Evaluation of different clay minerals as additives for soil water repellency alleviation

(2006) 238–248www.elsevier.com/locate/clay

Applied Clay Science 31

Evaluation of different clay minerals as additives for soil waterrepellency alleviation

Lubomir Lichner a,⁎, Pavel Dlapa b, Stefan H. Doerr c, Jorge Mataix-Solera d

a Institute of Hydrology, Slovak Academy of Sciences, Racianska 75, 83102 Bratislava, Slovak Republicb Department of Soil Science, Faculty of Natural Science, Comenius University, Mlynska dolina B-2, 842 15 Bratislava, Slovak Republic

c Department of Geography, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, UKd GEA — Grupo de Edafología Ambiental, Department of Agrochemistry and Environment, Universidad Miguel Hernandez,

Avda del Ferrocarril s/n, E-03202 Elche, Alicante, Spain

Received 29 July 2004; received in revised form 6 June 2005; accepted 17 October 2005Available online 23 November 2005

Abstract

The objective of this study was to evaluate the effectiveness of kaolinite, illite, Na- and Ca-montmorillonite in alleviating waterrepellency for a simple model soil material of known composition before and after wetting and drying phases. Sand was renderedwater repellent by adding 10 and 30 g kg−1 stearic acid followed by adding different amounts (1, 2 and 3 mass %) of respectiveclays. Treated and untreated control sand were wetted and exposed to prolonged drying phase at 50 °C to simulate the effects ofwetting followed by drying under a hot spell. The persistence of water repellency was measured with the water drop penetrationtime (WDPT) test. During the wetting/drying cycle, the control samples (stearic acid treated sand) and clay treated sands behavedin a fashion typically observed in water repellent soils: they were wettable above and water repellent below a critical water content,with repellency persistence (WDPT) increasing with decreasing water content. Kaolinite and Na-montmorillonite were found to bethe only clay minerals able to lower the persistence of repellency of the stearic acid sand. The difference between Ca- and Na-montmorillonite ability to alleviate water repellency is explained by the differences in inter-particle forces in the clay–exchangeable cation–water system. During the prolonged drying phase at 50 °C, the persistence of water repellency increased withthe duration of heating at 50 °C, with the increase being greatest during the first 48 h in the majority of the samples. While kaoliniteand Na-montmorillonite addition resulted in a reduction in the persistence of water repellency of the stearic acid sand, Ca-montmorillonite and illite addition increased the persistence of water repellency of the stearic acid sand. The net effect of claymineral addition is dependent on the amount of Ca2+ ions occurring at the surface of clay minerals. Pools of available(exchangeable) calcium decrease in the order as follows: Ca-montmorilloniteNN illiteNNNa-montmorillonite≈kaolinite.© 2005 Elsevier B.V. All rights reserved.

Keywords: Clay mineral; Soil water repellency; Wetting phase; Drying phase

⁎ Corresponding author. Tel.: +421 24 9268227; fax: +421 244259404.

E-mail address: [email protected] (L. Lichner).

0169-1317/$ - see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.clay.2005.10.012

1. Introduction

Soil water repellency (hydrophobicity) reduces theaffinity of soils to water such that infiltration orwetting may be delayed for periods ranging from afew seconds to hours, days or weeks. It is thought to

239L. Lichner et al. / Applied Clay Science 31 (2006) 238–248

be caused primarily by a coating of amphiphilic mole-cules on individual soil particles. These long-chainedhydrophobic organic substances may be released froma range of plants, organisms, and decaying organicmatter either naturally or during burning. The releaseof hydrophobic compounds by some plants is thoughtto be a competitive plant strategy. It may serve toreduce evaporation, to improve water conservation bychannelling water deep into the soil profile via pref-erential pathways, to reduce nutrient leaching from theroot zone during short-term, heavy rain events, and tosuppress the germination of competing vegetation(Novak, 1995; Ritsema and Dekker, 1996; Doerr etal., 2000; Gerke et al., 2001; Hallett et al., 2001;Lichner et al., 2002).

