Deploying a molecular copper catalyst for efficient degradation of commercial and industrial dyes under practical conditions

Abstract

Water pollution due to the discharge of inadequately treated contaminated water from industry creates an ecological imbalance posing several health hazards leading to the depletion of aquatic flora and fauna. Therefore, an immediate solution pertaining to stringent action and awareness is warranted to preserve natural water bodies from industrial effluent. Here, we report a two-step method for wastewater treatment, wherein the first step deploys an adsorption-based dye-treatment method (coagulation and flocculation). In contrast, the second step enforces chemical oxidation induced via a combination of a bio-inspired molecular copper complex and H₂O₂. This catalytic unit efficiently degrades a versatile array of toxic industrial dyes, with varying molecular templates, present in aqueous solution at room temperature. The dye treatment process was primarily monitored via optical spectroscopy as decolouration of the solution indicated oxidative degradation of the dye molecules. A complementary gas chromatography experiment established that CO₂ gas is produced as the major product of this process. Detailed mechanistic studies revealed that the chemical process proceeds via the formation of a Cu(III)-hydroxo intermediate, which actively destroys the aromatic backbone of the dyes without following traditional radical-based Fenton's chemistry. This two-step process was active over a broad pH range (pH 3–11) of aqueous solutions, while it exhibited excellent efficiency even during the degradation of actual industrial dyes in their original form without any pre-treatment. The capability of this dye-treatment process was successfully tested at 100 liter scale with industrial dyes, showcasing the high technology readiness level (TRL) of this process.

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Water impact

This article describes a cost-effective, efficient, and scalable dye effluent treatment process, which completely degrades aromatic dyes to carbonated water without forming any hazardous intermediates. This method can be deployed to treat contaminated wastewater containing a range of toxic aromatic dyes. This process was tested at a 100 liter scale without any drop in its efficiency.

Introduction

Organic dye manufacturing has emerged at the forefront of the chemical industry with a strong influence over several verticals, including textile, printing, polymer, cosmetics, pharmaceutical, food, and leather businesses. The advent of organic dyes provided several advantages over traditional colorants owing to their low-cost production, extended chemical stability, and inertness in biological and physicochemical conditions. A diverse range of organic dyes on a scale of 7 × 10⁷ tons is produced annually all over the world. Such large-scale production typically requires a copious amount of water in the manufacturing process, which inadvertently generates almost an equal amount of dye-contaminated waste.^1–3 The direct discharge of this wastewater containing polyaromatic organic pollutants adversely affects the surrounding aquifers, primarily due to the acute toxicity and hazardous and carcinogenic traits of the dye molecules. Colored wastewater directly hampers the natural growth of aquatic flora by severely limiting their photosynthetic capability. Additionally, the dye pollutants create an imbalance due to an abrupt decrease in the dissolved oxygen concentration and available total nutrients (nitrogen and potassium). Such alterations directly translate into a sharp drop in chemical and biochemical oxygen demand while increasing the total dissolved solids (TDS).⁴ Hence, the immediate release of this untreated dye industrial wastewater into natural water bodies explicitly poses a serious risk to human health and the neighboring ecosystem.^5,6 Therefore, the removal of the dye contaminants from industrial wastewater has been reckoned a major challenge to mitigate the negative environmental impact while ensuring the continuous growth of this economically viable sector.¹

