By Greg Reyneke, MWS
Arsenic is an abundantly occurring element that can be found at varying concentrations in the Earth’s outer crust around the world; it tends to be found at higher concentrations in areas where there are copper and lead ores. Arsenic is classified chemically as a metalloid since it exhibits properties of both a metal and a nonmetal, but we usually just call it a metal.
Elemental arsenic is a silver/gray solid, but it is rarely found like that in nature; it tends to combine with active elements like sulfur, chlorine and oxygen. Arsenic combined with these elements is called inorganic arsenic. When arsenic combines with carbon or organic ligands, is referred to as organic arsenic. Inorganic arsenic compounds (such as those found in water) are highly toxic while organic arsenic compounds (such as the arsenobetaine found in seafood) are generally less harmful to health.
Arsenic cannot ever be destroyed in the environment; it can only change its form. Most inorganic and organic arsenic compounds are white or colorless powders that do not evaporate, but can be attached to tiny particles that inevitably become airborne. When airborne particles are tiny enough, they can stay suspended by air currents for days and travel many thousands of miles from their original point of origin. These particles will eventually hit the ground as rain, snow, etc. Many common arsenic compounds can easily dissolve in water, so arsenic can easily get into lakes and rivers, and is not only found in groundwater.
We are exposed to arsenic every single day through fruit juice, vegetables, rice and grains, dust in the air, or even by contact with pressure-treated lumber. These sources of arsenic usually have no unique smell or taste, so you typically cannot tell if an arsenic compound is lurking in your food, water or air. Currently, the primary recognized cause of unintentional arsenic consumption is from drinking groundwater that contains arsenic. Surveys of US drinking water indicate that about 80 percent of water supplies have less than 2 ppb of arsenic, but two percent of supplies exceed 20 ppb of arsenic.
An underappreciated and growing threat
Both inorganic and organic arsenic compounds can be metabolized by your body to varying degrees and some will even be excreted in your urine. Most organic arsenic is expelled and a large amount of inorganic can leave your body after a few weeks, but some of that arsenic will remain in your body indefinitely, usually bonded to your bones, muscles and connective tissue. An elevated level of arsenic in the body is believed to interfere with cellular metabolism.
Arsenic poisoning produces gastroenteritis, esophageal pain, vomiting and violent diarrhea. Eventually the skin becomes cold and clammy, blood pressure drops and overall body weakness sets in. Death from circulatory failure is described as sweet release from the convulsions. Elevated doses of arsenic that aren’t fatal will cause restlessness, nausea, vomiting, headaches, dizziness, chills, cramps, irritability and variable levels of paralysis and neuropathy that may progress over several weeks. Even at extremely low levels, arsenic consumption has recently been linked to the development of diabetes, oxidative stress, DNA damage, skin damage and immune system malfunction in susceptible persons.
The United States Department of Health and Human Services (DHHS), the International Agency for Research on Cancer (IARC) and United States Environmental Protection Agency (US EPA) have classified arsenic as a human carcinogen. Chronic exposure to even low arsenic levels (less than 0.05 mg/L) has been linked to health complications, including cancer of the skin, kidney, lung and bladder, as well as other diseases of the skin, neurological and cardiovascular system. Historical studies indicate that skin absorption of arsenic is negligible, so handwashing and bathing do not appear to pose a known risk to human or animal health.
Know your enemy
There are seven common arsenic species that can be encountered in water (see Figure 1). Water exposed to dissolved oxygen will generally contain arsenic in the pentavalent +5 (oxidized) state, whereas hypoxic waters (low or no dissolved oxygen) will contain arsenic in the trivalent +3 (reduced) state. Both are quite toxic but the trivalent is more easily assimilated by humans, making it even more dangerous. Some species of bacteria derive energy by oxidizing various materials while reducing arsenates to form arsenites using enzymes known as arsenate reductases. There are also species of photosynthetic bacteria that can oxidize arsenites into arsenates, so remember to speciate whenever practical before attempting a treatment solution. Treatment methods vary in efficacy based on the oxidation-state of the arsenic, base pH and of course, the concentration of interfering contaminants.
Setting the limits and balancing cost
The path towards reducing arsenic content in drinking water has been slow and tortuous, since arsenic is found in so many places and the significant expense involved in treating it at a municipal level. The first national allowable arsenic level regulation in the US was 0.05 mg/L, as established under the 1975 National Interim Primary Drinking Water Regulations (NIPDWRs). As part of the 1996 Safe Drinking Water Act (SDWA) amendments, US EPA was directed to conduct health effects and cost-versus-benefit research to determine a new arsenic standard.
