Removal behaviors and mechanisms of orthophosphate and pyrophosphate by calcined dolomite with ferric chloride assistance

Chemosphere 235 (2019) 1015e1021

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Removal behaviors and mechanisms of orthophosphate and pyrophosphate by calcined dolomite with ferric chloride assistance Yunfeng Xu, Hui Hong, Fei Yang, Liang Zhang, Jiahui Xu, Li Dou, Ying Hao, Guangren Qian**, Jizhi Zhou* School of Environmental and Chemical Engineering, Shanghai University, No.99 Shangda Rd., Shanghai, 200444, PR China

h i g h l i g h t s
CaFe-LDHs and MgFe-LDHs were synthesized by calcined dolomite and ferric chloride.
Orthophosphate removal was due to precipitation by Ca2þ and adsorption by MgFe-LDHs.
Pyrophosphate was mainly removed by precipitation.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 February 2019 Received in revised form 22 June 2019 Accepted 2 July 2019 Available online 3 July 2019

Phosphate is one of the main contaminations in water, so an effective method of decreasing or removing phosphate is needed. The main purpose of this paper is to synthesize CaFe-LDHs and MgFe-LDHs from the mixture of calcined dolomite and ferric chloride to remove orthophosphate and pyrophosphate. The study showed that removal of orthophosphate was attributed to the precipitation by Ca2þ and adsorption by MgFe-LDHs, where the former played a main role. As for pyrophosphate, it was mainly removed by precipitation at the initial pyrophosphate concentration ranging from 3.228 to 17.04 mmol/L. When the initial concentrations became relatively higher, the removal efficiency of pyrophosphate decreased because the complexation effects by Fe3þ, Ca2þ and Mg2þ took place. © 2019 Elsevier Ltd. All rights reserved.

Handling Editor: Veeriah (Jega) Jegatheesan Keywords: Calcined dolomite CaFe-LDHs MgFe-LDHs Removal of orthophosphate and pyrophosphate

1. Introduction Although phosphorus is one of the basic elements for organisms, a large amount of phosphorus can cause a series of environmental problems, such as eutrophication (Fang et al., 2018; Wan et al., 2017), which is mainly in the forms of orthophosphates (OP) and pyrophosphates (PP) (Altundogan and Tumen, 2002). Currently, adsorption is recognized as an effective and promising method for removal of phosphates owing to its low cost and environmental friendliness (Hrenovic et al., 2008; del Rio et al., 2012; Li et al., 2017), particularly for low phosphates

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Zhou).

(G.

Qian),

https://doi.org/10.1016/j.chemosphere.2019.07.018 0045-6535/© 2019 Elsevier Ltd. All rights reserved.

[email protected]

concentrations (such as the phosphates concentrations were less than 30 mg/L) (Zhang et al., 2011). To find the superior adsorbents with high effects for phosphate removal from wastewater, natural materials (such as dolomite powder, montmorillonite and kaolinite)(Karageorgiou et al., 2007; Karaca et al., 2006; Zhu et al., 2009; Taylor et al., 2009) and metals oxides (or hydroxides)(Zeng et al., 2008; Tanada et al., 2003; Genz et al., 2004) have been studied. Moreover, even solid wastes have been utilized in some research, such as steel slag (Bowden et al., 2009) and fly ash (Xu et al., 2012), however, the adsorption capacity of phosphates from above adsorbents is low and unsatisfactory. Layered double hydroxides (LDHs), a desirable absorbent with remarkable selectivity to phosphate ions and high anionic exchange capacity, have attracted considerable attention (Wu et al., 2012; Zhou et al. 2011, 2012). In the past few years, different types of LDHs were synthesized for phosphate removal, the phosphate removal percentage remained above 92% at a pH range of

