A Detailed Insight into the Detrital and Diagenetic Mineralogy of Metal(oid)s: Their Origin, Distribution and Associations within Hypersaline Sediments

Abstract

Hypersaline environments are among the most vulnerable coastal ecosystems and are extremely noteworthy for a variety of ecological reasons. Comprehensive assessment of metal(oid) contamination in hypersaline sediments from Sečovlje (Northern Adriatic, Slovenia) was addressed by introducing the detrital and diagenetic mineralogy and geochemical properties within the solid sediment material. Close associations between Fe/Mn oxides and oxyhydroxides with As, Cr, Ni, Pb and Zn, and between organic matter with Cu, Pb and Zn were confirmed using X-ray powder diffraction, SEM-EDS and ICP emission spectrometry analysis. Possible incorporation or adsorption on the crystal lattices of clay minerals (As, Cr, Pb, Sn and Zn), halite (As) and aragonite/calcite (Cd, Cu, Pb, Sr and Zn) were also detected. All presented correlations were highlighted by various statistical analyses. The enrichment factor (EF) values showed a low degree of anthropogenic burden for As, Bi, Hg and Zn, while Cd, Cr, Cu, Ni, Pb, Sn and Sr originated from the geological background. These results emphasise that a detailed mineralogical and geochemical characterisation of solid (especially detrital and diagenetic) sediment particles is crucial in further understanding the metal(oid) translocation within the hypersaline ecosystems.

1. Introduction

Metal(oid)s are pervasive in all earthly environments, some (e.g., Fe, Ni, Cu and Zn) are essential for plant growth and human and animal health. However, some become highly toxic to the surrounding ecosystems when present in excessive amounts [1,2,3]. Metal(oid)s can enter into various surface and subsurface environments from both natural (via dissolution of metal(oid)-bearing minerals) and anthropogenic sources (waste materials, dredged materials, biosolids, fly ash, and atmospheric deposits) [1,4,5,6]. Consequently, they are marked as serious ecological pollutants due to their persistence and non-degradability in the environment [1,2,7,8,9,10,11,12].

By connecting land and sea, salt marshes (hypersaline environments) are among the most widespread, productive and vulnerable coastal ecosystems, providing a wide range of benefits to coastal populations and most ecological services such as: shoreline protection, fisheries support, water quality enhancement, maintenance of healthy marine ecosystems, habitat provision, wildlife conservation, key sink/source of organic material, nutrients and carbon sequestration [13,14,15]. Unfortunately, salt marshes also serve as repositories for pollutants from terrestrial runoff, e.g., nutrients, pesticides, halogenated hydrocarbons and metal(oid)s [14,16,17,18,19,20,21]. Salt marsh sediments are therefore highly enriched in metal(oid)s due to salinity, tides and occasional flooding [22,23,24,25]. They represent not only a sink, but also a source of metal(oid)s for all ecosystems (water, plants, animals and also humans) surrounding the salt marshes [16,17,20,23,24,25].

Depending on the chemical and geological conditions in the sediments, different forms of metal(oid)s are associated with a variety of inorganic and organic solid phases and dissolved in the pore water of the sediment [1,4,26,27]. In general, metal(oid)s are mainly concentrated in the fine-grained size of the solid phases [1,4,28,29]. Iron and manganese oxides/oxyhydroxides [30,31,32,33,34] and metal sulphides [31,34,35] are well-documented scavengers of metal(oid)s. Clay minerals are also one of the major carriers of metal(oid)s as they have enhanced sorption capacity especially during mobilization and diffusion processes [36]. Metal(oid)s can be bound to various forms of organic matter: living organisms, detritus and coatings on mineral particles, etc. [27,34,37].

In addition, early diagenetic release of metal(oid)s from sediments has been documented in studies of shallow coastal systems [38,39,40,41,42,43]. The diagenesis process, can cause solutes to become mineral saturated, resulting in the precipitation of authigenic sulphide, carbonate and phosphate minerals. These precipitates can sorb or co-precipitate metal(oid)s, and thus act as long-term sinks for pollutants in sediments. For example, Parkman et al. [43] and Pirrie et al. [44] demonstrated that early diagenetic sulphides act as sinks for Cu, Pb and Zn.

There are few studies addressing the mineralogy and chemistry of sedimental (detrital) grains and diagenetic mineral precipitates in coastal/salt marsh and—hypersaline environments [45,46]. The integrated mineral and chemical approach presented is necessary to identify the host species of metal(oid) minerals and the conditions that control the release of metal(oid)s into the system.

Understanding metal(oid) mineralogy in sediments is critical to predict and define metal(oid) translocation paths in target ecosystems. In terms of bioavailability (a prerequisite for translocation processes), a thorough understanding of metal(oid) associations in sediment solid components is necessary—whether they are bound to mineral constituents (and which mineral constituents) or organic matter. This is the first step to a comprehensive understanding of bioavailability and the transfer of metal(oid) to the system. Therefore, the following research objectives are pursued:

• Determine detailed mineralogy (at the submicron level) and the chemical composition of hypersaline sediment particles.
• Characterisation of metal(oid)-bearing minerals and their origin (geogenic and/or anthropogenic sources).
• To assess the metal(oid) distribution in the hypersaline system.

2. Materials and Methods

The Sečovlje Salina Nature Park (Figure 1) is located in the southwestern part of the Slovenian Adriatic coast, on the border with Croatia. It is the largest (an area of about 750 hectares) of the Slovenian coastal marsh wetlands and represents an ecologically unique environment with diverse ecosystems, including transitional forms between marine, brackish, freshwater and terrestrial ecosystems. These extreme environments of high salinity and aridity provide shelter to rare animal and plant species. Thus, salt-resistant or salt-tolerant plants (halophytes), also thrive in this saline environment. The area was designated a nature park in 1990. In 1993, it became the first Slovenian wetland to be included in the list of internationally important marshes under the auspices of the Ramsar Convention [47,48]. The Sečovlje salt-pans are among the few still active salt-pans in the Mediterranean and the first records of their operation date back to the 12th century. Since 2001, the Sečovlje salt-pans have been protected as a Nature Park by a special decree of the government of the Republic of Slovenia [47,48].

