Reactive Iron Oxide Research Articles

Iron-bound organic carbon and their determinants in peatlands of China

There is a strong correlation between iron oxide mineral and organic carbon (OC) concentrations. However, the role of iron oxide minerals in the preservation of OC in peatlands is poorly understood. In this study, we collected soil samples from 10 peatlands in central and west China to comprehensively investigate the concentration, characteristics, and spatial variability of iron oxide-bound OC (Fe-bound OC) concentrations and determine the impact of climatic factors and soil physicochemical properties on the correlation between OC and iron oxides. Our results showed that the concentration of Fe-bound OC ranged from 3.91 to 18.79 g kg−1. On average, Fe-bound OC accounted for 3.42 ± 1.32% of soil OC. The molar ratio of OC: Fe ranged from 2.17 to 12.18, indicating that coprecipitation was the dominant mechanism affecting the binding of OC to reactive iron oxides in peatlands. δ13C isotope analyses revealed that Fe-bound OC was relatively enriched in 13C in peatlands. Iron mineral sources, local vegetation, and the water table were responsible for differences in the concentration of Fe-bound OC identified among the 10 sites. Correlation analysis results indicated that total organic carbon (TOC) was the main determinant factor for the concentration of Fe-bound OC (r = 0.549, p < 0.01), and the binding mechanism was jointly regulated by TOC and reactive Fe content. The proportion of Fe-bound OC to TOC (fFe-bound OC) was positively correlated with the reactive Fe (r = 0.457, p < 0.05), indicating that the importance of binding with iron minerals in OC accumulation is regulated by the concentration of reactive Fe in peatlands. This study highlights the quantitative characteristics and spatial variability of Fe-bound OC in peatlands, providing important information for assessing and managing the carbon cycle in peatland ecosystems.

Immobilisation of Cadmium, Copper, Lead, and Nickel in Soil Using Nano Zerovalent Iron Particles: Ageing Effect on Heavy Metal Retention

Nano zerovalent iron (nZVI) is one of the most widely used nanotechnologies in soil remediation, but its long-term performance can be limited due to its high reactivity and rapid ageing. The aim of this study was to compare the immobilisation of Cd, Cu, Ni, and Pb in artificially contaminated soil samples using nZVI and to evaluate their retention over a 1-year period. Single-metal-contaminated soil samples were amended using 0–1.05% of nZVI. Leaching, using CaCl2 solution and a sequential extraction procedure, was performed to evaluate the immobilisation efficiency and fractionation of heavy metals before and after ageing. The results of the extractions showed that the application of nZVI initially reduced the exchangeable fraction of all heavy metals. The greatest immobilisation efficiency was achieved for Cu. The retention of heavy metals by nZVI was studied by constantly humidifying soil samples for a 12-month period. The results showed that over this period, the efficiency of nZVI treatment decreased, due to the decrease in the amount of reactive iron oxides in soil samples. Retention of immobilised heavy metals by nZVI strongly depended on their ionic radius. The highest retention results were obtained for Cu and Ni, whereas the lowest results were obtained for Cd and Pb.

The results prediction of iron oxides aluminothermy

"The paper presents the results and conclusions of the statistical processing of data on aluminothermic reduction reactions of iron oxides. Starting from the thermodynamic and chemical information regarding the aluminothermic reduction reactions of the iron oxide mixtures, their thermal and chemical effects were estimated and the information thus obtained was statistically processed. The results obtained by statistical processing of information obtained by theoretical reasons, presented in the form of regression equations or their graphical expressions, can be used to effectively determine the optimal phase composition of thermit kits based on iron oxides and forecast the value of reaction heat, resulting from the actual reactions development. The practical application of these results was ensured by the company QUARK IMPEX SRL, one of the main producers and users of thermit welding kits for rail joints by aluminothermal welding, in Romania. "

