Nowadays, the discharge of organic pollutants in wastewater has gradually become an environmental issue that has attracted much attention. Solving the pollution of water by organic dyes and antibiotics is one of the important challenges to be rose to urgently. Various important fields use organic dyes because of their color rendering properties such as cosmetics (Huang et al., 2015), papermaking (Katheresan et al., 2018) and leather (Zhao et al., 2011). Most organic dyes have complex aromatic structures and are refractory, carcinogenic, and mutagenic (Moussavi et al., 2016; Wang et al., 2016). Methylene blue (MB), as a common cationic dye, is widely applied in industrial and food production. Long-term exposure to MB-rich waste fluid will not only harm aquatic plants and organisms (Tan et al., 2015), but also cause serious damage to the human body, such as causing vomiting, allergic reactions and damage to the kidneys and liver (Jiao et al., 2022; Kundu et al., 2018). Similarly, antibiotics have attracted extensive concern in recent years because of their detection in surface water and food chain (Liu et al., 2017). Among them, tetracycline (TC) is extensively applied to animal husbandry and aquaculture. Nevertheless, most TC is difficult to metabolize through digestive systems of living organisms and can only be excreted in urine and feces, thus TC is released into the aquatic system in its original form (Leong et al., 2016). The TC released into the water environment will not only harm human health and ecological balance, but also cause drug resistance of micro-organisms (Yu et al., 2017). Therefore, how to efficiently and quickly remove organic pollutants in water environment has become the key of water pollution research.
At present, several methods including adsorption (Zhang et al., 2022), membrane separation (Zhang et al., 2019), photocatalytic degradation (Wu et al., 2022), biodegradation (Xiong et al., 2017) and redox (Kueasook et al., 2020) have been fully developed to treat wastewater rich in organic pollutants. Among them, adsorption method is regarded as an attractive approach to remove organic pollutants from wastewater because of its advantages of low energy consumption, simple operation, environmental friendliness and no secondary pollution (Tian and Zhou, 2022). Various materials have been developed to adsorb organic pollutants, such as graphene oxide (Salas et al., 2010), metal-organic frameworks (Wang et al., 2018), zeolite (Seredych and Bandosz, 2007) and carbon nanotubes (Cheng et al., 2019). However, these materials still face problems such as cumbersome preparation process, poor selectivity, insufficient adsorption capacity and secondary pollution (Chen et al., 2017). Compared with these adsorbents, porous carbon is regarded as the most effective adsorbent to remove organic pollutants due to its relatively low investment, high modifiability, huge surface area and abundant porous structure (Kundu et al., 2018). For example, Kueasook et al. (2020) prepared multi-layered porous carbon from bagasse with a maximum MB adsorption amount of 100 mg/g. Zhao C. et al. (2020) published an environmentally friendly preparation of reed bioporous carbon by doping sludge for TC removal in aqueous solution (qe max = 153.4 mg/g).
Lignin, as a special class of renewable polyhydroxyl aromatic polymers, is widely present in plant cell walls (Bz et al., 2020). The abundant oxygen-containing groups in lignin, such as hydroxyl, methoxy, carbonyl, etc., show broad application prospects in terms of designability, adsorption performance, dispersibility and affinity for organic pollutants (Cedap et al., 2020; Zhao et al., 2021). Nevertheless, almost all lignin has been burnt as fuel, which not only wastes resources, but also leads to a large number of greenhouse gas emissions (Ponomarev and Sillanpää, 2019). Therefore, it is a promising alternative to make better use of lignin resources and convert them into lignin-based high-efficiency adsorbents. For instance, Brazil et al. (2020) prepared activated carbon possessed large specific surface area by microwave pyrolysis of sulfate lignin, which has good adsorption performance for MB in aqueous solution.
Although lignin-based activated carbon has improved the dye adsorption capacity to a high level, the simultaneous adsorption of two organic pollutants (TC and MB) by lignin-based activated carbon is rarely reported. Therefore, it is urgent to design a lignin-based activated carbon to remove different organic pollutants. At present, most studies only pay attention to the large adsorption amount of adsorbent, but often ignore the key characteristic of adsorption rate. At the same time, nitrogen doping can endow the activated carbon surface with unique electronic properties and abundant functional groups, thereby accelerating the adsorption efficiency. Herein, we successfully prepared a lignin-based nitrogen-doped magnetic porous carbon via facile one-pot method, which is expected to achieve rapid and efficient removal of MB and TC in wastewater within a wide pH range. The structural characterizations of porous carbon were investigated by SEM, N2 physical adsorption, XRD, FT-IR and TG. The impacts of initial pH of the solution, pollutant concentration, adsorption time, system temperature and water quality were systematically studied. Ultimately, the removal mechanism was obtained through the adsorption kinetics, isotherm and thermodynamics research.