Jun 05, 2019

Talking Tech

Exploring arsenic removal technologies

Exploring arsenic removal technologies

Arsenic is one of the most abundant metalloid trace elements naturally found in the earth’s crust and is introduced into water streams by the dissolution of minerals, rocks, ores and industrial effluents. In water, arsenic exists as a part of organic compounds (organoarsenic) or inorganic species (oxyanions). The primary inorganic species represent the two oxidation states of arsenic: trivalent arsenic as arsenite and pentavalent arsenic as arsenate. 

Arsenic has been identified by the World Health Organization (WHO) as one of the ten toxic chemicals or groups of chemicals of major public health concern. Arsenic and inorganic arsenic have been established as a Class 1 human carcinogen by the International Agency for Research on Cancer. The primary route for human exposure is contaminated drinking water. Chronic exposure can cause skin lesions, diabetes, cardiovascular diseases and other ailments. Acute exposure to high concentrations can lead to muscle cramping and even death. Cancer caused by arsenic is difficult to detect, as symptoms can take 10 to 20 years to manifest. The toxicity of arsenic differs between arsenic species. 

In 2000, WHO decreased its limit for arsenic in drinking water from 200 ppb to 10 ppb. Shortly after, other regulatory organizations adjusted their limits as well, including the U.S. EPA, the Bureau of Indian Standards, the European Commission, and the National Health and Medical Research Council, Australia. Some countries still have higher limits, such as Mexico which has a limit of 25 ppb. 

Arsenic contamination in drinking water is an issue in many countries, including India, China, Mexico, Argentina, Brazil, Thailand and the U.S. More than 150 million people worldwide are estimated to be exposed to drinking water with arsenic concentration above the WHO guideline of 10 ppb. According to the latest data from the Ministry of Drinking Water and Sanitation of India, a population of 14.7 million is at risk of ingesting contaminated drinking water, with West Bengal leading in the number of areas affected by arsenic contamination. Millions of people in Bangladesh still are drinking arsenic contaminated water. Effective treatment of arsenic-contaminated water is crucial for public health and safety. 

 

Exploring Technologies

There are a number of available treatment technologies for reducing arsenic below regulatory levels. These technologies include, but are not limited to, oxidation, coagulation-flocculation, precipitation, adsorption, ion exchange, electrolysis and membrane techniques. In communities that use wells to remove arsenic from groundwater, coagulation-flocculation is the most widely used method. However, this method generates a significant amount of hazardous sludge wastes.

Membrane technologies, like reverse osmosis and nanofiltration, are increasingly used for arsenic removal. These techniques have high removal efficiency, easy operation and minimum sludge formation. However, the costs are relatively high and these methods may not be suitable for large-scale processes. The concentrated effluent in the reject leaves a waste stream that can be released into the environment. 

Adsorption often is a preferred method to treat arsenic contaminated waters because of its generally low cost, low energy, high efficiency, ease-of-operation and manageable waste. In the last 10 years, advances in material science and chemistry have provided new and novel adsorbents with unique size-shape and surface-active properties which selectively remove arsenic from aqueous sources. These materials include metal oxides, zeolites, activated carbons and titanium-based sorbents, among others.

Modified carbons. Among various adsorbent materials, arsenic adsorption onto surface modified activated carbon is gaining more attention. The main advantage of using activated carbon is its high surface area, well-developed super-microporous structure, relatively low cost, and the presence of a range of surface functional moieties. The surface of activated carbon is predominantly hydrophobic in nature, but also may contain appreciable amounts of heteroatoms formed during the activation process and can significantly influence the adsorptive properties of activated carbon.

Adsorption of arsenic by activated carbon is a mass transfer process in which soluble arsenic is transferred from the liquid phase to the surface of activated carbon and becomes bound either by physical or chemical interactions. Adsorption capacity depends on the activated carbon properties, adsorbate chemical properties, temperature, ionic strength, pH and more. Both granular activated carbon and powder activated carbon remove arsenic, however neither form of activated carbon has a high selectivity to arsenic when in the presence of competing. There is a need for improving arsenic adsorption affinity and selectivity by carrying out appropriate surface modification of pure activated carbon. 

