Chemosphere 80 (2010) 438–444

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Chemosphere

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Technical Note

Effects of liquefaction time and temperature on heavy metal removaland distribution in liquefied CCA-treated wood sludge

Hui Pan *

Calhoun Research Station, Louisiana State University Agricultural Center, 321 Highway 80E, Calhoun, LA 71225, United States

a r t i c l e i n f o a b s t r a c t

Article history:Received 28 February 2010Received in revised form 12 April 2010Accepted 13 April 2010Available online 11 May 2010

Keywords:CCA-treated wood recyclingWood liquefactionHeavy metal fractionationSequential extractionMetal recovery

0045-6535/$ - see front matter Published by Elsevierdoi:10.1016/j.chemosphere.2010.04.037

* Tel.: +1 318 6442662; fax: +1 318 6447244.E-mail address: hpan@agcenter.lsu.edu

Wood liquefaction was studied as a recycling method for chromated copper arsenate (CCA)-treated woodwaste. The effects of liquefaction temperature and time on the removal of the heavy metals and their dis-tribution in liquefied CCA-treated wood sludge were investigated. The residue content decreased as thetemperature increased from 120 to 180 �C regardless of the reaction time. It decreased gradually with theincrease of reaction time under liquefaction temperatures 120 and 150 �C. But it decreased as the reactiontime increased from 30 to 60 min then increased when the reaction time increased to 90 min under liq-uefaction temperature 180 �C due to the re-condensation of decomposed wood components. The totalconcentrations of arsenic, chromium, and copper in the sludge samples increased, while the percentageof the removed metals decreased, with increasing liquefaction temperature, which could be related to thechanges of wood residue content and the fate of the heavy metals under different liquefaction conditions.The exchangeable/acid extractable fraction of all three heavy metals decreased as the liquefaction tem-perature increased. At the same time, Cr and As increased in both oxidizable and reducible fractions. Theamount of Cr in the oxidizable fraction increased 40% as the liquefaction temperature increased from 120to 180 �C. The major change of Cu distribution was the increase in reducible fraction with the increase ofliquefaction time. The results of this study suggested that high liquefaction temperature tends to inhibitthe heavy metal recovery when liquefaction is used as a recycling method for CCA-treated wood waste.

Published by Elsevier Ltd.

1. Introduction

Preservative treatment significantly prolongs the useful life ofwood products exposed in extreme environments, thus markedlyreducing the need to harvest our forest while improving thereliability and safety of a variety of structures (Morrell, 2004).Chromated copper arsenate (CCA) was the most commonly usedwaterborne wood preservative in the past several decades due toits efficient protection performance yet inexpensive price. About168 mL of CCA preservative were used in the US in 2007 (Vlosky,2009). Although the useful life of a wood product is extended20–40 times by wood preservative, this durable material must beremoved from service eventually. It was estimated that the amountof spent CCA-treated wood will expand greatly from the currentamounts of 3–4 to around 12 Mm3 y�1 in the US and Canada withinthe next 15 y (Kazi and Cooper, 2006).

CCA-treated wood has been primarily disposed within con-struction and demolition debris landfills, with municipal solidwaste landfills as alternative disposal options. It was reported thatthe concentrations of arsenic and chromium were significantly

Ltd.

high in simulated landfill leachate (Jambeck et al., 2008). In addi-tion, federal regulatory changes led to the installation of landfillfacilities with liners and leachate collection systems, which sharplyreduced the number of available landfills in some regions (Morrell,2004). The development of an environmentally friendly technologyfor spent CCA-treated wood is of great importance to the environ-ment, the treated product industry, and the end-user of CCA-trea-ted wood products.

