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Published online 1 May 2008
Published in J Environ Qual 37:824-829 (2008)
DOI: 10.2134/jeq2007.0236
© 2008 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Organic Compounds in the Environment

Site-Specific Adsorption of 1,3-Dinitrobenzene to Bacterial Surfaces: A Mechanism of n–{pi} Electron-Donor-Acceptor Interactions

Xiaolei Qu, Lin Xiao and Dongqiang Zhu*

State Key Laboratory of Pollution Control and Resource Reuse, and School of the Environment, Nanjing Univ., Jiangsu 210093, China

* Corresponding author (zhud{at}nju.edu.cn).

Received for publication May 10, 2007.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 REFERENCES
 
Surface and subsurface contamination with nitroaromatic compounds (NACs) has drawn considerable attention, and biosorption may play an important role in the fate and transport of these compounds in the environment. We studied the sorption of polar 1,3-dinitrobenzene (DNB) as a representative NAC and 2,6-dichlorobenzonitrile and nonpolar phenanthrene and 1,2,4,5-tetrachlorobenzene from the aqueous phase to two common bacteria, gram-negative Escherichia coli and gram-positive Bacillus subtilis. Sorption of DNB is highly nonlinear and is well described by the Langmuir model and shows the highest capacity among all tested solutes (up to 2.4% of E. coli biomass and 7.6% of B. subtilis biomass by weight) despite the lowest solute hydrophobicity. These results indicate that strong specific sorptive interactions exist between DNB and bacterial surfaces. We propose a mechanism of n–{pi} electron-donor-acceptor interactions between the oxygen electron pairs of deprotonated carboxyl groups (electron donors) of bacterial surfaces and DNB (electron acceptor). Biosorption of DNB increases with deprotonation of functional groups as pH increases, which rules out hydrophobic effects and H-bonding as major sorption driving forces because they are both favored by protonation of functional groups as pH decreases.

Abbreviations: DNB, 1,3-dinitrobenzene • DNL, 2,6-dichlorobenzonitrile • EDA, electron-donor-acceptor • HOC, hydrophobic organic compound • NAC, nitroaromatic compound • PHEN, phenanthrene • TeCB, 1,2,4,5-tetrachlorobenzene


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 REFERENCES
 
NITROAROMATIC compounds (NACs) are a group of polar, hydrophobic organic compounds (HOCs) that have received considerable attention because of their frequent use as explosives, pesticides, and organic solvents (Rickert, 1985; Spain et al., 2000). One prominent example is the surface and subsurface contamination by 1,3-dinitrobenzene and nitroaromatic explosives such as 2,4,6-trinitrotoluene. To assess the mobility and reactivity of NACs in the environment, it is essential to elucidate the molecular-level interactions and factors that control the sorption behavior of NACs to natural solid phases. Previous studies of NAC sorption were focused on soil and mineral surfaces. Sorption of NACs to these sorbents is strong, nonlinear, and dependent on sorbent property (e.g., type of exchangeable cation) and NAC structure, suggesting that specific interactions may be involved (Hundal et al., 1997; Weissmahr et al., 1997; Weissmahr et al., 1999; Sheremata et al., 1999; Boyd et al., 2001; Li et al., 2004).

Biosorption refers to uptake and accumulation of chemicals by biomass. It is well documented that microorganisms (e.g., bacteria, fungi, and yeasts) have good sorptive affinities toward many organic pollutants, such as phenols, dyes, and pesticides (Ju et al., 1997; Aksu, 2005 and references therein; Nacèra and Aicha, 2006; Wu and Yu, 2006). To our knowledge, no direct attention has been paid to biosorption of NACs by microorganisms.

Biosorption is expected to play an important role in the environmental fate of organic pollutants. For example, soil is rich in microorganisms (e.g., bacteria, fungi, algae, and protozoa), of which bacteria occur in the largest amount, typically of 1 x 108 cells g–1 for organic-rich surface soils (Alexander, 1977). Biosorption to indigenous and exogenous (i.e., injected in a bioremediation operation) bacteria may profoundly affect the transformation and sequestration of organic pollutants in soils. Furthermore, bacteria are recognized as major colloidal constituents in surface and ground water aquifers (Schafer et al., 1998; Harter et al., 2000; Hijnen et al., 2005). Thus, bacteria may act as effective carriers for strongly sorbing pollutants (heavy meals and HOCs) and thereby accelerate their transport in the aquifer by reducing the retardation effect (Kim and Corapcioglu, 1997; Kim et al., 2003; Pang et al., 2005; Simunek et al., 2006).

