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Article

Efficient Photodegradation of Dyes from Single and Binary Aqueous Solutions Using Copper(II) Coordination Polymers

by
Ildiko Buta
,
Maria Andreea Nistor
and
Simona Gabriela Muntean
*
“Coriolan Drăgulescu” Institute of Chemistry, Romanian Academy, 24 Mihai Viteazu Bvd., 300223 Timisoara, Romania
*
Author to whom correspondence should be addressed.
Molecules 2025, 30(8), 1652; https://doi.org/10.3390/molecules30081652
Submission received: 13 March 2025 / Revised: 30 March 2025 / Accepted: 5 April 2025 / Published: 8 April 2025
(This article belongs to the Special Issue Study on Synthesis and Photochemistry of Dyes)
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Abstract

:
The present study reports the application of three copper(II) coordination polymers, namely 1∞[Cu3L2(N3)] CH3COO (CP1), 1∞[Cu3L2(NO3)]NO3·2CH3OH·2H2O (CP2), and 1∞[Cu3L2(H2O)](ClO4)2 (CP3), where H2L stands for N,N′-bis[(2-hydroxybenzilideneamino)propyl]-piperazine) as catalysts for photocatalytic degradation of Acid Orange 7 and Methyl Orange dyes from single and binary aqueous solutions. The influence of the photocatalyst nature, hydrogen peroxide presence, reaction time, dye concentration, and catalyst dose on the photodegradation efficiency was studied. Under visible light irradiation, complex CP1 demonstrated the highest photodegradation efficiency of 92.40% and 80.50% towards Acid Orange 7 and Methyl Orange, respectively. The kinetic studies indicated that the photodegradation process followed a pseudo-first-order kinetics. The highest rate of the degradation process was obtained when CP1 is used, and the necessary time for the degradation of the dyes increases with increasing concentration of the dye solutions. The degradation efficiency of more than 75% after five recycling/reuse cycles of CP1 and the yields higher than 72% obtained for the degradation of dyes from the binary system demonstrate the photocatalytic capacity of CP1. A photocatalytic oxidation mechanism was proposed and the stability of the CP1 complex before and after the photodegradation process of dyes, both from simple and binary solutions, was investigated and confirmed.

Graphical Abstract

1. Introduction

The rapid expansion of cities, consumerism, and the development of the textile industry in recent decades has caused serious environmental pollution. One of the most serious problems has become water pollution, which causes long-term damage to the environment [1]. Dyes are organic compounds widely used especially in the textile and food industries. The discharge of colored textile wastewater into collecting canals or even into rivers and streams has extended and become the main source of pollution worldwide [2]. The dye molecules do not degrade as colored wastewater is discharged into rivers and then into seas and oceans [3,4,5]. This causes the gradual accumulation of dyes in the water and inhibits sunlight from penetrating the surface of water. This fact affects the ability of plants to carry out photosynthesis, determining the reduction of the oxygen content in the water and thus causing the death of aquatic flora and fauna. For all these reasons, the treatment of colored wastewater is of great importance and interest both for water purification and for its reuse [6]. In the last decades, various methods of treating water contaminated with azo dyes have been investigated, including physical, chemical, biological techniques, and others [6,7,8].
Photodegradation of water pollutants represents advanced oxidation processes that include ultraviolet or visible light irradiation, successfully applied for the degradation of chemical substances; e.g., dyes from industrial effluents [9,10,11]. In the last years, the design and synthesis of new materials with good photocatalytic properties and visible light responsiveness have been highly demanded and developed. To address this challenging issue, many studies were carried out to improve the properties of existing photocatalytic materials or to develop new potential materials such as metal oxide nanocomposites [12], multi-component oxides [13] polymeric nanostructures [14], modified zeolitic imidazolate frameworks nanoparticles [15], geopolymers [16], etc. Colored industrial wastewater usually contains mixtures of dyes. In the last decade, studies have focused on the removal of dyes from multicomponent solutions by adsorption [17,18,19]. Based on data from the literature, we found that the potential application of a catalyst in a multipollutant system has been studied in few cases. Recently, different materials have been applied for the photocatalytic degradation of dyes from binary systems with some interesting results. For instance, Zhang and coworkers [20] reported the fabrication of iron-doped manganese oxide nanoparticles for the degradation of Indigo Carmine and Rhodamine B from a binary mixture under solar light irradiation. Ayodhya [21] reported the degradation of Fluorescein/Rhodamine B from a binary dyes system, in the presence of Ag-CuO composites, under visible light irradiation. Verma and coworkers [22] investigated the kinetics of photocatalytic degradation of single and binary dye mixtures regarding concentration, pH, and extent mineralization to identify the best working conditions for simultaneous degradation. Usman’s group reported selective photocatalytic degradation of Methylene Blue over Rhodamine B in the presence of an anatase TiO2 catalyst, under UV light irradiation [23]. However, to the best of our knowledge, no study on the photocatalytic degradation of dyes in multicomponent systems using coordination polymers is reported so far.
Coordination polymers (CP) are a class of compounds with extended structures obtained from the self-assembly of metal ions and organic bridging ligands [24,25,26]. CPs has aroused great interest due to their properties suitable for potential applications in fields like magnetism [27], luminescence [28], gas adsorption [29], chemical sensors [30], and lately for the promising results in sustainable polluted wastewater treatment [31,32,33,34]. These properties are strongly related to the topology of the CP’s structures that can be determined by an appropriate choice of the metal ion, the preorganization capacity of the ligand, and the nature of the counterions [25,26]. Several desirable properties, including semiconducting nature [35], visible light responsiveness [36], and tunable surface area [37], make CPs a promising photocatalyst. In the literature, there are several papers reporting the heterogeneous photodegradation of dyes using copper(II)-based coordination polymers [38,39,40], and by applying UV-Vis spectrometry as a viable method for studying photocatalysis over time [41,42]. Therefore, as a continuation of our work [43], in the present study, the photocatalytic properties of three CuII coordination polymers, namely 1∞[Cu3L2(N3)]CH3COO (CP1), 1∞[Cu3L2(NO3)]NO3·2CH3OH·2H2O (CP2), and 1∞[Cu3L2(H2O)](ClO4)2 (CP3), where H2L stands for N,N′-bis[(2-hydroxybenzilideneamino)propyl]-piperazine), have been reported. In the present study, the degradation capacity of CP1–CP3 was evaluated for the removal of two azo dyes from single and binary systems. Moreover, the factors that influence the degradation process were analyzed and the kinetic parameters of the process were determined.

