Comprehensive evaluation on a prospective precipitation-flotation process for metal-ions removal from wastewater simulants

Highlights: • Efficient removal for metal ions is obtained by enhanced precipitation flotation. • Low metal-ion concentration limits precipitate efficiency and flotation removal. • Generation and regulation of precipitate particle is controlled by humics and Fe³⁺. • The regulated marco-size precipitate is of higher fractal dimension. -- Abstract: Toxic metal pollutants threaten water environment. It exists undesirably metal-ion concentration limits with conventional precipitation flotation. An enhanced precipitation flotation system focusing on efficient removal for bivalent metal-ions was researched. The system involved the addition of humics and Fe³⁺ to generate and regulate the precipitates. The characteristics of precipitates were investigated by particle analysis, conditional stability constants and DLVO theory calculations, and SEM&TEM imaging. The results reveal that metal-ions chelate with humics at low metal-ion concentration, with generating the limited micro-size precipitates of <2.0 μm, fractal dimension of 1.60–1.80 and precipitate efficiency of <91.00%. By adding trivalent Fe³⁺, the macro-size precipitates are obtained with particle size of approximate 10.0 μm, fractal dimension of 1.50–1.60, and nearly-total flotation removal of precipitate. The chelating interaction of Fe³⁺ with humics is the mainly regulating mechanism, which could enhance the conditional stability constants and the precipitate efficiency of metal-ions at low concentration. The desired precipitate particles are finally obtained by breaking the limitations of metal-ion concentration. Finally, the flotation removal of metal-ions from single or mixed solutions is respectively 99.10 ± 0.10% for Cu²⁺, 99.60 ± 0.10% for Pb²⁺, and 94.30 ± 0.30% for Zn²⁺. Therefore, the enhanced precipitation flotation process is an efficient purification approach for metal-containing wastewaters.