Introduction Antimony mainly exists in the oxidation states of Sb(III) and Sb(V), where Sb(V) is the predominant species and exists as Sb(OH)6- in oxidic environments. The environmental contamination of Sb has received much attention due to the toxicity and adverse effect on the hematic, gastrointestinal, and respiratory systems of human-beings. Treatment of antimony-contaminated water is a prominent topic. Adsorption is considered as one of the most attractive and practical approaches for Sb remediation in aqueous media because of its operation simplicity, cost-effectiveness, high efficiency, and less sludge production. Magnetite (Fe3O4) as one of the most promising magnetic materials is extensively investigated. However, some previous studies showed that the removal capacity and reactivity for pollutants of the iron-based magnetic nanoparticles are size dependent. A high surface free energy results in a greater aggregation for nanoparticles. The formation of aggregates can decrease the surface area of the magnetic nanoparticles, thereby restricting the treatment performance for contaminants. Recent technologies are developed using some porous materials as mechanical supports to enhance the dispersibility of magnetic nanoparticles. Herein, acid-treatment sepiolite was prepared as support materials, and acid-sepiolite supported magnetite nanocomposite powder was obtained for Sb(V) adsorption. In addition, the effects of initial pH value, reaction time, initial mass concentration of Sb(V), adsorbent dosage, and coexisting ions (SO42-, Ca2+, Mg2+, Fe3+, and humic acid) on the adsorption of Sb(V) were also investigated. Methods For acid-treatment of sepiolite, all chemical reagents used were of analytical grade. Acid-treatment sepiolite was firstly prepared, i.e., 1.0 g of sepiolite was added into 100 mL of 1.2 mol/L HCl solution and reacted at 60 ℃. After 12 h of stirring, the suspension was centrifuged, and washed with deionized water until the pH value was 7.0. Finally, the precipitate was collected and dried at 50 ℃. For preparation of acid-sepiolite supported magnetite nanocomposite powder, the acid-sepiolite supported magnetite nanocomposite powder was prepared by a microwave-assisted reflux method. In the synthesis process, 1 mmol (0.27 g) of FeCl3·6H2O and 0.5 mmol (0.10 g) of FeCl2·4H2O were dissolved in 20 mL of ethylene glycol, and then 0.25 g of acid-treatment of sepiolite was dispersed in the solution under ultrasonication to form solution A. Afterwards, 0.16 g (4 mmol) of NaOH was dissolved in 3 mL of deionized water to form solution B. Solution B was introduced into solution A in a 100 mL round-bottomed flask. Subsequently, the round-bottomed flask with the reactants was equipped on the microwave reactor, and irradiated for 20 min at 80% of the full power (640 W). After cooling to room temperature, the precipitates were collected and washed with alcohol for several times, and finally dried in a vacuum oven at 60 ℃ for overnight. A stock solution containing Sb(V) (600.0 mg/L) was prepared via dissolving K[Sb(OH)6] with deionized water, and a series of solutions used were prepared via diluting the stock to the desired concentrations-actual concentrations measured by inductive coupled plasma-atomic emission spectroscopy (ICP-AES). In a typical adsorption run, 50 mg of adsorbent was added into 50 mL of solution containing 60.0 mg/L Sb(V) with constantly stirred at 180 r/min at 25 ℃. The initial pH of the solution was adjusted with HCl and/or NaOH solutions before the addition of adsorbent. The effect of pH value, contact time, adsorbent dosage, and initial concentration of Sb, coexisting anions (Ca2+, Fe3+, Mg2+, SO42-, and humic acid) on the Sb adsorption were investigated in the same procedures. After adsorption, the residual level of Sb in solution was determined by ICP-AES. Results and discussion After the microwave irradiation, the prepared Acid-Sep-Fe3O4 composite powder maintains a nanorod-like structure of sepiolite, and massive magnetite nanoparticles appear on the surface of the powder. The corresponding energy dispersive X-ray spectroscopy and X-ray diffraction patterns indicate the preparation of Acid-Sep-Fe3O4 composite powder. The BET surface area and total pore volume of the composite powder are 269.00 m2/g and 0.96 cm3/g, respectively. Moreover, the magnetic saturation of the powder is 10.4 emu/g. The obtained composite powder has high surface area and good magnetic nature, which are beneficial for Sb(V) adsorption and adsorbent recovery. The Sb(V) adsorption by nanocomposite powder is dependent on the pH value with the maximum adsorption under acidic conditions and decreases with the increase of pH value. The adsorption percentage of Sb(V) increases with the increase of adsorbent dosage, and over 85% of Sb(V) is removed at the adsorbent dosage of 50 mg/50 mL. The prepared powder has a great adsorption rate to Sb(V) due to its high specific surface area. 81.4% of Sb(V) is adsorbed after 240 min of agitation. Moreover, the maximum removal capacity of the powder toward Sb(V) is 92.3 mg/g, which is greater than that of the nanoscale magnetite (79.2 mg/g), acid-treatment sepiolite (42.3 mg/g), and raw sepiolite (35.6 mg/g). The results indicate that the adsorption capacity of magnetite toward Sb(V) is enhanced after the support of acid-treatment sepiolite. The presence of SO42-, Ca2+, Mg2+, and humic acid has a negligible effect on the adsorption of Sb(V), while the Sb(V) sorption process is enhanced with the addition of Fe3+, due to the formation of Fe precipitates. The adsorption behavior of nanocomposite powder toward Sb(V) fits the pseudo-second-order kinetic model and the Langmuir model. The XPS analysis reveal that the adsorption of composite powder toward Sb(V) mainly occurs via electrostatic interaction followed by chemical reactions. Conclusions Acid-treatment sepiolite supported Fe3O4 nanocomposite powder was prepared via acid-treatment of sepiolite and microwave-assisted reflux method with a natural nanorod-like sepiolite. The prepared composite powder exhibited a superior adsorption capacity toward Sb(V) with the maximum adsorption capacity of 92.3 mg/g, which was greater than that of the unsupported Fe3O4 nanoparticles (i.e., 79.2 mg/g), acid-treatment sepiolite (i.e., 42.3 mg/g), and raw sepiolite (i.e., 35.6 mg/g). The adsorption behavior of Sb(V) followed the pseudo-second-order kinetic model and Langmuir model. The Sb(V) adsorption by magnetic nanocomposites mainly occurred via electrostatic interaction followed by chemical reactions. The prepared acid-treatment sepiolite supported Fe3O4 nanocomposite powder could be used as an ideal adsorbent for Sb(V) due to the facile fabrication, efficient adsorption performance and easy magnetic separation.