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Desorption in a System Consisting of Humic Acid, Heavy Metals, and Clay Minerals
Journal of Colloid and Interface Science 218, 225–232 (1999) Article ID jcis.1999.6419, available online at http://www.idealibrary.com on
Adsorption/Desorption in a System Consisting of Humic Acid, Heavy Metals, and Clay Minerals Aiguo Liu and Richard D. Gonzalez 1 Chemical Engineering Department, Tulane University, New Orleans, Louisiana 70118 Received February 8, 1999; accepted July 12, 1999
Metal adsorption/desorption in a system consisting of humic acid, metal ions, and clay minerals is described. Montmorillonite and purified humic acid were selected as a prototype materials for this study. At a constant ionic strength, the amount of humic acid adsorbed on montmorillonite decreases when pH is increased. A slight increase in humic acid adsorption on montmorillonite is observed when there are bivalent metals present in the system. The metal adsorption on montmorillonite does not correlate to the amount of humic acid adsorbed on montmorillonite. Montmorillonite with preadsorbed humic acid does not show a significant change in the capacity of adsorbed metal ions. An increase in the ionic strength at a pH of 6.5 results in an increase in the adsorption of lead on montmotillonite in the presence of humic acid, while at a lower pH, the increase in ionic strength results in a decrease in metal adsorption. The bridging of bivalent metal ions between montmorillonite and humic acid is proposed as the dominant adsorption mechanism. © 1999 Academic Press Key Words: adsorption/desorption; humic acid; montmorillonite; heavy metals.
INTRODUCTION
The adsorption of heavy metals by sediments and natural organic materials (NOM) is an important mechanism that controls metal concentration in aqueous systems. Montmorillonite together with kaolinite and quartz are the primary constituents of sediments found in the lower Mississippi Delta (G. C. Flowers, personal communication, 1995). The substitution of Al 31 for Si 41 in the tetrahedral sheet and Mg 21 for Al 31 in the octahedral sheet induces a negative charge on montmorillonite (1). Typical montmorillonites, such as Wyoming montmorillonite, have a cation exchange capacity (CEC) around 80 meq/100g and a point of zero charge (PZC) of approximately 2.5 (2). Under general environmental conditions, its surface is always negatively charged. Humic substances are present in aqueous systems and can exist either in solution or as a precipitate mixed with sediments. They tend to associate into complex chemical structures that are more stable than the starting materials (3, 4). About 80% of the dissolved organic 1
To whom correspondence should be addressed. Fax: 504-8656744. E-mail: [email protected].
materials (DOM) consist of humic substances. In natural water, about 50% of the DOM consists of humic and fulvic acids (5). The ecotoxicology of heavy metals is determined by the pathway of decontamination in a natural aqueous system. Humic substances provide important sources of dissolved adsorbent organic ligands and, therefore, are expected to influence the bioavailibility and mobility of metals in soil, sediments, and aquatic systems. With respect to the potential toxicity of heavy metals, adsorption by humic substances is believed to reduce metal uptake by organisms. Extensive studies have been carried out on the adsorption of metals on sediments. Most of the laboratory studies performed in the last two decades have been focused on the adsorption of heavy metals on well-characterized particulates, especially the oxides (6 –13). Experimental studies of metal adsorption on various clay materials have been widely carried out. Humic acid and fulvic acid adsorb on the surface of particles and may alter their surface properties. They may also complex with the metal ions changing the interaction between the metal ions and solid particles. Numerous studies have shown the presence of a strong interaction between humic substances and clay minerals even though both are negatively charged under normal pH conditions (14 –17). Zhou et al. (12) found that the humic substance uptake by montmorillonite increases with ionic strength. The reasons for this increase in humic acid uptake were assumed to be the neutralization of surface charges and the compression of diffuse double layers. The latter was believed to be more important. Taylor and Theng (18) found that the sorption of cadmium by kaolinite increased with an increase in the humic acid concentration and solution pH. Maguire et al. (19), on the other hand, found that the addition of humic acid to the clay–Cs system depressed the sorption of Cs by clays and enhanced the desorption of Cs. The phenolic and carboxylic groups (-OH and -COOH) are believed to be the most active adsorption sites on humic and fulvic acid. Lo¨vgren and Sjo¨berg (20) studied Cu, Ca, and Hg complexation with NOM concentrated from bog-water at 0.1 M ionic strength. Kerndorff and Schnitzer (21) investigated sorption of various metal ions on humic acid at different pH, metal concentration, and HA concentration and found that the order of sorption efficiency is a function of pH.
