Oxyhydroxide Research Articles

Studies of the Morphology of Hematite Synthesized from Waste Iron Sulfate.

Microwave-based reactions have gained traction in recent years due to their ability to enhance reaction rates and yield while reducing energy consumption. Also, according to the conception of 'waste to materials', various waste feeds are intensively sought to be tested. The experimental setup of this study involved varying pH levels, oxidation agents, and precipitation agents to optimize the synthesis process of iron red based on waste iron sulfate. The selection of oxidation and precipitation agents was found to significantly influence the pigment synthesis process. Various oxidizing agents, including hydrogen peroxide and atmospheric air, were evaluated for their effectiveness in promoting the oxidation of ferrous ions to ferric ions, essential for pigment formation. Additionally, different precipitation agents such as sodium hydroxide and ammonia solution were assessed for their ability to precipitate iron hydroxides and facilitate pigment particle formation. The characterization of synthesized pigments revealed promising results in terms of quality and color properties. Helium Ion Microscopy (HIM) analysis confirmed the formation of well-defined pigment particles with controlled morphology. X-ray diffraction (XRD) studies provided insights into the crystalline structure of the pigments, indicating the presence of characteristic iron oxide phases. By improving this technology, waste iron sulfate can be efficiently transformed into valuable iron pigments, offering a sustainable solution for waste management while meeting the growing demand for high-quality pigments.

Exploring the influence of aquatic phosphate on Fe floc dynamics in water treatment

The formation of flocs is crucial in the coagulation process of water treatment. However, the nature of ligand exchange on the surface of primary nanoparticles (PNPs) during floc formation requires further investigation to enhance our understanding of the coagulation mechanism. Phosphate (P) is a ubiquitous nutrient ion in aquatic surface water, in this study, the impact of P on floc growth under different pH conditions were investigated. The results revealed that floc growth patterns depended on both P dosage and pH. The mode of ligand exchange between P and in-situ formed ferric hydroxide within a pH range of 5 to 10 was further explored, and remarkable disparities in pH changes induced by P addition were observed. At lower pH levels, OH− release occurred relatively slowly, stabilizing with continued P addition. At neutral pH, OH− release was comparatively higher with P addition, while under alkaline conditions, both the quantity of OH− and its release rate decreased. It was deduced that Fe–OH21/2+ sites function as "active sites," while Fe–OH1/2− sites act as "inert sites" on the surface of PNPs formed during flocculation. These sites are crucial in the interconnections between flocs formed during coagulation and in floc growth. Analyses of Fe PNPs by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM), with and without P addition, revealed that the introduction of P inhibits or interferes with the self-crystallization of Fe PNPs through chemical coordination reactions. The results offer deeper insights into the coagulation mechanism and the transformation of Fe flocs in raw waters containing P during water treatment practices.

Correction: The Biological Denitrification Using Ferric Hydroxide Desulfurized Waste as an Electron Donor

Correction: The Biological Denitrification Using Ferric Hydroxide Desulfurized Waste as an Electron Donor

Quantifying the mineral magnetic signature of petroleum systems and their source rocks: a study on the Inner Moray Firth, UK North Sea

