- Email: [email protected]
The role of humic acids from Tasmanian podzolic soils in mineral degradation and metal mobilization
Cfeochimica et Cosmochimica Acta,1973, Vol.37,pp.269to281.PergamonPress. Printedin NorthernIreland
The role of humic acids from Tasmanian podzolic soils in mineral degradation and metal mobilization. W. E. BAKER Department of Mines, Hobart, Tasmania, (Received 1 May 1972; accepted in revised form
Australia
15 August 1972)
Abstract-Humic
acid extracted from a podzolic soil developed under Eucalyptus dekgatenais and Pteridiurn aquilinum in northwestern Tasmania exhibits very strong solvent activity towards a number of minerals and metals. Aqueous solutions (0.1 per cent w/v) of this acid acting for 24 hours on mineral grains ranging in size from 297 to 590 p, extracted varying amounts of metal. Chaloopyrite yielded 140 ,ug Cu whereas chalcocite released 15,000 pg Cu. Some correlation is found between relative bond strengths of sulphides and their degradation by humic acids. For example, galena is far less stable than sphalerite. Haematite and pyrolusite are quite vulnerable to humic acid attack and this may be a factor in the lack of development of extensive gossans over mineralization in western Tasmania during the current erosional cycle. Metals are particularly strongly attacked with a maximum release of 291,000 ,ug Pb in 24 hours. Contrary to earlier Endings, Ag and Au were found to release 400 ,ug and 20 ,ug respectively of metal in a period of 6 weeks. Humic acid extracted from soils below other vegetation types in northwestern and western Tasmania are all active in mineral degradation. The variable effect of the acids is possibly the result of overall differences in complexing sites active under the conditions of experimentation Several examples of minor soil organic compounds show no greater and selective complexetion. activity than humic acids and on the basis of their very low content in the soils studied, they are Many metal humates display low soluconsidered inferior to the latter as agents of weathering. bility in water, but they are readily mobilized in the presence of humic acids. Humic acids developed under varying vegetation in a cool temperate climate are potentially very powerful metal humates suggests solvents in the weathering cycle. Their ability to mobilize precipitated that classical concepts of relative metal mobility may need modification in environments where appreciable concentrations of these substances are found.
INTRODUCTION THE POSSIFXLITY of reaction
between humic acid and minerals was considered early in the history of soil science. SPREN~EL (1826) and THENARD (1870) suggested that humic acids attacked silicates whilst J~LIEN (1879) proposed an almost universal solvent role for them in many geological processes. CLARKE (1911) dismissed the early statements through lack of supporting experimental evidence. More recently, GUILLIN (1928), GRAHAM (1941), MAREL (1949), KONONOVA et al. (1964) and PONOMAREVA and RA~IM-ZADE (1969) have presented data indicating reaction between silicates and humic acids, whereas BLANCK (1933), KRAUSKOPF (1967) and LOTJ~HNAN (1969) question the effectiveness of these acids in weathering. The action of humic acids on the minerals of economic deposits has not received the attention given to silicates. GRUNER (1922) demonstrated that water drawn through a bed of peat attacked silicates, iron oxides and siderite but had little effect on pyrite. FREISE (1931) published data that indicated considerable solubilisation of gold by humic acids. FETZER (1934) and ONG and SWANSON (1969) found no evidence for reaction between gold and humic acids. As a result of studies with a variety of minerals FETZER (1946) stated that humic acids were ineffectual solvents. The effectiveness of many naturally occurring organic substances in maintaining the solubility of relatively insoluble inorganic compounds has been demonstrated by 269
270
W.
E.
BAKER
DUFF et al. (1963), EVANS (1964) and others. SCH~TZ et al. (1954) and SCRALSCHA et al. (1967) have suggested that simple organic products of living and decaying vegetation may cause weathering by complexation. GINZBURCJet al. (1963) and HTJANGand KELLER (1970) have studied the decomposition of a number of silicates by simple organic acids known to occur in soils. However, the organic substances used in the former studies are found in only minor amounts in the biosphere. As the bulk of the organic reserves available to the weathering solutions occur as humic substances it is desirable that the role of this group be more critically evaluated. For this study, five soils of podzolic affinities developed over quartzose bedrock and supporting different types of vegetation were sampled from various localities in northwestern and western Tasmania. The solvent activity of the humic acids extracted from these soils (referred to in text as HA 1 to HA 5) towards a number of minerals and metals was compared with the activity of deionised water equilibrated with atmospheric carbon dioxide. Further comparison was made with the activity of a number of organic compounds which are minor constituents of soils. A limited investigation of the mobility of some metal humates in the presence of humic acid was also made. Since the writer’s interest is largely in exploration geochemistry, the selection of minerals is biased towards those which occur in economic deposits. EXPERIMENTAL TECHNIQUES Preparation
of humic acids and mineral samples
The organic matter was extracted from the minus 297 p fraction of the soil samples by intermittent shaking for 24 hours with 0.1 M Na4P,0,-NeOH under an atmosphere of nitrogen. The extract and several washings of the soil residue were treeted with ion exchange resins to remove the excess reagents (HORI and OEUDA, 1961). Fine suspended solids were removed by use of a super centrifuge. Humic acids were precipitated from the soil extract by acidification to pH = 2 with H,SO, (KONONOVA, 1966) and the flocculated acid collected with the super centrifuge. The humic acids were taken into solution in 0.1 M N&OH, purified by treatment with ion exchange resin and dried at 40°C under vacuum. The organic content of all soils used in this study was low ( <4 per cent) and 80 to 90 per cent of the extracted organic matter was recovered as humic acid. The ash content of the humic acid samples varied from 3 to 5 per cent. Sample location data, composition and vegetation types under which the humic acids developed are given in Table 1 and the trace element content in Table 2. In previous studies, such as those of FETZER (1946) and SCHALSCELA el al. (1967), finely pulverised (minus 74 ,u) minerals or rocks together with solution strengths in the range of 1 to 6 per cent (w/v) were used. For the present series of experiments, an effort was made to reduce the surface effect by using cosrser grained (297-590 ,u) mineral samples. The weight taken in grams was equivalent to the specific gravity of the mineral concerned and the strength of the aqueous solutions of humic acids was reduced to 0.1 per cent (w/v). Metal hum&es were prepared by the addition of 100 ml of 0.1 M solutions of the respective metals to 100 ml volumes of a 0.5 per cent solution of HA 1. The hum&es were washed by centrifuging with deionised water and then dialysed for 3 days. Humic acid-mineral
interaction 2yrocedures
The movement of near surface waters through fractured material was simulated by use of a vacuum operated perfusion unit (Fig. 1). This unit was modified after the design of AUDUS (1946). The use of flexible connections allows the flow rate to be adjusted. For the present study the flow was set at 25 l/day. The selected mineral was contained in a glass tube sealed at one end with nylon sifting cloth. This sample holder was suspended in the reservoir of the perfusion
The role of humic acids from Tssmanisn podzolic soils Table 1. Location and compositions
271
d&s, for humic acids pH of 0.1% Composition* H N 0
C :H
sc!%Jn
59.9
2.9
1.9
34.1
20.3
3.4
59.5
2.9
2.6
35.0
205
3-o
i~~s~ls~a e~e~~ol~~
55.0
4.3
36
36.8
12.8
3.2
~~~~sc~sn~ sphaerocephalzcs
55-5
4.2
36
36.6
13.2
3.0
-
-
Sample
Location
Vegetation type
C
HA 1
Oliver Hill 814000 m N 380000 m E Oliver Hill 815000 m N 380000 m E Lower Be&ah 815000 m N 400000 m E Savage River 815000 m N 302000 m E Kimberlay 820000 m N 400000 m E
Eucalyptus delegatensis and Pteridium aquilinuwa
HA 2
HA 3
HA 4
HA 5
Sphagnum
sp.
