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Leaching resistance of copper amine-treated Scots pine.

Abstract

The depletion of copper and amine from copper amine- and amine-treated Scots pine blocks was investigated. For copper amine-treated blocks, the greatest leaching resistance was found for copper monoethanolamine and copper polyimine treatments, which retained more than 80 percent copper when leached in water and more than 50 percent when leached with a citrate buffer. This was explained by the loss of precipitated amine-copper compounds during buffer leaching. Leached amine-treated blocks also showed residual amine, suggesting the formation of compounds with wood derivatives, possibly amine acid salts with acidic functionality in hemicellulose. The amine to copper mole ratio in leached wood was 1, confirming that considerable amine is lost during leaching, and that the resulting complexes in wood contain copper bound to amine.

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The recent changes in the wood preservation industry have seen ammoniacal amine copper formulations take on a new prominence in wood preservation in North America (Morris et al. 2002). However, there remains a need for a better understanding of the fixation of the copper and any co-biocides as this will impact preservative loss, which is important for two reasons. First, the degree of protection of the preservative declines, and second, it impacts the environmental acceptability of the preservative system. For copper-based preservatives, the amount of copper lost from the treated wood can be directly correlated with the degree of preservative fixation (Kamdem and Zhang 2000, Ruddick 1996). A relative assessment of leachability can be determined from laboratory leaching experiments (Zhang and Kamdem 1999). However, care must be exercised when extending these comparative results to what might occur under actual service conditions, since losses in the latter will be much lower.

Copper can form very stable complexes with amines (Manriquez et al. 1996). However, if amine copper preservatives are to fix in wood, a balance will be required between the copper to amine bonding, and complexes formed between copper and the wood components. If the copper to amine bonding is too strong, the preservative will remain unreacted with the wood and will tend to leach easily from the treated wood.

The objective of this study was to investigate the influence of different amines on the fixation of copper in wood, as well as to examine any reactions between the amine and the wood. In addition to the distilled water leaching, a citrate buffer solution was used as a highly aggressive leaching medium. This leaching environment was chosen to provide additional information on the relative stability of amine complexes and simple copper precipitates formed in wood.

Materials and methods

Treating solutions

The copper amine solutions, including copper monoethanolamine (Cu-MH), copper ethylenediamine (Cu-en), copper aminoethylethanolamine (Cu-AEEA), and copper polyethylenimine (mixed with 20% of monoethanolamine) (Cu-Polyim), were supplied by Dr. Wolman GmbH. The treating concentrations of copper (expressed as CuO) and mole ratio of nitrogen to copper are shown in Table 1. Monoenthanolamine (MH) and ethylenediamine (en) were obtained from Fisher Scientific Company and Matheson Coleman & Bell Manufacturing Chemists, respectively, as reagent grade solvents. They were diluted to 5 percent by weight with distilled water prior to use.

Sample preparation

Ovendried Scots pine (Pinus sylvestris) sapwood blocks (50 by 25 by 15 mm) with the largest dimension being along the grain were placed in a beaker in a desiccator and vacuum impregnated for 30 minutes using a laboratory vacuum pump. The treating solution was introduced and under vacuum and air allowed to enter the desiccator until atmospheric pressure was reached. The beaker was then allowed to stand at atmospheric pressure for one hour. The blocks were removed from the treating solutions and air-dried on a wire screen for two weeks at ambient condition, prior to leaching. Eight blocks were treated with each solution to provide three blocks for leaching in water or citrate buffer, with two blocks being retained for reference.

Leaching assessment

During leaching, each block was placed individually in a beaker and vacuum impregnated with 125 ml of either water or citric acid buffer. The buffered solution was prepared by dissolving 0.6 g sodium hydroxide and 1.6 g citric acid in 1 liter of distilled water (Cooper 1991). For initial leaching, the beakers were placed in an ultrasonic bath for 24 hours, after which they were removed and allowed to stand on the bench for three days. The leaching solution was then replaced with fresh solution (125 ml). The above procedures were repeated once, after which the beakers with the blocks were allowed to stand for a further three weeks with the leaching solution being replaced on a weekly basis. Samples of each leachate were collected and retained for copper analysis.

Analysis of copper and nitrogen

Analysis of copper in treated blocks. -- The analysis of copper in leached copper amine-treated blocks was conducted using an ASOMA[TM] X-ray fluorescence analyzer. The leached block was ground into 30-mesh sawdust for the X-ray analysis. In order to convert the X-ray data from weight percent to the more usual weight of chemical per volume of wood (kg/[m.sup.3]), the wood density of each block was determined from the initial ovendried weight and dimensions before treatment. The results of the copper retention were reported on an oxide (CuO) basis.

