Effect of aluminum and manganese ions on the chlorination of polyamide membranes

The deterioration of separation performance of polyamide membranes after chlorine exposure is a core issue in the application process of membrane technology. The co-existence of metal ions (i.e., Al3+ and Mn2+), which are ubiquitous in different types of water bodies, may complicate the chlorination process. This study systematically evaluated membrane degradation under the simultaneous presence of free chlorine and Al3+ or Mn2+. The membrane water flux after chlorine exposure of 2400 mg/L & sdot;h was decreased by 70 %, of which 49 % was resulted from chlorine exposure, attributing to competing effects of the decreased hydrophilicity (contact angle increased from 56 degrees to 68 degrees) and cross-linking degree (Onet/N ratio increased from 1.1 to 2.3). The changes in the physiochemical properties of membranes were due to chlorination and hydrolysis induced by chlorination. The addition of Al3+ or Mn2+ to chlorine resulted in a further reduction of 8 %-13 % for water flux, and a variation as high as 21 % (increase) and 15 % (decrease) for salt rejection compared to virgin membrane at different exposure periods. These cations similarly resulted in a more hydrophobic membrane surface with varied surface charge and the formation of deposition layers. In addition, Al3+ and Mn2+ did not affect chlorination degree (chlorine incorporation). However, Al3+ promoted chlorination-promoted hydrolysis, while Mn2+ inhibited, reflected by the opposite trends of Onet/N.