USDA, Forest Service, Forest Products LaboratoryMadison, WI 53726-2398E-mail: [email protected]
(Received September 2018)
Abstract. This article reports on the development of accelerated laboratory methods to allow estimation ofpreservative leaching from pressure-treated wood exposed to precipitation. End-matched lumber specimenswere pressure-treated with a boron–copper formulation and exposed to natural weathering for 1 yr, lab-oratory immersion protocols, or a laboratory-simulated rainfall protocol. The rainfall runoff or immersionwater was collected at intervals according to the method used and analyzed for concentrations of copper andboron. Of the laboratory methods evaluated, the simulated rainfall approach resulted in leaching patternsmost similar to outdoor exposure, especially in the case of copper. However, this method is relativelycomplex and not ideally suited for standardized use. Although the immersion methods evaluated initiallyexaggerated leaching, reasonable approximations of leaching from 1 yr of natural weathering were achievedwith accelerated testing. Models were developed to relate hours of immersion to millimeters of precipitation,and used to evaluate how well the immersion methods might predict leaching from natural weathering overmany years of exposure. One of the methods produced boron and copper leaching estimates that were within15% and 7%, respectively, of losses predicted for wood exposed to 5 yr of natural weathering. The resultsindicate that laboratory immersion methods have value in estimating long-term preservative leaching fromtreated wood products exposed to precipitation.
Keywords: Wood preservative, leaching, precipitation, accelerated test methods, immersion.
INTRODUCTION
Biocides and other constituents used to improvethe durability and performance of wood exposedoutdoors are subject to leaching from precipita-tion, standing water, or soil moisture. Evaluatingresistance to leaching is a critical step in deter-mining whether a test formulation is likely toprovide long-term protection. Similarly, quanti-fying the expected release of biocide from treatedwood into the environment is a key component of
evaluating a test formulation’s potential for en-vironmental impacts. Thus, durability concernsare focused on the quantity of biocide remainingin the wood, whereas environmental concerns arefocused on the quantity of biocide lost from thewood. This distinction has had practical conse-quences for the manner in which leaching isevaluated. Conventional standard leaching testswere developed to address durability concernsand designed to greatly accelerate leaching. Al-though these tests are valuable for ensuring long-term durability, they are less useful for evaluatingpotential environmental impacts.
The methods used to evaluate preservative leachingare discussed in Lebow (2014) and Lebow et al(2008, 2017) and will be only briefly summarizedhere. In North America, the most commonly usedstandardized leaching method for preservative-treated wood is American Wood Protection As-sociation (AWPA) Method E11-16, StandardMethod for Accelerated Evaluation of PreservativeLeaching (AWPA 2017). This method uses small(19 mm) blocks immersed for 14-17 d. Othercountries also use immersion of relatively smallspecimens when evaluating new preservatives forresistance to leaching (BSI 1997; CNS 2000; JSA2004). In each case, the method is intended togreatly accelerate leaching in an effort to evaluatethe potential for long-term protection.
Developing accelerated methods to provide rea-sonable estimates of environmental releases fromin-service products has proven challenging, es-pecially for most treated wood that is subject toleaching from precipitation. In one such effort,the Organization for Economic Cooperation andDevelopment (OECD) describes an approachinvolving a brief dip immersion using small (15by 25 by 50–mm) specimens (OECD 2009). Thedip immersions can be either three 1-min dips,two 1-h dips, or one 2-h dip per day for 19 d.Although intended to simulate in-service leach-ing, there is some concern that relatively shortimmersions approach may not represent com-mercially produced lumber (Baines 2005) or notproduce the moisture conditions reported forwood products exposed to natural weathering(Lebow et al 2008; Bahmani et al 2016). Onestudy which compared outdoor leaching with theOECD method concluded that the laboratorymethod risked underestimating in-service leach-ing (Morsing and Lindegaard 2004), whereasanother reported that the OECD method resultedin less leaching than other laboratory methods(Lesar et al 2008). Longer immersion times mayallow greater wetting of specimens, althoughspecimen dimensions must be considered. Arecent study noted that although the leachingfrom the small blocks used in the AWPA standardmethod was unrealistically high, employing thesame method with larger lumber specimens
appeared to underestimate copper losses whencompared with leaching observed in an outdoorexposure (Lebow et al 2017). These findingsindicate that there is potential for the use ofimmersion periods to simulate leaching fromprecipitation if a suitable combination of im-mersion period–specimen size can be identified.
