Speciation Of Trace Elements Research Articles

Speciation of trace elements in groundwater, surface water and sediments: a short review

In order to understand the interaction between groundwater, surface water and sediments, there is a need for speciation analysis of trace elements in the mentioned matrices. This could be done experimentally or by calculation with e.g. thermodynamic codes. This article will give a short overview of methods with some examples and results. Speciation in water could be determined by thermodynamic calculations with e.g. PHREEQC or Visual MINTEQ. Experimental speciation in water could be done by ultrafiltration or dialysis in situ. If applicable, in situ dialysis in the field is simple with little need of equipment. Speciation of trace elements in drinking water is of special interest to assess possible health effects. Speciation of trace elements in sediments and soils can be done by any one of the numerous selective extractions. Which speciation approach depends on which trace elements are of interest. A common need is to get information about plant uptake of specific elements.

Geochemical constraints on the role of tuffisite veins in degassing at the 2008–09 Chaitén and 2011–12 Cordón Caulle rhyolite eruptions

Hybrid activity during the rhyolitic eruptions of Chaitén (2008–09) and Cordón Caulle (2011–2012) in Chile has offered unprecedented insights into the enigmatic and complex degassing processes occurring during eruptions of silicic magma. Highly permeable, transient fracture networks within the conduit can act as outgassing channels. Their interaction with deeper volatile-rich melt can account for both punctuated explosive activity and large-scale degassing of the system, leading towards predominantly effusive behaviour. In this study we characterise trace element concentrations and 210Pb226Ra systematics within pyroclastic material from the recent eruptions at Chaitén and Cordón Caulle volcanoes. Results reveal how gas fluxing from deep, volatile-rich reservoirs to the surface, within magmatic conduits, can be recorded by trace elements and 210Pb226Ra disequilibria in tuffisite veins.Tuffisite veins (particle-filled fracture networks) are present in volcanic bombs from both eruptions. Trace element heterogeneity associated with tuffisites preserves evidence for degassing. At Chaitén, enrichments (e.g. Cu) and depletions (e.g. Mo, Li and Bi) are identified in vein material and clasts transported within veins, and record multiple degassing events. At Cordón Caulle, enrichments of volatiles in an early vein (e.g. Tl and Bi) and depletions in a later vein (e.g. Cd, In, Pb and Tl) reflect interactions between glassy clasts and the carrier gas phase that transported them. In contrast, 210Pb and 226Ra, which can be fractionated during degassing, are mostly in secular equilibrium. Modelling suggests that the disparity between the signals preserved in these two types of chemical signatures reflects the brevity of degassing events and the relative volumes of tuffisite veins and the bodies of degassing magma that they source gas from. The lack of preserved 210Pb enrichments in tuffisite veins at both volcanoes places an upper limit on the mass of deeper, bubble-rich magma outgassed via tuffisites during their lifetime.This study shows that both the presence, and absence, of sample-scale geochemical heterogeneity can be used to place constraints on syn-eruptive physical processes and underlines the value of analysing a wide suite of trace element species.

Investigating the use of microdialysis and SEC-UV-ICP-MS to assess iodine interactions in soil solution

