Leaching of organic carbon enhances mobility of biochar nanoparticles in saturated porous media

Abstract

Application of biochar materials generates large quantities of nanoparticles that possess relatively high mobility and can facilitate the transport of environmental contaminants. Here, we show that leaching of organic carbon (OC), an important weathering process of biochar, can markedly enhance the mobility of biochar nanoparticles. The nanoparticles of rice straw biochar and moso bamboo biochar receiving treatment to leach out OC exhibit a similar morphology and size distribution to the nanoparticles of the untreated biochar materials, but slightly different surface chemical properties, in particular, lower contents of carboxyl groups. Even though this alteration is too small to affect the surface charge negativity and hydrophobicity of the biochar nanoparticles, it considerably enhances the mobility of the biochar nanoparticles in artificial groundwater-saturated sandy soil, while having negligible effects on the mobility of the nanoparticles in purified quartz sand. Supplementary transport and particle remobilization experiments carried out in 20 mM NaCl and 0.68 mM CaCl₂, along with calculations of particle–collector interaction energy profiles based on extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory, verify that the OC-deficient nanoparticles are less affected by cation bridging between the nanoparticles and soil grains, owing to their lower abundance of carboxyl groups. The OC-leaching-induced mobility enhancement is more pronounced for low-temperature biochar materials (300 °C) than for high-temperature ones (500 °C), as the former can leach out greater amounts of OC. The findings call for further understanding of the effects of environmental aging processes on the fate and effects of biochar nanoparticles.