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Adsorption of cadmium and humic acid onto hematite
Chemosphere,Vol. 30, No. 2, pp. 243-256, 1995
Pergamon
0045-6535(94)00387-4
Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0045-6535/95 $9.50+0.00
ADSORPTION OF CADMIUM AND HUMIC ACID ONTO HEMATITE Allen P. Davis" and Vivek Bhatnagar Environmental Engineering Program Department of Civil Engineering University of Maryland College Park, MD 20742
(Received in Germany 28 July 1994; accepted 29 September 1994)
ABSTRACT Cadmium adsorption onto ct-Fe203 (hematite) is successfully described using a non-electrostatic surface complexation model (NEM). Humic acid (Aldrich) adsorption onto hematite decreases with increasing pH, exhibiting ligandexchange surface complexation characteristics. In ternary systems, the presence of humic acid (HA) leads to an increase in Cd(II) adsorption, dependent on HA concentration. The substrate loading order is observed to have noticeable effect on metal uptake at higher Cd(II) concentrations in the ternary systems; the general trend with respect to Cd0I) adsorption being: Cd(II) before HA > simultaneous adsorption ~ HA before Cd(lI). EDTA complexation significantly reduces the Cd(II) adsorption in the HA/hematite system.
INTRODUCTION Interactions of heavy metals with natural particulates typically control the fate and transport of metals through ground and surface water systems. For example, dissolved metals deposited in rivers and lakes bind to particles and ultimately accumulate in the sediments. As a result, it has become clear that sediment quality is important in defining the toxicological characteristics of aquatic environmental systems since numerous aquatic species live in direct contact with sediments. Additionally, transport of metals in groundwater systems is regulated by interactions with stationary and mobile aquifer material. One of the primary mechanisms by which heavy metals and interact with particulates is through adsorption. Sediments and soils are composed of various (hydr)oxides, clays, and other materials, typically possessing a coating of organic matter. These geochemical phases are responsible for the adsorption of metals from aqueous
* To whom correspondence should be addressed.
243
244 systems. In order to provide a better understanding of metal-particulate interactions, metal adsorption onto defined solids must be investigated. Fundamental characterization of interactions among individual system components can be applied to more complex natural systems for the modeling of heavy metal fates. Two important geochemical phases with respect to heavy metal interactions are iron oxides and organic matter. Humic substances with molecular weights ranging from several hundreds to millions constitute the bulk of natural organic matter and can appreciably affect the fate of heavy metals in the environment. Sequential extraction techniques have suggested that, in sediments with a high organic content, the organic matter is the most important constituent for copper binding (Luoma, 1986). Lion et al. (1982), also employing sequential extractions, noted that the majority of adsorbed Cd(ll) and Cu(II) could be attributed to the organic phase, while Pb(II) was mostly adsorbed onto metal oxides. Goethite and hematite are the most commonly occurring natural forms of iron oxide. The surface Fe atoms of these oxides are hydrated in aqueous solution by coordination with water, producing surface hydroxyl groups (~OH). Adsorption of heavy metals occurs through surface complexation reactions between the oxide surface and the dissolved metal (Stumm, 1992).
(1)
S-OH + M2* ,t S_._OM+ + H÷
A number of workers have examined the adsorption of cadmium and other metals onto iron (hydr)oxides (Johnson, 1990; Cowan et al., 1991). The sorption of humic acids (HA) onto metal oxides has been described as a ligand-exehange mechanism (Parfitt et al., 1977: Tipping, 1981a; 1981b', Tipping and Cooke, 1982). The following ligand-exchange sequence has been suggested (Murphy et aL, 1990):
S-OH
+
S-OH2+ +
H÷ a
_S-OH2+
Hu-C(O)O-
_S-OH2+OC(O)-Hu ~
(2a) =,
S-OH~+O-C(O)-Hu
_S-OC(O)-Hu +
H20
(2b) (2c)
where Hu-C(O)O- represents a humic acid carboxylic group. Protonation of the surface is postulated to be the first step, making the surface hydroxyl group more exchangeable. Initially, the humic carboxyl group forms an outersphere complex with the protonated hydroxyl group: a subsequent ligand exchange step yields an inner-sphere complex. The effect of humic acid on metal adsorption by hydrous oxides is not easily generalized.
In this ternary
system, humic acid can bind directly on thc oxide surface, and if similar sites are employed in metal adsorption, this competition and site blockage may decrease the metal adsorption. Additionally, complexation between transition metals and HA is favorable: complexes that remain in solution will prevent a fraction of the metal from adsorbing.
245 However, the formation of ternary HA-metal-surface complexes is possible as well. The overall effect on metal adsorption from the addition of HA will depend on the stability of the numerous interactions involved. Elliott and Huang (1979, 1981) noted significant adsorption of Cu(ll)-organic complexes onto y-Al203 and aluminosilicates.
Evidence suggested that electrostatic interactions are important in the adsorption of anionic metal
complexes. Adsorption of Ni(II)-EDTA complexes onto "[-AI203was found by Bowers and Huang (1986). At high concentrations of the complex, the adsorption was ligand-like, occurring at low pH. Little adsorption occurred on SIP2. Hydrogen bonding of the complex to protonated surface groups was suggested to dominate the adsorption. Huang et al. (1977) reported that Cd(ll), Pb(ll), and Cu(II) adsorption onto SiP 2 and AI203 is significantly enhanced by the presence of trace amounts of humic acid, NTA (nitrilotriacetic acid), glycine, and tartrate. The adsorption of metal ions is affected by the ligand/metal molar ratio in solution since this ratio controls the aqueous metal speciation and the resulting competition between the uncomplexed iigand and the metal ions or metai-ligand complexes for surface sites (EIliott and Huang, 1979). The authors noticed in their experiments with the Cu(II)-NTA/AI203 system that small concentrations of NTA initially increased the metal adsorption: this trend continued up to a NTA:Cu molar ratio of 1:1 and was attributed to the progressive increase in the formation of CuNTA- surface complexes at the expense of the non-adsorbable Cu2+(aq) ion. However, when the NTA:Cu ratio increased beyond 1: 1, the free NTA competed for the surface sites with CuNTA- leading to a decrease in the total Cu(II) adsorption. Elliott and Denneny (1982) concluded from their study using three northeastern U.S. soils and several organic ligands, that when the strength of the metal-ligand bond is stronger than the metal-soil interaction, adsorption of the metal (i.e., cadmium) is significantly inhibited as soluble complexes out-compete the soil sites for metal binding. Laxen (1985) reported that the sorption of trace concentrations of cadmium and nickel onto iron oxide is enhanced by the presence of humic substances. In this study, the adsorption of dissolved cadmium onto iron oxide as affected by the presence of humic acid is evaluated. The specific objectives are to experimentally evaluate and model cadmium uptake, observe the effect of the presence of humic acid, including loading order, and to evaluate the competition provided by EDTA (ethylenediaminetetraacetic
acid) to cadmium adsorption onto iron oxide, also in the presence of HA.
