(506d) Utilization of Insoluble Mg Minerals As an Alternative Mg Source for Struvite Crystallization from Wastewater

Introduction

The significant discharge of N and P nutrients from agricultural systems into wastewater has posed the challenge of sustainable treatment in order to mitigate significant environmental consequences. Nutrient laden wastewater can be treated as a feedstock and the NH₄⁺ and PO₄^3- nutrients can be captured and recycled back into agricultural systems via struvite crystallization. Struvite (MgNH₄PO₄.6H₂O) is a slow-release fertilizer that can be produced from wastewater by adding an Mg source. While soluble Mg sources such as MgCl₂ have been studied in previous literature reports, owing to cost of precursor production it is more sustainable and cost-effective to use naturally abundant insoluble Mg minerals.¹ Furthermore, detailed characterization of products as well as reaction intermediate studies using in situ spectroscopic methods are required in order to fully elucidate mechanistic insights and to asses product quality. Our studies have demonstrated that insoluble Mg minerals such as bulk periclase (MgO)¹, magnesite (MgCO₃)², and dolomite (CaMg(CO₃)₂)³ all result in struvite formation from simulated wastewater.

Materials and Methods

All bulk reagents were purchases from Millipore-Sigma. In situ Raman spectroscopy has been utilized to elucidate mechanistic insights of phosphate adsorption on the mineral surface. Aqueous PO₄^3- concentration was monitored using ion chromatography for kinetic studies. Ex situ Raman spectroscopy and powder X-ray diffraction were utilized to characterize all precursors and products. Simulated wastewater in all studies utilized 100 ppm NH₄⁺ and 500 ppm PO₄^3- to reflect reported agricultural wastewater. The total reaction time was 2 hours.

Results and Discussion

Figure 1(a) shows the nutrient removal efficiency for the optimal loading of each Mg source used in the struvite crystallization studies. Mg²⁺:NH₄⁺:PO₄^3- ratio of 1:1:1 or slightly higher Mg2+ loading was found to be preferable MgO and MgCO₃ both exhibit ~80% NH₄⁺ removal and over 90% PO43- removal, while producing pure struvite. Dolomite was less effective with 37% NH₄⁺ removal and a resulted in a mixture of struvite and amorphous hydroxyapatite. MgO exhibited a 2-regime behavior for struvite formation; a kinetic regime for low MgO loadings which resulted in struvite formation, and a thermodynamic regime where high MgO loadings resulted in NH₄⁺ adsorption and desorption (shown in Figure 1(b)) with the final product being Mg₃(PO₄)₂.22H₂O. Dolomite displayed a similar behavior while all MgCO₃ loadings resulted in struvite formation as shown in Figure 1(c). Ex situ Raman analysis of the intermediate reaction product after 5 minutes for MgO showed that the reaction intermediate is in the form of magnesium hydroxycarbonate. Furthermore, kinetic fitting of MgO concentration data showed the best fit for pseudo-second-order, indicating that a surface reaction is rate limiting for struvite formation. MgCO₃ kinetics followed pseudo-first-order. The change in order is hypothesized to be due to the fact that MgCO₃ carbonate surface is closer to the reaction intermediate observed in the MgO study. This further demonstrates that the oxide surface transformation to carbonate is of significance for phosphate adsorption. Figure 1(d) shows normalized Raman band intensity ratios for in situ monitoring of the calcined dolomite surface during struvite formation. This study demonstrates clearly that the MgO surface sites undergo significant transformation prior to struvite formation. In phase 1 of the reaction the oxide surface is transformed to a carbonate-rich surface by dissolved CO₂ reacting with the surface hydroxyl groups. These carbonate groups are replaced by HPO₄^2- and H₂PO₄^- group chains as shown by the increasing normalized Raman band intensity ratio. Finally, the phosphate chains transform to PO₄^3- groups that result in the struvite formation. Further work will include extending this time-resolved Raman study methodology to other Mg minerals to identify the surface reaction mechanism.

Figure 1. (a) Removal of NH₄⁺ and PO₄^3- for each Mg source. (b) NH₄⁺ concentration profile for each loading of MgO (c) pXRD patterns for reaction products for each loading of MgCO₃ (d) Normalized band intensity variation for struvite formation reaction on the calcined dolomite surface.

Significance

Modern agricultural practices, as well as overfertilization due to the increasing demand for food production has led to significant anthropogenic impact on the nitrogen and phosphorus cycles.⁴ Loss of ammonium and phosphate from agricultural systems in wastewater has led to significant environmental consequences such as eutrophication, which has been demonstrated to restrict access to freshwater resources, adversely affect aquatic ecosystems, and emit significant quantities of greenhouse gases. Utilization of low-cost and abundant natural Mg minerals have been demonstrated to be sustainable Mg sources for the struvite crystallization process, which allow for nutrient recycle and reuse.

References

(1) Kiani, D.; Sheng, Y.; Lu, B.; Barauskas, D.; Honer, K.; Jiang, Z.; Baltrusaitis, J. Transient Struvite Formation during Stoichiometric (1:1) NH4+ and PO43– Adsorption/Reaction on Magnesium Oxide (MgO) Particles. ACS Sustain. Chem. Eng. 2018, 7, 1545–1556, DOI: 10.1021/acssuschemeng.8b05318.

(2) Lu, B.; Kiani, D.; Taifan, W.; Barauskas, D.; Honer, K.; Zhang, L.; Baltrusaitis, J. Spatially Resolved Product Speciation during Struvite Synthesis from Magnesite (MgCO 3 ) Particles in Ammonium (NH 4 + ) and Phosphate (PO 4 3− ) Aqueous Solutions. J. Phys. Chem. C 2019, 123, acs.jpcc.8b12252, DOI: 10.1021/acs.jpcc.8b12252.

(3) Kiani, D.; Silva, M.; Sheng, Y.; Baltrusaitis, J. Experimental Insights into the Genesis and Growth of Struvite Particles on Low-Solubility Dolomite Mineral Surfaces. J. Phys. Chem. C 2019, DOI: 10.1021/acs.jpcc.9b05292.

(4) Smol, M. The importance of sustainable phosphorus management in the circular economy (CE) model: the Polish case study. Journal of Material Cycles and Waste Management. Springer Tokyo March 15, 2019, pp 227–238.