Wood liquefaction has been used as a method to recycle spentCCA-treated wood products (Lin and Hse, 2005). CCA-treated woodwas liquefied with an organic solvent under similar liquefactionconditions as untreated wood. After liquefaction, liquefied CCA-treated wood was diluted to facilitate the next precipitation step.A precipitant was then introduced to the diluted liquefied CCA-treated wood followed by stirring. Finally, the precipitated sludgecontaining the heavy metals was separated from the supernatantsolution by centrifugation. Precipitated CCA metals were mixturesof three metals and associated with other organic or inorganic sub-stances. Knowledge of the heavy metal species and oxidation statesis important for further separation or recovery of the metals forreuse. A modified three-step sequential extraction procedure wasapplied to fractionate the heavy metals removed by the liquefac-tion–precipitation method (Pan et al., 2009). The results showed

H. Pan / Chemosphere 80 (2010) 438–444 439

that, compared to original CCA-treated wood, the fractionation ofthe heavy metals changed markedly after liquefaction. Most ofthe heavy metals were moved from the oxidizable fraction to theexchangeable/acid extractable fraction, which indicated easieraccessibilities of the heavy metals after liquefaction. It is knownthat wood liquefaction comprises a complex set of reactions takingplace in the polymetric components of wood, including derivatiza-tion such as esterification or etherification of free hydroxyl groupsin cellulose or lignin as well as reactions that break the polymerchain of cellulose or lignin (Krzan and Kunaver, 2006). With theinvolvement of the major wood components (i.e., cellulose, hemi-celluloses, and lignin) and a large number of minor wood compo-nents (e.g., extractives), variation of liquefaction conditions, suchas reaction time and temperature, will cause different liquefactionresults.

The purpose of this study was to investigate the influences ofdifferent liquefaction times and temperatures on the removal ofthe heavy metals from liquefied CCA-treated wood and the frac-tionation of the heavy metals after liquefaction.

2. Materials and methods

2.1. Materials

CCA-treated wood waste was obtained from the same local pre-servative-treated wood product company as in our previous studybut from different product sources. All materials were prepared forliquefaction in the same manner as in our previous paper (Panet al., 2009).

2.2. Wood liquefaction and precipitation of the heavy metals

CCA-treated wood sawdust was liquefied as in our previous pa-per (Pan et al., 2009). Liquefaction solvent to wood ratio was 3/1(w/w). Concentrated sulfuric acid was added as a catalyst at 3%(w/w) of the liquefaction solvent. Three liquefaction temperatures(120, 150, and 180 �C) and times (30, 60, and 90 min) were inves-tigated in this study. Table 1 lists the labels for the liquefied woodfrom each liquefaction condition. Duplicate liquefaction was per-formed at each time–temperature condition. After liquefaction,about 5 g of liquefied mixture was measured out from the wholebatch to determine the residue content. It was diluted by 50 mLacetone/water (4/1, v/v) and followed by filtration. The solid partwas oven-dried over night and the dry residue was weighed. Theresidue content of each liquefied wood was then calculated asthe percentage of the weight of the oven-dried residue over theweight of initial wood used in liquefaction.

Preparation of precipitated liquefied wood sludge was the sameas in our previous paper (Pan et al., 2009).

2.3. Sequential extraction

A modified three-step sequential extraction procedure definedby the Community Bureau of Reference (Rauret et al., 2000) wasfollowed to fractionate the sludge from liquefied CCA-treatedwood. For each extraction specimen, about 0.5 g oven-dried sludgewas precisely weighed to three decimal places into a 50 mL

Table 1Liquefaction temperature and time and the assigned code to each condition.

Time (min) 120 �C 150 �C 180 �C

30 LW1 LW4 LW760 LW2 LW5 LW890 LW3 LW6 LW9

polyethylene centrifuge tube. The extraction procedures were thesame as in our previous paper. Most liquefied CCA-treated woodsludge samples turned to a clear solution after the H2O2 digestionat step 3. A few sludge samples with high wood residue contentwere still cloudy after the H2O2 treatment. 3 mL extra H2O2 wereadded to these samples to completely digest wood materials andto turn the solution into clear liquid. It was assumed that the extra3 mL H2O2 did not change the distribution of the heavy metals inthe sludge sample, based on the following two considerations:(1) H2O2 extraction was the last step of the sequential extractionprocedure in this study; (2) the successive fraction after H2O2

extraction (i.e., the residue fraction) of the heavy metals weremostly released by aqua regia or hydrofluoric acid. However, afairly large amount of residue remained after step 3 for originalCCA-treated wood samples, probably due to the crystalline cellu-lose. This residue was then subjected to hot nitric acid digestionand was denoted as step 4 for original CCA-treated wood waste.

