Coupled Hydro-Biogeochemical Processes Controlling Cr Reductive Immobilization in Columbia River Hyporheic Zone

Transformation of dissolved organic matter and related arsenic mobility at a surface water-groundwater interface in the Hetao Basin, China

Porewater arsenic mobility above the groundwater table has been recognized as a potential cause of arsenic-rich groundwater, but the processing pathways of dissolved organic matter (DOM) in that hyporheic zone and their effect on porewater arsenic release remain poorly understood. To address these issues, two porewater profiles were sampled in a surface water-groundwater interaction zone from the Hetao Basin, China, to monitor the porewater geochemistry and DOM molecular characteristics. The results show that the porewater arsenic, Fe(II), and DOC concentrations were all significantly higher than those of the intruding pond water, and were located above the conservative mixing model lines. This indicates a net release of these solutes from the sediment. By comparing the porewater with pond water DOM, we found that the carboxyl-rich alicyclic molecules (CRAM) were selectively preserved, carbohydrates and aliphatics/proteins were preferentially consumed, and low O/C-ratio compounds with high bioproduction index (I_bioprod) and terrestrial index (I_terr) were produced. The transformation of CHO to CHOS compounds also represented a pathway of recalcitrant DOM production. The produced recalcitrant organic compounds mostly contributed to the elevated porewater DOC concentrations, but their contribution decreased along the filtration path. The consumption of labile DOM compounds would be responsible for Fe(III) hydroxide reduction and arsenic release. The generated recalcitrant DOM may also be a driver of porewater arsenic mobility by acting as electron shuttles. This study highlights the importance of the hyporheic zone in shaping shallow groundwater DOM composition and the potential contribution to arsenic enrichment.

Delimiting conditions under which natural organic matter can control Fe speciation and size in freshwaters

Pollutant and nutrient mobility in natural waters is typically controlled by sorption onto the high surface area of colloidal particles, most of which may form by precipitation of Fe(III)(hydr)oxides. Therefore, prediction of the speciation and size of Fe is critical to managing water quality. Prediction from pH and dissolved oxygen (D.O.) saturation can fail because of Fe binding to natural organic matter (N.O.M.) in natural waters. We test the influence of environmental variables -temperature, illumination and mixing order of Fe and N.O.M. with D.O.- on the impact of N.O.M.. Differences in mixing order simulate Fe(II) mixing with N.O.M. in groundwater prior to emerging, in comparison to Fe(II) emerging into oxic surface waters containing N.O.M.. Fe speciation and size were measured in waters containing N.O.M. with and without D.O., but also a water to which N.O.M. and then D.O. were added sequentially. Without D.O. free Fe(II) bound to N.O.M. and became a filterable particle. Binding increased with pH and at 7.5 was sufficient for Fe speciation in oxic waters to become influenced by whether mixing had been sequential or simultaneous. Therefore, at high pH Fe speciation in oxic surface waters requires knowledge of N.O.M. content of this water and upstream groundwaters. Cold (10 °C) decreased anoxic binding of Fe(II) to N.O.M. and both cold and darkness alsodecreased Fe binding to N.O.M. under oxic conditions, because in both cases Fe(III)(hydr)oxide surfaces out-compete N.O.M. for binding Fe. Cold and darkness therefore overwhelm the effect of mixing order on oxic Fe speciation, and cold even makes the presence or absence of N.O.M. irrelevant. In the cold or dark, prediction of Fe speciation and size in surface waters may not require knowledge of N.O.M. content of upstream groundwaters. Furthermore, when cold, prediction may not even require knowledge of N.O.M. content of the surface waters.

