Consideration of Total Ion Composition in Designing Toxicity Tests Using Aluminum Salts and Mineral Acids

Abstract

Review of the current literature reveals that many researchers involved in the study of the effects of aluminum and pH on aquatic life are using aluminum salts, that is, sulfate or chloride forms, to obtain the desired aluminum concentration for their experiments. This technique raises some concern for soft water experiments due to the unknown effect of excessive concentrations (in relation to background) of the associated anions SO4= or Cl−. For example, if aluminum chloride (AlCl3) is employed to obtain a concentration of 0.5 mg/L aluminum, the solution also will be increased by 2.0 mg/L Cl−. Likewise, if aluminum sulfate [Al2(SO4)3] is used to achieve the 0.5 mg/L aluminum, SO4= will be increased 2.6 mg/L. For inland streams (that is, not affected by ocean salts), these values appear to be out of proportion. In some instances the use of aluminum salts can easily double or increase by the power of 10 the concentrations of associated anion. SO4= or Cl− concentrations are further increased by the use of sulfuric and hydrochloric acid to adjust pH. Since aluminum is an effective buffer below pH 5.0, solutions containing high concentrations of aluminum also will require larger volumes of acid to achieve pH values below this level, thus compounding the problem. The question of acid form, that is, sulfuric, hydrochloric, nitric, etc., in relation to toxicity leads to the consideration that these “strong acids” are only one component contributing to the total acidity of a system. In coniferous forests with deep acid soils, storm water runoff could contribute significantly to the acidity of streams due to fulvic acids. Their chelating ability also could enhance the mobilization of aluminum. How these weak organic acids affect or regulate pH or pH/aluminum toxicity in fish is also unknown. The paper discusses the importance of total solute concentration and the possible role specific anions play in mediating osmoregulation in acid- and acid/aluminum-stressed fish.