Amorphous Iron Oxyhydroxide Research Articles

Behavior of Prussian blue-based materials in presence of ammonia

Prussian blue and related materials, usually considered that behave as a zeolite for ammonia adsorption, when hydrated are unstable in the presence of this gaseous species. They remain stable only in the anhydrous form. In the decomposition products ammonium hexacyanometallates and an XRD amorphous iron(III) oxyhydroxide are detected. The crystallization and adsorbed water present in these solid materials participates in a decomposition reaction to give NH 4 + and OH −. The very basic OH − anion removes the iron(III) cations from the complex to form Fe(OH) 3 and finally FeOOH, while the formed NH 4 + appears as the salt of the complex anion. As reference reactions, the interaction of ammonia with ferrocyanic acid and ferric chloride, both in solid state, were studied where crystalline ammonium ferrocyanide and ammonium chloride, respectively, are formed.

Adsorption and Retention of U-238 and Th-232 from Groundwater Using BIOS

ABSTRACTBiofilms were grown in situ 296 metres below sea level in the Äspö Hard Rock Laboratory. The prominent organism in these biofilms wasGallionella ferruginea, which is an iron oxidising chemolithotrophic microorganism that grows in low oxygen conditions. This organism grows an organic stalk structure capable of binding and concentrating trace metals. This stalk structure also allows amorphous ferric iron oxyhydroxides, or BIOS, (bacteriogenic iron oxides) to precipitate. The pH of the groundwater within the system was between 7.4 and 7.6, with Eh potential between 150 and 190 mV and oxygen saturation between 3 and 15%. Biofilms developed within two weeks and were sampled every two weeks for three months. Cell number and stalk length was recorded for each sample. The concentration of Cr, Ni, Cu, Zn, Mo, REE (rare earth elements), U-238 and Th-232 was measured by ICP-MS. Early results suggested thatGallionellabiofilms and associated BIOS could potentially concentrate trace metals up to 1000 fold higher than levels within the host rock and over 1 000 000 times the levels in the groundwater over a period of years. These new experiments indicate thatGallionellabiofilms and BIOS can rapidly attenuate metals to levels over 1000 fold higher than the levels in the groundwater. This process can occur anywhere where reduced groundwater enters the waste repository tunnel, open cavities or where groundwater extrudes at the surface. Because of this, there is huge potential to use biofilms and BIOS for retention of radionuclides and pollution control.

Sequential extraction of sulfide-rich tailings remediated by the application of till cover, Kristineberg mine, northern Sweden

A sequential extraction has been carried out on sulfide-rich mine tailings. The purpose was to investigate how elements released by oxidation are secondarily retained in the tailings and the possible consequences of the remediation. After investigating the solid tailings, seven samples were chosen for sequential extractions. Two samples were oxidised, situated just above the oxidation front; two samples from just below the former oxidation front with increased concentrations of several elements; two unoxidised samples were from an intermediate depth, and the deepest sample was from the tailings–peat boundary at the bottom of the impoundment. Five phases were extracted: adsorbed/exchangeable/carbonate; labile organics; amorphous Fe-oxyhydroxides/Mn-oxides; crystalline Fe-oxides; and organics/sulfides. The addition from dried porewater to the extracted fractions has been calculated and considered as minor. In the oxidised tailings, the sulfide fraction still dominates for elements such as Fe, S, Cd, Co, Cu, Hg and Zn, although the concentrations are low compared to the unoxidised tailings. Generally, the second most important fraction is the adsorbed/exchangeable/carbonate fraction. Below the oxidation front, the sulfide content of the tailings sharply increases. In the secondary enrichment zone, the total element concentrations increase compared with the deeper unoxidised samples, mainly due to secondary retention. For some elements, secondary retention is greater than the total amount extracted for the deeper unoxidised samples. In the secondary enrichment zone the adsorbed/exchangeable/carbonate fraction represents approximately 20 wt.% or more for Cd, Co, Mn, Ni and Zn. The amorphous iron oxyhydroxide or the crystalline iron oxide fractions are less important at this level, although for As, Ba and Cu the amorphous iron oxyhydroxide fraction represents up to 20 wt.%. At the lower border of the enrichment zone, the total concentration for most metals is lower, but the importance of the adsorbed/exchangeable/carbonate fraction is further enhanced for Cd, Cu, Ni and Zn. These elements have 35–60 wt.% of the total amount from this fraction. For As, Cd, Cu, Ni and Pb, the secondary fractions extracted (extractions A–D) represent between 60 and 80 wt.% of the total content. At greater depth in the impoundment the relative importance of the adsorbed/exchangeable/carbonate fraction decreases, whilst the importance of amorphous iron oxyhydroxide and crystalline iron oxide fractions increases. The adsorbed/exchangeable/carbonate fraction is the most easily remobilised fraction. A raised groundwater table previously situated below the enrichment zone may result in the release of secondarily retained metals.

