Review
Overview of biochar production from preservative-treated wood with detailed analysis of biochar characteristics, heavy metals behaviors, and their ecotoxicity

https://doi.org/10.1016/j.jhazmat.2019.121356Get rights and content

Highlights

  • Recent applications of biochar from preservative-treated wood are introduced.

  • Fate of heavy metals during biochar preparation is discussed in-depth.

  • Environmental impact of heavy metals retained in biochar should be considered.

  • The challenges and future perspective of biochar utilization is proposed.

Abstract

Concerns over the disposal of preservative-treated wood waste and its related environmental problems are the main driving forces of research into the recycling of preservative-treated wood. Preservative-treated wood waste composed of cellulose, hemicellulose, and lignin with several types of heavy metals can be recycled in various ways, such as wood-based composites, heavy metal extraction, energy recovery, etc. In particular, thermochemical conversion has attracted considerable attention recently because energy can be recovered from biomass as liquid fuel and bio-oil, as well as produce bio-char with a high carbon content, which can be applied to valuable products, such as soil amendment, adsorbents, solid fuels, and catalyst supports. On the other hand, environmental issues, such as heavy metal volatilization and heavy metal leaching, are still a challenge. This review reports the state-of-the-art knowledge of biochar production from preservative-treated wood with the main focus on the feedstock, process technology, biochar characteristics, application, and environmental issues. This review provides important information for future studies into the recycling of preservative-treated woods into biochar.

Introduction

Wood is one of the most valuable natural resources and is used traditionally in outdoor applications and building interiors. On the other hand, wood is an organic material composed of cellulose, hemicellulose, and lignin, which are is prone to biological/chemical corruption by both living organisms and abiotic components. Ross reported that 10% weight loss from fungal attack could reduce the strength of wood by almost 50% (Ross, 2010). Consequently, many studies have been performed to develop wood preservatives focusing on durability improvement and replacement cost reduction (Teng et al., 2018).

Chromated copper arsenate (CCA)-treated wood impregnated with hexavalent chromium (Cr), copper (Cu), and pentavalent arsenic (As) is a representative preservative wood used widely in the industrial field, and is classified into three types depending on the metal content (Ohgami et al., 2015). Humar et al. reported that the disposal amount of CCA-treated wood will reach about 16 × 106 m3 in 2020 (Humar et al., 2004). Because CCA is a water-soluble heavy metal, soil and ground water can be polluted because of their leaching by exposure to rainwater (Aceto and Fedele, 1994). In particular, it has been reported that As can cause lung cancer in humans (Taylor et al., 1989). In addition, Cr can increase the risk of lung cancer (Luippold et al., 2003). Accordingly, CCA-treated wood was regulated in several countries approximately 30 years ago, which led to a rapid worldwide shift to copper-based wood preservatives (Hingston et al., 2001a).

Non-chromated arsenical water-borne preservatives, such as alkaline copper quaternary (ACQ), copper azole, copper citrate, and copper ethanolamine, have been used in numerous timber applications over the past decade (Bolin and Smith, 2011). Among them, ACQ, a mixture of quaternary ammonium compounds (QACs) and copper oxide, is commonly used because of its great biocidal activity against fungal or insect attack (Hata et al., 2003). Despite its relative environmentally friendly properties compared to CCA, it will eventually be thrown away at the end of its life time and its waste management will become a major issue in the near future. Hasan et al. reported that the amount of leached metals was proportional to the degree of retention of the treated wood with higher retention degrees resulting in higher quantities in leachates and more mass of metals lost. Accordingly, Cu leached from the ACQ-treated wood is up to 15 times higher than the Cu leached from CCA-treated wood, which might result in soil and water pollution by eluted copper (Hasan et al., 2010).

Preservative-treated wood waste can be recycled as an energy source or other valuable products via a thermochemical conversion process. Pyrolysis is a thermochemical conversion process performed near 400–500 °C under limited oxygen conditions, which generates bio-oil as the main liquid product along with biochar (solid) and non-condensable gas (Li et al., 2018). Various types of biomass, including wood waste, agricultural residues, forestry residues, municipal solid waste, and animal manures, have been used as feedstock for biochar production (Duku et al., 2011). Preservative-treated wood waste, such as CCA- and ACQ-treated wood, has been suggested as an alternative feedstock for bio-oil and biochar production (Koo et al., 2014; Kim et al., 2012). Recently, there has been increasing interest in understanding biochar as a whole, particularly its prospects and applications to environmental management as well as valuable products because it is a multifunctional material related to greenhouse gas reduction, carbon sequestration, soil fertilization, contaminant immobilization, and water filtration (Ok et al., 2015). According to previous research, the physicochemical characteristics of biochar are clearly influenced by the pyrolysis variables, including the feedstock type, temperature, residence time, and atmosphere (Li et al., 2018). Among them, the feedstock characteristics and temperature are the main parameters affecting the biochar yields and characteristics (Enders et al., 2012; Bird et al., 2011).

