Title: Photocatalytic Degradation of Lindane in Potable Water Systems Amanda M. Nienow, , Irene C. Poyer, J
1Photocatalytic Degradation of Lindanein Potable
Water SystemsAmanda M. Nienow,, Irene C.
Poyer, Juan Cesar Bezares-Cruz, Inez Hua, Chad
JafvertCivil and Environmental Engineering,
Purdue University, West Lafayette, IN
47907Advanced Concepts and Technologies,
International, Waco, TX 76710
ENVR 195
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OH QUANTIFICATION
EFFECT OF VARYING H2O2 CONCENTRATION
INTRODUCTION
- Increased terrorist activity in the United
States and throughout the world has prompted
concern over the security of the nations water
sources, purification and distribution systems
from possible chemical, biological, radiological,
or nuclear (CBRN) and/or toxic industrial
chemicals and material (TICs /TIMs)
contamination. Technologies, such as reverse
osmosis (RO), used in water purification systems
for monitoring and providing safe drinking water
are effective for most compounds at normal
operating conditions, but there are a number of
CBRN agents as well as toxic industrial chemicals
and materials (TICs and TIMs) that are not
effectively removed by reverse osmosis (RO). The
most promising removal technology to use in-line
as a replacement for current polishing
technologies have been identified and include
photochemical processes, such as photocatalytic
oxidation (PCO). These technologies will have
the benefit of enhancing performance, reducing
the logistical support requirements and
potentially enabling continuous polishing
treatment of the RO product water, thus reducing
the risk of exposure to CBRN, TICS and TIMS. -
- PCO can be broadly divided into direct or
indirect photolysis, and homogeneous (single
phase - UV/H2O2 or UV/O3) or heterogeneous (two
or more phases, e.g., UV/TiO2) systems. Direct
photolysis requires target contaminants to
possess a chromophore (a functional group on the
molecule) which directly absorbs light and
reacts. However, molecules without chromophores
may participate in secondary photochemical
reactions based on their interactions with
free-radicals. In the case of H2O2, free-radicals
can be generated during photolysis with UV light
to produce a highly reactive hydroxyl radical
(OH) -
- Similarly, O3 can decompose via photolysis or
acid-base reactions. The aqueous O3 reaction
mechanism varies with pH (more alkaline systems
favor ozone decomposition) and as a result
produces several different free-radicals -
-
- The efficacy of engineered photochemical
processes for destroying or transforming chemical
agents in a homogeneous system (UV/H2O2 or UV/O3)
has been investigated. Preliminary investigative
work was completed on the degradation rates of
the chlorinated pesticide lindane, one of the
most stable toxic industrial chemicals (TICs). - Results presented here are for lindane
degradation via UV/H2O2 testing effects of
Terephthalic acid is commonly used in sonolysis
to determine the concentration of OH 1. In
the presence of the radical, terephthalic acid is
transformed into 2-hydroxyterephthalic acid, a
compound that fluoresces when excited at 315 nm.
Terephthalic acid solutions, with the addition of
H2O2, were irradiated and the products were
detected with a SLM-Aminco Bowman Series 2
Luminescence Spectrophotometer. The concentration
of 2-hydroxyterephthalic acid was then used to
determine the OH concentration.
- The optimal H2O2 concentration with both 0.26 mM
and 13 mM Lindane was between 1 mM and 5 mM,
which correlates well with the formation of OH. - The drop in rate constants at higher H2O2
concentrations is likely due to recombination of
OH, also observed in the terephthalic acid
experiments (see OH Quantification Box).
Irradiated solutions with 5 mM H2O2 produced the
highest OH concentrations. The slower rate with
higher H2O2 additions is likely due to
recombination of OH.
