Environ. Sci. Technol. 2002, 36, 1485-1490
Heavy Metal Uptake by Lignin: Comparison of Biotic Ligand Models with an Ion-Exchange Process RAY H. CRIST,† J. ROBERT MARTIN,† AND D E L A N S O N R . C R I S T * ,‡ Messiah College, Grantham, Pennsylvania 17027, and Department of Chemistry, Georgetown University, Washington, D.C. 20057
Metal uptake by kraft lignin, hereafter referred to as lignin, occurs by displacement of protons or bound metals with equilibrium constants KexH and Kex, respectively. Values calculated for wide ranges of initial concentrations are reasonably constant, thereby demonstrating the validity of these displacement processes and proving that uptake in these systems is not simple adsorption. It was found that the stoichiometry for Sr and Cd uptake by Caloaded lignin is 1 mol of metal for 1 mol of Ca released. This observation for metals of very different binding strengths is difficult to rationalize with the biotic ligand model as generally applied but is in complete agreement with an ionexchange process. Binding strengths to lignin, which contains only oxygen ligands, follow the order Pb > Cu > Zn > Cd > Ca (strongest to weakest). For proton displacement, only more tightly bound metals such as Pb, Cu, Zn, and Cd can compete with protons for anionbinding sites at low pH, but at high pH, uptake of Ca, Sr, and Li can occur. An observed logarithmic decrease of KexH with pH can be explained by having only weaker acids available for proton displacement under more basic conditions. The advantages and disadvantages of using adsorption and biotic ligand models for an ion-exchange process are discussed.
Introduction Lignin is a major component of plants where it serves as a binding agent for cellulose and other materials. The structure of lignin is rather complex. A polymer of coniferin, it contains propane units having aldehyde, keto, hydroxy, methoxy, and phenolic groups (1). The kraft process of paper production (heating with alkali and sulfide) produces polyhydroxy phenolic, carboxylic acid, and sulfide functional groups in a soluble black liquor mixture (1, 2). Acidification precipitates this modified lignin, hereafter referred to as lignin, as a 2030% byproduct in the manufacture of paper having no use comparable to its availability. In recent years, some progress has been made determining the ability of this material to sorb toxic metals. An alkaline solution of lignin at high pH was investigated for Cu removal (3). Maximum Cu removal was obtained when the pH was lowered to 4.7 where the lignin had precipitated. Uptake was * Corresponding author phone: (301)654-0875; fax: (202)687-6209; e-mail:
[email protected]. † Messiah College. ‡ Georgetown University. 10.1021/es011136f CCC: $22.00 Published on Web 02/22/2002
2002 American Chemical Society
decreased at still lower pH and also at higher pH where the nature of functional groups had changed. In another study (4), it was found that lignin from black liquor showed an optimum uptake of lead at pH 5-7 and of zinc at pH 4-6. Data were analyzed by Freundlich and Langmuir isotherm equations. Although interesting thermodynamic results were presented, the process was taken to be one of simple “adsorption”. Sorption of uranium on a product of wood or crop residues and tannery waste containing lignin was studied for possible environmental applications (5). Kraft lignin in powder or bead form has been investigated for removal of Pb, Zn, Cr(III), and Cr(VI) from process waters (6). For quantitative treatments or sorption, two models are in current use: Langmuir adsortion and biotic ligand (712). Both involve metal ions in solution binding to sites on the solid surface, without any other change, and both can therefore be represented by eq 1 where species in parentheses represent nonaqueous phase species:
M + (S) a (MS)
K ) (MS)/(S)[M]
(1)
Recently Meyer (13) has used this model to account for competition of metals for surface sites. In his approach, biotic ligand models can accommodate this competition by assuming that two or more metals bind to a negative surface, as illustrated for divalent M and Ca ions in eqs 2 and 3, where anion sites of the surface S are represented as X-. Meyer combined equilibrium expressions analogous to these to give eq 4.
