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1. Recovery of struvite for organic production: Mineral-based magnesium supplementation and pH elevation

   https://doi.org/10.1016/j.jclepro.2024.142244  

   Iftikhar, Aysha; Tao, Wendong (May 2024, Journal of Cleaner Production)

   Phosphate in wastewater can be recovered in the form of struvite crystals for use as slow-release fertilizer. Currently, struvite recovery often requires supplementing magnesium ions and raising pH with chemicals, making the recovered struvite unfavorable for organic production. In an effort for cleaner production, this study developed a versatile approach employing two mineral products in variable combinations for optimal supplementation of magnesium and elevation of pH. Magnesite, a mineral of MgCO3, was ground and calcined without use of any catalysts. The magnesite calcined under the optimum conditions (800 ◦C for 30 min) can be dissolved in near-neutral filtrate of sludge digestate to supplement magnesium and raise pH for effective struvite formation. The OMRI-listed Epsom salts (MgSO4•7H2O), mineral-based water-soluble commercial products, can be used to supplement magnesium without changing pH of alkaline wastewater. Six-hour batch operation of an airlift crystallizer removed 85.7% and 94.7% of phosphate in hydrolyzed human urine when magnesium was amended to 1.2 × molar concentration of phosphate with calcined magnesite and an OMRI-listed Epsom salt, respectively. More than 98% of phosphate was removed from filtrate of sludge digestate in 3-h batch operation using calcined magnesite to raise pH to 8.5 and the Epsom salt for further magnesium supplementation. Struvite accounted for 85.7%, 90.5%, and 81.5% of the crystals recovered from urine with calcined magnesite, urine with Epsom salt, and filtrate with both mineral products, respectively. The material and energy costs of this green process were estimated to be $0.16/kg struvite from urine with calcined magnesite, $1.37/kg struvite from urine with Epsom salt, and $0.94/kg struvite from filtrate with both mineral products. This study proved the technical and economic feasibility of chemical-free magnesium supplementation and pH elevation, making the recovered struvite potentially certifiable for organic production. 

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2. Assessing the Intrinsic Strengths of Ion–Solvent and Solvent–Solvent Interactions for Hydrated Mg2+ Clusters

   https://doi.org/10.3390/inorganics9050031  

   Delgado, Alexis Antoinette; Sethio, Daniel; Kraka, Elfi (May 2021, Inorganics)

   Information resulting from a comprehensive investigation into the intrinsic strengths of hydrated divalent magnesium clusters is useful for elucidating the role of aqueous solvents on the Mg2+ ion, which can be related to those in bulk aqueous solution. However, the intrinsic Mg–O and intermolecular hydrogen bond interactions of hydrated magnesium ion clusters have yet to be quantitatively measured. In this work, we investigated a set of 17 hydrated divalent magnesium clusters by means of local vibrational mode force constants calculated at the ωB97X-D/6-311++G(d,p) level of theory, where the nature of the ion–solvent and solvent–solvent interactions were interpreted from topological electron density analysis and natural population analysis. We found the intrinsic strength of inner shell Mg–O interactions for [Mg(H2O)n]2+ (n = 1–6) clusters to relate to the electron density at the bond critical point in Mg–O bonds. From the application of a secondary hydration shell to [Mg(H2O)n]2+ (n = 5–6) clusters, stronger Mg–O interactions were observed to correspond to larger instances of charge transfer between the lp(O) orbitals of the inner hydration shell and the unfilled valence shell of Mg. As the charge transfer between water molecules of the first and second solvent shell increased, so did the strength of their intermolecular hydrogen bonds (HBs). Cumulative local vibrational mode force constants of explicitly solvated Mg2+, having an outer hydration shell, reveal a CN of 5, rather than a CN of 6, to yield slightly more stable configurations in some instances. However, the cumulative local mode stretching force constants of implicitly solvated Mg2+ show the six-coordinated cluster to be the most stable. These results show that such intrinsic bond strength measures for Mg–O and HBs offer an effective way for determining the coordination number of hydrated magnesium ion clusters. 

