Fabrication of recyclable Fe³⁺ chelated aminated polypropylene fiber for efficient clean-up of phosphate wastewater

Abstract

Herein, a Fe³⁺-loaded aminated polypropylene fiber has been reported as an efficient phosphate adsorbent. The remarkable phosphate removal ability of the fiber is due to Fe³⁺ immobilization, and it demonstrates a maximum adsorption capacity of 33.94 mg·P·g^–1. Adsorption experiments showed that the fiber is applicable over a wide pH range from 2 to 9. Furthermore, the adsorption kinetics and isotherm data were consistent with the pseudo-second-order and Langmuir adsorption models, respectively. The adsorption equilibrium of the fiber for phosphate was reached within 60 min, indicating an efficient monolayer chemisorption process. Moreover, the adsorbent maintained prominent phosphate removal in the presence of competitive ions such as NO₃^– and Cl^–, exhibiting high selectivity. More importantly, the fiber demonstrated excellent reusability (5 times) and low adsorption limit below 0.02 mg·P·g^–1. In addition, the phosphate removal efficiency of the fiber can exceed 99% under continuous flow conditions. The adsorption mechanism was studied by X-ray photoelectron spectroscopy, showing that the adsorption of phosphate on the fiber mainly depended on the chemical adsorption of the modified Fe³⁺. Overall, this study proves that the fiber possesses many advantages for phosphate removal, including high adsorption efficiency, lower treatment limit, good recyclability, and environmental friendliness.

Key words： phosphate adsorption; aminated polypropylene fiber; Fe³⁺; ligand exchange; reusability

1 Introduction

Phosphorus is an important non-renewable resource that is widely used in modern agriculture and in the chemical industry. Presently, phosphate reserves are of low quality and expensive to extract [1]. Furthermore, phosphorus enters the water environment through different routes during various application processes, causing loss of phosphorus resources and water eutrophication [2]. Naturally, phosphorus has diverse chemical forms in water, with phosphate as the main form, leading to water eutrophication. Currently, phosphorus is the limiting factor for water eutrophication, and pollution control in water has always been a global concern. Therefore, considering the economic and environmental benefits, the removal as well as recovery of phosphate from wastewater is of great significance.

To date, several techniques, such as chemical precipitation [3], membrane separation process [4], biological treatment [5], anion exchange [6], and adsorption [7] have been used to remove phosphate. Among them, the adsorption method demonstrates several advantages, such as high efficiency and convenient application, attracting the attention of many researchers. Diverse adsorption materials, such as natural minerals [8], activated carbon [9], and composite materials [10] exist. However, phosphate adsorbents in powder form are difficult to recover in water, which may cause secondary pollution problem, and some materials have the problems of harsh preparation conditions, high cost, and low removal ability. The preparation of new adsorbents by loading functional groups with specific affinity for phosphate on solid supports can overcome the aforementioned defects, which is one of the research hotspots in recent years [11]. Many researchers have used different metals, metal oxides, and their modified adsorbents for phosphate removal [12–14]. Among the different metals, iron ions have attracted much attention because of their low price, strong binding force with phosphorus, and several sources [15]. In recent years, significant progress has been made in the use of adsorbents, such as iron-modified biochar [16], chitosan [17], zeolite [18], metal organic framework [19], resins [20] and polymers [21] in phosphate removal. However, some of these supports have the disadvantages of difficult recovery, complex preparation, and poor repeatability. Hence, it is important to develop a support material to load iron for phosphate removal with a better performance.

Recently, textile fiber has attracted wide attention as a support material because of its ease of modification, chemical stability, and low cost [22–24]. Compared with other supports, the fibers are flexible and easy to weave, making them applicable to filtering devices in different scenarios [25–27]. Many researchers have successfully constructed functionalized fiber adsorbents for dyes, heavy metals, and phosphate removal, and the results demonstrate that these functionalized fibers have excellent recycling ability and high selectivity for pollutants [28–32]. Furthermore, polypropylene fiber (PPF) demonstrates more acid−alkali resistance, higher mechanical strength, and lower density, making it significant in the field of catalysis and adsorption [33–35]. For example, Zhang et al. [34] prepared alkylbenzene functionalized polypropylene to increase the removal ability of dioctyl phthalate, which demonstrated a high adsorption capacity of 272.4 mg·g^–1. Li et al. [25] constructed an ion-imprinted PPF for the selective removal of Cu²⁺, retaining almost the same adsorption capacity even after 10 cycles. Shi et al. [36] reported an ionic liquid-supported PPF for the conversion of fructose to 5-hydroxymethylfurfural with superior catalytic activity and excellent reusability of 10 times. However, the application of functionalized PPF in the purification and recovery of phosphate from water has rarely been reported. Furthermore, textile fibers have dual attributes of resources and pollution. The textile industry is the second-largest source of industrial pollution, accounting for 10% of global carbon emissions [37]. China is the largest fiber consumer with an annual output of approximately 26.1 million tons [38]. The unreasonable utilization of textile fibers causes serious waste of resources and environmental pollution. Therefore, the preparation of phosphate adsorption materials using textile fiber waste has high innovation and practical application value [39].

