The waste sulfuric acid lake of the TiO2-plant at Armyansk, Crimea, Ukraine. Part II. Modelling the chemical evolution with PHRQPITZ

The past and future evolution of a salt lake near Armyansk (Crimea, Ukraine), in which waste sulfuric acid has been discharged since 1969, is modelled using the equilibrium aqueous chemistry model PHRQPITZ. The initial lake is represented by a simplified salt solution, about 1.4 times concentrated average seawater. Since the waste acid contains iron and other metals, the effluent is modelled by an Fesingle bondSO4 solution of pH 1.04, containing minor amounts of Na and Mg. The PHRQPITZ model is adapted to describe precipitation of the trivalent Fe minerals natrojarosite and ferrihydrite. The yearly input of effluent constituents is assumed constant, and the input of water from the effluent is compensated by the yearly excess of evaporation over rainfall. According to the model, calcite dissolution from the underlying calcareous clays nearly buffers the increase in acidity of the lake by the effluent, maintaining pH at a value around 1.7, up to 16 a of operation (1986). Increase of Fe, Na, and SO4 is buffered by precipitation of natrojarosite and gypsum. At the direct contact with the sediments a relatively impervious Fe (hydr)oxide layer develops. This hampers further reaction with the sediments, which effectively stops around 1992, when pH has reached a value of 0.85. It is predicted that with further increase in concentration, either through continued acid input or by evaporation of the acid lake, dissolution of previously precipitated natrojarosite first acts as a relative H buffer, followed by resumed precipitation of natrojarosite when the potential H increase is counterbalanced by the SO4/HSO4 association equilibrium. The concept of buffering in its strict sense does not apply, as conditional equilibrium constants for the extremely acid and concentrated conditions in the lake are far from constant and may differ significantly from their theoretical value.