In Situ Nanoscale Imaging of Struvite Formation during the Dissolution of Natural Brucite: Implications for Phosphorus Recovery from Wastewaters

Optimizing phosphorus precipitation from acidic sewage sludge ash leachate: Use of Mg-rich mining by-products for enhanced nutrient recovery

Phosphorus recovery from Sewage Sludge Ashes (SSA) by wet chemical extraction followed by selective precipitation has gained great attention in recent years, attempting to reduce the anthropic pressure on natural reserves. This study investigates the selective precipitation process at lab- and small pilot-scales by means of two conventional and one innovative precipitating agents, the latter derived from a low-grade magnesium oxide mining by-product (LG-MgO named PC8), assessing the role of the most relevant operating parameters. Lab-scale experiments were performed on leachates obtained from bottom and fly ashes, in which several operating conditions were tested, differing in the type of precipitating agent, target pH and nutrient molar ratio. Based on experimental results, small pilot-scale experiments were conducted with Ca(OH)2 and PC8 at pH 7. Effective phosphorus precipitation was obtained at lab-scale at pH equal to 4 for high Al/P molar ratio, while SSA leachate with low Al/P molar ratio promoted improved phosphorus precipitation (>90%) only at pH higher than 8 with PC8. Small pilot-scale findings confirmed the effectiveness of PC8 in increasing simultaneously the pH and the nutrient content of the solid precipitate. The comprehensive assessment of the samples denoted compliance with the European Regulation (EU 2019/1009), which allows the formulation of different fertilizers with agronomic relevance. This is the first time that experiments from small pilot-scale tests in the field of phosphorus recovery from SSA were investigated using an innovative precipitant providing key information for the process scale-up.

Phosphorus recovery from sewage sludge via Mg-air battery system

A pressing issue in contemporary society is the resource scarcity of phosphorus. Operating on the principle of electrochemical reactions between Mg as the anode and oxygen from air as the cathode, Mg-air batteries (MAB) have been employed to provide new prospects for phosphorus recovery in struvite form. Different phosphorus concentrations and reaction time impact struvite generation in MAB systems; however, the exact mechanism has rarely been investigated. We investigated how varying the initial phosphorus concentration and the reaction time affects phosphorus recovery, electricity generation, and the efficiency of struvite production in MAB. Additionally, we examine the impact of solid carbon sources on phosphorus transformation in sludge. The findings revealed that the incorporation of solid carbon sources facilitated the release of phosphate by changing phosphorus speciation. The electrolyte derived from the conditioned sludge filtrate exhibited a remarkable phosphorus removal efficiency of 91.7 % within 1 h, yielding the highest struvite purity of ∼70 %, whereas that using raw sludge filtrate or extending the reaction time was found to be less effective, even reducing struvite formation. Furthermore, different electrolytes influence the system's ability to passivate anode, and electrolytes with higher phosphorus concentrations have better electricity production performance. The results by Visual MINTEQ model confirmed that longer reaction times and lower initial phosphorus concentrations can negatively affect struvite formation by introducing Mg3(PO4)2 and Mg(OH)2. The integration of agricultural waste as carbon sources with MAB for phosphorus recovery represents a potential methodology for struvite recuperation from sewage sludge, thereby heralding a sustainable strategy for resource recovery.

Wastewater Treatment for Carbon Dioxide Removal

Wastewater treatment is notorious for its hefty carbon footprint, accounting for 1-2% of global greenhouse gas (GHG) emissions. Nonetheless, the treatment process itself could also present an innovative carbon dioxide removal (CDR) approach. Here, the calcium (Ca)-rich effluent of a phosphorus (P) recovery system from municipal wastewater (P recovered as calcium phosphate) was used for CDR. The effluent was bubbled with concentrated CO2, leading to its mineralization, i.e., CO2 stored as stable carbonate minerals. The chemical and microstructural properties of the newly formed minerals were ascertained by using state-of-the-art analytical techniques. FTIR identified CO3 bonds and carbonate stretching, XRF and SEM-EDX measured a high Ca concentration, and SEM imaging showed that Ca is well distributed, suggesting homogeneous formation. Furthermore, FIB-SEM revealed rhombohedral and needle-like structures and TEM revealed rod-like structures, indicating that calcium carbonate (CaCO3) was formed, while XRD suggested that this material mainly comprises aragonite and calcite. Results imply that high-quality CaCO3 was synthesized, which could be stored or valorized, while if atmospheric air is used for bubbling, a partial direct air capture (DAC) system could be achieved. The quality of the bubbled effluent was also improved, thus creating water reclamation and circular economy opportunities. Results are indicative of other alkaline Ca-rich wastewaters such as effluents or leachates from legacy iron and steel wastes (steel slags) that can possibly be used for CDR. Overall, it was identified that wastewater can be used for carbon mineralization and can greatly reduce the carbon footprint of the treatment process, thus establishing sustainable paradigms for the introduction of CDR in this sector.

A review of struvite crystallization for nutrient source recovery from wastewater

Nutrient recovery from wastewater not only reduces the nutrient load on water resources but also alleviates the environmental problems in aquatic ecosystems, which is a solution to achieve a sustainable society. Besides, struvite crystallization technology is considered a potential nutrient recovery technology because the precipitate obtained can be reused as a slow-release fertilizer. This review presents the basic properties of struvite and the theory of the basic crystallization process. In addition, the possible influencing variables of the struvite crystallization process on the recovery efficiency and product purity are also examined in detail. Then, the advanced auxiliary technologies for facilitating the struvite crystallization process are systematically discussed. Moreover, the economic and environmental benefits of the struvite crystallization process for nutrient recovery are introduced. Finally, the shortcomings and inadequacies of struvite crystallization technology are presented, and future research prospects are provided. This work serves as the foundation for the future use of struvite crystallization technology to recover nutrients in response to the increasingly serious environmental problems and resource depletion.

