Phosphate loading alters schwertmannite transformation rates and pathways during microbial reduction

Facilitated transport of ferrihydrite with phosphate under saturated flow conditions

With increasing phosphate (P) entering the environment during agricultural application, the subsurface flow of particular P has been recently discussed as a vital P transport pathway. Iron (oxyhydr)oxide colloid-facilitated P transport is critical for iron and P biogeochemical processes in the subsurface. This study investigated the ferrihydrite colloid-facilitated P transport through adsorption and column experiments under different P concentrations and three pH conditions. Increased P loading on ferrihydrite colloids decreased the transport of ferrihydrite colloids (< 8.0%) under acid conditions through pore straining and irreversible attachment. Under neutral and alkaline conditions, ferrihydrite colloids exhibited more negative surfaces and smaller diameters with increasing P, which further enhanced ferrihydrite colloid transport (maximum to 95.6%). Ferrihydrite colloid-facilitated P transport was limited under acid conditions, and it was 10% - 57% enhancement under neutral and alkaline conditions with increasing P adsorption. Under neutral conditions, ferrihydrite colloid-facilitated P transport was strongest (maximum to 68.84%) because of its stronger ferrihydrite colloid transport than under acid conditions and larger P adsorption capacity than under alkaline conditions. Our findings indicate that the facilitated transport of ferrihydrite colloids in the presence of P may be appreciable in iron and phosphate-rich soil and subsurface systems, which is essential for evaluating the fate of iron and iron-facilitated P and potential environmental risks of P transport in the subsurface.

Divergent repartitioning of antimony and arsenic during jarosite transformation: A comparative study under aerobic and anaerobic conditions

Jarosite is the host mineral of Sb(V) and As(V) in mining environments. However, the repartitioning of Sb and As during its transformation is poorly understood. Additionally, the mutual effect between the redistribution behavior of As and Sb during jarosite conversion remains unclear. Here, we investigated the transformation of Sb(V)-, As(V)- and Sb(V)-As(V)-jarosite at pH 5.5 under aerobic and anaerobic conditions without a reductant. The results indicated that co-precipitated Sb(V) promotes jarosite dissolution, and the final products were mainly goethite and hematite. In contrast, the co-precipitated As(V) retarded jarosite dissolution and altered the transformation pathway, mainly forming lepidocrocite, which might be attributed to the formation of As-Fe complexes on the jarosite surface. The inhibiting or promoting effect increased with the increase in co-precipitated As or Sb concentration. In the treatment with Sb(V)-As(V)-jarosite, the inhibition effect of co-precipitated As(V) on mineral dissolution was predominant, but the end-products were mainly goethite and hematite. Compared with the aerobic system, the dissolution and transformation of jarosite in treatments in the anaerobic system occurred faster, although without a reductant, which was possibly associated with the reduced CO2 content in the reaction solutions after degassing. In all treatments, the release of Sb(aq) and As(aq) into the solution was negligible during jarosite transformation. The transformation processes drove As into the surface-bound exchangeable and poorly crystalline phases, while Sb was typically redistributed in the poorly crystalline phase. During the transformation of Sb(V)-As(V)-jarosite, the co-existence of As significantly increased the proportion of Sb distributed on the solid surface and in the poorly crystalline phase. These findings are valuable for predicting the long-term fate of Sb and As in mining environments.

Oxide- and Silicate-Water Interfaces and Their Roles in Technology and the Environment

