Towards Sustainable Struvite Production

Struvite may not be a household word, but it is all too familiar to the operators of wastewater treatment plants.

A crystalline mineral known chemically as magnesium ammonium phosphate hexahydrate, struvite occurs naturally in decomposing organic materials such as sludge from animal wastes and from treated wastewater.

When it crystallizes on equipment surfaces in treatment plants, says Jonas Baltrusaitis, struvite can clog pipes, requiring them to be chemically cleaned or replaced and, in some cases, forcing a plant to be shut down.

But there is a brighter side to the picture, says Baltrusaitis, an assistant professor of chemical and biomolecular engineering.

Struvite contains three nutrients vital to plant growth—nitrogen, phosphorus and magnesium. Treatment plant operators aim to recover these nutrients early in the wastewater treatment process and convert them to fertilizer. This must be done before the nutrients harm pipes and equipment and before they are discharged into the environment, where they pollute streams, rivers and lakes.

Baltrusaitis and his group have used advanced microscopy and spectroscopy techniques to study the formation of struvite crystals at the molecular level. Their work promises to lead to a cheaper, less energy-intensive conversion method and eventually to greater sustainability in wastewater treatment and in agriculture.

The group recently reported their results in ACS Sustainable Chemistry and Engineering, a journal of the American Chemical Society, in an article titled “Spectroscopic and Microscopic Identification of the Reaction Products and Intermediates during the Struvite (MgNH₄PO₄·6H₂O) Formation from Magnesium Oxide (MgO) and Magnesium Carbonate (MgCO₃) Microparticles.”

The article’s authors are Baltrusaitis, Criztel Navizaga ’18 and Hanyu Zhang of Lehigh, and Erica Kirinovic, Amanda R. Leichtfuss and Jennifer D. Schuttlefield Christus of the University of Wisconsin at Oshkosh. Navizaga is a chemical engineering major. Zhang received his M.S. in chemical engineering from Lehigh in 2016.

Most wastewater treatment plants, says Baltrusaitis, use insoluble iron or aluminum salts to recover phosphorus (phosphates) from sludge and then dispose of the nutrients in landfills. Water-soluble magnesium salts such as magnesium chloride (MgCl₂) are also used to recover the nutrients via struvite formation.

“The traditional way of making struvite is not very sustainable,” says Baltrusaitis. “Soluble magnesium salts are typically made from seawater or brine. Seawater has to be evaporated in order to recover struvite, and this requires a lot of energy.”

Baltrusaitis and his group are proposing to form struvite by using magnesium oxide (MgO), dolomite (MgCO₃*CaCO₃) or magnesium carbonate (magnesite, or MgCO₃) instead of magnesium obtained from seawater or magnesium chloride containing salt brines. MgO and MgCO₃ are naturally occurring abundant minerals.

“We have to react this magnesium with phosphorus and nitrogen contained in aqueous phosphate and ammonium ions,” he says. “This is a different chemical process from the one used in conventional struvite production, but the end product is the same.

“The chemical pathways for the two reactions are very different. Because magnesite and dolomite are insoluble in water, they require a heterogeneous pathway to form struvite. Magnesium chloride, on the other hand, is water-soluble, and the reaction process occurs through conventional homogeneous nucleation and precipitation.”

In their experiments, Baltrusaitis and his group utilized time-resolved, ex situ X-ray diffraction (XRD) and attenuated total-reflectance Fourier transform infrared spectroscopy (ATR-FTIR) to measure the potential of MgO and MgCO₃ to recover nutrients from wastewater.

They also employed scanning electron microscopy (SEM) to identify reactive intermediates and Raman spectroscopy to measure the relative kinetics of struvite formation.

“[We have] shown that well-defined struvite crystals can be grown on these virtually insoluble magnesium sources [MgO and MgCO₃], which indicates [that] these reactions have potential to be used for nutrient recovery from various sources of wastewater,” the group wrote in their paper.

The group also reported that Raman spectroscopy had identified a reactive intermediate “comprised of an amorphous structure that contains magnesium hydroxide structural units, implying a common reactive intermediate between homo- and heterogeneously nucleated struvite.

“This suggests that for a sustainable nutrient recovery using insoluble MgO and MgCO₃, additional preparative steps can be taken that result in an enhanced number of the surface magnesium hydroxide groups.

“These data provide newly identified mechanistic aspects of struvite growth using MgO and MgCO₃, which will be necessary if these materials are going to be used for efficient nutrient recovery from vast diversity wastewater streams.”

The work by his group, says Baltrusaitis, will lead not only to greater sustainability in the recovery of nutrients from wastewater but potentially to greener agricultural practices as well.

“Waste from wastewater treatment plants is going to grow proportionally as population grows,” he says. “The current technology, taking MgCl₂ from seawater, is less sustainable and is going to require more and more energy to recover nitrogen and phosphorus from animal waste and other waste.

“We believe we have devised a low-energy, low-environmental-impact technology that can potentially break this spiral.”

At the same time, says Baltrusaitis, the use of MgO and MgcO₃ to recover and convert nutrients could give farmers an alternative to conventional fertilizers that are wasteful.

“Manure consists of 70 to 80 percent water,” he says. “When you spray it on crops or on lawns as fertilizer, ammonia evaporates from it.

“We need to confine manure as a solid to keep ammonia from evaporating or leaching out. We can do this by using dolomite and magnesite and other inorganic materials to confine ammonia and phosphorus as struvite.”

The work in the Baltrusaitis lab is supported by the newly funded National Science Foundation grant # 1710120 “INFEWS N/P/H₂O: Chemical and structural transformations at low solubility magnesium mineral-wastewater interface during struvite formation and growth.”

Story by Kurt Pfitzer