(189ca) Molecular Simulation and Experimental Study of Oxalic Acid Adsorption on Water-Feldspar Interface

Molecular simulation and experimental study of
oxalic acid adsorption on water-feldspar interface

Xue
Xiaopeng (speaker),
Li Ping*, Yu Jianguo

State Key Laboratory of Chemical Engineering,
College of Chemical Engineering, East China University of Science and
Technology, Shanghai 200237, China

*Correspondence
author email: imusik@163.com, liping_2007@ecust.edu.cn

Abstract

Adsorption
of organic molecules on mineral-water interface plays a significant role in many
fields of application, such as minerals flotation with organic agents, bioleaching
minerals by organic metabolites of microorganisms, interaction between organic
acids and minerals for nutrient component acquisition in soils. Generally, there
are main two kinds of mechanism for organic molecules adsorption on mineral
surface, namely as inner-sphere adsorption and outer-sphere adsorption. For inner-sphere
adsorption, there exists the direct chemical bond adsorption between
organic ligands and cations of
mineral surface. For outer-sphere adsorption, organic molecules will interact with hydroxyl groups on
mineral surface through hydrogen bonding or electrostatic interaction.

During
bioleaching feldspar to extract potassium element as chemical fertiliser, it is
found that oxalic acid adsorption on water-feldspar
interface is important
for feldspar decomposition. Moreover, feldspar (KAlSi₃O₈)
is also most common mineral and accounts for 98% in earth¡¯s crust, and oxalic
acid is one of the most common microbial metabolites. In this work, both classical
molecular dynamic simulation and DFT calculation are adopted to investigate oxalic
acid adsorption on water-feldspar interface, and the predicted results are validated
with the detected ATR-FTIR
spectra.

Fig.1 Adsorption of H₂C₂O₄, HC₂O₄^-
and C₂O₄^2- on water-feldspar interface

Three
forms of oxalic acid interacted with feldspar are built, as shown in Fig. 1, at
adsorption equilibrium states obtained through l ns molecular dynamic
simulation. Adsorption energies for three system are calculated as 0.1 ev, 0.7ev and 1.3ev, respectively. According to the
simulation results, it is found that H₂C₂O₄ is
adsorbed by outer-sphere mechanism, while HC₂O₄^-
and C₂O₄^2- are absorbed by inner-sphere mechanism.

As
known, there are two sites, Al site and Si site, on feldspar surface for competitive
adsorption of organic
ligands, which site would
be prior to be occupied by HC₂O₄^- and C₂O₄^2-?
Here, we shall adopt both DFT simulation and ATR-FTIR detection to recognize
the characteristic bond frequencies, to validate oxalic acid adsorption on Al
site, not on Si site.

Fig.2 ATR-FTIR spectra and DFT predictions of frequency.

The detected ATR-FTIR spectra for H₂C₂O₄,
HC₂O₄^- , C₂O₄²,
feldspar, feldspar-H₂C₂O₄, feldspar-HC₂O₄^-
, feldspar-C₂O₄², are shown in Fig.2,
respectively, it is found two new peaks after adsorption of HC₂O₄^-
and C₂O₄^2-, one at 1617cm-¹ and
another at 1317cm^-1. Then, we make molecular simulation with HF/3-21g*
and DFT basis: B3LYP/6-31g* to predict frequencies of Al-oxa
and Si-oxa. It can be seen Al-Oxa
frequencies are more close to the detected frequencies. Therefore, HC₂O₄^-
and C₂O₄^2- are absorbed through interaction between
O atom of HC₂O₄^- (C₂O₄^2-)
and Al atom rather than Si atom, which is inner-sphere adsorption mechanism. H₂C₂O₄
is adsorbed through electrostatic force between O atom of feldspar and H atom
of H₂C₂O₄, which is outer-sphere adsorption
mechanism.