INFLUENCE OF ORGANIC ACIDS AND NATURAL CHELATING AGENTS ON COPPER, CHROMIUM, AND ARSENIC LEACHING FROM TREATED WOOD

 

Lisa Y. Blue, Ralph W. Sheets, Richard N. Biagioni (rnb266f@mail.smsu.edu), Chemistry Department, Southwest Missouri State University, 901 S. National Ave., Springfield, MO 65804, USA

 

ABSTRACT

Studies were carried out to assess the influence of naturally occurring complexing agents on the leaching of copper, chromium, and arsenic (CCA) from treated wood.  Ground CCA wood was leached with extracts from dried leaves of several species of trees and with solutions of salicyl alcohol and tannic, salicylic, phthalic, and oxalic acids.  Leaf extract leachates showed elevated levels of CCA  compared to controls.  CCA concentrations did not correlate with the leaching solutions= pH values but Cr and As concentrations were moderately correlated to the total acid content of the leaching solutions as measured by acid-base titrations.  Tannic acid, salicylic acid, oxalic, and phthalic acid leachates showed significant CCA elevation, supporting the conclusion that organic acids are primarily responsible for leaching by leaf extracts.  Leachings were also carried out using CaCl2 and MgCl2 solutions at concentrations greater than those found in leaf extracts.  Levels of Cu, but neither As nor Cr, were slightly elevated in these leachates, suggesting that ion exchange is not a major factor in fixation reversal.

 

INTRODUCTION

Copper chromium arsenate (CCA) treatment is widely used to reduce the susceptibility of softwoods to insect attack and fungal decay (Lebow 1993).  Field studies (Lebow 1993; Stillwell and Gorny 1997) have shown some losses of CCA components from treated lumber, suggesting questions about factors that reverse CCA fixation in environmental settings.

CCA components are fixed in wood in a variety of forms, including as precipitates of copper arsentates, chromium arsenates, and copper chromates, and as ion exchanged Cu2+ (and Cr3+ to a lesser extent) bound to lignin (Lebow 1993).  Reversal of fixation could therefore be influenced by dissolution of precipitates by H+ or complexing agents, and reversal of ion exchange by cation displacement.  Warner (Warner and Solomon 1990) observed that substantial amounts of CCA were leached from wood by acidic buffers, but Cooper (1991) showed that it was the citrate ion, rather than H+, in Warner=s  buffers that was primarily responsible for leaching.

Decaying plant matter is known to produce numerous acids ranging from high molecular mass humic and fulvic acids to low molecular mass acids that include both carboxylic and phenolic functional groups (Gaffney et al, 1996;  van Loon and Duffy, 2000).  Cooper=s demonstration that citrate, a complexing agent, enhanced leaching and our recognition that decaying leaves produce a variety of organic acids led to our evaluation of the potential for chemicals derived from leaves to influence retention of CCA.  In this study, ground CCA-treated lumber was leached with extracts from dried leaves to simulate conditions that might arise when treated lumber is in contact with moist decaying leaves.  Wood was also leached with solutions of several organic acids representative of the functional groups found in natural extracts, and with Mg2+ and Ca2+ solutions to evaluate ion exchange.

METHODS

Leaves from oak (Quercus velutina), sycamore (Platanus occidentalis), sassafras (Sassafras albidum), silver maple (Acer saccharinum), sugar maple (Acer saccharum), willow (Salix nigra), and walnut trees (Juglans nigra) were dried at 60EC.  Extract solutions were prepared by heating a mixture of 20.0 g dried leaves and 250 ml deionized water (40°C) for 24 hours and then filtering.  Solutions (10 mM) of oxalic acid, tannic acid, salicylic acid, salicyl alcohol, and potassium hydrogen phthalate (KHP) and a 1% (w/v) humic acid solution (sodium salt, Aldrich) were also prepared.

One-gram samples of ground CCA lumber were combined with 25 ml of leaching solutions (4 replicates for leaf extracts, 3 replicates for acids), mechanically shaken for 24 hours, and filtered.  Leaf extract leachings were acidified with nitric acid prior to analysis to prevent molding.  Wood samples were also leached with 10 mM CaCl2 and MgCl2 solutions.  Deionized water leachings (7 replicates) were used as a control, and samples of untreated wood were also leached for comparison.

Concentrations of arsenic, calcium, chromium, copper, and magnesium were analyzed using a Varian Liberty 150AX  ICP Spectrometer.  Leaf extract and humic acid solutions spiked with CCA were also measured.  Potentiometric titrations of 50-ml aliquots of leaf extract solutions with 0.10 M NaOH were also carried out.

