Lanthanum-doped activated carbon derived from municipal sludge for enhanced defluoridation: characteristics and...

The sewage sludge production has been increasing along with the ever-growing populations and wastewater treatment rate. Lanthanum-doped activated carbon (AC-La) was derived from municipal sludge via chemical activation and utilized for fluoride removal. Batch experiments were conducted to discuss the effect of lanthanum dosage, time and pH on the adsorption process. The results showed that 4 g/L AC-La exhibited a fluoride removal rate of 80.9% with 10 mg/L initial fluoride concentration, and the optimal pH range for adsorption was 3-10. X-ray fluorescence, scanning electron microscopy with energy-dispersive X-ray spectroscopy, Brunauer-Emmett-Teller, X-ray diffraction, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses were conducted to analyze the microstructure and chemical properties of sludge, unmodified activated carbon (AC) and AC-La. The results showed that with initial lanthanum dosage of 15 wt%, the final loading amount of La in AC-La was 13.8 wt%. After modification, the specific surface area of AC-La increased from 1.8 m²/g (sludge) to 133.0 m²/g. The removal mechanism of fluoride onto AC-La was mainly the inner-sphere complexation between lanthanum and fluoride, facilitated by exchange interaction with hydroxyls. A stability study showed that AC-La maintained a quite small dissolution and was safe in waters (La dissolution rate < 0.2‰).