Geochemical fractions of rare earth elements in two floodplain soil profiles at the Wupper River, Germany

Geoderma 228–229 (2014) 160–172

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Geochemical fractions of rare earth elements in two floodplain soil profiles at the Wupper River, Germany Julia Mihajlovic a, Hans-Joachim Stärk b, Jörg Rinklebe a,⁎ a b

University of Wuppertal, Department D, Soil- and Groundwater-Management, Pauluskirchstraße 7, D-42285 Wuppertal, Germany UFZ — Helmholtz Centre for Environmental Research, Department of Analytical Chemistry, Permoserstraße 15, 04318 Leipzig, Germany

a r t i c l e

i n f o

Article history: Received 30 November 2012 Received in revised form 22 November 2013 Accepted 11 December 2013 Available online 15 January 2014 Keywords: Scandium (Sc) Yttrium (Y) Lanthanum (La) Lanthanoides Sequential extraction Paddy soils

a b s t r a c t We aimed to determine the concentrations and geochemical fractions of rare earth elements (REEs) according to the genetic soil horizons of two soil profiles (Eutric Fluvisols) at the Wupper River, Germany. The concentrations were determined using aqua regia extraction and the geochemical fractions were assessed using a sequential extraction procedure developed by the Commission of the European Communities Bureau of Reference. Single REE concentrations varied from 0.09 mg kg−1 (Lu) to 40 mg kg−1 (Ce). We have detected small differences of the REE concentrations between the horizons that seem to be due to flooding and the linked homogenisation processes. Rare earth elements dominate in the residual fraction (73.5%), followed by reducible (19.6%), oxidisable (6.6%), and water soluble, exchangeable (0.4%) fraction (calculated from the sum of the fractions and mean of both soil profiles). The proportion of the residual fraction tends to decrease with increasing atomic number, whereas the proportions of the other three fractions increase. Rare earth elements with higher atomic number seem to take earlier the bonding-places in the first three fractions than REEs with lower atomic number and therefore, rather the latter are bound to residual fraction or occur as free species. Important factors that affect the geochemical fractions and mobility of REEs are the adsorption of REEs onto clay and amorphous Fe–Mn oxides as well as formation of phosphate or organic complexes with REEs. A low pH favours the releases of REEs from the soil. In future, the impact of flooding regime and physico-chemical soil properties on the concentrations, geochemical fractions, and release kinetics of REEs should be determined in frequently flooded soils around the globe to improve our understanding of the geochemical behaviour of REEs. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Rare earth elements (REEs; Sc, Y, and the 15 lanthanoides) are used in many industrial key technologies as well as microelement fertiliser or feed additive in agriculture. Through exploitation of REEs and disposal of used products containing REEs, the metals can enter into the soil and may be transferred into the food chain. Many rivers, estuaries, and paddy soils were contaminated from industrial, municipal, communal, and agricultural discharges of waste as well as geogenic sources (e.g., Du Laing et al., 2009a; Rinklebe et al., 2007). The anthropogenic microcontaminants La and Gd have already been detected in rivers, lakes, estuaries, coastal waters, groundwater, and tap water (Bau and Dulski, 1996; Hennebrüder et al., 2004; Kulaksiz and Bau, 2011). Gadolinium is used in contrast agents for magnetic resonance imaging and therefore, elevated Gd concentrations were especially determined near hospitals (Kulaksiz and Bau, 2011). Likewise, high La concentrations attain via the Rhine River into the

⁎ Corresponding author at: Soil- and Groundwater-Management, Department D, University of Wuppertal, Pauluskirchstraße 7, D-42285 Wuppertal, Germany. Tel.: + 49 202 439 4057; fax: + 49 202 439 4196. E-mail addresses: [email protected] (J. Mihajlovic), [email protected] (H.-J. Stärk), [email protected] (J. Rinklebe). 0016-7061/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geoderma.2013.12.009

North Sea (Kulaksiz and Bau, 2011). Several negative effects of REEs on organisms have been reported, for example Waring and Watling (1990) and Pairon et al. (1995) have documented that some inhaled REEs tend to accumulate in human lung and lymph nodes. Gadolinium complexes can be incorporated into bones of patients (Darrah et al., 2009) which finally lead to death (Kay, 2008). However, the mobility of REEs and their possible impacts on ecosystems are still relatively unknown; thus, potential risks for human health and environment can currently be inadequately estimated. The Wupper River (Germany), a tributary of the Rhine River, was highly polluted from discharges of textile and metal industry during the last centuries. Generally, pollutants have been transported with the river water and accumulate in floodplain soils during flooding events with low flow velocity (Rinklebe et al., 2007). Thus, certain areas of the floodplain soils along the Wupper River possess nowadays considerable amounts of metals (Frohne et al., 2011, 2012). Sedimentation as well as changes of water table level and resulting alterations between anaerobic (flooding) and aerobic conditions (aeration due to desiccation) have an impact on the content, geochemical fractions, and mobility of metals. The geochemical phase the metals occur is important for the mobility and potential toxicity of metals in soils, and factors such as sorption–desorption, redox processes, content of metal, organic material, clay minerals, carbonates, sulphur, iron

