Thermal activation and physicochemical characterization of lignite coal for enhanced adsorption of dyes and petroleum products from water

The increasing contamination of global water resources by persistent organic dyes and petroleum-derived hydrocarbons represents a critical environmental challenge with far-reaching ecological and public health implications.This comprehensive investigation evaluates Uzbek lignite coals as sustainable, low-cost adsorbents for aqueous pollutant remediation, focusing on raw coal and brown seed coal from the Angren deposit.Through systematic thermochemical activation (400-1000°C) coupled with advanced material characterization, we demonstrate how activation parameters govern the development of critical adsorbent properties.Proximate analysis revealed distinct compositional profiles, with raw coal containing 12.2% ash and 35% volatile matter versus BSC's 26.4% ash content and enriched oxygen-functional groups (1.7 mmol/g OH).Thermogravimetric analysis identified three-stage decomposition mechanisms, while XRD confirmed the formation of quartz (SiO2) and hematite (Fe2O3) crystalline phases in activated products.FTIR spectroscopy delineated temperature-dependent evolution of aromatic C-H (680-700 cm -1 ) and carbonyl (1550-1610 cm -1 ) functionalities, correlating with enhanced adsorption capacity.SEM imaging revealed RC's superior macroporous architecture (pore diameter >50nm) compared to BSC's denser morphology, directly impacting contaminant uptake kinetics.Our findings establish that controlled thermal activation transforms lignite into effective adsorbents, with RC demonstrating particular promise for large-scale water treatment applications due to its balanced porosity (22.8-32.5% mass loss during activation) and mineral composition.This work provides actionable insights for developing regionally-sourced, economically viable purification materials that address both environmental and resource-utilization challenges in Central Asia Keywords: lignite coal, thermal activation, adsorption, water purification, physicochemical characterization, activated carbon, dyes role in its industrial processing and transformation [1-5].Sub-bituminous (lignite) coal is a dense, carbon-mineral mass with a dark brown color.It contains 50-77% carbon, up to 40% moisture, and up to 50% volatile matter.It burns easily with a smoky flame, produces a dark brown liquid when treated with KOH, and releases ammonia and other volatile products during dry distillation [6].The global production of activated carbon (AC) has significantly increased between 2010 and 2020, particularly in China, India, and Indonesia.The primary raw materials for AC production are bituminous and sub-bituminous coal, due to their high yield and low ash content.Anthracite is minimally used because of its high density and low reactivity, while lignite is limited due to its high moisture and volatile content.Since 2012, the share of coal-based AC has increased from 26 to 61%, driven by stringent environmental regulations [7][8][9][10].China leads in coal-based AC production, and further growth is projected.The expansion of the activated carbon market is driven by strict environmental policies, particularly in the United States and China, as well as its widespread application in water treatment.The Asia-Pacific region dominates both production and consumption of activated carbon.However, market growth may be hindered by the high cost of coal and the development of alternative adsorbents, such as silica gel.In China, over 10,000 water treatment facilities are operational, while in the United States, federal water protection laws regulate wastewater treatment [11].Lignite remains an important resource due to its availability, diverse physicochemical properties, and broad industrial applications.Various activation techniques enhance the adsorption and catalytic properties of coal, making it effective for the purification of water, air, and soil.The relevance of research in this field is driven by the increasing demand for cost-effective sorbents.To improve efficiency, carbon materials are modified using chemical or physicochemical treatments, which increase specific surface area, reactivity, and introduce functional groups [12].Modern methods for producing activated carbon (AC) rely on thermal and chemical activation, which generate a well-developed porous structure.Steam-gas activation occurs at 500-1000 °C in the presence of steam, CO2 or air.

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