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Simultaneous removal of ammonia and phosphate using green synthesized iron oxide nanoparticles dispersed onto zeolite
Journal Pre-proofs Simultaneous removal of ammonia and phosphate using green synthesized iron oxide nanoparticles dispersed onto zeolite Qianyu Xu, Wenpeng Li, Li Ma, Dan Cao, Gary Owens, Zuliang Chen PII: DOI: Reference:
S0048-9697(19)34994-0 https://doi.org/10.1016/j.scitotenv.2019.135002 STOTEN 135002
To appear in:
Science of the Total Environment
Received Date: Revised Date: Accepted Date:
23 July 2019 13 October 2019 14 October 2019
Please cite this article as: Q. Xu, W. Li, L. Ma, D. Cao, G. Owens, Z. Chen, Simultaneous removal of ammonia and phosphate using green synthesized iron oxide nanoparticles dispersed onto zeolite, Science of the Total Environment (2019), doi: https://doi.org/10.1016/j.scitotenv.2019.135002
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Simultaneous removal of ammonia and phosphate using green synthesized iron oxide nanoparticles dispersed onto zeolite
Qianyu Xu1, Wenpeng Li1, Li Ma1, Dan Cao1, Gary Owens2 and Zuliang Chen1*
1School
of Environmental Science and Engineering, Fujian Normal University, Fuzhou
350007, Fujian Province, China. 2Environmental
Contaminants Group, Future Industries Institute, University of South
Australian, Mawson Lakes, SA 5095, Australia.
*Corresponding author address: School of Environmental Science and Engineering, Fujian Normal University, Fuzhou 350007, Fujian Province, China. Email: [email protected]
1
Abstract Since elevated levels of common nutrients, such as ammonia and phosphate, in natural water bodies (lakes and rivers) can lead to significant deterioration of pristine water ecosystems due to eutrophication, new and cost-effectiveness remediation strategies are urgently required. This work investigated the feasibility of using green synthesized iron oxide nanoparticles dispersed onto zeolite by eucalyptus leaf extracts (ELMNP@zeolite), to simultaneously remove ammonia and phosphate from aqueous solutions. SEM and XRD both showed that EL-MNP@zeolite had better stability and dispersity than unsupported zeolite. At an initial concentration of 10 mg L-1 each for the two co-existing ions the synthesized material removed 43.3% of NH4+ and 99.8% of PO43- . Removal of co-existing NH4+-PO43- was impacted by the ratio of zeolite to ELMNP, temperature, initial ion concentration and solution pH. Under optimium conditions the maximum adsorption capacity of EL-MNP@zeolite for NH4+ and PO43was 3.47 and 38.91 mg g-1, respectively. For both ions’ adsorption followed a pseudosecond-order kinetic reaction, confirming that the removal of ammonia and phosphate by EL-MNP@zeolite was via chemisorptions, where interaction between NH4+-PO43and EL-MNP@zeolite may be through either electrostatic adsorption or ligand exchange. Overall these results indicated that EL-MNP@zeolite had significant potential as a nano-remediation strategy to simultaneously remove cationic ammonium and anionic phosphate from wastewaters.
Keywords: ammonium; iron oxide nanoparticles; phosphate; zeolite.
2
1. Introduction Eutrophication is one of the most serious problems affecting aquatic ecosystems in China. Where serious outbreaks of cyanobacteria can not only affect the ecological function and water quality of rivers and lakes, but can also affect the safety of drinking water and consequently human health (Zhang et al., 2019). Unfortunately, rapid industrial development and changes in other human activities have led to a worldwide increase in the levels of nitrogen (N) and phosphorus (P) entering rivers, lakes and coastal waters, which has directly led to increased incidents of eutrophication and the frequency of toxic algal blooms (Sondergaard and Jeppesen, 2007). Whist many common methods exist for the removal of nitrogen and phosphorus; including adsorption, biological removal, crystallization, ion exchange and precipitation (Huo et al., 2012), and of all the current environmental remediation technologies, adsorption the most advantageous because it is economical, consumes only small amounts of energy and is relatively easy to practically implement (Zheng and Wang, 2010). At present, the natural adsorbents most widely used in wastewater treatment include activated carbon, clay minerals, iron oxides and zeolites (Bonetto et al., 2015). Of these adsorbents, zeolites are particular attractive because modified zeolite in combination with struvite crystals were shown to simultaneously remove both ammonia and phosphate from simulated aquaculture wastewater (Huang et al., 2014a). However, the modification of zeolites via the addition of secondary chemical reagents during the composite synthesis process is not always ideal as this can produce secondary pollution. Thus, many researchers have instead stressed the need to develop an effective “green” material for the simultaneous removal of both nitrogen and phosphorus (Wang and Peng, 2010; Wan et al., 2017; Liu et al., 2013; Gao et al., 2008). The green synthesis of magnetic iron oxide nanoparticles (MNPs) has garnered 3
increasing attention for the removal of phosphate (Wang et al., 2014b) due to this method’s biocompatibility, cost-effectiveness and eco-friendliness. Additionally, as shown in our previous work (Huang et al., 2014b, 2014c) high surface area iron oxides can easily be synthesized. Previous batch experiments have shown that the maximum adsorption capacity of zero-valent nano-iron for phosphate was 54.34 mg g-1 (Maamoun et al., 2018), while another showed > 90% nitrate removal efficiency from water using a laboratory-scale nZVI-based continuous processing system (Khalil et al., 2018). These studies indicated that nZVI is a viable adsorbent for removing both ammonia and phosphate from water. In addition, the green synthesis of nZVI using plant extracts has been more common and more widely applicable for the efficient removal of total nitrogen and phosphorus from waters (Devath et al., 2016; Padil and Cerník, 2013; Shameli et al., 2012; Lukman et al., 2011; Parsons et al., 2007). However, one of the issues limiting the practical application of nZVI for water treatment is that nZVI with high specific surface areas tend to agglomerate in aqueous solution which compromises both dispersibility and reactivity (Cao et.al, 2016). Many researchers have hence addressed this issue by decorating nZVI on a natural support. For example, using ordinary zeolite as an adsorbent support, a technique for preparing an iron-modified zeolite for arsenate removal was developed where the amount of arsenate adsorbed by the iron-modified zeolite depended on the concentration of iron ions on the adsorbent (Baskan and Pala, 2011). An iron-modified zeolite was also used to remove cyanide from water, where theiron-modified zeolite had a stronger adsorption capacity than the support zeolite and the maximum adsorption capacity reached 33.98 mg g-1 (Maulana and Takahashi, 2018). While these studies indicate that iron-modified zeolites have potential for environmental remediation, few studies have applied them to mixed nutrient rich (N and P) waste waters. 4
However, since natural zeolites were used to successfully remove ammonium (Wan et al., 2017; Huang et al., 2014a) and MNPs were used to successfully remove phosphate (Gao et al., 2016), it should be possible to disperse MNPs onto zeolites to form a hybrid material exhibiting enhanced reactivity and dispersibility of the MNPs and capacity to simultaneously remove both nitrogen and phosphorus. Based on this hypothesis, an iron oxide nanoparticle-zeolite hybrid (EL-MNP@zeolite) was green synthesized using eucalyptus leaf extract and the factors affecting the removal efficiency of ammonium and phosphate ions determined. Specifically, this study involved (1) characterization of EL-MNP and EL-MNP@zeolite before and after calcination so that any changes in surface and chemical species could be recognized; and (2) batch adsorption experiments to evaluate the absorption of NH4+-PO43- by EL-MNP@zeolite, supported by adsorption kinetic and thermodynamic studies.
2. Experimental 2.1 Materials Ferric trichloride hexahydrate (FeCl3·6H2O, Analytical Reagent Grade) and sodium acetate (CH3COONa, Analytical Reagent Grade) were purchased from Guangzhou Chemical Reagent Co., Ltd. (China) and used without further purification. Zeolite ((SiO2)x(Al2O3)y Analytical Reagent Grade) was bought from Sinopharm Chemical Reagent Co., Ltd. (China) and also used without further purification. Eucalyptus leaves were obtained from the campus of Fujian Normal University in Fujian, China. Deionized (DI) water was used throughout all experiments.
2.1.1 Preparation of EL-MNP and EL-MNP@zeolite The eucalyptus leaf extract (EL) was prepared as described previously (Wang et al., 5
2014a). In this traditional method, a blend of FeCl3·6H2O (0.02 mol) and CH3COONa (0.24 mol) was dissolved in eucalyptus leaf extract (120 mL) and continuously stirred for 2 hours at 70 oC to obtain EL-MNP. The EL-MNP was washed using anhydrous ethanol and deionized water three times and then dried at 45 oC under vacuum for 12 hours.
