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Article

Physical–Chemical and Thermal Properties of Clays from Porto Santo Island, Portugal

by
André Valente
,
Paula C. S. Carvalho
and
Fernando Rocha
*
GeoBioTec-GeoBioSciences, GeoTecnhologies and GeoEngineering Research Centre, Department of Geosciences, Campus de Santigo, University of Aveiro, 3810-193 Aveiro, Portugal
*
Author to whom correspondence should be addressed.
Appl. Sci. 2024, 14(19), 8962; https://doi.org/10.3390/app14198962 (registering DOI)
Submission received: 10 July 2024 / Revised: 13 September 2024 / Accepted: 1 October 2024 / Published: 5 October 2024
(This article belongs to the Section Earth Sciences)
Browse Figures
Versions Notes

Abstract

:
The use of clays for thermal treatments and cosmetic purposes continues to be a worldwide practice, whether through the preservation of native cultural traditions, pharmaceutical formulations or integrative health and well-being practices. Special clays, such as bentonites, are very common for healing applications due to their high cation exchange capacity (CEC), high specific surface area (SSA) and alkaline pH values and, therefore, are used in multiple therapeutic and dermocosmetic treatments. Numerous bentonitic deposits occur on Porto Santo Island with different chemical weathering degrees. This research evaluates which residual soils have the most suitable characteristics for pelotherapy. The texture of residual soils varies from silt loam to loamy sand and SSA between 39 and 90 m2/g. The pH is alkaline (8.7 to 9.6), electrical conductivity ranges from 242 to 972 µS/cm, and CEC from 50.4 to 86.8 µS/cm. The residual soils have a siliciclastic composition (41.36 to 54.02% SiO2), between 12.52 and 17.65% Al2O3 and between 52 and 82% smectite content, which are montmorillonite and nontronite. Specific heat capacity (0.5–0.9 J/g°C) and cooling kinetics (14.5–19 min) show that one residual soil has the potential to be suitable for pelotherapy according to the literature. Moreover, the residual soils have As, Cd, Co, Cr, Hg, Mn, Ni, Pb, Sb and V concentrations higher than the limits of guidelines for cosmetics and pharmaceutical products.

1. Introduction

Minerals have been used in the pharmaceutical and cosmetic industries and for therapeutic purposes [1], and most of these are clays and clayey minerals, known as healing clays [2]. One of those is bentonite, which is a special type of clay mainly composed of smectite [3]. Bentonite is an aluminium phyllosilicate clay, but it is distinguished from other clays, as it has high cation exchange capacity, high specific surface area and alkaline pH values (>7). Among therapeutic applications, the external use of bentonites is highlighted, known as pelotherapy, for osteopathic rheumatic disorders treatment, post-traumatic processes and sprains [2,4], or even in dermocosmetics and dermopharmaceuticals, due to the following characteristics: consistency and adhesiveness, high specific heat capacity, low cooling rate, high cation exchange capacity and finer and more regular grain, which results in a pleasant sensation [5,6]. For instance, a high cation exchange capacity is a way of compensating the human body with the supply of some vital chemical elements that are present in minerals [4,7]. Furthermore, a high specific surface area enables higher adsorption of toxins, impurities, bacteria and viruses [1,8].
On Porto Santo Island, these green-coloured clays composed of fine-grained particles (<63 μm) and whose most abundant mineral is smectite are one of the most characteristic geological materials on the island and are of scientific, touristic and educational interest [9,10,11,12,13]. The occurrence of several bentonite deposits is a source of resources that are used locally in the construction of Porto Santo’s typical parlour houses, in wine clarification and in medical geology applications.
However, there is a shortage of recent research about the textural, chemical, mineralogical, physicochemical, thermal, rheological and toxicological characterization and suitability of these clay materials from Porto Santo Island for pelotherapy applications. The thermal parameters, such as cooling rate and specific heat capacity, have an important role in clay materials’ behaviour as therapeutic agents [14,15]. Bentonites have the potential to retain heat while the therapeutic treatment occurs, which enables a greater effect on musculoskeletal diseases [2,15].
Therefore, the main goal of this research is to assess the potential of seven residual soil samples from Porto Santo Island for therapeutic and dermocosmetic treatments through empirical analysis. For this purpose, the following objectives have been defined: (1) to characterize the textural properties, (2) to characterize chemical and mineralogical composition, (3) to characterize thermal properties, (4) to compare the values obtained to previous research and pre-established parameters in the literature and (5) to identify the best bentonite outcrop with the most suitable characteristics for the proposed treatments.