Over the years, numerous physical, biological, and chemical wastewater treatment methods have been developed to alleviate the concentration of harmful dyes in wetlands.^7,8 Reportedly, a series of physical methods, such as coagulation, flocculation, adsorption, ion exchange, and filtration, were found useful for the rapid removal of dyes. However, these user-friendly methods necessitate the pre-treatment of wastewater in an attempt to attain the specific chemical conditions for the optimal execution of physical dye removal strategies. Generally, these methods involve phase conversion of the colored dye pollutants rather than their complete elimination. Therefore, expensive additional methods are necessarily coupled with physical methods for efficient wastewater treatment.^9–18 In nature, specific microbes are known for catalyzing dye removal and biodegradation by rationally employing their fascinating enzymatic systems with high catalytic efficiency. Such a revelation has inspired a closer look at employing existing biological machinery for dye effluent treatment. Albeit these microbes completely degrade the organic dyes, their activity is limited to narrow physiological conditions (near neutral pH, ambient temperature, even an anaerobic atmosphere in certain cases), and their activity prevails only for a handful of genres of organic dye. Hence, these biological methods do not offer a robust and general solution for the treatment of industrial effluent under practical conditions.^12–14 Furthermore, Fenton's chemistry is one of the most popular chemical methods that has been widely employed for dye degradation in wastewater. It involves the Fe(II/III)-catalyzed oxidation of organic compounds in the presence of H₂O₂, where the intermediate generation of hydroxyl radicals (OH˙) essentially depletes the organic dye molecules. This advanced oxidation process (AOP) exhibited the best results among all the available chemical methods for treating organic dyes. However, the effect of Fenton's chemistry is maximized in acidic conditions (pH 3–4), which has severely impeded its application with the need for pre-acidification of the dye effluents. Thus, the overall operational cost of such a chemical treatment process remains relatively high.^15–20 Several modifications have been executed to improve the efficiency of chemical processes. For instance, the use of an auxiliary photocatalytic system significantly enhanced the Fenton reaction rate. This strategy was specifically successful for a range of regular dyes with concomitant treatment with different metal complexes such as a hexanuclear Ni(II) complex, Ag-based compound, or lanthanide-containing molecules under UV illumination.^28–30 However, the necessary use of high-energy UV electromagnetic radiation to activate the photosensitized model has narrowed their practical applications. Recently, Ghosh and co-workers reported salen-ligated iron and copper complexes that extended the photo-driven chemical degradation of dyes even under visible light irradiation.²¹ Despite these success stories, the inclusion of a photo-irradiation system has significantly increased the operational cost, stalling its possible usage on a large-scale basis. Hence, the search for an effective, economical, and eco-friendly organic dye degradation method still continues.

Here, in this work, we have attempted to bridge the gap between biological and chemical dye degradation methods with the use of a bio-inspired synthetic catalyst. Cytochrome P450 is reckoned to be a versatile biocatalyst to lead the oxidative degeneration of foreign molecules in biology.^22,23 We have recently developed a copper-based molecular catalyst containing a flexible and redox-active ligand scaffold that can readily activate oxygen molecules in an aqueous solution.²⁴ Further investigation of this copper complex reveals that this copper complex can even mimic the oxidative catalytic activity of the cytochrome P450 enzyme. This copper-bound synthetic catalyst readily degrades an array of twelve different commercial dyes in the presence of H₂O₂ as a redox ally. This chemical catalytic process presumably proceeds through the formation of a highly oxidizing Cu(III)-hydroxo species (not via a typical Fenton-type radical pathway), and this reactivity remains intact over a wide range of pH conditions (pH 3.0–12.0). Therefore, this catalyst can be directly employed for dye degradation without any pre-adjustment of the acidity or alkalinity of the wastewater. This process was successful even for the treatment of four different real-life industrial effluents. Later, we deployed this chemical process in tandem with physical treatment (coagulation and flocculation) to degrade 100 liters of as-received dye industry effluent within six hours to obtain completely cleansed water where CO₂ was found to be the only terminal organic product. Here, this unique bio-inspired catalyst plays a pivotal role as an inexpensive material for a wide variety of industrial wastewater treatment methods to generate reusable water. Hence, this synthetic catalyst provides a new avenue for the chemical treatment of organic dye effluents without the need for any pre-treatment procedures or expensive auxiliary photocatalysts and negating the formation of highly reactive side products. Moreover, this modular chemical treatment method can be integrated with existing dye treatment processes to improve water recyclability from different industries while alleviating their negative environmental impacts.

Results and discussion

The synthesis and structural characterization of the copper complex (C1) were executed in detail earlier by our group.²⁴ The representative optical, vibrational (FTIR), and EPR data recorded for this complex are displayed in Fig. S1–S3.†C1 exhibited a reversible redox signal around −0.01 V (vs. FeCp₂^+/0) in an electrochemical experiment in organic media (Fig. S4†), which was stoichiometric in nature. This feature was probed further via spectroelectrochemistry that revealed a stark change around an LMCT band near 500 nm while altering the potential around the −0.01 V peak (Fig. S5†). This optical change specifically indicated a Cu(III/II) redox change.^25,26 All these data established Cu(II) as the resting state of the copper center in C1.

