Synthesis of nanoZrO₂ via simple new green routes and its effective application as adsorbent in phosphate remediation of water with or without immobilization in Al-alginate beads

Phosphate contamination of water bodies and its hazardous effects have been globally recognised and, in fact, some countries issued phosphate acts to control or ban the usage of phosphate in detergents and allied materials (Fadiran et al. 2008). The potential sources of phosphate pollution are: excessive application of fertilizers in agricultural fields; inadequately treated effluents from phosphate-based industries viz., detergents, drinks, food materials, etc. (Karageorgiou et al. 2007).

The presence of phosphate in water bodies causes eutrophication. This phenomenon of wild growth of aquatic plants results in depletion of dissolved oxygen (DO) content in water, generates offensive gases due to fermentation of decayed plant parts and increases acidity of water. Further, the sunlight will not reach to the needed depths in waters and thereby, the photosynthesis process is obstructed (Ames & Dean 1970; Batchelor & Dennis 1987). This causes stress to aqua plants and organisms and thereby, loss in eco-cycles. As per WHO, 0.1 ppm is the maximum limit of phosphate for discharging waters (Fadiran et al. 2008).

Removal of phosphate using conventional methods based on precipitation, biological degradation, ion-exchange and electrochemical process (Korngold 1973; PCA 2006; Lacasa et al. 2011; Yamashita & Yamamoto-Ikemoto 2014) are non-economical, involve complicating procedures and need expert supervision and are not adoptable in poor countries like India. Methods based on adsorption are attracting the researchers and in this aspect, many bio-materials are developed as effective adsorbents for the removal of phosphates (Jyothi et al. 2012; Rao & Ravindhranath 2015; Mohan et al. 2017; Sujitha & Ravindhranath 2017).

Developing adsorbents based on nano particles for water remediation, is one of the recent trend in the purification waters (Ozmen et al. 2010; Dehghani et al. 2015; Ghafar A et al. 2015; Ravindhranath & Mylavarapu 2017a, 2017b; Ravulapalli & Ravindhranath 2019). The nano particles are proving to be effective adsorbents in view of increase in surface area, quantum confinements and other morphological changes attributed to the nano particles.

One of the main disadvantages of nano-materials as adsorbents are percolation of water through them is low due to micro or nano-sized voids. This can be avoided by immobilizing the nano particle in beads. In fact, our research groups have successfully developed some composite adsorbents based on beads formed by cross linking of Sodium alginate with Ca²⁺ and Zn²⁺ (Mohan et al. 2017; Naga Babu et al. 2017; Sujitha & Ravindhranath 2017). The developing of adsorbents based on nano particles to have maximum sorption capacity and at the same to make easy the filtration process, is one of the potential aspects of water pollution control research (Ravindhranath & Mylavarapu 2017a, 2017b). Further, the production of nano particles via conventional methods using various materials as capping, reducing and stabilizing agents, are less encouraging as most of the chemicals used in these methods are toxic and not eco-friendly. Hence, green methods of synthesis are attractive. Using the green synthesized nano particles in the composite materials for being used as adsorbents, is one of the vital aspect of water remediation techniques.

In the present investigation, nano ZrO₂ particles are tried to be synthesized using green routes. Non-toxic and bio-degradable extract of seeds of Sapindus plant is used as capping and/or stabilizing agent. Reagent (OH⁻) is generated by urea hydrolysis method to prevent super-saturation. The mother liquid is a solvent blend of water and ethylene glycol (4:1). At these experimental conditions of high viscosity of media and slow generation of reagent, the nucleation and growth of the particle is slow and when the particle is reached to nano size, the capping agent prevents the further growth and stabilizes. This methodology is found to be successful in this investigation. Thus obtained green synthesized nZrO₂ particles are immobilized in the beads produced by cross linking sodium alginate with Al³⁺. The nZrO₂ and beads are investigated as adsorbents for the removal of phosphate from water. The cross linking with Al³⁺ against the conventional divalent Ca²⁺ has been done with an endeavour to increase the cumulative sorption nature of nano-ZrO₂ and Al-alginate in view of enhanced positive charge of the cross linking metal ion (Al3+) and binding capacity of the functional groups of the beads. The controlling parameters namely, pH, sorbent dosage, equilibration time, initial phosphate concentration and temperature are optimized for the maximum removal of phosphate from simulated water. Natures of sorption, kinetic and thermodynamic parameters have been investigated. The developed procedures are applied to the samples collected at affected lakes in Prakasham District of Andhra Pradesh, India.

