R O C Z N I K I G L E B O Z N A W C Z E T . X X X I V , N R 1—2, W A R S Z A W A 198S
A L E X A N D R A V A S U , C. N ED E A
THE USE OF Е-pH TERNARY DIAGRAMS IN SELECTING EXTRACTANTS OF AMORPHOUS IRON AND ALUMINIUM
COMPOUNDS IN SPODIC SOILS IN ROMANIA R esearch In stitu te for S o il S cien ce and A groch em istry,
B u ch arest
P o ly tec h n ica l In stitu te, B u ch arest
IN TRO DU CTIO N
Iron humus podzols, acid soils with low biological activity, developed in a humid temperate clim ate, in forest and alpine zones, on acid parent materials, are characterized by the presence of colloid gels formed by the hydration and polimerization of Fe and A1 oxides.
These have resulted from the chemical weathering of silicates main tained for a long time in an amorphous state in acid conditions by the organic matter which plays the role of a protection colloid or complexing agent, favouring the formation of amorphous, organo-mineral “com p lexes”, as micellar associations. The quantity and distribution of this active amorphous material [7] in the soil profile are specific to the ev o
lution stages of the present podzolization process [22].
For the study of amorphous soil material m any methods of differen tial solubilization have been used on a sim plifying hypothesis concerning the particle size, hydration degree, temperature, contact period etc. fac tors which can influence the kinetics of extraction reactions.
Due to the fact that the extraction based upon differential solubiliza tion is the only w ay used in practice for separating the amorphous ma terial — the elem ents dosed in extractants being of various origin, from hydrosoluble forms, com plexed ions and gels associated with organic molecules or crystallized minerals to products of the partial attack of crystallized minerals — in order to ensure a correct interpretation of analytical data it is required to find an objective criterion for appreciat ing the selectivity of extractants, according to the chemical form of diffe rent elem ents separated from soils.
Thus, the elaboration of the ЕчрН diagrams for ternary system s, ele- ment-water-ligand, had conducted to the outlining of the first objective
4 A. Vasu, C. Nedea
criterion for appreciating the selectivity of extractants used in soil analysis [23].
Being a condensed, logical, generally admitted form of presenting the data characterizing the equilibrium conditions of simultaneous ch e mical and electrochem ical reactions w ithin the lim its admitted for stan dard chemical potentials of components participating in the reactions con sidered, the Е-pH diagrams have been elaborated by P o u r b a i x [18] for the majority of binary elem ent-w ater system s. Over the last 15 years, binary diagrams have been used in soil science for studying hydromorp- hic soils. В o n n e a u et al. [1] recommends the Fe-water and Mn-water binary diagrams for the study of the solubility and m obility of these ele ments in soils.
The introduction into the elem ent-water binary system s of a third constituent which can form com plexes or insoluble salts, called for the elaboration of elem ent-water-ligand ternary system diagrams, represen ting modifications of those of the binary system s that present the stability conditions of the respective dissolved or solid compounds [11— 16].
The diagrams of the ternary system s permitted the extension of their applicability on to soil chem istry, as a first objective criterion for appre ciating the selectivity of the extractants used in soil analysis (28].
The paper presents the E-pII equilibrium diagrams of the Fe-water- -ligand (extractant) and A l-w ater-iigand (extractant) ternary system s, corroborated with the experim ental results and their use as thermody namical basis for proving the validity of the indexes established for the identification and characterization of the evolution stages of spodic soils [2, 21, 22].
M ETH OD S
In elaborating the diagrams of the ternary system s som e sim plifying hypotheses have been advanced: the thermodynamic soil system is con sidered as a sim ply (Fe, Al)-^water binary system , the kinetics of chemi cal reactions specific for the practical conditions in which selective extractions are made, as w ell as the reactivity difference of different soil Fe and A1 forms (ionic forms, amorphous hydroxides, oxohydroxo- ions w ith different polymerization degrees com plexed or uncom plexed by the organic m atter, crystallized hydroxides and oxides) have not been taken into consideration.
