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TL;DR: In what aspects are the products of the indigo vat dyeing process (sodium sulphate, sulphite and thiosulphate ions) harmful to the environment or to living beings?


I was recently looking into the wastewater created in the process of vat dying using indigo. I found out that since indigo is quite insoluble ($\pu{990\mu g}$ at $\pu{25^{\circ}C}$) in water for reasons of intra-molecular hydrogen bonding, a reducing agent like sodium dithionite ($\ce{Na2S2O4}$) is required, which gives leucoindigo by reducing indigo in an alkaline medium.

indigo intra-H-bonding

A related passage from a distantly related paper:

For this process, sodium dithionite ($\ce{Na2S2O4}$) has been considered as a best reducing agent in traditional and industrial dying but the reducing agent releases sodium sulphate, sulphite and thiosulphate ions into the media. Those ions not only cause environmental concerns as major pollutants of the modern textile industry waste waters but they present also no negligible risks of toxicity against living organisms.

A few papers suggest that the "wastewater" generated by the process is not suitable for disposal as such. But I still find no clarity, despite prolonged efforts at scouring journals, in why these exact ions cause the suggested adverse effects to the environment. I would be grateful if someone could please share any relevant knowledge.

These ions appear pretty harmless to me..

indigo dyeing


References:

  1. The role of natural indigo dye in alleviation of genotoxicity of sodium dithionite as a reducing agent, Bektas I, Karaman S, Diraz E, Celik M, Cytotechnology, 2016 Dec; 68(6): 2245–2255, DOI: 10.1007/s10616-016-0018-7

BONUS: Are there any other harmful substances or environmental hazards in this nice indigo broth?

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For this process, sodium dithionite (Na2S2O4) has been considered as a best reducing agent in traditional and industrial dying but the reducing agent releases sodium sulphate, sulphite and thiosulphate ions into the media. Those ions not only cause environmental concerns as major pollutants of the modern textile industry waste waters but they present also no negligible risks of toxicity against living organisms.

It is a very convoluted sentence with so many negations. My simple definition of environmentally dangerous stuff is that anything which is not found in nature is bad for the environment in excessive amounts. Sodium thiosulfate and sulfite are strong reducing agents, in excess amounts, they will slowly deplete the oxygen levels in water. Read the concept of "Chemical Oxygen Demand" in natural waters. These ions will heavily affect the COD, which is a measure of water quality. Sodium sulfate should be fine. You have to check the half-life of these ions (thiosulfate and sulfite) in natural waters.

Untreated textile waste is worse in the sense because it is colored. If large amounts of untreated textile waste, such as those of indigo, are discarded into natural streams they might block the light reaching the depths.

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