Activated Carbon for Bromate Reduction

Tech Update

Activated carbon is well-known for its ability to remove organic
compounds from water through a process known as adsorption, remove chlorine and
chloramine through various chemical reactions, and serve as a general filter
medium. However, its use for reduction of bromate is unclear. Various authors
have studied the use of activated carbon and for the most part have concluded
activated carbon is not a cost-effective solution. However, these authors have
failed to realize the limitations of carbon validation methods or the fact that
not all carbons are alike, especially when chemical reactions control the
process.

Regulations and Reactions

Bromate (BrO3-) is a disinfection byproduct formed by the
reaction of ozone and naturally occurring bromine in drinking water. Although
bromate is unlikely to be formed using standard chlorination disinfection,
there is some evidence that commercially available sodium hypochlorite
solutions may contain bromate as a contaminant.1 Bromate is a highly toxic
substance that causes irreversible renal failure, deafness and death and has been
linked to renal tumors in rats. As such, the American, Canadian and European
environmental protection agencies have designated 10 µg/L as the maximum
acceptable concentration or maximum contaminant level (MCL) in drinking water.

The important precursor to bromate formation in drinking
water is bromide. In the United States, the average bromide concentration in
drinking water is approximately 100 µg/L. Since bromate is 63 percent
bromide, only 6.3 µg/L of bromide needs to be converted to bromate upon
ozonation to exceed the MCL. Natural sources of bromine in groundwater are
saltwater intrusion and bromide dissolution from sedimentary rocks. Bromine
usually is present in drinking water as either hypobromous acid (HOBr-) or
hypobromite (OBr-). When exposed to ozonation, the bromide ion readily is
oxidized to aqueous bromine. In addition to bromate, aqueous bromine can cause
various types of brominated disinfection byproducts such as bromoform and
brominated haloacetic acids.

In order to understand the formation of aqueous bromate, a
corollary understanding of ozone decomposition is needed. Ozone can play a
direct (molecular ozone pathway) or indirect (hydroxyl radical pathway)
oxidative role in forming byproducts. Ozone reacts directly with the bromide ion
to form hypobromite and oxygen.

O3 + Br- ? O2 + OBr-

Two ozone molecules then react directly with the hypobromite
to form bromate and oxygen. Alternatively, the hypobromite can react with
multiple hydroxyl radicals created by the destruction of ozone.

2O3 (or OH*) + OBr- ? 2 O2 + BrO3

These reactions are generalized and not necessarily
balanced, but they give a good overview of the mechanisms at work in bromate
formation.

While the 10 µg/L MCL is anticipated to impact a
limited number of utilities currently using ozone as the primary disinfectant
to inactivate Giardia and viruses, a greater number of utilities will be
impacted by this MCL when compliance with the Long Term 2 Enhanced Surface
Water Treatment Rule (LT2ESWTR) is required. Compliance will mean continuance
of meeting filtration avoidance criteria of two-log Cryptosporidium
inactivation and overall inactivation requirements (three-log Giardia, four-log
viruses and two-log Cryptosporidium) using a minimum of two disinfectants.

Activated Carbon Research for Bromate Reduction

The use of activated carbon has been investigated by various
authors for the removal or reduction of bromate.2,3,4 The data to date have
been inconsistent and, in some cases, misleading due to the techniques used to
determine the applicability of activated carbon.5,6 It also is apparent that
the carbon selection process was overlooked, which has lead to generalizations
concerning the use of activated carbon for this application.

One paper however has focused on the effect of surface
properties on bromate removal.7 The data shows surface properties can and do
affect bromate removal performance. Other applications such as chloramine
removal in the liquid phase and hydrogen sulfide and sulfur dioxide oxidation
in the vapor phase also have been shown to be affected by surface properties.
Commercially available activated carbons produced for catalytic properties as
well as adsorptive properties do exist and have been investigated for bromate
reduction. However, the data is misleading due to testing conditions.

Activated Carbon for Bromate Reduction--Reaction Kinetics

Testing was conducted using a differential reactor to
determine the reaction rate for a standard bituminous coal-based granular
carbon and a catalytically enhanced carbon. Typical carbon properties are shown
in Table 1. Reaction rate data (Figure 1) show the reaction follows a first
order reaction and, more importantly, data show the reaction rate for the
catalytically enhanced carbon is 3.4 times faster than the standard carbon.

Analysis of the water confirms the reaction product from
bromate destruction is bromide (Figure 2). The faster reaction rate for the
catalytically enhanced carbon would allow shorter contact time systems to be
designed for full-scale use. Experimental design studies show typical
properties such as iodine number cannot be used to predict bromate reduction
performance, however, catalytic activity as measured by the peroxide number is
useful in determining the more applicable carbon.

Activated Carbon--Real World Application

Differential reactor studies are useful for the
determination of reaction rates. However, full scale testing is required to
verify the data. Studies published concerning the reduction of bromate have
utilized the Rapid Small Scale Column Test (RSSCT) procedure, which was
designed for adsorption applications and may not translate well to applications
where a different removal mechanism such as oxidation/reduction or ion exchange
exists. Data from the literature as well as the differential reactor work
conducted for this paper show the reaction to be a reduction of bromate;
therefore, the RSSCT column study procedure may not be accurate.

Column studies were conducted using actual particle size
carbons and full-scale contact times to verify performance. A column study
using a 30-minute contact time and catalytically enhanced 8¥30 mesh carbon
showed bromate could be successfully reduced from an average of 110 ppb bromate
to an average of less than 5 ppb (see Figure 3).

Conclusions

* Differential
reactor studies indicate the bromate reduction reaction to bromide to be first
order.

* Data
show standard carbon properties such as iodine number cannot be used to
indicate bromate reduction performance.

* Catalytic
activity as measured by the peroxide number does give some indication of
bromate reduction performance.

* Column
study data show activated carbon can be utilized to reduce bromate to
acceptable levels.

Kimberly Thompson is an inside technical sales representative at Calgon Carbon Corp. She holds a bachelor of science in biochemistry from Steon Hill College, Greensburg, Pa., and has five years of experience as a chemist at Calgon Carbon. She may be reached at 412-787-6315; thompson@calgoncarbon.com. Neal Megonnell is a corporate technical sales specialist at Calgon Carbon. He holds a bachelor of science in chemistry from the University of Pittsburgh and a masters of science in chemical engineering/colloids, polymers and surfaces from Carnegie Mellon University. He may be reached at 412-787-6638; megonnell@calgoncarbon.com.

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