1
Removing Arsenic Compounds from Water
Arsenic compounds in water
Arsenic is the 20th most abundant element in the earth’s crust, with an average concentration in rocks of about 2 mg kg-1. In soils, the average concentration is about 5 mg kg-1. Most of the naturally occurring compounds in surface rocks and soils are insoluble, as all of the soluble material has been washed away, resulting in a concentration in seawater of about 4 µg L-1. The concentrations in rivers and lakes ranges from 0.1 to 2 µg L-1. There is, of course, considerable variation in surface water concentrations, and waters impacted by relevant industrial activity or drainage through mine waste or unique geology may have concentrations as high as 200 mg L-1.
With regard to drinking water, the US Environmental Protection Agency has established a maximum contaminant level of 10 µg L-1 for municipal drinking water systems, all of which should have been in compliance by January 2006. Many countries still use 50 µg L-1as the action level, even though the WHO suggests that 10 µg L-1should be the target. The predominant chemical species as a function of pH and redox potential are shown in Fig. 1 [1]. Both arsenate and arsenite are the conjugate bases of weak acids. The pKa values of arsenate are 2.1. 6.7 and 11.2, and of arsenite are 9.1, 12.1 and 13.4. Water in contact with air will typically have an Eh value of 0.35 to 0.5 V, and so the most likely species in oxygenated natural waters is H2AsO4-.
Parts of SE Asia, notably Bangladesh and West Bengal, India have a major problem with groundwater contamination. The problem is so great because very many people, especially in rural areas, are drinking water that is easily obtained from a shallow aquifer via inexpensive tube wells. It has been estimated that 35 – 77 million people are exposed to arsenic-contaminated drinking water and that as many as a quarter of a million people will die from arsenic-induced cancers. There are a variety of mechanisms by which the arsenic gets into the ground water, but the predominant one by far is the dissolution of iron oxides that contain surface adsorbed arsenic as a result of bacterial action. The microbes that are feeding on dissolved carbon or buried peat are using the oxygen from the iron oxide, which produces soluble iron and arsenic as a byproduct of their metabolism. Concentrations can be in excess of 1000 µg L-1.
Bangladesh uses the 50 µg L-1action limit and has a program of testing in which wells are painted green if the concentration is less than 50 µg L-1and red if the concentration is greater than 50 µg L-1.
Remediation
There are three main strategies proposed for the “solution” to the problem of ground water: (a) purify the water before use, (b) use (suitably treated) surface water, and (c) drill into the deeper aquifer that is not (yet) contaminated with arsenic. A very large number of potential methods for purifying the well water by adsorption of the dissolved arsenic on a solid adsorbent material have been proposed. In a 2007 review article [2], entitled “Arsenic removal from water/wastewater using adsorbents,” 616 relevant articles were cited. Charcoal is a popular material and can be made from an very large number of starting materials including: bones, bagasse, bark, beat-sugar sludges, blood, blue dust, coal, coffee beans, coconut shell, coconut coir, cereals, carbohydrates, cottonseed hulls, corn cobs, distillery waste, fuller’s earth, fertilizer waste, slurry, fish, fruit pits, graphite, human hairs, jute stick, kelp and seaweed, lignin, lignite, lampblack, leather waste, municipal waste, molasses, nut shells, news paper, oil shale, olive stones, petroleum acid sludge, pulp-mill waste, palm tree cobs, petroleum coke, petroleum acid sludge, potassium ferrocyanide residue, rubber waste, rice hulls, refinery waste, reffination earth, scrap tires, sunflower seeds, spent fuller’s earth, tea leaves, wheat straw, and wood. In addition almost every kind of metal oxide, several minerals, and many biological materials (both live and dead) have been investigated. There is considerable interest in “zero valent” iron, as well as various iron-containing minerals and oxides as the most successful household devices have been based on passage through iron-bearing filter media [3].
In this experiment you will investigate the ability of your chosen material to remove the arsenic (in the form of arsenate) from a solution containing 100 µg L-1arsenic.