METHODS AND MATERIALS
Iron oxide (hematite, ct-Fe203-H20, 99.9%) was obtained from Alfa Products. X-ray diffraction confirmed the structure of the iron oxide as ct-Fe203. The specific surface area was determined as 12 m2/g using the EGME method (Carter et al., 1986): a pHp~¢of 7.2 was determined by electrophoretic mobility measurement in 10.2 M NaNO 3. For all adsorption experiments 125 mL plastic bottles were used. Bottles and all glassware were thoroughly acid washed and rinsed with deionized water from a Hydro-service reverse osmosis/ion exchange apparatus (Model LPRO-20). Cd(ll) concentrations (Cd(NO3)2,4H20, Fisher Sci.) ranging from 10~ to 10.4 M were mixed with the
246 iron oxide at 1 g/L. The ionic strength HNO3 and NaOH.
w a s 10 -2
M
NaNO 3
(Fisher Sci.); the suspension pH was adjusted using
The samples were shaken for 4 hours, the suspension pH was measured, and they were
subsequently centrifuged at 3200 rpm for 30 minutes. The supernatant was decanted into 20 mL vials and acidified to pH 1-2 using concentrated HNO3. Dissolved Cd(lI) concentrations were measured using a Varian Techtron Model AA-5 atomic absorption speetrophotometer.
These values, subtracted from the total initial metal concentration
provided the adsorbed metal concentrations. For investigation of humic acid adsorption, 1 g/L Fe203 was mixed with 5, 10, 15, and 20 mg/L Aldrich humic acid (I=10 2 M NaNO3). The initial pH of the samples was adjusted to 3 to 8: the pH was not increased beyond 8 to prevent hydrolysis ofhumic acid (Murphy et al., 1990). At~er 6 hours of shaking, the suspension pH was measured, the samples centrifuged and the supernatant was acidified to pH 2-3. The dissolved humic acid concentration was determined by measuring absorbance at 240 nm using a Beckman DU-2 UV-vis spectropbotometer and comparing to calibrated standards.
The difference between the initial and the equilibrium dissolved HA
concentration gave the quantity of humic acid adsorbed onto the iron oxide. Ternary mixtures of iron oxide, Cd(II), and humic acid were prepared and analyzed using the procedures above. A range of 106-10 4 M Cd(II) was investigated at 5 and 10 mg/L humic acid. Sequential adsorption of cadmium and humic acid was examined as follows: In one set of experiments, humic acid was first adsorbed onto iron oxide (5 and 10 rag/L) employing the method detailed above. Subsequently, cadmium was spiked into this humic acid-iron oxide equilibrated system (0.5 to 5 mL of 10.3 M Cd(NO3)2 into 50 mL) to produce Cd(ll) concentrations in the range of 106 to 104 M; the samples were next shaken for 4 additional hours. Solids separations and cadmium adsorption measurements were completed as above. In the second sequential experiment, cadmium was first adsorbed onto the iron oxide ([Cd(ll)] = 106-10 4 M) and subsequently humic acid was spiked to produce final concentrations of 5 and 10 mg/L into the cadmium-iron oxide system. These samples were shaken for 4 additional hours, the solids separated, and adsorbed cadmium was determined. Systems containing EDTA (Baker) in addition to Cd(II) and HA were also investigated. Three EDTA:Cd(II) ratios, 1:10, 1:2, and 1:1 were employed. In all of these multi-component mixtures, 10-~ M Cd(II) and 10 mg/L humic acid were added to 1 g/L iron oxide, adsorbed cadmium was monitored.
RESULTS AND DISCUSSION
Cadmium adsorption: Cadmium(II) adsorption onto 1 g/L hematite generally follows the expected metal adsorption behavior (Figure 1). The metal adsorption begins at pH 5.5-6 and rises steeply to essentially 100% within the next 2-3 pH units. With an increase in total Cd(II) concentration, the adsorption edge shiits to a higher pH region. A non-clcctrostatic surface complexation model (NEM) is used to model the Cd(ll) adsorption. In this model, the specific chemical interactions between the surface and the metal arc stressed and clectrostatic interactions arc neglected. Amphoteric reactions at the hematite/water interface include:
247
100 i
'1
• 2.9x10-6 M
80 + ~ 8.5x10-6 M /
o
I
• 1.6X10-5 M
60 ¢
4.1x10-5 M
( ~" to
j 40+
!
•
•
• 4.7X10-5M
A
A AA
~**
~ 9.3x10-5 M •
[]
[]
i
3
•
~
4
Z ~"f't
5
6
±
7
&
8
9
10
11
pH
Figure 1.
Adsorption of Cd(lI) onto ot-Fe20~. (1 g/L Hematite, I=102 M NaNO~.)
S_-OH2 = S_-OH + H÷
S--OH =S_-O+H ÷ Values of K~
= 1 0 -a'7
:
;
K~
(3a)
K~
(3b)
and K~ = 10"0.3 were obtained by Cowan et al. (1991) for amorphous iron oxyhydroxide using
the NEM model. Only monodentate surface species, S--OH and the free aquo metal ion, Cd2÷, are assumed to participate in the adsorption reaction.
S-OH + Cd 2+ =' S-OCd÷ + IF
;
K~(i.t)
(4)
The NEM has been successfully used by Park and Huang (1989) in modeling Co(IIL Ni(ll), and Zn(lI) adsorption onto hydrous cadmium sulfide, and has been found to be essentially equivalent to the triple-layer surface complexation model in adequately describing Cd(lI) adsorption onto Fe203"H20(am) (Cowan et al., 1991). The intrinsic constant, with the electrostatic correction factor taken as unity is then defined as (Park and Huang, 1989):
248
Table I. Stability Constants for Cd(ll) Surface Complexation on Hematite.
Cd(II) Concentration (M)
[
Range of K" between pH 4.5 to 10.0
[
Average Value of K'
2.9x 10.6
1.9x10 -3 _ 5.6x10 -5
2.6x10 ~
8.5x10 "6
8.8x10 -4 - 7.0x10 --~
1.0xl0 ~
1.6xl0 -5
4.8x10 -~ _ 1.4x10 -5
3.5x10 -5
4.1x10 --~
4.6x10 -5 _ 9.6x10 -6
2.5x10 -5
4.7x10 5
4.6x!0 5 _ 9.6x10 -6
2.5x10 --~
9.3x10 5
3.0x10 -5 _ 1.3xi0 --s
1.7x I 0 -~
[S-OCd÷] [W] K~0n~ =
=
Ks
(5)
[S-OH] [Cd 2+]
Employing a site density of 11 sites/nm2, the specific surface area of 12 m2/g, and the solids loading of 1 g/L, a value of 2.2x10 "4 M is calculated for the total number of surface sites. A balance for the surface sites of the hematite includes metal-occupied and un-occupied sites. Using [S-OCd÷] and [Cd2+] from experimental results, the adsorption stability constant (KS) is determined for each metal adsorption experiment. Cadmium hydrolysis is evaluated using "K~ = 10~~4 and "K2 = 10~44 (Sillen and Martell, 1971). The calculated surface complexation constant varies only slightly with pH and an average K s is obtained for each concentration (Table I). The pH independence of K' supports the use of the NEM and the selection of the monodentate surface species [S-OCd+) as
the primary adsorbed species (Park and Huang, 1989). The stability constant decreases slightly with increasing cadmium loading, which is normally attributed to site heterogeneity and preferential adsorption onto high-energy sites. Figure 2 demonstrates that the model provides a good representation of the adsorption data.
