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Journal of Resources and Ecology >

2022 , Vol. 13 >Issue 3: 537 - 546

DOI: https://doi.org/10.5814/j.issn.1674-764x.2022.03.018

Some Hot Topics in Ecology and Resources Use (Guest Editors: MIN Qingwen, SHI Peili)

Removal Arsenic(V) Efficiency and Characteristics Using Modified Basic Oxygen Furnace Slag in Aqueous Solution

  • YANG Liyun , 1, 2 ,
  • GAO Mengdan 1 ,
  • LV Yan 3 ,
  • LI Shaojie 1 ,
  • YANG Libing 3 ,
  • LI Shuwu , 1, *
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  • 1. School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China
  • 2. Department of Metallurgy, Graduate School of Engineering, Tohoku University, Sendai, Miyagi 980-8579, Japan
  • 3. Metallurgical Technology Institute, Central Iron and Steel Research Institute, Beijing 100053, China
*

YANG Liyun, E-mail:

Received date: 2021-08-06

  Accepted date: 2021-12-10

  Online published: 2022-04-18

Supported by

The Central Iron & Steel Research Institute(18161550A)

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Abstract

Basic oxygen furnace (BOF) slag, the solid waste produced in the steelmaking process, is reused in industry, agriculture and environmental treatment. However, as an adsorbent for wastewater, the removal effect of BOF slag on anionic pollutants needs to be improved. In this study, acid and alkali were used to modify BOF slag, and the removal efficiency and mechanism of arsenic(V) with modified BOF slag in solution were studied. The effects of the As(V) initial concentration, solution pH and reaction time on the removal efficiency were determined by batch experiments, and the removal mechanism of As(V) using modified BOF slag was studied by an adsorption kinetic model and isothermal adsorption model and the Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) spectral analysis. The results showed that the slag modified by 15% sulfuric acid had the best removal effect on As(V), while the removal effect of As(V) by alkali-modified slag was not ideal. The removal rate of As(V) by acid-modified slag increased with the increase in the initial concentration, decreased slowly with the increase in pH and reached equilibrium in 180 min. The adsorption kinetic model and isothermal adsorption model of As(V) by acid-modified BOF slag showed that the chemical adsorption was the limiting step. The FTIR and XPS analysis results showed that the silicate and ferrite in the acid-modified slag could remove As(V) in the solution by ion exchange to form an arsenate precipitate. Therefore, modified BOF slag can be used as a potential adsorbent for large scale arsenic polluted waterbody to realize the ecological utilization of industrial solid waste.

Key words： modified BOF slag; As(V); removal efficiency; removal mechanism

Cite this article

YANG Liyun , GAO Mengdan , LV Yan , LI Shaojie , YANG Libing , LI Shuwu . Removal Arsenic(V) Efficiency and Characteristics Using Modified Basic Oxygen Furnace Slag in Aqueous Solution[J]. Journal of Resources and Ecology, 2022 , 13(3) : 537 -546 . DOI: 10.5814/j.issn.1674-764x.2022.03.018

