Chemically modified biochar produced from conocarpus waste increases NO3 removal from aqueous solutions. 
Biochar has emerged as a universal sorbent for the removal of contaminants from water and soil.  However, its efficiency is lower than that of commercially available sorbents.  Engineering biochar by chemical modification may improve its sorption efficiency.  In this study, conocarpus green waste was chemically modified with magnesium and iron oxides and then subjected to thermal pyrolysis to produce biochar.  These chemically modified biochars were tested for NO3 removal efficiency from aqueous solutions in batch sorption isothermal and kinetic experiments.  The results revealed that MgO-biochar outperformed other biochars with a maximum NO3 sorption capacity of 45.36 mmol kg(-1) predicted by the Langmuir sorption model.  The kinetics data were well described by the Type 1 pseudo-second-order model, indicating chemisorption as the dominating mechanism of NO3 sorption onto biochars.  Greater efficiency of MgO-biochar was related to its high specific surface area (391.8 m(2) g(-1)) and formation of strong ionic complexes with NO3.  At an initial pH of 2, more than 89 % NO3 removal efficiency was observed for all of the biochars.  We conclude that chemical modification can alter the surface chemistry of biochar, thereby leading to enhanced sorption capacity compared with simple biochar.