Subsurface Interactions of Humic Substances with Cu( II)in Saturated Media Wllberl I. Oden,t Gary L. Amy,’i* and Martha Conkline
Department of Civil Engineerlng, Northern Arlzona University, Flagstaff, Arizona 8601 1, Department of Civil, Environmental, and Architectural Engineering, Unlverslty of Colorado, Boulder, Colorado 80309, and Department of Hydrology and Water Resources, University of Arizona, Tucson, Arlzona 8572 1 The interactions between natural organic matter (NOM) and Cu(I1)during transport through saturated media were investigated with column experiments. Six sources ’of NOM represented different humic fractions. Si02 and a-Al203 comprised the mineral surfaces. The experiments were conducted at pH 6.2 and 7.5. Total Cu(I1) was measured by atomic absorption spectroscopy; NOM was measured as UV absorbance at 254 nm. Results showed variation in NOM breakthrough as a function of source. The presence of Cu(I1) caused greater retardation of all NOM sources relative to the NOM-alone systems. The NOM effect on Cu(I1) transport was mixed, depending on the NOM source. At pH 7.5, NOM greatly facilitated Cu(11) transport. These mechanisms may influence NOM and Cu(I1) transport in groundwater: (1) competition between NOM and the mineral surface for Cu(I1) complexation, (2) formation of mixed ligand complexes, (3) complexation of Cu(I1) by adsorbed NOM, and (4)Cu(I1) bridging of NOM to mineral surfaces.
Introduction Trace metal transport in groundwater is controlled by metal interactions with the solid mineral phase and the speciation of the metal in the aqueous phase. In a typical aquifer matrix, the solid phase is comprised primarily of clay minerals (on the basis of surface area), which are dominated by silicates and metal hydroxides (Fe, Al, Mg). Metal adsorption on silicates can occur at edge sites, interlayer sites, or as a result of isomorphous substitution. The affinity of hydroxides for trace metals is largely a function of Ph, due to the amphoteric nature of the hydroxide minerals. A metal can interact with a surface oxygen or hydroxyl acting as a ligand (1). McBride ( Z ) , using electron spin resonance spectroscopy (ESR) to examine the nature of the Cu2+adsorption by crystalline and noncrystalline alumina, found that the adsorption mechanism involved immobilization of Cu2+by displacement of one or more HzO ligands or surface oxygen ions, forming at least one Cu-0-A1 bond.
* Corresponding author. t
Northern Arizona University.
* University of Colorado. University of Arizona.
0013-936X/93/0927-1045$04.00/0
0 1993 American Chemlcal Society
Dissolved organics in groundwater can significantly alter the metal-solid interaction by complexing the metal or by adsorbing onto the solid surface. Humic substances, if in sufficient concentration, can be important metalcomplexing agents, depending on the nature of the humic substances and the aqueous inorganic matrix. Factors that affect NOM complexation of metals are pH, ionic strength, NOM-source characteristics, and competing ligands. Humic substances may also significantly coat the solid mineral, controlling the electrochemistry of the solid phase. Davis (3)and Tipping (4)found that in surface waters virtually all suspended solid surfaces were coated by adsorbed organic matter. Groundwater NOM is typically distinguished from surface water NOM by a lower oxygen content, less color, and a lower hydrophobic/hydrophilic NOM ratio (5). Groundwater NOM is similar to surface water NOM in molecular weight, carboxyl content, and metal binding (5-7). Thurman suggests that the longer residence time of organic matter in groundwater results in the hydrophobic substances being either adsorbed to aquifer solids or biodegraded. Davis (3) showed that NOM is readily adsorbed by aluminum oxide in the pH range of natural waters and that adsorption occurs due to reaction between the weakly acidic functional groups of NOM with basic surface hydroxyls. Tipping and Cooke (8) and Davis (3) noted that adsorbed organic matter imparts a negative surface charge to metal oxides. Tipping and Cooke (8) suggest that the adsorbed humics cause the plane of electrokinetic shear to