The pollution of water resources has become a serious problem and is causing public concern. Chlorophenols (CPs) are one of various organic contaminants in the environment. These toxic compounds are widespread and suspected carcinogens ^[1]. CPs have been widely used as raw materials and synthetic intermediates in many industrial processes. Because of their broad-spectrum antimicrobial properties, CPs have also been widely used as preservatives and disinfectants. CPs are generated as side products in the disinfection of drinking water and bleaching of pulp with chlorine as well. CPs are resistant to biodegradation; therefore, discharge of large amounts of these compounds into the environment is a threat to human health and ecosystems ^[2].

The Fenton process is an effective method for degrading a wide range of organic pollutants in water. The Fenton reaction is based on the catalytic decomposition of H₂O₂ by Fe²⁺ ions in aqueous solution to produce HO^•, which can non-selectively oxidize most organic compounds. Catalysis by Fe²⁺ ions involves the Fe²⁺ ⇆ Fe³⁺ redox cycle shown in Eqs. (1) and (2) ^[3]:

However, because the reduction of Fe³⁺ to Fe²⁺ via Eq. (2) is much slower than the oxidation of Fe²⁺ to Fe³⁺ via Eq. (1), the traditional homogeneous Fenton process needs a high concentration of ferrous salt (50-80 mg/L). The subsequent treatment of iron-containing water and ferric hydroxide sludge therefore consumes large amounts of chemicals. Additionally, the homogeneous Fenton process typically works under acidic conditions (pH = 2.0-3.0), which is unfavorable in practice because of the costs of acidification during processing and neutralization after treatment ^{[4, 5, 6]}. To overcome these disadvantages of the homogeneous Fenton process, and to achieve catalyst recycling, much effort has been made to develop heterogeneous Fenton-like catalysts ^{[7, 8, 9, 10, 11, 12]}. However, many of the heterogeneous Fenton-like catalysts reported in the literature still require acidic pH conditions, and some of them show significant deactivation in consecutive reaction cycles ^{[13, 14, 15, 16, 17]}. The development of reusable and efficient heterogeneous Fenton-like catalysts with wide working pH ranges is therefore highly desirable.

Polyoxometalates (POMs), which are multi-metal oxygen clusters, have been intensively studied as catalysts because of their ability to undergo multi-electron transfer reversibly. Irradiation with light of energy higher than or equal to that of the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap of a POM results in charge transfer from O²⁻ to M⁶⁺ (M = W, Mo) at M-O-M bonds, leading to the formation of an excited POM with strong oxidizing power ^{[18, 19, 20, 21, 22]}. Recently, the use of POMs in Fenton or related processes has been attracting increasing attention. Choi’s group ^[23] reported that SiW₁₂O₄₀⁴⁻ or PW₁₂O₄₀³⁻ in the Fe⁰/O₂ system can mediate electron transfer from Fe⁰ to O₂ to produce Fe²⁺ and H₂O₂. Sedlak’s group ^[24] showed that addition of PW₁₂O₄₀³⁻ to the Fe³⁺/H₂O₂ homogeneous system extends the working pH range up to pH = 8.5. Taghdiri et al. ^[25] reported that silicotungstic acid catalyzes the oxidation of hexamethylenetetramine in the presence of H₂O₂ or H₂O₂/Fe²⁺. These studies suggest potential synergies between iron and POM for catalytic oxidation of pollutants by H₂O₂. It is therefore reasonable to infer that water-insoluble complexes containing both iron and heteropolyanions would be efficient heterogeneous Fenton-like catalysts. However, binary complexes of iron ions (Fe²⁺ or Fe³⁺) with many heteropolyanions (e.g., SiW₁₂O₄₀⁴⁻ and PW₁₂O₄₀³⁻) are soluble in water ^[24].

Recently, we found that some amino acids can link Fe³⁺ ions with various heteropolyanions in aqueous solution to form water-insoluble ternary complexes; these complexes show Fenton-like catalytic activity ^{[26, 27]}. Here, we report a new Fe^III-glutamate-silicotungstate ternary complex (Fe^IIIGluSiW), which acts as a highly active heterogeneous Fenton-like catalyst in the degradation of 4-chlorophenol (4-CP) at neutral pH value. The catalytic mechanism of Fe^IIIGluSiW was also investigated.

