Removal of harmful oxyanions from contaminated water by Donnan dialysis

The removal of oxyanions like nitrate, sulphate, phosphate, chromate and perchlorate was investigated in single, binary and multi-component feed systems using Donnan dialysis across two homogeneous anion exchange membranes, FAD PET 75 and FAS PET 75. The ion exchange isotherms were established for oxyanion - chloride binary systems with both anion exchange membranes. The effect of initial concentration, recirculation flow rate, and receiver salt concentration was examined with FAD PET 75. The overall kinetic coefficient was calculated from the flux equation and was used to compare the transport kinetics of different oxyanions. The kinetics of oxyanions removal followed the order of nitrate > perchlorate > chromate > sulphate > phosphate, which can be attributed to the hydrated radius and structure of the oxyanions. The presence of co-existing oxyanions did not show a significant effect on the removal of nitrate, perchlorate, and chromate while sulphate and phosphate were influenced by their presence. Based on the present study, FAD PET 75 was able to remove all the target oxyanions effectively whereas FAS PET 75 was ineffective in the removal of sulphate and phosphate because of the higher cross linkage of the matrix which was evident with its lower water content.