Langmuir 1993,9, 3057-3062
3057
Adsorption and Coprecipitation of Multiple Heavy Metal Ions onto the Hydrated Oxides of Iron and Chromium Russell J. Crawford,' Ian H. Harding, and David E. Mainwaring Centre for Applied Colloid and BioColloid Science, Department of Applied Chemistry, Swinburne University of Technology, P.O. Box 218, Hawthorn 3122, Australia Received January 11, 1993. I n Final Form: July 15, 199P The removal of metal ions from solution using adsorption and coprecipitationtechniques in the presence of more than one metal ion has been measured and modeled. The removal of all combinationsof aqueous metal ions, Cr(III),Ni(II),and Zn(II),using amorphous iron(II1)oxide has been measured and then modeled using the James-Healy method13 for metal ion adsorption. Similarly, the removal of aqueous Ni(I1) and Zn(II), both singly and in combination, using amorphous chromium(II1) oxide as the adsorbing or coprecipitating colloid has been measured and modeled. Simple competition between metal ions for surface sites can account for the results obtained in the simple systems involving Ni(I1) and Zn(I1) as adsorbing ions. The presence of Cr(II1)in adsorption and coprecipitationexperiments,however,enhanced the removal of both Ni(I1)and Zn(II),and so simple competitionfor surface sites cannot explain the results obtained. Two different mechanisms are discussed which could account for this enhancement. Firstly, in the presence of Cr(III), the surface chemical characteristics of the adsorbent surface change as the various metal ions adsorb or coprecipitate, and this influences the removal characteristics of subsequently adsorbing ions. Secondly,amixed hydroxide speciesforms in solution and this speciesis strongly adsorbed.
Introduction The adsorption of single metal ions onto colloidal surfaces has been extensively investigated- and models for this process have been proposed and applied to various systems.13vs11 The models available are relatively successful at predicting the removal characteristics of single metal ion species onto rigorously defined colloidalsurfaces; however less attention has been given to systems where the surface characteristics of the adsorbent change as the adsorption process proceeds, such as is possible in multiple metal ion adsorption.12 In this study, the influence of a second and third metal ion on the adsorption or coprecipitation of a given metal ion will be investigated, initially using amorphous iron(111)oxide and then amorphous chromium(II1) oxide as the adsorbing or coprecipitating colloid. The individual adsorption and coprecipitation removal characteristics of the single metals Cr(III), Ni(II), and Zn(II), using an amorphous iron(II1) oxide colloid, and of Ni(I1)and Zn(II),using an amorphous chromium(1II)oxide colloid, have been measured and modeled by the authors13 using the James-Healy model for metal ion adsorption.13 0
Abstract published in Advance A C S Abstracts, September 15,
1993.
(1) James, R. 0.; Healy, T. W. J. Colloid Interface Sci. 1972,40 (l), 42. (2)James, R. 0.; Healy, T. W. J. Colloid Interface Sci. 1972,40 (l), 53. (3)James, R. 0.; Healy, T. W. J. Colloid Interface Sci. 1972,40 (l), 65. (4)Harding, I. H.; Healy, T. W. J. Colloid Interface Sci. 1985,107 (2), 362. (5)Harding, I. H.; Healy, T. W. J. Colloid Interface Sci. 1985,107 (2), 371. (6)Benjamin, M.M.Enuiron. Sci. Technol. 1983, 17, 686. (7) Benjamin, M. M.; Leckie, J. 0. Enuiron. Sci. Technol. 1981, 15, 1050. (8)Benjamin, M. M.; Leckie, J. 0. Enuiron. Sci. Technol. 1982, 16, 162. (9)Davis, J. A.; James, R. 0.; Leckie, J. 0. J. Colloid Interface Sci. 1978,63,480. (10)S t u " ,
W.;Krummert, R.; Sigg, L. Croat. Chem. Acta 1980,53, 291. (11)Fueratenau, D. W.; Osseo-Asare,K. J. ColloidInterfaceSci. 1987, 118(2), 524. (12)Benjamin, M.M.; Leckie, J. 0. J. Colloid Interface Sci. 1981,83 (2),410.
