Enhancement of Phosphate Adsorption on Zirconium Hydroxide by

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Enhancement of phosphate adsorption on Zirconium hydroxide by ammonium modification Xubiao Luo, Xing Wu, Zhong Reng, Xiaoye Min, Xiao Xiao, and Jinming Luo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01523 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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Industrial & Engineering Chemistry Research

Enhancement of phosphate adsorption on Zirconium hydroxide by ammonium modification Xubiao Luo†, Xing Wu†, Zhong Reng†, Xiaoye Min†, Xiao Xiao†, Jinming Luo††* †

Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources

Recycle, Nanchang Hangkong University, Nanchang 330063, P.R. China ††

School of Civic and Environmental Engineering and Book Byers Institute for

Sustainable Systems, Georgia Institute of Technology, Atlanta, 30332, United States

ABSTRACT: Surface modification of adsorbents plays a crucial role in its adsorption performance. Furthermore, the most important step in modification is to design the modification groups based on target pollutants. In this study, the phosphate adsorption performance of zirconium hydroxide was enhanced by different ammonium modification as a result of increasing utilization efficiency of adsorption site. The maximum capacity is 155.04 mg/g from the zirconium hydroxide modified by dimethylamine. In comparison with undecorated zirconium hydroxide, the selectivity factors (α) for Cl-, SO42-, and NO3- are all raised by almost 2 times by dimethylamine modification. The mass transfer rate K2 also increase 6 times using N-methylaniline and N-ethylmethylamine. Results from Zeta-potential and FT-IR revealed that the enhanced adsorption capacity for phosphate were directly related to the inner-sphere complex and electrostatic interactions (quaternary ammonium groups and phosphate), ligand exchange (the hydroxyl groups of zirconium hydroxide and phosphate). Moreover, the utilization ratio of hydroxyl group on adsorbents were improved from 22.9% to 33.9% by dimethylamine modification, which is proved by XPS. At last, the modified zirconium hydroxide presents excellent performances of anti-interference in real wastewater. Overall, modified zirconium hydroxide is considered to have great potential for engineering application, and then the method of increase utilization of adsorption site gives a new route for other adsorption systems. 1

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1 Introduction As a result of the rapid development of agriculture and industry, the amount of waste water discharged is increasing. Phosphorus is one of the important elements in wastewaters, and it is necessary elements for aquatic organisms in water. But excessive amounts phosphorus in water system will cause serious eutrophication problems, which the nutrient exceeds the self-purification capacity of water ecosystem. [1] Therefore, it is very important to minimize the phosphorus content in wastewater to prevent eutrophication. [2] In the EU, the Water Framework Directive is likely to require discharge levels ranging between 0.1-0.5 mg P-PO43-/L. [3] Since the zirconium oxide has a remarkable affinity for the phosphate, [4] a great deal of research has been conducted on the removal of phosphate by zirconium-based materials, such as amorphous zirconium hydroxide, [5] mesoporous zirconium oxide, [6] amorphous zirconium oxide nanoparticles, [7] hydrated zirconium oxide [8,9] and so on. And zirconium is not a cheap material, it is significant to make full use of zirconium in phosphorus removal. In order to get better performance and reduce costs, some studies are carried out with zirconium as the main modification component, such as gel loaded with zirconium, [10] loading zirconium on monophosphonic acid resin, [11] hydrous zirconium oxide loaded with anion exchange resin, [12] biomass-based hydrous zirconium oxide nanocomposite, [13] fibers or activated carbon loaded with zirconium hydroxide, [14,15] binary oxides of zirconium with other metals (Mn-Zr) [16] and so on. To data, the maximum adsorption capacity of 2


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pure zirconium oxide nanoparticles and the zirconium-based adsorbent for phosphorus are 99.01 mg/g and 131.6 mg/g, respectively.[7,17] However, the enhancement extent of adsorption capacity is limited. The zirconium-based adsorbent is need to further design and increase the utilization efficiency of adsorption site. Researchers concluded that phosphate was adsorbed on the surface hydroxyl of zirconia. [18,19] Therefore, the first aim of this work is to use the surface hydroxyl of zirconia as full as possible to obtain higher adsorption capacity. On the other hand, lots of works reported surface modification could enhance effectively the adsorption capacity of phosphorus, such as La(III)-201(D-201, a anion exchange resin), [20] ACF(activated carbon fiber)-LaOH, [21] ACF-La-Fe, [22] E33(BayoxideE33)/Ag(II) [23] and so on. It is worth noting that, the raw material and the modified substances must co-operate in adsorption process, and show the most excellent performance. If they co-operate well, not only the adsorption capacity, but also the adsorption selectivity and speed will be improved. [24] The second aim of this work is to find a modified substance to promote the adsorption selectivity and speed. Inorganic ion exchangers are a good choice as modifiers for their specific selectivity against certain anions, [25] the kinetics can be improved by the incorporation of about 10% of quaternary ammonium groups, thus creating another mixed-ionogenic or polyfunctional anion exchanger. [26] As quaternary ammonium groups has good adsorption ability for phosphate, it can be expect that quaternary ammonium groups may enhance the adsorption performance of zirconium hydroxide. 3



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In addition, addition of charged surface groups onto the host materials is an effective approach to prevent nanocomposites from conglomerating. [27] Furthermore, selectivity can also be improved due to steric hindrance formed by quaternary ammonium groups. In this study, a novel phosphate-removing material was synthesized by bonding quaternary ammonium groups to the zirconium hydroxide surface with the help of epichlorohydrin. Herein, the objective of this study is to investigate the adsorption performance of phosphate by modified zirconium hydroxide. This research mainly focused on how the adsorption feature changed by surface modification. The isothermal adsorption, adsorption kinetic, coexisting anions, effects of pH were explored to check whether our aim is reached. The adsorption mechanism was revealed by Zeta potential, FTIR and XPS analysis combined adsorption data. The results from this study will provide useful information for phosphate removal by modified zirconium hydroxide.