For the synthesis of the hybrid material, FeCl3·6H2O (0.02 mol) and CH3COONa (0.24 mol) were initially added into eucalyptus leaf extract (120 mL) and stirred for 1 hour at 70 oC before being filtered. Subsequently, zeolite was added into the EL-MNP solution at three different volume ratios (zeolite: EL-MNP = 1:1, 2:1 and 3:1) to obtain ELMNP@zeolite (Fig. 1). The final EL-MNP@zeolite solid was filtered (0.45 μm) and washed with anhydrous ethanol and deionized water three times and dried in a vaccum oven at 45 oC s for > 12 hours. The resulting black powder was stored under vacuum prior to sieving to < 110 mesh.
Fig. 1 Synthesized mechanism of EL-MNP@zeolite Since zeolites have many different functional groups across their surface, and iron oxide nanoparticles are charged, iron oxide nanoparticles can be loaded onto zeolites by a combination of electrostatic and functional group interactions. 6
2.2. Characterization Morphology and particle size distributions of both EL-MNP and EL-MNP@zeolite were investigated by scanning electron microscope (SEM) (JSM 7500F, Japan). ELMNP@zeolite before and after calcination were characterized using an X-ray diffraction meter (XRD) with a high-power Cu Kα radioactive source (k=0.154 nm) at 40 kV and 40 mA, in the 2θ range 10-80° at a scanning rate of 3° min-1.
2.3. Batch experiments Batch adsorption experiments, studying the simultaneous removal of NH4+-PO43- by EL-MNP@zeolite, used the following set of four experiment conditions. For a fixed temperature (30 °C), initial concentration (10 mg L-1) and pH (6.35), the dose of ELMNP@zeolite was varied (4, 5, 6, 7, 8, 9 and 10 g L-1). For a fixed temperature (30 °C), EL-MNP@zeolite dose (7 g L-1) and pH (6.35), the initial concentrations were varied (5, 10, 20, 40, 60, 80 and 100 mg L-1). For a fixed EL-MNP@zeolite dose (7 g L-1), initial concentration (10 mg L-1), and pH (6.35), temperature was varied (20, 30 and 40 °C). For a fixed temperature (30 °C), EL-MNP@zeolite dose (7 g L-1), and initial concentration (10 mg L-1) pH was varied (4.00, 6.00, 8.00, 10.00 and 12.00). Unless otherwise stated above all other experimental conditions were maintained as follows: the required EL-MNP@zeolite (zeolite: EL-MNP = 2:1) dose was added to a mixed ammonium and phosphate stock solution (25 mL) in a 50 mL glass tube and placed in a shaker (250 r min-1) equilibrated to the desired temperature. After shaking for the defined equilibrium time, the adsorbent was filtered from the solution with a 0.45 um membrane filter and the residual concentration in the solution determined spectrophotometrically. All experiments were at least triplicated. 7
Following exposure to EL-MNP@zeolite for predetermined periods (5, 10, 15, 20, 30 and 60 min) the residual concentrations (Ce) of NH4+ and PO43- in the filtered solutions were measured using a UV-Spectrophotometer (752N, Shanghai, China) at 420 nm and 700 nm, respectively. The removal efficiencies were then calculated using the following equation (Weng et al., 2013): η=
C0 ― Ce C0
(1)
× 100%
Where η (%) was the nitrogen and phosphorus removal efficiency, C0 was the initial nitrogen and phosphorus concentration (mg L-1), and Ce was the residual nitrogen and phosphorus concentration after removal (mg L-1). Two parallel samples were set up for all experiments.
3. Results and Discussion 3.1 Characterizations SEM images of EL-MNP (Fig. 2a) revealed small spherical EL-MNP particles with an average size of 60 nm, which clearly indicated that the eucalyptus leaf extracts could reduce metallic ions to obtain EL-MNPs. The size distribution observed here was more uniform compared with iron nanoparticles synthesized using sodium borohydride as a reducing agent (Wang et al., 2014b; Chen et al., 2011; Chen et al., 2013). When ELMNPs were introduced into the zeolite (Fig. 2b) EL-MNP appeared to be spread across the surface and pore spaces of the zeolite, which was ascribed to the existence of many iron nanoparticles becoming embedded within the zeolite network (Zhang et al., 2011). Compared to the discrete EL-MNPs material, the produced hybrid material had a more dispersive which was more advantageous for removal of NH4+ and PO43-.
8
Fig. 2 Scanning electron microscopy images of EL-MNP (a) and EL-MNP@zeolite (b).
The XRD patterns of EL-MNP and EL-MNP@zeolite before and after synthesized were not dramatically different (Fig. 3). Since only a few characteristic peaks were observed this indicated that the crystal structure of the EL-MNP nanoparticles obtained by green synthesis was amorphous (Cao et al., 2016). The broad peak at 2θ = 26.4° was characteristic of zeolite, which was decreased in intensity following hybrid formation, suggesting that extract of both nano-iron and eucalyptus leaves were acting as capping agent/stabilizer by covering the surface of the zeolite thereby making the characteristic peaks of the zeolite rougher (Weng et al., 2013; Wang et al., 2014b). Characteristic peaks appearing at 2θ of 35.7°, 35.5° and 20-35° were attibuatble to maghemite (γFe2O3), magnetite (iron oxide) and iron hydroxide, respectively, which indicated that oxidation occurred during the formation of green tea synthesized iron nanoparticles (GT-Fe NPs) (Cao et al., 2016; Weng et al., 2013; Wang et al., 2014b; SuazoHernandez et al., 2019).
9
Fig. 3 XRD patterns of EL-MNP (lower trace) and EL-MNP@zeolite (upper trace).
3.2. Batch experiments As the ratio of zeolite to EL-MNP increased from 1:1, 2:1, to 3:1, both the ammonium and phosphate removal efficiencies increased from 11.9%, 37.8% to 42.3% and 96.9%, 98.0% to 98.6%, respectively (Fig. 4). The overall removal efficiency for both ions was thus enhanced by improving the ratio of zeolite to EL-MNP which reached a higher adsorption at 3:1. The removal of phosphate was consistently better than that of ammonium. Zeolites in the natural environment have a large specific surface area that is difficult to match with other mineral materials due to the aluminosilicates which: firstly, embrace many channels in their structure (Wan et al., 2017; Huang et al., 2014a); and secondly, improve the dispensability of iron nanoparticles. Furthermore, the total number of surface adsorptive sites of EL-MNP@zeolite increased when zeolite was added to the EL-MNP, leading to increased adsorption of both NH4+ and PO43- onto iron nanoparticles (Wan et al., 2017; Cao et al., 2016).
10
Fig. 4 Removal efficiency of ammonium and phosphate by EL-MNP@zeolite (zeolite: EL-MNP= 1:1, 2:1 and 3:1). Conditions: C (NH4+-PO43-) = 10 mg L-1; 30 ℃; dosage 7 g L-1; 30 min.
The removal kinetics of NH4+ and PO43- was extremely rapid (Fig. 5). Within the first two minutes, the removal efficiency of both NH4+ and PO43- rapidly increased to 33.2% and 95.1%, respectively, before gradually increasing more slowly over time, eventually rising to 40.2% and 98.8%, for NH4+ and PO43- respectively at equilibrium at 15 min.
11
Fig. 5 Removal of NH4+-PO43- Conditions: C (NH4+-PO43-) = 10 mg L-1; 30 ℃; dosage 7 g L-1, 30 min.