Details of the modeling are presented in this earlier paper. In summary, the free energy of adsorption is broken into three components, AG'wlvation,i, AG'Codombic,i, and AG'chemidi. The first two terms are fixed by the use of simple electrical double layer theory while the third term is effectivelya fitting parameter. The values of A G o h d d , j obtained for the single metal adsorption/coprecipitation systems stated above are given in Table I. The aim of this study is to determine the influence of a second or third metal ion on the resulting adsorption and coprecipitation profiles and particularly on the AGochemid,i appropriate to a given metal ion and a given substrate surface.
Experimental Section Details of the experimental parameters and conditions are given in our earlier paper.13 The differences between precipitation, adsorption,and coprecipitationhave also been discussed but should, however, be stressed. Precipitation implies the formation of a metal hydroxide or hydrous oxide simply by increasing the pH of the metal ion solution under investigation. Adsorption implies removal by a substrate which is separately prepared (by pH adjustment)and then mixed with the metal ion solution under investigation. Coprecipitationimplies removal by a substrate which is prepared in the presence of the metal ion under investigation. In thispaper,multiple metalions complicate the above definitionsin that two or more metal ions when added to a preformed colloid are considered to undergo adsorption; however they may also coprecipitate with each other. In this study, adsorption experiments are defined only in terms of the main adsorbing substrate so that the above coprecipitation processes may occur during "adsorption" experiments. Results The data resulting from the various adsorption and coprecipitation experiments are given in Figures 1-10. Listed with each figure are the A G ' c h e d d i values used in the James-Healy modeling for these systems. A single A G ' c h e f i d j value is given in the cases where adsorption or coprecipitation can only be occurring with one particular surface type; however also listed in some cases are the (13)Crawford, R. J.; Harding, I. Preceding paper in this issue.
H.; Mainwaring, D. E. Langmuir.
0743-7463/93/2409-3057$04.00/0 0 1993 American Chemical Society
Cruwford et al.
3058 Langmuir, Vol. 9, No. 11, 1993 100,
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Figure 1. Adsorption of Cr(II1) (50 ppm) with amorphous iron(111) oxide (250 ppm) alone and with either 50 ppm Ni(II), 50 ppm Zn(II), or both 50 ppm Ni(I1) and 50 ppm Zn (11): (1) A C o d e d ~ ,=i -50 kJ mol-', amorphous iron(II1) oxide surface.
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Figure 2. Coprecipitation of Cr(II1) (50 ppm) with amorphous iron(II1)oxide (250 ppm) alone and with either 50 ppm Ni (II), 50 ppm Zn(II), or both 50 ppm Ni(I1) and 50 ppm Zn(1I): (1) AGochemid,i = -61 kJ mol-*,amorphous iron(II1) oxide surface. AG",h,d&,i values which would result if adsorption or coprecipitation occurred with different surface types, these resulting from previous adsorption or coprecipitation of metal species in the multiple metal ion experiments. 1. ExperimentsInvolvingCr(II1) as the Adsorbing or Coprecipitating Metal Ion. The removal by adsorption of Cr(1II) onto amorphous iron(II1) oxide in various metal ion environments is given in Figure 1. The four adsorption curves follow the same removal profile and can be modeled using a single theoretical isotherm. The presence of Ni(I1) and/or Zn(I1) has no influence over the adsorption profile of Cr(II1) onto amorphous iron(II1) oxide. The removal by coprecipitation of Cr(II1) with amorphous iron(II1) oxide in various metal ion environments is given in Figure 2. The four coprecipitation curves again follow the same removal profile and can be modeled using a single theoretical isotherm. The presence of Ni(I1) and/ or Zn(I1) has no influence over the coprecipitation profile of Cr(II1) onto amorphous iron(II1) oxide. As shown in our earlier paper,13the extent of removal by coprecipitation is, a t any given pH, greater than the extent of removal by adsorption. 2. Experiments Involving Ni(I1) as the Adsorbing or Coprecipitating Metal Ion. The removal by adsorption of Ni(I1)ontoamorphous iron(II1)oxide in various
