Contrasting impacts of pH on the abiotic transformation of hydrochar-derived dissolved organic matter mediated by δ-MnO2

Abstract Hydrochar produced from hydrothermal carbonization of agricultural waste becomes a promising soil conditioner, and especially, its labile fraction, dissolved organic matter (DOM) may play an important role in improving soil fertility and quality. However, little is known about the interaction and transformation of hydrochar-derived DOM with soil reactive components after its amendment into soil. The abiotic transformation of hydrochar-derived DOM by a common soil metal oxide, δ-MnO2 at environmentally relevant pHs (4, 6 and 8) was investigated by using a combination of batch experiments and advanced spectroscopic techniques. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and fluorescence spectroscopy revealed that lignins and carboxyl-rich alicyclic molecules were predominant in hydrochar-derived DOM and phenolic fluorophores were the dominant fluorescent composition. In the presence of δ-MnO2, hydrochar-derived DOM was rapidly transformed via both decomposition and polycondensation, resulting in significant structure changes of hydrochar-derived DOM with increasing humic-like fraction similar to soil organic matter (SOM). The transformation process of hydrochar-derived DOM molecules was highly pH-dependent, with polycondensation occurring preferentially at pH 8 and more rapid oxidation at pH 4. Meanwhile, δ-MnO2 was reduced to Mn(II) and Mn(III) during the reaction, and the accumulation of Mn(III) in the solid phase of δ-MnO2 reached a maximum at pH 4. Collectively, the results characterized the transformation mechanism of DOM by δ-MnO2 at molecular level, which provided a new perspective for better understanding of the biogeochemical processes of biochar for its application in soil quality improvement and remediation.