Sorption of copper and cadmium by allophane-humic complexes

Developments in Soil Science, Volume 28A Editors: A. Violante, P.M. Huang, J.-M. Bollag and L. Gianfreda © 2002 Elsevier Science B.V. All rights reserved.

37

SORPTION OF COPPER AND CADMIUM BY ALLOPHANE- HUMIC COMPLEXES G. Yuan, H.J. Percival, B.K.G. Theng and R.L. Parfitt Landcare Research, Private Bag 11052, Pahnerston North, Nev^ Zealand

We have investigated the sorption of Cu and Cd by allophane and its complexes with a soil humic acid (HA), varying in carbon content from 14-123 g kg'\ The sorption of Cu and Cd was measured at pH 5.0 and 5.5, using the batch technique and a single concentration (2mM) of Cu or Cd in the presence of LiC104. The sorbed metals were partly desorbed by treatment with 0.1 M KNO3. Allophane sorbed 71 and 179mmolCukg"^ at pH 5.0 and 5.5, respectively. By comparison, only 3 mmol kg"^ of Cd was sorbed at pH 5.0 and 9 mmol kg"^ at pH 5.5. Irrespective of pH, complex formation with HA led to a marked increase in Cu and Cd sorption. Indeed, sorption increased linearly with the amount of HA sorbed. Less than 20% of the sorbed Cu, and 49-94% of sorbed Cd, was desorbable by KNO3. Copper appears to be specifically sorbed by allophane through coordination with exposed hydroxyl groups because sorption can occur even when allophane was positively charged. Moreover, the sorption capacity for Cu increased with pH, and the majority of the sorbed Cu was not desorbable. In contrast, the sorption of Cd by allophane largely occurs by electrostatic interactions. The relative contribution to metal sorption of allophane and HA in allophane-HA complexes was derivedfromthe linear relationship between sorption and HA content of the complexes. The HA content at which there is an apparently equal (50/50) contribution of allophane and HA to Cu sorption is 28 and 87 g C kg"^ allophane at pH 5.0 and 5.5, respectively. Above these apparently "equal-contribution indicator" values the HA component in the complex contributes more to sorption than does the allophane counterpart. For Cd the apparently "equalcontribution indicator" values are 1.9 and 5.2 g C kg'^ allophane at pH 5.0 at pH 5.5, respectively. This observation has implications for soil management in terms of permissible loads for heavy metals and practical means of increasing the sorption capacity of soil.

1. INTRODUCTION Copper (Cu) and cadmium (Cd) in soils may be derived from parent materials or introduced through such human activities as timber preservation [ 1 ] and the application of phosphate fertilizers [2]. The mobility of heavy metals in soil, and their availability to plants, are greatly influenced by sorption to soil constituents, of which clay and humus are the most active [3-4]. There is circumstantial evidence to indicate that the organic matter (humus) in soil plays an important role in heavy metal sorption [5-7]. However, as McBride et al. [8] have pointed out, metal sorption experiments often fail to reveal a strong correlation between organic matter content and either metal solubility or metal sorption capacity. This is possibly because the effect of organic matter is difficult to separatefromthat of pH, the soils used have a range of organic matter contents, and soil organic matter is chemically heterogeneous [8].

38 Often the dominant mineral constituent in the clayfractionof New Zealand soils derived from volcanic ash, allophane is a short-range ordered hydrous aluminosilicate mineral. Its primary or unit particle is a hollow spherule with an outer diameter of 3.5-5.0 nm and a wall thickness of ca 0.7 nm [9-10]. The spherule wall is composed of a curved A1-0,0H octahedral (gibbsitic) sheet to which orthosihcate [(O3) Si(OH)] groups are attached on the inside. Structural defects within the wall give rise to «0.3 nm wide perforations [11] where (0H)A1(H20) groups are exposed. These groups can acquire protons on the acid side, and lose protons on the alkaline side, of the point of zero charge, and hence are responsible for the pH-dependent variable-charge characteristics of allophane. Allophane can sorb metal cations, including Cu and Cd ions, through electrostatic (coulombic) interactions and by inner-sphere coordination involving exposed hydroxyl groups [12-13]. Since soil organic matter can also sorb metal ions, allophanic surface soils have a large propensity for taking up Cu and Cd ions [14]. Clay and organic matter in soil, however, are closely associated to form a clay-organic complex. As such, their individual involvement in heavy metal sorption, and contribution to the sorption capacity of soil, are difficuU to assess. Here we investigate the sorption of Cu and Cd by allophane and its complexes with humic acid (HA). By using allophane complexes with varied amounts of sorbed HA, we can evaluate the apparent contributions of allophane and HA in the allophane-HA complex to metal sorption based on HA quantity.

