Transport of Organic Phosphate in Soil as Affected by Soil Type1

D. L. HOFFMAN AND D. E. RoLSTON2

ABSTRACTMiscible displacement experiments were conducted on live

soils using 7.5-cm diam and either 2.4- or 5.1-cm long columns.Glycerophosphate (771-2,195 ppm-P) and chloride (100 meq/liter) were applied in a 50-ml pulse to soil columns maintainedat steady water contents slightly less than saturation. Twomathematical models were used in fitting the effluent con-centration curves. The first was an analytical solution whichconsisted of first-order hydrolysis kinetics and reversible, in-stantaneous, linear adsorption reactions. The second was anexplicit-implicit finite difference solution which consisted offirst-order hydrolysis kinetics and two types of adsorption sites,one of which was reversible, instantaneous, and nonlinear; theother was reversible, kinetic, and nonlinear. The values of thepartition coefficient and the exponent of the Freundlich iso-therm used in the two-site model were determined by batchadsorption isotherm experiments. The rate coefficients of theadsorption-desorption reactions and hydrolysis reactions as wellas the proportions of adsorption sites that were kinetic orequilibrium were determined by fitting calculated to measuredglycerophosphate elution curves. Both the linear adsorptionpartition coefficient and the hydrolysis rate coefficient weredetermined by fitting elution curves of glycerophosphate usingthe linear, equilibrium model. The fitted partition coefficientwas not in agreement with the batch adsorption isotherm in-dicating that the adsorption-desorption reactions were not in-stantaneous. Both mathematical models were capable of fittingthe chloride effluent curves of all five soils quite well. Thetwo-site model was capable of fitting the glycerophosphateeffluent curves of all five soils more closely than the linear,equilibrium model. The two-site model, though, was incapableof accurately describing the adsorption-desorption reactions intwo of the soils. This was apparently due to a wide range ofkinetic rates of the reactions in these two soils.

Additional Index Words: glycerophosphate, adsorption-de-sorption, phosphorus fertilization, phosphatase enzyme, reactionkinetics, hydrolysis, transport theory.

Hoffman, D. L., and D. E. Rolston. 1980. Transport of organicphosphate in soil as affected by soil type. Soil Sci. Soc. Am. J.44:46-52.

T TNDERSTANDING THE MOVEMENT of organic phos-*~J phates in soil is important in relation to the po-tential of phosphorus pollution of groundwater fromsuch sources as feedlots and industrial and municipalwaste dumping, and in relation to the possibility oforganic phosphates as fertilizers. Spencer and Stewart(1934), Hilbert et al. (1938), Conrad (1939), andPinck et al. (1941) showed increased movement oforganic phosphates over inorganic phosphates in soil.More recently Rolston et al. (1975) and Castro andRolston (1977) showed that the movement of organicphosphates with irrigation water is greatly affectedby soil type.

Hydrolysis and adsorption-desorption reactions con-trol the transport of organic phosphates in the soil

1 Contribution from the Department of Land, Air and WaterResources, University of California, Davis, CA 95616. Received9 Nov. 1978. Accepted 14 Sept. 1979.

a Research Assistant and Associate Professor of Soil Science,respectively.

solution. Organic phosphates are hydrolyzed in thesoil by the enzyme phosphatase. Brams et al. (1975)showed that first-order kinetics could be used to de-scribe the hydrolysis of nitrophenylphosphate.

Many different descriptions of the adsorption-de-sorption reactions of solutes in soil have been usedby various investigators. Castro and Rolston (1977)used a single linear-instantaneous term to describeadsorption-desorption reactions of glycerophosphate.The adsorption of pesticides, even at high concentra-tions has been well described by a Freundlich relation-ship by Rao and Davidson (1979). Mansell et al.(1977) studied the effect of using linear-instantaneous,nonlinear-instantaneous, and nonlinear-kinetic reac-tion terms for modeling the transport of phosphate.Cameron and Klute (1977) developed a model using acombination of linear-kinetic and linear-instantaneousterms. Selim et al. (1976) combined nonlinear-kineticand instantaneous terms.

