Disinfection of Irrigation Water using Titanium Electrodes

The microbiological safety of irrigation water is critical to the prevention of fresh produce contamination with human pathogens. This work reports on the electrochemical disinfection of natural irrigation water using an undivided batch cell assembled with titanium (Ti) electrodes as both anode and cathode. Although Ti is considered an inactive electrode material for most applications due to the presence of an oxide film on the surface, we show here that it can efficiently catalyze the electrochemical oxidation of a few ppm of chloride ions in natural irrigation water to free chlorine species, which in turn enable fast disinfection. Disinfection experiments were performed by applying a polarity-reversing direct current between the two Ti electrodes. Compared to applying a polarity-constant direct current, the polarity-reversal technique effectively inhibited extensive oxidation of the Ti electrode surface as indicated by a lower oxide/hydroxide content on the electrode surface, thus providing excellent stability during electrolysis. The natural irrigation water was collected from the Waiahole Ditch Irrigation System in Hawaii. The naturally occurring concentration of chloride in the water was 1.84 mg/L and no other chemicals were added. E. coli K12 ER2738 was selected as a model bacterium to evaluate the electrochemical cell’s disinfection capability. The applied current density was varied between 0 mA/cm2 and 2 mA/cm2, and the half-period (T/2) of the polarity-reversing direct current was varied between 5 s and 60 s. The best disinfection performance was achieved at 2 mA/cm2 and T/2 = 10 s, requiring only 5 min of treatment for the complete disinfection of E. coli (4-log reduction). The trends in the concentration of free and total chlorine in the solution matched very well with those of the disinfection efficiency, suggesting that E. coli was inactivated by free chlorine species electrogenerated at the Ti electrodes. Hydrogen peroxide, short-lived oxidants, and direct electron transfer most likely had minor contributions to the disinfection process. The presence of a volcano-shaped dependence of the free and total chlorine concentration and disinfection efficiency on T/2 suggests that the chloride oxidation activity of the Ti electrode was closely related to the oxidation state (or the thickness of the oxide layer) of the electrode surface, i.e., there exists an optimal oxide layer thickness at which the Ti electrode shows the highest electrocatalytic activity toward chloride oxidation. During the disinfection experiment, disinfection byproducts (DBPs) of chlorate, bromodichloromethane, dibromochloromethane, and bromoform were not detected in the solution. Chloroform was first seen in the solution at 10 min (2.5 μg/L), and then its concentration increased to 9.0 μg/L at 20 min. These values are significantly lower than recommendations for drinking water, and the timeframe of chloroform production suggests a 5 min disinfection treatment avoids significant DBP formation. As a comparison, Pt/Ti electrodes prepared by electrodeposition showed negligible disinfection at 2 mA/cm2, but the disinfection efficiency increased dramatically at 4 mA/cm2 and 6 mA/cm2. This phenomenon was most likely due to the Pt catalyst’s high activity, readily catalyzing side reactions such as oxygen evolution and oxidation of organic matter in the natural irrigation water. The Ti electrodes consumed less electrical energy than the Pt/Ti electrodes because they needed a lower current density to achieve similar disinfection efficiency. The current work demonstrates that the Ti electrode electrochemical cell can provide an efficient, robust, and cost-effective solution to irrigation water disinfection, and thus is well suited for agriculture sector applications.