『Abstract
Fe oxide-impregnated paper (Pi-paper) is used as an artificial
Phosphorus (P) sink to study P availability in soils and runoff.
Pi-papers were introduced as they would mimic P uptake by plant
roots by decreasing the P concentration in solution to negligibly
low levels and thus enhancing P desorption from the soils solid
phase. The rate of transfer of P from soil to Pi-paper would thus
be limited by the soil P desorption rate. The maximum desorption
rate is indeed achieved when the original methods is used in which
(at least) four Pi-papers per gram soil are placed in a soil suspension
(soil-solution ratio of 0.025 kg L-1). In several studies
this method has however been adapted depending on the research
question of interest without investigating the effect of this
adaptation to the processes involved in the transfer of P from
soil to Pi-paper. The aim of this study is to improve our understanding
of the processes that occur in the Pi-paper-solution-soil system
and so to extend the theoretical basis of this method. Insight
is gained in these processes by comparing the experimentally determined
P transfer from soil to Pi-paper with P transfer that is modeled
based on the measured P concentration in solution and the (kinetic)
Langmuir equation of the Pi-paper. P adsorption by a Pi-paper
from standard solutions is not instantaneous but can be described
with a kinetic Langmuir equation that is a characteristic of the
Pi-paper. Over time, the Pi-paper reaches equilibrium with the
solution, and the kinetic Langmuir equation can be rewritten to
a Langmuir equation. Regardless if there is equilibrium or not,
P adsorption to the Pi-paper is a function of the P concentration
in solution.
By adding Pi-paper to a soil suspension, a re-distribution of
P takes place between the reactive surface area of the soil and
the new reactive surface area of the Pi-paper that initially contains
no adsorbed phosphate. As opposed to the original method where
the desorption rate was limiting the overall P transfer, the adsorption
rate to the Pi-paper is limiting when one Pi-paper per gram soil
is placed in a soil suspension (soil-solution ratio of 0.1 kg
L-1). In this situation, the P concentration in solution
is found to be in equilibrium with the soil's solid phase. With
increasing contact time (＞〜24 h) the whole system approaches equilibrium.
With each successive Pi-paper newly added, more P is removed from
the soil system and the decrease of P in solution will be governed
by the soil P desorption isotherm.
Varying the number of Pi-papers and the soil to solution ratio
thus has a large effect on the transfer rate between soil and
Pi-paper and if either the soil desorption rate, the Pi-paper
adsorption rate or a combination limits this transfer rate. With
increased insight in the P transfer between soil and Pi-paper
sink, it becomes possible to tailor the experimental design to
help answer the research question one is interested in.
Keywords: Phosphorus; Fe-oxide-impregnated paper; Kinetic Langmuir
equation; Desorption; Rate 』
1. Introduction
2. Theory and data interpretation
3. Materials and methods
3.1. Preparation of Pi-papers
3.2. P adsorption to a Pi-paper from a standard solution
3.3. P adsorption to a Pi-paper from a soil suspension
3.4. Effect of number of Pi-papers and soil to solution ratio
on P adsorption
4. Results and discussion
4.1. P adsorption to a Pi-paper from a standard solution
4.2. P adsorption to a Pi-paper from a soil suspension
4.2.1. Cumulative P adsorption
4.2.2. P concentration in solution
4.2.3. Predicting P adsorption to a Pi-paper from a soil suspension
4.3. Considerations concerning the experimental setup
5. Conclusions
Acknowledgements
References