In this study, PPF was used as a support material to synthesize a novel Fe³⁺-supported functionalized fiber. First, the amide polypropylene fiber (PP_AMF) was obtained by a free radical polyreaction between PPF and acrylamide (AM), and ammoniated fiber (PP_AMAF) was obtained by an amine exchange reaction with hyperbranched polyethylene imine (HPEI). Finally, the PP_AMAF-Fe was formed by chelating Fe³⁺ on the ammoniated fiber. PP_AMAF-Fe was systematically characterized to confirm its successful preparation. The phosphate removal ability of the PP_AMAF-Fe was examined in detail. For example, different influencing factors, such as fiber modification degree, pH of the solution (wastewater), contact temperature, and coexisting anions, were studied for the removal of phosphate by the PP_AMAF-Fe. Moreover, the adsorption kinetics, isotherms, and thermodynamic models were utilized to illustrate the phosphate adsorption mechanism of PP_AMAF-Fe. Furthermore, the recycling performance and phosphate removal ability of the PP_AMAF-Fe in actual wastewater were studied to determine its practical application. Additionally, a continuous flow installation was attempted to remove phosphate using PP_AMAF-Fe in wastewater.

2 Experimental

2.1 Synthesis of PP_AMAF-Fe

Three-steps mechanism for PP_AMAF-Fe synthesis (Fig. S1, cf. Electronic Supplementary Material, ESM): (Step 1) Dried PP (1.0 g), AM (10.0 g), benzoyl peroxide (BPO, 0.03 g), toluene (5.0 g), (NH₄)₂Fe(SO₄)₂·6H₂O (0.08 g), and deionized water (20 mL) were mixed in a reaction kettle, and the reaction temperature was set to 85 °C for 1.5 h. After the reaction, the prepared fibers were repeatedly rinsed with water (70–80 °C). Subsequently, an amide fiber (PP_AMF) with a weight of 17.66% was obtained by drying in an oven at 60 °C overnight. (Step 2) The dried PP_AMF (0.2 g) and HPEI (2.0 g) were placed in a reaction kettle with 15 mL H₂O at 110 °C for 6 h. As in the previous step, after cooling, the fibers were washed and dried, and PP_AMAF with a weight gain of 13.5% were obtained. PP_AMAF with different degrees of modification was obtained by changing the amount of HPEI, more details are provided in Table S1 (cf. ESM). (Step 3) The dried PP_AMAF (100 mg) and 50 mL FeCl₃ (0.1 mol·L^–1) were subjected to a chelation reaction at room temperature. After 2 h of reaction, the fiber with attached Fe³⁺ was rinsed with deionized H₂O. After drying the fiber at 60 °C overnight, Fe³⁺-supported aminated polypropylene fibers were obtained.

2.2 General procedures of phosphate removal by the functionalized fiber

In a series of adsorption experiments, phosphate adsorption capability of PP_AMAF-Fe was measured by setting different solution pH value, temperature, adsorption time and phosphate concentration. When the adsorption reaction ends, the functionalized fiber was taken out from the adsorption system using tweezers, the remained phosphate concentration was determined at the wavelength of 700 nm of Vis spectrophotometer. All adsorption experiments will be conducted parallelly three times, the average value and standard deviation of the adsorption data were calculated.

The pH of KH₂PO₄ sample was adjusted to 3–9 with hydrochloric acid and sodium hydroxide. Then the 10 mg dried PP_AMAF-Fe were added to 15 mL of KH₂PO₄ (20 mg·P·L^–1) solutions of different pH and were stirred by a magnetic stirrer for 1.5 h under room temperature (25 °C).

Adsorption kinetics and adsorption thermodynamic were implemented with 15 mL of phosphate (20 mg·P·L^–1) with pH of 6. Different 10 mg dried PP_AMAF-Fe were added to the solution at different contact temperatures (288, 298 and 308 K). After the above mixed liquid was stirred for 2, 5, 10, 15, 30, 60, 90, 120, 150 and 180 min, the remained phosphate concentration was determined.

At 298 K and pH 6, PP_AMAF-Fe was immersed into 15 mL of KH₂PO₄ with different original concentration (1–50 mg·P·L^–1). After the adsorption equilibrium, the remained phosphate concentration was determined.

In order to reveal the effects of coexisting ion on phosphate adsorption. The Cl^–, NO₃^–, CO₃^2– and SO₄^2– solutions were prepared with the same initial concentrations (1 mmol·L^–1). Then the above anion solution was mixed with identical concentration KH₂PO₄ solution of 1 mmol·L^–1, respectively. Then 10 mg dried PP_AMAF-Fe was immersed into the coexisting ionic solution (15 mL) with electromagnetic stirring for 1.5 h at 25 °C.

