RETENTION OF C O U M A R I C ACID BY SOIL AND ITS C O L L O I D A L

C O M P O N E N T S

R I C C A R D O R I F F A L D I , A L E S S A N D R O SAVIOZZI , and RENATO L E V I - M I N Z I

Institute of Agricultural Chemistry, University of Pisa, Via S. Michele degli Scalzi 2, 56124 Pisa, Italy

(Received January 6, 1990; revised April 17, 1990)

Abstract. In this study of the behavior of coumaric acid added to soil, the disappearance of the acid was found to be due to adsorption plus microbial degradation, and was influenced by the concentration and contact time. Adsorption experiments set up with soils varying widely in their chemical and physical properties, showed that the Freundlich isotherm fits the data quite well and that among the different soil factors, only the pH was closely and negatively correlated with coumaric acid adsorption. Approximately at pH > 7 no adsorption occurred, perhaps due to the repulsion between the negatively charged soil colloids and the dissociated acidic groups of coumaric acid. Adsorption experiments carried out with different adsorbents showed that the hydroxy-Fe compound was the most effective in retaining coumaric acid, followed by humic acid, illite, kaolinite and vermiculite, in this order.

1. Introduction

Phenolic acids are ubiquitous in nature, occurring in plant tissues, microorganisms and soils. There is evidence that the phenolic acids in soils originate from the decomposition of plant residues and from synthesis by soil microorganisms (Ste- venson, 1982; Whitehead et al., 1981), and that the phenol composition of the soil is largely influenced by the vegetation (Whitehead et al., 1982), soil characteristics (Riffaldi et al., 1989; Kuiters and Denneman, 1987), the cropping systems (Maciak and Harms, 1986), and climatic conditions (Kuiters and Denneman, 1987). They are believed to be important intermediates in the formation of humic substances (Flaig et al., 1975). Their presence in the soil may have either detrimental or beneficial effects on germination and growth processes (Kuiters and Sarink, 1987). A complete review of the occurrence and chemistry of phenolic acids was recently published by Hartley and Whitehead (1985).

In the last few years, there has been growing interest in the recycling of organic wastes, whether agricultural, industrial or municipal in origin, as a means of replenishing the organic matter in soils. A major problem limiting the acceptance of these residue management methods is that the wastes, which contain appreciable quantities of phenolic acids (up to about 1900 p.p.m.d.w.; Genevini and Negri, 1986), particularly if shallowly incorporated, can often increase the phenolic acid level in the soil with a concomitant decrease in crop yield (Wang and Yang, 1967; Glass, 1976). In order to establish the significance and the behavior of phenolic compounds in soil, it is necessary to gain insight into the factors that influence their occurrence and disappearance.

The retention of phenolic acids in soils has been reported to occur by adsorption

Water, Air, and Soil Pollution 51: 307-314, 1990. © 1990 Kluwer Academic Publishers. Printed in the Netherlands.

308 RICCARDO RIFFALDI ET AL.

(Wang et al., 1971) and to be influenced more by hydroxy-Fe and A1 compounds

than by the different clay minerals (Huang et al., 1977). Shindo and Kuwat suka

(1976) failed to find a close relat ion between the free Fe content in soils and their

adsorp t ion capacity, while Wang et al. (1971) reported the probable role of organic

mat ter in the sorpt ion of phenolic acids. Lehman et al. (1987) showed that the

disappearance of phenol ic acids in the soil involved an oxidat ion of the acids, coupled

with a reduct ion in the soil Fe and Mn oxides. The possibili ty that soil could

inactivate these compounds and so reduce their al lelopathic potent ia l was postulated

by Yakle and Cruse (1984).

Because relatively little in fo rmat ion is available on just how quickly simple phenolic

acids are retained in the soil in compar i son with other c o m m o n organic compounds ,

this study was unde r t aken to investigate the behavior of phenolic acids in soil and

to evaluate the role of the different soil colloids in their retention.