The reactivity of iron oxides and hydroxide during low-temperature sulfation

Sulfation is a technique commonly applied to the extraction of mineral commodities, such as rare earths, lithium, nickel, and titanium, as a preparation stage for leaching. The behavior of iron compounds during sulfation/dissolution will directly impact acid consumption and downstream purification steps. This work shows the marked differences in the sulfation/dissolution of hematite (granular, specular, and one produced by pyrite roasting), goethite, and magnetite samples over increased temperature and time. At 80 °C, the solubilized iron fraction varies within 2% to 70% accordingly specular hematite < calcine hematite < granular hematite < magnetite << goethite. Sulfation increases over time, from 5 to 15 min, when it reaches a plateau, ascribed to the formation of a ferric sulfate product layer. Increase in temperature from 80 °C to 160 °C increases the sulfation of magnetite (42.0 to 95.0%), calcine hematite (16.0 to 55.5%), and specular hematite (2.0 to 13.0%), with small or negative effect on goethite (70.0 to 76.0%) and granular hematite (33.0 to 25.0%). The influence of the crystalline structure, porosity, and phase transformation on the sulfation behavior of the different iron oxides and hydroxide, and the possible implications on the selective leaching of metals, are discussed in this article.

The river–groundwater interface as a hotspot for arsenic release

Geogenic groundwater arsenic (As) contamination is pervasive in many aquifers in south and southeast Asia. It is feared that recent increases in groundwater abstractions could induce the migration of high-As groundwaters into previously As-safe aquifers. Here we study an As-contaminated aquifer in Van Phuc, Vietnam, located ~10 km southeast of Hanoi on the banks of the Red River, which is affected by large-scale groundwater abstraction. We used numerical model simulations to integrate the groundwater flow and biogeochemical reaction processes at the aquifer scale, constrained by detailed hydraulic, environmental tracer, hydrochemical and mineralogical data. Our simulations provide a mechanistic reconstruction of the anthropogenically induced spatiotemporal variations in groundwater flow and biogeochemical dynamics and determine the evolution of the migration rate and mass balance of As over several decades. We found that the riverbed–aquifer interface constitutes a biogeochemical reaction hotspot that acts as the main source of elevated As concentrations. We show that a sustained As release relies on regular replenishment of river muds rich in labile organic matter and reactive iron oxides and that pumping-induced groundwater flow may facilitate As migration over distances of several kilometres into adjacent aquifers. The interface between riverbed and aquifer is a biogeochemical reaction hotspot for arsenic release from river sediments, according to numerical simulations of groundwater flow and biogeochemical reaction processes.

Iron redox cycling, sediment resuspension and the role of sediments in low oxygen environments as sources of iron to the water column

Continental margin sediments represent a potentially large, but poorly constrained, source of iron to surface waters to support primary productivity. To investigate this problem, we examined iron redox cycling in the sediments of Santa Monica Basin (SMB), CA as it relates to benthic sources of iron to the water column. Our results show that iron redox cycling in SMB sediments results in the formation of a thin layer at the sediment surface (<2 cm thick) that is highly enriched in amorphous, reactive iron oxides. Such oxides, when resuspended into the water column, may be a source of bioaccessible iron to primary producers. Calculations show that the resuspension flux of reactive amorphous iron oxides from SMB sediments may be up to two orders of magnitude greater than benthic fluxes of soluble dissolved iron from most continental margin sediments. The factors which favor the formation of this surface sediment layer enriched in reactive amorphous iron oxides appear to be related to the dissolved O2 level in SMB bottom waters, and its impact on iron redox cycling and the occurrence of sediment bioturbation. These factors are not necessarily unique to SMB, and other sediment sites on the open continental margin that lie within the O2 minimum zone may also have similar properties. Furthermore, expansion of O2 minimum zones due to global warming could possibly increase the areal extent of such environments, and potentially act as a negative feedback on rising atmospheric CO2 by providing additional iron to stimulate primary production. Further work examining iron biogeochemistry in low O2 continental margin settings will be needed to more critically examine these possibilities, along with studies that better define the role sediment resuspension fluxes may play as an iron source to surface waters. Such efforts will ultimately lead to an improved understanding of the role that ocean deoxygenation may play in enhancing this sediment iron source to support surface ocean primary production.

