Water chemistry effects on arsenic removal

Oct. 13, 2010

Several technologies have been developed for addressing arsenic contamination, and several common point-of-use or point-of-entry options are currently available for removing arsenic. These developments stem from this background: Effective

Several technologies have been developed for addressing arsenic contamination, and several common point-of-use or point-of-entry options are currently available for removing arsenic.

These developments stem from this background:

Effective in January 2006, the US Environmental Protection Agency (EPA) reduced the maximum contaminant level (MCL) for arsenic in drinking water to 10 parts per billion (ppb). In addition to the estimated 4,000 community water systems impacted by this regulation, it is estimated that over 10 million private wells also exceed the new MCL.

Several states have proposed arsenic standards that are more stringent than the EPA’s ruling. New Jersey has set its MCL for arsenic at 5 ppb and several other states are evaluating whether to follow this example.

Drinking water with arsenic levels between the former MCL of 50 ppb and the new MCL of 10 ppb can contribute to serious negative health effects, which may include skin damage, circulatory problems and an increased risk of developing cancer.

Arsenic removal options

Reverse osmosis (RO) membranes offer the advantage of improving many water quality parameters simultaneously, as a point-of-use solution for arsenic contamination.

However, RO’s non-selectivity, its waste stream of rejected water and an inability to effectively remove the arsenite (As(OH)3) form of arsenic often preclude its choice as the sole treatment of arsenic.

RO still should be considered when there are multiple problems to be addressed (e.g., high salt content) and when only arsenate is present.

Ion exchange

Ion exchange (strong base anion resin) can, in principle, be a cost-effective point-of-entry means of reducing arsenic levels, but the lack of specificity can cause the resin to require frequent regeneration if other common competing ions, such as phosphates or sulfates, are present.

If the resin is not regenerated appropriately, arsenic can concentrate on the resin and then be released into the water at a higher concentration than is present in the influent water being treated.

Finally, as with RO membranes, strong base anion resins will not effectively remove the arsenite form of arsenic.

Selective adsorptive media

For the sole treatment of arsenic, a selective adsorptive media often is the best approach. A passive bed of adsorptive media can be used to effectively reduce arsenic to non-detectable levels, and the media can last up to several years depending on the water chemistry.

In cases where the water is relatively free of particulate, the bed may not even need backwashing or other service during its useful life. Adsorptive media is easy to use and easily disposed.

The most common media currently available are based on activated alumina, iron oxides and titanium oxides. A system’s efficiency at removing arsenic varies widely with the type of media used and the chemistry of the water being treated.

Different media properties

When selecting an adsorptive media for arsenic removal, there are several factors that should be considered.

One is that the lifetime of an adsorptive bed is inversely related to the concentration of arsenic. Activated alumina, while inexpensive and physically stable, has a relatively low capacity for arsenic, and as a result the system size required for appropriate removal with activated alumina can grow quite large if arsenic concentrations are high.

Iron- and titanium-based media can vary in their capacity to capture arsenic, and arsenic levels in the feed water will have a big effect on the best choice for a given system.

Oxidation states

It also is crucial to understand the oxidation state of the arsenic, since the different arsenic species are adsorbed with different efficiencies.

In many drinking water sources, arsenic is present in a +5 oxidation state as the arsenate (AsO4-3) anion. In environments where arsenic can be reduced further, it also can be found in the +3 oxidation state as arsenite, As(OH)3.

The arsenite species is typically more difficult to remove since it has no charge in neutral-pH water. Some adsorbents cannot remove it at all, and even the most selective ones, such as titanium oxide, remove arsenite with approximately half the efficiency of their removal of arsenate in typical water.

If the water source is high in arsenite, a pre-oxidation step may be necessary.

Impact of pH

The pH of the water also has a strong effect on adsorption efficiency. As pH changes, the charge associated with the arsenic anion also changes.

The H2AsO4- anion carries a single negative charge at or below pH 7, but it loses a proton at higher pH, resulting in a doubly charged anion, HAsO4-2. The singly charged anion is adsorbed more effectively from solution than the doubly charged species.

In a similar manner, the charge state on the surface of the adsorbent varies with pH also. Titanium oxide has a positive surface charge below a pH of 5, and a negative surface charge at higher pH. Likewise, an iron oxide adsorbent has a positive charge below a pH of about 7, and a negative charge at higher pH.

This variation in adsorbent surface charge can explain why the adsorption capacity for arsenic is so sensitive to pH changes in the common drinking water range of 6.5 to 8.5 (see accompanying graph).

Titanium oxide’s properties

It is easy to see how both effects — the doubly charged arsenic anion and the negative surface charge of the adsorbents — could work together to dramatically reduce efficiency at higher pH.

Using this rationale, you might expect titanium oxide to be a less effective adsorbent at normal drinking water pH conditions, since it is negatively charged at an even lower pH.

However, the opposite is true. Titanium oxide has a higher adsorption efficiency for arsenic in neutral pH water than iron oxides do. At higher pH, all adsorptive media lose efficiency, but the effect is less dramatic for titanium oxide.

Interfering silica

Another major factor to consider is the effect of interfering ions present in the water being treated.

Silica has proved to be the most common and problematic interferant for arsenic removal. There is some debate as to the relationship, but it is clear that high silica levels, especially those above 20 parts per million (ppm), result in dramatically reduced capacity for arsenic.

There is emerging evidence that the impact of silica is through simple interference, as opposed to a fouling mechanism. The evidence for this is the reversibility of the effect, as well as the minor silica build-up on adsorbent media over time.

The silica interference seems to affect every type of adsorbent used, though the most selective adsorbents, such as titanium oxide, are least affected.

Sulfate, phosphate interference

Other species can interfere with arsenic adsorption in various degrees:

  • Sulfate can interfere with very non-specific adsorbents such as anion exchange resins and activated alumina.
  • Phosphate can be problematic for these non-specific adsorbents as well, and can cause significant problems with adsorption using iron oxide-based media or moderate problems with titanium oxide media.
  • Vanadium and selenium adsorb via a similar mechanism as arsenic, so they can act as interferants as well.

Know your water chemistry

With so much to consider, it is important to have a good handle on the chemistry of the water being treated in order to select an appropriate point-of-entry or point-of-use solution. Understanding the importance of water chemistry will allow for effective system design, optimum operating conditions and the setting of proper performance expectations.

Factors such as the type of arsenic present, level of silica interference and water pH can be even more important in determining the lifetime of the adsorptive bed than the arsenic level itself. Any model for performance provided by a media supplier should incorporate these factors at a minimum.

Fredrick W. Vance, Ph.D. is a development specialist for Dow Water Solutions, a unit of The Dow Chemical Co. based in Midland, MI, that specializes in component technologies designed to advance the science of water purification. He holds a B.S. in chemistry from Hope College, and a Ph.D. in inorganic chemistry from Northwestern University. His recent work has focused on the development and application of Dow’s proprietary media for the selective removal of arsenic from drinking water. He can be reached by e-mail at: [email protected].

Alan Greenberg is a marketing development manager for Dow Water Solutions. He holds a B.S. in chemistry from Southern Illinois University-Carbondale, and an M.S. in analytical chemistry from Michigan State University. Greenberg has 25 years of experience working with ion exchange resins and other water treatment technologies and is leading the commercialization efforts for Dow-brand media for the selective removal of arsenic from drinking water.

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