Fig 1.
Photograph of (a) pine biochar; (b) mesquite biochar; (c) miscanthus biochar; (d) sewage waste biochar pre F-T cycling; and (e) pine biochar; (f) mesquite biochar; and (g) miscanthus biochar; (h) sewage waste biochar after 20 F-T cycles. These biochars were placed in a water bath for at least one day and sealed in cylinders without drainage pre F-T cycling. The images show that biochars made from different feedstocks are visually different. However, for the same biochar type there was no visual difference between pre- and post- F-T cycles. Biochar feedstocks were sieved between 2.36–3.35 mm.
Table 1.
Basic information of the undrained and drained experiments in this study.
Fig 2.
Grain size (at the shortest chord of a biochar grain projection, Dmin) distribution by volume for (a) pine; (b) mesquite; (c) miscanthus; and (d) sewage waste biochar from 0–20 F-T cycles in the undrained experiments. There were decreases in Dmin for pine, mesquite, and miscanthus biochars with increasing number of F-T cycles. However, distributions of Dmin for sewage waste biochar from 0–20 F-T cycles overlapped with each other, meaning that there is no change of Dmin distribution for sewage waste biochar. We didn’t observe any particle aggregation. Values and errors are mean and standard deviation of triplicate samples. Numerical data for individual samples are presented in S2 Table.
Fig 3.
Grain size distribution (at the maximum diameter of a biochar grain projection, Dmax) by volume for (a) pine; (b) mesquite; (c) miscanthus; and (d) sewage waste biochar from 0–20 F-T cycles in the undrained experiments. There were decreases in Dmax for pine, mesquite, and miscanthus biochars with increasing number of F-T cycles. However, distributions of Dmax for sewage waste biochar from 0–20 F-T cycles overlapped with each other, meaning that there is no change of Dmax distribution for sewage waste biochar. Values and errors are mean and standard deviation of triplicate samples. Numerical data for individual replicates are presented in S2 Table.
Fig 4.
Cumulative grain size (Dmin) distribution by volume at 0 and 20 F-T cycles and the best-fit modified hyperbolic model for (a) pine; (b) mesquite; (c) miscanthus; and (d) sewage waste biochar in the undrained experiment. Values and errors are mean and standard deviation of triplicate samples. Numerical data for individual replicates are presented in S2 Table.
Fig 5.
Cumulative grain size (Dmax) distribution by volume at 0 and 20 F-T cycles and the best-fit modified hyperbolic model for (a) pine; (b) mesquite; (c) miscanthus; and (d) sewage waste biochar in the undrained experiment. Values and errors are mean and standard deviation of triplicate samples. Numerical data for individual replicates are presented in S2 Table.
Table 2.
Modified hyperbolic model parameters and goodness of fit include: The weighting factors (w1), constants A and c for two sub grain size distribution curves, R-square (R2) and root-mean-square error (RMSE).
Fig 6.
Median grain size (a) at the shortest chord (Dmin50) and (b) at the maximum diameter (Dmax50) of all undrained experiment biochar samples from 0 to 20 F-T cycles. Note: X-axis is not on a linear scale. Values and errors are mean and standard deviation of triplicate samples. Numerical data for individual replicates are presented in S2 Table.
Fig 7.
(a) Aspect ratio (AR = Dmin50/Dmax50) of all the undrained experiment biochar samples from 0 to 20 F-T cycles and (b) AR of the undrained experiment pine, mesquite, and miscanthus biochars before (bulk) and after passing (fine) or being retained (coarse) by a U.S. standard #20 mesh (0.853 mm) at 20 F-T cycles. Note: X-axis is not on a linear scale. Symbols plotted are mean values. Errors are calculated using Eq 1. Numerical data for individual replicates are presented in S2 Table.
Fig 8.
(a) Skeletal density (ρs), (b) envelope density (ρe), and (c) intraporosity (ϕintra) of four types of biochar from 0–20 F-T cycles in the undrained experiment. For ρs and ρe, values and errors are mean and standard deviation of triplicate samples. For ϕintra, values are mean, and errors are calculated using Eq 4. Note: X-axis is not on a linear scale.
Table 3.
Skeletal density (ρs, kg/m3), envelope density (ρe, kg/m3), and intraporosity (ϕintra, m3/m3, volume of pores/envelope particle volume) pre- and post- F-T cycles in the undrained experiment.
For ρs and ρe, values and errors are mean and standard deviation of triplicate samples. For ϕintra, values are mean, and errors are calculated using Eq 4.
Fig 9.
Grain size (a) at the shortest chord (Dmin) and (b) at the maximum diameter (Dmax) distribution by volume of drained experiment mesquite biochars from 0 to 10 F-T cycles. Values and errors are mean and standard deviation of triplicate samples. Numerical data for individual replicates are presented in S2 Table.
Fig 10.
(a) Median grain size (Dmin50 and Dmax50) and (b) aspect ratio (AR) of drained experiment mesquite biochars from 0–10 F-T cycles. Values and errors are mean and standard deviation of triplicate samples. Numerical data for individual replicates are presented in S2 Table. Note: X-axis is not on a linear scale.
Fig 11.
Conceptual model of how the grain size of pine, mesquite, and miscanthus biochars with higher water content was reduced by F-T cycles through loss of thin layers from the particle surface.
(a) a biochar particle with its skeleton (black), connected intrapores filled with water (gray) and isolated, air-filled intrapores (white); (b) water-filled intrapores expand during freezing which increases intraporosity; and (c) biochar grain size decreases through loss of fine particles from surface of larger particles due to intrapores’ expansion.
Fig 12.
Positive Pearson correlation coefficients (R = 0.90) between decrease in median grain size and water content at 10 F-T cycles.
Values and errors are mean and standard deviation of triplicate samples. Numerical data for individual replicates are presented in S2 Table.