Environ. Sci. Technol. 2008, 42, 3739–3744
Extraction of Hexavalent Chromium from Chromated Copper Arsenate Treated Wood under Alkaline Conditions SUZANA RADIVOJEVIC AND PAUL A. COOPER* Faculty of Forestry University of Toronto 33 Willcocks St. M5S 3B3 Toronto, ON, Canada
Received November 22, 2007. Revised manuscript received January 17, 2008. Accepted February 26, 2008.
Information on chromium (Cr) oxidation states is essential for the assessment of environmental and health risks associated with the overall life-cycle of chromated copper arsenate (CCA) treated wood products because of differences in toxicity between trivalent [Cr(III)] and hexavalent [Cr(VI)] chromium compounds.HypotheticalCr(VI)fixationproductswereinvestigated in CCA type C treated sawdust of aspen and red pine during or following preservative fixation by extraction with Cr(VI)-specific extractants. Cr(VI) was found only in alkaline extracts of treated wood. A major source of Cr(VI) was method-induced oxidation of fixed Cr(III) during alkaline extraction, as confirmed by demonstrated oxidation of Cr(III) from CrCl3 treated wood. Oxidation of nontoxic and immobile Cr(III) to toxic and mobile Cr(VI) was facilitated by the presence of wood at pH > 8.5. Thermodynamic equilibrium between Cr(III) and Cr(VI) is affected by pH, temperature, rates of dissolution of Cr(III) compounds, and oxygen availability. Results of this study recommend against alkaline extraction protocols for determination of Cr(VI) in treated wood. This Cr oxidation mechanism can act as a previously unrecognized route for generation of hazardous Cr(VI) if CCA treated wood is exposed to alkaline conditions during its production, use, or waste management.
Introduction Use of chromated copper arsenate (CCA) wood preservative has been restricted, and in some countries completely abandoned during the past decade, because of health and environmental hazards posed by arsenic (As) and hexavalent chromium [Cr(VI)]. In the USA and Canada, CCA continues to be used for commercial and industrial products because of its superb performance against wood decaying organisms, and many CCA treated residential products placed in service prior to its phase-out for residential applications in 2004 still remain in use. Following wood treatment with waterborne CCA solution by vacuum-pressure impregnation, Cr(VI) in the treating solution is reduced to Cr(III) by organic wood constituents. Complete Cr(VI) reduction is of utmost importance because it supplies Cr(III) for As (V) precipitation, facilitates Cu(II) fixation by oxidizing wood components, and converts Cr from toxic and soluble hexavalent to nontoxic and sparingly soluble trivalent form (1–4). Although Cr reduction is generally considered complete with proper fixation and Cr(III) is regarded as the exclusive chromium * Corresponding author e-mail:
[email protected]. 10.1021/es702885f CCC: $40.75
Published on Web 04/09/2008
2008 American Chemical Society
oxidation state in treated wood and its water leachates (3, 4), it has been suggested that a reservoir of water insoluble Cr(VI) may be present (5–7), which could be released under alkaline conditions (8–12). Cr(III) is nontoxic when taken in moderate quantities, but Cr(VI) is carcinogenic, mutagenic, acutely toxic to humans, and harmful to plants and animals (13, 14). Because of remarkably different toxicities and bioavailabilities of Cr(III) versus Cr(VI), chromium speciation is critically important for the assessment of environmental and health risks related to manufacturing, consumer use, and waste management of CCA treated wood products. Dahlgren and Hartford (1, 15), suggested that stable Cr(VI) fixation products existed only as transient species such as chromic acid adsorbed on cellulose and chromium