2. MATERIALS AND METHODS Both the allophane and HA used in this study were obtained from New Zealand soils using conventional methods of clay separation and organic matter extraction. The allophanic clay was obtained from the C horizon of the One Tree Point Sand which classifies as an Aerie Alaquod in Soil Taxonomy [15] and as a Humus-pan Perch-gley Podzol in the New Zealand Soil Classification [16]. The humic acid was obtainedfromthe Koputaroa Sand which is a Typic Udipsamment in Soil Taxonomy or an Acidic Sandy Brown Soil in the New Zealand Soil Classification. Details of the location of the sampling sites, together with the procedures for clay separation and HA extraction/purification have been given by Yuan et al. [17]. The allophane content of the clay, estimated both by infrared spectroscopy and acid-oxalate extraction [18], was 850 g kg'^ The allophanic clay (subsequently referred to as "allophane") contained trace amounts of iron (3 g kg'') together with quartz and kandite impurities. The carbon content of the allophane was 57 g kg"' and the nitrogen content was 1.1 g kg'. The surface charge of the allophane was determined by ion adsorption [ 17] and the results are given later. The purified HA contained 530 g C kg"', 60.7 g N kg"', 8.3 g S kg"', 12 g ash kg', and had a COOH acidity of 260 cmolc kg"' as determined by direct titration [19]. Allophane-HA complexes with HA contents of 14-123 g OC kg"' were obtained by adding a solution of HA to a suspension of allophane in 2 mM CaCl2 [17]. Sorption of Cu and Cd by allophane and allophane-HA complexes was measured at pH 5.0 and 5.5, using the batch technique and a single concentration (2 mM) of Cu or Cd. Briefly, allophane (100 mg) or allophane-HA complexes containing 100 mg of allophane were mixed with 30 mL of 2mM Cu(C104)2 or 2mM Cd(C104)2, in the presence of 2mM LiC104 as background electrolyte. The pH of the suspensions was initially adjusted to 5.0 and 5.5 by adding HCIO4 or LiOH and re-adjusted twice during the 24-h period of shaking at 20 ± 1 °C. Adjustment of pH caused < 2% change of the initial 30 mL volume, and corrections for the quantity of Cu and Cd present in the final solution were made

39 accordingly. At the end of shaking, the suspension was centrifuged for 12 min at 18,000 rpm (38,700 X g). Concentrations of Cu and Cd in the supernatant solutions were determined by atomic absorption spectrophotometry (AAS). The amount of Cu and Cd sorbed were calculated from the difference between the amount of Cu and Cd initially added to, and that measured at equilibrium with, the allophane and allophane-HA complexes. Blanks at pH 5.0 and 5.5 were prepared to allow for any precipitation. Sorption experiments were conducted in triplicate. To assess the possibihty of formation of a hydroxide or hydroxy carbonate precipitate during sorption experiment, the logarithms of the Cu^^ and Cd^"^ activities were calculated using the extended Debye-Hiickel equation and were compared with the solubilities of several Cu and Cd minerals. Solubility lines of Cu and Cd minerals were determined from the data of Lindsay [20], assuming atmospheric partial pressure of CO2. Desorption was carried out (in triplicate) by dispersing the allophane and allophane-HA complexes manually in 30 mL of 0.1 M KNO3 solution, shaking for 24 h at 20 ± 1 °C, and centrifiiging the suspension for 12 min at 18,000 rpm (38,700 x g). The concentration of Cu and Cd in the supernatant solutions was determined by AAS, allowing the amount of Cu and Cd desorbed to be calculated after correcting for entrained Cu and Cd. The composition of the HA and the organic matter associated with the allophane sample was analysed by pyrolysis-field ionization mass spectrometry (Py-FMS) as described by Schulten and Leinweber [21]. Sample crucible was heated automatically with a heating rate of 10°C/scan in the temperature range from 100 to 700 °C. The thermal degradation of the sample is indicated by the thermogram which reflects the purity, thermal stability and weight loss (volatile matter). Ten classes of organic compounds (EXl - EX 10) were identified according to their marker signals (mass-tocharge ratios). The contents of each class was estimated from the respective ratio of ion intensity to total ion intensity.

