Why are bauxite and aluminum toxic to organisms?

We all know that aluminum has no apparent use for living beings. Its toxicity lies in its ability to solubilize and diffuse rapidly into the environment, entering and interfering with biomolecules in tissues and organs. One of the largest sources of aluminum is bauxite, which can release this element through acid mine drainage. Learn more about how this happens in this text.

Keywords: bauxite toxicity, aluminum toxicity, geobiology

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Any rock enriched with mostly aluminum hydroxides is called bauxite. These hydroxides follow the general formula of AlOx(OH)3-2x, according to Wikipedia, and the “x” should be equivalent to any value between 0 and 1. Honestly, I do believe this formula because it makes sense with all the minerals that can make up bauxite (at least from what I have managed to understand, I explain it below, sorry if my deductions are wrong). This formula is also equivalent to another one that is often circulating out there, Al2O3 · nH2O, but this one is somewhat incorrect because it oversimplifies the structure of bauxite minerals, and also makes it seem like they are oxides, not hydroxides.

There are 3 main aluminum hydroxides that can make up bauxite:

• Gibbsite: Al(OH)3, which according to the general formula, would be the same as: AlOo(OH)3-2(0), which by calculation, would mean that the oxygen in “AlO” does not really exist, and on the part of the hydroxide, since it is clear that “3 – 2(0) = 3 – 0 = 3”. And that is why it is simplified to Al(OH)3. But as I said, this deduction may be wrong. The following is more reliable: according to the oxide formula, it would be Al2O3 · 3H2O. If you do the multiplication in the water molecule, you would get that 3H2O = H6O3 = O3H6, to put it in hydroxide mode, and adapting it to the general formula, it would be: Al2O3 (O3H6). Even more simplified, it would be: Al2O6H6. Wanting to leave it as hydroxide: Al2 (OH)6. And we can summarize it even further, dividing the subscripts by the same value (in this case, 2), finally leaving: Al(OH)3.

• Bohmite and diaspore: γ-AlO(OH) and α-AlO(OH), respectively. But it's clear that the general formula for both is AlO(OH). The same logic applies: according to the oxide formula, it would be Al2O3 · H2O. Trying to add it all up, it would be: Al2O4H2. Expressed as hydroxide: Al2 (O4H2). If we divide all the subscripts by 2, we get: Al(O2H). And this way of expressing hydroxide is unconventional; ideally, it should remain as a clean "OH". What we can do to fix it is simply remove one of the oxygens and place it next to the aluminum. This gives us the formula under the general scheme: AlO(OH). If instead we wanted to arrive at that formula by starting directly with the general formula, I think it would be using x=1: AlO1(OH)3-2(1), which would give: AlO(OH)3-2, leaving again: AlO(OH).

But anyway, discussing the chemical formulas of aluminum hydroxides in bauxite was not the purpose of this article. Bauxite is dangerous to organisms primarily because of its aluminum, especially the Al3+ ion, which is highly reactive with the biomolecules of living organisms (meaning it can disrupt their order and cause their functions to fail). In wastewater treatment bacteria, its presence decreases the absorption of other essential metals such as calcium, magnesium, or iron, in addition to hindering cell membrane formation and slowing bacterial growth.

If you think about it, it's strange. Aluminum is one of the most abundant elements in the Earth's crust; in fact, it is the third most abundant and ubiquitous metal in the Earth's crust, accounting for 7% of its mass after oxygen and silicon. It has no known biological use, but it is clearly present in living beings simply because it is a common element in soils. In mineral terms, it is usually incorporated into soils in precipitated form as gibbsite (one of the components of bauxite) or in the form of harmless aluminosilicates. When soils become acidic (pH below 5), gibbsite and other ions can decompose and release Al ions, especially Al3+, which can then solubilize in the soil and disperse more quickly, expanding their toxic effect. This is why acidic soils with some degree of aluminum contamination are so detrimental to plant growth: dissolved aluminum quickly enters plant tissues, interfering with the cytosol, cytoplasm, and cell walls. The effects are usually noticeable in the roots, which do not grow as they should.

It seems that the problem that causes the increase in aluminum solubility is soil acidification. Mining is precisely one of the activities that most acidifies soil. Especially in those where sulfide-rich minerals are extracted, the oxidation of these compounds releases metals (many of them heavy) and sulfuric acid (responsible for acidification) into the soil, and, worse still, into nearby waters. This phenomenon is known as acid mine drainage (AMD). AMD can also be mediated by acidophilic bacteria naturally present in the environment. Acidity can drop to a pH of 3, which is definitely lower than the 5 required for aluminum's toxic effect.

Sulfides are common in almost all soils, one of the most abundant being pyrite (FeS2). The alteration of soils containing sulfides, for example, during mining by exposing them to air, makes them susceptible to oxidation, which results in AMD, as I mentioned in the previous paragraph. Sulfides can be associated with bauxite ores, such as pyrite itself (with gray bauxite, for example), and this is not only an environmental problem but also a technical one, because if it exceeds 0.8%, it already affects the quality of the alumina (Al2O3) obtained from bauxite using the Bayer process, which consists of dissolving bauxite in hot sodium hydroxide (NaOH).

In conclusion, bauxite mining can expose sulfides to oxidation, which causes AMD, which can then attack other compounds in the soil, for example bauxite itself, releasing Al3+ ions that are poisonous to organisms. Another toxic consequence of bauxite mining is the Bayer process used: in addition to the oxides, it also leaves behind a soluble solution of sodium aluminate [NaAl(OH)4], along with residual sludge rich in iron oxyhydroxide (BRR). Both increase alkalinity (pH from 11 to 13), reduce the solubility of essential plant nutrients, and generally create uninhabitable environments, in addition to the excessive amount of Na+ ions.