Natural reusable calcium-rich adsorbent for the removal of phosphorus from water: proof of concept of a circular economy

A calcium-rich rock (limestone) was used as adsorbent to remove phosphorus from water. Phosphorus could be subsequently desorbed from limestone at pH = 4, and potentially reused as fertilizer following pH neutralization. Sorption of phosphorus onto limestone was not affected by 100 mM KCl or by the nitrogen present in a commercial fertilizer, but it was hindered by 100 mM NaCl, urea and river water. The phosphorus removed was however never below ∼9 mg P/kg rock, and it increased with increasing phosphorus concentrations in water. Phosphorus removal increased with 100 mM CaCl2 at neutral pH, likely due to its precipitation. Mixing for 30 s enhanced phosphorus sorption. Desorption of phosphorus from limestone following sorption in deionized water was ∼50%, ∼22%, and ∼11% at pH = 4, pH = 7, and pH = 11, respectively. Phosphorus desorption was lower when sorption had occurred in river water than in deionized water or in 100 mM urea.