Preparation Of Coconut Shell-sludge Composite Hydrothermal Activated Carbon And Its Adsorption Of Dyes

The conversion of sludge into adsorbed carbon material is an ideal way,due to it resource disposal of sludge from municipal sewage treatment plant.The higher carbon content and functional group structure of raw materials can be conserved during hydrothermal carbonization.Meanwhile,this technology has simple operating conditions,mild reaction conditions and no pollution to the environment,thus it will have immeasurable application prospects in the field of waste disposal.In this work,the sludge from the municipal sewage treatment plant was used as raw material,and the biomass of coconut shell was added to enhance the carbon content of the raw materials.The biochar was synthesized by a new environmentally friendly and lower cost thermochemical technology.The preparation method included hydrothermal co-carbonization and chemical activation.The optimal conditions for the preparation of activated carbon by single factor experiment were as followed:the mass ratio of coconut shell to sludge was 1:1;hydrothermal temperature was 180℃;hydrothermal time was 4 h;the pH value of solution was 7;the activation temperature was 700℃;the activation time was 50 min;and the activator concentration was 2.0 mol/L.The adsorption capacity of MB and yield of activated carbon obtained in this conditions were 665.37 mg/g and 15.27%.When the mass ratio of coconut shell to sludge was 1:1,the synergistic effect of the coconut shell and sludge was the strongest,which significantly improved the yield of hydrochar and the adsorption capacity of activated carbon for MB,indicating that the mixture of various biomass might promote synthesis of activated carbon materials with excellent high-yield properties.The raw materials,hydrochar and activated carbon were characterized by infrared spectra analysis,elemental analysis,N2 physical adsorption,thermogravimetric analysis,phase composition and morphology.The experimental results showed that the heavy metal concentration of the activated carbon prepared by this method met the national standard,and the impact on the environment was not significant.With the preparation process proceeded,the content of C,H,O,N and S in the raw materials reduced significantly,while the aromatization of the solid strengthened,and the oxygen-containing functional groups reduced.After hydrothermal co-carbonization,a serried faveolate structure appeared on the surface of hydrochar,and KOH activation caused the hydrochar structure to continuously forming pores,resulting in a porous structure with irregular morphology.Hydrothermal process promoted the decomposition of R1-O-R2,-COOH and N-H structures in raw materials,while the activation process significantly weakened C-O,saturated alkyl and O-H structures.However,catalytic oxidation of KOH led to the appearance of new C=O structures in activated carbon,thus it could improve the adsorption capacity of pollutants;The specific surface area,pore volume and average pore diameter of activated carbon were 873.54 m2/g,0.57 cm3/g and 2.51 nm,respectively.The adsorption process of MB and CR dyes by activated carbon from coconut shell and sludge was not only affected by the internal diffusion process of activated carbon,but also the role of boundary layer.In addition,the adsorption behavior of the two types of dyes was in accordance with the pseudo-second kinetic model and the Langmiur isotherm model,indicating that MB and CR molecules absorbed on activated carbon was homogeneous in a single layer,which was mainly chemisorption process.With the increase of adsorption temperature,the adsorption capacity of activated carbon also increased sharply.The maximum adsorption capacities of MB and CR at 40℃ were 653.02 mg/g and 295.43 mg/g,respectively.In addition,thermodynamic experiments revealed that the transfer of MB and CR molecules to the surface of activated carbon was a spontaneous endothermic process with increasing randomness in the system.As for MB,the force between activated carbon and MB included the hydrogen bonds produced by N(CH3)2 and S heteroatom with hydroxyl on the surface of activated carbon,and π-π interaction generated by double bonds and aromatic rings in MB molecules with aromatic rings in activated carbon.The adsorption mechanism of CR was the interaction of-SO3-structure and positive sites on the surface of activated carbon,hydrogen bonds of N atoms and hydroxyl,and the attraction between delocalized π electrons of activated carbon and double bonds and aromatic rings of CR molecules.According to the adsorption experiment,activated carbon had strong adsorption capacity for various types of dyes,and its application in the practical dye wastewater was possible.