Pinus radiata
-
-
-
3.3
* Ash-free data based upon analyses by C.S.I.R.O. Microanalytical Service. Table 2. Trace element oontent of humic acids. (,ag/ml for 0.1 per cent w/v aqueous solutions)
cs Fe
Mg Ne K Al Si Mn, Ni, Co * Cu, Zn, Pb Us, Sb, Bi
HA1
Humic acid sample HA3 HA2
0.3 4.0 0.1 4-o 0.2 4.0 t5
0.9 1.8 0.1 7.8 0.3 2.0 <5
to.2
<0.2
HA4
HA5
1.2 4.7 0.1 9.5 0.2 4.0 <5
1.2 1.2 0.1 5.0 0.2 <1-O <5
0.7 26.4 0.3 4.8 0.4 14.0 <5
<0.2
<0.2
<0.2
* Range for these elements tO*Ol-0.18. unit so that the mineral wss immersed in the solvent under study. Circulation of the solvent from the reservoir through external tubing to the top of the sample holder was maintained by the spplicstion of a slight suction to the unit. In the initial study a large number of sulphides, oxides, gangue and secondary minerals and metals were exposed to the action of deionised water equilibrated with atmospheric CO, and a 0.1 per cent solution of HA 1. The standard running time wss 24 hours, although differences in the reectivity of the minerals required some variation iu the duration of experiments. From the results of this study, 10 minerals snd 2 metals were selected for use in comparative extraction studies. These samples were extracted with O-1per cent solutions of HA 2, HA 3, HA 4, HA 5, oxalic acid, salicylie acid, pyrogalloX and &nine. A few leeching experiments were carried out with precipitated sulphides and pulverised minerals. The samples were placed on a wad of macerated filter paper contamed in a buchner
272
W. E. BAKER
0.5 mmbore
capillary
tubing
2mm
bore capillary tubing
Fig 1. Perfusion apparatus. funnel connected to a receiver under slight suction. Circulation of 50 ml volumes of water and of 0.1 per cent solutions of HA 1 was maintained by manual transfer from receiver to funnel for a period of half an hour. Leaching of metal humates preparedfrom HA 1 was studied by a similar procedure. The humateswerespreadover a tied area of retentive flter paper which was placed on the macerated filter paper wad. Ten 50 ml volumes of water were drawn through each metal humate sample and the procedurethen repeated using a 0.1 per cent solution of HA 1. A limited study WBBmade of the stability of silicate minerals. Feldspar, biotite, enstatite. actinolite, and epidote were exposed to deionised water equilibrated with atmospheric CO, and 0.1 per cent solutions of HA 1 in the perfusion apparatus for a period of 6 days. Grain size and sample weights were the same aa for perfusion experiments with the other minerals. The reaction solutions together with deionised water washings of the mineral beds were stabilised by the addition of 20 ml of an ammoniacal solution of 6 per cent EDTA (pH = 9) All analytical data were acquired by the use of atomic absorption spectrophotometry and have been corrected for the trace element content of the solvents used.
RESULTSAND DISCUSSION The results of analysis of the water and O-1 per cent HA 1 solution experiments with a variety of minerals and metals are given in Table 3. The extraction values range from 120 ,ug/day Zn from sphalerite to 291,000 ,ug/day Pb from the pure metal. This humic acid preparation is a strong solvent of many minerals, as in all cases studied, the amount of metal extracted is far in excess of that extracted by water. These results are contrary to the findings of FETZER(1946) and the magnitude of variation can be seen from a comparison of the results for CuS (covellite). If the figures given by FETZER(1946) are reduced to a daily rate of solution of metal then distilled water and 1 per cent humic acid extracted 230 ,ug and 150 ,ugCu, respectively. In this study, the extraction values for deionised water and O-1 per cent humic acid were 20 ,ug and 4400 ,ug Cu, respectively. Detailed comparative study of the interaction between minerals and humic acids is made difficult by a number of factors. There is considerable uncertainty in
The role of humic acids from Tasmanian podzolic soils
273
Table 3. Reaction of water and HA 1 with minerals and metals Element determined
Sample Sulphides: Galena Sphalerite Chalcopyrite Bornite Covellite Chalcocite Pyrite Arsenopyrite Loellingite Bismuthinite Stibnite Heazlewoodite Pararammelsbergite Breithauptite Oxides : Haematite Pyrolusite Gangue Minerals: Calcite Fluorite Barite Secondary Minerals: Malachite Smithsonite Anglesite Cerussite Pyromorphite Crocoite Annabergite Metals: Iron Lead Copper Zinc
PbS ZnS CuFeSs Cu,FeS,
cus CL@ Fess FeAsS FeAsS, B&S, SbsSs N&S, N&3, NiSb
Pb Zn cu Fe cu Fe cu cu Fe Fe Fe Bi Sb Ni Ni Ni Fe MI3
CaCO, CaF, BaSO,
C~,P%C%
ZnCO, PbSO; PbCO, Pb,(PG,),Cl PbCrO, (Ni, Co),(As0,),8H,O
,ag metal extracted in 24 hrt H,O/atmos CO, 0.1% HA 1 30 20
3000 120 140 80 1800 800 4400 15000* 240 2700 3200 3800 750 43000 * 74000 * 59000 * 340 2100
1200 50
93000 *
Zn Pb Pb Pb Pb Ni
10 50 (10 (10
104000* 70000* 96000 * 218000* 4600 8200 5800
Fe Pb cu Zn
1200 50 10 160
133000* 291000* 46000* 37000*
Ca Ca
Be
cu
600 500
* Estimated from 2 hour run. 7 100 ml O-1% (w/v) HA 1 acting on weight in g equal to specific gravities of minerals of grain size 247 to 690 p.
procedures
for the calculation of bonding energies in many minerals (BACHINSKI, The humic acids are heteropolycondensates with structurally indeterminate arrays of reactive groups which may result in selective complexing properties. The relative stability of metal complexes formed by the action of humic acids would have a bearing on the competition of metals for various complexing sites. The literature data for stability constants of such complexes KHANNA and STEVENSON, 1962 ; SCHNIYZER and SKINNER, 1967) are not consistent and it is possible that both 1969;
NICKEL, 1969).
274
W. E. BAKER
the magnitude and relative order of values would vary for differing soil extracts. In the natural environment, the wide variability in crystallization of minerals results in the development of surfaces of differing character and hence reactivity towards solvents. Despite these difficulties, some correlation is evident between the extraction of metals from sulphides (Table 3) and relative bond strengths as given by BACHINSKI (1969) The low stability of galena compared with sphalerite and chalcopyrite is in accordance with the relative bond strengths of these minerals. Similarly the decreasing stability of chalcopyrite-bornite-covellite-chalcocite is as predicted by BACHINSKI (1969). However, contrary to the order of bond strengths is the unusually high solubility of the nickel minerals and of bismuthinite relative to stibnite. The reason for the differing behaviour of the latter two very similar minerals is not clear. The fact that Sb attains a pentavalent state more readily than Bi may Differing behaviour of the two metals towards sorption by possibly be a factor. humic substances was noted by SZALAY (1964) who classed Bi as huminophile and Sb as huminophobic. The stability of pyrite compared with that of arsenopyrite and loellingite is compatible with the structures of these minerals from the viewpoint of ligand field theory as discussed by NICKEL (1968). The solubility of haematite and pyrolusite may have some bearing on the fact that very few gossans have developed over mineral deposits of western Tasmania during the present erosional cycle. In many cases sulphides appear at or near the surface with practically no development of secondary minerals. The relatively high solubility of pyrolusite compared with haematite suggests that an alternative mechanism to pH and Eh might be available for the separation of Fe and Mn during the weathering of gossans. A study of the Fe and Mn content of Tertiary (?) gossan and associated soil in the area sampled for the production of HA 1 revealed that the Fe:Mn ratio increased markedly in the soil. Several of the gossan samples were extracted for 24 hours in the perfusion apparatus with water adjusted to pH = 3 and with a 0.1 per cent solution of HA 1. Whereas the extraction of Fe was <25 ,ug in all cases, the extraction of Mn was far higher and the humic acid was about twice as effective as water at pH = 3 (Table 4). Many of the gangue and secondary minerals are strongly attacked by HA 1. Ca is rapidly removed from calcite whilst the less soluble fluorite and barite are also affected. Calcite was also subjected to the action of water under an atmosphere which was 25 per cent (v/v) CO,. This resulted in the release of 2700 pg Ca in 24 hours which is still well below the value achieved in the same period by the humic acid. It thus seems likely that limestone and dolomite exposed to the action of humic acids would be subjected to particularly devastating erosion. There is marked contrast in the solubilities of the secondary Pb minerals; cerussite and anglesite show far less stability than pyromorphite and crocoite. This may be a factor in the rarity of occurrence of the former two minerals in the near-surface development of lead mines in the Zeehan and Dundas areas of western Tasmania. The action of HA 1 on metals is notably aggressive. In view of the reducing properties of humic acids, ONE and SWANSON(1969) queried the possibility that these substances could complex Au since the metal would first have to be oxidised. The data of Table 3 indicate that, for other metals, oxidation must have proceeded quite rapidly despite the presence of humic acid. Possibly humic acids are far better
The role of humic acids from Tasmanian Table 4. Analytical
data for gossan-soil pairs from Oliver Hill, NW and laboratory extraction of Mn
G.l s.l G.2 5.2 G.3 5.3 G.4 5.4 G.5 s.5 G.6 S.6 G.7 s.7 G.8 S.8 G.9 s.9 G.10 s.10
9.96 5.86 16.71 6.92 17.00 6.56 40.80 7.97 38.69 539 29.01 4.81 19.64 4.16 20.63 2.93 44.49 12.07 20.14 15.24
275 Tasmania
Laboratory extraction of Mn from gossan H,O at pH = 3 0.1% HA 1
Mn Sample No.
