Study of Sorption and Desorption of Atrazine , Ametryn and Metolachlor on Brazilian Soils

Os processos de adsorção e dessorção dos herbicidas ametrina, atrazina e metolaclor foram estudados em cinco so los brasileiros com diferentes conteúdos de matéria orgânica, utilizando-se cromatografia gasosa de alta resolução (HRGC). As linhas de regressão geradas apresentaram coeficiente de determinação > 0,964, indicando um bom ajuste dos da dos à equação de Freundlich. O equilíbrio de adsorção foi atingido em aproximadamente 6 h. Histerese foi observada nas isotermas de adsorção e dessorção. Correlações significativas foram encontradas en tre o coeficiente de adsorção, matéria orgânica e capacidade de troca de cátions. Não foram observadas correlações en tre o coeficiente de adsorção dos herbicidas (ametrina, atrazina e metolaclor) e solubilidade em água, pKa e massa mo lec u lar destes compostos, pois estes herbicidas não pertencem à mesma família química. Por este fato, as correlações en tre adsorção e propriedades dos herbicidas citadas acima foram estabelecidas somente en tre as triazinas. O coeficiente de adsorção das triazinas mostrou-se diretamente correlacionado com a solubilidade em água, pKa e massa mo lec u lar destes compostos nos solos avaliados.

Sorption is a general term that ain cludes adsorption (surface binding) and partitioning.Desorption is the reverse process of sorption3.As soil-applied herbicides are gen er ally con sid ered to en ter plant roots as sol utes with the soil wa ter, the de gree of ad sorp tion mod i fies the avail ability of her bi cide to plants or the ac tiv ity of the her bi cide.
Or ganic her bi cides ad sorb on both or ganic and in organic surfaces depending upon the chemical properties of the adsorbents and ad sor bates in volved.
Physical and chemical properties of the herbicides influencing adsorption and desorption processes are the chemical character, shape, configuration, acidity or basicity of molecule (pKa or pKb), water solubility, charge distribution on the organic cation, polarity, molec u lar size and polarizability 4 .
Some extensive adsorption studies involving a serie of analogous s-triazines have been realized.S-triazines are adsorbed by various soil constituents in strik ingly different degrees.Clay minerals, organic matter and pH are the soil properties mainly responsible for the adsorption process in soil [5][6][7][8] .The adsorption of the acetanilide her bi cides has been shown to in crease with in creases in the or ganic mat ter con tent of the soils, her bi cide con cen tration, and tem per a ture 9,10 .
This means that the adsorption process is dependent on the type of soil under consideration and the climatic conditions of a given location.The results obtained by researchers in this area are difficult to extrapolate to conditions distinct from those where the data were obtained.In addition, data concerning the adsorption processes of pesticides on Brazilian soils is very scarce 11, 12   .The in di ca tors of the en vi ron men tal qual ity pres ently in use were es tab lished with the view to at tend to eco log i cal char ac ter is tics ex ist ing in other coun tries, not nec es sar ily those pre dom i nant in Brazil.
The adsorption processes on soils has usually been determined by radio-tracer techniques (conventional method), us ing 14 C-labeled material diluted with pesticides and quan ti ta tive anal y sis by liq uid scin til la tion spectrom e try [13][14][15] .
Lanças et al. 16 re ported the vi a bil ity of the use of cap illary gas chro ma tog ra phy as an an a lyt i cal method to de termined the ad sorp tion of trifluralin in soils.
Thus, the pur pose of this pa per was to eval u ate the sorption and desorption of her bi cides, two s-triazines (atrazine and ametryn) and one ac et an i lide (metolachlor), on five Bra zil ian soils, un der lab o ra tory con di tions us ing high reso lu tion gas chro ma tog ra phy (HRGC).The re la tion ship among sorp tion, the physicochemical prop er ties of these her bi cides and soil com po nents was es tab lished.

Soils
Five types of soils with no re cent his tory of her bi cide ap pli ca tion and rep re sent ing a good range in or ganic mat ter con tent were used in the sorp tion stud ies.Each soil sam ple was ob tained by the mix ture of twenty dif fer ent col lec tion points from 0 to 20 cm depths.Be fore use, the soils were air-dried and ground to pass a 2.0 mm sieve.Ta ble 1 shows the char ac ter is tics of these soils.

