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Adsorption - desorption studies introduction J. Cornejo During the la st years it has been reported that pes ti cides have bee n found in groundwater, s urf ace waters, foods and even in animal and human ti ssues. The move ment of th ese che ml cals through lh e soil profile governs their potential to be transferred into waterbod- ¡es 3nd foodchains. The manageme nt and research models in describing lhe moveme nt and fate of pes ti cides in lhe soil ecosystem is largely depende nt on lhe diverse processes lhat in clude leaching, volatilisation, che mi cal, biological and photoche mi ca l degradati on and sorption-desorp- lion on soil par ti cles. Adsorption is probably lhe controlling step in lhe transparl of chem- icals along lhe soil profi le and sho ul d nOl be underest im ated. Biodegradation, fo r example, is generally more pronounced for molecules in lh e solution phase lhan when lhey are sorbe d. On lhe other hand, sorption of organic chemi ca ls to so il and so il co ll oids is an important interfa ce mass-transfer process. The description of such a process needs of inf ormation on lhe equilibrium ac hi eved between phases and th e rate on whi ch eqllilibrillm is approached. In any case this process may be affe cted by physicoche mi cal properties of th e pes ti cide, solid and liquid phase properties and climatic conditions. On th e basis of the above con- sideralion s th eir relative partitioning between the solid and solution phase can also deter- mine lhe behaviour of pes ti c id es in soils. In order to lInderstand th e sorp li on-desorption processes and lo obta in th e parameters needed fo r theirqllantification several recomme nd ati ons, guidelines, etc. have been devel- oped. In this sec ti on sorne me th ods have bee n prese nted taking into accounl th e ex isting gllide lin es. However, new melhods dealing wilh specific problems are prese nted. In lhe section 3. 1. two contributions are prese nt.ed related with th e fractionati on and char- acte ri sation of lhe humic subtances and methods 10 study th e pestic id e stabilisa ti on and bound residues forma ti on in so il s. In section 3.2. methods presented are related to th eadsorp- ti on capacity of soils and so il components by means of adsorption-desorption isotherms obta in ed in sta ti c o rd yna mi c systems and by calculation of adsorplion coe ffi c ien ts Kc lay and Koc. Me th ods prese nted in section 3.3. dealing with th e procedures toes tablish lheope rat in g mec ha ni sm during th e sorption process of pes ti c ides on soil and so il compone nl s. Preparation and characterization of samples The retention o fp es ti c id es by so il particles has bee ll extensively studied. However, many papers are published every year because is still necessary to obta in a be tt er kn owledge of sorption phenomena. So rption on so il colloids depends of several factors bUI und ou btl y is dependent on th e nature and prope rt ies ofth e surfaces avai lable fo r int. eracti on wilh mol- ec ules prese nt in th e interface (Co rn ejo and Her mosín 1996). Jt is genera ll y recog ni sed
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Adsorption - desorption studies introduction - CSIC

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Page 1: Adsorption - desorption studies introduction - CSIC

Adsorption - desorption studies introduction J. Cornejo

During the last years it has been reported that pes tic ides have been found in groundwater, surface waters, foods and even in animal and human ti ssues. The movement of these chemlcals through lhe soil profi le governs their potential to be transferred into waterbod­¡es 3nd foodchain s.

The management and research models in describing lhe movement and fate of pestic ides in lhe soi l ecosystem is largely dependent on lhe di verse processes lhat include leaching, volatilisation, chemical , biological and photochemical degradation and sorption-desorp­lion on soil partic les. Adsorption is probably lhe controlling step in lhe transparl of chem­icals a long lhe soil profi le and should nOl be underestimated. Biodegradation, fo r example, is generally more pronounced for molecules in lhe solution phase lhan when lhey are sorbed.

On lhe other hand, sorpt ion of organic chemicals to soil and soil colloids is an important interface mass-transfer process. The description of such a process needs of information on lhe equilibrium achieved between phases and the rate on which eqllilibrillm is approached. In any case this process may be affected by physicochemical properties of the pes ticide, solid and liquid phase propert ies and climatic conditions. On the basis of the above con­sideral ion s their relative partitioning between the sol id and solution phase can also deter­mine lhe behaviour of pestic ides in soi ls.