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For example, at pH 2.4, the sorption efficiency order is Hg . Fe . Pb . Cu 5 Al . Ni . Cr 5 Zn 5 Cd 5 Co 5 Mn, but at pH 5.8, the order is Hg 5 Fe 5 Pb 5 Al 5 Cr 5 Cu . Cd . Zn . Ni . Co . Mn. Hg(II) was the most easily adsorbed metal by HA, and Mn was the most difficult. Ca inhibits the adsorption of trace metals on HA more effectively than Mg (22). Electrochemical methods have been widely used to study metal adsorption on humic acid and fulvic acid. These methods include such techniques as ion-selective electrodes (ISE) and anodic stripping voltammetry (ASV) (22–26). The advantage of electrochemical methods of analysis is that one can make in situ measurements without the need to separate the complexed ions from the free metal ions. A major drawback of ISE, from our own experience, is that a steady reading is not easy to obtain. This may be due to the adsorption of humic acid or fulvic acid molecules on the surface of the electrodes. As for the ASV, the detection limit is not low enough to meet the usual conditions of metal concentration. Malcolm and MacCarthy (27) concluded that commercial humic acids are not representative of aquatic or soil humic acids. Following a comparison using calcium titration data for Aldrich and Suwannee Stream humic acids, Hering and Morel (28) concluded that the use of reprecipitated commercial humic acid in preliminary studies of metal complexation is adequate to within a first approximation. The purpose of this study is to obtain a better understanding of how humic substances influence metal adsorption/desorption under different environmental conditions. Montmorillonite and humic acid are selected as prototypes in this study. Emphasis is placed on the system of humic acid–metal ions and montmorillonite. EXPERIMENT
Materials and Supplies The sodium form of montmorillonite from Crook County, Wyoming, was supplied by the Source Clay Minerals Repository, University of Missouri, Columbia. After pretreatment with 0.1 M HCl and 0.5 M NaCl (29), the solution was allowed to settle. The larger particles which precipitated from the wash were discarded. The montmorillonite treated in this manner was retained as a slurry solution. The montmorillonite concentration in this stock solution was determined by drying a well-defined volume of the solution at 105°C until a constant weight was obtained and by measuring the difference in weight. The clay concentration of the stock solution was in the range 20 ; 30g/liter. Humic acid was prepared from sodium humate (Aldrich Chemical Co.). The commercial sodium humate was first dissolved in deionized water and precipitated by adjusting the solution pH to 1.5. The precipitate was redissolved in deionized water. This solution was further purified by an ultrafiltra-
tion process. Ultrafree-15 filtration units, provided by the Millipore Company, were made of polysulfone membranes with molecular weight cutoffs (MWCO) of 5,000 and 10,000. The wall of the units was made of polycarbonate. They could fit into 50-ml centrifuge tubes and achieve separation by centrifugal force. The concentrate from the 10,000 MWCO membrane filtration and the filtrate permeated through 5,000 MWCO membrane filtration was discarded. Following each filtration, the concentrate was washed three times with deionized water. The purified solution of humic acid was used as a stock solution. The concentration in these solutions is expressed in terms of total carbon content as determined by combustion analysis. A 1000 ppm Cd(NO 3) 2, Cu(NO 3) 2, and Pb(NO 3) 2 standard solution was purchased from Fisher Scientific. The concentration of this standard solution was guaranteed by the supplier and was used both as a standard and as the metal solution for the adsorption experiments. All other chemicals used were reagent grade or better. Adsorption Procedures Batch adsorption was carried out by using 50-ml scaled centrifuge tubes made out of polypropylene. As detailed in Fig. 1, the initial metal concentration was less than 5.0 3 10 25M. Following mixing of the desired volume of stock solution, the pH was adjusted by the addition of dilute HNO 3 or NaOH. The final volume was adjusted using deionized water. The samples were placed in an orbital shaker and shaken for 48 h at 25°C. The samples were then centrifuged at 11,000 rpm for 20 min. The supernatants were filtered using a 0.45-mm syringe filter. Ten-milliliter aliquots of filtrate from each sample were acidified using concentrated HNO 3 and stored in polypropylene centrifuge tubes for analysis by atomic absorption. Additional 10-ml aliquots of filtrate were transferred into the ultrafiltration unit equipped with a 5,000 MWCO polysulphone ultrafiltration membrane to separate complexed metal ions from uncomplexed ions. The separation was carried out by centrifugation at about 5,000 rpm (;2000 g). Humic acid concentrations were determined following 0.45-mm syringe filter filtration. To determine the reversibility of the adsorption of metal ions on montmorillonite with or without the presence of humic acid, the desorption properties were also tested. The montmorillonite samples were taken from centrifuged samples containing the adsorbed metals. The content of adsorbed metal was known from analysis of the concentration of supernatant solution and by calculations based on the mass balance. These solid samples were redistributed in 0.1 M NaNO 3 solution. The pH was adjusted to 4.0 through the addition of an NaAc–HAc buffer solution. After being shaken for 5 h at room temperature, the samples were centrifuged at 11,000 rpm for 15 min. The supernatant was collected in a 50-ml volumetric flask. The same procedure was repeated three times. Deionized water was added to the flask which contained the collected supernatant
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FIG. 1. Schematic representation of experimental procedures.
until the 50-ml mark was reached. The supernatant solution was analyzed using atomic adsorption to determine the metal concentration. The desorption ratio was defined as the amount of metal desorbed divided by amount of the metal originally adsorbed. Analyses The concentration of metals in the aqueous phase was determined by atomic absorption. A Perkin–Elmer model 500 Atomic Absorption Spectrophotometer was used in these measurements by following the standard procedures and methods established by the EPA. A calibration curve was determined using standard samples prior to each measurement. Humic acid concentration was measured using a UV160 spectrophotometer at a wavelength of 400 nm. Pettersson et al. (30) used the absorption at 250 nm to analyze the concentration of humic acid. However, at this wavelength the absorption is very sensitive to the NO 32 present in our samples. In order to eliminate the effect of pH on the absorption, samples (usually 5 ml) for humic acid concentration analysis were mixed with a 2-ml NaAc–HAc buffer solution to stabilize the solution pH to approximately 4.5. The concentration of humic acid was expressed as ppm carbon. The concentration in the stock solution of humic acid was determined by an EAGER 200 combustion elemental analyzer. The standard solution of humic acid was obtained by diluting the stock solution of humic acid with a known concentration. The amount of metal adsorbed on either
montmorillonite or humic acid was determined by mass balance. RESULTS
The results of the adsorption of humic acid as a function of solution pH and concentration are given in Tables 1 and 2. The
TABLE 1 The Adsorption of Humic Acid on Montmorillonite as a Function of pH a
Eq. pH 2.37 3.90 4.57 5.01 5.63 6.55 6.82 7.31 7.98
b
Eq. conc. of HA in solution c (mg/liter)
Adsorbed c (mg/g-mont)
Percentage of adsorbed HA (%)
9.42 26.18 33.88 41.28 51.93 59.58 63.28 66.64 70.64
26.19 22.84 21.30 19.82 17.69 16.16 15.42 14.75 13.95
93.29 81.36 75.87 70.60 63.01 57.56 54.93 52.54 49.69
a Ionic strength was 0.1 M NaNO 3. Initial concentrations of HA and clay were 140 mgC/liter and 5.0 g/liter, respectively. b Equilibrium pH variation was 60.1. c Average value of at least three measurements. The relative errors are less than 8.0%.