The Sečovlje Salina Park is located at the mouth of the Dragonja River into the Bay of Piran (Gulf of Trieste, northern Adriatic Sea). The catchment area of the Dragonja River is about 96 km² large and composed predominantly of Eocene flysch deposits characterised by a thin to medium bedded siliciclastic and carbonate-siliciclastic turbidite sandstones, marls, and meter-thick beds (megabeds) of calciturbidites. Cretaceous and Paleogene limestones are only found locally, near the outflow of the Dragonja River into the sea. Sediments in the Sečovlje Salina area originate from the geological hinterland of the Dragonja River [23,49,50,51].

The catchment area is basically a much closed geological area mainly towards the inland and thus we cannot find any anthropogenic sources from inland direction. The only possible pollution path from the inland could be through the air particles deposition—like this we can justify the presence of cinnabar or mercury particles originated from Idrija historical mine, which is almost 70 km by air from the Sečovlje Salina. Cinnabar or mercury small particles (e.g., μm-nm size) emitted into the air can travel thousands of miles in the atmosphere before they settle into the ground.

Conversely, we could trace the pollution from the sea side much more clearly. There are two large ports near the Sečovlje Salina: (1) Koper at the air distance of 14 km with cargo traffic and (2) Trieste at the air distance of 23 km with cargo and passenger traffic. On the other side of the Gulf of Trieste bay at the air distance of 28 km, is the mouth of the Soča/Isonzo River, which transports/transported different geogenic and anthropogenic material from inland, including the material from now long closed Idrija Hg mine and several small Cu deposits. However, the inflow of presented material into Sečovlje Salina area is strongly dependent on the currents in the Gulf of Trieste bay. In general, the currents are clock-wise (up along the Adriatic coast and down along the Italian coast), but it could be more complex, meaning that the currents could also bring the dispersed geogenic and anthropogenic material from Soča/Isonzo River, Trieste and Koper port.

Within the research area there are also many vineyards and orchards where CuSO₄ was used as a pesticide spray.

In April 2020, samples of underlying sediment, its corresponding rhizo-sediment and upper Salvia fruticosa plant were taken in the wider area of Sečovlje Salina (Figure 2,

Table 1. Sampling positions in the investigated area.

Sampling Location	Latitude	Longitude
1	45°29′52.67″	13°35′19.70″
2	45°29′44.73″	13°35′22.24″
3	45°29′46.01″	13°35′30.44″
4	45°29′17.91″	13°35′54.26″
5	45°29′13.60″	13°35′56.19″
6	45°29′16.59″	13°36′08.92″

); (1–3) from the Piccia sampling site, where seawater evaporates, (4 and 5) from the crystallisation basin and (6) from the Poslužnica sampling site, which is a reservoir or collector of concentrated brine in the crystallisation area. At the time of sampling, the sampling sites were not covered with water (marine or brine water). Sediment and rhizo-sediment samples were collected for this study in order to comprehensively identify the mineralogical and geochemical signature and metal(oid)-bearing minerals in the samples studied. A representative sample of underlying sediment and a representative sample of rhizo-sediment for each sampling site marked in Figure 2 consisted of 3 subsamples of underlying sediments and 3 subsamples of rhizo-sediment.

The underlying sediments were collected using plastic corers (a 15 cm long tube with an internal diameter of 10 cm) and packed in pre-cleaned plastic bags. The sediments were macroscopically homogeneous and bioturbated with a lighter grey colour due to oxidation. Samples of S. fruticosa plant and rhizo-sediment attached to the plant roots were also packed in pre-cleaned plastic bags. All samples were transported to the laboratory, where they were prepared for further analyses according to analytical protocols.

Underlying sediment (further expression used in the text: sediments (S)) samples were dried at 50 °C for 48 h. Ten grams of each sample was set aside for particle size analysis, and the remainder was sieved through a 0.315 mm polyethylene sieve to remove large organic and mollusk debris. For subsequent mineralogical and geochemical analysis, the samples (<0.315 mm) were quartered, milled and homogenised to a fine powder (<63 μm) using an agate mortar. For scanning electron microscopy energy dispersive spectrometer (SEM-EDS) analysis a portion of each quartered sample (<0.315 mm) was used to produce polished sections. The rhizo-sediment adhering to the plant roots was carefully washed with distilled water in a plastic container and left until all the particles had settled. The water was then removed and the samples were dried at 50 °C for 48 h. Further preparation of the rhizo-sediment samples was identical to that of the sediment samples. Two representative samples (15 g of sediment and rhizo-sediment) for the preparation of the clay fraction (<2 μm) were dispersed together with 200 mL of distilled water using a kitchen blender and an ultrasound. Sodium pyrophosphate was added to the suspension as a dispersant to promote dispersion and prevent flocculation. The suspension was gently stirred with a plastic spoon and allowed to stand for 45 min. After this time (Stokes’ law), the supernatant with a particle size of 2 μm was pipetted from the area 1 cm below the surface. The entire procedure was repeated three times for each sample to decant almost the entire fraction below 2 μm. The supernatants from 3 cycles were placed in a separate container and dried in an oven at 50 °C for 24 h. This dry clay fraction was used to produce a circular (diameter 2.5 cm) SEM-EDS polished section. The total thickness of the polished section was 1 cm.

The Slovenian salt production company (Soline Pridelava soli, d.o.o.), which manages salt production in the Sečovlje Salina Nature Park, issued a sampling permit. Sampling had no negative impact on endangered or protected species living in the Sečovlje Salina Nature Park.

The granulometric composition of the sediment samples studied was analysed using a laser granulometer Particle sizer (laser granulometry) and dynamic image analyser Fritch Analysette 22–28 with a measuring range from 40 nm to 2 mm (laser granolumetry) and 20 μm to 20 mm (dynamic image analyser). The sample was ultrasonically dispersed for 2 min before analysis. Grain-size frequency distribution plots were generated for each sample to check whether the distributions were polymodal. Numerical parameters for mean, median, primary mode, standard deviation, skewness, and kurtosis, percentage of clay, silt and sand were derived from the distribution data using the software provided. The position (i.e., size in μm) of a secondary mode was determined from the grain-size frequency plots. The image of individual grains were recorded by using an Analysette 28 Imagesizer (Fritsch) with a size range of 2800 to 20 μm.

