Influence of chromated copper arsenate type-C concentration on the quality of treated fence posts

This study aimed to reduce the concentration of chromated copper arsenate type-C (CCA-C) in preservative treatments of Eucalyptus sp. for use in applications with ground contact. The genetic materials used were Eucalyptus cloeziana of seminal origin and the clonal hybrid (E. urophylla x E. camaldulensis), which was derived from six year old crops at commercial plantations. The basic density, moisture content, percentage of sapwood, and wood treatable volume were determined, and the anatomical structures were characterized. The genetic materials were treated with concentrations of 1.2%, 1.3%, 1.4%, 1.5%, and 1.6% active ingredients (AI) of CCA-C. The quality of the preservative treatment was evaluated via penetration and retention tests. The treated wood volume of E. cloeziana exhibited 44% irregular partial penetration, 40% vascular penetration, 12% peripheral partial penetration, and 4% no penetration. For the hybrid, the wood volume exhibited 68% total penetration, 24% peripheral partial penetration, and 8% irregular partial penetration. The concentrations of 1.4% AI and 1.5% AI for E. cloeziana and of 1.4% AI and 1.6% AI for hybrid reached the minimum retention required by standard.


Introduction
Native forest woods are used in civil construction and rural buildings because they have natural resistance and durability. However, the use of this raw material source without proper replacement has led to scarcity. Therefore, it has been necessary to replace native woods with other rapidly grown materials from planted forests (Paes et al., 2001;Raminelli et al., 2019;Vivian et al., 2020).
Despite their abundance and rapid growth, reforestation woods have low natural durability, and treatment with preservatives is necessary to increase their resistance to attack by xylophagous organisms (Vivian et al., 2020).
In Brazil, the market for treated wood remains smaller than the pulp and paper, reconstituted panels, timber, and biomass energy industries. The production of wood intended for preservative treatment reached an estimated 1.4 million m 3 in 2018. The main products were fence posts, poles, and crossties (IBÁ, 2019).
Chromated copper arsenate type-C (CCA-C) is considered the best preservative and the most used in many countries.
In the United States, it dominated the wood market between 1970 and 2004. Since then, its use was restricted to industrial applications, and its retail trade was prohibited. In Brazil, it is applied to approximately 90% of treated wood because it has lower leaching and better field performance when compared to other preservatives available on the market (Lebow, 2010).
Several technical standards define quality parameters for product marketing, and they highlight the penetration and retention performance of the preservative (Hunt;Garratt, 1967). The Brazilian Technical Standard NBR 9480 (ABNT, 2009) states that the preservative should penetrate 100% of the sapwood, and the retention should be at least 4.0 kg m -3 , 6.5 kg m -3 , or 9.6 kg m -3 for components that do not have contact with the ground (construction wood), those that have contact with the ground (fence post), and utility poles and crossties, respectively.
Several studies have evaluated the quality of wood treated with CCA-C. Fence posts made of Eucalytpus urophylla clones aged nine years treated with 1.5% active ingredients (A.I.) and 1.7% AI of CCA-C and compressed at 12 kgf cm -2 for 60 min achieved deep and regular fence post penetration (Amaral, 2012). In addition, Schneid et al. (2013) produced poles of approximately 18-year-old E. saligna and E. cloeziana treated with 2.0% AI of CCA-C and exposed them to 60 min, 90 min, and 120 min of pressures of 10 kgf cm -2 and 12 kgf cm -2 . They observed that only E. saligna poles treated with a pressure of 12 kgf cm -2 for 120 min reached the minimum retention (9.6 kg A.I. m -³) required by NBR 9470 (ABNT, 2009). Valle et al. (2013) investigated the penetration and retention in E. urophylla wood aged 5.3 years, treated with 2.0% A.I. CCA-C, and exposed to 90 min of 12 kgf cm -² pressure. These procedures resulted in deep and regular penetration for all of the pieces treated with CCA-C and average retention above the minimum (6.5 kg A.I. m -3 ) required by the Brazilian standard NBR 9480 (ABNT, 1986) (Valle et al. 2013). In their investigation of the retention of CCA-C in fence posts of Corymbia torelliana (11 years old) and E. grandis × E. urophylla (9 years old) treated with a concentration of 2.0% A.I. of CCA-C and exposed to 60 min under pressure, Lopes et al. (2017) noted that the average retention for both genetic materials was higher than the minimum requirement by the standard (6.5 kg A.I. m -3 ).
To meet minimum retention requirements, higher CCA-C concentrations are used during treatment, which increases the price of treated wood because treatment usually constitutes 50% of the total production cost. As such, the preservative treatment is particularly costly. Research that aims to reduce the preservative concentration while meeting the requirements of relevant standards is essential both economically and environmentally. Therefore, the present study aimed to reduce the concentration of CCA-C in preservative treatments of Eucalyptus sp. for use in applications with ground contact.

