interactions between organic model compounds and pesticides with water-soluble soil humic substances

88 Acta hydrochim. hydrobiol. 29 (2001) 2-3, 88–99

© WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001 0323-4320/01/0208–0088 $ 17.50+.50/0

Interactions between Organic Model Compoundsand Pesticides with Water-soluble Soil HumicSubstances

Irene Franco a,Laura Catalano a,Marco Contin a,Maria De Nobili a

a Dip. Prod. Vegetale Tecnol.Agrarie, Universitá di Udine,Via delle Scienze 208,Udine 33100, Italy

Correspondence: M. De Nobili, E-mail: [email protected]

Interactions in solution of both water-soluble and sodium hydroxide extractable humic sub-stances with four model organic compounds of different polarity and four pesticides of differ-ent solubility were investigated by size-exclusion chromatography. At neutral or alkaline pH,association occurred only for the most hydrophobic compounds (acridine orange and atra-zine) and for bromocresol green, at low pH after charge suppression.The role of humic substances in promoting mobility of xenobiotic substances through the soiland their effect on interaction of xenobiotics with soil components was also examined. Dis-solved humic substances enhance removal of hydrophobic xenobiotic compounds from so-lution by aluminium and iron hydroxides. The presence of humic substances in the solidphase (humified organic matter) also favours sorption of hydrophobic compounds, whereaspredominance of non-humic soil organic matter components favour sorption of polar or neg-atively charged xenobiotic compounds.

Wechselwirkung organischer Modellsubstanzen und Pestizide mitwasserlöslichen Huminstoffen aus Böden

Die Wechselwirkung von Huminstoffen mit vier Modellsubstanzen unterschiedlicher Polari-tät (Acridinorange, 3,5-Dinitrobenzoesäure, Bromkresolgrün, Bromphenolblau) sowie vierPestiziden unterschiedlicher Wasserlöslichkeit (Atrazin, Metamitron, Linuron, 4,6-Dinitro-o-kresol) wurde mittels Größenausschlusschromatographie untersucht. Dabei wurden sowohlwasserlösliche und als auch mit Natronlauge extrahierbare Huminstoffe eingesetzt. Asso-ziation trat im neutralen und alkalischen Bereich nur für die am stärksten hydrophoben Ver-bindungen (Acridinorange und Atrazin) auf und für Bromkresolgrün bei niedrigem pH-Wertaufgrund der Neutralisation negativer Ladungen.Die Rolle der Huminstoffe bei der Mobilisierung von Xenobiotika im Boden und der Einflussder Huminstoffe auf die Wechselwirkung zwischen Bodenbestandteilen und Xenobiotikawurde ebenfalls untersucht. Durch gelöste Huminstoffe wird die Entfernung hydrophoberXenobiotika aus der wässrigen Phase durch Sorption an Aluminium- und Eisenhydroxidverbessert. Huminstoffe als Bestandteile der Festphase begünstigen ebenfalls die Sorptionhydrophober Verbindungen. Überwiegen beim organischen Bodenmaterial hingegen dieNicht-Huminstoffe, wird die Sorption polarer oder negativ geladener Xenobiotika begünstigt.

Keywords: Sorption, Mobility, Xenobiotics

Schlagwörter: Sorption, Mobilität, Xenobiotika

Interactions of Pesticides with Humic SubstancesActa hydrochim. hydrobiol. 29 (2001) 2-3, 88–99 89

