Preparation and characterization of carbonaceous adsorbents from sewage sludge using a pilot-scale microwave heating equipment

Microwave heating technology is considered as a cost-effective alternative way for preparing activated carbon from organic solid wastes. In this study, a pilot-scale microwave heating apparatus with the processing capacity of 5 kg solid waste was constructed to prepare carbonaceous adsorbents from sewage sludge, and the effects of important microwave processing parameters and chemical activation treatments on the quality and yield of resultant carbonaceous adsorbents were investigated with the aim of technological industrialization. The surface properties and chemical compositions of the carbonaceous adsorbents were characterized using nitrogen adsorption, iodine adsorption, scanning electron microscopy, X-ray fluorescence, and Fourier-transform infrared spectroscopy. The microwave heating process could be clearly classed into three stages, i.e. drying, low-temperature pyrolysis, and carbonization. The increase of carbonization temperature reduced the contents of oxygen-containing functional groups in sludge-based adsorbents. The optimum carbonization temperature was 600 °C, and a higher carbonization temperature (700 °C) gave rise to a detrimental impact on the quality of sludge-based adsorbents, in terms of specific surface area, total pore volume and iodine value. Activation treatment with KOH, ZnCl2 and H3PO4 achieved the better development of porous texture. The sludge-based adsorbent prepared via KOH activation at carbonization temperature of 600 °C presented the highest BET surface area of 130.7 m2 g−1 and the highest total pore volume of 0.13 mL g−1. However, according to the results of batch adsorption experiment, the sludge-based adsorbents prepared via H3PO4 activation acted as the best adsorbent to adsorb Cu2+ and Pb2+ from aqueous solutions.