Selective extraction of trace metals associated with hydrous aluminum oxides

JANECE SLAVEK A N D WILLIAM FREDERICK PICKERING Chemistry Department, Uriiversity of Newcastle, N.S. W., 2308, Australia

Received September 29, 1986

This paper is dedicated to Professor Douglas E. Ryarz on the occasiorz of his 65 th birthday

JANECE SLAVEK and WILLIAM FREDERICK PICKERING. Can. J . Chem. 65, 984 (1987). In soil/sediment analysis, subdivision of trace metal content into different categories is usually based on selective extraction

schemes. To assess the disposition of metal ions bound to aluminum hydrous oxides in such schemes, suspensions of AI(OH)3 gel, gibbsite, or alumina were loaded with up to 5 pmol of Cu, Pb, Cd, or Zn ions prior to being extracted for 24 h with one of fifteen different chemical solutions. The percentage of sorbed ion retrieved varied along the reagent sequence: NaCl, CaClz < MgClZ, NH4N03 < CH3COONH4, Na citrate, Na4P207 < EDTA, DTPA < CH3COOH, H2C204, HC1, HN03. In each system, the recovery value varied with the initial surface loading (a function of sorption pH) and reflected changes in metal species form, e.g., bonded M2', (MOH'), M(OH)2. With low loading levels up to 40% was displaced by salt solutions; with 1 to 2 pmol sorbed, as little as 10% was displaced by acids or complex formers but this increased to -90% with higher loadings. The relationship between sorption pH, amount sorbed, and extraction value was complex, and since in selective extraction schemes classification is based on recovery values, changes in initial retention parameters (e.g. system pH) lead to varying fractions of the different metal ions being classified as "ion exchangeable", "chemisorbed", and "incorporated in the lattice".

JANECE SLAVEK et WILLIAM FREDERICK PICKERING. Can. J. Chem. 65, 984 (1987) Lors de l'analyse des sols et des sCdiments, la subdivision des Cchantillons en diverses categories d'tlCments contenus a I'Ctat

de trace est gCnCralement basCe sur des schtmas d'extractions ~Clectives. Pour Cvaluer la disponibilitk des ions mCtalliques liCs a des oxydes hydratCs d'aluminium dans de tels schCmas, on a chargC des suspensions de Al(OH)3, de gibbsite ou d'alumine avec des quantitCs allant jusqu'i 5 pmol d'ions Cu, Pb, Cd ou Zn avant de les extraire pendant 24 h avec l'une des solutions chimiques dCcrites plus loin. Les pourcentages d'ions adsorbCs que I'on a pu rCcupCrer varient selon la skquence suivante des rCactifs : NaCI, CaCll < MgCI,, NH4N03 < CH3COONH4, citrate de Na, Na4P207 < EDTA, DTPA < CH3COOH, H2C204, HCI, HN03. Dans chacun des systkmes, la quantitC de produit qui est rCcupCrCe varie avec le chargement initial de la surface (qui est une fonction du pH de l'adsorption) et elle est un reflet des changements dans la forme des esptces mCtalliques, comme M2', (MOH') ou M(OH)2. Lorsque les quantitCs de mttaux qui sont chargees sont faibles, des niveaux allant jusqu'a 40%, les mCtaux sont dCplacCs par les solutions de sels. Lorsqu'on fait adsorber de 1 a 2 pmol, il n'y a que 10% des mCtaux qui sont dCplacCs par les acides ou par les composCs pouvant former des complexes; toutefois, cette quantitC croit jusqu'a 90% avec des chargements plus ClevCs. La relation entre le pH d'adsorption, la quantitC adsorbee et la valeur de rCcupCration est complexe; puisque la classification des schCmas d'extractions sClectives est bake sur les valeurs de rCcupCration, des changements dans les paramktres de rttention initiale (comme le pH du systkme) conduisent a des fractions variables d'ions mCtalliques diffkrents qui sont classifiCs comme "Cchangeables", "chemisorbCs" ou "incorporCs au rCseau".

