Preparation of activated carbon from carbonized rice husk by ozone activation for Cr(VI) removal

Activated carbon (AC) was prepared from carbonized rice husks using ozone as an activating agent. The AC was characterized by nitrogen adsorption, SEM-EDAX and FT-IR. The Brunauer-Emmett-Teller surface area of the carbons was increased from 20 to 380 m2/g by the activation. It was observed that the silica attached to the carbonaceous material is loosened, leading to a release of carbon during the ozone activation. Ozone exists as both molecular and atomic oxygen on the surface of carbon. Atomic oxygen, as a powerful oxidizing agent, oxidizes the carbon surface into acidic functional groups such as carboxylic, ketonic and phenolic. The rice husk AC was used for the adsorption of Cr (VI) ions. A maximum removal percentage (94%) of Cr (VI) ions was obtained for a 100 mg/L aqueous solution at the optimized conditions of pH value of 2.0, adsorbent dosage of 0.2g, time of 2.5 h and stirring speed of 300 r/min. Adsorption equilibrium and kinetic models were used to investigate the adsorption mechanism. It was found that the adsorption isotherms were well fitted by the Freundlich equation. The adsorption rate follows pseudo second order kinetics and the adsorption is spontaneous and exothermic. It is possible to recover Cr and carbon by NaOH stripping.