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Effect of microbial and othernaturally occurring polymers onmineral dissolutionSusan A. Welch a & Philippe Vandevivere ba College of Marine Studies, University of Delaware, Lewes,DE, 19958, USAb College of Marine Studies, University of Delaware, Lewes,Delaware, USAVersion of record first published: 28 Jan 2009.

To cite this article: Susan A. Welch & Philippe Vandevivere (1994): Effect of microbial andother naturally occurring polymers on mineral dissolution, Geomicrobiology Journal, 12:4,227-238

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Ceomicrobhlogy Journal. Volume 12, pp. 227-238 0149-0451 /94 J 10.00 + .00Printed in the UK. All rights reserved. Copyright © 1994 Taylor & Francis

Effect of Microbial and Other Naturally OccurringPolymers on Mineral Dissolution

SUSAN A. WELCHPHILIPPE VANDEVIVERE

College of Marine StudiesUniversity of DelawareLewes, Delaware, USA

Several naturally occurring polymers were tested for their effect on mineral dissolu-tion. Polymers composed primarily of neutral sugars had no effect on dissolution,even at concentrations 1000 times greater than average dissolved organic carbonconcentration in groundwater. In contrast, alginate, a polysaccharide composed oftwo uronic acids, inhibited dissolution by 80% at the highest concentration. A high-molecular-weight (26 kD) polyaspartate also inhibited dissolution, though a lowermolecular weight (6 kD) polyaspartate had no effect. Solutions of fresh microbial ex-tracellular polysaccharides (EPS) extracted from subsurface microbes increased thedissolution rate of feldspars, probably by forming complexes with framework ions insolution. However, EPS inhibited dissolution in experiments with both high- and low-molecular-weight microbial metabolites by irreversibly binding to mineral surfaces.

Keywords alginate, extracellular polysaccharides, feldspar dissolution, polymers,polyaspartate

Water-rock interactions are important for controlling the mineral and solution composi-tion of natural waters, soils, and aquifers. All of these environments contain bacteria withconcentrations ranging from 103 to 1010 cells/cm3 (Kampfer et al., 1991; Hicks & Freder-ickson, 1989; Webley et al., 1963; Albrechtsen & Winding, 1992). Recent studies on mi-crobial abundance in deep subsurface environments show that cell numbers are high, 105

to 108 cells/cm3, and that microbial abundance and diversity does not decrease with depth(Balkwill, 1989). Bacteria in the subsurface are viable, metabolically active, and able togrow on a large number of carbon substrates (Balkwill, 1989; Hazen et al., 1991; Freder-ickson et al., 1989; Madsen & Bollag, 1989; Sinclair & Ghiorse, 1989; Phelps et al.,1989). Some of the bacteria are free living, but most are attached to mineral surfaces andare therefore able to have a direct impact on water-rock interactions (Hazen et al., 1991).

Bacteria may affect mineral dissolution by producing inorganic or low-molecular-weight organic acids that interact with mineral surfaces. Abiotic studies of mineral dissolu-tion show that dissolution rates increase as pH decreases from neutral to acidic (Chou &Wollast, 1985; Brady & Walther, 1989; Welch & Ullman, 1993) and that rates are enhanced

Received 19 September 1994; accepted 21 November 1994.This work is one component of ongoing studies concerning the role of bacteria on mineral dis-

solution in subsurface environments and was performed under the supervision of Drs. William Ull-man and David Kirchman with funding provided by the U.S. Department of Energy SubsurfaceScience Program (grant DE-FG02-91ER61184).

Address correspondence to Susan A. Welch, College of Marine Studies, University ofDelaware, Lewes, DE 19958, USA.

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228 S. A. Welch and P. Vandevivere

by organic acids, particularly near neutral pH where the effect of proton-promoted dissolu-tion is small (Welch & Ullman, 1993; Wogelius & Walther, 1991). Mineral dissolution ex-periments with bacteria in a complex medium show that mineral dissolution rates increasecompared to abiotic controls when bacteria produce organic acids and significantly decreasesolution pH (Berthelin, 1971; Berthelin & Dommergues, 1972). Bacteria also enhanced min-eral dissolution rates in batch experiments by producing chelating organic ligands without asignificant change in the pH of the bulk solution (Vandevivere et al., 1994). Field evidencealso indicates that bacteria are important for increasing mineral dissolution: Recent studies ofmineral and glass surfaces show bacteria associated with etch features, sites of preferentialdissolution (Hiebert & Bennett, 1992; Bennett & Hiebert, 1992; Thorseth et al., 1992).

