contribution of adsorption phenomena to the degradation study of marble statues in the...

319

*Authors to whom all correspondence should be addressed. E-mail: [email protected] (Prof. F. Roubani-Kalantzopoulou);[email protected] (Dr. Eleni Metaxa)

Contribution of Adsorption Phenomena to the Degradation Study of MarbleStatues in the Archaeological Museum of Philippi, Greece

Vithleem Floropoulou, Triantafyllia Agelakopoulou, Eleni Metaxa*, Chaido-StefaniaKaragianni and Fani Roubani-Kalantzopoulou* School of Chemical Engineering, National Technical

University of Athens, 157 80 Zografou, Athens, Greece.

(Received 31 July 2008; revised form accepted 13 July 2009)

ABSTRACT: A description is provided of the contribution of time-resolvedanalysis in understanding adsorption phenomena and its contribution to anexamination of the overall degradation mechanism of authentic marble piecesbrought about by air pollution. As a consequence, a scientific answer has beenprovided to the question “how are materials of cultural heritage affected by airpollution?” The use of reversed-flow inverse gas chromatography (RF-IGC) hasallowed the determination of important local physicochemical quantities relatingto the influence of three light hydrocarbons, either in the presence or absence ofsulphur dioxide. A detailed analysis of the results as well as a correlation studyis given.

1. INTRODUCTION

A number of papers have been published which address the importance of the surroundingenvironment in stone alteration. Thus, the study of the decay of stone as a result of atmosphericpollution, carried out by LISA in Europe since the early 1980s, has been reviewed by Lef evre andAusset (2002). The authors have proposed an interesting explanation for two different types ofgypsum development, i.e. above and below the surface. Gypsum formation strongly depends on themineralogical composition and on the rock fabric. In compact limestone, gypsum appears only atthe surface, whereas in marble its presence seems to be more penetrative (Siegesmund et al. 2002).

In the building trade, the term “marble” is employed to describe any calcareous rock which iscapable of taking a good polish as well as being suitable for ornamental work or constructionpurposes. Appearance and strength are recognized as the most important properties of marble. Thecolouration in almost all marbles is primarily due to small amounts of aluminium, potassium, iron,nickel and copper; it is also enhanced by trace amounts of barium, titanium, chromium,manganese and lithium. Calcite and dolomite, the respective carbonates of calcium andmagnesium, are the major constituents of most varieties of marble. The proportion of calciumcarbonate in calcite marbles exceeds 96% and this is further increased to 99% for marble varietieswith pure white backgrounds. The strength of marble is attributed not only to the mineralconstituents and their grain arrangement, but also to its singular molecular structure. Impurestones are incapable of weathering uniformly or satisfactorily withstanding wear. As the unstableminerals break down, they leave weathered residues which stain the stone’s surface. The cohesivestrength of the stone is also reduced due to uneven thermal expansion of the mineral grains.

Loosely cemented marbles are porous, absorb dirt, weather rapidly and do not possess greatnatural strength. These conditions exist in most crystalline limestones and dolomites (Mies 2001).

The adsorption of air pollutants on carbonaceous surfaces has attracted much experimental andtheoretical interest during the past few decades (Gibson et al. 1997; Korpiel and Vidic 1997). Tounderstand “how materials function”, we must first understand “how molecules from theenvironment interact with their surfaces”. Both the investigation and elucidation of themechanism by which adsorption influences the reactivity of a solid are important for a number ofreasons. Firstly, they provide an understanding of reaction rate processes. At the same time, theyare of considerable technological interest, both in terms of the processing of the materials and theirperformance under service conditions. One such example is the selection of an appropriate coatingmaterial for protecting and preserving the surfaces of statues.

A basic phenomenon that relates to the whole of surface chemistry is the adsorption of chemicalspecies at interfaces, i.e. the two-dimensional surface of separation between regions of differentchemistry or states of orientation in engineering materials. Such adsorption may occur either bythe formation of strong chemical bonds (chemisorption) or via weaker physical attachment(physisorption) of the adsorbed molecules. In general, this leads to adsorbed films on an atomicscale (Hondros 1984).

The most stable surfaces are identified by the lowest surface energy with the equilibriummorphology being predicted by the construction of a Wulff plot. The energy of a surface is alwayshigher than that of the bulk. The periodic ordering of atoms or ions in the bulk of a crystal latticeis reflected by bulk-terminated crystal surfaces whose properties depend on the specific overallordering of the crystal, as well as on the direction in which the surface plane is orientated withrespect to this internal ordering. As a result, a surface may develop a square, hexagonal or even akinked structure (Cygan et al. 2002).

Calcite is the most commonly occurring mineral amongst carbonate materials, its surfacehaving been extensively studied using a number of surface analytical techniques. For example, acombination of XPS and LEED has been employed to study calcite surfaces exposed to air andaqueous solutions (Stipp and Hochella 1990; Stipp 1999). Often dipolar surfaces which arecomposed of alternating layers of calcium ions and carbonate groups occur, as in the case of the(001) calcite plane. Such surfaces require “reconstruction” or the addition of charged species inorder for the model surface to be stabilized (Van Beurden 2005).