chromates, which occur during early fixation but are ultimately converted to final and stable Cr(III) products by the end of fixation. In contrast, Pizzi (5, 7) and Ostmeyer et al. (6) suggested that Cr(VI) could be permanently fixed in wood as stable, insoluble Cr(VI)-lignin complexes. Subsequent studies of similar reaction products between chromic acid and lignin model compounds using magnetic susceptibility measurements demonstrated that these complexes contain Cr in trivalent, and not hexavalent, form (16). Reports on the Cr oxidation state also vary depending on the choice of experimental techniques. Solid-state analysis of Cr in treated wood by methods such as X-ray photoelectron spectroscopy (XPS) (17) and X-ray absorption spectroscopy (XAS) (18) offer strong evidence that only Cr(III) exists in CCA treated wood after fixation. Extractions with water show negligible water soluble Cr(VI) contents in fully fixed CCA treated wood. For example, Cooper et al. (4) reported Cr(VI) levels below 1 µg/L in water leachates containing 200-2700 µg/L of total Cr. However, extractions with alkaline extractants designed to dissolve water-insoluble Cr(VI) compounds from CCA treated wood were reported to extract significant amounts of Cr(VI). Nygren and Nylsson (8) found that, although there was no detectable water soluble Cr(VI) in the CCA treated samples they evaluated, up to 20% of the estimated Cr content was extracted with 7% Na2CO3; they assumed that all extracted Cr was hexavalent and suggested that it represented unfixed “weak alkali soluble Cr(VI)”. Similarly, Cruz et al. (12) assigned hexavalent oxidation state to all Cr determined by atomic absorption spectroscopy (AAS) in the extract of CCA treated wood extracted using Na2CO3 and NaOH in the presence of MgSO4 and suggested this method as “applicable to the determination of hexavalent chromium in CCA-treated building materials”. Alkaline digestion with Na2CO3/NaOH at 90–95 °C, prescribed by EPA method 3060A (19) for extraction of Cr(VI) from “soils, sludges, sediments and similar waste materials”, extracted up to 4% and 7% of total Cr in hexavalent form from new and weathered CCA treated wood, respectively, whereas no Cr(VI) was detected in corresponding extracts of water and buffer solutions with near neutral pH (10, 11). A series of extractions at different pH showed that Cr(VI) was consistently found in extracts of CCA treated wood at pH g 9 and that Cr(VI) contents increased with alkalinity of the solution (11). The inconsistencies regarding the presence of toxic Cr(VI) in fixed treated wood warrant further investigation of the analytical methods for its determination. This work addresses the suitability of several extractants to extract hypothetical Cr(VI) fixation products from unfixed and fixed CCA treated wood. Results of these extractions led to the hypothesis that Cr(VI) may be generated through method-induced oxidation of fixed in situ Cr(III) during alkaline extractions, including standard EPA method 3060A (19). No studies confirming VOL. 42, NO. 10, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Extraction of Cr(VI) from Aspen (CCA Retention 30 kg/m3) with H2O/H2O and H2O/Extractant at Different Stages of CCA Fixationa average Cr(VI), % of total Cr fixation time (h)
1st extraction (H2O)
2nd extraction
yield
0
81.4 (0.6)
6
54.5 (0.2)
6
55.0 (0.4)
46
24.7 (0.3)
46
24.1 (1.0)
H 2O KH2PO4/K2HPO4 H2O KH2PO4/K2HPO4 H2O Na2CO3 H2O Na2CO3 H2O NaOH
