Acid-modified coffee grounds biochar for nitrate and nitrite removal: an optimization via central composite design

Elevated concentrations of nitrate (NO3−) and nitrite (NO2−) are possibly the most widespread contaminant in groundwater, contributing to eutrophication. However, it is also a potential nutrient which can be recovered. The present work evaluates and optimizes the removal efficiency of NO3− and NO2 using acid-modified waste coffee grounds biochar (ACGB) in aqueous solution. Central composite design (CCD) was adopted to optimize the independent variables (acid molarity, heating temperature, and storage time) influencing the removal of NO3− and NO2−. The ANOVA analysis of the CCD suggested all the independent operating parameters had significant (p value < 0.05) impacts in the removal efficiency of NO3− and NO2− by the ACGB. The results exhibited optimal removal efficiencies of 83.8% and 73.3% for NO3− and NO2−, respectively, using the modification conditions of 0.8 M HCl, at 150 °C for 60 min. This work highlights removal of NO3− and NO2− onto an organic chemical free adsorbent. Point zero charge of ACGB presumes that electrostatic attraction of the positively charged surface of ACGB has mainly contributed to removal process via adsorption. However, adjustment of pH to alkaline condition negatively impacts the removal efficiencies of NO3− and NO2− which decreased to less than 50%. The spent nitrogen enriched, ACGB has high potential to be converted into soil fertilizer.