Mechanochemically synthesized Fe-Mn binary oxides for efficient As(III) removal: Insight into the origin of synergy action from mutual Fe and Mn doping

Insights into the stability assessment and reaction mechanisms of Mn-oxide-containing adsorbents for As(Ⅲ) removal in filter columns: Migration laws and stabilization mechanisms of Mn element

This study focused on two Mn-oxide-containing adsorbents for As(Ⅲ) removal, namely granular iron-manganese composite oxide (GFMO) and granular iron-manganese-copper composite oxide (GFMCO). The comparative experiments results demonstrated that GFMCO exhibited superior performance in As(Ⅲ) removal and a more obvious Mn(II) release compared to GFMO. Furthermore, this study explored the approaches for the control of manganese release during As(Ⅲ) removal, identifying sodium hypochlorite (NaClO) oxidation followed by manganese sand filtration as the most effective method for capturing released Mn(Ⅱ) in water. Manganese sand columns effectively captured released Mn(Ⅱ) from effluent, while chlorine oxidation significantly improved manganese removal. The positive effect of copper on Mn(Ⅱ) removal by oxidants was also assessed. In addition, the solution pH significantly impacted manganese removal efficiency, with alkaline conditions being the most conducive. Moreover, the presence of sulfite notably accelerated manganese release. Characterization results of the adsorption columns indicated that the manganese element undergoes release, migration, and speciation transformation within the filter systems, where redox reactions, adsorption processes, and autocatalytic oxidation processes were all involved. Not only NaClO oxidation but also autocatalytic oxidation with newly-formed Mn oxides contributed to the transformation from Mn(Ⅱ) to Mn oxides, promoting the stabilization of Mn element in manganese sand filtration columns. This study not only provides valuable insights into the stability of Mn-oxide-containing adsorbents for As(Ⅲ) removal in the filter systems and but also presents a scientific basis on engineered approaches to control the transformation and migration of released manganese ions.

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.

The influence of Mn(II) on transformation of Cr-absorbed Schwertmannite: Mineral phase transition and elemental fate

Schwertmannite (Sch) is considered as an effective remover of Chromium (Cr) due to its strong affinity for toxic Cr species. Since the instability of Sch, the environmental fate of Cr deserves attention during the transformation of Sch into a more stable crystalline phase. The ubiquitous manganese(II) (Mn(II)) probably affects the transformation of Sch and thus the environmental fate of Cr. Therefore, this study investigated the impact of Mn(II) on the transformation of Cr-absorbed Sch (Cr-Sch) and the associated behavior of SO42- and Cr. We revealed that the transformation products of Cr-Sch at pH 3.0 and 7.0 were goethite and Sch, respectively. The presence of Mn(II) weakened the crystallinity of the transformation products, and the trend was positively correlated with the concentration of Mn(II). However, Mn(II) changed the transformation products of Cr-Sch from hematite to goethite at pH 10.0. Mn(II) replaced Fe(III) in the mineral structures or formed Mn-O complexes with surface hydroxyl groups (-OH), thereby affecting the transformation pathways of Sch. The presence of Mn(II) enhanced the immobilization of Cr on minerals at pH 3.0 and 7.0. Sch is likely to provide an channel for electron transfer between Mn(II) and Cr(VI), which promotes the reduction of Cr(VI). Meanwhile, Mn(Ⅱ) induced more -OH production on the surface of secondary minerals, which played an important role in increasing the Cr fixation. In addition, part of the Mn(Ⅱ) was oxidized to Mn(Ⅲ)/Mn(Ⅳ) at pH 3.0 and pH 7.0. This study helps to predict the role of Mn(II) in the transformations of Cr-Sch in environments and design remediation strategies for Cr contamination.

Polyethyleneimine-modified iron-doped birnessite as a highly stable adsorbent for efficient arsenic removal

Remediation of arsenic contamination is of great importance given the high toxicity and easy mobility of arsenic species in water and soil. This work reports a new and stable adsorbent for efficient elimination of arsenic by coating polyethyleneimine (PEI) molecules onto the surface of iron-doped birnessite (Fe-Bir). Characterization results of surface microstructure and crystalline feature (scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectrometer (FTIR) and X-ray photoelectron spectroscopy (XPS), etc.) suggest that Fe-Bir/PEI possesses a fine particle structure, inhibiting the agglomeration of birnessite-typed MnO2 and offering abundant active sites for arsenic adsorption. Fe-Bir/PEI is capable of working in a wide pH range from 3 to 11, with an efficient removal capacity of 53.86 mg/g at initial pH (pH0) of 7. Meanwhile, commonly coexisting anions (NO3-, SO42-, and Cl-) and cations (Na+, K+, Ca2+ and Mg2+) pose no effect on the arsenic removal performance of Bir/PEI. Fe-Bir/PEI exhibits a good reusability for arsenic removal with low Mn and Fe ions leaching after 5 cycles. Besides, Fe-Bir/PEI possesses efficient remediation capability in simulated As-contaminated soil. The modification of PEI in Fe-Bir/PEI can adsorb newly formed As(V), which is impossible for the adsorbent without PEI. Further, the arsenic removal mechanism of Fe-Bir/PEI is revealed with redox effect, electrostatic attraction and hydrogen bonding.

