Applied Clay Science 104 (2015) 36–47
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Research Paper
Physicochemical characterization, elemental speciation and hydrogeochemical modeling of river and peloid sediments used for therapeutic uses Margaret Suárez Muñoz a,⁎, Clara Melián Rodríguez a, Alina Gelen Rudnikas a, Oscar Díaz Rizo a, Miren Martínez-Santos b, Estilita Ruiz-Romera b, Juan R. Fagundo Castillo c, Aurora Pérez-Gramatges d, Nadia V. Martínez-Villegas e, Dagoberto Blanco Padilla f, Rebeca Hernández Díaz g, Patricia González-Hernández c a
High Institute for Applied Sciences and Technology, Havana, Cuba University of the Basque Country (UPV), Spain c Faculty of Chemistry, Havana University, Cuba d Department of Chemistry, Pontifícia Universidade Católica do Rio de Janeiro (PUC-Rio), RJ, Brazil e Applied Geoscience Department, Institute for Scientific and Technological Research of San Luis Potosi (IPICYT), Mexico f Thermal Center “San Diego de los Baños”, Pinar del Río, Cuba g Geology Faculty, University of Pinar del Río “Hermanos Saíz”, Pinar del Río, Cuba b
a r t i c l e
i n f o
Article history: Received 3 September 2014 Received in revised form 11 November 2014 Accepted 22 November 2014 Available online xxxx Keywords: Sediment Peloid Elemental speciation Hydrogeochemical modeling Metals
a b s t r a c t The present study was conducted to characterize the river sediment and the final peloid from San Diego de los Baños Thermal Center (Pinar del Rio, Cuba), based on its original sedimentary geochemistry and composition, in order to establish the physicochemical and geochemical properties for its inorganic quality assessment in therapeutic uses. The original sediment was extracted from the estuary of San Diego River, and then maturated with thermal waters, yielding a peloid with known anti-inflammatory and dermatological properties. A comparative study of total content and geochemical speciation of seven transition metals (Cr, Cu, Fe, Ni, Mn, Pb and Zn) in the sediments was performed using a sequential extraction procedure and Inductively Couple Plasma Emission (ICP) techniques, as well as hydrogeochemical modeling to predict and correlate species under the physicochemical conditions measured for the river and peloid sediments. The results showed that the main differences originated from maturation process are closely related to the changes in electric conductivity (EC) and redox potential (Eh). These variations are reflected in the composition of major elements, and at a lesser extent, in the total content of the transition metals. Most of the elements studied in this investigation appeared in the less mobile fractions, which suggested low availability in the sediments, under the studied conditions. In the case of Mn, species are mainly located in the most leachable fractions, which together with its relatively high content, indicates a need for regular monitoring of this element in the peloids used in the Thermal Center. The findings were useful to predict the behavior of these transition metals regarding solubility, potential motility and availability in the river and final peloid sediments, and led to classify the San Diego de los Baños peloid as mud or fangi. It was concluded that most of the metals are strongly retained in the peloid through the maturation process, as many factors contribute to the low mobility, such as the nearly neutral pH, the presence of organic matter, redox conditions, and the presence of carbonates and salts. The sediments were finally characterized regarding possible contamination, according to USEPA and background criteria, and were considered as non-polluted/non-contaminated except for Mn, which showed a moderate contamination factor. The results from the present work show the relevance of physicochemical and elemental characterization to peloid classification and quality assessment. Moreover, it evidences the importance of performing geochemical speciation using both experimental and theoretical techniques, for proper assessment of mobility and distribution mechanisms of soluble and solid mineral species present in these sediments. © 2014 Elsevier B.V. All rights reserved.
1. Introduction ⁎ Corresponding author at: Department of Radiochemistry, High Institute for Applied Sciences and Technology (InSTEC), Ave. Salvador Allende, esq. Luaces, Plaza de la Revolución, La Habana, Cuba. Tel.: +53 7 8789851. E-mail address:
[email protected] (M. Suárez Muñoz).
http://dx.doi.org/10.1016/j.clay.2014.11.029 0169-1317/© 2014 Elsevier B.V. All rights reserved.
Thermal muds (peloids) are hydrothermal or hydrothermalized sediments produced by primary or secondary mixing of clay (geo) materials with salty thermo-mineral waters, accompanied by organic materials produced by the biological–metabolic activity of
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micro-organisms growing through years in the so-called “natural maturation process” (Veniale et al., 2007). The final product is a sediment containing micro-organisms and organic and inorganic compounds with biological activity that allow its application for treatment of different pathologies, usually related with stimulatory, antiphlogistic and analgesic actions (Veniale, 1997; Martin-Díaz, 1998; Carretero, 2001; Nappi, 2001; Tateo et al., 2005). The therapeutic use of peloids is called Pelotherapy, and it constitutes a typical procedure of medical hydrology for the treatment of the osteoarticular system, cutaneous diseases among others. Nowadays, the new frontier of pelotherapy (healthy therapies focused on wellness and relaxing) is impacting two main hindrances: (i) natural occurrences of thermal muds are going exhausted; and (ii) focused treatments of specific pathologies need the formulation of peloids possessing suitable properties. Therefore, many spa-centers currently prepare the thermal muds by “maturation” of sedimentary clays mixed with thermo-mineral waters, as shown in Fig. 1 (Veniale et al., 2007). The maturation process can be very complex since it is necessary to initially modify the water–sediment paste environment to create the appropriate conditions for developing a new-growth of micro-flora and fauna, as well as their metabolic products. The final product will depend not only on the conditions used in the maturation process (pH, Eh, temperature, light exposure, hydrologic regime) but also on the initial composition of water and sediment, such as biogenic elements and organic matter, as well as on the maturation time and the specific procedure. All these variables have a decisive effect in the chemical and biologic reactions that can occur during maturation, defining the properties of the final peloid (Carretero et al., 2010). During the last