METAL-COMPLEX FORMATION BY LICHEN COMPOUNDS

METAL-COMPLEX FORMATION BY LICHEN COMPOUNDS

I. K. ISKANDAR AND J. K. SYERS (Department of Soil Science, University of Wisconsin, Madison, Wisconsin)

Summary The formation of soluble complexes, frequently coloured, when solid lichen

compounds were shaken with water suspensions of biotite, granite, and basalt indicated that chemical weathering had occurred. The formation of colourless complexes and the adsorption of the dissolved lichen compound or complex by the silicate phase complicate the interpretation of the spectrophotometric analysis data. Lichen compounds invariably released greater amounts of Ca than of Mg, Fe, and A1 from the silicates and, for each lichen compound, the release of Ca was usually greater from biotite than from granite or basalt. Release of cations from the silicate materials resulted largely from metal-complex formation rather than from reactions directly involving hydrogen ions. Citric, salicylic, and p-hydroxybenzoic acids and EDTA, used as control organic acids, usually released considerably greater amounts of cations from the silicates than did the lichen compounds, consistent with the higher water solubility of the control organic acids. Similar amounts of Fe, All Ca, and Mg were released from the silicates by solutions of the lichen compounds and by solid lichen compounds. Lichen compounds are sufficiently soluble in water to form soluble metal complexes and to effect chemical weathering of minerals and rocks.

Introduction METAL-COMPLEX formation is recognized as a biochemical weathering factor in soil formation (Bloomfield, 1951 ; Swindale and Jackson, 1956; Davies et al., I 60). Duff and Webley (1959) and Henderson and Duff

bacteria, can dissolve silicate minerals. The metal-complexing ability of oly henolic compounds, particularly the o-diphenols (Hess et al., 19&], {as also been demonstrated, and their significance in soil profile deve opment discussed (Bloomfield, 1966; Davies et al., 1960).

The organic compounds which occur in lichens have been studied by chemists interested in chemical structures and biosynthesis (Asahina and Shibata, 1954; Shibata, 1958). These compounds are usual1 referred to as lichen acids, although not all of them are in fact acids. 8 is commonly held that lichen compounds are insoluble in water (Smith, 1921 ; Smith, 1962; Haynes, 1964; Culberson, 1970) and for this reason the role of lichens in the chemical weathering of rocks appears to have lar ely been discounted (S ers and Iskandar, 1972). Recent studies

(1963) have s R own that 2-ketogluconic acid, produced by certain

p. (Isiandar and S ers, 1971) B ave shown that lichen de sides and depsidones, common r y occurring lichen compounds, are s ightly soluble in water, The latter finding, in conjunction with the presence in the molecule of polar groups which are able to function as electron donors, ex lains the formation of soluble, coloured complexes when certain lic g en compounds or ground lichens are allowed to react with water suspensions of minerals and rocks (Schatz, 1963 ; Syers, 1969). In both of these studies, however, no data were obtained for the amounts of cations complexed. Journal of Sol1 Science. VoL 23, No. 3. 1972

5113.8 T

256 I. K. ISKANDAR AND J. K. SYERS The purpose of this paper is to re ort on the extent of metal-complex

formation when lichen compoun B s and control organic acids are allowed to react with biotite, granite, and basalt.

Materials and Methods Materials. The six lichen compounds investigated were salazinic and

stictic acids (depsidones of the 8-orcinol type) ; evernic and lecanoric acids (depsides of the orcinol-type); atranorin (a depside of the 8-orcinol

Depside of orcinol-type Depside of a-orcinol-type

Evernic acid Atranorin

Depsidone of a-orcinol-type

OH

Dibasic, aliphatic

HO H Salazinic acid Roccellic acid

FIG. I . Structural formulae (Culberson, 1969) of four of the lichen compounds investigated.

type); and roccellic acid (a dibasic aliphatic acid). The structural formulae (Culberson, 1969) of representative lichen compounds are shown in Fig. I. Citric, salicylic, and p-hydroxybenzoic acids, and EDTA were used as controls.

Biotite (Spruce Pine, North Carolina, U.S.A.), porphyritic biotite granite (St. Cloud, Minnesota, U.S.A.), and basalt porphyry (Boulder County, Colorado, U.S.A.) were crushed and ground to pass a 300-mesh sieve. The < z p m fraction of each was separated by centrifugation (Jackson, 1956 and discarded. The 0-2 pm fraction was used in all

readily soluble cations and extractable reaction products formed during grinding, washed well with distilled water, and dried.