Enhanced strategies for phosphate recovery from urine by magnesium galvanic process

Magnesium galvanic process (MGP) can be applied to recover phosphate from source-separated urine. However, information on how the urine matrix affects MGP performance is limited. Therefore, this study investigated the mechanism of phosphate recovery by MGP in synthetic and real urine matrixes. Our results showed that the major components in urine (i.e., NH4+, Cl-, and HCO3-) all exhibited acceleration effects on corrosion of Mg plate. However, the underlying action mechanism of each component was distinct. Ammonium facilitated the conversion from MgO to Mg(OH)2, chloride complexed with Mg2+ ions, and bicarbonate led to complexation as well as formation of MgCO3. Furthermore, our results revealed an interesting aspect where although bicarbonate alone accelerated the corrosion of Mg plate, its coexistence with other ions inhibited overall performance due to the blocking effect of formed MgCO3 on chloride penetration and reduction in free magnesium ion concentration. After elucidating the interaction of NH4+, Cl-, and HCO3- on the passive layer of the Mg plate, we proposed to pretreat urine with HCl, which resulted in a significant enhancement in current production and phosphate recovery. This improved MGP was further tested in a continuous flow reactor, which recovered over 95% of phosphate in real urine for more than 1 h. The phosphate precipitates were confirmed as high purity struvite. Generally, the improved MGP, which simultaneously produced Mg2+, dihydrogen, and electricity with no energy input, is a promising sustainable and green alternative for phosphate recovery from source-separated urine.

Evidence for liquid-liquid phase separation during the early stages of Mg-struvite formation

The precipitation of struvite, a magnesium ammonium phosphate hexahydrate (MgNH4PO4 · 6H2O) mineral, from wastewater is a promising method for recovering phosphorous. While this process is commonly used in engineered environments, our understanding of the underlying mechanisms responsible for the formation of struvite crystals remains limited. Specifically, indirect evidence suggests the involvement of an amorphous precursor and the occurrence of multi-step processes in struvite formation, which would indicate non-classical paths of nucleation and crystallization. In this study, we use synchrotron-based in situ x-ray scattering complemented by cryogenic transmission electron microscopy to obtain new insights from the earliest stages of struvite formation. The holistic scattering data captured the structure of an entire assembly in a time-resolved manner. The structural features comprise the aqueous medium, the growing struvite crystals, and any potential heterogeneities or complex entities. By analysing the scattering data, we found that the onset of crystallization causes a perturbation in the structure of the surrounding aqueous medium. This perturbation is characterized by the occurrence and evolution of Ornstein-Zernike fluctuations on a scale of about 1 nm, suggesting a non-classical nature of the system. We interpret this phenomenon as a liquid-liquid phase separation, which gives rise to the formation of the amorphous precursor phase preceding actual crystal growth of struvite. Our microscopy results confirm that the formation of Mg-struvite includes a short-lived amorphous phase, lasting >10 s.

Interaction of divalent metals with struvite: sorption, reversibility, and implications for mineral recovery from wastes

Phosphorus (P) recovered from wastewater as struvite (MgNH₄PO₄·6H₂O) can meet high P demands in the agricultural sector by reuse as a P fertiliser. Heavy metals are prevalent in wastewaters and are common fertiliser contaminants, therefore struvite as a sorbent for metals requires evaluation. Struvite sorption experiments were conducted in model solutions with cadmium (Cd), cobalt (Co), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) at 1-5 μM concentrations from pH 7-10. The struvite metal loading increased with dissolved metal concentration and pH, ranging from 2 to 493 mg kg^-1. Highest loadings were observed for 5 μM Pb, which exceeded the 120 mg kg^-1 European Union (EU) struvite fertiliser limit at all pH values. At 5 μM concentrations, Ni and Cd loadings exceeded EU limits of 100 mg kg^-1 at pH 10, and 60 mg kg^-1 at pH 8-10, respectively. In desorption experiments, 10-85% metal was released after resuspension in metal-free solutions, with a positive correlation between initial loading and amount desorbed. Distortions of the struvite phosphate band, by Fourier transformation infrared (FTIR) spectroscopy, indicated lowered symmetry of phosphate vibrations with metal sorption. X-ray absorption fine structure spectroscopy (XAFS) analysis of pH 9 solids indicated tetrahedral coordination for Cu and Zn, octahedral coordination for Co and Ni, and Pb in 9-fold coordination. Precipitation of Pb-phosphate minerals was a primary mechanism for Pb sorption. The results provide insight into metal contaminant sorption with struvite in wastewaters, and the potential for metal desorption after recovery.

Precipitation of struvite using MgSO4 solution prepared from sidestream dolomite or fly ash

Struvite (NH₄MgPO₄∗6H₂O) is a slow-release fertilizer produced from phosphorus and nitrogen-containing wastewater in the presence of Mg salts. Commercial Mg salts are the single most significant cost of struvite precipitation. In this study, H₂SO₄ formed as an industrial sidestream was used to prepare MgSO₄ solution from waste dolomite (DOL) and fly ash (FA). MgSO₄ solution was then used to precipitate struvite from a synthetic (NH₄)₂HPO₄ solution and from actual industrial process waters. The best results were obtained with real process waters where over 99% of phosphate and about 80% ammonium removals were achieved with both MgSO4 solutions after 30 min of reaction time. A higher molar ratio between Mg and P improved the phosphate removal efficiency, especially with DOL-based MgSO4 solutions; however, it had no practical effect on ammonium removal. The struvite content of precipitates was 75.49% with an FA-based chemical and 60.93% with a DOL-based chemical; other valuable nutrients (Ca, K, S, Fe, Mn, and Cl) were captured in the precipitates. The results indicate that both sidestream-based reagents perform well in struvite precipitation and that the formed precipitates could be used as fertilizers.

In Situ Kinetic Observations on Crystal Nucleation and Growth

Nucleation and growth are critical steps in crystallization, which plays an important role in determining crystal structure, size, morphology, and purity. Therefore, understanding the mechanisms of nucleation and growth is crucial to realize the controllable fabrication of crystalline products with desired and reproducible properties. Based on classical models, the initial crystal nucleus is formed by the spontaneous aggregation of ions, atoms, or molecules, and crystal growth is dependent on the monomer's diffusion and the surface reaction. Recently, numerous in situ investigations on crystallization dynamics have uncovered the existence of nonclassical mechanisms. This review provides a summary and highlights the in situ studies of crystal nucleation and growth, with a particular emphasis on the state-of-the-art research progress since the year 2016, and includes technological advances, atomic-scale observations, substrate- and temperature-dependent nucleation and growth, and the progress achieved in the various materials: metals, alloys, metallic compounds, colloids, and proteins. Finally, the forthcoming opportunities and challenges in this fascinating field are discussed.

