Synthesis, characterization, and methylene blue adsorption isotherms of hydrochars derived from forestry waste and agro-residues

Abstract

Mason pine-derived hydrochar (MPHC), cedarwood-derived hydrochar (CHC), bamboo-derived hydrochar (BHC), coconut shell-derived hydrochar (CSHC), pecan shell-derived hydrochar (PSHC), wheat straw-derived hydrochar (WSHC), maize straw-derived hydrochar (MSHC), and rice straw-derived hydrochar (RSHC) were synthesized by hydrothermal carbonization (HTC). The physicochemical properties of these hydrochars were characterized by various techniques, and the adsorption behavior of methylene blue (MB) on hydrochars from an aqueous solution was also investigated. The characterization results suggested that the hydrochars possessed various oxygen-containing functional groups (e.g., ether and hydroxyl groups, etc.). Thermodynamic parameters demonstrated that adsorption was spontaneous for all produced hydrochars. The adsorption was endothermic for CHC, BHC, CSHC, PSHC, WSHC, MSHC, RSHC, and exothermic for MPHC. The Langmuir model best described the adsorption process. MB adsorption capacity is ranked as MPHC > PSHC > CSHC > CHC > MSHC> WSHC > RSHC > BHC. The saturated adsorption value for MB on these hydrochars at 15 ℃ was 155. 14, 109.24, 93.15, 91.71, 88.11, 86.36, 70.01, and 64.43 mg/g, respectively; the difference in adsorption value indicates that the type of biomass affects MB adsorption. This high adsorption capacity for MB suggests that the produced hydrochars could be utilized as a promising new adsorbent in wastewater treatment.