Another approach to evaluating leaching fromwood exposed to precipitation is simulatedrainfall (Cooper and MacVicar 1995; Lebow et al2003; Lebow et al 2004; Morrell et al 2004;Mitsuhashi et al 2007; Mankowski and Manning2008; Lebow et al 2017). Simulated rainfall cancreate more realistic wetting conditions and al-lows some control over rainfall rates and fre-quency. Lebow et al (2017) reported that lumberspecimens exposed to simulated rainfall produceda pattern and quantity of leaching most similar tothat of natural exposure, especially for copper.However, the equipment required to simulaterainfall is more complex than that required forother laboratory leaching methods, and none ofthese approaches have been standardized.
Ideally, a standard method would be relativelysimple to describe and conduct, while still pro-viding meaningful results. In addition, the speci-men dimensions (surface area to volume ratio)would more closely relate to dimension lumber sothat the release rates could be applied to in-servicecommodities. Recent research indicated that thereis potential for using immersion of lumber-sizedspecimens to simulate aboveground leaching if theimmersion periods can be adjusted to simulatemoisture contents observed in wood productsexposed outdoors. In this article, we evaluatedextended laboratory immersion leaching methodsfor their ability to simulate moisture contents andleaching rates similar to that fromwood exposed tonatural precipitation. The results were comparedwith leaching from outdoor exposure tests andwith those of other accelerated tests reported inLebow et al (2017).
MATERIALS AND METHODS
The specimens leached in this study were end-matched and pressure-treated in the same charge
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as those evaluated in an earlier study (Lebow et al2017). Specimens were prepared from fivesouthern pine parent boards with dimensions of38 by 92 by 2438 mm (2 by 4 by 8 ft., nominal).Specimens were selected to be free of heartwood,knots, and other visible wood defects. Eightmatching 38 by 92 by 102-mm-long lumbersections were cut from each parent board (onlysix of these specimens were used in the currentstudy). One specimen cut from each board wasrandomly assigned to one of the six leachingconditions (described in the following) so thateach leaching condition had one replicate fromeach parent board. All specimens were conditionedto constant weight and approximately 10% MC at23°C and 55% RH before preservative treatment.The lumber specimens were end-sealed with twocoats of a neoprene rubber sealant to prevent end-grain penetration during preservative treatment andsubsequent leaching from the end-grain.
Preservative Treatment Process
The preservative evaluated in this study was analkaline borax–copper formulation containing1.3% elemental boron and 0.5% elemental cop-per. This formulation was selected because itcontains a readily leachable component (boron)and a less leachable component (copper). It isimportant to note that this formulation is notcurrently used for commercial pressure treat-ments and that the quantities of preservativeleached reported in this study are not directlyapplicable to any current commercial pressuretreatment preservatives. A full cell treatmentschedule was used to enhance uniformity oftreatment. An initial 30-min vacuum at �81 kPa(gauge) was followed by introduction of thetreatment solution and a 60-min pressure periodat 1034 kPa (gauge). The specimens wereweighed before and after treatment to allow thecalculation of preservative uptake. Because end-matched specimens were used and because allspecimens were treated in a single charge, re-tentions were similar between treatment groups(Table 1). Following treatment, the specimenswere stored in plastic bags for 1 wk to prevent
rapid drying and then reequilibrated in a roommaintained at 23°C and 55% RH.
