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Soil Science Society of America Journal 66:437-444 (2002)
© 2002 Soil Science Society of America

DIVISION S-2—SOIL CHEMISTRY

Sorption of Iron-Cyanide Complexes in Soils

Thilo Rennert and Tim Mansfeldt*

Arbeitsgruppe Bodenkunde und Bodenökologie, Fakultät für Geowissenschaften, Ruhr-Universität Bochum, D-44780 Bochum, Germany

* Corresponding author (tim.mansfeldt{at}ruhr-uni-bochum.de)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The Fe-cyanide complexes ferricyanide, [Fe(CN)6]3-, and ferrocyanide, [Fe(CN)6]4-, are of an anthropogenic source in soils. As the complexes are largely charged, sorption on the soil matrix is a possible retention mechanism for these anions. To evaluate soil properties controlling Fe-cyanide complex sorption, experiments were performed with 17 uncontaminated soil horizons by a batch technique. Soil organic matter (SOM) was destroyed in six horizons. The experiments were conducted at soil pH, reaction time of 24 h, and an ionic strength of 0.01 (NaNO3). The affinity of the Fe-cyanide complexes for the soil matrix differed, because 14 samples sorbed higher amounts of ferrocyanide than of ferricyanide. Calculated sorption maxima were quantitatively explained by physical and chemical soil properties using multiple regressions. The regression equations were checked by variance analysis. The regression equations for all samples showed that the sorption of both complexes depended on organic C (Corg), clay, and oxalate-extractable Fe (Feo). The sorption of the complexes on soils containing <10 g Corg kg-1 was governed by pH and clay contents. Clay and oxalate-extractable Al (Alo) were the most important properties influencing ferricyanide sorption on samples containing high amounts of Corg. On the same samples, ferrocyanide sorption was governed by Alo. Organic matter promotes the sorption of both complexes, especially on Fluvisol samples. Destruction of SOM of these samples minimized the sorption by up to 99%. Therefore organic matter in these soils may have a special affinity for Fe-cyanide complexes possibly because of the reaction between Fe-cyanide N and reactive groups of SOM.

Abbreviations: Corg, organic C • MGP, manufactured gas plant • subscript d, dithionite-citrate extractable • subscript O, oxalate extractable


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
CYANIDE IN THE FORM of the Fe-cyanide complexes ferricyanide and ferrocyanide are not present in natural soils (Fuller, 1985). Industrial activities of mankind have led to inputs of these compounds in the soil environment. Soils on sites of former manufactured gas plants (MGP) and coke ovens are commonly contaminated with cyanide (Shifrin et al., 1996), in which cyanide is present as the ferric ferrocyanide Berlin Blue, Fe4[Fe(CN)6]3 (Mansfeldt et al., 1998). On these sites, dissolved Fe-cyanide complexes are found in the soil solution and partially in the ground water (Meeussen et al., 1994). Berlin Blue and Na ferrocyanide are added to road salt as anticaking agents (Paschka et al., 1999). Hence, road salt is a potential source of Fe-cyanide complexes in soils. Furthermore, Fe-cyanide complexes are present in soils developed from blast furnace sludge deposits (Mansfeldt and Dohrmann, 2001). Both Fe(III) and Fe(II) form very stable complexes with cyanide and both Fe-cyanide complexes are nearly kinetically inert (Sharpe, 1976). However, the Fe-cyanide complexes are potentially hazardous, because they are converted to extremely toxic free cyanide, CN- and HCNg,aq, when transported to surface water and exposed to sunlight (Meeussen et al., 1992). All dissociation constants of ferricyanic acid are <1 and so are pK1 and pK2 of ferrocyanic acid. The dissociation constants pK3 and pK4 of ferrocyanic acid are 2.2 and 4.2 (Jordan and Ewing, 1962), respectively.

The mobility of dissolved Fe-cyanide complexes in soils on sites of former MGPs and coke ovens is controlled by precipitation and dissolution of Berlin Blue and adsorption on soil minerals, if the concentrations are too small for precipitation (Meeussen, 1992). Adsorption is a possible retention mechanism for Fe-cyanide complexes in uncontaminated soils as well, because the complexes have largely negative charges. Generally, anion sorption in soils depends on the contents of soil constituents bearing positive charges with the exception of anions which can form inner-sphere surface complexes on negatively charged surfaces. Apart from ionic strength, the variable charge is a function of the soil reaction. Thus, acid soils should sorb anions to a larger extent than neutral and alkaline soils. Possible sorbents in soils are Al and Fe (hydr)oxides and clay minerals (Scheffer and Schachtschabel, 1998). Depending on pH, the overall charge of SOM is either neutral or negative. Therefore, SOM may decrease anion sorption (Yu, 1997).

