Phosphate removal by Ion exchange in batch mode

Water with phosphate concentrations above 2 μM may adversely affect aquatic life and human health. In this study, the parameters affecting phosphate removal from aqueous solutions by ion exchange were investigated – contact time and temperature, initial pH, initial phosphate concentration and resin dosage, and the presence of other ions. The best phosphate removal (99%) from 100 mg-P/L initial solution was observed at pH 10 and 25 °C after 3 hours of contact time. No negative phosphate removal results were obtained from phosphate solutions containing sulfate, nitrate and ammonium ions; i.e., resembling real wastewater. Desorption (with NaOH or NaCl) and recovery (with CaO) studies of phosphate sorbed by resin were also carried out, as well as thermodynamic investigations. The proportional desorbability of phosphate from the resin with NaCl was 85.6%. The recovery efficiency of phosphate was 79.4%. The sorption process was spontaneous and endothermic. The ion exchange mechanism was determined using different internal and external diffusion models. The mechanism controlling the removal of phosphate from aqueous solution is both internal and external diffusion. Application of the Langmuir and Freundlich isotherm models showed that the experimental results fit well with the Freundlich model.


INTRODUCTION
The most common types of phosphates present in natural water environments and wastewater are orthophosphates and polyphosphates. Phosphate pollution in wastewater is mostly caused by fertilization in agricultural areas, detergents, animal wastes and industrial activities. Phosphate in wastewater is one of the main factors that leads to the eutrophication and deterioration of water bodies, which leads to short and long-term environmental problems (Kilpimaaa et al. 2014). In such an environment, the depletion of oxygen content, color change, increased turbidity, excessive phosphate accumulation at the bottom, reduction in the number of species, decomposition and fouling are observed, and the environment gradually becomes unusable (Minareci et al. 2009).
In water treatment applications, ion exchange involves the replacement of an ion in the aqueous phase with an ion from the resin phase. The ion exchanger is insoluble in water and can be a substance of natural origin such as zeolite or kaolinite, or a synthetic substance such as a polymer. These materials have fixed-charged functional groups on their internal and/or external surfaces, and the opposite charged (counter-) ions attached to these groups are found. If the functional group is negative, the counter-ion attached is a cation. If it is positive, the counter-ion is an anion and can exchange with another counter-ion in the aqueous phase. For most ion exchange applications in water treatment, synthetic organic resins are utilized because of their relatively large available exchange capacities and ease of regeneration (Helfferich 1962).
The aim of this work was to investigate the phosphate removal ability of Lewatit Monoplus M 600 anion exchange resin from aqueous solution in batch mode operation. Various sorption kinetic models for phosphate were investigated. Along with the sorption process, the effects of contact time and temperature, initial solution pH, and initial phosphate concentration and resin dosage, and the presence of other ions on phosphate removal efficiency were investigated. Isotherm studies were also done and the thermodynamic parameters for phosphate removal determined.

EXPERIMENTAL Materials
Lewatit MonoPlus M 600 was a strongly basic, gelular anion exchange resin with beads of uniform size based on a styrene-divinylbenzene copolymer. It was kindly supplied by Ökotek Chem., Turkey. The resin's characteristics are given in Table 1. A stock phosphate solution was prepared by dissolving anhydrous potassium dihydrogen phosphate (KH 2 PO 4 ) in distilled water to give a concentration of 5,000 mg-PO 4 3À /L. This was diluted when necessary.

