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Forest management practices in boreal peatlands increase nutrient export and suspended solids to watercourses calling for development of new water protection methods. One potential solution could be adsorption-based purification of runoff water using biochar. The aim of this study was to determine the adsorption rate and capacity for Norway spruce and silver birch biochars to design a biochar-filled reactor for a ditch drain. In a 10-day laboratory experiment, biochar was stirred with runoff water from a clear-cut peatland forest, and changes in water pH, total nitrogen, nitrate nitrogen, ammonium nitrogen, phosphorus, and total organic carbon concentrations were measured. Based on the concentration changes, adsorption was quantified and adsorption model containing the adsorption rate and capacity was fitted to the data. Our results indicate that biochar effectively adsorbs both inorganic and organic nitrogen from runoff water. Birch biochar had higher adsorption capacity of nitrogen than spruce biochar. This study demonstrates that the adsorption of nitrogen compounds onto biochar surfaces increases with increasing initial concentrations. Thus, aquatic ecosystems exposed to high nutrient loads from fertile peatlands would particularly benefit from biochar-based water purification.

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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. The aim of this study was to test a biochar reactor and to evaluate its efficiency in runoff water purification and consecutive nutrient recycling in clear-cut peatland forests. The goodness of the method was tested in a meso-scale (water volume thousands of liters) reactor experiment by circulating runoff water through wood biochar-filled columns and by determining water nutrient concentrations in the column inlet and outlet. The pseudo-first and second order kinetic models were fitted to the experimental data and the adsorption rate (Kad) and maximum adsorption capacity (Qmax) of the biochar reactor were quantified. The concentration of total nitrogen (TN) decreased by 58% during the 8-week experiment; the majority of TN adsorption occurred within the first 3 days. In addition, NO3-N and NH4-N concentrations decreased below the detection limit in 5 days after the beginning of the experiment. The maximum adsorption capacity of the biochar reactor varied between 0.03–0.04 mg g−1 biochar for NH4-N, and was equal to 0.02 mg g−1 biochar for TN. The results demonstrated that the biochar reactor was not able to adsorb TN when the water TN concentration was below 0.4 mg L−1. These results suggest that a biochar reactor can be a useful and effective method for runoff water purification in clear-cut forests and further development and testing is warranted. Unlike traditional water protection methods in peatland forestry, the biochar reactor can effectively remove NO3-N from water. This makes the biochar reactor a promising water protection tool to be tested in sites where there is the risk of a high rate of nutrient export after forest harvesting or drainage.