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Leaching studies and fungal resistance of potential new wood preservatives.

Abstract

New inorganic wood preservatives based on chromated copper arsenate (CCA) have been formulated by replacing arsenic with molybdenum (Mo), tungsten (W), or zinc (Zn) phytate. The new preservatives are expected to exhibit lower human toxicity than CCA. CCMo and CCW have been examined by leaching tests and fungal exposure. The leaching of all elements of the CCW wood preservative is similar to that of CCA on a molar basis. CCW or CCMo treatment of wood blocks rendered them as resistant or more resistant to a brown-rot fungus than CCA at similar concentrations. The formulation containing zinc phytate was deemed unacceptable for this use due to poor solution stability and high component leaching.

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Chromated copper arsenate (CCA) is a widely used waterborne wood preservative in the United States, but its production for use in residential structures is prohibited as of January 2004. While CCA-treated wood may still be used in certain non-residential applications, the wood preservation industry must turn to alternative preservatives for treating wood to be used in decking, play structures, picnic tables, residential fencing, walkways, and the like. Worldwide, several alternatives to CCA-preserved wood exist, including copper citrate, ACQ (alkaline copper quaternary ammonium salts), copper HDO (bis [N-cyclohexyl-diazeniumdioxy] copper), and copper azole. Only two of these, ACQ and copper azole, are available for consumer purchase in the United States. These alternative treated wood products are estimated to cost 10 to 30 percent more than comparable CCA-treated wood products (EPA 2003).

Given that much existing technology is geared toward pressure treatment of wood by CCA, a simple substitute for arsenic in this composition might be used with minimal equipment upgrades. Molybdenum, tungsten, and zinc were investigated as replacements for arsenic in CCA that could result in a formulation less toxic to humans. Molybdate and tungstate are both known to be toxic to termites (Brill and von Meyer 1985, Brill et al. 1987, Katsuta 1992, Gossett and Gossett 1994) and fungi, (Singh and Khanna 1969, Punja and Grogan 1982) and zinc has well-known anti-fungal properties. While toxicity studies of tungsten and molybdenum compounds have not been as extensive as those of arsenic, a comparison of the National Institute for Occupational Safety and Health (NIOSH) recommended exposure limits (REL) for compounds of these elements gives an idea of their relative toxicities. The REL for soluble molybdenum is 25 times that of arsenic (time-weighted average), and for soluble tungsten, the REL is 5 times that of arsenic. Zinc is an essential human nutrient.

In order to examine the potential use of copper and chromium based wood preservatives containing molybdenum, tungsten, or zinc instead of arsenic, we prepared new aqueous formulations with these elements. Since arsenic in CCA is speciated as arsenate (As[O.sub.4.sup.3-]), molybdenum-, tungsten-, and zinc-containing anions were selected for investigation. Molybdenum and tungsten in their highest and most stable oxidation state, VI, exist in aqueous solution at pH values above 9 as M[O.sub.[4.sup.-].sup.2]- (M = Mo or W). At lower pH values, molybdate and tungstate undergo condensation reactions and form larger complex oxoanions, but may still preserve their solubility (Pope 1983). Zinc predominantly exists as [Zn.sup.2+] at pH values below 8 (Baes and Mesmer 1976), so it was chelated with an anionic ligand, phytate, to form an anionic species. Phytate was chosen because phytic acid is an inexpensive chelant that was shown to interact well with a fiber matrix (Mancosky et al. 2002); it was hoped that this would lead to the stabilization of the zinc chelate in the wood samples.

The procedures for preparing the new wood preservative formulations are very similar to those used for preparing CCA solutions. Typical CCA formulations are commonly prepared by first dissolving chromic acid in water and then adding copper oxide and arsenic acid as solids to the chromic acid solution. In the new formulations presented here, copper oxide is dissolved in a solution of chromic acid. Tungstate and molybdate are prepared for addition to the chromium/copper solution by dissolving their sodium salts in water and acidifying the solution to form tungstate and molybdate polyanions. Zinc phytate is prepared by dissolving zinc sulfate in a solution of phytic acid. The tungstate, molybdate, or zinc solution is then added to the chromium/copper solution. In comparison to typical CCA formulation preparation, this may represent an additional dissolution step for the arsenic replacement element.

Wood blocks (southern yellow pine) were treated with the new preservative formulations and tested for resistance to leaching and a brown-rot fungus (Gloeophyllum trabeum).

Experimental

Preparation of treating solutions

The molar ratios of the treating solutions were based on that of CCA type C, where arsenate is replaced by molybdate, tungstate, or a zinc chelate (zinc phytate). CCA-C is composed of 18.5 percent CuO, 47.5 percent Cr[O.sub.3], and 34.0 percent [As.sub.2][O.sub.5] (molar ratio 1 Cu: 2.86 Cr: 1.78 As).

