Selective Sequential Extraction Research Articles

Distribution and Homogenization of Multiple Mercury Species Inputs to Freshwater Wetland Mesocosms.

Mercury (Hg)-impaired aquatic ecosystems often receive multiple inputs of different Hg species with varying potentials for transformation and bioaccumulation. Over time, these distinct input pools of Hg homogenize in their relative distributions and bioaccumulation potentials as a result of biogeochemical processes and other aging processes within the ecosystem. This study sought to evaluate the relative time scale for homogenization of multiple Hg inputs to wetlands, information that is relevant for ecosystem management strategies that consider Hg source apportionment. We performed experiments in simulated freshwater wetland mesocosms that were dosed with four isotopically labeled mercury forms: two dissolved forms (Hg2+ and Hg-humic acid) and two particulate forms (nano-HgS and Hg adsorbed to FeS). Over the course of one year, we monitored the four Hg isotope endmembers for their relative distribution between surface water, sediment, and fish in the mesocosms, partitioning between soluble and particulate forms, and conversion to methylated mercury (MeHg). We also evaluated the reactivity and mobility of Hg through sequential selective extractions of sediment and the uptake flux of aqueous Hg in a diffusive gradient in thin-film (DGT) passive samplers. We observed that the four isotope spikes were relatively similar in surface water concentration (ca. 3000 ng/L) immediately after spike addition. At 1-3 months after dosing, Hg concentrations were 1-50 ng/L and were greater for the initially dissolved isotope endmembers than the initially particulate endmembers. In contrast, the Hg isotope endmembers in surface sediments were similar in relative concentration within 2 months after spike addition. However, the uptake fluxes of Hg in DGT samplers, deployed in both the water column and surface sediment, were generally greater for initially dissolved Hg endmembers and lower for initially particulate endmembers. At one year postdosing, the DGT-uptake fluxes were converging toward similar values between the Hg isotope endmembers. However, the relative distribution of isotope endmembers was still significantly different in both the water column and sediment (p < 0.01 according to one-way ANOVA analysis). In contrast, selective sequential extractions resulted in a homogeneous distribution, with >90% of each endmember extracted in the KOH fraction, suggesting that Hg species were associated with sediment organic matter. For MeHg concentrations in surface sediment and fish, the relative contributions from each endmember were significantly different at all sampling time points. Altogether, these results provide insights into the time scales of distribution for different Hg species that enter a wetland ecosystem. While these inputs attain homogeneity in concentration in primary storage compartments (i.e., sediments) within weeks after addition, these input pools remain differentiated for more than one year in terms of reactivity for passive samplers, MeHg concentration, and bioaccumulation.

Sequential selective supercritical fluid extraction (S3FE) of triglycerides and flavonolignans from milk thistle (Silybum marianum L, Gaertn)

Supercritical fluid extraction with carbon dioxide is nowadays one of the best approaches on an economic and ecological point of view for the extraction of natural compounds from plants. Besides, many seeds or fruits contain both lipids, as triglycerides, which can be used for food or cosmetic industries, and polyphenols which do have many bioprotective effects for the human health. By using successively pure carbon dioxide and carbon dioxide added with a polar modifier, either pure ethanol or a ethanol/water mixture, we achieved first the extraction of triglycerides of the milk thistle seeds, followed by the extraction of the flavonolignans, which are bioactive compounds. Effects of pressure (15 and 25 MPa) and temperature (40 and 60 °C) were studied on the lipids extraction efficiency. For flavonolignans, the polar compounds, the impact of modifier amount (pure ethanol from 10% to 40%) and water percentage in ethanol (5–15%) were compared. The duration of the two sequential steps to achieve the complete seed defatting and to increase the flavonolignan recovery were also studied. The complete lipids extraction was achieved with pure CO2 at 40 °C, 25 MPa for 30 min whereas the extraction of flavonolignans was achieved with 20% of modifier (wate/ethanol 15/85) during 90 min. The analysis of the triglyceride composition of milk thistle seed were studied by using supercritical fluid chromatography (SFC), whereas the analysis of flavonolignans was carried out by an improved Ultra-high-pressure liquid chromatography (UHPLC) method.

