microbiological and physicochemical changes occurring in a contaminated soil after...

AUTHORS

R. Iturbe � Instituto de Ingenierıa, UniversidadNacional Autonoma de Mexico, Coordinacionde Ingenierıa Ambiental, Grupo Saneamiento deSuelos y Acuıferos, Apartado Postal 70-472,Coyoacan 04510, Mexico, D.F., Mexico

Rosario Iturbe received her Ph.D. on hydraulicengineering from UNAM. She has great ex-perience on contaminants migration; ground-waters contamination; and remediation,petroleum-contaminated soils characteriza-tion, and treatment by means of physicho-chemical and biological processes (i.e. in-situand ex-situ soil washing, biopiles, air soil vaporextraction, and surfactant-enhanced biodegra-dation of aged petroleum fractions). Cur-rently, she is a researcher and group leader ofthe Soil and Aquifers Remediation group atthe Environmental Engineering Departmentof the Engineering Institute, UNAM.

J. Lopez � Instituto de Ingenierıa, UniversidadNacional Autonoma de Mexico, Coordinacionde Ingenierıa Ambiental, Grupo Saneamientode Suelos y Acuıferos, Apartado Postal 70-472,Coyoacan 04510, Mexico, D.F., Mexico

Jessica Lopez studied environmental engineeringat UPIBI-IPN, Mexico. Afterward, she receivedher M.S. degree in environmental engineeringfrom UNAM, Mexico. Today, she works in theEnvironmental Engineering Department ofthe Engineering Institute as a specialized tech-nician in the Soil and Aquifers Remediationgroup. This work is part of her M.S. degree ex-perimental work.

L. G. Torres � Departamento de Bioproce-sos, Unidad Profesional Interdisciplinaria enBiotecnologıa, Instituto Politecnico Nacional,Av. Acueducto s/n, La Laguna, Ticoman,07340, Mexico, D.F., Mexico;[email protected]

Luis G. Torres has experience in industrialwastewaters’ biological treatment and char-acterization and remediation of metal and/orpetroleum-contaminated soils. Currently, hisinterest is on the surfactant’s application to en-vironmental problems. His main research linesare surfactant-enhanced biodegradation ofaged petroleum fractions in soils, in-situ andex-situ soil washing, preparation of petroleumfractions-surfactant-water emulsions as a first

Microbiological andphysicochemical changesoccurring in a contaminatedsoil after surfactant-enhancedsoil washingR. Iturbe, J. Lopez, and L. G. Torres

ABSTRACT

A soil contaminated with petroleum hydrocarbons, arising from

an old Mexican refinery, was previously characterized in terms of

total petroleum hydrocarbons (TPHs), diesel and gasoline fractions,

benzene-toluene-ethylbenzene-xylene (BTEX), polycyclic aromatic

hydrocarbons (PAHs), and some metals. A health risk analysis de-

termined that some PAHs should be reduced, and the Mexican stan-

dard indicates that high TPHs concentrations must be reduced to

2000 mg kg�1. This soil was submitted to a surfactant-enhanced

washing using sodium dodecyl sulfate (SDS) with and without NaCl,

polyethoxylated sorbitan monoleate (TW80), and polyethoxylated

nonylphenol (E600) (an anionic and two nonionic surfactants) at

different concentrations. Based on these experiments, it was decided

to wash soils with given doses of each surfactant in a continuous sys-

tem, where 9 kg of soil was washed alternatively with surfactant solu-

tions and plain water during 42 days. After this process, soils were

drained, dried, and milled. Parameters, such as microbial count,

total N, organic carbon and organic matter, available P, pH, dry and

bulk densities, electrical conductivity (as a measure of salts content),

Na, K, Ca, and Mg, were measured before and after the washing

process. In general, all parameters were affected by the soil washing,

but the most interesting changes were the increases in organic mat-

ter, electrical conductivity (for the experiments in which SDS was

used), phosphorus, and total nitrogen (except for the experiment with

TW80). Log total heterotrophs count was reduced to one half in most

experiments, and Na and Mg were, in general, diminished, whereas K

and Ca were augmented because of the surfactant soil washing. How-

ever, statistical analysis (analysis of variance [ANOVA]) indicated

that only electrical conductivity and phosphorus were significantly

affected (p < 0.05). Changes are discussed and compared with

Environmental Geosciences, v. 15, no. 4 (December 2008), pp. 173– 181 173

Copyright #2008. The American Association of Petroleum Geologists/Division of EnvironmentalGeosciences. All rights reserved.