Chromium copper molybdate (CCMo). -- Solutions of three different concentrations were prepared, and are referred to as 2.0X, 1.0X, and 0.5X. The amounts used in the 2.0X solution are given as an example; 1.0X is half the concentration of the 2.0X solution, and 0.5X is one quarter that concentration. Chromium oxide (Cr[O.sub.3], 1.50 g, 15.01 mmol) was dissolved in 50 mL of water with stirring. Copper oxide (CuO, 0.59 g, 7.36 mmol) was added to the chromate solution as a solid and the mixture was stirred vigorously until the CuO all dissolved, giving a yellow solution. Sodium molybdate dihydrate ([Na.sub.2]Mo[O.sub.4] * 2[H.sub.2]O, 2.26 g, 9.35 mmol) was dissolved in another container in 50 mL of water, and the pH of the solution was adjusted to 2 with 6 M HCl. The molybdate solution was then mixed with the chromium/copper solution. No precipitates were formed as a result of mixing the molybdate solution with the chromium/copper solution. The volume of this solution was increased to 250 mL in a volumetric flask.

Chromium copper tungstate (CCW). -- Prepared as described above for CCMo, except [Na.sub.2]W[O.sub.4] * 2[H.sub.2]O (3.08 g, 9.35 mmol, for 2.0X solution) was used in place of [Na.sub.2]Mo[O.sub.4] * 2[H.sub.2]O.

Chromium copper zinc phytate (CCZn). -- A solution of zinc phytate was prepared by first dissolving 11.79 mL (9.35 mmol) of a 40 percent solution (wt/wt) of phytic acid in 50 mL of water. Then zinc sulfate (ZnS[O.sub.4] * 7[H.sub.2]O, 2.69 g, 9.35 mmol) was added to the phytate solution. The zinc phytate solution was added to a chromium/copper solution, identical to that prepared for CCMo (above).

Wood block treatment and leaching

Pine cubes (19 mm) were vacuum-treated, conditioned, and leached in deionized water according to the American Wood-Preservers' Association (AWPA) standard E11-97. The three preservative compositions, CCMo (chromated copper molybdate), CCW (chromated copper tungstate), and CCZn (chromated copper zinc phytate), were each tested at three different retention levels. A CCA solution (60 wt%), obtained from Chemical Specialties, Inc., Charlotte, North Carolina, was diluted to achieve the desired concentration and used to treat similar wood blocks, and CCA-treated lumber was purchased from Home Depot and milled to appropriate size.

The aqueous preservatives (solutions corresponding to each of the three different retention levels) were introduced into the wood by placing the blocks under vacuum, and then allowing the aqueous solution to flow into the vessel containing the wood blocks while still under vacuum. After the blocks were completely submerged in the treating solution, the vacuum was released and the blocks were allowed to stand in the treating solution for 30 minutes. The treated blocks were then weighed, and the weight of solution taken up into each block was used to calculate the amount of preservative in each block (retention level). Blocks were conditioned by heating overnight at 50[degrees]C and then storing at 23[degrees]C and 50 percent humidity for 21 days.

After conditioning, the blocks were leached by first impregnating them with water (to prevent floating) using the same vacuum procedure just described. Then they were placed in screw-cap bottles containing a magnetic stir bar and plastic mesh (which separated the blocks from the stir bar). Leachate water was decanted and replaced with fresh distilled water (total of 300 mL for each six-block sample) at 6, 24, and 48 hours, and every 48 hours thereafter for 14 days. The leachates were analyzed for chromium, copper, and either molybdenum, tungsten, zinc, or arsenic by inductively coupled plasma spectrophotometry (ICP).

Fungal resistance testing

Southern yellow pine cubes (19 mm) were treated at three concentrations of wood preservative as just described with CCA, CCMo, and CCW. Unleached blocks were tested for resistance to a brown-rot decay fungus, Gloeophyllum trabeum (G. trabeum, strain ATCC 11539), according to AWPA standard E10-97, a soil-block test. The testing was performed at Mississippi State University. Five replicates were tested in each sample group.

Instrumental methods

Analysis of chromium, copper, arsenic, molybdenum, tungsten, and zinc in leachate samples was performed by ICP.

Results and discussion

Stability of preservative solutions

The new preservatives, CCMo, CCW, and CCZn, were all homogeneous aqueous solutions with pH values near 2. Both CCMo and CCW solutions were stable at room temperature; no discoloration or precipitation occurred when these solutions were stored in plastic screw-cap bottles for several months. However, the CCZn solution changed in color from yellow to dark green after standing for less than a week. This is probably indicative of the decomposition of the phytate ligand due to oxidation by [Cr.sup.+6], resulting in its reduction to green-colored [Cr.sup.3+].

