Adsorption interaction between natural adsorbents and textile dyes in aqueous solution

Agricultural wastes from maize culture fields were prepared and activated yielding the natural absorbents designated as SOMAP, SOMAP1, SOMAP2, HELAP, HELAP1, SAGAP, and SAGAP1. The samples were further characterized using microscopy, FTIR spectroscopy, titration, and adsorption from solution. The dry and wet methods of analysis confirmed the presence of functional groups (hydroxyl, carboxylic, phenolic, and lactonic) in the bulk and on the surface, respectively. Each prepared solid sample was tested for adsorption ability using aqueous dye solutions of basic blue 41 (BB41), acid blue 74 (AB74), and reactive black 5 (RB5). The adsorption affinity decreased according to BB41 > AB74 > RB5. For BB41 the relative adsorbent saturation followed the series: SOMAP2 < SOMAP1 < SOMAP, HELAP < HELAP1 and SAGAP < SAGAP1. Adsorption equilibrium concentrations were followed by HPLC with diode array detection. The equilibrium data fitted well with both Langmuir and Freundlich models of adsorption. The values of the Freundlich exponent 1/n correlated with the variance of the group distribution curves. The monolayer capacity of all studied adsorbents using the basic dye BB41 correlated acceptably with the corresponding amount of total acid groups on the adsorbent surface.