低温下Fe(II)对Ferrihydrite相转化的催化作用研究

研究了在低温、近中性条件下,在微量Fe(II)离子存在下Ferrihydrite(又称为水合氧化铁hydrousironoxide)的相转化过程.结果表明,微量Fe(II)离子的存在不仅可以加速Ferrihydrite的相转化过程,而且其相转化产物的组成也与没有Fe(II)离子存在时产物的组成有所不同,即除了α-FeOOH和α-Fe2O3外,还形成了γ-FeOOH;相转化过程既与阴离子的种类、反应温度、反应时间等因素有关,也与Fe(II)离子存在状态有关;Fe(II)离子通过催化Ferrihydrite的溶解过程,从而加速整个相转化过程.对该过程的深入研究将对认识和了解自然条件下铁氧化物的形成与相互转化具有重要意义.

Ferrihydrite的亚微观结构对反应活性的影响

以Fe(III)盐为原料、NaOH为沉淀剂、采用三种方法调控制备了ferrihydrite,借助X射线衍射(XRD)、红外光谱(IR)、差热分析(DTA)及其在稀盐酸中的溶解速率等手段对其结构进行了表征,探讨了ferrihydrite的形成环境对其亚微观结构及其反应活性的影响.结果表明,不同方法制备的ferrihydrite的亚微观结构不同,恒pH条件下制备的ferrihydrite结构与α-Fe2O3结构最为相似,更易转化为α-Fe2O3粒子.

Combined Effect of Arsenic and Cadmium on the Transformation of Ferrihydrite into Crystalline Products

Ferrihydrite, prepared in the presence of different amount of As and Cd in the solution, was used to study the combined effect of As and Cd coexisted in the same system on the transformation of ferrihydrite into crystalline products at pH & and pH 12. The data showed that there was apparent interaction between As and Cd in the transformation process. At pH 8, the transformation product was hematite with 1% As and different percent Cd (mole fraction, so as the follows), but the size of particles formed with different amount of Cd was different. At pH 12, the transformation products varied from sole hematite with 1% As and less than 2% Cd to a mixture of hematite and goethite with more than and equal to 2% Cd, and the percentage of goethite in the transformation products increased with the increasing level of Cd in the system. XRD (X-ray diffraction) and chemical analysis data showed that almost all As and part of Cd initially present in the system were retained in the crystalline products. The presence of As increased the amount of Cd retained in the structure of iron oxide. SEM (Scanning Electron Microscope) examination showed that the presence of As and Cd also altered the morphology of cry stalline products.

Enrichment of Phosphate on Ferrous Iron Phases during Bio-Reduction of Ferrihydrite

The reduction of less stable ferric hydroxides and formation of ferrous phases is critical for the fate of phosphorus in anaerobic soils and sediments. The interaction between ferrous iron and phosphate was investigated experimentally during the reduction of synthetic ferrihydrite with natural organic materials as carbon source. Ferrihydrite was readily reduced by dissimilatory iron reducing bacteria (DIRB) with between 52% and 73% Fe(III) converted to Fe(II) after 31 days, higher than without DIRB. Formation of ferrous phases was linearly coupled to almost complete removal of both aqueous and exchangeable phosphate. Simple model calculations based on the incubation data suggested ferrous phases bound phosphate with a molar ratio of Fe(II):P between 1.14 - 2.25 or a capacity of 246 - 485 mg·P·g-1 Fe(II). XRD analysis indicated that the ratio of Fe(II): P was responsible for the precipitation of vivianite (Fe3(PO4)2·8H2O), a dominant Fe(II) phosphate mineral in incubation systems. When the ratio of Fe(II):P was more than 1.5, the precipitation of Fe(II) phosphate was soundly crystallized to vivianite. Thus, reduction of ferric iron provides a mechanism for the further removal of available phosphate via the production of ferrous phases, with anaerobic soils and sediments potentially exhibiting a higher capacity to bind phosphate than some aerobic systems.

