Desalination combined with hexavalent chromium reduction in a microbial desalination cell

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Two reactors were operated for 20 desalination cycles (one desalination cycle contained 10 batch-fed operational cycles) with an initial Cr6+ concentration of 100 mg/L and pH values of 2.0 and 3.0, respectively. Curves of current density and cathode resistance are shown in Fig. 7 (A–B). As shown in Fig. 7A, the maximum current density of both reactors with pH 2.0 and 3.0 generally remains at a high level during the 10 desalination cycles. However, the current density dropped quickly from the 11th desalination cycle. This meant that the Cr2O3 deposition had affected the MDC performance. After 1200 h operation, the resistance of the cathodes in both of the two reactors increased rapidly from 7.4 Ω to 605.3 Ω and 5.2 to 1027 Ω for pH 2.0 and pH 3.0, respectively (Fig. 7A). As a result of the increase in cathode resistance, the maximum current density of the MDCs during one desalination cycle was reduced by 42% and 75%, from 785 mA/m2 to 452 mA/m2 for an initial pH of 2.0 and from 364 mA/m2 to 147 mA/m2 for an initial pH of 3.0, respectively. The greater current density reduction at pH 3.0 may be caused by more Cr2O3 being deposited on the cathode surface than under the pH 2.0 condition. As shown in Eq. (9), Cr2O3 could react with H+ and produce Cr3+ under a sufficiently acidic condition [28]. þ Cr2 O3 þ 6H ¼ 3Cr 3þ þ H2 O ð9Þ Moreover, the decline in the MDC performance after long-term operation is not only caused by an increase in cathode resistance but also by bio-fouling on the AEM facing toward the anode [31]. Further study of the removal of Cr2O3 deposition and improvement in MDC performance after long-term operation needs to be investigated in the next step in our research.