Arsenic in Water:
Arsenic is found in the environment as oxides, with two different oxidation states: trivalent arsenic oxide, As4O6, and pentavalent arsenic oxide, As4O10. Both oxides are acidic, when hydrolyzed they form oxo-acidic compounds. Similar to phosphorus chemistry, trivalent arsenic oxide is hydrolyzed to arsenious acid in the reaction:
As4O6 + 6 H2O → 4 H3AsO3
Arsenious acid dissociates into hydrogen ions and an Arsenite ion. The trivalent form of arsenic is also known as Arsenite.
The pentavalent arsenic anion – Arsenate – can occur from oxidation of the Arsenite ion. It is also formed by the hydrolysis of the pentavalent oxide in the reaction.
As4O10 + 6 H2O → 4 H3AsO4
As V is the form usually found in surface water and occasionally in ground water. As III is generally found in ground water sources.
Within the pH range of 6.5 – 8.5 for drinking water, arsenious acid begins to dissociate, but the predominant form is non-ionic, making it harder to remove.
Following effects of consuming Arsenic laden Water make Arsenic removal mandatory:
The effect on the human body to low-level exposure of arsenic is a matter of ongoing study. Long-term exposure to arsenic by drinking water is directly linked to cancer of the skin, lungs, urinary bladder and kidneys. It can cause acute gastrointestinal and cardiac damage, as well as vascular disorders such as Blackfoot disease. The poison passes through the placental membrane and is metabolized by the foetus. Sub-lethal effects include diabetes, keratosis,
heart disease and high blood pressure. Toxicity is dependent on diet and health, but is cumulative. Arsenic is very slowly excreted by the body through deposition in the hair and nails. Acute exposure can sometimes be addressed by chelation therapy, although this is not an option for long-term ingestion.
There are various methods for removal of Arsenic from Water. The method selection depends on various factors like the type of Arsenic in water, quantity to be treated, concentration of Arsenic, site conditions etc.
Granular Ferric Hydroxide (GFH):
Granular Ferric Hydroxide (GFH) is an absorptive medium designed for the removal of arsenic, phosphates, chromium and other heavy metals. Raw water pH and contaminant concentration (e.g. iron, manganese, chromium, organics, silica, phosphates, etc.) determine the life of the media. Preoxidation of raw water is not required and both arsenic valence states are removed. Periodic backwashing of the media is required depending on raw water quality. GFH is presently classified as a non-regenerative media that must be removed from the filter vessel when exhausted, and replaced with new media. Research is being conducted to determine the feasibility of regenerating GFH. Spent GFH media is disposed of in a landfill. State regulations should be consulted to verify acceptable testing methods and media disposal requirements.
Ion Exchange:
Capable of complete removal of all dissolved matter, including arsenic, from water, this process is widely used for the production of deionized water. One great advantage of Ion Exchange is that no pH adjustment is necessary. Recent advances in resin technology have replaced the weak-base anion resins with strong-base ones. Pentavalent arsenic (As+5), being present as the divalent anion HAsO4
-2, appears to have a greater affinity for this type of resin. Strong base resins permit the use of ordinary sodium chloride brine for regeneration, and eliminate the need for the use of strong acids. Regeneration is a slow and water-intensive process. Typically, columns are rinsed with 1-2 bed volumes to displace the regenerant. This is followed by a fast rinse for about 10 minutes at design flow. The used regeneration brine, containing arsenic, is a hazardous waste and must be disposed of accordingly.
Activated Alumina:
Activated alumina has a long history of use as an adsorptive treatment technology for arsenic removal. The media is a by-product of aluminium production. It is primarily an aluminium oxide that has been activated by exposure to high temperature and caustic soda. The material is extremely porous and has a high average surface area per unit weight (~350 m2/g). The capacity for arsenic removal by activated alumina is pH dependent, with the maximum removal capacity achieved at pH 5.5. Adjusting the pH of the source water, therefore, provides removal capacity advantages. As the pH deviates from the 5.0 – 6.0 range, the adsorption capacity for arsenic decreases at an increasing rate. It has been observed that that arsenic removal capacity has been reduced by more than 15% at pH 6.0 compared to that of pH 5.5.
