untitled BRITISH STANDARD BS EN 15192 2006 Characterisation of waste and soil — Determination of Chromium(VI) in solid material by alkaline digestion and ion chromatography with spectrophotometric det[.]
Summary of literature methods for Cr (VI) determinations in solids [6]
The initial attempts to establish an analytical protocol for detecting Cr(VI) in solid materials began in the late 1970s Since that time, numerous studies have emerged, leading to the development of new analytical methods for Cr(VI) analysis and related topics.
Cr speciation in solid matrices has been extensively studied, as highlighted in various sources [9] to [25] A comprehensive overview of Cr(VI) speciation in solid materials is provided in the state-of-the-art document CEN/TR 14589 Additionally, M Pettine et al [6] have reviewed the literature methods for determining Cr(VI) in solids.
The digestion procedure described in this standard is based on the USEPA method 3060A In 1996 USEPA
The revised Method 3060A for extracting Cr(VI) from soil, sludges, sediments, and solid wastes involves alkaline digestion at temperatures between 90 °C and 95 °C for 60 minutes This method, based on the research by James et al., requires 2.5 g of a field moist and homogenized sample in a 250 ml digestion vessel, along with 50 ml of a digestion solution (0.28 mol/l Na2CO3/0.5 mol/l NaOH), 400 mg of MgCl2, and 0.5 ml of a 1.0 mol/l phosphate buffer The inclusion of Mg²⁺ in the phosphate buffer is crucial as it mitigates the risk of Cr(III) oxidation, which can lead to an overestimation of Cr(VI), especially in samples with high Cr(III)/Cr(VI) ratios.
Theoretical kinetic background for Cr(III)-Cr(VI) inter-conversions [6]
The extraction of Cr(VI) from solid matrices is heavily influenced by the experimental conditions, which can affect the reliability of the results due to potential inter-conversions between Cr(VI) and Cr(III).
Chromium(VI) can react with various inorganic reductants, including Fe(II) and sulfide, as well as several organic compounds such as carboxylic acids, hydroxo-carboxylic acids, aldehydes, phenols, and humic acid Humic material and iron are prevalent in soil and sediments, and they can be readily released into solution when exposed to strong alkaline conditions The use of a 0.5 mol/l NaOH solution is effective in solubilizing humic substances Additionally, the solubility of Fe(III) significantly increases in strongly alkaline solutions (pH >10) due to the formation of Fe(OH)₄⁻ species.
Thermodynamic analyses indicate that various chemicals, such as molecular oxygen and Mn(IV) oxides, can act as potential oxidants for Cr(III) in both acidic and alkaline environments Additionally, hydrogen peroxide and Mn(III) oxides may function as either oxidants or reductants, depending on the pH levels.
The inter-conversion between Cr(III) and Cr(VI) can occur in the presence of reactants that either reduce Cr(VI) or oxidize Cr(III), provided that the operational conditions favor these redox reactions It is essential to evaluate the kinetic characteristics of these reactions, as they may thermodynamically influence the conversion of Cr(VI) to Cr(III) during digestion This evaluation is briefly outlined in the following sections.
Fe(II) is a prevalent reducing agent in solid matrices, and its interaction with Cr(VI) during extraction treatments results in Cr(VI) concentrations that are lower than actual levels In strongly alkaline conditions, the oxidation rate of Fe(II) by dissolved oxygen surpasses that of Cr(VI) Additionally, temperature significantly affects the oxidation rates of Fe(II) with O2 more than with Cr(VI) In highly alkaline, carbonate-rich environments, the presence of Fe(CO₃)₂²⁻ greatly accelerates the oxidation of Fe(II) compared to Fe(OH)₂.
Oxidation rates of Fe(II) with Cr(VI) remain unaffected by carbonate species A pH value of 10 or higher, combined with elevated carbonate concentrations and temperatures, can effectively prevent interference from Fe(II), as these conditions promote its oxidation by dissolved oxygen.
Alkaline digestion effectively reduces the likelihood of Cr(VI) reduction by minimizing reactions with sulfide, sulfite, humic materials, and other organic compounds The kinetic and thermodynamic properties of Cr(VI) reduction, along with the faster reaction of molecular oxygen with potential reductants, further decrease the risk of Cr(VI) reduction at pH levels above 10.
