Aqua Regia Extractable Content Research Articles

Geochemical analyses of metal(loid) fractions do not predict plant uptake behavior: Are plant bioassays better tools to predict mine rehabilitation success?

Management of metal(loid) tailings at historic sites presents environmental hazards usually requiring rehabilitation to mitigate pollution risks. Strategies employed include capping or establishing vegetation directly, which requires tailings assessments to determine suitable rehabilitation approaches. Assessments are typically geochemical analyses, but plant based approaches may provide a more accurate measure of revegetation success although they are often limited to germination indices. This study uses the plant bioassay (Rhizotest™) with common geochemical assessment to predict plant uptake of metal(loid)s and the subsequent likely rehabilitation success. Pb/Zn tailings from five legacy sites within the UK and Ireland were characterized for pH, EC, water soluble and CaCl2-extractable content and aqua regia extractable content. Uptake of Sb, As, Cd, Cu, Ca, Mg, Mn, Zn, Pb was determined in shoots and roots of Lolium perenne.Total Zn, Pb, Sb, Cd and As in tailings ranged from 694 to 2683 mg kg−1, 1252 to 8072 mg kg−1, 14 to 148 mg kg−1, 1.3 to 44 mg kg−1 and 1.3 to 45 mg kg−1, respectively. The only correlation found between total and water soluble or CaCl2-extractable metal(loid) contents was for Cd, where r = 0.8 for total and CaCl2-extractable fractions. Limited uptake and translocation risk was identified for major contaminants Zn and Pb in most tailings samples but in some cases exceedance of phytotoxic threshold values occurred that was not reflected in geochemical analysis. Crucially, although total Cd and Sb content was relatively low (< 20 mg kg−1) in some tailings, elevated plant content for some samples highlights phytotoxic risk from minor elements. Results indicate that screening based on geochemical content is not sufficiently predictive of metal(loid) phytoavailability to reliably inform mine rehabilitation strategies. We therefore strongly recommend that geochemical analyses are supplemented with plant based bioassay to plan mine tailings revegetation and reduce risk of wider ecosystem metal(loid) transfer.

Monitoring metal pollution in soils using portable-XRF and conventional laboratory-based techniques: Evaluation of the performance and limitations according to metal properties and sources

Large variability in the spatial distribution and content of metals is generally recognised in anthropogenically-polluted soils, hence, a detailed site investigation implying the collection and analysis of a large number of soil samples is often necessary. To this regard, the selection of a rapid, cost-effective and accurate analytical technique to assess the concentration of metals in soil is of paramount importance. The overall objective of this work was to evaluate the possibility of assessing the aqua regia-extractable (AR) content of metals in soil from the multi-element profile of the soil obtained by a portable X-ray fluorescence analyser (pXRF). To this objective, we attempted: (i) to establish, by simple linear regressions, the relations occurring between the metal contents measured by pXRF and AR in laboratory setting on air-dried and 2 mm-sieved soil samples from two case studies (A-agricultural and B-industrial sites); (ii) to define metal-based linear models predicting metal AR contents from pXRF measurements; (iii) to assess the influence of metal properties and sources on relations found between the two analytical methods. Very satisfying correlations (R2 > 0.90) were observed between the AR and pXRF contents of Ca, Cu, Cr, Ni, Pb and Zn in the site A, and of Cd, Cu, Pb and Zn in the site B. For the majority of metals, lower AR than pXRF contents were measured, as result of the AR incomplete dissolution of metal-bearing silicates. This was not observed when metals - of anthropogenic origin - occurred in soil in very high concentrations (i.e., Cr for A and Pb for B). In both sites, the comparison among different regression parameters revealed a strong metal-dependence. Moreover, for most of the metals, the parameters of each metal-regression line significantly differed between the two case studies, indicating site-dependence of regression fits.

Environmental availability of trace elements (Pb, Cd, Zn, Cu) in soil from urban, suburban, rural and mining areas of Attica, Hellas

The environmental availability of trace elements (Pb, Cd, Zn, Cu) in soil samples from the urban area of Athens and Piraeus is compared to that from samples of suburban, uncontaminated rural and the nearby historical mining area of Lavrion in Attica. These trace elements had been previously shown to be related to anthropogenic activities in the urban chemical environment of Athens. Trace element environmental availability is determined by using three single stage chemical extractions of (i) nitric acid (HNO3), (ii) ethylenediaminetetraacetic acid (EDTA) and (iii) acetic acid (CH3COOH), and the respective extractabilities are calculated as ratios relative to total element concentrations. All elements showed the same decreasing concentration trend in the order mining>urban>suburban≈rural soil categories. Median concentration, based on aqua regia dissolution, are 185, 173, 86, 0.4mg/kg and 2.26% for Pb, Zn, Cu, Cd and Fe, respectively, for the urban soil samples. Whereas, the corresponding median concentrations of Pb, Zn and Cd in Lavrion soil are over ten times higher. The ratios of the extractable concentrations of Pb, Zn and Cd, relative to the aqua regia extractable content in soil, are also higher for the mining soil category. Extractable concentrations for all the studied elements are highly dependent on their respective aqua regia extractable values in all land use categories; extractability ratios are also controlled by HNO3 extractable Fe in soil. For the two toxic elements, Pb and Cd, the data of this study suggest that environmental availability has to be assessed on a site-specific basis as local conditions, and in particular the mineralogical composition of soil, may influence their relative extractability.