In June 2000, US EPA proposed a revised arsenic MCL of 0.005 mg/L and requested public comment on alternative MCLs of 0.003, 0.010, and 0.020 mg/L. The agency published a final rule in the Federal Register in January 2001 (US EPA, 2001), establishing the maximum contaminant level (MCL) of arsenic in drinking water at 10 ppb (0.010 mg/L). This rule is enforced at over 60,000 community water systems in the United States and adherence to the rule is required by most counties before a home can be occupied if served by a private water source. According to the US Geological Survey (USGS), arsenic concentrations exceeding 10 μg/L appear to be far more frequently observed in the western United States (including Alaska) than in the eastern half.
Obviously, US EPA’s MCL for arsenic doesn’t represent a safe consumption level, but rather a level at which the risk to human health and safety is reduced and economically achievable by municipal water providers. Since arsenic is consumed from so many other non-water sources, we should strive to reduce the drinking water arsenic level to zero.
US EPA has identified the following as Best Available Technologies (BATs) for achieving compliance with the latest regulatory limitand are listed in the final rule and the Implementation Guidance for the Arsenic Rule as Small System Compliance Technologies (SSCT).:
- Activated alumina (AA)
- Electrodialysis reversal
- Enhanced coagulation/filtration
- Enhanced lime softening
- Ion exchange (IX)
- Oxidation with post-filtration
Additionally, a variety of specialized resins, titanium and other rare-earth metal media are available that show promising results in addressing arsenic. In the POE world, the most common methods of treatment include:
- Activated alumina
- Co-precipitation with manganous media
- Ion exchange
- Metal oxide adsorbants (sorbents) in tanks or cartridges
Fools rush in
The first rule when attempting to address arsenic contamination is to know and understand your limitations. If you’ve never done this before, don’t rush in and wing it; you will (at a minimum) embarrass yourself and possibly endanger the health and safety of your customer. Consult with industry experts and your equipment supplier(s) to ensure that you always protect your customer and put their needs first.
Testing is not optional
Whenever you are tasked with an arsenic reduction project, test the source water using a reputable NELAP ( National Environmental Laboratory Accreditation Program) accredited laboratory whose data will be accepted by your local regulating agency. Test for the following at a minimum: total arsenic (speciation is nice, but it doesn’t hurt to just assume that it is all As3 as a precaution), pH, total alkalinity, phosphate, silica, vanadium, selenium, molybdenum, iron and manganese. If there is hydrogen sulfide (rotten-egg) odor, then be sure to quantify this for the total oxidative demand calculations.
Knowledge is power
The Water Quality Research Foundation (WQRF) maintains a powerful resource in the Contaminant Occurrence Map (https:// www.wqrf.org/map.html, see Figure 1). Be sure to visit this site and research reported arsenic and other related contaminant levels in the area; this can help you with contingency-planning. The USGS also maintains and extensive GIS database. The map graphic image at https://water.usgs.gov/GIS/browse/arsenic_map.png is based on samples from 31,000 wells and springs in 49 states collected for studies on potable groundwater resources by the USGS, the Minnesota Pollution Control Agency, the Texas Water Development Board, the Wisconsin Department of Natural Resources and the six New England states: New Hampshire, Vermont, Maine, Massachusetts, Rhode Island and Connecticut. The tabular data and GIS files in this data set exclude arsenic concentrations from non-USGS sources. Estimation of the arsenic concentration in groundwater in any specific area must consider the following limitations and sources of variability:
- The data can include a variety of well types, including private wells, public supply wells and monitoring wells not used for water supply.
- These groundwater samples do not accurately represent drinking water served by public water supply systems because these utilities may treat or mix groundwater with high concentrations from individual wells to meet drinking water standards before delivering it to consumers.
- The appearance of the arsenic distribution is influenced by the order in which wells are plotted. Typically, wells with higher concentrations are drawn on top of those with more moderate concentrations. Over-plotting like this may exaggerate the frequency of high values in areas where wells are close together.
- Arsenic concentration may vary with depth within the same aquifer, or between aquifers that are stacked vertically – for example, a shallow sand and gravel aquifer can overlay a deeper bedrock aquifer.
With these qualifications in mind, data from resources like these provide an estimate of arsenic occurrence in the groundwater resource in general.
Developing a treatment strategy
The scope of this article is not to teach you exactly how to treat every situation, but rather to help you understand the treatment paradigm and possibly improve your technique. Some things to consider:
- All moving parts should be protected by a 100-micron filter at minimum.
- Arsenic V is typically easier and more cost effective to address than Arsenic III, so consider an oxidizing stage before any attempt at removal. Various oxidation techniques can be used, such as bulk aeration, ozonation, chlorination, permanganate addition, as well as to exposure to an oxidative media like manganese greensand or other manganous materials.
