Preparation, Characterization And Evaluation Of Optimal Activated Carbons Derived From Prosopis Africana Seed Hulls For The Removal Of Chlorophenols From Aqueous Solution
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Date
2015-12
Authors
Garba, Zaharaddeen Nasiru
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Publisher
Universiti Sains Malaysia
Abstract
Potentials of an agricultural waste in Nigeria as a new precursor were harnessed for preparation of activated carbon for the adsorption of chlorophenols from synthetic wastewaters. Optimal activated carbons were prepared by physicochemical activation from the seed hulls of Prosopis africana as a new precursor using potassium oxalate, sodium acetate and potassium carbonate as the chemical activating agents. The variables optimized were activation temperature, activation time and chemical impregnation ratio (IR); and the responses were yield and chlorophenols percentage adsorption. The optimal conditions were 780 oC, 3.19 IR and 63 min, 795 oC, 2.45 IR and 62 min as well as 800 oC, 2.70 IR and 58 mins for K2C2O4-AC, CH3COONa-AC and K2CO3-AC respectively, denoting activation temperature, impregnation ratio and activation time, respectively. The adsorbents were characterized via BET surface area analysis, scanning electron microscopy, Fourier transform infrared, elemental analysis and proximate analysis to determine their physical and chemical characteristics. The BET surface area of 1095.56, 1085.92 and 1071.89 m2 g-1 for K2C2O4-AC, CH3COONa-AC and K2CO3-AC respectively were obtained. A range of 30-350 mg L-1, 30-50 oC and pH 2-12 were used to study the effect of initial concentration, process temperature and pH on the batch adsorption experiments. An increase in the initial concentration of the chlorophenols and contact time increased the adsorption uptake of the adsorbents. Highest removal efficiency of the chlorophenols on to the adsorbents was at lower initial solution pH (pH ≤ 2) attributed to ease of ionization and surface chemistry variation experienced at that pH on surfaces of the ACs. Batch equilibrium isotherms and kinetics data were modeled, the mechanism of the process, thermodynamic and regeneration studies were evaluated. The equilibrium adsorption data were modelled using five various forms of the linearized Langmuir equations as well as Freundlich and Temkin adsorption isotherms. In comparing the suitability of each isotherm model, chi square (χ2) was incorporated with the correlation coefficient (R2) to justify the basis for selecting the best adsorption model. Langmuir-2>Freundlich>Temkin isotherms was the best order that described the equilibrium adsorption data. Pseudo-second-order kinetic model best described the entire CPs adsorption processes on all the ACs. Intraparticle diffusion model was also applied to identify the adsorption mechanism but from Boyd plot, it was found that the chlorophenols adsorption on all the ACs was mainly governed by particle diffusion. Adsorption thermodynamic studies revealed the adsorption processes to be endothermic, spontaneous and physical. The regeneration results revealed that the adsorbents were used successfully for five cycles of adsorption. Commercial activated carbon was also studied under same condition to compare the efficiency of the prepared adsorbents.
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Keywords
Optimal activated carbons