Mg Fe- layered double hydroxides (LDHs) modified electrode enhanced capacitive deionization for simultaneous phosphorus recovery and copper ions removal

Biogas slurry is a nutrient-rich substance with significant potential for resource utilization. However, its direct use in agricultural fields is limited due to the presence of heavy metal ions such as Cu2+. In this work, Mg Fe-LDHs modified activated carbon (AC) was prepared by a simple co-preparation method, and used as electrodes for capacitive deionization (CDI). The results revealed that the CDI system exhibited high efficiency in removing both phosphorus (P) and copper ions (Cu2+) under weakly acidic conditions (pH = 5). When the influent concentration was 500 mg NH4+-N /L, 20 mg PO43--P/L and 2 mg Cu2+/L, the adsorption capacity could reach 9.30 and 2.03 mg/g for P and Cu2+ with an applied voltage of 1.0 V, which was higher than that of pristine AC (1.13 mg/g for P and 1.35 mg/g for Cu2+). The main electrosorption mechanism for P in Mg Fe-LDHs/AC electrode is ion exchange, while Cu2+ ions are immobilized on the electrode surface. During the treatment of simulated biogas slurry, the CDI system can reduce the concentration of Cu2+ from 2.0 to 0.33 mg/L. P recovery was achieved by regenerating the electrode with 0.2 M NaOH to obtain a relatively pure P solution (17.3 mg/L).