Preparation of EDTA modified cotton fiber iron complex and catalytic properties for aqueous Cr(VI) reduction and dye degradation

With the development of industry, heavy metal ions in water have caused serious environmental pollution, and pose a significant threat to human and aquatic life and health. Hexavalent chromium (Cr(VI)) is a highly toxic heavy metal ion, which can cause DNA damage through food chain enrichment in humans as it is non-biodegradable. It can lead to mutagenesis and carcinogenesis in humans (Jing et al. 2017). Moreover, a maximum acceptable level for total Cr species in wastewaters has been mandated at 0.1 to 0.2 mg L⁻¹ in many countries to minimize their harm to both humans and the natural environment (Jing et al. 2017; Wang et al. 2017). Cr(VI) and trivalent chromium (Cr(III)) are the most stable forms in the aquatic environment, and Cr(VI) is about 1,000 times more poisonous than Cr(III). Since about 2000, many techniques such as adsorption (Mao et al. 2012), ion-exchange (Petruzzelli et al. 1995), chemical reduction (Lakshmipathiraj et al. 2008; Kim et al. 2019) and precipitation (Palmer & Wittbrodt 1991) have been employed for Cr species removal from aqueous media. Reduction of Cr(VI) with nano-TiO₂ or Fe ions and their complexes as photocatalysts has been a major focus of investigation, due to their excellent chemical and optical properties, and low cost (Marinho et al. 2016, 2017; Ye et al. 2017). Although they show strong Cr(VI) reduction ability, it is difficult to separate and recycle nano-TiO₂ particles or Fe ions from solution when the reaction is complete, which limits their application in actual wastewater treatment. Thus, there is real practical significance in searching for novel, bifunctional, heterogeneous photocatalysts with strong reduction and adsorption capacity, to improve chromium ion removal rates and reduce secondary environmental pollution.

The molecular structure of ethylenediaminetetraacetic acid (EDTA) contains four carboxyl and two amino groups, as coordination functional groups, has high affinity for many divalent or trivalent metal ions, and can form highly stable complexes (Yang et al. 2014). EDTA has, therefore, been used widely as a chelating agent for heavy metal ion removal from water. EDTA complexes are photoactive to UV and visible light, and Fe(III)-EDTA complexes are converted to Fe(II) after exposure to light and can be used as an efficient photocatalyst for Cr(VI) (Beiyuan et al. 2018). Studies have also shown that EDTA has poor biodegradability as a synthetic chelating agent, and the metal-EDTA complexes formed in wastewater treatment are difficult to remove by traditional biological or chemical processes, thus causing serious pollution problems (Klüner et al. 1998; Beiyuan et al. 2018). EDTA immobilization on fiber carrier surfaces not only yields good metal-ion adsorption performance but can also improve solid-liquid separation after reaction, avoiding secondary environmental pollution. Moreover, EDTA molecules not coordinated with Fe(III) on the fiber surface can effectively capture Cr(III) in aqueous solution, thereby removing Cr(III) from solution.

Fe(III)-EDTA can also be used as a heterogeneous reaction catalyst in the oxidative degradation of organic pollutants such as dyes. It is worth noting that cotton is a natural fiber offering the benefits of easy degradability, low cost, large specific surface and high porosity, which is advantageous for contact between pollutants and the fiber surface's active sites in the reaction process (Dong et al. 2020). On that basis, cotton fabric was modified with Na₂EDTA using a hydrothermal method to introduce surface hydroxyl groups, and subsequent coordination of Fe(III) ions to produce an Na₂EDTA-modified cotton-fiber-Fe complex for this study. This was used as a heterogeneous photocatalyst in Cr(VI) reduction of and the oxidative degradation of Reactive Red 195 to investigate its catalytic reduction and oxidation performance.

Commercially scoured, boiled and mercerized, 100% cotton plain fabric (156.8 g m⁻²) was employed. Na₂EDTA, NaH₂PO₄ (SHP), FeCl₃·6H₂O, Na₂S₂O₈ (SPS), K₂Cr₂O₇, diphenylcarbazide and NaOH were all analytical grade reagents, used as received without further purification. A commercially available azo dye, Reactive Red 195 (RR195), was used after purification by re-precipitation.

