Nutrient release and ammonium sorption by poultry litter and wood biochars in stormwater treatment

The feasibility of using biochar as a filter medium in stormwater treatment facilities was evaluated with a focus on ammonium retention. Successive batch extractions and batch ammonium sorption experiments were conducted in both deionized (DI) water and artificial stormwater using poultry litter (PL) and hardwood (HW) biochars pyrolyzed at 400 degrees C and 500 degrees C. No measureable nitrogen leached from HW biochars except 0.07 mu mol/g of org-N from 400 degrees C HW biochar. PL biochar pyrolyzed at 400 degrees C leached 120-127 mu mol/g of nitrogen but only 7.1-8.6 mu mol/g of nitrogen when pyrolyzed at 500 degrees C. Ammonium sorption was significant for all biochars. At a typical ammonium concentration of 2mg/L in stormwater, the maximum sorption was 150 mg/kg for PL biochar pryolyzed at 400 degrees C. In stormwater, ion competition (e.g. Ca2+) suppressed ammonium sorption compared to DI water. Surprisingly, ammonium sorption was negatively correlated to the BET surface area of the tested biochars, but increased linearly with cation exchange capacity. Cation exchange capacity was the primary mechanism controlling ammonium sorption and was enhanced by pyrolysis at 400 degrees C, while BET surface area was enhanced by pyrolysis at 500 degrees C. The optimal properties (BET surface area, CEC, etc.) of biochar as a sorbent are not fixed but depend on the target pollutant. Stormwater infiltration column experiments in sand with 10% biochar removed over 90% of ammonium with influent ammonium concentration of 2 mg/L, compared to only 1.7% removal in a sand-only column, indicating that kinetic limitations on sorption were minor for the storm conditions studied. Hardwood and poultry litter biochar pyrolyzed at 500 degrees C and presumably higher temperature may be viable filter media for stormwater treatment facilities, as they showed limited release of organic and inorganic nutrients and acceptable ammonium sorption. (C) 2016 Elsevier B.V. All rights reserved.