Microbial activity and community structure in the treatment of metal- and cyanide-containing wastewater

Abstract

A lab-scale upflow anaerobic bioreactor filled with granular sludge and cow manure was operated for 140 days to determine the mechanism of metal removal and the vertical distribution of metal precipitates. Heavy metal ions were removed in the order of $Cu^{2+}$, $Cd^{2+}$, $Zn^{2+}$, $Fe^{2+}$ and $Mn^{2+}$ with respect to the height in the reactor. The solid phase analysis showed that the heavy metals were mostly precipitated in the form of metal sulfides by sulfate reduction. The contents of metal precipitates in the reactor were as follows: (ⅰ) Cd and Zn were highest in the bottom, (ⅱ) Fe was highest at the low-middle layer, and (ⅲ) Mn was increased with the height in the reactor. The vertical distribution of metal sulfides in the reactor was directly related to the solubility product (Ksp). Results obtained in this study suggest a feasibility of the application to separate precipitation of metal ions and recovery of valuable metals from metal-containing wastewater. To remove metals and cyanide in sulfate-reducing condition, the toxicity of free cyanide (FC), zinc-complexed cyanide (ZC), nickel-complexed cyanide (NC) or copper-complexed cyanide (CC) was determined. FC at 1 mM decreased the initial sulfate reduction rate from 0.30 to 0.14 mmol $day^{-1} g^{-1}$ SS (suspended solid), whereas 0.5 mM cyanide had a minimal effect (0.25 mmol $day^{-1} g^{-1}$ SS). The order of toxicity of metal-complexed cyanides was as follows: ZC (most toxic) > FC = NC > CC (least toxic), which also corresponds with the order of the stability (dissociation) constants of the metal-cyanide complexes. Consortium degrading cyanide was enriched using NC as the sole nitrogen source. This consortium completely removed 0.5 mM of nickel-complexed cyanide under sulfate-reducing conditions in 11 days. Analysis of clone library of 16S rRNA genes shows that the consortium was composed of three major phylotypes including Desulfovibrio. To efficiently remove different types of cyanide, activat...