What Is a Biosolid?

Biological minerals are inorganic mineral materials in animals and plants, such as bones, teeth, mollusk shells, plant vascular bundles and so on. Biominerals are formed under specific biological conditions, and thus have special advanced structures and assembly methods. The main biological inorganic solids present in biological organs. Insoluble calcium salts, such as carbonates and phosphates, are found throughout the biological world. Many precipitates are used as support structures or special hard tissues, some of which appear in animal bones or other hard parts.

After 2 billion years of natural selection and optimization, the structure of the organism is almost perfect. In addition to the formation of biologically active complexes, metal ions that are ingested by organisms also become important components of hard tissues such as bones through the formation of biological minerals; such as hydroxyapatite and calcite. The rocks are the same and are therefore called "biominerals".

In nature, calcium was chosen to build the lithosphere, and the insoluble salts formed by calcium were used to support organisms. There are more than 60 types of minerals known in the body so far, and calcium-containing minerals account for about half of the total. Among them, carbonate is the most widely used inorganic mineral, followed by phosphate. Calcium phosphate (including hydroxyapatite, octacalcium phosphate and amorphous calcium phosphate) mainly constitutes the internal bones and teeth of vertebrates; calcium carbonate mainly constitutes the exoskeleton of invertebrates. Compared with natural minerals with the same composition, because biological minerals are controlled by special biological processes and special biological environments, they often have extremely high selectivity and directionality, so the resulting crystals exhibit special properties, such as having extremely High strength, good fracture toughness, shock absorption performance and special functions. In addition to the two basic functions of protecting and supporting biological minerals, there are many other special functions. For example, in calcium carbonate minerals, calcite is the photoreceptor of trilobites, and in mammals' inner ears, it acts as a gravity and motion sensor. Stone is used as a buoyancy device in cephalopod shells, but in most cases it is found in the exoskeleton of mollusks like calcite.

In addition to the composition of biological organisms, some biological minerals are the products of biological pathological processes, such as calcium oxalate is the main mineral component of human urinary stones. Only by understanding the process of calcium oxalate forming stones in the body, can we find a way to treat and even prevent the occurrence of urinary stones. ^[1]

The formation of biological minerals is very complicated, and many mechanisms, especially dynamic processes, are still unclear. We will discuss the formation reaction of hydroxyapatite using the principle of precipitation reaction.

Hydroxyapatite (1) Origin of Hydroxyapatite

Hydroxyapatite is a component of bones and teeth. So how does hydroxyapatite precipitate out of solution?

Under physiological conditions, the main forms of phosphate ions are HPO ₄ ^2- and H ₂ PO _4- .

H ₂ PO _4- + H ₂ O HPO ₄ ^2- + H ₃ O ⁺

In the system, HPO ₄ ^{2- is the} main existence form; therefore, HPO ₄ ^2- is called orthophosphate in biochemistry, abbreviated as Pi, and its possible reactions with Ca ²⁺ include the following.

Ca ²⁺ + HPO ₄ ^2- + 2H ₂ O CaHPO ₄ · 2H ₂ O

3Ca ²⁺ + 2HPO ₄ ^2- + 2OH- Ca ₃ (PO ₄ ) ₂ + 2H ₂ O

8Ca ²⁺ + 6HPO ₄ ^2- + 4OH-+ H ₂ O Ca ₈ H ₂ (PO ₄ ) ₆ · 5H ₂ O

10Ca ²⁺ + 6HPO ₄ ^2- + 8OH- Ca ₁₀ (OH) ₂ (PO ₄ ) ₆ + 6H ₂ O

The solubility of various forms of precipitates (expressed as Ca ²⁺ concentration) at different pH conditions can be calculated from K _sp precipitated by poorly soluble strong electrolytes, as shown in the figure.

Although the minerals contained in hard tissues are equivalent to natural minerals in terms of composition and crystallization. However, living hard tissues differ from inanimate minerals in the following ways.

(1) The hard tissue is highly ordered in structure.

(2) Minerals in hard tissues are formed in the organic matrix and embedded in the matrix.

(3) Minerals in hard tissues not only participate in calcification-decalcification balance, but also participate in cellular activities.

(4) Minerals in hard tissues are formed during the entire metabolic process of the organism, and they participate in the metabolic process. ^[2]

There are three types of biological minerals in hard tissues.

1. Calcium phosphates

The most important inorganic constituents in biological minerals are calcium phosphates. Among them, apatite hydroxyapatite [Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ ] is the most common, and its OH ^- is replaced by F ^-to form fluoroapatite [Ca ₁₀ (PO ₄ ) ₆ F ₂ ]. The organism continuously takes in calcium and phosphorus from the environment. It continuously accumulates Ca ²⁺ and PO ₄ ^3- in the body and outside the body. After reaching a certain concentration, it combines with H ⁺ , OH ^- and H ₂ O to form different types of phosphates. Common biophosphates are listed in the chart.

Common biophosphates

Different forms of calcium phosphate are basically the same as calcium phosphate in minerals. These different calcium phosphate salts can be converted into each other after they are formed. If all the calcium salts in the solution are supersaturated, the fastest crystallization rate will be precipitated first. Under normal physiological conditions, amorphous calcium phosphate is first formed and quickly converted to octacalcium phosphate, and finally converted to the most stable and stable hydroxyapatite with K _ap . Hydroxyapatite is the main phosphate component in biological minerals and is called biological apatite.

More than 90% of Ca and P in the human body are present in the bones, and there is a dynamic balance between calcification and decalcification in the blood:

10Ca ²⁺ + 6PO ₄ ^3- + 2OH- Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ (s)

This balance maintains the normal metabolism of calcium and phosphorus in the body, and once out of control will cause a variety of diseases. If excessive fluorine enters the body, it can combine with calcium to form CaF ₂ deposited in bone tissue, locally change the hydroxyapatite to fluoroapatite, destroy the balance of calcification and decalcification, and cause fluorosis (osteosclerosis and osteoporosis) Decreased blood calcium enhances parathyroid function, inhibits renal tubules from absorbing phosphorus, and a large amount of phosphorus is lost from urine, resulting in disorders of calcium and phosphorus metabolism.

2. Calcium carbonate

The calcium carbonate minerals existing in the biological world are aragonite and calcite, which are both typical ionic lattice structures. There are 6 CO ₃ ^2- oxygen coordinations around Ca ^{2+ in} calcite; and 9 CO ₃ ^2- oxygen coordinations around Ca ^{2+ in} aragonite. The mammal's otoliths are calcite single crystals, and the gastropod shell nacre has a aragonite structure.

3 Silica

Most of the silica in the mineral is crystalline silica, but in biology it is usually amorphous silica, which is a hydrate of silica. ^[2]