Effects of pH and Soil Minerals on Phosphorus Release from Agricultural Waste–Based Sorbents: A Continuous-Flow Column Study

Effective methods for the recovery and reuse of phosphorus are needed for sustainable agriculture. In this study, magnesium-amended corn cob chars prepared with brucite and sea bittern and calcium silicate hydrate (CSH) synthesized with hydrated lime and rice husk ash were tested for phosphorus recovery from a model animal wastewater at pH 8.0. After phosphorus uptake from the model wastewater, the two Mg-chars had a phosphorus concentration of $182–198\mathrm{mg}\mathrm{P}/\mathrm{g}$, and CSH had a phosphorus concentration of $46\mathrm{mg}\mathrm{P}/\mathrm{g}$. In continuous-flow column release studies, in which these materials were mixed with sand after phosphorus uptake, Mg-chars released over 80% of the recovered phosphorus within five pore volumes at pH 5.5, 7.0, and 8.5. Post-P-exposure CSH, on the other hand, required 335 pore volumes to release 90% of the recovered phosphorus at pH 8.5. The more rapid release at pH 5.5 and 7.0 compared to pH 8.5 for the spent Mg-chars is attributed to the greater solubility of magnesium phosphate minerals at lower pH values. When goethite or kaolinite was added to the sand columns containing spent Mg-char, the rate and extent of phosphorus release slowed significantly, especially for goethite versus kaolinite, and especially at pH 5.5. and 7.0 versus pH 8.5 for both minerals. This trend is attributed to the greater electrostatic driving force for the adsorption of phosphate to the mineral surface below the mineral point of zero charge (${\mathrm{pH}}_{\mathrm{pzc}}$).

Identifying ways to conserve and reuse phosphorus are important for several reasons: global reserves of phosphorus minerals are limited, phosphorus is essential for food production, anticipated population growth will require increasingly intensive food production in the coming decades, and uncontrolled release of phosphorus to the environment causes environmental degradation. For these reasons, novel materials were prepared from agricultural wastes and other low-cost materials and used to recover phosphorus from model animal wastewater. Then, the phosphorus-loaded treatment materials were added to soil columns to understand how they would release phosphorus when applied to crops as fertilizers. After exposure to the model wastewater, treatment materials made from magnesium-amended corn cobs had concentrations of phosphorus similar to commercial fertilizers and were also effective at phosphorus release, especially at low to neutral pH. Together, the results demonstrate the promise of these treatment materials for recovering phosphorus from concentrated wastewater and subsequent use as fertilizers to meet the challenges of sustainable food production. The treatment materials also have the potential for lower environmental impacts and reduced costs compared to commercial fertilizers.