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Table 1 Chemical properties of humin, HA, FA, and different types of lignin

From: Latest development in the fabrication and use of lignin-derived humic acid

Type of lignin

Solubility

Carboxylic acid group, mmol/g

Phenolic OH, mmol/g

Aliphatic OH, mmol/g

Molecular weight (Mw, g/mol)

C/N

Refs.

Humin

Not soluble

3–4

2

NA

> 300,000

NA

[69, 73,74,75]

HA

pH > 2

2–5

2–6

1–4

2000–1,000,000

8–61

[69, 73,74,75,76]

FA

soluble

8–9

3–6

3–5

600–900

6.7–9.2

[69, 73,74,75]

Kraft lignin (KL)

pH > 7

0.3

2.6

2.45

1000–15,000

135

[51, 77, 78]

Lignosulfonate (LS)

soluble

0.1–0.53

1.5–2

1.9–4

1000–50,000

240

[51, 78,79,80]

Organosolv lignin

pH > 7

0.05–0.25

2.6–5.1

1.3

500–5000

203

[53, 78, 79, 81]

Soda lignin

pH > 7

0.9–1

2.5–3.7

2.4

800–3000

68

[53, 78, 79, 82]