Applied Clay Science 83–84 (2013) 270–279

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Applied Clay Science

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Research paper

Composition and physico-chemical properties of peloids used in Spanish spas: Acomparative study

Manuel Pozo a,⁎, María Isabel Carretero b, Francisco Maraver c, Eduardo Pozo d, Isidoro Gómez b,Francisco Armijo c, Juan Antonio Martín Rubí d

a Dpto. Geología y Geoquímica, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spainb Dpto. Cristalografía, Mineralogía y Química Agrícola, Facultad de Química, Universidad de Sevilla, 41012 Sevilla, Spainc Escuela de Hidrología Médica, Facultad de Medicina, Universidad Complutense de Madrid, 28040 Madrid, Spaind Instituto Geológico y Minero de España (IGME), Tres Cantos, 28760 Madrid, Spain

⁎ Corresponding author. Tel.: +34 914974808; fax: +3E-mail addresses: manuel.pozo@uam.es (M. Pozo), car

fmaraver@med.ucm.es (F. Maraver), epozo@igme.es (E. Pfarmijoc@med.ucm.es (F. Armijo), ja.martin@igme.es (J.A.

0169-1317/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.clay.2013.08.034

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 March 2012Received in revised form 20 August 2013Accepted 25 August 2013Available online 14 September 2013

Keywords:PelotherapySpanish peloidsClay mineral propertiesInstrumental texture analysis

This study analyses and characterises the thermal and non-thermal properties of five peloids from differentSpanish locations. Three of the peloids designated as ARCH, ARN and RAP were made from medical mineralwater and clay. The peloid designated as BOI was made from medical mineral water and a mixture of peat andclay, and the fifth peloid designated as LOP was derived from a natural sedimentary environment (lagoon)wherein the clayey silt sediment was matured in sea water. The samples were compared by means of physicaland physico-chemical determinations namely, grain-size analysis, BET specific surface area, plasticity index,CEC and exchangeable cations, instrumental texture analysis and thermal parameters. The results showed signif-icant differences between the different peloids studied, especially with regard to their composition and somenon-thermal properties. The highest values for BET, plasticity index and CEC were observed in the sample com-posed of Na-saturated trioctahedral smectite. The mixing of peat with clay also favoured high values for CEC andplasticity index. The instrumental texture analysis showed similar values for cohesiveness and springiness in allthe peloids, but there were differences in hardness and adhesiveness. Regarding the thermal properties, thevalues were similar within a relative narrow range for retentivity, t37 parameter, and relaxation time. Despitethe differences in the composition of the peloids, the values achieved for instrumental texture analysis and ther-mal parameters were comparable with those of the reference TERDAX peloid.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

In a recent paper by Gomes et al. (2013) a new definition of peloidhas been proposed: “A peloid is a maturated mud or muddy dispersionwith healing and/or cosmetic properties, composed of a complex mix-ture of fine-grained natural materials of geologic and/or biologic origin,mineral water or sea water, and common organic compounds from bio-logical metabolic activity”.

Peloids have been routinely used as thermal agents in many spas,health resorts and medical centres since ancient times (Carreteroet al., 2006). Those peloids made with clay are the type that is most ex-tensively used in Mediterranean countries (e.g., France, Greece, Italy,Turkey, and the Dead Sea area), whilst peat based peloids are preferredin Northern and Central Europe (e.g., Czech Republic, Germany, and

4 914974900.re@us.es (M.I. Carretero),ozo), iangel@us.es (I. Gómez),M. Rubí).

ghts reserved.

Hungary) and peloids consisting of sulphur-rich compounds are usedin other places in the world (e.g., Argentina).

The maturation (ageing) process for the formation of peloid is com-plex, involving physical, chemical, physico-chemical and biological phe-nomena (Fig. 1). The process is dependent both on the nature and initialcomposition of themineral water and the claymaterial used, and on thecharacteristics of the clay–water mixture after maturation (pH, Eh, bio-genic elements, organic matter). The other factors involved in the mat-uration process are temperature, maturation time and the particularprocedure selected, i.e., continuous stirring, discontinuous stirring, orwithout stirring.

In recent years, several authors have made significant contribu-tions to the study of peloids (Gomes et al., 2013, and referencestherein). The therapeutic effects of peloids have been extensivelystudied, particularly in the fields of rheumatology and dermatology(Beer et al., 2003; Bellometti et al., 2005; Codish et al., 2005; Evciket al., 2007; Fioravanti et al., 2007; Forestier et al., 2010; Fraioliet al., 2011; Nappi et al., 1996; Sukenik et al., 1990). These effectsare based on the thermal-therapeutic properties of the peloids aswell as their chemical and biological actions which are mainly

Mineralwater

Paste or dispersionwith therapeutic action(healing mud)

Inorganic and/ororganic mud

MATURATION

PELOID

Staticorstirring

TemperatureSolid/water ratioTime

MUD WATER

BIOLOGICAL PRODUCTS

PRECIPITATED SALTS

Fig. 1. Scheme showing the peloidmanufacture. As can be seen the peloid is a complexmixture includingmud, water, precipitated salts and biological products. In addition to themineralwater and clay/organic mud composition the main controlling factors are temperature, solid/liquid ratio and time.

271M. Pozo et al. / Applied Clay Science 83–84 (2013) 270–279

associated with the components generated during the maturationprocess (Galzigna et al., 1996; Odabasi et al., 2008).

For a clay paste or peloid to be suitable for pelotherapeutic uses, itshould have several properties, such as a low cooling rate, high absorptioncapacity, high cationic exchange capacity, good adhesiveness, easy han-dling, and pleasant feel when applied to the skin (see review ofCarretero et al., 2006).

The present study analyses five peloids from different Spanish loca-tions. These peloids are currently in use and have a medically proventherapeutic effect on the treatment of rheumatic diseases (Maraveret al., 2004). The main aim of this study is to make a comparison oftheir composition andphysico-chemical properties in order to establish,

RAP

ARN

AREL RAPOSO

ARNED

LO P

SPAIN

Por

tuga

l

Fig. 2. Geographical location and appearance

if possible, reference values for thepreparation of peloids using differentclay–mudmaterials andmineral waters. These results will also allow anevaluation of the actual influence of the inorganic parameters on theirtherapeutic effects.

