Release of colloidal biochar during transient chemical conditions: The humic acid effect

Our understanding of colloidal biochar (CB) transport and release is largely unknown in environments with transient chemical conditions, e.g., ionic strength (IS), pH, and especially humic acid (HA). In this study, column experiments were conducted to investigate CB transport and retention in the presence and absence of HA, and CB release under transient IS and pH conditions in saturated sand. Step reductions in solution IS from 25 to 0.01 mM produced significant release peaks of CB due to a reduction in the depth of the primary minima on rough surfaces with small energy barriers. In contrast, step increases of solution pH from 4 to 10 only slightly increased CB release presumably due to the strong buffering capacity of CB. The CB retention was diminished by HA during the deposition phase. However, the release of CB with transients in IS and pH was not influenced much when deposition occurred in the presence of HA. These observations indicate that HA increased the energy barrier during deposition but did not have a large influence on the depth of the interacting minimum during transient release. Potential explanations for these effects of HA on CB retention and transient release include enhanced repulsive electrostatic interactions and/or altering of surface roughness properties. Our findings indicated that the release of retained CB is sensitive to transient IS conditions, but less dependent on pH increases and CB deposition in the presence of HA. This information is needed to quantify potential benefits and/or adverse risks of mobile CB in natural environments.