Co-Pyrolysis Biochar Derived from Rape Straw and Phosphate Rock: Carbon Retention, Aromaticity, and Pb Removal Capacity

Biochar has been recognized as a promising tool for carbon sequestration and heavy metal removal from contaminated water. To promote the application of biochar in environment improvement, the effects of exogenous minerals on carbon sequestration and Pb removal capacity of biochars were investigated. In this study, the original biochar and the co-pyrolysis biochars were prepared by co-pyrolyzing rape straw (RS) with phosphate rock (PR) in proportions of 1:0, 5:1, 2:1, and 1:1 (w:w) at 500 °C and maintaining them for 2 h, and the effects of co-pyrolysis of RS with PR on the carbon retention, aromaticity, graphitization, and Pb removal capacity of biochars were analyzed. Thermogravimetric analysis showed that PR might prevent the degradation of RS during pyrolysis. Compared with original biochar, the carbon retention of co-pyrolysis biochars increased by up to 27.5%, but the aromaticity and graphitization of co-pyrolysis biochars were reduced because the ratios of H/C and ID/IG (disordered peak/graphite peak) increased from 0.46 to 0.69 and from 1.59 to 1.82, respectively. Moreover, phosphate in both co-pyrolysis biochars could precipitate with Pb to form a more stable pyromorphite; a low additive amount of PR (RS/PR = 5:1) had a positive effect on Pb removal capacity, while the high additive amount of PR (RS/PR < 5:1) showed a negative effect on Pb removal capacity. Therefore, the co-pyrolysis of RS with low additive amount of PR would be a promising method to improve carbon sequestration and enhance Pb removal. These findings provide a theoretical basis for efficient use of co-pyrolysis biochars in the environment as a carbon sink and a heavy metal sorbent.