Electro-removal of copper, chromium, and arsenic from chromated copper arsenate treated waste wood

ABSTRACT Wood is a renewable material considered eco-friendly and used for various purposes. While wood treated with chromated copper arsenate (CCA) does not deteriorate, its final disposal may entail risks due to the concentration and toxicity of the components. The removal of CCA from wood can be achieved in different ways. This study focuses on the reduction of the concentrations of Cu, Cr, and As chemical species by the electro-removal technique, aiming to obtain biomass with low deleterious potential that would allow multiple uses or safe disposal in landfills. The analytical results showed reductions of 79.5, 87.4, and 81.3% in the mean concentrations of Cu, Cr, and As, respectively. It is worth mentioning the occurrence of the fungus Xylaria sp. after treatment 6 (60 min, 5 g, and 25 V), further suggesting that the method was effective. Samples of these fungi were identified from isolates by culture in medium, DNA extraction, and sequencing of the Internal Transcribed Spacer (ITS) region.


INTRODUCTION
In order to avoid deterioration and provide an extension to its useful life, wood is subjected to treatments with chemical preservatives, especially watersoluble preservatives composed of a combination of water-soluble salts (BABAEE; MULLIGAN;RAHAMAN, 2018;OHGAMI et al., 2015;MERCER;FROSTICK, 2014). Chromated copper arsenate (CCA) provides an insecticidal and fungicidal action when applied to the wood. It is currently considered to be the most efficient wood preservative, acting mainly on the permeable outer layers of wood and blocking deterioration by chemical, physical, and biological agents (FERNÁNDEZ-COSTAS et al., 2017;BOTOMÉ et al., 2017;VIDAL et al., 2015). However, concerns due to the high toxicity of the mixture of inorganic compounds with copper, chromium, and arsenic base, together with the possibility of leaching causing deleterious effects on the environment, led to the prohibition of CCA by countries such as the USA, England, Denmark, and Canada (OHGAMI et al., 2015;MERCER;FROSTICK, 2014;JANIN et al., 2012).
In Brazil, the Brazilian Institute of the Environment and Renewable Natural

