Phase transformation of Cr(VI)-adsorbed ferrihydrite 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.

Se(IV) oxidation by ferrate(VI) and subsequent in-situ removal of selenium species with the reduction products of ferrate(VI): performance and mechanism.

In order to treat selenium pollution, the study presents the use of potassium ferrate (K2FeO4) as an environmentally friendly agent for in situ removal of Se(IV) from aqueous media. Batch experiments were carried out to evaluate the influences of various factors including dosage of K2FeO4, ex-situ and in-situ adsorption, initial pH, and adsorption isotherms. The results showed that increasing dosage of K2FeO4 benefited the removal of total selenium with the efficiency up to 97.0% and Se(IV) removal significantly depended on pH, and as the pH increases, the decrease in Se(IV) adsorption efficiency is a general trend of pH dependence. The X-ray powder diffraction, Fourier transformed infrared spectrometer and high-resolution X-ray photoelectron spectroscopy analysis indicated that Se(IV) was removed from the aqueous solution by adsorbing on the surface of the decomposition products of K2FeO4 which are ferric oxide nanoparticles, and the selenium adsorbed on the generated ferric oxide nanoparticles existed in the forms of Se(IV) and Se(VI). Se(IV) and Se(VI) were adsorbed to the decomposition products of K2FeO4 by forming an inner-sphere complexes and an outer-sphere complexes, respectively.

Build-up of a continuous flow pre-coated dynamic membrane filter to treat diluted textile wastewater and identify its dynamic membrane fouling.

This research built up a continuous dynamic flow filter membrane to treat diluted textile wastewater and basically investigated dynamic membrane fouling mechanism. By pre-depositing particles activated carbon (PAC) on membrane support material (MSM), a thin layer was formed on its surface, which showed excellent results in removing organic pollutants from diluted textile wastewater. Experimental data were regressed by the Langmuir, Freundlich, Temkin, Dubinin-Radushkevich (D-R) and Sips isotherm models. The three two-parameter isotherms (Temkin, D-R and Freundlich) were the models that best fitted, with respectively 0.977, 0.975 and 0.973 regression coefficients. D-R model has registered the maximum calculated adsorption capacity Qmd, cal. = 45.499 mg/g and the mean energy which was required to adsorb 1 mol of MB dye by the DM layer E = 4.249 kJ/mol; indicating the energy distribution onto heterogeneous surface of a physical adsorption process. Furthermore, kinetic models results showed that MB adsorption onto PAC at different initial concentrations follows the pseudo-second order. The obtained results also indicated that a flexible DM layer with different thickness can be formed from different amount of PAC pre-deposited on MSMs, which demonstrated that it was convenient to adjust the required DM thickness to filtrate a known initial concentration for >99% organic pollutants removal efficiency rate. However, DM fouling occurred on small pores MSMs; which resulted in an increase of the filtration pressure what have affected the filtration performance. PAC and MSMs surface morphology and texture structure, before and after filtration, were visualized respectively by Field Emission Scanning Electron Microscopy (FESEM), Fourier Transform Infra-Red and Attenuated Total Reflectance (FTIR/ATR). From these experimental results, a sustainable flux (>6.85 × 10-5 m/s) was established to discriminate no fouling from fouling conditions based on flux and TMP trends variance.

Removal of nitrate from water by acid-washed zero-valent iron/ferrous ion/hydrogen peroxide: influencing factors and reaction mechanism.

In this paper, a system consisting of acid-washed zero-valent iron (ZVI), ferrous ion (Fe2+), and hydrogen peroxide (H2O2) was employed for the removal of nitrate (NO3-) from water, and the reaction mechanism for this is discussed. The effects of acid-washed ZVI, Fe2+, H2O2, and initial NO3- concentration on nitrate removal were investigated. Acid-washed ZVI before and after reaction with nitrate were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Results reveal that the combined system can enhance the corrosion of ZVI and facilitate aqueous nitrate reduction. The products of nitrate reduction are mainly ammonium, with some N2. The ZVI particles after reaction may have a core of ZVI with an oxidation layer mainly consisting of Fe3O4.

Chromium removal using resin supported nanoscale zero-valent iron.