2. Results and Discussion

2.1. Characterization of CP1–CP3

The detailed information about the synthesis and structural characterization of CP1–CP3 are presented in the previous paper [43]. Briefly, the CP1–CP3 complexes were obtained by reacting appropriate copper(II) salts with H2L in a 2:1 stoichiometric ratio, according to Figure 1. The crystallographic investigation revealed that CP1–CP3 consist of trinuclear complex entities, {Cu3L2}2+, connected via azido (CP1), nitrato (CP2), and phenoxido (CP3) bridges.

2.1.1. Thermal Properties of H2L and CP2

Due to the fact that the perchlorate and azide metal complexes are potentially explosive, the thermal properties were investigated only for H2L ligand and CP2 [44]. Thermogravimetric analysis measurements performed on both H2L ligand and CP2 in nitrogen/air conditions shows a relatively good thermal stability, with no decomposition processes below 200 °C (Figure 2). The decomposition of the H2L occurs in three stages. The first stage is found in the range of 25–350 °C, with the mass loss of 48.64% (calcd. 48.04%) corresponding to piperazine, two propyl chains and two nitrogen atoms. The second and third stages are observed in the range of 350–500 °C and 500–1000 °C and are associated with the weight losses of 29.79% (calcd. 29.42%) and 7.29% (calcd. 7.83%), respectively corresponding to the loss of aromatic fragments. For complete oxidation of the organic residues, the system was supplied with air and a final mass loss of 14.28% (calcd. 14.71%) was observed.
The thermogram of CP2 shows two major stages of decomposition and a dehydration process in the range of 30–80 °C of 2.93% (calcd. 3.02%) corresponding to the loss of two water molecules present in the outside coordination sphere. The first stage of 42.94% starts at 230 °C and continues up to 430 °C and the second one of 37.06% starts after 450 °C and completes at 800 °C. By supplying the system with air, complete oxidation of organic residues occurs and the resulting product coincides for three copper and three oxygen atoms (found 20.00%; calc. 19.55%).

2.1.2. Band Gap Analysis

The band gap of a semiconductor photocatalyst is an important parameter for determining its degradation capacity and refers to the energy difference between the highest valence band and the lowest conduction band [45]. In general, a decrease in the band gap increases the photocatalytic activity.
The band gaps for CP1–CP3 were calculated from UV-Vis absorption spectrum, using Tauc plots and Tauc’s Equation (1) [33]:
α h υ n = k h υ − E g
where α is the optical absorption coefficient (2.303A/t); A—absorbance; t—thickness of the material; hν—1240/wavelength; Eg—bandgap energy; and n is 2 for direct transition.
Representing the dependence of (αhν)2 as a function of the photon energy (hν) and extrapolating the linear portion of the curves to the zero absorption coefficient value, the values of the energy bands for the investigated materials were obtained. The Eg values were 3.09 eV for CP1, 3.15 eV for CP2, and 3.12 eV for CP3, comparable with other CPs, indicating that the CPs might be used as catalysts. The Eg value obtained for H2L ligand was 3.68 eV.

2.2. Photocatalytic Studies

The catalytic activities of the synthesized complexes (CP1–CP3), toward photodegradation of industrial dyes were explored. The influence of different process parameters such as nature and quantity of the catalyst and initial dye concentration on photodegradation efficiency were investigated.

