Pure and Applied Chemistry

Abstract: A former open-cast pit was used as the disposal site for wastewater from lignite low-temperature coking processes. The formed lake with a strong stratification had a final volume of 2 × 10⁶ m³. Its water was highly toxic and of dark brownish color (transparency about 3 cm). Traditional remediation methods were too expensive, and it was therefore necessary to develop a new, ecologically and economically acceptable remediation strategy. This strategy is based on a first precipitation step of the dark-brown-colored macromolecules as iron-humate flocs at pH 4.0. These macromolecules are similar to fulvic/humic acid and are formed abiotically by autoxidation of polyphenolic compounds. Thus, the dissolved organic matter (DOM, about 280 mg/l) was decreased by about 50 % and transparency improved. After a neutralization step with 2200 m³ of 20 % calcium carbonate suspension and a subsequent "fertilization" with 0.75 % phosphoric acid (3 × 0.8 m³) "enhanced natural attenuation" was initiated. Due to higher transparency, intense growth of algae ("blooming") and bacteria started in the upper zone of the lake. In winter, algae sedimented and one year after the precipitation step, the water of the surface zone was almost clear (transparency 1 m, after 10 years over 3 m) and odorless.