Quantification methods of pyrogenic carbon in soil with soil as a complex matrix: comparing the CTO-375 and Cr₂O₇ methods

Figures & data

Figure 1. PyC/OC ratio of each material (alone) after treatment using the CTO-375, original Cr₂O₇, and improved Cr₂O₇ methods. Error bar indicates 1 standard deviation (n = 3). ND: not detected

Figure 2. PyC/OC ratio of mixed samples after treatment using the CTO-375, original Cr₂O₇, and improved Cr₂O₇ method. For the kaolinite-melanoidin mixture, melanoidin was added to kaolinite at 4% TOC. For the kaolinite-soot mixture and the kaolinite-biochar mixture, pyrogenic carbons were added to kaolinite to set a 10% PyC/OC ratio in the case of the 4% TOC of kaolinite-melanoidin mixture. Values of OC content and PyC content of each material measured by the CTO-375 method were used to make the mixtures, except for the PyC content of biochar. For the content of biochar, the value measured by the Cr₂O₇ method was used. Error bar indicates 1 standard deviation (n = 3). ND: not detected

Figure 3. PyC/OC ratio of mixed samples designed to mimic environmental soil after treatment using the CTO-375, original Cr₂O₇, and improved Cr₂O₇ method. The TOC content and PyC/OC ratio of the mixture were set to 4% and 10%, respectively. The values of OC content and PyC content of each material measured by the CTO-375 method were used to make the mixtures, except for the PyC content of biochar. For the content of biochar, the value measured by the Cr₂O₇ method was used. The error bar indicates 1 standard deviation (n = 3) of each constituent material

Figure 4. Thermograms of mixture samples after CTO treatment. For the kaolinite-melanoidin mixture, the samples were prepared as 4% TOC. For the kaolinite-soot mixture and kaolinite-melanoidin-soot mixture, the samples were prepared as 4% TOC, and as theoretically 10% PyC/OC ratio at 375°C. OC and PyC contents for each material measured using the CTO-375 method were used to make the mixtures. The arrows shown in the figure indicate whether the legend depends on the value of the left or right axis. Error bar indicates 1 standard deviation (n = 3)

Figure 5. Thermograms of kaolinite-melanoidin mixtures after CTO treatment. Error bar indicates 1 standard deviation (n = 3)

Figure 6. PyC/OC ratios of melanoidin according to different K₂Cr₂O₇ concentrations using the Cr₂O₇ method. Error bar indicates 1 standard deviation (n = 3). ND: not detected

Table 1. PyC/OC ratio of volcanic ash soil, and the recovery rate of PyC of the soil mixed with soot or biochar, after treatment using the improved Cr₂O₇ method

Material	OC (%)	PyC/OC ratio (%)		Recovery rate of PyC (%)^2)
		Expected^1)	Measured
Volcanic ash soil	16.4 (0.32)		18.9 (4.38)
Volcanic ash soil with soot	18.1 (0.35)	22.9	22.6 (1.59)	98.8 (6.92)
Volcanic ash soil with biochar	18.6 (0.36)	22.7	23.3 (6.32)	102.5 (27.80)

¹⁾The expected PyC/OC ratio was calculated from the PyC content of in the volcanic ash, soot, and biochar measured by the improved Cr₂O₇ method.

²⁾The recovery rate of PyC was calculated by dividing the measured PyC/OC ratio by those expected values.

The values in parenthesis indicates 1 standard deviation (n = 3).

Figure 7. Scanning electron microscope micrographs showing the changes in surface morphology of the soot and biochar after treatment using the improved Cr₂O₇ method. (A) Soot before treatment. (B) Soot after HF treatment. (C) Soot after HF and K₂Cr₂O₇/H₂SO₄ treatment (D) Biochar before treatment. (E) Biochar after HF treatment. (F) Biochar after HF and K₂Cr₂O₇/H₂SO₄ treatment. The enclosed area indicates the regions in which EDS elemental analyses were performed by EDS

Table 2. EDS analyses showing the hanging O/C ratios in soot and biochar treated by HF and K₂Cr₂O₇/H₂SO_4.

	O/C ratio
		Treatment
Material	Before (None)	HF	HF and K2Cr2O7/H2SO4
Soot	0.188	0.149	0.159
Biochar	0.244	0.191	0.518