decades, several investigations have been done to get insights on the reactions occurring during the “maturation” process of the sediments (Veniale et al., 1999, 2004), which has evidenced the need for “certification” of the quality and suitability of peloids devised for specific therapies (Nicolini et al., 2004; Setti et al., 2004). Important reactions related with the maturation process have been associated with changes in the redox environment, which causes ions and other compounds to be released and incorporated into the original sediment. In particular, mobile and/or exchangeable toxic elements (e.g. Fe, Mn, Cu, Zn, Pb, Cr, As, Cd, Hg), are of special concern, since they can be scavenged by the skin sweat (Summa and Tateo, 1998; Summa et al., 2005; Tateo and Summa, 2007). These metals have therefore become important discussion issues in the “certification” of the quality of sediments for therapeutic uses (Nicolini et al., 2004; Setti et al., 2004). However, there are few comprehensive studies related to the presence and availability of these metals in sediments after maturation process, despite being a key knowledge to understand the possible beneficial and/or dangerous effects to human health (Carretero, 2001; Wilson,
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2003). In particular, there is a need for studies related to the geochemical composition of peloids used in balneotherapy due to the risk of heavy metals in peloids, the changes in the maturation process, the interaction between metals and other components of the peloid matrix, and with the human biomembranes, with possible re-adsorption through the skin (Mihelčić et al., 2012). The potential effect of metals in peloids can be assessed by total content determination and by speciation. Depending on chemical and geological conditions, metallic elements can be partitioned into different chemical forms that are associated with a variety of organic and inorganic phases (Pagnanelli et al., 2004; Clozel et al., 2006; Arain et al., 2007). Thus, speciation analysis of metallic elements might provide useful information regarding the potential mobility and bioavailability of a particular element, which consequently can offer a more realistic estimation of its effects (Yang et al., 2009). In Cuba, there is still a great reserve of natural peloids, used in pelotherapy. However, in the case of San Diego de los Baños peloid, the original sediment is extracted directly from the estuary of San Diego River, and enriched later with inorganic and organic components, as well as with the microbiota of calcic sulfated, fluoric, radionic and sulfurous mesothermal waters (33 °C) of San Diego de los Baños Thermal Center. Our group has systematically study these waters, which show low Eh values (− 226 to − 270 mV) and their EC values oscillate between 1480 and 2200 µS/cm (Fagundo et al., 2007). The maturation process of the sediment is done using a static, open method for 15 days, and under different environmental conditions. The peloid produced in this way is routinely used in the form of mud-bath or mudpatch to alleviate inflammatory (osteoarthritis, rheumatoid arthritis, bursitis) and dermatological (eczemas, psoriasis, cutaneous seborrhea and mycosis) processes, as an analgesic, in male and female infertility treatments, and as a cosmetic product (acne, keloids). In a previous study, we characterized the organic content of the original sediments and resultant peloids by chromatographic techniques, and found that there is relationship between these compounds and the potential therapeutic action of the thermal mud (Suárez et al., 2011). These organic compounds are present as a result of the organic matter decomposition and the rich biological activity in the peloid. The presence of bioglea in San Diego de los Baños has been well documented, and it is composed in the higher percent of cyanobacteria as the Leptothrix subtilissima, and in the lower percent by diatoms (Ex. Navicula sp.) (Pérez Loyola et al., 2003). On the other hand, the sanitary safety of the peloid is guaranteed by verifying or limiting the presence of total coliforms, Escherichia coli, fecal streptococcus, Pseudomona aeruginosa, Staphylococcus aureus, Clostridium perfringens and Salmonella (NC, 1998). The present study aims to characterize further the complex systems that compose the San Diego de los Baños peloid, in order to establish the physicochemical properties, geoavailability and inorganic content quality related to its therapeutic uses, and based on its original sedimentary geochemistry and composition. Total concentration and chemical speciation of Cr, Cu, Fe, Ni, Mn, Pb and Zn, will be determined in the different phases, by Inductively Couple Plasma Emission (ICP) techniques, which will lead to a partial assessment of the maturation process to determine the influence of this procedure in the incorporation, leaking or mobility of metals in the sediment.
2. Materials and methods 2.1. Field methodology
Fig. 1. The cycle for the preparation (maturation) of thermal mud (Veniale et al., 2007).
Samples of the original river sediment (RSD) were collected directly from the estuary of San Diego River, while peloid sediment (PSD) samples were collected from the maturation pool in San Diego de los Baños Thermal Center, located in Pinar del Rio, Cuba. In both cases, a composite sample was prepared from the different collected fractions. After sampling, sediment samples were sealed in clean polyethylene
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containers, placed in a cooler at 4 °C, and transported to the laboratory immediately for further analysis. “In situ” measurements of pH, oxidation–reduction potential (Eh), electric conductivity (EC), temperature and dissolved oxygen in river and peloid sediment samples were carried out using a HANNA HI-8424 pH/Eh meter, a WTW LF197 Conductimeter with internal temperature probe and a HANNA HI 914 oxymeter, respectively. 