Methods. To investigate the reaction of solid lichen compounds and control organic acids with water suspensions of biotite, granite, and basalt, a 20-mg sample of the lichen compound or control organic acid was shaken in an 18-1111 polypropylene tube with 250 mg of silicate

experiments. ’kh e samples were was fl ed with 0’002N HC1 to remove

METAL-COMPLEX FORMATION BY LICHEN COMPOUNDS 257 material and 10 ml of sterilized water on a rotary shaker for 96 h at 25 "C. In addition, 10 ml of solution, obtained by shaking 30 mg of lichen compound with 15 ml of sterilized water under the same conditions, was added to a separate series of 250-mg samples of the silicate materials and shaken as above. To isolate the effect of metal-complex formation from that of hydrogen ions on the release of cations from the silicate materials, zso-mg samples of silicate material were shaken with 10 ml of an extractant prepared from sterilized water by ad'usting the pH

The pH of the system was checked and ad'usted twice daily durin the

extracts collected by filtration through Whatman No. 50 tlter-papers followed by supercentrifugation at 15 ooo r. m. The pH of the extracts was determined using a glass electrode. $he extracts were analysed spectro hotometrically using a Unicam S.P. 800. For chemical analysis,

H,SO, and evaporated to dryness. Fo lowing ignition at 450 "C for 2 h, the residue was dissolved in 5 ml 0 . 5 ~ HC1 with heating and quanti- tatively transferred to a 25-ml volumetric flask. Iron was determined colorimetrically by the orthophenanthroline method (Jackson, 1956), A1 by the Ferron method (Rainwater and Thatcher, 1960), and Ca and Mg by atomic absorption spectrophotometry (in I per cent SrCI, solutions).

Curves were plotted for the release of each cation against pH of the extractant for the silicate materials in the absence of lichen compounds or control or anic acids. From the pH of the extracts obtained b

to certain values in the range of pH 3 to 8 by the addition of A C1 or NaOH.

96-h shaking period. After shaking, the tu L es were centrifu ed an 2 the

z ml o P the extract was digested with ml conc. HNO, and 2 ml conc. I

shaking solid fichen compounds or solutions of lichen com ounds wit i: the silicate materials, the release of each cation caused ! y reactions

values are subtracte B from the amounts of each cation released from the directly involving h drogen ions can be calculated. When the latter

silicate materials during shaking with lichen compounds (solids or solutions), it is possible to isolate the effect of metal-complex formation from that of hydrogen ions on cation release.

To test for the presence of micro-organisms in the solutions and extracts before and after equilibration with the silicate materials, a small volume of the li uid was streaked on sterilized nutrient agar contained in petri-plates w R ich were incubated at 37 "C for 4 days.

Results Experiments with solid lichen compounds and control organic acids

Four of the lichen compounds investigated formed soluble coloured complexes when shaken with water suspensions of biotite (Fig. 2A). The remaining two, roccellic acid and atranorin, did not yield coloured extracts (data not presented). Only the solution of salazinic acid, obtained by shaking the solid compound with water alone, absorbed radiation in the visible region (Fig. 2d), although the solutions of all lichen compounds absorbed in the ultraviolet region (Iskandar and Syers, 1971). The greater absorbance of the biotite-salazinic acid combination (Fig. zAd) compared to that of salazinic acid alone (Fig. 2Af) is attributed

258 I. K. ISKANDAR AND J. K. SYERS to the formation of a soluble coloured complex. The extract obtained from the reaction of lecanoric acid and biotite (Fig. 2Ad) was highly coloured and showed a maximum absorbance at a proximately 470 nm.

allowed to react with Ca salts is used as a specific colour test for this acid (Culberson, 1969).

The formation of a reddish-yellow complex w R en lecanoric acid is

o.8[ C

C

"I I

400 450 500 550 600 650 700 Wavelength (nm)

b

F I C . ~ ~ . Spectrophotometric analyses in the visible region of the extracts obtained by shaking water suspensions of (A) biotite, (B) granite, and (c) basalt with solid samples of (a) salazinic, (b) stictic, (c) evernic, (d) lecanoric, and (e) roccellic acids. Curve

(d) is for salazinic acid after shaking with water in the absence of biotite.