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(hydro-)oxides, and pH which control the mobility and bioavailability (Du Laing et al., 2009b). Generally, REEs from anthropogenic sources seem to be bound in biologically available forms (Rao et al., 2010), however, very little data concerning the concentrations and geochemical fractions of REEs in soils are available up to date. Therefore, it is currently difficult to determine the mobility of REEs in soils and to assess potential risks. Thus, our objectives are: i) to quantify the current concentrations of REEs in two Eutric Fluvisol soil profiles at the Wupper River, Germany; ii) to determine the geochemical fractions of REEs (exchangeable, reducible, oxidisable, residual fraction) as a function of depth; iii) to quantify relations between soil properties, and the concentrations and distribution of REEs within the different geochemical fractions. 2. Materials and methods 2.1. Study area and study sites The study area is located close to the confluence of the Wupper River into the Rhine River near Cologne in North Rhine-Westphalia, Germany (Fig. 1). The mean long-term annual precipitation is approximately 850 mm and the mean long-term annual air temperature is 10.8 °C (Deutscher Wetterdienst (Eds), 2012). The Wupper River is approximately 115 km in longitude with an average gradient of 0.4% (Wupperverband (Eds.), 2012). The discharge averages 15.4 m3 s−1 and the catchment area of the Wupper River comprises 815 km2 (Wupperverband (Eds.), 2012). The Wupper River is in large parts a stream of the average mountain until the inflow into the Rhine Valley. Furthermore, the Wupper River is rich in tributary streams and a gravel dominated river of the basement rock from the middle course to the water mouth. The geological parent material consists of sediments of the Rhine River (“Niederrheinische Bucht”), which is predominantly shale of Devonian origin (“Rheinisches Schiefergebirge”) (Wupperverband (Eds.), 2012). We selected two study sites in the study area and one soil profile was dug at each study site (profile 1: 51°5′4.1″N, 7°0′12.61″E; profile 2: 51°7′49.86″N, 7°1′35.10″ E). The study sites are used as grassland and are flooded seasonally by the Wupper River, usually in springtime. Both soils are classified as Eutric Fluvisol (IUSS Working Group WRB (Eds.), 2006). 2.2. Soil description, sampling, and sample preparation The soils were described in detail and classified according to “Bodenkundliche Kartieranleitung” (Ad-hoc-Arbeitsgruppe Boden,

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2005), and “World reference base for soil resources 2006” (IUSS Working Group WRB, 2006). Here, symbols of horizons are discussed according to IUSS Working Group WRB (2006). Soil samples have been collected in accordance to genetic horizons. Sampling was performed in four replicates of about 1 kg each, which were pooled to one sample. For chemical analysis, all visible roots, macro fauna, and fresh litter were removed from the samples and furthermore, soil material was homogenized, air dried for several days, and sieved to b 2 mm. Subsamples were ground using a metal free mortar. 2.3. Laboratory analyses 2.3.1. Soil properties Particle-size distribution was determined by wet sieving and sedimentation using the pipette sampling technique according to DIN ISO 11277 (2002). The pH was determined according to Blume et al. (2011). Total C (Ct) and N (Nt) were measured with dry combustion and thermal conductivity detection using a C/N/S-Analyser (Vario EL Heraeus, Analytik Jena, Germany). Inorganic C was quantified by dry combustion and IR-Detection (Blume et al., 2011) with a C-MAT 550 (Ströhlein). Soil organic C was calculated as the difference between Ct and inorganic C. Double lactate-soluble P (PDL) was determined as described by Verband Deutscher Landwirtschaftlicher Untersuchungsund Forschungsanstalten (Eds.) (1991). Concentrations of Al, Fe, and Mn in aqua regia were quantified by inductively coupled plasma optical emission spectrometry (ICP-OES) (Ultima 2, Horiba Scientific, Unterhaching, Germany). A four-point calibration was performed by diluting single standard and multi element solutions (CertiPur, Merck) with deionized water. Analyses were conducted in three replications. The relative standard deviation of replicate analysis was below 2%. Oxalate-extractable iron and manganese were measured according to Schwertmann (1964). Dithionite-extractable iron and manganese were determined as described by Mehra and Jackson (1960). Effective cation exchange capacity (CECeff) was calculated by using BaCl2 according to DIN ISO 11260 (2011). 2.3.2. REEs The samples were digested with aqua regia (DIN ISO 11466, 1997) to determine the concentrations of Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu as well as concentrations of Al, Fe, Mn, and S. These concentrations will be considered as ‘total concentrations’ in this study, ignoring that certain parts may remain in the residuum. However, it is well established that the aqua regia extraction is the best available technique to determine the ecologically relevant fractions (e.g. Löll et al., 2011; Rao et al., 2010; Rauret et al., 1999), although its

Fig. 1. Location of the study site in North Rhine-Westphalia, Germany.

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pertinence is case- and site-specific. In particular, this procedure is standard in DIN ISO 11466 (1997) and therefore recommended for evaluating the exposure pathway “soil-human” in the German Soil Protection Ordinance (BBodSchV, 1999). Sequential extraction was performed according to Rauret et al. (1999) to determine the geochemical fractions of the REEs. The extraction was developed by the Commission of the European Communities Bureau of Reference. Other authors, such as Rao et al. (2010) have also used this method. The term “geochemical fractions” is used as a synonym for “binding forms” here. The first step of the sequential extraction releases the water soluble/exchangeable/ carbonate bound fraction (F 1) and was carried out with acetic acid. The second step of the extraction (reducing step) was performed with hydroxylammonium chloride and digests the reducible/basically Fe−Mn oxide bound fraction (F 2). Hydrogen peroxide and ammonium acetate were used for the oxidisable fraction/the REEs which are bound to organic matter and sulphides (F 3) (third step). Finally an aqua regia step was carried out to extract the residual fraction (F 4). The analytical accuracy was achieved by the use of international certified reference soil material (GBW07407) for total concentration. According to Hornburg and Lüer (1999) we assume that 95% of the total metal concentrations are exploited using aqua regia extraction. Therefore, 95% of the total REE concentrations were used as a reference level for the recovery (percentage of the concentration-sum of the fractions on the total concentration of the reference soils). The recovery accounts in average 90 to 106%; except Sc with 126%, La with 85%, Yb with 81%, Lu with 76%, and Tm with 72%. Each soil sample was extracted in duplicate and the standard deviation was on average below 3.5%. Additionally, the sum of the concentrations of the four steps was compared with the concentration reached by the aqua regia extraction of a separate sample — as an internal check of the procedure. This recovery (percentage of the concentration-sum of the fractions on the concentration of the separate aqua regia extraction) amounts on average 90 to 101% depending on the element (only Sc has a recovery of 37% due to difficulties in measuring). However, the uncertainty of a sequential extraction procedure is often around 10% (e.g. Tessier et al., 1979). Concentrations of REEs were determined by inductively coupled plasma mass spectrometry (ICP-MS) with pneumatic nebulisation using an “ELAN-DRCe” quadrupol-based ICP-mass spectrometer (Perkin Elmer). Before measuring, the spectrometer was tuned (sensitivity, formation of oxide ions, formation of double charged ions) to achieve optimum conditions, and calibration was done with three standard solutions containing 1, 4 and 10 μg l− 1 of each REE, respectively. The three calibration standard solutions were prepared by dilution of ICP-MS reference standard “Multi-element Solution 1” (Certiprep®, SPEX, 10 mg l− 1 of each rare earth element) with 1% (v/v) nitric acid. Samples were diluted 1:10 with 1% (v/v) nitric acid prior to measurement. The used HNO3 was of ultrapure quality (Merck) and has been diluted with deionised water produced by a “Milli-Q Element” (Millipore/Merck). An addition of Rhodium to achieve [Rh] = 4 μg l−1 was made to every sample for internal standardisation. A standard was measured periodically between every five samples for control of instrumental drift. The following isotopes were used for analysis: 45-Sc (corrected for SiO+), 89-Y, 139-La, 140-Ce, 141-Pr, 143-Nd, 147-Sm, 151-Eu (corrected for BaO+), 157-Gd (corrected for CeO+), 159-Tb, 163-Dy, 165-Ho, 166-Er, 169-Tm, 172-Yb, 175-Lu and 103-Rh (as internal standard). Timing of measurement was a 32 ms dwell time by 32 sweeps, resulting in a 1.024 s integration time for every measured isotope in each of three replicate measurements. The detection limit (based on real lab blanks, 3 s-criterion) was 0.01 to 9.30 μg kg− 1 depending on the element and on the used extraction (Supplemental material 1). The relative standard deviation of replicate analysis was below 3%. The calculations were carried out by using MS Excel. Concentrations are given in relation to dry matter. The concentration-sums of the fractions were normalised to the upper continental crust (UCC) (Taylor and McLennan, 1985). Cerium anomalies were calculated as

