Layered Yttrium Hydroxide l-Y(OH)3 Luminescent Adsorbent for

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Layered Yttrium Hydroxide l-Y(OH)3 Luminescent Adsorbent for Detection and Recovery of Phosphate from Water over a Wide pH Range Minhee Kim, Hyunsub Kim, and Song-Ho Byeon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13437 • Publication Date (Web): 27 Oct 2017 Downloaded from http://pubs.acs.org on October 30, 2017

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ACS Applied Materials & Interfaces

Layered Yttrium Hydroxide l-Y(OH)3 Luminescent Adsorbent for Detection and Recovery of Phosphate from Water over a Wide pH Range Minhee Kim, ‡ Hyunsub Kim, ‡ and Song-Ho Byeon* Department of Applied Chemistry, College of Applied Science and Institute of Natural Sciences, Kyung Hee University, Gyeonggi 17104, Korea.

KEYWARDS: Layered hydroxide, Adsorbent, Phosphate, Luminescence, Rare earths

ABSTRACT

Layered yttrium hydroxide, l-Y(OH)3, has been explored as a representative member of the layered rare earth hydroxide family (l-RE(OH)3; RE = rare earths) for removal and recovery of phosphate from aqueous solution. Compared to the hexagonal form, h-Y(OH)3, which has a weakly positive surface charge only at low pH, the layered polymorph composed of hydroxocation layers exhibited a high point of zero charge (pHpzc ~ 11) and significantly enhanced adsorptive ability for anions over a wide pH range. The Langmuir isotherm model and

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pseudo-second-order kinetic model were adopted to explain the phosphate adsorption on lY(OH)3. This new adsorbent revealed high capacity, efficiency, stability, selectivity, and reusability in adsorption of phosphate from a single electrolyte as well as natural waters containing competing anions. Essentially complete phosphate recovery from aqueous solutions at low phosphate concentrations (2.0 mg-P/L) was demonstrated with an adsorbent dosage of 0.025 – 0.5 g/L. The adsorption of phosphate was accompanied by an increase in the solution pH, suggesting a release of OH− ions during the adsorption reaction. In particular, when Ce3+ and Tb3+ were co-doped (l-Y(OH)3:Ce,Tb), phosphate adsorption led to the characteristic 5D4 → 7FJ (J = 6, 5, and 4) emissions of Tb3+ under commercial 312 nm-UV irradiation. The photoluminescence of phosphate-adsorbed l-Y(OH)3:Ce,Tb provided evidence of the innersphere complexing mechanism involving the formation of Y(Ce, Tb)–O–P bonds through which the energy transfer can occur. The “luminescence-on” behavior of l-Y(OH)3:Ce,Tb by phosphate adsorption was employed to detect and recover phosphorus at low concentrations in deionized water, mineral water, tap water, and river water.

1. INTRODUCTION Phosphorus is well recognized as a leading cause of eutrophication in reservoirs, estuaries, lakes, rivers, and sea.1-3 Therefore, removal of phosphorus before or even after discharge into aquatic environments is required to prevent algae blooms caused by serious eutrophication, which depletes dissolved oxygen. On the other hand, phosphorus is an essential nutrient for all living organisms and an important element for cellular function as it occurs in deoxyribonucleic acid (DNA), adenosine triphosphate (AT), and adenosine diphosphate (ADP). The application of


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phosphorus as a fertilizer is also essential in agriculture, but most applied phosphate fertilizer is lost during food production and consumption.4 As the natural deposits of phosphorus are depleted and there is no substitute for phosphorus, significant efforts are necessary for the recovery and reuse of this nonrenewable resource. Among various technologies developed for removal and recovery of phosphate from wastewater, adsorption-based processes are considered the most promising route because of their simplicity, low-cost, high efficiency, and fast removal rate.2,5 Compared with precipitation and crystallization from water with relatively high phosphate concentrations, which usually has low selectivity, recovery processes of phosphates via ion-exchange and adsorption are relevant for surface water, especially at low phosphate concentrations.6 Although various types of adsorbents have been studied over the past decades, the development of adsorbents with higher capacity, performance, and reusability is still a challenge. The adsorption of anions on metal hydroxides is an important mechanism for decreasing their concentrations to acceptable levels.7,8 Metal oxides with a large number of surface hydroxyl groups and metal hydroxide-based adsorbents are therefore attractive candidates for recovery of phosphate and remediation of environmental water.9-13 However, conventional adsorbents show low selectivity for adsorption toward phosphate over common inorganic anions including SO42−, NO3−, Cl− and HCO3−.14 Unfortunately, most adsorbents based on metal hydroxides require acidic conditions for phosphate adsorption. Their adsorption capacity decreases significantly at higher than neutral pH because the surface charge of many metal hydroxide particles is negative over a wide pH range and weakly positive at only low pH. Layered double hydroxides (LDHs) have been frequently proposed as a promising alternative for selective phosphate exchangers and adsorbents even at high pH values.15-18 The adsorption and ion-exchange characteristics of LDHs



