A standard protocol for NaBH₄reduction of CDOM and HS

Abstract

Sodium borohydride (NaBH 4 ) selectively reduces ketones/aldehydes to alcohols and quinones to hydroquinones and has been used to reduce these carbonyl moieties within humic substances (HS) and chromophoric dissolved organic matter (CDOM) to gain insight into their contributions to the optical/photochemical properties of these materials. To further investigate the effects of borohydride reduction on the absorption and emission properties of HS and to develop a standard protocol for this reduction, the effects of increasing borohydride concentrations were examined over the course of 96 h. Time courses for the reduction revealed the presence of two kinetically distinguishable pools of carbonyls that were irreversibly reduced. A third pool appears to be reversibly reduced at a low mass excess of NaBH 4 , but contributes much less to HS absorption. Reduced samples can be adjusted to pH 7 and passed through a size exclusion column (Sephadex G-10) to remove borate salts, thus producing salt-free samples that can be used for further analyses. A 25-fold mass excess of NaBH 4 relative to the HS mass produced the largest changes in optical properties without generating excessive amounts of reaction products. Although smaller additional changes are observed at higher mass excesses, the removal of excess borate becomes substantially more difficult and time consuming. While most changes are complete by 48 h, our results suggest that the reaction should be allowed to proceed for 96 h in the dark at room temperature to ensure that additional changes in the optical signals are not observed.Sodium borohydride selectively reduces carbonyl-containing compounds such as ketones/aldehydes and quinones within HS to alcohols and hydroquinones, respectively (Johnson and Rickborn 1970;Leenheer et al. 1995;Rohm and Haas 2003;Tinnacher and Honeyman 2007). Previous work has shown that treatment of aquatic CDOM/HS with sodium borohydride produces a preferential loss of visible absorption and enhanced, blue-shifted fluorescence emission (Ma et al.

Optical Properties and Photochemical Transformation of the Dissolved Organic Matter Released by Sargassum

“…No changes in optical properties were observed, suggesting that our Sargassum SPE-DOM samples could not undergo further oxidation. Reduction of Sargassum SPE-DOM samples was performed using NaBH 4 following methods and recommendations reported previously (Schendorf et al, 2016). Briefly, SPE-DOM samples were adjusted to pH 10-11 and were reduced using a 25 times mass excess (based on the DOC concentration of the SPE-DOM sample) aliquot of NaBH 4 .…”

The One-Sample PARAFAC Approach Reveals Molecular Size Distributions of Fluorescent Components in Dissolved Organic Matter

“…44,45 However, verifying this assumption is difficult for datasets containing environmental samples. While multiple studies have questioned the superposition assumption due to electronic interactions between chromophores, [46][47][48][49] evidence of electronic interactions have mainly been observed under conditions of strong chemical oxidation, 50 although one study reported selfquenching of humic acid in HPSEC separations. 51 The extent to which electronic interactions undermine the use of PARAFAC under environmental conditions is still unknown.…”

Insights on Dissolved Organic Matter Production Revealed by Removal of Charge-Transfer Interactions in Senescent Leaf Leachates

“…Leaf leachates were adjusted to pH 7 and pH 10 using high purity NaOH (Fisher, Hampton, NH, USA), and the optical properties were recorded for each pH. A total of 17.0 mg of sodium borohydride (Acros Organics, Hampton, NH, USA) was added to 17.0 mL of leaf tea, and the reactions were allowed to proceed for 48 h [16]. Leachates were then filtered through 0.45 µm nylon filters and optical properties were again recorded at pH 10, pH 7 and natural pH.…”

“…Additions of HClO 4 were used to adjust pH (VWR). Results are reported at a neutral pH of 7, as recommended by [16]. All experiments were run with three analytical replicates.…”

“…Recent studies probing origins of aquatic DOM under the premise of the EI model have employed reduction of natural samples with sodium borohydride. Sodium borohydride causes selective reduction of carbonyl containing ketones and aldehydes, removing the electron acceptors of CT interactions [9][10][11][12][13][14][15][16]. The effects of borohydride treatment on the CDOM optical properties are two-fold: (i) the removal of CT interactions through the reduction of select electron acceptors decreases absorbance, and (ii) increases fluorescence by electron donors that are no longer quenched by CT relaxation pathways [13].…”