Analysis of copper in leachate of each leaching phase. -- The copper content in the leachate solutions was measured by an atomic absorption spectrometer (AA). Standard copper solutions (2 ppm, 4 ppm, 6 ppm, 8 ppm, 10 ppm) were prepared by serially diluting a commercial 1,000 ppm standard copper solution. All of the leachate solutions were diluted to provide a suitable copper concentration for the AA analysis. The initial retention of copper in blocks prior to leaching was calculated from the sum of the residual copper in the blocks, and the total copper determined from all of the leachates.

Analysis of nitrogen in the leached blocks and solid chemicals. -- A 0.1 g aliquot of treated sawdust was placed into a Kjeldahl digestion tube. To each sample a half of a tablet of Kjeldahl catalyst (5.0 g [K.sub.2][SO.sub.4], 0.15 g Cu[SO.sub.4], and 0.15 g Ti[O.sub.2]) was added. The tubes were set on the digestion rack in the heating mantle, and then 5.0 ml of 98 percent pure sulfuric acid was carefully added to each tube. After the sulfuric acid had been added, the samples were heated to about 400[degrees]C and the digestion continued for two hours. The endpoint was identified by the formation of a clear lime-green liquid which showed no effervescence. The digestion tubes were then removed from the heating mantle and allowed to cool to room temperature prior to being diluted.

The nitrogen analysis was performed with the Kjeldahl technique using a Lachat QuikChem[TM] Automated Ion Analyzer. Each digested solution (5 ml) was diluted to ensure that the acid concentration was lower than 4 percent.

Results and discussion

Recovery of amine during distilled water and citrate buffer leaching

Wood is slightly acidic, so it is expected that during amine treatment reaction with the more reactive carboxylic acid groups found in the hemicellulose to produce amine acid salts will cause some of the amine to bind to the wood. The remainder will be readily leached. The residual nitrogen retained in treated blocks (in terms of the ovendried weight of the blocks) after leaching with distilled water and citrate buffer leaching of amine-treated wood is shown in Table 2. The nitrogen remaining in the leached monoethanolamine treated blocks was much lower than that of the ethylenediamine-treated blocks. However, since ethylenediamine contains twice as much nitrogen as monoethanolamine, it can be concluded that the two amines reacted with the sapwood to a similar degree. It was interesting to note that a slight decrease in the nitrogen content was consistently observed after the citrate buffer leaching. This would suggest that the citrate buffer is able to remove some of the complexed amine from the wood.

Leaching of copper and amine from copper amine-treated wood

The copper retentions for the copper amine-treated blocks were approximately 11.0 to 12.0 kg/[m.sup.3] (Table 3). For the copper amine-treated samples, the two ultrasonic leaching phases removed most of the mobile copper (Fig. 1). After completion of the leaching, the blocks impregnated with Cu-MH and Cu-Polyim showed the greatest leaching resistance to distilled water, with approximately 91 percent and 83 percent of the copper remaining, respectively (Table 3). The loss of copper for the Cu-MH is similar to the observations of Zhang and Kamdem (1999) who reported about 88 percent retention in a dynamic leaching test in which three blocks were leached in 200 ml of distilled water. The citrate buffered leaching removed about the same amount of copper from the blocks treated with Cu-MH as those impregnated with Cu-Polyim. The formulation of Cu-Polyim actually contains 20 percent by weight of monoethanolamine solvent. The similar leaching behavior suggests that it is the monoethanolamine which is primarily responsible for the copper complexing or reacting with wood, while the polyimine acts to stabilize the Cu-MH complex in solution.

Of the four copper amine treatments, the Cu-en samples retained the lowest amount of copper after either water or citrate buffered leaching. This clearly demonstrates that the copper insolubilization/fixation reaction between Cu-en and wood is much less than that between other copper amine solutions and wood. This may be attributed to the formation of a very stable [bis(ethylenediamine) copper] (24) cation with a very polar interaction to wood components. In addition the highly basic nature of ethlyenediamine will tend to cleave acetyl groups from hemicellulose producing free acetate. This may react with copper to produce water-soluble copper acetate with or without complexed amine.

