An Investigation Into the Potential of Ionic Silver As a Wood Preservative

An Investigation into the Potential of Ionic Silver as a Wood Preservative Benjamin Dorau, Rachel Arango, and Frederick Green, III Abstract On December 3 1, 2003, the wood preservation in- explore the potential of silver-based biocides as wood dustry, in conjunction with the Environmental Protec- preservatives. We conclude that potential silver-based tion Agency, voluntarily phased out the use of biocides should be investigated further to determine chromated copper arsenate (CCA)-treated lumber for minimum inhibitory concentrations for fungal decay residential applications. This ended over 25 years of and termite damage. CCA as the predominant wood preservative in the United States. The first generation of replacement pre- Introduction servatives, alkaline copper quats (ACQ) and copper The wood preservation industry is currently in tran- azole (CA), has begun to replace CCA in 2004. Other sition. As of December 31,2003, the most used preser- potential replacements for CCA are under investigation vative in history, chromated copper arsenate (CCA), at the Forest Products Laboratory (FPL). One of these, has no longer been available for most residential appli- recently mandated by the 108th Congress, is the use of cations. This is due to health concerns about the envi- wood preservatives based upon silver (Ag) instead of ronmental toxicity of arsenic (As) and chromium (Cr- copper. Silver has been shown to have potential as a via- IV), two of the main components of CCA (1). Of pri- ble, safe, and cost-effective preservative. The FPL has mary concern is the possibility of increased cancer risk initiated a feasibility study of ionic silver salts for inhi- for children playing on CCA-treated playground bition of brown-rot fungi, termite damage, and mold equipment. contamination. To date, two silver formulations (~ 1% The currently approved substitutes for CCA, such as Ag) have been shown to be moderately resistant to alkaline copper quat (ACQ) and copper azole (CA), rely brown-rot decay, to inhibit termite damage with 100 on much greater levels of copper to combat attack by de- percent mortality of termites, and to resist colonization cay fungi and insects (1). Although these high copper by three common mold species. Additional testing, in- systems have relatively low mammalian toxicity, there cluding American Society for Testing and Materials leach testing and lower silver concentrations, further is some concern that copper, quaternary ammonium salts, and azoles may be harmful to aquatic biota. A preservative that minimizes toxicity concerns for both mammals and non-mammals would be preferable. One possibility is a wood preservative using low levels of Dorau: silver. Chemist This paper outlines the findings of a preliminary Arango: Biological Lab Technician study on the feasibility of silver-based wood preserva- Green: tives. It includes the results of termite feedings, leach- Ph.D., Research Microbiologist, USDA Forest Service, ing, boron fixation, resistance to fungal decay, and mold Forest Products Laboratory, Madison, WI growth inhibition. Environmental and economic con- Dorau, Arango, and Green, III 133 cerns of silver-based wood preservatives are also tions with complexed silver ions, neutral silver atoms addressed. were seen to be complexed by such ligands as EDTA (13). Reactive carboxyl groups, as found in EDTA, are Colloidal Silver and Ionic Silver also found in many components of wood and should The most widely publicized form of silver is “colloi- therefore bind individual reduced silver atoms dal silver.” An Internet search on colloidal or particu- effectively (12). late silver reveals thousands of pages filled with claims It is helpful at this point to discuss the actual proper- that are at best questionable and at worst deceptive. ties of a colloidal solution. Metal colloids are minute Among these claims are assertions that colloidal silver clusters of neutral metal atoms that are small enough to products can cure or treat over 650 diseases, ranging interact with polar solvents, such as water, forming an from cancer to diabetes (2). Others tout the antibacte- rial properties of colloidal silver products, claiming that electric double layer (7). The inner layer includes ab- + they kill all microbes and none is resistant to colloidal sorbed ions, which for metal colloids tend to be H ions silver (2). Little or no evidence is presented to support (1 4). The outer layer is a diffuse region in which counter - these claims. ions, in this case OH , are distributed according to Most colloidal silver products on the market use di- Brownian motion and electrostatic forces (7). This layer rect current on a silver electrode in aqueous solution. is what keeps colloids from aggregating under normal This is often referred to as “electrical silver.” When two conditions. When the liquid is removed, as in the case silver electrodes are immersed in aqueous solution and of drying after wood treatment, the