Mechanism of Calcium Ion Precipitation from Hard Water Using Pulsed Spark Discharges

Direct pulsed spark discharge treatment was found to be able to induce the precipitation of calcium ions in supersaturated hard water (Yang et al. in Water Res 44:3659, 2010). The present study investigated possible pathways for the plasma-induced precipitation. Both UV and reactive species were found not the major factors that stimulate the precipitation in present setup. A transient hot-wire method was used to investigate the effect of plasma-induced local micro-heating. Approximately 15% drop in the calcium-ion concentration was observed, indicating that the local micro-heating effect could be the major contributing factor. Additionally, a nanosecond pulsed corona discharge was used to investigate the non-thermal effect of plasma, and a maximum 7% drop in the calcium hardness was observed. Calcite with rhombohedron morphology was observed in both cases, similar to the structure observed in the spark discharge treatment case, indicating that calcium-ion precipitation process could be attributed to both the thermal and non-thermal effects produced by the plasma.