Interfacial reactions drive all elemental cycling on Earth and play pivotal roles in human activities such as agriculture, water purification, energy production and storage, environmental contaminant remediation, and nuclear waste repository management. The onset of the 21st century marked the beginning of a more detailed understanding of mineral aqueous interfaces enabled by advances in techniques that use tunable high-flux focused ultrafast laser and X-ray sources to provide near-atomic measurement resolution, as well as by nanofabrication approaches that enable transmission electron microscopy in a liquid cell. This leap into atomic- and nanometer-scale measurements has uncovered scale-dependent phenomena whose reaction thermodynamics, kinetics, and pathways deviate from previous observations made on larger systems. A second key advance is new experimental evidence for what scientists hypothesized but could not test previously, namely, interfacial chemical reactions are frequently driven by "anomalies" or "non-idealities" such as defects, nanoconfinement, and other nontypical chemical structures. Third, progress in computational chemistry has yielded new insights that allow a move beyond simple schematics, leading to a molecular model of these complex interfaces. In combination with surface-sensitive measurements, we have gained knowledge of the interfacial structure and dynamics, including the underlying solid surface and the immediately adjacent water and aqueous ions, enabling a better definition of what constitutes the oxide- and silicate-water interfaces. This critical review discusses how science progresses from understanding ideal solid-water interfaces to more realistic systems, focusing on accomplishments in the last 20 years and identifying challenges and future opportunities for the community to address. We anticipate that the next 20 years will focus on understanding and predicting dynamic transient and reactive structures over greater spatial and temporal ranges as well as systems of greater structural and chemical complexity. Closer collaborations of theoretical and experimental experts across disciplines will continue to be critical to achieving this great aspiration.

Effect of phosphate on ferrihydrite transformation and the associated arsenic behavior mediated by sulfate-reducing bacterium

Although PO₄^3- is commonly found in association with iron (oxyhydr)oxide, the effect of PO₄^3- on ferrihydrite reduction, mineralogical transformation, and associated As behavior in sulfate-reducing bacteria (SRB)-rich environments remains unclear. In this study, batch experiments, together with geochemical, mineralogical, and biological analyses, were conducted to elucidate these processes. The results showed that SRB can reduce ferrihydrite via direct and indirect processes, and PO₄^3- promoted ferrihydrite reduction by supporting SRB growth at low and medium PO₄^3- loadings. However, at high loadings, PO₄^3- stabilized the ferrihydrite. PO₄^3- shifted the transformation of ferrihydrite from magnetite and mackinawite to vivianite, which scavenges As effectively by incorporating As into its particle. In systems with 0.5 mM SO₄^2-, PO₄^3- exerted a weak effect on As mobilization. However, in systems with 10 mM SO₄^2-, substantial amounts of As were released into the solution, and PO₄^3- impacted As behavior strongly. Low PO₄^3- loadings increased the mobilization of As because of the competitive adsorption of PO₄^3- on mackinawite. Medium and high PO₄^3- loadings were beneficial for As immobilization because of the substitution of mackinawite by vivianite. These findings have important implications for understanding the biogeochemistry of iron (oxyhydr)oxide and As behavior in SRB-containing sediments.

Anaerobic syntrophic system composed of phosphate solubilizing bacteria and dissimilatory iron reducing bacteria induces cadmium immobilization via secondary mineralization

Secondary mineralization is a promising method for remediating cadmium (Cd) pollution in sediments, but the poor stability of Cd-containing secondary minerals is a bottleneck that limits the development of this approach. The existence of phosphate can enhance the formation of stable secondary minerals and points a new direction for Cd immobilization. In this research, a novel syntrophic system composed of phosphate solubilizing bacteria (PSB) and dissimilatory iron reducing bacteria (DIRB) was established and the effect and mechanism of Cd immobilization in the system were also explored. The results showed that under the conditions of DIRB:PSB (V:V)= 3:1, syntrophic bacteria dosage of 5% and glucose dosage of 5 g/L, Cd incorporated in the secondary minerals could account for about 60% of the total Cd. In the pH range of 5-9, alkaline environment was conducive to the immobilization of Cd and the percentage of combined Cd was up to 58%, while the combined Cd in secondary minerals decreased from 62% to 56% with the increase of initial Cd concentration from 0.1 to 0.3 mmol/L. In addition, XRD, XPS, Mössbauer and other characterization results showed that secondary minerals, such as Cd exchange hydroxyapatite (Cd-HAP) and kryzhanovskite (Fe₃(PO₄)₂(OH)₃) were formed in this new system. The established syntrophic system of PSB and DIRB is thus a prospective bioremediation technology for Cd immobilization in sediments and can avoid the potential risk might be caused by the addition of phosphorus-containing materials.

