Zinc hydroxide

Toxicity to microorganisms

Description of key information

A nitrification inhibition test with Zn sulfate on activated slugde originating from a municipal wastewater treatment plant was performed leading to a NOEC of 100 µg Zn/L.

Key value for chemical safety assessment

EC10 or NOEC for microorganisms:

100 µg/L

Additional information

The EU Risk Assessment (RA) on zinc (Munn et al. 2010) reviewed the available data for zinc toxicity to microorganisms to set the PNEC for Sewage Treatment Plants (STP). The most recent reference mentioned in the RA report was from 1997. A number of results on micro-organisms were considered not relevant in the RA process for reasons that are also applicable to the REACH analyses (ECHA 2008) e.g.: results on Pseudomonas fluorescens, Vibrio fischeri were considered less relevant. Results obtained on an industrial activated sludge were (and are for the present analysis) also not considered relevant for the setting of the PNEC-STP.  In the EU RA, a number of results obtained on protozoa were also reviewed, but for the setting of the PNEC-STP, preference was given to the activated sludge test.

According to the approach followed in the EU RA, results on activated sludge respiration inhibition are also used for the present analysis. The available literature on this endpoint was checked for information becoming available after 1997. In general, the aim of the STP- assessment is the protection of the degradation and nitrification functions, and the process performance and efficiency of domestic and industrial STPs. The PNEC should be derived from results obtained under the most sensitive system conditions (ECHA 2008). For metals in general, this is under conditions of the domestic STP; in industrial treatment systems, organisms are generally adapted to higher metal levels.

In addition to the data from the RA, the following information has been reviewed and identified as relevant: a) zinc toxicity in a nitrification inhibition test (Juliastuti et al 2003); b) zinc toxicity on activated sludge bacteria growth (Cabrero et al 1998); c) zinc toxicity on activated sludge respiration and nitrification (Dalzell et al 2002). ATP luminescence proved to be insensitive to zinc (Dalzell & Christophi 2002). According to the guidance, preference should be given to nitrification inhibition because respiration inhibition is a less sensitive endpoint.

The study conducted by Juliastuti et al. (2003) reported a NOEC of 0.1 mg Zn/L in an ISO 9509 nitrification inhibition test. According to the guidance for PNEC setting (ECHA R.10, table R.10.6) this result obtained on a standard nitrification test is considered most relevant for setting the PNEC STP. Applying the rules of PNEC setting, this NOEC results in a PNEC STP of 100 µg Zn/L (AF of 1 ; ECHA R.10, table R.10.6).

Several data were available for this endpoint. Formerly (ECB 2008), and in the REACH-registrations of a number of zinc substances of November 2010, a PNEC of 52 µg/L for STP was derived, based on the lowest EC50 of 5.2 mg Zn/L observed in a sludge respiration inhibition test (Dutka et al. 1983). According to the guidance (ECHA 7.8.17.1) preference should be given to nitrification inhibition because respiration inhibition is a less sensitive endpoint. A study conducted by Juliastuti et al. (2003) reported a NOEC of 0.1 mg Zn/L in an ISO 9509 nitrification inhibition test. Applying the rules for PNEC setting (ECHA R.10, table R.10.6) this result yields a PNEC STP of 100 µg Zn/L (AF of 1).