Effect of a Permanganate-Bearing Reactive Oxidant on Flocs in Electrocoagulation: Transformations and Interfacial Interactions

Importance of iron complexation and floc formation towards phosphonate removal with Fe-electrocoagulation

Phosphonates are widely used scale inhibitors, but the residual phosphonates in drainage are challenging to remove because of their chelating capacity and resistance to biodegradation. Here, we reported a highly efficient and robust Fe-electrocoagulation (Fe-EC) system for phosphonate removal. Surprisingly, we found for the first time that phosphonates like NTMP were more efficiently removed under anoxic conditions (80% of total soluble phosphorus (TSP) in 4 min) than oxic conditions (0% of TSP within 6 min) in NaCl solution. A similar phenomenon was observed when other phosphonates, such as EDTMP and DTPMP, were removed, highlighting the importance of iron complexation and floc formation toward phosphonate removal with Fe-EC. We also showed that the removal efficiency of NTMP by electrochemically in-situ formed flocs (97%) was much higher than post-adsorption systems (ex-situ, 40%), revealing that the growth of flocs consumed the active site for NTMP adsorption. Beyond the removal of TSP, 10 % of NTMP-P was also degraded after the electrolysis phase, evidenced by the evolution of phosphate-P. However, this did not happen in anoxic or chemical coagulation processes, which confirms the formation of reactive oxygen species via Fe(II) oxidation in the oxic Fe-EC system. The primary removal mechanism of phosphonates is due to their complexation with iron (hydr)oxide generated in the Fe-EC system by forming a Fe-O-P bond. Encouragingly, the Fe-EC system exhibits comparable or even better performance in treating phosphonate-laden wastewater (i.e., cooling water). Our preliminary cost calculation suggests the proposed system (€ 0.009/m3) has a much lower OPEX under oxic conditions than existing approaches. This study sheds light on the removal mechanism of phosphonate and the treatment of phosphonate-laden wastewater by playing with the iron complexion and flocs formation in classical Fe-EC systems.

A membrane fouling control strategy based on a combination of pre-treatment mitigation and in-situ membrane surface regulation using a composite coagulant

Ultrafiltration technology (UF) is efficient in surface water treatment, but its development and widespread application are limited by membrane fouling. Herein, an efficient and stable polymerized ferric titanium coagulant (PFTC) was synthesized and used as a UF pretreatment agent in actual lake water treatment. The control mechanism of PFTC on membrane fouling was investigated from the perspective of organic removal efficiency and in-situ membrane surface regulation. PFTC demonstrated a remarkable affinity for soluble metabolic intermediates and hydrophilic proteins through complexation and hydrogen bonding force, achieving removal efficiencies of 66.4 % for UV254 and 81.3 % for DOC, respectively. The hydrophilic pollutants with high molecular weight and non-saturated structure could be preferentially removed by PFTC due to its diverse hydrolysates including positively charged Fe-based hydrolysates, amorphous Ti-based hydrolysates, and highly polymerized Fe-Ti copolymers. The flocs generated by PFTC exhibited strong hydrophilicity, allowing for the formation of a loose porous cake layer on the ultrafiltration membrane, which acted as a hydrophilic layer to enhance the anti-fouling performance of ultrafiltration membrane. With its dual function of contaminant removal and in-situ membrane surface regulation, PFTC alleviated 98.9 % of membrane fouling. This study provides new insights into membrane fouling control by coagulation pretreatment and efficient treatment of surface water.

Enhanced electrooxidation/electrocoagulation-ultrafiltration membrane process with S2O42- for saline algae-containing surface water treatment: Purification and membrane performance

The surface water in coastal areas involving algae, is often affected by saline and emerging contaminants caused by saltwater intrusion, and expanding aquaculture industry. Therefore, it is necessary to conduct studies to address the issues that affect ecological safety and health of aquatic environments. This study presents the development of an enhanced electrooxidation/electrocoagulation-ultrafiltration (EO/EC-UF) membrane process using S2O42- (DTN@EO/EC-UF) for the treatment of saline water containing algae. Our results have shown that significant removal of NH3-N (95.1 %), UV254 (89.4 %) and algae (75.7 %) was achieved with the addition of S2O42- (DTN). Additionally, an optimal DTN dosage of 40 mg/L was used in the DTN@EO/EC process to enhance water purification, utilizing reactive species such as SO4·- and ·OH. After coupling with the ultrafiltration (UF) process, optimal operating conditions (DTN: 40 mg/L, current density: 4.65 mA/cm2, electrolysis: 60 s) were applied to treat the saline algae-containing surface water. The generated free chlorine, including NHCl2, accounted for approximately 22 % (0.14 mg/L). In addition, DTN significantly improved the ceramic membrane's permeability and anti-fouling characteristics, with a maximum increasing specific flux from 0.76 to 0.93, mainly attributing to the reduced the irreversible fouling resistance. Furthermore, we discovered that common membrane cleaning using acid or base enhanced the DTN@EO/EC-UF process. In conclusion, this study established an innovative DTN@EO/EC-UF process with excellent performance in terms of water purification and membrane self-cleaning. The results provided a promising alternative for treating saline algae-containing surface water.