Functions of phosphorus additives on immobilizing heavy metal cadmium in the char through pyrolysis of contaminated biomass

Pyrolysis was applied for post treatment of two crop residues contaminated by heavy metal cadmium (Cd) after biosorption, with phosphorus additives used for enhancing the recovery and stability of Cd in the char. The results indicate that phosphoric acid (PA) can increase the Cd recovery in char to a level of> 90% at the PA dosage of 71-143 mg/g biomass, when the plant residues were pyrolyzed at 350 degrees C. The pyrolysis with phosphorus additives reduced the bioavailable Cd to < 3.5% of total Cd, but the influence on the leachable Cd was varied with specific additive and crop species. The sequential extraction test indicated that phosphorus additives helped Cd in the char be transformed into more stable species. The scanning electron microscopy - energy-dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD) and infrared spectroscopy (IR) analyses indicated that immobilization of Cd in the char was related to formation of Cd phosphates and their linkage to carbon matrix of chars.