Theoretical Study of As₂O₃ Adsorption Mechanisms on CaO surface

Abstract

Emission of hazardous trace elements, especially arsenic from fossil fuel combustion, have become a major concern. Under an oxidizing atmosphere, most of the arsenic converts to gaseous As₂O₃. CaO has been proven effective in capturing As₂O₃. In this study, the mechanisms of As₂O₃ adsorption on CaO surface under O₂ atmosphere were investigated by density functional theory (DFT) calculation. Stable physisorption and chemisorption structures and related reaction paths are determined; arsenite (AsO₃³⁻) is proven to be the form of adsorption products. Under the O₂ atmosphere, the adsorption product is arsenate (AsO₄³⁻), while tricalcium orthoarsenate (Ca₃As₂O₈) and dicalcium pyroarsenate (Ca₂As₂O₇) are formed according to different adsorption structures.

1. Introduction

Arsenic is a hazardous element existing in fossil fuels such as coal and petroleum [1]. According to the properties of arsenic and its compounds, it has been classified as volatile trace element by Clark and Sloss [2]. During combustion or chemical industry processes, gaseous arsenic is released into the environment. Excess amounts of arsenic can pollute water and soil. The exposure of arsenic to human may lead to hyperpigmentation, keratosis, skin and lung cancers with high possibility [3,4]. Arsenic compounds (including inorganic arsine) have been identified as hazardous air pollutants by the US government since 1990 [5]. The concentration of atmospheric arsenic in China is 51.0 ng/m³, which is much higher than the limit of NAAQS (6.0 ng/m³, GB 3095-2012) and the limit of WHO (6.6 ng/m³, WHO) [6].

Combustion of fossil fuels, especially coal, is one of the main sources for anthropogenic emission of atmospheric arsenic [7]. It was estimated that 335.5 tons of atmospheric arsenic were emitted from Chinese coal-fired plants in 2010 [8]. In 2011, the US Environmental Protection Agency issued the Mercury and Air Toxics Standards (US, MATS, updated in 2016). An arsenic emission limit of 3.0 × 10⁻³ lb/MWh (approximately 0.41 μg/m³) was set for coal-fired power plants [9]. In Chinese coal-fired power plants, the control of arsenic still remains scarce, but there are increasing interests in understanding its transformation in flue gas and developing emission reduction techniques.

Under an oxidizing atmosphere, gaseous As₂O₃ should be the main form of arsenic combustion products [10]. It has been proven that CaO could adsorb As₂O₃ in the coal-fired flue gas, and the dominating products were arsenate (AsO₄³⁻) [11,12,13,14]. CaO component in fly ash leads to the enrichment of arsenic [15,16,17,18]. R.O. Sterling [11] found that CaO could effectively adsorb As₂O₃ at 600 °C and 1000 °C; the adsorption products were Ca₃As₂O₈ when O₂ existed. Jadhav [12] studied the adsorption products of As₂O₃ on a CaO surface under O₂ atmosphere between 300 °C and 1000 °C. X-ray photoelectron spectroscopy (XPS) and X-ray Diffraction (XRD) reflected that, when temperature was lower than 600 °C, the adsorption product was Ca₃As₂O₈; when temperature was between 700 °C and 900 °C, the adsorption products was Ca₂As₂O₇; and when temperature was as high as 1000 °C, the adsorption product was Ca₃As₂O₈. He also revealed that SO₂ and HCl played a weak role in adsorption. Li [13,14] studied the influence of CO₂ and SO₂ on the capture of As₂O₃ by CaO. The existence of SO₂ and CO₂ did not change the form of arsenic in adsorption products. The previous study certified the strong adsorption of As₂O₃ on CaO surface and the important role O₂ played in the reaction. However, the acute toxicity and low concentration of arsenic significantly limit the experimental research of As₂O₃ adsorption. The adsorption mechanisms still remain unclear, especially the composition of adsorption active sites and product structures.