2.2.1. Influence of the Nature of the Photocatalyst

To study the photodegradation of AO7 and MO dyes, we first investigated the influence of the nature of the material applied as a photocatalyst. Coordination polymers CP1–CP3 (1 g/L) were introduced into aqueous dye solutions (30 mg/L), photodegradation studies being performed at room temperature, solution pH, under visible light irradiation, for 6 h. In order to evaluate the photocatalytic capacities of the investigated coordination polymers, two additional control studies were performed working under the same conditions (irradiation with visible light, pH of the solution, room temperature); first, in the absence of CPs and second in the presence of the ligand (H2L). The obtained results are presented in Figure 3.
The results (Figure 3) showed that the degradation of AO7 and MO solutions under visible light irradiation in the absence of a photocatalyst (CPs) was not detected (R < 2%).
The ligand (H2L) exhibited 10.42 and 9.82% removal of AO7 and MO in 200 min, respectively, indicating that the catalyst’s properties of H2L were insufficient for the degradation of investigated dyes. At the same time, a significant degradation of the investigated dyes occurred in the simultaneous presence of CP1–CP3 and visible light.
After 150 min under irradiation at 546 nm, the degradation efficiency of AO7 dye was higher than that for MO degradation (Figure 3a). The degradation efficiencies of AO7 reached 92.40% for CP1, 71.80% for CP2, and 89. 30% for CP3. For the case of MO, the obtained values were 80.50% for CP1, 47.00% for CP2, and 57.45% for CP3. As can be seen the catalytic efficiency increased in the order CP2 < CP3 < CP1, consistent with the values obtained for the band gap energy (Eg), indicating that CP1 had the highest photocatalytic activity.
The use of H2O2 for the degradation of colored pollutants is useful as an auxiliary oxidant, due to the formation of additional highly reactive hydroxyl radicals [46]. Thus, 30 µL hydrogen peroxide 30% was added to the system as an oxidizing agent and the experiments were performed under the same conditions (pH, temperature, quantities). The obtained results under visible light irradiation with (Figure 4b) and without (Figure 4a) H2O2 highlights the fact that the removal efficiency of AO7 and MO increased slightly in the presence of hydrogen peroxide.
The removal efficiency of AO7 increased with the addition of H2O2 from 92.40% to 93.81% for CP1, from 71.80% to 76.60% for CP2, and from 89.03% to 91.10% for CP3 (Figure 4b). For the case of MO, the presence of hydrogen peroxide generated an increase in the degradation efficiency from 80.50% to 84.82%, from 47.00% to 51.70% and from 57.45% to 71.43% in the case of using CP1, CP2, and CP3, respectively. The enhancement of photodegradation efficiency may be attributed to the presence of a larger quantity of hydroxyl radicals from the photolysis of H2O2 that can react with the dye molecules.
As seen in Table 1, compared to the dye photodegradation activity of other recently reported coordination polymers (Table 1), the investigated complexes show similar or even higher photodegradation efficiency.
Overall, taking into account that the addition of hydrogen peroxide into the system slightly increased dye degradation by 6.70% for AO7 and 24.30% for MO along with the additional costs and possible pollution involved [51], we considered that it was more feasible to continue further studies in the absence of H2O2.

2.2.2. Evolution in Time of the Photodegradation Process

The degradation process in time was followed by measuring the intensity of the characteristic absorption band at 483 nm for AO7 and at 463 nm for MO, respectively. The changes over time, in the absorption spectra of the AO7 and MO dyes during the photocatalytic degradation process, under visible light irradiation using CPs as catalysts, are presented in Figure S1 Supplementary File. In the presence of the complexes, the intensity of the absorption peak of AO7 and MO decreased significantly in 180 min revealing that the photocatalytic reaction progressed. The change of color over time is also exemplified in Figure S2 Supplementary File. The clear solutions obtained in the case of using CP1 should be noted, this can be attributed to the greater photocatalytic degradation capacity of this compound.
As shown in Figure 5, during the first 30 min, adsorption occurs on the catalyst surface. Previous studies related to the dyes photodegradation have shown that a time of 30 min is suitable to reach equilibrium in the adsorption process [52]. After the photocatalytic reaction, in the presence of CPs, the concentration of dye solutions decreases significantly in 100–150 min, indicating the efficiency of photodegradation of AO7 and MO dyes from aqueous solutions.
In the presence of CP1–CP3, the intensity of the absorption peak of AO7 and MO decreased significantly in the time interval of 100–150 min revealing that the photocatalytic reaction progressed. As the photocatalytic degradation occurs, the concentration of the dye solutions slowly decreases, indicating a possible surface coverage of the catalyst in time which can hinder the exposure to light irradiation and the formation of radicals for further oxidation of the pollutants [53]. Control experiments (without CP or in the presence of ligand) showed no degradation of the dyes over time.

2.2.3. Effect of the Catalyst Dosage

Since the best results were obtained using CP1 as a catalyst, further studies were carried out using this compound. In order to highlight the efficiency of CP1 as a catalyst, the influence of the catalyst dosage on the dyes’ degradation efficiency was further investigated. For this study, amounts of 0.5, 1, 2, and 3 g/L were used, keeping all other operating conditions constant. Figure 6a illustrates the degradation efficiency of AO7 and MO dyes in the presence of different amounts of coordination polymer CP1. As was expected, the dye degradation efficiency increased with increasing CP1 dosage. The efficiency increased from 70.54% to 92.40% for AO7 and from 54.54% to 80.50% for MO, with the increase of CP1 dosage from 0.5 to 1 g/L, possibly due to the increase in the number of active sites on the catalyst surface available for dye molecules.
The use of amounts greater than 1 g/L of CP1 causes an insignificant increase in the degradation yield: ~2% for AO7 and 4.5% for MO, respectively. It can be assumed that an excess of CP1 beyond the optimum amount may induce a shading effect by lowering the light transparency of the solution and reducing the amount of sunlight that reaches the active sites of the catalyst [54]. Therefore, the optimal dose of CP1 catalyst selected for further studies was 1 g/L.

