Non-reactive phosphorus (e.g., phosphonate) contributes to eutrophication, while little attention has been paid to its control. The iron-based materials are highly efficient in orthophosphate removal from water, while the correlation between their physicochemical properties and selective phosphonate (e.g. Nitrilotri(methylphosphonic acid) (NTMP)) adsorption is still unclear. In this study, Fe 3 O 4 (magnetite), FeOOH (goethite) and Fe 2 O 3 (hematite) with different crystal structures, particle sizes and surface area were synthesized for NTMP adsorption. The physicochemical properties of the three kinds of iron oxides were characterized. The batch experiments were applied to explore the NTMP adsorption and desorption performance. The maximum adsorption capacity of NTMP on Fe 3 O 4 , FeOOH and Fe 2 O 3 were 4.14, 1.91 and 0.99 mg-P/g, respectively. Results implied the crystal structure and surface area of iron oxides determined the maximum NTMP adsorption capacity. The NTMP adsorption on the three iron oxides was spontaneous, endothermic and randomness increase. The iron oxides showed high selectivity towards NTMP adsorption in water containing anions, while the co-existing Ca 2+ and Mg 2+ remarkably increased the adsorption capacity. The successive adsorption-desorption cycles suggested a favorable reusability of the above iron oxides and a high NTMP desorption efficiency. The specific NTMP adsorption capacity followed the sequence of FeOOH > Fe 2 O 3 > Fe 3 O 4 , and was highly dependent on surface OH − proportion. The NTMP adsorption mechanism was mainly attributed to inner-sphere complexation. This study provides insights into the iron-based adsorbents design for selective phosphonate adsorption in the future.
Ze Liu,Peng Liao,Ao Qian,Zheng Wang,Bowen Yang,Jian Wang,Na Zhang,Shiwei Xie
The efficient decontamination and resource recovery of organophosphonate-laden wastewater remain a critical environmental challenge. In this study, a sequential “electrocoagulation-oxidation” coupled process integrating iron electrocoagulation (EC) with peroxymonosulfate (PMS) activation was developed for the degradation of nitrilotris(methylene phosphonic acid) (NTMP). The initial EC stage achieved near-complete (∼100% within 20 min) NTMP removal via adsorption, and subsequent PMS oxidation of the iron precipitates under optimal conditions (50 mA, Fe:PMS molar ratio of 3:1 transformed 53.20% of the adsorbed NTMP into orthophosphate. Notably, the sequential post-addition of PMS yielded significantly superior mineralization efficiency compared to the pre-addition strategy. Advanced solid-phase characterizations revealed that the activation mechanism is driven by in-situ generated structural Fe(II), predominantly hosted within magnetite formed under neutral initial pH. Mechanistic investigations using radical quenching and electron paramagnetic resonance (EPR) spectroscopy identified sulfate radicals (SO 4 • - ) as the dominant reactive species, with a minor contribution from Fe(IV). The oxidation proceeded primarily via surface-mediated C-P bond cleavage, resulting in the in-situ immobilization of the released orthophosphate on the iron mineral matrix. Crucially, phytotoxicity assays using perennial ryegrass confirmed that this oxidative treatment significantly reduced the ecotoxicity of the precipitates and converted the pollutants into a bioavailable phosphorus source. This work establishes the sequential EC-PMS process as a promising, sustainable strategy for the simultaneous decontamination of phosphonates and the recovery of phosphorus resources.
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/m 3 ) 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.
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