2. Geological Settings

Porto Santo Island, located in the North Atlantic Ocean (Figure 1a), with an area of 42 km2 and a length of 16 km in the NE–SW direction, is part of the Madeira archipelago in Portugal, formed by the Madeira and Selvagens islands [16]. Porto Santo Island (Figure 1b) is the oldest volcanic complex in the Madeira archipelago. The island corresponds to a volcanic edifice with a complex and heavily eroded structure, which has resulted in the exposure of rocks that bear witness to the submarine (lower unit), transitional and subaerial (upper unit) phases of formation [17,18].
At the NE and SW of this volcanic island, submarine sequences (lower unit) comprise basaltic and trachybasaltic flows interlayered with pyroclasites and hyaloclastites and other volcaniclastic deposits, which are associated with submarine acid outflows and acid domes [17,19]. The submarine basaltic rocks from lower unit, submarine volcanoclastic deposits (Figure 1c), also occur in the north-east sector, at Serra de Dentro and Serra de Fora, where bentonite deposits also crop out [20].
The upper (or subaerial) unit contains trachytes and basalt dykes, and basalt and gabbro plugs cut the various subaerial and submarine lava flows and occur in the north-eastern region of the island (Figure 1c) [17,19].
Once the volcanic structure was formed, the island began to be exposed to strong erosion, especially in the northern sector, that formed the Quaternary sedimentary deposits (Figure 1c), almost exclusively formed by calcareous sands and sandstones [19,20], which cover the central sector and south coast.

3. Materials and Methods

3.1. Materials Description

Seven residual soil samples (5, 14, 14A, 15A, 26, 36A and 119) were collected in the NE area of Porto Santo Island (Figure 1c). Samples 5, 14, 15A, 26 and 36A were collected in submarine volcanoclastic deposits (seamount phase), Sample 14A in submarine lava outcrops (island phase) and Sample 119 in acid domes (Figure 1c). The residual soil samples were collected up to 20 cm depth and sieved through a 2 mm sieve. At the laboratory, the samples were wet-sieved through a 63 μm sieve and dried at 40 °C. To perform complete and proper physical–chemical, granulometric, mineralogical and thermal characterization of the smectite-rich soils, we followed the procedures of Gomes [2], Bastos and Rocha [4], López-Galindo et al. [21], Rebelo et al. [22,23] and Quintela et al. [24].

3.2. Methods

3.2.1. Textural Analysis

The grain size distribution of the samples (<2 mm) was obtained by sieving, in accordance with NP EN 933-1:2014 norma, to classify the soil texture using the USDA Soil Texture Triangle [25]. The <2 μm fraction (clay fraction) was obtained by the sedimentation process following Stokes’ Law. The specific surface area (SSA) is determined using the BET method with the Micromeritics Gemini V 2380 equipment (manufactured by Micrometrics, in Norcross, Georgia), with N/He, precision and linearity around 0.5%, according to the protocol and standard used by the Department of Materials and Ceramics Engineering, University of Aveiro [22].

3.2.2. Chemical Analysis

The major elements (%) SiO2, Al2O3, Fe2O3, TiO2, CaO, MgO, K2O, Na2O and P2O5 were determined by X-ray fluorescence spectrometry (XRF), using the AXIOS PW4400/40 XRF-Panalytical equipment (manufactured by Marvel Panalytical, in Almelo, Netherlands), operating on a Rb tube under argon/methane. The loss on ignition (LOI) was also determined by heating the samples at 1000 °C for 2 h. Additionally, trace and minor elements were obtained from an aqua regia digestion of the samples and were determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) using Agilent Technologies 7700 Series equipment (manufactured by Agilent Technologies, in Santa Clara, CA, USA), following the method of Bastos and Rocha [4]. The detection limits in mg/kg were as follows: 0.01 for Cd, Ce, Hg, Mo and Y; 0.02 for In, Nd, Sb and Tl; 0.03 for Cs; 0.05 for Sn; 0.1 for As, Be, Co, Dy, Er, Eu, Gd, Ho, Li, Lu, Nb, Ni, Pb, Pr, Rb, Sc, Se, Tb, Th, Tm, U, W, Yb, Zn and Zr; 0.2 for Cu; 0.5 for Ba, La and Sr; 1.0 for B, Cr, Hg, Mn and V.

3.2.3. Mineralogical Analysis

The mineralogical analyses were obtained by X-ray diffraction (XRD) in non-orientated aggregates (<63 μm), separated by wet sieving, and in orientated aggregates (<2 μm). The oriented aggregates were treated with glycerol and exposed to heat treatment at 500 °C. The semi-quantification and identification of the minerals were determined using the criteria adopted from Brindley and Brown [26]. The method established the basal peak area (001) for each mineral, which was then weighted by reflection powers [27,28]. The results were obtained using a Philips/Panalytical power diffractometer, model X’Pert-PRO MPD (manufactured by Marvel Panalytical, in Almelo, Netherlands), carrying an automatic slit, using a KαCu (λ = 1.5405 Å) radiation operated at 50 kV and 30 mA, which allowed data to be collected from 2 to 70° 2θ with a step size of 1° and a counting interval of 0.02 s. The chemical composition and texture of the clay minerals were determined using a scanning electron microscope (SEM) with Hitachi S-4100 equipment, model VEGA LMU (manufactured by Tescan, in Brun, Czech), operating in high and low vacuums, capable of imaging through secondary and backscattered electron detectors and elemental chemical analysis by energy-dispersive spectroscopy (EDS) [23,29]. The mineralogical analyses were carried out at the Department of Geosciences, University of Aveiro.

