Comprehensive mineralogical and physicochemical characterization of recent sapropels from Romanian saline lakes for potential use in pelotherapy

Mineralogy and thermal properties

The bulk mineral composition of sapropels is detailed in Table 1. The XRD analysis indicates that Amara and Tekirghiol sapropels are enriched in silicates, i.e., quartz (30.8% and 29.1% respectively), plagioclase-albite (10.1% and 8.9%), carbonates, mainly calcite (6.8%) and aragonite (13.1%) in Amara, and calcite (8.7%) in Tekirghiol (Fig. 2). By contrast, Ursu sapropel contains lower concentrations of silicates, mainly quartz (15.4%), plagioclase (5.5% albite and 8% andesine), sulfides, i.e., pyrite (1.5%) and is enriched in halite (34.5%). The major clay components in the sapropels were 2:1 dioactahedral and 2:1 trioctahedral clays, representing 28.9%, 23.6% and 20.8% of clay minerals in Tekirghiol, Amara and Ursu samples, respectively. Muscovite was detected in similar concentrations in Tekirghiol (4.5%) and Amara (4.2%). Quantitative mineralogical clay composition of the fraction < 2 μm shows that these sapropels are defined by the presence of illite, smectite and interstratified illite/smectite as major components (> 90% in each sample), and kaolinite and chlorite as minor fractions (Table 2; Fig. 3).

Differential thermal analysis and thermogravimetric analysis

The thermal behavior of sapropel samples is presented in Table 3. Total mass loss ranged from 21.84% (Tekirghiol) to 24.72% (Ursu). TGA shows the main mass loss at temperatures between 200 and 700 °C for Amara sapropel (16.02%) and between 700 and 1000 °C for Ursu sapropel (15.76%), while stepwise significant mass loss was observed for Tekirghiol sapropel at temperatures between 200 and 1000 °C (10.23% at 200–700 °C and 9.32% at 700–1000 °C).

For all analyzed samples, the TGA/DSC curves (Fig. 4) indicate the removal of molecular water at lower temperatures (200 °C), followed by a second event causing substantial mass loss possibly due to dehydroxylation of illite and smectite, the main clay minerals in these sapropels. Decomposition of carbonates takes place above 650 °C (up to 900 °C), and is probably responsible for the considerable mass loss detected in Amara sapropel.

Physical characteristics

Particle size analysis revealed that all three sapropels can be defined as sandy silts (Table 4). Tekirghiol sapropel was 74.4% silt, 13% clay and 12.7% sand, compared to Ursu sapropel with 65.9% silt, 9% clay and 25% sand. The Amara sapropel contained the highest concentration of sand particles (35%), lowest concentration of clay (7.2%) and silt (57%). In Tekirghiol and Ursu samples the mud fraction (silt and clay) was considerably greater (87.5% and 74.9%, respectively) than the sand fraction as compared to Amara sample (64.4%). Similar CEC values were determined for Tekirghiol (27.2 cmol(+)/kg) and Amara (24 cmol(+)/kg). The sapropel from Ursu Lake featured highest density, humidity and cation exchange capacity, and lowest specific surface area (Table 4) among all analyzed sediments. To verify the influence of salt contents on the SSA, we performed the measurements both in raw (salted) and desalted sapropels. The highest increase of SSA after desalinization (more than 3 times, from 2.77 to 8.7 m² g⁻¹) was observed for hypersaline Ursu Lake sapropel. In Amara and Tekirghiol sediments, the SSA increase after desalinization was 1.6 times (from 8.3 to 13.7 m² g⁻¹) and 1.3 times (from 12.63 to 16.53 m² g⁻¹), respectively. The total pore volumes, which could be measured only after desalinization, showed similar values, ranging from 0.034 ml g⁻¹ in Ursu sapropel, to 0.031 ml g⁻¹ in Amara and 0.030 ml g⁻¹ in Tekirghiol. These findings suggest enhanced water entrapment corroborated by larger total pore volume and enrichment in NaCl (Table 5) which might explain the higher humidity of Ursu sapropel as compared to the other tested muds.

