Ligand-promoted reductive cleaning of iron-fouled membranes from submerged membrane bioreactors

Abstract

Four pathways for cleaning of iron oxide-fouled membranes, namely i) proton (acid)-assisted, ii) ligand-promoted (citric acid), iii) reductive (ascorbic acid), and iv) ligand-promoted reductive (ascorbic acid-citric acid-mediated) were examined and their effectiveness were compared in this study. The cleaning effectiveness under oxic conditions followed the order: proton-assisted < ligand-promoted < reductive < ligand-promoted reductive. Iron oxide dissolution rate initially increased with increase of citrate concentration in the ascorbic acid-citric acid solutions but declined at higher citrate concentrations indicating that an intermediate citrate concentration was required for optimal cleaning. The mechanism of ligand-promoted reductive dissolution mediated by ascorbate and citrate was investigated through studies of dissolution behaviour under both oxic and anoxic conditions with citrate shown to have a mitigating effect on the consumption of oxygen, apparently by reducing the iron catalyzed oxidation of ascorbate. Kinetic modelling showed that the dynamics of dissolution could be reasonably well simulated with the inclusion of a surface > Fe(III)-citrate-Fe(II) ternary complex which facilitates the detachment of surface Fe(II). Use of dual reagents (ascorbic acid and citric acid) under oxic conditions is recommended for the cleaning of iron-fouled membranes in view of the extreme cleaning effectiveness though it is critical that cleaning conditions be carefully optimised.