Bacterial cell walls are composed of amphiphilic polymerized structures (polysaccharide, cross-linked shot peptides, proteins, and organic acids) that are rich in functional groups (carboxyl, amine, phosphate, and hydroxyl) (Sherbert, 1978). The O-containing functional groups (mainly carboxyl and phosphate) could be deprotonated and accumulate negative charges on the bacterial surface. In response to the heterogeneous structure and chemistry of bacterial surfaces, organic biosorption may invoke various sorption driving forces, including hydrophobic effects, defined as combined entropic effects and van der Waals forces (Schwarzenbach et al., 2003), and other direct molecular sorptive interactions (e.g., dipole/polarizable interactions, H-bonding, and electron-donor-acceptor [EDA]) interactions (Zhu and Pignatello, 2005a).

Because of the relatively strong electron-withdrawing ability of the nitro group, NACs may act as strong electron {pi}-acceptors and interact with electron donors, if provided, via EDA interactions. Recently, Zhu and Pignatello (2005b) reported {pi}–{pi} EDA interactions between polycyclic aromatic structures ({pi}-donors) of wood-made charcoals and nonporous granite and NACs ({pi}-acceptors). To account for the strong affinities of NACs to smectite clay surfaces, Weissmahr et al. (1997) proposed n–{pi} EDA forces between oxygen electron pairs of the siloxane surface (electron n-donor) of clays and the NAC (electron {pi}-acceptor). Likewise, it is reasonable to assume that n–{pi} EDA interactions exist between oxygen electron pairs of deprotonated functional groups (electron donors) of bacterial surfaces and NACs (electron acceptors). To test this hypothesis, we studied sorption of a representative NAC, accompanied by three other polar and nonpolar HOCs, to two common bacteria, gram-negative Escherichia coli and gram-positive Bacillus subtilis, suspended in aqueous solutions by the batch technique. We compared sorptive affinities of the different solutes and examined the effects of pH and the presence of transition metal ions on 1,3-dinitrobenzene (DNB) sorption to better understand the underlying mechanism(s) of sorptive interactions.


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 REFERENCES
 
Materials
Gram-negative E. coli (ATCC 25922) and gram-positive B. subtilis (ACCC 11060) were initially cultured in 30 mL of Luria-Bertani broth for 12 h at 32°C in the exponential phase and then transferred to 3 L of fresh Luria-Bertani broth and grown for another 12 h in the stationary phase. The bacteria were separated from the growth medium by centrifugation (6000 rpm for 10 min), followed by washing three times with Type I water (distilled and deionized, electric conductivity >18.2 M{Omega} cm) and centrifugation (6000 rpm for 10 min). The supernatant was discarded after centrifugation in each step. The obtained bacteria pellets were mixed with 0.1 mol L–1 aqueous solution of NaCl, KCl, or CsCl to prepare a homogeneous bacteria suspension (cell concentration equivalent to an optical density of 0.87 at 460 nm). An aqueous stock solution of Cu2+ (320 mg L–1) or Fe3+ (2.1 x 104 mg L–1), if needed, was added directly to the bacteria suspension to give a metal concentration of 3 mg L–1. The pH of the bacteria suspension was unadjusted except where noted, when it was adjusted by 0.1 mol L–1 HCl and 0.1 mol L–1 NaOH. A known volume of bacteria suspension in Type I water was frozen and then freeze dried. The obtained dry biomass was weighed to calculate the biomass concentration of bacteria suspension used in batch sorption experiments (0.29 mg L–1 for E. coli and 0.17 mg L–1 for B. subtilis).

Sorbates included polar 1,3-dinitrobenzene (DNB) (Aldrich, St. Louis, MO) as a representative NAC, 2,6-dichlobenzonitrile (DNL) (Chem Service, West Chester, PA), nonpolar phenanthrene (PHEN) (Fluka, Riedel-de Haën, Seelze, Germany), and 1,2,4,5-tetrachlorobenzene (TeCB) (Aldrich). Water solubilities and n-octanol–water partition coefficients of the sorbates are listed in Table 1 .


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  		Table 1. Water solubility (SW) and n-octanol–water partition coefficient (KOW) for sorbates.