Sedimentation and Transport of Different Soil Colloids: Effects of Goethite and Humic Acid

Soil colloids significantly facilitate the transport of contaminants; however, little is known about the effects of highly reactive iron oxide and the most representative organic matter on the transport of soil colloids with different physicochemical properties. This study investigated the effects of goethite (GT) and humic acid (HA) on the sedimentation and transport of soil colloids using settling and column experiments. The stability of soil colloids was found to be related to their properties and decreased in the following order: black soil colloids (BSc) &gt; yellow soil colloids (YSc) &gt; fluvo-aquic soil colloids (FSc). Organic matter increased the stability of BSc, and ionic strength (Ca2+) promoted the deposition of FSc. Colloids in individual and GT colloids (GTc) coexistence systems tended to stabilize at high pH and showed a pH-dependence whereby the stability decreased with decreasing pH. The interaction of GTc and kaolinite led to a dramatic sedimentation of YSc at pH 4.0. HA enhanced the stability of soil colloids, especially at pH 4.0, and obscured the pH-dependent sedimentation of soil colloids. The transport ability of soil colloids was the same as their stability. The addition of GT retarded the transport of soil colloids, which was quite obvious at pH 7.0. This retardation effect was attributed to the transformation of the surface charge of sand from negative to positive, which increased the electrical double-layer attraction. Although sand coated with GT–HA provided more favorable conditions for the transport of soil colloids in comparison to pure sand, the corresponding transport was relatively slow. This suggests that the filtration effect, heterogeneity, and increased surface roughness may still influence the transport of soil colloids.

Effect of sulfur on hydrogen-reduction behavior of iron oxide during microwave heating

Hydrogen-reduction behavior of a high-sulfur magnetite sample was investigated in the presence of CaO under microwave irradiation at 1050 W and 2.45 GHz to clarify the effect of sulfur on the microwave-assisted reduction reaction of iron oxides. Results of the microscope observations showed a gap between the reduced metallic iron and its surrounding, which is a high-sulfur iron oxide. Formation of such a gap is likely attributed to a lower microwave absorbability of the reduced Fe than the sulfur-contained iron oxide which results in a different thermal expansion in the phases during microwave heating. Such a gap in the high-sulfur magnetite sample enhances the hydrogen-reduction reaction as compared with a pure magnetite sample owing to higher gas accessibility to un-reacted parts in the former sample.

Organo-mineral complexes protect condensed organic matter as revealed by benzene-polycarboxylic acids

Condensed organic matters (COM) with black carbon-like structures are considered as long-term carbon sinks because of their high stability. It is difficult to distinguish COM from general organic matter by conventional chemical analysis, thus the contribution by and interaction mechanisms of organo-mineral complexes in COM stabilization are unclear and generally neglected. Molecular markers related to black carbon-like structures, such as benzene polycarboxylic acids (BPCAs), are promising tools for the qualitative and quantitative analysis of COM. In this study, one natural soil and two cultivated soils with 25 y- or 55 y-tillage activities were collected and the distribution characteristics of BPCAs were detected. All the investigated soils showed similar BPCA distribution pattern, and over 60% of BPCAs were detected in clay fraction. The extractable BPCA contents were substantially increased after mineral removal. The ratios of BPCA contents before and after mineral removal indicate the extent of COM-mineral particle interactions, and our results suggested that up to 73% COM were protected by mineral particles, and more stronger interactions were noted on clay than on silt. The initial cultivation dramatically decreased COM-clay interactions, and this interaction was recovered only slowly after 55-y cultivation. Kaolinite and muscovite are important for COM protection. But a possible negative correlation between BPCAs and reactive iron oxides of the cultivated soils suggested that iron may promote COM degradation when disturbed by tillage activities. This study provided a new angle to study the stabilization of COM and emphasized the importance of organo-mineral complexes for COM stabilization.