2.2. Laboratory methodology River and peloid sediment samples were air-dried after collection, and then sieved in order to obtain a sediment fraction of size smaller than 63 μm, since metal concentrations in sediments are highly dependent on size. Total carbon (C), hydrogen (H), nitrogen (N) and sulfur (S) contents were first determined using a LECO CS2000 analyzer, and the results were expressed as dry weight percentage. Along with the elemental analysis, total metal contents were determined as in the following procedure: three subsamples (0.5 g each) were added into microwave tubes containing ultrapure HNO3 (65%) and concentrated HClO4 (60%), in a 3:1.5 ratio (Ruiz et al, 1991), and digested in a microwave oven (ETHOS 1, Millestone). The digested samples were heated by increasing the temperature to 180 °C for 10 min and kept at that temperature for an additional 25 min. After the digestion, all samples were filtered through a 0.45 μm Millipore nitrocellulose filter, and diluted to 50 mL with Milli-Q water. The final extracts were analyzed using an inductively coupled plasma optical emission spectrometer (ICP-OES, Perkin Elmer Optima 2000), looking for metals commonly present in sediments (Cu, Cr, Fe, Mn, Ni, Pb and Zn). The total metal concentrations were compared with results obtained from the sequential extraction procedure of the sediment samples. The latter consisted in a modified Tessier procedure to determine metal fractionation in the sediments (Tessier et al., 1979; Zimmerman and Weindorf, 2010; Oluwabukola et al., 2011). The extraction was carried out progressively on an initial dry weight of 1.0 g of river sediment and peloid samples, using three replicates. The error of the mean values was determined as the standard deviation. Table 1 shows the sequential extraction procedures used to obtain five different fractions: (i) exchangeable (EXC), (ii) bound to carbonates (CARB), (iii) bound to easily reducible oxides (ERO), (iv) bound to organic matter and sulfide (MO), and (v) residual (R-RES) fractions. After being treated with the extraction reagents, samples were centrifuged at 4000 rpm for 20 min. The supernatant was decanted and filtered through a 0.45 μm filter, before analysis using ICP-OES. Quality control was assured by the analysis of triplicate samples and standard reference materials. The standard reference material (MAG 1) was used for total element analysis; percentage recoveries ranged from 86% (for Cr) to 110% (for Cr). The same standard reference material was used to verify the accuracy of the sequential extraction method. The recovery rates for heavy metals ranged from 87% (for Pb in the MO fraction) to 116% (for Ni in the RES-R fraction). Analytical precision, expressed as relative standard deviation, was in general better than 10%. 2.3. Hydrogeochemical modeling The hydrogeochemical modeling was carried out using the theoretical Pourbaix Eh–pH diagram (Research Center for Deep Geological
Environments, 2005) and the PHREEQC software with the MINTEQ database (USGS, 2005). The theoretical Pourbaix Eh–pH diagram was used as a practical tool to establish the dominant aqueous species and the stable solid phases of each metal and sulfur in the sediments, defined by the Eh and pH values. Theoretical equilibrium models, based on equilibrium chemistry of aqueous solutions interacting with minerals, gases, solid, exchangers and sorption surfaces, were used in the PHREEQC software to calculate the distribution of aqueous species (metal speciation), using thermodynamic and physicochemical constrains. The saturation index (SI), e.g. the degree of saturation to determine if certain minerals have a tendency to dissolve or precipitate out of solution, in order to reach equilibrium, was calculated as: SI ¼ log
IAP KT
3
where IAP is the ion activity product and KT is the equilibrium constant, for the given material and at the given temperature. As all the chemical reactions occurring during the maturation process take place in an aqueous environment, a theoretical aqueous solution with the same chemical composition and physicochemical conditions than those determined for the sediments (RSD and PSD) was used as an approximation of the studied system for interpretation of the hydrogeochemical modeling. 2.4. Sediment quality assessment There are three methods or steps of assessing the sediment quality (ADEC, 2013). The first step is to compare the sample data to national or international sediment quality guidelines. These guidelines establish the minimum content above which elements can be considered as a risk. In the present work, both the San Diego river sediment and the San Diego de los Baños peloid sediment were considered and evaluated as generic sediments, using standard quality guidelines (USEPA, 1977) to establish chemical inorganic quality criteria, since it was not possible to find in the literature any specific requirements regarding metal content in peloids. The second step is to evaluate background concentrations and compare with the obtained data, to determine if the metal content in the analyzed sediment represents a natural concentration or if it has being affected by anthropogenic contamination. These values establish the minimum content above which elements can be considered as a risk as anthropogenic contamination compared to background levels (which is the one generally applied to metals). This approach is more realistic since it is based in real data from the studied place, and it allows for calculation of the contamination factor (Cf) and the contamination degree (Cd) (Kwon and Lee, 1998). The contamination factor for each i element is calculated as: i
Cf ¼
C i0−1 C in
1
where C i0−1 and Cin are the mean content and the background value for each metal, respectively; and n is the number of elements. The background value is defined as the original elemental concentration in the sediment sample, before possible anthropogenic or environmental contamination. In this work, metal background values were extracted
Table 1 Experimental conditions for sequential extraction procedures in analysis of sediments. Step
Extraction procedures
Fraction
1 2 3 4
12.5 mL (1 M NH4NO3), pH 7.0, 0.5 h shaking 12.5 mL (1 M Na-acetate), pH 5.0, 6 h shaking 12.5 mL (0.04 M hydroxylamine hydrochloride in 25% acetic acid), 6 h at 95 °C in water bath 1.5 mL (0.01 M HNO3) and 2.5 mL (30% H2O2), 5 h at 85 °C in water bath, followed by additional 1 mL of H2O2, 1 h at 85 °C in water bath, then 7.5 mL of 1 M NH4NO3, 10 min shaking at room temperature 12.5 mL (4 M HNO3), 16 h at 80 °C in water bath
Water-soluble/exchangeable fraction (EXC) Carbonate-bound fraction (CARB) Fe/Mn oxide-bound fraction (ERO) Organic matter/sulfide-bound fraction (MO)
5
Residual fraction (R-RES)
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from Díaz Rizo et al. (2011), where the metal concentrations were determined by X-ray fluorescence analysis. These values were selected using the earliest data available (1926–1928), which corresponds to a depth of 30–35 cm (Díaz Rizo et al., 2011). The contamination degree is calculated as:
Cd ¼
n −1 X
i
Cf ¼
i¼1
n −1 X i¼1
C i0−1 C 1n
2
