Hydrothermal Fabrication of Zirconium Oxyhydroxide Capped

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Materials and Interfaces

Hydrothermal Fabrication of Zirconium Oxyhydroxide Capped Chitosan/ Kaolin Framework for Highly Selective Nitrate and Phosphate Retention Ilango Aswin Kumar, and Natrayasamy Viswanathan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01859 • Publication Date (Web): 26 Sep 2018 Downloaded from http://pubs.acs.org on September 29, 2018

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Industrial & Engineering Chemistry Research

Hydrothermal Fabrication of Zirconium Oxyhydroxide Capped Chitosan/Kaolin Framework for Highly Selective Nitrate and Phosphate Retention Ilango Aswin Kumar and Natrayasamy Viswanathan* Department of Chemistry, Anna University, University College of Engineering - Dindigul, Reddiyarchatram, Dindigul -624 622, Tamilnadu, India.

*

Corresponding author. Tel.: +91-451-2554066; fax: +91-451-2554066.

E-mail address: [email protected] (N. Viswanathan)

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Industrial & Engineering Chemistry Research 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

Abstract In recent scenario, hydrothermal technology has paid more attention due to its unique advantages like large surface area and high porous nature. In the present study, chitosan (CS) embedded kaolin (KN) clay namely CSKN composite was prepared. To enhance the sorption capacity (SC) and selectivity, zirconium oxyhydroxide (ZrO(OH)2) was coated onto CSKN composite to get Zr@CSKN composite using in situ precipitation (In situ) and hydrothermal (Hydro) methods. The developed Zr@CSKN composite was utilized towards nitrate and phosphate removal which helps in the minimization of methemoglobinemia (blue baby syndrome) and eutrophication. The adsorbents were characterized using FTIR, XRD, SEM, BET and EDAX with mapping analysis. Various adsorption influencing factors like contact time, adsorbent dosage, pH, co-anions, initial ion concentration and temperature was optimized. The experimental data was fitted with various isotherms and thermodynamic parameters to find the nature of adsorption. The reuse and field studies of Zr@CSKN composite were also examined. Keywords: Chitosan; Kaolin; ZrO(OH)2; In Situ; Hydrothermal; Nitrate; Phosphate; Adsorption.


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1. Introduction Nitrogen and phosphorus are the significant nutrients for the growth and reproduction of plants and other living organisms.1 But the excess nitrate content leads to methemoglobinemia (blue baby syndrome) whereas excess phosphate and nitrate contents leads to eutrophication. The tolerance limit of nitrate and phosphate contents in the drinking water are 40 and < 0.5 mg L−1 respectively.2,3 The widespread use of nitrate and phosphate has tuned the researchers to develop the new approaches for the removal of nitrate and phosphate. Several strategies have been developed towards the proficient adsorption of nitrate and phosphate including chemical reduction,4 biological process,5 adsorption,6 bioelectrochemistry,7 membrane process,8 ionexchange9 and chemical precipitation.10 Among them, adsorption is mostly preferred due to their higher removal efficiency, low-cost economy and easily operated.11 In recent years, the commercial clay materials such as bentonite, fly ash, montmorillonite, kaolin, etc., have been applied for toxic ions removal.12,13 Kaolin(KN) is a low-cost and most abundant inorganic clay mineral having structural formula of Al2Si2O5(OH)4 and the elemental constituents are SiO2 (45.68%), Al2O3 (40.45%), and H2O (13.87%) respectively.14 The silicate layer of KN clay hold the isomorphous substitutions of Si4+ by Al3+ which helps in the removal of nitrate and phosphate. In addition, the interlayer sites of KN clay occupied by easily exchangeable OH groups.15 Nevertheless, KN clay has some bottlenecks like pressure drop during filtration, low specific surface area, and low adsorption capacity.16 To prevail such troubles, various biopolymer encapsulated clay composite have been prepared and utilized to remove the toxic contaminants from drinking/wastewater.17 Chitosan (CS) is a natural polysaccharide derived from the deacetylation of chitin.18 The functional groups of CS such as hydroxyl (-OH) and amino (-NH2) could act as the potential chelating sites to adsorb the various toxic ions.19,20 To improve the nitrate and phosphate sorption capacity (SC), CS undergoes 3 ACS Paragon Plus Environment


chemical

modifications

like

cross-linking,21 grafting,22

functionalization,23 metal

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ion

assimilation,24 biocomposite with porous framework,25 etc. Hence, kaolin clay was dispersed into chitosan matrix to form CSKN composite. Zirconium oxyhydroxide (ZrO(OH)2) is an inorganic material which is non-toxic, chemically stable and insoluble in water. The incorporation of ZrO(OH)2 on various biopolymers have been utilized for fluoride removal.26,27 In the literature, it is also reported that the higher valence metal ions like Zr4+, La3+, Ce3+, Fe3+, etc., have greater tendency to attract the negatively charged anions towards itself. The coating of ZrO(OH)2 on CSKN composite which gives Zr@CSKN composite with more available reactive sites for both nitrate and phosphate removal. In addition, Zr4+considered as hard acid whereas nitrate and phosphate are considered as hard bases. According to HSAB concept, the hard acid (Zr4+) present in Zr@CSKN composite gets readily binds with hard bases like nitrate and phosphate.28,29 Recently, the adsorbents synthesized by hydrothermal (Hydro) technique using autoclave possess unique advantages like high surface area, high micropore volume, highly porous nature, definite shape and surface which enhance the adsorption capacity of the adsorbents.30-32 Hence, the present investigations focus on the fabrication of Zr@CSKN composite by in situ precipitation and hydro methods for nitrate and phosphate removal. A comparative evaluation of the prepared adsorbents by both in situ and hydro methods was carried out. The experiments were performed to optimize the numerous adsorption affecting factors includes contact time, adsorbent dosage, solution pH, co-anions, initial adsorbate concentration and temperature. The recyclable performance of Zr@CSKN composite was carried out. The field trial aptness of the prepared Zr@CSKN composite by both methods was tested for nitrate and phosphate contaminated water sample and attains good results which ensuring its suitability at field conditions. 4 ACS Paragon Plus Environment

