Electrochemical Monitoring of Manganese in Drinking Water

Low levels of Mn are essential for many metabolic processes in the human body, yet excessive exposure to this heavy metal has been correlated to neurodegenerative diseases with Parkinson’s like symptoms in adults and neurological deficits in children. Erosion of Mn rich soil and rocks and human activities directly pollute groundwater which supplies drinking water for 51% of the total US population and 99% of the rural population. Hence contaminated drinking water represents one of the major sources of Mn intake. The non-enforceable updated guideline set by the EPA is 50ppb Mn. Trace metal analytical methods such as ICP-MS and AAS are limited to centralized laboratory testing and not feasible for point-of-care. Our work is aimed at the development of an electrochemical platform for efficient and rapid Mn detection in drinking water. The core technique is Square Wave Cathodic Stripping Voltammetry (SWCSV), carried out on disposable three-electrode sensors with a platinum working electrode, platinum counter electrode and an electroplated Ag/AgCl reference electrode. A 3D printed interface integrates a small vibration motor for effective agitation of the 12µL droplet sample during the deposition phase and allows connection to a commercial potentiostat. Figure 1(a) depicts the hardware components of the platform, with an onset on the sensor configuration. First, the system performances are characterized in standard solutions. Compared to peak amplitude, peak area is found to be a more suitable indicator of Mn concentration, with a linearity of R^2=0.998 in the range 5ppb-200ppb. The introduction of an embedded agitation mechanism allows for a reduction in the limit of quantitation (LOQ) from 5ppb to 3ppb and an approximate 2.3X increase in sensitivity from 9.09nC/ppb to 20.75nC/ppb. Analysis of tap water is then performed to confirm the capabilities of the electrochemical platform in more complex matrices. With optimal SWCSV parameters a limit of quantitation of 6ppb, a sensitivity of 26.45 nC/ppb and a linearity with R^2=0.995 are achieved. The developed protocol requires minimal sample preparation of the water samples, with a 2X dilution in 0.2M Acetate Buffer to provide enough ionic strength to sustain the faradaic reactions and buffer the pH at 4.7. 3-point standard addition is executed to determine the Mn concentration in unknown specimens and performances are compared to ICP-MS on the analysis of 78 tap water and well water samples from Holliston, MA with concentrations in the range 0.03ppb-5.3ppm. 100% agreement, 70% accuracy and 91% precision were obtained. Figure 1(b) shows typical voltammograms from the measurement of the unknown sample and the 3 spikes at known Mn concentrations to implement the 3-point standard addition. To enhance monitoring of water quality in local communities and rural areas, parallelization of the measurements can further reduce the analytical time and improvements in the interface can ultimately make the devised portable and easy-to-use electrochemical station soon deployable for practical on-the-field testing. Figure 1