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Removal of As(V) from aqueous solution by using cement-porous hematite composite granules as adsorbent
Results in Physics 11 (2018) 23–29
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Removal of As(V) from aqueous solution by using cement-porous hematite composite granules as adsorbent
T
⁎
Wangyang Xua, Bingqiao Yangb, , Feifei Jiaa,c, Tianxing Chena, Lang Yanga, Shaoxian Songa,c,d a
School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, 430070 Wuhan, Hubei, China School of Xingfa Mining Engineering, Wuhan Institute of Technology, Xiongchu Road 693, Wuhan 430073, Hubei, China c Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Luoshi Road 122, 430070 Wuhan, Hubei, China d Hubei Provincial Collaborative Innovation Center for High Efficient Utilization of Vanadium Resources, Wuhan University of Technology, Luoshi Road 122, 430070 Wuhan, Hubei, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Porous hematite Portland cement Granule As(V) adsorption Solid-liquid separation
The potential of using Portland cement-porous hematite composite granules (CHG) as a new adsorbent for the adsorption of As(V) from aqueous solution was investigated in this study. This research was performed through the measurements of adsorption isotherm, adsorption kinetics, X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), scanning electron microscope (SEM) and energy dispersive X-ray detector (EDX). The pore structure of CHG was extraordinary developed with the porosity, the total pore volume and the average pore diameter being 45.48%, 0.3586 mL/g and 62.7 nm, respectively. From the adsorption batch of As(V) on porous hematite and CHG, CHG exhibited a high adsorption capacity of 9.84 mg/g for As(V), slightly lower than the porous hematite adsorbent. It was observed that CHG kept greater than 83% adsorption capacity of the porous hematite. However, CHG was convenient in solid-liquid separation after adsorption benefited from its high mechanical strength, stability in solutions and big size, which could be separated directly from the water without any other equipment and technology. As a result, this work provides a theoretical basis for the practical application of granular adsorbents to the actual As(V) sewage.
Introduction Arsenic contamination in natural water has been a worldwide environmental problem as it provokes toxic effects on biota [1–4]. It has been implicated in accidents with human disease or death [5,6], contributing to the long-time exposure and consumption [7–10]. Therefore, the removal of arsenic contamination from aqueous solution is very crucial. Various technologies have been developed to remove arsenic from water [11,12], such as coagulation, adsorption [13–15], ion exchange, membrane filtration, capacitive deionization (CDI) [16–18] and bioremediation [19–22]. Among these technologies, adsorption is a dominant method due to its simple operation, low cost, regeneration capacity, and no chemical addition [16,23]. To enhance the removal efficiency, miscellaneous adsorbents has been researched [24,25]. Due to the low cost, strong affinity and high selectivity for inorganic arsenic species in sorption processes [26], iron minerals are considered as effective adsorbents for arsenic removal [27–30]. However, the low surface area and large dosage of the natural iron minerals limit the arsenic removal efficiency. Therefore, arsenic
⁎
adsorption with porous iron oxides has aroused more attentions recently, which can overcome the smaller specific surface and large dosage disadvantage of natural iron ore and the complex production process and high cost shortcoming of synthetic iron ore [31,32]. Thermal modification is one of economical and effective means [33,34]. It was reported that thermal decomposition of hydrous, hydroxy and carbonate minerals (e.g., limonite, goethite and siderite) was an effective method to produce porous hematite with a large surface area and uniform pores [35,36]. Nevertheless, much difficulty in adsorption process limited the applications of traditional porous hematite. As the traditional adsorbents are usually in powders with fine particle size, they can’t be applied directly in large-scale applications due to their drawbacks in separation and recycle. Granular-based adsorption gained impetus for pollutants removal toward industrial applications in recent years [37,38]. It is obvious that the adsorption of granule has a simple treatment process, less consumption of power and a convenience in separation when the granular adsorbents are being applied to the actual treatment, which can decrease the cost of its transportation, operation and storage [39].
Corresponding author. E-mail address: [email protected] (B. Yang).
https://doi.org/10.1016/j.rinp.2018.08.031 Received 29 June 2018; Received in revised form 26 July 2018; Accepted 15 August 2018 Available online 22 August 2018 2211-3797/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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then heated from room temperature to 500 °C at a rate of 5 °C/min in a muffle furnace (Vulcan3-550PD, UK). Afterwards, the temperature in the furnace was kept constant for 4 h to prepare the porous hematite [41,42]. The crucible then was removed from the furnace and cooled to room temperature. The disc pelletizer used in this paper was modified with a coater. After starting the pelletizer inclined to 30°, the porous hematite was constantly added and water mist was constantly sprayed to obtain a kind of hematite granules (HG) with certain size. The hematite granules were taken out to weigh the mass of the hematite. A part of the HG was placed in an oven at 105 °C as the sample to characterize the water content of HG. The mass of the porous hematite was calculated by using the mass of HG minus the mass of water. Then, the remaining HG was returned to the disc pelletizer. By adding Portland cement and spraying water mist continuously, a cement layer was attached to the surface of grains to form the Portland cement-porous hematite composite granules (CHG). Finally, the CHG should be cured under dripping water at room temperature at every 12 h for 7 d until the CHG reach a certain strength.
