HEXAVALENT CHROMIUM [Cr(VI)] IN DRINKING WATER OF GREECE – ESTIMATION OF THE ORIGIN M. MITRAKAS*, N.D. TZOUPANOS**, N. KAZAKISO, E. KAPRARA*, K. SIMEONIDIS*, P. SAMARASOO AND A.I. ZOUBOULIS** *Department of Chemical Engineering, Aristotle University of Thessaloniki, 54124, Thessaloniki, Greece **Laboratory of General and Inorganic Chemical Technology, Department of Chemistry, Aristotle University of Thessaloniki, 54124, Thessaloniki, Greece OLaboratory of Engineering Geology & Hydrogeology, Deptartment of Geology, Aristotle University of Thessaloniki, 54124, Thessaloniki, Greece OODepartment of Food Technology, ΑΤΕΙ of Thessaloniki, 57400 Thessaloniki SUMMARY: The increased reports of hexavalent chromium Cr(VI) presence in drinking waters all over the world as well as in Greece, the well-established and proved natural existence of Cr(VI) in waters and the expected re-evaluation of chromium limits in drinking water impose the systematic monitoring of Cr(VI) concentrations in natural waters intended for human consumption. In this study, samples were collected from the targeted regions of Greece, i.e. areas in which the geological background is predominated by ultramafic minerals and the water supply depends mainly on groundwater resources, and analyzed for Cr(VI). A wide range of Cr concentrations (2-105μg/L) was detected in the areas studied, with most of them ranging below the current (but questionable) limit of 50 μg/L, and the Cr(VI) concentration being more than 90% of the total, verifying also that chromium concentrations in natural waters should be attributed mainly to Cr(VI). 1. INTRODUCTION Hexavalent chromium [Cr(VI)] has long been recognized as a potential carcinogen via inhalation, whereas nowadays there is an increasing concern that Cr(VI) is also carcinogenic by the oral route of exposure. The adverse health effects due to chromium substances inhalation are well known; perforation of nasal septum, asthma, bronchitis, pneumonia, inflammation of the larynx and the liver as well as increased possibility of cancer development are between them. Skin contact can cause allergies, dermatitis and skin necrosis and apoptosis (Linos et al., 2011; Barceloux, 1999). In Greece, at the year 2006, the pollution of Asopos river with Cr(VI) was reported and two years after, systematic investigations regarding Cr(VI) presence in Asopos river and its basin appeared in the literature, showing that the Thiva-Tanagra-Malakasa basin was contaminated with Cr(VI) (Vasilatos et al., 2008). Concentrations of Cr(VI) up to 80 μg/L were detected in surface and ground waters used for public water supply, whereas according to other studies (Economou-Eliopoulos et al., 2011) a wide spatial variability of the total Cr content in groundwater in the aquifer of the Asopos basin was found, ranging from <2 to 180 μg/L. At first, it was thought that the intensive industrial activity utilizing large amounts of Cr in the area was responsible for increased Cr(VI) levels in waters. However, relatively recent publications proved the natural existence of Cr(VI) in waters, mainly due to the contact of water with ultramafic rocks such as serpentinite, dunites, ophiolites etc. (Cooper, 2002; Fantoni et al., 2002; Morisson et al., 2009), indicating that Cr(VI) presence in natural waters should be examined from a new perspective. The natural presence of Cr in the area is further supported by recent surveys, verifying the presence of Cr-containing minerals in the Asopos river and basin, e.g. chromite, Fe-chromite, Cr-bearing goethite and silicates (Economou-Eliopoulos et al., 2011; Moraetis et al., 2012). Considering also that the presence of increased levels of naturally occurring chromium should be attributed mainly to Cr(VI), since Cr(III) solubility at common pH range (i.e. pH 6.5-8.5) of natural waters is significantly lower than 5 μg/L, it is more than clear that the presence of Cr(VI) in natural waters of Greece is an emerging and essential issue. The World Health Organization (WHO/SDE/WSH/03.04/4) and the European Union- Drinking Water Directive (98/83/EC) have established a 50 μg/L drinking water standard for total Cr in European countries. However, due to the increased concern for Cr(VI) toxicity nowadays it is expected that Cr limits in drinking water will be re-evaluated in the near future. Considering also that in the Greek territory there are other regions than the Asopos river basin where ultramafic rocks can be found, the monitoring of drinking water quality by means of Cr(VI) presence in Greece is imposed. The aim of the study was to examine the possible natural occurrence of Cr(VI) in drinking waters around Greece. For this purpose, samples were collected from the targeted areas, e.g. areas in which the geological background is predominated by ultramafic rocks and the water supply depends mainly on groundwater resources and analyzed for Cr(VI) as well as several other parameters which are supposed to affect Cr(VI) presence in natural waters. 