1. Introduction

Forests are the dominant terrestrial ecosystem on Earth, accounting for 31% of the total land area [1]. The world’s forested area is 4.06 billion hectares. Tree growing is the main task of forestry, as a result of which a large amount of renewable raw materials are produced annually. Most of the world’s forests (54%) are located in the Russian Federation (815 million hectares), Brazil (497 million hectares), Canada (347 million hectares), the United States of America (310 million hectares), and China (220 million hectares), according to the data provided by the Food and Agriculture Organization of the United Nations (FAO) for 2020 [1].

The Krasnoyarsk Territory is 69.3% covered with forests and is one of the leading forested regions in the Russian Federation. The total forest resources of the Krasnoyarsk Territory amount to 164 million hectares [2]. The main forest-forming species in the Krasnoyarsk Territory are conifers; they occupy more than 75.9% of the forested area, which amounts to 9.7 billion m 3; the share of spruce and fir amounts to 16.0% (1.6 billion m 3) [3].

The most important feature of Siberian forests is the preservation of natural plantations in large areas that are relatively weakly exposed to human anthropogenic impact. Forest ecosystems have a natural (background) level of biodiversity and represent standards for population, species, and ecosystem diversity. These forests are fundamentally different from the “cultivated” forests represented in Western Europe and Russia by artificial plantations with reduced resistance to adverse environmental factors and climatic conditions [4].

Plant waste is a major source of environmental pollution. Millions of tonnes lay sitting in dumps for many years, creating additional pressure on the environment, as the bark decomposes slowly under natural conditions. In summer, bark dumps pose a fire hazard. In addition, dumping territories are excluded from the economic turnover. Burning the bark is not cost-effective due to its low heating value and its high ash and moisture content. It partially decomposes when stored for a long time, forming phenolic compounds, which are washed off by precipitation and meltwater. Pollution of the environment by these compounds can lead to disruption of the biological balance between individual links of biogeocenoses and thereby cause great damage to the national economy. At the same time, tree bark contains valuable extractives, and large-tonnage bark waste is a huge raw material resource for manufacturing expensive chemical products ^[5][7]. The problem of wood waste recycling is a weak point in integrated raw material processing ^[6][7][8][9][8,9,10,11].

2. Use of Coniferous Bark

Despite the high content of tanning substances in bark, barking waste from pulp and paper plants and wood processing plants is not used as raw material for tanning agents production due to the high content of wood impurities (up to 30%). This raw material composition leads to a decrease in quality of obtained extracts.

A modified type of bark is used for air purification as a biofilter, as well as for water purification, which allows for the binding of poisonous ions of lead, cadmium, mercury, and zinc ^[10][23].

In addition to the above, bark is a good lignin-carbohydrate complex that can be used as a sorbing agent for collecting oil products from the surface of polluted water bodies.

Bark can be used in new lignocellulosic composites ^[11][33]. Crushed bark is used as a filler in polylactic acid (PLA) composites, which have higher strength indicators and low moisture resistance compared to High Density Polyethylene (HDPE) composites ^[12][34]. In thermoplastic polymers, tree bark can be considered a filler, which leads to an increased heat capacity and thermal conductivity ^[13][35].

3. Chemical Composition of Coniferous Bark

The results presented in Table 1 indicate that the chemical composition of larch bark differs markedly from other softwood bark. First of all, this refers to the content of water-extractable substances (as well as suberin and phenolic acids). The content of these components in larch bark is 1.5–3.0 times higher than in other types of softwood. At the same time, it has a low content of easily hydrolyzable polysaccharides, 1.5–2.0 times lower than others.

Thus, bark contains a large number of elements that are necessary for tissue growth, with the exception of nitrogen and sometimes phosphorus.

The composition of extractive substances of the bark includes compounds of various classes: aliphatic and aromatic hydrocarbons, polyphenols, tannides, fatty acids, sterols, terpenoids, monosaccharides, pectin substances, and a number of other compounds.

The content of the main components in the bark of various types of larch is shown in Table 2 .

4. Extraction Processing of Coniferous Bark

The table shows that fir, larch, and cedar bark is the richest in phenolic compounds. Larch bark is represented by almost all classes of flavonoids, from the flavanone naringenin to bioflavonoids, proanthocyanidins, and condensed tannins. The phenolic complex of larch bark is represented by phenolic acids and their esters; it contains monomeric flavonoids, spiroflavonoids, and oligomeric and polymeric flavonoid compounds.

According to previous studies, it has been established that Siberian larch bark is a great source of unique biologically active phenolic compounds, the quantitative content of which can reach up to 8–12% of absolute dry matter. The phytocomplex extracted from larch bark shows an antioxidant activity 1.5 times higher than dihydroquercetin ^[20][71]. Toxicopharmacological evaluation has shown that the antioxidant complex extracted from larch bark with ethyl acetate has a pronounced capillary-strengthening effect, which surpasses that of dihydroquercetin.

The main task for tanning substance extraction is the correct selection of an extraction agent that maximizes the substance extraction from the raw material.

The use of monoethanolamine as an additive to the main extraction agent makes it possible to extract from softwood bark, for example, larch bark, up to 50% of phenolic substances, which makes the obtained extracts promising for further processing ^[21][96].