[Traduit par la revue]

Introduction Only a small fraction of the trace element content of soils and

sediments is liable to be "biologically active". In recognition of this, analysts have developed a series of chemical extrac- tion schemes which attempt to subdivide total contents into segments which bear labels such as "detrital (or residual)", "non-detrital", "ion exchangeable", "bound to carbonates", "organically bound", or "associated with hydrous oxides". In respect to the latter group, most of the extended "fractionation" procedures include steps which help identify the fraction of total element content associated with the Mn(1V) and/or Fe(II1) oxide components (1, 2). None of the analytical methods, however, attempt to identify the amount associated with hydrous aluminum oxides, a soil/sediment component formed during weathering of alumino-silicate minerals or via hydrolysis of leached aluminum salts. The Al-hydroxy species can control the pH of acid soils (3) and can sorb both metal ions (4, 5) and anions (e.g. F- (6)).

The omission of analytical procedures for identifying the Al-bound metal fraction could arise from the bland assumption that treatments which disrupt Fe(II1) or Mn(IV) oxides should simultaneously release ions bound to A1 oxides. Alternatively, it may arise from limited knowledge of sorbed metal ion behaviour.

The chemical and structural form of the A1 species found in environmental samples depends on the source, and the condi-

tions prevailing during the aging cycle. The hydrogel [Al(OH)3] formed by hydrolysis of Al(II1) salts (pH 3 to 9) spontaneously loses water to form the polyhydroxide known as p-aluminum hydroxide. On aging, this gradually converts to boehmite (A100H) and gibbsite (A1203.3H20) (7, 8). These transitions are accompanied by: ( i) a change from amorphous to crystalline structural form. During the structural rearrangement, incorpora- tion of favored ion (e.g. Mg, Ni, Zn) can occur, and it has been postulated (9) that this results in two types of cation adsorption, namely, specific and non-specific (i.e. displaceable by ~ a ~ + ) ; (ii) a significant decrease in acid or base solubility; and (iii) a reduction in the adsorptive capacity for metal ions (e.g. freshly precipitated gel's capacity for zn2+ (- 160 mmol/kg) was found to be tenfold that of aged oxide, and corresponded to a tenfold difference in surface area (10)).

During the sorption of metal ions H+ are displaced, and the specific sorption of divalent cations to fresh gel may involve coordination to two (or even three) surface-OH groups. Metal ion uptake tends to vary from very little to near total uptake over a narrow (1-2 unit) pH range, and if selectivity is defined in terms of the pH required for 50% retention (pH5o), then the sequence is Cu(4.8) > Pb(5.2) > Zn(5.6) > Ni(6.3) > Co(6.5) > Cd(6.6) > Mg(8. I ) > Ca > Sr(9.2) > Ba (4.5).

On a-A1203, sorption of Zn, Mn, Co, or Ba from < l o p 7 &I solutions follows a log adsorption isotherm, with the pH required for 50% retention being virtually independent of

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SLAVEK AND PICKERING

TABLE 1. Percentage of total A1 in substrate dissolved by extracting agents

Sorbed Metal Pb Cu Zn Cd

Substrate* A B A B A B A B

0.5 M HC1 75 1 M HN03 78 0.5 M CH3COOH 82 0.25 M CH3COOH/

0.25 M CH3COONH4 62 0.5 M (COOH)2 87 0.1 M (COOH)2/

0.18 M (COONH4)~ 80 0.1 M Na4P207 33 1.0 M Na3 citrate 59 0.005 M EDTA 27 0.005 M DTPA 24 1 M CH3COONH4 3 1 M NH4N03 1 1 M MgClz 2 0.05 M CaC12 1 0.5 M NaCl 3

*Substrates: A, AI(OH), gel, aged 24 h; B, a-A1203. With gibbsite, Al detected was extremely low; e.g. < 1% with acid solutions.

concentration (e.g. for Zn, pHso - 6.8) (1 1). With Cd sorption, the pHso value shifts to lower regions (e.g. 8.0 to 6.7) when the a-A1203 suspension concentration is increased (e.g. from 2 to >20g/L) (12).

y-A1203 uptake behaviour is altered by the presence of nitrolotriacetate (NTA), yielding an affinity sequence such as C u > Z n > Cd > Pb,Ni(13).

