Struvite

Struvite (magnesium ammonium phosphate) is a phosphate mineral with formula: NH₄MgPO₄·6H₂O. Struvite crystallizes in the orthorhombic system as white to yellowish or brownish-white pyramidal crystals or in platey mica-like forms. It is a soft mineral with Mohs hardness of 1.5 to 2 and has a low specific gravity of 1.7. It is sparingly soluble in neutral and alkaline conditions, but readily soluble in acid.

Struvite
Crystals of struvite from dog urine
General
Category	Phosphate mineral
Formula (repeating unit)	NH4MgPO4·6H2O
Strunz classification	8.CH.40
Crystal system	Orthorhombic
Crystal class	Pyramidal (mm2) H-M symbol: (mm2)
Space group	Pmn21
Identification
Color	Colorless, white (dehydrated), yellow or brownish, light gray
Crystal habit	Euhedral to platey
Twinning	On {001}
Cleavage	{100} perfect
Fracture	Uneven
Mohs scale hardness	1.5–2
Luster	Vitreous to dull
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	1.7
Optical properties	Biaxial (+) 2V Measured: 37°
Refractive index	nα = 1.495 nβ = 1.496 nγ = 1.504
Birefringence	δ = 0.009
Solubility	Slightly soluble, dehydrates in dry, warm air
Other characteristics	Pyroelectric and piezoelectric
References	[1][2][3]

Struvite urinary stones and crystals form readily in the urine of animals and humans that are infected with ammonia-producing organisms. They are potentiated by alkaline urine and high magnesium excretion (high magnesium/plant-based diets). They also are potentiated by a specific urinary protein in domestic cats.

Name

Struvite was first described in 1845 by the German chemist Georg Ludwig Ulex (1811–1883), who found crystals of struvite in what he surmised had once been a medieval midden in Hamburg, Germany; he named the new mineral after the geographer and geologist Heinrich Christian Gottfried von Struve (1772–1851) of Hamburg.[4][2]

Occurrence

Struvite is occasionally found in canned seafood, where its appearance is that of small glass slivers, objectionable to consumers for aesthetic reasons but of no health consequence.[5] A simple test can differentiate struvite from glass.[6]

Use of struvite as an agricultural fertilizer was in fact first described in 1857.

Struvite kidney stones

Struvite precipitates in alkaline urine, forming kidney stones. Struvite is the most common mineral found in urinary tract stones in dogs,[7] and is found also in urinary tract stones of cats and humans. Struvite stones are potentiated by bacterial infection that hydrolyzes urea to ammonium and raises urine pH to neutral or alkaline values. Urea-splitting organisms include Proteus, Xanthomonas, Pseudomonas, Klebsiella, Staphylococcus, and Mycoplasma.

Even in the absence of infection, accumulation of struvite crystals in the urinary bladder is a problem frequently seen in housecats, with symptoms including difficulty urinating (which may be mistaken for constipation) or blood in the urine (hematuria). The protein cauxin, a protein excreted in large amounts in cat urine that acts to produce a feline pheromone, has recently been found to cause nucleation of struvite crystals in a model system containing the ions necessary to form struvite. This may explain some of the excess struvite production in domestic cats.[8] In the past, surgery has been required to remove struvite uroliths in cats; today, special acidifying low magnesium diets may be used to dissolve sterile struvite stones.[9]

Upper urinary tract stones that involve the renal pelvis and extend into at least 2 calyces are classified as staghorn calculi. Although all types of urinary stones can potentially form staghorn calculi, approximately 75% are composed of a struvite-carbonate-apatite matrix.

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  Dog struvite bladder stones
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  Struvite crystals in a human urine sample with a pH of 9. Abundant amorphous phosphate crystals, several squamous and non-squamous epithelial cells and a few leukocytes can also be observed.
• 
  Another image from the same urine sample as with the image on the left.