Batch sorption of phosphate
In all experiments except the dosage study, a fixed amount of resin was contacted with 50 mL of 100 mg/L phosphate solution, in a capped flask shaken at 140 rpm in a water bath with a temperature-controlled shaker (Memmert). After the samples were filtered, the supernatant solution was analyzed in a spectrophotometer (Hach DR-2000) using the molybdovanadat method to determine the phosphate concentration (Köse & Kıvanç 2011). For the kinetic tests, samples were taken from the shaker water bath at fixed time intervals at 25, 35 and 45°C, and filtered, and the supernatant solution phosphate concentration determined. In order to see the effect of pH, the solution's initial pH was adjusted between 2 and 10.
For the sorption isotherm study and to investigate the effect of initial concentration on phosphate removal, solutions of different phosphate concentrations were contacted with different resin amounts for 3 hours at 25°C and the optimum pH.
The effect of ions such as sulphate, nitrate and ammonium, which are present with the phosphate ions in real wastewaters, on phosphate removal was investigated. K 2 SO 4 , NaNO 3 and NH 4 Cl were used to obtain the respective anions. Tests were carried out at 100 mg-PO 4 3À /L, with 10 or 50 mg/L concentrations of each ion. At the end of the sorption process performed at optimum values, the resin was separated from the solution by centrifugation and filtration, and shaken with 50 mL of 0.5 M NaOH or NaCl solution for 2 hours. The concentration of phosphate ions desorbed from the resin was determined. 4.4 mg of solid CaO was added to the desorption solution to recover the desorbed phosphate.
The amount of phosphate adsorbed, q (mg/g), was calculated using the following equation: The percentage of phosphate removed (%) from solution was calculated using the equation: where C o and C e are the initial and equilibrium phosphate concentrations (mg/L) respectively, m is the mass of resin (g), and V is the volume of the solution (L).

RESULTS AND DISCUSSION
Effect of contact time and temperature, and kinetic models of ion exchange The sorption of phosphate on resin was observed as a function of contact time to determine the equilibrium time at 25, 35 and 45°C. From Figure 1, the percent removal of phosphate increased with increase in contact time.
Phosphate removal was rapid initially then increased slowly until saturation. In the early stages, the increase in the phosphate concentration gradient between the solution and the resin increased the sorption rate. In addition, there were many vacant sites initially in the resin phase. Since there was no significant change in the phosphate concentration in the equilibrium, the percentage of phosphate removal remained constant. Equilibrium was attained within 3 hours for all temperatures.
The effect of temperature on phosphate sorption was investigated by varying the temperature from 25 to 45°C. Thermodynamically, the changes in standard free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) of sorption were calculated using Equations (3)-(5). The values obtained are summarized in Table 2.

DG ¼ ÀRTlnK
(3) where R is the gas constant, K is the equilibrium constant and T is the temperature in K. The equilibrium constant (K) is calculated according to Equation (4): where C s is the equilibrium concentration of phosphate on resin (mg/g), C e is the equilibrium concentration of phosphate in solution (mg/L). According to the van't Hoff equation: Figure 2 gives the linear graph of lnK versus 1/T. The line's slope gives ΔH°(kJ/mol) and the intercept on the xaxis gives ΔS°(J/mol K). Negative ΔG°values at different temperatures showed that the sorption process was spontaneous. Sorption is endothermic if ΔH°is positive, and a positive ΔS°value indicates increasing irregularity during sorption.
Determining the mass transfer parameters to explain the mechanism of the sorption process is important for process design. Mass transfer kinetics for the sorption process includes three steps. The first step is the external diffusion of the adsorbate in the liquid phase, which is transferred through the liquid film around the resin or adsorbent. The difference in concentration between the bulk solution and the surface of the adsorbent is the driving force of the external diffusion. The second step is the internal diffusion of the adsorbate, which is transferred to the pores and channels of the resin or adsorbent. The third step is binding of the adsorbate to the active sites of the resin or adsorbent. The slowest of these steps controls the sorption process and is the rate-limiting step. The prediction of the rate-limiting step is very important for the design of a sorption process. It's assumed that the third step is rapid when compared to the first two steps. Therefore, there are two main mass transfer mechanisms in a sorption process, internal and external diffusion (Figure 3). In order to explain the mechanism involved in the