Resources (Instituto Brasileiro do Meio Ambiente e dos Recursos Naturais
Renováveis -Ibama) allows the industrial use of the CCA with the formulation based on salt contents of chromium (45.7%), copper (18.5%), and arsenic (34. 0%), which is classified as CCA type C (CHAGAS et al., 2015;OHGAMI et al., 2015;VIDAL et al., 2015). Brazil consumes approximately 700,000 m 3 / year of treated wood, a large part of which comes from plantations of eucalyptus (93.5%) and pine (6.5%). Most of this wood is treated with CCA and applied in the production of stakes (62%), poles (30%), railroad ties (5%), and in civil construction (3%), demonstrating that CCA represents an excellent commercial alternative to increase the durability of wood (FERRARINI et al., 2016;VIDAL et al., 2015;. When removed from service life, a large proportion of copper (Cu), chromium (Cr), and arsenic (As) remains in treated wood and can enter the waste stream unless actions are taken to avoid it (FDEP, 2017;TANG et al., 2015;OHGAMI et al., 2015).
After its use, CCA-treated wood generates waste categorized in class I -hazardous, according to the Brazilian Association of Technical Standards (Associação Brasileira de Normas Técnicas -ABNT, 2004), NBR 10004, whose objective is to classify solid waste according to their potential risks to the environment and to public health so that they can be properly managed. The solid and semi-solid residues resulting from anthropogenic activities, including the liquid effluents and debris originated from the wood preservation process, must be subjected to treatments with economically viable techniques and good removal efficiency so they can be discarded in class II -non-hazardous landfill sites.
The final disposal of CCA-treated wood becomes a challenge to the environmental sciences as it is a solid residue with high concentrations of Cu, Cr, and As, which are toxic components with possible harmful interactions with local communities (MOHAJERANI; VAJNA; ELLCOCK, 2018;FDEP, 2017;OHGAMI et al., 2015). Incineration of those wastes is prohibited in many countries, such as Denmark and Brazil, and may pose a risk to human health (FERNÁNDEZ-COSTAS et al., 2017;BOTOMÉ et al., 2017;OHGAMI et al., 2015).
Although the specialized literature points to studies on wood preservatives that are less aggressive to the environment, there are currently no efficient and economically viable substitutes in relation to CCA, whose consumption should continue to increase in the coming decades (MOHAJERANI; VAJNA; ELLCOCK, 2018;VIDAL et al., 2015;. In Brazil, there is no regulation targeted at the disposal of these solid residues, and one of the current challenges is the removal of CCA from treated wood for reuse or recycling (FERRARINI et al., 2016;VIDAL et al., 2015;. According to the National Solid Waste Policy, Law Number 12.305/2010, waste management has the objective of protecting public health and environmental quality, whose responsibility falls on generators and public authorities (BRASIL, 2012). As stated by D' Almeida and Vilhena (2000), the management of solid waste, including its treatment and final disposal, is the responsibility of generators.
Therefore, it is important to develop methodologies for the removal of toxic elements in those types of waste, predicting the difficulties related to their use and proposing viable alternatives for reuse or disposal (SANTOS et al., 2018;TANG et al., 2015;FERRANI et al., 2015). Numerous researchers have turned their attention to the efficacy and use of electro-removal processes in CCA-treated wood compounds as well as avoiding the possible contamination of soils and water from the use of discarded CCA-treated wood at the end of its service life (FDEP, 2017;KUMPIENE et al., 2016;OHGAMI et al., 2015;. Studies in the last decades have shown concern over the remediation of CCA-treated wood by the use of different extractive solvents, such as oxalic acid, phosphoric acid, sulfuric acid, EDTA, hydrogen peroxide, and their mixture in different proportions (MOHAJERANI; VAJNA; ELLCOCK, 2018;KUMPIENE et al., 2016;JANIN et al., 2012). Janin et al. (2012) show that the best cost-benefit ratio lies in the use of sulfuric acid-based electrolytic extractive solutions, but state that oxalic acid has a higher extraction yield when compared to EDTA, phosphoric acid, or sulfuric acid alone. There is also a great interest in the development of effluent recovery technologies resulting from these extractive treatments through techniques, such as coagulation/precipitation with ferric chlorides, route-evaporation of compounds, and exchange of ions through ion exchange membranes (BABAEE; MULLIGAN; RAHAMAN, 2018;FERRARINI et al., 2016;COUDERT et al., 2014;JANIN et al., 2012).
The electro-removal technique is based on the emergence of an electric field in a conductive solution created by a voltage source between the electrode terminals. The electromotive force acting between the terminals of the graphite electrodes through the action of the voltage source constitutes an electric field capable of polarizing the CCA salts. This is achieved by the production of polarization voltages between the molecules and also with the acid electrolytic conductive medium, guiding the movement of ions in the solution and "pulling" the CCA from the wood (TICIANELLI; GONZALEZ, 2015;BRETT;BRETT, 2000). The reactions involved in the electron translation process are: This electric field is capable of mediating an electric force on the ions of the system, which provides electrokinetic movement (MERCER;FROSTICK, 2014;COUDERT et al., 2014;CHRISTENSEN et al., 2006). Thus, the cleaning agent used in the electro-removal process is provided by an electric current that transports the soluble ions (Cu, Cr, and As) that are present in the wood (SANTOS et al., 2018;MOHAJERANI;VAJNA;ELLCOCK, 2018;FERRARINI et al., 2016;TANG et al., 2015;CHRISTENSEN et al., 2006).
Energy dispersive X-ray fluorescence (EDXRF) is one of the techniques recommended by the American Association of Environmental Impact Researchers Electro-removal CCA for Treated Wood, at the Bill Hinkley Center for Solid and Hazardous Waste Management, for quality-quantification of CCA in wood samples (FDEP, 2017).
In this sense, the electro-turning technique should not be confused with the process of adsorption of metals in liquid medium by sawdust of wood since it deals precisely with the process of removal of Cu, Cr, and As metals from wood treated with CCA. The metals removed from the wood (solid material) pass into an electrolytic fluid and can be used again in the production of CCA for the preservation of new wood or to be recovered from the medium by different methods (BABAEE; MULLIGAN; RAHAMAN, 2018;FERRARINI et al., 2016;TANG et al., 2015;JANIN et al., 2012).
In this context, the objective of this study is to investigate the electroremoval of Cu, Cr, and As from CCA-treated wood waste by an electro-removal technique, aiming at the formation of biomass with low deleterious potential that allows multiple uses of the material or even its safe disposal in landfills.

