Binding of proton and iron to lignite humic acid size-fractions in aqueous matrix

作者：Begum Zinnat A^*; Rahman Ismail M M^*; Tate Yousuke; Ichijo Toshiharu; Hasegawa Hiroshi^*

来源：Journal of Molecular Liquids, 2018, 254: 241-247.

DOI：10.1016/j.molliq.2018.01.104

• Iron
• Lignite-derived humic acids
• Protonation
• Complexation
• Bioavailability

The bioavailability of trivalent iron (Fe3+) to plants can be enhanced using fertilizer solutions containing humic acids (HA) as manifested from the increased crop yield at an iron stress conditions. The lignite-derived HA (HA(lignite)) facilitates higher diffusion of Fe3+ between the soil layers as attributable to more number of reactive sites in the assemblage compared to those from other origins. In the current work, the proton-binding of HA(lignite) size-fractions (5-10, 10-30, 30-100, and >100 kDa), as segmented based on the molecular weight distribution, and their complexation with Fe3+ have been studied at varying pH ranging from low to high. The protonation or formation of Fe3+-complexes exhibited a comparable pattern despite the differences in the conformational distribution of HA(lignite) size-fractions. The protonation behavior specified that the behavior of HA(lignite) size-fractions has similarity with that of a dibasic acid. The results are interpreted using reactive structural units (RSU) concept to show that the carboxyl and phenolic-hydroxyl groups in the HA(lignite) size-fractions simultaneously available as the Fe3+-binding sites. The stability constants for larger MW fractions of HA(lignite) (>100 kDa) was the lowest, as attributed to the increased aggregation rate in an aqueous matrix. The trend in conditional stability constants of HA(lignite)-size fractions and other Fe-chelators point to a better Fe-binding capability of HAlignite (30-100 kDa) size-fraction than the biodegradable alternatives (GLDA, HIDS, EDDS, IDSA, or NTA), while the Fe-interaction was stronger with classical synthetic chelators (EDTA, DTPA, or EDDHA).

• 出版日期2018-3-15