Soil humic acid aggregation by dynamic light scattering and laser Doppler electrophoresis

Abstract

Humic acids (HAs), similar to other fractions of humic substances (HSs), have a large number of reactive functional groups enabling them to aggregate in solutions. Regardless of the origin of humic acid (aqueous or soil), this aggregation process is dependent on environmental conditions and strongly influences the mobility of soluble ionic and molecular pollutants. The aim of this work was to monitor the aggregation process of two humic acids isolated from different mineral soils (IHSS Elliot soil HA standard and Rendzic Leptosol HA) in the 2-11 pH range. Changes in aggregate size in HA sols were followed up using dynamic light scattering (DLS), while zeta potential (ZP) measurements in the same pH range were performed applying laser Doppler electrophoresis (LDE) technique. The effect of HA sol concentration and soil source on aggregation was examined as well. Besides, HA samples were characterized using Fourier transform infrared (FT-IR) spectroscopy. By inspecting HA-particle-size dependence on pH, it can be concluded that both HAs in corresponding sols behave as molecular aggregates or supramolecular structures, formed from small individual moieties (sizes < 10 nm) at higher pH values. The ZP vs. pH curve for both HAs revealed the ZP minimum in the 5-7 pH range, caused most likely by dissociation of acidic functional groups prevailing at lower pH values and deaggregation predominating over dissociation at higher pH values.

Properties of humic acids from copper tailings 20 years after reclamation

“…Relative band intensities (ATR-FTIR spectra) have been used to estimate relative abundances of broad classes of functional groups (OH, N-H, I 3300 ; alkyl, I 2923 ; carboxyl, I 1702 ; aromatic C, I 1620 ; carbohydrate-like structures/polysaccharides, I 1080 and I 1030 ). 11 These relative band intensities, as well as aromaticity index (I 1620 /I 2920 ) values 20,21 have been used to compare HA molecular characteristics. Relative band intensities and aromaticity index values obtained in this study are shown in Table III.…”

“…19,29 Humic substances are able to rearrange and restructure themselves in response to environmental changes such as pH, ionic strength, moisture as well as HS concentration. 21,30 Aggregation properties of HAs significantly influence their interactions in nature. For example metal complexation 31 is the reason why is important to study their colloidal character.…”

“…In this study, PSD and ZP were measured in highly alkaline solutions (pH 10), low HA concentration (0.02 g dm -3 ) and constant ionic strength (0.1 M NaCl solution) to obtain HA basic unit size. 21 Alkaline solutions were acidified by HCl to pH 3 to examine the HA reaggregation ability. Since PS is closely related to colloidal stability of humic particles, 21,32 ZP at both pHs was measured.…”

“…The measurements were performed using a Zeta-sizer Nano ZS with 633 nm He-Ne laser (Malvern, UK), and the data were analyzed by the Zetasizer software version 6.20 (Malvern, UK). Measurement details are given by Jovanović et al 21 Absorbances of alkaline and acid sols at 465 and 665 nm were recorded by UV-Vis spectroscopy to calculate the E 4 /E 6 index.…”

Characterization of pH-fractionated humic acids with respect to their dissociation behaviour

“…The increase in the humic acids yield with the increasing alkalinity of the solution applied was directly related to the capability of acid functional groups (carboxyl and phenol hydroxyl) for dissociation at definite pH values. Recent work of Jovanovic et al (2013) is focused on aggregation of humic acids in the 2-11 pH range. They concluded that humic acids behave as macromolecular aggregates or supramolecular structures, formed from small individual moieties (sizes < 10 nm) at higher pH values.…”