1 Introduction

It is well known that xenobiotic organic compounds, such aspesticides and organic pollutants spread by industrial activi-ties, are subject to complex sorptive interactions in soil andoften display a high affinity for soil organic matter [1, 2]. Alarge number of studies have examined the interactions be-tween xenobiotic chemicals and humic substances (HS) inthe solid phase. For sparingly soluble hydrophobic com-pounds these interactions can be described in most cases asa partition between a liquid and a solid organic phase and aregenerally well correlated with the hydrophobicity of the con-taminant expressed as the octanol-water partition coefficient,Kow. Mingelgrin and Gerstl [3] suggested that compounds oflower polarity tend to escape from a polar solvent as waterand to adsorb on hydrophobic surfaces such as organic mat-ter after removing solvent molecules from the surface itself.The behaviour of ionic, acidic, or basic compounds is howevermore complex because of the possibility of electrostatic inter-actions with charged soil components. In other cases, differ-ent kinds of chemical bonding are responsible for sorption oforganic chemical from the water phase [2]. Comparatively,fewer studies have dealt with the effects of dissolved HS onthe transport and distribution of these substances in the envi-ronment [4, 5]. It has been shown that dissolved or suspend-ed organic matter in the aqueous phase can increase the ap-parent solubility of organic pollutants [6] and reduce the avail-ability of toxic compounds to aquatic organisms [7]. Investiga-tion of interactions of selected xenobiotic organic substanceswith the water-soluble fraction of humic substances and of theinfluence of the latter on interactions of dissolved moleculeswith soil components can provide information needed to com-pile effective predictive models for the fate of pollutants in theenvironment.

The aim of this work was to examine the interactions in solu-tion of water-soluble humic substances (WSHS) and for com-parison of dissolved sodium hydroxide extractable humic ac-ids (HA) from the same organic soil, with four model organicmolecules of different polarity and four pesticides of differentsolubility and octanol-water partition coefficients. The fourmodel organic compounds were chosen either for their struc-tural similarity with pesticides and/or for displaying intenseand well defined absorption peaks in the visible region. Thisfeature allows an easy detection and quantification of theamount left in solution at equilibrium even with the back-ground absorption of dissolved humic materials. We also ex-amined the effect of interaction of xenobiotic chemicals in so-lution on sorption by soil components.

2 Materials and methods

Water-soluble humic substances were extracted from aLithuanian sphagnum moss peat with distilled water (1:20) for

24 h. The water extract was filtered on 0.2 µm membrane fil-ters and acidified to pH 2.0 with HCl; the slightly turbid waterextract was then applied to a XAD-8 column [8].Water-solublehumic substances were desorbed by elution with 0.1 MNaOH, saturated with H+ by treatment with a cation-exchangeresin in the H-form (Amberlite 120 H+) and then freeze-driedto yield low-ash WSHS. Following extraction with water ofWSHS, humic acids were afterwards extracted from the samepeat sample with 0.5 M NaOH, under nitrogen, for one hour.They were then precipitated with HCl, repeatedly washed withdistilled water and collected by centrifugation. Final solutionswere prepared immediately before each experiment, by dis-solving 0.5 g L–1 the freeze dried of WSHS and HA prepara-tions in 0.02 M sodium tetraborate or in distilled water (adjust-ing the pH to 7.0 with diluted NaOH) and eventually by mixing(1:1) with solutions containing 50 to 70 mg L–1 of organicmodel compounds or pesticide.

Size-exclusion chromatography was performed on SephadexG-50 superfine columns (1.3 cm i.d., 30 cm long) using dis-tilled water or 0.02 M sodium tetraborate at pH = 9.3 as elu-ents [9]. Eluate fractions (3 mL) were collected and their UV-vis spectra recorded to detect possible association of xenobi-otic compounds with HS.

In the adsorption/desorption experiments, different soil com-ponents were utilised as sorbents: a Na+-montmorillonite, alu-minium hydroxide (0.3 µm, α form), amorphous iron hydrox-ide, soil organic matter (SOM) from a sphagnum moss peat(41.3 % organic carbon content, 176.4 cmol kg–1 cation-ex-change capacity and pH 3.5) and non-humic soil organic mat-ter (NH-SOM).The latter was prepared from the same peat byremoving HS by extraction with 0.5 N NaOH, followed by re-peated washings and equilibration of the residue at the origi-nal pH, before air drying.