DOI:10.1306/eg.06060808002

changes occurring in soils caused by different nonanthropogenic

events such as acid rain, organic fertilization, reseeding with native

species, amending of soils with manure, etc.

INTRODUCTION

Surfactant-enhanced soil washing has become a very important

remediation technique for hydrocarbons and metal-contaminated

soils in Mexico. Because of its low cost and ease of application, it has

been used at laboratories, pilot plants, and full-scale levels by our

research group. Different surfactants and surfactant mixtures have

been employed for cleaning up soils contaminated by total pe-

troleum hydrocarbon (TPH)-diesel and TPH-gasoline fractions,

benzene-toluene-ethylbenzene-xylene (BTEX), polycyclic aromatic

hydrocarbons (PAHs), and metals, among other contaminants (Torres

et al., 2003, 2005a; Iturbe et al., 2004a).

Used surfactants are ionic and nonionic regarding their chemical

nature and synthetic and natural according to their anthropogenic

or nonanthropogenic origin. Some employed surfactants that have

been used are of food grade (i.e., the Tween and Span series, devel-

oped by Imperial Chemical Industry [ICI] company), but many others

are not considered in that group. Some surfactants have even been

studied because of the possible environmental disturbances that they

can produce on natural environments, such as the nonylphenol sur-

factants (Radix et al., 2002).

Mexican legislation (as many others in the world) considers soils

within three kinds of uses, and hence, three different sets of limits

are established in the standard NOM-EM-138-SEMARNAT/SS-

2003 (SEMARNAT, 2005). These uses are industrial, habitational,

and agricultural. The main restrictions for soils are TPH content, the

four BTEX, and six PAHs, i.e., benzo[a]pyrene, dibenzo[a,h]anthra-

cene, benzo[a]anthracene, benzo[b]fluoranthene, benzo[h]fluor-

anthene, and indene[1,2,3-cd]pyrene. The levels for TPHs, BTEX,

and PAHs are less restrictive for industrial use, followed by habita-

tional purposes, and finally for agricultural use.

Our research group has made some contributions regarding

these issues, studying the biodegradability of some surfactants, in

terms of total surfactant concentration (final biodegradation) and

total organic carbon (TOC) content (mineralization) (Torres et al.,

2005b), as well as on the use of natural surfactants such as guar and

locust bean gums (Torres et al., 2006) for surfactant-enhanced soil

washing procedures.

The questions remaining when washing contaminated soils with

surfactants are Do the physicochemical and microbiological char-

acteristics inherent to soils change because of the surfactant-aided

washing processes? What kind of changes occur and how determi-

nant will be those changes for soil fertility? These questions have not

been answered yet, but it is necessary to point out that the answers

are more important when soils have an agricultural use than when

their use is for habitational or industrial purposes.

step for biotreatments of fuels (e.g. biodesul-furation), and rheology and mixing of sludgesand suspensions applied to environmentalproblem solutions.

ACKNOWLEDGEMENTS

This work was conducted entirely at the Insti-tuto de Ingenieria, Universidad NacionalAutonoma de Mexico (UNAM). We thankA. Castro (II-UNAM) for his technical review.The authors also thank the help of the anon-ymous reviewers who enhanced the qualityof the article.

174 Microbiological and Physicochemical Changes Occurring in a Contaminated Soil

As far as we know, no publications focused on the

study of the changes in soils exist because of the

surfactant-enhanced soil washing processes. Two pub-

lications in the literature aimed their goals on the study

of changes in infiltration characteristics, effective suc-

tion at the wetting front, capillary rise and soil pene-

trability, and hydraulic conductivity changes caused by

surfactant application in agricultural soils (Liu and Roy,

1995; Abu-Zreig et al., 2003).