2.4. Determination of metal concentration

About 0.5 g original CCA-treated wood and sludge sample wasdigested with 10 mL nitric acid for the determination of total metalconcentration (AWPA A7-08, 2008). The concentrations of As, Cr,and Cu in each sample were determined by inductive coupled plas-ma atomic emission spectroscopy (ICP-AES). The calculation of per-centage heavy metal removed by liquefaction–precipitation wasthe same as in our previous paper (Pan et al., 2009).

2.5. Statistical analysis

Analysis of variance (ANOVA) was used to analyze the effects ofthe variables (i.e., liquefaction time and temperature) on the resi-due content, percentage of heavy metal removal, and the heavymetal distribution in liquefied CCA-treated wood sludge. The re-sults of the ANOVA are provided in the Supplementary material,SM (Tables SM-1 to SM-3, Fig. SM-1).

3. Results and discussion

3.1. Residue content of liquefied CCA-treated wood

Wood residue content is a direct measurement that indicatesthe extent of the liquefaction reaction. In general, higher liquefac-tion temperature, longer reaction time, and stronger acid catalystresult in less wood residue content, in other words, more completeliquefaction. The line in Fig. 1 shows the residue content of lique-fied CCA-treated wood at different liquefaction temperatures andtimes. The residue content decreased as the temperature increasedfrom 120 to 180 �C regardless of the reaction time. The ANOVA re-sult (Table SM-1) indicated that these two variables had a signifi-cant interaction. The residue content decreased gradually as thereaction time increased from 30 to 90 min at 120 and 150 �C. Butit first decreased when the reaction time was prolonged from 30to 60 min, and then increased when the reaction time further in-creased to 90 min at liquefaction temperature 180 �C. It is verylikely that the effect of the re-condensation reaction became moreprofound at higher liquefaction temperature. Wood lignin, a natu-ral aromatic polymer, has the tendency to undergo a secondarycondensation reaction as a result of relatively mild treatment(Sarkanen, 1963). Condensation reactions could occur among 5-hydroxymethylfurfura derivatives (cellulose degradation products)and aromatic derivatives (lignin degradation products) themselves,and in between, when liquefaction reaction was prolonged, andtherefore generate insoluble condensed residue (Yamada et al.,2001; Kobayashi et al., 2004).

440 H. Pan / Chemosphere 80 (2010) 438–444

3.2. Total concentration and percentage removal of CCA metals

The total metal concentration in liquefied CCA-treated woodsludge is exhibited in Fig. 1. In general, the concentrations of allthree heavy metals increased as the liquefaction temperature in-creased. This result could associate with the decrease of wood res-idue in liquefied CCA-treated wood sludge with increasingtemperature (line in Fig. 1). To avoid an extra processing step forfuture scale-up practice, wood residue was retained in diluted liq-uefied wood and co-precipitated with heavy metals in the sludge.As a result, precipitated sludge with higher wood residue contentcontains more wood, and thus lower relative heavy metal concen-tration. Liquefaction time had very similar effects on the concen-trations of all three heavy metals in the liquefied wood sludge.The concentrations of all three heavy metals increased as the reac-tion time prolonged from 30 to 90 min. However, the increasingrates were not parallel at different liquefaction temperatures,which was indicated by the ANOVA result (Table SM-2) as a signif-icant time and temperature interaction. Fig. SM-1 is a typical inter-action plot of the liquefaction time and temperature. It showed aslow increase of the heavy metal concentrations with the increaseof liquefaction time under lower liquefaction temperature. Then itincreased much faster with the increase of reaction time underhigher liquefaction temperature. This result was attributed to thechanges of residue content of liquefied CCA-treated wood at differ-ent liquefaction conditions. As shown in Fig. 1, the average residuecontent of liquefied CCA-treated wood dropped from around 63%to 25% when the liquefaction temperature increased from 120 to180 �C. As discussed earlier in this section, this drastic decreasecould in turn cause the dramatic increase of heavy metal concen-tration in precipitated sludge.