Humic acid adsorption: The adsorption of humic acid onto hematite ranges from 60-45% for 5 mg/L HA to 45-10% for 15-20 mg/L HA: the adsorption decreases almost linearly with an increase in pH (Figure 3), as was observed by Murphy et al. (1990). The higher adsorption at lower pH qualitatively supports the mechanism given by Eqn 2. These results are consistent with those of other workers, i.e., Tipping et al. (1983) for peat humic substances onto goethite, Tipping (1981b) for Esthwaite water humic substances onto both goethitc and hematite, and Parfitt et al. (1977) for humic and fulvic acids onto gibbsite, goethite, and imogolite. At higher concentrations,
the degree of adsorption may be limited due to blockage of sites by adsorbed humic acid molecules, since humic acid has a large molecular structure and each molecule may cover several possible complexation sites on the oxide surface.
With an increase in the concentration of humic acid, the fractional adsorption is found to decrease,
suggesting the saturation of surface sites.
Cadmium adsorption onto hematite in the presence of humic acid (ternary system): There are three possible roles for complexing ligands in ternary metal adsorption systems: Ligands can inhibit metal adsorption by
249
100
Z
) 80
£ 0 ID
¢J
~
i =~
•
8.5x10-6 M Cd(ll)
~
417X10-5
M
Cd(ll)
Modeled-8.SxlO-6
so +! ~
Modeled-4.7xl 0-5
40~
20
0
•
3
im
4
5
3 6
7
8
9
10
11
pH
Figure 2.
Adsorption Modeling of 8.5x10 6 M and 4.7x10 ~ M Cd(II) onto ct-Fe203 using a Non-Electrostatic Surface Complexation Model.
100
80 i "O
i 60 i
m~
0
i
"r
40 4i
•
•
•
• Z
5 mg/L
•
10 mg/L
20 i I
i:
• 15 mg/L
i
' 20 mg/L
I
o J 3
4
5
6
7
pH
Figure 3.
Adsorption of Humic Acid onto ct-Fe203. (1 g/L Hematite, I=10 .2 M NaNO~.)
8
250
100
8o 'U
0 W
(
8o! 40
[]
[]
I
•
[]
Cd/
•
•
I
/
5 mg/L HA
./
[]
5.0x10-6 M Cd/ 10 mg/L HA
20~ / 0
,
,
i
3
4
• 5.0x10-6 M Cd only I
5
6
7
8
9
10
11
pH
Figure 4.
Adsorption of 5x10 6 M Cd(ll) onto ~-Fe203 in the Presence of Humic Acid. 1 g/L Hematite, I=10 .2 M NaNO~. Cd(ll)-only Curve is Modeled using Constant from Table I.
forming non-adsorbable complexes with the metal, as well as by competing with the metal itself for available surface sites. Metal adsorption can be enhanced by the presence of a ligand due to the formation of metal-ligand ternary surface complexes. The details of such a system will depend on the adsorbing and complexing properties of all species involved. Cadmium adsorption onto hematite is found to increase in the presence of humic acid in the pH range of 5 to 8 (Figure 4). The degree of enhancement increases with increasing concentrations of HA at low Cd loadings, the difference between 5 and i0 mg/L HA is less apparent for [Cd(II)] greater than 5x10 ~ M. Also the greatest enhancement from HA addition is noted for the lowest Cd(II) concentrations. Laxen (1985) found that the adsorption of Cd(II) onto ferric oxide was enhanced in the presence of humic substances: however in his case the enhancement was essentially independent of humic concentration. explanation for the discrepancy may lie in the experimental conditions:
The
the study by Laxen used Cd(II)
concentrations of only 5xl0 9 M, thus available Cd(II) is likely to be a strongly limiting constituent. Davis and Leckie (1978) showed that Cu(II) adsorption onto amorphous iron oxide was increased by the presence of thiosulfate, glutamic acid and 2,3-pyrazinedicarboxylic acid. These authors explain that if the ligand's major coordinating functional groups are not used in surface binding and are directed outward towards solution, then
251 trace metal adsorption can be significantly enhanced. The enhanced adsorption can result from the adsorption of metal-ligand complexes or through trace metal complexation by the adsorbed ligands; in either case, the final configurations are identical. The functional groups of an adsorbed ligand may provide new adsorption sites for trace metals which may be more reactive than isolated oxide surface sites. If the functional groups of HA are available and are oriented towards the solution, then the metal uptake may be substantially enhanced by the humic substances. Davis and Leckie (1978) also state that since humic materials are large and have low solubility, they can accumulate at the solid/water interface, providing a coating on hydrous oxides and clays. Thus, the adsorption of metal ions may be dominated by the exposed functional groups of the adsorbed organics rather than those of the oxide. As a result, the adsorption of trace metals in natural waters may be governed by chelation reactions with these humic acid functional groups. Since the HA molecule is large, many metal binding sites are likely per molecule. Therefore, many Cd(ll) ions can bind to the HA, which consequently binds to the hematite surface.
(C(O)O-Cd÷L S_-OH2+ + Hu-(C(O)O-), + mCd2+ ,, S-OC(O)-Hu
+ H20
(6)
(C(O)OL,,
The modeling of ternary systems is very complex, involving multiple adsorbed and solution phase equilibria. Additional complexities result when employing humie acid due to the heterogeneity of this material.
Modeling
competitions among species may be compounded by metals, ligands, and metal-ligand complexes not preferentially adsorbing on the same type of surface sites. Effect of loading order (Sequential adsorption): Tests covering a range of Cd(ll) concentrations and two different humic acid concentrations were completed to observe the effect of substrate loading order on Cd(II) adsorption. Interesting, differing results are obtained depending on the adsorption sequence (Figures 5a-c). At the lower Cd(II) loadings, there is little difference among the three different sequences (i.e., Figure 5a). However, at the 7-10x10 5 M Cd(ll) loading, obvious divergence among the systems is apparent (Figures 5b&c). For these systems, addition ofhumic acid to a Cd(ll)/hematite equilibrated system (or "Cd first" system) leads to more Cd(II) adsorption than the simultaneous adsorption of Cd(ll) and HA. Cd(II) adsorption from a Cd(ll) spike into a humic acid/hematite equilibrated system (HA first) did not significantly vary from that of the simultaneous mode. Overall, Cd(II) adsorption is higher for 10 mg/L HA than for 5 mg/L for all cases and the trends with respect to sequential loadings are identical. Humic acid molecules each contain many functional groups. As humic acids adsorb and cover the oxide surface, they significantly alter the characteristics of the surface with respect to metal uptake. binding sites are likely blocked by the surface complexation of large HA molecules.
Hematite metal
However, this adsorption
should not affect exposed sites on the HA that are available for Cd(ll) complexation through the formation of ligandlike ternary complexes. During simultaneous adsorption, the humic acid may cover the surface and form ternary
252
100
80
÷
"0 m
-B 0
60
-~ 4.5x10-6 M Cd spike/ 5 mg/L HA
iO
'0
.< •
o
40
n
Z
• 6.7X10-6 M Cd/
~
5 mg/L HA spike g
20
• 7.1x10-6 M Cd/ 5 mg/L HA simultaneously
m~ i
0 3
4
5
7
6
8
9
10
11
pH
100
!
"0
80 ÷ )
60
" 7.1x10-5 M Cd spike/ 5 mg,/L HA
L
i
:1
•
,,m~
m
i 20 -~
•
• 6.7x10-.5 M Cd/ 5 mg/L HA spike
• •
;
0i
•
~:J 5
3
6
7
8
Cd/ 5 mg/L HA simultaneously
10x10-5 M
9
10
11
pH 100 • 80 t
m=
I
<
60 t
•
•
7.1X10-5 M Cd spike/ 10 mg/L HA
r
40 ~-
•
i
•
6.7x10-5 M Cd/ 10 mg,/L HA spike
•
10x10-5 M
-
I
20 +
*'~c~':'~ ~e
o
Cd/ 10 mg/L HA simultaneously
%o 0
~
3
4
•
5
6
7
8
9
10
11
pH
Figure 5.
