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1 Soil Science and Plant Nutrition ISSN: (Print) (Online) Journal homepage: Changes in Zero Point of Charge (ZPC), Specific Surface Area (SSA), and Cation Exchange Capacity (CEC) of kaolinite and montmorillonite, and strongly weathered soils caused by Fe and Al coatings Katsutoshi Sakurai, Atsushi Teshima & Kazutake Kyuma To cite this article: Katsutoshi Sakurai, Atsushi Teshima & Kazutake Kyuma (1990) Changes in Zero Point of Charge (ZPC), Specific Surface Area (SSA), and Cation Exchange Capacity (CEC) of kaolinite and montmorillonite, and strongly weathered soils caused by Fe and Al coatings, Soil Science and Plant Nutrition, 36:1, 73-81, DOI: / To link to this article: Published online: 04 Jan Submit your article to this journal Article views: 450 View related articles Citing articles: 23 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 04 December 2017, At: 10:17

2 Soil Sci. Plant Nutr., 36 (1), 73-81, Changes in Zero Point of Charge (ZPC), Specific Surface Area (SSA), and Cation Exchange Capacity (CEC) of Kaolinite and Montmorillonite, and Strongly Weathered Soils Caused by Fe and A1 Coatings Katsutoshi Sakurai 1, Atsushi Teshima 2, and Kazutake Kyuma Soil Science Laboratory, Faculty of Agriculture, Kyoto University, Sakyo-ku, Kyoto, 606 Japan Received February 3, 1989 In order to analyze the effects of A1 and Fe coatings on mineral grains, montmorillonite (Mt), kaolinite (Kt), and deferrated strongly weathered soils (T6B and T7B) were artificially coated with 2, 6, 10% of Fe or AI hydroxides. Changes in the values of the zero point of charge (ZPC), specific surface area (SSA), and cation exchange capacity (CEC) were examined for these samples. Furthermore, observations by X-ray diffraction (XRD) and scanning electron microsocpy (SEM) were made. The effect of sesquioxide coatings on the ZPC, SSA, and CEC values was found to be entirely dependent on the material coated and the type of coating material. In the case of Mt, Al coatings caused a decrease in the inter-lamellar spaces and CEC, and a shift of ZPC to a higher ph value, whereas Fe coatings caused no significant change except for the development of variable charges sufficient to induce a ZPC. In the case of Kt and the deferrated soil samples, Fe coatings brought about a shift of the ZPC to a higher ph value, an increase in the specific surface area (SSA) and a decrease in CEC, whereas A1 coatings caused similar changes except for SSA. The shift of ZPC to a higher ph value was caused by the high value of the ZPC of Fe and Al hydroxides and/or blocking of negative charges by A1 hydroxides. Decrease in CEC was caused by the addition of positive charges of hydroxides reflecting the higher ZPC value and, furthermore, through charge neutralization in the inter-lamellar spaces by the positive charges of A1 hydroxides. The SSA was increased by the addition of particles of Fe hydroxides, but remained almost constant when A1 hydroxides covered the whole mineral surfaces. In particular, A1 precipitates occupied the inter-lamellar spaces of Mt and caused a decrease in SSA. Fe oxides accumulated in the strongly weathered soils were considered to increase the ZPC value and enlarge the SSA, but the increase in SSA did not always result in an increase in the CEC. In contrast, the CEC was reduced through the ZPC shift toward a higher ph value. Key Words: clay minerals, SSA, strongly weathered soils, ZPC. Present addresses: ~ Faculty of Agriculture, Kochi University, Nankoku, 783 Japan; z Toray Silicon Co., Ltd., Ichihara, Japan.