Leaching experiments

Results for CCMo, CCW, CCZn (during the first 96 hr.) and CCA leaching are presented in Figures 1 to 5. The overall trends are similar for each preservative formulation: chromium is leached in the smallest amounts, copper leaching is slightly greater, and the third component (As, Mo, W, or Zn) is the most extensively leached metal. In several other reports examining CCA leaching, the order of leaching in seawater is Cu > Cr > As (Weis et al. 1991, Merkle et al. 1993, Putt 1993, Breslin and Adler-Ivanbrook 1998), but leaching proportions can vary with wood species and leaching media (Hingston et al. 2002). Southern yellow pine samples subjected to simulated rainfall (Lebow et al. 2003) showed the same relative order of leaching amounts as this study; that is, As > Cu > Cr. The rate and amount of arsenic leaching from both lab-treated CCA wood blocks and store-bought CCA-treated wood are similar. The shape of this curve (arsenic leaching versus time) in the leaching graphs is also very similar for molybdenum and tungsten leaching, though the gram amounts of molybdenum and tungsten leaching are greater. Figures 1 to 5 show representative leaching versus time data for each of the preservative compositions.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

For the CCMo-treated blocks, leaching of chromium is low while copper leaching is moderate and molybdenum leaching is greatest. However, the amounts of Mo leaching are about the same across the three different loading levels, so a smaller percentage of Mo was leached from the blocks at higher loading (Table 1). This could reflect the solubility of a Mo-containing compound formed in the fixation of the preservative to the wood. For the CCW-treated blocks, both chromium and copper leaching levels were low, and comparable to the values found for the CCA-treated blocks. Tungsten leaching was less severe than molybdenum leaching at the low and medium loading levels, but the two are similar for the high loading level. In comparison to arsenic leaching, the weight and percent values of tungsten leaching are greater (the percent values are estimates for the store-bought CCA wood). However, the molar amounts of tungsten and arsenic leaching are similar due to the high atomic weight of tungsten. At the 4.3 kg/[m.sup.3] loading level, 0.123 mmol of tungsten is leached from the CCW-treated blocks over the 14-day experiment. The lab-prepared CCA-treated blocks leached 0.116 mmol of arsenic over the experiment. Leaching of all three metals was extensive for the CCZn-treated blocks; therefore, the experiment was terminated after leaching for 96 hours, and this preservative composition was not examined further.

While the leaching procedure followed the AWPA standard, it would also be desirable to test the leachability of CCMo- and CCW-treated samples in different media, such as seawater or artificial seawater. It may also be possible to decrease the amount of molybdenum or tungsten leached from preservative-treated blocks by optimizing the formulations of the preservative solutions.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Fungal resistance

The results of exposing blocks treated with CCA, CCMo, and CCW at three concentrations in a soil-block test to the brown-rot fungus G. trabeum are shown in Figure 6 and Table 2. While the control (untreated) blocks lost an average of 62.8 percent of their weight to fungal decay, the treated blocks suffered much less fungal attack. For the CCW-treated blocks, fungal resistance was independent of preservative concentration, and their weight loss was equivalent to blocks treated with the highest level of CCA examined in this study. Blocks treated with CCMo at the medium and high levels matched the performance of the CCW-treated blocks. This shows that it may be possible to use a lower concentration of CCW or CCMo compared to CCA to achieve the same level of pest protection, which would lead to lower treating costs and less leaching. However, testing the resistance of CCMo- and CCW-treated wood blocks to other organisms including brown-rot and white-rot fungi (particularly copper-tolerant strains) and insects is desirable for estimating the efficacies of these preservatives.

Conclusions

New inorganic aqueous wood preservatives CCW and CCMo may be good candidates as alternatives to CCA. They are resistant to leaching out of test wood blocks after fixation, and protect wood blocks from attack by the brown-rot fungus G. trabeum.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]
Table 1. -- Metal leaching from preservative-treated wood blocks.

 Mo, W, Zn,
Preservative Treatment level Cr leached Cu leached or As
 (kg/[m.sup.3] (mg)

CCMo 2.8 1.43 1.93 (14.5) 19.13 (77.1)
 (6.2) (a)
 5.4 1.56 (3.6) 5.10 (20.1) 20.15 (41.1)
 11.4 0.79 (0.9) 3.94 (7.5) 23.60 (23.5)
CCW 4.3 0.29 (1.1) 0.49 (3.1) 22.61 (38.1)
 8.3 0.21 (0.4) 0.64 (2.1) 31.56 (27.5)
 17.1 1.02 (0.9) 1.06 (1.6) 60.30 (25.3)
CCZn (b) 2.7 7.13 (30.8) 5.31 (37.4) 8.24 (45.5)
 5.5 12.57 (26.6) 11.00 (39.2) 15.95 (43.2)
 10.9 19.96 (20.7) 21.24 (36.3) 30.00 (39.6)
CCA-lab (c) 5.8 1.57 (2.8) 0.72 (2.2) 8.68 (16.2)
CCA 1 (d) 6 (e) 0.59 (1.0) 0.72 (2.2) 8.58 (15.4)
CCA 2 (d) 6 (e) 0.53 (1.0) 0.74 (2.2) 9.14 (16.5)

