Amorphous Ferric Oxyhydroxide Research Articles

Redox Oscillations Activate Thermodynamically Stable Iron Minerals for Enhanced Reactive Oxygen Species Production.

Reactive oxygen species (ROS) play key roles in driving biogeochemical processes. Recent studies have revealed nonphotochemical electron transfer from redox-active substances (e.g., iron minerals) to oxygen as a new route for ROS production. Yet, naturally occurring iron minerals mainly exist in thermodynamically stable forms, restraining their potential for driving ROS production. Here, we report that tide-induced redox oscillations can activate thermodynamically stable iron minerals for enhanced ROS production. •OH production in intertidal soils (15.8 ± 0.5 μmol/m2) was found to be 5.9-fold more efficient than those in supratidal soils. Moreover, incubation of supratidal soils under tidal redox fluctuations dramatically enhanced •OH production by 4.3-fold. The tidal hydrology triggered redox alternation between biotic reduction and abiotic oxidation and could accelerate the production of reactive ferrous ions and amorphous ferric oxyhydroxides, making thermodynamically stable iron minerals into redox-active metastable iron phases (RAMPs) with reduced crystallinity and promoting surface electrochemical activities. Those RAMPs displayed enhanced redox activity for ROS production. Investigations of nationwide coastal soils verified that tide-induced redox oscillations could ubiquitously activate soils for enhanced ROS production. Our study demonstrates the effective formation of RAMPs from redox oscillations by hydrological perturbations, which provides new insights into natural ROS sources.

Phosphate Recovery from Aqueous Solutions via Vivianite Crystallization: Interference of FeII Oxidation at Different DO Concentrations and pHs.

Vivianite (Fe3(PO4)2·8H2O) crystallization has attracted increasing attention as a promising approach for removing and recovering P from wastewaters. However, FeII is susceptible to oxygen with its oxidation inevitably influencing the crystallization of vivianite. In this study, the profile of vivianite crystallization in the presence of dissolved oxygen (DO) was investigated at pHs 5-7 in a continuous stirred-tank reactor. It is found that the influence of DO on vivianite crystallization was highly pH-related. At pH 5, the low rate of FeII oxidation at all of the investigated DO of 0-5 mg/L and the low degree of vivianite supersaturation resulted in slow crystallization with the product being highly crystalline vivianite, but the P removal efficiency was only 30-40%. The removal of P from the solution was substantially more effective (to >90%) in the DO-removed reactors at pH 6 and 7, whereas the efficiencies of P removal and especially recovery decreased by 10-20% when FeII oxidation became more severe at DO concentrations >2.5 mg/L (except at pH 6 with 2.5 mg/L DO). The elevated degree of vivianite supersaturation and enhanced rate and extent of FeII oxidation at the higher pHs led to decreases in the size and homogeneity of the products. At the same pH, amorphous ferric oxyhydroxide (AFO)─the product of FeII oxidation and FeIII hydrolysis─interferes with vivianite crystallization with the induction of aggregation of crystal fines by AFO, leading to increases in the size of the obtained solids.

Enhancing effects of dissolved and media surface-bound organic matter on titanium dioxide nanoparticles transport in iron oxide-coated porous media under acidic conditions

Natural organic matter (NOM) and iron oxides have been proved to be crucial factors controlling the behaviors of nanoparticles in heterogenous environment. Here, we conducted experimental and modeling study on the transport of titanium dioxide nanoparticles (TiO2 NPs) in iron oxide-coated quartz in the presence of NOM under acidic conditions. Results showed the antagonistic effects of iron oxides and NOM on TiO2 NPs mobility. The inhibition of iron oxides coated on quartz was crystal form-dependent other than quantity-dependent. Amorphous ferric oxyhydroxide with higher specific surface area brought more positive charge and favorable deposition sites onto quartz, and induced more retention of nanoparticles than two crystalline iron oxides, goethite and hematite. Dissolved organic matter (DOM) facilitated TiO2 NPs transport in iron oxide-coated quartz. In comparation with the limited enhancing effects of DOM, the NOM coatings on media surface partially or largely offset the inhibition of goethite on nanoparticles mobility through direct occupation of attachment sites and sites screening due to the steric repulsion of the macromolecules. Owing to the higher steric hindrance, humic acid, both in dissolved and media surface-bound states, exerted stronger facilitating effects on TiO2 NPs mobility relative to fulvic acid.

