Study on removal of heavy metal ions from electroplating wastewater using red mud leachate

To address the dual challenges of heavy metal contamination in electroplating wastewater and red mud stockpiling, this study developed a novel red mud-based colloid (RM100) from red mud washing leachate. The colloid facilitates the efficient, simultaneous removal of Pb(II), Cu(II), Ni(II), Zn(II), and Ag(I) ions from electroplating wastewater. Characterization analyses revealed that RM100 exhibits excellent dispersion (D_mean = 180.99 nm), a highly negative surface charge (ζ = -32.03 mV), and abundant surface active sites. Heavy metal removal experiment at a 1:1 vol ratio (RM100 colloid to wastewater) demonstrated that over 90% of the mixed heavy metal ions were removed within 1 min, and complete flocculation and sedimentation were achieved within 10 min. The high removal efficiency is attributed to a dual synergistic mechanism: (1) negatively charged surface enriches metal cations through electrostatic attraction; (2) free alkali and chemically bound alkali collectively generate an alkaline environment that promotes metal hydroxide precipitation, wherein gel particles serve as heterogeneous nucleation sites, facilitating rapid solid-liquid separation. Despite significant competitive antagonism among ions (affinity order: Pb/Cu > Ag > Zn/Ni), RM100 exhibited preferential adsorption towards the highly toxic Pb(II) and Cu(II). This method directly utilizes red mud leachate and is characterized by a simple process and low cost. This technology achieves the simultaneous, efficient removal of multiple metals and rapid sludge settling within 30 min, offering a novel and industrially viable strategy for both red mud valorization and advanced electroplating wastewater treatment.