Mutagenic activities of biochars from pyrolysis
Graphical abstract
Introduction
The pyrolysis of biorefinery residuals (Hayes et al., 2005, Hayes, 2009) and of residues of bioenergy processes that utilise lignocellulosic feedstocks, agricultural residues and organic wastes gives a bio-oil, biochar, and syngas (Kwapinski et al., 2010). The biochars have the capacities to replenish soil carbon pools, to restore soil fertility, to sequester CO2, and to have plant growth promoting and soil amendment properties (in Lehmann and Joseph, 2015). The term “BIOCHAR” is used for the black carbon (BC) “produced by the thermal decomposition of biomass under limited or absent oxygen, and used as a soil amendment to increase fertility and sequester atmospheric CO2” (Mukherjee et al., 2011). Sohi et al. (2010) have suggested that the term “biochar” should be reserved for biomass-derived char produced specifically for application to soil.
Because the production and the applications of biochars are emerging globally as novel industrial products with commercial potential (in Lehmann and Joseph, 2015), it will be important to focus more attention on the sources and on the reserves of resources for the production of the biochars, and on biochar characterization and testing services.
Emphasis has focused on physicochemical parameters, such as sorption properties, surface area, and compositional and structural features of biochars. There is information about the formation and retention of various polyaromatic hydrocarbons (PAHs), ubiquitous environmental pollutants (Mayer et al., 2012). PAHs, and other newly formed molecular signatures not yet identified may be formed during pyrolysis. These might have mutagenic characteristics that could affect the DNA of environmental organisms that contact the biochar material (Furihata and Matsushima, 1986). The evolution of PAHs has been reported in the temperature range of 400–600 °C, and the recent publication by Madej et al. (2016) has shown `that, in research with a laboratory scale pyrolyser, PAH concentrations were low at temperatures of 500, 600, and 700 °C when carrier gas (N2) flow rates were high, irrespective of the O2 content (up to 2%), or of the feedstock. Bucheli et al. (2015) have pointed out that PAHs are predominantly pyrosynthesized in the gas phase. Hence at low carrier gas flow rates PCA molecules may be sorbed to the surface of the biochar or entrapped within the biochar matrix. Ke Sun et al. (2014) studied the sorption of dibutyl phthalate and of phenanthrene (PHE) by plant- and manure-derived biochars produced at 300 and 450 °C. Though the surface area of the plant-derived biochar was much higher than that of the manure-derived product, sorption was greater for the manure-derived biochar formed at the higher temperature. That was attributed to the higher surface C content of the animal–derived product compared with the bulk C; π–π interactions were considered to be the dominant mechanism of interaction in the case of the PHE sorptive at 450 °C.
It is important to develop and validate a series of suitable protocols for the ecological monitoring of the quality and properties of the pyrolysis biochar products, and to initiate the establishment of regulatory constraints and procedures required for implementation in the product characterization chain.
The objective of this study was to address potential adverse environmental impacts that pyrolysis processes and products such as biochar can have, and to validate an approach to detect mutagens in biochar products. A previous assessment was made by Anjum et al. (2014) of the mutagenic potential of pyrolysis biochars using Salmonella/mammalian-microsomal mutagenicity testing (the Ames test). Their results indicated that hemp biochar had significantly higher mutagenicity potential than wood biochar. Our study has examined the suitability of two of the Ames tester strains of Salmonella typhimurium, TA98 and TA100 (also used by Anjum et al.) for determining the mutagenic potential of biochars from a variety of different feedstocks. These strains were developed to contain mutations in the host histidine gene which make the strains autotrophic (require histidine for growth) and a series of other mutations such as uvrB which eliminates accurate excision repair of damaged DNA and rfa which eliminates the lipopolysaccharide coat of the tester strain making it permeable to bulky chemicals. The TA98 strain contains a hisD3052 (a − 1 frameshift mutation) while TA100 contains a hisG46 point mutation (GAG–leucine for a GGG–proline). In addition both strains contain the mutation enhancing plasmid pKM101 which enhances chemical mutagenesis in host Ames tester strains (Ames et al., 1975, Maron and Ames, 1983, Arimoto et al., 1981, Levin et al., 1982, Jurado et al., 1993, Mortelmans and Zeiger, 2000). The basic test examines reversion to histidine prototrophy by examining the potential of a test substance to revert the histidine mutations such that the tester strains may now grow in the absence of added histidine. Since mutagens on occasion may require metabolic activation, potential mutagenic substances are also tested following treatment with S9 liver mixture to assess the presence of so called pro mutagens (Mortelmans and Zeiger, 2000). We wished to apply the test to detect potential mutagenic components extracted from biochars with dimethyl sulfoxide (DMSO) as solvent with and without metabolic activation via the rat liver S9 mix. (Water might be regarded as a more appropriate solvent since it is the solvent that operates in the soil environment to which biochar would be applied. DMSO was chosen because it is often the solvent of choice in mutagenicity testing and it is a good extractant for lipid products. Such products can be enriched in incomplete combustion and in biochars formed at the lower pyrolysis temperatures. It can be predicted that DMSO will extract compounds of different polarities, etc., that will not dissolve in water, and some of these additional compounds may be mutagenic.) The use of DMSO as solvent would result in initial base line data for further environmental testing, monitoring, and evaluation of biochar quality and properties when produced on an industrial scale.
Section snippets
Biomass feedstocks, pyrolysis condition, and morphologies of biochars
Pig manure, cow manure, calf manure, sawdust, Miscanthus, (a C4 grass, Miscanthus × giganteus), and solid municipal waste were used to prepare biochars. A laboratory scale pyrolysis reactor (5 cm i.d.) was used to produce the biochar under slow pyrolysis conditions. Temperatures of 400 ± 10 °C; 600 ± 10 °C; 800 ± 10 °C, and residence times of 10, 30 and 60 min in an N2 atmosphere were used. After pyrolysis the biochar was allowed to cool, in an atmosphere of N2, in a cooler zone of the reactor. The
Results and discussion
Many of the studies on biochars have focused primarily on comparisons of biochars produced by different thermal processes, on operational conditions and feedstock varieties, and on formulations based on their physico-chemical properties and soil and plant growth benefits. Fig. 1 shows the morphologies of biochars produced from Miscanthus, and pig manure. In the case of Miscanthus, a high porosity, based on the plant cell structures, is evident. The manure biochar, where there is an absence of
Conclusions
Data have shown that the biochars produced from sawdust and Miscanthus × giganteus grass at 400 and 600 °C were not mutagenic under the test conditions, whereas the presence of frame-shift mutagen(s) which require metabolic activation, as well as deletion mutagens which do not require metabolic activation were detected in pig manure biochar produced under the same conditions. Consideration might be given to the incorporation of the methods described into routine evaluation of biochar products, and
Acknowledgements
The authors acknowledge financial support via a research grant scheme from the Science Foundation Ireland (SFI) (www.sfi.ie), the DIBANET project supported by European Union Seventh Framework Programme (FP7), HKPB Scientific Ltd (www.hkpb.ie) and Irish Research Council for Science, Engineering and Technology (IRCSET) (www.ircset.ie). They would like to acknowledge the helpful comments made by the referees.
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