particles will com- a direct current is applied, bubbles appear at one of the bine, forming larger clusters. This will decrease the sur- electrodes. This means that hydrogen in the water is be- face area and may reduce the efficacy of the silver as a ing reduced and the silver electrode is being oxidized to wood preservative. For these reasons, colloidal silver silver hydroxide. Thus, most colloidal silver products does not appear to have great potential as a wood pre- are actually suspensions of an ionic salt. servative. In contrast, preparations of colloidal silver found in Ionic silver salts appear to be a better alternative to the academic literature were not prepared using direct colloidal silver. Silver ions in solution offer a greater current or electricity but rather were reductions of solu- surface area because of the fact that they are individual ble silver salts, most using sodium borohydride (3-6). atoms, not clusters of atoms as seen in a colloid. This This very delicate and difficult process is not well- means they react more quickly and readily. Also, since suited to mass production. This process also yields very ions are relatively smaller than colloidal particles, they low concentrations, because metal colloids at high con- will penetrate wood more rapidly and effectively. Al- centrations tend to aggregate (7). though some of the silver ions used in a wood treatment Perhaps the best argument against colloidal silver is will most likely be reduced, they will form very small the only available scholarly article on its antimicrobial particles that can still be bound to wood components efficacy. Spratt and others (8) found colloidal silver to be and provide a great deal of reactive surface area. Finally, ineffective against bacteria when compared with more a counter ion for Ag+ can be chosen to reduce the poten- common antimicrobials such as sodium hypochlorite, tial for leaching. For these reasons, it appears that ionic chlorohexidine, and iodine. While this does not directly silver has more potential than colloidal silver as a wood relate to the ability of colloidal silver to kill fungi and preservative. termites, it does offer a fair estimate of its effectiveness. In addition, the leaching properties of a colloidal sil- Antimicrobial and Antifungal Effects of Silver ver treatment would most likely be poor. The sizes of Silver is a very effective bactericide and fungicide. the particles in a silver solution tend to be in the range The silver ion concentrations that exhibits antibacte- of 40 to 120 nm (9). These particles are too large to in- rial properties is roughly 0.1 µg/L, or 10–9 mol/L; the teract with the functional groups in cellulose or lignin, concentration for fungicidal activity is 1.9 µg/L (15, 16). which are less than 1 nm in size (10). Therefore, the Silver appears to have higher toxicity for bacteria than neutral silver particles would have little incentive to in- do more notorious antimicrobial metals such as mer- teract with the much smaller chemical constituents in cury, copper, lead, chromium, and tin (17). Its mamma- wood (11). Also, most of the silver particles would sim- lian toxicity, however, is relatively low. Human serum ply be excluded by the fibrous components and not pen- concentrations of up to 600 µg/L and urine concentra- etrate effectively, because wood has shown an ability to tions of 1,100 µg/24 h have been observed with no clini- filter out waterborne contaminants (12). On the other cal consequences (1 7). Perhaps most importantly, no hand, individual silver atoms, even uncharged, can in- mutagenic or carcinogenic activity has been reported teract with organic constituents. During redox reac- for silver (1 7). 134 Woodframe Housing Durability and Disaster Issues The mechanisms of the antimicrobial action of sil- Interaction of Silver Treatments with ver can be described as follows (17): Termites and Decay Fungi 1. Silver ions form insoluble compounds in the cell The digestive systems of termites contain protozoa wall with (and thereby inactivate) sulfhydryl that aid in the breakdown of cellulose into more easily groups, which are essential components of en- metabolized carbohydrates (20). Because of its anti- zymes responsible for transmembranous energy microbial properties, silver has the potential to dispose metabolism and electrolyte transport. The result of these organisms in the termite gut, disrupting the di- is a loss of fluids and electrolytes from the organ- gestive cycle and resulting in the death of the insect. isms, which dry out and die. This would require the release of silver ions, which come either from the ionic salt used in the treatment or 2. Silver ions block the respiratory chain of bacteria from oxidized silver metal. The solubility of the salt will in the cytochrome oxidase and NADH-succi- most likely determine the efficacy of the treatment as it nate-dehydroginase region. applies to termites, although all the silver salts in the 3.

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