Novel adsorbent synthesized from red mud and acid mine drainage for enhanced contaminant removal: industrial waste transformation, adsorbent performance and metal(loid) removal mechanisms

Antimony (Sb), arsenic (As), and lead (Pb) are toxic metal(loid)s that can cause serious environmental pollution. The above emphasizes the need for a development of efficient technologies for metal(loid) removal, especially from low-pH environments due to the increased mobility of the contaminants under acidic conditions. Herein, for the first time, removal of Sb(V/III), As(V/III), and Pb(II) from acidic waters is described, using low-cost adsorbents synthesized from red mud (RM; a solid waste from the alumina industry) under the catalysis by acid mine drainage (AMD; acidic effluents from mines). The reactions between RM and AMD were studied to describe Fe/S/Ca/Al/Si speciation transformations during the adsorbent formation. In addition, Sb/As/Pb adsorption behavior and the relationship between adsorbent properties and Sb/As removal efficiencies were investigated. The maximum removal capacities for Sb(V), Sb(III), As(V), As(III), and Pb(II) reaching 1,637.8, 80.2, 109.2, 16.4, and 518.7 mg/g, respectively. The primarily Ca2+ from the Ca-bearing compounds (determined in RA2, synthesized under SRM/LAMD 2:1) contributed to Sb(V) removal (via precipitation of CaSb2O5(OH)2 and formation of Sb-bearing precipitate, with silicate gel nanospheres securing high stability of the immobilized Sb). In the As immobilization process, Fe(III) (hydr)oxides (formed in RA4, synthesized under SRM/LAMD 4:1) such as goethite (α-FeOOH) and bernalite (Fe(OH)3) played a dominant role. Convertible sulfates (e.g., CaSO4 and Al2(SO4)3) in RA10 (synthesized under SRM/LAMD 10:1) ensured a high removal capacity for Pb(II) across a wide pH range (2 to 7) via PbSO4 precipitation. In contrast, the formation of PbCO3 and Pb3(CO3)2(OH)2 was the main mechanism for Pb(II) removal by unamended RM at pH 5.0, while no contaminant removal capacity was observed at pH 2.0.