Some heavy metals such as chromium will catalyze the oxidation of chlorine, which will cause the irreversible performance degradation of the diaphragm. This is because Cr6+ is less stable than Cr3+ in water. It seems that metal ions with high oxidation valence have stronger destructive effect. Therefore, the concentration of chromium should be reduced or at least Cr6+ should be reduced to Cr3+ in the pretreatment part.
