Precipitation Of Ferrihydrite Research Articles

Dissolution behavior of stone wool fibers in synthetic lung fluids: Impact of iron oxidation state changes induced by heat treatment for binder removal

Stone wool fiber materials are commonly used for thermal and acoustic insulation, horticulture and filler purposes. Biosolubility of the stone wool fiber (SWF) materials accessed through acellular in vitro dissolution tests can potentially be used in future as an indicator of fiber biopersistence in vivo. To correlate acellular in vitro studies with in vivo and epidemiological investigations, not only a robust dissolution procedure is needed, but fundamental understanding of fiber behavior during sample preparation and dissolution is required. We investigated the influence of heat treatment procedure for binder removal on the SWF iron oxidation state as well as on the SWF dissolution behavior in simulant lung fluids (with and without complexing agents). We used heat treatments at 450 °C for 5 min and 590 °C for 1 h. Both procedures resulted in complete binder removal from the SWF. Changes of iron oxidation state were moderate if binder was removed at 450 °C for 5 min, and there were no substantial changes of SWF’s dissolution behavior in all investigated fluids after this heat treatment. In contrast, if binder was removed at 590 °C for 1 h, complete Fe(II) oxidation to Fe(III) was observed and significant increase of dissolution was shown in fluids without complexing agent (citrate). PHREEQC solution speciation modeling showed that in this case, released Fe(III) may form ferrihydrite precipitate in the solution. Precipitation of ferrihydrite solid phase leads to removal of iron cations from the solution, thus shifting reaction towards the dissolution products and increasing total mass loss of fiber samples. This effect is not observed for heat treated fibers if citrate is present in the fluid, because Fe(III) binds with citrate and remains mobile in the solution. Therefore, for developing the most accurate SWF in vitro acellular biosolubility test, SWF heat treatment for binder removal is not recommended in combination with dissolution testing in fluids without citrate as a complexing agent.

The effect of biogeochemical redox oscillations on arsenic release from legacy mine tailings

Exposed and un-remediated metal(loid)-bearing mine tailings are susceptible to wind and water erosion that disperses toxic elements into the surrounding environment. Compost-assisted phytostabilization has been successfully applied to legacy tailings as an inexpensive, eco-friendly, and sustainable landscape rehabilitation that provides vegetative cover and subsurface scaffolding to inhibit offsite transport of contaminant laden particles. The possibility of augmented metal(loid) mobility from subsurface redox reactions driven by irrigation and organic amendments is known and arsenic (As) is of particular concern because of its high affinity for adsorption to reducible ferric (oxyhydr)oxide surface sites. However, the biogeochemical transformation of As in mine tailings during multiple redox oscillations has not yet been addressed. In the present study, a redox-stat reactor was used to control oscillations between 7 d oxic and 7 d anoxic half-cycles over a three-month period in mine tailings with and without amendment of compost-derived organic matter (OM) solution. Aqueous and solid phase analyses during and after redox oscillations by mass spectrometry and synchrotron X-ray absorption spectroscopy revealed that soluble OM addition stimulated pyrite oxidation, which resulted in accelerated acidification and increased aqueous sulfate activity. Soluble OM in the reactor solution significantly increased mobilization of As under anoxic half-cycles primarily through reductive dissolution of ferrihydrite. Microbially-mediated As reduction was also observed in compost treatments, which increased partitioning to the aqueous phase due to the lower affinity of As(III) for complexation on ferric surface sites, e.g. ferrihydrite. Oxic half-cycles showed As repartitioned to the solid phase concurrent with precipitation of ferrihydrite and jarosite. Multiple redox oscillations increased the crystallinity of Fe minerals in the Treatment reactors with compost solution due to the reductive dissolution of ferrihydrite and precipitation of jarosite. The release of As from tailings gradually decreased after repeated redox oscillations. The high sulfate, ferrous iron, and hydronium activity promoted the precipitation of jarosite, which sequestered arsenic. Our results indicated that redox oscillations under compost-assisted phytostabilization can promote As release that diminishes over time, which should inform remediation assessment and environmental risk assessment of mine site compost-assisted phytostabilization.