2 Experimental part 2.1 Materials

Zirconium oxychloride octahydrate (Zr-OHCl2•8H2O, 99.0%) was

used as the raw material, deionized water was used as the solvent, and sodium hydroxide (NaOH, AR) was used as the reagent and also to adjust solution pH. Epichlorohydrin (C3H5ClO, AR) was used as the medium in the quaternization

4


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process. Dimethylamine ((CH3)2NH, AR), N-Ethylmethylamine (CH3NHC2H5, AR), N-Methylaniline (CH3NHC6H5, AR), Diethylamine ((C2H5)2NH, AR) were used as the donor in the quaternization process. Sodium dihydrogen phosphate anhydrous (NaH2PO4, AR) was used to prepare phosphate absorption solution. Concentrated hydrochloride acid (HCl, 32-38%) were used to adjust solution pH. Sodium chloride (NaCl, 99.5%), sodium sulfate anhydrous (Na2SO4, 99%), and sodium nitrate (NaNO3, AR) were used in the competing ion effect experiment. All inorganic reagents were obtained from Xilong Chemical Co., Ltd (Guangzhou, China) and all organic reagents were obtained from J&K Scientific Ltd (Shanghai, China). 2.2 Preparation of zirconium hydroxide Zirconium hydroxide (Zr-OH) was prepared via zirconium oxychloride octahydrate and sodium hydroxide interreaction. NaOH solution was added dropwise to ZrOCl2•8H2O solution with vigorously stir. The white precipitate was appeared on dripping NaOH solution process. The precipitate was separated by filtration and washed with deionized water until neutral pH, and finally product was dried in air at 373.15 K. The drier sample was grind for the following experiment. 2.3 Modification of Zirconium Hydroxide

Zirconium hydroxide was modified

according to Scheme S1. Here, first epichlorohydrin (0.1 mol) and zirconium hydroxide (0.25 mol) were reacted at 303.15 K with stirring. After 1 h, dimethylamine (0.66 mol) was then added dropwise into the system and stirred for 5 h at 343.15 K. Especially, controlling the dripping speed of dimethylamine is very important because 5



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this chemical reaction is exothermic reaction. The whole products were centrifuge by ethanol and washed with deionized water, then, dry for 12 h at 353.15 K. The dry samples were then grinded and stored for use. And other quaternary modification processes were carried out under the same conditions at the same ratio. 2.4 Phosphate adsorption experiments The phosphate adsorption isotherms experiments were occurred at 298.15 K in 100 mL conical flask. Phosphate solution of 20 mL was used for each experiment with 20 mg Zr-OH or modified Zr-OH, and the solution pH was adjusted to desired value by dilute HCl and NaOH. Firstly, the mixture was dispersed by ultrasonic to make good dispersion of Zr-OH or modified Zr-OH in phosphate solutions before putting it in shaker. After the experiments, the solution was centrifuged to collect supernatant in order to analyze the remaining phosphate concentration. Phosphate concentration in the supernatant solution was analyzed on an ion chromatograph (Dionex ICS 1100 Ion Chromatography, Thermal Scientific, Sunnyvale, CA, U.S.A.). For the sake of the kinetic study of phosphate adsorption on pure Zr-OH and modified Zr-OH, the initial phosphate concentration of 200 mg/L was used, the solution pH was adjusted to about 7, and the contact time was from 0h to 3 h. For the equilibrium adsorption isotherm exploration, the reaction phosphate concentration was ranged from 20 to 350 mg/L, the solution pH was adjusted to about 6, and the contact time was 12 h. For the influence of pH on phosphate removal study, the initial phosphate concentration was 300 mg/L, the solution pH was adjusted from 2 to 10, and the 6


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contact time was 12 h. For the coexisting anion competing experiments, 0.05 g adsorbents (pure Zr-OH and after different amino modified Zr-OH) were added into 50 ml mixed solution containing known concentrations phosphate solution and SO42-, Cl-, NO3-. After ultrasonic processing, the solutions were then put into shaker with 298.15 K and 200 rpm for 12 h, and the supernatant solution were obtained to detect the concentration of these anions on the ion chromatography. And the experimental conditions for real wastewater (from Dongjiang Environmental Company Limited) are similar to those of isothermal experiments. 2.5 Material Characterization The Fourier transform infrared (FT-IR) spectra of adsorbents (pure Zr-OH and dimethylamine-Zr-OH) before and after phosphate adsorption were evaluated by FT-IR spectrometer (VERTEX 70 FTIR apparatus), and their zeta potential charge (pHzpc) were analyzed by an electrophoretic spectroscopy (JS84H, Shanghai Zhongchen Digital Instrument Co., Ltd, Shanghai, P.R. China ). The textural structure including specific surface area, pore volume and pore size was determined by nitrogen adsorption/ desorption isotherms according to the Brunauer– Emmett–Teller (BET) analyzer (JW-BK122W, Beijing JWGB Sci. & Tech. Co., Ltd., China). The semi-quantitative chemical composition and surface chemical states of the samples were examined by X-ray photoelectron Spectrometer (XPS, Japan, Shimadzu/Kratos Axis Ultra DLD).

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3 Results and discussion 3.1 Adsorption Isotherm The adsorption capacity of adsorbents (pure Zr-OH and after different amino modified Zr-OH) on phosphate was investigated in the shaker and the results are shown in Fig. 1.The adsorption capacity of adsorbents were obtained via the following equation: [28] Qe =

C0 -Ce V

(1)

m

where Qe (mg/g) is the adsorption capacity of adsorbents to phosphate at equilibrium; C0 and Ce (mg/L) represent the initial and the equilibrium concentrations of phosphate in the solution, respectively; V (L) is the volume of phosphate for adsorb; and m (g) is the dry mass of the adsorbents. In order to explore their adsorption type, Langmuir isotherm models, Freundlich isotherm models and Dubnin–Radushkevich adsorption isotherm (D-R isotherm) models were used to fit the adsorption data. The Langmuir equation and Freundlich equation can be written in the following form: [29] Qe =Qm 1+Kl C K

(2)

l e

Qe =Kf Ce n 1

(3)

This equation are often written in linear forms, respectively: [30] Qe =Kf Ce n 1

(4)

log Qe = log Kf + n log Ce (5) 1

where Qe and Ce represent the adsorption capacity of adsorbents from experiment and the equilibrium phosphate concentration (mg/L), respectively. Kf and n are the 8


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Freundlich constants. Kl is the Langmuir constant, and Qm is the max adsorption capacity by the Langmiur calculation. The Dubnin–Radushkevich equation can be provided in the following form: [29]

qe = qmax −

Ce1

(6)

This equation is often written in linear forms: [31] ln qe = ln qm − Kε2

(7)

qe is the molar concentration from experiment (mol/g) and qm is the max adsorption capacity (mol/g), ε (kJ mol−1) is the Polanyi potential and it can be calculated by the followed equation: ε = RT ln(1 +

