By Sally Brown, University of Washington
Abstracts of these resources are available in the searchable Information Portal offered to Northwest Biosolids members.
Molybdenum, Silver, Thallium and Vanadium
Uptake of thallium from naturally contaminated soils into vegetables
Selection of cultivars to reduce the concentration of cadmium and thallium in food and fodder plants
Intake and health effects of Thallium among a population living in the vicinity of a cement plant emitting Thallium containing dust
Geochemical and mineralogical data for soils of the conterminous United States
I got an email a few weeks back from a concerned citizen about Thallium in biosolids. He was considering using biosolids to fertilize his wheat but wanted to be sure that it was safe. My initial email response detailed the 503 risk assessment process and the two National Academy reports as proof of safety; Close but no cigar. He wanted detailed information. So I offered to do this library on the basics of thallium (Tl) in biosolids.
While you may not have been asked about Tl in biosolids, chances are pretty good that you have been asked about either heavy metals in general or a metal that is not included on the 503 list and so not regulated in biosolids. An aside here- typically the term heavy metals is more commonly used to signify stuff that you should be afraid of rather than elements that behave as metals with high molecular weights. Thallium falls right into that category. What I will try to do with this library is to give you tools to answer both the specific question about Tl as well as an approach that you can use to answer questions about metals in general.
The first article in the library is actually a book, Trace Elements in Soils. The book has a chapter on Molybdenum, Silver, Thallium and Vanadium. These types of books are great reviews of basic properties, uses and reactivity in soils for a range of elements. Turns out that Thallium is in some rare minerals that are sometimes associated with lead and zinc. It is also similar in size to potassium so can be found in some rocks rich in K. Coal can also have Tl with an average concentration of 0.05 ppm with values up to 7.5 ppm in coal from certain parts of China. Soil concentrations are typically under 1 ppm, depending on the type of rocks the soils came from. With that- sources of Tl in the environment include coal combustion and smelting of Pb and Zn ores. Currently Tl has some industrial applications including imaging for myocardial disease and making special types of glass. Its broadest application was banned in the 1970s- it was a major ingredient in rat poison. The article – as is typically the case, also mentions sludge as a source and references one study that I could not locate that measured Tl concentrations in some biosolids in Canada and found one with concentrations over 100 ppm.
This was one of the big problems with this particular request- there is next to nothing on Tl in biosolids. As with any trace element- I turned to the world expert who thankfully still replies to my emails despite having retired. Rufus Chaney shared his library on Tl with me. The library had a number of references some of which I’ll share here. The second paper in the library reports on a pot study from the Czech republic that looked at Tl uptake by a range of garden crops from naturally enriches soils. Some crops take up more than others. Point of this is to show that Tl uptake into the edible portion of plants can occur in garden vegetables. Third paper in the study compares plant uptake of Cd (a known and very well studied metal in biosolids) and Tl for two species and with different cultivars. Much like Cd, Tl uptake varies by type of plant and also by cultivar. If you are concerned about Tl uptake- don’t eat kale grown in Tl enriched soils.
So we know where Tl comes from. We know that it can be taken up by plants. What actually happens to people that are exposed to Tl. For that we go to the 4th paper in the library. Here a study was done comparing the health of residents from an area near a coal plant with elevated Tl concentrations and those in an urban area with no apparent source of Tl contamination. The researchers did find evidence of increased Tl exposure in both hair and urine samples in the people living near the coal plant. Health impacts included sleep disorders, headache and fatigue. The primary exposure pathway was determined to be eating vegetables from gardens in the area. In other words, Tl is not a good element from a human health perspective.
So let me talk a bit about what this means for biosolids. As we all know, for something to be detected in biosolids at concentrations that are of concern, it has to have a source. While the papers that are in the library give you basic information on Tl, there is no paper on concentrations and behavior in biosolids amended soils. If you go to the EPA Sewage sludge survey (https://www.epa.gov/sites/production/files/2015-04/documents/targeted_national_sewage_sluldge_survey_sampling_and_analysis_technical_report_0.pdf) you can find that Tl was detected in 80 of the 84 samples analyzed at concentrations ranging from 0.02- 1.7 mg kg. This suggests that biosolids generated in areas without coal burning or metal smelting facilities or highly specialized manufacturing would be very likely to fall within this range and not be a significant source of Tl to the environment. The literature also suggests that some areas may have soils enriched with Tl as a result of the soil parent material. If this is a concern, have your soil tested and based on the results of the test and your eating and gardening preferences, avoid growing leafy greens in your garden or at least avoid eating a significant amount of the leafy greens that you grow.
The final paper in the library is one that I have found very useful. It is from the USGS and reports on concentrations of a wide range of elements in soils in the US. This can be used as a baseline for questions and concerns about trace elements in biosolids. Hopefully this library will be useful both for concerns about Tl and any other elements.