Description
This paper examines the effects of the adsorption of silver nanoparticles (AgNPs) and toxicity of silver ions (Ag⁺) on Nitrosomonas europaea that is continuously cultured within a chemostat reactor. N. europaea, an ammonia oxidizing bacteria (AOB), is used in the removal of nitrogen from industrial and sewage waste through the oxidation of ammonia (NH₃) to nitrite (NO₂⁻). Additionally it is important to note that AgNPs and Ag⁺ are also commonly found in industrial and sewage waste and therefore the study of their adsorption and toxicity to N. europaea bacterium is crucial. In order to determine the influence of growth rates on AgNPs and Ag⁺ and their potential adsorption and toxicity to N. europaea cells, respectively, a small scale chemostat was built. This device continuously cultivates N. europaea cells inside of a reactor while constantly adding new growth media for the cells and extracting media at the same flow rate to regulate the internal volume of the reactor. Simultaneously, filtered air is bubbled into the system from a standard aquarium pump. Additionally a pH probe was installed to track the internal pH of the reactor. The oxidation of ammonia to nitrite results in the release of protons and a drop in pH. As the pH drops, the pH meter sends a signal to a peristaltic pump to add sodium carbonate to the system until the pH in the reactor is stable, (pH = 7.8). Once the desired pH is reached the pump shuts off. There are also ports on either side of the reactor that allow for the addition or extraction of media via a 30mL syringe. Flow in and out of the reactor is initially 0.3 mL/min for a first round of testing and is increased to 0.6 mL/min for a second round of testing, and the solution is constantly stirred to ensure the proper amount of agitation to prevent mass transfer limitations. After cells were grown and the contents of the reactor reached steady state conditions, (as determined by constant cell densities and NO₂⁻ concentrations), cells were withdrawn from the reactor for adsorption and toxicity testing. Adsorption testing was completed using AgNPs and toxicity testing was completed using Ag⁺. After the first round of testing the chemostat flow rate was raised and kept constant at 0.6mL/min and brought to steady state. Adsorption and toxicity testing was redone using the new cells.
