Synthesis and Characterization of MgO Modified ... - ACS Publications

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Synthesis and Characterization of MgO Modified Diatomite for Phosphorus Recovery in Eutrophic Water Peng Xia, Xuejiang Wang,* Xin Wang, Jing Zhang, Hao Wang, Jingke Song, Rongrong Ma, Jiayi Wang, and Jianfu Zhao State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, China ABSTRACT: The highly dispersed MgO-nanoflake modified diatomite (MgO−D) was synthesized by a two-step process and used for the adsorption of phosphate from wastewater. The adsorbents involved were characterized by Brunauer−Emmett−Teller (BET), X-ray diffraction (XRD), and field-emission scanning electron microscopy (SEM). Different conditions such as solution pH, contact time, temperature, and coexisting anions were investigated. MgO−D exhibited an excellent phosphate adsorption capacity within the pH range of 3−10 and approached its maximum of 104.94 mg g−1 at optimal conditions. The adsorption kinetics followed the pseudo-second-order kinetic model. The adsorption isotherms had a good fit with the Langmuir model, while the maximum adsorption capacity reached 137.93 mg g−1 at 298 K. The thermodynamics parameters indicated that the sorption process was spontaneous and exothermic. A mechanism that involved (i) hydroxylation of MgO nanoflakes, (ii) electrostatic attraction of negatively charged phosphate ions with hydroxylated adsorbent, and (iii) in situ chemical conversion, was proposed. Results presented here indicated the potential use of MgO−D for phosphate recovery in water.

1. INTRODUCTION It is acknowledged that phosphorus is an indispensable nutrient element for aquatic life,1 but an excessive input of phosphorus would trigger harzardous algal blooms.2 This causes a severe deterioration of water quality which poses a considerable threat to human health and living standards. The acceptable phosphorus concentration in water made by EPA criterion is 0.1 mg L−1 or less.3 Conversely, phosphate is a nonrenewable resource on Earth, while it is currently used as a nonsubstitutable fertilizer in agriculture.4 The proved phosphate reserves on Earth are about to run out in 50−100 years without any intervention measures. Therefore, phosphorus removal combined with its recovery from wastewater is of significant importance to ecological safety and sustainable development and advocated by many researchers.5 Several kinds of wastewater such as municipal wastewater, swine wastewater,6,7 human urine,8,9 and irrigation water contain a large amount of phosphorus. Theoretically, 15−20% of the phosphorus consumption demand could be satisfied by recovering phosphorus from domestic wastewater alone.10 Up to date, various technologies such as adsorption, chemical precipitation, and biological treatment have been utilized to wastewater before discharge.11 However, chemical precipitation is limited by overdose of chemicals and a generous amount of chemical sludge.12 Meanwhile, biological performance fluctuates under different biological conditions and wastewater composition11 and needs additional carbon sources.13 The adsorption of phosphorus has advantages of low cost, easy operation, and potential regeneration.3,14 Furthermore, the recycled adsorbents © XXXX American Chemical Society

loaded with phosphorus could be used as a soil conditioner and an agricultural fertilizer.15−17 Recently extensive research has been done to find low-cost and easily available materials for phosphate removal.18 Various adsorbents such as zeolite,19 biochar,20 ferrihydrite,21 clays,1,22 and layered double hydroxides4 have been investigated. Among them, metal oxides usually exhibit a much better performance in phosphate removal than other adsorbents.23 Particularly, the synthesized nanosize magnesium oxide with high ionic character, simple stoichiometry, and crystal structure has been applied in adsorption, catalysis, refractory, and superconductor products.24 Nanoscale MgO particles exhibit more reactive surfaces because of elevated concentrations of edge/corner sites and other defect sites.25 However, conventional nanosize magnesium oxide often faces the challenge of coaggregation26 and separation problems in aqueous solutions. Dispersing magnesium oxide nanoparticles on larger size supporting material during preparation process would successfully reduce coaggregation and promote separation. Diatomite is a natural fossil assemblage of ancient algaes called diatoms.27,28 which has low cost, porosity, small size, and superior thermal stability.29,30 As an easily available and highly porous mineral, diatomite could be used as a suitable carrier material for nanosized magnesium oxide.31 Our group recently published a paper concerning recovery of nutrients in wastewater by MgO modified diatomite.32 Received: July 11, 2016 Accepted: October 28, 2016

A

DOI: 10.1021/acs.jced.6b00616 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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2.2. Preparation of MgO−D. The preparation of MgO−D was described in our previous work.32 Briefly, Dt was washed and dried to remove impurities before modification; second, 10 g of Dt was first dispersed in 100 mL of 1.25 M MgCl2 under ultrasonic vibration for 30 min, and 100 mL of 2.25 M NaOH was added slowly under vigorous stirring for 12 h at room temperature; third, the mixture was aged for another 24 h, filtered, and washed to remove Cl−. After that, the residue was dried at 353 K and then calcined in a furnace at 723 K for 3 h. 2.3. Analytical Methods. The zero point of charge (pHzpc) of adsorbents was obtained by mass titration.33 Nitrogen adsorption−desorption isotherms method was used to detect BET surface area and pore size distribution (Micrometrics, ASAP 2020). The micrograph of adsorbents was observed through scanning electron microscopy (SEM, Zeiss Ultra 55). The crystalline structure was characterized by a D8 ADVANCE X-ray diffraction (XRD, Bruker AXS, Germany) operated at 40 kV and 40 mA (step size 0.1°) over the range 10° < 2θ < 90°. X-ray photoelectron spectroscopy (XPS, PHI 5000C ESCA) with Al Kα X-ray radiation was applied to probe the chemical composition of samples. 2.4. Batch Adsorption Experiments. Artificial phosphate stock solution (120 mg L−1) was prepared by dissolving quantitative analytically pure KH2PO4·6H2O in distilled water, and dilutions of the stock solution were used in next experiments. The initial pH was adjusted at a certain value by adding

