Simultaneous adsorption of a ternary mixture of brilliant green, rhodamine B and methyl orange as artificial wastewater onto biochar from cocoa pod husk waste. Quantification of dyes using the derivative spectrophotometry method

Abstract

Aiming to mitigate disposal and water pollution problems, biochars were prepared from cocoa pod husk (CPH) waste for application in artificial wastewater simulating real conditions of textile effluents. Therefore, the morphological properties of biochars obtained via one/two step NaOH activation were compared and characterized by XRD, Raman spectroscopy, FESEM/EDS and the BET technique. According to these results, biochar obtained from the one-step method (AQ₁) was chemically characterized by Boehm titration as well as the point of zero charge and used for the removal of a mixture of brilliant green (BG), rhodamine B (RB) and methyl orange (MO) as artificial wastewater. To evaluate the remaining concentration of each dye after adsorption without previous separation, a derivative spectrophotometry method was applied because some overlap will occur. In order to overcome this problem, adequate wavelengths for each dye in the mixture were established: BG = 631, RB = 592 and MO = 377 nm. Additionally, optimum conditions were determined such as pH 6 and a dosage of 50 mg and the effect of the initial dye concentration was studied in the range of 5–20 mg L⁻¹. The adsorption rate of the ternary dye mixture was fitted to the pseudo second-order model and the affinity of AQ₁ towards each dye was in the following order: BG > RB > MO. Furthermore, the adsorption isotherm data were described by multi-component Langmuir and Freundlich models. Finally, the presence of sodium chloride was explored and the synergetic effect in the removal of BG and MO was demonstrated, whereas the adsorption of RB was not promoted under the influence of this salt widely used in the textile industry.