Phosphorus recovery as K-struvite from a waste stream: A review of influencing factors, advantages, disadvantages and challenges

Currently, the depletion of natural resources and contamination of the surrounding environment demand a paradigm shift to resource recycling and reuse. In this regard, phosphorus (P) is a model nutrient that possesses the negative traits of depletion (will be exhausted in the next 100 years) and environmental degradation (causes eutrophication and climate change), and this has prompted the scientific community to search for options to solve P-related problems. To date, P recovery in the form of struvite from wastewater is one viable solution suggested by many scholars. Struvite can be recovered either in the form of NH₄-struvite (MgNH₄PO₄•6H₂O) or K-struvite (MgKPO₄•6H₂O). From struvite, K (MgKPO₄•6H₂O) and N (MgNH₄PO₄•6H₂O) are important nutrients for plant growth, but N is more abundant in the environment than K (the soil's most limited nutrient), which requires a systematic approach during P recovery. Although K-struvite recovery is a promising approach, information related to its crystallization is deficient. Here, we present the general concept of P recovery as struvite and details about K-struvite, such as the source of nutrients, factors (pH, molar ratio, supersaturation, temperature, and seeding), advantages (environmental, economic, and social), disadvantages (heavy metals, pathogenic organisms, and antibiotic resistance genes), and challenges (scale-up and acceptance). Overall, this study provides insights into state-of-the-art K-struvite recovery from wastewater as a potential slow-release fertilizer that can be used as a macronutrient (P-K-Mg) source for plants as commercial grade-fertilizers.

Spontaneous nutrient recovery and disinfection of aquaculture wastewater via Mg-coconut shell carbon composites

Aquaculture wastewater contained large amounts of pathogenic microorganisms, nitrogen (N) and phosphorus (P). In this study, the nutrient recoveries and wastewater disinfection were simultaneously achieved using Mg-coconut shell carbon (Mg-CSC). The composites were prepared by a ball milling method. The hydrogen peroxide (H₂O₂) was in-situ generated by the dissolved oxygen reduction driven by Mg corrosion on the CSC surface, which inactivated the microorganisms. Besides that, Mg corrosion provided sufficient Mg ions and appropriate pH conditions for struvite formation. The results show that 5.4-log E.coli removal was achieved under different conditions. Improving the Mg/CSC ratio and composite dosage could shorten the time required for disinfection. In addition to H₂O₂, singlet oxygen played a critical role. Reactive oxygen species destroyed the cellular structure and killed the bacteria. The recoveries of NH₄⁺-Nand P under certain conditions were about 60% and 91%, respectively. An increased composite dosage could improve the recovery ratio of P. Excessive dosages were not beneficial for removing NH₄⁺-N. The characterization result revealed that struvite crystals were the main precipitates on the CSC surface. The Mg-CSC composites also revealed satisfied nutrient recovery and disinfection performances in the real aquaculture wastewater treatment process.

Engineering Calcium-bearing Mineral/Hydrogel Composites for Effective Phosphate Recovery

Effectively recovering phosphate from wastewater streams and reutilizing it as a nutrient will critically support sustainability. Here, to capture aqueous phosphate, we developed novel mineral-hydrogel composites composed of calcium alginate, calcium phosphate (CaP), and calcium silicate (CSH) (CaP + CSH/Ca-Alg). The CaP + CSH/Ca-Alg composites were synthesized by dripping a sodium alginate (Na-Alg) solution with ionic precursors into a calcium chloride bath. To change the mineral seed's properties, we varied the calcium bath concentrations and the ionic precursor (sodium dibasic phosphate (NaH₂PO₄) and/or sodium silicate (Na₂SiO₃)) amounts and their ratios. The added CSH in the mineral-hydrogel composites resulted in the release of calcium and silicate ions in phosphate-rich solutions, increasing the saturation ratio with respect to calcium phosphate within the mineral-hydrogel composites. The CSH addition to the mineral-hydrogel composites doubled the phosphate removal rate while requiring lesser initial amounts of Ca and P materials for synthesis. By incorporating both CSH and CaP mineral seeds in composites, we achieved a final concentration of 0.25 mg-P/L from an initial 6.20 mg-P/L. Moreover, the mineral-hydrogel composites can remove phosphate even under CaP undersaturated conditions. This suggests their potential to be a widely applicable and environmentally-sustainable treatment and recovery method for nutrient-rich wastewater.

Carbonation, Cementation, and Stabilization of Ultramafic Mine Tailings

Tailings dam failures can cause devastation to the environment, loss of human life, and require expensive remediation. A promising approach for de-risking brucite-bearing ultramafic tailings is in situ cementation via carbon dioxide (CO₂) mineralization, which also sequesters this greenhouse gas within carbonate minerals. In cylindrical test experiments, brucite [Mg(OH)₂] carbonation was accelerated by coupling organic and inorganic carbon cycling. Waste organics generated CO₂ concentrations similar to that of flue gas (up to 19%). The abundance of brucite (2-10 wt %) had the greatest influence on tailings cementation as evidenced by the increase in total inorganic carbon (TIC; +0.17-0.84%). Brucite consumption ranged from 64-84% of its initial abundance and was mainly influenced by water availability. Higher moisture contents (e.g., 80% saturation) and finer grain sizes (e.g., clay-silt) that allowed for a better distribution of water resulted in greater brucite carbonation. Furthermore, pore clogging and surface passivation by Mg-carbonates may have slowed brucite carbonation over the 10 weeks. Unconfined compressive strengths ranged from 0.4-6.9 MPa and would be sufficient in most scenarios to adequately stabilize tailings. Our study demonstrates the potential for stabilizing brucite-bearing mine tailings through in situ cementation while sequestering CO₂.

