Removal of methylene blue from aqueous solution using common bean vine and cowpea vine biomass

Abstract

Agricultural waste materials from common bean (Phaseolus vulgaris L.) and cowpea (Vigna unguiculata subsp. unguiculata) vines were investigated as adsorbents to remove methylene blue (MB) from aqueous solution. Both vines exhibited efficient adsorption of MB. Characterization of the adsorbents was performed by infrared spectroscopy, scanning electron microscopy and N2 adsorption at 77 K. Composition was determined by the van Soest method. The adsorption isotherms of the vine adsorbents for MB fitted a Langmuir model well and the kinetics followed a pseudo-second-order model. The adsorption capacities of common bean and cowpea vines for MB were 181.82 and 144.93 mg/g, respectively, and the adsorption was a spontaneous and exothermic process. The high adsorption performance was mainly due to the hierarchical pore structure and abundant surface functional groups of the vine materials. The adsorption mechanism involved pore filling, electrostatic attraction, hydrogen bonding, π–π interactions, ion-exchange and n–π interactions. The results revealed that common bean and cowpea vines can be used as effective low-cost adsorbents in the treatment of contaminated wastewater.