Analytical grade chemicals were used in this work. Solutions were prepared with double distilled water.

The conventional capping and stabilizing agents used in the synthesis of nano ZrO₂ are toxic and costly. So investigations are being made to identify the plant extracts as substitutes. In this process, we identified that extract of Sapindus plant seed possesses good surfacting nature and it is employed in the present work. Further, the success of limiting the growth of particles to nano size depends upon particle growth conditions, mainly low super-saturation, viscosity of the mother liquor and entrapping ability of the capping agent (Vogel 1961). As per the classical theory of precipitations, the growth of particles comprises three important steps: super-saturation, nucleation and aggregation. When the growth of particle is at nano dimensions, the growth is to be arrested by employing a capping agent. Super-saturation influences particle size (Vogel 1961). The rate of nuclei formation is proportional to Q-S/S where Q = concentration of solute and S = equilibrium solubility. The speed of aggregation of molecules is to be retarded by increasing the viscosity of the medium. At low super-saturation and high viscosity of medium, the aggregation of particles is less and before the particles cross the nano size, the growth is to be stopped by capping agents (Vogel 1961; Sun et al. 2007; Pandian et al. 2015; Ravindhranath & Mylavarapu 2017a, 2017b).

In the present investigation, to achieve the low super saturation, we employed classical methods of homogenous precipitations (Vogel 1961). The precursor was zirconyl chloride and the precipitating agent was OH⁻ generated by the hydrolysis of urea. The viscosity of the mother liquid was enhanced by using ethylene glycol. With low super-saturation, high viscosity and by using an effective bio-surfactant derived from Sapindus plant seeds as capping and stabilizing agent, nano ZrO₂ were successfully synthesized in this work as is evident from the following narration.

Seeds of Sapindus plant were collected, washed and dried. The dried seeds were skinned out and the leathery bio-parts were dried at 90 °C. 5.0 g of the bio-material was extracted repeatedly with hot water. The extractions were combined, filtered and the filtrate was diluted to 500 mL. Thus obtained extract was preserved at 4 °C in a refrigerator.

Requisite amount of ZrOCl₂.8H₂O was dissolved in a solvent blend of water and ethylene glycol (4:1 V/V) to obtain 0.05 N solutions. To 100 mL of this solution taken in a 250 mL beaker, 20 mL of bio-extract obtained from the seeds of Sapindus plant was added and stirred for 30 min using a magnetic stirrer at 600 rpm. Then 10 g A.R. urea was added slowly and the resulting solution was heated to 80–90 °C with constant stirring until the pH was increased to pH 9. Then the heating was stopped and stirring was continued for 15 h. The solution was centrifuged and the particles were washed with distilled water and dried at 90 °C. Then the obtained material was calcinated at 500 °C for 6 h in a muffle furnace.

The experimental conditions adopted in this work for generating slowly the nuclei of metal hydroxide, provides sufficient time for capping agent to limit the growth of the particle to nano size. The particles were characterized with X-ray diffraction (XRD) and by using Scherrer formula (Dorofeeva et al. 2012), the average crystalline size for ZrO₂ sample was 10.91 nm.

In the present work, the nano ZrO₂ was immobilised in beads synthesized by cross linking sodium alginate with Al³⁺. The conventional beading with Ca²⁺ was replaced with Al³⁺ in this investigation with an aim to derive cumulative advantages of nano size of ZrO₂ and residual positive charges of Al³⁺ in enhancing the adsorption ability of the composite towards negatively charged phosphate ions. In this investigation, good beads of Al-alginate embedded with nano ZrO₂, were successively synthesized.