Equilibrium adsorption ability (q_e, mg·P·g^–1) of PP_AMAF-Fe, the adsorption capacity at a specific time (q_t, mg·P·g^–1), the desorption efficiency (R, %) of PP_AMAF-Fe-P (phosphate adsorbed by PP_AMAF-Fe) are calculated as follows:

(1)q_{\mathrm{e}}=\frac{\left(C_{\mathrm{o}}-C_{\mathrm{e}}\right)\times V}{m},

(2)q_{\mathrm{t}}=\frac{\left(C_{\mathrm{o}}-C_{\mathrm{t}}\right)\times V}{m},

(3)R=\frac{C_{\mathrm{o}}-C_{\mathrm{t}}}{C_{\mathrm{o}}}\times 100\mathrm{\%},

where C_o, C_e, C_t represent the initial concentration (mg·P·L^–1), equilibrium concentration (mg·P·L^–1) and the phosphate concentration (mg·P·L^–1) over a certain time. V (mL) is the solution volume and m is the mass (g) of the PP_AMAF-Fe.

3 Results and discussion

3.1 The preparation of PP_AMAF-Fe

The PP_AMAF-Fe synthesis process is shown in Fig. S1, and the fiber photos are presented in Fig. S2 (cf. ESM). PPF cannot react directly with polyethylenimine because of the lack of functional groups that can react with the amino groups [25]. The polymerization chain of PPF contains a tertiary carbon structure, which can easily generate tertiary carbon radicals; thus, AM can be grafted onto PPF by radical polymerization. In the first step of the grafting reaction, BPO generates free radicals (PhCOO·) catalyzed by Fe²⁺, which attack the PPF surface to produce tertiary carbon radicals, and the tertiary carbon radicals attack the carbon–carbon double bond (C=C) of AM to cause a polymerization reaction. The number of amide functional groups on PP_AMF can be controlled by the dosage of AM, and the weight gain of PP_AMF relative to PPF was 17.66% in this study. In the second reaction step, the amide groups on the surface of PP_AMF reacted with the polyethylene imine through ammonolysis. Therefore, after the above two-step reaction, the fiber surface was modified with amino functional groups, which can easily chelate with Fe³⁺ to prepare the PP_AMAF-Fe.

The optimization of the PP_AMAF-Fe synthesis process mainly includes optimizing the functional degree of ammoniated fibers and the concentration of iron to maximize the phosphate adsorption capacity of the functionalized fibers. In this work, the weight gain of the functionalized fiber was used to calculate the degree of modification [40], and the weight of the PP_AMAF increased after the amination reaction, proving that HPEI was actually present on the fiber surface. It is well known that HPEI demonstrates high hydrophilicity, hence, the hydrophilicity of the fiber surface is greatly improved after modification [41]. The hydrophilicity of the fiber was evaluated by the water uptake capacity, as shown in Table S2 (cf. ESM), and the water uptake of PP_AMAF (28.93%) was higher than that of PPF (4.74%). Furthermore, the amino functional groups on the fiber surface were favorable for chelating Fe³⁺ [42]. By controlling the concentration of the substrate (HPEI, Fe³⁺), the ammoniated fibers (PP_AMAF) with different weight gains and grafted with different iron contents were synthesized.

To select suitable functionalized fibers, the phosphate adsorption capacity of the PP_AMAF-Fe fibers prepared from PP_AMAF was analyzed (Fig. S3, cf. ESM). First, the effect of weight gain of aminated fibers chelated with Fe³⁺ on the adsorption of phosphate was tested. The results are presented in Fig. S3(a); the weight gain of the aminated PP_AMAF fiber increases with the phosphate purification capacity of PP_AMAF-Fe. When the weight gain of PP_AMAF was greater than 13.5%, the adsorption capacity of PP_AMAF-Fe for phosphate remained unchanged. This is because the chelation of Fe³⁺ occurred on the fiber surface, when the weight gain of PP_AMAF exceeded a certain value, the amino group was grafted inside the fiber skeleton and could not chelate Fe³⁺ [43]. In addition, a larger degree of modification could also destroy the structure of the fiber surface layer, thus affecting the adsorption capacity [44]. Therefore, a weight gain of 13.5% for PP_AMAF was selected.

To verify the role of Fe³⁺ modification, PP_AMAF chelated with different Fe³⁺ contents for phosphate removal was investigated. As demonstrated in Fig. S3(b), when compared with PP_AMAF, the phosphate adsorption ability of PP_AMAF-Fe evidently improved, proving that Fe³⁺ modification contributes positively to phosphate adsorption. Furthermore, when the initial concentration of Fe ion was higher than 0.1 mol·L^–1, the phosphate removal ability of PP_AMAF-Fe attained equilibrium, that is, the chelating ability of PP_AMAF to Fe³⁺ reached saturation. Therefore, 100 mg PP_AMAF chelated with 50 mL of Fe ion (0.1 mol·L^–1) was selected as the best synthesis condition for PP_AMAF-Fe. In this study, PP_AMAF-Fe was characterized and applied under this optimal condition.

