Influence of synthesis parameters on the properties of the composite adsorbents based on sepiolite and nano-zerovalent iron

Immobilizing nano-zerovalent iron particles (nZVI) on inert supporting materials of high surface area provides an easy operation for water contaminants removal and excellent adsorption/reduction ability of nZVI. In this dissertation, fibrous natural mineral sepiolite (SEP) with high specific surface area was used for nZVI immobilization, as well as partially acid-activated sepiolite (AAS) with even higher specific surface area. nZVI particles were loaded onto supports via Fe reduction by sodium borohydride. Different support/nZVI ratios (10:1, 5:1, 2.5:1 and 2:1) were applied in order to obtain composites with good nZVI dispersibility and high efficiency for the removal of Cd, CrO4 and dye C.I. Reactive Orange 16 from water. Besides, during the synthesis, the SEP/AAS suspension was treated by ultrasound probe for different period of time (10 or 30 min) to achieve an efficient de-aggregation of the sepiolite particles. Effect of temperature and contact time on adsorption was also investigated. Despite higher specific surface area of AAS than SEP, the dispersibility of nZVI particles was lower with AAS support, as acid activation created strong inter-fiber bonds, which could not be broken during ultrasound treatment. Prolonged ultrasound treatment improved de-aggregation of AAS, but caused shortening of the SEP fibers their re-aggregation. Good dispersibility of the spherical nZVI particles was achieved with SEP at the SEP:nZVI mass ratios ≥ 2.5:1 and with AAS only at the mass ratio AAS:nZVI = 10:1, for 10 min-ultrasound treatment. The presence of Fe in the composites was confirmed by X-ray diffraction (XRD) and differential-thermal/thermo-gravimetric (DTA/TGA) analyses. The results of DTA/TGA in air and nitrogen indicated that the composite with SEP contained more iron oxide formed by Fe oxidation during synthesis than the composite with AAS, at the same support/nZVI ratio. Therefore, SEP used as a support provided better nZVI dispersibility, but lower degree of oxidation during the synthesis was achieved using AAS. High values of point of zero charge of all the composites indicate the high basicity of the surface due to the presence of basic Fe–OH groups, as proven by infrared spectroscopy (FTIR analysis). X-Ray photoelectron spectroscopy (XPS) confirmed that the surface of nZVI was composed of oxidized iron species. Metallic iron was detected after sputtering, at a depth of 40 nm beneath the original surface. The content of Fe decreased after Cd adsorption as a result of iron oxidation during adsorption process. The XPS depth profile showed that cadmium was present at the surface and in subsurface region of the composite, whereas its concentration was higher on the surface. The adsorption capacities of the composites for Cd ions increased as the quantity of nZVI increased, regardless of the decrease in the specific surface area. The adsorption isotherms confirmed that the presence of SEP and AAS lowered the degree of agglomeration in comparison to pure nZVI, which provided higher adsorption capacities of nZVI in the composites than of pure nZVI. The results also showed that the prevention of both aggregation and oxidation during the synthesis was necessary for obtaining SEP/AAS–nZVI composite with high adsorption capacity. However, oxidation during adsorption was proven beneficial for Cd removal. Prolonged ultrasound treatment of the support suspension provided higher adsorption capacity for the SEP-based composite, regardless of worse textural and morphological properties while the opposite was found for AAS-based composite. According to the modeling of the adsorption data, the establishment of strong bonds between Cd and adsorbents sites of different energy and monolayer formation were proposed. Adsorption capacity of the composites obtained at support/nZVI ratio = 2.5:1 increased with temperature increase from 25 to 40 oC and then decreased with further temperature increase to 50 oC. The efficiency of CrO4 removal from water by the investigated composites was determined by the content of hosted nZVI. The efficiency of AAS–nZVI composites were higher than of SEP-nZVI, which shows that the degree of nZVI oxidation during synthesis is the factor that determines the adsorption capacity more than nZVI particles dispersivity. Possible mechanisms of CrO4 removal by the composites involve CrO4 reduction to Cr, followed by Cr adsorption or Cr(OH)3 and Fe(OH)3 coprecipitation, and electrostatic attraction between CrO4 and protonated functional groups of the composite. The efficiency of the composites with small content of nZVI for the removal of anionic dye C.I. Reactive Orange 16 was increased significantly when H2O2 was used to form Fenton reagens. On the other hand, the efficiency of composites with higher content of nZVI was much higher without H2O2, when reductive mechanism was dominant, especially at higher temperatures. The results showed that the in situ formed porous oxide layer on the nZVI surface was beneficial for the removal of Cd, while the detachment of the oxide layer from the nZVI particles was desirable for the reductive mechanism of the pollutant removal.