Biocides on Stone

Biological Growth such as algae, lichens, fungi and mosses are common on the façade of stone buildings and sculptures. They grow in patches where adequate light, temperature, humidity and moisture are available which would be conducive to its growth. These biological agents on the façade are sometimes detrimental to the structure of the stone but also sometimes act as protective coatings against the vicissitudes of nature. Some of the organisms, which grow such as the algae, are sticky and traps dust particles in the atmosphere leading to soiling of the surface of the stone. Generally when these organism are undesirable, they can be removed or prevented by control the surface of the stone- by providing for drainage of the water and encouraging the quick drying out of the stone. The most recent of interventions to prevent the growth of bio organisms on the stone is the application of biocides on the stone. The term biocide denotes any chemical that is able to kill or inhibit the growth of living organisms; it is most commonly used with regard to microorganisms and higher plants. The earlier interventions were generally physical interventions to the stone and its environs while the use of biocides as a means of control biological growth is a chemical process. The biocides used on the stone surfaces are generally effective for only a short period of time and due to its toxic nature, some of these have been discontinued in use owing to the health and safety regulations. The paper is an attempt to understand the process of bio deterioration of stone and some of the applications of biocides on the stone.


Bio deterioration of stone.

The term bio deterioration refers to any undesired change in the material properties due to the activity of microorganisms and/or organisms belonging to various systematic groups. Stones are constantly undergoing a transformation, which is a result of microorganisms and other organisms – be it in the form of weathering or the degradation of the same. This deterioration on significant works of art and architecture is detrimental to the aesthetics and threatens the conservation of these artworks. The nature of the deterioration of the stone is actively linked with various physical, chemical and various other processes. Bio deterioration of stone due to biological organisms (bio deteriogens) is linked to the environment as a whole. The processes of nature and the environment are very complex and determined by:


- Light (Primary energy input)
- Nutrition (Secondary energy input)
- Climate.


Light: The growth of the photosynthetic organisms is dependent on the Light energy as the primary source of energy. Various organisms have preferences for various types of radiations as determined by their photosynthetic pigments. Some bacteria and algae are limited in their growth in the absence of light.


Nutritive factors: The minerals present in the stone – in relation to the petrographic nature of the stone and the organic substances play a role in the growth of bio deteriogens. All organisms need mineral elements for photosynthetic processes. The petrography of the surface of the stone determines the pH of the substrate of the stone for the biological organism. For organism that can grow with a large tolerance of pH, this is not a consideration. Some organisms like bacteria grow in alkaline substrates, while others like fungi grow better in an acidic substrates. The growth of bio deteriogens is also enhanced by dry deposition of particle mass, protective wax treatments and the like.


Climate: The growth of the bio- organisms is hugely determined by the climate- the temperatures and humidity of the environment. High temperatures generally increase growth as it increases the rate of chemical reactions. The duration of the period of wetness increases the growth of the organisms.


Type of stone: Various characteristics of the stone determine the influence and effect that biological growth could have on the stone surface. Some of these include the surface texture, the porosity of the stone and the surface roughness.



Bio deterioration- the Processes.

The biological deterioration of the stone is both a physical and a chemical process. Physical processes are mechanical in nature. The growth of the roots of the organisms in the stone exerts a pressure on the stone. Microorganisms do not limit to superficial growth on the surface but penetrate into the stone structure, which could cause fracturing of the stone. The minute disintegration of microorganisms collectively leaves behind a soft and loose surface susceptible to attack from larger factors such as wind and rain. The affected stone surfaces are also more susceptible to the freeze/ thaw weathering due to water which gets trapped in the crevices.
Chemical processes act directly on the stone and lead to the production of organic and inorganic acids and the production of chelating substances. Acids may be strong and lead to the formation of salts on the substrates. All organic materials produce carbon dioxide, which dissolves to form a weak carbonic acid, and this dissolves lime in marbles and other carbonic rocks.


Types of Biological Growths.


The few classes of bio deteriogens that are found in stone are the following classes of plants:

1. Algae and Cyanobacteria.
2. Bacteria.
3. Fungi.
4. Lichens.
5. Mosses
6. Liverworts.


Algae and Cyanobacteria-Algae are simple plant forms without leaves, stems and roots. They exist as cells, which clump together and range from microscopic in size to the size of seaweeds. They grow across a range of pH, light intensity, moisture content and temperatures. Algal spores are very resistant to drying out and may remain viable for long periods of time. Cyano bacteria – or blue green algae are different from other algal groups in that their internal structure resembles bacteria but are smaller and have photosynthetic pigments. They exist as filaments or as single cells. Cyano bacteria grow on substrates, which are more alkaline based. These organisms are photosynthetic. Cyanobacteria can even tolerate lower light levels. They accelerate stone deterioration in that algae are water retentive and in some cases lead to the dissolution of material elements through mineral elements through acidic secretions, chemical and physical effects of mucilage.