Ce / Ce* = CeN / (LaN ∗ PrN)0.5 and Eu anomalies as Eu / Eu* = EuN/ (SmN ∗ GdN)0.5, where the subscript N indicates UCC normalised values. A value below “1” (negative anomaly) represents depletion, a value above “1” (positive anomaly) enrichment compared to the UCC. 2.4. Statistical analysis ORIGIN 8.0 was used for creating the figures. Pearson's correlation coefficients (r) were calculated using SPSS 13.0. According to Brosius (2002) the strength of the correlations was categorised as follows: r = 0 represents no correlation; 0 b r b 0.2 very weak correlation; 0.2 ≤ r b 0.4 weak correlation; 0.4 ≤ r b 0.6 modest correlation; 0.6 ≤ r b 0.8 strong correlation; 0.8 ≤ r b 1 very strong correlation; r = 1 perfect correlation. The correlations are significant on the level of confidence of 0.05 and 0.01 (two-tailed), respectively. 3. Results and discussion 3.1. Soils and vertical distribution of the sum of REE concentrations Both soil profiles are dominated by sand and silt (Table 1) and are CO3-free. The pH ranged between 5.2 and 6.9 in profile 1 and between 6.1 and 6.7 in profile 2. Oxalate- and dithionite-extractable Fe concentrations are higher in the upper soil horizons compared to the sub-soil horizons. The differentiation of the horizons of the Fluvisols is often weak since the soil development is interrupted through frequent flooding events and the linked processes such as sedimentation and erosion (e.g. Rinklebe et al., 2007; Shaheen and Rinklebe, 2014 -in this issue). The REEs are involved in those processes; therefore, we found only small differences concerning the vertical distribution of the REE concentrations (Figs. 2 and 3). The studied REEs show a similar pattern concerning their depth distribution. Nevertheless, we have detected differences between the horizons of oxidation (Bl) and reduction (Br) in soil profile 2. There, the minima REE concentrations were determined in the oxidation-horizons (Bl 2 and Bl 3) and a higher concentration in a horizon showing both, oxidative and reductive properties (Brsl) and thus, a high percentage of area with iron–manganese-compounds. The linkage between REEs and sesquioxides is important since REEs could coordinate as REE3 + or REE(OH)2 + with OH− on the Fe–Mn oxide surface (Fleet, 1984). Accordingly, most of the heavy REEs (HREEs, i.e. REEs having a mean atomic mass above ca. 153 u and an effective ion radius below 95 pm, i.e. Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu (Tyler, 2004)) as well as Y, and Eu (sum of the fractions) show a positive correlation to amorphous Fe- and Mn-oxides (Supplemental material 3). Moreover, the oxidation-horizon is rich in sand compared to the other horizons whereas the Brsl-horizon has less sand and is rich in silt (Table 1). Silty and clayey soils possess frequently higher REE concentrations than sandy soils (Tyler and Olsson, 2005) because especially clay minerals contain REEs and can act as carriers for them (Laveuf and Cornu, 2009). Thus, we found a positive correlation between REE concentrations (sum of the fractions) as well as clay (Table 3) and silt content (Supplemental material 2). Soil profile 1 demonstrates that fine particles are most important for REEs because the horizon with the highest clay content (Ah 1) exhibits the highest REE concentration/ lowest REE depletion relative to UCC (Table 2), and vice-versa. Thus, the particle-size distribution seems to play a crucial role for the concentrations of REEs in the soil profiles. Cerium is the most abundant REE which might be because the element can occur in + 3 and + 4 valence states (Cao et al., 2001). Cerium(IV) is less mobile than other REEs because sorption of Ce(IV) onto Fe/Mn-(hydr)oxides is stronger (oxidative scavenging process of Ce(III) by Fe/Mn-oxides which can be microbial catalysed; Moffett, 1990), and this element can accumulate as insoluble cerianite (CeO2) (Braun et al., 1990; Compton et al., 2003; Koppi et al., 1996). Furthermore, Ce can be oxidised by carbonate and Ce(IV) is subsequently

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Table 1 Properties of the two studied soil profiles. Depth

Symbol Symbol of horizonb Particle-size of horizona distribution [%]b sand

silt

pH Corg [CaCl2]

Nt

PDL

S

Al

[%]

[%]

[g kg−1] [mg 100 g−1]

8.26 7.80 7.09 8.13 7.19 3.68 1.50 0.90 1.12 5.02 5.15 4.32 4.74 3.72 2.15 2.41 1.01 0.62 0.73

0.45 0.31 0.25 0.31 0.26 0.13 0.08 0.06 0.07 0.32 0.29 0.18 0.19 0.16 0.11 0.13 0.07 0.05 0.06