In batch adsorption experiments, the removal efficiencies of NH4+ were significantly affected by the hybrid dose while the removal efficiency of phosphate remained largely unaffected (Fig. 6a) and this suggested that coacity for phosphate is large but the limitied amount of a binding sites for ammonium exist, so as increase dose more become available. Thus, removal efficiency of NH4+ ions increased from 1.40 to 56.57% while the removal efficiency of PO43- increased from 97.9 to 100%. As the amount of EL-MNP@zeolite increased the surface area available and the number of adsorption sites for interaction with both ions also increased, which enhanced the overall removal efficiency, but the adsorbent dose and adsorption capacity of the static unit volume declined. Typically for a constant concentration of target contaminant, an increase in adsorbent dose increased the the number of contact points and hence strengthened the adsorbability of EL-MNP@zeolite (Chen et al., 2013; Zhang et al., 2011). However, 12
increasing the adsorbent dose to peak value led to a reduction in adsorbent activity on the unit volume’s surface area, which in turn led to fewer binding sites. Therefore, the appropriate dosage of EL-MNP@zeolite was 7 g L-1. The removal efficiency of NH4+ and PO43- fell from 62.7 to < 0.5% and from 99.9 to 38.7%, respectively as the initial concentrations increased from 5 to 100 mg L-1 (Fig. 6b). This was due to a fixed number of adsorption sites on the surface of ELMNP@zeolite for a fixed adsorbent dose, so that as the concentration of contaminant increased, competitive adsorption between contaminants will increase, thus reducing the adsorption efficiency (Wan et al., 2017; Gao et al., 2016). At smaller initial contaminant concentrations, all of the available adsorption sites on EL-MNP@zeolite were available and involved in removing contaminant, resulting in overall adsorption being much more efficient. For a fixed adsorbent dose, as the initial concentration of NH4+-PO43- increased the occupancy of available adsorption sites increased and consequently the removal efficiency decreased (Chen et al., 2013; Zhang et al., 2011). For most practical applications, the optimum temperature for removal is an important consideration. The removal of NH4+-PO43- by EL-MNP@zeolite at various temperatures (20, 30 and 40 oC) is shown in Fig. 6c. After 2 minutes the removal efficiencies of NH4+ at these temperatures were 36.7, 33.0 and 24.9%, respectively and thereafter the reaction rate slowed down until equilibrium was reached at 30 min, and the removal efficiencies were stable at 43.3, 39.9 and 34.7%, respectively. The removal efficiency equilibrium of PO43- was much higher reaching 99.8, 99.7 and 99.5% in < 10 min. This temperature dependence confirmed NH4+ removal by EL-MNP@zeolite became less efficient as temperature increased suggesting that adsorption of NH4+ was not favored by elevating temperature and that the adsorption process was exothermic. In comparison temperature barely affected the adsorption of PO43-, although the 13
equilibration time greatly decreased with temperature. It is likely that the different valencies of the NH4+ and PO43- resulted in different variation sin surface charge and adsorption mechanisms (Zhang et al., 2011). The optimium temperature for removal of mixed solution of NH4+ and PO43- was 20 oC. The effect of pH on the removal efficiency of NH4+-PO43- (Fig. 6d) showed that there was no obvious impact on PO43- adsorption when the initial pH was < 8.00. However above pH 8.00, the removal efficiency of phosphate significantly decreased from 81.5 to 37.2% as the pH increased from 10.00 to 12.00. The removal efficiency of NH4+ was initially increased from 27.7 to 59.1% when the pH was increased from 2.00 to 4.00 (Fig. 6d) and therafter remained remained stable at 59% in the pH range from 4.00~10.00 range, before plummeting to 2.9% at pH 12.00. This variation in removal efficiency with pH was attributed to adsorption sites on the EL-MNP@zeolite being in more deprotonated state when the pH of the aqueous solution was between 4.00 and 10.00 via the mechanism as shown in equation 2 resulting in a more negative surface charge. ≡ 𝐹𝑒𝑂𝐻↔ ≡ 𝐹𝑒𝑂 ― + 𝐻 +
(2)
Thus, in this pH range the increased number of anion binding sites can interact more with NH4+. However, at very low pH (pH < 4.00) excessive amounts of H+ ions outcompeted NH4+ ions for adsorption sites, reducing the removal efficiency of NH4+ (Wan et al., 2017; Huang et al., 2014a). Furthermore, under extremely alkaline conditions (pH >10.00) competitive adsorption between OH- and PO43- and hydroxyl adsorption sites formed on the surface of the iron oxide nanoparticles is covered with Fe to reduce the adsorption site. Therefore, retaining a reasonable amount of control over pH iscrucial for improving the adsorption of both NH4+-PO43- by ELMNP@zeolite. 14
Fig. 6 Effect of adsorbent dose (a); initial concentration (b); temperature (c); and pH (d) on the removal efficiency of ammonium and phosphate ions by EL-MNP@zeolite (2:1)
3.3. Adsorption kinetics, isotherms and thermodynamic studies The temporal variation of removal efficiency at three different temperatures (20, 30 and 40 ℃) was monitored over an extended period (Fig. 7) in order to better calculate kinetic and adsorption isothermal models.
15
Fig. 7 Variation in removal efficiency of NH4+and PO43- by EL-MNP@zeolite at three different temperatures (20, 30 and 40 ℃). Conditions: initial concentration (20 mg L1),
dose (1g L-1) and pH (7.00).
Two of the most common kinetic models, the pseudo first (equation 1) and second order (equation 2) models, were applied here to better understand the adsorption mechanism of EL-MNP@zeolite (Chen et al., 2013; Zhang et al., 2011): ln (qe ― qt) = lnqe ― k1t t qt
1
(3)
1
(4)
= k q2 + qe 2 e
h = k2q2e
(5)
Where qe and qt (mg g-1) were the adsorption capacities at equilibrium and at time (min), respectively and k1 and k2 were the rate constant for the pseudo-first-order adsorption (min-1) and pseudo-second-order adsorption (g mg-1 h-1), respectively. The best fit values for all parameters are summarized in Table 1.
16
Table 1 Adsorption kinetics parameters for ammonium and phosphate co-adsorption on EL-MNP@zeolite under three different temperatures.
Pseudo-first-order
Pseudo-second-order
Temperature qe (K)
qe k1
(min-1)
(mg g-1)
NH4+
PO43-
k2
R2
R2 (mg g-1)
(g min-1 min-1)
293
37.99
0.378
0.927
3.93
0.0021
0.977
303
35.04
0.407
0.934
3.47
0.0022
0.978
313
5.19
0.291
0.725
2.84
0.0022
0.963
293
6310.69
1.365
0.544
38.61
0.0128
0.999
303
19753.78
1.544
0.607
38.91
0.0133
0.999
313
323191.40
1.849
0.827
37.88
0.0038
0.999
The pseudo-first-order kinetic correlation coefficients R2 of the composite material for removing NH4+ were 0.927, 0.934 and 0.725, respectively, while the pseudo-first-order correlation coefficients of composites for PO43- were 0.544, 0.607 and 0.827, respectively (Table 1). In comparison the pseudo-second-order kinetic correlation coefficients of the two pollutants demonstrated a much better correlation, with correlation coefficients of NH4+ being 0.977, 0.978 and 0.963, respectively, and the correlation coefficients of PO43- being 0.999, 0.999 and 0.999, respectively. Thus, the hybrids removal behavior was more consistent with a pseudo-second-order kinetic model.
The kinetic parameter coefficient R2 of NH4+and PO43- under the three temperatures 17
were consistently greater than 0.990, indicating that the simultaneous removal process was via chemical adsorption. Based on this, the interaction between NH4+-PO43- and EL-MNP@zeolite via either electron-sharing or ligand exchange, was expected to result in a good removal efficiency. For NH4+, as the temperature increased, the pseudosecond-order rate constants k2 remained largely unchanged being 0.0021, 0.0022 and 0.0022 g mg-1 min-1, respectively. In comparion, the pseudo-second-order rate constants for PO43- varied more significantly being 0.0128, 0.0133 and 0.0038 g mg-1 min-1, respectively. These rates of removal indicated that the adsorption of both NH4+ and PO43- would thus be relatively quick and hence practical for controlling complex pollution. The increase in temperature had little effect on the equilibrium rate of NH4+ adsorption indicating that ammonium adsorption by EL-MNP@zeolite was relatively stable under the three temperatures considered. For phosphate, as the temperature increased, the rate of removal initially increased before decreasing, which indicated that the adsorption process of PO43- by EL-MNP@zeolite for may be exothermic and the material was most stable at 30 ℃.