2.2.4. Effect of Initial Concentration of Dye Solutions: Kinetic Studies

Photodegradation is known to be influenced by the concentration of organic dye in a solution, which is closely related to dye aggregation [55]. As the dye concentration increases, the dye molecules form aggregates. The higher the concentration of the dye solution, the larger the size of the dye aggregates [56,57]. Aggregates are adsorbed on the substrate surface making it difficult to access the catalyst surface, leading to longer photodegradation times and yields.
Different concentrations were selected: 15, 30, 45, and 60 mg/L to investigate the influence of the initial dye concentration on the photodegradation efficiency, maintaining constants of temperature and solution pH. Figure 6b,c illustrate the photocatalytic degradation of the investigated dyes for different values of the initial concentration. As can be seen in Figure 6b,c and Table 2, the degradation efficiency increased slightly with the increase in the dye concentration from 15 to 30 mg/L but decreased significantly for a further increase in the concentration of the dye solutions.
This behavior is in agreement with reported studies for different types of catalysts and organic dyes [55] and indicates that, for a certain amount of catalyst, with increasing dye concentration the number of active sites available for photocatalysis decreases [58,59]. As the concentration increases, more dye molecules are adsorbed on the catalyst surface. Moreover, the large number of the adsorbed dye molecules hinders the absorption of visible light and, consequently, prevents the initiation of the photo-oxidation process [55].
Degradation yields of 92.40% (AO7) and 80.50% (MO) were obtained working under optimal conditions for 30 mg/L dyes solutions, this being considered as the optimal concentration for both investigated dyes. Additionally, the obtained results highlighted the fact that solutions with low dye concentrations (15 mg/L) can be discolored faster than the most concentrated ones (60 mg/L), the time required for degradation increasing with the increase in the initial concentration of the dye solutions (Table 2).
Kinetic studies on the photocatalytic degradation of AO7 and MO dyes over time were carried out using 10 mg of CPs catalyst in 10 mL of dye solutions. Kinetic studies were also performed using CP1 as a catalyst, for different dye concentrations of 15, 30, 45, and 60 mg/L. The kinetic of dye photodegradation was investigated using the Langmuir–Hinshelwood model, Equation (2) [60].
l n C C 0 = − k a p p t
where C0 is the initial dye concentration (mg/L), C is dye concentration at time t (mg/L), t is the irradiation time (min), and kapp is the first-order rate constant (min−1).
From the graphical representation of the experimental data as ln(C/C0) versus time (Figure S3 Supplementary File), the rate constant (k) was determined by linear fitting based on regression analysis, and the obtained data are presented in Table 3. For each dye and each compound included in this investigation, the obtained R2 and SD values are listed.
The obtained results showed that the degradation process of AO7 and MO using CP1, CP2, and CP3 followed a pseudo-first-order kinetic. The rate constants obtained indicated that the speed of degradation process increases in the order CP2 < CP3 < CP1.
The theoretical data obtained for the first-order rate constants (k) (Table 3) show that, with the increase in dye concentration, the necessary time for the photodegradation of investigated dyes increases, which confirms the results obtained experimentally (Table 2). With the increase in dye concentration, the values obtained for k decrease, indicating lower degradation process rates.

2.2.5. Recycling of Photocatalyst

To evaluate the stability of the CP1 catalyst during the dye degradation process and its reuse, five photocatalytic degradation cycles of AO7 and MO were performed. After each cycle, the catalyst was washed with ethanol, dried, and then reused.
As the number of cycles increases, the efficiency of the photocatalytic degradation decreases, due to the occupation of the active centers on the catalyst surface by the adsorption of dye molecules (Figure 7a). Qualitative estimation of dye degradation can be emphasized by the decoloration of the dye solutions highlighted in Figure S4 in the Supplementary File.
From the first to the third reutilization, a reduction in the photodegradation efficiency of 18.70% in the case of AO7 and 13.60% for MO (Figure 7a) was observed. From the third to the fifth cycle, the loss in efficiency was 13.95% for AO7 and 16.30% for MO, respectively. After five simultaneous adsorption–photocatalytic cycles, the capacity of CP1 was still considerable. The decrease in degradation yield after five reuses was about 30% for AO7 and 27% for MO, and the average degradation efficiency for the five cycles was 76.76% for AO7 and 67.53% for MO, respectively, demonstrating the reusability of CP1.

2.2.6. Photodegradation of Binary Dye Mixture

To highlight the efficiency of a catalyst, it is important to examine its performance in the decomposition of dyes in mixtures, which simulate a real textile effluent [46,61].
Based on our knowledge and the literature survey carried out, only a few researchers have investigated the potential of a dye degradation catalyst in a multicomponent system. Moreover, until now, there is no mention in the specialized literature related to the application of coordination polymers for the degradation of dyes from binary systems. Therefore, for future potential industrial applications, in addition to the regeneration and reuse studies of CP1–CP3, the degradation studies of dyes from multicomponent mixtures are very important. For these studies, the photocatalytic degradation of AO7 and MO dyes from binary solutions was evaluated, working under visible light irradiation, at a speed of 300 rpm, 25 °C, solution pH, using an amount of 10 mg of CP1–CP3, for a volume of 10 mL dye solution. The experiments were performed for a dye concentration of 30 mg/L, in a volume ratio of 1:1 with, and without the addition of 30 µL H2O2.
UV-Vis spectrophotometry was used to follow the decrease in absorbance over time (Figure S5 Supplementary File), and implicitly the discoloration of the dye solutions due to the degradation of the dyes. After 240 min, the absorbance decreased due to discoloration of the dye solution under visible light, using CP1.
The degradation of the mixture of binary dyes was evaluated based on the degradation efficiency (Equation (1)) calculated at the value of the maximum wavelength of the binary solution (474 nm) as well as at the wavelength of each dye: AO7 (483 nm) and MO (463 nm). For the binary solution, lower degradation efficiencies were obtained compared to the single solutions (Figure 7b, Table 4) for all tested compounds (CP1–CP3). This is due to the fact that the two azo dyes in the binary solution compete for the active centers on the CP1–CP3 surface, available for adsorption and degradation.
The degradation of the dyes in the binary system followed the same trends as in the single solutions. The highest degradation efficiency of 73.83% and 83.54% (H2O2 added), respectively, was obtained using CP1. It is notable that, also in the case of using CP3 as a catalyst, degradation efficiencies higher than 53% (H2O2 added) were obtained for the mixture of dyes from the binary system.
If we discuss from the point of view of the individual dyes in the binary system, yields higher than 72% were obtained for both dyes using CP1. When H2O2 was added, similar to single dye solutions, the degradation efficiency of both dyes in the binary system increased to over 81% using CP1 and over 50% using CP3, respectively (Figure 6b).
Overall, CP1 showed the highest photocatalytic degradation of both single dyes and the mixture of dyes in the binary system, compared to CP2 and CP3.
The obtained results are promising and encouraging, indicating that CP1 is a viable catalyst for subsequent application on colored industrial wastewater.