Assessment of the barrier function of Chernozem and Luvisol under their experimental contamination by copper ions

The aim of the work was to assess the ecological resistance of topsoils of Chernozem and Luvisol to modeled copper contamination. To determine the sorption capacity of soils and to obtain the required levels of contamination the experiment was carried out. Soils were contaminated by the Cu acetate solutions with concentrations from 20 to 200 mg/L, which corresponds to the input of 20–2 000 mg/kg of Cu. Approximation of Cu adsorption isotherms by the Langmuir equation was statistically significant. The obtained values of maximum adsorption (Cmax) and Langmuir constant (KL) for Chernozem were higher than for Luvisol. This indicates a higher adsorption capacity and stronger retention of adsorbed Cu in Chernozem compared with Luvisol. This fact can be explained by the higher content and composition of organic matter, the heavier texture and the neutral pH value of Chernozem. By the method of sequential selective extractions, it was found that in uncontaminated soils, more than 50% of Cu is strongly bounded to silicate fraction (&gt;50%). Assessment of Cu fractionation at different levels of contamination reveals a slight increase in the percentage of Cu bioavailable forms (exchangeable and acid-soluble) in Chernozem at 200 mg/kg and its significant increase in Luvisol at 200 mg/kg. At contamination of about 2 000 mg/kg Cu, both soils are characterised by a predominance of pools of bioavailable and bound to (hydr)oxide of Fe and Mn forms. The mobility factor does not exceed 10% for Chernozem and 25% for Luvisol in the native and contaminated by 200 mg/kg soils, which indicates low and medium bioavailability of Cu in them, respectively. Contamination by 2 000 mg/kg of Cu increased the mobility factor up to 25% in Chernozem and up to 40% in Luvisol, which means the medium and high risk of Cu migration into the adjacent environments.

Selective sequential extraction and microanalysis for revealing distribution, occurrence mode, and removal potential of arsenic in phosphate ore

Arsenic (As) is a common impurity extensively existing in phosphate ore and it has to be removed using complicated operations during the phosphorus chemical process. To investigate the occurrence mode of As would be beneficial to evaluate the separation of As from phosphate ore in the flotation process. In the current study, selective sequential extraction (SSE) and microanalysis were employed to reveal the distribution and occurrence modes of As in a phosphate ore sample. The SSE results indicated that As in the phosphate ore was dispersed in fractions of carbonate minerals (18.27 %), oxides/hydroxides (ca. 38 %), and sulfide minerals (22.07 %). In addition, 3.05 % of As could be extracted in the exchangeable and soluble fraction. Microanalysis of scanning electron microscope (SEM), transmission electron microscope (TEM), and electron probe micro-analyzer (EPMA) revealed the main arsenic-containing minerals by means of energy dispersive spectrometer (EDS) or wavelength dispersive spectrometer (WDS). Results showed that all of iron oxides/hydroxides phases, all of rhodochrosite particles, and parts of pyrite were arsenic-containing, with the average contents of As measured to be 0.142, 0.052, and 0.047 %, respectively. Consequently, arsenic amounts in the phosphate ore descended in the order of iron oxides/hydroxides > pyrite > rhodochrosite. These arsenic-containing minerals/phases could be removed through different beneficiation methods, such as flotation and chemical minerals processing. The findings of this study provide a mineralogical insight into developing potential techniques to remove As from phosphate ores.

