oxides. The aim of this study was to investigate the important hot topic regarding the fate and transformation of As during activated carbon (AC) facilitated microbial reduction of As-bearing ferrihydrite. Our results show that the rate and extent of Fe­(III) reduction in As-bearing ferrihydrite by <i>Shewanella oneidensis</i> MR-1 were accelerated by AC. The AC facilitated reduction caused the release of As­(III) into the solution, whereas it caused the preferential immobilization of As­(V) on the solid phase. Furthermore, AC accelerated the precipitation of vivianite and siderite in sequence during microbial reduction processes. Both of the formed vivianite and siderite had an insignificant capacity for capturing As­(III); however, As­(V) was selectively immobilized by vivianite compared to that of siderite. Taken together, our findings provide crucial insights into understanding the role of AC on the redox and immobilization of Fe and As in suboxic and anoxic environments and thus their environmental fate when pyrogenic carbons are employed for agronomic and environmental applications