In order to more accurately measure the adsorption capacity of the prepared hybrid for the two pollutants, the adsorption data was fit to both the Langmuir and Freundlich equations (Table 2) (Chen et al., 2013; Zhang et al., 2011). The liberalized version of the Freundlich model is given by equation 6: 1
(6)
ln qe = ln KF + nln Ce
Where qe (mg g-1) is the equilibrium absorption capacity and Ce (mg L-1) is the concentration of ammonium or phosphate at equilibrium, while KF (L g-1) and 1/n represent the Freundlich equilibrium constant and a characteristic constant related to adsorption capacity, respectively. 18
Table 2 Isotherm parameters for ammonium and phosphate co-adsorption on ELMNP@zeolite. Freundlich
Temperature (℃)
Langmuir
30
1/n
KF (mg g-1)
R2
Qm (mg g-1)
KL (L mg-1)
R2
NH4+
0.42
1.05
0.708
5.44
0.10
0.955
PO43-
0.58
14.26
0.955
59.88
0.38
0.977
The Langmuir isotherm model can be expressed via equation 7: Ce qe
Ce
1
(7)
= q0 + q0KL
Where q0 (mg g-1) is the maximum adsorption capacity and KL is the Langmuir adsorption equilibrium constant (L mg-1), which is related to energy of adsorption and enthalpy change. The best fit parameters of for each model are summaried in Table 2, where according to the biggest correlation coefficients R2, EL-MNP@zeolite adsorption of both NH4+ and PO43- was most consistent with the Langmuir type adsorption isotherm. This indicated that NH4+ and PO43- were adsorbed on the EL-MNP@zeolite via a monolayer adsorption process. More PO43- was absorbed by EL-MNP@zeolite than NH4+, because the adsorption was favoured by the higher valance negative charge. The value of 1/n was less than 1 for both NH4+ and PO43- under all conditions, suggesting that, consistent with the above kinetics results, the adsorption process was mainly involved chemical adsorption.
Thermodynamic parameters were determined from Kc (the thermodynamic distribution coefficient) and the standard Gibbs free energy ΔG0 (kJ mol−1), standard enthalpy 19
change ΔH0 (kJ mol−1), and standard entropy change ΔS0 (J mol−1 K−1) were calculated using equations 8 and 9: ln Kc = ―
ΔG0 RT
= ―
ΔH0 RT
+
ΔS0 R
(8)
ΔG0 = ― RTln Kc
(9)
Where T (℃) was the absolute temperature, and R was molar gas constant (8.314 J mol-1 K-1). The values of ΔG0, ΔH0 and ΔS0 were calculated from the slope and intercept of the plot of ln Kc versus 1/T (Table 3) Table 3 Thermodynamic parameters for ammonium and phosphate co-adsorption on EL-MNP@zeolite. △G0 (KJ mol-1) △H0 (KJ mol-1)
△S0 (KJ mol-1) 293k
303k
313k
NH4+
-3.40
0.0096
-0.603
-0.483
-0.413
PO43-
-10.51
0.0049
-9.084
-9.025
-8.987
For NH4+ adsorptions, ΔH0 and ΔS0 indicated that the process was exothermic, while the negative values of ΔG0 indicated NH4+ adsorption was spontaneous and favourable. For PO43-, ΔH0 and ΔS0 also indicated that adsorption was exothermic and the solidliquid interface degree of disorder decreased during the reaction. Since irrespective of temperature ΔG0 was less than zero, PO43- was subjected to spontaneous adsorption. The general decline in ΔG0 with increasing temperature indicated that elevated temperature was not conductive to adsorption.
Based on all of the above calculations and the conclusions drawn from individual 20
experiments, the following adsorption mechanism was proposed. The adsorption of NH4+ by EL-MNP@zeolite was mainly via physical adsorption. This is because the surface of the material is negatively charged in water, and NH4+ is electrostatically attracted to it. The adsorption of phosphate was mainly via chemical adsorption. This is because when EL-MNP@zeolite is added to water and surface iron is hydrolyzed to form a Fe-OH bond, which exchanges with the phosphate to form FePO42- (Cao et al., 2016; Tabrizi and Sepehr, 2015).
3.4 Comparison of adsorption capacities The adsorption capacities of several materials for ammonium and phosphate are shown in Table 4. The NH4+ adsorption capacity by EL-MNP@Zeolite was lower than that of all other materials, while PO43- was adsorbed by EL-MNP@Zeolite better than any other materials. This indicated that EL-MNP@Zeolite had selective absorbability for two kinds of pollutants, where the adsorption effect of PO43- was better that that of NH4+ After removing a mixed solution containing 20 mg L-1 NH4+ and 20 mg L-1 PO43-, the residual NH4+ was 16.53 mg L-1, and the residual PO43- was 1.6 mg L-1, and the mixed solution met the national water quality discharge standard.
Table 4 The maximum phosphate and ammonium adsorption capacities of various materials. Materials Natural zeolite
Phosphate adsorption
Ammonium adsorption
capacity (mg g-1)
capacity (mg g-1)
2.73
17.68
Wan et al., 2017
8.25
17.55
Huang et al., 2014a
References
Modified zeolite combined with struvite crystallization 21
Iron nanoparticles
-
9.70
Wang et al., 2014b
EL-MNP@Zeolite
38.91
3.47
-
4. Conclusions In this work, EL-MNP@zeolite was synthesized using green synthetic methods, where iron nanoparticles were successfully decorataed onto zeolite. Zeolite not only plays the role of an inorganic carrier to improve the dispersion of iron nanoparticles and prevent agglomeration and oxidation, but it also provides adsorption sites for NH4+ ions and promotes the simultaneous removal of NH4+-PO43-. The removal efficiency was positively correlated with the amount of adsorbent, but when the pH of the reaction solution was between 4.00-10.00, the adsorption effectwas reduced. The removal efficiencies of NH4+ and PO43- were 43.3% and 99.8%, respectively. Adsorption of NH4+ and PO43- was exothermic and low temperature was favorable for the simultaneous removal of both NH4+ and PO43- by EL-MNP@zeolite. The adsorption of NH4+-PO43- onto EL-MNP@zeolite was via chemical adsorption, which was consistent with the good fit to the pseudo-second-order kinetics.
Acknowledgment The Fujian Province Innovation and Entrepreneurship Talents, China is gratefully acknowledged for its assistance with this research.
Declaration of interests
22
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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synthesis of iron oxide nanoparticles and their magnetic properties. J. Phys. Chem. C 112, 10398-10401. Huang, H.M., Xiao, D. A., Pang, R., Han, C.C., Ding, L., 2014a. Simultaneous removal of nutrients from simulated swine wastewater by adsorption of modified zeolite combined with struvite crystallization. Chem. Eng. J. 256, 431-438. Huang, L.L., Weng, X.L., Chen, Z.L., Megharaj, M., Naidu, R., 2014b. Synthesis of iron-based nanoparticles using oolong tea extract for the degradation of malachite green. Spectrochim. Acta. A 117, 801-804. Huang, L.L., Weng, X.L., Chen, Z.L., Megharaj, M., Naidu, R., 2014c. Green synthesis of iron nanoparticles by various tea extracts: Comparative study of the reactivity. Spectrochim. Acta. A 130, 295-301. Huo, H.X, Lin, H., Dong, Y.B., Cheng, H., Wang, H., Cao, L.X., 2012. Ammonianitrogen and phosphates sorption from simulated reclaimed waters by modified clinoptilolite. J.Hazard. Mater 229-230, 292-297. Khalil, A.M.E., Eljamal, O., Saha, B.B., Matsunaga, N., 2018. Performance of nanoscale zero-valent iron in nitrate reduction from water using a laboratory-scale continuous-flow system. Chemosphere 197, 502-512. Liu, H.B., Chen, T.H., Zou, X.H., Xie, Q.Q., Qing, C.S., Chen, D., 2013. Removal of phosphorus using NZVI derived from reducing natural goethite. Chem. Eng. J. 234, 8087. Lukman, A.I., Gong, B., Marjo, C.E., Roessner, U., Harris, A.T., 2011. Facile synthesis, stabilization, and anti-bacterial performance of discrete Ag nanoparticles using 24
Medicago sativa seed exudates. J. Colloid Interface Sci. 353, 433-44. Maamoun, I., Eljamal, O., Khalil, A.M.E., Sugihara, Y., Matsunaga, N., 2018. Phosphate Removal Through Nano-Zero-Valent Iron Permeable Reactive Barrier; Column Experiment and Reactive Solute Transport Modeling. Transport Porous Med. 125, 395-412. Maulana, I., Takahashi, F., 2018. Cyanide removal study by raw and iron-modified synthetic zeolites in batch adsorption experiments. J. Water Process Eng. 22, 80-86. Padil, V.V., Cerník, M., 2013. Green synthesis of copper oxide nanoparticles using gum karaya as a biotemplate and their antibacterial application. Int. J. Nanomedicine 8, 889898. Parsons, J.G., Peralta-Videa, J.R., Gardea-Torresdey, J.L., 2007. Use of plants in biotechnology: Synthesis of metal nanoparticles by inactivated plant tissues, plant extracts, and living plants. Dev. Environ. Sci. 5, 463-485. Shameli, K., Ahmad, M.B., Zamanian, A., Sangpour, P., Shabanzadeh, P., Abdollahi. Y., Zargar, M., 2012. Green biosynthesis of silver nanoparticles using Curcuma longa tuber powder. Int. J. Nanomedicine 7, 5603-5610. Sondergaard, M., Jeppesen, E., 2007. Anthropogenic impacts on lake and stream ecosystems, and approaches to restoration. J. Appl. Ecol. 44, 1089-1094. Suazo-Hernandez, J., Sepulveda, P., Manquian-Cerda, K., Ramirez-Tagle, R., Rubio, M.A., Bolan, N., 2019. Synthesis and characterization of zeolite-based composites functionalized with nanoscale zero-valent iron for removing arsenic in the presence of selenium from water. J. Hazard Mater 373, 810-819. 25