When atoms or molecules are adsorbed onto surfaces, their chemistry (i.e., reactivity) towardsother atoms and molecules changes and the structure of the underlying substrate changes as well.The adsorbed species often form ordered overlying structures (due to their mutual interaction) overa wide range of temperatures and surface coverages, these structures possessing unit cells larger thanthe unit cell of the substrate. In most cases, however, there is still a simple ratio between the unitcells of the substrate and adsorbate (due to adsorbate–substrate interaction). Finally, the adsorbates(adspecies) can induce a “reconstruction” of the solid surface. The thermodynamic driving force foradsorbate-induced restructuring is the difference in the strength of the adsorbate–substrate bondsassociated with the reconstructed and unreconstructed surfaces. More specifically, the loss inadsorption energy is larger than the gain in energy associated with the reconstruction of the cleansurface. If massive diffusion-controlled atom transport does not occur along the surface, thetimescale of this adsorbate-induced restructuring may be quite short, i.e. of the same order as that ofthe adsorption time of a monolayer (10–6 s) (Van Beurden 2005; Christmann 1995; Hofmann 2008).

In the present work, the novel method of reversed-flow inverse gas chromatography (RF-IGC)(Katsanos et al. 1999a–c; Gavri et al. 1999; Roubani-Kalantzopoulou et al. 2001; Roubani-Kalantzopoulou 2004; Katsanos and Karaiskakis 2004; Metaxa et al. 2007; Agelakopoulou et al.2007, 2008) has been used to investigate quantitatively and to elucidate qualitatively the time

320 V. Floropoulou et al./Adsorption Science & Technology Vol. 27 No. 3 2009

progress of the action of three light hydrocarbons (ethane, ethylene and acetylene) on a sample ofan ancient statue in the interior of the Museum of Philippi, near Salonica, in Greece. Apart fromits low cost, simplicity, speed and accuracy, this method circumvents the difficult solution oftraditional integral equations involving three physicochemical functions used elsewhere in variousapproximations for estimating energy-distribution functions. Based on some explicit functions oftime, derived from a simple mathematical model, it is possible to determine all the physicochemicalquantities which play a crucial role in the degradation of authentic marble, including surfacediffusion coefficients, local adsorption parameters, etc. For the systems examined in the presentwork, six local physicochemical quantities have been determined, viz. the local adsorptionisotherm, θ, the local adsorption energy, ε, the density probability function, ϕ(ε,t), the concentrationof non-adsorbed gas, cy, the local monolayer capacity, c*

Smax and the adsorption entropy, ∆S. Asimple PC program has been used for the calculations†. The combination of the above variablescreates a domain of time-resolved surface chemistry which enables important conclusions to bereached regarding the conservation of ancient monuments.

2. THEORETICAL

The mathematical model used to calculate all the physicochemical quantities is based on well-known equations and has the advantage that it uses non-steady-state conditions — in contrast toa great number of other models based on equilibrium conditions. Only the essential relationshipsproviding an insight into the methodology used to extract the parameters in Figures 2–8 arementioned here, since a detailed theoretical analysis has already been published elsewhere(Katsanos et al. 1999b; Gavril et al. 1999; Katsanos and Karaiskakis 2004).

Equation (1) forms the core of the experimental and mathematical models:

H1/M = g c(l′,t) = A1 exp(B1t) + A2 exp(B2t) + A3 exp(B3t) (1)

This describes the set of physicochemical phenomena taking place inside the diffusion column,which is considered as the “chemical reactor” of the system [Figure 1(a)] since it contains the“gas–solid” system under study. All the corresponding calculated adsorption parameters areincorporated in the values of the A and B quantities in the algebraic sum of the exponential timefunctions; g [cm/(mol cm3)] is the calibration factor of the detector and c(l′,t) is the measuredsampling concentration of the gaseous analyzer at x = l′ or z = 0, expressed in mol/cm3.

The form of equation (1) is not an a priori assumption, but the result of the solution of a systemof three partial differential equations and a local isotherm, with certain initial and boundaryconditions. These four basic equations are as follows.

First, the local adsorption isotherm of the gas adsorbate studied:

(2)

where the symbols denote the following: c*

S adsorbed equilibrium concentration of gas adsorbate at time t (mol/g);mS initially adsorbed amount of gas adsorbate (mol);

cm

ay L

a

ak c dS

S

S

y

Sy

t∗ = −( ) + ∫δ τ2 1 0( )τ

Adsorption Phenomena in the Degradation of Marble Statues in Greece 321

†The PC program employed is available on request.


