Iron Transformation and Its Role in Phosphorus Immobilization in a UCT-MBR with Vivianite Formation Enhancement

Dual role of organic matter in Feammox-driven nitrogen and phosphate removal

Feammox is a novel microbial process that enables simultaneous nitrogen and phosphorus removal in wastewater treatment. This study investigated the role of organic matter in Feammox-driven nutrient removal during long-term bioreactor operation by gradually increasing the influent chemical oxygen demand (COD) concentration from 0 to 50, and then to 100 mg/L. The results revealed that the ammonium removal efficiency was reduced from 60.5 % to 20.7 % with COD concentration increasing from 0 to 100 mg/L. In contrast, organic matter enhanced nitrate removal through heterotrophic denitrification, which outcompeted nitrate-dependent Fe(II) oxidation. Phosphorus removal was increased up to approximately 90 % via Fe(II)-mediated precipitation, forming vivianite crystals, evidenced by X-ray diffraction analysis. Continuous addition of Fe(III) alleviated the inhibitory effect of organic matter on ammonia oxidation by serving as an alternative electron acceptor, reducing competition. Therefore, optimizing organic matter levels and ensuring sufficient Fe(III) availability are crucial for achieving efficient nutrient removal in Feammox systems, particularly for treating wastewater with a low carbon/nitrogen ratio.

Nanophase REE phosphate crystallization induced by vivianite oxidation: mechanistic insights and mineralogical implications

Our study investigates the interaction between multi-component rare earth element (REE; La, Ce, Pr, Nd, Dy)-bearing aqueous solutions and vivianite (Fe3 2+(PO4)2·8H2O) grains under hydrothermal conditions (50-165 °C). The results revealed the solution-mediated, progressive oxidation and dissolution of vivianite. This resulted in the formation of iron phosphates, metavivianite [Fe2+Fe2 3+(PO4)2(OH)2·6H2O], and giniite [Fe2+Fe4 3+(PO4)4 (OH)2·2H2O], iron oxide hematite [Fe2O3], and rare earth phosphates, rhabdophane [REE(PO4)·H2O] and monazite [(LREE)PO4]. The extent of the reactions was found to be dependent on temperature, pH, and the concentration and ionic radii of the rare earths in solution. The rate of vivianite oxidation and dissolution increased with increased temperature, with 50% of vivianite transformed after 32 days at 50 °C, and 100% transformed after 28 days and 4 hours at 90 and 165 °C respectively. The pH of the solutions at all three temperatures maintained the stability of rhabdophane, and only at the highest temperature of 165 °C it began to transform to monazite. Understanding the stability of iron phosphates, their transformation products, and their capacity to incorporate REEs is crucial for resource recovery, especially in the extraction of REEs from waste materials.

Efficient phosphorus recovery from waste activated sludge: Pretreatment with natural deep eutectic solvent and recovery as vivianite

Recycling phosphorus from waste activated sludge (WAS) is an effective method to address the nonrenewable nature of phosphorus and mitigate environmental pollution. To overcome the challenge of low phosphorus recovery from WAS due to insufficient disintegration, a method using a citric acid-based natural deep eutectic solvent (CA-NADES) assisted with low-temperature pretreatment was proposed to efficiently release and recover phosphorus. The results of 31P nuclear magnetic resonance (NMR) confirmed that low-temperature pretreatment promoted the conversion of organic phosphorus (OP) to inorganic phosphorus (IP) and enhanced the effect of CA-NADES. Changes in the three-dimensional excitation-emission matrix (3D-EEM) and flow cytometry (FCM) indicated that the method of CA-NADES with low-temperature thermal simultaneously release IP and OP by disintegrating sludge flocs, dissolving extracellular polymeric substances (EPS) structure, and cracking cells. When 5 % (v/v) of CA-NADES was added and thermally treated at 60 °C for 30 min, 43 % of total phosphorus (TP) was released from the sludge. The concentrations of proteins and polysaccharides reached 826 and 331 mg/L, respectively, which were 6.30 and 14.43 times higher than those of raw sludge. The dewatering and settling of the sludge were also improved. Metals were either enriched in the solid phase or released into the liquid phase in small quantities (most efficiencies of less than 10 %) for subsequent clean recovery. The released phosphorus was successfully recovered as vivianite with a rate of 90 %. This study develops an efficient, green, and sustainable method for phosphorus recovery from sludge using NADES and provides new insights into the high-value conversion of sludge.