However, the common method of granulation for iron ore powder is to add a certain amount of binder to assemble the power together to form the grain. The binders can be either inorganic or organic, including bentonite, various polymeric materials, etc [40]. However, the binder tends to block the pores of porous hematite and weaken the adsorption capacity. In this work, an innovative granulation method was studied to produce porous hematite granular adsorbents with certain strength and high specific surface area for arsenic removal. To achieve this purpose, the batch test including experimental equilibrium adsorption capacities, adsorption isotherm and kinetics studies was conducted in As(V) solution to compare the adsorption capacity of CHG and the porous hematite. In order to further understand the As(V) adsorption sites on porous hematite granular, the SEM-EDX spectra were also investigated. The research extended the knowledge into providing a new method to develop the granular adsorbent, reducing the loss of adsorbent capacity. It had important practical significance and application value for removal of arsenic in water. Experimental
Arsenic adsorption Materials Batch tests of As(V) adsorption on porous hematite and CHG were carried out in a conical flask. All the tests were carried out in triplicate, and the average values were given. In the experiments of the porous hematite, 0.1 g products were added to 100 mL As(V) solutions with different concentration in the Erlenmeyer flasks for 24 h to investigate adsorption isotherm. For the adsorption kinetics, 1 g adsorbents were added into 1 L As(V) solution with an initial concentration of 5 mg/L. In the experiments of CHG, the CHG contained about 0.1 g of porous hematite was added to 100 mL As(V) solutions with different concentration in the Erlenmeyer flasks for 72 h to investigate adsorption isotherm. For the adsorption kinetics of the CHG, the CHG contained about 1 g of porous hematite was added into 1 L As(V) solution with an initial concentration of 5 mg/L. All the initial pH of the solution was adjusted to 7.0 by using HCl or NaOH solutions. The adsorption experiments continued at 25 °C with an agitation rate of 150 rpm. After the adsorption, the mixture was filtered through a 0.22-μm membrane filter. The As(V) concentration of filtrate was measured by molybdenum blue colorimetric method [43,44] at 880 nm (Orion Aquamate, Thermo Scientific, USA). To achieve a well understanding of the effect of the innovative granulation method on the adsorption capacity of the powder adsorbent, a different method was used to calculate the adsorption capacity of CHG. The mass of the porous hematite contained in CHG took the place of the total mass of CHG to simplify operations by neglecting the weeny adsorption of the surface of CHG to calculate:
The siderite sample used in this work was collected from the Liupanshui siderite roasting plant located in Guizhou province, China. It was firstly ground, then purified with high-intensity magnetic separation. The mean particle size (D50) was of 48.63 μm for siderite after 3 min of ball milling. The purities of the samples were of 77.22% FeCO3 for siderite based on chemical analysis. Ordinary Portland cement (P.O 42.5) was used in this work, adhering to Chinese national standard 1752007 with a compressive strength of 42.5 MPa at 28 d (SAC, 2007). The D50 were of 20.95 μm for ordinary Portland cement. Disodium hydrogen arsenate heptahydrate (Na2HAsO4·7H2O), sodium hydroxide (NaOH), hydrochloric acid (HCl) were from Sinopharm Chemical Reagent Company. All chemicals were of analytical grade. All the water used in this work was produced from the Millipore Milli-Q Direct 8/16 water purification system. Preparation of the granular adsorbents The preparation scheme of the CHG was presented in Fig. 1. The siderite concentrate sample was firstly placed in a ceramic crucible, and
qCHG =
Vo (Co−Ce ) m
where qCHG was the adsorption capacity on CHG, Vo was the initial volume of As(V) solution, Co and Ce was the initial and equilibrium arsenic concentrations of the solution, respectively, m is the porous hematite weight contained in CHG used in the adsorption test. Measurement The particle size distribution of the materials was characterized with water as the dispersed phase on a Mastersizer Particle Size Analyzer (Malvern APA 2000). The Brunauer-Emmett-Teller (BET) specific surface area and pore diameter were determined by using gas adsorption analyzer. The X-ray diffraction (XRD) patterns of the materials were obtained on a D8 Advance X-ray diffractometer (BrukerAXS) with Cu Kα radiation. The mercury intrusion porosimetry (MIP) was used to obtain the pore size and specific surface area of CHG on a mercury pressure meter (AutoPore IV9500). The scanning electron
Fig. 1. Schematic illustration of preparation of the granular adsorbents. 24
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Fig. 2. The typical morphology appearance of CHG. (a) The cross-section of CHG; (b) The appearance of CHG.
microscope (SEM, Zeiss Merlin Compact) was used to analyze the morphology of the particles, while the arsenic distribution after adsorption on the materials was recorded with energy dispersive X-ray detector (EDX, Oxford X-MAX). Results and discussion Characterization of CHG The typical morphology appearance of CHG was shown in Fig. 2. As can be seen from Fig. 2(a), there was obvious boundary to divide CHG into the interior of CHG and the external of CHG. The interior of CHG was composed by porous hematite. The interior of CHG performed powder after drying, because porous hematite was only temporarily granulated due to the capillary force of water and mechanical force in the granulating process. Meanwhile, the external of CHG represented a stable layer to enwrap the interior of CHG, which indicated Portland cement reacted with water in hydration and hardening process. Then, the CHG had a certain strength and stability to keep granular in room temperature (Fig. 2(b)). Fig. 3(a) showed the XRD patterns of the siderite concentrate and the interior of CHG, in which S and H labeled on the characteristic peaks represented siderite and hematite respectively. At 500 °C, it was obvious that there was no siderite peak but hematite peaks, indicating that the siderite had completely transformed into hematite at the calcining temperature. The siderite decomposed into porous hematite for the reaction that the ferrous carbonate reacted with oxygen at 500 °C and decomposed to produce iron oxide and carbon dioxide gas [45,46]. The reaction is:
Fig. 3. X-ray diffraction patterns of the siderite and the interior of CHG (a) and the Portland cement and the external of CHG (b). Indexes: H-Hematite; SSiderite; P-Portlandite; Hy-Calicium silicate hydrate; T-Tricalcium silicate; CCalcite; D-Dicalcium silicate.