The aim of this work is to show quantitatively

the main microbiological and physicochemical changes

occurring in a soil after the application of surfactant-

enhanced soil washing. Parameters, such as microbial

count, total N, organic carbon and organic matter, P, pH,

dry and bulk densities, electrical conductivity (as a mea-

sure of salt content), Na, K, Ca, and Mg, were measured

before and after the remediation process.

MATERIALS AND METHODS

Soil Characterization

The soil employed in this work was obtained from an

old refinery site located in northern Mexico (Iturbe et al.,

2004b). In particular, this soil sample was obtained from

a zone very close to the coking plant. This soil is also

very similar to that used in our previous work (Lopez

et al., 2004). For the initial characterization of soil,

the United States Environmental Protection Agency

(USEPA) 418.1 method was applied to determine if

TPH and PAH were measured using the USEPA

8310 method. Dry and bulk densities were measured

according to American Society for Testing and Materials

(ASTM) D854-83 and probe methods, respectively. Po-

rosity was calculated correlating both. The organic car-

bon fraction was determined using the Walkley-Black

method. The pH value was measured according to the

ASTM D4972-89 technique. Total nitrogen was eval-

uated using the USEPA 351.1 method. Phosphorus was

measured according to the Bray-Kurtz I method. Elec-

trical conductivity was measured using the potentiomet-

ric method. The Na, K, Ca, and Mg were measured in

accord to USEPA techniques (EPA 6010). Heterotro-

phic bacteria were counted using the technique re-

ported by Torres et al. (2005a), which consists basically

of platting bacteria dilutions in agar Petri dishes.

Surfactants

Commercial surfactants employed along this work

were Canarsel TW80 (TW80), Emulgin 600 (E600),

and SDS. TW80 is an ethoxylated sorbitan monoleate

(Canamex SA de CV, Mexico), E600 is an ethoxy-

lated nonylphenol (Polaquimia, Mexico), and SDS is a

sodium dodecyl sulfate (Sigma Products, United States).

The two nonionic products were characterized by Torres

et al. (2003). E600 is the same as Surfacpol 906 used in

that work. The general properties of the three surfac-

tants are summarized in Table 1. The solutions were

prepared in distilled water and stored at 4jC until used,

except those used for the continuous assessments that

were prepared in tap water and used immediately. The

NaCl was of analytical grade (J. T. Baker, Mexico).

Soil Washing TPH Removals (Batch Assessments)

Soil samples (6 g) were placed in 40-mL vials contain-

ing 20-mL water; water + NaCl or a fixed concentra-

tion of a surfactant solution (with or without NaCl)

was added. The assessed surfactant concentrations were

0.1, 0.5, and 1%. Assessed NaCl (JT Baker, Mexico)

concentrations were 1, 2, and 3%. The flasks were kept

at room temperature during 20 hr under reciprocating

Table 1. CMC and Other Physical-Chemical Characteristics of the Surfactants

Commercial Surfactant

Name Chemical Name Type

Molecular

Weight

CMC*

(mg L�1)

MST**

(din cm� 1) HLBy

Canarsel TW80yy Ethoxylated sorbitan monooleate Nonionic 1308 65.4 43 15.0

Emulgin 600yy Ethoxylated nonylphenol Nonionic 483 45.06 30 11.0

SDSz Sodium dodecyl sulfate Ionic 288 1586 41 40.0

*CMC = critical micellar concentration.**MST = minimum surface tension in water.yHLB = hydrophilic-lipophilic balance.yyFrom Torres et al. (2003).zFrom Li-zhong and Chiou (2001).

Iturbe et al. 175

agitation. After this period, the flasks were allowed to

settle for 1 hr, and samples were taken for TPH analysis.