2. M a t e r i a l s a n d M e t h o d s

S O I L S

Four teen soils f rom various areas of Tuscany, varying widely in their chemical

properties, were examined. The top soil samples were air dried, homogenized and

passed through a 0.5 m m sieve. Various soil characteristics (Table I) were determined

by s tandard methods (Page, 1982). The an ion exchange capacity (A.E.C.) was

TABLE I

General characteristics of the soils

Soil pH Sand Silt Clay Org .C Org.N C/N Free Iron A.E.C. CaCO 3 No H20 % % % % %o % meq/100g %

1 6.9 70.0 20.7 9.3 2.4 2.2 11 0.70 6.5 - 2 8.2 56.3 33.1 10.6 1.7 1.8 9 1.16 5.4 5.9 3 8.6 46.5 37.5 16.0 0.7 0.9 8 1.28 8.1 24.5 4 8.5 24.6 53.3 22.1 1.2 1.2 10 2.20 9.7 13.9 5 5.7 55.6 31.9 12.5 1.2 1.4 9 1.56 9.5 - 6 6.0 63.0 24.7 12.3 1.3 1.5 9 1.40 12.2 - 7 5.3 69.9 22.0 8.1 1.6 1.9 8 1.55 14.1 - 8 4.9 80.7 7.2 12.1 0.1 0.1 10 1.74 7.7 - 9 7.0 45.1 31.6 23.3 0.9 1.4 6 2.15 8.8 -

10 8.1 38.9 48.5 12.6 2.2 2.8 8 1.63 5.6 2.6 11 8.5 53.1 30.0 16.9 0.5 0.7 7 1.68 6.1 9.0 12 5.5 31.2 54.1 14.7 1.6 1.7 9 2.55 14.0 - 13 6.1 61.1 8.5 30.4 0.6 0.7 9 1.80 7.2 - 14 8.3 60.2 29.4 10.4 1.1 1.1 10 1.54 5.7 6.4

Mean 7.0 54.0 30.9 15.1 1.2 1.4 8.8 1.60 8.6 - S.D. 1.4 15.6 14.4 6.3 0.6 0.7 1.3 0.5 3.0 -

RETENTION OF COUMARIC ACID BY SOIL AND ITS COLLOIDAL COMPONENTS 309

determined according to the method of Mehlich (1953). Soil sterilization was

accomplished by autoclaving three times at 2-day intervals, for 3 hr at 120 °C and

1.4 bar. Previous studies have shown that this procedure eliminates microbial growth. Acidification of the alkaline soil No. 14 was obtained by treating portions of

the soil with different amounts of diluted HC1. After equilibrating for 24 hr the samples were air dried. The silt-loam soil No. 14 was chosen because it is a representative soil in the permanent-pasture regions of Tuscany.

C O L L O I D S

The clay minerals used were kaolinite from Bath (South Carolina), illite from Fithian

(Illinois), and vermiculite from Libby (Montana). The humic acid was from K. & K. Laboratories (Lot. 8985A) and the hydroxy-Fe compound was prepared according to Huang et al. (1977). Only the ~ 2 ~tm fractions of the different materials

were used.

R E T E N T I O N O F C O U M A R I C A C I D

Retention experiments were set up using the batch technique: solutions of analytical grade p-coumaric acid were prepared at concentrations of 30 to 1000 ppm in 10-2M CaCI> Five milliliters of standard solution were added to 1.0 g of soil, or to 50 mg of colloids, in 25 mL glass tubes. Rather high concentrations were chosen to simulate industrial pollution plus the addition of organic wastes to the soil. Coumaric acid was chosen for this experiment because it occurs widely in both the free and combined form in higher plants, and reacts readily with soil (Huang et al., 1977) but at rate moderate enough to be followed by our procedures. The suspensions were shaken for periods ranging from 1 to 48 hr, centrifuged at 4000 rpm, filtered on cellulose acetate (0.2 Ixm) and analyzed for phenolic acid using a Folin-Ciocalteus reagent, following the method of Kuwatsuka and Shindo (1973). The amount of coumaric acid removed from solution was considered to be equivalent to the amount adsorbed by the soil and the different colloids, or degraded by soil microorganisms. All the experiments were carried out in duplicate at 25 °C and the results were averaged.