Quantum chemistry calculation based on density functional theory (DFT) has become an effective method to simulate structures [19] and surface reaction of volatile trace elements [20]. For example, the adsorption of As⁰ on a CaO (001) surface has been effectively studied by Zhang [21]. In this study, the adsorption structures and the detailed adsorption steps between the CaO surface and As₂O₃ (under O₂ atmosphere) have been studied by advanced DFT calculation, with the aim to offer microscopic information about critical reactions, and thus, to provide guidance to develop more efficient adsorbents and related control technologies.

2. Methods and Modeling

2.1. Methods

The material studio CASTEP [22,23] module was applied in the DFT calculation. The GGA (Generalized Gradient Approximation) and PBE [24] (Perdew-Burke-Ernzerhof) were chosen to describe the exchange and correlation interactions. The electronic wave functions were expanded on a plane wave basis with cut-off energy of 380 eV. The ultra-soft pseudo potential was referred to describe the interactions between electrons and the ionic cores [25]. ‘The spin-polarized’ option was selected for ‘spin-unrestricted’ calculations [26]. The BFGS (Broyden-Flechter-Goldfarb-Shanno) optimization algorithm was chosen for geometry optimization [27]. The transition state and reaction path (intermediate states) was determined by using the complete Linear Synchronous Transit/Quadratic Synchronous Transit (LST/QST) method [28] and confirmed by the Nudged-Elastic Band (NEB) method [29].

The convergence criteria of geometry optimization included: (a) self-consistent field (SCF) of 5.0 × 10⁻⁷ eV/atom; (b) energy of 5 × 10⁻⁶ eV/atom; (c) displacement of 5 × 10⁻⁴ Å; (d) force of 0.01 eV/Å; and (e) stress of 0.02 GPa. The convergence of complete LST/QST method (RMS, Root Mean Square) was set to 0.05 eV/Å. The convergence criteria of NEB included: (a) energy of 1.0 × 10⁻⁵ eV/atom; (b) max force of 0.05 eV/Å; and (c) max displacement of 0.004 Å.

The adsorption energy (E_ads) was defined as follows:

E_ads = E_pro − (E_slab + E_adsorbate)

(1)

where E_pro was the total energy of adsorption product, E_slab was the total energy of the slab model, and E_adsorbate was the total energy of isolated adsorbate As₂O₃ or O₂ at its equilibrium geometry. A negative E_ads value represented a stable adsorption system.

2.2. Modeling

The energy of CaO crystal cell was converged with 6 × 6 × 6 k points in the Monhorst-pack grid [30]. The equilibrium geometry of As₂O₃ and O₂ was examined in a cell of 20 × 20 × 20 ų periodic box. As shown in

Table 1. Calculated lattice parameters, bond lengths, and bond angles.

Substance	Previous Data	Simulated Data
CaO [31,32]	4.836 Å/4.807 Å	4.837 Å
As2O3 [33]	As–O bond 1.794 Å	As–O bond 1.814 Å
	As–O bond 1.610 Å	As–O bond 1.622 Å
	O–As–O angle 106.3°	O–As–O angle 111.2°
	As–O–As angle 133.8°	As–O–As angle 141.8°
O2 [34]	O–O 1.210 Å	O–O 1.240 Å

, the values of the calculated bond lengths, angles, and lattice parameters are consistent with the data reported from the previous study, indicating the reliability of the calculation.

In our previous study, the CaO(001) slab model has been widely used for CO₂ [35], Se⁰ [36] and SeO₂ [37] heterogeneous adsorption reaction, in which the good consistency with experimental work has been proven. Similarly, a 4-layer 3 × 3-surface CaO (001) slab was modeled to describe the CaO surface between CaO and As₂O₃ in this study. The superficial two layers of atoms were relaxed while the rest layers were fixed [38]. The vacuum region between slabs was set to 10 Å to avoid interactions among periodic images [39]. The energy of slab models and related adsorption structures were converged with 2 × 2 × 1 k points in the Monhorst-pack grid. The detailed modeling process was put in the Supplementary Materials (Optimization of slab model section: Tables S1 and S2).