2.0% w/v sodium alginate solution in distilled water was heated slowly to reach 80 °C to obtain a homogenous gel. To this gel, 1.5 g of nano ZrO₂ was added with constant stirring. The resulting solution was stirred (600 rpm) until a homogeneous solution was obtained. Then the solution was cooled to room temperature and was added to 3% acidic solution of aluminium chloride in HCl (2N) at 5 °C taken in a beaker, drop wise using a dropper having a good capillary delivery tube. The moment the drops were touched to Al³⁺ solution, excellent beads were developed. The formed beads were allowed to be in contact with the mother liquid for an overnight for the complete digestion to occur. Then beads were filtered, dried at 70 °C for 5 h. The beads obtained were named as nZrO₂-Al-alig(nano zirconium oxide embedded aluminium alginate). The synthesis of the beads is graphically summarized in Figure 1.

XRD, Fourier transform infrared (FTIR), field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray (EDX) investigations were made to study the nature of adsorption of the composite material: nZrO₂ and ‘nZrO₂-Al-alig’ towards phosphate.

X-ray powder diffract meter with CuKα (λ= 0.154 nm) radiation over the 2θ range of 20–80 °C was used to record XRD patterns of nZrO₂. FESEM images of nZrO₂ and nZrO₂-Al-alig were taken at 500 to 10,000× magnifications at the acceleration voltage of 15.0 kV. FTIR and EDX spectra of nZrO₂ and nZrO₂-Al-alig before and after adsorption of phosphate were recorded. The results are presented in Figures 2–5.

Batch adsorption experiments were employed as described in literature (A.R.K. 1995; Metcalf Eddy 2003). 100 mL of phosphate solutions of concentration: 50 mg/L, were taken into 250 mL conical flasks and to them, different quantities of nZrO₂ and nZrO₂-Al-alig (0.025 to 0.2 g) were added. Then, initial pHs of solutions were adjusted from 2 to 12 by using 0.1 M HCl/0.1 M NaOH. Then the flasks were agitated in orbital shaker for desired time at 300 rpm at room temperature (30 °C). After a definite time of agitation, the solutions were centrifuged/filtered. The residual phosphate in the filtrates was analysed spectrophotometrically adopting ‘Molybdenum Blue’ method (APHA (American Public Health Association) 1985) using UV-Visible-159 model spectrophotometer (ELICO). The percentage removal and adsorbed amount of phosphate (adsorbent capacity) were calculated by the equations: % removal = and qe = where m= mass of adsorbent (g), V= volume of the solution (L), C₀ and C_i are the initial and final concentrations (mg/L) of phosphate ions (Dehghani et al. 2018a). The influence of pH, sorbent dosage, initial concentration of phosphate ions, co-ions and temperature on the % removal of phosphate were assessed. The results are presented in Figures 6–9.

XRD pattern of nano ZrO₂ is depicted in Figure 2. The XRD pattern shows cubic phase as per JCPDS card No. 27-0997. The broadening of XRD peaks indicates fine size for ZrO₂ particles. The average size of ZrO₂ particles was calculated using Scherrer formula (Dorofeeva et al. 2012). The average size of ZrO₂ particles is: 10.91 nm. This reflects the successful adoption of the present green route of synthesis and the classical homogenous method of precipitations in obtaining nanoZrO₂.

The FTIR spectra for nZrO₂ and ‘nZrO₂-Al-alig’ are presented in Figure 3(a) and 3(b). IR spectrum of nZrO₂ (prepared from green routes) resembles that of previously reported in literature (Bi et al. 2017). It is interesting to note the difference between before and after spectra. The frequencies at 745, 930, 1,072.6 and 1,146 cm⁻¹of nZrO₂ are shifted to 751, 937, 1,075, 1,153.6 cm⁻¹ with emphatic increase in intensities after adsorption of phosphate. A new peak in the after-adsorption spectrum at 2,352.2 cm⁻¹ is due to phosphate.

In the case of nZrO₂-Al-alig, there is clear evidence that phosphate is adsorbed. The spectrum before adsorption has a broad peak between 3,036 to 3,275 cm⁻¹ with an apex at 3,150 cm⁻¹ pertains to –OH stretching. The broadness indicates hydrogen bonding. In addition to this peak, additional sharp peaks of different intensities are observed at 3,415 cm⁻¹, 3,490 cm⁻¹ and 3,556 cm⁻¹ in after-IR adsorption spectrum. These are due to interactions of phosphate with the adsorbent. Further, a new additional peak at 2,049.8 cm⁻¹ (after adsorption) is due to the surface structural changes caused by the adsorption of phosphate. Small peaks before adsorption pertaining to –C-O stretching at 1,038 cm⁻¹ and 1,102 cm⁻¹ are developed into a sharp merged peak with improving intensity at 1,143 cm⁻¹. This is due to the adoption of HPO₄²⁻. A sharp small peak at 778 cm⁻¹ before adsorption pertains to Zr-O (stretching), has been reduced to a very small peak due to the formation of ‘Zr-PO₄³⁻’. Consequently a new sharp peak with medium intensity is appeared at 630.7 cm⁻¹. It is due to asymmetric stretching of ‘Zr/Al- PO₄³⁻. All these findings emphatically prove the adsorption of phosphate.