Environmental mobility of antimony around mesothermal stibnite deposits, New South Wales, Australia and southern New Zealand

Antimony (Sb) occurs principally in stibnite (Sb 2S 3) in mesothermal vein deposits hosted in low-grade metamorphic belts of eastern Australia and southern New Zealand. Stibnite is commonly associated with gold. Many deposits have been mined historically, with one large deposit, at Hillgrove, New South Wales, being mined recently. Natural outcrops in the relatively rugged terrains are oxidised under humid to semiarid conditions and stibnite transforms to oxides including valentinite, senarmontite, stibiconite, and rarely cervantite. Oxidation of stibnite and associated arsenopyrite and pyrite causes local acidification; however, acid is readily neutralised by carbonates in mineralised zones and host rocks, and associated waters are near neutral (pH 6–8.5). Stibnite dissolves readily in moderately oxidised waters as SbO 3 −, in conjunction with antimony oxide formation. Experimental stibnite oxidation yielded up to 37 ppm dissolved Sb, and mine and tailings dam waters at Hillgrove have up to 55 ppm dissolved Sb. Natural stibnite can contain >5000 ppm As in solid solution. Dissolution of stibnite, arsenopyrite and arsenian pyrite releases arsenic, and resultant dissolved As concentrations are up to 3.6 ppm (experimental) and up to 7.2 ppm (mine and tailings dam seepages). Mine and tailings discharge waters have elevated Sb and As where they emerge, but attenuation occurs by deposition of the metals onto amorphous iron oxyhydroxides which can contain >10% each of Sb and As. Historic disposal of mineralised waste rock material into the stream system at Hillgrove has caused strong contamination of stream sediments with Sb and As. Equilibration of stream water with contaminated stream sediment, as well as additions from erosion of natural outcrops and mine and tailings dam seepages, has led to the main drainage system (Bakers Creek) containing strongly contaminated water (up to 1.8 mg/l Sb and 0.3 mg/l As) for 20 km until its junction with the Macleay River. Environmentally high values of dissolved Sb (and As), are inevitable in waters associated with mesothermal stibnite (–gold) deposits.

Microbial and hydrothermal aspects of ferric oxyhydroxides and ferrosic hydroxides: the example of Franklin Seamount, Western Woodlark Basin, Papua New Guinea