Iron-retrofitted anaerobic baffled reactor system for rural wastewater treatment: Stable performance of nutrients removal with phosphorus recovery and minimal sludge production

An iron-retrofitted anaerobic baffled reactor (ABR) system was developed for the effective treatment of rural wastewater with reduced maintenance demand and aeration costs. Average removal efficiencies of chemical oxygen demand, total nitrogen and total phosphorus of 99.4%, 62.7% and 92.6% were achieved respectively, when the ABR system was operating at steady state. With effective bioreduction of FeIII in the anaerobic chambers, phosphorus was immobilized in the sludge as vivianite, the sole phosphorus-carrying mineral. The FeIII in the recirculated sludge induced Feammox in the ABR reactor, contributing 14.8% to total nitrogen removal. Biophase separation and enrichment of microorganisms associated with iron and nitrogen transformations were observed in the system after Fe dosing, which enhanced the removal of pollutants. The coupling of Feammox and vivianite crystallization to remove nitrogen and phosphorus in an iron-retrofitted ABR would appear to be a promising technology for rural wastewater treatment.

Efficacy and mechanism of enhanced Sb(V) removal from textile wastewater using ferric flocs in aerobic biological treatment

Antimony contamination from textile industries has been a global environmental concern and the existing treatment technologies could not reduce Sb(V) to meet the discharge standards. To overcome this shortcoming, ferric flocs were introduced to expedite the biological process for enhanced Sb(V) removal in wastewater treatment plant (WWTP). For this purpose, a series of laboratorial-scale sequential batch reactor activated sludge processes (SBRs) were applied for Sb(V) removal with varied reactor conditions and the transformation of Fe and Sb in SBR system was investigated. Results showed a significant improvement in Sb(V) removal and the 20 mg L-1 d-1 iron ions dosage and iron loss rate was found to be only 15.2%. The influent Sb(V) concentration ranging 153-612 μg L-1 was reduced to below 50 μg L-1, and the maximum Sb(V) removal rate of the enhanced system reached about 94.3%. Furthermore, it exhibited high stability of Sb(V) removal in the face of antimonate load, Fe strike and matrix change of wastewater. Sludge total Sb determination and capacity calculation revealed decreasing in Sb adsorption capacity and desorption without fresh Fe dosage. While sludge morphology analysis demonstrated the aging and crystallization of iron hydroxides. These results verify the distinct effects of fresh iron addition and iron aging on Sb(V) removal. High-throughput gene pyrosequencing results showed that the iron addition changed microbial mechanisms and effect Fe oxidized bacterial quantity, indicating Sb(V) immobilization achieved by microbial synergistic iron oxidation. The present study successfully established a simple and efficient method for Sb(V) removal during biological treatment, and the modification of biological process by iron supplement could provide insights for real textile wastewater treatment.

Strategies for optimizing biovivianite production using dissimilatory Fe(III)-reducing bacteria

Vivianite (Fe3(PO4)2·8H2O), a sink for phosphorus, is a key mineralization product formed during the microbial reduction of phosphate-containing Fe(III) minerals in natural systems, and also in wastewater treatment where Fe(III)-minerals are used to remove phosphate. As biovivianite is a potentially useful Fe and P fertiliser, there is much interest in harnessing microbial biovivianite synthesis for circular economy applications. In this study, we investigated the factors that influence the formation of microbially-synthesized vivianite (biovivianite) under laboratory batch systems including the presence and absence of phosphate and electron shuttle, the buffer system, pH, and the type of Fe(III)-reducing bacteria (comparing Geobacter sulfurreducens and Shewanella putrefaciens). The rate of Fe(II) production, and its interactions with the residual Fe(III) and other oxyanions (e.g., phosphate and carbonate) were the main factors that controlled the rate and extent of biovivianite formation. Higher concentrations of phosphate (e.g., P/Fe = 1) in the presence of an electron shuttle, at an initial pH between 6 and 7, were needed for optimal biovivianite formation. Green rust, a key intermediate in biovivianite production, could be detected as an endpoint alongside vivianite and metavivianite (Fe2+Fe3+2(PO4)2.(OH)2.6H2O), in treatments with G. sulfurreducens and S. putrefaciens. However, XRD indicated that vivianite abundance was higher in experiments containing G. sulfurreducens, where it dominated. This study, therefore, shows that vivianite formation can be controlled to optimize yield during microbial processing of phosphate-loaded Fe(III) materials generated from water treatment processes.

