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·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° to 68°) and cross-linking degree (O_net/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 O_net/N.