Computational insights into the adsorption mechanism of gallic acid-bearing reagents on calcium-bearing mineral surfaces
Graphical abstract
Introduction
Gallic acid (GA, 3,4,5-trihydroxybenzoic acid) is commonly the hydrolysis product of tannic acid (TA) and tannins (Bacelo et al., 2016). GA containing reagents have attracted much attention in mineral processing and secondary resource recovery, especially in the field of flotation, owing to its good affinity to metal sites on mineral surfaces (Chen et al., 2017, Gao et al., 2019, Rutledge and Anderson, 2015). After heating, GA can produce pyrogallic acid (pyGA). GA/pyGA possess three phenolic hydroxyl groups that are supposed to bond with the surface metal ions (Gao et al., 2019). TA also possess similar phenolic hydroxyls. A previous report had shown that TA could be used to depress calcite more strongly than fluorite (Zhang et al., 2018). Conversely, propyl gallate (propyl 3, 4, 5-trihydroxybenzoate, a derivative of GA) had been reported to have a stronger affinity for fluorite than calcite (Gao et al., 2019). These results suggest that the adsorption behavior of TA on the calcite surface is completely different from that of GA. However, the inner mechanism of the different flotation performance of TA and GA is still not well understood.
In this communication, multiscale theoretical calculations under the vacuum phase have been adopted to investigate the interaction mechanisms of GA-bearing reagents with fluorite and calcite surfaces, focusing on the spatial effects of GA-bearing reagents.
Section snippets
Models
Small molecules in an implicit solvation model were pre-optimized by using the quantum chemistry software package (More details can be found in the supportting materials). Fluorite (1 0 0) and calcite (1 0 4) surfaces were investigated as the common exposed surface (Hossain et al., 2009, Hu et al., 2012). Periodic slabs with three layers of CaCO3 and CaF2 structures were created. A vacuum depth of 30 Å was added to each slab. The the constructed slab size for the adsorption of small molecule
GA and pyGA adsorption on mineral surfaces
Solution species analysis of GA at the whole range of pH (Fig. S1), and first-principles cluster calculations, and the deprotonation site results (Fig. S2 and S3) have consistently shown that GA with deprotonated 4-OH and pyGA with deprotonated 2-OH are the major species in aqueous solution. Thus, both GA with deprotonated 4-OH and pyGA with deprotonated 2-OH have been adopted in the following interface simulations.
Fig. 1(a) shows that GA− can adsorb on the calcite surface via a single
Conclusions
First-principles calculations have revealed that GA− or pyGA− possess a fairly stronger adsorption affinity to the fluorite surface than on calcite. MD simulation results have shown that TA spreading is better on the calcite surface than on the fluorite surface due to the interaction difference of TA with the mineral surfaces. The selectivity is attributed to the balance of the chemical force and the repulsive vdW force as a result of spatial effects based on the vacuum phase calculations.
CRediT authorship contribution statement
Jianyong He: Conceptualization, Methodology, Validation, Investigation, Data curation, Writing - original draft, Visualization. Yuehua Hu: Supervision, Funding acquisition. Wei Sun: Conceptualization, Methodology, Funding acquisition. Chenyang Zhang: Conceptualization, Methodology, Validation, Investigation, Writing - review & editing. Chenhu Zhang: Methodology, Formal analysis, Writing - review & editing. Shangyong Lin: Formal analysis, Writing - review & editing. Jingxiang Zou: Software,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the China Postdoctoral Science Foundation (No. 2018M642988); the Natural Science Foundation of China (No. 51704330); the National Key Research and Development Program (No. 2016YFE0101300); the National 111 Project (No. B14034), Collaborative Innovation Center for Clean and Efficient utilization of Strategic Metal Mineral Resources.
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