INFLUENCE OF POLLUTION LEVEL ON HEAVY METALS MOBILITY IN SOIL FROM NW ROMANIA

To assess the influence of pollution degree on the heavy metals (Cu, Cd, Zn, Pb) mobility in soil, total metals content obtained by extraction in aqua regia and extractable contents in a buffered diethylenetriaminepentaacetic acid (DTPA) solution were measured using inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS). Soil samples were collected from two areas from NW Romania with different pollution levels: a mining area (Baia Mare) and a rural area (Iclod). High pollution level with heavy metals was found in Baia Mare area, whereas contents of metals in Iclod did not exceed the alert limits established by Romanian legislation. The percentages of extractable metals were calculated, and correlations between soils pollution levels and heavy metals mobility were made. The results showed that despite the significant difference between the total metal content in soil from the two regions, estimated as the aqua regia extractable content, the percentages of DTPA extractable metals were comparable for Pb and Zn, while those for Cu and Cd were higher in the polluted region. However, significant correlation coefficients were found between total and mobilisable metals content in unpolluted site compared to the polluted site.

Weathering of sulphide minerals and trace element speciation in tailings of various ages in the Guanajuato mining district, Mexico

We studied the behavior of the trace elements (TE) As, Cu, Pb and Zn associated with sulphide minerals in tailings of different age at the Guanajuato mining district, Mexico. The objective was to determine the dominant fractions into which the TE move as tailings age and to identify how much time is needed until the dominant metal fractions approach equilibrium. We collected 21 samples from the surface of mine waste deposits of different ages (0, 2 , 4, 16, 70, 75, and 100 years), and measured their aqua regia extractable contents of As, Cu, Pb and Zn. We also applied a sequential extraction procedure to quantify water soluble TE as well as TE associated with carbonates, iron oxides, sulphides and the residual fraction. The mineralogy was analysed by X-ray diffraction, and selected samples were studied on polished specimens through a stereoscopic microscope. The TE in samples extracted with aqua regia ranged between 10 and 168 mg kg − 1 for As, 12 to 194 mg kg − 1 for Cu, 31 to 308 mg kg − 1 for Pb, and 122 to 1129 mg kg − 1 for Zn, and varied in a wide range within each age group of tailings. Water soluble Cu, Pb and Zn contents were below detection limits in almost all samples, which was attributed to the alkaline pH (7.17 to 8.61) in the tailings. Water extractable As was detected only in tailings older than 16 years, and concentrations ranged between 0.06 and 7.58 mg kg − 1 . The proportion of TE associated with sulphides decreased in the tailings as they age, while the proportion of TE associated with iron oxides increased with time of exposure to the atmosphere, approximating equilibrium after 60 years for As, Cu and Zn, and after 40 years for Pb. Observations of polished specimens suggests that oxidation proceeds until coatings of secondary minerals cover and protect sulphide mineral grains from further weathering. First order rate equations were adjusted to the proportions of TE associated with either sulphides or iron oxides. Assuming that the TE in sulphide fractions correspond to arsenopyrite (FeAsS), chalcopyrite (CuFeS 2), galena (PbS) and sphalerite (ZnS), the relative oxidation rate of sulphides followed the order: PbS > ZnS > FeAsS ≈ CuFeS 2, while the relative affinity of the elements with iron oxides followed the sequence Cu ≈ Zn > As > Pb.

The utilization of modified BCR three-step sequential extraction procedure for the fractionation of Cd, Cr, Cu, Ni, Pb and Zn in soil reference materials of different origins

A modified three-step sequential extraction procedure for the fractionation of heavy metals, proposed by the Commission of the European Communities Bureau of Reference (BCR) has been applied to the Slovak reference materials of soils (soil orthic luvisols, soil rendzina and soil eutric cambisol), which represent pedologically different types of soils in Slovakia. Analyses were carried out by flame or electrothermal atomic absorption spectrometry (FAAS or ETAAS). The fractions extracted were: exchangeable (extraction step 1), reducible-iron/manganese oxides (extraction step 2), oxidizable-organic matter and sulfides (extraction step 3). The sum of the element contents in the three fractions plus aqua-regia extractable content of the residue was compared to the aqua-regia extractable content of the elements in the origin soils. The accuracy obtained by comparing the determined contents of the elements with certified values, using BCR CRM 701, certified for the extractable contents (mass fractions) of Cd, Cr, Cu, Ni, Pb and Zn in sediment following a modified BCR-three step sequential extraction procedure, was found to be satisfactory.

Certification of the extractable contents of Cd, Cr, Cu, Ni, Pb and Zn in a freshwater sediment following a collaboratively tested and optimised three-step sequential extraction procedure.