Ad sorp tion equi lib rium
To establish when herbicide adsorption reached equilibrium in the soil systems, 10 mL of the herbicide solution, 20 µg/mL (in 0.01 Maqueous CaCl2) was added to 2 g of soils no. 1 and 5 in glass tubes.Du pli cate sam ples plus blanks (no soil) used as con trol stan dards were shaken for 0.25, 1, 3, 6, 12, 24 h.The tubes were cen tri fuged for 15 min at 4000 rpm.The supernatant was fil tered.Suit able aliquots were taken for gas chro mato graphic anal y sis.

Ad sorp tion
Stock solutions of herbicides were made in 0.01 M CaCl2 at concentrations be low that cor re spond ing to their max i mum sol u bil ity in wa ter (Ta ble 2).The stock so lu tions were di luted with 0.01 M CaCl2 to yield five con cen tra tions (1, 5, 10, 20, 30 µg/mL) that were used in sub se quent exper i ments.Two grams of soil were shaken with 10 mL of each her bi cide so lu tion in a glass tube for 6 h at room temper a ture (25± 2 °C).All treat ments were per formed in dupli cate, and ap pro pri ate blanks (no soil) at each de sired con cen tra tion were run.The tubes were cen tri fuged at 4000 rpm for 15 min and aliquots of the supernatant were fil tered and taken for anal y sis.The amount ad sorbed (mi cro grams of her bi cide per gram of soil) was de ter mined by the dif ference be tween the blank and treat ment val ues; i.e.
where A is the amount ad sorbed, v the so lu tion vol ume (mL), w is the mass of soil in grams, C B is the equi lib rium con cen tra tion of blank (µg/mL), and Ce is the equi lib rium con cen tra tion in the supernatant (µg/mL).

Desorption
Soil sam ples were shaken with so lu tions of atrazine, ametryn and metolachlor as de scribed for ad sorp tion.Af ter centrifugation and re moval of aliquots for gas chro matographic anal y sis, the re main ing supernatant was care fully de canted.Ten mil li li ters of 0.01 M CaCl2 were added to the tubes, and the sam ples were shaken for 6 h.The tubes were cen tri fuged, and aliquots of supernatant were fil tered and re moved for anal y sis.Desorption, ex pressed as mi crograms ad sorbed per gram of soil, was de ter mined by dif ference, tak ing to in ac count the so lu tion re main ing in the soil af ter the supernatant was poured off.

Her bi cide anal y sis
The her bi cide con cen tra tions in the supernatant were de ter mined by high res o lu tion gas chro ma tog ra phy (HRGC) us ing the fol low ing con di tions: Ametryn: SRI -8610 chromatograph equipped with flame ion iza tion de tec tor (FID); fused sil ica col umn, LM 1 (100% cross-linked dimethyl polysiloxane) -25 m x 0.53 mm x 1 µm; tem per a tures: col umn = 190 °C-6 °C/min -240 °C; in jec tor = cold "on col umn" in jec tor, op er ated at the am bi ent tem per a ture (25 °C), de tec tor = 300 °C; car rier gas, H2 at a lin ear ve loc ity of 53.50 cm/s.

Gas chro mato graphic anal y sis
In lit er a ture there are few ar ti cles us ing chro ma tog raphy meth ods in ad sorp tion stud ies and the ma jor ity of these, use re verse-phase HPLC 10,20,26,28 .The chro ma tog raphy meth ods are more sen si tiv ity (lower de tec tion lim its) when com pared with ra dio-tracer tech niques (con ventional method) and other dis ad van tages of the con ven tional meth ods are the ne ces sity of the use of 14 C la belled pes ticides with high radiochemical pu rity, es pe cially in Brazil, where these com pounds are ex pen sive, slow to ob tain due to de pend ence on im por ta tion, and are nec es sary ad e quate dis card.
No deg ra da tion of the her bi cides was ob served in the ab sence of ad sor bent ma te rial (soil), since a con stant concen tra tion of her bi cide was ob served through out the ex peri ment, and no new peak ap peared on the chromatograms.The chromatograms re fer ring to the ad sorp tion phe nom enon for each in di vid ual her bi cide are shown in Fig. 1.