Tabrizi, A.B., Sepehr, B., 2015. Extraction of ammonia and nitrite using modified magnetite iron oxide nanoparticles before spectrophotometric determination in different water samples, Int. J. Anal. Chem. 95 (9), 833-846. Wan, C.L., Ding, S.A., Zhang, C., Tan, X.J., Zou, W.G., Liu, X., Yang, X., 2017. Simultaneous recovery of nitrogen and phosphorus from sludge fermentation liquid by zeolite adsorption: Mechanism and application. Sep. Purif. Technol. 180, 1-12. Wang, S., Peng, Y., 2010. Natural zeolites as effective adsorbents in water and wastewater treatment. Chem. Eng. J. 156, 11-24. Wang, T., Jin, X.Y., Chen, Z.L., 2014a. Green synthesis of Fe nanoparticles using eucalyptus leaf extracts for treatment of eutrophic wastewater. Sci. Total Environ. 466, 210-213. Wang, T., Lin, J.J., Chen, Z.L., Meghara, M.j, Naidu, R., 2014b. Green synthesized iron nanoparticles by green tea and eucalyptus leaves extracts used for removal of nitrate in aqueous solution. J. Clean. Prod. 83, 413-419. Weng, X.L., Huang, L.L., Chen, Z.L., Megharaj, M., Naidu, R., 2013. Synthesis of iron-based nanoparticles by green tea extract and their of malachite. Ind. Crop. Prod. 51, 342-347. Zhang, R., Qi, F., Liu, C., Zhang, Y., Wang, Y., Song, Z., 2019. Cyanobacteria derived taste and odor characteristics in various lakes in China: Songhua Lake, Chaohu Lake and Taihu Lake. Ecotoxicol Environ Saf. 181, 499-507. Zhang, X., Lin, S., Chen, Z.L., Megharaj, M., Naidu, R., 2011. Kaolinite-supported nanoscale zerovalent iron for removal of Pb2+ from aqueous solution: reactivity, 26
characterization and mechanism. Water Res. 45, 3481-3488. Zheng, Y., Wang, A.Q., 2010. Preparation and ammonium adsorption properties of biotite-based hydrogel composites. Ind. Eng. Chem. Res. 49, 6034-6041.
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Research highlights
► A hybrid iron oxide nanoparticle- zeolite were synthesized. ► Green EL-MNP@zeolite has better stability and dispersity. ►Ammonia and phosphate was simultaneously removed. ► NH4+ and PO43- follows the pseudo-second-order kinetic model.
28
Graphical abstract
29
S0048-9697(19)34994-0 https://doi.org/10.1016/j.scitotenv.2019.135002 STOTEN 135002
To appear in:
Science of the Total Environment
Received Date: Revised Date: Accepted Date:
23 July 2019 13 October 2019 14 October 2019
Please cite this article as: Q. Xu, W. Li, L. Ma, D. Cao, G. Owens, Z. Chen, Simultaneous removal of ammonia and phosphate using green synthesized iron oxide nanoparticles dispersed onto zeolite, Science of the Total Environment (2019), doi: https://doi.org/10.1016/j.scitotenv.2019.135002
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© 2019 Elsevier B.V. All rights reserved.
Simultaneous removal of ammonia and phosphate using green synthesized iron oxide nanoparticles dispersed onto zeolite
Qianyu Xu1, Wenpeng Li1, Li Ma1, Dan Cao1, Gary Owens2 and Zuliang Chen1*
1School
of Environmental Science and Engineering, Fujian Normal University, Fuzhou
350007, Fujian Province, China. 2Environmental
Contaminants Group, Future Industries Institute, University of South
Australian, Mawson Lakes, SA 5095, Australia.
*Corresponding author address: School of Environmental Science and Engineering, Fujian Normal University, Fuzhou 350007, Fujian Province, China. Email: [email protected]
1
Abstract Since elevated levels of common nutrients, such as ammonia and phosphate, in natural water bodies (lakes and rivers) can lead to significant deterioration of pristine water ecosystems due to eutrophication, new and cost-effectiveness remediation strategies are urgently required. This work investigated the feasibility of using green synthesized iron oxide nanoparticles dispersed onto zeolite by eucalyptus leaf extracts (ELMNP@zeolite), to simultaneously remove ammonia and phosphate from aqueous solutions. SEM and XRD both showed that EL-MNP@zeolite had better stability and dispersity than unsupported zeolite. At an initial concentration of 10 mg L-1 each for the two co-existing ions the synthesized material removed 43.3% of NH4+ and 99.8% of PO43- . Removal of co-existing NH4+-PO43- was impacted by the ratio of zeolite to ELMNP, temperature, initial ion concentration and solution pH. Under optimium conditions the maximum adsorption capacity of EL-MNP@zeolite for NH4+ and PO43was 3.47 and 38.91 mg g-1, respectively. For both ions’ adsorption followed a pseudosecond-order kinetic reaction, confirming that the removal of ammonia and phosphate by EL-MNP@zeolite was via chemisorptions, where interaction between NH4+-PO43and EL-MNP@zeolite may be through either electrostatic adsorption or ligand exchange. Overall these results indicated that EL-MNP@zeolite had significant potential as a nano-remediation strategy to simultaneously remove cationic ammonium and anionic phosphate from wastewaters.
Keywords: ammonium; iron oxide nanoparticles; phosphate; zeolite.
2
1. Introduction Eutrophication is one of the most serious problems affecting aquatic ecosystems in China. Where serious outbreaks of cyanobacteria can not only affect the ecological function and water quality of rivers and lakes, but can also affect the safety of drinking water and consequently human health (Zhang et al., 2019). Unfortunately, rapid industrial development and changes in other human activities have led to a worldwide increase in the levels of nitrogen (N) and phosphorus (P) entering rivers, lakes and coastal waters, which has directly led to increased incidents of eutrophication and the frequency of toxic algal blooms (Sondergaard and Jeppesen, 2007). Whist many common methods exist for the removal of nitrogen and phosphorus; including adsorption, biological removal, crystallization, ion exchange and precipitation (Huo et al., 2012), and of all the current environmental remediation technologies, adsorption the most advantageous because it is economical, consumes only small amounts of energy and is relatively easy to practically implement (Zheng and Wang, 2010). At present, the natural adsorbents most widely used in wastewater treatment include activated carbon, clay minerals, iron oxides and zeolites (Bonetto et al., 2015). Of these adsorbents, zeolites are particular attractive because modified zeolite in combination with struvite crystals were shown to simultaneously remove both ammonia and phosphate from simulated aquaculture wastewater (Huang et al., 2014a). However, the modification of zeolites via the addition of secondary chemical reagents during the composite synthesis process is not always ideal as this can produce secondary pollution. Thus, many researchers have instead stressed the need to develop an effective “green” material for the simultaneous removal of both nitrogen and phosphorus (Wang and Peng, 2010; Wan et al., 2017; Liu et al., 2013; Gao et al., 2008). The green synthesis of magnetic iron oxide nanoparticles (MNPs) has garnered 3
increasing attention for the removal of phosphate (Wang et al., 2014b) due to this method’s biocompatibility, cost-effectiveness and eco-friendliness. Additionally, as shown in our previous work (Huang et al., 2014b, 2014c) high surface area iron oxides can easily be synthesized. Previous batch experiments have shown that the maximum adsorption capacity of zero-valent nano-iron for phosphate was 54.34 mg g-1 (Maamoun et al., 2018), while another showed > 90% nitrate removal efficiency from water using a laboratory-scale nZVI-based continuous processing system (Khalil et al., 2018). These studies indicated that nZVI is a viable adsorbent for removing both ammonia and phosphate from water. In addition, the green synthesis of nZVI using plant extracts has been more common and more widely applicable for the efficient removal of total nitrogen and phosphorus from waters (Devath et al., 2016; Padil and Cerník, 2013; Shameli et al., 2012; Lukman et al., 2011; Parsons et al., 2007). However, one of the issues limiting the practical application of nZVI for water treatment is that nZVI with high specific surface areas tend to agglomerate in aqueous solution which compromises both dispersibility and reactivity (Cao et.al, 2016). Many researchers have hence addressed this issue by decorating nZVI on a natural support. For example, using ordinary zeolite as an adsorbent support, a technique for preparing an iron-modified zeolite for arsenate removal was developed where the amount of arsenate adsorbed by the iron-modified zeolite depended on the concentration of iron ions on the adsorbent (Baskan and Pala, 2011). An iron-modified zeolite was also used to remove cyanide from water, where theiron-modified zeolite had a stronger adsorption capacity than the support zeolite and the maximum adsorption capacity reached 33.98 mg g-1 (Maulana and Takahashi, 2018). While these studies indicate that iron-modified zeolites have potential for environmental remediation, few studies have applied them to mixed nutrient rich (N and P) waste waters. 4
However, since natural zeolites were used to successfully remove ammonium (Wan et al., 2017; Huang et al., 2014a) and MNPs were used to successfully remove phosphate (Gao et al., 2016), it should be possible to disperse MNPs onto zeolites to form a hybrid material exhibiting enhanced reactivity and dispersibility of the MNPs and capacity to simultaneously remove both nitrogen and phosphorus. Based on this hypothesis, an iron oxide nanoparticle-zeolite hybrid (EL-MNP@zeolite) was green synthesized using eucalyptus leaf extract and the factors affecting the removal efficiency of ammonium and phosphate ions determined. Specifically, this study involved (1) characterization of EL-MNP and EL-MNP@zeolite before and after calcination so that any changes in surface and chemical species could be recognized; and (2) batch adsorption experiments to evaluate the absorption of NH4+-PO43- by EL-MNP@zeolite, supported by adsorption kinetic and thermodynamic studies.