The objectives of this research were to investigatethe movement of glycerophosphate in several soil typesusing miscible displacement techniques and to com-pare measured elution curves to those calculated fromusing displacement techniques and organic phosphateconcentrations in the range that might occur duringapplication of an organic phosphate fertilizer with ir-rigation water.

THEORYIn general form, the equation describing the movement of

a reactive solute in a uniform porous media may be written

e at at a* [i]where p is the bulk density (g cnr8), e is the volumetric watercontent (cm3 cm'3); S is the amount of solute adsorbed on theporous media (/<g g'1 soil); C is the concentration of the solutein solution (/jg cm'3 soil solution); D is the convective diffusioncoefficient (cm2 hour'1); v is the average pore water velocity(cm hour"1); 0 is the rate of removal of solute from solution byirreversible reactions (jug g"1 hour'1) such as hydrolysis, trans-formation, or precipitation; t is time (hour); and x is the spacevariable (tm).

In this study, two mathematical models were used to describethe movement of glycerophosphate (GP) in soil. The first modelassumes a single type of adsorption site, at which the reactionis assumed to be linear and instantaneous with the adsorbedphase in equilibrium with the solution phase, and hydrolysisof the glycerophosphate obeying first-order kinetics. Thislinear, equilibrium model is described by the equation:

n«JT = D * SC A-*- ' ~ A [2]

where D* = D(\ + R)~\ v* = v(\ + R)-\ and A'* = kff^ (1 +R)~l, R — p9'^R', k (houi-1) is the hydrolysis rate constant re-sulting from the assumption that the hydrolysis of GP followsfirst-order kinetics (Brams et al., 1975), and R' (cm3 solutiong"1 soil) is the adsorption-desorption partition coefficient. Inthis model, R' is derived from the Freundlich equation

S = R'CX [3]

with N — 1.The solution of Eq. [2] for a semi-infinite medium with zero

initial concentration of solute at x = 0 and t = 0, solute con-centration of C0 for t between 0 and t,, and zero concentrationof solute for t > ^ is given by Eq. [3a] of Misra et al. (1974).

46

HOFFMAN & ROLSTON: TRANSPORT OF ORGANIC PHOSPHATE IN SOIL AS AFFECTED BY SOIL TYPE 47

Parlange and Starr (1975) have shown that the calculatedcurves given by the solution for a semi-infinite column areessentially the same as the elution curve from finite columns ifthe Peclet numbers (vL/D) are > 4, where L is column length.The Peclet numbers were > 4 for all experiments discussedhere.

The other model used in this study assumes two types ofadsorption sites. The two site types will be denoted Si andSn. Sites Si are characterized by kinetic, nonlinear adsorption-desorption reactions. The adsorption-desorption reactions atSites Sr are described by the equation

dt p 1 'where S: is the amount of solute adsorbed at sites of type Si(/<g GP g"1 soil), fej is the adsorption rate coefficient (hour"1),and k2 is the desorption rate coefficient (hour-1). When equili-brium exists between the concentration of GP in solution andthe amount adsorbed on the soil, then

S, = fi',C-v = JR'C» = ~ • C*P «2

[5]

where / represents the fraction of the total sites which are oftype Si, R' is the partition coefficient of the combined sites,and .R'i is the partition coefficient of sites S:.

Sites Sn are characterized by instantaneous, nonlinear adsorp-tion-desorption reactions (Eq. [3] for N ^ 1). These reactionsare described by the equation

where Sn is the amount of GP adsorbed at Sites Sn (yug GPg-1 soil), and (l—f)R' = R'n is the partition coefficient ofSites S,,.

Substituting the adsorption terms from Eq. [5] and [6] anda first-order sink terms into Eq. [1] yields

I? = D -ac

t7]

Equation [7] was solved for a column of length L subject tothe following initial and boundary conditions:

C = 0

S, = 0

S,, = 0

vC — D — = t/C0

vC — D — = 0

ac7T = 0

x = 0

x = 0

x = L

0 < t

t >

t > 0

[8a][8b]

[8c]

[8d]

[8e]

[8f]

using the explicit-implicit finite difference approximation meth-od (Carnahan et al. 1969; Salvador! and Baron, 1961). The

finite difference solution of Eq. [7] and [8] is the same asthat of Selim et al. (1976) except a sink term for hydrolysiswas added.

