nanoparticles of various size (~ 25.8 to 49.7 nm), obtained by a facile oxidation process using Ni(NO 3 ) 2 .6H 2 O, NaOH and sodium hypochlorite as precursor materials at various temperatures (0 o , 25 o , 50 o and 70 o C), is found to remove toxic Cr(VI) from aqueous solution (20 g L -1 ). The structural, morphological, surface charge and chemical compositions of the synthesized samples were characterized by XRD, TEM, zeta potential and EDX respectively. Adsorption capacity is found to be strongly dependent on the size and surface heterogeneity of the synthesized particles and the plausible mechanism for such significant adsorption efficacy is attributed to the sorbate -sorbent electrostatic interaction and shielding of Cr(VI) ions. The adsorption mechanism fits with the Langmuir isotherm model with maximum 60% Cr(VI) removal capacity (20.768 mg g -1 (calculated) and 20.408 mg g -1 (predicted from isotherms)) corresponding to Ni 2 O 3 nanoparticles, prepared at 70 0 C in 3 hours at room temperature. Thermodynamic parameters, obtained from fitting, demonstrate that the adsorption process being endothermic in nature follow pseudo-secondorder kinetic model. The spontaneity of the adsorption process gets reduced with increasing particle size. pH of the solution is observed to have a remarkable effect on the adsorption, giving maximum adsorption at pH = 6. .
Please do not adjust marginsPlease do not adjust margins commercially used material for this purpose is the activated carbon or mesoporous carbon due to their high surface porosity. 8, 9 Apart from them, bio-adsorbents are often used as adsorbents in waste water treatment. 10-13 However, they have weak mechanical strength, poor separation capacity and hence they are not very effective in this field. Recently, alternative materials including various oxide based nanocomposites and nanostructure have been synthesized as potential adsorbent of heavy metal ions. 14,15 Thus, with the advancement of technology, newer materials with improved adsorption capacities are being developed.In this context, it has to be mentioned that metal oxides are known to have high adsorption capacity of toxic metals in nanodimensions. Iron and aluminium oxide have been identified to adsorb toxic substances from waste water effectively due to their high surface adsorption ability. [16][17][18] Among different transitional materials, developed so far, NiO has proven itself as an another promising material that shows potential application in battery electrodes, super-capacitors, smart windows, catalysis etc. [19][20][21][22] Nickel, being a transitional metal, possesses variable oxidation states, but most of them are found to be abundant in normal conditions. 23,24 In this context, it has to be mentioned that pseudo hydrate of Ni(III) oxide was reported but their anhydrous form was not observed due to easy transformation of them into NiO on heating. 25 On the other hand, it was established that these higher oxides may possess many interesting properties if obtained in anhydrous form.Accor...