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Article Cite This: ACS Omega 2018, 3, 6931−6944
Differentiating Inorganics in Biochars Produced at Commercial Scale Using Principal Component Analysis Joyce S. Clemente,* Suzanne Beauchemin, Yves Thibault, Ted MacKinnon, and Derek Smith Natural Resources Canada, CanmetMINING, 555 Booth Street, Ottawa, Ontario, Canada K1A 0G1
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ABSTRACT: Characterizing the inorganic phase of biochar, beyond determining element concentration, is needed for appropriate application of these materials because mineral forms also influence element availability and behavior. Inorganics in 13 biochars (produced from Poultry litter, switchgrass, and different types of wood) were characterized by proximate analysis, chemical analysis, powder X-ray diffraction (XRD), and scanning electron microscopy with energy-dispersive X-ray (SEM-EDX) spectroscopy. Principal component analysis (PCA) was used to compare biochars and characterize associations between elements. The biochars were produced using commercial-scale reactors and represent materials with properties relevant to field application. Bulk inorganic concentration and composition were responsible for differentiating biochars after PCA of chemical data. In comparison, differentiation based on PCA of diffractogram fingerprints was more nuanced. Here, contributions from cellulose and turbostratic crystalline C influenced separation between samples. It was also sensitive to mineral forms of Ca (whewellite and calcite). Differences in crystalline C and Ca minerals separated two biochars generated from the same willow feedstock using the same pyrolysis conditions at different temperatures. PCA of 606 SEM-EDX point scans revealed that inorganics belong to four main clusters containing Ca, Fe, [Al, Si], and [Cl, K, Mg, Na, P, S] consistent with XRD identification of calcite, magnetic Fe-oxide, silicates, and sylvite. It further suggested that amorphous Pcontaining minerals associated with Ca (not identified through XRD) were constituents of willow and poultry litter-derived biochars. However, unlike PCA of XRD, it was not able to differentiate the two biochars derived from willow. The three analysis methods provided different perspectives on the properties of the biochar inorganic phase. Combining information from multiple methods is needed to better understand the inorganic composition of biochars.
1. INTRODUCTION Biochars are carbonaceous residues produced by pyrolysis of biomass and are known to increase moisture retention, improve air permeability, and improve soil structure.1 Biochars contain an organic C (OC) and an inorganic (ash) component. Biochar OC is defined by its condensed aromatic structure content, and this property is rightly the subject of extensive study.2,3 However, there have been fewer studies dealing with biochar ash and it is important to further understand the inorganic components of biomass pyrolysis products as these materials become more widely used. Biochar ash is important because of its acid neutralization potential and because it may influence agrochemical retention, nutrient levels, and microbial populations in soils.4 Inorganics in biochars may come from the feedstock or may be introduced during collection, processing, pyrolysis, or postproduction.5 During pyrolysis, ash concentrations are enriched up to ∼600 °C, above which compounds such as CaCO3 begin to decompose.6 Biochars derived from woody material usually have low ash content (between 0.9 and 4.4 wt % ash), but high ash content of up to 70 wt % has been reported in biochars from cow manure.6−8 It has been demonstrated that considering ash content alone is not sufficient at explaining the behavior of biochars. The © 2018 American Chemical Society
concentration and element composition of inorganics in biochars influence their potential to immobilize environmental contaminants. For example, although removal of Cr(III) was related to ash content, improved removal of Cr(VI) was more specifically attributed to Fe-oxide concentrations rather than total ash.9 In addition, phosphates and carbonates in biochar produced from manure precipitated Pb from solution.10 In a recent study, Clemente et al. found that at acidic pH, removal of cations, such as Cd and Ni, from solution was facilitated by the presence of compounds containing Ca, Fe, Mg, P, and S, whereas Mn interfered with Cd and Ni removal.11 The removal of Pb and Cd from solution by manure-derived biochar modified with MnO2 was shown to be pH-dependent and due to different adsorption mechanisms that included electrostatic interactions, binding to −O and −OH groups, and ion exchange.12 Furthermore, a recent study by Limwikran et al. demonstrated variations in the water solubility and soil diffusivity of K and P, which the authors attributed to variations in the crystallinity of K- and P-bearing phases.13 Therefore, it appears that mineral forms and not just element Received: March 19, 2018 Accepted: June 6, 2018 Published: June 26, 2018 6931
DOI: 10.1021/acsomega.8b00523 ACS Omega 2018, 3, 6931−6944
Article
ACS Omega Table 1. Summary of Feedstock and Conditions Used To Produce Biochars and Wood Ash Feedstock
4
Softwood Airex 1 Airex 2 Pyrovac Hardwood BassWd 400
5 6 7 8
BassWd 450 Willow 450 Willow 500 BasquesC
basswood sawdust willow biomass willow biomass maple trunks and branchesa
9
BasquesF
maple trunks and branchesa
1 2 3
Pyrolysis and combustion conditions
softwood sawdust spruce sawdust softwood bark
fast pyrolysis at 370−425 °C, cooled by water atomization fast pyrolysis at 425 °C, cooled by water atomization fast pyrolysis at 475 °C, 20 kPa reactor pressure24
basswood sawdust
fast pyrolysis at 400 °C, steel shot heat carrier,
