Removal of cadmium and (radio)toxic metal ions using activated sludge-based sorbents

Biochars produced from sewage sludge were investigated towards their adsorption capacity for cadmium and (radio)toxic metals. Samples of waste sewage sludge were subjected to pyrolysis at 350, 450 and 800 °C. The resulting biochars were characterized by using techniques like Fourier-transform infrared spectroscopy and scanning electron microscopy. Their specific surface area and pore size were determined using the BET method, and the point of zero charge was assessed via zeta-potential analysis. The application of biochars and dry sludge in removing Cd(II) from aqueous solutions was demonstrated considering the effect of various factors including metal concentration, temperature, pH, and presence of competing ions. Adsorption isotherms were modeled using Freundlich, Langmuir and Temkin models, while adsorption kinetics were assessed through the pseudo-first and pseudo-second-order as well as Elovich kinetic models, along with the corresponding thermodynamic data. In addition, the adsorption capacity was tested for Cs(Ι), Ba(ΙΙ), Eu(ΙΙΙ) and U(VΙ) ions. Dry sludge and the biochar produced after pyrolysis at 800 °C proved to be the most effective materials for removing cadmium as well as cesium, barium, europium and uranium from aqueous solutions. The evaluation of equilibrium and kinetics data indicated that activated sludge products could serve as effective sorbents for retention of hazardous metals.