2.3. Material Stability

The stability of a photocatalyst is crucial for the practical applications in environment remediation. Therefore, in order to estimate the stability of the studied material, the IR spectra of CP1 before and after photocatalytic reactions were recorded (Figure 8).
The IR spectrum of the solid residue of CP1 isolated from the reaction mixture via centrifugation and dried in air exhibits a very strong absorption band situated at 2034 cm−1 characteristic to the N3− anion and a sharp signal at 1624 cm−1 assigned to the ν(C=N) stretching vibrations. The presence of vibrational bands at 1539 and 1401 cm−1 is typical of antisymmetric and symmetric ν(COO−) stretching frequencies, respectively, while the difference ∆ν of 138 indicates the uncoordinated mode of the carboxylate group [62]. The band at 1328 cm−1 is assigned to the νaryl-O vibration. The medium band observed at 759 cm−1 corresponds to the C-H out-of-plane bending vibration of the substituted phenyl ring. The IR spectra of the CP1 after photodegradation of AO7 and MO from a single and binary system display the same pattern with the original one, demonstrating the stability of the complex in the photocatalytic process.
The confirmation of CP1’s stability along with the good results from the presented recycle studies (see Section 2.2.5) recommend CP1 as an efficient catalyst for the degradation of dyes from aqueous solutions.

2.4. Photocatalytic Mechanism

The possible mechanism for the photocatalytic process is based on the semiconductor’s capacity to generate electron–hole pairs able to promote oxidation and reduction reactions followed by organic dyes degradation [63]. Electron–hole pairs are generated upon visible light irradiation; thus, (i) electrons from the valence band are excited to the conduction band leaving holes (h+) with oxidizing character capable of generating hydroxyl radicals (•OH); (ii) in the conduction band, photoinduced electrons (e−) with reducing character are formed, capable of generating superoxide radical anions (•O2−) (Figure 9) [64,65,66].
In order to confirm the assumed radical-mediated path, a trapping experiment with disodium ethylenediaminetetraacetate (EDTA-2Na) as holes h+ scavenger was performed. Upon addition of 10 mg EDTA in the photocatalytic reaction, the degradation rate of AO7 and MO in the presence of CP1–CP3 decreased to 2–16%, suggesting only a very small quantity of h+ in the system (Figure S6, Supplementary File). There are two possibilities for photodegradation inhibition: directly, by blocking the attack of the holes on the pollutant and indirectly by blocking the formation of •OH radicals which oxidize the pollutant. However, the experiment shows that in this case, h+ and •OH radicals are the main active species in the AO7 and MO photodegradation. Based on these results, a possible mechanism that follows the photocatalytic oxidation pathway can be proposed [67,68,69,70]. The reactive oxygen species generated could oxidize the AO7 and MO dyes. The possible degradation mechanism consists of several steps involving cleavage of the azo bond (–N=N–) generating β-naphthol, N,N-dimethyl-p-phenylenediamine, and sulfanilic acid [71,72], followed by further degradation of the intermediates to carbon dioxide (CO2), water (H2O), and other small organic molecules.

3. Materials and Methods

3.1. Synthesis of Coordination Polymers

Different solutions of H2L (1.22 mmol) in CHCl3/CH3OH (1:1 v/v, 60 mL) were reacted with Cu(NO3)2·3H2O (2.44 mmol) or Cu(ClO4)2∙6H2O (2.44 mmol) or Cu(CH3COO)2∙H2O (2.44 mmol) and NaN3 (2.44 mmol) dissolved in CH3OH (40 mL) and Et3N (3.7 mmol). Dark green single crystals suitable for X-ray analysis were obtained by slow evaporation of the mother liquor [43]. Elemental analysis calcd. for CP1: C50H63Cu3N11O6 (1104.73) C, 54.36; H, 5.75; N, 13.95%. Found: C, 54.72; H, 5.79; N, 13.76%; CP2: C50H72Cu3N10O14 (1227.79) calcd.: C, 48.91; H, 5.91; N, 11.41%. Found: C, 48.49 H, 5.78 N, 11.95%, CP3: C48H62Cl2Cu3N8O13 (1220.57): C, 47.23; H, 5.12; N, 9.18%. Found: C, 47.81, H, 5.23; N, 8.92%.