Additionally, we applied the novel material for phosphate removal in this work. The objectives of this study were to develop a novel adsorbent for phosphate removal. The performance under various laboratory conditions was investigated. The mechanism of adsorption of phosphate was discussed by the characterization of materials.

2. MATERIALS AND METHODS 2.1. Materials. The raw diatomite (Dt) used in this study was obtained from Hengxing Chemical Co., Ltd., Tianjin, China. Table 1 gives the chemical constitutent. All of the chemical Table 1. Constitutent of the Raw Diatomite (wt %) constituent

value (%)

SiO2 Al2O3 Fe2O3 CaO MgO others

85.51% 5.32% 3.49% 1.56% 0.36% 3.76%

reagents (including MgCl2·6H2O, NaOH, NH4Cl, KH2PO4· 6H2O) were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd., China.

Figure 1. SEM images of (a) raw diatomite and (b−c) MgO−D; (d) EDS spectrum of MgO−D. B



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Figure 2. (a) Effect of initial pH on the retention of phosphate at dosage 0.3 g L−1, initial concentration 40 mg L−1, T = 298 K. (b) Determination of the point of zero charge of raw diatomite and MgO−D. Conditions: 0.01 mol/L KNO3, T = 298 K.

(Figure 1d) analysis confirmed that the deposited nanoflakes was mainly composed of magnesium and oxygen, which is the chemical composition of MgO. 3.2. Effect of Solution pH. The adsorption of phosphate by Dt and MgO−D as a function of pH was given in Figure 2a. Dt exhibited a relatively low adsorption capacity of phosphate at a wide pH range of 3−10. The adsorption capacity of MgO−D decreased from 104.65 to 72.91 mg g−1 as pH increased from 3 to 10. The uptake of phosphate by MgO−D reached 104.65 mg g−1 at pH 3, which is 25 times greater comparing to Dt. Nevertheless, the rise of solution pH showed a negative effect on the sorption of phosphate by both Dt and MgO−D. For the surface charge of adsorbent and the existence form of adsorbate could be easily influenced by pH, it is considered as a negligible parameter in adsorption process.35 The pHzpc of Dt and MgO−D was experimentally measured to be at 11.07 and 5.60 (Figure 2b). Due to the high reactivity of MgO nanoplates, the zeta potential of MgO−D would change at different solution pH: positive at pH below 11.07 and negative at pH above 11.07. While Dt has a relatively stable surface, the surface charge could unlikely be influenced by solution pH. Additionally, as shown in Figure 3, the orthophosphate

dilute NaOH and HCl solutions. Adsorption experiments were conducted by shaking the mixture of adsorbent and phosphate solution in a thermostatic shaker at a constant temperature at a speed of 150 rpm for a predetermined time. Supernatant was filtered through a 0.22 μm microbrane filter, and the concentration of phosphate in the supernatant was analyzed spectrophotometrically by molybdenum blue method at 700 nm on a UV−vis−NIR scanning spectrophotometer (Shimadzu UV-2550, Japan).

3. RESULTS AND DISCUSSION 3.1. Characterization. The BET and XRD characterizations could be found in our previous work.32 The N2 adsorption/ desorption isotherms and pore size distribution of MgO−D consisted with type IV isotherms of H3 hysteresis loops, corresponding to calippary condensation. MgO−D exhibited a narrow hysteresis loop without limiting adsorption at relatively high p/p0 between 0.9 and 1.0, indicating the textural pores between aggregates of plate-like particles. The pore size distribution curve of MgO−D was broad and multimodal with small pores and larger ones, reflecting porosous structure in stacked nanoflakes.34 As compared to Dt, the BET surface area of MgO− D increased from 4.478 to 34.780 m2 g−1, and total pore volume increased from 0.011 to 0.195 cm3 g−1. The introduction of MgO significantly changed the pore structure and surface area of Dt. Similar results have been reported in a former work.29 In XRD determination, SiO2 (JCPDS 39-1425) were observed according to two characteristic diffraction peaks of 2θ at 21.7° (101) and 35.9° (200) in Dt, intermediate product Mg(OH)2−D, and MgO−D. After calcination, two new diffraction peaks at 42.9° (200) and 62.3° (220) were observed, indicating MgO (JCPDS 65-0476). Furthmore, characteristic diffraction peaks of Mg(OH)2 at 18.6° (001), 38.1° (101), 50.8° (102), and 58.6° (110) (JCPDS 07-0239) disappeared, suggesting that Mg(OH)2 was transformed into MgO completely.23 The typical surface micrograph of Dt and MgO−D was observed by SEM. As shown in Figure 1a, the morphology of Dt was shower-like. The SEM micrograph of MgO−D (Figure 1b) reveals that the original geometry of pores disappeared due to the deposition of magnesium, and the surface of MgO−D was rough and porous due to magnesium oxide deposits. Highmagnification SEM images (Figure 1c) indicate that MgO was mainly existed as nanoflakes and deposited uniformly at the surface of diatomite without any notable aggregation.29 The EDS