Iron-cyanide complexes are sorbed by various mechanisms on inorganic soil constituents. Ferricyanide was mobile in soils, the factors influencing its sorption were pH and the contents of Fe oxides and clay minerals (Fuller, 1985). Decreasing soil pH increased ferrocyanide sorption by soils (Ohno, 1990). Neither ferricyanide nor ferrocyanide were retarded by sandy aquifer material as judged from column experiments (Ghosh et al., 1999). Similar to sulfate, ferricyanide was predicted to form outer-sphere and weak inner-sphere surface complexes on goethite, {alpha}-FeOOH, whereas ferrocyanide was sorbed inner-spherically and by precipitation of a Berlin-Blue-like phase (Rennert and Mansfeldt, 2001). Outer-sphere surface complexation of ferricyanide on goethite was reported by Theis et al. (1988). Both anions formed outer-sphere surface complexes on {gamma}-Al2O3 (Cheng and Huang, 1996). They were interlayered in clay minerals such as hydrotalcite (Hansen and Koch, 1994), and ferricyanide was not adsorbed on negatively charged kaolinite (Stein and Fitch, 1996).

The objective of this paper was to investigate the sorption behavior of Fe-cyanide complexes in uncontaminated soils by batch experiments. Physical and chemical properties of the soil samples were used to explain the extent of sorption by multiple regressions. Therefore, the statistical evaluation should yield soil properties which are important for the sorption of Fe-cyanide complexes in soils. The term sorption used here includes both two-dimensional adsorption and three-dimensional processes such as diffusion into the interior of minerals and surface precipitation (Scheidegger and Sparks, 1996).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Characterization of Soil Samples
Seventeen soil horizons were investigated. According to the World Reference Base for Soil Resources (Deckers et al., 1998), the soils were classified as Gleysols, Fluvisols (developed from recent fluviatile and marine sediments), Podzols, Cambisols, Ferralsols, and Phaeozems. The samples were collected in excavated pits, freeze-dried, and sieved to 2 mm. Particle-size distribution was analyzed according to Schlichting et al. (1995) by sieving and sedimentation. Contents of dithionite-citrate-extractable (Fed) and Feo were determined following Mehra and Jackson (1960) and Schwertmann (1964). Contents of amorphous Al (hydr)oxide, Alo, were determined by oxalate-extraction after Schlichting et al. (1995). The extracts were analyzed for Fe and Al by atomic absorption spectroscopy (PE 3100, Perkin Elmer, Überlingen, Germany). Organic C, Corg, was calculated from the difference between total and inorganic C. Total C was determined by dry combustion at 1473 K (Coulomat, Deltronik, Düsseldorf, Germany) and subsequent coulometric detection of CO2 which was absorbed in alkaline solution. Inorganic C was determined with the same equipment by adding 15% (vol./vol.) HClO4 to the preheated sample (333 K). Soil pH was measured potentiometrically with a WTW pH 90 (Wissenschaftlich-Technische Werkstätten, Weilheim, Germany) and a INLAB 406 electrode (Ingold, Steinbach, Germany) in 0.01 M CaCl2 with a soil/solution ratio of 1:2.5. Soil organic matter was destroyed in six samples by treating them with 30% (vol./vol.) H2O2 at 333 K for at least 21 d. These samples were also used in sorption experiments. Altogether the sorption of Fe-cyanide complexes by 23 samples was investigated.