In the corundum-type structure of a-A1203 a layer of A1 atoms lies between two layers of oxygen atoms; the y modification has a spinel-type structure in which the 0 atoms have an approximate face-centred cubic order with A1 atoms occupying half of the octahedral interstices.

It has been proposed (14) that synthetic Fe and A1 hydrous oxides serve as useful models for some of the species found in the environment, and in view of the role that these materials can play in soil/sediment chemistry, improved understanding of metal ion desorption processes was deemed to be highly desirable. The results obtained in systematic examinations of the Fe and Mn systems have been published (15,16). This paper outlines the extraction behaviour observed using hydrous A1 oxides loaded with varying amounts of Cu, Pb, Cd, or Zn ions, and a selection of the reagents used in analytical speciation schemes.

Experimental Adsorbents

A suspension of AI(OH)3 was prepared (as required) by slowly adding NaOH solution to vigorously stirred M I 6 0 AI(N03)3 until the pH was -7. The suspension was kept well stirred and aliquots taken for transfer to vials ( - 13 mg solid/ 10 cm3).

A suspension of A.R. aluminum hydroxide powder (May and Baker, 36% H 2 0 ) containing 2 g L-' was prepared as required. X R D examination of the powder showed it to be predominantly gibbsite.

Suspensions of alumina (2 g L-I) were prepared from chromatog- raphy grade A1203, supplied by May and Baker.

of the 5 pmol of M 2 + subsequently added (as 10.0 cm3 of 5 X M Me(N03)2 solution). With fresh gel and alumina there was a marked pH effect on uptake in the vicinity of and accordingly the amounts sorbed varied between individual vials. Each test solution was diluted to 25.0 cm3 with demineralised water and the vial was capped before being mixed overnight in an end-over-end stirring unit. The pH of the systems stabilized after about 12 h of mixing.

After this equilibration period, the solid phase was collected on a 0.45 p m (25 rnm diameter) membrane (using an Amicon Vacuum manifold unit) and washed with 5.0 cm3 water. The pH of each aqueous phase was measured using a Philips@ Meter, Model 9407, and the Pb, Cu, Zn, or Cd content was determined using a Varian@ AA875 Atomic Absorption Spectrometer and appropriate C2H2, air flame conditions. (For A1 determinations a C2H2-N20 flame was utilised.)

Each filter membrane plus transferred solid was returned to the original vial, and 30.0 cm3 of extracting solution (selected from the range listed on Table 1) was then added. Care was taken to ensure minimal loss of solid to the walls of the filter vessel. To compensate for random losses or experimental variations, each extraction study was replicated (i.e. more than six samples per reagent). To identify loading effects, most studies were repeated using different initial NaOH additions (i.e. system pH values), in order to vary the amount of MZf sorbed by the substrates. As the pH required for onset of significant adsorption varied with substrate nature and the metal ion involved, suitable conditions had to be determined from trial runs.

The vials containing the solid, membrane, and extractant solution were equilibrated overnight at 25OC in the end-over-end mixer before being centrifuged (and filtered when necessary) to yield a clear aqueous phase which could be analysed for metal content. The AAS unit was calibrated by adding known amounts of metal ion to the sameextractant solution as involved in the test study.

The percentage recovery of metal ion was calculated by comparing the pmol M" found in the extractant against the pmol sorbed in the uptake stage.