In order to lInderstand the sorpli on-desorpt ion processes and lo obtain the parameters needed for theirqllantification several recommendations, guidelines, etc. have been devel­oped . In this section sorne methods have been presented taking into accounl the ex isting gllidelines. However, new melhods dealing wilh specific problems are presented.

In lhe section 3. 1. two contributions are present.ed related with the fractionation and char­acterisation of lhe humic subtances and methods 10 study the pestic ide stabilisation and bound residues formation in soils. In section 3.2. methods presented are related to theadsorp­tion capacity of soils and soil components by means of adsorption-desorption isotherms obta ined in static ordynamic systems and by calculation of adsorplion coeffic ients Kclay and Koc. Methods presented in section 3.3. dealing with the procedures toestablish lheoperating mechanism during the sorption process of pestic ides on soil and soil componenls.

Preparation and characterization of samples

The retention ofpes ticides by soil particles has bee ll extensively studied. However, many papers are published every year because is still necessary to obta in a better knowledge of sorption phenomena. Sorption on soil colloids depends of several factors bUI undoubtly is dependent on the nature and propert ies ofthe surfaces avai lable for int.eraction wilh mol­ecules present in the interface (Cornejo and Hermosín 1996). Jt is generally recognised

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that limited attention has been paid to relate the properties of the soil organic maner with the sorption of pesticides in soil as compared to similar studies with inorganic soil col­loids. Thi s faet must be attributed to the complexity of the structure of so il organic matter that has not been totally elucidated so faf (Saiz limenez, 1996).

In Sectio l1 3.1. Piccolo el al. provide several methods to qualitati ve ly ¡solate humic, ful vic acids and humin from soils. Extraction procedure is very datailed explained and later on pro­cedures to purify humic extracts fram silicate and metal contaminants are al 50 reported. In a rder to assess the molecular dimensions of humic extracts from soils, the Soil Exclusioll Chromatography (SEC) id widely used. This method is quite useful todetermine the weight­average molecular weight (Mw) and the number-average molecular weight (Mn). On the other hand, these authors also show the advantages of Nuclear Magnetic Resonance Spec­troscopy (NM R) for assessing the structural features of soil organic matter, indicating thedif­ference between liquid and solid-state NMR spectroscopies. Another technique recommended for analysis of natural organic matter is the Infrared Spectroscopy, in the modern vers ion of Fourier Transform IR (FfIR), perfonned by either Ihe KBr pellet teeh­nique in the transmittance mode or by KBr mixtures on the diffuse reflectance mode (DRIFf).

Sorption-desorption ofmany pesticides by soils are usually not complete ly reversible and the reversibility often decreases when the chemical res idence time in soil increases. This behaviour is leading to the stabilization ofthe pesticide under less available and less biode­gradable form s. Ban·iuso el al. present a methodology fol' the application of so il fraction ­ation techniques and to isolate soil organic constituents of different properties to determine their capacity lO establish intel'actions with pesticides and 10 stabil ize them as bound resi­dues. Methods forpl'eparation c1ays and soil clay minel'a ls for pes ticide sorption studies are presented in Section 3.3, where they are al so shown from a mechanistic point ofview.

Sorption of pesticides on soil

From a broad point of view adsorption describes the process of enrichment (positive adsorption) or depletion (negative adsorption) of one or more chemical species in an inter­face region. However, it is the free energy balance G = H - T S of the adsorption process wh ich finall y determines the relative bonding (H) of the sorbate and sorbent and the order of the system (S). Although it is generally aecepted the importanee of sorption-desorption process, considerable controversy exists lo explain lhe behaviour of ol'ganic compounds in soils. On lhe olher hand, it is necessary lo have lhe reliable parameters to make the proper prediction of sorption behaviour of these chemicals in soils. Most of these predictions techniques are related to estimate Koc, assuming the organic carbon content (OC) as the single best soil characteristic determining rhe pesticide adsorplion. The use of the octanol­water pmtition coefficient (Kow) of a given compound and its water solubility has been proposed by Chiou el al. (1979) to explajn the behaviour of nonionic organic compound in soils. However, these parameters have been considered of limited use even for nonionic organie chemieals (Sehwartzenbaeh and Westall, 1981 , Weber 1986, Weidenhaupt el al. - 1997). Further difficulties exist when the partition theory is tried 10 be appl ied to the sorption in low organic matter soils. Many papers have demonstrated very good con·ela­tions between sorption of pesticides and soil mineral components content. Evenmore, the association between organic matter and c1ay mine rals, especia lly in low organic matter soil s and most of subsoils change drastically ilS behaviour on organic chemicals sorption

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Adso rption - desorption studies/lnlroductio1/. 99

(Cornejo and Hermosin, !996). !n many cases the pesticides whose sorption capacity has been related to soil c1ay content were polar, ionic or ionizable characters.