Protocol of clean techniques and preparation of solutions
Clean technique protocols were adopted for the cleaning of glassware and the preparation of storage containers. These were treated in Extran (10%, 24 h), triple washed with distilled water (Quimis ® , Q341-25), bathed with nitric acid (10%, 24 h), and triple washed with ultrapure water. In accordance with the cleaning techniques protocol, the samples and other materials were manipulated in a granite worktop after being cleaned in order to minimize the effects of possible contaminations by chemical species according to the clean techniques protocol.
All reagents used were of analytical grade, and ultrapure water was used for the preparation of solutions (Millipore equipment, Simplicity -UV).

Collection and reparation of samples
The samples destined for the electro-removal treatment were obtained from a public lighting pole donated by the Procel Company of the Frederico Westphalen City. The CCA-treated wood from Eucalyptus grandis has been in use for over 20 years for electric power transmission lines. With the aid of an electric chain saw (Electric chain 2200 W Saber 18"45 cm -Tekna) the material was fractionated into smaller parts and sent to the joinery, where the chip reduction (< 2 cm in length) was performed with the aid of an equipment for shredding organic waste (Trapp brand, TR 200 model), as recommended by Christensen et al. (2006).
Parts of these samples were sent directly to perform the electro-removal process in different experimental conditions according to the factorial design.
The remainder were stored in a double Ziplock (2 L) polypropylene bag, identified with labels (external/internal) made in pencil, and sent for further quantification of the average concentrations of the chemical species of interest.

Electro-removal treatments
The electro-removal treatment system used in this study was mounted on the bench scale ( Figure 1) and operated through the use of a voltage source (Ametek, Sorensen, XPD 120-4.5). During the experiment, different electrical potentials were applied between the electrode terminals of graphite immersed in an electrolytic conducting solution, between which some mass of CCA-treated wood was deposited. Graphite was chosen as the base material of the conduction electrodes because it is an excellent conductive material to use in electrodes, it is cheap when compared to other types of materials and it also enables the reuse of the electrode without the loss of its capacity (MANOSSO, 2006).
The base of the system was formed by a magnetic stirrer that provided smooth movement of the electrolytic conductive solution in order to avoid excessive heating during the treatment. This study opted for the formulation proposed by Christensen et al. (2006), a mixture of oxalic acid and phosphoric acid, which is considered the one with the highest extraction yield.
For each test, samples were prepared in volumes of 450 mL using ultrapure water, 150 mL of oxalic acid at 5% concentration, and 300 mL of phosphoric acid at a concentration of 0.5 mol L -1 . Separate batches of wood samples were packed in a porous polypropylene filter system aligned between the terminals in order to prevent the free dispersion of the sample in the conductive solution.
Quantification of Cu, Cr, and As concentrations Two control samples were obtained by the mechanical pruning of branches using a metal blade (machete), with a diameter greater than 50 and 250 mm in length, of a tree from the Eucalyptus grandis forest in the Universidade Federal de Santa Maria (UFSM) area. These were taken to the laboratory, where they were devoid of the internal and external peels and dried in a circulation oven at 50°C for 120 h. They were then fragmented into powder fractions (sawdust) and sent to mechanical screening in a sieve agitator system, with only fine fractions (< 0.063 mm) being used as white control (without CCA) and in production of reference material in this study.
For the analysis, data were taken in triplicate, with samples of 1 g of mass.
Samples were fragmented in a knife mill equipment (Solab brand, SL 31 model),

