Fe(II) bio-oxidation mediates red mud transformations to form Fe(III)/Al (hydr)oxide adsorbent for efficient As(V) removal under acidic conditions

Acidic wastewater rich in arsenic (As) is one of the most hazardous types of liquid waste and has caused severe environmental pollution over the past few decades due to a lack of cost-effective and environment-friendly disposal technologies. Here, a new strategy for the efficient immobilization of As(V) under acidic conditions is
presented. The approach uses Fe(II) bio-oxidation by the acidophilic bacterium Sulfobacillus (Sb.) thermosulfidooxidans to transform red mud into a low-cost Fe(III)/Al (hydr)oxide adsorbent for As(V) removal. The mechanisms of the formation of the adsorbent and subsequent adsorption of As onto its surface were investigated, together with the relationship between the adsorbent structure and As adsorption efficiency. The [Fe(II)]ini greatly affected the growth of Sb. thermosulfidooxidans, Fe/Al speciation, and surface structure of the adsorbent, as well as the As removal capacity. The aluminosilicates in RM provided numerous sites for Fe(III) adsorption, inhibiting the transformation of amorphous Fe(III) (hydr)oxides to crystalline Fe(III) minerals. The newly formed Fe(OH)3 and schwertmannite played a dominant role in As(V) immobilization. The transformation of the amorphous Fe(III) (hydr)oxides to more crystalline Fe(III) minerals, caused by an excess of Fe(II) (2 ≤ [Fe(II)]ini ≤ 4 g/L), resulted in a significant decrease in the As adsorption capacity (reduce ratio: 29.3–53.5%). The Fe(III)/Al (hydr)oxide adsorbent bio-synthesized under [Fe(II)]ini of 1 g/L showed the highest As(V) adsorption efficiency (89.9 mg/g; at pH 2.0), which was significantly higher than that of the original RM under the same conditions (5.7 mg/g). The adsorption process followed the pseudo-second-order kinetic and Langmuir isotherm models.