3.2 Characterization of the prepared fibers

3.2.1 Scanning electron microscope (SEM)

Fig.1 depicts the SEM images of the original PPF, PP_AMF, PP_AMAF, PP_AMAF-Fe, and PP_AMAF-Fe-P fibers. All fibers were continuous and complete when the image was magnified 200 times, indicating that the fiber structure was well maintained. When the SEM of the fibers was magnified by 2000 times, the surface of the functionalized fiber (Fig.1(c) and 1(d)) was slightly rougher than that before modification, which may be related to the grafting of the monomer to the fiber surface [45]. The diameter expansion of the fibers (Fig.1(c) and Fig.1(d)) was due to swelling during the reaction process [46]. A crack appeared on the fiber surface (Fig.1(a–c)) under high magnification (20000 times), which could be beneficial for phosphate adsorption owing to the increase in contact area [25]. After the adsorption of phosphate, the surface of PP_AMAF-Fe-P (Fig.1(e)) demonstrated no obvious damage. The results illustrate that the fiber possessed a good structure after use.

Fig.1 SEM images of (a) PPF, (b) PP_AMF, (c) PP_AMAF, (d) PP_AMAF-Fe, and (e) PP_AMAF-Fe-P; the image is magnified 200, 2000, and 20000 times.

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3.2.2 Fourier transform infrared spectroscopy (FTIR)

The PPF and functionalized fibers were characterized using FTIR, and the results are presented in Fig.2. Typical peaks of PPFs (Fig.2(a)) were concentrated at 1630–1654, 997, and 840 cm^–1. The characteristic absorption bands of –C–H for anti-symmetric –CH₃, anti-symmetric –CH₂, and symmetric –CH₂ were observed at 2958, 2920, and 2838 cm^–1, respectively [47]. The functionalized fibers exhibited –C=O tensile bands in the 1630–1654 cm^–1 region, demonstrating the successful grafting of AM onto the fiber (Fig.2(b)). Furthermore, the wide peak concentrated at 3431 cm^–1 corresponds to the –N–H asymmetric stretching mode of amine, indicating that HPEI was present on the fiber surface (Fig.2(c)). After the grafting of Fe³⁺ and the phosphate adsorption (Fig.2(d) and Fig.2(e)), the peak value and frequency band displacement changed slightly.

Fig.2 FTIR spectra of (a) PPF, (b) PP_AMF, (c) PP_AMAF, (d) PP_AMAF-Fe, and (e) PP_AMAF-Fe-P.

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3.2.3 Elemental analysis

The elemental analysis of the samples is shown in Table S3 (cf. ESM). PPF (Table S3, entry 1) possesses 1.46% nitrogen, which is due to the nitrogen-containing surfactant during the production procedure [35]. The N content of PP_AMF (Table S3, entry 2) increased significantly compared with that of PPF (Table S3, entry 1) because of the introduction of amide N (19.7%). Furthermore, the C content of PP_AMAF (Table S3, entry 3) decreased and the N content increased because the C content of HPEI (C₂H₅N)_n (55.81%) was lower than that of PP_AMF (74.23%). Similarly, the N content of HPEI (11.63%) is higher than that of PP_AMF (6.33%). Moreover, the N, C, and H contents of PP_AMAF-Fe and PP_AMAF-Fe-P were reduced (Table S3, entries 4–5), which may be attributed to the introduction of Fe and P. The elemental analysis results indicate that the changes in the content of various elements in the modification process of the fiber are in line with expectations, proving the successful preparation of PP_AMAF-Fe-P.

3.2.4 X-ray diffraction spectroscopy (XRD)

The crystal structure of the fiber was characterized by XRD (Fig.3). As shown in Fig.3(a), the 2θ values of the original PPF were 14°, 16°, 17°, and 18° [48]. The results demonstrate that PPF possessed many crystal line types. After chemical grafting, PP_AMF, PP_AMAF, and PP_AMAF-Fe demonstrated characteristic peaks similar to those of PPF, indicating that the fiber structure remained intact after modification. Some peaks of pure PPF were slightly shifted, showing successful amination versus iron grafting process (Fig.3(b–d)). Moreover, PP_AMAF-Fe-P retained its excellent crystal structure after adsorption (Fig.3(e)). A series of results showed that PP_AMAF-Fe has high structural stability.

Fig.3 XRD images of (a) PPF, (b) PP_AMF, (c) PP_AMAF, (d) PP_AMAF-Fe, and (e) PP_AMAF-Fe-P.

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3.3 Effect of pH

The pH of the solution is an important external factor affecting the phosphate removal ability of the functionalized fiber because the pH affects the form of phosphate protons and the surface structure of the fibers simultaneously. Hence, the adsorption capacity of PP_AMAF-Fe in solutions over a wide pH range of 2–9 was investigated (Fig.4). The phosphate adsorption capacity of PP_AMAF-Fe increased gradually between pH 2 and 6, reached the maximum level at pH 6, and then decreased as the pH increased. Furthermore, adsorption is mainly related to the morphology of phosphate and the charge characteristics of the functional groups on the adsorbent [41]. H₃PO₄ is the primary form when the pH of the phosphate solution is lower than 2, which is difficult to bind to Fe³⁺ [42]; therefore, the adsorption capacity is low. At pH values greater than 6, there is competition between OH^–, PO₄^3–, and HPO₄^2–, which further reduces phosphate adsorption. When the pH was approximately 6, H₂PO₄^– and HPO₄^2– were the main forms of phosphate [49], which could easily be combined with Fe³⁺ sites to achieve the best adsorption effect. As the pH between natural water and wastewater is mostly between 5 and 7, PP_AMAF-Fe demonstrated high phosphate removal capacity in the range of pH 5–7 and possessed optimal removal at pH 6, indicating that PP_AMAF-Fe has good practical application ability.

