Characterization and geochemistry of technogenic magnetic particles (TMPs) in contaminated industrial soils: Assessing health risk via ingestion

Geoderma 295 (2017) 86–97

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Characterization and geochemistry of technogenic magnetic particles (TMPs) in contaminated industrial soils: Assessing health risk via ingestion Anna Bourliva a,⁎, Lambrini Papadopoulou a, Elina Aidona b, Katerina Giouri a, Konstantinos Simeonidis c, George Vourlias c a b c

Department of Mineralogy-Petrology-Economic Geology, Faculty of Geology, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece Department of Geophysics, Faculty of Geology, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece Department of Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece

a r t i c l e

i n f o

Article history: Received 17 August 2016 Received in revised form 3 November 2016 Accepted 2 February 2017 Available online xxxx Keywords: Technogenic magnetic particles (TMPs) Industrial soils Heavy metals Magnetic properties Oral bioaccessibility

a b s t r a c t The objective of this study was the detailed characterization of “technogenic” magnetic particles (TMPs) separated from contaminated industrial soils. Moreover, non-carcinogenic health risk posed by “technogenic” metals Pb and Zn via the ingestion exposure pathway was carried out. Volume magnetic susceptibility (κ) was measured directly in the field around a chemical industry in Sindos industrial area, Northern Greece. Representative soil samples were collected from the sites where elevated κ values were recorded. Mass specific magnetic susceptibility (χlf) depended on the content of TMPs and ranged from 52.6 × 10−8 m3 kg−1 to 821.2 × 10−8 m3 kg−1 with the maximum values detected in the immediate vicinity of the industrial unit where the most contaminated soils were also identified. Mineralogically the TMPs exhibited a dominant iron spinel phase, while other iron-bearing phases such as hematite and lepidocrocite were detected. Morphologically, Fe-rich spherules and irregularshaped particles were the most commonly observed particles in TMPs, revealing various Fe contents often associated with elevated heavy metal contents. TMPs exhibited significantly higher concentrations of trace elements compared to non-magnetic fractions (NMFs) indicating that potentially harmful elements (PHEs) are preferentially enriched in the TMPs. Furthermore, for the first time, a health risk assessment study for the incidental soil magnetic fractions (MFs) ingestion of the “technogenic” metals Pb and Zn was carried out based on U.S. Environmental Protection Agency (U.S.E.P.A.) guidelines, incorporating oral bioaccessibility measurements using the BARGE Unified Bioaccessibility Method (UBM). Significant fractions (N50%) of Pb and Zn occur in bioaccessible forms in the TMPs and both children and adults are experiencing potential health risk since determined HQs values were significantly higher than safe level (=1). © 2017 Elsevier B.V. All rights reserved.

1. Introduction Τhe global expansion of industrialization and urbanization has resulted in the emission of significant quantities of industrial and urban dusts into the environment which contain relatively large amount of magnetic particles (Jordanova et al., 2006; Lu et al., 2009; Magiera et al., 2011, 2013, 2015, 2016; Rachwał et al., 2015; Szuszkiewicz et al., 2015). The term “technogenic” magnetic particles (TMPs) has been widely used in order to describe these anthropogenic magnetic particles originating from a wide variety of high temperature technological processes such as metallurgy, fuel combustion, ceramics, cement production, coke production, etc. (Catinon et al., 2014; Lu et al., 2016; ⁎ Corresponding author at: Department of Mineralogy-Petrology-Economic Geology, School of Geology, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece. E-mail addresses: [email protected], [email protected] (A. Bourliva).

http://dx.doi.org/10.1016/j.geoderma.2017.02.001 0016-7061/© 2017 Elsevier B.V. All rights reserved.

Magiera et al., 2011; Rachwał et al., 2015; Szuszkiewicz et al., 2015, 2016). As a result, the deposition and input of these particles on the surface soils could increase the values of magnetic susceptibility mainly in the immediate vicinity of industrial sites due to long term accumulation of pollutants resulting from industrial activity (e.g. Catinon et al., 2014; Jordanova et al., 2013; Šajn et al., 2013; Sarris et al., 2009; Xia et al., 2014). Technogenic magnetic particles (TMPs) comprise mainly iron oxides with ferrimagnetic or antiferromagnetic properties (Brown et al., 2011; Catinon et al., 2014; Jabłońska et al., 2010; Magiera et al., 2011; Magiera and Strzyszcz, 2000). Such physical properties enable physical separation, facilitating in that way their characterization and investigation. An additional characteristic feature of TMPs is their co-occurrence with anthropogenic forms of iron oxides and heavy metals (Hansen et al., 1981; Hulett et al., 1980; Yu and Lu, 2016). Thus, significant correlation has been observed by many researchers between magnetic