Enhancement of dissolution may not be the only possible effect of bacteria at the min-eral-water interface. Microbes produce extracellular polysaccharides (EPS), which help themattach to mineral surfaces. Production of EPS can be enhanced by attachment (Vandevivere &Kirchman, 1993). These compounds are composed primarily of sugars such as glucose, galac-tose, mannose, rhamnose, fucose, or Af-acetylglucosamine, and sugar acids such as glucuronicacid and galacturonic acid (Sutherland, 1977; Christensen & Characklis, 1990). This microbialslime, or glycocalyx, covers a much larger surface area than the bacteria alone and may inhibitwater-rock interaction by forming a diffusion inhibiting layer or by irreversibly binding to re-active sites on the mineral surface. Mineral dissolution may alternatively be enhanced by thesecompounds where they form soluble complexes with mineral framework elements such as Al,Fe, and Si, and other metal ions as well (Mera & Beveridge, 1993; Geesey & Jang, 1989).

The purpose of this study is to demonstrate that microbial EPS and other naturally oc-curring polymers have an effect on mineral dissolution. This hypothesis was first tested withsimple analogs of the more complex polymers, primarily polysaccharides but also somepoly-amino acids, to see if certain structures or functional groups affect reactions at the min-eral-water interface. Further experiments were performed with uncharacterized extracellularpolysaccharides that were extracted from cultures of three subsurface microbial strains.

Materials and Methods

Minerals

Labradorite and bytownite bulk rock samples were purchased from Ward's Natural Sci-ence Establishment. Bulk rock samples were crushed, ground, and sieved to obtain the125-250 nm size fraction. Mineral grains were then washed and sonicated repeatedly (—20times) until the supernatant was clear. Samples were then further purified with a Frantzisodynamic magnetic separator. Previous dissolution experiments with minerals treatedthis way showed rapid and irreproducible initial dissolution rates, presumably due to thedissolution of fine particles or very reactive sites produced during the crushing procedure(Welch, 1991; Welch & Ullman, 1993). Therefore, mineral samples were washed in 0.1mM HC1 (pH 4) for several hours to remove very reactive material prior to the experimen-tal manipulations. This treatment is not expected to significantly alter mineral surfacechemistry, since the release of framework elements (Al, Si) is approximately stoichiomet-ric at this pH (Welch, 1991; Welch & Ullman, 1993).

Polymers

Polymers used in experiments listed in Table 1 were purchased from either Fisher Scien-tific or Sigma Chemical. Polymers were dissolved in weakly buffered 0.5 mM NaHCO3

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Polymers and Mineral Dissolution 229

Table 1Impact of polymers on the dissolution rate of plagioclase feldspar compared to

control at near neutral pH

Polymer

CelluloseStarchPolysucroseGum xanthan

Commercial bacterialpolysaccharide

Polyaspartate (6 kD)Polyaspartate (26 kD)Alginate

Monomer

GlucoseGlucoseSucroseGlucose, mannose,

glucuronic acid

AspartateAspartateMannuronic acid,

glucuronic acid

Concentration(g/L)

0.10.001-10.001-10.001-1

0.001-1

0.50.5

S0.010.1-1

Impact ondissolution"

0000

0

0-0—

Note. Most of the polymers sampled have a negligible impact on feldspar dissolution even atfairly high concentrations (1 g/L).

"0, No net impact; -, dissolution inhibition.

solutions with polymer concentration ranging from 0.001 to 1 g/L. Initial pH was ad-justed with HC1 between 6.5 and 7, where feldspar dissolution rates should be lowest andindependent of pH (Welch & Ullman, 1993; Chou & Wollast, 1985).

Microbial polymers were obtained from mucoid-producing isolates B0693, B0428,and B0577 from the Subsurface Microbial Culture Collection (Florida State University,D. Balkwill, curator). Each strain was grown in a complex peptone-yeast extract-glucosemedium and harvested by low speed centrifugation (5000,g). Cells were resuspended indistilled water and centrifuged at high speed (36,000,?) to strip the exopolymers. The su-pernatant phase was then mixed with 3 volumes of cold isopropanol, kept overnight at4°C, and centrifuged at SOQOg to recover the EPS. The EPS were then redissolved, dia-lyzed for 24 h with an 8000 molecular weight cut-off membrane, and lyophilized to dry-ness. EPS solutions for dissolution experiments were made by dissolving the EPS in 10mM PIPES buffer solution, pH 6.8. PIPES buffer was used in our microbial experimentsbecause it has a much higher buffering capacity than the bicarbonate buffer and it doesnot appear to significantly affect mineral dissolution compared to distilled water controls.