1

Ce 1

)

(8)

Ce1 need to be converted to molar concentration (mol/L), E is adsorption mean free energy (kJ/mol). These fitting parameters from the experimental data were shown in Table 1. (The linear fitting curves show in Figure. S1) From these experimental data, the Qm of dimethylamine-Zr-OH, pure Zr-OH, N-methylaniline-Zr-OH, N-ethylmethyl amine-Zr-OH, diethylamine-Zr-OH are 155.04 (mg/g), 118.91 (mg/g), 89.3 (mg/g), 88.0 (mg/g), 66.89 (mg/g) based on Langmiur model, respectively. Clearly, the zirconium hydroxide modified by dimethylamine has the largest adsorption capacity for phosphate, whereas the other quaternized zirconium hydroxide has less phosphate adsorption capacity than pure zirconium hydroxide. This may ascribe to the shortest carbon-chain length of dimethylamine in these modified quaternary ammonium groups. However, a 9



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quaternary ammonium group having a large ionic radius can block the adsorption of phosphate to the surface of the zirconium hydroxide, and thus it may also have an inhibitory effect on other anions. And the Qm of dimethylamine-Zr-OH is higher than the value reported in most of the previous literature. Table 2 summarizes some of the adsorption capacities of the relevant adsorbents for phosphate in previous literature. Furthermore, the Qm of dimethylamine-Zr-OH was higher than that of other zirconium-based materials. Compared with the Freundlich model, their adsorption process is more consistent with the Langmuir model (R2 >0.982), which revealing that the adsorption process belongs to monolayer adsorption. From D-R model fitting data, the E values are all above 8 and lower than 16. It was well demonstrated that adsorption energy is closely associated with the adsorption process: (i) when E is less than 8 kJ/mol, physical adsorption is the major process; (ii) when E is in the range of 8–16 kJ/mol, the adsorption behavior is dominated by ligand exchange; (iii)when E is in the range of 20–40 kJ/mol, chemisorption is predominant in the adsorption procedure. [32] It was indicating that the adsorption of phosphate to adsorbents (pure Zr-OH and after different amino modified Zr-OH) can be described as a strong interaction of the phosphate with the adsorbent, which is ligand exchange. Moreover, as shown in Fig. 1, the quaternary ammonium groups modified zirconium hydroxide for phosphate has a stronger removal effect at low concentration, which can be ascribed to the ammonium groups. Dimethylamine-Zr-OH were chose for further experiment as the best modified adsorbent. 10


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3.2 Influence of pH on Phosphate Removal The influence of pH is significant for the adsorption of phosphate solution onto the Zr-OH and dimethylamine-Zr-OH as shown in Fig. 2, since the adsorption of phosphate on the surface of the Zr-based adsorbents affected by pH were reported before. [12] The results were shown in Fig. 2. It was evidence that phosphate adsorption capacity rapidly reduces from 130.38 to 37.81 mg/g and 100.71 to 17.54 mg/g with the solution pH increased from 2 to 11. Clearly, the removal of phosphate was more efficient in acidic solution. The adsorption of phosphate onto adsorbents (Zr-OH, dimethylamine-Zr-OH) is pH-dependent. This can be explained as that, the surface of Zr-OH and dimethylamine-Zr-OH aggregate more negatively charged because of existed OHions at high pH values, then cause a strong competition between phosphate and OHions for surface active sites of adsorbents. Certainly, dimethylamine-Zr-OH can also promote the adsorption of phosphate by electrostatic interaction at low pH. [7] The phosphate is present with H3PO4/H2PO4-/HPO42-/PO43- in the solution and change with the change of pH. As we known that the pKa values of phosphate are 2.12, 7.2 and 12.36(Fig. 2). The dominant species of phosphate are HPO42− and H2PO4− under low pH range (pH≈7). At this pH range, the mechanism might display with ligand exchange between phosphate hydrolysis products (H2PO4−, HPO42−) and the surface of adsorbents (Zr-OH) as follows (Eqs. (9) and (10)): [6] Zr–OH + H2PO4− = Zr(H2PO4) + OH−

(9)

2Zr–OH + HPO42− = Zr2(HPO4) + 2OH−

(10)

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Similar tendency was observed on dimethylamine-Zr-OH which contains Zr-P and N-P electrostatic interactions. This phenomenon indicate that the dimethylamine was successfully bonded to the zirconium hydroxide and worked for the adsorption process. 3.3 Adsorption Kinetics The kinetics data of the phosphate adsorption onto adsorbents (pure Zr-OH and after different amino modified Zr-OH) at room temperature (298.15K) and the pH about 7.0 are revealed in Fig. 3. The kinetic processes of phosphate adsorption onto adsorbents (pure Zr-OH and after different amino modified Zr-OH) can be divided into two stages: a relatively rapid adsorption step, and a slower adsorption step. However, compared with pure Zr-OH, amino modified Zr-OH was more efficient for phosphate removal at a short initial period. For example, at 0.5 minutes after the start of the experiment, the adsorption amount of dimethylamine-Zr-OH for phosphate reached 50 mg/g, which is 50% of the final adsorption capacity obtained in the kinetic experiment. Correspondingly, the adsorption amount of pure Zr-OH was only 11.39 mg/g at same time, which is 15% of the final adsorption capacity. Such an excellent adsorption effect may be due to the quaternary ammonium groups on the amino modified Zr-OH surface and large surface area (BET=166.40 cm2/g, Size=526.8nm) (Figure. S2). The data of BET provide a good contact efficiency between amino modified Zr-OH and the phosphate in the solution, which facilitates the better contact and reaction of the phosphate ions with the active sites on the amino modified Zr-OH. Although the specific surface area of 12


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amino modified Zr-OH is slightly smaller than pure Zr-OH (BET=186.68 cm2/g, Size=327.4nm) (Figure. S2), the quaternary ammonium groups will block the surface vacancies of the zirconium hydroxide, but the quaternary ammonium groups could provide more rapid adsorption by electrostatic interaction. In order to further explore the adsorption mechanism, the adsorption kinetic data was fitted with the pseudo-first-order kinetics model and the pseudo-second-order kinetics model. [33] The pseudo-first-order kinetics model and can be written in the following form, respectively: dpt dt dpt dt

=k1 (qe − qt )