Chemical composition of sapropels

The overall chemistry of the explored sapropels and overlying water is highlighted in Tables 5, 6 and Supplementary Table 1, respectively. The salt contents and pH ranged from sulfate-enriched, low-saline and slightly alkaline (Amara), to chloride-dominated, moderately saline and pH-neutral (Tekirghiol) or hypersaline and slightly acidic (Ursu). PCA (Principal Component Analysis) further differentiates the three lakes sapropels, as Tekirghiol seems to be defined by the inorganic carbon and metal concentrations (i.e., Al, Mg, Fe, K), Amara by higher calcium and sulfates contents (Fig. 5), and hypersaline Ursu sediment by Na⁺, Cl⁻, dissolved organic carbon (DOC) and phosphorous. Except for Sr, detected in higher concentrations (124 mg kg⁻¹) mainly in Tekirghiol sapropel compared to Amara (47 mg kg⁻¹) and Ursu (43 mg kg⁻¹), potentially toxic elements such as As, Co, Cr, Cd, Cu, Ni, Pb, are present in low concentrations (Table 4). The predominant rare earth elements shared by all samples are cerium and neodymium (~ 20 and ~ 10 mg kg⁻¹ in Tekirghiol and Amara; ~ 5 and ~ 3 mg kg⁻¹ in Ursu Llake, respectively). Ursu Lake sediment has substantially (i.e., ~ fourfold) lower lanthanide concentrations that Tekirghiol and Amara (Supplementary Table 2).

Raman spectroscopy

FT-Raman spectra of the bulk samples (Fig. 6) showed main bands characteristic to the C=C skeletal stretching mode of carotenoids (1512–1516 cm⁻¹), with highest intensity in Ursu sapropel. Substantial presence of methylated compounds was indicated by bands at 1433–1445 cm⁻¹ and 2933 cm⁻¹. The broaden band at around 1600 cm⁻¹ suggested an intricate overlap of organic compounds. For Tekirghiol and Amara sapropels confocal Raman spectra (532 nm excitation) pointed on the presence of β-carotene most probably originating from cyanobacteria (Supplementary Figures S1 and S2). The main bands specific to carotenoids were obtained from “dark green” spots of about 1–3 μm and recorded at 1518–1520 (C=C), 1156 (C–C) and 1004–1006 cm⁻¹ (C–CH₃). Noteworthy, in Tekirghiol sample, the signal assigned to kerogen has been recorded from multiple “black” spots and recognized according to the characteristic D and G bands at 1360 and 1561–1601 cm⁻¹ respectively (Supplementary Figure S1). Additional spurious bands were associated with traces of organic matter (amide of proteins at 1657 cm⁻¹, S–S and C–S at 561 and 645 cm⁻¹ respectively).

Two types of carotenoid signatures were distinguishable in Ursu Lake sapropel, tentatively associated with the main stretching modes of β-carotene (1516 cm⁻¹) in Cyanobacteria and Archaea and fucoxanthin (1528 cm⁻¹) in diatoms (Supplementary Figure S6). The SERS test revealed only the enhancement of the carotenoids resonance Raman bands and a considerable decrease of the background intensity level (Supplementary Fig. S3, B). Contrary to the expectations, humic substances were not detectable.

FT-IR spectroscopy and SEM

SEM–EDX analyses of Amara and Tekirghiol sapropels showed substantial presence of O, Al, Si and metals like Mg, Fe, K (Supplementary Figure S4), and high content of Na and Cl (21.59% and 31.38% relative concentration, respectively) in Ursu Lake sapropel, demonstrating the substantial discrepancy in salinity compared to Tekirghiol (Na—2.83%, Cl—2.74%) and Amara sapropels (Na—1.6%, Cl—0.69%). The Amara sapropel appeared as the most enriched in Ca²⁺ followed by Tekirghiol (6.91%) and Ursu sapropels (1.69%). Trace amounts of titanium were detected in all samples, while trace amounts of fluorine were detected only in Tekirghiol and Ursu. Amara Lake sapropel shows higher S content (1.49%) compared to Tekirghiol (0.88%) and Ursu sapropels (0.93%). S-rich inorganic complexes were detected by EDX elemental mapping and FT-IR spectroscopy analyses in Amara and Ursu samples (Supplementary Figure S5) whereas FT-IR spectra of Amara sample showed distinguishable absorption bands associated to –SH (at 2590 cm⁻¹), O=S=O (1140 cm⁻¹) and S–O stretching modes (600–700 cm⁻¹)¹⁹ (Supplementary Figure S3). The FT-IR analyses of dry and wet sediments pointed on the presence of absorption bands at 2950 and 2850 cm⁻¹ that are associated with –CH₃ and –CH₂– groups, confirming the presence of organic matter²⁰ (Supplementary Figure S6). The FT-IR spectra of Amara sample show a detectable signal at 1725–1700 cm⁻¹ that is characteristic for carbonyl groups in aldehydes, ketones and carbonic acids. The absorption bands in the spectral region of 1690–1500 cm⁻¹can be associated with vibrations the carbonyl (C=O) and amino (–N–H) bonds of amides present in proteins, a finding that is corroborated by spectrophotometric protein quantification (Table 6). The aluminosilicates were detectable in all samples by the HO– specific broad FT-IR band between 3600 and 3650 cm⁻¹ and absorption bands corresponding to Si–O–Si stretching (1040 and 860 cm⁻¹) and Si–O–Si bending vibrational modes (465 cm⁻¹) in addition to overtone of Al–O in Si cage (TO₄) (1347–1360 cm⁻¹) and Al–OH signals (891–936 cm⁻¹). The doublet at 780–798 cm⁻¹ is due to Si–O–Si inter tetrahedral bridging bonds in SiO₂ and OH deformation band.