Ferric chloride hexahydrate, glutamic acid, silicotungstic acid, sodium silicate, sodium tungstate, and H₂O₂ (30%) were provided by the Sinopharm Chemical Reagent Beijing Co., Ltd. 4-CP was purchased from the Tianjin Guangfu Fine Chemical Industry Research Institute. All reagents were analytical grade and used without further purification.

Fe^ШGluSiW was synthesized by precipitation in aqueous solution at ambient temperature. Ferric chloride hexahydrate (0.27 g) and glutamic acid (0.15 g) were dissolved in water (25 mL) to form a red soluble complex of Fe³⁺ ions and glutamate. A solution of silicotungstic acid (25 mL, 0.02 mol/L) was added dropwise, precipitating Fe^ШGluSiW. The precipitate was aged for 24 h at room temperature, and red-brown Fe^ШGluSiW powder was obtained by washing the precipitate with water and drying at 50 °C under vacuum.

The prepared Fe^IIIGluSiW was characterized using inductively coupled plasma atomic emission spectroscopy (ICP-AES; Shimadzu ICPS-7510), thermogravimetry (TG; Seiko TG/DTA6300), Fourier-transform infrared (FT-IR) spectroscopy (Shimadzu IRAffinity-1), ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS; Shimadzu UV2550), X-ray diffraction (XRD; XD-2, Cu K_α radiation), and field-emission scanning electron microscopy (FE-SEM; Hitachi S-4800).

Degradation of 4-CP was performed at 25 ± 2 °C in an Fe^IIIGluSiW/H₂O₂ system under both dark and irradiated conditions. All experiments were performed in a cylindrical quartz tube under magnetic stirring. A 400 W high-pressure mercury lamp was used as the irradiation source; it was surrounded by a quartz jacket to enable water cooling. A cutoff filter was placed around the lamp to remove radiation below 350 nm. In a typical procedure, Fe^IIIGluSiW powder was added to 4-CP solution (100 mg/L) with stirring to maintain a uniform suspension. The suspension pH was adjusted using dilute H₂SO₄ or NaOH solution, and then H₂O₂ was added. No pH control was applied during the process. In the photo-assisted Fenton-like process, H₂O₂ addition and irradiation of the reaction system were performed simultaneously. Specimens were withdrawn at regular time intervals and analyzed immediately after filtration through a 0.22 μm filter to remove the Fe^IIIGluSiW powder. Experiments were performed in triplicate, and all the results were expressed as mean values.

The 4-CP concentration was determined using a high- performance liquid chromatography (HPLC) system (Shimadzu LC-20AD) equipped with C18 reverse-phase column and UV detector. The mobile phase was a mixture of methanol (65%) and 1% acetic acid aqueous solution (35%) at a flow rate of 0.5 mL/min. The total organic carbon (TOC) content was determined using a TOC analyzer (Shimadzu TOC-V CPH).

The ICP-AES, TG, and FT-IR results show that the chemical formula of Fe^ШGluSiW is [Fe(C₅H₈NO₄)(H₂O)]₂SiW₁₂O₄₀∙13H₂O. The ICP-AES data for the Fe, Si, and W contents in Fe^ШGluSiW are close to the values calculated based on the suggested chemical formula. Calcd for Fe^IIIGluSiW (%): Fe 3.15, Si 0.79, W 62.2; found (%): Fe 3.62, Si 0.75, W 61.1.

The TG curve of Fe^IIIGluSiW in Fig. 1 shows a multi-step mass loss in the range 30-425 °C. The 6.4% mass loss in the first step, from 30 to 174 °C, arises from loss of the 13 crystallization water molecules (anal., calcd: 6.6%). The 9.5% mass loss in the subsequent steps, from 174 to 425 °C, is caused by loss of the coordinated water molecule and stepwise destruction of the glutamate moiety (anal., calcd: 9.3%). The TG data are in agreement with the proposed chemical formula.

Fig. 1. TG and DTG curves of FeШGluSiW.

The FT-IR spectrum of a POM reflects the structure of the heteropolyanion, i.e., the primary structure. The FT-IR spectrum of Fe^ШGluSiW is shown in Fig. 2(1) and the assignments of the main absorption peaks, based on literature values ^{[28, 29]}, are listed in Table 1. The FT-IR spectrum of Fe^ШGluSiW has the characteristic peaks of Keggin-structured SiW₁₂O₄₀⁴⁻ at 986, 920, 881, and 793 cm^-1; these are respectively attributed to the stretching vibrations of terminal W=O bonds, tetrahedral Si-O bonds, and two types of bridging W-O-W bonds in SiW₁₂O₄₀⁴⁻. Structural information on the glutamate moiety can also be obtained from the FT-IR spectrum. The peaks at 1630 and 1617 cm⁻¹ correspond to the asymmetric vibrations of two COO⁻ groups of glutamate. The absorption at 1507 cm⁻¹ is assigned to the bending vibration of NH₃⁺. The CH₂ and CH bending modes are observed at 1459, 1437, and 1349 cm⁻¹. The FT-IR data verify the presence of Keggin-structured SiW₁₂O₄₀⁴⁻ and glutamate moiety in Fe^ШGluSiW.