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properties and heavy metal concentrations, indicating TMPs as an important source of heavy metal contamination in the topsoils of urban and industrial regions (Blaha et al., 2008; Lu et al., 2012; Zhang et al., 2011; Zhu et al., 2013). Consequently, TMPs are used as tracers of industrial pollution since their presence, even in trace amounts, can be easily detected by magnetic measurements. Moreover, the coexistence of TMPs and trace metals in contaminated soils has developed magnetic separation as an alternative treatment method and physical technique in soil cleaning (Dermont et al., 2008; Mercier et al., 2002b; Rikers et al., 1998). On the other hand, the mobility and fate of metals associated with TMPs are of great environmental concern. Since metallic toxicants have a specific nature and behavior in the environment (e.g. persistent, non-biodegradable and easily bioaccumulated in the trophic chain), their presence has attracted more attention, concerning how it can affect human health (Liao et al., 2005; Qing et al., 2015; Zhao et al., 2014). Metals can be easily transferred into humans through various exposure routes such as ingestion, inhalation, and dermal contact. As a result, in many cases adverse health effects have been observed in both the residents of inhabited areas which are adjacent to industrial areas and in their employees (Peña Fernández et al., 2014; Wang et al., 2011; Yousaf et al., 2016). As demonstrated by numerous studies metals could accumulate in the fatty tissues, and subsequently affect the functions of the organs, as well as disrupt the nervous or endocrine system (Waisberg et al., 2003; Wang, 2013). However, the actual health risks of metals depend strongly on the ingested soil components, since ingestion has been consecutively reported as the main exposure pathway (Chabukdhara and Nema, 2013; Kelepertzis, 2014; Li et al., 2015; Luo et al., 2012; Pan et al., 2016; Salmani-Ghabeshi et al., 2016). They also depend on the fraction of the total soil metal contents, which is soluble in the gastrointestinal tract available for absorption and thus, is human accessible (Oomen et al., 2002; U.S.E.P.A., 2007). Many tests and protocols have been successfully developed with the Unified Bioaccessibility Method (UBM) which is a two stage in vitro protocol, being developed by the Bioaccessibility Research Group of Europe (BARGE) and validated to estimate the oral metal bioaccessibility metals in soils (Patinha et al., 2014; Reis et al., 2013, 2014). The magnetic properties, morphology, mineralogy, and chemical composition of TMPs isolated from contaminated soils, industrial and urban dusts have been extensively studied (Aidona et al., 2016; Bhattacharjee et al., 2011; Bourliva et al., 2016; Iordanidis et al., 2008; Lu et al., 2016; Magiera et al., 2011, 2013, 2016; Sokol et al., 2002; Wang, 2014; Yang et al., 2014; Zhao et al., 2006). However, detailed studies considering the enrichment and mobility of potentially harmful elements (PHEs) in TMPs separated from heavily contaminated locations are scarce (Lu et al., 2012). Furthermore, despite the fact that to the best of our knowledge recently there has been growing interest in the human health risk assessment studies, there is a research gap considering the oral bioaccessibility and the impact of PHEs accompanying the TMPs on human health of both adults and children. In the present study a detailed mineralogical, morphological and geochemical characterization of TMPs along with oral bioaccessibility data were investigated. The objectives of the study were (1) to characterize the enrichment of PHEs in TMPs separated from contaminated industrial soils; (2) to estimate the oral bioaccessibility of trace metals which presented elevated anthropogenic contents, in the TMPs of the contaminated soils; and (3) to evaluate the health risk from incidental ingestion of soil TMPs. 2. Materials and methods 2.1. Study area The study area is located in Sindos Industrial Area (SIA) which is extended in a flat area at the west of the city of Thessaloniki, Northern Greece and hosts the largest part of the industrial activity in the region

87

(Fig. 1). In particular, the sites selected for measuring and sampling in the study area, were focalized in abandoned meadows around a chemical industry (electrolytic treatment of pyrolusite-MnO2). In detail, the study area is situated in the easternmost sector of the Thessaloniki coastal plain and developed above loose Quaternary formations. These formations, unconsolidated to partly consolidated marine-lacustrine sediments consist mainly of sand and black silty clays (Andronopoulos et al., 1991; Kockel et al., 1978; Rozos et al., 2004). Geotectonically, the broader area belongs to the Axios (Vardar) zone and more specifically in the Peonia subzone, which is characterized by the presence of low to medium grade metamorphic rocks and a few exposures of ophiolitic bodies (Andronopoulos et al., 1991; Mercier, 1966). The soil materials of the entire area are dominated by Fluvisols according to the Soil Atlas of Europe (European Commission, 2005). The prevailing surface level winds in the study area during the winter period are of NNW origin. During the rest of the year northern winds prevail in the morning and night hours, while at midday and afternoon the predominance of southern winds is apparent, mostly due to sea breeze (Tsilingiridis et al., 1992). Regarding the occurrence frequency of all wind directions in the broader area, the annual percentage of southern winds is limited to about 6%, while the percentages of calms and northern winds reach 50% and 30%, respectively (Moussiopoulos et al., 2009). On this basis, the study sites were selected due to the general influence by the SIA during the occurrence of the later ones and also by taking into account the possible transport of pollutants from the investigated area to Sindos town, a suburb of the city of Thessaloniki, which is in the immediate vicinity and has a population of 8228 inhabitants. 2.2. Field measurements Previous to sampling, field measurements of volume magnetic susceptibility (κ) were carried out on June 2015, in the area around the above mentioned chemical industry. Values were measured using a Bartington MS2 susceptibility meter equipped with a MS2D field loop sensor. The measurements were taken in open spaces every 25–50 m (where possible) up to a distance of ~500 m from the borders of the industrial unit. At each measuring point (a total of 24 individual points, Fig. 1) the κ value (expressed in × 10− 5 SI units) was calculated as a mean value of 10–15 readings on a square of ~2 m2. Field measurement of volume magnetic susceptibility (κ) ranged from 28 to 669 × 10−5 SI with the highest values recorded at sites adjacent to the industrial unit. Based on these measurements, volume magnetic susceptibility spatial distribution pattern (ArcGIS, inverse distance interpolation method) was depicted so as the most representative locations to be selected (Fig. 1). 2.3. Samples collection, preparation, magnetic separation A total of 13 topsoil (soil horizon A, depth 0–20 cm) samples were collected, about 1–1.5 kg each, using a plastic spade (carefully cleaned after each sampling), mainly from locations where elevated κ values were recorded. Soil samples were air-dried in an oven at 40 °C and sieved to provide the b 2 mm and b 250 μm size fractions. A representative amount of the b 2 mm fraction was grounded in order to get a fine powder and subsequently used for the determination of the soils physico-chemical properties. The b250 μm size fraction was used for the rest of the analytical determinations, as it appears to be the most commonly used soil size fraction in oral bioaccessibility and health risk studies. This occurs since it closely represents the soil particles more available for incidental ingestion due to hand-to-mouth contact behavior (Liu et al., 2016; Pan et al., 2016; Ruby and Lowney, 2012). The extraction of the TMPs was obtained by using a neodymium magnet sealed with a double polyethylene bag. The extraction procedure was applied in a subset of soil