Fluoride, selenium, and other inorganic ions and organic molecules also are removed by the same pH adjustment activated alumina process. The process, however is preferential for arsenic at the optimum pH level of 5.5. Other ions that compete with arsenic for the same adsorptive sites at other pH levels are not adsorbed in the pH range of 5.0 – 6.0. Included are silica and hardness ions that are adsorbed in the pH range of 7 – 10. Activated alumina either can be regenerated or can be replaced with new media when the selected breakthrough point is reached. At the optimum pH for arsenic removal, fluoride, selenium, some organic molecules, and some trace heavy metal ions are adsorbed; however, these are also completely regenerated along with arsenic. Because these ions compete for the same adsorptive sties with arsenic, their presence might deplete the alumina capacity for arsenic. When excess fluoride and arsenic are present in the water supply, a special treatment technique is required.
The adsorptive capacity of many adsorptive media, particularly activated alumina, is pH sensitive; removal capacity increases with decreasing pH. Employing pH adjustment, therefore, generally provides cost advantages regardless of whether the media is regenerated or replaced. Because the pH adjustment chemicals are usually the same chemicals that are used for regeneration, it is generally advantageous to couple regeneration with pH adjustment systems when the media can be regenerated
Lime Softening:
Excess Lime Softening is the addition of a sufficiently high lime dosage, at times in excess of 1 gram per liter, to obtain a pH greater than 11.5. It has long been used for the removal of calcium and magnesium carbonate hardness, and is also capable of the removal of approximately 90% of any arsenic that may be present. The removal of trivalent arsenic appears to be dependent upon the precipitation of
magnesium hydroxide {Mg(OH)2}. While this is an old tried and true process, and while apparently quite capable of arsenic removal, the process remains plant and chemical intensive, requires the re-carbonation of the water, and produces large volumes of sludge. For these reasons, unless there is also a demonstrated need for softening, the process is not economically viable.
Coagulation Filtration:
Conventional coagulation/flocculation/filtration, using iron salts, is effective in the removal of up to 90% of arsenate (As+5) at pH levels of 7 or less. Above a pH of 7 flocs from iron salts effectively remove arsenic. Iron coagulants will remove about 50% of trivalent arsenic (As+3). Thus, it is very important to fully oxidize As+3 to As+5 with chlorine or another strong oxidant prior to coagulation.
Electrodialysis (ED) and Electrodialysis Reversal (EDR)
ED is an electrochemical membrane process initially developed for the treatment of saline or brackish waters. Instead of hydrostatic pressure, the process uses an applied direct current (DC) voltage to move dissolved anions and cations from alternate cells through semi-permeable membranes. This purifies a portion of the feed water, while concentrating another. While capable of removing arsenic to low levels, the process is equipment, energy and labour intensive. It also creates a concentrate which must be disposed of, and is wasteful of water.
EDR is an ED process which reverses the polarity of the electrodes on a controlled time cycle, which reverses the direction of ion movement in a membrane stack. Reversing polarity provides automatic flushing of scale forming minerals from the surface of the membrane. EDR typically requires little or no pre-treatment to minimize fouling of the membrane. ED/EDR systems are not considered to be economically viable for any but very small installations.
Nano Filtration:
This process, also known as “membrane softening” uses an ultra-low-pressure membrane designed to allow only passage of particles less than 1 nanometer (10 Angstroms) in size.
It is, thus, very efficient in the removal of dissolved matter, but is, of course, not selective for arsenic only. Like all other membrane processes, extensive pre-treatment is necessary to prevent fouling of the delicate and expensive membranes caused by particulate matter, scaling, or biofouling.
Reverse Osmosis:
Reverse Osmosis or hyper filtration as it is called due to ability of membranes to filter out the ions. The membranes are not ion selective though and will remove almost all the salts. The salt rejection can be achieved up to 99.5 % in some membranes & hence can be successfully implemented for Arsenic removal also. The only flip side of RO process is the extensive pre-treatment & power required to force the pure water through the membranes.