Resin supported nanoscale zero-valent iron (R-nZVI) was synthesized by the borohydride reduction method. Batch experiments were conducted to evaluate the factors affecting Cr(VI) removal. It was found that nZVI loads, resin dose, pH value and initial concentration of Cr(VI) were all important factors. Scanning electron microscopy showed that the nZVI particles in R-nZVI became sphere after reacting with Cr(VI). This phenomenon was attributed to the co-precipitation of Cr(III) and Fe(III) on the surface of resin. X-ray diffraction pattern confirmed that Fe(0) diminished after the reaction. At optimum conditions, the Cr(VI) removal efficiency was 84.4% when the initial concentration of Cr(VI) was 20.0 mg/L. Regeneration of R-nZVI and resin was possible. R-nZVI can also remove Cr(III) efficiently. However, the removal mechanisms of Cr(VI) (anion) and Cr(III) (cation) are different. The former is chemical reduction, while the latter is ion exchange at pH below 6.3 and precipitation at pH above 6.3. This study demonstrates that R-nZVI has the potential to become an effective agent for treating wastewater containing Cr(VI) and Cr(III).

Influence of montmorillonite incorporation on ferrihydrite transformation and Cr(VI) behaviors during ferrihydrite-Cr(VI) coprecipitates aging.

Hexavalent chromium (Cr(VI)) is a pollutant with high migration ability, and the destiny of Cr(VI) is highly correlated with ferrihydrite (Fh). Montmorillonite (Mt) is a clay mineral abundantly presents in nature. Although Cr(VI) adsorption on montmorillonite or ferrihydrite has been studied, Cr(VI) behaviors during the Fh-Cr-Mt coprecipitates transformation still remain unknown. In this study, calcium montmorillonite (Ca-Mt) or sodium montmorillonite (Na-Mt) was coprecipitated with ferrihydrite and Cr(VI). Effect of Ca-Mt (or Na-Mt) incorporation on coprecipitates transformation and Cr(VI) behaviors during aging were investigated. The results showed that Ca-Mt or Na-Mt incorporation inhibited the transformation of ferrihydrite in Fh-Cr-Ca-Mt or Fh-Cr-Na-Mt at the initial pH of 5.0, 7.0 and 9.0. During aging, two kinds of Mt were supposed to interact with Fh to form the FeOSi and FeOAl bonds, and thus the formation of hematite and goethite were limited. By testing the Cr(VI) distribution in each phase of coprecipitates during transformation, delay on Cr(VI) migration and redistribution could be found in systems added with montmorillonite, and Cr(VI) was retained in coprecipitates to a greater extent compared with the systems without montmorillonite addition. The results of this study contribute to increasing our knowledge about the role of clay minerals on the coprecipitates transformation when they coexist at different pH values. It is also significant for the heavy metals polluted sites repairing.

Enhanced immobility of Pb(II) during ferrihydrite-Pb(II) coprecipitates aging impacted by malic acid or phosphate.

Metastable ferrihydrite is omnipresent in environments and can influence the fate of Pb(II) during ferrihydrite transformation. Ferrihydrite is rarely pure and often coexists with impurities, which may influence the mineralogical changes of ferrihydrite and Pb(II) behavior. In this work, we investigated the effect of malic acid or phosphate on Pb(II)-ferrihydrite coprecipitates (Fh-Pb) transformation and the subsequent fate of Pb(II) during the 10-day aging of Fh-Pb. Results showed that both malic acid and phosphate retarded Fh-Pb transformation and prevented the release of Pb(II) from Fh-Pb back into solutions. Pb(II) was beneficial to goethite formation by inhibiting hematite formation while both malic acid and phosphate inhibited goethite formation since they could act as templates of nucleation. Besides, malic acid and phosphate improved the proportion of non-extracted Pb(II) during Fh-Pb transformation, indicating that Pb(II) was incorporated into secondary minerals. Pb(II) could not replace Fe(III) within the crystal lattice due to its large radius but was occluded into pores and defect structures within the secondary mineral lattices. This work can advance our understanding of the influences of malic acid and phosphate on Pb(II) immobility during Fh-Pb aging.