diffraction peaks of C-S-H was identified. From the above analysis, it concluded that a considerable degree of cement hydration reaction took place during the preparation process of CHG. Fig. 4(a) showed the SEM image of the interior of CHG, indicating that the porous hematite was featured by micron-scale granular structures. Due to the fact that the particle size didn’t change during calcination, the granular structures could be considered to be porous hematite crystals. And, it could be seen that the surface of the particle was uneven with a large amount of voids. This observation was attributed to the elimination of carbon oxide in the siderite crystals, leaving a welldeveloped pore construction. For the external of the CHG presented in Fig. 4(b), various hydration products in the shape of coral reefs had rich developed pores. It considered that the capillary pressure and mechanical force weakened the adhesion between cement powder during the pelleting process, which made the cement particles not close tightly. Because of the fact, the C-SH gel formed by hydration was loose and tended to present a porous structure after hardening. This was critical for the cement layer of CHG to have good permeability. The mercury intrusion-extrusion curve of CHG shown in Fig. 5. Mercury began to intrude into the sample in the low pressure, indicating that there was an amount of pores with prodigious aperture. As
500oC
FeCO3 + O2 → Fe2 O3 + CO2 The decomposition removed CO2 and CO from siderite crystal structure, forming hematite with uniform pores and large surface area, which was customarily referred to as the porous hematite. The XRD patterns of the Portland cement and the external of CHG was demonstrated in Fig. 3(b). Compared with the XRD curve of Portland cement powder, the XRD curve of the external of CHG showed that the intensity of the diffraction peak of P (portlandite) increased obviously, indicating great amount of portlandite was produced after granulation. Moreover, the intensity of diffraction peak of T (tricalcium silicate: the most principal constituent of Portland cement) decreased distinctly, implying that a considerable tricalcium silicate was involved in reaction as reactant during granulation. Cement was highly complex material whose constituents reacted with water to form various hydration products such as C-S-H (calcium silicate hydrate), portlandite [47]. The main reaction is:
3CaO∙SiO2 + nH2 O → xCaO∙SiO2 ∙ (n−3 + x ) H2 O + (3−x ) Ca (OH )2 As the C-S-H generated was amorphous, there was no new 25
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Table 1 The specific surface area, pore volume and pore dimension of the CHG measured by MIP. Sample
Total Intrusion Volume (mL/g)
Total Pore Area (m2/ g)
Median Pore Diameter (Volume) (nm)
Median Pore Diameter (Area) (nm)
Average Pore Diameter (4 V/A) (nm)
Porosity (%)
CHG
0.36
22.86
5108.1
6.1
62.7
45.48
that there were a large number of bottleneck holes in CHG. The existence of the bottleneck hole indicated that the actual pore size distribution would be larger than the measured pore size distribution. It was inferred from the data of Table 1 that the pores of the samples were well developed with the porosity up to 45.48%. This was supported by the SEM image of the surface of the CHG given in Fig. 4. The total pore volume was 0.36 mL/g and the median pore diameter calculated by volume datum was up to 5108.1 nm, which was consistent with the observation from the mercury intrusion-extrusion curve in Fig. 5 with the low pressure. Studies demonstrated that the median pore diameter calculated by volume was close to the median pore diameter calculated by area for homogeneous matter. Taking into account that CHG was made up of internal porous hematite and external cement hydration layer, it was easy to explain the great difference between the median pore diameter calculated by volume and the median pore diameter calculated by area in Table 1, being 5108.1 nm and 6.1 nm respectively. On the basis of these results, it concluded that CHG was a macroporous material with good permeability, having the average pore diameter in Table 1 with the value of 62.7 nm. The schematic illustration of solid-liquid separation for the CHG was exhibited in Fig. 6. Although many kinds of powder materials were proven to be effective adsorbents, they couldn’t be used directly in large-scale applications due to the difficulty of solid-liquid separation. CHG had good stability and maintained complete morphology after the adsorption of 72 h in the solution. And the mixed solution was basically clarified without powder dispersion and would be convenient in solidliquid separation. Unlike the CHG, the porous hematite powder was sufficiently dispersed in solution before and after adsorption and difficult for solid-liquid separation.
Fig. 4. Scanning electron microscopy image of the CHG: (a) the interior of CHG; (b) the external of CHG.
