Influences and Isotherm Models on Phosphorus Removal from Wastewater by Using Fe³⁺-Type UBK10 Cation Exchange Resin as Absorbent

Abstract

Phosphorus is a nutrient that is required for life. Eutrophication, on the other hand, is caused by an overabundance of phosphorus in the hydrosphere. Eutrophication is a form of water pollution that can be solved by removing phosphorus from the environment. Adsorption with cation exchange resin is a more practical method for removing phosphate ions at low concentrations than traditional approaches. The column approach is good for recovering phosphate effectively. As a result, a superior adsorption ability of the column and a practical regeneration process are critical. Accordingly, the goal of this study is to design a phosphate ion recovery system using a Fe-type cation exchange resin (Fe³⁺-type UBK 10) column. The batch approach was used to investigate the characteristics influencing the adsorption of phosphate ions on Fe-type UBK 10 in order to better comprehend the mechanism of adsorption. The number of phosphate ions adsorbed increased with increasing reaction time, according to the findings. The best results were achieved using 3 g of resin in 0.05 M NaCl at pH 6. The best fit was found in the Langmuir isotherm using equilibrium data.

1. Introduction

Phosphorus is extracted from P-rich rocks and is a non-renewable material. Approximately 240 million tons of the world’s mined phosphate rock are now produced from phosphorus-rich ores [1]. Phosphorus is being extensively employed in agriculture, the chemical industry, and medicine as human activities become more intense. Excessive phosphorus enters the environment and natural water bodies as a result of the direct discharge of sewage following the use of high-phosphorus washing powder and the restricted usage of phosphorus in the soil. Phosphorus content is rising steadily over time. Phosphate pollution poses a serious threat to ecosystems. Therefore, phosphorus presents a fundamental challenge in the field of resource recovery, and better technologies are urgently needed to advance toward a circular economy and to address the problem of soil depletion [2]. Excess phosphorus is a key factor contributing to the eutrophication [3] (one of the trophic states showing the range of total phosphorus around 30–100 mg/L [4]) of water bodies receiving wastewater treatment effluents. Due to the excessive development of autotrophic organisms (such as algae) and the inhibition of deep aquatic photosynthesis, eutrophication of the water body lowers the concentration of dissolved oxygen, which results in hypoxia. The ecological equilibrium is disturbed and the quantity of aquatic species is significantly reduced [5]. Phosphate levels of 1 mg L⁻¹, for example, are sufficient to trigger algae blooms in lakes, rivers, and reservoirs [6]. Humans are also unable to absorb the element through drinking water [7]. According to the World Health Organization (WHO), 10 g/L of phosphate is the upper limit for first-class water quality in surface waters. According to the U.S. Environmental Protection Agency (USEPA), the amount of total phosphorus in flowing waterways such as rivers, streams, and lakes should not be more than 0.10 mg/L or 0.05 mg/L in any stream that reaches lakes or reservoirs [8]. Phosphate levels in water that are higher than the recommended level (10 g/L) should be reduced. Moreover, many nations around the world control and limit the total phosphate concentration in water. In China, the limit is 0.5 mg P L⁻¹, while it is 0.1–0.5 mg P L⁻¹ for the EU, and 0.05 mg P L⁻¹ for the USA [9]. For the conservation of water resources, research on a strategy for removing phosphate ions from polluted water is necessary.

Up to this point, phosphorus removal has been accomplished using a number of technological methods, such as chemical precipitation, biological methods, ion exchange, electrodialysis, and adsorption [10,11,12]. However, current research efforts are concentrated on the creation of high-quality, high-performance adsorbents that enable regeneration and swift ultimate disposal. For phosphate removal, adsorption is a more suitable, cost-effective, simple, and economical operation [10]. Since adsorption does not generate sludge and has minimal disposal problems, it is stable [13], low cost, and pollution free. As a result, research into phosphate ion removal techniques from polluted water is critically necessary for the protection of natural resources. When choosing the most effective phosphorus removal technique, it is crucial to take into account the individual properties of the wastewater and the adsorbent material as well as the overall treatment goals. The technique of choosing should be determined by a thorough analysis of variables including cost, efficiency, scalability, and environmental effect. Several materials such as anion exchange resin have been investigated as adsorbents for phosphate removal [6,8,9,10,14,15,16,17,18,19,20,21]. Adsorption is a technology that has promise for removing macro- and trace nutrients from wastewater. As an adsorbent, several materials have been employed. The removal of certain chemicals, such as phosphorus, is not selectively accomplished by these materials. Therefore, a specific phosphorus removal technique seems appealing. Ion exchangers are made of an insoluble substance that contains ions that may be swapped with other ions in solutions. Adsorption with cation exchange resin is a more practical method for removing phosphate ions at small concentrations than the traditional approach [22,23]. Cation exchange resins that adsorb ferric ions have been demonstrated to be effective in removing anions in recent times [10].

