Experimental and computational studies on activated Bambara groundnut (Vigna subterranean) hulls for the adsorptive removal of herbicides from aqueous solution

Abstract

Abstract Background The excessive usage of herbicides to control herbs by farmers has become an issue of interest to the environmentalist due to the threat posed by such act to the ecosystem, and therefore, there is the need to curb such practices. The efficiency of Bambara groundnut shell for the removal of pendimethalin (PE) and paraquat dichloride (PQ) herbicides from aqueous solution was established in this work. The activated carbon was prepared via chemical activation method using trioxonitrate (v) acid by determining its void volume, moisture content, bulk density and Fourier transform infrared (FTIR) and scanning electron microscopy (SEM) methods. Batch adsorption techniques were set to optimize the adsorption parameters such as solution pH, adsorbate concentration, contact time, adsorbent dosage and temperature in order to depict the best optimal conditions for the adsorption process. The adsorption process was examined in terms of its equilibrium data, kinetics, thermodynamics involved in the adsorption process as well as computational quantum chemical parameters evaluation. Results The batch adsorption experiments revealed that the amounts of PE and PQ adsorbed were found to vary with the contact time, adsorbent dosage, pH and initial concentration. The adsorption of PE and PQ decreased with increasing adsorbent dose but increases with increasing initial concentration of the PE and PQ solution. Isotherm studies revealed that the equilibrium data fitted to both Langmuir and Freundlich model with R-squared values of 0.976, 0.993 and 0.909, 0.978 for PE and PQ, respectively, which implied that Langmuir isotherm had a better fit. This was also found to be an indication that the uptake of PE and PQ by ACBGNS occurred through monolayer adsorption on identical homogenous sites. Also, kinetic modeling results obtained showed that the pseudo-second-order model explained the adsorption kinetics of PE and PQ by ACBGNS best, which meant that chemisorption was the slowest step and, thus, the rate determining step. The positive value of ΔH and the positive value of ΔG show the endothermic and spontaneous nature of adsorption of PE and PQ ACBGNS. Conclusion Batch adsorption experiment and characterization of the ACBGNS have indicated that Bambara ground nut shell can be used to produce activated carbon that can be applied effectively for adsorption of PE and PQ from an aqueous solution. Computational studies results obtained from quantum chemical analysis are consistent with the experimental results obtained from this study.