Figure 3. Adsorption of Ni(I1) (50 ppm) with amorphous iron(111)oxide (250 ppm) alone and with either 50 ppm Cr(III), 50 ppm Zn(II), or both 50 ppm Cr(II1) and 50 ppm Zn(I1): (1) AGO&&, = -46 kJ mol-', amorphous iron(II1) oxide surface, AGochedw = -52 kJ mol-', amorphous chromium(II1) oxide surface, A G o ~ d =j -51 kJ mol-', amorphous zinc(I1) oxide surface;(2) AGO&& = -33 kJ mol-', amorphous iron(II1)oxide surface, AG',hdw = -39 kJ mol-', amorphous zinc(I1) oxide surface. metal ion environments is given in Figure 3. The presence of Zn(I1)has little effect on the adsorption of Ni(II), despite the expectation that Zn(I1) would have almost completed its adsorption at the onset of Ni(I1) adsorption. There is a common argument (see, for example, Netzer and HughesI4) that metal ions compete for surface sites and that adsorption of one metal ion should inhibit the adsorption of a second. Benjamin and Leckie12 on the other hand have argued that different metals compete for different surface sites and that adsorption of one metal can have little influence over later adsorption of a second metal. The data shown in Figure 3 do support this latter approach although the lack of effect could be simply due to insensitivity of changes in surface area (see Figure 8 in our previous paper). Whatever the case, adsorption of Zn(I1) appears to have little effect over the adsorption of Ni(I1) onto amorphous iron(II1) oxide. The presence of Cr(III),however,considerably enhances the adsorption of Ni(I1). Enhancement of adsorption is rarely seen in metal ion adsorption experiments and even in this study the enhancement may reflect some coprecipitation (which we have shown to be more efficient than adsorption) as well as adsorption as the removal mechanism. Cr(II1) coprecipitates with Ni(I1) to remove Ni(11) at a lower pH than Ni(I1) would be removed by adsorption onto iron(II1) oxide. The datashown inFigure 3 probably reflect coprecipitation of Cr(II1) with Ni(I1). The four adsorption curves shown in Figure 3 can be modeled in several different ways, as will be discussed shortly. The data follow two distinct isotherms: one in the presence and one in the absence of Cr(II1). The removal by coprecipitation of Ni(I1) with amorphous iron(II1) oxide in various metal ion environments is given in Figure 4. The data again follow two distinct isotherms; however the difference is not simply the presence of Cr(II1). In contrast to the adsorption experiment, Ni(1I) coprecipitation in the presence of both Cr(111)and Zn(I1) displays a profile which is similar to that observed for Ni(I1) coprecipitation with neither Cr(II1) nor Zn(I1) present, yet the presence of Cr(II1) (without Zn(I1)) still enhances removal. ~
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(14)Netzer, A.; Hughes, D.E. Water Res. 1984,18,927.
Adsorption of Heavy Metal Ions
Langmuir, Vol. 9, No. 11,1993 3059
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zinc(I1) oxide surface.
The removal by adsorption of Ni(I1) onto amorphous chromium(II1) oxide, both alone and in the presence of Zn(II),is given in Figure 5. Ni(I1) adsorption in the absence of Zn(I1) follows the expected isotherm shape; however the isotherm corresponding to Ni(I1) removal in the presence of Zn(I1) is of an unusual shape. At a pH less than approximately 6, Ni(I1) removal is enhanced by the presence of Zn(I1); however at higher pH values Ni(I1) removal is, if anything, slightly inhibited. The enhancement seen is very unusual and difficult to attribute to simple coprecipitation of Ni(I1) with Cr(II1) since that isotherm is shown (in Figure 5 ) to be a t higher pH, or of simple coprecipitation of Ni(I1) with Zn(1I) since no such enhancement was seen in the case when amorphous iron(111)oxide was used as the substrate. The enhancement of metal ion adsorption is well understood as a surface precipitation phenomenon with species such as surfactants.15 Anion adsorption is also understood to be enhanced, under appropriate conditions, (15) Ananthapadmanabhan,K.P.;Somasundaran, P.Colloids Surf. 1986, 13, 151.