3 74 K. SAKURAI, A. TESHIMA, and K. KYUMA In soils, mineral grains are usually coated to a greater or lesser extent by the products of pedogenesis. Often these coatings consist of amorphous materials released from primary minerals by weathering or those translocated and deposited. In the presence of these coatings, the surface charge depends +not only on the charge of the mineral grains themselves but also on the charge developed by the amorphous coating materials. Therefore the relationship between the charge developed by the mineral grains, especially by clay minerals, and the variable charge developed largely by the amorphous coatings should be studied in more detail. In our previous work (Sakurai et al. 1989a), crystalline Fe oxides coexisting with kaolinite were considered to be the dominant components of the variable charge in the strongly weathered soils, and amorphous alumino-silicates (allophane and allophane-like materials) were the dominant components of the variable charge in the volcanic ash soils. Although the amounts of Fe and Al oxides were correlated well with the value of the ZPC of the strongly weathered soils and volcanic ash soils, the amount of organic matter and the permanent negative charge also affected the ZPC values. The dependence of the ZPC value on the oxide content should be better analyzed quantitatively in a more simplified system. Hendershot and Lavkutich (1983) attempted to evaluate the effect of sesquioxide coatings on the ZPC using the potentiometric titration method. They used five standard mineral samples and three C horizon samples artificially coated with pure iron or aluminum hydrous oxides. Since they used only one level of the coating materials, the changes in the physical and chemical properties could not be adequately evaluated systematically. Thus, in the present paper, as a function of the amount of the Fe and A! coatings, the changes in the values of the zero point of charge (ZPC), specific surface area (SSA), and cation exchange capacity (CEC) were studied. METHODS Sample preparation. Two mineral samples (Georgia kaolin and Black Hills bentonite (BH200), abbreviated as Kt and Mt, respectively) and two B horizon samples of strongly weathered soils from Thailand (T6B and T7B, classified as oxisol and ultisol, respectively) were used. Georgia kaolin is composed of well-crystallized kaolinite with small amounts of quartz, and Black Hills bentonite is composed of Na-Montmorillonite with small amounts of cristobalite and quartz. T6B and T7B contained 12.3 and 9.2% of total iron oxides, and 9.5 and 8.2% of free oxides (dithionite-citrate-bicarbonate extractable), respectively. Some other characteristics were described elsewhere (Sakurai et al. 1989b). After being ground to pass a 0.50 mm mesh sieve, the soil samples were treated twice with a 30% H202 solution (at 100~ to remove organic matter, and then deferrated twice with dithionite-citrate-bicarbonate at ph 7.3 (Mehra and Jackson 1960). The Kt and Mt samples were washed twice by N HC1 before use. The coated samples were produced by taking subsamples of this cleaned material and precipitating Fe or A1 hydroxide on them as follows: 30 g samples were placed in a 250 ml polypropylene beaker with 100 ml of 0.i M HC1, and a sufficient amount of FeCl3 or A1C13 solution was added to reach a value of approximately 2% Fe2Oa or A12Oa by weight. The samples were then titrated slowly with 1 M NaOH to ph 7.0, and this ph value was maintained for 1 week by daily retitration. The samples were dialyzed against distilled water until the conductivity of the suspension became close to that of deionized water. The water was changed twice daily. The samples were then dried at 40oc. The material was aged by