Deposits of Fe-Si-Mn oxyhydroxides are commonly found on the seafloor on seamounts and mid-ocean spreading centers. At Franklin Seamount located near the western extremity of Woodlark Basin, Papua New Guinea, Fe-Si-Mn oxyhydroxides are being precipitated as chimneys and mounds upon a substrate of mafic lava. Previous studies have shown that the vent fluids have a low temperature (20–30°C) and are characterized by a total dissolved iron concentration of 0.038 mM kg-1, neutral pH (6.26) and no measurable H2S. The chimneys have a yellowish appearance with mottled red–orange patches when observed in situ from a submersible, but collected samples become redder within a few hours of being removed from the sea. The amorphous iron oxyhydroxides, obtained from active and inactive vents, commonly possess filamentous textures similar in appearance to sheaths and stalks excreted by the iron-oxidizing bacteria Leptothrix and Gallionella; however, formless agglomerates are also common. Textural relationships between apparent bacterial and non-bacterial iron suggest that the filaments are coeval with and/or growing outwards from the agglomerates. The amorphous iron oxyhydroxides are suggested to precipitate hydrothermally as ferrosic hydroxide, a mixed-valence (Fe2+-Fe3+) green–yellow iron hydroxide compound. Consideration of the thermodynamics and kinetics of iron in the vent fluid, suggest that the precipitation is largely pH controlled and that large amounts of amorphous iron oxyhydroxides are capable of being precipitated by a combination of abiotic hydrothermal processes. Some biologically induced precipitation of primary ferric oxyhydroxides (two-XRD-line ferrihydrite) may have occurred directly from the fluid, but most of the filamentous iron micro-textures in the samples appear to have a diagenetic origin. They may have formed as a result of the interaction between the iron-oxidizing bacteria and the initially precipitated ferrosic hydroxide that provided a source of ferrous iron needed for their growth. The processes described at Franklin Seamount provide insight into the formation of other seafloor oxyhydroxide deposits and ancient oxide-facies iron formation.

Fate of pyrene in contaminated soil amended with alternate electron acceptors

Creosote-contaminated soil samples from the Libby Ground Water Contamination Superfund Site in Libby, MT, were amended with the potential alternate electron acceptors (AEA) nitrate (KNO 3), manganese oxide (MnO 2), and amorphous iron oxyhydroxide (FeOOH) and incubated at low oxygen tensions (0–6% O 2). The fate of 14C-pyrene was evaluated with respect to the different soil amendments. The fate of 14C from the radiolabeled pyrene with regard to mineralization and bound residue formation within soil humic fractions was not significantly different from controls for the iron and manganese amended soils. Nitrate amendments appeared to stimulate 14C-pyrene mineralization at a level of 170 mg NO 3–N kg −1, and inhibit mineralization at 340 mg NO 3–N kg −1. The stimulatory effect did not appear to be the result of nitrate serving as an electron acceptor. Although AEA amendments did not significantly affect the rate or extent of 14C-pyrene mineralization, results of oxygen-deprived incubations (purged with N 2) indicate that AEA may be utilized by the microbial community in the unsaturated contaminated soil system.

Accumulation of a ferric mineral in the biofilm of Montacuta ferruginosa (Mollusca, Bivalvia). Biomineralization, bioaccumulation, and inference of paleoenvironments

Infrared absorption spectra of the amorphous and phosphorus-rich ferric mineral associated with the biofilm of Montacuta ferruginosa, a marine bivalve, were obtained by applying the KBr disc method. Phosphate absorption bands due to PO stretching vibrations were observed at 1100 and 1020 cm −1. This result, as well as the similarity of spectra with other well known (bio)minerals, suggests that the mineral is an amorphous iron oxyhydroxide gel with phosphate sorbed on its surface rather than a pure ferric phosphate. It is suggested that phosphate ions are of microbial origin because phosphate-producing enzymes were detected in vivo. Apart from iron, the mineral phase is devoid of other heavy metals. Despite its similarity with other abiotically precipitated phases, it is argued that the mineral is the result of in situ microbial biomineralization processes in the biofilm and not the result of a simple bioaccumulation process. This is supported by microscopic observations. A geological implication is that the simple presence of such iron minerals in ancient sedimentary environments, where microbes have not been fossilized, might be considered as an indicator of microorganisms performing biomineralization. By comparison with the present-day environment of M. ferruginosa, precise paleoenvironmental conditions may be inferred.