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Fig. 1. Localization of studied sites in Sindos area, Northern Greece (up) and spatial distribution of volume magnetic susceptibility (κ) measured in situ (down).

samples and was run continuously until no magnetic particles were attached to the magnet. Samples selection was based on the magnetic signal and the Pollution Load Index-PLI introduced by Tomlinson et al. (1980) (unpublished data). It involved samples with both elevated magnetic susceptibility and total pollution load so as to ensure a “technogenic” load of pollutants in the selected samples used for

further analyses. The lowest values recorded in the study area were determined in a site which was considered as non-polluted. Hence, the sample collected from this site was regarded as background sample (Table 1). However, the elevated regional background values for As are due to geochemical anomalies attributed to the lithology of the broader study area (Salminen et al., 2005) The extracted

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89

Table 1 Background concentrations of the measured major and trace elements along with mass specific magnetic susceptibility. Comparison with the background values for upper continental crust and the average European soil levels is also given. Al

Ca

Fe

K

Mg

Na

Ti

As

Bi

Cd

Co

Cr

Cu

Upper Continental Crusta Average European Soil Levelsb Regional Background values a a

8.04

3

3.5

2.8

Ni

Pb

Mn

Mo

Sb

Sn

V

W

Zn

mg kg-1

% 1.33 2.89

0.3

1.5 0.127 0.098 7

5.53 1.34 2.26 0.72 0.71 1.82 0.25

15

0.145 0.3

0.1

xlf (×10-8 m3 kg-1)

10

35

25

20

20

600 1.5

0.2

8

60

13

18

23

0.6

0.6

5.5 60 3

60

2

71

b5

52

10.2 60 32.4 27.7 32.4 636 0.1 b0.1 2.7 41 0.5 54

56

Rudnick and Gao, 2003 Salminen et al., 2005

magnetic fractions (MFs) and the residue (hereafter called nonmagnetic fractions, NMFs) were collected and weighed. 2.4. Analytical determinations 2.4.1. Physico-chemical properties of soils Soil pH was determined as pH (CaCl4) according to the ISO10390:1994 protocol. Soil organic matter (SOM) was determined by loss-on-ignition, at 430 °C for about 16 h (Schumacher, 2002). Elemental analysis of carbon and sulfur (C and S) were determined by element analyzer (Eltra CS-2000). The soil organic carbon (OC) content was determined using a non-dispersive infrared analysis (NDIR) method. Cation exchange capacity (CEC) and the exchangeable cations were measured according to the ISO13536-1995 protocol. These soil properties were determined in the b2 mm soil size fraction, which is usually used in environmental studies. 2.4.2. Magnetic measurements The mass specific magnetic susceptibility (χ) of soil samples was measured at low (0.46 kHz) and high (4.6 kHz) frequency using a Bartington MS2 laboratory magnetic susceptibility meter (Bartington Ltd., UK), equipped with a dual frequency MS2B sensor. Magnetic susceptibility value provides an indication of the concentration within the sample of strongly ferrimagnetic minerals, such as magnetite. Frequency-dependent magnetic susceptibility (χfd) was defined as χfd(%) = [(χlf − χhf)/χlf] × 100, where χlf and χhf represent magnetic susceptibility values at 0.46 and 4.6 kHz, respectively. Magnetic susceptibility measurements were carried out at the Department of Geophysics of the Aristotle University of Thessaloniki on bulk soil samples and the extracted MFs. Magnetic hysteresis loops were recorded by a 1.2H/ CF/HT Oxford Instruments Vibrating Sample Magnetometer (VSM) at room temperature under a maximum applied field of 1 T. These magnetic measurements were carried out at the Physics Department, Aristotle University of Thessaloniki on bulk soil samples. 2.4.3. Mineralogical and morphological analysis Mineralogical characterization of soil samples was performed by XRay powder diffraction (XRPD) using a water-cooled Rigaku Ultima + powder diffractometer with CuKa radiation, a step size of 0.05o and a step time of 3 s, operating at 40 kV and 30 mA. A scanning electron microscope (JEOL JSM-840A) was used to analyze the overall size distribution and morphology of soil particles, while their elemental composition was determined using an X-ray energy dispersive spectrometer-EDX (INCA 300). The elemental analyses were performed in a “spot mode” in which the beam is localized on a single area manually chosen within the field of view. 2.4.4. Heavy metal contents Total heavy metal contents of the bulk soil samples and both magnetic (MFs) and non-magnetic (NMFs) fractions were determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) at the