Determination of the A1 content of extractant solutions provided an indication of the solubility of the substrate in the various reagents, and these results are summarized on Table 1.

Extraction procedure Results Aliquots (10 cm3) of Al(OH)3 gel or solid suspension (i.e. 13 mg

AI(OH)~ gel or 20 mg crystalline solids) were transferred to acid- With Al(OH)3 gel and A1203, the effect of pH on uptake washed glass vials. An appropriate volume of 0.01 M NaOH was then followed the pattern reported by earlier workers, that is, uptake added to raise the system pH and ensure adsorption of a-high proportion increased from low values io near total retention over a narrow

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986 CAN. J . CHEM. VOL. 65, 1987

C 0 + u 80 - m L + X 6 0 7 0

W _ - - a',

y m o l M e t a l I o n Sorbed FIG. 1. Effect of amount of metal ion sorbed by substrates on the

percentage recovered by 24 h extraction at 25°C. A , Cd displaced by NH4N03 ( a , 0, 0) or CH3COONH4 (A, A, A). Symbol shading represents data using gibbsite, A1203 or Al(OH)3 gel, respectively. B, Zn displaced using different acid solutions, e.g. HC1 ( W , I]), HN03 (*, O) , CH3COOH ( A , A), or (C00H)2 (V, V). The extraction values found using Al(OH)3 gel as substrate fell within the zone defined by sloped shading. C, Cu displaced using different complex forming extractants e.g. EDTA (a, O) , DTPA ( A , A), or sodium citrate (U, 0). The recoveries obtained using the same reagents on AI(OH)3-sorbed metal ion fell within the area outlined by vertical shading.

pH range. However, in the presence of the aluminum substrates, metal ion hydrolysis continued overnight, as shown by a drop in pH which ranged from - 1 pH unit using gel to -0.3 pH units with suspensions. These changes, plus aging effects caused the pHso values to vary marginally from those quoted in the Introduction. With gibbsite, slow partial dissolution of the solid (possibly accompanied by secondary formation of gel) led to wide variations (e.g. 15%) in uptake at most pH values. Hence plots of amount sorbed versus pH yielded broad zones rather than steeply sloping curves.

Due to the significant effect of pH on metal ion uptake in all of the systems studied, each replicate tended to adsorb different amounts of metal ion, and this was found to lead to variations in the amount of sorbed metal ion recovered bv extraction.

Figure 1 shows the effect of amount sorbed on the percentage recovered in three different systems, and highlights the distinct minimum in uptake values discerned in some of the extraction studies. This minima is considered to correspond to maximum uptake of a MOH+ species. At higher loadings increasing amounts of M(OH), should exist. With lead sorbed on Al(OH)3, the minimum release zone was followed at higher coating levels by an abrupt increase in the percentage extracted. In many of the copper and zinc studies, the percentage of metal ion retrieved increased almost linearly as coating levels increased. By contrast, recovery values using the chloride salts

E X T R A C T A N T S

FIG. 2. Block diagram showing the effect of substrate and extrac- tant type on the percentage of sorbed Cu or Cd recovered. Each rectangle indicates the spread of recovery values obtained (covers different amounts sorbed) and shading indicates the substrate: vertical ( I I I I I ) , A1203; horizontal (E), AI(OH)3 gel; and open box, gibbsite. Reagent code: 1, 0.5 M HCl; 2 , 1 M HN03; 3 , 0.5 M CH3COOH; 4 , 0.25 M CH3COOH/0.25 M CH3COONH4; 5 , 0.5 M HzC204; 6, 0.1 M H2C204/0.18 M (NH4)?C204; 7, 0.005 M EDTA; 8, 0.005 M DTPA; 9, 1 M sodium citrate; 10, 0. l M Na4P207; 11, 1 M CH3COONH4; 12, 1 M NH4N03; 13, 1 M MgC12; 14, 0.5 M NaCl; and 15, 0.05 M CaCl?.

of Na, Ca, or Mg, tended to continually decline as the amount of metal ion initially sorbed increased, that is, there was a gradual change from the maximum values shown on Figs. 1 and 2 (reagents 13 to 15) to the lower limits indicated by the rectangles.