Sediment pH was measured in situ using a portable pH meter (EUTECH Instruments) and further premeasured in the laboratory according to ISO standard 10390:2005 (suspension of sediment in water). Total organic carbon (TOC) content was determined using an Elementar Vario Micro CHNS elemental analyser. Prior determination, freeze-dried powdered samples were acidified with 6 M HCl to remove inorganic carbon [52]. The precision of the method was 3%. To calibrate the analytical system the Calibration Standard sulfanylamide (with theoretical values N—16.27%, C—41.82%, H—4.68%, O—18.58%, S—18.62%) was used.

The mineral assemblage of the samples was determined using X-ray powder diffraction (XRD) Philips PW3710, equipped with Cu Kα radiation and a secondary graphite monochromator. Data were acquired at 40 kV, and a current of 30 mA over a range from 2 to 70° 2θ, at a rate of speed of 3.4θ/min. The oriented clay mineral aggregates were prepared by a combination of ultrasound dispersion, salt removal by centrifugation (3 × 3 min at 2500 rpm) and the glass slide method. Afterwards, the samples were treated with ethylene glycol-solvated condition and exposed to the reagent vapour for at least 24 h at 70 °C. Diffraction patterns were validated with the X’Pert HighScore Plus diffraction software version 4.6 using PAN-ICSD powder diffraction files and the Rietveld method, a full-pattern fit method used to compare the measured and calculated profiles.

SEM analysis was performed on ThermoFisher Scientific Quattro S using a Schottky effect field emission electron source (FEG-SEM) equipped with an Oxford Instruments UltimMax 65 energy-dispersive spectrometer (EDS). Microscopy and chemical analyses were conducted at 10 mm working distance, 20 kV acceleration voltage and 10 nA beam current for large grains (>10 μm), while acceleration voltage was reduced (10–15 kV) for clay minerals and particles smaller than 10 μm to achieve better spatial resolution. The instrument was calibrated with pure silicon and cobalt standards for light and heavy elements, respectively. The error in wt.% concentration at the 2σ level ranged from 0.05–0.25 for light elements and 0.30–0.40 for heavy elements (

Table 2. Uncertainty and internal standards for SEM-EDS analysis.

Element	X-ray Line Type	Standard	Error wt.% (2σ)
Na	K series (Kα)	Albite	±0.04
Al	K series (Kα)	Al2O3	±0.08
Si	K series (Kα)	SiO2	±0.12
K	K series (Kα)	KBr	±0.09
S	K series (Kα)	FeS2	±0.25
Fe	K series (Kα)	Fe	±0.14
Mn	K series (Kα)	Mn	±0.11
Ca	K series (Kα)	Wollastonite	±0.22
Mg	K series (Kα)	MgO	±0.04
Ti	K series (Kα)	Ti	±0.18
Cu	L series (Lα)	Cu	±0.28
Pb	M series (Mα)	PbTe	±0.38
Hg	M series (Mα)	HgTe	±0.31
Zn	L series (Lα)	Zn	±0.34

). The EDS spectra were acquired over a period of 60 s. Before starting the analyses, the samples were coated with a thin film of amorphous carbon to ensure the electrical conductivity of the sample and prevent charge build-up.

Major oxides and trace metals content was obtained by ICP emission spectrometry, after lithium metaborate/tetraborate fusion and dilute nitric acid digestion (BV Mineral Lab—Acme, Vancouver, BC, Canada). The quality of the laboratory tests and objectivity was assured by the use of neutral laboratory tests. The accuracy of the analytical method, estimated by calculating the relative systematic error between the measured and recommended values of the reference materials STD DS11 and STD OREA262, was in the range of 2–13% with a median value of 7%. The precision of the analytical method, expressed as the relative percentage difference (% RPD) between the sample S6 duplicate measurements was in the range of 0–7% with a median value of 3%.

Basic statistical parameters for each element and multivariate statistical parameters were implemented using Statistica VII and Grapher 8 statistical software. The dissimilarity between objects was calculated using the Euclidean distance, and the objects were clustered using both the average linkage and Ward’s method (Cluster analysis (CA)). The Euclidean distance was adopted as it provides a greater emphasis to larger differences between variables. Principal component analysis (PCA) was used to disclose mineralogical and geochemical correlations within the data set.

3. Results

3.1. Basic Sediment Properties

The grain-size distribution data are presented in ternary diagram (Figure 3) and in Supplementary Materials. The selected samples (sample S3 and sample RS3) are a good representation of the overall grain size pattern of the studied samples and this is generally true for the Sečovlje Salina area [23,49,53]. The sediment samples are silts with minor amounts of clay and sand: sample S3 is slightly clayey silt and sample RS3 is slightly sandy clayey silt. The samples are very poorly sorted and polymodal. Sample S3 has two closely spaced modes at about 2–3 μm and 9–10 μm and a primary mode of 600–700 μm. Sample RS3 has two modes, a primary at about 2–3 μm and a secondary centred at 300 μm (Supplementary Materials).

Laser granulometry analysis clearly demonstrates that the sediments studied are a mixture of two principle grain-size populations, one dominated by silt and clay and the other by medium and coarse sand. The fine population originates from hinterland flysch rocks and consists mainly of clay minerals, while the coarse population particles originate from background flysch rocks (predominantly weathering resistant terrigenous particles such as quartz) and calciturbidite megabeds (carbonate lithic grains and bioclasts).

3.2. Sediment pH and Total Organic Carbon Content

The pH values of the sediment samples ranged from 7.3 to 7.9 (Supplementary Materials) and can be compared with previous research carried out in the Sečovlje salt pans [23,49,53] and other studies on the distribution of trace metals in salt marshes [17,25,54]. The total carbon content in all samples exhibit values ranging from 0.85 to 1.96%, with a mean value of 1.14% (Supplementary Materials). The reported values are within those for the Marano and Grado Lagoon salt marshes [25] and the Rosário salt marsh [17]. The highest pH values and total organic carbon content were observed in samples from the crystallisation basin and the Poslužnica area.

