Trace Element Contamination: Findings of Studies on the Cleaning of Membrane Filters and Filtration Systems
In Reply Refer To: July 17, 1992
Mail Stop 412
OFFICE OF WATER QUALITY TECHNICAL MEMORANDUM 92.13
Subject: Trace Element Contamination: Findings of Studies on
the Cleaning of Membrane Filters and Filtration
Systems
PURPOSE
In 1989 and 1990 Art Horowitz conducted a series of experiments
with emphasis on the cleaning of filters and filtration systems.
This memorandum describes the results and conclusions from those
experiments. Building on the information in Office of Water
Quality (OWQ) Technical Memorandum 91.10, this memo provides
background for some of the decisions that the OWQ is making to
provide a supportable parts-per-billion (ppb) protocol for
dissolved trace elements.
STUDY COMPONENTS
Figure 1 provides information on the various experimental designs
and identifies the three filter brands tested--MFS, Millipore, and
Nuclepore. These brands were selected because Millipore and MFS
represent approximately 80 percent of the Division's usage, and
Nuclepore is the filter preferred by research chemists. All
experiments were made on 142-mm (millimeter) diameter filters.
Four experiments were conducted:
1. Cleaning of membrane filters for major and trace elements:
A. Concentration of elements in successive wash aliquots of
50, 50, 100, and 200 milliters (mL), which correspond to
cumulative wash volumes of 50, 100, 200, and 400 mL.
B. Comparison of deionized water (DIW) versus 2 percent HNO3.
C. Comparison of element concentrations in the 400-mL wash
volume to National Stream-Quality Accounting Network
(NASQAN) reporting limits (RLs).
D. Comparison of filter brands.
2. Sample dilution effect of cleaning filters with DIW.
3. Carryover contamination in equipment: Laboratory study.
4. Carryover contamination in equipment: Field study.
Table 1 provides information on the National Water Quality
Laboratory's (NWQL) RLs for each experiment and for NASQAN in
1990. Reporting levels changed from experiment to experiment
depending on the analytical method used; namely, inductively
coupled plasma (ICP) atomic emission spectroscopy, or graphite
furnace atomic absorption spectroscopy (GFAAS).
CLEANING OF MEMBRANE FILTERS FOR MAJOR AND TRACE ELEMENTS
Five individual filters from a single batch of each of the three
brands were tested. This designation, together with the four wash
volumes (50, 100, 200, and 400 mL) and the two wash solutions
(DIW and 2 percent HNO3) gave a design of 120 data points
(5 x 3 x 4 x 2). Results were compared by computing mean
constituent concentrations and the standard deviations for the
five filters for each combination of brand, volume, and wash
solution. Ranks of constituent concentrations and standard
deviations were also computed. The rank results are not reported
in this memo, but are cited in several places.
Analyses were made for 19 major and trace elements. Of these,
11 showed either: (a) no detectable concentrations throughout the
entire experiment, or (b) concentrations below the study's
reporting level before or in the aliquot corresponding to the
200-mL cumulative wash volume. The 200-mL volume is important
because it has been the Division's "rule of thumb" in cleaning
filters prior to collecting the filtrate for dissolved trace-
element analysis. Elements in these two categories included
silver (Ag), barium (Ba), beryllium (Be), cobalt (Co), lithium
(Li), molybdenum (Mo), manganese (Mn), sodium (Na), strontium
(Sr), vanadium (V), and zinc (Zn). The remaining eight elements--
which are the focus of this memo--were cadmium (Cd), copper (Cu),
lead (Pb), nickel (Ni), iron (Fe), silicon (Si), magnesium (Mg),
and calcium (Ca). For these elements, concentrations in the wash
solutions were quite erratic, giving rise to high standard
deviations (Tables 2, 3, and 4). Thus, although differences based
on means alone were observable for many tested comparisons, only a
few differences were statistically significant. The erratic
results appear to have arisen from: (a) non-uniformity of
contamination between individual filters in a batch, (b)
differences in flow paths by which cleaning solutions passed
through the 142-mm filters, and (c) in certain cases,
contamination arising during the experiment or subsequent
laboratory analyses.
Comparison of 400-ML versus 200-ML Wash Volumes
In this and following discussions, mean and standard deviations
are described. Five observations were available for each brand of
membrane filter, wash volume and wash solution. To compute means
and standard deviations, "less than" values were assigned a value
of one-half the reporting limit. Table 2 shows the computed means
and standard deviations for the eight elements for the DIW washes
(by filter brand and wash volume), whereas Table 3 shows the
comparative values for the 2 percent HNO3 washes. From these
tables, the following was observed:
1. Elemental concentrations generally showed a decay function,
with low concentrations reached by the 200-mL cumulative wash
volume.
2. In nearly two-thirds of 48 cases (8 elements x 3 filter brands
x 2 wash solutions), the mean elemental concentrations were less
in the 400-mL versus the 200-mL cumulative wash volume. This
trend was evident for both DIW and 2 percent HNO3 for the three
filter brands, and for most of the elements. Furthermore,the
observation of lower concentrations in the 400- versus the 200-mL
cumulative volume was confirmed by the mean of ranks. Although
the mean concentrations were typically less in the 400-mL wash
volume, the difference between the 200-mL and 400-mL wash volumes
tended to be small. For example, in DIW washes, the difference
for Cd, Pb, and Cu was