Sequential Adsorption of 10s M Cd(II) onto a-FcqO3 in the Presence of 5 mg/L Humic Acid a) 4.5=7.1x10 ~s M Cd(ll), 5 mg/L Humic Acid b) 6.7-10x10 s M Cd(II), 5 mg/L Humic Acid c) 6.7-10x10 s M Cd(II), 10 mg/L Humic Acid
253
100
80 ¢1 0
tJ
60
•
NO EDTA
""
lx10-6 M EDTA
•
5x10-6 M EDTA
, ~
lx10-5 M EDTA
.j
40 ~-
20
%E
0 3
4
5
6
7
pH
Figure 6.
Effect of EDTA on Adsorption of 10'~ M Cd(II) onto ot-Fe203 (1 g/L) in the Presence of 10 mg/L Hurnic Acid (I = 10.2 M NaNO3).
complexes with some Cd(lI), and also some Cd(lI) may be adsorbed directly onto the hematite. However, in the "Cd first" mode, Cd(II) ions initially adsorb onto the hematite surface. At high Cd(lI) loadings, the addition of HA allows the formation of ternary surface complexes, both on the hematite surface, and with adsorbed Cd. Thus adsorbed Cd may be physically covered by subsequently adsorbed HA. This sequential attachment may produce an overall enlaancement of Cd(ll) removal since the blocked hematite sites in this case already contain adsorbed Cd(II). As a result, for large ligands with multiple metal binding sites, attachment order may be important in determining metal adsorption capacity onto solids. Competition with EDTA (Quaternary Systems):
The effect of a second ligand, EDTA, a strong
complexing agent with a cadmium stability constant of 1017'2 (Sillen and Martell, 19717, on the ternary adsorption system was studied (Figure 6). There is no noticeable effect from 10~ M EDTA since a maximum of only 10% of the total Cd(II) could be complexed. Increasing the EDTA concentration to 5xl 0 .6 and 10-5 M significantly reduces the Cd(ll) adsorption, most likely due to the competitive formation of non-adsorbing Cd(II)-EDTA complexes. Elliott and Denneny (1982) state that in natural systems, organic ligands can out-compete soil humic materials for metals, lfthe metal-ligand stability constant is greater than the humic-metal complexation constant, a reduction in metal uptake by the solid results upon ligand addition. They found that Cd(ll) sorption onto soils was reduced
254 by added ligands, proportional to the Cd-L stability constant. Similar results are observed here, where EDTA outcompetes both the humic acid and the oxide surface sites for complexation with the cadmium. The ratio of reduction in the adsorption of Cd(II) to added EDTA concentration is less than 1:1 in spite of the large Cd(II)-EDTA stability constant. Cadmium equal to about 45% of the EDTA concentration is prevented from adsorbing in these systems. Various competitive reactions contribute to this low ratio. The sorption of EDTA onto hydrous oxide surfaces can occur through the formation of surface complexes (Rubio and Matijevid, 1979; Bowers and Huang, 1985); EDTA uptake by 13-FeOOH occurred from pH 2.5 to 9 (Rubio and Matijevid, 1979). Although the formation of ternary metal-EDTA-surface complexes is possible (Bowers and Huang, 1986), it is apparently negligible here.
SUMMARY AND CONCLUSIONS The modeling of Cd(ll) adsorption onto hematite by the use of a non-electrostatic surface complexation model was successful and describes the data well. The stability constants determined from the NEM varied little with pH, supporting the application of the NEM. Humic acid adsorption onto hematite demonstrates characteristics of a general ligand-exchange mechanism pattern, adsorption density decreasing with increasing pH. The presence of humic acid increases Cd(II) uptake by hematite. Humic acid apparently becomes sorbed onto the oxide surface, allowing other HA sites with a high affinity for Cd(II) to be available for the formation of ternary adsorption complexes.
For the concentrations examined, Cd(lI) adsorption is found to increase with
increased concentrations of humic acid. The order of loading of substrates is found to have a noticeable effect on the final Cd(II) uptake for high Cd(ll) concentrations. The Cd(lI) uptake is observed to vary in the manner: humic acid spike > humic acid/Cd(II) simultaneously = Cd(II) spike.
This implies that adsorption interactions among the aquatic substrates can be
dependent on their order of loading, and the various reaction mechanisms may not be considered equivalent. The presence of EDTA reduces the Cd(lI) adsorption significantly, competing with the surface-bound humic acid by complexing Cd(II): Cd-EDTA complexes are weakly, if at all, adsorbed. In spite of the large metal complex stability constant of Cd(II)-EDTA, the ratio of additional Cd(I1) dissolved to added EDTA is observed to be less than 1:1; this may be due to some EDTA being adsorbed to the hematite surface. Dissolved and sorbed organic matter is important in controlling metal sorption characteristics onto particulates. Humic acid can enhance metal adsorption onto oxides, the degree of enhancement may depend on the structure of the surface complexes and the sequential order of sorption.
ACKNOWLEDGEMENT A University of Maryland General Research Board Summer Research Grant to A.P.D. is gratefully acknowledged.
255
REFERENCES Bowers, A.R., and Huang, C.P. (1985) "Adsorption characteristics of polyacetie amino acids onto hydrous ?-A1203," .1. Colloid lnterface Sci., 105, 197-215. Bowers, A.R., and Huang, C.P. (1986) "Adsorption characteristics of metal-EDTA complexes onto hydrous oxides," .I. Colloid Interface Sci., 110, 575-590. Carter, D.L., Mortland, M.M., and Kemper, W.D. (1986) "Specific Surface," In Methods of Soil Analysis, 2nd ed., Klute, A. (ed.), Soil Sci. Soc. Am., Madison, WI, Part I, 413-422. Cowan, C.E., Zachara, J.M., and Resch, C.T. (1991) "Cadmium adsorption on iron oxides in the presence of alkalineearth elements," Environ. Sci. Technol., 25, 437-446. Davis, J.A., and Leckie, J.O. (1978) "Effect of adsorbed complexing ligands on trace metal uptake by hydrous oxides," Environ, Sci. Technol., 12, 1309-1315. Elliott, H.A., and Denneny, C.M. (1982) "Soil adsorption of cadmium from solutions containing organic ligands," J. Environ. Qual., 11,658-662. Eiliott, H.A., and Huang, C.P. (1979) "The adsorption characteristics of Cu(ll) in the presence of chelating agents," J. Colloid Interface Sci., 70, 29-45. Elliott, H.A., and Huang, C.P. (1981) "Adsorption characteristics of some Cu(ll) complexes on aluminosilicates," Water Res., 15, 849-855. Huang, C.P., Elliott, H.A., and Ashmead, R.M. (1977) "Interfacial reactions and the fate of heavy metals in soilwater systems," ,/. Wat. Pollut. Control Fed., 49, 745. Johnson, B.B. (1990) "Effect of pH, temperature, and concentration of the adsorption of cadmium on goethite," Environ. Sci. Technol., 24, 112-118. Laxen, D.P.H. (1985) "Trace metal adsorption/coprecipitation on hydrous ferric oxide under realistic conditions: The role of humic substances," Water Res., 19, 1229-1236. Lion, L.W., Altmann, R.S., and Leckie, J.O. (1982) "Trace-metal adsorption characteristics of estuarine particulate matter: evaluation of contributions of Fe/Mn oxide and organic surface coatings," Environ. Sci. Technol., 16, 660666. Luoma, S.N. (1986) "A comparison of two methods for determining copper partitioning in oxidized sediments," Mar. Chem., 20, 45-59. Murphy, E.M., Zachara, J.M., and Smith, S.C. (1990) "Influence of mineral-bound humic substances on the sorption of hydrophobic organic compounds," Envh'on. Sci. Technol., 24, 1507-1516. Parfitt, R.L., Fraser, A.R., and Farmer, V.C. (1977) "Adsorption on hydrous oxides. III. Fulvic acid and humic acid on goethite, gibbsite, and imogolite," J. Soil Sci., 28, 289-296. Park, S.W., and Huang, C.P, (1989) "The adsorption characteristics of some heavy metal ions onto hydrous CdS surface," J. Colloid Interface Sci., 128, 245-257. Rubio, J., and Matijevi6, E. (1979) "Interactions of metal hydrous oxides with chelating agents. I. I~-FeOOH-EDTA," Colloid Interface Sci., 68, 408-42 I.
