Phosphorus recovery from animal manures through pyrolysis: phosphorus transformations, release mechanisms, and applications of manure biochars in agriculture

Abstract

Phosphorus (P) is a vital element to enhance crop growth, but the excessive application of water-soluble P fertilizers has led to dwindling global P resources and elevated P levels in surface and ground waters. At the same time, high levels of P are excreted by livestock and poultry industries. These animal manures present an attractive source of secondary P, but the direct application of manures to farmlands may cause issues with P losses and environmental and health risks. To overcome this, pyrolysis (the thermal conversion of a biomass in oxygen-poor conditions) has been used in some situations without a full understanding of the impacts of the pyrolysis process on P forms and availability in the manure. This article critically reviews the use of pyrolysis to recover P from three types of animal manures (cow, swine, and poultry) in the form of biochars for applications in agriculture. Specific emphasis is paid to the P species in manures and their transformations during the pyrolysis process with the help of spectroscopic techniques (e.g., ³¹P NMR and XANES) and P fractionation schemes. The P concentrations, species, and availability are highly dependent on manure composition and especially pyrolysis conditions. During pyrolysis, the P is concentrated in the solid phase (biochar) and transformed into more inorganic (orthophosphate) and more crystalline forms as the pyrolysis temperature increases. Higher pyrolysis temperatures reduce the P extractability, which lowers the risk for P losses but may also affect plant P uptake. Strategies to modify P availability are presented and critical perspectives are given on the risks and limitations of manure-derived biochar application in agriculture.