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TABLE 2 The Adsorption of Humic Acid on Montmorillonite as a Function of the Equilibrium Concentration of Humic Acid a Eq. conc. of HA in solution b (mg/liter)
pH
4.35 8.67 15.85 19.97 31.08 40.47 69.09 99.13
6.01 5.84 5.92 6.05 5.88 5.97 6.02 5.93
c
Adsorbed b (mg/g-mont)
Percentage of adsorbed HA (%)
3.79 7.62 13.32 14.72 16.94 19.98 23.62 26.97
81.33 81.48 80.78 78.66 73.16 71.17 63.09 57.64
a
Ionic strength was 0.1 M NaNO 3. Clay concentration was 5.0 g/liter. Average value of at least three measurements. The relative errors are less than 8.0%. c The pH variation was 60.1. b
concentration of montmorillonite was 5.00 g/liter for all the data if not specified. The ionic strength was 0.1 M NaNO 3 for all samples. The amount of adsorbed humic acid on the montmorillonite decreased with increasing pH. The percentage of adsorbed HA was calculated by taking the adsorbed amount of humic acid and dividing it by the total amount of humic acid initially added. The percentage of adsorbed humic acid decreased from about 93% at pH 2.37 to about 50% at a pH of about 8.0.
FIG. 2.
The results of the isothermal adsorption of humic acid on montmorillonite at a constant pH of about 6.0 and ionic strength of 0.1 M are given in Table 2. An increase in the equilibrium concentration of humic acid results in an increase in the amount of adsorbed humic acid on montmorillonite. Figure 2 shows the results of the adsorption of metals on montmorillonite as a function of humic acid concentration in the solid phase. This experiment was performed by first adsorbing humic acid on the montmorillonite. The humic acid adsorption was carried out by adding different amounts of humic acid to the montmorillonite suspension. Initial concentrations of HA for a series of samples were in the range 10 to 300 mgC/liter. The pH and ionic strength were controlled at 3.0 and 0.1 M, respectively. After separation by centrifugation, the amount of humic acid adsorbed on montmorillonite was determined and the precipitate was redistributed in the metal solution. The pH and the ionic strength in the process of metal adsorption were controlled at 6.0 and 0.1 M, respectively. The desorption of humic acid under such conditions was not detectable by the UV-VIS method. Thus the amount of humic acid desorption was neglected. A slight increase in the amount of adsorption was observed for Cd and Cu when the adsorbed humic acid was increased. But no measurable change in the amount of adsorbed Pb on montmorillonite was observed. Two different adsorption sequences, preadsorption and coadsorption, were examined. Preadsorption refers to the process by which the montmorillonite used in the metal adsorption was
Metal adsorption on montmorillonite in the presence of humic acid. The pH and ionic strength were controlled at 6.0 and 0.1 M, respectively.
ADSORPTION/DESORPTION ON CLAY MINERALS
FIG. 3. Metal adsorption on montmorillonite as a function of adsorbed humic acid. The pH and ionic strength were controlled at 6.0 and 0.1 M, respectively. (top) Pb adsorption, (middle) Cd adsorption, (bottom) Cu adsorption. (F) preadsorption, (E) coadsorption.
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preequilibrated with the humic acid and the amount of humic acid adsorbed was known. Coadsorption refers to the process by which the metal, humic acid, and montmorillonite stock solutions were mixed together simultaneously. The results of the adsorption sequences on the metal adsorption are shown in Fig. 3. No obvious difference between the two adsorption processes is observed for Pb and Cd. For Cu, there is a slightly higher amount of adsorption for the coadsorption process than the preadsorption process. The adsorption of humic acid and metals on montmorillonite as a function of ionic strength is shown in Fig. 4. The set of data were obtained by adding a different amount of 5 M NaNO 3 solution to the samples containing the same initial concentration of metals and humic acid. The equilibrium concentrations of metals and humic acid in the supernatant were analyzed following centrifugation. An interesting observation is that in the presence of humic acid, the amount of adsorbed Pb on montmorillonite increases with increasing ionic strength. Figure 5 shows the results of Pb adsorption as a function of ionic strength at two different equilibrium pH values. It shows that at a relatively high pH, e.g., when pH . 6.0, the amount of adsorbed Pb increases with increasing ionic strength, which is similar to the results shown in Fig. 4. But at a lower equilibrium pH, an increase in the ionic strength results in a decrease in the amount of adsorbed Pb on montmorillonite. The effect of metals on the adsorption of humic acid on montmorillonite is shown in Fig. 6. The adsorption of humic
FIG. 4. Adsorption of Pb and humic acid on montmorillonite as a function of ionic strength. Ionic strength was controlled with NaNO 3. Equilibrium pH was 6.5. Initial concentrations of HA and lead were 104 mgC/l and 5 3 10 25 M, respectively.
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FIG. 5. Effect of ionic strength on Pb adsorption in the presence of humic acid.
acid on montmorillonite is compared for the three different metals studied. A slight increase in the amount of humic acid adsorption on montmorillonite is observed when there are metals present in the system. Desorption of Cd, Pb, and Cu from montmorillonite as a function of the adsorbed amount of humic acid are shown in Fig. 7. The original adsorption was carried out at pH 6.0 6 0.1 and an ionic strength I 5 0.1 M. The solid phase obtained from centrifugation with a known amount of adsorbed metal and humic acid was used to carry out the desorption experiment. The solid was redistributed in a solution of I 5 0.1 M and buffered at pH 3.8. No obvious effect of adsorbed humic acid on the desorption of the three metals tested was observed.
FIG. 6. Effect of metal ions on humic acid adsorption on montmorillonite. Initial metal concentrations were 5 3 10 25 M for all samples. Equilibrium pH was controlled at 6.0 6 0.1.
DISCUSSION
Humic Acid Adsorption on Montmorillonite As shown in Table 1, an increase in pH from ;2.3 to ;8.0 results in a 50% decrease in the amount of humic acid adsorbed on montmorillonite. In this pH range, the surface of montmorillonite is negatively charged. The results of the zeta-potential measurements show that the negative charge density on montmorillonite will increase with increasing pH. The same trend
FIG. 7. Effect of humic acid on the desorption of metal ions from montmorillonite. Adsorption was carried out at a pH 6.0 6 0.1 and ionic strength I 5 0.1 M. The solid phase obtained from centrifugation with a known amount of adsorbed metal was used to carry out the desorption experiment. The solid was redistributed in a buffered solution at pH 3.8 and I 5 0.1 M.
