Fe–Al binary composite filled dialysis membrane tubes (DMT-HFAO): a modified method for assessment of phosphate desorption from aqueous and soil solutions

Abstract

Phosphorus (P) limits plant growth particularly in strongly acidic soils due to P fixation. P availability to a plant is a functional concept of time rather than a measurable quantity. Therefore, a method that can estimate P availability over time is required. This research work was intended to synthesize a nanocomposite material that can monitor soil P desorption kinetics. To this effect, a binary sorbent system filled in a dialysis membrane tube was developed. Accordingly, calcined and amorphous powder samples of Fe–Al binary mixed oxides were synthesized by a gel-evaporation method and characterized by XRD, FTIR, TGA-DTA, SEM-EDX and BET techniques. The performance, as a phosphate sink, of crystalline hydrous ferric aluminum oxide (HFAO) and hydrous amorphous ferric aluminum oxide (HAFAO) each filled in a dialysis membrane tube (DMT) was evaluated. A single hydrated ferric oxide (HFO) suspension filled in dialysis membrane tubes (DMT) designated DMT-HFO was used as a benchmark. For the aqueous system, the sorption capacity of the DMT-HFAO was found to be 260% (mg mg⁻¹ phosphate) whereas the amorphous congener (DMT-HAFAO) was approximately 200% (mg mg⁻¹ phosphate) times that of DMT-HFO during the 24 h equilibration. For the soil solution system, the phosphate desorbed by the DMT-HFAO was about 520% (mg mg⁻¹ phosphate) compared with a single system, DMT-HFO, in 168 h. For the desorption experiment carried out with soil solution, the data fitted fairly well with first order kinetics for both sorbents (R² = 0.946–0.998), the amount adsorbed by DMT-HFAO being greater than DMT-HFO. The soil data fitted an intra-particle diffusion model fairly well for both sorbents (R² = 0.98–0.992) with rate constants, k_p, following the order: DMT-HFAO > DMT-HAFAO > DMT-HFO. The DMT-HFAO approach also showed better fit to the two component first order model (R² = 0.994 & 0.997) indicating that the modified method has promising potential for a long-term phosphate desorption kinetics study from soil, the implication of which is important both from agricultural and environmental perspectives. However, correlation of the P adsorbed by this sink method with actual plant P uptake in various soils should be carried out to validate the universality of this technique.