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Abstract
Single-step hydrothermal carbonization (HTC) was carried out using Pumpkin Seed (PS) biomass (LCMs) where water as green catalyst was used. The sample was placed inside the autoclave reactor at 150 °C and the hydrochar (PSHC) obtained was activated using Na2CO3 where the impact of activation process parameter as well as adsorption process parameters were observed using Plackett–Burman design (PBD). After activation, the surface area of PSHC was increased significantly to produce PSAC. In PBD Design, eleven input/independent variables, such as activation temperature (A) in °C, temperature ramping (B) °C/min, activation time (C) in min., impregnation ratio (D), gas flow rate (E) in ml/min, ball milling time (F) in hrs, mesopore volume percentages (G), HTC pre-carbonization temperature (H), ionic strength of solution using NaCl (I), competitive phenol solution (J) in mg/L, 4-Nitrophenol (4-NP) (K) solution in mg/L were selected and output variables were selected for 4-NP removal percentages (%R1) and apparent porosity (R2) percentages. PBD will give information about the single factors’ impact on removal percentages of 4-NP in multi-solute system. The final activated carbon PSAC and hydrothermally (HTC) carbonized char PSHC were characterized to observe its removal efficiency for 4-NP in a single solute and multi-solute system. The results revealed that, the optimum conditions to obtain highest removal percentages were associated with apparent porosity and were dependent on the physio-chemical properties of the starting materials, which is a lignin based hard precursors of PS under analysis. The results showed significant improvement in the surface area and pore size distribution characteristics of the hydrochar after the activation process. This specified the promising potential of this biomass as precursors for preparing adsorbent materials for removing 4-NP from wastewaters. Furthermore, the adsorption isotherms and kinetics parameters were evaluated in a single solute system of 4-NP. The activated sample of PSAC showed better affinity towards 4-NP than Phenol (P) itself due to its’ pKa values. Adsorption data were fitted with Langmuir, Freundlich and Temkin isotherm model for 4-NP removal. Additionally, the Langmuir separation factor (RL) and the Freundlich factor (1/n) were both below one, indicating a favorable sorption process. These results highlight the potential of PS lignocellulosic materials (LCMs) for activated carbon-based adsorbent (PSAC) formation as effective adsorbents for 4-NP removal from wastewater.
Keywords: Hydrothermal Carbonization, 4-Nitrophenol (4-NP), Nitrophenol (P), Competitive Adsorption, Plackett-Burman Design, Isotherm