Bacteria-They are single- celled organisms invisible to the naked eye. They can thrive in varied physical conditions though some bacteria require organic material for growth. Secretions by bacteria cause some stones to decay . Some species of bacteria secrete acids such as sulphuric acids which cause severe damage to calcium based stones.


Fungi-Fungi cells grow as long strands or hyphae. They have a diameter of 5-9 microns and can grow to lengths of several meters. They are very resistant to desiccation and can withstand extreme conditions for long periods of time. They require organic matter as nutrients and cause decay to stones by physical penetration into and around the grains.


Lichens-Lichens are plant forms created through a symbiotic relationship between algae and fungi. They affect the stones by causing bleaching and blistering of the surface through physical processes.


Mosses-These are small plant structures and appear on stone surfaces generally around cracks. They require much more water for nutrition and photosynthesis than all the other smaller plants and have a high capacity to hold water.


Liverworts-These are small plants, which are found in stone areas where there is a lot of moisture and humidity. No effects of liverworts have been reported on stone.


Identification of common biological growths on stone:

Growth Colour Appearances Habitat

Algae Green- sometimes black, red, orange or yellow

• Can appear as uniform patches or streaks on the stone surface.
• Sometimes slimy to touch.
• Favors areas that retain moisture and light.


Blue green algae Green, blue green, grey, black.

• Can appear as dark mats, patches or streaks on stone surfaces.
• Sometimes slimy to touch
• Favors humid environment – requires less light than algae.


Bacteria Not visible to naked eye.

• Microscopic determination required to distinguish from algae.
• Grows where water is present.
• No sunlight required.

Fungi Depends on species.

Long strands of filaments massed together.
Requires moisture but not sunlight.


Lichens Many colors ( white, grey, orange, red, black, yellow and green)

Flat crusts close to the surface growing away from the surface. Grows in areas too dry for other organisms to grow.


Mosses Mainly green or red.

Leaf region of the cells and root grow together in small lumps loosely attached to the surface. Requires a very damp environment, sunlight and soil.

Liverworts Green

Flat lobed thallus with primitive roots – loosely attached to the surface.
Requires a very damp environment, sunlight and soil.

( Source: Cameron, Urquhart, Wakefiled, Young. Biological Growths on Sandstone buildings. Historic Scotland.)


The identification of the type of biological growth on the substrate of the rock is very important as the biocidal treatment on each of them varies.


Biocides:

The Biological growth on the stone is treated with biocides generally for aesthetic reasons. Streaks of growing fungi and mosses on stone structures detract from the aesthetic values. Algae are primarily the first microorganisms to attack the surface of the stone, which paves the way for other higher organisms to follow. The prevention of the growth of algae could reduce the potential for other organism to attack the surface. Most organisms require water for growth. Keeping the stone dry could help in the process of prevention of biological growth. As some organisms have the potential of withstanding these phenomena, biocides are used to eliminate these organisms.


Biocides act on contact. They have a capacity to inhibit the biosynthetic process such as the synthesis of amino acids, lipids, and proteins and can interrupt the connection between respiration and ATP synthesis, block photosynthesis or interfere with the growth processes. Biocides do not have a lasting effect and their effect could last from a few months to a few years.


There are numerous factors, which have to be kept in mind:

1. The treatment of the biocides must kill all the biological organisms but should not affect human health. The toxicity of the biocide is generally quantified by using the LD-50( Lethal Dose –50) values as indices. It is a measure of the amount of active substance that can be expected to cause death to a group of experimental animal species. ( See Annexure 1 for information biocides and their toxicity)

2. It should be effective for a considerable period of time.

3. It should not leave behind deposits on the stone and maintain the original color and texture of the stone.



Some types of Biocides:

Early studies showed that the biocides generally belonged to the following chemical groups :

1. Quaternary ammonium compounds.

2. Borates.

3. Metals- such as copper.

4. Chlorophenols.

5. Phenoxides.


Organo tin compounds are no longer used. Sodium hypochlorite is another effective biocides, which is no longer used due to the sodium salts that cause efflorescence. Biocides generally work in two ways- by inhibiting the growth of the organisms and a irreversible lethal biocidal effect.


