Nonlinear regression analysis of the sorption of crystal violet and methylene blue from aqueous solutions onto an agro-waste derived activated carbon

Sorption of synthetic dyes on low-cost solid sorbents is a simple technique for their removal from wastewater. Recent initiatives in the sorption process have sought the use of activated carbon derived from agricultural wastes as it provides an attractive and cheaper alternative to commercial activated carbon, which is usually expensive. This research investigates the sorption kinetics and equilibrium of two synthetic cationic dyes, crystal violet and methylene blue from aqueous media using activated carbon prepared from an agro-waste, Millettia thonningii seed pods. Sorption experiments were carried out using the batch process. The kinetic data were analyzed using the pseudo-first-order, pseudo-second-order, and intraparticle diffusion models while the equilibrium data were analyzed using the Langmuir, Freundlich, and Redlich–Peterson isotherm models. Nonlinear regression method was used to fit the data to the isotherm models in order to determine model parameters and the best-fit isotherms. Thus, three error functions; coefficient of determination, Chi-square statistic test, and the sum of error squares were applied to evaluate the sorption data. The pseudo-second-order model best described the sorption kinetics of both dyes while the Redlich–Peterson model described the equilibrium data the most, followed closely by the Freundlich isotherm model indicating a heterogeneous sorbent surface. The experimental results indicate that the agro-waste derived activated carbon is a viable adsorbent for the remediation of dye-contaminated water.