Spectral change of dissolved organic matter after extracted by solid-phase extraction and its feasibility in predicting the acute toxicity of polar organic pollutants in textile wastewater.

Spectroscopic parameters can be used as proxies to effectively trace the occurrence of organic trace contaminants, but their suitability for predicting the toxicity of discharged industrial wastewater with similar spectra is still unknown. In this study, the organic contaminants in treated textile wastewater were subdivided and extracted by four commonly-used solid-phase extraction (SPE) cartridges, and the resulting spectral change and toxicity of textile effluent were analyzed and compared. After SPE, the spectra of the percolates from the four cartridges showed obvious differences with respect to the substances causing the spectral changes and being more readily adsorbed by the WAX cartridges. Non-target screening results showed source differences in organic micropollutants, which were one of the main contributors leading to their spectral properties and spectral variations after SPE in the effluents. Two fluorescence parameters (C1 and humic-like) identified by the excitation emission matrix-parallel factor analysis (EEM-PARAFAC) were closely correlated to the toxicity endpoints for Scenedesmus obliquus (inhibition ratios of cell growth and Chlorophyll-a synthesis), which can be applied to quantitatively predict the change of toxicity effect caused by polar organic pollutants. The results would provide novel insights into the spectral feature analysis and toxicity prediction of the residual DOM in industrial wastewater.

Minimizing the creation of spent pickling liquors in a pickling process with high-concentration hydrochloric acid solutions: mechanism and evaluation method.

The purpose of this investigation is to propose a strategy for minimizing the creation of spent pickling liquors through the synergistic corrosion inhibition of OP-10 and potassium iodide, thus facilitating a cleaner production process for acid pickling of metals with a high-concentration solution (6.0 mol/l) of hydrochloric acid. Results obtained with the methods of weight loss and electrochemical polarization showed that adding KI and OP-10 could enhance the energy barrier of the corrosion reaction and improved the corrosion inhibition for mild steel in high concentration of HCl solutions. A synergistic effect was identified when KI and OP-10 were present in suitable proportions. The results of the electrochemical experiments and scanning electron microscope (SEM) observations showed that the complex inhibitor was a mixed-type inhibitor and it formed a compact film on the metal surface, thus providing an effective protection for the metal in the aggressive solutions, which significantly minimized the creation of spent pickling liquors. A simple and convenient method was also proposed for the quantificational evaluation of the inhibition degree in the creation of spent pickling liquors.

Properties and mechanism of Cr(VI) adsorption and reduction by K2FeO4 in presence of Mn(II).

To efficiently treat hexavalent chromium (Cr(VI)) wastewater, K2FeO4 was used to remove and reduce Cr(VI) in presence of Mn(II) in this paper. Batch removal experiments were carried out to study the effect of Fe/Mn molar ratios, initial pH, in-situ and ex-situ and co-existing ions on Cr(VI) removal. The results showed the removal efficiency of Cr(VI) was 97.7% for the initial Cr(VI) concentration of 10.0 mg/L at Fe/Mn molar ratio of 2:3 and initial pH 8.0. Meanwhile, the high removal efficiency of Cr(VI) had been maintained throughout the pH range of 3.0-8.0 in the experimental study. Moreover, the removal process was relatively stable regardless of in-situ and ex-situ, and co-existing ions such as Ca2+ and low concentration of HCO3- had no intense effect on Cr(VI) removal, while SO42- inhibited Cr(VI) removal in the reaction system. To investigate the removal mechanism of Cr(VI) by K2FeO4 in presence of Mn(II), the reaction products were characterized by the Fourier transformed infrared spectrometer, X-ray powder diffraction, Transmission electron microscopy and the high-resolution X-ray photoelectron spectroscopy. The results indicated the ferrate decomposition products of γ-FeOOH/γ-Fe2O3 had the ability to adsorb Cr(VI) and react with Mn(II) to form γ-Fe2O3-Mn(II) complex to adsorb and reduce Cr(VI).

Fate of metal-EDTA complexes during ferrihydrite aging: Interaction of metal-EDTA and iron oxides.