Oxidation of antimony (III) by amorphous iron and manganese oxyhydroxides

Amorphous forms of natural and synthetic Fe oxyhydroxides and synthetic Mn oxyhydroxides were used to study the oxidation of antimonite, Sb(III), at different pH values. Sb species were measured by differential pulse adsorptive stripping voltametry (DPACSV). The oxidation of Sb(III) to Sb(V) was always rapid and completed after a few days following pseudo-first order rate laws. A slightly slower oxidation rate was observed at lower pH and was attributed to the lower stability of the oxyhydroxide compounds under mildly acidic conditions. The oxidation pseudo-first order rate of Sb(III) by natural Fe oxyhydroxides was also slower when compared to synthetic Fe compounds. This could be explained by a slight crystallinity of the natural oxides (when compared to amorphous synthetic compounds) and by their more complex chemical composition which can include adsorbed ions and organic matter. The study confirms that amorphous Fe and Mn oxyhydroxides present in natural waters and sediments can play a detoxifying role by adsorbing and oxidizing the more toxic Sb(III) into Sb(V).

Crystal chemistry of suspended matter in a tropical hydrosystem, Nyong basin (Cameroon, Africa)

Suspended matter (SM) from the Nyong basin (Cameroon, Africa), a tropical watershed, was collected by tangential flow ultrafiltration to separate particulate (>0.45 μm) and colloidal (<0.45 μm; >20 kDa) fractions. In this basin, two distinctive systems in a selected small catchment (Nsimi–Zoétélé) of the Nyong river basin have been considered: (i) colourless water (groundwater and spring) with a low suspended load (<3 mg/l) and a low total organic carbon content (TOC<1 mg/l) and (ii) coloured water (Mengong brook and Nyong river), which is organic rich (TOC>10 mg/l) and contains higher amounts of SM (10–20 mg/l) than the colourless water. Freeze-dried samples of SM have been analysed by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), electron paramagnetic resonance spectroscopy (EPR), and visible diffuse reflectance spectroscopy (DRS). Colourless water mainly contains mineral phases, such as poorly ordered kaolinite, plus quartz and goethite in the particulate fraction, and euhedral kaolinite plus amorphous iron oxyhydroxides in the colloidal fraction. In contrast, the SM in coloured water is mainly organic in nature. The mineral phases in the particulate fraction are similar to those from clear water, but with additional phytoliths and diatom frustules composed of biogenic opal. In the colloidal fraction, complexation of Fe 3+ and Mn 2+ with organic matter is evidenced by EPR, together with significant occurrence of Fe oxyhydroxides associated with organic matter. The sites of Al, Si, Fe, Mn in colloidal fractions derived from spectroscopic analyses are discussed with reference to chemical analyses performed by inductively coupled plasma mass spectrometry. Most of the observed solid phases or species correspond to those expected from published thermodynamic calculations for the same hydrosystem, except the colloidal iron oxyhydroxides in the coloured water. The presence of such iron phases is emphasised since they are expected to have large sorption capacities for numerous trace elements. The crystal chemistry of SM is used to discuss the origin of the mineral particles transported from the soil to the main rivers in terms of mechanical and chemical erosion processes.

The Differential Geochemical Behavior of Arsenic and Phosphorus in the Water Column and Sediments of the Saguenay Fjord Estuary, Canada

The distribution and partitioning of dissolved and particulate arsenic and phosphorus in the water column and sediments of the Saguenay Fjord in Quebec, Canada, are compared. In addition, selective and/or sequential extractions were carried out on the suspended particulate matter (SPM) and solid sediments to contrast their geochemical behaviors in this natural aquatic system. Results of our analyses show that both arsenic and soluble reactive phosphate are actively scav- enged from the water column by settling particles. Upon their accumulation at the sediment-water interface some As and P may be released to porewaters following the degradation of organic matter to which they are associated. The porewater concentrations are, however, limited by their strong affinity for authigenic, amorphous iron oxyhydroxides which accumulate in the oxic sediments near the sediment-water interface. The geochemical behavior of arsenic and phosphorus diverge most strikingly upon the development of anoxic conditions in the sediments. Following their burial in the anoxic zone, amorphous iron oxyhydroxides are reduced and dissolved, releasing phosphate and arsenic to the porewaters. We observed, however, that porewater arsenic concentrations increase at shallower depths than phosphate in the sediments. The reduction of arsenate, As(V), to arsenite, As(III), and its desorption prior to the reductive dissolution of the carrier phase(s) may explain this observation. Driven by the strong concentration gradient established in the suboxic zone, phosphate diffuses up towards the oxic layer where it is readsorbed by authigenic iron oxyhydroxides. In the organic-rich and rapidly accumulating sediments at the head of the Fjord, porewater sulfate depletion and the resulting absence of a sulfide sink for Fe(II), may lead to the formation of vivianite in the fermentation zone, a potential sink for phosphate. Arsenite released to the porewaters in the suboxic and anoxic zones of the sediments diffuses either down, where it is adsorbed to or incorporated with authigenic iron sulfides, or up towards the oxic boundary. Arsenite appears to migrate well into the oxic zone where it may be oxidized by authigenic manganese oxides before being adsorbed by iron oxyhydroxides present at the same depth. Whereas, in the absence of authigenic carbonate fluorapatite precipitation, the ability of oxic sediments to retain mineralized phosphate is a function of their amorphous iron oxyhydroxide content, arsenic retention may depend on the availability of manganese oxides, the thickness of the oxic layer and, its co-precipitation with iron sulfides at depth.