podzolic soils
(%I
Fe:Mn
30.05 0.45 17.94 1.42 20.63 0.21 6.73 0.33 3.47 0.24 12.56 0.15 21.08 0.21 13.90 0.13 1.29 0.13 11.66 0.21
0.3 13.0 0.9 4.6 0.8 31.2 6.1 24.2 11.2 22.5 32.3 32.1 0.9
&g/24 hr) 620
1290
1300
2160
-
-
570
910
-
-
1420 -
2380 -
19.8 1.5
-
22.5 34.5
-
-
290
740
92.9 1.7 17.2
Normal oxidation of the metals in an complexing agents than they are reductors. aqueous environment could be expected to accelerate in the presence of a complexing agent which removed ions from the metal surface as they were formed. The results obtained with other metals encouraged an examination of the behaviour of Ag and Au. As only small quantities of coarse-grained samples of these metals were available, weights equal to l/lOth the respective speci& gravities were placed in flasks with 50 ml volumes of 0.5 per cent solutions of HA 1 and these were shaken intermittently during 6 weeks. Analysis at the end of this period revealed that 400 pugof Ag and 20 ,ug of Au had been taken into solution. This is contrary to the findings of FETZER (1934, 1946), and 0~a and SWANSON (1969). The results of reaction of the finely divided (minus 74 ,u) samples with water and HA I, over a period of half an hour are given in Table 5. These are seen to be generally consistent with the data of Table 3. PbS, CuS and NiS display high solubility relative to ZnS. The solubility of Bi,S, is far in excess of Sb,S, and the same situation is found for the pulverised bismuthinite and stibnite. Compared to the results for the 297-590 ,u mineral samples (Table 3), extraction of Fe from haematite has increased relative to Mn from pyrolusite. This is probably due to the fact that whereas the surfaces of the coarser grains of haematite are parts of single crystals, for pyrolusite they consist of an aggregate of finer grains. Thus the effect of grain size reduction is more marked for the haematite.
W. E. BAKER
276
Table 6. Action of water and HA.1 on precipitated sulphides and pulverized minerals Element determined
Sample Lead sulphide Zinc sulphide Cupric sulphide Nickel sulphide Bismuth sulphide Antimony sulphide Bismuthinite Stibnite Haematite Pyrolusite
pug metal extracted in + hr 0.1% HA 1 H,O/atmos CO,
Pb Zn
<3 75
CU
Ni Bi Sb Bi Sb Fe Mn
425 <5 <5 <5 t5
2100 95 530 1440 70 <5 435 25 340 310
Table 6 gives details of the action of the humic acids from under different vegetation cover during 1 hour perfusion runs on a selection of samples. These results show that whilst all the humic acids are active in mineral degradation there is considerable variation in the extent of reaction. The variation shows no consistent pattern and it is likely that some specificity is operating in addition to changes in the total content of reactive groups. This feature is apparent in the low activity of HA 4 with haematite whilst it is the most aggressive with calcite. The high solubihty of pyrolusite relative to haematite is not uuiversal, since of the acids studied to date, only HA 1,HA 2 and HA 4 show this tendency. The reaction of all acids with bismut~i~ is greater than with stibnite, although HA 2 has an appreciable effect on the latter mineral. Table 6. Action of various humic acids with minerals and metals
Sample Galena Sphderite Born&e Ch&ocite Bismuthinite Stibnite P~rar&~e~ber~~ Hsematite Pyrolusite Calcite Copper Lead
Element determined Pb Zn cu
Water atmos CO, 1
CU
10
Bi Sb Ni Fe Mn Ca cu Pb
HA l* 200 30 190 3800 550 46 9800 470 1000 10500 5700 27400
,ug metal extracted in 1 hour HA 2 HA 3 HA 4 210 90 230 5100 1600 340 9600 90 1800 9600 5800 39400
80 80 180 1300 410 10 5400 410 290 6400 3200 23000
230 40 200 5600 1000 50 9200 <5 1040 11100 6100 33000
HA5 110 30 130 2700 460 <5 6000 490 440 730 4800 24000
* All extractants 0*1% w/v.
most of the earlier studies which dispute the activity of humic acids (HARRAB, and LOVE, 1929; FETZER, 1946),it is considered that the low molecular weight acids and other minor organic substances are the effective agents of mineral degradation. KRAUSKOPF (1967) questions the role of humic acids in geological processes and suggests that the acidity of natural solutions is more likely to be In
1929; MURRAY
The role of humic a&Is from Tasmaman pods&c soils
277
due to the simple organic acids. In the past, the acidic properties of humic substances have been emphasized, and the appeal made by Scn,Arz et aE. (1954 to consider humic substances as complexing agents does not appear to have penetrated the geological literature to any great extent. Since the low molecular weight organic oompounds constitute only a minor proportion of the soil organic matter, they would need to be extremely active to have a potential in the weathering processes comparable with that of the humic substances. The activity of several of the simple organic compounds found in soils is compared with that of HA 1 in Table 7. These results show that the simple compounds exhibit an activity of the same order as that of the humic acids and hence, because of the low concentrations, their effectiveness in weathering processes would be limited. Table 7. Action of various organic compounds with minerals and metals
Sample Galena Sphalerite Born&e Chalcocite Bismuthinit,e Stibnite Par~a~e~sbergi~ Haematite Pyrolusite Calcite Copper Lead
Element determined
HA 1*
Pb Zn cu CU Bi Sb Ni Fe Mu Ca cu Pb
200 30 190 3800 550 45 9800 470 1000 10500 6700 27400
pg metal extracted in 1 hour Oxalic Salioylio acid Pyrogallol Alanine acid 130 30 260 4450 180 <5 10500 <3 4200 11900 5500 41800
95 20 650 9750 4820 580 7620 80 15500 980 2620 660
35 8 55 920 1640 <6 2380 20 5150 2040 1190 1470
<5 20 15 1530 55 <5 1730 20 520 1400 700 240
* All extra&ants O-1% w/v.
The results of the reaction of HA 1 with the silicate minerals are given in Table 8. Whilst these silicates are more resistant to attack than the other minerals studied, the amount of metal removed by the humic acid is impressive. A direct comparison of the results with earlier work is not possibIe, but the high extraction of Na, K, Mg, Ca and Fe relative to Si and Al supports the claim of ScE~sa~ et aE. (1967) that the metals are removed from silicate frameworks by ~mplexation. The results obtained by PONOXARETA and RAGIM-ZADE (1969) for 6ne grained samples of feldspar and biotite showed that removal of K, Na, Fe and Mg by fulvic and humic acids exceeded Si and Al, although considerable amounts of the latter elements were found in the leach solutions. l?or the present series of experiments, with coarser grained samples under perfusing solutions, far higher relative extraction of metals other than Si and Al has been found, In view of the effect of humic acid on silicates, it is pertinent to consider the effect they might have on structural materials such as portland cement. A crushed sample of this material subjected to a 24 hour perfusion run with HA 1 released 5300 ,ug Ca. Taking this result in conjunction with the data for metals, it is possible that such ma~rials as reinforced concrete piping would have a
278
W. E. BAKER
comparatively short life if bnried in an env~onment where tihese substances are active. The activity of humic acids with respect to metals could also offer a supplementary mechanism to bacterial corrosion for their destruction. Table 8. Reaction of water and HA 1 with silicates Mineral Feldspar (perthite) (Na, K)AlSi,O,
Biotite WMg. Fe), WSk&,) fO%
Element determined
pg metal extmcted in 5 days H,O/atmos CO, 0.1% HA 1
N-a K Al Si
340 190
1260 640 220 t5
K
60 20 <5
100 90 240 300 6
xg Fe Al Si
Enstatite (Mg, Fe”)SiO,
Mg FO Si
<5 <5
620 220 <5
Actinolite CaWg, Fe”& (Si*O~~)(OH, Ffa
his F% Ca Si
25 <6 760 <5
960 2900 40
Epidote Ca F”Al,O.OH (Si&%)WOJ
Ct.3 F% Al Si
1800 10 40 <6
10700 1300 320 5
80
80
The metal contents and leaching characteristics of 10 humates are shown in Big. 2. The quantity of metal in the humates varies widely and this feature is being studied further. It has been found that a Ni humate prepared by the action of HA 1 on heazlewoodite contained 12.7 per cent Ni compared with 3.5 per cent Ni for the humate prepared by mixing solutions of HA 1 and a Ni salt. It is possible that in the presence of the strong salt solution, colloidal properties of the humic acid result in coagulation before many functional groups react with Ni ions. Whilst most of the metal humates display their classical ‘insolubility’ in water, they are readily mobilized by a O-1 per cent solution of HA 1. Where appreciable metal was mobilized by water (Cu, Ni, Mg, Ca), the solution was coloured by the humate, indicating that it was the metal complex that was moving. The base metals are all highly mobile in the presence of humic acid and this could affect their dispersion patterns. The data also has a bearing on geomedical studies such as those of WARREN et al. (1967)) who found a higher than average incidence of multiple sclerosis in regions where Pb in particular was highly mobile. The leaching characteristics of the Fe and MYnhumates are very similar and thus the preferential mobilization of m found for the gossan at Oliver Hill is limited to the mineral de~adation phase of the hnmic acid activity.
