Removal Of Mn Research Articles

Comprehensive treatment of waste salts from electrolytic manganese metal industry and recovery of manganese, magnesium, and ammonium.

A large amount of water-soluble waste salts is produced from traditional electrolytic manganese metal industry (EMMWS), and landfill treatment of the EMMWS increases the risk of pollution of the surrounding water and soil. The EMMWS mainly comprises sulphates with highly active components that can be recycled as secondary resources. Herein, a simple and efficient process comprising four-stage precipitation and separation, two-stage crystallisation, molten salt treatment, and water-leaching is proposed for comprehensive treatment of the EMMWS, as well as the recovery of manganese, magnesium, and ammonium, systematically investing the feasibility of this strategy. The results indicate that 95.3% Mn in the saturated EMMWS solution containing NH3 can be selectively oxidised to Mn oxides by O2(g), and the residual Mn can be thoroughly removed to 0.31mg/L with (NH4)2S2O8 oxidation. 92.78% Mg in the solution after Mn removal can be precipitated as 4MgCO3·Mg(OH)2·4H2O with NH4HCO3 and NH3·H2O, and the 98.81% of residual Mg can be removed as struvite with concentrated H3PO4. (NH4)2SO4 product can be obtained by crystallisation after Mn and Mg separation. 97.49% Mn in the Mn oxides can be converted into MnSO4 via (NH4)2SO4 molten salt treatment, and a light pink MnSO4·H2O product is obtained with water-leaching and crystallisation operations. Following the proposed process, the Mg, Mn, and NH4+/NH3 in EMMWS are recycled as 4MgCO3·Mg(OH)2·4H2O, struvite, MnSO4·H2O, and (NH4)2SO4 products. All the products obtained meet the relevant national standards. Economic analysis shows that a profit of US$282.45 might be achieved for each ton of EMMWS treated. This process is of great significance for the effective treatment of EMMWS and the recovery of manganese, magnesium, and ammonium resources.

Microbial-induced reassembly of phosphate sol: New insight into the removal and fixation of divalent manganese in natural aquatic environment.

The commonly used precipitation method struggles to effectively remove low-concentration heavy metals from water. Herein, we demonstrate that the formation of non-settleable phosphate sols at a low low-concentration is the main reason by using phosphate precipitation as an example and report a new method called microbial-induced reassembly (MIR) of the phosphate sols for the removal and fixation of divalent manganese (Mn(II)) from low-concentration wastewater under neutral conditions. Under the induction of microorganisms, the Mn3(PO4)2 sols formed in low Mn(II) concentration could be reassembled into larger and flower-like precipitates with good settleability, allowing for the removal and fixation of low-concentration Mn(II) through natural settlement. Moreover, MIR is able to reduce Mn(II) levels from 10mg/L to as low as 0.1mg/L, achieving nearly 100% removal. The interaction between bacterial protein functional groups and Mn2+ ions drives the reassembly of Mn3(PO4)2 sols. MIR of phosphate sol is applicable to both Gram-negative bacteria, such as Escherichia coli, and Gram-positive bacteria, such as Staphylococcus aureus, as well as mixed aerobes. It is also suitable for the removal and fixation of other heavy metals like copper and zinc. This study offers a novel approach for the removal of low-concentration heavy metals from water, more importantly provides a new insight into the migration and fixation of heavy metals in the form of phosphate precipitates induced by microbes in natural aquatic environments.