Two methods are presented in section 3.2 in con nection with the delerminalion of adsorption coeffic ient Koc by K6rdel and the introduction by Hermosín el al. of a organic chemical adsorption coefficient on a c1ay fracuon basis, Kc1ay, defined in a similar way thal Koc is. The aUlhors present a detailed review of several cases described in lhe litera­ture where the soil c1ay fraction acls as determin ing factor in pesticides-soi l sorplion pro­cesS. The vaLidation of the melhod was made using two herbicides and several soi l c1ay frac tions separated from soils used in previously pesticides sorplion studies. On lhe other hand, K6rdel presenls an HPLC screening method for the determination ofthe adsorption­coefficient Koc. This method is very simple servi ng as a reasonable compromise between batch equilibrium in soil column experiments and estimations of Koc from the octano 1/ water parti tion coefficient Kow. The values obtai ned are good enough compared with lhe correspondi ng results from batch equ ilibrium studies.

Batch experiments used in lhe Iiterature to determine sorption of pesticides by soils and soi! components generaUy fo LLow lhe OECD GuideLine for Tesling of Chemica!s, 106: Adsorption-Desorplion (OECD, 198 1) and OECD Gu ide!i nes for Testing of Chemicals: Adsorption/desorption using a batch equilibrium melhod. Gu ide!ine 106 (OECD, 1996). This technique is widely used lo obtain the sorplion parameters after appl icatioll the appro­priated mechanism equation, ei ther for soils or its conslituenls. Drevellkar el al. describe a method to obtain the sorption coefficienl of several sorbenls with different type of organic matter contento

Molioari el al. describe a simplified and fasl method for the screen ing of chemical sub­stances on the basis of their kinetics and isotherms of adsorption-desorption process on dry soil. This method is suggested when a small volume of liqu id phase is used in final analysis.

A flow equil ibration method is described by Garcia-ValcarceJ el al. in which a pes ticide solution of known concentratiol1 is passed through a column of so il until the eftluent reaches the same concentration as the input solution. This method is quite simple and unexpensive nOl requ iring especial equ ipmenl simulating field conditions. From another poinl of view, Walker has developed a method lo measure sorplion of pesticides by soi !s in a static system at realistic soil : solution ratio. As the Garcia-Vaicarcel method, this is a very simple and unexpensive one. T his method consists in the use of a centrifugation tech­nique combined with a filtrat ion system in order to obtain the soil solution and the corre­sponding pesticide concentratioll.

A different technique is proposed by Deleu and Copio being designated by lhe authors as "A vailability Test" . The aim of this test is to estimate the pesticide concentration of the soil solution in situation close to fi eld conditions. The test is based 0 0 the determination of the amount of cOlllpound, which is soluble in differenl volu llles of CaCI2 after adsorp­lion of the pesticidc 0 11 identical weights of soil.

Mechanisms of adsorption

It is nol really easy lo determ ine lhe adsorption Illcchanism of pesticides by soil particles in fate and transporl models. Several mechanisms may operate during the ill teract ion between gi ven pesticide and soil particles depending on the nature of lhe organo-mineral association. The overall sorption process is often observed lo be init ially fast, but then to

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100 LaboratOlY tests

decrease lo a slower sustained rate that may require long periods af time lo reach lhe equi­librium. The low rate of adsorption is attributed lo interparticJe diffusion of salute to inter­nal surface af soil components and to diffusion af salute through their matrices. Slow kinetic studies can help to understand lhe true extensiol1 af sorption. It is necessary to paint out that 1110St studies have approached lhe sorption-desorption problem fmm lhe sorption step alone. W hen desorption studies have been undertaken hysteresis effects have often been observed. This phenomenon is sometimes described as nonequilibrium, nonsingu­larity, slowly reversible or irreversible or resistant sorption. Many disc Llss ions have appeared in the last years 011 lhis process based 0 11 nonequilibrium sorptiol1 behaviour and slow ki netics of several chemicals in soils.