Sorption Experiments
For the sorption experiments K3[Fe(CN)6] and K4[Fe(CN)6] (reagent grade, Riedel-de Haën, Germany) were used. From these salts, stock solutions containing 2000 mg CN- L-1 were prepared. All sorption experiments were carried out by a batch technique in 50-mL polyethylene bottles at a temperature of 283 K in duplicate. About 1.25 g of soil was suspended in 25 mL H2O, and ionic strength was adjusted to 0.01 using 0.1 M NaNO3. Then an aliquot ranging from 0 to 12.5 mL of a Fe-cyanide complex solution was added to gain initial concentration ranging from 0 to 37.4 M [Fe(CN)6]. The range of concentrations differed between the samples. The highest concentrations were used with the Fluvisol samples. All experiments were conducted at soil pH. The samples were shaken horizontally for 24 h in darkness at 150 oscillations per min. This reaction time is sufficient for both complexes to equilibrate on goethite (Theis et al., 1988; Rennert and Mansfeldt, 2001), on Al oxide (Cheng and Huang, 1996), and on soils (Ohno, 1990). After the experiments, the phases were separated by membrane filtration (cellulose nitrate, 0.45-µm filter). The filtrates were digested and distilled with a microdistiller (Eppendorf-Netheler-Hinz, Hamburg, Germany) and cyanide concentrations were subsequently determined spectrophotometrically at 600 nm using a Lambda 2 spectrophotometer (Perkin Elmer, Überlingen, Germany). The whole procedure is described in detail by Mansfeldt and Biernath (2000). As Fe-cyanide complexes were the only cyanide species present in the filtrates, cyanide concentrations were recalculated to Fe-cyanide complex concentrations. From each set of experiments, we took three random samples which were analyzed for free cyanide. Free cyanide was never found. All samples were stored in darkness.

Sorption Isotherms
The sorption experiments were interpreted using the Langmuir-Freundlich isotherm (Sips, 1948; cited after Kümmel and Worch, 1990)

[1]
where S is the sorbed amount [mmol Fe(CN)6 kg-1]; Smax, the maximum sorption [mmol Fe(CN)6 kg-1]; b and n are constants; and c is the final solution concentration [mM Fe(CN)6]. We use the sorption maximum as a measure of affinity, using the parameter b as such a measure may lead to misapplications (Harter and Baker, 1977).

Statistical Evaluation
All statistical calculations were performed with SPSS 9.0 (SPSS, Inc., Chicago, IL). Soil properties were correlated linearly with each other and with the sorption maxima calculated following Eq. [1]. Multiple linear regressions were conducted with Smax as dependent variable and soil properties as independent variables. In a first step, both logarithmic and original values of the variables were used. The contents of clay and Corg were logarithmized after being recalculated to grams per kilograms. The significance of the regression equations was checked by variance analysis (F-test). The significance and the importance as explanatory variables of the parameters were checked using T-tests. Theoretical values for F and T, which are necessary to decide whether an equation or a variable can be accepted or has to be refused due at a given confidence level, were adapted from Backhaus et al. (1996). The confidence level used for variance analysis was at least 0.975. The confidence level for the T-tests varied between 0.5 and 0.999, as described below. Regression equations were recalculated without those variables which were not significant or important or both as explanatory variables because of the results of the T-tests. Multiple regressions were performed with all soil samples (n = 17), with the samples containing >10 g Corg kg-1 (n = 8), and with the samples containing >10 g Corg kg-1 (n = 9). The six samples treated with H2O2 are artificial substrates and therefore not included in the statistical evaluation.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Chemical and physical properties of the soils are presented in Table 1. The soils were acid to neutral with pH values ranging from 3.1 to 7.0. Clay contents varied between 21 and 638 g kg-1. Most of the samples contained <10 g Corg kg-1, the overall range was 1 to 25 g kg-1. The samples contained between 1 and 49 g pedogenic Fe (Fed) kg-1. The contents of AlO varied between 0.1 and 2.5 g kg-1. Inorganic C was present in one sample only (#14), it contained 3 g inorganic C kg-1.


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  		Table 1. Chemical and physical properties of the soil horizons.

 
The sorption of Fe-cyanide complexes in soil samples containing >10 g Corg kg-1 is presented in Fig. 1 . All sorption experiments were fitted to Eq. [1]. Mostly L-type isotherms were observed, although four S-type isotherms were found (ferricyanide: Samples #05 and #07; ferrocyanide: Samples #04 and #07). S-type isotherms are characterized by very small sorption at low concentrations. However, the adherence of sorption data to an isotherm provides no evidence of the actual sorption mechanism (Sposito, 1984). Isotherm parameters for these and all other experiments are given in Table 2. For both complexes, two groups of samples differing in their affinities for Fe-cyanide complexes based on Smax values could be distinguished. Three samples (#01, #04, and #06) sorbed both complexes to a very small extent. Ferricyanide was sorbed on the Samples #02, #03, #05, and #09 to a small extent. Ferrocyanide sorption on the Samples #05, #07, and #09 was larger. Both complexes were sorbed to the largest extent on #08. This sample was acid and contained large amounts of clay and Fe oxides. In general, ferricyanide was sorbed to a smaller extent than ferrocyanide. The ratio of the sorption maxima [Smax (ferrocyanide)/Smax (ferricyanide) ranging from 1.2 to 6] was mostly larger than the ratio of the complex charges, the Samples #02 and #06 were exceptions.