Two-stage catalytic setup for the treatment of commercial dyes and industrial effluents

A two-stage catalytic setup was devised here to implement an efficient dye-degradation strategy (Fig. 1). In the first step, a couple of adsorption techniques (flocculation and coagulation) were deployed sequentially for the rapid precipitation of insoluble and sparingly soluble dye components present in the dark-colored effluent. Abundant and inexpensive metal salts, such as FeSO₄ and CaO, were utilized in this step. The resultant turbid solution was filtered to obtain a transparent brown-colored solution containing water-soluble dye molecules. Next, the solution was exposed to an advanced oxidation process (AOP) in the presence of a mixture of C1 and H₂O₂. The initial dye degradation experiment was investigated with a commercially available Rh-6G dye (12, Fig. 2) under variable pH conditions (pH 3–11) (Fig. S6 and S7†). Dye degradation in this process was followed by a gradual decrease in the signature optical bands in the visible region for the dye solution with varying amounts of C1 and H₂O₂ (Fig. 3A). An optimized ratio of 2 : 5 for the primary catalyst C1 and the auxiliary redox partner H₂O₂ was established during this optical spectroscopy experiment (Fig. S7b†). A series of control experiments were also performed either in the presence of a mixture of CuCl₂/H₂O₂ or H₂O₂ alone, which displayed significantly less dye degradation compared to the C1/H₂O₂ combination (Fig. S8 and S9†). This data clearly indicates the role of C1 in this dye fragmentation process. The pH of the reaction mixture emerged as a key factor where the maximum activity for Rh-6G dye degradation (12) was observed under neutral (pH 7.0, 94%) to basic conditions (pH 11.0, 90%) compared to an acidic medium (pH 3.0, 57%) (Fig. S9b†). We attempted to gain an insight into the possible formation of intermediates during degradation of the dye (Rhodamine-6G) via mass spectroscopy at different times during the experiment. The initial data showcases the presence of only one species, the precursor Rhodamine-6G, without any other mass fragments (Fig. S10†). Next, we recorded the mass data 30 minutes after adding the catalyst C1/H₂O₂ mixture into the same solution of Rhodamine-6G. Now, the primary peak of Rhodamine-6G was not observed, while multiple peaks of lower fragments of the molecule were noticed (Fig. S11†). These data indicated that Rhodamine-6G is getting degraded following chemical treatment with the catalyst C1/H₂O₂ mixture. Interestingly, after 1.0 hour of treatment, we did not observe any significant mass signal with an m/z value >60. The complementary HPLC data also did not display any organic fragment, while the gas chromatography (GC) data highlighted the presence of CO₂ in the headspace. This data presumably indicates that the majority of the dye has been converted to CO₂ while smaller fragments of the dye were generated during the process. However, at the end of the experiment, no detectable amount of these highly reactive fragments remains in the solution. As the majority of the dye molecules are converted into CO₂, the treated solution does not pose any immediate toxicity threat.

Fig. 1 A schematic representation of the two-stage setup for a standard and industrial dye degradation system, where the initial physical treatment (stage I) precipitates insoluble dye components via coagulation and flocculation adsorption methods, while advanced oxidation triggered by copper complex C1 and H₂O₂ degrades the soluble dye molecules (stage II).

Fig. 2 The chemical structures of the commercially available dye molecules employed in this study along with their corresponding maximum observed degradation (%) and reaction time.

Fig. 3 (A) Optical spectra of Rh-6G (20 μM) in the presence of complex C1/H₂O₂ under neutral aqueous conditions (black, red, blue, green, and violet traces recorded for Rh-6G before treatment, and after 2 h, 4 h, 6 h, and 8 h of treatment, respectively). (B) Optical spectra of DE-2 (200 μl) in the presence of complex C1/H₂O₂ under neutral aqueous conditions (black, red, blue, and green traces for DE-2 before treatment, and after 30 min, 60 min, and 90 min, respectively).

The corresponding mass spectral data of the dye sample pre- and post-C1/H₂O₂ treatment also supported the loss of the precursor Rh-6G molecule during this process (Fig. S10 and S11†).

Next, the degradation process triggered by this unique two-stage dye treatment process was investigated on an array of eleven other commercially available dye molecules, covering a wide variety of popular organic pigment templates, including azo, xanthene, and carmine (Fig. 2).²⁷ The chemical oxidation driven by the C1/H₂O₂ blend continued to favorably disintegrate all the dyes in comparison to CuCl₂/H₂O₂ or H₂O₂ (Fig. S12–S22†). Interestingly, the azo dyes (1, 5, 9, 10, 11 in Fig. 2) displayed a swift degradation pattern, while the xanthene-based pigments (2, 3, 12 in Fig. 2) followed relatively slow kinetics (Table S1†). The successful and almost complete removal of dye molecules by the copper complex and H₂O₂ mix showcased the potency of this chemical dye treatment process (Fig. 4). Herein, complex C1 settled at the bottom of the container at the end of the chemical process, which was collected and recycled for subsequent treatment of the dyes.