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2e10 by vegetable biochar/Mg-Al LDOs (Zhang et al., 2019). In practical applications, however, a cheap raw material for LDHs synthesis is needed. Calcined dolomite is a mixture of CaO and MgO with low solubility, which is cheap and easy to get (Nagase et al., 2014). It can be used as a source of divalent metal, and other metal sources can be added to synthesize the corresponding LDHs (Karaca et al., 2004; Xu et al., 2010). Researcher synthesized MgCaFeCl-LDHs and examined its removal effect on tripolyphosphate. The results showed that the removal efficiency of tripolyphosphate from MgCaFeCl-LDHs was higher than that of CaFeClLDHs alone (Zhou et al., 2011). Besides, different ratios of different metal molar could lead to the differences in the removal efficiency of phosphate (Iftekhar et al., 2018). In this study, CaFe-LDHs and MgFe-LDHs system was constructed by calcined dolomite and ferric chloride at the different ratios (M2þ: M3þ ¼ 2, 3, and 4), the removal behaviors and mechanisms of OP and PP at different initial concentrations were investigated. It was expected that this work would be helpful in the actual application of calcined dolomite and contribute to phosphates removal in wastewater. 2. Materials and methods 2.1. Preparation of LDHs Dolomite powder was firstly calcined by muffle furnace at 1000  C for 1 h. Hereafter, calcined dolomite was grinded and sieved through 100 meshes. According to the ratio between bivalent metals and Fe contents (M2þ: M3þ ¼ 2, 3, and 4), 0.057 g of calcined dolomite powder was mixed with 0.153 g, 0.102 g and 0.077 g FeCl3$6H2O respectively. The mixtures were then put into 100 ml of redistilled water. Magnetic stirring was conducted at room temperature for 48 h. After the reaction ended, the products were centrifuged at the speed of 4500 r/min and washed twice. The obtained solid products were dried by 105  C, and then grinded and sieved through 100 meshes. The obtained solid products (2 g/L) were put into a closed container with redistilled water and shaken for 24 h at room temperature, where washing solid products were generated. Calcined dolomite or solid products (100 mg) were dissolved in 30 ml of aqua regia. After cooling to ambient temperature, the solution was diluted up to 50 ml with redistilled water and then centrifuged at the speed of 4500 r/min. The concentrations of metals in the solution were detected.

selective electrode was devoted to determine the concentrations of free calcium ions in solutions. 2.3.2. Characterization methods The powder X-ray diffraction (XRD) patterns for the solid samples were registered on a Dmax/RB diffractometer (Rigaku Co.) with Cu Ka radiation (l ¼ 0.15406 nm) at 40 kV and 100 mA. Speed of Angular speed device was 8 /min. The initial and terminal scanning angles were 5 and 80 , respectively. Analysis of main reflection profile of XRD was studied by Jade software. 3. Results and discussion 3.1. Characterization of calcined dolomite by XRD According to the results of ICP, concentrations of Ca2þ and Mg2þ were 9.48 mmol/g and 10.28 mmol/g in calcined dolomite, respectively. The XRD pattern of calcined dolomite is shown in Fig. 1. It could be seen that there were six strong reflection and the corresponding values are at 2q of 32.36 , 37.49 , 54.02 , 29.4 , 43.02 , and 62.37. Among them, three characteristic reflections of CaO were found from the XRD patterns at 2q of 32.36 , 37.49 and 54.02 . The remaining reflections were consistent with the XRD patterns of CaCO3 at 2q of 29.4 , which indicated that CO2 contamination existed in the samples. In addition, the remaining reflections corresponded to MgO. 3.2. Analysis of synthesized products 3.2.1. Analysis of speciation XRD patterns of pure LDHs and solid products synthesized by calcined dolomite powder and FeCl3$6H2O (M2þ: M3þ ¼ 2, 3, and 4) are displayed in Fig. 2. According to the characteristic reflection of raw LDHs, the characteristic reflection of MgFe-LDHs and CaFeLDHs simultaneously appeared in the synthesized products at different ratios of M2þ/M3þ. XRD patterns of the solid products before and after washing at different ratios of M2þ/M3þ is present in Fig. 3. The characteristic reflection of CaFe-LDHs at 2q of 30.58 disappeared after washing. The disappearance of this reflection by the washing process was mainly because CaFe-LDHs were unstable and prone to hydrolysis. Based on this phenomenon, it was preferable proved that the

