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ARS Home » Northeast Area » Beltsville, Maryland (BARC) » Beltsville Agricultural Research Center » Environmental Microbial & Food Safety Laboratory » Research » Publications at this Location » Publication #309703
Title: Probing the pH dependent optical properties of aquatic, terrestrial and microbial humic substances by sodium borohydride reduction

Author
item Schmidt, Walter
item HEIGHTON, L.P. - University Of Maryland

Submitted to: Journal of Geography and Geology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/27/2014
Publication Date: 9/1/2014
Publication URL: http://handle.nal.usda.gov/10113/4114166
Citation: Schmidt, W.F., Heighton, L. 2014. Probing the pH dependent optical properties of aquatic, terrestrial and microbial humic substances by sodium borohydride reduction. Journal of Geography and Geology. 6:214-227.

Interpretive Summary: Soils are a complex matrix consisting of minerals, microorganisms, and a variety of organic polymers, including humic substances (HA) and fulvic acid (FA) substances. These relatively stable polymers are the consequence of microbial degradation of plant-derived compounds. HA and FA have traditionally been differentiated from each other by extractability as a function of pH, i.e. acid soluble versus base soluble. Characterization of each by chemical and spectroscopic means is highly uncertain because the samples themselves and/or analytical methods are too variable. In this paper, we describe the effect of treating HA and FA with a reducing agent (sodium borohydride), to simplify the chemical structure. We observed that HA and FA were more readily characterized using spectroscopic methods after sodium borohydride treatment. Furthermore, our findings indicate that humic substances are the result of microbial degradation of cellulosic materials, whereas fulvic substances are the result of microbial degradation of proteinaceous materials. These findings will of of interest to other researchers interested in identification and origins of soil organic compounds.

Technical Abstract: Chemically reducing humic (HA) and fulvic acids (FA) provides insight into spectroscopically identifiable structural moieties generating the optical properties of HA/FA from aquatic, microbial and terrestrial sources. Sodium borohydride reduction provides targeted reduction of carbonyl groups. The contrast between the pH induced optical changes of untreated, reduced and reoxidized HA/FA highlights differences in the quantity, and physicality of structural components generating optical properties associated with HA/FA. Because borohydride reactions alter pH, pH re-adjustment to the original pH is required. Careful titrations of selected HA/FA provided the 'mole H+ g-1 HA/FA required to titrate reduced and reoxidized HA/FA from pH 2-11; and the pH dependent spectral slope (S) at low (pH 2-3), neutral (pH 7-7.5) and high (pH 11.0). Molar extinction coefficients (') (Lmg-1cm-1) at pH 7.6 and 350 nm provide a point of consistency linking intrinsic pH dependent optical properties to the concentration of material used for titrations. Soil derived humic acids differ from aquatic humic acids in the heterogeneity of optically identifiable group as well as the overall concentration of those groups. The microbial source of FA has a limited concentration of homogeneous titratable groups when compared to aquatic FA generated terrestrially. Microbial FA exhibits pH linked optical recovery upon reoxidation when compared to aquatic FA which is consistent with the presence of quinones in microbial FA.