Kinetics and molecular structure of the binding process between coal-based fulvic acid and zinc ions

The fulvic acid (FA) extracted by lignite oxygenolysis contains a variety of reactive functional groups that leading to trace element zinc (Zn) can be chelated at these sites, and has great potential in life science and environmental remediation. The interaction mechanism of FA with Zn ions (Zn(II)) was studied by reaction kinetics. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were applied in products characterization. The three-dimensional molecular structures of the zinc fulvate (FA-Zn) were explored by using quantum chemical calculations. The results show that the chelation reaction of FA with Zn(II) had pseudo-second-order reaction kinetics; the carboxyl, hydroxyl, carbonyl, amino and mercapto groups in FA act as monodentate or multidentate ligands to form chelates with Zn(II). The molecular structure formation of FA-Zn was related to its specific functional group binding sites and binding energy. when FA bound Zn(II) intramolecularly, the model was most stable when the ortho-dicarboxylic acid site was bound, the minimum energy of this model was −4095.9342 a.u. and the bond lengths were 1.87 Å and1.86 Å; when FA bound Zn(II) intermolecularly, among the models without water, the model with carbonyl and phenolic hydroxyl sites binding was the most stable, the minimum energy of this model was −6108.2298 a.u. and the bond lengths were 1.97 Å and1.89 Å; when Zn(II) was combined with two water molecules, the model with binding at the amino site was the most stable, the minimum energy of this model was −6261.0946 a.u. and the bond lengths were 1.93 Å and 2.04 Å. [Display omitted] •Environmentally friendly chelated Zn fulvate (FA-Zn) was successfully prepared.•Zn(II) are bound to oxygen-, nitrogen- and sulfur-containing functional groups.•The chelation reaction of FA with Zn(II) conforms to a quasi-secondary kinetic model.•Optimal models for FA-Zn via intra- and inter-molecular chelation was constructed.