Improved barium removal and supersaturation depletion in wastewater by precipitation with excess sulfate

Abstract

Barium ions found in wastewaters cause incrustation on membrane separation equipment used in desalination systems. In this study barium removal by precipitation is addressed, considering excess sulfate addition as a means of reducing barium concentration in solution and depleting BaSO4 supersaturation. Precipitation is conducted with synthetic wastewater in semicontinuous mode. For low excess sulfate, an induction time of a few hours is observed. As the excess sulfate is increased and/or as barium sulfate seeds are added, precipitation proceeds within a few minutes. Besides, the excess sulfate improves barium ion removal due to the common-ion effect. Residual supersaturation ratios were found to lie within the range of 1.1–3. These values were associated with a fourth order dependency of the molecular growth rate with the supersaturation ratio. Calcium carbonate and calcium sulfate dihydrate were found to be ineffective heterogeneous seeds to barium sulfate precipitation. Calcium ions were found to inhibit BaSO4 precipitation, blocking the process at a high residual supersaturation ratio of 4–5. For a sufficiently large initial supersaturation, the solution approaches equilibrium after 180 min.