Experimental planning, statistical analysis, and graphic constructions
A factorial scheme depends, in part, on the range of factors investigated. Thus, differences at levels large enough to cause alterations in the responses were   Table 2.

Identification of the fungal isolate
It is worth mentioning that the onset of the fungus Xylaria sp. in the samples occurred after treatment 6 (60 min, 5 g, 25 V) and 7 (20 min, 15 g, 25 V), suggesting that the method may be effective. The methodology for extracting DNA from isolates was described by Vicente et al. (2008).
The material was triturated and chloroform was added. The aqueous phase was collected, transferred to a new tube, and had ethanol added to precipitate the DNA. The DNA obtained was quantified by spectrophotometry (260 nm) and its integrity verified in 0.8% agarose gel.
Amplicons were purified with Exonuclease I and Alkaline Phosphatase

Analytical figures of merit and mean concentrations of analytes obtained by dispersive energy x-ray fluorescence spectrometry
The EDXRF technique was demonstrated to meet the requirements of the analytical applications according to the merit figures, ensuring the reliability of the results for the Cu, Cr, and As chemical species. Table 3 shows the results of the mean, confidence interval, precision CV (%), error (%), the determination limit of the method (DML), and the Z-Score test obtained from data on the standard sample.
The results presented in Table 3 show a good agreement between the material's reference and tracing, and also point to the precision of the chemical species analysis by the EDXRF technique, which was inferior to 20%, an acceptable standard for environmental samples (INMETRO, 2018). The error was estimated to be less than 10%, while the limit of determination of the method was compatible with the mean concentrations of the chemical species in the reference material (INMETRO, 2018;ROUSSEAU, 2013).

Experimental planning of the electro-removal technique
The best treatment for the removal of the chemical element chromium (Table 2) was the sixth treatment, which was performed with the longer time (60 min), the lower mass (5 g), and the higher electrical voltage between the terminals (25 V). This treatment resulted in an mean concentration of 15.9 mg kg -1 , providing greater removal of chromium than the control (mean concentration of 124.1 mg kg -1 ), i.e., a reduction of 87.2% over the initial concentration. The eighth electro-removal treatment, performed with the use of the longest time factor (60 min), the greatest mass (15 g), and the highest voltage between the terminals (25 V), provided an average after-removal concentration of 16.6 mg kg -1 .
Finally, the seventh electro-removal treatment, performed with the shortest time (20 min), the greatest mass (15 g), and the highest tension (25 V), reduced the mean concentration to 17.2 mg kg -1 .
For Cu, it is highlighted that the seventh electro-removal treatment presented a mean concentration of 15.6 mg kg -1 after the treatment ( Table 2). The treatment had shorter applied time (20 min), lower mass (5 g), and higher tension between the terminals (25 V). In relation to the control, the best electro-removal treatment for copper was the sixth. This treatment used the greatest time factor (60 min), the smallest mass (5 g), and the highest voltage between the terminals (25 V) and resulted in a mean concentration of 12.1 mg kg -1 in the treated wood, leading to a greater copper removal than the control (mean concentration of 59.1 mg kg -1 ), i.e., a reduction of 79.5% in relation to the initial concentration.
The highest reduction of mean arsenic concentration in the samples (Table 2) et al. (2000) obtained yields of CCA removal from treated wood in the order of 93% for Cu, 95% for Cr, and 99% for As due to process optimization of experimental conditions with the use of membranes and ion-exchange electrodes.
The factorial design shows that the treatment conditions for Cu, using greater time, lower mass, and higher tension obtained a better agreement, with a mean of 15.9 mg kg -1 . The best treatment for Cr, also due to the factors mentioned above, obtained a mean of 12.1 mg kg -1 . For As, the best conditions were also represented by greater time, lower mass, and higher tension. Considering these results, it can be concluded that the treatment conditions of the sixth electroremoval treatment were ideal for the removal of Cu, Cr, and As. and As, respectively, all related to treatment 6.
In addition, the study emphasizes the efficiency of the electro-removal treatment of chemical species by reducing Cu (by more than four times in the best treatment in relation to the untreated control), Cr (by more than five times in the best treatment in relation to the untreated control), and As (by more than ten times in the best treatment in relation to the untreated control). The ANOVA test (Table 4) demonstrates the existence of significant differences (p < 0.05) between the different treatments when considering each chemical species in relation to the untreated sample.

