Stable and Affordable Phosphonates Removal by Iron Scrap Packed-bed Anode Electrocoagulation under Realistic Conditions: Mechanism and Passivation Mitigation over Long-term Operation

Abstract

Iron electrocoagulation (Fe-EC) exhibits broad application in water remediation towards various pollutants, including emerging organic phosphorus compounds (i.e., phosphonates). However, it suffers relatively high costs due to the frequent replacement of iron anode consumables, particularly electrode fouling. Here we report an iron scrap packed-bed (ISPB) anode electrocoagulation (EC) system for efficiently removing phosphonate. In Na₂SO₄, NaCl and NaHCO₃ electrolytes, the ISPB-EC system effectively removed 39-99% of nitrilotrimethylene triphosphonic acid (NTMP) with 0.1 mM total soluble phosphorus (TSP) concentration at a coulombic dosage of 144 C/L. In contrast, only 2-23% NTMP was eliminated with conventional Fe-EC under identical conditions. We also found the partial conversion of NTMP to inorganic phosphate, primarily attributed to the formation of HO· and Fe(IV)O²⁺ during the oxidation of Fe²⁺ in the ISPB-EC system. We further validated the adaptability and robust efficacy of ISPB-EC in realistic conditions, including actual cooling water (ACW). Our cost calculation suggests that the new system achieves a lower cost (€0.0067/m³) in treating NTMP-loaded ACW than the traditional Fe-EC system (€0.009/m³). Moreover, we addressed the scaling issue in the newly developed ISPB-EC system. We did not notice apparent cathode scaling over short-term batch tests. However, orange-red scales gradually formed on the cathode in the continuous flow experiment, accompanied by an increased cell voltage. To this end, we proposed and validated the strategy of periodic polarity reversal in alleviating the cathode scaling. Notably, the voltage can be reduced to the initial level by refilling the iron scrap after eliminating cathode fouling through polarity reversal, realizing the long-term stable operation of the ISPB-EC system over 336 hours. Our work established an affordable, highly efficient electrocoagulation system using cheap waste iron scrap electrodes to treat phosphonates-contained wastewater.