Shuzhen Zhang

Soil organic matter (SOM) comprises a continuum of organic materials from granular organic debris to small organic molecules and contains more organic carbon than global vegetation and the atmosphere combined. It has remarkable effects on soil ecological functions and the global carbon cycle as well as the fate of pollutants in the terrestrial ecosystem. Therefore, characterization of SOM is an important topic in soil science, ecology, and environmental science. Chemical complexity and spatial heterogeneity are by far the two biggest challenges to our understanding of SOM. Recent developments in analytical techniques and methods provide the opportunity to reveal SOM composition at the molecular level and to observe its distribution in soils at micro- and nanoscales, which have greatly improved our understanding of SOM. This paper reviews the outstanding advances in SOM characterization regarding these two issues from target and nontarget analyses comprising molecular marker analysis, ultrahigh-resolution mass spectrometry analysis, and in situ microscopic imaging techniques such as synchrotron-based spectromicroscopy, nanoscale secondary ion mass spectrometry, and emerging electron and optical microscopic imaging techniques. However, current techniques and methods remain far from unlocking the unknown properties of SOM. We systematically point out the limitations of the current technologies and outline the future prospects for comprehensive characterization of SOM at the molecular level and micro- and nanoscales, paying particular attention to issues of environmental concern.

The selective sorption of dissolved organic matter (DOM) on minerals is a widespread geochemical process in the natural environment. Recent studies have explored the influence of this process on the molecular fractionation of DOM at water–mineral interfaces. However, it remains unclear how molecular fractionation affects the photochemistry of DOM. Here, we demonstrate that the adsorptive fractionation of DOM on ferrihydrite greatly reduces its photoproduction of reactive oxygen species (ROS) including ¹O₂, O₂^•–, and •OH normalized to organic carbon (ROS_OC). The ROS_OC for ¹O₂, O₂^•–, and •OH were positively correlated with the abundances of polyphenols and oxygenated polycyclic aromatics, which were also observed using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis to be preferentially sequestered by ferrihydrite. The molecules that preferentially remained in the solution after adsorption displayed low levels of ROS_OC. The molecular fractionation of DOM induced by adsorption on ferrihydrite therefore influenced the molecular components and also significantly reduced the photoreactive fractions of DOM in waters. These results are very important in promoting our understanding of the effects of molecular fractionation on the biogeochemical features, behaviors, and implications of DOM in the environment.

Due to its complex composition and structure, many of the properties of natural organic matter (NOM) are poorly understood. In this study, the oxidization-induced chemiluminescence (OCL) of NOM was investigated, and a flow-injection OCL method was developed using alkaline persulfate-H2O2 as oxidizing agent. The method is suitable for the direct analysis of NOM in both homogeneous and heterogeneous samples without isolation or concentration. A strong linear relationship (p ＜ 0.001) was found between the normalized organic carbon OCL (OCLOC) and the percentage of aromatic carbon in standard NOM and soil samples, suggesting that OCLOC can be used as an empirical indicator to assess the aromaticity degree of NOM in both homogeneous and heterogeneous samples. By using this method, the percentages of aromatic carbon in a forest soil profile with low organic carbon content were estimated, and a decrease in the degree of aromaticity in deeper soil was observed. Considering the high sensitivity (lower than 0.1 mgC L-1) and throughput (13 s per detection), and low sample consumption (less than 1 mg) of the method, the proposed OCLOC indicator shows great promise for the high-throughput evaluation of the aromaticity degree of NOM for a wide variety of environmental and geochemical samples.

Reduction of ferric iron (Fe(III)) to ferrous iron (Fe(II)) by dissimilatory iron-reducing bacteria is widespread in anaerobic environments. The oxidation of Fe(II) in aerobic environments has been found to produce hydroxyl radical (·OH); however, the role of iron-reducing bacteria in the process has not been well understood. Here, Shewanella oneidensis MR-1 mediated redox transformation of four typical iron (oxyhydr)oxides and the production of reactive oxygen species were investigated. The results showed that the production of ·OH was mainly determined by the insoluble Fe(II) formed during microbially mediated reduction and also mediated by the mineralogical phase. Moreover, this study for the first time observed the exogenetic iron-independent production of ·OH by Shewanella oneidensis MR-1, and the integrated pathway of ·OH generation during iron redox process was revealed. Superoxide (O₂^·-) was indicated as a key intermediate species that was produced by both abiotic and biotic pathways, and ·OH was generated by both the exogenetic iron-dependent Fenton-like reaction and exogenetic iron-independent pathways. Shewanella oneidensis MR-1 played a pivotal role in both the reduction of Fe(III) and the production of O₂^·-. These findings contribute substantially to our understanding of the generation mechanism of reactive oxygen species at oxidation-reduction boundaries in the environment.