All assessments were conducted in duplicate, and the

average of the two tests is reported. Differences between

duplicates were always less than 5%. After Soxhlet ex-

traction, a gravimetric method was used. This method

has been used traditionally to assess the performance

of different surfactants and mixtures in washing contam-

inated soils and to establish the optimal surfactant doses,

and was used in this work with that purpose, but to stimu-

late the entire soil washing process, we decided to per-

form a continuous assessment of soil flushing.

In-Situ Soil Washing (Continuous Assessments)

Metal boxes and pipe connections were employed for

the continuous washing assessments. Contaminated soil

(9 kg) was added to each box (four treatments in du-

plicate: eight total). Boxes were fed with surfactant solu-

tions controlling the venoclisis valves. Surfactant solu-

tions were prepared by mechanical mixing and stored in

10-L plastic tanks. Washing cycles of 24 hr were applied

to soils as follows: during 12 hr, the surfactant solution

was fed (7 mL min�1, 5 L a day). By opening the upper

valves, the dirty solution was drained during a 12-hr pe-

riod at the same feed rate. Every other day, the soil was

washed with surfactant solutions, and every other day,

the soil was rinsed with the same amount of tap water to

that used for surfactant washing.

At the end of 42 days, all solutions and water were

drained, and the soil was dried, milled, and evaluated in

terms of pH, dry and bulk density, porosity, phospho-

rus, total nitrogen, electric conductivity, organic matter

(and organic fraction), granulometry, total heterotrophs,

K, Mg, Na, and Ca. Measurements were performed in

each soil box, including the nontreated soil sample.

Statistical Analysis

An analysis of variance (ANOVA) was used to ana-

lyze the effect of treatments (different surfactants as

well as the addition of NaCl) on the different pa-

rameters measured before and after the soil washing

process.

RESULTS

Soil Characterization

Figure 1 shows a general chromatogram for the soil.

The mixture of products covers the diesel retention

Figure 1. General chromatogram forthe contaminated soil. Retention time vs.abundance.


time and 20 min more. The soil contaminant concen-

tration for TPH/kg soil is 17 200 mg kg�1. Besides,

the 16 PAHs reported by USEPA as priority pollut-

ants were measured, and the following were found

in small amounts: benzo(a)anthracene (0.2855 mg

kg�1), benzo(a)pyrene (0.24 mg kg�1), chrysene

(0.2317 mg kg�1), fluoranthene (0.142 mg kg�1),

phenanthrene (0.04 mg kg�1), pyrene (0.0317 mg kg�1),

benzo(b)fluoranthene (0.0234 mg kg�1), benzo(k)fluor-

anthene (0.0196 mg kg�1), anthracene (0.0073 mg kg�1),

and naphthalene (0.0044 mg kg�1). Note that benzo

(a)anthracene, chrysene, benzo(a)pyrene, and benzo

(b and k)fluoranthene are PAHs considered as carcino-

genic (Lopez et al., 2004). These PAH concentrations

are quite low if compared with reports of a soil obtained

from a manufacturing gas plant (Yeom et al., 1996) where

PAH concentrations were in the range of 32 mg kg�1

(for indeno[1,2,3-cd]pyrene) and 2219 mg kg�1 for

naphthalene.

Table 2 shows the soil characterization results. Note

that the soil is slightly acidic, permeable, and with low P

and N levels. The carbon organic fraction (fco) was

normal for a soil of this kind. The Na, K, Ca, and Mg

levels are quite noticeable because monovalent ele-

ments are present in a high proportion in relation to

the divalent ones, although it is possible to find other

alkaline-earth metal species in the soil such as Be, Sr,

and Ba. Soil showed a low electrical conductivity (non-

saline soil).

At this point, it is important to mention that, in

our previous work (Iturbe et al., 2004a), we stated that

because of the contamination at the site from which

the soil was collected, a health risk assessment was

performed, and the results of the study suggested that

benzene concentrations must be reduced in 8 of the 16

studied refinery zones to 0.0074–0.0078 mg kg�1. Va-

nadium concentration must also be reduced in two zones

to a concentration of 100 mg kg�1. Benzo(a)pyrene

concentration must be reduced to 0.1 mg kg�1 only in

one of all the studied sites. In addition, the Mexican

standard NOM-138-SEMARNAT/SS-2003 (SEMAR-

NAT, 2005) suggests a maximum of 500 (light fraction)

to 6000 (heavy fraction) mg kg�1 for industrial soils.

