Polyphosphoric acids

Toxicity to microorganisms

Description of key information

A waiver is proposed to cover this endpoint, and supporting evidence with a K4 study (Ishii et al., 1981) is provided.  

Additional information

In accordance with Annex XI, section 1.5 of Regulation (EC) No 1907/2006 (REACH) a read across approach may be used when substances have similarities based on the likelihood of common breakdown products via physical and biological processes, which result in structurally similar chemicals. As mentioned in the hydrolysis summary, polyphosphoric acid is hydrolysed to orthophosphate in environmental conditions. Thus a read across from orthophosphoric acid to pyrophosphoric acid is justified. In addition, studies performed with pyrophosphate salts can also be used. Pyrophosphate salts are ionic in nature and therefore dissociate readily into cations and anions in water. Cations as potassium and sodium are essential micronutrients that are ubiquitous in the enviroment. As such, their uptake is tightly regulated and is therefore not considered to pose a risk for ecotoxicity. In environmental conditions, the pyrophosphate anion is unstable and a number of different processes result to an ultimate breakdown product of orthophosphate.

According to REACH Annex VIII, this endpoint does not need to be fulfilled if there are mitigating factors indicating that microbial toxicity is unlikely to occur. Phoshoric acid in water will dissociate in phosphate anions and H+ ions, the latter causing a decrease of pH. However, pH levels in wastewater are typically adjusted in wastewater treatment plants to ensure a neutral discharge to the receiving water (e.g., pH between 6-9) and in order to prevent inhibitory effects on the growth of microorganisms. Therefore, the microorganisms are essentially not exposed to phosphoric acid, but to phosphate instead. According to REACH Annex VIII column 2, the study would not need to be conducted. Furthermore, phosphate is an essential nutrient for activated sludge systems, as it comprises part of the biomass in the activated sludge. Typical influent values in domestic sewage are as follows: Total phosphate as P: 4 – 15 mg/L; Inorganic phosphate as P: 3-10 mg/L. When influent phosphate levels are at such levels that phosphate could pass through a treatment plant and result in excessive levels in a receiving water, additional unit operations are added to a treatment plant (e.g., precipitation) to remove excess phosphate. In conclusion, both the pH and the phosphate concentrations will be kept at an acceptable level within a sewage treatment plant to prevent adverse effects to microorganisms and the receiving waters.

As a supportive evidence, Ishii et al. (1981) assessed the toxicity of phosphoric acid to microorganisms by using the oxygen absorption rate of activated sludge. The toxicity of phosphoric acid to protozoa was measured by looking at mortality. The IC50 values were 270 and 240 mg/L, respectively. It can be concluded that phosphoric acid is of low toxicity to microorganisms.

In addition, a toxicity test performed on STP microorganisms with a pyrophosphate salt ( disodium dihydrogenpyrophosphate) has been investigated according to OECD TG 209. An EC50_3h of greater than 1000 mg/L and a NOEC of 1000 mg/L were determined (Clarke, 2010).