Nitrogen (N) and phosphorus (P) nutrient recovery in a solid form from wastewater streams is of utmost importance while managing their global cycles. Here, we show that water insoluble MgO precursor can be utilized to synthesize MgNH 4 PO 4 •6H 2 O (struvite). Hence, water-soluble magnesium precursors, such as MgCl 2 , that require expensive synthesis routes can be substituted with more sustainable, naturally occurring but less soluble magnesium precursors, such as MgO. Time-resolved analysis of solid and liquid products showed two MgO concentration regimes during struvite formation, e.g. a kinetically controlled one at [Mg:NH 4 + :PO 4 3− ] ratio of 4.80:1:1 and an equilibrium limited one at [Mg 2+ ]: [NH 4 + ]:[PO 43− ] = 1.44:1:1. In both cases, >70% of NH 4 + and PO 4 3− species were removed from solution with the pseudo-second-order dependence of the reaction rate on the starting MgO concentration. The dual adsorption/reaction behavior of both ions needed in equimolar amounts to form struvite resulted in heterogeneous product distribution with magnesium phosphate as the dominant component at equilibrium. Spatially resolved Raman and pXRD analyses showed that crystalline struvite (MgNH 4 PO 4 •6H 2 O), magnesium phosphate (Mg 3 PO 4 •22H 2 O) and complex amorphous species are present. The Raman spectral footprint of the intermediate component was assigned to dypingite, Mg 5 (CO 3 ) 4 (OH) 2 •4H 2 O. This suggests that parallel hydration reaction takes place, under ambient conditions, to form magnesium hydroxycarbonates when excess MgO is available. In turn, this implies that various carbonates of magnesium can potentially be utilized with improved conversion during struvite precipitation from aqueous N and P precursor solutions. Lastly, sustainability and economic analysis of struvite synthesis from MgO clearly showed that employing MgO instead of MgCl 2 can significantly decrease overall energy requirements, and the resulting carbon footprint by a factor of ∼3.

While population growth necessitates a significant increase in crop production, stringent environmental regulations require that it be done using sustainable nutrient sources. Nutrients in the form of NH4 + and PO4 3– are recovered from wastewater streams via precipitation, using water-soluble magnesium ions to form sustainable, slow-release fertilizer, struvite (MgNH4PO4·6H2O). However, the magnesium needed is mainly incorporated in the crystal lattices of very low-solubility minerals. This work utilizes a combination of powder X-ray diffraction (pXRD) and ex situ Raman and energy-dispersive X-ray spectroscopies combined with ion chromatography to characterize transformation products of low-solubility MgCO3 particles in NH4 +- and PO4 3–- containing aqueous solutions with and without Ca2+ ions present. Although pXRD showed struvite as the predominant solid product for the molar ratio [Mg2+/NH4 +/PO4 3–] of [0.2:1:1] and higher, ex situ Raman spectra evidenced formation of a dypingite-like phase along with struvite. Single-crystal Raman spectroscopy in combination with scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy showed Ca2+ incorporation into the structure of struvite crystals as submicron crystallites at the Ca2+/Mg2+ ratio of 0.2, from both CaCO3 and CaCl2 and at the Ca2+/Mg2+ ratio of 1, in the case of CaCO3. Moreover, distinct solid product speciation was observed when Ca2+ was present in aqueous solutions when using CaCl2; for example, hydroxyapatite was observed for Ca2+/Mg2+ = 1 when CaCl2 was used. The results reported here unravel the effect of the physicochemical solution parameters, such as concentration of MgCO3, pH, Ca2+ concentration, and solubility of Ca-containing precursors, on the formation of struvite crystals. This shows that recovery of nutrients containing N and P from wastewater streams is possible in the form of a slow-release fertilizer (struvite) using low-solubility, abundant magnesium-containing minerals.

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A gravimetric gas detection device based on surface functionalized Capacitive Micromachined Ultrasound Transducers (CMUTs) was designed, fabricated and tested for detection of carbon dioxide (CO2) and sulfur dioxide (SO2) mixtures in nitrogen. The created measurement setup of continuous data collection, integrated with an in-situ Fourier Transform Infrared (FT-IR) spectroscopy, allows for better understanding of the mechanisms and molecular interactions with the sensing layer (methylated poly(ethylene)imine) and its need of surface functionalization for multiple gas detection. During experimentation with CO2 gases, weak molecular interactions were observed in spectroscopy data. Linear sensor response to frequency shift was observed with CO2 concentrations ranging from 0.16 vol % to 1 vol %. Moreover, the Raman and FT-IR spectroscopy data showed much stronger SO2 and the polymer interactions, molecules were bound by stronger forces and irreversibly changed the polymer film properties. However, the sensor change in resonance frequency in the tested region of 1 vol % to 5 vol % SO2 showed a linear response. This effect changed not only the device resonance frequency but also affected the magnitude of electroacoustic impedance which was used for differentiating the gas mixture of CO2, SO2, in dry N2.

A capacitive micromachined ultrasonic transducer (CMUT)-based sensor modified with methylated poly­(ethylenimine) (mPEI) was designed and tested for the detection of two acidic gases: carbon dioxide (CO2) and sulfur dioxide (SO2). Combined gas sensing and Fourier transform infrared spectroscopy of the adsorbed products allowed to simultaneously and in situ determine the types and strength of the molecular interactions responsible for sensing. For CO2, the limit of detection was calculated to be 0.011 CO2 vol % and the limit of quantification was calculated to be 0.033%. For SO2, the limit of detection was calculated as 0.232 SO2 vol % and the limit of quantification was 0.704%. The sensing system exhibited a linear response at lower concentrations for CO2 and linear response for all the tested concentrations of SO2. In situ IR and ex situ Raman showed that CO2 was observed to undergo weak molecular coordination with mPEI while SO2 bound strongly and irreversibly degraded the thin mPEI films.

We manufactured and tested a capacitive micromachined ultrasound transducer (CMUT)-based sensor for CO2 detection at environmentally relevant concentrations using polyethylenimine as a CO2 binding material. The assembly of a sensing chip was 10 × 20 mm, and up to 5 gases can potentially be detected simultaneously using a masking technique and different sensing materials. The limit of detection was calculated to be 0.033 CO2 vol % while the limit of quantification was calculated to be 0.102%. The sensor exhibited a linear response between 0.06% and 0.30% CO2 while concentrations close to those in flue gas can also be measured using dilution with inert gas.

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