Restrictor orseparationcolumn

Referenceinjector

DetectorGasinjector

Solid bed

Diffusioncolumn

Sampling column

Carrier gasinlet

(a)

Four-portvalve

y = L2

z = L1

z = 0

y = 0

x = 0 x = l' x = l' + l

y

z

x

(b)

400

350

300

250

200

150

100

50

0

400

350

300

250

200

150

100

50

0

0 50

Samplepeaks

Baseline

Time (min)

Vol

tage

(m

V)

100 150

Figure 1. (a) Schematic representation of the RF-IGC set-up employed for surface characterization via gas diffusion and(b) a typical chromatogram obtained using this set-up.

αS amount of solid material per unit length of column bed (g/cm);δ(y – L2) Dirac’s delta function for the initial condition of the bed, when the gas adsorbate

is introduced as an instantaneous pulse at the point y = L2 (cm–1);y length coordinate along the section L2 (cm);ay cross-sectional area of the void space in the region y (cm2);k1 local adsorption parameter (s–1);

cy gaseous concentration of adsorbate as a function of time t and coordinate y alongthe column (mol/cm3);

τ dummy variable for time.Secondly, the mass balance equation for the gas adsorbate in the gaseous region, z, of the

diffusion column:

(3)

the additional symbols contained in this equation being defined as: cz gaseous concentration of gas adsorbate as a function of time, t, and length

coordinate, z, along the column (mol/cm3);D1 diffusion coefficient of the gas adsorbate into the carrier gas (nitrogen) (cm2/s);kapp apparent rate constant of the first-order or pseudo-first-order reaction of the gas

adsorbate in the gaseous phase (s–1).Thirdly, the mass balance equation of the same gas adsorbate in the region y of the diffusion

column, filled with the solid material under study:

(4)

where:D2 diffusion coefficient of the gas adsorbate into the gaseous phase in section y (cm2/s);k–1 rate constant for desorption of the solute from the solid bulk (s–1);cS concentration of gas adsorbate adsorbed onto the solid at time t (mol/g).Finally, the rate of change of the concentration of adsorbate adsorbed:

(5)

where k2 (s–1) is the rate constant of a possible first-order or pseudo-first-order surface reaction ofthe adsorbed solute.

By the insertion of the initial conditions and cS(0,y) = 0, where m

is the amount (mol) of gas adsorbate introduced as a pulse at y = L2, it is possible to determine thephysicochemical quantities that characterize the gas–solid interaction. These are calculated fromthe experimental data — the pairs (H,t) [Figure 1(b)] and the various geometrical characteristicsof the diffusion column and the solid bed — on the basis of the following equations, by means ofa suitable PC program based on non-linear least-square regression analysis (Katsanos et al.1999a–c; Gavril et al. 1999; Roubani-Kalantzopoulou et al. 2001; Roubani-Kalantzopoulou 2004;Katsanos and Karaiskakis 2004; Metaxa et al. 2007; Agelakopoulou et al. 2007, 2008):

(a) the local adsorption energy, ε:

ε = RT[ln(KRT) – ln(RT) – ln K0] (6)

(b) the local adsorption equilibrium concentrations, cS* and cy:

(7)c kL

gD

A

BB tS

y

S z

i

ii

i

∗

=

= ( ) −∑αα

ν1

1

1

3

1[exp ]

c ym

ay Ly

y

( , )0 2= −( )δ

∂∂c

tk c c k cS

S S S= −( ) −−1*

2

∂∂

∂∂

c

tD

c

yk

a

ac c k cy y S

yS S app y= − − −−2

2

2 1 ( )*

∂∂

∂∂

c

tD

c

zk cz z

app z= −1

2

2


(8)

(c) the local adsorption isotherm, θt:

(9)

(d) the local monolayer capacity, c*Smax:

(10)

(e) the probability distribution function for adsorption energies, ϕ(ε,t):

(11)

(f) the local adsorption entropy, ∆S–

0ads(local):

(12)

All these relationships are based on the Jovanovic local isotherm, viz.:

θ(P,T,ε) = 1 – exp(–KP) (13)

where

K = K0(T) exp(ε/RT) (14)

with R being the gas constant, and

(15)

where m is the molecular mass of the adsorbate, k is the Boltzmann constant, h is the Planckconstant and the ratio υS(T)/bg(T) is that of two partition functions, viz. that of the adsorbedmolecule, υS(T), and that of the rotations/vibrations in the gaseous phase, bg(T). This ratio hasbeen taken as being approximately unity, as assumed elsewhere (Katsanos et al. 1999a–c; Gavrilet al. 1999; Roubani-Kalantzopoulou et al. 2001; Roubani-Kalantzopoulou 2004; Katsanos andKaraiskakis 2004; Metaxa et al. 2007; Agelakopoulou et al. 2007, 2008).