Materials
- Protective gloves, goggles, and lab coats or aprons
- Reaction vessels with removable screw caps
- Test strips
- Reagent packets #1 and #2
Experimental Precautions
The sensing pad on the strip contains mercuric bromide. All mercury compounds should be considered to be potentially hazardous. Do not touch the pad and dispose of used strips in the package provided. Hydrogen and arsine gases are generated. Hydrogen is potentially flammable and arsine is extremely toxic. Do not smell the gases that are evolved, and remove the cap from the vessel at the end of the experiment in a well-ventilated area (a fume hood, if possible).
It is important to:
- Wear protective gloves, goggles, and a lab apron or lab coat
- Work on a surface that can easily be cleaned.
- Work in a well-ventilated area.
When you have completed your analysis of a water sample for arsenic compound concentrations is it important that you:
- Continue to wear your gloves, goggles, and lab coat or apron as you dispose the arsenic test strips, empty reagent packets, and contents of the reaction bottles.
- Dispose the arsenic test strips and water samples in containers provided by your teacher.
- Put the reaction vessels in a location provided by your teacher.
Experimental Procedure:
Prepare a sufficient volume of 100 µg L-1As solution so that after interaction with the absorbing medium, there is at least 50 mL of solution to test with the Hach kit. Place this volume in a suitable beaker with the known mass of your chosen absorbent and stir, agitate, swirl or otherwise ensure that the contents of the solution come into contact with as much of the surface of the material as possible. Note the time for which the solution is in contact with the medium. If the result of a recent analysis of a 100 µg L-1solution is not available, set up two test vessels: one for the “purified” water, and one for the original “contaminated” water. Decant 50 mL of the “purified” water into the reaction vessel. Measure the arsenic content as usual with the Hach kit.
- Lift the flap on the black cap and gently slide a test strip into the groove so that the reactive pad faces down toward the small opening and completely covers the opening. Close the black flap.
- Add a water sample to the reaction vessel up to the 50-mL mark.
- Add the content of one reagent Packet #1 to the water and swirl until it has dissolved.
- Add the content of one reagent Packet #2 to the vessel.
- Immediately cap the vessel but do not over tighten. Do not shake or invert the vessel. Swirl gently for 5 to 10 s. Do not splash the test strip.
- Let the reaction proceed for 40 min. Swirl the solution at least twice.
- Remove the test strip and place on the sheet of standard spots and photograph with a digital camera.
- Transfer the image to a computer and analyze with the ADI software. Plot the relevant R, G or B value as a function of the arsenic concentration and interpolate the arsenic concentration corresponding to your sample.
- Calculate the efficiency of your selected arsenic-removal procedure as a percentage, and the capacity of the adsorbent in µg g-1.
Camera Operation
Any digital camera can be used; a Pentax Optio W90, a 12.1 megapixel, camera is suggested. The key components are: 1) Turn off the flash. 2) Check the ISO values, the default on cameras can be as high as 800. Change the default to the lowest setting. The camera used in the development of this experiment was set to ISO 100; ISO 80 also works. 3) Place it into the microscope mode if the camera has this setting. 4) Hold the camera straight over the picture. The camera should be 2-3 inches above the picture to be taken; it depends on the area of the picture to be taken. 5) Check the picture after the shot to make sure it is in focus and clear. Other key components of the camera used for this lab were: image tone was natural (camera default), white balanceawb (camera default), sensitivity auto (camera default)sharpness, saturation, and contrast set in the middle (camera default.)
References.
1. B.W. Vink, “Stability relations of antimony and arsenic compounds in the light of revised and extended Eh-pH diagrams,” Chemical Geology, 130 (1996) 21-30.
2. D. Mohanand C. U. Pittman Jr. “Arsenic removal from water/wastewater usingadsorbents—A critical review,”Journal of Hazardous Materials, 142 (2007) 1–53.
3.The Grainger Prize, National Academy of Engineering, 2007
(accessed June 21st 2011)