10.6 12.5 10.4 7.1 2.0 0.6 0.6 1.8 0.6 10.1 10.9 8.4 5.9 2.9 1.9 2.1 0.6 0.6 1.9

Fe

Mn

Feo

Fed

Mno Mnd CECeff

clay

2–0.063 63–2 b2 [cm] Soil profile 1 0–10 10–32 32–41 41–60 60–76 76–104 104–140 140–155 155–170 Soil profile 2 0–10 10–28 28–58 58–68 68–90 90–100 100–118 118–140 140–170 170–200

Ah 1 Ah 2 Ah-Bg 1 Ah 3 Ah-Bg 2 C1 C2 Cq C3 Ah 1 Ah 2 Ah-Bl 1 Ah-Bl 2 Bl 1 Bl 2 Bl 3 Bhrl Brsl Br

aoAh 1 aoAh 2 aGw+oAh aorAh aGw+M+rAh aM 1 aM 2 ailC aM 3 aoAh 1 aoAh 2 aGo-roAh-M 1 aGo-roAh-M 2 aGo-M aGo 1 aGo 2 aGhro aGrso aGr

[mm]

[μm]

54 56 61 50 50 50 66 92 63 25 33 9 23 24 26 42 23 8 22

35 36 31 42 45 47 29 6 30 66 58 83 68 67 65 47 69 80 66

11 8 8 8 5 3 5 2 7 9 9 8 9 9 9 11 8 12 12

5.21 6.39 6.64 6.74 6.85 6.72 6.69 6.67 6.63 6.17 6.10 6.35 6.62 6.74 6.71 6.73 6.66 6.62 6.50

0.67 0.57 0.46 0.46 0.33 0.12 0.07 0.03 0.06 0.46 0.41 0.29 0.31 0.26 0.15 0.20 0.08 0.06 0.47

11.22 11.21 11.21 11.21 11.21 11.94 11.93 11.93 11.93 11.94 11.94 11.94 11.94 11.93 12.29 12.08 12.11 12.14 12.16

[cmol+ kg−1]

[%] 14.82 14.82 14.82 14.82 14.82 15.75 15.75 15.75 15.75 15.33 15.32 15.32 15.32 15.32 15.87 15.85 15.86 15.87 15.85

1.04 0.68 0.58 0.64 0.75 0.92 0.76 1.01 0.80 0.72 0.72 0.58 0.60 0.57 0.45 0.56 0.49 0.49 0.55

1.38 2.00 1.51 1.64 0.91 0.38 0.29 0.13 0.32 0.64 0.84 0.96 1.06 0.86 0.45 0.41 0.35 0.31 0.45

2.62 3.29 2.76 3.10 2.35 1.88 1.58 1.74 1.81 1.61 1.77 2.01 2.12 1.82 1.19 1.14 1.05 1.14 0.80

0.09 0.06 0.05 0.06 0.07 0.08 0.05 0.03 0.05 0.06 0.06 0.05 0.05 0.05 0.04 0.05 0.04 0.04 0.05

0.10 0.08 0.06 0.06 0.07 0.07 0.03 0.06 0.07 0.07 0.07 0.06 0.05 0.05 0.04 0.05 0.04 0.04 0.02

13.0 14.4 13.3 16.6 15.1 10.3 8.2 3.4 8.0 16.3 13.4 10.9 11.6 10.6 9.2 11.3 6.5 6.1 5.5

PDL: Double lactate-soluble P; Fed: Fe, dithionite-extractable; Feo: Fe, oxalate-extractable; Mnd: Mn, dithionite-extractable; Mno: Mn, oxalate-extractable; and CECeff: effective CEC. a IUSS Working Group WRB (Ed.) (2006). b German Soil Classification System (Ad-hoc Arbeitsgemeinschaft Boden, 2005).

preferentially adsorbed onto humic acids (Pourret et al., 2008). However, these processes can lead to a preferential removal of Ce, and might explain the very slightly negative Ce anomaly in both soil profiles (Table 2). Lutetium is the rarest REE. Generally, the concentrations of REEs tend to decrease with increasing atomic number, whereas REEs with even atomic numbers are more frequent than their neighbours with odd atomic numbers, according to the Oddo–Harkins rule (Laveuf and Cornu, 2009): Ce N Nd/La N Y N Pr N Sm N Gd N Dy N Er N Sc N Yb N Eu N Tb N Ho N Tm N Lu. The order of magnitude of the REE concentrations is within the range for the earth's crust as described by Taylor and McLennan (1985). All studied REEs are depleted in comparison to the UCC, especially Sc, Y, Ho, Er, Tm, Yb, and Lu (Table 2). This might be due to leaching during flooding or high groundwater levels. Most HREEs are more depleted than light REEs (LREEs, i.e. REEs having a mean atomic mass lower than ca. 153 u and an effective ion radius above 95 pm, i.e. La, Ce, Pr, Nd, Sm, and Eu (Tyler, 2004)) relative to UCC. Heavy REEs form easier soluble complexes (Braun et al., 1990) and therefore, they are more leachable than LREEs, which rather occur as free species (Cantrell and Byrne, 1987). Eighty-five percent of the sum of REEs extracted with the sequential extraction procedure belong to the group of the LREEs (data not shown). The enrichment of LREEs compared to HREEs is in agreement with the typical pattern of continental crust. The LaN/YbN-ratio expresses the fractionation between LREEs and HREEs. This ratio indicates a gentle slope of the REE pattern with values varying from 1.7 to 2.5, tending to be lower in the upper than in the deeper horizon (Table 2). Negative Eu anomalies occur in both profiles (Table 2) and may be attributed to temporary reducing conditions and to the associated presence of Eu(II). The negative Eu anomalies are stronger in soil profile 2 than in soil profile 1 since this soil seems to be wet for longer periods (field observations). 3.2. Proportion and vertical distribution of the geochemical fractions of REEs The geochemical fractions of the REEs differ slightly within each soil profile (Figs. 4 and 5). The majority of the REEs (three-quarter) have