Ad sorp tion iso therms
On the ba sis of the ki netic ex per i ments, the ad sorp tion of her bi cides by the soils reached an ap par ent equi lib rium af ter 6 h.
The em pir i cal Freundlich re la tion ship was used to describe the re sults of ad sorp tion.The lin ear form of this equa tion is as fol lows: where x/m is the con cen tra tion of the ad sor bate per unit amount of ad sor bent (µg /g), Ce is the ad sor bate con cen tration in so lu tion af ter equil i bra tion (µg/mL), and K and 1/n are con stants rel a tive to the af fin ity of ad sor bent for ad sorbate and to the de gree of cur va ture of the iso therm, re spectively.The K value can be used to com pare sorp tion of dif fer ent her bi cides on var i ous soils at unit con cen tra tion 15 .
Log-log plots of her bi cide (ametryn, atrazine, and metolachlor) ad sorp tion on each soil type yielded straight, par al lel lines (Fig. 2).The gen eral shapes of the ad sorp tion iso therms were very sim i lar, sug gest ing that sim i lar adsorp tion mech a nisms were in volved.
Freundlich con stants K and 1/n and the cor re spond ing co ef fi cient of de ter mi na tion (r 2 ) are given in Ta ble 3. Most of the re gres sion lines gen er ated had a co ef fi cient of de termi na tion (r 2 ) > 0.964 which in di cated a good fit of the data by the Freundlich equa tion 20 .
No lim it ing ad sorp tion was ob served within the concen tra tions range used.As the con cen tra tion of her bi cides was in creased, the amount ad sorbed in creased in al most lin ear fash ion with these con cen tra tions.
Desorption iso therms rep re sent the mi cro grams of herbi cide still ad sorbed per gram of soil as a func tion of equilib rium con cen tra tion af ter one desorption cy cle.Ad sorp tion was par tially re vers ible, the desorption val ues were smaller than those for ad sorp tion (Ta ble 3).Noncoincidence of ad sorp tion and desorption iso therms is called hys ter esis 21 and it in di cates that a dif fer ent range or spec trum of forces are in volved in ad sorp tion than in desorption.Hys ter esis has been ob served in other her bicide ad sorp tion stud ies [22][23][24] .The re vers ibil ity of ad sorp tion plays an im por tant role in her bi cide con trol in ag ri cul ture.

Soil prop er ties af fect ing the ad sorp tion phe nom ena
The re la tion ship be tween sorp tion and soil prop er ties was de ter mined through sim ple cor re la tion anal y sis (Ta ble 4).Strong cor re la tions were found among or ganic mat ter Ta ble 3. Freundlich con stants and co ef fi cient of de ter mi na tion (r 2 ) for her bi cide ad sorp tion and desorption.  .re quire higher rates of her bi cide ap pli ca tion to main tain a given level of her bi cidal ac tiv ity than do soils with lower val ues of these prop er ties.The im por tance of or ganic mat ter in ad sorp tion processes was also re ported by sev eral au thors.Sequi et al. 25 re lated that the sorp tion abil ity for atrazine and triluralin were in creased consideralably in soils with high or ganic mat ter con tent.The ad sorp tion of ametryn was pos i tively cor re lated with or ganic mat ter and silt con tent 26 .These data agree with cor re la tion ob served for this triazine (Ta ble 4).