Struvite enteroliths

Struvite is a common mineral found in enteroliths (intestinal concretions) in horses.[10]

Wastewater treatment

Struvite can be a problem in sewage and waste water treatment, particularly after anaerobic digesters release ammonium and phosphate from waste material. Struvite can form a scale on lines and belts, in centrifuges and pumps, clog system pipes and other equipment including the anaerobic digester itself. Struvite, also referred to as MAP, forms when there is a mole to mole to mole ratio (1:1:1) of magnesium, ammonia and phosphate in the wastewater. The magnesium can be found in soil, seawater as well as drinking water. Ammonia is broken down from the urea in wastewater, and phosphate, which is found through food, soaps and detergents. These elements in place, struvite is more likely to form in a high pH environment, where there is higher conductivity, lower temperatures, and higher concentrations of magnesium, ammonia and phosphate. Recovery of phosphorus from wastestreams as struvite and recycling those nutrients into agriculture as fertilizer appears promising, particularly in agricultural manure and municipal waste water treatment plants.

Having struvite scale in a wastewater treatment system can lead to great inefficiency within the plant or operation due to clogging of the pipes, pumps and equipment. There have been a few options to solve this issue, including replacing the pipes, or using a hydro-jetter or a mechanical grinder to clear them. But many lines can be underground and either of these options implies considerable downtime and labor. Chemical cleaning is now predominately used to clear systems of struvite. Chemical cleaning products have been developed to remove and prevent struvite with minimal downtime. Even a chemical-free, electric method of removing and preventing struvite has been developed and tested successfully at wastewater treatment plants in the USA. The electronic sine wave it produces is sent through the water in the pipe and is therefore effective on underground piping as well. [11] [12][13]

Uses

There is considerable interest in the utility of urine-derived struvite as a fertilizer in austere situations. [14] [15]

References

1. "Struvite" (PDF). Handbook of mineralogy.
2. "Struvite Mineral Data". Webmineral.
3. "Struvite". Mindat.
4. Ulex GL (1845). "On struvite, a new mineral". Memoirs and Proceedings of the Chemical Society. 3: 106–110. doi:10.1039/mp8450300106.
5. Thompson C (2011-03-10). "Suspected "glass" in canned fish actually natural crystal". Komo News. Retrieved 2016-07-21.
6. "Struvite Crystals, the facts ..." (PDF). Hambleton District Council.
7. "Uroliths". Shiloh Shepherd Genetic Task Force. February 2014. Retrieved 2014-02-07.
8. Matsumoto K, Funaba M (February 2008). "Factors affecting struvite (MgNH4PO4.6H2O) crystallization in feline urine". Biochimica et Biophysica Acta. 1780 (2): 233–9. doi:10.1016/j.bbagen.2007.09.013. PMID 17976920.
9. Smith BH, Stevenson AE, Markwell PJ (December 1998). "Urinary relative supersaturations of calcium oxalate and struvite in cats are influenced by diet". The Journal of Nutrition. 128 (12 Suppl): 2763S–2764S. doi:10.1093/jn/128.12.2763S. PMID 9868260.
10. Blue MG, Wittkopp RW (July 1981). "Clinical and structural features of equine enteroliths". Journal of the American Veterinary Medical Association. 179 (1): 79–82. PMID 7251465.
11. Burns RT, Moody LB, Walker FR (November 2001). "Laboratory and in-situ reductions of soluble phosphorus in swine waste slurries" (PDF). Environmental Technology. 22 (11): 1273–8. doi:10.1080/09593332208618190. PMID 11804348. S2CID 25433506. Archived from the original (PDF) on 2012-03-27.
12. Hume M. "Sewage plant carries sweet smell of phosphorus". The Globe And Mail. Retrieved 2014-07-08.
13. Morton B. "Reclaiming minerals from waste water to make fertilizer". The Vancouver Sun. Retrieved 2013-06-05.
14. Antonini, Samantha (2012). Nutrient recovery from human urine: Treatment options and reuse potential (PDF) (PhD). University of Bonn, DE.
15. Pradhan, Surendra K; Mikola, Anna; VahalaPublished, Riku (May 2017). "Nitrogen and Phosphorus Harvesting from Human Urine Using a Stripping, Absorption, and Precipitation Process". Environmental Science & Technology. 51 (9): 5165–5171. Bibcode:2017EnST...51.5165P. doi:10.1021/acs.est.6b05402. PMID 28409915.

External links

• Website of the European Phosphorus Platform about Phosphorus Recovery