A comparison of nanosilver and silver ion effects on bioreactor landfill operations and methanogenic population dynamics

Abstract

Silver nanoparticles (AgNPs, nanosilver) and silver ions released from industry and various consumer products are eventually disposed in sanitary landfills. To compare the effects of these two forms of silver on landfill bioreactor operations with leachate recirculation, municipal solid waste (MSW) in six identical 9-L bioreactors was exposed to AgNPs (stabilized with 0.06% polyvinyl alcohol) or Ag+ at a silver concentration of 10 mg/kg solids. The landfill anaerobic digestion was operated and monitored for more than 200 days. There was no significant difference in cumulative methane volume or methane production rate between the groups of control and 10 mg/kg Ag+. However, MSW treated with 10 mg/kg AgNPs resulted in a reduced methane production (by up to 80%) and accumulation of volatile fatty acids in the leachates. This was accompanied by higher leachate Ag concentrations (an average of 3.7 ± 0.3 mg/L) after day 132 as compared to those in the groups of control and 10 mg/kg Ag+ at 0.7 ± 0.4 and 1.1 ± 0.3 mg/L, respectively. Quantitative PCR targeting 16S rRNA genes of methanogens indicated reduced methanogenic growth in the bioreactors exposed to nanosilver. The peak values of total methanogens in leachates were (1.18 ± 0.09) × 1010, (4.57 ± 2.67) × 1010 and (7.72 ± 0.78) × 108 (cells/mL) for the groups of control, Ag+ and AgNPs, respectively. The results suggest that silver ions have minimal impact on landfill methane production at the concentration of 10 mg/kg. However, nanosilver inhibits methanogenesis and is more toxic than its counterpart, likely due to slow and long-term Ag+ release from nanosilver dissolution yielding more bioavailability in landfill leachates.