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

Authors: WANG, YANG - China Agricultural University; Bradford, Scott; SHANG, JIANYING - China Agricultural University

Submitted to: Environmental Pollution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/23/2020
Publication Date: 1/29/2020
Citation: Wang, Y., Bradford, S.A., Shang, J. 2020. Release of colloidal biochar during transient chemical conditions: The humic acid effect. Environmental Pollution. 260. https://doi.org/10.1016/j.envpol.2020.114068.
DOI: https://doi.org/10.1016/j.envpol.2020.114068

Interpretive Summary: Biochar is increasingly being applied to soils to sequester carbon, improve fertility, and/or enhance contaminant remediation. However, little research has previously investigated the influence of changes in soil solution chemistry such as ionic strength (IS), pH, and organic carbon on the mobility of small biochar particles. Our results show that a reduction in IS induced biochar particle release, whereas an increase in pH or organic carbon increased the initial transport of biochar particles but did not significantly increase the release of previously retained particles. This behavior was explained by consideration of the adhesive interaction of biochar particles with the soil. This information will be of interest to scientists and government regulators that are concerned with carbon sequestration and soil remediation using biochar.

Technical Abstract: 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 deposition in the presence of HA. This information is needed to quantify potential benefits and/or adverse risks of mobile CB in natural environments.