Experimental

Dissolution experiments were performed in batch reactors. For the experiments with thesimple polysaccharides and poly-amino acids, 1 g feldspar was added to 100 ml of solu-tion in a polycarbonate bottle. Three replicate experiments were performed for each treat-ment. Sample bottles were placed on a shaker table (100 rpm) in a 25°C incubator.Aliquots were removed from each bottle daily for up to 5 days and analyzed potentiomet-rically for pH and for dissolved Si using a Technicon AutoAnalyser II (Glibert & Loder,1977). Sample pH did not drift significantly during the experiments. A recent study(Gaffney et al., 1989) suggests that a systematic problem occurs when using the silico-

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230 S. A. Welch and P. Vandevivere

molybdate blue method on samples that have a high organic content. However, standardaddition experiments show no apparent matrix effect in these experiments.

Dissolution experiments using the microbial EPS were also performed in batch reac-tors. However, the solution to solid ratio was lower (50 ml: 1 g) since it was difficult toisolate large quantities of EPS. Experiments were sampled only once after 4-7 days forthe determination of pH and dissolved silica.

Results

Dissolved silica release to solution was used as an indicator of overall mineral dissolu-tion. The other major ions, Al, Ca, and Na, were not measured in these experiments. Pre-vious experiments with feldspars indicate that dissolution is nonstoichiometric (Welch,1991; Welch & Ullman, 1993) with nonframework cations (Ca, Na) preferentiallyleached from the mineral surface compared to the framework elements (Al, Si). Theredoes not appear to be any systematic variation in cation leaching with changes in solutionchemistry. The relative rates of detachment of framework elements (Al, Si), however, areaffected by solution chemistry. Si was preferentially removed from mineral surfaces inexperiments similar to these (Welch, 1991; Welch & Ullman, 1993), leaving an Al-richresidual material. Al release only contributes significantly to total net mineral dissolutionin experiments with low solution pH (pH < 5) or with high concentrations of complexingorganic ligands.

In the first set of experiments, five of the polysaccharides tested (cellulose, starch,polysucrose, gum xanthan, and a commercial bacterial polysaccharide) had no significantimpact on the dissolution of feldspar (Table 1). Even gum xanthan, which forms a vis-cous (almost gel-like) solution at the highest concentration tested (1 g/L), had no effect(Figure 1). Alginate, a polymer of mannuronic and guluronic acids, inhibited dissolutionwhen concentrations were above 0.1 g/L; lower alginate concentrations had no significanteffect on silica release from feldspars (Figure 2).

Two aspartate polymers were also tested for their possible effects on mineral dissolu-tion. The low-molecular-weight polymer (6 kD) had no significant effect on Si releasefrom feldspar. However, the high-molecular-weight polymer (26 kD) significantly de-creased dissolution compared to the control (Figure 3).

In the second set of experiments using microbial metabolites, the extracellular poly-saccharides (EPS) extracted from three strains of bacteria enhanced bytownite dissolution(Figure 4). Silica release from the feldspars increased from 20 to 35% compared to thecontrol.

A series of abiotic batch dissolution experiments were performed with solutions ofgluconic acid, a low-molecular-weight metabolite produced by all three cultured strains ofbacteria (Vandevivere et al., 1994), and the fresh EPS extracts. Vandevivere et al. (1994)demonstrated that gluconic acid substantially increases Si release from feldspars at nearneutral pH. In these experiments, dissolved Si concentration increased approximately200% in solutions with gluconate compared to the PIPES buffer control, while the freshEPS extracts alone only increased dissolution 20-35% (Figure 4). Batch dissolution exper-iments with gluconate and EPS were performed to determine if the enhancement effects ofboth types of microbial compounds would be additive. However, bytownite dissolution insolutions of EPS and gluconate was actually inhibited by the EPS compared to gluconatealone (Figure 4), though dissolution was still greater than treatments with no added micro-bial metabolites. Increasing EPS concentration from 0.05% to 0.1% while keeping glu-conate concentration constant (10 mM) further inhibited dissolution (Figure 5).