The percentages of heavy metal removed from CCA-treatedwood waste by liquefaction–precipitation are demonstrated inFig. 2. The ANOVA result (Table SM-3) showed that only liquefac-tion temperature had a significant effect on the removal of the hea-vy metals. There was neither significant effect of liquefaction timenor interaction of these two variables. On average, the percentageof all three elements removed decreased with increasing liquefac-

Fig. 1. Wood residue content in liquefied CCA-treated wood and total metal concentrliquefaction temperatures and times of LW1–9).

tion temperature. The major reason that could affect the removalof all three heavy metals is likely associated with the wood residuecontent at different liquefaction conditions. Many studies haveshown that a large variety of biomass can be used as adsorbentsto remove As (Ghimire et al., 2003; Guo and Chen, 2005), Cr (Tor-resdey et al., 2000; Aravindhan et al., 2004; Ahalya et al., 2007),and Cu (Hammaini et al., 2007). During the liquefaction reaction,oxidative degradation of the wood structure is likely to releasethe heavy metal cluster from its attachment to the wood polymers(Nico et al., 2004). Concurrently, the acidic reaction conditions dur-ing liquefaction could be viewed as the reverse reaction to the CCAfixation process. Therefore, most heavy metals that have been de-tached from wood polymers stayed in liquefied wood solution. Ourprevious study has supported this result (Pan et al., 2009). Duringthe subsequent precipitation process, wood residue could act as anadsorbent to the released metals. As the amount of wood residuedecreased with the increasing liquefaction temperature, fewermetals were captured during the precipitation process.

In addition, changes in speciation of the metals during liquefac-tion may play an important role. For instance, in the case of Cr,Cr(VI) in the original treating solution was believed to reduce toCr(III) during the post-treatment period of CCA-treating processon wood products (also called fixation process). At the same time,it precipitates inorganic materials on the wood cell walls and pro-motes interaction between As and Cu with binding sites in wood(Bull et al., 2000). It was reported that Cr(VI) was not present infully fixed timber and Cr(III) was the dominant chromium speciesin treated wood after fixation (Bull et al., 2000; Bull, 2001).Although Cr(III) could be oxidized to Cr(VI) during the service timeof treated wood products under certain circumstances, Cr(VI)would be easily leached out into the environment due to its bettermobility than Cr(III). Therefore, it could be assumed that Cr(III) isstill the dominant species in CCA-treated wood waste. When thiswood waste was subjected to liquefaction, the condition couldfavor the oxidation reaction of Cr(III) to Cr(VI). Besides the hightemperature, concentrated sulfuric acid was used as the catalystfor liquefaction. It is also known as an oxidizing agent. Therefore,it is possible that a small amount of Cr(III) was oxidized to Cr(VI)

ation in liquefied CCA-treated wood sludge (RC = residue content; see Table 1 for

Fig. 2. Percentage removal of heavy metals from CCA-treated wood waste (see Table 1 for liquefaction temperatures and times for LW1–9).

H. Pan / Chemosphere 80 (2010) 438–444 441

and this portion increased as the liquefaction temperature in-creased. Cr(VI) could escape from subsequent Ca(OH)2 precipita-tion and remain in the solution rather than associate with thesludge. The thermochemical decomposition of CCA-treated woodis complicated and difficult to determine. Chemical speciation ofthe heavy metal after liquefaction is an ongoing study in our laband the result will be reported in future publications.

3.3. Distribution of heavy metal species in original CCA-treated woodwaste and liquefied wood sludge

Fig. 3 shows the metal distribution in the original CCA-treatedwood waste. The majority of As and Cr existed in the oxidizable

Fig. 3. Distribution of heavy metal species i

fraction, likely bound to wood components. The amounts of Aspresent in the exchangeable/acid extractable fraction and the easyreducible fraction were very close (20%). Cr has the least portion inthe exchangeable/acid extractable fraction (6%) yet the highest inthe oxidizable fraction (80%) among these three heavy metals. Cuoccurred principally in the exchangeable/acid extractable fraction(61%). Only about 4% of Cu was present in the oxidizable fraction.All three heavy metals were present in very small amounts (<1%) inthe residue fraction, which is supposed to exist in the crystal struc-tures of primary and secondary minerals such as silicates. How-ever, with treated wood waste, it is more likely that the heavymetals in this fraction were associated with crystalline celluloseand therefore was the most difficult portion to access or be

n the original CCA-treated wood waste.

