Metal speciation distribution of anaerobic fermentation with alfalfa grass harvested from abandoned iron mine and the influence of metals addition

Abstract

Metal-contaminated plants after phytoremediation provide organic matter and trace metals for anaerobic fermentation. In this study, alfalfa straw harvested from an abandoned iron mine was used for anaerobic fermentation. The variation of metal speciation during the anaerobic fermentation process and the influences on fermentation parameters were investigated. The results showed that the group adding alfalfa increased the peak daily biogas yield by 33.40 % compared to the group with cow dung only. Fe in the fermenter was predominated by the soluble and the residual fraction. Cu was transformed from soluble Cu to residual Cu with the fermentation progressed. On the 13th day of fermentation, the percentage of soluble Fe decreased while the percentage of soluble Cu, Ni and Zn increased which induced the daily biogas yields peak. Metal speciation was correlated to the fermentation parameters. Daily biogas yields were negatively correlated to the acid-extractable fraction of Ni and Zn (p < 0.05). pH was negatively correlated to the soluble Cu, the acid-extractable fraction of Cu and Zn (p < 0.05), while positively correlated to the reducible and oxidizable Fe (p < 0.01). Further adding metals promoted the biogas production of alfalfa by bringing forward the daily biogas peak and increasing the peak yields. In particular, adding Cu only produced more biogas than adding other metals (single or multiple). Exogenous metals addition generally resulted in more acid-extractable fraction of Cu, Ni and Zn, i.e. the mobility of these metals increased. 19 % of Fe was activated into acid-extractable fraction after the addition of Ni. The results of this study are expected to provide references for the reutilization of contaminated biomass and optimizing the metal contained fermentation process.