Current research focuses on extracting humic acid (HA)
compounds
from low-rank coals to obtain high value-added products. In this study,
HAs with high purity and low heavy metal content were obtained from
lignite by combining acid pretreatment with hydrothermal treatment.
Scanning electron microscopy, elemental analysis (EA), Fourier transform
infrared (FTIR) spectroscopy, X-ray diffraction, and inductively coupled
plasma optical emission spectrometry (ICP–OES) were used to
analyze raw lignite and HAs. The effects of acid and hydrothermal
treatments on the inorganic elements, functional groups, and yield
of HAs were examined. The results showed that acid treatment reduced
the ash content of lignite from 20 to 9%, and hydrothermal treatment
increased the yield of HAs from 36 to 68%. The chemical properties
of HAs exhibited an increase in molecular weight and improved aromaticity
after acid and hydrothermal treatments. The results of ICP–OES
analysis suggested that the combined method of acid and hydrothermal
treatments resulted in a significant reduction of heavy metal elements
in HAs. FTIR analysis confirmed the results and demonstrated that
the extracted HA from nitric acid pretreated and hydrothermal generation
of lignite PHA was rich in carboxyl and phenolic functional groups.
PHA was applied to biochar as an activator for the adsorption of heavy
metal ions. The experimental results showed that PHA was successfully
loaded onto biochar and introduced a large number of functional groups,
and the adsorption capacity of the modified biochar for Pb2+ was effectively improved.
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