Upcycling olive oil cake through wet torrefaction to produce hydrochar for water decontamination

Abstract

Oil extraction from drupes generate tons of olive oil cake (OOC) as a solid waste residue, raising alarming concern towards its safe and cost-effective disposal. Herein, an approach converting waste OOC to a value-added hydrochar (HC) for aqueous environment remediation, was proposed. Among synthesized HC, the one produced at 150 °C/6 h (OOCHC-150) displayed a most favorable yield (70.29%) and methylene blue (MB) uptake (24.24 mg/g), being selected for subsequent chemical modification. After testing different chemical agents, NaOH was chosen as modifier, since the resulting material (NaOH@OOCHC-150) displayed the best MB removal performance. Infra-red and X-ray photoelectron spectroscopic analyses revealed dominance of oxygen-containing functionalities over NaOH@OOCHC-150 surface, which played a critical role in MB adsorption through electrostatic and coordinate interactions. Three weight loss zones were observed during OOCHC-150 thermal degradation, which reduced to two zones for NaOH@OOCHC-150 due to the rupture of outer lignin layer, consequently exposing cellulose and hemicellulose. This was supported by NaOH@OOCHC-150 XRD pattern, which displayed multiple peaks between 14.8 and 23.9° (of OOCHC-150 pattern) and also for amorphous hemicellulose (at 15.8°) and crystalline cellulose (at 22.8, 26.4, and 34.5°). Presence of Na over NaOH@OOCHC-150 and N together with S traces on MB saturated NaOH@OOCHC-150 surfaces, as observed during elemental analysis, respectively confirmed successful chemical modification and MB adsorption. About 70–90% adsorption at varied initial concentration (Co) values was accomplished within 10 min. Adsorption isotherm and kinetic data were fitted to Temkin and pseudo-second-order (PSO) models. MB uptake over NaOH@OOCHC-150 was endothermic (depicted by positive ΔH°) and favorable (affirmed by negative ΔG°).