Elsevier

Science of The Total Environment

Volume 661, 15 April 2019, Pages 685-695
Science of The Total Environment

Physicochemical property and colloidal stability of micron- and nano-particle biochar derived from a variety of feedstock sources

https://doi.org/10.1016/j.scitotenv.2019.01.193Get rights and content

Highlights

  • Micron- and nano-particle biochars have more O-containing functional groups and minerals than bulk biochar.

  • Plant waste micron- and nano-particle biochars have more functional groups and aromatic rings than municipal waste biochars.

  • Municipal waste micron- and nano-particle biochars are rich with carbonates, phosphates, and aluminosilicate.

  • Colloidal stability of micron- and nano-particle biochars increases with reduced particle sizes and is affected by pH and IS.

Abstract

The ever-increasing land application of biochar may raise the environmental issue of micronparticle (MP) and nanoparticle (NP) biochars for their high mobility or as a carrier to facilitate transport of contaminants in soil. In this study, a variety of biochars were produced from pyrolysis of nine biomass sources and then subjected to the extraction of MP and NP biochars. The diverse physicochemical properties and electrokinetic stability of MP and NP biochars were further investigated. MP and NP biochars accounted for 1.43–20.5% and 0.99–15.3% of bulk biochar and had colloidal particle diameters mainly smaller than 1 μm and 100 nm, respectively. The MP and NP biochars contained more O-containing functional groups and mineral components but less aromatic clusters than bulk biochar. The yield of MP/NP biochars derived from plant sources such as woods, herbs, and agricultural waste was positively linear to the ash content of their bulk biochars but this relationship wasn't applied to the municipal sourced biochar such as manure and sewage sludge. More condensed aromatic rings and functional groups were found in MP/NP biochar from plant biomass than municipal sourced biochar. However, the latter was rich with minerals like carbonates, phosphates, and silicates. Higher functional groups in the plant sourced MP/NP wheat straw biochar accounted for the extremely high stability to resist the whole range of ionic strength studied, while the municipal sourced MP/NP dairy manure biochar with less functional groups and more minerals were readily destabilized, with the Critical Coagulation Concentration (CCC) values of 75 mM and 100 mM, respectively. Overall, this study revealed the size-dependent characteristics of composition and structure as well as high colloidal stability of MPs and NPs which are helpful for prediction of their environmental fate and risk.

Introduction

Biochar is a carbonaceous material generated from the pyrolysis of biomass at relatively low temperatures (Lehmann, 2007; Tack et al., 2019). Characterized by condensed aromatic structures, high specific surface areas, high multiple porosity, and abundant functional groups, biochar has been proven to effectively serve as recalcitrant carbon pool for carbon sequestration (Zimmerman et al., 2011) and soil amendments for pollution remediation (Chen et al., 2008; Ahmad et al., 2014) or fertility promotion (Jeffery et al., 2011; Novak et al., 2009).

However, due to the highly heterogeneous properties stemming from different feedstock and production conditions, biochar may challenge to the ecological system (Lian and Xing, 2017). Once applied into soil, biochar can be lost from soils laterally to surface runoff and vertically to deeper soil or groundwater, even coupled to a common marine fate via fluvial and atmospheric transport (Qi et al., 2017; Schmidt and Noack, 2000). Especially, colloidal biochar and dissolved biochar with micron or even nanometer diameters have significant mobility in subsurface and downward migration of biochar in soils and aquifers (Qu et al., 2016). Distinguished from the bulk biochar, these fine fractions of biochar are very reactive component participating in earth's carbon-cycling and some terrestrial chemical processing (Major et al., 2010). Colloidal sized biochars are mainly formed by pore collapse and matrix fracture during biomass charring or by breakup due to grinding (Liu et al., 2018), and would release and subsequently transport by fluvial processes. Rapid mass loss of the carbon in biochar (<5%) were observed over an 8.5-year laboratory incubation by Kuzyakov et al. (2014), which might be attributed to the mineralized reactive component of biochar like colloidal biochars. Spokas et al. (2014) suggested that physical deterioration in natural environment like sorption of water and water vapor could stress the physical structure of biochar and led to the release of fine biochar particles. However, under field conditions, the release of these fine particles would vary with climatic conditions, soil hydrology, and surrounding solution chemistry (Spokas et al., 2014; Xu et al., 2017a), which make release and transport of micronparticle (MP) and nanoparticle (NP) biochar unpredictable.