where the sum of all contamination factors is taken into account to evaluate the combined risk. The third step of the sediment quality assessment consists in quantifying the incidence of adverse biological effects in aquatic environment, within the range of chemical concentrations (Kwon and Lee, 1998; ADEC, 2013). In this work, only the two first steps were used, since this third approach was beyond the purposes of the investigation. 3. Results 3.1. Physicochemical and elemental characterization of San Diego river sediment (RSD) and San Diego de los Baños peloid sediment (PSD) Sediment sample analysis and characterization was divided in three main parts, namely, the physicochemical characterization, the elemental analysis (C, H, N, S), and the metal content (major and minor elements). The results obtained for physicochemical parameters show very low (negative) oxidation–reduction (Eh) potential, low dissolved oxygen (DO) content, and rather neutral pH values, in both the river and the peloid sediments (Table 2). On the other hand, the changes after the maturation process were significant for Eh and electric conductivity (EC), where differences in 0.120 V and 13 mS/cm, respectively, were observed. The elemental analysis showed that C, H, N and S composition was very similar in both sediments, being slightly higher in RSD than in PSD, for all the elements. Finally, the analysis of major and minor elements (metal content) of the river (RSD) and the peloid (PSD) sediments showed that Ca, Na, Mg, and K are the main cations that contribute to sediment composition, and are also responsible for the significant differences in chemical composition after maturation process (Table 2). The concentration of these four major elements was found in the range of 6.5 to 41 mg/L, in the case of RSD, and in the range of 5.4 to 33 mg/L, in PSD. Iron and Mn were also
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found to contribute significantly, in concentrations between 0.77 and 0.84 mg/L for Mn, and between 26 and 27 mg/L for Fe. Other metals (Cr, Cu, Ni, Pb and Zn) appeared at lower concentrations (9–73 μg/g), and these results were similar in both RSD and PSD samples. For better assessment of the possible influence of maturation process in the sediment chemical composition, an analysis of major and minor element composition was made, by comparing its concentrations in RSD and PSD samples (Fig. 2a and b). Most major (Ca, Na, Mg, Mn and K) elements exhibited a decrease in their concentration in the PSD in relation with the values found for the RSD; meanwhile, Fe showed no significant changes, as well as most of the minor elements (Cr, Ni, Pb and Zn). In the case of Cu, there was a modest increase in concentration after the maturation process of the river sediment. 3.2. Speciation of transition metals in river and peloid sediments To analyze the potential mobility and, therefore, availability of transition metals, the distribution of their cations was determined in the different extraction fractions, in RSD and PSD samples (Fig. 3a and 3b, respectively). In general, the sum of the distribution in MO and RES-R fractions in both sediments represents more than 80% in all the studied metals, with the exception of Mn (distributed mostly in the CARB, ERO and EXC fractions) and Pb (distributed 100% in the MO fraction). Regarding speciation in the two sediment samples, Fe, Mn, Ni, Pb, and Zn exhibited similar chemical partitioning among the fractions in RSD and PSD. In the case of Cu, chemical partitioning in PSD was slightly shifted to the MO fraction, also showing the appearance of a small but significant percentage (2.4%) for the EXC fraction in the peloid sediment. Differences between RSD and PSD were mostly observed in the distribution of Cr, where the overall distribution in the MO and R-RES fractions accounts for nearly 95% of the total content. Nevertheless, there is an evident decrease of Cr in the R-RES fraction (67.6% in RSD to 51.3% PSD), while a significant increase was obtained in the percentage of Cr in the MO fraction (27.8% in RSD to 44.9% in PSD). 3.2.1. Hydrogeochemical modeling Other results that contribute to understand the potential mobility of metals are the distribution of metal species obtained from the analysis
Table 2 Physical-chemical parameters and elemental composition obtained in the analysis of San Diego river (RSD) and San Diego de los Baños peloid (PSD) sediments. Results were the average of three determinations. Parameter
San Diego river sediment (RSD)
San Diego de los Baños peloid sediment (PSD)
pH EC (mS/cm) Eh (V) DO (mg/L) T (°C) %C %H %N %S μg/g
7.35 ± 0.02 21.35 ± 0.02 −0.220 ± 0.002 0.80 ± 0.06 25.7 ± 0.1 4.62 ± 0.07 2.18 ± 0.08 0.39 ± 0.01 1.98 ± 0.06 52.9 ± 0.6 23.5 ± 0.4 34.1 ± 0.9 9±2 73 ± 2 41 ± 1 27.0 ± 0.9 6.5 ± 0.2 7.28 ± 0.08 0.84 ± 0.01 17 ± 2
7.61 ± 0.02 7.67 ± 0.02 −0.337 ± 0.002 0.60 ± 0.06 23.6 ± 0.1 3.91 ± 0.04 1.90 ± 0.04 0.33 ± 0.02 1.88 ± 0.02 51.7 ± 0.9 25.9 ± 1.2 32.3 ± 0.5 8±2 73.1 ± 0.6 32.8 ± 0.5 26.2 ± 0.4 5.4 ± 0.2 6.41 ± 0.07 0.77 ± 0.01 11.4 ± 0.5
mg/g
Cr Cu Ni Pb Zn Ca Fe K Mg Mn Na
Fig. 2. Metal concentration in RSD and PSD sediments. (a) Major elements in mg/L and (b) minor elements in μg/L.
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Fig. 3. Distribution of transition metals in the different extraction fractions of RSD sediment (a) and PSD sediment (b).
of the theoretical Pourbaix Eh–pH diagrams (Fig. 4a–h). The sulfur species were also modeled due to the relevance for metal mobility in highly reductive environments. According to the results from this model, Fe, Mn, Ni, Pb and Zn, are in the form of soluble species in both sediment samples (RSD and PSD). While Mn, Ni and Zn are in ionic (2+) form, Pb and Fe are in the form of the ionic complexes PbOH+ and FeOH+, respectively. On the other hand, Cr is present in the ionic (3+) form, as the soluble complex species (CrOH2+) in RSD, but as the insoluble oxide Cr2O3 in PSD. Of all the elements analyzed in the sediments, Cu species are the only predicted to be in solid form in both sediments (RSD and PSD). In the case of sulfur species (S), it appears in the form of HS−. To complement these results with thermodynamic data in the hydrogeochemical model, theoretical chemical equilibrium models were run using the PHREEQC software. The theoretical species predicted as single cations according to this model were Cu+, Fe2+, Mn+2, and Zn2 +, in both RSD and PSD samples (Table 3). On the other hand, in the case of Cr, Ni and Pb, there were more than one species predicted 2+ , for each metal in both sediments, namely, Cr(OH)3, Cr(OH)+ 2 , Ni 2+ + Ni(OH)2, PbOH and Pb3(OH)4 . The tendency to dissolve or precipitate out of solution was calculated as the saturation index (SI), to predict
the possible mineral phases formed in RSD and PSD (Table 3). In general, it can be seen that, in both sediments, the major mineral phases are oxides, sulfides and hydroxides for most metals. The exception was Mn, where no mineral phases reached significant saturation under the conditions of hydrogeochemical modeling used in this work.
