Understanding Nitrilotris(methylenephosphonic acid) reactions with ferric hydroxide

Stable and affordable phosphonates removal by iron scrap packed-bed anode electrocoagulation under realistic conditions: Mechanism and passivation mitigation over long-term operation

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 h. Our work established an affordable, highly efficient electrocoagulation system using cheap waste iron scrap electrodes to treat phosphonates-contained wastewater.

Glyphosate is a transformation product of a widely used aminopolyphosphonate complexing agent

Diethylenetriamine penta(methylenephosphonate) (DTPMP) and related aminopolyphosphonates (APPs) are widely used as chelating agents in household and industrial applications. Recent studies have linked APP emissions to elevated levels of the herbicide glyphosate in European surface waters. However, the transformation processes and products of APPs in the environment are largely unknown. We show that glyphosate is formed from DTPMP by reaction with manganese at near neutral pH in pure water and in wastewater. Dissolved Mn2+ and O2 or suspended MnO2 lead to the formation of glyphosate, which remains stable after complete DTPMP conversion. Glyphosate yields vary with the reaction conditions and reach up to 0.42 mol%. The ubiquitous presence of manganese in natural waters and wastewater systems underscores the potential importance of Mn-driven DTPMP transformation as a previously overlooked source of glyphosate in aquatic systems. These findings challenge the current paradigm of herbicide application as the sole source of glyphosate contamination and necessitate a reevaluation of water resource protection strategies.

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.

Complexation-based selectivity of organic phosphonates adsorption from high-salinity water by neodymium-doped nanocomposite

Organic phosphonates have been widely used in various industries and are ubiquitous in wastewaters, and efficient removal of phosphonates is still a challenge for the conventional processes because of the severe interferences from the complex water constitutions. Herein, an Nd-based nanocomposite (HNdO@PsAX) was fabricated by immobilizing hydrated neodymium oxide (HNdO) nanoparticles inside a polystyrene anion exchanger (PsAX) to remove phosphonates from high-salinity aqueous media. Batch experiments demonstrated that HNdO@PsAX had an excellent adsorption capacity (∼90.5 mg P/g-Nd) towards a typical phosphonate (1-hydrox-yethylidene-1,1-diphosphonic acid, HEDP) from the background of 8 g/L NaCl, whereas negligible HEDP adsorption was achieved by PsAX. Attractively, various coexisting substances (humic acid, phosphate, citrate, EDTA, metal ligands, and anions) exerted negligible effects on the HEDP adsorption by HNdO@PsAX under high salinity. FT-IR and XPS analyses revealed that the inner-sphere complexation between HEDP and the immobilized HNdO nanoparticles is responsible for HEDP adsorption. Fixed-bed experiments further verified that HNdO@PsAX was capable of successively treating more than 4500 bed volumes (BV) of a synthetic high-salinity wastewater (1.0 mg P/L of HEDP), whereas only ∼2 BV of effective treatment capacity was received by PsAX. The exhausted HNdO@PsAX was amenable to a complete regeneration by a binary NaOHNaCl solution without significant loss in capacity. The capability in removing other organic phosphonates and treating a real electroplating wastewater by HNdO@PsAX was further validated. Generally, HNdO@PsAX exhibited a great potential in efficiently removing phosphonates from high-salinity wastewater.

Transformation of Iminodi(methylene phosphonate) on Manganese Dioxides - Passivation of the Mineral Surface by (Formed) Mn2

Aminopolyphosphonates (APPs) are strong chelating agents with growing use in industrial and household applications. In this study, we investigated the oxidation of the bisphosphonate iminodi(methylene phosphonate) (IDMP) - a major transformation product (TP) of numerous commercially used APPs and a potential precursor for aminomethylphosphonate (AMPA) - on manganese dioxide (MnO2). Transformation batch experiments at pH 6 revealed AMPA and phosphate as main TPs, with a phosphorus mass balance of 80 to 92% throughout all experiments. Our results suggest initial cleavage of the C-P bond and formation of the stable intermediate N-formyl-AMPA. Next, C-N bond cleavage leads to the formation of AMPA, which exhibits lower reactivity than IDMP. Reaction rates together with IDMP and Mn2+ sorption data indicate formation of IDMP-Mn2+ surface bridging complexes with progressing MnO2 reduction, leading to the passivation of the mineral surface regarding IDMP oxidation. Compound-specific stable carbon isotope analysis of IDMP in both sorbed and aqueous fractions further supported this hypothesis. Depending on the extent of Mn2+ surface concentration, the isotope data indicated either sorption of IDMP to the mineral surface or electron transfer from IDMP to MnIV to be the rate-limiting step of the overall reaction. Our study sheds further light on the complex surface processes during MnO2 redox reactions and reveals abiotic oxidative transformation of APPs by MnO2 as a potential process contributing to widespread elevated AMPA concentrations in the environment.

