Surface characterization and dye adsorptive capacities of char obtained from pyrolysis/gasification of sewage sludge

Sewage sludge was used to develop a potential adsorbent for dye removal by pyrolysis under either N2 or CO2 atmospheres. The results showed that the surface area of the char increases as the pyrolysis temperature increase under the CO2 atmosphere. The maximum surface area of the char is achieved with pyrolysis at 750 °C under the CO2 atmosphere, 60.7 m2 g−1 with mainly mesopores. The FT-IR spectra of the char prepared under both N2 and CO2 atmospheres indicate a decrease in –OH, –NH and CO functionalities with increasing the pyrolysis temperature, corresponding to a decrease in the acidity of the char. The maximum adsorption capacities of acid and basic dyes were found to increase with an increase in the pyrolysis temperature while that of the reactive dye possessed no correlation. The adsorption mechanism is governed by the combination of the electrostatic interactions and dispersive interactions. The equilibrium data fit well with the Langmuir model of adsorption suggesting a monolayer coverage of dye molecules at the outer surface of sewage sludge derived chars. The maximum adsorption capacities of acid yellow 49, basic blue 41 and reactive red 198 dyes are reported at 116, 588 and 25 mg g−1 of char, respectively.