System-level model and experiments for irrigation water alkalinity reduction and enrichment using an atmospheric...

The treatment of distilled water with varying amounts of dissolved sodium bicarbonate (representing alkalinity) is considered using an atmospheric pressure electrical discharge. The discharge ignited between a capillary tube (used as powered electrode) and a ground electrode wrapped around the beaker holding the treated water consists of streamers propagating in ambient air and striking the water surface. The streamer interaction with water is shown to lead to a decrease in pH and an increase in nitrate concentration. The pH variation with time is shown to be similar to a titration curve for acid-base neutralization with final pH values around 3 for 22 min of treatment. The nitrate ion concentration increase with time is consistent with a two-rate system-level model that is characterized by two asymptotic rates for NO₃^- creation by the plasma. The two asymptotic rates are calibrated to be about 2.7 μmol/min and 22.5 μmol/min with the transition between the two rates occurring at the breakeven time that is representative of the time required for all dissolved bicarbonate to be consumed by the plasma treatment. The increase in rate of NO₃^- creation at the breakeven time is attributed to the increase in conductivity of the treated solution once all bicarbonate is consumed thereby modifying the plasma properties. Another system-level model that is based on the observed pH variation is also considered for comparison with measured data. While both system-level models have some discrepancies with the measurements, the two-rate model based on the nitrate ion concentration is concluded to be more useful for determining the NO₃^- formation rates in the context of irrigation water enrichment. The discrepancies are attributed to the simplicity of the system-level models considered here where the effect of the plasma is completely represented by the creation of just one chemical species in HNO₃ thereby neglecting potentially important species such as HNO₂ and H₂O₂. Nevertheless, the proposed system-level model could greatly assist in the design of plasma treatment systems with specified alkalinity, pH and nitrate ion levels for irrigation water.