2. Materials and methods

2.1. Materials

The peloid samples ARCH, ARN, BOI and RAP were obtained fromfour Spanish spas; Archena (Murcia), Arnedillo (Logroño), Caldas deBoí (Lérida) and El Raposo (Badajoz), respectively. The fifth sample

BOI

ARCH

CALDAS DE BOI

CHENA

ILLO

AGAN

France

N

LOP

of the different Spanish peloids studied.

272 M. Pozo et al. / Applied Clay Science 83–84 (2013) 270–279

(LOP) is a natural healing mud consisting of muddy sediments from asea water lagoon located in south eastern Spain (Lo Pagán, Murcia).The location of samples and their appearance in spa centres are shownin Fig. 2. The samples were collected in their place of origin. A homoge-neous portion of the peloid was placed in a plastic tube and stored in arefrigerator at 4 °C until the time of analysis.

2.2. Analytical methodology

A sample from each of the five peloids was removed and the follow-ing physical parameters were measured: colour, percentages of waterand solids, densities of the water and solids, peloid density, percentageof residue at 850 °C, consistency (liquid limit, plastic limit, plasticityindex), instrumental analysis of texture (TPA), grain-size distributionand thermal parameters. Using dried peloids, an investigation was car-ried out by means of SEM–EDX. Using a powdered sample, the specificsurface area (BET), CEC and abrasiveness were determined. The miner-alogical and chemical characterizations of these peloids have been re-ported by Carretero et al. (2010), who carried out experiments usingartificial sweat which showed that potentially toxic elements were notleached out from the peloids.

The textural features and microfabric were examined using a PHILIPSSEM XL-30 electron microscope equipment with EDX analytical system(EDAX DX4i). The samples were desiccated and the undisturbed sampleswere lightly gold sputter coated prior to examination.

The plasticity index was calculated based on Atterberg limits(liquid limit and plastic limit), following the UNE 7-377-75 and UNE103-104-93 standards. The grain-size distribution was determined bywet sieving for coarser fractions (N100 μm) and photo sedimentation(MicromeriticsR SediGraph 5100 ET) in the 0.5 to 100 μm range.

The cationic exchange capacity (CEC) was determined followingthe Tuckermethod (Tucker, 1974), wherein the clay sample is saturatedwith 1 N ammonium chloride at pH 8.2. After which the presenceof ammonium is measured using a Kjeldahl distillation apparatusand exchangeable Ca2+, Mg2+, Na+ and K+ cations determined byatomic absorption spectrophotometry using a PerkinElmer AAnalist-100spectrophotometer.

To obtain the thermal parameters and to define the cooling curves ofthe peloids the procedure of Rambaud et al. (1986) and Armijo (1991)was followed. The heat capacity (volume), thermal conductivity coeffi-cient and thermal retentivity were obtained from calculations. The t37parameterwas determined from the cooling curve,which is the time re-quired for the peloid to cool from 45 °C to 37 °C. Finally, the relaxationtime (Armijo and Armijo, 2006) was ascertained to quantify the heattransfer rate. The exponential temperature decay of a peloid is definedas the time necessary for 63.22% of the whole to decrease in tempera-ture from 45 to 39.9 °C.

The abrasivity of the peloid dispersionswas determined by themassloss (mg) of a bronze wire, before and after rotation in the dispersionusing an Einlehner abrasion tester (model no. AT 1000).

The instrumental texture analysis was performed using a Stevenstexture analyzer (model no. QTS 25). This analysis is primarily usedfor the evaluation of themechanical characteristics of amaterial, usuallyfood. Thematerial is subjected to a controlled force fromwhich a defor-mation curve of its response can be generated. The texture profile anal-ysis (TPA) values and the resulting curve are shown in Fig. 3. A scientifictexture analysis can provide quantifiable, repeatable and accurate dataabout the physical properties of food, cosmetics, pharmaceutical andchemical products. For the purposes of this study, the characteristicsanalysed were hardness, springiness, adhesiveness and cohesiveness(Bourne, 1978; Szczesniak, 1963).

Hardness (popular nomenclature: soft, firm, hard) is defined here asthe compression force necessary to attain a given deformation measuredin grammes (g), and the instrumental definition is the peak force of thefirst compression cycle (see Fig. 3). Springiness (popular nomenclature:plastic, elastic) is the rate at which a deformed material returns to its

original condition after the deforming force has been removed, and ismeasured in millimetres (mm). The instrumental definition of springi-ness is the height that the material recovers during the time betweenthe end of the first compression and the start of the second compression.Adhesiveness (popular nomenclature: sticky, tacky, gooey) is the force re-quired to pull amaterial away froma surface and ismeasured in grammesper second (g ∙s). The instrumental definition of this force is the negativearea for the first compression. Cohesiveness (popular nomenclature:crumbly, crunchy, brittle) is the strength of the internal bonds whichform the body of the material (dimensionless); the instrumental defini-tion is the ratio between thepositive force during the second compressioncycle and that of the first.

Thematerial to be tested was placed in a plastic vessel 8 cm in heightand 6 cm in diameter. After homogenising the sample, it was levelledwith the top of the vessel and a test probe ball (1 cm in diameter)was inserted into the sample to a depth of 20 mm, at a velocity of30 mm/min and a trigger of 5 g, applying TPA (test profile analysis)through two consecutive cycles.

The results of the instrumental texture and thermal properties of thereference peloid (TERDAX) from a French thermal centre in Dax werealso analysed for use as a comparison.

3. Results

3.1. Mineralogy

The mineralogy of the peloids has been studied by Carretero et al.(2010) and is summarised in Table 1. Themineralogical analysis revealedsignificant differences in the composition of both the bulk sample andclay fraction. All the peloids studied had a clay mineral content higherthan 40%, although the BOI sample had lower percentages as a result ofits organic matter (peat) and the LOP sample because of its muddy–siltycomposition. All the peloids had more than 10% calcite, with the excep-tion of ARCH (b5%) which was also the only peloid with a significantpercentage of dolomite (2%). The detrital mineral content (quartz andfeldspars) was very low in the ARCH and BOI samples, but was approxi-mately 10% in the ARN and LOP samples, even reaching 14% in the RAPsample.

