Performance and mechanisms of emerging animal-derived biochars for immobilization of heavy metals

Abstract

Animal-derived biochars (ADBs) are a series of newly emerging biochars from the pyrolysis of animal carcasses. Their elemental composition and structural properties would be quite different to those of plant-based biochars and/or manure-derived biochars (MDBs). In this work, ADBs derived from cattle and swine carcasses were applied to investigate their sorption potentials and mechanisms for immobilization of Pb(II), Cd(II), Cu(II), and Hg(II). ADBs showed high calcium (Ca; >27%) and phosphorus (P; >15%) contents and relatively low carbon (C; <8%) content. Ca and P were present in the skeleton as calcium–phosphate complexes, dominantly as hydroxyapatite (HAP), which was identified by CP–MAS 31P NMR analysis. By contrast, the reference biochars derived from cattle, swine, and poultry manures possessed low Ca (<13%) and P (<4%) contents and relatively high C (>30%) content. Sorption experiments showed that ADBs were more effective in immobilizing Pb(II), Cd(II), and Cu(II) than MDBs. However, the affinities of ADBs for Hg(II) were lower than those of MDBs. Characterization analyses indicated that Pb(II) was effectively immobilized via precipitation, whereas ion exchange was the dominant mechanism in combining Cd(II) or Cu(II) with ADBs. ADBs are unsuitable for immobilizing Hg(II) because of the lack of oxygen-containing functional groups on the C surface, which can induce the reduction of Hg(II) to form Hg2Cl2. The immobilized heavy metals were difficult to be rereleased from ADBs in simulated acid rain environment. Hence, ADBs are promising biochars for immobilizing Pb(II), Cd(II), and Cu(II).