be moved some distance from the oxide surface, and the observed potential is due to humic functional groups not involved in the adsorption reaction. Stevenson (9) suggested mechanisms involved in humic substance adsorption: (1) physical adsorption, (2) electrostatic attraction, (3) hydrogen bonding, and (4)coordination complexes. When clays and humic substances are similarly charged, NOM adsorption may be facilitated by ‘bridging’ associated with polyvalent cations. Greenland (10)emphasizes the importance of Ca(II), Fe(III1, and Al(II1) in binding organic matter to clays. He found Ca2+-facilitated adsorption to be reversible,whereas Fe(II1) and Al(III), which can form strong complexes with organic matter, are not Environ. Scl. Technol., Vol. 27, No. 6, 1993
1045
readily desorbed. Innskeep and Baham (11)showed that the presence of Cu(I1) can increase the adsorption of dissolved humics to clays. Holm and Curtiss (7) showed that organically bound Cu(I1)was the predominant Cu(I1)speciesin groundwaters where the total Cu(1I) concentration was less than organic ligand concentration. Innskeep and Baham (11) found that at higher pH values (7.5-8.0) the complexation of Cu(I1) by dissolved humic substances can decrease the amount of Cu(I1) adsorbed to a clay surface. Allard et al. (12) noted that, when humics were present, americium adsorption on (r-Alz03decreased at high pH and increased at low pH. At high pH, the humic molecules complexed Am3+much more strongly than the negatively charged alumina surface sites. Davis and Leckie (13)examined the effect of adsorbed simple organic acids on Cu(I1) and Ag(1) uptake by aluminum oxide. They chose organic acids to vary in structure (aliphatic and aromatic) and number and distribution of coordinating functional groups. The organic adsorbate resulted in either a decrease, no affect, or an increase in Cu(I1) uptake. For example, picolinic acid (one COO- group) and 2,3-pyrazinedicarboxylicacid (two COO- groups) (2,3-PCDA) had similar adsorption isotherms, but picolinic acid decreased Cu(I1) uptake and 2,3-PDCA increased it. The extra carboxyl group not involved in adsorption was available for metal complexation. Elliot and Huang (14) investigated the adsorption of Cu(I1) on A1~03(g)in the presence of the chelating agents nitriloacetate (NTA), aspartate, and glycine and found that complexation can enhance total Cu(I1)uptake. They concluded that electrostatic interaction between the Cu complexes and the alumina surface is an important mechanism but noted that specific chemical interactions occur since adsorption of Cu(I1) was observed at the pH,,, of the A12O3(g). Micera et al. (15) studied the adsorption of Cu(1I) complexes of glutamic and aspartic acids onto aluminum hydroxide and found that the presence of adsorbed ligands increased the adsorption of metal at low pH while at high pH Cu(I1) adsorption was inhibited due to complexation by the excess ligand in solution. Aqueous-phase chemical reactions of interest in this study are shown below. The symbol L can refer to a single ligand or a binding group on a ligand. For simplicity, a divalent ligand is considered: aCu2++ bOH-- Cua(OH)b'2a-b' CcU2+
+ dL2--
CU,Ld(2C'2d)
(1)
(2)
Surface reactions of interest follow. The symbol >Xi refers to a surface site on the mineral phase:
+ + + + + + -
>XiOH + H+ >X,OH >X,OH
>&OH
>xiL-
>XiOH2+
>XiO-
Cu2+ L2-
cu2+
H+
(3)
(4)
>XiOCu+ H+
(5)
>XiL-
(6)
OH-
>X,LCU+
1046 Envlron. Scl. Technol., Vol. 27, No. 6, I993
(7)
Materials and Methods
Materials. All chemicals were obtained as analytical reagent grade and used without further purification. Nitrogen gas was purified prior to use by stripping through a 2 M NaOH solution to remove COZand rehydrated by bubbling through distilled deionized water. Solutions were prepared as needed and stored for no more than 2 weeks in the dark at room temperature. Column experiments were performed with silica beads (am-SiOz) and a-A1203. The silica beads, obtained from Potter's Industries, were in the size range of 425-600-rm diameter. The (r-A1203 was obtained from Union Carbide (product name Linde SF-6). The manufacturer's literature reported nominal