4 Changes in ZPC, SSA, and CEC by Fe and AI Coatings 75 alternately saturating the samples with distilled water and drying them nine more times. After being ground to pass a 0.50 mm mesh sieve, some of them underwent similar treatments stepwise to obtain 6% coating and then 10% coating. The highest level (10%) of coating was obtained to simulate the free Fe oxide contents of strongly weathered soils. Uncoated samples were prepared in an identical manner only without the addition of FeCls or A1Cls. Analytical methods. Potentiometric titration curves, ZPC and o'p values were obtained using the STPT methods, described by Sakurai et al. (1988). The cross-over point of two acid or base titration curves at different concentrations of the electrolyte is defined as the ZPC at which the adsorption of H + and OH- on the variable charge components is balanced. ~rp represents the magnitude of the net adsorption of H + or OH- at the ZPC and the magnitude of the permanent charge revealed at the ZPC (Sakurai et al. 1989a). Specific surface areas (SSA) were obtained from Ca-saturated samples using two methods; ethylene glycol mono-ethyl ether (EGME) adsorption method (Eltantawy and Arnold 1973) and low-temperature nitrogen adsorption examined in a Quantasorb (Yuasa- Quantachrome). Cation exchange capacity (CEC) was determined by the ammonium acetate (ph 7.0) method. RESULTS AND DISCUSSION Surface observation by scanning electron microscopy (SEM) Figure la to c are examples of the SEM photographs of Mt taken before and after coating with 10% of A1 or Fe hydroxides. Regardless of the variability in the charge properties of the samples before the coating treatment, when the coating material was identical, coated samples showed no significant differences. A1 oxides gave a smooth surface on Mr, Kt, and deferrated T6B and T7B, although Hendershot and Lavkulich (1983) reported tl~at A1 oxide coating made a smooth surface on quartz grains but not on illite. Main observations were as follows: 1) Clean surface was dominant in the uncoated samples. There were some small particles, about 50 to 200,~ in diameter, on the clay particles more than 500 A in diameter. We used the term "small" for the particles 50 to 200 A in diameter and "larger" for those with a diameter exceeding 200 A. 2) When 2% coating with A1 hydroxides was made, the frequency of the small particles increased. At the same time, the number of large particles more than 200,~ in diameter also increased. Once the A1 hydroxide content increased to 6 and 10%, the surface became smooth with a smaller number of these small and large particles. As was suggested by Oades (1984), smoothing by A1 may have proceeded as a result of the adsorption of the planar AI polycations onto the surface. 3) As the amount of the Fe coating increased, the number of both small and large particles increased proportionately. No smoothing on the surface was observed. Oades (1984) also confirmed the existence of Fe hydroxides on kaolinite as spherical particles 10 to 100 A in diameter by electron microscopy. X-ray diffraction analysis X-ray diffractograms of the Na-saturated powder samples were taken for all the samples using Ni-filtered CuK~ radiation. Clay minerals of the T6B and T7B soils included kaolins and hematite with a small amount of quartz. After deferration, the hematite peak disappeared. As the amount of A1 hydroxides increased, the intensity of the (001) reflection of

5 Fig. 1. SEM photograph of montmorillonite (Mt). a, montmorillonite coated with 10% Fe; b, montmorillonite coated with 10% AI; c, montmorillonite uncoated...., CJ'l?" CIl :> ;;0:: c ::0.~ :> >-! t'i1 CIl :r: ~? I>l ;:l 0. ;;0:: ;;0:: -< C ~ :>

6 Changes in ZPC, SSA, and CEC by Fe and A1 Coatings 77 Table 1. c-axis spacing (/k) of montmorillonite (Na saturated). Sample 25*C 150*C Uncoated A12Oa 2% % % FezO3 2% % % OH-!; 4-0 /- 4 STPT-Z PC~ /~(/ STPT- / ~" _ '-~~ Pc 8 -I- ~ p / =Kt uncoated 7 ph Fig. 2. Potentiometric titration curves of kaolinite (Kt) obtained by the STPT method. bentonite became weaker, while that of kaolinite did not change. Gibbsite was formed even in the samples coated with 2% of A1 and its formation increased with the amount of coating. On the other hand, Fe coating did not yield any definite peak presumably because Fe hydroxides still occurred as X-ray amorphous materials even after the aging treatments adopted in the sample preparation. The interlayering of the coating materials was also examined (Table 1). Before heating, Mt coated with A1 showed the strong (001) reflection at 15.