The role of humic acids from Tasmanian podzolic soils Pb
Zn
600
7.7
b i‘ L I, I_L 2300
(5
279
10
3200
_---
lmr
0
F&II)
~33
250
(5
0
670
$’
(5
660
--
0
1000
0
1000
,N;f
600
i: 530
:
\\,__<
0
1000
KEY
3
100
--_--
0
1000
TO DIAGRAMS
%Metolin
L
0
humate
~~~Extrocted
yg.fxtracted
750
1000
0
Mls.leoch solution
1000
Fig 2. Leaching characteristics of metal humates.
The reducing properties of soil organic matter are held to effect the enrichment of Al in some bauxite formation by selective removal of Fe (HARDEN and BATESON, 1963). The leaching experiments indicate that the Al humate has a far lower mobility than that of Fe and this could also possibly be a factor in some enrichments of Al. The marked difference in mobility of the Ni and Co humates could also lead to differing behaviour of these elements in some surficial processes. 7
280
W. E. BAKER
CONCLUDINGREMARKS The data presented in this paper establishes that humic acids form the bulk of the organic matter present in many Tasmanian soils of podzolic affinities. These acids are powerful solvents of a number of minerals and could be very active in the weathering cycle. It is possible that they have maximum effect under a cool temperate, high rainfall climate such as has existed in western Tasmania since the Pleistocene glacial epoch. The rapid solution of minerals associated with economic deposits by humic acids and the mobility of the metal humates formed could explain the rarity of gossans and appearance of sulphides at the surface in western Tasmania. In cases where the rate of leaching exceeds the physical lowering of the surface, secondary dispersion patterns may be weakened or obliterated and geochemical prospecting would need to be based upon the search for primary halos in the country rock. Acknowkdgmenta-This study is part of a joint Department of Mines-University of Tasmanie investigation and the writer is indebted to Mr. J. SYMONS,Director of Mines and Professor S. W. CAREYfor their support. Dr. J. WILMS~~~RST kindly arranged for the analyses of the humic acids whilst Dr. E. WILLIA~~~and Dr. J. VAN MOORTaffordedthe author much useful discussion. I also wish to thank Mr. C. WEBB for laboratory assistance, Mrs. B. WALTERSfor typing the manuscript and Mr. H. MACKINNON for drafting the diagrams. REFERENCES AUDUSL. J. (1940) A new soil perfusion apparatus. Nature 158,419. BA~HINSKID. J. (1969) Bond strength and sulfur isotropic fractionation in coexisting sulfides. Econ. Beol. 64, M-65. BLANCKE. (1933) Die sogennante humussiiureverwitteringim lichte neuester bodenforschung. Ernlihr. Pjl. 29,41-43. CLARKE F. W. (1911) The data of geochemistry, 2nd ed. U.S. Geol. Susv. Bull. 491. DUFF R. B., WEBLEY D. M. and SCOTTR. L. (1963) Solubilization of minerals and related materials by 2.ketogluconic acid-producing baoteria. Soil. Sci. 95, 104114. EVANSW. D. (1964) The organic solubilizationof mineralsin sediments. In Advo~ceein Organic Cteochemistry (editors Colombo U. and Hobson G. D.), pp. 263-270. Monograph 15. Earth Science Series. Pergamon. FETTERW. G. (1934) Transportation of gold by organic solutions. Econ. Geol. 29, 599-604. FETZERW. G. (1946) Humia acids and true organic acids as solvents of minerals. Econ. Gwl. 41, 47-56. FREISEF. W. (1931) The transportation of gold by organic undergroundsolutions. Econ. Gwl. 26, 421431. GINZBUR~I. I., YASHINA R. S., MAT~EEVAL. A., BELYATSKIIV. V. and NUZHDELOVSKAYA T. S. (1963) Decomposition of certain minerals by organic acids. In Chemistry of the Earth’8 Crust (editorVinogradov A. P.) Vol. 1, pp. 304-320. Israel Program for ScientiilcTranslation, Jerusalem, 1966. GRAHAME. R. (1941) Colloidal organic acids aa factors in weathering of anorthite. SoiESci. 52, 291-295. GRUNERJ. W. (1922) The origin of sedimentary iron formations: the Biwabik Formation of the Meaabi Range. Econ. Beol. 17, 407-460. GUILLINR. (1928) Dissociationintigrale dee silcates par l’acide carboniqueet les acideshumiquea et reactions annexes. Compt. Rend. 187, 673-675. HARDENG. and BATJ~SON J. H. (1963) A geochemicalapproach to the problem of bauxite genesis in British Guiana. Econ. Geol. 52, 1301-1308. HARRARN. J. (1929) Solvent effects of certain organic acids upon oxides of iron. Econ. Bwl. 24, 50-61.
The role of humic acids from Tasmenien podzolic soils
281
HORI and OKUI)AA. (1961) Purification of humic acid by the use of ion exchange resin. Sodb Sci & P&pat N&r. Tokyo, 7,4. HUAN~ W. H. and KE~~EB W. D. (1970) Dissolution of rock-forming s&ate mineralsin organio acids: simulated first-stage weatheringof fresh mineralsurfaces. Am. Mineral. 5&2076-2094. JULIENA. A. (1879) On the geological action of the humus acids. Proc. Am. Assoc. ,4dv Soi. 28, 311-410. KEANNA S. S. and STEVENSON F. J. (1962). Metallo-orpic complexes in soil: 1. Potentiometric titration of some soil organio matter isolates in the presence of transition metals. Soil: Sci. B&298-305. KONONOVAM. M. (1966) Soil OrgunicMutter. 2nd English edition. Pergamon. KONONOVAM. M., A.L~~ANDROVA I. V. end TITOVAN. A. (1964) Decomposition of silicates by soil organic matter. Pochv~~~~~e 10,I-12. English translation in Rusiarr Soil Sci. 1964, 1005-1014.
KYRAUSKOPF K. B. (1967) ~~t~~~~~ to ~wc~~~~~. ~cGr&w-~11. LOU~HNANF. C. (1969) C&r&uz~ ~~~~~ of the Silicate Minerals. Eisevier. MAnEL H. VAN DER (1949) Minerelogicol composition of a heath podzol profile. So4 Sci. 67, 193-207. MURKY A. N. and Love W. W. (1929) Action of organic acids upon limestone. Bull. Amer. Assoc. Petrol. Gwl. 13,1467-1475. NICKIXLE. H. (1968) Structuml stability of minerals with the pyrite, marcasite, arsenopyrito and ldllingite structures. Can. Mineral. 9, 311-321. NICKEL E. H. (1969) Bond strength and sulfur isotopio fra&ionation in coexisting sulfides. Ecorr. Geol. 84, 934-935. ONG H. L. and SWANSONV. E. (1969) Nature1 organic mids in the transportation, deposition end concentration of gold. Q. Golo. Sch. i@imes 04, 395-425. PONOXA~EVAV. V. and RAGIIM-ZADE A. I. (1969) Comparative study of fulvic and humic acids English translation as the agents of silicate mineral decomposition. P~hvov~e~~~e 8,26-36. in ~~8~~~ Soil Sei. 1969, 157-166. SCHALSCHA E. B., APP~LT H. and SCEATZA. (1967) Chelation ss a weathering mechanism. l-Effect of complexing agents on the solubilizstion of iron from minerals and grsnodiorite. Uwchim. cosmochim. Acta 31, 587-596. SCFIATZ A., CHERONXS N. D., SCHULTZ V. and TREWLAWNEYG. S. (1954) Chelation (sequestration) as a biological weathering factor in pedogenesis. Pean. Acud. Sci. 28, 4P51. SCHNITZER M. and SKINNERS. I. M. 1967 Orgeno-metallic interactions in soils: 7. Stability oonstants of Pb2f, X2+, Mn*, Cow, Gas+ and Mgw-fulvic acid complexes. Soil Sci. 103, 247-252.
SPREN~ELC. (1826) Uber pflanzenhumus, humussaure und humussaure s&e. Kastlzers Arch @es. ,VaturZehre. 8, 145-220. SZALAYA. (1964) Cation exchange properties of humio acids and their importance in the geoehemicLa1 enrichment of UOs* and other cations. &ochim. ~o~eh~m. Acta 28,1605-1614. THENA~DP. (1870f Observations sur le Memoire de M. Friedel. Conapt.Bad. 70, 1412-1414. WA~LREN H. V., DEVAULTR. E. end CROSSC. H. (196’7) Possible correlationsbetween geology and some disease patterns. Ann. N.Y. Acud. Sci. 188, 657-710.