Figure 3. PO4 speciation under different solution pH values.

speciation existing in the solution is pH-related.36 H3PO4 was the prime species at pH < 2.5; the uncharged H3PO4 molecule could hardly be captured by the charged surface. H2PO4− and HPO42− became the dominant species at pH 2.5−10; strong electrostatic attraction could be found between the negatively charged phosphate anions and the positively charged surface. C



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Figure 4. (a) Effect of contact time on the sorption of phosphate. (b) The pseudo-first-order kinetics. (c) The pseudo-second-order kinetics. (d) The intraparticle diffusion kinetics (adsorbent dosage 0.3 g L−1, pH 5, T = 298 K).

Table 2. Pseudo-First-Order and the Pseudo-Second-Order Kinetic Model Parameters under Different Initial Phosphate Concentrations pseudo-first-order

pseudo-second-order

C0 (mg L−1)

qe,exp (mg g−1)

qe,cal (mg g−1)

K1 (min−0.5)

R2

qe,cal (mg g−1)

K2 (mg g−1 min−0.5)

R2

20 40 60

56.57 102.97 124.93

25.49 39.40 44.35

0.0113 0.0109 0.0104

0.9501 0.9261 0.9315

61.31 105.15 126.42

1.13 × 10−3 7.30 × 10−4 6.99 × 10−4

0.9997 0.9997 0.9998

A lower surface area and more likely negatively charged surface were attributed to the weaker adsorption of phosphate by Dt. The variation tendency of phosphate sorption as a function of pH could be explained by more OH− competing with PO4 (H2PO4−, HPO42−, and PO43−) and less protonated adsorbent surface. 3.3. Adsorption Kinetics. Adsorption kinetics describes the adsorption rate and reveals when the equilibrium was reached. The kinetic parameters help to understand the mechanism of adsorption and is useful for the prediction of possible application. Figure 4a illustrated the adsorption kinetics of MgO−D under different initial concentrations. The equilibrium adsorption capacity was dependent on the initial phosphate concentration, and the equilibrium adsorption capacity increased from 59.91 mg g−1 to 124.01 mg g−1 as the initial phosphate concentration increased from 20 mg L−1 to 60 mg L−1. Moreover, the sorption of phosphate increased with contact time and reached 90% of maximum within 120 min, followed by a slowly increasement to reach equilibrium. In this study, pseudo-first-order and pseudo-second-order kinetic models6,37,38 were used to fit the kinetics data. The linear form of the pseudo-first-order model is given as

ln(qe − qt ) = ln qe − k1t

(1)

where qt (mg g−1) and qe (mg g−1) stand for the amounts of phosphate adsorbed at time t (h) and equilibrium, respectively; k1 (min−1) is the rate constant of pseudo-first-order models; k1 and qe can be calculated from the linear plot of log(qe − qt) versus t (Figure 4b). Second, the pseudo-second-order model equation is expressed as t 1 t = + 2 qt qe k 2qe

(2)

where qt (mg g−1) and qe (mg g−1) are the amounts of phosphate adsorbed at time t (h) and equilibrium, respectively; k2 (g mg−1 min−1) is the pseudo-second-order rate constant; k2 and qe can be calculated from the linear plot of t/qt versus t (Figure 4c). The fitting results were summarized in Table 2. The correlation coefficient values for the pseudo-first-order kinetic model were relatively small (0.9501, 0.9261, and 0.9315 for 20, 40, and 60 mg L−1), and the calculated qe,cal values did not match the D



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Table 3. Intraparticle Diffusion Parameters at Different Initial Phosphate Concentrations Region 1

Region 2

C0 (mg L−1)

ki,1 (mg g−1 min−0.5)

C1

R12

ki,2 (mg g−1 min−0.5)

C2

R22

20 40 60

6.53 10.84 13.58

1.92 7.37 9.67

0.9966 0.9916 0.9857

0.38 0.53 0.56

50.96 90.28 110.70

0.8328 0.8536 0.8798

Figure 5. (a) Effect of temperature on the sorption of phosphate. Linear plots of the (b) Langmuir and (c) Freundlich equations (adsorbent dosage 0.3 g L−1, pH 5).

qt = kit 0.5 + C

experimental qe values well, indicating that the pseudo-firstorder kinetic model was not applicable for the adsorption process. Conversely, the pseudo-second-order model fitted the experimental data well with a correlation coefficient R2 higher than 0.99, and the calculated qe,cal values show a good agreement with the experimental qe values, suggesting the sorption process was mainly controlled by chemical action. Similar results were reported in the study of adsorbents such as layered double hydroxides,35 Fe3O4,39 and MgO microspheres34 for phosphate removal. Furthermore, the rate constant K2 decreased with the increase of initial phosphate concentration. It gives information that the sorption rate was higher at the lower initial phosphate concentration. The adsorption process in aqueous solution usually consists of multiple steps like bulk diffusion, adsorption of adsorbate onto external surface of adsorbent, and the adsorbed adsorbate moving into the inner pores. Accordingly, the intraparticle diffusion model based on the theory proposed by Weber and Morris could identify the rate-controlling steps of the adsorption process and is given as

