Iron phosphate (FePO₄) was prepared by mixing 25 mL of 0.83 M Iron(III) chloride hexahydrate and 25 mL of 0.83 M dibasic anhydrous sodium phosphate in a 50-mL conical tube. After mixing, the solution was left to react for 30 min, and the white precipitate was recovered by centrifugation at 4,000 rpm for 10 min. The precipitate was washed several times with deionized water to remove the remaining ions and then dried in an oven at 60°C.

The mineralogy was confirmed using X-ray diffraction (Rigaku, JP/SmartLab, 9 kW). The surface structure and composition were analyzed using scanning electron microscopy combined with an energy-dispersive X-ray spectroscopy (EDS) (Zeiss Ultra Plus). Specific surface areas were measured using the Brunauer–Emmett–Teller (BET) protocol (Micromeritics, ASAP 2020).

Metal removal experiments were conducted in single and mixed solutions of As and Pb. The As(V) stock solution was prepared from sodium arsenate heptahydrate (Na₂HAsO₄·7H₂O). The As(III) stock solution was prepared from sodium arsenite (NaAsO₂). The Pb stock solution was prepared from lead nitrate [Pb(NO₃)₂]. The As concentration in the solutions ranged from 0.5 to 10 mg/L, and the concentration for the Pb solutions was 5–100 mg/L. Solutions containing both As and Pb were prepared by mixing stock solutions to adjust the final concentration to be the same as for a single-element solution. To evaluate the pH effect on metal or As immobilization, the solution pH was adjusted using acetate buffer, Tris-HCl buffer, and glycine buffer to achieve pH 5, 7, and 10, respectively.

The experiment was conducted by mixing 0.05 g of iron phosphate and 20 mL of elemental solution in a 50-mL conical tube and shaking it for 24 h at 180 rpm (Kim and Park, 2023). The suspension was filtered using a 0.45 μm syringe filter, and the concentrations of As, Pb, Fe, and P were measured using inductively coupled plasma optical emission spectroscopy (ICP-OES, PerkinElmer, Avio 500).

Through the calculation of the adsorption isotherm, the chemical reactions and properties of the minerals and solutions can be determined (Olsen and Watanabe, 1957). The data on the immobilization of As by iron phosphate were analyzed using the Langmuir and Freundlich isotherm models. The Langmuir (Eq. 1) and Freundlich (Eq. 2) isotherms can be written as follows: q e = b Q m C e 1 + b C e  Langmuir isotherm (1) q e = K f C e n  Freundlich isotherm (2) where, q e (mg/g) is the amount of As adsorbed per gram of iron phosphate, Q_m (mg/g) is the maximum amount of the adsorbed As, and C_e (mg/L) is the equilibrium concentration of As. b (L/mg) is the Langmuir adsorption constant, which is related to the affinity of the binding sites. K_f ((mg/g)(L/mg)^1/n) is the Freundlich adsorption constant, and n is related to the adsorption intensity.

Loamy sand soil contaminated with metal(loid)s such as As, Cd, Cu, Pb, and Zn was collected around an abandoned metal mine, dried at room temperature, and sieved to collect particles <2 mm in size. The pH and EC of the soil were measured after shaking 5 g of soil in 25 mL of deionized water for 30 min. The organic matter in the soil was analyzed using the Walkley–Black method (Walkley and Black, 1934). The total element concentrations were determined by digesting the soil with aqua regia. The chemical properties of the soil are presented in Supplementary Table S1. To immobilize the As, Cd, Cu, Pb, and Zn with iron phosphate, 10 g of mine soil was mixed with different amounts of iron phosphates (5, 10, and 20 g/kg). The water content of the soil was kept at 15% by adding 2 mL of deionized water and then incubating it in an incubator at 25°C for 7 days. To evaluate the immobilization of the target elements in the soil, they were extracted from soil samples in 25 mL of deionized water and 50 mL of 0.05 M (NH₄)₂SO₄ for 2 h at 180 rpm in a shaker (Taghizadeh-Toosi et al., 2012). The supernatant was separated by centrifugation at 4,000 rpm for 10 min and then filtered through a 0.45-μm syringe filter. Then, the bioavailable metal and As concentrations were analyzed using ICP-OES.

XRD and SEM-EDX analysis showed amorphous iron phosphate with fine particles (Supplementary Figures S1, S2). The BET specific surface area of the iron phosphate was measured and found to be 72.2 ± 0.2 m²/g due to the small size of the nanoparticles.