SUMMARY This study aims to expand on existing connections between magnetic minerals and hydrocarbons within petroleum systems. Previous studies have focussed on single-source petroleum systems whereas this study, for the first time, analyses a multi-source petroleum system to investigate potential correlations between different kerogen type source rocks and magnetic minerals. To do this, the study investigates the magnetic mineral characteristics of the Inner Moray Firth (IMF), UK North Sea, through room-, low- and high-temperature techniques, and correlates this to published basin and petroleum systems modelling results that show a three-source hydrocarbon mix. Magnetic mineral analysis identifies extensive evidence for magnetite, goethite and siderite, alongside more minor lepidocrocite and iron sulphides. Although we find that magnetite is ubiquitous within the IMF, its abundance is relatively low, and, in contrast, the relatively magnetically weak goethite is more likely the most abundant magnetic mineral throughout the IMF. In agreement with previous studies, we find magnetic enhancement at oil-water contacts (OWCs); however, here, we identify two different magnetic enhancement processes at OWCs in wells, which are dependent on the amount of sulphur available in the local environment. Wells with low levels of sulphur have increasing levels of magnetite towards the OWC, with the magnetic enhancement occurring at the top of the water-saturated section. Sulphur-rich environments display an increase in iron sulphides near the OWC at the bottom of the oil-saturated sediments. Additionally, we confirm the presence of siderite as indicator of upward vertical migration. Combining with petroleum system model predictions, we find direct links between iron hydroxide presence and Type I and II–III kerogen source rocks, and iron sulphide presence with Type II kerogen source rocks. This study shows the potential for further utilization of magnetic mineral analysis within hydrocarbon exploration and petroleum system definition.

Improved bacterial elimination in wastewater through electrocoagulation: hydrogen generation, adsorption of colloidal bacteria-flocks, and electric field bactericidal action

ABSTRACT Electrocoagulation (EC) has emerged as a promising method for wastewater treatment, offering efficient removal of various contaminants, including bacteria. This study investigates the mechanisms underlying bacterial removal in EC processes, mainly the hydrogen-mediated foam, the effect of operational parameters, including initial pH, current density, and reaction time, and evaluates the associated energy consumption. The EC reactor employed aluminum electrodes and operated at a current intensity of 3.0 A. It demonstrated a notable bacterial removal efficiency, with 120.102 UFC ml−1 of mesophyll floral aerobic bacteria removed through colloid bacteria-flocs precipitation, 440.102 UFC ml−1 via bacterial-bulls flotation from foam, and 117.102 UFC ml−1 through attraction at the electrodes’ plate surface. We found that the EC process leads to the formation of aluminum hydroxide and ferrous hydroxide precipitates, which adsorb bacteria and facilitate their removal from the wastewater via electrostatic forces with an energy consumption of 45 kWh/m3. Overall, this research provides valuable insights into the mechanisms governing bacterial removal in EC and highlights the importance of energy consumption analysis for optimizing wastewater treatment processes.

Integrating multispectral remote sensing and geological investigation for gold prospecting in the Borongo-Mborguene gold field, eastern Cameroon

Multispectral remote sensing data has great promise for mineral prospecting via regional-scale mapping of alteration zones and geological structures, once the method is calibrated using in-situ geological observations. The Borongo-Mborguene area, located in eastern Cameroon at the transition from sub-tropical to arid climates, hosts several alluvial gold deposits. In this investigation, maps developed by processing of Landsat-8 OLI images, ASTER satellite images, and SRTM DEM data were compared to results of a comprehensive field survey to locate zones of hydrothermal alteration and regional structural lineaments, and to verify the association of these features with gold mineralization. Specific Landsat-8 and ASTER band ratios, as well as principal component analysis of the multispectral images, were selected to isolate spectral signals that characterize particular groups of alteration minerals, such as clays, hydrated carbonates, hydrated sulphates, iron oxides, and iron hydroxides. Lineaments and their preferred orientations were extracted by filtering of the digital elevation data. A fuzzy logic approach was subsequently applied to the ensemble of resulting maps to highlight five areas of concentrated hydrothermal alteration and structural complexity as potential gold prospects (Mborguene, Boutila, Nandoungue, Borongo and Ouaden villages). Petrographic study of samples collected from the remotely-identified alteration zones revealed a hydrothermal ore assemblage of gold, galena, pyrite and hematite in mineralized quartz veins. Structural control of the alteration zones in the Borongo-Mborguene gold field was confirmed by field mapping along a shear zone. Hence, our petrographic and field studies indicate that the multispectral remote sensing data and data processing methods developed herein are appropriate for mineral exploration in remote sub-tropical areas of Africa. More generally, the computationally efficient and simple procedure developed here should yield accurate preliminary maps of the spatial distribution of hydrothermal halos and related mineral deposits in challenging mixed-type vegetated terrain around the globe.