(3)

where ki is the intraparticle diffusion rate constant (mg g−1 min0.5) and C is a constant related to the thickness of the boundary layer. According to this model, if intraparticle diffusion is the only ratelimiting step of the whole sorption process, the plot of qt vs t0.5 would yield a straight line acrossing through the origin.35 The plot of qt versus t0.5 (Figure 4d) was a two-stage type, revealing a two-step adsorption processthe first stage depicting the mass transfer from the bulk solution to the surface of adsorbent and the second indicating the intraparticle diffusion. The deviation of straight lines from the origin was indicative of that the pore diffusion was not the sole rate-controlling step for the whole reaction. The values of rate parameters (ki,1 and ki,2) were listed in Table 3. It could be found that ki,2 values for the second region were quite lower than ki,1 values for the first region, representing that the intraparticle diffusion controls the adsorption rate, which is the slowest step of the sorption process.24,40 On the first stage, the adsorption rate was high due to the boundary layer effect. In addition, the rate constants E



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Table 4. Langmuir and Freundlich Isotherm Parameters under Different Temperatures Langmuir

Freundrich

T (K)

qm (mg g−1)

KL (L mg−1)

RL

R2

n

KF (mg g−1)

R2

288 298 308

151.51 137.93 133.33

0.6748 0.5074 0.3348

0.0162 0.0201 0.0321

0.999 0.999 0.995

3.60 3.37 2.93

59.8 48.7 39.5

0.752 0.786 0.742

(ki,1, ki,2) and constants (C1, C2) increased significantly as the initial phosphate concentration increased from 20 to 60 mg L−1. Apparently, a higher initial concentration produced a greater boundary layer effect and driving force for the uptake of phosphate.35 3.4. Adsorption Isotherms. Adsorption isotherms correlate the adsorption capacity (qe) with the equilibrium concentration of adsorbate (Ce) and are of importance to investigate the interaction between adsorbate species and adsorbent surface sites. Based on the pH and kinetic studies, equilibrium experiments were conducted under optimal conditions with different initial phosphate concentrations (10−90 mg L−1) at temperatures of 288, 298, and 308 K as depicted in Figure 5a. A fast increase of the amount of phosphate adsorbed was observed at low solution concentrations, indicating good affinity between phosphate ions and adsorbent surface, and it reached a plateau at higher concentrations, revealing a finite adsorption capacity. The equilibrium adsorption capacity decreased from 147.52 to 125.91 mg g−1 as the temperature increased from 15 to 35 °C, indicating that the adsorption process favored lower temperatures.35 In this study, the obtained isotherm data were fitted to the Langmuir41 (Figure 5b) and Freundlich42 (Figure 5c) isotherm models. The Langmuir isotherm model assumes monolayer adsorption onto adsorbent surface which contains a finite number of uniform adsorption sites with no transmigration of adsorbed molecules on the surface.43 The Freundlich isotherm model is an empirical relationship describing the adsorption of solutes from a liquid to a solid surface and assumes a monolayer adsorption with a heterogeneous energetic distribution of active sites, accompanied by interactions between adsorbed molecules.44 The equations are given as Ce C 1 = e + qe qm KLqm

1 ln qe = ln KF + log Ce n

The essential characteristics of the Langmuir equation can be expressed as a dimensionless separation factor RL,45 which is given as 1 RL = 1 + KLC0 (6) where C0 is the highest initial concentration of phosphate (mg L−1) and KL is the Langmuir isotherm model constant (L mg−1). The values of RL determine whether the adsorption isotherms are favorable (0 < RL < 1), unfavorable (RL > 1), linear (RL = 1), or irreversible (RL = 0). From Table 4, it could be found that RL ranged from 0.3348 to 0.6748 L mg−1, indicating that the sorption characteristics of phosphate onto MgO−D was favorable within the experimental conditions.46 3.5. Thermodynamics. To further evaluate the adsorption process, thermodynamic characteristics such as exothermic or endothermic and spontaneous or not were investigated. Thermodynamic parameters including ΔG°, ΔS°, and ΔH° were calculated using the isotherms data by the following van’t Hoff equation: ΔG° = ΔH ° − T ΔS° = −RT ln Kd

ln Kd =

(7)

ΔS° ΔH ° − R RT

(8) −1

−1

where R is the universal gas constant (8.314 J mol K ), T is the absolute temperature (K), and Kd (L mol−1) is the thermodynamic equilibrium constant calculated from Langmuir equilibrium constant. The values of ΔH° and ΔS° were obtained from the linear plot of ln Kd vs 1/T and summarized in Table 5. Table 5. Thermodynamic Parameters for the Adsorption of Phosphate on MgO−D

(4)

temperature (K)

Kd (L mol−1)

ΔG° (kJ mol−1)

ΔS° (J mol−1 K−1)

ΔH° (kJ mol−1)

22918 16886 10389

−24.04 −23.94 −23.68

−17.3

−29.09

(5)