3. RESULTS Figure 1 shows that all metal-allophane/complex systems were undersaturated with respect to the least soluble hydroxide or hydroxy carbonate phase, precluding the possibility of precipitate formation involving Cu or Cd hydroxide or hydroxy carbonate. This conclusion was fiirther supported by the lack of a detectable change in metal concentrations of the blanks. Figure 2 shows that allophane by itself can sorb much Cu. As pH increased from 5.0 to 5.5, sorption increased from 71 to 179 mmol kg'^. Irrespective of pH, complex formation with HA led to a marked increase in sorption. Indeed, sorption (S) increased linearly as the amount of HA sorbed increased from 14 to 123 g organic carbon (OC) kg'^ allophane. The best-fit relations were: S (mmol Cu kg"^ allophane) = 65.4 + 2.38 OC (at pH 5.0, R^ = 0.992, P<0.01)

(1)

S (mmol Cu kg"^ allophane) = 192.6 + 2.22 OC (at pH 5.5, R^ = 0.992, P<0.01)

(2)

Figure 3 shows that much less Cd than Cu was sorbed by allophane and its complexes under the same experimental conditions. For allophane by itself, 3.1 mmol kg"^ of Cd was sorbed at pH 5.0

40

and 9.2 mmol kg '^ at pH 5.5. These values are more than one order of magnitude smaller than for Cu. The effect of pH on Cd sorption was also relatively small. The linear relationships between Cd sorption and the amount of HA complexed with allophane may be expressed by Equations (3) and (4): S (mmol Cd kg"^ allophane) = -5.1 + 1.61 OC (at pH 5.0, R^ = 0.996, P<0.01)

(3)

S (mmol Cd kg "^ allophane) = 10.1 + 1.78 OC (at pH 5.5, R^ = 0.998, P<0.01)

(4)

CU3(0H)2(C03)2 Cu(0H)2

CU2(0H)2C03 ^ -2

4.5

5.0

5.5

6.0

5.0

4.5

pH

5.5

6.0

pH

Figure 1. Solubility of Cu and Cd (log(M^^) after 24-h equihbration of allophane and allophane-HA complexes with (a) 2mM Cu(C104)2, and (b) 2mM Cd(C104)2.

500

250 T

20

40 60 80 100 120 Sorbed HA (g OC/kg allophane)

Figure 2. Cu sorption by allophane and allophane-HA complexes with various amounts of sorbed humic acid.

20

40 60 80 100 120 Sorbed HA (g OC/kg allophane)

140

Figure 3. Cd sorption by allophane and allophane-FLA complexes with various amounts of sorbed humic acid.

41 Figure 4 shows that part of the sorbed Cu and Cd can be displaced (desorbed) from allophane and allophane-HA complexes by treatment with KNO3. However, the ratios of desorbed to sorbed metal ions were substantially different for Cu and Cd. In the case of Cu less than 20% was desorbable by this means, whereas 49-94% of sorbed Cd could be displaced. For Cu there was a clear effect of pH, with a lower desorption ratio at higher pH, whereas with Cd the pH effect was minimal. The desorption ratio of Cd decreased as the HA content increased (from A to F) in allophane-HA complexes.

25

] pH 5 . 0 | ^ pH 5.5

-20

.215

so

110 •6 5

allophane

allophane

B C D Allophane and complexes

Figure 4. Desorption of Cu and Cd from allophane and its complexes with various amounts of sorbed humic acid (Cu and Cd were previously sorbed at pH 5.0 and 5.5); A, B, C, D, E, F refer to complexes with sorbed humic acid content of 13.9, 27.9, 55.1, 81.7, 104 and 123 g OC/kg allophane, respectively.