Selection of the Surfactants to be Employed in theContinuous Assessments

Soil washing assessments were previously performed

to determine the surfactant concentrations to be em-

ployed in the box assessments in continuous mode

(Lopez et al., 2004). Surfactants assessed were SDS,

EW600, and TW80 at concentrations of 0.1, 0.5, and

1.0%. For the dose of 1%, removals were as follows:

E600, 65%; TW80, 60.3%; and SDS 56%. Based on the

results of the referred work, it was decided to assess

the use of NaCl to improve the ionic surfactant’s per-

formance. Because of the presence of Ca and Mg, the

surfactant-enhanced soil washing performance of an-

ionic surfactants, such as SDS, is reduced according to

the following reactions.

2NaDS þ Caþþ ! CaðDSÞ2 þ 2Naþ ð1Þ

2NaDS þ Mgþþ ! MgðDSÞ2 þ 2Naþ ð2Þ

As noted, Ca and Mg present in soils interact with

SDS (in this case, expressed as Na-DS), forming the

complex Ca(DS)2 and liberating Na+. This is an un-

desired reaction but can be handled through one of the

following approaches (Torres et al., 2005a): (1) using mix-

tures of surfactants, i.e., anionic + nonionic; (2) addi-

tion of NaCl or other Na+-containing salt; and (3) using

a Ca++ sequestrant, i.e., sodium metasilicate, zeolites,

etc. In this case, NaCl was used at concentrations of

1 and 2%. Different results were obtained; for 0.1%

concentration, the highest removal was observed for

TW80 (69.4%), followed by E600 (53.53%), and finally

for SDS (45.86%). For the 0.5% concentrations, best

results were observed for E600 (55.56%) and then for

Table 2. Soil Physical-Chemical Characteristics

Parameter (units) Values Method

Dry density (g cm�3) 1.4 ASTM d854-83

Bulk density (g cm�3) 2.5 Probe method

Porosity 0.4 Calculated from dry

and bulk density

pH 6.37 ASTMD4972-89

Electrical conductivity (mS) 2.2 Potentiometric method

Total nitrogen (mg kg�1) 582.15 USEPA 351.1

Available phosphorus

(mg kg�1)

4.8 Bray-Kurtz method

Organic carbon (%) 1.3 Walkley-Black method

Sodium (mg kg� 1) 27,567 Extraction: USEPA 6010B

Potassium (mg kg�1) 3240 Analysis: USEPA 6010

Calcium (mg kg�1) 7168

Magnesium (mg kg�1) 1621

Total of Na + K + Ca +

Mg metals (mg kg� 1)

39,696

Iturbe et al. 177

TW80 (49.08%) and at the end for SDS (16.1%). Fi-

nally, for the 1% solutions, the best case was found for

E600 (65.09%) followed by TW80 (60.27%) and SDS

at the end (55.98%). From these results, it was decided

to use 0.5% SDS with 1% NaCl and without any, 1%

EW600, and 0.5% TW80. With these assessments, the

use of two different nonionic surfactants (one nonyl-

phenol, one sorbitan monoleate, both polyethoxylated)

and an anionic one, with and without NaCl addition,

was covered.

Washing of Soils in Continuous Mode (Boxes)

Soil washing assessments were performed in duplicate.

After 42 days of operation, equipment was stopped and

soil was drained, dried, and milled. The TPH removal

efficiencies for each surfactant or mixture employed

were as follows: 0.5% SDS alone induced a removal

of 38.46%, whereas addition of the same amount of

SDS plus 1% NaCl induced a TPH removal of 48.76%

(a 10.1% difference). When using TW80 at 0.5%, the

TPH removal was lower, i.e., 39.24%, and finally, when

using E600, the TPH removal reached the lowest value

found in this work (32.38%). Results are the average of

duplicate assays.