(a) Values in parentheses are percent of total.
(b) 96-hour leaching results.
(c) Blocks treated with CCA-C solution in the laboratory.
(d) Blocks cut from store-bought CCA-treated lumber.
(e) Estimated.

Table 2. -- Results of wood block exposure to G. trabeum.

Preservative Treatment level Avg. weight loss Standard deviation
 (kg/[m.sup.3] (%)

CCMo 2.7 6.1 1.2
 5.2 4.6 0.7
 10.7 4.8 0.3
CCW 3.7 4.2 0.7
 7.5 4.4 0.6
 15.2 4.5 0.3
CCA 4.1 10.8 3.5
 9.2 6.3 0.9
 18.9 3.9 0.2
Control 0 62.8 2.5


Literature cited

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Breslin, V.T. and L. Adler-Ivanbrook. 1998. Release of copper, chromium and arsenic from CCA-C treated lumber in estuaries. Estuarine Coastal and Shelf Sci. 46:111-125.

Brill, W.J. and W.C. von Meyer. 1985. Method and composition for control of termite and shipworms. U.S. Patent 4,504,468.

__________, S.W. Ela, and J.A. Breznak. 1987. Termite killing by molybdenum and tungsten compounds. Naturwissenschaften 74(10):94-495.

Environmental Protection Agency (EPA). 2003. Response to requests to cancel certain chromated copper arsenate (CCA) wood preservative products and amendments to terminate certain uses of other CCA products. Federal Register 68(68):17366-17372.

Gossett, J.S. and B.N. Gossett, 1994. Control of undesirable organisms. International Patent WO 94/10836.

Hingston, J.A., A. Bacon, J. Moore, C.D. Collins, R.J. Murphy, and J.N. Lester. 2002. Influence of leaching protocol regimes on losses of wood preservative biocides. Bull. Environ. Contam. Toxicol. 68:118-125.

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Lebow, S., R.S. Williams, and P. Lebow. 2003. Effect of simulated rainfall and weathering on release of preservative elements from CCA treated wood. Environ. Sci. Technol. 37: 4077-4082.

Mancosky, D., L. Lucia, B. Brogdon, and R. Braun. 2002. Application of nontraditional technologies to improve the efficiency for the hydrogen peroxide bleaching of pulps. In: Proc. TAPPI Fall Technical Conference & Trade Fair, San Diego, CA. TAPPI, Norcross, GA. pp. 299-308.

Merkle, P., D.L. Gallagher, and T.N. Soldberg. 1993. Leaching rates, metals distribution and chemistry of CCA treated lumber: Implications for water quality monitoring. In: Proc. Environmental Consideration in the Use of Pressure Treated Wood. Forest Prod. Soc., Madison, WI. pp. 69-78.

Pope, M.T. 1983. Heteropoly and Isopoly Oxometalates. Springer-Verlag, New York.

Punja, Z.K. and R.G. Grogan. 1982. Effects of inorganic salts, carbonate-bicarbonate anions, ammonia, and the modifying influence of pH on sclerotial germination of Sclerotium rolfsii. Phytopathology 72(6):635-639.

Putt, A.F. 1993. Sediment bound CCA-C leachate 10 day repeated exposure toxicity to Amplisca abdita under static conditions. Springborn Laboratories Inc., Wareham, MA.

Singh, R.S. and R.N. Khanna. 1969. Effect of certain inorganic chemicals on growth and spore germination of Alternaria tenuis, the fungus causing core rot of mandarin oranges in India. Mycopathol. Mycol. Appl. 37(1): 89-96.

Weis, P., J.S. Weis, and L.M. Coohill. 1991. Toxicity to estuarine organisms of leachates from chromated copper arsenate treated wood. Archives of Environmental Contamination and Toxicology 20:118-124.

The authors are, respectively, Assistant Scientist and Professor, Inst. of Paper Science and Technology and School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 500 Tenth Street NW, Atlanta, GA 30332-0620. This work was funded by the member companies of the Institute of Paper Science and Technology. This paper was received for publication in November 2003. Article No. 9796.
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Title Annotation:chromated copper arsenate
Author:Cowan, Jennifer; Banerjee, Sujit
Publication:Forest Products Journal
Geographic Code:1USA
Date:Mar 1, 2005
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