3.2.2. Quality assessment Due to the therapeutic use of peloids, a further analysis of the concentration values of transition metals (Cr, Cu, Fe, Mn, Ni, Pb and Zn) regarding possible contamination was made by comparing with the limits reported by the USEPA (Fig. 5). It can be seen that in RSD and PSD, Cu, Cr and Ni values were within the limits reported by the USEPA, while Fe and Mn concentrations were above the higher limit. Lead and Zn concentrations fell below the USEPA lower limit. These elements were also analyzed regarding possible contamination during maturation process of the sediment, using data from the literature for the background levels (USEPA, 1977; Kwon and Lee, 1998; Díaz Rizo et al., 2011). The results showed a low contamination factor (Cf) for Cu, Fe, Ni and Pb, and moderate Cf for Cr, Mn and Zn, in both RSD and PSD (Table 4). On the other hand, in
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RSD and PSD, only Mn exhibited a moderate degree of contamination (Cd), while the rest of the transition metals showed low Cd values. 4. Discussion 4.1. Physicochemical and elemental characterization of San Diego river sediment (RSD) and San Diego de los Baños peloid sediment (PSD) 4.1.1. Physicochemical characterization The physicochemical parameters will be analyzed throughout this discussion as they are relevant to the mobility of metals in sediments and to the maturation process to obtain the final peloid. These parameters show that both sediment samples (RSD and PSD) have anoxic media (low Eh and DO values), saline conditions, and rather neutral pH values, at the temperatures between 24 and 26 °C, found in the samples (Table 2). As can be observed, the most important changes
41
occurring during the maturation process, from RSD to PSD, are related to the EC and the Eh values. The peloid obtained by this process had lower EC and Eh than the original sediment (RSD), showing that there was a modification of the water–sediment paste environment. Both EC and Eh variations can have a decisive effect in the chemical and biological reactions occurring during maturation, which are responsible for the properties of the final peloid, since they can affect metal mobility due to changes in metal-binding capacity of humic materials, insoluble metal sulfide formation (or sulfide oxidation), and changes in Fe/Mn-oxyhydroxides (Laing et al., 2009; Carretero et al., 2010). 4.1.2. Elemental analysis The elemental composition (C, H, N, S) results did not show great variations between river and peloid sediment samples, although there seems to be an overall tendency to decrease from RSD to PSD (Table 2). Therefore, we can conclude that the maturation process did
Fig. 4. Pourbaix Eh–pH diagrams of the transition metals (Cr (a), Cu (b), Fe (c), Mn (d), Ni (e), Pb (f), Zn (g)) and sulfur (h) speciation in RSD and PSD sediments.
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Fig. 4 (continued).
not induce significant changes in the organic matter and sulfur content, which are some of the key elements to define the therapeutic nature of peloids. In addition, the H/C ratio can be calculated for RSD and PSD, giving values of 0.47 and 0.49, respectively. These low values indicate that the main source of organic matter in the sediments is through terrestrial material contribution (Schmidt et al., 2009). The estuarine origin of the river sediment justifies that the major contribution of organic matter is due to the material dragged by the San Diego River and not from the seawater input. Based on the physicochemical parameters and elemental composition obtained, we can now attempt to classify the peloid sediment from San Diego de los Baños, and to infer the distribution of organic compounds in this peloid. In general, peloid sediments can be classified into two groups, according to their organic matter composition and maturation waters: one comprises the peloids with high mineral composition and low organic matter (1%–5%), which are known as
mud or fangi (hydrothermal water) and limus (seawater), while the other includes those with higher content of the organic matter (N 5%) and water from different sources (e.g., peat, bioglea, sapropel) (San Martín, 1994; NC, 1998; Bergel, 2000). In this work, the total carbon (organic and inorganic) content found in the peloid sediment has values of approximately 4%, indicating low organic matter content. Based on this value, and the maturation water origin (hydrothermal), the San Diego de los Baños peloid sediment can be classified as mud or fangi. In addition, the reductive conditions (low oxidation–reduction potential of −0.34 V) and the high presence of sulfides (S content near to 2% of the total composition), suggest a low quantity of polar compounds in the peloid, which means that the humic fractions are less oxidized or humified. Previous results from organic characterization of this peloid, reported by Suárez et al. (2011), showed a 5.3% of organic matter and only 1% of n-heptane removable substances (Suárez et al., 2011).
M. Suárez Muñoz et al. / Applied Clay Science 104 (2015) 36–47 Table 3 Possible soluble species and mineral phases calculated using PHREEQC hydrogeochemical modeling of San Diego river (RSD) and San Diego de los Baños peloid (PSD) sediments. (SI: saturation index). Element
Soluble species
Mineral phases
SI
Cr Cu
Cr(OH)+ 2 , +
Cu
Fe
Fe2+
Mn Ni
Mn2+ Ni2+, Ni(OH)2
Pb Zn
Pb(OH)+, Pb3(OH)2+ 4 Zn2+
Cr2O3 Covellite (CuS) Chalcocite (Cu2S) Chalcopyrite (CuFeS2) Cu metal Djurleite, anilite, blaublei (CuxS) Hematite (Fe2O3) Magnetite (Fe3O4) Goethite (FeOOH) Mg-ferrite (MgFe2O4) – Millerite (NiS) Ni(OH)2 Bunsenite (NiO) Galena (PbS) Sphalerite (ZnS) Wurtzite (ZnS) Zincite (ZnO) Zn(OH)2
18.9 11.7 30.1 19.8 14.3 14–29 10.5 16.3 3.4 4.6 – 7.2 6.5 4.8 10.6 7.9 6.0 3.1 2.8
Cr(OH)3
4.1.3. Major and minor elements In sediments, chemical elements can exist in significant quantities due to the lithological origin resultant from the geochemical cycle in the ecosystem (Galán and Romero, 2008; Zvinowanda et al., 2009). Inorganic elements can be classified as major, minor and trace elements, based on their elemental abundance in the sediments. Major elements in the
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Table 4 Contamination factors and contamination degrees for transition metals in RSD and PSD sediments (calculated using background data from Kwon and Lee, 1998; USEPA, 1977 and Díaz Rizo et al., 2011). San Diego river sediment (RSD) Element Contamination factor (Cf) Cr 1.0 Cu 0.6 Fe 0.6 Mn 1.3 Ni 0.6 Pb 0.5 Zn 1.0
San Diego de los Baños peloid sediment (PSD) Contamination degree (Cd) 6.1 3.1 3.2 7.1 3.2 3.1 5.6
Contamination factor (Cf) 1.0 0.6 0.5 1.2 0.5 0.5 1.0
Contamination degree (Cd) 5.4 3.3 2.9 6.3 3.0 2.8 5.4
Notes: Cf b 1 low contamination factor; 1 ≤ Cf b 3 moderate contamination factor; 3 ≤ Cf b 6 considerable contamination factor; Cf ≥ 6 very high contamination factor. Notes: Cd b 6 low degree of contamination; 6 ≤ Cd b 12 moderate degree of contamination; 12 ≤ Cd b 24 considerable degree of contamination; Cd ≥ 24 very high degree of contamination.