3. Results and Discussion

The relationship between the disappearance of the coumaric acid, added at concentrations of from 100 to 1000 ppm to soil sample No. 14 and the length of time for which the compound was in contact with the soil is shown in Table II. The disappearance of the coumaric acid in solution should represent the cumulative amounts of the compound adsorbed by the soil and microbially degraded. Scott et al. (1982) had already designated the disappearance of phenols in soil as the 'relative apparent adsorbed phenol'. The fact that coumaric acid was totally recovered even after 24 hr of contact time at concentrations of 5 mg g-i indicates that it was not strongly sorbed into the soil. Since the relative loss decreased as

TABLE II

Relationships between coumaric acid disappearance from soil and equilibrium shaking time as influenced by concentration of phenolic acid

Contact time (hr)

1 8 24 48

25

Coumaric acid Total coumaric acid added (mg g-l) Disappearance % - - disappeared (mg g-l)

0.5 0 8 42 96 0.48 1 0 2 19 60 0.60 1.5 0 0 15 42 0.63 2.5 0 0 13 33 0.82 5 0 0 0 8 0.4O

the concen t ra t ion o f coumar i c acid increased, and since at the highest concen t ra t ion

the coumar i c acid levels decreased (both in abso lu te and percentage terms), it wou ld

seem to indicate tha t processes o ther than adso rp t i on were p r o b a b l y involved; this

is conf i rmed by the failure of the re ta ined + so lu t ion phases to reach equi l ibr ium.

F igure 1 shows the influence of the coumar ic acid concen t ra t ion on the lag-

phase , which represents the t ime dur ing which no phenol ic acid loss occurred . The

l inear increase (r = 0.97, P = 0.01) in the length o f lag-phase was significant , and

suppor t s the hypothes is of a b io logica l deg rada t i on of coumar ic acid; the a m o u n t

of deg rada t ion at the highest concen t ra t ion (5 mg g- l ) was d rama t i ca l l y reduced

20

0 0 1 2 3 4 5

coumaric acid concenl/ration, mg/g soil

Fig. 1. Influence of coumaric acid concentration on the time of the lag-phase for soil No 14.

e -

i15 ¢0 c- O.

-~ 10

3 l0 R I C C A R D O R I F F A L D I ET AL.

RETENTION OF COUMARIC ACID BY SOIL AND ITS COLLOIDAL COMPONENTS 311

(Table II), thus showing the toxic effect of high levels of coumaric acid. Further evidence for the involvement of microorganisms in coumaric acid degra-

dation was obtained by recovery experiments performed for the same contact times and phenolic acid concentrations as in the above experiment, and using the same soil (No. 14) but this time autoclaved at 120 °C to inhibit microbial activity. The recovery of coumaric acid in the supernatant liquid after contact with the soil, was 100% in every case, indicating that no acid was adsorbed by the sterile soil.

The inability of the sterile soil to remove coumaric acid from solution could be due to the pH of the soil-phenolic acid solution system. Since this pH, even at the maximum concentration of 5 mg g-1 of soil, was always higher than 8 and since the pK a value of coumaric acid is 4.63, it can be deduced that at pH > 8 coumaric acid is present almost entirely in the form of a negatively charged species which is repelled by the negatively charged soil; thus adsorption is inhibited. Supporting evidence was obtained by reacting 5 mL of the 1000 ppm coumaric acid solution with 1 g soil No. 14 that had been previously acidified and sterilized. The soil-acid suspensions were shaken for 1 hr, the amount of time established by preliminary experiments to be sufficient for the system to reach equilibrium, and the pattern of disappearance from the solution phase of the coumaric acid in this soil suspensions under different pH conditions is presented in Figure 2. It can be seen the adsorption of phenolic acid by the soil is pH dependent with

30

2 0 - U e"

¢0

e~

._~

10

0 -_ ' _ I 9 8 7 6 5 4 3 2

pH Fig. 2. P a t t e r n o f % d i s a p p e a r a n c e o f c o u m a r i c ac id f r o m s u s p e n s i o n o f soil No . 14 acidi f ied a n d

steril ized.

312 R I C C A R D O RIFFALDI ET AL.

a maximum at pH <~4.5, a value which is very close to the pK a of coumaric acid;

indeed, a lower pH lessens the anionic character of the organic acid and the negative

charge on the soil colloids. Furthermore, below the pK a of the acidic group some

adsorption is possible through H-bonding and Van der Waals forces. Huang et

al. (1977) reported that the adsorption capacity for phenolic acids is caused by

the interaction of negatively charged COO- and phenolate O- groups with positively

charged hydroxy-A1 and Fe components, these being much more responsible for

the phenolic acid adsorption in soil than the clay minerals.