288 298 308

−1

The ΔG° values were negative at different temperatures, indicating that the adsorption process was favorable and thermodynamically spontaneous. Moreover, it increased from −24.04 to −23.68 kJ mol−1 as the temperature increased from 288 to 308 K, indicating a lower spontaneity at higher temperatures. Generally, ΔG° values between −20 kJ mol−1 and 0 kJ mol−1 suggest a physisorption process, while this suggests a chemisorption process if the ΔG° values are in the range of −40 kJ mol−1 and −80 kJ mol−1.46 Herein, the values of ΔG° were neither in the range of physisorption or chemosorption process. It could be assumed that,more mechanisms were involved such as electrostatic interaction, ion exchange, and precipitation.46,47 In Table 4, a negative value of ΔH° could be observed, revealing the exothermic nature of phosphate adsorption on MgO−D, which was in accordance with the decrease of adsorption capacity at higher temperatures depicted

where Ce (mg L ) is the equilibrium concentration of phosphate, qe (mg g−1) is the equilibrium amount of phosphate adsorbed, qm (mg g−1) is the theoretical maximum monolayer adsorption capacity (mg g−1), KL is the Langmuir sorption constant (L mg−1), KF is the Freundlich model constant, and 1/n is an empirical parameter or heterogeneity factor which measures the adsorption intensity. The Langmuir and Freundlich model parameters along with correlation coefficients (R2) were present in Table 4. According to the correlation coefficients, the adsorption process was more satisfied by the Langmuir equation with R2 higher than 0.99, which indicates the monolayer sorption of phosphate on the surface of adsorbent. The maximum adsorption capacities predicted by Langmuir equation were also in agreement with the experimental data. The values of 1/n were lower than 1 which means that the sorption of phosphate was favorable. F



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by the isotherms. The negative value of ΔS° demonstrated a decreased randomness in the sorption process. 3.6. Effect of Coexistent Substances. Various coexisting anions in water could interfere the sorption of phosphate due to competitive adsorption. Four commonly exist anions such as Cl−, NO3−, CO32−, and SO42− were investigated at three different concentrations, and the results were illustrated in Figure 6.

Giving the above analysis, the removal of phosphate by MgO−D could be not only a simple adsorption process. When MgO−D was added in aqueous solution, the porous MgO nanoflakes with high reactivity on the surface of diatomite could be hydroxylated in the presence of water:49 MgO(s) + H 2O(l) → MgOH+(surface) + OH−(aqueous)

(9)

According to the hydroxylation equation, the formation of MgOH+ on the surface of diatomite depends on pH values. Considering the very high pHzpc of MgO−D, the hydroxylation process could be easily accomplished under experimental conditions. Thus, the in situ formed MgOH+ brought more active sites for phosphate adsorption. As described in part 3.2, negatively charged HPO42− and PO43− could rapidly be captured by protonated MgO−D due to electrostatic attraction. This sorption behavior above is also validated in the kinetic and isotherm studies. Meanwhile, the surface complexation of PO4 and MgOH+ would react with each other and in situ convert into crystal structure compound of Mg3(PO4)2 and MgHPO4 as follows: MgOH+ + HPO4 2 − ⇒ MgHPO4

(10)

MgOH+ + PO4 3 − ⇒ Mg3(PO4 )2

(11)

This could immediately reduce the density of MgOH+ which will be beneficial to the reaction of MgO hydroxylation. Consequently, the MgO nanoflakes on the surface of diatomite would be converted into MgOH+ completely,23 and the continuously reaction between PO4 and MgOH+ was attributed to the exceptional phosphate sorption capacity. 3.8. Economic Evaluations. Increasing numbers of clay adsorbents for the removal of phosphates have been used as listed in Table 6. The phosphate adsorption capacity of MgO−D is much greater than former reports with qm,exp of 147.52 mg g−1. There are lots of advantages using MgO−D as a phosphate removal adsorbent. First, diatomite is cheap and abundant in China, while the conventional adsorbents such as polymer and activated carbon take a risk of high cost. Furthermore, the post-adsorption MgO−D could be used as fertilizer in agriculture because of its high content of phosphorus, while the regeneration of conventional adsorbents brings more cost. Yao et al.17 explored the potential application of Mg-rich biochar prepared to recoverand reuse phosphate (P) from aqueous solution; the results showed that most of the P reclaimed was bioavailable and could stimulate grass seed germination and growth.

Figure 6. Effect of coexisting anions on phosphate adsorption (adsorbent dosage 0.3 g L−1, C0 40 mg L−1, pH 5, T = 298 K).

Cl−, NO3−, and SO42− had little negative influence on the sorption process and followed in the order Cl− < NO3− < SO42−. The presence of CO32− had a significantly negative effect on the sorption of phosphate. For example, the sorption capacity decreased from 87.12 to 57.40 mg g−1 when the concentration of CO32− increased from 50 to 200 mg L−1. This may be attributed to a high affinity between CO32− and surface adsorption sites. Furthermore, the sorbed CO32− could form complex with MgO and block the sorption sites. Although CO32− inhibited the sorption of phosphate, more than 65% of phosphate ions can be removed under coexisting ions. This indicated that MgO−D has excellent sorption selectivity toward phosphate ion. 3.7. Mechanism of Sorption. In order to understand the mechanism of sorption of phosphate onto MgO−D, adsorbent after sorption was separated, washed, and dried for characterization. Figure 7a gives the FTIR spectra of MgO−D before (A) and after (B) sorption; the adsorption peak at 500 cm−1 corresponding to the vibration of MgO was weakened, and a new peak appearing at 1069 cm−1 which is associated with phosphate was found. The XRD patterns of post-adsorption MgO−D was given in Figure 7b. A series of peaks of 2θ at 11.09°, 14.68°, 16.66°, 27.94°, and 33.69° were found, which have high affinity with Mg3(PO4)2·22H2O crystal structure (JCPDS 31-0805). Figure 7c shows the micrograph of MgO−D after sorption. Hexagonal crystal was found on the surface of diatomite and the EDS spectrum (Figure 7d) revealed that Mg, O, and P are the principal elements of the crystal. The peak of high-resolution XPS spectrum of P 2p was divided into two separate peaks, 132.9 and 133.8 eV, corresponding to PO43− and HPO42−, respectively (Figure 7e).48 It demonstrated that PO43− and HPO42− existed in the surface of diatomite. Furthermore, the XPS spectra of Mg 2p (Figure 7f) were tested and divided at 51.8 and 52.6 eV, respectively, which probably confirmed the presence of MgHPO4 and Mg3(PO4)2.48