S ectrophotometric analysis data for the extracts obtained by shaking

basalt (Fig. 2c) were qualitatively similar to those for biotite except that soluble coloured corn lexes were not obtained with lecanoric acid and

r n i t e &ig. 2 ~ a ) than with biotite (Fig. reacted more strongly with 2 ~ a ) or basalt (Fig. 2ca). T e solutions obtained by shaking biotite, granite, and basalt with water did not absorb radiation in the visible

soli c?l lichen compounds with water suspensions of granite (Fig. 2 ~ ) and

granite (Fig. 2 ~ d ) or l asalt (data not resented), and that salazinic acid

METAL-COMPLEX FORMATION BY LICHEN COMPOUNDS 259 region. There was no microbial growth, as determined by the agar plate test, in any of the solutions or the extracts when lichen com ounds were present, which is not su rising in view of the well-esta g fished

Culberson, I 969). Consequently, any increase in absorbance in the visible region when lichen compounds are allowed to react with water suspensions of minerals and rocks indicates the formation of soluble coloured complexes and shows that chemical weathering has occurred.

antimicrobial properties of lic 'K en compounds (Laakso et al., 1952;

9 0.4 \

= c ~ a e O '\

The fact that a coloured complex is not formed does not necessarily indicate that complex formation has not occurred, since the complex may be colourless or may be adsorbed by the silicate phase (discussed below). This is well illustrated by spectrophotometric analyses of the extracts obtained by the reaction of citric acid and EDTA with water suspensions of granite (Fi . 3). In each case, no significant increase in absorbance was obtained % etween 450 and 700 nm, even though both cam ounds are capable of forming soluble metal complexes. Similar resu ts were obtained with biotite and basalt (data not presented . Salicylic acid, on the other hand, reacted with biotite, granite, and basa t to form a soluble, coloured com lex which showed a maximum absorb-

dilute ferric chloride solution to salicylic acid produces a complex whic shows maximum absorbance at a similar wavelength.

The pH of the extracts obtained by shaking the lichen compounds alone in water ranged from 5-4 for lecanoric acid to 6-1 for atranorin, whereas that for the control organic acids varied between 2-9 and 4.8 (Table I). For biotite, granite, and basalt alone in water, the pH values of the solutions were 8.0, 6- lichen compounds or acids were shaken with water suspensions of the H values of the extracts were intermediate between those obtained or the lichen com ounds or control organic acids alone and the silicate materials alone. uch intermediate pH values are to be anticipated for the lichen compound-silicate

1 X ance at approximately 5 10 nm ( P ig. 3 for granite) ; the addition of a ve

P

6.5, respectively. When the solid

f ;P

260 I. K. ISKANDAR AND J. K. SYERS mixtures in view of the low solubility of the former and the buffer ca acity of the latter.

%he amounts of Fe, Al, Ca, and Mg released from biotite, granite, and basalt by extractants prepared by adjusting the H of sterilized water to certain values in the range of 3 to 8 using HC P and NaOH were as

compound Biotite

TABLE I

Ganite Basalt

p H of solutions obtained by shaking solid lichen compounds, control organic acids, or silicate materials alone with water, and p H of extracts obtained by reaction of solid lichen compounds and control orpanic acids with water

6-6 6.8 6.7 6.2 6.7 7'6

Compound

5'7 5'9 5'7 5'6 6.0 6.0 5'5 5'7 5'8 6.0 6- I 6.3

Salazinic Stictic Evernic Lecanoric Roccellic Atranorin

3'7 3.8

6.5 5 '0

Citric Salicylic p-hydroxy

benzoic EDTA

3' 2 5'5 3'2 4'8

5.6 6.5 4'0 4' 3

8.0 6.4

5'6

5.8 5 '7

5 '4 5'7 6. I

2'9 2'9

3'2 4'8

. . 6.5

expected (Table 2). With the exception of the release of Ca from biotite, very low amounts of Fe, Al, Ca, and Mg were released from the silicate materials in contact with an extractant of pH 5 or higher.

The release of varying amounts of Fe, Al, Ca, and Mg when solid lichen com ounds were shaken with water suspensions of biotite,

The values reported in Table 3 have been corrected for the release of cations from the silicate materials due to reactions directly involving hydrogen ions. Also, the lichen com ounds did not contain detectable

evernic acids with basalt, the lichen compounds re eased greater amounts of Ca (129 to 1100 pg/g silicate material) than of Mg, Fe, and Al from biotite, ranite, and basalt. The release of Ca, with one exce tion, was

%y lecanoric acid from biotite ~ I I O O pg/g biotite) is consistent with the formation of a strongly coloured complex (Fig. 2 ~ d ) . For biotite, the release of cations was generally in the order Ca > A1 > Fe > Mg.

granite, an a basalt (Table 3) is attributed to metal-complex formation.

amounts of Fe, Al, Ca, and Mg. up ith the exce tion of salazinic and

reater B rom biotite than from ranite or basalt. The large re P ease of Ca

P

METAL-COMPLEX FORMATION BY LICHEN COMPOUNDS 261 The amounts of Fe, Al, Ca, and Mg released when the control organic

acids were shaken with water suspensions of the silicate materials (Table 3) were usually a reciably greater than the amounts of the cations released when soli8Echen compounds were shaken with the same silicate

