Concentration affects significantly the capacity of biochar to adsorb nitrogen from forest runoff water

In boreal peatland forests, drainage and harvesting increase nitrogen (N) export to watercourses and the highest N concentration in runoff water occurs outside the growing season when traditional water protection methods based on biological activity are inefficient. In these conditions, water purification based on adsorption could offer a solution. Biochar can be an effective sorbent material for removal of nutrients from water due to its high specific surface area, porous structure and high cation and anion exchange capacity. We tested adsorption capacity for total N (TN) of spruce and birch biochar using water collected from ditch drains of boreal harvested peatland. The water was collected outside the growing season when TN concentration was 4.6 mg L^-1. During the growing season, TN concentration varies from 0.5 to 2 mg L^-1. To study the effect of TN concentration on adsorption capacity, we diluted water samples to concentrations 1, 2, 3 and 4 mg L^-1. We added 5 g of biochar to 1 L of water and shook the samples for 180 h. TN adsorption capacity increased monotonously from the smallest to the highest concentration. Adsorption capacity was 2.6 and 3.7 times greater in the highest concentration compared to the lowest concentration in spruce and birch, respectively. This indicates that concentration affects significantly the capacity of biochar to adsorb N from forest runoff water. Therefore, biochar can be an effective water protection tool in areas, which have high TN concentration, and it can be a complementary method supporting water purification outside the growing season.