particle diameters of 1 f 0.3 pm. The reported surface area is 6-8 m2/g. The minerals were cleaned by soaking in NaOH solutions overnight, followed by rinsing in distilled deionized water and soaking overnight in HN03 solutions, and by repeated rinising until the rinse solution pH was approximately 9.0. The surface charge and pH,,, of the a-Al203 particles were determined by measurement of electrophoretic mobility of the particles at various pHs, The particles were conditioned overnight in a suspension containing 25 mg/L a-Al2O3and 0.01 M NaN03. The effect of various buffers and buffer concentrations on the particle pH,,, were determined by preparing parallel suspensions in closed flasks with variable concentrations of HC03-, MES (4-morpholineethanesulfonic acid), and HEPES [N-(2hydroxyethyl)piperazine-N'-2-ethanesulfonic acid]. After being conditioned overnight, the suspensionswere adjusted from pH 3 to pH 10, allowing 15 min of conditioning at the different pHs prior to mobility measurements. The pHzpcsfor the particles with no buffer, HC03-, MES, and HEPES were determined to be 8.5, 7.2, 8.2, and 7.8, respectively. Chowdhury (16) also found that HC03decreased the pHzpcof aluminum hydroxide colloids. Five sources of natural organic matter were used in this study: Biscayne Aquifer NOM, Orange County groundwater NOM, Suwannee River fulvic acid, Suwannee River humic acid, and soil fulvic acid. The Orange County groundwater and Biscayne Aquifer samples were used as received after 0.45-pm filtration. Hardness for these two waters were 18.9 and 236 mg/L as CaC03. Orange County groundwater NOM was also used to produce an extracted humic substance fraction (XAD-8 isolate); the humic substance fractionation was done followingthe procedure described by Thurman and Malcolm (17). The Suwannee River fulvic and humic acids were obtained from the International Humic Substances Society (IHSS); the soil fulvic acid was obtained from Contech E.T.C. Limited (Ottawa). Natural groundwater samples were characterized by dissolved organic carbon (DOC) and UV absorbance at 254-nm wavelength at pH 7.0 (1-cm pathlength). Dry humic substance sourceswere also characterized by specific absorbance (i,e., DOC/UV abs) after dissolution in a 0.01 M NaN03 adjusted to pH 7.0. The humic fraction of the total NOM of the Biscayne Aquifer and Orange County groundwater samples was determined on the basis of a DOC mass balance after the humic substance extraction procedure mentioned above. The average molecular weights of the NOM and humic substances were determined by ultrafiltration in pressurized stirred cells, Samples were filtered in parallel
Table I. Characterization of NOM Sources source
av mol w t
Orange County groundwater NOM Orange County groundwater XAD-8 isolate Biscayne Aquifer NOM Contech soil fulvic acid Suwannee River fulvic acid Suwannee River humic acid
1900
carboxylic acidity (mg/g of C)
DOC (mg/L)
UV abs (crn-')
20.0 19.3 13.0 17.0 14.0 12.0
5.4 5.0 5.6 5.0 5.0 5.0
0.238 0.235 0.118 0.445 0.430 0.575
1790 4950 1910 4390
through membranes (Amicon YM and YC series) with nominal molecular weight cutoffs of 500,1000,5000,lO 000, and 30 000. Filtrates were collected in sequential 5-mL aliquots and analyzed for UV absorbance which were plotted against the respective cumulative filtrate volumes following Logan and Jiang's (18) development of membrane permeation coefficients. Permeate data were analyzed using experimentally determined permeation coefficients to correct for the rejection properties of the ultrafiltration membranes. Carboxylic acidity was determined for each humic source using potentiometric titration. Carboxylic acidity is defined as the milliequivalents of base to titrate the sample from pH 3 to pH 8. Titrations were controlled by an IBM XT-compatible computer connected to a Metrohm 665Dosimat automatic titrator. The pH was measured using an Orion combination glass electrode connected to a Fisher 950 Accumet pH meter. Prior to the titrations, samples were purged with N2 gas at pH 3.0 for 12 h to remove inorganic carbon. Column Experiments. The column apparatus consisted of two feed reservoirs, a high-pressure Eldex 