Solid-state ¹³C- and ¹H-NMR

The ¹³C ss-NMR spectra recorded by the CP-MAS technique (Supplementary Figure S7) indicate that the carbonyl/keto peak at 182 ppm, the carboxyl at 171.5 ppm, and the aliphatic peaks at 23.3 and 26.7 ppm, most probably associated to CH₃/CH₂ groups, are common to all three samples. Other aliphatic carbons peaking at 44.7 ppm were observed in Tekirghiol and Amara sapropels. The Tekirghiol sapropel is distinguished by the small peaks at 39.6 and 54.1 ppm assignable to a CH/CH₂ group, the high intensity line at 33.7 ppm in which might be generated by molecules with many CH₂ groups, for instance a fatty acid, and a low intensity NMR line, assignable to an aromatic carbon at 129.6 ppm. The ¹H ss-NMR spectra (Supplementary Figure S7), are dominated by protons associated with the inorganic component of the sample (i.e., OH peak at − 3.4 ppm, the lines in the 4.7–5.2 ppm spectral range associated with water molecules in various binding environments). These lines can be correlated with the clay fraction identified in all the sediment samples by the mineralogical investigation. The ¹H ss-NMR lines of the organic component are associated to the aliphatic signals in the 1–2 ppm range, the line at 8.9 ppm (hydrogen bonded carboxyl/amine), and possibly an aromatic proton at 5.8 ppm. The later can only be distinguished in the Tekirghiol sapropel, as there is no complete overlapping with the water NMR line.

Isotope analysis and elemental analysis

Total organic carbon analysis indicated OC contents ranging from 2.09% (Tekirghiol) to 3.24% (Ursu). Total nitrogen (TN) concentrations are similar in Tekirghiol and Ursu sapropels (0.25 and 0.26%) and slightly higher (0.35%) in Amara sediment. C/N ratios (% w/w of combusted sediments) vary from 5.61% in Ursu, to 7.35% in Amara and 8.49% in Tekirghiol sample (Table 6). The values of δ¹³C_OC of organic matter in the carbonate-free sapropels were similar in Amara and Ursu lakes (− 27.77‰ and – 27.63‰, respectively) and slightly higher (− 23.87‰) in Tekirghiol Lake while δ¹⁵N values in bulk sapropels were 12.38, 5.13 and 4.98 inTekirghiol, Amara and Ursu lakes, respectively (Table 6). A large δ³⁴S difference from sulfides to sulfates was identified in Tekirghiol and Ursu sediment pore waters (29.49‰ and 22.63‰, respectively), which is corroborated by soluble sulfides (HS– or H₂S) detected in these sediments (Table 5).

Pigment and lipid composition

Chlorophyll concentration was found to be highest in Ursu Lake (5490.86 µg L^–1), compared to Tekirghiol Lake (3011.52 µg L⁻¹) and Amara Lake (2348.8 µg L⁻¹) (Table 6). Significant differences were detected between the fatty acids (FAs) profiles of the water samples as compared to sapropels, for each of the three investigated lakes (Supplementary Table 3). The prevalent saturated FAs were C16:0 (31.2–43.9%) and C18:0 (4.5–3.6%), along with the abundant unsaturated FAs C16:1 n-9 (1.4–23.4%, highest in Ursu Lake surface), C18:1 n-9 (7–23.5%, highest in Tekirghiol water), C18:2 n-6 (3.0–10.5%, highest in Amara water and sapropel), respectively C18:3 n-3 (1.4–24.2%, highest in Amara water). The monounsaturated C16:1v7 isomer, characteristic of bacterial sources was abundant in Ursu sapropel.

Source: Ecology - nature.com