Synthesis of functional hydrochar from olive waste for simultaneous removal of azo and non-azo dyes from water

Industrial wastewater effluent containing dyes and hazardous waste, such as olive waste, pose serious threat to health and environment. This work investigated the removal of azo and non-azo dyes from a single and binary aqueous system using hydrochar produced from hazardous olive waste using hydrothermal carbonization (HTC). Optimisation suggested hydrochar obtained at 250°C (HC-9) exhibited best physiochemical, structural properties and around 300% increase in carboxylic and carbonyl groups indicated by nuclear magnetic resonance spectroscopy. HC-9 removed ∼100% of Methylene blue and Congo red in single (120 and 180 min) and binary system (60 min), respectively. Ex situ X-ray photon electron spectroscopy and surface area analysis after adsorption suggested chemisorption supported by pseudo-second order reaction, Langmuir, and Redlich-Peterson model. HC-9 exhibited around 80% reusability in three adsorption cycles. Thus, better working efficiency of our hydrochar in the complex wastewater make it a potential candidate to be employed in commercial plants. •∼300% increase in alkyl C, aromatic, carboxylic, and carbonyl groups in hydrochar.•H/C and O/C atomic values for hydrochar were better than for brown coal or lignite.•Hydrochar had surface zeta potential of -38.4 mV.•Hydrochar removed ∼100% Methylene blue and Congo red in 2–3hr.•Hydrochar removed both dyes simultaneously in 1 hour in binary system. [Display omitted]