Iron (Fe) is ubiquitously present in soil, predominantly in the form of goethite (FeOOH), an oxyhydroxide, as well as hematite (Fe₂O₃) and magnetite (Fe₃O₄). Iron-based materials have received a lot of attention as one of the favored adsorbents due to the selective phosphorous adsorption [24]. As per a previous publication [25], it has the ability to effectively adsorb Se. The utilization of iron-type resin is being employed. The column method is considered to be a more favorable approach in order to prevent the production of a substantial quantity of sludge. In order to employ the column technique for the elimination of deleterious constituents for future work, it is imperative to fabricate the requisite adsorbent. The process of ferric ions being adsorbed onto cation exchange resin is accompanied by the adsorption of the anion, which is commonly referred to as “coadsorption” in the literature [25].

This study aimed to investigate the potential of a cation exchange resin that adsorbs ferric ions (Fe³⁺-type) for the removal of P. At first, the adsorption behavior of P onto the Fe³⁺-type resin was investigated through a batch method. The study’s final aim was to employ isotherm models to more thoroughly describe the adsorption mechanism.

2. Materials and Methods

2.1. Formulation of Fe³⁺-Type UBK 10

The UBK 10 (Na type, Mitsubishi Chemical Co. Ltd., Tokyo, Japan) strong acid-type cation exchange resin was employed in this work. Crosslinking 10% and 2.2 meq/cm³ cation exchange capability were the values. By mixing 40 g of the Na-type UBK 10 with 500 cm³ of 0.1 mol/L FeCl₃ solution (pH 1.5) and shaking the suspension for 24 h, the cation exchange resin that could adsorb ferric ions was created. The resin underwent three days of air drying after filtering via a 0.45 m membrane filter. The quantity of Fe adsorbed to the UBK 10 resin was calculated to be 0.75 mmol/g dry resin based on measurements made using an atomic absorption spectrophotometer (AAS, iCE 3300, Thermo Fisher Scientific Inc., Waltham, MA, USA) to measure the Fe content in the filtrate. The resin in question will henceforth be referred to as Fe³⁺-type UBK 10. The Fe³⁺-type UBK 10 experiment was utilized to produce the ferric ion absorption cation ion exchange resin, widely used for phosphate removal [22]. KH₂PO₄ was bought from Wako Pure Chemicals as a source of PO₄³⁻. The investigation’s reagents were of the analytical variety. Utilizing ultrapure water, the sample solutions were created. By dissolving KH₂PO₄ in ultrapure water, the stock solutions of PO₄³⁻ were created.

2.2. Adsorption of Phosphate Ions to the Fe³⁺-Type UBK 10 by the Batch Method

Utilizing potassium dihydrogen phosphate, a stock solution of phosphate solution (3.06 ppm as PO₄³⁻) with or without NaCl was generated (KH₂PO₄). The effectiveness of the resin’s ability to remove phosphorus was assessed after the initial pH of synthetic phosphorus effluent was adjusted to 2–9 using HCl or NaOH solution. Analytical grade reagents were implemented in this research. Fe³⁺-type UBK 10 was added to phosphate solution (500 cm³) in varied quantities, with 1 ppm as P. At room temperature, the suspensions were stirred continuously for a contact time. Using a syringe, approximately 10 cm³ of the sample solution is obtained at suitable intervals (specified time). The suspension is screened through a 0.45 m membrane and placed in a 10 cm³ volumetric flask, which is then filled with a 0.4 cm³ molybdate solution and a 0.1 cm³ L-ascorbic acid solution. P content in filtrates was evaluated using the molybdate blue phosphorus approach in combination with a UV-visible spectrophotometer (SHIMADZU, Tokyo, Japan) using absorbance at 880 nm. Adsorption is essential for removal, but recovery and reusing the adsorbate are just as crucial for making the entire process affordable, environmentally friendly, and sustainable. Desorption is most often carried out in phosphorus investigations using NaOH and NaCl.