3.2. Materials and Physical Measurements

All reagents were used as purchased from commercial suppliers, without further purification. N,N′-bis[(2-hydroxybenzilideneamino)-propyl]-piperazine (H2L) was synthetized following the procedure described earlier [73]. Caution: Salts of perchlorate and azide and their metal complexes are potentially explosive and should be handled with great care and in small quantities.
In order to identify the synthesized samples, elemental analyses (C, H, N) were performed on an Elementar UNICUBE CHNS analyzer (Elementar Analysensysteme GmbH, Langenselbold, Germany), with helium as the carrier gas. To identify the functional groups of the material before and after photodegradation, Fourier-Transform Infrared (FTIR) spectra were recorded on a Cary 630 FTIR from Agilent Technologies (Santa Clara, CA, USA), using KBr pellets, in the range of 400 to 4000 cm−1. The TG data were obtained using a Discovery TGA 5500 analyzer from TA Instruments (New Castle, DE, USA) under a continuous flow of N2, heating rate of 10 °C/min, starting from 25 °C up to 1000 °C for H2L and from 25 °C up to 800 °C for CP2. After that, isothermal heating under an air flow for 10 min was maintained.
Two anionic azo dyes, Acid Ornge 7 (AO7; C16H11N2NaO4S, C.I. 15510) and Methyl Orange (MO; C14H14N3NaO3S), were selected as potential colored pollutants. Acid Orange 7 is widely used in the textile industry for dyeing wool, silk, and polyamide fibers in the leather and paper-processing industry. It can also be used both as an indicator and for biological dyeing. Methyl Orange is one of the most widely used pH indicators and titration indicators in the pharmaceutical industry and in analytical chemistry. The wide use of these dyes does not mean that they are not toxic or potentially hazardous compounds. Azo dyes contain aromatic groups (benzene rings) and azo groups in their molecules, which are toxic, carcinogenic, and teratogenic with a negative impact on the environment [74,75]. In addition, the presence of these dyes in wastewater discharged into sewers or rivers causes their coloring and therefore the deterioration of water quality [76]. The structures, the characteristics, and the toxicity of the selected dyes are presented in Table 5.

3.3. Photocatalytic Studies

The photocatalytic performances of the complexes CP1–CP3 were investigated for the degradation of two organic dyes at room temperature, in a UV chamber with a 500 W Hg lamp providing 546 nm irradiation. Dye solutions of different concentrations, 15, 30, 45, 60 mg/L, were prepared by dilutions from a stock dye solution with a concentration of 200 mg/L. Experiments were performed at the natural pH of the dye solutions: 6.8 for AO7, and 6.4 for MO. The photocatalyst was dispersed in an aqueous solution of dye and stirred at 200 rpm in the dark for 30 min, to reach the adsorption–desorption equilibrium. Then, the mixture was irradiated using visible light under continuous magnetic stirring, at room temperature, in order to accomplish the dye photodegradation. At different time intervals, samples of solution were taken, and the catalyst particles were separated by centrifugation. Using a UV-vis spectrophotometer (ABLE-JASCO, Cluj, Romania), the absorbance at the maximum wavelength was measured: 483 nm for AO7, 463 nm for MO, and the concentration of the investigated dye in solution at different time periods was determined. Using the obtained data, the degradation efficiency was calculated with Equation (3):
R = C 0 − C C 0 ⋅ 100
where C0 and C represents the concentration of dye at the beginning, and after photodegradation (mg/L).
The control experiments were carried out under the same working conditions (natural solution pH, room temperature), but without the addition of the catalyst and also in the presence of the ligand.
The regeneration and reuse experiments of the investigated coordination polymers were carried out over five cycles under similar reaction conditions.

4. Conclusions

Three coordination polymers CP1–CP3 were used in the photocatalytic process of AO7 and MO dyes from single and binary aqueous solutions. The obtained experimental data show that the photocatalytic activity increases in the following order: CP2 < CP3 < CP1, for both investigated dyes, consistent with the CP’s band gaps. In the larger context of dye removal efficiency studies, CP1 stands out. This coordination polymer has an excellent ability to almost completely remove azo dyes from aqueous solution, the degradation efficiency being reduced by increasing the dye concentration and decreasing the amount of CP1 used. Degradation efficiencies of 93.81% and 84.82% were obtained in the presence of H2O2, respectively, and over 90% for AO7 and 80% for MO in the absence of H2O2, indicating the high degradation capacity of CP1 under simple and normal working conditions. The catalytic kinetics were described by a pseudo-second-order kinetic model, and the highest speed of degradation process was obtained for CP1.
An interesting aspect of this study was the application of coordination polymers for the degradation of selected azo dyes from the binary solution (1:1). It is noteworthy that degradation efficiencies higher than 72%, and over 81% in the presence of H2O2, respectively, were obtained for both dyes in the binary solution in 240 min, using CP1.
The recycling of compound CP1 for the degradation of AO7 and MO dyes in five cycles of adsorption/degradation demonstrates its efficient reuse. The yield decrease was approximately 30% for AO7 photodegradation and 27% for MO after five reuses, which once again proves the efficiency of CP1 as a catalyst in the dye degradation process.
In addition, the excellent stability of CP1 after the photodegradation process of dyes from both simple and binary solutions was demonstrated by the consistency of the initial and final structure of the complex.
A photocatalytic oxidation mechanism was proposed, where h+ and •OH radicals are the main active species in the AO7 and MO photodegradation.
All the obtained results indicate that CP1 can be used as an environmentally friendly and efficient photocatalyst for the degradation of dyes and consequently for the depollution of wastewater containing dyes.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/molecules30081652/s1, Figure S1: Time-dependent UV-vis spectra of aqueous solutions of Acid Orange 7 (a) and Methyl Orange (b) upon irradiation with visible light using CP1; Figure S2: Changing the color of the dye solutions over time, during the photodegradation process; Figure S3: Kinetics plots and the fitting curves for the photodegradation of AO7 (a, c) and MO (b, d) using CP1; Figure S4: Initial dye solutions and the dye degradation process using CP1, during five photodegradation cycles; Figure S5: Time-dependent UV-vis spectra of binary dyes mixture with (a) and without (b) adding H2O2; Figure S6: Degradation efficiency of AO7 and MO dyes using CP1–CP3, with and without adding EDTA.