Hydrothermal Mineralizing Processes and Associated Sedimentation in the Santorini Hydrothermal Embayments

Seven sediment cores from the Palaea and Nea Kameni hydrothermal embayments (PK and NK) on Santorini have been studied by using sediment smear slides, X-ray diffraction, and radiocarbon techniques to determine their lithology, mineralogy, and age, respectively. The cores have been analyzed chemically for a suite of elements, including selective chemical leaching, to determine the partition of elements between coexisting phases. Lithologically and chemically, the sediments from the two embayments are very different. Sediments from PK are oxidized in the upper parts of the cores and reduced at depth; they comprise mainly hydrothermal pyritiferous diatomaceous ooze with minor Fe oxyhydroxide, volcanic debris, gypsum, and siderite. Sediments from NK are oxidized throughout and comprise predominantly amorphous iron oxyhydroxides and goethite; they are thought to have formed by direct precipitation from solution. In PK, biogenic, detrital seawater evaporate, and hydrothermal sediment phases have been identified but only the detrital inputs can be correlated between cores. Mn shows a pronounced enrichment towards the outer part of the embayment because of its dispersion down the embayment and precipitation in the more-oxidizing sediments towards the open sea. In NK, the sediments contain detrital, seawater evaporate, and hydrothermally precipitated and scavenged phases. The presence of substantial amounts of organic matter in the PK sediments but not in the NK sediments is probably the main reason why geochemical processes in the two embayments are different. Age dates obtained from the cores by using the 14C technique indicate that sedimentation in the PK embayment commenced at ∼ 50 AD, whereas in the NK embayment it commenced at ∼ 1900 AD, which is consistent with the known history of Santorini. Based on these dates, calculated metal accumulation rates in both embayments are higher than in any other dated hydrothermal metalliferous sediments with the exception of those in the Atlantis II Deep of the Red Sea. Sediment ponding in the embayments is the principal cause of these results.

Surfactant separation as a technique for physical and chemical characterization of ore processing residues

When characterizing multi-phase materials, several authors have recommended separating the material into various fractions consisting of a reduced number of minerals. However, the common separation techniques are of limited value when treating ultra-fine-grained materials. We present in this paper a separation technique using tensio-active solutions which break up any particle agglomerates which may be present and facilitates sedimentation of component particles according to grain size. These fractions have a distinctive mineralogy and chemistry. The technique has been applied to two selected samples from two mining residue sites in Germany. Results from Bielatal, a tin residue, show that a large proportion of the arsenic and heavy metals are concentrated in a suspension fraction, which consists of clay minerals and amorphous iron oxyhydroxides. Silicates such as a micas and feldspar settle out quite rapidly, whereas, other minerals, such as hematite take longer. In the case of Dänkritz, a uranium mine waste, the same technique was applied and similar results were obtained for the heavy metals distribution. Additional study indicated that radium is with barite and other radio-isotopes were associated with the fine suspension fraction.