The EDTA modified cotton-fiber-Fe complex was prepared according to previous research (Dong et al. 2019). Typically, 1.0 g of cotton fiber was immersed for 20 minutes in a mixed solution containing specified concentrations of Na₂EDTA and SHP, then transferred to a 100 mL, Teflon-lined, stainless-steel autoclave and heated at 200 °C (except for special instructions) for 1 hour, before cooling to room temperature. The modified cotton fiber (EDTA-Cotton) obtained was then placed in 50 mL of an aqueous solution containing 0.10 mol L⁻¹ Fe(III) ion, held at 55 °C and stirred continuously for 3 hours, to prepare the EDTA-Cotton Fe complex (Fe-EDTA-Cotton). Finally, the carboxylic acid content (Q_COOH, mmol g⁻¹) on the EDTA-Cotton surface and the Fe content (Q_Fe, mmol g⁻¹) of the resulting Fe-EDTA-Cotton were calculated using the method described by Dong et al. (2019).

The cotton fabric was analyzed by SEM, FTIR and DRS, before and after modification and coordination. For SEM observation, a Zeiss Gemini 500 scanning electron microscope operating at 15 kV was used to investigate the sample surface structure. For FTIR analysis, a Nicolet iS50 FT-IR spectrometer was used to determine surface chemical constituents. A Hitachi UH4150 UV-vis-NIR spectrometer was used to assess the light adsorption ability of fibrous samples by measuring their diffuse reflectance UV-Vis spectra (DRS), with barium sulfate as the reference.

To investigate the effect of reaction conditions on the EDTA-Cotton surface Q_COOH, the concentrations of EDTA and SHP and the hydrothermal reaction temperature were changed respectively, and the cotton fabrics were modified by the method noted above to obtain EDTA-Cotton with different Q_COOH values. The results – that is, reaction conditions vs Q_COOH value – are shown in Figure 1.

Figure 1(a) shows clearly that the Q_COOH value on the EDTA-Cotton surface increases significantly with increasing Na₂EDTA concentration, indicating that raising the Na₂EDTA concentration can significantly promote cross-linking with the cotton fiber. In part, this is because a pair of adjacent carboxyl groups in Na₂EDTA's molecular structure will dehydrate to form cyclic anhydride, with high reaction activity, when the temperature and pressure are high, which reduces the esterification reaction activation energy between Na₂EDTA and hydroxyl groups in the cellulose molecular structure and promotes esterification between them, introducing carboxyl groups onto the fabric surface (Júnior et al. 2009; Gan & Dong 2019) – see Equation (3). The probability of contact between Na₂EDTA molecules and hydroxyl groups in the cellulose molecular structure and the diffusion rate of Na₂EDTA into the cotton fiber also increase, increasing the number of molecules involved in cross-linking, which increases Q_COOH. It is noted that, during the cross-linking reaction between polycarboxylic acid and cellulose, NaH₂PO₄ can significantly promote the reaction between carboxyl and hydroxyl groups.

Figure 1(b) shows a gradual rise in the Q_COOH value as the SHP concentration increases, which means that increasing the SHP concentration can promote the esterification reaction. This is mainly because SHP can promote the dehydration of adjacent carboxyl groups in the Na₂EDTA molecular structure to form cyclic anhydride (Mao et al. 2012), which is conducive to esterification. It is noted that when the SHP concentration exceeds 0.32 mol L⁻¹, the EDTA-Cotton surface's Q_COOH value no longer increases significantly, and the modification reaction tends to be balanced. There are two reasons: (1) high concentrations of SHP will cause the ester bond generated during esterification to break and reduce the amount of Na₂EDTA esterified with the fiber macromolecules; and (2) the increase in SHP concentration will promote the esterification of more carboxyl groups in the EDTA-Cotton molecular structure to form ester bonds, reducing the number of free carboxyl groups and preventing the EDTA-Cotton surface's Q_COOH value from increasing. Therefore, it can be considered that the optimal SHP concentration for EDTA-Cotton preparation is 0.32 mol L⁻¹.