Microbial reduction of schwertmannite by co-cultured iron- and sulfate-reducing bacteria

Schwertmannite (Sch) is an iron-hydroxysulfate mineral commonly found in acid mine drainage contaminated environment. The transformation mechanism of Sch mediated by pure cultured iron-reducing bacteria (FeRB) or sulfate-reducing bacteria (SRB) has been studied. However, FeRB and SRB widely coexist in the environment, the mechanism of Sch transformation by the consortia of FeRB and SRB is still unclear. This study investigated the Sch reduction by co-cultured Shewanella oneidensis (FeRB) and Desulfosporosinus meridiei (SRB). The results showed that co-culture of FeRB and SRB could accelerate the reductive dissolution of Sch, but not synergistically, and there were two distinct phases in the reduction of Sch mediated by FeRB and SRB: an initial phase in which FeRB predominated and Fe³⁺ in Sch was reduced, accompanied with the release of SO₄^2-, and the detected secondary minerals were mainly vivianite; the second phase in which SRB predominated and mediated the reduction of SO₄^2-, producing minerals including mackinawite and siderite in addition to vivianite. Compared to pure culture, the abundance of FeRB and SRB in the consortia decreased, and more minerals aggregated inside and outside the cell; correspondingly, the transcription levels of genes (cymA, omcA, and mtrCBA) related to Fe³⁺ reduction in co-culture was down-regulated, while the transcription levels of SO₄^2--reducing genes (sat, aprAB, dsr(C)) was generally up-regulated. These phenomena suggested that secondary minerals produced in co-culture limited but did not inhibit bacterial growth, and the presence of SRB was detrimental to dissimilatory Fe³⁺ reduction, while existed FeRB was in favor of dissimilatory SO₄^2- reduction. SRB mediated SO₄^2- reduction by up-regulating the expression of SO₄^2- reduction-related genes when its abundance was limited, which may be a strategy to cope with external coercion. These findings allow for a better understanding of the process and mechanism of microbial mediated reduction of Sch in the environment.

Mineral transformation and dissolution of jarosite coprecipitated with hazardous oxyanions and their mobility changes

Jarosite coprecipitation with hazardous oxyanions can attenuate the concentrations of these elements in acid mine drainage. However, jarosite can be easily transformed to goethite with changes in geochemical conditions. Consequently, the released oxyanions can greatly affect environments. The changes in the mineralogy and mobility of five oxyanions, namely AsO₄, SeO₃, SeO₄, MoO₄, and CrO₄, which were coprecipitated with jarosite, are investigated herein during the mineral transformation. Our results show that the oxyanion species and the pH values greatly affect the mineral transformation and dissolution rates of jarosite-containing oxyanions. The transformation and dissolution rates of the jarosite samples at pH 8 are noticeably higher than those at pH 4. The X-ray diffraction results show that the CrO₄ and SeO₄ jarosites are as effectively transformed to goethite as the jarosite without oxyanions, while the SeO₃ and AsO₄ jarosites are least transformed, resulting in different sulfate and oxyanion concentrations in the solution. The oxyanions in jarosite are the main controlling factor in the mineral transformation and dissolution rates. In acid mine drainage, although CrO₄ is easily attenuated by the jarosite precipitation, it has the highest mobility during the goethite transformation. On the contrary, AsO₄ shows the opposite case.