Simultaneous removal of Cd(Ⅱ)and As(Ⅴ)by ferrihydrite-biochar composite:Enhanced effects of As(Ⅴ)on Cd(Ⅱ)adsorption

The coexistence of cadmium(Cd(Ⅱ))and arsenate(As(Ⅴ))pollution has long been an environmental problem.Biochar,a porous carbonaceous material with tunable functionality,has been used for the remediation of contaminated soils.However,it is still challenging for the dynamic quantification and mechanistic understanding of the simultaneous sequestration of multi-metals in biochar-engineered environment,especially in the presence of anions.In this study,ferrihydrite was coprecipitated with biochar to investigate how ferrihydritebiochar composite affects the fate of heavy metals,especially in the coexistence of Cd(Ⅱ)and As(Ⅴ).In the solution system containing both Cd(Ⅱ)and As(Ⅴ),the maximum adsorption capacities of ferrihydrite-biochar composite for Cd(Ⅱ)and As(Ⅴ)reached 82.03μmol/g and 531.53μmol/g,respectively,much higher than those of the pure biochar(26.90μmol/g for Cd(Ⅱ),and 40.24μmol/g for As(Ⅴ))and ferrihydrite(42.26μmol/g for Cd(Ⅱ),and 248.25μmol/g for As(Ⅴ)).Cd(Ⅱ)adsorption increased in the presence of As(Ⅴ),possibly due to the changes in composite surface charge in the presence of As(Ⅴ),and the increased dispersion of ferrihydrite by biochar.Further microscopic and mechanistic results showed that Cd(Ⅱ)complexed with both biochar and ferrihydrite,while As(Ⅴ)was mainly complexed by ferrihydrite in the Cd(Ⅱ)and As(Ⅴ)coexistence system.Ferrihydrite posed vital importance for the co-adsorption of Cd(Ⅱ)and As(Ⅴ).The different distribution patterns revealed by this study help to a deeper understanding of the behaviors of cations and anions in the natural environment.

Dynamic coupling of ferrihydrite transformation and associated arsenic desorption/redistribution mediated by sulfate-reducing bacteria

Sulfate-reducing bacteria play an important role in the geochemistry of iron(oxyhydr)oxide and arsenic(As)in natural environments;however,the associated reaction processes are yet to be fully understood.In this study,batch experiments coupled with geochemical,spectroscopic,microscopic,and thermodynamic analyses were conducted to investigate the dynamic coupling of ferrihydrite transformation and the associated As desorption/redistribution mediated by Desulfovibrio vulgaris(D.vulgaris).The results indicated that D.vulgaris could induce ferrihydrite transformation via S^(2-)-driven and direct reduction processes.In the absence of SO_(4)^(2-),D.vulgaris directly reduced ferrihydrite,and As desorption and re-sorption occurred simultaneously during the partial transformation of ferrihydrite to magnetite.The increase in SO_(4)^(2-)loading promoted the S^(2-)-driven reduction of ferrihydrite and accelerated the subsequent mineralogical transformation.In the low and medium SO_(4)^(2-)treatments,ferrihydrite was completely transformed to a mixture of magnetite and mackinawite,which increased the fraction of As in the residual phase and stabilized As.In the high SO_(4)^(2-)treatment,although the replacement of ferrihydrite by only mackinawite also increased the fraction of As in the residual phase,22.1%of the total As was released into the solution due to the poor adsorption affinity of As to mackinawite and the conversion of As^(5+)to As^(3+).The mechanisms of ferrihydrite reduction,mineralogy transformation,and As mobilization and redistribution mediated by sulfate-reducing bacteria are closely related to the surrounding SO_(4)^(2-)loadings.These results advance our understanding of the biogeochemical behavior of Fe,S,and As,and are helpful for the risk assessment and remediation of As contamination.

Ferrihydrite transformation impacted by coprecipitation of lignin:Inhibition or facilitation?

Lignin is a common soil organic matter that is present in soils,but its effect on the transformation of ferrihydrite(Fh)remains unclear.Organic matter is generally assumed to inhibit Fh transformation.However,lignin can reduce Fh to Fe(Ⅱ),in which Fe(Ⅱ)-catalyzed Fh transformation occurs.Herein,the effects of lignin on Fh transformation were investigated at 75℃ as a function of the lignin/Fh mass ratio(0-0.2),pH(4-8)and aging time(0-96 hr).The results of Fh-lignin samples(mass ratios=0.1)aged at different pH values showed that for Fh-lignin the time of Fh transformation into secondary crystalline minerals was significantly shortened at pH 6 when compared with pure Fh,and the Fe(Ⅱ)-accelerated transformation of Fh was strongly dependent on pH.Under pH 6,at low lignin/Fh mass ratios(0.05-0.1),the time of secondary mineral formation decreased with increasing lignin content.For high lignosulfonate-content material(lignin:Fh=0.2),Fh did not transform into secondary minerals,indicating that lignin content plays a major role in Fh transformation.In addition,lignin affected the pathway of Fh transformation by inhibiting goethite formation and facilitating hematite formation.The effect of coprecipitation of lignin on Fh transformation should be useful in understanding the complex iron and carbon cycles in a soil environment.