Figure 1(c) shows that increasing the hydrothermal temperature also increases the Q_COOH value – that is, increasing the temperature can promote modification significantly. This is mainly because increasing the hydrothermal temperature promotes the dehydration of the adjacent carboxyl group pairs in EDTA's molecular structure to form anhydride (Júnior et al. 2009), accelerating the esterification rate between EDTA and the cotton fiber, so that the fabric surface contains more carboxyl groups. The temperature increase also increases the degree of swelling and wetting of the cotton fiber, so that EDTA molecules immerse more easily in the fiber (Júnior et al. 2009). However, although the Q_COOH value of the EDTA-Cotton surface at 200 ° is high, the fiber's mechanical properties are reduced severely due to the high temperature and pressure conditions. Therefore, it is considered that the best hydrothermal temperature for preparing EDTA-Cotton is 180 °C.

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EDTA-Cotton was coordinated with Fe(III) under specific conditions and, after the reaction was complete, the Q_Fe values on the Fe-EDTA-Cotton surface were measured – see Figure 2.

Figure 2(a) shows that Q_Fe increases initially but then decreases with increasing Q_COOH, indicating that increasing Q_COOH is a benefit and increases Q_Fe. However, strong concentrations lower the Fe(III)/cotton coordination. The mechanism is that higher Q_COOH concentration on the EDTA-Cotton surface improves coordination with Fe(III), yielding higher Fe(III) concentrations on the cotton fiber surface. However, excess coordination tightens the cotton fiber surface structure. The resulting steric effect blocks the carboxyl/Fe(III) coordination reaction and lowers the cotton surface's hydrophilicity (Liu et al. 2018), which also blocks Fe(III) permeation. Temperature and Fe(III) concentration were the two main factors in the coordination reaction – see Figure 2(b). Obviously, Q_Fe generally increases with C_Fe,0 at the same temperature, so C_Fe,0 increases the coordination reaction, mainly because increasing C_Fe,0 enhances Fe(III) expansion on the cotton fiber surface and its permeability, benefiting the coordination reaction between them. Equally, Q_Fe increases with reaction temperature at the same C_Fe,0, indicating that raising the reaction temperature enhances Fe(III)/EDTA-Cotton coordination. The increased temperature accelerates the swelling of the cotton fiber, exposing more internal carboxyl groups, which enhances coordination. With the temperature increase, however, the Brownian movement of the liquid molecules will also increase, leading the spreading of Fe(III) internally and externally to the cotton fiber surface, which is good for coordination with the cotton fiber. While a higher reaction temperature will increase Q_Fe, it will also cause explosive hydrolysis to generate FeOOH, so it should not be too high.

SEM images (Figure 3(a)–3(c)) show that folds on the cotton surface are covered by heterogeneous, raised, soil-like material, which is thought to be caused by the Fe(III) on the cotton fiber surface (Liu et al. 2018; Dong et al. 2019). This indicates that the cotton fiber surface is the main location of the coordination reaction between EDTA-Cotton and Fe(III). It is noted that no obvious change is detected and the fiber appearance is preserved, after the EDTA modification and Fe(III) coordination reactions.

Figure 3(d) shows that the characteristic spectral line peaks of cotton fiber all migrate after EDTA modification. More importantly, a new peak has appeared at around 1,745 cm⁻¹. This might be a carboxyl and ester characteristic peak after EDTA-Cotton modification. This indicates that esterification happened between the hydroxyl in the cotton-fiber-molecule structure and carboxyl in the EDTA-molecule structure (Dong et al. 2019). The migration and lower magnitude of the characteristic spectral line peaks for Fe-EDTA-Cotton also mean that the coordination reaction between EDTA-Cotton and Fe(III) and its complex has shown up.

Figure 3(e) shows a strong characteristic peak in the EDTA-Cotton UV area, arising from π-π transition of the unsaturated bond in its macro-molecular structure. Fe-EDTA-Cotton has higher absorption capability than EDTA-Cotton in the UV and Vis spectral region, and higher Q_Fe can improve this further. In other words, introducing Fe(III) will promote the light absorption properties of cotton fiber significantly. The strong visible light absorption performance of Fe-EDTA-Cotton indicates that the utilization rate of solar or LED light can be improved, accelerating the reduction of Cr(VI) and/or the oxidation removal of dyes.