Electrochemical recovery of phosphorus from wastewater using tubular stainless-steel cathode for a scalable long-term operation

Phosphorus (P) is an irreplaceable element, playing a vital role in living organisms, yet has limited earth reserves. The possibility of P recovery from wastewaters by electrochemically-induced calcium phosphate precipitation (ECaPP) was demonstrated previously. The current study presents a novel scalable prototype consisting of a column-shaped electrochemical reactor, a tubular stainless-steel cathode, and a Pt coated Ti anode. The adhesion of solids to the cathode, important for product recovery, was shown not to be negatively impacted by electrodes' vertical placement. The influence of current (density), hydraulic retention time (HRT), and initial phosphate concentration in this prototype were examined under continuous flow operation. The system accomplished the highest P removal rate (1267 mg/day) at 1.5 d HRT and 800 mA in treating undiluted cheese wastewater with 48.5 kWh/kg P. Moreover, the prototype showed high stability and efficiency (> 50%) over 173 days of continuous operation without performing maintenance. After turning off the current (0 mA), the system realized a surprising P removal jump up to 97.3%, revealing the delayed diffusion of hydroxide ions by the deposition layer. The calculation of CAPEX and OPEX of ECaPP in treating 100 m³ cheese wastewater per week indicates that the ECaPP plant can realize net-positive from the 12^th year. The recovered solids have relatively high P content (> 9wt%) and insignificant contamination of heavy metals. Overall, the proven suitability of the scalable prototype can pave the way towards the actual adoption of the ECaPP process.

Crystallization kinetics and growth of struvite crystals by seawater versus magnesium chloride as magnesium source: towards enhancing sustainability and economics of struvite crystallization

The recycling of nutrients from wastewater and their recovery in the form of valuable products is an effective strategy to accelerate the circular economy concept. Phosphorus recovery from wastewater by struvite crystallization (MgNH₄PO₄·6H₂O) is one of the most applied techniques to compensate for the increasing demand and to slow down the depletion rate of phosphate rocks. Using low-cost magnesium sources, such as seawater, improves the financial sustainability of struvite production. In this study, the potential of seawater for struvite crystallization versus the commonly used magnesium source, MgCl₂, was tested by crystal growth and kinetic experiments. The impact of ammonium concentration, magnesium concentration and pH on the growth kinetics of struvite in synthetic and real reject water were studied. The results showed that simultaneous precipitation of calcium phosphate was insignificant when using seawater, while presence of struvite seeds diminished it further. Among the supersaturation regulators, pH had the most significant effect on the struvite growth with both MgCl₂ and seawater, while high N:P molar ratios further improved the struvite crystal growth by seawater. The N:P molar ratios higher than 6 and Mg:P molar ratios higher than 0.2 are recommended to improve the crystal growth kinetics. It was concluded that seawater is a promising alternative magnesium source and the control of supersaturation regulators (i.e., Mg:P, N:P and pH) is an effective strategy to control the reaction kinetics and product properties.

Performance evaluation of microbial fuel cell for landfill leachate treatment: Research updates and synergistic effects of hybrid systems

Over half of century, sanitary landfill was and is still the most economical treatment strategy for solid waste disposal, but the environmental risks associated with the leachate have brought attention of scientists for its proper treatment to avoid surface and ground water deterioration. Most of the treatment technologies are energy-negative and cost intensive processes, which are unable to meet current environmental regulations. There are continuous demands of alternatives concomitant with positive energy and high effluent quality. Microbial fuel cells (MFCs) were launched in the last two decades as a potential treatment technology with bioelectricity generation accompanied with simultaneous carbon and nutrient removal. This study reviews capability and mechanisms of carbon, nitrogen and phosphorous removal from landfill leachate through MFC technology, as well as summarizes and discusses the recent advances of standalone and hybrid MFCs performances in landfill leachate (LFL) treatment. Recent improvements and synergetic effect of hybrid MFC technology upon the increasing of power densities, organic and nutrient removal, and future challenges were discussed in details.

Dissolution and Precipitation Dynamics at Environmental Mineral Interfaces Imaged by In Situ Atomic Force Microscopy

Chemical reactions at the mineral-solution interface control important interfacial processes, such as geochemical element cycling, nutrient recovery from eutrophicated waters, sequestration of toxic contaminants, and geological carbon storage by mineral carbonation. By time-resolved in situ imaging of nanoscale mineral interfacial reactions, it is possible to clarify the mechanisms governing mineral-fluid reactions.In this Account, we present a concise summary of this topic that addresses a current challenge at the frontier of understanding mineral interfaces and their importance to a wide range of mineral re-equilibration processes in the presence of a fluid aqueous phase. We have used real-time nanoscale imaging of liquid-cell atomic force microscopy (AFM) to observe the in situ coupling of the dissolution-precipitation process, whereby the dissolution of a parent mineral phase is coupled at mineral interfaces with the precipitation of another product phase, chemically different from the parent. These nanoscale observations allow for the identification of dissolution and growth rates through systematically investigating various minerals, including calcite (CaCO₃), siderite (FeCO₃), cerussite (PbCO₃), magnesite (MgCO₃), dolomite (CaMg(CO₃)₂), brushite (CaHPO₄·2H₂O), brucite (Mg(OH)₂), portlandite (Ca(OH)₂), and goethite (α-FeOOH), in various reacting aqueous fluids containing solution species, such as arsenic, phosphate, organo- or pyrophosphate, CO₂, selenium, lead, cadmium, iron, chromium, and antimony. We detected the in situ replacement of these parent mineral phases by product phases, identified through a variety of analytical methods such as Raman spectroscopy, high-resolution transmission electron microscopy, and various X-ray techniques, as well as modeling by geochemical simulation using PHREEQC. As a consequence of the coupled processes, sequestration of toxic elements and hazardous species and inorganic and organic carbon, and limiting or promoting recovery of nutrients can be achieved at nano- and macroscopic scales.We also used in situ AFM to quantitatively measure the retreat rates of molecular steps and directly observe the morphology changes of dissolution etch pits on calcium phosphates in organic acid solutions present in most rhizosphere environments. By molecular modeling using density functional theory (DFT), we explain the origin of dissolution etch pit evolution through specific stereochemistry and molecular recognition and provide an energetic basis by calculating the binding energies of chemical functionalities on organic acids to direction-specific steps on mineral surfaces. In addition, we further quantified precipitation kinetics of calcium phosphates (Ca-P's) on typical mineral surfaces at the nanoscale in environmentally relevant solutions with various organic molecules, by measurements obtained from sequential images obtained by liquid-cell AFM. In situ dynamic force spectroscopy (DFS) was used to determine binding energies of single-molecules with different chemical functionalities found in natural organic matter at mineral-fluid interfaces. Quantifying molecular organo-mineral bonding or binding energies mechanistically explains phosphate precipitation and transformation. From DFS measurements, molecular-scale interactions of mineral-natural organic matter (DNA, proteins, and polysaccharides) associations were determined. With this powerful tool, single-molecule determinations of polysaccharide-amorphous iron oxide or hematite interactions provided the mechanistic origin of the phase- or facet-dependent adsorption. These systematic investigations and findings significantly contribute to a more quantitative prediction of the fate of nutrients and contaminants, chemical element cycling, and potential geological carbon capture and nuclear waste storage in aqueous environments.