0.09 (0.03) 0.08 (0.02) 0.08 (0.01) 0.08 ( 11), less Cr(VI) was released because the unfixed Cr(VI) was almost completely reduced to Cr(III) (data not shown). Hightemperature alkaline extractions of samples containing reducing agents have been reported to lead to a methodinduced Cr(VI) reduction (24, 25). Multistep extractions of fixed CCA treated samples, aimed to ensure complete removal of available Cr(VI), with 0.1 M Na2CO3, 0.1 M NaOH, and 1 M NaOH are compared to water extraction and are presented as cumulative values of eight consecutive extraction steps for Cr(VI), total Cr, and As in Table 2. The pH of the extracts did not deviate significantly from the reference values. Although Cr(VI) was not detected in water extracts, its cumulative contents in alkaline extracts ranged from about 8 to 26% based on the total Cr in the samples. Cr(VI) was detected in all alkaline extracts even after repeated extractions, although its availability decreased over the extraction time, as indicated by slopes of cumulative Cr(VI) extractions curves for 0.1 M NaOH (Figure 1). This trend was more pronounced for CCA retention of 6.4 kg/m3 than that of 30 kg/m3, which indicates that there is more easily oxidizable Cr with the higher retention treatment. Extraction efficiencies for all components decreased in the order of 1 M NaOH > 0.1 M NaOH > 0.1 M Na2CO3, suggesting that extractability increased with the strength and pH of the extractant (Table 2). Although relative Cr(VI) extraction yields were somewhat higher in red pine than in aspen treated to 6.4 kg/m (3) (Table 2), the absolute extraction yields were comparable (Figure 1), suggesting similar Cr(VI) availability in the two wood species. Total soluble Cr was consistently higher than Cr(VI), ranging between approximately 10 and 46% of estimated Cr content. High solubility of Cr(III) and VOL. 42, NO. 10, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Extraction of Cr(VI) and Total Cr According to EPA Method 3060A from CCA Treated Aspen (Fixed) and CrCl3 Treated Aspena sample
treatment
aspen
CCA 30
aspen
CrCl3
no substrate
CrCl3
Cr(VI)
Cr total
MgCl2 addition
%
ppm
SD (ppm)
%
ppm
SD (ppm)
N Y N Y N Y
1.4 0.6 3.0 0.3 BDL BDL
1.9 0.72 1.5 0.14 BDL BDL
0.1 0.09 0.1 0.04
4.0 0.5 9.3 0.2
5.4 0.7 4.8 0.10
0.1 0.1 0.1 0.02
kg/m3
a
Mean Cr extraction yields (for three replicates) given as %, based on the amount in the sample, and as concentrations in the solution (ppm). BDL, below detection limit.
of 1.4 and 3%, respectively of Cr in hexavalent form (Table 3). Cr(VI) was not observed in control samples in the absence of wood. Addition of MgCl2 to prevent method-induced oxidation suppressed Cr(III) solubility and considerably decreased the level of Cr(III) oxidation but did not prevent it (Table 3). EPA Method 3060A suggests that Cr(VI) extracts of soil samples should be stable for 168 h following the extraction, but we observed that aging of both filtered and unfiltered extracts at ambient temperature led to continued dissolution and oxidation of Cr, as illustrated in Figure 3 by the change in Cr(VI) contents over time in unfiltered samples. The effects of extraction conditions employed by the EPA protocol on Cr and As solubility and Cr oxidation state were evaluated by comparing extraction yields from incompletely fixed CCA treated aspen (retention 6.4 kg/m3) in 0.5 M NaOH/ 0.28 M Na2CO3 at 23 and 93 °C (Table 4). Extractions were performed at the point when 26% of Cr and 9.5% of As was water soluble and unfixed. Total Cr, Cr(VI), and As were 6.6, 2, and 8%, respectively, higher for alkaline extraction at 23 °C than for water extraction. Based on the confirmed Cr(III) oxidation in alkaline solutions, these results suggest dissolution of Cr(III) and As, which are fixed at this stage (i.e., insoluble in water) and subsequent oxidation of Cr(III) to Cr(VI). Conversely, extraction at 93 °C led to a decrease in Cr solubility and rapid Cr(VI) reduction, similar to the earlier results with 0.1 M Na2CO3 at 90–95 °C, resulting in 3 and 3.7% of Cr(VI) in the absence and in the presence of MgCl2, respectively.