3.2.4. Physical–Chemical Analysis

The pH was determined in accordance with the International Organization for Standards (ISO) [30], using calibrated Hanna Instruments equipment, model HI 9126, with an accuracy of ±0.05, in a 1:5 soil–distilled water suspension. Electrical conductivity (EC) was determined using calibrated Hanna Instruments equipment, model HI 9033 Multi Range, with an accuracy of ± 0.05, in accordance with ISO [31], in a 1:5 soil–water suspension [24,32]. Cation exchange capacity (CEC) is divided into saturation, filtration, distillation and titration, and through this testing, it is possible to determine the exchange ions. Initially, a saturation solution of ammonium acetate (CH3COONH4) was used; the solution was filtered through Macherey-Nagel MN640d filter paper under vacuum extraction to determine the exchange cations (Na+, K+, Mg2+ and Ca2+), using Agilent Technologies 7700 Series, by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) equipment (manufactured by Agilent Technologies, in Santa Clara, California, USA). The excess of ammonium acetate was cleaned with ethanol and tested with Nessler’s reagent. The distillation process begins with the addition of magnesium oxide (MgO), and the material is distilled into a flask with a 4% solution of boric acid (H3BO3) and the bromocresol indicator (0.1%). Finally, the sample was titrated with 0.1N hydrochloric acid (HCl), according to the procedure in Bastos and Rocha [4].

3.2.5. Thermal Analysis

The cooling kinetics is a parameter determined by a Dual Thermometer LT Lutron Tm-906A (manufactured by Lutron, in Coopersburg, Pennsylvania, USA) that calculates the decreasing rate in temperature between 60 °C and 30 °C at room temperature. The specific heat capacity was determined by differential scanning calorimetry (DSC) in a Shimadzu DSC-50 calorimeter (manufactured by Shimadzu, in Tokyo, Japan), with scanning speeds ranging from 0.1 to 99.0 °C/min and temperatures from ambient to 725 °C. The heat flux measurement range is from 0.01 mW to 100 mW. Thermal analyses were carried out according to the protocols and norms of the Department of Geosciences and the Department of Chemistry, University of Aveiro, respectively [4,22,24].
The Chemical Index Alteration (CIA) is used to evaluate the residual soil maturation intensity resulting from the alteration of volcanic rocks. According to Goldberg and Humayun [33], this index includes the most common major elements in volcanic rocks, agreeing with the following formula: CIA = Al2O3/ (Al2O3 + CaO + Na2O + K2O) × 100.

4. Results and Discussion

4.1. Morphology and Grain Size Analysis

The seven residual soil samples from Porto Santo Island show different grain size distributions (Table 1). The results indicate that Samples 5, 26, 36A and 119 contain 81%, 76%, 67% and 50%, respectively, of silt + clay fraction (<63 μm), which means fine granularity and better profitability compared to Samples 14, 14A and 15A, which contain sand fraction concentrations above 50%. The concentrations of fine particle fraction (silt + clay) in Samples 5, 26, 36A and 119 are lower than the samples of Porto Santo Island studied by Gomes et al. [13], which in their case was 90%. According to Veniale et al. [5] and Gomes and Silva [7], samples suitable for application in thermal treatments and dermocosmetics must have a high fine-particle fraction concentration. Studies carried out in the Spanish Thermal Centre show that in most of these places, the healing clays applied in pelotherapy have around 57–70% of particles sized between 2 and 20 μm [34].
Moreover, the samples should have specific surface area values greater than 10 m2/g [35], also found in residual soils from Porto Santo Island, which present specific surface area values ranging between 39 and 90 m2/g (Table 1). Samples 14, 15A, 26 and 36A show the highest values of the specific surface area from 89 m2/g to 90 m2/g (Table 1). These samples have similar values to the samples studied by Rebelo et al. [35] in same region, which in their case was 79 m2/g, although these values are significantly lower when compared to Silva et al. [11] and Gomes et al. [13], who obtained 119 m2/g. In addition, the specific surface area values are also within the range of values obtained from previous studies about thermal treatments and dermocosmetics [36,37,38]. These values are due to the highest clay particle fraction, and thus the presence of clay minerals. Samples 26 and 36A have a high ratio between the content of fine particles fraction and specific surface area, although it is advisable that these samples undergo a maturation process or mechanical grinding to obtain a finer and more regular grain in order to be suitable for therapeutic applications.
The SEM images of residual soil samples from Porto Santo Island show two different morphologies with different shapes and sizes (Figure 2a,b). Samples with less than 50% silt–clay fraction (14, 14A and 15A) exhibit a heterogeneous morphology with different particle sizes, with some grains more rounded and others more elongated (Figure 2a). Furthermore, the samples with a higher fine-particle fraction concentration (5, 26, 36A and 119) exhibit a more homogeneous morphology, with the fine particles displaying a scaly arrangement (Figure 2b).