Arsenate was immobilized by iron phosphate, and the immobilization rate ranged from 50.7% to 63.7%, depending on the initial As(V) concentration. As the concentration of As in the initial solution increased, the phosphorus (P) concentration increased in the final solution (Table 2). This suggests that As(V) substitutes for phosphate in iron phosphate (Tawfik and Viola, 2011). Arsenate, as the phosphate analog, induced the release of phosphate because of competition for sorption sites (Lambkin and Alloway, 2003). However, Fe was not released during As(V) immobilization, indicating that As(V) reacted with Fe and was removed from the solution.

The immobilization data were well-fitted to both the Langmuir and Freundlich isotherms. The Freundlich adsorption isotherm better explained As(V) adsorption (R ² = 0.99) than the Langmuir adsorption isotherm did (R ² = 0.96) (Figure 1; Table 1). The absence of a plateau in Figure 1 further supports the conclusion that the Freundlich adsorption isotherm is a more suitable model for explaining the adsorption behavior observed in the study. The maximum adsorption amount (Q_m) calculated based on the Langmuir isotherm model was 5.12 mg/g (Table 1). The Langmuir isotherm implies monolayer adsorption, whereas the Freundlich adsorption isotherm shows multilayer adsorption (Priya et al., 2022). A better fit to the Freundlich adsorption isotherm suggests that the adsorbent has a higher adsorption capacity (Zhou et al., 2017). Although the immobilization of As(V) and As(III) by iron phosphate was explained by the Freundlich adsorption isotherm, the surface precipitation of As by iron phosphate cannot be excluded. Ardes et al. (2013) showed that surface precipitation might result from the kinetics of adsorption/desorption, which can be recognized as ternary adsorption. Tiwari and Pandey (2013) also explained that Fe(III) and As(V) were precipitated, followed by the complexation of As with ferrihydrite.

The precipitation reaction requires partial dissolution of Fe from minerals to induce surface precipitate. When iron phosphate reacted with As, dissolved Fe resulted in the formation of various precipitates associated with As(V) (Lenoble et al., 2005). Iron was released in the solution at a pH higher than 7, which indicates the possibility of surface precipitation in alkaline conditions (Table 3). Under the experimental condition (pH 3), As exists as H₃AsO₃ and Fe is in the form of Fe²⁺ according to the Eh–pH diagram of As and Fe, which might contribute to both adsorption and surface precipitation (Figure 2A). However, Fe²⁺ was not detected in the experimental solution, suggesting that As sorption was mainly attributed by the adsorption process (Aredes et al., 2013). Gallegos-Garcia et al. (2012) also reported that As showed a high affinity for iron (hydr)oxide minerals, and ion exchange adsorption of As on the mineral surface was the As removal mechanism when Fe was not eluted.

To evaluate the possibility of As precipitation with iron phosphate, geochemical modeling using PHREEQC was implemented with the following solution properties: temperature, 25°C; pH, 3, 5, 7, and 10; pe, 4; solution density, 1; As(V), 10 mg/L; sodium (Na), 9.7 mg/L; Fe, 0, 0, 29.3, and 14.6 mg/L; P, 10.3, 9.1, 42.1, and 96.6 mg/L; and strengite (FePO₄·2H₂O), 2.5 g/L, respectively. The results showed that the saturation index of FeAsO₄·2H₂O ranged from −5 to −7 according to pH, indicating that there is a possibility of As precipitation with Fe if the condition changes (Figure 3). At pH 7, the saturation index of As₂O₃ and native As increased, which is related to the release of Fe from iron phosphate, resulting in the possibility of As mineral precipitation (Figure 3).

Arsenite was not immobilized by iron phosphate at the initial low As(III) concentration and was only slightly removed at a higher As(III) concentration (Figure 4). Because As(III) has higher mobility than As(V), it is estimated that the immobilization rate was lower (Oremland and Stolz, 2003). Therefore, it is necessary to oxidize As(III) and adsorb it as As(V) in the real environment.

Lead was not detected in the solution, and the concentration of released P decreased as the Pb concentration increased because of the precipitation of P with Pb (Table 2). Because almost 100% of Pb was removed, an adsorption isotherm model was not applied for Pb removal. According to the Eh–pH diagram of Pb and phosphate, Pb and P exist as Pb²⁺ and H₂PO₄ ⁻, respectively, in the experimental condition, precipitation might be the main mechanism of Pb removal in the solution (Figure 2B). The phosphate concentration increased with increasing pH in the solution, showing that there is a possibility of Pb precipitation with phosphate (Table 3). Cao et al. (2003) also reported that Pb removal from the solution by phosphate primarily involves the precipitation of metal phosphate along with some ion exchange processes and surface complexation reactions on the phosphate rock.

