Release of free-state ions from fulvic acid-heavy metal complexes via VUV/HO photolysis: Photodegradation of fulvic acids and recovery of Cd and Pb stripping voltammetry currents.

Fulvic acid (FA), a ubiquitous organic matter in the environment, can enhance the mobility and bioavailability of Cd and Pb through competitive complexation to form FA-heavy metal ions (FA-HMIs) complexes with excellent solubility. Because FA-HMIs are electrochemically inactive, square wave anodic stripping voltammetry (SWASV) cannot accurately detect the content of bioavailable Cd and Pb in soils and sediments. This study ostensibly aimed to efficiently recover SWASV signals of Cd and Pb in FA-HMIs by disrupting FA-HMIs complexes using the combined vacuum ultraviolet and HO (VUV/HO) process. Essentially, this study explored the photodegradation behavior and photolysis by-products of FA and their effects on the conversion of FA-HMIs complexes to free-state Cd and Pb using multiple characterization techniques, as well as revealed the complexation mechanism of FA with Cd and Pb. Results showed that reactive groups such as carboxyl and hydroxyl endowed FA with the ability to complex Cd and Pb. After FA-HMIs underwent VUV/HO photolysis for 9 min at 125 mg/L of HO, FA was decomposed into small molecular organics while removing its functional groups, which released the free-state Cd and Pb and recovered their SWSAV signals. However, prolonged photolytic mineralization of FA to inorganic anions formed precipitates with Cd and Pb, thereby decreasing their SWSAV signals. Moreover, the VUV/HO photolysis significantly improved the SWASV detection accuracy toward the Cd and Pb in real soil and sediment samples, verifying its practicality.