Polyaromatic Hydrocarbons in Biochar

I have recently found some rather disturbing information that biochar contains polyaromatic hydrocarbons, a known carcinogen. I am also aware it adsorbs polyaromatic hydrocarbons from the soil as well as all sorts of other nasty stuff. The factors at play determining wether it leaches PAHs to my understanding, are out of the control of the average gardener (specific soil organisms, highly controlled and low temperature pyrolysis, specific combustion chambers) and even if you could control these things you would have very little way to know if what you did worked without a laboratory to test your end product. This makes me very reluctant to use any more biochar on my soils considering there could be as much risk as benefit. Knowing this what would you do?

Sunlight is also a known carcinogen, but I'm happy to make use of that while taking some precautions.

I don't have the biochemistry background to have really informed opinions, but reviewing the Wikipedia article on terra preta, it seems like the aspect you're worried about is touted as the engine of that material's success.

The chemical structure of charcoal in terra preta soils is characterized by poly-condensed aromatic groups that provide prolonged biological and chemical stability against microbial degradation; it also provides, after partial oxidation, the highest nutrient retention.

From other bits in that article, it doesn't seem like it's really a too-exacting process. Making charcoal in a too-hot system, for instance, burns off the potentially risky/beneficial stuff leaving stuff that's too inert to really help, but also sort of impossible to do any harm. Assuming that terra preta is man-made, I think we have to assume that people of the stone age could make biochar just fine, so how finnicky could it be?

I make biochar pretty haphazardly, but I think I'll continue to do so until I see stronger contrary evidence.

I think this is pretty good evidence that what I’m saying happens and is a risk.
https://www.sciencedirect.com/science/article/pii/S0160412018311000

“ The total ILCR for adults was above 10−6, which suggests a risk to human health from direct exposure to PAHs in vegetables grown in biochar-amended soil. These results demonstrate that biochar application may lead to contamination of plants with PAHs, which represents a risk to human health”

https://www.sciencedirect.com/science/article/pii/S0160412018311000 wrote:The total ILCR values of vegetables grown in biochar-amended soil in the greenhouse were around 10^−6; the highest value was 6.57 × 10^−6. ILCR values <10^−6 are indicative of no risk to human health, values of 10^−6 to 10^−4 indicate a low risk, and ILCR values >10^−4 are indicative of a high risk (US-EPA, 1996). The ILCR values for 10% of carrot, 40% of Chinese cabbage, 40% of water spinach, and 5% of spinach samples were 10^−6 to 10^−4, which suggests a low risk to health. The ILCR values of pakchoi, pterocladia tenuis, lettuce, and cherry radish were lower than 10^−6. The total ILCR values of the eight vegetables grown in 80% biochar-amended soils were 10^−6 to 10^−4, which indicates a low

Figuring this out required a bunch of reading and would require a bunch more to be really comfortable with the subject. Some responses:
-  In general, sources say that 10^-6 represents no additional risk. So the results of this study cluster around the lowest bound for additional risk. It seems like it's probably reasonable to assume there is a very low, but real, increased risk when using biochar the way most people do.
- They show the quantity of PAHs decreasing with a half-life of about 35 months, so to be much more careful you could e.g. inoculate your char for three years before working it into your beds. (Or maybe apply it immediately before a fallow rotation.)
- It seems like biochar increases grain yield by about 15% and vegetable yield by about 25%. If those yield gains mean that less land needs to be cultivated, or that land can be cultivated without machines,  or that farmers can spend less time absorbing the sun's dangerous radiation, that will help to offset some of the ILCR.
- It definitely seems like systems that keep pyrolysis long and cool produce safer char.
- That study also cites several ways that biochar can reduce the amount of other toxic compounds that make it into vegetables, so that has to be put in the balance as well.

I look forward to more research being done along these lines. In the meantime, we each have to figure out our own risk aversion. I'm in a position where my default is to favor microbial diversity and volume and biochar helps with that. But I also value rational assessment and scientific advance, so my mind can be changed. I guess this makes me think about being more cautious but isn't powerful enough to steer me away from biochar. You might perform the risk calculations a little differently and land on no biochar until we know more and we'll both have made reasonable decisions.

I believe, as with many things in life, there is a risk assessment that everyone needs to make for themselves.

I don't think it is a stretch that in the process of combustion, there are things that can be created that are not intended simply due to life does not occur in a vacuum. We have as much control as we have and we can always work towards a better process.

In the industrial safety world, there is a thing referred to as "The Hierarchy of Controls" when assessing possible ways of reducing risk for a task.



Ideally, we can completely remove a potentially hazardous task so nobody has the chance to be hurt. In reality, this doesn't always happen. So what is there to do? We do our best to mitigate and manage the risk.

For me, when I produce biochar, I make liberal use of personal protection equipment. I utilize gloves to prevent skin contact with biochar as well as utilize a mask to deal with dust. After I am done working with biochar, I ideally will take a shower to wash away any residual dust that might be on me.

