Interaction of humic acids with soil minerals: adsorption and surface aggregation induced by Ca2+

Abstract

Environmental context Humic acids, important components of natural organic matter in soils, sediments and aquatic media, can interact with the surface of minerals affecting key environmental processes. In the presence of calcium, humic acids can also interact among themselves leading to molecular aggregates. We demonstrate that a solid mineral surface facilitates the formation of humic acid aggregates, and thus surface aggregation occurs under conditions where normal aggregation in solution does not occur. Abstract Humic acids (HAs) interact with the surface of mineral particles leading to the formation of clay–humic complexes that affect the transport of nutrients and contaminants in the environment, soil structure, soil erosion and carbon sequestration by soils. The interaction is influenced by the presence of multivalent ions, such as Ca2+, which enhances the uptake of HAs by the particles. This article reports the effects of Ca2+ on the interaction between a HA and a soil clay fraction, both obtained from the same soil sample. The study was performed by using zeta potential measurements, HA adsorption isotherms, Ca2+ adsorption isotherms and microscopy. The results show that at low HA concentrations and low Ca2+ concentrations HA adsorption takes place, but that at high concentrations surface aggregation and precipitation also takes place, a process that is seldom reported or analysed in the literature. HA adsorption isotherms only give the overall HA uptake by the solid but they do not allow differentiation of HA adsorption from surface aggregation. However, HA adsorption v. Ca2+ concentration plots and Ca2+ adsorption isotherms at different HA concentrations can distinguish these two processes quite clearly. In addition, surface aggregation could be undoubtedly observed with optical microscopy. Surface aggregation starts to take place at a 0.7mM Ca2+ concentration, which is lower than the Ca2+ concentration needed to start HA aggregation in solution. This indicates that the surface of soil minerals acts as a nucleation centre for HA aggregation.

Effect of alginate on the aggregation kinetics of copper oxide nanoparticles (CuO NPs): bridging interaction and hetero-aggregation induced by Ca2+

“…According to the literature, Ca 2+ has the ability of producing aggregation of alginate molecules above a certain cation concentration due to the complexation of the carboxylic groups (Kloster and Avena 2015). Thus, the above quantitative comparison of the amounts of adsorbed alginate and the unadsorbed alginate allows us to hypothesize a heterocoagulation mechanism between the alginate-covered CuO NPs and the unabsorbed alginate under high concentration of Ca(NO 3 ) 2 in this study.…”

Effects of pH Conditions and Application Rates of Commercial Humic Substances on Cu and Zn Mobility in Anthropogenic Mine Soils

“…Humic and fulvic acids possess similar structures, but have different molecular weights and contents in functional groups; humic acids have higher molecular weight and less oxygen-containing functional groups than fulvic acids [9]. These chemical differences make humic acids less soluble than fulvic acids, decreasing their solubility at higher polyvalent cation contents, and lower pH due to aggregation of humic molecules themselves by cation bridging and their adsorption onto soil particles by reduced electrostatic repulsion [12][13][14][15]. Humic and fulvic acids present in humic substances contain a great variety of functional groups that are very active adsorption sites-especially carboxyl and phenolic OH (acidic groups)-and have strong complexation ability with metal ions [8,9,11,16].…”

Humik Asit Uygulamalarının Toprak Özellikleri ile Makro Besin Elementi, Mikro Besin Elementi ve Ağır Metal İçerikleri Üzerine Etkisi

“…The decrease in calcium content is in accordance with the decrease in lime content. Kloster and Avena (2015) reported that the addition of a small dose of humic acid resulted in a little increase in calcium content but that adding a large dose of humic acid decreased the calcium content.…”