FIGURE 3. Cr(VI) monitoring in aged extracts following extraction according to EPA Method 3060A. As may originate from the dissolution of Cr-As fixation products under alkaline conditions. The persistence of Cr(VI) in extracts of completely fixed sawdust suggested possible oxidation of fixed Cr(III) during alkaline extractions at ambient temperature. Detection of Cr(VI) in alkaline extracts of aspen sawdust treated with CrCl3 as a source of trivalent chromium confirmed that Cr(III) was oxidized to Cr(VI) above pH 8.5 and that the extent of oxidation increased with alkalinity (Figure 2). Control samples of CrCl3 solution mixed with each extractant had less than 0.07% of total Cr oxidized, which may be attributed to the oxidation with dissolved oxygen (32). These findings confirm that Cr(III) oxidation depended on the presence of wood components. It was also affected by the oxygen availability, because only 0.46% (SD ) 0.01%, N ) 3) of Cr(III) in the samples was oxidized with 0.1 M NaOH in helium compared to 2.1% (SD ) 0.2%, N ) 3) in air. Alkaline digestion of fully fixed CCA treated aspen and CrCl3 treated aspen with 0.28 M Na2CO3/0.5 M NaOH at 90–95 °C according to EPA Method 3060A (19) resulted in extraction
Discussion Results presented here demonstrate spontaneous oxidation of Cr(III) to Cr(VI) under moderate to strong alkaline conditions (pH g 8.5) in the presence of wood. Although Cr(VI) is considered the thermodynamically stable form of Cr under alkaline and oxidizing conditions (33), Cr(III) oxidation is very slow in the absence of strong oxidizing species and has not been recognized as a spontaneous reaction in treated wood. Oxidation of Cr(III) from CCA treated wood and sludges has been proposed only in relation to the use of strong oxidizing agents such as sodium
TABLE 4. Extraction of CCA Components at 23°C (Ambient Temperature) and 93°C from Incompletely Fixed CCA Treated Aspen (6.4 kg/m3)a method of extraction
a
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extracted component (%)
extractant
temperature (°C)
MgCl2
Cr (VI)
Cr
Cu
As
H2O 0.5 M NaOH/0.28 M Na2CO3 0.5 M NaOH/0.28 M Na2CO3 0.5 M NaOH/0.28 M Na2CO3
23 23 93 93
N N N Y
26.8 28.8 3.0 3.7
26.0 33.6 17.4 3.8
6.1 3.7 23.0 4.1
9.5 17.5 51.4 21.5
Relative standard deviations (RSD) for mean values (of three replicates) were between 0.1 and 2%.
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hypochlorite (9, 20, 34), hydrogen peroxide (21), sodium percarbonate (34), and sodium peroxydicarbonate (9). However, our results suggest that, in alkaline solutions, wood constituents supply effective oxidizing species, which can cause substantial Cr(III) oxidation at pH g 8.5. This is in agreement with reported findings of Cr(VI) in extracts of CCA treated wood at pH g 9 in the absence of confirmed oxidizing agents (9–12). The Cr(III) oxidation rates are affected by the type of alkali, pH, reaction temperature, and oxygen availability, which is in agreement with James et al. (24) who suggested that the oxidation potential of Cr(III) in soil depends on the Cr(III) form, the oxidizing and reduction potential of the matrix, and the pH. In addition to directly affecting the Cr(III) oxidation, these factors also influence the rates of Cr(III) dissolution, which have been positively correlated to the rates of Cr oxidation (35). The excess of total Cr to that of Cr(VI) in alkaline extracts implies that Cr(III) dissolution is a critical step for Cr oxidation. Chromium fixation products that comprise amorphous chromium arsenates and chromium hydroxide (3), or recently proposed complexes of Cr(III) dimers bridged by an As oxyanion and bound to the wood through Cr (18), are likely solubilized in alkaline solutions and form reactive species such as Cr(OH)4- (36). Although Cr(III) oxidation was observed only in the presence of wood, the rates of oxidation decreased in the absence of atmospheric O2, suggesting that wood may act as a catalyst for oxidation with O2 or that oxidizing species in wood are formed only in the presence of oxygen. Alkaline degradation of wood can result in the formation of soluble oxidizing species, including a variety of free radicals originating from lignin (37), cellulose (38), or other dissolved organic matter (39). Cr(III) oxidation by free radicals was observed in 0.5 M NaOH (40), and hydroxyl