Silicic acid (H4SiO4), zirconium(4+) salt (1:1), reaction products with sodium hydroxide

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

15 mg/L

PNEC freshwater (intermittent releases):

15 mg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

2 mg/L

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

2 000 mg/L

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

no hazard identified

Sediment (marine water)

Hazard assessment conclusion:

no hazard identified

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

no hazard identified

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

The substance reacts with water. The water soluble reaction product is a multi-constituent substance: Reaction mass of disodium metasilicate and sodium hydroxide (EC 910-245-3).

Sodium Hydroxide: A generic PNEC cannot be derived from single-species toxicity data for NaOH, as the pH of natural waters as well as the buffer capacity of natural waters show considerable differences and aquatic organisms/ecosystems are adapted to these specific natural conditions, resulting in different pH optima and pH ranges that are tolerated. There is a lot of information available about the relationship between pH and ecosystem structure and also natural variations in pH of aquatic ecosystems have been quantified and reported extensively.

Disodium Metasilicate:

Residual insoluble zirconium hydroxide and unreacted zircon sand:

PNEC values for the aquatic compartment cannot be derived. The available acute ecotoxicity tests in fish and daphnids show EC50 or LC50 values which are higher than 100 mg/L (based on added test substance) or > 100% v/v saturated solution. When zirconium dichloride oxide is dissolved in a buffered aqueous solution (such as a natural surface water) precipitation of zirconium as zirconium hydroxide/zirconium dioxide (pH dependent), zirconium carbonate (pH dependent) and/or zirconium phosphate will occur. The precipitation of zirconium phosphate in algal test media seems to result in some growth inhibition due to phosphate deprivation (i.e., a secondary effect). This was demonstrated in algal growth inhibition experiments with read across substances. The fact that in an algal growth inhibition test with zirconium dichloride oxide no measurable zirconium concentrations > LOQ (11 µg Zr/L) could be detected in any of the treatments whereas significant reduction of growth was observed in the 100% v/v saturated solution supports the assumption that the observed effects are not due to primary exposure to bioavailable zirconium, but rather due to a secondary effect such as phosphate deprivation. This is further supported by the study of Kumar and Rai (1978), in which it was demonstrated that additional phosphate dosing countered the effect on algal growth. The phosphate deprivation effect is not considered environmentally relevant as it may only occur extremely locally. Overall, in view of the extremely low bioavailability of zirconium in environmentally relevant media at environmentally relevant conditions, it can be concluded that zirconium from zirconium dichloride oxide is not toxic to aquatic organisms.

Similarly, micro-organisms in a sewage treatment plant are not expected to be exposed to zirconium (dichloride oxide), as zirconium will have been removed from the water column through hydrolysis and carbonate and/or phosphate complexation before reaching the biological treatment step. Often a pH increase step is included for metal precipitation as one of the (first) waste water treatment steps in on-site waste water treatment plants. If such as step is included the removal efficiency will be 100%. Moreover, no adverse effects have been observed in an activated sludge respiration inhibition test with the read across substance zirconium acetate, another 'water soluble' zirconium compound with similar behaviour in the aquatic environment as zirconium dichloride oxide. Therefore no PNEC needs to be derived.

As no PNEC aquatic could be derived, no PNEC values for soil and sediment can be derived either by using the equilibrium partitioning method. No toxicity data are available for sediment or soil organisms, except for a short-term toxicity study with terrestrial plants, yielding only unbound NOEC values. Therefore, no PNEC values for soil and sediment can be derived applying the assessment factor method either. Since zirconium dichloride oxide is not considered hazardous to the environment, no chemical safety assessment needs to be conducted and therefore no PNECs need to be derived for these compartments.

Conclusion on classification

As PNECs could not be derived for the sodium hydroxide and insoluble zirconium salts, the PNECs used for the substance are based upon those for disodium metasilicates after correction for their approximate content of the substance (maximum 50%) after it has reacted with water.