Determining the transport behaviors of biochar nano-particles in porous media

Abstract

The transport of biochar nano-particles (BCNPs) influenced by complex physicochemical factors is systematically investigated through the transport experiments combined with the interfacial chemistry theory and numerical modeling methods. The two-dimensional (2D) distributions of Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energy are obtained and a new indicator of DLVO barrier (ξDB) is proposed to identify the inflection point related to agglomeration and deposition of BCNPs. Results suggest the mobility of BCNPs in porous media decreases with ionic strength, and the inhibitory effect of divalent cations on BCNPs transport is stronger than that of monovalent cations. However, the mobility of BCNPs is enhanced by the increases in pH and humic acid concentrations. In addition, the higher the concentration of BCNPs, the higher the retention of BCNPs in saturated porous media. The mobility of BCNPs is enhanced under the conditions of high flow velocity. A combined value of DLVO energy barriers is calculated and results suggest the decision coefficient (R2) decreases with the fraction of DLVO energy barrier of BCNPs-BCNPs, indicating that the interaction between BCNPs and quartz sand (QS) plays a major role in the transport of BCNPs in porous media. Afterward, the continuous values of two-site kinetic transport parameters of BCNPs in porous media as a function of hydrochemical factors can be predicted by the combined value of DLVO energy barriers. Simultaneously, the critical hydrochemical conditions related to BCNPs mobility can be quantitatively determined by a combination of column experiment, DLVO interaction energy and transport model. The critical hydrochemical conditions are IS(Na+)= 29.3–29.4 mM, IS(Ca2+)= 2.50–2.55 mM, pH= 3.36–3.37, HA concentration= 3.68–5.84 mg·L−1. The findings are important for understanding the transport behavior and mechanism of BCNPs in porous media, and accurate prediction and assessment of BCNPs transport patterns in soil-groundwater systems.