Phosphorus recovery from an igneous phosphate rock using organic acids and pyrolysis condensate

Abstract

Directly applied igneous phosphate rocks (PR) are agronomically ineffective because they are intrinsically unreactive. The main process by which PRs are solubilised in soils by microorganisms and plants is through the exudation of low molecular weight organic acids, which have a short lifespan in soils, thereby limiting the period of PR dissolution and agronomic effectiveness. This study measured the amount of P recovered from Dorowa PR (DPR) by suspension in water and sequential leaching with citric, acetic, oxalic acids, and pyrolysis condensate at various pH values. Of the dilute acids tested, oxalic acid was more effective at solubilising P from the DPR, cumulatively liberating 46% of total P from the DPR after 3 extractions at pH 3. However, aqueous phase pyrolysis liquid from maize stover pyrolysis was less effective at recovering P, yielding less than 14% of the total P despite maintaining a pH range of 3 to 3.8 in leachates after sequential extractions. The poor solubilisation was due to the low concentrations of chelating and complexing acids in the pyrolysis liquid, with the dominant acid being acetic acid and the high Ca content. Further research on improving the dissolution potential of pyrolysis liquid by removing cations in the liquid before leaching and other alternative processes to produce complexing acids such as oxalic acid is recommended.