The widespread presence of ferrihydrite in the environment makes many contaminants including metal-EDTA complexes being adsorbed on it. However, the fate of metal-EDTA complexes during the transformation of ferrihydrite was poorly understood. Understanding the migration and speciation changes of metal-EDTA adsorbed on ferrihydrite during the transformation was helpful to predict its fate in some natural and engineering environments. In this work, the interaction of the two metal-EDTA complexes (Ni(II)-EDTA and Ca(II)-EDTA) and ferrihydrite during the 9-day transformation of ferrihydrite at different pH values was studied. The results showed that part of EDTA complexing metals changed to non-complexed metals during the ferrihydrite transformation, which was due to the fact that metal in the metal-EDTA exchanged with Fe(III) on ferrihydrite. Besides, different speciation of metal ions migrated during the transformation of ferrihydrite. Meanwhile, Fe(III)-EDTA formed in this process, and the exchange of metal in Ca(II)-EDTA with Fe(III) in ferrihydrite was faster than that of Ni(II)-EDTA. Besides, the presence of metal-EDTA affected the transformation rate of ferrihydrite under neutral and alkaline condition, and metal-EDTA accelerated the dissolution of ferrihydrite to form goethite. Therefore, ferrihydrite and metal-EDTA influenced each other during the transformation of ferrihydrite. The results of this work revealed that the process of metal-EDTA dissolving ferrihydrite not only included the dissociation of metal-EDTA, but also involved the migration of metal ions and affected the transformation of ferrihydrite.

Interaction between Se(IV) and fulvic acid and its impact on Se(IV) immobility in ferrihydrite-Se(IV) coprecipitates during aging.

Selenium (Se) is regarded as a trace element for humans, but it is toxic in excess. In natural environments, the mobility of Se is dominantly controlled by the Se oxyanions with high solubility such as selenite (Se(IV)). Se(IV) is often associated with the omnipresent ferrihydrite and coexisting organic matter. However, there is little information on the dynamic interactions among Se(IV), fulvic acid, and ferrihydrite. This study investigated the influence of fulvic acid on ferrihydrite-Se(IV) coprecipitates (Fh-Se) transformation for 8 days and the subsequent behavior of Se(IV) at varied pH (5.0, 7.5, and 10.0). Results showed that fulvic acid had different effects on Fh-Se transformation at varied pH values. Fh-Se transformation was promoted by fulvic acid at initial pH 5.0 whereas it was inhibited at initial pH 10.0. Interestingly, at initial pH 7.5, Fh-Se transformation was promoted at a low C/Fe ratio while it was suppressed at a high C/Fe ratio. Besides, fulvic acid induced the generation of more extractable Se(IV) at initial pH 5.0 and more coprecipitated Se(IV) at initial pH 7.5 and blocked the release of Se(IV) at initial pH 10.0. Fulvic acid possibly interacted with Se(IV) via carboxyl complexation and weakened the inhibition of Se(IV) on Fh-Se transformation. Thus, fulvic acid increased the transformation rate of Fh-Se. These findings help to uncover the environmental behavior of Se(IV) and organic matter during ferrihydrite transformation.

Fate of Cr(VI) during aging of ferrihydrite-humic acid co-precipitates: Comparative studies of structurally incorporated Al(III) and Mn(II).

Ferrihydrite-humic acid co-precipitates have impacts on the adsorption and reduction of Cr(VI) in the natural environment. Besides, ferrihydrite-humic acid co-precipitates usually coexist with foreign metal cations like Al(III) and Mn(II), which may change the properties of ferrihydrite and affect the fate of Cr(VI). In this work, structurally incorporated Al(III) or Mn(II) in ferrihydrite-humic acid co-precipitates with Cr(VI) (Fh-HA-Cr-Al or Fh-HA-Cr-Mn) were prepared, and the behavior and phase transformation of co-precipitates were explored via the characterization analyses of samples during aging for 10 days. This study showed that partial adsorbed Cr(VI) was reduced to Cr(III) in the presence of humic acid, thereby reducing the toxicity of Cr(VI). Interestingly, two different results occurred because of the incorporation of Al(III) and Mn(II). Al(III) hindered the transformation of ferrihydrite and changed the aging products by inhibiting the dissolution of ferrihydrite, which decreased Cr to incorporate iron minerals. By contrast, doping of Mn(II) accelerated the phase transformation of co-precipitates, and was more conducive to the encapsulation and fixation of Cr. The results of this study can facilitate the understanding of the effects of Al(III) and Mn(II) on Cr(VI) fixation during the aging of Fh-HA-Cr.