Study on the Mechanism of Carbon Steel Sparks

Abstract Spark testing is an effective and convenient method that can be used be identify general classification of the steel material by observing the sparks projected by the grinding wheel. In order to more accurately identify the type of spark, the spark streams of carbon steels are described and illustrated by a high-speed camera. The appearance and phase analysis of the residues collected from the spark streams are studied separately by scanning electron microscope and X-ray diffractometer. Results show that the spark streams are the trajectory of the movement of carbon steel wear debris. The luminescence of the debris is provided by the heat released by the iron oxidation reaction. The carbon burst is caused by the explosion of carbon monoxide (CO) on the surface of carbon steel wear debris, and CO is produced by the reduction reaction of carbon and iron oxides. The main shape of the residues is flaky and spherical. There are some holes caused by carbon burst on the surface of the spherical residues. For the qualitative phase analysis of the residues, the main phase is composed of iron, ferrous oxide (FeO), ferric oxide (Fe2O3), and ferroferric oxide (Fe3O4). As the increase of the carbon content of steel, the quantity of iron phase is greatly reduced, and the quantity of Fe2O3 and Fe3O4 phase is gradually increased. Moreover, the amount and density of carbon burst is increased, and the number of the holes in the spherical residues are also increased.

Corrosion behavior of SA508 low alloy steels exposed to aerated boric acid solutions

The corrosion rates of the reactor pressure vessel materials of SA508 Grade 3 were measured using a weight loss method in aerated boric acid solutions to simulate the evaporation of leaked PWR primary water in an ambient environment. The corrosion behavior and products were examined using X-ray diffraction and electron microscopy. SA508 showed typical general corrosion characteristics. The corrosion rate increased steadily as the boron concentration was increased. As the immersion time elapsed, the corrosion rate slowly or rapidly decreased according to the oxidation reaction of iron. The corrosion rate showed a complicated pattern depending on the temperature; it increased gradually and then rapidly decreased again when reaching a certain transition temperature. The corrosion products of SA508 were found to be FeO(OH), Fe2O3, and Fe3O4. As the boron concentration decreased and the temperature was increased, the formation of Fe3O4 was more favorable as compared to the formation of FeO(OH) and Fe2O3. Consequently, the changes of the corrosion rate and behavior were closely related to the oxidation reaction of iron on the surface. The corrosive damage to SA508 appears to be most severe when the oxidation reaction is such that Fe2O3 forms as a corrosion product.

Kinetics of Ni reaction with organic matter-ferrihydrite composites: Experiments and modeling

The interactions between organic matter (OM) and minerals are among the most important environmental processes, which significantly affect the fate of carbon and contaminants in the environment. How the interactions between OM and ferrihydrite (Fh), one of the most reactive iron oxides, affect the kinetic reactions of heavy metals with both OM and iron oxides, however, are still largely unknown. In this study, two typical OM, humic acids (HA) and fulvic acids (FA), were used to synthesize OM-Fh co-precipitation composites. We investigated the interactions between OM and ferrihydrite and their impact on kinetic reactions of nickel (Ni) with OM-Fh composites using spherical aberration corrected scanning transmission electron microscopy (Cs-STEM), batch and kinetic experiments, and X-ray adsorption spectroscopy (XAS). Cs-STEM results, at both nano and sub-nano scales, showed three typical distributions of OM on ferrihydrite, adsorbed on ferrihydrite surfaces, accumulated as OM nanoparticles, and being blocked in tiny pores in composites. Batch experiments and modeling quantified the effects of OM and ferrihydrite interactions, which was supported by XAS results. The kinetics model developed in this study integrated both WHAM 7 and CD-MUSIC models and successfully predicted the kinetics of Ni adsorption and desorption on OM-Fh composites under the impact of OM-Fh interactions. The model can quantitatively assess the dynamic changes of Ni speciation on different OM and Fh binding sites during the kinetic processes. Our results contribute to developing a comprehensive model for predicting the dynamic metal behavior in the environment when multiple adsorbents are present.