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2. Experimental Section 2.1. Materials Chitosan (85% of deacetylated) with average molecular weight of 25 kDa was purchased from Pelican Biotech and Chemicals Labs, India. Kaolin clay, ZrOCl2·8H2O (98.0%), CH3COOH (99-100%), HCl (35-38%), NaOH (≥98.0%), ammonium heptamolybdate tetrahydrate (≥99.0%) and ammonium metavanadate (98.0%) were procured from Merck, India. The standard nitrate and phosphate stock solution was prepared by dissolving about 1.6305 g of anhydrous potassium nitrate (≥98.0%) and 1.4329 g of anhydrous potassium dihydrogen phosphate (≥98.0%) in 1000 mL of double distilled water individually. All other chemicals were applied in AR grade. 2.2. Synthesis of the Adsorbents 2.2.1. Synthesis of Zirconium Oxyhydroxide (ZrO(OH)2) Briefly, 2 g of ZrOCl2·8H2O was dissolved in 100 mL of double distilled water. About 50 mL of 0.1 M NaOH was taken in the burette was slowly dropped into Zr4+ solution and magnetically stirred for 15 min. After reaching basic pH~10, it produces a white precipitate of ZrO(OH)2 and the resultant suspension was aged for 24 h. For hydrothermal synthesis, ZrO(OH)2 slurry was transferred into teflon covered autoclave by maintaining temperature at 120 °C for 3 h. The obtained ZrO(OH)2 precipitate by both in situ and hydro methods was dried in hot air oven at 70 °C for 2 h followed by grinding into fine particles using ball mill (IKA, Germany) for nitrate and phosphate adsorption studies. 2.2.2. Synthesis of Chitosan/Kaolin (CSKN) Composite About 2 g of chitosan (CS) was dissolved using 2% of glacial CH3COOH. About 10 g of kaolin (KN) clay was dispersed in the small quantity of double distilled water and then poured into CS solution. The mixture was stirred vigorously for 2 h in order to get chitosan/kaolin 5 ACS Paragon Plus Environment


(CSKN) composite slurry. Then, CSKN composite slurry was aged for 24 h in the mother liquid. For hydrothermal synthesis, CSKN composite slurry was transferred into teflon covered autoclave and heated to 120 °C by maintaining the same temperature for 6 h. The obtained CSKN composite by both in situ and hydro methods was dried in hot air oven at 70 °C for 2 h. The dried CSKN composite was crushed into fine particles using ball mill (IKA, Germany) and utilized for nitrate and phosphate adsorption studies. 2.2.3. Synthesis of ZrO(OH)2 Coated CSKN (Zr@CSKN) Composite About 2% of ZrOCl2·8H2O solution was slowly added into CSKN composite slurry and continuously stirred for 2 h. Further, 5 mL of 0.1 M NaOH was slowly added into Zr4+ surrounded CSKN composite solution till pH~10 in order to get Zr@CSKN composite. The obtained Zr@CSKN composite slurry was aged for 24 h in the mother liquid. For hydrothermal synthesis, Zr@CSKN composite slurry was transferred into teflon covered autoclave and heated to 120 °C by maintaining the same temperature for 6 h. The obtained Zr@CSKN composite by both in situ and hydro methods was dried in hot air oven at 70 °C for 2 h. The dried Zr@CSKN composite was crushed into fine particles using ball mill (IKA, Germany) and used for nitrate and phosphate adsorption studies. 2.3. Batch Adsorptive Studies The nitrate and phosphate adsorption experiments were made using batch studies. About 0.1 g of adsorbent was added into 50 mL of 100 mg L-1 of initial ion concentration of nitrate/phosphate solution taken in the iodine flask. The reaction content was shaken using mechanical shaker by diverged time period of 10-60 min followed by the filtered adsorbate concentration was analyzed using UV-Visible spectrophotometer. The various adsorption influencing factors such as contact time, co-anions, dosage, solution pH, initial ion concentration and temperature was carried out to optimize the maximum nitrate and phosphate SC. To 6 ACS Paragon Plus Environment

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determine the optimum pH for the adsorption studies, the pH of nitrate and phosphate solution was adjusted between 3 and 11 using 0.1 M of HCl/NaOH. The adsorption isotherms were examined with four typical initial ion concentration of nitrate and phosphate solution as 80, 100, 120 and 140 mgL-1 with the assorted reaction temperature at 303, 313 and 323 K. The nitrate and phosphate SCs of the sorbent can be estimated by the mass balance eq 1 as follows sorption capacity (SC) =

C i − Ce V mg g-1 m

(1)

where Ci and Ce is the initial and final concentration of nitrate and phosphate at equilibrium rate (mg L-1), V is the solution volume (L) and m is the adsorbent dosage (g). 2.4. Analysis The

nitrate

and

phosphate

concentration

was

analyzed

using

UV-Visible

spectrophotometer kit (model: Spectroquant Pharo 300, Merck) at 202 and 400 nm respectively. The ammonium metavanadate and ammonium heptamolybdate tetrahydrate were used as the reagents to find the phosphate concentration in water. Thermo Orion Benchtop multiparameter kit (model VERSA STAR92) with pH electrode was used to measure the pH of solution. The pH drift method was conducted to find out the pH at zero point charge (pHzpc) of the adsorbent.33 The standard methods was followed to examine the other water quality parameters of drinking water like total hardness, chloride, dissolved oxygen and total dissolved solids.34 2.5. Characterization Studies The functional groups prediction analysis of the adsorbents was recorded using Fourier transform infrared (FTIR) spectrometer with a JASCO-460 plus model. BET surface analyzer with a NOVA 1000 model was applied to find out the textural properties of the adsorbent under nitrogen atmosphere condition. X-ray diffraction (XRD) study was carried out to find the crystalline nature of the adsorbent materials using X’pert 173 PRO model PAN-alytical 7 ACS Paragon Plus Environment


