Sequential Extraction of Phosphorus from Soil: A

Geoarcheology Application

Ariel Atkinson, A.R. Smith Department of Chemistry, Appalachian State University, Boone, NC 28607, Senior Research Final Presentation under the supervision of Carol M. Babyak

Why?-50 soil samples from Fayetteville, NC

-New I-95 loop will be built

-Surfer contour plot will be constructed

-Historical land use will be

recorded

http://www.ssg-surfer.com/ssg/detailed_description.php?products_id=135

Surfer Contour Plot

Inorganic phosphorus in soil can be extracted in three different fractions

The amount of phosphorus in each fraction is expressed as a percent of total phosphorus in the soil

Used to determine what the land was used for historically

Quantified either through

colorimetry or ICP-OES

Overview

Phosphorus

Abundant in the environment

Comes from a variety of human activities:

-farming -cleaning -food preparation

-medical care -waste

Stable in soil can reveal the historical land use patterns.

Forms of P

Operational Definition of Phosphate Fractions in Soil

Fraction INon-occluded

Fraction IIOccluded

Fraction IIIBound to calcium

Al

Al

OH

crystal surface

O

O

P

O

OH

Eidt, Science 1977, 197, 1327; Holliday, V.T. J. Archeological Science 2007, 34, 301.

Ca10(PO4)6F2

Ca10(PO4)6(OH)2

Al

Al

OH

O

O

P

O

O Al

Al

OH

Phosphate is part of an iron or aluminum hydroxide mineral

Phosphate is part of a calcium mineralPO4

3- is weakly adsorbed

Correlation Between Fractionation of Phosphorus and Land Use Source FrI

%FrII%

FrIII%

Total (ppm)

Land Use

Norway 84 7 9 1256 Mixed veg. farming

Germany 82 10 8 78 Mixed veg. farming

Colombia 85 11 4 206 Mixed veg. farming

Germany 44 49 7 43 Forest (pine)

Wisconsin 34 54 12 315 Forest (maple)

Argentina 38 28 34 2324 Residential (abnd)

Wisconsin 40 20 40 1393 Residential(modern)

(Eidt, 1974)

Goals

Extract inorganic phosphorus from soil samples Quantify phosphorus fractions in Fayetteville soil

using Murphy-Riley colorimetry Quantify fractions in soil using ICP-OES Compare colorimetry to ICP-OES Possibly decide which method is superior

Soil Preparation and Extraction

Fraction Extraction Solvent

Fraction 1- Loosely Bound Phosphorus

NaOH/NaCl-shake 12 hrs

Na3Cit/HCO3- heat 30 mins.

Fraction 2- Occluded Phosphorus Na3Cit/HCO3-heat

Na-dithionite-let oxidize for 8 days

Fraction 3-Calcium Phosphorus HCl-shake 4 hrs

1) Air dry overnight

2) Cone and Quarter

3) Sieve

4) Extraction on 2 g of soil

http://www.bridgewater.edu/~koverway/courses/CHEM320/ppts/Section2Sampling.pdf

Cone and Quartering

Extraction Solvents for each Fraction

Methods of Quality Control1) A sample duplicate every ten samples

2) A standard reference soil purchased from NIST

3) A calibration curve is produced using prepared phosphate standards every time colorimetry or ICP-OES is performed

Murphy-Riley ColorimetryMolybdenum is added to the extract

A Molybdophosphoric acid is formed, and then reduced by sodium citrate or ascorbic acid to form a blue color

