Spectral Characteristics of Dissolved Organic Matter in Hydrochars and Hydrothermal Aqueous Phases Derived from Typical Coastal Beach Plants at Various Hydrothermal Temperatures

Hydrochars were prepared using coastal beach plants (Phragmites australis, Aeluropus littoralis, and Spartina alterniflora) at 180, 220 and 260 ℃. Fractional properties and contents of dissolved organic matter derived from hydrochars (HDOM) and that from hydrothermal aqueous phases (ADOM) at various hydrothermal temperatures were compared through spectroscopic techniques. The results show that the dissolved organic carbon (DOC) contents of hydrochars ranged from 18.04 to 50.13 mg·g^-1, and the DOC concentrations of hydrothermal aqueous phases ranged from 2.67 to 5.46 g·L^-1, with both DOC contents decreased with the increase of hydrothermal temperatures. Both HDOM and ADOM contained two humic-like substances (C1 and C2) and two protein-like substances (C3 and C4), and the content of the total protein-like fractions of both HDOM and ADOM increased with increasing hydrothermal temperatures, whereas the total humic-like fractions showed the opposite trend. The protein-like proportions of HDOM and ADOM were generally higher than humic-like substances except in the case of Spartina alterniflora-derived HDOM and ADOM. The autochthonous source fractions of HDOM and ADOM were evident, and both exhibited characteristics of high bioavailability and low humification. HDOM was more aromatic, hydrophobic, and had a higher relative molecular weight compared to ADOM. In addition, the relative molecular weights of both HDOM and ADOM decreased with increasing hydrothermal temperatures. The aromaticity and hydrophobicity of ADOM decreased with the increase of hydrothermal temperatures, whereas the aromaticity and hydrophobicity of HDOM derived from various plants displayed diverse change patterns with the increase of hydrothermal temperatures. The primary functional groups of HDOM and ADOM included CO of carboxyl compounds, CC of aromatic, -OH, C-O of polysaccharides or C-O-C of ethers. These functional groups were significantly affected by hydrothermal temperatures and plant materials. This study provides crucial theoretical support for the determination of the application amounts of hydrochar and hydrothermal aqueous phases in agricultural application, water quality management of receiving water bodies, as well as for the assessments of environmental contaminants migration and transformation in farmland.