Leaching Conditions Evaluated
For this study, the research described in Lebowet al (2017) was expanded by conducting an ad-ditional trial of outdoor leaching under naturalexposure along with two laboratory immersionapproaches. For comparison, this article also pres-ents data from earlier research with a simulatedrainfall method and amethod similar to the existingAWPA Standard E11 (AWPA 2017). The leachingmethods are summarized in Table 1. The laboratorymethods were conducted at room temperature,whereas the temperature of the outdoor specimensvaried with weather conditions.
E11Immerse (modification of AWPA Stan-dard E11). This laboratory immersion method ispatterned after AWPA Standard E11-16, Stan-dard Method for Accelerated Evaluation ofPreservative Leaching (AWPA 2017). AWPAStandard E11 is currently the most commonlyused standardized leaching method in NorthAmerica but uses small 19-mm blocks and isintended to greatly accelerate leaching. In thisstudy, single 38 by 92 by 102-mm lumberspecimens were used instead of multiple smallerblocks. The end-grain of the specimens had alsobeen sealed with a neoprene rubber coating tolimit leaching to the radial and tangential sur-faces. A larger leaching container was used, andthe volume of leaching water was increased to600 mL in proportion to the increased surfacearea. As prescribed in the method, the specimenswere vacuum impregnated with deionized waterbefore immersion (also in deionized water). Theywere immersed for a total of 16 d, with watercollections at 0.25, 1, 2, 4, 7, 9, 11, 14, and16 d (Table 1).
100hrImmerse (each immersion period100 h). The Lebow et al (2017) research in-dicated that the E11Immerse method resulted inless copper leaching than was observed forspecimens exposed outdoors. It was hypothesized
Lebow et al—LABORATORY IMMERSION METHOD FOR PREDICTION OF PRESERVATIVE LEACHING 3
that this was a result of the shorter leachingduration, which may have limited the amount ofsoluble copper that had time to diffuse to thesurface from the interior of the specimens. In thisversion, the method was similar to E11Immerseexcept that the immersion periods between watercollections were extended to approximately 100 hto allow more time for diffusion to occur. Inaddition, the specimens were not initially im-pregnated with water to more closely simulate themore gradual wetting that occurs in natural ex-posures. Although water collections were tar-geted for 100-h intervals, allowances were madefor worker convenience, and some intervals wereslightly more than or less than 100 h (Table 1).The total immersion time using this method was910 h or 37.9 d.
7dayImmerse (each immersion period 7d). This method was identical to that of the100hImmerse, except that the collection intervalswere extended to 7 d. Again, the intent of thelonger interval is to allow more time for solu-bilized copper to diffuse from the interior to theexterior of the blocks. In this case, the totalimmersion time is approximately four timesgreater than that of the AWPA E11 method. Thetotal immersion time using this method was 63 d.
SimRain (simulated rainfall). This methodused simulated rainfall to leach specimens, whichwere placed separately into stainless steel traysthat were slightly wider and longer (98 by108 mm) than the specimens. The specimenswere supported on a plastic grid so that they wereabove the tray outlet drain. Runoff from thespecimens drained through the tubing intopolyethylene collection containers below. Sim-ulated rainfall (RO water) was applied from arotating fan-spray nozzle mounted 1 m above thespecimens. The rate of rainfall was controlled at8 mm/h by the speed of the nozzle sweep and bycycling the nozzle off and on during rainfallevents. Daily rainfall was applied at 60-min in-tervals (60 min on, 60 min off) over 13 h (a totalof 7 h of rainfall per day). Rainfall was applied4 d per week (Monday-Thursday). The runofffrom the specimens was collected twice per week,after 112 mm of rainfall had accumulated. Thispattern was repeated for 4 wk, yielding a total of896 mm of rainfall and eight leachate collections.
Outdoor1 (exposure tonatural precipitation). Thismethod assessed leaching under natural exposureconditions. The lumber specimens were placedinto stainless steel trays in the manner similar tothe SimRain method and exposed outdoors from
Table 1. Leaching methods employed, and initial boron and copper content in specimens based on uptake during pressuretreatment.
Designation Leaching method
Treatment uptake (g)
Boron Copper
Mean Stdev Mean Stdev
E11Immerse Vacuum impregnated and immersed according toAmerican Wood Protection Association E11 with ninewater collections at 0.25, 1, 2, 4, 7, 9, 11, 14, and 16 d
2.90 0.29 1.11 0.11
100hrImmersea Immersed with nine water collections at 100-h intervals(4.2, 8.3, 12.6, 17, 21.2, 25.3, 29.6, 34.0, and 37.9 d)
2.87 0.30 1.11 0.12
7dayImmersea Immersed with nine water collections at weekly intervals(7, 14, 21, 28, 35, 42, 49, 56, and 63 d)
2.90 0.30 1.12 0.11
SimRain Simulated rainfall with water collections at 2, 4, 9, 11, 16,18, 23, and 25 d
2.90 0.29 1.12 0.11
Outdoor1 Outdoor year 1, natural rainfall with eight watercollections at 39, 67, 103, 116, 151, 182, 224, and 268 d(based on rainfall received)
2.98 0.24 1.15 0.09
Outdoor2 Outdoor year 2, natural rainfall with eight watercollections at 41, 79, 97, 114, 164, 186, 207, and 250 d(based on rainfall received)
2.89 0.29 1.11 0.11
a Each immersion period was 100 h or 7 d for these methods, respectively.
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March 6 to November 29, 2014, at a site nearMadison, WI. A weather station installed adja-cent to the specimens collected rainfall data at 15-min intervals. The specimens were exposed to868 mm of rainfall, and the leachate was collectedeight times, after rainfall accumulations of 113,105, 114, 121, 174, 102, 92, and 45 mm. Theintent was to collect rainfall after approximately112 mm of accumulation, corresponding to thecollection intervals used in the simulated rainfallmethodology. However, the final leaching periodwas curtailed because of sustained subfreezingtemperatures.
Outdoor2 (exposure tonatural precipitation). Thistrial was similar to Outdoor1, but in this case thespecimens were exposed from March 6 to No-vember 11, 2015. The specimens were exposedto a total of 835 mm of rainfall, and the leachatewas collected eight times after rainfall accu-mulations of 99, 100, 114, 102, 103, 104, 118,and 96 mm.
MC Measurements
A resistance-type moisture meter was used toevaluate the internal MC of Outdoor1 and Out-door2 specimens. Because electrical resistancedrops rapidly when free water is present in celllumens, resistance-type moisture meters loseaccuracy when the wood MC exceeds the FSP(approximately 26-28%). However, some changein resistivity does occur at higher moisturecontents, and researchers have recently presenteddata indicating that measurements greater than30% MC can be at least semiquantitative if theelectrodes are glued or screwed into the wood(Brischke and Lampen 2014; Lebow and Lebow2016). The moisture meter used in this study wasa General Electric Protimeter Timbermaster(General Electric Sensing, Danbury, CT), whichdisplays MC readings between 7% and 100%.The internal calibration recommended for southernpine was used in this study. Stainless steel screwswere used as electrodes because preliminary trialsindicated that the pin electrodes tended to yieldlower, and more variable, MC readings. Initially,
10-mm diameter holes were drilled to a depth of19 mm into the center of a narrow face of eachspecimen and filled with silicone sealant. Afterthe sealant dried, trim head wood screws (#7,76 mm length) were driven through the sealantand into the specimen until they extended towithin 19 mm of the opposite narrow face. Pilotholes were used to ensure that the screwsremained aligned as they were driven into thewood. The two screws, spaced 25 mm apart,were thus measuring the MC in an internal zonethat was approximately 39 mm from each endand 19 mm from the wide and narrow faces ofthe specimens (Fig 1).