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3;55

O 1 g/1• 0.1 g/1V 0.01 g/1T control

10 20 30

time (hours)

40 50

Figure 1. Silica release from labradorite feldspar versus time in solutions of gum xanthan. Gumxanthan has no significant effect on feldspar dissolution, even at concentrations of 1 g/L, where itforms a viscous, almost gel-like solution.

D control

T 0.001 g/1

V 0.01 g/1

353 • 0.1 g/1

O 1 g/1

10 20 30

time (hours)

40

Figure 2. Silica release from labradorite versus time in alginate solutions. Alginate concentrationsgreater than 0.1 g/L inhibited dissolution. Lower alginate concentrations had no significant effect.

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232 S.A. Welch and P. Vandevivere

O control• 6 kDV 26 kD

20 30

time (hours)

40 50

Figure 3. Silica release from labradorite in solutions of polyaspartate, 0.5 g/L. The 26-kD polyas-partate inhibited dissolution while the 6-kD polymer had no significant effect.

Discussion

Several factors may affect adsorption of polymers onto mineral surfaces, and thereforemineral-water interactions. These factors include polymer composition (and types offunctional groups), polymer size, structure, and shape (Christensen & Characklis, 1990;Marshall, 1985). Polymer composition will determine the type or strength of bonding andnumber of possible bonding sites. For example, compounds that have acid functionalgroups available are more likely to form strong metal-ligand bonds to the mineral sur-face, while compounds that have hydroxy or aldehyde groups are more likely to formweaker hydrogen bonds or van der Waals bonds to surfaces.

With respect to polymer size, smaller polymers are less likely to irreversibly bind toa surface because they have fewer sites available for binding, while larger polymers aremore likely to form many bonds, binding irreversibly to surfaces. Davis and Gloor (1981)found that naturally occurring high-molecular-weight compounds from a lake adsorbedirreversibly to alumina particles, whereas lower molecular weight compounds were onlyweakly adsorbed.

Bonding also depends on structure and shape, which, in turn, depend on the orienta-tion of the functional groups, as groups that are held out from the bulk of the polymermolecule are more likely to react with a mineral surface than those that are turned in to-ward the polymer, because of steric hindrance. The structure and shape of a polymer alsoplay a role in adsorption. Polymers can be long linear compounds or can be branched andcross-linked. In solution and at the mineral-water interface, polymers can be tightlycoiled like a sphere, uncoiled like a stiff rod, or randomly coiled (Christensen & Charack-lis, 1990). Uncoiled polymers have the highest potential for irreversibly adsorbing to a

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Polymers and Mineral Dissolution 233

CO

180

160

140

120

100

80

60

40

20

n

i i ii — i

-

-

-

-

-

T

11J

--|1 I1

-------con 693

^ H buffer

428 577

] gluconate

Figure 4. Dissolved silica concentration after 5 days in buffered solutions of 0.1% EPS (black bar)and EPS with 10 raM gluconate (white bar). The numbers 693, 428, and 577 refer to 3 mucoid-pro-ducing microbial strains from the DOE subsurface microbiology culture collection. Feldspar disso-lution increased 20-35% in solutions of EPS compared to the control. However, in experimentswith EPS and gluconate, the EPS inhibited dissolution approximately 25% compared to the glu-conate control.

mineral surface, since the whole molecule can come in contact with the surface. Sphericalpolymers have the least contact with the mineral surface per unit molecular weight andare, therefore, likely to have less effect on mineral-water reactions than a comparable un-coiled compound. Most microbial polysaccharides are either stiff rods, random coils, orhave some intermediate shape (Christensen & Characklis, 1990) and are therefore likelyto be able to interact with a large mineral surface area when adsorbed.

In our first set of experiments we wanted to determine if certain compositions orfunctional groups are important in mineral-water interaction. Presumably, compoundsthat are more strongly adsorbed should have a larger impact on mineral-solution interac-tions, including dissolution. Polymers of well-known composition were chosen, but otherfactors (such as size, shape, and structure) that can be important in dissolution enhance-ment or inhibition were uncontrolled in these experiments.