Hydrolysis of norfloxacin in the hyporheic zone: kinetics and pathways

Understanding the hydrolysis behavior and pathway of norfloxacin (NOR) in the hyporheic zone (HZ) is important for predicting its environmental persistence. Therefore, the effects of different environmental factors on NOR hydrolysis were investigated, and the hydrolysis pathway of NOR in the HZ was determined by DFT calculations and UPLC/TOF-MS. The hydrolysis process of NOR was consistent with the first-order kinetic. The experiment of environmental factors showed that DO was an important factor to affect NOR hydrolysis, and its hydrolysis rate was positively correlated with DO concentration. The superoxide radical (·O2-) was the main active species for NOR hydrolysis. The hydrolysis rates of NOR under neutral and alkaline conditions were higher than that under acidic conditions in both aerobic and anoxic environments. The ions of Ca2+, Mg2+, HCO3-, CO32-, and NO3- in simulated water samples inhibited the hydrolysis of NOR, while Cl- promoted its hydrolysis. In addition, the electronegativity of NOR was determined by DFT calculations, and it was speculated that the active sites of NOR hydrolysis were mainly located in the piperazine ring and quinolone ring. The main hydrolysis pathway of NOR in aerobic environment was piperazine ring cracking and quinolone ring decomposition, and that in anoxic environment was piperazine ring cracking. The results are of great significance to evaluate the environmental fate of NOR in the HZ and provide a theoretical basis for further understanding the degradation and governance of fluoroquinolones in water environment.

A process-based model for describing redox kinetics of Cr(VI) in natural sediments containing variable reactive Fe(II) species

Sediment-associated Fe(II) is a critical reductant for immobilizing groundwater contaminants, such as Cr(VI). The reduction reactivity of sediment-associated Fe(II) is dependent on its binding environment and influenced by the biogeochemical transformation of other elements (i.e., C, N and Mn), challenging the description and prediction of the reactivity of Fe(II) in natural sediments. Here, anaerobic batch experiments were conducted to study the variation in sediment-associated Fe(II) reactivity toward Cr(VI) in natural sediments collected from an intensive agricultural area located in Guangxi, China, where nitrate is a common surface water and groundwater contaminant. Then, a process-based model was developed to describe the coupled biogeochemical processes of C, N, Mn, Fe, and Cr. In the process-based model, Cr(VI) reduction by sediment-associated Fe(II) was described using a previously developed multirate model, which categorized the reactive Fe(II) into three fractions based on their extractabilities in sodium acetate and HCl solutions. The experimental results showed that Fe(II) generation was inhibited by NO₃^- and/or NO₂^-. After NO₃^- and NO₂^- were exhausted, the Fe(II) content and its reduction rate toward Cr(VI) increased rapidly. As the Fe(II) content increased, the three reactive Fe(II) fractions exhibited approximately linear correlations with aqueous Fe(II) concentrations ( [Formula: see text] ), which was probably driven by sorptive equilibrium and redox equilibrium between aqueous and solid phases. The model results indicated that the reaction rate constants of the three Fe(II) fractions (k_n) significantly increased with incubation time, and log(k_n) correlated well with [Formula: see text] [ [Formula: see text] , [Formula: see text] and [Formula: see text] ]. The numerical model developed in this study provides an applicable method to describe and predict Cr(VI) removal from groundwater under dynamic redox conditions.

Effects of chronic exposure of antibiotics on microbial community structure and functions in hyporheic zone sediments

Microbial communities in hyporheic zones (HZ) provide vital biogeochemical functions such as contaminant degradation for river ecosystems. Antibiotics are contaminants that have been increasingly detected in HZ sediments. In this study, sediments from different HZ locations in a contaminated river, Maozhou river, China were sampled and analyzed using qPCR and high-throughput sequencing to investigate the effect of antibiotic contamination on microbial community structures and functions in HZ sediments. Results indicated that types and concentrations of antibiotics in HZ sediments were heterogeneously distributed that were largely consistent with the distribution of antibiotic sources. Sediments near animal farm and hospital contained higher antibiotic concentrations compared with those from mainstream. The distribution of ARGs was well correlated with antibiotics. Bacterial indicator genera indicating differences between mainstream area and other sampling areas were positively correlated with antibiotics, suggesting the influences of antibiotics on reshaping microbial community structures. PICRUSt revealed positive relationships between antibiotics and predicted functional genes involved in defense, signal transduction, and recombination and repair. This imply the defensive response of microbial communities on antibiotic attack. These results indicated that antibiotic contamination in the watershed posed a potential risk on HZ microbial community structures and functions, which may further threaten river ecosystem functions.