FESEM images of nZrO₂ and ‘nZrO₂-Al-alig (beads)’ are presented in Figure 4. The image before adsorption has cavities, small pores, and irregular surface. These have been converted to smooth surfaces after adsorption. These morphological changes reveal phosphate adsorption ‘onto’ nZrO₂ and nZrO₂-Al-alig.

The EDX spectra of nZrO₂ and nZrO₂-Al-alig before and after phosphate adsorption are depicted in Figure 5. It is clearly seen that phosphate peaks are noticed in the EDX spectra taken after adsorption (Figure 5(b) and 5(d)) and they are missing in the EDX spectra taken before adsorption (Figure 5(a) and 5(c)). Hence, it may conclude that the phosphate is adsorbed ‘onto’ the surface of both the adsorbents.

Various experimental conditions were optimized for the maximum extraction of phosphate. Investigations were carried out by varying one parameter while maintaining the other experimental conditions constant. Investigations were made by varying the pH from 2 to 12, agitation time: 5 to 60 min, adsorbent dosage from 0.025 to 0.2 g/100 mL, at initial concentration: 50 mg/L and rpm: 300 and temperature 30 °C for ascertaining the optimum extraction conditions. The effect of initial concentration of phosphate and temperature on the adsorption efficiency of the two adsorbents was also investigated by maintaining all other parameters at optimum levels while varying only the parameter under study.

The effect of pH on the adsorption of phosphate onto the surface of nZrO₂ and nZrO₂-Al-alig are depicted in Figure 6(c). The maximum removal of 93.0% for nZrO₂ and 99.5% for nZrO₂-Al-alig are observed at pH 7 and above or below, the removal is less favoured. H₃PO₄ is a tri-basic acid and it has pKa values: 2.14, 7.2, 12.4. So below pH 3, the main species is H₃PO₄, between pH 3–9, H₂PO₄⁻ and HPO₄²⁻ co-exist and between pH 9–12, HPO₄²⁻ and PO₄³⁻ co-exist and above pH 12, main species is PO₄³⁻ (Vogel 1961). At pH 7, the H₂PO₄⁻ and HPO₄²⁻ are presented in almost equivalent concentrations and in fact, these ions form a buffer and the buffering capacity is maximum at pH ∼7. pH 0 values for ZrO₂ and ZrO₂-Al-alig are at pH 6.9 and 7.1, respectively (Figure 6(a) and 6(b)). So, above these pH values, the surface of the adsorbents possesses negative charge. As the charge of phosphate species H₂PO₄⁻ and HPO₄²⁻ is also negative, the repulsion is caused and so, there is less adsorption. When pH of solution is below 7, the surface hydroxyl groups of both the adsorbents are protonated and attain positive charge. So they attract the negative species of phosphate ions. At pH 7, both the negative species are in equimolar proportions and hence, there is maximum interaction of the species towards the surface. So, the extraction is at its maximum at pH ∼7. With the decrease in pH of the solution below 7, doubly charged HPO₄²⁻ species are converted to mono charged negative H₂PO₄⁻. This results in less interaction of phosphate species with the positively charged surface when compared to the species at pH 7. So, adsorption decreases as pH decreases from 7. For maximum adsorption, the surface of the adsorbents must be charged positively while the phosphate species possess high cumulative negative charge. This condition is acquired by the two adsorbents at pH 7 and so, maximum adsorption is observed.