Deep insight on mechanism and contribution of arsenic removal and heavy metals remediation by mechanical activation phosphogypsum

Arsenic-containing wastewater and arsenic-contaminated soil can cause serious environmental pollution. In this study, phosphogypsum with partial mechanical activation of calcium oxide was used to prepare a new phosphogypsum-based passivate (Ca-mPG), and its remediation performance on arsenic-contaminated soil was evaluated in terms of both effectiveness and microbial response. The results showed that the optimum conditions for the preparation of the passivate were optimized in terms of single factor and response surface with a ball milling speed of 200 r/min, a material ratio of 6:4 and a ball milling time of 4 h. Under these conditions, the adsorption capacity was 37.75 mg/g. The leaching concentration of arsenic (As) in the contaminated soil after Ca-mPG modification decreased from 25.75 μg/L to 5.88 μg/L, which was lower than the Chinese national standard (GB/T 5085.3-2007); Ca-mPG also showed excellent passivation effect on other heavy Metals (copper, nickel, cadmium, zinc). In addition, As-resistant bacteria and passivators work together to promote the stabilization effect of contaminants during the remediation of As-contaminated soil. The mechanisms of Cu, As(III)/As(V), Zn, Cd, and Ni removal were related to ion exchange, electrostatic adsorption of substances on heavy metals, calcium binding to other substances to produce precipitation; and microbially induced stabilization of HMs, oxidized. Overall, this study demonstrates an eco-friendly "waste-soil remediation" strategy to solve problems associated with solid waste reuse and remediation of HM-contaminated soils.

Ice-templated synthesis of tungsten oxide nanosheets and their application in arsenite oxidation

Tungsten oxide (WO₃) nanosheets were prepared as catalysts to activate hydrogen peroxide (H₂O₂) in arsenite (As(III)) oxidation. Ice particles were employed as templates to synthesize the WO₃ nanosheets, enabling easy template removal via melting. Transmission electron microscopy and atomic force microscopy revealed that the obtained WO₃ nanosheets were plate-like, with lateral sizes ranging from dozens of nanometers to hundreds of nanometers and thicknesses of <10 nm. Compared to that of the WO₃ nanoparticle/H₂O₂ system, a higher efficiency of As(III) oxidation was observed in the WO₃ nanosheet/H₂O₂ system. Electron spin resonance spectroscopy, radical quenching studies, and As(III) oxidation experiments under anoxic conditions suggested that the hydroperoxyl radical (HO₂^●) acted as the primary oxidant. The WO₃ nanosheets possessed numerous surface hydroxyl groups and electrophilic metal centers, enhancing the production of HO₂^● via H₂O₂ activation. Various anions commonly present in As(III)-contaminated water exhibited little effect on As(III) oxidation in the WO₃ nanosheet/H₂O₂ system. The high oxidation efficiency was maintained by adding H₂O₂ when it was depleted, suggesting that the catalytic activity of the WO₃ nanosheets did not deteriorate after multiple catalytic cycles.

In-situ production of iron flocculation and reactive oxygen species by electrochemically decomposing siderite: An innovative Fe-EC route to remove trivalent arsenic

The removal of trivalent arsenic (As (III)) from water has received extensive attention from researchers. Iron electrocoagulation (Fe-EC) is an efficient technology for arsenic removal. However, electrode passivation hinders the development and application of Fe-EC. In this work, an innovative Fe-EC route was developed to remove As (III) through an electrochemical-siderite packed column (ESC). Ferrous ions were produced from siderite near the anode, and hydroxide was generated near the cathode during the electrochemical decomposition of siderite. As a result, an effect of Fe-EC-like was obtained. The results showed that an excellent removal performance of As (III) (>99%) was obtained by adjusting the parameters (As (III) concentration at 10 mg/L, pH at 7, Na₂SO₄ at 10 mM and the hydraulic retention time at 30 min) and the oxidation rate of As (III) reached 84.12%. The mechanism analysis indicated that As (III) was oxidized to As (Ⅴ) by the produced active oxide species and electrode, and then was removed by capturing on the iron oxide precipitates. As (III) was likely to be oxidized in two ways, one by the reactive oxygen species (possibly •OH, Fe(IV) and •O₂^- species), and another directly by the anode. The long-term effectiveness of arsenic removal demonstrated that ESC process based on the electrochemical-siderite packed column was an appropriate candidate for treating As (III) pollution.

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.

Rapid purification of As(III) in water using iron-manganese composite oxide coupled with sulfite: Importance of the SO5•- radicals

Manganese (Mn)-containing composite metal adsorbents are very effective at removing arsenite (As(III)) from contaminated water, however, the low removal speed and oxidation efficiency have limited their further application. In this study, a nonhomogeneous catalytic oxidation-adsorption system was constructed by coupling iron-manganese composite oxide (FeMnO_x) with sulfite (S(IV)) to enhance the recovery of oxidative capacity and accelerate the removal of As(III). Experimental results showed that the FeMnO_x/S(IV) system decreased the As(III) concentration from 1079 to <10 µg/L within 10 min and almost completely oxidized As(III) to As(V). In contrast, FeMnO_x alone removed only 82.4% of As(III) within 30 min, and 60.0% of the adsorbed As(III) was not oxidized. Meanwhile, the adsorption capacity of FeMnO_x/S(IV) system for As(III) was considerably higher than that of the only-FeMnO_x system (76.5 > 46.3 mg/g). The efficient and fast As(III) removal was attributed to the SO₅^•- radical generated by S(IV) acting as the driving force for the redox cycle between As(III) and Mn(II/III/IV). Several environmental factors (e.g., solution pH and inorganic anions) and the reusability and practicality of FeMnO_x were systematically investigated, and the results further confirmed the superiority of the FeMnO_x/S(IV) system in As(III) removal. In particular, the proposed FeMnO_x nanocellulose aerogel effectively purified arsenic-contaminated groundwater using a fixed-bed column. Thus, FeMnO_x-S(IV) coupling is very promising for the purification of arsenic-contaminated water bodies.