Low-molecular-weight compounds that have multiple functional groups, particularlywhen they are adjacent to each other, such as catechol or a-keto acids, have very stronginteractions with mineral surfaces, forming stable bidentate complexes (Welch & Ullman,1992, 1993; Kummert & Stumm, 1980; Stumm & Wieland, 1990; Wieland et al., 1988).These compounds can enhance mineral dissolution by forming metal-ligand bonds at thesurface that weaken the metal-oxygen bonds between the mineral surface and the bulk. Incontrast, the neutral sugars, even though they have multiple adjacent hydroxy groups, can-

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234 S. A. Welch and P. Vandevivere

120

100

80

60

40

20

n

i i t i t

T

-

-

-

-

-

1

T

1 l

-

T

-

-

-

1 2 3 4 5

Figure 5. A repeat of experiment in Figure 4 using a different bytownite feldspar and only one ofthe microbial EPS. Bars represent dissolved silica concentration in different solutions at the end ofa 5-day-long feldspar dissolution experiment: (1) 10 mM PIPES buffer control, pH 6.8; (2) 10 mMPIPES + 0.05% EPS; (3) 10 mM PIPES + 10 mM gluconate; (4) 10 mM PIPES + 0.05% EPS + 10mM gluconate; (5) 10 mM PIPES + 0.1% EPS + 10 mM gluconate. Both EPS (2) and gluconate (3)alone enhanced bytownite dissolution compared to the PIPES buffer control (1). Bytownite dissolu-tion was inhibited in solutions with both gluconate and EPS (4, 5) compared to gluconate controls(3). Increasing EPS concentration from 0.05% (4) to 0.1% (5) further inhibited dissolution.

not easily form bidentate surface complexes. The adjacent hydroxy groups in neutral sug-ars do not generally lie in the same plane because of steric inhibition of rotation. There-fore, these compounds only have weak interactions with mineral surfaces, by hydrogenbonding or van der Waals forces, and have almost no effect on dissolution. Previous ex-periments showed that 10 mM glucose solutions have no detectable effect on mineral dis-solution at neutral pH compared to an inorganic control, while 1 mM solutions of oxalateor catechol increased rates by an order of magnitude (Welch & Ullman, 1993; Welch,1991; Vandevivere et al., 1994).

Our experiments showed that the polymers composed primarily of neutral sugars,starch, cellulose, gum xanthan, and polysucrose had no detectable effect on mineral dis-solution, even when concentrations reached 1 g/L, 1000 times greater than average dis-solved organic carbon (DOC) in groundwater, approximately 1 ppm (Thurman, 1985).These polymers are composed of oligosaccharide compounds linked by glycosidic bonds.The hydroxy functional groups of the individual sugar monomers are rigidly held in ei-ther axial or equatorial positions. These hydroxyls may be able to form hydrogen bondswith oxygens at the mineral surface, weakly adsorbing to the surface. However, they donot remove any surface framework elements (Al, Si) when they desorb. Because the hy-drogen bonds are easily broken and reformed, these molecules may not be irreversiblysorbed to mineral surfaces.

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Polymers and Mineral Dissolution 235

Only one of the simple polysaccharides tested, alginate, inhibited dissolution. Algi-nate has a structure similar to the other polysaccharides and is composed primarily of twosugar acids, mannuronic and guluronic acids, that are linked by glycosidic bonds. How-ever, unlike the other polysaccharides tested, which were composed mainly of neutralsugars, alginate has acid functional groups (R-COOH) in axial and equatorial positionsoff the main ring. In addition to hydrogen bonding and van der Waals bonding, thesegroups should be able to form complexes between the acid functional groups and frame-work elements at the mineral surface in a manner similar to the low-molecular-weight or-ganic acids. If many of the acid functional groups bind to the mineral surface, the poly-mer will become irreversibly attached and dissolution will be inhibited.

One of the poly-amino acid compounds tested in these experiments, the 26-kDpolyaspartate compound, inhibited dissolution like the alginate. This compound is com-posed of approximately 200 linked monomers and also has acid functional groups avail-able for forming metal-oxygen bonds to the mineral surface. Its not clear why this highermolecular weight polymer inhibited dissolution while the lower molecular weight polyas-partate, which is composed of approximately 50 linked monomers, had no apparent effect.It is possible that the 26-kD polyaspartate is irreversibly bound to reactive sites on themineral surface while the 6-kD polyaspartate is reversibly sorbed and that the effect ispurely due to molecular size and number of reactive sites per molecule. Davis and Gloor(1981) also found that naturally occurring higher molecular weight organic material wasmore strongly bound to mineral surfaces than lower molecular weight material. Other fac-tors, such as polymer structure (branched vs. unbranched) or polymer shape (coiled vs. un-coiled) may be affecting adsorption and mineral-water interaction in these experiments.