Fig. 2. FT-IR spectra of (1) FeШGluSiW, (2) FeШGluSiW used under irradiation, and (3) FeШGluSiW used in the dark.

Table 1 Main relevant FT-IR data (in cm−l) of FeIIIGluSiW and their assignments. FeIIIGluSiW FeIIIGluSiW used in the dark FeIIIGluSiW used under irradiation Assignment 793 795 797 νa(W-O-W) 881 883 883 νa(W-O-W) 920 918 920 νa(Si-O) 986 986 986 νa(W=O) 1349 (m) 1349 (m) 1349 (m) δ(CH) 1437 (w) 1438 (w) 1437 (w) δ(CH2) 1459 (w) 1459 (w) 1459 (w) δ(CH2) 1507 (w) 1507 (w) 1507 (w) δ(NH3+) 1617 (sh) 1617 (sh) 1617 (sh) νa(CO2-) 1630 (vs) 1630 (vs) 1630 (vs) νa(CO2-) m: medium, w: weak, sh: shoulder, vs: very strong.

Table 1 Main relevant FT-IR data (in cm−l) of FeIIIGluSiW and their assignments.

The UV-Vis DR spectra of silicotungstic acid and Fe^ШGluSiW are shown in Fig. 3. The spectrum of silicotungstic acid has a wide absorption band in the region 200-450 nm, with peaks at 210 and 265 nm; these are attributed to charge transfer from O²⁻ to W⁶⁺ in the Keggin unit for W=O and W-O-W bonds, respectively ^[30]. It is worth noting that the spectrum of Fe^ШGluSiW shows a clear red shift of the absorption onset compared with that of silicotungstic acid. This may be favorable for photocatalysis because of the relatively narrow HOMO- LOMO gap of Fe^ШGluSiW.

Fig. 3. UV-Vis DRS spectra of (1) silicotungstic acid and (2) FeШGluSiW.

The XRD patterns of H₄SiW₁₂O₄₀·xH₂O and Fe^ШGluSiW are shown in Fig. 4. The XRD pattern of a POM reflects its secondary structure, i.e., the three-dimensional arrangements of the heteropolyanions, cations, and other components. Usually, the primary structure of POM is stable (Fig. 2); however, the secondary structure is variable. As shown in Fig. 4, H₄SiW₁₂O₄₀·xH₂O is highly crystalline, whereas Fe^ШGluSiW is less crystalline and possibly contains an amorphous phase.

Fig. 4. XRD patterns of (1) silicotungstic acid and (2) FeШGluSiW.

The SEM image of Fe^ШGluSiW in Fig. 5 shows irregular particles of size 100-200 nm.

Fig. 5. SEM image of FeШGluSiW.

Zero-valent iron (ZVI) has been used as a heterogeneous Fenton-like catalyst for the degradation of 4-CP at ambient temperature ^[13]. For comparison, the catalytic activity of Fe^ШGluSiW was evaluated under similar conditions: 4-CP 100 mg/L, H₂O₂ 20 mmol/L, Fe^ШGluSiW 1.0 g/L, and temperature 25 ± 2 °C. The natural starting pH was 6.5, and no pH control was applied throughout the process. All experiments were conducted under dark and irradiated conditions separately.

Control experiments indicated that no significant 4-CP degradation occurred in 40 min when Fe^ШGluSiW or H₂O₂ was used alone. However, as shown in Fig. 6, rapid degradation was observed when both Fe^ШGluSiW and H₂O₂ were present, and the reaction was significantly accelerated by irradiation. The time required to degrade 4-CP completely was ca. 40 min in the dark and ca. 15 min under irradiation. Similar to most heterogeneous Fenton-like systems, the reaction rate is also influenced by experimental parameters such as catalyst dosage, H₂O₂ concentration, and initial pH value; these issues will be discussed in Section 3.4.