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  		Fig. 1. (a) Ferricyanide and (b) ferrocyanide sorption in soil horizons containing >10 g Corg kg-1 (#01 C; #02 E; #03 Bsh; #04 stagnic 2Bg; #05 Bw; #06 gleyic Bg; #07 BC; #08 ferralic B; #09 stagnic Bg). Isotherms are plotted following Eq. [1].

 

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  		Table 2. Parameter values of Langmuir-Freundlich isotherms of the sorption of Fe-cyanide complexes on soils.

 
The sorption of Fe-cyanide complexes in soils containing >10 g Corg kg-1 is shown in Fig. 2 . Again, the sorbed amounts between the samples varied, but these samples had a higher affinity for the complexes, because the sorbed amounts at Smax were larger compared with those shown in Fig. 1. The isotherms were linear over a wide range of equilibrium concentrations. For larger concentrations saturation plateaus were found. Only Samples #12 and #13 sorbed more ferrocyanide than ferricyanide at Smax. The other samples had a higher affinity for ferricyanide. Especially Samples #10 to #12, #15, and #16 sorbed large amounts of Fe-cyanide complexes. Samples #13, #14, and #17 sorbed Fe-cyanide complexes to a distinctly smaller extent.



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  		Fig. 2. (a) Ferricyanide and (b) ferrocyanide sorption in soil horizons containing >10 g Corg kg-1 (#10 Ap; #11 Ah; #12 fluvic Ah; #13 Ah; #14 Ap; #15 fluvic Ah; #16 fluvic Bg; #17 mollic Ah). Isotherms are plotted following Eq. [1].

 
The sorption of the complexes on soils which were treated with H2O2 is shown in Fig. 3 . The destruction of SOM resulted in a large decrease in the sorption of both complexes. The decrease in Corg was in the range of 84 to 98% (calculated from data in Table 1), and it consisted roughly with the decrease in Smax (77 to 99%, calculated from Table 2). As a consequence, most of the samples used here sorbed small amounts of Fe-cyanide complexes, because their sorption maxima were in the range of 0.2 to 2.6 mmol [Fe(CN)6] kg-1. This was in the range of the results of the samples shown in Fig. 1. Hence, these treated samples showed a similar behavior as samples which were naturally poor in Corg. However, there were four samples showing sorption maxima in the range of 6 mmol [Fe(CN)6] kg-1. Hence, organic matter had an influence on the extent of sorption of Fe-cyanide complexes on the samples used here.



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  		Fig. 3. (a) Ferricyanide and (b) ferrocyanide sorption in soil horizons after treatment with H2O2 (#10 Ap; #11 Ah; #12 fluvic Ah; #15 fluvic Ah; #16 fluvic Bg; #17 mollic Ah). Isotherms are plotted following Eq. [1].

 
Correlation coefficients of the relations between variables, including sorption maxima and soil properties, are presented in Table 3. The sorption maxima correlated with each other and with Corg. Furthermore, the various Fe fractions correlated and so did pH and Corg. Therefore, the combination of the soil parameters was used to explain the sorption maxima calculated following Eq. [1] using multiple regressions. We are aware of the fact that the following regression equations are only valid for the soils used here. It is not likely that they can be transferred to all soils quantitatively. However, they yield soil parameters qualitatively which are important for the sorption of Fe-cyanide complexes in soils. As noted before, both logarithmic and original data were used. In the calculations, the results obtained from using logarithmic values were less significant compared with those with the original data. Therefore, only the results using the original data are presented. For both complexes the relation between sorption maxima and soil parameters of the 17 original soil samples was significant. For ferricyanide the corrected square of the correlation coefficient r was 0.478, and for ferrocyanide r2 was 0.41. The regression equations for all soil samples are for ferricyanide

[2]
and for ferrocyanide

[3]


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  		Table 3. Correlation coefficients of soil properties and sorption maxima of the sorption of Fe-cyanide complexes in soils (n = 17).