In the adsorption experiments, solutions, containing acridineorange (AO), a zinc chloride double salt of acridine base,3,5-dinitrobenzoic acid (DNB), DNOC (4,6-dinitro-o-cresol),metamitron (4-amino-3-methyl-6-phenyl-1,2,4-triazin-5(4H)-one), linuron (3-(3,4-dichlorophenyl)-1-methoxy-1-methyl-urea), atrazine (2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine), bromophenol blue (3,3',5,5'-tetrabromodiphenol-sulfonephthalein; BPB), bromocresol green (3,3',5,5'-tetra-bromodicresolsulfonephthalein; BCG), were equilibrated withan amount of SOM or NH-SOM, equivalent to 1 g of oven drymaterial. Concentrations were respectively 1 mg L–1,2 mg L–1, 5 mg L–1, 10 mg L–1, and 20 mg L–1 for BPB andBCG; 10 mg L–1, 20 mg L–1, 30 mg L–1, 40 mg L–1, and50 mg L–1 for DNOC; 10 mg L–1, 20 mg L–1, 50 mg L–1,100 mg L–1, and 200 mg L–1 for DNB and 100 mg L–1,200 mg L–1, 300 mg L–1, 400 mg L–1, and 500 mg L–1 for AO.

The aqueous solubilities of organic compounds were deter-mined in triplicate adding an excess amount of the compoundto a volumetric flask containing distilled water and shaking at

I. Franco et al.90 Acta hydrochim. hydrobiol. 29 (2001) 2-3, 88–99

Table 1: Structures and chemical properties of the model compounds studied.Struktur und chemische Eigenschaften der untersuchten Modellsubstanzen.

Name Structuralformula

Molecularweight

Wavelengthof absorption

maximumnm

Solubility

mg L–1

Kow

(non-ionized form)Kow

(ionized form)

Acridine orange(zinc chloride doublesalt of acridine)AO

265.36 490 873(1)

(20°C)803.5(1) –

3,5-Dinitrobenzoic acidDNB

229.12 331 986(1)

(20°C)11.22(1) 0.028(1)

Bromophenol blue(3,3',5,5'-tetrabromo-diphenol-sulfonephthalein)BPB

691.97 590 4000*(25°C)

8.12(1) 0.00085(1)

Bromocresol green(3,3',5,5'-tetrabromo-dicresol-sulfonephthalein)BCG

720.02 615 4000*(25°C)

48.97(1) 0.00129(1)

* Water solubilities cited in Merck index and CRC Handbook of Pesticides(1) Determined in this work

(20 ± 2) °C during 24 h until saturation was reached. Themaximum concentration reached in the water phase wasmeasured by UV-vis spectrophotometry on solutions filteredat 0.2 µm.

When not available in literature, the octanol-water partitioncoefficients were also determined in triplicate dissolving aknown quantity of the chemical in a 1:1 octanol/water mixtureand shaking for 2 h to reach equilibrium [10]. After separatingthe two phases, the concentration of the chemical in eachphase was measured by UV-vis spectrophotometry.The octa-nol-water partition coefficient (Kow) was calculated accordingto the equation Kow = co/cw where co is the concentration ofchemical in the organic phase (octanol) and cw is the concen-tration in the water phase at equilibrium. Some chemical char-acteristics of the compounds examined are reported inTables 1 and 2.

Adsorption and desorption of AO from soil components inthe presence of HS was performed according to a batchmethod [11]. Briefly, adsorption was carried out in triplicateon 1 g oven dried equivalents of sorbent treated with100 mL of solution containing 125 mg L–1 acridine orangeand 500 mg L–1 WSHS or HA. The suspensions were shak-en for 4 h at room temperature to achieve equilibrium andthen centrifuged at 4000 rpm for 10 minutes and filteredthrough 0.45 µm Whatman nitrocellulose filters. Blanks wereprepared shaking 1 g of each material with 100 mL of dis-tilled water; different controls containing only the acridineorange solution but no humic substances and vice versawere also prepared. The amount of adsorbed chemical wascalculated as the difference between the original concentra-tion of the solution and that remaining at the end of theequilibration time.