ADSORPTION/DESORPTION ON CLAY MINERALS
occurs with the molecules of humic acid. The higher the pH, the greater will be the dissociation of the functional groups OCOOH and OCOH to OCOO 2 and OCO 2. The net effect will result in an increase in negative charges on humic acid. The electrostatic interaction between the charges will repel the humic acid from the surface of montmorillonite. There are three possible mechanisms governing the adsorption of humic acid on montmorillonite. They are cation bridging, water bridging, and H-bond complexation. At low pH, H-bond complexation could be the dominant mechanism. At higher pH, because of the dissociation of OCOOH and OCOH, the molecules of humic acid become more hydrophilic. At the same time, the adsorption of cations becomes more significant at high pH, so that cation bridging becomes the main mechanism. The mechanism of water bridging could exist under conditions of both low and high pH. Although the desorption of humic acid from montmorillonite was not systematically investigated in this study, it has been found that at a pH of ;4.0 and an ionic strength of 0.1 M, the desorption of humic acid from montmorillonite is so small that the UV-VIS method used to analyze the humic acid concentration could not detect it. This observation illustrates either a strong bond (e.g., H-bond) existing between the humic acid and montmorillonite or that the humic acid molecules have entered into the space between the layers of montmorillonite. The Effect of Humic Acid on the Adsorption of Metals on Montmorillonite The effect of humic acid on metal adsorption was studied by relating the amount of metal adsorbed on montmorillonite to the amount of humic acid adsorbed. Only a slight increase in the amount of metal adsorbed on montmorillonite was observed for Cd and Cu when the amount of humic acid adsorbed was increased (Fig. 2). Generally, this is contrary to the results that minerals coated by humic acid have a higher capacity in the adsorption of metal ions. Experimental results are available for minerals such as kaolinite (11, 16, 31). However, no direct data are available in the case of montmorillonite for comparison purposes. Using the results from the kaolinite studies as a comparison with these from montmorillonite might be inappropriate due to differences in the structure of kaolinite and montmorillonite. As proposed by Leckie (32) and Stumm (2), there are two possible structures for the adsorption of the metal and organic complex compounds on mineral surfaces. One is the S–Me–HA and the other is the S–HA–Me, where S represents the adsorption site on the solid surface and Me is the metal ion. From the adsorption data, HA has almost an order of magnitude higher adsorption capacity than montmorillonite. As shown in Fig. 3, there is no significant difference in metal adsorption between adsorption sequences, i.e., preadsorption and coadsorption. The adsorption/desorption data in Fig. 6 prove that the existence of bivalent metal ions enhances the
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adsorption of humic acid on montmorillonite. The desorption rates of the three metals from montmorillonite have no significant change when the amount of humic acid adsorption is increased (Fig. 7). Combining the overall data, the most possible surface structure should be S–Me–HA given sufficient adsorption time and at a relatively high pH. Two possible reasons might be proposed to explain the experimental results. The first is that the metal ions are acting like a bridge (2) between the humic acid and the montmorillonite. And the second is that montmorillonite has a stronger affinity for the bivalent metal ions than humic acid so that even the sites pre-occupied by humic acid in montmorillonite could be finally intercalated by metal ions. The Effect of Ionic Strength on the Adsorption of Metal Ions in the Presence of Humic Acid Figure 4 demonstrates a simultaneous adsorption of Pb 21 and humic acid on montmorillonite as a function of ionic strength at a pH of 6.5. It shows that increasing the ionic strength results in an increase in the amount of lead adsorbed, at the same time, resulting in a decrease in the amount of humic acid adsorbed. This trend is the opposite of results previously observed in this research. Other investigators (6 –11) have observed that the amount of metal adsorbed on montmorillonite decreased with increasing ionic strength. The adsorption of humic acid decreased when ionic strength was increased. At a lower pH, e.g., at a pH of 4.0 as shown in Fig. 5, the trend of lead adsorption is reversed. The increase in ionic strength results in a decrease in lead adsorption. At a higher pH, the humic acid tends to be hydrophilic. An increase in the ionic strength results in the reduction of the hydrodynamic thickness of adsorbed humic acid (2) and compresses the thickness of the double layer. The net effect would be beneficial for the penetration of metal ions into the inner layer and adsorbtion directly on the montmorillonite surface. On the other hand, there will be more functional groups in humic acid protonated at lower pH and the molecules of humic acid become more hydrophobic. In this situation, H-bonding might be the principle mechanism for humic acid adsorption. Therefore, it would be more difficult for the metal ions to intercalate. Increasing the ionic strength will only cause more monovalent ions (Na 1 in this study) to exist in the outer layer and cause a decrease in the electrostatic potential to further decrease the adsorption of the bivalent metal ions. More detailed experiments are needed to fully recognize the environmental significance of this result, which means that the presence of natural organic materials might be environmentally beneficial in keeping harmful metals adsorbed on the sediments rather than remaining in solution. This might be an important factor to be included in the investigation of water pollution in estuaries.
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CONCLUSIONS
Bivalent metal adsorption on montmorillonite remains unchanged in the presence of humic acid, except in the case of adsorption as a function of ionic strength. An increase in lead adsorption on montmorillonite was observed when the ionic strength was increased in the presence of humic acid. From the overall data, bivalent metal ions are most likely bridging between the adsorption sites on montmorillonite and humic acid molecules. Further studies on the system of metal ions, humic acid, and clay minerals concerning the aggregation properties as a function of pH and ionic strength are necessary to properly estimate the rate and distribution of heavy metal pollution in the estuary regions.
9. 10. 11. 12. 13. 14. 15. 16. 17.
18. 19.