Sequential extraction procedures have been used during the last decades to study and determine trace element mobility in aquatic sediments. However, the results obtained are operationally defined and, therefore, the significance of the analytical results is related to the extraction scheme used. So, there is a need for suitable reference materials for this type of study to enable the validation of the methodologies and to control the quality of the measurements. This paper describes the preparation of a sediment reference material, BCR-701, the homogeneity and stability studies and the analytical work performed for the certification of the extractable contents of some trace elements, following a collaboratively tested and optimised sequential (three-step) extraction procedure. The paper contains all the results and gives the methods used to certify the extractable contents (mass fractions expressed as mg kg-1 on a dry matter basis) of Cd, Cr, Cu, Ni, Pb and Zn in all steps. Moreover, indicative values are given for the aqua regia extractable contents of the six elements in the sediment and the residue obtained after sequential extraction.

Application of a modified BCR sequential extraction (three-step) procedure for the determination of extractable trace metal contents in a sewage sludge amended soil reference material (CRM 483), complemented by a three-year stability study of acetic acid and EDTA extractable metal content.

This paper provides additional data on a sewage sludge amended soil certified reference material, CRM 483, which was certified in 1997 for its EDTA and acetic acid extractable contents of some trace metals, following standardised extraction procedures. The additional work aimed to test the long-term stability of the material and the applicability of an improved version of the BCR three-step sequential extraction procedure on the sewage sludge amended soil (CRM 483). The paper demonstrates the CRM 483 long-term stability for EDTA and acetic acid extractable contents of Cd, Cr, Cu, Ni, Pb and Zn and gives the results (obtained in the framework of an interlaboratory study) for the extractable contents of the same elements in the CRM 483, following the BCR three-step sequential extraction scheme. The aqua regia extractable contents following the ISO 11466 Standard are also given. The data are given as indicative (not certified) values.

Solid sampling electrothermal vaporization inductively coupled plasma mass spectrometry for the direct determination of Hg in sludge samples

The present paper reports on the use of solid sampling electrothermal vaporization–inductively coupled plasma mass spectrometry (ETV–ICP-MS) for the direct determination of Hg in sludge samples. For `real-life' samples, the pretreatment was limited to drying, sieving and finally homogenization by means of shaking. One milligram amounts of the samples of interest were inserted into a graphite furnace coupled to a quadrupole-based ICP-mass spectrometer for subsequent analysis. The temperature programme of the graphite furnace was selected such that Hg was vaporized and transported into the plasma at a relatively low temperature (≈700°C). Under these conditions, co-volatilization of considerable amounts of the matrix, leading to signal suppression and hence, curvature of the mass response curve, could be avoided. Quantification was accomplished by means of external calibration, using a (solid) certified reference material, with a matrix composition comparable to that of the samples. The accuracy attainable was evaluated by means of (i) analysis of a sludge certified reference material and (ii) comparison of the ETV–ICP-MS result for a `real-life' sample with the aqua regia extractable Hg content, as determined by pneumatic nebulization ICP-MS after sample processing according to the German standard DIN 38414-S7. Although in all cases, the precision was observed to be limited by the inhomogeneous distribution of the analyte element over the matrix, the ETV–ICP-MS result obtained for the sludge reference material showed an excellent agreement with the certified value, while for the `real-life' sample, the average ETV–ICP-MS result agreed within 10% with the aqua regia extractable content. The approach developed was finally used for the analysis of sludges sampled at different locations in Limburg (Flanders, Belgium). For all but one sample, the Hg content was seen to be below the maximum allowable content, as specified in Flanders.

Geochemistry and acidity of sulphide-bearing postglacial sediments of western Finland

The geochemistry, mineralogy, acidity and electric conductivity of sulphide-bearing postglacial sediments located on the coastal plains of western Finland were studied. Grain size and mineralogical analysis (eight samples studied) indicated that the sediments are silts dominated by primary minerals (quartz, feldspars), and that the clay fraction ( 13%) of the aqua-regia extractable contents of several elements (Co, Mn, Ca, Ni, Zn, Sr, Na and B) are mobilised at oxidation. Because of the high quantities of acidity and metals liberated at oxidation, it is argued that stream-water quality, which is poor in the study area, will not improve unless artificial draining is restricted in areas of sulphide sediments.

Extractability of Polluting Elements Cd, Cr, Ni and Pb of Soil with Three Methods

Abstract A laboratory study was conducted to evaluate three different chemical methods in extracting Cd, Cr, Ni and Pb of soils with wide range in their properties. The methods studied were aqua regia, 0.5 N HNO3 and acid ammonium acetate-EDTA, pH 4.65 (AAAc-EDTA). The mean values of Cd, Cr, Ni and Pb extracted by aqua regia were 0.22, 53.2, 35.4 and 23.9 mg/kg. The 0.5 N HNO3 extracted of the aqua regia extractable cadmium on an average 77%, of lead 54%, of nickel 8% and of chromium 3%. The respective percentages in case of the AAAc-EDTA extraction were 64% for cadmium, 34% for lead, 4.5% for nickel and 0.26% for chromium. Highest percentage of cadmium and lead were extracted of soils where the aqua regia extractable contents were highest. This may be considered as an indication that 0.5 N HNO3 and AAAc-EDTA could be successfully used in pollution studies.