442 H. Pan / Chemosphere 80 (2010) 438–444

extracted. Another piece of evidence indicating this conclusion isthat this portion of metals no longer existed in the liquefied CCA-treated wood sludge samples.

The distribution of As, Cr, and Cu in the sludge samples are plot-ted in Fig. 4a–c, respectively. Compared to their distributions in theoriginal CCA-treated wood waste shown in Fig. 3, the most drasticchanges were the increase of the exchangeable/acid extractablefraction in most sludge samples. In addition, liquefaction tempera-ture showed an obvious effect on the metal distribution in lique-fied CCA-treated wood sludge. As shown in Fig. 4a, the amounts

Fig. 4. Distribution of heavy metal species in liquefied CCA-treated wood sludge: (a) As,

of As in the exchangeable/acid extractable fraction were all above75% in the sludge samples, compared to only 20% in the originalCCA-treated wood waste. However, this fraction decreased as theliquefaction temperature increased. In accordance with the de-crease of the exchangeable/acid extractable fraction, the reducibleand oxidizable fraction increased.

Fig. 4b shows the distribution of Cr in the sludge samples. Sim-ilar to As, the most apparent change of Cr distribution was the in-crease of the exchangeable/acid extractable fraction as comparedto that in original CCA-treated wood waste. The exchangeable/acid

(b) Cr, and (c) Cu (see Table 1 for liquefaction temperatures and times for LW1–9).

H. Pan / Chemosphere 80 (2010) 438–444 443

extractable fraction slowly increased as the reaction time increasedfrom 30 to 90 min at 120 �C. But it decreased dramatically withprolonged reaction time at 150 �C. This fraction continued decreas-ing at a higher temperature 180 �C then changed little as the reac-tion time increased. The amount of Cr in this fraction decreased byabout 40% when the liquefaction temperature increased from 120to 180 �C. Along with the decrease in the exchangeable/acidextractable fraction was the large increase in the oxidizable frac-tion. The reducible fraction also increased about 10% with increas-ing liquefaction temperature from 120 to 180 �C.

Fig. 4c demonstrated the distribution of Cu in the sludge sam-ples from different liquefaction conditions. Unlike As and Cr, theamount of Cu changed profoundly in both the exchangeable/acidextractable and reducible fraction with changes in liquefactiontemperature and time. The exchangeable/acid extractable fractionincreased gradually when the liquefaction time increased from 30to 90 min at 120 �C. It started to decrease as the temperature in-creased to 150 �C. It kept decreasing as the temperature rose to180 �C and the amounts of Cu in this fraction were even lower thanthat in the original CCA-treated wood waste. The reducible fractionchanged concurrently with the exchangeable/acid extractable frac-tion, however, in the opposite direction. The amounts of Cu in theoxidizable fraction were small and remained at 2–4% in all sludgesamples from different liquefaction conditions.