1 Introduction

Basic oxygen furnace (BOF) slag is an industrial solid waste produced in the steelmaking process, and its main chemical composition is CaO, SiO2, FeO and MgO (Belayachi et al., 2016). In China, the annual output of BOF slag is more than 100 million tons, accounting for 24% of the total industrial solid waste, but the utilization rate is less than 25% (Bidone et al., 2016; Chen et al., 2019). The resource utilization of BOF slag can not only decrease the environmental cost of steel plants but also effectively reduce the environmental problems such as land occupation and environmental pollution caused by BOF slag storage. At present, BOF slag is used mainly in building materials (such as paving, cement raw material, and slag brick), mine filling materials, desulfurizer in the desulfurization process or wastewater adsorbent (Ni2+, Cd2+, etc.) (Chen et al., 2003; Duan et al., 2014). In Japan, BOF slag can also be used as agricultural fertilizer and soil amendment (Fan et al., 2016; Gao et al., 2016).
BOF slag is an efficient adsorbent for heavy metal ions. The OH- produced during slag hydrolysis has strong acid neutralization properties and can perform chemical precipitation with heavy metal ions. Guo et al. (2018) studied the removal efficiency of Pb(II), Cu(II) and Cd(II) by BOF slag through batch experiments and showed that the removal rate of Pb2+ and Cu2+ by BOF slag was approximately 100%, and the removal rate of Cd2+ was approximately 85%. Ho et al. (2006) studied the removal mechanism of BOF slag for Cr(III) and Zn(II) using a column test and showed that the BOF slag removed mainly heavy metal ions through ion exchange and coprecipitation. However, in addition to heavy metal cations, there are also some harmful anions such as AsO43- in polluted water and soil. Therefore, when wastewater containing cations and anions is treated using BOF slag, the OH- produced by slag hydrolysis may compete with anions, and the removal effect of the anion is not obvious. Thus, modification methods are generally used to enhance the removal efficiency of BOF slag for anions. For example, Kang et al. (2019) modified BOF slag with Al2O3, and the modified slag could effectively remove nitrate in aqueous solution. Liu et al. (2020) modified BOF slag with nitric acid, which significantly improved the saturated adsorption capacity of BOF slag for phosphate.
Arsenic is the 20th most abundant element in the earth's crust, with a special abundance near 1.5-3.0 mg kg-1 (Mandal et al., 2002). More than 100 countries in the world, including Bangladesh, India, China, Thailand, Japan and other populous countries with high arsenic concentration in groundwater (Poh et al., 2006), have suffered from As pollution in their water environments. As element can cause a variety of damage to human health (for example, cancer, cardiovascular and cerebrovascular diseases, liver diseases, and hearing disorders). As pollution in the water environment has become a serious problem in the past decades (Saleh et al., 2011; Sharf et al., 2019). There are two forms of As in a polluted water environment: trivalent (AsO-) and pentavalent (AsO2-) As. Due to the poor affinity between AsO- and adsorbent, it is necessary to convert AsO- with strong toxicity into AsO2- in the treatment process and then remove the AsO2- by flocculation, sedimentation, adsorption, electrochemistry and other methods (Tae et al., 2017; Turkdogan et al., 2000). If BOF slag could effectively remove the AsO2- in water after modification, it would provide not only a new resource utilization method for BOF slag but also a cost- effective material for large areas of As-polluted water.
Therefore, in this study, BOF slag was modified with aluminum hydroxide and acid (sulfuric acid and hydrochloric acid), and the composition, pore structure and elemental composition of the modified slag were analyzed. The removal efficiency and mechanism of AsO2- using modified BOF slag were studied.

2 Materials and methods

2.1 Materials and instruments

The BOF slag used in this study was from the Meishan iron and steel plant, Nanjing, China. After crushing, grinding and sieving (60 mesh), the BOF slag was bagged and sealed for subsequent experiments. The main chemical composition of BOF was CaO, Fe2O3 and SiO2, which accounted for more than 85% of the total content of BOF slag (Table 1). The aluminum hydroxide, hydrochloric acid (36%-38%), and sulfuric acid (98%) were all chemically analytically pure. The aqueous solution containing As5+ was prepared by dilution of AsO43- (1000 μg mL-1) (purchased from China standard material network) with deionized water.
Table 1 Main chemical composition of raw BOF slag (Unit: %)
Main chemical composition CaO Fe2O3 SiO2 MgO MnO Al2O3 TiO2 Other
Raw BOF slag 40.81 27.59 18.56 4.53 2.73 2.81 0.68 2.30
Scanning electron microscopy (SEM-EDS, S-4800, Hitachi, Japan) was used to characterize the samples. The chemical composition of BOF slag was determined by X-ray fluorescence spectroscopy (XRF-1800, Shimadzu, Japan). X-ray diffraction (XRD, D8 ADVANCE, Bruker, Germany) was used to analyze the mineral composition of the BOF slag. Fourier transform infrared spectroscopy (FTIR, Thermo Scientific, Waltham, MA, USA) was used to analyze the functional groups on the surface of raw slag, modified slag and slag after the removal of the pollutant. X-ray photoelectron spectroscopy (XPS, Perkin Elmer PHI5600) was used to analyze the changes in the surface composition of the modified slag and the slag after the removal of the pollutant.

2.2 Modification

2.2.1 Alkali modification

The different mass ratios of raw slag: water: aluminum hydroxide (3:2:0.3, 3:2:0.5, 3:2:0.7, 3:2:0.9) were set, and mixtures were vibrated at the speed of 200 r min-1 at 25 ℃ for 180 min and then allowed to stand overnight. Subsequently, the mixtures were put into an electric blast drying oven and dried at 100 ℃ for 24 hours (Wang et al., 2008). Then, the mixtures were heated at 700 ℃, 800 ℃ and 900 ℃ for 2 hours for modification. Twelve groups of alkali-modified slag were obtained. Each modified experiment was set up with two repetitions.

