Nitrate contamination of groundwater is a global issue that has stimulated significant research interest. Understanding the prevailing processes of nitrate removal from groundwater, influences of process conditions and the performance of various approaches are vital for successfully dealing with nitrate remediation. Natural and engineered processes of nitrate removal from groundwater are discussed with focus on the performance of natural and engineered innovative techniques for nitrate removal from groundwater. Nitrate removal from groundwater can be accomplished through microbial autotrophic and heterotrophic mechanisms in the subsurface under natural and engineered conditions. The application of these processes to in-situ and ex-situ nitrate removal is also discussed in this review. Further, the effect of temperature, pH, electron donor and biogeochemistry on groundwater nitrate removal are elaborated. While natural attenuation processes result in small amounts of nitrate removal from groundwater, engineered processes are able to achieve significantly higher nitrate removal from groundwater. However, the challenges of secondary pollution need to be addressed in adopting these technologies for groundwater treatment for potable use.
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 Fouriertransform infrared spectroscopy (FTIR), thermogravimetric and differential thermal analysis (TGA/DTA), scanning electron microscopy (SEM) and the Brunauer-Emmett-Teller (BET) surface area (S BET ) 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 (S BET ) and micropore volume of the prepared GAC from coconut husk was 1120 m 2 /g and 0.392 cm 3 /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.
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