4.2. Chemical and Mineralogical Analysis

The residual soil samples from Porto Santo Island show SiO2 concentrations ranging from 41.36% (Sample 14A) to 54.02% (Sample 5) (Table 2); therefore, all samples have a siliciclastic composition, which is coherent with their mineralogical composition (Table 3). The phyllosilicate contents (smectite) are higher than 50% in all samples, with Sample 36A having the highest concentration (82%) and Sample 14A the lowest concentration of phyllosilicates (55%) (Table 3), which is in line with the predominant fine-grained samples (Figure 2). According to Antunes et al. [39] and Rebelo et al. [22,35], the mineralogical composition of the samples collected on Porto Santo Island shows predominant phyllosilicate contents, above 50%, and in some cases, these contents reach 70% to 90%. Al2O3 varies from 12.52% (Sample 14A) to 17.65% (Sample 5) (Table 2), associated with the hydrated phyllosilicates, specifically smectite, which is present in all samples, and traces of kaolinite also found in Sample 14A (Table 3). The soil samples from Porto Santo Island also have in their mineralogical composition K-feldspars, plagioclases, calcite, Fe-oxides (magnetite/maghemite and hematite), opal, anatase and quartz (Table 3), in accordance with Antunes et al. [39] and Rebelo et al. [22,35]. The concentrations of CaO vary between 4.20% (Sample 36A) and 6.12% (Sample 26) and are related with the presence of calcite (Table 2 and Table 3), as is their occurrence in the structure of phyllosilicates as exchange cations. CaO and Na2O (which vary from 4.20% to 9.94% and 1.57% to 2.62%, respectively) are present in the structure of the plagioclases, but also in smectite. MgO has concentrations between 3.19% (Sample 26) and 6.01% (Sample 14A) and can be found in the structure of phyllosilicates (smectite) (Table 3) in the octahedral sheet or as exchange cations. Previous studies carried out on Porto Santo Island [12,13,22] support the major presence of SiO2 (45% to 55%), Al2O3 (17% to 19%), Fe2O3 (8% to 12%), CaO (2% to 6%) and MgO (3% to 4%) in the chemical composition of the samples.
The hydrolysis of aluminous silicates (including feldspars) corresponds to the chemical reaction between water and the mineral with the occurrence of cationic substitutions in the mineral structure by hydrogen ions (H+ and OH−) and is the process responsible for the formation of phyllosilicates [3,40]. Therefore, most residual soil samples that have higher content in phyllosilicates (14, 15A, 36A and 119) present lower content in feldspars and thus a higher degree of maturity than those with higher feldspar contents (5, 14A and 26). The CIA values (Table 2) of the samples are between 49.29% (14A) and 70.33% (36A), which means that the sample with the highest value (36A) is richer in clay minerals and Fe-oxides/hydroxides than the sample with the lowest CIA value (Sample 14A), as evidenced by the mineralogical composition of the samples (Table 3). However, in the case of the sample with lowest CIA value (14A), it is richer in primary minerals such as feldspars. These values are slightly lower when compared to other studies on volcanic soils [29].
The presence of magnetite/maghemite and hematite in these samples explains the concentrations of Fe2O3 (8.20–12.50%) (Table 2 and Table 3), as well as the reddish colour of the samples. Opal is present in most samples (except in Sample 36A). According to Ferreira and Serrano [41], the common clays formed by submarine diagenesis do not have quartz in their mineralogical composition, as it can be seen in XRD graphs (Figure 3a,b), although Sample 26 is the only one with minor concentrations of quartz.
In the study of the clay fraction (<2 μm), through the appropriate treatments of orientated aggregates (natural, glycerol and 550 °C), minerals from the smectite group are the main components of residual soil samples from Porto Santo Island, with characteristic reflections between d001 = 14.66 Å and d001 = 15.11 Å. The crystallinity index, which reveals the structural order–disorder of smectite, was determined using the ratio between the width (measured at half height) and the height of reflection (001). Therefore, the smectite with the highest crystallinity (0.24, 0.22 and 0.23; Samples 5, 14 and 119, respectively) are those with the lowest values of this index (Table 3); these index values are lower (0.22–0.54) than those studied by Ferraz et al. [9] and Antunes et al. [39], which are, in first study, around 0.84, and in second one, between 0.13 and 1.61. The smectite from residual soil samples collected in this study displays a dioctahedral type, with d060 values ranging from 1.48 Å to 1.50 Å, with the exception of Sample 14A, which displays a trioctahedral type (1.52 Å) (Table 3).
In addition, through the EDS analysis, it was possible to determine the presence of clay minerals from the smectite group, namely montmorillonite (Figure 4) and nontronite, with sodium (Na) being the main exchange cation, followed by calcium (Ca), magnesium (Mg) and potassium (K). Nearly twenty points were selected and analysed by SEM/EDS from each of the seven residual soil samples from Porto Santo Island. The most frequent chemical analyses by EDS are those shown in Table 4 (EDS1 and EDS2). These two examples describe two clay minerals from the smectite group and are represented by the following crystallochemical formula:
EDS1—(Si3.7Al0.3)(Al1.3Fe0.1Mg0.6)(Ca0.1Na0.7)O10(OH)2(H2O) can be considered as montmorillonite.
EDS2—(Si2.9Ti0.3Al0.8)(Al0.3Fe1.5Mg0.2)(Ca0.3Na0.2K0.1)O10(OH)2(H2O) can be considered as nontronite.
As can be seen in the crystallochemical formula of smectite, despite montmorillonite containing a low iron (Fe) content, it is rich in aluminium (Al) and magnesium (Mg); on the other hand, nontronite contains a significant content of iron in the octahedral layer.
The residual soil samples from Porto Santo Island show higher concentrations of Ba (barium), Cr (chromium), Cu (copper), Mn (manganese), Ni (nickel), V (vanadium) and Zn (zinc) compared to the other minor and trace elements (Table 5). These concentrations vary between 46 and 177 ppm in Ba, 5 and 57 ppm in Cr, 8 and 70 ppm in Cu, 572 and 3900 ppm in Mn, 4 and 114 ppm in Ni, 4 and 177 ppm in V and 86 and 108 ppm in Zn, which is in line with Rebelo et al. [23]. According to Rebelo et al. [23], the higher concentration was obtained for Ba (504 ppm), Cr (79 ppm), Cu (90 ppm), V (188 ppm) and Zn (99 ppm) and can be associated with the bedrock and chemical element mobility [30,42]. In agreement with the United States Pharmacopeia (USP 40-NF 35) [43], these elements, except for Ba and Zn, are classified as “Class 2” in terms of toxicity level, and it is recommended that their concentrations do not exceed the accepted limits. Therefore, according to the U.S. Pharmacopeia, a set of standards and methods is needed to guarantee the quality of raw materials, medicine drugs and other substances for curative usage [44]. Health Canada [45], the European Commission (EC No. 1223/2009) [46] and the European Medicine Agency (EMEA) [47] (Table 5) established the accepted limits that these elements must not exceed in order for them to be viable for therapeutic purposes.
According to Health Canada [45], Ba has a maximum limit of 1300 ppm. Moreover, in line with the EMEA [47], the maximum limit for Cr is 25 ppm, for Cu is 250 ppm, for Mn is 250 ppm, for Ni is 25 ppm, for V is 25 ppm and for Zn is 1300 ppm. However, the chemical composition of residual soil samples can contain impurities that are harmful to health, known as potentially toxic elements (PTEs): antimony (Sb), arsenic (As), cadmium (Cd), mercury (Hg) and lead (Pb), among others [48] and references therein], which are classified as “Class 1” in terms of toxicity level [43]. These elements commonly appear in the chemical composition of clay materials because they occur in nature, but they can also result from anthropogenic causes such as car traffic, agricultural production or mining activity [49]. Among other problems, the PTEs can cause breast cancer and pancreatic cancer, respectively [50]. For this reason, the European Commission [46] developed a list of substances prohibited in cosmetic products (EC No. 1223/2009), which includes As, Cd, Hg, Pb and Sb.
Samples 5, 14, 14A, 15A, 26, 36A and 119 present concentrations of As, Cd, Hg, Pb and Sb (“Class 1”) and Co, Cr, Mn, Ni and V (“Class 2”) higher than the acceptable limits.
Bearing in mind that “Class 1” elements are prohibited in cosmetic products, thus an acceptable limit of 0 mg/kg was considered [46].
All residual soil samples have an As, Cd, Hg, Pb and Sb content slightly above 0 (mg/kg), required by the European Commission [46]. Furthermore, Samples 5, 14, 14A, 15A, 26, 36A and 119 show concentrations of Co and Mn higher than the acceptable limit established by Health Canada [45] and EMEA [47], respectively. In the case of Samples 14A and 119, they contain concentrations of Cr and Ni, respectively, above the acceptable limit of EMEA [47]. Despite the concentration of As, Cd, Co, Cr, Hg, Mn, Ni, Pb, Sb and V being higher than the acceptable limit for therapeutic purposes, it is recommended, in future works, to determine the dermal bioaccessibility of these elements to understand how easily they are introduced into the human body.