Fig. 6. 4-CP removal in the FeIIIGluSiW/H2O2 system. Except for the investigated parameter, others fixed on FeIIIGluSiW 1.0 g/L, H2O2 20 mmol/L, and pH = 6.5.

The catalytic performance of Fe^ШGluSiW at initial pH = 6.5 was also evaluated based on TOC removal. The results (Fig. 7) show that TOC removal after 2 h was 27% in the dark and 72% under irradiation. The Fe^ШGluSiW ternary complex therefore showed high Fenton-like catalytic activity in the degradation of 4-CP, without any pH adjustment (at natural pH = 6.5), and the reaction rate clearly increased under irradiation. In contrast, it has been reported that ZVI has no catalytic activity in 4-CP degradation at pH > 4.0 ^[13].

Fig. 7. TOC removal from 4-CP solution in FeIIIGluSiW/H2O2 system. Reaction conditions: FeШGluSiW 1.0 g/L, H2O2 20 mmol/L, pH = 6.5.

The stability of a solid heterogeneous catalyst during reactions is crucial to its reusability. In solution, SiW₁₂O₄₀⁴⁻ can be hydrolyzed to SiO₃²⁻ and WO₄²⁻ at neutral or alkaline pH values ^[18]. In reaction systems of initial pH = 6.5, Fe^ШGluSiW is therefore expected to be unstable and prone to decomposition through hydrolysis of SiW₁₂O₄₀⁴⁻ in Fe^ШGluSiW. Furthermore, the glutamate moiety in Fe^ШGluSiW is possibly oxidized during this procedure, which also results in the decomposition of Fe^ШGluSiW. If Fe^ШGluSiW decomposes during 4-CP degradation, Fe, Si, and W at appropriate concentrations should be detected in the bulk solution by ICP-AES. The stability of Fe^ШGluSiW during 4-CP degradation at pH = 6.5 was investigated by separating the Fe^ШGluSiW powder from the solution after the reaction (recovery yield ca. 80%-90%). The concentrations of Fe, Si, and W in the solution were ca. 0.018, 0.0086, and 0.096 mmol/L, respectively, for the dark system, and ca. 0.030, 0.012, and 0.14 mmol/L, respectively, for the irradiated system. This shows that in addition to the unavoidable loss of catalyst during separation, ca. 5% of Fe^ШGluSiW decomposed during the reaction. The separated Fe^ШGluSiW was characterized using FT-IR; the results are shown in Fig. 2 and Table 1. There are no obvious differences between the FT-IR spectra of the freshly prepared Fe^ШGluSiW and used Fe^ШGluSiW samples. Experiments confirmed that there were no significant differences in the Fe^ШGluSiW activity after three cycles for both the dark and irradiated systems. The Fe^ШGluSiW catalyst is therefore stable during 4-CP degradation.

SiW₁₂O₄₀⁴⁻ immobilized in water-insoluble Fe^ШGluSiW is more resistant to hydrolysis than SiW₁₂O₄₀⁴⁻ dissolved in water. This is similar to the reported observation that PW₁₂O₄₀³⁻ immobilized in a SiO₂ matrix is stable in a neutral pH medium ^[31]. Also, in contrast to the fast oxidation of 4-CP, oxidation of the glutamate moiety in the Fe^ШGluSiW catalyst is retarded. The exact reasons for the stabilities of the SiW₁₂O₄₀⁴⁻ and glutamate moiety in Fe^ШGluSiW remain unknown, and further investigation is needed.

The widely accepted catalytic mechanism in heterogeneous Fenton-like systems using iron-based solid catalysts is similar to that in homogeneous Fe²⁺/H₂O₂ or Fe³⁺/H₂O₂ systems; these involve a redox cycle between Fe³⁺ and Fe²⁺ ions on the catalyst surface and production of HO^• from H₂O₂ decomposition. However, previous studies have indicated the formation of ferryl (Fe^IV) species under some conditions; this is a weaker oxidant than HO^• and cannot oxidize aromatic compounds ^[32]. It has been suggested that in POM photocatalysis, HO^• can be formed through H₂O activation by an excited POM ^[33].

The oxidative intermediate species formed in the Fe^IIIGluSiW/H₂O₂ system were detected by performing control experiments involving addition of 0.3 mol/L n-butanol (an HO^• scavenger) to the reaction system at pH = 6.5. The results show that 4-CP degradation was significantly retarded in both the dark and irradiated systems, indicating that HO^• is the dominant oxidative intermediate species in the Fe^IIIGluSiW/H₂O₂ system.