radicals ( · OH) are known to affect Cr redox chemistry even at low alkalinity (39). Quinone intermediates, which are known as strong oxidants, are also formed during alkaline degradation of lignin. It should be noted that Cr(VI) was also observed in leather originally free of Cr(VI), which was, among other factors, related to high pH exposures and possible action of free radicals formed from the organic constituents of leather in the presence of oxygen and UV light (41, 42). The complex effect of temperature on the thermodynamic equilibrium between Cr(III) and Cr(VI) is reflected in the relative proportions of Cr(III) and Cr(VI) in the extracts at the ambient and digestion temperatures, respectively. Although conversion of Cr(III) to Cr(VI) was a dominant redox reaction at ambient temperature, Cr(VI) in unfixed samples was rapidly reduced to Cr(III) at digestion temperature. Nevertheless, low Cr(VI) levels were observed in all samples of fixed and unfixed wood extracted at the digestion temperature. Cr(III) oxidation at digestion temperature may be, among other factors, inhibited by low solubility of Cr(III), which decreases considerably with increasing temperature. Our results show that alkaline extraction techniques do not preserve the oxidation state of Cr and, therefore, cannot be recommended as valid approaches for qualitative and quantitative investigation of insoluble forms of Cr(VI) in CCA treated wood. EPA Method 3060A (19) did not meet criteria for analysis of CCA treated wood, because Cr(III) oxidation was observed in all instances and was not completely suppressed even with addition of MgCl2. Alkaline extractions at ambient temperature led to even more extensive Cr oxidation than high temperature digestion methods and should be recognized as a source of method induced Cr oxidation in earlier studies reporting incidence of Cr(VI) in alkaline extracts of treated wood (8, 10–12). Quantification of Cr(VI) in alkaline extracts, which is sometimes based on the assumption that they contain only Cr(VI) (8, 12), should instead be performed using Cr(VI)-specific analytical tech-
niques, because our results confirm variable but significant contribution of soluble Cr(III) in alkaline extracts. No recommendations for modification of alkaline extractions to avoid method-induced Cr(III) oxidation can be made before this reaction mechanism is fully understood. Nondestructive techniques such as XAS and magnetic susceptibility should be considered more appropriate methods for the assessment of Cr(VI) in CCA treated wood. In the light of significant hazards posed by Cr(VI), exposure conditions and recycling options for CCA treated wood that encounter high pH should be carefully evaluated with regard to potential Cr(III) oxidation. Cr(III) oxidation can be a possible reason for elevated Cr(VI) leaching from cementwood composites manufactured from CCA treated wood under alkaline conditions (4) and from CCA decking material treated with deck-washing solutions with pH higher than 8 (9, 34). Cr(VI) could be created in landfills from waste CCA treated wood if it is mixed with alkaline building wastes such as concrete, gypsum board, and cement debris (11).
Literature Cited (1) Dahlgren, S. E.; Hartford, W. H. Kinetics and mechanism of fixation of Cu-Cr-As wood preservatives. Pt. II. Fixation of Boliden K33. Holzforschung 1972, 26, 105–113. (2) Smith, D. N. R.; Williams, A. I. The effect of composition on the effectiveness and fixation of copper/chrome/arsenic and copper/chrome preservatives. Part II: Selective absorption and fixation. Wood Sci. Technol. 1973, 7, 142–150. (3) Bull, D. C. The chemistry of chromated copper arsenate II. Preservative — wood interactions. Wood Sci. Technol. 2001, 34, 459–466. (4) Cooper, P. A.; Jeremic, D.; Ung, Y. T. Effectiveness of CCA fixation to avoid hexavalent chromium leaching. Forest Prod. J. 2004, 54, 56–58. (5) Pizzi, A. J. The Chemistry and kinetic behavior of Cu-Cr-As/B wood preservatives. I. Fixation of chromium on wood. Polym. Sci. Chem. Ed. 1981, 19, 3093–3121. (6) Ostmeyer, J. G.; Elder, T. J.; Littrell, D. M.; Tatarchuk, B. J.; Winandy, J. E. Spectroscopic analysis of Southern pine treated with chromated copper arsenate. I X-Ray Photoelectron Spectroscopy (XPS). J. Wood Chem. Technol. 