The role of humic acids from Tasmanian podzolic soils in mineral degradation and metal mobilization. W. E. BAKER Department of Mines, Hobart, Tasmania, (Received 1 May 1972; accepted in revised form
Australia
15 August 1972)
Abstract-Humic
acid extracted from a podzolic soil developed under Eucalyptus dekgatenais and Pteridiurn aquilinum in northwestern Tasmania exhibits very strong solvent activity towards a number of minerals and metals. Aqueous solutions (0.1 per cent w/v) of this acid acting for 24 hours on mineral grains ranging in size from 297 to 590 p, extracted varying amounts of metal. Chaloopyrite yielded 140 ,ug Cu whereas chalcocite released 15,000 pg Cu. Some correlation is found between relative bond strengths of sulphides and their degradation by humic acids. For example, galena is far less stable than sphalerite. Haematite and pyrolusite are quite vulnerable to humic acid attack and this may be a factor in the lack of development of extensive gossans over mineralization in western Tasmania during the current erosional cycle. Metals are particularly strongly attacked with a maximum release of 291,000 ,ug Pb in 24 hours. Contrary to earlier Endings, Ag and Au were found to release 400 ,ug and 20 ,ug respectively of metal in a period of 6 weeks. Humic acid extracted from soils below other vegetation types in northwestern and western Tasmania are all active in mineral degradation. The variable effect of the acids is possibly the result of overall differences in complexing sites active under the conditions of experimentation Several examples of minor soil organic compounds show no greater and selective complexetion. activity than humic acids and on the basis of their very low content in the soils studied, they are Many metal humates display low soluconsidered inferior to the latter as agents of weathering. bility in water, but they are readily mobilized in the presence of humic acids. Humic acids developed under varying vegetation in a cool temperate climate are potentially very powerful metal humates suggests solvents in the weathering cycle. Their ability to mobilize precipitated that classical concepts of relative metal mobility may need modification in environments where appreciable concentrations of these substances are found.
INTRODUCTION THE POSSIFXLITY of reaction
between humic acid and minerals was considered early in the history of soil science. SPREN~EL (1826) and THENARD (1870) suggested that humic acids attacked silicates whilst J~LIEN (1879) proposed an almost universal solvent role for them in many geological processes. CLARKE (1911) dismissed the early statements through lack of supporting experimental evidence. More recently, GUILLIN (1928), GRAHAM (1941), MAREL (1949), KONONOVA et al. (1964) and PONOMAREVA and RA~IM-ZADE (1969) have presented data indicating reaction between silicates and humic acids, whereas BLANCK (1933), KRAUSKOPF (1967) and LOTJ~HNAN (1969) question the effectiveness of these acids in weathering. The action of humic acids on the minerals of economic deposits has not received the attention given to silicates. GRUNER (1922) demonstrated that water drawn through a bed of peat attacked silicates, iron oxides and siderite but had little effect on pyrite. FREISE (1931) published data that indicated considerable solubilisation of gold by humic acids. FETZER (1934) and ONG and SWANSON (1969) found no evidence for reaction between gold and humic acids. As a result of studies with a variety of minerals FETZER (1946) stated that humic acids were ineffectual solvents. The effectiveness of many naturally occurring organic substances in maintaining the solubility of relatively insoluble inorganic compounds has been demonstrated by 269
270
W.
E.
BAKER
DUFF et al. (1963), EVANS (1964) and others. SCH~TZ et al. (1954) and SCRALSCHA et al. (1967) have suggested that simple organic products of living and decaying vegetation may cause weathering by complexation. GINZBURCJet al. (1963) and HTJANGand KELLER (1970) have studied the decomposition of a number of silicates by simple organic acids known to occur in soils. However, the organic substances used in the former studies are found in only minor amounts in the biosphere. As the bulk of the organic reserves available to the weathering solutions occur as humic substances it is desirable that the role of this group be more critically evaluated. For this study, five soils of podzolic affinities developed over quartzose bedrock and supporting different types of vegetation were sampled from various localities in northwestern and western Tasmania. The solvent activity of the humic acids extracted from these soils (referred to in text as HA 1 to HA 5) towards a number of minerals and metals was compared with the activity of deionised water equilibrated with atmospheric carbon dioxide. Further comparison was made with the activity of a number of organic compounds which are minor constituents of soils. A limited investigation of the mobility of some metal humates in the presence of humic acid was also made. Since the writer’s interest is largely in exploration geochemistry, the selection of minerals is biased towards those which occur in economic deposits. EXPERIMENTAL TECHNIQUES Preparation
of humic acids and mineral samples
The organic matter was extracted from the minus 297 p fraction of the soil samples by intermittent shaking for 24 hours with 0.1 M Na4P,0,-NeOH under an atmosphere of nitrogen. The extract and several washings of the soil residue were treeted with ion exchange resins to remove the excess reagents (HORI and OEUDA, 1961). Fine suspended solids were removed by use of a super centrifuge. Humic acids were precipitated from the soil extract by acidification to pH = 2 with H,SO, (KONONOVA, 1966) and the flocculated acid collected with the super centrifuge. The humic acids were taken into solution in 0.1 M N&OH, purified by treatment with ion exchange resin and dried at 40°C under vacuum. The organic content of all soils used in this study was low ( <4 per cent) and 80 to 90 per cent of the extracted organic matter was recovered as humic acid. The ash content of the humic acid samples varied from 3 to 5 per cent. Sample location data, composition and vegetation types under which the humic acids developed are given in Table 1 and the trace element content in Table 2. In previous studies, such as those of FETZER (1946) and SCHALSCELA el al. (1967), finely pulverised (minus 74 ,u) minerals or rocks together with solution strengths in the range of 1 to 6 per cent (w/v) were used. For the present series of experiments, an effort was made to reduce the surface effect by using cosrser grained (297-590 ,u) mineral samples. The weight taken in grams was equivalent to the specific gravity of the mineral concerned and the strength of the aqueous solutions of humic acids was reduced to 0.1 per cent (w/v). Metal hum&es were prepared by the addition of 100 ml of 0.1 M solutions of the respective metals to 100 ml volumes of a 0.5 per cent solution of HA 1. The hum&es were washed by centrifuging with deionised water and then dialysed for 3 days. Humic acid-mineral
interaction 2yrocedures
The movement of near surface waters through fractured material was simulated by use of a vacuum operated perfusion unit (Fig. 1). This unit was modified after the design of AUDUS (1946). The use of flexible connections allows the flow rate to be adjusted. For the present study the flow was set at 25 l/day. The selected mineral was contained in a glass tube sealed at one end with nylon sifting cloth. This sample holder was suspended in the reservoir of the perfusion
The role of humic acids from Tssmanisn podzolic soils Table 1. Location and compositions
271
d&s, for humic acids pH of 0.1% Composition* H N 0
C :H
sc!%Jn
59.9
2.9
1.9
34.1
20.3
3.4
59.5
2.9
2.6
35.0
205
3-o
i~~s~ls~a e~e~~ol~~
55.0
4.3
36
36.8
12.8
3.2
~~~~sc~sn~ sphaerocephalzcs
55-5
4.2
36
36.6
13.2
3.0
-
-
Sample
Location
Vegetation type
C
HA 1
Oliver Hill 814000 m N 380000 m E Oliver Hill 815000 m N 380000 m E Lower Be&ah 815000 m N 400000 m E Savage River 815000 m N 302000 m E Kimberlay 820000 m N 400000 m E
Eucalyptus delegatensis and Pteridium aquilinuwa
HA 2
HA 3
HA 4
HA 5
Sphagnum
sp.