Boyd's diffusion equations
Boyd's external model has been applied to describe the diffusion of adsorbate through a bounding liquid film (Boyd et al. 1947): where q t and q e are the amounts of phosphate sorbed (mg/g) at any time t and equilibrium time (min), respectively, and k b is the Boyd constant (min À1 ). Boyd et al. (1947) represented an internal diffusion model as follows: B is a parameter defined as : where r is the radius of the resin particle (cm), D eff is the effective intraparticle diffusion coefficient of adsorbate in the adsorbent phase (cm 2 /min), and z is an integer. For values of F ranging from 0 to 0.85: while for values ranging from 0.86 to 1: The plots for diffusion coefficient determination are shown in Figure 4(a) and 4(b) with the linear fitting equations and regression coefficients (R 2 values). The Boyd constant (k b ) was calculated from the slope of the plot of Àln(1 À F) versus time (Figure 4(a)). Figure 4(b) showed that experimental data were plotted according to Boyd's internal diffusion model. If the plot of Bt versus t passed through its origin, it could be said that the mechanism controlling the sorption process is internal diffusion (Yakub et al. 2019;Wang & Guo 2020). Therefore, Figure 4 shows that external diffusion was as important as internal diffusion for the sorption mechanism of phosphate onto ion exchange resin. According to Table 3, R 2 values for both Boyd's diffusion models at different temperatures vary between 0.88 and 1. The magnitude of D eff is 10 À5 -10 À12 cm 2 /s sorption type can be explained by chemisorption (Girish & Ramachandra 2016). Apart from the correlation coefficient (R 2 ), the chi-square (X 2 ) test was used to measure the goodness-of fit. The chi-square test defined by Equation (12) (Das et al. 2021): where q exp and q cal are the experimental and calculated values, respectively. Smaller chi-square values indicate better curve fitting.

Frusawa and Smith external diffusion model
The Furusawa and Smith model was used to analyse the effect of external mass transfer resistance on sorption rate. This model is expressed by Equation (13) where C t is the concentration at time t (mg/L), C 0 is the initial phosphate concentration (mg/L), m is the adsorbent mass per unit volume of the phosphate solution (g/L), K L is the Langmuir constant (obtained by multiplying Qo and b) (L/mg), β L is the mass transfer coefficient (m/s) and Ss is the outer surface of the adsorbent per unit volume of phosphate solution (m À1 ). A linear plot of Y(t) ¼ ln(C t =C o À 1=1 þ mK L ) versus time was used to find the coefficient β L (Figure 5). External mass transfer resistance is generally related to the mass transfer coefficient, and increasing this coefficient decreases the resistance. As can be seen from Table 3, the mass transfer coefficients are large and the external mass transfer rate is accordingly high for all temperatures.

Weber-Morris internal diffusion model
If the rate-limiting step in the sorption process is intraparticle diffusion, the Weber-Morris diffusion model is often used. It is represented by Equation (14) (Ho et al. 2000;Yakout & Elsherif 2010;Velempini et al. 2017;Jia et al. 2020) where C (mg/g) is a parameter that indicates the boundary layer effects and k i is the intraparticle diffusion rate constant (mg/g min 1/2 ), and is related to the intraparticle diffusivity D id (cm/min) as described by Equation (15) (Teixeira et al. 2013).
where d p is the particle diameter (cm) and q e is the equilibrium sorption capacity (mg/g). k i and C can both be determined from the slope and intercept of Figure 6. If intraparticle diffusion is involved in the sorption process, a plot of q t versus t 1/2 would be a good linear relationship and pass through the origin (Poots et al. 1976). As seen from Figure 5, deviation of the line from the origin indicates that the rate limiting step is not only intraparticle diffusion. This implies that phosphate sorption was a multistage process. Figure 6 had two linear parts. The initial part represents external diffusion and the second intraparticle diffusion. The parameters of intraparticle diffusion are shown in Table 3. The value of R 2 . 0.98 in this model justifies the mechanism for the diffusion of phosphate. The experimental and calculated quantities sorbed at equilibrium (q eexp and q ecal ) from the model agreed closely given the low values for the chi square test (X 2 ).