Impact of Physico-Chemical Heterogeneity on Arsenic Sorption and Reactive Transport under Water Extraction

Heterogeneity in physical and chemical properties is a common characteristic in a subsurface environment. This study investigated the effect of physico-chemical heterogeneity on arsenic (As) sorption and reactive transport under water extraction in a layered system with preferential flow paths. A flume experiment was performed to derive the spatio-temporal data of As reactive transport. The results indicated that the heterogeneous system significantly accelerated downward (vertical direction) As migration as a coupled effect of physical and chemical heterogeneity that led to fast As transport with low As sorption along the preferential flow paths. The results also indicated that such a heterogeneity effect was driven by water extraction that enhanced the downward groundwater flow along the preferential flow paths. Numerical simulations were performed by matching the experimental results to provide insights into the dominant processes controlling the As migration in the heterogeneous systems. The simulation results highlighted the importance of the kinetic oxidation of mineral-bonded Fe(II) to Fe(III) in the clay matrix that dynamically increased As sorption affinity and retarded As reactive transport. A coupled model of reactive transport along the preferential flow paths, sorption-retarded diffusion from the preferential flow paths into the clay matrixes, and reactions that change sorption affinity in the matrix was required to describe the As reactive transport systems with physico-chemical heterogeneities. The results have strong implications for understanding and modeling As downward migration from shallow to deep aquifers under groundwater pumping conditions in field systems with inherent heterogeneity.

Reduced NOM triggered rapid Cr(VI) reduction and formation of NOM-Cr(III) colloids in anoxic environments

Natural organic matter (NOM) can influence the toxicity and speciation of chromium (Cr) in subsurface through redox reactions and complexation. Under anoxic conditions, NOM can be reduced by microorganisms or geochemical reductants, and the reduced NOM (NOM_red) represents a large reservoir of organic matter observed in anoxic sediments and water. While the current body of work has established the kinetic of Cr(VI) reduction by oxidized NOM (NOM_ox) under oxic conditions, much less is known about the rates and mechanisms of Cr(VI) reduction triggered by NOM_red under anoxic conditions and the colloidal properties of the reaction products. This study provided new information regarding the NOM_red-mediated Cr(VI) reduction and colloidal stability of reduced Cr(III) particles over a wide range of environmentally relevant anoxic conditions. We show that under dark anoxic conditions reduced humic acid (HA_red) moieties (e.g., quinone) can quickly reduce Cr(VI) to Cr(III), and the reduced Cr(III) can subsequently complex with carboxyl groups of HA leading to the formation of stable HA-Cr(III) colloids. Rates of Cr(VI) reduction by HA_red are 3-4 orders of magnitude higher than those by oxidized HA (HA_ox) due primarily to the higher reducing capacity of HA_red. The stable HA-Cr(III) colloids are formed across a range of HA concentrations (8-150 mg C/L) and pH conditions (6-10) with hydrodynamic diameter in the range of 210-240 nm. Aberration-corrected scanning transmission electron microscopy (Cs-STEM) and X-ray photoelectron spectroscopy (XPS) confirmed that the particles are composed of HA-Cr(III). The high colloidal stability of HA-Cr(III) particles could be attributed to the enhanced electrosteric stabilization effect from free and adsorbed HA, which decreased particle aggregation. However, the presence of divalent cations (Ca²⁺ and Mg²⁺) promoted particle aggregation at pH 6. These new findings are valuable for our fundamental understanding of the fate and transport of Cr in organic-rich anoxic environments, which also have substantial implications for the development and optimization of subsurface Cr sequestration technology.

Formation and Transport of Cr(III)-NOM-Fe Colloids upon Reaction of Cr(VI) with NOM-Fe(II) Colloids at Anoxic-Oxic Interfaces

Natural organic matter-iron (NOM-Fe) colloids are ubiquitous at anoxic-oxic interfaces of subsurface environments. Fe(II) or NOM can chemically reduce Cr(VI) to Cr(III), and the formation of Cr(III)-NOM-Fe colloids can control the fate and transport of Cr. We explored the formation and transport of Cr(III)-humic acid (HA)-Fe colloids upon reaction of Cr(VI) with HA-Fe(II) colloids over a range of environmentally relevant conditions. Cr(VI) was completely reduced by HA-Fe(II) complexes under anoxic conditions, and the formation of Cr(III)-HA-Fe colloids depended on HA concentration (or molar C/Fe ratio) and redox conditions. No colloids formed at HA concentrations below 3.5 mg C/L (C/Fe ratio below 1.6), but Cr(III)-HA-Fe colloids formed at higher HA concentrations. In column experiments, Cr(III)-HA-Fe(III) colloids formed under oxic conditions were readily transported through sand-packed porous media. Colloidal stability measurements further suggest that Cr(III)-HA-Fe colloids are highly stable and persist for at least 20 days without substantial change in particle size. This stability is attributed to the enrichment of free HA adsorbed on the Cr(III)-HA-Fe colloid surfaces, intensifying the electrostatic and/or steric repulsion interactions between particles. The new insights provided here are important for evaluating the long-term fate and transport of Cr in organic-rich redox transition zones.