ACKNOWLEDGMENTS The authors acknowledge financial support from the U.S. Department of Energy as a part of the Tulane/Xavier joint program on Hazardous Wastes in the Aquatic Environment of the Mississippi River Basin.
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Adsorption/Desorption in a System Consisting of Humic Acid, Heavy Metals, and Clay Minerals Aiguo Liu and Richard D. Gonzalez 1 Chemical Engineering Department, Tulane University, New Orleans, Louisiana 70118 Received February 8, 1999; accepted July 12, 1999
Metal adsorption/desorption in a system consisting of humic acid, metal ions, and clay minerals is described. Montmorillonite and purified humic acid were selected as a prototype materials for this study. At a constant ionic strength, the amount of humic acid adsorbed on montmorillonite decreases when pH is increased. A slight increase in humic acid adsorption on montmorillonite is observed when there are bivalent metals present in the system. The metal adsorption on montmorillonite does not correlate to the amount of humic acid adsorbed on montmorillonite. Montmorillonite with preadsorbed humic acid does not show a significant change in the capacity of adsorbed metal ions. An increase in the ionic strength at a pH of 6.5 results in an increase in the adsorption of lead on montmotillonite in the presence of humic acid, while at a lower pH, the increase in ionic strength results in a decrease in metal adsorption. The bridging of bivalent metal ions between montmorillonite and humic acid is proposed as the dominant adsorption mechanism. © 1999 Academic Press Key Words: adsorption/desorption; humic acid; montmorillonite; heavy metals.
INTRODUCTION
The adsorption of heavy metals by sediments and natural organic materials (NOM) is an important mechanism that controls metal concentration in aqueous systems. Montmorillonite together with kaolinite and quartz are the primary constituents of sediments found in the lower Mississippi Delta (G. C. Flowers, personal communication, 1995). The substitution of Al 31 for Si 41 in the tetrahedral sheet and Mg 21 for Al 31 in the octahedral sheet induces a negative charge on montmorillonite (1). Typical montmorillonites, such as Wyoming montmorillonite, have a cation exchange capacity (CEC) around 80 meq/100g and a point of zero charge (PZC) of approximately 2.5 (2). Under general environmental conditions, its surface is always negatively charged. Humic substances are present in aqueous systems and can exist either in solution or as a precipitate mixed with sediments. They tend to associate into complex chemical structures that are more stable than the starting materials (3, 4). About 80% of the dissolved organic 1
To whom correspondence should be addressed. Fax: 504-8656744. E-mail: [email protected].
materials (DOM) consist of humic substances. In natural water, about 50% of the DOM consists of humic and fulvic acids (5). The ecotoxicology of heavy metals is determined by the pathway of decontamination in a natural aqueous system. Humic substances provide important sources of dissolved adsorbent organic ligands and, therefore, are expected to influence the bioavailibility and mobility of metals in soil, sediments, and aquatic systems. With respect to the potential toxicity of heavy metals, adsorption by humic substances is believed to reduce metal uptake by organisms. Extensive studies have been carried out on the adsorption of metals on sediments. Most of the laboratory studies performed in the last two decades have been focused on the adsorption of heavy metals on well-characterized particulates, especially the oxides (6 –13). Experimental studies of metal adsorption on various clay materials have been widely carried out. Humic acid and fulvic acid adsorb on the surface of particles and may alter their surface properties. They may also complex with the metal ions changing the interaction between the metal ions and solid particles. Numerous studies have shown the presence of a strong interaction between humic substances and clay minerals even though both are negatively charged under normal pH conditions (14 –17). Zhou et al. (12) found that the humic substance uptake by montmorillonite increases with ionic strength. The reasons for this increase in humic acid uptake were assumed to be the neutralization of surface charges and the compression of diffuse double layers. The latter was believed to be more important. Taylor and Theng (18) found that the sorption of cadmium by kaolinite increased with an increase in the humic acid concentration and solution pH. Maguire et al. (19), on the other hand, found that the addition of humic acid to the clay–Cs system depressed the sorption of Cs by clays and enhanced the desorption of Cs. The phenolic and carboxylic groups (-OH and -COOH) are believed to be the most active adsorption sites on humic and fulvic acid. Lo¨vgren and Sjo¨berg (20) studied Cu, Ca, and Hg complexation with NOM concentrated from bog-water at 0.1 M ionic strength. Kerndorff and Schnitzer (21) investigated sorption of various metal ions on humic acid at different pH, metal concentration, and HA concentration and found that the order of sorption efficiency is a function of pH.