42

The surface charge characteristics of the allophane are shown in Figure 5. At pH 5.0, the allophane had a positive charge of 73 and a negative charge of 9 mmolc kg"^, giving a net (positive) surface charge of 64 mmolc kg'^. At pH 5.5, the positive charge decreased to 57, while the negative charge increased to 12 mmolckg'^, resulting in a net (positive) charge of 45 mmolc kg"^ The point of zero charge (PZC) of allophane in 0.01 M NaCl was (pH) 6.4. Table 1 summarizes the composition of the HA and the organic matter associated with the allophane derived from Py-FMS. The organic matter associated with the allophane contained proportionally more lipids and alkylaromatics but less N-compounds and fatty acids than the HA sample. 100 n

1

PZC=6.4

s

Figure 5. Surface positive and negative charge of allophane in 0.01 M NaCl solution, and the point of zero charge (PZC). Table 1 Composition of humic acid and organic matter in allophanic clay Humic acid Organic matter associated with allophane Volatile matter (%) 63.8 46.2 % of total ion intensity EXl - carbohydrates 2.0(4) 5.3(11) EX2 - phenols 9.0(18) 8.1 (16) EX3 - hgnin dimers 5.8(11) 3.5 (7) EX4-lipids 7.8 (16) 13.3(25) EX5 - alkylaromatics 9.0(18) 16.6(32) EX6 - N-compounds 7.6(15) 4.0 (8) EX7 - sterols 1.3(3) 0.5(1) EX8 - peptides 1.2(2) 3.1 (6) EX9 - suberin 0.0 (0) 0.1 (0) EXIO - fatty acid 0.8 (2) 2.8(6) Sum-EXl-10 49.5 52.3 numbers in parentheses refer to percentages of assigned mass signals.

43

4. DISCUSSION 4.1. Sorption characteristics of allophane and its complexes for Cu and Cd Cu^^ is under-saturated with respect to the least soluble hydroxide and hydroxy carbonate phases (Figurel). Although it seems unlikely that precipitation of these phases contributes to the sorption of Cu at pH 5.0 and 5.5, the possibility of layered double hydroxide formation as suggested by Sparks [22] for systems of hydrolysed Al^^ and Ni^"^, Co^^, or Zn^"^, cannot be excluded. Furthermore, sorption increases with pH (Figure 2) and far exceeds the amount of negative charge on allophane at either pH (Figure 5). These observations strongly suggest that Cu is specifically sorbed by allophane. At pH < 5.5, Cu^^ is the dominant species in solution. Charge balance would require two units of negative surface charge for one Cu^^ ion. The allophane has negative surface charge of 9 and 12 mmolc kg'^ at pH 5.0 and 5.5, respectively. The negative charge at pH 5.0 (9 mmolc kg'^) can only account for 6% of the Cu^^ sorbed (equivalent to 142 mmolc kg'^). The corresponding value at pH 5.5 is 3% of the sorbed Cu^^(equivalent to 358 mmolc kg"^). Thus, cation exchange is not amajor process responsible for Cu sorption by allophane. Instead, inner-sphere coordination of Cu^^ by surface AlOH groups apparently plays a key role in sorption. Clark and McBride [12, 23] have suggested that Cu is preferentially coordinated to adjacent AlOH groups, exposed on the surface of allophane particles, through a binuclear (bidentate) mechanism, although weaker binding to isolated AlOH and SiOH groups is possible. The pH-dependence of Cu sorption by allophane (Figure 2) is consistent with this suggestion, since an increase in pH would facilitate neutralisation of the protons released when Cu^"^ coordinates with surface hydroxyl groups of allophane. This type of coordination also accounts for the low percentages (< 20 %) of Cu desorbable by KNO3 (Figure 4) since K^ does not coordinate with surface AlOH groups. Similarly, the sorption of Cu^"^ by humic substances occurs through coordination with oxygencontaining functional groups (e.g., COOH, phenolic OH, C=0). Amino and imino groups may also be involved [24]. Because of the coordinating ability of HA with metals, it is expected that Cu sorption by allophane-HA complexes would increase with the HA content of the complexes. The COOH content of the HA is 2600 mmol kg"\ This is equivalent to 4.91 mmol COOH g'^ C, as the HA has a C content of 530 g kg"^ If we assume that Cu coordinates with COOH in a bidentate manner, this would equate to Cu sorption of 2.45 mmol g"^ C, a value in close agreement with the coefficients of 2.22 and 2.38 in Equations (1) and (2). This is good evidence for a copper-organic bidentate formation. Under the same experimental conditions allophane and its complexes sorb smaller amounts of Cd (Figure 3) than Cu. In other words, the affinity of allophane and its complexes for Cd is lower than for Cu. The same sequence of affinity was observed for Al hydroxide [25] and humic acid [26]. Allophane sorbed 3.1 mmol kg'^ of Cd at pH 5.0 and 9.2 mmol kg'^ at pH 5.5. Being divalent at pH < 5.5, these values correspond to 6.2 and 18.4 mmolc kg"^ of Cd^^ sorbed, close to the negative surface charge of allophane (9 mmolc kg"^ at pH 5.0 and 12 mmolc kg'^ at pH 5.5). This leads to the conclusion that, in contrast to the situation for Cu, electrostatic interaction (cation exchange) is mainly responsible for Cd sorption by allophane. Using a synthetic allophane Denaix et al. [ 13] have also proposed that Cd sorption at pH < 7 occurs by cation exchange. The high ratio of desorbed to sorbed Cd (94% at pH 5.0 and 93% at pH 5.5, as shown in Figure 4), and the minimal effect of pH on this ratio, suggest that Cd is not strongly bound by allophane, in keeping with the electrostatic interaction mechanism. Because the negative surface charge increases with pH so does the sorption of Cd. This extra Cd, however, is also desorbable by KNO3.