Pinus radiata
-
-
-
3.3
* Ash-free data based upon analyses by C.S.I.R.O. Microanalytical Service. Table 2. Trace element oontent of humic acids. (,ag/ml for 0.1 per cent w/v aqueous solutions)
cs Fe
Mg Ne K Al Si Mn, Ni, Co * Cu, Zn, Pb Us, Sb, Bi
HA1
Humic acid sample HA3 HA2
0.3 4.0 0.1 4-o 0.2 4.0 t5
0.9 1.8 0.1 7.8 0.3 2.0 <5
to.2
<0.2
HA4
HA5
1.2 4.7 0.1 9.5 0.2 4.0 <5
1.2 1.2 0.1 5.0 0.2 <1-O <5
0.7 26.4 0.3 4.8 0.4 14.0 <5
<0.2
<0.2
<0.2
* Range for these elements tO*Ol-0.18. unit so that the mineral wss immersed in the solvent under study. Circulation of the solvent from the reservoir through external tubing to the top of the sample holder was maintained by the spplicstion of a slight suction to the unit. In the initial study a large number of sulphides, oxides, gangue and secondary minerals and metals were exposed to the action of deionised water equilibrated with atmospheric CO, and a 0.1 per cent solution of HA 1. The standard running time wss 24 hours, although differences in the reectivity of the minerals required some variation iu the duration of experiments. From the results of this study, 10 minerals snd 2 metals were selected for use in comparative extraction studies. These samples were extracted with O-1per cent solutions of HA 2, HA 3, HA 4, HA 5, oxalic acid, salicylie acid, pyrogalloX and &nine. A few leeching experiments were carried out with precipitated sulphides and pulverised minerals. The samples were placed on a wad of macerated filter paper contamed in a buchner
272
W. E. BAKER
0.5 mmbore
capillary
tubing
2mm
bore capillary tubing
Fig 1. Perfusion apparatus. funnel connected to a receiver under slight suction. Circulation of 50 ml volumes of water and of 0.1 per cent solutions of HA 1 was maintained by manual transfer from receiver to funnel for a period of half an hour. Leaching of metal humates preparedfrom HA 1 was studied by a similar procedure. The humateswerespreadover a tied area of retentive flter paper which was placed on the macerated filter paper wad. Ten 50 ml volumes of water were drawn through each metal humate sample and the procedurethen repeated using a 0.1 per cent solution of HA 1. A limited study WBBmade of the stability of silicate minerals. Feldspar, biotite, enstatite. actinolite, and epidote were exposed to deionised water equilibrated with atmospheric CO, and 0.1 per cent solutions of HA 1 in the perfusion apparatus for a period of 6 days. Grain size and sample weights were the same aa for perfusion experiments with the other minerals. The reaction solutions together with deionised water washings of the mineral beds were stabilised by the addition of 20 ml of an ammoniacal solution of 6 per cent EDTA (pH = 9) All analytical data were acquired by the use of atomic absorption spectrophotometry and have been corrected for the trace element content of the solvents used.
RESULTSAND DISCUSSION The results of analysis of the water and O-1 per cent HA 1 solution experiments with a variety of minerals and metals are given in Table 3. The extraction values range from 120 ,ug/day Zn from sphalerite to 291,000 ,ug/day Pb from the pure metal. This humic acid preparation is a strong solvent of many minerals, as in all cases studied, the amount of metal extracted is far in excess of that extracted by water. These results are contrary to the findings of FETZER(1946) and the magnitude of variation can be seen from a comparison of the results for CuS (covellite). If the figures given by FETZER(1946) are reduced to a daily rate of solution of metal then distilled water and 1 per cent humic acid extracted 230 ,ug and 150 ,ugCu, respectively. In this study, the extraction values for deionised water and O-1 per cent humic acid were 20 ,ug and 4400 ,ug Cu, respectively. Detailed comparative study of the interaction between minerals and humic acids is made difficult by a number of factors. There is considerable uncertainty in
The role of humic acids from Tasmanian podzolic soils
273
Table 3. Reaction of water and HA 1 with minerals and metals Element determined
Sample Sulphides: Galena Sphalerite Chalcopyrite Bornite Covellite Chalcocite Pyrite Arsenopyrite Loellingite Bismuthinite Stibnite Heazlewoodite Pararammelsbergite Breithauptite Oxides : Haematite Pyrolusite Gangue Minerals: Calcite Fluorite Barite Secondary Minerals: Malachite Smithsonite Anglesite Cerussite Pyromorphite Crocoite Annabergite Metals: Iron Lead Copper Zinc
PbS ZnS CuFeSs Cu,FeS,
cus CL@ Fess FeAsS FeAsS, B&S, SbsSs N&S, N&3, NiSb
Pb Zn cu Fe cu Fe cu cu Fe Fe Fe Bi Sb Ni Ni Ni Fe MI3
CaCO, CaF, BaSO,
C~,P%C%
ZnCO, PbSO; PbCO, Pb,(PG,),Cl PbCrO, (Ni, Co),(As0,),8H,O
,ag metal extracted in 24 hrt H,O/atmos CO, 0.1% HA 1 30 20
3000 120 140 80 1800 800 4400 15000* 240 2700 3200 3800 750 43000 * 74000 * 59000 * 340 2100
1200 50
93000 *
Zn Pb Pb Pb Pb Ni
10 50 (10 (10
104000* 70000* 96000 * 218000* 4600 8200 5800
Fe Pb cu Zn
1200 50 10 160
133000* 291000* 46000* 37000*
Ca Ca
Be
cu
600 500
* Estimated from 2 hour run. 7 100 ml O-1% (w/v) HA 1 acting on weight in g equal to specific gravities of minerals of grain size 247 to 690 p.
procedures
for the calculation of bonding energies in many minerals (BACHINSKI, The humic acids are heteropolycondensates with structurally indeterminate arrays of reactive groups which may result in selective complexing properties. The relative stability of metal complexes formed by the action of humic acids would have a bearing on the competition of metals for various complexing sites. The literature data for stability constants of such complexes KHANNA and STEVENSON, 1962 ; SCHNIYZER and SKINNER, 1967) are not consistent and it is possible that both 1969;
NICKEL, 1969).
274
W. E. BAKER
the magnitude and relative order of values would vary for differing soil extracts. In the natural environment, the wide variability in crystallization of minerals results in the development of surfaces of differing character and hence reactivity towards solvents. Despite these difficulties, some correlation is evident between the extraction of metals from sulphides (Table 3) and relative bond strengths as given by BACHINSKI (1969) The low stability of galena compared with sphalerite and chalcopyrite is in accordance with the relative bond strengths of these minerals. Similarly the decreasing stability of chalcopyrite-bornite-covellite-chalcocite is as predicted by BACHINSKI (1969). However, contrary to the order of bond strengths is the unusually high solubility of the nickel minerals and of bismuthinite relative to stibnite. The reason for the differing behaviour of the latter two very similar minerals is not clear. The fact that Sb attains a pentavalent state more readily than Bi may Differing behaviour of the two metals towards sorption by possibly be a factor. humic substances was noted by SZALAY (1964) who classed Bi as huminophile and Sb as huminophobic. The stability of pyrite compared with that of arsenopyrite and loellingite is compatible with the structures of these minerals from the viewpoint of ligand field theory as discussed by NICKEL (1968). The solubility of haematite and pyrolusite may have some bearing on the fact that very few gossans have developed over mineral deposits of western Tasmania during the present erosional cycle. In many cases sulphides appear at or near the surface with practically no development of secondary minerals. The relatively high solubility of pyrolusite compared with haematite suggests that an alternative mechanism to pH and Eh might be available for the separation of Fe and Mn during the weathering of gossans. A study of the Fe and Mn content of Tertiary (?) gossan and associated soil in the area sampled for the production of HA 1 revealed that the Fe:Mn ratio increased markedly in the soil. Several of the gossan samples were extracted for 24 hours in the perfusion apparatus with water adjusted to pH = 3 and with a 0.1 per cent solution of HA 1. Whereas the extraction of Fe was <25 ,ug in all cases, the extraction of Mn was far higher and the humic acid was about twice as effective as water at pH = 3 (Table 4). Many of the gangue and secondary minerals are strongly attacked by HA 1. Ca is rapidly removed from calcite whilst the less soluble fluorite and barite are also affected. Calcite was also subjected to the action of water under an atmosphere which was 25 per cent (v/v) CO,. This resulted in the release of 2700 pg Ca in 24 hours which is still well below the value achieved in the same period by the humic acid. It thus seems likely that limestone and dolomite exposed to the action of humic acids would be subjected to particularly devastating erosion. There is marked contrast in the solubilities of the secondary Pb minerals; cerussite and anglesite show far less stability than pyromorphite and crocoite. This may be a factor in the rarity of occurrence of the former two minerals in the near-surface development of lead mines in the Zeehan and Dundas areas of western Tasmania. The action of HA 1 on metals is notably aggressive. In view of the reducing properties of humic acids, ONE and SWANSON(1969) queried the possibility that these substances could complex Au since the metal would first have to be oxidised. The data of Table 3 indicate that, for other metals, oxidation must have proceeded quite rapidly despite the presence of humic acid. Possibly humic acids are far better
The role of humic acids from Tasmanian Table 4. Analytical
data for gossan-soil pairs from Oliver Hill, NW and laboratory extraction of Mn
G.l s.l G.2 5.2 G.3 5.3 G.4 5.4 G.5 s.5 G.6 S.6 G.7 s.7 G.8 S.8 G.9 s.9 G.10 s.10
9.96 5.86 16.71 6.92 17.00 6.56 40.80 7.97 38.69 539 29.01 4.81 19.64 4.16 20.63 2.93 44.49 12.07 20.14 15.24
275 Tasmania
Laboratory extraction of Mn from gossan H,O at pH = 3 0.1% HA 1
Mn Sample No.
