Date of Degree


Document Type


Degree Name



Earth & Environmental Sciences


Ashaki A. Rouff

Subject Categories

Environmental Sciences | Geochemistry


adsorption, arsenic, coprecipitation, P recovery, Struvite, wastewater


The formation of struvite, MgNH4PO4*6H2O (MAP), from wastes is one of the methods that can be used to recover P from wastes efficiently. However, since there are usually toxic components in the wastes, like arsenic (As), the possibility of having toxic contaminants in MAP is a big concern. So, the interaction between As and MAP during coprecipitation (CPT) and adsorption (ADS) processes were studied at pH 8-11. MAP precipitated without As at pH 8-11 was also characterized.

During CPT process, the MAP was precipitated from a MgCl2-(NH4)2HPO4-NaCl-H2O system spiked with As at an initial pH (pHi) of 8-11. The batch experiments showed that more As was found in the solids at higher pH and pentavalent As, As(V), was the favorable As oxidation state in the solids. The results from X-ray absorption fine structure spectroscopy (XAFS) indicated that As(V) was incorporated in the solids; whereas the trivalent As, As(III), was adsorbed on the mineral surface. The solids precipitated at pHi 8-11 without the addition of As were also characterized. The 31P nuclear magnetic resonance spectroscopy (NMR) confirmed that over 95% of phosphate was in MAP at pHi 8-10, with newberyite (MgHPO4∙3H2O) as a minor crystalline phase. At pHi 11, only 60% of the phosphate was in MAP, with 22% and 18% in an amorphous phase and sodium phosphate, respectively.

The As adsorption using MAP and hydroxylapatite (HAP), Ca5(PO4)3OH, studied in a MgCl2-(NH4)2HPO4-NaCl-H2O system with pH fixed at 8-11. As the pH was increased, more As(V) was removed from the solution for both MAP and HAP; for As(III), the removal by either MAP or HAP were limited for pH 8-11. Using radioactive 33P, a faster phosphate exchange at the mineral-water interface was found on HAP. The XAFS results showed the adsorbed As(V) formed monodentate mononuclear surface complexes on MAP and, for HAP, As(V) was incorporated near the mineral surface.

The studies in this thesis about the interaction between MAP and As can be helpful in producing low-As MAP with maximum recycle of P from wastes with high As concentration and using MAP as an adsorbent to remove As from phosphate-rich wastes.



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