Systematic Evaluation of Dissolved Metals Loss During Water Sample Filtration

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INNOVATIVE RESEARCH FOR A SUSTAINABLE FUTURE
A Systematic Evaluation of Dissolved Metals Loss During Water Sample Filtration
Risk Management Research Project - Addressing Challenges through Science and Innovation
This research study is examining the impact of water quality and filtration approach on
the loss of soluble lead and copper on filtration devices, including commonly used
capsule and disc syringe filters. A variety of commercially available filter materials are
being tested, including polyvinylidene fluoride, polytetrafluoroethylene, nylon,
polypropylene, and mixed cellulose acetate. Only filters with a pore size of 0.45 u.m are
being examined. The importance of water quality parameters including pH, alkalinity,
and phosphate on sorption losses is being examined. The impact of filtration approach,
including flow rate and sequential sampling, is also being explored.

Background
EPA method 16691 provides specifications for the collection and filtration of water
samples prior to the analysis of dissolved and particulate trace elements and metals in
surface waters. Filtration apparatus specifications include the use of a disposable,
tortuous path, capsule and disc filters with an effective pore size of 0.45 u.m and a
diameter of 15 mm or larger.
Similar filtration approaches are commonly used to separate soluble and particulate constituents in drinking water samples,
bench- and pilot-scale water treatment studies, and fundamental drinking water research studies. Syringe filters are
typically used in such cases; given the ease of utilization. The State of Texas's environmental regulatory enforcement
agency noted differences in dissolved metal levels in water samples when different filter types were employed for
particulate filtration in the field. Specifically, discrepancies were found in dissolved metal fractions between those samples
filtered using a low-volume syringe filter and those filtered through an in-line capsulated cartridge filter. Similar
inconsistencies have been noted by EPA researchers performing filtration separations during fundamental drinking water
research investigations. In the case of Texas, many reported considerably higher lead and aluminum values for water
samples filtered using syringe filters, than those reported when utilizing in-line capsule filters; often within similar water
types found in similar geographic areas. This action can confound a state's ability to confirm the relative quality of its data
and the subsequent impairment status of its waters. The findings have clear implications on the validity of dissolved metals
measurements reported by a state's 303(d)2 lists, and the reliability of research data reported to have used similar filtration
procedures.
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Research Approach
This study is being carried out using EPA method 16691 by preparing
aqueous solutions containing a combination of copper and lead or
solutions of the individual metals. The solutions contain deionized water
adjusted to a specific pH (7) and dissolved inorganic carbon concentration
(50 mg/L). Analyte concentrations are prepared to levels conservatively
below the theoretical solubility to ensure that no particles are formed. The
water is then passed through a filter in a series of sequential volumes at a
uniform flow rate of 50 mL/min by a syringe pump. Each filter (Table 1) is
used for seven volumes of 15-20 ml and then discarded. Table 2
summarizes the conditions and number of the experimental test runs.
Office of Research and Development
Risk Management Research

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Research Goals
1. Based on EPA method 16691, determine if the type of filter material used to separate soluble from particulate fractions
creates significant losses of dissolved metals (lead and copper) in water during filtration due to adsorption losses.
2. Evaluate the impact of water chemistry and other factors on sorption losses during water filtration.
3. Identify the best filtration material and protocol for separating soluble and particulate metals in water samples.
Results to Date
To date, results have shown a bias associated with various
types of dissolved metals and filters. Figure 1 illustrates a
significant loss in lead concentration for a glass
fiber/nylon/glass microfiber syringe filter; trials averaged a loss
of 99.6% of lead concentrations. Cellulose ester syringe filters
show losses in lead concentrations between 5.8% and 7.6%
(Figure 2) which is within the uncertainty of the analytical lead
determinations. Some of the filter results, such as the
polytetrafluoroethylene syringe filter (Figure 3), showed initial
losses of nearly 30% after the first 15 ml of solution was
pushed through. However, as the volume increased, the
average loss of lead leveled out to 10.8%.
Table 2. Number of Runs for Lead and Copper
Table 1. Types of Filters Examined
Number of Runs
3
7
27
3
33
Pb (ug/L)
0
0
50
100
100
Cu (ug/L)
50
100
0
0
75
Filter Type
Glass Fiber/Nylon
Glass Fiber/Nylon/Glass Microfiber
Hydrophilic Polytetrafluoroethylene
Hydrophobia Polytetrafluoroethylene
Mixed Cellulose Esters
Nylon
Polyethersulfone
Polypropylene
Polypropylene/Nylon
Polypropylene/Polyethersulfone
Polytetrafluoroethylene
Polyvinylidene Fluoride
Glass Fiber/Glass Microfiber
8 of Different
Filters
1
1
2
2
1
2
1
1
1
1
1
1
1
Two additional proprietary filters were tested but are not listed
above
Volume Passed Through Filter (ml)
Figure 1. Three trials of a Glass Fiber/Nylon/Glass
Microfiber syringe filter tested with 50 ug/L Pb, 50
mg/L C (NaHCO3) at pH 7.
«j 20
Volume Passed Through Filter {ml_f

Figure 2. Two trials of a Mixed Cellulose Esters
syringe filter tested with 50 ug/L Pb, 50 mg/L C
(NaHCO3) at pH 7.
fj.
&
Volume Passed Through Filter (mL)

Figure 3. Two trials of a Hydrophilic
Polytetrafluoroethylene syringe filter tested with
50 ug/L Pb, 50 mg/L C (NaHCO3) at pH 7.
Contacts
Principal Investigators: Darren Lytle, (513) 569-7432, lytle.darren@epa.gov | Keith Kelty, (513) 569-7414, kelty.keith@epa.gov | Maily
Pham, (513) 569-7212, pham.maily@epa.gov
Communications: Michelle Latham, (513) 569-7601, latham.michelle@epa.gov

Footnotes
1EPA Method 1669, Sampling Ambient Water for Trace Metals at EPA Water Quality Criteria Levels; U.S. Environmental Protection Agency, 1996.
2Clean Water Act Section 303(d); 33 U.S.C. §1251 et seq.; U.S. Environmental Protection Agency, 1972.
Office of Research and Development
Risk Management Research
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