256 Sillen, L.G., and Marteil, A.E. (1971) Stabilitv Constants, The Chemical Society, London, Special Publication No. 25. Stumm, W. (1992) Chemist~. of the Solid-Water Interface, Wiley, New York. Tipping, E. (1981a) "The adsorption of aquatic humic substances by iron oxides," Geochim. Cosmochim. Acta, 45, 191-199. Tipping, E. (1981b) "Adsorption to goethite (Qt-FeOOH) of humic substances from different lakes," Chem. Geol., 33, 81-89. Tipping, E., and Cooke, D. (1982) "The effects of adsorbed humic substances on the surface charge of goethite (ctFeOOH) in freshwaters," Geochim. Cosmochim. Acta, 45, 191-199. Tipping, E., Gfiffifth, J.R., and Hilton, J. (1983) "The effect of adsorbed humic substances on the uptake of copper(II) by goethite," Croat. Chem. Acta, 56, 613-621.
Pergamon
0045-6535(94)00387-4
Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0045-6535/95 $9.50+0.00
ADSORPTION OF CADMIUM AND HUMIC ACID ONTO HEMATITE Allen P. Davis" and Vivek Bhatnagar Environmental Engineering Program Department of Civil Engineering University of Maryland College Park, MD 20742
(Received in Germany 28 July 1994; accepted 29 September 1994)
ABSTRACT Cadmium adsorption onto ct-Fe203 (hematite) is successfully described using a non-electrostatic surface complexation model (NEM). Humic acid (Aldrich) adsorption onto hematite decreases with increasing pH, exhibiting ligandexchange surface complexation characteristics. In ternary systems, the presence of humic acid (HA) leads to an increase in Cd(II) adsorption, dependent on HA concentration. The substrate loading order is observed to have noticeable effect on metal uptake at higher Cd(II) concentrations in the ternary systems; the general trend with respect to Cd0I) adsorption being: Cd(II) before HA > simultaneous adsorption ~ HA before Cd(lI). EDTA complexation significantly reduces the Cd(II) adsorption in the HA/hematite system.
INTRODUCTION Interactions of heavy metals with natural particulates typically control the fate and transport of metals through ground and surface water systems. For example, dissolved metals deposited in rivers and lakes bind to particles and ultimately accumulate in the sediments. As a result, it has become clear that sediment quality is important in defining the toxicological characteristics of aquatic environmental systems since numerous aquatic species live in direct contact with sediments. Additionally, transport of metals in groundwater systems is regulated by interactions with stationary and mobile aquifer material. One of the primary mechanisms by which heavy metals and interact with particulates is through adsorption. Sediments and soils are composed of various (hydr)oxides, clays, and other materials, typically possessing a coating of organic matter. These geochemical phases are responsible for the adsorption of metals from aqueous
* To whom correspondence should be addressed.
243
244 systems. In order to provide a better understanding of metal-particulate interactions, metal adsorption onto defined solids must be investigated. Fundamental characterization of interactions among individual system components can be applied to more complex natural systems for the modeling of heavy metal fates. Two important geochemical phases with respect to heavy metal interactions are iron oxides and organic matter. Humic substances with molecular weights ranging from several hundreds to millions constitute the bulk of natural organic matter and can appreciably affect the fate of heavy metals in the environment. Sequential extraction techniques have suggested that, in sediments with a high organic content, the organic matter is the most important constituent for copper binding (Luoma, 1986). Lion et al. (1982), also employing sequential extractions, noted that the majority of adsorbed Cd(ll) and Cu(II) could be attributed to the organic phase, while Pb(II) was mostly adsorbed onto metal oxides. Goethite and hematite are the most commonly occurring natural forms of iron oxide. The surface Fe atoms of these oxides are hydrated in aqueous solution by coordination with water, producing surface hydroxyl groups (~OH). Adsorption of heavy metals occurs through surface complexation reactions between the oxide surface and the dissolved metal (Stumm, 1992).
(1)
S-OH + M2* ,t S_._OM+ + H÷
A number of workers have examined the adsorption of cadmium and other metals onto iron (hydr)oxides (Johnson, 1990; Cowan et al., 1991). The sorption of humic acids (HA) onto metal oxides has been described as a ligand-exehange mechanism (Parfitt et al., 1977: Tipping, 1981a; 1981b', Tipping and Cooke, 1982). The following ligand-exchange sequence has been suggested (Murphy et aL, 1990):
S-OH
+
S-OH2+ +
H÷ a
_S-OH2+
Hu-C(O)O-
_S-OH2+OC(O)-Hu ~
(2a) =,
S-OH~+O-C(O)-Hu
_S-OC(O)-Hu +
H20
(2b) (2c)
where Hu-C(O)O- represents a humic acid carboxylic group. Protonation of the surface is postulated to be the first step, making the surface hydroxyl group more exchangeable. Initially, the humic carboxyl group forms an outersphere complex with the protonated hydroxyl group: a subsequent ligand exchange step yields an inner-sphere complex. The effect of humic acid on metal adsorption by hydrous oxides is not easily generalized.
In this ternary
system, humic acid can bind directly on thc oxide surface, and if similar sites are employed in metal adsorption, this competition and site blockage may decrease the metal adsorption. Additionally, complexation between transition metals and HA is favorable: complexes that remain in solution will prevent a fraction of the metal from adsorbing.
245 However, the formation of ternary HA-metal-surface complexes is possible as well. The overall effect on metal adsorption from the addition of HA will depend on the stability of the numerous interactions involved. Elliott and Huang (1979, 1981) noted significant adsorption of Cu(ll)-organic complexes onto y-Al203 and aluminosilicates.
