Release of free-state ions from fulvic acid-heavy metal complexes via VUV/H2O2 photolysis: Photodegradation of fulvic acids and recovery of Cd2+ and Pb2+ stripping voltammetry currents

Abstract

Fulvic acid (FA), a ubiquitous organic matter in the environment, can enhance the mobility and bioavailability of Cd2+ and Pb2+ 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 Cd2+ and Pb2+ in soils and sediments. This study ostensibly aimed to efficiently recover SWASV signals of Cd2+ and Pb2+ in FA-HMIs by disrupting FA-HMIs complexes using the combined vacuum ultraviolet and H2O2 (VUV/H2O2) 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 Cd2+ and Pb2+ using multiple characterization techniques, as well as revealed the complexation mechanism of FA with Cd2+ and Pb2+. Results showed that reactive groups such as carboxyl and hydroxyl endowed FA with the ability to complex Cd2+ and Pb2+. After FA-HMIs underwent VUV/H2O2 photolysis for 9 min at 125 mg/L of H2O2, FA was decomposed into small molecular organics while removing its functional groups, which released the free-state Cd2+ and Pb2+ and recovered their SWSAV signals. However, prolonged photolytic mineralization of FA to inorganic anions formed precipitates with Cd2+ and Pb2+, thereby decreasing their SWSAV signals. Moreover, the VUV/H2O2 photolysis significantly improved the SWASV detection accuracy toward the Cd2+ and Pb2+ in real soil and sediment samples, verifying its practicality.