The clay mineral assemblages consisted of smectite and subordinat-ed illite in the ARCH sample, but comprised a mixture of illite and/orsmectite with subordinated kaolinite and chlorite in the other peloids(see Table 1). Smectite was present in all the clay fraction sampleswith the exception of the LOP sample. The highest smectite contentwas found in the ARCH and RAP samples, which were also the richestin clay minerals. It should be noted that the ARCH sample containedtrioctahedral smectite (Na-saturated) but in the other samples theclay minerals were dioctahedral. Illite was the second most abundantclay mineral, with the exception of the LOP sample wherein illite–mica predominated. Kaolinite and chlorite were also found, but inlower proportions. The presence of gypsum and halite salts in thepeloids is related to the chemical properties of the maturation water,and is particularly marked in strongly mineralized waters such asthose belonging to Archena, Lo Pagán, and Arnedillo (Carretero et al.,2010). The presence of halite and gypsum after the maturation of claywith sea water or mineral-medicinal water has been observed byCarretero et al. (2007) and Tateo et al. (2010), respectively.

3.2. Peloid microfabric

The SEM–EDX study of the air-dried samples revealed that the con-stituents were arranged with clastic textures in a compact matrix-typeskeletal and glomerular microfabric (Grabowska-Olszewska et al.,1984), in which the orientation of the components was determined bythe size and morphology of their constituents. Only the ARCH sampleshowed differing features as a result of its specific composition.

Load

(g)

Time (s)

A1

A2

A3

H

S

H = HARDNESSA2/A1 = COHESIVENESSA3 = ADHESIVENESSS = SPRINGNESS

A B

Fig. 3. Textural profile analysis. A. Typical TPA graph illustrating the procedure to obtain hardness, cohesiveness, adhesiveness and springiness values. B. Image of the texture analyzer usedin this work.

273M. Pozo et al. / Applied Clay Science 83–84 (2013) 270–279

The ARCH sample appeared as dense aggregates of fine smectite par-ticles (b5 μm) which contained localised euhedral dolomite (Fig. 4A).The smectite showed a characteristic wavy and crenulated appearance,occasionally with a laminar to turbulent microfabric and a magnesium-rich composition (EDX analysis: 64.23% SiO2, 29.61% MgO, 3.71% Al2O3,0.68% K2O, 0.67% CaO, 1.09% Fe2O3).

The ARN sample showed a clastic texture in which laminar (clayminerals) and non-laminar components, mostly carbonates and silicates(terrigenous components), were mixed (Fig. 4B). Within the laminarcomponents (b10 μm) the face–edge and edge–edge orientations haddeveloped in such a way as to generate interparticle porosity. Localisedaggregates of laminar particles were coating the silicate and carbonategrains displaying turbulent microfabric (Fig. 4C).

The BOI sample was characterised by the presence of an abun-dance of plant biological remains, as well as large calcareous bioclasts(N500 μm) and fine-grained siliceous deposits (20 μm) which corre-spond to shells and diatom frustules, respectively (Fig. 4D). It is worthnoting the polymineralic character of the sample, and its porosity

Table 1Mineralogical composition of Spanish peloids.Adapted from Carretero et al. (2010).

Samples ARCH ARN BOI RAP LOP

Bulk mineralogy (%w/w)Phyllosilicates 95 60 42 65 41Quartz 2 10 6 10 9Calcite 1 30 15 20 40Dolomite 2 0 0 0 IdPlagioclase 0 Id 0 3 IdK-feldspar 0 Id 0 0 0Gypsum 0 0 1 0 4Halite 0 Id 0 0 6Others Tr. (bloedite) 0 1% (cri) 0 Tr. (arag)Organic matter (OM) 0 0 35 2 Id

Clay mineralogy (%w/w)Smectite 94 (89) 38 (23) 50 (21) 70 (45) 0Illite 6 (6) 52 (32) 40 (17) 28 (19) 85 (35)Kaolinite 0 9 (5) 5 (2) 2 (1) 10 (4)Chlorite 0 1 (b1) 5 (2) 0 5 (2)Mixed layers 0 Id Id 0 0d (060) Ǻ 1.526 1.504 na 1.506 na

which have resulted from the varying dimensions and morphologiesof the constituents.

The RAP sample had a porous appearance with sub-rounded inclu-sions dispersed throughout the clay groundmass wherein claymineralsand carbonates were mixed, giving rise to a glomerular microfabric(Fig. 4E). The clay aggregates exhibited both the typical crenulatedsmectite particles and sheet micamorphologies, aswell as clastic grainsof varied composition.

The LOP sample was characterised by a clastic texture wherein thegrains of carbonate and silicate were included in a clayey-silt matrixand precipitated salts, mostly halite (Fig. 4F).

3.3. Non-thermal physical and physico-chemical properties

The values obtained for both physical and physico-chemical proper-ties have been summarised in Table 2. The consistency tests revealedthat the plasticity index was highest in the ARCH sample (227%) andlowest in the LOP sample (20%) (Fig. 5A). In the other samples theindex ranged between 30 and 60% (BOI N RAP N ARN).

The grain-size analysis revealed the predominance of a silt fractionin all the samples with the exception of the ARCH sample wherein theclay fraction reached 59% (Table 2). The highest sand fraction wasobserved in the LOP sample (close to 19%)whereas in the other samplesit was lower than 5%. The cross plot of the clay fraction percentageagainst the plasticity index (activity chart) revealed a very high levelof activity in the ARCH and BOI samples (IP/% clay ratio N3) (Fig. 6).The remaining samples (RAP, ARN and LOP) showed a medium levelof activity (IP/% clay ratio b 1).

The results of BET specific surface areawere consistentwith the grain-size distribution, especially in the clay fraction content (Fig. 5B). Thus, thehighest values (N50 m2/g)were found in the ARCH and RAP samples, andthe lowest (6 m2/g) in the BOI sample. Intermediate valueswere found inthe LOP and ARN samples. The values observed were consistent withthose found by Sparks (2005) and Carretero and Pozo (2009).

