Mercury is considered one of the most toxic elements in almost all forms even in low concentrations as a result of bioavailability, mobility and high bioaccumulation factor (biomagnification factor up to 106 in the food chain) [1–3]. For this reason its determination and speciation is of great interest in all environmental compartments, such as soil, airborne particulate matter and dust, sediment, water, waste, air and biological samples [4–10]. Natural sources of Hg emission account for 5207 Mg yr-1, while the anthropogenic contribution is estimated to account for 2320 Mg yr-1, of which more than 95% has been released during the last century [11, 12]. The main natural sources of Hg relate to evasion from marine surface waters, biomass burning and volcanoes emission making Hg contamination a global concern. The anthropogenic occurrence of Hg from local sources can create hotspots as is the case of fossil-fuel fired power plants and recently biocombustible, mining (cinnabar, gold, polymetallic ores), processing of non-ferrous metals, cement plants, municipal and medical waste incinerators, chlor-alkali facilities. The diffuse anthropogenic sources of Hg include traffic, human crematories, biomass and coal burning for domestic heating and uncollected waste products (fluorescent lamps, batteries, thermometers, non- and biodegradable packaging materials, etc.). As a result, the studies related to total Hg concentration in soil and plants, as well as the distribution of its species in soil nearby chlor-alkali facilities [13], cinnabar mine [14, 15], urbane areas [16], agricultural and forest zones [17] are of great interest. Only monitoring total Hg in environment gives limited data and speciation analysis is mandatory as it provides more useful information related to anthropogenic sources, distribution of Hg forms, potential toxicity and health risk. The non-chromatographic methods are useful tools in environmental studies for providing operationally-defined fractionation of Hg species following single or sequential extraction in specific reagents [13–15, 18, 19].
Determination of Hg in environmental solid samples involves cold vapor (CV) generation from digested samples in acidic media and detection by atomic fluorescence spectrometry (CV-AFS) and optical emission spectrometry or mass spectrometry in inductively coupled plasma (CV-ICP-OES, CV-ICP-MS) [13, 20–22]. Alternatives to these conventional methods are direct release of Hg vapor by thermal desorption from solid sample and detection by atomic absorption spectrometry [4, 14, 16]. The development of methods for Hg determination meeting green analytical chemistry demands such as microplasma sources/microtorches of low power and low Ar consumption equipped with microspectrometers has became in recent years an innovative field [23–25]. In line with this trend a miniature equipment with a capacitively coupled plasma microtorch and detection by optical emission spectrometry was developed in our laboratory and successfully applied for Hg determination in different materials after digestion and CV generation (CV-μCCP-OES) [26, 27]. Recently it has been demonstrated that the analytical technique based on CV-μCCP-OES provides figures of merit for Hg determination in soil similar to the standardized CV-AFS [28].
The aim of this study was the assessment of soil contamination with Hg by determining total content as well as water-available, mobile, semi-mobile and non-mobile fractions of Hg in samples collected from an area under the influence of a former chlor-alkali plant in Romania.
Experimental
Site description and sample collection
The case study refers to the Turda town, a former industrial center in north-western Romania. The local economy was based mainly on chemical industry, building materials (cement), glass, porcelain and metallurgy. The Turda Chemical Plant founded in 1911 and closed more that 15 years ago generated an important contamination of soil with Hg from chlor-alkali electrolysis, but also with other metals such as Cu and Zn. The manufactured compounds were sodium hydroxide, chlorine, hydrochloric acid, copper pesticides, Fe, Zn, Na and K salts, and Ca hypochlorite. After 1998 the industrial facilities were closed and partially demolished. No measures were undertaken for soil remediation so that Hg has remained a pollutant of concern in the area of the former chemical plant and perhaps also in the residential zone. Currently, the cement factory is also closed and only a distribution unit has remained in the zone. Several existing manufacturing units related to gypsum cardboard and adhesives do not represent pollution sources.
A number of 38 soil samples were collected from a depth between 20 and 30 cm during May 2013 from the perimeter of the former chlor-alkali plant (7) and waste landfills (5), and residential area (26). Samples were transported in polyethylene bags to laboratory.
Reagents, standard solutions and CRMs
Nitric acid, 65% ultrapure, hydrochloric acid, 37% ultrapure, sulfuric acid 98%, ultrapure and ethanol for chromatography (Merck, Darmstadt, Germany) were used for soil sample preparation. Standard solution of 1000 mg/l Hg (Merck, Darmstadt, Germany) was used to prepare working standards in the range 0.1 – 10 ng/ml stabilized in 5% (v/v) HCl. Stannous chloride dihydrate for mercury determination (Merck, Darmstadt, Germany) served to prepare 20% (w/v) SnCl2 in 15% (v/v) HCl as derivatization reagent. The BrCl solution prepared by dissolution of 1.50 g KBr (99.9 +% p.a.) and 1.08 g KBrO3 (99.9 +% p.a.) in 100 ml concentrated HCl was used for oxidation of the organic matter in aqueous extracts, while the 12% (w/v) hydroxylamine in water for reducing the excess of BrCl. The ICP multielement standard solution IV 1000 mg/l (Merck, Darmstadt, Germany) was used to prepare multielement working standards in the range 0 – 25 μg/ml by dilution with 2% HNO3 necessary in the determination of Al, Ba, Ca, Cr, Cu, Fe, K, Li, Mg, Mn, Na, Sr and Zn by ICP-OES. Solutions of 0.1 N K2Cr2O7 and 0.2 N Mohr΄s salt were prepared for the determination of organic matter. Solutions necessary for the determination of leachable content of Cl-, NO3- and SO42- by high performance liquid chromatography were prepared according to SR ISO 10304-1:2007. All solutions were prepared with Milli-Q (18 MΩ/cm) water obtained in laboratory (Millipore Corp., Bedford, USA).
