Adsorption of methylene blue dye in aqueous solutions using NiS₂-NiS nanocrystalline composite synthesized by mechanochemistry

Full text: NiS₂-Nis composite was successfully synthesized by mechanochemistry in argon atmosphere at room temperature starting from Ni34S66 powder mixtures. X-ray powder diffraction (XRPD) patterns showed that 9 h of milling are enough to allow the formation of the NiS₂ phase (83%), which is accompanied by a small fraction of the hexagonal NiS (17%) phase. Rietveld analysis indicates that the average crystallite size is about 22 nm for NiS₂ and 12 nm for NiS. Transmission Electron Microscopy (TEM) analyses confirms the nanometric size of the crystalline domains and reveal that the nanocrystals are aggregated in large particles of hundreds of nanometers. The prepared material slowly reacts with air through a complex phase transition scene, being mainly converted to nickel sulfate hexahydrate after long term storage (tens of months) as given by XRPD analyses using synchrotron radiation and conventional diffractometer. The fresh and aged composite was used for the adsorption of methylene blue (MB) from aqueous solutions. The effect of different amounts of NiS2–NiS nanocrystalline composite, adsorption time for dye removal and dye concentration were evaluated, as well as the effect of aging in the adsorption process. Both the fresh and aged samples showed good adsorption kinetics of MB from aqueous solution, and about 40% of the dissolved dye is adsorbed in less than 1 min when 1.88 g L^-1 of adsorbent is used, reaching 94% after 120 min. Adsorption of MB was faster in the aged material probably due to its higher surface area when compared to the fresh sample. (author)