Kraft pulping, which constitutes approximately 90 % of the pulping process, produces over 1300 million tons of black liquor (BL) annually, whose management and disposal pose some serious environmental concerns [1]. During this procedure, the wood undergoes extraction of cellulose using a heated solution comprising sodium sulfide (Na2S) and sodium hydroxide (NaOH), resulting in the production of a large amount of BL as a waste by product. This BL contains organic and inorganic substances such as lignin, hemicellulose, carboxylic acids, phenolics, NaOH, Na2S2O3, and Na2SO4. [2,3]. According to a statistical data report, approximately 5000 tons of lignin, along with BL, is discharged into waterbodies yearly [4]. The haphazard or incineration discarding of BL leads to the emission of toxic gases, negatively impacts the environment [5]. Traditionally, it has been used for combustion in kraft mills to generate heat and energy. However, the requirement for supplementary auxiliary fuel and its relatively low calorific values make it inconvenient to use [6]. Therefore, efforts are being made to innovate alternative approaches to turn this carbon-rich waste material into valuable products.

In recent years, there has been a significant upsurge in scholarly interest in discovering optimal strategies for the efficient utilization of BL. This has resulted in diverse research efforts involving the recovery of lignin monomeric compounds [7], synthesis of graphene sheets [8], porous polymeric materials [9], production of tall oil [10], recovery of carboxylic acids [11], and biofuels production [12]. However, all these methods entail high processing costs and the usage of chemicals. An alternative viable option for utilizing BL is the conversion into activated carbon (AC). AC is the class of porous material known for its excellent adsorption capacity, and it has been widely studied due to its easy preparation. To date, various waste materials have been utilized to produce AC, such as used tires [13], municipal solid waste [14], waste office paper [15], various types of biomasses [[16], [17], [18]], waste plastic [19] and explored their applications in different areas.

The conversion of BL into AC is especially appealing due to its abundant carbon content, largely attributed to the presence of lignin, a carbon-rich component [20]. This has led researchers to investigate the feasibility of producing AC by extracting lignin from BL. Numerous studies have reported the successful preparation of AC by extracting lignin from BL [4,21,22]. Nevertheless, it is imperative to highlight that the extraction of lignin from BL necessitates the usage of hazardous chemicals, which not only pose an environmental risk but also add to the overall cost of producing AC.

In this regard, the direct utilization of BL for AC preparation can compensate for financial outflows associated with extracting and processing lignin separately while minimizing the involvement of chemicals. Moreover, the parent pulping process imparts a substantial amount of alkali content (sodium and potassium) to it, thereby enhancing the activation process. It is worth noting that potassium hydroxide (KOH) and sodium hydroxide (NaOH) are well-known activating agents in the synthesis of activated carbon due to their role in enhancing reactivity and porosity, which results in a greater surface area [23]. However, despite these potential advantages, few studies have used the direct approach to prepare activated carbon from lignin. The study conducted by Zhao et al. focused on the preparation of activated carbon directly from the BL and evaluated the effect of carbonization temperature, activation temperature, and activation time on the electrochemical properties of ACs [24]. Tian et al. utilized activated carbon derived from BL for wastewater treatment [25]. Volperts et al. Prepared N-doped AC from BL having 2672 m2/g surface area and reported a comparable oxygen reduction activity to the commercially available 20 % Pt/C catalyst [26]. Zhu et al. utilized rice husk-derived biochar to synthesize nitrogen-doped activated carbon with an exceptional surface area of 2646 m2/g and a specific capacitance of 337 F/g in a 6 M KOH electrolyte [27]. However, these studies involved an additional carbonization step to achieve the high surface area of the activated carbon, which necessitates supplementary energy consumption.

The present study involves the direct conversion of BL into AC using a hydrothermal-assisted activation, which eliminates the high-temperature pre‑carbonization step. The properties and performance of BLAC were systematically compared with ACs derived from BL extracted lignin and commercially available kraft lignin as part of the investigation.

Various characterization techniques, including X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM), N2 adsorption-desorption isotherms, and X-ray photoelectron spectroscopy (XPS) were employed to assess the structural, textural, and chemical properties of ACs The electrochemical activities of the ACs were analyzed via cyclic voltammetry (CV), galvanic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). The findings concluded that BLAC demonstrated superior electrochemical properties due to its higher surface area, which resulted from the activation process facilitated by the internal inorganic salts and alkalis. By addressing the carbon footprint and reducing additional energy demands in inherent chemical conversion processes and pre‑carbonization, this study adopted a carbon-neutral endeavor, rendering the entire process both eco-friendly and sustainable.