The copper monoethanolamine is not able to produce stable cationic species such as ethylenediamine, but forms instead more covalently bonded copper-amine complexes. In solution, the excess ethanolamine stabilizes the [Cu-(MH)[.sub.2]][.sup.2+] cationic species. However, once in contact with the wood, this readily decomposes forming an ethanolamine-copper-wood complex, a copper-wood compound or simple copper precipitates, (such as copper hydroxide or carbonate). Previous research has suggested that such precipitates are readily removed by citrate buffered leaching (Jiang and Ruddick 1997). From the data in Table 3, it may be concluded that about 35 percent of the copper is present in Cu-MH-treated wood as a simple precipitate. The 50 percent resisting leaching in the buffer solution is considered to arise from more insoluble ethanolamine-copper-wood complexes.

[FIGURE 1 OMITTED]

The stability of copper-chelated complexes is affected by two factors, the donor atom on the ligand and the size of the ring formed during chelation. It is well established that nitrogen forms stronger donor bonds to copper than oxygen and that five membered chelating ligands are more stable than six membered rings (Cotton and Wilkinson 1962). The depletion of the copper from the Cu-AEEA-treated samples was similar to that recorded for copper ethylenediamine. AEEA contains a primary amine and hydroxyl groups on the end of the molecule, with a secondary amine in the center of the ligand. Because of the stability of the five and six membered rings, chelation of the copper involving the hydroxyl group and the primary amine is unlikely. This leaves the possibilities of diamine chelation from the primary and secondary amine, or the chelation of the secondary amine and the hydroxyl group. With the known requirement of a high excess monoethanolamine to stabilize the copper-MH complex in solution, the stability of this Cu-AEEA suggests that the diamine chelation is the major formation. In this case, the behavior of Cu-AEEA would be similar to Cu-en rather than Cu-MH. The results are consistent with this interpretation. The polyethyleneimine ligand clearly would be unable to form stable copper complexes due to the bulky nature of the ligand. However, Cu-Polyim-treated wood had the same leaching resistance as Cu-MH, implying that in the Cu-Polyim solution, the dominant reaction with wood involves Cu-MH. The higher nitrogen content of the leached Cu-Polyim-treated wood, however, also confirms that at least some of the Polyim ligand is retained.

The copper remaining in the treated wood after leaching with the buffered solution ranged from 13 to 51 percent of the initial retention and was significantly lower than that retained after distilled water leaching (41% to 86%) (Table 3). As noted earlier this may be explained by the buffered leaching effectively removing simple precipitated copper. From the data in Table 3, the amount of copper precipitated in wood for the copper-amine treatment can be estimated at about 26 to 36 percent.

Plotting the amine content after buffer and water leaching for the six simple solutions (excluding the mixed Cu-Polyim) against the residual copper confirmed a trend of higher nitrogen content associated with increasing copper present in the treated wood. It might be expected that the amount of amine that reacts with wood and copper may vary between the individual amines. However, the linearity of the plot (y = 0.958x + 0.0936 [r.sup.2] = 0.9328) suggests that the mole ratios of the amine to copper are similar for the amines evaluated. More importantly, the slope of the best linear fit of the amine to copper ratio is approximately 1. Thus during the process of fixation and leaching, three of the original amine molecules have been lost. In addition, only one of the amine molecules is retained when sapwood is impregnated with amine-copper solutions with the ratios shown in Table 1 and leached as described earlier. This result is consistent with a crystal structure of di(ethanolamine)bis(vanillinato) di-copper(II) (Ruddick et al. 2001). The intercept of 0.09 mmol/g, for zero moles of copper is close to the background concentration of nitrogen in wood and may suggest that no residual monoethanolamine remained after leaching. The linearity of the plot combined with the small value of the free nitrogen, suggests that most of the copper remaining after leaching was complexed with amine.

Conclusions

1. Of the four copper amine treatments, wood impregnated with Cu-MH and Cu-Polyim had the highest leaching resistance to distilled water or citrate buffer leaching.

2. In copper amine-treated wood, leaching using citrate buffer removed significantly more copper and nitrogen than distilled water.

3. After leaching of amine-copper treated wood, the mole ratio of amine to copper was almost unity, suggesting that most of the amine was leached under the conditions of the experiment.
Table 1. -- Concentration of copper amine solutions after dilution. (a)

Solution ID Cu-MH Cu-en Cu-AEEA Cu-Polyim

Concentration of CuO (%) 1.49 1.50 1.52 1.53
N:Cu (mole ratio) 4.0:1 3.9:1 4.0:1 3.9:1

(a) Cu-MH = copper monoethanolamine; Cu-en = copper ethylenediamine; Cu-
AEEA = copper aminoethylethanolamine; Cu-Polyim = copper
polyethylenimine mixed with 20 percent of monoethanolamine.