The CEC values varied between 11 and 112 cmol(+)∙kg−1 (Fig. 7A).The highest value was observed in the ARCH sample, exceeding100 cmol(+)∙kg−1, followed by the BOI sample (55 cmol(+)∙kg−1).Values between 10 and 30 cmol(+)∙kg−1 were obtained for the otherpeloids. The percentage of the exchangeable cations in each of the sam-ples is displayed in Fig. 7B. Na+ predominated in the ARCH and ARN

CaCa

P

P

B

Do

Sm

A

Ca

P

C

OM

D

Ca

Gy

Q

H

HG

E F

Fig. 4. SEM images of the peloids. A. Sample ARCH showing smectite aggregates (Sm) and euhedral dolomite inclusions (Do). B. Sample ARN displaying clastic texture with interparticleporosity (arrows) consisting of a clay groundmass wherein calcite (Ca) and terrigenous grains and occasional sheet minerals (P) are recognised. C. Higher magnification of sample ARNshowing a turbulent laminar microfabric around detrital grains (arrows). D. Sample BOI exhibits a mixture of organic matter from plant debris (OM) and clay. Broken carbonate shells(arrow) and silica bioclasts (rectangle) are identified. E. Close-up image of a silica diatom frustule from sample BOI. F. General texture of sample RAP. Highlighting the existence of glo-merular morphologies and coated grains (arrows) in a clay groundmass. G. Higher magnification of sample RAP showing coated grains and moulds (arrow). H. Sample LOP showing aclastic texture wherein calcite (Ca), quartz (Q), gypsum (Gy) and halite (Ha) are recognised.

274 M. Pozo et al. / Applied Clay Science 83–84 (2013) 270–279

samples at over 50%, and Ca2+ in the BOI sample. In the LOP and RAPsamples Na+ and Ca2+ predominated but in proportions below 50%.

From the results obtained by determining the instrumental texture(Table 2, Fig. 8) it is clear that the ARCH sample was the peloid withthe lowest instrumental hardness (132 g), whereas the highest valueswere detected in the LOP and ARN samples (461–462 g). With respectto cohesiveness, the ARCH and RAP samples produced the highestvalues and consequently the greatest cohesion. The adhesiveness wasgreatest in the LOP and RAP samples, which required the most effort

in removing them from the skin. However, the ARCH and BOI samplesgave the lowest cohesion values (b3500 g ∙s). The values obtained forthe springiness were similar in the BOI, RAP and LOP samples (around19.65 mm), with the minimum value recorded in the ARN sample andthe intermediate in the ARCH sample.

An analysis of the abrasivity of the samples (Fig. 9A) indicated thatthe LOP sample was themost abrasive (91.2 mg), whichwas consistentwith its high level of instrumental hardness. For the ARN and RAP sam-ples, slightly lower values were recorded (70–85 mg). However, in the

Table 2Physical and physico-chemical properties of Spanish peloids.

SAMPLES ARCH ARN BOI RAP LOP

Colour (Munsell) 5 Y 7/2 5 YR 4/4 N1 5 YR 4/1 5 Y 6/4Water % (w/w) 76.64 31.43 56.69 39.59 34.32Solids % (w/w) 25.36 68.57 44.31 60.41 65.68Dry residue (850 °C) % (w/w) 23.31 64.50 22.73 53.23 56.16Density (kg/m3) 1114 1562 1193 1427 1494Plastic limit (%) 95 11 90 18 24Liquid limit (%) 322 43 144 60 44Plasticity index (%) 227 32 54 42 20Sand (%) 0.80 0.40 18.70 2.40 1.20Silt (%) 40.20 61.60 69.90 53.80 77.90Clay (%) 59.00 38.00 11.30 43.80 20.80BET (m2/g) 97 19 6 54 14Hardness (g) 132 462 263 394 461Cohesiveness 0.80 0.50 0.66 0.80 0.50Adhesiveness (g∙s) 2491 4962 3284 7102 6966Springiness (mm) 18.73 17.56 19.68 19.62 19.68Abrasiveness (mg) 7.20 72.80 44.80 84.60 91.20CEC (cmol(+)/kg) 112 24 55 30 11Heat capacity (vol) (×106 J/m3 K) 3.80 2.97 3.52 3.04 3.16Thermal conductivity coeff. (W/m K) 0.45 0.53 0.40 0.49 0.48Thermal retentivity (106 s/m2) 8.33 5.59 8.71 6.21 6.60t37 parameter (minutes) 43.40 20.20 46.40 28.50 21.90Relaxation time (minutes) 9.89 4.60 10.06 6.51 4.99

0

50

100

150

200

250

0 10 20 30 40 50 60 70

% CLAY

PL

AS

TIC

ITY

IND

EX

VERY HIGH

HIGH

MEDIUM

LOW

BOI

LOP

ARCH

ARN

RAP

Fig. 6. Clay fraction against plasticity index of peloids (activity chart).

100

120

CEC % SmectiteA

275M. Pozo et al. / Applied Clay Science 83–84 (2013) 270–279

BOI sample, this value dropped to 44.8 mg as a result of the organicmatter in the peloid. In the ARCH sample the high smectite contentresulted in a low level of abrasiveness of only 7.2 mg. The abrasivityvalues show a positive correlation with the measured instrumentalhardness (Fig. 9B).

3.4. Thermal parameters and cooling test

The results revealed that the highest volumetric heat capacity wasexhibited in the ARCH and BOI samples (3.80–3.52106 J/m3 K) followedby the RAP and LOP samples, with the lowest values recorded in theARN sample (Table 2). Therefore, the highest thermal conductivity

0.00

50.00

100.00

150.00

200.00

250.00

300.00

350.00

%

SAMPLES

Pw Lw Pi

0

20

40

60

80

100

120

ARCH ARN BOI RAP LOP

ARCH ARN BOI RAP LOP

BE

T(m

2/g

)

SAMPLES

specific surface area %clayB

A

Fig. 5. Plasticity and BET specific surface of peloids. A. Plastic limit, liquid limit and plasticityindex. B. BET specific surface values (m2/g) and a comparison with clay content (%).

coefficients were recorded in the ARN, RAP and LOP samples, andthe lowest in the ARCH and BOI samples. As a consequence, theheat retention (retentivity) was the highest in the ARCH and BOIsamples (N8106 s/m2), whilst the ARN, RAP, LOP samples rangedbetween 5.59 and 6.60106 s/m2 (Fig. 10A).