Исследование дезинфицирующей способности феррата натрия

More and more attention has been recently paid to the disinfection of industrial wastewater. Food, petrochemical, pharmaceutical, and other industries cannot directly dump wastewater into the environment, therefore, the wastewater is fed to the deep biological treatment lines. A high level of biological threat and the expanding list of potential pathogens cause the need to search for new highly efficient disinfecting agents. The use of ferrates can be a new step in the improvement of the wastewater disinfection process due to the combination of processes of inactivation of pathogenic microflora and the reactant coagulation on ferric hydroxides generated during decontamination (oxidation). Within the framework of the study, the potential use of sodium ferrate as a disinfecting agent obtained by electrochemical dissolution of iron electrodes in alkali solutions has been confirmed. It has been established that sodium ferrate can effectively inactivate conditionally pathogenic cultures A. niger, B. subtillis which are part of activated sludge symbiosis. The efficient dosing of ferrate for complete inhibition of B. subtillis is 0.5 % mass, whereas for A. Niger it is 15 % mass. It has been proved that microflora inhibition occurs directly under the action of sodium ferrate and not due to the alkali reaction of the medium. Industrial tests of the disinfecting ability of ferrates in relation to wasted activated sludge of industrial wastewater deep biological treatment systems have confirmed the results obtained for individual objects (A. niger, B. subtillis). Based on the results, a conclusion about the potential use of ferrates in the deep biological treatment processes for highly bacteriologically contaminated industrial wastewater has been made. The main advantages of the suggested agent include the low toxicity of metabolites, minimum secondary contamination of treated water, and possible reactant advanced treatment (dephosphorization).

In Ukrainian

The relevance of obtaining new samples of nanoscale materials and researching ways to control these processes is supported by constant progress in the fields of nanotechnology, semiconductors, biomedicine, wastewater treatment etc. The Fe0(Steel3)-H2O-O2 system, which is considered promising for use in the specified industries, allows both obtaining a number of nanoscale phases of iron oxygen-containing compounds (IOCs), in particular magnetite, cobalt ferrous ferrite, iron(III) hydroxide oxides, and controlling these processes due to changes in the physicochemical conditions of the process. Taking into account the permanent change in the specified system of phase composition on the metal surface, the method of X-ray diffraction analysis (XRD) was applied using the created additional device for the goniometric attachment GP-13 for the in situ study of IOCs formation processes on the surface of a steel disc in the system Fe0(St3)-H2O-O2. This made it possible to investigate the mechanisms and optimal conditions for the formation of individual ultradisperse and nanosized phases of IOCs under a number of physicochemical conditions. By calculating the area under the characteristic peaks on the in situ XRD patterns, quantitative ratios of IOCs formed under certain physicochemical conditions were found, and the average size of crystallites was calculated according to Scherer's formula, which provided additional information for the analysis of IOCs formation mechanisms. Along with this, the method of scanning electron microscopy (SEM) and the XRD without an additional in situ device were used. Based on the analysis of the obtained kinetic regularities, conclusions were made about the formation of IOCs by the formation of iron (II-III) hydroxycarbonate and hydroxysulfate layered double hydroxides I and II types, followed by their transformation to the final nanoscale phases of magnetite, cobalt ferrous ferrite, and iron(III) hydroxide oxides. The formation and accumulation of ferrous hydroxide phase was not recorded by the XRD in situ, however, in the Fe0(St3)-H2O-O2 system, the formation of iron (II-III) layered double hydroxides based on its monolayers is not excluded. The preference of carbonate anions over chloride anions in the formation processes of iron layered double hydroxides is shown.

Enhanced Arsenate Immobilization by Kaolinite via Heterogeneous Pathways during Ferrous Iron Oxidation.

