METALS IN MEDICINE AND THE ENVIRONMENT

Arsenic is well known as a toxic element.  The World Health Organization (WHO) has set a recommended limit of 10 mg/L (reduced from 50 mg/L in 1993) concentration of arsenic in drinking water (1, 2).  The increased knowledge of arsenic toxicity, particularly carcinogenesis, led to the change.  Not until 2001 did the United States follow WHO by also reducing the acceptable arsenic concentration to 10 mg/L.  In many countries an acceptable arsenic level remains 50 mg/L.   This is in part due to inadequate testing facilities and equipment.   In some areas of the world, arsenic is present in the groundwater at concentrations higher than the set provision (1).  Natural and unnatural arsenic in the environment can lead to groundwater contamination.  Figure 1 shows areas where arsenic is present in aquifers.

Figure 1.  Map illustrating arsenic-affected aquifers (1).

The two common forms of arsenic are As(III), arsenite, and As(V), arsenate.  Arsenite is more mobile and toxic than As(V).   Arsenic can be released from sediments under anaerobic reducing conditions in the form of arsenite, and under oxidizing conditions where the pH is high, in the form of arsenate (HAsO4­­­2-) (1, 3, 4).

There are a variety of symptoms resulting from chronic arsenic exposure.  Skin lesions and hyperkeratosis are signs of arsenic poisoning (Figure 2).  These telltale signs can take as long as 10 years to appear (2, 5, 6).  Skin cancer is the most common cancer with a connection to arsenic poisoning.  Cancers of the bladder, kidney, and lung are being linked to excessive arsenic exposure.   Low dose exposure to arsenic may lead to neurological disease, cardiovascular disease, or diabetes (6).


Figure 2.  A woman shows signs of arsenic poisoning.

Arsenic in Bangladesh

Dr. Alan Smith, director of the arsenic research program at the University of California, Berkeley, states, “The contamination of groundwater by arsenic in Bangladesh is the largest poisoning of a population in history…” (6).
Beginning in the 1970’s the United Nations Children’s Fund (UNICEF) partnered with the Department of Public Health and Engineering to install tube-wells to provide a safe source of drinking water.  These wells were to replace water supplies contaminated with bacteria and responsible for causing large outbreaks of disease.  During the time the wells were installed, arsenic was not a known concern and was therefore not tested.  Today, there are between 8-12 million tube wells and 90% of the population receives their drinking water from a well (2, 6). 

In the mid-1980’s there was increasing evidence that arsenic was hazardous.  However, the fact was not acknowledged until 1993 when WHO decreased the acceptable concentration (6).  The issue in Bangladesh was not addressed until 1998 when the World Bank attempted to initiate a program to screen the countries tube wells (5).  

Arsenic concentrations in the range of <0.5 µg/L to 3200 µg/L were found in groundwater across Bangladesh (2, 7).  Figure 3 shows a map of Bangladesh and the concentration of arsenic in groundwater.  The estimated number of people exposed to arsenic concentrations greater than 50 µg/L is 28-35 million and the number of those exposed to more than 10 µg/L is 46-57 million (2).   Chakraborti et al. reported that of 27,000 wells sampled in Bangladesh, 59% of them exceeded 50 µg/L and 73% exceeded 10 µg/L (8).  According to some approximations, arsenic in drinking water will cause 200–270 thousand deaths from cancer in Bangladesh alone. (5). 


Figure 3. Map showing concentrations of arsenic in groundwater (1).

The problem in Bangladesh is further detrimental due to the economic status of the country.  Many of the people are living in poverty and are unaware of the contamination, and cannot afford healthcare. 

Linking Arsenic in the U.S. to Chickens

It is reported that the poultry industry in the United States supplements chicken feed with roxarsone, 3-nitro-4-hydroxybenzene arsonic acid.  The organic arsenic compound promotes growth and controls intestinal parasites.  Nearly all the arsenic consumed by the chickens is excreted in the manure.  Manure has been used as a nitrogen containing fertilizer for centuries and is spread onto farmland (9).   A large percentage of the nations poultry comes from the Delmarva Peninsula, with Sussex County, Delaware being the largest producing county of broiler chickens in the United States (10). Chicken manure introduces huge quantities of arsenic to agricultural land.  Poultry litter is spread on land at the rate of 9 to 20 metric tons per hectare. Each year, it is estimated, 20 to 50 metric tons of roxarsone in chicken litter is applied to fields on the Delmarva Peninsula (11).


 
Figure 4.   Roxarsone

Roxarsone itself is not toxic, but the compound is converted to inorganic arsenate fairly quickly.  The organic form is soluble in water can be easily leeched into groundwater or surface water. The genus Clostridium is responsible for the conversion to As(V).  In anaerobic reducing conditions, the As(V) is further converted to As(III), a more mobile and toxic form.  The processes involved in the transformation have not been investigated to date (12).  In Figure 5 the path from chicken feed to water and beyond is illustrated (13). 