Composite Treatment Module for Removing Acidity and Metal (Loid)S from Acid Rock Drainage

This study evaluates a two-stage approach for remediating acid rock drainage (AMD) generated from an abandoned coal mine site. The method combines the adsorption and chemical precipitation properties of drinking water treatment residue (WTR) in a continuous flow-through fluidized filter bed with hydroponic phytoremediation using Vetiver grass to remove dissolved metal(loid)s. In the first stage, the filter demonstrated exceptionally high removal efficiencies for several metals, including up to 99.67% of Al (<1 mg/L), 99.46% of Fe (<2 mg/L), and 92.48% of As. Significant removal efficiencies were also achieved for Zn (69.24%) and Cu (62.85%). However, relatively low removal efficiencies were observed for Mn (12.50%), Ni (26.74%), and Co (21.36%). Additionally, the system effectively reduced acidity, increasing the pH from 2.67 to a near-neutral level of 6.13. The second stage, a 30-day Vetiver grass treatment, further reduced Mn concentrations by 56.9%, improving upon the limited Mn removal observed in the fluidized filter column. This underscores the complementary role of hydroponic phytoremediation in enhancing the overall metal removal process. This innovative and cost-effective system demonstrates significant potential for AMD remediation, achieving simultaneous removal of metal(loid)s and neutralizing acidity in contaminated mine water.

Toward one-step As(III) removal in ultrafiltration with in situ BioMnOx cake layer: Mechanism and feasibility insights.

Inorganic arsenic (As) is one of the most significant chemical contaminants in drinking water worldwide. Although membrane-based technologies are commonly used for As removal, they often encounter challenges including complex operation, high energy consumption, and the need for chemical addition. To address these challenges, we proposed a one-step ultrafiltration (UF) process empowered by in situ biogenic manganese oxides (BioMnOx) cake layers without any additional chemicals, to treat source water contaminated with both As and manganese (Mn). During the filtration, BioMnOx continuously generated on the membrane surface with the oxidation of Mn2+ by Mn-oxidizing bacteria. The in situ generated BioMnOx cake layer exhibited a heterogeneous structure, high specific surface area, and significant catalytic activity. Notably, with this BioMnOx cake layer, the one-step UF successfully achieved a nearly 100% removal of As(III). This high efficiency is due to the catalytic oxidation of As(III) to As(V) by BioMnOx, followed by the adsorption of As(V) onto the BioMnOx surface. With the removal of Mn2+, new BioMnOx was continuously formed, which provided new catalytic and adsorption sites, thereby enabling a self-sustained removal of As(III). In addition to the advantages of simple operation and chemical free, the process also exhibited a good economic feasibility with a low energy consumption (0.078 kWh/m3) and a low operating cost (0.229 CNY/m3). Our study provides an example to show that cake layers on membranes are not inherently detrimental and can be beneficial in specific applications.

CTAB-modified alkali-activated binder derived from Favia corals and glass waste: A novel bio-based adsorbent for effective removal of Mn(VII) ions from aqueous solutions

CTAB-modified alkali-activated binder derived from Favia corals and glass waste: A novel bio-based adsorbent for effective removal of Mn(VII) ions from aqueous solutions

Influence of filter backwashing on iron, manganese, and ammonium removal in dual-media rapid sand filters used for drinking water production

Iron (Fe), manganese (Mn), and ammonium (NH4+) removal from groundwater using rapid sand filtration is a widely employed method in drinking water production. Over time, Fe and Mn oxides accumulate in the filter, which necessitates frequent backwashing to avoid clogging. In this study, we investigated the impact of backwashing on the microbial community and filter chemistry in a dual-media filter comprising anthracite and sand layers. Specifically, we focused on the removal of Fe, Mn, and NH4+ over the runtime of the filter. With increasing runtime, depth profiles of dissolved and particulate Fe revealed the buildup of Fe oxide flocs, causing Fe2+ and Mn2+ oxidation and nitrification to occur at greater depths within the filter. Towards the end of the filter runtime, breakthrough of suspended Fe oxides was observed, likely due to preferential flow. Backwashing effectively removed metal oxide flocs and restored the Fe removal efficiency in the top layer of the filter. While the two layers remained separate, the anthracite and sand layers themselves fully mixed during backwashing, leading to a homogenous distribution of the microbial community within each layer. Methyloglobulus and Gallionella were the predominant organisms in the anthracite layer, likely catalyzing methane and Fe2+ oxidation, respectively. The nitrifying community of the anthracite consisted of Nitrosomonas, Candidatus Nitrotoga, and Nitrospira. In contrast, the nitrifying community in the sand layer was dominated by Nitrospira. Backwashing minimally affected the microbial community composition of the filter medium except for Gallionella, which were preferentially washed out. In conclusion, our research offers a molecular and geochemical basis for understanding how backwashing influences the performance of rapid sand filters.