Iron (Fe) speciation in size-fractionated aerosol particles in the Pacific Ocean: The role of organic complexation of Fe with humic-like substances in controlling Fe solubility

Abstract. Atmospheric deposition is one of the main sources of dissolved iron (Fe) in the ocean surfaces. Atmospheric processes are recognized as controlling fractional Fe solubility (Fesol%) in marine aerosol particles. However, the impact of these processes on Fesol% remains unclear. One of the reasons for this is the lack of field observations focusing on the relationship between Fesol% and Fe species in marine aerosol particles. In particular, the effects of organic ligands on Fesol% have not been thoroughly investigated in observational studies. In this study, Fe species in size-fractionated aerosol particles in the Pacific Ocean were determined using X-ray absorption fine structure (XAFS) spectroscopy. The internal mixing states of Fe and organic carbon were investigated using scanning transmission X-ray microscopy (STXM). The effects of atmospheric processes on Fesol% in marine aerosol particles were investigated based on the speciation results. Iron in size-fractionated aerosol particles was mainly derived from mineral dust, regardless of aerosol diameter, because the enrichment factor of Fe was almost 1 in both coarse (PM>1.3) and fine aerosol particles (PM1.3). Approximately 80 % of the total Fe (insoluble + labile Fe) was present in PM>1.3, whereas labile Fe was mainly present in PM1.3. The Fesol% in PM>1.3 was not significantly increased (2.56±2.53 %, 0.00 %–8.50 %, n=20) by the atmospheric processes because mineral dust was not acidified beyond the buffer capacity of calcite. In contrast, mineral dust in PM1.3 was acidified beyond the buffer capacity of calcite. As a result, Fesol% in PM1.3 (0.202 %–64.7 %, n=10) was an order of magnitude higher than that in PM>1.3. The PM1.3 contained ferric organic complexes with humic-like substances (Fe(III)-HULIS, but not Fe-oxalate complexes), and the abundance correlated with Fesol%. Iron(III)-HULIS was formed during transport in the Pacific Ocean because Fe(III)-HULIS was not found in aerosol particles in Beijing and Japan. The pH estimations of mineral dust in PM1.3 established that Fe was solubilized by proton-promoted dissolution under highly acidic conditions (pH < 3.0), whereas Fe(III)-HULIS was stabilized under moderately acidic conditions (pH 3.0–6.0). Since the observed labile Fe concentration could not be reproduced by proton-promoted dissolution under moderately acidic conditions, the pH of mineral dust increased after proton-promoted dissolution. The cloud process in the marine atmosphere increases the mineral dust pH because the dust particles are covered with organic carbon and Na. The precipitation of ferrihydrite was suppressed by Fe(III)-HULIS owing to its high water solubility. Thus, the organic complexation of Fe with HULIS plays a significant role in the stabilization of Fe that was initially solubilized by proton-promoted dissolution.

Saltwater intrusion weakens Fe-(oxyhydr)oxide-mediated (im)mobilization of Ni and Zn in redox-fluctuating soil–groundwater system

Iron in the form of (oxyhydr)oxides plays a profound role in the (im)mobilization of heavy metals in environmental geochemical processes occurring in the soil–groundwater system. Here, the influence of saltwater intrusion on Fe-(oxyhydr)oxide-mediated (im)mobilization of Ni(II) and Zn(II) in redox-fluctuating shallow aquifers was evaluated by chemical extraction, μ-XRF-XANES analysis, and 16S rRNA high-throughput sequencing. In phreatic water, the ferrihydrite-bound Ni/Zn (Fh-Ni/Zn) in soils contributed to a 12%–17% increase in carbonate-bound Ni/Zn (Cb-Ni/Zn) due to its own reductive dissolution, whereas the illite-adsorbed Ni/Zn (illite-Ni/Zn) only contributed 6%, 7%. The relative abundance of non-salt tolerant anaerobic Herbaspirillum and iron-reducing associated Ralstonia in soils accounted for nearly 50%. During the oxidation stage, the dissolved ferrihydrite reprecipitated to bind free Ni/Zn. However, saltwater invasion strongly weakened the dissolution–precipitation of ferrihydrite by inhibiting the growth of non-salt tolerant anaerobes and iron-reducing bacteria, and highlighted the contribution of illite-Ni/Zn. Under brackish water intrusion, illite-Zn contributed to a 12% increase in Cb-Zn, thereby surpassing the contribution of Fh-Zn (8%). Under seawater invasion, the dissolution–precipitation of ferrihydrite hardly occurred and the anaerobic salt-tolerant Bacillus (> 95%) prevailed. Therefore, the increase of Cb-Ni/Zn (7%–15%) in the reduction stages was contributed by illite-Ni/Zn. However, in the oxidation stages, the carbonate replaced the original role of reprecipitated ferrihydrite to bind the free Ni/Zn in solutions. These newly recognized mechanisms may be the key to predicting the mobility of toxic elements and developing appropriate remediation techniques of permeable reactive barriers under salinity stress.

