2, 4, 6-trinitrotoluene (TNT) is an aromatic explosive widely used in military activities, explosives, construction, dyes, and mining (Luan et al. 2017; Dasary et al. 2009; Ayoub et al. 2010). During the purification of 2, 4, 6-trinitrotoluene (TNT) with sodium sulfite, a dark red colored wastewater is produced, which is commonly known as TNT red water (Jiang et al. 2018; Dan et al. 2012). TNT red water is highly hazardous and toxic, and has been shown to be mutagenic and carcinogenic to both humans and animals (Zhang et al. 2011; Gumuscu and Tekinay 2013; Park 2007). Even a small amount of TNT, such as 1–2 g, can be lethal if ingested by humans (Deng et al. 2020). Nowadays, the maximum permissible concentration of TNT in wastewater discharge is restricted to 0.5 mg/L or less, however, the actual concentration of TNT in produced wastewater often exceeds this limit. Direct discharge of TNT red water into the environment can cause severe water and soil pollution (Pouretedal et al. 2016; Bhanot et al. 2020). Therefore, it is crucial to properly treat TNT red water before it is released into the environment.
Physical, chemical and biological methodologies are often applied to treat TNT red water. Incineration (Ayoub et al. 2010) and distillation (Zhao et al.2010; Wang et al. 2008) can effectively remove the pollutant in TNT red water, however, these methods may generate harmful by-products, causing second pollution (Ayoub et al. 2010; Rodgers and Bunce 2001), and often have high demands to equipment. Biological treatment has been widely used in wastewater treatment (Snellinx et al. 2002; Esteve-Nunez et al. 2001; Mercimek et al. 2015; Chatterjee et al. 2017). But it is not very effective against TNT red water, which is highly toxic to microorganisms and contains high concentrations of compounds. Therefore, a cost-effective pre-treatment process is necessary (Zhang et al. 2011). Recently, advanced oxidation processes (AOPs) such as photocatalysts have shown promise in treating TNT red water (Pouretedal et al. 2016; Zhu et al. 2012; Bui and Minh 2020). For instance, Zhu et al. synthesized cuprous oxide on acid-activated sepiolite and obtained a Cu2O/AS sample with excellent photocatalytic performance. The Cu2O/AS was able to degrade 87.0% of TNT red water, and most organic molecules can be effectively degraded (Zhu et al. 2012). Dinh et al. investigated the treatment of TNT red water using various advanced oxidation processes and their combinations. The study found that the Fenton/TiO2/O3/UV process was the most effective for TNT red water treatment, reducing COD by 99% after 30 h of treatment (Bui and Minh 2020). Nevertheless, high costs and difficulties in dealing with high concentrations of pollutants limit the use of these technologies. In contrast, adsorption is a more economical method for treating TNT red water (Deng et al. 2020; Zhang et al. 2011). Many adsorbents have been designed and fabricated for this purpose (Dan et al. 2012; Pouretedal et al. 2016; Zhang et al. 2012; Tu et al. 2015; Zhao et al. 2013; Hu et al. 2017; Meng et al. 2012). Unfortunately, most of them cannot balance the adsorption efficiency and cost (Deng et al. 2020), and have difficulties in practical application. Therefore, the future development of new, efficient adsorbents should focus on improving their adsorption capacity for pollutants while reducing treatment costs.
Biochar is a carbonaceous material that is generated by heating biomass under limited oxygen or anaerobic conditions, resulting in a highly porous structure (Inyang and Dickenson 2015; Liu et al. 2015). Due to its excellent adsorption capacity, low cost, and widespread availability, biochar has emerged as a promising material for environmental remediation and water treatment (Tan et al. 2015; Mohan et al. 2014; Lyu et al. 2020). Some studies have even shown that biochar outperforms commercial activated carbon in terms of adsorption and binding affinity for specific organic contaminants (Heo et al. 2019; Ahmad et al. 2012; Kearns et al. 2014), making it a viable alternative. The adsorption effects of biochar on organic compounds can be attributed to several factors, including pore-filling, hydrophobic effect, electrostatic attraction, and hydrogen bonding (Inyang and Dickenson 2015; Tan et al. 2015). By adjusting the activation method and controlling the pyrolysis temperature, the surface characteristics and adsorption performance of biochar can be optimized. Recent studies have shown that the presence of aromatic rings and electron conjugation can further enhance the adsorption of nitrobenzene compounds (Deng et al. 2020). Thus, biochar holds great potential as an adsorbent for treating TNT-contaminated water. However, there is a lack of recent research in this area.
In the present study, we utilized hydrothermal carbonization and chemical activation with KOH to produce highly effective adsorption of TNT red water from rice straw. To thoroughly characterize the synthesized biochar, we employed multiple techniques to analyze its surface properties, functional groups, and organic structures. We conducted studies on the adsorption kinetics and isotherms of the prepared biochar to determine its performance in adsorption. Additionally, we investigated the effects of solution pH and absorbent concentration. To evaluate the reusability of the biochar, we performed 5 consecutive adsorption-desorption cycles. Finally, we discussed the possible mechanism of adsorption.