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

ABSTRACT

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.

GRAPHICAL ABSTRACT

KEYWORDS:

• Struvite
• phosphorus
• metals
• FTIR
• XAFS

Acknowledgements

The authors thank Dr. Bruce Ravel, Lead Beamline Scientist of 6-BMM, National Synchrotron Light Source-II (NSLS-II), Brookhaven National Laboratory (BNL), Upton New York, for access to the beamline, technical support and assistance with XAFS data collection. The authors also thank Dr. Evert J. Elzinga for use of ATR-FTIR.

Data availability statement

Data can be obtained by emailing the authors.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

This research was supported by the National Science Foundation under Grant EAR 1506653 and 1530582.