Watershed-scale distributions of heavy metals in the hyporheic zones of a heavily polluted Maozhou River watershed, southern China

Hyporheic zone (HZ) sediments in river systems are often contaminated with heavy metals as a legacy of natural processes and anthropogenic activities. The geochemical behaviors of heavy metals in the HZ sediments at the laboratory scale have been extensively studied. However, the watershed-scale distributions of heavy metals in the HZ sediments and the processes controlling their distributions have not been well studied. Here, we report a watershed-scale study of heavy metals (i.e., Cr, Ni, Cu, Zn, Cd, and Pb) distributions in the HZ of the Maozhou River watershed, a heavily polluted area within the Pearl River Delta, southern China. Statistical analysis revealed that the spatial distribution of studied heavy metal concentrations was highly correlated with that of the sediment-associated sulfide at the watershed-scale. Metal extraction analysis and double-spherical aberration-corrected scanning transmission electron microscope imaging (Cs-STEM) further confirmed the strong association of heavy metals with sulfur. These observations demonstrated that the formation of metals-sulfide precipitates was the key process controlling the watershed-scale distributions of heavy metals (especially for Cr, Ni and Zn) in the HZ sediments. Additionally, high permeability of the HZ sediments may prevent Ni, Zn, Cd and Pb accumulation in sediments. Specially, Cu distribution was mainly affected by organic-Cu complexation. In the estuary area, salinity input likely affected the distributions of Ni, Zn and Cd through cation exchange processes. The findings improved our understanding of the distributions of heavy metals and the processes controlling their distributions at the watershed-scale, and have implications for remediating and managing contaminated HZ sediments.

Cr(III) Adsorption by Cluster Formation on Boehmite Nanoplates in Highly Alkaline Solution

The development of advanced functional nanomaterials for selective adsorption in complex chemical environments requires partner studies of binding mechanisms. Motivated by observations of selective Cr(III) adsorption on boehmite nanoplates (γ-AlOOH) in highly caustic multicomponent solutions of nuclear tank waste, here we unravel the adsorption mechanism in molecular detail. We examined Cr(III) adsorption to synthetic boehmite nanoplates in sodium hydroxide solutions up to 3 M, using a combination of X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), scanning/transmission electron microscopy (S/TEM), electron energy loss spectroscopy (EELS), high-resolution atomic force microscopy (HR-AFM), time-of-fight secondary ion mass spectrometry (ToF-SIMS), Cr K-edge X-ray absorption near edge structure (XANES)/extended X-ray absorption fine structure (EXAFS), and electron paramagnetic resonance (EPR). Adsorption isotherms and kinetics were successfully fit to Langmuir and pseudo-second-order kinetic models, respectively, consistent with monotonic uptake of Cr(OH)₄^- monomers until saturation coverage of approximately half the aluminum surface site density. High resolution AFM revealed monolayer cluster self-assembly on the (010) basal surfaces with increasing Cr(III) loading, possessing a structural motif similar to guyanaite (β-CrOOH), stabilized by corner-sharing Cr-O-Cr bonds and attached to the surface with edge-sharing Cr-O-Al bonds. The selective uptake appears related to short-range surface templating effects, with bridging metal connections likely enabled by hydroxyl anion ligand exchange reactions at the surface. Such a cluster formation mechanism, which stops short of more laterally extensive heteroepitaxy, could be a metal uptake discrimination mechanism more prevalent than currently recognized.