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.

Selective Leaching of Lithium and Beyond: Sustainable Eggshell-Mediated Recovery from Spent Li-Ion Batteries

This study introduces an innovative strategy for the selective leaching of lithium from spent Li-ion batteries. Based on thermodynamic assessments and exploiting waste eggshells as a source of calcium carbonate, an impressive 38% of lithium was dissolved selectively through mechanical milling and water leaching, outperforming conventional thermochemical methods. Afterwards, a hydrogen peroxide-assisted sulfuric acid leaching was also implemented to solubilize targeted elements (Mn, Co, Ni, and Li), with an exceptional 99% efficiency in Mn removal from the leachate using potassium permanganate and a pH range of 1.5 to 3.5. Selective separations of Co and Ni were then facilitated utilizing CYANEX 272 and n-heptane. This comprehensive study presents a promising and sustainable avenue for the effective recovery of Li and associated co-elements from spent lithium batteries.

Predicting removal of arsenic from groundwater by iron based filters using deep neural network models

Arsenic (As) contamination in drinking water has been highlighted for its environmental significance and potential health implications. Iron-based filters are cost-effective and sustainable solutions for As removal from contaminated water. Applying Machine Learning (ML) models to investigate and optimize As removal using iron-based filters is limited. The present study developed Deep Learning Neural Network (DLNN) models for predicting the removal of As and other contaminants by iron-based filters from groundwater. A small Original Dataset (ODS) consisting of 20 data points and 13 groundwater parameters was obtained from the field performances of 20 individual iron-amended ceramic filters. Cubic-spline interpolation (CSI) expanded the ODS, generating 1600 interpolated data points (IDPs) without duplication. The Bayesian optimization algorithm tuned the model hyper-parameters and IDPs in a Stratified fivefold Cross-Validation (CV) setup trained all the models. The models demonstrated reliable performances with the coefficient of determination (R2) 0.990–0.999 for As, 0.774–0.976 for Iron (Fe), 0.934–0.954 for Phosphorus (P), and 0.878–0.998 for predicting manganese (Mn) in the effluent. Sobol sensitivity analysis revealed that As (total order index (ST) = 0.563), P (ST = 0.441), Eh (ST = 0.712), and Temp (ST = 0.371) are the most sensitive parameters for the removal of As, Fe, P, and Mn. The comprehensive approach, from data expansion through DLNN model development, provides a valuable tool for estimating optimal As removal conditions from groundwater.

Evaluating manganese removal in groundwater using pilot scale biofilters: The role of filter media characteristics during start-up

This study investigated the influence of filter media characteristics on manganese (Mn) removal in groundwater biofilters during the start-up phase. Six pilot scale biofilters containing three different granular activated carbons (GAC), two anthracite, and one sand media were run for 133 days to examine their Mn removal performance at a drinking water utility, while also monitoring ATP and bacterial growth as indicators of biological activity. Key findings demonstrate the critical role of media characteristics, especially for GAC media. Initial Mn adsorption on GAC, with its higher surface area, higher macropore volume, and surface charge, promoted physicochemical and biological oxidation, thus contributing to the early onset of Mn removal during the start-up of the GAC biofilters. In contrast, biological processes dominated Mn removal on anthracite and sand biofilters during start-up. As expected, the presence of Mn-oxidizing bacteria was detected in biofilters, and ATP levels were correlated to Mn removal until the biofilters were acclimated, showing the potential of ATP as an acclimation monitoring metric. Once acclimated, all biofilters consistently reduced Mn levels from 60.9 ± 4.5 µg/L to below 5 µg/L (>90 % removal), while concurrently removing iron, thereby highlighting the biofilter's effectiveness at low water temperatures (<15 °C). This study demonstrates the advantage of GAC media for Mn removal in biofilters during start-up in regions with lower water temperatures.