instrument. Scanning electron microscope (SEM) with a Vega3 Tescan model was used to notice the surface behavior of the prepared adsorbents. Energy dispersive X-ray analyzer (EDAX) with a Bruker Nano GMBH model was used for the detection of various elemental constituents of hydrothermal assisted Zr@CSKN composite and their nitrate/phosphate adsorbed composite. 2.6. Statistical Tools The Microcal Origin (version 15) software was utilized to compute all the experimental data. The significance of data inclination and integrity fit of the appropriate model was observed by standard deviation (sd), chi-square analysis (χ2) and regression correlation coefficient (r). 3. Results and Discussion 3.1. Effect of Contact Period of Time The effect of reaction contact time is the main function for the adsorption process in which the lowest time required for highest SC was identified. This study was investigated by adding 0.1 g of adsorbent into 50 mL of 100 mg L-1 nitrate and phosphate solution taken in iodine flask. Subsequently, the reaction mixture was shaken using mechanical shaker over the time period of 10 to 60 min followed by their filtrate concentration was analyzed using UVVisible spectrophotometer and the results are shown in Figures 1A and B. It was observed that initially the nitrate and phosphate SCs was increased rapidly by increasing the reaction time and then attains equilibrium. From Figure 1A, the nitrate SCs of ZrO(OH)2 (In situ), ZrO(OH)2 (Hydro), CS, KN clay, CSKN composite (In situ), CSKN composite (Hydro) and Zr@CSKN composite (In situ) was observed as 2.93, 3.58, 3.99, 13.84, 18.77, 21.34 and 29.81 mg g-1 at 40 min of equilibrium time. However, the phosphate SCs was observed as 5.99, 7.10, 7.82, 19.99, 24.70, 29.94 and 35.17 mg g-1 with the same 40 min equilibrium time are shown in Figure 1B. However, within 30 min, Zr@CSKN composite (Hydro) have reached saturation and possess an enhanced nitrate and phosphate SCs of 34.62 and 40.58 mg g−1 respectively. The 8 ACS Paragon Plus Environment

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results concluded that Zr@CSKN composite (Hydro) attains the equilibrium time rapidly with enhanced SC due to its high surface area, better pore size and pore volume.35 In addition, Zr@CSKN composite (In situ) also possess the appreciable SC. Taking these important concerns, further nitrate and phosphate adsorption studies have been focused on both Zr@CSKN composite (In situ) and Zr@CSKN composite (Hydro) with the equilibrium contact time of 40 and 30 min respectively. 3.2. Influence of Adsorbent Dosage The influence of adsorbent dosage on the adsorption of nitrate and phosphate was performed using 50 mL of 100 mg L-1 nitrate and phosphate solution with the varied dosage (0.25 to 0. 5 g) of Zr@CSKN composite prepared by both in situ and hydro methods with fixed contact time of 40 and 30 min respectively. The graphical results of the effect of adsorbent dosage on nitrate and phosphate uptake are shown in Figure 1C. It was observed that, the increase in SC with rise of adsorbent dosage from 0.25 to 1.5 g will enhance the number of active sites of the adsorbent.36 Nitrate removal was increased from 6.81 to 31.84 mg g1

andphosphate removal was increased from 10.42 to 37.77 mg g-1 for Zr@CSKN composite (In

situ). However there is a superior increase in nitrate and phosphate SC was observed from 7.93 to 36.41 mg g-1 and 13.85 to 42.16 mg g-1 for Zr@CSKN composite (Hydro). Besides, on increasing the adsorbent dosage after 0.1 g, there was no further significant uptake of both nitrate and phosphate was observed. Consequently, 0.1 g dosage of the prepared Zr@CSKN composite by both methods was utilized as optimum dosage for further adsorption studies.



Figure 1. Effect of contact time of (A,B) ZrO(OH)2 (In situ), ZrO(OH)2 (Hydro), chitosan, kaolin clay, CSKN composite (In situ), CSKN composite (Hydro), Zr@CSKN composite (In situ) and Zr@CSKN composite (Hydro) on the nitrate and phosphate SC respectively; Effect of the nitrate and phosphate SC on (C) adsorbent dosage; (D) initial adsorbate strength; (E) solution pH; and (F) co-ions of Zr@CSKN composite (In situ) and Zr@CSKN composite (Hydro). 10 ACS Paragon Plus Environment

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3.3. Influence of Initial Adsorbate Strength The influence of changing initial concentration of nitrate and phosphate in the range of 20 to 140 mg L−1 was investigated. A direct proportional correlation between the SC and the initial adsorbate concentration is shown in Figure 1D. The prepared Zr@CSKN composite by in situ and hydro methods exhibit an increased nitrate and phosphate SCs with increasing initial adsorbate concentration until the equilibrium was reached. The results showed that nitrate and phosphate SCs was increased from 7.87 to 31.35 mg g-1 and 11.78 to 38.17 mg g-1 for Zr@CSKN composite (In situ) similarly for Zr@CSKN composite (Hydro), the nitrate and phosphate SCs was increased 9.12 to 36.73 mg g-1 and 14.53 to 42.98 mg g-1 by raising the initial ion concentration from 20 to 140 mg L-1 respectively. It was mainly considered that the adsorption occurred rapidly upto the concentration ranges of 20 to 140 mg L-1 and almost reached the maximum SC value at 140 mg L-1. This performance may be attributed that the higher initial adsorbate concentration would readily fill up the available active sites of the adsorbent surface. This is because of the extend of mass transport energetic power during the increase of initial adsorbate concentration thereby nitrate and phosphate SCs increases.37 At the same time, it was observed that, most of the nitrate and phosphate species was occupied in all the existing active sites of the adsorbents with in 100 mg L-1. Further, increasing the adsorbate concentration after 100 mg L-1 only a slight increase in SC was observed. Hence, 100 mg L-1 of the initial nitrate and phosphate concentration was taken as optimum initial concentration. 3.4. Influence of pH The pH study was carried out to examine the influence of initial pH of nitrate and phosphate solution on its SC using the prepared Zr@CSKNcomposite by both methods. About 0.1 g of adsorbent was added into 50 mL of 100 mg L-1 nitrate and phosphate solution at different 11 ACS Paragon Plus Environment