Clay minerals are ubiquitous in subsurface environments and have long been recognized as having a limited or negligible impact on the fate of arsenic (As) due to their negatively charged surfaces. Here, we demonstrate the significant role of kaolinite (Kln), a pervasive clay mineral, in enhancing As(V) immobilization during ferrous iron (Fe(II)) oxidation at near-neutral pH. Our results showed that Fe(II) oxidation alone was not capable of immobilizing As(V) at relatively low Fe/As molar ratios (≤2) due to the generation of Fe(III)-As(V) nanocolloids that could still migrate easily as truly dissolved As did. In the presence of kaolinite, dissolved As(V) was significantly immobilized on the kaolinite surfaces via forming Kln-Fe(III)-As(V) ternary precipitates, which had large sizes (at micrometer levels) to reduce the As mobility. The kaolinite-induced heterogeneous pathways for As(V) immobilization involved Fe(II) adsorption, heterogeneous oxidation of adsorbed Fe(II), and finally heterogeneous nucleation/precipitation of Fe(III)-As(V) phases on the edge surfaces of kaolinite. The surface precipitates were mixtures of amorphous basic Fe(III)-arsenate and As-rich hydrous ferric oxide. Our findings provide new insights into the role of clay minerals in As transformation, which is significant for the fate of As in natural and engineered systems.

Comparative evaluation of diffusion kinetics of arsenic adsorption on iron (oxy) hydroxides containing biopolymer nanocomposite beads

Exposure to arsenic-contaminated water poses great health risks to mankind. Nanocomposite beads have been proven to be an effective alternative for large-scale treatment of arsenic-loaded water. The larger size of the beads makes the adsorbent retrieval process less energy-intensive and prevents accidental leaching of nanoparticles into the surrounding environment. Adsorption using macroporous beads is predominantly a diffusion-controlled process. Therefore, in the present study, the shrinking core model (SCM) was used to compare and evaluate the diffusion kinetics of arsenic adsorption using in-situ precipitated iron (oxy)hydroxide alginate (IIAB) and chitosan beads (IICB). Simulated dynamic nondimensional liquid phase arsenic concentration profiles fit well with the experimental data (SSE = 0.018–0.025). External mass transfer coefficient (kf) and effective diffusivity coefficient (De) for both kinds of beads at different initial arsenic concentrations (C0 ∼ 1 mg.L−1 and 5 mg.L−1) were derived. By comparison of diffusivity values and the simulated concentration front profiles, it was observed that chitosan provided a better porous polymer template with more easily accessible adsorption sites covering up to 85 % of the beads’ volume. Comparison with progression conversion models has also been made to test the effectiveness of SCM fitting.

The Biological Denitrification Using Ferric Hydroxide Desulfurized Waste as an Electron Donor

The Biological Denitrification Using Ferric Hydroxide Desulfurized Waste as an Electron Donor

Effect of Al Substitution on Visible Short-Wave Infrared Reflectance Spectroscopy (VSWIR) of Goethite and Ferrihydrite

Goethite and ferrihydrite are the two major iron hydroxides, essential mineral constituents in the terrestrial surface system. Aluminum (Al) is the most common substituent in iron hydroxides, and it may significantly change the bulk and surficial physicochemical properties of iron hydroxides. Consequently, a practical and convenient approach is needed to efficiently identify the Al substitution degrees of iron hydroxides in natural occurrences. This study presents a comprehensive investigation of the VSWIR characteristics of laboratory-synthesized Al-substituted goethite and ferrihydrite, to establish diagnostic VSWIR parameters for the identification and quantification of Al substitution levels in iron hydroxides. The findings revealed that Al substitution can affect the band positions (P) of goethite and ferrihydrite at ~650 nm, ~900 nm, and ~1400 nm. The relationships between the Al substitution of ferrihydrite and VSWIR parameters can be expressed as P900 = −0.43 × Al(%) + 931 and P1400 = −0.07 × Al(%) + 1428, while that of goethite can be expressed as P650 = 0.42 × Al(%) + 657 and P900 = 2.29 × Al(%) + 936. The peak fitting results showed that the absorption intensity at 480–550 nm linearly decreases with increased Al substitution. The obtained VSWIR spectra of Al-substituted goethite and ferrihydrite provide a critical supplement to the spectral library for (Al) iron hydroxides, and these VSWIR parameters can be utilized for the semi-quantitative determination of Al substitution in natural iron hydroxides

