Environmental impacts and remediation of dye-containing wastewater

Synthetic dyes are used in various industries including textile processing, food production and the pharmaceutical sector. Yet, approximately 80% of the dye-containing wastewaters produced are often released untreated into waterways or used directly for irrigation, causing detrimental impacts on human health and ecosystems. In this Review, we discuss the environmental impact of dye-containing wastewater and explore the chemical, biological and physical mitigation strategies used to treat and decontaminate dye-containing wastewaters. Untreated synthetic dyes cause coloration of receiving water bodies, hindering the degree of visible light reaching the photic ozone, and have carcinogenic, mutagenic and teratogenic properties that have toxic impacts on plants, animals and humans. Chemical treatment methods such as coagulation are the most widely adopted treatment methods; however, they require careful sludge management to be effective. Biological degradation, involving the implementation of enzymes, microorganisms and plants, is an environmentally friendly and energy-efficient approach for dye degradation, but it requires lengthy reaction times and the use of selective bio-organisms for target dyes. Advanced membrane-based physical separation can achieve effective removal of dyes and inorganic salts from highly saline dye-containing wastewater, while also enabling their recovery and reuse. Strengthened regulatory requirements and development of non-toxic dyes are required in conjunction with these remediation treatments to effectively mitigate dye-related pollution. Wastewater dyes from textile, food and pharmaceutical industries are a major environmental concern. This Review discusses the environmental impacts of dye-containing wastewater and explores both conventional and emerging remediation strategies. Untreated synthetic dyes released into aquatic environments reduce the light available for photosynthesis by primary producers, with consequential impacts for the whole food chain. In addition, dyes are also directly harmful to plants, animals and humans, with human health implications including increasing allergy and cancer risk.Chemical coagulation and electro-coagulation are widely adopted methods of dye removal. However, coagulant efficacy and sludge management are crucial for efficient removal of dyes.Advanced oxidation processes, which encompass chemically mediated advanced oxidation processes, photocatalysis and electrocatalysis, have been proven effective at dye degradation. To ensure these methods are sustainable and safe, it is crucial to minimize chemical and energy consumption and monitor and manage the toxic by-products that can be generated during the process.Biological degradation is an eco-friendly and energy-efficient method for dye removal through the utilization of enzymes, microorganisms (bacteria, fungi, yeast and algae) and plants, but these methods require a long reaction time, owing to slow kinetics during dye degradation.Emerging membrane-based physical separation techniques, including tight ultrafiltration, loose nanofiltration and electro-driven nanofiltration, show a great potential in fractionation of dyes and salts from highly saline dye-containing wastewater. These methods also enable efficient recovery of dyes and salts to promote a circular economy in the textile sector.To achieve sustainable and safe synthetic dye use, these advanced remediation technologies must be implemented in combination with proper regulation of dye-containing wastewater discharge in collaboration between governing bodies and industry stakeholders.