Ferruginous coated grains of microbial origin from the Lower Devonian (Pragian) of the Prague Basin (Czech Republic) – Petrological and geochemical perspective

A unique occurrence of ironstone lenses represented by ferruginous oncoids is reported from the offshore Řeporyje Limestone (Lower Devonian, Pragian) from the Prague Basin (the Stydlé Vody Quarry). The oncoidal cortices show distinct arrangement of hematite/chamosite and/or iron-bearing calcite laminae, which are irregular, wavy and with a relatively high degree of inheritance of shape of the topography of the underlying laminae. Micro-domes and bulges, comparable to those observed in stromatolites and laminae overgrowing topographic irregularities represented by agglutinated foraminifers cemented to oncoid surfaces, were fairly commonly observed. The foraminifers probably represent the earliest representatives of agglutinated foraminifera encrusting surfaces of ferruginous grains. The surfaces of oncoids are wavy and often wrinkled. Although no microbial remains that could have been directly associated with oncoidal growth were recorded, we regard above mentioned features as indicators of biogenicity of the precipitates. We could only speculate the nature of the microbial consortia, but we regard the alternation of Fe2+ (chamosite, iron-bearing calcite) and Fe3+ (hematite) phases as a result of coupled oxidation - reduction processes related to passive and/or active microbial metabolic activities (precipitation of ferrihydrite – hematite precursor) and passive mineral authigenesis mediated by microorganisms (precipitation of chamosite precursor). Spatially restricted occurrence of the oncoidal lenses along with Al/Al + Fe + Mn vs. Fe/Ti, Nd vs. Ce/Ce*, Eu/Sm vs. Sm/Yb and other crossplots suggest that hydrothermal fluids were the most probable source of ferrous iron. Since most of the known oolitic or oncolitic ironstones represent rather large sedimentary bodies deposited around storm wave base to shallow subtidal environments under influence of upwelled anoxic watermasses or continental runoff, the reported oncolitic mounds represent a specific example of ironstones formation.

Carbon isotope stratigraphy of Precambrian iron formations and possible significance for the early biological pump

The origin of Precambrian iron-formations (IF) remains contentious, particularly with respect to the mineralogy of primary precipitates and the exact processes and conditions leading to their formation. Despite the uncertainties, prevailing hypotheses range from biological precipitation of ferrihydrite to abiotic water-column formation of greenalite. By contrast, iron carbonate minerals (siderite, ankerite) in IF have traditionally been attributed to diagenetic origins based on textural and isotopic relationships. Recent studies on IF from the Neoarchaean-Paleoproterozoic Transvaal Supergroup of South Africa have revealed evidence for apparently primary, low-δ13C, Fe/Mn-bearing Mg calcite as precursor to iron carbonate formation and as a potentially underestimated pathway of isotopically light carbon burial during IF deposition. Here, we present whole-rock δ13C data and carbonate-specific geochemical analyses for samples from five drill cores that capture the entire stratigraphic extent of the Kuruman and Griquatown IF of the Transvaal Supergroup. Our results demonstrate remarkable consistency in stratigraphic profiles among the locations for the trends and magnitudes of bulk δ13C values that are independent of paragenetic association, modal mineralogy, and chemical composition of the bulk carbonate fraction of each sample. We interpret these records as resulting from water-column abiotic carbonate formation that was accompanied by kinetic isotopic effects associated with fluctuating conditions (pH, alkalinity) controlling carbonate supersaturation in ambient seawater. Although our interpretation provides strong support for abiotic, anoxic models for IF genesis prior to the Great Oxidation Event (GOE), it does not entirely preclude additional biological mechanisms of primary ferric oxyhydroxide formation and its possible role in an early biological pump.

