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a Connecticut Agric. Exp. Stn., P.O. Box 1106, New Haven, CT 06504
b Dep. of Plant Science, U-4067, Univ. of Connecticut, Storrs, CT 06269
* Corresponding author (c.schulthess{at}uconn.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: AcO, acetate • ATR-FTIR, attenuated total reflectance Fourier transformed infrared • CV, coefficient of variation • DRIFT, diffuse reflectance infrared Fourier transformed • EM, electrophoretic mobility • IC, ion chromatography • TIC, total inorganic C • TOC, total organic C • 
i, amount of i adsorbed
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The interaction of different oxyanions with each other for sorption at mineral surfaces affects the bioavailability and mobility of these ions in soils and water (Melamed et al., 1995; Ali and Dzombak, 1996; Geelhoed et al., 1997). As one of the major solute species in soils, carbonates can interfere with various other ions for adsorption onto soil minerals. A limited number of studies that included carbonate species showed that they can compete with other anions for sorption on metal oxides, especially at high concentrations of CO3 (Zachara et al., 1987; Balistrieri and Chao, 1987; Schulthess and McCarthy, 1990; Van Geen et al., 1994; Duff and Amrhein, 1996). In contrast to this competitive effect, adsorption data for Al oxide showed that the presence of CO3 markedly promotes the adsorption of SeO4 and SO4 on Al oxide between pH 6 and 8 (Wijnja and Schulthess, 2000a). Low-molecular-weight organic anions, namely formate and AcO, also had a promotive effect. The finding of these promotive interaction effects on Al oxide raises the question if this is also the case with the interactions of anions on other metal (hydr)oxides. Goethite is the most common Fe (hydr)oxide in soils (Schwertmann and Cornell, 1991) and was therefore selected as the adsorbent in the present study.
Van Geen et al. (1994) determined the adsorption behavior of CO2 on goethite and observed an adsorption envelope between pH 2 and 9 with a maximum adsorption around pH 6. They observed a decrease in CrO4 adsorption with increasing CO2 adsorption in the goethite suspensions. Zachara et al. (1987) also observed a competitive effect of CO2 at elevated pressures on CrO4 adsorption on amorphous Fe oxyhydroxide. Selenite (SeO3) adsorption on goethite was reduced only at CO3 concentration levels that were more than 1000 times higher than SeO3 (Balistrieri and Chao, 1987). Chromate and SeO3 show adsorption edges at relatively high pH and reach complete adsorption at approximately pH 8 (Van Geen et al., 1994; Hayes et al., 1988). In this pH range, the CO3 adsorption intensity is relatively low (Van Geen et al., 1994; Villalobos and Leckie, 2000) and, hence, less competitive. In contrast, SeO4 and SO4 show adsorption edges at pH <7 on goethite (Zhang and Sparks, 1990a, b). In this pH range, CO3 adsorption is very high and potentially has a substantial effect on the adsorption of SeO4 and SO4.
The adsorption mechanism of CO3 and overall adsorption reaction at goethite–water interface were determined based on in situ attenuated total reflectance Fourier transformed infrared (ATR-FTIR) and diffuse reflectance infrared Fourier transformed (DRIFT) spectroscopy, and proton coadsorption (Wijnja and Schulthess, 2001). The ATR-FTIR data indicated monodentate inner-sphere CO3 surface complexes at the goethite–water interface, which was recently also suggested by Villalobos and Leckie (2001). Furthermore, DRIFT spectroscopy and proton coadsorption data indicated that extra protonated surface groups coexisted with the adsorbed CO3 anion (Wijnja and Schulthess, 2001). The same surface speciation was found for the CO3 adsorption on Al oxide (Wijnja and Schulthess, 1999). Based on second-order desorption kinetics and working with dried samples at high temperatures, Bhattacharyya (1989) proposed that adsorbed CO2 on a mica surface existed as a CO3-surface species associated with a proton that had migrated to a more stable surface site; desorption occurs when the carbonate and the migrating proton species recombine. The coexistence of extra protonated surface groups with adsorbed CO3 may provide extra reactive sites for sorption of other oxyanions. This mechanism was proposed for the promotive effect of CO3 on the adsorption of SO4 and SeO4 on Al oxide (Wijnja and Schulthess, 2000a). It is, therefore, hypothesized that CO3 adsorption also promotes the adsorption of SeO4 and SO4 on goethite.
