Efficient Removal of Phosphate from Aqueous Solutions Using Corundum- hollow-spheres Supported Caclined Hydrotalcite Porous Thin Films

LIU Yuncai, ZHU Chen

(School of Architecture and Materials Engineering, Hubei Second Normal University, Wuhan 430205, China)

Abstract: Phosphate was removed from aqueous environment by corundum-hollow-spheres supported caclined hydrotalcite (cHT) thin films.Mg-Al-CO3 hydrotalcite (HT) thin films were deposited on corundumhollow-sphere substrates by hydrothermal homogeneous precipitation at 120 ℃ for 30-240 min and cHT thin films were obtained by annealing of the HT thin films at 500 ℃ for 180 min.Their crystal phase, morphology and microstructure were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM).The results show that homogeneous, well-crystallized and hierarchical flower-like thin films were deposited firmly on the surface of the corundum.The mechanism of nucleation and growth of the HT thin films was fitted well with the anion coordination polyhedron growth unit model.To determine the absorption of phosphate by this adsorbent, different bed depth (10 - 30 cm) and flow rate (1.0 - 3.0 mL/min) were examined by column experiments.The highest removal efficiency of phosphate amounted to 98.5 % under optimum condition (pH =7.2).The adsorption capacity increased as the bed depth increased and decreased as the flow rate increased.

Key words: adsorption; phosphate; hydrotalcite; corundum-hollow-sphere; hydrothermal homogeneous precipitation; thermal anneal

1 Introduction

In many countries, eutrophication has occurred frequently in reservoirs, lakes, and coastal areas[1],and the main reasons is that the industrial sewage and domestic wastewater contains excessive amounts of phosphate[2-4].In order to prevent further deterioration of phosphorus chemical pollution, phosphorus removal must be carried out before sewage or wastewater is discharged into natural waters.Many treatment techniques of wastewater containing phosphate,including physical, chemical and biological treatment method, have been developed[5-7].Among these methods, adsorption method is considered as one of the most promising methods because it has advantages such as less investment, easy operation, high efficiency and environmental friendliness.

The success of an adsorption technology depends largely on the physical and chemical properties of an adsorbent.Up to date, numerous materials as adsorbent for phosphate removal have been investigated,including carbon-based materials, natural or modified clay mineral materials, and hydrotalcites materials[8-10].Activated carbon has been widely used for treatment of wastewater pollution[11].However, it is necessary to modify the surface of activated carbon to improve its adsorption ability of phosphate in water[12].Natural clay minerals used in pollution control have increasingly received attention[13,14], but their adsorption capacity is still low because of the small specific surface area.Hydrotalcite (HT)-like compounds have layered structure and typical properties on large surface area with high anion-exchange capacity, memory effect and adjustable element compositions, which makes them as potential high efficient adsorbents for removing anionic species from wastewater[15,16].However, their dispersion in aqueous solution is poor because they are waterinsoluble crystalline, which affects their adsorption capacity.

The hydrated carbonate anions in the HT interlayer space can be removed by calcination at appropriate temperature.Their calcination products exhibit promising adsorptive properties because the anionic group may enter the interlayer space during the reconstruction of the layered structures.The adsorbents must be subjected to repeated adsorption/desorption cycles.However, it is difficult and time consuming to separate the powdery adsorbents from the waste drainage after adsorption process.

The basic methods and procedures of this research were indicated in Fig.1.The cHT porous thin films were deposited on corundum-hollow-sphere substrates by hydrothermal homogeneous precipitation method and thermal annealing.The characteristics of the thin films, their growth process, adsorbent properties and regeneration process were investigated.When the corundum-hollow-sphere with cHT porous thin film was used as an adsorbent, it had two attracting advantages.One is that it had a higher cycle life,because the film was formed on a spherical surface and was more difficult to peel off from the substrate.The other is that the adsorbent was retrieved easily from aqueous solution, because it floated on the surface of aqueous solution.So this paper will provide a new insight into the design and fabrication of advanced adsorption materials for water pollutant removal.

Fig.1 Schematic diagram of the phase evolution of the cHT thin films on corundum-hollow-spheres during calcinations, adsorption and regeneration

2 Experimental

2.1 Reagents

Analytical grade chemicals were used without further purification.All solutions were freshly prepared with deionized water as the solvent.Mg(NO3)2·6H2O,Al(NO3)3·9H2O, CO(NH2)2, KH2PO4, NaOH, and Na2CO3were supplied by Beijing Chemical Reagent Company (Beijing, China).