Fine-grained sediments deposited and accumulated in shallow flat zones (such as salt marshes, hypersaline environments, lagoons) usually show a significant positive correlation between total organic carbon (TOC) and clay content, confirming that high organic matter content is usually associated with fine-grained sediments [46,55].

3.3. Mineralogy (XRD Analysis)

The main mineral phases are presented in the integrated diffractograms (Figure 4) for all samples. The mineral composition of the sediments studied consists of high quantities of quartz, calcite and illite/muscovite, followed by albite, aragonite, clinochlore, goethite, pyrite and halite (

Table 3. Mineralogical composition of the investigated samples in %.

Minerals [%] ± σ	Quartz	Calcite	Halite	Albite	Illite/Muscovite 2M1	Goethite	Clinochlore 2M	Kaolinite 1A	Aragonite	Pyrite
S1	30.1 ± 0.4	29.6 ± 0.4	0 ± 0	3.5 ± 0.7	34.1 ± 0.6	0.2 ± 0	0.8 ± 0.1	1.2 ± 0.1	0.2 ± 0.1	0.2 ± 0
S2	31.6 ± 0.4	29.7 ± 0.4	0.1 ± 0	3.5 ± 0.6	32.2 ± 0.3	0.2 ± 0.1	0.8 ± 0.1	1.3 ± 0	0.4 ± 0.2	0.1 ± 0.1
S3	30.9 ± 0.2	29 ± 0.2	0.1 ± 0.1	6.6 ± 0.2	29.9 ± 0.4	0.1 ± 0	1 ± 0	1.6 ± 0.1	0.9 ± 0.3	0.1 ± 0
S4	31.7 ± 0.2	18.7 ± 0	0.6 ± 0.1	4.8 ± 0.6	34.5 ± 0.9	0.3 ± 0	1 ± 0.1	1 ± 0.1	7.3 ± 0	0.1 ± 0
S5	31.1 ± 0.6	18.3 ± 0.2	0.7 ± 0	4.9 ± 0.5	33.9 ± 0.4	0 ± 0	0.9 ± 0	1.5 ± 0.1	8.7 ± 0.1	0.1 ± 0
S6	31.1 ± 0.2	18.6 ± 0	0.4 ± 0.1	6.9 ± 0.2	34.5 ± 0.6	0.1 ± 0.1	0.7 ± 0.1	1 ± 0.2	6.5 ± 0.2	0.2 ± 0.1
RS1	38.6 ± 0.3	28.1 ± 0.3	0 ± 0	6.9 ± 0.2	23.8 ± 0.9	0.2 ± 0	1.1 ± 0	0.7 ± 0.1	0.6 ± 0.2	0.1 ± 0
RS2	39 ± 0.3	30.7 ± 0.2	0 ± 0	9.2 ± 0.2	18.4 ± 0.6	0.3 ± 0	0.7 ± 0	1.2 ± 0.1	0.2 ± 0	0.2 ± 0
RS3	35.6 ± 0.5	33.5 ± 0.4	0.1 ± 0.1	7.6 ± 0.2	20.8 ± 1.1	0.1 ± 0	0.9 ± 0	0.9 ± 0.1	0.3 ± 0.1	0.2 ± 0.1
RS4	31.9 ± 0.2	20.6 ± 0	0.5 ± 0.1	4.2 ± 0.7	30.7 ± 0.8	0 ± 0	1.1 ± 0	1.1 ± 0.1	9.8 ± 0.1	0.1 ± 0
RS5	32 ± 0.4	18.4 ± 0.2	0 ± 0	6.7 ± 0.2	32.4 ± 0.9	0.2 ± 0.1	1.1 ± 0.1	1.6 ± 0.1	7.4 ± 0.3	0.2 ± 0.1
RS6	36.3 ± 0.6	18.3 ± 0.2	0 ± 0	6.7 ± 0.2	29.8 ± 1.1	0.5 ± 0.1	1.1 ± 0.1	1.1 ± 0.1	6.1 ± 0.1	0.2 ± 0

). The clay mineral phases revealed with XRD analysis (oriented preparation) are kaolinite (Figure 5, Table 3) and low quantities of chlorite, interstratified layers of illite/smectite (I/S), illite and smectite.

In principle, the mineral composition between sediments and rhizo-sediments is very similar, but minor differences can be observed, which are primarily related to the sampling locations. The rhizo-sediments from the Piccia area show an inversely proportional relationship between illite/muscovite, kaolinite and quartz, i.e., lower amounts of illite/muscovite and kaolinite with higher amounts of quartz.

The Piccia area is located closer to the sea and is not part of the salt processing crystallisation area with crystallisation basins and Poslužnica: the Piccia area is therefore very intact and the sediments have not been redeposited as they have been in the active crystallisation area. The occurrence of halite minerals is also slightly higher in the sediments from the active crystallisation basin and Poslužnica area. Higher quantities of aragonite (from 6.1% to 9.8%) and subsequently lower quantities of calcite (from 18.3% to 20.6%) were determined only in sediment and rhizo-sediment samples from the crystallisation basin and Poslužnica area. This is due to the mineral differences of the biogenic particles: in the crystallisation basin and in the Poslužnica area there are more aragonite skeletons, while calcite residues predominate in Piccia.

In their studies, Faganeli et al. [56], Glavaš [53], Glavaš et al. [49], Kovač et al. [23] and Ogorelec et al. [50] report a very similar mineralogical composition of the sediments in the Sečovlje Salina area. The only exceptions are the presence of goethite and the absence of gypsum in this study. The latter can be explained by the fact that the sediment was not covered with seawater or brine, which means that it is slightly oxidised. These are favourable conditions for the formation of Fe oxides/hydroxides (goethite FeOOH) [4]. On the other hand, gypsum (CaSO₄) is usually formed by evaporation processes in the main salt production season [53,56], which only starts towards the end of May.

Minor differences in the quantities and ratios of minerals are also very well represented by CA (Figure 6). The tree diagram clearly indicates two groups, separated by locations; samples of sediments and rhizo-sediments from the Piccia area form the first group, and samples of sediments and rhizo-sediments from the crystallisation basin and Poslužnica areas form the second group. Within the first group two small subgroups were recorded, the first representing the sediment samples and the second representing the rhizo-sediment samples, which is due to the increased quantities of quartz and the reduced quantities of illite/muscovite and kaolinite in the rhizo-sediment samples.

