Synthesis of Mesoporous Graphite Functionalized by Nitrogen for Efficient Removal of Safranin Dye Utilizing Rice Husk Ash; Equilibrium Studies and Response Surface Optimization

Mesoporous graphitic carbon (PG) was successfully synthesized from alkaline treated rice husk ash through chemical activation by phosphoric acid at 800 °C for 1 h and modified by nitric acid to produce porous graphite with nitrogen functional group (N.PG). The morphology and structure of N.PGC were characterized by XRD, SEM and Micromeritics ASAP2010 analyzer at 77 K. N.PG was applied as an adsorbent material for safranin-O dye from aqueous solution. The removal of safranin dye by the synthetic porous graphite with nitrogen functional groups shows higher capacity as compared to the pure phase of porous graphite. The adsorption process was investigated as a function of contact time, adsorbent mass, pH, initial dye concentration and ionic strength. The kinetic studies revealed that the adsorption equilibrium was reached after 480 min and the obtained data well fitted with the pseudo-second-order kinetic model and Elovich kinetic model. The equilibrium adsorption isotherm of safranin by the synthetic N.PG was described with Langmuir isotherm model, and the calculated qmax is 20.66 mg/g. The removal process is highly dependent on the pH value of the solution, and the optimum pH for maximum removal of safranin-O is pH 6. The response surface methodology in conjunction with the central composite rotatable design was used to optimize the sorption process. From the second order polynomial model, the predicted optimum conditions for maximum removal of safranin (100%) are 365 min contact time, 0.3 g dose, 5 g/l NaCl and pH 6 at initial concentration 127 mg/l.