4. CONCLUSIONS The MgO-nanoflake modified diatomite was developed by a chemical precipitation process followed by a calcination process and applied for the adsorption of phosphate from aqueous solution. The as-prepared MgO−D had a relatively larger specific surface area of 45.17 m2 g−1 comparing to raw diatomite. MgO−D exhibited a good affinity to phosphate in the pH range of 3−10, and the maximum adsorption capacity reached 104.94 mg g−1 at adsorbent dosage of 0.3 g L−1, pH 3, and initial phosphate concentration of 40 mg L−1. The kinetic and isotherm results indicated that the adsorption process followed the pseudo-second-order kinetic model and the Langmuir isotherm model with a maximum adsorption capacity of 137.93 mg g−1 at 298 K. The thermodynamics parameters indicated that the sorption process was spontaneous and exothermic. Coexisting anions showed a negative effect and followed the order G



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Figure 7. (a) FTIR spectra of MgO−D before (curve A) and after (curve B) sorption, (b) XRD patterns, (c) SEM image, (d) EDS spectrogram, and XPS spectra of (e) P 2p and (f) Mg 2p after sorption.

CO32− > SO42− > NO3− > Cl−. The present study demonstrated that MgO−D could be applied as an efficient adsorbent for the removal and recycle of phosphate from aqueous solution.

Table 6. Comparison of Adsorption Capacities of Different Adsorbents for PT and TAN adsorbents MDC (magnetic diatomite) ferrihydrite-modified diatomite synthetic zeolite MKC (Mg−Al hydrotalcite loaded kaolin clay) Fe modified bentonite MgO−D

qm (mg g−1) 11.89 37.3 52.9 11.85 11.15 147.52

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reference 1 11 22 35

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

This work was supported by the National Natural Science Foundation of China (No. 51678421 and 41571301) and International S&T Cooperation Program of China (No. 2014DFA91650).