3.4. Geochemical Analysis

The values of the major oxides are summarised in

Table 4. Major chemical composition of sediment samples (in %), including mean, standard deviation, minimum and maximum values, and standards data.

Major Oxides (%)	SiO2	TiO2	Al2O3	Fe2O3	MnO	MgO	CaO	Na2O	K2O	P2O
Samples
Sediment
S1	45.45	0.58	12.80	4.30	0.07	2.21	16.10	1.17	2.16	0.10
S2	45.74	0.59	12.72	4.41	0.07	1.84	16.15	0.95	2.14	0.12
S3	45.68	0.57	12.57	4.40	0.06	2.06	15.28	1.03	2.17	0.05
S4	44.31	0.57	11.50	4.36	0.09	3.16	13.12	1.02	2.10	0.08
S5	42.61	0.53	10.86	4.17	0.10	3.04	13.58	1.11	2.03	0.06
S6	44.30	0.54	11.44	4.30	0.09	3.18	12.72	1.17	2.09	0.13
Rhizo-sediment
RS1	47.49	0.55	11.18	3.76	0.06	1.90	17.10	1.12	1.79	0.05
RS2	48.28	0.59	9.95	3.34	0.08	1.79	18.74	1.04	1.48	0.09
RS3	46.41	0.53	10.96	3.75	0.07	1.97	19.37	1.11	1.66	0.06
RS4	42.17	0.51	10.22	3.82	0.10	2.76	14.36	1.32	1.95	0.13
RS5	46.25	0.57	11.52	4.32	0.09	3.16	13.13	1.07	2.07	0.09
RS6	43.84	0.52	10.08	3.20	0.09	2.82	12.05	0.98	1.31	0.11
Mean	45.18	0.55	11.28	3.99	0.08	2.43	14.97	1.09	1.89	0.08
Median	45.57	0.56	11.31	4.24	0.09	2.49	14.82	1.09	2.05	0.09
Std.Dev.	1.84	0.03	0.99	0.43	0.01	0.58	2.40	0.10	0.29	0.03
STD DS11	46.68	0.152	2.19	4.4	0.1	1.38	1.46	0.1	0.47	0.17
STD DS11 Exp.	46.78	0.163	2.13	4.43	0.11	1.41	1.48	0.09	0.48	0.16
STD OREAS262	60.36	0.003	2.55	4.6	0.052	1.94	3.98	0.1	0.39	0.084
STD OREAS262	61.13	0.005	2.46	4.7	0.053	1.94	4.17	0.1	0.38	0.091
Std. Dev. (95%)	0.218	0.003	0.04	0.03	0.003	0.01	0.05	0.003	0.005	0.004

and reflect a direct relationship to the samples’ mineral compositions. The abundance of major elements in the sediment and rhizo-sediment samples is very similar. The values of trace elements are summarised in

Table 5. Content ranges (mg/kg) for trace elements in the studied sediment samples, including mean, standard deviation; minimum and maximum values; standards data; critical threshold values for contaminants in soils according to Slovenian legislation ^a; sediment quality guidelines threshold effect limit (TEC) ^b; sediment quality guidelines probable effect limit (PEC) ^c.

Trace Elements (mg/kg)	Cr	Ni	Cu	Zn	As	Sr	Cd	Pb	Sn	Hg	Bi
Sediment samples
S1	69	56	21.67	66.20	6.10	219	0.17	12.10	0.56	0.045	0.16
S2	83	45.50	20.56	53.70	6.20	334	0.19	13.97	0.72	0.037	0.15
S3	65	47.90	23.40	51.50	5.90	310	0.20	12.45	0.79	0.034	0.21
S4	104	61.30	33.34	87.40	8.40	897	0.23	25.63	2.60	0.041	0.30
S5	112	65.40	31.20	93.70	9.20	765	0.19	24.61	1.10	0.035	0.29
S6	99	63.40	35.60	89.20	10.10	870	0.18	27.83	1.20	0.039	0.31
Rhizo-sediment samples
RS1	58	62.30	27.30	54.20	6.70	266	0.16	12.24	0.78	0.046	0.21
RS2	74	48.60	19.61	44.90	5.40	316	0.21	10.47	0.81	0.033	0.17
RS3	70	57.30	22.53	48.80	5.70	335	0.19	10.68	0.68	0.037	0.16
RS4	93	67.90	27.50	85.40	9.00	822	0.21	27.72	3	0.047	0.28
RS5	101	65.40	26.91	94.30	9.40	877	0.19	27.92	1.50	0.046	0.26
RS6	103	74.70	30.22	83.60	8.40	873	0.20	27.38	1.90	0.049	0.28
Mean	85.92	59.02	26.19	68.51	7.37	499.27	0.19	17.89	1.12	0.04	0.23
Median	88	61.80	27.11	74.90	7.55	550	0.19	19.29	0.96	0.04	0.24
Std.Dev.	18.23	8.86	5.21	19.50	1.70	292.79	0.02	7.87	0.80	0.01	0.06
STD DS11	61.3	80.2	143.95	339.5	42.1	66.3	2.18	140.36	6.08	0.279	11.59
STD DS11 Exp.	61.5	77.7	149	345	42.8	67.3	2.37	138	7.2	0.260	12.2
STD OREAS262	43.3	64.1	115.87	162.8	35.6	35.9	0.63	55.84	2.11	0.163	1
STD OREAS262 Exp.	41.7	62	118	154	35.8	36	0.61	56	3.39	0.170	1.03
Std. Dev. (95%)	0.45	1.15	1.80	3.58	0.23	0.275	0.05	0.63	0.6	0.01	0.16
Limit value a	100	50	60	200	20	/	/	85	/	/	/
Limit value b	52.3	15.6	18.7	124	7.24	/	0.7	30.2	/	0.13	/
Limit value c	160	42.8	108	271	41.6	/	4.2	112	/	0.7	/

and point to significant spatial differences between samples: sediment and rhizo-sediment samples from the crystallisation basin and Poslužnica exhibit a higher content of all trace elements compared to samples from Piccia.