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pH ranges from 3 to 11 using HCl/NaOH solution by keeping all other experimental conditions as constant. The results depicted in Figure 1E clears that the adsorption of nitrate and phosphate was highly dependent on the solution pH. The maximum adsorption behavior of the prepared Zr@CSKN composite by both methods for nitrate was occurred at pH 5. Nitrate is a univalent anion which could easily adsorb onto the protonated reactive cites of the composite surface via., electrostatic attraction. However, the higher nitrate adsorption was not supported at lower solution pH which is due to the presence of Cl- ions from the addition of HCl to alter the solution pH to acidic thereby it can easily compete with nitrate.38 After pH 7, the nitrate adsorption was decreased due to the existence of dominant hydroxyl (OH-) ions at basic pH condition. In the case of phosphate, the SC was initially increased by adjusting the pH from 3 to 7 and followed by a decrease in SC after pH 7. This is mainly due to the polyvalent nature of aqueous phosphate such as H3PO4 (pH < 2), H2PO4− (pH 2-7), HPO42− (pH7-11) and PO43− (pH > 11).39 The formation of aquatic phosphate in both acidic and basic environments are given by eq 2, 3, 4 and 5 as follows H2PO4- + H+

H3PO4 (pH < 2)

(2)

HPO42- + H+

H2PO4- (pH ~2-7)

(3)

H2PO4- + OH-

HPO42-+ H2O (pH ~7-11)

(4)

HPO4- + OH-

PO43-+ H2O (pH > 11)

(5)

Amongst, H2PO4− was primarily observed in acidic pH condition by the chemical reaction of HPO42− and H+ ion (cf. eq 3). In this situation the electrostatic binding attraction was formed between the protonated adsorbent surface and H2PO4-. Although, both PO43- and HPO42- are the most living species at basic pH condition, the presence of dominant OH- ions could act as the competitor for phosphate adsorption. The pHzpc of Zr@CSKN composite (In situ) and Zr@CSKN composite (Hydro) was found to be 5.21 and 5.92 which are shown in Figure S1.This 12 ACS Paragon Plus Environment

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result concludes that, if solution pH < pHzpc may increase the protonated active sites of Zr@CSKN composite surface which leads to the electrostatic attraction between the negatively charged both nitrate and phosphate with protonated Zr@CSKN composite. However, if solution pH > pHzpc, the formation of protonation of the adsorbent surface was declined which causes the electrostatic repulsion between negatively charged basic OH- ions with that of both nitrate and phosphate thereby the SC decreases. After the adsorption process, the equilibrium pH was tending towards neutral which shows the aptness of the prepared Zr@CSKN composite by both methods at varied pH situations. 3.5. Effect of Reaction Temperature In order to study the temperature effect of nitrate and phosphate solution, about 0.1 g of the prepared Zr@CSKN composite by in situ and hydro methods, was suspended into 50 mL of 100 mg L-1 individual nitrate and phosphate solution taken in iodine flask placed in the water bath shaker in the temperature ranges of 303, 313 and 323 K with fixed reaction contact time of 40 and 30 min respectively. The result depicted in Figure S2 suggests that the SC of the prepared composite was highly dependent on the nitrate and phosphate solution temperature. It was mainly identified that nitrate SC was slightly decreased from 29.81 to 26.83 mg g-1 and 34.62 to 32.74 mg g-1 on increasing the reaction temperature which indicates the exothermic nature of nitrate adsorption onto Zr@CSKN composite (In situ) and Zr@CSKN composite (Hydro) respectively.40 However, the slight increase in phosphate SC from 35.17 to 37.83 mg g-1 and from 40.58 to 43.01 mg g-1 on increasing the reaction temperature from 303 to 323 K which supports the endothermic nature of phosphate adsorption for both Zr@CSKN composite (In situ) and Zr@CSKN composite (Hydro) respectively.41



3.6. Influence of Rival Anions The various co-existing anions such as sulphate (SO42-), bicarbonate (HCO3-) and chloride (Cl-) in the natural water bodies are expected to often afflict with the reactive sites of the adsorbent and hence it influences during the adsorption of nitrate and phosphate. Herein, about 0.1 g of the prepared Zr@CSKN composite by in situ and hydro methods was added into 50 mL mixture containing 100 mg L-1 of respective nitrate and phosphate with that of 200 mg L-1 of respective SO43-, HCO3- and Cl- co-anions was taken in the iodine flask by keeping all other parameters as constant. The results pointed out that the co-existing anions had an evident effect on the nitrate and phosphate adsorption excluding bicarbonate in which its competing effect was found to be negligible which are shown in Figure 1F. Among the co-anions, the bonding electron charge cloud is maximum for sulphate results in the greater affinity to adsorb active sites of the adsorbent than nitrate. In the case of phosphate, both the chloride and sulphate possesses the competing effect and gets surrounded to the reactive sites of the adsorbent surface instead of phosphate. In addition, the similar anionic size of sulphate with that of phosphate results in the decrease of phosphate adsorption in water.42 3.7. Characterization Studies of the Adsorbents 3.7.1. FTIR Analysis The significant surface functional predictions of chitosan (CS), kaolin (KN) clay, hydrothermal assisted ZrO(OH)2, CSKN composite, Zr@CSKN composite after their nitrate and phosphate adsorption was corroborated by FTIR study and the spectral results are shown in Figures 2A and B. In FTIR spectra of chitosan, the predominant overlapping zone of a symmetrically stretching amine (-NH2) group with that of a stretching hydroxyl (-OH) group was appeared as a broad spectrum between 3605-3151 cm−1.43 The diminutive characteristic vibrant frequency bands of both asymmetric and symmetric C-H groups of chitosan was emerged at 14 ACS Paragon Plus Environment

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2929 and 2836 cm−1.44 The reactive functional groups of chitosan such as carbonyl (C=O) of the secondary amide (-CONHR) and also the protonated amine (-NH3+) possess the strong stretching frequencies at 1650 and 1415 cm−1.45 In addition, the characteristic stretching mode of C=O bounded both C-O-H and C-O-C linkages in chitosan was perceived at 1257 and 1026 cm−1 which are shown in Figure 2A.46

Figure 2.FTIR spectra of (A) chitosan, ZrO(OH)2, KN clay, hydrothermal assisted CSKN and Zr@CSKN; and (B) CSKN, nitrate sorbed CSKN, phosphate sorbed CSKN, Zr@CSKN, nitrate sorbed Zr@CSKN and phosphate sorbed Zr@CSKN composites. In FTIR spectra of fresh kaolin (KN) clay, the strong and sharp hydroxyl (-OH) stretching vibration bands at 3677 and 3429 cm-1 was observed which owed that -OH group is connected to Al octahedron plate of KN clay.47 The stretching vibration modes of Si-O and Si-OSi in KN clay was recorded at 1608 and 1049 cm-1. Furthermore, the bending vibration mode of OH in Al-OH was observed at 943 cm-1.48 The bands at 1391 and 1278 cm-1 are due to the symmetric and asymmetric stretching frequencies of Zr-O-H while 1030 cm-1 could be assigned to the bending vibrations of Zr-O and O-H in ZrO(OH)2. The main characteristic bands of