1/8-in. diameter piston pump, a Spectrum glass column, an Eldex fraction collector, a conductivity meter, a Perkin-Elmer Model 200 UV-Vis spectrophotometer equipped with a Hellma flow-throughcell, and associated valves and tubing. The flow rate was maintained at 0.5 mL/min (Darcy velocity = 0.009 cm/s). The column material consisted of 100mg (f3 ?6) (u-Al203 with approximately 13.8g of silica. This resulted in a silica: a-Al203surface ratio of 1:lO (silica bead surface area calculated geometrically) and a porosity of 0.37-0.38. The column eluent was monitored continuously for UV absorbance or conductivity and recorded on a Perkin-Elmer Model 200 chart recorder. Samples were collected with the fraction collector for total Cu(1I) measurements at intervals of 5 or 10 min, depending on the status of the breakthrough. Breakthrough curves (BTCs) were constructed based on normalized effluent concentrations (CeffluenJCinput)and effluent flows (flux-averaged volume of solution pumped per column pore volume). One pore volume is defined by the time a conservative (nonadsorbing) tracer reaches C/C, = 0.5, where C is the observed concentration and C, is the influent concentration. Retardation factors were determined by
R = JTm(l - C/C,) d T where C is the column effluent concentration, C, is the column influent concentration, T is the number of pore volumes, and TMis the total number of pore volumes displaced when C = C,. R describes the area above the BTC up to C/C, = 1.0. Mass balances (mass recoveries) are calculated by comparing the total solute mass flowing into the column to the total solute mass eluted. The column experiments investigated source variation of NOM, NOM/Cu(II), and Cu(I1) at pHs of6.2 and 7.5.
humic content ( % of DOC) 80 50
Table 11. Integrated Retardation Factors and Mass Recoveries
column run silica, CUT mixed bed, CUT Orange County groundwater NOM Orange County groundwater XAD-8 isolate Biscayne Aquifer NOM soil fulvic acid Suwannee River fulvic acid Suwannee River humic acid Orange County NOM/ Cu(II),CuT Orange County NOM/Cu(II), UV Orange County XAD/ Cu(II),CuT Orange County XAD/Cu(II), UV Biscayne Aquifer/Cu(II), CUT Biscayne Aquifer/Cu(II), UV soil fulvic/Cu(II), CUT soil fulvic/Cu(II), UV Suwannee fulvic/Cu(II), CUT Suwannee fulvic/Cu(II), UV Suwannee humic/Cu(II), CUT Suwannee humic/Cu(II), UV mixed bed, CUT,pH 7.5 Biscayne Aquifer NOM, pH 7.5 Orange County NOM, pH 7.5 Orange County NOM/ Cu(II), CUT,pH 7.5 Biscayne Aquifer/ Cu(II), CUT,pH 7.5
mass recovery integrated (% of adsorbed retardation mass recovered during desorption) factor 2.4 4.9 5.4
100 78 95
2.3
45
1.9 5.5 3.0 5.9 4.4
88
47 36 35 100
9.8 12.2
100 100
6.8 2.8 2.2 8.4 8.5 7.2 7.3 9.5 9.0 120.4 3.9 7.8 14.0
40.7
'
45 90 54 100 37 100
59 100 35 62 70 86 39
Solution conditions were 0.01 M NaN03 and 1mM buffer (MES or HEPES). Prior to the start of column experiments, inorganic carbon was removed by degassing with N2 or He for 12 h. During the column runs, a positive helium atmosphere was maintained in thereservoirs. NOM breakthrough was monitored using a continuous-flow cell coupled with a UV detector (at 254 nm). Bulk Orange County groundwater NOM and Biscayne Aquifer NOM solutions were 5.4 and 5.6 mg/L DOC, respectively, while solutions of other NOM isolates were 5.0 mg/L DOC. Influent Cu(I1) concentrations were 0.05 mM in all runs involving Cu(I1). Analytical Methods. Total Cu(I1)measurements were made with a Perkin-Elmer Model 360 atomic absorption spectrophotometer equipped with an HGA-400 graphite furnace. Wall or platform atomization of the Cu(I1) in a graphite tube was used with a Cu(I1) specific hollow cathode lamp from Varna. Dissolved organic carbon was measured with either a Dohrmann DC-80 total organic carbon analyzer or a Shimadzu Model TOC-500 organic carbon analyzer. UV absorbance was measured using a Shimadzu UV-16OA recording spectrophotometer or a Environ. Sci. Technol., Vol. 27,
No. 6, 1993
1047
x Orange Counly Groundwater NOM 0
Biscayne Aquifer NOM
Pore Volumes
Figure 1. Breakthrough of Orange County groundwater NOM and Blscayne Aquifer NOM on mixed bed, pH 6.2. DOC = 5.4 and 5.6 mg/L, respectively.