2. Experimental 2.1 Materials Ferric trichloride hexahydrate (FeCl3·6H2O, Analytical Reagent Grade) and sodium acetate (CH3COONa, Analytical Reagent Grade) were purchased from Guangzhou Chemical Reagent Co., Ltd. (China) and used without further purification. Zeolite ((SiO2)x(Al2O3)y Analytical Reagent Grade) was bought from Sinopharm Chemical Reagent Co., Ltd. (China) and also used without further purification. Eucalyptus leaves were obtained from the campus of Fujian Normal University in Fujian, China. Deionized (DI) water was used throughout all experiments.
2.1.1 Preparation of EL-MNP and EL-MNP@zeolite The eucalyptus leaf extract (EL) was prepared as described previously (Wang et al., 5
2014a). In this traditional method, a blend of FeCl3·6H2O (0.02 mol) and CH3COONa (0.24 mol) was dissolved in eucalyptus leaf extract (120 mL) and continuously stirred for 2 hours at 70 oC to obtain EL-MNP. The EL-MNP was washed using anhydrous ethanol and deionized water three times and then dried at 45 oC under vacuum for 12 hours.
For the synthesis of the hybrid material, FeCl3·6H2O (0.02 mol) and CH3COONa (0.24 mol) were initially added into eucalyptus leaf extract (120 mL) and stirred for 1 hour at 70 oC before being filtered. Subsequently, zeolite was added into the EL-MNP solution at three different volume ratios (zeolite: EL-MNP = 1:1, 2:1 and 3:1) to obtain ELMNP@zeolite (Fig. 1). The final EL-MNP@zeolite solid was filtered (0.45 μm) and washed with anhydrous ethanol and deionized water three times and dried in a vaccum oven at 45 oC s for > 12 hours. The resulting black powder was stored under vacuum prior to sieving to < 110 mesh.
Fig. 1 Synthesized mechanism of EL-MNP@zeolite Since zeolites have many different functional groups across their surface, and iron oxide nanoparticles are charged, iron oxide nanoparticles can be loaded onto zeolites by a combination of electrostatic and functional group interactions. 6
2.2. Characterization Morphology and particle size distributions of both EL-MNP and EL-MNP@zeolite were investigated by scanning electron microscope (SEM) (JSM 7500F, Japan). ELMNP@zeolite before and after calcination were characterized using an X-ray diffraction meter (XRD) with a high-power Cu Kα radioactive source (k=0.154 nm) at 40 kV and 40 mA, in the 2θ range 10-80° at a scanning rate of 3° min-1.
2.3. Batch experiments Batch adsorption experiments, studying the simultaneous removal of NH4+-PO43- by EL-MNP@zeolite, used the following set of four experiment conditions. For a fixed temperature (30 °C), initial concentration (10 mg L-1) and pH (6.35), the dose of ELMNP@zeolite was varied (4, 5, 6, 7, 8, 9 and 10 g L-1). For a fixed temperature (30 °C), EL-MNP@zeolite dose (7 g L-1) and pH (6.35), the initial concentrations were varied (5, 10, 20, 40, 60, 80 and 100 mg L-1). For a fixed EL-MNP@zeolite dose (7 g L-1), initial concentration (10 mg L-1), and pH (6.35), temperature was varied (20, 30 and 40 °C). For a fixed temperature (30 °C), EL-MNP@zeolite dose (7 g L-1), and initial concentration (10 mg L-1) pH was varied (4.00, 6.00, 8.00, 10.00 and 12.00). Unless otherwise stated above all other experimental conditions were maintained as follows: the required EL-MNP@zeolite (zeolite: EL-MNP = 2:1) dose was added to a mixed ammonium and phosphate stock solution (25 mL) in a 50 mL glass tube and placed in a shaker (250 r min-1) equilibrated to the desired temperature. After shaking for the defined equilibrium time, the adsorbent was filtered from the solution with a 0.45 um membrane filter and the residual concentration in the solution determined spectrophotometrically. All experiments were at least triplicated. 7
Following exposure to EL-MNP@zeolite for predetermined periods (5, 10, 15, 20, 30 and 60 min) the residual concentrations (Ce) of NH4+ and PO43- in the filtered solutions were measured using a UV-Spectrophotometer (752N, Shanghai, China) at 420 nm and 700 nm, respectively. The removal efficiencies were then calculated using the following equation (Weng et al., 2013): η=
C0 ― Ce C0
(1)
× 100%
Where η (%) was the nitrogen and phosphorus removal efficiency, C0 was the initial nitrogen and phosphorus concentration (mg L-1), and Ce was the residual nitrogen and phosphorus concentration after removal (mg L-1). Two parallel samples were set up for all experiments.
3. Results and Discussion 3.1 Characterizations SEM images of EL-MNP (Fig. 2a) revealed small spherical EL-MNP particles with an average size of 60 nm, which clearly indicated that the eucalyptus leaf extracts could reduce metallic ions to obtain EL-MNPs. The size distribution observed here was more uniform compared with iron nanoparticles synthesized using sodium borohydride as a reducing agent (Wang et al., 2014b; Chen et al., 2011; Chen et al., 2013). When ELMNPs were introduced into the zeolite (Fig. 2b) EL-MNP appeared to be spread across the surface and pore spaces of the zeolite, which was ascribed to the existence of many iron nanoparticles becoming embedded within the zeolite network (Zhang et al., 2011). Compared to the discrete EL-MNPs material, the produced hybrid material had a more dispersive which was more advantageous for removal of NH4+ and PO43-.
8
Fig. 2 Scanning electron microscopy images of EL-MNP (a) and EL-MNP@zeolite (b).
The XRD patterns of EL-MNP and EL-MNP@zeolite before and after synthesized were not dramatically different (Fig. 3). Since only a few characteristic peaks were observed this indicated that the crystal structure of the EL-MNP nanoparticles obtained by green synthesis was amorphous (Cao et al., 2016). The broad peak at 2θ = 26.4° was characteristic of zeolite, which was decreased in intensity following hybrid formation, suggesting that extract of both nano-iron and eucalyptus leaves were acting as capping agent/stabilizer by covering the surface of the zeolite thereby making the characteristic peaks of the zeolite rougher (Weng et al., 2013; Wang et al., 2014b). Characteristic peaks appearing at 2θ of 35.7°, 35.5° and 20-35° were attibuatble to maghemite (γFe2O3), magnetite (iron oxide) and iron hydroxide, respectively, which indicated that oxidation occurred during the formation of green tea synthesized iron nanoparticles (GT-Fe NPs) (Cao et al., 2016; Weng et al., 2013; Wang et al., 2014b; SuazoHernandez et al., 2019).
9
Fig. 3 XRD patterns of EL-MNP (lower trace) and EL-MNP@zeolite (upper trace).
