Effect of pH on Speciation and Toxicity of Aluminum to Rainbow Trout (Salmo gairdneri)

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Abstract
Alum (aluminum sulphate) is used in many water treatment plants as a flocculant. The resultant settled floe (called alum sludge) may be discharged into receiving waters. This study reports the effect of pH on the speciation of released aluminum (Al) from fresh sludge and Al uptake and toxicity to fish in river water in the presence of abiotic substrates enclosed in dialysis sacs. This study shows that freshly precipitated alum sludge holds Al effectively within a wide range of pH (4.50–10.0) and that abiotic substrates compete with the biota as represented by fish for Al. Fish mortalities observed could be due to colloidal particles and acid- and Al-induced stress at pH 4.50, to colloidal particles at pH 6.00, and to high alkalinity at pH 10.0. At pH 7–9, Al in water, present essentially as filterable nonexchangeable Al (FNEX-Al), was not lethal to fish.

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