Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Full Length Article DFT study on the influence of sulfur on the hydrophobicity of pyrite surfaces in the process of oxidation Peng Xi a, ⁎ , Changxing Shi a , Pingke Yan a, ⁎ , Wenli Liu b , Ligang Tang a a Department of Environmental Engineering, North China Institute of Science and Technology, Beijing 101601, China b School of Chemical and Environmental Engineering, China University of Mining and Technology (Beijing), Beijing 10083, China ARTICLE INFO Keywords: Pyrite Sulfur Hydrophobicity Quantum chemistry Density functional theory ABSTRACT Using the first-principles method of density functional theory (DFT) and the plane-wave ultra-soft pseudopo- tential method, the adsorption process of water on a perfect pyrite surface and a surface with an adsorbed sulfur atom was calculated and simulated. The mechanistic role of sulfur formed during pyrite surface oxidation in the hydrophobicity of pyrite surfaces was explored. The simulation results showed that compared with the perfect pyrite surface, the surface with an adsorbed sulfur atom had an absorption energy that went from a negative to a positive value. The strong FeeS covalent bond was formed first between the surface pyrite iron atom and the adsorbed sulfur atom. Following this, a weak SeO bond was formed between the sulfur and oxygen atoms of the water molecule. The adsorption of H 2 O on the pyrite surface was hindered by the generation of sulfur so that the surface with an adsorbed sulfur atom became hydrophilic. The simulation results are consistent with the results of wetting heat. The influence of sulfur adsorption on pyrite hydrophobicity was mechanistically revealed from the view of quantum chemistry. 1. Introduction The electrochemical properties of the pyrite surface are a key factor in determining its floatability. To explore its mechanism, the electro- chemical properties of the pyrite surface and the influence on float- ability have been experimentally studied by both domestic and overseas scholars. Hamilton and Woods et al. [1] found that the initial oxidation products on the pyrite surface were sulfur and some sulfate. Backley, Woods et al. [2], Yoon et al. [3] and Zhu et al. [4] considered that more sulfur or sulfur-like material was produced on the pyrite surface after its initial oxidation and the oxides were hydrophilic. Pyrite is the most widely distributed sulfide mineral in nature. Thus, the adsorption of organic and inorganic molecules (such as water, oxygen, hydroxyl calcium and hydroxyl) on the pyrite surface is a sig- nificant area of interest for researchers at home and abroad. The first principles calculation method based on density functional theory (DFT) can be used to analyse the mechanism of water molecule adsorption on the surface of pyrite at the atomic level. Previous studies [5,6] used density functional theory to study the effects of carbon atoms’ in- corporation and adsorption on the hydrophobicity of pyrite surfaces. The adsorption behaviour of water molecules on the surface of coal pyrite was revealed at the atomic level. However, the effect of sulfur produced in the oxidation process of pyrite on the mechanism and inherent laws of hydrophobicity was not reported at the atomic level. As the most widely distributed sulfide minerals, the adsorption of or- ganic and inorganic molecule (O 2 ,H 2 O, CaOH, OH - ) on the surface of pyrite is of interest. Chen et al. [7,8] and Li et al. [9,10] studied the adsorption mechanism of water molecules, calcium hydroxide, and oxygen molecules on the surface of pyrite and showed that density functional theory was appropriate to model pyrite surface adsorption. In this paper, the pyrite (1 0 0) surface taken as the object research and was used to calculate and simulate the adsorption of water mole- cules on the surface of ideal pyrite and pyrite covered by sulfur. The adsorption difference of the water molecule on different pyrite surfaces was compared by examining the adsorption energy, Mulliken bond population, electron density and electronic density of states. The me- chanism and inherent laws surrounding the hydrophobicity of pyrite surfaces in the initial oxidation process were revealed at the atomic level. 2. Methods and model 2.1. Model parameter selection and calculation method Structural optimizations and electronic and optical calculations were performed using CASTEP, GGA-PW91 [11,12]. The crystal https://doi.org/10.1016/j.apsusc.2018.09.197 Received 15 April 2018; Received in revised form 20 September 2018; Accepted 24 September 2018 ⁎ Corresponding authors. E-mail addresses: pengxi@ncist.edu.cn (P. Xi), yanpingke@126.com (P. Yan). Applied Surface Science 466 (2019) 964–969 Available online 24 September 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved. T