Ferruginous casts of bromalites in kaolin beds: microbial ferrihydrite‐goethite transformation as early stage taphonomy in lacustrine and riparian sediments

Two spatiotemporally distant deposits in western North America on differing sides of the K-Pg boundary are recognized to have accumulated innumerable ferruginous faecal-like specimens consisting of 3-D casts suspended in kaolin clay fills of lacustrine and riparian areas proximal to fluvial channels. Evidence presented interprets these specimens as organic in origin, consisting of coprolites and other bromalites (cololites), at the Readlyn deposit of the Whitemud Formation (Maastrichtian) in southern Saskatchewan and the Salmon Creek deposit of the Wilkes Formation (Miocene) near Toledo, Washington State. Comparable taphonomic processes at each deposit are suggested by preservation of identical, metre-long, enigmatic casts of the digestive system (cololites) of an unknown animal associated with the innumerable coprolites. Micro-fabrics of specimen interiors suggest that microbial mats entombed faecal and other organic entities in kaolin-rich lacustrine muds. This research proposes that microbial-induced ferrihydrite precipitation rapidly transformed into thin encrustations of goethite, encasing faecal droppings and other organic remains. Ferrihydrite-goethite-hematite biomineral sequencing cemented kaolin and quartz silt grains at the interface with the organic remains, precluding significant decay and thereby preserving external surface morphologies as encrustations. The mm-thick layers of the cemented sediment enveloped the organic droppings as rigid goethite moulds, preventing the collapse of the encasement morphologies as the organic residues were replaced by biominerals. Concentric growth layers of microbial mat-induced ferrihydrite-goethite cement progressed inward, resulting in 3-D casts. Only external morphologies were preserved, but often with finely detailed surface textures. The interiors of the casts preserve evidence of interconnected fabrics of pseudomorphed bacterial cell walls consisting of radially arranged jackets of acicular (Readlyn deposit) or platy (Salmon Creek deposit) goethite crystallites. Concretionary siderite replacement fabrics in some specimens resulted in the obliteration of the earlier microbial induced biomineral fabrics. Previous interpretations that the Wilkes Formation specimens resulted from inorganic processes only, thereby pseudo-coprolites, have been reappraised as faecal droppings comparable to coprolites of the Whitemud Formation.

Impact of temperature on the leaching of sulphate, Co, Fe, Mn, Ni and Zn from the Ballangen tailings deposit, Norway: a laboratory column experiment

Abstract Temperature is an important factor affecting the leaching of contaminants from waste deposits, especially in the Nordic region where temperature change is more drastic than other areas. In this study, the impact of temperature variation in the leaching of sulphate, Co, Fe, Mn, Ni and Zn from the Ballangen tailings deposit, northern Norway, was investigated using a column leaching experiment. Unoxidized tailings were fed into four columns, which were subsequently put into four wine fridges set at 5, 10, 14 and 18 °C, respectively. The columns were filled with 600 mL of deionized water from the top every second week. Leachate was collected at the bottom and tested for pH, conductivity and concentrations of , Co, Fe, Mn, Ni and Zn. The saturation index for ferrihydrite and the activity of Fe2+ in the leachate were calculated with PHREEQC. The results showed that the conductivity and leachate concentrations of , Co, Fe, Mn and Ni were highest at 14 and 18 °C, and lowest at 5 °C, which showed high tailings oxidation and subsequent leaching of contaminants at higher temperatures. X-ray photoelectron spectroscopic (XPS) analysis of the residual material confirmed the oxidation of sulphides and leaching of many elements. Ferrihydrite was supersaturated in the leachate from the 14 and 18 °C columns, which showed the oxidation of pyrrhotite and olivine and the precipitation of ferrihydrite. The cumulative mass of Zn leached out was highest at 10 °C, which might be the threshold temperature for the leaching of Zn.