Performance of electrokinetic remediation system for mercury contaminated marine sediment: Roles of electrode spacing and electrode configuration

Understanding mercury (Hg) species existing after electrokinetic remediation (EK) for marine-sediment remediation is limited. Herein, the Hg fraction removal of EK from contaminated marine sediment was investigated appertaining to bipolar electrode settings along with the effects of electrode spacing and configuration considered. Based on the selective sequential Hg extractions (Hg SSE), much of the Hg in the sediment originated from F5 (i.e., HgS) and F4 (i.e., HgO). The F5 fraction removed by EK was about 30 %, while the F4 fraction had a slight increase of about 34 %. When it comes to electrode configuration, a hexagonal pattern has a higher Hg removal performance than that of a rectangular shape. The addition of anodes increases the remediation surface area, thus allowing superior Hg removal. This study indicates that the electrode spacing significantly affects the mercury removal and the remediation time. Determining suitable spacing enhances the electrical potentials in the migration flux.

Supercritical Fluid Extraction (SFE) of Polar Compounds from Camellia sinensis Leaves: Use of Ethanol/Water as a Green Polarity Modifier.

The use of bioactive plant extracts in cosmetic products is a common practice. Most of these extracts are obtained by maceration in organic solvents, and depending on which solvents are used, the polarity and the structure of the target molecules will vary. Polyphenols are polar compounds that often display antioxidant and/or antibacterial activities. To extract them, ethanol/water mixtures are usually selected as green solvents. This solid-liquid extraction (assisted or not) requires the use of high volumes of solvents and many additional steps like mixing, agitation, filtration, and evaporation. Alternatively, supercritical carbon dioxide (SC-CO2) offers many benefits for plant extraction: economical, non-toxic, and naturally concentrated extracts. However, its low polarity is not suitable to solubilize polar compounds. In this study, an experimental design was used to optimize supercritical fluid extraction (SFE) of caffeine and catechins from Camellia sinensis. Catechins are recognized for skin care use (antioxidant) and caffeine is also used for its skin care properties and to prevent excess storage of fat in cells. The temperature, modifier content, and water additive percentage were used as independent variables. The results showed that while the temperature was an insignificant parameter, a higher percentage of water (up to 20% in ethanol) and modifier favored the extraction of the polar target molecules. Additionally, the SFE results were compared with ultrasound-assisted extraction (UAE). Finally, a sequential selective extraction of caffeine from catechins is also presented.

An investigation on mobility of heavy metals for assessing the reusability of soil-like material reclaimed from mining of municipal solid waste dumpsites

Landfill mining, often referred to as “bio-mining”, enables the recovery of resources, including combustible, compostable, and recyclable fractions from landfills. However, most of the materials mined from old landfills mainly consist of soil-like materials (SLM). The reuse of SLM depends on the concentration of contaminants, such as heavy metals, soluble salts, etc. A sound risk assessment requires sequential extraction to determine the bioavailability of heavy metals. This study focuses on the mobility and chemical speciation of heavy metals in SLM from four old municipal solid waste dumpsites in India by performing selective sequential extraction. Additionally, the study compares the results with those of four previous investigations to identify international similarities. It has been observed that Zn was mainly available in the reducible phase (average 41%), whereas Ni and Cr proved to have the highest distribution in the residual phase (64% and 71%, respectively). Pb analysis showed a large portion in the oxidizable phase (39%), while Cu was mainly present in the oxidizable (37%) and residual (39%) phases. Similarities with previous investigations were observed for Zn (primarily reducible 48%), Ni (residual 52%), and Cu (oxidizable 56%). Correlation analysis showed that Ni correlated with all heavy metals (ρ = 0.71–0.78), except with Cu. The present study suggested that Zn and Pb are associated with a high risk of pollution due to their maximum distribution in the bioavailable phase. The findings of the study can be used to assess the heavy metal contamination potential of SLM prior to its reuse in offsite applications.