TABLE 2

Amounts of Fe, Al, Ca, and Mg released from silicate materials as a function of p H by an extractant prepared by adjusting the p H of sterilized

water

Mg

312 74 14

--

~~

Biotite

Fe

765 126 14

5 5 4 4 4 4

Fe A1 --

5 5 3 3 2 1 2 1

Fe A1 Ca Mg Fe A1 Ca M g Fe A1 Ca

1 Gramte 1 Basalt

Mg

77 182 78 175

146 I 4 4

.. .. 4 6 30 43 3 20 2 22 40 3 16 2 I 10 I 2 I 3

40 88

176 85

i 57 49

All in pg/g silicate material.

89 63 211

48 67 28

TABLE 3 Amounts of Fe, AI, Ca, and Mg released from water suspenswns of silicate materials by solid lichen compounds and control organic acids, corrected for

release of cations due to reactions directly involving hydrogen iotls

93 121 175 52 93 149 183 58 97 150 188 53 63 159 172 73 61 122 176 52 65 150 188 47

Compound


Citric Salicylic p-hydroxy

benzoic EDTA

27 94

49 1x3 40 37

89 123

62 56

37 66

Biotite I Grade I Bmalt

6x5

777 1100

129

784

270

90 73 75 52 94 34

93 90

100

52 90 63

102 3590 1880 170 1980 1010

133 398 421 192 1756 616

419 243 IOO 184

964 112 2010 153

3040 1150

240 2950

All in pg/g silicate material.

materials, a findin consistent with the higher water solubility of the

reater amounts of Ca were released by the control organic acids from

affnity of Ca ions to form artially dissociated com lexes in solution with anions of carboxylic aci s. With the exception o the data obtained for the release of Ca from biotite, citric acid released more Fe and Al

control organic aci f s. As was usually the case for the lichen compounds,

3480 to 7280 pg/g) than from basalt (1180 to 3150 pg/g) or 421 t0.1875 pg/g). Joseph (1946) has commented on the strong

a P

2890 4410

I550 1260

262 I. K. ISKANDAR AND J. K. SYERS than Ca and Mg from the three silicate materials; the order of release for both granite and basalt was Fe > A1 > Ca > Mg. There was no consistent order in the release of

Fe and A1 than did the other acids tion, p-hydroxylbenzoic acid

the greater distance between the

0.41zkb-- 0.2

400 450 500 550 600 650 700 Wavelength (nm).

FIG. 4. Spectrophotometric analyses in the visible range of the extracts obtained by shaking (A) biotite and (B) basalt with solutions of (b) salazinic and (c) stictic acids.

Curve (Aa) represents a solution of salazinic acid.

groups in p-hydroxybenzoic acid and the smaller size of the trivalent cations. For EDTA, the release of cations was essentially in the order Ca > Fe > Al > Mg.

Experiments with Solutions of Lichen Compozrmis The absorbance in the visible region of the extracts resulting from the

interaction of solutions of lichen compounds with the silicate materials was lower than that obtained for the reaction of solid lichen compounds with water suspensions of the silicate materials. Salazinic acid was adsorbed by biotite, as shown by the decrease in absorbance of the extract resulting from the reaction of a solution of salazinic acid with biotite (Fig. q b ) compared with that of a solution of salazinic acid alone (Fig. q a ) . Basalt (Fi .4sb) also adsorbed salazinic acid but to a lesser extent

occurred in the experiment with the solid compound, but once the adsorption capacity of biotite was saturated, more salazinic acid could be released into solution from the solid hase. The extent to which other

than biotite. A f sorption by biotite of dissolved salazinic acid probably

lichen compounds are adsorbed by t R e silicate phases is not known

METAL-COMPLEX FORMATION BY LICHEN COMPOUNDS 263 because their solutions do not absorb radiation in the visible region. The lower absorbance of the extract obtained from the reaction of a solution of stictic acid with biotite (Fig. q c ) , compared to that of the reaction of solid stictic acid with a water suspension of biotite (Fig. zAb), may suggest either a greater extent of metal-complex formation or less adsorption of the complex in the system where the solid lichen compound was used. Adsorption of the dissolved lichen compound or the complex formed by the reaction of the dissolved lichen compound with cations released from the silicate materials complicates the interpretation of the spectrophotometric analysis data.

TABLE 4 Amounts of Fe, Al, Ca, and M released from silicate materials by

solutions1 of compounds

I Biotite I Granite I Basalt


5 5 61 45 54 29 38

40 30 36 36 46 24

I 28 I 12

65 65 94 93

I 82 191 280

169 I 68 191

89 I 1 8 49 61 66 68

All in pg/g silicate material. I Solution cbtained by shaking 30 mg of lichen compound with 15 ml of sterilized water for 96 hours.