Struvite crystallization by using raw seawater: Improving economics and environmental footprint while maintaining phosphorus recovery and product quality

Seawater, as an alternative magnesium source, has the potential to improve the overall economics and environmental footprint of struvite production compared to the use of pure magnesium salts. However, the dilution effect and the presence of other ions in seawater can reduce the phosphorus recovery potential and the simultaneous precipitation of other compounds may reduce the quality of the produced struvite. This work presents a comparative study of seawater and MgCl₂ by performing a series of thermodynamic equilibrium modeling and crystallization experiments. The results revealed that acceptable phosphorus recovery (80-90%) is achievable by using seawater as the magnesium source for struvite precipitation. Further, the simultaneous precipitation of calcium phosphates was successfully controlled and minimized by optimum selection of reaction pH and seawater volume (i.e. Mg:P and Mg:Ca molar ratios). The increase of temperature from 20 °C to 30 °C reduced the phosphorus recovery by 15-20% while it increased the particle size by 30-35%. The presence of suspended solids in reject water did not have significant effects on phosphorus recovery but it made the struvite separation difficult as the obtained struvite was mixed with suspended solids. The experimental results and economic evaluation showed that the use of seawater can reduce the chemical costs (30-50%) and the CO₂-footprint (8-40%) of struvite production. It was concluded that seawater is a potential alternative to pure magnesium sources in struvite production, while studies in larger scale and continuous mode are needed for further verification before full-scale applications.

Time-Resolved Dynamics of Struvite Crystallization: Insights from the Macroscopic to Molecular Scale

The crystallization of magnesium ammonium phosphate hexahydrate (struvite) often occurs under conditions of fluid flow, yet the dynamics of struvite growth under these relevant environments has not been previously reported. In this study, we use a microfluidic device to evaluate the anisotropic growth of struvite crystals at variable flow rates and solution supersaturation. We show that bulk crystallization under quiescent conditions yields irreproducible data owing to the propensity of struvite to adopt defects in its crystal lattice, as well as fluctuations in pH that markedly impact crystal growth rates. Studies in microfluidic channels allow for time-resolved analysis of seeded growth along all three principle crystallographic directions and under highly controlled environments. After having first identified flow rates that differentiate diffusion and reaction limited growth regimes, we operated solely in the latter regime to extract the kinetic rates of struvite growth along the [100], [010], and [001] directions. In situ atomic force microscopy was used to obtain molecular level details of surface growth mechanisms. Our findings reveal a classical pathway of crystallization by monomer addition with the expected transition from growth by screw dislocations at low supersaturation to that of two-dimensional layer generation and spreading at high supersaturation. Collectively, these studies present a platform for assessing struvite crystallization under flow conditions and demonstrate how this approach is superior to measurements under quiescent conditions.

Electrochemically mediated calcium phosphate precipitation from phosphonates: Implications on phosphorus recovery from non-orthophosphate

Phosphonates are an important type of phosphorus-containing compounds and have possible eutrophication potential. Therefore, the removal of phosphonates from waste streams is as important as orthophosphate. Herein, we achieved simultaneously removal and recovery of phosphorus from nitrilotris (methylene phosphonic acid) (NTMP) using an electrochemical cell. It was found that the C-N and C-P bonds of NTMP were cleaved at the anode, leading to the formation of orthophosphate and formic acid. Meanwhile, the converted orthophosphate reacted with coexisting calcium ions and precipitated on the cathode as recoverable calcium phosphate solids, due to an electrochemically induced high pH region near the cathode. Electrochemical removal of NTMP (30 mg/L) was more efficient when dosed to effluent of a wastewater treatment plant (89% in 24 h) than dosed to synthetic solutions of 1.0 mM Ca and 50 mM Na₂SO₄ (43% in 168 h) while applying a current density of 28 A/m² and using a Pt anode and Ti cathode. The higher removal efficiency of NTMP in real waste water is due to the presence of chloride ions, which resulted in anodic formation of chlorine. This study establishes a one-step approach for simultaneously phosphorus removal and recovery of calcium phosphate from non-orthophosphates.

Enhanced electrochemical phosphate recovery from livestock wastewater by adjusting pH with plant ash

In the field of environmental wastewater treatment, it is a very meaningful topic to recover phosphate from swine wastewater in the form of struvite precipitation. The solution pH is one of the important influencing factors in the process of struvite precipitation. In this paper, an attempt was made to recover the phosphate from swine wastewater by adding plant ash. Experimental results have revealed that aeration can be replaced by optimal plant ash adding mode to increase the phosphate recovery efficiency. With the dosages of plant ash and magnesium metal were respectively 11.66 and 3.33 g/L the phosphate recovery efficiency reached 97.69% in 60 min. The efficiency was still above 95% after repeatedly using magnesium pellet for 3 times. The economic evaluation further revealed that the recovery cost of the proposed method was 0.62 $/kg PO₄-P.