3. Results and Discussions

According to the spatial position of As₂O₃ and surface atoms distribution, three groups, including twenty-one possible As₂O₃ structures, were first modeled as the initial structures for optimization (provided in Figure S1). After the geometric optimization of the initial structures, plenty of adsorption structures were validated, then the possible reaction paths were calculated. Based on the minimal point of the reaction paths, additional stable structures were acquired. Most of the physisorption structures were similar in terms of structural pattern and close in terms of energy level; thus, three representative physisorption structures (adsorption energy higher than −100 kJ/mol [40]) were determined. Additionally, ten chemisorption structures (adsorption energy lower than −100 kJ/mol [40]) were identified. Based on these structures, various adsorption paths were finally confirmed. For briefness, the n^th physisorption structure was abbreviated as P_n, while the n^th chemisorption structure was abbreviated as C_n.

3.1. Stable Sorption Structures

3.1.1. Stable Physisorption Structures

Three representative physisorption structures have been shown in

Table 2. Stable physisorption structures, adsorption energy, electron density cloud, and E_ads.

Name	Top View	Front View	Electron Density Cloud	Structure Details	Eads
P1				Bond12: 2.450 Å Bond34: 2.469 Å	−65.8 kJ/mol
P2				Bond12: 2.380 Å	−62.6 kJ/mol
P3				Bond12: 2.876 Å Bond34: 2.539 Å	−58.4 kJ/mol

. The dominating differences are the number of As₂O₃’s O bonded with superficial Ca and the distribution of the superficial Ca occupied by As₂O₃’s O. Three types of physisorption follow the crystal orientation <100>, <110> and <110>, respectively. Two or three superficial Ca is close to As₂O₃’s O, and the bond length is about 2.380 Å to 2.876 Å. The corresponding adsorption energy ranges from −65.8 kJ/mol to −58.4 kJ/mol. Based on electron density cloud, physisorption active sites are composed of superficial Ca atoms that interact with O of As₂O₃.

3.1.2. Stable Chemisorption Structures

Ten chemisorption structures were obtained, with E_ads ranging from −198.5 kJ/mol to −391.4 kJ/mol, which implies strong chemisorption. Superficial Ca is close to As₂O₃’s O, the bond length is about 2.269 Å to 2.528 Å, while superficial O is close to As₂O₃’s O, the bond length is 1.788 Å to 2.086 Å. According to electron density cloud and bong length, chemisorption active sites are superficial O atoms that interact with As of As₂O₃. According to the adsorption energy and structure (i.e., the positions of As and O), four categories were classified in

Table 3. Chemisorption structures, adsorption energy, electron density cloud and E_ads.

Category	Name	Top View	Front View	Electron Density Cloud	Structure Details	Eads
I	C1				Bond12: 2.635 Å Bond14: 2.086 Å Bond35: 2.360 Å	−198.5 kJ/mol
II	C2				Bond12: 1.858 Å Bond34: 2.360 Å Bond56: 2.386 Å	−222.1 kJ/mol
II	C3				Bond12: 2.424 Å Bond34: 1.815 Å Bond56: 2.314 Å Bond78: 2.490 Å	−274.4 kJ/mol
II	C4				Bond12: 2.269 Å Bond34: 1.949 Å Bond56: 2.391 Å	−292.0 kJ/mol
II	C5				Bond12: 2.293 Å Bond34: 1.943 Å Bond56: 2.298 Å	−315.1 kJ/mol
III	C6				Bond12: 1.815 Å Bond34: 2.355 Å	−302.3 kJ/mol
III	C7				Bond12: 1.788 Å Bond34: 2.528 Å Bond56: 2.514 Å Bond78: 2.422 Å	−314.0 kJ/mol
IV	C8				Bond12: 2.357 Å Bond34: 1.901 Å Bond56: 2.298 Å	−381.7 kJ/mol
IV	C9				Bond12: 2.472 Å Bond34: 1.869 Å Bond56: 2.503 Å	−388.6 kJ/mol
IV	C10				Bond12: 2.382 Å Bond34: 1.894 Å Bond56: 2.299 Å Bond78: 2.392 Å	−391.4 kJ/mol

:

• Category I: As₂O₃’s As is located on the hollow site
• Category II: All of As₂O₃’s O is located on or close to superficial Ca top site
• Category III: As₂O₃ transforms into a spoon-shaped structure
• Category IV: All of As₂O₃’s As is located on two neighboring superficial O top site

3.2. Adsorption Process

Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

3.2.1. Transformation Process of Physisorption Structures to Chemisorption Structures

In the following part, the transition state number n is abbreviated as TS_n, and the intermediate position number n is abbreviated as IP_n, for short.

