On Formation and Decomposition of Bentonite - Humic and Kaolinite Humic Aggregates in Diluted Suspensions Dragoljub D. Bilanovic1, Steven A. Spigarelli2, and Timothy J. Kroeger3 Center for Environmental, Earth and Space Studies, Bemidji State University, Sattgast Hall 107, Bemidji, MN 56601, U.S.A., Phone (218) 755 2801, Fax (218) 755 4107, 1
[email protected],
[email protected], 3
[email protected] ABSTRACT Clay minerals are of importance in soil/water chemistry but also in retention/ transport of chemicals through the environment. Aggregates containing clays and humic substances are, either directly or indirectly, associated with almost all biological, chemical and physical phenomena in waters and soils. Formation and decomposition of aggregates made of humic substances and either bentonite or kaolinite were studied in diluted mixtures by tracing distribution of aggregate sizes and their counts. These mixtures are unstable in time with respect to: i) number of aggregates found at a particular dilution, and ii) distribution of aggregate sizes at a given dilution. Successive sequences of decomposition/ aggregation and aggregation/ decomposition events were observed in this and previously reported studies (Bilanovic and Spigarelli 2002a and 2002b, Bilanovic and Spigarelli 2004). The sequences are speculated to be caused by: i) exposure of fresh clay surface upon decomposition of existing aggregates, ii) partitioning of adsorbed humic substances between the old and freshly exposed clay surfaces, iii) adsorption of dissolved humics on a fresh clay surface, and iv) decomposition of the large humic units into smaller ones, a process probably catalyzed by clays during which the ionic makeup of aggregates' hydration shells change. Qualitative and quantitative changes in the composition of aggregates may have profound effects on the availability, transport, and fate of chemicals in both aquatic and terrestrial systems. Experiments in progress are focused on adsorption/release of nutrients and contaminants on the aggregates prepared in diluted, fresh and aged mixtures. INTRODUCTION Clays are a common inorganic part in natural aggregates found in waters and soils. Catalytic and adsorption properties of clays make them important in soil/water-chemistry but also in retention/transport of organic and inorganic chemicals through the environment (Kersting et. al., 1999; Plaschke et. al., 2001; Schwartzen and Matijevic, 1974). A huge amount of
bentonite clay (B), primarily made of montmorillonite, is used each year in various industries (Lagaly and Ziesmer 2003). Montmorillonite, [(1/2Ca, Na)0.7 (Al, Mg, Fe)4 (Si, Al)8 O20 (OH)4 7nH2O), is found in both poorly drained soils and those of arid regions (Donahue, Miller and Shickluna, 1983). Montmorillonite has a substantial internal surface area, elevated ion-exchange capacity, and expands up to
ten times its dry volume upon contact with water by crystalline and osmotic swelling (Luckham and Rossi, 1999; Norrish, 1954). Montmorillonite is a three-layer clay in which an octahedral sheet is inserted between two tetrahedral sheets made of silicate (SiO4). Charge on sheets stems from: a) defects in crystal lattice, b) broken bonds at the corners and edges of the crystal, and c) isomorphous substitution of Si4+ with Al3+, Fe3+/Fe2+, or Mg2+ (Norrish, 1954). The charge is compensated by Na+, Ca+2, Mg2+ and other ions, which are sorbed with their hydration shell; water molecules also participate in charge neutralization. Large quantity of kaolinite clay (K) is consumed in paper, porcelain and other industries. Kaolinite, Al2Si2O5(OH)4, forms in warm humid climates, but also in cool temperate climates with sufficient rainfall (Sumner 2000). Kaolinite is a twolayer clay consisting of an octaedral and a tetrahedral sheet (Schulze 1989). Kaolinite is characterized by i) small surface charge, and ii) small surface area, as a consequence its cation exchange capacity is up to fifty times smaller than that of montmorillonite (Sondi and Pravdic 1998, Sumner 2000). Humic substances (HS) exist in dissolved and particulate forms in sediments, soils and waters. They are reported to: a) facilitate transport of contaminants to lower soil horizons and within aquifers, b) participate in removal and release of organic and inorganic compounds to/from bulk media, and c) influence availability of inorganic phosphate (Alvarez et. al., 2004; Fitch and Du, 1996; Tipping, 2002). HS are formed by decomposition of biological material but organics in secondary effluents are quite similar to HS in soil (Rebhun and Manka, 1971 ). HS contain alcohol, amino, carboxyl, and other functional groups (Stevenson, 1994;