50 present study H



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Notes

solution by a hydrated aluminum oxide modified natural zeolite. Chem. Eng. J. 2015, 271, 204−213. (20) Yao, Y.; Gao, B.; Inyang, M.; Zimmerman, A. R.; Cao, X.; Pullammanappallil, P.; Yang, L. Removal of phosphate from aqueous solution by biochar derived from anaerobically digested sugar beet tailings. J. Hazard. Mater. 2011, 190, 501−507. (21) Wang, H.; Zhu, J.; Fu, Q.-L.; Xiong, J.-W.; Hong, C.; Hu, H.-Q.; Violante, A. Adsorption of Phosphate onto Ferrihydrite and Ferrihydrite-Humic Acid Complexes. Pedosphere 2015, 25, 405−414. (22) Hamdi, N.; Srasra, E. Removal of phosphate ions from aqueous solution using Tunisian clays minerals and synthetic zeolite. J. Environ. Sci. 2012, 24, 617−623. (23) Liu, Y.; Li, Q.; Gao, S.; Shang, J. K. Exceptional As(III) Sorption Capacity by Highly Porous Magnesium Oxide Nanoflakes Made from Hydrothermal Synthesis. J. Am. Ceram. Soc. 2011, 94, 217−223. (24) Pei, Y.; Wang, M.; Tian, D.; Xu, X.; Yuan, L. Synthesis of core− shell SiO2@MgO with flower like morphology for removal of crystal violet in water. J. Colloid Interface Sci. 2015, 453, 194−201. (25) Klabunde, K. J.; Stark, J.; Koper, O.; Mohs, C.; Park, D. G.; Decker, S.; Jiang, Y.; Lagadic, I.; Zhang, D. Nanocrystals as Stoichiometric Reagents with Unique Surface Chemistry. J. Phys. Chem. 1996, 100, 12142−12153. (26) Yuan, P.; Liu, D.; Fan, M.; Yang, D.; Zhu, R.; Ge, F.; Zhu, J.; He, H. Removal of hexavalent chromium [Cr(VI)] from aqueous solutions by the diatomite-supported/unsupported magnetite nanoparticles. J. Hazard. Mater. 2010, 173, 614−621. (27) Erdem, E.; Colgecen, G.; Donat, R. The removal of textile dyes by diatomite earth. J. Colloid Interface Sci. 2005, 282, 314−319. (28) Al-Degs, Y.; Khraisheh, M. A. M.; Tutunji, M. F. Sorption of lead ions on diatomite and manganese oxides modified diatomite. Water Res. 2001, 35, 3724−3728. (29) Xie, F.; Wu, F.; Liu, G.; Mu, Y.; Feng, C.; Wang, H.; Giesy, J. P. Removal of phosphate from eutrophic lakes through adsorption by in situ formation of magnesium hydroxide from diatomite. Environ. Sci. Technol. 2014, 48, 582−90. (30) Chao, Y.; Zhu, W.; Chen, F.; Wang, P.; Da, Z.; Wu, X.; Ji, H.; Yan, S.; Li, H. Commercial Diatomite for Adsorption of Tetracycline Antibiotic from Aqueous Solution. Sep. Sci. Technol. 2014, 49, 2221− 2227. (31) Khraisheh, M.; Aldegs, Y.; McMinn, W. Remediation of wastewater containing heavy metals using raw and modified diatomite. Chem. Eng. J. 2004, 99, 177−184. (32) Xia, P.; Wang, X.; Wang, X.; Song, J.; Wang, H.; Zhang, J.; Zhao, J. Struvite crystallization combined adsorption of phosphate and ammonium from aqueous solutions by mesoporous MgO−loaded diatomite. Colloids Surf., A 2016, 506, 220−227. (33) Fiol, N.; Villaescusa, I. Determination of sorbent point zero charge: usefulness in sorption studies. Environ. Chem. Lett. 2009, 7, 79−84. (34) Zhou, J.; Yang, S.; Yu, J. Facile fabrication of mesoporous MgO microspheres and their enhanced adsorption performance for phosphate from aqueous solutions. Colloids Surf., A 2011, 379, 102− 108. (35) Deng, L.; Shi, Z. Synthesis and characterization of a novel Mg− Al hydrotalcite-loaded kaolin clay and its adsorption properties for phosphate in aqueous solution. J. Alloys Compd. 2015, 637, 188−196. (36) Karageorgiou, K.; Paschalis, M.; Anastassakis, G. N. Removal of phosphate species from solution by adsorption onto calcite used as natural adsorbent. J. Hazard. Mater. 2007, 139, 447−452. (37) Weber, W. J.; Morris, J. C. Proceedings of the International Conference on Water Pollution Symposium; Pergamon, Oxford, 1962; Vol. 2, pp 231−266. (38) Datta, D.; Uslu, H.; Kumar, S. Adsorptive Separation of Cu2+from an Aqueous Solution Using Trioctylamine Supported Montmorillonite. J. Chem. Eng. Data 2015, 60, 3193−3200. (39) Yan, L. G.; Yang, K.; Shan, R. R.; Yan, T.; Wei, J.; Yu, S. J.; Yu, H. Q.; Du, B. Kinetic, isotherm and thermodynamic investigations of phosphate adsorption onto core-shell Fe(3)O(4)@LDHs composites

The authors declare no competing financial interest.

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REFERENCES

(1) Chen, J.; Yan, L.-g.; Yu, H.-q.; Li, S.; Qin, L.-l.; Liu, G.-q.; Li, Y.-f.; Du, B. Efficient removal of phosphate by facile prepared magnetic diatomite and Illite clay from aqueous solution. Chem. Eng. J. 2016, 287, 162−172. (2) Wang, Z.; Lin, Y.; Wu, D.; Kong, H. Hydrous iron oxide modified diatomite as an active filtration medium for phosphate capture. Chemosphere 2016, 144, 1290−1298. (3) Ramasahayam, S. K.; Guzman, L.; Gunawan, G.; Viswanathan, T. A Comprehensive Review of Phosphorus Removal Technologies and Processes. J. Macromol. Sci., Part A: Pure Appl.Chem. 2014, 51, 538− 545. (4) Yu, Q.; Zheng, Y.; Wang, Y.; Shen, L.; Wang, H.; Zheng, Y.; He, N.; Li, Q. Highly selective adsorption of phosphate by pyromellitic acid intercalated ZnAl-LDHs: Assembling hydrogen bond acceptor sites. Chem. Eng. J. 2015, 260, 809−817. (5) van der Houwen, J. A.; Valsami-Jones, E. The application of calcium phosphate precipitation chemistry to phosphorus recovery: the influence of organic ligands. Environ. Technol. 2001, 22, 1325− 1335. (6) Fang, C.; Zhang, T.; Li, P.; Jiang, R. F.; Wang, Y. C. Application of magnesium modified corn biochar for phosphorus removal and recovery from swine wastewater. Int. J. Environ. Res. Public Health 2014, 11, 9217−9237. (7) Huang, H.; Xu, C.; Zhang, W. Removal of nutrients from piggery wastewater using struvite precipitation and pyrogenation technology. Bioresour. Technol. 2011, 102, 2523−2528. (8) Ban, Z.; Dave, G. Laboratory studies on recovery of N and P from human urine through struvite crystallisation and zeolite adsorption. Environ. Technol. 2004, 25, 111−121. (9) Ganrot, Z.; Dave, G.; Nilsson, E. Recovery of N and P from human urine by freezing, struvite precipitation and adsorption to zeolite and active carbon. Bioresour. Technol. 2007, 98, 3112−3121. (10) Yuan, Z.; Pratt, S.; Batstone, D. J. Phosphorus recovery from wastewater through microbial processes. Curr. Opin. Biotechnol. 2012, 23, 878−883. (11) Xiong, W.; Peng, J. Development and characterization of ferrihydrite-modified diatomite as a phosphorus adsorbent. Water Res. 2008, 42, 4869−4877. (12) Jacobs, J. F.; Hasan, M. N.; Paik, K. H.; Hagen, W. R.; van Loosdrecht, M. C. M. Development of a bionanotechnological phosphate removal system with thermostable ferritin. Biotechnol. Bioeng. 2010, 105, 918−923. (13) Rittmann, B. E.; Mayer, B.; Westerhoff, P.; Edwards, M. Capturing the lost phosphorus. Chemosphere 2011, 84, 846−853. (14) Zhu, Z.; Zeng, H.; Zhu, Y.; Yang, F.; Zhu, H.; Qin, H.; Wei, W. Kinetics and thermodynamic study of phosphate adsorption on the porous biomorph-genetic composite of α-Fe2O3/Fe3O4/C with eucalyptus wood microstructure. Sep. Purif. Technol. 2013, 117, 124−130. (15) de-Bashan, L. E.; Bashan, Y. Recent advances in removing phosphorus from wastewater and its future use as fertilizer (1997− 2003). Water Res. 2004, 38, 4222−4246. (16) Sengupta, S.; Pandit, A. Selective removal of phosphorus from wastewater combined with its recovery as a solid-phase fertilizer. Water Res. 2011, 45, 3318−3330. (17) Yao, Y.; Gao, B.; Chen, J.; Yang, L. Engineered Biochar Reclaiming Phosphate from Aqueous Solutions: Mechanisms and Potential Application as a Slow-Release Fertilizer. Environ. Sci. Technol. 2013, 47, 8700−8708. (18) Karaca, S.; Gurses, A.; Ejder, M.; Acikyildiz, M. Kinetic modeling of liquid-phase adsorption of phosphate on dolomite. J. Colloid Interface Sci. 2004, 277, 257−263. (19) Guaya, D.; Valderrama, C.; Farran, A.; Armijos, C.; Cortina, J. L. Simultaneous phosphate and ammonium removal from aqueous I