accredited Activation Laboratories (ActLabs.), Canada. The extraction of the metals was performed via a multi acid digestion procedure Specifically, about 0.25 g of the prepared soil sample was heated in a concentrated HF-HNO3-HClO4 mixture to fuming and taken to dryness. The residue was dissolved in HCl. Quality assurance and quality control (QA/QC) included reagent blanks, analytical duplicates and analyses of in house reference materials provided by Activation Laboratories (multi-element soil standards GXR-1, GXR-4, GXR-6, SDC-1, OREAS 45d and SdAR-M2) (QA/QC report is provided in Supplementary material). Results of the method blanks were always below detection limits. The recovery rates were estimated within ±10% of the certified value, and analytical precision was better than ±5%. 2.4.5. In vitro oral bioaccessibility The up to now assumptions that 100% of soil-borne contaminants are bioavailable lead to overestimations of health risks and therefore, made the estimation of pollutant oral bioaccessibility more important in terms of health risk assessment and remediation efforts. The in vitro oral bioaccessibility determinations of the “technogenic” metals lead (Pb) and zinc (Zn) were carried out in GeoBioTec, University of Aveiro, Portugal. Bioaccessible concentrations (mg kg−1) were determined in the b 250 μm fraction of a subset of soil samples using the Unified Bioaccessibility Method (UBM), developed by the Bioaccessibility Research Group of Europe (BARGE). The UBM simulates the leaching of a solid matrix in the human GI tract (Wragg et al., 2011). It is about a two stage in vitro simulation that represents residence times and physico-chemical conditions associated with both the Gastric tract (G phase) and the GastroIntestinal tract (GI phase). Standard operation procedure for the validated BARGE UBM method is available online (BARGE, 2010) and has been also fully described by Wragg et al. (2011). The methodology has been validated against a swine model for arsenic (As), cadmium (Cd) and lead (Pb) in soils (Denys et al., 2012). Analyses of the bioaccessible extracts were performed by ICPMS. Duplicate analysis, blanks, and the certified reference material BGS102 were extracted with every batch of UBM bioaccessibility extractions for quality control. The method-specific certified reference soil BGS102 (an ironstone soil from Lincolnshire, UK), prepared at the British Geological Survey (BGS), provides certified UBM values for As in both G and GI extraction phases and Pb in the G phase (Wragg et al., 2011). Recently reproducible UBM guidance values for fifty-five additional elements outside of the material's certification have also been reported (Hamilton et al., 2015). The results obtained by the analysis of the blanks were below the detection limit, while the recovery rates for BGS102 were estimated within ± 5% of the certified value (Hamilton et al., 2015). Mean repeatability (expressed as RSD %) was b10% for Pb and Zn in the gastric phase, while duplicate analysis in the gastrointestinal phase exhibited non acceptable values (RSD N 20%) (QA/QC report is provided in Supplementary material). Precipitation phenomena probably occurring in GI phase due to centrifugation in the UBM procedure, prevents correct analysis. However the concentrations used in this study were those reported in the G-phase as they presented the highest

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values from the available compartments (Denys et al., 2012; Wragg and Cave, 2003; Wragg et al., 2009) and also are considered to provide a more conservative estimate of risk (Farmer et al., 2011). In addition to phase-specific bioaccessible concentrations, the percentages of bioaccessible fractions (%BAF) which reveal the actual concentration of the element in the extract (Environmental Agency, 2007) and are highly dependent on the determined total concentration, were calculated as follows: BAFð%Þ ¼

ElementBioaccessible  100; ElementTotal

ð1Þ

where ElementBioaccessible and ElementTotal are the element bioaccessible concentration (mg kg−1) obtained from either the G or GI phase and total concentration (mg kg− 1), respectively, that was assessed for each element. 2.4.6. Exposure and health risk assessment In order to evaluate the potential health risk of adults and children through the exposure via ingestion of TMPs the major indices i.e. Average Daily Intake (ADI, mg kg−1 day−1) and Hazard Quotient (HQ) for Pb and Zn were employed according to the models developed by the United States Environmental Protection Agency (U.S.E.P.A., 2002). For the exposure assessment model, average daily intake of metals from incidental ingestion of soil was calculated according to the following formula: ADI ¼

C  IR  ED  EF BW  TA

ð2Þ

where ADI is the average daily intake (mg kg−1 day−1); IR is the ingestion rate (200 mg day−1 for children and 100 mg day−1 for adults; EF is the exposure frequency of 350 days' year−1; ED is the exposure duration (6 years for children and 26 years for adults; BW is the average body weight (15 kg for children and 80 kg for adults; AT is the averaging time (for non-carcinogens, AT = ED × 365 days). Exposure data were adapted from US Environmental Protection Agency guidelines (U.S.E.P.A., 2014), however regional or individual differences may introduce some uncertainties in the procedure. The concentration Cexp in Eq. (1) is the exposure point concentration for each element corresponding to the reasonable maximum exposure and is defined as the upper limit of the 95% (95% UCL) confidence interval for the mean (Hu et al., 2012; Kelepertzis, 2014; Zheng et al., 2010). The potential non-carcinogenic risk of metals in soils is expressed as a Hazard Quotient (HQ), as suggested by the US EPA guidelines when a reliable site-specific bioaccessible (bioaccessible fraction of the examined element of concern in the solid-phase) value is available (U.S.E.P.A., 2007). Therefore, the exposure estimate (i.e., ingested dose) is adjusted when calculating the Hazard Quotient (HQ): HQ ¼