MATERIALS AND METHODSMiscible Displacement

Crushed soils were sieved through a 3-mm screen and poured,in increments, into 7.5-cm diam, acrylic plastic columns (2.4-to 5.1-cm long) to the bottom of which was attached a porous,fritted glass plate. Basic properties of the five soils used areshown in Table 1. With the addition of each increment ofsoil, the soil in the column was stirred to assure uniform ag-gregate distribution. The exterior of the column was tappedto cause the soil to settle, producing a reasonable bulk density.After leveling and smoothing the soil surface, another frittedglass plate was attached to the top of the column. The assem-bled column was then attached to two influent constant headburets at the top and to an effluent fraction collector at thebottom.

In order to establish steady-state flow in the columns, 0.01NCaSO4 solution was supplied to the top of the column from oneof the constant head burets until the influent and effluent flowrates had stabilized. This was followed by the application, attime t = 0, of a 50-ml pulse of glycerophosphate (GP) and Cl(100 meq liter-1). In the columns containing Columbia, Pano-che, and Redding soil, the head at both the top and bottom ofthe column was maintained at —5 cm of water. For the Aikenand Sacramento soil columns, the heads at the top and thebottom were maintained at 0 and —10 cm of water, respectively,in order to achieve a larger flow rate than was possible withequal heads at either end.

The effluent from the column was collected in samples ofabout 8 ml each in an automatic fraction collector. The Cl con-centration in the effluent samples was determined using aBuchler Cl analyzer. The concentration of inorganic P wasfound to be insignificant (< 1 ppm) in the effluent of all col-umns. Therefore, a total P determination was used to measurethe concentration of organic P.

The total P concentration in the effluent was determined by,first, digesting an aliquot of each sample with HC1O4, and thenanalyzing the digest for inorganic P. The Murphy and Rileymethod (1962) was used for all inorganic P determinations.

Batch Adsorption IsothermsBatch adsorption isotherms for GP were conducted on auto-

claved, duplicate samples of each of the soils in order to pre-vent hydrolysis during the adsorption experiments. Five gramsof autoclaved soil were shaken for 2 hours in 50 ml of a solu-tion containing a known amount of GP. Four concentrations ofsolutions in the range of those expected within the soil solu-tion of displacement experiments were used for each soil withthe greatest being 1,200 ppm. The disadvantage of using suchhigh concentrations is that small changes in concentration aredifficult to measure accurately.

Immediately after shaking, 0.5 ml of 25% K.C1 solution wasadded to each flask and soil was filtered from the solution usingWhatman no. 40 filter paper. The filtrate was then analyzedfor total P using the same method described for the columneffluent samples.

Table 1—Basic soil properties.Aiken Columbia Panoche Redding Sacramento

Classification

pHCEC, meq/100 gOrganic carbon, %Specific surface area, m' g~'Particle size analysis, %

ClaySiltSand

Clayey,kaolinitic,MesicXericHaplohumults

6.315.53.00

161

213643

Coarse-loamy,mixed,nonacid,' thermicAquicXeroflu vents

6.412.00.46

66

122563

Fine-loamy,mixed (calcareous),thermicTypicTorriorthents

7.529.50.83

197

294328

Fine,kaolinitic,thermicAbrupticDurixeralfs

5.012.5

2.0779

104050

Very-fine,montmorillonitic,thermicVerticHaplaquolls

6.060.0

4.98164

513613

48 SOIL SCI. SOC. AM. J., VOL. 44, 1980

Samples of each soil were subjected to identical treatment butwithout the addition of GP to the flask to make certain thatno interference would result from the release of native phos-phates due to autoclaving or K.C1 addition. No significantamounts of organic or inorganic P were found in the filtrates.