2. BACKGROUND 2.1 Origin and forms of chromium in natural waters The sources of Cr in natural waters can be anthropogenic, i.e. from industrial activities utilizing large amounts of chromium, such as leather tanneries, textile industry, cooling tower blow-down, plating factories, etc. Regarding the natural origin of Cr in waters, minerals’ leaching is the main source. The prevalent forms of Cr in waters are the two most common oxidation states, i.e. trivalent [Cr(III)] and hexavalent chromium ([Cr(VI)]. Chromium exists in its minerals in the trivalent oxidation state and this is the initial form in which Cr enters the water phase. However, Cr(III) solubility at the common pH range 7-8.5 of natural waters has been determined around 10-7 M (~5μg/L) (Rai et al., 1987), therefore higher naturally occurring chromium concentrations are attributed to Cr(VI) (Gozalez et al., 2005), due to natural oxidation of Cr(III) in ultramafic derived soils (Cooper, 2002) and ophiolitic rocks (Fantoni et al. 2002). Ultramafic soils and ophiolitic rocks contain grains (0.2-1 mm) of Cr2O3 in combination with relatively high pH values. Thermodynamically, these parameters favor, although they do not warrant (cross lines), the Cr(VI) formation (Figure 1). However, oxidation CRETE 2012 2 of Cr(III) to Cr(VI) is relatively poor with dissolved oxygen (Gallios and Vaclavicova, 2008), while it is very effective in the presence of MnO2 (Eary and Rai, 1997). It is suggested that MnΟ2 is likely to be responsible for most Cr(III) oxidation. Under oxidizing conditions (redox values 200-350 mV) and at the usual pH range of natural waters (i.e. pH 7 – 8.5), Cr(VI) exists - 2- in ultramafic aquifers mainly in the anionic forms HCrO4 , and CrO4 , depending on the pH (Guertin et al., 2005). o Figure 1. Eh-pH diagram for aqueous Cr species in the system Cr-H2O-magnetite at 25 C and 1 bar (Galios and Vaclavicova, 2008). Spotted area represents the thermodynamic characteristics of natural waters (redox 200-350mV, pH 7-8.5) in ultramafic aquifers. 2.2 Geological background In Greece, the chromium deposits occur in mantle origin peridotites and ophiolite complexes. The ophiolite complexes are formed when two oceanic crusts converge, important part of the oceanic crust is destroyed and the remainder rises to the surface and creates the ophiolite complexe. Ophiolite rocks in Greece record a Mesozoic history of subduction, accretion and obduction during the closure of Tethys and paleo Tethys Sea. The ophiolite complexes in Greece form two distinct, parallel lanes, with a general NW-SE direction as shown in the map of IGME (1983) (Figure 2). The internal (IRO) ophiolite complex along Axios zone and the external (ERO) ophiolite complex along sub pelagonic and Pindos zone (Mountrakis, 1986, Robertson et al. 1991). The worldwide average of Cr content in peridotites is 1800 mg/kg (Faure 1992). High Cr concentrations are found in minerals such as magnesio-chromite and chromite, members of the spinel group and in magnetite. Very poor in Cr (<70 mg/kg) are the olivines, whereas the content of Cr in serpentine is 100 mg/kg. 3. MATERIALS AND METHODS The map of Figure 2 presents the ophiolites-ultramafic rocks distribution in the Greek territory; the specific map along with the geological information presented in Section 2.2 were the guides for the water samples collection. Samples were collected from public drinking water distribution CRETE 2012 3 networks which depend on groundwater resources. A part of sample was acidified at pH<2 using HCl and a part stored at 4 oC if not analyzed immediately. All analyses were conducted according to Standard Methods for the Examination of Water or Wastewater (Clesceri et al., - 1989). The parameters pH, electric conductivity (EC) and HCO3 , were analyzed according to the methods 4500-H+, 2520 B, 2320-B, respectively. The metals Na, K, Ca and Mg were analyzed according to the methods 3500-Na, 3500-K, 3500-Mg, 3500-Ca by flame atomic absorption spectrophotometry (FAAS) using a Perkin Elmer instrument, model AAnalyst 800. Total Cr was determined by graphite furnace atomic absorption spectrophotometry (GFAAS) using a Perkin Elmer instrument, model AAnalyst 800. The detection limit of the method, calculated from 7 replicates of 2-5 μg Cr/L, was estimated to be 0.8 μg/L. The Cr(VI) concentration of the samples was determined by the diphenylcarbazide method [3500-Cr D] using a Perkin Elmer Lambda 2 UV/VIS spectrophotometer version 3.7 equipped with 10 cm path-length measurement cells. The detection limit of the method, calculated from 7 replicates of 2-5 μg Cr(VI)/L, was estimated to be 1.4 μg/L. The relative standard deviation (RDS) at concentration range between 5.9 and 69 μg/L (Table 1) calculated from 5 replicates was ranged between 1.8 and 6.5% for total chromium and between 2.4 and 6.2% for Cr(VI) determination.