4.3. Physicochemical Properties

The pH values are between 8.7 (Sample 119) and 9.6 (Samples 14 and 36A) (Table 6). Thus, it is considered an alkaline to very alkaline pH, which is in agreement with Lou et al. [51], who mentioned that alkaline pH is typical of soil samples with the presence of smectite. The skin pH slightly differs in different areas of the body, but in general, it is between 4.7 and 5.8 [52], meaning that the skin is acidic and acts as a protective barrier: neutralizing aggressors, inhibiting the growth of bacteria and restoring and maintaining the skin’s ideal acidic environment [4]. People with skin diseases have low pH values, so it is essential to apply an alkaline product to stabilize the skin’s pH levels [53]. The pH values of these samples are acceptable for skin therapeutic purposes, because, according to Quintela et al. [24], normally, samples considered healing clays and applied for pharmaceutical and cosmetic uses have a pH higher than 6. Regarding electrical conductivity, the results are between 242 µS/cm (Sample 26) and 972 µS/cm (Sample 15A), which are within the values appropriate for thermal treatments (10 to 1000 µS/cm) [6]. The cation exchange capacity values of residual soil samples vary between 43.8 meq/100 g (26) and 86.8 meq/100 g (36A) (Table 6), which is in line with Silva et al. [11] and Gomes et al. [13], who obtained around 80 meq/100 g in samples of the same region. The highest CEC values occur in the samples with the highest smectite content (15A and 36A) (Table 3), which are due to the high specific surface area of smectite. Moreover, these results are similar to other healing clays with the same application purposes [14,29]. According to Rebelo et al. [35], CEC must be higher than 10 meq/100 g in clays for pelotherapy applications; therefore, the soil samples from Porto Santo Island meet the minimum requirements to be considered as healing clays for pelotherapy. The concentration of exchangeable cations obtained in the residual soil samples is for Na+ (2.9–185.4 mg/L), Ca2+ (4.3–131.2 mg/L), Mg2+ (7.6–80.9 mg/L) and K+ (3.9–39.7 mg/L), with the highest concentrations of Na+ in Samples 14, 15A, 36A and 119, of Mg2+ in Samples 14A and 26, and of Ca2+ in Sample 5 (Table 6). The high CEC is a way of balancing the body with the supply of some vital chemical elements which belong to minerals [54], which is one of the main reasons why healing clays are used in thermal treatments and dermocosmetics [4,8].

4.4. Thermal Characterization

The residual soil samples have specific heat capacity values that are in the range of 0.6–0.9 J/g°C (Table 7). According to Rebelo et al. [35], the specific heat capacity of the samples should be higher than 0.5 J/g°C, which is a benefit for the pelotherapy treatments, as it requires less energy to heat. Therefore, it leads to the conclusion that they have acceptable values to be considered as healing clays, although these values are slightly lower compared to other samples from Porto Santo Island [11,35], where the values were 2.46 and 3.55 J/g°C, respectively, and other healing clays [55]. In terms of cooling kinetics, it is one of the fundamental properties of healing clays concerning applications in pelotherapy treatments; on the other hand, it is not a main characteristic in dermocosmetics. According to Gomes et al. [2], for a treatment to be effective, the material must maintain a temperature of over 30 °C for 20–30 min. From the residual soil samples from Porto Santo Island, Sample 36A is the only one that comes close to this time interval (19.0 min), which means that this sample takes longer to dissipate heat compared to others, like Sample 5, which has the lowest time (14.5 min). Sample 36A has the highest smectite contents (Table 3), which is in line with the thermal properties obtained. Studies conducted by Silva et al. [11] and Gomes et al. [13] in Porto Santo Island show cooling time values around 38 min, which means that the values of this research (14.5–19.0 min) are also slightly lower compared to other healing clays [29,55]. Because the measurement of the cooling kinetics was made on dry samples, an increase in cooling time with the wetting of residual soil samples is predicted [4,5,55]. Nevertheless, if we correlate the two thermal properties, specific heat capacity and cooling kinetics, it is possible to conclude that overall samples with longer a cooling time (extra time to dissipate heat) have higher specific heat capacity than those samples that take less cooling time.

5. Conclusions

The residual soil samples from Porto Santo Island are composed of smectite, feldspars (K-feldspars and Ca/Na plagioclases), Fe-oxides/hydroxides (magnetite/maghemite and hematite), calcite and opal, with a CIA that varies from 49.29% to 70.33%. From the seven residual soil samples studied, one sample (36A) is composed mainly of smectite (82%), has a pH of 9.6, an EC of 772 µS/cm and a CEC of 86.8 meq/100 g. This sample also presents the highest CIA (70.33%) and has an SSA of 89 m2/g and 67% of silt plus clay grain size particles. The thermal properties are also good for use in pelotherapy, with the specific heat capacity of 0.9 J/g°C and the cooling kinetics of 19 min; however, measurement of moisture content should be considered. Moreover, in Sample 36A, the concentrations of PTEs (As, Cd, Co, Cr, Hg, Mn, Ni, Pb, Sb and V) did not meet the requirements for cosmetic and pharmaceutical purposes, and their dermal bioaccessibility should be assessed in future studies. Furthermore, it is proposed to enlarge the sampling in this bentonitic deposit, the rheological characterization and the maturation process of the residual soils.