been found in the residual fraction (F 4). One-fifth is basically bound to Fe–Mn oxides (reducible fraction; F 2), one fifteenth to organic matter and sulphides (oxidisable fraction; F 3), and a very small part is water soluble, exchangeable (F 1). Davranche et al. (2011) reported similar results in wetland soils. However, Šmuc et al. (2012) have determined a smaller F 4 than F 3. Those differences seem to be due to the parent material and its properties such as pH, clay content, carbonate, and organic matter (Tyler, 2004). The low percentage of REEs bound to F 1 demonstrates that only a few REEs are highly mobile, which is important for plant-uptake (Šmuc et al., 2012). The REEs bound to F 2 and F 3 can become available under reducing (F 2) or oxidising (F 3) conditions. In wetland soils, the redox conditions can change drastically which in turn can affect the geochemical fractions of REEs. For instance, the REEs bound to Fe–Mn oxides and organic matter/ sulphides can be mobilised. In contrast, F 4 is hard to release and therefore regarded as non-available (Lijun et al., 1998). Consequently, a high percentage of REEs is unlikely to be released under the predominant environmental conditions. Therefore, the potential risk of transfer of REEs into groundwater, adjacent surface water, and plants seems to be relatively small at this site. The differences of the geochemical fractions are in average small between the two soil profiles. However, in soil profile 1, F 4 increases with increasing depth, whereas the other three fractions (especially F 2) decrease (Fig. 4). This might be partly attributed to the fact that the deeper, older sediments have had more time to build up stable binding forms. In addition, the REEs in the more mobile fractions may be dissolved and leached during high groundwater level and the stronger bounded REEs remain in the soil. Soil wetness and/or flooding cause reducing conditions in the subsoil and thus, there might exist less amorphous Fe (hydr-)oxides which are able to bind the REEs in reducible form. In consequence, this could be an explanation for the decreasing of F 2 with increasing depth in soil profile 1. However, in soil profile 2, the vertical distribution seems to be similar between the horizons (Fig. 5) because this soil seems to be wet for longer periods in comparison to soil profile 1 (field observations). While the oxalateextractable Fe concentrations in profile 1 are lower in the subsoil than in the upper horizons this seems not that obviously in profile 2

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Fig. 2. Vertical distribution of REEs in soil profile 1.

(Table 1). Thus, the distribution of the fractions is relatively homogeneous within soil profile 2. However, the sequential extraction procedure is not able to capture small differences certainly.

3.3. Relations of the geochemical fractions of REEs to soil properties In wetland soils, the mobility of an element is controlled by factors such as clay minerals, pH, organic material, cation exchange capacity, and Fe/Mn-(hydr)oxides (Du Laing et al., 2009b). A periodic change

between reducing and oxidising conditions often occurs. Therefore, we focus on the relations of REEs in F2 and F3 to soil properties (Table 3). 3.3.1. Clay content Clay content reveals a modest to strong positive correlation to the reducible form of REEs (except Sc) (Table 3). This might be because amorphous Fe–Mn (hydr)oxides have formed coatings on clay minerals which might contribute to an increasing sorption of REEs on clay minerals. In consequence, the clay rich horizons reveal relatively high

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165

Fig. 3. Vertical distribution of REEs in soil profile 2.

concentrations of most REEs in F 2. Scandium is the only element in F 2 which correlates very strong negative with clay. However, we could not determine a significant correlation between clay and REEs of F 3. 3.3.2. Soil reaction (pH value) We found modest to strong negative correlations between the pH value and most REEs in F 2 and F 3 (except Eu in F 2, and Sc) (Table 3) since a low pH favours the mobility of REEs. The surface of soil particles becomes more negatively charged with increasing pH and this

facilitates the adsorption of REEs because dissolved REE ions can easily form complexes with negatively charged groups (Zhu et al., 1993). Especially F 3 reacts to changing pH. Sulphides are more soluble at low pH and the negative surface charge of organic matter decreases with decreasing pH (Du Laing et al., 2009b). Additionally, there exists a higher concentration of REEs in F 2 at low pH. Amorphous Fe/Mn(oxyhydr)oxides become dissolved with lowering pH and cause a release of REEs (Cao et al., 2001). Cao et al. (2001) detected a change of the La, Ce, Gd, and Y species with altering pH. The REEs in the first

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Table 2 Sum of REE concentrations of the fractions of both soil profiles relative to the concentration of the upper continental crust. LREE Symbol of horizona ScN Soil profile 1 Ah 1 Ah 2 Ah-Bg 1 Ah 3 Ah-Bg 2 C1 C2 Cq C3 mean Soil profile 2 Ah 1 Ah 2 Ah-Bl 1 Ah-Bl 2 Bl 1 Bl 2 Bl 3 Bhrl Brsl Br Mean Mean of both profiles

0.07 0.11 0.12 0.13 0.07 0.05 0.00 0.00 0.00 0.06 0.02 0.00 0.00 0.12 0.12 0.11 0.09 0.10 0.11 0.09 0.08 0.07

HREE

YN

LaN

CeN

PrN

NdN

SmN EuN

GdN

TbN

DyN

HoN

ErN

TmN YbN

LuN

Total REEsN LaN/YbN Ce/Ce* Eu/Eu*

0.43 0.42 0.36 0.39 0.35 0.32 0.29 0.30 0.31 0.35 0.40 0.41 0.38 0.38 0.35 0.32 0.31 0.33 0.35 0.34 0.36 0.35

0.65 0.60 0.55 0.57 0.60 0.61 0.56 0.48 0.60 0.58 0.66 0.68 0.67 0.65 0.63 0.59 0.55 0.66 0.68 0.68 0.65 0.62

0.64 0.62 0.56 0.58 0.61 0.62 0.58 0.51 0.62 0.59 0.67 0.68 0.67 0.65 0.64 0.60 0.58 0.68 0.70 0.69 0.66 0.63

0.69 0.66 0.61 0.62 0.64 0.67 0.63 0.56 0.66 0.64 0.72 0.72 0.72 0.69 0.67 0.62 0.60 0.72 0.75 0.74 0.69 0.67

0.74 0.71 0.65 0.66 0.68 0.71 0.67 0.61 0.70 0.68 0.74 0.76 0.76 0.73 0.71 0.67 0.64 0.77 0.80 0.76 0.73 0.71

0.81 0.79 0.74 0.74 0.78 0.79 0.76 0.72 0.79 0.77 0.85 0.85 0.83 0.81 0.78 0.74 0.72 0.84 0.88 0.85 0.81 0.79