The objective of this study was to determine the effect of the presence of CO3 on the adsorption of SeO4 and SO4 on goethite. Acetate and formate were included in this study to check for similarities in effects of inorganic C and low-molecular-weight organic acids as was also observed in the study with Al oxide (Wijnja and Schulthess, 2000a). The experimental approach was similar to the study with Al oxide (Wijnja and Schulthess, 2000a). The adsorption edges of SeO4 and SO4 were determined in batch adsorption experiments in the absence and presence of CO3 or organic anions. The CO3 adsorption was measured to determine the adsorption affinities of the various different anions in the absence and presence of SeO4 and SO4. In addition, the effects of anion adsorption on the surface charge were determined by electrophoresis. Possible mechanisms for the observed interaction effects are discussed based on the knowledge of the CO3 adsorption reaction and its effect on goethite (Wijnja and Schulthess, 2001) and the surface charge effects of anion adsorption on goethite provided by electrophoretic mobility measurements.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Adsorption Experiments
The anion adsorption edges and isotherms were determined in batch adsorption experiments. Aliquots of the goethite suspension (containing 0.3057 g of goethite) were transferred into a 50-mL (nominal) polyallomer centrifuge tube. To each sample, a specific volume of deionized water was added to achieve a final volume of 35 mL. A specific volume of 0.5 M NaCl was added to achieve a final background salt concentration of 0.011 M NaCl. That is, the concentration of total added Cl (NaCl + HCl) was 0.011 M in all samples. If additions of NaOH were required to achieve a desired pH, specific volumes of 0.192 M NaOH were added at this point in the sample preparation procedure. Specific volumes of either 0.1 M Na2–SeO4 or 0.1 M Na2SO4 were added to achieve the desired initial concentrations of SeO4 or SO4. Next, specific volumes of either 0.1 M NaHCO3 or 0.1 M solutions of the sodium salts of AcO or formate were added to achieve the desired initial concentration. The range of total inorganic C (TIC) concentrations was 0.2 to 3 mM, which is in the lower end of the range found in natural soils, such as during the cooler months of the year. Finally, a specific volume of 0.2 M HCl was added if needed for the pH adjustment.
Stock solutions of the Na salts of the anions used as adsorbates in this study were prepared using CO2-free Milli Q water and analytical grade reagents. All stock solutions were kept in a glass bottle and the small aliquots of solution that were needed were added using an autodispenser (Metrohm Dosimat 665, Brinkmann Instruments, Westbury, NY) with accuracy of ±0.001 mL. Contamination of stock solutions from atmospheric CO2 was prevented by protection from the air using a CO2 scrubber (ascarite II, Fisher Scientific, Pittsburg, PA). Atmospheric exposure of the suspensions was minimized during sample preparation by keeping the tubes either capped or covered with parafilm while adding the various solutions. Solutions of CO2-free NaOH were prepared from stock solutions of saturated NaOH, which causes the CO3 to precipitate out of solution as Na2CO3(s).
Adsorption edges of SeO4 and SO4 were determined at an initial concentration of 0.2 and 1 mM. The pH in these samples ranged from 5 to 8. The CO3 or organic anion was concurrently added at an initial concentration of 0.2 to 3 mM in the binary-anion systems. Adsorption values at pH 6.6 (pH of optimum impact) were also collected or, when necessary, were linearly extrapolated to pH 6.6 from the adsorption isotherm data near the pH 6.6 conditions. To study the effect of SO4 on the SeO4 adsorption and vice versa, a series of samples were prepared with initial concentrations of 1 to 3 mM of both anions. The adsorption of CO3 in single adsorbate systems was determined to evaluate the effect of SeO4 on the adsorption of CO3. The initial concentrations in these samples was 1 mM for both CO3 and SeO4.