New insight into the changes in metal-phosphonate complexes from the addition of CaCO3 to enhance ferric flocculation for efficient phosphonate removal

Due to the stable chelating effect of organic phosphonates in wastewater, phosphonates with increasing emission are difficult to be removed effectively by traditional ferric salt flocculation, which has posed tough challenges for reducing total phosphorus pollution in recent years. In this work, calcium carbonate (CaCO3) was introduced to work together with the widely investigated flocculant of ferric chloride (FeCl3) to realize an efficient removal of nitrilotrismethylenephosphonic acid (NTMP) at much lower dosage of FeCl3. With an aid of synergy effect from together use of CaCO3 and FeCl3, the remaining concentration as low as 0.16 mg-P/L, far below the sewage discharge limit (0.5 mg-P/L), was simply obtained with a significantly reduced Fe/P molar ratio at only 4, resulting from calcium source donor to form more stable Fe-Ca-P tridentate bridging complexes, high affinity towards ferric ions on CaCO3 surface and slow-release alkaline from CaCO3. A comparison among sodium hydroxide (NaOH), calcium hydroxide (Ca(OH)2) and CaCO3 as additives, was carried out to highlight the advantages of using CaCO3 and clarify the mechanism for the greatly improved performance by a set of characterizations including XRD, FTIR, Zeta potential, XPS, SEM-EDS and TG analyses. The addition of CaCO3 in ferric flocculation resulted in further obvious advantages such as 75% shortened settling time and only one-third of sludge volume of the precipitant, beneficial to the sample handling in engineering application. The proposed new approach has been further confirmed to work efficiently on real phosphonate-containing wastewater. Discussion on the interaction between CaCO3 and ferric salts in phosphonate solutions shed new insights into the working mechanism of using CaCO3 for the treatment of phosphonates-containing wastewater.

Two Pathways Compete in the Mn(II)-Catalyzed Oxidation of Aminotrismethylene Phosphonate (ATMP)

Mn(II)-catalyzed oxidation by molecular oxygen is considered a relevant process for the environmental fate of aminopolyphosphonate chelating agents such as aminotrismethylene phosphonate (ATMP). However, the potential roles of Mn(III)ATMP-species in the underlying transformation mechanisms are not fully understood. We combined kinetic studies, compound-specific stable carbon isotope analysis, and equilibrium speciation modeling to shed light on the significance of such Mn-ATMP species for the overall ATMP oxidation by molecular oxygen. The fraction of ATMP complexed with Mn(II) inversely correlated with both (i) the Mn(II)-normalized transformation rate constants of ATMP and (ii) the observed carbon isotope enrichment factors (ε_c-values). These findings provide evidence for two parallel ATMP transformation pathways exhibiting distinctly different reaction kinetics and carbon isotope fractionation: (i) oxidation of ATMP present in Mn(III)ATMP complexes (ε_c ≈ -10 ‰) and (ii) oxidation of free ATMP by such Mn(III)ATMP species (ε_c ≈ -1 ‰) in a catalytic cycle. The higher reaction rate of the latter pathway implies that aminopolyphosphonates can be trapped in catalytic Mn-complexes before being transformed and suggests that Mn(III)ATMP might be a potent oxidant also for other reducible solutes in aqueous environments.

Sorption of recalcitrant phosphonates in reverse osmosis concentrates and wastewater effluents - influence of metal ions

Magnetic microparticles functionalized with tailored ZnFeZr oxyhydroxide adsorbent were used for the reversible sorption of orthophosphate and recalcitrant organo-phosphonates from wastewater. The loaded particles were harvested magnetically from water, regenerated in an alkaline solution and reused numerous times. The applicability of the technology to treat brackish water reverse osmosis concentrates was tested under controlled synthetic conditions by investigating the influence of typical metals (Ca²⁺, Pb²⁺, Cu²⁺) on the removal of common phosphonates (HEDP, NTMP, EDTMP), and vice versa. When present at equimolar concentrations, metal cations enhanced the adsorption of phosphonates and were co-adsorbed at pH 4.0-4.5 (with removals of 83-93% for Pb²⁺ and 53-73% for Cu²⁺), likely through ternary complex formation. In the absence of metals, at pH > pH_PZC ∼ 7 (the material point of zero charge), a drop in adsorption efficiency was observed for orthophosphate and all phosphonates. Thus, at pH 7, an increased adsorbent dose (>0.1 g/L) was necessary to remove 1 mg/L NTMP-P in 30 min. The reusability and effluent polishing potential of the ZnFeZr particles was demonstrated in a pilot test with municipal wastewater throughout 55 adsorption/desorption cycles without any drop in performance. Consistent removal of the non-reactive phosphorus species to ultra-low concentrations (<0.05 mg/L P_tot) and complete orthophosphate elimination (<0.005 mg/L PO₄-P) was maintained under optimal conditions.