=k2 (qe − qt )2

(11) (12)

The pseudo-first-order kinetics model is often written in linear forms: − ! " = − # $

(13)

And the pseudo-second-order kinetics model is also often written in linear forms: t qt

=

1 k2 qe

2+

t

(14)

qt

ho =k2 qe 2

(15)

Where qt and qe (mg/g) are the adsorption capacities of phosphate at time t and at equilibrium, respectively; k1 and k2 (g/mg∙min) are the adsorption rate constant of the pseudo-first-order equation and the pseudo-second-order equation, respectively; ho(mg/min∙g) is the initial adsorption rate. The kinetics correlation coefficients are summarized in Table 3 from the fitting data by pseudo-first-order kinetics model and the pseudo-second-order kinetics model. Clearly, the kinetics data of the adsorbents 13



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(pure Zr-OH and different amino modified Zr-OH) are more suitable the pseudo-second-order kinetics model with R2 higher than 0.99, compare with the R2 of the pseudo-first-order kinetics model. The pseudo-second-order rate model reveals a chemisorptions occurred between phosphate and the adsorbents (pure Zr-OH and after different amino modified Zr-OH).[34] The k value of amino modified Zr-OH is also much larger than that of pure Zr-OH. The enhancement of mass transfer rate is likely derived from incorporation of more than 10% quaternary ammonium group. [26] And the difference in the k value among the zirconium hydroxide modified by the quaternary ammonium groups may be due to the difference in the number of quaternary ammonium groups actually bonded to the surface of the zirconium hydroxide. However, the quaternary ammonium groups may cover a large number of adsorption sites of zirconium hydroxide itself if it exceeds a certain value and will have a large effect on the adsorption capacity, which is consistent with the isothermal experimental results. So, the kinetic effects of quaternary ammonium groups on the adsorption of phosphate on zirconium hydroxide are very significant, but there is a need to control the amount of quaternary ammonium groups. 3.4 Effects of Coexisting Anions There were many ions exist in industrial wastewater, such as Cl-, NO3- and SO42-, which may present at high levels. The presence of these ions can interfere with the adsorption of the target ion (PO43-) on adsorbents (pure Zr-OH and after different amino modified Zr-OH) by competitive adsorptions. And this is also a commonly used method to evaluate the performance of 14


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adsorbents by competitive ion adsorption. Here, the influence of coexisting anions on the phosphate adsorption process were examined in two ways. First, the binary solution of equimolar concentration was prepared ( PO43-/Cl-, PO43-/NO3-, PO43-/SO42-)as the adsorption solution, since the same number of ions are present in equimolar concentrations of the binary solution, and the result depicted in Fig. 4. The concentration of phosphate was 300 mg/L, and the concentration of coexisting anions (Cl-, NO3-, SO42-) are 112 mg/L, 195 mg/L, 300 mg/L by equimolar concentration conversion, respectively. Before and after the addition of the coexisting ions (Cl-, NO3-, SO42-), it can be seen that the influence on the adsorption capacity onto adsorbents (pure Zr-OH and after different amino modified Zr-OH) is slight. The adsorption driven by outer-sphere association through electrostatic forces is strongly sensitive to electrolytes addition, however, the formation of inner-sphere complex always tends to show the little influence to common anions interference. [35,36,37] It is also shown that the adsorption of zirconium hydroxide to phosphate is inner-sphere complex. [7] And compared with the effect of chloride ion and nitrate ion, the effect of sulfate ion on the adsorption capacity of amino modified Zr-OH was the greatest, because the presence of sulfate can not only compete with phosphate but also inhibit the activity of quaternary ammonium groups. [5,38] The anti-interference ability of the adsorbent (pure Zr-OH and after different amino modified Zr-OH) was also examined by increasing the concentration of interfering ions and the results were shown in Fig. 5. The increasing ionic strength does not have an obvious impact on the 15



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adsorption capacity of phosphate on different amino modified Zr-OH. And the data in Fig. 5 show that the adsorption capacity of amino modified Zr-OH is larger than that of pure Zr-OH when increase the concentration of coexisting. This further suggests that amino modified zirconium hydroxide has better anti-interference performance. The activity of the quaternary ammonium group will be great inhibited when the concentration of the sulfate in the solution is 500 mg/L from the literature. [39] Yet, the presence of quaternary ammonium groups also acts as a hindrance to other coexisting ions close to zirconium hydroxide. It is well known that quaternary ammonium groups are not specific for coexisting anions, but still exhibit a stronger adsorption capacity for phosphate. This may ascribe to the quaternary ammonium group cooperated with Zr-OH surface to form a “specific” adsorption site for phosphate. To quantify the selectivity of the adsorbents (pure Zr-OH and after different amino modified Zr-OH), the distribution coefficient KD (L/g) and the selectivity factor of phosphate with respect to the coexisting ions (α), which were determined by the following equations and the results were listed in Table 4. q

KD = Ce

(16)

e

K&

α= K

(17)

&'

where qe and Ce are the adsorption capacity and the equilibrium concentration of phosphate, respectively. KD1and KD2 are the distribution coefficient of phosphate and D2 (D2=Cl-, NO3- and SO42-); α is the selectivity coefficient. The value of KD should 16


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be related to the adsorption capacity, but the zirconium hydroxide modified by N-ethylmethylamine and N-methylaniline has a larger KD value than that of pure Zr-OH, which may be due to the presence of quaternary ammonium groups that prevent the coexisting ions from approaching the adsorption sites of zirconium hydroxide. It is clear from the value of α that the zirconium hydroxide after the amino modification has a better selectivity than the pure zirconium hydroxide as the concentration of coexisting ions increases, although the α value of pure Zr-OH is not low. Isothermal adsorption and coexisting anions experiments proved dimethylamine modified zirconium hydroxide has a very good performance and has great application prospect. However, the adsorption mechanism of the adsorption of phosphate by dimethylamine modified zirconium hydroxide needs further investigation 3.5 Mechanism Analysis The isoelectric point (IEP) of adsorbents (Zr-OH, dimethylamine-Zr-OH) were show on Fig.6. Without quaternary ammonium modifications, the IEP of Zr-OH was about pH 4, while the IEP was increased to about pH 5 after dimethylamine modified. From the opposite point of view, this result was supported by negative shift of the IEP with the addition of anions. [40] The presence of protonation and deprotonation can have an effect on the IEP of the metal oxide. The difference in the isoelectric point of the adsorbents (Zr-OH, dimethylamine-Zr-OH) was caused by the addition of quaternary ammonium groups, this situation enhances protonation by replacing the hydroxyl groups on the surface of the zirconium hydroxide with positively charged quaternary ammonium groups. Of 17



