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DIVISION S-2-SOIL CHEMISTRY

Subsurface Migration of Copper and Zinc Mediated by Soil Colloids

^a Contribution from the Dep. of Agronomy, Univ. of Kentucky Agr. Exp. Station, Lexington, KY 40546 (Journal Article no. 97-06-170) USA

akaratha{at}ca.uky.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Colloid migration in subsurface environments has attracted special attention lately because of its suspected role in facilitating transport of contaminants to groundwater. This study investigated the potential role of water-dispersible soil colloids (WDCs) with variable composition in transporting Cu and Zn through undisturbed soil columns. Copper or Zn solutions without colloids (controls) and combined with suspensions of montmorillonitic, mixed, illitic, and kaolinitic colloids with a range of surface properties were applied at a constant flux into undisturbed soil columns. The soil columns represented upper solum horizons of Maury (fine, mixed, mesic Typic Paleudalf) and Loradale (fine-silty, mixed, mesic Typic Argiudoll) soils with contrasting porosities and organic C (OC) contents. Colloid and metal recoveries in the eluent varied with metal, colloid, and soil properties. The presence of colloids typically enhanced metal transport by 5- to 50-fold over that of the control treatments, with Zn being consistently more mobile than Cu. The greatest metal transport potential was shown by colloids with high negative surface charge and OC content and the lowest by colloids with large particle size, low negative surface charge, and high Fe- and Al-hydroxyoxide contents. Although the dominant transport mechanism was metal sorption by colloids and cotransport, the additional soluble Cu and Zn transported in the presence of colloids suggests involvement of physical exclusion, competitive sorption, or increased metal solubilization processes. Increased amounts of OC content in the soil column appeared to overshadow the effects of macroporosity on the transport of both metals, especially Cu. These findings have important ramifications on the use of contaminant transport prediction models and the application of efficient remediation technologies.

Abbreviations: BTCs, breakthrough curves • CEC, cation-exchange capacity • EM, electrophoretic mobility • OC, organic C • PVC, polyvinyl chloride • WDC, water-dispersible colloids

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
IN RECENT YEARS, increased anthropogenic inputs of heavy metals in terrestrial environments have caused considerable concern relative to their impact on groundwater contamination (Alloway, 1995). Although most heavy metals are generally considered to be relatively immobile in most soils for short periods, their mobility under certain solution- and soil chemical conditions may exceed ordinary rates and pose a significant threat to groundwater quality (Scokart et al., 1983; Jorgensen, 1991; Maskall et al., 1995). This threat has been substantiated by recent research evidence showing that water-dispersed colloidal particles migrating though soil macropores and fractures can significantly enhance metal mobility, causing dramatic increases in transported metal load and migration distances (Mills et al., 1991; Puls and Powell, 1992; Liang and McCarthy, 1995; Ouyang et al., 1996; Ryan and Elimelech, 1996).

Buddemeier and Hunt (1988) found that virtually all the activity of Mn, Co, Sb, Cs, Ce, and Cu in groundwater samples of underground nuclear test cavities at the Nevada site was associated with colloidal particles. Laboratory column leaching studies of Sandhu and Mills (1987) showed that 90% of the eluted Cr and As was associated with colloidal Fe- and Mn-oxides. Nelson et al. (1985) determined that colloidal OC was a major factor controlling the distribution of plutonium between solid and dissolved phases. Laboratory tests of Eichholz et al. (1982) also showed that cationic nuclides were competitively adsorbed on suspended clay particles capable of traveling at bulk water flow velocity in porous mineral columns. Other studies have shown as much as 0.2 mg L^-1 of Cu, Pb, and Cd to be associated with colloidal material in freshwater samples (Tillekeratne et al., 1986). Kaplan et al. (1995) also reported a significant association of Cr, Ni, Cu, Cd, Pb, and U with groundwater colloids recovered from an acidified sandy aquifer, with the metal–colloid association increasing with pH. However, because of low colloid concentrations in the aquifer, their role in the colloid-facilitated metal transport process was assumed to be minor. In transport experiments with sand-packed laboratory columns, Roy and Dzombak (1997) observed colloid-enhanced mobilization of Ni, which was controlled by pH and influent metal concentrations.