Arsenic adsorption Fig. 7 illustrated the adsorption kinetics of As(V) on the porous hematite powder and the CHG. The adsorption capacity reached 85% in about 10 h with the porous hematite as adsorbent, whereas it reached 80% in nearly 30 h with the CHG. These results revealed that As(V) adsorption was slower on the CHG. This was in anticipation and conformity with the theory that the adsorption rate of granular adsorbent was slower than that of powder adsorbent. Compared with the powder, the granular adsorbents had less contact with the solution, which must lead to more times to reach adsorption equilibrium. Nevertheless, the adsorption equilibrium time of CHG was only double times that of the
Fig. 5. The mercury intrusion-extrusion curve of the MIP of the CHG.
the test was governed by the Washburn-Laplace equation in which the size of intruded pore accesses, assimilated to cylindrical capillaries, were inversely proportional to the applied pressure:
P=
4γcosθ d
where P was the mercury injection pressure (Pa), γ was the surface tension of mercury (N/m), θ was the contact angle between solid and mercury (o), and d was the pore access diameter (m) [48]. Moreover, the cumulative intrusion volume of mercury increased rapidly to approximate the maximum under low pressure, inferring that pore diameter of pores were very high. Meanwhile, the mercury extrusion curve obviously lagged and the end point didn’t return to zero point, showing
Fig. 6. The morphologies diagram of the CHG and the porous hematite powders in the As (V) initial solution and after the adsorption for 72 h. 26
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constant of the first-order kinetic model (h−1), k2 was the rate constant of the second-order kinetic model (g·h−1·g−1). As it was expected, the rate constants of As(V) adsorption on the CHG were slower than on the porous hematite for the two models. The adsorption isotherm of As(V) on the porous hematite and the CHG were illustrated in Fig. 8. It was evident that the adsorption capacity increased with increasing As(V) equilibrium concentration for CHG and porous hematite. Moreover, the adsorption capacity didn’t reach the maximum at a considerable concentration. The data was further simulated with the Langmuir and Freundlich models. The results displayed that the adsorption fitted the Langmuir isotherm, shown in Table 3. Theoretically, the value of the constant K was related to the adsorption energy of the adsorbate on adsorbent surfaces. The larger the K value, the stronger the adsorption [41]. Therefore, the results shown in Table 3 suggested that adsorption of As (V) species on the CHG was approximate as strongly as porous hematite powder. At the same time, the qm of CHG was close to that of porous hematite, being 9.84 mg/g and 11.81 mg/g, respectively. The result indicated that CHG retained the most of adsorption capacity of internal porous hematite, which showed the innovative granulation method to produce CHG was rational.
Fig. 7. Adsorption kinetics of As(V) on minerals: the porous hematite and the CHG.
Distribution of arsenic on CHG
Table 2 The parameter of adsorption kinetics models for As(V) adsorption on the porous and the CHG. Sample
qm(mg/g)
k1(h−1)
R12
k2(g h−1g−1)
R22
Porous hematite CHG
4.75 2.23
0.35 0.05
0.82 0.99
0.108 0.015
0.92 0.99
To figure out the adsorption sites of the As(V) on CHG, SEM-EDX spectra was adopted to present the difference of elemental distribution outside the external of CHG (cement hydration layer) and the interior of CHG (porous hematite), respectively. Fig. 9 showed the SEM images and EDX spectra of the external of CHG and the interior of CHG respectively after As(V) adsorption at initial arsenic concentration of 50 mg/L. Here, EDX spectrum was used to identify if there was any arsenic on the surfaces of the two components of CHG. As it was shown, there were several peaks attributed to arsenic in the EDX spectrum for the interior of CHG, indicating that there was much arsenic adsorbed on the interior of CHG. However, there was no peak corresponding to arsenic in the case of the external of CHG, indicating that the concentration of arsenic might be lower than the limitation of the EDX or negligible arsenic was adsorbed on the external of CHG, which assumed that cement was only served as a matrix coating outside the iron-oxide [49]. These results suggested that the As(V) adsorption capacity of the CHG came from the interior porous hematite. According to the results above in the test analysis, it could assume that the cement hydration layer with developed pore construction had no adsorption on arsenic, which only acted as a framework to immobilize porous hematite powder. Compared with the SEM-EDX spectra, SEM-EDX elemental mapping could be more intuitive to perform the distribution of several elements, especially for the elements required special attention. Fig. 10 showed the morphological image of the interior of CHG, EDX elemental mapping of As and Fe on the surfaces respectively after As(V) adsorption at initial arsenic concentration of 50 mg/L. It was distinct that a good deal of As elements distributed in the internal of CHG, indicating a good adsorption of As(V) in the internal of CHG. Meantime, it exhibited that the distribution of arsenic and iron had considerable overlap, which could speculate the aqueous arsenic oxyanions such as H2AsO4- and HAsO42- undergo a ligand exchange reaction with iron species and form an inner-sphere surface complex [42,50,51].
Fig. 8. Adsorption isotherm of As(V) on minerals: the porous hematite and the CHG. Table 3 The parameter of Langmuir isotherm for As(V) adsorption on the porous and the CHG. Sample
qm(mg/g)
K(L/g)
R2
Porous hematite CHG
11.81 9.84
0.18 0.12
0.94 0.95
Conclusion CHG exhibited excellent adsorption capacity of As(V) (9.84 mg/g), which was main attributed to the good affinity of porous hematite to As (V). Meantime, the cement hydration layer on the external of CHG performed negligible adsorption, which only acted as a framework to immobilize porous hematite. Simultaneously, the high mechanical strength stability in solutions and millimeter size brought the easy
porous hematite powder, also showing the developed pore structure and good permeability of CHG from the side. The data shown in Table 2 were further simulated with the Pseudofirst-order kinetics and Pseudo-second-order kinetics models, where qm was the mass of As(V) adsorbed at equilibrium (mg/g), k1 was the rate 27
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Fig. 9. SEM images and EDX spectrum of the different components of the CHG with As(V) adsorption. (a) the interior of CHG; (b) the external of CHG. Fig. 10. SEM image of the CHG after As(V) adsorption. (a) SEM image of the interior of CHG; (b) EDX elemental mapping image of As; (c) EDX elemental mapping image of Fe.
solid-liquid separation after adsorption, which gave a prospect for the practical application of adsorbents in the As(V) sewage.