 
Batch Sorption
Batch sorption experiments were performed in 22-mL EPA vials equipped with Teflon-lined screw caps. Vials received sufficient volume of bacteria suspension followed by sorbate in methanol carrier, which was kept below 0.1% by volume to minimize co-solvent effects. Isotherms on duplicates were done for DNB in NaCl and KCl solutions, and single-point data on three or four replicates were done for other solutes and for DNB with other electrolytes. The samples were shaken by an orbital shaker and incubated at 20 ± 0.5°C for at least 18 h. This period was sufficient to reach apparent sorption equilibrium (no further uptake) for organic solute and transition metal based on previous kinetic studies of phenanthrene sorption (Xiao et al., 2007) and Cd2+ sorption (Yee and Fein, 2001) to E. coli. Changes in bacterial activity and growth curve are expected to be small during sorption experiments in the static phase, and any effects on sorption would be offset for the same batch in comparison studies. Afterward, the samples were centrifuged at 4000 rpm for 20 min to allow complete separation between gel-like bacteria and solution (i.e., the supernatant showed no detectable optical density at 460 nm). Morphology inspection of E. coli cells at the end of sorption experiments by scanning electron microscopy (data not shown) indicated that the bacteria remained intact (i.e., no lysis) throughout the sorption experiment.

Solute TeCB was extracted from an aliquot of the supernatant with hexanes and analyzed by gas chromatography with electron-capture detection using a 60 m x 0.25 mm DB-1 capillary column (J&W Scientific, Folsom, CA). Other solutes in aqueous solutions were analyzed directly by high-performance liquid chromatography with a UV detector using a 4.6 x 150 mm HC-C18 column (Agilent, Santa Clara, CA). Isocratic elution was performed under the following conditions: 60% methanol:40% water (v/v) with a wavelength of 238 nm for DNB; 80% methanol:20% water (v/v) with a wavelength of 254 nm for PHEN; and 75% methanol:25% water (v/v) with a wavelength of 210 nm for DNL. Transition metal concentration in the supernatant was measured by an atomic absorption spectrometer (Hitachi Z-8100; Hitachi, Japan). To take account for solute loss from processes other than biosorption (e.g., metal sorption to glassware and organic sorption to septum and glassware), calibration curves were obtained separately from controls receiving the same treatment as the sorption samples but no bacteria. Calibration curves included at least seven standards over the test concentration range of samples. When electron-capture detection was used, calibration curves were fit to a power law expression to account for the detector response nonlinearity. Adsorbed mass was assumed equal to the difference between added mass and mass in aqueous solution. The pH of the bacteria suspension at sorption equilibrium was measured at the end of sorption experiments. In general, the pH was close to 6.0 and varied by less than 0.2 units between suspensions of different solute/electrolyte combinations.


   Results and Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
REFERENCES
 
Characterization of Bacteria
Elemental analysis on freeze-dried biomass of bacteria gave C (43.1%), H (6.5%), O (30.3%), N (12.8%), and S (0.5%) for E. coli and C (43.3%), H (6.6%), O (37.2%), N (9.3%), and S (0.5%) for B. subtilis on a weight basis. The polarity index, measured as the atomic ratio of (O + N)/C, is 0.78 for E. coli and 0.83 for B. subtilis, indicating generally greater contents of polar functional groups in bacterial biomass relative to soil organic matter (i.e., C [52–58%], H [3.4–4.8%], O [34–39%], and N [3.7–4.1%]) (Sparks, 2003), by which the (O + N)/C ratio was calculated to be 0.65 to 0.83. Moreover, bacterial surfaces are negatively charged under the tested condition. The surface charge, measured as zeta potential, vs. pH and concentration of transition metal (Cu2+, Fe3+) is given elsewhere (Xiao et al., 2007).

Batch Sorption
Sorptive Characteristics
Sorption data, plotted as sorbed concentration (q) vs. aqueous phase concentration (Ce) at sorption equilibrium, are given in Fig. 1 and Fig. 2 for different solute/electrolyte combinations. Results of replicate samples are shown as individual data points with SDs. Sorption isotherms conducted for DNB in NaCl and KCl solutions were fit to the Langmuir model:

Formula 1[1]
where KL (L mmol–1) is the Langmuir affinity coefficient, and q0 (mmol kg–1) is the adsorptive capacity. The fitting curves are also shown in Fig. 2, and the parameters are summarized in Table 2 .


Figure 1
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  		Fig. 1. Equilibrium sorbed concentration (q) vs. aqueous phase concentration (Ce) shown on logarithm scales for different solutes to bacteria in 0.1 mol L–1 NaCl. Closed circles, 1,3-dinitrobenzene (DNB); closed triangles, phenanthrene; closed diamonds, 1,2,4,5-tetrachlorobenzene; closed squares, 2,6-dichlorobenzonitrile. Isotherms on duplicates were done for DNB, and single-point data on three replicates were used for other solutes. Bidirectional error bars, in most cases smaller than the symbols, represent SDs.