2.2 Substrate material

The corundum hollow spheres were purchased from the Sinosteel Luoyang Institute Refractories Research Co.Ltd (Luoyang, China).The morphology of the corundum hollow spheres is showed in Fig.2.The average of outside diameter of the hollow spheres is 5.5 mm, and the average of thickness of the shell is 0.2 mm.The hollow spheres were ultrasonically cleaned in ethanol for 15 min, and then dried by warm air.

Fig.2 SEM images of the appearance of the partially broken corundum-hollow-sphere

2.3 HT and cHT thin film preparation

The HT thin films were synthesized by a hydrothermal homogeneous precipitation method using Mg(NO3)2·6H2O, Al(NO3)3·9H2O and CO(NH2)2as raw materials.Firstly, 1 M aqueous solution with molar ratio Mg/Al/urea of 5:2:14 was transferred into a Teflon-lined autoclave, then the pretreated hollow spheres were immersed in the solution, and finally the autoclave was heated at a temperature of 393 K for 30,60 and 240 min, respectively.The collected spheres were rinsed with deionized water and dried at room temperature.The cHT thin films were prepared by calcining HT at a temperature of 500 ℃ for 3 h in a muffle furnace.

2.4 Thin film characterization

Rigaku model and D/Max-2500PC X-ray diffractometer (XRD) were applied to analyze the crystal structure and phase composition of the thin films.When using CuKαradiation (λ= 0.154 050 nm,30 kV, 60 mA), the scan rate was 1.20 (°)//min and the 2θrange was 5°-70°.The surface morphologies of the spheres coated with HT thin film and cross section of the thin film were characterized when using a Hitachi model S-4800 field-emission scanning electronic microscope (FE-SEM) working an accelerating voltage of 10 kV.

2.5 Continuous column experiments

Phosphate in the solution could exist in several forms: H3PO4, H2PO4-, HPO42-, and PO43-, depending on the pH value of solution.According to the ionization equilibrium constant (pKa1=2.15, pKa2=7.20,and pKa3=12.33) of phosphate in aqueous solution,phosphate ions in solution mainly existed in the form of H2PO4-within a pH range of 2.15-7.20.Potassium dihydrogen phosphate (KH2PO4) solution at 0.12 mmol/L was used in this study, and the pH value of the solution was adjusted to 7.2 by adding 0.1 mol/L NaOH solution before the adsorption experiments.Most of the anions present in the solution under this pH condition were H2PO4-.

The continuous column tests of phosphate adsorption by cHT were carried out in an organic glass cylinder with an inner diameter of 5 centimeters and a height of 60 centimeters.In the cylinder, the adsorbent bed was supported by rock wool to ensure the uniformity of the velocity distribution of the solution.Experiments were performed at adsorbent bed depths of 10, 20 and 30 centimeters, and influent flow rates of 1, 2, and 3 milliliter per minute, respectively.

The influent phosphate solution of predetermined concentration (C0= 0.12 mmol/L) was pumped into the cylinder by the peristaltic pump, and passed through the adsorbent bed in a up-flow mode.Solutions overflowing out at the outlet of the column were collected at regular intervals, and the residual phosphate concentration (C) of the solutions was determined using a Hitachi model U-3900 UVVisible spectrophotometer at a wavelength 700 nm by the molybdate blue spectrophotometric method.All continuous column experiments were performed at solution pH = 7.2 under room temperature.

2.6 cHT thin film regeneration

The regeneration performance of adsorbent will determine its service life and cost directly.Generally,the performance of adsorbent was studied by repeated adsorption/desorption cycles experiments.The regeneration process should not damage the structure of the adsorbent; otherwise, the reuse would be affected in successive removal of phosphate.

In the present study, the mixture of corundum hollow spheres coated with rHT thin films and Na2CO3solution (0.2 mol/L) were shaken horizontally on a rotary shaker at 120 rpm at 25 ℃ for 3 h to reach the completely replacement of H2PO4-by CO32-.After that, the corundum hollow spheres coated with HT thin films were removed, washed with doubly distilled water, and calcined at a temperature of 500 ℃ for 3 h in a muffle furnace.The regeneration efficiency was studied up to three adsorption/ desorption cycles.

