The regeneration of P-loaded iron coated sand
The process is applicable for most sorbents of which the active components are iron or aluminum (hydr)oxides as it makes use of the pH-dependent adsorption behavior.At high pH, both the sorbent and phosphate species in solution are negatively charged (e.g., HPO42-), providing an unfavorable condition for sorption. Furthermore, the increased number of OH-ions competes with phosphate for the sorption site, thereby replacing phosphate ions on the sorbent and regenerating the sorbent.
Within the regeneration process, an alkaline solution (e.g. NaOH, KOH) is circulated over the sorbent column and more base is added to keep a constant pH in the solution. After the desorption step, a neutralization step is performed. This step ensures that the effluent during the sorption step will have a pH that allows direct discharge in soil and surface water. Furthermore, some potential precipitation products (e.g., calcium carbonates and calcium phosphates) that were formed during the desorption step could be removed with this neutralization solution.
To avoid the need to discharge the regeneration solution, KOH and H2SO4 are used as the desorption and neutralization agents, respectively. The produced solution, containing phosphate, potassium, iron, and sulfate, can be used as a nutrient solution in agriculture.
VITO also developed an excel tool to estimate the economic feasibility of the regeneration process (download the tool in our Document Library) performed by farmers. In the case of smaller installations, it might be economically more interesting to dispose of the loaded sorbent and purchase new ones. However, the investments in a regeneration set-up are worthwhile for bigger filter installations.