 

RESULTS

Concentrations of As, Cr, Cu, Mg, and Ca in leachates:  Summaries of the leaching results are shown in Table 1.  All results are corrected for any background levels of CCA components in the leaching solutions.  Spiking studies showed significant signal suppression (typically 20 - 30%) for leaf extracts (Cr only) and humic acid (Cr and Cu), and results were corrected for these effects.  CCA levels were insignificant for untreated wood and for the original leaf extracts.

Potentiometric titrations:  Titrations of the leaf extract solutions produced broad, nearly featureless titration plots without any discernible equivalence point, as expected for high molecular mass polyelectrolytes with a variety of acidic moieties in a range of environments (Gamble and Schnitzer 1973).  Nevertheless, it was clear that some of the leaf extracts contained higher concentrations of acidic groups.  The volume of titrant required to reach pH 10 was arbitrarily chosen as providing a measure of the relative amounts of acid present in each solution. These values ranged from approximately 6 ml to 27 ml of 0.10 M NaOH per 50 ml sample, corresponding to 12 to 54 meq acid per liter of extract.  There was no statistical correlation between initial pH and the titration volume.

Leaf extract leachates:  With the exception of arsenic levels for sassafras and sycamore extracts, CCA levels were elevated above the deionized water controls at the 0.01 significance level based on one sided t-tests and pooled standard deviations. 

CCA levels appeared totally uncorrelated to the initial pH of the leaf extract solutions, consistent with Cooper=s conclusion that in the pH range investigated here, H+ is not a critical factor affecting leaching.  However, As levels show a significant correlation to total acidity (as measured by titrant volume required to reach pH 10) with R2 = 0.82 (0.0051 significance level), while Cr levels show a weaker correlation with R2 = 0.53 (0.064 significance level).  No correlation was observed between Cu levels and total acidity.  Correlations between As, Cr, and Cu were relatively weak.  Cu vs. Cr correlated with R2 = 0.63 (0.033 significance level), and Cr vs. As with R2 = 0.66 (0.027 significance level), but there was no significant correlation of Cu vs. As (R2 = 0.19 with " = 0.33). 

 

 

Text Box: Table 1. Summary of results for leachates Extracting solution pHinit dVpH 10 (ml) eAs (ppm) eCr (ppm) eCu (ppm) eMg (ppm) eCa (ppm) aOak 4.0 10.7 9.2 ± 0.3 3.5 ± 0.2 10.1 ± 0.4 25 ± 3 44 ± 7 aSassafras 4.9 6.3 7.4 ± 1.5 3.7 ± 1.0 12.6 ± 1.7 38 ± 8 61 ± 15 aSilver Maple 4.6 22.9 10.1 ± 0.5 4.8 ± 0.4 11.5 ± 0.6 46 ± 8 61 ± 9 aSugar Maple 3.3 26.6 11.6 ± 0.5 4.2 ± 0.3 10.1 ± 0.3 47 ± 8 130 ± 24 aSycamore 4.9 9.3 6.8 ± 0.1 3.0 ± 0.0 9.9 ± 0.6 72 ± 13 153 ± 23 aWalnut 4.6 22.0 11.7 ± 0.4 5.6 ± 0.1 17.9 ± 0.9 61 ± 11 4 ± 1 aWillow 4.0 15.5 9.1 ± 0.7 3.9 ± 0.3 10.5 ± 1.0 56 ± 11 75 ± 12 bSalicylate 17.0 ± 0.3 5.6 ± 0.2 8.4 ± 0.2 bTannate 12.2 ± 0.2 2.9 ± 0.1 2.0 ± 0.01 bOxalate 27.4 ± 2.1 13.6 ± 1.2 16.1 ± 1.2 bPhthalate 16.9 ± 0.9 6.4 ± 0.4 8.6 ± 0.9 bSalicyl alcohol 7.6 ± 0.2 2.3 ± 0.1 1.1 ± 0.1 cHumic acid 6.8 ± 0.4 5.6 ± 0.4 19.0 ± 0.4 bMgCl2 8.0 ± 0.5 2.2 ± 0.1 2.3 ± 0.1 bCaCl2 6.8 ± 0.8 1.4 ± 0.3 2.5 ± 0.4 DI water 7.8 ± 0.4 2.3 ± 0.1 1.0 ± 0.1 0.2 ± 0.1 0.7 ± 0.3 a Prepared from 20 g dried leaves in 250 ml water. b 10 mM solutions c 1% w/v solution d Volume of 0.10 M NaOH required to react pH 10 in titration of 50 ml extractant. e Uncertainties shown are standard deviations for replicate measurements.