 

Figure 2
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  		Fig. 2. Equilibrium sorbed concentration (q) vs. aqueous phase concentration (Ce) shown on linear scales for 1,3-dinitrobenzene (DNB) to bacteria in different electrolyte solutions. Closed circles, 0.1 mol L–1 NaCl only; closed triangles, 0.1 mol L–1 KCl only; open diamonds, 0.1 mol L–1 CsCl only; open triangles, 0.1 mol L–1 NaCl plus 3 mg L–1 Fe3+; open squares, 0.1 mol L–1 NaCl plus 3 mg L–1 Cu2+. Isotherms on duplicates were done for NaCl and KCl solutions, and single-point data on four replicates were done for other electrolytes. Bidirectional error bars, in most cases smaller than the symbols, represent SDs. Lines represent Langmuir fitting curves to sorption isotherms. Arrows point to samples spiked with equal DNB concentrations.

 

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  		Table 2. Langmuir model parameters (KL ± SD and q0 ± SD){dagger} for isotherms measured for sorption of 1,3-dinitrobenzene to bacteria in 0.1 mol L–1 NaCl or KCl solution.

 
A striking observation made from Fig. 1 is that DNB shows much larger sorptive affinities to E. coli and B. subtilis than other tested solutes (PHEN, TeCB, DNL) despite the lower hydrophobicity, as justified by n-octanol–water partition coefficients and water solubility values (Table 1). With the studied concentrations, sorption of DNB is more than one order of magnitude greater than that of DNL and TeCB and more than two orders of magnitude greater than that of PHEN. With the studied biomass to solution ratio, the sorptive capacity of DNB by weight is equivalent to 2.4% of E. coli biomass and 7.6% of B. subtilis biomass; the removal efficiency of DNB from the aqueous phase is more than 99% when Ce is less than 0.1 mg L–1. The range of measured solid-to-solution distribution coefficient (Kd = q/Ce) of DNB is 1.6 x 103 to 2.8 x 105 L kg–1 for E. coli and 1.4 x 104 to 4.3 x 105 L kg–1 for B. subtilis. On a weight basis of sorbent, the DNB sorption to bacterial surfaces is remarkably stronger than that to smectite clays, which are well documented in literature for strong abilities to retain NACs (i.e., Kd is up to an order of magnitude 102 L kg–1 for DNB adsorption to K+-exchanged smectite clays) (Johnston et al., 2001; Li et al., 2004). The DNB sorption to bacterial surfaces is highly nonlinear (shown more apparently in Fig. 2 on linear scales). The sorption reaches the maximum capacity at low solute concentrations, and Kd varies by an order of 2 from the very high to very low concentration (0.04–14.5 mmol L–1 for E. coli and 0.01–6.5 mmol L–1 for B. subtilis). The strong sorptive affinity and nonlinearity suggest site-specific sorptive interactions of DNB on bacterial surfaces.

Mechanisms of specific sorptive interactions have been advanced for adsorption of NACs by smectite clays and carbon materials. Previous studies found that NACs are strongly retained by smectite clays, especially those saturated with weakly hydrated cations (e.g., K+ and Cs+) (Hundal et al., 1997; Weissmahr et al., 1997; Boyd et al., 2001; Johnston et al., 2002; Li et al., 2004). To take account for the observations, mainly two different mechanisms have been proposed: (i) n–{pi} EDA interactions between oxygen electron pairs of the siloxane surface (electron donor) of clays and the NAC (electron acceptor) (Haderlein et al., 1996; Weissmahr et al., 1997) and (ii) direct complexation between the exchangeable cation and the nitro group of NACs (Boyd et al., 2001; Johnston et al., 2001). Weakly hydrated cations favor NAC adsorption because of less steric effects to allow better formation of inner-sphere EDA complexes (mechanism 1) or less competitive reactions of hydration for cations in NAC complexation (mechanism 2). Nevertheless, these two mechanisms have to entail specific interactions that are induced by the strong electron-withdrawing ability of the nitro group. Recently, Zhu and Pignatello (2005b) found that when hydrophobic effects are normalized to the same level, the adsorption to nonporous graphite shows a decreasing order of trinitrotoluene > dinitrotoluene > nitrotoluene, which correlates with {pi}-acceptor strength of solute. The investigators proposed {pi}–{pi} EDA interactions between polycyclic aromatic units (electron donors) of graphite and NACs (electron acceptors).