4
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Figure 6. Coprecipitationof Ni(I1) (50 ppm) with amorphous chromium(II1)oxide (250 ppm) alone or with 50 ppm Zn(I1): (1) precipitation isotherm for 250 ppm Cr(II1);(2) A G o - a = -64 kJ mol-', amorphous chromium(II1)oxide surface; AGO-& = -63 kJ mol-', amorphous zinc(I1) oxide surface.
by the presence of an adsorbing metal ion.6 I t has also been postulated that a mixed hydroxy chloride complex can enhance adsorption of a metal ion.16 There is no current theory, however, to explain how the adsorption of one given metal ion can be enhanced by the presence of a second adsorbing metal ion other than the aforementioned coprecipitation which falls short of a complete description. The data in Figure 5 can be modeled assuming standard solution chemistry by selecting the AGOchemi4.i found in the absence of Zn(I1) to be -44 kJ mol-'; however the fit is less satisfactory than that obtained when modelingsingle metal systems.13 Improvement to the model requires the postulation of unusual solution chemistry (see later discussion) or the postulation of two different surfaces involved in Ni(I1) adsorption. These two surfaces are illustrated in Figure 5 by amorphous chromium(II1) oxide with a AGochemicd,i of -52 kJ mol-' and amorphous zinc(I1) oxide with a A G o c h e m i 4 i of -41 kJ mol-'. The surface area of a colloid coated with Zn(II), which now resembles amorphous zinc(I1) oxide, is assumed to be that of the original substrate. The experimental points can be seen to be modeled by pH-dependent proportionation of the two theoretical curves. The removal by coprecipitation of Ni(I1) with amorphous chromium(II1) oxide, both in the presence and absence of Zn(II), is given in Figure 6. In both cases the removal of Ni(I1) follows initially the precipitation isotherm for the Cr(II1) alone. Subsequent removal follows an isotherm possessing a shape characteristic of an adsorption or coprecipitation process. Zn(I1) has little, if any, effect on the percentage removal by coprecipitation of Ni(I1) with amorphous chromium(II1) oxide. 3. Experiments Involving Zn(I1)as the Adsorbing or Coprecipitating Metal Ion. The removal by adsorption of Zn(I1) onto amorphous iron(II1)oxide in various metal ion environments is given in Figure 7 and shows similar phenomenon to that given in Figure 3 for Ni(I1). The presence of Ni(I1) has little effect on the adsorption of Zn(I1); however the presence of Cr(II1) considerably enhances the adsorption of Zn(I1). The four adsorption curves can be successfully modeled in several different (16) Spark, K.;Johnson, B. B.; Wells,J. D.A u t . J. Chem. 1990,43, 749.
(17) Krupanidhi, S. B.;Sayer, M. J. Appl. Phys. 1984,66 (Il), 3308.
Crawford et al.
3060 Langnuir, Vol. 9,No. 11, 1993
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Figure 7. Adsorption of Zn(I1) (50 ppm) with amorphous iron(111)oxide (250 ppm) alone and with either 50 ppm Cr(III), 50 ppm Ni(II), or both 50 ppm Cr(1II) and 50 ppm Ni(I1): (1) A G o c b d d j = -55 kJ mol-', amorphous iron(II1) oxide surface, AGO*&& = -63 kJ mol-', amorphous chromium(II1) oxide surface; (2) AG0bedd, = -45 kJ mol-', amorphous iron(II1)oxide
Figure 9. Adsorption of Zn(I1) (50 ppm) with amorphous chromium(II1)oxide (250 ppm) alone or with 50 ppm Ni(I1): (1) AGO*& = -62 kJ mol-', amorphous chromium(II1) oxide
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Figure 8. Coprecipitationof Zn(I1) (50 ppm) with amorphous iron(II1)oxide (250 ppm) alone and with either 50 ppm Cr(III), 50 ppm Ni(II),or both 50 ppm Cr(II1)and 50 ppm Ni(I1): (1) A G o b d d j = -45 kJ mol-', amorphous iron(II1)oxide surface; A G o m d , i = -54 kJ mol-', amorphous chromium(II1) oxide
surface.
