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Preparation of high surface area sludge-based activated hydrochar via hydrothermal carbonization and application in the removal of basic dye

Khoshbouy, Reza, Takahashi, Fumitake, Yoshikawa, Kunio
Environmental research 2019 v.175 pp. 457-467
Fourier transform infrared spectroscopy, adsorption, aqueous solutions, ash content, biomass, carbon, carbon dioxide, cost effectiveness, drying, endothermy, hydrochars, hydrothermal carbonization, kinetics, methylene blue, microspores, models, pH, potassium hydroxide, scanning electron microscopy, sludge, sorption isotherms, surface area, temperature, titration, value-added products, wastewater, water content, zeta potential
Conventional methods to produce sludge activated carbon require high-energy due to high moisture content and poor dewaterability of the wet sludge. Recently hydrothermal carbonization (HTC) has been emerged as a promising thermo-chemical wet biowaste conversion technology for the valorization by converting these precursors into high value-added products such as a nanostructured carbon. In this study, sludge-based activated hydrochars (SAC) were prepared from high moisture content (88.9%) wastewater sludge (without pre drying) via HTC followed by physical (P-SAC) and chemical (C-SAC) activation with CO2 and KOH, respectively. Further, the effects of HTC temperatures (170, 200, 230 and 260 °C) and the activation methods on the physicochemical characteristics of hydrochar and SACs were investigated through ultimate analysis (CHNO), SEM, BET, BJH, FT-IR, Boehm back titration and zeta potential. The result showed that KOH-activation at 700 °C could significantly enhance the surface area of hydrochars (from 6.3 to 1613.9 m2/g) compared with CO2-activation even at 900 °C (261.6 m2/g). The mesopore P-SAC prepared from hydrochar at lower HTC temperature due to the presence of ash content. While, the negatively charged microspores C-SAC (VTotal: 0.88 cm3/g and Vmicro/VTotal: 73.3%) with high SBET was synthesized at higher HTC temperature. Prepared hydrochars were applied for the removal of a basic dye (such as methylene blue, MB) from aqueous solution. Based on Langmuir isotherm model (R2>99.4%), the maximum monolayer MB adsorption capacity of hydrochar, P-SAC and C-SAC were 63.3, 122.4, and 588.2 mg/g, respectively at pH > 8.0. The MB adsorption on C-SACs followed the pseudo-second-order kinetic model and a spontaneous endothermic reaction from 298 to 328 K. The commercial ACs also was compared with our materials and found that produced activated hydrochars showed superior results for MB removal. Therefore, HTC can be the potential carbonization method for wet biomass conversion to valuable carbonaceous material in a cost-effective way.