M2+ + 2(X-) a (MX2)
KM ) (MX2)/[M2+] (X-)2 (2)
Ca2+ + 2(X-) a (CaX2)
KCa ) (CaX2)/[Ca2+] (X-)2 (3)
[Ca2+](MX2)/[M2+](CaX2) ) K
K ) KM/Kca
(4)
Extensive studies on diverse biological systems such as humic materials (14), algae (15-18), and peat moss (18, 19) have shown that metal uptake occurs by ion exchange of aqueous metal ions with anion sites associated with metals such as Ca (eq 5) and by proton displacement (eq 6). Since ions are released from the dense phase upon metal uptake, these processes are fundamentally different from the adsorption-type equations represented by eqs 1-3:
M2+ + (CaX2) a (MX2) + Ca2+ Kex ) [Ca2+](MX2)/[M2+](CaX2) (5) M2+ + 2(HX) a (MX2) + 2H+
KexH ) [H+]2(MX2)/[M2+](HX)2 (6)
A cursory inspection of the equilibrium relationships 4 and 5 suggests that there is no difference in these approaches. However, in the biotic ligand model, metal ions are assumed to act independently, and the ratio of CaOFF to MON is determined by the ratio of association constants KM/Kca.. Specifically, this adsorption model does not predict the 1:1 ratio of metal OFF to metal ON observed for the ion-exchange systems studied. The work noted above has given impetus to approaches for metal biosorption by modified lignins and for a use of this byproduct of paper manufacture. We now report results of experiments to determine whether uptake of toxic metals VOL. 36, NO. 7, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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by kraft lignin (hereafter referred to as lignin) can be described by ion-exchange equilibria.
TABLE 1. Proton Exchange Constants KexH for Uptake of Pb by NLga by Pb2+ + 2(HX) / (PbX2) + 2H+
Materials and Methods Kraft pine lignin (Indulin AT) was kindly provided by Westvaco, Charleston Heights, SC. As received, it is a powder with a particle size of 200-300 µm containing a small amount of Na and Ca. It was used in this form, referred to as native lignin (NLg) for sorption of metals at the lower pH range of 4-6 where the reaction was one of proton release by the sorbed metal. For metal uptake with Ca release, NLg was first converted to the acid form of lignin (HLg) by treating the powder with nitric acid at pH 1.5 for 30 min. Addition of calcium hydroxide to the desired pH gave CaLg, which was used as a suspension of 20 mg in 10 mL of H2O. The stability of CaLg depends on pH. For example, a sample of HLg was treated with Ca(OH)2 taking the pH to 9.0, and that pH was maintained by the addition of Ca(OH)2 until equilibrium was achieved. If the sample was recovered by centrifugation, quickly washed, and returned to water, Ca was released slowly over about 30 min when the system became stable. However, if this process was repeated, the sample again slowly lost Ca when returned to pure water. It was found that systems at ca. pH 6.5 give the most stable CaLg preparation, and this was the material used for the stoichiometry and alkali metal displacement experiments described below. All M(II) ions were provided as nitrate salts, and all M(I) ions were provided as chlorides (Fisher Scientific). Concentrations were determined by atomic absorption (AA) with a Perkin-Elmer model 2380 equipped with an acetylene flame instrument and utilizing the following wavelengths (in µm): Ca, 422; Sr, 460; Ba, 553; Pb, 217; Cs, 324; Cd, 228; and Zn, 213. After equilibration, a mixture was centrifuged and the supernatant liquid was analyzed for aqueous metal concentrations. For amounts of sorbed metals, the residual solid was treated with 10 mmol L-1 HNO3 for 30 min followed by centrifugation and AA analysis of this aqueous layer for released metals. For experiments to determine stoichiometry, the solid sample was treated with concentrated nitric acid and AA done on the resulting solution. All errors are given as average deviations of the mean. Metal-Proton Exchange with Native Lignin. A sample of 20 mg of NLg suspended in 10 mL was treated with a Thomas tissue grinder. This procedure involves pulling the suspension up and down a tube with a closely fitting, rotating plunger, which produced 10-70 µm particle size. Some of the contained Na and Ca were released when the suspension