Fungal Mobilization of Selenium in the Presence of Hausmannite and Ferric Oxyhydroxides.

Bioleaching of mineral phases plays a crucial role in the mobility and availability of various elements, including selenium. Therefore, the leachability of selenium associated with the surfaces of ferric and manganese oxides and oxyhydroxides, the prevailing components of natural geochemical barriers, has been studied in the presence of filamentous fungus. Both geoactive phases were exposed to selenate and subsequently to growing fungus Aspergillus niger for three weeks. This common soil fungus has shown exceptional ability to alter the distribution and mobility of selenium in the presence of both solid phases. The fungus initiated the extensive bioextraction of selenium from the surfaces of amorphous ferric oxyhydroxides, while the hausmannite (Mn3O4) was highly susceptible to biodeterioration in the presence of selenium. This resulted in specific outcomes regarding the selenium, iron, and manganese uptake by fungus and residual selenium concentrations in mineral phases as well. The adverse effects of bioleaching on fungal growth are also discussed.

Dissimilatory Iron-Reducing Microorganisms Are Present and Active in the Sediments of the Doce River and Tributaries Impacted by Iron Mine Tailings from the Collapsed Fundão Dam (Mariana, MG, Brazil)

On 5 November 2015, a large tailing deposit failed in Brazil, releasing an estimated 32.6 to 62 million m3 of iron mining tailings into the environment. Tailings from the Fundão Dam flowed down through the Gualaxo do Norte and Carmo riverbeds and floodplains and reached the Doce River. Since then, bottom sediments have become enriched in Fe(III) oxyhydroxides. Dissimilatory iron-reducing microorganisms (DIRMs) are anaerobes able to couple organic matter oxidation to Fe(III) reduction, producing CO2 and Fe(II), which can precipitate as magnetite (FeO·Fe2O3) and other Fe(II) minerals. In this work, we investigated the presence of DIRMs in affected and non-affected bottom sediments of the Gualaxo do Norte and Doce Rivers. The increase in Fe(II) concentrations in culture media over time indicated the presence of Fe(III)-reducing microorganisms in all sediments tested, which could reduce Fe(III) from both tailings and amorphous ferric oxyhydroxide. Half of our enrichment cultures converted amorphous Fe(III) oxyhydroxide into magnetite, which was characterized by X-ray diffraction, transmission electron microscopy, and magnetic measurements. The conversion of solid Fe(III) phases to soluble Fe(II) and/or magnetite is characteristic of DIRM cultures. The presence of DIRMs in the sediments of the Doce River and tributaries points to the possibility of reductive dissolution of goethite (α-FeOOH) and/or hematite (α-Fe2O3) from sediments, along with the consumption of organics, release of trace elements, and impairment of water quality.

Assessment of Aspergillus niger Strain's Suitability for Arsenate-Contaminated Water Treatment and Adsorbent Recycling via Bioextraction in a Laboratory-Scale Experiment.

In this work, the viability of bioaccumulation and bioextraction processes for arsenic removal from contaminated waters, as well as the recycling of arsenate-treated amorphous ferric oxyhydroxide adsorbent (FeOOH) were evaluated using the common soil microscopic filamentous fungus Aspergillus niger. After treating the contaminated arsenate solution (100 mg As L−1) with FeOOH, the remaining solution was exposed to the growing fungus during a static 19-day cultivation period to further decrease the arsenic concentration. Our data indicated that although the FeOOH adsorbent is suitable for arsenate removal with up to 84% removal efficiency, the fungus was capable of accumulating only up to 13.2% of the remaining arsenic from the culture media. This shows that the fungus A. niger, although highly praised for its application in environmental biotechnology research, was insufficient for decreasing the arsenic contamination to an environmentally acceptable level. However, the bioextraction of arsenic from arsenate-treated FeOOH proved relatively effective for reuse of the adsorbent. Due to its production of acidic metabolites, which decreased pH below 2.7, the fungal strain was capable of removing of up to 98.2% of arsenic from the arsenate-treated FeOOH adsorbent.