Fig.4 Effect of pH on phosphate adsorption capacity of PP_AMAF-Fe.

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3.4 Adsorption kinetics and adsorption thermodynamics

Contact time and temperature are important factors affecting the adsorption process, and adsorption kinetics can critically reflect the adsorption efficiency of the fiber adsorbent. The adsorption kinetics and thermodynamics of the PP_AMAF-Fe for phosphate are presented in Fig.5. Fig.5(a) shows the adsorption kinetics of PP_AMAF-Fe for phosphate removal at different temperatures (288, 298, and 308 K). PP_AMAF-Fe reached half its saturation level for phosphate in only 5 min, and reached adsorption equilibrium within 60 min, indicating high adsorption efficiency and sensitivity. To better understand the phosphate adsorption by PP_AMAF-Fe over time, the pseudo-first-order kinetic model (4) and pseudo-second-order kinetic model (5) were applied to fit the phosphate adsorption data. The fitting equations are as follows:

Fig.5 (a) Phosphate adsorption kinetics of PP_AMAF-Fe under different temperatures; (b) linear fitting diagram of first-order model; (c) linear fitting diagram of second-order model; (d) the linear equations of lnK_c and 1/T and the linear equations of lnK₂ and 1/T.

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(4)\mathrm{l}\mathrm{o}\mathrm{g}\left(q_{\mathrm{e}}-q_{\mathrm{t}}\right)=\mathrm{l}\mathrm{o}\mathrm{g}{q}_{\mathrm{e}}-\frac{k_{1}t}{2.303},

(5)\frac{t}{q_{\mathrm{t}}}=\frac{1}{k_2{q_{\mathrm{e}}}^2}+\frac{t}{q_{\mathrm{e}}},

where q_t is the removal capacity (mg·P·g^–1) at time t, and q_e is the removal capacity (mg·P·g^–1) at equilibrium time (min). k₁ and k₂ are the adsorption rate constants. Linear maps of the dynamic pattern are presented in Fig.5(b) and 5(c), and the adsorption kinetic parameters are listed in Table S4 (cf. ESM). The results confirm that the phosphate adsorption data of PP_AMAF-Fe were more consistent with the second-order model in terms of the correlation coefficient (Table S4). The first-order kinetic model was found to be in agreement with physical adsorption, while the second-order kinetic model tends to describe chemical adsorption [41], hence the removal of phosphate by PP_AMAF-Fe is more inclined to the chemisorption mechanism.

The phosphate removal ability of PP_AMAF-Fe increased (Fig.5(a)) with an increase in the temperature of the solution, indicating that phosphate removal by PP_AMAF-Fe is an endothermic process. To better reveal the influence of temperature, some typical thermodynamic parameters, such as Gibbs free energy (∆G⁰, kJ·mol^–1), standard enthalpy (∆H⁰, kJ·mol^–1), and standard entropy (∆S⁰, kJ·mol^–1), were used to provide information on thermodynamic parameters during the adsorption process. The thermodynamic parameters were calculated as follows:

(6)K_{\mathrm{c}}=\frac{C_{\mathrm{A}\mathrm{e}}}{C_{\mathrm{e}}},

(7)\mathrm{\Delta }{G}^{\mathrm{o}}=-\mathrm{R}T\mathrm{I}\mathrm{n}{K}_{\mathrm{c}},

(8)\mathrm{I}\mathrm{n}{K}_{\mathrm{c}}=\frac{\mathrm{\Delta }{S}^{\mathrm{o}}}{\mathrm{R}}-\frac{\mathrm{\Delta }{H}^{\mathrm{o}}}{\mathrm{R}T},

where K_c is the distribution coefficient of adsorption, C_Ae is the adsorbed phosphate on PP_AMAF-Fe (mg·P·L^–1), R is the ideal gas constant (8.314 J·mol^–1·K^–1), and T is temperature (K).

Table S5 (cf. ESM) lists the thermodynamic parameters of the functionalized fibers. ∆G⁰ with a negative value proves that phosphate removal by PP_AMAF-Fe is spontaneous, and the greater the absolute value of ∆G⁰, the higher the driving force and phosphate adsorption capacity. The positive value of ∆H⁰ (9.64 kJ·mol^–1) provides evidence for the endothermic reaction of PP_AMAF-Fe with phosphate. Furthermore, ∆S⁰ with a positive value illustrates that the randomness of the system increases during phosphate adsorption process by PP_AMAF-Fe [34]. In addition, the activation energy E_a = 22.27 kJ·mol^–1 calculated by the linear equations of lnK₂ and 1/T (Fig.5(d)), further indicating that chemisorption is the dominant adsorption mechanism of phosphate by PP_AMAF-Fe.

