Abstract
Humic acids are known as natural substances of a supramolecular nature. Their self-assembly ability can affect the migration of heavy metals and other pollutants in nature. The formation of metal-humic complexes can decrease their mobility and bioavailability. This study focuses on metal ions diffusion and immobilization in humic hydrogels. Humic acids were purchased from International Humic Substances Society (isolated from different matrices—peat, soil, leonardite, water) and extracted from lignite mined in Czech Republic. Copper(II) ions were chosen as a model example of reactive metals for the diffusion experiments. The model of instantaneous planar source was used for experimental data obtained from monitoring the time development of copper(II) ions distribution in hydrogel. The effective diffusion coefficients of copper(II) ions showed the significant dependence on reaction ability of humic hydrogels. Lower amounts of the acidic functional groups caused an increase in the effective diffusion coefficient. In general, diffusion experiments seem to act as a valuable method for reactivity mapping studies on humic substances.
Highlights
Reactivity and transport properties of metal ions are important both for evaluating and understanding the role of humic acids in natural systems and human-driven applications in solving their structural questions
Some results [13] showed that the associations have polar surfaces and unpolar cores, polar surfaces of humic particles are in contact with soil solution, less polar subunits are located in the inner layers, and unpolar structures are accommodated in the core of humic association, outward, towards the surrounding free soil environment
It was confirmed that humic acids influenced the diffusivity of metal ions in hydrogels
Summary
Reactivity and transport properties of metal ions are important both for evaluating and understanding the role of humic acids in natural systems and human-driven applications in solving their structural questions. Humic acids are recognized as a component of natural organic matter that plays a key role in the self-detoxification of soils and sediments. Their self-assembly and complexation ability can result in the reduction of the mobility of toxic metal ions (and other pollutants), biological uptake and bioaccumulation of toxic chemicals in plants as well as the pollution of the underground water supplies [1,2,3,4,5]. The concept of humic acids having a supramolecular structure is widely accepted This concept presumes that humic substances are associations of small molecules self-assembled by weak forces and hydrogen bonds [6,7,8,9,10,11,12,13]. A similar break in humic properties at a certain humic content in the studied system was observed in other works [8,27,28,29,30] and the high concentration region is often considered to be a pseudo-micellar organization phase [28]
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