In-situ growth of layered double hydroxide films on biomedical magnesium alloy by transforming metal oxyhydroxide

Layered double hydroxide (LDH) films have been widely used for corrosion protection and medical applications. The most common synthesis methods for in-situ growth of LDH films on magnesium (Mg) alloys are hydrothermal treatment and steam coating. However, both the methods are mainly used for preparing MgAl LDH films and have limitations to in-situ fabricate other Mg-based LDH films which are suitable for biomedical application. Herein, a novel synthesis strategy for in-situ growth of LDH films on Mg alloys is proposed. Firstly, amorphous ferric oxyhydroxide (FeOOH) film was developed on Mg-Nd-Zn-Zr (JDBM) alloy by electrodeposition technique. Then, the as-prepared FeOOH films were transformed into MgFe LDH film via hydrothermal treatment in water. In the LDH formation process, Mg2+ would enter into amorphous FeOOH and replace the Fe3+ quickly to form LDH. The thickness of the LDH films can be easily controlled by adjusting electrodeposition time. Electrochemical tests and cells culture experiments indicated that the as-prepared MgFe LDH films improved corrosion resistance and biocompatibility of JDBM alloy. In addition, MgMn LDH can also be prepared from amorphous manganese oxyhydroxide (MnOOH) via this newly developed two-step method. The present work provides a new strategy for preparation of LDH films on Mg alloys surface.

Selenite Distribution in Multicomponent System Consisting of Filamentous Fungus, Humic Acids, Bentonite, and Ferric Oxyhydroxides

Selenite is an environmental toxicant whose mobility is affected in soil by various natural components, including humic acids, clay minerals, amorphous ferric oxyhydroxides [FeOx(OH)y], and microorganisms. However, interactions of selenite with these components are usually evaluated separately. Therefore, we addressed selenite behavior in multicomponent system in this study with emphasis on its immobilization and fungal accumulation. Our results highlighted significant acidification of culture medium by common soil fungus Aspergillus niger’s production of protons in selenite presence which affected selenium immobilization. While bentonite and humic acids did not enhanced immobilization efficiency significantly, composite of FeOx(OH)y/humic acids was extremely successful in selenium immobilization. This was most likely by enhanced redox transformation of selenite into elemental selenium as the contribution of each component was synergistic. Subsequently, selenium bioavailability in culture medium decreased and negatively affected bioaccumulation efficiency by fungus. Our results highlighted the significance of multicomponent systems in more realistic evaluation of selenium mobility and transfer to microorganisms.

Comparison of Arsenic Adsorption on Goethite and Amorphous Ferric Oxyhydroxide in Water

Except for the specific surface area and pore size, the hydroxyl groups on the surface of the ferric oxides were determined as the key factor in arsenic adsorption in this study. Two synthetic mesoporous ferric oxides, amorphous ferric oxyhydroxide (AFO) and goethite, were used to adsorb As(III) and As(V) in aqueous solution. The experimental results showed that the AFO had a higher hydroxyl group density, resulting in a higher arsenic adsorption capacity than that on the goethite for both As(III) and As(V). Also, it was found that the adsorption of As(III) on both the goethite and AFO was faster than that of As(V), and the adsorption rate fitted the pseudo-second-order kinetics. The findings indicated a promising modification of adsorbents for arsenic remediation.

Rapid oxidation and immobilization of arsenic by contact glow discharge plasma in acidic solution