The prepared samples were mixed for 19 to 24 h on a hematology mixer at 20°C, and then centrifuged for 20 min at 20000 x g. An aliquot of the supernatant was then collected for the anion measurements. The pH was measured in the supernatant using a combination calomel electrode (Accu-pHast, Fisher Scientific, Pittsburg, PA).
The SeO4 and SO4 concentrations in the supernatants were determined using a Dionex-300 ion chromatograph (IC) (Dionex Corp., Sunnyvale, CA) with a Dionex IonPac AS4A SC column connected to an anion self-regenerating suppressor (ASRS) and a conductivity detector (CDM-3). The eluent was 1.8 mM CO3/1.7 mM HCO3 with a flow rate of 2 mL min-1. Peak areas were measured with a Dionex 4400 integrator (Dionex Corp., Sunnyvale, CA). The retention times were 3.9 min for SO4 and 4.7 min for SeO4. The supernatants were diluted 20 to 40 times for the measurement on the IC. The coefficient of variance (CV) in the SeO4 and SO4 measurements was typically <4%.
The concentrations of CO3 and organic anions were measured using a total organic C (TOC)/TIC analyzer (TOC-5000, Shimazdu Corp., Braintree, MA) according to the procedure outlined by Schulthess and McCarthy (1990) and Schulthess et al. (1998). The CV values for the TIC measurements were <2% and for the TOC measurements <1%. The TOC/TIC analyzer was also used to confirm low levels of CO3 concentrations in the blank solutions.
Propagation-of-error analysis indicated that the variance in the IC- and TIC/TOC- measurements were the major source of experimental error in the obtained adsorption data. The error in the adsorption data was typically ±0.01 µmol m-2 for SeO4, SO4, and CO3. The uncertainty in the pH measurements of the samples without CO3 or organic anions was 0.05 to 0.10 pH units. The samples with CO3 or organic anion were more stable and the uncertainty in pH of these samples was typically <0.02 pH units.
The Fe concentration in the supernatants was measured to determine the effect of the anions on the dissolution of goethite. The supernatants were filtered (0.1-µm Supor membrane filter, Gelman Sciences, Ann Arbor, MI), acidified with concentrated HNO3, and analyzed for Fe by inductively coupled plasma atomic emission spectroscopy (ICP-AES).
The electrophoretic mobility (EM) of goethite was measured using a Laser Zee meter (Model 501, Pen Kem, Bedford Hills, NY). To perform EM measurements, a 0.25-mL aliquot of a batch adsorption sample was diluted in 
30 mL of its own supernatant resulting in a solid concentration of 0.09 g L-1.
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RESULTS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The Effect of Anions on Selenate Adsorption
The SeO4 adsorption on goethite at 0.2 mM SeO4 initial concentration in the absence and presence of 0.2 mM CO3 are shown in Fig. 1 . The adsorption edges are shifted to a higher pH in the presence of CO3 in the pH range of 6 to 8. The maximum effect is around pH 6.9, where the adsorption of SeO4 increased from 0.10 to 0.18 µmol m-2 in the presence of CO3. The promotive effect diminished at pH <6.5 where adsorption was approaching 100%, and at pH >7.3, where the adsorption of SeO4 and CO3 were low.
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0.11 µmol m-2 at pH 6) compared with CO3 (0.9 µmol m-2 at pH 6; Fig. 4)
. Consequently, competitive effects may not occur with AcO in this concentration range.
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For comparison reasons, the interaction effect between SeO4 and SO4 is shown in Fig. 5 . The presence of 1 mM SO4 has no effect at pH >6.5, but seems to become competitive at pH <6.5. The presence of 3 mM SO4 has a much stronger and more obvious competitive effect on SeO4 adsorption. Comparing the adsorption intensity of SeO4 in the presence of 3 mM CO3 (Fig. 2) and in the presence of 3 mM SO4 (Fig. 5) indicates that SO4 is more competitive with SeO4 than CO3.