Compositional changes of dissolved organic carbon during its dynamic desorption from hyporheic zone sediments

Dissolved organic matter (DOM) is an important driver for biogeochemical reactions that affect microbial community function, and regulate changes in porewater chemical composition and redox properties in the environment. This study investigated the variation in DOM molecular composition during the detachment of organic matter (OM) from hyporheic zone (HZ) sediments using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR-MS). Diffusive mass transfer and microbial degradation were the two primary processes controlling the rate of OM release and molecular composition changes during the detachment from sediments. The diffusive mass transfer process limited the rate of OM release from the sediments, but had negligible effect on the molecular signature of the released OM. Microbial degradation on the other hand preferentially consumed the protein- and lipid-like fractions of the DOM, characterized by lower nominal oxidation states of carbon (NOSC), lower molecular weight, and a higher saturation of chemical bonds. The results have strong implication to the organic carbon dynamics and related microbial activities and contaminant transformation in hyporheic zones, an important critical area in river systems.

Structural characteristics of cake layer in membrane bioreactor with chromate exposure

Chromate (CrO₄^2-) exposure, especially high concentration (mg/L), still probably occurs in the industrial and mining area due to industrial accidents or even illegal discharge, though CrO₄^2- has been restricted to be discharged into wastewater treatment system (WWTS). Therefore, this study was applied to better understand the structural characteristics of cake layer in membrane bioreactor (MBR), which is one of best alternative for WWTS of industrial or mining area, with CrO₄^2- exposure. Three submerged MBRs with CrO₄^2- exposure (10 mg/L was normal high concentration CrO₄^2-; 50 mg/L as extreme level for better identification; 0 mg/L as control condition) were applied in this study. Results showed that CrO₄^2- exposure caused an obvious variation of cake layer structure. Because of organic component variation, cake layer structure with CrO₄^2- exposure was re-constructed into loose and porous with biomicromolecules, and resulted in the rapid cake layer thickness increase, finally leading to severe membrane biofouling. Additionally, CrO₄^2- distributed evenly along the cross-sectional cake layer. CrO₄^2- only induced the inorganic structure variations of cake layer, but without any obvious effects on the other inorganic elements structure. CrO₄^2- exposure induced the bacterial community structure variation and led to tolerated-CrO₄^2- microorganisms as the majority in cake layer community, but had no obvious effects on the population diversity.

A Generalized-Rate Model for Describing and Scaling Redox Kinetics in Sediments Containing Variable Redox-Reactive Materials

This study developed a generalized modeling approach for describing and scaling redox reactions from reactive components to the sediments and their assemblages, using Cr(VI) reduction as an example. Batch experiments were performed to characterize the rates of Cr(VI) reduction in four Fe(II)-containing sediments and their assemblages. The experimental data were first used to calibrate a generalized-rate model of Cr(VI) reduction with generic rate parameters. The generalized-rate model was then used to describe the kinetics of Cr(VI) reduction in the sediment assemblages by linearly scaling the rate parameters from the individual sediments. Via comparison with the experimental results, this study found that the generalized-rate model with generic rate parameters can describe Cr(VI) reduction in individual sediments and their assemblages with different redox reactivity toward Cr(VI) reduction. The sediment-associated Fe(II) and its reactivity were found to be the key variables in the generalized model for describing the Cr(VI) reduction in the studied sediments. A three-step extraction method was subsequently developed to estimate the rate-specific Fe(II) pools that can facilitate the application of the scaling approach in field systems.

Effect of Water Chemistry and Hydrodynamics on Nitrogen Transformation Activity and Microbial Community Functional Potential in Hyporheic Zone Sediment Columns

Hyporheic zones (HZ) are active biogeochemical regions where groundwater and surface water mix. N transformations in HZ sediments were investigated in columns with a focus on understanding how the dynamic changes in groundwater and surface water mixing affect microbial community and its biogeochemical function with respect to N transformations. The results indicated that denitrification, DNRA, and nitrification rates and products changed quickly in response to changes in water and sediment chemistry, fluid residence time, and groundwater-surface water exchange. These changes were accompanied by the zonation of denitrification functional genes along a 30 cm advective flow path after a total of 6 days' elution of synthetic groundwater with fluid residence time >9.8 h. The shift of microbial functional potential toward denitrification was correlated with rapid NO₃^- reduction collectively affected by NO₃^- concentration and fluid residence time, and was resistant to short-term groundwater-surface water exchange on a daily basis. The results implied that variations in microbial functional potential and associated biogeochemical reactions in the HZ may occur at space scales where steep concentration gradients present along the flow path and the variations would respond to dynamic HZ water exchange over different time periods common to natural and managed riverine systems.