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Biosolids, byproducts of wastewater treatment, can serve as an alternative source of nutrients in crop production. When applied to the soil, these materials enable nutrient recycling while providing a solution for waste management. However, the nutrients could be detrimental if their release exceeded the soil’s capacity to retain them. Biosolids conversion to biochar, a material obtained by heating in the absence (or low concentration) of oxygen, could convert nutrients to more stable forms that are less readily transported to water bodies where they could foster eutrophication. Our objective was to identify phosphorus (P) release and associations in biosolids from various places (Florida & Chicago in USA, 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, and 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 water quality risk they would pose when land applied. Therefore, in addition to soil property considerations, the rate of biosolids or biochar applications should take into account the properties of the amendments that vary geographically base on source and processing differences. |