Selection of Biocides:

1. Bacterial or fungicidal compounds.

The same compounds are generally used for controlling the growth of bacteria and fungi. They consist of derivatives of carbamic acid, bezothriazoles, benzalkonium chlorides, isotiazoline chlorides and formaldehydes. Quaternary compounds of ammonia are also used.


2. Algicidal compounds.

Algae, lichens and mosses are generally controlled by compounds which inhibit or prevent photosynthetic processes. Spraying or painting the compounds often achieves this. The most often used compounds include sodium salts of dimethyliocarbamic acid and mercaptobenzothriazoles, borates, uracils, derivatives of urea, pyridil mercuric acetate, and cupritetramine sulphate, complexes of copper and hydrazine and quaternary ammonium compounds.


3. Herbicides.

Herbicides are used rarely on stones.


Treatment:

The treatment of the affected stones with biocides requires consideration. A biocide, which works effectively with one type of organism, may not work at all with the other. It thus becomes important to recognize the type of bio organism on the stone substrate. Stone cleaning affects the stone surface and the near surface characteristic of the stone and this may influence the re growth of organisms on the stone surface. Chemical cleaning may affect the stone surface – roughness. Some biocides use acids, which could have affected the calcite-based stones. It becomes necessary to identify the nature of the biological growth and test various biocidal treatments. There are many ways in which biocidal treatments are applied to the surface of the stone. Some of these processes involve spraying using pneumatic sprayers and adjustable nozzles, as cellulose poultices, injection in the stumps of higher plants etc.

Spores produced by bacteria, fungi and some algae are very resistant to biocides and can remain dormant for long periods of time. Most spores are resistant to biocides that contain quaternary ammonium compounds, disinfectants, alcohols, solvents, phenoxides and hypo chlorides. Biocide compounds that are effective include ethylene oxide, gluteraldehyde, formaldehyde, halogens (chlorine, iodine and bromine), hydrochloric acid and hydrogen peroxide. They are mostly formulated as fungicides. Biocide products keep changing constantly both in the product name and in the compounds used in its formulation. It becomes difficult to identify the active ingredients in the compounds. Biocides are tested in the laboratory on liquid cultures of the target organisms. There are often cases when the reaction with the target compounds on a substrate is totally different.


The effective lifetime of the different biocides depend on:


• Type of biocide.

• Concentration of the biocide.

• Nature of the organic growths.

• pH of the environment.

• Temperature.

• Microclimate.

• Exposure of the treated areas.

• Type of stone.


Regular re-application of the biocide on the surface of the stone is required. Minor change sin the concentration could affect the efficacy of the biocide. Compounds such as phenols and alcohols are generally more biocidal at lower concentrations than the quaternary compounds. The presence of dirt and other organic debris can reduce the effectiveness of the biocide. Biocide could be lost from the surface of the stone due to the washing out action of rain. Porous stones can hold biocides in its pores and are less susceptible to washing out than non-permeable rocks.


Effects of Biocides:

One of the most significant side effects of biocide application on the stone surface is the ability of the biocide to alter the color of the stone. The other effect that is of concern is the possibility of salt crystallization due to the compounds used in the biocide. Deposits in stone from salts, which expand due to crystallization could have devastating results in the stone. Some biocides can cause changes in the stone surface altering the water absorption and water penetration in the stone.


Application of Biocides:

Choosing the period of time for the application.

The best period of time would be in the summer after a dry spell of time. Organic growths are most susceptible during the wet period but the biocide gets washed off more easily. The substrate has to be maintained dry to ensure that the biocide does not run off. The biocide should be well brushed in. After the initial application, the dead organic material should be removed as the organism could absorb much of the biocide.


Some tests done on biocides:

The following is a test done by Claudia Pincki, Riccardo Balzarotti and Reiner Mansch.

Summary of the test.

They took four biocides and tested their efficacy against nitrifying bacteria isolated from building stone.