Element cycling in the terrestrial environment is heavily reliant upon processes that occur in soil solution. Here we present the first application of microdialysis to sample iodine from soil solution. In comparison to conventional soil solution extraction methods such as Rhizon™ samplers, centrifugation, and high-pressure squeezing, microdialysis can passively sample dissolved compounds from soil solution without altering the in-situ speciation of trace elements at realistic soil moisture conditions. In order to assess the suitability of microdialysis for sampling iodine, the permeability factors and effect of perfusion flowrate on I− and IO3− recovery was examined in stirred solutions. Furthermore, microdialysis was used to sample native soluble iodine at a range of water contents and iodine-enriched soils to investigate iodine soil dynamics. Total iodine concentrations were measured using ICP-MS. Inorganic species and the molecular weight distribution of organically bound iodine were determined by anion exchange and size exclusion chromatography (SEC) coupled to an ICP-MS, respectively. The most effective recovery rates in stirred solution were observed with the slowest perfusion flowrate yielding 66.2 ± 7.1 and 70.5 ± 7.1% for I− and IO3−, respectively. Microdialysis was proven to be capable of sampling dissolved iodine from the soil solution, which accounted for <2.5% of the total soil iodine and speciation followed the sequence: organic-I > I− > IO3−. The use of SEC coupled to (i) UV and (ii) ICP-MS analysis provided detail regarding the molecular weight distribution of dissolved org-I compounds. Dissolved org-I was detected with approximate molecular weights between 0.1 and 4.5 kDa. The results in this study show that microdialysis is a suitable technique for sampling dissolved iodine species from soils maintained at realistic moisture contents. In addition, inorganic iodine added to soils was predominately bound with relatively low molecular weight (<4.5 kDa) soluble organic matter.

Welcome to the New L&amp;O: Methods Associate Editors!

Welcome to the New <i>L&amp;O</i>: <i>Methods</i> Associate Editors!

Distribution trend of trace elements in digestate exposed to air: Laboratory-scale investigations using DGT-based fractionation

The use of digestate as amendment for agricultural soils has already been proposed as an alternative to mineral fertilizers or undigested organic matter. However, little information is available concerning the effect of digestate atmospheric exposure on trace elements speciation and, consequently, on their mobility and bio-accessibility when digestate is stored in open tanks or handled before land spreading. In this study, we investigated at laboratory-scale the effect of digestate aeration on the distribution of Al, As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Sb, Se and W using the diffusive gradients in thin films technique (DGT)-based fractionation. For this purpose, experiments were performed to assess the variation in distribution between the labile, soluble and particulate fractions over time in digested sewage sludge during passive and forced aeration. Results showed that aeration promoted a dissolution of Al, As, Co, Cr, Cu, Fe, Mn, Mo and Pb, suggesting a possible increase in their mobility that may likely occur during storage in open tanks or handling before land spreading. Labile elements’ fraction increased only during forced aeration (except for Fe and Mn), suggesting that their short-term bio-accessibility can increase only after significant aeration as the one assumed to occur when land spreading takes place.

Environmental significance and geochemical speciation of trace elements in Lower Baram River sediments

The geochemistry and distribution of major, trace and rare earth elements (REE's) was studied in the surface sediments of the Lower Baram River during two seasons: the Monsoon (MON) and Post – monsoon (POM). The major geochemical processes controlling the distribution and mobility of major, trace and REE's in the Lower Baram River surface sediments was revealed through factor analysis. The risk assessment of major and trace element levels was studied at three specific levels; i.e. the enrichment level [Contamination Factor (Cf), with the geo-accumulation index (Igeo)], the availability level [metals bound to different fractions, risk assessment code (RAC)], and the biological toxicity level [effect range low (ERL) and effect range medium (ERM)]. The results of all the indices indicate that Cu is the element of concern in the Lower Baram River sediments. The geochemical fractionation of major and trace elements were studied through sequential extraction and the results indicated a higher concentration of Mn in the exchangeable fraction. The element of concern, Cu, was found to be highly associated in the organic bound (F4) fraction during both seasons and a change in the redox, possibly due to storms or dredging activities may stimulate the release of Cu into the overlying waters of the Lower Baram River.