The rate of cooling between 45 °C and 36 °C (t37) showed significantdifferences in the time taken to reach 37 °C (Fig. 10B). The optimal

0

20

40

60

80

ARCH ARN BOI RAP LOP

CE

C (

cmo

l(+)

/kg

)

SAMPLES

0

10

20

30

40

50

60

70

80

90

100

ARCH ARN BOI RAP LOP

Exc

han

gea

ble

cat

ion

s (%

)

SAMPLES

Ca Mg Na KB

Fig. 7. A. Cation exchange capacity (cmol(+)∙kg−1) and smectite content (%). B. Percentageof exchangeable cations (Na+, K+, Ca2+, Mg2+).

0

1000

2000

3000

4000

5000

6000

7000

8000

0 100 200 300 400 500

HARDNESS (g)

AD

HE

SIV

EN

ES

S (

g.s

)

ARCH ARN BOI

RAP LOP TERDAX

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

17 17.5 18 18.5 19 19.5 20

SPRINGNESS (mm)

CO

HE

SIV

EN

ES

S

ARCH ARN BOI

RAP LOP TERDAX

A

B

Fig. 8. Instrumental texture values of peloids compared with the TERDAX peloid as areference. A. Adhesiveness (g ∙s) vs. hardness (g) graph. B. Cohesiveness vs. springiness(mm) graph.

050

100150200250300350400450500

ARCH ARN BOI RAP LOP

H(g

)-A

(mg

)

SAMPLES

Hardness Abrasiveness

R² = 0.911

0102030405060708090

100

0 100 200 300 400 500A

bra

sive

nes

s (m

g)

Hardness (g)

Hardness-abrasiveness

A

B

Fig. 9. A. Abrasiveness (mg) of peloids. B. Bivariate diagram showing the correlationbetween abrasiveness and TPA hardness.

05

1015

202530

3540

4550

ARCH ARN BOI RAP LOP

SAMPLES

Min

ute

s

t37 Rtime

0

1

2

3

4

5

6

7

8

9

10

ARCH ARN BOI RAP LOP

106

J/m

3 K -

106

s/m

2

SAMPLES

Retentivity HeatcapvolA

B

Fig. 10. Thermal properties of peloids. A. Retentivity (106 s/m2) and volumetric heatcapacity (106 J/m3 K). B. Parameter t37 (minutes) and relaxation time (minutes).

276 M. Pozo et al. / Applied Clay Science 83–84 (2013) 270–279

thermal behaviour was observed in the ARCH and BOI samples whichtook over 40 min to cool to 37 °C. The ARN, RAP and LOP samples cooledmore rapidly, between 20 min and 30 min. With regard to the relaxa-tion time, the BOI and ARCH peloids showed the highest values (around10 min) and therefore the slowest heat transfer.

4. Discussion

4.1. Interpretation of non-thermal physical and physico-chemical properties

The compositional and textural differences observed in the peloidsstudied here account for the variations in the physical and physico-chemical properties observed. Therefore, the predominance of Na-saturated trioctahedral smectite in the ARCH peloid favoured highwater retention (76.64%) and the lowest density (1114 kg/m3), the rela-tively low clay content can be explained by the state of aggregationwhichbentonite dispersions undergo in the presence of NaCl (Luckham andRossi, 1999), which noticeably increases the silt fraction. In the caseof the RAP sample, the predominance of Ca2+ in the maturation waters(Carretero et al., 2010) also facilitates the aggregation of clay particles(Kjellander et al., 1988), which explains the development of the glomer-ular morphology seen on the SEMmicrographs.

The highest sand fraction content was observed in the BOI samplewhich was related to the presence of organic debris and brokenbioclasts, which were identified by SEM. The predominance of the siltfraction and 35% organicmatter content could account for a BET specificsurface area of only 6 m2/g. The low values for BET specific surface area in

materialswith high percentages of organicmatter have been explained asa result of the adsorption of the organic matter by the clay mineralsurfaces, thereby diminishing the availability of their external surfaces(Kaiser and Guggenberger, 2008).

Table 3Summary of physical and physico-chemical properties of the Spanish and the TERDAXpeloids.

Peloid type Clay (ARCH, ARN, RAP, LOP) Clay + peat (BOI) TERDAX

Lowest value Highest value Value Value

% Water 31.43 76.64 56.69 46.13% Solids 25.36 68.57 44.31 53.87% Dry residue (850 °C) 23.31 64.5 22.73 50.24Density 1114 1562 1193 1356Hardness (TPA) 132 461 263 138Cohesiveness (TPA) 0.5 0.8 0.66 0.65Adhesiveness (TPA) 2491 7102 3284 2646Springiness (TPA) 17.56 19.68 19.68 18.8Heat capacity (vol) 2.97 3.8 3.52 3.31Thermal conductivity 0.45 0.53 0.4 0.47Thermal retentivity 5.59 8.33 8.71 6.99t37 value 20.2 43.4 46.4 30.7Relaxation time 4.60 9.89 10.06 7.0

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In addition to the distribution of the particle sizes, the presence andtype of smectite, either trioctahedral or dioctahedral, could account forthe variations observed in the plasticity index, activity and cationicexchange capacity in the samples studied (Grim, 1962).

Regarding the parameters of consistency, the Atterberg limits rose asthe smectite content increased, and reached their highest values in theARCH samples followed by the RAP and ARN samples. The content ofsmectite in the ARCH sample was almost double to that observed inthe RAP sample, but the plasticity index was almost 5.5 times greater.In addition to the difference in smectite content, other influencescan be explained by the fact that the ARCH sample smectite was Na-saturated, contained a higher proportion of clay fraction, and includedsalts related to the chemical composition of the maturation water,which in the ARCH sample was sodium chloride, but was calcium bicar-bonate in the RAP sample. According to Grim (1962) the high limitvalues for smectite-rich clays (bentonites) are a consequence of theirability to disperse into extremely small particles with a tremendousamount of potent adsorbing surface. The extremely high values forNa-bentonites are a consequence of the greater dispersing action ofthis cation which permits the breakdown into flakes approaching theunit-cell thickness. The lower limit values for the bentonites carryingthe other cations are a consequence of the greater bonding action ofthese cations and the fact that they favour the development of waterlayers of only a limited thickness.