Kh

m kT

T

b TS

g

03

3 2 5 22

=( ) ( )π

υ/ /

( )

( )•

∆S R Sads local transg

( ),ln0

1= −

−

−θ

θ0

ϕ εθ

,/ /

( )/

/

max

tc RT

KRT c t c c t

KRT T

c c

S

S S y S y( ) =∂ ∂( ) + ∂ ∂ ∂

∂ ∂−

∂ ∂∗

∗ ∗ ∗2

KKRT

c cc c

KRTS S

S y

max

/∗ ∗∗

= +∂ ∂

θϑϑ

• •t = = −∗

∗

∗c

c c KRT

c

cS

S

S

ymax Smax

11 1

∗

cL

gDA B ty i

i

= ( )=∑ν 1

1 1

3

expi


3. EXPERIMENTAL

Chromatographic measurements were carried out using a Shimadzu Gas Chromatograph model 8A,having a temperature control accuracy of ± 1 K which was equipped with an FID detector. Theexperimental arrangement (accommodated within the oven of the chromatograph) was inaccordance with those described elsewhere (Katsanos et al. 1999a–c; Gavril et al. 1999; Roubani-Kalantzopoulou et al. 2001; Roubani-Kalantzopoulou 2004; Katsanos and Karaiskakis 2004;Metaxa et al. 2007; Agelakopoulou et al. 2007, 2008). This is shown diagrammatically in Figure 1(a).Each experiment involved injecting a small quantity of gas pollutant into column L2. In theexperiments aimed at evaluating the synergistic effect of sulphur dioxide, this second pollutantwas injected through the same column. A typical example of the chromatogram obtained is shownin Figure 1(b). Nitrogen of high purity (99.999%), supplied by Air Liquide Ltd., was used as thecarrier gas. Prior to use, this was dried by passage over silica gel.

The principal novelty of the RF-IGC method, which is a modified version of inverse gaschromatography, is the exploitation of gaseous diffusion currents (inside the diffusion column)perpendicular to the classical chromatographic current (inside the sampling column), whereasthe main role of the carrier gas in GC is neglected [cf. Figure 1(a)]. The flow of carrier gasthrough the sampling column is reversed for a short time interval (shorter than the dead time inthe sampling column) by turning the valve from one position [Figure 1(a), solid lines] to theother [Figure 1(b), broken lines] and vice versa, thus creating an extra chromatographic peak(or more than one) which represents the concentration at x = l′ [Figure 1(b)] at the moment ofreversal.

4. RESULTS AND DISCUSSION

Besides their chemico-mineralogical composition and texture, what characterizes natural stonesand geomaterials is their wide heterogeneous and anisotropic structures. The latter originate fromvarious multiphase formation routes (e.g. crystallization from the fused state, sedimentation,diaenesis, metamorphism and deformation) over long geological times, i.e. millions of years. Asfar as chemical reactivity in polycrystalline materials is concerned, what really sets them apartfrom classical homogeneous reactions is that their reactivity is dominated by their heterogeneousnature (Hondros 1984). Because of their crystallographic properties, natural stones exhibitdifferent types of structural surfaces — each with its own adsorption energy distribution — whichdetermine the variations in the observed weathering, deterioration patterns and processes of thesematerials. Knowledge of the properties of geomaterials, their weathering processes, as well as thesubsequent material changes is a basic requirement in understanding the complex mechanismsinvolved in their resulting degradation.

4.1. Time-resolved analysis of all calculated physicochemical quantities in the presence orabsence of sulphur dioxide

4.1.1. C2H2, C2H2/SO2

From Figure 2(a), it will be seen that, in the presence or absence of SO2, desorption of C2H2commenced within a very short time from the start of the experiment. However, in the absence ofSO2, desorption occurred at a slightly smoother rate.


On the other hand, the plot of the variation of the adsorption energy, ε, versus time depicted inFigure 2(b) shows that a slight decrease in the value of the maximum energy occurred in thepresence of SO2, this being observed at an earlier time than in the absence of SO2.

The corresponding plot of the energy distribution function, ϕ(ε,t), depicted in Figure 2(c) showsfour peaks which correspond to the appearance of active sites, both in the presence or absence of


(a)

Ads

orpt

ion

isot

herm

, θ(d

imen

sion

less

)

(b)

(c)

1.0

0.8

0.6

0.4

0.2

00 10 20 30 40

C2H

2/L1991 Statue of Philippi

C2H

2/SO


50

Time (min)

60 70 80 90

100

90

80

70

60

500 10 20 30 40 50 60 70 80

Ads

orpt

ion

ener

gy, ε

(kJ/

mol

)

90

C2H


C2H

2/SO


Time (min)

Ene

rgy

dist

ribut

ion

func

tion,

φ(ε

,1)

[mol

/(kJ

min

)]

0.6

0.5

0.4

0.3

0.2

0.1

00 10 20 30 40 50

Time (min)

60 70 80 90

C2H


C2H

2/SO


A

B CD

Figure 2. Plots against time of (a) the local adsorption isotherm, θ, (b) the local adsorption energy, ε, and (c) the energydistribution function for adsorption energy, ϕ(ε,t), for the C2H2/(SO2)/L1991 Statue of Philippi systems at 323.2 K.