Evidence suggested that electrostatic interactions are important in the adsorption of anionic metal
complexes. Adsorption of Ni(II)-EDTA complexes onto "[-AI203was found by Bowers and Huang (1986). At high concentrations of the complex, the adsorption was ligand-like, occurring at low pH. Little adsorption occurred on SIP2. Hydrogen bonding of the complex to protonated surface groups was suggested to dominate the adsorption. Huang et al. (1977) reported that Cd(ll), Pb(ll), and Cu(II) adsorption onto SiP 2 and AI203 is significantly enhanced by the presence of trace amounts of humic acid, NTA (nitrilotriacetic acid), glycine, and tartrate. The adsorption of metal ions is affected by the ligand/metal molar ratio in solution since this ratio controls the aqueous metal speciation and the resulting competition between the uncomplexed iigand and the metal ions or metai-ligand complexes for surface sites (EIliott and Huang, 1979). The authors noticed in their experiments with the Cu(II)-NTA/AI203 system that small concentrations of NTA initially increased the metal adsorption: this trend continued up to a NTA:Cu molar ratio of 1:1 and was attributed to the progressive increase in the formation of CuNTA- surface complexes at the expense of the non-adsorbable Cu2+(aq) ion. However, when the NTA:Cu ratio increased beyond 1: 1, the free NTA competed for the surface sites with CuNTA- leading to a decrease in the total Cu(II) adsorption. Elliott and Denneny (1982) concluded from their study using three northeastern U.S. soils and several organic ligands, that when the strength of the metal-ligand bond is stronger than the metal-soil interaction, adsorption of the metal (i.e., cadmium) is significantly inhibited as soluble complexes out-compete the soil sites for metal binding. Laxen (1985) reported that the sorption of trace concentrations of cadmium and nickel onto iron oxide is enhanced by the presence of humic substances. In this study, the adsorption of dissolved cadmium onto iron oxide as affected by the presence of humic acid is evaluated. The specific objectives are to experimentally evaluate and model cadmium uptake, observe the effect of the presence of humic acid, including loading order, and to evaluate the competition provided by EDTA (ethylenediaminetetraacetic
acid) to cadmium adsorption onto iron oxide, also in the presence of HA.
METHODS AND MATERIALS
Iron oxide (hematite, ct-Fe203-H20, 99.9%) was obtained from Alfa Products. X-ray diffraction confirmed the structure of the iron oxide as ct-Fe203. The specific surface area was determined as 12 m2/g using the EGME method (Carter et al., 1986): a pHp~¢of 7.2 was determined by electrophoretic mobility measurement in 10.2 M NaNO 3. For all adsorption experiments 125 mL plastic bottles were used. Bottles and all glassware were thoroughly acid washed and rinsed with deionized water from a Hydro-service reverse osmosis/ion exchange apparatus (Model LPRO-20). Cd(ll) concentrations (Cd(NO3)2,4H20, Fisher Sci.) ranging from 10~ to 10.4 M were mixed with the
246 iron oxide at 1 g/L. The ionic strength HNO3 and NaOH.
w a s 10 -2
M
NaNO 3
(Fisher Sci.); the suspension pH was adjusted using
The samples were shaken for 4 hours, the suspension pH was measured, and they were
subsequently centrifuged at 3200 rpm for 30 minutes. The supernatant was decanted into 20 mL vials and acidified to pH 1-2 using concentrated HNO3. Dissolved Cd(lI) concentrations were measured using a Varian Techtron Model AA-5 atomic absorption speetrophotometer.
These values, subtracted from the total initial metal concentration
provided the adsorbed metal concentrations. For investigation of humic acid adsorption, 1 g/L Fe203 was mixed with 5, 10, 15, and 20 mg/L Aldrich humic acid (I=10 2 M NaNO3). The initial pH of the samples was adjusted to 3 to 8: the pH was not increased beyond 8 to prevent hydrolysis ofhumic acid (Murphy et al., 1990). At~er 6 hours of shaking, the suspension pH was measured, the samples centrifuged and the supernatant was acidified to pH 2-3. The dissolved humic acid concentration was determined by measuring absorbance at 240 nm using a Beckman DU-2 UV-vis spectropbotometer and comparing to calibrated standards.
The difference between the initial and the equilibrium dissolved HA
concentration gave the quantity of humic acid adsorbed onto the iron oxide. Ternary mixtures of iron oxide, Cd(II), and humic acid were prepared and analyzed using the procedures above. A range of 106-10 4 M Cd(II) was investigated at 5 and 10 mg/L humic acid. Sequential adsorption of cadmium and humic acid was examined as follows: In one set of experiments, humic acid was first adsorbed onto iron oxide (5 and 10 rag/L) employing the method detailed above. Subsequently, cadmium was spiked into this humic acid-iron oxide equilibrated system (0.5 to 5 mL of 10.3 M Cd(NO3)2 into 50 mL) to produce Cd(ll) concentrations in the range of 106 to 104 M; the samples were next shaken for 4 additional hours. Solids separations and cadmium adsorption measurements were completed as above. In the second sequential experiment, cadmium was first adsorbed onto the iron oxide ([Cd(ll)] = 106-10 4 M) and subsequently humic acid was spiked to produce final concentrations of 5 and 10 mg/L into the cadmium-iron oxide system. These samples were shaken for 4 additional hours, the solids separated, and adsorbed cadmium was determined. Systems containing EDTA (Baker) in addition to Cd(II) and HA were also investigated. Three EDTA:Cd(II) ratios, 1:10, 1:2, and 1:1 were employed. In all of these multi-component mixtures, 10-~ M Cd(II) and 10 mg/L humic acid were added to 1 g/L iron oxide, adsorbed cadmium was monitored.
RESULTS AND DISCUSSION
Cadmium adsorption: Cadmium(II) adsorption onto 1 g/L hematite generally follows the expected metal adsorption behavior (Figure 1). The metal adsorption begins at pH 5.5-6 and rises steeply to essentially 100% within the next 2-3 pH units. With an increase in total Cd(II) concentration, the adsorption edge shiits to a higher pH region. A non-clcctrostatic surface complexation model (NEM) is used to model the Cd(ll) adsorption. In this model, the specific chemical interactions between the surface and the metal arc stressed and clectrostatic interactions arc neglected. Amphoteric reactions at the hematite/water interface include:
247
100 i
'1
• 2.9x10-6 M
80 + ~ 8.5x10-6 M /
o
I
• 1.6X10-5 M
60 ¢
4.1x10-5 M
( ~" to
j 40+
!
•
•
• 4.7X10-5M
A
A AA
~**
~ 9.3x10-5 M •
[]
[]
i
3
•
~
4
Z ~"f't
5
6
±
7
&
8
9
10
11
pH
Figure 1.
Adsorption of Cd(lI) onto ot-Fe20~. (1 g/L Hematite, I=102 M NaNO~.)
S_-OH2 = S_-OH + H÷
S--OH =S_-O+H ÷ Values of K~
= 1 0 -a'7
:
;
K~
(3a)
K~
(3b)
and K~ = 10"0.3 were obtained by Cowan et al. (1991) for amorphous iron oxyhydroxide using
the NEM model. Only monodentate surface species, S--OH and the free aquo metal ion, Cd2÷, are assumed to participate in the adsorption reaction.
S-OH + Cd 2+ =' S-OCd÷ + IF
;
K~(i.t)
(4)
The NEM has been successfully used by Park and Huang (1989) in modeling Co(IIL Ni(ll), and Zn(lI) adsorption onto hydrous cadmium sulfide, and has been found to be essentially equivalent to the triple-layer surface complexation model in adequately describing Cd(lI) adsorption onto Fe203"H20(am) (Cowan et al., 1991). The intrinsic constant, with the electrostatic correction factor taken as unity is then defined as (Park and Huang, 1989):
248
Table I. Stability Constants for Cd(ll) Surface Complexation on Hematite.