Phosphorus recovery through struvite crystallisation: Recent developments in the understanding of operational factors

Phosphorus is an essential element for life and is predicted to deplete within the next 100 years. Struvite crystallization is a potential phosphorus recovery technique to mitigate this problem by producing a slow release fertilizer. However, complex wastewater composition and a large number of process variables result in process uncertainties, making the process difficult to predict and control. This paper reviews the research progress on struvite crystallization fundamentals to address this challenge. The influence of manipulated variables (e.g. seed material, magnesium dosage and pH) and sources of variation on phosphorus removal efficiency (e.g. organics and heavy metal concentration) and product purity were investigated. Recently developed models to describe, control and optimize those variables were also discussed. This review helps to identify potential challenges in different wastewater streams and provide valuable information for future phosphorus recovery unit design. It therefore paves the way for commercialization of struvite crystallization in the future.

Calcium Carbonate Packed Electrochemical Precipitation Column: New Concept of Phosphate Removal and Recovery

Phosphorus (P) is a vital micronutrient element for all life forms. Typically, P can be extracted from phosphate rock. Unfortunately, the phosphate rock is a nonrenewable resource with a limited reserve on the earth. High levels of P discharged to water bodies lead to eutrophication. Therefore, P needs to be removed and is preferably recovered as an additional P source. A possible way to achieve this goal is by electrochemically induced phosphate precipitation with coexisting calcium ions. Here, we report a new concept of phosphate removal and recovery, namely a CaCO₃ packed electrochemical precipitation column, which achieved improved removal efficiency, shortened hydraulic retention time, and substantially enhanced stability, compared with our previous electrochemical system. The concept is based on the introduction of CaCO₃ particles, which facilitates calcium phosphate precipitation by buffering the formed H⁺ at the anode, releases Ca²⁺, acts as seeds, and establishes a high pH environment in the bulk solution in addition to that in the vicinity of the cathode. It was found that the applied current, the CaCO₃ particle size, and the feed rate affect the removal of phosphate. Under optimized conditions (particle size, <0.5 mm; feed rate, 0.4 L/d; current, 5 mA), in a continuous flow system, the CaCO₃ packed electrochemical precipitation column achieved 90 ± 5% removal of phosphate in 40 days and >50% removal over 125 days with little maintenance. The specific energy consumptions of this system lie between 29 and 61 kWh/kg P. The experimental results demonstrate the promising potential of the CaCO₃ packed electrochemical precipitation column for P removal and recovery from P-containing streams.

Fractionating various nutrient ions for resource recovery from swine wastewater using simultaneous anionic and cationic selective-electrodialysis

Different from current nutrient recovery technologies of recovering one or two nutrient components (PO₄^3- or NH₄⁺) from wastewater, this study aimed to fractionate various nutrient anions and cations simultaneously, including PO₄^3-, SO₄^2-, NH₄⁺, K⁺, Mg²⁺ and Ca²⁺, into several streams. The recovered streams could be further paired together to produce high-value products. A novel electrodialysis process was developed by integrating monovalent selective anion and cation exchange membranes into an electrodialysis stack. Results revealed that nutrient recovery was achieved effectively by fractionating PO₄^3- and SO₄^2- into the anionic product stream, whereas bivalent cations (Mg²⁺ and Ca²⁺) were extracted in the cationic product stream and the monovalent cations (K⁺ and NH₄⁺) were concentrated in the brine stream. For the permeation capabilities of anions, SO₄^2- and Cl^- possessed the higher preference, whereas PO₄^3- permeated the membrane more difficult. As to the cations, the permeation sequence was: NH₄⁺≈K⁺ >Ca²⁺>Mg²⁺≈Na⁺. Enhancing voltage values not only promoted ion migration rates, but also led to the increase of energy consumption. Although elevating initial phosphate concentration in the anionic product streams from 60 mg/L to 470 mg/L did not influence phosphate fractionation significantly, the current efficiency decreased from 3.55% to 0.65% and a remarkable increased of energy consumption from 29.42 kWh/kg NaH₂PO₄ to 160.13 kWh/kg NaH₂PO₄ was observed. Further experiments were conducted for phosphorus recovery by pairing two recovered product streams, which revealed that phosphate precipitation could be achieved by using inherent Ca²⁺ and Mg²⁺ in the wastewater without dosing external cation sources.

Struvite Crystallisation and the Effect of Co2+ Ions

The controlled crystallisation of struvite (MgNH4PO4∙6H2O) is a viable means for the recovery and recycling of phosphorus (P) from municipal and industrial wastewaters. However, an efficient implementation of this recovery method in water treatment systems requires a fundamental understanding of struvite crystallisation mechanisms, including the behavior and effect of metal contaminants during struvite precipitation. Here, we studied the crystallisation pathways of struvite from aqueous solutions using a combination of ex situ and in situ time-resolved synthesis and characterization techniques, including synchrotron-based small- and wide-angle X-ray scattering (SAXS/WAXS) and cryogenic transmission electron microscopy (cryo-TEM). Struvite syntheses were performed both in the pure Mg-NH4-PO4 system as well as in the presence of cobalt (Co), which, among other metals, is typically present in waste streams targeted for P-recovery. Our results show that in the pure system and at Co concentrations < 0.5 mM, struvite crystals nucleate and grow directly from solution, much in accordance with the classical notion of crystal formation. In contrast, at Co concentrations ≥ 1 mM, crystallisation was preceded by the transient formation of an amorphous nanoparticulate phosphate phase. Depending on the aqueous Co/P ratio, this amorphous precursor was found to transform into either (i) Co-bearing struvite (at Co/P < 0.3) or (ii) cobalt phosphate octahydrate (at Co/P > 0.3). These amorphous-to-crystalline transformations were accompanied by a marked colour change from blue to pink, indicating a change in Co2+ coordination in the formed solid from tetrahedral to octahedral. Our findings have implications for the recovery of nutrients and metals during struvite crystallisation and contribute to the ongoing general discussion about the mechanisms of crystal formation.