A Facile Synthesis of Photocatalytic Fe(OH)3 Nanoparticles for Degradation of Phenol

AbstractIron hydroxide (Fe(OH)3) nanoparticles have garnered considerable attention for their effectiveness in water treatment. This paper reviews recent studies on the photocatalytic degradation of phenol using Fe(OH)3, detailing the synthesis procedure, photocatalytic reaction conditions under UV‐C and catalyst amount optimization. Iron hydroxide was synthesized using a facile precipitation method, and the produced catalyst was thoroughly characterized using XRD, FTIR, UV‐DRS, DTA/TGA and SEM. Characterization results indicated that the catalyst exhibited very low crystallinity, a 1.84 eV band gap, and very small particle sizes. The study investigated the degradation performance of phenol for different amounts of iron hydroxide, ranging from 0.1 g to 1 g, and determined the optimum catalyst amount to be 0.7 g. The catalyst demonstrated a degradation efficiency for phenol exceeding 90 % at the end of a 180 minute photocatalytic reaction. The paper offers a comprehensive analysis of Fe(OH)3 as a photocatalyst for phenol degradation, highlighting its potential for future applications in water treatment.

Oxygen vacancy-enhanced Ni3FeN/NF nanoparticle catalysts for efficient and stable electrolytic water splitting

The highly effective and economical electrocatalysts hold great importance in the context of water electrolysis. In this work, a nanosheet catalyst (Ni3FeN/NF) composed of nanoparticles for comprehensive water splitting was synthesized through a facile hydrothermal technique followed by a subsequent nitridation process. This catalyst is endowed with an abundance of oxygen vacancies, thereby conferring a richer array of active sites. Therefore, the catalyst demonstrates a markedly low overpotential for the OER of 271 mV at 50 mA cm−2 and an equally low overpotential for the HER of 35 mV at 10 mA cm−2. Serving as a dual-function electrode, this electrocatalyst is employed in overall water splitting in alkaline environments, demonstrating impressive efficiency of 10 mA cm−2 at a cell voltage of 1.45 V. In-situ Raman spectroscopy was employed to elucidate the active phase and dynamic surface structure of the Ni3FeN/NF catalyst by conducting measurements at intervals of 0.1 V. At open circuit voltage (OCV), Ni3FeN/NF exhibited pronounced peaks at 532 and 702 cm−1. Notably, at potentials exceeding 1.40 V, distinct peaks were observed at 473 and 551 cm−1. The manifestation of these dual peaks indicates that γ-Ni(Fe)OOH serves as the predominant active species in the oxygen evolution reaction (OER) mechanism for Ni3FeN/NF.

Ferromagnetic PEO coatings on titanium as photo-Fenton-like heterogeneous catalysts