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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CO3 
0.9 µmol m-2 versus SeO4 0.30 µmol m-2 and SO4 0.38 µmol m-2. Based on this, a competitive effect of CO3 on SeO4 and SO4 would be expected in this pH range. However, the interaction effect of CO3 with SeO4 and SO4 adsorption on goethite appears to be promotive at relatively low concentrations of CO3, and becomes gradually more competitive with higher CO3 concentrations (Fig. 1, 2, 6, and 7). The promotive interaction effect of CO3 on the adsorption of other anions by goethite is a novel observation. Conversely, the competitive effect of CO3 observed in this study at the higher concentrations has also been observed in studies on the interaction of adsorbed carbonate with CrO4 adsorption on goethite (Zachara et al., 1987; Van Geen et al., 1994). Also contrasting, the competitive interaction effect between SeO4 and SO4 is consistent with the commonly observed competitive interaction effect between different oxyanions (Manning and Goldberg, 1996; Geelhoed et al., 1997).
The presence of 1 to 3 mM acetate resulted in 50% increase in the adsorption of SeO4 (Fig. 1 and 3) and in the presence of 1 mM AcO 25% increase of SO4 adsorption (Fig. 6 and 7) on goethite at pH 6.6. Unlike CO3, AcO maintained an enhancing effect and did not become competitive at the higher concentration levels. Conversely, other studies on the interaction of AcO with the adsorption of oxyanions on metal (hydr)oxides show little or no effect (Earl et al., 1979; Dynes and Huang, 1997).
The observed trends in the interaction effects between CO3, AcO, and formate with the adsorption of SeO4 and SO4 are generally the same as those observed with Al oxide (Wijnja and Schulthess, 2000a). The promotive effect of CO3 appears to be smaller on goethite, and it even becomes more competitive than promotive at the higher CO3 concentrations on goethite. This may be the result of the higher total anion adsorption density on goethite compared with the systems in the experiments with Al oxide. For example, the total anion adsorption (SeO4 + CO3) in systems with 1 mM SeO4 and 1 mM CO3 at pH 6.6 is 0.92 µmol m-2 on goethite versus 0.79 µmol m-2 on Al oxide. Expressed as percentage of the total surface site density (3.45 singly coordinated sites per square nanometer or 5.7 µmol m-2 for goethite) (Hiemstra and Van Riemsdijk, 1996, 1999), the surface coverage on goethite is 16%. The Al oxide, whose surface was found to have transformed into the bayerite polymorph upon aging (Wijnja and Schulthess, 1999), is expected to have a higher site density: 8.35 sites nm-2 or 13.9 µmol m-2 for pure bayerite (Hiemstra et al., 1999). The resulting surface coverage for Al oxide is then only 6%. The higher anion adsorption density on goethite could possibly result in more competitive interaction effects as discussed in more detail below.
Figure 2 clearly indicates that there is an optimum CO3 adsorption level for the promotive effect on the SeO4 adsorption. Under the conditions in Fig. 2, the optimum CO3 adsorption level for the promotive effect is at 0.2 mM total concentration, or at 0.07 mM aqueous concentration at equilibrium and 0.18 µmol m-2 adsorbed CO3, which corresponds to 3% of the total estimated surface site coverage. The total adsorption level for CO3 on a similar goethite was estimated at 1.2 to 1.4 µmol m-2 (Villalobos and Leckie, 2000). Hence, 0.18 µmol m-2 corresponds to 14% of the total CO3 sites. This implies that there is a relative small fraction of specific sites for CO3 adsorption that are effective in promoting the SeO4 or SO4 adsorption. The promotive effect of CO3 may suggest that the presence of adsorbed CO3 must cause an alteration of surface protonation or surface charge that favors the adsorption of the coadsorbing anion. Additional CO3 adsorption sites apparently do not result in promotive effects, but rather result in direct competition with SeO4 or SO4 for sites or in indirect competition through electrostatic effects.