SO2. In addition, in the presence of SO2, a small shift to smaller times occurs for the maxima inϕ(ε,t), accompanied by a decrease in the values of these maxima, particularly with respect to thesecond, third and fourth maximum. For the maxima obtained in the presence of SO2, the first peakassociated with chemisorption (at the start of the adsorption experiment) is absent, other than itsborder with the second peak. This contrast with the situation when SO2 is absent; in this case, thecorresponding area (A) is clearly defined. More specifically, the value of the second maximumdecreases from 0.162 mol/(kJ min) (at 12 min) to 0.0926 mol/(kJ min) (at 10 min), that of the third


8

6

4

2

00 10

Non

-ads

orbe

d ga

seou

sco

ncen

trat

ion,

cy (

mol

/cm

3 )

20 30 40 50 60 70 80 90

C2H


C2H

2/SO


Time (min)

(a)

15

Mon

olay

er c

apac

ity, c

*(m

mol

/cm

3 ) 10

5

00 10 20 30 40 50 60 70 80 90

C2H


C2H

2/SO


Time (min)

(b)

0

–50

–100

–150

–200

–250

–300

0 10 20 30 40 50 60 70 80 90

C2H


C2H

2/SO


Time (min)

Ads

orpt

ion

entr

opy,

∆S

[J/(

mol

K)]

(c)

Sm

ax

Figure 3. Plots against time of (a) the non-adsorbed gaseous concentration, cy, (b) the local monolayer capacity, cSmax, and(c) the local adsorption entropy, ∆S, for the C2H2/(SO2)/L1991 Statue of Philippi systems at 323.2 K.

maximum decreases from 0.0623 mol/(kJ min) (at 20 min) to 0.0615 mol/(kJ min) (at 16 min) and,finally, the fourth maximum decreases from 0.0315 mol/(kJ min) (at 32 min) to 0.0312 mol/(kJ min)(at 30 min). Indeed, graphic analysis of the areas under the peaks depicted in the plots of Figure 2has shown that the number of active sites in this area covered with C2H2 decreased by 71% due tothe presence of SO2.

Similarly, in the plots of cy versus time depicted in Figure 3(a), a slight increase in themaximum appears in the presence of SO2, accompanied by its earlier appearance relative to thesituation for the plot of cy versus time in the absence of SO2. In contrast to the above behaviour,the curves of c*

Smax versus time depicted in Figure 3(b) show an obvious slight time delay in thepresence of SO2.

Coincidence of the maximum and minimum occurs in the plots of ∆S versus time in Figure 3(c),either in the absence or presence of SO2, except that once again a shorter length of time wasnecessary for their appearance when SO2 was present in the system. In both cases, the slopes ofthese curves are almost identical.

4.1.2. C2H4, C2H4/SO2

Compared to the situation depicted in Figure 2, the situation is reversed in the plots of theadsorption isotherm, θ, versus time shown in Figure 4(a), with the data for the system


C2H4/L1991 Statue of Philippi

C2H4/SO2/L1991 Statue of Philippi

Time (min)

560 10 20 30 40 50 60 70 80 90

66

76

86

96

Ads

orpt

ion

ener

gy, ε

(kJ/

mol

)

00 10 20 30 40 50

Time (min)

60 70 80 90

0.2

0.4

0.6

0.8

1.0C2H4/L1991 Statue of Philippi


Ads

orpt

ion

isot

herm

, θ

(dim

ensi

onle

ss)

(a)

(b)

Figure 4. Plots against time of (a) the local adsorption isotherm, θ, and (b) the local adsorption energy, ε, for theC2H4/(SO2)/L1991 Statue of Philippi systems at 323.2 K.

C2H4/SO2/L1991 Statue of Philippi now being slightly delayed in comparison the data obtained inthe absence of SO2. In particular, the adsorption process started virtually instantaneously andsmoothly in both cases, although it was even smoother when SO2 was present.

The plots of the adsorption energy, ε, versus time depicted in Figure 4(b) show that a slightdecrease occurred in the maximum value of the energy in the presence of SO2, but that anydifference in the time at which this maximum occurred was absent in this particular case. To bemore specific, the value of εmax in the presence of SO2 was 92.2 kJ/mol (at 10 min) and, similarly,εmax in the absence of SO2 was 95.2 kJ/mol (at 10 min).

Although only one peak (B) is apparent in the plots of ϕ(ε,t) versus time shown in Figure 5(a),four kinds of active sites could actually be detected for both systems provided that a separatediagram employing a suitable time scale was developed [cf. Figure 5(b)]. Moreover, in the presenceof SO2, a large increase occurred in the maximum value of peak B depicted in Figure 5(a). Thus,the value of the maximum of peak B increased to 36.8 mol/(kJ min) (at 12 min) from a value of0.638 mol/(kJ min) (at 12 min) obtained in the absence of SO2. Graphic calculation of the areaunder peak B in each case, to obtain the corresponding number of active sites, showed that thepresence of SO2 caused a huge increase in the number of active sites covered with C2H4. Indeed,these increased by a factor of ca. 50 relative to the situation identified in the absence of SO2.


