ARTICLE pubs.acs.org/est
Novel System for Controlled Investigation of Environmental Partitioning of Hydrophobic Compounds in Water Luca Nizzetto,*,† Rosalinda Gioia,‡ Claire L. Galea,‡ Jordi Dachs,§ and Kevin C. Jones‡ †
Norwegian Institute for Water Research, Gaustadalleen 21, NO-0349, Oslo, Norway Centre of Chemicals Management, Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ , U.K. § Department of Environmental Chemistry, IDAEA-CSIC, Jordi Girona 18-24 barcelona 08034, Catalunya, Spain ‡
bS Supporting Information ABSTRACT: Partitioning behavior of hydrophobic and semivolatile chemicals (such as many POPs and PAHs) in water is key in controlling their environmental distribution and fate. A new equilibrium method is presented here which allows determination of the equilibrium partition coefficient of hexachlorobenzene with suspended particle (KSPM≈ 337 L gOC1) in a complex bulk water sample by correcting for a number of sampling artifacts and for the presence of dissolved matter. The method provides simultaneous experimental determination of the fraction of chemical truly dissolved in water (representing in this case about 54% of the bulk water concentration) and that associated to DM (21%). The Henry’s law constant was also experimentally determined during the KSPM measurements, providing information on the occurrence of partitioning equilibrium in the system for each single observation. Results showed that the high level of quality control and accuracy provided confidence intervals for the KSPM estimates within 1 order of magnitude.
’ INTRODUCTION A wide range of chemicals classified as priority environmental pollutants are hydrophobic and semivolatile organic compounds. Examples are many of the persistent organic pollutants (POPs) and substances such as polycyclic aromatic hydrocarbons (PAHs), with demonstrated potential for environmental multiphase partitioning, including bioconcentration. The processes controlling the fate of these chemicals in aquatic ecosystems are fundamental for determining their overall environmental distribution and exposure of biota. Hydrophobicity promotes partitioning with particulate1,2 and dissolved r 2011 American Chemical Society
organic matter3 present in the water column. Partitioning, directly or indirectly, has implications for key fate processes, such as the degradation rate,4 the rate of sedimentation,5,6 leaching and runoff,7 and bioavailability in the water column and sediments.8,9 Partition coefficients, defined as the ratio between the concentration of the chemical in suspended Received: May 26, 2011 Accepted: August 8, 2011 Revised: August 3, 2011 Published: August 08, 2011 7834
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Environmental Science & Technology particulate matter (SPM) or dissolved matter (DM) and that in the truly dissolved (TD) phase, at equilibrium, are fundamental metrics for the assessment and prediction of exposure. Additionally, partitioning into living biomass under steady state conditions can be described by the bioconcentration factor (BCF). This is a useful metric for determining concentrations in organisms at the base of the aquatic food chain, often used in regulatory bioaccumulation assessments.10 Obtaining reliable measurements of these coefficients is technically challenging for hydrophobic chemicals. In the laboratory there are difficulties associated with partitioning of the test chemical to the test system materials. For example, partitioning of the chemical can occur between the TD phase and the filter or filter holder surfaces,11 or flask surfaces, potentially generating significant artifacts in the measurements. The presence of dissolved organic matter (naturally occurring in field samples or derived from cell exudates or culture media), can also significantly affect the measurements.12 These effects cannot be easily quantified without adequate quality control measures. Arnot and Gobas13 have reviewed a large number of reports on BCF assessment and found that about 45% of them did not fulfill at least one of the quality control criteria. Here we present and test a novel benchtop experimental system (later referred to as a Multi Media Test Chamber, MMTC), in which simultaneous determination of chemical concentrations in TD, DM, SPM phases and the head space gas phase (GP) can be performed in a closed and controlled system. The MMTC is fully portable and can be used in the field. The approach, based on “single equilibrium” multimedia partitioning was designed to allow determination of equilibrium conditions, correction for major sampling artifacts, and mass balance based intrinsic control on the quality of data. This study is presented in two parts: experiment 1, conducted to demonstrate the suitability of the method by determining the dimensionless air water equilibrium partition coefficient (KAW) of HCB for Milli-Q, synthetic freshwater and seawater, and natural seawater in laboratory and field conditions; and experiment 2, conducted to assess the suitability of the test system for determining SPM/ water partition coefficients in a complex bulk water sample.