Control experiments were performed in two homogeneous mixed solutions, each containing 100 mg/L of 4-CP, to investigate possible degradation of 4-CP in the bulk solution. One solution contained 0.028 mmol/L Fe³⁺ (ferric chloride), 0.014 mmol/L SiO₃²⁻ (sodium silicate), and 0.168 mmol/L WO₄²⁻ (sodium tungstate), and the other contained 0.028 mmol/L Fe³⁺ and 0.014 mmol/L SiW₁₂O₄₀⁴⁻ (silicotungstic acid). The two aqueous solutions are equivalent to the bulk solution phase of the Fe^ШGluSiW/H₂O₂ system, on the basis that 5% of the catalyst decomposes during the reaction, as mentioned in Section 3.3. The former solution corresponds to the case in which Fe^ШGluSiW decomposes through hydrolysis of SiW₁₂O₄₀⁴⁻, and the latter represents decomposition of Fe^ШGluSiW through oxidation of glutamate. Both solutions were adjusted to pH = 6.5, followed by addition of H₂O₂ to a final concentration of 20 mmol/L. The changes in 4-CP concentration were monitored. The results show that 4-CP degradation was negligible under both dark and irradiated conditions. The formation of HO^• and degradation of 4-CP in the heterogeneous Fe^ШGluSiW/H₂O₂ system therefore probably occur on or near the Fe^ШGluSiW surface.

For a heterogeneous Fenton-like process, the adsorption of H₂O₂ on the catalyst surface is crucial to the formation of HO^•. Tang’s group ^[34] reported that ethylenediaminetetraacetic acid (EDTA) can significantly enhance the catalytic activity of nano-BiFeO₃ for oxidative degradation of bisphenol A. The carboxyl and amino groups in EDTA can concentrate H₂O₂ within local areas on the catalyst surface by hydrogen bonding with H₂O₂; this facilitates HO^• formation. In the case of Fe^IIIGluSiW, because of the presence of glutamate moieties and SiW₁₂O₄₀⁴⁻ heteropolyanions on the catalyst surface, H₂O₂ is also prone to adsorption on the surface via hydrogen bonding. This may be an important reason for the high catalytic activity of Fe^IIIGluSiW.

We propose a possible mechanism for Fe^ШGluSiW catalysis, based on our experimental results and those reported in the literature. When the reaction is performed in the dark, the catalytic mechanism for Fe^ШGluSiW is similar to those for iron-based materials, in which H₂O₂ reacts with iron ions on the catalyst surface (denoted by ≡Fe^III and ≡Fe^II), resulting in the formation of HO^• and oxidation of 4-CP (Eqs. (3)-(5)) ^{[13, 14, 15, 16, 17]}:

The hydrogen-bonded H₂O_{2sub> on the Fe^ШGluSiW surface is highly reactive with Fe^III and Fe^II, resulting in fast production of HO^•, and therefore rapid degradation of 4-CP.}

When the reaction is conducted under irradiation, the reduction of ≡Fe^III to ≡Fe^II via Eq. (3) is accelerated, resulting in enhanced production of HO^• via Eq. (4) ^[35]. More importantly, 4-CP is also oxidized through a photocatalytic route mediated by SiW₁₂O₄₀⁴⁻ in Fe^ШGluSiW. For POM photocatalysis, the oxidation of organic compounds is suggested to occur via two parallel reaction routes, i.e., oxidation by an excited POM (POM) and by HO^• produced via activation of H₂O by an excited POM ^[33]. The reduced POM (POM⁻) is oxidized by O₂ to close the catalytic cycle. Photocatalysis by Fe^ШGluSiW of the degradation of 4-CP is expressed by Eqs. (6)-(8):