Ferromagnetic ‘Ti/Fe- and/or Co-containing TiO2-layer’ composites formed by plasma electrolytic oxidation (PEO) in electrolytes with dispersed particles of iron and/or cobalt hydroxides exhibited photocatalytic activity in methyl orange (MO) degradation under UV and visible light in the presence of hydrogen peroxide. The influence of Fe/Co ratio and additional annealing was studied on their chemical composition, optical, photocatalytic, and adhesion properties. With the proportion of cobalt in the coatings, their adhesion to titanium increased. After annealing, coatings with a higher proportion of iron had higher adhesion. The optical band gap energy Eg determined for direct allowed transitions was 1.37 eV and 1.61 eV for individual Fe-containing and Co-containing composites, respectively. In mixed Fe- and Co-containing PEO-composites, two direct allowed transitions were detected with Eg1 = 1.68 eV and Eg2 = 2.21–2.54 eV. Among initial PEO-coated samples, the most active and stable are Fe- and Co-containing PEO layers with a predominance of cobalt: MO degradation under 3-h irradiation with UV and visible light was 64 and 21 %, respectively. According to X-ray photoelectron spectroscopy, their surface layers contain a higher percentage of Fe2+ and Co2+ compared to Fe3+ and Co3+. Annealing the samples in air led to an increase in Eg values and a leveling off of photocatalytic activity as a result of the formation of crystalline NaTi2(PO4)3, FePO4, or CoPO4 phases and an increase in the proportion of oxidized iron. The main role in the degradation of MO was found to be played by hydroxyl radicals formed as a result of the interaction of hydrogen peroxide with photogenerated electrons.

Sn-doped cobalt–iron hydroxide nanoarrays for enhanced electrocatalytic oxygen evolution in oilfield wastewater systems

Developing efficient electrocatalysts suitable for oilfield wastewater is considered the most crucial aspect of advancing photoelectrochemical water splitting for hydrogen production in arid regions. In this work, Sn-doped cobalt iron hydroxide (SnCoFe-LDH/NF) with a nanoarray structure was successfully prepared in situ on nickel foam using a simple electrodeposition method. The catalyst exhibits high oxygen evolution reaction (OER) activity in 1.0 mol L−1 KOH, requiring only an overpotential of 239 mV to provide a current density of 10 mA cm−2, with a Tafel slope as low as 37.8 mV dec−1. More importantly, it also maintains excellent electrocatalytic activity in 1.0 mol L−1 KOH+oilfield wastewater. The enhancement of electrocatalysis performance can be attributed to the incorporation of Sn ions and layered structure of hydroxide, which increases the inherent activity of the materials, speeds up the kinetics, and improves the synergy between the hydroxides. This study proposes a simple and efficient strategy to prepare cheap and efficient electrocatalysts operating in complex water systems.

Mechanism of Germanium Adsorption by Iron Hydroxide Colloids during the Leaching Process of Secondary Zinc Oxide.

In the leaching process of secondary zinc oxide, there is a problem of germanium loss caused by the colloidal adsorption of germanium by iron hydroxide (Fe(OH)3) formed by Fe3+ hydrolysis. In response to this, this article elucidates the hydrolysis conditions of Fe3+ and the adsorption mechanism of the Fe(OH)3 colloid on germanium through theoretical analysis and simulation of the adsorption process. The coexistence of Fe3+ and H2GeO3 requires high acidity conditions (pH < 1.53 at 25 °C). The adsorption of germanium by the Fe(OH)3 colloid is a spontaneous exothermic entropy reduction process, which conforms to a pseudo-second-order kinetic model and includes three stages: fast, slow, and equilibrium. In addition, the adsorption process can be fitted by the Langmuir isotherm adsorption model, mainly consisting of monolayer and chemical adsorption. The Fe(OH)3 colloid has a great adsorption capacity for germanium at 328 K, and the equilibrium adsorption capacity is 261.15 mg/g in 40 min. During leaching, the adsorption of germanium by Fe(OH)3 colloids can be inhibited by increasing the reaction temperature, controlling the pH value of the solution system, and suppressing the formation of Fe3+ at the source. This study provides direction for how to suppress the adsorption of germanium by Fe(OH)3 colloids during the leaching process of secondary zinc oxide, which is of great significance for improving the germanium leaching efficiency and fully utilizing limited germanium resources.

Alkali Extraction of Arsenic from Groundwater Treatment Sludge: An Essential Initial Step for Arsenic Recovery.