ADI  BAF% R f Do

ð3Þ

where ADI is the average daily intake (mg kg− 1 day− 1), BAF% is the

bioaccessible metal fraction and RfDo is the oral reference dose (mg kg−1 day−1), which corresponds to the maximum allowable daily oral dose that is not likely to cause any deleterious effects on human health and is 3.50 × 10− 3 for Pb and 3.00 × 10−3 for Zn (U.S.E.P.A., 2010, 2013; Gu et al., 2016). The ratio of average daily intake to the reference dose can be applied to estimate the non-cancer risk on humans: HQ b 1 indicates no adverse health effects, while HQ N 1 indicates likely adverse health effects (Man et al., 2010). 2.4.7. Statistical analysis Analysis of variance (ANOVA) was used to identify significant differences among the bioaccessible concentrations and the HQs obtained for the two fractions (MF and NMF) extracted from the studied soil samples. 3. Results and discussion 3.1. Soil properties The results of soil properties that were determined in a subset of five samples are presented in Table 2. The pH values of the studied soils were neutral/near neutral with a range from 6.6 to 7.0, while the SOM ranged between 1.27% and 5.29%. Samples were characterized by OC values in the range of b 0.5 to 2.1% indicating that almost all the soil samples have an important amount of inorganic carbon. Considerable CEC was revealed by the studied soils with values ranging from 22.4 to 28.1 cmol kg− 1. The most abundant exchangeable cation was Ca exhibiting a mean value of 21.6 cmol kg−1. The concentrations of TC and TS (expressed as relative proportions) were in the ranges of 1.28–3.69% and 0.03–0.11%, respectively. 3.2. Magnetic properties 3.2.1. Magnetic properties and content of TMPs The magnetic properties of the bulk soil samples and the extracted MFs along with the content of TMPs in the studied bulk industrial soils are shown in Table 3. The values of mass specific magnetic susceptibility (χlf) in the bulk soil samples ranged from 103.5 × 10−8 to 821.2 × 10−8 m3 kg− 1 with a mean of 349.8 × 10− 8 m3 kg−1 well above the regional background value (52.6 × 10−8 m3 kg−1) (Table 1). The elevated values indicated that the accumulation of TMPs resulted in enhanced magnetic content in topsoils. Even though direct comparison of magnetic susceptibility values of soils from different regions is difficult, the χlf values recorded were higher than those reported for industrially derived contaminated soils (Lu et al., 2012). Moreover, the χlf values determined in the extracted MFs exhibited a range of 5458 × 10− 8–7570.5 × 10− 8 m3 kg−1, values substantially higher than the corresponding ones in the bulk soil samples (Table 3). On the other hand, the low values of frequency dependence magnetic susceptibility (χfd b 5%) for both soil fractions, bulk and magnetic, indicated an almost complete lack of superparamagnetic magnetite grains (grain size b 0.01 μm) within the soil samples and a large contribution of anthropogenic related magnetic phases (Dearing et al., 1996;

Table 2 Soil properties determined in a subset of five selected soil samples. Sample

pH

SOM

OC

CEC

NaEXCH

Soil 1 Soil 2 Soil 3 Soil 4 Soil 5 Mean ± SDa Median a

7.0 6.6 6.8 6.7 7.0 6.8 ± 0.2 6.8

SD: standard deviation.

2.7 1.75 5.29 1.27 3.40 2.88 ± 1.58 2.70

KEXCH

MgEXCH

CaEXCH

TC

cmol kg−1

% 0.9 0.6 2.1 b0.5 1.2 1.2 ± 0.6 1.0

28.1 24.9 22.4 25.1 22.4 24.6 ± 2.4 24.9

0.6 b0.1 0.3 b0.1 0.3 0.4 ± 0.2 0.3

0.5 0.3 1.6 0.3 2.0 0.94 ± 0.80 0.5

TS %

1.3 0.9 2.5 1.5 1.8 1.6 ± 0.6 1.5

25.6 23.5 17.8 23.0 18.1 21.6 ± 3.5 23.0

1.98 2.11 3.69 1.28 2.21 2.25 ± 0.88 2.11

0.11 0.03 0.07 0.03 0.04 0.06 ± 0.03 0.04

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Table 3 Magnetic properties and mass of magnetic fractions (MFs) in the soil samples. Values for references soils are also listed. Bulk

Min Max Mean ± SDa Median a

MFs

χlf (×10−8 m3 kg−1)

χfd (%)

χlf (×10−8 m3 kg−1)

χfd (%)

103.5 821.2 349.8 ± 300.7 221.3

0.25 1.97 1.97 ± 0.69 1.38

5458 7570.5 6556.2 ± 879.6 6598.05

3.68 4.84 4.36 ± 0.49 4.45

Fe (%)

MFs (%)

1.59 6.57 3.58 ± 1.28 3.36

3.69 13.75 7.52 ± 2.36 6.23

SD: standard deviation.