Structure–reactivity relationships in the removal efficiency of catechol and hydroquinone by structurally diverse Mn-oxides

Catechol and hydroquinone are widely present hydroxybenzene isomers in the natural environment that induce environmental toxicities. These hydroxybenzene compounds can be effectively removed by manganese (Mn)-oxides via sorption and oxidative degradation processes. In the present study, we investigated the structure–reactivity relationships in the sorption and oxidation of catechol and hydroquinone on Mn-oxide surfaces. Two widely present Mn-oxides, including hydrous Mn oxide (HMO) and cryptomelane, comprised of layer and tunnel structures, respectively, are specifically studied. Effects of Mn-oxide structures and environmental pH conditions on the removal efficiency of these hydroxybenzene compounds, via sorption and oxidative degradation, are investigated. Cryptomelane, which has a higher specific surface area than HMO, possesses a higher sorption and oxidation capacity. The complexation mechanisms of catechol and hydroquinone vary due to their structure-induced difference in reactivity. Catechol reduced and dissolved more Mn from Mn-oxides than hydroquinone, accompanied by a higher C loss of catechol-C, suggesting a higher reactivity of catechol. Structural changes occurred in the Mn-oxides resulting from reaction with catechol and hydroquinone: reduction of Mn(IV), corresponding formation of Mn(III) and Mn(II) in the mineral, and free Mn2+ ions released into the suspension. These insights could help us better understand and predict the fate of hydroxybenzene compounds in Mn-oxide-rich soils and wastewater treatment systems that generate Mn-oxides via Mn removal and the associated environmental toxicity.

Removal of Impurities from Nickel and Cobalt in Mixed Hydroxide Precipitate with Solvent Extraction Using Di(2-Ethylhexyl) Phosphoric Acid

ABSTRACT This study investigates the removal of major impurities, namely Fe, Al, Zn, and Mn, in a solution produced by sulfuric acid leaching of mixed hydroxide precipitate from the Ni and Co via solvent extraction using di(2-ethylhexyl) phosphoric acid (D2EHPA). The results of extraction experiments showed that the extracted Ni, Co, Fe, Al, Zn, Mn, and Mg increased as the equilibrium pH, D2EHPA concentration, and O/A ratio increased. The optimum extraction condition was achieved at a pH of 3.5, a 20% (v/v) D2EHPA concentration, and an O/A ratio of 1, with a complete (>99.9%) extraction of Al, Fe, Zn, and 97.33% Mn with co-extracted Ni and Co of 22.32% and 29.13%, respectively, in a single extraction stage. Complete Mn removal can be achieved in two extraction stages. Co-extracted Ni and Co can be scrubbed out using a mild sulfuric acid solution at a pH of 1.2 with only less than 2% of the Al, Fe, Zn, and Mn co-scrubbed. McCabe-Thiele diagrams of the scrubbing processes were constructed, and the results showed that Ni and Co can be scrubbed out using a mild acidic solution at a pH of 1.2, O/A ratio of 1, and temperature of 40°C for 10 min of mixing. Zn and Mn can be stripped completely using 0.5 M sulfuric acid solution at an O/A of 1 and temperature of 40°C for 10 min of mixing, while Fe and Al remained unstripped. Fe and Al can subsequently be stripped via reductive stripping.

Fe and Mn Removal from Acid Mine Drainage by Utilizing Chlorella sorokiniana and Monoraphidium neglectum as Biosorbent