was brought to the desired pH (a value between 4 and 6). A solution of M(II) nitrate (Pb, Cu, Sr, Ca) was then added to give a set of concentrations 0.1, 0.2, 0.4, and 0.6 mmol L-1 before any reactions (including precipitations) occur. Protons were released lowering the pH, which however was held constant by addition of 10 mmol L-1 LiOH. The amount of LiOH used is the measure of the protons released. After being centrifuged, the liquid was analyzed by AA for M. The lignin sample was then analyzed for the sorbed (MX2). As illustrated by data for Pb in Table 1, proton exchange constants for the reaction Pb2+ + 2(HX) f 2H+ + (PbX2) were calculated from eq 6, where [H+] is given by the pH and Pb species as described above. The amount of active acid sites (HX) is given by (HX) ) (HX)o - HOFF where (HX)o is the total acid sites available for the system. This value, the capacity (C), is first estimated from extrapolation of the curve for HOFF ) (PbX2) uptake vs [Pb2+], followed by trial to give a minimum error for calculated KexH values. Stoichiometry for Ca Release on Uptake of Sr or Cd. CaLg was prepared by adding 20 mg of Ca(OH)2 to a suspension of 0.10 g of HLg in 10 mL of H2O. After 1 h for equilibration, the sample was removed by centrifugation and 1486
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[Pb2+] initial, mequiv L-1 [Pb2+] final, mequiv L-1 (PbX2), µmol (HX),b µmol KexH average
0.2 0.03 1.7 3.2 0.55
0.4 0.8 0.12 0.43 2.8 3.7 2.2 1.2 0.48 0.60 0.56 (0.05
1.2 0.79 4.1 0.9 0.62
a Data for a suspension of 20 mg of NLg in 10 mL of water at pH 5.0. Remaining acid sites. These values are (HX) ) (HX)0 - HOFF, where HOFF is the amount of LiOH needed to maintain the pH at 5.0 and (HX)0 ) 5.0 µmol is the capacity, determined as described in Materials and Methods. b
washed twice with water. It was then suspended in 50 mL of H2O taken to pH 6.5. To four 10-mL aliquots were added 0.04, 0.06, 0.08, and 0.10 mL of 10 mmol L-1 Sr. After being stirred with a magnetic bar for 30 min, the suspensions were filtered, and AA of the filtrate gave [Ca2+] and [Sr2+] in mmol L-1. The filter paper was dried and then in small pieces treated with 2 mL of concentrated HNO3 and heated on a hot plate for about 5 min. The lignin completely dissolved, releasing Sr and Ca for AA to give (CaX2) and (SrX2) in mmol/20 mg. These data gave the number of moles of Sr sorbed relative to the number of moles of Ca displaced. They were also used to calculate Kex by eq 5. Similar experiments were done for Cd displacing Ca at pH 6.0. Metal-Metal Exchange with CaLg. A suspension of 20 mg of HLg, initially at pH ca. 5, was taken to the desired pH with Ca(OH)2 giving a calcium salt component CaLg. When Pb was added, Ca was released. Protons were also released from unchanged HLg by reaction 6, and the extent of Pb sorbed by this process was measured by the amount of 10 mmol L-1 LiOH required to hold pH constant. Ca release was obtained by AA analysis of the filtrate, and the moles of sorbed (PbX2) was given by the sum, (mol of CaOFF) + (mol of HOFF)/ 2. Values of this (PbX2), as well as (CaX2) by AA analysis of a nitric acid-treated sample, and aqueous values of [Ca2+] and [Pb2+] by AA of the solution after centrifugation were used to calculate metal-metal exchange constants Kex from eq 5. Since the pH of these experiments was between 6 and 9, Pb was mostly present as the precipitated hydroxide, Pb(OH)2. However, in these cases, and for other metals with small solubility products, the species in eq 5 are the free aqueous ion concentrations, and those are the quantities measured by AA analysis of a centrifugate. Alkali Metal Ion Displacement of Ca from CaLg. In a process taking about 1 h, HLg was taken to pH 6.5-7.0 with Ca(OH)2 thereby producing CaLg. After washing, a 0.1-g sample was suspended in 50 mL of H2O at pH 6.5 and then 10 mL of the suspension added to beakers containing 0.1, 0.3, 0.6, and 1.0 mL of 0.1 M LiCl. Ca is released in a relatively fast process, 15-30 min, so that the dense phase remains fairly stable. After being stirred for 30 min, the solution was analyzed, after centrifugation, for Ca released. The procedure was repeated for Cs.