Sorption of Cu(II) by Al hydroxide organo–mineral coprecipitates: Microcalorimetry and NanoSIMS observations

The Al constituents of soils and sediments play an important role in the bioavailability and fate of trace heavy metals. Pure Al precipitates rarely exist; instead, Al3+ is commonly coprecipitated with organics to produce organo–Al hydroxide composites. Herein, we replicate the Al hydroxide organo−mineral composite formation in soils and sediments by synthesizing composites using humic acid (HA) during coprecipitation with Al hydroxide (Al(OH)3). The mineralogy and morphology results indicate the formation of amorphous and nanosized pure Al(OH)3 and Al(OH)3HA composites, while HA reduces the SSA and aggregation of the pure mineral. The sorption of Cu is enhanced on the Al(OH)3HA composite over pure Al(OH)3 at pH < 6, which is attributable to the additional binding sites present in the HA molecule. The isothermal titration calorimetry (ITC) results show that Cu sorption is endothermic on Al(OH)3 and exothermic on HA, and the sorption sequence is HA > Al(OH)3. The Cu uptake by the Al(OH)3HA composite is an energy-favorable process accomplished by an increase in entropy, signifying the formation of Cu-ligand inner-sphere complexes. Nano secondary ion mass spectrometry (NanoSIMS) results demonstrate that Cu is mainly sorbed on the HA fraction of the Al(OH)3–HA composite. This study therefore has important implications for predicting the cycling of trace heavy metals in Al-rich geologic environments, and sheds light on the combined use of ITC and NanoSIMS to uncover the distribution and speciation of trace elements in complex mineral–organic composites.

Redox-stat bioreactors for elucidating mobilisation mechanisms of trace elements: an example of As-contaminated mining soils

The environmental fate of major (e.g. C, N, S, Fe and Mn) and trace (e.g. As, Cr, Sb, Se and U) elements is governed by microbially catalysed reduction-oxidation (redox) reactions. Mesocosms are routinely used to elucidate trace metal fate on the basis of correlations between biogeochemical proxies such as dissolved element concentrations, trace element speciation and dissolved organic matter. However, several redox processes may proceed simultaneously in natural soils and sediments (particularly, reductive Mn and Fe dissolution and metal/metalloid reduction), having a contrasting effect on element mobility. Here, a novel redox-stat (Rcont) bioreactor allowed precise control of the redox potential (159 ± 11 mV, ~ 2 months), suppressing redox reactions thermodynamically favoured at lower redox potential (i.e. reductive mobilisation of Fe and As). For a historically contaminated mining soil, As release could be attributed to desorption of arsenite [As(III)] and Mn reductive dissolution. By contrast, the control bioreactor (Rnat, with naturally developing redox potential) showed almost double As release (337 vs. 181 μg g−1) due to reductive dissolution of Fe (1363 μg g−1 Fe2+ released; no Fe2+ detected in Rcont) and microbial arsenate [As(V)] reduction (189 μg g−1 released vs. 46 μg g−1 As(III) in Rcont). A redox-stat bioreactor thus represents a versatile tool to study processes underlying mobilisation and sequestration of other trace elements as well.

Measuring the distribution of trace elements amongst dissolved colloidal species as a fingerprint for the contribution of tributaries to large boreal rivers

Organic and inorganic colloids play important roles governing the speciation, transport, and bioaccessibility of trace elements in aquatic systems. These carriers are especially important in the boreal zone, where rivers that contain high concentrations of iron and organic matter are prevalent. The distribution of trace elements amongst different colloidal species (or “speciation profile”) can therefore be useful as a fingerprint to detect different trace element sources and for tracking colloid transformations, with implications for bioaccessibility. Asymmetrical flow field-flow fractionation coupled to an inductively coupled plasma mass spectrometer was applied to detect the source of trace elements based on their speciation profile along a 125-km stretch of a large river in the Canadian boreal forest. Both the concentration and proportion of bound trace elements were increased by tributary inputs: bound As, Co, Fe, Mn, Pb, U, and Zn increased monotonically from upstream to downstream, increasingly resembling the speciation profile of tributaries. Principal component (PC) analysis also revealed tributary contributions of bound Cu, Ni, Th, V, and Y reflecting their higher concentrations in tributaries, and PC scores also increased monotonically from upstream–downstream. Monotonically decreasing concentrations of mainly ionic and small (i.e. <ca. 300 Da) As, Ba, Mo, and U species were also observed from upstream–downstream.