The mining industry generates acid mine drainage (AMD) characterized with a low pH value and high dissolved metal concentration that leads to the negative impacts on the environment and human health. The objectives of this research were to investigate the growth response of mixed culture of microalgae Chlorella sorokiniana and Monoraphidium neglectum in a liquid media contaminated with AMD; generate the optimum environmental conditions (pH value and contact time) to determine the efficiency biosorption of iron and manganese contained in the solution of AMD into the consortium of microalgae; and quantify the maximum removal amount of iron and manganese contained in the solution of AMD by utilizing microalgae consortium of Chlorella sorokiniana and Monoraphidium neglectum as biosorbent. AMD used in this research was characterized with a pH value of 1.65 with iron and manganese concentrations of 8.28 mg/L and 4.57 mg/L. The research of biosorption was conducted in 150 rpm with pH level variations of 4, 5, and 6, and contact time variations of 60, 120, and 180 min. The maximum value of iron and manganese removals occurred when pH level reached 5 at 180 min of contact time with removal efficiency of 89.73% for iron and 94.53% for manganese. The results proved that the mixed culture of microalgae namely Chlorella sorokiniana and Monoraphidium neglectum can be utilized to remove iron and manganese contained in acid mine drainage.

Development and validation of a 1D gas–liquid model for dissolved Mn(II) removal by oxidation process in a square bubble column

Modeling multiphase reactors requires an in-depth multidisciplinary analysis of various phenomena including the chemical kinetics, oxygen mass transfer, as well as the simulation of flows within these contactors. In this study, a 1D model of two-phase flow for Mn(II) removal from drinking water by aeration process is developed and validated. This model is based on the Eulerian description of two-phase gas–liquid flow with the comparison between a one-medium bubble size and a two-bubble class description to represent the bubble population in the heterogeneous flow regime. All the relevant chemical species are simulated by coupling hydrodynamics, gas–liquid mass transfer, and reaction kinetic. A pseudo-first-order model accounting for homogeneous and heterogeneous kinetics is considered for Mn(II) oxidation. The performance of the developed 1D model is compared and validated with experimental data. The 1D model was able to predict quantitatively Mn oxidation as a function of pH, initial Mn(II) and initial Mn(IV) concentrations.

Coupled effects of iron (hydr)oxides and clay minerals on the heterogeneous oxidation of aqueous Mn(II) and crystallization of manganese (hydr)oxides

The formation of nanominerals and mineral nanoparticles (NMMNs) has drawn broad attention due to their high reactivity and omnipresence in the environment. While the heterogeneous formation of NMMNs on surfaces of various minerals has been extensively studied, there is limited understanding of how mineral heteroaggregates influence this process. In this study, we investigated how heteroaggregates of iron (hydr)oxides and clay minerals affect the heterogeneous oxidation of aqueous Mn(II) and crystallization of manganese (hydr)oxides (MnOx). Our results revealed that iron (hydr)oxides (ferrihydrite) and clay minerals (kaolinite or montmorillonite) in heteroaggregates exerted coupled effects on these processes, dictating the distribution of Mn and the morphology of MnOx. Specifically, ferrihydrite catalyzed gradual oxidative removal of Mn(II) and triggered MnOx nucleation; in contrast, kaolinite/montmorillonite rapidly adsorbed Mn(II) but hardly catalyzed its oxidation. These reactions collectively resulted in fast adsorption and gradual oxidation of Mn(II) on the heteroaggregates. Further, MnOx nanoparticles formed on ferrihydrite surfaces migrated to kaolinite/montmorillonite surfaces, leading to interactions between MnOx and various component minerals within the heteroaggregates. This significantly altered the subsequent growth pathways and the eventual morphology of MnOx. Consequently, while MnOx nanoparticles in the ferrihydrite-only system aggregated freely and formed well-extended nanowires, those in the ferrihydrite-kaolinite system predominantly became short nanorods due to the immobilization by kaolinite surfaces; in the ferrihydrite-montmorillonite system, considerable MnOx nanoparticles attached to montmorillonite surfaces due to strong electrostatic attraction, and subsequently grew into blocky particles via particle attachment. These findings illustrate that surface reactivities of heteroaggregated ferrihydrite and kaolinite/montmorillonite are coupled when they interact with aqueous Mn(II) or MnOx. Our work exemplifies, for the first time, the cooperation between surfaces of various minerals during the heterogeneous formation of NMMNs. Findings from this study also enhance our understanding of MnOx formation on surfaces with diverse atomic structures, and contribute to the knowledge of Mn cycling in the environment.

Removal of Mn(II), Cd(II) and Ni(II) metal ions from aqueous solutions using modified chitosan by simple method

Removal of Mn(II), Cd(II) and Ni(II) metal ions from aqueous solutions using modified chitosan by simple method

Sr, Mn, and Zn extraction and separation from zinc electrolyte crystallization sediment with Ca and Sr sulfate cocrystallization structure: Reduction roasting reconstruction mechanism

Zinc electrolyte crystallization sediment (ZECS) generated in the zinc hydrometallugy industry contains valuable metals of strontium (Sr), manganese (Mn), and zinc (Zn), which has economical value but poses potential detrimental to the environment. However, the extraction and recovery of Sr is extremely difficult due to the thermodynamically stable Ca and Sr sulfate cocrystallization structure in ZECS. The destruction of refractory sulfate cocrystallization structure is a prerequisite for Sr extraction, and the carbothermal reduction roasting method is demonstrated to be efficient for the phase reconstruction. In this study, a stepwise Sr, Mn, and Zn recovery process from ZECS via carbothermal reduction roasting-acid leaching-selective precipitation-multistage extraction was proposed. In the reduction roasting procedure, all the sulfates including CaSO4, SrSO4, and Ca0.7Sr0.3SO4 are reduced to soluble sulfides. Meanwhile, most MnO2 is converted to a low-valence Mn product (MnO) that is soluble in acid, and the reduced Zn(g) is volatilized which can be collected from the flue dust. In the acid leaching procedure, the leaching efficiencies of Sr, Ca, and Mn are as high as 97.34 %, 97.44 %, and 95.49 % respectively, while 99.50 % Si in the form of silica is enriched in the leaching residue. After that, high-purity Sr solution was obtained after selective precipitation for Mn removal and D2EHPA multistage extraction for Ca removal. 97.41 % Mn in the leaching solution is precipitated by adjusting the pH value to 9.5 with NaOH solution. After four-stage extraction, 79.60 % Sr and only 0.35 % Ca are retained in the aqueous phase. The purified solution contains 2.88 g/L Sr, 0.052 g/L Ca, and 0.014 g/L Mn, which can be used to prepare industrial SrCO3 product via the carbonation operation. Thermodynamic behavior and reduction roasting reconstruction mechanism of Ca and Sr sulfate cocrystallization structure in ZECS were discussed in this work. The distributions of main metal elements (Sr, Ca, Mn, and Zn) in ZECS during the carbothermal reduction roasting-acid leaching-selective precipitation-multistage extraction process were also systematically studied.

Zeolite functionalized with macroalgae as novel material for Fe and Mn removal from real acid mine drainage

The treatment of heavy metals such as Fe and manganese Mn remains a critical global issue, necessitating solutions that are not only effective and efficient but also cost- effective and easy to implement. Adsorption has been identified as one of the most efficient techniques for removing heavy metals. This study investigates adsorption method applied to real AMD, characterized by high levels of heavy metal contamination and low pH. Key parameters analyzed include adsorption performance, isotherm and kinetic models, thermodynamic properties, and adsorption characteristics. The adsorbent used was zeolite functionalized with carboxyl, hydroxyl, and sulfate groups derived from macroalgae. Material characterization was conducted using SEM, XRD, and FTIR. The results indicate that zeolite functionalized with carboxyl, hydroxyl, and sulfate groups from macroalgae effectively reduced Fe and Mn concentrations by up to 99 % within just five minutes. Furthermore, the adsorption process followed a pseudo-second-order kinetic model and conformed to the Langmuir isotherm model.