Arsenic (As)-bearing Fe(III) precipitate groundwater treatment sludge has traditionally been viewed by the water sector as a disposal issue rather than a resource opportunity, partly due to assumptions of the low value of As. However, As has now been classified as a Critical Raw Material (CRM) in many regions, providing new incentives to recover As and other useful components of the sludge, such as phosphate (P) and the reactive hydrous ferric oxide (HFO) sorbent. Here, we investigate alkali extraction to separate As from a variety of field and synthetic As-bearing HFO sludges, which is a critical first step to enable sludge upcycling. We found that As extraction was most effective using NaOH, with the As extraction efficiency increasing up to >99% with increasing NaOH concentrations (0.01, 0.1, and 1 M). Extraction with Na2CO3 and Ca(OH)2 was ineffective (<5%). Extraction time (hour, day, week) played a secondary role in As release but tended to be important at lower NaOH concentrations. Little difference in As extraction efficiency was observed for several key variables, including sludge aging time (50 days) and cosorbed oxyanions (e.g., Si, P). However, the presence of ∼10 mass% calcite decreased As release from field and synthetic sludges considerably (<70% As extracted). Concomitant with As release, alkali extraction promoted crystallization of poorly ordered HFO and decreased particle specific surface area, with structural modifications increasing with NaOH concentration and extraction time. Taken together, these results provide essential information to inform and optimize the design of resource recovery methods for As-bearing treatment sludge.

Exploring Rare Earth Element behavior in the Mount Etna volcanic aquifers (Sicily)

This study presents the first data on REY (Rare Earth Elements plus Yttrium) in the aquifer of Mount Etna (Sicily, Italy). Patterns normalized to chondrites indicate strong water–rock interaction, facilitated by a slightly acidic pH resulting from the dissolution of magma-derived CO2. REY patterns provide insights into the processes of both mineral dissolution and the formation of secondary phases. The relative abundance of light to heavy rare earth elements is compatible with the prevailing dissolution of ferromagnesian minerals (e.g., olivine or clinopyroxenes), reinforced by its strong correlation with other proxies of mineral dissolution (e.g., Mg contents). Pronounced negative Ce anomalies and positive Y anomalies demonstrate an oxidizing environment with continuous formation of secondary iron and/or manganese oxides and hydroxides. The Y/Ho fractionation is strongly influenced by metal complexation with bicarbonate complexes, a common process in C-rich waters. In the studied system, the measured REY contents are always below the limits proposed by Sneller et al. (2000, RIVM report, Issue 601,501, p. 66) for surface water and ensure a very low daily intake from drinking water.

Global sensitivity analysis with deep learning-based surrogate models for unraveling key parameters and processes governing redox zonation in riparian zone

The riparian zone constitutes an intricate redox environment, giving rise to distinct redox zones characterized by dissolved oxygen (DO), nitrate, manganese dioxide (Mn (IV)), iron hydroxide (Fe(III)), and sulfate. Processes encompassing river fluctuation, groundwater flow, microbial growth and death, as well as solute reactive transport, exert substantial influences on the spatiotemporal zonation of these redox zones in the riparian zone. Nonetheless, understanding remains elusive regarding how these processes govern the thickness of these zones. In this study, we built a one-dimensional (1-D) model which is adapted from Zhu et al. (2023) that integrates these processes to simulate the temporal variation of the thicknesses of different redox zones in riparian zone. Sobol’s sensitivity analysis method and the hierarchical sensitivity analysis framework based on Bayesian Networks (BNs) were then employed to quantify the sensitivities of the 17 selected parameters and the four generalized processes governing redox zonation, respectively. To mitigate the computational costs, multi-layer perceptron (MLP) was employed to establish the surrogate models, efficiently predicting model outputs for sensitivity analysis. The results of sensitivity analysis highlight the groundwater flow and reactive transport processes as the most important processes affecting thickness variations of the redox zones. Parameters such as the initial hydraulic conductivity (Ks0) and DOC concentration (CDOC) play a predominant role in governing the thickness variations. Parameters linked to river fluctuation and microbe growth processes exhibit very limited effects on redox zonation. These findings offer valuable insights into the factors controlling redox zones in the riparian environment.