0.83 0.83 0.73 0.78 0.78 0.74 0.69 0.68 0.74 0.76 0.82 0.82 0.81 0.80 0.75 0.70 0.67 0.75 0.81 0.77 0.77 0.76

0.67 0.67 0.60 0.62 0.59 0.57 0.54 0.53 0.57 0.60 0.65 0.67 0.65 0.63 0.60 0.54 0.54 0.60 0.62 0.63 0.61 0.60

0.57 0.56 0.49 0.52 0.49 0.47 0.42 0.44 0.45 0.49 0.54 0.54 0.53 0.52 0.49 0.44 0.44 0.46 0.49 0.48 0.49 0.49

0.44 0.43 0.38 0.41 0.36 0.34 0.31 0.34 0.33 0.37 0.41 0.42 0.40 0.40 0.37 0.33 0.33 0.35 0.37 0.36 0.37 0.37

0.43 0.44 0.37 0.39 0.37 0.34 0.32 0.34 0.34 0.37 0.41 0.41 0.39 0.40 0.36 0.32 0.33 0.34 0.36 0.36 0.37 0.37

0.39 0.37 0.31 0.35 0.30 0.29 0.26 0.30 0.27 0.32 0.33 0.35 0.32 0.33 0.30 0.28 0.27 0.28 0.31 0.29 0.31 0.31

0.36 0.34 0.29 0.30 0.27 0.27 0.23 0.26 0.25 0.29 0.29 0.32 0.29 0.30 0.28 0.24 0.25 0.27 0.28 0.27 0.28 0.28

0.60 0.58 0.52 0.54 0.55 0.56 0.52 0.46 0.55 0.54 0.60 0.61 0.60 0.59 0.58 0.54 0.51 0.60 0.63 0.61 0.59 0.57

0.78 0.75 0.73 0.73 0.73 0.73 0.69 0.73 0.74 0.73 0.70 0.74 0.72 0.70 0.66 0.59 0.60 0.64 0.70 0.69 0.68 0.70

0.37 0.36 0.30 0.32 0.29 0.27 0.25 0.29 0.27 0.30 0.32 0.33 0.31 0.31 0.29 0.26 0.26 0.27 0.29 0.28 0.29 0.30

1.76 1.70 1.83 1.81 2.06 2.23 2.22 1.69 2.18 1.92 2.07 2.03 2.12 2.10 2.21 2.31 2.15 2.48 2.37 2.38 2.21 2.07

0.96 0.97 0.97 0.98 0.98 0.98 0.97 0.97 0.98 0.97 0.98 0.97 0.97 0.97 0.98 1.00 1.01 0.98 0.98 0.98 0.98 0.98

0.95 0.93 0.99 0.96 0.93 0.96 0.95 1.04 0.96 0.96 0.85 0.88 0.88 0.87 0.86 0.83 0.86 0.81 0.83 0.85 0.85 0.90

LREE: REEs having a mean atomic mass lower than ca. 153 u and an effective ion radius above 95 pm; HREE: REEs having a mean atomic mass above ca. 153 u and an effective ion radius below 95 pm (Tyler, 2004). The subscript N in the text indicates to the upper continental crust (Taylor and McLennan, 1985) normalised values. Ce/Ce* = CeN / (LaN ∗ PrN)0.5; Eu / Eu* = EuN / (SmN ∗ GdN)0.5. a IUSS Working Group WRB (Ed.) (2006).

three fractions migrate into the aqueous phase with decreasing pH and vice versa, the REEs in F 4 increase with increasing pH. We found relatively high percentages of REEs in F 2 and F 3 in the horizons with the lowest pH values, however, not vice versa. Obviously other factors such as Corg and CECeff might play an important role here. The sorption of REEs in soils with low ionic strength primarily depends on cation exchange rather than on pH (Coppin et al., 2002). However, the available REE concentrations are generally higher in acid than in calcareous soils (Hu et al., 2006). 3.3.3. Content of soil organic carbon and cation exchange capacity Total soil organic carbon (Corg) reveals a very strong positive correlation with REEs (except Sc) in F 3 and a modest to strong positive correlation with REEs (except Sc) in F 2 (Table 3). One reason for this is that REEs can form chelate with Corg or organic sulphide. Those dissolve under strong oxidising conditions and release the REEs (Lijun et al., 1998). According to this, we calculated strong to very strong positive correlations between REEs (except Sc) in F 3 and cation exchange capacity (CEC) (Supplemental material 2) which seem to be due to those adsorption-places which result from organic matter. Furthermore, Fe and Al can form complexes with Corg and sulphides (FeS) what is indicated by the close relation to F 2. We observed the maximum of REEs bound to F 3 and F 2 in the horizons with the highest content of Corg and vice versa which is confirmed via the correlations. 3.3.4. Content of phosphorus The PDL content and REEs (except Sc) in F 2 and F 3 correlate strong to very strong positive (Table 3) which may result from the strong affinity of REEs to phosphated compounds and therefore, phosphates naturally contain significant amount of REEs (Tyler, 2004). Rare earth element phosphates have a very low solubility which leads to low leaching and high accumulation of REEs in soils (Zhang et al., 2006). Furthermore, (amorphous) Al- and Fe-(hydr)oxides can adsorb phosphate-ions which can form complexes with organic matter (Scheffer and Schachtschabel, 2010). This might explain the positive correlation between PDL content as well as REEs in F 2, and F 3.

Accordingly, we determined relatively high concentrations of REEs in these fractions in the horizons with a high PDL content, and vice versa.