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Figure 1. MMTC design and concept.
multimedia single equilibrium partitioning. At the end of the incubation time, samples of the different media in the microcosm are collected and the concentrations in the probe (CP, μg L1), gas phase (C G , μg L 1 ), and bulk phase (C Bulk , μg L 1 ) are determined. At this stage, assuming that the chemical activity in the probe and the truly dissolved phase of the water sample are equivalent, it is: CP ¼ CTD
where CTD (μg L ) is the concentration in the truly dissolved phase of the bulk sample. This relationship stands if (i) the thermodynamic activities of the chemical in the freely dissolved solute fraction, the reversibly bound solute of suspended or dissolved fractions and the fraction in the TD phase are identical at a given temperature;4 and (ii) the volatile exchangeable fraction of the natural dissolved organic matter in the bulk phase has a negligible capacity for adsorbing the test chemicals. Based on these assumptions the following coefficients can be determined:
’ EXPERIMENTAL SECTION MMTC Description and Principles. The microcosms used in this study consist of 2-L glass bottles with valved polytetrafluoroethylene (PTFE) screw caps (Figure 1). The caps have a small metal clamp holding a 25-mL glass vial positioned in the head space. This vial, hereafter called the “probe”, is filled with a phase which is equivalent to the inorganic truly dissolved fraction of the water sample placed in the bottom of the bottle (in this study, pure seawater, fresh water, or Milli-Q water, depending on the test to be performed). Test chemicals are added to the MMTC before the sample is introduced, using a carrier solvent (e.g., toluene) with a vapor pressure several orders of magnitude higher than the chemical. After the solvent is evaporated, the sample (e.g., bulk water including suspended particles and dissolved organic matter) is introduced in the system. During the incubation, the chemical undergoes partitioning between the different media (e.g., glasswarewaterdissolved and suspended matter gas phase); for example it may degas through the water/headspace interface, and from the gas phase into the probe medium. This continues until a steady state is reached. Assuming no degradation or other losses of the chemical, this state corresponds to the
ð1Þ 1
CG ¼ KAW CTD
ð2Þ
CBulk CTD ¼ KBW CTD
ð3Þ
where KBW is the dimensionless equilibrium partition coefficient between the bulk phase dissolved or suspended in the water sample (e.g., suspended particulate + dissolved organic matter) and the TD phase in water. The experimental value of KAW (obtained for each individual test) can be compared with separate measurements performed using the pure water as the bulk phase and the probe, to ensure equilibrium partitioning is achieved between the probe and the GP and to validate eq 1. The occurrence of equilibrium between the probe and gas phase provides information on the state of equilibrium partitioning for the whole system. This assumes that the equilibration time between the probe and the GP is longer than the glasswaresurface water and the waterair exchange, as supported by previous results.12,14 Additionally, assuming that the concentration of the chemical associated with the SPM (CSPM) can be reliably measured through filtration or other techniques, it is possible to solve the 7835
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mass balance of the chemical in the bulk water phase and determine the concentration associated only with the DM as follows: CDM ¼ CBulk CSPM CTD
ð4Þ
MMTC Preparation. All glassware employed in this study was thoroughly solvent cleaned and rinsed with Milli-Q water before use. HCB was chosen as the test chemical for this study given its hydrophobic, semivolatile, and persistent character.15 To reduce analytical effort and improve precision, radioisotope techniques were employed. 14C12HCB (Institute of Isotopes Co., Ltd., Budapest, Hungary) was used as the only source of HCB. The chemical and radiochemical purity were >95%. The specific radioactivity was 2.58 KBq/μg. For the air/water partitioning test (experiment 1) about 38 000 Bq of 14C12HCB (corresponding to 13.7 μg) was dissolved in 2 mL of toluene and introduced into a 1-L Duran glass bottle. The bottle was kept open and gently swirled to let the solvent evaporate while distributing the chemical uniformly on the inner glass wall. After the solvent completely evaporated, 1 L of Milli-Q water was added. The bottle was sealed with a cap carrying a PTFE insert and the solution was stirred through the experiments using a glass-coated stirring bar. This process generates a saturated solution of the 14C12HCB,18 hereafter called the stock solution. Aliquots of the stock solution were used to spike the water sample in the MMTC. The spiking method described here provided levels of 14 C12HCB in the test chamber considerably below saturation and prevented the presence of significant residues of the organic solvent in the microcosm during measurements. This allowed the fulfilment of fundamental quality criteria for BCF determination.13 Experiment 1: AirWater Partitioning Test. This test was performed under a range of temperatures and salinities, both in laboratory (Experiment 1a) and in controlled field conditions (Experiment 1b). The rationale for this was (i) to demonstrate the reliability of the method and robustness of assumptions under a simplified scenario, and (ii) to provide information on KAW under the conditions set for the later partitioning experiments on a complex water matrix. In the laboratory experiment (Experiment 1a), NaCl was added to Milli-Q water to obtain salinities similar to those of fresh water (0.5 g/L) and seawater (35 g/L). The microcosms were spiked with 125 mL of the stock solution and diluted with Milli-Q water, pseudo-fresh-water or pseudo-seawater, to reach the final volume of 1 L. Five mL of the respective water phases water were used as probe. Observations were performed in triplicate. The microcosms were kept in a thermostatic chamber on an orbital shaker at 100 rpm for 72 h before sampling. Observations were performed at a range of temperatures between 30 and 0.5 °C. For the controlled field test (Experiment 1b), the same set up as for the experiment 1a was used. Samples of surface seawater were collected on board the RV Hesperides in JanuaryMarch 2009 from 10 stations in proximity to the Antarctic Peninsula, using Niskin bottles. Water samples were prefiltered through a 0.5-μm screen and successively processed through tangential ultrafiltration using two Vivaflow 200 (Sartorius Stedim Biotech) units assembled in parallel and equipped with membranes with a cut off of 5000 Da, to eliminate most of the colloids, viruses, and small bacteria. Microcosms were prepared as described for the laboratory experiments. Incubation was carried out in a pool