Table 3 and Figure 2 show the soil parameter values

before and after soil washing. Results are the average

of two independent assessments. Regarding pH (ini-

tial value of 6.3 units), it can be said that soil washing

caused a slight increment in that parameter with all the

surfactants assessed. Nevertheless, according to Jones

and Wolf (1984), soils changed from slightly acid values

(6.1–6.5 pH units) to neutral values (6.6–7.3 pH units).

The changes were between 6.7 (SDS + NaCl) and 12.4%

(E600). Dry and bulk soil densities were also affected

by soil washing. Changes in dry density were between

10.7 (TW80) and 19.3% (SDS), whereas for bulk den-

sity, changes were between 0.02 (TW80) and 2.8%

(E600). Porosity was another parameter affected by soil

washing, i.e., changes were in the range of 22.5 (SDS)

and 25% (the rest of the assessments).

Regarding salinity, carbon, nitrogen, and phospho-

rus levels in the soil before and after the washing pro-

cess, the salinity level (measured as the electrical conduc-

tivity in mS) showed a big increment for the experiments

with SDS with and without salt (319 and 228%), where-

as for nonionic surfactants, salinity augmented 76.1%

(with TW80) and diminished with E600 (3.6%). The

biggest change was observed in the assessment with

only SDS. This change is related with the Na+-Ca++

interchange as stated in reactions 1, 2, and 3.

At the beginning of the process, the soil contained

approximately 580 mg kg�1 of available P, and washing

the soils with the different surfactants produced sub-

stantial increments. Soil changed from a low P con-

centration (<15 mg kg�1) to medium (1–30 mg kg�1)

Table 3. Some Microbiological and Physical-Chemical Changes in Soil Caused by the Washing Process*

Parameter

Initial

Soil

SDS

0.5%

Difference

(%)

SDS 0.5% +

NaCl 1%

Difference

(%) E600 1%

Difference

(%)

TW80

0.5%

Difference

(%)

pH (units) 6.30 6.74 6.98 6.69 6.19 7.08 12.38 6.84 8.57

Dry density (g cm�3) 1.40 1.13 �19.29 1.23 �12.14 1.21 �13.57 1.25 �10.71

Bulk density (g cm�3) 2.50 2.47 �1.20 2.46 �1.60 2.43 �2.80 2.50 0.00

Porosity 0.40 0.49 22.50 0.50 25.00 0.50 25.00 0.50 25.00

P (mg kg� 1) 4.80 41.40 762.50 30.54 536.25 22.60 370.83 28.51 493.96

Ntotal (mg kg� 1) 582.15 614.00 5.47 221.2 �62.00 721.54 23.94 677.7 16.41

Electrical conductivity (mS) 418.00 1751.00 318.90 1370.25 227.81 403.00 �3.59 736.25 76.14

Organic matter (%) 2.30 6.03 162.17 6.52 183.48 7.09 208.26 6.29 173.48

Organic carbon (%) 1.30 3.51 170.00 3.79 191.54 4.11 216.15 3.65 180.77

Total heterotrophs

CFU g soil� 1

6.6E06 5.5E05 �91.67 7.5E05 �88.64 5.1E05 �92.27 1.1E06 �83.33

Log total heterotrophs 15.70 7.11 �54.71 6.96 �55.67 7.25 �53.82 7.31 �53.44

Na (mg kg�1) 27,657 1831 �93.38 1777 �93.57 1876 �93.22 1715 �93.80

K (mg kg� 1) 3240 4242 30.93 4422 36.48 4441 37.07 4486 38.46

Ca (mg kg� 1) 7168 15,781 120.16 14,525 102.64 13,640 90.29 14,329 99.90

Mg (mg kg�1) 1621 675 �58.36 625 �61.44 615 �62.06 765 �52.81

*All values are an average of duplicates.


and high P concentrations (>30 mg kg�1) (NOM-021-

RECNAT-2001).

The reason for these changes could be that some

P-related complexes were mobilized by the surfactants,

which made them more available for measurement.