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For example, at pH 2.4, the sorption efficiency order is Hg . Fe . Pb . Cu 5 Al . Ni . Cr 5 Zn 5 Cd 5 Co 5 Mn, but at pH 5.8, the order is Hg 5 Fe 5 Pb 5 Al 5 Cr 5 Cu . Cd . Zn . Ni . Co . Mn. Hg(II) was the most easily adsorbed metal by HA, and Mn was the most difficult. Ca inhibits the adsorption of trace metals on HA more effectively than Mg (22). Electrochemical methods have been widely used to study metal adsorption on humic acid and fulvic acid. These methods include such techniques as ion-selective electrodes (ISE) and anodic stripping voltammetry (ASV) (22–26). The advantage of electrochemical methods of analysis is that one can make in situ measurements without the need to separate the complexed ions from the free metal ions. A major drawback of ISE, from our own experience, is that a steady reading is not easy to obtain. This may be due to the adsorption of humic acid or fulvic acid molecules on the surface of the electrodes. As for the ASV, the detection limit is not low enough to meet the usual conditions of metal concentration. Malcolm and MacCarthy (27) concluded that commercial humic acids are not representative of aquatic or soil humic acids. Following a comparison using calcium titration data for Aldrich and Suwannee Stream humic acids, Hering and Morel (28) concluded that the use of reprecipitated commercial humic acid in preliminary studies of metal complexation is adequate to within a first approximation. The purpose of this study is to obtain a better understanding of how humic substances influence metal adsorption/desorption under different environmental conditions. Montmorillonite and humic acid are selected as prototypes in this study. Emphasis is placed on the system of humic acid–metal ions and montmorillonite. EXPERIMENT
Materials and Supplies The sodium form of montmorillonite from Crook County, Wyoming, was supplied by the Source Clay Minerals Repository, University of Missouri, Columbia. After pretreatment with 0.1 M HCl and 0.5 M NaCl (29), the solution was allowed to settle. The larger particles which precipitated from the wash were discarded. The montmorillonite treated in this manner was retained as a slurry solution. The montmorillonite concentration in this stock solution was determined by drying a well-defined volume of the solution at 105°C until a constant weight was obtained and by measuring the difference in weight. The clay concentration of the stock solution was in the range 20 ; 30g/liter. Humic acid was prepared from sodium humate (Aldrich Chemical Co.). The commercial sodium humate was first dissolved in deionized water and precipitated by adjusting the solution pH to 1.5. The precipitate was redissolved in deionized water. This solution was further purified by an ultrafiltra-
tion process. Ultrafree-15 filtration units, provided by the Millipore Company, were made of polysulfone membranes with molecular weight cutoffs (MWCO) of 5,000 and 10,000. The wall of the units was made of polycarbonate. They could fit into 50-ml centrifuge tubes and achieve separation by centrifugal force. The concentrate from the 10,000 MWCO membrane filtration and the filtrate permeated through 5,000 MWCO membrane filtration was discarded. Following each filtration, the concentrate was washed three times with deionized water. The purified solution of humic acid was used as a stock solution. The concentration in these solutions is expressed in terms of total carbon content as determined by combustion analysis. A 1000 ppm Cd(NO 3) 2, Cu(NO 3) 2, and Pb(NO 3) 2 standard solution was purchased from Fisher Scientific. The concentration of this standard solution was guaranteed by the supplier and was used both as a standard and as the metal solution for the adsorption experiments. All other chemicals used were reagent grade or better. Adsorption Procedures Batch adsorption was carried out by using 50-ml scaled centrifuge tubes made out of polypropylene. As detailed in Fig. 1, the initial metal concentration was less than 5.0 3 10 25M. Following mixing of the desired volume of stock solution, the pH was adjusted by the addition of dilute HNO 3 or NaOH. The final volume was adjusted using deionized water. The samples were placed in an orbital shaker and shaken for 48 h at 25°C. The samples were then centrifuged at 11,000 rpm for 20 min. The supernatants were filtered using a 0.45-mm syringe filter. Ten-milliliter aliquots of filtrate from each sample were acidified using concentrated HNO 3 and stored in polypropylene centrifuge tubes for analysis by atomic absorption. Additional 10-ml aliquots of filtrate were transferred into the ultrafiltration unit equipped with a 5,000 MWCO polysulphone ultrafiltration membrane to separate complexed metal ions from uncomplexed ions. The separation was carried out by centrifugation at about 5,000 rpm (;2000 g). Humic acid concentrations were determined following 0.45-mm syringe filter filtration. To determine the reversibility of the adsorption of metal ions on montmorillonite with or without the presence of humic acid, the desorption properties were also tested. The montmorillonite samples were taken from centrifuged samples containing the adsorbed metals. The content of adsorbed metal was known from analysis of the concentration of supernatant solution and by calculations based on the mass balance. These solid samples were redistributed in 0.1 M NaNO 3 solution. The pH was adjusted to 4.0 through the addition of an NaAc–HAc buffer solution. After being shaken for 5 h at room temperature, the samples were centrifuged at 11,000 rpm for 15 min. The supernatant was collected in a 50-ml volumetric flask. The same procedure was repeated three times. Deionized water was added to the flask which contained the collected supernatant
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FIG. 1. Schematic representation of experimental procedures.
until the 50-ml mark was reached. The supernatant solution was analyzed using atomic adsorption to determine the metal concentration. The desorption ratio was defined as the amount of metal desorbed divided by amount of the metal originally adsorbed. Analyses The concentration of metals in the aqueous phase was determined by atomic absorption. A Perkin–Elmer model 500 Atomic Absorption Spectrophotometer was used in these measurements by following the standard procedures and methods established by the EPA. A calibration curve was determined using standard samples prior to each measurement. Humic acid concentration was measured using a UV160 spectrophotometer at a wavelength of 400 nm. Pettersson et al. (30) used the absorption at 250 nm to analyze the concentration of humic acid. However, at this wavelength the absorption is very sensitive to the NO 32 present in our samples. In order to eliminate the effect of pH on the absorption, samples (usually 5 ml) for humic acid concentration analysis were mixed with a 2-ml NaAc–HAc buffer solution to stabilize the solution pH to approximately 4.5. The concentration of humic acid was expressed as ppm carbon. The concentration in the stock solution of humic acid was determined by an EAGER 200 combustion elemental analyzer. The standard solution of humic acid was obtained by diluting the stock solution of humic acid with a known concentration. The amount of metal adsorbed on either
montmorillonite or humic acid was determined by mass balance. RESULTS
The results of the adsorption of humic acid as a function of solution pH and concentration are given in Tables 1 and 2. The
TABLE 1 The Adsorption of Humic Acid on Montmorillonite as a Function of pH a
Eq. pH 2.37 3.90 4.57 5.01 5.63 6.55 6.82 7.31 7.98
b
Eq. conc. of HA in solution c (mg/liter)
Adsorbed c (mg/g-mont)
Percentage of adsorbed HA (%)
9.42 26.18 33.88 41.28 51.93 59.58 63.28 66.64 70.64
26.19 22.84 21.30 19.82 17.69 16.16 15.42 14.75 13.95
93.29 81.36 75.87 70.60 63.01 57.56 54.93 52.54 49.69
a Ionic strength was 0.1 M NaNO 3. Initial concentrations of HA and clay were 140 mgC/liter and 5.0 g/liter, respectively. b Equilibrium pH variation was 60.1. c Average value of at least three measurements. The relative errors are less than 8.0%.
