Yuanming Pan has discovered arsenic in some supposedly “green” fertilizers

Researchers at the University of Saskatchewan are concerned that so-called “green” fertilizers may not be as safe as they sound.

Geological sciences professor Yuanming Pan says naturally-sourced fertilizers can potentially leach arsenic into the soil, water and crops.

“About 10 years ago people around the world began to recover struvite from waste water and use it as fertilizer,” Pan said. Struvite, the active ingredient in green fertilizers, is a mineral that accumulates in sewer pipes. The compound tends to clog the pipes and prompt expensive repairs.

Major cities around the world including Vancouver, Edmonton and Saskatoon extract these struvite precipitates from waste water pipes and convert them into a fertilizer that is meant to be envi- ronmentally friendly.

“This is a common phenomenon and has been known for a long time,” Pan said. “On the one hand, you solve the problem of these pipes being clogged and on the other hand our lakes and rivers become less polluted because you take out the phosphate and ammonium.”

Pan’s team found that certain natural fertilizers, while effective, pick up arsenic molecules and release them into the soil, potentially contaminating water and crops.

“Phosphate, or PO4, is a tetrahedron in geometry,” said Ning Chen, one of the researchers on Pan’s team. “Arsenate also has this kind of shape, which is how it gets into the crystal structure of fertilizer.”

The team collaborated with the Canadian Light Source and the university’s Structural Science Center to synthesize struvite crystals, then used a technique called x-ray absorption spectroscopy to view the compounds up close.

“Our PhD student Jinru Lin is very good at synthesizing crystals, so she did the synthesis and all of the analysis,” Pan said. “The crystals she grew were a few milli- meters in size and from chemical analysis we found that struvite can pick up hundreds of PPMs of arsenic.”

Long-term exposure to arsenic has been linked to several forms of cancer, according to Health Canada. Exposure to high levels of arsenic can also cause nausea, diarrhea, vomiting, numbness and skin problems.

However, Pan isn’t suggesting struvite fertilizers should come off the market. He merely cautions that all countries should have strict regulations on their production.

“Countries like Canada, most of the European countries and the States have very strict regulations for waste water so it’s not an issue because the waste water is treated beforehand,” Pan said. “This isn’t true of every country. In this case it would be a problem.”

Struvite fertilizers made in Sas- katoon claim to have an arsenic content below detectable limits. Ostara Nutrient Recovery Technologies, one company which produces green fertilizers in Saskatoon, boasts a product which has an arsenic level 100 times lower than conventional fertilizers.

The team’s research began in 2009. Pan says he decided to look into phosphates because “they have a huge impact on the world.”

Pan attributes the success of the project to the tools at the CLS and the Structural Science Centre.

“We are able to do analysis that other people can’t. That makes the work even more interesting,” Pan said. The team used Chen’s Hard X-ray Micro-Analysis beamline to view the compounds.

“Without the synchrotron you wouldn’t see anything,” Chen said. “Chemical analysis tools are very accurate but they won’t tell you where the arsenic is. They can give you chemistry information but not structure information. We knew there was arsenic in [the fertilizers]. The question was where they are located in the structure.

“You have to know where the arsenic is before you can deal with it. Other technologies cannot give you this information. The CLS played a key role in this particular project.”

Pan says he has already started to follow up on this research.

“We have received funding from the federal government to do sequestration of radionuclides,” Pan said. “To remove these radionuclides from contaminated water would be a huge benefit. We think these phosphates can be used to precipitate radionuclides, but we would have to use the highly sen- sitive techniques at the CLS.”

Green fertilizers have been around since humans first began growing food. One hundred years ago, Chile harvested enough bird and bat guano to make it one of its top exports. But today, with limited availability of guano-based fertilizers, more countries are switching over to producing struvite fertilizers.

“As long as existing regulations are followed, [struvite fertilizers] are safe to use,” Pan said. “We know from our experimental results that struvite can pick up arsenic. If the waste water is already treated then it shouldn’t be a problem, but if it’s not treated then it becomes a concern.”

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Photo: Rainer Kocsis/Associate News Editor