Environmental context Antimony is classified as a priority pollutant and is increasingly detected in the environment as a result of various anthropogenic activities. Natural secondary iron minerals are important sinks for many toxic elements. We found that antimony promotes structural transformations in secondary iron minerals, and that antimony is then further immobilised by the newly formed iron mineral phases. Secondary iron minerals (SIMs), naturally formed in acid mine drainage (AMD), can serve as sinks to remove antimony (Sb) from the environment. In this study, we investigated the relationship between the structural characteristics and Sb(III)or Sb(V)sorption behaviours of two different SIMs (neo-formed SIM (n-SIM) and mature SIM (m-SIM)). The results showed that both m- and n-SIMs were composed of mixed crystal phases, mainly schwertmannite, jarosite, goethite and ferrihydrite. Despite similar elemental compositions, they had different microstructures. m-SIM was a regular spheroid with a close-packed arrangement, while n-SIM was an incomplete irregular spheroid with ordered channels and a random loose arrangement. The initial Sb(III)or Sb(V)sorption rate order was n-SIM > m-SIM. The maximum sorption capacities of n- and m-SIMs were 219.8 (Sb-III) or 366.3 (Sb-V) mg g(-1)and 160.3 (Sb-III) or 114.9 (Sb-V) mg g(-1)respectively, and n- and m-SIMs could both effectively remove Sb from natural wastewaters, while trace Sb levels remained. Sb immobilisation on SIMs occurred as a rapid endothermal and homogeneous sorption process. After Sb sorption, the SO(4)(2-)and total Fe((aq))concentrations in the sample solutions significantly changed. X-ray diffraction (XRD) and Raman analyses confirmed that n- or m-SIM had transformed in the presence of Sb, even without Fe-(aq)(II), which indicated an important SIMs transformation-Sb immobilisation relationship. These findings are valuable for better understanding the Sb behaviours associated with natural SIMs and the short-term fate of Sb in the environment.

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