3.5 The effect of phosphate initial concentration

Adsorption isotherms help determine the largest adsorption ability of an adsorbent and explain the principle of adsorption [43]. The phosphate removal ability of PP_AMAF-Fe at different initial phosphate concentrations was studied at 298 K. As depicted in Fig.6, the adsorption capacity of PP_AMAF-Fe increased with increasing initial phosphate concentration; at the initial stages of the phosphate concentration increase, the amount of phosphate adsorption increased rapidly until adsorption equilibrium was reached. The maximum phosphate adsorption capacity of PP_AMAF-Fe was 32.42 mg·P·g^–1. This is because a high phosphate concentration improved the recognition and binding of phosphate to Fe³⁺ sites, and the phosphate removal ability of the fiber gradually attained saturation with the depletion of Fe³⁺ sites on the fiber surfaces [41].

Fig.6 (a) Effect of phosphate concentrations on the phosphate removal capacity of PP_AMAF-Fe; (b) linear fitting of the Langmuir isotherm model of PP_AMAF-Fe for phosphate adsorption.

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To better describe and analyze the adsorption isotherms, the most common isotherm models, Langmuir (9) and Freundlich (10), were used. The Langmuir model assumes equal availability of adsorption sites, monolayer surface coverage, adsorption uniformity, and non-interactions between adsorption molecules [50]. Meanwhile, the Freundlich model was used to explain heterogeneous adsorption, which describes reversible adsorption and is not limited to monolayer adsorption [47]. The linearized Langmuir and Freundlich models are as follows:

(9)\frac{C_{\mathrm{e}}}{q_{\mathrm{e}}}=\frac{C_{\mathrm{e}}}{q_{\mathrm{m}\mathrm{a}\mathrm{x}}}+\frac{1}{K_{\mathrm{l}}\times q_{\mathrm{m}\mathrm{a}\mathrm{x}}},

(10)\mathrm{l}\mathrm{o}\mathrm{g}{q}_{\mathrm{e}}=\mathrm{l}\mathrm{o}\mathrm{g}{K}_{\mathrm{f}}+\frac{\mathrm{l}\mathrm{o}\mathrm{g}{C}_{\mathrm{e}}}{n},

where q_max (mg·P·g^–1) is the maximum adsorption capacity and K_l (L·g^–1) and K_f (mg·g^–1) are the Langmuir and Freundlich adsorption constants, respectively. By fitting the two models to the experimental data, Fig.6(b) and S4 (cf. ESM) were obtained, and the corresponding coefficients are listed in Table S6 (cf. ESM).

Comparing the Langmuir isotherm model (R² = 0.98266) and Freundlich model (R² = 0.82777), the former demonstrates a better linear regression correlation. In other words, the Langmuir isotherms can better describe the equilibrium data. Hence, it can be concluded that the Fe³⁺ responsible for phosphate adsorption is uniformly distributed on the surface of the functionalized fiber, and that the adsorption of phosphate by PP_AMAF-Fe is monolayer adsorption on the fiber surface. Theoretically, the maximum removal ability of the PP_AMAF-Fe for phosphate is 33.94 mg·P·g^–1 calculated by the Langmuir isotherm, which is superior to the reported phosphate adsorbent materials, such as Fe(III) loaded chitosan-biochar composite fiber (19.24 mg·P·g^–1) [16], crosslinked Fe(III)-chitosan (10.2 mg·P·g^–1) [17], and Fe-La/MgO nanosheets (12.28 mg·P·g^–1) [51]. Therefore, PP_AMAF-Fe is an excellent adsorption material with a high phosphate removal capacity.

3.6 Adsorption selectivity and adsorption limit

Adsorption selectivity or the competitive ion effect is a key factor in evaluating the adsorption ability of adsorbents [52]. To assess the selective removal of phosphate by PP_AMAF-Fe in the presence of other anions-coexisting in water bodies, identical concentrations of Cl^–, CO₃^2–, NO₃^–, and SO₄^2– were added to the phosphate solution (1 mmol·L^–1), and the result is shown in Fig.7. The order of influence of coexisting ions was SO₄^2– > CO₃^2– > NO₃^– > Cl^–, indicating that Cl^– and NO₃^– had little hindrance to the adsorption of phosphate by PP_AMAF-Fe (Fig.7(a)). However, the CO₃^2– and SO₄^2– affected the decrease of phosphate adsorption from 21.99 to 6.60 and 5.81 mg·P·g^–1, respectively, which may be owing to the strong affinity of the two ions for the Fe³⁺ on PP_AMAF-Fe, thus competing with phosphate ions [42]. Despite the influence of the coexisting anions, an adsorption capacity of more than 5 mg·P·g^–1 was obtained, which proves that PP_AMAF-Fe has good adsorption selectivity and highly practical.

Fig.7 (a) Phosphate removal ability by PP_AMAF-Fe in competitive ions solution; (b) breakthrough curves of phosphate solutions.