ways, as will be discussed shortly. The data follow two distinct trends: one in the presence and one in the absence of Cr(II1). The removal by coprecipitation of Zn(I1) onto amorphous iron(II1) oxide in various metal ion enviroments is given in Figure 8. It can be seen that a t higher removals the four isotherms are approximately equivalent. There is a deviation from a single isotherm at lower removals. Again, the presence of Cr(II1) enhances removal; however in this case (Zn(I1) in contrast to the case of Ni(II)), the enhancement is small and occurs only at low percentage removal. The removal by adsorption of Zn(I1) onto amorphous chromium(II1) oxide, both alone and in the presence of Ni(I1); is given in Figure 9. In contrast to the adsorption of Ni(I1) onto amorphous chromium(II1) oxide (Figure 5), there is no an~malousisotherm shape, although the gradient of the isotherm is steeper than seen in most other cases. The presence of Ni(I1) does not affect the removal of Zn(I1) by adsorption onto amorphous chromium(II1) oxide. I t is important to note that at pH 5.5, where Zn(I1) adsorption has commenced, Cr(II1) is not fully precipi-
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Figure 10. Coprecipitationof Zn(I1) (50 ppm) with amorphous chromium(II1)oxide (250 ppm) alone or with 50 ppm Ni(I1): (1) precipitation isothermfor 250 ppm Cr(II1);(2) AGO,+,&& = -75 kJ mol-', amorphous chromium(II1) oxide surface.
tatedl3 and this would inhibit removal at low pH. When the data are modeled, the reduction in overall surface area caused by incomplete formation of the amorphous chromium(II1) oxide was taken into account. The removal by coprecipitation of Zn(I1) with amorphous chromium(1II) oxide, both alone and in the presence of Ni(II), is given in Figure 10. The removal profile can be modeled in two ways, either by assuming that the Zn(11)is physically removed as the Cr(II1) precipitates by simple entrainment or by using the James-Healy model. Both theoretical isotherms are almost identical and model accurately the experimental isotherms in both the presence and absence of Ni(I1). The presence of Ni(I1) has no effect on the removal of Zn(I1)by coprecipitation with amorphous chromium(II1) oxide.
Discussion 1. The James-Healy Model. Details of the JamesHealy model and the parameters required are given in our earlier ~ a p e r . 1In~ short, the important parameters used in modeling adsorption and coprecipitation data were surface area, isoelectric point, and dielectric constant of the solid substrate and A G ' c h e d d i of the metal ion and its hydrolysis products. The A G o c h e d d , i values selected on the basis of experiments in the presence of single adsorbent systems are given in Table I. The respective
Adsorption of Heavy Metal Ions
Langmuir, Vol. 9, No. 11, 1993 3061
Table I. Chemical Free Energy Components of Adsorption and CoDreciDitation ~~
~
(kJ mol-') adsorption coprecipitation AGO-
metal ion Cr(II1) Ni(II) Zn(I1)
colloid type iron(II1) oxide iron(II1) oxide chromium(II1)oxide iron(1II) oxide chromium(1II)oxide
-50
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-45 -75
Table 11. Physical Properties of Solid Substrates surface area isoelectric dielectric adsorbate (m2 L-1) point (pH) constant (e) iron(II1) oxide 60 8.2 14.2 chromium(II1)oxide 53 9.2 8.0 9.8 1l.W zinc(I1) oxide Reference 17.