The effects of pH regulation upon the release of sulfate from ferric precipitates formed in acid mine drainage

`Batch experiments' (25:1 v:w) were used to test the effects of pH upon the release of SO −2 4 from ferric precipitates formed in acid mine drainage (AMD) in southeastern Kentucky. Analytical grade CaO [`quicklime'], Ca(OH) 2 [hydrated lime] and CaCO 3 [referred to as `limestone'] were used as alkalinity generating agents and were mixed with ferric precipitates composed of amorphous iron oxyhydroxides, jarosite and goethite. Regression analyses indicated that aqueous SO −2 4 concentrations increased linearly with pH between values of 2.5 and 9.0. At pH=7, 30–45% of the oxalate soluble SO −2 4 was released from the ferric precipitates. The 3 types of alkaline neutralizing agents released similar, but not identical, quantities of SO −2 4 at a given pH. The results of these experiments suggest that AMD remediation schemes should consider raising the pH of waste streams only to values necessary to precipitate and/or sorb metals, in order to minimize SO −2 4 desorption. Iron oxyhydroxides that precipitate within AMD often sorb high concentrations of SO −2 4; a significant percentage of which will be released in streams draining carbonate bedrock.

Mechanisms Controlling Release of Colloids to Groundwater in a Southeastern Coastal Plain Aquifer Sand

The release of colloids from a shallow Southeastern Coastal Plain deposit, in response to treatment with solutions that perturbed electrostatic and cementitious associations among the clay-size constituents, suggested that these constituents are predominantly chemically bound within the interstitial matrix. Sediment-packed columns subjected to a sequence of high/low ionic strength solutions, acidic pH (2.6) solutions, and anionic surfactant solutions yielded undetectable colloid releases. Treatment with elevated pH (8.8) solutions and solutions containing phosphate resulted in mobilization of colloids only partially representative of the bulk matrix. Organic acids, which both (i) initiated dissolution of the amorphous iron oxyhydroxide intermediaries among kaolinite books and (ii) imparted negative charge to the iron oxyhydroxide surfaces to generate sufficiently repulsive forces among the juxtaposed matrix constituents, appeared to induce mobilization of colloids compositionally representative of the ent...

Ferromanganese crust and nodule deposits from the continental margin south and west of Tasmania

West of Tasmania and on the South Tasman Rise, from 41°S to 50°S, rocky outcrops in water depths of 1500–4500 m are commonly coated with ferromanganese crusts up to 20 cm thick. These crusts are unusually thick as compared to those in most parts of the world ocean. Manganese nodules of similar chemical composition are often associated with the crusts. The results from 43 samples analysed here build on previous work on a few samples that showed most crusts and nodule deposits to be of low grade. Both types of deposits had been shown previously to be dominantly ferruginous vernadite and X‐ray amorphous iron oxyhydroxide, and of hydrogenetic origin. The Tasmanian nodules are exceptionally large in places and roughly spherical (up to 12 cm in diameter). Unusually, many irregular nodules are cored by granite or other basement rocks. The nodules, excluding nuclei, average 12.1% Mn, 16.1% Fe, 0.14% Cu, 0.29% Ni and 0.20% Co. The three valuable metals (Cu, Ni, Co) total 0.63% on average. These contents are not commercially interesting as compared to those of the deep‐ocean nodules from the central Indian Ocean and the northeast Pacific Ocean (high Ni and Cu), and the Cook Islands Exclusive Economic Zone (high Co). The crusts average 16.2% Mn, 17.2% Fe, 0.08% Cu, 0.32% Ni and 0.34% Co, the latter three valuable metals together averaging 0.74%, grades that are not of commercial interest. Calculated growth rates apparently vary from about 0.5 mm/106 y above 2000 m water depth to about 20 mm/106 y below 3000 m. Six bulk analyses of crusts taken at three stations within the lower part of the oxygen minimum zone (1500–2000 m water depth) average 0.79% cobalt, contents comparable to those in high‐grade crusts from the Marshall Islands, indicating that these shallow‐water crusts have long‐term economic potential. But the shallow‐water crusts sampled are only 2–4 cm thick (approximately the cut‐off grade for economic deposits). Further seabed sampling in the oxygen‐minimum zone might reveal thicker high‐grade crusts.