Insights into the fouling layer of flat-sheet membrane and its development in an integrated oxidation ditch-membrane bioreactor.

This work revealed the characteristics of fouling layer on the flat-sheet membranes and its development in an integrated oxidation-ditch membrane bioreactor. During the operation period (130 days), the reactor performed very well in removing pollutants. As the operation proceeded, membrane fouling occurred on the flat-sheet membranes and trans-membrane pressure showed a cyclical variation. The experimental results showed that the process of membrane fouling appeared successively in two different structures: biofilm (BF) and sludge fouling (SF). The substances causing membrane fouling were mainly organic foulants and a small amount of inorganic metal compounds, especially the protein-like and fulvic acid-like substances in loosely bound extracellular polymeric substances (LB-EPS). The analysis of microbial communities revealed that SF and BF had very different microbial properties. Although most membrane foulants could be removed by physical and chemical cleaning methods, the protein-like and fulvic acid-like substances in BF were contribute much to causing irreversible membrane fouling.

Towards deep purification of secondary textile effluent by using a dynamic membrane process: Pilot-scale verification.

The present investigation used regular powered activated carbon (PAC) as the dynamic membrane (DM) material and successfully built-up a pilot-scale DM system for deep purification of the secondary textile effluent, which aimed at verifying the technical and economic feasibility of the DM with real secondary textile effluent. The hydrodynamic experiments indicated that the filtration resistance gradually increased along with the operation of DM system, and among which, the PAC size was the most important influencing factor. More dosage and smaller sized PAC were beneficial to enhance the purification effect of micro-organic pollutants, but they simultaneously improved the operational costs, which implied that the adoption of DM materials should comprehensively consider the removal results and the type and dosage of DM materials for obtaining an optimal result, and the operational costs would be drastically reduced by regenerating the wasted PAC. More than 50% residual micro-organic pollutants were further removed by the system, and they were mainly some aliphatic and aromatic compounds, which were the main refractory organic pollutants in most textile effluents. It was also proved by the pilot-scale DM study that the removed residual pollutants from the secondary textile effluent were mainly aromatic protein II. Due to the contained complex functional groups in their molecular structure, soluble microbial metabolites were relatively easier to be removed by the DM layer.

Rapid granulation of aerobic granular sludge and maintaining its stability by combining the effects of multi-ionic matrix and bio-carrier in a continuous-flow membrane bioreactor.

The present investigation aimed at providing a novel approach to promote the rapid granulation and stability of aerobic granular sludge (AGS) in a continuous-flow membrane bioreactor (MBR). By operating two identical MBRs with or with no bio-carrier for 125 days, it was found that the combination of multi-ionic matrix and bio-carrier could promote the rapid formation and maintain the long-term stability of AGS. The primary AGS was first observed inside the reactor on day 14, and the mature AGS appeared soon and kept stable for more than 4 months (its average size still was about 800 µm on day 125). Suitable filling ratio of bio-carrier was beneficial to form a stable and regular circulating water flow inside, and adding divalent metal ions quickly reduced the negative charges of tiny sludge particles, which were two essential factors leading to the rapid granulation of AGS and maintaining its stability. The multi-ionic matrix not only enhanced the biological aggregation process, but also facilitated the expansion of the cultivated AGS into a new multi-habitat system of Mn-AGS, in which, complex microbial communities with rich bio-diversity robustly promoted the efficient removal of organic pollutants and nutrients.

Distribution characteristics of phosphorus-containing substances in a long running aerobic granular sludge-membrane bioreactor with no sludge discharge.