2.2.2 Acid modification

The BOF slag was impregnated with sulfuric acid (10.00%, 15.00%, 20.00%, 25.00%, 30.00%) and hydrochloric acid (7.35%, 11.03%, 14.70%, 18.38%, 22.05%) at room temperature for 12 hours. The samples were washed several times with deionized water, placed in a drying oven to dry at 110 ℃ (± 5 ℃) for 2 hours and then cooled (Wu et al., 2018). After cooling, the samples were ground and screened for subsequent experiments. There were 10 group samples in total, and two repetitions for each group.

2.3 Adsorption experiment

After the best modification conditions were determined, the following adsorption experiments were completed with the modified slag under the best conditions. The adsorption efficiencies of AsO2- by modified slag at different initial As(V) solution concentrations (1 mg L-1, 2 mg L-1, 3 mg L-1, 4 mg L-1, 5 mg L-1), pH values (2, 3, 4, 5, 6) and reaction times (60 min, 120 min, 180 min, 240 min, 300 min) were studied. There were 15 groups in the adsorption experiment and 2 replicates in each group. All experiments were carried out in a 100 mL conical flask. Modified BOF slag (2.00 g) was added into 20 mL of AsO43- solution and then shaken in the constant temperature shaking box. After the adsorption reaction, the solution was filtered by centrifuge, and the content of As in the solution was determined by inductively coupled plasma atomic emission spectrometer (ICP-AES; Optima 7000DC, PerkinElmer Instruments (Shanghai, Co., Ltd. China).
The adsorption rate was calculated by Eq. 1.
$\rho =\frac{{{C}_{0}}-{{C}_{t}}}{{{C}_{0}}}\times 100%$
where ρ is the adsorption rate (%), C0 is the initial As(V) concentration (mg L-1), and Ct is the residual As(V) concentration at time t (mg L-1).
The adsorption capacity was calculated by Eq. 2.
$q=\frac{({{C}_{0}}-{{C}_{t}})\times V}{m}$
where q is the adsorption capacity (mg kg-1), V is the As(V) concentration in solution (L), and m is the mass of modified BOF slag (kg).

2.4 Data analysis

2.4.1 Adsorption kinetics

In the quasi first- and quasi second-order kinetic models of Lagergren, the quasi first-order kinetic equation shows that the main speed control step in the removal process is liquid film diffusion (Xu et al., 2015), and the quasi second-order kinetic equation shows that the removal process is controlled by chemical adsorption (Ho et al., 2006). The two kinetic models are as follows:
$\ln ({{q}_{e}}-{{q}_{t}})=\ln {{q}_{e}}-{{K}_{1}}t$
$\frac{t}{{{q}_{t}}}=\frac{1}{{{K}_{2}}{{q}_{e}}^{2}}+\frac{t}{{{q}_{e}}}$
where qe is the adsorption capacity of a unit of adsorbent at reaction equilibrium (mg kg-1), qt is the adsorption capacity of the unit adsorbent at time t (mg kg-1), K1 is the pseudo first-order adsorption rate constant (g (mg min)-1), and K2 is the pseudo second-order adsorption rate constant (g (mg min)-1).
According to the kinetic model, we can determine whether the removal of As(V) by modified slag is controlled by chemical adsorption, and the corresponding kinetic parameters can be determined.