- If the water contains enough iron, a manganous-media oxidizing iron remover can possibly remove significant amounts of arsenic along with the precipitated iron.
Here are some general guidelines:
- High iron levels (> 0.3 mg/L) and high iron to arsenic ratio (20:1). Iron removal processes can be used to promote arsenic removal from drinking water via adsorption and co-precipitation. Source waters with this ratio are potential candidates for arsenic removal by iron removal.
- High iron levels (> 0.3 mg/L) and low iron to arsenic ratio (< 20:1). If the iron to arsenic ratio in the source water is less than 20:1, then a modified treatment process such as coagulation/filtration with the addition of iron salts should be selected.
- Low iron levels (< 0.3 mg/L). Technologies such as adsorptive media and ion exchange are best suited to sites with relatively low iron levels in their source waters (less than 0.3 mg/L, the secondary MCL for iron). Above this level, taste, odor and color problems can occur in the treated water, along with increased potential for fouling of system components with iron particulates.
- After the removal of iron and arsenic with manganous media, adsorbents (sorbents) are currently the most cost-effective method for reducing arsenic in residential and small commercial systems. Commonly used sorbents are made from titanium and other rare-earth metals.
- Always deploy a dual-pass arsenic sorbent technique – plan for redundancy.
- Anion exchange is an effective technique for dropping alkalinity and reduction of competing ions in the source water that can significantly prolong the service-life of sorbent media beds.
- If utilizing non-backwashing columns of arsenic removal media, or whole-house media cartridges, remember to address sediment >5 micron upstream to prevent fouling and inevitable channeling.
- Sub-micron post-polishing can be a cost-effective technique to protect from downstream passage of fines from media attrition.
See Figure 3 for an example of combining manganous media with dual-pass sorbents along with pre- and post-filtration.
Figure 3. Arsenic treatment system, containing combined manganous media with dual-pass sorbents along with pre- and post-filtration.
Whenever possible, pilot test the proposed treatment approach, or at the least, bench to test the efficacy of your approach. It is so much cheaper and easier to adjust your methodology at this point than to install full-size equipment and have to make changes later at full-scale.
Managing expectations and planning maintenance
Carefully educate the prospective client about what your arsenic reduction system can and can’t do, especially if there are other non health-related contaminants like hardness, sulfates and TDS that will not be addressed. Equally important is to plan and document a routine maintenance plan for prefilters, oxidizers, sorbet tanks/cartridges and other related components; your system will only perform properly when properly maintained.
Arsenic levels continue to rise and we’re finding it in more places than ever before. As more research data is developed, we are realizing that the only acceptable level of arsenic in drinking water is ZERO. With global climate change and exploding populations, groundwater resources are increasingly strained and underground water dynamics are changing. We don’t have access to the same quality or quantity of water that we used to and probably won’t ever have it again. You need to be prepared to address arsenic in places and at levels that you’ve never had to address before.
Additional reading resources
“Association of arsenic with adverse pregnancy outcomes/infant mortality: a systematic review and meta-analysis.” Quansah R, Armah FA, Essumang DK, Luginaah I, Clarke E, Marfoh K, et al. Environ Health Perspect. 2015;123(5):412-21.
Arsenic in tube well water in Bangladesh: health and economic impacts and implications for arsenic mitigation. Flanagan, SV, Johnston RB and Zheng Y (2012). Bull World Health Organization 90:839-846.
“Effects of Arsenic Toxicity at the Cellular Level: A Review.” Vigo, J. B. & J. T. Ellzey (2006). Texas Journal of Microscopy. 37 (2): 45–49.
“The role of biomethylation in toxicity and carcinogenicity of arsenic: A research update.” Stýblo, M.; Drobná, Z.; Jaspers, I.; Lin, S.; Thomas, D. J. (2002). Environmental Health Perspectives. 110 (Suppl 5): 767–771
Treatment Technologies for Arsenic Removal. US EPA Office of Research and Development, National Risk Management Research Laboratory
About the author
Greg Reyneke has almost three decades of ongoing experience in the management and growth of water treatment dealerships. His expertise spans the full gamut of residential, commercial and industrial water quality management applications. A recipient of the Ray Cross and Regents Awards, Reyneke has been active in the WQA since 2004 and has served on numerous committees and task forces. He is past-President of the Pacific Water Quality Association and serves on the WQA Board of Directors and Board of Governors. Reyneke writes prolifically and travels worldwide, helping to improve human life through better water quality. You can follow him on his blog at www.gregknowswater.com.