2.2. Removal of orthophosphate and pyrophosphate

2.3. Characterization 2.3.1. Determination of phosphate and metal ions The concentrations of OP and PP were measured by the ICS-1100 ion chromatograph (Dionex). ICP atomic emission spectrometer (LEEMAN LABS, USA) was used for determination of metal ions. PCa-1-01 Rex calcium ion

—MgO —CaO —CaCO3

Relative intensity (a.u.)

The mixtures were severally put into 100 ml of 40 mg/L and 400 mg/L sodium phosphate solutions. NaOH and HNO3 were used to adjust pH to 11 and then magnetic stirring was conducted at room temperature for 48 h. The solutions of mixed systems were filtrated by 0.45 mm filter membranes, and the concentrations of OP, Ca2þ, Mg2þ, and Fe3þ were measured. The experimental methods for 100 ml of sodium pyrophosphate solution were the same as that for OP. The concentrations of sodium pyrophosphate were 3.228 mmol/L, 6.655 mmol/L, 17.04 mmol/L, 27.51 mmol/L, and 33.64 mmol/L, respectively.

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Fig. 1. XRD pattern of calcined dolomite powder.

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Fig. 2. XRD patterns of pure LDHs and solid products synthesized by calcined dolomite powder (M2þ) and FeCl3$6H2O (M3þ) at different ratios (M2þ/M3þ ¼ 2, 3, and 4).

Fig. 4. The concentrations of metal ions in solid products synthesized by calcined dolomite powders (M2þ) and FeCl3$6H2O (M3þ) at different ratios (M2þ/M3þ ¼ 2, 3, and 4).

Relative intensity (a.u.)

washing-4:1

4:1 washing-3:1 3:1 washing-2:1 2:1 5

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2 (degree) Fig. 3. XRD patterns of solid products synthesized by calcined dolomite powder (M2þ) and FeCl3$6H2O (M3þ) at different ratios (M2þ/M3þ ¼ 2, 3, and 4) before and after washing.

Fig. 5. The concentrations of metal ions released from solid products synthesized by calcined dolomite (M2þ) and FeCl3$6H2O (M3þ) at different ratios (M2þ/M3þ ¼ 2, 3, and 4) after the washing process.

increased with the increasing ratio of M2þ/M3þ, namely, more Ca2þ was lost from the synthesized solid products.

synthesized intercalation compounds by calcined dolomite and FeCl3$6H2O were mixture of MgFe-LDHs and CaFe-LDHs.

3.3. Removal effects and mechanisms of orthophosphate

3.2.2. Analysis of metals The concentrations of metal ions in the synthesized solid products are displayed in Fig. 4. Judged from the computed results, the actual ratio of M2þ/M3þ was 1.87, 2.76, and 3.72, which were less than the theoretical ratio of 6.5%, 8%, and 7%, respectively. The release of metal ions during the washing process is shown in Fig. 5. As the ratio of M2þ/M3þ increased, the dissolution of Mg2þ and Fe3þ was relatively lower than Ca2þ and almost no obvious changes took place. The phenomenon that Mg2þ and Fe3þ maintained constant concentrations illustrated that MgFe-LDHs could steadily exist in the solid products and would not be hydrolyzed after the washing process. On the contrary, much Ca2þ was released during the washing process, which was mainly because CaFe-LDHs were unstable and prone to hydrolysis. Additionally, the higher the ratio of M2þ/M3þ is, the more easily CaFe-LDHs compounds can be formed. So, the concentrations of Ca2þ in washing solutions

