Adsorptive behavior of phosphorus onto recycled waste biosolids after being acid leached from wastewater sludge

A vast amount of phosphorus is being wasted or inefficiently utilized in wastewater treatment sludge worldwide. This paper investigates the adsorptive loading of phosphorus from the sludge on different biosolid materials for potential recovery and after use. The phosphorus was leached with acid from wastewater sludge from a chemical P removal process and adsorbed onto four different waste-based biosolid materials. The four biosolids were biochar, commercial lignin, sludge char (pyrolyzed wastewater treatment sludge), and humus (extracted from black liquor). Among the studied biosolids, loaded sludgechar had the highest phosphorus content, yet all materials performed well in P-adsorption. Optimal leaching and adsorption conditions were identified as pH 3 and adsorbent dosage between 0.5 g/L and 0.61 g/L for all biomaterials. The highest adsorption capacity value reached 400–500 mg/g with temperature-dependence. Biosolid materials were characterized with FT-IR, SEM-EDS, XRF, XRD, and XPS. Mathematical modeling through kinetic adsorption models showed that all biomaterials obey a pseudo first order kinetic model, and pore and intra-particle diffusion contribute to the adsorption mechanisms. The isotherm models suggest that the adsorbents are heterogeneous, and the adsorbate physiochemically bond with the functional groups of adsorbents with different adsorptive energies. The process is temperature-dependent and endothermic. XPS and XRD analyses showed that phosphorus adsorbed on the materials is mostly phosphate bound with Fe and Ca. Overall recovery efficiency was 21% (P bound on biosolids / P in sludge before leaching). Such phosphorus-loaded biomaterials are promising for use as feasible slow-release fertilizers.