Assessments of Heavy Metals Accumulation, Bioavailability, Mobility, and Toxicity in Serpentine Soils

Accumulation of heavy metals is a concerning issue due to their known persistence in the ecosystem, and there are standard limits established for their maximum allowable concentrations in soils. However, heavy metal accumulation coming from serpentinite soils often exceeds the regulatory values, and there is a lack of knowledge regarding their bioavailability, mobility, and toxicity in the environment. This research applied novel selective sequential extraction and leaching procedures to assess the gaps in knowledge regarding heavy metals accumulation on serpentinite topsoil derived from a few states in Peninsular Malaysia. Based on the total digestion method, the concentration of all studied heavy metals except Mn exceeded the site screening levels issued by the Department of Environment, Malaysia (DOEM). The Geo-accumulation Index categorized Cr, Cd, Ni, and Co as extreme contamination and Cu, Pb, Zn, and Mn as unpolluted to moderate contamination. From the extraction results, Cd was found bounded 100% to a residual fraction. Meanwhile, Ni, Co, and Cr were mostly (≥92%) found to be bound to a residual fraction, with the remaining percentages distributed within non-bioavailable fractions (crystalline Fe oxides, poorly crystalline Fe oxides, and Mn oxides). Nevertheless, Cu, Pb, Zn, and Mn contaminants showed an increase (1–9%) in bioavailability and mobility fractions (soluble–exchangeable, surface-adsorbed, and organic matter) which pose a threat to the environment. The toxicity of the heavy metals greatly surpassed the DOEM standards; however, it was still below the global USEPA toxicity control. This research concluded that, even though the toxicity level of the topsoils had not exceeded the global toxicity limit, the accumulation of heavy metals in the serpentinite soils needs to be addressed due to its high concentration and its being potentially bioavailable and mobile in the environment.

Stability and trace element composition of natural schwertmannite precipitated from acid mine drainage

Schwertmannite is an iron hydroxo sulfate secondary mineral (Fe8O8(OH)8-2x(SO4)x•nH2O; 1 ≤ x ≤ 1.75) that commonly precipitates in sulfate-rich acid and metalliferous drainage as Fe(II) oxidises to Fe(III). It displays a high capacity to incorporate a range of elements through substitution of metal cations for Fe and anionic species for inner-sphere and outer-sphere sulfate groups, and surface adsorption. Precipitates from the base to top of sediment profiles immediately downstream from the historic Sunny Corner sulfide deposit are dominated by schwertmannite, indicating the stability of this mineral over decadal time frames in the low pH and high sulfate conditions. Various mineral characterisation methods, coupled with hydroxylamine hydrochloride based sequential selective extractions, reveal two forms of schwertmannite co-existing and in similar proportions throughout the profiles. The first form of schwertmannite has a high S/Fe ratio and the pattern of release of Cu, Pb, Zn and As relative to Fe indicates such trace elements are mostly adsorbed to outer-sphere sites. By contrast, the second and more crystalline schwertmannite has lower S/Fe ratios and requires higher concentrations of hydroxylamine hydrochloride to dissolve. The second schwertmannite displays substantial substitution of arsenate for sulfate, with As/(As + S) molar ratios ranging from 0.1 to 0.6, and substantially higher concentrations of trace elements. Sequential precipitation of the two schwertmannites from the AMD appears more likely than transformation of one form to the other. At sites such as Sunny Corner, pH neutralisation or removal of sulfate from the AMD stream may result in destabilisation of the schwertmannite and release of associated trace metals.

Water-Rock Interaction Processes: A Local Scale Study on Arsenic Sources and Release Mechanisms from a Volcanic Rock Matrix.