00

Time (min)

0.2

0.4

0.6

0.8

10 20 30 40 50

A

B

C D

60 70 80 90

C2H


(b)

00

Time (min)

10

20

30

40

10 20 30 40 50 60 70 80 90

Ene

rgy

dist

ribut

ion

func

tion,

φ (ε

,1)

[mol

/(kJ

min

)]E

nerg

y di

strib

utio

n fu

nctio

n,φ

(ε,1

) [m

ol/(

kJ m

in)]

C2H


C2H

4/SO

2/L1991 Statue of PhilippiB

(a)

Figure 5. (a) Plots against time of the energy distribution functions for the adsorption energy, ϕ(ε,t), for theC2H4/(SO2)/L1991 Statue of Philippi systems at 323.2 K and (b) time-resolved analysis plots of the data depicted in (a).

In the plots of cy versus time in Figure 6(a), a slight decrease in the maximum value wasobserved in the presence of SO2, viz. from 7.11 mol/cm3 to 6.87 mol/cm3 at the same time (10 min).This is exactly the opposite of that observed in the presence of acetylene [cf. Figure 3(a)]. Figure 6(b)shows the time-resolved analysis of the (local) monolayer capacity, cSmax, which demonstrates asimilar behaviour to that observed with acetylene [see Figure 3(b)], the only difference being that,in this case, the increase in the presence of SO2 was less. Furthermore, the values of cSmax for theadsorption of C2H4 onto Statue L1991 of Philippi, in the absence of SO2, were lower than thecorresponding ones for the system C2H2/Statue L1991 of Philippi [in Figure 3(b)].


(a)

Non

-ads

orbe

d ga

seou

s

conc

entr

atio

n, c

y (m

ol/c

m3 )

00

Time (min)

2

4

6

8

10 20 30 40 50 60 70 80 90

C2H


C2H

4/SO


(b)

Mon

olay

er c

apac

ity, c

*

(mm

ol/c

m3 )

0

5

10

15

0

Time (min)

10 20 30 40 50 60 70 80 90

C2H


C2H

4/SO


(c)

Ads

orpt

ion

entr

opy,

∆S

[J/(

mol

K)]

Time (min)

00

–50

–100

–150

–200

–250

–300

10 20 30 40 50 60 70 80 90

C2H


C2H

4/SO


Sm

ax

Figure 6. Plots against time of (a) the non-adsorbed gaseous concentration, cy, (b) the local monolayer capacity, cSmax and(c) the local adsorption entropy, ∆S, for the C2H4/(SO2)/L1991 Statue of Philippi systems at 323.2 K.

The plot of ∆S versus time in Figure 6(c) clearly shows that the minima coincide irrespectiveof whether SO2 is absent or present in the system. Moreover, the presence of SO2 in the systemled to an increase in the rate at which the entropy increased with time.

4.1.3. C2H6, C2H6/SO2

The plots depicted in Figure 7(a) are based on the time-resolved analysis of the (local) adsorptionisotherm, θ. It is obvious that, either in the presence or absence of SO2, instant desorption ofethane occurred within a very short time period from the start of the experiment, although such


1.0

0.8

0.6

0.4

0.2

00 10 20 30 40 50

Time (min)

60 70

(a)

80 90

Ads

orpt

ion

isot

herm

, θ(d

imen

sion

less

)

A

B

C

(b)

0 10 20 30 40 50

Time (min)

60 70 80 90







A

B

C60

70

80

90

100

Ads

orpt

ion

ener

gy, ε

(kJ/

mol

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A

B

C D

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2.0

1.6

1.2

0.8

0.4

0Ene

rgy

dist

ribut

ion

func

tion,

φ (ε

,1)

[mol

/(kJ

min

)]

Figure 7. Plots against time of (a) the local adsorption isotherm, θ, (b) the local adsorption energy, ε, and (c) the energydistribution functions for the adsorption energy, ϕ(ε,t), for the C2H6/(SO2)/L1991 Statue of Philippi systems at 323.2 K.

desorption was smoother in the case where SO2 co-existed in the system. In both cases, desorptionof ethane appeared to be virtually complete. However, in the absence of SO2, the abrupt desorptionof C2H6 from the surface of the statue was followed by a slight re-adsorption, as indicated by theslight increase in the resulting plot. This was probably due to a “reconstruction” of the solidsurface, as implied indirectly by the corresponding energy plot depicted in Figure 7(b). Theassumption of such a “reconstruction” is not unreasonable for this type of behaviour, since thesolid sample of Statue L1991 of Philippi has been shown by XRD analysis (Agelakopoulou et al.2007, 2008) to be “pure calcite”, without any traces of dolomite or gypsum. As already mentionedelsewhere, calcite surfaces are highly susceptible to reconstruction (Cygan et al. 2002; VanBeurden 2005).