The trends were complex, and explanation difficult, hence detailed consideration of this aspect has been deferred to a later publication.

The loading effect contributed significantly to the range of recovery values reported in Figs. 2 and 3. On these diagrams the spread of values obtained is indicated by the length of the rectangular plots. Figure 2 highlights the effect (if any) of substrate nature on the extractability of sorbed copper and cadmium ions (Pb and Zn responses were generally similar to one or other of these two studies).

Figure 3 summarises the effect of metal ion on the amount recovered by different extractants using partially aged Al(OH)3 as substrate.

Other factors, besides "loading effects", contributed to the broad spread of results. Perusal of the points shown on Fig. 1, shows that recoveries varied widely even when the amount sorbed by replicates varied little. Partly to blame are experi-

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SLAVEK AND PICKERING 987

EXTRACTANTS

FIG. 3. Diagrams showing the ability of different extractants to retrieve metal ions sorbed on Al(OH)3 gel. Metal ion code: Pb, a; Cu, B;Cd, 0 ;andZn, H. Reagentcode, I to 15, sameasforFig. 2.

mental design limitations (e.g. gel filtration problems, colloid loss during substrate isolation, aging effects: and calculations based on analysis differences). Similar problems were encoun- tered in earlier studies using Fe(II1) and Mn(IV) oxides (15, 16), but in these investigations precision was far better (e.g. 2 5 to 10% deviation from a mean value).

The "solubility" of the substrate also contributed to system behaviour. As noted in the Introduction, changes in gel structure during aging reduces solubility. In our study, aging (-48 h total) plus chemical interaction with metal ions at the surface reduced gel solubility in acids by about 25% (cf. Table 1) . The values quoted are the mean of 5 to 20 determinations, with reproducibility being around 2 10%. Variations in matrix dissolution could have contributed to varying amounts of "occluded and/or chemi-sorbed metal ion" being retrieved.

The alumina solubility was quite low, and with gibbsite suspensions (which only slowly reach saturation values (17)), dissolved A1 was close to the AAS detection limit. With these materials, metal ion recovery levels were often lower than observed with gel, but loading effects blurred any substrate based trends.

Discussion Selective extraction of metal iotzs sorbed on cllumitzum hydrous

oxides A primary aim of this study was to elucidate the conditions

required for release of any metal ion associated with aluminum oxide or hydroxide species in soils and/or sediments.

The diverse results obtained in this controlled study indicate, however, that sorbed metal ions are bonded by several different mechanisms, and the initial loading was only occasionally fully retrieved (cf. upper limits of rectangles drawn on Figs. 2 and 3).

The reagents used included strong acids and EDTA, systems recommended for distinguishing between detrital and non- detrital metal fractions.

The detrital fraction consists mainly of fragments of the original ground rock (in which metals are usually present in lattice structures); the non-detrital material contains weathering products of fine particle size with a high sorptive capacity for metal ions. The sorbed ions may be bound through electrostatic attraction (ion exchangeable), by less definable weak bonding or by compound formation (chemisorption). The reagents tested included many recommended for identifying ion exchangeable fractions, weakly sorbed material and bound to the hydrous oxides of Fe and Mn(IV).

\ ,

In terms of extraction behaviour, the responses could be grouped under four major headings, namely, acids, complex forming ligands, ammonium salts, and chloride salts.

The last two groups embrace reagents used (by some) to evaluate the ion exchange capacity of soils or to estimate the amount of ion exchangeable metal ion present in sediments. A study of Figs. 2 and 3 show that up to 80% of sorbed Pb, Cu, or up to 60% of Cd was occasionally displaced by these reagents. The recovery value varied with the nature of the substrate (Fig. 2), element of interest (Fig. 3), the amount initially sorbed (Fig. 1) and the reagent selected. One ion in this group ( M ~ ~ + ) was very effective in displacing Pb2+, a result that suggests that a high proportion of sorbed lead was non-specifically bonded.