surface areas, isoelectric points, and dielectric constants used are given in Table 11. 1A. Experiments Involving Cr(II1)as the Adsorbing or Coprecipitating Metal Ion. The AGochemicd,i value used for modeling Cr(II1) adsorption onto amorphous iron(II1) oxide was -50 kJ mol-'. As shown in Figure 1, this value is unaffected by the presence of Ni(I1) and/or Zn(I1). The AGochemid,i value used for modeling Cr(II1) coprecipitation with amorphous iron(II1) oxide was -61 kJ mol-'. As shown in Figure 2, this value is also unaffected by the presence of Ni(I1) and/or Zn(I1). The lack of influence of other metals on the removal of Cr(II1) by amorphous iron(II1) oxide is not surprising since Cr(II1) adsorbs and/or coprecipitates a t a considerably lower pH than either Zn(I1) or Ni(I1). 1B. Experiments InvolvingNi(I1)as the Adsorbing or Coprecipitating Metal Ion. Ni(I1) adsorption onto amorphous iron(II1) oxide has been shown in our earlier paperI3 to be best modeled with a A G o h e m i d , i of -33 kJ mol-'. As is shown in Figure 3, the presence of Zn(I1) has no effect on the A G o c b e m i d i required to model the data well. However,when Cr(II1) is also present, the AGo-d,i required to model the data well is considerably different. While this may be simply due to a coprecipitation mechanism between Ni(I1) and Cr(II1) (see later discussion), it is noteworthy that the James-Healy model is still capable of fitting the data if a larger negative AGochemid,i is applied (see Figure 3). Ni(I1) coprecipitation onto amorphous iron(II1) oxide has been shown in our earlier paperI3 to be best modeled with a AGochemid,i of -38 kJ mol-'. As is shown in Figure 4, the presence of Zn(I1) has no effect on the A G o C h e m i d j required to model the data well. However, when Cr(II1) is present, the required AGochemid,i to obtain a good fit is considerably different. Again this may be simply due to a coprecipitation mechanism between Ni(I1) and Cr(II1). In this case, however AGochemid,i for Ni(I1) coprecipitation onto amorphous iron(II1) oxide is altered by the presence of Cr(III), but must either revert back to the original value when Zn(II), as well as Cr(II1) is present, or the adsorbate surface may have changed to that resembling amorphous zinc(I1) oxide. If Ni(I1) coprecipitation with amorphous iron(II1) oxide in the presence of Zn(I1) and Cr(II1) must be modeled assuming an amorphous zinc(I1) oxide surface, one would expect the same trend for simple adsorption. The optimal A G o c h e G d , i values, and their resultant theoretical curves assuming an amorphous zinc(I1) oxide surface are given with Figure 3. Clearly, if AGochemid,i can be varied at will, the experimental data can be modeled assuming any surface type. Although these data are successfullymodeled assuming adsorption processes only,
this is only possible if one assumes that A G o h m i & is a variable dependent on the solution chemistry of other species present in the system. Ni(I1) adsorption and coprecipitation using amorphous chromium(II1) oxide is even more complex. The AGOchemidi value obtained for adsorption, -44 kJ mol-', remains the optimal value for the model when Zn(I1) is also present (Figure 5). However, the experimental points are considerably underestimated at low pH and overestimated a t high pH. An alternative is to continue the proposition of a surface modification process whereby the chromium(II1) oxide surface changes to resemble that of zinc(I1) oxide due to adsorption of Zn(I1) ions. This is illustrated in Figure 5 by a theoretical curve using initially a A C o h e m i d j of -52 kJ mol-' for a chromium(II1) oxide surface and later a AG'chemidj of -41 kJ mol-' for a zinc(11)oxide surface. For this to be true, the AGohe,,,i& for Ni(I1) adsorption onto amorphous chromium(II1) oxide must again be a variable dependent on the solution chemistry of other ions present. In the earlier case of Ni(I1) adsorption onto amorphous iron(II1) oxide, in the presence of Zn(II), the effect was not obvious; it would not be expected to be obvious, however, since (as shown in Figure 3)there is little difference between Ni(II) adsorption onto an amorphous iron(II1) oxide surface using a AGoch-id,i of -46 kJ mol-l and