The remaining peloids showed a plasticity index lower than 60%,the BOI sample being remarkable in that it reached 54%, a typicalvalue for materials of high plasticity rich in organic compounds(Day, 1999).

The high clay fraction content and the Na-saturated character of thesmectite comprising the ARCH sample account for the greater activity ofthis peloid, which reached a value of 3.84, considerably higher than thatof the ARN, RAP and LOP samples (b1). This activity is defined as theratio of the plasticity index to the abundance of the clay fraction(Skempton, 1953). According to Grim (1962) one would generally ex-pect that active clayswould have a relatively highwater-holding capac-ity and high CEC and therefore properties at variance with the nature ofthe exchangeable cations. On the basis of plasticity index–clay ratio,Skempton (1953) established five groups ranging from inactive clays(b0.75) to active clays (N1.25), wherein bentonites (especially Na-bentonites) exhibited the highest activity values. Grain-size distribu-tion, soluble salts and organicmatter are all factors which affect activity.Smaller grain-sizes and consequently an increased surface area cause anincrease in the Atterberg limits (chiefly Lw) and an increase in activity(Platen and Winkler, 1958; White, 1949). All these factors account forthedifferences observedbetween theARCHsample and the other peloids.

The BOI sample showed high activity (4.7), which is consistent withSkempton's findings for sediments containing organic material.

As expected, the bentonite in the ARCH sample gave it a higher cation-ic exchange capacity (CEC), above 100 cmol(+)∙kg−1, which was withinthe typical range of values for smectite (Grim, 1968) and higher thanthose corresponding to the ARN and RAP samples, which had a lowersmectite content. The BOI sample reached a CEC of 55 cmol(+)∙kg−1, aresult of the presence of both smectite and organic matter (Tan andDowling, 1984).

The rheological behaviour of a peloid is correlated to its physico-chemical and visco-elastic properties. Some parameters related to thevisco-elastic behaviour of the samples were obtained from a texturalprofile analysis (TPA). Thus, the ARCH and RAP samples had a similarcohesiveness which was superior to the other peloids, a fact whichcan be explained by the high content of minerals not belonging to theclay minerals, which would have produced a reduction in the internalforce which binds the constituents of the peloid together. With respectto adhesiveness, the lower density and percentage of solids in the ARCHsample accounted for a lower value than that observed in the other sam-ples. A comparison of the range of values obtained in the samples studiedand the reference TERDAX peloid is shown in Table 3, where other

physical parameters are also shown. It can be seen that the values forthe reference peloid fall within the range observed in the Spanish peloids.

The grain size features and especially the lower hard detrital mineralcontent (quartz and feldspar) produced a very low abrasivity in theARCH sample, which was higher in the ARN, LOP and RAP samples.According to Rapp and Laufmann (1995) a high content of particlesb2 μm in the dispersion results in lower abrasivity. Grain shape is alsoan important factor, and the presence of sharp-edged quartz or feldspargrains as accessory minerals in clays is known to influence abrasion.Therefore, the abrasiveness depends on the size of the grain and thehard mineral content, as well as its degree of roundness. It has been ob-served that the smaller the grain size of the hard minerals (abrasive),the lower its abrasivity, being negligible in sizes smaller than 20 μm(Klinkergerg et al., 2009). In the BOI peloid the abrasivity is intermediatebetween the ARCH sample and the other samples. This is as a conse-quence of the presence of organic matter and a low hard detrital mineralcontent.

4.2. Interpretation of thermal properties

The thermal behaviour of peloids is probably one of themost impor-tant therapeutic effects in pelotherapy. The thermal properties includespecific heat, thermal conductivity, retentivity, cooling rate, t37 parame-ter and relaxation time (Skauge et al., 1983; Rambaud et al., 1986;Armijo, 1991; Ferrand and Yvon, 1991; Veniale et al., 2007; Legidoet al., 2007; Ortiz de Zarate et al., 2010; Casas et al., 2011, 2013). Of allof the above properties, the cooling rate is one of themost critical, becausethe heat contributed by the peloid also plays a role as a therapeutic agent.Lewis (1935) was the first author to study the thermal properties ofpeloids. This author suggested that the specific heat increased as the per-centage of water increased, whilst the thermal conductivity decreased.Therefore, retentivity is equally increased. Berbenni (1965) studied theheat retention rates of some Italian peloids and concluded that eachmud had an optimum water content corresponding to its maximumheat retention capacity, and that the smaller the clay particle, the lowerits cooling rate. According to Veniale et al. (2004) the thermal behaviourvaries according to the thermal water used during the maturation pro-cess, whilst the temperature reached by the peloid twenty minutes afterapplication depends on its water retention capacity.

Among the samples studied, the ARCH and BOI samples were thosethat showed the greatest heat capacity and thermal retentivity, but alower coefficient of thermal conductivity. Similarly, the higher smectitecontent of the RAP sample produced better thermal properties than theARN sample. The variation in the thermal parameters of these peloids ismainly a function of their water retention capacity, with fine-grainedclay minerals playing a fundamental role, especially those with a sorptivecapacity such as smectite (Sala and Tessier, 1994; Yvon and Ferrand,1996). In smectite, its laminar charge value determines the hydration of

278 M. Pozo et al. / Applied Clay Science 83–84 (2013) 270–279

the interlayer cations and therefore the degree ofmoistness (Laird, 1999).Similarly, water retention by the organic matter was responsible for thethermal characteristics of the BOI sample. Organic matter being capableof holding up to 20 times its weight in water (Stevenson, 1982).

Both times during which the peloid releases heat between 45 and37 °C (t37) and the relaxation time are of great importance from atherapeutic point of view. The highest value for the t37 parameter wasobserved in the BOI and ARCH samples, and the lowest values weremeasured in the RAP, ARN and LOP samples. With respect to relaxationtime, the ARCH and BOI samples were the best peloids from a thermalperspective. Obviously, as stated above, the capacity to retain water(assisted by the smectite and organic matter) plays a fundamental rolehere. Compared with the Spanish peloids, the TERDAX peloid (Table 3)showed lower values for retentivity, t37 parameter and relaxation timethan the BOI and ARCH samples, which were close to the RAP sample,and higher than the LOP and ARN samples.