Article

with easy magnetic separation assistance. J. Colloid Interface Sci. 2015, 448, 508−516. (40) Oladoja, N. A.; Aboluwoye, C. O.; Oladimeji, Y. B. Kinetics and isotherm studies on Methylene Blue adsorption onto ground palm kernel coat. Turk. J. Eng. Environ. Sci. 2008, 32, 303−312. (41) Langmuir, I. THE CONSTITUTION AND FUNDAMENTAL PROPERTIES OF SOLIDS AND LIQUIDS. PART I. SOLIDS. J. Am. Chem. Soc. 1916, 38, 2221−2295. (42) Weber, T. W.; Chakravorti, R. K. Pore and solid diffusion models for fixed-bed adsorbers. AIChE J. 1974, 20, 228−238. (43) Chao, Y.; Zhu, W.; Wu, X.; Hou, F.; Xun, S.; Wu, P.; Ji, H.; Xu, H.; Li, H. Application of graphene-like layered molybdenum disulfide and its excellent adsorption behavior for doxycycline antibiotic. Chem. Eng. J. 2014, 243, 60−67. (44) Li, S.; Yang, Q.; Zhong, Y.; Chen, F.; Xie, T.; Yao, F.; Sun, J.; Jiang, C.; Li, X.; Zeng, G. Adsorptive Bromate Removal from Aqueous Solution by Commercial Strongly Basic Resin Impregnated with Hydrated Ferric Oxide (HFO): Kinetics and Equilibrium Studies. J. Chem. Eng. Data 2016, 61, 1305−1312. (45) Tan, I. A.; Ahmad, A. L.; Hameed, B. H. Adsorption of basic dye on high-surface-area activated carbon prepared from coconut husk: equilibrium, kinetic and thermodynamic studies. J. Hazard. Mater. 2008, 154, 337−346. (46) Huang, W.-Y.; Li, D.; Liu, Z.-Q.; Tao, Q.; Zhu, Y.; Yang, J.; Zhang, Y.-M. Kinetics, isotherm, thermodynamic, and adsorption mechanism studies of La(OH)3-modified exfoliated vermiculites as highly efficient phosphate adsorbents. Chem. Eng. J. 2014, 236, 191− 201. (47) Zhou, Q.; Wang, X.; Liu, J.; Zhang, L. Phosphorus removal from wastewater using nano-particulates of hydrated ferric oxide doped activated carbon fiber prepared by Sol−Gel method. Chem. Eng. J. 2012, 200−202, 619−626. (48) Song, Y.; Shan, D.; Chen, R.; Zhang, F.; Han, E.-H. A novel phosphate conversion film on Mg−8.8Li alloy. Surf. Coat. Technol. 2009, 203, 1107−1113. (49) Rocha, S. D. F.; Mansur, M. B.; Ciminelli, V. S. T. Kinetics and mechanistic analysis of caustic magnesia hydration. J. Chem. Technol. Biotechnol. 2004, 79, 816−821. (50) Zamparas, M.; Gianni, A.; Stathi, P.; Deligiannakis, Y.; Zacharias, I. Removal of phosphate from natural waters using innovative modified bentonites. Appl. Clay Sci. 2012, 62−63, 101−106.

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