Arsenic is a potentially toxic element (PTE) that is widely present in groundwater, with concentrations often exceeding the WHO drinking water guideline value (10.0 μg/L), entailing a prominent risk to human health due to long-term exposure. We investigated its origin in groundwater in a study area located north of Rome (Italy) in a volcanic-sedimentary aquifer. Some possible mineralogical sources and main mechanisms governing As mobilization from a representative volcanic tuff have been investigated via laboratory experiments, such as selective sequential extraction and dissolution tests mimicking different release conditions. Arsenic in groundwater ranges from 0.2 to 50.6 μg/L. It does not exhibit a defined spatial distribution, and it shows positive correlations with other PTEs typical of a volcanic environment, such as F, U, and V. Various potential As-bearing phases, such as zeolites, iron oxyhydroxides, calcite, and pyrite are present in the tuff samples. Arsenic in the rocks shows concentrations in the range of 17–41 mg/kg and is mostly associated with a minor fraction of the rock constituted by FeOOH, in particular, low crystalline, containing up to 70% of total As. Secondary fractions include specifically adsorbed As, As-coprecipitated or bound to calcite and linked to sulfides. Results show that As in groundwater mainly originates from water-rock interaction processes. The release of As into groundwater most likely occurs through desorption phenomena in the presence of specific exchangers and, although locally, via the reductive dissolution of Fe oxy-hydroxides.

Process intensification of element extraction using centrifugal contactors in the nuclear fuel cycle

This review focuses on consolidating solvent extraction performed in the process intensification equipment known as Centrifugal Contactors (CCs), implemented in Spent Nuclear Fuel (SNF) reprocessing and radioactive waste processing. Recovery of valuable actinides is important from sustainability perspectives as it is a source of metals of technological interest from SNF, specifically the recovery of fissile and fertile material, and can also be employed in the processing of Waste Electrical and Electronic Equipment (WEEE). Solvent extraction (also referred to as liquid-liquid extraction, or aqueous separation), is employed in the separation of f-block elements and fission products in SNF. The sequential isolation using different flowsheets has been performed on a range of scales using CCs. However, solids, either present in the feed solution or formed in situ, are always cited as a concern for the operability of CCs, and their extraction efficiencies. This review quantifies the unexpected solid arisings and accumulation during operation in the presence and absence of highly radioactive isotopes from bench to plant scale. The review concludes with techniques implemented for the removal of solids from CCs.

Behavior of iron and other heavy metals in passivated sediments and the coupling effect on phosphorus

In situ passivation, which is easy to operate and affordable, is one of the most commonly used methods for sediment phosphorus (P) remediation. Understanding the behavior of iron and other heavy metals in passivated sediments is important for alleviating lake eutrophication and for ensuring drinking water safety. In this study, we investigated the behavior of P, Fe, Mn, Cd, Co, and Pb in lanthanum modified bentonite (LMB, Phoslock®) and polyaluminum chloride (PAC)-passivated sediments using intact sediment cores. Rhizon sampler and diffusive gradients in thin films technology (DGT) were respectively used to collect soluble and labile substances in sediment; a modified sequential selective extraction method was used to characterize metal forms. Results showed that LMB reduced soluble reactive phosphorus (SRP) at sediment depths of 0 ~ −15 mm and DGT-labile P flux at 0 ~ −50 mm. Correlation between DGT-labile P and Fe (R2 = 0.71) indicated that P mobility in the LMB group was affected by the behavior of Fe. PAC decreased SRP at sediment depths of 0, −5, −10, −15, −20, −25, and −50 mm with removal rates of 100%, 90%, 45%, 35%, 81%, 89%, and 100%, respectively. DGT-labile P flux was decreased by PAC at 0 ~ −10 mm and −50 ~ −110 mm, but increased at −10 ~ −50 mm; this is a result of synthetical effect by Al flocs adsorption and Fe(III) reductive dissolution. LMB decreased Cd, Co, and Pb in LMB layer in carbonate, reducible, and oxidizable forms. PAC decreased Cd mobility but caused the transformation of Co and Pb from reducible to other forms because of Fe(III) reductive dissolution. Those results indicate that sedimentary Fe plays an important role in in situ passivation. We suggest modifying passivators to Fe(II) adsorbents and increasing DO permeability of sediment to promote the formation of an Fe(III) passivation layer and hence the effectiveness of P control.