Cd(II) Concentration (M)
[
Range of K" between pH 4.5 to 10.0
[
Average Value of K'
2.9x 10.6
1.9x10 -3 _ 5.6x10 -5
2.6x10 ~
8.5x10 "6
8.8x10 -4 - 7.0x10 --~
1.0xl0 ~
1.6xl0 -5
4.8x10 -~ _ 1.4x10 -5
3.5x10 -5
4.1x10 --~
4.6x10 -5 _ 9.6x10 -6
2.5x10 -5
4.7x10 5
4.6x!0 5 _ 9.6x10 -6
2.5x10 --~
9.3x10 5
3.0x10 -5 _ 1.3xi0 --s
1.7x I 0 -~
[S-OCd÷] [W] K~0n~ =
=
Ks
(5)
[S-OH] [Cd 2+]
Employing a site density of 11 sites/nm2, the specific surface area of 12 m2/g, and the solids loading of 1 g/L, a value of 2.2x10 "4 M is calculated for the total number of surface sites. A balance for the surface sites of the hematite includes metal-occupied and un-occupied sites. Using [S-OCd÷] and [Cd2+] from experimental results, the adsorption stability constant (KS) is determined for each metal adsorption experiment. Cadmium hydrolysis is evaluated using "K~ = 10~~4 and "K2 = 10~44 (Sillen and Martell, 1971). The calculated surface complexation constant varies only slightly with pH and an average K s is obtained for each concentration (Table I). The pH independence of K' supports the use of the NEM and the selection of the monodentate surface species [S-OCd+) as
the primary adsorbed species (Park and Huang, 1989). The stability constant decreases slightly with increasing cadmium loading, which is normally attributed to site heterogeneity and preferential adsorption onto high-energy sites. Figure 2 demonstrates that the model provides a good representation of the adsorption data.
Humic acid adsorption: The adsorption of humic acid onto hematite ranges from 60-45% for 5 mg/L HA to 45-10% for 15-20 mg/L HA: the adsorption decreases almost linearly with an increase in pH (Figure 3), as was observed by Murphy et al. (1990). The higher adsorption at lower pH qualitatively supports the mechanism given by Eqn 2. These results are consistent with those of other workers, i.e., Tipping et al. (1983) for peat humic substances onto goethite, Tipping (1981b) for Esthwaite water humic substances onto both goethitc and hematite, and Parfitt et al. (1977) for humic and fulvic acids onto gibbsite, goethite, and imogolite. At higher concentrations,
the degree of adsorption may be limited due to blockage of sites by adsorbed humic acid molecules, since humic acid has a large molecular structure and each molecule may cover several possible complexation sites on the oxide surface.
With an increase in the concentration of humic acid, the fractional adsorption is found to decrease,
suggesting the saturation of surface sites.
Cadmium adsorption onto hematite in the presence of humic acid (ternary system): There are three possible roles for complexing ligands in ternary metal adsorption systems: Ligands can inhibit metal adsorption by
249
100
Z
) 80
£ 0 ID
¢J
~
i =~
•
8.5x10-6 M Cd(ll)
~
417X10-5
M
Cd(ll)
Modeled-8.SxlO-6
so +! ~
Modeled-4.7xl 0-5
40~
20
0
•
3
im
4
5
3 6
7
8
9
10
11
pH
Figure 2.
Adsorption Modeling of 8.5x10 6 M and 4.7x10 ~ M Cd(II) onto ct-Fe203 using a Non-Electrostatic Surface Complexation Model.
100
80 i "O
i 60 i
m~
0
i
"r
40 4i
•
•
•
• Z
5 mg/L
•
10 mg/L
20 i I
i:
• 15 mg/L
i
' 20 mg/L
I
o J 3
4
5
6
7
pH
Figure 3.
Adsorption of Humic Acid onto ct-Fe203. (1 g/L Hematite, I=10 .2 M NaNO~.)
8
250
100
8o 'U
0 W
(
8o! 40
[]
[]
I
•
[]
Cd/
•
•
I
/
5 mg/L HA
./
[]
5.0x10-6 M Cd/ 10 mg/L HA
20~ / 0
,
,
i
3
4
• 5.0x10-6 M Cd only I
5
6
7
8
9
10
11
pH
Figure 4.
Adsorption of 5x10 6 M Cd(ll) onto ~-Fe203 in the Presence of Humic Acid. 1 g/L Hematite, I=10 .2 M NaNO~. Cd(ll)-only Curve is Modeled using Constant from Table I.
forming non-adsorbable complexes with the metal, as well as by competing with the metal itself for available surface sites. Metal adsorption can be enhanced by the presence of a ligand due to the formation of metal-ligand ternary surface complexes. The details of such a system will depend on the adsorbing and complexing properties of all species involved. Cadmium adsorption onto hematite is found to increase in the presence of humic acid in the pH range of 5 to 8 (Figure 4). The degree of enhancement increases with increasing concentrations of HA at low Cd loadings, the difference between 5 and i0 mg/L HA is less apparent for [Cd(II)] greater than 5x10 ~ M. Also the greatest enhancement from HA addition is noted for the lowest Cd(II) concentrations. Laxen (1985) found that the adsorption of Cd(II) onto ferric oxide was enhanced in the presence of humic substances: however in his case the enhancement was essentially independent of humic concentration. explanation for the discrepancy may lie in the experimental conditions:
The
the study by Laxen used Cd(II)
concentrations of only 5xl0 9 M, thus available Cd(II) is likely to be a strongly limiting constituent. Davis and Leckie (1978) showed that Cu(II) adsorption onto amorphous iron oxide was increased by the presence of thiosulfate, glutamic acid and 2,3-pyrazinedicarboxylic acid. These authors explain that if the ligand's major coordinating functional groups are not used in surface binding and are directed outward towards solution, then
251 trace metal adsorption can be significantly enhanced. The enhanced adsorption can result from the adsorption of metal-ligand complexes or through trace metal complexation by the adsorbed ligands; in either case, the final configurations are identical. The functional groups of an adsorbed ligand may provide new adsorption sites for trace metals which may be more reactive than isolated oxide surface sites. If the functional groups of HA are available and are oriented towards the solution, then the metal uptake may be substantially enhanced by the humic substances. Davis and Leckie (1978) also state that since humic materials are large and have low solubility, they can accumulate at the solid/water interface, providing a coating on hydrous oxides and clays. Thus, the adsorption of metal ions may be dominated by the exposed functional groups of the adsorbed organics rather than those of the oxide. As a result, the adsorption of trace metals in natural waters may be governed by chelation reactions with these humic acid functional groups. Since the HA molecule is large, many metal binding sites are likely per molecule. Therefore, many Cd(ll) ions can bind to the HA, which consequently binds to the hematite surface.
(C(O)O-Cd÷L S_-OH2+ + Hu-(C(O)O-), + mCd2+ ,, S-OC(O)-Hu
+ H20
(6)
(C(O)OL,,
The modeling of ternary systems is very complex, involving multiple adsorbed and solution phase equilibria. Additional complexities result when employing humie acid due to the heterogeneity of this material.
