Removal of sulfate ions by dissolved air flotation (DAF) following precipitation and flocculation

• An Al/SO42 − molar ratio of 4:1 was found best for effective sulfate precipitation with polyaluminum chloride.
• Flocculation was performed using 20 mg·L− 1 cationic polyacrylamide.
• Dissolved air flotation (DAF) assisted by nanobubbles showed a high flotation kinetics (k = 4.03 min− 1).
• Results were validated using a coal acid mine drainage after metal ions precipitation.
• DAF has potential for the treatment of sulfate-bearing effluents, at high superficial rates.

The removal of sulfate ions from wastewater is an environmental challenge faced by several industrial sectors, such as the mining, metallurgical, chemical and petrochemical industries. Most existing options are inefficient and costly, particularly for sulfate-bearing acid mine drainage (AMD; coal and metal sulfides). This work focused on the precipitation of sulfate with polyaluminum chloride at pH 4.5 at the bench scale and their separation by flotation. However, these hydrophilic precipitates did not float via dissolved air flotation (DAF) and needed to be flocculated with cationic polyacrylamide. The best removal of sulfate-sodium salts from solution was obtained using an Al/SO42 − molar ratio of 4:1 and 20 mg·L− 1 of polymer flocculant. The sulfate-bearing flocs were readily removed from the aqueous solutions by microbubbles (MBs, 30–100 μm) and nanobubbles (NBs, 150–800 nm). The separation was very rapid and followed a first-order flotation kinetics model with a high rate constant of 4.1 min− 1. The results were validated using AMD generated by a coal mine with a sulfate concentration of 1753 mg·L− 1, and the anion concentration was rapidly reduced to below the World Health Organization (WHO) standard of 500 mg·L− 1. Attempts to improve the removal efficiency with sodium oleate flotation collector and conditioning with NBs were not successful. The maximum removal percentage (80–82% of the feed content) appears to be limited by the efficiency of the DAF process and the chemical equilibrium of the precipitates, which leaves some soluble sulfate in solution. Bubbles readily attach to the flocs and become entrained and/or entrapped in the flocs, creating aerated flocs. Because all of these mechanisms operate simultaneously, the flotation of the flocs is very rapid, as indicated by the high kinetics rate constant. We concluded that the DAF of sulfate-loaded flocs has potential for the treatment of voluminous sulfate-bearing effluents, including coal AMD, at high superficial rates.

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