Ammonia oxidation process of entrapped and suspended cell inhibited by silver nanoparticles

Abstract

Silver nanoparticles (AgNPs) are widely used in commercial products because of their excellent antimicrobial activity. Entrance of AgNPs and its released Ag ions (Ag+) into wastewater treatment plants could harm ammonia oxidation (AO) process resulting in environmental problems. This study aimed to study inhibitory effect of AgNPs and Ag+ on AO process in suspended cells of ammonia oxidizing cultures enriched at initial ammonia concentrations of 0.5 and 30 mM (called NAS 0.5 mM and NAS 30 mM, respectively) and then mitigation measures using entrapment technique were tested. The results indicated that AgNPs and Ag+ were higher toxic to NAS 0.5 mM than NAS 30 mM due to different proportion of active ammonia oxidizers and communities. Silver nanoparticles of 1-100 mg/L inhibited AO activity by 90.6 ± 8.6% to 94.8 ± 4.3% and 44.9 ± 2.4% to 73.8 ± 1.5%, whereas Ag+ concentration of 0.05- 0.50 mg/L inhibited by 86.3 ± 0.8% to 93.4 ± 1.24% and 52.7 ± 2.14% to 93.9 ± 1.89% for NAS 0.5 mM and NAS 30 mM, respectively. The inhibition result suggested that the AgNP toxicity mainly derived from the liberated Ag+. The primary mechanism for toxicity of AgNPs and Ag+ caused microbial cell death. Cell entrapped in barium alginate (BA), polyvinyl alcohol (PVA), and the mixture of PVA-BA used to mitigate negative influence of AgNPs and Ag+. The results showed that BA, PVA, and PVA-BA- entrapped cells remained AO 81-100%, 57-97%, and 75-100% at AgNPs 1-100 mg/L whereas the entrapped cells remained AO 98-100%, 61-99%, and 79-100% under Ag+ 0.05-0.50 mg/L, respectively. Less damage of membrane integrity in the entrapped cells experiments with AgNPs and Ag+ indicated that the entrapped cells were less exposed to silver since the entrapment matrices could limit mass transfer of silver into the gel beads. BA and PVA-BA were highly recommended for further study in the mitigating toxicity of AgNPs and Ag+ since they remained high cell viability and more stable in synthetic wastewater.