We herein propose a mechanism of n–{pi} EDA interaction between DNB (electron acceptor) and oxygen electron pairs of the deprotonated carboxyl groups (electron donors) of bacterial surfaces. Based on potentiometric titration of bacterial surfaces, previous studies showed that the pKa of the carboxyl site is 4.87 for E. coli and 4.82 for B. subtilis, and the pKa of the phosphate site is 6.9 for B. subtilis, where carboxyl also greatly surpasses phosphate in concentration (Yee and Fein, 2001). The tested bacteria, E. coli and B. subtilis, do not excrete much extracellular content (e.g., polysaccharides and organic acids) (Bergey et al., 1994), which should have been largely removed by repeated washing and centrifugation. The pH ranges from 5.8 to 6.2 in this study, and thus deprotonated carboxyl groups are the primary component responsible for the negative charge on bacterial surfaces.

Free carboxylate anions fully solvated in aqueous solution would not induce strong n–{pi} EDA interactions with NACs. This is evidenced by the fact that the activity coefficient of DNB, measured from its aqueous solubility, keeps constant (SD = 0.01) when CH3COONa concentration changes from 0.1 to 0.3 mol L–1 (solubility data of DNB vs. [CH3COONa]; not shown). In the aqueous phase, the activity coefficient of an organic solute is inversely proportional to its aqueous solubility (Schwarzenbach et al., 2003). Formation of n–{pi} EDA complexes between CH3COO– and DNB in aqueous solution is too energy costly due to "desolvation penalty" from solvation of CH3COO– (H-bonding and polar interactions with water) and "entropy penalty" (i.e., the association process to form n–{pi} complexes leads to loss of conformational freedom). On the bacterial surface, one can imagine that the deprotonated carboxyl groups are somewhat imbedded within structures of hydrophobic subunits (e.g., aliphatic chains), which relieves the "desolvation penalty" of these ionized groups and thus facilitates n–{pi} EDA interactions. Furthermore, compared with the free solution phase, the bacterial surface enables a more ordered and compact arrangement of deprotonated carboxyl groups, therefore thermodynamically allowing better formation of n–{pi} EDA complexes on the bacterial surface by decreasing the "entropy penalty." Figure 3 gives an illustrative delineation of n–{pi} EDA interactions between DNB and the negatively charged bacterial surface.


Figure 3
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  		Fig. 3. Illustrative schema for n–{pi} electron-donor-acceptor interactions between 1,3-dinitrobenzene (DNB) (electron acceptor) and negatively charged bacterial surfaces (electron donor).

 
Effects of Metal Ions
Single-point data of DNB sorption with presence of Cs+, Fe3+, and Cu2+ are shown in Fig. 2, together with sorption isotherms of DNB in NaCl and KCl solutions for comparison. For Cs+, Fe3+, and Cu2+ solutions, DNB was spiked at the same concentration, which was included in the spiking series for Na+ and K+ solutions (arrows in Fig. 2). In the given ionic strength of 0.1 mol L–1, sorption of DNB shows a decreasing order of Na+ > K+ > Cs+, which positively correlates with hydrated radius of a cation. A cation with a smaller hydrated radius (e.g., Cs+) would cause larger electronic shielding on the negative charge of bacterial surfaces due to stronger electrostatic forces, thereby more effectively suppressing n–{pi} EDA interactions. Sorption of the base cations to bacterial surfaces is analogized by the process of cation exchange with negatively charged mineral surfaces: Where a cation with the smallest hydrated radius is preferentially sorbed (i.e., for the Group I elements), the general order of selectivity is Cs+ > Rb+ > K+ > Na+ > Li+ > H+ (Sparks, 2003). In contrast to the observed cation effect on DNB sorption to bacterial surfaces, previous studies showed that significantly stronger adsorption of NACs occurs on smectite clays exchanged with weakly hydrated cations (K+, Cs+) relative to the strongly hydrated cations (Na+, Ca2+) (Boyd et al., 2001; Johnston et al., 2002). The investigators applied the mechanism of direct cation complexation with the nitro group, which is favored when the cation is less hydrated. This mechanism can be ruled out for sorption of DNB to bacterial surfaces due to the opposite cation effect.

