Environmental Engineering Reference
In-Depth Information
1. Extraction with salt solutions (e.g., KCl): This extraction removes soluble and
exchangeable P which are considered bioavailable.
2. Extraction with alkali: Residue from Step 1 is treated with 0.1 M NaOH which
removes P bound to Fe and Al, and some organic P. This P pool is generally
considered to be unavailable but Fe bound P may become available under
anaerobic conditions. A more complex extraction sequence can be used to
separate Al-P from Fe-P and organic P.
3. Extraction with acid: Residue from Step 2 is treated with 0.5 M HCl which
removes Ca and Mg bound P which is considered to be unavailable. The residue
contains organic P and stable forms of inorganic P that are considered
unavailable.
Soluble P in water and soil extracts can be determined without pretreatment
colorimetrically or through instrumentation methods including inductively coupled
plasma (ICP) spectrometry and ion chromatography. Ion chromatography measures
OP; ICP measure inorganic P and organic P and it is used for soil extraction
extracts. Colorimetric methods measure primarily OP. Most commonly used is
the ascorbic acid method which measures OP and a small fraction of the condensed
phosphates; standard methods calls them “reactive phosphorous”. In the ascorbic
acid method (Eaton et al.
2005
), OP reacts with ammonium molybdate and potas-
sium antimonyl tartrate in an acid medium to form phosphomolybdic acid, which is
reduced to an intensely colored molybdenum blue by ascorbic acid. The intensity of
the blue color is measure on a spectrophotometer and the concentration is deter-
mined from an individually prepared calibration curve.
7.6.4 Phosphorus Fractionation Methods for Reduced Soils
Phosphorous fractionation methods were developed primarily for agricultural soils in
an attempt to estimate fertilizer needs. Wetland soils may present an additional
challenge because P chemistry in soils and sediments is strongly influenced by the
oxidation-reduction status (redox potential). Ferric and manganic oxides and
hydroxides provide major adsorption sites for P under oxidized conditions. In addi-
tion, ferric and manganic phosphate minerals which form and persist under oxidized
conditions become unstable under reduced conditions releasing soluble P into the soil
solution (Moore and Reddy
1994
). Oxidation of anaerobic sediments results in the
rapid conversion of Fe
2+
to Fe
3+
. Within minutes, Fe(OH)
3
precipitates out of solution
and has a tremendous capacity to sorb P, causing soluble P levels in the porewater to
be reduced by orders of magnitude. Therefore, it is critical that samples collected
from reduced soils and sediments are kept reduced during the sampling procedure,
transport to the lab, storage, and the initial phases of P fractionation. Moore and Coale
(
2009
) present methods that specifically address these concerns.