Phosphorus recovery from waste activated sludge by sponge iron seeded crystallization of vivianite and process optimization with response surface methodology

As a novel phosphorus recovery product, vivianite (Fe₃(PO₄)₂·8H₂O) has attracted much attention due to its enormous recycling potential and foreseeable economic value. Taking sponge iron as seed material, the effect of different reaction conditions on the recovery of phosphorus in waste activated sludge by vivianite crystallization was studied. Through single factor tests, the optimal conditions for vivianite formation were in the pH range of 5.5-6.0 with Fe/P molar ratio of 1.5. Scanning electron microscopy (SEM), powder X-ray diffraction (XRD), and energy dispersive spectroscopy (EDS) were used to analyze the components of the crystals. The results showed that the vivianite produced by sponge iron as the seed crystal were larger and thicker (300-700 μm) than other seed (200-300 μm) and without seed (50-100 μm). Moreover, vivianite, which was synthesized with sponge iron as seed, was obviously magnetic and could be separated from the sludge by rubidium magnet. The Box-Behnken design of the response surface methodology was used to optimize the phosphorus-recovery process with sponge iron (maximum phosphorus recovery rate was 83.17%), and the interaction effect of parameters was also examined, pH had a significant effect on the formation of vivianite. In summary, this research verifies the feasibility of using sponge iron as the seed crystal to recover phosphorus in the form of vivianite from waste activated sludge, which is conducive to the subsequent separation and utilization of vivianite.

Factors and pathways regulating the release and transformation of arsenic mediated by reduction processes of dissimilated iron and sulfate

The driving process and explanatory factors regulating the transformation and migration of arsenic (As) mediated by dissimilatory iron reducing bacteria (DFeRB) and sulfate reducing bacteria (SRB) remain poorly understood. The novelty of this study is to explore the driving process and key environmental factors governing As mobilization mediated by DFeRB and SRB based on continuous As speciation and environmental parameter monitoring in a sediment-water system. The results illustrate the reduction process mediated by DFeRB and SRB significantly promotes the reduction of As(V) and the endogenous release of As. However, in the DFeRB and SRB mediated reductions, the main driving process and key explanatory factors that dominate the As mobility are significantly different. DFeRB has significant effects on the reductive dissolution and re-distribution of Fe(III) oxyhydroxides and As-containing Fe(III) minerals and on adsorption-desorption, which in turn influenced the transformation of iron species and the release and ecotoxicity of As. Meanwhile, the environmental factors that affect As mobility depend on Fe²⁺ and Fe³⁺ in DFeRB-induced reduction, presenting two main pathways: the process of As mobilization mediated by DFeRB, and the process influenced by the inorganic phosphorus involved in the competitive adsorption and anion exchange. Significantly different from DFeRB, the effects of SRB on As behavior mainly occur by influencing the adsorbed As, pyrite, and As sulfides in the sediments and through the formation of sulfides during the sulfate reduction. The main pathways of As mobilization reflect the direct effects of SRB, S^2-, and Fe²⁺. In addition, the role of NH₄⁺-N in the driving process of As mobility is more pronounced in SRB-induced reduction. NO₃^--N is an essential factor affecting As mobility, but the effects of NO₃^--N on As lead to non-significant pathways. This work provides insights into the environmental effects of DFeRB and SRB on the biogeochemical cycle of As.

Biosynthesis of vivianite from microbial extracellular electron transfer and environmental application

Vivianite (Fe₃(PO₄)₂·8H₂O) is a common hydrous ferrous phosphate mineral which often occurs in reductive conditions, especially anoxic non-sulfide environment containing high concentrations of ferrous iron (Fe²⁺) and orthophosphate (PO₄^3-). Vivianite is an important product of dissimilatory iron reduction and a promising route for phosphorus recovery from wastewater. Its formation is closely related to the extracellular electron transfer (EET), a key mechanism for microbial respiration and a crucial explanation for the reduction of metal oxides in soil and sediments. Despite of the natural ubiquity, easy accessibility and attractive economic value, the application value of vivianite has not received much attention. This review introduces the characteristics, occurrence and biosynthesis of vivianite from microbial EET, and systematically analyzes the application value of vivianite in the environmental field, including immobilization of heavy metals (HMs), dechlorination of carbon tetrachloride (CT), sedimentary phosphorus sequestration and eutrophication alleviation. Additionally, its potential functions as a slow-release fertilizer are discussed as well. In general, vivianite is expected to make more contributions to the future scientific research, especially the solution of environmental problems. Overcoming the lack of understanding and some technical limitations will be beneficial to the further application of vivianite in environmental field.