This works for me, even with the acknowledgement of potential hazard, and I think others might feel similarly.

In my mind, do what is right for you and yours!

This is a relatively well-researched subject in the world of biochar. Short answer: yes, biochar production can result in the formation of PAHs. There is a correlation between temperature and PAH formation, and for most of us using artisan methods like flame cap this is good news, because we're not achieving the higher temperatures where the gas phase reactions really start to happen. I guess there is also a risk of contamined feedstock, but again most of us aren't putting known hazardous materials into our burns.

Harm is in the dose. Standards for biochar destined for animal feeds are typically under 10 mg/kg and applications to soil or water under 20. But the more important attribute is bioavailabilty, and this is why most of us don't get too worked up about it. Organic contaminants are adsorbed onto the biochar substrate, and they hold on tight because they are such a good fit chemically with the graphene nanostructure. This happens during the process of pyrolysis, so by the time you're quenching the PAH molecules are basically immobilised and the only way to knock them loose is with organic solvents like ketones.

Fossil biochars from millions of years ago have been tested and the PAHs that formed alongside them were still bound to the carbon matrix. The concentrations measured are very similar to what we see in biochar made in the present. So that says to me that it's always been around, and the environments where biochar plays a role have evolved in its presence. So I'd call it low risk on that basis.

Well thank you very much to all of you! How very informative and useful. I will now continue to happily use my biochar.

We haven't delved into biochar in our growing paradigm and I'm not well-familiar with its pros and cons.  As Phil S. has mentioned, there is a fair amount of research done on the issue of PAH production not only during deliberate biochar formation, but also from natural prairie and forest fire.  The linked review deals more with microbial formulations to deal with the bioremediation of soils contaminated with pollutants, including PAH's, but upon reading it I found myself wondering how far out from time zero the studies on biochar have been done when analysing PAH content in the soils.  Across different environments and microbial populations, how stable are the PAH's in their toxic form over time and might they undergo microbial degradation or modification to lesser toxicity/carcinogenicity in a soil-specific manner?  

https://pmc.ncbi.nlm.nih.gov/articles/PMC11434427/pdf/microorganisms-12-01814.pdf

Do the benefits outweigh the negativity?

Did you see Phil Stevens' post above? These are good questions to ask, IMHO.  If the biochars in soil are present but not really leaching, it doesn't appear that they are affecting us. Biochar has been known to absorb and adsorb some pretty nasty chemicals. It's why it's used in filters.  One of my main thoughts on this topic is about how humans have had fires go through their living areas since time began.  Tribal indigenous societies have in general, allowed fire and seen it as part of nature.  The Indigenous at Terra Preta weren't harmed by their soil. They thrived.  Diseases of the White Man killed them.  Since they don't have homes that cost $400, 000, they just move during the fire and perhaps afterwards. Burned organic material has been in the soil forever.  Much recent literature suggests that it was at least tolerated and perhaps done intentionally.  The Native Americans where I live intentionally burned the valley floor every year.   It seems to me that all of the recent science is revealing how indigenous people have generally understood the value of the soil and of nature, such as the books 1491 and 1492. it appears to me that they mostly have and still do understand the value of dynamics such as biochar and soil microbiology.  Rather than freaking out and retreating into stopping something that has benefited so many people, with so few downsides, I am going with the likelihood that, as Rachel Carson said, the balance of nature will take care of it.

John S
PDX OR

Interesting to see surprise here.  My understanding from the first that what made char durable in the soil was those very same hydrocarbons keeping it from rotting.  My understanding is most of it is tied up in the physical and chemical bonds in the char itself.  This is why the char can be effective for millennia.

Interesting.  My (basic) understand is that biochar is mostly of benefit in marginal fertility soils?

For the science part of biochar, this is one study that could be referenced from 2021.

9. Conclusions
Biochar can improve agricultural soils in a variety of ways. These methods include, but are not limited to, improving water holding capacity, improving soil stability through addition of aggregates and solids, increasing microbiome populations and controlling fungi populations, reducing the need for fertilizer and reduce fertilizer leaching.

I would like to emphasis this part of the study conclusion as well.

It is essential for the future of biochar application to learn how to manipulate biochar properties so as to tailor the amendment to each region, climate, crop type and soil. Overall, the literature indicates that biochar has beneficial effects on soil quality and crop yields, but possible constraints need to be explored.  

While this is old 'technology', this is still new to the rigors of scientific study and requires folks to gradually unravel the mystery of the complexities of it's function. Biochar isn't just biochar. How the biochar is made, from what it is made, and how it is handled can potentially lead to different properties is my interpretation.

I'm personally comfortable utilizing it in my garden experiments.

Wonder how it compares to the BBQ I make/eat?

I agree with Timothy Norton. Biochar helps clay soils to drain better. It helps in areas of low rainfall, bcause it absorbs 6 times its volume of water. It improves the flow of nutrients thru the mycelium of particular nutrients. It is housing for microbes can balance the action in soils.

John S
PDX OR