Author Contributions

Conceptualization, A.V., P.C.S.C. and F.R.; Methodology, A.V., P.C.S.C. and F.R.; Validation, F.R.; Formal analysis, A.V., P.C.S.C. and F.R.; Investigation, A.V., P.C.S.C. and F.R.; Resources, P.C.S.C. and F.R.; Data curation, A.V., P.C.S.C. and F.R.; Writing—original draft, A.V., P.C.S.C. and F.R.; Writing—review & editing, P.C.S.C. and F.R.; Visualization, P.C.S.C.; Supervision, F.R.; Project administration, F.R.; Funding acquisition, F.R. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the Fundação para a Ciência e Tecnologia (FCT), through national funds, by the projects UIDB/04035/2020—GeoBioTec-GeoBioSciences, GeoTechnologies and GeoEngineering, funded by FCT, FEDER funds through the Operational Program Competitiveness Factors—COMPETE.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

We are grateful to Denise Terroso for the XRD data and Cristina Sequeira for the XRF, SEM and grain size distribution data. As well as Celeste Azevedo from the Department of Chemistry for specific heat data and Célia Miranda from the Department of Materials and Ceramic Engineering for specific surface area data, all from the University of Aveiro. We thank the anonymous reviewers for their careful reading of our manuscript and valuable suggestions that helped improve it.

Conflicts of Interest

The authors declare that they have no competing financial interests or personal relationships that may have influenced the work reported in this study.