The m ain extractants recommended in soil science literature for extracting free (nonsilicate) forms of Fe and A1 (associations of acids w ith a complexing action and a reducting agent, the latter not being taken into consideration) have been studied from the thermodynamic and experi mental points of view [28, 29]. These extractants are: natrium pyrophosp hate 0.1 M at pH 10 (P) — D u с h a u f o u r and J a с q u i n [5]; natrium
The use of E^pH ternary diagrams 5
pyrophosphate 0.2 M at pH 7.3 w ith natrium dithionite (P-D) — F r a n - z m e y e r et al. [7]; natrium citrate 0.2 M at pH 7.3 w ith natrium dit hionite (C-D) — De С o n i n с к et al. [3] m odified by V a s u [21]; oxalic acid 0.4 M — ammonium oxalate bufferd at pH 3.5 w ith natrium dithio
nite (O-D) — D u c h a u f o u r and S o u c h i e r ,[6] and, for Al, natrium
hydroxide 0.5 M — H a s h i m o t o and J а с к s o n [9].
The Е-pH ternary diagrams of Fe(Al)-ligand (pyrophosphate, citrate, oxalate)-HOH system s have been calculate and plotted taking into con sideration the chemical and electrochem ical reactions which can take place when Fe(Al) — especially the amorphous hydroxides and oxohy- droxoions respectively, the components for which the selectivity of e x tractants is considered — are in the presence of the ligand. Thus, the thermodynamic conditions of the relative predominance of the compo nents which can be formed in the respective system s, are established as w ell as their evolution as a function of the concentration in which they occur.
In order to facilitate the comparison of the diagrams, the concentra tions of Fe3+ and A l3^ have been maintained constant in the system —
for Fe3+ 0.015 mol. I-1 and for A l3+ 0.01 mol. I“ 1. These correspond to
the experim ental conditions, expressed at 100 g soil: 1.5% Fe extracted w ith P solution, 2.3% Fe and 1.2% Al extracted w ith P-D solution, 3.8°/'o
Fe and 1.2% Al w ith C-D solution, 4.2% Fe and 1.35% Al w ith 0--D
solution and 2.7% Al w ith NaOH solution.
The highest stability of the com plexes formed with different ligands (extractants) is about pH ± 1 units corresponding to the m iddle of the stability area of the com plex formed b y the, action of the extractant (complexing agent) upon the studied elem ent.
R ESU L TS A N D D ISC U SSIO N
The analysis of superposed Fe-wàter-ligànd \ extractants) diagrams
shows the follow ing (Fig. 1): natrium pyrophosphate at pH 10 can extract
the smallest amounts of Fe, predom inantly corresponding to the organo- - mineral compounds (a conclusion also emphasized by comparing the amounts of С extracted by P ofc by P-D, which are practically the same); pyrophosphate-dithionite at pH 7.3 can extract somewhat larger amounts, predom inantly corresponding to the active amorphous Fe compounds (ionic forms, amorphous hydroxides, oxohydroxoions w ith different poly merization degrees — recent gels, maintained in. this form in acid m e dium by the organic matter having a protection colloid or com plexing agent role); dithionite-oxalic buffer and dithioiiite-citrate can extract the largest amount of Fe, including, besides the ionic and amorphous forms, cryptocrystalline forms, as w ell as crystallized hydroxides and axides..
6 A. V asu, C. N edea
Besides the selectivity of the extractants studied comparatively the Е-pH ternary diagrams show the uselessness of adding the natrium dit- hionite as a reducting agent to the extracting solutions. The stability lim its of the com plexes formed w ith Fe3+ and their compounds drop much lower than 1/2 in the stability area of Fe2+ from the diagrams of the Fe-HOH system , especially when the oxalic buffer is concerned.