All of the experiments were conducted at a constant tem-perature of 25 °C.

RESULTS AND DISCUSSIONUsing the two-site model, both R' and N from the

Freundlich equation, must be included with the inputdata for the computer simulation. Batch adsorptionisotherms for GP were run in order to determinethese parameters. Figure 1 shows the experimentalpoints for each soil from the batch studies, as well asthe curves representing the Freundlich equation fittedto the data points. Data fitting was done using aleast squares error method.

Figure 1 shows that Aiken loam, a soil with a con-siderable amount of Fe and Al oxides, had the highestadsorptive capacity of the five soils tested. TheFreundlich parameters, R' and N, fitted for the Aikenisotherm were 71.2 cm3 solution g"1 soil and 0.45,respectively. Sacramento clay, a soil with relativelyhigh organic matter content, showed the next highestadsorptive capacity, with R' and N values of 7.92 cm3

solution g"1 soil and 0.70, respectively. Of the threeremaining soils, Panoche clay loam showed a slightlyhigher adsorptive capacity than Redding loam which,in turn, had an adsorptive capacity slightly greaterthan that of Columbia sandy loam. The R' and 2Vvalues for Panoche and Redding were 3.16 cm3 solu-tion g"1 soil and 0.72 and 12.75 cm3 solution g"1 soiland 0.47, respectively. The coefficients of determina-tion (r2) for Aiken, Sacramento, Panoche, and Red-ding soils were all > 0.91. The fact that the glycero-phosphate adsorption isotherms were well describedby the Freundlich equation over a wide concentrationrange suggests that adsorption sites were not saturatedat any concentration used in this study. For Columbia,however, the r2 was very small (0.59) indicating that

• AIKEN ——» COLUMBIA» PANOCHE° REDDING• SACRAMENTO

200 400 600 800 1000EQUILIBRIUM SOLUTION C fag P/cm3 solution)

1200

Fig. I—Two-hour adsorption isotherms for GP with each ofthe five soils. The symbols represent measured values. Singledata points at nearly the same solution concentration in-dicate equal adsorption for duplicate samples. The curvesrepresent the least squares error fit of the Freundlich equa-tion (except Columbia).

a Freundlich relationship did not describe adsorp-tion of GP on Columbia or that the errors in deter-mining very small amounts of GP adsorbed from largesolution concentrations were large. The values of R'and IV (33.6 and 0.26) for Columbia were very muchdifferent than those of the other soils. An attempt touse these values of R' and N in the two-site model re-sulted in complete inability to fit the calculated tothe measured curve regardless of the values of f, k\,and k2 used. Consequently, a value of 2V of 0.7 wasselected which compared more reasonably with theother soils of similar physical and chemical characteris-tics. A Freundlich relationship was then fitted byeye to the Columbia data of Fig. 1 to give an R' valueof 1.5. This value of R' was also more in line with thevalues of R' determined from the linear, equilibriummodel. The curves of Fig. 1 and 3 for Columbia werecalculated using R' and N values of 1.5 and 0.7, re-spectively.

The relative concentration distribution of GP inthe effluent of the Panoche column is shown in Fig.2. The closed circles are the concentrations of GPin the column effluent. The solid curve representsthe calculated concentration of a reactive solute pro-duced by the linear, equilibrium model fitted to thecolumn effluent data. The broken curve representsthe calculated concentration of a reactive solute pro-duced by the two-site model, fitted to the columneffluent. The dashed curve represents the calculatedcurve produced by the two-site model when all ad-sorption sites are assumed to be of type Sn (nonlinear,equilibrium). The value of D determined in the fittingof the chloride effluent data by each model was used asthe D describing the displacement of GP in the simula-tion produced by that model. The values of D de-termined by the two models for each soil differedslightly due to the different boundary conditions used.