2.4.2 Isothermal adsorption model

The Langmuir isothermal adsorption model, Freundlich isothermal adsorption model and Temkin isothermal adsorption model were used to fit the experimental data. The Langmuir model generally assumes that the adsorption site is on an adsorbent surface, and each site can adsorb a molecule, which shows the adsorption is a single molecular layer adsorption (Xue et al., 2005). The Freundlich model is a semi-empirical equation, which can be used for surface adsorption and multimolecular layer adsorption under various non-ideal conditions (Yang, 2015). The Temkin model assumes that the decrease in adsorption heat is linear. The value of the Temkin constant b is less than 8 kJ mol-1, and this value indicates that the adsorption process is physical adsorption. If the value of the Temkin constant b is more than 8 kJ mol-1, this value indicates that the adsorption process is chemical adsorption (Yang et al., 2017). The formulas of the three models are as follows:
$\frac{1}{{{q}_{e}}}=\frac{1}{{{K}_{L}}{{q}_{m}}{{C}_{e}}}+\frac{1}{{{q}_{m}}}$
$\lg {{q}_{e}}=\frac{1}{n}\lg {{C}_{e}}+\lg {{K}_{F}}$
$~~{{q}_{e}}=\frac{RT}{b}\text{ln}{{K}_{T}}+\frac{RT}{b}\text{ln}{{C}_{e}}$
where qe is the equilibrium adsorption capacity (mg kg-1); qm is the maximum adsorption capacity (mg kg-1); Ce is the equilibrium concentration of solute in solution (mg L-1); KL is the equilibrium constant related to adsorption heat (L mg-1); and KF and n are adsorption equilibrium constants related to adsorption capacity and adsorption strength, respectively. KT and b are the Temkin adsorption coefficient and the Temkin constant, respectively, which are related to the adsorption heat (J mol-1). R is the gas constant under standard conditions; T is the reaction temperature (K).

3 Results and discussion

3.1 Adsorption efficiency of modified BOF slag on As(V)

3.1.1 Alkali modification

To determine the As(V) adsorption effect of the slag after alkali modification, 2 mg L-1 of initial solution concentration and 3 hours of reaction time were used in the adsorption experiment. The result indicated that the alkali-modified BOF slag showed no obvious improvement in adsorption efficiency on As(V) in the solution (Table 2). In addition, the higher the modified temperature was, the worse the As(V) adsorption effect of the modified slag was. Yang et al. (2019) showed that when BOF slag was modified by Al2O3, the removal efficiency of nitrate could be significantly improved in solution. When Al2O3 is used to modify BOF slag, the Ca(OH)2 in BOF slag reacts with the Al(OH)3 at high temperature to generate calcium silicate (Eq. 8), which contains active Al2O3. The Al2O3 is complexed with the water molecule to form an Al-OH group, and the nitrate can be removed by exchanging the hydroxyl in the group. However, arsenate does not react in the same way.
2Al(OH)3+Ca(OH)2→CaO∙Al2O3+4H2O
Table 2 As(V) content after adsorption using alkali-modified slag
Mass ratio of
raw slag : water
: Al(OH)3		 Modified
temperature (℃)		 As(V)
(mg L-1)		 Removal rate (%) Adsorption capacity
(mg kg-1)
3 : 2 : 0.3 700 0.41±0.02 80 15.90
800 0.48±0.02 76 15.20
900 0.50±0.01 75 15.00
3 : 2 : 0.5 700 0.46±0.04 77 15.40
800 0.49±0.05 76 15.20
900 0.56±0.03 72 14.40
3 : 2 : 0.7 700 0.45±0.02 78 15.60
800 0.53±0.04 74 14.80
900 0.56±0.04 72 14.40
3 : 2 : 0.9 700 0.44±0.02 78 15.60
800 0.50±0.03 75 15.00
900 0.49±0.02 76 15.20
Raw slag 0.51±0.05 75 14.90

3.1.2 Acid modification

To determine the As(V) adsorption effect by the slag after acid modification, the initial As(V) concentration and reaction time were set as in the adsorption experiment described above (3.1.1). The result showed that when HCl was used for modification, the As(V) adsorption rate by modified slag decreased with the increase in the HCl concentration used for modification. However, when H2SO4 was used for modification, the removal rates first increased and then decreased with the increase in the H2SO4 concentration used for modification, reaching the maximum (90%) at 15% H2SO4 (Table 3). Therefore, the slag modified using 15% H2SO4 was used in the subsequent adsorption batch test.
Table 3 Removal rate of As(V) by acid modified slag
Modified
type		 Modified
concentration
(%)		 Concentration of
solution after
adsorption (mg L-1)		 Removal
rate (%)		 Adsorption capacity
(mg kg-1)
HCl 7.35 0.36±0.04 82 16.40
11.03 0.38±0.02 81 16.20
14.70 0.87±0.08 57 11.40
18.38 1.54±0.09 24 4.80
22.05 1.62±0.12 19 3.80
H2SO4 10.00 0.28±0.03 86 17.20
15.00 0.21±0.02 90 18.00
20.00 0.30±0.05 85 17.00
25.00 0.37±0.05 82 16.40
30.00 0.45±0.05 78 15.60
Raw slag 0.51±0.05 75 14.90

