Orthophosphoric acid

Toxicity to microorganisms

Description of key information

Activated sludge respiration inhibition test: EC50 (3h) >1000 mg/L for activated sludge of a predominantly domestic sewage (OECD TG 209, static, Klimisch reliability 1)

Key value for chemical safety assessment

EC50 for microorganisms:

1 000 mg/L

EC10 or NOEC for microorganisms:

1 000 mg/L

Additional information

According to Annex VIII, section 9.1.4, column 2 of Regulation (EC) No. 1907/2006 (REACH) a study for activated sludge respiration inhibition testing does not need to be performed if there are mitigating factors indicating that microbial toxicity is unlikely to occur. Phosphoric acid will dissociate in water to phosphate anions and H+ ions, the latter causing a decrease of pH. However, pH levels in wastewater are typically adjusted in the wastewater treatment plant 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.

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.

A study on dipotassium orthophosphate was available and as such in accordance with Annex XI, Section 1.5 of REACH which states that read-across may be used in the case of substances whose physicochemical, toxicological and ecotoxicological properties are likely to be similar. Similarities may be based on:

1) A common functional group

2) The common precursors and/or the likelihood of common breakdown products via physiological and biological processes, which result in structurally similar chemicals; or

3) A constant pattern in the changing of the potency of the properties across the category

Orthophosphoric acid and dipotassium orthophosphate have the following similarities:

1). Both substances share the PO₄^3-anion as common functional groups.

2) Both substances will ultimately dissociate into the common breakdown products of the PO₄^3-anion.

The supporting study (Ishii et al) assessed the toxicity of phosphoric acid itself 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.