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Environmental fate & pathways

Adsorption / desorption

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Data is provided on the surrogate substance manganese sulphate. Sorption strength increases with pH. However, in several soils, residual 54Mn in solution was below values expected for sorption, suggesting oxidation. The Kd median result is considered to be sufficiently representative at present to use in risk assessment.

At 3 days incubation the median Kd was 1598 L/kg. At 100 days incubation the median Kd was 650 L/kg and at 180 days incubation the median Kd was 601 L/kg.

Key value for chemical safety assessment

Additional information

This proposal addressed the solid–liquid partitioning of manganese (Mn) in soils. To be most relevant for the risk assessment, the ‘adsorption Kd’ was measured in soils. No such Mn adsorption studies have been made for European soils before and no comparison has been found between adsorption (of added Mn) and desorption (of native Mn) Kdvalues.

Manganese in soil is present in different redox states, most commonly +2, +3 or +4. In soil, MnIIIand MnIVoccur as insoluble oxides. At low pH or under reduced conditions, MnIIis the most stable form and this species has much higher solubility than MnIII/IV. The Mn redox chemistry therefore plays an important role in mobility and availability of Mn. The Kdconcept that is used in traditional exposure estimates for risk assessment, refers to adsorption reactions, for which solution concentrations increase as the solid-phase concentrations increase. For Mn, this Kdconcept does not hold when MnIIIand MnIVare present as precipitates (oxides), because the solubility of a precipitate does not depend on the amount of precipitate. Mathematically, the Kdcan still be derived in case of precipitation reactions, but this value increases with increasing total concentration once the precipitate is formed. For pragmatic reasons for risk assessment, we have derived the Kdvalues in this study, even when precipitates formed. Practically, this means that Kdvalues of Mn in soil not only depend on pH or redox but also on the total Mn in soil.

Thirty-five European soils were selected from our database to make a comprehensive assessment of the Kdvalues in European soils and to encompass the range of properties that may affect the Kdvalues, i.e. the cation exchange capacity, pH and organic matter content. Soils were amended with (carrier free)54MnII(a Mn2 +salt) and subsamples were extracted with 10 mM CaCl2at 3, 100 and 180 days after spiking. The adsorption Kdvalues ranged from 1.0 to 20550 L/kg at 3 days and from 1.0 to 6516 L/kg at 180 days. The geometric mean Kdincreased 1.2-fold between day 3 and day 180, from 334 to 424 L/kg. On a restricted set of soil, MnO2was added. The added Mn reached the same speciation as native Mn in most soils by day 100.

The fraction of total Mn that is present as MnII was estimated by extraction with 1 M NH4Ac. At 180 days, the partitioning of the isotope and the soil Mn between extractable (MnII) and non-extractable (MnIII/IV) pools was similar for the isotope and the native Mn, indicating that the isotope nearly fully equilibrated within this period. Effectively, this means that the solid-liquid distribution of native Mn in soil may be used to predict the Kdof added Mn.

 

For risk assessment, regression models with R2of 0.64-0.65 were derived from which appropriate Kdvalues can be selected that are valid for Mn added to aerobic soil as MnIIsalts or MnO2. The Kdin low pH soils (pH<5.6) is strongly related to pH and did not depend on the total Mn concentration in soil. In high pH soils (pH>5.6), both pH and total Mn concentration affects the partitioning of Mn in the soil. Practically, the Kdvalues can be implemented in the EU risk assessment schemes if generic values of soil pH and total Mn are selected. A look-up table allows selecting appropriate Kdvalues depending on pH and total Mn. These predictions are only valid for aerobic soils. Under saturated conditions, reduction of MnIII/IVoxides may occur, resulting in an increase in MnIIconcentration and dissolved Mn concentrations.