Valorization of Mango Peels as Low-Cost Biosorbent in Methylene Blue Adsorption:Kinetic and Equilibrium Study

摘要

Mango peels,which constitute a significant proportion of urban waste,have been modified with phosphoric acid for use as a biosorbent in the removal of methylene blue from wastewater.The characterization of the obtained biosorbent showed that cellulose is the primary constituent followed by lignin and hemicellulose.The high water content and the low value of ash content indicate that the studied biosorbent is a porous material containing a low proportion of inorganic,inert,amorphous and unusable part for biosorbent production.The zero charge point(pHpzc)assessment showed that the overall surface charge of the biosorbent is negative and therefore plays a key role in the adsorption process.The adsorption of methylene blue by mango peels biosorbent is a two-step process:a rapid first step in which over 90%methylene blue is removed in less than 10 min followed by a slowdown of the adsorption rate when approaching the adsorption equilibrium.Among pseudo-first,pseudo-second order and intraparticle diffusion kinetics models studies,pseudo-second order was the best applicable to describe methylene blue adsorption,suggesting a two-step mechanism:the transfer of methylene blue molecules from the solution to the mango peels biosorbent surface,followed by the interaction between adsorbates and surface.The equilibrium adsorption data were analyzed by Langmuir,Freundlich and Temkin isotherms models.Among them,Langmuir was the best model to describe adsorption,indicating the existence of homogeneous distribution of adsorption sites on mango peels biosorbent surface and a mono-layer adsorption of methylene blue molecules.The low value of Temkin’s constant B relative to the interaction energy between methylene blue molecules and the surface of the biosorbent shows that the adsorption involved is a physisorption process.