44

The HA in allophane-HA complexes contributes to further sorption of Cd by allophane (Figure 3). However, the contribution of HA to Cd sorption is substantially lower than that to Cu sorption because the slopes in Equations (3) and (4), 1.78 and 1.61, are smaller than the corresponding values for Cu in Equations (1) and (2). This is in agreement with the higher affinity of Cu for HA than that of Cd for HA [26]. The Cd sorbed by the HA in allophane-HA complexes is less desorbable than that sorbed by allophane itself, as indicated by the decreasing desorption/sorption ratio of Cd (Figure 4). This suggests that Cd may also coordinate with HA. 4.2. Relative contribution of HA and allophane to Cu and Cd sorption Understanding the relative contributions of organic matter and inorganic components of soil to contaminant sorption is important. It provides a means of predicting how a potential change in soil attributes, as a result of environmental change and/or soil management practice, would affect the performance of the soil as an environmental filter for contaminants. Because of the intrinsic complexity of the clay-HA interaction the question whether the sorption of metal cations to humic acid-modified clay conforms to an additive, competitive, or synergistic model has not been resolved. The linear additivity model in which sorption by clay-organic complex reflects the sum of its component parts has been favoured by some workers while alternative sorption models have been advocated by others [27, and references therein]. Little information is available on the sorption capacity of allophane-humic complex for heavy metals. Because of its relative simplicity, however, the linear additivity model may serve as a reference model for the sorption of metal ions by clay-humic complexes. On this basis, we derive an empirical indicator to compare the apparent contributions of humic acid and allophane to the sorption of metal ions by the allophane-HA complex. Because both components of the complex contribute to Cu and Cd sorption, there is a certain value of HA for which the allophane and HA makes an apparently equal (50/50) contribution to the sorption of Cu and Cd by the complexes, hi other words, the allophane-HA complex has twice capacity as allophane for sorbing metal ions. Below this "equal-contribution indicator" value allophane contributes more to the sorption than does HA, whereas above this value the opposite is true. The equal-contribution indicator value for Cu at pH 5.0 and 5.5 is 28 and 87 g OC kg'^ allophane, respectively. These values were obtained from Figure 2 by doubling the value of Cu sorption by allophane itself and then finding out the corresponding X-axis value through the regression line. Similarly, the equal-contribution indicator value for Cd is 1.9 g OC kg"^ allophane at pH 5.0 and 5.2 at pH 5.5. The equal-contribution indicator value for Cu is one order of magnitude higher than that for Cd because the amount of Cu sorbed by allophane is one order of magnitude greater than that of Cd under the same conditions (and the slopes in Equations (1) to (4) are comparable). Although organic matter is capable of sorbing much Cu and Cd there is often no strong correlation between the organic matter content of soils and their metal sorption capacity [8,28]. This might be because the composition of organic matter varies from soil to soil. Thus, empirical relations between sorption capacity and certain soil attributes obtained for a particular soil should not be extrapolated to other soils without due caution. The large difference in equal-contribution indicator values for Cu and Cd has implications for soil management. It would be much more difficult to increase the sorption capacity of allophanic soils for Cu by increasing the organic matter content, than it would be for Cd since the equalcontribution indicator value for Cu is much higher than that for Cd. This is particularly true at the higher pH of 5.5. Thus, prevention of acidification may be more effective than organic matter addition in maintaining the sorption capacity of allophanic soils for Cu. On the other hand, the large