The presence of transition metal ion (Cu2+ or Fe3+) at a low initial concentration of 3 mg L–1 dramatically suppresses DNB sorption to bacterial surfaces, with stronger effects observed on Cu2+ (Fig. 2). With the same loading of Cu2+ and Fe3+, sorption of PHEN and TeCB is increased by 1.5 to 4 times to E. coli (Xiao et al., 2007). One cause for this observation is that binding of transition metals to deprotonated functional groups (mainly carboxyl) neutralizes the negative charge and thereby increases the hydrophobicity of bacterial surfaces to favor hydrophobic partition. Alternatively, coordination of transition metals greatly reduces electron-donor abilities of the deprotonated groups due to electronic coupling and thus seriously impairs n–{pi} EDA complexation. Competition for the deprotonated groups by transition metals should be strong because the metals are present in concentrations comparable with that of DNB (the initial concentration is about 0.05 mmol L–1 for Cu2+ and Fe3+ and 0.01–0.12 mmol L–1 for DNB) but are of much higher affinities due to the inner-sphere complexation. Compared with Cu2+, Fe3+ is expected to sorb at the hydroxyl forms with lower binding affinities to bacterial surfaces (Xiao et al., 2007) and thus is less effective to inhibit DNB sorption. The strong inhibitive effect on DNB sorption by coexisting transition metals at low levels also disfavors the mechanism of direct complexation of the nitro group with earth or alkaline cations retained on bacterial surfaces.

Effects of pH
The result of pH effect on DNB sorption to E. coli, plotted as q vs. pH, is shown in Fig. 4 . Our previous studies demonstrated that sorption of PHEN and TeCB to E. coli increases as pH decreases mainly due to the enhanced hydrophobicity of bacterial surfaces with protonation of the ionized groups (i.e., transition from the –COO– group to the –COOH group) (Xiao et al., 2007). In this study, sorption of DNB to E. coli increases with deprotonation of the O-containing groups as pH increases. From pH 3.7 to 5.9, Kd increases from 270 ± 20 L kg–1 to 1200 ± 100 L kg–1 (SDs calculated from triplicates). To our knowledge, similar results have not been reported for biosorption of nonionic HOCs or for sorption of HOCs to natural soils and sediments and humic substances. Moreover, such a pH effect on DNB sorption cannot be attributed to speciation (e.g., protonation) or hydrolysis reactions of solute because sorption of DNB to polyethylene beads (Aldrich), a sorbent dictated by inert methylene structures, keeps nearly constant within a similar pH range between 3.3 and 6.4 (data not shown). The observed pH effect on biosorption of DNB also rules out hydrophobic effects and H-bonding (between–NO2 and H-donor groups) as major sorption driving forces because they are both favored by protonation of functional groups. The results strongly suggest that n–{pi} EDA interactions dominate DNB sorption. As more O-containing groups become deprotonated with increasing pH, the bacterial surface becomes more negatively charged, providing more electron-donor sites for EDA interactions.


Figure 4
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  		Fig. 4. Equilibrium sorbed concentration (q) vs. pH for sorption of 1,3-dinitrobenzene (spiked at 0.086 mmol L–1) to E. coli in 0.1 mol L–1 NaCl. Bidirectional error bars represent SDs calculated from three replicates.

 
Environmental Implications
This study demonstrated the extraordinarily strong binding affinities of NACs to bacterial surfaces. In addition to the well known smecite clays, bacteria may be another type of naturally occurring adsorbents that play an important role in the environmental fate of NACs. Because of their ubiquitous existence and great mobility, bacteria may be effective carriers to greatly accelerate the transport of NACs in the surface and subsurface environments. The high sorption affinities and capacities of NACs by bacteria may find promising applications in remediation of NAC contamination. The antagonism pH effects on biosorption between NACs and other HOCs can be explored in practice by selectively sorbing/desorbing NACs (i.e., apply an alkaline condition to favor the sorption process while applying an acidic condition to favor the desorption process). The presence of transition metals (Cu2+ and Fe3+) at trace levels (a few milligrams per liter or less) was found to dramatically inhibit NAC biosorption. The results indicate that identification and quantification of the transition metals within biosorbents is a prerequisite to better monitor the biosorption of NACs. Furthermore, the presence of other environmentally important metal ions (e.g., Al3+) and positively charged mineral surfaces (e.g., aluminum and iron oxides/hydroxides at pHs below their points of zero charge) with strong electron coupling abilities would likely affect the biosorption of NACs. This is an area that deserves more research.


   ACKNOWLEDGMENTS
 
This work was supported by the China National Science Foundation (Grant 20637030 and Grant 20647002). We thank Mrs. Lina Yuan for assisting with the scanning electron microscopy analysis.


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