2.3. Molybdenum Blue Phosphorus Approach with a UV-Visible Spectrophotometer

Ammonium molybdate solution was generated by mixing 12 g of (NH₄)₆Mo₇O₂₄ of and 0.048 g of C₈H₄K₂O₁₂Sb₂ into 60 mL deionized water for use in the Molybdenum blue phosphorus technique utilized in this investigation. Using a volumetric flask, the solution was mixed with 32 mL of 9 mol dm⁻³ H₂SO₄ and diluted to 100 mL 3.6 g of L-ascorbic acid was dissolved in 50 mL deionized water to produce the L-ascorbic acid solution, which should be kept refrigerated at 10 °C. The solution is shown in blue color. The levels of color depended on concentration of P in the solution.

2.4. Equilibrium Studies

A specified quantity of resin (3 g) was placed into 500 mL beakers under various concentrations of P (0–6 mmol/L).

The speed at 200 rpm was used as the agitation speed while the beaker was shaken. The final phosphorus concentration was measured using UV/VIS spectrophotometer with a wavelength of 880 nm.

Different beginning phosphorus concentrations (0–6 mmol/L) were used to establish the equilibrium period. To make sure that all isotherms reached equilibrium, the agitation duration for the isotherm research was set at 48 h.

It is evident that understanding the interactions between adsorbents and adsorbates requires the use of adsorption isotherms. Isotherms, or the time-course of the quantity of adsorbate on the adsorbent at a constant temperature, are commonly used to characterize adsorption [26]. Equation (1) was used to determine the quantity of phosphate adsorption at equilibrium, q_e [27,28] (mg P g⁻¹).

q_e = (C₀ − C_e)V/M

(1)

where C₀ and C_e (mg P L⁻¹) are the initial and equilibrium phosphate concentration, respectively, V (L) is the volume of the solution, and m (g) is the mass of the adsorbent.

To characterize the experimental data from the equilibrium investigation, the Langmuir, Freundlich, and Temkin isotherms were employed.

2.4.1. The Langmuir Model

According to the Langmuir model, there is no adsorbate transmigration in the plane during homogeneous energies of adsorption onto the surface [26]. The adsorbed layer is one molecule in thickness or monolayer adsorption [29]. The Langmuir equation, as opposed to the Freundlich equation, was created from a theoretical perspective to simulate the adsorption of gas molecules on surfaces. Later, it was used for the ion adsorption from solution on mineral surfaces. Equation (2) below illustrates the Langmuir isotherm for non-linear monolayer adsorption.

q_e = (q_m bC_e)/1 + bC_e

(2)

Equation (3) below reveals the linear form of Langmuir isotherm for non-linear monolayer adsorption [29].

C_e/q_e = C_e/q_m + 1/q_mb

(3)

where ${\mathrm{q}}_{\mathrm{e}}$ represents the balanced amount of adsorbate (mg/g), ${\mathrm{C}}_{\mathrm{e}}$ is the balanced dilution of adsorbate (mg/L), ${\mathrm{q}}_{\mathrm{m}}$ is maximum adsorption capacity, and $\mathrm{b}$ is the Langmuir constant. Linear plots are ${\mathrm{C}}_{\mathrm{e}}/{\mathrm{q}}_{\mathrm{e}}$ and ${\mathrm{C}}_{\mathrm{e}}$. ${\mathrm{q}}_{\mathrm{m}}$ and $\mathrm{b}$ are given by the slope and intercept, respectively.

In the Langmuir model, the R_L parameter, which is the separation factor, is an essential aspect of the adsorption process shown in Equation (4). The values for R_L are shown in

Table 1. Values of separation factor for the Langmuir model.