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are useful for environmental applications to remove hazardous anions such as chromate, phosphate, arsenate, and selenate from aqueous solutions.19 In contrast, although Zr-doped LDHs show an improved selectivity for phosphate ions,20 many LDH sorbents are still unsatisfactory for phosphate and adsorb competing anions such as Cl−, NO3−, SO3−, CO32−, SO42−, and many organic carboxylates, sulfonates, and sulfates by the ion-exchange reactions in the interlayer of LDHs.21 Furthermore, the intercalation of phosphate anions into the gallery of LDHs must be accompanied by the release of other anions from the interlayer to the surrounding water. Apart from improving the adsorption efficiency and capacity, it is also important that the adsorbents are reusable via processes like desorption and regeneration. Compared with other metals, rare earths have some attractive advantages such as high chemical stability, low toxicity, and biocompatibility.22 Rare earth oxides with high adsorption capacities for various hazardous anions (i.e. fluoride, bichromate, and arsenate) have been employed for environmental remediation applications.23-26 Rare earth orthophosphates (REPO4; RE = rare earths) generally exhibit extremely low water-solubility and excellent chemical/thermal stability.27,28 The use of lanthanum for adsorption provides the advantage of high affinity for phosphate because the lanthanum–phosphate complex forms even at low concentrations of phosphate. Lanthanum adsorbents loaded on a mesoporous silica SBA-15 showed high selectivity for phosphate removal by the formation of LaPO4 species with a very low Ksp.29 Some lanthanum-containing adsorbents achieved both high adsorption capacity and a wide operating pH range at a low phosphate concentration.30-33 Cerium-containing materials were also proposed for efficient removal of oxyanions including phosphate.34-36 Although some developed sorbents containing rare earth species showed a large adsorption capacity of phosphate, their structures have not been well defined.37,38 Such amorphous sorbents might have


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problems in terms of the reproducibility and reliability of their performance for both phosphate removal and recyclability of the sorbent itself. In addition, the leaching of rare earth species impregnated on sorbents cannot be ignored. Therefore, the development and improvement of rare earth-based adsorbents with high adsorption efficiency, capacity, selectivity, and recyclability have garnered increasing attention for the recovery of phosphate in recent years. Recently, we developed a series of layered polymorph (l-RE(OH)3) of rare earth hydroxides through the topotactic exchange reaction between Cl– and OH– ions in the interlayer gallery of layered rare earth hydroxychlorides, RE2(OH)5Cl∙nH2O (RE = rare earths).39 In contrast to the conventional hexagonal form (h-RE(OH)3) of rare earth hydroxides, the structure of these layered polymorphs is composed of alternately stacked rare earth hydroxocation layers and interlayer OH– groups. Consequently, the rare earth hydroxide layers have a positive charge, which is favorable for electrostatic interactions with negatively charged ions. This structural feature is similar to LDHs, showing a high positive charge over a wide pH range.40 However, the anion adsorption behavior on l-RE(OH)3 has not yet been studied. Therefore, we explored the possible applications of l-Y(OH)3, a member of l-RE(OH)3 family, as adsorbents for the phosphate recovery from water in this work. l-RE(OH)3 has no interlayer anions such as Cl– and NO3– that could be released during adsorption reactions of different anions. High selectivity was also expected because these materials exhibit no exchange reactivity with other organic and inorganic anions in the interlayer galleries. These features provide important advantages for l-RE(OH)3 over other metal hydroxide adsorbents including LDHs. More importantly, we could endow l-Y(OH)3 with a ‘luminescence-on/off’ capability by doping activator ions such as Ce3+ and Tb3+ into matrices. As a representative example, the photoluminescence properties of Ce3+ and Tb3+ co-doped-layered yttrium hydroxide (l-



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Y(OH)3:Ce,Tb) were then employed for systematic evaluation of phosphate adsorption and desorption.

2. EXPERIMENTAL SECTION 2.1. Preparation of Y2(OH)5NO3∙nH2O and Y1.8Ce0.2-2xTb2x(OH)5NO3∙nH2O for Structural and Photoluminescence Studies. Well-crystallized layered yttrium hydroxynitrate (Y2(OH)5NO3∙nH2O; LYH) and Ce,Tb-co-doped layered yttrium hydroxynitrates (Y1.8Ce0.22xTb2x(OH)5NO3∙nH2O;

x = 0.0, 0.025, 0.05, 0.075, and 0.1) were prepared in a

hexamethylenetetramine ((CH2)6N4; HMTA) solution. Typically, a stoichiometric amounts of Y(NO3)·6H2O, Ce(NO3)·6H2O, and Tb(NO3)·6H2O (total 10 mmol), NaNO3 (130 mmol), and HMTA (10 mmol) were dissolved in 100 mL deionized water. The resulting aqueous solution was heated at 90°C with mild stirring (350 rpm) for 12 h and was then cooled to room temperature. The precipitate was filtered, washed with a copious amount of distilled water several times, and dried at 60°C. 2.2. Preparation of Y2(OH)5NO3∙nH2O and Y1.8Ce0.1Tb0.1(OH)5NO3∙nH2O for Recovery of Phosphorus. LYH and Y1.8Ce0.1Tb0.1(OH)5NO3∙nH2O (LYH:Ce,Tb) precursor powders were prepared for the synthesis of phosphate adsorbents with a large surface area in aqueous solutions at room temperature. Y(NO3)·6H2O (5 mmol) was dissolved in deionized water (30 mL). For the Ce,Tb-co-doped compositions, stoichiometric amounts of Ce(NO3)·6H2O and Tb(NO3)·6H2O were additionally dissolved. A KOH (20 mmol) solution was then added drop-wise to the prepared mixture (OH− : (Y,Ce,Tb)3+ ratio = 2 : 1) with vigorous stirring at room temperature. The resulting solution was aged at room temperature for 12 h. After the reaction was complete,


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the solid products were collected by several repeated centrifugations and washing steps with distilled water several times to completely remove Na+ and NO3− ions. The solids were then dried at 60 C for 6 h. 2.3. Preparation of Y(OH)3∙H2O and Y0.9Ce0.1-xTbx(OH)3∙H2O (Layered Yttrium Hydroxide and Ce,Tb-Co-Doped Layered Yttrium Hydroxides). LYH and LYH:Ce,Tb crude powders were dispersed in aqueous 0.1M NaOH solutions (pH ~ 13) for the ion-exchange reaction between interlayer NO3− ions and OH− ions. After stirring (750 rpm) these mixtures for 24 h at room temperature, the resulting layered hydroxide precipitates, Y(OH)3∙H2O (l-Y(OH)3) and Y0.9Ce0.05Tb0.05(OH)3∙H2O (l-Y(OH)3:Ce,Tb) were collected by filtration, washed with copious amounts of water to remove residual salts, and then dried at 60 ºC for one day. Hexagonal Y(OH)3 (h-Y(OH)3) was also prepared for comparison. YCl3·6H2O (0.01 mol) was dissolved in 40 mL deionized water and NaOH (0.04 mol) was subsequently added with stirring (750 rpm). After stirring for 30 min, the resulting solution was put into a Teflon-lined stainless steel autoclave with a capacity of 100 mL at room temperature. The autoclave was then sealed and maintained at 160 °C for 12 h. The solution was continuously stirred (400 rpm) during the hydrothermal treatment. After the reaction was complet, the solid product was collected by filtration, washed with distilled water, and dried at 60 °C for 12h. 2.4. Phosphate Adsorption Reactions on l-Y(OH)3 and l-Y(OH)3:Ce,Tb. Phosphate stock solutions of 500 mg-P/L were prepared by dissolving AR grade KH2PO4 in deionized water, tap water, river water, and mineral water. All working solutions were freshly taken from the stock solutions and diluted to desired concentrations. To understand the influence of pH on adsorption, the initial solution pH was adjusted to 5, 7, and 9 by adding appropriate amounts of 0.1 M HCl or



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0.1 M KOH solution. All adsorption experiments were repeated 3 times, and the mean values of analysis data were adopted. The phosphate (as total P) adsorption experiments were conducted at 26 ± 2 ºC in 50 mL ~ 1 L polypropylene bottles by adding various doses of l-Y(OH)3 and l-Y(OH)3:Ce,Tb into 20 ‒ 500 mL of a phosphate solution with an initial total P concentration of 2 ‒ 500 mg-P/L and initial pH values of 5, 7, and 9. These mixtures were stirred (200 rpm) for 1 min to 24 h. After the adsorption reaction, the solution pH was measured, and then the supernatant was collected through filtration using a syringe filter (0.45 µm Whatman filter disc). The phosphate concentration remaining in solution was measured using the molybdenum blue-ascorbic acid method.41 The absorbance of the intense blue phosphate-molybdate complex (formed by the reaction of orthophosphate ions and ammonium molybdate in the presence of ascorbic acid) was measured at a wavelength of 880 nm. The amount of phosphate adsorbed on the adsorbents was calculated from the difference between the initial concentration and the equilibrium concentration in the solutions. Adsorption kinetic experiments were performed in a 500 mL bottle with 200 mL of aqueous solution at an initial phosphate concentration of 200 mg-P/L. After adding 0.1 g of l-Y(OH)3 adsorbent to the phosphate solutions at initial solution pH values of 5, 7, and 9, the resulting suspensions were constantly stirred (750 rpm) for appropriate adsorption times. The supernatant was then collected using a 0.45 µm syringe filter and was analyzed to determine the residual phosphate concentration. Experiments to determine the adsorption isotherms were also performed at initial pH values of 5, 7, and 9. The initial concentration of the phosphate solution was varied from 2.5 to 100 mg-


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P/L. Then, 0.1 g l-Y(OH)3 was mixed into 200 mL of phosphate solutions of different initial concentrations. These mixtures were stirred at a speed of 750 rpm for 24 h and filtered prior to measuring the phosphate concentration remaining in the solutions. A set of adsorption experiments was performed to investigate the effect of common anions on phosphate recovery by l-Y(OH)3 in aqueous solutions containing Cl−, NO3−, SO42−, and HCO3− anions. The initial phosphate concentration was 0.065 mM (2.0 mg-P/L) and the concentrations of coexisting anions were 0.065, 0.325, and 0.65 mM, respectively. The adsorption reaction was performed for 1 h with stirring (400 rpm). The change of Cl−, NO3−, SO42−, and HCO3− anion concentrations before and after adsorption reactions were measured by ion chromatography (IC) and total inorganic carbon (TIC) analyses. To understand the interference on the adsorption of phosphate under conditions similar to real environmental water streams, l-Y(OH)3 was added into 2.0 mg-P/L phosphate solutions prepared using deionized water, tap water, mineral water, and river water. The adsorbent dosage was fixed at 0.5 g/L. After adsorption for 1 h, the corresponding filtrates were analyzed to determine the residual phosphate concentration in solutions. 2.5. Phosphate Desorption and Regeneration of l-Y(OH)3:Ce,Tb. Phosphate desorption/regeneration behavior was examined to assess the recyclability of l-Y(OH)3:Ce,Tb adsorbent. First, 0.1 g of l-Y(OH)3:Ce,Tb was reacted in a 200 mg-P/L phosphate solution (pH ~ 7) at room temperature for 24 h. Phosphate saturated l-Y(OH)3:Ce,Tb was then filtered and washed 3 times with deionized water to remove any unadsorbed phosphate. Subsequently, the phosphate desorption from l-Y(OH)3:Ce,Tb was performed in a 1.0 M NaOH solution with stirring (500 rpm) at room temperature for 48 h and at 100 ºC for 2 h. Then, supernatant was



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collected by filtration (0.45 μm PTFE) to measure the amount of released phosphate. The recovered adsorbent was washed and dried at 60 ºC for the next cycle of adsorption and desorption. The adsorption and regeneration cycles of l-Y(OH)3:Ce,Tb were repeated 5 times. The regenerated adsorbent was weighed again for every adsorption experiment. 2.6. Characterization. The formation of single-phased layered hydroxynitrates, layered hydroxides, and their Ce3+ and Tb3+-co-doped forms was confirmed by XRD patterns recorded with a Bruker D8 Advance diffractometer. The sizes of the adsorbents were compared using field emission scanning electron microscopy (FE-SEM) performed with a Carl Zeiss LEO SUPRA 55 electron microscope operating at 30 kV. Specimens for the FE-SEM were coated with Pt-Rh for 180 s under vacuum. Thermogravimetric (TG) curves to determine the amount of interlayer water were recorded in air at a heating rate of 5 °C/min using a Seiko Instruments TG/DTA320. To estimate the point of zero charge (pHpzc), l-Y(OH)3 and h-Y(OH)3 powders were suspended in deionized water. A 20 mL suspension was then adjusted to various pH values by adding NaOH or HCl solution with stirring (600 rpm). Then, 1.0 mL of the resulting solution was placed into a disposable folded capillary cell (DTS1070, Malvern), and the zeta (ζ) potentials were measured using a Zetasizer Nano ZS90 (Malvern). The absorption spectra of phosphate solutions were measured by employing UV-vis spectrophotometers (Shimadzu Multispec-1501 and a LAMBDA 35). Photoluminescence spectra of l-Y(OH)3:Ce,Tb before and after phosphate adsorption, desorption, and regeneration were recorded on an FP-6600 spectrophotometer (JASCO) equipped with a Xenon flash lamp.


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3. RESULTS AND DISCUSSION 3.1. Characterization of l-Y(OH)3. A powder X-ray diffraction (XRD) pattern of Y2(OH)5NO3∙1.5H2O (LYH) is displayed in Figure S1a. A series of strong (00l) reflections is a characteristic of the layered structure and agrees well with previously reported patterns.42 As compared in Figure S1b, a systematic shift of (00l) reflections toward higher reflection angles after reaction of LYH in a 0.1 M aqueous NaOH solution was observed. This was attributed to the formation of layered yttrium hydroxide (Y(OH)3∙nH2O; l-Y(OH)3) by the exchange reaction between NO3− and OH− ions in the interlayer space without direct transformation to hexagonal yttrium hydroxide. The observed interlayer distance of ~ 7.57 Å and an n value of ~ 1.0 determined by thermogravimetric (TG) analysis are close to those of previously reported Y(OH)3∙H2O.39 The XRD pattern of this phase is distinctly different from that of the conventional hexagonal form (h-Y(OH)3) shown in Figure S1f. Thus, in contrast to the hexagonal structure composed of tri-capped trigonal prismatic yttrium hydroxide polyhedra, the structure of the resulting l-Y(OH)3 remains topologically identical to the LYH precursor (i.e., typical layered structure with interlayer water molecules). The surface charge plays an important role in the adsorption process because it determines the strength of electrostatic interaction between adsorbates and the solid surface of the adsorbent. An important advantage provided by the maintenance of layered structure is that this hydroxide can have a positive surface charge originating from the hydroxocation layers in the structure. For instance, measured zeta (ζ) potentials of layered rare earth acetates and chlorides are both highly positive (30 – 50 mV).43,44 Layered double hydroxides (LDHs) with similar structural characteristics also show highly positive ζ potentials over a wide pH range, and their points of zero charge (pHpzc) are close to ~11.16 In contrast, the surface charge of many conventional



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hydroxides such as α-Al(OH)3 and Fe(OH)3 is weakly positive only at low pH range.45,46 The surface ζ potentials of l-Y(OH)3 and h-Y(OH)3 are compared as a function of solution pH in Figure 1. Compared to that (pHpzc < 4) of h-Y(OH)3, the point of zero charge of l-Y(OH)3 is significantly higher (pHpzc ~ 11). Thus, the surface charge of l-Y(OH)3 is strongly positive up to pH ~ 11, whereas h-Y(OH)3 exhibits a weakly positive ζ potential only for pH < 4. As the cationic nature of the hydroxide layers is expected to result in effective electrostatic interactions with anions, it is challenging to make l-Y(OH)3 an effective phosphate adsorbent over a wide pH range. 3.2. Reaction of l-Y(OH)3 in Aqueous Phosphate Solutions. To evaluate the phosphate uptake behaviors, the adsorption reaction of l-Y(OH)3 (0.5 g/L) was performed in 500 mg-P/L aqueous phosphate solutions at initial pH values of 5, 7, and 9. XRD patterns of l-Y(OH)3 before and after reactions for 24 h are shown in Figures S1b – S1e. Compared with the (00l) reflections of lY(OH)3 before the reaction, no shift in position was observed after the adsorption reaction at different pHs, indicating that the basal spacing of l-Y(OH)3 remains unchanged after the uptake of phosphate. If the hydroxide ion was replaced by the larger phosphate ion in the interlayer space, the basal spacing would have increased. Accordingly, a simple exchange reaction between two types of anions in the interlayer space of l-Y(OH)3 does not explain why the interlayer distance remains unchanged. Although exchange reactions with organic and inorganic anions generally occur in the interlayer galleries of LDHs and LRHs, we did not find any direct evidence in this work that proves inclusion of phosphate anions in the interlayer space of lY(OH)3. This result agrees with a previous report that the layered polymorph of rare earth hydroxides shows no exchange reactivity with other organic and inorganic anions.39 The release of unnecessary and potentially toxic anions, which can accompany phosphate adsorption, is


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undesirable for environmental applications. Therefore, the lack of topotactic incorporation into the interlayer space is one of the important advantages of l-Y(OH)3. We therefore supposed that the phosphate uptake by l-Y(OH)3 takes place through both (i) the formation of outer-sphere complexes by strong electrostatic interactions between phosphates and yttrium hydroxide layers with high positive charge density and (ii) the formation of inner-sphere (i.e., chemisorbed) complexes, in which surface Y–OH bonds are broken and new Y–O–P bonds are formed, as schematically illustrated in Figure 2. Phosphate adsorption onto many metal oxides and metal oxide hydroxide adsorbents has been explained by a similar surface complexation mechanism.11,47,48 The change in solution pH was monitored to confirm the release of OH− groups during the phosphate adsorption reaction. As shown in Figure 3a, when 0.5 g/L of l-Y(OH)3 was used, the solution pH only slightly increased after the 24 h adsorption reaction in 500 mg-P/L phosphate solutions at initial pH values of 5, 7, and 9. However, when 1.5 g/L of l-Y(OH)3 was used, the solution pH increased up to ~ 10 after reaction in the same phosphate solutions regardless of initial pH. Thus, when the amount of released OH− ions exceeded the buffer capacity of the phosphate solution, the solution pH significantly increased as the sorption reaction continued (Figure 3b). This observation supports the inner-sphere complexation mechanism, which causes a release of OH− ions into the solution. Although μ3-OH groups in an edge-sharing octahedral network (as in FeOOH) are unfavorable for exchange because three Fe–O bonds should be broken for adsorption,49 the high Lewis acidity and high coordination number polyhedral network consisting of relatively weak Y–OH bonds (as in l-Y(OH)3) could facilitate the formation of inner-sphere complexes via ligand exchange reactions at the surface μ3-OH sites (Figure 2a). Despite this structural difference, phosphate adsorption on l-Y(OH)3 by μ3-OH



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exchange at the surface was expected to be slower and more gradual than μ2-OH or -OH exchange. 3.3. Adsorption Isotherms and Kinetic Studies. l-Y(OH)3 adsorbents for practical applications were prepared in aqueous solutions at room temperature according to our previous work.42 This room temperature route could be adopted as an effective method of large-scale production of layered rare earth hydroxide adsorbents. The adsorption isotherms of phosphate on l-Y(OH)3 at different pHs are presented in Figure 4. Observed curves at initial pH values of 5, 7, and 9 were analyzed using a Langmuir model:50 qe = qmKLCe/(1 + KLCe) and Freundlich model:51 qe = KFCe1/n Here, Ce (mg/L) and qe (mg/g) are the equilibrium concentration and amount of phosphate adsorbed, respectively. KL(L/mg) is the constant related to the affinity for binding sites, qm (mg/g) is the maximum adsorption capacity per unit mass of adsorbent to form a complete monolayer on the surface, KF((mg/g)(L/mg)1/n) is the Freundlich constant (related to adsorption capacity of the adsorbent), and n is the adsorption density. The parameters obtained by fitting the experimental data are listed in Table S1. Their fitting curves are compared in Figure 4. The regression coefficients indicated that phosphate adsorption on l-Y(OH)3 follows the Langmuir isotherm model at all tested pH values, which assumes a monolayer adsorption process. The maximum phosphate monolayer adsorption capacities (qm) for l-Y(OH)3 derived from the Langmuir model are 90.8, 75.9, and 49.6 mg-P/g at initial pH values of 5, 7, and 9, respectively. These values are


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close to those of Cl−-type MgAl-LDH with low selectivity toward phosphate ions and poor recyclability.21,52,53 Considering very low solubility of YPO4, very high affinity for phosphate could be one of reasons for the large adsorption capacity of l-Y(OH)3. Based on the preliminary experiment, the nature of the rare earth in the hydroxocation layer is crucial for determining the phosphate adsorption reaction rate as well as the capacity. For example, the obtained qm of lGd(OH)3 (~ 170 mg-P/g) at initial pH = 5 was much larger than that of l-Y(OH)3 at the same pH value. Comparative phosphate adsorption studies will be extended using l-RE(OH)3 series and reported elsewhere. Batch experiments were performed to evaluate the rate of phosphate adsorption on l-Y(OH)3. Figure 5 shows the adsorption behavior of l-Y(OH)3 (dose = 0.1 g) as a function of reaction time in 200 mg-P/L aqueous phosphate solutions at different initial pHs. The kinetic curves show a fast initial phosphate uptake followed by a slower process to reach complete equilibrium after 24 h regardless of initial pH. This curve shape agrees with the general time-dependent adsorption behavior of various adsorbents. The experimental amount of phosphate adsorbed on l-Y(OH)3 near the plateau is around 86, 78, and 52 mg-P/g at initial pH values of 5, 7, and 9, respectively. These values are consistent with maximum adsorption capacities expected by the Langmuir isotherm model (Table S1). Although HPO42− ions (rather than H2PO4− ions) are the primary species in solutions in the 7 to 12 pH range,54 the adsorption process occurs efficiently despite the relatively high pHs. Thus, it is proposed that the high pHpzc (~ 11) of l-Y(OH)3 enhances the phosphate adsorption ability over a wide range of pH even in high phosphate concentrations. The pseudo-first-order model: qt = qe – qe-k1t



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and pseudo-second-order model:55 qt = k2qe2t/(1 + k2qet) were applied to fit the kinetic data of phosphate adsorption on l-Y(OH)3, where qe and qt are the amount of phosphate adsorbed on an adsorbent (mg/g) at equilibrium and at reaction time t (h), respectively, and k1(h−1) and k2(g/mg/h) are the adsorption rate constants for the pseudo-firstorder model and the pseudo-second-order model. The obtained kinetic parameters are summarized in Table S2, and the corresponding fitting curves are displayed in Figure 5. The regression coefficients indicate that the kinetic data are better described by the pseudo-secondorder kinetic model, and consequently the replacement of –OH by phosphate ion to coordinate to yttrium (chemisorption process) would dominate the adsorption of phosphate on l-Y(OH)3. The initial phosphate adsorption rate (h = k2qe2) was higher at lower pH, as is generally observed. 3.4. Photoluminescence Spectra of l-Y0.9Ce0.1-xTbx(OH)3 before and after Phosphate Adsorption. Compared with other metal hydroxides developed for phosphate adsorption, the lRE(OH)3 (RE = rare earths) series provides the advantage of a characteristic luminescence depending on the nature of doped-activator ions and the polyhedral surroundings of the rare earth elements. Therefore, we propose that photoluminescence spectroscopy can be a useful technique with which to investigate the phosphate adsorption mechanism. Furthermore, the variation in luminescence associated with phosphate adsorption could be used as a detection method to monitor and maintain phosphorus concentrations in environmental waters at lower than desired levels. If we consider a possible inner-sphere complexing mechanism by the ligand exchange reaction between a surface –OH group and phosphate anions, the adsorption reaction should be


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accompanied by the formation of a Y3+–O–P covalent bond. To verify the influence of such bonds on the luminescence characteristics, Ce3+ and Tb3+ co-doped YPO4 was prepared and examined by measuring its photoexcitation and emission spectra. The 4f-5d transition of the Ce3+ ion is frequently used for sensitization to overcome the weak parity forbidden intra-4f transition of the Tb3+ ion.56,57 Energy transfer between Ce3+ and Tb3+ ions occurs effectively in many host materials such as borates, silicates, and phosphates.58,59 As shown in Figure S2, when monitored at 544 nm (5D4 → 7F5 emission of Tb3+), Y0.9Ce0.05Tb0.05PO4 showed strong f-d transitions of Ce3+ in the 250 – 350 nm range in addition to weak f-f transitions of Tb3+ ions in the 350 – 400 nm range. Observation of the broadband Ce3+ excitation in the spectrum monitored at Tb3+ implies an efficient energy transfer from Ce3+ to Tb3+ ions in the YPO4 matrix.60 The green emission of Tb3+ originating from Ce3+-Tb3+ energy transfer excitation (λex = 312 nm) is much stronger than that from the forbidden f-f transitions of Tb3+. Therefore, we envisioned that Ce3+ and Tb3+ co-doped l-Y(OH)3 would show a green luminescence if a Y–O–P bond is formed during the phosphate adsorption. In this work, l-Y0.9Ce0.1-xTbx(OH)3 solid solutions (x = 0.0, 0.025, 0.05, 0.075, and 0.1) were prepared, and their photoluminescence spectra were compared as a function of Ce3+/Tb3+ ratio before and after the adsorption reaction in aqueous phosphate solutions. No difference was observed in XRD patterns of l-Y0.9Ce0.1-xTbx(OH)3 solid solutions (Figure S3), confirming that no significant structural modification was induced by Ce3+ and Tb3+ doping. As shown in Figure S4, essentially no emission was observed in the spectra of lY0.9Ce0.1-xTbx(OH)3 despite a slight variation depending on the amount of doped Tb3+ (due to the forbidden f-f transition of Tb3+). In contrast, after the phosphate adsorption reaction, the excitation of Ce3+ ions using a commercial 312 nm UV-lamp resulted in a series of characteristic 5

D4 → 7FJ (J = 6, 5, and 4) emissions of Tb3+ ions in the emission spectra of l-Y0.9Ce0.1-



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xTbx(OH)3.

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This green emission, which was attributed to the energy transfer from Ce3+ to Tb3+

ions in the same matrix, can be facilitated by the phosphate groups in the coordination sphere. The simple electrostatic interaction between phosphate anions and hydroxocation layers of lY0.9Ce0.1-xTbx(OH)3 would not explain the observed green emission after the adsorption reaction. This ‘luminescence-on’ behavior consequently indicates that phosphate is directly coordinated to Y3+ (Ce3+ and Tb3+) by replacement of hydroxyl groups on the l-Y0.9Ce0.1-xTbx(OH)3 surface. A similar replacement of surface hydroxyl groups by phosphates was also confirmed by previous FT-IR spectroscopic studies for various adsorbents.61-63 Thus, the photoluminescence spectroscopic studies further support the inner-sphere complexing mechanism involving the formation of Y(Ce, Tb)–O–P bonds through which the energy transfer can occur. 3.5. Application of l-Y(OH)3:Ce,Tb to Detect Phosphate at Low Concentrations. If the coordination of phosphate ions to RE3+ is accomplished, adsorption-sensitive variation in photoluminescence could be employed for environmental water remediation. As the maximal difference in emission intensity before and after phosphate adsorption was observed at x ~ 0.05 (Figure S4b), the ‘luminescence-on’ behavior of l-Y0.9Ce0.05Tb0.05(OH)3 (l-Y(OH)3:Ce,Tb) by phosphate adsorption was systematically evaluated to detect phosphorus at low concentrations in aqueous solution. When different doses (0.05 – 0.5 g/L) of l-Y(OH)3:Ce,Tb adsorbent were examined in a 2.0 mg-P/L solution (20 mL), which is in the concentration range of typical raw wastewaters,64,65 phosphate removal was efficient regardless of solution pH. More than 98% of phosphate was adsorbed within 1 h, with practically complete removal of phosphates at pH < 9. For instance, a low adsorbent dose such as 0.05 g/L could reduce the phosphate concentration in effluent to lower than 10 μg-P/L at pH = 7. The photoluminescence spectra of l-Y(OH)3:Ce,Tb powders filtered from mixtures were compared to assess the emission behavior as a function of


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adsorbed concentration of phosphates. The relationship between the dosage and the emission intensity of l-Y(OH)3:Ce,Tb at pH = 7 is shown in Figure 6. It is evident that the phosphate adsorption causes the ‘luminescence-on’ of l-Y(OH)3:Ce,Tb adsorbent. Even emission intensity after only 4 mg-P/g adsorption (0.5 g/L dose) is distinguished from that before adsorption. When we increased the adsorbed phosphate/adsorbent weight ratios, the emission intensity was continuously enhanced in proportion to phosphate concentrations until the surface adsorption sites were saturated (~80 mg/g at pH = 7). Further increase of this weight ratio led to a constant intensity within experimental error. When we used a commercial 312 nm-UV lamp, the ‘luminescence-on’ of l-Y(OH)3:Ce,Tb adsorbent by phosphate adsorption was visually observable, as demonstrated in Figure 6. While l-Y(OH)3:Ce,Tb adsorbent showed no discernible color before phosphate adsorption, a characteristic green light was emitted after the adsorption reaction of 0.05 – 0.1 g/L adsorbent in 2.0 mg-P/L aqueous phosphate solution, whose brightness increased as a function of the adsorbed phosphate/adsorbent weight ratio. Therefore, this simple and convenient ‘luminescence-on’ system could be exploited as a detector to simply ascertain the presence or the approximate concentration of phosphorus. 3.6. Interference of Common Anions. The effect of coexisting anions on phosphate recovery by l-Y(OH)3 was examined in aqueous solutions containing Cl−, NO3−, SO42−, and HCO3− anions commonly found in most wastewaters. In particular, these anions are generally sensitive to the ion-exchange reaction in the interlayer galleries of LRHs. Therefore, if we used RE2(OH)5X·nH2O (X− = Cl− and NO3−) as adsorbents for phosphate removal or recovery, we could not avoid the release of X− anions from the interlayer space by the ion-exchange reaction with these common anions. Similarly, even though LDHs show high phosphate adsorption capacity, they often have low selectivity toward phosphate ions because a considerable interlayer



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exchange reaction occurs with other common anions.66,67 One of the important advantages of lRE(OH)3 is that no interlayer reaction with common organic and inorganic anions (and consequently no release of X− anions) occurs during the operation. To ensure high selectivity of l-Y(OH)3 for phosphates, the influence of competing anions on phosphate adsorption was evaluated by adding the same and 5 and 10 times larger molar amounts of NaCl, NaNO3, Na2SO4, and NaHCO3 into the KH2PO4 (2.0 mg-P/L or 0.065 mM) solution at pH ∼7. As shown in Figure 7, no significant difference in phosphate adsorption was observed in the presence of the tested anions even at 10 times higher concentrations than phosphate. The phosphate recovery was higher than 98.7% of the value obtained in the solution containing phosphate only. Even in the simultaneous presence of all tested anions, the phosphate adsorption was practically complete (Table S3). In contrast, the concentration differences of competing anions before and after adsorption reactions were less than 2% even in solution containing 10 times larger molar amounts than phosphate, indicating high selectivity of l-Y(OH)3 toward phosphate. Considering that phosphate adsorption can transform rare earth adsorbents to corresponding phosphates with very low solubility,68 a high affinity for phosphate would allow lY(OH)3 to exhibit a high selectivity toward phosphate ions in a solution containing many interfering anions. Furthermore, the unavailability of interlayer exchange reaction with competing anions could be also responsible for the high selectivity of l-Y(OH)3. As a consequence, coexisting anions do not significantly interfere with phosphate recovery when using l-Y(OH)3 adsorbent. The phosphate recovery efficiency of l-Y(OH)3 was also compared in deionized water (DW), mineral water (MW), tap water (TW), and river water (RW). The pH of all water samples was in


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the neutral range of 6.45 – 7.2, and 0.5 g/L of l-Y(OH)3 adsorbent was used. Essentially no phosphate was detectable in tap water and mineral water (< 10 μg-P/L), whereas the concentration of phosphate in river water was relatively high and close to 102 μg-P/L. When the adsorption of l-Y(OH)3 was performed in 2.0 mg-P/L solutions prepared with corresponding waters, practically complete recovery of phosphate (99.5 – 105%) was successfully accomplished in all solutions in 1 h, as shown in Figure 8. The phosphate concentration in filtrate from a river water solution was also below the experimental error level of the present method, indicating the recovery of even 102 μg-P/L pre-existing phosphate. The luminescence spectral selectivity for phosphate was also assessed by measuring the emission spectra to compare the relative 5D4 → 7FJ emission intensities of Tb3+ ions after reacting l-Y(OH)3:Ce,Tb in 2.0 mg-P/L solutions prepared with deionized water, tap water, river water, and mineral water. As shown in Figure 9a, the emission from l-Y(OH)3:Ce,Tb was significantly enhanced (‘luminescence-on’) after adsorption reactions in all kinds of solutions, and their intensities were quite similar to one another. The green color of phosphate adsorbed lY(OH)3:Ce,Tb under a commercial 312 nm-UV lamp was sufficiently distinguished with the naked eye (Figure 9b). In conclusion, no significant interference was caused even in tap water or river water. Thus, l-Y(OH)3 and l-Y(OH)3:Ce,Tb exhibited great potential for efficient recovery and monitoring of phosphate anions in environmental waters. 3.7. Desorption and Regeneration Studies. Although several advantages such as costeffectiveness, simplicity, and flexibility in industrial operations make the adsorption method one of the best candidates in comparison with other available technologies, used adsorbents must be regenerated and recycled several times. However, the desorption process is often unsuccessful to restore the adsorbents to their original states, and many sorbent materials were found to be



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unsatisfactory in terms of reusability. Complete desorption of phosphates from l-Y(OH)3 powder was also difficult even in concentrated NaOH solutions at room temperature, but the desorption efficiency was close to 96% after 48 h and > 98% after 4 days in preliminary experiments. To assess reusability, the phosphate-adsorbed l-Y(OH)3:Ce,Tb samples were prepared by reacting in 200 mg-P/L phosphate solutions at initial pH ~ 7. In recycling experiments, desorption reactions were performed in a 1.0 M NaOH solution for 48 h at room temperature. After filtration, lY(OH)3:Ce,Tb powder was reused. Figure 10a shows the variation in adsorption and desorption efficiency during the recycle processes at room temperature. Although the capacity of phosphate adsorption was somewhat decreased with increasing adsorption/desorption processes, 5 times regenerated adsorbent still exhibited ~ 90 % adsorption efficiency. While the strong interaction between yttrium and phosphate ions is beneficial for capacity and selectivity, the high affinity toward phosphate ions would be unfavorable for the desorption of phosphates to regenerate lY(OH)3 adsorbent. Thus, desorption of the phosphate coordinated to rare earths in mild conditions is rather slow. Fortunately, the stable characteristics of l-Y(OH)3 under basic conditions allow the desorption of phosphate at higher temperature without any loss or modification of its structure. Figure 10b compares the amount of adsorbed phosphate over 5 adsorption/desorption cycles when desorption reaction was performed at 100°C for 2h. In addition to the fairly improved desorption efficiency, the adsorption efficiency of ~ 94 % observed after 5 cycles indicates no significant performance degradation. As shown in Table S4, essentially no metal leakage from adsorbent was observed in ICP analyses for filtered solutions after adsorption and desorption reactions. Good reusability of l-Y(OH)3:Ce,Tb adsorbent was also demonstrated in the photoluminescence spectra measured during the recycling test. The similar ‘luminescence-on’ and ‘luminescence-off’ intensities before and after 5 regenerations


