Acid rain effects on stone monuments

Prévia do material em texto

edited by: 
MICHAEL R. SLABAUGH 
HELEN J. JAMES 
Weber State College 
Ogden. Utah 84408 
A. Elena Charola 
Metropolitan Museum of Art', New York, NY 10028 
One of the first reactions taught in most elementarychem- 
istry courses is the neutralization reaction in which the di- 
rection depends on the relative strengths of the species in- 
volved, that is, the reaction goes from a "strong acid" to a 
"weak" acid: 
acid 1 + base 2 F. acid 2 + base I 
Yet few people are capable of identifying this type of reac- 
tion when they see it occurring in everyday life. One of the 
unfortunate examples is the reaction produced by "acid 
rain" on marble monuments. 
What Is "Acid Rain"? 
Rain is naturally slightly acidic because of the presence of 
CO, in the air that dissolves in the water to form carbonic 
acid. The reaction can he written in a simplistic way as: 
H,O + CO, e H,CO, a Ht +HCO,- (1) 
"Natural" rain has a pH of about 5.13.~ 
In urban or highly industrialized areas, other gases of an 
acidic nature are produced from combustion of fuels. Of 
these, the most common are the oxides of sulfur and nitro- 
gen. In the case of sulfur, SO2 is the gas most commonly 
produced. Under the influence of UV light and through 
catalytic action of dust particles, building surfaces, and oth- 
er agents, SO, can he oxidized to SO3 by the oxygen or ozone 
in the air. The SO3 then dissolves in water to produce sulfu- 
ric acid. The reaction can be written as: 
H,O + SO, e H,SO, F! Ht + HS0,- (2) 
The pH of this rain depends on the concentration of SOs; the 
' Current address: ICCROM, 13, via di San Michele. 00153 Roma, 
Italy. 
The pH of "natural" rain can be calculated from the following 
equilibria ( 15): 
CO, (g) + H,O F! H&O, logK = -1.46 
H&03 z= Ht + HC0,- lag K, = -6.36 
and knowing that the atmospheric level of Con is 0.0003 aim. 
From the first equation the concentration of dissolved carbonic 
acid can be calculated: 
Knowing this concentration the pH can be calculated from the 
second equilibrium: 
[HtI2 = K, X 1.04 X 10@ = 4.54 X 10-l2 
pH = 5.6 
rain will have a lower pH where this concentration is higher: 
urban areas. hieh traffic areas. and industrialized areas. The 
pH will chakggduring the time the rain falls. At the begin- 
ning of the rain, the concentration of SO3 in the air will he 
highest, and rain will have its lowest pH value. Values as low 
as pH 3 or lower have been actually measured. This repre- 
sents a thousandfold increase of H+ concentration with re- 
spect to normal rain! Similar reactions could he written for 
the case of the nitrogen oxides. but this discussion will he 
limited to those of t h i sulfur oxides mentioned above. 
The term "acid rain" was coined as a conseauence of the 
observed lowering of the pH. The problem, though, is not so 
simple. and the currentlv preferred term is "acid ~ r e c i ~ i t a - 
tioi2'. This term is more comprehensive and inciudei not 
only "wet deposition", the deposition of pollutants in the 
presence of water, as when i t rains or snows; but also "dry 
deposition", the settling of gaseous or solid pollutants, such 
asaerosols and dust in the absence of water: 
Whlch Stones Are Used to Make Monuments? 
The stone usually associated with monuments is marble, a 
calcareous stone. made from large crvstals of calcite. the 
thermodynamically stable mineray form of CaC03. ~ a r b l e is 
a metamorphic rock, that is, it is a rock that has undergone 
recrystallization a t high temperatures and under great pres- 
sures. Before this process, the stone existed as a sedimentary 
rock, called limestone. Limestone is made up of much small- 
er crvstals of calcite that were "sedimented" toaether, usual- 
ly under water. Chemically, marble and limestone are identi- 
cal, hut morphologically they differ in crystal size and poros- 
ity. Limestone is made from smaller crystals and is therefore 
more porous than marble. This difference affects the ap- 
pearance and "workability" of the stone. Because marble 
can attain amuch higher polish than limestone, the former is 
preferred for monuments. Limestone, however, is used ex- 
tensively in buildings. For example, Rockefeller Center in 
New York Citv is constructed of Indiana limestone, one of 
the most popuiar limestones in the United States. 
