Fates of Heavy Metals in Anaerobically Digesting the Stover of Grain Sorghum Harvested from Heavy Metal-Contaminated Farmland

Two varieties of grain sorghum, including Chuannuo (CN) and Luzhouhong (LZH), were attempted on heavy metal (HM−) contaminated farmland. HMs distribution in the harvested biomasses, including grain and stover (leaf + stalk), were analyzed. The mostly HM-accumulated part was anaerobically digested to check the biogas releasing performances, and the fates of HMs in digestion. Results indicated the bioconcentration factors were lower than 1, suggesting these investigated varieties were characterized by lower accumulation on HMs. The accumulated HMs mainly existed in stover, in detail, 80.9% (Cu), 71.4% (Pb), 97.8% (Cr) and 99.1% (Cd) were in CN, and the corresponding HM was 78.4%, 85.1%, 98.9% and 97.8%, respectively, in LZH. Anaerobic digestion of stover were performed well with methane yield of 173.78 mL/g VS and 144.73 mL/g VS for CN and LZH, respectively. After digestion, more than 99% HMs were retained in solid residues, in which the residual state HMs were increased by 7–38%. Besides, the decreased mobility (except Cu) and bioavailability after anaerobic digestion implied these HMs tended to be more stable in the digested solid residues. Graphical Abstract