Effect of K⁺-modified biochar on the adsorption performance of methylene blue

Using corn stalks as raw materials, modified corn stalk biochar (hereafter referred to as biochar) was prepared by phosphoric acid (H₃PO₄) activation modification, potassium hydroxide (KOH) loading K⁺ modification and carbonization. The structure of the biochar was characterized using SEM, BET, XRD, ICP, XPS, and FT-IR. The prepared biochar was used as adsorbent to determine its adsorption performance on methylene blue (MB) in the simulated wastewater. The results showed that the K⁺-modified biochar had a columnar lamellar structure with richer pores and contained the oxygen-containing functional groups such as —OH and —COOH on the surface, and the oxygen-containing functional groups showed a trend of specialization after K⁺ modification. The maximum adsorption capacity was 222.93 mg/g, which was 226.02% and 23.51% higher than that of unmodified biochar (68.38 mg/g) and the phosphoric acid (H₃PO₄)-activated modified biochar (180.49 mg/g), respectively. The kinetic and isotherm models were consistent with the proposed secondary kinetic model and the Langmuir model, respectively, and the adsorption process was unimolecular layer adsorption and was subject to both physical and chemical adsorption, with chemical adsorption dominating in the adsorption process. The liquid film diffusion model simulation and the intraparticle diffusion model simulation were carried out for the diffusion process in adsorption. The slope of the liquid film diffusion model curve was 0.6, so the liquid film diffusion was not the main control step of the diffusion process, and the intraparticle diffusion process was divided into the membrane diffusion stage (0−25 min) and the intraparticle diffusion stage (25−140 min). The intraparticle diffusion stage is significantly lower than the membrane diffusion stage in terms of adsorption rate (k_id2=6.453 5<k_id1=32.266 9), and this model fitting curve does not pass through the origin, and the intraparticle diffusion stage is the key stage for rate control. The thermodynamic parameters of the adsorption of MB on the K⁺-modified biochar were obtained by thermodynamic analysis, in which the separation coefficients RL were all between 0 and 1, ΔH=7.611 2 kJ/mol>0, and ΔG were all negative, indicating that the adsorption of MB on the biochar is reversible and spontaneous heat absorption, and the warming helps the adsorption to proceed. ΔS=30.041 5 J/(mol·K)>0, indicating the increase of randomness at the solid-liquid interface during the adsorption process.