Chemical speciation and leaching characteristics of hazardous trace elements in coal and fly ash from coal-fired power plants

This work reports the chemical speciation and leaching characteristics of seven hazardous trace elements (HTEs, including Hg, As, Cr, Cd, Ba, Mn, and Pb) in the coal and fly ash samples collected from four coal-fired power plants in China. The physical structure and chemical composition of the fly ash were characterized by the scanning electron microscope (SEM) and the energy dispersive X-ray spectrometer (EDX). The chemical speciation of HTEs in the coal and fly ash was measured by the modified three-step sequential extraction method, proposed by the European Community Bureau of Reference (BCR). Leaching characteristics of HTEs in the fly ash were investigated by a single batch leaching test. The concentration of HTEs in solid and liquid samples was determined by the direct mercury analyzer DMA 80 (Hg in solid) and the inductively coupled plasma-mass spectrometry (ICP-MS). Results show that for the coal, concentration of Hg, As, Cr, Cd, Ba, Mn, and Pb is 0.06–0.22, 0.63–4.01, 8.91–13.09, 0.06–0.15, 108.67–229.21, 49.94–100.24 and 6.74–26.38 mg/kg, respectively. Mercury is mainly in the residual form while Cd and Ba are primarily in reducible form. Compared with other HTEs, manganese in water/acid soluble and exchangeable fraction has the large percentage. For the fly ash, the concentration of Hg, As, Cr, Cd, Ba, Mn and Pb is 0.17–1.26, 5.15–25.74, 43.25–64.61, 0.56–0.70, 777.05–970.70, 163.83–831.47 and 28.94–119.57 mg/kg, respectively. Mercury and chromium are mainly in the residual speciation. Arsenic and manganese in water/acid soluble and exchangeable form have high ratio with value of 7.27–58.60% and 6.14–62.27%, respectively. Cadmium and barium are primarily in the reducible form. Based on the risk assessment code, manganese in the coal can pose high or very high risk on the environment. Leaching concentration of Cr in some fly ash is higher than permissible limits and the pH value of leaching solution for the fly ash is alkaline. Considering huge fly ash production from coal combustion and complex landfill conditions, some suitable disposal measures to minimize the risk on the environment are needed.

Thermal stability, chemical speciation and leaching characteristics of hazardous trace elements in FGD gypsum from coal-fired power plants

Hazardous trace elements (HTEs) may release from flue gas desulfurization (FGD) gypsum in the utilization concerning heat treatment or landfills, which cause great harm on the environment. Thermal stability, chemical speciation and leaching characteristics of seven HTEs (Hg, As, Cr, Cd, Ba, Mn and Pb) in the FGD gypsum from three power plants equipped with air pollution control devices (APCDs) in China were investigated in this work. Results show that concentration of Hg, As, Cd, Pb, Cr, Ba, and Mn in the three FGD gypsums is 0.20–3.85, 1.04–2.87, 0.02–0.69, 3.50–7.01, 9.19–21.32, 14.84–55.94, and 45.95–69.85 mg/kg, respectively. The evaporation rate (ER) of HTEs in the three FGD gypsums increases as the thermal treatment temperature rises. More attention should be given to Hg in the thermal disposal process of gypsum, because its thermal stability is worst with ER higher than 94% at the temperature higher than 350 °C. Hg and Ba in the three FGD gypsums are mainly existed in residual form with ratio of 73.11–86.69% and 72.97–82.46%, respectively. The easily soluble As in the three FGD gypsums has the minimum percentage of 2.75–4.78%. The sum of Cr in water/acid soluble and exchangeable fraction (F1), reducible fraction (F2), and oxidizable fraction (F3) in the three FGD gypsums has a considerable ratio of 37.88–53.84%. Compared with other HTEs in the three FGD gypsums, Mn in the easily soluble form occupies largest share of 40.64–55.43%, which will be leached out from gypsum and bioavailable to the environment under typical environmental condition. The HTEs in the three FGD gypsums are in different risk levels, namely, low risk (including Hg, As, Cr, Ba), medium risk (including Cd and Pb (except FGD-G1)), high or very high risk (Mn), which should take Mn in serious consideration once disposing FGD gypsum based on the RAC. Leaching characteristics test indicates the three FGD gypsums belong to non-hazardous wastes. Several suitable disposal measures to minimize the risk of HTEs release on the environment are needed considering the bulk production of gypsum and complex landfill conditions, etc.

