Abstract

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 (Fe-EDTA-Cotton). After characterization by SEM and FTIR, Fe-EDTA-Cotton was used as a heterogeneous photocatalyst to reduce Cr(VI) and degrade Reactive Red 195 oxidatively, to evaluate its catalytic activity. The results indicated that high Na2EDTA and/or NaH2PO4 concentrations, and/or high hydrothermal temperature increased the carboxyl group content (QCOOH) of the cotton fiber; high Fe content and elevated temperature could enhance coordination. Fe-EDTA-Cotton had a significant photocatalytic function in reducing Cr(VI). Increasing the QFe value of the complex or stronger irradiation favored photocatalytic reduction of Cr(VI) species. Increasing the system's pH did not help the reduction reaction. Fe-EDTA-Cotton could also remove Cr(VI) and Reactive Red 195 simultaneously from aqueous solution, and showed strong photocatalytic capacity. The reduction or oxidation reaction could be regulated by changing Na2S2O8 concentration in the reaction system, which might provide a new way to treat wastewater containing organic pigments and Cr(VI) species.

Highlight

  • Hydrothermal process enhanced EDTA modification of cotton fabric

  • EDTA-modified cotton fiber Fe complex was prepared

  • Cr(VI) removal was improved by its synergistic effect

  • Fe complexes can remove Cr (VI) and organic dyes from aqueous solution in one bath

  • The work might provide a new way to purify the wastewater containing organic pigments and Cr (VI) species.

Graphical Abstract

Graphical Abstract
Graphical Abstract

INTRODUCTION

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.

METHODS

Reagents and materials

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.

Preparation of Fe complex on modified cotton fiber

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 (QCOOH, mmol g−1) on the EDTA-Cotton surface and the Fe content (QFe, mmol g−1) of the resulting Fe-EDTA-Cotton were calculated using the method described by Dong et al. (2019).

Characterization

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.

Photocatalytic experiments

The photocatalytic measurements were carried out using Cr(VI) and RR195 as target pollutants under visible light irradiation from a 100 W LED lamp (Guangdong Ant Illumination Co., China). In a typical procedure, 1.0 g Fe-EDTA-Cotton was dispersed in each of 100 mL Cr(VI) solution (0.04 mmol L−1) and 100 mL RR195 (0.05 mmol L−1), the latter including SPS (1.5 mmol L−1). The test solution was sampled at fixed time intervals and the sub-sample separated by centrifugation. The Cr(VI) concentration was determined with a UV-2401 Shimadzu spectrophotometer using a colorimetric method based on the 1,5-diphenylcarbazide reaction at 540 nm (maximum absorption wavelength). The total concentration of the remaining Cr species in the test solution was measured using a PinAAcle 900H atomic absorption spectrometer (Perkin-Elmer Inc., USA). The proportional removal of Cr(VI) (R%) was calculated using Equation (1). Total Cr species removal efficiency (η%) was expressed using Equation (2). The concentration of RR195 remaining in solution was measured during the reaction by a method reported in Dong et al. (2010).
formula
(1)
formula
(2)
where C06 and Ct6 were the initial and residual concentrations of Cr(VI), respectively, and C0t and Ctt the initial and residual concentrations of total Cr species during the reaction.

RESULTS AND DISCUSSION

Preparation of EDTA-cotton

To investigate the effect of reaction conditions on the EDTA-Cotton surface QCOOH, 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 QCOOH values. The results – that is, reaction conditions vs QCOOH value – are shown in Figure 1.

Figure 1

Relationship between CEDTA, CSHP, hydrothermal temperature and QCOOH value.

Figure 1

Relationship between CEDTA, CSHP, hydrothermal temperature and QCOOH value.

Figure 1(a) shows clearly that the QCOOH 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 QCOOH. 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 QCOOH 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 QCOOH 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 QCOOH 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 QCOOH 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 QCOOH 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.

(3)

Preparation of Fe-EDTA-Cotton

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

Figure 2

Coordination reaction between Fe(III) and EDTA-Cotton under various conditions.

Figure 2

Coordination reaction between Fe(III) and EDTA-Cotton under various conditions.

Figure 2(a) shows that QFe increases initially but then decreases with increasing QCOOH, indicating that increasing QCOOH is a benefit and increases QFe. However, strong concentrations lower the Fe(III)/cotton coordination. The mechanism is that higher QCOOH 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, QFe generally increases with CFe,0 at the same temperature, so CFe,0 increases the coordination reaction, mainly because increasing CFe,0 enhances Fe(III) expansion on the cotton fiber surface and its permeability, benefiting the coordination reaction between them. Equally, QFe increases with reaction temperature at the same CFe,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 QFe, it will also cause explosive hydrolysis to generate FeOOH, so it should not be too high.

Catalyzer characterization

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

SEM (a) Cotton fiber, (b) EDTA-Cotton, (c) Fe-EDTA-Cotton, and (d) FTIR and (e) DRS spectra of cotton fiber, EDTA-Cotton and Fe-EDTA-Cotton.

Figure 3

SEM (a) Cotton fiber, (b) EDTA-Cotton, (c) Fe-EDTA-Cotton, and (d) FTIR and (e) DRS spectra of cotton fiber, EDTA-Cotton and Fe-EDTA-Cotton.

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 QFe 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.

Heterogeneous photocatalytic reduction of aqueous Cr(VI)

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)).

Figure 4

Reduction efficiency of Fe-EDTA-Cotton for Cr(VI).

Figure 4

Reduction efficiency of Fe-EDTA-Cotton for Cr(VI).

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).

