Effect of Hydrochar Modification on the Adsorption of Methylene Blue from Aqueous Solution: An Experimental Study Followed by Intelligent Modeling

Abstract

Wheat straw, which is a carbon-rich precursor and a common agriculture waste in Sanandaj, was modified to produce hydrochar with high adsorption capacity by the hydrothermal carbonization (HTC) method. The hydrochars were tested as adsorbents to remove methylene blue (MB) from aqueous solution, and the effects of various interfering parameters, including pH, MB concentration, and adsorbent dosage, were investigated using artificial neural networks (ANNs) on adsorption modeling. Adsorption isotherms and kinetics were studied to explain the MB adsorption process. The prepared hydrochars were characterized using Brunauer–Emmett–Teller (BET), scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDAX), and high-performance liquid chromatography (HPLC) instruments. The maximum MB removal efficiency achieved by hydrochar modified by KOH (0.1 M) and adsorption data fitted well with the Langmuir isotherm and pseudo-second-order kinetics. In terms of elemental composition, the hydrochar sample contained 52.19% carbon (C), 3.37% hydrogen (H), 0.1% nitrogen (N), 0.15% sulfur (S), and 35.66% oxygen (O). The ash content in the sample was 8.50%. The recorded hydrogen-to-carbon ratio (H/C) and oxygen-to-carbon ratio (O/C) indicated a shift towards humification, implying the influence of KOH addition during the hydrochar production process. Additionally, the specific surface area of the hydrochar, as measured by the BET method, was found to be 11.54 m²/g. Among the aromatics, a significant presence of hydroxymethylfurfural (HMF) was detected, with a concentration of 4.70 g/kg DM. The modeling results demonstrated that the concentration of MB had the most substantial impact on the predicted removal, followed by pH, adsorbent dosage, and contact time.