Table 2: Structures and chemical properties of herbicides and pesticides studied.

Struktur und chemische Eigenschaften der untersuchten Herbizide und Pestizide.

Name Structuralformula

Molecularweight

Wavelengthof absorption

maximumnm

Solubility

mg L–1

Kow

Linuron(3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea)

249.11 247 81*(25°C)

1010(2)

Metamitron(4-amino-3-methyl-6-phenyl-1,2,4-triazin-5(4H)-one)

202.12 302 47.9*(25°C)

680(1)

Atrazine(2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine)

215.68 268 28*(25°C)

219(2)

DNOC(4,6-dinitro-o-cresol)

198.06 363 130*(15°C)

0.48(1)

* Water solubilities cited in Merck index and CRC Handbook of Pesticides(1) Kow determined in this work(2) Kow found in literature

Desorption of acridine orange was determined by replacingequilibrium adsorption solutions with 100 mL of distilled waterand shaking for 4 h. After filtration, the concentration releasedinto solution (desorbed chemical) was determined by UV-visspectrophotometry using a Perkin-Elmer Lambda-5 spectro-photometer at the adsorption maxima given in Tables 1 and 2.The amount of irreversibly adsorbed chemical was calculatedfrom the difference between the amount initially adsorbed andthat passed into solution at the end of the equilibration time.

3 Results and discussion

Size-exclusion chromatography (SEC) is a relatively rapidand convenient method for estimating the partitioning or bind-ing of organic compounds to humic substances in solution. Inparticular SEC can give information either on the position ofthe equilibrium interaction, when the eluent contains a fixedconcentration of the interacting chemical, or on the liability of

the interaction itself as in the present work. In size-exclusionchromatography, differences in pH between sample and elu-ent must be avoided if the separations required must bebased on molecular size differences. Infact, because of the in-teractions occurring at acidic pH between humic substancesin particular, and phenolic substances in general with the dex-tran gel, any change in the chromatographic behaviour of sol-utes, which follows a change in the pH of the sample, cannotbe interpreted in terms of changes in molecular weight. In thiswork, however, SEC was used only as a generic mean ofseparation, without making any assumption about molecularsizes, or molecular size variations. By comparing, under thesame experimental conditions, the chromatographic behav-iour of mixtures of humic substances and xenobiotics withthat of humic substances and xenobiotics applied separatelyto the SEC column, it is possible to investigate reciprocal in-teractions.

The SEC chromatograms (Figs. 1, 2, 3, and 4) and the UV-vis-ible spectra (Fig. 5) of fractions collected during SEC chroma-


Fig. 1: Size-exclusion chromatography of HA, BCG, and HA+ BCG applied together as 1:1 mixture, at pH 7 (above) andpH 2.7 (below). Eluent: distilled water.

Größenausschlusschromatogramme von Huminsäure (HA),Bromkresolgrün (BCG) sowie von HA + BCG, als 1:1-Mi-schung aufgegeben; bei pH = 7 (oben) und pH = 2.7 (unten).Eluent: destilliertes Wasser.

tography of both WSHS and HA with bromocresol green(BCG), bromophenol blue (BPB), linuron, metamitron, and at-razine did not show any interaction either at high (0.02 M bo-rate solution) and low (distilled water) ionic strength. When

Fig. 2: Size-exclusion chromatography of HA, BPB, and HA+ BPB. Eluent: distilled water.