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3. Binding energies of hydrated cobalt( ii ) by collision-induced dissociation and theoretical studies: evidence for a new critical size

   https://doi.org/10.1039/C7CP05828D  

   Coates, Rebecca A.; Armentrout, P. B. (January 2018, Physical Chemistry Chemical Physics)

   The experimental sequential bond energies for loss of water from Co 2+ (H 2 O) x complexes, x = 5–11, are determined by threshold collision-induced dissociation (TCID) using a guided ion beam tandem mass spectrometer with a thermal electrospray ionization source. Kinetic energy dependent TCID cross sections are analyzed to yield 0 K thresholds for sequential loss of neutral water molecules. The thresholds are converted from 0 to 298 K values to give hydration enthalpies and free energies. Theoretical geometry optimizations and single point energy calculations at several levels of theory are performed for the reactant and product ion complexes. Theoretical bond energies for ground structures are used for direct comparison with experimental values to obtain structural information on these complexes. In addition, the dissociative charge separation process, Co 2+ (H 2 O) x → CoOH + (H 2 O) m + H + (H 2 O) x−m−1 , is observed at x = 4, 6, and 7 in competition with primary water loss products. Energies for the charge separation rate-limiting transition states are calculated and compared to experimental threshold measurements. Results suggest that the critical size for which charge separation is energetically favored over water loss is x crit = 6, in contrast to lower values in previous literature reports. 

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4. Low concentrations of Cu ²⁺ in synthetic nutrient containing wastewater inhibit MgCO ₃ -to-struvite transformation

   https://doi.org/10.1039/D0EW01035A  

   Barčauskaitė, Karolina; Drapanauskaitė, Donata; Silva, Manoj; Murzin, Vadim; Doyeni, Modupe; Urbonavicius, Marius; Williams, Clinton F.; Supronienė, Skaidrė; Baltrusaitis, Jonas (March 2021, Environmental Science: Water Research & Technology)

   null (Ed.)

   Simultaneous major nutrient nitrogen (N) and phosphorus (P) recovery from wastewater is key to achieving food–energy–water sustainable development. In this work, we elucidate the reaction kinetics, crystalline structure and chemical composition of the resulting solid precipitate obtained from simulated N and P containing wastewater solution using widely abundant low solubility magnesite (MgCO 3 ) particles in the presence of common transition metal ions, such as zinc (Zn 2+ ) or copper (Cu 2+ ). We show that up to 100 ppm Zn 2+ from the simulated wastewater can be incorporated into the struvite lattice as isolated distorted Zn 2+ while even at very low concentrations of ∼5 ppm Cu 2+ ions almost completely inhibit struvite crystal formation. The resulting solid precipitate distinctly affects soil microbial biomass carbon and soil dehydrogenase enzyme activity. These results show a cautionary case where abundant natural mineral MgCO 3 exhibits very different chemistry in Cu 2+ containing simulated wastewater and does not readily adsorb or retain NH 4 + and PO 4 3− ions, unlike less sustainable but more water-soluble magnesium sources, such as MgCl 2 , at the equivalent [Mg 2+ ] : [NH 4 + ] : [PO 4 3− ] molar ratio of 1.4 : 1 : 1. 

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5. Molecular-level origin of the carboxylate head group response to divalent metal ion complexation at the air–water interface

   https://doi.org/10.1073/pnas.1818600116  

   Denton, Joanna K.; Kelleher, Patrick J.; Johnson, Mark A.; Baer, Marcel D.; Kathmann, Shawn M.; Mundy, Christopher J.; Wellen Rudd, Bethany A.; Allen, Heather C.; Choi, Tae Hoon; Jordan, Kenneth D. (July 2019, Proceedings of the National Academy of Sciences)

   We exploit gas-phase cluster ion techniques to provide insight into the local interactions underlying divalent metal ion-driven changes in the spectra of carboxylic acids at the air–water interface. This information clarifies the experimental findings that the CO stretching bands of long-chain acids appear at very similar energies when the head group is deprotonated by high subphase pH or exposed to relatively high concentrations of Ca 2+ metal ions. To this end, we report the evolution of the vibrational spectra of size-selected [Ca 2+ ·RCO 2 − ] + ·(H 2 O) n =0 to 12 and RCO 2 − ·(H 2 O) n =0 to 14 cluster ions toward the features observed at the air–water interface. Surprisingly, not only does stepwise hydration of the RCO 2 − anion and the [Ca 2+ ·RCO 2 − ] + contact ion pair yield solvatochromic responses in opposite directions, but in both cases, the responses of the 2 (symmetric and asymmetric stretching) CO bands to hydration are opposite to each other. The result is that both CO bands evolve toward their interfacial asymptotes from opposite directions. Simulations of the [Ca 2+ ·RCO 2 − ] + ·(H 2 O) n clusters indicate that the metal ion remains directly bound to the head group in a contact ion pair motif as the asymmetric CO stretch converges at the interfacial value by n = 12. This establishes that direct metal complexation or deprotonation can account for the interfacial behavior. We discuss these effects in the context of a model that invokes the water network-dependent local electric field along the C–C bond that connects the head group to the hydrocarbon tail as the key microscopic parameter that is correlated with the observed trends. 

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