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TABLE 2 The Adsorption of Humic Acid on Montmorillonite as a Function of the Equilibrium Concentration of Humic Acid a Eq. conc. of HA in solution b (mg/liter)
pH
4.35 8.67 15.85 19.97 31.08 40.47 69.09 99.13
6.01 5.84 5.92 6.05 5.88 5.97 6.02 5.93
c
Adsorbed b (mg/g-mont)
Percentage of adsorbed HA (%)
3.79 7.62 13.32 14.72 16.94 19.98 23.62 26.97
81.33 81.48 80.78 78.66 73.16 71.17 63.09 57.64
a
Ionic strength was 0.1 M NaNO 3. Clay concentration was 5.0 g/liter. Average value of at least three measurements. The relative errors are less than 8.0%. c The pH variation was 60.1. b
concentration of montmorillonite was 5.00 g/liter for all the data if not specified. The ionic strength was 0.1 M NaNO 3 for all samples. The amount of adsorbed humic acid on the montmorillonite decreased with increasing pH. The percentage of adsorbed HA was calculated by taking the adsorbed amount of humic acid and dividing it by the total amount of humic acid initially added. The percentage of adsorbed humic acid decreased from about 93% at pH 2.37 to about 50% at a pH of about 8.0.
FIG. 2.
The results of the isothermal adsorption of humic acid on montmorillonite at a constant pH of about 6.0 and ionic strength of 0.1 M are given in Table 2. An increase in the equilibrium concentration of humic acid results in an increase in the amount of adsorbed humic acid on montmorillonite. Figure 2 shows the results of the adsorption of metals on montmorillonite as a function of humic acid concentration in the solid phase. This experiment was performed by first adsorbing humic acid on the montmorillonite. The humic acid adsorption was carried out by adding different amounts of humic acid to the montmorillonite suspension. Initial concentrations of HA for a series of samples were in the range 10 to 300 mgC/liter. The pH and ionic strength were controlled at 3.0 and 0.1 M, respectively. After separation by centrifugation, the amount of humic acid adsorbed on montmorillonite was determined and the precipitate was redistributed in the metal solution. The pH and the ionic strength in the process of metal adsorption were controlled at 6.0 and 0.1 M, respectively. The desorption of humic acid under such conditions was not detectable by the UV-VIS method. Thus the amount of humic acid desorption was neglected. A slight increase in the amount of adsorption was observed for Cd and Cu when the adsorbed humic acid was increased. But no measurable change in the amount of adsorbed Pb on montmorillonite was observed. Two different adsorption sequences, preadsorption and coadsorption, were examined. Preadsorption refers to the process by which the montmorillonite used in the metal adsorption was
Metal adsorption on montmorillonite in the presence of humic acid. The pH and ionic strength were controlled at 6.0 and 0.1 M, respectively.
ADSORPTION/DESORPTION ON CLAY MINERALS
FIG. 3. Metal adsorption on montmorillonite as a function of adsorbed humic acid. The pH and ionic strength were controlled at 6.0 and 0.1 M, respectively. (top) Pb adsorption, (middle) Cd adsorption, (bottom) Cu adsorption. (F) preadsorption, (E) coadsorption.
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preequilibrated with the humic acid and the amount of humic acid adsorbed was known. Coadsorption refers to the process by which the metal, humic acid, and montmorillonite stock solutions were mixed together simultaneously. The results of the adsorption sequences on the metal adsorption are shown in Fig. 3. No obvious difference between the two adsorption processes is observed for Pb and Cd. For Cu, there is a slightly higher amount of adsorption for the coadsorption process than the preadsorption process. The adsorption of humic acid and metals on montmorillonite as a function of ionic strength is shown in Fig. 4. The set of data were obtained by adding a different amount of 5 M NaNO 3 solution to the samples containing the same initial concentration of metals and humic acid. The equilibrium concentrations of metals and humic acid in the supernatant were analyzed following centrifugation. An interesting observation is that in the presence of humic acid, the amount of adsorbed Pb on montmorillonite increases with increasing ionic strength. Figure 5 shows the results of Pb adsorption as a function of ionic strength at two different equilibrium pH values. It shows that at a relatively high pH, e.g., when pH . 6.0, the amount of adsorbed Pb increases with increasing ionic strength, which is similar to the results shown in Fig. 4. But at a lower equilibrium pH, an increase in the ionic strength results in a decrease in the amount of adsorbed Pb on montmorillonite. The effect of metals on the adsorption of humic acid on montmorillonite is shown in Fig. 6. The adsorption of humic
FIG. 4. Adsorption of Pb and humic acid on montmorillonite as a function of ionic strength. Ionic strength was controlled with NaNO 3. Equilibrium pH was 6.5. Initial concentrations of HA and lead were 104 mgC/l and 5 3 10 25 M, respectively.
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FIG. 5. Effect of ionic strength on Pb adsorption in the presence of humic acid.
acid on montmorillonite is compared for the three different metals studied. A slight increase in the amount of humic acid adsorption on montmorillonite is observed when there are metals present in the system. Desorption of Cd, Pb, and Cu from montmorillonite as a function of the adsorbed amount of humic acid are shown in Fig. 7. The original adsorption was carried out at pH 6.0 6 0.1 and an ionic strength I 5 0.1 M. The solid phase obtained from centrifugation with a known amount of adsorbed metal and humic acid was used to carry out the desorption experiment. The solid was redistributed in a solution of I 5 0.1 M and buffered at pH 3.8. No obvious effect of adsorbed humic acid on the desorption of the three metals tested was observed.
FIG. 6. Effect of metal ions on humic acid adsorption on montmorillonite. Initial metal concentrations were 5 3 10 25 M for all samples. Equilibrium pH was controlled at 6.0 6 0.1.
DISCUSSION
Humic Acid Adsorption on Montmorillonite As shown in Table 1, an increase in pH from ;2.3 to ;8.0 results in a 50% decrease in the amount of humic acid adsorbed on montmorillonite. In this pH range, the surface of montmorillonite is negatively charged. The results of the zeta-potential measurements show that the negative charge density on montmorillonite will increase with increasing pH. The same trend
FIG. 7. Effect of humic acid on the desorption of metal ions from montmorillonite. Adsorption was carried out at a pH 6.0 6 0.1 and ionic strength I 5 0.1 M. The solid phase obtained from centrifugation with a known amount of adsorbed metal was used to carry out the desorption experiment. The solid was redistributed in a buffered solution at pH 3.8 and I 5 0.1 M.