The enhanced ability of ammonium salts to effectively displace sorbed metal ions may be due to p~eferential sorption of the small NH4+ ion (ionic radius - 2.5 A), complemented by the tendency of Cu, Cd, and Zn to form ammine complexes (with H+ transfer to surface hydroxyl groups). With acetate salts, formation of aceto-complexes may also contribute to recovery values.

The effective stability of any metal complexes formed, relative to substrate bonding strength, strongly influenced the amount recovered, as shown by the variable responses observed using chelating agents (e.g. EDTA, DTPA, citrate) and complex formers (e.g. oxalate, pyrophosphate). All these reagents form complexes whose structure, charge, and effective stability is a function of system pH and ligand concentration. Some products had limited solubility (e.g. Zn2+ with citrate, Pb2+, and (COOH),). As shown in Table 1, these reagents also partially dissolved gel substrates.

If one accepts the oft assumed premise that complex-forming ligands release both displaceableand chemi-sorbed metal ions, while chloride salts release only the exchangeable fraction, then comparison of the recoveries listed on Figs. 2 and 3 indicates that up to half of the Cu, Pb, and Zn was chemi-sorbed; with Cd non-specific sorption appears to have predominated.

Acid reagents also had the potential to release both weakly held and chemi-sorbed ions, through a combination of substrate dissolution and preferential sorption of H f (aided by metal complex formation when using acetic and oxalic acid systems). The total recovery predicted was not achieved, however, due to part of the sorbed material becoming incorporated in a crystal lattice or converted into a sparingly soluble product. The fraction in this category was influenced by the amount of metal ion initially sorbed (cf. Fig. 1). -.

Specific sorption persists down to quite low initial metal ion concentrations, with at least three bonding modes still involved. In anodic voltammetry studies, using p g L-' levels of metal ion and acetate buffer solutions, it was found (18) that added Al(OH)3 gel sorbed significant amounts of the nanomoles of metal ion present (even though it existed in solution primarily as aceto-complexes). Sorption by a-Al2o3 from <lo-' M

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988 CAN. 1. CHEM. VOL. 65, 1987

solutions (1 1) and uptake by y-A1203 in the presence of the chelating agent NTA (13) was noted earlier.

With metal ions sorbed on alumino-silicates (e.g. kaolin, illite, montmorillonite) extraction behaviour resembled that reported in this paper in many respects. Most of the sorbed metal ion was strongly held, with pH 3 ammonium oxalate being the most efficient releasing agent, followed by EDTA and acid solutions ( 19).

Analytical schemes The absence of specific mention of aluminum-bound metal

ion in any of the published selective extraction procedures (cf. refs. 1 and 2) is now understandable, since no single reagent appears to have the capacity to consistently achieve optimum release of this particular sub-category.

Treatment of sediments by any of the published sequential procedures would lead to A1 associated material appearing in different segments of the scheme, e.g., a significant part could appear with the ion exchangeable fraction, some would be classified as weakly sorbed, part would be released with the Fe, Mn hydrous oxide sediment, and varying amounts would be detected in the "residual" sub-group.

At this point, it should also be noted that satisfactory procedures for differentiating between extractable forms of A1 have yet to be developed. It has been reported, for example, that the approach used for extractable Fe (based on oxalate, dithionite, and pyrophosphate extractions) was less useful for distinguishing between the various forms of A1 in soils (20). A more recent study demonstrated that a 4 h extraction with acidified (pH 3) ammonium oxalate was the most efficient single procedure for assessing the amount of translocated Al, Fe, and Si present as inorganic compounds in the non-crystalline weathering products found in podzol B horizon samples (2 1). A pyrophosphate extraction provided an estimate of the organic bound A1 fraction. For evaluating the amount of poorly ordered alumina gels in samples, extraction with cold 5% NaZC03 has been proposed (22).

