Implications for the fate of green fertilizers

The use of green fertilizers is a practice that has been around since humans first began growing food, but researchers are warning that modern techniques for the creation of these fertilizers could have implications on soil toxicity.

One hundred years ago, a country such as Chile harvested enough bird and bat guano to make it one of the top exports. But today, with a limited availability of guano-based fertilizers, new techniques have been developed to both produce a popular green fertilizer known as struvite, and to treat waste water at the same time.

Struvite is an ammonium phosphate mineral in crystalized form that can be found in guano and has proven to be an effective natural fertilizer. However struvite is also a major cause of kidney stones in humans, dogs and cats, and is also a major problem is sewage and waste water systems as it can clog pipes and water systems.

For these reasons many cities like Edmonton, Saskatoon and others are experimenting with large reaction tanks that convert the waste water and remove struvite crystals that are then used as fertilizers.

But researchers at the University of Saskatchewan and the Canadian Light Source synchrotron are concerned that struvite fertilizer is not necessarily as green as it might sound.

"Some of the commercially available green fertilizers are labeled that they have low toxins," said U of S geological scientist Dr. Yuanming Pan. "The point we are realizing from our research is that if struvite is not treated properly during the process, then however low the toxins are, over time, the accumulation will be significant."

Pan said this is "not something we can take for granted," and one of the major findings from research recently published in the scientific journal Environmental Science and Technology.

"From the data we collected during this experiment, we can draw a number of conclusions," said Pan. "If struvite recovered from the waste water is used on acidic soil, it will transform into something called newberyite, but newberyite will effectively keep all of the arsenic in it. If this dissolves in the soil, it's a contaminant."

Using the powerful HXMA X-ray beamline, Pan and CLS scientists Drs. Ning Chen and Jinru Lin probed the atomic arrangement of a number of struvite and newberyite samples using X-ray absorption spectroscopy. The researchers discovered that while struvite and newberyite are important green fertilizers, they are capable of sequestering significant amounts of arsenic.

The researchers said they were surprised to find that the speciation of arsenic is different between struvite and newberyite, even though both of them are phosphate minerals. Newberyite is capable of sequestering both As5+ and As3+. This is the first report of As3+ at the P site in phosphate minerals.

Pan said that like any research, they are having more questions than answers, but the issue is one that should not be ignored. The capacities of struvite and newberyite for sequestering significant amounts of arsenic also make them potentially useful for the remediation of arsenic contamination, an area requiring further research.