Investigations were made to optimize the adsorbent concentration for maximum extraction of phosphate. The concentrations were varied from 0.025 to 0.2 g/100 mL while maintaining other extraction conditions at optimum levels. Observations are presented in Figure 6(d). Adsorption is almost linearly increased up to 0.1 mg/100 mL for nZrO₂ and 0.08 mg/100 mL for nZrO₂-Al-alig. After that dosage, a steady state is resulted. The maximum removal is found to be 93.1% for nZrO₂ and 99.5% for nZrO₂-Al-alig. The linear increase in the adsorption with progressive raise in concentration of adsorbents is owing to the increase in sorption sites. But at high concentrations, active sites prevailing on surface are blocked due to agglomeration and hence, steady state.

After equilibration of the phosphate solutions at different time intervals with the two adsorbents while maintaining constant the other optimum conditions of extractions, the samples were assayed for the residual phosphate. The findings are depicted in Figure 6(e). 99.5% extraction was observed after 35 min of equilibration for nZrO₂-Al-alig and 93.1% after 30 min for nZrO_2.

As is viewed from the Figure 6(e), the adsorption is progressively increased with an increase in time. After 30 min for nZrO₂ and 35 min for nZrO₂-Al-alig, steady states are reached. Initially many sorption sites are available for the adsorption of phosphate. So, the adsorption is maximum at the beginning. But with time, the active sites are progressively used up. As the amount of adsorbent is fixed, the sites available are also limited. Hence, with time, the rate of adsorption is decreased. After certain time, all sites are engaged in the equilibrium state of adsorption/desorption and hence steady state is resulted (Krishna Mohan et al. 2017).

The initial concentration of phosphate solution has influence on the adsorption of phosphate ions. It was investigated by varying the initial concentrations from 60 to 150 mg/L while keeping constant (optimum) all other extraction conditions. The results are depicted Figure 6(f). With the raise in phosphate concentration from 60 to 150 mg/L, the adsorption falls from 92.37 to 72.24% for nZrO₂ and 97.5 to 79.6% for nZrO₂-Al-alig(bead).

As the adsorbent concentration is fixed (0.1 g/100 mL for nZrO₂ and 0.08 g/100 mL for nZrO₂-Al-alig), the active sites are limited. At low initial concentration of phosphate (adsorbate) active sites available per one molecule of phosphate are many and probabilities of interactions are more and so, adsorption is more. As the phosphate concentrations are increased, the sites offered per molecule of phosphate for adsorption process to occur is decreased and hence, % removal is less.

Temperature influences the adsorption process profoundly. Its effect was studied at temperatures 303, 313, 323 and 333 K on the adsorption of phosphate onto nZrO₂ and nZrO₂-Al-alig by equilibrating 100 mg/L of phosphate solution at other optimum conditions of extraction. The results are depicted in Figure 6(g). As the temperature increases from 303 to 333 K, the percentage removal of phosphate is increased from 86.3 to 92.2% for nZrO₂ and 88.2 to 97.3% for nZrO₂-Al-alig.

The rise in percentage removal with rise in temperature indicates the adsorption process is favourable at high temperatures. At elevated temperatures, the pores in the adsorbents are enlarged and, moreover, some new sites may be formed. These changes in surface morphology create additional paths for the phosphate molecules to reach the inner laying active sites. Thereby, hidden active sites in the folds of layers of the adsorbents are open for the adsorption process.

Elevated temperatures help phosphate ions to overcome activation energy barrier at the interface between the solution and the adsorbent. This enhances the mobility or diffusion of phosphate into the surface layers of the adsorbents. Due to these effects, the adsorption is more with raise in temperature (Mansooreh et al. 2014).

To assess the nature of adsorption process, various thermodynamic parameters namely, ΔH, ΔS and ΔG were evaluated as per the procedures in literature (Fan & Zhang 2018). The pertaining equations are: ⁠, and where, qe = amount of phosphate adsorbed at equilibrium, Kd = distribution coefficient, Ce = equilibrium concentration of phosphate, R = gas constant and T = temperature (Kelvin), ln KD vs. (1/T) plots, Figure 6(h), were used to evaluate ΔH and ΔS. Values obtained for all the parameters are presented in Table 1.

Negative values for ΔG signify the spontaneity of adsorption process for both the adsorbents under investigation towards adsorbate (phosphate). Further, the increase in negative values of ΔG with the rise in temperature reflects the more spontaneity of the adsorption process at elevated temperatures.