Insights from modern diffuse-flow hydrothermal systems into the origin of post-GOE deep-water Fe-Si precipitates

Post-GOE deep-water Fe-Si precipitates associated with volcanogenic massive sulfide deposits are an important feature of the Proterozoic rock record. Although it is clear that these enigmatic deposits formed in oxygen-deficient and hydrothermally influenced deep-water settings, the oxidation mechanism(s) resulting in their precipitation remain(s) unclear. Whilst existing genetic models typically couple direct and/or bacterially-mediated iron oxidation with abiotic silica precipitation, low temperature diffuse hydrothermal fluids offer a potential mechanistic alternative to explain the observed layering. Herein, via combination of petrographic observations with elemental and isotopic data (Fe, Si, and O), we explore the genesis of the primary mineral phases present within recent Fe-Si precipitates obtained from the Southwest Indian Ridge. Formation of ferrihydrite and opal-A in these precipitates provides insight into the genesis of widely invoked precursor minerals to post-GOE deep-water Fe-Si precipitates. Specifically, we find that the mineralogical layers that typify these Fe-Si precipitates may have originated via biologically mediated ferrihydrite precipitation and abiotic precipitation of opal-A from oversaturated fluids during diffuse flow. By analogy, we propose that diffuse hydrothermal fluid flow played an important role in the formation of post-GOE deep-water Fe-Si precipitates.

Arsenate removal by resin-supported ferric ions: Mechanism, modeling, and column study

Removal of 10 mg/dm3 of As(V) by resin-supported Fe(III) was investigated in batch and column studies. The best As(V) removal was achieved at pH 3, and a high sorption density of 0.40 mmol-As/mmol-Fe was obtained after 96 h in a batch study. This sorption density could not be explained only by As(V) surface complexation with ferrihydrite. X-ray absorption near edge structure (XANES) analysis at the As K-edge suggested that over 90% of the As(V) was removed by surface precipitation as poorly crystalline ferric arsenate and the remainder was removed by surface complexation with ferrihydrite. XANES analysis at Fe K-edge suggested that 14%–35% of Fe(III) in the resin was used in the ferrihydrite and ferric arsenate. Kinetic modeling of the precipitation of ferrihydrite and ferric arsenate combined with a surface complexation model for As(V) and ferrihydrite successfully reproduced the batch experiment results. The kinetic constant for precipitation of ferrihydrite and ferric arsenate obtained by fitting of the batch experiment results successfully reproduced the column experiment results. The ratio of surface precipitation of ferric arsenate and As(V) surface complexation with ferrihydrite obtained by the constructed model was the same as in the XANES results. In the column study, a slow flow was advantageous for As(V) removal because surface precipitation of ferric arsenate took a long time.

Biochar-templated surface precipitation and inner-sphere complexation effectively removes arsenic from acid mine drainage

Treatment of aqueous leachate from acid mine tailings with pristine biochar (BC) resulted in the removal of more than 90% of the dissolved arsenic with an attendant rapid and sustained pH buffering from 3 to 4. Pine forest waste BC was transformed to a highly effective adsorbent for arsenic remediation of acid mine drainage (AMD) because the dissolved iron induced “activation” of BC through accumulation of highly reactive ferric hydroxide surface sites. Physicochemical properties of the BC surface, and molecular mechanisms of Fe, S, and As phase transfer, were investigated using a multi-method, micro-scale approach (SEM, XRD, FTIR, XANES, EXAFS, and STXM). Co-located carbon and iron analysis with STXM indicated preferential iron neo-precipitates at carboxylic BC surface sites. Iron and arsenic X-ray spectroscopy showed an initial precipitation of ferrihydrite on BC, with concurrent adsorption/coprecipitation of arsenate. The molecular mechanism of arsenic removal involved bidentate, binuclear inner-sphere complexation of arsenate at the surfaces of pioneering ferric precipitates. Nucleation and crystal growth of ferrihydrite and goethite were observed after 1 h of reaction. The high sulfate activity in AMD promoted schwertmannite precipitation beginning at 6 h of reaction. At reaction times beyond 6 h, goethite and schwertmannite accumulated at the expense of ferrihydrite. Results indicate that the highly functionalized surface of BC acts as a scaffolding for the precipitation and activation of positively charged ferric hydroxy(sulf)oxide surface sites from iron-rich AMD, which then complex oxyanion arsenate, effectively removing it from porewaters.Graphical abstract

Mobilization of arsenic from coal fly ash in the presence of dissolved organic matter