3.3.1. Analysis of solid products XRD patterns of solid products generated from OP removal process at initial OP concentrations of 40 mg/L and 400 mg/L are shown in Fig. 6. When the ratio of M2þ/M3þ was 3 (Fig. 6 (a)), the characteristic reflection of MgFe-LDHs appeared. According to the calculation results, the interlayer spacing distance of MgFe-LDHs before and after OP removal process by calcined dolomite and FeCl3$6H2O was 7.69 Å and 7.80 Å, separately. No obvious change in interlayer spacing could be observed, which indicated that OP ions were not intercalated into the synthesized intercalation compounds. In other words, interlayer anion exchange did not happen during the OP removal process. On the contrary, these characteristic reflections were not observed at OP concentrations of 400 mg/ L (Fig. 6 (b)). This result might lie in that OP ions were precipitated with metals in the order of Ca2þ, Mg2þ, and Fe3þ. When the initial OP concentration was relatively low (40 mg/L), OP was precipitated

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(a)

Relative intensity (a.u.)

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Calcined Dolomite 40mg/L P

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Fig. 6. XRD patterns of solid products generated from orthophosphate removal process at initial orthophosphate concentrations of 40 mg/L (a) and 400 mg/L (b) at different ratios (a: ratios of M2þ/M3þ was 3; b: ratios of M2þ/M3þ were 2, 3 and 4).

with Ca2þ firstly. Meanwhile, MgFe-LDHs were formed by Mg2þ and Fe3þ in alkaline condition. Therefore, the removal of OP could be attributed to precipitation by Ca2þ and adsorption by MgFeLDHs. Thereinto, precipitation by Ca2þ played a major role. However, it couldn't be ignored that OP was also precipitated by Mg2þ when a high initial OP concentration (400 mg/L) was involved. A lot of Mg2þ was needed and consumed under this condition, which prevented the formation of MgFe-LDHs. At the same time, large amounts of amorphous solid products were generated, which also affected the formation of MgFe-LDHs. Hence, there was no generation of MgFe-LDHs at the initial OP concentrations of 400 mg/L. 3.3.2. Analysis of solution components The concentrations of Ca2þ, Mg2þ, Fe3þ, and PO3 4 in the solution treated by calcined dolomite and FeCl3$6H2O are shown in Fig. 7. At the initial OP concentration of 40 mg/L (Fig. 7(a)), PO3 was 4 completely removed under four different treatment conditions. When calcined dolomite was solely applied without FeCl3$6H2O, the dissolution of Ca2þ and Mg2þ from calcined dolomite was very little, the concentrations of which was only 0.58 mmol/L and 1.82 mmol/L, respectively. However, when FeCl3$6H2O was added into the solution together, the released amounts of Ca2þ and Mg2þ significantly increased, the maximum concentrations of dissolved