Arsenic is a priority pollutant in aquatic ecosystem and therefore the remediation of arsenic-bearing wastewater is an important environmental issue. This study unprecedentedly reported simultaneous oxidation of As(III) and immobilization of arsenic can be achieved using contact glow discharge process (CGDP). CGDP with thinner anodic wire and higher energy input were beneficial for higher As(V) production efficiency. Adding Fe(II) in CGDP system significantly enhanced the oxidation rate of As(III) due to the generations of additional OH and Fe(IV) species, accompanied with which arsenic can be simultaneously immobilized in one process. Arsenic immobilization can be favorably obtained at solution pH in the range of 4.0–6.0 and Fe(II) concentration from 250 to 1000μM. The presence of organics (i.e., oxalic acid, ethanol and phenol) retarded the arsenic immobilization by scavenging OH or complexing Fe(III) in aqueous solution. On the basis of these results, a mechanism was proposed that the formed ionic As(V) rapidly coprecipitated with Fe(III) ions or was adsorbed on the ferric oxyhydroxides with the formation of amorphous ferric arsenate-bearing ferric oxyhydroxides. This CGDP-Fenton system was of great interest for engineered systems concerned with the remediation of arsenic containing wastewater.

Biologically Induced Mobilization of Arsenic Adsorbed onto Amorphous Ferric Oxyhydroxides in Aqueous Solution During Fungal Cultivation

This paper evaluates heterotrophic leaching of arsenic (As) pre-adsorbed onto amorphous ferric oxyhydroxides (FeOx) and its subsequent biovolatilization under laboratory conditions during Aspergillus niger static cultivation. With initial 90 mg.L−1 As concentration and absence of FeOx, the biomass As accumulation capacity attained 1.4 mg.g−1 on the 15th day of cultivation. While FeOx suppressed As biomass accumulation up to 0.23 mg.g−1, it did not influence biovolatilization activity. After 15-day cultivation, almost 1.8 mg As was released into the surrounding culture media, accumulated and subsequently transformed into its volatile derivatives, regardless of FeOx presence or absence. The A. niger strain was able to enhance As release from the FeOx surfaces; the total As medium concentration increased to 3.1 mg.L−1 on the 15th cultivation day, and this amount was considerably higher than the 0.128 mg.L−1 As concentration leached from FeOx in fungal absence. These observations indicate that complex mutual interactions between As immobilized in FeOx and filamentous fungi are environmentally significant regarding As mobility and transformation in oxic environments.

Cleaning strategies for iron-fouled membranes from submerged membrane bioreactor treatment of wastewaters

Cleaning strategies for iron-fouled membranes from submerged membrane bioreactors (MBRs) dosed with iron salts are investigated here. Severe membrane fouling resulting from iron dosing was observed with amorphous ferric oxyhydroxide particles and gelatinous assemblages containing Fe(III) bound to polysaccharide materials responsible particularly for irreversible membrane fouling. Sodium hypochlorite (NaOCl) and citric acid, the most commonly used chemical cleaning agents, were not particularly effective in removing iron species from the membrane surface while the Fe(III) reducing agents ascorbic acid and sodium dithionite were very effective in removing iron-containing deposits under the conditions used in this study. In the presence of oxygen, the Fe(III)-catalysed oxidation of ascorbic acid and direct oxygen-mediated oxidation of dithionite control the dissolution rate constant. Use of NaOCl followed by ascorbic acid at concentrations of 10–20mM is recommended as a reasonable balance between cleaning effectiveness and cost though further investigation of cleaning of fouled membranes from full scale MBRs is required.

Synthesis of Gold@Iron Oxide Core-Shell Nanostructures via an Electrochemical Procedure