Translocation, accumulation and bioindication of trace elements in wetland plants

This study aimed to shed further light on the capacity of macrophytes to translocate, accumulate and bioindicate the levels of trace elements present in contaminated water and sediments. Specifically, this study aimed to find evidence whether translocation, accumulation and bioindication are dependent on the kind of trace element and plant species. To investigate the correlation between trace elements in plants and in the environment, the concentrations of As, Cd, Cr, Cu, Hg, Mn, Ni, Pb, and Zn were analyzed in twenty different wetland plants, and in water and sediments from a wetland area affected by urban and industrial pollutants. Results showed that wetland plants share some common characteristics such as high tolerance to toxic element levels, capacity of phytostabilization and different element concentrations in the various organs. Moreover, element translocation from sediments to roots seems more influenced by the kind of plant species and trace element, whereas translocation across the various organs seems mainly species-specific. No clear patterns of trace element translocation were identified according to plant life forms.

Leaching characteristic of toxic trace elements in soils amended by sewage sludge compost: A comparison of field and laboratory investigations

A 3-years field test and laboratory leaching test have been conducted to assess the environmental impact of land application of sewage sludge compost in conjunction with wheat and rice crops. Considering the complexity and variability of field conditions, we compared the result of laboratory test with the field test to understand the accuracy and uncertainty associated with using the laboratory test to evaluate the field scenario. The laboratory test with cycling of compost additions and water percolation was a high time-efficient and feasible method to simulate the annually repeated additions of compost in the field application scenario. The results of laboratory test were congruent to the 3-years field test regarding the leaching characteristics and geochemical speciation of toxic trace elements. Both the laboratory and the field test showed that repeated additions of compost to soils can increase leaching concentrations of toxic trace elements at neutral to alkaline pH. Increased toxic trace elements leaching was caused by the increase of organic matter from compost application and organic matter dissolution at alkaline pH. Uncertainties of the laboratory test mainly included the negligibility of crop growth and the strongly reducing condition formed with continuous percolation procedure.

Energy-Dispersive Total-Reflection Resonant Inelastic X-ray Scattering as a Tool for Elemental Speciation in Contaminated Water.

This work presents a state-of the-art analytical methodology, by which chemical state information on metallic elements is obtained for liquid samples in a fast and simple manner. This method overcomes limitations of conventional X-ray techniques, such as X-ray absorption spectroscopy, by applying resonant inelastic X-ray scattering under total reflection geometry (TRIXS). TRIXS is particularly applicable for the analysis of small quantity of liquid samples deposited on polished reflectors. This feature is relevant for the chemical speciation of metallic trace elements contained in water samples, since the degree of their toxicity depends crucially on the concentration of specific chemical species included. The analytical merits of the proposed methodology were studied at Elettra Sincrotrone Trieste and at the Brazilian Synchrotron Light Laboratory. Contaminated water samples with low concentration of different chromium and manganese compounds were measured. Results prove the analytical potential of the TRIXS technique in characterizing different chemical species of metallic elements in water samples.