Ca2þ and Mg2þ are 8.85 and 5.96 mg/L respectively (Fig. 7(a)). Moreover, as the ratio of M2þ/M3þ increase, the dissolution of Ca2þ and Mg2þ decrease. For calcined dolomite powder, most of the calcium and magnesium existed in the forms of MgO and CaO. The addition and dissolution of FeCl3$6H2O reduced the solution pH, which promoted the dissolution of calcium and magnesium oxides in calcined dolomite. Afterwards, PO3 4 was precipitated by the dissolved Ca2þ preferentially at low OP concentrations, and the dissolved Mg2þ was easy to form MgFe-LDHs in OH rich environment. So, the formation of MgFe-LDHs and CaFe-LDHs led to the decrease of the dissolved Mg2þ and Ca2þ. This also explained why there was no layered structure when the ratio of M2þ/M3þ was 2, but it appeared when the ratio reached 3 and 4. When the ratio of M2þ/M3þ was 2, more FeCl3$6H2O was added resulted in a lower pH of the solution. At this moment, the requirement of OH for the formation of layered compounds couldn't be realized. When the ratio rises to 3 and 4, adequate OH is conducive to the formation of a layered structure. When the ratio of M2þ/M3þ was 2, the leaching amounts of Ca2þ by theoretical calculation was 8.85 mmol/L, while 9.50 mmol/L Ca2þ was remained in the actual calcined dolomite, indicating that only 0.65 mmol/L Ca2þ was utilized. Compared to the result that 1.29 mmol/L OP was removed, it could be concluded that about half of OP was removed by Ca2þ in theory. Therefore, the removal of OP was not only due to the precipitation by Ca2þ but also the surface adsorption by MgFe-LDHs. In the reaction equilibrium, the pH of the solution is 8.0e9.0, and the OP mainly exists in the form of HPO2 4 .At this moment, the amorphous calcium hydrogen phosphate (CaHPO4) was formed by the precipitation of calcium and OP. In a word, 0.65 mmol/L Ca2þ could be precipitated by 0.65 mmol/L 2 HPO2 4 , while the remaining HPO4 was removed by the effect of surface adsorption. The concentrations of Ca2þ, Mg2þ, Fe3þ, and PO3 4 in the solution treated by calcined dolomite and FeCl3$6H2O at the initial OP concentrations of 400 mg/L (12.90 mmol/L) are shown in Fig. 7(b). It could be seen that the concentrations of metal ions in solution were relatively low since iron oxides or hydroxides were stable. The Ca2þ and Mg2þ reacted with excess PO3 4 to form precipitates of calciumphosphate or magnesium-phosphate compounds. Theoretical calculation showed that the removal amounts of PO3 4 were about 3.3 mmol/L when calcined dolomite was solely added. Since FeCl3$6H2O was introduced, the adsorption capacity of PO3 4 significantly increased, which illustrated that the addition of FeCl3$6H2O was conducive to OP removal. Moreover, the more FeCl3$6H2O was added, the higher removal efficiency of OP was achieved. When the ratio of M2þ/M3þ was 2, the removal amount of OP could reach 9.69 mmol/L. The reason was that the pH was relatively high (11.0e12.0) in the OP removal process at the presence of calcined dolomite. Besides, the dissolved Ca2þ precipitated with PO3 4 to form hydroxyapatite (Fig. 6(b)). After the addition of FeCl3$6H2O, the pH of the solution was significantly reduced (8.0e9.0), where OP was mainly in the form of HPO2 4 . Hence, CaHPO4 was the main solid amorphous precipitation. The ratio of Ca2þ/PO3 4 was 5/3 in hydroxyapatite, while it was 1/1 in calcium hydrogen phosphate. So FeCl3$6H2O led to the decrease of pH and then changed the form of phosphate, which finally resulted in the increase of the removal rate. As mentioned above, 9.48 mmol/g Ca2þ was contained in calcined dolomite. 9.48 mmol/L HPO2 4 would be consumed if all Ca2þ participated in precipitation reactions, while the actual removal of phosphate was 9.69 mmol/L. Besides, no layered structure appeared at the concentrations displayed in Fig. 6(b). Therefore, the removal of a small portion of phosphate might be contributed by surface adsorption of iron hydroxide (L. Zhang et al., 2016). In general, the experimental results showed that the removal of

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(a)

(b)

2þ 3þ 2þ 3þ Fig. 7. Concentrations of Ca2þ, Mg2þ,Fe3þ and PO3 ¼ 2, 3, and 4) (The initial 4 in the solutions treated by calcined dolomite (M ) and FeCl3$6H2O (M ) at different ratios (M /M OP concentrations: 40 mg/L (a), 400 mg/L (b)).