Ni(I1) adsorption onto amorphous zinc(I1) oxide surface having the same surface area and A G o c h e m i d j of -39 kJ mol-'. The above arguments can be extended to Ni(I1) coprecipitation with amorphous chromium(II1) oxide in the presence and absence of Zn(I1) (Figure 6); however the AGochemid,i term for coprecipitation of Ni(I1) onto a zinc(11)oxide substrate must be increased to a value of -63 kJ mol-'. The early part of the removal profile is not modeled well simply because neither Zn(1I) nor Cr(II1) have themselves been completely removed from solution. The larger negative A G o h e m i d , i required is consistent with all other systems which show coprecipitation to be more effective, at any given pH, than adsorption. 1C. Experiments Involving Zn(I1)as the Adsorbing or Coprecipitating Metal Ion. The A G o h e m i d i used for Zn(I1) adsorption onto amorphous iron(II1) oxide is -45 kJ mol-'. Closer inspection of the data in Figure 7 shows that there is a slight decrease in Zn(I1) removal in the presence of Ni(II), probably simply due to competition for surface sites and subsequent lowering of surface area available for adsorption. Experimental accuracy, however, does not allow a genuine distinction to be made. Qualitatively the pattern observed for Zn(I1) removal in the presence of Ni(I1) and Cr(II1) is similar to the pattern observed for Ni(I1) removal in the presence of Zn(I1) and Cr(II1). The A G o c h w i d , i for Zn(I1) coprecipitation with amorphous iron(II1) oxide used was -45 kJ mol-' as shown in Figure 8. This is not significantly affected by the presence of Ni(I1) or Cr(II1). Since Cr(II1) is removed at a pH much lower than Zn(I1) (see Figure 11, it is likely that the iron(111) oxide surface will be modified by the presence of Cr(II1). To model the data assuming the surface has been modified to that of amorphous chromium(II1) oxide requires a AGochemid,i of -54 kJ mol-', which is not consistent with adsorption or coprecipitation of Zn(I1)with amorphous chromium(II1) oxide (no iron). The A G o c b m i d , i for Zn(I1) adsorption onto amorphous chromium(II1) oxide used was -62 kJ mol-' as shown in Figure 9. The presence of Ni(I1) has no significant effect on this value.
Crawford et
3062 Langmuir, Vol. 9,No. 11, 1993 The A G o c h e d d , j for Zn(I1) coprecipitation with amorphous chromium(II1) oxide was found in our earlier paper to be -75 kJ mol-l; however the isotherm is also equivalent to the precipitation curve for 250 ppm Cr(1II). As shown in Figure 10, the presence of Ni(I1) has no effect. 2. Coprecipitationversus Adsorption. Much of the unusual enhancement of adsorption data shown earlier is clearer to understand when coprecipitation of adsorbing metal ions is considered. It is important to note that adsorption studies involving several metal ions could also involve coprecipitation of those metal ions since it is only the main substrate (iron@) or chromium(1II) oxide) which is preformed during these adsorption experiments. It is also important to note that the species available for coprecipitation in multiple metal systems may be quite different to those involving a single adsorbent. In this way, the need to arbitrarily consider AGOchemidj of a given metal ion a function of the solution chemistry of the other metal ion(@may be alleviated. Ni(I1) coprecipitation with amorphous iron(II1) oxide in the presence and absence of Zn(I1) follows essentially two different isotherms as illustrated in Figure 4. It is tempting to suggest that Zn(I1) has a direct effect on the interaction between Ni(I1) and Cr(II1). Zn(I1) is removed at a lower pH than Ni(II), however, and this may simply result in less Cr(II1) being present and thus less ability to coprecipitate the Ni(I1). A similar difference in the coprecipitation of Zn(I1) is not seen simply because it is the Zn(I1) which interacts with Cr(II1) before the Ni(I1). Ni(1I) adsorption and coprecipitation with amorphous chromium(II1) oxide is considerably more difficult to explain on the basis of simple adsorption and coprecipitation processes. In the presence of Zn(II), Ni(I1) adsorption is considerably enhanced a t low pH as shown in Figure 5. The presence of Zn(I1) may in fact lower the available chromium(II1)oxide surface area and/or lower any soluble Cr(II1) present. In either case the adsorption isotherm would be moved to a higher pH values, which is the opposite of that observed. Without invoking unusual solution chemistry, the data shown in Figure 5 cannot be modeled using the James-Healy model without treating ACochemid,i as a variable dependent on other metal ions present. 