The environmental issues resulting from preservative-treated, wood-based biochar production should be considered because trace amounts of heavy metals, particularly As, can volatilize into the atmosphere when pyrolyzed. Most heavy metals are retained in biochar at even higher concentrations than in general soil or contaminated soil (Huang et al., 2018). Therefore, heavy metals can leach into the soil and affect the ecosystem, particularly biomass growth (Li et al., 2019a). When heavy metals are absorbed and transferred into biomass, they can cause metabolic disturbances, arrest cell division, cell death, or alter the structure and functions of various membranes or enzymes (Dadrasnia and Emenike, 2013; Singh et al., 2013).

Biochar application into valuable products, such as adsorbents, catalyst support, and batteries, has undergone rapid development recently (Gu et al., 2015; Mohan et al., 2014; Lee et al., 2017; Qian et al., 2015; Li et al., 2019b). Nevertheless, few review papers have introduced the behavior of heavy metals in preservative wood during biochar production as well as efficient methods to handle these metals. This paper introduces the state-of-the-art knowledge of the fate of heavy metals in preservative-treated wood during biochar production, including (1) the feedstock characteristics, (2) catalytic effect of heavy metals on the thermal degradation behavior of preservative-treated wood, (3) biochar characteristics from preservative-treated wood, (4) risks that heavy metals pose when they escape into the ecosystem during the biochar production process, and (5) heavy metal removal method from biochar. Furthermore, the challenges and future prospects for the efficient and eco-friendly utilization of biochar are discussed. This report is expected to offer easy accessibility to an extensive readership and will be an influential reference for future directions of eco-friendly biochar production from preservative-treated wood.

Section snippets

Chemical composition of preservative-treated wood

All woody biomass is composed of holocellulose (cellulose and hemicellulose) and lignin. Cellulose is a homogeneous polysaccharide comprised of ringed glucose through covalent bonding between the oxygen of the C1-hydroxyl group of glucose and the C4 of the adjoining glucose, called a β-1,4 glycosidic bond and is a major constituent of the primary cell walls of lignocellulosic biomass (Jarvis, 2003). Hemicellulose is heterogeneous polysaccharide with diverse structures composed of xyloglucans,

Thermochemical conversion process for biochar production

Biochar can be produced from a range of thermochemical processes, involving fast/slow pyrolysis and gasification. Each process is distinguished by different reaction temperatures, heating rates, residence times of the volatiles, and atmosphere (N2, O2, and air) (Meyer et al., 2011). Hydrochar is another carbon-rich material possibly produced from lignocellulosic biomass or agricultural residue via a hydrothermal carbonization process (HTC) (Oliveira et al., 2013). This process uses water as the

Fate of heavy metals and their ecotoxicity

Generally, biochar could improve the soil quality and reduce soil ecotoxicity by adsorbing potentially toxic trace elements (As, Cd, Cr, Cu, Hg, Ni, and Zn) and organic contaminants (agro-chemicals, antibiotics, and other hydrocarbons) from soil or water (Palansooriya et al., 2019; Ahmad et al., 2012; Inyang et al., 2016; Liang et al., 2017; Beesley et al., 2011; Tang et al., 2012; Yang et al., 2015; Wang et al., 2015, 2017). Environmentally persistent free radicals (EPFRs), which have an odd

Heavy metal removal process from biochar

As mentioned above, heavy metals, which are generally used in wood preservatives, exhibit ecotoxicity and have a high boiling point. Cu or Cr are inevitably retained in biochar (small amounts of As can be volatilized) (Kim et al., 2012; Jones and Quilliam, 2014). In particular, according to a previous study, liquefaction effectively removes most of the metals (98% As, 92% Cr, and 83% Cu) from the CCA-treated wood, with only approximately 2, 6, and 7% of As, Cr, and Cu, respectively, remaining

Challenges and future perspectives

Recently, biochar obtained from HTC or pyrolysis has been studied widely because pyrolysis or incineration is effective in generating energy (Lucchini et al., 2014). The physical (e.g., water holding capacity, O2 content, and moisture level), chemical (e.g., pollutant immobilization and carbon sequestration), and biological (e.g., microbial abundance, diversity, and activity) properties of the soils can be improved synergistically by adsorbing various toxic compounds by biochar (Gul et al., 2015

Conclusions

The disposal of spent preservative-treated wood becomes more expensive because of strict regulations. Therefore, an economical way to recycle waste preservative-treated wood should be investigated. Generally, carbonaceous biochar, which is produced from pyrolysis, can adsorb heavy metals leached in soil or water, and reduce the ecotoxicity of contaminated sites, simultaneously enhancing the bioavailability of contaminated sites. On the other hand, when the preservative-treated wood is

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2018R1A2B2001121).

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