Results Metal-Proton Exchange with Native Lignin. Proton release from NLg by Cu are shown in Figure 1 for various pH ranging from 4.5 to 5.5. The capacity (C) for Cu is 2.0 µmol/20 mg at pH 4.5 and 6.0 µmol/20 mg at pH 5.5 Similar results are shown for Pb in Figure 2, with capacities of 1.5->10 µmol/ 20 mg for pH 4.0-5.5. Values of the proton exchange constant KexH, calculated by eq 6 for data points from the pH 5 curve of Figure 2 and given in Table 1, are constant over a 6-fold change in Pb concentration at that pH within experimental error, demonstrating validity of eq 6 at a given pH. However, it was
TABLE 2. Stoichiometry for Ca Released on Sr Uptake by CaLga and Ion-Exchange Constants Kex initial Sr, mmol L-1 [Ca2+], mmol L-1 [Sr2+], mmol L-1 (SrX2), µmol (CaX2), µmol
0.05 0.022 0.027 0.35 1.33
0.10 0.053 0.049 0.40 1.41
0.20 0.076 0.152 0.64 1.11
0.40 0.089 0.311 0.87 1.05
calculated Kex
0.214
0.306
0.288
0.242
0.262 ( 0.036
average
FIGURE 1. Protons released from native lignin when Cu is added to a suspension of 20 mg of NLg in 10 mL of water.
stoichiometry CaOFF/SrON, µmol/µmol molar ratio
0.22/0.35 0.53/0.40 0.76/0.64 0.89/0.87 0.63
1.32
material balance total Sr added, 0.50 µmol ([Sr2+] + (SrX2)), 0.62 µmol a
1.18
1.02
1.04 ( 0.21
average 1.0
2.0
4.0
0.89
2.16
3.98
Data for a 20-mg suspension of CaLg in 10 mL of water at pH 6.5.
TABLE 3. Stoichiometry for Ca Released on Cd Uptake by CaLga and Ion-Exchange Constants Kex FIGURE 2. Protons released from NLg when Pb is added to a suspension of 20 mg of NLg in 10 mL of water.
initial Cd, mmol L-1 [Ca2+], mmol L-1 [Cd2+], mmol L-1 (CdX2), µmol (CaX2), µmol
0.04 0.017 0.00066 0.271 1.39
0.06 0.027 0.00185 0.344 1.28
0.08 0.031 0.00274 0.422 1.21
0.10 0.042 0.00436 0.436 1.31
calculated Kex
5.02
3.92
3.94
3.20
4.02 ( 0.51
average stoichiometry CaOFF/CdON, µmol/µmol molar ratio
0.17/0.27 0.27/0.34 0.31/0.42 0.42/0.44 0.63
0.79
material balance total Cd added, 0.4 µmol ([Cd2+] + (CdX2)), 0.33 µmol a
FIGURE 3. Dependence of log KexH on pH for displacement of protons from NLg by Pb, Cu, Sr, and Ca. found that KexH values depend strongly on pH and are therefore conveniently represented as logarithms. A plot of log KexH for Cu, Pb, Sr, and Ca releasing protons from lignin vs pH is shown in Figure 3. Because of hydroxide precipitation, Pb and Cu are studied at the lower pH. Stoichiometry for Ca Release on Uptake of Sr or Cd. When Sr was added to a suspension of CaLg in water, Ca was released. After 30 min for equilibration, Ca2+ and Sr2+ in the filtrate and (CaX2) and (SrX2) in the sample were analyzed by AA. Values for the ratio (µmol of CaOFF):(µmol of SrON) shown in Table 2 are unity within experimental error (1.04 ( 0.21), thereby demonstrating that Sr uptake is accompanied by a stoichiometric release of Ca. The data give a material balance, since the amount of Sr added is the same as the sum of that found in solution and sample after equilibration. Finally, the data can be used to calculate Kex. These values are constant over an 8-fold change in initial
0.74
0.95
0.78 ( 0.09
average 0.6
0.8
1.0
0.52
0.69
0.88
Data for a 20-mg suspension of CaLg in 10 mL of water at pH 6.0.