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Applsci 14 08962 g001
Figure 1. The geographic settings of Porto Santo Island and the Madeira archipelago (a,b) and the geological setting of Porto Santo Island with the sampling distribution (c).
Figure 1. The geographic settings of Porto Santo Island and the Madeira archipelago (a,b) and the geological setting of Porto Santo Island with the sampling distribution (c).
Applsci 14 08962 g001
Applsci 14 08962 g002
Figure 2. SEM images (ampliation 150× and resolution 15 keV) of Sample 14 (a), with different particles sizes and shapes, and Sample 36A (b), with a homogeneous morphology.
Figure 2. SEM images (ampliation 150× and resolution 15 keV) of Sample 14 (a), with different particles sizes and shapes, and Sample 36A (b), with a homogeneous morphology.
Applsci 14 08962 g002
Applsci 14 08962 g003
Figure 3. XRD graphs of samples: (a) 14A and (b) 36A.
Figure 3. XRD graphs of samples: (a) 14A and (b) 36A.
Applsci 14 08962 g003
Applsci 14 08962 g004
Figure 4. Photography of a fine particle (<2 μm) of montmorillonite (ampliation 150× and resolution 15 keV).
Figure 4. Photography of a fine particle (<2 μm) of montmorillonite (ampliation 150× and resolution 15 keV).
Applsci 14 08962 g004
Table 1. Grain size distribution and specific surface area of residual soil samples from Porto Santo Island as well as theoretical values for smectite and pelotherapy applications.
Table 1. Grain size distribution and specific surface area of residual soil samples from Porto Santo Island as well as theoretical values for smectite and pelotherapy applications.
Soil Samples% Clay
(<2 μm)		% Silt
(<63 μm)		% Sand
(>63 μm)		Soil Texture (USDA, 2017)Specific
Surface Area (m2/g)
5146719Silt loam83
1463064Sandy loam90
14A22078Loamy sand39
15A83062Sandy loam90
26156124Silt loam90
36A115633Silt loam89
11954550Sandy loam76
Smectite *----150–800
Pelotherapy Applications **---->10
* In Gomes et al. [10]; ** pelotherapy applications, in Rebelo et al. [35]; - not defined.
Table 2. Chemical analyses (%) of major elements of residual soil samples from Porto Santo Island as well as Chemical Index Alteration (%).
Table 2. Chemical analyses (%) of major elements of residual soil samples from Porto Santo Island as well as Chemical Index Alteration (%).
51414A15A2636A119
SiO254.0248.0441.3646.5646.9247.9651.21
TiO21.932.882.042.882.402.602.10
Al2O317.6516.1312.5216.2216.4015.6516.31
Fe2O38.2012.5012.2812.4911.2211.649.86
MgO2.813.886.014.233.194.253.29
CaO4.644.339.944.646.124.204.93
Na2O2.621.571.691.781.971.592.62
K2O0.921.031.250.821.530.810.83
P2O50.571.180.581.780.911.521.56
SO30.020.040.030.050.090.030.09
LOI7.707.9013.857.9013.859.426.13
CIA68.3369.9549.2969.1263.0370.3366.05
Table 3. Semi-quantification (%) and mineralogical qualification of residual soil samples from Porto Santo Island as well as crystallinity index and d060.
Table 3. Semi-quantification (%) and mineralogical qualification of residual soil samples from Porto Santo Island as well as crystallinity index and d060.
51414A15A2636A119
Smectite61685273568268
Kaolinite--3----
Plagioclases191313714512
K-Feldspars6331924
Calcite33172223
Hematite--2----
Magnetite/maghemite510461196
Opal63666-7
Anatase---5---
Quartz----2--
Smectite crystallinity index0.240.220.360.390.540.370.23
d0601.501.501.521.501.491.501.49
- Not detected.
Table 4. EDS analysis of two fine particles (1 and 2).
Table 4. EDS analysis of two fine particles (1 and 2).
AlCaClFeMgOKSiNaTi
EDS14.70.40.00.41.879.00.111.52.10.1
(Si3.7Al0.3)(Al1.3Fe0.1Mg0.6)(Ca0.1Na0.7)O10(OH)2(H2O)
EDS27.01.90.210.41.354.51.019.81.62.3
(Si2.9Ti0.3Al0.8)(Al0.3Fe1.5Mg0.2)(Ca0.3Na0.2K0.1)O10(OH)2(H2O)