3.2 Characteristics of acid-modified slag and its adsorption efficiency for As(V)

3.2.1 Characteristics of acid-modified slag

The raw BOF slag was a small granule with a rough and porous surface. However, after modification by H2SO4, the BOF slag presented as a long grain, and the particle surface became rougher, which indicated that the specific surface area became larger (Fig. 1). H2SO4 could effectively remove the impurities in the pores of the BOF slag, resulting in a large number of pores. The specific surface area and adsorption site of modified BOF slag improved the adsorption efficiency of modified slag on As(V).
Fig. 1 SEM of raw BOF slag (a) and BOF slag modified by H2SO4 (b)
The results of the XRF analysis showed that the main components of the modified slag were CaO and Fe2O3, the content of which was 62.43%, and the others were oxides of Si, Mn, Mg, Al, etc. (Table 4). Compared with that of the raw slag (Table 1), the CaO content of the modified slag decreased from 40.81% to 32.19%, but the content of the other oxides (Fe2O3, SiO2 and MnO) increased in the modified slag. The total content of Fe2O3, SiO2 and MgO, which were closely related to heavy metal removal in the modified slag, was 54.66%. They can remove heavy metal ions through flocculation in aqueous solution (Yang et al., 2017).
Table 4 Main chemical composition of BOF slag modified by H2SO4 (Unit: %)
Sample CaO Fe2O3 SiO2 MgO MnO Al2O3 TiO2 Other
Modified BOF slag 32.19 30.24 21.47 4.09 3.25 2.95 0.71 5.10
The main crystalline phase of BOF slag is Ca2SiO4, Ca3SiO5, Ca2Fe2O3, Ca4Al2O3, CaAl12O19 and CaCO3. In the process of acid modification, CaCO3 reacted with H2SO4. Therefore, the crystalline phases of the modified slag were mainly Ca2SiO4, Ca3SiO5 and Ca2Fe2O5 (Fig. 2), and the content of CaO in the modified slag decreased (Table 4).
Fig. 2 XRD of the raw BOF slag (a) and BOF slag modified by H2SO4 (b)

3.2.2 As(V) adsorption efficiency by modified slag

The As(V) removal rate by the modified slag increased with the increase in the initial concentration, and then the sample increased slowly and reached equilibrium at 94%, and the maximum removal efficiency attached 21.51 mg kg-1 (Fig. 3a), which was better than other adsorbents (Table 5). When the As(V) initial concentration was low, the adsorption site of the adsorbent was sufficient; with the increase in the initial concentration, the adsorption site was gradually saturated. Therefore, the removal rate showed a gradual linear upward trend.
Fig. 3 Batch treatment experiments under different conditions. (a) As(V) adsorption rate and adsorption capacity at different initial concentrations; (b) As(V) adsorption rate and adsorption capacity at different pH; (c) As(V) adsorption rate and adsorption capacity at different reaction time.
Table 5 Adsorption capacity of different modified materials for As(V)
Adsorbent raw material Maximum adsorption capacity (mg kg-1) References
Red mud (activated by hydrochloric acid) 8.86 Altundoğan et al., 2002
Blast furnace slag 1.40 Kanel et al., 2017
Magnetic Fe3O4 0.40 Akin et al., 2012
River sand 0.44 Chen et al., 2021
Nano Fe2O3 Modified Diatomite 12.38 Yu et al., 2020
Cucurbit peel modified by FeCl3 1.86 Wang et al., 2017
Iron loaded with waste cork particles 4.90 Ariana et al., 2018
Acid modified slag 21.51 This study
When the pH was between 2 and 4, the As(V) removal rate was above 80% (Fig. 3b). In this range, the pH value had little effect on the removal rate. The removal rate decreased with the increase in pH and decreased sharply when the pH exceeded 5. When the pH increased, H+ decreased, and OH- increased gradually, which was not beneficial to As(V) removal.
When the initial As(V) concentration of solution was 2 mg L-1, the pH was 4.0, and the amount of modified BOF slag was 2 g, the As(V) adsorption rate by modified slag increased first and then tended to be stable with the increase in time. The adsorption rate was approximately 82.9% and reached equilibrium at 180 min (Fig. 3c). Therefore, the removal rate of As(V) by modified slag increased with the increase in the initial concentration, decreased slowly with the increase in the pH and reached equilibrium in 180 min.