Sequential separation and selective extraction of uranium and thorium from monazite sulfate leach liquor using dipropylamine extractant

Monazite mineral is the main source of uranium and thorium, which are mainly associated with rare earth elements (REEs). So, their separation is the crucial task to satisfy their application for nuclear fuel. Dipropylamine was used for the first time in the selective extraction and separation of uranium and thorium from Monazite sulfate leach liquor. The extraction of thorium (98%) was achieved using dipropylamine in kerosene at pH 1; it was followed by selective extraction of uranium (97%) by Dipropylamine at pH 3. The thermodynamic analysis demonstrated that uranium (VI) and thorium (IV) extraction by dipropylamine is an exothermic process. From the stripped liquor (1 M H2SO4), Th and U were precipitated as oxalate and peroxide, respectively, for technological application. Thus, marketable pure thorium and uranium products were prepared in addition to rare earth concentrate.

Mercury vertical and horizontal concentrations in agricultural soils of a historically contaminated site: Role of soil properties, chemical loading, and cultivated plant species in driving its mobility

The long term vertical and horizontal mobility of mercury (Hg) in soils of agricultural areas of a historically contaminated Italian National Relevance Site (SIN Brescia-Caffaro) was investigated. The contamination resulted from the continuous discharge of Hg in irrigation waters by an industrial plant (Caffaro S.p.A), equipped with a mercury-cell chlor-alkali process. The contamination levels with depth ranged from about 20 mg/kg dry weight (d.w.) of soil in the top (plow) layer to less than 0.1 mg/kg d.w. at 1 m depth. The concentrations varied also spatially, up to one order of magnitude within the same field and showing a decreasing trend from the Hg source (i.e., irrigation ditches). The concentration profiles and gradients measured were explained considering Hg loading, soil properties, such as the texture, organic carbon content, pH and cation exchange capacity. A Selective Sequential Extraction (SSE) was also applied on soil samples from an ad hoc greenhouse experiment to investigate the role of different plant species in influencing Hg speciation in soils. Although most of the extracted Hg was included in scarcely mobile or immobile forms, some plant species (i.e., alfalfa) showed to importantly increase the soluble and exchangeable fractions with respect to the unplanted control soils, thus affecting mobility and potential bioavailability of Hg.

Can Sediments Contaminated by Mining be a Source of Mercury in the Coastal Environment Due to Dredging? Evidence from Thermo-Desorption and Chemical Speciation

The sediments in the Gulf of Trieste (northern Adriatic Sea, Italy) are contaminated by mercury (Hg) due to historic mining which took place in Idrija (Slovenia). Despite many studies having been done regarding Hg, no information is available on the potential impact of dredging required along the main channel approaching the Port of Monfalcone. Sixteen surface sediment samples were collected along the channel to determine both total Hg concentration and chemical species using the thermo-desorption (TD) technique. Six samples were also chosen to apply a selective sequential extraction (SSE). The TD technique showed the maximum Hg release approximately at 260 and 335°C, corresponding to metacinnabar (β-HgS) and cinnabar (α-HgS), respectively. The SSE demonstrated that Hg was mainly associated with poorly soluble or insoluble compounds (98.7%). A resuspension event over a limited period of time can be considered of negligible impact to the water column due to the scarce Hg mobility from sediments.

Environmental Impact Assessment in the Former Mining Area of Regoufe (Arouca, Portugal): Contributions to Future Remediation Measures.