Author Contributions

Conceptualization, S.G.M.; investigation, I.B. and M.A.N.; methodology, I.B. and S.G.M.; formal analysis, I.B., M.A.N. and S.G.M.; writing—original draft, I.B. and S.G.M.; visualization, I.B., M.A.N. and S.G.M.; supervision, S.G.M.; validation, I.B. and S.G.M.; writing—review and editing, S.G.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The authors declare that the data supporting the findings of this study are available within the paper and its Supplementary Information Files. CCDC 2006145 (CP1), 2006144 (CP2), and 2006146 (CP3) contain the supplementary crystallographic data for this paper [43]. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/structures/ (accessed on 1 April 2025) (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)1223-336-033; or deposit@ccdc.cam.uk).

Acknowledgments

This work was implemented within Projects 2.2 and 4.1.2 of the “Coriolan Drăgulescu” Institute of Chemistry, and RO-OPENSCREEN project, MySMIS code: 127952, Contract no. 371/20.07.2020, co-financed by European Regional Development Fund through the Competitiveness Operational Program 2014–2020.

Conflicts of Interest

The authors declare no conflict of interest.

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Molecules 30 01652 g001
Figure 1. Synthesis of the coordination polymers: CP1–CP3.
Figure 1. Synthesis of the coordination polymers: CP1–CP3.
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Figure 2. Thermal decomposition of (a) H2L and (b) CP2.
Figure 2. Thermal decomposition of (a) H2L and (b) CP2.
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Figure 3. Removal efficiency of CP1–CP3 for the photodegradation of (a) Acid Orange 7 and (b) Methyl Orange dyes.
Figure 3. Removal efficiency of CP1–CP3 for the photodegradation of (a) Acid Orange 7 and (b) Methyl Orange dyes.
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Figure 4. Degradation efficiency of investigated dyes using CP1–CP3, with (a) and without (b) adding H2O2; 1 g/L CPs, 30 mg/L initial dye concentration, 27 °C, solution pH.
Figure 4. Degradation efficiency of investigated dyes using CP1–CP3, with (a) and without (b) adding H2O2; 1 g/L CPs, 30 mg/L initial dye concentration, 27 °C, solution pH.
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Figure 5. Photodegradation of (a) Acid Orange 7 and (b) MO dyes in time; under visible light (546 nm), with and without complexes, and in the presence of ligand H2L.
Figure 5. Photodegradation of (a) Acid Orange 7 and (b) MO dyes in time; under visible light (546 nm), with and without complexes, and in the presence of ligand H2L.
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Figure 6. Influence of catalyst quantity (a), initial concentration of Acid Orange 7 (b), and MO (c) dyes on the photodegradation process.
Figure 6. Influence of catalyst quantity (a), initial concentration of Acid Orange 7 (b), and MO (c) dyes on the photodegradation process.
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Figure 7. Degradation efficiency (a) of investigated dyes in five consecutive cycles; (b) of investigated dyes from binary solution using CP1–CP3, without and with adding H2O2.
Figure 7. Degradation efficiency (a) of investigated dyes in five consecutive cycles; (b) of investigated dyes from binary solution using CP1–CP3, without and with adding H2O2.
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Figure 8. IR spectrum of CP1 before and after photodegradation of (a) single AO7 and MO dyes and (b) binary system AO7-MO.
Figure 8. IR spectrum of CP1 before and after photodegradation of (a) single AO7 and MO dyes and (b) binary system AO7-MO.
Molecules 30 01652 g008
Molecules 30 01652 g009
Figure 9. Schematic representation of decomposition mechanism of organic dyes under visible irradiation in the presence of CP1–CP3 and possible intermediate products.
Figure 9. Schematic representation of decomposition mechanism of organic dyes under visible irradiation in the presence of CP1–CP3 and possible intermediate products.
Molecules 30 01652 g009
Table 1. Photocatalytic efficiency (%) of different coordination polymers for removal of dyes under visible light irradiation.
Table 1. Photocatalytic efficiency (%) of different coordination polymers for removal of dyes under visible light irradiation.
DyePhotocatalystR (%)R with H2O2 (%)Ref.