Russell et al. (1975) proposed a specific site for CO2 adsorption on the goethite surface. The carbonate anion surface specie was believed to be formed by a reaction with a surface oxygen located in the trough of the (100) face and believed to provide a route for charge redistribution and H-bonding on the surface. More recent research has revealed that the 110-face is predominant on synthetic goethite (Hiemstra and Van Riemsdijk, 1996). Although, the structural arrangement of the various types of surface groups on the 110-face is different from the 100-face, adsorbed CO3 may still provide such charge redistribution at specific sites on the 110-face. Detailed molecular-scale information about this adsorption mechanism is currently not available.
The high proton stoichiometry of CO3 compared with other divalent oxyanions (Wijnja and Schulthess, 2001) may also indicate such a mechanism. At low surface densities (<10%), carbonate anion adsorption at specific sites of this type may prevent mass action and electrostatic interactions and, thereby, also prevent competitive effects between the different anions (Mesuere and Fish, 1992). The significant CO3 adsorption in the pH range of 6 to 8 (Fig. 4) (surface coverage 10% of total sites), may have occurred at these specific sites and, thereby, did not compete with the adsorption of SeO4 or SO4.
One should note that the adsorbed CO3 caused not merely the absence of competitive interactions but also the presence of a promotive effect on SeO4 or SO4 adsorption, which cannot be adequately explained by the specific site theory alone. Regardless of the mechanism(s) involved at low CO3 adsorption levels, competitive effects were shown for SeO4 in Fig. 2 at higher CO3 adsorption levels. At the higher CO3 adsorption densities, steric and electrostatic interactions may begin to play a role and decrease the SeO4 adsorption.
The DRIFT spectra of goethite with adsorbed CO3 indicated that adsorbed CO3 is accompanied by extra protonated surface groups on goethite (Wijnja and Schulthess, 2001). The overall effect of the CO3 adsorption on the surface charge of goethite can be provided by EM data. The EM data for goethite in the absence and presence of various anions are shown in Fig. 8 . These data indicate that the presence of CO3 decreases the net positive charge of goethite particles compared with the system with only NaCl. At pH <5.5, the EM of goethite with adsorbed CO3 increases sharply with decreasing pH. This sharp increase in EM coincides with the decrease in CO3 adsorption at this pH (Fig. 4).
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0.9 µmol m-2, Fig. 4) compared with on Al oxide (0.4 µmol m-2; Wijnja and Schulthess, 2000a). Consequently, CO3 affects the surface charge of goethite more than that of the Al oxide. The EM data for the systems with SeO4 and SO4 in Fig. 8 show that the effect of these anions on the EM of goethite is much larger than the effect of CO3 at pH <7.5. Adsorption of SeO4 (Fig. 2) and SO4 (Fig. 7) resulted in a much larger decrease of EM, even though their adsorption intensity is lower than CO3 at pH >5.5 (Fig. 4). These data clearly show the difference in effects between CO3 and SeO4 or SO4 adsorption on goethite. The differences in EM effect between these oxyanions also parallel the differences in proton coadsorption data observed for these oxyanions (Wijnja and Schulthess, 2001). The proton-to-carbonate stoichiometry appeared to be higher for CO3 compared with other bivalent oxyanions. Recently, Villalobos and Leckie (2000) also found a relatively high proton-to-carbonate stoichiometry for CO3 adsorption on goethite that is comparable with the value found by Wijnja and Schulthess (2001).
The smaller surface charge effect of CO3 adsorption (Fig. 8) may be explained by the extra protonated surface groups that coexist with adsorbed CO3 and neutralize, to some extent, the negative surface charge effects of the adsorbed CO3 anion. The larger surface charge effects of SeO4 and SO4 (Fig. 8) may explain why the CO3 adsorption decreases in the presence of SeO4 or SO4 (Fig. 4). That is, the less positive surface in the presence SeO4 or SO4 is less favorable for CO3 adsorption.