Several different reaction systems containing 100 ml 0.04 mmol L⁻¹ Cr(VI) solution were set up at pH 6.0 and 25 °C. The results – Figure 4 – show that R% values within the first 60 minutes were below 5% when the original cotton fiber or EDTA-Cotton were used, with or without irradiation (curves i to iii), indicating that Cr(VI) is reduced very little under these conditions. The R% value increased gradually to 13.75% at 60 minutes for EDTA-Cotton with irradiation (curve iv), which may be due to the weak light absorption properties of EDTA-Cotton in the UV and Vis regions (Figure 3(e)).

Light irradiation can promote reduction by the reducing groups C-H and C = O on the EDTA-Cotton surface with Cr(VI) in aqueous solution (Deng et al. 2017), causing some Cr(VI) reduction – Equations (4) and (5). R% is only 8.03 after 60 minutes in the dark (curve v). When irradiation is introduced into the system, however, (curve vi), R% increases sharply to 89.53 at 60 minutes – i.e., the catalytic reduction of Cr(VI) by Fe-EDTA-Cotton is highly dependent on light irradiation. Several similar results have been reported for the homogeneous photocatalytic reduction of Cr(VI) in the presence of Fe(III) and organic acids – for example, oxalic and citric – in water (Marinho et al. 2016; Ye et al. 2017). The main reason is that Fe(III) on the Fe-EDTA-Cotton surface absorbs incident photons and is converted to Fe(II) due to LMCT (ligand to metal charge transfer) effects under light radiation. The Fe(II) generated on the Fe-EDTA-Cotton surface can act as an effective reducing agent to convert Cr(VI) to Cr(III) through a series of single electron transfer processes (Luo et al. 2017). At the same time, Fe(II) is oxidized to Fe(III), and the cyclic Fe(III)/Fe(II) reaction is realized – Equations (6) and (7).

Figure 5(a) shows that k increases significantly with increases in Q_Fe, indicating that the increase in Q_Fe can significantly accelerate the Cr(VI) reduction reaction, when catalyzed by Fe-EDTA-Cotton. This is because the number of active sites on the catalyst surface increases, so that more Cr(VI) ions are adsorbed on the catalyst surface where reduction occurs, which is conducive to the conversion of Cr(VI) to Cr(III).

Figure 5(b) shows the effect of pH on k. As can be seen, k is significantly higher at pH 3 and 6 than at pH 9 – that is, acidic and neutral conditions are favorable for the reduction reaction. Generally, Cr(VI) exists mainly in aqueous media in the forms of H₂CrO₄, CrO₄⁻ and CrO₄²⁻, while HCrO₄⁻ is its principal form in the pH range 1.0 to 6.8 (Deng et al. 2017). The -N = functional group on the Fe-EDTA-Cotton surface is also protonated at low pH levels, promoting the adsorption of negatively charged Cr(VI) through electrostatic attraction. This is conducive to the rapid reduction of Cr(VI) species, as shown in Equation (8) (Li et al. 2009). Equations (9) and (10), however, show that high H⁺ concentrations in solution can also promote rapid Cr(VI) reduction (Marinho et al. 2016; Luo et al. 2017).

As pH levels increase in the reaction system, the concentration of H⁺ falls, making the Fe(III) at the active site interact easily with OH⁻ in the solution to form an insoluble precipitate, thus reducing the number of active sites and reducing the catalyst performance for Cr(VI). Figure 5(c) shows that k increases significantly with increasing reaction temperature, indicating that higher reaction temperatures are beneficial to the catalytic reduction reaction. With the temperature increase, the Brownian motion of the molecules in solution intensifies, and the average kinetic energy increases, intensifying the thermal motion of Cr(VI) in the solution. The fibers also swell to a greater extent, helping to improve contact probability between Cr(VI) and the catalyst, thus enhancing Cr(VI) reduction.

The Arrhenius equation (Zhang et al. 2018) shows that the apparent activation energy of the Cr(VI) reduction reaction in this system is 25.45 KJ mol⁻¹. As can be seen in Figure 4, irradiation is necessary for catalytic reduction reaction. Figure 5(d) shows that k under the Hg lamp is significantly higher than with the other three LED light sources. In other words, although the intensity of visible light from the Hg lamp is lower than that of LED-h, the former can improve the catalytic reduction performance of Fe-EDTA-Cotton for Cr(VI) more effectively. It is important that the increase in LED light radiation intensity is conducive to the increase in k, as it means that the increase in visible light intensity from LEDs can promote the Cr(VI) reduction reaction, and that LEDs with high light radiation intensity can replace Hg lamps, which consume more energy, in the context of the catalytic reduction of Cr(VI) by Fe-EDTA-Cotton.