Mechanistic insight into Cr(Ⅵ)retention by Si-containing ferrihydrite

Hexavalent chromium[Cr(Ⅵ)]causes serious harm to the environment due to its high toxicity,solubility,and mobility.Ferrihydrites(Fh)are the main adsorbent and trapping agent of Cr(Ⅵ)in soils and aquifers,and they usually coexist with silicate(Si),forming Si-containing ferrihydrite(Si-Fh)mixtures.However,the mechanism of Cr(Ⅵ)retention by Si-Fh mixtures is poorly understood.In this study,the behaviors and mechanisms of Cr(Ⅵ)adsorption onto Si-Fh with different Si/Fe molar ratios was investigated.Transmission electron microscope,Fourier transform infrared spectroscopy,X-ray diffraction,X-ray photoelectron spectroscopy,and other techniques were used to characterize Si-Fh and Cr(Ⅵ)-loading of Si-Fh.The results show that specific surface area of Si-Fh increases gradually with increasing Si/Fe ratios,but Cr(Ⅵ)adsorption on Si-Fh decreases with increasing Si/Fe ratios.This is because with an increase in Si/Fe molar ratio,the point of zero charge of Si-Fh gradually decreases and electrostatic repulsion between Si-Fh and Cr(Ⅵ)increases.However,the complexation of Cr(Ⅵ)is enhanced due to the increase in adsorbed hydroxyl(A-OH-)on Si-Fh with increasing Si/Fe molar ratio,which partly counteracts the effect of the electrostatic repulsion.Overall,the increase in the electrostatic repulsion has a greater impact on adsorption than the additional complexation with Si-Fh.Density functional theory calculation further supports this observation,showing the increases in electron variation of bonding atoms and reaction energies of inner spherical complexes with the increase in Si/Fe ratio.

The role of organic and inorganic substituents of roxarsone determines its binding behavior and mechanisms onto nano-ferrihydrite colloidal particles

The retention and fate of Roxarsone(ROX)onto typical reactive soil minerals were crucial for evaluating its potential environmental risk.However,the behavior and molecular-level reaction mechanism of ROX and its substituents with iron(hydr)oxides remains unclear.Herein,the binding behavior of ROX on ferrihydrite(Fh)was investigated through batch experiments and in-situ ATR-FTIR techniques.Our results demonstrated that Fh is an effective geo-sorbent for the retention of ROX.The pseudo-second-order kinetic and the Langmuir model successfully described the sorption process.The driving force for the binding of ROX on Fh was ascribed to the chemical adsorption,and the rate-limiting step is simultaneously dominated by intraparticle and film diffusion.Isotherms results revealed that the sorption of ROX onto Fh appeared in uniformly distributed monolayer adsorption sites.The twodimensional correlation spectroscopy and XPS results implied that the nitro,hydroxyl,and arsenate moiety of ROX molecules have participated in binding ROX onto Fh,signifying that the predominated mechanisms were attributed to the hydrogen bonding and surface complexation.Our results can help to better understand the ROX-mineral interactions at the molecular level and lay the foundation for exploring the degradation,transformation,and remediation technologies of ROX and structural analog pollutants in the environment.

In situ speciation analysis and kinetic study of arsenic adsorption on ferrihydrite with surface-enhanced Raman spectroscopy