~, and after heating to 150~ the strong reflection of Mt coated with 2, 6, and 10% A1 hydroxides collapsed to 11.2, 13.0, and 14.2,~, respectively. On the contrary, the strong reflection of Mt coated with Fe collapsed from 13.5 to 10.0./~ by the heating treatment to 150~ These results suggest that only the AI coating occup!ed the inter lamellar spaces and induced a more stable structure resistant to heating in proportion to the increase of the amount of A1. Studies on the removal of free iron oxides from a number of red soils (Deshpande et al. 1964) showed that most of the iron was present in small, discrete particles which did not affect appreciably the physical and chemical properties of the soil, whereas aluminum oxides may be present as finely divided surface-absorbed layers carrying positive charges. Greenland and Oades (1968) and Rengasamy and Oades (1977) also confirmed that Fe hydroxides occurred as discrete particles on kaolinite. Titration curves and ZPC values The STPT curves of Mt and Kt are shown in Figs. 2 and 3, where the 10% Fe and Al-coated samples and untreated samples are dealt with. The examination of the curves indicates that in some cases the coatings caused important changes in the slope and shape of the curves as well as changes in the ZPC. The slope of the titration curves of uncoated Kt was very gentle, reflecting the capacity to adsorb H + or OH- (potential determining ions, PDI), whereas after coating with Fe or A1, the slope became steeper, indicating an increase in PDI adsorption. Especially, the amount of the PDI adsorbed by the A1 coating was very large around ph 5.4. On the other hand, the Mt uncoated and coated with 10% of Fe showed a very similar charge property except at a ph value below 3, where Mt coated with Fe adsorbed a larger amount of H +. The amount of PDI adsorbed by Mt coated with A1 hydroxides was extremely

7 78 K. SAKURAI, A. TESHIMA, and K. KYUMA o.- I ' 1 I ~ ph' E ~ H + 9 v7 y o.=.,e0 // ~ f o., oo=e~ w..,o~o Fo II Fig. 3. Potentiometric titration curves ofmontmorillonite (Mt) obtained by the STPT method. Table 2. STPT-ZPC and O'p values of the samples treated with various amounts of AI and Fe. Sample Uncoated Al-coating Fe-coating 2% 6% 10% 2% 6% 10% STPT-ZPC Kt Mt T6B T7B , STPT ap (meq/100 g) Kt Mt T6B T7B I All the values are the means of duplicated determinations. --, not detectable. large below ph 4. A1 dissolution accompanied by a consumption of H + ions was not significant even at ph 3 (less than 5% of the total amount of the A1 coating). As compared with Kt, the adsorption of PDI by Mt was much larger. It is obvious from Fig. 3 that Mt has a large permanent negative charge, and A1 hydroxide adds a significant variable charge to enable the release of H + ions, acting as a buffer to a ph rise. ZPC values were also markedly affected by the coatings of Fe and A1 hydroxides. A greater ZPC shift was brought about by A1 for Mr, T6B, and T7B, whereas by Fe for Kt (Table 2). The O'p value is a measure of the permanent charge at the ZPC (Sakurai et al. 1989a); its positive value indicates the presence of a negative charge at the ZPC and vice versa. As the amount of the coating materials increased, the value of ap decreased leading to a small positive value or even to a negative value, which indicated a decrease in the amount of negative charges. As was observed earlier in the SEM photographs, Fe hydroxide seemed to occur as discrete particles on the clay surface rather than as a smooth layer, whereas A1 hydroxide appeared to cover the whole surfaces. Hence, Fe hydoxides through the formation of polymeric species may not affect the charge originating from the clays, whereas A1 hydroxide may neutralize the charge originating from the clay minerals. Oades (1984) recognized that A1 polycations were much more efficient in blocking negative charges than Fe polycations, and the effect of the blocking was due not only to the higher charge of the A1 polycations but also to their shape. Spherical Fe hydroxides may increase the amount of positive charges in the clay- or soil-hydroxide system, but their effect