Table 2. -- The nitrogen content (mmole/g of wood) in leached amine and
copper amine-treated Scots pine blocks.

 Nitrogen content (mmole/g of wood)
Treatment group (a) Water leaching Buffer leaching

MH 0.282 0.207
En 0.520 0.464
Cu-MH 0.299 0.241
Cu-en 0.342 0.278
Cu-AEEA 0.186 0.136
Cu-Polyim 0.416 0.316

(a) MH = monoenthanolamine; En = ethylenediamine; Cu-MH = copper
monoethanolamine; Cu-en = copper ethylenediamine; Cu-AEEA = copper
aminoethylethanolamine; Cu-Polyim = copper polyethylenimine mixed with
20 percent of monoethanolamine.

Table 3. -- Copper content in copper amine-treated Scots pine blocks
before and after the completion of leaching. (a)

 CuO remaining
 Initial retention after
Treatment group (b) Leaching solution of CuO (c) leaching
 (kg/[m.sup.3]) (%)

Cu-MH Water 11.2 (0.4) 90.6 (1.5)
Cu-MH Buffer 11.9 (0.3) 50.6 (2.3)
Cu-en Water 11.3 (0.3) 42.4 (1.6)
Cu-en Buffer 11.0 (0.2) 13.6 (1.7)
Cu-AEEA Water 11.7 (0.6) 46.7 (2.1)
Cu-AEEA Buffer 11.3 (1.0) 20.4 (0.3)
Cu-Polyim Water 12.0 (0.5) 81.2 (1.3)
Cu-Polyim Buffer 11.2 (0.9) 51.3 (1.5)

(a) Values in parentheses are standard deviations.
(b) Cu-MH = copper monoethanolamine; Cu-en = copper ethylenediamine; Cu-
AEEA = copper aminoethylethanolamine; Cu-Polyim = copper
polyethylenimine mixed with 20 percent of monoethanolamine.
(c) Initial retention determined by sum of X-ray analysis of leached
blocks and AA analysis of leachate.


Literature cited

Cooper, P.A. 1991. Leaching of CCA from treated wood: pH effects. Forest Prod. J. 41(1):30-32.

Cotton, F.A. and G. Wilkinson. 1962. Advanced Inorganic Chemistry. Interscience Publishers, a divison of John Wiley & Sons. 546 pp.

Jiang, X, and J.N.R. Ruddick. 1997. A comparison of the leaching resistance of diammine-copper complexes and copper carbonate precipitated in wood. Int. Res. Group. on Wood Pres. Doc. No. IRG/WP/97-30158.

Kamdem, P.D. and J. Zhang. 2000. Contribution of wood components on the absorption of copper amine. Int. Res. Group. on Wood Pres. Doc. No. IRG/WP/00-30216.

Manriquez, V., M.C. Vallette, N. Lara, N.G. Tejeda, O. Wittke. G. Diaz, S. Munoz, and L. Kriskovic. 1996. Crystal structure and vibrational spectra of polyamine-copper (II) complexes. J. Chem. Crystal. 26(1):15-22.

Morris, P.I., S.M. McFarling, and A. Zahora. 2002. Treatability of refractory species with amine and amine/ammoniacal formulations of ACQ. Forest Prod. J. 52(10):37-42.

Ruddick, J.N.R. 1996. The fixation chemistry of ammoniacal copper wood preservatives. Proc. Amer. Wood Pres. Assoc. 92:32-49.

________, C. Xie, and F.G. Herring. 2001. Fixation of amine copper preservatives. Part 1. Reaction of vanillin, a lignin model compound with monoethaolamine copper sulphate solution. Holzforschung. 55(6):585-589.

Zhang, J. and P. D. Kamdem. 1999. Interaction of copper-amine complexes with wood: Influence of copper source, amine ligands and amine to copper ratio on copper retention and leaching. Int. Res. Group. on Wood Pres. Doc. No. IRG/WP/99-30203.

Xiao Jiang*

John N. R. Ruddick*

The authors are, respectively, Research Scientist, Lonza Inc., 79 Route 22 E, Annandale, NJ 08801 and Professor, Dept. of Wood Science, Forest Sciences Centre, Univ. of British Columbia, 4041-2424 Main Mall, Vancouver, BC, Canada V6T 1Z4. This paper was received for publication in August 2003. Article No. 9739.

*Forest Products Society Member.
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Author:Jiang, Xiao; Ruddick, John N.R.
Publication:Forest Products Journal
Geographic Code:1USA
Date:Dec 1, 2004
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