Antagonistic Effects of Humic Acid and Iron Oxyhydroxide Grain-coating on Biochar Nanoparticle Transport in Saturated Sand

Biochar land application may result in multiple agronomic and environmental benefits (e.g., carbon sequestration, improving soil quality, and immobilizing environmental contaminants). However, our understanding of biochar particle transport is largely unknown in natural environments with significant heterogeneity in solid (e.g., patches of iron oxyhydroxide coating) and solution chemistry (e.g., the presence of natural organic matter), which represents a critical knowledge gap in assessing the environmental impact of biochar land application. Transport and retention kinetics of nanoparticles (NPs) from wheat straw biochars produced at two pyrolysis temperatures (i.e., 350 and 550 °C) were investigated in water-saturated sand columns at environmentally relevant concentrations of dissolved humic acid (HA, 0, 1, 5, and 10 mg L(-1)) and fractional surface coverage of iron oxyhydroxide coatings on sand grains (ω, 0.16, 0.28, and 0.40). Transport of biochar NPs increased with increasing HA concentration, largely because of enhanced repulsive interaction energy between biochar NPs and sand grains. Conversely, transport of biochar NPs decreased significantly with increasing ω due to enhanced electrostatic attraction between negatively charged biochar NPs and positively charged iron oxyhydroxides. At a given ω of 0.28, biochar NPs were less retained with increasing HA concentration due to increased electrosteric repulsion between biochar NPs and sand grains. Experimental breakthrough curves and retention profiles were well described using a two-site kinetic retention model that accounted for Langmuirian blocking or random sequential adsorption at one site. Consistent with the blocking effect, the often observed flat retention profiles stemmed from decreased retention rate and/or maximum retention capacity at a higher HA concentration or smaller ω. The antagonistic effects of HA and iron oxyhydroxide grain-coating imparted on the mobility of biochar NPs suggest that biochar colloid transport potential will be dependent on competitive influences exerted by a number of environmental factors (e.g., natural organic matter and metal oxides).