Membrane-based technology in water and resources recovery from the perspective of water social circulation: A review

In this review, the application of membrane-based technology in water social circulation was summarized. Water social circulation encompassed the entire process from the acquirement to discharge of water from natural environment for human living and development. The focus of this review was primarily on the membrane-based technology in recovery of water and other valuable resources such as mineral ions, nitrogen and phosphorus. The main text was divided into four main sections according to water flow in the social circulation: drinking water treatment, agricultural utilization, industrial waste recycling, and urban wastewater reuse. In drinking water treatment, the acquirement of water resources was of the most importance. Pressure-driven membranes, such as ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) were considered suitable in natural surface water treatment. Additionally, electrodialysis (ED) and membrane capacitive deionization (MCDI) were also effective in brackish water desalination. Agriculture required abundant water with relative low quality for irrigation. Therefore, the recovery of water from other stages of the social circulation has become a reasonable solution. Membrane bioreactor (MBR) was a typical technique attributed to low-toxicity effluent. In industrial waste reuse, the osmosis membranes (FO and PRO) were utilized due to the complex physical and chemical properties of industrial wastewater. Especially, membrane distillation (MD) might be promising when the wastewater was preheated. Resources recovery in urban wastewater was mainly divided into recovery of bioenergy (via anaerobic membrane bioreactors, AnMBR), nitrogen (utilizing MD and gas-permeable membrane), and phosphorus (through MBR with chemical precipitation). Furthermore, hybrid/integrated systems with membranes as the core component enhanced their performance and long-term working ability in utilization. Generally, concentrate management and energy consumption control might be the key areas for future advancements of membrane-based technology.

The activation of peroxymonosulfate by biochar derived from anaerobic and aerobic iron-containing excess sludge

The excess sludge from municipal sewage treatment plants is rich in Fe (III) due to chemical dephosphorization. The activation of peroxymonosulfate (PMS) by biochar derived from anaerobic and aerobic iron-containing excess sludge was studied systematically in this research. Fe (III)-containing excess sludge was cultured in an anaerobic environment for conversion of partial Fe (III) to Fe (II), which was further carbonized to prepare biochar labeled AnSx@Fe. Meanwhile, aerobic sludge with different Fe (III) content was directly carbonized to produce biochar labeled AeS@Fe. For biochar (AnS20@Fe-15%) prepared from 15% Fe(III)-containing anaerobic cultured 20 days sludge, the relative contents of Fe (III) and Fe (II) were 21.26% and 78.74%, which were 31.03% and 68.97% for biochar (AeS@Fe-10%) prepared from 10% Fe (III)-containing aerobic sludge. Fe (III) can be reduced to Fe (II) by both anaerobic culture and carbonization. Their removal rates of tetracycline (TC) through 60 min PMS activation were 97% and 98%, with TOC (Total organic carbon) removal of 61.8% and 53.4% respectively. The reactive species including sulfate radical [Formula: see text], hydroxyl radical (·OH) and singlet oxygen (¹O₂) were produced during PMS activation. After O₂-aeration treatment of both AeS@Fe and AnSx@Fe, the relative content of Fe (II) was decreased and group C = O was disappeared, which resulted in reduction of [Formula: see text], ·OH and ¹O₂. The generation of [Formula: see text] and ·OH was dominated by the Fe (II) activation and the ¹O₂ generation was originated from graphite type N and C = O. Direct carbonization of aerobic and anaerobic sludge is a feasible method to produce biochar for PMS activation.

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.

