Magnesium activation affects the properties and phosphate sorption capacity of poultry litter biochar

Authors: Padilla, Joshua - Josh; Watts, Donald - Don; NOVAK, JEFFREY - Retired ARS Employee; CERVEN, VASILE - Non ARS Employee; IPPOLITO, JAMES - Colorado State University; Szogi, Ariel; JOHNSON, MARK - Us Environmental Protection Agency (EPA)

Submitted to: Biochar
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/22/2023
Publication Date: 10/7/2023
Citation: Padilla, J.T., Watts, D.W., Novak, J.M., Cerven, V., Ippolito, J.A., Szogi, A.A., Johnson, M.G. 2023. Magnesium activation affects the properties and phosphate sorption capacity of poultry litter biochar. Biochar. 5. Article 64. https://doi.org/10.1007/s42773-023-00263-5.
DOI: https://doi.org/10.1007/s42773-023-00263-5

Interpretive Summary: Poultry litter (PL) is a mixture of chicken excreta, feathers, bedding material, and spilled feed. Because it is an excellent source of plant nutrients, PL is commonly applied to agricultural fields as a low-cost fertilizer. However, the overapplication of PL can result in soils saturated in phosphorus (P), which can negatively affect water quality due to high concentrations of P in agricultural runoff. Rather than using PL as fertilizer, we propose converting PL into biochar, a charcoal-like substance made from exposing biomass to high heat without oxygen (pyrolysis). This biochar can then be used as a soil amendment to reduce P in runoff from P-saturated soil. Biochar made from PL is naturally high in P, requiring chemical modification to enhance its ability to bind P in soils. We investigated the effect of magnesium activation (PL soaked in solutions containing up to one molar concentration of magnesium), pyrolysis temperature (500-900°C), and PL age (1, 3-5, or 7-9 years old) on PL-derived biochar properties and P binding capabilities. We found that biochars activated with magnesium levels greater than or equal to 0.25 mols and made at temperatures greater than or equal to 700°C had low concentrations of P in the solution. This biochar material can bind up to one percent P by weight. Our results indicate that the magnesium activation of PL-derived biochars is an effective strategy for producing biochars with the potential to bind P in soils.

Technical Abstract: Biochars with a high affinity for phosphorus (P) are promising soil amendments for reducing P in agricultural runoff. Poultry litter (PL) is an abundant biochar feedstock, however, PL-derived biochars are typically high in soluble P and therefore require chemical modification to become effective P sorbents. This study investigated the effect of magnesium (Mg) activation on contents of extractable P (EP) and P sorption capacities of PL-derived biochars. Biochar was produced at 500-900°C from PL activated with 0-1 M Mg. Three differentially aged PL feedstocks were evaluated (1-, 3-5-, or 7-9-year-old). Increased Mg activation level and pyrolysis temperature both resulted in EP reductions from the biochars. Specifically, biochars produced at temperatures >=700°C from PL activated with >=0.25 M Mg had negligible EP contents. X-ray diffractograms indicated that increased Mg loading favored the formation of stable Mg3(PO4)2 phases while increasing temperature favored the formation of both Mg3(PO4)2 and Ca5(PO4)3OH. Maximum P sorption capacities (Pmax) of the biochars were estimated by fitting Langmuir isotherms to batch sorption data and ranged from 0.66-10.35 mg g-1. Average Pmax values were not affected by PL age or pyrolysis temperature; however, biochars produced from 1 M Mg-activated PL did have significantly higher average Pmax values (p<0.05), likely due to a greater abundance of MgO. Overall, results demonstrated that Mg activation is an effective strategy for producing PL-derived biochars with the potential ability to reduce P loading into environmentally sensitive ecosystems.