Waste tyre powder-based activated carbons by Co2 activation for methylene blue and phenol removal

The development of industries in Malaysia has led to water pollution. Adsorption is an effective technique in the treatment of wastewater as it utilizes low cost adsorbent, no sludge generation, and simple to operate. Pyrolysis waste tyre powder is a suitable precursor of activated carbon due to its high carbon content, less commercial value and abundantly available. This work aims to evaluate the adsorption properties of activated carbons prepared from pyrolysed waste tyre powder for the removal of positively charged methylene blue and negatively charged phenol from water. The activated carbons were prepared by physical activation using carbon dioxide at activation temperatures of 900 to 1000 °C for 2 to 8 h. The resultant activated carbons were modified with hydrofluoric acid followed by nitric acid, and were characterized for Brunauer–Emmett–Teller surface area, pH value at the point of zero charge, scanning electron microscope, functional groups and thermal gravimetric analysis. The adsorption of methylene blue and phenol were studied at varying concentrations (5 - 200 mg/L), contact times (5 min - 72 h) and temperatures (30 - 60 oC). The isotherm, kinetics and thermodynamics models were employed to describe the adsorption data. The specific surface area of activated carbons increased with activation temperature and time. The maximum adsorption of methylene blue is 132 mg/g. The oxidized activated carbon showed an increase of methylene blue adsorption from 102 mg/g (bulk) to 107 mg/g. However, the phenol adsorption decreased from 48 to 39 mg/g after oxidation. The equilibrium data of methylene blue adsorption fitted well with the Langmuir and Redlich-Peterson models, while that of phenol adsorption obeyed the Freundlich isotherm. The kinetics data of both model pollutants could be described by the pseudo-second-order model. The rate-limiting step in the adsorption of methylene blue and phenol could be dominated by pore diffusion. The positive enthalpy change and entropy change indicate that the adsorption of methylene blue is endothermic and spontaneous at high temperatures, while the phenol adsorption is exothermic and spontaneous at low temperatures. In conclusion, the activated carbons prepared are feasible to be used as an adsorbent.