Chemical Aging Changed Aggregation Kinetics and Transport of Biochar Colloids

Little is known about aggregation and transport behaviors of aged biochar colloids in the terrestrial environment. This study investigated aggregation kinetics and transport of biochar colloids from aged (HNO₃ treatment) and pristine pinewood biochars pyrolyzed at 300 and 600 °C (PB300 and PB600) in NaCl and CaCl₂ solutions. In NaCl solutions, critical coagulation concentrations (CCCs) of aged PB300 and PB600 colloids (540 mM and 327 mM) were much greater than the CCCs of pristine biochar colloids (300 mM and 182 mM). This is likely due to substantial increase of negatively charged oxygen-containing functional groups (primarily carboxyl) on aged biochar surfaces. Intriguingly, in CaCl₂ solutions the CCCs of the aged PB300 and PB600 colloids decreased to 25.2 mM and 32.1 mM from 58.6 mM and 41.7 mM for the pristine colloids, respectively. This probably resulted from greater surface charge neutralization and Ca²⁺ bridging for the aged biochar colloids. In salt solutions (e.g., 10 and 50 mM NaCl and 1 and 10 mM CaCl₂), the aged biochar colloids showed higher mobility in porous media than the pristine biochar colloids. This study demonstrated that pristine and aged biochar colloids were stable in the solutions with environmentally relevant ionic strength, and the aging process might substantially increase their mobility in the subsurface.