5. Conclusions

The Spanish peloids studied in this work have different mineralogicalcompositions, including different percentages of clay minerals, carbon-ates, terrigenous minerals, salts and organic matter. Taking into accountthe mineralogical differences observed, a wide range of values wasobserved for several physical and physico-chemical values including theplasticity index (32–227%) and CEC (24–112 cmol(+)∙kg−1), beingcloser in the case of thermal retentivity (5.59–8.71106 s/m2). These sam-ples had in common a high content of fine-grained fraction (clay + silt)reaching percentages greater than 95% (although the BOI sample wasan exception), and a variable smectite content showing thermal parame-ters which were close to those of the reference TERDAX peloid.

A textural profile analysis (TPA) is an easy, fast and reproduciblemethod for determining the mechanical properties of peloids. Despitethe small number of samples, the range of values obtained for hardness(133–462 g), cohesiveness (0.5–0.8), adhesiveness (2491–7102 g ∙s)and springiness (17.56–19.68 mm) can be used as reference values.These values are also useful in determining the most appropriateway of using peloids made in spas, either as pastes or dispersions. Theparameters obtained from the TERDAX peloid are included within theproposed reference values.

These results indicate that peloids with different compositions anda wide range of non-thermal physico-chemical properties can be usedsuccessfully in pelotherapy. In addition, the comparatively similar valuesfor thermal properties are in agreement with the importance of peloidsfor thermotherapy purposes.

Acknowledgements

This workwas funded by the Research Group of the Andalusia BoardRNM-349 andUCM911757 (Universidad ComplutenseMadrid,MedicalHydrology).

References

Armijo, F., 1991. Propiedades térmicas de los peloides. Bol. Soc. Esp. Hidrol. Med. 6, 151–157.Armijo, F., Armijo, O., 2006. Curva de enfriamiento de los peloides españoles —

propiedades térmicas. An. Hidrol. Med. 1, 97–110.Beer, A.M., Junginger, H.E., Lukanov, J., Sagorchev, P., 2003. Evaluation of the permeation

of peat substances through human skin in vitro. Int. J. Pharm. 253 (1–2), 169–175.Bellometti, S., Richelmi, P., Tassoni, T., Bertè, F., 2005. Production of matrix metallopro-

teinases and their inhibitors in osteoarthritic patients undergoing mud bath therapy.Int. J. Clin. Pharmacol. Res. 25, 77–94.

Berbenni, P., 1965. L'indice di capacità di retenzione di calore, caratteristica chimico–fisicaessenziale nella scelta di un peloide. Thermae 2, 13–21.

Bourne, M.C., 1978. Texture profile analysis. Food Technol. 32 (7), 62–66 (72).Carretero, M.I., Pozo, M., 2009. Clay and non-clayminerals in the pharmaceutical industry.

Part I. Excipients and medical applications. Appl. Clay Sci. 46, 73–80.Carretero, M.I., Gomes, C., Tateo, F., 2006. Clays and human health. In: Bergaya, F., Theng,

B.K.G., Lagaly, G. (Eds.), Handbook of Clay Science. Elsevier, Amsterdam, pp. 717–741.

Carretero, M.I., Pozo, M., Sánchez, C., García, F.J., Medina, J.A., Bernabé, J.M., 2007. Compar-ison of saponite andmontmorillonite behaviour during static and stirring maturationwith sea water for pelotherapy. Appl. Clay Sci. 36, 161–173.

Carretero, M.I., Pozo, M., Martín-Rubí, J.A., Pozo, E., Maraver, F., 2010. Mobility of elementsin interaction between artificial sweat and peloids used in Spanish spas. Appl. ClaySci. 48, 506–515.

Casas, L.M., Legido, J.L., Pozo, M., Mourelle, L., Plantier, F., Bessieres, D., 2011. Specific heatof mixtures of bentonitic clay with sea water or distilled water for their use inthermotherapy. Thermochim. Acta 524, 68–73.

Casas, L.M., Pozo, M., Gómez, C.P., Pozo, E., Bessieres, D., Plantier, F., Legido, J.L., 2013.Thermal behaviour of mixtures of bentonitic clay with different salinity waters.Appl. Clay Sci. 72, 18–25.

Codish, S., Abu-Shakra, M., Flusser, D., Friger, M., Sukenik, S., 2005. Mud compresstherapy for the hands of patients with rheumatoid arthritis. Rheumatol. Int. 25(1), 49–54.

Day, R.W., 1999. Geotechnical and Foundation Engineering. McGraw-Hill.Evcik, D., Kavuncu, V., Yeter, A., Yigit, I., 2007. The efficacy of balneotherapy andmud-pack

therapy in patients with knee osteoarthritis. Joint Bone Spine 74, 60–65.Ferrand, T., Yvon, J., 1991. Thermal properties of clay pastes for pelotherapy. Appl. Clay Sci. 6,

21–38.Fioravanti, A., Perpignano, G., Tirri, G., Cardinale, G., Gianniti, C., Lanza, C.E., Loi, A., Tirri, E.,

Sfriso, P., Cozzi, F., 2007. Effects of mud-bath treatment on fibromyalgia patients:a randomized clinical trial. Rheumatol. Int. 27, 1157–1161.

Forestier, R., Desfour, H., Tessier, J.M., Françon, A., Foote, A.M., Genty, C., Rolland, C.,Roques, C.F., Bosson, J.L., 2010. Spa therapy in the treatment of knee osteoarthritis:a large randomised multicentre trial. Ann. Rheum. Dis. 69, 660–665.

Fraioli, A., Serio, A., Mennuni, G., Ceccarelli, F., Petraccia, L., Fontana, M., Grassi, M.,Valesini, G., 2011. A study on the efficacy of treatment with mud packs and bathswith Sillene mineral water (Chianciano Spa, Italy) in patients suffering from kneeosteoarthritis. Rheumatol. Int. 31, 1333–1340.

Galzigna, L., Moretto, C., Lalli, A., 1996. Physical and biochemical changes of thermal mudafter maturation. Biomed. Pharmacother. 50, 306–308.