Tailored synthesis from rhombic dodecahedron to spherical ordered mesoporous carbon nanoparticles via one-step strategy

Owing to the shallow understanding of the formation mechanism of ordered mesoporous carbon nanoparticles (OMCNs), there are still many obstacles to the synthesis of OMCNs with controllable morphology by a simple and efficient method. Herein, we first propose a universal mechanism for OMCNs formation, that is, the composite micelle units self-assembly mechanism. Under the guidance of this mechanism, the inexpensive F127 was selected as the soft template, resorcinol and hexamethylenetetramine were used as the carbon source, and a series of dispersions were synthesized for the first time under acidic condition by the one-step aqueous solution method. Interestingly, OMCNs could be transformed from spherical to rhombic dodecahedron by adjusting the amount of F127. Moreover, the particle size of the obtained polymer nanoparticles (PNs) exhibited a linear relationship with the amount of F127 in a wide range (WF127 = 0.25 g–0.70 g), and the linear equation is y=−1523.6x+1055.8 (R2 = 0.99), which can realize the customized synthesis of PNs. As a demonstration of their versatility, a simple and novel method was developed to prepare magnetic Fe-OMCNs by iron doping of PNs by oxidation reaction of iron. These findings have pioneering implications for promoting the development of OMCNs.

Arsenic adsorption and removal by a new starch stabilized ferromanganese binary oxide in water

A new starch stabilized ferromanganese binary oxide (starch-FMBO) with an Fe/Mn ratio of 1.00–2.00 and a synthetic pH of 2 was prepared using an organic polymer (starch) as the stabilizing dispersant. The maximum arsenic adsorption capacity of starch-FMBO was 161.29 mg/g. Adsorption optimization was also conducted, which revealed that starch-FMBO had a high adsorption capacity over a wide pH range (pH 3.0 to pH 11.0) and in the presence of some common anions (HCO3−, SO42−, Cl−, and NO3−). Arsenic removal by FMBO and starch-FMBO followed pseudo-second-order dynamics (R2 ≥ 0.99), indicating that the adsorption rate depended on the chemical adsorption process. Through the analysis of X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR), it was shown that hydroxyl was continuously produced and complexated with As(III) and As(V) during the adsorption process. Thus, the reaction of iron oxide and manganese oxide with arsenic was inferred and explained. The developed starch-FMBO shows promise for its application in the treatment of arsenic-contaminated water.

Carbon, iron and sulphur cycling in the sediments of a Mediterranean lagoon (Ghar El Melh, Tunisia)

Coastal lagoon sediments are important for the biogeochemical carbon cycle at the land-ocean transition, as they form hotspots for organic carbon burial, as well as potential sites for authigenic carbonate formation. Here, we employ an early diagenetic model to quantify the coupled redox cycling of carbon, iron and sulphur in the sediments of the shallow Ghar El Melh (GEM) lagoon (Tunisia). The model simulated depth profiles show a good correspondence with available pore water data (dissolved inorganic carbon, NH4+, total alkalinity, Ca2+, Fe2+ and SO42−) and solid phase data (organic matter, pyrite, calcium carbonate and iron (oxyhydr)oxides). This indicates that the model is able to capture the dominant processes influencing the sedimentary biogeochemical cycling. Our results show that sediment of the GEM lagoon is an efficient reactor for organic matter breakdown (burial efficiency < 10%), with an important role for aerobic respiration (32%) and sulphate reduction (61%). Despite high rates of sulphate reduction, free sulphide does not accumulate in the pore water, due to a large terrestrial input of reactive iron oxides and the efficient sequestration of free sulphide into iron sulphide phases. High pyrite burial (2.2 mmol FeS2 m−2 d−1) prevents the reoxidation of reduced sulphide, thus resulting in a low total oxygen uptake (4.7 mmol m−2 d−1) of the sediment and a relatively high oxygen penetration depth. The formation of pyrite also generates high amounts of alkalinity in the pore water, which stimulates authigenic carbonate precipitation (2.7 mmol m−2 d−1) and leads to alkalinity release to the overlying water (3.4 mmol m−2 d−1). Model simulations with and without an N-cycle reveal a limited influence of nitrification and denitrification on overall organic matter diagenesis. Overall, our study highlights the potential role of coastal lagoons for the global carbon and sulphur cycle, and their possible contribution to shelf alkalinity, which increases the buffering capacity of the coastal ocean for CO2 uptake.

Electrochemical Analysis of Changes in Iron Oxide Reducibility during Abiotic Ferrihydrite Transformation into Goethite and Magnetite.