The plots depicted in Figure 7(b) show that the presence of SO2 in the system led to a smallincrease in the maximum value of the adsorption energy — from 93.9 kJ/mol to 96.7 kJ/mol —during the first 10 min of the experiment. Desorption followed shortly afterwards (branch B), thisbeing more abrupt in the absence of SO2 but lasting longer when SO2 was present. In the absenceof SO2, desorption occurred via a single-layer process, followed by re-adsorption attributed to a“reconstruction” of the surface of the statue surface which was compatible with chemisorption, asdemonstrated by the high energy values obtained. In this way, a steady state was established, asreflected by the stabilization of the adsorption energy values.

The above-mentioned “reconstruction” mechanism may be attributed to one or a combinationof more than one of the following reasons.

(i) An increase in entropy. Close approach between the molecules of some adsorbates leavesmore surface space available for occupation by other species. This allows other adsorbatesto form various configurations, thus increasing the surface entropy (Jansen 2008).

(ii) The creation of sub-surface states (sss) which have no effect upon the free energy of thesurface (Christmann 1995).

(iii) Variation in both the chemisorption and physisorption mechanisms in such a way that theadsorbing/desorbing system attains a state of thermodynamic equilibrium.

Plots of ϕ(ε,t) versus time for the C2H6/L1991 Statue of Philippi both in the presence andabsence of SO2 are depicted in Figure 7(c). Three clearly distinguishable adsorption areas (A, B, C)are visible, but one extra peak (D) which is not visible also exists, as can be revealed by“unfolding” these plots through the use of a better scale for the time axis. Thus, four peaks arepresent in both systems. However, the heights of the peaks corresponding to the C2H6/L1991Statue of Philippi system were higher than the corresponding ones in the presence of SO2; this wasparticularly noticeable in the case of peak B. Specifically, the magnitude of this peak diminishedfrom a value of 2.13 mol/(kJ min) in the absence of SO2 to 0.306 mol/(kJ min) in its presence.This kind of behaviour could be attributed to the presence of SO2 in the system causing a decreasein the repulsive lateral interactions responsible for ethane desorption, probably through a“perturbation” in the existing force field caused by the polar nature of SO2. As a consequence, thedesorption process is moderated (cf. the smoother slope of the descending branch of thecorresponding curve in Figure 7) and a “new field” of attractive lateral forces is established. This“new force field” contributes positively to physisorption. Indeed, the assumption of this kind of“van der Waals force redistribution” is not unreasonable since van der Waals forces are related tomobile adsorption processes compatible with physisorption (Adamson 1990).

Since the areas under the above peaks represent the number of active sites of substratesusceptible to adsorption at the corresponding times, the influence of SO2 co-existing with C2H6in the system is more apparent if the relative difference in these populations in the presence or


absence of SO2 is calculated. This method of determining the “relative difference of populations”is useful when the above-mentioned “synergistic effect” is substantial, as in the case of area B inFigure 7(c). In particular, by means of a graphical calculation of this specific area, it has beenshown that this “relative difference” led to a reduction rate of 83.2% when SO2 was present.


8

6

4

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00 10

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-ads

orbe

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ncen

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(mol

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3 )

20 30 40 50 60 70 80 90Time (min)

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(b)

2.252.001.751.501.251.000.750.500.25

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er c

apac

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m3 )

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C2H6/SO2/L1991 Statueof Philippi

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–200

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Ads

orpt

ion

entr

opy,

∆S

[J/(

mol

K)]

(c)

Sm

ax

Figure 8. Plots against time of (a) the non-adsorbed gaseous concentration, cy, (b) the local monolayer capacity, cSmax, and(c) the local adsorption entropy, ∆S, for the C2H6/(SO2)/L1991 Statue of Philippi systems at 323.2 K.

As far as the “synergistic effect” of SO2 on areas C and D in Figure 7(c) is concerned, this leadsnot only to a decrease in the maximum ϕ-values, but also to a time delay in the presence of SO2.Indeed, the maximum value of ϕ for area C diminished from 0.066 mol/(kJ min) at 16 min to0.0591 mol/(kJ min) at 20 min.

A slight decrease in the maximum of the curve of cy versus time depicted in Figure 8(a) mayalso be observed in the presence of SO2, viz. from 6.95 mol/cm3 to 6.54 mol/cm3. In the absenceof SO2, a maximum appears at 28 min in the curves of cSmax versus time depicted in Figure 8(b),i.e. at the point when monolayer coverage is complete [cf. Figure 7(b)], corresponding to a valueof 2.09 mmol/cm2. This maximum disappears in the presence of SO2, thereby indicating that SO2molecules are those first preferably adsorbed onto the statue surface.

The presence of such a sharp increase in the curve obtained in the absence of SO2 was notobserved in the other hydrocarbon systems studied, either in the absence of presence of SO2 [cf.Figures 3(b) and 6(b)]. This sharp curve may be attributed to complete monolayer surfacecoverage of the statue under these conditions. Actually, this fact is confirmed by the differentshapes of the curves of local adsorption isotherm and local adsorption energy, versus t, depictedin Figures 6(b) and (c). This is particularly true for the latter stages of these curves which weredifferent when SO2 was present. The fact that this sharp increase was missing from thecorresponding curves of the other systems examined in the present study also supports thisinterpretation.

