Insights into manganese removal from mine water by Chlorella vulgaris

Microalgae have been extensively applied on bioremediation processes and biosorption is usually considered the main mechanism describing metal removal from wastewaters. Nevertheless, the water chemistry is modified during algal growth and the increase in both pH and oxygen concentrations result in metal precipitation. In this regard, the aim of the current study is to investigate how these two phenomena promote metal removal from waters, applying Chlorella vulgaris and high concentrations of manganese (50 mg/L)—one of the most common and difficult metals to be removed from fresh- and wastewaters alike. Triplicate experiments were carried out at 25 °C and 150 min −1 under 24 h/day of artificial light and revealed that the growth of C. vulgaris was associated with a reduction in Mn 2+ concentration to values below 1 mg/L, as the pH changed from 6.2 to 9.6, within 14 days. Biosorption as a removal mechanism appears not to be relevant as the manganese uptake by the C. vulgaris cells was actually reduced from 10.9 mg/g (10 min) to 0.9 mg/L, within 24 h of testing, indicating desorption of the manganese initially adsorbed. Furthermore, the formation of brown precipitates containing Mn 3+ /Mn 4+ oxy-hydroxides was observed, which was confirmed upon reaction with leucoberbelin blue (LBB) solutions. In addition, no evidence of the participation of extracellular materials on Mn 2+ removal was observed. These outcomes support the hypothesis that an increase in pH, resulting from the growth of C. vulgaris , is responsible for increasing pH to values above 8.0, values in which Mn 2+ oxidation is catalysed.