Water repellency of soils has major repercussions forplant growth, surface and subsurface hydrology, and forsoil erosion. Enhanced surface runoff, which can pro-mote soil erosion, may occur if heavy rainstorms followprolonged dry periods, a combination of events that isincreasing in frequency and intensity as a consequenceof climate change (Shakesby et al., 2000; Hardy, 2003;Holko and Miklanek, 2003; Zhang et al., 2004). Soilwater repellency generally increases during dry wea-ther, while it is reduced or completely eliminated afterprolonged and/or heavy precipitation (Doerr et al.,2000). This reduction in water repellency is thought tobe accompanied with a change in orientation of organicmaterials with amphiphilic structure (Ma'shum andFarmer, 1985). Re-establishment of water repellencymay be associated with the energy input during heatingor a new input of hydrophobic substances (Doerr andThomas, 2000).

An effective method to prevent soil water repellency,which is often used in sport turfs to prevent “dry patch”,or “localized dry spot”, is to irrigate frequently, thusmaintaining high soil moisture levels (Cisar et al.,2000). Other methods to mitigate water repellency in-clude the use of surfactants as commonly applied to golfgreens (Cisar et al., 2000; Kostka, 2000), and the addi-tion of clay to top soil (claying) either by deep plough-ing into a clayey subsoil, or by top dressing with clay asused successfully in some regions in Australia (McKis-sock et al., 2000). In addition to increasing the surfacearea of soils, clay is thought to be effective in amelio-rating repellency by masking hydrophobic surfaces inthe soil matrix and exposing hydrophilic clay surfaces(Ward and Oades, 1993).

Laboratory investigations have shown that the effi-ciency of clay additions may depend on clay mineralo-gy and the wetting history of soils, however, resultshave not always been in agreement. For example, dur-

ing repeated wetting and drying phases, the clays dom-inated by kaolinite were much more effective inreducing water repellency than their montmorillonite-rich counterparts (McKissock et al., 2000, 2002). How-ever, in sand in which repellency was artificially in-duced by cetyl alcohol, montmorillonite was moreeffective in reducing water repellency than kaolinite(Ward and Oades, 1993), and kaolinite was more effec-tive than illite (Ma'shum et al., 1989).

The aim of this study is to address the conflictingnature of previous findings by eliminating some of theuncertainties arising from the study of relatively com-plex particle mixtures. We therefore evaluated the abil-ity of four key clay minerals (kaolinite, illite, Na- andCa-montmorillonite) to alleviate water repellency byusing a highly water repellent model soil material witha well-known and physically and chemically simplecomposition. The model soil material was producedby adding a simple hydrophobic compound (stearicacid) to clean quartz (silica) sand. This model soil wastreated with clay additions and, together with untreatedcontrol model soil, was subjected to a wetting phase anda prolonged drying phase at 50 °C to simulate theeffects of a wetting phase followed by a drying phaseunder a hot dry spell. The effects of the various treat-ments on the persistence of water repellency wereexamined.

2. Materials and methods

Clean silica sand of medium particle size composition(with the particles b1 mm, and with 86.3% of particles inthe range of 400–800 μm) was used as the basis for the modelsoil. Water repellency of the silica sand was induced byaddition of stearic acid (Aldrich, purity 95%, molecularweight 284.5), resulting in a homogenous and highly repellent“model soil”. This was achieved by dissolving stearic acid indiethylether (Riedel–de Haën, p.a.), which was then added tothe silica sand and mixed up to total volatilization of diethy-lether. In this way, samples of stearic acid sand containing 10and 30 g kg−1 stearic acid were prepared.

The clay mineral materials (kaolinite KGa-1b, illite Imt-1,Na-montmorillonite Swy-2, and Ca-montmorillonite Saz-1)were purchased from the Society's Source Clay Repository(University of Missouri, Columbia, USA). Their Source Clays(kaolinite KGa-1b, Na-montmorillonite Swy-2, and Ca-mont-morillonite Saz-1) are derived from large, reasonably homog-enized stocks, and their pre-treatment involved low-temperature, steam-fired tray drying, and mill pulverization.Illite Imt-1 was homogenized by mill pulverization beforeapplication by us, as no attempt had been made to homogenizeor beneficiate the Special Clays by the producer. During thisoperation we found that it contained about 30% (vol.) ofquartz particles. The physical, chemical, and mineralogical

Table 1Water drop penetration time (WDPT) values of samples before thewetting and drying treatment

Sample WDPT (s)

10 g kg−1 stearicacid


Stearic acid sand (control) 6540 10,860Stearic acid sand+1%kaolinite

263 6400

Stearic acid sand+2%kaolinite

283 5660


880 6800

Stearic acid sand+1% illite 13,820 15,020Stearic acid sand+2% illite 14,240 17,360Stearic acid sand+3% illite 14,580 15,840Stearic acid sand+1% Ca-montmorillonite

8280 7300

Stearic acid sand+2%Ca-montmorillonite

8660 6920


6900 8000

Stearic acid sand+1%Na-montmorillonite

10,380 17,080


9280 14,060


6580 16,200

240 L. Lichner et al. / Applied Clay Science 31 (2006) 238–248

characteristics of the clay minerals used in this study aredescribed in Van Olphen and Fripiat (1979).