3.2. Batch experiments As the ratio of zeolite to EL-MNP increased from 1:1, 2:1, to 3:1, both the ammonium and phosphate removal efficiencies increased from 11.9%, 37.8% to 42.3% and 96.9%, 98.0% to 98.6%, respectively (Fig. 4). The overall removal efficiency for both ions was thus enhanced by improving the ratio of zeolite to EL-MNP which reached a higher adsorption at 3:1. The removal of phosphate was consistently better than that of ammonium. Zeolites in the natural environment have a large specific surface area that is difficult to match with other mineral materials due to the aluminosilicates which: firstly, embrace many channels in their structure (Wan et al., 2017; Huang et al., 2014a); and secondly, improve the dispensability of iron nanoparticles. Furthermore, the total number of surface adsorptive sites of EL-MNP@zeolite increased when zeolite was added to the EL-MNP, leading to increased adsorption of both NH4+ and PO43- onto iron nanoparticles (Wan et al., 2017; Cao et al., 2016).
10
Fig. 4 Removal efficiency of ammonium and phosphate by EL-MNP@zeolite (zeolite: EL-MNP= 1:1, 2:1 and 3:1). Conditions: C (NH4+-PO43-) = 10 mg L-1; 30 ℃; dosage 7 g L-1; 30 min.
The removal kinetics of NH4+ and PO43- was extremely rapid (Fig. 5). Within the first two minutes, the removal efficiency of both NH4+ and PO43- rapidly increased to 33.2% and 95.1%, respectively, before gradually increasing more slowly over time, eventually rising to 40.2% and 98.8%, for NH4+ and PO43- respectively at equilibrium at 15 min.
11
Fig. 5 Removal of NH4+-PO43- Conditions: C (NH4+-PO43-) = 10 mg L-1; 30 ℃; dosage 7 g L-1, 30 min.
In batch adsorption experiments, the removal efficiencies of NH4+ were significantly affected by the hybrid dose while the removal efficiency of phosphate remained largely unaffected (Fig. 6a) and this suggested that coacity for phosphate is large but the limitied amount of a binding sites for ammonium exist, so as increase dose more become available. Thus, removal efficiency of NH4+ ions increased from 1.40 to 56.57% while the removal efficiency of PO43- increased from 97.9 to 100%. As the amount of EL-MNP@zeolite increased the surface area available and the number of adsorption sites for interaction with both ions also increased, which enhanced the overall removal efficiency, but the adsorbent dose and adsorption capacity of the static unit volume declined. Typically for a constant concentration of target contaminant, an increase in adsorbent dose increased the the number of contact points and hence strengthened the adsorbability of EL-MNP@zeolite (Chen et al., 2013; Zhang et al., 2011). However, 12
increasing the adsorbent dose to peak value led to a reduction in adsorbent activity on the unit volume’s surface area, which in turn led to fewer binding sites. Therefore, the appropriate dosage of EL-MNP@zeolite was 7 g L-1. The removal efficiency of NH4+ and PO43- fell from 62.7 to < 0.5% and from 99.9 to 38.7%, respectively as the initial concentrations increased from 5 to 100 mg L-1 (Fig. 6b). This was due to a fixed number of adsorption sites on the surface of ELMNP@zeolite for a fixed adsorbent dose, so that as the concentration of contaminant increased, competitive adsorption between contaminants will increase, thus reducing the adsorption efficiency (Wan et al., 2017; Gao et al., 2016). At smaller initial contaminant concentrations, all of the available adsorption sites on EL-MNP@zeolite were available and involved in removing contaminant, resulting in overall adsorption being much more efficient. For a fixed adsorbent dose, as the initial concentration of NH4+-PO43- increased the occupancy of available adsorption sites increased and consequently the removal efficiency decreased (Chen et al., 2013; Zhang et al., 2011). For most practical applications, the optimum temperature for removal is an important consideration. The removal of NH4+-PO43- by EL-MNP@zeolite at various temperatures (20, 30 and 40 oC) is shown in Fig. 6c. After 2 minutes the removal efficiencies of NH4+ at these temperatures were 36.7, 33.0 and 24.9%, respectively and thereafter the reaction rate slowed down until equilibrium was reached at 30 min, and the removal efficiencies were stable at 43.3, 39.9 and 34.7%, respectively. The removal efficiency equilibrium of PO43- was much higher reaching 99.8, 99.7 and 99.5% in < 10 min. This temperature dependence confirmed NH4+ removal by EL-MNP@zeolite became less efficient as temperature increased suggesting that adsorption of NH4+ was not favored by elevating temperature and that the adsorption process was exothermic. In comparison temperature barely affected the adsorption of PO43-, although the 13
equilibration time greatly decreased with temperature. It is likely that the different valencies of the NH4+ and PO43- resulted in different variation sin surface charge and adsorption mechanisms (Zhang et al., 2011). The optimium temperature for removal of mixed solution of NH4+ and PO43- was 20 oC. The effect of pH on the removal efficiency of NH4+-PO43- (Fig. 6d) showed that there was no obvious impact on PO43- adsorption when the initial pH was < 8.00. However above pH 8.00, the removal efficiency of phosphate significantly decreased from 81.5 to 37.2% as the pH increased from 10.00 to 12.00. The removal efficiency of NH4+ was initially increased from 27.7 to 59.1% when the pH was increased from 2.00 to 4.00 (Fig. 6d) and therafter remained remained stable at 59% in the pH range from 4.00~10.00 range, before plummeting to 2.9% at pH 12.00. This variation in removal efficiency with pH was attributed to adsorption sites on the EL-MNP@zeolite being in more deprotonated state when the pH of the aqueous solution was between 4.00 and 10.00 via the mechanism as shown in equation 2 resulting in a more negative surface charge. ≡ 𝐹𝑒𝑂𝐻↔ ≡ 𝐹𝑒𝑂 ― + 𝐻 +
(2)
Thus, in this pH range the increased number of anion binding sites can interact more with NH4+. However, at very low pH (pH < 4.00) excessive amounts of H+ ions outcompeted NH4+ ions for adsorption sites, reducing the removal efficiency of NH4+ (Wan et al., 2017; Huang et al., 2014a). Furthermore, under extremely alkaline conditions (pH >10.00) competitive adsorption between OH- and PO43- and hydroxyl adsorption sites formed on the surface of the iron oxide nanoparticles is covered with Fe to reduce the adsorption site. Therefore, retaining a reasonable amount of control over pH iscrucial for improving the adsorption of both NH4+-PO43- by ELMNP@zeolite. 14
Fig. 6 Effect of adsorbent dose (a); initial concentration (b); temperature (c); and pH (d) on the removal efficiency of ammonium and phosphate ions by EL-MNP@zeolite (2:1)
3.3. Adsorption kinetics, isotherms and thermodynamic studies The temporal variation of removal efficiency at three different temperatures (20, 30 and 40 ℃) was monitored over an extended period (Fig. 7) in order to better calculate kinetic and adsorption isothermal models.
15
Fig. 7 Variation in removal efficiency of NH4+and PO43- by EL-MNP@zeolite at three different temperatures (20, 30 and 40 ℃). Conditions: initial concentration (20 mg L1),
dose (1g L-1) and pH (7.00).
Two of the most common kinetic models, the pseudo first (equation 1) and second order (equation 2) models, were applied here to better understand the adsorption mechanism of EL-MNP@zeolite (Chen et al., 2013; Zhang et al., 2011): ln (qe ― qt) = lnqe ― k1t t qt
1
(3)
1
(4)
= k q2 + qe 2 e
h = k2q2e
(5)
Where qe and qt (mg g-1) were the adsorption capacities at equilibrium and at time (min), respectively and k1 and k2 were the rate constant for the pseudo-first-order adsorption (min-1) and pseudo-second-order adsorption (g mg-1 h-1), respectively. The best fit values for all parameters are summarized in Table 1.
16
Table 1 Adsorption kinetics parameters for ammonium and phosphate co-adsorption on EL-MNP@zeolite under three different temperatures.