Größenausschlusschromatogramme von Huminsäure (HA),Bromphenolblau (BPB) sowie von HA + BPB. Eluent: destil-liertes Wasser.

the pH of the solution containing the model substance waslowered to 2.7 before mixing with HA or WSHS, a weak andreversible interaction occurred between humic substancesand the non-ionised form of BCG. Part of the added BCG wasin fact eluted earlier than free BCG as evidenced from chang-es in the elution profile and from the UV-vis spectra of collect-ed fractions (Fig. 5). Changes in the elution behaviour do notmean, in this case, the occurrence of changes in molecularsize, but are the result of eventual interactions with the solidphase (pure model compounds) and with humic substances(1:1 mixtures). In Figure 5, it is possible to compare the spec-tra of corresponding fractions from different chromatogramsobtained from humic acids applied to the column alone at twodifferent pH values, those of the humic acids mixed with BCGor bromophenol blue (BPB), and then of these two modelcompounds under the same conditions. The spectra of humicsubstances eluted in fraction VI, which corresponds to the ex-cluded peak, do not show (Fig. 5) co-elution of BPB or BCG,whereas co-elution is observed in fraction X (Kav = 0.2...0.3)for samples of acidic pH. Most BCG was co-eluted with the


Fig. 3: Size-exclusion chromatography of linuron, metam-itron, atrazine, and of these compounds mixed with HA.Eluent: distilled water.

Größenausschlusschromatogramme von Linuron, Meta-mitron und Atrazin sowie von diesen Verbindungen im Ge-misch mit Huminsäure (HA). Eluent: destilliertes Wasser.

more retained humic acids, at a modified Kav with respect tothe chromatographic behaviour of the pure compound. TheKav of BPB was not modified. In the case of metamitron(Fig. 4) and BPB, no interactions were observed even at lowpH, where BPB was in a non-ionised state. This is testified bythe lack of the typical BPB and BCG absorption maxima infractions collected at elution volumes different from that corre-

Fig. 4: Size-exclusion chromatography of WSHS, metamitron,and WSHS+metamitron. Eluent: distilled water.

Größenausschlusschromatogramme von wasserlöslichenHuminstoffen (WSHS) und Metamitron sowie von Metamitronim Gemisch mit WSHS. Eluent: destilliertes Wasser.

sponding to the elution of the pure compound. BPB and BCGare very similar in structure but differ for the presence of twoCH3 groups on the BCG molecule. These groups provide tobromocresol green a higher lipophilicity, suggesting that hy-drophobic bonding might be responsible for the interactionobserved with HA in spite of the rather similar solubility dis-played by the two compounds. Dinitrobenzoic acid, DNOC,metamitron, and linuron were also separated from the bulk ofWSHS and HA during SEC. The monitoring of the UV-visspectra of fractions collected at 3 mL intervals did not revealco-elution of even small amounts of these chemicals togetherwith humic substances. All these polar compounds were notadsorbed, during elution, on the stationary phase of the col-umn and their chromatographic behaviour was not modifiedby the presence of humic substances. Even if, under differentand more favourable pH or concentration conditions, thesesubstances could theoretically interact with HS in the soil so-


Fig. 5: UV-vis spectra of correspondingfractions collected during elution in dis-tilled water of humic acids applied to thecolumn alone or mixed with BCG orBPB at pH 7.0 or 2.7. Above: fraction VI,corresponding to the excluded peak(Kav = 0); middle: fraction X (Kav =0.25...0.3); below: fraction XX (Kav =0.8).

UV-vis-Spektren unterschiedlicherFraktionen bei der Größenausschluss-chromatographie. Oben: Fraktion VI,entspricht dem Ausschlussvolumen(Kav = 0); Mitte: Fraktion X (Kav =0.25...0.3); unten: Fraktion XX (Kav =0.8). Aufgegeben wurde Huminsäureallein oder Huminsäure im Gemisch mitBromkresolgrün (BCG) oder Bromphe-nolblau (BPB) bei pH = 7 oder pH = 2.7.Eluent: destilliertes Wasser.


Fig. 6: Size-exclusion chromatography of WSHS, HA, WSHS+ bentonite, HA + bentonite, WSHS + acridine, HA + acridine,WSHS + bentonite + acridine, and HA + bentonite + acridine.