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To verify the phosphate removal by PP_AMAF-Fe in real water, natural lake water (Chaohu Lake) was selected for the phosphate adsorption experiments. The main anions in Chaohu Lake are shown in Table S7 (cf. ESM), which shows that the composition of the water body is complex, and the contents of several anions are higher than that of phosphate. To highlight the practical application of PP_AMAF-Fe, the phosphate concentration in the real water was adjusted to 4510 µg·P·L^–1. In 15 mL of the above-mentioned water specimen, different qualities of PP_AMAF-Fe were magnetically stirred for 24 h; the details are shown in Table S8 (cf. ESM). With a PP_AMAF-Fe content of more than 50 mg, the phosphate concentration was reduced to the strictest international standard of a maximum concentration of 10 µg·P·L^–1 required to avoid water eutrophication [53]. Therefore, PP_AMAF-Fe has good application prospects in the prevention of water bodies eutrophication.

3.7 Application of PP_AMAF-Fe in continuous flow model

We constructed a flow adsorption model using a silicone tube (100 mm in length and 5.6 mm in diameter) and a peristaltic pump to verify the performance of PP_AMAF-Fe in removing phosphate under continuous flow conditions. The specific operation was to fill 300 mg PP_AMAF-Fe into a silicone tube, and then the phosphate solution (2 mg·P·L^–1) was injected into the peristaltic pump at a flow rate of 0.8 mL·min^–1. Finally, the phosphate concentration in the outflow was measured at intervals to obtain the phosphate breakthrough curve (Fig.7(b)). The phosphate removal rate was greater than 99% when the outflow liquid product was less than 1700 mL, demonstrating the good removal capacity of PP_AMAF-Fe under continuous flow conditions. Because of the flexibility and plasticity of the fiber materials, they can be woven into different shapes to match the treatment of various water bodies, showing broad application prospects for PP_AMAF-Fe.

3.8 Desorption and reusability studies

The reusability of a sorbent material is an important parameter in practical applications. Therefore, the adsorption-desorption process was repeated to evaluate the reusability of PP_AMAF-Fe. After optimized comparison, 30 mL of 0.1 mol·L^–1 ethylene diamine triacetic acid (EDTA) was selected as the optimum desorption condition (Table S9, cf. ESM). As displayed in the Fig. S5 (cf. ESM), the phosphate ions on PP_AMAF-Fe were desorbed by a 0.1 mol·L^–1 EDTA solution with a desorption rate of 99%. When the adsorption−desorption process was repeated five times, the phosphate removal rate was 99%, indicating that PP_AMAF-Fe has excellent tolerance and reusability.

3.9 Adsorption mechanism

The phosphate adsorption mechanism of PP_AMAF-Fe was analyzed using energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS), the results are described in Fig.8. The contents and distributions of the elements obtained by EDS are shown in Fig.8(a) and Table S10 (cf. ESM). The results show that the original PPF contained mainly C. Compared with PPF, new N and O peaks appeared after the two-step amination modification of PP_AMAF, indicating that amines were successfully grafted onto PP_AMAF (Fig.8(a), traces I–II). Moreover, after chelating Fe³⁺, new Fe and Cl peaks appeared in the PP_AMAF-Fe spectrum, revealing that Fe³⁺ was successfully loaded onto the fiber (Fig.8(a), trace IV). When the adsorption reaction ended, the spectrum of PP_AMAF-Fe-P showed a new P peak, and the Cl peak decreased (Fig.8(a), trace V). This phenomenon indicates that PP_AMAF-Fe can successfully adsorb phosphate, and that the binding between Fe and P is accomplished by ligand exchange between Cl^– and phosphate anions.

Fig.8 (a) EDS spectra of (I–V) PPF, PP_AMF, PP_AMAF, PP_AMAF-Fe, and PP_AMAF-Fe-P; (b) XPS survey of PPF, PP_AMF, PP_AMAF, PP_AMAF-Fe, and PP_AMAF-Fe-P; (c) high resolution spectrum of Cl 2p for PP_AMAF-Fe; (d) high resolution spectrum of P 2p for PP_AMAF-Fe-P; (e) high resolution Fe 2p spectrum of PP_AMAF-Fe and PP_AMAF-Fe-P; (f) high resolution N 1s spectrum of PP_AMAF-Fe and PP_AMAF-Fe-P.

Full size|PPT slide

XPS characterization of the fibers was performed to prove the phosphate adsorption mechanism of PP_AMAF-Fe. The spectra and corresponding atomic compositions of the XPS survey are presented in Fig.8(b) and Table S11 (cf. ESM), respectively. As seen in Fig.8(b), new element peaks of Fe and Cl were found in PP_AMAF-Fe, which were similar to the EDS results. These results indicate that PP_AMAF-Fe was successfully constructed. Furthermore, the high-resolution spectrum of Fe 2p for PP_AMAF-Fe could be decomposed into 2p_1/2 at 725.536 eV and 2p_3/2 at 711.731 eV (Fig.8(e)). When the adsorption reaction ended, the binding energy of the Fe 2p peak shifted to 725.531 and 711.915 eV, and the Cl content of PP_AMAF-Fe-P decreased significantly, illustrating that phosphate and Cl^– ions exchange, and the valence band of Fe 2p may undergo electron transfer and form Fe–O–P bonding to successfully adsorb phosphate [54], which was also demonstrated in a previous study of an iron-modified SMER for phosphate removal [55].