Batch Studies of Phosphonate and Phosphate Adsorption on Granular Ferric Hydroxide (GFH) with Membrane Concentrate and Its Synthetic Replicas

Phosphonates are widely used as antiscalants for softening processes in drinking water treatment. To prevent eutrophication and accumulation in the sediment, it is desirable to remove them from the membrane concentrate before they are discharged into receiving water bodies. This study describes batch experiments with synthetic solutions and real membrane concentrate, both in the presence of and absence of granular ferric hydroxide (GFH), to better understand the influence of ions on phosphonate and phosphate adsorption. To this end, experiments were conducted with six different phosphonates, using different molar Ca:phosphonate ratios. The calcium already contained in the GFH plays an essential role in the elimination process, as it can be re-dissolved, and, therefore, increase the molar Ca:phosphonate ratio. (Hydrogen-)carbonate ions had a competitive effect on the adsorption of phosphonates and phosphate, whereas the influence of sulfate and nitrate ions was negligible. Up to pH 8, the presence of Ca^II had a positive effect on adsorption, probably due to the formation of ternary complexes. At pH > 8, increased removal was observed, with either direct precipitation of Ca:phosphonate complexes or the presence of inorganic precipitates of calcium, magnesium, and phosphate serving as adsorbents for the phosphorus compounds. In addition, the presence of (hydrogen-)carbonate ions resulted in precipitation of CaCO₃ and/or dolomite, which also acted as adsorbents for the phosphorus compounds.

Batch studies of phosphonate adsorption on granular ferric hydroxides

Phosphonates are widely used in various industries. It is desirable to remove them before discharging phosphonate-containing wastewater. This study describes a large number of batch experiments with adsorbents that are likely suitable for the removal of phosphonates. For this, adsorption isotherms for four different granular ferric hydroxide (GFH) adsorbents were determined at different pH values in order to identify the best performing material. Additionally, the influence of temperature was studied for this GFH. A maximum loading for nitrilotrimethylphosphonic acid (NTMP) was found to be ∼12 mg P/g with an initial concentration of 1 mg/L NTMP-P and a contact time of 7 days at room temperature. Then, the adsorption of six different phosphonates was investigated as a function of pH. It was shown that GFH could be used to remove all investigated phosphonates from water and, with an increasing pH, the adsorption capacity decreased for all six phosphonates. Finally, five adsorption-desorption cycles were carried out to check the suitability of the material for multiple re-use. Even after five cycles, the adsorption process still performed well.

Stable carbon isotope analysis of polyphosphonate complexing agents by anion chromatography coupled to isotope ratio mass spectrometry: method development and application

Compound-specific carbon isotope analysis (carbon CSIA) by liquid chromatography/isotope ratio mass spectrometry (LC-IRMS) is a novel and promising tool to elucidate the environmental fate of polar organic compounds such as polyphosphonates, strong complexing agents for di- and trivalent cations with growing commercial importance over the last decades. Here, we present a LC-IRMS method for the three widely used polyphosphonates 1-hydroxyethane 1,1-diphosphonate (HEDP), amino tris(methylenephosphonate) (ATMP), and ethylenediamine tetra(methylenephosphonate) (EDTMP). Separation of the analytes, as well as ATMP and its degradation products, was carried out on an anion exchange column under acidic conditions. Quantitative wet chemical oxidation inside the LC-IRMS interface to CO₂ was achieved for all three investigated polyphosphonates at a comparatively low sodium persulfate concentration despite the described resilience of HEDP towards oxidative breakdown. The developed method has proven to be suitable for the determination of carbon isotope fractionation of ATMP transformation due to manganese-catalyzed reaction with molecular oxygen, as well as for equilibrium sorption of ATMP to goethite. A kinetic isotope effect was associated with the investigated reaction pathway, whereas no detectable isotope fractionation could be observed during sorption. Thus, CSIA is an appropriate technique to distinguish between sorption and degradation processes that contribute to a concentration decrease of ATMP in laboratory batch experiments. Our study highlights the potential of carbon CSIA by LC-IRMS to gain a process-based understanding of the fate of polyphosphonate complexing agents in environmental as well as technical systems.