sediments studied in the present work included the alkaline and alkaline-earth metals Ca, Na, Mg, and K. The main constituent is Ca, because of the contribution of the karstic rocks and the estuarine environment in the RSD, and in the case of PSD, also due to the maturation process in a calcium sulfated water environment. High Na concentrations are also a consequence of the estuarine environment, but as a more soluble ion, it tends to remain in the liquid phases, and therefore its concentration in sediments is lower than that found for Ca. Calcium and Na, and in a lesser extent, Mg and K, showed a significant decrease in the peloid sediment compared to the original river sediment (Table 2). This
Fig. 5. Comparison of transition metal concentrations in RSD and PSD sediments with USEPA limits (USEPA, 1977; Kwon and Lee, 1998). The left scale represents the concentration of the Cr, Cu, Ni, Pb and Zn; the right scale represents Fe and Mn concentration.
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behavior is probably due to washup of the sediment salts by the maturation waters, and the changes in the chemical equilibrium leading to the dissolution of minerals. Overall, these processes change the composition of the original river sediment (collected near the sea) to a peloid (PSD) with less mineral composition, which agrees with the changes observed for EC, as previously discussed. In addition, significant concentrations of Fe and Mn (the order of mg/g), and at the trace level (μg/g), other transition metals such as Cr, Cu, Ni, Pb and Zn were also found. The concentration of most of these transition metals remained practically constant during the maturation process of the sediment (Table 2 and Fig. 2a and b). The two exceptions were Cu, which concentration showed a modest increase, and Mn, with a slight decrease. In the case of Mn, the highly reductive conditions measured in the peloid sediment after maturation can be the cause for the slight decrease observed in its content. In such environment, it can be expected to have extensive reduction of Mn(IV) to Mn(II) before other metals such as Fe and Cu. The soluble and mobile Mn2+ species, as will be discussed further in this section, can then migrate out of the sediment into the water, decreasing in this way the concentration in the final peloid. On the other hand, the increase observed in Cu levels is likely a consequence of the contribution of Cu-leaking from water pipes. San Diego de los Baños Health Center was built in the 50s where Cu plumbing was commonly used. The sulfide waters from the thermal sources are so aggressive that it can attack the metals in the drain pipe, dragging the CuS formed into the water flow, and thus the solid can be deposited in the final peloid. Therefore, regardless of the ubiquitous presence of Cu in the environment, even considered as an essential nutrient for normal growth and metabolism of living organisms, its levels should be regularly measured, since abnormal levels of Cu intake induce nutritional deficiency to levels as high as to be acutely toxic (Eisler, 2000). 4.2. Chemical speciation of transition metals in river and peloid sediments The potential environmental risk of transition metals in sediments is associated with both their total content and their speciation (Yang et al., 2009). Therefore, a detailed characterization has become a crucial aspect in the “certification” of the quality and suitability of peloids, and sediments in general, for therapeutic uses (Nicolini et al., 2004; Setti et al., 2004; Summa et al., 2005). This is the main reason for metal content speciation before and after the maturation process of peloids, in addition to sediment characterization regarding physicochemical parameters and elemental composition. 4.2.1. Metal distribution Although total metal distribution can be used as a preliminary approach to evaluate quality and suitability of peloids and their original sources for therapeutic uses, chemical speciation is of paramount importance to elucidate possible mechanisms of distribution and mobility in these complex systems. The analysis of the different fractions obtained after sequential extraction showed the distribution of the species according to their affinity with the different phases (Fig. 3a and b). There is a predominance of Cr, Cu, Ni, Pb and Zn in the less leachable fractions (MO and R-RES). These transition metals are likely to have a very low mobility, being highly immobilized in both sediments. Some of the transition metals, like Cr, Cu and Pb, showed a high affinity for the organic matter, being 100% in the case of Pb. Lead has been reported to form strongly bounded complexes with the organic matter, and that slighted alkaline pH inhibits the mobility of this element (Strawn and Sparks, 2000; Brown et al., 2003; Basta et al., 2005; González et al., 2011). Therefore, despite Pb being dangerous to human health in most of its chemical forms (Eisler, 2000), it was verified in the present investigation that, under the studied conditions, Pb is highly immobilized in both sediments, and the maturation process does not contribute to increase Pb level or its mobility in the final peloid. On the other hand, Ni and Zn, are mostly located in the R-RES phase
(60–70%), in agreement with Izquierdo et al. (1997), who reported 70.5% of the Ni in the Cádiz Bay (Spain), associated to the RES-R fraction, a behavior similar to that described by other authors for noncontaminated sediments. The difference between Ni and Zn lies in the different percentages obtained for both metals in the organic matter fraction. This behavior can be explained by the tendency of Ni to have less affinity for MO (Kabata-Pendias and Pendias, 2000; Sparks, 2003; Martínez et al., 2011), while Zn can form soluble chelates with the humified (mainly fulvic acids) and non-humified (aminoacids) organic matters (Kiekens, 1990). For both metals, it is expected that brusque changes in sediment conditions would only lead to a slight redistribution between the MO and ERO fractions (Yong, 2001), and not to their release into the solution. These results are somehow in contrast with the values found by Yang et al. (2009) in the analysis of heavy metals in sediments from the mainstream, tributaries and lakes of the Yangtze River (China), who obtained much larger recoveries of Cu and Zn species in the CARB and ERO fractions, concluding that the presence of these metals in the sediments posed some ecological risk. A special case in the study of the possible metal mobility in the sediments is the distribution of Fe and Mn, which were distributed in all the fractions. However, while Fe species reside mostly in the MO fraction, Mn species are extracted at high percentages in the leachable fractions (80% among EXC, CARB and ERO). Both metals appear in relatively high concentrations, typical of hydrothermal sediments. The difference lies on the tendency of Mn species to be mobilized from the sediment solid phase, increasing its potential bioavailability (Yang et al., 2009). Some authors, such