Abstract:
The mining of valuable minerals from wastewater streams is attractive as it promotes a
circular economy, wastewater beneficiation, and valorisation. To this end, the current study evaluated
the rapid removal of aqueous Cr(VI) by polycationic/di-metallic Fe/Al (PDFe/Al) adsorbent
recovered from real acid mine drainage (AMD). Optimal conditions for Cr(VI) removal were 50 mg/L
initial Cr(VI), 3 g PDFe/Al, initial pH = 3, 180 min equilibration time and temperature = 45 C.
Optimal conditions resulted in 95% removal of Cr(VI), and a maximum adsorption capacity of
Q = 6.90 mg/g. Adsorption kinetics followed a two-phase pseudo-first-order behaviour, i.e., a fast
initial Cr(VI) removal (likely due to fast initial adsorption) followed by a slower secondary Cr(VI)
removal (likely from Cr(VI) to Cr(III) reduction on the surface). More than 90% of adsorbed Cr(VI)
could be recovered after five adsorption–desorption cycles. A reaction mechanism involving a rapid
adsorption onto at least two distinct surfaces followed by slower in situ Cr(VI) reduction, as well
as adsorption-induced internal surface strains and consequent internal surface area magnification,
was proposed. This study demonstrated a rapid, effective, and economical application of PDFe/Al
recovered from bona fide AMD to treat Cr(VI)-contaminated wastewater.