3.3.5. Content of oxalate-extractable iron and manganese Oxalate-extractable Fe correlates strongly to very strongly positive with the REEs (except Sc) in F 2 and F 3 (Table 3). Oxalate-extractable Mn is especially for REEs (except Sc) in F 3 important (strong to very strong positive correlation). Yttrium, La, Gd, Ho, Er, Tm, and Lu in F 2 correlate moderately positive with Mno. Rare earth elements might be released when amorphous Fe–Mn (oxyhydr)oxides dissolve under reducing conditions what is confirmed by the correlations coefficients. It seems that mainly the iron rich parts of the oxides are able to bind the REEs instead of the manganese-oxides because the correlation is closer between the reducible form of REEs and Feo than to Mno. Metal–organic complexes and metal sulphides seem to lead to strong correlations between REEs (except Sc) in oxidisable form and Feo/Mno. We observed a strong correlation between Feo and REEs. This might indicate that the younger, amorphous Fe is relevant for retaining the REEs. Yan et al. (1999) found lower REE concentrations in crystalline Fe-oxides compared to amorphous ones. This may result from the restricted lattice incorporation of REEs in crystalline Fe-oxides due to differences in ion radius between Fe/Mn and REEs (Braun et al., 1993). Šmuc et al. (2012) found that F 2 is linked with amorphous Fe–Mn hydroxides. This is may be due to their irregular structure which might cause the REEs to be more easily embedded as well as released compared to the regular structure of the older Fe–Mn oxides. In various cases, we found relatively high percentages of REEs bound in F 2, and F 3 in the horizons with relatively high concentration of Feo, and Mno, and vice versa. Scandium reacts differently to the soil properties than Y, and the lanthanoides, e.g. we could not find a statistical relation between Sc and pH, Corg, and PDL. This may be explained by the fact that Scandium is no lanthanoide and therefore, it reveals different properties such as lower mean atomic mass, smaller effective ion radius, which leads to different geochemical reactions and a different distribution of the geochemical fractions in the two soil profiles.

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Fig. 4. A: Geochemical fractions of REEs and their vertical distribution in soil profile 1. B: Geochemical fractions of REEs and their vertical distribution in soil profile 1.

167

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Fig. 4 (continued).

J. Mihajlovic et al. / Geoderma 228–229 (2014) 160–172

Fig. 5. A: Geochemical fractions of REEs and their vertical distribution in soil profile 2. B: Geochemical fractions of REEs and their vertical distribution in soil profile 2.

169

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Fig. 5 (continued).

J. Mihajlovic et al. / Geoderma 228–229 (2014) 160–172

171

Table 3 Correlation coefficients (Pearson) between REEs as well as clay, pH, Corg, PDL, Feo, and Mn.

LREE

HREE

Sc Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sum REEs

Clay

pH

F2

F2

F3

F2

F3

F2

F3

F2

F3

F2

F3

−0.968⁎ 0.545⁎ 0.485⁎ 0.612⁎⁎ 0.599⁎⁎ 0.611⁎⁎ 0.662⁎⁎ 0.594⁎⁎ 0.508⁎ 0.584⁎⁎ 0.599⁎⁎ 0.597⁎⁎ 0.592⁎⁎ 0.591⁎⁎ 0.607⁎⁎ 0.636⁎⁎ 0.573⁎

n.s. −0.571⁎ −0.686⁎⁎ −0.616⁎⁎ −0.626⁎⁎ −0.585⁎⁎ −0.494⁎

n.s. −0.702⁎⁎ −0.728⁎⁎ −0.665⁎⁎ −0.698⁎⁎ −0.702⁎⁎ −0.654⁎⁎ −0.606⁎⁎ −0.650⁎⁎ −0.696⁎⁎ −0.671⁎⁎ −0.699⁎⁎ −0.698⁎⁎ −0.690⁎⁎ −0.759⁎⁎ −0.694⁎⁎ −0.627⁎⁎

n.s. 0.762⁎⁎ 0.786⁎⁎ 0.663⁎⁎ 0.678⁎⁎ 0.642⁎⁎ 0.498⁎ 0.496⁎ 0.713⁎⁎ 0.652⁎⁎ 0.680⁎⁎ 0.700⁎⁎ 0.699⁎⁎ 0.701⁎⁎ 0.700⁎⁎ 0.696⁎⁎ 0.719⁎⁎

n.s. 0.865⁎⁎ 0.890⁎⁎ 0.896⁎⁎ 0.890⁎⁎ 0.873⁎⁎ 0.873⁎⁎ 0.881⁎⁎ 0.891⁎⁎ 0.865⁎⁎ 0.882⁎⁎ 0.857⁎⁎ 0.831⁎⁎ 0.834⁎⁎ 0.820⁎⁎ 0.812⁎⁎ 0.889⁎⁎

n.s. 0.832⁎⁎ 0.887⁎⁎ 0.815⁎⁎ 0.840⁎⁎ 0.818⁎⁎ 0.664⁎⁎ 0.631⁎⁎ 0.806⁎⁎ 0.788⁎⁎ 0.788⁎⁎ 0.784⁎⁎ 0.795⁎⁎ 0.778⁎⁎ 0.801⁎⁎ 0.744⁎⁎ 0.846⁎⁎

n.s. 0.811⁎⁎ 0.875⁎⁎ 0.836⁎⁎ 0.841⁎⁎ 0.832⁎⁎ 0.786⁎⁎ 0.709⁎⁎ 0.788⁎⁎ 0.775⁎⁎ 0.780⁎⁎ 0.778⁎⁎ 0.805⁎⁎ 0.761⁎⁎ 0.805⁎⁎ 0.752⁎⁎ 0.782⁎⁎

n.s. 0.810⁎⁎ 0.811⁎⁎ 0.736⁎⁎ 0.766⁎⁎ 0.744⁎⁎ 0.667⁎⁎ 0.670⁎⁎ 0.819⁎⁎ 0.779⁎⁎ 0.785⁎⁎ 0.781⁎⁎ 0.788⁎⁎ 0.795⁎⁎ 0.794⁎⁎ 0.783⁎⁎ 0.791⁎⁎

n.s. 0.726⁎⁎ 0.804⁎⁎ 0.797⁎⁎ 0.775⁎⁎ 0.742⁎⁎ 0.715⁎⁎ 0.690⁎⁎ 0.735⁎⁎ 0.710⁎⁎ 0.723⁎⁎ 0.726⁎⁎ 0.705⁎⁎ 0.713⁎⁎ 0.697⁎⁎ 0.704⁎⁎ 0.805⁎⁎

n.s. 0.530⁎ 0.532⁎ n.s. n.s. n.s. n.s. n.s. 0.500⁎ n.s. n.s. 0.493⁎ 0.473⁎ 0.497⁎

n.s. 0.762⁎⁎ 0.694⁎⁎ 0.718⁎⁎ 0.736⁎⁎ 0.748⁎⁎ 0.789⁎⁎ 0.828⁎⁎ 0.784⁎⁎ 0.793⁎⁎ 0.790⁎⁎ 0.762⁎⁎ 0.763⁎⁎ 0.754⁎⁎ 0.756⁎⁎ 0.756⁎⁎ 0.696⁎⁎

