Abstract
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.
Highlights
30% of the land area in Finland is covered by peatlands [1] and more than half of the peatlands (4.7 Mha) have been ditch drained for forestry [2]
total nitrogen (TN) concentration increased when 850 L of new water was added into the water tank feeding the biochar reactor (Figure 2a)
The TN concentration decreased from 0.8 mg L−1 to 0.35 mg L−1 in the first phase of the experiment, corresponding to 56% purification efficiency
Summary
30% of the land area in Finland is covered by peatlands [1] and more than half of the peatlands (4.7 Mha) have been ditch drained for forestry [2] Forestry operations such as clear-cutting, ditch network maintenance and fertilization affect approximately 2–3% of forest areas in Finland. Nitrogen (N) concentration and export load increase in the receiving waters after the clear-cut as tree removal decreases water and nutrient uptake and increases runoff [8,9,10].
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