In addition, there were three main types of nitrogen in the high-resolution spectrum of PP_AMAF-Fe (Fig.8(f)): amide O≡C–N (401.49 eV), –NH–/–NH₂ (399.83 eV), and tertiary amine –NR₂ (399.13 eV) [30]. The binding energies of the N 1s spectrum increased to 401.60, 399.95 and 399.16 eV separately after the adsorption reaction was terminated, which was caused by the protonation of the amine groups [56]. Protonated amines, such as –NH₃⁺, can absorb phosphate ions through electrostatic attraction [57]. By analyzing and summarizing the above, a feasible adsorption mechanism was proposed, as shown in Fig. S6 (cf. ESM). The phosphate adsorption mechanism of PP_AMAF-Fe may be the coordination reaction between Fe and phosphorus and the electrostatic attraction of protonated amines and phosphate ions.

3.10 Comparison with other iron modified adsorbents for phosphate removal

The maximum adsorption ability, equilibrium time, and repeatability of PP_AMAF-Fe were compared with those of other reported phosphate adsorbents modified with iron. As shown in Tab.1, PP_AMAF-Fe exhibited a higher phosphate adsorption ability than most reported adsorbents. Furthermore, the adsorption equilibrium time of PP_AMAF-Fe for phosphate was shorter than that of many other adsorbents; hence, the functionalized fiber showed excellent adsorption sensitivity. Importantly, PP_AMAF-Fe exhibited excellent performance in removing phosphate efficiently after five cycles. In addition, as a very mature synthetic fiber, the cost of commercial PPF is low, and PP_AMAF-Fe can also be prepared using PPF waste, which provides a new method for the resource utilization of fiber solid waste. Therefore, the removal of phosphate by PP_AMAF-Fe is highly cost-effective and environmentally friendly. In summary, PP_AMAF-Fe has the advantages of low cost, low removal limit, high adsorption capacity, excellent recyclability, and environmental friendliness.

Tab.1 Different iron modified adsorbents for phosphate removal

Adsorbents	qe/(mg·P·g–1)	Equilibrium time	Run	Ref.
Fe(III) loaded chitosan-biochar composite fibers	19.24	240 min	–	[16]
Calcium-modified iron-based adsorbents (Fe/CaCl2)	16.97	20 min	–	[58]
Crosslinked Fe(III)-chitosan (CTS-Fe-CL)	10.20	300 min	5	[17]
Fe(III)-doped chitosan (CTS-Fe)	15.70	300 min	5	[17]
Fe-La/MgO nanosheets	12.28	80 min	–	[51]
Eggshell functionalized with iron oxyhydroxide	21.98	240 min	–	[59]
microporous Fe2O3/g-C3N4	17.11	15 min	5	[60]
PPAMAF-Fe	34.42	60 min	5	This study

4 Conclusions

Phosphorus pollution is a significant bottleneck that restricts water quality safety and has become a topic of global concern. In this study, PPF was used as a support to prepare a novel adsorbent to remove phosphate from wastewater, providing an important approach for the reutilization of fiber waste and the purification of phosphate. First, PP_AMAF-Fe was developed and optimized via a three-step synthesis process. The phosphate adsorption capacity was significantly enhanced (33.94 mg·P·g^–1) by the modification of Fe³⁺. According to the phosphate adsorption experiment, PP_AMAF-Fe showed wide pH adaptability, with an optimal pH of 6 and high adsorption efficiency within an adsorption equilibrium time of 60 min. Furthermore, PP_AMAF-Fe exhibited excellent reusability of five times and high selectivity for phosphate in the presence of competing ions such as Cl^– and NO₃^–. In addition, PP_AMAF-Fe exhibited a prominent removal efficiency of 99% under continuous flow conditions, proving its applicability to flowing water. More importantly, PP_AMAF-Fe can reduce the phosphate in simulated sewage from 4500 µg·P·L^–1 to below 10 µg·P·L^–1, which is lower than the international standard of minimum P concentration that causes eutrophication. The dynamics and isotherm adsorption data were more consistent with the pseudo-second-order kinetic and Langmuir linear equations, proving that the adsorption mechanism is more inclined toward monolayer chemical adsorption. The thermodynamic parameters indicated that the activation energy of phosphate removal by PP_AMAF-Fe was 22.27 kJ·mol^–1, further illustrating a chemisorption process. Furthermore, the XPS proves that the interaction between phosphate and Fe³⁺ is mainly responsible for the adsorption process. In brief, owing to the advantages of the fiber support, PP_AMAF-Fe exhibited competitive characteristics, including high efficiency, good selectivity, and excellent reusability, as well as economic and environmental benefits.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 22208003), Natural Science Foundation of Anhui Province (No. 1908085QB87), Major Science and Technology Projects in Anhui Province (No. 202103a06020012).

Electronic Supplementary Material

Supplementary material is available in the online version of this article at https://dx.doi.org/10.1007/s11705-022-2253-0 and is accessible for authorized users.

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