Fig. 4 The percentage degradation of all dyes and industrial effluents has been calculated based on UV spectra after treatment with C1/H₂O₂ for (1) AB-10B, (2) BBG, (3) BG, (4) CR, (5) DB-71, (6) FS, (7) IC, (8) MBH, (9) MO, (10) MR, (11) RB-5, (12) Rh-6G, (13) DE-1, (14) DE-2, (15) DE-3, and (16) DE-4 (details of the dyes are given in Fig. 2).

Subsequently, the efficiency of this newly designed process was investigated on a series of four different industrial dye effluents (DE 1–4) that were collected from various sources as follows: home textile effluents (DE-1), denim industry effluents (DE-2), creation industry effluents (DE-3), and textile industry effluents (DE-4). All these effluents contain a mixture of different types of proprietary organic dye. Therefore, the complete treatment of such as-received industrial dye effluents remains a challenging task and they have rarely been processed properly with existing chemical methods.²⁸ However, the currently described two-stage dye treatment method was able to decompose even these industrial dye solutions (DE 1–4) completely without any prior dilution. The use of C1/H₂O₂ (2 : 5) is vital for this remarkable as-received industrial dye treatment that is achieved in a short timespan (20 min–2 hours) under ambient conditions (Table S1†). The steady disappearance of the optical bands of the dye solutions (DE 1–4) was followed to monitor the industrial sample disintegration process (Fig. 3B). A minimal drop in the optical absorbance data during the respective control experiments without C1 again confirmed the significance of the bio-inspired copper complex in this process (Fig. S23–S26†).

Next, we investigated the possible product profile of these chemically oxidized dye solutions. During the initial optical experiments, strong effervescence was spotted in the pigmented aqueous solution during the treatment with C1/H₂O₂. To explore this further, the dye degradation experiment was performed in a gas-tight container for 8 continuous hours, and the components of the head-space were probed using a gas chromatography (GC) instrument at regular intervals. As depicted in Fig. 5A, the addition of C1/H₂O₂ to Rh-6G dye solution produced CO₂ as the major product with minimal CO generation. Again, the addition of CuCl₂/H₂O₂ alone or H₂O₂ alone showcased a significantly lower amount of CO₂ that matched their inferior dye degradation property observed earlier. This trend of substantial CO₂ evolution as the chief product during the chemical oxidation process, triggered by C1/H₂O₂, remained the same with other commercial (Fig. S27–S38†) and as-received industrial dyes (Fig. 5B and S39–S42†).

Fig. 5 The variation of head-space CO₂ and CO gas observed during continued incubation of (A) Rh-6G and (B) industrial dye sample DE-2 in the presence of various oxidizing agents: H₂O₂ (CO₂: red trace; CO: black trace); CuCl₂/H₂O₂ (CO₂: green trace; CO: blue trace); C1/H₂O₂ (CO₂: brown trace; CO: violet trace).

The production of CO₂ as the product reflects the complete oxidation of the organic dye molecules during the chemical oxidation process. The C1/H₂O₂ mixture ensures almost complete oxidation of the dye molecules as measured from the decaying optical spectral data (Fig. 3 and S12–S26†). The conglomeration of the adsorption and chemical oxidation strategies resulted in a stark improvement in water quality in terms of total dissolved solids (TDS), chemical oxygen demand (COD), and pH. The adsorption process primarily elicits TDS descent as it actively removes the insoluble and partially soluble dye ingredients. The loss of the reduced organic molecules in the oxidation process was also corroborated by measuring the COD. A sharp drop in the COD (8000–20 000 ppm to <200 ppm) was observed across all the dye solutions with the C1/H₂O₂ treatment (Fig. S43†). This data indicated that the organic dye molecules are converted into CO₂ during the chemical oxidation process. The evolution of acidic CO₂ gas also buffered the reaction mixture with the concurrent generation of carbonate and bicarbonate ions. The CO₂-inflicted pH adjustment was especially evident for the industrial dyes (DE 1–4), where the pH of the initial basic dye solutions (pH >10) shifted towards neutrality (pH ∼8.3–8.5) (Table S2†).