Four certified reference materials, RTC-CRM048-50G Trace Metals Sand 1, RTC-CRM025-050 Soil (Sandy loam-Metals), LGC6141 Soil contaminated with clinker ash and LGC6135 Soil-Hackney Brick Works (LGC Promochem, Wesel, Germany), were analyzed to check the accuracy of the Hg measurements by CV-μCCP-OES.
Soil sample preparation and characterization
Besides Hg, other soil chemical characteristics (pH, organic matter, total content of 13 metals and water leachable content of Cl-, NO3- and SO42-) were considered. The pollution with Hg was assessed by soil ranking on different categories related to the alert, intervention levels and contamination factor. Considering the contaminated soil as waste a classification on three categories was also made based on the leachability assay.
Samples were dried at room temperature to avoid Hg loss and the soil moisture was determined on a parallel sample at 105 ± 5°C. For the determination of total Hg and the other metals the soil samples were mineralized with aqua regia as specified in SR ISO 11466:1995. An amount of 250 mg test soil sample ground and sieved to < 250 μm was subjected to microwave-assisted digestion with 12 ml aqua regia using the program given in ref. [28]. The digest was filtered and diluted to 100 ml ultrapure water. Soil samples were subjected to water leaching following the procedure SR ISO 12457-1:2003 at a liquid-to-solid ratio of 2:1 to determine the Hg available fraction and concentration of anions (Cl-, NO3-, SO42-). An amount of wet sample sieved through the 4 mm sieve corresponding to 175 g dry sample was leached in the REAX 20 shaker (Heidolph, Schwabach, Germany) for 24 ± 0.5 h at room temperature (20 ± 5ºC) with a volume of water corresponding to a liquid-to-solid ratio of 2:1. Mercury species fractionation as: (i) mobile; (ii) semi-mobile and (iii) non-mobile involved a 3-step sequential extraction according to EPA 3200 scheme [19]. The mobile fraction contains organic (CH3HgCl) and inorganic Hg2+ species (chloride, nitrate, sulfate, oxide and hydroxide). The semi-mobile Hg species in soil relates mainly to elemental Hg and possibly amalgams, while the non-mobile fraction to sulfide (HgS) and calomel (Hg2Cl2).
Step 1. Mobile Hg species fraction: 1.5 g test soil sample was subjected to 3×2.5 ml 2% (v/v) HCl and 10% (v/v) ethanol ultrasound assisted extraction at 60 ± 2°C for 7 min each time. After each extraction the supernatant was separated by centrifugation at 3100 rpm for 5 minutes. After the last extraction the residue was washed with 2.5 ml ultrapure water by manual shaking for 1 min, then the supernatant was separated by centrifugation. The extracts and rinse were combined.
Step 2. Semi-mobile Hg species fraction: the residue from the first step was washed with portions of 5 ml warm water of 60 ± 2°C in the ultrasound bath for 5 min each time until the supernatant was free of Cl-. Rinses were discarded. Extraction was performed twice with 5 ml 1:2 HNO3 on water bath at 95 ± 2°C for 20 min. The residue was rinsed with 5 ml ultrapure water in ultrasound bath for 1 min. Extracts and rinse were combined.
Step 3. Non-mobile Hg species fraction: The residue from the previous step was extracted twice with 5 ml 1:6:7 (v/v/v) HCl:HNO3:H2O for 20 min at 95 ± 2°C on water bath. Supernatants were separated by centrifugation, while the residue was washed by manual shaking with 5 ml ultrapure water for 1 min. Extracts and rinse were combined.
Appropriate aliquot volumes from each extract were subjected to oxidation with 500 μl BrCl solution and the excess was reduced with 500 μl 12% hydroxylamine. Then 2.5 ml concentrated HCl were added and the sample was diluted to 50 ml with water.
For the determination of organic matter an amount of 0.5 g soil was oxidized at 100°C with 10 ml 0.1 N K2Cr2O7 solution in the presence of 20 ml 98% H2SO4. After cooling, the excess of K2Cr2O7 was back titrated with a solution of 0.2 N Mohr΄s salt in the presence of o-phenanthroline as indicator [29].
For the determination of soil pH a 1:5 (volume fraction) suspension of soil in water was prepared according to ISO 10390:2005.
The concentrations of anions in the water leachate were determined following the procedure given in ISO 10304-1:2007 for water quality control.