In this study, we examined the influence of dissolved organic matter (humic and fulvic acids) on arsenic (As) mobilization from two Class F coal fly ashes under oxic and anoxic conditions. Batch leaching experiments were conducted to examine As leaching behavior and changes in As aqueous and solid phase speciation after exposure to humic and fulvic acids for up to 4 weeks (ash/buffer ratio = 0.01 g/mL). Under oxic conditions, mobilized As concentrations increased compared to the organic-free controls and were positively correlated with percent aliphatic carbon and oxygen/carbon ratio of the humic and fulvic acids, suggesting both non-specific hydrophobic association of the organic matter with the surface of the fly ash particles and competitive desorption between negatively charged organic functional groups and As anions adsorbed to mineral surfaces. Under anoxic conditions, organic matter enhanced the reductive dissolution of As-bearing iron oxides such as hematite, resulting in increased As mobilization. X-ray absorption spectroscopy indicated that the majority of As in the leached and unleached fly ash was As(V) (≥90%), with As(V)-ferrihydrite precipitation occurring under all leaching conditions tested. Our results demonstrated that humic and fulvic acids are capable of mobilizing As from fly ash under both oxic and anoxic conditions, though redox appeared to be a larger driver of As mobilization compared to dissolved organic matter (DOM). DOM is a parameter which should be considered in risk assessment strategies for fly ash that is exposed to water (e.g., subsurface infiltration) or released to aquatic systems.

The influence of metal mobility on resource potential in circumneutral pH iron-rich copper mine waste rocks

Waste rocks from iron-oxide copper-gold (IOCG) deposits are dominated by Fe-bearing silicates and oxides, with only minor sulphides and potentially no pyrite. Hence, waste rock piles typically have circumneutral pH. However, oxidation of minor sulphides and Fe-bearing minerals can cause localised acidification at the mineral grain scale, especially where Fe oxidation and ferrihydrite precipitation is facilitated by bacteria. This study characterises the general biogeochemical system of waste rock piles at the Salobo IOCG mine, Brazil using a combination of biotic and abiotic leaching experiments on typical, weakly mineralised rocks, and observations of mineralogical changes and associated metal mobility in incipiently weathered, Cu-bearing overburden. Biotic experiments used a strain of the Fe- and S-oxidising Acidithiobacillus ferrooxidans subsp. cultured from the Salobo mine site. Most of the acid generated during oxidation of Salobo rocks is neutralised by alteration of labile Fe-silicates, particularly biotite, chlorite, olivine and amphiboles. Copper in our laboratory experiments was mobilised into solution (up to 1000 mg/l) from disseminated Cu-sulphide minerals during this acidification, and re-precipitated nearby (on the m-scale) within the rock mass during neutralisation. Much of this Cu precipitated with the abundant ferrihydrite, with some Cu precipitating locally with Mn-oxides, suggesting that secondary ferrihydrite and Mn-oxides represent possible exploration targets associated with comparable near-surface, weathered targets. Primary microparticulate gold was found encapsulated in magnetite and Cu-sulphides, with the sulphide-bearing portion of gold liberated during oxidation. Magnetite only demonstrated minor alteration (μm-scale) during short-term acidic oxidation (months). All these alteration processes are expected to occur within the waste piles during storage at the site, and could enhance the metallurgical leachability of these low-grade rocks for future re-processing. In the meantime, the lack of large-scale acidification, and adsorption of mobilised trace metals to the abundant ferrihydrite will ensure that the waste rocks remain environmentally benign.

Performance improvement of aqueous zinc-iron flow batteries through organic ligand complexation of Fe(II)/Fe(III)

In aqueous iron-based redox flow batteries (RFBs), there occurs a fatal performance degradation due to the formation of ferrihydrite via Fe(III) hydrolysis during long-term cycling. This paper describes an organic-ligand complexation of Fe(II)/Fe(III), which allows a high stability and reversibility of the Fe(II)/Fe(III) redox couple in aqueous zinc-iron RFBs. The remarkable effectiveness of pyridine chosen as a best complexing ligand is compared to seven organic ligands through various experiments including cell-cycling tests. It is experimentally confirmed that the pyridine initially coordinated to a Fe(II) cation forms a much strong bonding despite pH < 2, which leads to best electrochemical performances containing improved reaction kinetics and electron transfer. It also exhibits a slight discharge-capacity loss of only 2.9 % vs. 1st cycle at 100th cycle as compared to the pristine with an enormous loss of 78.7 % due to ferrihydrite precipitation, resulting in dynamically improved current efficiency with 73.34 % higher than the pristine one. Consequently, these results indicate that the pyridine as a complexing ligand agent can not only inhibit ferrihydrite during long-term cycling, but also enhance electrochemical stability and reversibility of the Fe(II)/Fe(III) redox couple.

