Proteus vulgaris Microbial System for Urea Removal and Sensing in Synthetic Urine

Abstract

One of the most challenging problems when trying to recycle urine for different purposes is the removal of urea. In this project we studied an ureolysis system using the bacterium Proteus vulgaris for the transformation of urea to ammonia and its subsequent oxidation to nitrogen at a Pt electrode. Our system was tested under different conditions, e.g. pH, microbial reaction time, and urea and bacteria concentrations. Our results indicate that a pH 8 is optimal for the Proteus vulgaris urease activity and the ammonia oxidation reaction at a Pt electrode. The reaction time and concentration dependence on the produced ammonia oxidation reaction current densities was studied. Results also showed limited ammonia oxidation under high urea concentrations in ~ 2.5 x 109 cfu/mL Proteus vulgaris in synthetic urine. A sensing device was developed using the microbial reaction at a Pt microchip. This work demonstrates the possibility of P. vulgaris being used for an ureolysis system for water recovery from synthetic urine (see Figure 1). The urea in urine solution was converted to ammonia by the bacterial urease. The ammonia oxidation reaction was detected by cyclic voltammetry. The optimal conditions for both processes, the urea to ammonia microbial conversion and ammonia oxidation electrochemical reaction, were determined. High concentrations of urea in the synthetic urine system may reduce the Pt electrode oxidation efficiency by urea poisoning. The bacteria concentration in the ureolysis system is crucial to avoid this ammonia oxidation reaction efficiency problem. For the average urea concentration in urine of 0.22 M, the current ammonia oxidation peak obtained may be improved by increasing the exposure time of the bacteria with the urine before the ammonia oxidation, or by increasing the bacteria concentration in order to increase the amount of urea converted to ammonia in an interval of time. Further improvements are needed to obtain ammonia oxidation reaction currents from the system in order to make a self-sustainable ureolysis system. An electrochemical whole cell sensor for the detection of urea in synthetic urine was also developed. For this, a custom-made platinum miniature microchip electrode onto a Cyclic Olefin Copolymer (COC) substrate, utilizing lift-off photolithography technique was done. P. vulgaris was placed at the Pt electrode surface and exposed to different urea concentration in synthetic urine for urea electrochemical determination. Urea was indirectly monitored by electrochemically measuring the ammonia electro-oxidation liberated by the microbial urease. Results showed that a calibration curve was possible after measuring the ammonia oxidation peak density current vs. the urea concentration in synthetic urine solution Figure 1