as Lovley and Phillips (1988), also reported that the reduction of Fe3+ does not begin before all Mn4+ is depleted, thus Mn would always be more soluble and Fe will tend to be more immobilized under highly reductive conditions, such as those found for the river and peloid sediments in this work. The Mn species in the CARB fraction are related to HCO− 3 in the sediments, and therefore its release into solution would be pH dependent (Yang et al., 2009). The distribution obtained in the ERO fraction, also suggests an Eh dependence of the Mn species. These results indicate the importance of the control of physicochemical parameters (pH and Eh) in the sediments to minimize the mobility of Mn species into solution during therapeutic application of peloids. Although there were not significant differences in the metal distribution from river to peloid sediments, it is important to note that Cr and Cu distributions among the different fractions seem to change during the maturation process (RSD to PSD). In the case of Cr, the distribution between the MO and R-RES phases was shifted towards the former (28:68 in RSD to 45:52 in PSD). This behavior could be explained by the variation of salinity during the maturation process: in RSD, the higher salinity makes Cr less bounded to MO, as reported by Lores and Pennock (1998), that found that binding of Cr to organic acids being less than 20% at high salinity, in PSD the affinity to MO is higher due to the decrease of salinity and therefore, an increase in binding of Cr to organic humic acids. In contrast, although Cu complexes with organic molecules show a high stability, not being observed significant changes in the MO:R-RES fraction ratio between the two sediment samples, a significant change occurs in the distribution in the ERO fraction. The content of Cu in this phase, which is related to species sensitive to redox changes, decreased from RSD to PSD, leading to an increase in the more labile EXC fraction (from 0 to 2%). An increase in the Cu concentration in the EXC fraction indicates an increase in the mobility of the Cu species incorporated through the maturation process, as discussed above. Since both Cr and Cu are metals which toxicity is known to be highly dependent on its concentration, and in the case of Cr, also on its chemical form (Eisler, 2000), it is essential to follow its concentration and distribution to evaluate peloid quality for therapeutic applications. Therefore, other hydrogeochemical studies, able to predict the chemical species likely to exist in the sediments under these thermodynamic conditions, become necessary to better assess the potential risks of these metals in therapeutic applications.
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4.2.2. Hydrogeochemical modeling The results from calculations based on theoretical hydrogeochemical models using Pourbaix Eh–pH diagrams and PHREEQC modeling showed the soluble species and mineral phases likely to exist in the sediments studied in this work (Fig. 4a–h and Table 3). As can be seen, Mn and Zn can only exist as cations (+2) in their soluble forms, while Cr and Pb are mostly predicted as hydroxyl complexes. All these species are typical of an environment with low (negative) Eh values and neutral/slightly alkaline pH, such as the San Diego de los Baños peloid, favoring the reduction of metal species, whenever a suitable electron donor is present (e.g., Fe(II), sulfur compounds and bacteria acting as catalysts) (Guo et al., 1997; Simpson et al., 2000). In the case of Cr and Pb, these conditions promote the hydrolysis of the metal cations, with a tendency to a rapid reduction from the very toxic Cr6+ to less toxic/essential nutrient trivalent form (Cr3+) (Eisler, 2000; Laing 2 et al., 2009), and in Pb, to the complexes PbOH+ and Pb3(OH)+ 4 , which can have direct impact on sorption and desorption mechanisms (Yong, 2001). Nickel and Fe species have contributions of both the +2 cation and the hydroxyl complex. Meanwhile, the most probable soluble species for Cu are Cu(I) and Cu(0), which are the species formed at higher reduction potentials, and therefore more likely to be reduced in these conditions. All these predicted soluble species of transition metals did not change under the conditions assumed for the maturation process of the river sediment (RSD) to obtain the final peloid (PSD), with the exception of Cr, which showed no soluble species in the PSD. On the other hand, the PHREEQC equilibrium models predicted the oxides and hydroxides of reduced Fe2 + (e.g. hematite, magnetite, goethite) and Cr3 + (as Cr2O3) as the possible solid mineral phases with tendency to precipitate out of solution, being the oxides those with higher saturation index (Table 3). These solids can be mainly distributed in the MO and R-RES fractions (N 80%), decreasing the potential mobility of these heavy metals, as discussed above (Fig. 3a and b). Sulfur can form precipitates with many transition metals due to the presence of S2− and HS− species in the sediments, as predicted by the hydrogeochemical models. Sulfides are the main mineral species expected to precipitate as solids for Cu and Pb, with very high saturation indexes, which coincides with the high percentages found for these elements in the fraction associated to organic matter (MO). At lower values of SI compared to Cu, nickel and Zn are predicted as well to precipitate as sulfides, associated to the same MO fraction, in agreement with reports from other authors (Eggleton and Thomas, 2004). However, the models also predict the existence of mineral phases as oxides and hydroxides of Ni and Zn, which would be mainly located in the R-RES fractions (60–70% of total distribution of both metals in the RSD and PSD). Finally, it was found that Mn has no tendency to precipitate due to the highly reductive environment, (negative Eh values) found in both sediments, which is in agreement with the findings regarding total content and distribution analysis of this element. These results are also in accordance with the study that refers that soluble manganese is generally found in sediments as Mn2+ ion, and that it is very mobile (Manahan, 2001). lthough Eh and pH dependence lead us to suggest a careful study of the peloid application forms, which can have a direct impact in absorption through the skin, dermal absorption of this metal is considered minimal, and not a risky exposition pathway in normal conditions (ATSDR, U.S., 2000). Nevertheless, since some of the Mn could be released from processes associated with “natural changes” in the pH and redox environments or the changes in contact with skin and sweat, its contents and distribution should be routinely followed in quality control protocols of these peloids. The trends predicted for the mineral phases of the transition metals, under the simulated conditions, did not show any differences between the two samples studied (RSD and PSD), suggesting that the maturation process is not affecting significantly the distribution of soluble species and mineral phases in the sediments.