Copper and Zn are two metals that are consistently added to soils in increasing quantities in the form of fertilizers, pesticides, livestock manures, sewage sludges, and industrial emissions (Adriano, 1989). Both of these metals show moderate mobility under relatively acid soil conditions (pH 5–7) because of increased solubility and formation of soluble complexes with organic ligands (Elliott et al., 1986; Stevenson and Fitch, 1986; Klamberg et al., 1989). For a humic sandy podzolic soil with pH 5, Jorgensen (1991) reported a greater mobility for Zn than for Cu because of stronger binding of Cu by humus. However, similar mobilities were observed in a loamy soil with low OC or when the metals were leached as soluble EDTA chelates. Buchter et al. (1989) found that distribution coefficients (K_d) for Cu retention by 11 soils from 10 soil orders increased with cation-exchange capacity (CEC) and pH and were consistently higher for Cu than for Zn. Greater sorption affinity for Cu than for Zn was also observed by Elliott et al. (1986) and Narwal and Singh (1995) on different types of mineral soils with varying amounts of OC content under acidic conditions. Increased levels of OC limited the mobility of both metals, especially that of Cu.

However, the above metal retention and mobility patterns have been established considering only soluble species. Because of their small size, large surface area, and charge properties, mobile soil colloids (mineral and organic) can potentially have greater metal sorption affinity per unit mass than the soil matrix. Therefore, some fraction of the soluble Cu and Zn species migrating through soil pores could be preferentially bound to mobile colloid surfaces rather than soil pore walls and enhance the mobility and transport of Cu and Zn to greater soil depths. The objective of this study was to assess the magnitude and mechanisms of colloid-mediated transport of Cu and Zn by soil colloids with diverse physicochemical and mineralogical composition under laboratory intact soil column leaching conditions.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Metal Solutions
Aqueous solutions (10 mg L^-1) of Cu and Zn were prepared from CuCl₂ and ZnCl₂ reagents (>99% purity, Aldrich Chemicals, Milwaukee, WI). These solutions were used as controls and in mixtures with 300 mg L^-1 colloid suspensions in the leaching experiments. The same CuCl₂ and ZnCl₂ reagents were used to prepare the equilibrium solutions for the adsorption isotherm experiments.

Colloid Fractions
Water-dispersible colloids were fractionated from upper Bt horizons of six soils representing the series: Beasley silt loam (fine, montmorillonitic, mesic Typic Hapludalfs), Loradale silt loam (fine-silty, mixed, mesic Typic Argiudolls), Maury silt loam (fine, mixed, mesic, Typic Paleudalfs), Shrouts silty clay loam (fine, mixed, mesic Typic Hapludalfs), Smithdale loam (fine-loamy, siliceous, thermic Typic Hapludults), and Waynesboro silt loam (clayey, kaolinitic, thermic Typic Paleudults). The extraction of the WDC fractions (2 µm) was accomplished by mixing 10 g of soil with 200 mL of deionized H₂O (without addition of dispersing agent) in plastic bottles, shaking overnight, centrifuging at x 130 g (750 rpm) for 3.5 min, and decanting. The concentration of the colloid fraction was determined gravimetrically, and before it was stored as a stock suspension, 0.002% (by weight) of NaN₃ was added to suppress microbial activity. Subsamples of stock colloid suspensions were air-dried, gently crushed, and passed through a 0.23-mm opening diameter sieve for characterization. Physicochemical and mineralogical properties of the colloid fractions are shown in Table 1 (Seta and Karathanasis, 1997a).

Leaching Experiments
Prior to setting up the leaching experiment, the intact soil columns were trimmed flat from both ends and saturated from the bottom upward with deionized H₂O (immersed by about 9/10 in open buckets) to remove air pockets. Then the columns were set up in stands and about five pore-volumes of deionized H₂O containing 0.002% (w/w) of NaN₃ were introduced into the top of each column (downward vertical unsaturated flow) with a peristaltic pump at a constant flux (2.21 cm h^-1) to remove loose material from the pores of the soil columns. No additional colloid generation from the soil matrix was detected in the eluent after five pore volumes, suggesting attainment of a steady-state colloid elution concentration of near zero prior to the initiation of the colloid tracer experiments. At that point, a set of duplicate columns was designated for each of the following solution and/or suspension leaching treatments:

1. 10 mg L^-1 of Cu or Zn (as CuCl₂ or ZnCl₂) and 300 mg L^-1 of Beasley colloid
   
2. 10 mg L^-1 of Cu or Zn and 300 mg L^-1 of Shrouts colloid
   
3. 10 mg L^-1 of Cu or Zn and 300 mg L^-1 of Loradale colloid
   
4. 10 mg L^-1 of Cu or Zn and 300 mg L^-1 of Maury colloid
   
5. 10 mg L^-1 of Cu or Zn and 300 mg L^-1 of Smithdale colloid
   
6. 10 mg L^-1 of Cu or Zn and 300 mg L^-1 of Waynesboro colloid
   

Therefore, a total of 28 soil columns for each soil were used in the experiment [2 metals x (6 colloids + 1 control) x 2 replicates].

The leaching solutions/suspensions were applied to the top of each column through a continuous step input of 2.21 cm h^-1 controlled with a peristaltic pump. This rate was tested in earlier experiments and found to provide consistent free flow conditions without ponding on the top of the columns. All input mixtures were allowed to equilibrate for 24 h before application. For 10 d, eluents were monitored periodically with respect to volume, colloid, and metal concentration. Breakthrough curves (BTCs) were constructed based on reduced metal and colloid concentrations (ratio of effluent concentration to influent concentration = C/C_o) and pore-volume (flux averaged volume of solution pumped per column pore volume). An additional BTC was constructed from reduced Cl^- concentrations of the eluted control solutions to compare elution patterns of the conservative Cl^- vs. the interactive Cu and Zn solutes.

Colloid concentrations in the eluent were determined with a Bio-Tek multichannel (optical densitometer with fiber-optic technology; Bio-Tek Instruments, Winooski, VT) microplate reader precalibrated with known concentrations of each colloid at 540 nm. Total metal concentration in the eluents was allocated to solution phase and colloidal phase (colloid-bound contaminant). The eluent samples were centrifuged for 30 min at x 2750 g (3500 rpm) to separate the soluble contaminant fraction from the colloid-bound contaminant fraction. The absence of colloidal material in the supernatant solution was verified by filtration through a 0.2-µm membrane filter. The soluble metal (Cu, Zn) fractions were analyzed by atomic absorption- (concentrations >1 mg L^-1) or inductively coupled plasma (ICP) spectrometry (concentrations <1 mg L^-1). The reproducibility between replicate columns was within ±10%.

Metal Adsorption Isotherms
For eluted samples with moderate to high colloid concentration (>50 mg L^-1), the eluted colloid-bound Cu or Zn was extracted with 0.5 M MgCl₂ and 1 M (HNO₃ + HCl) solutions and analyzed by atomic absorption spectroscopy. However, for the eluted samples with colloid concentration <50 mg L^-1, the experimental and analytical uncertainty was too high to rely on direct extraction determinations. Therefore, for these samples the colloid-bound Cu or Zn fraction was calculated from adsorption isotherms generated from batch experiments. The amount of the colloid-bound metal eluted in the column leaching experiments was calculated by extrapolating the metal equilibrium concentration in the eluent to the adsorption capacity of the colloid and multiplying by the colloid concentration in the eluent. Agreement between extracted and isotherm-estimated Cu and Zn on low colloid–concentration eluted samples was verified with extractions of selected samples following a concentration pretreatment (Fig. 1) .

View larger version (22K): [in this window] [in a new window]   Fig. 1 Relationship between colloid-bound metals extracted by 0.5 M MgCl2 or 1 M (HNO3 + HCl) and estimated from adsorption isotherms

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Adsorption Isotherms
Isotherms of Cu and Zn sorption by soils and colloid fractions were prepared by plotting equilibrium solution concentrations of the metals against the calculated amount adsorbed to the solid phase. The data comformed well to the Freundlich equation (Table 2) . The Loradale soil with mixed mineralogy but four times higher OC content than the Maury soil showed higher sorption affinity for both Cu and Zn. The sorption affinity of the colloids followed the sequence: Loradale > Beasley > Maury > Smithdale > Shrouts > Waynesboro for Cu, and Loradale > Beasley > Maury > Shrouts > Waynesboro > Smithdale for Zn.

