Effect of freeze-thaw cycling on grain size of biochar

Abstract

Biochar may improve soil hydrology by altering soil porosity, density, hydraulic conductivity, and water-holding capacity. These properties are associated with the grain size distributions of both soil and biochar, and therefore may change as biochar weathers. Here we report how freeze-thaw (F-T) cycling impacts the grain size of pine, mesquite, miscanthus, and sewage waste biochars under two drainage conditions: undrained (all biochars) and a gravity-drained experiment (mesquite biochar only). In the undrained experiment plant biochars showed a decrease in median grain size and a change in grain-size distribution consistent with the flaking off of thin layers from the biochar surface. Biochar grain size distribution changed from unimodal to bimodal, with lower peaks and wider distributions. For plant biochars the median grain size decreased by up to 45.8% and the grain aspect ratio increased by up to 22.4% after 20 F-T cycles. F-T cycling did not change the grain size or aspect ratio of sewage waste biochar. We also observed changes in the skeletal density of biochars (maximum increase of 1.3%), envelope density (maximum decrease of 12.2%), and intraporosity (porosity inside particles, maximum increase of 3.2%). In the drained experiment, mesquite biochar exhibited a decrease of median grain size (up to 4.2%) and no change of aspect ratio after 10 F-T cycles. We also document a positive relationship between grain size decrease and initial water content, suggesting that, biochar properties that increase water content, like high intraporosity and pore connectivity large intrapores, and hydrophilicity, combined with undrained conditions and frequent F-T cycles may increase biochar breakdown. The observed changes in biochar particle size and shape can be expected to alter hydrologic properties, and thus may impact both plant growth and the hydrologic cycle.

Environmental behavior of engineered biochars and their aging processes in soil

“…Some natural nano-sized particles may be mixed with biochar particles, wedging into their curvature surface, or even the porous structures. Because of their different expansion properties, biochars may be broken into smaller particles during the freeze-thawing cycles or dry-wet cycles (Liu et al 2018).…”

Chemical Aging Changed Aggregation Kinetics and Transport of Biochar Colloids

“…The time scale of biochar aging in the field varies from a few weeks to more than 10 years. ,,− To accelerate and simulate the aging process, microbial, physical, and chemical aging , have been used previously. Microbial aging is carried out in both aerobic and anaerobic conditions, resulting in the release of mineralizable organic compounds that can be a carbon source for microorganisms. , Physical aging is typically achieved through fragmentation, erosion, air-drying, and alternating cycles of wetting–drying or freezing–thawing, , which leads to formation of fresh reactive surfaces and a decrease of biochar particle size.…”

“…The time scale of biochar aging in the field varies from a few weeks to more than 10 years. ,,− To accelerate and simulate the aging process, microbial, physical, and chemical aging , have been used previously. Microbial aging is carried out in both aerobic and anaerobic conditions, resulting in the release of mineralizable organic compounds that can be a carbon source for microorganisms. , Physical aging is typically achieved through fragmentation, erosion, air-drying, and alternating cycles of wetting–drying or freezing–thawing, , which leads to formation of fresh reactive surfaces and a decrease of biochar particle size. Chemical aging often contributes to an increase of labile organic molecules, oxidation of certain minerals (e.g., magnetite), and/or oxygen-containing functional groups in biochar. ,, Previous studies showed that these aging methods changed the physicochemical properties of biochar, including labile organic compounds, specific surface area and porosity, element composition and molecular structure, anion/cation exchange capacity, , surface roughness and hydrophilicity, and enrichment of oxygen-containing functional groups .…”

Mercury Removal from Contaminated Water by Wood-Based Biochar Depends on Natural Organic Matter and Ionic Composition

“…High pyrolytic temperature in biochars generally translates to high pore volume and water holding capacity. 36 SEM imaging of SWP700 shows a highly porous framework consisting of macro-(Figures 1 and S2), meso-, and micropores, which could be penetrated by water transporting dissolved Hg(II) species. Recent studies using the same biochar show that 95% of the total porosity in SWP700 corresponds to a pore width of <3 μm, 37 with 1.4 and 7.8% consisting of micro-(<2 nm) and mesopores (2−50 nm), respectively.…”