Air-dry model soil (i.e. stearic acid treated silica sand) wasplaced in a glass vial, and the relevant clay mineral was addedto obtain samples with the mass of 5 g and the clay contents of0%, 1%, 2%, and 3% (mass). The vial was gently shaken end-over-end for 2 min at 30 rpm to mix the clay with the sand.The samples were then placed in Petri dishes, and the persis-tence of water repellency was estimated with the commonlyused water drop penetration time (WDPT) test in all thesamples. This test determines how long water repellency per-sists on a porous surface and is thought to be most indicativeof the hydrological consequences of water repellency in soilsas it relates to the time required for raindrops to infiltrate(Doerr, 1998).

A wetting phase was imposed through adding 2 g ofdistilled water on the surface of each sample, and allowing itto penetrate for three days. Then the samples were gentlymixed to obtain identical moisture in the whole volume. Itshould be noted that water remained on the surface of themixtures of the repellent sands containing 30 g kg−1 stearicacid and treated with Ca-montmorillonite even after threedays. A drying phase was subsequently induced by dryingthe samples at 50 °C. During the drying phase, water repel-lency was measured repeatedly using the WDPT test in all thesamples. After reaching a water content near zero, the heatingof samples was continued to simulate a prolonged hot dryspell. Control samples (i.e. sand treated only with stearic acid)were heated for more than 2500 h, samples with kaoliniteadditions for about 1300 h, and those with illite, Na- andCa-montmorillonite additions for about 600 h. The tempera-ture of 50 °C was chosen to represent conditions that maycommonly be reached in some agricultural regions at thesurface of a bare soil during a hot summer (Doerr and Thomas,2000).

It should be mentioned that water content of the systemswith Na- and Ca-montmorillonite was not equal to zero at thestart of our experiments. The Na- and Ca-montmorillonitecontain one or two water layers in the interlamellar space atthe beginning, and this water cannot be removed by drying at20 °C. The Na- and Ca-montmorillonite water content were3.27% and 13.63%, respectively, as measured with the gravi-metric method (drying at 105 °C) by ISO 11465 (1993). As aresult, the water content of the samples with 3% Na- and Ca-montmorillonite additions at the start of the experiments were0.1% and 0.4%, respectively. Water content desorbed by dry-ing at 50 °C was 2.03% and 5.93% for Na- and Ca-montmo-rillonite, respectively.

The WDPT test involved placing three drops of distilledwater from a medicinal dropper onto the sample surface andrecording the actual time required for complete droplet infil-tration. In subsequent processing, an average of three WDPTvalues was used. The volume of water in a droplet was 58±5μL. A standard droplet release height of approximately 10 mmabove the soil surface was used to minimise the crateringeffect on the soil surface (Wylie et al., 2001). The followingclasses of the persistence of water repellency were distin-

guished: wettable or non-water-repellent soil (water drop pen-etration time WDPTb5 s); slightly (WDPT=5–60 s), strongly(WDPT=60–600 s), severely (WDPT=600–3600 s), and ex-tremely (WDPTN3600 s) water repellent soil (Dekker andRitsema, 1995).

3. Results and discussion

The silica sand, kaolinite, illite, Na- and Ca-mont-morillonite used in experiments were originally wetta-ble with WDPTb1 s. After treatment with stearic acidthe sand was extremely water repellent for both treat-ments with 10 and 30 g kg−1 stearic acid, with WDPTsof 6540 and 10,860 s, respectively. The effect of addedclay minerals on the results of the WDPT test for thesample mixtures before wetting or drying are listed inTable 1. It is surprising that an addition of wettable clayminerals (with the exception of kaolinite into 10 g kg−1

stearic acid sand, as well as kaolinite and Ca-montmo-rillonite into 30 g kg−1 stearic acid sand) resulted in anincrease in WDPT in both types of water repellentsands. The water content in the interlamellar space ofmontmorillonites was influenced by exchangeablecations with the order Na+bCa2+, because it increaseswith increasing the hydration energy of exchangeablecation (Dontsova et al., 2004).