Bacterially-mediated supergene alteration and redistribution of copper in mineralised rocks at the Salobo IOCG deposit, Brazil

The primary ore at the giant Salobo iron oxide copper-gold (IOCG) deposit in Brazil is dominated by magnetite and Fe-rich silicates, with minor (~1%) disseminated Cu-Fe sulphides and a distinctive absence of pyrite. Supergene Fe-silicate and sulphide oxidation has been mediated by bacteria, commonly within micrometre-scale acidic environments, but silicate-dominated acid neutralisation has maintained ambient circumneutral pH. Supergene Cu mobility has been controlled by the highly localised Fe-dominated biogeochemical processes, with Cu redistribution on the micrometre to metre scales and negligible supergene Cu enrichment. Silicates, especially the phyllosilicates biotite and chlorite, have variably altered to the clay minerals vermiculite and/or smectite, and ultimately to kaolinite. Leaching of Mn, Fe, Mg and Si accompanied silicate alteration, with minor addition of Cu to clay mineral structures and along cleavage planes. Iron mobilised by bacterial oxidation processes initially precipitated as ferrihydrite (with up to ~10 wt% Cu). Most Cu and Mn, mobilised in localised acidic environments, were precipitated nearby (metre scale) at circumneutral pH and reflect the immediate supergene biogeochemical environments: Cu-carbonates, pseudomalachite, chrysocolla, and Cu-Mn oxides (= “copper wad”). The early stages of these natural supergene processes were simulated in laboratory leaching studies using a range of fresh rocks inoculated with an endemic iron-oxidising strain of Acidithiobacillus ferrooxidans previously cultured from the Salobo site. Bacterially-mediated precipitation of ferrihydrite, commonly Cu-bearing, in these experiments strongly resembles that seen in the natural outcrops. These leaching experiments confirmed the effectiveness of silicate-dominated acid neutralisation, especially when accompanied by bornite oxidation, which is also acid-consuming. In contrast, chalcopyrite oxidation is acid-producing and leads to more effective localised Cu mobility in solution. Our combination of field and laboratory observations at the Salobo IOCG deposit shows that the biogeochemical environment dominated by Fe-silicates with little sulphides, typical for these systems, has led to an entirely different style of supergene Cu redistribution from that seen at many other Cu deposits, such as chalcopyrite and pyrite-rich porphyry copper deposits, where substantial supergene Cu enrichment has occurred.

Plutonium(IV) Sorption during Ferrihydrite Nanoparticle Formation.

Understanding interactions between iron (oxyhydr)oxide nanoparticles and plutonium is essential to underpin technology to treat radioactive effluents, in cleanup of land contaminated with radionuclides, and to ensure the safe disposal of radioactive wastes. These interactions include a range of adsorption, precipitation, and incorporation processes. Here, we explore the mechanisms of plutonium sequestration during ferrihydrite precipitation from an acidic solution. The initial 1 M HNO3 solution with Fe(III)(aq) and 242Pu(IV)(aq) underwent controlled hydrolysis via the addition of NaOH to pH 9. The majority of Fe(III)(aq) and Pu(IV)(aq) was removed from solution between pH 2 and 3 during ferrihydrite formation. Analysis of Pu–ferrihydrite by extended X-ray absorption fine structure (EXAFS) spectroscopy showed that Pu(IV) formed an inner-sphere tetradentate complex on the ferrihydrite surface, with minor amounts of PuO2 present. Best fits to the EXAFS data collected from Pu–ferrihydrite samples aged for 2 and 6 months showed no statistically significant change in the Pu(IV)–Fe oxyhydroxide surface complex despite the ferrihydrite undergoing extensive recrystallization to hematite. This suggests the Pu remains strongly sorbed to the iron (oxyhydr)oxide surface and could be retained over extended time periods.