[Sr2+ ], in agreement with the ion-exchange equilibrium. Similar data and results are given for Cd in Table 3 where the molar ratio of CaOFF:CdON is also considered to be unity (average of 0.78 ( 0.09, with the more reliable three higher concentrations cases giving 0.82). Metal-Metal (Ca) Ion Exchange. Heavy metal uptake on CaLg was studied under a range of pH conditions where, in addition to existing sorbed Ca, protons from weaker acids were released, as indicated for Cd sorption in Figure 4. The amount of (MX2) formed by proton release for a given [M2+ ] was determined by measuring the amount of protons released and utilizing the reaction M2+ + 2(HX) f (MX2) + 2H+. The total binding capacity is given by the maximum of the sum (HOFF/2) + (CaOFF). To examine applicability of ion exchange eq 5, [Ca2+] and [M2+] were obtained by AA analysis of the filtrate, (CaX2) was from analysis of the lignin sample, and the value for (MX2) was given by the sum (HOFF/2) + (CaOFF). Data for displacement of Ca by Cu are given in Table 4, with Kex values calculated from eq 5. The fact that these values are constant for a 10-fold variation in [Cu2+ ] demonstrates ion-exchange equilibrium 5, which is valid even in the presence of considerable amounts of the metal in the form of its solid hydroxide. VOL. 36, NO. 7, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 5. Metal-Metal Ion-Exchange Constants Kex for Displacement of Ca from CaLga at Various pH by M2+ + (CaX2) / Ca2+ + (MX2) pH metal
6
7
8
9
Kexb
Cd Zn Cu Pb
2.1 4.8 9.2 13.6
4.3 6.6 5.4 14.9
5.8 5.0 5.6 11.2
4.0
4.0 ( 1.0 5.5 ( 0.6 6.7 ( 1.2 13.2 ( 1.3
a A suspension of 20 mg of CaLg in 10 mL of H O is treated with 2 various amounts of 0.1 mmol L-1 Cd, for example. After equilibration and centrifugation, [Ca2+] and [Cd 2+] in the supernatant solution and (CaX2) in the sample are analyzed by AA. (CdX2) is given by the sum, CaOFF + HOFF/2. b Values of Kex are averaged for the pH shown.
FIGURE 4. Release of protons and Ca from calcium-loaded lignin by Cd at various pH. Cd is added to a suspension of 20 mg of CaLg in 10 mL of water at various pH. In each case, the curve for equivalents of CdON was one calculated from equivalents of CaOFF + equivalents of HOFF.
TABLE 4. Metal-Metal Ion-Exchange Constants Kex for Cu Uptake by CaLga by Cu2+ + (CaX2) / Ca2+ + (CuX2) initial Cu, mmol L-1 Ca2+, mmol L-1 Cu2+, mmol L-1 HOFF/2, µmolb CaOFF, µmol (CuX2), µmolc (CaX2), µmol calculated Kex average
0.10 0.65 0.081 0.30 0.47 0.77 1.25 4.9
0.30 0.60 1.07 0.13 0.274 0.312 0.60 0.90 0.89 0.95 1.49 1.85 1.13 1.10 5.1 6.0 5.4 ( 0.4
1.0 1.27 0.50 1.10 1.00 2.19 1.00 5.5
a Data for 20 mg CaLg in 10 mL H O at pH 7.0 with addition of Cu2+. 2 µmol of H displaced by Cu and measured by the amount of LiOH needed to maintain the pH at 7.0. HOFF/2 in µmol is the amount of Cu sorbed by displacing protons. c Values given by the sum, CaOFF + HOFF/2. b
Initially, it may seem surprising that one can obtain meaningful results when a significant amount of added metal precipitates in the form of hydroxides. This impression arises because one often uses the initial amount of metal to deduce concentrations of other species by mass balance, and if there is considerable precipitation one cannot readily do this. Also, one might be accustomed to measuring the total amount of metal on the solid, and in the case of precipitation this amount includes the hydroxide in addition to other sorbed species. However, to test the ion-exchange equation, we do not use mass balance to get other concentrations by difference. Three of the four species, [M2+], [Ca2+], and (CaX2), are determined by direct measurement, and (MX2), the amount of metal sorbed specifically at anion sites, is obtained from amounts of Ca and protons released. “Initial concentrations” in the tables thus represent only how solutions are made up and not free ion concentrations. Values of Kex for displacement of Ca from CaLg by various metals over a range of pH 6-9 are given in Table 5. For each metal they are reasonably constant showing independence of pH, as expected since the Kex equation does not contain H+. For the several metals Kex gives an indication of the relative binding strength and follows the order Pb > Cu > Zn > Cd > Ca (strongest to weakest). This order may seem surprising in view of the usual trend Cu > Pb > Zn > Cd. However, the usual trend includes amino ligands that bind stronger to Cu than Pb. For lignin, which does not have nitrogen-containing groups, complexation occurs at oxygen 1488
