Changes in speciation, mobility and bioavailability of Cd, Cr and As during the transformation process of pig manure by black soldier fly larvae (Hermetia illucens)

Abstract

The black soldier fly larvae (BSFL) (Hermetia illucens) are a good candidate for poultry and livestock manure treatment. The harvested insect bodies and feces can be used as animal feed and organic fertilizer. However, heavy metals have a negative impact on the transformation process of the manure by BSFL. Here we evaluated the effects of Cd, Cr and As in pig manure on the growth of BSFL, as well as the mobility and changes in speciation of the metals during the transformation process. The results showed that As significantly reduced the weight of BSFL, but Cr and Cd had no significant effects on BSFL growth. The bioaccumulation factors (BAFs) at various concentrations of Cd (2.8–3.7) were significantly higher than those of Cr (0.44–0.62) and As (0.43–0.45). The heavy metals in pig manure were mainly transferred to BSFL feces, and the distribution percentages of Cd, Cr and As in the BSFL feces were 52.6–62.0%, 90.3–94.2% and 93.0–93.3%, respectively. Cd concentrations in BSFL feces were significantly lower than those in the pig manure, while there were no significant differences in the concentrations of Cr and As between BSFL feces and pig manure (except for the treatment with the addition of 150 mg Cr kg−1). The metal speciation (weak acid soluble, reducible, oxidizable and residual fractions) in BSFL feces obviously changed when compared with pig manure. In BSFL feces, the reducible proportion of Cr decreased while the oxidizable proportion increased. Cd mainly existed in the weak acid soluble and reducible states in pig manure and BSFL feces. For As, the proportions of weak acid soluble and reducible states decreased in BSFL feces. After the BSFL transformation process, the bioavailable fraction of Cr decreased by 17.3–23.1%, but those of Cd and As did not change significantly, except for As in the CK group. These findings contribute to our understanding of the roles of BSFL during the biotransformation process of livestock manure as well as the safe utilization of transformed products.