Distribution and transformation behavior of heavy metals and phosphorus during sewage sludge pyrolysis
Mengyan Lv, Tingting Zhao, Jingjing Chen, Lizhi Tong, Zhuobiao Ni, Qingqi Lin, Zhepu Ruan, Rongliang Qiu
Abstract
Pyrolysis of sludge is an effective technique for the solidification of heavy metals and recovery of phosphorus, a critical resource. This study examined the effects of pyrolysis temperatures (300, 500, and 700 °C) and heating rates (5, 10, and 20 °C/min) on the distribution and transformation of heavy metals and phosphorus in sludge pyrochar, as well as their implications for resource utilization. With an increase in both pyrolysis temperature and heating rate, there was a decrease in the content of C, H, and N, while the total phosphorus (TP) content in the pyrochar increased. The pyrolysis process facilitated the decomposition of organic phosphorus, leading to the production of PO 4 3 − ions and the subsequent formation of calcium phosphorus (Ca-P). Notably, at 700 °C, the proportion of AlPO 4 in the pyrochar was 12 % lower compared to that derived from 300 °C. An increase in pyrolysis temperature enhanced the conversion of Al-P to Ca-P, resulting in higher formation of Ca 3 (PO 4 ) 2 and Ca(H 2 PO 4 ) 2 . The optimal conditions for apatite phosphorus enrichment were identified at a pyrolysis temperature of 700 °C and a heating rate of 10 °C/min, yielding the highest content of 10.48 mg/g. According to the BCR method, 80–98 % of heavy metals (As, Cr, Pb) in the pyrochar were present in the stabilized fractions (F3 +F4) when pyrolyzed at 500 °C with a heating rate of 10 °C/min. This study thus provides a theoretical foundation for assessing both the phosphorus recovery potential and the heavy metal stabilization in sludge pyrochar. • Studied P and HMs transformation in sludge pyrolysis. • Pyrolysis temperature was key, over heating rate in HMs and P transformation. • Pyrochar produced at 500°C effectively retained As, Cr, Pb. • Pyrolysis boosts bioavailable P from 5.36 to 10.82 mg/g. • Optimal pyrolysis condition: 500°C, 10 °C/min pyrolysis yields 54 % Ca-P.