The four biocides were:

- Algophase applied in organic solvent (isopropylic alcohol + 20% acetone) (PHASE, Firenze,Italy)

- PH025/d a new water based formulation of Algophaseâ (PHASE, Firenze, Italy)

- Kathon WT a derivate of isothiazolinone (Rohm & Haas, Frankfurt, Germany)

- Benzalkonium chloride (Sigma chemicals, No. B 6295)


From the natural building stone – nitrifying bacteria were isolated. All biocides were screened over a wide range (0.00001-12%) of concentrations in liquid culture using sterile media for nitrifiers as diluent. Tubes without the addition of biocides were used as controls. Specimens of Ihrlersteiner green sandstone and Sander sandstone were inoculated with ammonia- and nitrite-oxidizers as well as chemoorganotrophic bacteria and fungi isolated from mortar [3]. The specimen were regularly supplied with ammonium chloride and incubated for seven month at 28 °C and high stone moisture. Sub samples of grounded stone material were incubated with the biocides for 30 min, washed and cell numbers were determined by a most-probable number test (MPN) and agar plates respectively. Samples without the addition of biocides were used as control.


Efficacy against mixed populations on natural stone –

After incubation for 30 min, Algophaseâ as well as PH025/d killed all nitrifying bacteria, hemoorganotropic bacteria and fungi grown in a biofilm on natural stone at MIC (Table 2). In contrast to this, Kathon and benzalkonium chloride just reduced the numbers of bacteria and fungi even at 10 times MIC ( minimum inhibitory concentration). Benzalkonium chloride was more effective than Kathon.


Conclusions

• Algophase and PH025/d with their low toxicity and high efficacy are promising biocides.
Algophase( 2,3,5,6-tetrachloro-4( methylsuphonyl) pyridine).

Algophase belongs to the group of biocides that are applied as they have a preventive effect to reduce in time the recolonisation of the restored surfaces. It is generally applied at the end of the conservative treatments and is effective against algae, fungi and cyanobacteria . It has an effective activity against fungi and lichens for up to 5-6 years. The tests at the Templete de Mudejar showed that eight years after the application of the biocide Algophase, organic growth was beginning to appear. It was concluded that the biocide Algophase though it dramatically reduced the test samples of the organic materials in the laboratory were successful in the field to a lesser degree. On the field, test samples showed the absence of the biocide on the façade after 8 years, which pointed towards the washing off of the biocide from the façade in the interim. The data showed that Algophase was able to inhibit the growth of micro organisms for a period of 5-6 years.


Other research on Biocides:

Another interesting experiment in recent years is the tests done by Masonry Conservation Research Group in the Robert Gordon University, Scotland. They assessed the efficacy of various biocides on sandstone.

For the experiment, they used six different types of sandstones and the effect of three different biocides. The biocides chosen for evaluation contained:

(1) alkylaryl trimethyl ammonium chloride,

(2) Dodecylamine lactate and dodecylamine salicylate and

(3) A copper salt.

The aim of the experiment was to determine the effective life spans of some typical biocides when applied to sandstones under field conditions and to investigate whether sandstone type had any influence on the rate of algal growth and/or the effective life span of the biocides investigated. The rate of green algal growth on six different sandstone types, both fresh (control) and biocide treated, was monitored using a chroma meter. It was found that the growth of algae on these stones which were of different porosities differed and also that the effect of the biocide was dependent not only on the biological growth and the porosity of the stone but also on the mineralogy of the stone. The presence of clays in the stone affected the efficacy of the biocides; quartz and feldspars had no effect. Bio efficacy depended on the stone types- sandstones required higher concentrations than limestones. (See Annexure 2.)


Alternative methods of Control:

Copper strips: The water run off from the copper flashing strips produces a mildly toxic biocidal wash. This however leaves behind a slightly greenish stain, which is sometimes difficult to remove.

Brushing and washing: The organic growth can be physically removed by brushing and scraping. This is to be followed by washing down with de- ionized water. The physical action leaves behind bristle marks on the surface of the stone.

Water repellants and consolidants: Water repellents have been used to prevent growth of organic materials in porous stones. This could sometimes lead to the crystallization of salts in the salts and the entrapment of moisture in the stone due to the waterproofing layer on the outside.
Ultraviolet light: Ultraviolet light is used in the control of growth of algae. UV light can kill photosynthetic organisms and does not have the risks which are attached to the other processes.

Health and Safety:

Biocides are hazardous compounds, which can be irritant to both the skin and the eye. They should be used with extreme care and not ingested. Precautionary measures would include providing personal protection, and controlling exposure and also preventing exposure.


Concerns:

1. Potential damage. -There has been general concern in recent years that the addition of biocides has become a major concern that the removal of the natural coating of the stone could accelerate future weathering and decay.

2. Aesthetic. -The application of biocides in some cases change the color of the stones and hence could not affect the look of the building.

3. Other considerations. -When biocides are applied to organism, they are destroyed and leave behind a black residue, which takes a long time to get removed.