Rapid Recovery of Metals in CCA-treated Wood

 ABSTRACT

The recovery of heavy metals from chromated copper arsenate (CCA)-treated southern pine wood particles was investigated using binary acid solutions consisting of two of acetic, oxalic, and phosphoric acids in a microwave reactor. Formation of an insoluble copper oxalate complex in the binary solution containing oxalic acid was the major factor for low copper removal. Furthermore, the possible complexation of acetic/oxalic acid in the organic phase, the decomposition of oxalic acid in acetic acid at high temperatures, and the promotion of the formation and precipitation of the copper oxalate by phosphoric acid may induce an antagonistic effect which adversely influenced the effectiveness of the copper extraction. It was found that the addition of acetic acid into phosphoric acid enhanced the chromium recovery rate of the mixed acid solution. This synergistic effect of mixed acetic acid and phosphoric acids is considered one of the most interesting and significant discoveries in the study. The minimal reaction conditions for extracting the maximum percentage of metals was 2.75% phosphoric acid, 0.5% acetic acid, and 130°C. The total recovery rate approached 100% for arsenic, 96.7% for chromium, and 98.6% for copper in a one step process.

INTRODUCTION

Preservative-treated wood products are well known to significantly prolong service life, and thereby extend the forest resource and enhance its sustainability. Inevitably, however, the treated products will become unserviceable either due to mechanical damage or failure, biological deterioration, or obsolescence. It is estimated that approximately 12 million m3 per year of spent CCA treated wood will be removed from service in the United States and Canada in the next 20 years (Kazi and Cooper 2006). Disposal of this material has become a major concern because of its residual toxic chromated copper arsenate (CCA) content, in particular the arsenic and chromium. Previous studies have shown that CCA compounds can be gradually leached into the environment (Townsend et al. 2005; Moghaddam and Mulligan 2008). Conventional waste disposal options for spent preserved wood, such as land-filling, are becoming more costly or even impractical because of increasingly strict regulatory requirements and liability concerns. The burning of treated wood may be extremely dangerous to the environment and human health, particularly if the wood has been treated with CCA. There is an urgent requirement for the development of techniques to effectively recycle decommissioned CCA-treated wood.

Fixation is a chemical process in which the preservative chemically bonds to the wood. It is well recognized that exposure of CCA-treated wood to an acid solution can re-oxidize the chromium thereby converting the CCA elements into their water-soluble form. Thus, acid extraction using different acids and wide ranges of reaction conditions has been extensively studied for removal of CCA from out-of service CCA-treated wood (Shiau et al. 2000; Clausen 2004; Humer et al. 2004; Kazi and Cooper 2006; Gezer et al. 2006; Kakitani et al. 2007; Janin et al. 2009). These studies have shown that the recovery of CCA elements from CCA-treated wood can be obtained with many organic acids, such as oxalic, acetic, citric, and formic acids; however, the acid extraction process is slow. Therefore, cost-effective acid extraction methods are lacking.

Our recent studies on removal of CCA elements from spent CCA-treated wood have focused on the application of the microwave energy to facilitate acid extraction. A preliminary study (Yu et al. 2009) has shown: (1) microwave-assisted acid extractions with oxalic, acetic, and phosphoric acids have reduced the reaction times from hours to minutes compared to the conventional methods, (2) oxalic acid effectively removed arsenic and chromium but not copper, (3) acetic acid extraction was highly effective for the removal of arsenic and copper but not for chromium, and (4) extraction using phosphoric acid was less effective as compared to both oxalic and acetic acids. These results indicated that none of the individual acids were able to effectively remove all three CCA elements simultaneously, but showed a potential complementary effect for extraction. For instance, oxalic acid removed chromium very effectively but not copper, and acetic acid effectively extracted copper but not chromium. The results strongly suggested the opportunity for a two-acid process by either a synergistic extraction effect of the combined acids or a two-step process of consecutive acid extraction. In this study, two acids were mixed, and the extraction potential was evaluated. The acids studied were oxalic, acetic, and phosphoric. The objective of this study was to develop a cost-effective microwave-based dual acids extraction system to maximize removal of CCA elements from spent-CCA-treated wood. This was addressed by optimizing of the acid combinations and concentration, extraction times, and reaction temperature to minimize any environment impact.

