Investigation of the adsorption performance of acid treated lignite coal for Cr (VI) removal from aqueous solution

Abstract

Several methods have been used to reduce the negative contribution of heavy metal contaminants effluents in the environment. The method studied in this research was the use of activated carbon derived from Ethiopian lignite for hexavalent chromium adsorption from aqueous solution. The adsorption study was evaluated as the function of initial Cr (VI) concentration, adsorbent dosage and pH. All the batches' tests were conducted by using synthetic wastewater in this study. General Factorial Design (GFD) of Response surface methodology (RSM) was used in this batch test for analysis of experimental. The effects of independent variables on various initial Cr (VI) concentrations between 20 ppm and 100 ppm were evaluated. According to ANOVA analysis, each and combined term is statistically significant. A Cr (VI) removal efficiency of 98 % was achieved at pH 1 with 3.5 g of LAC in 200 ml of solution at 4 h of contact time using initial Cr (VI) concentration of 20 ppm. Removal increased up to 96.54% when Cr (VI) concentration decreased to 20.5 ppm, and adsorbent dose increased to 3.01 g at pH 1. 49.94% chrome removal was achieved at Cr (VI) concentration of 100 ppm and pH 5 with an adsorbent dose of 3.5 g for the combined effect of initial Cr (VI) concentration and pH, which was decreased with increased both initial Cr (VI) concentrations and pH value. With the combined effect of pH and adsorbent dosage the maximum percentage removal of Cr (VI) ion was obtained at lower pH and higher adsorbent dosage (96.5%). The adsorption of Cr (VI) was evaluated using batch adsorption equilibrium isotherms (Langmuir and Frendlich) and kinetics reaction rate (first and second order). The Frendlich isotherm model was the best one for the description of adsorption of Cr (VI) onto LAC heterogeneous surface energy. The adsorption kinetics of Cr (VI) onto LAC followed the second order kinetic model, suggesting that the adsorption reaction rate depends on two parameters, which might be the Cr (VI) concentration and adsorbent dosage. These results support similar conclusions mentioned by several authors in the literature.