As pH levels rise, the risk of oxidative processes that convert Cr(III) to Cr(VI) increases, despite the reduced risk of Cr(VI) reduction Additionally, the aging of Cr(III) is significantly influenced by higher pH and temperatures, which ultimately decreases the potential for Cr(III) oxidation.
Molecular oxygen and manganese oxides may act as oxidants in solid digestion processes The USEPA method 3060A acknowledges the potential oxidation of native Cr(III) in solid matrices under alkaline conditions To mitigate this oxidation, it recommends the addition of Mg²⁺ to the alkaline extracting solution This suppression is believed to occur either through the coprecipitation of Cr(III) with Mg²⁺ or through the sorption of Mg²⁺ on manganese oxides, which reduces their ability to oxidize Cr(III).
Mg²⁺ significantly inhibits the oxidation rates of Cr(III) with H₂O₂ by affecting its aging process This inhibition is attributed to the formation of a solid phase, CrₓMg(1−ₓ)₁.₅(OH)₃, which regulates the solubility of chromium(III), similar to the mixed solid phase CrₓFe(1−ₓ)₁.₅(OH)₃ The influence of Mg²⁺ is likely also present in the oxidation of Cr(III) with O₂ and MnO₂, supporting the USEPA's decision to add this ion to mitigate Cr(III) oxidation during alkaline digestion of solids Additionally, a comparable effect on Cr(III) aging has been observed with carbonate.
To minimize the risk of Cr(III) conversion to Cr(VI) during the digestion of solid samples, it is essential to maintain a pH around 10, along with high temperatures and elevated concentrations of Mg²⁺ and carbonate ions.
The procedure outlined achieves maximal dissolution of all forms of Cr(VI) in solid samples while minimizing method-induced oxidation and reduction; however, species transformation may still occur To address this issue in the analysis of Cr(VI) in solid samples, speciated isotope dilution mass spectrometry (SIDMS) can be employed, as detailed by D Huo and H.M “Skip” Kingston The EPA RCRA Method 6800 specifically addresses the correction for such degradations or conversions.
Special needs for Cr(VI) determination in soil extracts [7]
The diphenylcarbazide (DPC) method is widely used for detecting Cr(VI) in aqueous solutions, but it faces challenges due to interfering compounds such as molybdenum, mercury, iron, and vanadium, which can lead to positive interference Additionally, reductants like hydrogen peroxide, Fe(II), sulfide, and sulfite can reduce Cr(VI) to Cr(III) under acidic conditions, resulting in underestimation of Cr(VI) levels While significant concentrations of these reductants are rare in aqueous sample analysis, their presence is more likely when applying the DPC method to soil extracts.
Strongly alkaline conditions are essential for effectively digesting solids, as they significantly reduce the unwanted interconversions between Cr(III) and Cr(VI) These conditions enhance the dissolution of Fe(III) species and humic-like matter, which can interfere with Cr(VI) determination using the DPC method The dissolution of Fe(III) occurs through the formation of negatively charged hydrolysis products, such as Fe(OH)₄⁻, while the release of humic matter is associated with the solubility of humates in strong alkaline environments.
Zhilin et al [41] highlighted the limitations of the spectrophotometric DPC method in the presence of certain interferences, leading to the proposal of a post-column derivatization technique for Cr(VI) to effectively separate it from positive interferences [42] This ion chromatographic (IC) protocol is documented as the USEPA 7199 method [43] The IC method addresses many challenges associated with heavy metals by diluting the sample with an eluent stream of ammonium sulfate and ammonium hydroxide at a pH of 9.0 to 9.5, utilizing a guard column to eliminate organics, and achieving Cr(VI) separation on an anion exchange column.
Based on these considerations the use of the ion chromatography method is needed to overcome interferences from reductants when derivatization of Cr(VI) with DPC is used
Test results from over 1,500 field soil samples revealed the dissolution of both soluble and insoluble Cr(VI) spikes using the alkaline digestion method Acceptable recoveries of Cr(VI) spikes were achieved in soils with Cr(VI) and in most aerobic soils lacking native Cr(VI) However, auxiliary parameters such as oxidation-reduction potential, pH, sulfide, and total organic carbon indicated that strongly reducing samples cannot sustain Cr(VI) in laboratory matrix samples Therefore, it is crucial to correctly interpret poor Cr(VI) spike recovery and avoid labeling these results as unacceptable without considering the characterization of auxiliary parameters in such samples.