As shown in Figure 1, when As₂O₃ approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P₁ to C₇ (Figure 1a), P₂ to C₈ (Figure 1b) and P₃ to C₈ (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As₂O₃ is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

3.2.2. Transformation Process of Chemisorption Structures

Chemisorbed As₂O₃ gradually transforms into more stable structures. Different possible reaction paths were calculated. The four categories of chemisorption structures can be sorted by the E_ads of each as Category IV < Category III ≈ Category II < Category I.

Category I has relatively high energy, i.e., relatively low stability, its transformation to Category II, III and IV could be triggered by molecular thermal vibration.

The pathway that Category I transforms to Category II is shown in Figure 2. Firstly, C₁ transforms into C₆ (Category II) and then C₅ (Category III), with the energy barrier of 10.8 kJ/mol, 16.7 kJ/mol, and 6.7 kJ/mol, respectively. As shown in Figure 2, Category I transforms into Category IV along with another reaction path, the related energy barrier is 7.4 kJ/mol. The relatively low energy barrier suggests that Category I is not stable enough, and could easily transform to Category II, III and IV.

The reaction path of Category II is shown in Figure 3. C₃ firstly transforms into intermediate and then converts to C₉. The corresponding energy barrier is 16.1 kJ/mol and 83.0 kJ/mol, proving that Category II transforms to Category IV with the special direction of thermal vibration.

Structures of Category III can transform into Category II, as shown in Figure 4. The spoon-shaped structure of As₂O₃ in C₇ disappears and then overcomes a 48.3 kJ/mol energy barrier to transform to C₅, indicating the conversion of Category III to Category II.

Category IV is the most stable category. C₈, C₉, and C₁₀ can transform into each other (shown in Figure 5). As₂O₃’s As does not move during the transformation. When all of As₂O₃’s O in C₈ vibrate, C₈ converts to C₉, and the energy barrier is 41.6 kJ/mol (Figure 5a). When one of As₂O₃’s O in C₈ vibrates, C₈ converts to C₁₀, and the energy barrier is 153.3 kJ/mol (Figure 5b). When one of the oxygen atoms of As₂O₃ in C₉ vibrates, C₉ converts to C₁₀, and the energy barrier is 154.4 kJ/mol (Figure 5c). The difference in energy barrier is caused by the movement distance of As₂O₃’s O being motivated by thermal vibration. In the first reaction, the movement distance of As₂O₃’s O is shorter than that in the second and third reactions, which demands relatively lower energy to overcome the energy barrier.

3.3. Path of the Reaction

According to above-mentioned processes, the reaction paths can be concluded as follows; firstly, the isolated As₂O₃ is physisorbed on a CaO surface (As₂O₃’s O weakly interacts with superficial Ca); secondly, the physisorbed As₂O₃ transforms to chemisorbed As₂O₃. (As₂O₃’s As interacts with superficial O); and thirdly, due to thermal vibration, the chemisorbed As₂O₃ transforms into more stable chemisorbed As₂O₃ (the position of As₂O₃’s O changed).

The adsorption path of As₂O₃ was summarized as the process shown in Figure 6. These reactions could be classified as three types according to the energy barrier with the aim to reflect the intensity of the required reaction temperature. The number of superficial CaO occupied by As₂O₃ is also considered in order to describe the adsorption reaction equation.

Blue arrow: energy barrier is in the range of 0–40 kJ/mol, suggesting that reaction is likely to occur under a relatively low-temperature condition.

Yellow arrow: energy barrier is in the range of 40–100 kJ/mol, suggesting that reaction is likely to occur under a relatively medium-temperature condition.

Red arrow: energy barrier is in the range of 100–200 kJ/mol, suggesting that reaction is likely to occur under a relatively high-temperature condition.

Figure 6 reveals that three main reaction paths may exist:

• As₂O₃ → P₁ → C₇ → C₉ →C₁₀;
• As₂O₃ → P₂ or P₃ → C₈ → C₉ → C₁₀;
• As₂O₃ → P₂ or P₃ → C₈ → C₁₀.