The highest increment was observed with the SDS

assessment with an increase of 762.5% followed by

SDS + NaCl with a change of 536.5%, then by E600

(494%), and E600 (371%).

Total nitrogen changes caused by the soil washing

process were more marked when washing with E600

(a 24% increase), followed by TW80 with 16.4%, and

SDS with 5.5%, whereas for SDS + NaCl, they resulted

in a decrease of about 62%. Regarding this parameter,

soils changed from rich soils (>0.22%) to medium-

rich soils (0.158 to 0.221%) and medium soils (0.126–

0.158%) (Moreno, 1978). Finally, both organic mat-

ter and organic carbon changes were more pronounced

when washing with E600 (an increment of 208.3 and

216.2%, respectively). Lower changes were observed

for SDS + NaCl (increments of 183 and 191% for or-

ganic matter and organic carbon, respectively). Regard-

ing organic matter, soil changed form medium soil

(1.81–2.4%) to extremely rich soils (>4.21%).

Regarding total heterotrophs before and after the

soil washing process, soils washed with SDS with and

without salts, as well as with E600, were affected in

their total count with a diminution of one order of mag-

nitude (from E06 to E05 carbonate fabric unit [CFU] g

soil�1), whereas the soil washed with TW80 was di-

minished in a factor of 1/6.

Regarding Na, K, Ca, and Mg (after soil washing),

changes were as follows. A clear diminution in the Na+

content of approximately 93% for the different surfac-

tants occurred. This is caused by reactions 1, 2, and 3. A

higher diminution was observed for TW80, but differ-

ences among assessments are very low. The K+ in-

creased in soils washed with all surfactants (about 31–

38%). The maximal increment was observed for TW80

(38.4%). Calcium content in soil increased in about

100% for experiments where SDS, TW80, or E600 was

used. When SDS + NaCl was used for soil washing,

Ca++ increased almost 120%. This fact was obviously

caused by the Na+-Ca++ interchange previously men-

tioned. With respect to Mg, when using E600, a 62%

diminution was observed, followed by SDS + NaCl

(61.4%), SDS (58%), and TW80 (53%).

DISCUSSION

Liu and Roy (1995) studied the changes in the hydrau-

lic conductivity of soils caused by the flow of ionic and

Figure 2. Changes in different parameters caused by the surfactant washing of soil. fco = carbon organic fraction.

Iturbe et al. 179

nonionic surfactants. They studied both surfactant ad-

sorption and precipitation phenomena in the presence

of anionic surfactants. These authors concluded that the

precipitation of the divalent electrolyte SDS seems to

be the prevalent mechanism influencing the change

in hydraulic conductivity. This change is related with

the Na+-Ca++ interchange according to

Soil � Caþþ þ 2Naþ ! Soil � Na2 þ Caþþ ð3Þ

Abu-Zreig et al. (2003) studied the effect of ap-

plying surfactants (sulfonic acid LS, Rexonic N25-7,

and Rexol 25/7) on the hydraulic properties of soils.

These authors used surfactants as soil conditioners to

improve its hydrophysical characteristics and soil struc-

ture and infiltration, as well as to control soil erosion.

They found that the anionic surfactant (sulfonic acid)

had a significant effect on the hydraulic properties of two

studied soils. Applications of sulfonic acid caused de-

creases in capillary rise and penetrability and an increase

in the solid-liquid contact angle, shape factor, and sorp-

tivity. Except for a slight decrease in hydraulic conduc-

tivity resulting from the application of Rexonic and

Rexol, the nonionic surfactants did not reveal a signifi-

cant impact on the hydraulic characteristics of test soils.

Gardner and Arias (2000) studied clay swelling and

formation, as well as permeability reductions induced

by a nonionic surfactant (sorbitan monoleate poly-

ethoxylated, called TW80 in this work). They investi-

gated the loss of permeability on flushing with solutions

containing TW80. The permeability change was cor-

related with the colloid transport in the column with

the highest clay concentration, although the effect was

transient. Clay swelling was postulated as the primary

mechanism for the permeability reductions.