The calculated curve produced by the linear, equi-librium model was fitted to the elution data by alter-ing the partition coefficient, R', and the irreversiblesink rate coefficient, k. The value of R' mainly af-fects the position of the pulse in the column effluentwith respect to the volume eluted, while k mainly

0.4

ui 0.3oi-

0.2

0.1

I————I————I———PANOCHE CLAY LOAM

—— Linear, equilibrium—— Non-linear, equilibrium—— Non-lineor, two-site

2 3PORE VOLUMES

Fig. 2—Elution curve for GP resulting from a pulse applicationto a column containing Panoche clay loam. The solid, dashed,and broken curves were calculated from the linear, equilib-rium model; the nonlinear, equilibrium model; and the non-linear, two-site model, respectively.

HOFFMAN & ROLSTON: TRANSPORT OF ORGANIC PHOSPHATE IN SOIL AS AFFECTED BY SOIL TYPE 49

affects the area under the curve. The adsorption anddesorption rate coefficients, &t and kz, respectively, thefraction of total adsorption sites that are in equilib-rium, fj and the irreversible sink rate coefficient, k,are adjusted when fitting the two-site model to theelution data. Parameters kt and k% affect both theposition and shape of the calculated curve, while /primarily affects the position. The ratio of the valuesof ki and k2 is constant for any one soil as is shownin Eq. [5]. The value of &2 was calculated by the two-site model computer program according to the value ofki supplied. As with the linear, equilibrium model,the value of k affects the area bounded by the calcu-lated curve.

The values of k and R', determined by fitting theGP effluent data from the Panoche column using thelinear, equilibrium model, were found to be 0.01hour"1 and 0.28 cm3 solution g"1 soil, respectively.Since the values of R' and N were determined frombatch adsorption isotherms for the two-site model,rather than being fitted by the model, the parametersto be determined by curve fitting were ki, kz, f, and k.The fitted values of k\, k2, f, and k for the Panochecolumn were determined to be 20.0 hour"1, 6.39hour"1, 0.40, 0.01 hour"1, respectively. A compilationof fitted parameter values from both models appearsin Table 2.

Comparing the fit of the calculated curves to thedata of Fig. 2, it is readily observed that the two-sitemodel described the elution curve more closely thandid the linear, equilibrium model. The calculatedcurve generated by the two-site model reproduced theslopes of both the leading edge and the trailing edgeaccurately. The two-site model also produces a slowlydecreasing tailing of the relative concentration at thebase of the following edge, which is indicated on theGP elution curves of every one of the five soils. Thelinear, equilibrium model does not produce this tailingof the calculated curve. Both the nonlinearity of theadsorption terms and the addition of kinetic sites foradsorption contribute to these features of the calcu-lated curve. The calculated curve produced by thenonlinear-equilibrium model shows that the use ofinstantaneous adsorption reactions overestimates theretardation of GP.

Figures 3, 4, 5, and 6 show the relative concentrationdistribution of GP and the calculated curves for theColumbia sandy loam, Redding loam, Aiken loam,and Sacramento clay columns, respectively. In all casesthe two-site model proves to be better able to repro-duce the tailing shown by the elution data than doesthe linear, equilibrium model. The rapid rise of theelution data from the Columbia (Fig. 3) and Redding(Fig. 4) columns is also better predicted by the two-site model. The curves calculated by the two-sitemodel for Aiken (Fig. 5) and Sacramento (Fig. 6)appear to rise too rapidly. This may be due to ex-perimental problems caused when initiating the pulsewith very short columns. The Aiken and Sacramentocolumns were 2.4 cm in length — about half the lengthof the other columns.

While the linear, equilibrium model appears to becapable of describing the GP elution curves of somesoils fairly well (see Fig. 2 and 6), the fitted valuesof R' indicate that the assumption of instantaneousequilibrium between the concentration of GP in solu-tion and the amount sorbed on the soil is not valid.If the equilibrium assumption were valid, the fittedR' should be the value of the slope of a linear iso-therm describing the batch isotherm for that soil.Figure 7 shows a comparison between the batch ad-sorption data for Panoche clay loam and a linearisotherm with a slope equal to the value of R' asfitted by the linear, equilibrium model. At all con-centrations less than or equal to the concentrationof the GP pulse (C0 = 771 ppm), the calculated iso-therm, shown by the solid line, is well below the batchadsorption data points, represented by the solid cir-cles. Data from the Panoche column were used asan example because it had a Peclet number well inexcess of four, as required for the use of semi-in-finite boundary conditions, and because the GP elu-tion curve was well described by the calculated curveof the linear, equilibrium model (Fig. 4). In fact,all of the soils used, with the exception of Sacramentoclay which had the lowest Peclet number (5.58), showsimilar results. The fact that the linear isothermwith a slope of the fitted R' shows lower sorptionthan does the batch adsorption data indicates that thesolution and sorbed phases of GP were not in equilib-