F ig. 1. E q u ilib rium diagram s E—p H of sy stem s
1 — F e -H O H , e i — F e -H O H p y r o p h o s p h a t e 0,1 M , e i v —? F e - H O H p y r o p h o s p h a t e 0,2 M ,
e u x — F e - H O H o x a l a t e , e u — F e - H O H c i t r a t e
The use of Е-pH ternary diagrams... 7 In chem ical literature no com plexes of Fe2+ w ith the com plexing agents (extractants), excepting citrate, are taken into consideration. The experi mental results of extractions w ith or without adding dithionite confirm the conclusions deduced therm odynam ically (Fig. la).
The analysis of the superposed A l-water-ligand (extractant) and Al- -water diagrams (Fig. 2) shows that dithionite-citrate at pH 7.3 extracts predominantly active amorphous Al. Because for natrium pyrophosphate
Fig. 2. E q u ilib rium diagram s Е-pH of sy stem s
1 — A l-H O H , е ц — A l- H O H o x a l a t e , e j A l-H O H c i t r a t e , e i n — A l-H O H -N a O H
8 A. V asii, C. N edea
the constants of the Al com plex are not mentioned in chemical literature, a comparison of the experim ental results w ith those obtained for citra- te-dithionite has been resorted to. It has become evident that natrium pyrophosphate at pH 10 can extract somewhat lower amounts than citra- te-dithionite at pH 7.3, probably the A l form the organo-mineral com pounds (the С amounts extracted by P and P-D are practically identical); the fact that pyrophosphate-dithionite at pH 7.3 extracts amounts of Al identical w ith those extracted by C-D (Fig. 2a) and the indirect evidence given by F r a n z m a y e r et al. [7] enables us to infer tha P-D extracts
Fig. 3. Fe, A l q u a n tities ex tra cted w ith d ifferen t ex tra cta n ts from rep resen ta tiv e cam bic, spodic and andic soils
predominantly active amorphous Al. The oxalic-dithionite buffer can e x tract larger Al amounts, nam ely predominantly active amorphous Al and aged gels; natrium hydroxide 0.5 M solution extracts total amorphous Al and cryptocrystalline forms (including the compounds of the allophanic type), as w ell as slightly crystallized silicates (having different ordina
The use of Е-pH ternary diagram s.. 9
tion degrees) of the halloizit and m ethahalloizit type [8], a fact also con
firm ed-by H é t i e r [10].
The experim ental results obtained on Mount Fagaraç spodic soils evolved on chlorit-sericite schists under beech, beech m ixed with coni fers, spruce or meadows, at 650— 2240 m a.s.l. w ith an annual average rainfall of 750— 1250 mm and an annual average temperature of 7 to — 1.2°C and Mount Harghita spodic and cambic soils of andic character evolved on andezites w ith pyroxens under conifers m ixed with beech, spruce or meadows, at 950— 1750 m a.s.l. w ith an annual average rainfall of 730— 1300 mm and an annual average tem perature of 1.5 to 5°C, con firm the pracitcal validity of the suppositions deduced thermodinamically (Fig. 3). This attests the accuracy of the interpretation of Е-pH diagrams of the Fe(Al)-ligand ternary system s as an objective criterion for the selectivity of the extractants, for w hich only indirect evidence has been available [24] up to the present.
Thus, it is proved that the hypothesis used over the last 12 years is valid, nam ely that the P-D or P solution extracts predominantly active amorphous Fe and Al, these being the specific constituents of the con temporary iron-humus illuvial process. The spodic character is estimated by the corroboration of morphological soil features with the lim its con cerning the active amorphous Fe and Al amounts and dynamics (expres sed by the maximum value of the soil and, within the same lim its, to the nature of parent material (Table 1).