Methodology
The genetic materials used were Eucalyptus cloeziana of seminal origin and the clonal hybrid (E. urophylla × E. camaldulensis), which was derived from six year old crops at commercial plantations. For each genetic material, 25 fence posts of 2.20 m in length and from 8 cm to 10 cm in diameter were collected randomly from the drying stacks of a forestry company's yard in Martinho Campos, Minas Gerais, Brazil.
From each fence post, 3 cm (base and top) were discarded to avoid impurities and cracks. A 5-cm portion was also removed at one of the ends to determine the basic density according to NBR 11941 (ABNT, 2003) and the moisture content as NBR 7190 (ABNT, 2010), and the sapwood percentage, treatable wood volume, vessel diameter, and frequency were measured.
To determine the percentage of sapwood and treatable wood volume, cross measurements of the total diameter and the heartwood diameter were performed on each disk with the aid of a ruler with a precision of 1 mm. The length of the fence posts was also measured. From these measurements, the percentage of sapwood and the treatable volume of fence posts were calculated. It was not possible to visually differentiate the heartwood from sapwood in E. cloeziana.
To determine the diameter and frequency of the vessel, temporary slides were made with histological sections of the analyzed material. The measurement of anatomical characteristics was performed according to the IAWA Committee (1989), using an Olympus BX41 microscope with an attached camera and WincellPRO image analysis software.
The resultant fence posts containing the same characteristics were treated with CCA-C in an autoclave. The product used was LIFEWOOD 60 composed of 28.5% hexavalent chromium (CrO3), 11.10% bivalent copper (CuO), 20.40% pentavalent arsenic (As2O5), and 40% water, supplied by Koppers Performance Chemicals Brasil Comércio de Preservantes LTDA. For each preservative treatment, five fence posts of each genetic material were selected for quality testing. Each batch of fence posts was treated with a certain A.I. concentration (Table 1).   For the retention test, 5 equidistant increments were removed from each fence post (Figure 2). They were then oven-dried at 103 ± 2 °C for 6 h. They were weighed (dry mass obtained through a precision scale) and the length and diameter (0.01 mm) were measured (using a digital caliper) for volume determination to calculate the anhydrous density (g cm -³).
Subsequently, the incremental positions were ground and sifted. The 40 mesh and 60 mesh samples were oven-dried (103 ± 2 °C) until reaching a constant weight. Then, they were transferred to plastic cuvettes and mounted with Mylar film TF-160 manufactured by Premier Lab Supply located in the city of Port St. Lucie, FL, USA, for X-ray analysis. CCA-C

Irregular Partial Penetration
Peripheral Partial Penetration

Total Penetration
Research, Society andDevelopment, v. 11, n. 4, e21411427088, 2022 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v11i4.27088 5 retention and the concentration of each active ingredient (copper, chromium, and arsenic) of the product in the analyzed samples were obtained by the Lab-X 3500 spectrometer manufactured by Oxford Instruments located in the city of Witney, Oxon, UK, using the anhydrous density as the input variable. CCA-C retention and the concentrations of chromium, copper, and arsenic were indicated by the spectrometer readings. The physical parameters, anatomical parameters, and CCA-C penetration data were subjected to descriptive analysis.
The CCA-C retention data were subjected to analysis of variance (ANOVA) and, when significant, the regression equations were fitted for the variables as a function of the CCA-C concentrations. The correlation between CCA-C retention and wood characteristics was also investigated.
Before the ANOVA, the normality of the data was verified via Kolmogorov-Smirnov test, and the homogeneity of the variances was verified via Cochran test. Statistical analyses were performed using the R software, version 3.2.4. (R Core Team, Vienna, Austria).