${\mathbf{R}}_{\mathbf{L}} \mathbf{V}\mathbf{a}\mathbf{l}\mathbf{u}\mathbf{e}\mathbf{s}$	Adsorption Process for the Langmuir Model
${\mathrm{R}}_{\mathrm{L}}=0$	Irreversible
${\mathrm{R}}_{\mathrm{L}}=1$	Linear
$0<{\mathrm{R}}_{\mathrm{L}}<1$	Favorable
${\mathrm{R}}_{\mathrm{L}}>1$	Unfavorable

.

R_L = 1/(1 + bC₀)

(4)

The R_L value assists in categorizing the adsorption process into several groups. R_L < 1 indicates favorable adsorption. As the initial concentration of the solute rises, so does the adsorption capacity. R_L = 1 represents linear adsorption. As the initial concentration of the solute rises, the adsorption capacity stays constant. R_L > 1 suggests unfavorable adsorption. As the solute’s initial concentration rises, so does its ability to be adsorbed. R_L = 0 indicates irreversible adsorption. This improbable scenario means that all adsorption sites have already been occupied and that no more adsorption will take place.

2.4.2. The Freundlich Model

The Freundlich equation, which lacks a theoretical foundation and was derived experimentally, seems to be effective for explaining processes that lead to surface precipitation and chemical adsorption of ions [30,31]. In non-ideal, reversible, and heterogeneous systems, where (1) adsorption is not restricted to a monolayer coverage and (2) adsorption capacity rises as input adsorbate concentration rises, the Freundlich isotherm model is frequently employed to explain the processes [32]. It seems advantageous for modeling ion sorption and surface precipitation reactions through chemical adsorption [33]. The non-linear form of the Freundlich equation is given in Equation (5). A high adsorbate affinity is distinguished by a high value of K_f [26]

q_e = K_f C_e^1/n

(5)

The linear form of Freundlich isotherm is quantified by Equation (6) [29]

Log q_e = log K_f + 1/n log C_e

(6)

where $K_f$ and $n$ are the Freundlich constant acquired from the slope and intercept. Sorption data can be established to follow the Freundlich model by plotting $\log q_e$ against $\log C_e$. The $\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$n$}\right.$ parameter, a heterogeneity factor, is a gauge of the adsorbate concentration on the adsorbent and should range between 0.1 and 1. The values for n are given in

Table 2. Values of separation factor for the Freundlich model.

$\mathbf{n} \mathbf{V}\mathbf{a}\mathbf{l}\mathbf{u}\mathbf{e}\mathbf{s}$	Adsorption Process for the Freundlich Model
$n=0$	Irreversible
$n=1$	Linear
$0<n<1$	Favorable
$n>1$	Unfavorable

.

2.4.3. The Temkin Model

Additionally, for heterogeneous adsorption of an adsorbate on a surface, the Temkin isotherm is accessible. The Temkin model is utilized, and Equation (7) depicts the Temkin model in its non-linear version [34].

q_e = RT/b₁ ln(k_tC_e)

(7)

In Equation (8), the Temkin model is shown in its linear form.

q_e = B₁ lnk_t + B₁lnC_e

(8)

where ${\mathrm{B}}_1$ is adsorption heat (kJ mol⁻¹), and ${\mathrm{k}}_{\mathrm{t}}$ is the equilibrium binding constant (L g⁻¹). ${\mathrm{B}}_1$ and ${\mathrm{k}}_{\mathrm{t}}$ can be determined from the slope and the intercept of the curve, respectively, by plotting ${\mathrm{q}}_{\mathrm{e}}$ versus $\ln {\mathrm{C}}_{\mathrm{e}}$. Rapid sorption of adsorbate at the initial stage is shown by a high ${\mathrm{b}}_1$ value [26].

In the Harkin-Jura model, the existence of a heterogeneous pore distribution may be used to explain how the isotherm equation, which accounts for multilayer adsorption, works. In the Halsey model, the fitting of experimental data to this equation confirms that the adsorbent is heteroporous and that this equation is appropriate for multilayer adsorption [26].

