Improving earthworm quality and complex metal removal from water by adding aquatic plant residues to cattle manure

Abstract

Vermicomposting is an economical and environmentally friendly process. However, related knowledge of vermicomposting aquatic plant residues (APRs), earthworm quality, and mechanisms for metal removal from water is still lacking. Nelumbo and Oenanthe javanica residues and their mixture were treated with Eisenia foetida and cattle manure for 45 days. Compared with the control comprising only cattle manure, addition of the APR mixture improved earthworm quality, mainly for low crude ash, high alkaloid compounds and different fat compositions in the Nelumbo residue and the balanced protein proportion of the APR mixture. All the vermicompost especial O. javanica residue added (VO) played efficient roles in removing metals from water initially containing 2.0 mg Cu L^-1 and 8.0 mg Zn L^-1. There were higher removal efficiencies (E_rs) at the dosage of 4 g L^-1 with a small microbial contribution. VO significantly increased E_rs, which could be from the decrease of phylum Firmicutes (especial Bacteroides) abundance, stronger CH₂, C = O, and CH, the addition of COOH groups, and higher organic matter and total phosphorus contents. The combination of VO and Hippuris vulgaris was optimized as an ecological and economical method for treating complex-metal polluted water. Moreover, our study widened the route for APR reuse.

Keywords: Aquatic plant residues; Heavy metal; Oenanthe javanica; Vermicompost; Water quality.