Electron transfer to ferric iron in (oxyhydr-)oxides (hereafter iron oxides) is a critical step in many processes that are central to the biogeochemical cycling of elements and to pollutant dynamics. Understanding these processes requires analytical approaches that allow for characterizing the reactivity of iron oxides toward reduction under controlled thermodynamic boundary conditions. Here, we used mediated electrochemical reduction (MER) to follow changes in iron oxide reduction extents and rates during abiotic ferrous iron-induced transformation of six-line ferrihydrite. Transformation experiments (10 mM ferrihydrite-FeIII) were conducted over a range of solution conditions (pHtrans = 6.50 to 7.50 at 5 mM Fe2+ and for pHtrans = 7.00 also at 1 mM Fe2+) that resulted in the transformation of ferrihydrite into thermodynamically more stable goethite or magnetite. The changes in iron oxide mineralogy during the transformations were quantified using X-ray diffraction analysis. MER measurements on iron oxide suspension aliquots collected during the transformations were performed over a range of pHMER at constant applied reduction potential. The extents and rates of iron oxide reduction in MER decreased with decreasing reaction driving force resulting from both increasing pHMER and increasing transformation of ferrihydrite into thermodynamically more stable iron oxides. We show that the decreases in iron oxide reduction extents and rates during ferrihydrite transformations can be linked to the concurrent changes in iron oxide mineralogy.

Simultaneous desulfuration and denitrogenation of model diesel fuel by Fe-Mn microwave modified activated carbon: Iron crystalline habit influence on adsorption capacity

Nowadays, diesel desulfuration is of the utmost importance for the petrochemical industry in order to achieve efficient technologies for the production of clean fuels. Adsorption under normal conditions has the potential to produce ultra-low sulfur diesel if advanced materials capable of simultaneously removing both sulfur and nitrogen can be designed. This work contributes to the technology of clean fuels by using iron oxide-activated carbons (AC) that were produced with a microwave-assisted technique in the simultaneous adsorption of sulfur (dibenzothiophene and 4,6-dimethyldibenzothiophene) and nitrogen (indole and quinoline) compounds from model diesel fuel. Crystal habit was tailored with the addition of Mn-ions. The materials were characterized and their capacity was measured at batch and dynamic conditions. The use of Mn during Fe synthesis led to the deposition of reactive iron oxide twinning particles, while keeping most of the pore structure of the activated carbon. This modification allowed a higher total and simultaneous denitrogenation and desulfurization capacity and faster adsorption kinetics unlike iron and manganese modifications of activated carbon. Column dynamic experiments demonstrated that it is feasible to remove the adsorbates up to 200 bed volumes without reaching saturation of the adsorbent. The results demonstrated that twinned iron oxide particles deposited onto activated carbon were more reactive towards sulfur and nitrogen compounds. These particles were only obtained when Mn was present.

Dissolution, Adsorption, and Redox Reaction in Ternary Mixtures of Goethite, Aluminum Oxides, and Hydroquinone

To better understand the oxidative reactivity of iron oxides in the fate of contaminants in acidic environments, we examined the reactivity of goethite in binary mixtures with Al2O3 by carrying out oxidation experiments of hydroquinone (HQ) in the presence of goethite and/or Al2O3 at pH 3. Kinetic results revealed inhibiting effects of 0.2–20 g/L of three different types of Al2O3 on the oxidative reactivity of goethite. Surprisingly, soluble Al ions of 0.18–18 mM had a negligible impact on the reactivity. It turned out that the Fe3+ dissolved from goethite partly contributed to the observed HQ oxidation and the Al2O3 adsorbed the Fe3+ to lead to the slower HQ oxidation. The observed pseudo-first-order rate constants in HQ oxidation had a strong linear correlation with Fe3+ concentration in various goethite and Al2O3 mixtures. Separate experiments confirmed the reactivity of Fe3+ toward HQ and the linear correlation between [Fe3+] and HQ oxidation reactivity. Finally, sedimentation experiments showed negli...