To ascertain the relative importance of the various soil components in the retention

of phenolic acids, excluding the factor of microbial decomposition, a series of 1

g of soil samples, which differed to a large extent in their chemical and physical

properties (Table I) were sterilized as described above and equilibrated for 1 hr

with 5 mL of CaClz 10 -2 M solutions containing initial concentrations of coumaric

acid ranging from 30 to 300 ppm. A parallel experiment to study the relative

magnitude of adsorption by the humic acid, the hydroxy-Fe compound and various

clay minerals was carried out using the same procedure, except that 50 mg of

adsorbent was used. The results confirm that soils with an alkaline pH do not retain coumaric acid,

while acid soils and the various colloids studied adsorb phenolic acid according

to the empirical Freundlich equation (expressed in linear form): log Q = log k +

1/n log Ce, where Q is the amount of coumaric acid adsorbed per unit of adsorbent

(~tg g-l), Ce is the equilibrium adsorbate concentration in solution (~g mL ~) and

TABLE III

Freundlich adsorption isotherm constants by coumaric acid for different acid soils and adsorbents

Sample k l/n r Equilibrium Degree of solut ion dissociation pH CaC12 (a%)

Soil No 5 8.3 0.89 0.95 a 4.8 60 6 4.0 0.93 0.99 a 5.1 75 7 15.5 0.74 0.99 a 4.6 48 8 24.5 0.45 0.97 a 4.0 19 9 0.3 1.16 1.00 a 6.2 97

12 2.5 1.04 0.99 a 4.8 60 13 4.6 0.78 0.99 a 4.9 65

Kaolinite 9.8 0.61 0.99 a 3.7 11 Vermiculite 3.7 0.69 0.96 a 7.2 100 Illite 21.4 0.74 0.99 a 3.8 13 Humic acid 148 0.67 0.98 a 4.7 54 Hydroxy-Fe 2630 0.56 0.99 a 3.3 4

a Significant at 1% level.

RETENTION OF C O U M A R I C ACID BY SOIL AND ITS COLLOIDAL COMPONENTS 313

k (adsorption capacity) and 1/n (adsorption intensity) are regression parameters which characterize each adsorbent. For the above experimental conditions, k (mL g-1 adsorbent) represents the quantity of coumaric acid adsorbed for Ce=l p.g mL -I, and 1/n reflects the degree of non-linearity of the adsorption. The Freundlich adsorption isotherm constants of coumaric acid for acid soils and for the various adsorbents are given in Table III. In this study, the values k and 1/n were evaluated empirically and used to compare the coumaric acid adsorption affinity of the material being tested. Comparing the data from Tables I and III, limited to samples that showed adsorption, significant correlations could be found only between the soil k and 1/n, and the solution pH. The experimental data suggest an inverse function between k and pH CaC12 (r=-0.80, P=0.05) which can be attributed to the fact that as the pH decreases, the percentage of negatively charged molecules of coumaric acid, expressed as degree of dissociation (Table III), also decreases (r=0.96, P-0.01), and thus the molecules are less repelled by the negatively charged soil. The constant 1/n (reflecting the rate at which the amount adsorbed increases as the concentration of the coumaric acid in solution increases) was well correlated with the pH CaC12 at equilibrium (r= 0.86, P= 0.05), indicating that as the pH increases, the increase in the affinity of the soil for coumaric acid will be more pronounced. These findings are in contrast with those of Shindo and Kuwatsuka (1976) who found that the pH values of a soil had little influence on the adsorption of phenolic substances, but they are in agreement with the results of Huang et al. (1977) who reported that the adsorption of phenolic acid was greater under acidic conditions. In this experiment in contrast with the findings of some authors (Huang et al., 1977; Wang et al., 1971) coumaric acid adsorption was not closely related to other soil properties such as its clay, Fe oxide and organic C contents. However, the data reported for the adsorption of coumaric acid by adsorbents show that the retention sequence is as follows (in decreasing order of adsorption): hydroxy-Fe compound > humic acid > illite > kaolinite > vermiculite. The significantly higher adsorption capacity of the hydroxy-Fe compound (more than 100 times greater than the most adsorbent clay minerals) is essentially attributable to the high chemical reactivity of its positively charged Fe-OH °" 5+ group (Huang et al., 1977), while the relatively low adsorption capacity of vermiculite can be attributed to the relatively high pH of its equilibrium solution (pH 7.2) where the completely ionized coumaric acid molecules are repelled by the interlayers of this clay mineral.