Manure is spread onto fields and rainwater causes run-off into rivers, streams, and lakes.  Studies report that elevated concentrations of arsenic was found in the Pocomoke River near areas spread with poultry manure.  Another study, by the Maryland Geological Survey, found arsenic levels violating federal health standards in 11 percent of 250 drinking wells sampled on the Eastern Shore and elsewhere in the state (14).  There is not a large amount of evidence linking these higher arsenic levels to chicken litter, but it cannot be ruled out. Arsenic may have an impact on aquatic life.   Arsenic represses the immune system in fish and may be contributing to the fish-kill in the Shenandoah River (15).  An additional manner of arsenic pollution is through pellets sold as garden fertilizer.  This can lead to exposure by dust (9).  



Figure 5. Arsenic from chicken feed to water (13).

 

Arsenic Remediation

Due to the high solubility in water across a wide pH range, an oxidation step is required to reduce both solubility and mobility.  Manganese(IV) oxides represent one of the main redox catalysts in the environment, while also extensively adsorbing a number of anions and cations.  Their highly reactive surfaces allow manganese oxides, even in low concentrations, to oxidize metals such as arsenic from arsenite to arsenate, a much less reactive and mobile form (Figure 6). However, several variables can influence the oxidation reaction: a variety of particle sizes can be found in nature ranging from a nanometer to micrometer size particles, thus altering the exposed surface area. Iron oxides will also reduce arsenic, but much slower.  The released Mn2+ ions adsorb onto the manganese dioxide, giving it a positive surface charge and leading to an enhancement in the removal of arsenate produced as a result of arsenite oxidation (3).

H3AsO3 + MnO2 --> HAsO42- + Mn2+ + H2O


Figure 6. The reduction of arsenite by manganese dioxide (3).

This treatment could be applied to water filtration.  Bajpai and Chaudhuri constructed a home arsenic removal unit that was affordable and effective.  They made manganese dioxide coated sand and used it to efficiently remove arsenic from a water sample.  The unit cost approximately five U.S. dollars (4).

Resources

World Health Organization – Arsenic in drinking water

Wikipedia – Arsenic contamination of groundwater

Image Sources

Poisoned woman

Roxarsone

References

(1) Smedley, P. L. and Kinniburgh, D. G.  A review of the source, behaviour and distribution of arsenic in natural waters.  Applied Geochemistry. 2002, 17:517-568.

(2) World Health Organization.  Arsenic in drinking water. May 2001. Accessed October 26, 2008.

(3) Driehaus, W., Seith, R., and Jekel, M. Oxidation of arsenate(III) with manganese dioxides in water treatment.  Wat. Res. 1995, 29(1):297-305.

(4) Bajpai, S. and Chaudhuri, M.  Removal of arsenic from ground water by manganese dioxide – coated sand.  J. Envir. Eng.  1999, 125(8):782-784.

(5) Pearce, F.  Bangladesh’s arsenic poisoning: who is to blame?  The UNESCO CourierJanuary 2001. Accessed October 26, 2008.

(6) Smith, A. H., Lingas, E. O., and Rahman, M.  Contamination of drinking-water by arsenic in Bangladesh: a public health emergency.  Bulletin of the World Health Organization2000.  78(9):1093-1103.

(7) Vu, K. B., Kaminski, M. D., and Nunez, L.  Review of arsenic removal technologies for contaminated groundwater.  2003. Accessed October 26, 2008.

(8) Chakraborti, D. et al.  Characterization of arsenic based sediments in the Gangetic Delta of West Bengal, India.  Arsenic Exposure and Health Effects IV2001.  27-52.

(9)  Christen, K.  Chicken poop and arsenic.  Environmental Science and Technology Online. March 29, 2006. Accessed October 26, 2008.

(10) Delmarva Poultry Industry.  Delaware broiler chicken production. May 2008. Accessed October 26, 2008.

(11) Hileman, B.  Arsenic in chicken production. Chemical and Engineering News. April 7, 2007.  85(15):34-35. Accessed October 26, 2008. 

(12)  Stolz, J. F., Perera, E., et al.  Biotransformation of 3-nitro-4-hydroxybenzene arsonic acid (roxarsone) and release of inorganic arsenic by Clostridium species.  Environ. Sci. Technol. 2007, 41:818-823.

(13) Denver, J., Ator, S. W., et al.  Water quality in the Delmarva Peninsula, Delaware, Maryland, and Virginia, 1999-2001. U. S. Geological Survey. 2004. Accessed October 26, 2008.

(14) Pelton, T. Arsenic's use in chicken feed troubles health advocates.  March 10, 2007. Accessed October 26, 2008.

(15) Something fishy about the Shenandoah River. March 2008. Accessed October 26, 2008.

Author: Joseph Houck