Analysis of wood characteristics
The minimum, average, and maximum values of the anatomical and physical parameters for the studied woods are presented in Table 2.
Eucalyptus cloeziana had the lowest average moisture content and the highest basic density, while the hybrid had the highest moisture content and the lowest basic density. The average moisture values found for both materials are by the recommendations of authors such as Santini (1988).
It was not possible to distinguish heartwood and sapwood in Eucalyptus cloeziana. Regarding the diameter of the vessels, the results found in this study for both materials are in agreement with results observed by other authors such as Evangelista (2011). However, to the frequency of vessels, the results found here for the analyzed materials differ from the results of the cited author. This difference was probably due to the particularities of the materials, such as, for example, the water regime of the place where the evaluated genetic materials were planted. The values in parentheses are the standard deviations (SD); N.D. = Not Distinct. Source: Authors.

Penetration test of the CCA-C in wood
The wood volume of E. cloeziana exhibited 44% irregular partial penetration, 40% vascular penetration, 12% peripheral partial penetration, and 4% no penetration, which was in accordance with the patterns reported by Sales-Campos et al. (2003). These results are similar to those found by Chagas et al. (2015) in Tectona grandis wood.
However, these results differed from those observed by Schneid et al. (2013)  For the clonal hybrid, the wood volume exhibited 68% total penetration, 24% peripheral partial penetration, and 8% irregular partial penetration, which was similar to the results obtained by Amaral (2012) and Valle et al. (2013) for a clone of E. urophylla.
The insufficient penetration in E. cloeziana may have been due to its higher vessel frequency per square millimeter, as its vessels showed a smaller average diameter ( Table 2). The reuse of preservative solution increases impurities in the CCA-C solution, which can produce a large number of particles that make penetration difficult.

Retention test of the CCA-C in wood
For E. cloeziana, the lowest (4.3 kg m -3 ) and the highest retention values (7.5 kg m -3 ) were observed at concentrations of 1.3% and 1.5% A.I. of CCA-C, respectively. For the clonal hybrid, the lowest (4.6 kg m -3 ) and the highest retention values  (Table 3). The values in parentheses are the standard deviations (SD). Source: Authors. For both genetic materials, the retentions below the minimum stipulated by NBR 9480 (ABNT, 2009) may have been due to poor distribution or penetration of preservative in the fence posts.

Functional relation between retention and concentration of the CCA-C
When the equations fitted for E. cloeziana were analyzed, the linear model ( Figure 3) was found to be significant, and it showed the best data behavior. The determination coefficient (R²), indicated that approximately 15% of the total retention variation could be explained by variation in the CCA-C concentration.
The fitted equations for the clonal hybrid showed that the relationship between retention and concentration exhibited better behavior with the quadratic model ( Figure 3). The determination coefficient indicated that 82% of the total variation in the CCA-C retention could be explained by variation in the CCA-C concentration. Analysis of the correlations between retention and moisture content and between retention and wood basic density for E. cloeziana and the clonal hybrid found no significant correlations among the variables. However, for the clonal hybrid, correlation coefficients were 0.44 and 0.47 when the retention was associated with the percentage of sapwood and with the treatable volume, respectively.

Conclusion
For the E. cloeziana fence posts treated with concentrations of 1.4% and 1.5% A.I. of CCA-C, the retentions were satisfactory for use in applications with soil contact according to NBR 9480 Standard (ABNT, 2009). In addition, for the clonal hybrid, woods treated with 1.4% and 1.6% A.I. of CCA-C also meet the requirements for use in applications with ground contact as per the standard. The other concentrations used reached the minimum stipulated for use in structures without contact with the ground according to the standard.
It is recommended to carry out tests with xylophagous organisms using the same genetic materials used in this study and with the same treatment parameters to evaluate the toxicity conferred on wood by the preservative products to these organisms. Moreover, it is recommended preservative treatment in woods with greater moisture and diameter variation to observe the influence of these variables on solution retention.