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Direct Hydrodynamic Radius Measurement on Dissolved Organic Matter in Natural Waters Using Diffusion NMR Gang Zheng* and William S. Price Nanoscale Organization and Dynamics Group, School of Science and Health, University of Western Sydney, Penrith NSW 2751, Australia ABSTRACT: Dissolved organic matter from natural waters is a complex mixture of various chemical components, which play vital roles in many environmental processes such as the global carbon cycle and the fate of many key anthropogenic pollutants. Despite its environmental significance, dissolved organic matter in natural form has never been studied using nuclear magnetic resonance based hydrodynamic radius measurements due to its extremely low concentration (e.g., a few mg/L) in natural waters. In this study, NMR-based hydrodynamic radius measurements were performed directly on unconcentrated pond, river, and sea waters. The key chemical components of the dissolved organic matters from different sources were identified as carbohydrates, carboxyl-rich alicyclic molecules, and aliphatic molecules. By using the Stokes−Einstein−Sutherland equation, the average hydrodynamic radii of the three key components were calculated.

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INTRODUCTION Dissolved organic matter (DOM) from natural waters1−3 is a complex mixture of various chemical components, which play vital roles in many environmental processes such as the global carbon cycle4 and the fate of many key environmental pollutants5−10 (e.g., polychlorinated dibenzodioxins (PCDDs) and polybrominated diphenyl ethers (PBDEs)). The effective hydrodynamic radius (i.e., Stokes radius, rs) of DOM directly affects many DOM-related environmental processes such as the fate of key anthropogenic pollutants11 and light absorption in natural waters.12 Despite the environmental significance of the rs of DOM, there has never been a well developed technique that can provide accurate hydrodynamic and chemical information of DOM in natural form (e.g., unconcentrated natural waters) at the same time so that the rs of each key DOM component can be studied in detail. This is mainly due to the extremely low concentration (e.g., a few mg/L) of DOM in natural waters although a few techniques (e.g., ultrafiltration,13−17 dynamic light scattering (DLS),12 and highperformance size exclusion chromatography (HP-SEC))18−21 have been intensively applied to unconcentrated natural waters. Neither ultrafiltration nor DLS can provide chemically specific hydrodynamic information of DOM (i.e., rs of specific chemical component of DOM) and for HP-SEC the natural chemical structure and oligomeric state of DOM can be easily changed by the use of eluent (e.g, sodium acetate solution) other than pure water and the calibration of the HP-SEC column is also difficult due to the largely unknown chemical structure of DOM. PGSTE-WATERGATE22,23 is the method of choice for nuclear magnetic resonance (NMR) based rs measurements (e.g., pulsed gradient spin−echo (PGSE) translational (or self-) diffusion experiments24−30) on extremely dilute aqueous © 2012 American Chemical Society

samples (e.g., unconcentrated natural waters) owing to its noninvasiveness (i.e., solutes being kept in their natural form), information-richness (i.e., providing chemical profiles of solutes) and, more importantly, the highly efficient solvent (i.e., water) signal suppression31−37 it provides. Water signal suppression is vital for dealing with extremely dilute aqueous samples because the gigantic water signal makes observation of weak solute signals impossible by both overlapping such signals and preventing their adequate digitization due to the low preamplification required to prevent saturation of the NMR receiver by the water signal. In this study, rs measurements were performed directly on the DOM in unconcentrated natural waters by the use of a modified PGSTE-WATERGATE method (Figure 1), and the rs values of the major DOM components from pond, river, and sea waters were obtained.

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MATERIALS AND METHODS Sample Collection and Preparation. Natural waters were collected from a pond (University of Western Sydney, Campbelltown, New South Wales, Australia) in March 2011, the Georges River (East Hills, New South Wales, Australia) in December 2011, and the Tasman Sea (Coogee, New South Wales, Australia) in November 2011. The dissolved organic carbon (DOC) concentrations of the pond, river, and sea waters were measured as 8, 3, and 1 mg/L, respectively, by the use of an Analytik Jena Multi N/C 3100 total organic carbon

Received: Revised: Accepted: Published: 1675

August 24, 2011 January 2, 2012 January 3, 2012 January 3, 2012 dx.doi.org/10.1021/es202809e | Environ. Sci. Technol. 2012, 46, 1675−1680

Environmental Science & Technology

Article

acquisition parameters were as follows: spectral width 10 kHz; free induction decay (FID) being digitized into 9 k data points; π/2 pulse length 16.75 μs; NS = 2048; number-of-dummyscans = 128; recycle-delay = 1.7 s. Diffusion measurements were performed by linearly incrementing the gradient strength (g1) from 0.25 to 0.54 T m−1 with g2 = 0.24 T m−1, a gradient length (δ) of 0.003 s, and a diffusion time (Δ) of 0.03 s. Maple 15 (Maplesoft) was used for the Stejskal and Tanner analysis25 of the modified PGSTE-WATERGATE method and thus the derivation of eq 1. OriginPro 8 (OriginLab) was used for all diffusion data analysis.

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Figure 1. The modified PGSTE-WATERGATE method. π/2 Pulses and selective π pulses are represented by bars and bar groupings, respectively; g1 (with a duration of δ), g2, and g3 are pulsed magnetic field gradients; the gradient-filled, black, and gray rectangles represent diffusion weighting, spoiler, and compensating gradients, respectively; τ1, τ2, δ1, and δ2 are delays. The gray g1, g2, and g3 pulsed gradients applied before the first π/2 pulse were utilized to cancel the dephasing effects on the NMR lock signal (i.e., deuterium resonance from D2O) of the gradient filled g1 and g2 gradients in the first 2τ1 period and the black g3 gradient in the τ2 period. The gray g1 and g2 gradients right before the third π/2 pulse were utilized to cancel the dephasing effects of the gradient filled g1 and g2 gradients in the second 2τ1 period. By the use of lock-balancing pulsed gradients, the deuterium lock resonance was kept strong and therefore the drifting of resonances due to poor field-frequency locking was minimized and thus a higher signal-to-noise ratio was achieved.

THEORETICAL BASIS Modified PGSTE-WATERGATE. By the use of the modified PGSTE-WATERGATE method, the water signal was manipulated to see a much higher diffusion-based attenuation factor than the DOM signals, and thus be suppressed. The diffusionbased signal attenuation (E) for the new method can be written as22 ⎧⎡ ⎛ ⎞ 4 E = exp⎨⎢ − γ2δ2⎜Δ − δ − 2δ2⎟(g1 − g2)2 ⎝ ⎠ 3 ⎩⎣ ⎤ ⎫ − b′⎥D⎬ ⎦ ⎭

(1)

where γ is the gyromagnetic ratio, b′ stands for the attenuation factor which can be ignored when Δ ≫ δ, δ1, and δ2, D is the self-diffusion coefficient, and all the other parameters are defined in Figure 1. Stokes−Einstein-Sutherland Equation. The self-diffusion coefficient and rs of molecules or colloids can be related by the Stokes−Einstein−Sutherland equation38−40

analyzer (Analytik Jena, Germany). The sampled waters were filtered using a 0.2-μm cellulose acetate filter. Filtered/ unconcentrated pond water (0.36 mL) and 0.04 mL of D2O were dispensed into a magnetic susceptibility-matched (to D2O) NMR tube (Shigemi). Filtered river water (0.54 mL) or seawater (0.54 mL) and 0.06 mL of D2O were dispensed into a normal NMR tube (Wilmad). Pond water was resampled in November 2011. The sampled water was filtered using a 0.2-μm cellulose acetate filter. Fifteen mL of filtered pond water was freeze-dried and the product was (partly) dissolved in 1 mL of Milli-Q water resulting in concentrated DOM solution. The concentrated DOM solution (0.54 mL) and 0.06 mL of D2O was dispensed into a normal NMR tube (Wilmad). 1 H PGSE NMR Analysis. For the filtered/unconcentrated water samples, 1H PGSE NMR diffusion experiments were performed on a Bruker Avance 500 spectrometer at 500 MHz using a TXI high-resolution probe equipped with a gradient coil at 25 °C by the use of the modified PGSTE-WATERGATE method (Figure 1). Typical acquisition parameters were as follows: spectral width 10 kHz; free induction decay (FID) being digitized into 9 k data points; π/2 pulse lengths 8.15 μs (pond water), 9.75 μs (river water), and 13.25 μs (seawater); number-of-scans (NS) = 16 384 (pond water), 32 768 (river water), and 76 800 (seawater); number-of-dummy-scans = 128; recycle-delay = 1.7 s. Diffusion measurements were performed by linearly incrementing the gradient strength (g1) from 0.25 to 0.58 T m−1 with g2 = 0.21 T m−1, a gradient length (δ) of 0.003 s, and a diffusion time (Δ) of 0.03 s. Please refer to refs 22 and 23 for detailed information on setting up PGSTE-WATERGATE experiments. For the concentrated pond DOM solution, 1H PGSE NMR diffusion experiments were performed on a Bruker Avance 400 spectrometer at 400 MHz using a BBO high-resolution probe equipped with a gradient coil at 25 °C by the use of the modified PGSTE-WATERGATE method (Figure 1). Typical