located on the ship main deck constantly flushed with seawater. Therefore, equilibration occurred at the same temperatures measured in surface water. A net was used to cover the pool to reproduce radiation conditions similar to those for 15 m depth in the ocean. A total of 10 observations were carried out. Experiment 2: ParticleWater Partitioning Test. The applicability of the method to measure partitioning of 14C12HCB in complex water samples, e.g., in presence of suspended and dissolved organic matter, was tested. For this test, each microcosm bottle was spiked directly with 10 000 Bq of 14C12HCB (corresponding to 3.88 μg), dissolved in 1 mL of dichloromethane. After complete evaporation of the solvent, 700 mL of seawater previously filtered through a 0.45-μm polycarbonate membrane was added to each microcosm. Samples of SPM were collected and concentrated in the middle part of Køngsfjord, Svalbard, Arctic (78.93 N, 11.93 E) using a phytoplankton net with a 25-μm nylon mesh. Samples were immediately transferred to the Kings Bay AS marine and gently screened through a 95-μm laboratory in Ny-alysund nylon mesh, to isolate the 2595 μm fraction of the total SPM. An aliquot of this fraction was transferred into the triplicate microcosms previously spiked (as described above). The samples were then diluted to a final volume of 1 L using the filtered seawater. The final particle concentration in the microcosms was quantified based on the particulate organic carbon (POC) concentration, which was 1.08 ( 0.16 mg OC/L (error represents 2 SD). The probe solution consisted of 5 mL of the same water used as sample but previously processed through tangential filtration as described earlier. Incubation was carried out over 96 h at 0 °C, before the first sampling occurred. Sampling from the Microcosm. The gas phase in the microcosm head space was sampled using a Sep-Pak Light C-18 cartridge (Waters, Milford, MA, USA), connected through a PTFE luer lock adaptor to one of the valves of the screw cap. Gas phase was sampled (after venting the second valve of the cap) using a small pump running at a calibrated flow rate of 60 ( 2 mL min1 at atmospheric pressure. Total sampled volume was 600 ( 20 mL. Immediately after sampling, the cartridges were eluted 3 times with 2 mL of dichloromethane directly into 25-mL scintillation vials and mixed with scintillation liquid (SL) (Ultima Gold XR). The last eluted fraction was measured separately to ensure complete recovery of the chemical from the sorbent. Scintillation counting was performed over 10 min, using a Canberra Packard Tri Carb 2300 TR, UK, and standard calibration and quench correction techniques. After sampling of the head space was carried out, the microcosms were opened and the probe was recovered. SL was added directly into the probe vial for counting. Collection of the sample from the microcosm was carried out using a class A graduated glass pipet (previously rinsed with acetone and Milli-Q water). Five mL of the sample was collected, transferred to a scintillation vial, and mixed with SL for counting. In experiment 2, 20-mL samples were collected through a glass syringe and filtered through a 13-mm diameter glass fiber filter (GFF) (nominal pore size was 0.6 μm). Syringe filtration was performed gently using a stainless steel filter holder assembled with two PTFE O-rings. The last 5 mL of the filtrated liquid was transferred to a scintillation vial to count the radioactivity in the total dissolved fraction to obtain the value of the filtrate concentration (CF, ng mL1). After filtration, GFFs were carefully removed from the filter holder and transferred to a scintillation vial for the measurements of the particle bound 7836
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fraction. Sampling of SPM was repeated 3 times at daily intervals from each microcosm. Characterization and Correction of Filtration Artifacts. Adsorption on GFFs and filter holder wet surfaces can represent a considerable source of artifacts, especially because PTFE O-rings have a non-negligible sorptive capacity for hydrophobic chemicals in the truly dissolved phase.16 This was controlled by performing an adsorption test on a set of 6 replicates under the same temperature conditions used for experiment 2. The test was performed by sampling from a positive control (e.g., a test performed with no addition of SPM) using the same procedure and materials as for the real samples. Filter adsorption was defined in terms of the ratio (KGFF-W, L) between the mass of the chemical adsorbed on the filter membrane (after filtering the volume (VS = 20 mL) of the water sample) and the TD phase concentration, calculated as follows: KGFFW ¼
AGFFC CP
ð5Þ
where AGFFC is the mass μg) of the chemical measured on the filter surfaces. The concentration of 14C12HCB associated with suspended particles (CSPM, μg L1) in experiment 2 was therefore calculated by correcting for filter adsorption effects as follows: CSPM ¼
ðAGFFS ðCP 3 KGFFW ÞÞ VS
ð6Þ
Quality Assurance/Quality Control. All media and reagents were measured for background β decay emissions. These values were used for blank correction. The effects of sorption of HCB on glassware surfaces on water concentrations was monitored and the results are reported in text SI1 in the Supporting Information. Equilibration time between the glass surfaces and the water phase was