On the basis of the results obtained, it can be argued that the persistence in the soil of compounds such as coumaric acid, that may have either detrimental or beneficial effects on plants (Kuiters and Sarink, 1987), is mainly affected by the microbial degradation rate. Unlike in alkaline soils, in acid soils sorption is enough that coumaric acid would exist in association with soil particles rather than in solution.

314 RICCARDO RIFFALDI ET AL.

4 . C o n c l u s i o n s

On the basis of the above results it can be concluded that: - the disappearance of the coumaric acid in soil caused by microbial degradation is rapid at low phenolic acid concentrations, drops as the concentration increases and increases with exposure time; - among the different physical and chemical soil factors analysed, only the pH appears to significantly influence the adsorption in sterile soil; - at approximately pH > 7 no adsorption occurs, probably because above pH 7 the dissociated acidic groups of coumaric acid are repelled by negatively charged soil colloids; - among the adsorbents tested, the following sequence of adsorption capacities was found: hydroxy-Fe compound > humic acid > illite > kaolinite > vermiculite.

A c k n o w l e d g m e n t s

This study was carried out with the financial support of the Italian National Research Council.

R e f e r e n c e s

Flaig, W., Beutelspacher, H., and Rietz, E.: 1975, Soil Components, Vol. 1, Springer-Verlag, Berlin. Genevini, P. L. and Negri, M. C.: 1986, Processing and Use of Organic Sludge and Liquid Agricultural

Wastes, Kluwer Acad. Publ., Dordrecht. Glass, A. D. M.: 1976, Can. J. Botany 54, 2440. Hartley, R. D. and Whitehead, D. C.: 1985, Soil Organic Matter and Biological Activity, Martinus Nijhoff,

Dordrecht. Huang, E M., Wang, T. S. C., Wang, M. K., and Hsu, N. W.: 1977, SoilSci. 123, 213. Kuiters, A. T. and Denneman, C. A. J.: 1987, SoilBiol. Biochem. 19, 765. Kuiters, A. T. and Sarink, H. M.: 1987, Zeitseh. Pflanzen. Bodenk. 15, 84. Kuwatsuka, S. and Shindo, H.: 1973, Soil ScL andPlant Nutr. 19,219. Lehman, R. G., Cheng, H. H., and Harsh, J. B.: 1987, SoilSci. Soc. Am. J. 51,352. Maciak, F. and Harms, H.: 1986, Plant and Soil 94, 171. Mehlich, A.: 1953, J.A.O.A.C. 36,445. Page, A. L.: 1982, Methods of SoilAnalysis, Am. Soc. Agron. Monogr., Madison. Riffaldi, R., Saviozzi, A., and Levi-Minzi, R.: 1989, Agrochimica 33, 386. Scott, H. D., Wolf, D. C., and Lavy, T. L.: 1982, J. Environ. Qual. 11, 107. Shindo, H. and Kuwatsuka, S.: 1976, Soil Sci. andPlant Nutr. 22, 23. Stevenson, F. J.: 1982, Humus Chemistry, J. Wiley & Sons, New York. Wang, T. S. C. and Yang, T. K.: 1967, Ann. Rept. Taiwan SugarExp. St. 184. Wang, T. S. C., Yeh, K. L., Cheng, S. Y.0 and Yang, T. Z.: 1971, Biochemical Interactions among Plants,

Nat. Acad. Sci., Washington, D. C. Whitehead, D. C., Dibb, H., and Hartley, R. D.: 1981, SoilBiol. Biochem. 13, 343. Whitehead, D. C., Dibb, H., and Hartley, R. D.: 1982, J. Appl. Ecol. 19, 522. Yakle, G. A. and Cruse, R. M.: 1984, SoilSci. Soc. Am. J. 48, 1143.