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Chemical speciation can fundamentally affect the potential toxicity and bioavailability of heavy metals. The transformation of heavy metal speciation and change of bioavailability were investigated in an anaerobic digestion (AD) system using four different substrates (pig manure (PM), cattle manure (CAM), chicken manure (CHM) and rice straw (RS)). The results obtained indicated that the total contents of heavy metals in PM, CHM and CAM were higher than in RS and decreased in the order Zn > Cu > Ni > Pb > As > Cd in all substrates. Moreover, the speciation with the largest proportion for each heavy metal was the same both in the different substrates and the biogas residues. Among them, Zn, Ni, Cd and As were mainly in the reducible fraction (F2), while Cu was mainly in the oxidizable fraction (F3) and Pb occurred predominantly in the residual fraction (F4). Our results further indicated that the AD process had a greater effect on the speciation of heavy metals in CHM and PM, but less on CAM and RS. The rates of change in bioavailability followed the order PM > CHM > CAM > RS. Changes in organic matter, humic acid or local metal ion environment as a result of AD were inferred as likely mechanisms leading to the transformation of heavy metal speciation. These results enhanced our understanding of the behavior of heavy metals in AD and provided a new perspective for the treatment and disposal of the substrates.

The speciation of heavy metals, besides the total concentrations, urgently need to be considered when assessing the eco-toxicity and the bioavailability of heavy metals in environment. This paper aims to investigate the distribution and chemical speciation (e.g. the acid extractable fraction (F1), the reducible fraction (F2), the oxidizable fraction (F3), and the residual fraction (F4)) of heavy metals during the anaerobic digestion process of swine manure. The majority of six heavy metals from the manure was located in biogas residue in the order of decreasing concentration Zn > Cu > Ni > As > Pb > Cd. The transformation of heavy metals among four fractions was observed during the digestion process, and the change of bioavailable fraction of Zn, Cu, Ni, Cd, As and Pb were 9.71%, −6.04%, −19.24%, 13.62%, −16.48% and −7.22%, respectively. The heat map of correlation coefficients and the stepwise linear regressions model were established to describe the correlation between the bioavailability of the metals and the given digestion variables to predict the influence of the selected variables on the bioavailability of heavy metals. The variations of heavy metal bioavailable fractions are attributed to three key digestion variables, NH4+-N concentration, CH4% in biogas daily yield and pH. These results provide a new perspective for analysis and control of heavy metals during the anaerobic digestion process.