Tipping, 2002). HS molecular weights are in the range of 600 to more than 200,000 (Stevenson, 1994; Tipping, 2002) with highest values, probably, a consequence of aggregation of smaller units into larger ones (Stevenson, 1994). HS are polyelectrolytes containing both hydrophilic and hydrophobic moieties (Schulten and Schnitzer, 1995; Schulten and Gleixner 1999) so they behave like surfactants (Rebhun, de Smedt, Rwetabula, 1996). Different research schools are currently arguing if HS are: 1) macromolecules randomly coiled in a solution, 2) associations of relatively small molecules held together by weak interactions, or 3) micellar or pseudomicellar structures (Clapp and Hayes, 1999). Knowledge of sizes and shapes of HS in a solution could improve understanding of their behavior in both soils and waters. Particles, made of clays and HS, are frequently carriers of atrazine (Lesan and Bhandari, 2000), mercury (Amirbehman et al. 2002; Bilanovic and Spigarelli 2002b), plutonium (Kersting et al 1999) and other pollutants (Rebhun et al, 1992). For example, the amount of mercury associated with the colloidal phase can be as high as 72% of total Hg (Babiarz et al 2001). Clays and humic substances are, either directly or indirectly, associated with almost all biological, chemical and physical phenomena in waters and soils. Quantitative and qualitative advances in understanding of mobilization and deposition of colloidal aggregates made of humic substances and clays are needed for better understanding of: a) soil erosion, b) transport and fate of pollutants in soils and waters, c) irrigation, b) in situ remediation and other processes (Kretzschmar et al, 1999); the same is true for micron-size aggregates made of humic substances and clays.
HS are generally considered to be "cementing" compounds in aggregates (Donahue, Miller and Schikluna, 1983) but within a certain concentration range, HS disperse the aggregates (Tarchitzky and Chen, 2002; Visser and Callier, 1998). Aggregate formation may lead to the removal of chemicals from a given environment while decomposition probably releases chemicals to the environment (Bilanovic and Spigarelli, 2002a). Processes of aggregate formation/ decomposition are triggered by a change of: i) pH, ii) ionic strength, iii) redox potential, iv) concentration and form of aggregate fractions, v) fractions chemistry and vi) other bio-geo-chemical factors. A change in concentration of clay(s) and/or HS can be: a) irregular, like during flooding, and b) regular, like seasonal change. Bentonite (B) and kaolinite (K) are of different: i) chemical, ii) structural and iii) physical properties implying that Humic Acid - Bentonite (HAB) and Humic Acid Kaolinite (HAK) suspensions should differ with respect to: i) number of aggregates, and ii) distribution of aggregate sizes. These differences may have profound effects on availability, transport and fate of chemicals in the environment. The study reported here is centered on formation and decomposition of micron-size aggregates in two model systems made of: a) Humic Acids (HA) and Bentonite (B), and b) Humic Acids (HA) and Kaolinite (K). Aggregation/ decomposition processes are followed by tracing distribution of aggregate sizes and their count: i) at different HA:B and HA:K ratios, and ii) different time intervals. MATERIALS AND METHODS Humic acid sodium salt was purchased from Aldrich Chem. Comp. and used
without further purification; double distilled water was used to prepare HA stock suspension ([HA]0 = 100.0 mg L-1); HCl and NaOH where used to adjust pH to 6.35 + 0.05. Bentonite (B) and Kaolinite (K) were purchased from Fisher Sci. Comp. USA. One gram of clay was added to 500 ml double distilled water and stirred on a magnetic stirrer for 24 hours; after centrifugation the supernatant was decanted. The stirring, centrifuging, decanting cycle was repeated three times. No attempt was made to eliminate particles > 2.0 µm. HCl and NaOH were used to adjust pH to 6.35 + 0.05. The concentration of the clays in their stock suspensions was adjusted to 100 mg B L-1 and 100 mg K L-1. Double distilled water was used to prepare the following control suspensions of: B5 = 5.0 mg B L-1, B0.05 = 0.05 mg B L-1, K5 = 5.0 mg K L-1, K0.05 = 0.05 mg K L-1, and HA5 = 5.0 mg HA L-1, HA0.05 = 0.05 mg HA L-1. Double distilled water was also used to make the following mixtures: HAB5-5 = 5.0 mg B L-1 + 5.0 mg HA L-1, HAB 0.055 = 5.0 mg B L-1 + 0.05 mg HA L-1, HAB 5-0.05 = 0.05 mg B L-1 + 5.0 mg HA L-1, HAK 5-5 = 5.0 mg K L-1 + 5.0 mg HA L1 , HAK 0.05-5 = 5.0 mg K L-1 + 0.05 mg HA L-1, and HAK 5-0.05 = 0.05 mg K L-1 + 5.0 mg HA L-1. To avoid microbial growth the suspensions and the mixtures were exposed to microwave radiation for 2 min d-1 for 3 consecutive days. A laser particle counter (Spectrex = PC 2000) and accompanying software (Spectrex -“Spercount”) were used to count the number of aggregates and determine the distribution of their sizes in each suspension and mixture. Readings were conducted in triplicate with the threshold set on ten and count time t = 10 seconds. Standards provided by the
manufacturer were used to calibrate the instrument. Throughout the work average count for standard-one was 475 + 46 particle mL-1 and for standard-two, it was 1085 + 80 particle mL-1; according to the manufacturer the standards are stable for > ten yr. Minimum detection limit of the counter used is 500nm; for this reason aggregates of diameter < 1.0 µm were reported in the 1.0 µm class throughout the work. Measurements were conducted at 23 + 0.5oC. Prior to reading, samples were shaken on a reciprocal shaker for 5 minutes at 250 strokes per minute then left to rest for an additional 5 minutes prior to determination of particle counts and sizes. The count and sizing were conducted at 0.16 (in further text referred to as “0”), 24, and 500 hours; in between control suspensions and mixtures were stored in a drawer. RESULTS AND DISCUSSION At zero time the mixture made of 5.0 mg B L-1 and 5.0 mg HA L-1 (HAB5-5) contained 1122 aggregates cm-3; aggregate diameter was in the range 1 to 8 µm, median diameter was 4 µm. In control suspensions (i.e. 5.0 mg B L-1 and 5.0 mg 610 HA L-1) a combined total of aggregates cm-3 existed. The average volume of aggregate in HAB5-5 mixture was 48.46 µm3 or 34.5 percent smaller than the average volume of aggregate in the control suspensions (HAB5-5 vs. Control at 0 h in Figure 1). After twenty-four hours there were 1090 aggregates cm-3 in HAB5-5 mixture with aggregate diameter in range 1 to 7 µm. The average volume of aggregate in HAB5-5 mixture was 36.24 µm3 or 7.2 percent larger than the average volume of aggregate in the control suspensions. (HAB5-5 vs. Control at 24 h in Figure 1)
After five hundred hours there where 1108 aggregates cm-3 in HAB5-5 mixture with aggregate diameter in range 2 to 8 µm. The average volume of aggregate in HAB 5-5 mixture was 88.62 µm3 or 80.5 percent larger than the average volume of aggregate in the control suspensions. (HAB5-5 vs. Control at 500h in Figure 1) After one thousand hours there where 1172 aggregates cm-3 in HAB5-5 mixture with aggregate diameter in the range 3 to 10 µm. The average volume of aggregate in HAB5-5 mixture was 52.45 µm3 or 22.9 percent smaller than the average volume of aggregate in the control suspensions. (HAB5-5 vs. Control at 1000h in Figure 1). At zero time the mixture made of 5.0 mg K L-1 and 5.0 mg HA L-1 (HAK5-5) showed a total count of 1116 aggregates cm-3; aggregate diameter was in the range 3 to 10 µm; median diameter was 6 µm. In control suspensions (i.e. 5.0 mg K L-1 and 5.0 mg HA L-1) there were 888 aggregates cm-3. The average volume of aggregate in HAK 5-5 mixture was 114.11 µm3 or 92.8 percent larger than the average volume of aggregate in the control suspensions (HAK5-5 vs. Control at 0 h in Figure 2). After twenty-four hour there where 858 aggregates cm-3 in HAK5-5 mixture with aggregate diameter in range 1 to 6 µm. The average volume of aggregate in HAK5-5 mixture was 7.75 µm3 or 75.9 percent smaller than the average volume of aggregate in control suspensions. (HAK5-5 vs. Control at 24 h in Figure 2). After five hundred hours there where 1152 aggregates cm-3 in HAK5-5 mixture with aggregate diameter in range 3 to 10 µm. The average volume of aggregate in HAB 5-5 mixture was 132.7 µm3 or 182.0 percent larger than the average volume of aggregate in control suspensions. (HAK5-5 vs. Control at 500 h in Figure 2)
After one thousand hours there where 1172 aggregates cm-3 in HAK5-5 mixture with aggregate diameter in range 3 to 10 µm. The average volume of aggregate in
HAK5-5 mixture was 52.45 or 22.9 percent smaller than the average volume of aggregate in control suspensions. (HAK5-5 and Control at 1000h in Figure 2).