OP by calcined dolomite was due to the formation of hydroxyapatite by the precipitation of Ca2þ and OP. When ferric chloride was added into the system, the removal efficiency of OP was observably increased, especially it was increased by three times when the ratio of M2þ/M3þ was 2 (Fig. 7(b)). The addition of ferric chloride could decrease solution pH and increase the dissolution of Ca2þ and Mg2þ from calcined dolomite. The more ferric chloride was added, the more OP was removed. The binding strength of the metal ions with OP was in the order: Ca2þ> Mg2þ> Fe3þ(Xu et al., 2010). When the concentrations of OP were relatively low (40 mg/L), it could be completely removed. More specifically, the removal of OP was mainly due to the precipitation by Ca2þ and surface adsorption by

MgFe-LDHs generated in an alkaline environment, where the former was the main factor. When the concentrations of OP were relatively high (400 mg/L), Ca2þ and Mg2þ competitively participated with HPO2 4 to form calcium hydrogen phosphate or magnesium hydrogen phosphate. Furthermore, MgFe-LDHs or CaFeLDHs compounds could not be formed since a lot of Mg2þ was consumed and large amounts of amorphous solid products were generated. Thus, no MgFe-LDHs were produced at high initial OP concentrations. Based on above results, the combination of precipitation and surface capture enhanced OP removal effects by calcined dolomite, which made it potential materials to remove OP efficiently.

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amounts of PP began to decline. This result was mainly due to the complexation of PP by Fe3þ, Ca2þ, and Mg2þ in the order. Under this condition, Fe3þ, Ca2þ, and Mg2þ all existed in the complexation ions in the solution. Moreover, as shown in Fig. 8(a), the removal amounts of PP decreased and the concentrations of Ca2þ and Mg2þ in the solution began to increase when the CaFe-LDHs and MgFeLDHs began to dissolve, by which the complexation effects could also be proved. According to the computation results, with the increase of initial PP concentrations, the removal efficiency of PP decreased all along of 100%, 94.4%, 25.0%, 8.0%, and 6.7%, respectively. When the ratio of M2þ/M3þ were 3 and 4 (Fig. 8(b) and (c)), the removal amounts of PP and concentrations of metals in treated solution were similar to that in Fig. 8(a). When the ratio of M2þ/M3þ was 3, the removal efficiency of PP was 100%, 100%, 4.6%, 3.0%, and 1.4%, respectively. When the ratio of M2þ/M3þ was 4, the removal efficiency of PP with five initial PP concentrations decreased to 99.7%, 49.1%, 4.1%, 1.2%, and 0.7%, respectively. By comparing Fig. 8(a) with (b) and (c), it was found that the complexation only worked when Fe began to dissolve. When the

3.4. Removal efficiency and mechanisms of pyrophosphate When the ratio of M2þ/M3þ were 2, 3, and 4, the removal amounts of PP and concentrations of metals in treated solution are shown in Fig. 8. When the ratio of M2þ/M3þ is 2 (Fig. 8(a)), the removal amounts of PP increased firstly and then decreased with the increase of initial concentrations of PP. On the contrary, Ca2þ and Mg2þ declined firstly and then rose to the maximum amounts of 3.17 mmol/L and 3.43 mmol/L, respectively. The concentrations of Fe3þ increased continuously and reached the maximum amount of 4.62 mmol/L. In PP removal, it has been reported that CaFeLDH have effect on PP removal, especially the initial PP ranging from 1.61 to 4.84 mmol/L (Wu et al., 2012). The dissolution amount of Cl does not change with the change of initial PP concentration, indicating that the process of removing PP is not ion exchange, but surface adsorption (Xu et al., 2012). At low concentrations of PP, major PP was precipitated by Ca2þ as Ca2P2O7$2H2O, which was proved by previous research (Zhou et al., 2015). In addition, the removal mechanisms of PP were similar to OP. When the initial concentrations of PP increased to 16.49 mmol/L, the removal

The removal amounts of PP and the release of Ca2+, Mg2+ and Fe3+ (mmol/L)

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Fig. 8. The removal amounts of pyrophosphate and concentrations of Ca2þ, Mg2þ and Fe3þ in the solutions treated by calcined dolomite (M2þ) and FeCl3$6H2O (M3þ) at different ratios (M2þ/M3þ ¼ 2(a), 3(b), 4(c)) (The initial PP concentrations: 3.228 mmol/L, 6.655 mmol/L, 17.04 mmol/L, 27.51 mmol/L and 33.64 mmol/L).

