Influence of thermal air oxidation on the chemical composition and uranium binding property of intrinsic dissolved organic matter from biochar

In this work, uranium (U(VI)) binding characteristics of the intrinsic dissolved organic matters (DOM) from the biochars prepared under thermal air oxidation (TAO) and non-TAO conditions were studied using synchronous fluorescence spectra (SFS) and Fourier transform infrared (FTIR) in conjunction with the general two-dimensional correlation spectroscopy (2D-COS), heterospectral 2D-COS and moving-window (MW) 2D-COS. The chemical compositions of the intrinsic DOMs from biochars with/without TAO were investigated by Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS). Results showed that the preferential binding of U(VI) to functional groups followed the order: 937 (carboxyl γC–OH), 981 (carboxyl γC–OH), 1511 (aromatic vC = C), 1108 (esters or ethers vC–O), 1282 (esters or carboxyl vC–O), 1698 (saturated carboxylic acid or ketone vC = O) cm−1 for biochar DOM after TAO (OB600), and 937 (carboxyl γC–OH), 1484 (lipids δC–H or phenolic vC–O), 1201 (esters or carboxyl vC–O), 1112 (esters or ethers vC–O), 1706 (saturated aldehyde, carboxylic acid or ketone vC = O), 1060 (phenolic, esters or ethers vC–O), 1014 (phenolic, esters or ethers vC–O) cm−1 for the pristine biochar (B600). Fulvic-like substances at 375 nm in the biochar DOM showed a preferential binding with U(VI) after TAO, while humic-like substances played a more critical role in the U(VI) complexation with biochar DOM obtained from non-TAO condition. The results also indicated that TAO increased the content of fluorescent DOM and the chemical stability of DOM-U(VI) complexes. The FT-ICR MS results showed an increase in the relative abundance of protein-like, carbohydrates-like, tannins-like, unsaturated hydrocarbons, and condensed aromatic structure and a decrease in the relative abundance of lipid-like and lignin-like after TAO. Consequently, although biochar after TAO had a much poorer content of intrinsic DOM, its intrinsic DOM showed a much higher capacity in U(VI) precipitation. Therefore, the TAO substantially changed the chemical composition, binding property and environmental behavior of intrinsic DOM from biochar.