45 sorption capacity of allophane with respect to Cu implies that allophanic soils can take up large amounts of Cu. In addition, these soils are capable of retaining sorbed Cu against leaching. In contrast, allophane is not very effective in sorbing Cd. However, the sorption capacity of allophanic soils for Cd can be quickly raised by addition of organic matter, although much of the sorbed Cd is desorbable. The sorption behaviour of allophane and its HA complexes towards Cu and Cd may have other implications for soil management. Since allophanic soils may retain much more Cu than their negative charge would indicate, the use of a charge-related parameter such as cation exchange capacity (CEC) to indicate permissible Cu loadings may be inappropriate. Our data suggest that the allophane and organic carbon content (of the soil) would provide a better index of its capacity to sorb Cu.

5. CONCLUSIONS Allophane can sorb much greater amounts of Cu than Cd under similar conditions. Cu that is sorbed is also strongly retained, probably because of coordination with active surface groups. In contrast, Cd is relatively weakly held by allophane because it is largely sorbed by a cation exchange mechanism. Cu and Cd sorption by allophane-HA complexes increases linearly with HA content. At a certain pH value, the relative contribution of the allophane and HA components to metal sorption may be evaluated using an empirical indicator value that denotes an equal (50/50) contribution of allophane and HA to metal sorption. Above this value, the HA in the complexes contributes more to sorption than the allophane component. The equal-contribution indicator value for Cu is 28 g C kg'^ allophane at pH 5.0, and 87 g C kg"^ at pH 5.5. The corresponding values for Cd are only 1.9 g C kg'^ at pH 5.0 and 5.2 g C kg"^ at pH 5.5. Thus, it would be much more difficult to increase the sorption capacity of allophanic soils for Cu by addition of organic matter than it would be for Cd. As allophane can sorb much more Cu than its negative surface charge would indicate, the use of a charge-related parameter (e.g., CEC) as a guide to permissible Cu loadings for allophanic soils might not be appropriate.

ACKNOWLEDGEMENTS This research was supported by the New Zealand Foundation for Research, Science and Technology (C09811). We are grateful to Professors H.-R. Schulten and P. Leinweber of the University of Rostock, Germany, for the Py-FMS analyses. We thank Matthew Taylor of Landcare Research, Hamilton, for valuable discussions. The constructive comments by two anonymous reviewers are appreciated.

REFERENCES 1. Carey, P.L., McLaren, R.G., Cameron, K.C., Sedcole, J.R., 1996. Leaching of copper, chromium, and arsenic through somefree-drainingNew Zealand soils. Aust. J. Soil Res. 34, 583-597. 2. Loganathan, P., Hedley, M. J., Gregg, P.E.G., Currie, L.D., 1996. Effect of phosphate fertilizer