As shown in our investigation, the precision of determina- tions based on "selective extraction" can be quite poor. With real samples, this variability may be matched by sampling errors or sample heterogeneity. In addition, only a small fraction of the trace element content may be bonded to A1 species.

Bonding modes Factors which contribute to the observed extraction behav-

iour include the different types of bonding sites at the surface of aluminium oxide/hydroxide substrates, variations in the chemical form of the sorbed metal ion, ion diffusion into gel structures, and surface charge. Freshly precipitated A1 gel has a zero point charge value of around 9.4 (4) while the value quoted for a-A1203 is 8.8 (1 1) and since the system pH in the sorption studies was lower than these values the substrates should have maintained a net positive charge during the various sorption cycles.

As pH increases (and more metal ions are sorbed) chemi- sorption (involving bonding to Al-OH sites) becomes more pronounced. This is preceded or accompanied by proton loss from coordinated water molecules, yielding hydroxy species in which the average O H - : M ~ + ratios vary from near zero to two (when all the initial M Z f has been sorbed or precipitated). To explain the minima in extraction values noted in some studies (cf. Fig. I), the solubility of one of the intermediate species (e.g. SMOH where3 is substrate surface) must be lower than that of other species.

In the presence of kaolinite, illite, or montmorillonite clay particles the term p M + pOH remained virtually constant, up to the pH5, point (23). This suggested that the sorbed species was probably a polymer having the general formula (MOH),r. When over half of the total metal content had been sorbed/precipitated the termpM + 2pOH became constant, which implied that both sorbed metal ion and precipitated excess had the empirical formula M(OH),. The presence of particulate matter tends to lower the pH riquired fo r metal-hydroxide formation, and the pH associated with 50% precipitation (pHSo) varies with substrate composition (for example, using M solutions Pb (or Cd) pH,, values range from 3.1 (or 5.8) on hydrous Fe oxide (5) to 5.9 (or 7.3) on clay minerals (19)).

Terminology in extraction studies For quantitative recovery of sorbed metal ions, an extractant

must have the capacity to dissolve aged metal hydroxy polymers, disrupt chemisorption bonds, release occluded material, and displace electrostatically attracted species.

Any reagent capable of performing some, of all, of these tasks in respect to aluminum bound species, will undoubtedly have the potential to displace the same element from other sub- strates, particularly clays and hydrous iron oxides. Accordingly selective extraction of the aluminum bound fraction appears to be an unattainable goal.

This conclusion highlights the inappropriateness of some of the terminology used in chemical extraction studies. Categories such as hydrous oxide bound, organic bound have limited significance (due to varied bonding modes) and it would be preferable to standardise on a reagent based system (e.g. acetate-buffer soluble; released by pH 3 ammonium oxalate; released by HN03, H Z 0 2 attack; and so on).

For reasons outlined in review articles, it is equally impor- tant to closely specify experimental conditions, for example, reagent concentration, solid-to-liquid ratios, time of contact, and temperature.

Selective extraction procedures have useful applications, but their limitations have also to be recognised.

Acknowledgement This investigation received financial support from the AustTalian

Research Grants Scheme, and this assistance is acknowledged with thanks.

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20. J. A. MCKEAGUE, J. E. BRYDON, and N. M. MILO. Soil Sci. Soc. Am. Proc. 35, 33 (1971).

21. V. C. FARMER, J . D. RUSSELL, and B. F. L. SMITH. J. Soil Sci. 34, 571 (1983).

22. V. C. FARMER, B. F. L. SMITH, and J . M. TAIT. Clay Miner. 12, 195 (1977).

23. H. FARRAH and W. F. PICKERING. Chem. Geol. 25,317 (1979).

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