Li Hang, Hanson Blair, McKay Garrett
Zachry Department of Civil & Environmental Engineering, Texas A&M University, College Station, Texas 77843, United States.
Department of Civil, Environmental, and Architectural Engineering and Environmental Engineering Program, University of Colorado Boulder, Boulder, Colorado 80303, United States.
ACS Environ Au. 2024 Dec 20;5(2):197-210. doi: 10.1021/acsenvironau.4c00075. eCollection 2025 Mar 19.
Chemical reduction with sodium borohydride has been used for over four decades to probe the presence and function of carbonyl-containing moieties in dissolved organic matter (DOM). One of these structure-property relationships is the attenuation of UV-visible absorbance after borohydride reduction, an effect that has been observed universally across DOM of different origins. We previously demonstrated that DOM with similar bulk physicochemical properties exhibits bifurcating reactivity with borohydride depending on the source (i.e., soil vs. aquatic), as judged by the kinetics of fractional absorbance removal during reduction at a fixed borohydride:DOM mass ratio. This result and data from other studies suggest that a portion of borohydride-reducible chromophores in DOM may be inaccessible to the water solvent, explaining the incomplete absorbance attenuation even at very high borohydride mass excesses. Here, we study the reactivity of five DOM isolates with sodium borohydride via size exclusion chromatography coupled to total organic carbon, absorbance, and fluorescence detectors. Reduction with sodium borohydride resulted in quantifiable yet exceedingly small decreases in DOM molecular weight, suggesting that the reduction of carbonyl groups to alcohols does not markedly impact the DOM secondary structure. Interestingly, higher molecular weight DOM exhibited the most prominent changes in optical properties after reduction, suggesting that larger molecules contain a high proportion of borohydride-reducible moieties. Optical surrogates were inversely correlated to molecular weight across a single isolate, both native and reduced. However, the correlation broke down at lower molecular weights, wherein optical surrogates remained constant with continued decreases in elution volume, suggesting that there is an intrinsic lower limit to the ability of optical surrogates to capture further decreases in molecular weight. Overall, these results provide insights into the DOM structure and help inform future applications of sodium borohydride for studying the DOM source and reactivity.
四十多年来，硼氢化钠化学还原法一直被用于探究溶解有机物（DOM）中含羰基部分的存在及功能。这些结构-性质关系之一是硼氢化钠还原后紫外-可见吸光度的衰减，这一效应在不同来源的DOM中普遍存在。我们之前证明，具有相似整体物理化学性质的DOM根据来源（即土壤与水体）与硼氢化钠表现出不同的反应性，这是通过在固定硼氢化钠：DOM质量比下还原过程中吸光度去除分数的动力学来判断的。这一结果以及其他研究的数据表明，DOM中一部分可被硼氢化钠还原的发色团可能无法被水溶剂接触到，这解释了即使在硼氢化钠质量过量很高的情况下吸光度衰减仍不完全的现象。在此，我们通过尺寸排阻色谱结合总有机碳、吸光度和荧光检测器来研究五种DOM分离物与硼氢化钠的反应性。硼氢化钠还原导致DOM分子量有可量化但极其微小的降低，这表明羰基还原为醇类对DOM二级结构没有显著影响。有趣的是，较高分子量的DOM在还原后光学性质变化最为显著，这表明较大的分子含有高比例的可被硼氢化钠还原的部分。在单个分离物（天然的和还原后的）中，光学替代物与分子量呈负相关。然而，在较低分子量时这种相关性消失了，此时随着洗脱体积持续减小，光学替代物保持不变，这表明光学替代物捕捉分子量进一步降低的能力存在内在下限。总体而言，这些结果为DOM结构提供了见解，并有助于为未来硼氢化钠在研究DOM来源和反应性方面的应用提供参考。