ZrO(OH)2 shows the appearance of a shoulder at 645 and 480 cm–1 which can be attributed for the vibrations of Zr-OH and Zr-O-Zr linkages correspondingly.49 All the individual FTIR bands of chitosan, KN clay and ZrO(OH)2 was retained in the hydrothermal prepared Zr@CSKN composite which are shown in Figure 2A. The significant shrink in the intensities of FTIR characteristic bands of both nitrate and phosphate sorbed CSKN and Zr@CSKN composites may suggest the occurrence of nitrate and phosphate adsorption which are shown in Figure 2B. In addition to the altered FTIR signals, the new bands was observed at 1032 and 560 cm−1 which may due to the asymmetric stretching and bending modes of the phosphate in phosphate sorbed CSKN and Zr@CSKN composites which also confirms the occurrence of phosphate adsorption in the prepared adsorbents.50 3.7.2. SEM with BET and EDAX with Mapping Images The surface nature of the prepared adsorbents was analyzed using SEM and EDAX with mapping studies and the results are revealed in Figures 3, 4 and S3. The average particle size of the prepared Zr@CSKN composite by in situ and hydro methods was measured at 50 µm using SEM and it was found to be 2.83 and 1.02 µm which denotes that Zr@CSKN composite (Hydro) possess lower particle size with higher specific surface area compared to Zr@CSKN composite (In situ) (cf. Figures 3A and B). Figure 3A represents the SEM image of irregular, less prickly and inflexed surface of Zr@CSKN composite (In situ) with less porous projection. However, Figure 3B shows the high prickly surface with narrow and needle like structure of Zr@CSKN composite (Hydro) with higher porous projection indicates the modification in the preparation of Zr@CSKN composite by hydro method. The significant changes in the morphologies with respect to the smoother surface of both nitrate and phosphate sorbed Zr@CSKN composite (Hydro) may confirms the occurrence of nitrate and phosphate adsorption onto Zr@CSKN composite (Hydro) (cf. Figures 3C and D). 16 ACS Paragon Plus Environment

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Figure 3. SEM images of (A) Zr@CSKN composite (In situ); (B) Zr@CSKN composite (Hydro); (C) nitrate sorbed Zr@CSKN composite (Hydro); (D) phosphate sorbed Zr@CSKN composites (Hydro); (E) N2 adsorption/desorption isotherm of Zr@CSKN composite (Hydro) at 77 K; and (F) pore size distribution of Zr@CSKN composite (Hydro). BET studies was carried out to identify the surface textural properties such as specific surface area, average pore width and total pore volume of Zr@CSKN composite by both methods and their results are given in Table 1. In addition, BET plots of N2 adsorption/desorption isotherm curve of Zr@CSKN composite (Hydro) at 77 K and its pore size distribution results are shown in Figures 3E and F respectively. It was mainly observed from Table 1, Zr@CSKN composite (Hydro) possesses larger specific surface area, pore volume and pore width compared to Zr@CSKN composite (In situ). 17 ACS Paragon Plus Environment


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Table 1. BET Textural Properties of the Adsorbents. Zr@CSKN composite textural property

hydrothermal method

in situ precipitation method

BET surface area (m2 g-1)

85.50

63.91

average pore width (nm)

4.43

3.57

total pore volume (cm3 g−1)

0.05

0.03

Figure 4. EDAX spectra of hydrothermal assisted (A) Zr@CSKN; (B) nitrate sorbed Zr@CSKN; and (C) phosphate sorbed Zr@CSKN composites. 18 ACS Paragon Plus Environment

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The elemental analysis of Zr@CSKN composite by hydro method and after their nitrate and phosphate was examined using EDAX analysis is shown in Figure 4. It was clear that the existence of the important elements of Zr@CSKN composite (Hydro) includes C, N, O, Zr, Si and Al with better intensities may denotes the good formation of Zr@CSKNcomposite (Hydro). The percentage of N (6.02%) is low in Zr@CSKN composite (Hydro) (cf. Figure 4A). However in Figure 4B, the increase in the percentage of N (8.46%) was observed in nitrate sorbed Zr@CSKN composite (Hydro) may confirms the occurrence of nitrate adsorption. In addition, the appeared new phosphorus peak with higher intensity (3.74%) in EDAX spectra of phosphate sorbed Zr@CSKN composite (Hydro) denotes the occurrence of phosphate adsorption which is illustrated in Figure 4C. The coloring nature of the individual elements are present in Zr@CSKN composite (Hydro) and after their nitrate and phosphate sorption are investigated using mapping images which are shown in Figures S3A, S3B and S3C. The various elements present in Zr@CSKN composite (Hydro) with their mapping intensity are shown in Figure S3A. It was observed from Figure S3B that, the appearance of highly intense mapping image of elemental nitrogen in nitrate sorbed Zr@CSKN composite (Hydro) compared to the mapping intensity of nitrogen present in Zr@CSKN composite (Hydro) which may confirms the occurrence of nitrate sorption (cf. Figures S3A and S3B). In addition, the appearance of new and deep blue colored mapping image of highly intense phosphorus in the phosphate sorbed Zr@CSKN composite (Hydro) may confirms the occurrence of phosphate sorption (cf. Figure S3C). 3.7.3. XRD Analysis XRD images of hydrothermal assisted ZrO(OH)2, CSKN composite, Zr@CSKN composite and chitosan are shown in Figure S4. The amorphous and semi-crystalline nature of chitosan was identified from its two characteristic broad diffraction peaks at 19.92° and 25.26° 19 ACS Paragon Plus Environment


