Roles of Mg-Al layered double hydroxides and solution chemistry on P transport in soil

Abstract

Mg-Al layered double hydroxides (LDHs) were synthesized and applied to soils to control phosphorus (P) loss. Saturated soil column experiments and numerical modeling were conducted to investigate the transport, retention, and release behavior of P in natural soil with and without mixing LDHs under various solution chemistries. Retention of P in the soil increased with the addition of 0.5% LDHs and in the presence of Ca2+ in comparison to K+. The simulated results showed that irreversible retention of P is greater in the presence of Ca2+ than K+ in soil without LDHs and in the presence of 0.5% LDHs in comparison to the absence. The increase of ionic strength (IS) for K+ from 1 to 100 mM resulted in increased P retention in LDH-soil system due to more inner-sphere complexes and increased adhesive force. Near equilibrium retention on the reversible site occurred for all these experiments. The P was desorbed and the transport of dissolved P was improved with the reduction of IS in the presence of K+. On the other side, larger ion exchange and reduction of IS in the presence of Ca2+ induced the release of Fe/Al oxides which brought transport of P absorbed on those minerals, suggesting the potential for Fe/Al oxides-facilitated transport of P under the change of soil solution chemistry. Additionally, the transport and stability of LDHs in soil column were also investigated. The dissolution and transport of LDHs could be impeded in presence of high cation valence and IS.