ADSORPTIVE REMOVAL OF Ni 2+ IONS FROM AQUEOUS SOLUTIONS BY NODULAR SEPIOLITE (MEERSCHAUM) AND INDUSTRIAL SEPIOLITE SAMPL
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ADSORPTIVE REMOVAL OF Ni2+ IONS FROM AQUEOUS SOLUTIONS BY NODULAR SEPIOLITE (MEERSCHAUM) AND INDUSTRIAL SEPIOLITE SAMPLES FROM ESKIŞEHIR, TURKEY İLKER KIPÇAK *, EMEL KURTARAN ERSAL, AND MINE ÖZDEMIR 1
Department of Chemical Engineering, Eskişehir Osmangazi University, 26480 Eskişehir, Turkey
Abstract—Heavy metals in the environment are a problem due to their toxicity and bioaccumulation. Adsorptive removal of heavy metals by clay minerals has garnered increasing attention due to the abundance, low cost, and exceptional physicochemical properties of the clays. The purpose of the present study was to investigate the utilization of two Turkish sepiolite samples, nodular sepiolite (NS) and industrial sepiolite (IS), as adsorbents in removing Ni2+ ions from aqueous solutions. The specific surface areas of NS and IS are 182.19 m2 g–1 and 63.78 m2 g–1, respectively. The effects of adsorbent dosage, initial pH, contact time, initial concentration, and temperature on the adsorption of Ni2+ ions onto the sepiolite samples were investigated using a batch method. The optimum adsorbent dosage was determined as 0.6 g/50 mL of adsorbent and the optimum pH value was 6.0, for both sepiolite samples. The adsorption process obeyed the Freundlich isotherm model (KF: 4.89–9.73 mg1–1/n L1/n g–1 for NS and 4.27–6.42 mg1–1/n L1/n g–1 for IS) and the pseudo-second order kinetics model (k2: 0.0049–0.0397 g mg–1 min–1 for NS and 0.0688–0.1195 g mg–1 min–1 for IS). The adsorption process was spontaneous and endothermic, and the randomness increased. The samples exhibited large adsorption capacities through three cycles of adsorption-desorption tests. The results revealed that the sepiolite samples are promising as cost-effective adsorbents for the removal of Ni2+ ions from aqueous solutions. Keywords—Adsorption . Isotherm . Kinetics . Ni removal . Sepiolite . Thermodynamics INTRODUCTION Sepiolite is a fibrous clay mineral comprising tetrahedral and octahedral oxide/hydroxide sheets. The chemical formula is Mg8Si12O30(OH)4(H2O)4.8H2O. It has molecule-sized channels (0.37 nm×1.06 nm) oriented along the longitudinal direction of the fiber (Brauner and Preisinger 1956). A microscopic and sometimes macroscopic level of fibrous morphology differentiates the mineral from laminar clay minerals. Its fibrous morphology, small particle size, and intra-crystalline channels result in large specific surface areas. The BET specific surface area for sepiolite was reported to be in the range ~80 to 350 m2 g–1 (Suarez and Garcia-Romero 2012). Such a large BET surface area allows sepiolite to adsorb ions, polar liquids, water, and also molecules such as drugs or insecticides. Oxygen atoms in the tetrahedral sheet, water molecules coordinated with Mg2+ ions at the edge of the structure, and silanol groups resulting from the breakdown of Si–O–Si bonds are active adsorption centers on the sepiolite surface and provide a large adsorption capacity to the mineral (Yebra-Rodriguez et al. 2003). Sepiolite may appear in two forms: α-sepiolite which occurs as large bun
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