Sandstone also has been used for monuments. but in the 
United States it has been used mainly for buildings, an 
example of which is the New York City brownstone. Sand- 
stone is a sedimentary rock made of sand, which-for the 
purposes of this discussion-will be limited to mean quartz, 
the thermodynamically stable mineral form of SiO,. The 
grains of sand can he cemented together by either more 
quartz, or by calcareous material. In the latter case, the stone 
would be called a calcareous sandstone. 
Granite is the third of the important stones used in monu- 
ment construction. This rock is made up of three main min- 
erals: quartz, mica, and feldspar. Mica is actually a group of 
minerals composed of silicates of aluminum, potassium, 
436 Journal of Chemical Education 
magnesium, and iron. The feldspars are aluminosilicates of 
calcium, sodium, and/or potassium. Usually granites are 
large-grained, and the different minerals can be identified 
visually: the transparent quartz, the white or pinkish feld- 
soar. and the black mica. The porosity of granite is extreme- 
. . 
Iy low; consequently, it is usually used as a damp-proof 
barrier i n the Iuwer C U U ~ S C S of buildings. 
How Are the Stones Affected by the Acld Rain? 
Stone monuments most susceptible to acid rain are those 
made of the calcareous ones: marble, limestone, and calcare- 
ous sandstones. Pure sandstones and granites are not affect- 
. 
ed by acid rain. 
The reaction between a calcareous stone and acid rain can 
be written in this simplified form: 
H,SO, + CaCO, e Ca2+ + S02- + H20 + C02 (3) 
The dissolution of calcite involves more equilibria than 
the simple one represented by eq 3. The evidence suggests 
that three reactions occur simultaneously (I): 
CaCO, + H' E? Ca2+ + HCO,? (4) 
CaCO, + H2C0, G Ca2+ + 2 ~ ~ 0 , - (5 ) 
CaCO, + H20 a Ca2+ + HC03- + OH- (6) 
The different mechanisms controlling the dissolution of 
calcite depend on several conditions: pH, flow hydrodynam- 
ics, etc. Below pH -4 the dissolution is transport controlled, 
and the rate is proportional to the H+ activity in the solution 
(1-4). The dissolution is based mainly on the pH-dependent 
mechanism represented in eq 4. The higher the concentra- 
tion of acid (actually the activity) the higher the dissolution 
rate. 
Between pH -4 and 6, the rate of dissolution is less than 
that expected for transport control, and the reaction would 
appear to be controlled by surface kinetics (1,5-7). Hydro- 
dynamic flow conditions, the pressure of COz, and the homo- 
geneous reactions of this compound in the hulk fluid and/or 
boundary layer (eq 1) begin to assume important roles. 
As the pH rises above 6, the reaction tends to become pH- 
independent mechanism and the rate is surface controlled 
(8) . 
For this discussion, the interesting range is the intermedi- 
ate one, roughly between pH 4 and 6. As previously stated, 
several mechanisms affect the dissolution rate of calcite, and 
the dominant mechanism depends on the specific conditions 
in a given location. Inasmuch as conditions can vary drasti- 
cally even on the surface of a single monument, several 
mechanisms can he operating simultaneously on areas close 
to each other. 
The marble columns of the Roman emperors Marcus Aur- 
elius and Trajan, in Rome, illustrate the point well. The 
columns are carved in bas-relief bands that spiral around the 
column representing the conquests and victories of these 
emperors. Some areas of these monuments receive directflow of rainwater over them, and the damage observed is 
greatly increased by the hydrodynamic factors that com- 
pound the simple chemical dissolution of the marble. Other 
areas are more protected, due to wind effects induced by the 
buildings around the column, and these surfaces are still in 
extremely good condition, even though the column has been 
standing for over 1800 years! 
It is important to point out that the damage resulting from 
chemical dissolution of calcium carbonate is mainly a sur- 
face phenomenon. This surface damage is of special concern 
when the details on the finely worked relief are only some 
centimerrrs in depth. The strktural integrity of the marhle 
drums is not affected, however, by this surface dissolution. 