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FIGURE 5. Release of Ca from calcium-loaded lignin by alkali metals. Li (b) or Cs (9) is added to a suspension of 20 mg of CaLg in 10 mL of water at pH 6.5. functions such as carboxylates, phenolates, and enolates, and Pb is known to complex stronger to oxygen functions (for example, acetates) than Cu. Alkali Metal Displacement of Ca from CaLg. Ca was displaced from CaLg by Cs and Li, with Cs displacing almost twice as much Ca at 10 mmol L-1 concentrations of Cs or Li (see Figure 5). One explanation for this interesting result is based on the higher charge density (Z2/r) of the hydrated ions. The smaller Cs(H2O)6+ has a higher charge density of 0.0044 as compared to 0.0029 for the larger Li(H2O)22+. The greater effectiveness for Cs uptake may therefore result from easier transport of Cs with its higher charge density.
Discussion Proton Displacements. All metals studied showed a similar logarithmic decrease of KexH with pH, as shown in Figure 3. One explanation for this decrease is that Ka for a given type of acid decreases with pH. Because of a higher negative charge buildup at high pH, it is harder to remove a proton from a remaining acid group, the process involved in a metal displacement of protons. This explanation has been examined in more detail previously (18). An additional factor is that at higher pH, weaker acids are the ones still in acid form. Their lower Ka values mean that loss of a proton is less favored, also leading to lower observed KexH. Stoichiometry and Ion Exchange. The stoichiometry for the uptake of Sr and Cd is that 1 mol of Ca is released when 1 mol of the metal is taken up by lignin, as shown for Sr (Table 2) and for Cd (Table 3). Thus the actual process involved in initial uptake of these metals is one of ion exchange (shown in reaction 7 for Cd):
Cd2+ + (CaX2) a (CdX2) + Ca2+ Kex ) [Ca2+](CdX2)/[Cd2+](CaX2) (7)
The present results cannot readily be explained by the biotic ligand model in which metals act independently. According to the biotic ligand model, 1 mol of Cd could be sorbed without release of 1 mol of Ca. Also, this model does not predict the 1:1 ratio observed for both Cd and Sr but rather that the amount of Ca released would be very different for these metals of contrasting bonding characteristics. Equilibrium expression 7 can be derived from kinetics in a way similar to that for the Langmuir adsorption isotherm but with the rate in each direction proportional to the fraction of reactive sites for that direction. Setting the forward rate (eq 8) equal to the reverse rate (eq 9) and canceling the common denominator gives equilibrium expression 7, with Kex ) kf/kr:
Rf ) kf[Cd2+]
[ [
Rr ) kr[Ca2+]
] ]
(CaX2)
(CaX2) + (CdX2) (CdX2)
(CaX2) + (CdX2)
(8)
(9)
Although the kinetics have not been measured, this approach includes in a satisfactory way the fact that metal uptake does not occur in a homogeneous phase but rather with aqueous ions and a solid material. Data that show the constancy of Kex values in Tables 2-4 demonstrate the validity of the ionexchange process for metal uptake on Ca-pretreated lignin. An important point to note is that the ion-exchange expression is maintained even when most of the heavy metal is present as its solid hydroxide. Lignin with sorbed Ca can be used for metal-metal (Ca) exchange in the pH range of 6-9, but in the case of Pb, for example, there is only a very small amount of free [Pb2+] present at the higher pH. Nevertheless, the three equilibria are maintained: precipitation, Pb2+ + 2(OH-) a Pb(OH)2; proton displacement, Pb2+ + 2(HX) a (PbX2 ) + 2 H+; and ion exchange, Pb2+ + (CaX2) a (PbX2) + Ca2+. The presence of solid Pb(OH)2 does not interfere with analytical determinations of the quantities that appear in Kex: after equilibration, the suspension is centrifuged with AA analysis of the filtrate giving Ca2+ and Pb2+; the sample on acidification gives (CaX2); since some Pb(OH)2 is included in the