Transport and retention of biochar nanoparticles in a paddy soil under environmentally-relevant solution chemistry conditions

Highlights

• Transport of wood chip biochar nanoparticles is significant in a paddy soil.

• Humic acid enhanced the transport of biochar nanoparticles in the soil.

• Transport of biochar nanoparticles in the soil should take into account the surface charge heterogeneity and roughness of soil.

• Transport of biochar nanoparticles in the soil can be well interpreted by a two-site kinetic retention model.

Abstract

Land application of biochar has been increasingly recommended as a powerful strategy for carbon sequestration and soil remediation. However, the biochar particles, especially those in the nanoscale range, may migrate or carry the inherent contaminants along the soil profile, posing a potential risk to the groundwater. This study investigated the transport and retention of wood chip-derived biochar nanoparticles (NPs) in water-saturated columns packed with a paddy soil. The environmentally-relevant soil solution chemistry including ionic strength (0.10–50 mM), electrolyte type (NaCl and CaCl₂), and natural organic matter (0–10 mg L⁻¹ humic acid) were tested to elucidate their effects on the biochar NPs transport. Higher mobility of biochar NPs was observed in the soil at lower ionic strengths, with CaCl₂ electrolyte being more effective than NaCl in decreasing biochar NPs transport. The retained biochar NPs in NaCl was re-entrained (∼57.7%) upon lowering transient pore-water ionic strength, indicating that biochar NPs were reversibly retained in the secondary minimum. In contrast, negligible re-entrainment of biochar NPs occurred in CaCl₂ due to the primary minimum and/or particle aggregation. Humic acid increased the mobility of biochar NPs, likely due to enhanced electrosteric repulsive interactions. The transport behaviors of biochar NPs can be well interpreted by a two-site kinetic retention model that assumes reversible retention for one site, and irreversible retention for the other site. Our findings indicated that the transport of wood chip biochar NPs is significant in the paddy soil, highlighting the importance of understanding the mobility of biochar NPs in natural soils for accurately assessing their environmental impacts.

Introduction

Biochar is a pyrogenic carbon produced from bioenergy feedstocks or agricultural wastes in limited oxygen at relatively low temperatures (Lehmann, 2007). Recently, land application of biochar has been widely implemented for carbon sequestration, soil amendment, nutrient retention, and contaminant immobilization (Atkinson et al., 2010, Abit et al., 2014). The biochar particles applied in soils have a wide size range. Depending on the feedstock sources and pyrolysis temperature, the weight of the biochar nanoparticles (NPs) ranges from 1.6% to 2.6% of the total generated biochar particles (Wang et al., 2013a). Although the biochar NPs have a relatively smaller portion among the biochar populations compared to those of larger ones, these small biochar particles are reactive and will horizontally leach into surface water via runoff, drainage, or irrigation (Zhang et al., 2010, Guggenberger et al., 2008). The biochar NPs may likely migrate down along the soil profile via infiltration and enter the groundwater system, posing potential environmental risks because the pyrolyzed biochar could be intrinsically enrich in organic contaminants (PAHs and dioxins) (Hale et al., 2012, Oleszczuk et al., 2013). Furthermore, the biochar NPs that inherently exhibit high sorption affinities for a wide array of environmental contaminants (e.g., inorganic, organic, and pathogenic microorganism) (Cao et al., 2009, Chen et al., 2008, Hale et al., 2011), are likely to expedite the transport and wide dissemination of sorbed contaminants via ‘colloid-facilitated transport’ (Fang et al., 2016). Therefore, a full understanding of the fate and transport of biochar NPs is critical to better optimize their benign use in agronomic and environmental advantages, but also to evaluate and minimize potential negative impacts of biochar NPs and biochar-facilitated transport of contaminants in the subsurface environments.

A few studies have been done to reveal the transport and retention of biochar particles including NPs in water-saturated porous media (Wang et al., 2013a, Wang et al., 2013b, Zhang et al., 2010). The findings indicate that the mobility of biochar particles depends on both the physicochemical properties of surrounding environmental conditions (e.g., ionic strength, pH, and natural organic matter) and the innate properties of biochar (particle size and pyrolysis temperature). For instance, Wang et al. (2013a). reported that the biochar NPs produced at lower pyrolysis temperatures exhibit higher mobility in saturated sand columns due to stronger Lewis acid-base repulsive interactions. Larger biochar micrometer particles transport less through sand columns than smaller NPs due, in part, to greater physical straining and surface charge heterogeneity among micrometer particle populations (Wang et al., 2013a). The authors also found the antagonistic interplay of humic acid and iron oxide grain-coating on the transport of biochar NPs (Wang et al., 2013b). In comparison, Zhang et al. (2010). found that the transport of biochar bulk particles (<75 μm) reduces with decreasing pH and increasing ionic strength. Please note that, however, all previously published studies focus solely on the transport of biochar particles in well-defined model porous media, i.e., cleaned quartz sand or iron oxide-coated sand grains; and the knowledge on the transport of biochar NPs in more realistic subsurface environments (i.e., natural soils) is fairly limited (Leifeld et al., 2007, Major et al., 2010).

Natural soils are highly complicated and heterogeneous in terms of their physicochemical and biological characteristics, and thus the actual transport behaviors of biochar NPs in natural soils would be distinct from those obtained in well-defined model porous media (Sun et al., 2015, Quevedo and Tufenkji, 2012, Zhang et al., 2012a). The variety of soil properties such as pH, pore-water ionic strength and cation type, surface roughness, particle size distribution, preferential flow pathway, surface chemical heterogeneity, and organic matter can play a role in the transport of biochar NPs, which is not captured by model porous media (Jaisi and Elimelech, 2009, Sagee et al., 2012, Wang et al., 2014a, Fang et al., 2009). Therefore, the transport of NPs in model porous media may be less applicable to predict the actual mobility of NPs in natural soil (Wang et al., 2010). Because the soil pore-water often includes different types of cations with varying concentrations, and NPs could be surface-modified with humic acid during transport, resulting in complex transport behaviors of NPs in natural soil (Jaisi and Elimelech, 2009, Sagee et al., 2012). However, to our knowledge, no studies have been done unraveling the mobility of biochar NPs in natural soils.

The objective of this study is to elucidate the transport and retention of biochar NPs in a natural paddy soil at environmentally-relevant solution chemistry including ionic strength, cation type, and humic acid concentrations. The mechanisms governing biochar NPs’ transport and retention in the soil are interpreted using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory in combination with colloid transport model that includes different retention rate coefficients for biochar NPs in the soil.