Increased content of organic matter, clay minerals and sulphides in the sediments enhance the accumulation rate/capacity of metal(oid)s within the sediment material [31,34]. This is also the case of the sediment samples from the crystallisation basin and Poslužnica area, in which we found slightly higher values of organic matter, clay minerals and sulphide content, and consequently, enhanced metal(oid) content. Samples from the crystallisation basin and Poslužnica also comprise a slightly higher amount of halite minerals which contain a low content of trace metals [23]. In addition, higher Sr content was detected in the crystallisation basin and Poslužnica samples, showing a general presence and association of Sr with calcite/aragonite in marine sediments [57]. The similarity in the ionic radius allows for the substitution of Ca in carbonates [58] and therefore could be incorporated into CaCO₃, or more specifically, into aragonite [59].

The As, Bi, Cd, Cr, Cu, Hg, Ni, Pb, Sn and Zn values were compared to the critical threshold values for contaminants in soils as established by Slovenian legislation in regulations on critical, warning and limit emission values of hazardous substances in soils [60], which are in accordance with the optimum and action values of the new Dutch list [61]. Concerning the permitted levels of these elements, all correspond to the proposed levels except Cr (112 mg/kg) and Ni (74.70 mg/kg) which had slightly increased values. Additionally, the obtained values were also compared to consensus-based sediment quality guidelines (SQGs) [62] (Table 5). The TEL value is a sediment metal(oid) content at which a toxic response has started to be observed in benthic organisms. The PEL value is the metal(oid) content at which a large percentage of the benthic population shows a toxic response. A comparison of sediment metal(oid) content with the consensus-based TEC and PEC values revealed that only Ni content is higher than both the TEC and PEC special values, while Cr content distinctly exceeds and Cu and As contents slightly exceed the proposed TEC values.

The values of the investigated elements were consistent with those measured in surface sediments from the central Adriatic Sea [63], Bay of Piran [64], and Bay of Koper [65] and in two saltmarshes located in the Marano and Grado lagoon [66]. In contrast, salts marsh sediments from Tijuana Estuary [67], Rosario Estuary [68] and Northern Europe [69] carried much higher values of Cd, Cu, Pb and Zn. Very similar results for Sečovlje sediments were also reported by Ogorelec et al. [50], Glavaš et al. [53] and Kovač et al. [23]. The main reason for the increase in Cr and Ni values is thought to be the geological background associated with the inland Eocene flysch basin [23,50,53,65].

3.5. SEM-EDS Analysis

For a detailed characterisation of (1) sediment and rhizo-sediment composition (detrital grains and diagenetic precipitates) (2) and metal(oid)-bearing minerals and their origin (geogenic and/or anthropogenic sources), SEM-EDS analysis was employed. SEM-EDS results also confirmed the XRD analysis and geochemical results, and will complement the overall composition of the investigated samples.

The samples of salina sediment and rhizo-sediment are extremely alike and composed predominantly of a fine-grained matrix dominated by clay minerals, larger particles (clasts) of terrigenous and biogenic origin and other authigenic components (Figure 7).

The clay minerals in the matrix chemically correspond to chlorites and interstratified layers of illite-smectite (I/S). The composition of chlorite group minerals corresponds to that of chamosite. Chlorite and I/S particles are present in distinct populations, the first, detrital, forming large platy grains up to 100 μm in diameter (Figure 7b), and the second, probably authigenic (transformed and/or neoformed), forming the matrix and rarely exceeding a crystallite size of 5 μm (Figure 7c). The detrital clay particles are derived from the weathering of flysch rocks and erosion of soils from the catchment area [23,49,51,53,56] and accumulated in the wider Sečovlje area. Authigenic clays generally occur in sediment/soil profiles in (slightly) alkaline-saline environments and arid to semi-arid conditions [70,71]. They can originate from various diagenetic processes (transformed clays) or precipitate directly from solutions (neoformed clays) [72,73]. Authentic clay minerals can also include trioctahedral smectites, dioctahedral smectites, various mixed-layer minerals, as well as illite formed in illitisation processes [70,71,73,74].

The most common terrigenous particles detected in the samples are quartz (Figure 7), calcite/carbonate grains (Figure 7), potassium feldspar, plagioclase, amphibole, rutile, iron oxide and xenotime (Figure 8). K-feldspars represent tabular, elongated grains. The composition of K-feldspars corresponds to An0.12–9.47Ab8.11–19.38Or74.60–91.76 and plots near the orthoclase-rich corner of the feldspar discrimination ternary diagram (Figure 9). The composition of plagioclase grains (An0.64–34.47Ab65.34–99.35Or0.00–2.87) forms two separate clusters in the diagram. One group corresponds to nearly pure albite, while the other group plots between oligoclase-andesine in the composition area of intermediate plagioclase (Figure 9). Chemical composition of amphibole correlates to Ferro- hornblende (

Table 6. Elemental composition of analysed amphibole grains. Data given in wt.%.

	Amp-1	Amp-2	Amp-3	Amp-4	Amp-5
O	44.35	43.84	41.82	41.07	42.80
Mg	8.56	8.17	8.28	8.30	7.73
Al	7.93	8.26	8.03	8.21	8.41
Si	16.49	16.39	17.08	17.23	17.20
Fe	22.68	23.34	24.78	25.19	23.86

). The composition of Fe-oxides is in line with the composition of hematite (69.94 wt.% Fe). Xenotime grain, typical Y-rich variety, contains Gd (3.09 wt.%), Dy (4.71 wt.%), Er (3.16 wt.%), and Yb (3.36 wt.%).

The biogenic component is dominated by patches of amorphous organic material (Figure 7e) and planktonic foraminifera (Figure 7d). Foraminiferal tests are slightly magnesium CaCO₃ (0.26–0.47 wt.% Mg), while the chambers are filled in by diagenetic calcite cement containing a higher content of Mg (0.81–1.00 wt.%), Mn (0.26–0.49 wt.%) and Fe (2.34–3.27 wt.%), possibly suggesting the mobility of these elements during burial and diagenesis.