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Removal of As(V) from aqueous solution by using cement-porous hematite composite granules as adsorbent
T
⁎
Wangyang Xua, Bingqiao Yangb, , Feifei Jiaa,c, Tianxing Chena, Lang Yanga, Shaoxian Songa,c,d a
School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, 430070 Wuhan, Hubei, China School of Xingfa Mining Engineering, Wuhan Institute of Technology, Xiongchu Road 693, Wuhan 430073, Hubei, China c Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Luoshi Road 122, 430070 Wuhan, Hubei, China d Hubei Provincial Collaborative Innovation Center for High Efficient Utilization of Vanadium Resources, Wuhan University of Technology, Luoshi Road 122, 430070 Wuhan, Hubei, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Porous hematite Portland cement Granule As(V) adsorption Solid-liquid separation
The potential of using Portland cement-porous hematite composite granules (CHG) as a new adsorbent for the adsorption of As(V) from aqueous solution was investigated in this study. This research was performed through the measurements of adsorption isotherm, adsorption kinetics, X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), scanning electron microscope (SEM) and energy dispersive X-ray detector (EDX). The pore structure of CHG was extraordinary developed with the porosity, the total pore volume and the average pore diameter being 45.48%, 0.3586 mL/g and 62.7 nm, respectively. From the adsorption batch of As(V) on porous hematite and CHG, CHG exhibited a high adsorption capacity of 9.84 mg/g for As(V), slightly lower than the porous hematite adsorbent. It was observed that CHG kept greater than 83% adsorption capacity of the porous hematite. However, CHG was convenient in solid-liquid separation after adsorption benefited from its high mechanical strength, stability in solutions and big size, which could be separated directly from the water without any other equipment and technology. As a result, this work provides a theoretical basis for the practical application of granular adsorbents to the actual As(V) sewage.
Introduction Arsenic contamination in natural water has been a worldwide environmental problem as it provokes toxic effects on biota [1–4]. It has been implicated in accidents with human disease or death [5,6], contributing to the long-time exposure and consumption [7–10]. Therefore, the removal of arsenic contamination from aqueous solution is very crucial. Various technologies have been developed to remove arsenic from water [11,12], such as coagulation, adsorption [13–15], ion exchange, membrane filtration, capacitive deionization (CDI) [16–18] and bioremediation [19–22]. Among these technologies, adsorption is a dominant method due to its simple operation, low cost, regeneration capacity, and no chemical addition [16,23]. To enhance the removal efficiency, miscellaneous adsorbents has been researched [24,25]. Due to the low cost, strong affinity and high selectivity for inorganic arsenic species in sorption processes [26], iron minerals are considered as effective adsorbents for arsenic removal [27–30]. However, the low surface area and large dosage of the natural iron minerals limit the arsenic removal efficiency. Therefore, arsenic
⁎
adsorption with porous iron oxides has aroused more attentions recently, which can overcome the smaller specific surface and large dosage disadvantage of natural iron ore and the complex production process and high cost shortcoming of synthetic iron ore [31,32]. Thermal modification is one of economical and effective means [33,34]. It was reported that thermal decomposition of hydrous, hydroxy and carbonate minerals (e.g., limonite, goethite and siderite) was an effective method to produce porous hematite with a large surface area and uniform pores [35,36]. Nevertheless, much difficulty in adsorption process limited the applications of traditional porous hematite. As the traditional adsorbents are usually in powders with fine particle size, they can’t be applied directly in large-scale applications due to their drawbacks in separation and recycle. Granular-based adsorption gained impetus for pollutants removal toward industrial applications in recent years [37,38]. It is obvious that the adsorption of granule has a simple treatment process, less consumption of power and a convenience in separation when the granular adsorbents are being applied to the actual treatment, which can decrease the cost of its transportation, operation and storage [39].
Corresponding author. E-mail address: [email protected] (B. Yang).
https://doi.org/10.1016/j.rinp.2018.08.031 Received 29 June 2018; Received in revised form 26 July 2018; Accepted 15 August 2018 Available online 22 August 2018 2211-3797/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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then heated from room temperature to 500 °C at a rate of 5 °C/min in a muffle furnace (Vulcan3-550PD, UK). Afterwards, the temperature in the furnace was kept constant for 4 h to prepare the porous hematite [41,42]. The crucible then was removed from the furnace and cooled to room temperature. The disc pelletizer used in this paper was modified with a coater. After starting the pelletizer inclined to 30°, the porous hematite was constantly added and water mist was constantly sprayed to obtain a kind of hematite granules (HG) with certain size. The hematite granules were taken out to weigh the mass of the hematite. A part of the HG was placed in an oven at 105 °C as the sample to characterize the water content of HG. The mass of the porous hematite was calculated by using the mass of HG minus the mass of water. Then, the remaining HG was returned to the disc pelletizer. By adding Portland cement and spraying water mist continuously, a cement layer was attached to the surface of grains to form the Portland cement-porous hematite composite granules (CHG). Finally, the CHG should be cured under dripping water at room temperature at every 12 h for 7 d until the CHG reach a certain strength.