D=

kT f

(2)

where k is the Boltzmann constant, T is temperature, and f is the friction coefficient. Although the friction coefficient takes different forms depending on the shape of the diffusing molecule or colloid,41 for a spherical molecule or colloid with a Stokes radius rs, which normally increases with the increase of molecular weight (Mw), in a medium of viscosity η, the friction factor is given by

f = χπηrs

(3)

where χ = 4 (“slip” (boundary) condition) or 6 (“stick” (boundary) condition). In the slip condition, it is assumed that there is no interaction between the molecules or colloids of interest and the solvent molecules; in the stick condition, it is assumed that there are strong interactions between the molecules or colloids of interest and the solvent molecules so that the solvent layer closest to the molecular or colloidal surface moves at the same velocity as the molecule or colloid. Depending on whether the slip or stick boundary condition is assumed the Stokes radius calculated can differ considerably.

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RESULTS AND DISCUSSION Simpson and co-workers are pioneers in the application of water signal suppression NMR to the DOM in unconcentrated natural waters36 and they have also performed NMR-based rs measurements on DOM solutions,42 which were prepared from freeze-dried DOM, but not on unconcentrated natural waters. 1676

dx.doi.org/10.1021/es202809e | Environ. Sci. Technol. 2012, 46, 1675−1680


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Figure 2. (a) 1H 500 MHz spectrum of the unconcentrated pond water without water signal suppression (the spectrum is dominated by the huge water signal and the DOM signals are not visible). (b) 1H 500 MHz spectra of the unconcentrated pond water obtained using the modified PGSTEWATERGATE method at 25 °C with g2 = 0.21 T m−1, δ = 0.003 s, Δ = 0.03 s, NS = 16384, and different g1 values. The DOM signals are clearly visible and the water signal is highly suppressed.

Figure 3. 1H 500 MHz spectra of the unconcentrated pond water (a), river water (b), and seawater (c), with g1 = 0.25 T m−1, g2 = 0.21 T m−1, δ = 0.003 s, Δ = 0.03 s. 1H 400 MHz spectrum of the concentrated pond DOM solution (d) with g1 = 0.21 T m−1, g2 = 0.18 T m−1, δ = 0.003 s, Δ = 0.03 s. All the spectra were obtained using the modified PGSTE-WATERGATE method at 25 °C.

Here, NMR-based rs experiments were performed directly on the unconcentrated natural waters providing the hydrodynamic information of the DOM in natural form. Suppression of the dominant water signal is the most challenging task in the PGSE NMR (i.e., NMR-based rs) experiments directly on unconcentrated natural water samples due to the extremely high concentration of water (e.g., 9.9 × 105 mg L−1) compared to DOM. Most of the current water suppression PGSE NMR experiments have been designed for biological samples with relatively high concentrations (e.g., 1000 mg L−1); however, the DOM concentration in natural waters can be as low as a few mg L−1 (e.g., ref 36). Therefore, even after the application of these water suppression techniques, the observation of DOM signals can still be significantly hampered by the residual water signal, which interferes with the DOM signals close to water signal and also makes it impossible to set a sufficiently high NMR receiver gain for the proper amplification of the weak DOM signals. The modified PGSTE-WATERGATE (Figure 1) method contains lock-balancing gradient pulses (represented by gray rectangles) (e.g., ref 43) for the enhancement of signal-to-noise ratio. As shown in Figure 2, the DOM signals are clearly visible owing to the exceptional water suppression performance of the

modified PGSTE-WATERGATE method and the water signal has been totally removed when g1 = 0.30 T m−1, suggesting the modified PGSTE-WATERGATE method is suitable for studying the chemical profile of DOM with g1 = 0.30 T m−1, g2 = 0.21 T m−1, δ = 0.003 s, and Δ = 0.03 s. The major DOM components have been identified as aliphatic compounds, carboxyl-rich alicyclic molecules (CRAMS), and carbohydrates on the basis of their chemical shifts. Aromatic/amide resonances were observed for DOMs from different sources;44−47 however, no aromatic/amide resonance was visible in Figure 2b due to the low DOM concentration and thus the poor signal-to-noise ratio. The 1H water-suppressed spectra of the filtered/unconcentrated pond, river, and sea waters and concentrated pond DOM solution are compared in Figure 3. The aromatic/amide resonances (6.5−8.1 ppm) are clearly visible in the spectrum of the concentrated pond DOM solution while no aromatic/ amide resonance is visible in the spectra of filtered/ unconcentrated waters due to the extremely low concentration of DOM. Although the concentrations of DOM in pond, river, and sea waters differ significantly from each other as indicated by different DOCs and NSs, the spectra (Figure 3a, b, and c) share a common pattern indicating that aliphatic compounds, 1677



