Oxalate Ion Decomposition under UV Light from Low Pressure Mercury Vapor Lamps

Abstract

Kinetics of oxalate ion decomposition under UV light from low pressure mercury vapor lamps (LPMVL) was studied in a batch reactor. The effects of UV light intensity (1.38×10⁻⁶ to 5.27×10⁻⁶ EL⁻¹s⁻¹, where E: Einstein or 1 mole of photons), temperature (15−35°C), initial oxalate concentration ((2.05−21.1) × 10⁻⁵ M), initial pH (5.45−8.94) and alkalinity (0–50 mg L⁻¹ as CaCO₃) on the photodecomposition kinetics of oxalate in de-ionized water were investigated. Oxalate decay followed split-rate pseudo-first-order kinetics. The decay rate constants decreased with increasing initial oxalate concentration, initial pH, alkalinity and temperature, but increased with UV light intensity. Solution pH increased during oxalate decomposition and reached a plateau as oxalate reached the analytical detection limit in de-ionized water. Addition of carbonate alkalinity virtually eliminated the pH profile. Time-dependent profiles for non-purgeable organic carbon (NPOC) and total carbon (TC) showed that the carbon not accounted for in NPOC is likely to have been converted to CO₂. The pH profile of oxalate decay was estimated using closed system carbonate equilibrium analysis. The dissolved oxygen (DO) utilization during oxalate decay ranged between 0.3–0.8 mol O₂ / mol oxalate. The effect of DO and the decay of natural dissolved organic carbon (DOC) were also explored. Natural DOC retarded oxalate photodecomposition. The decay rate constants were slightly lower in the absence of DO.

Keywords:

• UV irradiation
• Oxalate
• Photodecomposition
• Water Purification
• Kinetics