The mentioned criteria have been also compared w ith both criteria given in soil Taxonomy, one for identifying the spodosols and the other for the spodic character of andosols, which cannot be used in estimating
T a b l e 1
S p e c i f i c v a lu e e bad v a r i a b i l i t é in d e x a e f o r activ c- eiuorphous m a t e r i a l i h sp o d ic and c a n b ic o o i l s /com pariD on t e r m /
S o i l s u b ty p e Pô A l % Pc+Al+C Maximum X xV c la y Pe A l Pe+Al Pe+Al+Cc la y A cid 0 . 4 - G .I - < 0 .1 5 1 .1 - t . 1 - 1 . 1 - 1 .0 -brown s o i l 0 .8 О .4 0 .0 4 —0 ,1 0 1,3 1 .3 1 .6 1 .4 C ry p to p o d z o lic 0 . 5 - о.з- > 0 .1 5 1 .1 - 1 .4 - 1 .3 - 1 .4 -brown e o i l e 1 .0 1 .0 0 .1 6 - 0 .5 2 2 .5 2 .2 1 .9 2 .0 P o d z o lic brovm 0 . 4 - 0 . 3 - > 0 .1 5 1 .6 - 1 .3 - 1 .2 - 1 .4 - • s o i l s • 1 .7 0 ,8 0 .2 1 - 0 .6 2 2 .3 2 .0 1 .8 2 .0 Brô^n-. .p o d zo ls ■0.2- Ou 2 - > 0 .1 5 1 .5 — 2 . 0 - • 1 .8 - 2 . 1 - ■ 2 .0 1 .0 0 .2 7 - 1 .0 0 4 .0 5 .0 3 .3 3 .0 P o d z o ls * 0 . 5 - 0 .Ö 4 - > 0 .1 5 > 4.1 > 4 .5 > 3 .4 > 3 .1 5 .0 1 .8 0 .4 0 - 1 .0 0 4 .1 - , . 4 . 5 - , • ■ 3 . 3 - 3 . 1 -50 20 25 15 Z ' I V - v a r i a b i l i t y index* r a t i o o f s p e c i f i o v a lu e s f o r each h o r iz o n to th e e l u y i a :t e d h o r i z o n s ; 0 and BD h o r i z ç n s .a re n o t ta k e n i n t o c o n s i d é r a t i o n # minimum v alLues f o r
10 A. Va-su, C. Nedea
the evolution stages of spodosols (Guide Book of National Conferenc? for Soil Science — Braçov, 1979).
The experim ental results accumulated have confirmed the validity of the established indexes. Over the last 12 years of comparative studies
of spodic, cambic, argillic and andic soils more than 100 soil profiles
have been analyzed from the point of view of the amounts and dynamics of different Fe, A1 and free Si forms. These soils have been developed on various crystalline schists, sandstones, conglomerate, marls, limestones, andesites and pyroclastites, under beech, beech m ixed w ith conifers, spruce and meadows, in the Southern and Eastern Carpathians, at of 600— 2240 m a.s.l., an annual average rainfall of 700— 1250 mm and an
annual temperatures of 8 to — 2°C [2, 21, 22, 25]. The results have been
confirmed also by other studies carried out in Romania and abroad [4, 17, 20]. Moreover, these results have proved that the criteria established on the basis of active amorphous material dynamics have a general va lidity not only for spodic soils with obvious spodic morphological featu res, as for instance podzolic brown soils, brown podzols and iron-humus podzols, but also for spodic soils with obscure spodic morphological fea
tures, such as the brown crypthopodzolic soils (with evolution stages similar to podzolic brown soils or even brown podzols), as w ell as for soils whose spodic character is superposed on the andic one [25] Fig. 4. The exam ples given in Fig. 4 show the same dynam ics of active am orphous materials, as expressive of the iron-humus podzolic process of spodosols developed on chlorite-sericitic schists, as for those developed on andezites w ith pyroxens *. For the former ones the only amorphous .naterial, there occur aged gels and cryptocrystalline or crystalline tran- podzolic process, while in the latter soils, besides the active amorphous material, there occur aged gels and cryptocrystalline or crystalline tran sitions w ith different ordination degrees, characteristic of the andic soil- - forming process.