Core-shell nanostructures have attracted considerable interest as a new class of nanomaterials due to the fascinating physical and chemical characteristics relative to their single-component counterparts. Herein, a facial and low-cost approach was reported for the synthesis of core-shell nanostructures. By using the citrate-stabilized gold nanoparticles as seeds, the gold@iron oxide/oxyhydroxide nanostructures were prepared via an electroxidation of iron nail at 1.0 V between the electrodes in aqueous solution. The possible formation processes were as follows:O2 + 4 e− + 2 H2O → 4 OH− (Cathode)Fe → Fe2+ + 2 e− (Anode)In the presence of dissolved oxygen, 4 Fe2+ + O2 → 4 Fe3+ + 2 O2 − amorphous ferric-oxyhydroxides and/or ferric oxide were obtained as precipitate. Fe3+ + 3 H2O ⇌ Fe(OH)3 + 3 H+ Fe(OH)3 ⇌ FeO(OH) + H2O2 FeO(OH) ⇌ Fe2O3 + H2OGold nanoparticles, then act as seeds for the growth of iron oxide/oxyhydroxide shells during the electrolytic process. Parameters such as pH values, buffer composition and reaction time all play important roles in growing different shell thicknesses of gold@iron oxide core-shell nanostructures. The appropriate shell thicknesses were further optimized to yield the greatest surface-enhanced Raman scattering (SERS) effects in Raman molecules. Together, an efficient and reproducible method has been developed to control the shell thicknesses of gold@iron oxide/oxyhydroxide nanostructures. The integration of SERS reporters onto the single gold@iron oxide/oxyhydroxide nanoparticle also allows effective imaging in complex biological system.Two iron nails were inserted into the gold nanoparticles solution (10 mL) as two electrodes placed parallel to each other. Potential of 1.0 V were applied to the electrodes using a direct current (DC) power supply. A detailed illustration of the experimental device was demonstrated in Figure 1. After 0 to 420 min continuous stirring, the anode iron nail corroded, and a gradual growth of a dense shell appeared on each gold nanoparticle. The solution was then centrifuged at 1,500g for 30 min to remove the free iron oxide/oxyhydroxide precipitate. The obtained core-shell nanoparticles were re-suspended in 1.0 mM citrate buffer (pH 6.8) for further experiments.As depicted in Figure 1, a simple electrochemical setup was used to synthesize the gold@iron oxide/oxyhydroxide core-shell nanostructures. The iron nails were used as both the anode and cathode, 0.38 mM citrate buffer (pH 4.3) as the electrolyte. With the reaction time increases from 0 to 420 min, the shell coating proceeds. The surface plasmon resonance peaks of the core-shell nanoparticles became more prominent and red-shifted form 530 nm to 560 nm (Figure 2A). Transmission electron microscopic (TEM) images in Figure 2B further confirm the as-synthesized core-shell structures. The diameter of the gold core was about 49 nm, while the shell thickness was 0, 5, 10, 13, and 16 nm through adjusting the reaction time, respectively. The lattice spacing of the core structure in HRTEM was around 0.22 nm, corresponding to Au (111) fcc planes. These nanoparticles were coated with an amorphous layer on their surface. The iron oxide/oxyhydroxide shell structure was characterized by XPS analysis (data not shown).Due to the electromagnetic field enhancement surrounded the metallic surfaces under laser excitation, gold nanoparticles are very attractive SERS substrates. Figure 3 shows the SERS spectrum of 2-naphthalenethiol (NTP)-labeled gold nanoparticles (Au-NTP). In the experiments, the laser at 785nm was used as an excitation light. All the SERS signals are characteristic of the NTP molecules on the gold substrates. After the deposition of the outer iron oxide/oxyhydroxide shell, SERS signals were retained well after the introduction of the amorphous shell on the NTP-labeled gold nanoparticles.In conclusion, we have demonstrated a facile synthesis route for the preparation of gold@ iron oxide/oxyhydroxide core-shell nanostructures. With the incorporation of SERS reporter, these nanocomposites displayed good SERS performance, which has great potential for multiplexed imaging in living cellsThis work was supported by National Tsing Hua University (101N7046E1) and the Ministry of Science and Techonology (NSC 102-2113-M-007-005-MY3, 102-2627-M-007-006-) of Taiwan, ROC.

An experimental investigation of reaction zone controls on fluid flow and mass transport

This study examines homogeneous and heterogeneous mass transport in porous media featuring a solid-phase reaction zone that evolves and influences mass transport. It uses both flow-tank experiments and 2D reactive transport modeling to elucidate plume and reaction-zone behavior in situations with inherent complexity due to flow and transport and geochemical variability. The system is created by pumping a dilute Fe(ClO4)3 solution through a medium of glass beads and crushed calcite. As pH increases through the dissolution of calcite, amorphous ferric oxyhydroxide precipitates. The distribution of Fe, dissolved carbonate species, and pH are strongly coupled to the reaction zone because of the dissolution of CaCO3 and the precipitation of Fe(OH)3. Plugging influences the transport of dissolved constituents because it promotes fingering, hydraulic fracturing and flow bypassing. With the homogeneous systems, differences in the fingering produced by changing CaCO3 contents produced a heterogeneous distribution of Fe(OH)3(s) and dissolved Fe. As a result, a reaction zone with a complex shape developed due to the heterogeneity in hydraulic conductivity and CaCO3 content.

