An assessment of the environmental sensitive geochemical anomalies in Cenozoic lignite deposits of Sonari, Barmer Basin, Rajasthan (India) using advanced technologies

Deep underground in the arid landscape of western India lies a layer of earth that tells a story of ancient seas, shifting continents, and the slow, quiet transformation of plant matter into fuel. This is the Barmer Basin, a geological depression where, millions of years ago, the land was a wet, swampy environment teeming with life. Over time, layers of sediment buried this organic material, squeezing and heating it until it became lignite, a soft, brown coal. While lignite is a valuable energy source for a developing nation like India, it is not just a pile of fuel. It is a complex mixture of carbon, water, and a wide array of other elements trapped within its structure. Some of these elements are harmless, but others are toxic. When coal is mined, transported, or burned, these hidden ingredients can escape into the air, water, and soil, posing risks to the plants, animals, and people living nearby. Understanding exactly what is inside a specific coal deposit, and how those ingredients are held together, is essential for managing the environmental impact of using this resource.

In a recent study, a team of researchers from several Indian institutions turned their attention to the Sonari lignite mine in the Barmer district of Rajasthan. Their goal was to create a detailed chemical and physical map of this specific deposit. They wanted to know not only how much energy the coal could provide, but also which specific elements were present, in what quantities, and how they were distributed throughout the rock. By examining the coal from the very top of the seam to the very bottom, the scientists aimed to uncover the history of its formation and predict how it might behave if used in power plants or left exposed in a mine.

The researchers began by collecting fifty-three distinct samples from the working faces of the mine, taking care to capture the full vertical range of the coal seam, which stretches about fourteen meters thick and sits roughly seventy to one hundred meters below the surface. They grouped these samples into fourteen composite bands based on their visual appearance, treating each band as a single representative sample for analysis. The coal itself appeared as stratified, matrix-rich layers, mostly black in color, though one band contained visible resin. Under the microscope, the team observed that the coal was primarily made of huminite, a type of organic matter derived from plant tissue, which made up more than half of the material. The rest consisted of smaller amounts of other organic components and inorganic minerals like pyrite, a shiny iron sulfide often called "fool's gold," and various clay minerals.

When the team analyzed the chemical composition, they found the lignite to be of moderate quality. It contained a significant amount of volatile matter, which is the gas and vapor released when the coal is heated, making it relatively easy to ignite. The ash content, which is the non-combustible residue left behind after burning, was relatively low, averaging around thirteen percent. However, the sulfur content was moderate, which is a critical factor because burning sulfur releases gases that can contribute to acid rain. The most striking findings, however, concerned the trace elements—tiny amounts of metals and minerals embedded within the coal. The researchers compared their findings to the average composition of brown coal found around the world and discovered that the Sonari lignite was unusually rich in several specific elements.

The concentrations of cobalt, chromium, copper, nickel, vanadium, and zinc were significantly higher than the global average. In some cases, the levels were nearly forty times higher than what is typically found in world coal. For instance, the amount of cobalt was found to be about thirty-eight and a half times the world average, while zinc was nearly eight times higher. Other elements like tin, vanadium, and nickel were also present in quantities several times greater than the norm. These elements are not just numbers on a chart; they are chemically active. When coal is burned, these metals can be released into the atmosphere as fine particles or fly ash, which can travel long distances before settling on the ground or into water bodies. Once released, elements like chromium and nickel can be toxic to plants, affecting their growth and photosynthesis, while others like lead and cadmium can accumulate in the human body, potentially causing damage to the nervous system and kidneys.

The study also revealed how these elements are held within the coal. Some are tightly bound to the organic plant matter itself, while others are trapped inside tiny mineral grains like pyrite, clay, or calcite. The researchers found that elements such as cobalt, chromium, and zinc were largely associated with the organic material or with sulfide minerals. This distinction is important because it influences how easily these elements can be released. If an element is locked inside a stable mineral grain, it might stay put during mining, but if it is bound to the organic matter, it could be more readily released during combustion. The team also identified specific minerals like illite, kaolinite, quartz, and gypsum within the coal, confirming that the coal formed in an environment where seawater had influenced the sediment, bringing in salts and minerals that became part of the rock.

The implications of these findings are significant for the region. The Sonari lignite deposit is a major source of energy, but its unique chemical fingerprint means that its use requires careful management. The high levels of certain toxic metals suggest that if this coal is burned without proper filtration and pollution control, it could release hazardous amounts of these elements into the environment. The study highlights that the coal is not a uniform substance; its composition changes from the top of the seam to the bottom, meaning that different layers might pose different risks. The researchers concluded that while the coal is a valuable resource, its extraction and use must be accompanied by a deep understanding of its environmental sensitivity. By knowing exactly what is inside the rock and how it is arranged, engineers and policymakers can make better decisions to protect the local ecosystem and public health, ensuring that the energy gained does not come at an unacceptable cost to the living world.