Harvesting of freshwater microalgae Scenedesmus obliquus and Chlorella vulgaris using acid mine drainage as a cost effective flocculant for biofuel production

Graphical abstract: Schematic presentation of coagulation/flocculation of microalgal biomass using AMD. - Highlights: • AMDs containing high Fe²⁺/Al³⁺ improved the settling kinetics of microalgal biomass. • The highest k₂ value was 40 × 10⁻² L mg⁻¹ min⁻¹ for C. vulgaris with AMD (1). • With AMD (2), k₂ was 4.0 × 10⁻² L mg⁻¹ min⁻¹ for both C. vulgaris and S. obliquus. • The highest FE (93%) and CF (29) for C. vulgaris was achieved with AMD (1). • AMD (1) removed 99.80% of Fe³⁺ and 99.99% of Al³⁺ from the supernatant. - Abstract: Development of a low-cost harvesting technology could be an effective approach for making microalgal biofuel commercially feasible. The use of acid mine drainage (AMD) to coagulate/flocculate biomass is a cost-effective strategy for addressing this challenge. Here, settling kinetics, flocculation efficiency (FE), and concentration factor (CF) of two morphologically different microalgae species, Scenedesmus obliquus and Chlorella vulgaris, were investigated with respect to AMD dosage (5% and 10%) and medium pH (7 and 9). AMD was collected from two different sites, AMD (1) and AMD (2), and increasing its dosage to 10% improved the settling rate, FE, and CF of the floc. At 10% AMD (1) dosage and pH 9, the highest rate constants (k₂) for the second order equations were 6.65 × 10⁻² and 40 × 10⁻² L mg⁻¹⋅min⁻¹ for S. obliquus and C. vulgaris, respectively; at 10% AMD (2), k₂ values were 4.22 × 10⁻² and 4.76 × 10⁻² L mg⁻¹ min⁻¹, respectively. Similarly, FE/CF values were 89%/25 for S. obliquus and 93%/29 for C. vulgaris with 10% AMD (1); and 81%/17 and 79%/17, respectively, with 10% AMD (2). AMD effectively removed 99.80% of Fe³⁺, 99.99% of Al³⁺, 94% of Ca²⁺, 84% of Mg²⁺ and all of Na⁺ and K⁺ ions from the supernatant. The results of kinetics, EF, and CF measurements indicate that AMDs, naturally rich in iron and aluminum ions, could provide a feasible option for the harvesting of microalgal biomass for biofuel generation.