Understanding the origins of N2O decomposition activity in Mn(Fe)CoAlOx hydrotalcite derived mixed metal oxides

The catalytic decomposition of N2O was studied over a series of calcined Mn(Fe)CoAl hydrotalcite-like compounds. The precursors were prepared by coprecipitation and characterized by XRD and TGA. The mixed metal oxides derived after calcination at 600 °C were characterized by XRD, N2 adsorption, H2-TPR and XPS. Moreover, in situ XAFS measurements over selected mixed metal oxides were performed. Such investigations under relevant reaction conditions are rare, while a comprehensive understanding of the involved active species may facilitate a knowledge-based catalyst optimization. The activity of the CoAlOx (Co/Al = 3/1, mol.%) catalyst varied depending on the loading of Mn or Fe (0.0575, 0.0821, 0.1150, 0.1725, 0.2300, mol.%). In the investigated series, Mn0.1725Co3AlOx reached the highest activity with T50 of about 305 and 376 °C under N2O/N2 and N2O,NO,O2/N2 feed, respectively. In situ X-ray absorption experiments over Mn0.1725Co3AlOx suggested that MnxCoyO4 spinels undergo reduction to CoO and MnO upon heating up to 600 °C in He. Under N2O/He conditions, initial reoxidation of cobalt species began at 350 °C. The lower activity obtained for Fe0.1725Co3AlOx is explained by the fact that the majority of Fe was not incorporated into the Co3O4 structure but instead formed less reactive iron oxide clusters.

Benthic fluxes of metals into the Pearl River Estuary based on 224Ra/228Th disequilibrium: From alkaline earth (Ba) to redox sensitive elements (U, Mn, Fe)

We extended the 224Ra/228Th disequilibrium approach to examine benthic fluxes of a variety of metals, ranging from alkaline earth (Ba) to redox sensitive elements (U, Mn, and Fe), into the Pearl River Estuary (PRE), China. Depth profiles of 224Ra and 228Th in bulk sediment, as well as dissolved 224Ra and trace metals in porewater were measured along a transect within the estuary in July 2015. Significant deficit of 224Ra relative to 228Th was commonly observed in the upper 0–15 cm sediment. We took advantage of the 224Ra/228Th disequilibrium in the bottom sediments to construct a full mass balance of 224Ra in the overlying water column. We demonstrated that porewater exchange (PEX) processes with scale lengths of several centimeters are the predominant mechanism for solute transport between sediments and overlying waters in the PRE. In contrast, deep porewater flow or submarine groundwater discharge (SGD) with scale lengths of “meters to kilometers” are a negligible component in the water column budget of 224Ra.Strong correlations between dissolved 224Ra and trace metals (Ba, U, Mn, and Fe) in porewater were frequently observed in the study region. This likely reflects a fact that geochemical cycling of alkaline earth elements (e.g., Ra and Ba) and redox sensitive elements (like U) in sediments was closely linked to diagenetic reactions of manganese and iron oxides. This linkage makes it possible to quantify benthic fluxes of alkaline earth and redox sensitive metals using 224Ra/228Th disequilibrium in sediments. Benthic Ba fluxes based on 224Ra/228Th disequilibrium were found to vary from virtually nil to 320 μmol m−2 d−1 within the PRE. The highest flux was identified at salinity = 3.0–7.8 and could lead to an elevation of 54 nmol Ba l−1 in the water column, which well reproduced the Ba excess frequently observed in the low salinity domain of the estuary. Benthic fluxes of redox sensitive U ranged from −0.42 (“−” denotes flux into sediment) to 1.3 μmol m−2 d−1. This could only cause a change of −0.1 to 0.3 nmol U l−1 in the water column, which is very small when compared to the U concentration of 13–14 nmol l−1 in the northern South China Sea. We therefore predicted that water column U in the PRE must behave conservatively during mixing. This prediction is consistent with historical measurements of water column U concentration within the PRE. Large benthic fluxes of Mn and Fe were generally acquired with the 224Ra/228Th disequilibrium method. They varied from virtually nil up to 97 mmol m−2 d−1, and from zero to 27 mmol m−2 d−1, respectively. These estimates are 1–2 orders of magnitude higher than historical measurements based on the traditional incubation method in other coastal settings. Nonetheless, they are in agreement with a simple consideration of Mn and Fe mass balances in the sediment. An important implication of this study is that the role of coastal sediments in estuarine geochemistry of trace metals may need to be re-evaluated.