Table 2Critical water contents for samples with kaolinite, illite and Ca-montmorillonite addition

Sample Critical water content (%)



Stearic acid sand (control) 0.01 2.8Stearic acid sand+1%

kaolinite0.1 0.7

Stearic acidsand+2% kaolinite

2.6 0.8

Stearic acidsand+3% kaolinite

3.5 2.8

Stearic acid sand+1% illite 2.2 4.7Stearic acid sand+2% illite 5.3 11.8Stearic acid sand+3% illite 7.1 14.1Stearic acid sand+1%

Ca-montmorillonite1.3 4.0


3.4 6.9


4.7 9.1


3.1. Effect of wetting/drying cycle on water repellency

After wetting and mixing the fully or partially wettedsamples, the water repellency disappeared in all thesamples indicating that the water contents achievedwere higher than the critical level for repellency to be

Fig. 1. Effect of 1% clay addition on the relationship between

exhibited. The critical water content wk is defined asthe upper water content, above which a soil will notexhibit water repellency. To account for the effects ofhysteresis, Dekker et al. (2001) suggested to use theterm transition zone, above which samples are wetta-ble, within which repellency may or may not occur,and below which susceptible samples will exhibitrepellency.

During the drying phase, the water repellency did notre-appear until the water content decreased below acritical water content. The critical water contents forthe model soil with and without the 1–3% (weight)addition of kaolinite, illite, and Ca-montmorillonite arelisted in Table 2. The Na-montmorillonite treated sam-ples are discussed separately below. It is evident that thecritical water content is higher for the higher clay con-tents for all the three clay minerals. For the same claycontent, the critical water content for illite and Ca-montmorillonite is higher than that for kaolinite. Exceptfor the 2% and 3% kaolinite additions, the critical watercontent was higher for the treatments with 30 g kg−1

stearic acid compared to those with 10 g kg−1 stearicacid for the samples of stearic acid sand with and with-out addition of kaolinite, illite, and Ca-montmorillonite.

The relationships between WDPT and moisture w forthe model soil with and without the 1–3% (weight)addition of kaolinite, illite, Ca- and Na-montmorillonite

the water drop penetration time WDPT and moisture w.

Fig. 2. Effect of 2% clay addition on the relationship between the water drop penetration time WDPT and moisture w.

Fig. 3. Effect of 3% clay addition on the relationship between the water drop penetration time WDPT and moisture w.



are presented in Figs. 1–3. The model soil samplesalone and samples treated with kaolinite, illite, andCa-montmorillonite behave as one would expect fromtypical water repellent soils. They are wettable for watercontents above, and repellent for water contents belowwk, with persistence of water repellency (WDPT) in-creasing with decreasing water content below wk.

As for the model soil treated with Na-montmoril-lonite, all the samples were wettable during the dryingcycle. The water repellency did not re-appear until thewater content decreased to nearly 0%. AlthoughWDPTs were greater than 5 s (indicating repellentconditions), in the case of higher content of this claymineral (2% and 3%), this cannot be attributed to thewater repellency. Water repellency is considered to bepresent when the sessile drop contact angle is greaterthan 75–90° (King, 1981), however, in our samplesthe drop spread out over the surface and its contactangle was well below 10°. Given this inconsistency,we used three replicate samples for placing three dropsof distilled water onto the soil surface. The WDPT vs.w relationships (Figs. 1–3) are thus constructedfrom the averages of three WDPT and w values,respectively.

We aimed to achieve a WDPTb5 s in an additionalexperiment with Na-montmorillonite and, therefore,added 3 g of distilled water onto the surface of each

Fig. 4. Effect of 1% clay addition on the relationship between the water drop

sample to obtain moisture w=60%. Small particlesspread out on the surface of surplus water after mixing,because the retention capacity of the porous materialwas surpassed. In this case, the water drop slightlypenetrated the layer of small particles resulting in aWDPTb1 s. This phenomenon was not observed inthe samples with Ca-montmorillonite, and therefore, itis unlikely to be representative for the field soils, inwhich Ca-ions are highly prevalent in comparison withNa-ions. This can be expected to be the case even forslightly saline soils.