Release of Sorbed Sulfate from Iron Oxyhydroxides Precipitated from Acid Mine Drainage Associated with Coal Mining

Batch experiments were used to investigate the release of sulfate sorbed on X-ray amorphous iron oxyhydroxide precipitates that formed in acid mine drainage (AMD) (pH = 2.8-3.2) from the Stearns Coal Belt in southeastern Kentucky. The sediments were characterized by high sulfate concentrations (∼600-1000 mmol/kg) and iron/sulfate ratios that ranged from 6.6to 8.6 as determined by dissolution in 10 M hydrochloric acid. The results of a ligand exchange experiment (using separate preparations of 0-0.25 N sodium nitrate, chloride, phosphate, bicarbonate, and oxalate) indicated that ∼60-70% of the total sulfate will be retained in the presence of monovalent ligands. This may be indicative of a bidentate bridging mechanism bonding iron and sulfate. Most of this sulfate would likely be stable upon the secondary iron oxyhydroxides associated with acid mine drainage. Sulfate desorption increased directly with pH. At neutral pH, approximately 33-50% of the total sulfate present in these precipitates was released to solution. The bicarbonate ion released ∼60% of the total sulfate from one of the AMD precipitates. These results imply that acid neutralization methods (e.g., application of crushed limestone) used to stabilize metals in AMD can have the unwanted effect of raising sulfate concentrations within impacted watersheds.

An AEM-TEM study of nanometer-scale mineral associations in an aquifer sand: Implications for colloid mobilization

Analytical and transmission electron microscopy (AEM-TEM) techniques were used to identify mineral juxtapositions at the nanometer-scale in the interstitial matrix of a shallow, Southeastern Coastal plain aquifer sand (Georgetown, South Carolina, USA). In doing so, we sought to infer particle-particle interaction mechanisms holding the matrix intact. The aquifer is a fine-to-medium quartz sand with approximately 12% by weight <63 μm size fraction composing the interstitial matrix. The clay-size fraction contains kaolinite, goethite, gibbsite, and vermiculite. The arrangement of the clay minerals is that of a framework of face-associated domains. Selective extraction revealed that goethite constituted 95% by weight of the free iron oxyhydroxides in the <63 μm size fraction, but AEM-TEM and high resolution TEM (HRTEM) indicated that the goethite occurred only in discrete aggregates among the clays. Conversely, the remaining 5% of the free iron oxyhydroxides comprised an amorphous iron phase which was found to be distributed throughout the matrix and directly associated with the clay particles. This evidence suggests that the amorphous iron oxyhydroxide phase could act as an effective binding intermediary among the clay-clay associations, possibly electrostatically or through bond linkages with the clay surfaces. In addition, HRTEM indicated the presence of another amorphous phase which appeared to hold the clay particle aggregates in a cementitious web. AEM suggested that this amorphous phase was silicon enriched, probably biogenic opal. This evidence suggests that instigating dissolution of the opaline silica phase may be necessary to induce substantial colloid mobilization in this aquifer sediment.

Application of a sequential extraction scheme to ten geological certified reference materials for the determination of 20 elements

Methods are described for a sequential extraction scheme to dissolve selectively elements bound in soils and sediments in the following nominal forms: (1) adsorbed, exchangeable, carbonate (AEC); (2) amorphous iron oxyhydroxide (am Fe ox), including manganese oxides; (3) crystalline iron oxides (cry Fe ox); (4) organics and sulfides; and (5) residual (mainly silicates). This scheme has been applied in triplicate to a suite of ten international CRMs, i.e., soils SRM 2709–2711 and the SO-1–4 series, marine mud MAG-1, lake sediment LKSD-4 and the till sample TILL-2. Elements determined comprise: As (by HG–quartz tube AAS, HG–QTAAS); Be, Ca, Co, Cr, Cu, Ni, P, Ti and V (by ICP-AES); Mn, Fe and Zn (by FAAS); and Cd, Ce, La, Li, Tl, Pb and U (by ICP-MS). The precision obtained is excellent, generally in the range 2–10% RSD, at concentrations 10 × higher than detection limits. Most results for the element concentrations summed over the five phases agree, within statistical uncertainties, with the recommended total values for the CRMs. Those where recoveries are significantly below 90% are for elements such as Cr and V, which are known to be present in refractory minerals and would require fusion for complete dissolution. The results presented herein for samples SO-1–4 and MAG-1 do not agree well with those recently published using a scheme purported to dissolve similar phases. This highlights the need to be more definitive in describing the nature and extent of the phases actually extracted so that comparisons can be made between different laboratories and studies.