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4.3. Quality assessment of river and peloid sediments from San Diego de los Baños Where a substance is present in the environment at a concentration above natural background levels the term ‘contaminant’ is used. The term ‘pollutant’ is used when a contaminant can be shown to have a deleterious effect on the environment (Andrews et al., 2004). The total concentration values of transition metals determined in this work in river and peloid sediments allow for a primary broad classification of the samples according to USEPA standards (Fig. 5). Regarding the contents of Cu, Pb and Zn, both sediments were classified as ‘non-polluted’, while considering Cr and Ni values, the USEPA limits rate them as ‘moderately polluted’. In the case of Fe and Mn, the higher total content found for these metals sets the classification of the river and peloid sediments as ‘heavily polluted’, since they exceed the higher limit. In this analysis, it should be considered that USEPA metal quality guidelines actually reflect worldwide data of diverse environmental samples (USEPA, 1977). Therefore, although it could be useful as a general guide for alerting about possible contamination, in the particular case of the sediments studied in this work, Cf and Cd values offer a more realistic approach, since they are based in the original composition of the sediment, and it also accounts for the combined effects of metals in the sample. Consequently, Cf and Cd criteria were followed in this work, using more accurate data from San Diego river sediments before substantial anthropogenic contamination (Díaz Rizo et al., 2011). Using this approach, we can conclude that Cr, Cu, Fe, Ni, Pb and Zn have low contamination factors and low degree of contaminations, while regarding Mn, the samples are considered as moderately contaminated (Table 4). As can be seen, Fe contamination is not longer observed, since the relatively high levels of Fe can be explained considering that Cuban soil composition has high levels of this metal, and in particular, ferritic (Ferralsol) soils are described in the Pinar del Rio region, where the sediments were sampled (Hernández and Moreno, 2010). However, in the case of Mn, contamination factor and degree have unusually higher values, compared to the rest of the transition metals studied in the sediment samples. Manganese is not considered a heavy metal “per se”, and its concentration in natural environments may be due to the content of different minerals or anthropogenic sources. In the case of RSD and PSD, although Mn values exceed the USEPA limits and background limits, suggesting an anthropogenic origin, no contamination sources (e.g. wastewater discharges, sewage sludge, mining and mineral processing, combustion of fossil fuels) have been reported or identified for this element in the zone. On the other hand, it should be mentioned that dissolved metals in seawater, such as those found in the estuarine water from where the river sediment is extracted, can be affected by the dissolution of redox-sensitive metals from reducing ocean floor and mid-ocean ridge hydrothermal sediments, which are typically iron- and manganese-rich (Andrews et al., 2004). Therefore, the presence of relatively high concentrations of soluble Mn is expected as a consequence of the disposal of San Diego hydrothermal waters in the river estuary and nearby pools (Peláez et al., 1990). Nevertheless, special care should be paid to the chemical equilibrium and distribution of this metal in the sediment samples, as well as to the conditions of application of the final peloid, since it was shown that Mn species are highly soluble and mobile, which can increase its bioavailability. Summarizing, the river and peloid sediments studied in this work can be considered as non-contaminated and suitable as prime material and medicinal peloids, paying attention to the previous warning related to Mn results. Other results of some metals (i.e. Co, Cu, Ni, Pb and Zn) in San Diego river sediments have been reported by Díaz Rizo et al. (2013), using instrumental nuclear analytical techniques. The results were compared with the concentration values in healing muds used worldwide (Spain, Italy and Croatia). Using threshold effect level (TEL) and probable effect level (PEL) criteria, the river sediment was classified by the authors as non-contaminated and suitable for medical purposes.
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5. Conclusions A physicochemical and geochemical characterization of the river sediment and the final peloid from San Diego de los Baños Thermal Center was done based on its original sedimentary geochemistry and composition, to contribute for its inorganic quality assessment in therapeutic uses. The results of the physicochemical analysis showed that the main differences between the two sediments, likely originated from the maturation process, are closely related to the decrease in electric conductivity and more negative redox potential (highly reductive) found in the final peloid. The comparative study of total content and geochemical speciation of seven transition metals (Cr, Cu, Fe, Ni, Mn, Pb and Zn) in the different fractions indicated that most of these elements appeared in the less mobile fractions, which suggest low availability in the sediments, under the studied conditions. In addition, the hydrogeochemical models predicted that the precipitation of most mineral species was determined by the saturation index based on oxide, hydroxide and sulfide species, under the physicochemical conditions measured for the river and peloid sediments. As a result, the San Diego de los Baños peloid was classified as mud or fangi, with low contamination factors, and containing most metals strongly retained in the peloid through the maturation process. The exception was Mn, which was mainly extracted in the most leachable fractions, in agreement with the predicted soluble species (Mn2+). The relatively high content and mobility found for this metal in the sediments suggest that regular monitoring should be done for this element content in the peloids used in the Thermal Center. The results from the present investigation show the relevance of physicochemical and elemental characterization to peloid classification and quality assessment, although complementary studies (e.g. membrane and sweat–peloid interactions) are suggested to corroborate the mobility and bioavailability of these metals from the sediment to the biological fluids and barriers. Moreover, it evidences the importance of performing geochemical speciation using both experimental and theoretical techniques, for proper assessment of mobility and distribution mechanisms of soluble and solid mineral species present in these sediments.
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