Under a relatively low-temperature condition (blue arrow, 0–40 kJ/mol), the main products are C₇ and C₈ (blue grid). Three superficial Ca and one or two superficial O are involved in the reaction, representing three CaO participates in the adsorption. The adsorption equation could be written as:

As₂O₃ + 3 CaO → Ca₃As₂O₆

(2)

Under a relatively medium-temperature condition (yellow arrow, 40–100 kJ/mol), the main products are C₉. Two superficial Ca and two superficial O participate in the structure. The adsorption equation could be written as:

As₂O₃ + 2 CaO → Ca₂As₂O₅

(3)

Under a relatively high-temperature condition (red arrow, 100–200 kJ/mol), the main product is C₁₀. Three superficial Ca and two superficial O are involved in the reaction (hollow Ca represents 1/2 Ca atom). The adsorption equation could be written as:

As₂O₃ + 3 CaO → Ca₃As₂O₆

(4)

With the reaction temperature increases, adsorption product changes from Ca₃As₂O₆ to Ca₂As₂O₅ and back to Ca₃As₂O₆ again. Different microcosmic adsorption structures lead to different macroscopic products and reaction equation.

Besides, as shown in Figure S2, the paths of C₁ transforming to other structures have been also been found. However, no possible paths which isolated or physisorbed As₂O₃ transforms to C₁ has been found, implying C₁ is unstable or nonexistent.

3.4. Partial Density of States (PDOS)

The change PDOS of As₂O₃ and CaO was put in the Supplementary Materials (Figure S3). For As₂O₃, the PDOS of physisorption structure 1, 2, and 3 are similar to each other. As the physisorption structure transforms to C₇, the p state orbitals near Fermi level (from −0.6 eV to 1.9 eV) drift to lower energy level, meanwhile get energy splitting and orbital reorganization, caused by the changing of As₂O₃ structure and the combination between As₂O₃’s As and superficial O. When C₇ transforms to C₅, s state orbital (−17.2 eV) energy level splits into two peaks of −18.0 eV and −16.9 eV, which is caused by the As-O bond breaking and the bonding between As and superficial O. When C₅ transform to C₈, the p state orbital (3.7 eV) and s state orbital (−17.9 eV) energy level both split slightly. This is the result of the slight change in the surface distribution of As₂O₃. As the adsorption products have close energies and structures, PDOS of C₉ and C₁₀ are basically similar to C₈.

For CaO slab surface, when an As₂O₃ molecule is physisorbed on the surface, little change of PDOS is detected. When As₂O₃ is chemisorbed, it can be seen that the superficial p orbitals around Fermi level (from −2.7 eV to 0.4 eV) drift to a lower energy range (from −5.8 eV to 0.2 eV). Moreover, a small peak (−16.8 eV) is separated from s orbitals (peak at −14.6 eV), proving that s orbitals participate in the chemisorption to some extent. Superficial p state orbitals near Fermi level play an important role in the chemisorption of As₂O₃. It suggests that the CaO surface’s property of capturing As₂O₃ might be improved by increasing the quantities of superficial p orbitals near Fermi level.

3.5. Influence of O₂ on Adsorbed As₂O₃

Under the flue gas atmosphere, especially O₂-containing atmosphere, O₂ reacts with chemisorbed As₂O₃; i.e., arsenite (AsO₃³⁻) is oxidized to arsenate (AsO₄³⁻). As an example, two stable chemisorption structures (C₅, C₉) identified previously were presented in

Table 4. Stable chemisorption structures under O₂ atmosphere, adsorption energy, electron density cloud and E_ads.

Name	Top View	Front View	Electron Density Cloud	Structure Details	Eads
C5 under O2				Bond12: 1.764 Å Bond34: 1.452 Å Bond56: 2.263 Å	−165.2 kJ/mol
C8 under O2				Bond12: 1.763 Å Bond34: 1.599 Å Bond56: 2.427 Å	−174.4 kJ/mol

. The distance between As₂O₃’s As and O₂’s O is 1.763–1.764 Å, which is close to the As-O bond length of As₂O₃ (1.628 Å). The distance between O₂’s O and superficial Ca is 2.247–2.263 Å. According to the electron density cloud, one of O₂’s O overlaps with As₂O₃’s As. The other O of O₂ overlaps slightly with the superficial Ca.