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Figure 6(a) shows the effect of Q_Fe on the catalytic oxidation performance of Fe-EDTA-Cotton. It is obvious that the dye decolorization rate (D%) is only about 14% after 90 minutes, when Q_Fe = 0 mmol L⁻¹. This is mainly due to the limited adsorption of dyes on cotton fibers, and the weak and slow oxidation of dyes by SPS. When Q_Fe >0 mmol L⁻¹, D% increases significantly with increasing reaction time, indicating that Fe(III) on the fiber surfaces is the key factor in Fe-EDTA-Cotton's photocatalytic oxidation activity, mainly because the dye molecules in aqueous solution are excited when exposed to visible light radiation (Equation (11)). The excited dye molecules transfer their own electrons to Fe(III) ions on the catalyst surface, reducing them to Fe(II) (Equation (12)) (Gao et al. 2016), and then reacting with S₂O₈²⁻ adsorbed onto the surface active site, or dissolved oxygen in the water, to produce and ⁠, which are used for oxidative dye degradation (Equations (13) and (14)) (Leng et al. 2014). At the same time, the generated can interact with OH⁻ to produce (Equation (15)). The synergistic effect of the three free radicals (⁠⁠, and ⁠) leads to the degradation of dye molecules in the aqueous solution. In addition, Fe(II) ions on the catalyst surface can react with S₂O₈²⁻ in the system to be oxidized to Fe(III), thus promoting the Fe(III)/Fe(II) cyclic reaction (Equation (16)) (Leng et al. 2014), and accelerating the production of active species, thereby improving catalytic oxidation performance significantly.

Figure 6(b) shows that the photocatalytic oxidation activity of Fe-EDTA-Cotton is significant under acidic and neutral conditions, but is reduced significantly under alkaline conditions. On the one hand, the reaction of S₂O₈²⁻ with water to form and (Equation (17)) can be promoted under alkaline conditions (Matzek & Carter 2016; Huang et al. 2017), generating further to oxidize dyes (Equation (18)) (Furman et al. 2010). However, the system's H⁺ concentration decreases and the Fe(III) ions in the active sites interact easily with the OH⁻ in solution to form insoluble precipitates, while the formation rate of active free radicals is reduced. On the other hand, the dye molecule adsorption on the catalyst surface decreases under alkaline conditions.

Fe-EDTA-Cotton can be prepared by modification of Na₂EDTA with cotton fiber and an Fe(III) coordination reaction. The carboxylic acid content of the EDTA-Cotton surface can be increased significantly by increasing the Na₂EDTA and NHP concentrations, and the hydrothermal reaction temperature. Increasing the Fe(III) concentration and the coordination reaction's temperature can increase the Fe(III) content on the surface of the complex.

SEM and FTIR analysis showed that EDTA could react successfully with hydroxyl groups on a cotton fiber surface. Carboxyl groups can be introduced onto cotton fiber surfaces, and Fe-EDTA-Cotton can be prepared by coordination reaction with Fe(III).

Fe-EDTA-Cotton, a heterogeneous catalyst, has a significant catalytic effect on Cr(VI) reduction and the reaction can be described by the pseudo first-order kinetic model.

Increasing Q_Fe, reaction temperature and/or light radiation intensity, and/or lowering pH can increase the reduction rate significantly. The complexes obtained can remove Cr(VI) and organic dyes from aqueous solution in one stage, with strong photocatalytic effects on both processes. Although the increase in oxidant concentration is beneficial to dye degradation, it is not conducive to Cr(VI) reduction.

This research was supported by the Educational Commission of Anhui Province of China (KJ2020A0354), High-level Scientific Research Foundation for the introduction of talent (2020YQQ045), Foundation of Anhui Polytechnic University (Xjky2020039), Research funding project for academic and technological leaders in Anhui Province (2020H218), Academic funding project for young and middle-aged top talents in disciplines (majors) of colleges and universities in Anhui Province (gxbjZD2020075), Key Research and Development Projects of Anhui Province (202004a06020055), and Wuhu Science and Technology Plan Project (2020yf51).

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