MONITORING BY-PRODUCTS
TESTING EFFECTS OF pH and NOM
1. pH
No buffer
- The fastest photodegradation reaction rates
occurred between pH 5 and pH 7, conditions most
closely simulating those of natural groundwater. - At pH 9, completed without buffer, the pH dropped
throughout the course of the reaction. Due to the
change in pH, the observed reaction rate under
these conditions is not necessarily first order. - At pH 11, Lindane undergoes hydrolysis. However,
hydrolysis rate constants are an order of
magnitude lower than the rate constants obtained
in these experiments, suggesting that the PCO
rate constants can be accurately determined by
preparing basic solutions of Lindane immediately
prior to use. (Note Upon sitting for several
days, hydrolysis products were observed in the
basic Lindane solutions).
- Complete dechlorination of the parent compound
was confirmed and quantitated based on the known
moles of Lindane and the expected moles of
chloride. - The formation of an unidentified organic acid was
observed during chloride analysis and suggests
incomplete carbon mineralization. - pH dropped significantly suggesting formation of
H. - Additional experiments with longer exposure time
are scheduled.
EXPERIMENTAL METHODS
A Rayonet RPR-100 Photochemical Reactor
(right) is used to irradiate the aqueous samples.
The reactor uses up to 16 lamps with a wavelength
of 254 nm. Eight lamps were used in the
experiments presented here. The photon flux,
determined by chemical actinometry, is 7 ? 10-6
einstein/sec. A 660 mL quartz tube is placed
inside the photochemical reactor. Aqueous
solutions of Lindane ( 0.1 mg/L or 4 mg/L) are
added to the tube and irradiated for up to 20
minutes. Some solutions were buffered to pH
values of 2.8, 7, or 11.2 with phosphate buffers.
5 mL of solution is removed at a series of
reaction times and the contents are either
extracted with an organic solvent (for analysis)
or sacrificed to measure the pH of the solution.
The concentration of the residual parent compound
is determined through gas chromatographic
analysis. Identification of by-products was
accomplished by ion chromatography.
Top View
KEY FINDINGS/CONCLUSIONS
2. Natural Organic Matter (as Humic and Fulvic
Acids)
- Lindane is almost completely mineralized after 45
minutes of irradiation at 254 nm (with a photon
flux of 7 ? 10-6 einstein/sec) to form chloride
ions and small organic acids. - Lindane does not degrade via direct photolysis or
by reaction with H2O2 or O3 alone. - The optimal conditions for removal of Lindane by
UV/H2O2 are a near-neutral pH, 1 mM H2O2, and
minimal amounts of dissolved organic matter. - H2O2 photocatalysis is a viable pathway for
degrading and removal of organic contaminants
from potable water.
REFERENCES/ACKNOWLEDGEMENTS
Rayonet RPR-100 Reactor
References 1 Mason, T.J., Lorimer, J.P.,
Bates, D.M., Zhao, Y. Dosimetry in
sonochemistry the use of aqueous
terephthalate ion as a fluorescence monitor.
Ultrason. Sonochem., 1994, 1(2), S91-94. 2
Larson, R. A., Zepp, R. G., Reactivity of the
carbonate radical with aniline derivatives.
Environ. Tox. Chem., 1988, 7, 265-274. 3
Haag, W. R., Yao, C. C. D., Rate constants for
the reaction of hydroxyl radicals with several
drinking water contaminants. Environ. Sci.
Technol., 1992, 26, 1005-1013. Acknowledgements
Advanced Concepts and Technologies,
International and TARDEC (U.S. Army Tank
Automotive Research, Development and Engineering
Center) for funding, and Dr. Changhe Xiao for
assisting with organic synthesis and luminescence
spectrophotometer analysis.
- Humic and fulvic acids slow the photodegradation
of lindane at 19.2 mg/L total humic and fulvic
acids, the reaction is just slightly faster than
the direct photolysis of lindane. - Humic acid has a larger effect on the rate
constant than fulvic acid. - Light attenuation and the scavenging of OH by
the humic and fulvic acids are the major causes
of the drop in reaction rate constants. Note
2 and
3.