n.s. −0.559⁎ −0.541⁎ −0.537⁎ −0.560⁎ −0.537⁎ −0.552⁎ −0.560⁎ −0.544⁎ −0.609⁎⁎

Corg

PDL

Feo

Mno

n.s. 0.471⁎ 0.463⁎

F 2: reducible fraction; F 3: oxidisable fraction; Pearson's correlation coefficient r. LREE: REEs having a mean atomic mass lower than ca. 153 u and an effective ion radius above 95 pm; HREE: REEs having a mean atomic mass above ca. 153 u and an effective ion radius below 95 pm (Tyler, 2004); Corg: organic carbon; CECeff: effective cation exchange capacity; PDL: Double lactate-soluble P; Feo: iron, oxalate-extractable; and Mno: manganese, oxalateextractable. n.s.: not significant. Number of data: n = 19 (except Sc F2, where n = 4 and Sc F3, where n = 14). ⁎p b 0.05; ⁎⁎p b0.01; significant correlations are given in bold.

The percentage of each fraction on the sum of REE concentrations differs for the two groups of REEs (Figs. 4 and 5). The proportion of the residual fraction decreases from LREEs to HREEs in both soil profiles. Light REEs seem to form faster stable bindings and thus might be more easily bound to F 4. Heavy REEs have a higher percentage of the other three fractions. A possible explanation could be that HREEs can easily form soluble complexes e.g. with Fe/Mn-oxides, and organic matter, and tend to be more mobile than LREEs (Braun et al., 1990). They might take earlier the bonding-places of the first three fractions than LREEs and therefore, rather LREEs than HREEs migrate into F 4 or occur as free species. Within this context Sonke and Salters (2006) mentioned that organic matter complexes with HREEs are more stable than with LREEs. Bau (1999) noted that rather HREEs than LREEs are sorbed onto Fe oxy-hydroxides. We found that mainly HREEs reveal a close positive relation with Corg, and amorphous Fe–Mn oxides (sum of the fractions; see Supplemental material 2 and 3) which leads to the assumption that HREEs form rather complexes with Corg, and amorphous Fe–Mn oxides than LREEs. Heavy REEs seem to have a higher potential availability than LREEs and might be preferentially leached due to their lower proportion of residual fraction compared to LREEs. On the other hand Taylor and McLennan (1985) mentioned that as LREEs have a larger effective ion radius than HREEs they can tend to be less incorporated in silicate mineral structures. However, aqua regia cannot digest REEs which are embedded in the lattice of silicates. Summarizing, each single REE reveals an element-specific geochemical fractionation in the studied floodplain soil profiles.

4. Conclusions The order of magnitude of the REE concentrations in the studied soils is within the range of the typical pattern of the earth's crust. The characteristic processes of Fluvisols, the frequent flooding and the linked homogenisation might have significantly contributed to the relatively homogeneous vertical distribution of the REE concentrations within the soil profiles. Rare earth elements can easily form complexes with or adsorb on clay minerals, organic matter, phosphate, and Fe–Mn oxides. The pH has a significant relation to the mobility of REEs; the mobility tends to increase with decreasing pH. The water soluble, exchangeable fraction was very low and the residual fraction relatively

high in the studied soils what may indicate that only a few REEs are available. Heavy REEs seem to take earlier the bonding-places in the more available fractions than LREEs and therefore, rather LREEs than HREEs are bound to residual fraction. Likewise, the fate of REEs is affected by the water content because the first three fractions may be dissolved and transported with water, and only the stronger bounded REEs remain in the soil profile. In future, the impact of flooding regime and physico-chemical soil properties on the concentrations, geochemical fractions, and release kinetics of REEs should be elucidated to improve our understanding of the geochemical behaviour of REEs in paddy soils around the globe. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.geoderma.2013.12.009. Acknowledgements We thank Mrs. T. Frohne, Mr. C. Vandenhirtz and Mrs. U. Winkler for their assistance. The first author thanks the University of Wuppertal, Germany, for providing the doctoral scholarship. References Ad-hoc Arbeitsgruppe Boden, 2005. In: Bundesanstalt für Geowissenschaften und Rohstoffe und Staatliche Geologische Dienste der BR Deutschland (Ed.), Bodenkundliche Kartieranleitung, 5th ed. (Stuttgart). Bau, M., 1999. Scavenging of dissolved yttrium and rare earths by precipitating iron oxyhydroxide: experimental evidence for Ce oxidation, Y-Ho fractionation, and lanthanide tetrad effect. Geochim. Cosmochim. Acta 63 (1), 67–77. Bau, M., Dulski, P., 1996. Anthropogenic origin of positive gadolinium anomalies in river waters. Earth Planet. Sci. Lett. 143, 245–255. BBodSchV, 1999. Federal Soil Protection and Contaminated Sites Ordinance. Blume, H.-P., Stahr, K., Leinweber, P., 2011. Bodenkundliches Praktikum. Berlin. Braun, J.-J., Pagel, M., Muller, J.-P., Bilong, P., Michard, A., Guillet, B., 1990. Cerium anomalies in lateritic profiles. Geochim. Cosmochim. Acta 54, 781–795. Braun, J.-J., Pagel, M., Herbillon, A., Rosin, C., 1993. Mobilization and redistribution of REEs and thorium in a syenitic lateritic profile — a mass-balance study. Geochim. Cosmochim. Acta 57 (18), 4419–4434. Brosius, F., 2002. SPSS 11. mitp-Verlag, Bonn. Cantrell, K.J., Byrne, R.H., 1987. Rare earth element complexation by carbonate and oxalate ions. Geochim. Cosmochim. Acta 51 (3), 597–605. Cao, X., Chen, Y., Wang, X., Deng, X., 2001. Effects of redox potential and pH value on the release of rare earth elements from soil. Chemosphere 44, 655–661. Compton, J.S., White, R.A., Smith, M., 2003. Rare earth element behaviour in soils and salt pan sediments of a semi-arid granitic terrain in the Western Cape, South Africa. Chem. Geol. 201, 239–255.

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