Recently, an emerging attention has been turned to the potential environmental risks of MP and NP biochar (Chen et al., 2017; Wang et al., 2013a; Wang et al., 2013b; Joseph et al., 2014) resulting from not only transport in soil and groundwater, but also as a carrier to facilitate the transport and transformation of soil contaminants (Wang et al., 2013a; Zhang et al., 2010). Wang et al. (2013a) extracted MP and NP biochar from wheat straw and pine needle biochars with two pyrolysis temperatures (350 and 550 °C) by settling and filtration, and conducted column experiments to investigate the transport and retention of MP and NP biochar in water-saturated quartz sand. They reported greater mobility for the biochars of lower pyrolysis temperatures and smaller particle sizes. Chen et al., 2017, Chen et al., 2018 found that nano-size wood chip biochar had a great mobility in paddy soil and enhanced phosphorus transport in alkaline soils. However, fundamental studies on the physicochemical property and electrokinetic stability of MP and NP biochar are scarce.

Qian et al. (2016) investigated the structure and removal effect of Cr(VI) and Cd(II) of colloidal biochars derived from rice straw at 100–700 °C and found that lower temperature biochar colloids contain more carboxyl and hydroxyl which showed the higher reduction efficiency for Cr(IV). Liu et al. (2018) extracted and characterized the property of NP biochar from four biomass sources of agricultural waste polysized at 300–600 °C and further investigated colloidal processes of peanut shell NP biochar. These researches somewhat filled in the gap of fundamental study of structural and colloidal property of biochar fine particles and revealed the inherent connection between bulk biochar and the chemical features of the fraction of MP and NP biochar (Qu et al., 2016; Wang et al., 2013a; Qian et al., 2016). Yet, studies on the effect of feedstock on MP and NP biochar properties and colloidal behaviors are fairly limited. It has been widely reported that composition and chemical structure of bulk biochars were influenced by feedstock sources (Kloss et al., 2012; El-Naggar et al., 2018). Different biomass sources contained various amounts of cellulose, lignin, and minerals, and therefore resulted in various carbon contents, chemical structures and ash contents of biochars (Zhao et al., 2013; Han et al., 2018). For example, manure or sludge biochar always had high ash content and low carbon fraction while the opposite trend was observed for the crop residue biochar (Zhao et al., 2013; Sun et al., 2017). Inherent-minerals in biochar showed varied affinity to organic and inorganic containments, which further affected any particular application for pollution remediation (Xu et al., 2017b). Thus, systematical investigations on how MP and NP biochar vary from different feedstock sources and further influence their environmental behaviors.

The main objective of this study was to explore the difference in composition and conformation between MP or NP biochar and bulk biochar as a function of feedstock. The structure and property of MP and NP were examined by combined techniques, including elemental analysis, X-ray fluorescence spectroscopy, X-ray diffraction, and Fourier transform infrared spectroscopy. The size distribution and morphology of MP and NP biochar were measured by dynamic light scattering and transmission electron microscope. The electrokinetic properties and colloid stability were also investigated through zeta potential measurements and aggregation kinetics.

Section snippets

Preparation of biochar and micron/nano-particle biochar

A total of 9 biochars were produced from five types of feedstock sources through a slow pyrolysis process. The five types of feedstock included woody biomass (pine wood and wood chips), herbaceous residues (barley grasses and wheat straws), agricultural wastes (peanuts shells and rice husks), animal manures (dairy manures and pig manures), and sewage sludge, which could be further divided into two categories with possible consistent property for each: the first three were referred to as the

Fractionation and yield of MP and NP biochar

Extracted suspensions of MP and NP biochars and their corresponding size distributions are shown in Fig. 1. The MP suspensions looked dark-brown while NP suspensions had light-brown colors (Fig. 1a and b). Generally, darker color indicated higher yield of MP/NP in biochars. So, MP biochars accounted for 1.43%–20.5%, while NP biochars were only between 0.99% and 15.3% (Table 1). Among all MP biochars, the barley grass (BG) MP biochar showed the darkest color, possessing the largest percentage of

Conclusion

This study investigated the composition and properties of micron- and nano-particle biochar and derived from a variety of feedstock sources. Although these fine particle biochars accounted for much less portion of the bulk biochar, they may raise environmental concern when large-scale applied in field. Compared with bulk biochars, the micron- and nano-particle biochars were abundant in mineral and in O-containing groups composition, high in alkalinity, and strong in dynamical stability,

Acknowledgments

This work was in part supported by the National Natural Science Foundation of China (No. 21537002, 21777095, 41701571), National Key R&D Program of China (No. 2018YFC1800600), and Shanghai Science and Technology Committee (No. 17DZ1202302).

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