Effect of alkali and alkaline earth metals accumulation in process water on humic acids formation during the circulating hydrothermal carbonization of Chinese medicine residues

Abstract

Circulating hydrothermal carbonization (HTC) has emerged as a sustainable way for enhancing the formation of humic acids (HAs) and mitigating the concerns of process water (PW) treatment. However, the accumulation of alkali and alkaline earth metals (AAEMs) in PW occurs synchronously. Herein, the influence of AAEMs valence state and concentration on HAs formation was examined. The accumulation of divalent AAEMs in PW significantly increases the hydrochar (HC) yield, while all AAEMs enhance the HAs yield. Among them, Na⁺ shows the best promotion effect on HAs yield of 9.22%. FTIR analysis shows that the monovalent AAEMs at 1 g L⁻¹ promote the retention of aliphatic structures in HAs, while the aromatic C=C intensity reduces slightly. More aliphatic and aromatic structure in HAs is observed by divalent AAEMs at 10 g L⁻¹. The divalent AAEMs cause a slight blue shift of HAs in 3D EEM, indicating lower aromatic condensation and molecular weight. Parallel factor analysis suggests that the accumulation of Ca in PW decreases the fluorescence intensity of HAs, further causing a shift from high aromatic and large molecular Component 1 to Component 2 with less condensed and more oxygenated fluorophores, indicating lower aromaticity and degree of condensation. This study provides a theoretical insight into the influence of AAEMs in PW on HA formation in the circulating HTC.