Environmental Engineering Reference
In-Depth Information
7.6.7.2 Anion Exchange Resins
Anion exchange resins represent the most common P-sink approach for soils.
Anion exchange resins remove dissolved phosphates from the soil solution via
surface adsorption (Kuo
1996
). The rate of P adsorption is a function of solution
P concentration. The resin promotes a low level of solution P concentration to
maintain P desorption from the soil. Anion exchange resins can be added directly to
soil suspensions, placed in a polyester bag, or the resin may be impregnated onto a
plastic membrane. Typically, the procedure utilizes chloride-saturated resin, bicar-
bonate (HCO
3
) saturated resin, or a combination of the two. The soil sample and
resin are shaken together in deionized water or weak electrolyte 16-24 h. Adsorbed
P is extracted with HCl and the P concentration is determined by the ascorbic acid
method. For details on this procedure see Kuo (
1996
).
7.6.7.3
Iron Oxide Method-Runoff
The FeO method is a unique approach to assess the potential for runoff to increase
fresh-water eutrophication. The use of iron-oxide (FeO) coated paper to test soil
was first reported by Sissingh (
1983
), who sought to develop a P test to estimate
plant-available P in tropical soils without mobilizing other forms of phosphates.
Sissingh (
1983
) created a strip of filter paper impregnated with iron hydroxide
which adsorbed mobile P from solution. The main advantage of this approach over
standard soil P tests is that the FeO paper functions as an ion sink and doesn't react
with soil as will chemical extractants. It has been shown that P extracted by this
method (FeO-P) from runoff sediment is correlated to algal growth (Sharpley
1993a
,
b
). A major benefit of the FeO method is its capability to differentiate
between soluble inorganic P from FeO-P in sediment of runoff. Sediment FeO-P
is considered bioavailable particulate P (BPP) and is calculated as follows: BPP
¼
total BAP
SP; where total BAP is total FeO-P from unfiltered runoff, and SP is
soluble inorganic P in filtered runoff (0.45-
μ
m filter).
The FeO method has a stronger theoretical justification for estimating P avail-
ability of runoff for plants and algae than do chemical methods (Sharpley
1993a
)as
chemical methods may mobilize additional forms of P which are not available to
plants or algae. Although algae uptake of P is restricted to OP, organic forms of
P can undergo mineralization and become available (Correll
1998
) and thus be
considered a latent source of BAP. There has been discussions focused on methods
to restrict hydrolysis of organic P adsorbed onto FeO paper (Robinson and Sharpley
1994
). However, adsorption and hydrolysis of organic P is not considered a problem
with the FeO method as organic P may be classified as latent BAP which may be
readily mineralized, thereby becoming available for algal use.
The FeO paper is made from filter paper immersed first in FeCl
3
·6H
2
O and
then in NH
4
OH. Paper preparation is described in detail by Myers et al. (
1997
).
The water of interest is sampled and taken to the lab. FeO paper is enclosed within