Größenausschlusschromatogramme von wasserlöslichenHuminstoffen (WSHS), Huminsäuren (HA), WSHS +Bentonit, HA + Bentonit, WSHS + Acridin, HA + Acridin,WSHS + Bentonit + Acridin sowie HA + Bentonit + Acridin.

lution, the interaction is therefore labile and a simple changein concentration would be sufficient to shift the equilibrium to-wards the free compounds. Interaction with dissolved humicsubstances is therefore not important for the mobilisation ofneutral or negatively charged polar or highly soluble com-pounds and will have little effect on sorption by soil compo-nents, in spite of the wide variety of possible chemical bond-ings, that could be formed.

On the contrary, compounds such as atrazine or acridine or-ange were partially (atrazine) or completely (AO) sorbed onthe stationary gel phase during elution through the SephadexG-50 column. AO is a weak base that bears a positive chargeonly at pH below 3.5 [12], determining the presence of AOmolecules as monocations at diluted concentrations (ca.20 µmol L–1). The higher concentrations used in our experi-ments may have induced the formation of dimers or oligomersbut precipitation was never observed and the pH of the solu-tions tested were such that AO was not protonated. Sorption

Fig. 7: UV-visible spectra of fractions X (Kav = 0.25...0.3) col-lected respectively during elution of HA and HA + acridine.

UV-vis-Spektren der eluierten Fraktion X (Kav = 0.25...0.3) beider Größenausschlusschromatographie von Huminsäure(HA) bzw. von Huminsäure im Gemisch mit Acridin.

Fig. 8: UV-visible spectra of fractions XXV (Kav = 1) collectedduring elution of HA, WSHS, HA + atrazine, and WSHS +atrazine.

UV-vis-Spektren der eluierten Fraktion XXV (Kav = 1) bei derGrößenausschlusschromatographie von Huminsäure (HA),wasserlöslichen Huminstoffen (WSHS), HA im Gemisch mitAtrazin sowie WSHS im Gemisch mit Atrazin.


on the gel matrix was irreversible both in distilled water andsodium tetraborate solution. In SEC the stationary phase, be-ing made of hydrated polymerised dextran, is not hydrophobicand sorption cannot be described as a partition to an organicphase.When, on the contrary, these substances were appliedto the column, after mixing with humic substances, a smallfraction of both AO (Figs. 6 and 7) and atrazine (Fig. 8) wereeluted together with humic substances: the interaction withWSHS can therefore contribute to the mobilisation of atrazinethrough the soil. UV-vis spectra of fractions collected duringsize-exclusion chromatography showed that association be-tween acridine orange or atrazine and HS throughout the elu-tion profile of WSHS and HA. The concentration of AO in thedifferent fractions was proportional to the amount of humicsubstances, whereas that of atrazine was larger in the moreretained fractions. The fact that, for polar molecules, whichare otherwise known to be able to bind to HS in the solidphase [2], the solute-solvent interactions are evidently strong-er than humic substances-solute interactions would also indi-cate that the driving force of sorption of dissolved xenobioticcompounds by soil organic matter is their weak free energy ofsolution, and not the energy gain of the bonds that the organicchemicals can form with the humic substances once parti-tioned in the solid phase [2, 3].

4 Effect of interactions with HS in solutionon the mobility of hydrophobic pollutantsthrough the soil

SEC chromatography results showed that the potential migra-tion of hydrophobic xenobiotics as AO or atrazine in soil andaquifer systems can be favoured by humic substances, sinceinteraction of these contaminants with humic molecules pre-vented their complete adsorption on the stationary phase.The nature of the sorbent is however important: different de-grees of interactions can be expected from different soil com-ponents.