An extremely important authigenic mineral in the investigated samples is pyrite. It forms pyrite framboids 5–30 μm in diameter (Figure 7f) or irregular polycrystalline aggregates composed of nearly nano-scale crystallites. Pyrite is often encased in layers of Fe-rich chlorite mica. The occurence of framboidal pyrite in the surface sediments suggests the authigenic precipitation and intense activity of sulphate-reducing bacteria or a possible local anoxic environment. It is also commonly reported and accepted that diagenetic pyrite is considered a sink for metal(oid)s in sediments [75,76,77,78,79]. No metal(oid)s have been correlated to diagenetic pyrite with the EDS in this study.

3.5.1. The Potential Metal(oid)s Mineral Carriers

The presented terrigenous minerals and rock fragments form the geological lithology of the Sečovlje Salina hinterland. Detrital mineral assemblages include chlorites (chamosite) [80] and amphiboles, which could contain (or are enriched) traces of Cr and Ni [81]. Further on, Ivanovič [82] reported chromite, Cr–spinel and Fe–Cr minerals in carbonate-siliciclastic sandstones containing enhanced Cr content. Therefore, the above stated minerals could be considered potential carriers of metal(oid)s (Cr and Ni) mainly due to their chemical composition and environmental stability in relation to weathering (chlorites, amphiboles, chromite, Cr–spinel, Fe–Cr minerals weather easily).

3.5.2. Detrital Mineral Carriers of Metal(oid)s

Cinnabar, CuS_x and Bi-oxide were discovered in the size range between 500 nm and 1 μm and consistently validated throughout the sample sets (Figure 10). Metal(oid)s carriers were observed as individual anhedral grains or as very fine-grained dispersed particles incorporated into organic material (≈250 nm). The elemental composition of the analysed cinnabar grain are given in

Table 7. Elemental composition of analysed cinnabar grains. Data given in wt.%.

	Cin-1	Cin-2	Cin-3
Hg	85.67	84.23	84.90
S	14.33	15.77	15.10

, respectively. The range of composition is close to the ideal stoichiometric values of cinnabar. Two Cu-containing sulphide phases also disclosed in the studied sample set. The first consists of 65.30 wt.% Cu and 34.60 wt.% S, which corresponds to a nearly ideal composition of covellite (CuS). The second phase consists of 61.05 wt.% Cu, 12.20 wt.% Fe and 26.75 wt.% S, which is within the compositional range of bornite (Cu₅FeS₄). The elemental composition of the bismuth oxide phase is 79.30 wt.% Bi and 20.66 wt.% O.

The metal sulphides, cinnabar, covellite and bornite, were probably introduced into the Gulf of Trieste by the rivers from the hinterland. The Soča/Isonzo River and its tributaries transport material from areas where notable mineral deposits were located, such as the Idrija Hg mine [83] and several Cu deposits in the Middle to Late Permian Gröden formation [84]. In addition, several mineralisations in the Permian strata contain bornite [84]. Minerals carrying Hg and Cu can be explained as geogenic and anthropogenic detrital input, mainly depending on whether these minerals are present in the studied sediments due to the weathering of ore-enriched rocks or due to mining activities in the ore areas. Bi₂O₃ is one of the most industrially important insoluble bismuth compounds, widely used in low melting point alloys, fire detection, catalysts, cosmetics, the pharmaceutical industry, batteries, magnets and dentistry [85]. The source of the Bi₂O₃ could be the results of cargo ship traffic from the nearby ports of Koper and Trieste.

3.5.3. Metal(oid)s in the Fine Matrix of Organic Matter and Clay Minerals

The content of Cu, Pb and Zn (

Table 8. Elemental composition of the organic matter-clay mineral mixture. The data are in wt.%.

	Org-1	Org-2	Org-3	Org-4	Org-5	Org-6
O	30.32	45.63	45.24	32.86	33.17	25.89
Mg	1.60	1.69	1.96	1.77	1.61	2.11
Al	2.16	1.24	2.96	1.65	1.87	10.51
Si	4.81	4.16	6.17	4.78	8.08	22.93
S	6.44	2.29	6.96	4.64	6.08	4.70
Ca	31.01	38.55	17.51	35.23	31.40	16.19
Fe	7.52	2.65	5.05	5.91	5.48	7.16
Cu	0.00	0.00	3.37	3.81	3.90	0.00
Zn	0.00	0.00	0.00	2.25	0.00	0.00
Pb	9.49	3.79	8.68	7.09	6.96	5.48

) were denoted in the mineral mixture of organic matter and clay minerals distributed among the terrigenous quartz, carbonate, amphibole and plagioclase grains (Figure 11). The abundance of Cu in this fine-grained material is closely related to the fact that Cu is extensively complexed by humic material (a fraction of the natural organic matter) in comparison to other metals [4,86]. Pb has been shown to exhibit a strong affinity to clays and organic matter through specific adsorption reactions [4,37,86]. Positive clay-bonding with Zn (especially 2:1 clay such as illite) has also been reported [4].

4. Discussion

The distribution and the fate of metal(oid)s in salt marsh/saline sediments cannot be understood without knowledge regarding the deposited mineral particles. The geological background with the associated flysch rocks represents the primary source for the sediments and the detrital particles trapped within the sedimentary material represents the particles of the background rocks. Further on, primary and secondary clay minerals tend to bind free ions from the solution. The properties of the deposited mineral particles have a tremendous influence on the binding ability of metal(oid)s to mineral surfaces or incorporation into the crystalline structure of newly formed authigenic minerals [87,88,89]. Moreover, particle size distribution is one of the most important factors affecting the ability of sediment particles to accumulate metal(oid)s. Metal(oid)s contents in surface sediments generally increase with decreasing grain size because of the affinity of metal(oid)s to bind to finer particles such as clay minerals [46,90]. Organic matter in the sediments could also contribute to the formation of metal-organic complexes [4,34].

In order to confirm the geogenic and anthropogenic origin of the Sečovlje Salina sediment material, the values of metal(oid)s in the investigated sediment samples were normalised to the values of Al. The enrichment factors (EF) were calculated using the normalised content in sediments and the average normalized contents of representative rock samples from the Sečovlje Salina geological background.

