Macromolecular Structure of a Commercial Humic Acid Sample

Abstract

The molecular structure of a commercial sample of humic acids (HA) was investigated by membrane dialysis experiments (MD) and low-pressure size-exclusion chromatography (LP-SEC). MD showed that HA molecules were retained by dialysis membrane with a cut-off of 6-8 kDa, independently from HA concentration (15 or 150 mg L −1 ), NaHCO 3 concentration (0.005-2.0 mol L −1 ), and from propan 2-ol (0-5 v/v %). SEC experiments at low pressure gave chromatograms with a broad peak, with an elution volume between those of the globular proteins bovine serum albumin (molecular weight = 66.5 kDa) and lysozyme from egg (molecular weight = 14.4 kDa). The pattern of the chromatogram did not vary with HA concentration, and second-run chromatograms of single eluted fractions showed relatively sharp peaks. From these data, we reveal that the commercial HA sample analysed has a macromolecular structure rather than being a supramolecular aggregate of relatively small molecules, as recently proposed for some samples of HA obtained from different sources.

Kinetic, Isothermal, and Thermodynamic Analyses of Adsorption of Humic Acid on Quaternized Porous Cellulose Beads

“…In addition, they adsorbed little HA at pH values higher than 6. HA is a macromolecular compound [32,33], and the chemical properties are different depending on the source and extraction method. In common, an HA molecule carries multiple carboxy and phenolic hydroxy groups, although its structure varies with the geographical origin, age, climate, and biological conditions [6,[34][35][36].…”

Humic Substances as Microalgal Biostimulants—Implications for Microalgal Biotechnology

“…In the “crowded” cellular environment with a high level of reactive oxygen species, HA exert antioxidant activity. The HA have a strong redox buffering activity due to the switch between oxidized quinone and reduced phenol [ 23 , 31 ].…”

“…The phenolic and carboxylic groups are responsible for the HS weak acid behavior and pH buffering [ 30 ]. The quinones–phenols switch is involved in redox buffering activity [ 25 , 31 ]. The coexistence of the hydrophobic and hydrophilic portions in HS supramolecular structure determines several properties of HS, such as the (pseudo)emulsifier effect and tendency to form micelles [ 32 , 33 ] and potential encapsulation of labile hydrophobic compounds in the hydrophobic pocket [ 34 ].…”

Separation approaches towards understanding supramolecular aggregate formation of humic acid

“…This is because the formation process is always accompanied by a change in molecular size. Based on the sharp peaks which appeared in the results of experiments on fractionated samples re-injected into low-pressure-SEC, Capasso et al inferred that commercial HA samples have a macromolecular structure rather than a supramolecular aggregate [8]. Pontoni et al conducted a high performance SEC, followed by an excitation-emission-matrix (EEM), 1 H-NMR, 1 H- 13 C heteronuclear single quantum coherence NMR, GC-MS and SEM to obtain the structural information of a natural organic matter (NOM) sample extracted from a peat soil [9].…”