The results from the sequential extraction strongly suggestedthat high liquefaction temperature would reduce the accessibilityor easy of recovery of the heavy metals in CCA-treated wood wasteby lowering the fraction in the exchangeable/acid extractable frac-tion of all three metals while increasing the oxidizable fraction inthe case of As and Cr and reducible fraction of Cu. Several reportshave been found on the study of metal behaviors in CCA-treatedwood waste when they are subjected to thermochemical treat-ment. The closest research area found to wood liquefaction waslow temperature pyrolysis of CCA-treated wood waste. Kakitaniand coworker (2004) reported that pyrolysis markedly loweredthe solubility of all CCA elements in pyrolysis residues comparedto un-pyrolyzed samples. Another group of researchers applied asequential extraction analysis on low temperature pyrolysisCCA-treated wood residues and found that As and Cu predomi-nantly existed in acid soluble and reducible fractions while Crwas predominantly present in the oxidizable and reducible frac-tions (Broeck et al., 1997). The result of our sequential extractionexperiment also exhibited similar trends regarding the distribu-tions of these three elements in liquefied CCA-treated wood sludge.As reported in our previous paper (Pan et al., 2009), most CCA com-pounds were released from bonds with wood components duringthe liquefaction reaction. However, the exact species of these ele-ments in liquefied CCA-treated wood and their fate during the liq-uefaction and precipitation are as yet unknown. According to someliterature, CCA elements mainly interact with wood lignin and cel-lulose through complexation, precipitation, and adsorption (Pizzi,1982). When the liquefaction temperature is increased and thereaction time prolonged, previously released heavy metals couldre-complex with decomposed wood components. Especially inthe case of Cr, being served as a fixation agent in the originalCCA-treating solution, Cr is supposed to easily form a complex withwood components. As a result, more Cr was re-complexed into oxi-dizable fraction than As and Cu. Metals presented in the reduciblefraction are considered to bind to oxidized compounds, such as Feand Mn oxides. The ICP-AES results showed that the average con-centrations of Fe and Mn in both original CCA-treated wood wasteand liquefied wood sludge samples were about 126 and 114 ppm,respectively, which could be from naturally uptake by trees. Theincreasing amount of the reducible fraction could be due to there-adsorption on Fe/Mn oxides as the redox potential changes dur-ing the increase of liquefaction temperature (Kelderman and Os-

man, 2007). Considering that the total amount of CCA metals wasconstant in the liquefaction–precipitation process, the increase inoxidizable and reducible fractions would in turn result in a de-crease in the exchangeable/acid extractable fraction.

4. Conclusions

Liquefaction temperature and time have significant effects onthe characteristics of liquefied CCA-treated wood sludge. Un-lique-fied wood residue content of liquefied CCA-treated wood de-creased as the liquefaction temperature increased. The reactiontime interacted with reaction temperature to affect the residuecontent. The residue content decreased as the reaction time in-creased under lower liquefaction temperature (120 and 150 �C)but increased under higher temperature (180 �C) probably due tothe re-condensation of decomposed wood components.

The total metal concentration in the sludge sample increasedwith the increase of liquefaction temperature and time. The in-crease rate was higher under higher liquefaction temperature.The highest percent of heavy metals removed from CCA-treatedwood wasted was 97% of As, 88% of Cr, and 89% of Cu and wasachieved at a liquefaction time of 90 min under 120 �C. And thepercentage of heavy metal removal decreased with increasing liq-uefaction temperature. Liquefaction time did not show a signifi-cant effect on the heavy metal removal.

The most obvious change on the distribution of heavy metalspecies after liquefaction was the increase of the exchangeable/acid extractable fraction of all three heavy metals. Liquefactiontemperature also showed an obvious effect on the metal distribu-tion in the sludge samples. The exchangeable/acid extractable frac-tion of all three heavy metals decreased as the liquefactiontemperature increased. At the same time, Cr and As increased inboth oxidizable and reducible fractions. The amount of Cr in theoxidizable fraction increased 40% as the liquefaction temperatureincreased from 120 to 180 �C. The major change of Cu distributionwas the increase in the reducible fraction with the increase of liq-uefaction time.

The results from this study suggested that high liquefactiontemperature tends to inhibit the heavy metal recovery because itlowers the percentage removal of heavy metal and the accessibilityof the removed heavy metals. Although with higher heavy metalremoval and the amount of heavy metal that is easily recoverable,low liquefaction temperature results in higher wood residue con-tent. From a practical point of view, higher wood residue contentwill increase the cost of heavy metal recovery by input of more ef-forts to remove wood residue. Therefore, consideration of bothpoints is of great importance for further study of application of thismethod to CCA-treated wood waste recycling.

Acknowledgements

The author gratefully acknowledges the financial support of thePilot Fund from the Louisiana Board of Regents and the NationalScience Foundation.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.chemosphere.2010.04.037.

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