Antimony (Sb) is classified as one of the 13 priority pollutant metals and is increasingly detected in the environment mainly due to various mining activities and industrial applications. Sb is a non-essential trace metal in human body and similar to arsenic (As) in chemical properties and toxicity, however, little was known about environmental behaviors of Sb and associated pollution cleanup. Owing to the unique chemical properties of iron, many researches reported on the Sb removal by chemosynthesized iron oxyhydroxides or Fe/Mn bimetal oxides, but few was about natural particles. Secondary iron minerals (SIMs), such as schwertmannite, jarosite and goethite, commonly occur in acid mine drainage environments and are considered an important sink for toxic metals. In this study, two natural sediments from the acid coal drainage, including the newborn secondary iron mineral (n-SIM) and mature one (m-SIM), were used to remove Sb(III/V), and the adsorption behaviors and surface structures of Sb(III/V) on secondary iron minerals were comprehensively investigated.
The natural sediments m-SIM and n-SIM were mainly composed of Fe, O and S, and their EDS-mapping analyses indicated that the morphologies of m-SIM and n-SIM were dependent on the distribution of O. The results of XRD analyses implied that both similar crystalline and amorphous iron oxide minerals coexisted in these natural sediments. However, m-SIM and n-SIM had different microstructures. The particle size of m-SIM was larger than that of n-SIM, and m-SIM was spheroids with a close-packed arrangement and regular sharp, while n-SIM had two morphologies: sea urchin-like and pompon-like with a random loose arrangement and non-uniform sizes. Batch experimental results showed that the effect of solution pH on Sb(III) adsorption was more remarkable than that of Sb(V) and the optimal pH was 7 and 3, respectively. The adsorption behavior of Sb(III/V) on n-SIM and m-SIM were spontaneous, endothermic, controlled by chemical adsorption and followed the monolayer Langmiur isotherm model. m-SIM had a larger adsorption capacity to Sb(III) than Sb(V), while n-SIM had a reverse trend but its maximum adsorption capacities of Sb(III) (219 mg/g) and Sb(V) (366 mg/g) were higher than those of m-SIM. The presence of Na+ promoted the Sb(III/V) adsorption while Ca2+ suppressed Sb(V) adsorption on SIMs. Characterization techniques of FTIR, Raman and XPS indicated the changes of functional groups, elemental compositions and binding energy regions after the adsorption of Sb(III/V) on SIMs, and X-ray absorption near edge structure (XANES) analyses showed that Sb(V) was the dominant species adsorbed on SIMs. These results revealed that the adsorption mechanism of Sb(III/V) on SIMs involved in the electrostatic force, coordinated action, oxidation and ion exchange reaction. This study contributes to better understanding for Sb adsorption mechanism on natural secondary iron minerals, and also provides potential materials for environmental remediation.