Addition of plantation waste to the bioconversion of pig manure by black soldier fly larvae: Effects on heavy metal content and bioavailability

During the conversion of pig manure by black soldier fly larvae (BSFL), the accumulation and speciation changes of heavy metals (HMs) have adverse effects on the environment. In this study, corn straw, rice straw, bamboo chips (BC), wood chips, and rice husk char were added to a bioconversion system to study the accumulation, migration, speciation changes, and microbial correlations of HMs. The results indicated that the addition of BC was most beneficial for the accumulation of HMs (47-72 %) in the BSFL body. In the BC group, the accumulation effect of the BSFL body on zinc (Zn) and arsenic (As) was the most evident (72 and 71 %, respectively). The results of linear fitting (R-2 > 0.90) and redundancy analysis (RDA; 90 %) indicated that the bacterium Bacillaceae (Bacillus) was beneficial for increasing the larval weight (LW) of BSFL, and a higher LW accumulated HMs. The addition of BC helped reduce the total amount (6-51 %) of available states (weak acid extraction and reducible states) in the BSFL residue. The RDA results indicated that bacteria (55-92 %) affected the transformation of HM speciation. For example, Zn and cadmium were mainly affected by Firmicutes, whereas copper and chromium were affected by Bacteroidetes. Proteobacteria and Pseudomonas formosensis affected the conversion of lead and As. This study provides important insights into the adsorption of HMs from pig manure by BSFL.