The apparent water repellency of model soils(WDPTN5 s) treated with Na-montmorillonite at mois-ture above 0% was most probably caused by the forma-tion of surface clay crusts. They were observed with thenaked eye on the model soil surface but only in experi-ments with Na-montmorillonite. The physicochemicaldispersion leads to migration of the very fine clayparticles with water toward the soil surface and causedrearrangement of soil particles and formation of clay“skin” at the soil surface during evaporation. The sur-face clay crust reveals smaller porosity than the bulksoil and causes the decrease in soil permeability forwater (increase in WDPT). The decrease in WDPTbelow 5 s was observed only when the water contentdecreased to nearly 0% due to disintegration of surfacecrust as the result of clay shrinking during desiccation.

penetration time WDPT and duration t of drying and heating at 50 °C.

Fig. 5. Effect of 2% clay addition on the relationship between the water drop penetration time WDPT and duration t of drying and heating at 50 °C.

Fig. 6. Effect of 3% clay addition on the relationship between the water drop penetration time WDPT and duration t of drying and heating at 50 °C.


Table 3Water drop penetration time (WDPT) values for the surfaces ofundisturbed and mixed samples after the wetting and drying cycle

Sample WDPT (s)

10 g kg−1 stearic acid 30 g kg−1 stearic acid

Undisturbedsample

Mixedsample

Undisturbedsample

Mixedsample

Stearic acid sand(control)

3780 15,300 9580 17,400


147 3180 257 5880


350 6360 134 1980


660 12,600 148 2100

Stearic acid sand+1%illite

8300 11,400 7060 15,900


5180 10,860 5740 14,400


4120 10,560 4020 11,700


11,400 16,200 15,400 16,500


14,600 16,800 15,900 17,400


16,400 17,400 18,400 19,800


10 220 5 250


16 240 15 280


23 280 9 310


3.2. Effect of a hot dry spell on water repellency

It was found after approaching w=0 that the per-sistence of water repellency was not re-established topre-wetting levels. The persistence of water repellencyincreased with duration of incubation at the tempera-ture of 50 °C, and relative increase was greatestduring the first 48 h in the majority of model soilsamples (Figs. 4–6).

During the hot dry spell simulation, the kaoliniteand Na-montmorillonite addition resulted in alleviat-ing the water repellency. In all cases the WDPTs ofmixtures with kaolinite and Na-montmorillonite werelower than the WDPT of the stearic acid sand. Incontrast, the WDPTs of mixtures with Ca-montmoril-lonite was higher than the WDPT of control modelsoil. In most cases, the illite addition increased thepersistence of water repellency, although not as mark-edly as the Ca-montmorillonite addition. It should bementioned that during the WDPT test on the mixtureswith illite, the water drops were covered with fine-grained material in a few minutes, making the mea-surements difficult.

The results obtained on simple model soil in thisstudy are in agreement with those obtained on fieldsoils by Ma'shum and Farmer (1985) in that oven-dry-ing can re-establish water repellency to some extent,although not to its initial level. After repeated wettingand drying phases, McKissock et al. (2002) observed adecrease in water repellency for field soils with no clayadditions and for soils with added Georgia kaolinite, butan increase in water repellency after addition of Wyom-ing bentonite. It has been suggested by Doerr andThomas (2000) that a new input of hydrophobic sub-stances may be required to fully restore water repellen-cy after its cessation during wetting. It has beenspeculated that repellency re-establishment during theheating involves a molecular alignment process with thepolar (hydrophilic) groups of organic molecules beingre-oriented towards the mineral surfaces, and the non-polar (hydrophobic) parts oriented towards the freespace (air) (Doerr et al., 2000). If this model applies,the rate of transition would depend, amongst otherfactors, on the activation energy required to instigatethe molecular re-organisation process leading to repel-lency re-establishment. This energy barrier must beovercome by supplied energy.

The persistence of water repellency was measured onundisturbed and mixed samples at the end of heating,and the results of WDPT tests are presented in Table 3.The re-organisation of stearic acid molecules coveringsand particles, as speculated above, would have taken

place during wetting, drying, and heating at 50 °C,resulting in different WDPTs as measured on undis-turbed and mixed samples.