Perkin-Elmer Model 200 UV-Vis spectrophotometer. pH measurements were made using a Fisher Accumet 950 meter equipped with an Orion combination glass pH electrode. Mobility measurements were performed on a Rank Brothers Mark I1 electrophoresis instrument equipped with a flat cell.
Pore Volumes
Flgure 2. Breakthrough of Contech soil fulvlc acid, Suwannee River fulvic acid, and Suwannee River humlc acid on mixed bed, pH 6.2. DOC = 5 mg/L, all sources. (a)
8
8 *: o Suwannee Humic Acid
I' x
0.5
Experimental Results
Suw Hum/Cu(Il), UV
Pore volumes
Pore Volumes (b)
Table I presents characterizations of the NOM sources used in this study. Continuous-flow column experiments were conducted to study the behavior of NOM and Cu(I1) in a simulated groundwater environment. All experiments were done using a glass column which was packed with silica and a-Al203. The flow rate was maintained at 0.5 mL/min (Darcy velocity = 0.009 cm/s) at ambient temperatures of 22 f 1OC, with pH kept at either 6.2 or 7.5. The following systems were investigated: (1)conservative tracer (0.01 M NaN03)
(2)Cu(I1) alone (3)NOM alone (4)Cu(I1) with NOM (preequilibrated) The breakthrough of NOM on a mixed bed at pH 6.2 was observed for Orange County groundwater NOM, Biscayne Aquifer NOM, Orange Country groundwater XAD-8 isolate, Suwannee River fulvic and humic acids, and soil fulvic acid. Table I1 presents the integrated retardation factors and mass balance (percent recovery of adsorbate on desorption limb) for the column runs (pH 6.2 unless otherwise specified). Figure 1 shows the breakthrough of Biscayne Aquifer NOM and Orange County groundwater NOM on a mixed bed. The Orange County groundwater NOM is more strongly adsorbed than the Biscayne Aquifer NOM. Figure 2 shows the breakthrough curves for the Contech soil fulvic acid and the Suwannee River fulvic and humic acids. The soil fulvic acid, Orange County groundwater NOM, and the Suwannee River humic acid show similar retardation factors (5.46, 5.38, and 5.88, respectively) while the Suwannee River fulvic acid and Orange County groundwater extracted humic material are less strongly adsorbed with retardation estimates of 2.97 and 2.29. The Orange County groundwater NOM and Biscayne Aquifer NOM are almost completely desorbed, with a mass recovery of 95 and 8896, respectively,while the other NOM sources' mass recoveries 1048 Environ. Sci. Technol., Vol. 27, No. 6, 1993
r
I
Pore Volumes
1
Pore volumes
Figure 3. Adsorption and desorption curves on mixed bed at pH 6.2 for (a) UV absorbance for Suwannee Rlver humic acid alone and Suwannee River humic acid/Cu( II), and (b) total Cu(II) for Cu(I I) alone and Suwannee River humlc acld/Cu(II) systems. DOC = 5 mg/L, Cu(I1) = 0.05 mM.