Table 2—Pertinent physical parameters of soil columns.

Parameter Panoche Columbia Redding Aiken Sacramento

Column length, cmC0, ppm-PWater content, cm* cnrj

Bulk density, g cm~3

Pore volume, cm'Water flux, cm hour'1Pore water velocity, cm hour'T,, hour

.D cm* hour1

R ' , cm1 solution g"1 soilk, hour

5.1771

0.4931.22

111.20.110.23

10.12Linear,

0.030.280.01

5.1771

0.5051.30

113.90.641.241.08

5.1771

0.4561.39

100.00.471.062.50

2.42,195

0.6021.08

62.00.721.201.58

2.4868

0.6420.85

68.00.220.355.05

equilibrium model parameters0.160.210.76

0.190.320.25

0.470.640.38

0.151.160.19

Nonlinear, two-site model parametersD, cm' hour'R ' , cm3 solution g-1 soil*„ hour'1k,, hour"1

Nfk, hour

0.043.16

20.06.390.720.600.01

0.161.505.04.320.700.700.76

0.2112.750.500.0260.470.380.20

0.6971.20

0.600.01340.450.650.32

0.157.92

15.03.160.700.550.17

500.3

S 0.2CCl-zllJ<Jzoo

SOIL SCI. SOC. AM. J., VOL. 44, 1980

I r I ICOLUMBIA SANDY LOAM——— Linear, equilibrium

Non-linear, equilibrium— -— Non-linear, two-site

0 1 2 3 4PORE VOLUMES

Fig. 3—Elution curve for GP resulting from a pulse applicationto a column containing Columbia sandy loam. The solid,dashed, and broken curves were calculated from the linear,equilibrium model; the nonlinear, equilibrium model; andthe nonlinear, two-site model, respectively.

rium. The pulse was moving too rapidly for sorptionsites to be filled to the capacity indicated by batchstudies. The use of the partition coefficient deter-mined from a batch adsorption experiment in a modelwhich uses an equilibrium adsorption term would re-sult in an overestimation of the retardation of thereactive solute. The dashed curves shown in Fig. 2,3, 4, 5, and 6 show that using a nonlinear adsorptionisotherm with instantaneous adsorption does not cor-rect this discrepancy. These curves were calculatedusing the R' and N values determined from the batchadsorption experiments. Again, as with the linear,equilibrium model, the value of R' would have to bedecreased in order to shift the calculated curve to theleft to fit the effluent data.

Since the values of both R' and AT as used in thetwo-site model were predetermined, the calculatedcurves were fitted to the elution data by altering thekinetic rate coefficients (ki and k2), and the fractionof sites in equilibrium (/). Using the two-site model,both the shape of the calculated curve and its positionwith respect to the volume of effluent can be adjusted.Changing &i and A2 primarily alters the shape of thecurve. As &i and kz become smaller, the curve risesmore rapidly and falls more gradually. The two soilswith high sesquioxide contents, Aiken and Redding,

0.2

0.1

UJo:

i i iAIKEN LOAM

———— Lineor, equilibrium _——-~ Non-lineor, equilibriumr—- — Non- linear,.two-site

0.06

0.04

0.02

<_i

REDOING LOAM

Linear, equilibriumNon-linear, equilibrium

— -—Non-linear, two-site

"= "0 I 2 3 4PORE VOLUMES

Fig. 4—Elution curve for GP resulting from a pulse applicationto a column containinng Redding loam. The solid, dashed, andbroken curves were calculated from the linear, equilibriummodel; the nonlinear, equilibrium model; and the nonlinear,two-site model, respectively.