Magiera et al., 2011). Therefore, it is verified that the enhanced magnetic signal was attributed to the presence of “technogenic” magnetic particles in the topsoils (Magiera et al., 2013). The amount of extracted TMPs was quite variable ranging from 3.69 to 13.75 wt%. 3.2.2. Magnetic properties of TMPs Hysteresis loops up to 1 T maximum applied field were analyzed for the extracted TMPs of the studied industrial soils and are illustrated in Fig. S1 (Supplementary Material). Parameters related to the domain state such as saturation magnetization Ms., saturation remanent magnetization Mrs., coercive force Hc and coercivity of remanence Hcr, were obtained from the corrected hysteresis loops and plotted on the Day diagram (Day et al., 1977), modified by Dunlop (2002). As seen in Fig. 2, the data points occupy the area of pseudo-single domain (PSD) range which is observed quite frequently in polluted soils (Jordanova et al., 2013; Meena et al., 2011). Clustering of the experimental data in the specific range implies a relatively uniform magnetic mineralogy, however, since the Day plot was designed originally for a certain range of titanomagnetites (Day et al., 1977; Dunlop, 2002), it is not used here in terms of magnetic granulometry. Detailed magnetic grain-size analyses performed by Szuszkiewicz et al. (2015) on TMPs separated from industrial dusts and fly ashes recorded also the dominance of PSD grains. 3.3. Mineralogical and microstructural characterization of TMPs

with various proportions of maghemite (γ-Fe2O3) and magnesioferrite (MgFe2O4). The identification between those three ferrospinel phases (e.g. magnetite, maghemite, magnesioferrite) is difficult since their diffraction peaks in XRD patterns are overlapping each other. Other iron containing phases such as hematite (a-Fe2O3) and lepidocrocite (γFeOOH) were also detected. In spite of magnetic separation, quartz and calcite contents were still elevated, while minor amounts of plagioclase were also detected. From a morphological perspective, SEM observations revealed TMPs with a large heterogeneity of particle sizes and shapes. Iron spherules along with angular/aggregate particles were the most commonly observed particles (Fig. 4). Moreover, observations on single grains of TMPs indicated their large size (diameter N 150 μm) suggesting that these particles originate from very close vicinity and they cannot be transported from distant sources. The ferrospheres detected are considered as characteristic particles deriving from combustion processes (Spiteri et al., 2005) operating in the adjacent industrial unit and exhibited various surface morphologies (Fig. 4a, b). On the other hand, irregular, angular/aggregate Fe-rich magnetic particles with elevated heavy metal contents were also observed (Fig. 4c, d). Generally, the angular magnetic particles are mainly released from vehicles via exhaust emission as well as the abrasion/corrosion of the vehicle engine and body work (Bućko et al., 2011; Hoffmann et al., 1999). Despite the absence of major transport network in the immediate neighborhood of the industrial unit, the heavy truck traffic occurring on the small local roads which serve the industrial area probably resulted in the release of these traffic related magnetic particles.

The mineralogical components of the TMPs were investigated using XRD and representative patterns of soil MFs are given in Fig. 3. The results exhibited a strong peak around 2.51–2.52 Å attributed to a dominant iron spinel phase, mainly magnetite (Fe3O4) probably mixed

Fig. 2. Day plot of the magnetic fractions (MFs) separated from industrial soils, Sindos area, Greece.

Fig. 3. Representative X-ray diffraction patterns of the magnetic fractions (MFs) separated from industrial soils, Sindos area, Greece. The most intense reflections of respective minerals are labeled. Fe-Spinel: iron spinel phase, Ht: hematite, Q: quartz, C: calcite, Lep: lepidocrocite.

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3.4. Interpretation of geochemical data 3.4.1. Geochemical results of bulk soil samples and association with soil properties The bulk soil samples were dominated by Al (mean 5.78%), Fe (mean 4.13%) and Ca (mean 3.51%), while from the determined trace elements Mn, Zn and Pb presented the higher concentrations with mean values of 1911 mg kg−1, 1171 mg kg−1 and 411.7 mg kg−1, respectively. Compared to regional background values (Table 1), the studied soils demonstrated a relative enrichment in “technogenic” metals such as Cd, Pb and Zn (Rachwał et al., 2015) indicating a “technogenic” load of heavy metals in the studied industrial soils. Pearson correlation coefficients between the determined elemental concentrations and soil properties were established and the results are

presented in Table S1 (Supplementary material). Strong correlation coefficients (p b 0.01) were revealed between several metal pairs (Table S1), while the significant (p b 0.05) correlation of Bi (0.700), Co (0.924), Cr (0.687), Cu (0.586), Ni (0.991), Pb (0.568), Sb (0.907), Sn (0.891), and Zn (0.965) with Fe suggested their relation with ferrimagnetic soil particles. On the other hand, no significant correlations were observed among major and trace elements, while elements regarded mainly of crustal origin, such as Cr, were strongly correlated with Cu (0.902) and Pb (0.973), typical “technogenic” contamination indicators, implying limited crustal influence in the studied soils. Regarding the association of trace elements to the soil properties, no significant correlation coefficients were observed for any pair of elemental contents with pH, SOM, OC or TC. A relatively high correlation coefficient that was revealed for TC with Co (0.911) was an exception.

Fig. 4. SEM images and representative EDX spectra of (a), (b) angular/aggregate magnetic particles and (c) and (d) spherules detected in magnetic fractions (MFs) separated from industrial soils, Sindos area, Greece.

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On the contrary, strong association between TS and PHEs such as Bi, Cd, Cr, Pb, Sb, Sn, W, and Zn contents imply their “technogenic” origin since the anthropogenic inputs of sulfur in soils are confirmed (Guo et al., 2016). Specifically, the deposition of SO2 emissions deriving mainly from industrial activities (i.e. combustion of fossil fuels) lead to elevated total sulfur contents in soils. Moreover, the activities of the adjacent chemical industry involve emissions of hydrogen sulfide (H2S), a gas that generates during the digest and purification processes.

