Effect of soluble calcium on enhancing nitrate retention by biochar

Batch experiments were conducted to test the hypothesis that nitrate (NO₃^-) could be immobilized by biochar via adsorption of CaNO₃⁺ to the negatively charged biochar surfaces. The results show that addition of soluble Ca in both aqueous and soil systems enabled NO₃^- retention by the biochar material. Increase in the added Ca enhanced the retention rate and the optimal NO₃^- retention was gained at a Ca/NO₃ molar ratio of 2 for the aqueous system. For the soil system, the Ca/NO₃ molar ratio required to attain the optimal NO₃^- retention was much greater due to competition of other soil-borne ligands and soil colloids for the available Ca. At the same level of added Ca, the amount of NO₃^- being retained tended to increase with increasing dose of the biochar. More NO₃^- was retained in the soil system than in the aqueous system at the same dosage level of biochar due to additional adsorption of CaNO₃⁺ by negatively changed soil organic and inorganic colloids. The findings obtained from this study have implications for developing effective methods for reducing NO₃^- leaching from soils.