Rhodamine B{[Zn3(L)(4,4′-bpy)]}n (GTU-3)76.50 [31]
Methylene Blue86.20
Acid Red 1752.80
Crystal Violet{[Cu2(Or)2(Bimb)3]·4H2O}n75.80 [32]
MethylViolet76.80
Rhodamine 6G 86.50
Rose Bengal76.10
Methylene Blue17.80
Congo Red40.20
Methyl blue [Cd(bpyp)(nba)2] 29.24 [33]
Methyl orange 35.44
Rhodamine B 95.52
Methyl violet 58.92
Rhodamine B{[Cu(L’)Cl]·2.25H2O}n6.1091.20[47]
Methylene Blue28.3091.90
Methyl orange{[Cd3L2(H2O)5]·H2O}n 59.80[48]
{[Cd3L2(hbmb)-(H2O)2]·2.5H2O}n 47.30
{[Cd3L2(btbb)(H2O)2]·2EtOH·1.5H2O}n 51.40
{[Cd6L4(bipy)2(H2O)6]·3H2O}n 80.00
Rhodamine-B GO@CuO nanocomposite 84.77 [49]
Malachite green 87.81
Methyl OrangeFe3O4@SiO2@ZnO 96.00[50]
Acid Orange 7CP-192.4093.81This work
CP-271.8076.60
CP-389.0391.10
Methyl OrangeCP-180.5084.82This work
CP-247.0051.72
CP-357.4571.43
H6L1 = hexakis(methyl-2-(4-phenoxyphenyl)acetatebenzene)cyclotriphosphazene; 4,4′-bpy = 4,4′-bipyridine; Bimb = 1,4-bis[(1H-imidazol-1-yl)methyl]benzene; OrK = potassium orotate; bpyp = 2,5-bis(pyrid-4-yl)pyridine; Hnba = 4-nitrobenzoic acid; L′ = 4-(2,6-di(pyrazin-2-yl) pyridin-4-yl)benzoate; H3L = 3,4-bi(4-carboxyphenyl)-benzoic acid; hbmb = 1,1′-(1,6-hexane) bis(2-methylbenzimidazole); btbb = 1,4-bis(2-(4-thiazolyl)benzimidazole-1-ylmethyl)benzene; 4,4′-bipy = 4,4′-bipyridine).
Table 2. Influence of initial concentration on dye removal by photodegradation.
Table 2. Influence of initial concentration on dye removal by photodegradation.
DyeC0
(mg/L)		R
(%)		Time
(min)
AO71590.93130
3092.40140
4579.46180
6056.32210
MO1574.29170
3080.50180
4570.51220
6056.61240
Table 3. Kinetic parameters for AO7 and MO photodegradation by coordination polymers CP1–CP3, and by CP1 at different concentrations.
Table 3. Kinetic parameters for AO7 and MO photodegradation by coordination polymers CP1–CP3, and by CP1 at different concentrations.
CPsAO7MO
Conc.
(mg/L)		k × 103
(min−1)		R2SDConc.
(mg/L)		k × 103
(min−1)		R2SD
CP11516.400.97160.15511511.280.92740.1483
3013.070.86310.3401309.330.92470.1707
458.570.92630.1687456.070.96570.0906
603.200.99180.0329603.940.96480.0581
CP2305.250.92870.1201305.820.92200.0694
CP3309.760.92460.2319306.700.88020.1156
Table 4. The photocatalytic degradation efficiency of AO7 and MO dyes from binary solution using CP1–CP3 under visible light irradiation.
Table 4. The photocatalytic degradation efficiency of AO7 and MO dyes from binary solution using CP1–CP3 under visible light irradiation.
R (%)
DyeAO7MOAO7 + MO
SingleBinarySingleBinary
-H2O2-H2O2-H2O2-H2O2-H2O2
CP192.4093.8174.6484.0580.5084.8272.0581.4673.8383.54
CP271.8076.6030.1233.2947.0051.7227.3529.0229.9831.34
CP389.0391.1043.0355.0054.4571.4334.3950.5141.7053.71
Table 5. Characteristics and toxicological information of the dyes.
Table 5. Characteristics and toxicological information of the dyes.
DyeMolecular Weight
(g/mol)		StructureToxicity
AO7350.32Molecules 30 01652 i001Dangerous for the aquatic environment in the long term. Strong irritant to skin and eyes, and may cause allergic reactions in sensitive people [77,78].
MO327.33Molecules 30 01652 i002Harmful to the environment, biology, and human health. May cause respiratory tract irritation and skin irritation [79]. It is a toxic, carcinogenic, tumorigenic, mutagenic, and genotoxic azo dye [80].
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Buta, I.; Nistor, M.A.; Muntean, S.G. Efficient Photodegradation of Dyes from Single and Binary Aqueous Solutions Using Copper(II) Coordination Polymers. Molecules 2025, 30, 1652. https://doi.org/10.3390/molecules30081652

AMA Style

Buta I, Nistor MA, Muntean SG. Efficient Photodegradation of Dyes from Single and Binary Aqueous Solutions Using Copper(II) Coordination Polymers. Molecules. 2025; 30(8):1652. https://doi.org/10.3390/molecules30081652

Chicago/Turabian Style

Buta, Ildiko, Maria Andreea Nistor, and Simona Gabriela Muntean. 2025. "Efficient Photodegradation of Dyes from Single and Binary Aqueous Solutions Using Copper(II) Coordination Polymers" Molecules 30, no. 8: 1652. https://doi.org/10.3390/molecules30081652

APA Style

Buta, I., Nistor, M. A., & Muntean, S. G. (2025). Efficient Photodegradation of Dyes from Single and Binary Aqueous Solutions Using Copper(II) Coordination Polymers. Molecules, 30(8), 1652. https://doi.org/10.3390/molecules30081652

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