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Table 1. Average Diffusion Coefficients (D, × 10−10 m2 s−1) and Hydrodynamic Radii (rs, × 10−10 m) of the Major DOM Componentsa concentrated pond DOM solution

pond water

river water

sea water

D

rs

D

rs

D

rs

D

rs

carbohydrates

4.5 ± 0.1

4.0 ± 0.4

Aliphatic Compounds

5.3 ± 0.1

5.1 (stick) 7.7 (slip) 4.7 (stick) 7.1 (slip) 5.1 (stick) 7.7 (slip)

4−10

5.9 ± 0.1


4.8 ± 0.4

CRAMS


2−6 (stick) 4−9 (slip) 4−8 (stick) 5−12 (slip) 5−24 (stick) 7−37 (slip)

major DOM component

4.3 ± 0.1 4.3 ± 0.2

5.2 ± 0.1 4.8 ± 0.1

3−7 1−5

The average hydrodynamic radius was calculated for both “stick” and “slip” conditions. For sea water, only the ranges for D and rs are given due to the extremely low concentration of DOM. a

Figure 4. Fitting eq 1 to the integrals of the carbohydrate (a), CRAMS (b), and aliphatic (c) peaks in Figure 2.

filtered/unconcentrated pond water, which highlighted the importance of the study of hydrodynamics of DOM in filtered/ unconcentrated natural waters. The average rs values of major DOM components (Table 1) were calculated using eqs 2 and 3. In the pond water, carbohydrates have a larger average rs than CRAMS and aliphatic compounds, which is in line with previous findings.21,42 With this initial application of diffusion NMR to the DOM in unconcentrated natural waters, it has been shown that the modified PGSTE-WATERGATE method is a powerful tool for studying the chemically specific hydrodynamics of environmentally important molecules and colloids due to its noninvasiveness and information-richness. The self-association of DOM and the interactions between DOM and persistent organic pollutants (e.g., PBDEs) will be studied using the proposed technique in future works.

carboxyl-rich alicyclic molecules (CRAMS), and carbohydrates are the major DOM components for not only pond water but also river and sea waters. Because each major DOM component was composed of molecules and colloids with different hydrodynamic radii, only an average diffusion coefficient was able to be obtained for each DOM component (Table 1) by fitting eq 1, a monoexponential function, to the diffusion attenuation data (i.e., integrals) of each DOM component peak at different g1 gradient strengths. The diffusion attenuation data of the filtered/unconcentrated pond water sample (Figure 4) and the river water sample (data not shown) were well-described by eq 1, however, the fit to the seawater sample was poor due to the extremely low signal-tonoise ratio (data not shown). Due to the limited signal-to-noise ratio in all spectra, it was not possible to differentiate between monoexponential decay and multiexponential decay that closely approximates monoexponential decay. It was reported that carbohydrates, CRAMS, and aliphatic compounds were the three major DOM components in natural waters; 36 however, it was never clear whether these components were present as a complex or as nonassociated species. The PGSTE-WATERGATE measurements revealed that carbohydrates, CRAMS, aliphatic compounds had distinctly different diffusion coefficients (Table 1) in pond water, indicating that they mainly occurred as three nonassociated species. In the river water, CRAMS and aliphatic compounds had distinctly different diffusion coefficients indicating they were not associated to each other; for the seawater, only the possible diffusion coefficient ranges are given due to the extremely low signal-to-noise ratio. In the concentrated pond DOM solution, the three major DOM components had similar diffusion coefficients indicating possible associations among the components. DOM aggregation was also clearly indicated by the smaller diffusion coefficients compared to those of the DOM components in

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AUTHOR INFORMATION

Corresponding Author

*Phone: +61 2 4620 3729; fax: +61 2 4620 3025; e-mail: g. [email protected].

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ACKNOWLEDGMENTS Mr. Scott Willis is thanked for valuable discussions on the calculation of hydrodynamic radius. Prof. Janice Aldrich-Wright, Mr. Benjamin Harper, and Mr. Ben Garbutcheon-Singh are thanked for their help with the utilization of freeze-dryer. The University of Western Sydney is thanked for financial support.

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REFERENCES

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