ADSORPTION/DESORPTION ON CLAY MINERALS
occurs with the molecules of humic acid. The higher the pH, the greater will be the dissociation of the functional groups OCOOH and OCOH to OCOO 2 and OCO 2. The net effect will result in an increase in negative charges on humic acid. The electrostatic interaction between the charges will repel the humic acid from the surface of montmorillonite. There are three possible mechanisms governing the adsorption of humic acid on montmorillonite. They are cation bridging, water bridging, and H-bond complexation. At low pH, H-bond complexation could be the dominant mechanism. At higher pH, because of the dissociation of OCOOH and OCOH, the molecules of humic acid become more hydrophilic. At the same time, the adsorption of cations becomes more significant at high pH, so that cation bridging becomes the main mechanism. The mechanism of water bridging could exist under conditions of both low and high pH. Although the desorption of humic acid from montmorillonite was not systematically investigated in this study, it has been found that at a pH of ;4.0 and an ionic strength of 0.1 M, the desorption of humic acid from montmorillonite is so small that the UV-VIS method used to analyze the humic acid concentration could not detect it. This observation illustrates either a strong bond (e.g., H-bond) existing between the humic acid and montmorillonite or that the humic acid molecules have entered into the space between the layers of montmorillonite. The Effect of Humic Acid on the Adsorption of Metals on Montmorillonite The effect of humic acid on metal adsorption was studied by relating the amount of metal adsorbed on montmorillonite to the amount of humic acid adsorbed. Only a slight increase in the amount of metal adsorbed on montmorillonite was observed for Cd and Cu when the amount of humic acid adsorbed was increased (Fig. 2). Generally, this is contrary to the results that minerals coated by humic acid have a higher capacity in the adsorption of metal ions. Experimental results are available for minerals such as kaolinite (11, 16, 31). However, no direct data are available in the case of montmorillonite for comparison purposes. Using the results from the kaolinite studies as a comparison with these from montmorillonite might be inappropriate due to differences in the structure of kaolinite and montmorillonite. As proposed by Leckie (32) and Stumm (2), there are two possible structures for the adsorption of the metal and organic complex compounds on mineral surfaces. One is the S–Me–HA and the other is the S–HA–Me, where S represents the adsorption site on the solid surface and Me is the metal ion. From the adsorption data, HA has almost an order of magnitude higher adsorption capacity than montmorillonite. As shown in Fig. 3, there is no significant difference in metal adsorption between adsorption sequences, i.e., preadsorption and coadsorption. The adsorption/desorption data in Fig. 6 prove that the existence of bivalent metal ions enhances the
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adsorption of humic acid on montmorillonite. The desorption rates of the three metals from montmorillonite have no significant change when the amount of humic acid adsorption is increased (Fig. 7). Combining the overall data, the most possible surface structure should be S–Me–HA given sufficient adsorption time and at a relatively high pH. Two possible reasons might be proposed to explain the experimental results. The first is that the metal ions are acting like a bridge (2) between the humic acid and the montmorillonite. And the second is that montmorillonite has a stronger affinity for the bivalent metal ions than humic acid so that even the sites pre-occupied by humic acid in montmorillonite could be finally intercalated by metal ions. The Effect of Ionic Strength on the Adsorption of Metal Ions in the Presence of Humic Acid Figure 4 demonstrates a simultaneous adsorption of Pb 21 and humic acid on montmorillonite as a function of ionic strength at a pH of 6.5. It shows that increasing the ionic strength results in an increase in the amount of lead adsorbed, at the same time, resulting in a decrease in the amount of humic acid adsorbed. This trend is the opposite of results previously observed in this research. Other investigators (6 –11) have observed that the amount of metal adsorbed on montmorillonite decreased with increasing ionic strength. The adsorption of humic acid decreased when ionic strength was increased. At a lower pH, e.g., at a pH of 4.0 as shown in Fig. 5, the trend of lead adsorption is reversed. The increase in ionic strength results in a decrease in lead adsorption. At a higher pH, the humic acid tends to be hydrophilic. An increase in the ionic strength results in the reduction of the hydrodynamic thickness of adsorbed humic acid (2) and compresses the thickness of the double layer. The net effect would be beneficial for the penetration of metal ions into the inner layer and adsorbtion directly on the montmorillonite surface. On the other hand, there will be more functional groups in humic acid protonated at lower pH and the molecules of humic acid become more hydrophobic. In this situation, H-bonding might be the principle mechanism for humic acid adsorption. Therefore, it would be more difficult for the metal ions to intercalate. Increasing the ionic strength will only cause more monovalent ions (Na 1 in this study) to exist in the outer layer and cause a decrease in the electrostatic potential to further decrease the adsorption of the bivalent metal ions. More detailed experiments are needed to fully recognize the environmental significance of this result, which means that the presence of natural organic materials might be environmentally beneficial in keeping harmful metals adsorbed on the sediments rather than remaining in solution. This might be an important factor to be included in the investigation of water pollution in estuaries.
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CONCLUSIONS
Bivalent metal adsorption on montmorillonite remains unchanged in the presence of humic acid, except in the case of adsorption as a function of ionic strength. An increase in lead adsorption on montmorillonite was observed when the ionic strength was increased in the presence of humic acid. From the overall data, bivalent metal ions are most likely bridging between the adsorption sites on montmorillonite and humic acid molecules. Further studies on the system of metal ions, humic acid, and clay minerals concerning the aggregation properties as a function of pH and ionic strength are necessary to properly estimate the rate and distribution of heavy metal pollution in the estuary regions.
9. 10. 11. 12. 13. 14. 15. 16. 17.
18. 19.
ACKNOWLEDGMENTS The authors acknowledge financial support from the U.S. Department of Energy as a part of the Tulane/Xavier joint program on Hazardous Wastes in the Aquatic Environment of the Mississippi River Basin.
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