Effects of Fe(III) Oxide Mineralogy and Phosphate on Fe(II) Secondary Mineral Formation during Microbial Iron Reduction

The bioreduction of Fe(III) oxides by dissimilatory iron-reducing bacteria may result in the formation of a suite of Fe(II)-bearing secondary minerals, including magnetite (a mixed Fe(II)/Fe(III) oxide), siderite (Fe(II) carbonate), vivianite (Fe(II) phosphate), chukanovite (ferrous hydroxy carbonate), and green rusts (mixed Fe(II)/Fe(III) hydroxides). In an effort to better understand the factors controlling the formation of specific Fe(II)-bearing secondary minerals, we examined the effects of Fe(III) oxide mineralogy, phosphate concentration, and the availability of an electron shuttle (9,10-anthraquinone-2,6-disulfonate, AQDS) on the bioreduction of a series of Fe(III) oxides (akaganeite, feroxyhyte, ferric green rust, ferrihydrite, goethite, hematite, and lepidocrocite) by Shewanella putrefaciens CN32, and the resulting formation of secondary minerals, as determined by X-ray diffraction, Mössbauer spectroscopy, and scanning electron microscopy. The overall extent of Fe(II) production was highly dependent on the type of Fe(III) oxide provided. With the exception of hematite, AQDS enhanced the rate of Fe(II) production; however, the presence of AQDS did not always lead to an increase in the overall extent of Fe(II) production and did not affect the types of Fe(II)-bearing secondary minerals that formed. The effects of the presence of phosphate on the rate and extent of Fe(II) production were variable among the Fe(III) oxides, but in general, the highest loadings of phosphate resulted in decreased rates of Fe(II) production, but ultimately higher levels of Fe(II) than in the absence of phosphate. In addition, phosphate concentration had a pronounced effect on the types of secondary minerals that formed; magnetite and chukanovite formed at phosphate concentrations of ≤1 mM (ferrihydrite), <~100 µM (lepidocrocite), 500 µM (feroxyhyte and ferric green rust), while green rust, or green rust and vivianite, formed at phosphate concentrations of 10 mM (ferrihydrite), ≥100 µM (lepidocrocite), and 5 mM (feroxyhyte and ferric green rust). These results further demonstrate that the bioreduction of Fe(III) oxides, and accompanying Fe(II)-bearing secondary mineral formation, is controlled by a complex interplay of mineralogical, geochemical, and microbiological factors.

Coprecipitation of Phosphate and Silicate Affects Environmental Iron (Oxyhydr)Oxide Transformations: A Gel-Based Diffusive Sampler Approach

Sorption of nutrients such as phosphate (P) and silicate (Si) by ferric iron (oxyhydr)oxides (FeOx) modulates nutrient mobility and alters the structure and reactivity of the FeOx. We investigated the impact of these interactions on FeOx transformations using a novel approach with samplers containing synthetic FeOx embedded in diffusive hydrogels. The FeOx were prepared by Fe(III) hydrolysis and Fe(II) oxidation, in the absence and presence of P or Si. Coprecipitation of P or Si during synthesis altered the structure of Fe precipitates and, in the case of Fe(II) oxidation, lepidocrocite was (partly) substituted by poorly ordered FeOx. The pure and P- or Si-bearing FeOx were deployed in (i) freshwater sediment rich in dissolved Fe(II) and P and (ii) marine sediment with sulfidic pore water. Iron(II)-catalyzed crystallization of poorly ordered FeOx was negligible, likely due to surface passivation by adsorption of dissolved P. Reaction with dissolved sulfide was modulated by diffusion limitations and therefore the extent of sulfidation was the lowest for poorly ordered FeOx with high reactivity toward sulfide that created temporary, local sulfide depletion (Fh < Lp). We show that coprecipitation-induced changes in the FeOx structure affect coupled iron-nutrient cycling in aquatic ecosystems. The gel-based method enriches our geochemical toolbox by enabling detailed characterization of target phases under natural conditions.