3.4.2. Enrichment of PHEs in TMPs The range and mean concentrations of major and trace elements in various fractions (MF, NMF and bulk sample) are listed in Table S2 (Supplementary material), while their ranges are illustrated in Fig. 5. As shown in Table S2, Fe was enriched in MFs and depleted in NMFs as expected, exhibiting total Fe contents in MFs and NMFs in the range of 21.30–33.80% (mean 25.62%), and 2.52–4% (mean 3.19%), respectively (Fig. 5). Titanium was also rather enriched in the soil MFs, exhibiting slightly higher contents in the MFs (mean 0.51%) compared to the corresponding in the NMFs (mean 0.22%). Calcium, Mg and K revealed similar enrichment in the soil MFs and NMFs, indicating their presence and relevance in both fractions. Nevertheless, Al and Na appeared to be slight enriched in NMFs, indicating their association with the nonmagnetic soil particles mainly of crustal origin. Considering the potentially harmful elements (PHEs), they were highly enriched in the MFs. Specifically, Cr, Mn, Pb and Zn presented their higher concentrations in the MFs with mean values of 1680.5, 4403.3, 1058.7 and 3841 mg kg−1, respectively. Chromium exhibited extremely elevated concentrations in the MFs ranging between 436 mg kg− 1and 5090 mg kg−1 in the MFs compared to those determined in the NMFs which ranged between 46 mg kg− 1 and 130 mg kg− 1. Likewise, for Mo, whose concentrations in the MFs were N20-times higher than the corresponding ones in the NMFs (3.6–44 mg kg−1 and 0.2–1.5 mg kg−1, respectively). The Enrichment/Depletion Ratios (EDRs) of elements, which are defined as the concentrations ratios of elements in the magnetic (MF) and non-magnetic (NMF fraction, are presented in Fig. 6. The mean

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enrichment ratio of Fe in MFs was calculated to be 8.24 (Fig. 6). The mean EDRs for PHEs ranged between 2.52 (Mn) and 56.56 (Mo) and followed the order Mo N W N Cr N Sb N Sn N Cd N V N Ni N Zn N Cu N Bi N Co N Pb N As N Mn. The obtained EDRs were significantly higher than those reported by Lu et al. (2012) for industrially derived contaminated soils. Moreover, similarities were observed in the decreasing order of EDRs determined for urban road dusts (Bourliva et al., 2016) verifying the existence of traffic related magnetic particles in the studied soil samples as supported by SEM observations. The significantly higher EDRs for PHEs in the studied soil MFs suggested that Fe and the rest of the elements are preferentially enriched in the technogenic magnetic particles (TMPs), while the significant correlations (p b 0.01) observed between PHEs, especially Mo, Zn, Ni, Sb and Sn, and mass specific magnetic susceptibility (χlf) verified their association with those particles (unpublished data). Taking into account this fact and having in mind that MFs comprised a considerable proportion of the bulk soil samples, their removal would reduce significantly the amount of heavy metals in the soil residues (NMFs). As it is shown in Fig. 6, most of the PHEs mean concentrations in the NMFs were lower than those exhibited by the bulk samples indicating magnetic separation as an alternative treatment method for the remediation of industrial soils exhibiting elevated concentrations in “technogenic” metals.

3.5. Oral bioaccessibility of PHEs from TMPs Bioaccessibility is a key factor limiting bioavailability and can be used as a conservative substitute of bioavailability for risk assessment purposes. Particularly, oral bioaccessibility refers to the fraction of a contaminant that is soluble by GI fluids and represents the maximum amount of contaminant that is available for intestinal absorption (Ruby et al., 1999; Rodriguez et al., 1999). The bioaccessible concentrations (mg kg−1) and the bioaccessible fractions (BAF %) of Pb and Zn in the MFs and residual NMFs (for comparison reasons) of the studied industrial soils are presented in Table S3 (Supplementary material), while their mean values are demonstrated in Fig. 7.

Fig. 5. Range of major and trace elements concentrations in various fractions (bulk, nonmagnetic and magnetic) of industrial soils, Sindos area, Greece.

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Fig. 6. Enrichment/Depletion Ratios (EDRs) of major and trace elements of industrial soils, Sindos area, Greece.

In all cases it was observed that bioaccessible concentrations (mg kg−1) and BAF% for both Pb and Zn was always higher in the G phase. This observed pattern of consistently lower bioaccessibility in the GI phase is in accordance with previous studies (Rodriguez et al., 1999; Wragg et al., 2011; Zia et al., 2011; Reis et al., 2013). Additionally, it was revealed by the results that Pb and Zn concentrations (mg kg−1) extracted within the simulated G and GI phases were higher in the MFs of the studied industrial soils compared to the corresponding ones in the residual NMFs. For example, bioaccessible Pb concentrations (G-phase) in the soil MFs ranged from 492.52 mg kg− 1 to 2079.52 mg kg−1 (mean 1089.11 mg kg−1), while in the NMFs from 248.83 mg kg−1 to 1079.83 mg kg−1 (mean 529.36 mg kg−1), respectively. Similarly, the extractable Zn concentrations in the G-phase of the soil MFs exhibited a range of 956.88–4711.30 mg kg− 1 (mean 3441.54 mg kg−1), while in the NMFs their range was 393.21–2134.41 mg kg−1 (mean 1464.55 mg kg−1), respectively. Despite the meaningful differences, comparison (ANOVA analysis, p b 0.05) between the two fractions (MF and NMF) demonstrated that they were not statistically significant (p b 0.05 in all cases). However, the small sample size could probably affect the significance level and maybe the results would have been significant if there were more cases. Regarding the corresponding bioaccessible fractions (BAF %) in the G phase, they were lower for the soil MFs compared to those of the residual NMFs. According to this fact, it is suggested that the highest determined bioaccessible fractions do not correspond to the highest bioaccessible concentrations. However, Pb and Zn were still considered as significantly bioaccessible in the studied TMPs since a percentage of 52.34–62.64% (mean 56.43%) of the total Pb and 42.91–69.08% (mean 57.41%) of the total Zn were bioaccessible in the G phase for the soil MFs. According to Lu et al. (2012), lower values were reported in the MFs of industrially derived contaminated soils. More specifically, 13% of the total Pb and about 20% of the total Zn were characterized as bioaccessible when evaluated by GJST test (Gastric Juice Simulated TestGJST). The Pb fraction that was extracted from industrial soils MFs by the same test was 22.3% (Lu et al., 2009). Furthermore, significant correlations were evident for Pb and Zn between their total, and bioaccessible concentrations (Fig. 8), while no linear association was observed between their total concentrations and the bioaccessible fraction (BAF%)

