What is the environmental impact of FAS?

There is an impact, but it is manageable, and it is generally lighter than a conventional sodium hydrosulfite process. FAS is not a "pollution-free" raw material. It decomposes in water: the sulfur ultimately becomes sulfate and the nitrogen becomes urea. The parameters that need to be controlled in wastewater are COD and total nitrogen; no heavy metals and no chlorinated organics are involved.

In practice, the FAS dosage is about one-fifth to one-quarter of sodium hydrosulfite. For the same reductive clearing or reductive bleaching result, the total mass entering the wastewater system is correspondingly smaller. For textile dyeing and paper mills, that gap is a more useful reference than a single toxicity figure.

On contact with water, under alkaline conditions or on heating, the C-S bond in the molecule breaks and the reaction follows two branches.

On the sulfur side, the sulfoxylate group is first converted to dithionite (under aerobic conditions), then oxidised to sulfite, and ends as sulfate. On the nitrogen side, urea is formed; it stays in the water and counts towards total nitrogen and ammonia nitrogen.

The end products contain no heavy metals and produce no chlorinated compounds. This is the fundamental difference between FAS and chlorine- or chromium-based reducing agents in terms of environmental fate.

Aquatic toxicity is low to moderate; under international criteria it falls into the "harmful to aquatic life" category. It is not highly toxic.

Of the three endpoints, algae are the most sensitive: EC50 32 mg/L (72 h, OECD 201). Daphnia 390 mg/L (24 h); zebrafish 416 mg/L (96 h). The activated sludge 28-day NOEC is not lower than 100 mg/L, indicating limited inhibition of biological treatment micro-organisms at normal concentrations.

The German water hazard class is WGK 1 (slightly hazardous). It is not classified as PBT or vPvB.