Ca2+-controlled Mn(II) removal process in Aurantimonas sp. HBX-1: Microbially-induced carbonate precipitation (MICP) versus Mn(II) oxidation

The application of manganese-oxidizing bacteria (MnOB) to produce manganese oxides (MnOx) has been widely studied, but often overlooking the concurrent formation of MnCO3. In this study, we found Ca2+ plays a crucial role in controlling Mn(II) removal in the bacterium Aurantimonas sp. HBX-1. Under conditions with 6.8 mM Ca2+ and without adding Ca2+, 100 μM Mn(II) was removed by 96.96 % and 38.28 % within 8 days, respectively. X-ray photoelectron spectroscopy (XPS) showed that adding Ca2+ increased the average oxidation state (AOS) of the solid products from 2.05 to 2.37. X-ray absorption fine structure (XAFS) analysis revealed the product proportions as follows: under Ca2+-supplemented condition, the ratio of MnOx (1 < x ≤ 2) to MnCO₃ was 52 % to 28.1 %, while under Ca2+-free condition, the ratio shifted to 4.6 % for MnOx (1 < x ≤ 2) and 55.2 % for MnCO₃. Urease activity assay and proteomic analysis confirmed the expression of urease and carbonic anhydrase, leading to the formation of MnCO3. Additionally, animal heme peroxidase (AHP) in strain HBX-1 was found to be responsible for Mn(II) oxidation through superoxide production, with Ca2+ addition promoting its expression level. Given the widespread presence of Ca2+ in wastewater, its potential impact on the biogeochemical Mn(II) cycle driven by bacteria should be reconsidered.

Homogeneous versus heterogeneous Mn(II) oxidation in peroxymonosulfate assisting chlorination: Synergistic role for enhanced Mn(II) oxidation in water treatment

Removal of Mn(II) is an essential step for addressing water discoloration in water treatment utilities worldwide. However, conventional chlorination suffers from poor oxidation of Mn(II) due to its low homogeneous oxidation kinetics. This study explored the oxidation capability of a new chemical dosing strategy employing peroxymonosulfate (PMS) to assist the chlorination process (PMS@Cl2) for effective Mn(II) oxidation. The study comprehensively explored both oxidation kinetics and underlying mechanisms associated with homogeneous and heterogeneous oxidation within the PMS@Cl2 system. At an [Mn(II)]0 of 1 mg/L, chlorination demonstrated inability in oxidizing Mn(II), with <10 % oxidation even at an elevated [Cl2] of 150 μM (∼10 mg/L). By contrast, PMS completely oxidized 100 % Mn(II) within a 30-minute reaction at a much lower [PMS] of 60 μM (kobs = 0.07 min−1 and t1/2 = 9 min), demonstrating its superior Mn(II) oxidation kinetics (over one order of magnitude faster than conventional chlorine). PMS@Cl2 exhibited an interesting synergistic benefit when combining a lower dose PMS with a higher routine dose Cl2 (loPMS@hiCl2), e.g. [PMS]:[Cl2] at 15:30 or 30:30 μM. Both conditions achieved 100 % Mn(II) oxidation, with even better values of kobs and t1/2 (0.16–0.17 min−1 and ∼4 min) relative to PMS alone at 60 µM. The synergic benefit of PMS@Cl2 was attributed to distinct functions played by PMS and Cl2 in both homogeneous and heterogeneous oxidation processes. Reactive species identification excluded the possible involvement of SO4•−, OH•, or chlorine radicals in the homogeneous oxidation of the PMS@Cl2 system. Instead, the dominant species was O2•− radical generated during the reaction of Mn(II) and PMS. Furthermore, the heterogeneous oxidation emphasized the important role of combining Cl2 dosing, which demonstrated an increased reactivity and electron transfer with the Mn−O−Mn complex, surpassing PMS. Overall, heterogeneous oxidation accelerated the oxidation kinetics of the PMS@Cl2 system by 1.1–2 orders of magnitude relative to the homogeneous oxidation of Cl2 alone. We here demonstrated that PMS@Cl2 could offer a more efficient mean of soluble Mn(II) mitigation, achieved with a relatively low routine dose of oxidant in a short reaction period. The outcomes of this study would address the existing limitations of traditional chlorine oxidation, minimizing the trade-offs associated with high residual chlorine levels after treatments for soluble manganese-containing water.