Ametryn
Weber and Pe ter 27 dem on strated that or ganic mat ter was the pri mary ad sorb ing sur face in soils for ac et an i lide her bi cides.Obrigawitch et al. 22 cal cu lated the co ef fi cient of de ter mi na tion (r 2 ) val ues for per cent or ganic car bon and clay ver sus the metolachlor Freundlich ad sorp tion con stant (K) for three soils.Or ganic car bon ver sus K re sulted in r 2 = 0.92, and clay ver sus K re sulted in r 2 = 0.68.The co ef fi cient of de ter mi na tion (r 2 ) value be tween or ganic mat ter con tent and metolachlor K is very sim i lar to the cor re la tion co ef ficient r = 0,947 calculed in this work for the same compound.
Sig nif i cant cor re la tion was also ob served among organic mat ter and clay con tent of other Bra zil ian soils (from the "cerrado"of the State of São Paulo) and sorp tion co ef ficient of aldrin, mal a thion, and para thion 28 .
The re gres sion equa tions be tween de gree of ad sorp tion (K) of the her bi cides and the con sid ered prop er ties of soils were: Ametryn If the K value in the Freundlich equa tion is re placed by the one found from equa tions 3 to 5, a sim ple re la tion ship may be es tab lished be tween her bi cide (ametryn, atrazine and metolachlor) ad sorp tion and soil prop er ties.This may be use ful for cal cu la tion of the quan tity of her bi cides to be ap plied in or der to achieve greater ef fi ciency 29 .
Sorp tion re la tion ships be tween her bi cides were il lustrated in a dif fer ent man ner by nor mal iz ing the Freundlich K con stants to a 1 g or ganic mat ter ba sis.The new con stant, Kom, was av er aged for each her bi cide over the five soil types (Ta ble 3).The or der of in creas ing Kom was sim i lar to that for the sim ple K con stants.
As suming that the her bi cides were ad sorbed mainly by or ganic mat ter, the change in free en ergy for ad sorp tion was cal cu lated for her bi cides by us ing the mean value for K`om 10 .
where ∆G is the free en ergy change (kcal / mol), R is the gas con stant (1.986 cal/Kmol), and T is ab so lute tem per ature.All ∆G for her bi cide ad sorp tion were neg a tive, as expected for a spon ta ne ous re ac tion (Ta ble 3).In gen eral the nu mer i cally smaller val ues of ∆G cor re sponded to the samples with the lower ad sorp tion ca pac ity.Freundlich iso therms (Fig. 4) showed dif fer ences in adsorp tion be tween her bi cides in dis tinct soils.Sorp tion of lindane var ied in dif fer ent Bra zil ian soils 12 .Felsot and Dahm 15 re lated that the vari able Freundlich iso therms for dif fer ent pes ti cides-soil sys tem in di cate that sorp tion is a com plex phe nom e non in volv ing dif fer ent types of ad sorption sites with dif fer ent sur face en er gies Ametryn (methlymercapto-triazine) was ad sorbed by all the soils more than all oth ers her bi cides in ves ti gated (Ta ble 3).

Her bi cide prop er ties af fect ing ad sorp tion
Weber and Pe ter 27 found two to three fold higher adsorp tion of prometryn (methylmercapto-triazine) than for the ac et an i lide her bi cides: alachlor, acetolachlor, and metolachlor.
No re la tion ship was found be tween sorp tion of her bicides (ametryn, atrazine, and metolachor) and their wa ter sol u bil ity, pKa, and mo lec u lar mass, Ta ble 2. This is what would be ex pected since these her bi cides do not be long to the same chem i cal fam ily.On ac count of this, the re la tionship be tween sorp tion and her bi cide prop er ties cited above will be es tab lished only for s-triazines.
The s-triazine ad sorp tion abil ity showed a di rect re lation ship with wa ter sol u bil ity, pKa and mo lec u lar mass of these componds in the soils eval u ated.This re sult sup ports the prem ise that the ad sorp tion in creases with the mo lec ular mass of ad sor bate in the same chem i cal fam ily 10 .
This study sug gests that atrazine (chloro-derivatives) would be ad sorbed in lower amounts by the stud ied soils than ametryn (methylmercapto-derivatives).Anal o gous re sults were ob tained in pre vi ous stud ies when s-triazines were ad sorbed on soil or ganic mat ter 7 .Ametryn, show ing a pKa value of 3.12, pres ents a more ba sic char ac ter than atrazine, pKa 1.68.The in creas ing ba sic ity can be explained in terms of the pres ence of more nucleophilic groups on the mol e cule.The ba sic ity ap pears to be im portant in the over all ad sorp tion-desorption re ac tion, es pecially if the ad sorbed ma te rial pres ents a ba sic char ac ter.Pro ton trans fer from protonated spe cies ad sorbed on a soil sur face to an un charged mol e cule at the soil sur face has been shown to be de pend ent upon the rel a tive basicities of two in ter act ing com pounds as well as the rel a tive con centra tions or ac tiv i ties of re ac tants and prod ucts 4 .
In con clu sion, the af fin ity be tween mol e cules of the s-triazines stud ied and soil par ti cles showed to be de pendent upon the chem i cal prop er ties of the soils (or ganic matter, and cat ion-exchange ca pac ity) and the chem i cal prop er ties (wa ter sol u bil ity, pKa, mo lec u lar mass) of the s-triazines in volved.

Fig ure 2 .
Fig ure2.Freundlich iso therms for ametryn ad sorp tion and desorption on five soils.

Ta ble 1. The
chem i cal and phys i cal prop er ties of the soils eval u ated.
Ta ble 2. Prop erties of the her bi cides stud ied.
Ta ble 4. Sim ple cor re la tion co ef fi cients (r) be tween K val ues and soil prop er ties de ter mined.