The W-Sn Regoufe mine, closed since the 1970s, was once intensively exploited for tungsten concentrates. Throughout its activity, considerable amounts of arsenopyrite-rich mine wastes were produced and, to this day, are still exposed to weathering conditions. Thus, this work aims at assessing soil contamination, using a combination of chemical, physicochemical and mineralogical analyses and sequential selective chemical extraction of the main potentially toxic elements (PTEs) in topsoils. Results show that Regoufe soils are enriched in most of the PTEs associated with the ore assemblage, but As and Cd contents far outstrip both international and national guidelines. The estimated contamination factor reveals that 67% of soil samples are classified as highly to ultra-highly contaminated. Similar distribution patterns, with the main focus around the unsealed mine adits, are observed when spatially projecting the modified degree of contamination (mCd) and arsenic contents. Fe-oxyhydroxides and organic matter demonstrate to have a preponderant role in the retention of Cd and As. In fact, despite the high PTE contents in soils, local surface waters are characterised by low metal(loid) contents and nearly neutral pH, with PTE concentrations below national thresholds for irrigation waters.

Assessment of selective sequential extraction procedure for determining arsenic partitioning in copper slag

Abstract An optimized selective sequential extraction (SSE) procedure was developed to assess the arsenic (As) partitioning in copper slag. The potential As species in copper slag are partitioned into the readily soluble As, dissolvable arsenates, sparingly soluble arsenates, As residing in sulfides, arsenopyrite and metal As, as well as As incorporated into glassy silicates. The inductively coupled plasma atomic emission spectrometry (ICP-AES), scanning electronic microscopy (SEM), transmission electron microscope (TEM), X-ray diffractometry (XRD) and Fourier transform infrared spectroscopy (FTIR) were used to characterize the leachates and residues produced from the operation scheme. The selectivity and recovery of extractants were evaluated through single-phase extraction procedures. Partitioning data of As in slag samples show good agreement with the reported works and the total As recovery of each operation is over 90%. This suggests that the optimized SEE scheme can be reliably employed for As partitioning in As-bearing byproducts from copper smelting.

Evolution of the Speciation and Mobility of Pb, Zn and Cd in Relation to Transport Processes in a Mining Environment.

Elements in mining extracts can be potentially toxic if they are incorporated into soils, sediments or biota. Numerous approaches have been used to assess this problem, and these include sequential extractions and selective extractions. These two methods have limitations and advantages, and their combined use usually provides a rough estimate of the availability or (bio)availability of potentially toxic elements and, therefore, of their real potential as toxicants in food chains. These indirect speciation data are interesting in absolute terms, but in the work described here, this aspect was developed further by assessing the evolution of availability-related speciation in relation to the transport processes from the emission source, which are mainly fluvial- and wind-driven. This objective was achieved by characterizing tailings samples as the source of elements in soils and sediments at increasing distances to investigate the evolution of certain elements. The standard procedures employed included a sequential five-step extraction and a selective extraction with ammonium acetate. The results show that the highest percentages of Zn and Pb in tailings, soils and sediment samples are associated with oxyhydroxides, along with a significant presence of resistant mineralogical forms. In the case of Cd, its association with organic matter is the second-most important trapping mechanism in the area. The physicochemical mechanisms of transport did not transform the main mineralogical associations (oxyhydroxides and resistant mineralogical forms) along the transects, but they produced a chaotic evolution pattern for the other minor matrix associations for Zn and a decrease in exchangeable and carbonate-bound forms for Pb in soils. Interestingly, in sediments, these mobile forms showed a decrease in Zn and a chaotic evolution for Pb. The most probable reason for these observations is that Zn2+ can form smithsonite (ZnCO3) or hydrozincite (Zn5(CO3)2(OH)6), which explains the retention of a carbonate-bound form for Zn in the soil transect. In contrast, Pb and Cd can appear as different mineral phases. The order of (bio)availability was Pb > Zn > Cd in tailings but Cd > Pb > Zn in soils. The physicochemical processes involved in transport from tailings to soils produce an increase in Cd (bio)availability. The trend is a decrease in bioavailability on moving away from the source (tailings), with maximum values obtained for Cd near to the source area (200–400 m).

