Effects of soil grain size and solution chemistry on the transport of biochar nanoparticles

Abstract

Biochar nanoparticles (BC-NP) have attracted significant attention because of their unique environmental behavior, some of which could potentially limit large-scale field application of biochar. Accurate prediction of the fate and transportability of BC-NP in soil matrix is the key to evaluating their environmental influence. This study investigated the effects of soil grain size and environmentally relevant solution chemistry, such as ionic strength (cation concentration, 0.1 mM–50 mM; cation type, Na+, and Ca2+), and humic acid (HA; 0–10 mg/L), on the transport behavior of BC-NP via systematic column experiments. The transportability of BC-NP in the soil-packed column decreased with decreasing soil grain size and was inversely proportional to soil clay content. At low cation concentrations (0.1–1.0 mM), a considerable proportion of BC-NP (15.95%–67.17%) penetrated the soil columns. Compared with Na+, Ca2+ inhibited the transportability of BC-NP in the soil through a charge shielding effect. With increasing HA concentration, the transportability of BC-NP increased, likely due to an enhanced repulsion force between BC-NP and soil particles. However, at a high HA concentration (10 mg/L), Ca2+ bridging reduced the transportability of BC-NP in the soil. Breakthrough curves of BC-NP were explained by the two-site kinetic retention model. The antagonistic effects of ionic strength and HA indicated that the transport behavior of BC-NP in the soil was governed by competitive effects of some environmental factors, including soil grain size, environmental solution chemistry, and natural organic matter content.

Migration Rules and Mechanisms of Nano-Biochar in Soil Columns under Various Transport Conditions

“…Since the diffusion coefficient of nanoparticles is small, they are not easy to diffuse into small pores with low concentrations and slow flow rates. Therefore, consistent with previous studies, the results of this study show that the ability of nanoparticles to migrate downward is relatively weak in fine-textured soils (silt loam), while the migration ability of NBC gradually increases in loam and sandy loam [ 26 , 50 , 51 ]. In addition, soil properties such as pH, CEC , and negative Zeta potential can alter the interaction energy and adsorption capacity between particle systems.…”

“…Chen et al found that by suddenly reducing the ionic strength of the carrier solution by five times (from 50 mM to 10 mM), a large number of BC colloidal particles retained on the surface of soil particles were released [ 25 ]. Additionally, the type of ions also affects the migration properties of NBC: divalent cations (Ca 2+ ) exhibit greater efficacy in inhibiting the migration of NBC in soil than monovalent cations (Na + ) [ 26 ]. This is mainly due to the fact that higher-valent cations are more likely to form bridges with NBC, leading to NBC aggregation.…”