Changes in Dissolved Natural Organic Matter Composition Induced by Reaction with U(VI) at Acidic and Neutral pH

Understanding oxidized uranium [U(VI)] reactions with natural organic matter (NOM) is necessary to predict the solubility and mobility of U and NOM in waters from organically rich geologic uranium deposits influenced by natural and anthropogenic processes. Here, we investigated the changes in the chemical composition of NOM resulting from its reaction with U(VI) under acidic and neutral pH under controlled laboratory conditions. We employed a multianalytical approach, including X-ray photoelectron spectroscopy (XPS), Fourier transform ion cyclotron resonance-mass spectrometry (FTICR-MS), transmission electron microscopy (TEM), and X-ray absorption spectroscopy (XAS). Results revealed that U(VI) reaction at acidic pH led to a decrease in the relative content of alcohols, carboxylic functional groups, and compounds with high O/C ratios in NOM. Our integrated analyses suggest that NOM's molecular composition is altered by two primary mechanisms: (1) selective aqueous complexation of U with alcohol and carboxylic groups in NOM and (2) U adsorption onto particulate organic matter (POM). These findings provide insights into NOM chemical transformations due to U interactions under pH conditions relevant to acid mine drainage, natural geologic deposits, and surface waters affected by legacy uranium mining.