Unlike the simple polymers tested, which did not increase mineral dissolution, thefresh microbial extracellular polysaccharides (EPS) caused an increase in the dissolutionof bytownite compared to controls. This increase could be attributed to at least two mech-anisms: (1) Soluble organic ligands can indirectly increase the dissolution rate by com-plexing with ions in solution, increasing the apparent solubility of the mineral, thus in-creasing the dissolution rate. This process is probably most important when solutions areclose to equilibrium saturation. (2) Organic compounds also directly increase mineral dis-solution rate by forming metal-organic complexes at the mineral surface, weakeningmetal-oxygen bonds to the bulk mineral, thus catalyzing the dissolution reaction.

High-molecular-weight compounds like EPS probably act indirectly on dissolutionby complexing with Al and Si in solution. Because of their large size and steric hindrancewith the surface, it is unlikely that they can attach to a mineral surface, form a complexwith a surface metal ion, and then detach with that framework ion. These compounds areonly able to have a relatively small enhancement effect on dissolution. Low-molecular-weight metabolites, however, are able to directly attack mineral surfaces and substantiallyenhance mineral dissolution rates (Welch, 1991; Welch & Ullman, 1993; Vandevivere etal., 1994).

Not all microbial EPS enhance dissolution, however. The commercial microbialpolysaccharide used in our experiments had no effect on dissolution. A polysaccharideisolated from Bacillus mucilaginosus also had no enhancement effect on the dissolutionof silicate minerals when it was in solution alone (Malinovskaya et al., 1990). However,combinations of EPS and organic acids in solution substantially increased silicate mineraldissolution compared to controls with only organic acids or organic free controls.

If the only effect of the natural microbial polysaccharide was to increase dissolution,then EPS and low-molecular-weight metabolites together should further enhance dissolu-tion, as did the EPS from B. mucilaginosus (Malinovskaya et al., 1990). This did not

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236 S. A. Welch and P. Vandevivere

occur in our experiments, however. Solutions with EPS and gluconate inhibited bytown-ite dissolution compared to controls with gluconate alone. There are several possible rea-sons why the EPS could inhibit dissolution. Adsorbed EPS can create a diffusion-inhibit-ing layer at the mineral surface, inhibiting both transport of ligands to the surface andtransport of framework ions away from the surface. Diffusion coefficients in biofilms areapproximately 80% of the values in water (Christensen & Characklis, 1990), so this maypartially account for the dissolution inhibition. However, since the EPS did not inhibitdissolution alone, and gum xanthan also did not inhibit dissolution, it seems unlikely thata decrease in diffusivity could account for the decrease in dissolution observed. This maynot be the case for natural environments, however, where water flow past mineral grainsis much lower.

Another possibility is that adsorbed EPS changes surface chemistry and surfacecharge. Incoming ligands may be repelled by the new surface if charges are the same. Inaddition, the low-molecular-weight organic ligands may preferentially react with the ad-sorbed organics and therefore be unable to react with the mineral surface.

Dissolution reactions are nonhomogeneous over the mineral surface. Features suchas cracks, ledges, and edges dissolve preferentially over the bulk surface. These high-en-ergy sites are likely to be preferentially attacked by ligands and protons because theyhave a higher density of bonding sites available. EPS can also bind to these sites, possiblystabilizing them, and then making them unavailable for further reaction.

Conclusions

Natural polymers can affect mineral-water interactions. Polymers that have negativelycharged functional groups, such as acids, that can bind directly to mineral surfaces havethe largest effect. Polymers that only form weak bonds (hydrogen bonding or van derWaals) had little to no effect on dissolution.

Microorganisms attached to mineral surfaces may be able to both increase and in-hibit the rates of mineral dissolution by producing high- and low-molecular weightmetabolites at the mineral surface. The microbial EPS examined in this study both in-creased and inhibited dissolution under different conditions. EPS can complex with ionsin solution, increasing the apparent solubility of the mineral. EPS can also inhibit dissolu-tion by forming a diffusion-inhibited layer or by irreversibly binding to reactive sites onmineral surfaces, making them unavailable for further reaction.

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