Evaluation of the structure and properties of humic acids from Yunnan lignite transformed by Penicillium ortum MJ51

Abstract

Biotransformation of low-rank coal into humic acids (HAs) has received increasing attention. In this study, Penicillium ortum MJ51, isolated from coal, was used to transform Yunnan lignite into HAs. Differences in the yield, structure and properties were evaluated for HAs produced from the biodegradation of nitric acid pretreated-lignite (MHA), and those produced via chemical extraction from raw lignite (CHA₁) and nitric acid pretreated-lignite (CHA₂). The results show that with the progression of degradation time, the MHA yield gradually increased and peaked on day 12 at 62.39 %, which was 199.60 % and 40.68 % higher than that of CHA₁ and CHA₂, respectively. Meanwhile, the contents of macro-micro nutrients and active groups (including carbonyl, carboxyl, methoxy, and N-alkyl), as well as the anti-flocculation ability and aliphaticity of MHA also gradually increased. However, the contents of C, H, N, heavy metals and ash, particle size, relative molecular weight, aromaticity, and hydrophobicity of MHA gradually decreased. On days 12–16 of degradation, the MHA had lower heavy metals, rare earth elements and ash content, higher macro-micro nutrients content, lower relative molecular weight and aromaticity, higher anti-flocculation ability and bioactivity than those extracted via the chemical method, making it suitable for agricultural applications.