Arsenic pollution poses a serious threat to human health,and is one of the most concerning environmental problems worldwide.The adsorption,fixation,and dissolution behaviors of arsenic on the surface of iron-(hydr-)oxides influence the environmental routes of arsenic cycle geochemistry.Both inner-sphere and outer-sphere adsorption configurations of arsenic on iron oxides have been proposed based on X-ray adsorption spectra.However,there is no systematic study on the in situ speciation analysis and adsorption kinetics of these species at such interfaces,because of the lack of an efficient monitoring strategy.The correlation of surface speciation and environmental stability is still unknown.Here,a shell-isolated SiO_(2)@Ag@Au-based surface-enhanced Raman spectroscopy(SERS)platform was developed for speciation analysis of the adsorbed arsenic species by eliminating the chemical interaction between arsenic and silver.Using ferrihydrite as a typical iron oxide,the intrinsic Raman spectra of the inner-sphere(~830 cm^(−1))and outer-sphere(~660 cm^(−1))complexes at the adsorption interface were identified.For the first time,the in situ kinetic monitoring of the formation and transformation of these species was realized.By correlating the speciation to the sequential extraction results,the environmental stability of arsenic on ferrihydrite was shown to be closely related to the adsorption configuration.It was shown that stability can be significantly promoted by transforming loosely bonded species(outer-sphere complexes)into inner-sphere structures.Our work demonstrated the applicability of SERS with shell-isolated plasmonic particles for arsenic geochemical cycle monitoring and mechanism studies.It also provided a convenient tool for developing effective strategies for arsenic pollutant control and abatement.

Maghemite(γ-Fe_2O_3) nanoparticles enhance dissimilatory ferrihydrite reduction by Geobacter sulfurreducens: Impacts on iron mineralogical change and bacterial interactions

Microbially mediated bioreduction of iron oxyhydroxide plays an important role in the biogeochemical cycle of iron.Geobacter sulfurreducens is a representative dissimilatory ironreducing bacterium that assembles electrically conductive pili and cytochromes.The impact of supplementation withγ-Fe_2O_3 nanoparticles(NPs)(0.2 and 0.6 g)on the G.sulfurreducens-mediated reduction of ferrihydrite was investigated.In the overall performance of microbial ferrihydrite reduction mediated byγ-Fe_2O_3 NPs,stronger reduction was observed in the presence of direct contact withγ-Fe_2O_3 NPs than with indirect contact.Compared to the production of Fe(Ⅱ)derived from biotic modification with ferrihydrite alone,increases greater than 1.6-and 1.4-fold in the production of Fe(Ⅱ)were detected in the biotic modifications in which direct contact with 0.2 g and 0.6 gγ-Fe_2O_3 NPs,respectively,occurred.X-ray diffraction analysis indicated that magnetite was a unique representative iron mineral in ferrihydrite when active G.sulfurreducens cells were in direct contact withγ-Fe_2O_3 NPs.Because of the sorption of biogenic Fe(Ⅱ)ontoγ-Fe_2O_3 NPs instead of ferrihydrite,the addition ofγ-Fe_2O_3 NPs could also contribute to increased duration of ferrihydrite reduction by preventing ferrihydrite surface passivation.Additionally,electron microscopy analysis confirmed that the direct addition ofγ-Fe_2O_3 NPs stimulated the electrically conductive pili and cytochromes to stretch,facilitating long-range electron transfer between the cells and ferrihydrite.The obtained findings provide a more comprehensive understanding of the effects of iron oxide NPs on soil biogeochemistry.

Extraction of electrons by magnetite and ferrihydrite from hydrogen-producing Clostridium bifermentans by strengthening the acetate production pathway

Conductive mineral nanoparticles, such as magnetite, can promote interspecies electron transfer between syntrophic partners.However, the effect of magnetite has only been inferred in intraspecific electron output. Herein, a hydrogen-producing strain,namely, Clostridium bifermentans, which holds several electron output pathways, was used to study the effect of magnetite on the intraspecific electron output manner. Additionally, insulated amorphous ferrihydrite, which was used as an extracellular electron acceptor, was selected to compare with magnetite. Electrons, which were originally used to generate hydrogen, were shunted with the addition of magnetite and ferrihydrite, which resulted in the reduction of hydrogen production and accumulation of Fe(II). Interestingly, more electrons(39.7% and 53.5%) were extracted by magnetite and ferrihydrite, respectively, which led to less production of butyrate and more acetate. More importantly, the increased electron extraction efficiency suggested that electroactive microorganisms can switch metabolic pathways to adapt to electron budget pressure in intraspecific systems. This work broadens the understanding of the interaction between iron oxides and fermentative hydrogen-producing microbes that hold the capacity of Fe(III) reduction.