Hansen and Strawn (2003) studied the kinetics of

phosphorus release from a manure-amended alkaline

soil in comparison with an unamended soil. They found

that Ca-P species (such as octacalcium phosphate) min-

erals control P release. Once these species were dis-

solved, a slow release of P occurs that is controlled by

less soluble P minerals, such as hydroxypatite. Changes

in Ca concentrations could affect the P changes accord-

ing to these findings, but more research is necessary to

reach a conclusion.

Lopez-Hernandez et al. (2004) studied the changes

in soil properties and earthworm populations induced

by the long-term organic fertilization of a sandy soil in

the Venezuelan Amazonia. They reported changes in

pH; organic matter; total nitrogen; total phosphorus;

microbial C, N, and P; phosphatase activity; and urease

activity caused by the fertilization program. Besides, sig-

nificant changes in K, Ca, and Mg were observed. These

changes were in the order of 62% for organic matter,

39% for total nitrogen, and 57.2% for total phosphorus.

Undoubtedly, changes in soil caused by surfactant-

enhanced washing will be very dependent on the kind

of soil. Main parameters affecting the changes would

be textural and chemical issues. As stated by Abu-Zreig

et al. (2003), the magnitude of changes depends on the

soil textural class and the surfactant characteristics and

their concentrations.

Although changes in soils caused by the type of sur-

factant were very dependent on the parameter under

discussion, Table 3 shows that more parameters with

the highest percent change were observed with E600

(bulk density, organic matter, total nitrogen, organic

carbon, and Mg). The rest of the assessments showed

three parameters with the highest percent change, i.e.,

dry density, electrical conductivity, and total count for

SDS; porosity, phosphorus, and Ca for SDS + NaCl;

and pH, K, and Ca for TW80.

The statistical analysis (ANOVA) with a confidence

value of 95% (i.e., p < 0.05), applied to the raw data,

indicated significant differences only for P and electrical

conductivity among the different treatments (i.e., em-

ployed surfactants).

As observed, changes caused by surfactant-enhanced

washing do not disturb drastically the soil’s microbio-

logical and physical-chemical characteristics. Changes

caused by acid rain, application of a pesticide, organic

fertilization, reseeding with native species, fire, amend-

ing of soils with manure, application of surfactants as soil

enhancers, etc., could produce bigger changes in soils if

compared with those affected by soil washing.

CONCLUSIONS

Regarding pH (initial value of 6.3 units), soil washing

caused a slight increment in it with all the surfactants

assessed. The change was between 6.2 (SDS + NaCl)

and 12.4% (E600). Dry and bulk soil densities were also

affected by soil washing. Changes in dry density were

between 10.1% (SDS + NaCl) and 19.3% (SDS), where-

as for bulk density, changes were between 0.02% (TW80)

and 2.8% (E600). Porosity was another parameter af-

fected by soil washing, i.e., changes were in the range of

22.5 (SDS) and 25% (the rest of surfactants).

Regarding salinity, carbon, nitrogen, and phosphorus

levels in the soil before and after the washing process, it


can be underlined that the salinity level (measured as

the electrical conductivity in mS) showed an increment

for the experiments with TW80 (76%) and SDS (319%).

At the beginning of the process, soil contained ap-

proximately 580 mg kg�1 of available P, after washing

the soils with the different surfactants, substantial in-

crements were observed. Probably some P-related com-

plexes were mobilized by the surfactants and made them

readily available for measurement. The highest incre-

ment was observed for the SDS assessment, with an

increase of 762%, followed by SDS + NaCl (536%),

TW80 (493%), and E600 (371%). Basically, this con-

stant increment in P levels is caused by the surfactant-

enhanced soil washing.

Changes caused by surfactant-enhanced washing

do not disturb drastically the soil’s microbiological and

physical-chemical characteristics. Changes caused by acid

rain, application of a pesticide, organic fertilization, re-

seeding with native species, fire, amending of soils with

manure, application of surfactants as soil enhancers, etc.,

could produce bigger changes in soils if compared with

those induced by soil washing.

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