We noted that the Fe^ШGluSiW powder changed to dark brown from red-brown during the irradiated reaction. This confirms reduction of SiW₁₂O₄₀^4- during the reaction. It is worth noting that most POMs by themselves are not good photocatalysts because of slow oxidation of POM⁻ by O₂ (Eq. (9)) ^[18]. This may be why 4-CP degradation under irradiation is slow when Fe^ШGluSiW is used alone without addition of H₂O₂. However, in the presence of both Fe^ШGluSiW and H₂O₂, H₂O₂ can oxidize SiW₁₂O₄₀⁵⁻ to SiW₁₂O₄₀⁴⁻, producing additional HO^• species (Eq. (10)) ^[25]. This proposal is supported by the fact that the redox potential of H₂O₂ to HO^• and H₂O (+0.71 V versus the normal hydrogen electrode ^[36]) is more positive than that of SiW₁₂O₄₀^4- to SiW₁₂O₄₀^5- (+0.057 V versus the normal hydrogen electrode ^[37]). Additionally, the hydrogen bonding of H₂O₂ on the Fe^ШGluSiW surface may promote oxidation of SiW₁₂O₄₀⁵⁻ by H₂O₂. The photocatalytic activity of Fe^ШGluSiW is therefore enhanced by the presence of H₂O₂. In summary, the increased rate of 4-CP degradation under irradiation is ascribed to simultaneous oxidations of 4-CP through Fenton-like and photocatalytic routes mediated by ferric iron and SiW₁₂O₄₀⁴⁻ heteropolyanions, respectively, in Fe^ШGluSiW.

The proposed catalytic mechanism explains the effects of catalyst dosage, H₂O₂ concentration, and initial pH value. As shown in Fig. 6(a) and (d), 4-CP degradation was hindered by decreasing the catalyst dosage from 1.0 to 0.2 g/L, and did not change significantly with increasing dosage from 1.0 to 2.0 g/L. The formation of HO^• and oxidation of 4-CP mainly occur on the catalyst surface. A lower catalyst dosage (e.g., 0.2 g/L) provides fewer active sites; therefore, smaller amounts of H₂O₂ and 4-CP are attached to the catalyst surface, resulting in a lower reaction rate. However, addition of an excess of catalyst to the reaction system (e.g., 2.0 g/L) leads to considerable agglomeration of catalyst powder, reducing the increase in the number of active sites caused by increasing the catalyst dosage. When the reaction is carried out under irradiation, the presence of excess catalyst reduces light penetration through water, and this retards the photocatalytic degradation of 4-CP.

Fig. 6(b) and (e) shows that decreasing the H₂O₂ concentration from 20 to 10 mmol/L inhibited the reaction. A lower H₂O₂ concentration results in decreased adsorption of H₂O₂ on the catalyst surface, leading to reduced production of HO^• via Eqs. (3) and (4), and slower oxidation of SiW₁₂O₄₀⁵⁻ by H₂O₂ via Eq. (10). The oxidation of 4-CP through both the Fenton-like and photocatalytic routes is therefore hindered. However, the reaction rate did not increase correspondingly when the concentration of H₂O₂ was increased from 20 to 30 mmol/L because of scavenging of HO^• by excess H₂O₂ ^{[7, 13]}. Furthermore, coverage of too many active sites on the catalyst by H₂O₂ molecules, i.e., a too high H₂O₂ concentration, restrains the adsorption of 4-CP, which is unfavorable for 4-CP oxidation.

The degradation of 4-CP was enhanced by lowering the initial pH from 6.5 to 3.0, and was significantly retarded by increasing the initial pH to 8.0 (Fig. 6(c) and (f)). This is because of the higher redox potential of HO^• and less decomposition of H₂O₂ to O₂ and H₂O at lower pH ^{[14, 38, 39]}. Although a low pH (e.g., 3.0) favors 4-CP degradation in the Fe^IIIGluSiW/H₂O₂ system, the reaction rate at an initial pH = 6.5, with no pH adjustment, is still considerable. Fe^IIIGluSiW is therefore a promising heterogeneous Fenton-like catalyst for the treatment of CPs.

An iron-glutamate-silicotungstate ternary complex, Fe^IIIGluSiW, was synthesized from ferric chloride, glutamic acid, and silicotungstic acid. The chemical formula of Fe^IIIGluSiW is [Fe(C₅H₈NO₄)(H₂O)]₂SiW₁₂O₄₀∙13H₂O, with glutamate moiety and Keggin-structured SiW₁₂O₄₀⁴⁻ heteropolyanion. The prepared Fe^ШGluSiW showed high catalytic activity in 4-CP oxidation by H₂O₂ at neutral pH under both dark and irradiated conditions. The high catalytic activity of Fe^ШGluSiW is related to the formation of hydrogen-bonded H₂O₂ on the catalyst surface. The enhanced degradation of 4-CP under irradiation is ascribed to the synergetic effect of Fenton-like and photocatalytic oxidations catalyzed by ferric iron and SiW₁₂O₄₀⁴⁻ heteropolyanions, respectively, in Fe^ШGluSiW.