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course, the IEP also revealed that quaternary ammonium groups have been in the surface of Zr-OH. The IEP and the data of pH effect can support each other, the adsorption capacity of the adsorbent (Zr-OH, dimethylamine-Zr-OH) changed with the change of pH. If the pH is below the isoelectric point, then the surface hydroxyl of adsorbents is protonated and positively charged. [12] Electrostatic attraction will then occur between the negative phosphate anion and the surface of the positively charged adsorbent (Zr-OH, dimethylamine-Zr-OH) (Eq. (18), (19) and (20)). [6] Zr–OH + H+ = Zr–OH2+

(18)

Zr–OH2++ H2PO4− = (Zr–OH2)+(H2PO4)−

(19)

2Zr−OH2+ + HPO42− = (Zr–OH2)2+(HPO4)2−

(20)

It can also be seen that the effect of the adsorbent on phosphate removal is significant at pH below 5. Certainly, the amino group also plays an important role in this process. [13] R−NH+(CH3)2+HxPO4(3-x)-(aq)→[R−NH+(CH3)2][HxPO4(3-x)-]

(21)

The mechanism of phosphate adsorption by dimethylamine-Zr-OH was further studied

by

the

FTIR

spectrum of microscopic.

The

FTIR

spectra

of

dimethylamine-Zr-OH were exhibited in Fig.7 (before and after phosphate adsorption). Before the phosphate adsorption, dimethylamine-Zr-OH had a strong hydroxyl stretching peak at 3442 cm-1 [41](O−H stretching vibration) and 2700 cm-1(N-H stretching vibration). The 1109 cm-1 were C-O stretching vibrations from the open-ring epichlorohydrin and 470 cm-1 is the stretching vibration of C-C also from 18


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the carbon chain on the open-ring epichlorohydrin. The 1570 cm-1(C=N stretching vibration) derived from the product of dimethylamine and epichlorohydrin during the modification. After the phosphate adsorption, it is clear that the peak at 1340 cm-1 disappears and that two peak appears at 1037cm-1 and 1456 cm-1. The disappearance of the peak at 1340 cm-1 represents the absence of a peak belonging to -OH from zirconium hydroxide, [6] which means that the -OH of zirconium hydroxide was effectively utilized during the adsorption of phosphate onto the dimethylamine-Zr-OH. However, the peak at 1037 cm-1 is attributed to the asymmetric vibration of P-O. [7] By the change of the two peaks, it can be concluded that the surface hydroxyl groups are react with phosphate by ligand exchange (described in Eq. (9), (10)). The peak at 3442 cm-1 was weakened and shift to lower wavenumber after the adsorption, which ascribe to adsorbed phosphate on the hydroxyl group. After adsorption of phosphate, the peak at 2700 cm-1 was significantly shifted to 2815 cm-1, it is an evidence that quaternary ammonium group participate the adsorption process. The hydroxyl of phosphate adsorption was further explored by XPS. The spectra of adsorbents (Zr-OH, dimethylamine-Zr-OH) before and after phosphate adsorption were analyzed and were showed in Fig. 8. As shown in Fig. 8a, the peaks corresponding to P 2p,N 1s , Zr 3d, and O 1s are clearly identified in the survey scan spectrum. With the advent of N peaks further confirmed that zirconium hydroxide was successfully modified by dimethylamine, and the presence of P peaks in the adsorbed product further confirms the adsorption of P on the adsorbent (Zr-OH, 19



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Page 20 of 45

dimethylamine-Zr-OH). From Fig. 8b, the O1s peak could be best fitted with three overlapped peaks: oxide oxygen, hydroxyl group (-OH), and adsorbed water. [7] It is important that the -OH percentage was different, the content ratio of hydroxyl peaks in 1, 2, 3 and 4 is 55.7%, 42.9%, 44.4% and 36.8% (Table S1), respectively. After dimethylamine modification, they can be calculated the utilization ratio of hydroxyl ( )('

group after dimethylamine modification increase from 22.9% (

(

( )(*

) to 33.9% (

(

)

and 11% hydroxyl group was replaced by dimethylamine. 11% dimethylamine group provides a direct evidence for explaining enhancement of the mass transfer rate because K.H. Leaser think also incorporation of about 10% quaternary ammonium group can improve the mass transfer rate of phosphate. [26] And the decrease of the hydroxyl group was caused by ligand exchange (described in Eq. (9), (10)) in the phosphate adsorption process, which is consistent with the results of the FTIR study. 3.6 Application to Real Wastewater Compared with simulated wastewater, the real wastewater has a more complex substrate environment. The adsorption experiment under real wastewater can provide important reference information for assessing the application prospect of the material. The real wastewater (pH=6.3, TSS=326 mg/L, COD=1179 mg/L) used in this study is from Dongjiang Environmental Company Limited, which contains anions such as phosphate (500 mg/L), chlorine (850 mg/L), Nitrate (630 mg/L), Sulfate (1120 mg/L). The results (Table S2 and Figure. S4) show that the adsorption capacity of dimethylamine-Zr-OH for phosphate is 76.22 mg/g, which is twice as much as 37.08 mg/g of Zr-OH. And the 20


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phosphate concentration dropped to the discharge standard (0.5 mg / L) in the 20 mL real wastewater by adding 230 mg adsorbent. These data not only confirm the potential of dimethylamine-Zr-OH in industrial applications, but also provide an important idea for the industrial use of materials-modified by quaternary ammonium groups.

4 Conclusion In this work, a high efficient, high selective adsorbent for phosphate was synthesized by modification of zirconium hydroxide with ammonium. The adsorption capacity of modified zirconium hydroxide was 155.04 mg/g, which was one of the highest adsorption capacity of zirconium-based materials in published literature so far. In addition to adsorption capacity, the modified zirconium hydroxide has a much better performance in kinetics and selectivity experiments. The excellent performance attributed to the collaboration of the hydroxyl groups and the quaternary ammonium group which verified by Zeta-potential and FT-IR. The effect of hydroxyl groups on the surface of zirconium hydroxide was further confirmed by XPS calculation. The application to wastewater verified the treatment ability of this ammonium modified Zr-OH. This study proved that the use of quaternary ammonium groups for zirconium hydroxide modification for phosphate is a very effective approach. The high adsorption speed and selectivity ability will lead to its outstanding in engineering applications and high commercial value.