This work aimed at revealing the distribution characteristics of phosphorus (P) containing substances in an aerobic granular sludge-membrane bioreactor (AGS-MBR). During the long running period (180 days) with no sludge discharge, AGS was successfully cultivated on day 20, and the system performed well in removing organic pollutants and total nitrogen (TN). However, the removal of total P (TP) showed a fluctuant tendency, and P was found to distribute in all the phases of the system. In the intracellular phase, it occupied the largest ratio all through the period. In AGS, inorganic P (IP) was measured to be about 74.4-77.8% of TP, with non-apatite IP (NAIP) composing 57.5-69.6%, while in organic P (OP), the ratio of monoester and diester phosphate was in the range of 19-26.9% and 12-13.5%, respectively. The presence of highly releasable and bioavailable P (NAIP + OP) in AGS implied that it might be a potential P resource for utilization.

Removal of heavy metal ions from wastewaters: a review.

Heavy metal pollution has become one of the most serious environmental problems today. The treatment of heavy metals is of special concern due to their recalcitrance and persistence in the environment. In recent years, various methods for heavy metal removal from wastewater have been extensively studied. This paper reviews the current methods that have been used to treat heavy metal wastewater and evaluates these techniques. These technologies include chemical precipitation, ion-exchange, adsorption, membrane filtration, coagulation-flocculation, flotation and electrochemical methods. About 185 published studies (1988-2010) are reviewed in this paper. It is evident from the literature survey articles that ion-exchange, adsorption and membrane filtration are the most frequently studied for the treatment of heavy metal wastewater.

Mn-incorporated ferrihydrite for Cr(VI) immobilization: Adsorption behavior and the fate of Cr(VI) during aging.

Chromium(VI) (Cr(VI)) is an environmental priority pollutant, and its mobility in natural environment is strongly controlled by ferrihydrite. Ferrihydrite always contains various ions, which may change the properties of ferrihydrite, thereby affecting the behavior of pollutants. This study aims to investigate the adsorption of Cr(VI) by Mn-incorporated ferrihydrite and the mobility behavior of Cr(VI) during aging. Results showed that the incorporation of Mn enhanced the adsorption of Cr(VI) on ferrihydrite, and the adsorption performance increased with the increase of Mn content. The maximum adsorption capacity for Cr(VI) reached to 48.5 mg/g with molar ratio of Mn/Fe 5%, while it was 36.1 mg/g for pure ferrihydrite. After aging for 7 days, ferrihydrite transformed into goethite and hematite. The adsorbed Cr(VI) on the surface of ferrihydrite was released into the solution during aging. The incorporation of Mn retarded the transformation of ferrihydrite, which inhibited the migration of adsorbed Cr(VI). Nevertheless, the incorporation of Mn resulted in the transformation of adsorbed Cr(VI) to non-desorbed Cr(VI), thereby enhancing the retention of Cr(VI). Our results suggest that the incorporation of Mn into ferrihydrite has an important role on the mobility of Cr(VI), which enhances our understanding of the behavior of Cr(VI) in the environment.

Effects of oxalate and citrate on the behavior and redistribution of Cr(VI) during ferrihydrite-Cr(VI) co-precipitates transformation.

Understanding the influence of organic matters on the fate of Cr(VI) during ferrihydrite-Cr(VI) (Fh-Cr) co-precipitates transformation helps to study the retention of Cr(VI) by iron oxides in the environment. In this paper, Fh-Cr was prepared by co-precipitation and the redistribution of Cr(VI) in the oxalate or citrate system during the transformation of Fh-Cr was studied. X-ray diffraction, Fourier transform infrared spectroscopy, Transmission electron microscopy, and X-ray photoelectron spectroscopy were used to characterize Fh-Cr for aging 7 days at 70 °C. Results showed that both oxalate and citrate could hinder the release of Cr(VI) from Fh-Cr and abate the harm of Cr(VI). Oxalate improved the transformation from Fh-Cr to hematite and promoted Cr(VI) to be enfolded into the secondary minerals to further immobilize Cr at initial pH of 5.0 and 7.0, while citrate evidently reduced the release of Cr(VI) through stabilizing Fh-Cr at initial pH of 9.0. Besides, reduction of Cr(VI) by oxalate and citrate was through forming the surface complexes that promoted electron transfer from oxalate or citrate to Cr(VI), which can effectively abate the harm of Cr(VI). The findings of this study can promote understanding of the influences of organic matters on Cr(VI) immobilization during transformation of iron oxides in nature.