 
The sorption maxima of both complexes depended on the contents of clay, Corg, and Feo. They differed in the value of the constant and in that ferrocyanide sorption additionally depended on pH. As could be expected, clay and Feo had a positive effect, and pH had a negative effect on the sorption of Fe-cyanide complexes in soils. High contents of Corg promoted the sorption of both anions. The correlation coefficients of the Eq. [2] and [3] were relatively small, therefore multiple regressions were performed with two groups of samples differing in their Corg contents. The two following regression equations were only valid for samples containing >10 g Corg kg-1. The regression equation for ferricyanide is

[4]
and is for ferrocyanide

[5]

Compared with the results of all soils, the correlation coefficient for both equations increased. Clay contents and pH influenced the sorption of both complexes, whereas Corg became irrelevant. Furthermore, the ferricyanide sorption maximum depended on Alo. However, as indicated in Eq. [4], Feo had a negative effect on ferricyanide sorption. Compared with Eq. [2] and [3], multiple regressions with samples containing >10 g Corg kg-1 yielded large correlation coefficients. For these samples the regression equation for ferricyanide is

[6]
and is for ferrocyanide

[7]

In both equations, the sorption maximum was affected positively by Alo and negatively by Fed-Feo. Only ferrocyanide sorption was promoted by Corg.

All regression equations (Eq. [2]–[7]) were checked by variance analysis to evaluate the level at which the equation was significant. As checked by F-tests, all regression equations were significant at various confidence levels, as shown in Table 4. A regression equation is significant at a given confidence level, if the empirical F value, Femp, is larger than the theoretical one, Ftheo. The smallest level was 0.975. The statistical checks of the significance and the importance of any individual variable of the Eq. [2] through [7] using T-tests are shown in Table 5. A variable influences the sorption maximum, if the absolute value of the empirical T, Temp, is larger than the theoretical T, Ttheo. The value of Ttheo depends on the number of the degrees of freedom and on the confidence level at which a variable is valid. Only the confidence levels are presented in Table 5, the respective values of Ttheo can be found in various statistical textbooks. The importance of a parameter as a explanatory variable and its direction is given by the beta values in Table 5. The larger is the absolute value of beta, the larger is the explanation of this variable, and thus its importance. Therefore, we can see that sorption of both complexes on all samples (Eq. [2] and [3]) was determined by the contents of Corg and clay. For samples containing >10 g Corg kg-1 (Eq. [4] and [5]), the clay contents and pHs were most important for the sorption of both complexes. Unlike for the Eq. [2] through [5], the results for the samples containing more than 10 g Corg kg-1 differed between the complexes. Clay contents were most important for ferricyanide sorption in these soils, for ferrocyanide sorption it were the contents of Alo.


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  		Table 4. Variance analytical interpretation of the regression Eq. [2] through [7].

 

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  		Table 5. Significance and importance as explanatory variables of the regression variables checked by T-tests.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
As noted before, the mechanism, by which an Fe-cyanide complex is sorbed, depends on the nature of the sorbent. The mechanisms by which Fe-cyanide complexes are sorbed on inorganic soil constituents such as clay minerals and Al and Fe oxides are largely clear, as presented in the introduction. The interactions between Fe-cyanide complexes and SOM have not yet been investigated. Schenk and Wilke (1984) showed that the cyanide ion, CN-, forms charge transfer complexes via cyanide N with quinone groups of humic acids using infrared spectroscopy. Both Fe-cyanide complexes have an octahedral structure with N located in the edges. Hence, Fe-cyanide N is potentially able to form the same complexes with quinone groups as cyanide N. Quinone groups are oxidation products of phenols and various polycyclic aromatic hydrocarbons. They are present in soil humic acids (Stevenson, 1994). Quinone groups can react with N-containing compounds such as amino acids (Stevenson, 1994), and a reaction between quinone groups and Fe-cyanide is possibly given by a nucleophilic addition reaction. This reaction type includes charge transfer complexes between cyanide N and quinone groups, as proposed by Schenk and Wilke (1984). Hence, this reaction might explain the positive influence of SOM on Fe-cyanide complex sorption. As the formation of quinones by oxidation of phenols occurs preferably in neutral and alkaline soils, it becomes clear that the qualitative composition of SOM and not the Corg content alone governs the extent of the reaction between Fe-cyanide complexes and SOM, as proposed above. However, it is possible that there are further functional groups in SOM which may be able to sorb Fe-cyanide complexes, e.g., positively charged functional groups such as R–NH+3.