which assisted by the plane of (110) and (130) respectively [JCPDS File No. 35-1974].51 The significant characteristic peaks at 12.30°, 21.92°, 23.98°, 34.02°, 36.80°, 42.98° and 61.95° are attributed to kaolin clay in the planes of (001), (111), (021), (102), (200), (041) and (002) respectively [JCPDS File No. 78-2110].52 Further, it was observed that XRD peaks of chitosan and kaolin was retained in both CSKN and Zr@CSKN composites which indicates the strong interaction between chitosan and kaolin in the respective composites. In addition, ZrO(OH)2 possess the broad and strong peaks were observed at 30.01°, 35.55°, 50.09° and 61.02° with the crystalline plane of (100), (102), (130) and (120) correspondingly.53 The result observed with the shifted intensities in the XRD peaks of individual chitosan, kaolin and ZrO(OH)2 in Zr@CSKN composite (Hydro) may confirms the good formation of Zr@CSKN composite (Hydro) for the removal of nitrate and phosphate. 3.8. Adsorption Isotherms Study The investigation in the isotherm equilibrium study of Zr@CSKN composite (Hydro) is very important since, it is used to understand the fundamental mechanism which is involved in the nitrate and phosphate adsorption. Langmuir,54 Freundlich55 and Dubinin-Radushkevich (DR)56 isotherm models were performed to predict the sorption isotherm nature at solid/liquid interphase. The isotherm experiment was examined by the addition of 0.1 g of Zr@CSKN composite (Hydro) into the discrete nitrate and phosphate solution with dissimilar initial concentration ranges of 80, 100, 120 and 140 mg L-1 at 303, 313 and 323 K. The results of relevant parameters and data of the isotherms are illustrated in Table 2. Langmuir isotherm assumes that there was a monolayer adsorption was observed at active sites of the adsorbent surface and it was attained by the plot of Ce vs Ce/qe.57 Freundlich isotherm develops the multilayer adsorption and it was obtained by the linear plot of log qe vs log Ce.58 The result of this linear plot gives Freundlich empirical constants like 20 ACS Paragon Plus Environment

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kF and n which are correlated to the nitrate and phosphate SC. In addition, Freundlich isotherm gives the conditions as in the values of n and 1/n should be around 1 to 10 and 0 to 1 for the feasible adsorption process to occur. From Table 2, it is clear that the value of n in Freundlich model falling between in the range of 1and10 and the value of 1/n observed between in the range of 0 and 1 indicates the feasible nature of nitrate and phosphate adsorption onto Zr@CSKN composite (Hydro). Table 2. Isotherm Results of Hydrothermal Assisted Zr@CSKN Composite for Nitrate and Phosphate Adsorption.

isotherms

Freundlich

Langmuir

Dubinin Radushkevich

nitrate adsorption

parameters

phosphate adsorption

303 K

313 K

323 K

303 K

313 K

323 K

1/n

0.194

0.198

0.210

0.375

0.379

0.381

n

2.846

2.849

2.848

4.709

4.711

4.719

kF (mg g-1) (L mg-1)1/n

31.83

31.07

30.98

38.34

38.91

39.62

r

0.887

0.885

0.888

0.990

0.992

0.991

sd

2.086

2.091

2.093

0.007

0.009

0.012

χ2

0.274

0.276

0.277

0.002

0.007

0.009

Qo (mg g-1)

34.62

33.42

32.74

40.58

41.98

43.01

b (L g-1)

0.854

0.895

1.064

0.995

1.003

1.009

RL

1.643

1.645

1.647

5.978

5.981

5.983

r

0.981

0.980

0.982

0.796

0.799

0.800

sd

1.095

1.098

2.001

0.289

0.291

0.290

χ2

0.048

0.055

0.059

0.131

0.140

0.141

kDR (mol2 J2 -1)

2.89E-02

2.73E-02

2.55E-02

6.81E-02

6.84E-02

6.90E-02

Xm (mg g-1)

27.94

27.18

26.90

31.34

32.09

32.45

E (kJ mol-1)

7.993

8.014

8.102

9.391

9.402

9.417

r

0.791

0.792

0.795

0.875

0.874

0.876

sd

2.967

2.971

2.972

0.789

0.791

0.803

χ2

1.997

2.007

2.010

1.643

1.645

1.651



D-R isotherm assumes that the characteristics of the adsorption are mainly related to the porosity of the adsorbents. The linearized form of D-R isotherm was obtained by the plot of ln qe vs ε2.59 The important parameters of D-R isotherm such as KDR, E, Xm, r and χ2 are given in Table 2. It was observed that the mean adsorption energy (E) value of Zr@CSKN composite (Hydro) between 7 and 10 kJ mol-1 may suggests that the both nitrate and phosphate adsorption system followed the ion-exchange and as well as physisorption mechanism.60 The experimentally measured raw isotherm (c vs q) data were given in the supporting information file as Figures S5 and S6, where the experimental data was compared with various data of isotherms. Moreover, the comparison values of various parameters such as r, sd and χ2 assert the suitable isotherm model of the adsorption process. From the result of lower sd and χ2 values and higher r value concluded that the nitrate adsorption onto Zr@CSKN composite (Hydro) follows Langmuir isotherm whereas phosphate adsorption follows Freundlich isotherm model. It is worthwhile to perceive the comparison in the adsorption capacity of various adsorbents prepared in this study with other reported adsorbents accessible in the market. The results exemplified in Table S1 pointed that among the listed adsorbents, Zr@CSKN composite (Hydro) was found to possess much higher nitrate and phosphate adsorption capacity compared to other adsorbents which declares its selectivity towards nitrate and phosphate. 3.9. Adsorption Thermodynamics Study The sorption behavior of nitrate and phosphate onto the active surface sites of Zr@CSKN composite (Hydro) was assessed through the various thermodynamic parameters include Gibbs free energy change (∆G°), standard enthalpy change (∆H°) and standard entropy change (∆S°). From the results illustrated in Table S2, the negative ∆G° value validates the spontaneity and feasible nature of nitrate and phosphate adsorption onto Zr@CSKN composite (Hydro). The gradual increase in negative ∆Gº value with elevation of reaction temperature demonstrates that 22 ACS Paragon Plus Environment

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this adsorption does proceed well at higher temperature of the adsorption system.72 The negative ∆H° (-2.89 kJ mol-1) spots the exothermic nature of nitrate adsorption whereas the positive ∆H° (7.37 kJ mol-1) points the endothermic nature of phosphate adsorption. Moreover, the positive ∆S° value demonstrates the increased randomness at the interface of nitrate and phosphate with that of Zr@CSKN composite (Hydro) surface.61 3.10. Sorptive Reaction Mechanism The significant reaction mechanism of nitrate and phosphate adsorption was supported by electrostatic attraction, surface complexation and ion-exchange which are illustrated in Figure 5. It was examined that the protonation of the prepared Zr@CSKN composite by both in situ and hydro methods may generating the positively charged adsorbent surface of metal hydroxide (MOH2+) and metal oxyhydroxide (M-O-OH2+) leads to stimulate the diffusion of both nitrate and phosphate from solution to the reactive sites of protonated sorbent surface. This result suggests that the formation of surface complexation mechanism of phosphate with that of the protonated Zr-O-OH2+, Si-OH5+ and Al-OH4+ in the prepared Zr@CSKN composite by both methods and the chemical formation of surface complexation reaction73 are given as follows Zr-O-OH2+ + H2PO4- (pH ~ 2-7)

ZrOPO4H2 + H2O

(6)

Si-OH5+ + H2PO4- (pH ~ 2-7)

SiPO4H2 + H2O

(7)

Al-OH4+ + H2PO4- (pH ~ 2-7)

AlPO4H2 + H2O

(8)

In addition, the univalent nitrate and multivalent phosphate can easily binds with protonated sorbent surface by Zr-O-OH2+, Si-OH5+ and Al-OH4+ viz., electrostatic attraction mechanism.74 This is further supported by pH study where the electrostatic binding attraction of the protonation of adsorbent surface with that of nitrate and phosphate was explained in detail (cf. Section 3.4). Next, there is also the possibility of nitrate and phosphate to get exchange



readily with interlayer OH- ion of kaolin clay of Zr@CSKN composite by ion-exchange mechanism.75

Figure 5. The feasible mechanism of nitrate and phosphate removal by in situ and hydro assisted CSKN and Zr@CSKN composites. 3.11. Field Test The evaluation in the adsorption performance of the prepared Zr@CSKN composite by in situ and hydro methods was verified at field conditions. The collected field water sample from nearby area of Dindigul district was treated with 0.1 g of the prepared Zr@CSKN composite by both methods were shaken using mechanical shaker with the optimized contact time of 30 and 40 min followed by the filtrate concentration was analyzed using UV-Visible spectrophotometer. The initial nitrate and phosphate concentration in the collected real wastewater sample was found to be 21.93 and 27.80 mg L-1 respectively which are shown in Table 3. After the treatment with the prepared Zr@CSKN composite by both methods, it was found that the final nitrate and phosphate concentration is found to be almost nil. 24 ACS Paragon Plus Environment

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Table 3. Field Suitability Studies of Zr@CSKN Composite.

before treatment

water quality parameters

after treatment Zr@CSKN composite (hydro)

Zr@CSKN composite (in situ)

initial nitrate concentration (mg L-1)

21.93

nil

nil

initial phosphate concentration (mg L-1)

27.80

nil

nil

pH

6.19

6.98

6.43

387

198

211

dissolved oxygen content (mg L )

4.52

7.98

7.72

total hardness (mg L-1)

602

476

483

total dissolved solids (mg L-1)

495

208

219

-

-1

Cl (mg L ) -1

This is because of the presence of higher valence metal ions like Zr4+, Si4+ and Al3+ in the prepared Zr@CSKN composite by both methods could easily attracts the negatively charged both nitrate and phosphate in field water. In addition, the other water quality parameters in the collected field water such as chloride, total hardness, dissolved oxygen and total dissolved solid are also shown in Table 3. From the above considerations, the use of prepared Zr@CSKN composite by both methods can be successfully applied for the uptake of nitrate and phosphate from the collected field wastewater effluent which ensuing its suitability at field conditions. 3.12. Regeneration Study The recycling and reuse of the prepared Zr@CSKN composite by in situ and hydro methods was performed using NaOH as eluent in order to reduce the cost of the adsorbent.76 The regeneration experiment was carried out by addition of 0.1 g of nitrate and phosphate adsorbed Zr@CSKN composite by both methods into 50 mL of 0.025 M NaOH solution. Afterward, the adsorbent was filtered, dried in hot air oven at 70 ºC for 3 h then added with nitrate and phosphate solution thereby shaken in mechanical shaker with the optimized contact time of 30 and 40 min followed by the filtrate concentration was analyzed in UV-Visible 25 ACS Paragon Plus Environment


spectrophotometer. The filtered adsorbent was reused again for adsorption studies. The adsorption-desorption system was replicated for six cycles, each cycles with new solution in the similar conditions and the results observed are shown in Figure S7. In first cycle, the nitrate and phosphate removal percentage was observed as 97.04 and 98.54% for Zr@CSKN composite (In situ). However, Zr@CSKN composite (Hydro) possess the slight increase in removal efficiency of 98.97 and 99.89%. Furthermore, the nitrate and phosphate removal percentage was reduced to 54.97% and 58.06% over the sixth cycles for Zr@CSKN composite (In situ). Similarly, it was also decreased to 63.29% and 67.56% for Zr@CSKN composite (Hydro). This consistent decrease in the removal percentage may be governed by the contending effect of OH- ions which engage the surface reactive sites of the prepared Zr@CSKN composite by both methods instead of nitrate and phosphate. From Figure S7, it was concluded that there is a significant loss in the nitrate and phosphate removal percentage was observed after fourth and fifth cycles for Zr@CSKN composite by in situ and hydro methods respectively. This result reveals that the prepared Zr@CSKN composite by both methods could be reused as the efficient recyclable adsorbent up to four and five cycles for wastewater treatment which helps to reduce the cost of the composite. 4. Conclusions In this work, Zr@CSKN composite was prepared by in situ and hydro methods for nitrate and phosphate retention. It was found that the hydrothermal assisted Zr@CSKN composite exhibits an enhanced SC than individual raw materials. The enhanced nitrate and phosphate SCs of 34.62 and 40.58 mg g-1 was observed for Zr@CSKN composite (Hydro) within 30 min. The nitrate and phosphate adsorption with that of the prepared adsorbents was affected by solution pH. The highly selective removal of nitrate and phosphate was observed over other anions such as chloride and bicarbonate except sulfate. The functional groups, surface and adsorption 26 ACS Paragon Plus Environment

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characteristic of the adsorbent materials was studied using FTIR, SEM, BET, XRD and EDAX with mapping analysis. The isotherm results of Zr@CSKN composite (Hydro) follows Langmuir and Freundlich isotherm models towards nitrate and phosphate adsorption respectively. The thermodynamic studies reveals that the adsorption of nitrate and phosphate on Zr@CSKN composite (Hydro) was feasible and spontaneous in nature. The negative ∆H° value of Zr@CSKN composite (Hydro) indicates the exothermic nature of nitrate adsorption whereas the positive ∆H° value of Zr@CSKN composite (Hydro) denotes the endothermic nature of phosphate adsorption. The possible adsorption mechanism of nitrate and phosphate was followed by ion-exchange, surface complexation and electrostatic attraction. The prepared Zr@CSKN composite by in situ and hydro methods can be easily regenerated and reused up to four and five cycles respectively. The tested Zr@CSKN composites at field conditions show its suitability and eco-friendly nature towards nitrate and phosphate adsorption from drinking/wastewater. Supporting Information Effect of pHzpc on the nitrate and phosphate SCs of Zr@CSKN composite (In situ) and Zr@CSKN composite (Hydro) was shown in the supporting information file as Figure S1. Effect of temperature on the nitrate and phosphate SCs of Zr@CSKN composite (In situ) and Zr@CSKN composite (Hydro) was shown in the supporting information file as Figure S2. Mapping images of hydrothermal assisted (A) Zr@CSKN; (B) nitrate sorbed Zr@CSKN; and (C) phosphate sorbed Zr@CSKN composites were shown in the supporting information file as Figure S3. XRD images of hydrothermal assisted ZrO(OH)2, CSKN, Zr@CSKN composites and chitosan were shown in the supporting information file as Figure S4.



The experimentally measured raw isotherm (c vs q) data of the prepared Zr@CSKN composite (Hydro) was given in the supporting information file as Figures S5 and S6 where the experimental data was compared with various isotherms towards nitrate and phosphate removal. The comparison results in the adsorption capacities of the various adsorbents prepared in this study with other reported adsorbents accessible in the market are given in the supporting information file as Table S1. Thermodynamic parameters of the hydrothermal assisted Zr@CSKN composite for nitrate and phosphate adsorption are shown in the supporting information file as Table S2. The regeneration performance of the prepared Zr@CSKN composite by in situ and hydro methods was given in the supporting information file as Figure S7. Acknowledgement The authors were grateful to University Grants Commission (F. No. 43-179/2014(SR)), New Delhi, India, for provision of financial support to carry out this research work. References (1) Aswin Kumar, I.; Viswanathan, N. Development and Reuse of Amine-Grafted Chitosan Hybrid Beads in the Retention of Nitrate and Phosphate. J. Chem. Eng. Data 2018, 63, 147-158. (2) Ward, M. H.; Dekok, T. M.; Levallois, P.; Brender, J.; Gulis, G.; Nolan, B. T.; Vanderslice, J. Workgroup Report: Drinking-Water Nitrate and Health-Recent Findings and Research Needs. Environ. Health Perspect. 2005, 113, 1607-1614. (3) Galalgorchev, H. WHO Guidelines for Drinking-Water Quality. IWSA Specialized Conference on Quality Aspects of Water Supply. 1992, 11, 1-16. (4) Bhatnagar, A.; Sillanpaa, M. A Review of Emerging Adsorbents for Nitrate Removal from Water. Chem. Eng. J. 2011, 168, 493-504.


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(31) Wang, W.; Jiao, T.; Zhang, Q.; Luo, X.; Hu, J.; Chen, Y.; Peng, Q.; Yan, X.; Li, B. Hydrothermal Synthesis of Hierarchical Core-Shell Manganese Oxide Nanocomposites as Efficient Dye Adsorbents for Wastewater Treatment. RSC Adv. 2015, 5, 56279-56285. (32) Periyasamy, S.; Viswanathan, N. Hydrothermal Synthesis of Hydrocalumite Assisted Biopolymeric Hybrid Composites for Efficient Cr (VI) Removal from Water. New J. Chem. 2018, 42, 3371-3382. (33) Lopez-Ramon, M. V.; Stoeckli, F.; Moreno-Castilla, C.; Carrasco-Marin, F. On the Characterization of the Acidic and Basic Surface Sites on Carbons by Various Techniques. Carbon 1999, 37, 1215-1221. (34) APHA. Standard Methods for the Examination of Water and Wastewater; American Public Health Association: Washington, DC, USA, 2005. (35) Li-Feng, C.; Hai-Wei, L.; Yang, L.; Chun-Hua, C.; Shu-Hong, Y. Synthesis of An Attapulgite Clay at Carbon Nanocomposite Adsorbent By a Hydrothermal Carbonization Process and Their Application in the Removal of Toxic Metal Ions from Water. Langmuir 2011, 27, 8998-9004. (36) Rajput, S.; Pittman Jr, C. U.; Mohan, D. Magnetic Magnetite (Fe3O4) Nanoparticle Synthesis and Applications for Lead (Pb2+) and Chromium (Cr6+) Removal from Water. J. Colloid Interface Sci. 2016, 468, 334-346. (37) Gao, Y.; Chen, C.; Tan, X.; Xu, H.; Zhu, K. Polyaniline-Modified 3D-Flower-Like Molybdenum Disulfide Composite for Efficient Adsorption/Photocatalytic Reduction of Cr(VI). J. Colloid Interface Sci. 2016, 476, 62-70. (38) Jozwiak, T.; Filipkowska, U.; Szymczyk, P.; Kuczajowska-Zadrozna, M.; Mielcarek, A. The Use of Cross-Linked Chitosan Beads for Nutrients (Nitrate and Orthophosphate) Removal from a Mixture of P-PO4, N-NO2 and N-NO3. Int. J. Biol. Macromol. 2017, 104, 1280-1293. 32 ACS Paragon Plus Environment

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Hydrothermal Fabrication of Zirconium Oxyhydroxide Capped

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