The recognition of Fe and A1 forms w ith the aid of Е-pH diagrams, thus constitutes an objective argument for identifying the chemical con stituents characteristic of different pedogenetic processes, even if they coexist as a result of soil evolution.
The recognition of Fe and A1 also substantiates and confirms the vali dity of the research work on the role of amorphous materials in charac terizing soil trophicity, for instance, as a source of pH dependent acidity [27], as an ecological determ inant w ith a lim itative role in spruce growth [26] or in phosphate adsorption {19].
1 T he so ils are described in the G uide B ook of N ation al C on feren ce for S o il S cien ce, B rasov, 1979.
F& g. 4 . F e a n d A l d y n a m i e i n c a m b i c , a n d i c a n d s p o d i c s o i l s
Pe — c r y p t o p o d z o l i c s i l i c a t e s o il, P 7 — p o d z o l, P8 — p o d z o lic b r o w n s o il, P 9 — p o d z o l i c b r o w n s o il, P 10 — c r y p t o p o d z o l i c b r o w n s o il, * 1 1 — c r y p t o p o d z o l i c b r o w n s o il, P 17 — a n d i c a c i d b r o w n s o il. p 18 — c r y p t o p o d z o l i c a n d o s o l , P2\ — a n d i c a c i d b l a c k s o il, P 32 —
a n d i c p o d z o lic b r o w n s o ll, P3 3 — a n d i c p o d z o lic b r o w n s o il, P200 — a n d i c b r o w n p o d z o l.
T h e us e of Е -p H ternary d ia g r a m s ...
12 A. Vaisu, C. Nedea C O N C LU SIO N S
In conclusion, the Е-pH ternary diagrams, used as an objective crite
rion for selecting extractants, provide a real physical sense for the a n a
lytical results obtained by conventional methods. Consequently, they
also ensure the validity of their interpretation in soil system atics, a s
w ell as in estimating soil production capacity. By comparing the d a ta
on the chemical forms of nutrients obtained in this way with phytophy- siological studies the diagrams can be also used in future for establishing new methods of soil analysis as w ell as for studying soil-plant relation ships.
R EFEREN CES
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The use of E^pH ternary diagrams 13
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[26] V a s u A., D u l v a r a E., S p i r e s c u M. , M o i s e E.: C on trib u tii la cu n oaç- terea fa cto rilo r lim ita tiv i la in rad acin area m o lid u lu i. A n. Inst. C ercet. P edol. çi A grochim ., X L I, 1975.
[27] V a s u A. , Z a n e l l i S., D u l v a r a E., I l i e М., I о n é s e u M.: C o m p o n e n t ai a cid itä tii çi im p o r t a n t lor in caracterizarea ecop ed ologicä a p rin cip alelor solu ri de m un te. An. In st. C ercet. P ed o l. çi A grochim ., X L II, 1976.
[28] V a s u A., N e d e a C.: A p p lica tio n de diagram es ten sio n -p H dans Г a n a ly se du sol. B ul. de l ’A S A F 9, 1979.
