Weathered coal, commonly known as outcrop coal, generally refers to lignite, bituminous coal, and anthracite coal close to or exposed to the surface. It is a product formed by the penetration and weathering of air, sunlight, rain, snow, wind, sand, ice, etc. China has abundant reserves of weathered coal, and weathered coal resources are available in most areas across the country.
Weathered coal has high oxygen content and low calorific value, and has lost its value as power fuel and coking coal. However, generally weathered bituminous coal contains a large amount of regenerated humic acid (HA) and a variety of oxygen-containing active functional groups, such as carboxyl groups, phenolic hydroxyl groups, quinone groups, alcoholic hydroxyl groups, etc. Generally, the humic acid content of good quality weathered coal can reach more than 50%. , thus giving new utilization value to weathered coal. Humic acid is an important organic part of most soils. At present, the overall soil quality in many places around the world is low, and soil degradation is serious. Not only has the degraded area been large, but also soil erosion, soil desertification, acidification, and salinization are continuing to expand. As industrial and urban pollution intensifies, soil heavy metal pollution and organic matter pollution are also becoming increasingly serious. Applying weathered coal to soil improvement can not only turn waste into treasure, but also effectively improve soil degradation and pollution. Currently, research in this direction has made great progress in China. Weathered coal humic acid is a mixture of various condensed aromatic hydrocarbon-based and carboxylic acid-based compounds that are soluble in dilute caustic alkali (KOH, NaOH) solutions formed by coal-forming substances through complex biochemical reactions or oxidation (including weathering). Its components are neither thermoplastic nor elastic, neither solubilizing nor crystallizing. It is an amorphous polymer colloid, mostly in a black or brown colloid state. When dry, its appearance has a shell-like fracture, and its color and density increase with the degree of coalification. Observed under a 12,000x electron microscope, humic acid is an agglomerate made up of extremely small spherical particles connected into threads, similar to grape bunches. The elemental composition of humic acid is similar to that of coal, that is, it is mainly composed of carbon, hydrogen, oxygen, nitrogen, sulfur and other elements; its molecule is a macromolecule composed of several similar structural units. Each structural unit is composed of three main parts: aromatic core, bridge bond and active group. Aromatic core: The aromatic core of humic acid is mostly composed of 5 to 6 aromatic rings or heterocyclic rings, such as benzene, naphthalene, anthracene, anthraquinone, pyrrole, furan, thiophene, pyridine and other rings. Bridge bonds: There are single bridge bonds and double bridge bonds connecting the aromatic core of humic acid. Bridge bonds include -CH2-, -O-, -S- bonds, etc. Active groups: The aromatic core of humic acid has several active groups, mainly including phenolic hydroxyl group, alcoholic hydroxyl group, carboxyl group, methoxy group, etc. The existence of these groups determines the properties of humic acid such as acidity, hydrophilicity, cation exchange performance, and metal ion complexing.