After the aboye considerations it is clear that very careful kinetic sludies must be under­taken together with sorption isolherms in static or dynamic systems as previous steps to describe the proper mechani slll.

Two contributions inc luded in section 3.3. are based on the descript ion of sample preparat ion and techniques used for sorpl ion ll1echani sms studied. Fusi el al. present ll1ethods lo study peslic ide inle raction with clays in water and organic solvents. Quanti­tati ve ll1easurements are conducled in water and qualitati ve studies by FT-IR in organic solvents in order to identify the functi onal groups and type of bonds in volved. The authors describe procedures for deterrninat ion of kinelic and adsorption isotherms and the ways lo fit data so obtained [ O adsorption models. They also indicate lhe calculation method of the rmodynamic parameters from the aboye ll1entioned experimental measure­me nLs.

Romero Taboada el al. ind icate similar methods than FlIsi el al., applied to the stlldy of eyanazine, earbendazime and ethirimol adsorption on soil , peat and montmorillonite.

Further research needs

The sorption of pestie ides by soi ls has been shown lo generally involve multiple reaclivity located on o rganic and inorganie surfaees and matrices o F d iffe rent size panicles or aggregates. Most o F the expressions used are considered lInder eqllilibrium eondi tions but very often slow kinetics oceur. Tt shou ld be convenient lO lake into aecount the mechani sms of diffusion through the organic malter matrices and that occllrring through the inte rpartic le pores. The methods given in seclion 3 (Adsorption - desorption studi es) show a wide spectrum of procedures lo lInderstand the sorption­desorption process of pestic ides in soils and soil components. Most of lhese methods a re new approaches or modified methodologies 10 he lp lo understand the so rption process in situations as c10se as poss ible to the fie ld conditions. However, lhere a re some gaps in the actual methodologies that need LO be improved. Sorplion shows sometimes anomalous behaviour that are nOl yet salisfactory ex pl ai ned. FlIrthermore, lhere a re questions on re lationship between soil properties and sorpt ion-desorption process that need appropri ated answers.

Some areas where we need more research and new methods 011' sorption of pestic ides in the soil environment are suggested:

- Procedures to dete rm ine the association degree of inorgan ic and organic colloids in soi l versus pesticides sorption.

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AdsOJIJtion - desorption studies/111lroductiofl 101

- Determination of reactivity of adsorptioll sites both in illorganic and organi c colloids surface and their corresponding matrices especially when they are associated as micro­aggregates. - Studies are needed to understand the mechanisms of slow adsorption and desorption kinetics . - Satisfactory methods for study bound or reca1ci trant residues.

References

Cornejo J. and Hermosin M.C (1996). ¡nteraction of humic substances and soi l clays. In: Humic Substances in Terrestrial Ecosystems, Chapo 15, A.Piccolo. EIsev ier Amsterdam. p 595-624.

Chiou eT., Pelers LJ. and Freed V.H. ( 1979). A physical concept of soil -water equilibria for nonionic orgallic compounds. Science, 206: 83 1-832.

OECD ( 198 1). OECD Guidelinesfol" Tesling ofChel1licals, 106: Adsorplion-DesOIplion. OECD, Paris.

OECD ( 1996) . OECD Guidelines fOI" Tesling of Chemica/s, lO6. AdsOIplion/desOIplion using a ba/h equilibrium method. OECD, Paris.

Saiz Jimenez e (1996). The Chemical Structure of Humic Substances: Recent Advances. In: Humic Substances in Terrestrial Ecosystems, Chap l. A. Piccolo. EIsevier Amster­dam. p 1-44.

Schwarzenbach R.P. and Westall J. ( 1981 ). Transport of nonpolar organic componnds from surface water to groundwater. Enviran.. Sci. Teclmol., 15: 1360-1370.

Weber J.B ., Shea P.H. and Weed S.B. (1986). Flnoridona retention and release in soils. Soil Sci. Soco AIIl.J. 50: 582-588.

Weidenhaupt A., Arnold e, Mü ller S.R. HederIein S.B. and Schwarzenbach R.P. ( 1997). Sorption of organotill biocides to mineral surfaces. Environ. Sci. TeclIna!.3 1: 2603 -2609.