However, the common method of granulation for iron ore powder is to add a certain amount of binder to assemble the power together to form the grain. The binders can be either inorganic or organic, including bentonite, various polymeric materials, etc [40]. However, the binder tends to block the pores of porous hematite and weaken the adsorption capacity. In this work, an innovative granulation method was studied to produce porous hematite granular adsorbents with certain strength and high specific surface area for arsenic removal. To achieve this purpose, the batch test including experimental equilibrium adsorption capacities, adsorption isotherm and kinetics studies was conducted in As(V) solution to compare the adsorption capacity of CHG and the porous hematite. In order to further understand the As(V) adsorption sites on porous hematite granular, the SEM-EDX spectra were also investigated. The research extended the knowledge into providing a new method to develop the granular adsorbent, reducing the loss of adsorbent capacity. It had important practical significance and application value for removal of arsenic in water. Experimental
Arsenic adsorption Materials Batch tests of As(V) adsorption on porous hematite and CHG were carried out in a conical flask. All the tests were carried out in triplicate, and the average values were given. In the experiments of the porous hematite, 0.1 g products were added to 100 mL As(V) solutions with different concentration in the Erlenmeyer flasks for 24 h to investigate adsorption isotherm. For the adsorption kinetics, 1 g adsorbents were added into 1 L As(V) solution with an initial concentration of 5 mg/L. In the experiments of CHG, the CHG contained about 0.1 g of porous hematite was added to 100 mL As(V) solutions with different concentration in the Erlenmeyer flasks for 72 h to investigate adsorption isotherm. For the adsorption kinetics of the CHG, the CHG contained about 1 g of porous hematite was added into 1 L As(V) solution with an initial concentration of 5 mg/L. All the initial pH of the solution was adjusted to 7.0 by using HCl or NaOH solutions. The adsorption experiments continued at 25 °C with an agitation rate of 150 rpm. After the adsorption, the mixture was filtered through a 0.22-μm membrane filter. The As(V) concentration of filtrate was measured by molybdenum blue colorimetric method [43,44] at 880 nm (Orion Aquamate, Thermo Scientific, USA). To achieve a well understanding of the effect of the innovative granulation method on the adsorption capacity of the powder adsorbent, a different method was used to calculate the adsorption capacity of CHG. The mass of the porous hematite contained in CHG took the place of the total mass of CHG to simplify operations by neglecting the weeny adsorption of the surface of CHG to calculate:
The siderite sample used in this work was collected from the Liupanshui siderite roasting plant located in Guizhou province, China. It was firstly ground, then purified with high-intensity magnetic separation. The mean particle size (D50) was of 48.63 μm for siderite after 3 min of ball milling. The purities of the samples were of 77.22% FeCO3 for siderite based on chemical analysis. Ordinary Portland cement (P.O 42.5) was used in this work, adhering to Chinese national standard 1752007 with a compressive strength of 42.5 MPa at 28 d (SAC, 2007). The D50 were of 20.95 μm for ordinary Portland cement. Disodium hydrogen arsenate heptahydrate (Na2HAsO4·7H2O), sodium hydroxide (NaOH), hydrochloric acid (HCl) were from Sinopharm Chemical Reagent Company. All chemicals were of analytical grade. All the water used in this work was produced from the Millipore Milli-Q Direct 8/16 water purification system. Preparation of the granular adsorbents The preparation scheme of the CHG was presented in Fig. 1. The siderite concentrate sample was firstly placed in a ceramic crucible, and
qCHG =
Vo (Co−Ce ) m
where qCHG was the adsorption capacity on CHG, Vo was the initial volume of As(V) solution, Co and Ce was the initial and equilibrium arsenic concentrations of the solution, respectively, m is the porous hematite weight contained in CHG used in the adsorption test. Measurement The particle size distribution of the materials was characterized with water as the dispersed phase on a Mastersizer Particle Size Analyzer (Malvern APA 2000). The Brunauer-Emmett-Teller (BET) specific surface area and pore diameter were determined by using gas adsorption analyzer. The X-ray diffraction (XRD) patterns of the materials were obtained on a D8 Advance X-ray diffractometer (BrukerAXS) with Cu Kα radiation. The mercury intrusion porosimetry (MIP) was used to obtain the pore size and specific surface area of CHG on a mercury pressure meter (AutoPore IV9500). The scanning electron
Fig. 1. Schematic illustration of preparation of the granular adsorbents. 24
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Fig. 2. The typical morphology appearance of CHG. (a) The cross-section of CHG; (b) The appearance of CHG.
microscope (SEM, Zeiss Merlin Compact) was used to analyze the morphology of the particles, while the arsenic distribution after adsorption on the materials was recorded with energy dispersive X-ray detector (EDX, Oxford X-MAX). Results and discussion Characterization of CHG The typical morphology appearance of CHG was shown in Fig. 2. As can be seen from Fig. 2(a), there was obvious boundary to divide CHG into the interior of CHG and the external of CHG. The interior of CHG was composed by porous hematite. The interior of CHG performed powder after drying, because porous hematite was only temporarily granulated due to the capillary force of water and mechanical force in the granulating process. Meanwhile, the external of CHG represented a stable layer to enwrap the interior of CHG, which indicated Portland cement reacted with water in hydration and hardening process. Then, the CHG had a certain strength and stability to keep granular in room temperature (Fig. 2(b)). Fig. 3(a) showed the XRD patterns of the siderite concentrate and the interior of CHG, in which S and H labeled on the characteristic peaks represented siderite and hematite respectively. At 500 °C, it was obvious that there was no siderite peak but hematite peaks, indicating that the siderite had completely transformed into hematite at the calcining temperature. The siderite decomposed into porous hematite for the reaction that the ferrous carbonate reacted with oxygen at 500 °C and decomposed to produce iron oxide and carbon dioxide gas [45,46]. The reaction is:
Fig. 3. X-ray diffraction patterns of the siderite and the interior of CHG (a) and the Portland cement and the external of CHG (b). Indexes: H-Hematite; SSiderite; P-Portlandite; Hy-Calicium silicate hydrate; T-Tricalcium silicate; CCalcite; D-Dicalcium silicate.
