University of Tasmania

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Low toxicity preservatives for production of roundwood posts and poles

posted on 2023-05-27, 19:23 authored by Joseph Hartley
A preservative that could be used to treat wood on farms would greatly increase the utility of farm timber and the profitability of timber production. Current preservatives are for the most part too toxic for use outside specialised treatment plants, or too expensive, or ineffectual. Many experimental preservatives appear to have potential, but remain untested or undeveloped. To test wood preservatives and develop wood preservatives for use on farms, an array of wood rotting fungi was grown from fruiting bodies collected on rotting wood and living trees. This process resulted in some new knowledge about the identity and habits of Tasmanian fungi. Putative brown rots were tested for copper tolerance, and an Antrodia isolate was identified as being very tolerant. This isolate was therefore used to test preservatives that used metals as fungicides. Two vigorous white rots were also used. A preservative system using silicate as a fixative for boron was identified as being likely to have easily developed potential, so a study was undertaken to clarify the effects of different concentrations of silicate, (as waterglass), and different ratios of silicate/boron on the leaching of boron. It was found that a fungicidal dose of boron could be fixed with a much lower dose of silicate than had previously been tried, and that the concentrations of boron being used were wastefully high, resulting in more than 90% of the boron being leached out. The order of treatment was found to be very important to both the initial concentration of boron in the wood and the percentage lost, through effects of treatment solution osmolarity and the affinity of silicate for boron. These effects hadn‚ÄövÑv¥t been described by previous authors and are very important to the practical use of this preservative system. An upper limit of waterglass concentration was identified, above which alkaline damage to the wood negates the positive effects of the greater concentration of silicate, (resulting in increased leaching). Another preservative system identified as having undeveloped potential was a waterglass and aluminium system, which had previously only been tested against two wood rot isolates, neither of which had proven tolerance of any metal. This preservative, (modified with the addition of a naturally occurring polysaccharide), was therefore tested against one of the same species of fungus that had been used in previous tests, (Trametes versicolor), along with another vigorous white rot, (Omphalotus nidiformis), and the copper tolerant Antrodia. The previously tested white rot was effectively suppressed. The other white rot showed no signs of suffering aluminium toxicity and grew healthily on the treated wood, but was unable to cause decay. Presumably the wood was protected by the polysaccharide, blocking access to lignin, (the preferred food of white rots). The copper tolerant Antrodia appeared to be susceptible to aluminium toxicity because it was unable to colonise the treated wood until late in the test, and then was unable to cover the wood with its usual exploratory growth but rather was restricted to defined patches of dense mycelium. Antrodia presumably required a dense population of mycelium to produce enough oxalic acid to detoxify the aluminium. The polysaccharide appeared to feed rather than inhibit Antrodia. Three new preservatives were also conceived and tested. The first used copper that was buffered so it could be combined in solution with a silicate fixative. Preliminary studies on leaching behaviour showed that fixation of copper was inhibited by excess buffer, such that practically attainable concentrations of silicate couldn‚ÄövÑv¥t fix high concentrations of copper (1% w/v in the treatment solution). Leaching resistance was improved by addition of boric acid to the solution, (or secondary treatment with boric acid), and also by secondary treatment with extra fixative, or drying the treated wood at 100¬¨‚àûC instead of at room temperature. The effect of the latter is probably due to thermal decomposition of buffer. In decay tests it was seen that even a weak formulation of copper-silicate completely protected wood from white rot. The same formulation wasn‚ÄövÑv¥t entirely effective against Antrodia on pine, but a formulation of similar strength completely protected alder, because Antrodia had a poor ability to attack even untreated alder. There is thus a possibility that copper-silicate without any additional fungicide might be an effective treatment for hardwood, which in any case covers the vast majority of standing timber on Australian farms. Further tests on the hardwoods in question, with other species of brown rot and also with soft rots, are thus warranted. A copper-silicate formulation that had the copper concentration reduced to 0.25% w/v, and a low concentration of boric acid added, proved to have high resistance to leaching and was completely impervious to Antrodia, (despite the strong resistance of this fungus towards copper). The other two new preservative concepts needed neither copper nor boron as fungicides, but depended on synergy and mutual precipitation of weaker natural fungicides, (entirely organic in one case). These preservatives successfully protected wood against vigorous white rots, and are predicted to be similarly successful against brown rots. In summary, three new preservatives have shown strong efficacy and have potential for treating timber on farms. In addition, the combination of waterglass and boric acid is a simple preservative that also appears practically useful, since it was demonstrated to be capable of fixing a fungicidal dose of boron, with a low dose of waterglass. The combination of waterglass and aluminium, on the other hand, is less reliable than previously supposed: one white rot showed no sign of aluminium toxicity when growing on the treated wood, and a brown rot that did show signs of toxicity was never the less able to detoxify the aluminium and attack the wood.



Tasmanian Institute of Agriculture

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