3.3 As(V) adsorption mechanism by modified BOF slag

3.3.1 Adsorption kinetics and isotherms

Based on the hypotheses of the adsorption kinetic model, the rate-limiting step of the pseudo first-order kinetic model is mainly diffusion (membrane diffusion or internal diffusion), and that of the pseudo second-order kinetic model is chemical adsorption through the valence force (Xue et al., 2005). Fitting the straight line with ln (qe-qt) and time (min) (Fig. 4a), the values of parameters K and qe in the pseudo-first-order dynamic equation can be obtained according to the intercept of the straight line (Table 5). Similarly, fitting t/qt and time (min) to determine a straight line (Fig. 4b), the values of parameters K and qe in the pseudo-second-order dynamic equation can be obtained according to the intercept of the straight line (Table 5). According to the correlation coefficient (R2) of the fitting curve and the comparison between the theoretical adsorption capacity (qe) and the actual adsorption capacity (qe exp, 20.79 mg kg-1), we could conclude that the pseudo-second-order model was more suitable for the adsorption of As(V) by the modified slag, which showed that the adsorption rate-limiting step was controlled by the chemical adsorption (Yang et al., 2017).
Fig. 4 The pseudo-first-order model (a) and (b) the pseudo-second-order model
The As(V) removal rate by the modified slag with the change in the initial concentration was used to fit the adsorption isotherms. Through the fitting of the ce/qe and the initial concentration ce to determine a straight line (Fig. 5a), the values of parameters K and qe in the Langmuir isotherm equation could be obtained according to the intercept and slope of the straight line (Table 6); through the fitting of lnqe with initial concentration lnce to make a straight line (Fig. 5b), the values of parameters K and 1/n in the Freundlich isotherm equation could be obtained according to the intercept and slope of the straight line (Table 6); and through the fitting of qe and the initial concentration lnce to make a straight line (Fig. 5c), the values of parameters K and B in Temkin's isothermal equation could be obtained according to the intercept and slope of the straight line (Table 6).
Fig. 5 The model of (a) Langmuir isotherm, (b) Freundlich isotherm and (c) Temkin isotherm.
Table 6 The parameters of adsorption kinetics
Quasi first-order model Quasi second-order model
qe (mg kg-1) K R2 qe (mg kg-1) K R2
13.460 0.034 0.965 21.510 0.006 0.999
According to the correlation coefficient (R2) of the fitting curve (Table 6), the Langmuir model was the best fit for the As(V) adsorption by the modified BOF slag. In the Freundlich model, the larger the KF value is, the larger the adsorption capacity is. n is the adsorption strength, which represents the influence of concentration on the adsorption effect. The smaller 1/n is, the modified BOF slag has a greater removal effect in the case of a smaller As(V) concentration (Yu et al., 2012). n>1 indicates a strong interaction between adsorbent and adsorbate (Zeng et al., 2005). In Table 5, 1 < n < 10 showed that the adsorption process was favorable. In the Temkin model, the value of the Temkin constant B was 8.94 kJ mol-1 (> 8 kJ mol-1), indicating that the adsorption of As(V) in the solution with the modified BOF slag was chemical adsorption.
Table 7 The parameters for adsorption kinetics
Langmuir Freundlich Temkin
qe K R2 1/n K R2 b K R2
11.810 0.210 0.992 0.420 0.430 0.915 8.94 4.393 0.918
According to the adsorption kinetics and isotherms, the adsorption of As(V) in solution by modified slag was mainly chemical adsorption.