Based on Figure 6 and Table 4, the reaction equation of adsorption under O₂ atmosphere can be written as Equations (5)–(7), corresponding to low-temperature, medium-temperature, and high-temperature adsorption, respectively.

3CaO + As₂O₃ + O₂ → Ca₃As₂O₈

(5)

2CaO + As₂O₃ + O₂ → Ca₂As₂O₇

(6)

3CaO + As₂O₃ + O₂ → Ca₃As₂O₈

(7)

With the increase of reaction temperature, adsorption product changed from Ca₃As₂O₈ to Ca₂As₂O₇ and then to Ca₃As₂O₈ in an O₂-containing atmosphere. According to this research, the product under low-temperature and high-temperature conditions is Ca₃As₂O₈ with different structures, i.e., crystalline form. Under a medium-temperature condition, the main product is Ca₃As₂O₇.

Previous experimental research consistently reflected that the adsorption product with O₂ existence is AsO₄³⁻, while different opinions existed regarding the adsorption structures. The study of Jadhav [12] found that the adsorption product obtained under 500 °C was mainly Ca₃As₂O₈ (JCPDS No.01-0933). Under 700 °C and 900 °C, the product was Ca₂As₂O₇ (JCPDS No.17-0444). When the temperature increased to 1000 °C, the reaction product was Ca₃As₂O₈ (JCPDS No.26-0295). Mahuli [41] (600 °C and 1000 °C) and Sterling [11] (800 °C) found that the adsorption product was Ca₃As₂O₈ (JCPDS No. 26-0295), while the sorbent used by Mahuli was Ca(OH)₂. Li [13] found that the product obtained under 600 °C mainly belonged to Ca₃As₂O₈ crystal structure (JCPDS No. 01-0933), and another kind of Ca₃As₂O₈ crystal (JCPDS No. 73-1928) was identified for the products obtained under 800 °C and 1000 °C.

The role of temperature on adsorption product transformation is qualitatively described. The more detailed description of the product layer development is associated with many other factors, such as the concentration and flow rate of As₂O₃ and O₂, and the quantity and granular size of CaO. The quantitative description of the adsorption process is still a very difficult challenge. Nevertheless, the DFT calculation findings revealed by this study could directly explain the experimental results obtained by previous researchers, which might provide some meaningful insight to understand the process of As₂O₃ adsorption on CaO.

4. Conclusions

The mechanisms of As₂O₃ adsorption on a CaO surface have been studied by using DFT calculation; conclusions are as follows:

(1)

Physisorption active sites are composed of superficial Ca atoms that interact with O of As₂O₃. Chemisorption active sites are superficial O atoms that interact with As of As₂O₃;

(2)

The adsorption process can be described as follows: the isolated As₂O₃ molecule is firstly adsorbed on the CaO surface by physisorption, and then physisorbed As₂O₃ will transform to chemisorbed As₂O₃. Due to thermal vibration, the chemisorbed As₂O₃ would overcome the energy barrier and transform to a more stable chemisorbed As₂O₃ state. The adsorption product is AsO₃³⁻;

(3)

The adsorption products of As₂O₃ under an O₂-containing atmosphere are AsO₄³⁻. The adsorption product’s structure is influenced by the adsorption temperature. Under relatively low-temperature, the product is Ca₃As₂O₈; under relatively medium-temperature, the product is Ca₃As₂O₇; and under relatively high-temperature, the product is Ca₃As₂O₈.

The consistency between DFT calculation and the previous experiments proves high possibilities to design and optimized the CaO-based adsorbents by modifying O sites or other elements. Besides, other flue gases such as SO₂ or CO₂ can be involved in the following study to achieve materials design under real flue gas conditions. The optimized CaO-based adsorbents should be of high industrial value, could be applied in the injection of limestone into the furnace, CaO looping reactor, and dry desulfurization, etc.