Examination of the plots of ∆S versus time in Figure 8(c) indicates the presence of a slightlygreater minimum of –291 J(mol K) at 12 min when SO2 was absent compared to the value of–285 J/(mol K) at 10 min in the presence of SO2. In addition, the slope of the curve relating todesorption — the ascending branch — was greater in the absence of SO2 and was followed by theappearance of a maximum of –107 J/(mol K) at 28 min accompanied by a slight decrease in the ∆Svalues which continued until stabilization was attained. This “maximum” value at 28 min in the ∆Splot in the absence of SO2 corresponds to the point at which “reconstruction” of the statue surfaceoccurred, as mentioned above in relation to the energy plot depicted in Figure 7(b). Indeed, one ofthe possible interpretations concerning “surface reconstruction”, the subsequent “surface energystability” and also the “constant adsorption energy values” has to involve entropy considerations.This interpretation is supported by the observed “maximum” in the ∆S plot values reported here.

4.2. The influence of the hydrocarbon bond on the adsorption phenomenon

4.2.1. In the absence of sulphur dioxide

The basic difference between the three examined systems of aliphatic hydrocarbon/(SO2)/L1991Statue of Philippi is that only in the absence of SO2 and the presence of ethane — a saturatedhydrocarbon — was monolayer surface coverage of the statue accomplished. This situation didnot occur in the other cases examined [cf. Figures 2(b), 4(b) and 7(b)].

Moreover, in the absence of SO2, the adsorption of acetylene onto the statue surface was greaterthan that of ethylene, as demonstrated for example by the data depicted in Figures 2/4 and 3/6.This indicates that, at equivalent concentrations, acetylene molecules were desorbed from theadsorbent with greater difficulty. These findings are in complete agreement with those found byYun et al. (1996), who applied the same method (RF-IGC) for the determination of the adsorptionequilibrium constants of ethane, ethylene, propylene and acetylene onto Al2O3 at differenttemperatures. These workers interpreted their observations as being due to the much larger


electrical density associated with the triple bond in acetylene causing stronger van der Waalsattractions and hence better physisorption properties.

4.2.2. In the presence of sulphur dioxide

A stronger positive synergistic effect was observed with respect to the adsorption of ethylenerelative to that of acetylene in the presence of SO2. More specifically, co-existent SO2 deceleratedthe desorption of ethylene from the statue surface, probably since a greater number of active siteswere preferentially covered with SO2 molecules. This may be explained by the possibility that thepresence of SO2 admolecules led to a decrease in the repulsive lateral interactions between theadsorbed ethylene molecules, which were consequently more widely separated. It should be notedthat the synergistic effect of SO2 on the adsorption of ethane onto the statue surface wasintermediate between those for the two unsaturated hydrocarbons studied.

More explanations concerning such adsorption processes and a comparison with related resultsobtained from the application of other techniques may be found in a recently published review(Roubani-Kalantzopoulou 2009).

5. CONCLUSIONS

The conservation of cultural heritage is important both in preserving the past and recognizing itsimportance in history. Cultural heritage materials are subject to continuous chemical and physicalchanges brought about by the environment in which they are situated. An assessment of the criticalrelationships between environmental factors and the damage they cause cannot be simply based onequilibrium measurements of the various physicochemical quantities pertaining to the damage. Onlytime-resolved measurements can give information on the actual mechanism of the damage. This iseasily achieved by the use of reversed-flow inverse gas chromatographic methods to the study ofsuch physicochemical quantities, as has been described in several original papers, reviews andbooks.

In the present work, the method of reversed-flow inverse gas chromatography coupled withtime-resolved analysis has been applied to the measurement of various important local adsorptionparameters describing the action of binary systems of air pollutants on the heterogeneous surfaceof a statue sample taken from the interior of the Museum of Philippi, near Salonica, in Greece.

In summary, in the presence of SO2, the effect of ethylene on this statue surface is moredeleterious than the corresponding effects of ethane and acetylene. Moreover, as indicated by theadsorption energy and adsorption entropy values obtained, the composition of the statue surfaceexamined appeared to favour the exclusive adsorption of ethane in the absence of SO2, obviouslythrough an adsorption-induced reconstruction of the statue surface.

All the above findings provide valuable information about the susceptibility of the examinedstatue to environmental pollutants.

ACKNOWLEDGEMENTS

The Archaeological Museums of Kavala and Philippi are especially thanked for the kind supplyof statue samples, by permission of the Greek Ministry of Culture.


REFERENCES

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“XRD–Raman–SEM Analysis of Marbles Exposed to Gas Pollutants”, submitted for publication. Agelakopoulou, T., Margariri, S., Bassiotis, I., Siokos, V., Metaxa, E., Karagianni, Ch.-S. and Roubani-

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