Preparation of silicon-doped ferrihydrite for adsorption of lead and cadmium:Property and mechanism

In this study,Si-doped ferrihydrite(Si-Fh) was successfully synthesized by a simple coprecipitation method for removal of heavy metals in water.Subsequently,the physicochemical properties of Si-Fh before and after adsorption were further studied using several techniques.The Si-Fh exhibited good adsorption capacity for heavy metal ions such as Pb(II) and Cd(II).The maximum adsorption capacities of lead and cadmium are respectively 105.807,37.986 mg/g.The distribution coefficients of the materials for Pb(II) and Cd(II) also showed a great affinity(under optimal conditions).Moreover,it was found that the adsorption fit well with the Freundlich isotherm and pseudo-second-order kinetic model which means this was a chemical adsorption process.It can be conducted from both characterization and model results that adsorption of Pb(II) and Cd(II) was mainly through the complexation interaction of abundance oxygen functional groups on the surface of Si-Fh.Overall,the Si-Fh adsorbents with many superiorities have potential for future applications in the removal of Pb(II) and Cd(II) from wastewater.

Synthesis of Mg(Ⅱ)doped ferrihydrite-humic acid coprecipitation and its Pb(Ⅱ)/Cd(Ⅱ)ion sorption mechanism

A magnesium doped ferrihydrite-humic acid coprecipitation(Mg-doped Fh-HA)was synthesized by coprecipitation method.The removal of heavy metals such as Pb(Ⅱ)and Cd(Ⅱ)was assessed.The isotherms and kinetic studies indicated that the Mg-doped Fh-HA exhibited a remarkable Pb(Ⅱ)and Cd(Ⅱ)sorption capacity(maximum 120.43 mg/g and 27.7 mg/g,respectively.)in aqueous solution.The sorption of Pb(Ⅱ)and Cd(Ⅱ)onto best fitted pseudo-second-order kinetic equation and Langmuir model.The adsorption mechanism of Mg-doped Fh-HA on Pb(Ⅱ)and Cd(Ⅱ)involves surface adsorption,surface complexation and surface functional groups(such as carboxyl group,hydroxyl group).In addition,ionexchange and precipitation cannot be ignored.The Mg-doped Fh-HA is a low-cost and high-performance adsorption material and has a wide range of application prospects.

Short-Range-Order Minerals and Dominant Accessory Properties Controlling P Sorption in Tropical Tephra Soils of the Cameroon Volcanic Line

Knowledge on soil properties likely to influence P sorption in tephra soils is very important for sustainable management of available P. Sorption studies on six tephra soils were conducted to relate P sorption to soil characteristics in order to identify those with potential influence on P sorption. Four equilibrium-based sorption models commonly encountered in soil studies (Langmuir, Freundlich, Temkin, and Van Huay) were used to describe P sorption in the soils. P sorption was determined by measuring the residual P content of a clarified equilibrating solution of 0.02 N KCl containing varying concentrations (0, 5, 10, 15, 30, 40, 50, 60, 80, and 100 mg/L) of P as KH2PO4 after mixing with 1 g of soil in duplicates for 16 hours at 25°C. Maximum amount of P sorbed for the varying P concentrations used ranged from 2080 to 5402 mg/kg with a potential for greater P sorption maxima at higher P solution concentrations. P sorption in these soils was best described by the Langmuir and Freundlich models. Allophane and ferrihydrite are the principal species controlling the high P sorption in these soils. pH-NaF proved to be a potentially reliable test for assessing the presence of allophanic materials and thus for estimating P sorbed. This work would guide both effective and efficient P fertilizer management with economic implications for both the study area and similar environments.

Unveiling the crucial role of iron mineral phase transformation in antimony(V)elimination from natural water

Antimony(Sb)in natural water has long-term effects on both the ecological environment and human health.Iron mineral phase transformation(IMPT)is a prominent process for removing Sb(V)from natural water.However,the importance of IMPT in eliminating Sb remains uncertain.This study examined the various Sb–Fe binding mechanisms found in different IMPT pathways in natural water,shedding light on the underlying mechanisms.The study revealed that the presence of goethite(Goe),hematite(Hem),and magnetite(Mag)significantly affected the concentration of Sb(V)in natural water.Elevated pH levels facilitated higher Fe content in iron solids but impeded the process of removing Sb(V).To further our understanding,polluted natural water samples were collected from various locations surrounding Sb smelter sites.Results confirmed that converting ferrihydrite(Fhy)to Goe significantly reduced Sb levels(<5μg/L)in natural water.The emergence of secondary iron phases resulted in greater electrostatic attraction and stabilized surface complexes,which was the most likely cause of the decline of Sb concentration in natural water.The comprehensive findings offer new insights into the factors governing IMPT as well as the Sb(V)behavior control.

