Effects of combined composting and vermicomposting of waste sludge on arsenic fate and bioavailability.

Composting and vermicomposting are traditional processes for the treatment of sludge. During these processes, the humification of organic matter has a significant effect on the physicochemical form and distribution of heavy metals. In this study, industrial sludge (groundwater treatment waste) contaminated by arsenic (396 ± 1 mg kg(-1)) was used. Such sludge poses a significant challenge with respect to effective treatment. Composting, vermicomposting (with Eisenia fetida), and the combined approach of composting and vermicomposting were performed to determine the evolution of arsenic speciation, mobility and bioavailability. The composting/vermicomposting was done with sludge, horse manure, and grass in the ratios of 3:6:1. A solution of 0.1M NH4COOCH3 was used as a single extraction solvent for determination of the mobile arsenic pool and targeted arsenic species (As(III), As(V), monomethylarsenic acid - MMA(V), dimethylarsenic acid - DMA(V)). The analysis of arsenic in the extracts was carried out by means of HPLC-ICP-MS spectrometry. In addition, the earthworm species E. fetida was used for bioaccumulation tests that followed the compost and vermicompost processes. The obtained results indicate a reduction in arsenic mobility and bioavailability in all matured composts and vermicomposts. The combined process exhibited a greater effect than compost or vermicompost alone.