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ratio of M2þ/M3þ was 2, the origin point of Fe dissolution was at the initial PP concentrations of 17.04 mmol/L, while it was at lower 6.655 mmol/L when the ratio of M2þ/M3þ was 4. This result might be aroused by the difference of FeCl3$6H2O addition. The same amounts of calcined dolomite under various ratios, more FeCl3$6H2O was added when the ratio of M2þ/M3þ was 2 than 4. Thereafter, the pH of the solution decreased more, which led to more Ca2þ dissolved from calcined dolomite and precipitated with PP. Hence, when the ratio of M2þ/M3þ was 2, the complexation effects came up at higher initial PP concentrations. In summary, the removal mechanism of PP was mainly its precipitation by Ca2þ at low concentrations, which was similar to OP removal. Nevertheless, the complexation effects by Fe3þ, Ca2þ, and Mg2þ affected and decreased the PP removal efficiency at higher concentrations. According to the results mentioned above, calcined dolomite was fit for PP removal at relatively lower concentrations. If the synthesized CaFe-LDHs and MgFe-LDHs are applied, the concentration of PP in waste water is a key problem. The concentration of phosphates in the wastewater varies greatly, and in the process of high concentration of PP, the removal effect of the formed LDHs is not particularly desirable. 4. Conclusions In this study, CaFe-LDHs and MgFe-LDHs synthesized by calcined dolomite with ferric chloride assistance had a good performance on phosphate removal. The removal of OP by CaFe-LDHs and MgFe-LDHs system was attributed to the precipitation of Ca2þ and the surface adsorption of MgFe-LDHs, where the former played a dominating role. With regard to the removal of PP, it was mainly precipitated by Ca2þ at the initial pyrophosphate concentration ranging from 3.228 to 17.04 mmol/L. Besides, the complexation effects by Fe3þ, Ca2þ and Mg2þ could decrease the PP removal efficiencies at higher concentrations. Acknowledgements This work was supported by the National Natural Science Foundation of China under Grant [number 41472312] and the Program for Innovative Research Team in University under Grant [number IRT13078]. References Taylor, R.W., Bleam, W.F., Ranatunga, T.D., Schulthess, C.P., Senwo, Z.N., Ranatunga, D.R.A., 2009. X-ray absorption near edge structure study of lead sorption on phosphate-treated kaolinite. Environ. Sci. Technol. 43 (3), 711e717. https://doi.org/10.1021/es8020183. Altundogan, H.S., Tumen, F., 2002. Removal of phosphates from aqueous solutions by using bauxite. I: effect of pH on the adsorption of various phosphates. J. Chem. Technol. Biotechnol. 77 (1), 77e85. https://doi.org/10.1002/Jctb.525. Bowden, L.I., Jarvis, A.P., Younger, P.L., Johnson, K.L., 2009. Phosphorus removal from waste waters using basic oxygen steel slag. Environ. Sci. Technol. 43 (7), 2476e2481. https://doi.org/10.1021/es801626d. del Rio, A.V., Morales, N., Figueroa, M., Mosquera-Corral, A., Campos, J.L., Mendez, R., 2012. Effect of coagulant-flocculant reagents on aerobic granular biomass. J. Chem. Technol. Biotechnol. 87 (7), 908e913. https://doi.org/10.1002/ jctb.3698. Fang, L., Wu, B., Chan, J.K.M., Lo, I.M.C., 2018. Lanthanum oxide nanorods for enhanced phosphate removal from sewage: a response surface methodology study. Chemosphere 192, 209e216. https://doi.org/10.1016/ j.chemosphere.2017.10.154.

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