Usually, reaction conditions have the greatest impact on catalytic performance, so the reaction conditions were optimized and R% under different conditions obtained. The data generated were used in the pseudo-first-order reaction kinetics equation and the results showed that it could be used to describe the Cr(VI) reduction reaction. The reduction reaction rate constants (k) are shown in Figure 5.
formula
(4)
formula
(5)
formula
(6)
formula
(7)
Figure 5

Photocatalytic reduction of Cr(VI) with Fe-EDTA-Cotton under various conditions (LED-L:1.87 mW cm−2, LED-M:5.10 mW cm−2 and LED-H: 14.95 mW cm−2 at 400–1,000 nm).

Figure 5

Photocatalytic reduction of Cr(VI) with Fe-EDTA-Cotton under various conditions (LED-L:1.87 mW cm−2, LED-M:5.10 mW cm−2 and LED-H: 14.95 mW cm−2 at 400–1,000 nm).

Figure 5(a) shows that k increases significantly with increases in QFe, indicating that the increase in QFe 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.

(8)

formula
(9)
formula
(10)

Heterogeneous photocatalytic oxidation of RR195

Figure 6(a) shows the effect of QFe 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 QFe = 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 QFe >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.

Figure 6

Catalysis of Fe-EDTA-Cotton during dye degradation under differing conditions.

Figure 6

Catalysis of Fe-EDTA-Cotton during dye degradation under differing conditions.

Figure 6(c) shows that the oxidation effect of SPS is significantly better than that of H2O2. This may be due to the fact that SPS system not only contains S2O82− with oxidation ability, but can also produce with stronger oxidation ability and with longer half-life, after being activated by Fe-EDTA-Cotton (Vicente et al. 2011). SPS is thus a safe alternative oxidant, replacing H2O2 in environmental remediation.
formula
(11)
formula
(12)
formula
(13)
formula
(14)
formula
(15)
formula
(16)
formula
(17)
formula
(18)

Heterogeneous photocatalytic treatment of wastewater contain Cr(VI) and RR195

Previous research has shown that Fe-EDTA-Cotton has both a catalytic effect on the oxidative degradation of organic dyes in water and excellent catalytic reduction performance for Cr(VI) in aqueous solution (Dong et al. 2020). However, wastewater composition is often complex, containing both organic dyes and inorganic heavy metal ions. There is thus great practical significance in investigating the removal performance of Fe-EDTA-Cotton for mixed pollutants. On this basis, a mixed solution containing Cr(VI) and RR195 was simulated, at natural pH level and 20 °C, to assess Fe-EDTA-Cotton's photocatalytic performance of (QFe = 0.83 mmol g−1) in treating different pollutant categories simultaneously under Hg lamp radiation. As shown in Table 1, the values of R120% and η120% decrease significantly with increasing initial SPS concentration. In other words, SPS has a significant inhibitory effect on the Cr(VI) reduction reaction in mixed solution. This is mainly because S2O82− in aqueous solution can react with Fe(II) to form and (Equations (13) and (15)), which will react with the dissolved dye molecules via oxidative degradation reactions and, at the same time, react with Cr(III) in the system to produce Cr(VI) (Equations (19) and (20)). This leads to a significant decrease in R120%, which affects the adsorption and removal of Cr(III) on the catalyst surface (Dong et al. 2019), and causes η120% to decrease slowly. The competitive adsorption of dye molecules and Cr(VI) on the catalyst surface also reduce R120%. It is noted in this context that R120% increases gradually when the SPS concentration exceeds 2.0 mmol L−1. So, the increase in the S2O82− concentration in the system can promote the Fe(III)/Fe(II) cyclic reaction (Equation (16)), which accelerates the Cr(VI) reduction reaction in the system, and increases R120%. On the other hand, excess SPS will become a radical quencher and capture the active radicals (Equations (21) and (22)) generated in the reaction system (Li et al. 2017), reducing the free radical content in solution, and thus the occurrence of reactions (19) and (20). Further, both Equation (15) and Gao et al.’s research results (2016) show that as the SPS concentration increases in the system, the pH of the solution falls gradually. The acidic condition is conducive to Cr(VI) photoreduction (Equations (9) and (10)). Despite this, a literature survey (Gao et al. 2016) has shown that acidic conditions are not conducive to Cr ion removal, which is the main reason that η120% decreases with increasing SPS concentration. D60% increased significantly, after 60 minutes, in the mixed pollutants, with the increased SPS concentration in the system. This is mainly because the SPS concentration increase raises the number of reactive radicals generated, oxidizing more dye molecules. Azo dyes and Cr(VI) can still be removed by redox in the mixed pollutant system, which means that Fe-EDTA-Cotton, as a heterogeneous catalyst, can be applied not only to the removal of a single pollutant in water, but also in the treatment of wastewater containing organic dyes, Cr(VI) and other pollutants, and still achieve the desired effect.
formula
(19)
formula
(20)
formula
(21)
formula
(22)
Table 1

D60%, R120% and η120% in the presence of Fe-EDTA-cotton

SPS conc. (mmol L−1)00.21234.5
R12095.43 86.35 60.57 47.27 52.71 58.57 
η12073.28 53.77 39.51 30.22 24.79 1.65 
D602.01 12.92 42.05 63.43 70.56 89.39 
SPS conc. (mmol L−1)00.21234.5
R12095.43 86.35 60.57 47.27 52.71 58.57 
η12073.28 53.77 39.51 30.22 24.79 1.65 
D602.01 12.92 42.05 63.43 70.56 89.39 

CONCLUSIONS

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 QFe, 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.

ACKNOWLEDGEMENT

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).

DATA AVAILABILITY STATEMENT

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

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Author notes

Hongfang Wu and Peng Wang contributes equally to this work.

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