Anal Bioanal Chem (2010) 397:1097–1111 DOI 10.1007/s00216-009-3417-1
REVIEW
Chromium speciation in solid matrices and regulation: a review N. Unceta & F. Séby & J. Malherbe & O. F. X. Donard
Received: 20 October 2009 / Revised: 10 December 2009 / Accepted: 16 December 2009 / Published online: 23 January 2010 # Springer-Verlag 2010
Abstract In recent years, the extensive use of chromium in industrial processes has led to the promotion of several directives and recommendations by the European Union, that try to limit and regulate the presence of Cr(VI) in the environment and to protect industrial workers using chromium and end-users of manufactured products. As a consequence, new standard methods and analytical procedures have been published at the EU level for Cr(VI) determination in soil, sludge, sediment, and similar waste materials, workplace atmospheres, cement, packaging materials, industrially produced samples, and corrosionprotection layers on some components of vehicles and electrical and electronic equipment. The objective of this article is to summarize the different directives and recommendations and to critically review the currently existing standard methods and the methods published in the literature for chromium speciation in the above mentioned
solid matrices, putting the emphasis on the different extraction procedures which have been developed for each matrix. Particular attention has been paid to Cr(III) and Cr (VI) inter-conversions that can occur during extraction and efforts to minimize these unwanted reactions. Although the use of NaOH-Na2CO3 solutions with hot plate extraction seems to be the more widespread procedure, species transformation can still occur and several studies suggest that speciated isotope-dilution mass spectrometry (SIDMS) could be a suitable tool for correction of these interconversions. Besides, recent studies have proved the role of Cr(III) in chromium toxicology. As a consequence, the authors suggest an update of standard methods in the near future. Keywords Hexavalent chromium . Cr legislation . Solid matrix . Solid-liquid extraction
Introduction N. Unceta (*) Department of Analytical Chemistry, Faculty of Pharmacy, University of the Basque Country, Paseo de la Universidad 7, 01006 Vitoria-Gasteiz, Spain e-mail:
[email protected] F. Séby Ultra Traces Analyses Aquitaine (UT2A), Hélioparc Pau-Pyrénées, 2, avenue du Président Angot, 64053 Pau Cedex 9, France J. Malherbe : O. F. X. Donard Laboratoire de Chimie Analytique Bio-Inorganique et Environnement, IPREM, UMR CNRS 5254, Université de Pau et des Pays de l’Adour, Hélioparc Pau-Pyrénées, 2, avenue du Président Angot, 64053 Pau Cedex 9, France
Chromium (Cr) is widely used in the chemical industry for different applications such as pigments, metal plating, or leather tanning and in chemical production such as chemical synthesis and as catalysts. As a result, different species of chromium can be released into the environment (soil, surface, and ground waters) and is then available to humans. Chromium can exist in several chemical forms between 0 and VI. However, only trivalent and hexavalent chromium are stable enough to occur in the environment. Cr(III) is considered an essential micronutrient in the human diet and is widely used as a nutritional supplement for humans and animals. Nevertheless, it has been demonstrated that Cr(III) is capable of eliciting eczema at low concentrations [1] and of causing DNA damage in cell-culture systems [2].
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In contrast, Cr(VI) is much more toxic than Cr(III) for both acute and chronic exposure and its compounds are regulated through the Dangerous Substances Directive (67/548/EEC). It is suspected of being extremely toxic after inhalation and oral exposure with effects on the respiratory tract, liver, kidney, gastrointestinal and immune systems, and possibly the blood, and dermal exposure may cause contact dermatitis, sensitivity, and ulceration of the skin [3]. Cr(VI) has been recognised as a highly toxic species and classified as a human carcinogen by the EPA and as a class I human carcinogen by the International Agency for Research on Cancer (IARC) [4, 5]. The toxicological disparity between Cr(III) and Cr(VI) is closely related to the chemical characteristics of each species, which also conditions the stability, mobility, and bioavailability of these species in the environment [6]. As presented in Fig. 1, the behaviour of Cr(III) and Cr(VI) in aqueous media is mainly affected by redox potential and pH. In acidic media, the high redox potential of the Cr (VI)/Cr(III) couple favours Cr(III) stabilization. In contrast, under alkaline conditions the redox potential decreases, which indicates stabilization of Cr(VI) under such conditions. Thus, thermodynamically, Cr(VI) could only be present at relatively high pH. Cr(III) can exist as several forms as a function of pH (Fig. 1). At pH between 0 and 4, trivalent chromium tends to form hexacoordinate complexes with complexing agents such as water, ammonia, sulfate, urea, and organic acids. Within pH 4–6, Cr(III) tends to form hydrolysis products simply abbreviated as Cr(OH)2+, Cr (OH)2+, and Cr(OH)3°. At pH higher than 6, Cr(III) precipitates to form Cr(OH)3(s) which is a stable species and one of the dominant forms of Cr(III) in the environment. Above pH 9 this precipitate is transformed into the soluble Cr(OH)4− complex. Because of the high E° value of the Cr(VI)/Cr(III) redox couple, the oxidation of Cr(III) to Cr (VI) by natural oxidants is not usual, only manganese oxide seems to be an effective oxidant in the environment [7].
N. Unceta et al.
Depending on pH and their concentrations, chemical species of Cr(VI) range from chromate (CrO42−) (pH 6.5–14) through hydrogen chromate (HCrO4−) and dichromate (Cr 2 O 7 2− ) (pH 0.7–6.5) to chromic acid (H2CrO4) (pH
