Fractionation of humic acid and its effects on the photodegradation of amitriptyline in water.

Humic acid (HA) plays a critical role in pollutants' photolysis, but due to its structural complexity, it is difficult to identify the structural features with high photocatalytic activity. Herein, HA was fractionated on a silica gel column to get three fractions, i.e. F_A, F_B and F_C, according to the polarity. After grading, humic- and fulvic-like components were enriched, while the protein-like component with a relatively small molecular weight was lost. Among the three fractions, the strongest polar fraction, F_A, is rich in carboxyl groups, with a low formation rate of ˙OH but a high quantum yield of the excited triplet state (³HA*). F_B has the strongest aromaticity and the highest formation rate of ³HA*. F_C, with weaker polarity, is rich in aromatic proteins, and has a high quantum yield of ³HA* and ˙OH. All HA fractions promoted the photodegradation of amitriptyline (AMT), and the promotion effect of bulk HA and F_B was greater than F_A and F_C, and ³HA* and ˙OH were the main active species. Product analysis indicated that the electron transfer of AMT and ³HA* and the subsequent hydroxylation were the main reaction pathways. HA fractions generated reactive halogen species (RHS) through ³HA* in the presence of halides, which obviously promoted AMT decomposition, and the role of RHS was significantly correlated with the quantum yield of ³HA*. In the presence of Fe³⁺, HA promoted AMT degradation by forming a complex of Fe-HA. This study is helpful for better understanding the relationship between the structural properties and photochemical activity of different fractions of HAs.