4. Conclusions
The study profiled the fluctuating heavy metal concentrations
in leachate during anaerobic digestion of shredded MSW in land-
fill bioreactors (1000 t per reactor) homogenised by the flood and
drain process. pH played an important role in distribution and
interactions of the dissolved heavy metals in leachate. The study
demonstrated that the pH variation in leachate caused mainly by
the accumulation and consumption of volatile fatty acids during
AD of shredded MSW governed various processes, including dissolution
and precipitation of various metal mineralogical phases.
In addition, metal complexation with DOM and metal adsorption
to MSW are important physical, chemical and biological processes
affecting dissolved metal concentration in the leachate.
Metal adsorption to MSW may result in the higher dissolved metal
S. Xie et al. / Journal of Hazardous Materials 299 (2015) 577–583 583
concentrations at the lower pH due to the electrostatic interaction
at the complex MSW surface. Strong negative correlations were
observed between pH and Zn, Cu, Ni, Pb and Cd, indicating that
adsorption to solid materials was the dominant process. A strong
positive correlation between pH and Cr indicated that complexation
with dissolved organic matter was the dominant process here.
Chromium complexation with DOM resulted in lower dissolved
concentrations at the lower pH due to decreased dissolution of
Cr(III)-DOM complexes. Both processes (adsorption and complexation)
might contribute to metal distributions for As and Co, as they
displayed a weak correlation to pH and As and Co. Data from this
study indicates thepossibility of correlating certaindissolvedheavy
metal concentrations to specific anaerobic degradation stages.