Removal of phosphonates from synthetic and industrial wastewater with reusable magnetic adsorbent particles

This work proposes a technology for phosphonate removal from wastewater using magnetically separable microparticles modified with a tailored ZnFeZr-oxyhydroxide adsorbent material which proved to be highly efficient, reaching a maximum loading of ∼20 mg nitrilotrimethylphosphonic acid-P/g (215 μmol NTMP/g) at room temperature, pH 6 and 30 min contact time. The adsorption process at pH < 7 was fast, following the pseudo-second-order kinetics model. Furthermore, NTMP adsorption onto ZnFeZr-oxyhydroxide proved to be endothermic. At pH > pH_pzc ≈7 (point of zero charge of the material) a drop in adsorption efficiency was observed for phosphate and for five different investigated phosphonates. Adsorption of NTMP could not be detected at pH > 8, however, the presence of more than 0.5 mM Ca^II improved significantly the adsorption efficiency. Successful reusability of the engineered particles was demonstrated throughout 30 loading cycles by changing the operational conditions (dose, pH) to optimize the performance. At optimal conditions, complete phosphonate removal was observed even after 30 cycles of particles' reuse in a synthetic NTMP-solution and DTPMP-rich membrane concentrate. In each cycle, phosphorus was desorbed and concentrated in a 2 M NaOH. Industrial phosphonate-containing wastewaters rich in calcium, e.g. membrane concentrates, proved to be especially suitable for treatment with the particles.

Optimized Procedure for Determining the Adsorption of Phosphonates onto Granular Ferric Hydroxide using a Miniaturized Phosphorus Determination Method

This paper introduces a procedure to investigate the adsorption of phosphonates onto iron-containing filter materials, particularly granular ferric hydroxide (GFH), with little effort and high reliability. The phosphonate, e.g., nitrilotrimethylphosphonic acid (NTMP), is brought into contact with the GFH in a rotator in a solution buffered by an organic acid (e.g., acetic acid) or Good buffer (e.g., 2-(N-morpholino)ethanesulfonic acid) [MES] and N-cyclohexyl-2-hydroxyl-3-aminopropanesulfonic acid [CAPSO]) in a concentration of 10 mM for a specific time in 50 mL centrifuge tubes. Subsequently, after membrane filtration (0.45 µm pore size), the total phosphorus (total P) concentration is measured using a specifically developed determination method (ISO_mini). This method is a modification and simplification of the ISO 6878 method: a 4 mL sample is mixed with H₂SO₄ and K₂S₂O₈ in a screw cap vial, heated to 148-150 °C for 1 h and then mixed with NaOH, ascorbic acid and acidified molybdate with antimony(III) (final volume of 10 mL) to produce a blue complex. The color intensity, which is linearly proportional to the phosphorus concentration, is measured spectrophotometrically (880 nm). It is demonstrated that the buffer concentration used has no significant effect on the adsorption of phosphonate between pH 4 and 12. The buffers, therefore, do not compete with the phosphonate for adsorption sites. Furthermore, the relatively high concentration of the buffer requires a higher dosage concentration of oxidizing agent (K₂S₂O₈) for digestion than that specified in ISO 6878, which, together with the NaOH dosage, is matched to each buffer. Despite the simplification, the ISO_mini method does not lose any of its accuracy compared to the standardized method.

B-CD8+ T Cell Interactions in the Anti-Idiotypic Response against a Self-Antibody

P3 is a murine, germline, IgM mAb that recognizes N-glycolylated gangliosides and other self-antigens. This antibody is able to induce an anti-idiotypic IgG response and B-T idiotypic cascade, even in the absence of any adjuvant or carrier protein. P3 mAb immunization induces the expression of activation markers in a significant percentage of B-1a cells in vivo. Interestingly, transfer of both B-1a and B-2 to BALB/Xid mice was required to recover anti-P3 IgG response in this model. In fact, P3 mAb activated B-2 cells, in vitro, inducing secretion of IFN-γ and IL-4, although this activation was not detected ex vivo. Interestingly, naïve CD8+ T cells increased the expression of activation markers and IFN-γ secretion in the presence of B-1a cells isolated from P3 mAb-immunized mice, even without in vitro restimulation. In contrast, B-2 cells were able to stimulate CD8+ T cells only if P3 was added in vitro. Using bioinformatics, a MHC class I-binding peptide from P3 VH region was identified. P3 mAb was able to induce a specific CTL response in vivo against cells presenting this peptide. Both humoral and CTL anti-idiotypic responses could be mechanisms to protect against the self-reactive antibody, contributing to keeping the tolerance to self-antigens.