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(Figure S5) would ensure the remaining good performance of l-Y(OH)3:Ce,Tb powder for phosphate adsorption/desorption recycling.

4. CONCLUSIONS We demonstrated the high performance of l-Y(OH)3 as a representative example of the layered rare earth hydroxide family for removal of phosphate from aqueous solutions. l-Y(OH)3 has desirable properties for water treatment such as (1) a high capacity for phosphate anions over a wide pH range, (2) no leakage of rare earth or other anions during operation, (3) high affinity of rare earths for binding phosphate anions, (4) high efficiency phosphate desorption to regenerate the adsorbent, and (5) performance that is insensitive to the presence of common anions in the environment (high selectivity). In particular, the sensitive “luminescence-on” behavior of lY(OH)3:Ce,Tb during phosphate adsorption is very useful to monitor the phosphate concentration in water to a limited level. Furthermore, the simple and low cost ‘aqueous solution at room temperature’ route for the synthesis of l-Y(OH)3 could be exploited as a means of large scale production in real waste and surface water treatment applications.



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FIGURES

Figure 1. pH dependence of the zeta potentials of (a) h-Y(OH)3 and (b) l-Y(OH)3.

Figure 2. Comparison of idealized structures of (a) l-Y(OH)3 and (b) h-Y(OH)3. Schematic representation of the adsorption of (c) H2PO4− and (d) HPO42− ions on l-Y(OH)3 sheets by complexation with release of OH−.


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Figure 3. (a) Solution pHs after the phosphate adsorption reaction on 0.5 g/L and 1.5 g/L of lY(OH)3 at initial pH values of 5, 7, and 9 for 24 h. (initial [P] = 500 mg/L) (b) Change of solution pH during the phosphate adsorption reaction on l-Y(OH)3 for initial pH values of 5, 7, and 9 for 24 h (initial [P] = 500 mg/L, adsorbent dose = 1.5 g/L).



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Figure 4. Adsorption isotherms of phosphate on l-Y(OH)3 at different pHs. Dashed and solid lines are the corresponding fitting curves based on the Freundlich and Langmuir equations, respectively. (initial [P] = 2.5 – 100 mg/L; adsorbent dosage = 0.1 g/L; solution volume = 200 mL; contact time = 24 h).

Figure 5. Adsorption kinetics on l-Y(OH)3 in 200 mg-P/L aqueous phosphate solutions at initial pH values of 5, 7, and 9. Dashed and solid lines are the corresponding fitting curves based on the pseudo-first-order and pseudo-second-order equations, respectively.


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Figure 6. Photoemission spectra and photographs under day light and 312 nm-UV irradiation of l-Y(OH)3:Ce,Tb (a) before and after phosphate adsorption reactions with (b) 0.5, (c) 0.1, (d) 0.075, and (e) 0.05 g/L dosages in a 2.0 mg-P/L solution.

Figure 7. Interference effects of common anions on the phosphate adsorption on l-Y(OH)3 adsorbent. The residual phosphate concentrations in filtrates were measured after adsorption reactions in 0.065 mM (2.0 mg-P/L) KH2PO4 solutions containing (a) 0.065, (b) 0.325, and (c) 0.65 mM Cl−, NO3−, SO42−, and HCO3− competing anions for 1 h.



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Figure 8. Phosphate adsorption efficiency in 2.0 mg-P/L solutions prepared with deionized water (DW), tap water (TW), mineral water (MW), and river water (RW) for 1 h.

Figure 9. (a) Photoemission spectra and (b) photographs under day light and 312 nm-UV irradiation of l-Y(OH)3:Ce,Tb before and after phosphate adsorption reaction in 2.0 mg-P/L phosphate solutions prepared with deionized water (DW), tap water (TW), mineral water (MW), and river water (RW).


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Figure 10. Amount of phosphate (q) adsorbed on l-Y(OH)3:Ce,Tb over 5 adsorption/desorption cycles when the desorption process was performed in a 1.0 M NaOH solution (a) for 48 h at room temperature and (b) for 2 h at 100℃. (initial [P] = 200 mg/L, adsorbent dose = 0.5 g/L, pH = 7).



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ASSOCIATED CONTENT Supporting Information. Tables of isotherm and kinetic parameters. Tables of analytical data. Excitation and emission spectra of Y0.9Ce0.05Tb0.05PO4 and Y0.9Ce0.1-xTbx (OH)3 solid solutions. XRD patterns of l-Y0.9Ce0.1-xTbx (OH)3 solid solutions. Emission spectra after 1st and 5th regenerated of l-Y(OH)3,:Ce,Tb. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions ‡ These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (No. NRF2017R1A2B4007178).


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SYNOPSIS The “luminescence-on” behavior of layered yttrium hydroxide, l-Y(OH)3:Ce,Tb, by phosphate adsorption was employed to detect and recover phosphorus at low concentrations in deionized water, mineral water, tap water, and river water.


Layered Yttrium Hydroxide l-Y(OH)3 Luminescent Adsorbent for

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