pH effect in aquatic fulvic acid from a Brazilian river

Abstract

Estudos do comportamento morfológico dos ácidos fúlvicos foram realizados utilizando a técnica de microscopia de força atômica aliada às técnicas de potencial zeta e espectroscopia de fluorescência. Os resultados mostraram que o ácido fúlvico (AFA) de um rio tropical mudou a sua morfologia quando variado o pH da solução aquosa dos AFA. Em pH ácido a morfologia é mais granular formando aglomerados de altura entre 10-55 nm. Em pH alcalino os valores foram de 2,5-4,0 nm, sendo que a morfologia em condições alcalinas é aberta formando estruturas não granulares. Estes resultados sugerem que para pH 3,5 as interações eletrostáticas são fracas e as ligações de hidrogênio são responsáveis pela formação de agregados, enquanto em pH 9,0 as interações eletrostáticas são fortes devido ao aumento da ionização de grupos fenólicos e ocorrem poucas ligações hidrogênio formando estruturas mais abertas. Os resultados de espectroscopia de fluorescência mostraram mudança na estrutura do AFA de rio tropical em pH alcalino em comparação com a estrutura em pH ácido, corroborando com os resultados obtidos pela AFM.Studies of the morphological behavior of fulvic acids were carried out using the Atomic Force Microscopy technique together with zeta Potential techniques and fluorescence spectroscopy. Results showed that the aquatic fulvic acid (AFA) of a Brazilian River changed its morphology when the AFA aqueous solution had its pH changed. In acid pH the morphology is granular forming 10-55 nm high agglomerates. In alkaline pH values were 2.5-4.0 nm, and the morphology forms non granular structures. These results suggest that for pH 3.0 electrostatic interactions are weak and hydrogen bonding is responsible for the aggregates formation; while at pH 9.0 electrostatic interactions are strong due to the increase in phenolic groups ionization and low hydrogen interaction forming open structures. Fluorescence spectroscopy results confirmed the AFM, that is, a change in the AFA in tropical river in alkaline pH compared to the structure in acid pH.

Aspects of protonation and deprotonation of humic scid durface on molecular conformation

“…These results corroborate with Saab et al 28 and Zara et al 12 which suggested that in acid pH weak electrostatic interactions and hydrogen bonding are responsible for aggregates formation while in alkaline pH electrostatic interactions are strong due to the increase in groups ionization, resulting in more flexibles structures by dispersive forces.…”

Water Dynamics and Its Role in Structural Hysteresis of Dissolved Organic Matter

“…52 Under the experimental conditions used here (i.e., acidic pH), the phenolic groups are protonized and may form H-bonds with the deprotonized carboxylic groups. 58 For this reason, we suggest that the Hbonds play a more important role in SRFA aggregates stabilization as compared to that in PPFA.…”

Unrefined Humic Substances as a Potential Low-cost Remediation Method for Groundwater Contaminated with Uranium in Acidic Conditions

“…Therefore, removal of Huma-K at pH 4 may be attributed to sorption as well as surface-induced and homogeneous precipitation. Similar results have been reported in studies of aquatic fulvic acid: at pH 3, fulvic molecules formed agglomerates at the surface of a mica sheet because most of the fulvic molecules were uncharged (Saab et al, 2010). Balnois et al (1999) studied the agglomeration of a hydrophilic humic acid from the Suwannee River and a peat humic acid on a surface of muscovite mica.…”