3 Results and discussion

3.1 Structural characterization of the thin films

The XRD patterns of the initial HT thin film,cHT thin film, rHT (after phosphate adsorption) thin film and the substrate are shown in Fig.3.Before being calcined, the HT thin film showed a crystallized hydrotalcite-like structure with an R3m symmetry(Fig.3(b)).The sharper and more symmetric reflections of the (003), (006), (110), and (113) planes, and more broad and asymmetric reflections of the (015), and(018) planes could be observed.The (009) reflection overlapped with the (012), resulting in a broad signal.

Fig.3 XRD patterns of substrate, HT film, cHT film and rHT film

The unit cell parameters of a crystal can be determined by the location of its diffraction peaks.For rhombohedral crystals, the cell parametersaandccould be calculated when using the following formula:a= 2d110,c= 3d003, in whichais the distance between the two cation ions in the unit cell andcis unit cell thickness[17].In this paper, the lattice parameters of HT and rHT have been calculated, as listed in Table 1.The averaged basal spacingd003of the HT was around 0.78 nm, which is consistent with the presence of carbonate intercalated anion.

Table 1 Crystallographic data of synthesized HT and rHT

After calcination at 500 ℃ for 3 h, all diffraction peaks representing the hydrotalcite structure disappeared completely (Fig.3(c)), at the same time, weak diffraction peaks appeared at 43° and 62° (2θ), which indicates that the interlayer water, CO32-and partial OH-in layer have been removed in H2O and CO2, and the mixed oxide Mg(Al)O has formed.When the cHT was placed in potassium dihydrogen phosphate solution at room temperature, the layered hydrotalcite-like structure was reconstructed (Fig.3(d)), which suggests that H2PO4-anions in solution were adsorbed onto the positive layer and formed the new negative layer.The positions of (003), (006), and (009) diffraction lines shifted towards lower 2θvalue, because the interlayer H2PO4-anions was larger than the CO32-anions.In addition, the intensity of (009) diffraction peak was obviously enhanced, which indicates that H2PO4-anions was arranged in a certain form between layers, which changed the density of interlayer electron cloud[18].However, the positions of the (110) diffraction peak of HT and rHT were essentially the same (61.3°), which indicates that the structure of the brucite-like sheets remained unchanged after the anion-exchange.The unit cell parameters of rHT were calculated by software Jade5.0, as listed in Table 1.

3.2 The morphology of the thin films

The SEM images of the as-prepared HT film on corundum-hollow-spheres are shown in Fig.4.Fig.4(a)demonstrates that the HT thin film was uniform over the whole corundum hollow sphere substrate, and the HT thin film possessed a homogeneous, wellcrystallized and platelet-like microstructure[19].The detailed morphologies indicate that the HT nanosheets grew perpendicular to the substrate surface and crosslinked each other to form a honeycomb structure layer.

Fig.4 SEM images of as-prepared HT thin film (a) and crosssectional view (b)

Further SEM observation on the cross section(Fig.4(b)) demonstrates the HT film was strongly adhered to corundum hollow sphere substrate, and the HT film consisted of a compact inner layer and a porous outer layer.The thickness of the porous layer was approximately 2 times of the thickness of the compact layer, and they were 1.15 and 0.60 μm respectively.The dense layer at the bottom of the film gave a higher bonding strength between the film and the substrate, so that the film was not easy to peel off from the substrate during repeated use.The porous layer at the top of the film made the film have a larger specific surface area,which was very beneficial to improve the adsorption capacity of the film.

3.3 The formation mechanism of the thin films

The growth mechanism model of the membrane was built based on growth unit model of anion coordination polyhedron.When the urea underwent hydrolysis under high pressure and high temperature in the autoclave at a temperature of 393 K, the CO32-and OH-ions continuously entered into the system, and the pH value of solution in the autoclave increased gradually as the reaction progressed, and Mg2+and Al3+ions in solution gradually translated into [Mg(OH)6]4-and [Al(OH)6]3-anion coordination polyhedron.More stable HT layer structure was formed by dehydration reaction of the anion coordination polyhedrons.Meanwhile, the OH-ions in the interlayer were exchanged by CO32-ions to form Mg6Al2(OH)16CO3·4H2O tiny particles.

Fig.5 shows the surface morphology of uncoated and HT coated corundum hollow sphere in the asreceived condition, and after 0, 30, 60 and 240 min of reaction, respectively.In the as-received condition corundum hollow sphere showed an uneven surface morphology that was due to consisting of cuboidal grains (Fig.5(a)).