Next-Generation Multifunctional Carbon-Metal Nanohybrids for Energy and Environmental Applications

Nanotechnology has unprecedentedly revolutionized human societies over the past decades and will continue to advance our broad societal goals in the coming decades. The research, development, and particularly the application of engineered nanomaterials have shifted the focus from "less efficient" single-component nanomaterials toward "superior-performance", next-generation multifunctional nanohybrids. Carbon nanomaterials (e.g., carbon nanotubes, graphene family nanomaterials, carbon dots, and graphitic carbon nitride) and metal/metal oxide nanoparticles (e.g., Ag, Au, CdS, Cu₂O, MoS₂, TiO₂, and ZnO) combinations are the most commonly pursued nanohybrids (carbon-metal nanohybrids; CMNHs), which exhibit appealing properties and promising multifunctionalities for addressing multiple complex challenges faced by humanity at the critical energy-water-environment (EWE) nexus. In this frontier review, we first highlight the altered and newly emerging properties (e.g., electronic and optical attributes, particle size, shape, morphology, crystallinity, dimensionality, carbon/metal ratio, and hybridization mode) of CMNHs that are distinct from those of their parent component materials. We then illustrate how these important newly emerging properties and functions of CMNHs direct their performances at the EWE nexus including energy harvesting (e.g., H₂O splitting and CO₂ conversion), water treatment (e.g., contaminant removal and membrane technology), and environmental sensing and in situ nanoremediation. This review concludes with identifications of critical knowledge gaps and future research directions for maximizing the benefits of next-generation multifunctional CMNHs at the EWE nexus and beyond.

Effect of ultrasound on the dissolution of magnesium hydroxide: pH-stat and nanoscale observation

It has been previously confirmed that the dissolution of magnesium hydroxide was a crucial procedure in its application process. Note that the ultrasound is an effective method for enhancing solid dissolution. In this study, the enhancement of ultrasound on the dissolution of magnesium hydroxide was investigated by the pH-stat method. The titrating results indicated that the promotion effect of ultrasound was only be observed at some certain cases, such as lower pH value. Meanwhile, the activation energy data obtained based on Shrinking Core Model for sonication case ranged from 6.56 to 49.13 kJ/mol, implying that magnesium hydroxide dissolution proceeds at sonication case cannot be explained well by this model. Nanoscale observation was conducted to identify the crystal surface variations during the dissolution process by using SEM and AFM. The analysis results indicated that the dissolution behaviors of magnesium hydroxide in the absence and presence of ultrasound were quite different. In the silent case, the dissolution followed the famous Stepwave model. As for the sonication case, the larger particle was broken into flake by the ultrasound firstly. Then, the heterogeneous global dissolution accompanied by the Stepwave model drove the dissolution process. The analysis results of nanoscale observation provided a reasonable explain for the kinetics research results on micro-level.

Electrochemical acidolysis of magnesite to induce struvite crystallization for recovering phosphorus from aqueous solution

A novel struvite crystallization method induced by electrochemical acidolysis of cheap magnesite was investigated to recover phosphorus from aqueous solution. Magnesite was confirmed to continuously dissolve in the anolyte whose pH stabilized at about 2. Driven by the electrical field force, over 90% of the released Mg²⁺ migrated to the cathode chamber via passing through the cation exchange membrane. The pH of the phosphate-containing aqueous solution in the cathode chamber was elevated to the appropriate pH fit for struvite crystallization. The products were identified as struvite crystals by scanning electron microscopy and X-ray diffraction. Increasing the magnesite dosage from 0.83 to 3.33 g L^-1 promoted the phosphorus recovery efficiency from 2.2% to 78.3% at 3 d, which was attributed to sufficient Mg²⁺ supply. Increasing the applied voltage from 3 to 6 V improved the recovery efficiency from 43.6% to 76.4% at 1 d, since the enhanced current density of the electrochemical system markedly accelerated both the magnesite acidolysis and the catholyte pH elevation. The initial catholyte pH between 3 and 5 was found to benefit the phosphorus recovery due to the final catholyte pH fit for the struvite crystallization.

Fractionating magnesium ion from seawater for struvite recovery using electrodialysis with monovalent selective membranes

As the consumption of global phosphorus reserves accelerates, recovering phosphorus as struvite (MgNH₄PO₄·6H₂O) from wastewater is an important option for phosphorus recycling. However, magnesium source is one of the major limiting factors for struvite recovery. In this work, different from previous studies where seawater was used directly as magnesium source in struvite precipitation, an electrodialysis stack equipped with monovalent selective cation-exchange membranes was designed to fractionate Mg²⁺ from seawater for struvite recovery. Results revealed that Mg²⁺ fractionation was achieved effectively. The comparison on applying the driving force for ionic transport showed that constant voltage was more preferable than constant current due to its higher Mg²⁺ separation efficiency, current efficiency and lower energy consumption. Increasing voltage from 7 V to 13 V would improve Mg²⁺ permeation ratio from 72.9% to 80.5% into the product stream but simultaneously increased the energy consumption from 5.40 (kWh/kg MgCl₂) to 11.69 (kWh/kg MgCl₂). In addition, the investigation on the influence of Ca²⁺ co-existence and further struvite recovery experiments revealed that the variation of Ca²⁺ concentrations in seawater did not influence Mg²⁺ fractionation significantly, nevertheless it might reduce struvite recovery efficiency through forming calcium phosphate.

Interaction of calcium, phosphorus and natural organic matter in electrochemical recovery of phosphate

To address the issues of eutrophication and the potential risk of phosphorus (P) shortage, it is essential to remove and recover P from P-containing streams to close this nutrient cycle. Electrochemical induced calcium phosphate (CaP) precipitation was shown to be an efficient method for P recovery. However, the influence of natural organic matter (NOM) is not known for this treatment. In this paper, the behavior of NOM and its effect on CaP precipitation was studied. In contrast to studies where NOM hindered CaP precipitation, results show that the interaction of NOM with CaP improves the removal of P, independent of the types of NOM. The P removal at the average increased from 43.8 ± 4.9% to 58.5 ± 1.2% in the presence of 1.0 mg L^-1 NOM. Based on the yellow color of the CaP product, NOM is co-precipitated. The bulk solution pH with and without buffers has totally different effects on the precipitation process. Without buffer, CaP precipitates on the cathode surface in a wide pH range (pH 4.0-10.0). However, the precipitation process is completely inhibited when the bulk solution is buffered at pH 4.0 and 6.0. This is probably due to neutralization of OH^- by the buffers. Regardless of the presence or absence of NOM and solution pH, the recovered products are mainly amorphous CaP unless the electrolysis time was increased to seven days with 4.0 A m^-2, in which crystalline CaP formed. These findings advance our understanding on the interaction of Ca, P and NOM species for the application of electrochemical method for P recovery from real wastewater.

