Phosphorus Release Behavior of Biosolids and Corresponding Biochars.

Biosolids, the byproducts of wastewater treatment, have been suggested as a valuable soil amendment. When applied to the soil, these materials enable nutrient recycling while providing a solution for waste management. However, the nutrients could be detrimental to the environment if their release exceeds the soil’s capacity to retain them. Biosolids conversion to biochar, a material obtained by combustion in the absence (or low concentration) of oxygen, could convert nutrients to more stable forms, which ultimately could benefit the environment. Our objective was to evaluate phosphorus (P) associations and desorption from biosolids from various places (Florida, Chicago, Spain and Brazil), as well as their corresponding biochars. Materials were assessed using solid-state and solution chemistry techniques along with desorption experiments to identify the mechanisms of P retention and release. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses indicated contrasting compositions among biosolids, with variations that related to processing and origin. Most of the biochar samples showed Ca- and Mg- P associations. The conversion of biosolids into biochar modified the mineralogy of some materials due to the presence of thermally unstable minerals (e.g., struvite). Chemical analyses such as water soluble P, pH, Mehlich 3-extractable P and other elements, total carbon, total Kjeldahl nitrogen confirmed differences between biosolids and their corresponding biochars. Data suggest that biosolids from different locations, as well as their corresponding biochars, would differ markedly in nutrient release behavior and in the level of environmental risk they would pose if land applied.