Colloid Transport
Most colloids showed gradual but asymmetric BTCs, which are typical of extensive solute dispersion, nonlinear sorption, or time-dependent processes. However, considering the highly reactive nature of the metals involved in the study, extensive ion exchange interaction with increasing saturation of the soil matrix should contribute greatly to the asymmetry of the BTCs. This contrasts the rapid breakthrough of the conservative Cl^- tracer from the control CuCl₂ and ZnCl₂ solutions (Fig. 2) , and that of herbicide–colloid suspensions observed in previous work (Seta and Karathanasis, 1997b). Copper-saturated colloid recoveries after 8 to 10 pore volumes of leaching ranged from 15 to 70% through Loradale columns (Fig. 2a) to 2 to 78% through Maury columns (Fig. 2b), following the sequence: Loradale > Beasley > Maury > Shrouts > Smithdale > Waynesboro. The respective recoveries for Zn-saturated colloids ranged from 25 to 99% through Loradale columns (Fig. 2c) to 3 to 98% through Maury columns (Fig. 2d).

View larger version (32K): [in this window] [in a new window]   Fig. 2 Breakthrough curves for Cu and Zn colloids eluted from Loradale and Maury soil columns

Colloid-Mediated Transport of Heavy Metals
Figures 3 and 4 show selected BTCs for Cu and Zn eluted in the absence (control) and presence of colloids. Metal elution in the presence of colloids is plotted separately for the soluble fraction and as the sum of the soluble and the colloid-bound fraction. Most BTCs showed considerable asymmetry attributed not only to preferential flow but to extensive chemical interaction with the soil matrix. This interaction is anticipated considering the variable affinities of the metals for the soil matrices and the different colloids (Table 2).

View larger version (31K): [in this window] [in a new window]   Fig. 3 Breakthrough curves for Cu and Zn eluted in the presence or absence of colloids from Loradale soil columns

View larger version (29K): [in this window] [in a new window]   Fig. 4 Breakthrough curves for Cu and Zn eluted in the presence or absence of colloids from Maury soil columns

The range of metal transport increase in the presence of WDCs was 5- to 50-fold more than that of the control treatment (absence of colloids), but it was colloid-type, soil, and metal specific. Metal cotransport in the presence of colloids was highest in the Loradale columns, where the higher OC content of the soil matrix apparently increased colloid mobility by forming soluble metal–organic complexes. This was more evident with the kaolinitic (Waynesboro) colloid, which showed a 3- to 10-fold increase in metal transport compared with that of the Maury column. Evidence of increased metal solubility and mobility through organic complex formation (Stevenson and Fitch, 1986; Klamberg et al., 1989) is also shown in some Cu-control BTCs (Beasley, Shrouts, Waynesboro), where increasing amounts of Cu are recovered in latter stages of elution.

The amount of extra metals transported in the presence of colloids varied with colloid type and transportability, but it was highest with the Beasley and Loradale colloids and lowest with the Waynesboro colloid. The increased cotransportability of the Beasley and Loradale colloids is associated with either high smectite or high OC content, respectively, which vest larger surface area, smaller particle size, and increased mobility to the colloids, resulting in reduced residence time within the columns. In contrast, the large kaolinite particles and low surface charge of the Waynesboro colloid may increase straining and deposition within the column, thus limiting pore conductivity, colloid mobility, and metal transport.

Unlike the herbicide migration in the presence of colloids (Seta and Karathanasis, 1997b), where the dominant transport mechanism was exclusion of soluble species from matrix surface sites blocked by colloids, enhanced metal transport was mainly due to colloid–metal binding and cotransporting mechanisms. In most cases, between 50 and 90% of the increase in metal transport in the presence of colloids was due to colloid-bound metal species (Fig. 3 and 4). Exceptions were the cotransport of Cu in the presence of Loradale and Shrouts colloids, where a major portion of the increased transport was due to increases in the soluble metal fraction apparently from organic complex formation. Eluted Zn in the presence of colloids was consistently higher than Cu, following the trends established by metal–soil affinity (Table 2) and colloid mobility and recovery sequences (Fig. 2). The amount of Zn eluted in the presence of colloids was 1.2 to 25 times higher than that of Cu, with the greatest differences observed with the Loradale and Shrouts colloids eluted through Maury columns. Respective differences in the Loradale columns were apparently moderated by the effect of metal–organic complexes. This is a consequence of the higher affinity of the metals for the colloids than for the soil matrix (Table 2).