The optical and GC experiments have unequivocally established that C1 is a vital component for exhibiting the chemical oxidation process in the presence of H₂O₂. The potent dye degradation by the C1/H₂O₂ mixture compared to CuCl₂/H₂O₂ suggests that a strong oxidizing agent is generated from C1. The optical absorbance of C1 was monitored following the addition of H₂O₂ to capture any possible oxidizing intermediates. This experiment interestingly resulted in the emergence of a unique optical peak ∼985 nm (Fig. S44†). Such a signature in the NIR region typically signals the formation of a highly oxidizing Cu(III)-oxo species that was presumably favored due to the presence of a flexible ligand scaffold in C1 (Scheme 1).^24,29 The formation of the Cu(III)-hydroxo intermediate was also corroborated from the comparative optical spectra recorded in the visible region before and after the dye treatment studies (Fig. S45†). Next, X-ray photoelectron spectroscopy (XPS) was performed to further corroborate the formation of the Cu(III)-hydroxo species. Here, the Cu 2p XPS data clearly indicated the emergence of an oxidized copper species following the reaction. In addition, a new signature is observed in the O1s XPS region, which was assigned to the hydroxide species linked to oxidized copper species (Fig. S46†).

Scheme 1 The possible catalytic mechanism for oxidative degradation of dye molecules by C1.

The formation of such a Cu(III)-oxo molecule occurs via a peroxo-bridged di-copper intermediate [Cu(II)–μ-O₂²⁻–Cu(II)], which was supported by theoretical calculations.²⁴ This Cu(III)-hydroxo species can drive the chemical oxidation of organic dyes that will primarily follow a non-Fenton pathway. A control experiment of dye degradation was performed in the presence of isopropyl alcohol (IPA) to further corroborate the proposed pathway. A typical Fenton's pathway progresses with the generation of hydroxyl radicals (OH˙), which can be scavenged by IPA. Hence, the presence of IPA in the reaction would limit OH˙ formation to display a plunge in dye degradation activity, which was actually the scenario when the experiment was performed with CuCl₂/H₂O₂. However, the dye degradation activity remained almost independent of IPA in an analogous experiment with C1/H₂O₂ (Fig. S47†). Here, the progress of dye depletion was followed by CO₂ generation via GC. Here, it is worth mentioning that H₂O₂ is relatively unstable in alkaline conditions compared to neutral and acidic conditions.³⁰

Conclusion

Dye industries are a major consumer of water where they generate a copious amount of wastewater that typically flows through the surrounding water bodies. Such a release adversely affects the biosphere and, hence, the pursuit for an efficient, eco-friendly, and economical wastewater treatment method remains active. Here, in this work, we have developed a two-stage organic dye treatment strategy involving initial adsorption followed by a unique chemical method. The chemical process is spearheaded by a copper-based molecular complex C1 that is designed based on the blueprint of the strongly oxidizing cytochrome P450 enzyme active site. An appropriate mixture of C1 and H₂O₂ originates a highly oxidizing Cu(III)-hydroxo intermediate that rapidly depletes an array of organic dye frameworks while producing CO₂ as the major product. Here, the C1/H₂O₂ blend follows a non-radical oxidizing pathway that restricts the generation of toxic side products during dye degradation. This process was equally successful in treating as-received industrial dye variants as this process significantly improved the water quality parameters (COD, TDS, pH, and transparency). The initial studies were performed on a small scale, which was later expanded to a full pilot-scale mode (at 100.0 liter volume of industrial dye) with analogous efficiency (Video S1†). Here only 400.0 mg of C1 was required in addition to 300.0 mL of H₂O₂, 100.0 g of CaO, and 100.0 g of FeSO₄. The majority of C1 employed in the chemical step was recoverable and was recycled in subsequent chemical oxidation. Hence, this physico-chemical process was adaptable for cleansing large-scale industry-grade dye wastewater, generating reusable, clean water while minimizing the environmental impact. This process does not require any expensive auxiliary processes (such as UV irradiation) to display remarkable dye degradation properties, and it can be deployed as a template method in divergent dye industries.

Author contributions

Conceptualization: AA, AD. Methodology: AA, BVM, CG, AD. Investigation: AA, BVM, NAS, CG, AD. Visualization: AA, AD. Funding acquisition: CG, AD. Project administration: AD. Supervision: CG, AD. Writing – original draft: AA, AD. Writing – review & editing: AA, BVM, NAS, TK, CG, AD.

Conflicts of interest

AA, CG, and AD have filed a patent application together lodged with IIT Gandhinagar (Indian Provisional Patent Application No: 201921045779) based on this work.

Acknowledgements

The authors would like to acknowledge the support from DST, India (DST/TMD(EWO)/OWUIS-2018/RS-05(G)) for this research activity. The authors would also like to thank IIT Gandhinagar and IIT Bombay for the research facilities.

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental methods, figures, and tables. See DOI: https://doi.org/10.1039/d3ew00185g

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