Table 5. Concentration of minor and trace elements (mg/kg) in residual soil samples from Porto Santo Island and limits for therapeutic purposes from some European guidelines.
Table 5. Concentration of minor and trace elements (mg/kg) in residual soil samples from Porto Santo Island and limits for therapeutic purposes from some European guidelines.
51414A15A2636A119HC (2012)EC
(2009)		EMEA (2008)
As<0.10.1<0.10.1<0.10.20.730-
B42511950315134---
Ba46.168.175.859.195.674.41771300--
Be1.91.21.21.51.41.53.4
Cd0.10.10.20.10.20.10.230-
Co7.425.237.617.622.713.016.35--
Cr5.05.057.06.020.04.07.0--25
Cs0.20.20.20.30.40.30.5
Cu14.422.170.110.719.78.411.9--250
Ga10.41313.113.19.6313.511
Hg0.040.040.050.050.030.070.0330-
In0.090.070.050.060.080.080.08
Li4.86.111.279813.5
Mn5721070143088113009403900--250
Mo0.290.560.670.360.760.250.97--25
Nb<0.1<0.10.9<0.10.7<0.10.3
Ni5.520.41148.224.44.244.4--25
Pb3.12.12.82.84.63.34.8100-
Rb15.39.78.99.112.711.510.1
Sb0.050.050.070.040.080.020.0950-
Sc5.711.410.79.88.78.26.1
Se<0.1<0.10.40.10.2<0.1<0.117--
Sn1.971.381.131.391.371.481.87
Sr75.7177169183207126125
Th8.75.13.15.43.66.88.3
Tl0.040.030.040.030.050.020.140.8--
U0.91.60.51.30.71.01.8
V631771211302047--25
W<0.10.10.20.10.20.10.3
Y28.625.718.326.923.430.551.5
Zn93.890.98785.887.996.4108--1300
Zr2.51.63.22.13.42.31.6
Ce16196.571.497.690.9127160
Dy6.96.24.16.45.76.98.8
Er3.12.92.03.12.73.45.1
Eu3.12.81.72.72.23.13.4
Gd9.78.25.68.37.39.711.3
Ho1.21.10.71.11.01.21.7
La74.846.63447.443.16280.1
Lu0.40.30.20.30.30.40.6
Nd64.845.931.245.541.955.966.8
Pr17.111.3811.410.314.417.2
Sm11.58.56.79.47.910.612.1
Tb1.31.10.81.11.01.31.6
Tm0.40.30.20.30.30.40.6
Yb2.52.21.62.22.02.63.8
- Not defined; HC—Health Canada; EC—European Commission; EMEA—European Medicines Agency.
Table 6. Physico-chemical parameters and concentration of exchangeable cations in residual soil samples from Porto Santo Island.
Table 6. Physico-chemical parameters and concentration of exchangeable cations in residual soil samples from Porto Santo Island.
Soil SamplespHElectrical Conductivity
(µS/cm)		CEC
(meq/100 g)		Exchange Ions
(mg/L)
Na+K+Mg2+Ca2+
59.371050.493.521.657.3131.2
149.673074.0155.620.578.56.5
14A9.364660.22.93.97.64.3
15A9.597274.6168.524.17.66.4
268.824243.810.925.965.67.4
36A9.677286.8185.418.358.68.0
1198.794059.8139.729.780.975.8
Smectite *--60–150----
Pelotherapy applications **-->10----
* In Gomes et al. [10]; ** pelotherapy applications, in Rebelo et al. [35]; - not defined.
Table 7. Thermal results of residual soil samples from Porto Santo Island as well as theoretical values for pelotherapy applications.
Table 7. Thermal results of residual soil samples from Porto Santo Island as well as theoretical values for pelotherapy applications.
Specific Heat Capacity
(J/g°C)		Cooling Kinetics
(min)
50.614.5
140.715.5
14A0.818.0
15A0.816.5
260.915.5
36A0.919.0
1190.817.0
Pelotherapy *0.5-
Pelotherapy **-~20.0
* In pelotherapy applications, in Rebelo et al. [35]; ** Gomes et al. [2]; - not defined.
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Valente, A.; Carvalho, P.C.S.; Rocha, F. Physical–Chemical and Thermal Properties of Clays from Porto Santo Island, Portugal. Appl. Sci. 2024, 14, 8962. https://doi.org/10.3390/app14198962

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Valente A, Carvalho PCS, Rocha F. Physical–Chemical and Thermal Properties of Clays from Porto Santo Island, Portugal. Applied Sciences. 2024; 14(19):8962. https://doi.org/10.3390/app14198962

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Valente, André, Paula C. S. Carvalho, and Fernando Rocha. 2024. "Physical–Chemical and Thermal Properties of Clays from Porto Santo Island, Portugal" Applied Sciences 14, no. 19: 8962. https://doi.org/10.3390/app14198962

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