Dynamics and Molecular Mechanism of Phosphate Binding to a Biomimetic Hexapeptide

Phosphorus (P) recovery from wastewater is essential for sustainable P management. A biomimetic hexapeptide (SGAGKT) has been demonstrated to bind inorganic P in P-rich environments, however the dynamics and molecular mechanisms of P-binding to the hexapeptide still remain largely unknown. We used dynamic force spectroscopy (DFS) to directly distinguish the P-unbound and P-bound SGAGKT adsorbed to a mica (001) surface by measuring the single-molecule binding free energy (Δ G_b). Using atomic force microscopy (AFM) to determine real-time step retreat velocities of triangular etch pits formed at the nanoscale on the dissolving (010) face of brushite (CaHPO₄·2H₂O) in the presence of SGAGKT, we observed that SGAGKT peptides promoted in situ dissolution through an enhanced P-binding driven by hydrogen bonds in a P-loop being capable of discriminating phosphate over arsenate, concomitantly forming a thermodynamically favored SGAGKT-HPO₄^2- complexation at pH 8.0 and relatively low ionic strength, consistent with the DFS and isothermal titration calorimetry (ITC) determinations. The findings reveal the thermodynamic and kinetic basis for binding of phosphate to SGAGKT and provide direct evidence for phosphate discrimination in phosphate/arsenate-rich environments.

Metal Sequestration through Coupled Dissolution–Precipitation at the Brucite–Water Interface

The increasing release of potentially toxic metals from industrial processes can lead to highly elevated concentrations of these metals in soil, and ground- and surface-waters. Today, metal pollution is one of the most serious environmental problems and thus, the development of effective remediation strategies is of paramount importance. In this context, it is critical to understand how dissolved metals interact with mineral surfaces in soil–water environments. Here, we assessed the processes that govern the interactions between six common metals (Zn, Cd, Co, Ni, Cu, and Pb) with natural brucite (Mg(OH)2) surfaces. Using atomic force microscopy and a flow-through cell, we followed the coupled process of brucite dissolution and subsequent nucleation and growth of various metal bearing precipitates at a nanometer scale. Scanning electron microscopy and Raman spectroscopy allowed for the identification of the precipitates as metal hydroxide phases. Our observations and thermodynamic calculations indicate that this coupled dissolution–precipitation process is governed by a fluid boundary layer at the brucite–water interface. Importantly, this layer differs in composition and pH from the bulk solution. These results contribute to an improved mechanistic understanding of sorption reactions at mineral surfaces that control the mobility and fate of toxic metals in the environment.

Is There a Precipitation Sequence in Municipal Wastewater Induced by Electrolysis?

Electrochemical wastewater treatment can induce calcium phosphate precipitation on the cathode surface. This provides a simple yet efficient way for extracting phosphorus from municipal wastewater without dosing chemicals. However, the precipitation of amorphous calcium phosphate (ACP) is accompanied by the precipitation of calcite (CaCO₃) and brucite (Mg(OH)₂). To increase the content of ACP in the products, it is essential to understand the precipitation sequence of ACP, calcite, and brucite in electrochemical wastewater treatment. Given the fact that calcium phosphate (i.e., hydroxyapatite) has the lowest thermodynamic solubility product and highest saturation index in the wastewater, it has the potential to precipitate first. However, this is not observed in electrochemical phosphate recovery from raw wastewater, which is probably because of the very high Ca/P molar ratio (7.5) and high bicarbonate concentration in the wastewater resulting in formation of calcite. In the case of decreased Ca/P molar ratio (1.77) by spiking external phosphate, most of the removed Ca in the wastewater was used for ACP formation instead of calcite. The formation of of brucite, however, was only affected when the current density was decreased or the size of cathode was changed. Overall, the removal of Ca and Mg is much more affected by current density than the surface area of cathode, whereas for P removal, the reverse is true. Because of these dependencies, though there is no definite precipitation sequence among ACP, calcite, and brucite, it is still possible to influence the precipitation degree of these species by relatively low current density and high surface area or by targeting phosphorus-rich wastewaters.

Electrochemical Induced Calcium Phosphate Precipitation: Importance of Local pH

Phosphorus (P) is an essential nutrient for living organisms and cannot be replaced or substituted. In this paper, we present a simple yet efficient membrane free electrochemical system for P removal and recovery as calcium phosphate (CaP). This method relies on in situ formation of hydroxide ions by electro mediated water reduction at a titanium cathode surface. The in situ raised pH at the cathode provides a local environment where CaP will become highly supersaturated. Therefore, homogeneous and heterogeneous nucleation of CaP occurs near and at the cathode surface. Because of the local high pH, the P removal behavior is not sensitive to bulk solution pH and therefore, efficient P removal was observed in three studied bulk solutions with pH of 4.0 (56.1%), 8.2 (57.4%), and 10.0 (48.4%) after 24 h of reaction time. While P removal efficiencies are not generally affected by bulk solution pH, the chemical-physical properties of CaP solids collected on the cathode are still related to bulk solution pH, as confirmed by structure characterizations. High initial solution pH promotes the formation of more crystalline products with relatively high Ca/P molar ratio. The Ca/P molar ratio increases from 1.30 (pH 4.0) to 1.38 (pH 8.2) and further increases to 1.55 (pH 10.0). The formation of CaP precipitates was a typical crystallization process, with an amorphous phase formed at the initial stage which then transforms to the most stable crystal phase, hydroxyapatite, which is inferred from the increased Ca/P molar ratio from 1.38 (day 1) to the theoretical 1.76 (day 11) and by the formation of needle-like crystals. Finally, we demonstrated the efficiency of this system for real wastewater. This, together with the fact that the electrochemical method can work at low bulk pH, without dosing chemicals and a need for a separation process, highlights the potential application of the electrochemical method for P removal and recovery.