Chemical speciation and complexation modeling of trace and rare earth elements in groundwater of Oban Massif and Mamfe mMbayment southeastern Nigeria

ABSTRACTChemical speciation was carried out on trace and rare earth elements on trace and rare earth elements data on water samples in Oban Massif and Mamfe Mbayment southeastern Nigeria. Product moment correlation showed significant correlation at r>0.7 between REEs Pr and Nd, Pr and Lu, Sm and Nd, Sm and Pr, Tb and Nd, Tm and Tb, and Yb and Nd. Speciation modeling result showed that free metal ionic species dominated boreholes and spring water samples. The predominant trace metal complex is the OH- ligand. Trace elements mineral saturation indexes in boreholes at oversaturation (S>1) predicted more mineral species than equilibrium saturation (S=0). Rare earth elements mineral oversaturation (S>1) in boreholes predicted least mineral species. Undersaturation (S<1) predicted highest range of rare earth elements minerals. Springs and streams at S>1, predicted more mineral range than equilibrium (S=O) while mineral prediction at S<1 were the highest. Rare earth elements are precipitated at undersaturated condition in the study area.

Periphyton and abiotic factors influencing arsenic speciation in aquatic environments.

Benthic periphytic biofilms are important food sources at the base of aquatic ecosystems. These biofilms also sit at the interface of oxic waters and hypoxic sediments, and can be influenced by or influence trace element speciation. In the present study, we compared arsenic (As) enrichment in periphyton exposed to arsenate (As[V]) or arsenite (As[III]) (20 μg/L, static renewal, 7 d), and we found similar accumulation patterns of total As (101 ± 27 and 88 ± 22 mg kg-1 dry wt, respectively). Periphyton As was 6281- and 6684-fold higher than their aqueous exposures and occurred primarily as As(V). When these biofilms were fed to larval mayflies, similar total As tissue concentrations (13.9 and 14.6 mg kg-1 dry wt, respectively) were observed, revealing significant biodilution (∼ 10% of their dietary concentrations). Finally, we investigated the influence of aeration and periphyton presence on As speciation in solutions and solid phases treated with As(III). Predominantly As(III) solutions were slowly oxidized over a 7-d time period, in the absence of periphyton, and aeration did not strongly affect oxidation rates. However, in the presence of periphyton, solution and solid-phase analyses (by microscale x-ray absorption spectroscopy) showed rapid As(III) oxidation to As(V) and an increasing proportion of organo-As forming over time. Thus periphyton plays several roles in As environmental behavior: 1) decreasing total dissolved As concentrations via abiotic and biotic accumulation, 2) rapidly oxidizing As(III) to As(V), 3) effluxing organo-As forms into solution, and 4) limiting trophic transfer to aquatic grazers. Environ Toxicol Chem 2018;37:903-913. © 2017 SETAC.

Experimental Column Setup for Studying Anaerobic Biogeochemical Interactions Between Iron (Oxy)Hydroxides, Trace Elements, and Bacteria.

Fate and speciation of trace elements (TEs), such as arsenic (As) and mercury (Hg), in aquifers are closely related to physio-chemical conditions, such as redox potential (Eh) and pH, but also to microbial activities that can play a direct or indirect role on speciation and/or mobility. Indeed, some bacteria can directly oxidize As(III) to As(V) or reduce As(V) to As(III). Likewise, bacteria are strongly involved in Hg cycling, either through its methylation, forming the neurotoxin monomethyl mercury, or through its reduction to elemental Hg°. The fates of both As and Hg are also strongly linked to soil or aquifer composition; indeed, As and Hg can bind to organic compounds or (oxy)hydroxides, which will influence their mobility. In turn, bacterial activities such as iron (oxy)hydroxide reduction or organic matter mineralization can indirectly influence As and Hg sequestration. The presence of sulfate/sulfide can also strongly impact these particular elements through the formation of complexes such as thio-arsenates with As or metacinnabar with Hg. Consequently, many important questions have been raised on the fate and speciation of As and Hg in the environment and how to limit their toxicity. However, due to their reactivity towards aquifer components, it is difficult to clearly dissociate the biogeochemical processes that occur and their different impacts on the fate of these TE. To do so, we developed an original, experimental, column setup that mimics an aquifer with As- or Hg-iron-oxide rich areas versus iron depleted areas, enabling a better understanding of TE biogeochemistry in anoxic conditions. The following protocol gives step by step instructions for the column set-up either for As or Hg, as well as an example with As under iron and sulfate reducing conditions.