The promotive effect of SOM was shown by the regression equations for all samples and by the decrease of sorption of the Fluvisol samples which were treated with H2O2. However, there were samples rich in Corg sorbing smaller amounts of Fe-cyanide complexes (Samples #07, #14, and #17). Hence, SOM of Fluvisols has perhaps a special affinity for Fe-cyanide complexes, i.e., this SOM contains functional groups, which can react with Fe-cyanide complexes, to a larger extent. In other studies dealing with the sorption of Fe-cyanide complexes in soils (Fuller, 1985; Ohno, 1990; Ghosh et al., 1999), the contents of Corg were not mentioned. A positive influence of SOM on anion adsorption on soils cannot be expected because of the overall neutral or negative charge of SOM, if we reduce anion sorption solely to electrostatic attraction. However, we could show that SOM promotes the sorption on some samples, hence the content of Corg is a soil property possibly enhancing the sorption of Fe-cyanide complexes on soils.

Furthermore, the sorption of Fe-cyanide complexes in soils is influenced by pH, clay, Feo, and Alo. The importance of these parameters for the sorption of inorganic anions on soils has been reported in the literature. Sorption maxima of P sorption by wetland soils were predicted by the contents of Feo and Alo (Reddy et al., 1998). Agbenin and Tiessen (1994) found that the phosphate sorption maxima of Brazilian soils could be explained by the contents of pedogenic Fe and Al and fine clay. Sulfate sorption in forest soils was governed by the contents of Feo, Alo, and pH (MacDonald and Hart, 1990). They used logarithmic values in multiple regressions. Based on the regression Eq. [3] through [5] and [7], sorption of Fe-cyanide complexes is enhanced at low pH values. However, innersphere surface complexation of an anion on a mineral surface is possible at a pH above the point of zero charge of the mineral. As pointed out in the introduction, innersphere surface complexation of Fe-cyanide complexes at high pH is known for ferrocyanide sorption on goethite only.

Maximum sorption of anions such as silicate or borate at high pH is caused by the fact that maximum sorption occurs near the pKa values of the conjugate acids (Hingston, 1981). However, all pKa values of ferricyanic and ferrocyanic acid are 4.2 or clearly lower (Jordan and Ewing, 1962). Hence, the negative effect of pH on Fe-cyanide complex sorption in the regression equations is consistent with the known sorption mechanisms and the concept of the effect of pKa values of conjugate acids. The sorption of Fe-cyanide complexes on soils containing <10 g Corg kg-1 could be explained by the soil properties mentioned above. Sorption of anions such as Fe-cyanide complexes on clay minerals and sesquioxides is given by the ability of the anions to form inner- or outersphere surface complexes or both. Therefore, anion sorption in soils generally depends on the contents of the mineral constituents mentioned above and pH. The extent of ferrocyanide sorption on neutral samples is not larger than that of ferricyanide. The opposite could be inferred from the pH-dependency of Fe-cyanide complex sorption on goethite (Rennert and Mansfeldt, 2001). Therefore, only the combination of soil properties may explain the extent of sorption suitably, because the mechanisms of Fe-cyanide complex sorption on mineral constituents differ. The lower affinity of samples containing small amounts of Corg for Fe-cyanide complexes cannot be explained by sorption kinetics, because it was shown that the sorption on Fe and Al (hydr)oxides is finished after a reaction time of 24 h (Cheng and Huang, 1996; Rennert and Mansfeldt, 2001).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Sorption of ferricyanide and ferrocyanide in soils seems to be more complex than the sorption of other inorganic anions because of the promoting influence of SOM. The sorption capacities of the soil samples differed over a wide range. This is important for the mobility of Fe-cyanide complexes which have been introduced into the natural soil environment. Soils containing high amounts of Corg and Alo and acid soils containing high amounts of Alo should sorb Fe-cyanide complexes to a large extent, and should thus minimize the translocation of these anions into the ground water. Most Fluvisols investigated here have a high sorption capacity for Fe-cyanide complexes. This capacity is caused by the presence of organic matter having a high affinity for Fe-cyanide complexes. Further research is necessary to enlighten the nature of the interactions between Fe-cyanide complexes and soil organic matter as well as to identify the functional groups which are responsible for these interactions.


   ACKNOWLEDGMENTS
 
This paper represents publication no. 163 of the Priority Program "Geochemical processes with long-term effects in anthropogenically affected seepage- and groundwater". Financial support was provided by Deutsche Forschungsgemeinschaft. We thank Jörg Rinklebe, UFZ Leipzig-Halle, Germany, for contributing four Fluvisol samples and the Phaeozem sample. Dr. M.W.I. Schmidt, Universität zu Köln, Germany, contributed one sample. Assistance in the laboratory was given by Carolin Kaufmann, Willi Gosda, and Frank Wellerdt, Ruhr-Universität Bochum.

Received for publication March 21, 2001.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome