New Method for Assessment of Long‐Term Phosphate Desorption from Soils

Abstract

A new method was developed to study P desorption kinetics. This new technique uses dialysis membrane tubes, filled with hydrous ferric oxide (ferrihydrite) acting as an “infinite” P sink. This system is mechanically stable for very long reaction periods, provided that a microbial inhibitor, e.g., chloroform, is added to the soil suspension to prevent hydrolysis of the membrane. The pH of the soil solution during desorption remains almost constant. After the desired time of contact between soil suspension and P sink, the sink can be easily separated from the soil suspension with practically no loss of soil material. As such, the new technique has important advantages to the Fe oxide impregnated filter paper P extraction method. The system is capable of maintaining a constant low P activity in solution, necessary to study long‐term P desorption kinetics of soils. This method was tested on six sandy soil samples and a comparison made with the amount of P desorbed by a single Fe oxide impregnated filter paper extraction (Pi). An important finding from this experiment was that P desorption continues for long periods. No desorption maximum was reached within 500 h, as is often suggested by desorption results based on repeated extractions with Fe‐impregnated filter paper. Furthermore, relatively large differences were observed between different soils with respect to the quantity of oxalate‐extractable phosphate released by the soils after a specified time of desorption.

Effects of Shaking Time on Long‐term Phosphorus Desorption using Dialysis Membrane Tubes filled with Hydrous Iron Oxide

“…Long-term P desorption study was done on the selected samples using dialysis membrane tubes filled with hydrous ferric oxides as described by Freese et al (1995). The dialysis membrane tubes filled with hydrous ferric oxide were placed in a 200-ml polyethylene bottle containing 1 g of soil and 80 mL of 2 mM calcium chloride (CaCl 2 ) and 0.3 mM KCl solution.…”

“…where SP is solid phase P, P sol is P in solution, P HFO is P adsorbed by HFO, k T is the rate constant of P transport through the membrane (0.09 ¡ 0.01h 21 ; Freese et al 1995), and k R is the rate constant of P release (de Jager and Claassens 2005). The presence of two pools is assumed: the pools with the fast release kinetics, pool A (SP A ), and the pool with the slow release kinetics, pool B (SP B ).…”

Distinguishing Adsorption and Surface Precipitation of Phosphate on Goethite (α-FeOOH)

“…First, some P is removed from exchange with solution by burial within the surface precipitate and thus is no longer part of the equilibrium process. It is clear from isotopic exchange studies, as well as from desorption studies, that much of the P on the goethite surface is not in rapid equilibrium with the solution P (8,12,15,16). Precipitation may be contributing to that lack of exchange.…”

Phosphorus removal from soil using Fe oxide-impregnated paper: Processes and applications

“…Van der Zee et al (1987) proposed to use Fe oxide-impregnated filter paper (Pi-paper) which was found to sorb P selectively and strongly. For the assessment of long-term P desorption kinetics, dialysis membrane tubes filled with hydrous ferric oxide have been proposed by Freese et al (1995).…”