21



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AUTHOR INFORMATION Corresponding author *Jinming Luo: E-mail: [email protected].

ACKNOWLEDGMENTS This study was financially supported by the National Science Fund for Excellent Young Scholars (51422807), the Natural Science Foundation of China (51238002, 51678285), the Key Project of Science and Technology Department of Jiangxi Province (20143ACG70006) and the Cultivating Project for Academic and Technical Leader of Key Discipline of Jiangxi Province (20153BCB22005).

ASSOCIATED CONTENT Supplementary Information Schematic route for the synthesis; the data of O 1s XPS; detailed information about application to real wastewater; the linear fitting curves and the BET data.

References [1] Aguiar, V. M. D. C.; Neto, J. A. B.; Rangel, C. M., Eutrophication and hypoxia in four streams discharging in Guanabara Bay, RJ, Brazil, a case study, Mar. Pollut. Bull. 2011, 62, 1915. [2] Kwon, H. B.; Lee, C. W.; Jun, B. S.; Yun, J. D.; Weon, S. Y. B., Koopman, Recycling waste oyster shells for eutrophication control, Resour. Conserv. Recy. 2004, 41, 75. 22


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[3] Blaney, L. M.; Cinar, S.; SenGupta, A. K., Hybrid anion exchanger for trace phosphate removal from water and wastewater, Water Res. 2007, 41, 1603. [4] Yeon, K. H.; Park, H.; Lee, S. H.; Iwamoto, M., Zirconium mesostructures immobilized in calcium alginate for phosphate removal, Korean J. Chem. Eng. 2008, 25, 1040. [5] Chitrakar, R.; Tezuka, S.; Sonoda, A.; Sakane, K.; Ooi, K., T Hirotsu, Selective adsorption of phosphate from seawater and wastewater by amorphous zirconium hydroxide, J. Colloid Interface Sci. 2006, 297, 426. [6] Liu, H.; Sun, X.; Yin, C.; Hu, C., Removal of phosphate by mesoporous ZrO2, J. Hazard. Mater. 2008, 151, 616. [7] Su, Y.; Cui, H.; Li, Q.; Gao, S.; Shang, J. K., Strong adsorption of phosphate by amorphous zirconium oxide nanoparticles, Water Res. 2013, 47, 5018. [8] Acelas, N. Y.; Martin, B. D.; López, D.; Jbfferson, B., Selective removal of phosphate from wastewater using hydrated metal oxides dispersed within anionic exchange media, Chemosphere. 2015, 119, 1353. [9] Lin, J.; Zhan, Y.; Wang, H.; Chu, M.; Wang, C.; He, Y.; Wang, X., Effect of calcium ion on phosphate adsorption onto hydrous zirconium oxide, Chem. Eng. J. 2017, 309, 118-129. [10] Biswas, B. K.; Inoue, K.; Ghimire, K. N.; Haraed, H.; Ohto, K.; Kawakite, H., Removal and recovery of phosphorus from water by means of adsorption onto orange waste gel loaded with zirconium, Bioresour. Technol. 2008, 99, 8685. 23



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[11] Awual, M. R.; El-Safty, S. A.; Jyo, A., Removal of trace arsenic (V) and phosphate from water by a highly selective ligand exchange adsorbent, J. Environ. Sci. 2011, 23, 1947. [12] Chen, L.; Zhao, X.; Pan, B.; Zhang, W.; Hua, M., Preferable removal of phosphate from water using hydrous zirconium oxide-based nanocomposite of high stability, J. Hazar. Mater. 2015, 284, 35. [13] Qiu, H.; Liang, C.; Zhang, X.; Chen, M.; Zhao, Y.; Tao, T.; Xu, Z.; Liu, G., Fabrication of a Biomass-Based Hydrous Zirconium Oxide Nanocomposite for Preferable Phosphate Removal and Recovery, ACS Appl. Mater. Inter. 2015, 7, 20835. [14] Awual, M. R.; Jyo, A.; Ihara, T.; Seko, N.; Tamada, M.; Lim, K. T., Enhanced trace phosphate removal from water by zirconium(IV) loaded fibrous adsorbent, Water Res. 2011, 45, 4592. [15] Okumura, M.; Fujinaga, K.; Seike, Y.; Hayashi, K., A simple in situ preconcentration method for phosphate phosphorus in environmental waters by column solid phase extraction using activated carbon loaded with zirconium, Anal. Sci. 1998, 14, 417. [16] Long, F.; Gong, J. L.; Zeng, G. M.; Chen, L.; Wang, X. Y.; Deng, J. H.; Niu, Q. Y.; Zhang, H. Y.; Zhang, X. R., Removal of phosphate from aqueous solution by magnetic Fe–Zr binary oxide, Chem. Eng. J. 2011, 171, 448. [17] Zong, E.; Wei, D.; Wan, H.; Zheng, S.; Xu, Z.; Zhu, D., Adsorptive removal of 24


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phosphate ions from aqueous solution using zirconia-functionalized graphite oxide, Chem. Eng. J. 2013, 221, 193. [18] Zhang, C; Li, Y.; Wang, F.; Yu, Z.; Wei, J.; Yang, Z.; Ma, C.; Li, Z.; Xu, Z.; Zeng, G., Performance of magnetic zirconium-iron oxide nanoparticle in the removal of phosphate from aqueous solution, Appl. Surf. Sci. 2016, 396, 1783. [19]Velazquezjimenez, L. H.; Hurt, R. H.; Matos, J.; Rangelmendez, J. R., Zirconium-carbon hybrid sorbent for removal of fluoride from water: oxalic acid mediated Zr(IV) assembly and adsorption mechanism, Environ. Sci. Technol. 2014, 48, 1166. [20] Zhang, Y.; Pan, B.; Shan, C.; Gao, X., Enhanced Phosphate Removal by Nanosized Hydrated La(III) Oxide Confined in Crosslinked Polystyrene Networks, Environ. Sci. Technol. 2016, 50, 1447. [21] Zhang, L.; Zhou, Q.; Liu, J.; Chang, N.; Wan, L.; Chen, J., Phosphate adsorption on lanthanum hydroxide-doped activated carbon fiber , Chem. Eng. J. 2012, 186, 160. [22] Liu, J.; Zhou, Q.; Chen, J.; Zhang, L.; Chang, N., Phosphate adsorption on hydroxyl–iron–lanthanum doped activated carbon fiber, Chem. Eng. J. 2013, 215, 859. [23] Lalley, J.; Han, C.; Li, X.; Dionysiou, D. D.; Nadagouda, M. N., Phosphate adsorption using modified iron oxide-based sorbents in lake water: Kinetics, equilibrium, and column tests, Chem. Eng. J. 2015, 284, 1386. [24] Guaya, D.; Valderrama, C.; Farran, A.; Armijos, C.; Cortina, J. L., Simultaneous 25