Phosphate Recovery from Aqueous Solutions via Vivianite Crystallization: Interference of FeII Oxidation at Different DO Concentrations and pHs

Vivianite (Fe₃(PO₄)₂·8H₂O) crystallization has attracted increasing attention as a promising approach for removing and recovering P from wastewaters. However, Fe^II is susceptible to oxygen with its oxidation inevitably influencing the crystallization of vivianite. In this study, the profile of vivianite crystallization in the presence of dissolved oxygen (DO) was investigated at pHs 5-7 in a continuous stirred-tank reactor. It is found that the influence of DO on vivianite crystallization was highly pH-related. At pH 5, the low rate of Fe^II oxidation at all of the investigated DO of 0-5 mg/L and the low degree of vivianite supersaturation resulted in slow crystallization with the product being highly crystalline vivianite, but the P removal efficiency was only 30-40%. The removal of P from the solution was substantially more effective (to >90%) in the DO-removed reactors at pH 6 and 7, whereas the efficiencies of P removal and especially recovery decreased by 10-20% when Fe^II oxidation became more severe at DO concentrations >2.5 mg/L (except at pH 6 with 2.5 mg/L DO). The elevated degree of vivianite supersaturation and enhanced rate and extent of Fe^II oxidation at the higher pHs led to decreases in the size and homogeneity of the products. At the same pH, amorphous ferric oxyhydroxide (AFO)─the product of Fe^II oxidation and Fe^III hydrolysis─interferes with vivianite crystallization with the induction of aggregation of crystal fines by AFO, leading to increases in the size of the obtained solids.

Vivianite recovery from high concentration phosphorus wastewater with mine drainage as iron sources

High concentration phosphorus wastewater has attracted much attention due to the safety of water ecology and the potential crisis of phosphorus resource, which is caused by large amounts of phosphorus discharging into natural water bodies. Vivianite (Fe₃(PO₄)₂·8H₂O) crystallization has been considered as an effective technology for phosphorus recovery. In this study, we develop a potentially low-cost, sustainable approach to recover phosphorus from high concentration phosphorus wastewater using mine drainage as iron source. Inoculated with both sewage and Geobacter, mine drainage was suitable for vivianite recovery from high concentration phosphorus wastewater with PO₄^3- concentration between 6 and 18 mM. When the PO₄^3- concentration increased gradually, both phosphorus removal efficiency (R_P) and vivianite recovery efficiency (R_V) decreased significantly. The highest R_V of 48 % was obtained with 9 mM PO₄^3- in Geobacter batches (CJ2 batches), which was 15 % higher than that in the paralleled sewage batches (33 % in HJ2). Simultaneously, vivianite accounted for 91 % of the solid phosphate compounds in CJ2 batches due to the enhancement of Geobacter.

Significantly enhanced P release from vivianite as a fertilizer in rhizospheric soil: Effects of citrate

The use of vivianite (Fe₃(PO₄)₂∙8H₂O) as a slow-release P fertilizer in agriculture could be a promising way for the utilization of the recovered vivianite products from sewage treatment systems but the efficiency of vivianite-P release in the rhizospheric soil was yet unclear. In this work the dissolution of vivianite was investigated under anoxic and aerobic conditions with the focus on the effects of citrate as a common organic matter in the rhizosphere by tracking the kinetics of P release and the variations of aqueous and solid phases. The results show that citrate effectively induced the dissolution of vivianite particles at pH 6 with simultaneous release of Fe and PO₄-P. The enhancement of vivianite dissolution was positively correlated to the concentrations of citrate with complete dissolution observed when citrate was above 6 mM. Compared with anoxic conditions, aerobic conditions further enhanced the dissolution of vivianite to some extent, which could be partially attributed to the oxidation and removal of aqueous Fe^II in the solution that drove the equilibrium towards dissolution. In the presence of 2 mM citrate, the decrease in pH from 6.0 to 4.0 enhanced the vivianite-P release by 56.1%, indicating the pH dependence of the citrate-induced vivianite dissolution. This study has shown that the efficiency of P release from vivianite products as a fertilizer varies largely under different physico-chemical conditions in the rhizospheric microenvironment, which is critical for determining the dosage of vivianite for a specific soil.

