Tea Bag Microplastics
Some very interesting studies of tea bag microplastics have been performed more recently very close to where you live in Cornwall, UK (Deer Park Farm) on tea bags purchased from numerous stores across Plymouth, UK over the period of July–October 2021.
https://www.sciencedirect.com/science/article/pii/S004896972402953X
I spend a lot of time reading scientific papers, but I must start by disclosing that I endorse none of them. I'm fascinated by the rigor and methods they utilize, but I will try to avoid my criticisms in this answer where they do not directly pertain to your question.
The study cited above discusses alternative plastics that have been used since the market was incentivized to switch to bioplastics years ago. The focus is on growth and reproduction of earthworms (Eisenia fetida) grown in the plastic that remains after tea bags have been buried 10 centimeters deep for 7 months. There is a lot of information there that you can read for yourself, so I won't try to reproduce it here.
I will, however, point out that the newer plant based tea bag plastics did not decompose significantly under those conditions in that time frame. Those conditions do not represent how those plastics may decompose, what secondary compounds they might produce, and how they may impact the environment when exposed to common composting methods. They buried them in some dirt. They didn't compost them. The temperature was too low and the moisture content wasn't even measured.
Although I'm very interested in microplastics and the environment, I'm temped to dismiss the tea bag plastic issue as a non-issue, or as somewhat ridiculous considering the tiny quantities involved. I would have no problem with accepting every last one of the 167 million teabags consumed in the UK annually and digging them by hand directly into my soil. What I think should be of more concern is what compounds get ingested through the guts of Brits when they make the tea and drink it.
Other Concerns regarding Cadmium
Although cadmium can be a very potent toxin and contaminant in soil, the amount you are adding when the polypropylene in your teabags degrades is considerably less than what you are probably adding from other sources. Fertilizers, tap water, and composted vegetables will add cadmium, but in most garden ecosystems that cadmium is leaving faster than it is entering.
Most of it leeches out when it rains, especially in well drained garden soil. The PH of your garden is probably slightly acidic, which also helps it to move. The organic matter, living microbes, and some fungi, as well as certain root exudates from your plants, cause the cadmium to be more bioavailable to be taken up into plants.
Although your plants don't need it for anything, they do absorb it. Wheat, lettuce, spinach, and parsley are all great for reducing the cadmium in soil. They are bioaccumulators just like your body is. This might sound horrifying to you but it's not as bad as it sounds. Cadmium is in the food chain in much higher concentrations in commercial products than in your particular garden food chain. If you regularly eat your own vegetables then your accumulated cadmium is probably much lower than those who rely on supermarkets for their food.
With two acres for you to farm, it's worth it to have a complete soil assay performed every few years to assess the content of nutrients, trace minerals, and heavy metals. You are likely to find that cadmium is either undetectable or in levels much lower than most agricultural soils.
If for any reason it is higher than you prefer it, then just make sure your PH is below 6 and grow a crop of wheat to pull it out, or simply over-water it once a year or a few times between plantings. This will of course wash out some of your nutrients temporarily, but it will also clear out accumulated salts and other contaminants that you will be glad to be rid of. If on the other hand you want to reduced the cadmium that your plants absorb, then simply raise the PH above 6.
The polyethylene in your tea bags is just a very thin film which will break down much faster than the solid plastic that contributes to the microplastic issues. It's biodegraded by microorganisms into basic hydrocarbons that will eventually improve your soil infinitesimally. More importantly, because you spread it regularly in such small amounts, it will actually contribute to the survival of necessary microbiology (fungi and microbes) that breaks down other, much more harmful, hydrocarbon compounds that may inadvertently enter your soil.