Figure 1. Number of Aggregates and Distribution of Their Sizes in HAB 5-5 Mixture at 0, 24, 500 and 1000 hours; Control is the sum of counts in B5 and HA5 suspensions.
Figure 2. Number of Aggregates and Distribution of Their Sizes in HAK 5-5 Mixture at 0, 24, 500 and 1000 hours; Control is the sum of counts in K5 and HA5 suspensions. Other mixtures tested showed qualitatively similar responses on aging to those of HAB5-5 and HAK5-5 mixtures as illustrated in Table 1 for mixtures HAB5005 and HAK5-005.
TIME (h) 0 24 500 1000
HAB 5-005 Aggregates/cm3 AVA* (µm3) 1026 17.96 1006 10.39 1063 13.71 1088 14.37
1
Control Aggregates/cm3 AVA* (µm3) 456 8.70 60 1.42 105 0.63 94 0.79
2 HAK 5-005 Control Aggregates/cm3 AVA* (µm3) Aggregates/cm3 AVA* (µm3) 787 9.67 630 11.33 0 873 3.98 111 5.38 24 802 2.41 51 0.60 500 1045 6.84 71 2.43 1000 Table 1. Aggregate Count and the Average Volume of Aggregate in HAB 5-005 and HAK 5-0.05 Mixture at 0, 24, 500 and 1000 hours. 1Control is the sum of counts in B005 and HA5 suspensions. 2Control is the sum of counts in K005 and HA5 suspensions. * AAV = The Average Volume of Aggregate
TIME (h)
Humic substances are heterogeneous, relatively large, polyelectrolytes characterized by numerous and different functional groups (Stevenson, 1994; Tipping, 2002). In freshly prepared HAB and HAK suspensions HA covers a portion of clay (C) surfaces (Figure 3 a & b). The fraction of the surface covered changes with change in [HA]:[C] ratio and may lead to decomposition of HAC aggregates. Decomposition of aggregates also exposes fresh, HA unoccupied, clay surface (Figure 3c). This triggers redistribution/ readsorption of previously sorbed HA between old and freshly available C surface (Figure 3c and 3d). Likewise dissolved and previously un-adsorbed HA will adsorb on a fresh C surface (Figure 3c and 3d). Clays are known for their catalytic properties; large HA units may be decomposed into smaller ones upon close contact of C and HA. Changes in ionic composition of hydration shells of aggregates are also probable. The consequence will be sequences of aggregations and decompositions accompanied by fluctuations in qualitative
and quantitative composition of HAC aggregates as observed in aged HAB and HAK mixtures (Figures 1 and 2) and reported previously (Bilanovic and Spigarelli 2004, 2002a, 2002b); earlier Narkis and Rebhun (1970) reported that humates affect association-dissociation equilibria of montmorilonite. Fluctuations in qualitative and quantitative composition of micron size HAC aggregates should affect adsorption/desorption of various chemicals present in both aquatic and terrestrial systems. Fluctuations in qualitative and quantitative composition of micron size HAC aggregates indicate that both qualitative and quantitative composition of colloidal HAC aggregates fluctuate in response to aging and/or environmental changes. Such fluctuations could profoundly affect availability, transport, and fate of chemicals in both aquatic and terrestrial systems. Experiments in progress are focused on adsorption/ desorption of nutrients and other chemicals in diluted HAB and HAK suspensions of different HA:C ratios and different ages.
a) Addition of HA to C at t = 0
+ Dissolved HA Clay (C)
b) Dissolved HA and HAC Aggregate at t = 0
+
+ ∆t
c) Decomposition of HAC Aggregate; Fresh Surface Available for Adsorption / Readsorption
+
+ ∆t
+
d) Formation of New Aggregates and Adsorption of Dissolved HA
Figure 3. Aggregate Formation and Decomposition in Diluted Humic Acid (HA) Clay (C) System.
ACKNOWLEDGEMENT This research was supported in part through grants by Minnesota Space Grant Consortium and Bemidji State University Professional Improvement Grant.
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