Nitrate removal from groundwater using chemically modified coconut husk based granular activated carbon: characterization of the adsorbent, kinetics and mechanism

Abstract

In this study, a highly porous chemically activated granular activated carbon (GAC) was prepared from coconut husk and tested as an adsorbent to remove nitrate from contaminated groundwater. The prepared GAC was characterized by Fourier-transform infrared spectroscopy (FTIR), thermogravimetric and differential thermal analysis (TGA/DTA), scanning electron microscopy (SEM) and the Brunauer–Emmett–Teller (BET) surface area (SBET) analysis. The effects of various process parameters such as initial nitrate concentration, contact time and adsorbent dose on nitrate removal efficiency (response) by the modified GAC were investigated using the statistically significant response surface methodology and Box–Behnken design of experiments. The experimental data were fitted to well-known adsorption isotherms and kinetic models to ascertain the mechanism of the adsorption process. Analysis of variance (ANOVA) was performed to determine the significance of the individual and the interactive effects of process variables on the response. The BET surface area (SBET) and micropore volume of the prepared GAC from coconut husk was 1120 m2/g and 0.392 cm3/g, respectively. The experimental results showed that physisorption was the main adsorption mechanism governing the process, while the rate of adsorption was limited at initial nitrate concentrations > 10 mg/L. The Langmuir mono-layer adsorption isotherm best fitted the experimental data with a maximum adsorption capacity of 6.0 ± 1.3 mg/g (~ 92.5%) with an adsorbent dose of 0.1 g/50 mL, an equilibrium time of 6 h at 28 ± 2 °C, and at pH 7.6 (± 0.2). Among the tested process variables, the adsorbent dose and initial nitrate concentration showed significant effects on the nitrate removal efficiency.