range from 35 to 47%. Triplicate runs on the Orange County groundwater NOM system showed an error in retardation factor values of k0.3 (f5.5%). The breakthrough of Cu(I1) on silica and silica/a-A1203 beds was investigated at pH 6.2 and on the silica/a-A1203 bed a t pH 7.5. Mass balances for the mixed bed and silica systems were 78 and loo%,respectively, indicating almost complete removal of Cu(I1) from the mineral surfaces. Integrated retardation factors for these systems are 4.93 and 2.35, respectively. Triplicate runs were performed on the mixed column at pH 6.2. The error in retardation factor values is f0.2 (f4%). The estimated retardation factor for the Cu(I1) breakthrough curve on the mixed bed at pH 7.5is estimated at 120. Cu(I1)was not detected until about 80 pore volumes at this pH. Complete breakthrough was not achieved in this column run. The breakthrough of preequilibrated NOM/Cu(II) was investigated for all of the NOM sources on mixed beds at pH 6.2 and for Orange County groundwater NOM and Biscayne Aquifer NOM on mixed beds at pH 7.5. Figures 3-5 show the adsorption and desorption curves for the Suwannee River humic acid, Orange County groundwater XAD-8 isolate, and soil fulvic acid systems, and the
(a) x Orange County NOM/Cu(II), CUT
o Biscayne NOM/Cu(ll),
1-
s
CUT
x OC XAD/Cu UV
05
i 50
OO
IO0
0
Pore Volumes
50 Pore volumes
100
(b)
x OC XAD/Cu(II). CUT
0.5
e
3
x OC XAD/Cu(ll), Cu
0.5
1
50 Pore Volumes
0
100
0 0
i
a m
,
50 Pore volumes
I
Pore Volumes
100
Flgure 4. Adsorption and desorption curves on mixed bed at pH 6.2 for (a) UV absorbance for Orange county XAD-8 isolate alone and Orange County XAD isolate/Cu(II) systems, and (b) total Cu(I1) for Cu(I1) alone and Orange County XAD lsolate/Cu(II) systems. DOC = 5 mg/L, CuQII) 0.05 mM.
Figure 6. Breakthrough of total Cu(I1) In the Orange County NOM/ Cu(II), Biscayne Aqulfer NOM/Cu(II), and Cu(I1) alone systems, pH 7.5. Cu(I1) = 0.05 mM, DOC = 5.4 and 5.6 mg/L, respectlvely. I
-
(a)
0 0
-
Pore Volumes
5
IO
15
20
5
IO
15
20
25 30 Pore Volumes
35
40
45
50
30
35
40
45
50
Pore volumes
(b)
3
05
I 0'
o Cu(I1) Alone, CUT
0
50
Pore Volumes
100
0
50 Pore volumes
100
Flgure 5. Adsorption and desorption curves on mlxed bed at pH 6.2 for (a) UV absorbance for Contech soil fulvlc acM alone and Contech fulvlc/Cu(II)systems,and(b)TotalCu(II)forCu(II)aioneandContech fulvlclCu(I1) systems. DOC = 5 mg/L, Cu(I1) = 0.05 mM.
25
Pore volumes
x Soil FA/Cu(lI). CUT
Figure 7. Adsorption and desorption limbs, Orange County NOM/ Cu(I1) system on mixed bed, pH 6.2. DOC = 5.4 mg/L, Cu(I1) = 0.05 mM.
/
0.51
0
Suwannee Humic Acid/Cu(II), CUT
x Suwannee Humic/Cu(lI), UV
91
corresponding NOM/Cu(II) systems on a mixed bed at pH 6.2. All three systems show greater retardation of UV absorbance and total Cu(I1) than in the NOM- and Cu(11)-only systems as did the Suwannee River fulvic acid/ Cu(I1) system. Orange County groundwater NOM and Biscayne Aquifer NOM both slightly facilitated Cu(I1) transport. Table I1 summarizes retardation factors and mass balances for the systems studied. Figure 6 shows the total Cu(I1) breakthrough curves for Orange County groundwater NOM/Cu(II) and Biscayne Aquifer NOM/ Cu(I1) systems on a mixed bed a t pH 7.5. Both NOM sources facilitate Cu(I1) transport at this pH. Discussion
The NOM breakthrough curves on the mixed bed at pH 6.2 show significant differences which are not explained by variations in molecular weight or carboxylic acidity. Source-related variations, including the stereochemistry of functional groups, may account for the different adsorption behaviors. The Orange County groundwater NOM and Biscayne Aquifer NOM sourcesreflect different
O O