3. Results and Discussions

3.1. Influence of Fe³⁺-Type UBK 10 Resin Quantity on Phosphate Ion Adsorption

In the range of 1–6 g L⁻¹, the influence of resin dosage was assessed. With a higher adsorbent dosage, Figure 1 indicates a rise in adsorption effectiveness. At 4 g, 5 g, and 6 g of Fe³⁺-type UBK 10 resin for 72 h, the phosphate adsorption percentage rose by more than 90%. Because adsorption occurs around the resin’s surface, a larger surface area can improve the odds of an adsorbed response. Due to an increase in sorption surface area, proportional phosphate removal increased with increasing resin dose [35]. The increase in adsorbent concentration translates into an increase in active exchangeable adsorption sites [36]. Increasing the concentration of the adsorbent material accounts for these outcomes. These findings are explained by the fact that raising the adsorbent will result in more contact surface area and adsorption sites; thus, increasing the removal amount will result in more contact surface area and adsorption sites, and so, the rises. In contrast, as the amount of adsorbent removed increased, the adsorption capacity dropped. The inverse link between the adsorption capacity and sorbent dosage, which is due to the inverse relationship between the adsorption and the mass of the adsorbent, is responsible for the adsorption capacity decreasing with increasing adsorbent dosage [37].

3.2. Impact of Contact Time on Phosphate Ion Adsorption

The contact time significantly affects the adsorption process [36]. At varied resin compositions, the influence of contact time on phosphorus adsorption effectiveness was examined. Figure 2 indicates that as reaction times rose, the phosphorus adsorption percentage increased. There is more than 95 percent adsorption in the case of 6 g, which is visible after 48 h. After 96 h, the phosphate adsorption percentage is enhanced by more than 90% at 4–6 g of Fe³⁺-type UBK 10 resin. Due to the purpose of economics, the quantity of Fe³⁺-type UBK 10 resin and reaction time are major aspects to develop. Early on, the sorption rate was accelerated by an increase in the gradient in phosphate concentration between the solution and the resin. Additionally, the first resin phase included a lot of empty locations. The equilibrium’s phosphate content did not significantly alter; therefore, the proportion of phosphate removal did not change either. Within 60 h, equilibrium had been reached at 6 g/L of resin mass.

3.3. Impact of NaCl Concentration on Rate of Phosphate Adsorption

Natural water contained many compositions including chloride. It could interfere with the phosphate removal through competitive adsorption [16]. With a higher salt concentration, the adsorption rate can be enhanced instantaneously [25]. Since the presence of NaCl accelerated the adsorption of P to the Fe³⁺-type UBK 10, the impact of the NaCl concentration was investigated. The variability in the P adsorption part as the NaCl concentration varied from 0 to 0.1 mol/dm³ is seen in Figure 3. The phosphate adsorption % rose swiftly with the rising NaCl concentration in 5 min, implying that the adsorption rate can be raised suddenly with the increasing NaCl concentration. The percentage of P that is adsorbed in the absence of NaCl is close to 50% with 0.05 and 0.1 mol/dm³ NaCl. At a NaCl concentration of 0.1 mol/dm³, the fraction of P quickly rose to around 95% in 5 min. A small rise in the adsorption ratio was observed, with values exceeding 0.01 mol/dm³. Above 0.05 mol/dm³ NaCl, the adsorption fraction remained steady at more than 90%. Adsorption is often slowed down with higher concentrations of the supporting electrolyte in the electrostatic adsorption of cations and anions on the surface of metal oxides and hydroxides [38]. Na⁺ ions may obstruct the negative charge of the SO³⁻ functional group. This was the reason for the rising adsorption amount of P on the Fe³⁺-type UBK 10 in the presence of NaCl. When the Fe³⁺-type cation exchange resin column can eliminate phosphate ions, the NaCl concentration will be an essential factor because boosting the NaCl concentration can enhance the process’s adsorption rate.

For the aqueous solution, the factors influencing phosphate removal by the resin were examined. In actual wastewaters, there are other ions in addition to the phosphate anion. As foreign ions, sulfate, nitrate, and ammonium ions were employed, and the effects of their presence on phosphate removal will be investigated in future work.