Sorption of arsenic by iron oxides and oxyhydroxides in soils

The concentration of arsenic has been determined in natural iron oxyhydroxides and oxide minerals extracted from soils at the Ashanti mine, Ghana, and was found to vary from 2 to 35,600 mg/kg. The highest concentration of arsenic in these phases occurred in neutral-pH oxidized clay-rich soils, up to 35,600 mg/kg, and in the oxidized surface portion of mine tailings. Where highly acidic soils occurred or reducing conditions were prevalent, lower concentrations of arsenic were measured in the iron oxyhydroxide and oxide fraction (up to 433 mg/kg). Lower concentrations of arsenic in these phases were also recorded from minerals in the organic-rich soils. The proportion of arsenic associated with amorphous iron oxyhydroxides was much greater than that associated with crystalline iron oxyhydroxide and oxide minerals within the studied sample set. In order to study the effects of pH and soil organic acids on arsenic sorption, the sorbed concentrations of As(V), As(III), monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA) on goethite, hematite and lepidocrocite were measured. The sorption decreases in the order as (V) > DMAA = MMAA > As(III) below pH 7 and As(V) > As(III) > MMAA = DMAA above pH 7. Sorption decreased in the order goethite > lepidocrocite > hematite. Increased fulvic acid concentration reduced arsenic sorption on all the iron minerals. Therefore, both a drop in pH and an increase in fulvic acid increases arsenic leaching while greater sorption occurs in solutions in the pH range 4–8. From the agreement between field observations and laboratory experiments, arsenic sorption by iron oxyhydroxide and oxide minerals would appear to be controlled by Eh and pH and in the presence of other potential sorbents such as organic acids. This has important consequences for the disposal of arsenic in mine tailings where reducing conditions are common and influence the stability of As(III) in place of As(V). This leads to lower sorption and greater leaching of arsenic. Additionally, in organic-rich soils, the competition for binding sites on mineral surfaces between organic acids and arsenic species may also lead to lower levels of arsenic retention.

Particle recycling in the TAG hydrothermal plume

Analysis of Th isotope distributions in the TAG hydrothermal plume yields evidence for pronounced 234Th/ 230Th isotope fractionation. This fractionation can be attributed to a combination of four processes: (1) particle ageing; (2) differential scavenging of 234Th and 230Th; (3) differential focusing of 234Th and 230Th and (4) recycling of old particles. We present arguments for the importance of the last of these processes, particle recycling. Particles in the TAG hydrothermal plume exhibit grain sizes which fall in the range 1–10 μm. Applying the range of densities reported for amorphous iron oxyhydroxide and iron oxide mineral phases, Stokes' Law predicts that these particles should achieve terminal settling velocities ranging between 4 and 8 m d −1 for coarse-grained (5 μm radius) particles and 0.2–0.3 m d −1 for fine-grained particles (1 μm radius). These velocities have been used in a physical model of re-entrainment and recycling in neutrally buoyant plumes. Our results indicate that particle recycling and re-entrainment in hydrothermal plumes may be important over length scales of 1–10 km. The model predicts that the proportion of particles re-entrained is independent of particle size but increases with plume strength. For plumes with volumetric fluxes greater than 10 3 m 3 s −1, the proportion of particles re-entrained exceeds 50%, in agreement with previous calculations based upon 230Th distributions. Sampling strategies in the past have typically concentrated upon processes operating within ⩽ 1 km of active vent fields and will need to be revised accordingly in the future.