Potential environmental applications of Helianthus annuus (sunflower) residue-based adsorbents for dye removal in (waste)waters

Abstract

Under the framework of Circular Economy, agricultural residues that once were considered an unwanted byproduct, are currently gaining popularity as novel bio-based products for environmental applications. This chapter evaluates the potential of Helianthus annuus (sunflower) biomass residues for (waste)water dye removal considering the factors regulating its subsequent chemical processes and equilibria. The initial pH, which affects both the dye speciation in solution and the surface charge of the adsorbent, is one of the most critical parameters and determines the optimum conditions for efficient dye removal. The soluble dyes, that present a high affinity for water, are generally ionized (e.g., possess positive or negative charge), and therefore, the sorption capacities of an adsorbent are strongly affected by the initial pH of the dye solution. The equilibrium data regarding the removal of various dyes from aqueous solutions using sunflower-based adsorbents (in pristine or chemically modified form) are usually better described by the Langmuir equation; however, in some cases, the Freundlich adsorption model seems to better fit the experimental data. The majority of the kinetic data associated with the removal of dyes using sunflower-based adsorbents follows the pseudo-second-order model. In addition to the thermodynamic adsorption data, this chapter summarizes and discusses also data of desorption experiments performed using dye-loaded adsorbents.