Table 2 shows the data related to the E4/E6 ratio and the D log K
coefficient for HA samples. As can be observed, the E4/E6 ratio
ranges from 5.64 to 7.80. This parameter is used to indicate humification
degree, decreasing the ratio value, the degree of condensation
increases [37,40,45–49]. The data obtained show that HA-2
and HA-4 are richer in aromatic structures than the HA-1 and
HA-3 samples.