An EXAFS study of arsenic bonding on amorphous aluminium hydroxide

Arsenic-contaminated waters can be remediated by adsorption of the As onto an amorphous Al(OH)3 sludge precipitated using poly-aluminium chloride, which is a widely used, very effective polymeric coagulant. Investigation of the atomic near-range order and structure of As adsorbed to this sludge using X-ray Absorption Spectroscopy showed that the EXAFS spectrum of the sludge samples is very similar to that of mansfieldite (AlAsVO4·2H2O), indicating that As in the sludge is present as AsV, and that the atomic near-range structure of As is similar in the two materials. In the sludge, the first As–O coordination shell is populated with ∼4 oxygen neighbours at 1.70±0.01Å, and As has ∼2 Al neighbours at 3.18±0.02Å, consistent with bidentate binuclear As–O–Al bonding (two O atoms from an AsO4 unit bonded individually to two Al atoms). Previous studies have shown that As bonding on crystalline Al oxides and hydroxides is also bidentate binucleate, contrasting with the variable As bonding on ferric oxy-hydroxides. In particular, the bidentate mononucleate bonding reported for As sorption on amorphous ferric oxy-hydroxide sludges suggests that As is more strongly bonded to Al(OH)3 sludges. The latter may, therefore, have an advantage over ferric oxy-hydroxide coagulants for treatment of As-enriched waters.

Factors Influencing Iron Reduction–Induced Phosphorus Precipitation

Abstract An iron-amended anaerobic sequencing batch reactor, fed with synthetic sewage, was used to evaluate the factors influencing iron reduction–induced phosphorus precipitation from a simulated septic system. Results revealed that soluble phosphate could be effectively removed from the reactor when the added amorphous ferric oxyhydroxide (α-FeOOH) was increased from a molar ratio of 1.5 to 3. Whereas cycle time, substrate concentration, and temperature did not have a noticeable effect on iron reduction and the subsequent phosphorus removal, agitation was found to be an important factor, enabling better contact between α-FeOOH and the microorganisms during phosphorus removal. Precipitates collected from the reactor were analyzed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). Results demonstrated that the precipitate was mainly vivianite [Fe3(PO4)2·8H2O], which suggests that phosphorus was removed through iron reduction–induced precipitatio...

Effects of sulfate ligand on uranyl carbonato surface species on ferrihydrite surfaces

Understanding uranium (U) sorption processes in permeable reactive barriers (PRB) are critical in modeling reactive transport for evaluating PRB performance at the Fry Canyon demonstration site in Utah, USA. To gain insight into the U sequestration mechanism in the amorphous ferric oxyhydroxide (AFO)-coated gravel PRB, U(VI) sorption processes on ferrihydrite surfaces were studied in 0.01 M Na 2SO 4 solutions to simulate the major chemical composition of U-contaminated groundwater (i.e., [ SO 4 2 - ] ∼13 mM L −1) at the site. Uranyl sorption was greater at pH 7.5 than that at pH 4 in both air- and 2% pCO 2-equilibrated systems. While there were negligible effects of sulfate ligands on the pH-dependent U(VI) sorption (<24 h) in both systems, X-ray absorption spectroscopy (XAS) analysis showed sulfate ligand associated U(VI) surface species at the ferrihydrite–water interface. In air-equilibrated systems, binary and mono-sulfate U(VI) ternary surface species co-existed at pH 5.43. At pH 6.55–7.83, a mixture of mono-sulfate and bis-carbonato U(VI) ternary surface species became more important. At 2% pCO 2, there was no contribution of sulfate ligands on the U(VI) ternary surface species. Instead, a mixture of bis-carbonato inner-sphere (38%) and tris-carbonato outer-sphere U(VI) ternary surface species (62%) was found at pH 7.62. The study suggests that the competitive ligand (bicarbonate and sulfate) coordination on U(VI) surface species might be important in evaluating the U solid-state speciation in the AFO PRB at the study site where pCO 2 fluctuates between 1 and 2 pCO 2%.