The results presented above demonstrate that claymineralogy is the most important factor determiningthe range of changes in water repellency persistence.The results presented here confirm the high effective-ness of clay minerals from the kaolinite group in allevi-ating the water repellency as suggested previously byMa'shum et al. (1989) and McKissock et al. (2002). Thisis the case despite the comparatively small surface areaand large particle size of kaolinite. An important factordetermining clay effectiveness can be the difference inthe ability of clay minerals to mask hydrophobic sur-faces and to facilitate water penetration. In this sense,kaolinite could be more effective than Ca-montmorillon-ite after the wetting and drying because Ca-montmoril-lonite tends to swell and aggregate when wetted, whichis associated with the uptake of interlamellar water.Kaolinite, on the other hand is non-swelling and thusremains dispersed when wetted, allowing it to retain an


effective covering of hydrophobic surfaces (McKissocket al., 2002).

Montmorillonite in the mixture with sand tends toaggregate even when it is initially dispersed (McKis-sock et al., 2000). Flocculation and dispersion dependon the crystal structure and crystal chemistry of clayminerals. Clay minerals with high surface charge tendto flocculate whilst clay minerals with low chargeremain dispersed on sand surface. Flocculation anddispersion is also strongly affected by exchangeablecations present. Tendency to flocculation will increasewith increasing cationic charge and with decreasingconcentration of soil solution. For these reasonschemical properties of soil can markedly affect thebehaviour of clay minerals added for the purpose ofmitigation of water repellency. This may be a reasonwhy the results of this present study, in which stearicacid was used are in part contrasting to the results inthe study by Ward and Oades (1993) outlined in theIntroduction, in which cetyl alcohol was used to im-part water repellency.

Differences in the effectiveness of clay minerals inmasking of hydrophobic surfaces (presented clearly byscanning electron micrographs at Fig. 9 (c, d) in McKis-sock et al. (2002)) and in their behaviour in watersuspensions can be explained by the differences ininter-particle forces in the clay–exchangeable cation–water system. Repulsive forces leading to dispersion ofclay particles in water suspensions result from an ex-tended double layer. Whenever clay particles approacheach other, repulsion between particles occurs becausethe outer parts of the double layer have the same type ofcharge—positive (Tan, 1993). The extent of doublelayer in dilute solutions is strongly dependent on theexchangeable cation adsorbed on siloxane surface ofmontmorillonite. In the case of monovalent sodiumcation the extent of double layer will be much higherthan in the case of divalent calcium cation (Van Olphen,1963). The effect of exchangeable cation is so high thatsodium cation will cause dispersion of montmorillonitewhile presence of calcium cation will cause coagula-tion. Coagulation of clay particles occurs when van derWaals attraction overcomes the repulsive forces due tocompression of the double layer (Tan, 1993). Whensodium is the main exchangeable cation, particles inwater suspension will be formed by individual mont-morillonite crystal platelets. At low electrolyte concen-tration the decay length of electrostatic force is greaterthan 60 nm (Sposito, 1984). In contrast, Ca-montmoril-lonite forms packets of clay platelets condensed togeth-er with low external surface area in water suspensions(Bradford and Blanchar, 1999).

In a previous paper (Dlapa et al., 2004), we haveproposed an additional mechanism to explain the effectof clay mineral additions on the persistence of waterrepellency, which is related to the water adhesion forces.Hydrophobic or hydrophilic properties result from thebalance between cohesion and adhesion forces. Cohe-sion originates from hydrogen bonding between watermolecules. Adhesion is attraction of water by solidsurfaces. Strong adhesion originates from the hydrogenbonding of water molecules to mineral surfaces. Slightadhesion occurs between water and the hydrophobicorganic surfaces, where the Lifshitz–van der Waalsforces predominate (Michel et al., 2001). The hydrophi-licity of minerals increases with the density of theircharges and polar groups, mainly OH−, on their surfaces(Bachmann and van der Ploeg, 2002). Mineral structureof kaolinite with hydroxyl groups on the basal plane,resulting in the higher density of hydroxyl groups on thekaolinite surface probably makes kaolinite more hydro-philic than Ca-montmorillonite, irrespective of the waterbinding capacity. We speculated that the difference inthe ability to attract water (hydrophilicity) is also a keyfactor affecting the ability of clay minerals to reduce thewater repellency in soils (Dlapa et al., 2004). Thus, themore hydrophilic kaolinite is more effective in allevia-tion of water repellency than the less hydrophilic Ca-montmorillonite. Low affinity for water of Ca-montmo-rillonite probably can lead to a displacement of watermolecules at mineral surface by amphiphilic organiccompounds, which in may be a mechanism to explainthe unusual finding of an increase in WDPT the claytreated model soil. As their hydrophobicity is due tosiloxane groups (Si–O–Si), the difference in hydrophi-licity could be explained by different ions responsiblefor hydrophilicity of these clay minerals. In kaolinite,the Al-surface carries OH− groups and is hydrophilic,while in Ca-montmorillonite hydrophilicity is almostentirely due to the exchangeable cations Ca2+. Amphi-philic cations will be strongly adsorbed on Ca-montmo-rillonite by an ion-exchange reaction in the interlamellarspace. For kaolinite there is no ion exchange; so physi-sorption will occur on the external surface only (Brind-ley and Robinson, 1946).

