Spontaneous aggregation of humic acid observed with AFM at different pH. (November 2015)

• Record Type: Journal Article
• Authors: Colombo, Claudio; Palumbo, Giuseppe; Angelico, Ruggero; Cho, Hyen Goo; Francioso, Ornella; Ertani, Andrea; Nardi, Serenella
• Is Part Of: Chemosphere. Volume 138(2015)
• Page Start: 821
• Page End: 828
• Publication Date: 2015-11
• Subjects: Humic acid -- Molecular conformation -- Particle size -- Leonardite -- Atomic force microscopy (AFM) -- Lignin–carbohydrate complex (LCC) model; Pollution -- Periodicals; Pollution -- Physiological effect -- Periodicals; Environmental sciences -- Periodicals; Atmospheric chemistry -- Periodicals; 551.511
• DOI: 10.1016/j.chemosphere.2015.08.010
• Languages: English
• ISSNs: 0045-6535

Abstract

Graphical abstract: A lignin–carbohydrate complex (LCC) model was proposed to explain the HA ring-like morphology conformation versus pH. Highlights: AFM was used to investigate the molecular dynamics of humic acid (HA). HA nanoparticles formed at pH 2–12 were observed by AFM. HA prepared at pH 5 forms ring-like conic particles. A lignin–carbohydrate complex model explains the HA ring morphology. The LCC model suggests a structure with 45–50 helical molecules looped around a cavity. Abstract: Atomic force microscopy in contact (AFM-C) mode was used to investigate the molecular dynamics of leonardite humic acid (HA) aggregate formed at different pH values. HA nanoparticles dispersed at pH values ranging from 2 to 12 were observed on a mica surface under dry conditions. The most clearly resolved and well-resulted AFM images of single particle were obtained at pH 5, where HA appeared as supramolecular particles with a conic shape and a hole in the centre. Those observations suggested that HA formed under these conditions exhibited a pseudo-amphiphilic nature, with secluded hydrophobic domains and polar subunits in direct contact with hydrophilic mica surface. Based on molecular simulation methods, a lignin–carbohydrate complex (LCC) model was proposed to explain the HA ring-like morphology. The LCC model optimized the parameters of β-O-4 linkages between 14 units of 1–4 phenyl propanoid, and resulted in an optimized structure comprising 45–50 linear helical molecules looped spirally around a central cavity. Those results added new insights on the adsorption mechanism of HA on polar surfaces as a function of pH, which was relevant from the point of view of natural aggregation in soil environment.