8 Changes in ZPC, SSA, and CEC by Fe and A1 Coatings 79 on the blocking of the negatively charged sites may be limited. Planar polycations of A1, however, may cover a large area and also occupy inter-lamellar spaces, as indicated by the XRD analysis, and, thus, neutralize and block a larger number of negatively charged sites. In the case of T6B and T7B, when the amount of AI coating increased from 6 to 10%, no change or no shift of ZPC was recognized. Furthermore, the o'p value showed a decrease in the amount of positive charges. Thus, as was evident from the occurrence of gibbsite in the X-ray diffractogram, a longer aging period in the sample preparation for the 10% than for the 6% coating may induce the formation of a more crystalline A1 hydroxide and a loss of positive charges. Specific surface area (SSA) SSA measured by the EGME retention method could be evaluated as the "total" surface area (hereafter denoted as EGME-SSA), including the inter-lamellar spaces between adjacent alumino-silicate sheets of montmorillonite, i.e., "inner" surface, (Eltantawy and Arnold 1973). In contrast, the low temperature N2 adsorption technique evaluated only the "outer" surface (hereafter denoted N2-SSA) where Nz gas was accessible. Thus these two methods were applied to differentiate the nature of the coating by Fe and A1. In the Fe-coated samples, the values of both Nz-SSA and EGME-SSA of Kt, T6B, and T7B increased with the increasing amounts of iron, whereas for Mt, the Nz-SSA value increased while the EGME-SSA value did not change (Fig. 4). As was shown in the SEM photographs, since Fe hydroxide mainly occurred as discrete particles on mineral grains, the outer surface of the samples increased independently of the underlying mineral grains. The increase in the surface area of Kt, T6B, and T7B associated with the Fe coating indicates that the Fe hydroxides increased the value of EGME-SSA from 300 to 500 m2/g. Greenland and Oades (1968) also reported that the specific surface areas (N2-SSA) of Fe-coated Kt increased almost linearly with increasing amounts of iron, suggesting the apparent independence of the kaolinite and Fe hydroxides. For the Mt samples coated with Fe, the increase in the outer surface and decrease in the inner surface were observed. Blocking of the inner surface by the coating materials and addition of new hydroxides to the surface exerted an antagonistic effect without causing any change in the total surface area (EGME-SSA). In the Al-coated samples, the values of both N2-SSA and EGME-SSA did not change for Kt, T6B, and T7B, whereas for the Mt samples the value of N2-SSA increased and that of EGME-SSA markedly decreased (Fig. 4). The SSA of the Al-coated samples was generally smaller than that of the Fe-coated samples presumably because the A1 coating gave a smooth surface layer on the mineral grains, which would have blocked inter-lamellar spaces in the case of montmorillonite. Thus, for the Kt, T6B, and T7B samples with little or no inter-lamellar spaces, only the original surface may have been thickened by the A1 coating and no significant changes were observed in the values of both N2- and EGME-SSA. Oades (1984) who noted that the sorption of the added A1 polycations did not change the surface area of the kaolinite, suggested that the A1 chains or sheets were adsorbed onto the surfaces of kaolinite. CEC (cation exchange capacity) CEC values of the uncoated samples always decreased by the coating treatment (Fig. 5). As the amount of coating increased, the CEC values decreased or remained almost constant, At first, hydroxides with a positive charge may have eliminated the negative charge on the mineral grains. This phenomenon was observed for Kt, T6B, and T7B coated with Fe up to

9 80 K. SAKURAI, A. TESHIMA, and K. KYUMA m~ EGME--SSA OC o Mt 9 Kt T6B 9 T7B meq/ 1009 CEC ~ Mt 9 Kt 40 ~ T6B 9 T7B 6 ~ ~ 1'o6 i ~ 1'o m, t N2--SSA 100 (3' 65 g lbo 5 g 1'0 AI coating % Fe coating % Fig. 4. Specific surface area (SSA) of the samples measured by the EGME and N2-gas adsorption methods. Fig. 5. r, o 2 6 lb io O A! coating./o Fe coatin 0 % CEC of the samples measured at ph %. However, after the surface of the deferrated samples was covered, no further decrease in the amount of negative charges occurred for Kt, T6B, and TTB coated with A1. CEC value of the Mt samples showed a similar change to that of the EGME-SSA values (Figs. 4, 5). Blocking of the negative charges should have caused a reduction in the CEC values for both the A1- and Fe-coated samples. However, the CEC values of the Fe-coated samples did not change. Fe oxides as discrete particles contributed to the increase of the amount of positive charges and SSA, but did not affect significantly the negative charges. However, a reduction in the O'p value was also observed (Fig. 3) because the variable negative charge of the Fe coatings compensated the H consumption by the clay materials at the ZPC, which was much lower than the ph 7.0 used for the CEC measurement. On the other hand, the A1 coatings covered the Mt surface and affected its charge properties considerably. These observations are in agreement with the former assumption that the Fe coatings can not occupy the inter-lamellar spaces, unlike the A1 coatings.. In the absence of significant negative charges (Kt, T6B, and T7B), a shift of ZPC to a higher ph value after coating resulted in a smaller difference between ph 7 and ZPC. This phenomenon may lead to a decrease in the CEC value measured at ph 7 due to the decrease in the variable negative charge component. Surface charge properties of the strongly weathered soils, T6B and T7B, are listed in Table 3, for the untreated original materials, those deferrated but uncoated, and those deferrated and subsequently coated with 10% of Fe oxides. Compared with the original materials, the artificially coated samples showed higher ZPC, ap, and CEC values, but a similar SSA value. Since the surface area was identical, the nature of the Fe oxides, especially their crystallinity, may have caused such a significant difference. As shown in the XRD data, hematite was detected in the original materials, whereas only amorphous

10 Changes in ZPC, SSA, and CEC by Fe and A1 Coatings 81 Table 3. Surface charge properties of T6B and T7B untreated, deferrated and uncoated, and deferrated and coated with 10% of Fe (as Fe203). O'p EGME-SSA CEC Sample Treatment STPT-ZPC (meq/100 g) (m2/g) (meq/100 g) T6B Untreated Uncoated Fe coated (10%) T7B Untreated Uncoated Fe coated (10%) materials were found in the coated samples. A higher ZPC value for amorphous materials was reported by Parks (1965). The deferrated, uncoated material showed a lower ZPC value and a higher o'p value than the original material (Table 3). After the deferrated samples were coated with Fe, the ap value became higher in comparison with that of the untreated soils, whereas the ZPC value increased even more than the original level. As suggested previously, the Fe coating treatment could replace ferric oxides effectively not in the interlayer space but the external surface and its charge characteristics. Further aging treatment for the formation of crystalline Fe oxides may enable to simulate the charge characteristics of the strongly weathered soils. REFERENCES Deshpande, T.L., Greenland, D.J., and Quirk, J.P. 1964: Influence of iron and aluminium oxides on the charges of soil and clay materials. In Transactions of 8th Congress of International Soil Science, Bucharest, Vol. 3, p Eltantawy, I.M. and Arnold, P.W. 1973: Reappraisal of ethylene glycol mono-ethyl ether (EGME) method for surface area estimations of calys. J. Soil Sci, 24, Hendershot, W.H. and Lavkulich, L.M. 1983: Effect of sesquioxide coatings on surface charge of standard mineral and soil samples. Soil Sci Soc. Am. J., 47, Greenland, D.J. and Oades, J.M. 1968: Iron hydroxides and clay surfaces. In Transactions of 9th Congress of International Soil Science, Adelaide, Vol. 1, p Mehra, O.P. and Jackson, M.L. 1960: Iron oxide removal from soils and clays by a dithionite-citrate system buffered with sodium bicarbonate. Clays Clay Miner., 7, Oades, J.M. 1984: Interactions of polycations of aluminum and iron with clays. Clays Clay Miner., 32, Parks, G.A. 1965: The isoelectric points of solid oxides, solid hydroxides, and aqueous hydroxo complex systems. Chem. Rev., 65, Rengasamy, P. and Oades, J.M. 1977: Interaction of monomeric and polymeric species of metal ions with clay surfaces. II. Changes in surface properties of clays after addition of iron(iil). A ust. J. Soil Res., 15, Sakurai, K., Nakayama, A., Watanabe, T., and Kyuma, K. 1989b: Influences of aluminum ions on the determination of ZPC (zero point of charge) of variable charge soils. Soil Sci. Plant Nutr., 35, Sakurai, K., Ohdate, Y., and Kyuma, K. 1988: Comparison of salt titration and potentiometric titration methods for the determination of zero point of charge (ZPC). Soil Sci. Plant Nutr., 34, Sakurai, K., Ohdate, Y., and Kyuma, K. 1989a: Factors affecting zero point of charge (ZPC) of variable charge soils. Soil Sci. Plant Nutr., 35, 21-31