The EF values (Supplementary Materials) obtained were similar for most of the metal(oid)s determined in the samples and pointed to two main sources of the sediment particles in the investigated area. Since the median EF values calculated for Sn (0.23), Cd (0.46), Ni (0.64), Cr (1.2), Cu (1.2) and Pb (1.3) ranged from 0.2 to 1.3, this supports the assumption that weathering of rocks from the hinterland is the predominant geogenic source [91] of material deposited in the Sečovlje Salina area. Hg EF values (0.95) pointed to the Idrija Hg mine and air deposition of cinnabar or mercury particles. As (2.57), Bi (2) and Zn (2.74) EF values were higher than 1.5, which according to Szefer et al. [92] and Chen et al. [93] reflect anthropogenic contamination of the metal(oid) in question, e.g., minor enrichment. However, if we consider the calculated EF values with respect to different sampling locations, the results are slightly different. Samples from the Piccia area reveal lower EF values (1.76) and the samples from the crystallisation basin and Poslužnica have higher values (2.94). The mentioned difference results from the fact that the samples from the crystallisation basin and Poslužnica contain more salt (halite minerals), in which traces of As have already been identified [94]. The occurrence of Bi was already defined as anthropogenic due to its elevated industrial application and the proximity of the studied area to ports, while Zn is a very well-known essential element for humans and animals [3], and is therefore also quantitatively abundant in any ecosystem.

PCA (Figure 12) explained 84.63% of the data variance in the first two ordination axes, highlighting significant positive correlations between the samples from the crystallisation basin and Poslužnica due to their higher metal(oid) and Sr content. A pattern of individual correlations among metal(oid) and Sr distribution between the samples from crystallisation basin and Poslužnica is also recognised. Samples from Piccia (S1, RS1, S2, RS2, S3 and RS3) clearly show lower metal(oid) and Sr content, thus we can track down negative connections or no connections with the samples from crystallisation basin (S4, RS4, S5 and RS5) and Poslužnica area (S6 and RS6).

As, Bi, Cr, Cu, Pb, Sn, Sr, Pb and Zn form a large integrated group, nevertheless, we can emphasise and define the differences in origin, distribution and association of the elements within this group. As, Pb and Zn could be associated with Fe/Mn oxides and oxyhydroxides (goethite and hematite in our study) [30,89,95,96], or incorporated into crystal lattices of clay minerals (especially illite) [4] and into halite minerals [94]. Cu is generally closely bound to organic matter [4] and could be adsorbed to aragonite [97]. In the following order, Zn > Pb > Cu display very high sorption capacities for aragonite and calcite [4,97,98], and hence the proximity and interrelation with Sr. Bi exhibits many chemical properties similar to those of As, and is generally accompanied by Pb ores in small amounts [85]. These could be the reasons for its positioning near As and Pb, although we have identified its anthropogenic origin. The distribution of Cr is influenced by geogenic sources, but, in sediments it could be (especially Cr(III)) rapidly and specifically adsorbed by Fe and Mn oxides and clay minerals [4]. This adsorption increases with increasing pH and organic matter content, which is the case in the Sečovlje sediments [4]. Clay minerals are assumed to be the sources of Sn and the measured Sn contents is very low. We cannot ignore the presence of tributyltin oxide in the surrounding waters, which is widely used as an antifouling paint for wood preservation in boats and ships due to its biocide effect [44,88].

Cd, Hg and Ni are located as individual outliers (Figure 12). Cd predominantly forms precipitates with carbonates, including biogenic particles, but Cd adsorption is strongly controlled by the presence of competing cations, such as divalent Zn and Cu [4]. Cinnabar grains (HgS) are strictly anthropogenic in origin and originate from the Idrija Hg mine. Ni is incorporated into detrital sediment particles and its accumulation is closely governed by Mn oxides and oxyhydroxides [4,89].

Whereas the investigated sediment samples are dominated by slightly oxic conditions (an active oxic/anoxic sedimentary boundary) and neutral to low alkaline pH, the precipitation of Fe and Mn oxides/oxyhydroxides, the formation of metal-organic complexes and framboidal pyrite production are preferred. Consequently, this also affects the metal retention. Changes in pH and salinity would undoubtedly alter the influence of the relative role of the absorption phases. For instance, an increased pH would favour the ionisation of carboxyl functional groups on organic matter and surface hydroxyl group on oxides [45,96] and releasing of metal(oid)s into the surrounding system, on the other hand, pyrite production would not occur.

5. Conclusions

This study demonstrates that understanding the origin and incorporation of metal(oid)s within different solid sediment particles (such as detrital particles, anthropogenic particles, clay minerals, organic matter and diagenetic minerals) is critical to identify their distribution, accumulation, retention and further transfer into a vulnerable and ecologically important ecosystem such as salt marshes.

The results suggest that the detrital particles from the geological hinterland, e.g., the type of rock fragments and minerals, the various anthropogenic sources, the particle size relative to the amount of clay minerals, the abundance of organic matter and early diagenetic precipitates, significantly affect the metal(oid) accumulation and current retention in corresponding sediment and rhizo-sediment content of the Sečovlje Salina. The metal(oid)s, e.g., As, Bi, Cd, Cr, Cu, Hg, Ni, Pb, Sn, Sr, Pb and Zn display many differences in the origin, distribution and association within the sediment components.

According to the results of XRD, ICP-ES, SEM-EDS and various statistical analyses, the studied elements were mainly associated with Fe/Mn oxides and oxyhydroxides (As, Cr, Ni, Pb, Zn), incorporated into or adsorbed onto the crystal lattices of clay minerals (As, Cr, Pb, Sn, Zn), halite (As) and aragonite/calcite (Cd, Cu, Pb, Sr, Zn) and associated with organic matter (Cu, Pb and Zn). Only As, Bi, Hg and Zn were recognised as anthropogenic, although it would be difficult to determine anthropogenic sources for As and Zn, as their abundance is more likely due to processes occurring in the sedimentary basin. Traces of As were found in halite minerals, while Zn is an essential element for all living beings and is therefore also present in larger quantities. BiO industrial compounds are very common cargo found in the nearby ports and Hg presence is the result of almost 500 years of historical mining activity in Idrija.