Promotion of phosphate release from humic acid-iron hydroxide coprecipitates in the presence of citric acid

Phosphate (P) resources are expected to be depleted within a century. Therefore, promoting balanced phosphorus fertilizer use and understanding phosphorus dynamics in soils containing iron (III), organic acids, and iron (III)-organic molecule particulates is crucial. This study investigated the sorption of citric acid onto humic acid-iron hydr(o)xide coprecipitate (HAFHCP) and the reciprocal effects of citric acid and P sorption on HAFHCP with different C/Fe ratios. The results showed that the maximum sorption capacity (MSC) of citric acid on HAFHCP decreased with increasing C/Fe ratios in the HAFHCP. The P sorption on HAFHCP pre-sorbed with citric acids (denoted as C-P) decreased by 50% compared with that of the MSC on FH. However, citric acids could only reduce P sorption by 20% when P was pre-sorbed on HAFHCP (denoted as P-C). The results suggested that upon the formation of HAFHCP, citric acids might increase P availability, especially in the C-P system. Although citric acids initially inhibited P sorption on HAFHCP in the P-C system, P sorption increased with prolonged reaction time. The exposures of new sorption sites upon dissolution of Fe from HAFHCP by citric acids or/and the formations of Fe bridge between P and organic domains of HAFHCP might contribute to these results. Additionally, a number of large HAFHCP aggregates became smaller while sorbing P due to the increasing electric repulsion on the surfaces of FH, enabling the subsequent dissolutions of more Fe by citric acids from HAFHCP in the P-C system. By integrating these innovative and sustainable strategies, the recycling and reuse of P can be optimized, thereby minimizing the reliance on synthetic fertilizers and mitigating environmental impacts. This approach fosters the efficient utilization of phosphorus resources, improves soil fertility, and enhances the overall resilience of agricultural systems and ecosystems.

Phosphate Removal in Relation to Structural Development of Humic Acid-Iron Coprecipitates

Precipitation of Fe-hydroxide (FH) critically influences the sequestration of PO₄ and organic matter (OM). While coatings of pre-sorbed OM block FH surfaces and decrease the PO₄ adsorption capacity, little is known about how OM/Fe coprecipitation influences the PO₄ adsorption. We aimed to determine the PO₄ adsorption behaviors on humic acid (HA)-Fe coprecipitates in relation to surface and structural characteristics as affected by HA types and C/(C + Fe) ratios using the Fe and P X-ray absorption spectroscopy. With increasing C/(C + Fe) ratios, the indiscernible changes in the proportion of near-surface C for coprecipitates containing HA enriched in polar functional groups implied a relatively homogeneous distribution between C and Fe domains. Wherein PO₄ adsorbed on FH dominated the P inventory on coprecipitates, yielding PO₄ sorption properties nearly equivalent to that of pure FH. Structural disruptions of FH caused by highly associations with polar functional groups of HA enhanced the C solubilisation. While polar functional groups were limited, coprecipitates consisted of core FH with surface outgrowth of HA. Although surface-attached HA that was vulnerable to solubilisation provided alternatively sites for PO₄ via ternary complex formation with Fe bridges, it also blocked FH surfaces, leading to a decrease in PO₄ adsorption.