The remaining increase of metal transport in the presence of colloids can be explained by at least three mechanisms: (i) physical exclusion of soluble metal species from pore-wall attachment resulting from temporary blockage by mobile colloid particles or permanent blockage of micropore entries by already attached colloid particles that deny accessibility to active micropore wall sites, (ii) competitive sorption of colloid-bound charged metals that may reduce the available surface charge of the soil matrix, and (iii) increased metal solubility induced by the presence of colloids—especially those organically enriched or with organic coatings. The above mechanisms, although secondary, may contribute in various extents to the overall metal transport enhancement. Therefore, the findings of this study suggest that colloids act in the transport process mainly as metal carriers and secondarily as metal transport facilitators.

Effect of Colloid, Soil, and Metal Concentration on Transport
The extent of colloid-induced metal transport was dependent on colloid type, metal type and concentration, and soil column properties. Generally, increased colloid surface area and charge, pH, OC, and EM tended to facilitate transport, while large colloid size, Fe- and Al-oxyhydroxides, and kaolinite appeared to be inhibiting. However, the quantitative statistical correlations between these parameters and metal–colloid cotransportability, expressed as the average C/C_o during the last two pore volumes of elution, were not always consistent (Table 3) . The inhibiting effects of colloid size were especially evident in the transport of Cu through Loradale columns and Zn through Maury columns. The negative correlations of metal transport with quartz are probably accessory to the large particle size of this mineral, while those of extractable Fe and Al are associated with their strong flocculating capacity (Goldberg et al., 1990). The positive effect of pH is associated with greater colloid stability and increased metal sorption capacity due to increased pH-dependent charges (Harter, 1983). Similar effects by OC, CEC, surface area, extractable bases, and expandable 2:1 minerals are associated with an increase in negative charges on colloid surfaces, and more repulsive forces driving the particles apart, thus inducing greater colloid stability and transportability (Goldberg et al., 1990). Somewhat surprising was the negative correlation of mica for metal transport through Maury columns, but the positive relationship with the Loradale columns. The latter was apparently induced by colloid–organic interactions. At first glance, the negative correlations of EM with metal transport are misleading because the more negative the EM value, the higher the mobility of the colloid. Therefore, high colloid EM values enhanced colloid-facilitated metal transport.

In order to isolate covariant effects of soil colloid properties on colloid-mediated transport, a multiple regression analysis with step-wise elimination of parameters not significant at the level was performed. The regression models (Tables 4 and 5) confirmed the effects of the properties signified by the single correlation analysis. They also corrobated the synergistic behavior of certain colloid properties on metal transport by appearing in different variable combinations with positive or negative effects. Generally, the predictions were better (higher R²) for colloid-bound than for total metal transport, except for Zn. Among the independent variables with the greater influence on the best predictive models were: surface area, OC, pH, EM, CEC, and kaolinite for Zn; and surface area, EM, kaolinite, and quartz for Cu.

Finally, increasing metal concentrations in influent colloid suspensions beyond 10 mg L^-1 drastically inhibited colloid and metal transport in the presence of colloids due to coagulation, flocculation, flow retardation, and pore clogging (Fig. 5) . This suggests that metal-mediated transport by soil colloids may be more critical in low ionic strength subsurface environments, typical in subtropical humid regions of the USA. Under these conditions, it is highly unlikely that Cu or Zn equilibrium solution concentrations higher than 10 mg L^-1 will be supported by any stable mineral controlling their solubility in contaminated soils or sediments.

View larger version (24K): [in this window] [in a new window]   Fig. 5 The effect of Cu concentration in the influent colloid suspension on the elution of (a) Shrouts colloids and (b) Cu in the presence of Shrouts colloids from Loradale soil columns

	Conclusions

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

Received for publication November 14, 1997.

	REFERENCES

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 

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