Pseudo-first-order
Pseudo-second-order
Temperature qe (K)
qe k1
(min-1)
(mg g-1)
NH4+
PO43-
k2
R2
R2 (mg g-1)
(g min-1 min-1)
293
37.99
0.378
0.927
3.93
0.0021
0.977
303
35.04
0.407
0.934
3.47
0.0022
0.978
313
5.19
0.291
0.725
2.84
0.0022
0.963
293
6310.69
1.365
0.544
38.61
0.0128
0.999
303
19753.78
1.544
0.607
38.91
0.0133
0.999
313
323191.40
1.849
0.827
37.88
0.0038
0.999
The pseudo-first-order kinetic correlation coefficients R2 of the composite material for removing NH4+ were 0.927, 0.934 and 0.725, respectively, while the pseudo-first-order correlation coefficients of composites for PO43- were 0.544, 0.607 and 0.827, respectively (Table 1). In comparison the pseudo-second-order kinetic correlation coefficients of the two pollutants demonstrated a much better correlation, with correlation coefficients of NH4+ being 0.977, 0.978 and 0.963, respectively, and the correlation coefficients of PO43- being 0.999, 0.999 and 0.999, respectively. Thus, the hybrids removal behavior was more consistent with a pseudo-second-order kinetic model.
The kinetic parameter coefficient R2 of NH4+and PO43- under the three temperatures 17
were consistently greater than 0.990, indicating that the simultaneous removal process was via chemical adsorption. Based on this, the interaction between NH4+-PO43- and EL-MNP@zeolite via either electron-sharing or ligand exchange, was expected to result in a good removal efficiency. For NH4+, as the temperature increased, the pseudosecond-order rate constants k2 remained largely unchanged being 0.0021, 0.0022 and 0.0022 g mg-1 min-1, respectively. In comparion, the pseudo-second-order rate constants for PO43- varied more significantly being 0.0128, 0.0133 and 0.0038 g mg-1 min-1, respectively. These rates of removal indicated that the adsorption of both NH4+ and PO43- would thus be relatively quick and hence practical for controlling complex pollution. The increase in temperature had little effect on the equilibrium rate of NH4+ adsorption indicating that ammonium adsorption by EL-MNP@zeolite was relatively stable under the three temperatures considered. For phosphate, as the temperature increased, the rate of removal initially increased before decreasing, which indicated that the adsorption process of PO43- by EL-MNP@zeolite for may be exothermic and the material was most stable at 30 ℃.
In order to more accurately measure the adsorption capacity of the prepared hybrid for the two pollutants, the adsorption data was fit to both the Langmuir and Freundlich equations (Table 2) (Chen et al., 2013; Zhang et al., 2011). The liberalized version of the Freundlich model is given by equation 6: 1
(6)
ln qe = ln KF + nln Ce
Where qe (mg g-1) is the equilibrium absorption capacity and Ce (mg L-1) is the concentration of ammonium or phosphate at equilibrium, while KF (L g-1) and 1/n represent the Freundlich equilibrium constant and a characteristic constant related to adsorption capacity, respectively. 18
Table 2 Isotherm parameters for ammonium and phosphate co-adsorption on ELMNP@zeolite. Freundlich
Temperature (℃)
Langmuir
30
1/n
KF (mg g-1)
R2
Qm (mg g-1)
KL (L mg-1)
R2
NH4+
0.42
1.05
0.708
5.44
0.10
0.955
PO43-
0.58
14.26
0.955
59.88
0.38
0.977
The Langmuir isotherm model can be expressed via equation 7: Ce qe
Ce
1
(7)
= q0 + q0KL
Where q0 (mg g-1) is the maximum adsorption capacity and KL is the Langmuir adsorption equilibrium constant (L mg-1), which is related to energy of adsorption and enthalpy change. The best fit parameters of for each model are summaried in Table 2, where according to the biggest correlation coefficients R2, EL-MNP@zeolite adsorption of both NH4+ and PO43- was most consistent with the Langmuir type adsorption isotherm. This indicated that NH4+ and PO43- were adsorbed on the EL-MNP@zeolite via a monolayer adsorption process. More PO43- was absorbed by EL-MNP@zeolite than NH4+, because the adsorption was favoured by the higher valance negative charge. The value of 1/n was less than 1 for both NH4+ and PO43- under all conditions, suggesting that, consistent with the above kinetics results, the adsorption process was mainly involved chemical adsorption.
Thermodynamic parameters were determined from Kc (the thermodynamic distribution coefficient) and the standard Gibbs free energy ΔG0 (kJ mol−1), standard enthalpy 19
change ΔH0 (kJ mol−1), and standard entropy change ΔS0 (J mol−1 K−1) were calculated using equations 8 and 9: ln Kc = ―
ΔG0 RT
= ―
ΔH0 RT
+
ΔS0 R
(8)
ΔG0 = ― RTln Kc
(9)
Where T (℃) was the absolute temperature, and R was molar gas constant (8.314 J mol-1 K-1). The values of ΔG0, ΔH0 and ΔS0 were calculated from the slope and intercept of the plot of ln Kc versus 1/T (Table 3) Table 3 Thermodynamic parameters for ammonium and phosphate co-adsorption on EL-MNP@zeolite. △G0 (KJ mol-1) △H0 (KJ mol-1)
△S0 (KJ mol-1) 293k
303k
313k
NH4+
-3.40
0.0096
-0.603
-0.483
-0.413
PO43-
-10.51
0.0049
-9.084
-9.025
-8.987
For NH4+ adsorptions, ΔH0 and ΔS0 indicated that the process was exothermic, while the negative values of ΔG0 indicated NH4+ adsorption was spontaneous and favourable. For PO43-, ΔH0 and ΔS0 also indicated that adsorption was exothermic and the solidliquid interface degree of disorder decreased during the reaction. Since irrespective of temperature ΔG0 was less than zero, PO43- was subjected to spontaneous adsorption. The general decline in ΔG0 with increasing temperature indicated that elevated temperature was not conductive to adsorption.
Based on all of the above calculations and the conclusions drawn from individual 20
experiments, the following adsorption mechanism was proposed. The adsorption of NH4+ by EL-MNP@zeolite was mainly via physical adsorption. This is because the surface of the material is negatively charged in water, and NH4+ is electrostatically attracted to it. The adsorption of phosphate was mainly via chemical adsorption. This is because when EL-MNP@zeolite is added to water and surface iron is hydrolyzed to form a Fe-OH bond, which exchanges with the phosphate to form FePO42- (Cao et al., 2016; Tabrizi and Sepehr, 2015).
3.4 Comparison of adsorption capacities The adsorption capacities of several materials for ammonium and phosphate are shown in Table 4. The NH4+ adsorption capacity by EL-MNP@Zeolite was lower than that of all other materials, while PO43- was adsorbed by EL-MNP@Zeolite better than any other materials. This indicated that EL-MNP@Zeolite had selective absorbability for two kinds of pollutants, where the adsorption effect of PO43- was better that that of NH4+ After removing a mixed solution containing 20 mg L-1 NH4+ and 20 mg L-1 PO43-, the residual NH4+ was 16.53 mg L-1, and the residual PO43- was 1.6 mg L-1, and the mixed solution met the national water quality discharge standard.
Table 4 The maximum phosphate and ammonium adsorption capacities of various materials. Materials Natural zeolite
Phosphate adsorption
Ammonium adsorption
capacity (mg g-1)
capacity (mg g-1)
2.73
17.68
Wan et al., 2017
8.25
17.55
Huang et al., 2014a
References
Modified zeolite combined with struvite crystallization 21
Iron nanoparticles
-
9.70
Wang et al., 2014b
EL-MNP@Zeolite
38.91
3.47
-
4. Conclusions In this work, EL-MNP@zeolite was synthesized using green synthetic methods, where iron nanoparticles were successfully decorataed onto zeolite. Zeolite not only plays the role of an inorganic carrier to improve the dispersion of iron nanoparticles and prevent agglomeration and oxidation, but it also provides adsorption sites for NH4+ ions and promotes the simultaneous removal of NH4+-PO43-. The removal efficiency was positively correlated with the amount of adsorbent, but when the pH of the reaction solution was between 4.00-10.00, the adsorption effectwas reduced. The removal efficiencies of NH4+ and PO43- were 43.3% and 99.8%, respectively. Adsorption of NH4+ and PO43- was exothermic and low temperature was favorable for the simultaneous removal of both NH4+ and PO43- by EL-MNP@zeolite. The adsorption of NH4+-PO43- onto EL-MNP@zeolite was via chemical adsorption, which was consistent with the good fit to the pseudo-second-order kinetics.
Acknowledgment The Fujian Province Innovation and Entrepreneurship Talents, China is gratefully acknowledged for its assistance with this research.
Declaration of interests
22
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Research highlights
► A hybrid iron oxide nanoparticle- zeolite were synthesized. ► Green EL-MNP@zeolite has better stability and dispersity. ►Ammonia and phosphate was simultaneously removed. ► NH4+ and PO43- follows the pseudo-second-order kinetic model.
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Graphical abstract
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