3.3.2 Adsorption mechanism

With the above results of the adsorption kinetics and isotherm models, the changes in the functional groups of modified BOF slag and the valence of As(V) before and after adsorption were analyzed by FTIR and XPS to discuss the adsorption mechanism of As(V) by modified BOF slag.
By comparing the FTIR spectra of the raw BOF slag and the modified BOF slag, the vibration frequency of the functional groups (Fe2O34-, SiO34-, OH-, CO32-) related to the adsorption capacity in the modified BOF slag was significantly increased (Fig. 6). After adsorption, the peaks at 599 cm-1 and 661 cm-1 from the Fe-O bond and the Si-O bond, respectively, showed stretching vibrations, which indicated that Fe2O34- and SiO44- in the modified BOF slag might have exchanged with AsO43- in the aqueous solution to form CaAs2O6. The absorption peak at 1130 cm-1 came from sulfate, and the symmetrical vibrational shrinkage peak might due to the combination of some sulfate and cation to form sulfate. At 1620 cm-1, the strength of the asymmetric stretching vibration peak increased slightly, indicating that CO32- in the modified slag might have participated in the reaction and been related to its hydrolysis (Zhao et al., 2013). The stretching vibration peak of the O-H bond appeared at 3550 cm-1, which might be related to the hydrolysis of basic oxides and silicates in modified BOF slag.
The XPS spectrum was used to identify the changes in the valence state of the surface components of the modified slag before and after adsorption. The As peak appeared in the full scan after the BOF slag adsorbed the As(V) in solution, indicating that the modified BOF slag combined with as As(V) (Fig. 7a). In addition, the Fe spectrum of the modified slag before and after adsorption did not change, indicating that the valence state of Fe remained unchanged during the whole adsorption process. The 3d spectrum of As in the XPS, the peak of 44.5 eV (As(V)) appeared (Fig. 7b), but there was no As(III) (44.78 eV)(Zhu et al., 1998), which showed that there was no oxidation-reduction reaction in the
Fig. 6 FTIR spectrum of raw BOF slag, modified BOF slag and modified BOF slag after adsorption.
Fig. 7 XPS spectrum (a) the full scanning XPS spectrum of the modified slag before and after adsorption and (b) the As 3d spectrum after adsorption
adsorption process of BOF slag on As(V), and As still existed in the BOF slag in the pentavalent form, also proving the possible ion exchange reaction mentioned above. The reaction equation was as follows:
3Ca2SiO4+4AsO43-=2Ca3(AsO4)2+3SiO44-
3Ca2Fe2O5+4AsO43-=2Ca3(AsO4)2+3Fe2O54-
Therefore, the adsorption of As(V) in aqueous solution by modified slag may have contained the following reaction mechanisms. Due to the large specific surface area and pores of the modified BOF slag, there was an interaction force (Van der Waals force) between the modified BOF slag and As(V), which was physical adsorption (Fig. 8 (1)). After modification, there were more adsorption sites of silicate (SiO44-) and ferrite (Fe2O54-) (Fig. 2), SiO44- and Fe2O54- in the modified slag that exchanged with AsO43- to form Ca3(AsO4)2 precipitate (Fig. 8 (2) - (3)), which was not easy to dissolve and relatively stable in the solution. In addition, a small amount of Fe2O3 contained in the BOF slag was hydrolyzed to produce the Fe(OH)3 colloid, which might have coprecipitated with As(V) (Fig. 8 (4)-(6)). Through these adsorption processes, the removal rate of As(V) by the modified slag could reach more than 90%. After adsorption, the precipitation formed by As(V) adsorption could be removed through acid cleaning, and some Ca2+ and Fe3+ ions would exposed from the modified slag, which could still be reused to adsorb As(V) in solution (Hao et al., 2020).
Fig. 8 The diagram of adsorption mechanism

4 Conclusions

In this study, the BOF slag was modified by alkali and acid to remove As(V) in the solution. The results showed that the BOF slag modified by 15% sulfuric acid had the best removal efficiency on As(V). The adsorption batch experiments showed that the removal rate of As(V) increased with the increase in the initial concentration and reached equilibrium at 94%; the removal rate first decreased slowly with the increase in the pH and then decreased sharply when the pH exceeded 5. The adsorption rate of As(V) by modified slag basically reached equilibrium at 180 min of reaction time.
The surface area and positive charge of the BOF slag increased after acid modification, which provided more adsorption sites for As(V). The adsorption of As(V) by modified BOF slag was consistent with the pseudo second-order kinetic model that indicated the As(V) adsorption rate-limiting step by the modified slag was controlled by the chemical adsorption. The Temkin isotherm model also showed that the adsorption of As(V) by the modified slag in the solution was mainly chemical adsorption. Based on a comparison of FTIR and XPS before and after the adsorption of modified slag, As(V) in the solution could exchange with silicate and ferrite to form Ca3(AsO4)2.

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