To read more please visit our publication: Rapid Recovery of Metals in CCA-treated Wood

Todd Shupe is the President of DrToddShupe.com and is a well recognized expert on wood-based housing and building materials, wood decay and degradation, and wood science. Shupe worked as a professor and lab director at LSU for over 20 years. He is active in several ministries including his Christian blog ToddShupe.com. Todd is the President of the Baton Rouge District of United Methodist Men, and Board Member for Gulf South Men and a Team Leader for The Kingdom Group. He is a volunteer for the Walk to Emmaus, Grace Camp, and Iron Sharpens Iron. Todd is a Men’s Ministry Specialist through the General Commission of United Methodist Men and is in training to be a Certified Lay Minister through the Louisiana Conference of the United Methodist Church.

Influence of solvent type on microwave-assisted liquefaction of bamboo

Influence of solvent type on microwave-assisted liquefaction of bamboo

Microwave-assisted liquefaction of bamboo in glycerol, polyethylene glycerol (PEG), methanol, ethanol, and water were comparatively investigated by evaluating the temperature-dependence for conversion and liquefied residue characteristics. The conversion for the liquefaction in methanol, ethanol, and water increased with an increase in reaction temperature, while that for liquefaction in glycerol and PEG was converse. The results of Fourier transform-infrared spectra for the liquefied residues revealed that cellulose was the main resistance to bamboo liquefaction in methanol, ethanol, and water. Glycerol could be selected as a commendable liquefacient for the solvolysis of bamboo components at low temperature using microwave energy. Moreover, liquefaction behaviors in glycerol and methanol under different temperatures were also distinguished by scanning electron microscopy images.

Bamboo has become one of the most important non-timber forest products in China and other Asian countries. This is primarily due to its rapid growth rate, availability, renewable nature, high productivity, short maturity cycle, and multiple uses. Currently, bamboo has been used in the preparation of high-value added products, such as panel, parquets, furniture, and structural composites. However, in the manufacturing of bamboo-based materials, the epidermis and wax layer of bamboo are usually split off. This is because of the poor wettability or penetration of these portions for subsequent treatments, for example, coating and preservative treatments, etc. Thus, large quantities of bamboo processing residues, such as epidermis, are cast aside as waste.

Recent achievements in techniques for converting woody materials into value-added liquid products under mild conditions using organic solvent and an acid catalyst have stimulated certain studies focused on evaluating bamboo as a raw material for manufacturing bio-products. Several studies have been conducted to formulate liquefied bamboo for bio-polyols and polyurethane foams (Yip et al. 2009; Zhang et al. 2013; Liu et al. 2008; Gao et al. 2010). Although pilot-scale evaluation of liquefied bamboo as chemical feedstocks for the preparation of polyurethane foams have shown encouraging results, an economically viable bamboo waste conversion technology is yet to be realized because of the high cost of the liquefaction process. Alcohols have been proven to be effective solvents for liquefaction of lignocellulosic biomass (Xu et al. 2013; Toor et al. 2013). The benefit from using alcohols in liquefaction is that they can be easily recovered after liquefaction.

Microwave energy has recently been applied in the liquefaction of lignocellulosic biomass (Pan et al. 2012; Li et al. 2013; Xiao et al. 2013). In a microwave heating system, microwave energy penetrates and produces a volumetrically distributed heat source; heat is generated throughout the material and leads to faster heating rate and improved kinetics as compared to conventional heating. However, microwave dielectric heating is based on the ability of a specific reagent or substance to absorb such radiation and convert it into heat at a given frequency (Cinta et al. 2014). Thus, solvents with different specificities that will be applied in a microwave-assisted liquefaction system may have an influence on the liquefaction behavior of biomass. In this study, liquefaction of bamboo with five different solvents (glycerol, PEG, methanol, alcohol, and water) using microwave energy was systematically investigated. The chemical structure and the surface morphology of the liquefied residues from different reaction conditions were comparatively analyzed. The specific objective of the study is to provide a primary understanding of the influence of solvents on the extent of liquefaction with microwave as heating energy.

Todd Shupe is the President of DrToddShupe.com and is a well recognized expert on wood-based housing and wood science.  Shupe worked as a  professor and lab director at LSU for over 20 years. He is active in several ministries including his Christian blog ToddShupe.com. Todd is the Secretary of the Baton Rouge District of United Methodist Men, Database Coordinator for Gulf South Men, and volunteer for the Walk to Emmaus, Grace Camp, Iron Sharpens Iron, Open Air Ministries, HOPE Ministries food pantry. Todd is currently preparing to be a Men’s Ministry Specialist through the General Commission of United Methodist Men.