Fig.5 SEM images of the uncoated corundum hollow sphere in the as-received condition (a) and the HT films formed after (b) 30 min, (c) 60 min and (d) 240 min of reaction at 120 ℃

The porous HT thin film was formed after 30 min of reaction, as shown in Fig.5(b).The HT tiny particles in the hydrothermal solution were adsorbed on the surface of the spheres and became crystalline nucleus.The crystal structure of HT was different from that of the corundum, so HT would tend to precipitate by three-dimensional heterogeneous nucleation on the corundum and some pore space would be generated at the interface[20].

After 60 min, as shown in Fig.5(c), a few plateletlike polycrystals grew out on the top of porous HT film.According to the coordination polyhedron law concerning growth habit, the growth rate of the crystal face depends on the kinds of element of coordination polyhedron present on the interface, and the more the number of element of coordination polyhedron in a direction of the crystal, the faster the crystal growth rate in the direction.After a long enough period of growth, the plate-plate overlapping of HT crystallites film was formed (Fig.5(d)), and at the same time, the bottom of the film was gradually densified.

3.4 Phosphate adsorption of the cHT film

The breakthrough curves were plotted by the relationship between the ratio (Ct/C0) of ion concentration (Ct) at timetto initial concentration(C0) and the effluent volume.At a constant flow rate of 2 mL / min, the effect of the bed depth (10, 20, 30 cm) on the breakthrough curve was shown in Fig.6.The three breakthrough curves had similar shapes, and it was observed that the smaller the river bed depth was, the shorter the breakthrough and exhaustion time was.There were more active sites that could adsorb phosphate in the high depth adsorbent bed filled the corundum hollow spheres coated with cHT film, so it took longer to reach the saturation point.Fig.6 also shows that the increase in bed depth increased the throughput volume because of higher contact time between the solution and the adsorbent,which is consistent with the study reported by other researchers[21].

Fig.6 The breakthrough curves of phosphate adsorption on the corundum hollow spheres coated with cHT film bed at various bed depths

Fig.7 shows the effect of flow rate (1, 2, 3 mL/min) on the breakthrough curve of the corundum hollow spheres coated with cHT film bed.As shown in Fig.7, the breakthrough time value reduced with increase of flow rate, and the slope of the breakthrough curves for phosphate adsorption onto cHT film increased with increase of flow rate at a constant bed depth.As the solution flowed continuously in the cylinder, the adsorbent in the fixed bed was saturated with the adsorbate and the effluent concentration was close to the influent concentration.

Fig.7 The breakthrough curves of phosphate adsorption on the corundum hollow spheres coated with cHT film bed at various flow rates

According to the fluid mechanics, the difference of the flow velocity of the fluid will lead to the important changes in the Reynolds number.As the flow rate increased, this number also increased, and the fluid tended to turbulent flow.When a fluid flew in a turbulent flow, its motion elements were very random.At a higher flow rates, the contact time between the solution and the adsorbent was not sufficient to establish chemisorption equilibrium, so the solution left the column before equilibrium and reached an earlier breakthrough time, which means that lower flow rates and longer contact times are beneficial for the removal of phosphate in the column.

3.5 Regeneration

The exchangeability of interlayer anions of HT and the layered structural reconstruction of cHT provide the possibility for the regeneration of rHT.The regeneration efficiency was studied up to three adsorption/desorption cycles.It is found that about 95 % and 92 % of the initial adsorption capacities were obtained after a second desorption cycle and a third desorption cycle, respectively.As the number of regeneration increased, the phenomenon of slight decreased in adsorption capacity, which may be attributed to structural disintegration of CHT during repeated calcination.

4 Conclusions

a) when using hydrothermal homogeneous precipitation method to investigate thein-situgrowth of HT thin films on corundum hollow spheres, the membrane possesses a homogeneous, well-crystallized and platelet-like microstructure.

b) The formation mechanism of the HT film was discussed.Firstly, Mg2+and Al3+ions in solution translated into [Mg(OH)6]4-and [Al(OH)6]3-anion coordination polyhedron.Secondly, HT tiny particles formed by dehydration reaction of the anion coordination polyhedrons and the OH-ions in the interlayer was replaced by CO32-ions.Finally, with the growth of crystalline nucleus on corundum hollow spheres, the porous HT film formed.

c) The performance of corundum-hollow-spheres coated with cHT film to remove phosphate in aqueous solutions was examined by column experiments.The adsorption capacity of the fixed bed increased as the bed depth increased and decreased as the flow rate increased.It may be used an adsorbent for the continuous phosphate removal in the future.

Conflict of interest

All authors declare that there are no competing interests.