diffraction peaks of C-S-H was identified. From the above analysis, it concluded that a considerable degree of cement hydration reaction took place during the preparation process of CHG. Fig. 4(a) showed the SEM image of the interior of CHG, indicating that the porous hematite was featured by micron-scale granular structures. Due to the fact that the particle size didn’t change during calcination, the granular structures could be considered to be porous hematite crystals. And, it could be seen that the surface of the particle was uneven with a large amount of voids. This observation was attributed to the elimination of carbon oxide in the siderite crystals, leaving a welldeveloped pore construction. For the external of the CHG presented in Fig. 4(b), various hydration products in the shape of coral reefs had rich developed pores. It considered that the capillary pressure and mechanical force weakened the adhesion between cement powder during the pelleting process, which made the cement particles not close tightly. Because of the fact, the C-SH gel formed by hydration was loose and tended to present a porous structure after hardening. This was critical for the cement layer of CHG to have good permeability. The mercury intrusion-extrusion curve of CHG shown in Fig. 5. Mercury began to intrude into the sample in the low pressure, indicating that there was an amount of pores with prodigious aperture. As
500oC
FeCO3 + O2 → Fe2 O3 + CO2 The decomposition removed CO2 and CO from siderite crystal structure, forming hematite with uniform pores and large surface area, which was customarily referred to as the porous hematite. The XRD patterns of the Portland cement and the external of CHG was demonstrated in Fig. 3(b). Compared with the XRD curve of Portland cement powder, the XRD curve of the external of CHG showed that the intensity of the diffraction peak of P (portlandite) increased obviously, indicating great amount of portlandite was produced after granulation. Moreover, the intensity of diffraction peak of T (tricalcium silicate: the most principal constituent of Portland cement) decreased distinctly, implying that a considerable tricalcium silicate was involved in reaction as reactant during granulation. Cement was highly complex material whose constituents reacted with water to form various hydration products such as C-S-H (calcium silicate hydrate), portlandite [47]. The main reaction is:
3CaO∙SiO2 + nH2 O → xCaO∙SiO2 ∙ (n−3 + x ) H2 O + (3−x ) Ca (OH )2 As the C-S-H generated was amorphous, there was no new 25
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Table 1 The specific surface area, pore volume and pore dimension of the CHG measured by MIP. Sample
Total Intrusion Volume (mL/g)
Total Pore Area (m2/ g)
Median Pore Diameter (Volume) (nm)
Median Pore Diameter (Area) (nm)
Average Pore Diameter (4 V/A) (nm)
Porosity (%)
CHG
0.36
22.86
5108.1
6.1
62.7
45.48
that there were a large number of bottleneck holes in CHG. The existence of the bottleneck hole indicated that the actual pore size distribution would be larger than the measured pore size distribution. It was inferred from the data of Table 1 that the pores of the samples were well developed with the porosity up to 45.48%. This was supported by the SEM image of the surface of the CHG given in Fig. 4. The total pore volume was 0.36 mL/g and the median pore diameter calculated by volume datum was up to 5108.1 nm, which was consistent with the observation from the mercury intrusion-extrusion curve in Fig. 5 with the low pressure. Studies demonstrated that the median pore diameter calculated by volume was close to the median pore diameter calculated by area for homogeneous matter. Taking into account that CHG was made up of internal porous hematite and external cement hydration layer, it was easy to explain the great difference between the median pore diameter calculated by volume and the median pore diameter calculated by area in Table 1, being 5108.1 nm and 6.1 nm respectively. On the basis of these results, it concluded that CHG was a macroporous material with good permeability, having the average pore diameter in Table 1 with the value of 62.7 nm. The schematic illustration of solid-liquid separation for the CHG was exhibited in Fig. 6. Although many kinds of powder materials were proven to be effective adsorbents, they couldn’t be used directly in large-scale applications due to the difficulty of solid-liquid separation. CHG had good stability and maintained complete morphology after the adsorption of 72 h in the solution. And the mixed solution was basically clarified without powder dispersion and would be convenient in solidliquid separation. Unlike the CHG, the porous hematite powder was sufficiently dispersed in solution before and after adsorption and difficult for solid-liquid separation.
Fig. 4. Scanning electron microscopy image of the CHG: (a) the interior of CHG; (b) the external of CHG.
Arsenic adsorption Fig. 7 illustrated the adsorption kinetics of As(V) on the porous hematite powder and the CHG. The adsorption capacity reached 85% in about 10 h with the porous hematite as adsorbent, whereas it reached 80% in nearly 30 h with the CHG. These results revealed that As(V) adsorption was slower on the CHG. This was in anticipation and conformity with the theory that the adsorption rate of granular adsorbent was slower than that of powder adsorbent. Compared with the powder, the granular adsorbents had less contact with the solution, which must lead to more times to reach adsorption equilibrium. Nevertheless, the adsorption equilibrium time of CHG was only double times that of the
Fig. 5. The mercury intrusion-extrusion curve of the MIP of the CHG.