We therefore determined the sorption of acridine orange frommixed solutions of WSHS or HA (500 mg L–1) plus acridine or-ange (125 mg L–1) on various soil components (Fig. 9). Bind-ing of acridine orange to HS (Table 3) influenced sorption onaluminium hydroxide: removal of AO from solution was muchlarger (about + 55 %) in the presence of both WSHS and HA.This was caused by the high affinity of HS for the polar surfac-es of these inorganic compounds and/or by their insolubilisa-tion. At a concentration of 500 mg L–1 both kinds of HS were infact completely removed from solution by solid aluminium hy-droxide; the same happened when HS solutions were treatedwith amorphous iron hydroxides, which, however adsorbed alarge amount of acridine (Table 3). On iron hydroxide (Fig. 9c),acridine orange was nearly completely sorbed only in thepresence of HA, whereas interaction of AO with WSHS had

Fig. 9: UV-vis spectra of equilibrium solutions of AO, AO +WSHS and OA + HA after adsorption by a) aluminium hydrox-ide, b) sphagnum peat, c) amorphous iron oxide, and d) Na+-montmorillonite.

Adsorptionsexperimente von Acridinorange (AO), AO im Ge-misch mit wasserlöslichen Huminstoffen (WSHS) sowie AOim Gemisch mit Huminsäuren (HA) an a) Aluminiumhydroxid,b) Moostorf, c) amorphem Eisenoxid und d) Natriummontmo-rillonit. UV-vis-Spektren der Gleichgewichtslösungen.


Table 3: Adsorption, desorption, and irreversible adsorption percentages (with respect to the initial solution concentration) ofacridine orange on different soil components, alone and in the presence of humic substances.

Anteil des adsorbierten, des wieder desorbierten und des irreversibel adsorbierten Acridinorange (bezogen auf die Konzentrationder Ausgangslösung) an verschiedenen Bodenbestandteilen; für Acridinorange alleine (AO) und für Acridinorange im Gemisch mitHuminstoffen (WSHS: wasserlösliche Huminstoffe, HA: Huminsäuren).

Na+ montmorillonite Aluminium hydroxideAdsorbed

%

Desorbed

%

Irreversiblyadsorbed

%

Adsorbed

%

Desorbed

%


%

AO 78.6 ± 2.5 17.7 ± 1.2 60.9 ± 3.7 28.9 ± 1.5 7.2 ± 0.2 21.7 ± 1.8AO + WSHS 88.2 ± 2.5 19.2 ± 1.0 69.0 ± 3.5 83.9 ± 2.6 3.1 ± 0.7 80.8 ± 3.2AO + HA 73.8 ± 3.0 19.4 ± 1.0 54.4 ± 4.0 99.9 ± 3.2 3.1 ± 0.4 96.8 ± 3.4

Iron hydroxide Organic matter (sphagnum peat)Adsorbed

%

Desorbed

%


%

Adsorbed

%

Desorbed

%


%

AO 81.1 ± 2.4 0.8 ± 0.1 80.2 ± 2.6 100 ± 1.5 0.0 ± 0.3 100 ± 1.71AO + WSHS 79.2 ± 1.7 0.0 ± 1.3 79.2 ± 2.7 97.1 ± 2.1 0.0 ± 1.8 97.1 ± 2.32AO + HA 99.3 ± 3.2 0.0 ± 0.2 99.3 ± 3.4 100 ± 1.9 0.0 ± 1.9 100 ± 3.14

no effect. Inorganic sorbents, especially aluminium hydrox-ides and more selectively iron oxides, are able to remove dis-solved organic matter efficiently from soil solution [13, 14].Hence, hydrophobic organic chemicals, which interact withdissolved humic molecules, will be less mobile in soils with ahigh content of oxides and hydroxides. Oxide and hydroxiderich soil horizons could represent effective barriers to inputsof dissolved humic substances and their interacting mole-cules to groundwaters. On the contrary interaction with HAslightly decreased sorption of the model compound by clay.

SOM (Fig. 9b) showed, as could be reasonably expected thehighest efficiency in removing hydrophobic xenobiotic com-pounds from solution and its sorption capacity was not affect-ed by solute-sorbate interactions of HA or WSHS with thesorbate.