The use of operationally-defined sequential Fe extraction methods for mineralogical applications: A cautionary tale from Mössbauer spectroscopy

Reactive iron minerals are crucial components of global nutrient cycles, directly controlling carbon transport and storage in marine sediments. Sequential selective extraction is frequently used for quantitatively characterising, and chemically isolating, individual Fe mineral phases. Reagent-specific mineral solubility is fundamental to the success of any sequential extraction, but is strongly affected by the varying physical and chemical morphology intrinsic to natural mineral samples. Natural sediment, rock, and soil samples often contain a mineral mixture, which further modifies solvent efficacy. 57Fe Mössbauer spectroscopy only probes the hyperfine interactions between next-nearest neighbouring atomic nuclei in the crystal lattice and is less affected by variation in mineral grain size and crystallinity than conventional, X-ray-based methods. In this study, we used Mössbauer spectroscopy in a novel context to cross-calibrate and optimise a popular, but frequently misused, sequential Fe extraction protocol. Our results showed that incomplete and premature removal of the target Fe minerals could occur at nearly every stage of the extraction and, in many cases, the leachate Fe content did not represent the target phase at all. Crystalline, natural siderite and amorphous, synthetic goethite were detected in the Mössbauer spectrum of the ammonium oxalate extraction for magnetite, after which all reactive Fe minerals should have been removed. Consistent with previous studies, and unlike many other clay minerals, nontronite was extracted as part of the highly reactive Fe pool, and in fact our data indicate that this mineral was extracted by the initial Na-acetate extraction that targets ‘carbonate-bound Fe’. Matrix effects appeared to cause variable yield efficiencies: synthetic goethite was successfully removed when present as an individual mineral yet persisted beyond its target extraction when present in an Fe mineral mixture. Although suitable for the quantification of operationally-defined Fe pools, we caution the unverified use of sequential Fe extraction protocols for mineral specific applications. The application of sequential Fe extractions to define the reactive Fe pools as a paleoredox proxy of depositional conditions appears relatively robust. The premature removal of 2-line ferrihydrite observed in this study (due to the use of the more aggressive Na-acetate extraction for crystalline siderite), does not limit the quantitative use of the sequential Fe extraction in ancient sediments, where such ‘easily reducible’ oxides are unlikely to persist. In contrast, attributing the outcomes of operationally-defined Fe pools to specific Fe minerals is precarious and potentially entirely erroneous. Where Fe mineral specificity or separation is required, we recommend post-extraction validation by another secondary technique. Mössbauer spectroscopy offers such a method that can independently verify extraction stages and assess mineral specificity.

Redistribution and speciation of elements in gold-bearing sulfide mine tailings interbedded with natural organic matter: case study of Novo-Ursk deposit, Kemerovo Region, Siberia

The paper presents data on two cores from the dispersion train of gold-bearing sulfide tailings in the Novo-Ursk deposit (Kemerovo region, Russia), where organic-bearing mud and natural organic matter are interbedded with cyanide tailings and acid mine drainage (AMD). The study focuses on speciation of Au, Ag and potentially toxic elements (PTE), including Fe, Cu, Zn, As, Se and Pb according to selective sequential extractions. Since large contents of elements are found in water solution and bound to Fe(III) compounds, sulfides and/or organic matter in both cores, the organic material behaves as a natural biogeochemical barrier for potentially toxic elements and precious metals released from AMD. The interaction with organic matter results in biogeochemical sorption and immobilization of elements and is favourable for secondary enrichment in gold and silver. It leads to immobilization of elements that further form new mineral phases or become partitioned between existing phases. On the other hand, very high concentrations of water-soluble and oxidizable species of PTE and Au, which are unstable in surface conditions and may become remobilized when the environment changes, make the organic material less workable and even potentially hazardous. Thus, both positive (secondary enrichment) and negative (pollution) effects should be taken into account in the planning and design of systems for purification, disposal of cyanide tailings and recovery of useful components from tailings using natural organic matter as a geochemical barrier.