Fig. 7. Mean values and range of total and bioaccessible Pb and Zn concentrations in nonmagnetic and magnetic fractions separated from industrial soils, Sindos area, Greece.

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Fig. 8. Linear correlations of total concentrations versus bioaccessible concentrations in the G phase for Pb and Zn (up) and total concentrations versus bioaccessible fractions % (down).

(Fig. 8). It is apparent though, that bioaccessibility does not depend on total elemental contents, indicating that the total concentration is not a reliable indicator of the associated health risk. 3.6. Health risk via ingestion of TMPs In this study, health risk via ingestion was assessed for each soil fraction (NMF and MF) in order to evaluate the hazardousness of the various soil fraction components to the health of both adults and children. The Hazard Quotients (HQs) calculated for soil NMFs and MFs are

presented in Table S4 (Supplementary material) and are illustrated in Fig. 9. In general, the non-carcinogenic HQs obtained from the soil MFs were constantly higher compared to those from the NMFs, however no significant differences among the two fractions were observed (ANOVA, p b 0.05). On the other hand, the ANOVA test revealed that HQ values for Pb were significantly higher (at level p b 0.01) compared to the ones calculated for Zn. Considering the potential noncarcinogenic risk for children, in all cases, the HQs determined according to Eq. (3) were far above the safe level (=1), indicating adverse health effects. For example, HQs for children obtained from soil MFs exhibited a range of 16.97–684.40 (mean 218.25) and 1.32–18.91 (mean 9.40) for Pb and Zn, respectively. Since these values are above the safe limit (=1), significant health risks were suggested for the children of any adjacent residential areas (e.g. Sindos town) from the incidental ingestion of soil magnetic components. Likewise, elevated HQs (N1) were calculated for adults and in the case of Pb for the MFs their values ranged between 1.59 and 64.16 for the TMPs. Considering the results clearly above the levels of concern, it should be noted that the estimations using the USEPA models are often conservative and hence, an overestimation of the potential health risks is possible. For that reason, uncertainty analysis which is fundamental, should be performed in order to quantify the uncertainties in the exposure parameters and provide considerable information about the variability and sensitivity of the calculated results.

4. Conclusions

Fig. 9. Hazard Quotients (HQs) for adults and children through metal (Pb and Zn) exposure to “technogenic” magnetic particles (TMPs) separated from industrial soils, Sindos area, Greece. The red line indicates the safe level. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

A detailed physicochemical characterization in relation with oral bioaccessibility data of TMPs was carried out in this study. A dominant iron spinel phase was detected in TMPs. It consisted mainly of magnetite probably mixed with various proportions of maghemite which governed the magnetization of the TMPs.

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Hysteresis ratios for all studied samples clustered in the pseudosingle domain (PSD) range. Characteristic morphological features were observed with spherical and angular/aggregate particles dominating the soil MFs. This is an indication, that the main sources of the TMPs are various technogenic processes such as coal-burning, industrially derived dusts and vehicle emissions. Elevated enrichment ratios of PHEs in the soil MFs revealed their preference to focalize in the TMPs which could be regarded as the main carriers of potentially toxic metals in the industrial soils. Additionally, the fact that PHEs content was remarkably depleted in the residual NMFs when compared to the corresponding in the bulk samples, allows the recommendation of magnetic separation as a probable alternative physical soil remediation method. Moreover, in the study of oral bioaccessibility of “technogenic” metals (Pb and Zn) from TMPs, it was observed that bioaccessible concentrations were higher from the soil MFs compared to the NMFs and were proportional to the total metal concentrations in the different soil fractions (magnetic and non-magnetic). On the contrary, the Pb and Zn bioaccessible fractions were lower from the TMPs, and yet major fractions (N 50%) were still in bioaccessible forms. The absence of any linear correlations between total concentrations and bioaccessibility (BAF %) verified that the total metal contents are not indicative of the health risk. Finally, despite the lower bioaccessible fractions of Pb and Zn in TMPs, the HQs determined for exposure through ingestion of the TMPs were higher than the nonmagnetic soil components and were in most cases actually far above the safety level exhibiting non-carcinogenic adverse health effects.

Acknowledgements The first author, Dr. Anna Bourliva, acknowledges financial support from the State Scholarships Foundation (IKY) through the operational program "IKY Fellowships of Excellence for Postgraduate Studies in Greece-Siemens Program". Also, the authors thank Mrs. Carla Patinha from GeoBioTec, Department of Geosciences, University of Aveiro, Portugal for her help in the bioaccessibility tests. The constructive comments and suggestions of the editor and three anonymous reviewers are highly acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.geoderma.2017.02.001.

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