ΔH values for both the adsorbents are positive and the value for nZrO₂ (16.77) kJ/mol is less than nZrO₂-Al-alig (33.305 kJ/mol). The positive values suggest the endothermic nature (Ngah & Hanafiah 2008). At high temperatures, the conditions are conducive for adsorbate (phosphate) to overcome the resistance at the solution–adsorbent interface and thereby, accelerating the adsorption process. Further, the magnitude of ΔH values indicates the nature of sorption process: physical adsorption 2.1–20.9 kJ/mol; chemical adsorption: 20.9–418.4 kJ/mol (Sun et al. 2012). As the values of ΔH are 16.77 kJ/mol and 33.305 kJ/mol for nZrO₂ and nZrO₂-Al-alig, respectively, the adsorption of phosphate is more inclined towards chemical in nature.

ΔS values are positive. This signifies the more disorder at the solid–liquid interface. Further higher values for nZrO₂-Al-alig than nZrO_2, reflect more disorderliness in the former than the latter. If the disorder is more, the randomness in the movement of phosphates (adsorbate) at the surface is more. This enables the phosphate molecules to penetrate deep into the layers of adsorbents. So, the adsorption is expected to be more in the case of nZrO₂-Al-alig than nZrO₂ (Borah et al. 2015).

The sorption nature was assessed by applying various adsorption isotherm models as has been described elsewhere (Freundlich 1906; Langmuir 1918; Temkin & Pyzhev 1940; Dubinin 1947). The results are presented in Table 2. The R² values fall in the order: Freundlich (0.99) > Temkin (0.985) > Langmuir (0.97) > Dubinin-Radushkevich (0.78) for nZrO₂; Freundlich (0.997) > Langmuir (0.965) > Temkin (0.963) > Dubinin-Radushkevich (0.56) for nZrO₂-Al-alig. The values indicate that Freundlich model (Figure 7(a)) is fit to understand the adsorption nature of both the adsorbents and, further, it indicates the heterogeneous and multilayer of adsorption (Dehghani et al. 2016). K_f and n values were noted from the plots. As 1/n for nZrO₂ (0.303) and nZrO₂-Al-alig (0.248) are in the range: 0.1 < 1/n < 1, the adsorption of phosphate onto the surface of the said adsorbents is favourable.

From the Langmuir isotherms (Figure 7(b)), RL (dimensionless separation factor) values were calculated by using R _L= 1 (1 + a_LC₀). The R_L values are 0.058 for nZrO₂ and 0.0121 for nZrO₂-Al-alig. As the R_L values are falling in the range: 0 < R_L< 1, the adsorption is favorable (Dehghani et al. 2018b).

Further, linear form of Temkin equation and Dubinin-Radushkevich equations were applied (Figure 7(c) and 7(d)). Isothermal constants along with the correlation coefficient values were presented in Table 2. The mean free energy (E) and heat of sorption (B) were calculated. As B value for the adsorbents: 24.14 for nZrO₂ and 22.81 for nZrO₂-Al-alig are more than 20 kJ/mol, the adsorption is more oriented towards chemical interactions than physical interactions.

The results are depicted in Figure 8(a)–8(d) and Table 3. On perusal of the R² values, pseudo-second-order is a better fit to explain the kinetics of adsorption. This model is followed by Elovich model, then by Bangham's pore diffusion model and lastly by pseudo-first-order mode in the case of both the adsorbents.

The interferences caused by commonly present co-anions (Cl⁻, F⁻, HCO₃⁻ NO₃⁻ and SO₄²⁻) in water were assessed. For this, five-fold excess of the said ions was added to the known amount of phosphate solutions. Thus prepared simulated solutions were subjected to extraction under the optimum conditions established in this investigation with both the adsorbents. Percent removals were calculated. The results are presented in Figure 9(a). It can be seen from the figure that chlorides, fluorides, nitrate and bicarbonate have marginal effect; sulphate has some interference.

To achieve cost effectiveness, the spent adsorbents are to be regenerated. The conditions to activate the spent adsorbents are to be optimized. For this object, many eluents at various concentrations were tried. 0.1 N Na₂SO₄ was a successful regenerating agent. The spent adsorbents were soaked in 0.1 N Na₂SO₄ solutions overnight. The adsorbent is filtered, washed with distilled water and dried at 105 °C. Thus regenerated adsorbents were employed for investigations. This cycle of regeneration and reuse was continued a number of times. The results are presented in Figure 9(b).

The efficiency of adsorption is marginally affected up to five cycles for both the adsorbents. For nZrO₂, percent removal has been decreased from 92.4 (first) to 80.9 after the fifth cycle of regeneration. In the case of nZrO₂-Al-alig, percent removal has decreased from 99.5 (first) to 90.3 (fifth). It may be concluded that by repetitive use of the adsorbents, complete removal of phosphate from water can be achieved.

The adoptability of nZrO₂ and nZrO₂- Al-alig as adsorbents was assessed with respect to the samples collected from different lakes in Prakasham District, A.P., India. The phosphate pollution problem is severe in these localities. So, the present developed new adsorbents were applied to different water samples collected in various localities in the district. The results are presented in Table 4. From the data, it may be concluded that both the adsorbents have effectively reduced the concentrations. It is more than 97.0% with nZrO₂ and complete removal is observed with nZrO₂- Al-alig.

The adsorbents developed in this investigation are compared with nano adsorbents reported in literature for the removal of phosphate from water. The data are summarized in Table 5.

It may be inferred from the table that the sorbents developed in this investigation, namely nZrO₂ and nZrO₂-Al-alig, have higher adsorption capacities than hitherto reported in literature.

In the present investigation, nano particles of zirconium oxide of average size, 10.91 nm, are successfully synthesized via green routes. In this synthesis, bio-extracts of seeds of Sapindus plant are employed as stabilizing and capping agents. Water: ethylene glycol (4:1 v/v) media is used to slow down the nucleation of ZrO₂ particles. Further, a homogenous method of precipitation is adopted while generating slowly the reagent, OH⁻ by urea hydrolysis. At these experimental conditions of low super-saturation and high viscosity of medium, the aggregation of particles is less and before the particles acquire nano size, the growth is prevented by the capping agent.

Two adsorbents are prepared: nZrO₂ and nZrO₂-Al- alig. These adsorbents are investigated for their phosphate absorptivity by varying extraction conditions using batch methods. The optimum conditions established for the maximum removal of phosphate are: pH 7; sorbent dosage: 0.1 g/100 mL (nZrO₂₎ and 0.08 g/100 mL (nZrO₂-Al- alig); equilibration time: 30 min (nZrO₂) and 35 min (nZrO₂-Al-alig); initial concentration of phosphate: 50 mg/L; temperature: 30 ± 1 °C; 300 rpm.

The adoption capacity for nZrO₂ is 126.2 mg/g and for nZrO₂-Al- alig is 173 mg/g. These are higher than hitherto reported investigations. The beads, besides facilitating easy filtration, enhance the adsorption capacity. The increase in the adsorption capacity may be due to the residual positive charges of Al³⁺ which is used as crossing agent and also due to the binding capacity of functional groups pertaining to the Al-alginate beads.

The sorption of phosphate ‘onto’ the surface of nZrO₂ and nZrO₂-Al- alig are characterized by various techniques including XRD, FTIR, FESEM and EDX. The sorption mechanism is investigated using various isotherm models. Freundlich model fits well for both the adsorbents. Thermodynamic studies reveal the endothermic and spontaneous nature of sorption. Pseudo-second-order model of kinetics fits well for both the adsorbents.

Spent adsorbents can be regenerated. Even after five cycles of regeneration of the adsorbents, a significant amount of phosphate is removed to an extent of 80.9% for nZrO₂ and 90.3% for nZrO₂-Al-alig. The beads' structure is so strong that even after five regenerations, they are not deteriorated.

One of the major advantages of these two adsorbents nZrO₂ and nZrO₂-Al- alig, is that they show substantial phosphate adsorptivity in neutral pH (∼7). Generally, the polluted lake waters or natural waters have pH 6–8 and, hence, the present developed adsorbents can be applied directly without adjusting the pH of water samples and, thereby, avoid one step in the purification of water. The adsorbents nZrO₂ and nZrO₂-Al- alig, are successfully applied to remove phosphate from water samples of polluted lakes.

All relevant data are included in the paper or its Supplementary Information.