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phosphate and ammonium removal from aqueous solution by a hydrated aluminum oxide modified natural zeolite, Chem. Eng. J. 2015, 271, 204. [25] Suzuki, T. M.; Bomani, J. O.; Matsunaga, H.; Yokoyama, T., Preparation of porous resin loaded with crystalline hydrous zirconium oxide and its application to the removal of arsenic, React. Funct. Polym. 2000, 43, 165. [26] Leaser, K.H., in: K. Dorfner (Ed.), Ion Exchangers, Walter de Gruyter, 1991, p. 33. [27] Zhang, Q.; Du, Q.; Hua, M.; Jiao, T.; Gao, F.; Pan, B., Sorption enhancement of lead Ions from water by surface charged polystyrene-supported nano-zirconium oxide composites Environ. Sci. Technol. 2013, 47 ,6536. [28] Ling, C.; Li, X.; Zhang, Z.; Liu, F.; Deng, Y.; Zhang, X.; Li, A.; He, L.; Xing, B., High Adsorption of Sulfamethoxazole by an Amine-Modified Polystyrene– Divinylbenzene Resin and Its Mechanistic Insight, Environ. Sci. Technol. 2016, 50, 10015. [29] Febrianto, J.; Kosasih, A.N.; Sunarso, J.; Ju, Y. H.; Indraswati, N.; Ismadji, S., Equilibrium and kinetic studies in adsorption of heavy metals using biosorbent: a summary of recent studies, J. Hazard. Mater. 2009, 162, 616. [30] Gautam, R. K.; Mudhoo, A.; Lofrano, G.; Chattopadhyaya, M.C., Biomass-derived biosorbents for metal ions sequestration: Adsorbent modification and activation methods and adsorbent regeneration, J. Environ. Chem. Eng. 2014, 2, 239. 26


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[31] Dubinin, M.; Radushkevich, L., Equation of the characteristic curve of activated charcoal, Chem. Zentr. 1947, 1, 875. [32] Han, C.; Li, H.; Pu, H.; Yu, H.; Deng, L.; Huang, S.; Luo, Y., Synthesis and characterization of mesoporous alumina and their performances for removing arsenic(V), Chem. Eng. J. 2013, 217, 1. [33] Gupta, S. S.; Bhattacharyya, K. G.,

Kinetics of adsorption of metal ions on

inorganic materials: a review, Adv. Colloid Interf. 2011, 162, 39. [34] Ho, Y. S.; Mckay,G., Pseudo-second order model for sorption processes, Process Biochem. 1999, 34, 451. [35] Bastin, O.; Janssens, F.; Dufey, J., A. Peeters, Phosphorus removal by a synthetic iron oxide-gypsum compound, Ecol. Eng. 1999, 12, 339. [36] Jia, K.; Pan, B.; Zhang, Q.; Zhang, W.; Jiang, P.; Hong, C.; Pan, B.; Zhang, Q., Adsorption of Pb2+, Zn2+, and Cd2+ from waters by amorphous titanium phosphate, J.Colloid Interf. Sci. 2008, 318, 160. [37] Zhang, Q.; Teng, J.; Zou, G.; Peng, Q.; Du, Q.; Jiao, T.; Xiang, J., Efficient Phosphate

sequestration

for

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purification

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Sandwich-like

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F., Selective removal of phosphate in waters using a novel of cation adsorbent: Zirconium phosphate (ZrP) behavior and mechanism, Chem. Eng. J. 2013, 221, 315. [40] Dou, X.; Mohan, D.; Jr, C. U. P.; Yang, S., Remediating fluoride from water using hydrous zirconium oxide, Chem. Eng. J. 2012, 198, 236. [41] Peng, Q.; Guo, J.; Zhang, Q.; Xiang, J.; Liu, B.; Zhou, A.; Liu, R.; Tian, Y., Unique Lead Adsorption Behavior of Activated Hydroxyl Group in Two-Dimensional Titanium Carbide. J. Am. Chem. Soc. 2014, 136, 4113. [42] Su, Y.; Yang, W.; Sun, W.; Li, Q.; Shang, J. K., Synthesis of mesoporous cerium– zirconium binary oxide nanoadsorbents by a solvothermal process and their effective adsorption of phosphate from water, Chem. Eng. J. 2015, 268, 270.

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List of Figures 1 Phosphate adsorption isotherms of adsorbents at 298.15K and pH about 7.0 2 The speciation of phosphate (A) and influence of pH on the adsorption capacity of Zr-OH and dimethylamine-Zr-OH for phosphate (B). 3 Effect of equilibrium time on adsorption of adsorbents (phosphate concentration: 200mg/L, pH ± 7.0,298.15K) 4 Effect of coexistent ions (Cl-, NO3- and SO42-) in equimolar solution on adsorption of phosphate at 298.15K and pH about 7.0. 5 Effect of coexistent ions (Cl-, NO3- and SO42-) in different concentration on adsorption of phosphate at 298.15K and pH about 7.0. 6 Plot of the zeta potential of Zr-OH and dimethylamine-Zr-OH as a function of pH. 7 FTIR spectra (KBr pellets) of dimethylamine-Zr-OH and after adsorption dimethylamine-Zr-OH. 8 XPS analysis of Zr-OH and dimethylamine-Zr-OH before or after phosphate adsorption .(a) XPS survey scan of Zr-OH and dimethylamine-Zr-OH before or after phosphate adsorption; (b)The surface O1s spectra of adsorbents (1) Zr-OH, and (2) after adsorption Zr-OH, and (3) dimethylamine-Zr-OH, and (4) after adsorption dimethylamine-Zr-OH.