Rut the oroblem of the deterioration of monuments is - -. . - - - L~~~ ~ ~ 
even more complicated than the dissolution of calcite. Equa- 
tion 3 shows that the calcium carbonate of the stone reacts to . ~ ~ - ~ - ~ ~ 
produce calcium sulfate. This compound can crystallize as a 
dihydrate salt, CaSOc2H20, a mineral called gypsum. Gyp- 
sum is fairly soluble in water and therefore only accumulates 
on those surfaces of the reacted stone over which there is no 
direct, substantial flow of water. Limestone buildings and 
marble statues develop a layer of gypsum in those areas 
protected from rain, s;ch as under eaves or overhangs. Be- 
cause urban air is polluted by dust, carbon particles, fly-ash, 
and other combu&ion products (dry deposition), this layer 
of gypsum blackens. After years in an urban environment, 
limestone buildings present a characteristic black and white 
appearance: white where the rain washes off the reaction 
product (gypsum) from the stone surface regularly; black 
where the rain cannot wash away the dark gypsum layer that 
has incorporated into itself the dry deposition. 
Acid rain will not react with sandstone buildings or the 
granite courses on buildings and monuments. However, dry 
deposition still occurs, and this will adhere to the stone 
causing a uniform black color to develop. 
The soluble sulfate and nitrate salts that form in the 
reaction of calcareous stones with acid rain subsequently 
dissolve in the same rainwater. This solution penetrates the 
stone by capillary action. When the stone dries out, the salts 
crvstallize in the nore svstem. The pressures exerted by - ~ ,~~~ 
rhesecrystallizing saltsare sufficient todiarupt the matrixof 
the $tone rnechanicalls (12-14). IT is e\.ident then, that the 
porosity of a stonr is one of the important factors determin- 
ing itsdurability since it nmtrols theam~,untufs(~lution chat 
wzl penetrate the pore system. The damage produced by 
this type of mechanical action can he more serious than that 
produced by simple chemical dissolution of the stone. 
Conclusions 
The deterioration of calcareous stones occurs mainly 
through two mechanisms: 
. the chemical dissolurionofcalcire 
. the mechanical damage x hen wluble salts formed dllr- 
ing cliss.,lurrun ~uhs~~uenrly rccn.srnlli,e in the pores oi the 
stone. 
The first mechanism is important when surface detail is to 
he preserved on a sculpture. The second mechanism can 
contrihute significantly to the structural deterioration of the 
stone. Current research involves the difficult task of deter- 
mining the extent to which each of these factors contribute 
to the overall decay of a statue or monument. 
DC. 1979; Pp 537-573. 
3. caner. E. N.: smloy, N.J. ~ ~ ~ ~ ~ ~ d i n g r of the ~ h i r d lnrernotionol congress an the 
D~fsr iorol ion and Preservation o/Sfone: (publisher needed) Venim, 1979; pp 107- 
129. 
4. Wey1.P. K. J. G d . 1958,66,163-176. 
5. Plummer, L.N.; Wig1ey.T. M. L. Ceochirn. Camochirn. Aefa 1976,40,191102. 
6. ~url,~.~.S~~~tion~inoti~~~/~nleit~:~hoFou~hInternationslCongrosaofSpsleol- 
agy: Yugoslavia, 1968: pp 6146. 
7. Guidobaldi, F. In The Conssr"alionof SLoneII: Ro~si-Manare& R., Ed.;Centroper 1s 
Conservezione delle Sculture all' Apertm Bologna, 1981; pp 483-497. 
8. Sjoherg. E. L. Oeochirn. Cosrnachim. Acla 1976.40.441-447. 
9. CUIPIOI, R. L.; RathernVJne,J. D. B.Science 1981,213,1018-1019. 
10. Rands, D. G.: Rmenov, J. A.; Laughlin. J. S. Proreedings of the Confersnes on 
D<grod.fion nfMofrr iuls due to Acid Rain; American Chemical society: Washing- 
ton, DC, in press. 
11. Charuls. A. E.: Koestler. R. J. wiener Belichtp ubpr Nduruissensehofl in dar Kunat, 
in press. 
12. Lewin. S. 2. Conservolion o f Historic Stone Buildings ond Monummts; National This brief introduction to the topic has not taken into 
account other phenomena: the reprecipitation of calcite near 
the surface (9) or the effect of high ionic strength (10) and 
particle size distribution (11) on the solubility of calcite. 
Volume 64 Number 5 May 1987 437