We did not find any appreciable effect of both kinds of HS onthe reversibility of adsorption from any of the soil componentsexamined (Table 3), but, because of selective sorption by soilcomponents after interaction with hydrophobic compounds,qualitative changes were induced in the part of WSHS left insolution (Fig. 6).

Interactions with AO modified the behaviour of WSHS, caus-ing a selective removal of low-molecular-weight WSHS fromsolution by montmorillonite with respect to a control without

acridine.This was coherent with the fact that AO did not affectthe behaviour of HA, which were mostly constituted by high-molecular-weight HS that were excluded from the column andeluted at Kav = 0 and were not sorbed by the clay.

A parallel adsorption experiment on the effect of organic mat-ter quality on the potential mobility of xenobiotic moleculesthrough the soil was carried out using, as a solid phase, twokinds of soil organic matter, i.e. sphagnum peat (SOM) andthe same after removal of humic substances (NH-SOM). Ad-sorption isotherms for AO and DNOC on SOM and for BPB onNH-SOM are reported in Figure 10. All other isotherms notreported in figure could not be determined because completesorption occurred at all, but the highest concentration tested.When HA were removed from SOM by extraction with NaOHunder N2, the hydrophobic character of the material changedand the original behaviour was not regained even after exten-sive washing and re-equilibration at the original pH. Progres-sive humification in fact increases the hydrophobic characterof peat [15] and surface hydrophobicity was therefore re-duced by extraction of HS. As expected, the intensity of AOadsorption at the highest concentration tested and the onlyone for which an equilibrium concentration of AO with SOMcould be measured, was ten times larger on SOM than onNH-SOM. DNOC was again preferentially sorbed by SOM(1.5 times the amount sorbed by NH-SOM at the highest con-centration tested).


Fig. 10: Sorption isotherms of acridine orange (AO) andDNOC on SOM and of bromophenol blue (BPB) on NH-SOM.

Sorptionsisothermen von Acridinorange (AO) und 4,6-Dinitro-o-kresol (DNOC) an bodenorganischen Stoffe (SOM) und vonBromphenolblau (BPB) an den Nicht-Huminstoff-Anteil derbodenorganischen Stoffe (NH-SOM).

Adsorption isotherms of compounds of much lower hydropho-bicity, such as DNB, which is only in part ionised at the pH ofthe peat’s surface, and even of BCG, which is fully ionised, butpartly hydrophobic, were not modified by extraction of HS.These results agree with the behaviour observed for partitionbetween solution and solid phase on two peats of differenthumification degree [16]. Removal of HS from peat resultedon the contrary in an increased retention of the most polar

and fully ionised BPB. The amount sorbed on NH-SOM wasabout 9 times larger than that sorbed by SOM. The degreeand type of adsorption appear therefore to be a function ofboth the lipophilicity of organic chemicals and of peat proper-ties, such as the degree of humification.These results can bereasonably extrapolated to soil organic matter in general andshow that predictions based on solute properties alone, suchas octanol-water partition coefficients only, cannot be accu-rate.

5 Conclusions

Dissolved humic substances potentially enhance risk oftransport of hydrophobic xenobiotic compounds by drainagewater, but have no effect on that of polar xenobiotic com-pounds. Effective risk enhancement is however strongly de-pendent on subsoil composition, as interactions with dis-solved humic substances favour adsorption of xenobioticcompounds on aluminium or iron hydroxides. Interaction withHS can therefore reduce at the end the mobility of hydropho-bic pollutants in spite of increased solubilisation. The qualityof dissolved humic substances and the humification degree ofsoil organic matter also plays a role in the balance betweenmobilisation and retention and cannot be overlooked. Remov-al of hydrophobic xenobiotic compounds from solution is larg-er on humified soil organic matter, whereas non-humic soilorganic matter components enhance sorption of polar xeno-biotic compounds.

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[Received: 3 November 1997; accepted: 9 May 2001]

interactions between organic model compounds and pesticides with water-soluble soil humic substances

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