29



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List of Tables 1 Langmuir, Freundlich and D-R models constants of phosphate adsorption on adsorbents. 2 Phosphate adsorption capacities of various adsorbents about Zr. 3 Pseudo-first-order and pseudo-second-order kinetic constants of phosphate adsorption on Zr-OH and different amino modified Zr-OH 4 The effects of common anions on the selectivity factor (α) of phosphate onto pure Zr-OH and different amino modified Zr-OH at 298 K.

30


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Table 1. Langmuir, Freundlich and D-R models constants of phosphate adsorption on adsorbents.

Adsorbent

Pure-Zr-OH Dimethylamine (Zr-OH) N-Methylaniline (Zr-OH) N-Ethylmethylamine (Zr-OH) Diethylamine (Zr-OH)

Langmuir isotherm

Freundlich isotherm

parameters

parameters

D-R isotherm parameters

KL

(mg/g)

(L/g)

118.91

0.02

0.994

1.86

6.22

0.920

4.2210-3

9.30

0.964

155.04

0.04

0.982

2.75

21.5

0.981

3.4510-3

11.9

0.992

89.3

0.01

0.988

1.81

4.30

0.939

3.6410-3

8.91

0.964

88.0

0.01

0.986

1.85

4.69

0.896

3.6310-3

9.00

0.927

66.89

0.02

0.985

3.16

11.9

0.744

1.6710-3

11.3

0.779

2

n

R

KF

2

R

31


Qm

E

Qmax

(mol/L)

(KJ/

R2

mol)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 45

Table 2. Phosphate adsorption capacities of various adsorbents about Zr.

Adsorbent

pH

Adsorption

Ref.

capacity (mg/g) Mesorporous ZrO2

6.8

91.05

[6]

Amorphous ZrO2

6.2

99.01

[7]

Ws(wheatstraw)-N(quaternary -aminated)-Zr(ZrO2)

7.0

97.75

[13]

GO(Graphite oxide)-Zr

7.0

131.6

[17]

Ce0.8Zr0.2O2

7.0

112.23

[42]

Dimethylamine-Zr-OH

7.0

155.04

Present study

32


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Table 3. Pseudo-first-order and pseudo-second-order kinetic constants of phosphate adsorption on Zr-OH and different amino modified Zr-OH Pseudo-first-order kinetics Adsorbent

K1

Qe -1

Pure-Zr-OH Dimethylamine (Zr-OH) Diethylamine (Zr-OH) N-Methylaniline (Zr-OH) N-Ethylmethylamine (Zr-OH)

(g mg

(mg

min-1)

g-1)

0.036

58.51

0.024

R12

Pseudo-second-order kinetics K2

h0

Qe -1

-1

(mg g-1

R22

(min )

(mg g )

0.984

0.001

79.13

11.27

0.996

40.21

0.748

0.004

110.37

48.56

0.998

0.030

24.32

0.777

0.004

37.95

5.515

0.995

0.067

22.66

0.925

0.006

34.72

8.122

0.997

0.023

22.31

0.641

0.006

51.84

17.09

0.998

33


min-1)


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Page 34 of 45

Table 4. The effects of common anions on the selectivity factor (α) of phosphate onto pure Zr-OH and different amino modified Zr-OH at 298 K.

equimolar coexisting anions

adsorbent

Diethylamine (Zr-OH) N-Ethylmethylamine (Zr-OH) Cl-

Pure-Zr-OH N-Methylaniline (Zr-OH) Dimethylamine (Zr-OH) Diethylamine (Zr-OH) N-Ethylmethylamine (Zr-OH)

NO3-

Pure-Zr-OH N-Methylaniline (Zr-OH) Dimethylamine (Zr-OH) Diethylamine (Zr-OH) N-Ethylmethylamine (Zr-OH)

SO42-

Pure-Zr-OH N-Methylaniline (Zr-OH) Dimethylamine (Zr-OH)

at different competing anions contents in solution

(PO43-= 300 mg/L)

300 mg/L

1000 mg/L

2000 mg/L

α

α

α

α

5.95

6.64

21.1

40.4

10.5

10.1

25.7

48.9

6.25

6.76

17.8

36.3

9.92

9.70

26.6

58.5

14.6

12.5

32.7

76.9

12.2

23.1

27.2

37.7

23.2

30.5

39.5

54.6

12.4

15.9

20.9

36.8

23.2

30.8

39.5

53.2

33.5

41.8

55.1

72.0

7.12

7.12

16.8

30.5

12.2

12.2

27.3

44.5

6.76

6.76

16.4

25.0

12.1

12.1

35.5

38.4

15.0

15.0

31.5

46.7

34


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Fig. 1. Phosphate adsorption isotherms of adsorbents at 298.15K and pH about 7.0.

35



100

H3PO4

H2PO4

2-

HPO4

PO4

3-

(A) Phosphate speciation(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 45

50

7.2

2.12

12.36

0 2

4

6

8

pH

36


10

12

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Fig. 2 The speciation of phosphate (A) and influence of pH on the adsorption capacity

of Zr-OH and dimethylamine-Zr-OH for phosphate (B).

37



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Fig. 3 Effect of equilibrium time on adsorption of adsorbents (phosphate

concentration: 200mg/L, pH ± 7.0,298.15K)

38


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Fig. 4 Effect of coexistent ions (Cl-, NO3- and SO42-) in equimolar solution on

adsorption of phosphate at 298.15K and pH about 7.0.

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Fig. 5 Effect of coexistent ions (Cl-, NO3- and SO42-) in different concentration on

adsorption of phosphate at 298.15K and pH about 7.0.

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Fig. 6 Plot of the zeta potential of Zr-OH and dimethylamine-Zr-OH as a function of

pH.

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Fig. 7 FTIR spectra (KBr pellets) of dimethylamine-Zr-OH and after adsorption

dimethylamine-Zr-OH.

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Fig. 8 XPS analysis of Zr-OH and dimethylamine-Zr-OH before or after phosphate

adsorption .(a) XPS survey scan of Zr-OH and dimethylamine-Zr-OH before or after phosphate adsorption; (b)The surface O1s spectra of adsorbents (1) Zr-OH, and (2) after adsorption Zr-OH, and (3) dimethylamine-Zr-OH, and (4) after adsorption dimethylamine-Zr-OH.

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Graphical Abstract/TOC：


Enhancement of Phosphate Adsorption on Zirconium Hydroxide by

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