show kinetic rate coefficients which are much lowerthan the other three soils. The values of these co-efficients are given in Table 2. Assuming that theGP molecules are bonding to soil constituents, such asFe(OH)3, through the phosphate group (Sprankle etal., 1975) by a strong double bond formed by the re-placement of two singly coordinate OH groups (Rus-sell et al., 1974; Parfitt et al., 1975), it is reasonablethat desorption be slow. It is also well documentedthat the retention of phosphate by clays is increasedas the proportion of exchangeable Fe and Al is in-creased (Coleman, 1944a, b; Ellis and Truog, 1955;Hemwell, 1957). Sprankle et al. (1975) showed thatkaolinite, in particular, retains large amounts of GPwhen saturated with Fe and Al. The low values of&i and k2 used for the Aiken and Redding soils allowthe calculated curve to simulate the extensive tailingproduced by the very slow desorption of GP. Thegreatly skewed calculated curves with which the elu-tion curves of Aiken (Fig. 5) and Redding (Fig. 4)are fitted show the effect of using small rate coeffi-cients.

Altering the value of / in the two-site model pri-marily moves the elution curve forward or backwardwith respect to the effluent volume displaced, ratherthan changing the shape of the curve. Generally, withall other parameters held constant, the more sites thatare in equilibrium (as / approaches 1.0), the more

cuzo

CUO -

2 0.08zUJo 0.06 -o

w 0.04 ->

< 0.02 -UJtt

SACRAMENTO CLAY———— Linear, equilibrium

Non-linear, equilibrium-̂ l ———— Non-linear, two-site

2 3PORE VOLUMES

2 3PORE VOLUMES

Fig. 5—Elution curve for GP resulting from a pulse applicationto a column containing Aiken loam. The solid, dashed, andbroken curves were calculated from the linear, equilibriummodel; the nonlinear, equilibrium model; and the nonlineartwo-site model, respectively.

Fig. 6—Elution curve for GP resulting from a pulse applica-tion to a column containing Sacramento clay. The solid,dashed, and broken curves were calculated from the linear,equilibrium model; the nonlinear, equilibrium model; andthe nonlinear, two-site model, respectively.

HOFFMAN & ROLSTON: TRANSPORT OF ORGANIC PHOSPHATE IN SOIL AS AFFECTED BY SOIL TYPE 516OO

_8400

inSaooCDITo</>o<

PANOCHE CLAY LOAM

-S = 0.275 C

200 400 600 800 1000 1200EQUILIBRIUM SOLUTION C (/j.q P/cm solution)

Fig. 7—Two-hour adsorption isotherm for GP with Panoche ClayLoam. The closed circles are measured values. The line rep-resents a linear relationship using the value of R" determinedby fitting the elution data with the linear, equilibrium model.

retarded the calculated curve would be. All of thesoils, except Redding, show similar values of / rangingfrom 0.55 for Sacramento to 0.70 for Columbia (Ta-ble 2). The value of / fitted for the Redding columnwas 0.38. If a large proportion of the reactive sitesin the Redding soil are on the surface of the oxidespresent, which seem to release adsorbed GP very slow-ly, it is reasonable to expect that the sites would bemainly kinetic in character. Although Aiken also hasconsiderable oxide compounds present, it also has ahigh clay content which may provide a large reactivesurface area to provide the high proportion of moreequilibrium sites.