Environmental management and potential valorization of wastes generated in passive treatments of fertilizer industry effluents

A phosphogypsum stack located in SW Spain releases highly acidic and contaminated leachates to the surrounding estuarine environment. Column experiments, based on a mixture of an alkaline reagent (i.e., MgO or Ca(OH)₂) dispersed in an inert matrix (dispersed alkaline substrate (DAS) technology), have shown high effectiveness for the treatment of phosphogypsum leachates. MgO-DAS and Ca(OH)₂-DAS treatment systems achieved near total removal of PO₄, F, Fe, Zn, Al, Cr, Cd, U, and As, with initial reactive mass:volume of leachate treated ratios of 3.98 g/L and 6.35 g/L, respectively. The precipitation of phosphate (i.e., brushite, cattiite, fluorapatite, struvite and Mn₃Zn(PO₄)₂·2H₂O) and sulfate (i.e., despujolsite and gypsum) minerals could control the solubility of contaminants during the treatments. Therefore, the hazardousness of these wastes must be accurately assessed in order to be properly managed, avoiding potential environmental impacts. For this purpose, two standardized leaching tests (EN-12457-2 from the European Union and TCLP from the United States) were performed. According to European Union (EN-12457-2) regulation, some wastes recovered from DAS treatments should be classified as hazardous wastes because of the high concentrations of SO₄ or Sb that are leached. However, according to United States (US EPA-TCLP) legislation, all DAS wastes are designated as non-hazardous wastes. Moreover, the solids generated in the DAS systems could constitute a promising secondary source of calcite and/or P. This research could contribute to worldwide suitable waste management for the fertilizer industry.

Quantitative determination of vivianite in sewage sludge by a phosphate extraction protocol validated by PXRD, SEM-EDS, and 31P NMR spectroscopy towards efficient vivianite recovery

Vivianite (Fe₃(PO₄)₂⋅8H₂O) is a potential phosphorus (P) recovery product from wastewater treatment plants (WWTPs). However, routine methods for quantification of vivianite bound P (vivianite-P) are needed to establish the link between vivianite formation and operating conditions, as current approaches require specialized instrumentation (Mössbauer or synchrotron). This study modified a conventional sequential P extraction protocol by insertion of an extraction step (0.2% 2,2'-bipyridine + 0.1 M KCl) targeting vivianite-P (Gu et al., Water Research, 2016, 103, 352-361). This protocol was tested on digested and dewatered sludge from two WWTPs, in which vivianite (molar Fe:P ratios of 1.0-1.6) was unambiguously identified by optical microscopy, powder X-ray diffraction, and scanning electron microscopy with energy dispersive X-ray spectroscopy. The results showed that vivianite-P was separated from iron(III)-bound P (Fe(III)-P) in the sludge. Vivianite-P constituted about half of the total P (TP) in the sludge from a Fe dosing chemical P removal (CPR) WWTP, but only 16-26% of TP in the sludge from a WWTP using a combination of Fe dosing CPR and enhanced biological P removal (EBPR). The modified protocol revealed that Fe-bound P (Fe-P, i.e., vivianite-P + Fe(III)-P) was the dominant P fraction, in agreement with quantitative ³¹P nuclear magnetic resonance (NMR) experiments. Moreover, it was shown that the conventional P extraction protocol underestimated the Fe-P content by 6-35%. The established protocol represents a reliable in-house analytical method that can distinguish and quantify vivianite-P and Fe(III)-P in sludge, i.e. facilitate optimized vivianite production at WWTPs.

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.

Roles of Phosphate Solubilizing Microorganisms from Managing Soil Phosphorus Deficiency to Mediating Biogeochemical P Cycle

Phosphorus (P) is a vital element in biological molecules, and one of the main limiting elements for biomass production as plant-available P represents only a small fraction of total soil P. Increasing global food demand and modern agricultural consumption of P fertilizers could lead to excessive inputs of inorganic P in intensively managed croplands, consequently rising P losses and ongoing eutrophication of surface waters. Despite phosphate solubilizing microorganisms (PSMs) are widely accepted as eco-friendly P fertilizers for increasing agricultural productivity, a comprehensive and deeper understanding of the role of PSMs in P geochemical processes for managing P deficiency has received inadequate attention. In this review, we summarize the basic P forms and their geochemical and biological cycles in soil systems, how PSMs mediate soil P biogeochemical cycles, and the metabolic and enzymatic mechanisms behind these processes. We also highlight the important roles of PSMs in the biogeochemical P cycle and provide perspectives on several environmental issues to prioritize in future PSM applications.