Application of Mössbauer spectroscopy to the study of tannins inhibition of iron and steel corrosion

The inhibitory effect of tannins was investigated using, among others, potentiodynamic polarizations and Mossbauer spectroscopy. These techniques confirmed that the nature, pH and concentration of tannic solution are of upmost importance in the inhibitory properties of the solutions. It is observed that at low tannin concentration or pH, both, hydrolizable and condensed tannins, effectively inhibit iron corrosion, due to the redox properties of tannins. At pH ≈ 0, Mossbauer spectra of the frozen aqueous solutions of iron(III) with the tannin solutions showed that iron is in the form of a monomeric species [Fe(H2O)6]3 + , without coordination with the functional hydroxyl groups of the tannins. The suspended material consisted of amorphous ferric oxide and oxyhydroxides, though with quebracho tannin partly resulted in complex formation and in an iron (II) species from a redox process. Other tannins, such as chestnut hydrolysable tannins, do not complex iron at this low pH. Tannins react at high concentrations or pH (3 and 5) to form insoluble blue–black amorphous complexes of mono-and bis-type tannate complexes, with a relative amount of the bis-ferric tannate generally increasing with pH. Some Fe2 + in the form of hydrated polymeric ferrous tannate could be obtained. At pH 7, a partially hydrolyzed ferric tannate complex was also formed. The latter two phases do not provide corrosion protection. Tannin solutions at natural pH react with electrodeposited iron films (approx. 6 μm) to obtain products consisting only on the catecholate mono-complex of ferric tannate. Some aspects of the mechanism of tannins protection against corrosion are discussed.

Industrial Atmospheric Corrosion Resistance of P-RE Weathering Steel

The corrosion behavior of low-carbon steel (CS), P-bearing steel (PS) and P-RE weathering steel (P-REWS) exposed for two years in Jiangjin of China was investigated. The results showed that during 2-year exposure test, corrosion data of the experimental steels followed the bilogarithmic equation, and the average corrosion depth of PS and P-REWS was decreased by 19.5% and 28.2% respectively compared with that of CS. Scanning electron microscope, electrochemical impedance spectroscope and Fourier transform infrared spectroscope were used to characterize the corrosion products. The research results showed that P in steel could promote the formation of an amorphous ferric oxyhydroxide layer near the substrate. The addition of RE could effectively increase the charge transportation resistance of rust.

Formation, aggregation and reactivity of amorphous ferric oxyhydroxides on dissociation of Fe(III)–organic complexes in dilute aqueous suspensions

While chemical reactions that take place at the surface of amorphous ferric oxides (AFO) are known to be important in aquatic systems, incorporation of these reactions into kinetic models is hindered by a lack of ability to reliably quantify the reactivity of the surface and the changes in reactivity that occur over time. Long term decreases in the reactivity of iron oxides may be considered to result from changes in the molecular structure of the solid, however, over shorter time scales where substantial aggregation may occur, the mechanisms of reactivity loss are less clear. Precipitation of AFO may be described as a combination of homogeneous and heterogeneous reactions, however, despite its potentially significant role, the latter reaction is usually neglected in kinetic models of aquatic processes. Here, we investigate the role of AFO in scavenging dissolved inorganic ferric (Fe(III)) species (Fe′) via the heterogeneous precipitation reaction during the net dissociation of organically complexed Fe(III) in seawater. Using sulfosalicylic acid (SSA) as a model ligand, AFO was shown to play a significant role in inducing the net dissociation of the Fe–SSA complexes with equations describing both the heterogeneous precipitation reaction and the aging of AFO being required to adequately describe the experimental data. An aggregation based mechanism provided a good description of AFO aging over the short time scale of the experiments. The behaviour of AFO described here has implications for the bioavailability of iron in natural systems as a result of reactions involving AFO which are recognised to occur over time scales of minutes, including adsorption of Fe′ and AFO dissolution, precipitation and ageing.