the test was governed by the Washburn-Laplace equation in which the size of intruded pore accesses, assimilated to cylindrical capillaries, were inversely proportional to the applied pressure:
P=
4γcosθ d
where P was the mercury injection pressure (Pa), γ was the surface tension of mercury (N/m), θ was the contact angle between solid and mercury (o), and d was the pore access diameter (m) [48]. Moreover, the cumulative intrusion volume of mercury increased rapidly to approximate the maximum under low pressure, inferring that pore diameter of pores were very high. Meanwhile, the mercury extrusion curve obviously lagged and the end point didn’t return to zero point, showing
Fig. 6. The morphologies diagram of the CHG and the porous hematite powders in the As (V) initial solution and after the adsorption for 72 h. 26
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constant of the first-order kinetic model (h−1), k2 was the rate constant of the second-order kinetic model (g·h−1·g−1). As it was expected, the rate constants of As(V) adsorption on the CHG were slower than on the porous hematite for the two models. The adsorption isotherm of As(V) on the porous hematite and the CHG were illustrated in Fig. 8. It was evident that the adsorption capacity increased with increasing As(V) equilibrium concentration for CHG and porous hematite. Moreover, the adsorption capacity didn’t reach the maximum at a considerable concentration. The data was further simulated with the Langmuir and Freundlich models. The results displayed that the adsorption fitted the Langmuir isotherm, shown in Table 3. Theoretically, the value of the constant K was related to the adsorption energy of the adsorbate on adsorbent surfaces. The larger the K value, the stronger the adsorption [41]. Therefore, the results shown in Table 3 suggested that adsorption of As (V) species on the CHG was approximate as strongly as porous hematite powder. At the same time, the qm of CHG was close to that of porous hematite, being 9.84 mg/g and 11.81 mg/g, respectively. The result indicated that CHG retained the most of adsorption capacity of internal porous hematite, which showed the innovative granulation method to produce CHG was rational.
Fig. 7. Adsorption kinetics of As(V) on minerals: the porous hematite and the CHG.
Distribution of arsenic on CHG
Table 2 The parameter of adsorption kinetics models for As(V) adsorption on the porous and the CHG. Sample
qm(mg/g)
k1(h−1)
R12
k2(g h−1g−1)
R22
Porous hematite CHG
4.75 2.23
0.35 0.05
0.82 0.99
0.108 0.015
0.92 0.99
To figure out the adsorption sites of the As(V) on CHG, SEM-EDX spectra was adopted to present the difference of elemental distribution outside the external of CHG (cement hydration layer) and the interior of CHG (porous hematite), respectively. Fig. 9 showed the SEM images and EDX spectra of the external of CHG and the interior of CHG respectively after As(V) adsorption at initial arsenic concentration of 50 mg/L. Here, EDX spectrum was used to identify if there was any arsenic on the surfaces of the two components of CHG. As it was shown, there were several peaks attributed to arsenic in the EDX spectrum for the interior of CHG, indicating that there was much arsenic adsorbed on the interior of CHG. However, there was no peak corresponding to arsenic in the case of the external of CHG, indicating that the concentration of arsenic might be lower than the limitation of the EDX or negligible arsenic was adsorbed on the external of CHG, which assumed that cement was only served as a matrix coating outside the iron-oxide [49]. These results suggested that the As(V) adsorption capacity of the CHG came from the interior porous hematite. According to the results above in the test analysis, it could assume that the cement hydration layer with developed pore construction had no adsorption on arsenic, which only acted as a framework to immobilize porous hematite powder. Compared with the SEM-EDX spectra, SEM-EDX elemental mapping could be more intuitive to perform the distribution of several elements, especially for the elements required special attention. Fig. 10 showed the morphological image of the interior of CHG, EDX elemental mapping of As and Fe on the surfaces respectively after As(V) adsorption at initial arsenic concentration of 50 mg/L. It was distinct that a good deal of As elements distributed in the internal of CHG, indicating a good adsorption of As(V) in the internal of CHG. Meantime, it exhibited that the distribution of arsenic and iron had considerable overlap, which could speculate the aqueous arsenic oxyanions such as H2AsO4- and HAsO42- undergo a ligand exchange reaction with iron species and form an inner-sphere surface complex [42,50,51].
Fig. 8. Adsorption isotherm of As(V) on minerals: the porous hematite and the CHG. Table 3 The parameter of Langmuir isotherm for As(V) adsorption on the porous and the CHG. Sample
qm(mg/g)
K(L/g)
R2
Porous hematite CHG
11.81 9.84
0.18 0.12
0.94 0.95
Conclusion CHG exhibited excellent adsorption capacity of As(V) (9.84 mg/g), which was main attributed to the good affinity of porous hematite to As (V). Meantime, the cement hydration layer on the external of CHG performed negligible adsorption, which only acted as a framework to immobilize porous hematite. Simultaneously, the high mechanical strength stability in solutions and millimeter size brought the easy
porous hematite powder, also showing the developed pore structure and good permeability of CHG from the side. The data shown in Table 2 were further simulated with the Pseudofirst-order kinetics and Pseudo-second-order kinetics models, where qm was the mass of As(V) adsorbed at equilibrium (mg/g), k1 was the rate 27
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Fig. 9. SEM images and EDX spectrum of the different components of the CHG with As(V) adsorption. (a) the interior of CHG; (b) the external of CHG. Fig. 10. SEM image of the CHG after As(V) adsorption. (a) SEM image of the interior of CHG; (b) EDX elemental mapping image of As; (c) EDX elemental mapping image of Fe.
solid-liquid separation after adsorption, which gave a prospect for the practical application of adsorbents in the As(V) sewage.
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