Multi-Analytical Approach and Geochemical Modeling for Mineral Trace Element Speciation in MSWI Bottom-Ash

The present research focused on the behavior of five mineral trace elements (MTE): Cr, Pb, Sb, Zn, and Ni. These MTE and their concentration levels are critical factors in bottom-ash (BA) reuse. Because of their multiple uses, these elements are present in all municipal solid waste. Moreover, because of their low concentrations in BA, traditional analysis tools such as XRD and FT-IR are not sufficiently accurate to detect them. This study aimed to define the speciation of the five main MTE through a multi-analytical approach. Several methods were used, each providing complementary information. Some of them are standard solid direct analysis; others are indirect leaching methods. A sequential extraction protocol was also conducted and the leaching behavior as a function of pH was modeled using the PHREEQCI® software. Among the five MTE studied, Cr (with high total content) was well stabilized in the matrix, while Ni (with low total content) was only leached at low pH values. Sb was chemically stable at pH > 9. Regarding Pb and Zn, all the methods used confirmed that leaching occurs in a wide range of pH values, except in neutral conditions where amphoteric behavior limits the leaching process. Among the five elements studied, Pb leaching showed the most restrictive behavior for MSWI BA valorization. The results obtained through the analytical methods applied are consistent and complement each other. They provide a good understanding of MTE chemical speciation and behavior.

Chemical Speciation of Potentially Toxic Trace Metals in Coal Fly Ash Associated with the Kingston Fly Ash Spill

Coal ash released into the environment may release toxic trace elements into water, sediments, and soils. The objective of this study was to characterize the chemical speciation of As, Se, Cu, Zn, Cr, and U in coal fly ash samples related to the 2008 Kingston ash spill. Three ash samples were analyzed using X-ray absorption spectroscopy (XAS) to determine oxidation states or dominant species of trace elements, which were previously found to range in concentration from 8 to 20 mg kg–1. Linear combination fitting (LCF) of X-ray absorption near edge structure (XANES) spectra from ash samples indicated that both reduced and oxidized forms of the trace elements were present in the fly ash samples. We used the mineralogical composition of the fly ash to select the most relevant standards for LCF fitting of XANES spectra, which included metal-doped glasses, trace elements sorbed to iron oxy(hydroxides), and pure mineral phases for each element. Arsenic K-edge XANES spectra were best fit as oxidized As(V) (95–100...

Trace element (Hg, As, Cr, Cd, Pb) distribution and speciation in coal-fired power plants

This work reports the distribution of selected trace elements, including Hg, As, Cr, Cd and Pb, from different sampling locations in a 300MW coal-fired power plant. The plant is equipped with low NOx burners, a selective catalytic reduction (SCR) denitrification system, an electrostatic precipitator (ESP) and a wet flue gas desulphurization system (WFGD). It was found that Cd is approximately evenly distributed between the bottom ash, fly ash and gypsum. Pb, Cr and As are mostly concentrated in the fly ash. Mercury, being the most volatile element, has the lower concentration in the solid form among the five target elements. The concentrations of As, Cr and Pb are higher in fly ash than in the gypsum, in contrast to Hg and Cd. FGD gypsum samples and fly ash samples from four coal-fired power plants were collected. Samples were leached and analyzed for the speciation of trace elements by a five-step extraction procedure modified from the Community Bureau of Reference (BCR) method. The speciation study indicates that more residual fraction for Hg, As and Pb than that of effective fraction in both fly ash and gypsum. However, Cr and Cd have more effective fraction than that of residual fraction in both fly ash and gypsum. In fly ash, 60 to 90% of the trace elements exist in the residual fraction. The stability of trace elements in the gypsum is weaker than that of the fly ash in the environmental resource utilization. This may mean that the trace elements should be retained in the fly ash as much as possible.