Effects of adsorbed phosphate on jarosite reduction by a sulfate reducing bacterium and associated mineralogical transformation

Jarosite is one of the iron oxyhydroxysulfate minerals that are commonly found in acid mine drainage (AMD) systems. In natural environments, phosphate and sulfate reducing bacteria (SRB) may be coupled to jarosite reduction and transformation. In this research, the effect of phosphate on jarosite reduction by SRB and the associated secondary mineral formation was studied using batch experiments. The results indicated that Fe³⁺ is mainly reduced by biogenic S^2- in this experiment. The effect of PO₄^3- on jarosite reduction by SRB involved not only a physico-chemical factor but also a microbial factor. Phosphate is an essential nutrient, which can support the activity of SRB. In the low PO₄^3- treatment, the production of total Fe²⁺ was found to be slightly larger than that in the zero PO₄^3- treatment. Sorption of PO₄^3- effectively elevated jarosite stability via the formation of inner sphere complexes, which, therefore, inhibited the reductive dissolution of jarosite. At the end of the experiment, the amounts of total Fe²⁺ accumulation were determined to be 4.54 ± 0.17^a mM, 4.66 ± 0.22^a mM, 3.91 ± 0.04^b mM and 2.51 ± 0.10^c mM (p < 0.05) in the zero, low, medium and high PO₄^3- treatments, respectively, following the order of low PO₄^3- treatment > zero PO₄^3- treatment > medium PO₄^3- treatment > high PO₄^3- treatment. PO₄^3- loading modified the transformation pathways for the jarosite mineral, as well. In the zero PO₄^3- treatment, the jarosite diffraction lines disappeared, and mackinawite dominated at the end of the experiment. Compared to PO₄^3--free conditions, vivianite was found to become increasingly important at higher PO₄^3- loading conditions. These findings indicate that PO₄^3- loading can influence the broader biogeochemical functioning of AMD systems by impacting the reactivity and mineralization of jarosite mineral.

Biogenic iron mineralization of polyferric sulfate by dissimilatory iron reducing bacteria: Effects of medium composition and electric field stimulation

Polyferric sulfate (PFS) is a coagulant widely used for removing contaminants from the aqueous phase; however, PFS destabilizes and recrystallizes in the solid phase in the presence of dissimilatory iron reducing bacteria (DIRB), which has a profound influence on the cycle of Fe and the fate of the associated pollutants. Our objective is to investigate the combined effects of medium composition and electric field stimulation on the biomineralization of PFS. Batch experiments were conducted with PFS and the DIRB Shewanella oneidensis MR-1 under anoxic conditions to examine the microbial reduction of PFS to Fe(II) and its subsequent biotransformation. The high concentration of phosphorous in phosphate buffer solution (PBS) is responsible for slower and less extensive Fe(II) generation compared to the lower concentration of phosphorous in a medium of 1,4-piperazinediethanesulfonic acid (PIPES). The PBS system induces the formation of green rust (SO₄^2-) and vivianite as the major minerals; in contrast, magnetite is the predominant end product in the PIPES system. The application of an anodic potential of 0.2 V significantly stimulates Fe(II) release from PFS, leading to precipitation and transformation of more crystalline minerals in increased quantities. The results demonstrate that Fe(II) catalyzes biomineralization of PFS to a variety of secondary products; this electron transfer process is highly dependent on the rate and magnitude of PFS reduction and the surface reaction with the host compound and adsorbed ions.