The more intense the blue color, the more phosphorus present

Immediately quantified using a spectrophotometer

http://www.umaine.edu/SECRL/photos.htm

y = 0.2479x + 0.0004

R2 = 1

0

0.05

0.1

0.15

0.2

0.25

0.3

0 0.2 0.4 0.6 0.8 1 1.2

y = 0.0411x - 0.0057

R2 = 0.9979

-0.005

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0 0.2 0.4 0.6 0.8 1

y = 0.2429x + 0.0018

R2 = 0.9986

0

0.05

0.1

0.15

0.2

0.25

0.3

0 0.2 0.4 0.6 0.8 1 1.2

Frac.IA: June 20, 2007 Frac. IB: July 6, 2007

Frac. 2: July 30, 2007 Frac. 3: July 17, 2007

y = 0.0317x - 0.004

R2 = 0.9984

0

0.005

0.01

0.015

0.02

0.025

0.03

0 0.2 0.4 0.6 0.8 1

Colorimetry Standard Curves

Colorimetry Results

%Fraction1 %Fraction 2 %Fraction 3average % 38.32 57.12 4.57

stnd dev. 16.90 17.59 4.25

min 11.49 16.21 0.75

max 79.21 85.86 29.23

All extractions and colorimetry of soil samples were performed successfully from May to July 2007. The results are highly variable

Quality Control Results

A rigorous quality control was followed May to July: 1) Percent differences for the duplicates ranges widely:

Fr1:24.4%Fr2:17.5%,Fr3:38.2%.

2)The SRM, certified by NIST to contain 860 mg of phosphate per kg of soil, measured to contain an average of 612 mg/kg. 28.8% difference

3) The average R-squared value, for the calibration curves, was 0.9932

Colorimetry Pros and Cons-Inconsistencies between duplicates inhomogeneous soil or non-reproducible extraction

-Requires a lot of preparation

-Time Consuming

-Fairly Good Calibration Curves

-Few Interferences

ICP-OESInductively coupled plasma-optical emission spectroscopy

Extract is aspirated into the torch of the ICP, where it is atomized

Atoms become excitedemitting photons at a characteristic wavelength

Intensity of the emission is correlated with the concentration of the phosphorus in the sample http://www.icp-oes.net/images/torche2.jpg

ICP results

Standard Calibration

y = 598.18x + 1.0436R2 = 0.999

0

50100

150

200

250300

350

0 0.2 0.4 0.6

Concentration of P (ppm)

Inte

nsity

(CPU

)

LOD=0.007 ppm

Sample Fraction Conc. w/ ICP-OES (ppm)

Conc. w/ Colorim. (ppm)

13 3 0.143 4.794

61 2 0.822 287.764

55 2 0.659 355.491

55 3 0.128 7.893

27 1b 0.809 174.810Standard Calibration Curve

What Happened?Interferences

-unlikely:ICP should atomize completely

-matrix spike

Bind to colloidal soil particles in solution

-heating

-nitric acid digestion

Bind to walls of container

Further Work

Storage Time Study Method development to

determine phosphorus bonded to bottle walls

Running fresh extracts on ICP-OES and colorimetry

Construction of surfer plots

Summary- > 50 samples from Fayetteville were sequentially extracted.

- > 200 extracts were analyzed using M-R colorimetry.

-Colorimetry data will be used to construct Surfer plots.

-Due to sample storage, it is impossible to say whether ICP-OES or colorimetry is the best method.

Acknowledgements

I would like to acknowledge the following people for their help:

-Dr. Carol M Babyak

-Dr. Keith Seramur

-Dr. Shea Tuberty

-Dr. Lynn Siefferman

-The A.R. Smith Department of Chemistry

References Eidt, Robert C. 1977.Detection and Examination of Anthrosols

by Phosphate Analysis. Science. 197. 4311: p. 30-34 Eidt, Robert. Woods. 1974. Theoretical and Practical

Considerations in the Analysis of Anthrosols. Abandoned Settlement Analysis. 1st ed, p.155-189.

Murphy, J. Riley,J.P.1962. A MODIFIED SINGLE SOLUTION METHOD FOR THE DETERMINATION OF PHOSPHATE IN NATURAL WATERS. Anal.Chim.Acta, 27: 31-36.

Sobeck, Ebeling.2007. “Mass Spectrometric Analysis for Phosphate in Soil Extracts.”Analytical Sciences Digital Library E-UGR. <http://www.asdlib.org/articles.php?type=eUGH>

Standard Methods for the Examination of Water and Wastewater. 1999. p.4-139 – 4-147.