Determination of Cr(VI) in glass
A reference method for determining Cr(VI) in glass has been established by the International Commission on Glass Technical Committee 2 This procedure involves digesting the glass sample with a mixture of sulfuric acid and ammonium hydrogen fluoride at room temperature Subsequently, diphenylcarbazide is added to create a violet complex, which is then measured using a spectrophotometer The method is sensitive enough to detect Cr(VI) levels as low as 2 mg/kg of glass.
Determination of Cr(VI) in air particulate matter
The International Organisation for Standardisation (ISO/TC 146 SC 2) has established a reference method for determining Cr(VI) in air particulate matter, outlined in ISO 16740:2005 This standard specifies a procedure for measuring the time-weighted average mass concentration of hexavalent chromium in workplace air Additionally, it details distinct sample preparation methods for extracting both soluble and insoluble forms of hexavalent chromium.
Before organizing the interlaboratory comparison, a robustness study was conducted to evaluate various digestion equipment, including hot plates, heating blocks, and ultrasonic baths Additionally, the study assessed different measurement methods such as ion chromatography with spectrophotometric detection, IC-ICP-MS, ICP-AES, AAS, and direct spectrophotometry.
The analysis involved three soils contaminated with low and high levels of Cr(VI) and three types of waste materials: fly ash, filter cake, and paint sludge Based on these analyses, several conclusions were drawn.
Hot plate and heating block digestions produced similar results across all samples when temperature was regulated and continuous stirring was applied In contrast, ultrasonic bath extraction at both 25°C and 60°C resulted in significantly lower recoveries of Cr(VI) content in all samples.
The incorporation of magnesium into a phosphate buffer effectively inhibits the oxidation of Cr(III) in soil samples Results from Cr(III) spiking indicate that the filter cake exhibits an oxidizing tendency Additionally, drying the sample at varying temperatures (40°C, 60°C, 80°C, and 105°C) leads to an increase in Cr(VI) content, suggesting a heightened oxidation potential associated with the drying process.
Ion chromatography with direct spectrophotometric detection and post-column derivatization using 1,5-diphenylcarbazide yielded similar results The total chromium content in alkaline digestion solutions was effectively determined using AAS and ICP-AES, provided that dilution or matrix matching was applied However, for certain materials, direct spectrophotometric analysis of the alkaline digestion solution may be hindered by co-extracted interfering substances, making it inadvisable.
In December 2005 and January 2006, an interlaboratory comparison was conducted by CEN/TC 292 WG 3, involving participants from seven member countries The study focused on two polluted topsoils and two waste materials, chosen based on a robustness study that identified varying levels of Cr(VI) Performance characteristics were documented in Table E.1, with repeatability and reproducibility calculated in accordance with ISO 5725 standards.