46 type on the accumulation and plant availability of cadmium in grassland soils. Nutrient Cycling in Agroecosystems 46,169-178. 3. McBride, M.B., 1989. Reactions controlling heavy metal solubility in soils. Adv. Soil Sci. 10, 1-56. 4. Alloway, B.J., 1995. Heavy Metals in Soils. 2nd ed. Blackie Academic and Professional, London, 5. Elhot, H.A., Liberati, M.R., Huang, C.P., 1986. Competitive adsorption of heavy metals by soils. J. Environ. Qual. 15,214-219. 6. Yuan, G., Lavkulich, L.M., 1997. Sorption behaviour of copper, zinc, and cadmium in response to simulated changes in soil properties, Commun. Soil Sci. Plant Anal. 28, 571-587. 7. Zhang, M., Alva, A.K., Li, Y.C., Calvert, D.V., 1997. Chemical association of Cu, Zn, Mn, and Pb in selected sandy citrus soils. Soil Sci. 162, 181-188. 8. McBride, M., Sauve, S., Hendershot, M., 1997. Solubility control of Cu, Zn, Cd, and Pb in contaminated soils. Eur. J. Soil Sci. 48, 337-346. 9. Hall, P.L., Churchman, G.J., Theng, B.K.G., 1985. Size distribution of allophane unit particles in aqueous suspensions. Clays Clay Miner. 33, 345-349. 10. Wada, K., 1989. Allophane and imogolite. In: Dixon, J.B., Weed, S.B. (Eds.), Minerals in Soil Environment. 2nd ed. Soil Sci. Soc. Am., Madison, WI. pp.1051-1087. 11. Wada, S.-L, Wada, K., 1977. Density and structure of allophane. Clay Minerals. 12,289-298. 12. Clark, C.J., McBride, M.B., 1984. Chemisorption of Cu(II) and Co(II) on allophane and imogolite. Clays Clay Miner. 32, 300-310. 13. Denaix, L., Lamy, I., Bottero, J.Y., 1999. Structure and affinity towards Cd^^, Cu^"", Pb^"" of synthetic colloidal amorphous aluminosilicates and their precursors. Colloids and Surfaces A: Physicochem. Eng. Aspects. 158, 315-325. 14. Abd-Elfattah, A., Wada, K., 1981. Adsorption of lead, copper, zinc, cobalt, and cadmium by soils that differ in cation-exchange materials. J. Soil Sci. 32, 271-283. 15. Soil Survey Staff, 1998. Keys to Soil Taxonomy. 8th ed. Natural Resources Conservation Service, United States Department of Agriculture, Washington, D.C. 16. Hewitt, A.E., 1998. New Zealand Soil Classification. 2nd ed. Manaaki WhenuaPress-Landcare Research, Lincoln, New Zealand. 17. Yuan, G., Theng, B.K.G., Parfitt, R.L., Percival, H.J., 2000. Interactions of allophane with humic acid and cations. Eur. J. Soil Sci., 51, 35-41. 18. Parfitt, R.L., 1990. Allophane in New Zealand - A review. Aust. J. Soil Res. 28, 343-360. 19. Swift, R.S., 1995. Organic matter characterization. In: Sparks, D.L. (Ed.), Methods of Soil Analysis: Part 3, Chemical Methods. Soil Science Society of America, Madison, WI. pp.10111070. 20. Lindsay, W.L., 1979. Chemical Equilibria in Soils. John Wiley & Sons, New York. 21. Schulten, H.-R., Leinweber, P., 1999. Thermal stability and composition of mineral-bound organic matter in densityfractionsof soil. Eur. J. Soil Sci. 50, 237-248. 22. Spark, D.L. 2000. Advances in understanding the kinetics and mechanisms of metal sorption at the mineral/water interface. In: Violante A., Gianfreda, L. (Eds.) Soil Mineral-Organic MatterMicroorganism Interaction and Ecosystem Health. Abstracts of 3^^^ ISMOM, May 2000, Naples, Italy, p.34. 23. Clark, C.J., McBride, M.B., 1985. Adsorption of Cu(II) by allophane as affected by phosphate. Soil Sci. 139,412-421. 24. Stevenson, F.J., 1982. Humus Chemistty: Genesis, Composition, Reactions. John Wiley &

47

Sons, New York. 25. Kinniburgh, D.G., Jackson, M.L., Syers, J.K., 1976. Adsorption of alkaline earth, transition, and heavy metal cations by hydrous oxide gels of iron and aluminum. Soil Sci. Soc. Am. J. 40,796799. 26. Stevenson, F. J., 1977. Nature of divalent transition metal complexes of humic acids as revealed by a modified potentiometric titration method. Soil Sci.123,10-17. 27. Murphy, E.M., Zachara, J.M. 1995. The role of sorbed humic substances on the distribution of organic and inorganic contaminants in groundwater. Geoderma 67,103-124. 28. Gray, C.W., McLaren, R.G., Roberts, A.H.C., Condron, L.M., 1999. Solubihty, sorption and desorption of native and added cadmium in relation to properties of soils in New Zealand. Eur. J. Soil Sci. 50,127-137.