[29] V a s u A. , N e d e a C.: D iagram ele tern are ten siu n e-p H , criteriu o b iectiv de se le c tiv ita te a ex tra cta n jilo r acide. V ol. C onf. NaJ. de p ed ologie, B raçov 1979, 1980. A. ВАСУ, Ц. НЕДЕА ПРИМЕНЕНИЕ ТРЕХКОМПОНЕНТНЫХ ДИАГРАМ М Е-pH ПРИ ОТБОРЕ РАСТВОРИТЕЛЕЙ АМОРФНЫХ СОЕДИНЕНИЙ ЖЕЛЕЗА И АЛЮ МИНИЯ В СПОДИКОВЫХ ПОЧВАХ РУМЫНИИ Институт почвоведения и агрохимии в Бухаресте Политехнический институт в Бухаресте Р е з ю м е В статье представлена возможность использования трехкомпонентных диаграмм типа элемент-вода-лиганд и двухкомпонентных типа элемент-вода для установления предела относительного преобладания в почвенном экстракте определенных соединений железа
14 A. Vasu, C. iNedea и алюминия. На диаграммах проанализированы двухкомпонентные системы Fe(Al) вода и трехкомпонентные Fe-во да-лиганд (пирофосфат, цитрат, оксалат), А1-вода-лиганд (цитрат, о к салат), а также AI-вода-гидроокись натрия. Результаты проведенных исследований под твердили пригодность разработанных диаграмм для подбора самых благоприятных условий экстракции и, вопреки пред-писаниям некоторых авю ров, показали ненужность добавления дитионита в экстракционные растворы при извлечении из почвы несиликатных форм железа и алюминия. Использование диаграмм и соответствующий подбор условий экстракции дают возможность установить химические критерии, позволяющие объективно разграничить криптоподзолистые, подзолисторжавые, ржаво-подзолистые и подзолистые почвы, а также почвы, в которых процессы подзолообразования сильно маскируются другими процессами (нпр. почвы с признаками андосолей). Подчеркивается роль аморфных веществ в формировании трофических свойств почв. A . V A S U , C. N E D E A
ZASTO SO W A N IE TR Ó JSK Ł A D N IK O W Y C H DIAG R A M Ó W Е -p H PRZY W Y B O RZE EK ST R A H EN TÓ W A M O R FICZN Y C H ZW IĄZKÓW ŻELA ZA I G LIN U
W SPOD IK O W YC H G LEBA C H R U M U N II
In sty tu t G leb o zn a w stw a i C h em ii R olnej w B u k a reszcie P o litech n ik a w B u k areszcie
S t r e s z c z e n i e
P rzed sta w io n o m o żliw o ść w y k o rzy sta n ia diagram ów tró jsk ła d n ik o w y ch typu p ierw ia stek —w o d a — ligan d i d w u sk ła d n ik o w y ch typ u p ierw ia stek —w od a do u sta la nia zak resów w zględ n ej d om in acji w ek stra k cie g leb o w y m ok reślon ych zw iązk ów
żelaza i glinu. N a diagram ach p rzean alizow an o u k ła d y d w u sk ła d n ik o w e F e(A l) — w od a oraz tró jsk ła d n ik o w e F e -w o d a -lig a n d (pirofosforan, cytryn ian , szczaw ian ) i A l-w o d a -lig a n d (cytranian, szczaw ian ), a tak że A l-w o d a -w o d o ro tlen ek sodu. W y n i ki p rzep row ad zon ych badań p o tw ierd ziły p rzyd atn ość op racow an ych diagram ów do w yboru n a jo d p o w ied n iejszy ch w a ru n k ó w ek stra k cji i w b rew zalecen iom n ie k tó rych au torów w y k a za ły zbędność d od aw an ia pod siarczym i (dithionitu) do roztw orów ek stra k cy jn y ch przy w y d z ie la n iu z g leb y n iek rzem ia n o w y ch form żelaza i glinu.
K orzystając z diagram ów i od p ow ied n io dob ierając w a ru n k i ek stra k cji u zysk an o m ożliw ość u stalen ia ch em iczn ych k ry terió w p o zw a la ją cy ch na ob iek ty w n e rozgra n iczen ie gleb sk ry to b ielico w y ch , b ielico w o -rd za w y ch , rd za w o -b ielico w y ch i bielic, a także gleb, w k tórych p rocesy b ielico w a n ia są siln ie m a sk o w a n e przez inne p roce sy (na przyk ład g leb y z cech am i andosoli).
P od k reślon o rolę su b sta n cji b ezp o sta cio w y ch w k szta łto w a n iu troficzn ości gleb.
D r A l e k s a n d r a V a s u R e s e a r c h i n s t i t u t e f o r S o il S c i e n c e and. A g r o c h e m i s t r y B u c h a r e s t
