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ION EXCHANGE APPLICATIONS IN WATER TREATMENT 12.17

senic removal is occurring and to prevent unsafe levels of arsenic in the treated water
from being delivered to the distribution system.
Oxidation of arsenite to arsenate with chlorine occurs at a very rapid rate. Test results
from an EPA-sponsored study showed that over 95% of the arsenite was converted to ar-
senate in less than 5-s contact time with 1 ppm residual chlorine. These tests showed that
the reaction was insensitive to pH in the range of 6.5 to 9.5. However, the reaction rate
decreased substantially outside this range.
The presence of other oxidizable substances must be accounted for, as they will con-
sume chlorine and may act as reverse catalysts for the oxidation of arsenite. For example,
the presence of 5 ppm of TOC in one case slowed the reaction rate so that almost 50% of
the arsenite remained unoxidized after 30 s. In this case more than 30 min was required to
reach 80% oxidation. The presence of chloramine has also been shown to reduce the rate
of oxidation substantially. Since both TOC and chloramines can be present (ammonia is
converted to chloramine by chlorine), each contaminated water being considered for this
type of treatment should be thoroughly evaluated at the pilot plant stage to ensure that this
critical step is carried out effectively and that all arsenites are converted to arsenates.
The relative affinity and operating capacity of strongly basic anion exchange resins is
markedly higher for arsenate than for arsenite. Sulfates and chlorides are preferred over
arsenites, but not over arsenates regardless of the type of media. This means that they are
removed before the arsenites, and therefore both of these ions would be included as ex-
changeable ions, which increases the loading. The arsenate ion, however, has a somewhat
higher affinity for the strongly basic anion resins higher than chloride but lower than sul-
fate. This means that arsenate dumping can occur if the resin column is inadvertently over-
run. Surface fouling due to naturally occurring organics reduces the affinity of the resins
for all forms of arsenic, resulting in reduced run lengths when fouling occurs. Improved
ion exchange processes using multiple beds in round-robin fashion show promise in avoid-
ing arsenic dumping and increasing regenerant efficiency (also reducing brine discharge).
They work on the principle of overrunning the first stage past the sulfate break and us-
ing the second stage to catch the arsenic. The sulfate-loaded resin is regenerated and placed
into the second-stage position while the arsenic loaded resin takes on the primary posi-
tion. Since the arsenic is only present at trace levels, the regeneration of both beds to re-
move the arsenic is not required on a regular basis. This approach, while interesting, is
not yet offered commercially.

Activated Alumina. Activated alumina has been used successfully to remove both forms
of arsenic. However, control of pH within a fairly narrow range and careful monitoring
of water chemistry are required for good success. Preoxidation of arsenic to the arsenate
form and pH control to the 5.5 to 6.0 range are highly recommended.
Regeneration of activated alumina is a two-step process. Following the backwash cy-
cle, it is first regenerated with sodium hydroxide and rinsed; next it is washed with a di-
lute acid such as sulfuric, and then the bed is rinsed down to the proper pH before being
returned to service.
Strong base ion exchange resins remove arsenic rapidly and to low levels, and can op-
erate over a wide pH range. However, only the arsenate can be removed without inter-
ference from other ions commonly found in potable water, so oxidative pretreatment to
convert arsenite to arsenate whenever arsenite is present is a must. Additionally, strong
base resins have a higher affinity for sulfates than arsenates, and sulfates are typically
present at concentrations orders of magnitude greater than that of arsenic. This requires
close attention to effluent monitoring to avoid the potential chromatographic dumping of
arsenic at concentrations much higher than that in the influent. If an ion exchange resin
is run past the arsenic breakthrough all the way to the sulfate breakthrough, no arsenic
will be left on the resin.