Curve fitting the GP elution curves using the twomathematical models resulted in two different valuesof the irreversible sink term, k, for four of the fivesoils. The difference in the values of k produced bythe two models was partially due to the different boun-dary conditions used for the two models, but adsorp-tion-desorption model type also affected the resultingk values. This is because the area under the calculatedcurve, which represents the amount of GP in the ef-fluent, changes as the shape of the effluent curvechanges, even though the peak height of the curveremains essentially the same. The value of k wasdetermined by matching the peak height of the calcu-lated curve to that of the elution curve. The peakheight was used in fitting k, rather than the percentagerecovery, because the slow release of GP from someof the soils resulted in incomplete recovery of unhy-drolyzed GP. The model that produces a largerbounded area when the calculated curve has beenfitted to the peak height will produce a smaller valueof k. The tailing produced by the two-site model re-sults in a larger area under the calculated curve at agiven peak height. The more tailing in the calculatedcurve of the two-site model, the greater the differencebetween the values of k as fitted by the two models.The soils which produced highly skewed elution curvesshowed larger differences in the value of k. The kfitted by the linear, equilibrium model was greaterthan that fitted by the two-site model by 18% forAiken loam and 23% for Redding loam. Panoche clayloam and Columbia sandy loam, on the other hand,

produced quite symmetrical elution curves and showeddifferences in k of only 4 and 0%, respectively.

The activity of phosphatase enzyme, which is in-volved in the hydrolysis of GP, is known to be pH-dependent. Rogers et al. (1940) found the maximumactivity of phosphatase to occur at pH 4.0. Phos-phatase activity is also affected by the mineral com-position of the soil. Saltzman et al. (1974) found thatkaolinite catalyzed the hydrolysis of an organic phos-phate. Of the three most acidic soils [Redding (pH5.0), Sacramento (pH 6.0), and Aiken (pH 6.3)],Redding and Aiken were kaolinitic. These two soilsshowed the highest fitted k values of the five soils. Thevalues of k fitted using the two-site model for Aikenand Redding were 0.32 and 0.20 hour"1, respectively.It is possible that precipitation of relatively insolublecompounds of the organic phosphate with free Feand Al in the Redding and Aiken soils may increasethe apparent rate of hydrolysis in these soils. Sacra-mento clay, with a pH of 6.0, had a fitted k value of0.17 hour"1. This was the third largest value of k,and was only slightly less than that fitted for Redding.Of the two remaining soils, Columbia (pH 6.4) has afitted k value of 0.76 hour"1, while Panoche, a mont-morillonitic soil with a pH of 7.5, had the lowest kvalue, at 0.01 hour"1.

SUMMARYThe value of the partition coefficient for glycero-

phosphate (GP) determined by simulating effluentdata, using a mathematical model which assumes linearequilibrium adsorption-desorption reactions, is notrepresentative of the partition coefficient determinedby batch adsorption techniques. The displacement ofGP in soil can be closely described with a mathematicalmodel which uses both kinetic and equilibrium-non-linear sites. However, due to the range of kineticrates in the complex adsorption-desorption reactionsof many soils, this two-site model will not describe thedisplacement of GP in all soils equally well. The hy-drolysis rate of GP in soils appeared to be satisfactorilydescribed using first-order reaction kinetics.

The infiltration of GP into soil is controlled by bothadsorption-desorption reactions and hydrolysis to in-organic P. In some soils, such as the Redding sandyloam used in this research, hydrolysis appears to bethe predominant factor limiting the movement of GP.In the case of Panoche clay loam, on the other hand,the adsorption of GP limited the rate of movementwhile hydrolysis was continuing very slowly. Theother three soils showed a more balanced relationshipbetween the effects of adsorption-desorption reactionsand hydrolysis.

ACKNOWLEDGMENTThe authors wish to express their appreciation to H. M. Selim,

P. S. C. Rao, J. M. Davidson, and R. S. Mansell for providingthe two-site computer program used in this research.

52 SOIL SCI. SOC. AM. J., VOL. 44, 1980

ERRATA

Transport of Organic Phosphate in Soil as Affectedby Soil Type

D. L. HOFFMAN AND D. E. ROLSTONSoil Sci. Soc. Am. J. 4:46-52 (Jan.-Feb. 1980 issue)

In the left hand column on page 47 (section onTheory), the following changes should be made in theparagraph preceding Eq. [4] and continuing throughEq. [7]:

a) Site type Si should be Su,b) Site type Sn should be Si,c) The fraction of total sites / should be (1 — /),d) (1 - /) should be /,e) The partition coefficients RI and RU should be

RU and RI, respectively.