Modeling
competitions among species may be compounded by metals, ligands, and metal-ligand complexes not preferentially adsorbing on the same type of surface sites. Effect of loading order (Sequential adsorption): Tests covering a range of Cd(ll) concentrations and two different humic acid concentrations were completed to observe the effect of substrate loading order on Cd(II) adsorption. Interesting, differing results are obtained depending on the adsorption sequence (Figures 5a-c). At the lower Cd(II) loadings, there is little difference among the three different sequences (i.e., Figure 5a). However, at the 7-10x10 5 M Cd(ll) loading, obvious divergence among the systems is apparent (Figures 5b&c). For these systems, addition ofhumic acid to a Cd(ll)/hematite equilibrated system (or "Cd first" system) leads to more Cd(II) adsorption than the simultaneous adsorption of Cd(ll) and HA. Cd(II) adsorption from a Cd(ll) spike into a humic acid/hematite equilibrated system (HA first) did not significantly vary from that of the simultaneous mode. Overall, Cd(II) adsorption is higher for 10 mg/L HA than for 5 mg/L for all cases and the trends with respect to sequential loadings are identical. Humic acid molecules each contain many functional groups. As humic acids adsorb and cover the oxide surface, they significantly alter the characteristics of the surface with respect to metal uptake. binding sites are likely blocked by the surface complexation of large HA molecules.
Hematite metal
However, this adsorption
should not affect exposed sites on the HA that are available for Cd(ll) complexation through the formation of ligandlike ternary complexes. During simultaneous adsorption, the humic acid may cover the surface and form ternary
252
100
80
÷
"0 m
-B 0
60
-~ 4.5x10-6 M Cd spike/ 5 mg/L HA
iO
'0
.< •
o
40
n
Z
• 6.7X10-6 M Cd/
~
5 mg/L HA spike g
20
• 7.1x10-6 M Cd/ 5 mg/L HA simultaneously
m~ i
0 3
4
5
7
6
8
9
10
11
pH
100
!
"0
80 ÷ )
60
" 7.1x10-5 M Cd spike/ 5 mg,/L HA
L
i
:1
•
,,m~
m
i 20 -~
•
• 6.7x10-.5 M Cd/ 5 mg/L HA spike
• •
;
0i
•
~:J 5
3
6
7
8
Cd/ 5 mg/L HA simultaneously
10x10-5 M
9
10
11
pH 100 • 80 t
m=
I
<
60 t
•
•
7.1X10-5 M Cd spike/ 10 mg/L HA
r
40 ~-
•
i
•
6.7x10-5 M Cd/ 10 mg,/L HA spike
•
10x10-5 M
-
I
20 +
*'~c~':'~ ~e
o
Cd/ 10 mg/L HA simultaneously
%o 0
~
3
4
•
5
6
7
8
9
10
11
pH
Figure 5.
Sequential Adsorption of 10s M Cd(II) onto a-FcqO3 in the Presence of 5 mg/L Humic Acid a) 4.5=7.1x10 ~s M Cd(ll), 5 mg/L Humic Acid b) 6.7-10x10 s M Cd(II), 5 mg/L Humic Acid c) 6.7-10x10 s M Cd(II), 10 mg/L Humic Acid
253
100
80 ¢1 0
tJ
60
•
NO EDTA
""
lx10-6 M EDTA
•
5x10-6 M EDTA
, ~
lx10-5 M EDTA
.j
40 ~-
20
%E
0 3
4
5
6
7
pH
Figure 6.
Effect of EDTA on Adsorption of 10'~ M Cd(II) onto ot-Fe203 (1 g/L) in the Presence of 10 mg/L Hurnic Acid (I = 10.2 M NaNO3).
complexes with some Cd(lI), and also some Cd(lI) may be adsorbed directly onto the hematite. However, in the "Cd first" mode, Cd(II) ions initially adsorb onto the hematite surface. At high Cd(lI) loadings, the addition of HA allows the formation of ternary surface complexes, both on the hematite surface, and with adsorbed Cd. Thus adsorbed Cd may be physically covered by subsequently adsorbed HA. This sequential attachment may produce an overall enlaancement of Cd(ll) removal since the blocked hematite sites in this case already contain adsorbed Cd(II). As a result, for large ligands with multiple metal binding sites, attachment order may be important in determining metal adsorption capacity onto solids. Competition with EDTA (Quaternary Systems):
The effect of a second ligand, EDTA, a strong
complexing agent with a cadmium stability constant of 1017'2 (Sillen and Martell, 19717, on the ternary adsorption system was studied (Figure 6). There is no noticeable effect from 10~ M EDTA since a maximum of only 10% of the total Cd(II) could be complexed. Increasing the EDTA concentration to 5xl 0 .6 and 10-5 M significantly reduces the Cd(ll) adsorption, most likely due to the competitive formation of non-adsorbing Cd(II)-EDTA complexes. Elliott and Denneny (1982) state that in natural systems, organic ligands can out-compete soil humic materials for metals, lfthe metal-ligand stability constant is greater than the humic-metal complexation constant, a reduction in metal uptake by the solid results upon ligand addition. They found that Cd(ll) sorption onto soils was reduced
254 by added ligands, proportional to the Cd-L stability constant. Similar results are observed here, where EDTA outcompetes both the humic acid and the oxide surface sites for complexation with the cadmium. The ratio of reduction in the adsorption of Cd(II) to added EDTA concentration is less than 1:1 in spite of the large Cd(II)-EDTA stability constant. Cadmium equal to about 45% of the EDTA concentration is prevented from adsorbing in these systems. Various competitive reactions contribute to this low ratio. The sorption of EDTA onto hydrous oxide surfaces can occur through the formation of surface complexes (Rubio and Matijevid, 1979; Bowers and Huang, 1985); EDTA uptake by 13-FeOOH occurred from pH 2.5 to 9 (Rubio and Matijevid, 1979). Although the formation of ternary metal-EDTA-surface complexes is possible (Bowers and Huang, 1986), it is apparently negligible here.
SUMMARY AND CONCLUSIONS The modeling of Cd(ll) adsorption onto hematite by the use of a non-electrostatic surface complexation model was successful and describes the data well. The stability constants determined from the NEM varied little with pH, supporting the application of the NEM. Humic acid adsorption onto hematite demonstrates characteristics of a general ligand-exchange mechanism pattern, adsorption density decreasing with increasing pH. The presence of humic acid increases Cd(II) uptake by hematite. Humic acid apparently becomes sorbed onto the oxide surface, allowing other HA sites with a high affinity for Cd(II) to be available for the formation of ternary adsorption complexes.
For the concentrations examined, Cd(lI) adsorption is found to increase with
increased concentrations of humic acid. The order of loading of substrates is found to have a noticeable effect on the final Cd(II) uptake for high Cd(ll) concentrations. The Cd(lI) uptake is observed to vary in the manner: humic acid spike > humic acid/Cd(II) simultaneously = Cd(II) spike.
This implies that adsorption interactions among the aquatic substrates can be
dependent on their order of loading, and the various reaction mechanisms may not be considered equivalent. The presence of EDTA reduces the Cd(lI) adsorption significantly, competing with the surface-bound humic acid by complexing Cd(II): Cd-EDTA complexes are weakly, if at all, adsorbed. In spite of the large metal complex stability constant of Cd(II)-EDTA, the ratio of additional Cd(I1) dissolved to added EDTA is observed to be less than 1:1; this may be due to some EDTA being adsorbed to the hematite surface. Dissolved and sorbed organic matter is important in controlling metal sorption characteristics onto particulates. Humic acid can enhance metal adsorption onto oxides, the degree of enhancement may depend on the structure of the surface complexes and the sequential order of sorption.
ACKNOWLEDGEMENT A University of Maryland General Research Board Summer Research Grant to A.P.D. is gratefully acknowledged.
255
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