3.4. Influence of pH on Phosphate Ion Adsorption

The adsorption of cation and anion is widely recognized to be altered by pH. One of the most critical factors that determine adsorption at water-adsorbent interfaces is pH [39]. For the adsorption of phosphate, several iron oxide and hydroxide species were examined, primarily from solutions with pH 2 or even buffered to pH = 7 [40]. Since metal oxides and hydroxides have an isoelectric point, the adsorption of cations and anions is typically regulated by pH in the hydrosphere and soil system, and FeOOH is one of the most significant adsorbents of PO₄³⁻ [41]. As a result, it was examined how the pH affected the adsorption of P throughout a pH range of 3 to 9. The relationship between the pH and the P adsorption is depicted in Figure 4. Figure 3 shows the influence of varied pH on adsorption efficiency at various contact periods. With increasing pH in the range of 3–9, the proportion of PO₄³⁻ adsorbed diminished. The phosphate adsorption % increased with increasing pH and peaked at pH 6 for all durations of contact.

When the pH is lower than 6, the surface of the material has a positive charge, which facilitates the adsorption of negatively charged phosphates in the forms of HPO₄²⁻ and H₂PO₄⁻, and which are maintained by iron oxyhydroxide by creating a complex inside the sphere. When the pH is lowered, more phosphate remains in the form of H₂PO₄⁻, which has a lower negative charge, making the adsorption process more challenging since HPO₄²⁻ forms stronger amphoteric complexes than H₂PO₄⁻. Additionally, by changing the negative charge of phosphate from divalent to monovalent, the low pH solution lessens the electrostatic interaction between the positively charged iron oxyhydroxide groups and the phosphate ions [42]. On the other hand, the protonation of phosphate ions depends on pH [43]. A concentration rise above pH 6 may be as a result of the rising competition between OH⁻ groups and phosphate species PO₄³⁻ for the adsorption sites [44]. FeOOH becomes deionized and negatively charged when the pH is greater than 6 [42].

As a result, a pH of 6 was chosen for phosphate adsorption on Fe³⁺⁻type UBK 10 resin since this pH is adequate for removing phosphate from natural water of similar pH without the requirement to adjust the pH.

3.5. Isotherm Model

The isotherm study is one of the important factors required for analysis and design of adsorbent adsorbate system [45]. Adsorption experiments were performed at various P concentrations (0–6 mmol/dm³) in order to create an adsorption isotherm. In Figure 5, the adsorption isotherm is presented. As illustrated in Figure 6, adsorption isotherm models such as the Langmuir, Freundlich, and Temkin models were investigated.

Table 3. Isotherm parameter values for isotherm models.

Isotherm Models	Isotherm Parameters	Values	${\mathit{R}}^2$
Langmuir Model	qm	2.854	0.99
	b	0.164
Freundlich Model	n	4.053	0.92
	kf	−1.736
Temkin Model	B1	16.645	0.88
	kt	−5.948

shows the values for the isotherm factors. The Langmuir isotherm model fit the data the best with R_L < 1, indicating favorable adsorption. From the result, R_L indicated that the adsorption capacity increases as the initial concentration of the solute increases. The correlation coefficient was higher than 0.98. According to the findings, the optimum phosphate ion adsorption volume for the Fe³⁺-type UBK 10 could be calculated at 2.85 mmol g⁻¹.

The preferred adsorption process was highlighted by the Langmuir separation factor ($R_L)$ and the Freundlich heterogeneity factor (n). Therefore, the comparison of isotherm best fits of three isotherm models from experimental data in this study is the Langmuir, Freundlich, and Temkin model.

4. Conclusions

The results and analysis of the adsorption performance of phosphate ions on the Fe³⁺-type UBK 10 resin are presented in the conclusion. Initially, the effects of pH, adsorbent dose, reaction time, and NaCl concentrations on phosphate ion adsorption were investigated. Consequently, the adsorption performance was pH dependent, with an enhanced adsorption rate as the amount of Fe³⁺-type UBK 10 resin was increased. The rate was modest, and the rate of adsorption grew steadily as reaction time reached up to 120 h. With higher NaCl content, the phosphorous adsorption percentage increased sharply. The ideal parameters were 1 ppm phosphorous to 3 g Fe³⁺-type UBK 10 resin in 500 mL under 0.1 M NaCl at pH 6. Ultimately, an isotherm-based description of adsorption behavior was given. With a correlation coefficient of 0.98, the Langmuir model provided the best match, and the maximum adsorption capacity of phosphate ions for the Fe³⁺-type UBK 10 was calculated to be 2.85 mmol g⁻¹.