Additionally, the smaller surface charge effects of CO3 (Fig. 8) and the presence of additional protonated surface groups associated with the adsorbed CO3 on goethite may enhance the adsorption of SeO4 and SO4 at low surface coverage as was proposed for the interaction between these anions on Al oxide (Wijnja and Schulthess, 2000a). The CO3 ligand exchange reaction and proton coadsorption reaction can be represented by the following reaction equations (Wijnja and Schulthess, 2001):
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Selenate and SO4 have adsorption edges below pH 7 in the absence of CO3 (Fig. 1 and 6). At pH >6.5, where the SeO4 and SO4 adsorption intensities are relatively low, significant CO3 adsorption occurs (Fig. 4). The additional protonated surface groups that coexist with adsorbed CO3 can, in this case, provide additional complexation sites and result in the enhanced adsorption of SeO4 and SO4 (Fig. 1 and 6). At pH >6.5, SeO4 and SO4 form predominantly outer-sphere surface complexes on goethite (Wijnja and Schulthess, 2000b). This suggests that the increased SeO4 and SO4 adsorption also occurs through outer-sphere complexation of these anions with the additional protonated surface sites generated by CO3 adsorption.
At pH <6, it is the increased surface protonation that occurs solely because of the lower pH that becomes significant and provides sufficient sorption sites for SeO4 and SO4. In relative terms, the extra protonation due to adsorbed CO3 is not significant under these low pH conditions and no increase in the high SeO4 or SO4 adsorption values can be observed (Fig. 1 and 6).
The promotive effect of AcO and formate on SeO4 and SO4 adsorption may also be the result of increased surface protonation. Acetate is known to be a relative weak adsorbing anion, forming mainly outer-sphere surface complexes with mineral surfaces (Ward and Brady, 1998; Persson et al., 1999). The proton coadsorption of AcO adsorption on goethite was measured according to the pH-stat procedure described in Wijnja and Schulthess (2001) at pH 6. The proton/AcO adsorption stoichiometry was approximately 1:1.2 in the presence of 11 mM NaCl. This ratio suggests that extra protonated surface groups may also exist in the presence of adsorbed acetate. As with CO3, the enhanced surface protonation may, in turn, promote the adsorption of SeO4 and SO4. As mentioned earlier, competitive effects in binary sorbate systems with acetate are absent presumably because of the much lower adsorption affinity of AcO compared with CO3.
Zachara et al. (1987) and Van Geen et al. (1994) observed a competitive effect of CO3 on the adsorption of CrO4 on Fe (hydr)oxides. The total CO3 concentration in their systems was in the range of 1 to 25 mM CO3. At these CO3 concentrations, the competitive interaction is likely to be dominant. The data in this study indicate that competitive effects begin to play a role with the increasing CO3 concentrations, especially at concentrations >1 mM CO3. In addition, the promotive effect of adsorbed CO3 may be ineffective in the pH range of 7.5 to 9, where CrO4 adsorption edges occur and CO3 adsorption is relatively low.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Based on the mechanistic information from Wijnja and Schulthess (2001), the proposed mechanism for the promotive effect of CO3 is the generation of extra sorption sites by extra protonated surface groups that coexist with adsorbed CO3. Similarly, AcO adsorption may also enhance the surface protonation and enhance the SeO4 and SO4 adsorption. At higher CO3 adsorption densities, competitive interactions begin to play a role and the overall interaction effect becomes neutral or even competitive.
The data of this study suggest that the surfaces of goethite present in natural environments may have substantial amounts of CO3 adsorbed. Depending on the CO3 levels, which varies seasonally in soils, this may enhance, decrease or have a neutral effect on the adsorption of moderate affinity anions such as SeO4 and SO4. Although CO3 appears to adsorb in significant amounts on goethite, its combined effects of promotive and competitive interactions may render it less competitive than was commonly believed. Similarly, the presence of AcO and formate may be less competitive with other anions due also to their promotive interaction effect.
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NOTES |
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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-FeOOH) surfaces, and its implications for anion adsorption. J. Chem. Soc. Faraday Trans. I, 71:1623–1630.
-Al2O3/water interface. Spectrochim. Acta 55:861–872.This article has been cited by other articles:
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  S. M. Garman, T. P. Luxton, and M. J. Eick Kinetics of Chromate Adsorption on Goethite in the Presence of Sorbed Silicic Acid J. Environ. Qual., September 1, 2004; 33(5): 1703 - 1708. [Abstract] [Full Text] [PDF]
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