sample, (PbX2 ) is taken as the sum of Ca and protons released (see Figure 4, for example). Experiments with solid hydroxides present demonstrate that metal-metal exchange is independent of pH, as expected since H+ is not in the expression for Kex and as shown in Table 5. Potential Misuse of Adsorption and Biotic Ligand Models. Metal uptake has been shown to occur by an ionexchange process for humic materials (14), algae (15-18), peat moss (18, 19), trout gills (20), and, in the present study, lignin. The question arises as to what are the consequences if data for such ion-exchange systems are treated by an adsorption-type model (eqs 1-3). As previously reported (17, 20), values of the binding constant K calculated by the Langmuir model for actual metal-metal ion-exchange data are not constant, independent of [M] as expected for an equilibrium constant. Instead, K increases markedly at low concentration of added M. Apparent K values are reasonably constant only under two conditions: (i) when the added metal concentration is very high, as is the case in some biosorption applications, and (ii) when the added metal is very low in concentration, but the amount of aqueous Ca displaced by the added metal is small as compared to aqueous Ca already present. Under other conditions, variation of K with concentration may be due to the fact that the back reaction of ion exchange is not taken into account in the Langmuir model and cannot be neglected.
Comparison of Biotic Ligand Models and Ion Exchange. As pointed out by a reviewer, a considerable advantage of biotic ligand models for uptake data is that they “fit easily into off-the-shelf computer models such as MINEQL+ and MINTEQ, which run on molar units and log K values. These models have many interactions of metals with inorganic and organic ligands built into them”. However, the biotic ligand model requires a critical, additional assumption, namely, that activities of solid materials are related to concentrations, which are problematic to define for solids. For example, in some calculations, the number of binding sites per gram of solid substrate are taken as the number of sites in the volume of solution used. However, clearly the sites do not exist in that volume. The main potential weakness of this model or the related Langmuir model is possible misuse when the reverse reaction is not properly taken into account as described above. When applicable, the Langmuir relation is useful, however, as it requires less data than the ion-exchange approach and, under the restrictions described above, can be applied to complex systems for which one cannot determine the stoichiometry. The main advantage of the ion-exchange relationship is that, for the systems studied, it represents the actual process: a chemical reaction of defined stoichiometry. Values of Kex are small, dimensionless numbers that give the relative binding of two ions rather than the very large constants with reciprocal concentration units generated by application of the biotic ligand model with no back reaction. Also, as pointed out by a reviewer, the ion-exchange relationship can rigorously accommodate reactions of different charge types, as seen in the present work for protons and divalent metals, or in potential toxicity studies involving univalent ions such as Ag or trivalent Al with divalent Ca or Mg, while some biotic ligand models do not explicitly consider charge. Two disadvantages of the ion-exchange approach are that the concentrations of more species need to be measured to demonstrate ion exchange and determine Kex and that the method is not readily applicable to software modeling at this point. Instead of calculating free ion concentrations under a variety of conditions of pH, hardness, and complexing agents, those concentrations are measured.However, once Kex for a given metal with some biomaterial has been determined, MINEQL+ should be useful in predicting various environmental scenarios.
Acknowledgments We express our appreciation to a reviewer for suggestions about comparison of models and other helpful comments.
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Received for review July 16, 2001. Revised manuscript received November 29, 2001. Accepted January 9, 2002. ES011136F