In the mixed solution of As(V) and Pb, the initial As(V) and Pb concentrations decreased, indicating that As(V) reacted with Pb and precipitated. When soil is contaminated with both As and Pb and lead arsenate is formed, their bioavailability is reduced compared to when As and Pb are present separately in the soil. Thus, in this case, having both reduces their environmental impact (Gamble et al., 2018; Li et al., 2019).

In the simultaneous removal of As(III) and Pb by iron phosphate, As(III) did not react with Pb. Arsenite removal was higher in the mixed solution than in the single-element solution, and 100% of Pb was removed by iron phosphate (Figure 4). The reason for the higher removal of both As(III) and Pb in the mixed solution can be attributed to the oxidation of As(III) by Fe(III), followed by phosphate substitution and the reaction of Pb with the released phosphate (Lenoble et al., 2005). The Fe released from the reaction of Pb and phosphate might participate in the reaction of Fe with As(III), leading to a higher As(III) removal rate in mixed solution than in single element solution. Arsenite adsorption in the mixed solution was better explained by the Freundlich isotherm than by the Langmuir isotherm (Figure 1).

As the pH increased, As(V) immobilization decreased, while As(III) immobilization showed the opposite tendency (Figure 5). Decreased As(V) immobilization can be explained by negatively charged As species predominant in the pH range of 2–12 and an increase in negative mineral surface at a higher pH than pH_ZPC (Mamindy-Pajany et al., 2011). Arsenic is oxidized at low pH and mainly exists in the form of As(V) in the natural environment (Bissen and Frimmel, 2003; Tabelin et al., 2020). The removal rates of As(V) and As(III) are contradictory because As(V) competes with hydroxide ions, which promotes desorption when the pH increases, thereby increasing mobility (Kim et al., 2019).

The iron concentration was analyzed and found to be the highest at pH 7.8 in the adsorption experiment. This might be because iron is oxidized and precipitated as a hydroxide as the pH increases (Table 3) (Liu et al., 2005). The phosphorus concentration also increased as the pH rose due to an increase in the substitution of As(V) and OH⁻ for phosphate (Table 3) (Borgnino et al., 2006; Barrow, 2017). The phosphorus concentration in the As(III)-dissolved solution also increased with increasing pH because of the substitution of phosphate by OH⁻ (Deng et al., 2018). As the pH increased, more phosphate dissolved in the solution, which enhanced the rate of Pb removal (Table 3; Figure 5). However, Pb has a high removal rate (>90%) at all pH levels, so a slight effect of pH on Pb removal was observed (Figure 5). This is mainly because Pb has a high removal rate compared to other elements and less adsorption competition with hydrogen ions (Weng et al., 2011; Kocabaş-Ataklı and Yürüm, 2013).

The pH effect on the simultaneous removal of As and Pb by iron phosphate in a mixed solution was evaluated. However, the initially measured As and Pb concentrations were low in various pH ranges because lead arsenate was formed (Figure 5). The co-precipitation of As(III) and Pb increased with increasing pH, and As(III) further increased in the mixed solution compared to the single As(III) solution (Figure 5). The iron concentration increased to pH 7 and decreased after that, which might react with As(III) (Table 3).

As the amount of added iron phosphate increased, the concentration of extracted As increased (Figure 6A). Free phosphate ions existed in the iron phosphate substituted for As in the soil and thus increased As mobility. Although bioavailable As increased with increasing iron phosphate, the Pb concentration in the extraction decreased with increasing application of iron phosphate (Figure 6B). Because the reactivity of phosphate for immobilization with Pb (and for As substitution) is higher than that of iron for immobilization with As, the extracted As was not immobilized by the released Fe (Figure 4). In addition, the relatively high solubility product of iron phosphate might cause the substitution of arsenate by phosphate because phosphate is a chemical analog of As(V) (Strawn, 2018).

Lenoble et al. (2005) showed that As was removed by iron phosphate in solution by solid dissolution and phosphate/arsenate exchange. However, they did not test As immobilization in soil. In our study, As mobility increased with iron phosphate, even though As was removed by iron phosphate in solution. In addition, a variety of minerals and competing ions in soil might result in different stabilizations of As in the solution and soil. Therefore, the reactivity of phosphate should be considered for the simultaneous immobilization of As and Pb using iron phosphate compounds in soil. With the reduced reactivity of phosphate, the substitution of As by phosphate will be less, and the simultaneous immobilization of both As and metals can be achieved. For example, oxidizing a part of pyrite (FeS₂) and coating the surface of pyrite with phosphoric acid to produce iron phosphate (FePO₄) decreased As and some of the metals in soil (Fytas and Evaneglou, 1998). Although the immobilization rate of iron phosphate varies from metal to metal, it is possible to simultaneously adsorb As, Cd, and Pb (Yuan et al., 2017).