podzolic soils
(%I
Fe:Mn
30.05 0.45 17.94 1.42 20.63 0.21 6.73 0.33 3.47 0.24 12.56 0.15 21.08 0.21 13.90 0.13 1.29 0.13 11.66 0.21
0.3 13.0 0.9 4.6 0.8 31.2 6.1 24.2 11.2 22.5 32.3 32.1 0.9
&g/24 hr) 620
1290
1300
2160
-
-
570
910
-
-
1420 -
2380 -
19.8 1.5
-
22.5 34.5
-
-
290
740
92.9 1.7 17.2
Normal oxidation of the metals in an complexing agents than they are reductors. aqueous environment could be expected to accelerate in the presence of a complexing agent which removed ions from the metal surface as they were formed. The results obtained with other metals encouraged an examination of the behaviour of Ag and Au. As only small quantities of coarse-grained samples of these metals were available, weights equal to l/lOth the respective speci& gravities were placed in flasks with 50 ml volumes of 0.5 per cent solutions of HA 1 and these were shaken intermittently during 6 weeks. Analysis at the end of this period revealed that 400 pugof Ag and 20 ,ug of Au had been taken into solution. This is contrary to the findings of FETZER (1934, 1946), and 0~a and SWANSON (1969). The results of reaction of the finely divided (minus 74 ,u) samples with water and HA I, over a period of half an hour are given in Table 5. These are seen to be generally consistent with the data of Table 3. PbS, CuS and NiS display high solubility relative to ZnS. The solubility of Bi,S, is far in excess of Sb,S, and the same situation is found for the pulverised bismuthinite and stibnite. Compared to the results for the 297-590 ,u mineral samples (Table 3), extraction of Fe from haematite has increased relative to Mn from pyrolusite. This is probably due to the fact that whereas the surfaces of the coarser grains of haematite are parts of single crystals, for pyrolusite they consist of an aggregate of finer grains. Thus the effect of grain size reduction is more marked for the haematite.
W. E. BAKER
276
Table 6. Action of water and HA.1 on precipitated sulphides and pulverized minerals Element determined
Sample Lead sulphide Zinc sulphide Cupric sulphide Nickel sulphide Bismuth sulphide Antimony sulphide Bismuthinite Stibnite Haematite Pyrolusite
pug metal extracted in + hr 0.1% HA 1 H,O/atmos CO,
Pb Zn
<3 75
CU
Ni Bi Sb Bi Sb Fe Mn
425 <5 <5 <5 t5
2100 95 530 1440 70 <5 435 25 340 310
Table 6 gives details of the action of the humic acids from under different vegetation cover during 1 hour perfusion runs on a selection of samples. These results show that whilst all the humic acids are active in mineral degradation there is considerable variation in the extent of reaction. The variation shows no consistent pattern and it is likely that some specificity is operating in addition to changes in the total content of reactive groups. This feature is apparent in the low activity of HA 4 with haematite whilst it is the most aggressive with calcite. The high solubihty of pyrolusite relative to haematite is not uuiversal, since of the acids studied to date, only HA 1,HA 2 and HA 4 show this tendency. The reaction of all acids with bismut~i~ is greater than with stibnite, although HA 2 has an appreciable effect on the latter mineral. Table 6. Action of various humic acids with minerals and metals
Sample Galena Sphderite Born&e Ch&ocite Bismuthinite Stibnite P~rar&~e~ber~~ Hsematite Pyrolusite Calcite Copper Lead
Element determined Pb Zn cu
Water atmos CO, 1
CU
10
Bi Sb Ni Fe Mn Ca cu Pb
HA l* 200 30 190 3800 550 46 9800 470 1000 10500 5700 27400
,ug metal extracted in 1 hour HA 2 HA 3 HA 4 210 90 230 5100 1600 340 9600 90 1800 9600 5800 39400
80 80 180 1300 410 10 5400 410 290 6400 3200 23000
230 40 200 5600 1000 50 9200 <5 1040 11100 6100 33000
HA5 110 30 130 2700 460 <5 6000 490 440 730 4800 24000
* All extractants 0*1% w/v.
most of the earlier studies which dispute the activity of humic acids (HARRAB, and LOVE, 1929; FETZER, 1946),it is considered that the low molecular weight acids and other minor organic substances are the effective agents of mineral degradation. KRAUSKOPF (1967) questions the role of humic acids in geological processes and suggests that the acidity of natural solutions is more likely to be In
1929; MURRAY
The role of humic a&Is from Tasmaman pods&c soils
277
due to the simple organic acids. In the past, the acidic properties of humic substances have been emphasized, and the appeal made by Scn,Arz et aE. (1954 to consider humic substances as complexing agents does not appear to have penetrated the geological literature to any great extent. Since the low molecular weight organic oompounds constitute only a minor proportion of the soil organic matter, they would need to be extremely active to have a potential in the weathering processes comparable with that of the humic substances. The activity of several of the simple organic compounds found in soils is compared with that of HA 1 in Table 7. These results show that the simple compounds exhibit an activity of the same order as that of the humic acids and hence, because of the low concentrations, their effectiveness in weathering processes would be limited. Table 7. Action of various organic compounds with minerals and metals
Sample Galena Sphalerite Born&e Chalcocite Bismuthinit,e Stibnite Par~a~e~sbergi~ Haematite Pyrolusite Calcite Copper Lead
Element determined
HA 1*
Pb Zn cu CU Bi Sb Ni Fe Mu Ca cu Pb
200 30 190 3800 550 45 9800 470 1000 10500 6700 27400
pg metal extracted in 1 hour Oxalic Salioylio acid Pyrogallol Alanine acid 130 30 260 4450 180 <5 10500 <3 4200 11900 5500 41800
95 20 650 9750 4820 580 7620 80 15500 980 2620 660
35 8 55 920 1640 <6 2380 20 5150 2040 1190 1470
<5 20 15 1530 55 <5 1730 20 520 1400 700 240
* All extra&ants O-1% w/v.
The results of the reaction of HA 1 with the silicate minerals are given in Table 8. Whilst these silicates are more resistant to attack than the other minerals studied, the amount of metal removed by the humic acid is impressive. A direct comparison of the results with earlier work is not possibIe, but the high extraction of Na, K, Mg, Ca and Fe relative to Si and Al supports the claim of ScE~sa~ et aE. (1967) that the metals are removed from silicate frameworks by ~mplexation. The results obtained by PONOXARETA and RAGIM-ZADE (1969) for 6ne grained samples of feldspar and biotite showed that removal of K, Na, Fe and Mg by fulvic and humic acids exceeded Si and Al, although considerable amounts of the latter elements were found in the leach solutions. l?or the present series of experiments, with coarser grained samples under perfusing solutions, far higher relative extraction of metals other than Si and Al has been found, In view of the effect of humic acid on silicates, it is pertinent to consider the effect they might have on structural materials such as portland cement. A crushed sample of this material subjected to a 24 hour perfusion run with HA 1 released 5300 ,ug Ca. Taking this result in conjunction with the data for metals, it is possible that such ma~rials as reinforced concrete piping would have a
278
W. E. BAKER
comparatively short life if bnried in an env~onment where tihese substances are active. The activity of humic acids with respect to metals could also offer a supplementary mechanism to bacterial corrosion for their destruction. Table 8. Reaction of water and HA 1 with silicates Mineral Feldspar (perthite) (Na, K)AlSi,O,
Biotite WMg. Fe), WSk&,) fO%
Element determined
pg metal extmcted in 5 days H,O/atmos CO, 0.1% HA 1
N-a K Al Si
340 190
1260 640 220 t5
K
60 20 <5
100 90 240 300 6
xg Fe Al Si
Enstatite (Mg, Fe”)SiO,
Mg FO Si
<5 <5
620 220 <5
Actinolite CaWg, Fe”& (Si*O~~)(OH, Ffa
his F% Ca Si
25 <6 760 <5
960 2900 40
Epidote Ca F”Al,O.OH (Si&%)WOJ
Ct.3 F% Al Si
1800 10 40 <6
10700 1300 320 5
80
80
The metal contents and leaching characteristics of 10 humates are shown in Big. 2. The quantity of metal in the humates varies widely and this feature is being studied further. It has been found that a Ni humate prepared by the action of HA 1 on heazlewoodite contained 12.7 per cent Ni compared with 3.5 per cent Ni for the humate prepared by mixing solutions of HA 1 and a Ni salt. It is possible that in the presence of the strong salt solution, colloidal properties of the humic acid result in coagulation before many functional groups react with Ni ions. Whilst most of the metal humates display their classical ‘insolubility’ in water, they are readily mobilized by a O-1 per cent solution of HA 1. Where appreciable metal was mobilized by water (Cu, Ni, Mg, Ca), the solution was coloured by the humate, indicating that it was the metal complex that was moving. The base metals are all highly mobile in the presence of humic acid and this could affect their dispersion patterns. The data also has a bearing on geomedical studies such as those of WARREN et al. (1967)) who found a higher than average incidence of multiple sclerosis in regions where Pb in particular was highly mobile. The leaching characteristics of the Fe and MYnhumates are very similar and thus the preferential mobilization of m found for the gossan at Oliver Hill is limited to the mineral de~adation phase of the hnmic acid activity.