1988, 8, 413–439. (7) Pizzi, A. Chromium interactions in CCA/CCB wood preservatives Part II. Interactions with lignin. Holzforschung 1990, 44, 419– 424. (8) Nygren, O.; Nilsson, C. A. Determination and speciation of chromium, copper and arsenic in wood and dust from CCAimpregnated timber. Analysis 1993, 21, 83–89. (9) Maas, R. P.; Patch, S. C.; Stork, A. M.; Berkowitz, J. F.; Stork. G A. Release of total chromium, chromium VI and total arsenic from new and aged pressure treated lumber. Technical Report 02–093; University of North Carolina-Asheville Environmental Quality Institute: 2002. http://www.bancca.org/CCA_References/ EQI_Arsenicreport.pdf. (10) Solo-Gabriele, H.; Townsend, T.; Cai. Y.; Khan, B.; Song, J.; Jambeck, J.; Dubey, B.; Jang, Y. Arsenic and Chromium Speciation of Leachates from CCA -Treated Wood. Final Technical Report #03–07; Florida Center for Solid and Hazardous Waste Management: Gainesville, Florida, 2003. (11) Song, J.; Dubey, B.; Jang, Y-C.; Townsend, T.; Solo-Gabriele, H. Implication of chromium speciation on disposal of discarded CCA-treate wood. Hazard.J. Mater. 2006, B128, 280–288. (12) Cruz, F. G.; Katz, S. A.; Milacic, R. Determination of hexavalent chromium in CCA-treated building timbers. J. Environ. Sci. Health,PartA Environ. Sci. Eng. 1995, 30, 299–306. (13) Toxicological Review of Hexavalent Chromium (CAS No. 18540– 29–9); U.S. Environmental Protection Agency: Washington, DC, 1998. (14) Kimbrough, D. E.; Cohen, Y. E.; Winer, A. M.; Creelman, L. M; Mabuni, C. M. A. critical assessment of chromium in the environment. Crit. Rev. Env. Sci. Technol. 1999, 29, 1–46. (15) Dahlgren, S. E.; Hartford, W. H. Kinetics and mechanism of fixation of Cu-Cr-As wood preservatives. Part I. pH behaviour and general aspects on fixation. Holzforschung 1972, 26, 62–69. (16) Schmalzl, K. J.; Forsyth, C. M.; Evans, P. D. The reaction of guaiacol with iron III and chromium VI compounds as a model for wood surface modification. Wood Sci. Technol. 1995, 29, 307–319. VOL. 42, NO. 10, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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(17) Kaldas, M.; Cooper, P. A.; Sodhi, R. Oxidation of wood components during chromated copper arsenate (CCA-C) fixation. J. Wood Chem. Technol. 1998, 18, 53–67. (18) Nico, P. S.; Fendorf, S. E.; Lowney, Y. W.; Holm, S. E.; Ruby, M. V. Chemical structure of arsenic and chromium in CCAtreated wood: Implications of environmental weathering. Environ. Sci. Technol. 2004, 38, 5253–5260. (19) SW846 Method 3060A, Test Methods for Evaluating Solid Waste, 3rd ed.; U.S. Environmental Protection Agency: Washington, DC, 1996. (20) Kazi, F. K. M.; Cooper, P. A. Rapid-extraction oxidation process to recover and reuse copper chrome arsenic from industrial wood preservative sludge. Waste Manage. 2002, 22, 293–301. (21) Kazi, F. K. M.; Cooper, P. A. Method to recover and reuse chromated copper arsenate (CCA) wood preservative from spent treated wood. Waste Manage. 2006, 26, 182–186. (22) AWPA. Standard P5–99. Standard for Waterborne Preservatives; American Wood Preservers’ Association Book of Standards: Granbury, Texas, 1999. (23) Radivojevic, S.; Cooper, P. A. Effects of CCA-C preservative retention and wood species on fixation and leaching of Cr, Cu, and As. Wood Fiber Sci. 2007, 39, 591–602. (24) James, B. R.; Petura, J. C.; Vitale, R. J.; Mussoline, G. R. Hexavalent chromium extraction from soils: A comparison of five methods. Environ. Sci. Technol. 1995, 29, 2377–2381. (25) Vitale, R. J.; Mussoline, G. R.; Rinehimer, K. A.; Petura, J. C.; James, B. R. Extraction of sparingly soluble chromate from soils: Evaluation of methods and Eh-pH and Eh-pH effects. Environ. Sci. Technol. 1997, 31, 390–394. (26) Potgieter, S. S.; Potgieter, H. Hexavalent chromium speciation in cement matrix. Am. Lab. 2003, 35, 26–28. (27) Coggins, C.; Hiscocks, P. Chromium on the surface of CCA treated wood. Int. Res. Group Wood Preserv. 1978. Doc. IRG/WP 386. (28) Arar, E. J.; Long, S. E; Pfaff, J. D. Method 218.6. Determination of dissolved hexavalent chromium in drinking water, groundwater, and industrial wastewater effluents by ion chromatography; U.S. Environmental Protection Agency: Cincinnati, Ohio, 1991. (29) AWPA. Standard A21–93. Standard method for the analysis of wood and wood treating solutions by inductively coupled plasma
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ES702885F
