Sorption of Chromium(VI), Cadmium(II) Ions and Methylene Blue Dye by Pristine, Defatted and Carbonized Nigella sativa L. Seeds from Aqueous Solution

Present study reports on the sorption study of chromium(VI), cadmium(II) ions and methylene bluedye by pristine, defatted and carbonized Nigella sativa L. seeds from aqueous solution. The removalof oil from pristine Nigella sativa L. (PNS) seeds was carried out by defatting the Nigella sativa withacetone and N,N-dimethylformamide and then labelled ANS and DNS, respectively. Thereafter thedefatted ANS and DNS adsorbents were carbonized at 600 ºC for 2 h under nitrogen and labelled asCANS and CDNS. The results of pristine, defatted and carbonized seeds were compared. The removalof Cr(VI), Cd(II) and methylene blue dye from aqueous solutions was investigated by varying adsorbateconcentration, solution pH, reaction contact time and temperature of the solution. The SEM imagesindicated that the surface morphology of PNS was irregular, whilst ANS and DNS had pores andcavities. CANS and CDNS was heterogeneous and had pores and cavities. FTIR spectroscopy showedthat the adsorbents surfaces had bands that indicated a lot of oxygen containing groups. The pH of thesolution had an influence on the removal uptake of Cr(VI), Cd(II) and methylene blue. The sorption ofCr(VI) decreased when pH of the solution was increased due to different speciation of Cr(VI) ionswhilst the removal of Cd(II) and methylene blue increased when solution pH was increased. Pseudofirst order kinetic model well described the adsorption of Cr(VI), Cd(II) and methylene blue ontoPNS. On the other hand, the kinetic data for ANS, CANS, DNS and CDNS was well described bypseudo second order. Furthermore, the removal mechanism onto PNS and ANS was better describedby Freundlich multilayer model. The CANS, DNS and CDNS fitted Langmuir monolayer model.Thermodynamic parameters indicated that the sorption processes of Cr(VI), Cd(II) and methyleneblue was endothermic and effective at high temperatures for all adsorbents. The ΔSº and ΔHº hadpositive values this confirmed that the sorption of Cr(VI), Cd(II) and methylene blue onto all adsorbentswas random and endothermic, respectively. The values of ΔGº confirmed that the sorption of Cr(VI),Cd(II) and methylene blue on all adsorbents was spontaneous and predominated by physical adsorptionprocess. The CANS had highest adsorption capacity of 99.82 mg/g for methylene blue, 96.89 mg/g forCd(II) and 87.44 mg/g for Cr(VI) followed by CDNS with 93.90, 73.91 and 65.38 mg/g for methyleneblue, Cd(II) and Cr(VI), respectively. The ANS capacities were 58.44, 45.28 and 48.96 mg/g whilstDNS capacities were 48.19, 32.69 and 34.65 mg/g for methylene blue, Cd(II) and Cr(VI), respectively.PNS had the lowest sorption capacities at 43.88, 36.01 and 19.84 mg/g for methylene blue, Cd(II) andCr(VI), respectively.