Catalytic remediation of dyes by tailored carbon dots in water – efficiency, ecotoxicity and microbial community aspects
Creators: name: Beker, Sabrina
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 4104
subject: Environmental management
classificationCode: 410404
subject: Environmental management

description: Dye pollution has become one of the main contributors to water contamination across the globe. The World Bank estimates that the dyeing industry contribute up to 17-20 % of total industrial water pollution. Synthetic dyes are extensively used in the textile, paper, food, and leather industries, and it has been widely reported that many of them are toxic and can cause irreversible damage to both environment and human health. Of the seventeen Sustainable Development Goals set by United Nations (2020), the sixth goal is to “ensure availability and sustainable management of water and sanitation for all” by 2030. The achievement of this goal has become a critical challenge for water resources management. Therefore, the scientific community is urged to research and develop cost-effective, sustainable, and efficient water treatment technologies to remove these hazardous industrial pollutants from water before their safe release into the natural environment.

Several approaches have been previously studied to remove dyes from water including physico-chemical and biological treatments. Despite these efforts to improve the efficiency of such systems, driven by increasingly stringent legislation, conventional water and wastewater treatments can be limited, costly and inefficient in terms of the treatment of these hazardous compounds. Recently, technologies like carbon-based nanomaterials have been shown to be both efficient and cost-effective water purification methods; however, what is currently unknown is whether these new technologies represent sustainable and environmentally friendly approaches. Significant advances have been made on the utilisation of carbon-based nanoparticles for water treatment; nevertheless, there is limited knowledge on the potential of such technology as catalysts to treat dye-containing water and their eventual fate in the environment. Notably, the use of low toxic carbon-based nanoparticles such as emergent carbon dots (CDs) might overcome the drawbacks of most used water remediation technologies, which are not able to reduce the toxicity of the effluents thereby necessitating additional processing.

Therefore, the overarching aim of this thesis was to examine the use of novel CDs as catalysts in the remediation of dyes using a Fenton-like approach to increase the degradation of dyes in water while minimising the effluents toxicity and its environmental impacts.

With the aim of investigating the catalytic activity of green synthesised CDs in the removal of dyes from water considering CDs key properties and factors that influence the catalytic reaction, the first research chapter of this thesis involved firstly synthesising CDs from cactus plant extract (Opuntia) and subsequent assessing their ability to act as Fenton-like catalysts. Secondly, to study the key properties involved in the CDs catalytic behaviour, a set of multiple characterisation techniques were employed including Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Zeta potential, ultraviolet–visible spectroscopy (UV-Vis) and transmission electron microscopy (TEM). The results of Chapter 3 showed that after only 60 minutes, approximately 90 % of a mix of three common cationic dyes (crystal violet, basic fuchsin and methyl green) from 16 to 22 mg L-1 were degraded in water at 20 °C and pH 7.5 in the presence of H2O2 at 0.5 % without any additional energy source. Characterisation revealed dots in the size range of 13-20 nm, negatively charged, rich in nitrogen and oxygen surface functionalities. Introducing a free radical quenching technique into the catalytic reaction, revealed that reactive oxygen species (ROS) such as hydroxyls, superoxide ions and singlet oxygens were generated due to the decomposition of H2O2 on the surface of the CDs. This chapter demonstrated that green synthesised CDs could be used as efficient catalysts in the Fenton-like degradation of a blend of commercial cationic dyes under optimised and mild conditions. The ROS along with the rich functionalities such as nitrogen and oxygen groups on the surface of the dots played key roles in the catalytic behaviour of the system, and the main catalytic active sites were detected as O–C=O and C–O/C–N via XPS and FTIR analyses.

To further investigate the effects of surface chemistry of CDs on the degradation of dyes in water, the second research chapter of this thesis (Chapter 4) aimed at functionalising CDs and testing their efficiency in the degradation of toxic methylene blue dye while analysing ecotoxicological aspects of the treated effluents. Carbon dots doped with nitrogen and iron with different concentrations were green synthesised and comprehensively characterised. Chapter 4 revealed that 7 nm, negatively charged nanodots rich in iron and nitrogen surface functionalities were able to degrade 100 % of methylene blue (MB at 20 mg L-1) under optimised conditions (at 50 °C, pH 8, H2O2 147 mM, and a nanocatalyst concentration of 0.5 mg mL-1) in only 60 minutes. In addition, based on spectroscopic analyses, the mechanism of action of iron and nitrogen-containing CDs in the degradation of MB was likely due to the catalytic functionalities of the surface groups of the CDs since nitrogen, oxygen and iron groups participated as potential active sites during the catalytic reaction. This innovative green remediation technology significantly reduced the ecotoxicity of the treated effluents. Remediated samples by the iron and nitrogen-containing CDs showed a half maximal effective concentration (EC50) of 97.8 % whilst the control remained highly toxic (EC50 of 1.2 %). This confirmed that catalytic degradation, performed by the iron and nitrogen-containing CDs did not generate harmful by-products to aquatic organisms, and therefore represents an alternative treatment for wastewaters containing cationic dyes and other pollutants, significantly reducing the toxicity of the effluent.

Finally, the third research chapter (Chapter 5) aimed at evaluating the remediation of MB contaminated Australian groundwater using the proposed technology in this thesis and analysing the impacts on the groundwater's microbiota in microcosms to better understand the changes in microbial community structure, diversity, and composition upon CDs remediation technology. The outcomes of Chapter 5 revealed that 78-82% of MB were removed from the groundwater samples over 25 h and the toxicity showed a 4-fold reduction when using CDs/H2O2. 16S rRNA amplicon-based sequencing revealed a predominance of at least 60 % Proteobacteria phylum in all samples where diversity and evenness were kept throughout the remediation process demonstrating that CDs could be a suitable remediation approach with minimal environmental impact.

Overall, this thesis research has demonstrated the use of effective and efficient eco-friendly CDs as Fenton-like catalysts in the remediation of harmful commercial dyes not only under optimised aqueous medium but also in Australian groundwater samples. This research provided encouraging results and numerous insights into the use of CDs as an eco-friendly option for the treatments of dye-contaminated water, which might be exploited for future environmental remediation applications.