The pH values of the extracts obtained by shaking solutions of the lichen compounds with the silicate materials were equal to, or only slightly lower than, those obtained by shaking the silicate materials alone with water (data not presented). Consequently, on the basis of the data re orted in Table 2, a very low proportion of the total cation release by

gen ions, metal-complex formation accounting for virtually all of the observed cation release from the silicate materials. The data resented

formation. In general, similar amounts of Fe, Al, Ca, and Mg were released from

biotite, granite, and basalt by solutions of the lichen compounds (Table 4) and by solid lichen corn ounds added to water suspensions of the

trend, namely the lower amounts of Fe released from granite and basalt, the higher amounts of Mg released from biotite, and the lower amounts of calcium released from biotite by the solutions of lichen compounds. In each case, solutions of the lichen compounds released greater amounts of Ca than of Fe, Al, and Mg. A lower amount of Ca was released from biotite by a solution of lecanoric acid than by the solutions of evernic, salazinic, and stictic acids, a finding which contrasts with the data obtained with solid lichen compounds (Table 3). For thirteen of the eighteen cases (six acids and three silicate materials), the order of release was Ca > A1 > Mg > Fe. It could be argued that the higher pH of

so P utions of lichen compounds may be attributed to the effect of hydro-

in Table 4 represent the release of cations attributable to meta P -complex

same silicate materials (Tab P e 3). There were several exceptions to this

264 I. K. ISKANDAR AND J. K. SYERS the extracts obtained by shaking solutions of the lichen compounds with the silicate materials could result in more metal-com lex formation

consistent relationship was apparent to substantiate this hypothesis. because of the greater extent of deprotonation of pheno P ic groups. No

Discussion Although the lichen compounds investigated are only slightly soluble

in water (Iskandar and Syers, 1971), it is apparent that these compounds are sufficiently soluble to function as metal-complexing agents and to cause chemical weatherin of silicate materials. This has been demon-

Al, Ca, and Mg when solid lichen compounds or solutions of lichen corn ounds are shaken with water suspensions of biotite, granite, and basapt. The effect of complex formation on cation release was isolated from that of hydrogen ions by subtracting the cation release attributable to the effect of hydrogen ions from the total cation release. Schatz (1963) and Syers (1969) have suggested previously that the release of cations from silicates by lichen compounds is due largely to complex formation and not to reactions directly involving hydrogen ions. Control organic acids usually released considerably greater amounts of Fe, Al, Ca, and Mg from the silicate materials than the lichen compounds, a finding consistent with the higher water solubility of the former.

Lichen compounds frequently contain certain polar groups, such as -OH, -CHO, -COOH (Fi . I), which can function as electron donors.

solubility of the molecule (Iskandar and Syers, 1971). Occurrence of two such donor groups in ortho (adjacent positions, for example -OH

the complexing of cations. The importance of ortho -OH and -COOH groups in the decom osition of silicates by organic compounds has been emphasized by einzburg et al. (1963), cited by Kononova et al.

Depsides and depsidones, similar to five of the six lichen com ounds used in this study, are the most abundant lichen compounds [Smith, 1962), which may account for up to 8 per cent of the dry weight of the lichen (Smith, 1921). The release of usnic acid from Cludonia spp., obtained by spraying with water in the field (Malicki, 1965), suggests that lichen compounds may be released b the action of rainwater. A

soluble complexes with metals is consequently in close proximity to the rock surface on which the crustose lichen is growing.

As pointed out by Smith (1962 and Syers and Iskandar (1972), the

under field conditions. If such complexes form, then the fact that they are water soluble suggests that they can be removed from the site of weathering.

Acknowledgements Research su ported by the College of Agricultural and Life Sciences,

University of bisconsin, Madison, and by a grant from the Research

strated in the present stu f y by the results obtained for the release of Fe,

The number and nature o P the polar groups largely determine the water

and -COOH in evernic acid and -OH an d -CHO in atranorin, favours

( 1 964)-

reserve of slightly soluble compounds w & ch are capable of forming

formation of soluble metal comp 1' exes has not yet been demonstrated

METAL-COMPLEX FORMATION BY LICHEN COMPOUNDS 265 Committee of the Graduate School, University of Wisconsin. We are

rateful to Drs. C. F. Culberson, S. Huneck, and C. A. Wachtmeister for the lichen compounds.

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METAL-COMPLEX FORMATION BY LICHEN COMPOUNDS