Low-cost eggshell-fly ash adsorbent for phosphate recovery: A potential slow-release phosphate fertilizer.

• A novel adsorbent was prepared from eggshell and fly ash for p adsorption. • The proportion of eggshells affects the final product of the adsorption process. • EFG has an excellent phosphorus adsorption capacity of up to 49.92 mg P/g. • The EFG remains intact morphological structure after the adsorption process. Fly ash (FA) and eggshells (ES) are common solid wastes with significant potential for the recovery of phosphorus from water. This study focuses on synthesizing a low-cost and environmental-friendly phosphate adsorbent called eggshell-fly ash geopolymer composite (EFG) using eggshells instead of chemicals. The CaO obtained from the high-temperature pyrolysis of eggshells provides active sites for phosphate adsorption, and CO 2 serves as a pore-forming agent. The phosphate adsorption performance of EFG varied with the eggshell-fly ash ratios and achieved a maximum of 49.92 mg P/g at an eggshell-fly ash ratio of 40 %. The adsorption process was well described by the pseudo-second-order model and the Langmuir model. EFG also exhibited a good regeneration performance through six-cycle experiments and achieved the highest phosphate desorption at pH 4.0. The results of the column experiment showed that EFG can be used as a filter media for phosphorus removal in a real-scale application with low cost. Soil burial test indicated saturated EFG has a good phosphate slow-release performance (maintained for up to 60 days). Overall, EFG has demonstrated to be a promising adsorbent for phosphorus recovery. [Display omitted] [ABSTRACT FROM AUTHOR]