Removal and fixation of arsenic by forming a complex precipitate containing scorodite and ferrihydrite

For the metallurgical industries, the safe disposal of arsenic-bearing waste is animportant issue. In this study, a two-stage arsenic precipitation process was proposed, which consists of formation of scorodite at lower pH followed by precipitation of arsenical ferrihydrite at higher pH. The experimental results show that arsenic can be removed effectively from a solution with arsenic concentration of 5 g/L as precipitate of crystalline scorodite through oxidizing ferrous ions with air stream at pH of 2at first. Then, the pH value was raised to 3 and 4, and almost all the residual arsenic was removed from the solution due to adsorption onto amorphous ferrihydrite at the molar ratio of Fe/As between 2 and 4. The United States Environmental Protection Agency's Toxicity Characteristic Leaching Procedure tests demonstrate that the complex precipitates generated under these conditions (Fe/As ratio of 4, final pH between 3 and 4) were very stable and no leached arsenic can be detected using acetic acid buffer solution (pH 4.93 ± 0.05) in tests of 72 h. Experimental results suggest that a high Fe/As ratio was necessary, so that the excess iron promotes the formation a product of good stability, arsenical ferrihydrite (AsFH: AsO43−·Fe(OH)(H2O)1+x) under a relatively high pH in acidic solution.

Study of pyrite oxidation with chlorine dioxide under mild conditions

Pyrite, usually associated with gold deposit, is disadvantageous to gold leaching. In this study, an environmentally friendly oxidizing agent, chlorine dioxide (ClO2), was used for the first time to efficiently oxidize pyrite under mild experimental conditions, so as to remove the adverse effects of pyrite on gold leaching. Firstly, with established content determination method, the prepared pure ClO2 solutions with different concentrations were investigated on their impacts on pyrite oxidation efficiency. Secondly, solid NaClO2 was used to oxidize pyrite under acidic condition, providing a new oxidation approach for practical application. In fact, the key of the approach is the use of solid NaClO2 in acidic environment to generate ClO2 and then oxidize pyrite. For the pyrite pulp with 400 mL volume with an initial concentration of 0.025 M, the oxidation efficiencies of Fe and S in pyrite reached 99.8% and 98% at a NaClO2 concentration of 0.125 M and C4H6O6 concentration of 0.0025 M with magnetic stirring for 1 h at 30 °C in a constant temperature water bath. The addition of C4H6O6 can increase the acidity of the system and form a complex with iron ions, thereby preventing the precipitation of ferrihydrite from acidity changes caused by overdose NaClO2. After redox reaction, the acidity and oxidation–reduction potential (ORP, Eh) of the system were noticeably increased. The results of chemical composition analysis and X ray diffraction (XRD) analysis of pyrite oxidation residue confirmed that pyrite was oxidized to ions by ClO2, and no solid by-products such as Fe2O3 or Fe(OH)3 were formed. The scanning electron microscope (SEM) analysis of residual pyrite indicated that small pyrite particles were oxidized first, and then oxidants eroded the defects of large particles, gradually oxidizing all pyrites. This method is expected to be used as a pretreatment approach for pyrite oxidation before gold leaching process.

Hydrochemical changes induced by underground pumped storage hydropower and their associated impacts

Underground pumped storage hydropower (UPSH) using abandoned mines is an alternative system to manage electricity production in flat regions. Water from an underground reservoir is pumped to a surface reservoir to store electricity in the form of potential energy. Later, water is discharged through turbines into the underground reservoir to produce electricity when demand increases. During this operation, the water hydrochemistry continuously evolves. It varies in order to reach chemical equilibrium with the atmosphere (in the surface reservoir) and with the surrounding porous medium and groundwater (in the underground reservoir). The hydrochemical variations may lead to reactions in the reservoirs and in the surrounding porous medium, causing potentially negative consequences for the environment and the system efficiency, especially when pyrite is present in the surrounding porous medium. In this case, pyrite oxidation leads to a decrease in pH and the precipitation of goethite or schwertmannite in the surface reservoir. The decrease in pH is mitigated when calcite is present in the porous medium. However, other concerns may arise, such as slight increases in pH, the precipitation of ferrihydrite and calcite in the surface reservoir, and the oxidation of pyrite and dissolution of calcite in the surrounding porous medium. Understanding the pH variations and the precipitation/dissolution of minerals is of paramount importance in terms of the environmental impacts and system efficiency. For this reason, this work investigates the main hydrochemical changes and their associated consequences when abandoned deep mines are used for UPSH by means of numerical modelling. The main objective is to highlight the importance of considering hydrochemical aspects when designing future UPSH plants.

Impurity-bearing ferrihydrite nanoparticle precipitation/deposition on quartz and corundum

During ferrihydrite precipitation, metal ions can be sequestered in it to form impurity-bearing ferrihydrite (IBF).