 


Organic acid leachates:  With the exception of arsenic levels for the humic acid leachate and all levels for the salicyl alcohol leachate, CCA levels were elevated for each of the organic acid solutions.  Correlations between each of the CCA components were strong (R2 > 0.95 with significance level £ 0.005) for the low molecular mass acid solutions, but ratios of components for the humic acid extract differed markedly from those for the other acids. 

MgCl2 and CaCl2 leachates:  Leaf extract leachates had Mg and Ca concentrations up to 72 ppm (3 mM) and 153 ppm (4 mM) respectively, so the 10 mM MgCl2 and CaCl2 solutions employed here represent metal ion concentrations somewhat higher than those found in the leaf extracts.  Cu levels were elevated above the controls at the 0.01 significance  level for each of these, but levels of As and Cr were not.  However, there was no statistically significant correlation between CCA concentrations and either Mg or Ca in the leaf extract leachates.

 

DISCUSSION

 

In principle, reversal of CCA fixation could occur by any of three mechanisms:  (1)  dissolution of inorganic salts arising from protonation of arsenate and chromate by H+;  (2)  dissolution of inorganic salts from complexation of Cu2+ and/or Cr3+;  (3)  displacement of Cu2+ (and to a lesser extent Cr3+) bound to lignin by ion exchange with H+, Mg2+, or Ca2+. 

Both Cooper’s work and our own show that CCA leaching does not correlate with solution pH, so the first mechanism is insignificant, at least for solutions within the pH range studied.

MgCl2 and CaCl2 leachates showed only small increases in Cu levels compared to controls;  Cr and As levels were not elevated.  These observations are consistent with descriptions  of CCA binding that indicate significant amounts of Cu, but not Cr or As, bind to lignin by ion exchange. However, given that the relatively high levels of Mg and Ca employed in these studies (roughly three times greater than the highest levels found in any of the leaf extracts) produced only slightly elevated leaching of Cu, it is likely that ion exchange is of minimal significance to leaching under normal environmental conditions.

The enhanced leaching of CCA both by leaf extracts and by most organic acids, and the correlations observed between leaching and total acid content for the leaf extracts, suggest that mobilization of salts by complexation of metal ions is a key factor in releasing CCA components. The highest levels of leaching were observed for oxalic acid, the lowest levels for tannic acid and salicyl alchol, and intermediate levels for phthalic and salicylic acids.  Data on complex ion formation (Martell, 1977) suggest that at neutral pH, formation constants for oxalate with Cu2+ vary as oxalate > phthalate . salicylate (data for tannic acid and salicyl alcohol not available).  Formation constants for Cr3+ with these anions were not available, but formation constants for Al3+, a reasonable surrogate for Cr3+, vary in the same manner.  Hence, it is possible that strength of complexation accounts for the observed variations in leaching.  Complexation by phenolic compounds should be least favorable at neutral to mildly acidic conditions, consistent with our observation that tannic acid and salicyl alcohol caused the least leaching.  However, ionic sizes vary in the order oxalate < phthalate . salicylate < tannate, so the rate at which anions diffuse into and complex ions diffuse out of wood fibers could also be a contributing factor.  We are planning additional studies to investigate this issue.

We note that none of the low molecular mass acids mimic the behavior of the leaf extracts or the humic acid with respect to ratios of the three CCA components extracted.  The role of molecular size and specific complexation requires further investigation.

 

ACKNOWLEDGMENTS

 

We are grateful to Heather Parker (SMSU Biology Department) for help in identifying leaves.  We acknowledge Southwest Missouri State University for providing funding.

 

REFERENCES

 

Cooper PA (1991) Forest Prod. J.  41(1): 30-32.

Gaffney JS, Marley NA, Clark SB, Editors (1996) Humic and Fulvic Acids:  Isolation, Structure, and Environmental Role, ACS Symp. Series 651, Washington DC, Amer. Chem. Soc.

Gamble DS, Schnitzer M (1973), in Trace Metals and Metal-Organic Interactions in Natural Waters, PC Singer, Editor, Ann Arbor MI, Ann Arbor Publishers, p. 265-302.

Lebow S, Leaching of Wood Preservative Components and Their Mobility in the Environment (1993) Gen.Tech Rep. FPL-GTR-93, Forest Service Lab., US Dept. Agr.

Martell AE, Smith RM (1977) Critical Stability Constants Vol. 3, New York, Plenum Press

Stillwell DE, Gorny KD (1997) Bull. Environ. Contam. Toxicol. 58:  22 – 29

van Loon GW, Duffy SJ (2000) Environmental Chemistry, New York, Oxford Press

Warner JG, Solomon KR (1990) Environ. Toxicol. and Chem. 9: 1331-1337