Effect of initial pH
The effect of pH on phosphate sorption is shown in Figure 7. At pH 2, the sorption percentage of phosphate onto resin was 80%. At pH 10, the phosphate sorption percentage was 99%, the highest achieved. At low pH values, phosphate exists predominantly as H 3 PO 4 , which is neutral and has difficulty entering resin pores because of its relatively large size. With increasing pH values, the major phosphate species are H 2 PO 4À , HPO 4 2À and PO 4 3À , which can sorb onto the resin (Ruixia et al. 2002;Taleb et al. 2008).

Effect of initial phosphate concentration and resin dosage
Phosphate sorption onto the resin was investigated as a function of resin dosage (between 0.1 and 1 g/50 mL) at five different initial phosphate concentrations (between 50 and 200 mg/L). The resin's removal efficiency varied approximately from 71 to 99.8% with increasing resin concentration, for all initial phosphate concentrations ( Figure 8). Proportional phosphate removal increased with increasing resin dosage due to the increasing sorption surface area.  The experimental data were applied to the Langmuir and Freundlich isotherm models. The linear forms of the isotherm equations are given by Equations (16) and (17) (Weber 1972): where C e is the equilibrium concentration (mg/L), q e is the amount of phosphate sorbed at equilibrium (mg/g), and Q o and b are the Langmuir constants related to sorption capacity and energy of sorption, respectively. Also, K and n are the Freundlich constants, being indicative of the sorption capacity and intensity of sorption, respectively. The isotherm model constants are given in Table 4 and a graph of the linear Freundlich equation is shown in Figure 9. According to the correlation coefficients, the Freundlich equation fits the experimental data better than the Langmuir equation, signifying heterogeneous surface binding.

Effect of other ions
The parameters affecting phosphate removal by the resin were investigated for the aqueous solution. However, there are other ions besides the phosphate anion in real wastewaters. Sulphate, nitrate and ammonium ions were used as foreign ions, and the effects of their presence on phosphate removal studied. The experimental data are shown in Figure 10, which shows that, from solutions of 10 or 50 mg/L of sulfate, nitrate and ammonium ions coexisting with 100 mg-phosphate/L, phosphate removal did not change significantly.

Desorption and recovery studies
Desorption and recovery processes are important in terms of resin reusability and cost. The experiments were conducted for contact time of 3 hours using 0.5 g-resin/50 mL and 100 mg-P/L at pH 10. After three hours sorption, the phosphate-loaded resin was filtered from the aqueous solution. The desorption experiments were carried out in 0.5 M NaOH and 0.5 M NaCl solutions, in batch mode, to recover the phosphate from the resin.  Proportional desorption was calculated using Equation (18): Desorbability(%) ¼ Amount of desorbed to the elution medium Amount of ion sorbed on the adsorbent Â 100 The proportional desorbability of phosphate was determined as 70.5 and 85.6% with NaOH and NaCl, respectively. Solid calcium oxide was added to the NaCl solution to recover the desorbed phosphate as calcium phosphate, giving a yield of 79.4%.

CONCLUSION
Use of Lewatit Monoplus M600 anion exchange resin for phosphate removal from aqueous solutions was investigated The amount of phosphate sorbed from aqueous solution increased with contact time, equilibrium being achieved within 180 min. No significant temperature effect on phosphate removal was observed. The optimum resin dose for phosphate removal was 0.5 g/50 mL. The values of ΔG°and ΔH°show that phosphate sorption on the resin is spontaneous and endothermic. The kinetic results show that external and internal diffusion are equally important for phosphate sorption onto the resin. The Freundlich isotherm model showed the best fit for the experimental results. Resin removal efficiency was high (99%) at initial pH 10. The presence of foreign  ions had no significant effect on phosphate removal. The desorption study showed that phosphate is desorbed easily with NaCl. Precipitate recovered in the form of calcium phosphate can be used as fertilizer or soil conditioner.

AUTHOR CONTRIBUTION STATEMENT
T.E.Bektas: design and conduct of experiments, creation of the article. B. Kıvanç Ugȗrluoglu: experimental work, B.Tan: editing. All authors discussed the results and contributed to the final manuscript.

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