Electro-removal post-treatment biological responses
After the wood samples dried out in the circulation oven, it was observed that the formation of fungi occurred in treatment 6, a fact that may be associated with a biological response to the greater efficiency of this treatment in the removal of highly toxic elements present in the samples. Figure 3 shows the formation of fungi in the samples (image taken with a manual microscope; Wireless Microscope, magnification of 50-500x).
Based on the sequence analysis of the internal transcribed spacer (ITS) regions of the rDNA gene, it was possible to identify the isolated fungus Xylaria sp. This region is widely used for the identification of fungi because it is highly intra-specifically conserved but variable among different species, which makes

Electro-removal CCA
The Xylariaceae family is the largest of the Ascomycota phylum, comprising 86 genera, seven of which are uncertain, and more than 1,343 species (LUMBSCH;HUHNDORF, 2010;STADLER et al., 2013). In Brazil, 24 generic names, 213 specific epithets, and 10 infra-specifics ones were listed (PEREIRA, 2015). Members of Xylariaceae play a functional role in terrestrial ecosystems, decomposing a wide range of substrates such as wood, leaves, seeds, and fruits.
The great majority live as saprophytes soon after tree deaths, while other species act as phytopathogenic and endophytic (ROGERS, 1979;WHALLEY, 1985).
Included in the order Sphaeriales and the family Xylariaceae, Xylaria is classified as a white-rot fungus due to the degradation of the lignin present in the chemical constitution of the wood. It can sometimes be described as soft rot, requiring concise macroscopic and microscopic analyses and chemical characteristics to define the type of degradation.
In general, Xylaria sp. has been described as wood-degrading due to its habit of colonizing decomposing material and/or living as saprophytes of woody plants (ALEXOPOULOS;BLACKWELL, 1996). As a general characteristic, the fungi of the species Xylaria sp. have black and rough clavulated stromas, with a diameter and length ranging from 2 to 5 mm and 5 to 7 cm, respectively, simple or branched, and sooty and white dotted surfaces.

CONCLUSIONS
The best Cr removal conditions were obtained with the highest stress, lowest mass, and longest time (60 min, 5 g, and 25 V) of treatment. After the treatment, a contaminated chemical residue composed of Cu, Cr, and As is generated. This residue can be processed for its reuse in the form of CCA preservative, which allows the restart of the treatment cycle of the wood.
Electro-removal, including the use of alternative materials that provide good results, produced reductions in Cu, Cr, and As mean concentrations of 79.5, 87.4, and 81.3%, respectively, than in other techniques commonly used for the removal of CCA. The electro-removal of the CCA preservative present in treated wood proved to be technically feasible. The results showed the proliferation of the fungus Xylaria sp. in the wood samples from treatment six, as well as the satisfactory removal of chemical species from the wood.
This study could represent the basis for other studies aiming to develop a pilot plant that could allow the study of the best treatments for larger amounts of CCA-treated wood.