含钙铝铁水解聚合产物的矿物学研究Ⅲ:陈化对形态的影响

用透射电镜、扫描电镜、热分析及矿物谱学等手段对含钙铝铁水解聚合产物(含微量硅)合成初期样品及其陈化9年后样品组成物相的形态演化进行了观察对比研究。结果表明,含钙铝铁溶液合成初期迅速形成类钙矾石双羟合结构体,其结构中部分Al3+被Fe3+取代、SO42-阴离子被Cl-离子替代,无定形的铝铁共聚物、铝硅共聚物和多种铝、铁、钙氢氧化物(氧化物)微晶竞争存在。缺氧条件下铝铁水解产物的演化由于氯、钙离子的存在而与文献的报道有所不同。陈化改变了含钙聚合铝(铁)溶液中的长程有序结构。其中的类钙矾石双羟合结构体在高Cl-、低pH环境下长期陈化过程经历了溶解-再结晶作用。大量的铁微晶相溶解形成富铝水羟合铁(ferrihydrite)胶体结构,Cl-与Ca2+以键合或共沉淀方式进入富铝水羟合铁(ferrihydrite)相。富铝水羟合铁(ferrihydrite)胶体最终的结晶相还是β-FeOOH。

Formation and transformation of schwertmannite in the classic Fenton process

The massive amount of sludge generated by the classic Fenton process, which has often been hypothesized to consist of ferric hydroxide, remains a major obstacle to its large-scale application. Therefore, reutilization of Fenton sludge has recently gained more attention.Understanding the formation, transformation, and properties of Fenton sludge combined with the stages of the Fenton reaction is pivotal, but not well illustrated yet. In this study,SEM-EDS, FT-IR, XRD, and XPS were applied to study the morphology, crystallinity,elemental composition, and valence state of Fenton sludge. The authors report that schwertmannite and 2-line ferrihydrite were generated and transformed in the oxidation phase and the neutralization phase of the Fenton process. SO_4^(2-) in the solution decreased by8.7%–26.0% at different molar ratios of Fe(II) to H_2 O_2; meanwhile, iron ion precipitated completely at pH 3.70 with the formation of schwertmannite containing sulfate groups in the Fenton sludge. The structural sulfate(Fe-SO_4) in schwertmannite was released from the precipitate with the addition of OH-, and the production of Fenton sludge decreased with increasing pH when pH > 3.70. Goethite was found to form when the final p H was adjusted to 12 or at a reaction temperature of 80°C. Moreover, the possible thermal transformation to goethite and hematite indicated that Fenton sludge can be reused as a raw material for synthesizing more stable iron(hydro)oxides. The results provide useful insights into the formation and transformation of Fenton sludge, with implications for regulating the crystal type of Fenton sludge for further reuse.

Phase transformation of Cr(Ⅵ)-adsorbed ferr ihydr ite in the presence of Mn(Ⅱ): Fate of Mn(Ⅱ)and Cr(Ⅵ)

Ferrihydrite is an important sink for the toxic heavy metal ions, such as Cr(Ⅵ). As ferrihydrite is thermodynamically unstable and gradually transforms into hematite and goethite, the stability of Cr(Ⅵ)-adsorbed ferrihydrite is environmentally significant. This study investigated the phase transformation of Cr(Ⅵ)-adsorbed ferrihydrite at different pH in the presence of aqueous Mn(Ⅱ), as well as the fate of Mn(Ⅱ) and Cr(Ⅵ) in the transformation process of ferrihydrite. Among the ferrihydrite transformation products, hematite was dominant, and goethite was minor. The pre-adsorbed Cr(Ⅵ) inhibited the conversion of ferrihydrite to goethite at initial pH 3.0, whereas little amount of adsorbed Mn(Ⅱ) favored the formation of goethite at initial pH 7.0. After the aging process, Cr species in solid phase existed primarily as Cr(Ⅲ) in the presence of Mn(Ⅱ) at initial pH 7.0 and 11.0. The aqueous Mn concentration was predominantly unchanged at initial pH 3.0, whereas the aqueous Mn(Ⅱ) was adsorbed onto ferrihydrite or form Mn(OH)_(2) precipitates at initial pH 7.0 and 11.0, promoting the immobilization of Cr(Ⅵ). Moreover, the oxidation of Mn(Ⅱ) occurred at initial pH 7.0 and 11.0, forming Mn(Ⅲ/Ⅳ)(hydr)oxides.