Surface modifications result in a change in the surface reactivity, chemical, physical, and structural properties of activated carbon. Positively charged activated carbon surfaces have a better affinity towards arsenic adsorption. The most commonly used activated carbon in water purification is coal and coconut-shell based carbon. Coconut-shell based activated carbons are highly microporous and hard, which makes them ideal for water purification.

Activated carbons treated with iron salt solutions have been explored and have been shown to improve arsenic adsorption in aqueous waste streams. These surface modified carbons showed enhanced removal because of the formation of a stable iron-arsenate complex. When used, care should be taken to ensure that carbon modifications do not leach into water, preventing secondary contamination. 

Titanium-based adsorbents. Titanium-based adsorbents have been evaluated for arsenic removal over the last decade. Several reports have determined that these materials have high arsenic capacities and low leachabilities. This means that they effectively can remove arsenic from waste streams in a variety of conditions and can be disposed of safely. While the cost of the media as a consumable is slightly higher than other technologies, the use of titanium-based sorbents can be more economical because the media lasts longer, does not need additional post-treatment and is considered safe for disposal.

 

Case Study

Well water supplying an elementary school in Michigan contained 33 µg/l (ppb) of naturally occurring arsenic. This is more than three times the level allowed by EPA’s Safe Drinking Water Act. The school had an initial arsenic treatment system installed in 2008. That system required chemical injection, three tanks each containing 32 cu ft of media, a 500-gal backwash tank and the onsite storage of arsenic waste until it could be removed for disposal. The operation of this treatment system was labor intensive. Daily service of more than an hour was required to start the system each morning. 

In spring of 2015, the arsenic concentration of the effluent water from the school’s treatment system was beginning to approach EPA’s 10 ppb arsenic limit. The school operations staff determined that an alternative system was necessary to remove arsenic in a more economical way. 

A full water chemistry analysis was obtained so that a new system could be designed efficiently. It was determined that twin 150,000 grain softeners followed by arsenic reduction filters containing Metsorb HMRG, a titanium-based adsorbent, would be ideal for solving the water treatment needs of the school. The softeners would remove the soluble iron and hardness present in the water, which was not treated with the original system. The reaction kinetics of the adsorbent would allow a reduction in the treatment media volume from 96 to 40 cu ft, processing at a flow rate of 50 to 100 gpm.

The operating costs of the process were reduced because there was no longer a need to store backwashed iron solids that could leach arsenic back into the water. Backwash cycles were extended to an automated system controlled monthly. These changes reduced the time needed to monitor and operate the arsenic reduction process.

The implementation of the system using the adsorbent was successfully completed. The arsenic concentration in the school’s drinking water was analyzed and was lower than the limit of detection for this testing, at less than 2 ppb. After four years of operation, the effluent remains less than the minimum contaminant level for arsenic.

 

Lessons Learned

Water contaminated by arsenic continues to be a health hazard globally. There are many available technologies that can remove arsenic from contaminated waters. Technology selection is driven by capital expenditure costs, operating costs, waste stream costs, ease-of-use and aqueous chemistries. Ongoing research and development is focused on increasing the selectivity or capacity of adsorbents, efficiencies of reverse osmosis systems and providing better technical solutions for consumers. 

About the author

Jola J. Solomon is manager for Aquaworks, a Filtrex Technologies group. Nabin K. Pal is research scientist for Aquaworks. Nichole Pennisi is business manager for Graver Technologies. Joshua Mertz is director-adsorbents for Graver Technologies. Solomon can be reached at [email protected] Pal can be reached at [email protected] Pennisi can be reached at [email protected] Mertz can be reached at [email protected]

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