Gomes, C., Carretero, M.I., Pozo, M., Maraver, F., Cantista, P., Armijo, F., Legido, J.L., Teixeira,F., Rautureau, M., Delgado, R., 2013. Peloids and pelotherapy: Historical evolution,classification and glossary. Applied Clay Science 75–76, 28–38.

Grabowska-Olszewska, B., Osipov, V., Sokolov, V., 1984. Atlas of theMicrostructure of ClaySoils. Panstwowe Wydawnictwo Navkowe, Varsow, Poland.

Grim, R.E., 1962. Applied Clay Mineralogy. McGraw-Hill.Grim, R.E., 1968. Clay Mineralogy. McGraw-Hill.Kaiser, K., Guggenberger, G., 2008. Mineral surfaces and organic matter. Eur. J. Soil Sci. 54,

219–236.Kjellander, R., Marcelja, S., Quirk, J.P., 1988. Attractive double-layer interactions between

calcium clay particles. J. Colloid Interface Sci. 126, 194–203.Klinkergerg, M., Rickertsen, N., Kaufhold, S., Dohrmann, R., Siegesmund, S., 2009.

Abrasivity by bentonite dispersions. Appl. Clay Sci. 46, 37–42.Laird, D.A., 1999. Layer charge influence on the hydration of expandable 2:1 phyllosilicates.

Clays Clay Miner. 47, 630–636.Legido, J.L., Medina, C., Mourelle, M.L., Carretero, M.I., Pozo, M., 2007. Comparative study

of the cooling rates of bentonite, sepiolite and common clays for their use inpelotherapy. Appl. Clay Sci. 36, 148–160.

Lewis, J., 1935. Thermal properties of peloids. Part II. Arch. Med. Hydrol. 13, 56–57.Luckham, P.F., Rossi, S., 1999. The colloidal and rheological properties of bentonite sus-

pensions. Adv. Colloid Interface Sci. 82, 43–92.Maraver, F., Corvillo, I., Palencia, V., Armijo, F., 2004. Therapeutic muds in Spain. Proceed-

ings of the 3rd Symposium on Thermal Muds in Europe; Nov 25–27; Dax, France,pp. 23–27.

Nappi, G., Masciocchi, M.M., De Luca, S., Calcaterra, P., Carrubba, I.G., 1996. Le muffetermali solfuree nel tratamento de la psoriasi cutanea. Med. Clin. e Term. 34,7–18.

Odabasi, E., Turan, M., Erdem, H., Tekbas, F., 2008. Does mud pack treatment have anychemical effect? A randomized controlled clinical study. J. Altern. Complement.Med. 14, 559–565.

Ortiz de Zarate, J.M., Hita, J.L., Khayet, M., Legido, J.L., 2010. Measurement of the thermalconductivity of clays used in pelotherapy by the multi-current hot-wire technique.Appl. Clay Sci. 50, 423–426.

Platen, H., Winkler, H.G.F., 1958. Plastizitat und Thixotropie von FractioniertenTonmineralien. Kolloid-Z 158, 3–22.

Rambaud, A., Rambaud, J., Berger, G., Pauvert, B., 1986. Mesure et étude du comportementthermique des boues thermales. J. Fr. Hydrol. 17, 293–302.

Rapp, H.U., Laufmann,M., 1995. Siebabrasion und Seine potentiellen Ursachen.Wochenbl.Pap. 18, 803–812.

Sala, G.H., Tessier, D., 1994. Water retention by clayey materials. Significance and predic-tion. C.R. Acad. Sci. Paris 318/II, 381–388.

Skauge, A., Fuller, N., Hepler, L.G., 1983. Specific heats of clay minerals: sodium and calciumkaolinites, sodium and calcium montmorillonites, illite, and attapulgite. Thermochim.Acta 61, 139–145.

Skempton, A.W., 1953. The colloidal “activity of clay”. Proc. Third Internat. Conf. Soil.Mech., I, pp. 57–61.

Sparks, D.L., 2005. Toxic metals in the environment: the role of surfaces. Elements 1,193–197.

Stevenson, F.J., 1982. Humus Chemistry. John Willey and Sons.Sukenik, S., Buskila, D., Neumann, L., Kleiner-Baumgarten, A., Zimlichman, S., Horowitz, J.,

1990. Sulphur bath and mud pack treatment for rheumatoid arthritis at the Dead Seaarea. Ann. Rheum. Dis. 49, 99–102.

Szczesniak, A.S., 1963. Classification of textural characteristics. J. Food Sci. 28 (385),389.

279M. Pozo et al. / Applied Clay Science 83–84 (2013) 270–279

Tan, K.H., Dowling, P.S., 1984. Effect of organic matter on CEC due to permanent andvariable charges in selected temperate soils. Geoderma 32 (2), 89–101.

Tateo, F., Agnini, C., Carraro, A., Giannossi, M.L., Margiotta, S., Medici, L., Finizio, F.E.,Summa, V., 2010. Short-term and long-term maturation of different clays forpelotherapy in an alkaline–sulphate mineral water (Rapolla, Italy). Appl. Clay Sci.50, 503–511.

Tucker, B.M., 1974. Laboratory procedures for cation exchange measurements on soils.Division of Soils Technical Paper No. 23.Commonwealth Scientific and IndustrialResearch Organization.

Veniale, F., Barberis, E., Carcangiu, G., Morandi, N., Setti, M., Tamanini, M., Tessier, D., 2004.Formulation of muds for pelotherapy: effects of “maturation” by different mineralwaters. Appl. Clay Sci. 25, 135–148.

Veniale, F., Bettero, Jobstraibizer, A., Setti, M., 2007. Thermal muds: perspectives of innova-tions. Appl. Clay Sci. 36, 141–147.

White, W.A., 1949. Atterberg plastic limits of clay minerals. Am. Mineral. 34, 508–512.Yvon, J., Ferrand, T., 1996. Preparation ex-situ de peloids: propriétés thermiques,

mécaniques et d'échange. In: Veniale, F. (Ed.), Atti Convegno “Argile Curative”. SaliceTerme/PV, Gruppo Ital. AIPEA, pp. 67–74.