Table E.1 — Performance characteristics of an international interlaboratory comparison on Cr(VI) determination (calculations according to ISO 5725) sample N Nres w(Cr(VI))
[mg/kg] soil 1 15 45 1,69 0,43 25,19 0,22 13,08 1,18 0,61 soil 2 19 57 2 007 205 10,22 88 4,36 568 242 waste 1 19 57 11 360 1 308 11,51 788 6,94 3 622 2 183 waste 2 13 39 12,90 8,97 69,55 1,59 12,31 24,85 4,40
N res number of accepted results w(Cr(VI)) mean content of Cr(VI) calculated from N data sets, in mg/kg dry matter
VR relative reproducibility standard deviation
Vr relative repeatability standard deviation
In Tables E.2 to E.5 an overview of the Cr(VI) determination is given per sample and per combination of digestion and detection method:
Method A: Hot plate digestion and ion chromatography with direct spectrophotometric detection
Method B: Hot plate digestion and ion chromatography with spectrophotometric detection after post-column derivatisation with 1,5-diphenylcarbazide
Method C: Heating block digestion and ion chromatography with direct spectrophotometric detection
Method D: Heating block digestion and ion chromatography with spectrophotometric detection after post-column derivatisation with 1,5-diphenylcarbazide
Table E.2 — Data for Cr(VI) determination and spike recoveries on soil 1 (low contaminated topsoil) method N Nres w(Cr(VI))
SD rec Cr(VI) [%] rec Cr(III) [%]
Table E.3 — Data for Cr(VI) determination and spike recoveries on soil 2 (high contaminated topsoil) method N Nres w(Cr(VI))
Table E.4 — Data for Cr(VI) determination and spike recoveries on waste 1 (paint sludge) method N Nres w(Cr(VI))
SD rec Cr(VI) [%] rec Cr(III) [%]
Table E.5 — Data for Cr(VI) determination and spike recoveries on waste 2 (fly ash) method N Nres w(Cr(VI))
Nres number of accepted results w(Cr(VI)) mean content of Cr(VI) calculated from N laboratory means, in mg/kg dry matter
SD w standard deviation calculated from N laboratory means
CV w coefficient of variation of laboratory means rec Cr(VI) mean recovery of Cr(VI) spike
SDrec.Cr(VI) standard deviation of recoveries of Cr(VI) spike rec Cr(III) mean recovery of Cr(III) spike detected as Cr(VI)
SD rec Cr(III) standard deviation of recoveries of Cr(III) spike detected as Cr(VI)
The performance characteristics for determining Cr(VI) in both soils and waste 1 are satisfactory However, the significant relative reproducibility standard deviation observed in waste 2 indicates pronounced matrix effects, highlighting the necessity for additional quality control data to validate analytical results in unknown matrices.
The spike recoveries for the four methods demonstrate high effectiveness, with Cr(VI) recovery exceeding 95% and Cr(III) recovery below 5% for both soil samples Method B, which utilizes hot plate digestion and ion chromatography with spectrophotometric detection after post-column derivatization with 1,5-diphenylcarbazide, yields the most reproducible results across both soils This is particularly notable for soil 1, which is less contaminated, due to the enhanced sensitivity of the detection method employed.
The spike recoveries for Cr(VI) and Cr(III) in paint sludge are significant, but the variability in results is largely due to the sample matrix's reducing nature, as indicated by tests using isotopically enriched chromium species This suggests that sample heterogeneity is not the primary reason for the poor recoveries Consequently, valid Cr(VI) content cannot be reported for the fly ash sample, and the test report should note the recoveries of the spiked samples Further investigation into the sample matrix's reducing and oxidizing tendencies is recommended.
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[50] ISO 5725, Accuracy (trueness and precision) of measurement methods and results
[51] CEN/TR 14589:2003, Characterization of waste - State of the art document - Chromium VI speciation in solid matrices
[52] EN ISO 11885:1997, Water quality — Determination of 33 elements by inductively coupled plasma atomic emission spectroscopy (ISO 11885:1996)
[53] EN 12506:2003, Characterization of waste — Analysis of eluates — Determination of pH, As, Ba, Cd, Cl - ,
Co, Cr, Cr VI, Cu, Mo, Ni, NO 2 -, Pb, total S, SO 4 2- , V and Zn
[54] prEN 14346, Characterization of waste — Calculation of dry matter by determination of dry residue and water content
[55] EN ISO 15586:2003, Water quality — Determination of trace elements using atomic absorption spectrometry with graphite furnace (ISO 15586:2003)
[56] EN ISO 17294-2:2003, Water quality — Application of inductively coupled plasma mass spectrometry (ICP-MS) — Part 2: Determination of 62 elements (ISO 17294-2:2003)
[57] ISO 11465, Soil quality — Determination of dry matter and water content on a mass basis — Gravimetric Method
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— Part 3: Determination of chromate, iodide, sulfite, thiocyanate and thiosulfate
[59] ISO 11083:1994, Water quality — Determination of chromium(VI) — Spectrometric method using 1,5- diphenylcarbazide
[60] ISO 16740:2005, Workplace air — Determination of hexavalent chromium in airborne particulate matter — Method by ion chromatography and spectrophotometric measurement using diphenyl carbazide