Utilization of nanosize spent oil shale for water treatment: application of top-down nanonization technology for...

Since the dawn of nanoscience, producing nanomaterials in a simple, low-cost, and high-yield manner has been a major issue. For the commercial manufacturing of nanomaterials, various bottom-up and top-down methodologies have been established. High-energy dry ball milling is widely used for the production of diverse nanomaterials, nanograins, nanoalloys, and nanocomposites. Physical grinding of inorganic solid waste into nanosize (1-100 nm) has improved their industrial applications particularly as water adsorbent. Application of nanosize spent oil shale by top-down methodology as adsorbent for phenol is addressed in the current research. The collected spent oil shale (SiO₂ and Al₂O₃ making 45% of the material) has microstructure with average particle size of 56.6 μm. The dry grinding was performed in a vibrating ball mill at various grinding time (5-150 min) while keeping the grinding parameters constant including number of balls, ball size, total mass, vibration frequency, and amplitude. Upon grinding, the mean particle diameter of the bulk material was reduced to 191.8 and 85.2 nm using 0.5 and 0.1 mm grinding balls, respectively. The effect of grinding time on particle size and surface area was investigated; both particle size and surface area were not affected after 60 min of grinding. Physical grinding by 0.1 mm balls has notably improved surface area and total pore volume by 52% and 62%, respectively. Although nanosize particles (85.2 nm) perform better than bulk material for phenol uptake, they underwent serious aggregation at pH > 2 and ionic strength > 1.0 mM. Hence, the 191.8-nm size is selected to assess the effect of mechanical grinding on adsorption rate and equilibrium capacity for phenol as a common pollutant. Upon nanonization, adsorption rate of phenol was highly increased from 0.39 to 4.43 mg g^-1 min^-1 as analyzed by pseudo-second-order model. Adsorption isotherms were adequately presented by Sips model (prediction error 5.4-7.2%) with a maximum phenol retention capacity of 39.29 mg/g after nanonization compared to 10.71 mg/g for the raw material. The performance of the nanosize spent oil shale for phenol retention was promising when compared with advanced adsorbents like multiwalled carbon nanotube.