3. Mixed Hydroxides in Solution. In our earlier paper we suggested that the presence of mixed metal hydroxides in solution may be required to fully explain the removal of single adsorbing metal systems. In this paper we have shown that an understanding of processes involving multiple adsorbing metal systems is even more complex. If, for example, Ni(II), Zn(II), and Cr(II1) are able to form a mixed hydroxide, whether in solution or as an insoluble precipitate, much of the anomalous behavior shown to date may be explained. When Ni(I1) is adsorbed onto amorphous iron(II1) oxide in the presence of Zn(II), such a complex cannot occur because Fe(II1) is completely precipitated well before (i.e. at a lower pH) any removal of Ni(I1) or Zn(I1) is seen. By contrast, when Ni(I1) is adsorbed onto amorphous chromium(II1) oxide in the presence of Zn(II), such a mixed
41.
complex could occur, but only a t pH values below 6 where Cr(II1) is not completely precipitated. Thus the isotherm shown in Figure 5 would reflect the formation of such a mixed hydroxide a t low pH, but not at high pH. Furthermore, the enhancement of removal of Ni(I1) by such a complex coprecipitate is consistent with the pattern of coprecipitation being more efficient than adsorption-a complex coprecipitate would be expected to be even more efficient. A similar complex may form during coprecipitation of Ni(I1) with chromium(II1)oxide in the presence of Zn(I1); however no enhancement would be seen since removal in this case can already be considered to be a maximum, reflecting the earliest stages of precipitation for Cr(II1) (see Figure 6). Similarly, such complexes should enhance the adsorption and coprecipitation of Zn(II), but again this would be obscured by the precipitation edge for Cr(II1) alone. Coprecipitation of Zn(I1) in the presence of Ni(I1) and Cr(II1) with amorphous iron(II1) oxide may also show evidence for enhanced adsorption (see Figure a), however experimental accuracy prevents us from placing too great a significance on these data.
Conclusion Adsorption and coprecipitation of Cr(II1) with amorphous iron(II1) oxide is independent of the presence of Ni(I1) and/or Zn(I1) since Cr(II1) precipitates at a lower pH than either of these metals and apparently at a lower or similar pH than any mixed precipitate involving any combination of these three metals. Zn(I1) adsorption and coprecipitation with both amorphous iron(II1) and chromium(II1) oxides are relatively unaffected by the presence of Ni(II), although there may be a slight suppression of removal, presumably due to simple competition for surface sites. Ni(I1) adsorption onto amorphouschromium(II1)oxide, however, followsan unusual isotherm shape in the presence of Zn(I1). Adsorption is enhanced at low pH but not at high pH. Ni(I1) adsorption onto amorphous iron(1II)oxide in the presence of Zn(I1) does not show this unusual isotherm shape. The James-Healy model for describing metal ion adsorption from the thermodynamics involved can be applied to all of the data presented in this paper. However, particularly in the case of Ni(1I) adsorption onto amorphous chromium(II1) oxide in the presence of Zn(II), it is necessary to alter the specific chemical free energy term (AGochemid,i) for the adsorbing metal and its hydrolysis species depending on whether or not other solution species are present. A more attractive proposal is that a complex mixed precipitate, involving Ni(II), Zn(II), and Cr(II1) forms during, for example, adsorption of Ni(I1) onto amorphous chromium(II1)oxide in the presence of Zn(II), and that this complex shows greater removal characteristics, at any given pH, than coprecipitates involving only two metals.
Acknowledgment. The authors thank Professor Tom Healy from the University of Melbourne for fruitful discussions regarding this research work.