The copper hydroxide and borax retention in thespecimens was relatively high, and preliminarytrials indicated than an adjustment was needed tocorrect the resistance readings. The adjustmentwas developed using a method previously de-scribed in Lebow and Lebow (2016). In brief, thinstrips (3 mm thick by 10 mm wide by 47 mmlong) of southern pine sapwood were vacuum-impregnated with preservative and then spread ona drying rack under ambient laboratory condi-tions. After 1, 2, 3, 4, or 5 h of air drying, pre-selected sets of specimens were removed from thedrying rack and individually wrapped in plasticfilm to prevent further drying. After 72 h, thespecimens were unwrapped, weighed, and theirresistance MC recorded. The specimens werethen oven-dried at 104°C to allow determination
Figure 1. Top view of specimen showing location of screwsused as electrodes. Depicts specimens set up for outdoorrainwater collection.
Lebow et al—LABORATORY IMMERSION METHOD FOR PREDICTION OF PRESERVATIVE LEACHING 5
of gravimetric MC. Matched specimens treatedwith deionized water were included for com-parison. The data were used to develop a seg-mented linear regression model, with censoringand heterogeneity between segments. The MCreadings reported in this article have been ad-justed according to this model. This procedurealso indicated that adjusted moisture meterreadings greater than 40% were poorly correlatedto gravimetric oven-dry MC.
Temperature may also affect resistance readings,and a temperature correction was developed forthe outdoor specimens. In this case, the leachingspecimens were used to develop the correction.The specimens were wrapped in plastic to preventdrying and equilibrated at set temperaturesranging from 4 to 31°C. The readings were thenadjusted to those obtained at 20°C.
Analysis of Leachate Solutions
The collected leachate was acidified to less thanpH 2 with nitric acid to maintain copper solubilityand analyzed by inductively coupled plasma emis-sion spectrometry (Horiba Instruments, UltimaII, Edison, NJ).
Statistical Methods and Analysis
Cumulative leaching was modeled using non-linear mixed effect models (Pinheiro and Bates2000) with the nlme package (version 3.1-128,Pinheiro et al 2016) in the statistical package R(version 3.3.1, R Core Team 2016). The non-linear relationships assumed were asymptoticregressions with offsets. The expected cumula-tive leached amount was modeled mathematicallywith a general form:
y¼ β1ð1� expð� β2ðx� β3ÞÞÞ;
where
y ¼ cumulative leached amount,x ¼ cumulative rain (mm) or time exposure (h),β1 ¼ asymptote,β2 ¼ rate constant, and
β3 ¼ offset (value of x at y ¼ 0, which implieseither an initial pulse or an initial delayedrelease).
However, the models also included random ef-fects and dependencies to capture the within-specimen dependencies over exposure periodsand the within–parent board dependencies acrossthe different treatment conditions; these wereassociated with the asymptote and rate constantparameters. Boron leaching was fit with onemodel, with separate parameter estimates for theasymptotes, rate constants, and offsets for each ofthe leaching conditions. Copper leaching was fitto a similar model. Long-term extrapolations forprediction of leaching were based on hypotheticalexposures to 500, 1000, 2000, and 5000 mm ofrain or hours of exposure, depending on theleaching condition. Population prediction intervalswere derived using simulation as described inBolker (2008). Extrapolations were made fromequations using parameters generated fromrandom samples (n ¼ 1000) from multivariatenormal distributions based on the parameterand variance–covariance matrix estimates ofthe statistical models. The 95th lower pre-diction interval is given as the 0.025 quantile ofthe extrapolations and the 95th upper predic-tion interval is the 0.975 quantile of the extrapo-lations. These intervals include within-exposurecondition variation but not between-exposure con-dition variation.
RESULTS AND DISCUSSION
In this study, the outdoor leaching trials wereintended to serve as the benchmark of actualleaching under “real-world” conditions. For treatedwood exposed aboveground, leaching is a functionof amount of precipitation and the resulting woodMC. The pattern of rainfall and internal MC of thespecimens for two separate 1-yr exposures areshown in Fig 2. The pattern of moisture gain andloss in the specimens was fairly similar eachyear, although the specimens gained moisturemost rapidly in year 1. In both years, specimensdried somewhat during some parts of the sum-mer, before regaining moisture and remaining
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consistently wet throughout the fall. Less dryingbetween rainfall events may have occurred in thefall because of lower temperatures and less di-rect sun exposure. The sustained fall moisturecontents (Fig 2) may have facilitated migrationof solubilized boron and copper to the woodsurface, resulting in an increase in the amount ofleaching per unit rainfall. In general, the twoseparate years of outdoor leaching resultedin remarkably similar quantities and patterns
of leaching (Figs 3 and 4), which providessome confidence in the use of these values asthe benchmark for comparison with laboratorymethods.
The manner in which leaching results are cal-culated and expressed can substantially affectinterpretation of the data. The quantity leachedcan be calculated as a percentage of the originalpreservative retention or on the basis of the amountof preservative released per unit surface area.
Figure 2. Rainfall and median interior MC for specimens exposed outdoors for year 1 (top) or year 2 (bottom). MC is cappedat 40% because of method limitations. Individual markers show rainfall amounts in 15-min intervals.
Lebow et al—LABORATORY IMMERSION METHOD FOR PREDICTION OF PRESERVATIVE LEACHING 7
Calculation of leaching as a percentage of theoriginal preservative retention is easily un-derstood, accounts for differences in initial load-ings, and provides an indication of the quantity ofpreservative remaining for future leaching or forefficacy against wood-degrading organisms. How-ever, the amount of preservative released perunit surface area may be more applicable whenattempting to estimate environmental releasesfrom a treated wood structure. In addition, thequantity leached can be expressed as a function
of leaching interval, leaching time, or amount ofprecipitation. In this article, we report and dis-cuss the results in several ways to allow a betterunderstanding of how the accelerated methodscompare with leaching under natural conditions.The primary objective of this research was todevelop an accelerated laboratory method thatcan be used to estimate leaching per unit surfacearea as a function of amount of precipitation.
When expressed as cumulative percentage leachedover exposure periods, the specimens immersed
Figure 3. Cumulative percent of boron (top) or copper (bottom) leached by leaching interval. Error bars show 1 standarddeviation from the mean.
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for weekly intervals (7dayImmerse) had the greatestleaching, followed by the specimens immersed for100-h intervals (Fig 3). Lumber specimens leachedusing the AWPA E11 method (AWPA 2017) hadthe least leaching of the accelerated methods,particularly in the case of copper.
Because copper is the primary active agent in themost common types of treated wood, it is im-portant that the leaching method does not greatlyunderestimate copper losses. It is worth notingthat calculating leaching solely by leaching in-terval obscures the large differences in duration of
leaching. In this study, the E11 method had theshortest intervals between water collections,whereas the outdoor exposures had the longestintervals. On the basis of percent leached perday, the E11 method had the greatest copperleaching. However, assuming that time is requiredfor solubilized copper to move from the interior ofthe specimens to the surface, the relatively lowtotal percentage of copper leaching from theE11 method is likely a result of shorter exposuretime. This observation during the earlier study(Lebow et al 2017) led to the decision to evaluate
Figure 4. Leaching as a function of amount of rainfall or hours of immersion for boron (top) and copper (bottom).
Lebow et al—LABORATORY IMMERSION METHOD FOR PREDICTION OF PRESERVATIVE LEACHING 9
more lengthy immersion periods in the currentresearch.
Because the objective of this study was to de-velop a laboratory leaching method that moreclosely simulated actual leaching outdoors, it wasnecessary to be able to relate the results of thelaboratory methods to quantity of precipitation.Although this was readily accomplished with thesimulated rainfall method, relating the immersionmethods required finding a relationship betweentime of immersion and volume of precipitation. InFig 4, leaching is expressed as quantity (mg)leached either per hour (immersed specimens) orper millimeter rainfall (outdoor and simulatedrainfall specimens) to allow comparison. Notethat in this case, because all specimens had thesame dimensions, there is no need to compareleaching on the basis of surface area.
When expressed on a per hour basis, releasesfrom the E11Immerse specimens are initially veryhigh because of the short intervals between watercollections and because they were initially im-pregnated with water (Fig 4). As the leachingcontinued, the more leachable components nearthe surface were depleted and a rapid decrease inleaching was observed for the E11Immersespecimens, especially in regard to copper. Al-though the pattern and quantity of leaching ob-served with the E11Immerse specimens do notcorrespond well to those observed in outdoorleaching, the results obtained with the 100hrIm-merse, and to some extent 7dayImmerse, methodsare more promising. The 100hrImmerse methodalso initially caused boron and copper lossesgreater than that observed outdoors but moreclosely mimicked outdoor leaching as the expo-sure continued. As shown in the leaching data inFig 5, leaching of copper from the 100hrImmersespecimens relates surprisingly well to that of theoutdoor specimens assuming that 1 h of immersionis equivalent to 1 mm of rainfall. Leaching ofboron from the 100hrImmerse specimens is greaterthan that observed for outdoor specimens, but thisdifference is primarily associated with the initialleaching period. In subsequent leaching intervals,1 h of immersion relates well to 1 mm of rainfall(Figs 4 and 5). The greater release observed during
the first interval with the immersionmethod is likelya result of more rapid initial wetting. The medianinteriorMC of the outdoor specimens remained lessthan 30% during the first interval (Fig 2), and thismay have limited the diffusion of copper and boronfrom the interior of the specimens to the surface.
The reason that the use of the 100-h immersionintervals caused 1 h of immersion to be ap-proximately equal to 1 mm of rainfall for copperleaching is unclear. The relationship may simplybe coincidental. However, it does provide aconvenient way to relate the accelerated im-mersion testing to typical volume of rainfall at aspecific location. Having this relationship isnecessary to relate leaching by immersion toleaching from precipitation. Further analysis isunderway to better understand and characterizethis relationship. By assuming that 1 h of im-mersion is equivalent to 1 mm of rainfall for the7dayImmerse, E11Immerse, and 100hrImmersemethods, the observed leaching was modeled toevaluate how well the accelerated leachingmethods might predict the amount of boron andcopper released during more prolonged in-serviceexposures. Leaching for specimens exposedoutdoors for 1 yr were also modeled to estimateleaching over longer exposures, and these esti-mates were compared with those of the laboratorymethods. Parameter estimates for the models areshown in Table 2. Using these models, Table 3shows howwell the laboratory methods estimatedoutdoor leaching when extrapolated over 0.5, 1,2, and 5 yr of exposure, assuming a hypotheticallocation receives 1000 mm of precipitation peryear. As expected, predicted leaching with theSimRain method most closely matched that ofoutdoor leaching for both boron and copper.However, the 100hrImmerse method also per-formed reasonably well in predicting leaching,especially over the longer term. It initiallyoverpredicts leaching because of the greaterinitial release (approximately three times as muchat 50 mm of rainfall, and 1.7 times as much at100 mm of rainfall) but stays within about 7-15%of the outdoor methods for longer exposures. The100hrImmerse method did particularly well inpredicting the long-term release of copper, which
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is commonly used in exterior use wood pre-servatives and may be more representative thanboron for leaching from exterior wood pre-servatives. By contrast, the E11Immerse methodperformed fairly well in predicting long-termboron leaching but substantially underestimatedlong-term copper loss. As a result of the very highleaching observed during the first week of im-mersion, the 7dayImmerse method overestimatedlong-term depletion of both boron and copper,even after 5 yr of rainfall.
Of the laboratory methods evaluated, the simu-lated rainfall approach resulted in leaching pat-terns most similar to outdoor exposure, especiallyin the case of copper. However, an immersionmethod would be more practical if duration ofimmersion can be related to amount of rainfall.The results of this study do indicate that relativelysimple immersion methods using lumber-sizespecimens have the potential to provide reason-able estimates of long-term leaching from treatedwood exposed to precipitation. The key to this
Figure 5. Comparison of laboratory leaching methods to outdoor exposure, assuming 1 h of immersion ¼ 1 mm of rainfall.
Lebow et al—LABORATORY IMMERSION METHOD FOR PREDICTION OF PRESERVATIVE LEACHING 11
approach appears to be optimizing the leachingintervals because the E11Immerse method sub-stantially underestimated long-term copper leach-ing, whereas the 7dayImmersemethod overestimated
both copper and boron leaching. The 100hrImmersemethod appears promising, although further im-provementmay be possible by adjusting the length ofthe initial intervals. By using lumber-size specimens,
Table 2. Model parameter estimates.
Parameter estimates
Leaching method Asymptote (mg), β1 Rate constant (h�1 or mm�1), β2 Offset (h or mm), β3
and by relating hours of immersion to amount ofprecipitation, these immersion methods provide areasonable means of estimating long-term leachingper unit surface area as a function of cumulativerainfall. This contrasts with the current AWPA E11small cube method which causes rapid initialleaching that has no clear relationship with expectedlosses from wood products exposed to naturalweathering. Although the total duration of the100hrImmerse method is longer than that of theAWPA E11 method (approximately 38 d vs 14-17d), no additional labor is required, and the time frameremains short when compared with that of standardlaboratory methods for evaluating resistance towood-attacking organisms.
CONCLUSIONS
The pattern and quantity of preservative leachedfrom pressure-treated wood are a function ofmany factors, including preservative chemistry,wood species, wood dimensions, and the char-acteristics of the leaching environment. It is thelatter factor that is most difficult to simulate,especially for wood exposed aboveground andsubjected to intermittent wetting from pre-cipitation. Ideally, an accelerated leaching methodwould allow estimation of preservative leaching asa function of the amount of rainfall at a specificlocation or climatic zone. Simulated rainfall is alogical approach, and our research has shown thata simulated rainfall method can closely emulatethe quantity of preservative leached from lumberspecimens exposed outdoors. However, thatmethod is complex and may not be well suited fortypical laboratory use or for standardization. Thisstudy demonstrated that much simpler immersionmethods can also be used to provide useful esti-mates of leaching from wood products exposed tonatural precipitation. The key features of theseimmersion methods are the use of larger specimensizes that simulate lumber, and the extension ofleaching intervals to allow wetting of the largerspecimens and time for solubilized preservativecomponents to diffuse to the wood surface. Anal-ysis and modeling of data from the immersionmethods indicate that time of immersion can berelated to volume of precipitation, thus allowing
prediction of leaching based on the rainfall char-acteristics of specific location or region. This studyemployed outdoor weathering near Madison, WI,and an alkaline borax–copper preservative; thus, thefit of the prediction will be somewhat influenced bythe pattern of rainfall at a site (fewer, high-volumerainfall events vs extended periods of slow rain-fall) and by characteristics of the preservativechemistry. It would be beneficial to have immersionleaching periods compared with natural leachingfor other locations and other types of preservativechemistries.
ACKNOWLEDGMENTS
The authors acknowledge Steven Halverson,Keith Bourne, and Randall Wruck of the USDAForest Products Laboratory for their assistance inpreparing test specimens and for design andconstruction of the rainfall simulator.
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