Fluorescence, absorbance, and ion exchange: coupling analysis with water treatment for improved insight on copper complexation with natural organic matter

Groundwater high in dissolved organic carbon (DOC) was spiked with 0–120 μmol/L of copper(II) (Cu) at pH 4, 6, and 8 to observe changes in fluorescence and UV absorbance of natural organic matter (NOM). The groundwater had a fluorescence excitation–emission matrix (EEM) characteristic of humic-like substances. Addition of Cu to the groundwater resulted in fluorescence quenching of EEM peak intensity and increase in UV absorbance at 254 nm (UVA₂₅₄). Both fluorescence quenching and increase in UVA₂₅₄ suggested the complex formation between Cu and NOM. Importantly, DOC measurements showed that physical loss of NOM did not play a measureable role in contributing to the observed fluorescence quenching and increase in UVA₂₅₄, which supports the complex formation of Cu–NOM. The conditional stability constant for Cu–NOM was estimated using both fluorescence quenching and UVA₂₅₄ increase at each pH and ranged from 4.26 to 5.77. As a follow up, anion exchange and cation exchange batch experiments were conducted using the groundwater spiked with 120 μmol/L Cu at pH 8 to gain insight on the behavior of Cu–NOM, and attempt to give physical meaning to the fluorescence quenching and UVA₂₅₄ increase. The ion exchange results suggested the possibility of a neutral or partially negatively charged Cu–NOM complex. The significance of this work resides in the idea that fluorescence quenching and UV absorbance of NOM can be used as a tool to evaluate the co-removal of contaminants through water treatment processes.