The role of humic acids from Tasmanian podzolic soils Pb
Zn
600
7.7
b i‘ L I, I_L 2300
(5
279
10
3200
_---
lmr
0
F&II)
~33
250
(5
0
670
$’
(5
660
--
0
1000
0
1000
,N;f
600
i: 530
:
\\,__<
0
1000
KEY
3
100
--_--
0
1000
TO DIAGRAMS
%Metolin
L
0
humate
~~~Extrocted
yg.fxtracted
750
1000
0
Mls.leoch solution
1000
Fig 2. Leaching characteristics of metal humates.
The reducing properties of soil organic matter are held to effect the enrichment of Al in some bauxite formation by selective removal of Fe (HARDEN and BATESON, 1963). The leaching experiments indicate that the Al humate has a far lower mobility than that of Fe and this could also possibly be a factor in some enrichments of Al. The marked difference in mobility of the Ni and Co humates could also lead to differing behaviour of these elements in some surficial processes. 7
280
W. E. BAKER
CONCLUDINGREMARKS The data presented in this paper establishes that humic acids form the bulk of the organic matter present in many Tasmanian soils of podzolic affinities. These acids are powerful solvents of a number of minerals and could be very active in the weathering cycle. It is possible that they have maximum effect under a cool temperate, high rainfall climate such as has existed in western Tasmania since the Pleistocene glacial epoch. The rapid solution of minerals associated with economic deposits by humic acids and the mobility of the metal humates formed could explain the rarity of gossans and appearance of sulphides at the surface in western Tasmania. In cases where the rate of leaching exceeds the physical lowering of the surface, secondary dispersion patterns may be weakened or obliterated and geochemical prospecting would need to be based upon the search for primary halos in the country rock. Acknowkdgmenta-This study is part of a joint Department of Mines-University of Tasmanie investigation and the writer is indebted to Mr. J. SYMONS,Director of Mines and Professor S. W. CAREYfor their support. Dr. J. WILMS~~~RST kindly arranged for the analyses of the humic acids whilst Dr. E. WILLIA~~~and Dr. J. VAN MOORTaffordedthe author much useful discussion. I also wish to thank Mr. C. WEBB for laboratory assistance, Mrs. B. WALTERSfor typing the manuscript and Mr. H. MACKINNON for drafting the diagrams. REFERENCES AUDUSL. J. (1940) A new soil perfusion apparatus. Nature 158,419. BA~HINSKID. J. (1969) Bond strength and sulfur isotropic fractionation in coexisting sulfides. Econ. Beol. 64, M-65. BLANCKE. (1933) Die sogennante humussiiureverwitteringim lichte neuester bodenforschung. Ernlihr. Pjl. 29,41-43. CLARKE F. W. (1911) The data of geochemistry, 2nd ed. U.S. Geol. Susv. Bull. 491. DUFF R. B., WEBLEY D. M. and SCOTTR. L. (1963) Solubilization of minerals and related materials by 2.ketogluconic acid-producing baoteria. Soil. Sci. 95, 104114. EVANSW. D. (1964) The organic solubilizationof mineralsin sediments. In Advo~ceein Organic Cteochemistry (editors Colombo U. and Hobson G. D.), pp. 263-270. Monograph 15. Earth Science Series. Pergamon. FETTERW. G. (1934) Transportation of gold by organic solutions. Econ. Geol. 29, 599-604. FETZERW. G. (1946) Humia acids and true organic acids as solvents of minerals. Econ. Gwl. 41, 47-56. FREISEF. W. (1931) The transportation of gold by organic undergroundsolutions. Econ. Gwl. 26, 421431. GINZBUR~I. I., YASHINA R. S., MAT~EEVAL. A., BELYATSKIIV. V. and NUZHDELOVSKAYA T. S. (1963) Decomposition of certain minerals by organic acids. In Chemistry of the Earth’8 Crust (editorVinogradov A. P.) Vol. 1, pp. 304-320. Israel Program for ScientiilcTranslation, Jerusalem, 1966. GRAHAME. R. (1941) Colloidal organic acids aa factors in weathering of anorthite. SoiESci. 52, 291-295. GRUNERJ. W. (1922) The origin of sedimentary iron formations: the Biwabik Formation of the Meaabi Range. Econ. Beol. 17, 407-460. GUILLINR. (1928) Dissociationintigrale dee silcates par l’acide carboniqueet les acideshumiquea et reactions annexes. Compt. Rend. 187, 673-675. HARDENG. and BATJ~SON J. H. (1963) A geochemicalapproach to the problem of bauxite genesis in British Guiana. Econ. Geol. 52, 1301-1308. HARRARN. J. (1929) Solvent effects of certain organic acids upon oxides of iron. Econ. Bwl. 24, 50-61.
The role of humic acids from Tasmenien podzolic soils
281
HORI and OKUI)AA. (1961) Purification of humic acid by the use of ion exchange resin. Sodb Sci & P&pat N&r. Tokyo, 7,4. HUAN~ W. H. and KE~~EB W. D. (1970) Dissolution of rock-forming s&ate mineralsin organio acids: simulated first-stage weatheringof fresh mineralsurfaces. Am. Mineral. 5&2076-2094. JULIENA. A. (1879) On the geological action of the humus acids. Proc. Am. Assoc. ,4dv Soi. 28, 311-410. KEANNA S. S. and STEVENSON F. J. (1962). Metallo-orpic complexes in soil: 1. Potentiometric titration of some soil organio matter isolates in the presence of transition metals. Soil: Sci. B&298-305. KONONOVAM. M. (1966) Soil OrgunicMutter. 2nd English edition. Pergamon. KONONOVAM. M., A.L~~ANDROVA I. V. end TITOVAN. A. (1964) Decomposition of silicates by soil organic matter. Pochv~~~~~e 10,I-12. English translation in Rusiarr Soil Sci. 1964, 1005-1014.
KYRAUSKOPF K. B. (1967) ~~t~~~~~ to ~wc~~~~~. ~cGr&w-~11. LOU~HNANF. C. (1969) C&r&uz~ ~~~~~ of the Silicate Minerals. Eisevier. MAnEL H. VAN DER (1949) Minerelogicol composition of a heath podzol profile. So4 Sci. 67, 193-207. MURKY A. N. and Love W. W. (1929) Action of organic acids upon limestone. Bull. Amer. Assoc. Petrol. Gwl. 13,1467-1475. NICKIXLE. H. (1968) Structuml stability of minerals with the pyrite, marcasite, arsenopyrito and ldllingite structures. Can. Mineral. 9, 311-321. NICKEL E. H. (1969) Bond strength and sulfur isotopio fra&ionation in coexisting sulfides. Ecorr. Geol. 84, 934-935. ONG H. L. and SWANSONV. E. (1969) Nature1 organic mids in the transportation, deposition end concentration of gold. Q. Golo. Sch. i@imes 04, 395-425. PONOXA~EVAV. V. and RAGIIM-ZADE A. I. (1969) Comparative study of fulvic and humic acids English translation as the agents of silicate mineral decomposition. P~hvov~e~~~e 8,26-36. in ~~8~~~ Soil Sei. 1969, 157-166. SCHALSCHA E. B., APP~LT H. and SCEATZA. (1967) Chelation ss a weathering mechanism. l-Effect of complexing agents on the solubilizstion of iron from minerals and grsnodiorite. Uwchim. cosmochim. Acta 31, 587-596. SCFIATZ A., CHERONXS N. D., SCHULTZ V. and TREWLAWNEYG. S. (1954) Chelation (sequestration) as a biological weathering factor in pedogenesis. Pean. Acud. Sci. 28, 4P51. SCHNITZER M. and SKINNERS. I. M. 1967 Orgeno-metallic interactions in soils: 7. Stability oonstants of Pb2f, X2+, Mn*, Cow, Gas+ and Mgw-fulvic acid complexes. Soil Sci. 103, 247-252.
SPREN~ELC. (1826) Uber pflanzenhumus, humussaure und humussaure s&e. Kastlzers Arch @es. ,VaturZehre. 8, 145-220. SZALAYA. (1964) Cation exchange properties of humio acids and their importance in the geoehemicLa1 enrichment of UOs* and other cations. &ochim. ~o~eh~m. Acta 28,1605-1614. THENA~DP. (1870f Observations sur le Memoire de M. Friedel. Conapt.Bad. 70, 1412-1414. WA~LREN H. V., DEVAULTR. E. end CROSSC. H. (196’7) Possible correlationsbetween geology and some disease patterns. Ann. N.Y. Acud. Sci. 188, 657-710.