The fact that Ca-montmorillonite and illite treat-ments increased water repellency in our experiments isnoteworthy and may also be related to the formation ofcalcium stearate complex. During the wetting phase,Ca2+ ions available at the surface of clay mineralsinteract with stearic acid to produce an insoluble calci-um stearate complex, which collects at the air/waterinterface. Formation of calcium stearate complex pro-duces hydrophobicity, which may reduce or overcome


hydrophilic effect of clay mineral addition. The net effect ofclay mineral addition is dependent on the amount of Ca2+

ions occurring at the surface of clay minerals. Pools ofavailable (exchangeable) calcium decrease in the order asfollows: Ca-montmorilloniteNN illiteNNNa-montmorilloni-te≈kaolinite. As a result, the net effect of Ca-monmorillo-nite addition was to increase WDPT, illite additions causeda less marked increase in WDPT, and only the treatmentswith Na-montmorillonite and kaolinite reduced WDPT ofthe model soils, prior to the wetting a drying cycle.

4. Conclusions

Water repellency in soils may be suppressed by add-ing clay to the soil matrix. Amongst illite, Na- and Ca-montmorillonite, and kaolinite, we found that the latterwas the only clay mineral able to lower the persistenceof repellency of model soil (sand treated with the stearicacid) following a wetting/drying cycle.

The samples of stearic acid with and without addi-tions of kaolinite, illite, and Ca-montmorillonite be-haved similar to typical water repellent soils. Theywere wettable for water contents higher than the criticalwater content, and the persistence of water repellency(WDPT) increased with decreasing water content. It wasfound that the critical water content increased with claycontent for all the three clay mineral types. For the sameclay content, the critical water content for illite and Ca-montmorillonite was higher than that for kaolinite,which may be explained by the formation of a calciumstearate complex from available (exchangeable) calciumoccurring at the surface of clay minerals.

The samples of stearic acid with Na-montmorillonitebehaved differently to other samples during a wetting/drying cycle. Apparent water repellency of those sam-ples (WDPTN5 s) at moisture contents above 0% wascaused by formation of surface clay crusts. The decreasein WDPT below 5 s was observed only when the watercontent decreased to nearly 0% due to disintegration ofsurface crust as the result of clay shrinking during des-iccation. This behaviour is suggested to be due to differ-ences in inter-particle forces in the clay–exchangeablecation–water system.

During the prolonged drying phase at 50 °C, thepersistence of water repellency increased with durationof heating at the temperature of 50 °C, and the relativeincrease was greatest during the first 48 h in the major-ity of the samples. While kaolinite and Na-montmoril-lonite addition resulted in a reduction in the persistenceof water repellency of the stearic acid sand, Ca-mont-morillonite and illite addition increased the persistenceof water repellency of the stearic acid sand.

Acknowledgement

The authors would like to thank the anonymousreviewers for making a number of helpful suggestionson the original manuscript. The financial support fromthe Science and Technology Assistance Agency ProjectNo. APVT-51-006502, the Slovak Scientific GrantAgency Projects Nos. 2/3032/23 and 1/0619/03, theNatural Environment Research Council Advanced Fel-lowship NER/J/S/2002/00662, and the Spanish–SlovakProject No. 2004SK0003 is gratefully acknowledged.

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Evaluation of different clay minerals as additives for soil water repellency alleviation

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