Visible-Light-Induced Photocatalytic Degradation of Methyl Orange by Cobalt (II) Thiosemicarbazone Complexes

Abstract

A series of p-substituted isonitrosoacetophenone-4-phenylthiosemicarbazone cobalt(II) complexes were synthesized and investigated for their photocatalytic activity toward the degradation of methyl orange (MO), a common azo dye pollutant in industrial wastewater. The synthesized complexes were characterized using elemental analysis, molar conductance, magnetic susceptibility, UV–Visible spectroscopy, FTIR spectroscopy, and TG–DTA. Photocatalytic degradation studies were carried out under visible-light irradiation using a tungsten filament lamp. The effects of irradiation time, catalyst dosage, and catalyst reusability were systematically evaluated. The cobalt(II) complexes exhibited efficient photocatalytic activity, resulting in significant degradation of methyl orange under the optimized reaction conditions. The enhanced catalytic performance is attributed to the favourable redox properties of cobalt(II) and the chelating nature of the thiosemicarbazone ligands, which facilitate the generation of reactive oxygen species during the photocatalytic process. The catalysts also demonstrated good stability and reusability over successive cycles. These findings indicate that the synthesized cobalt(II) complexes are efficient and environmentally benign photocatalysts with potential applications in the treatment of dye-contaminated wastewater.

Introduction

The text investigates the use of cobalt(II) thiosemicarbazone complexes as photocatalysts for removing methyl orange (MO) dye from contaminated water. Synthetic dyes from industries such as textiles, paper, leather, plastics, and pharmaceuticals are difficult to degrade and can harm aquatic ecosystems. Methyl orange is particularly resistant because of its stable azo bond and aromatic structure.

Research Objective

The study aims to develop efficient and recyclable photocatalysts based on cobalt(II) complexes for the degradation of methyl orange under visible light. Six complexes (CL1–CL6) containing different substituted thiosemicarbazone ligands were investigated.

Methodology

The cobalt(II) complexes were synthesized by reacting cobalt chloride with isonitrosoacetophenone phenyl thiosemicarbazone ligands in a 1:2 metal-to-ligand ratio. The complexes were characterized using techniques including:

• UV–Visible spectroscopy
• FT-IR spectroscopy
• Thermal analysis (TG-DTA)
• Magnetic susceptibility measurements
• Molar conductance
• Cobalt-content analysis

A 10 ppm methyl orange solution was used for photocatalytic experiments. The dye has a maximum absorption wavelength (λmax) of 480 nm. Degradation was monitored by measuring the decrease in absorbance at 480 nm under tungsten-lamp irradiation.

Photocatalytic Mechanism

When the cobalt complexes absorb light, they generate electron–hole pairs. These interact with water and oxygen to produce reactive oxygen species such as hydroxyl radicals (•OH) and superoxide radicals (O?•?). These reactive species attack the azo bond and aromatic structure of methyl orange, progressively breaking the dye into smaller compounds and ultimately promoting mineralization into less harmful products.

Effect of Irradiation Time

Photocatalytic degradation increased consistently with irradiation time for all six complexes. After 120 minutes:

• CL1: 92.38% degradation
• CL2: 93.33%
• CL3: 94.28%
• CL4: 95.23%
• CL5: 96.19%
• CL6: 97.14%

Thus, CL6 showed the best photocatalytic performance, while CL1 showed the lowest activity. CL6 contains a nitro (–NO?) substituent, which appears to improve the electronic properties and charge-transfer behavior of the complex.

Effect of Catalyst Dosage

Increasing the catalyst amount from 25 to 100 mg improved methyl orange degradation for all complexes. For example, CL6 increased from 37.14% degradation at 25 mg to 95.23% at 100 mg.

This improvement is attributed to the greater number of active photocatalytic sites and increased production of reactive oxygen species. No decrease in activity was observed within the tested dosage range, making 100 mg the most effective dosage under the reported conditions.

Overall Conclusion

The study demonstrates that cobalt(II) isonitrosoacetophenone phenyl thiosemicarbazone complexes are promising visible-light photocatalysts for methyl orange degradation. Their performance improves with both irradiation time and catalyst dosage. Among the six complexes, CL6 (the nitro-substituted complex) was the most effective, achieving 97.14% degradation after 120 minutes and 95.23% degradation at 100 mg catalyst dosage. The findings suggest that these cobalt complexes could potentially be developed as efficient and environmentally friendly materials for treating dye-contaminated industrial wastewater.

Conclusion

The photocatalytic degradation of Methyl Orange (MO) was examined using a series of cobalt(II) complexes derived from p-substituted isonitrosoacetophenone phenyl thiosemicarbazone ligands. The synthesized complexes showed notable photocatalytic performance, with degradation of more than 85% of MO achieved after 120 min of irradiation. Among the studied complexes, CL6, incorporating the p-nitro-substituted ligand, showed the maximum degradation efficiency. The enhanced photocatalytic behaviour of CL6 can be associated with the strong electron-withdrawing effect of the nitro group, which may facilitate charge separation and decrease the probability of electron–hole recombination. Consequently, the formation of reactive oxygen species may be enhanced, contributing to the effective breakdown of the MO molecules. CL5, containing the p-chloro substituent, also exhibited relatively high photocatalytic activity compared with the unsubstituted and electron-donating derivatives. The complexes containing electron-releasing groups, namely methyl, methoxy, and hydroxyl substituents, exhibited comparatively reduced photocatalytic efficiencies. These groups increase electron density in the ligand system and may not promote charge separation as effectively as electron-withdrawing substituents. Among the series, the unsubstituted complex CL1 showed the lowest degradation efficiency. The results clearly indicate that the nature of the substituent attached to the ligand has a considerable influence on the photocatalytic performance of the cobalt(II) complexes. Electron-withdrawing groups, particularly the nitro group, appear to favour enhanced photocatalytic degradation of MO. Hence, the synthesized cobalt(II) complexes may be considered potential photocatalysts for the removal of organic dyes from contaminated water and for environmental remediation applications.

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Copyright

Copyright © 2026 Shivani B. Malap, Payal M. Ramnani, Raju M. Patil. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.