Fig. 1 (a) The traditional van Krevelen diagram; (b) The modified van Krevelen diagram; (c) The modified van Krevelen diagram associated with NOSC; (d) The modified van Krevelen diagram associated with NOSC’. (In (a) and (b), the red dots symbol the compounds with H/C<1.5, the green dots are the compounds with 1.5≤H/C<2.0, and the blue dots are the compounds with H/C≥2.0; NOSC: the nominal oxidation state of carbon; NOSC’: the adjusted nominal oxidation state of carbon)

Fig. 1(c) and (d) demonstrate the modified van Krevelen diagrams associated with NOSC and NOSC’, respectively. It can be seen that the compounds with NOSC’>0 are slightly higher than those with NOSC>0. Among the 2235 organics in the database, approximately 21.3% with NOSC<0 in Fig. 1(c) change into that with NOSC’≥0 in Fig. 1(d). Generally, compounds with NOSC>0 are more oxidized and less degradable, while those with NOSC<0 are more likely degraded. When the impact of halogen is not considered, the oxidation state of organic carbon is underestimated, and the degradation potential of organic matter is overestimated.

Three methods were used for calculating the aromaticity index, including AI, AImod ⁴ and Xc.⁵ Of them, AI was suggested for compounds in which oxygen was mainly present as carbonyl functional groups; AImod was adjusted on the basis of AI when oxygen could also be bound with π-bonds in carboxyl groups;¹²and Xc was aimed at aromatic compounds with long alkylation.⁵ Considering the contribution of halogen elements to the saturation of organic matter, we adjusted these three indexes based on the original equations and put forward them as AI’, AImod’ and Xc’, respectively. The equations used to calculate the tree adjusted indexes are as follows: