By Frank Baumann, PE
Chemistry and toxicity
Because of its appearance, arsenic is considered a metalloid and is not a true metal. Arsenic exists in four valence (oxidation) states. Valence is defined as the combining power of an element with other elements or compounds. These four states are:
- Arsenate (As+5)
- Arsenite (As+3)
- Elemental arsenic (As0)
- Arsine gas (As-3)
In water, if arsenic is present, it is predominantly in the As+5 (arsenate) form. Arsenite (As+3), when present, is readily oxidized to arsenate in aerobic waters at pH values above 7.0. Conversely, arsenate (As+5) can be reduced to arsenite at low pH values.
Of the two predominant species, the trivalent form, arsenite (As+3) is considerably more toxic than the pentavalent (As+5) form, although it has been demonstrated that As+5 is better absorbed by the human body because it tends to react less with the membranes in the gastrointestinal tract.1
Arsenic metabolism is a two-step process: arsenate (As+5), entering a cell, is reduced to arsenite (As+3), which is then methylated to arsenate. The methylation is a detoxification step, since neither MMA or DMA are as toxic to the system as is inorganic arsenic, although the chronic effects of MMA or DMA are not known.2
Arsenic removal processes
The U.S. EPA will identify a number of treatment options for the removal of arsenic as best available technology (BAT). Discussed below, in no particular order of applicability, preference, efficiency or treatment cost, are some of the processes under consideration.
This membrane process is primarily designed for the desalting of saline or brackish waters by the application of hydrostatic pressure.3 This overcomes osmotic pressure and drives the water to be treated through a semi-permeable membrane designed to allow passage of water, but not of dissolved contaminants. The process requires expensive and fragile membrane stacks, either cellulose-acetate or thin film composite. Cellulose-acetate membranes can be operated at up to 400 psi, but are subject to biological attack and hydrolysis. They also allow the salt passage to double after a service life of about three years. The more expensive thin film composite membranes are capable of the same or greater flux rate, but at half the applied pressure. These allow only a less than 30 percent increase in salt passage after three years. Both require considerable pre-treatment to prevent scaling, plugging and colloidal or biological fouling of the membranes.
Since the recovery of product water, as a percentage of feed water, is a function of applied hydrostatic pressure (up to 400 psi or more), the process tends to be quite energy intensive. Most reverse osmosis plants are designed for 75-80 percent recovery; i.e., up to 25 percent of the flow must be disposed of as a concentrated, possibly hazardous, waste.3 Reverse osmosis is quite capable of the removal of arsenic to very low levels. Process operation and maintenance costs, as well as labor intensity, will tend to rule out its application for all but small volume treatment systems.
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. Toxic characteristic leaching procdure (TCLP) testing is a federal test commonly used to determine if the spent media is classified as hazardous or non-hazardous. State regulations should be consulted to verify acceptable testing methods and media disposal requirements.
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 has a long history of use as an adsorptive treatment technology for arsenic removal. The media is a by-product of aluminum production. It is primarily an aluminum 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. Process demonstrations have shown that arsenic removal capacity has been reduced by more than 15 percent at pH 6.0 compared to that of pH 5.5.
Fluoride, selenium and other inorganic ions and organic molecules are also 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 can either be regenerated or 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 sites 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 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.4
Excess lime softening is the addition of a sufficiently high lime dosage (at times in excess of one 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 percent of any arsenic that may be present.5
The removal of trivalent arsenic appears to be dependent upon the precipitation of magnesium hydroxide [Mg(OH)2]. The addition of powdered activated carbon, though apparently not absolutely necessary, appears to enhance removal efficiency. 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 recarbonation of the water and produces large volumes of sludge. For these reasons, unless there is also a demonstrated need for softening, the process may not be deemed economically viable.
Conventional coagulation/flocculation/filtration, using iron salts, is effective in the removal of up to 90 percent of arsenate (As+5) at pH levels of seven or less.6 Above a pH of seven flocs from iron salts effectively remove arsenic. Iron coagulants will remove about 50 percent of trivalent arsenic (As+3)3. Thus, it is very important to fully oxidize As+3 to As+5 with chlorine or another strong oxidant prior to coagulation.
High arsenic concentrations are frequently found in anaerobic waters. These same waters are generally high in ferrous iron (Fe+2) and manganous manganese (Mn+2), necessitating their removal. Iron and manganese removal processes relying on the oxidation and subsequent precipitation of the metals as hydroxides, will also effectively remove soluble arsenic by co-precipitation and/or adsorption reactions. It is almost as though the presence of the iron and manganese to be removed is analogous to a natural coagulant addition, as it facilitates the removal of arsenic. Removals of soluble arsenate (As+5) during ferrous iron oxidation and precipitation processes are very significant. This is not likely the case during soluble manganese (Mn+2) oxidation alone. In such cases, the addition of iron salts is indicated.
Of the arsenic removal processes available and discussed in this article, iron coagulation, whether practiced primarily for the removal of arsenic or also of iron and manganese, appears to be the most promising and proven process available. This process is capable of the removal of 90 percent or more of any arsenic present. Furthermore, many such plants are already in existence and operating efficiently, albeit they were initially designed for the removal of iron and manganese only.
Some iron and manganese removal processes rely on a proprietary adsorptive media for their removal efficiencies. In these systems, the iron/manganese (along with any arsenic present) are oxidized, then the iron and manganese are precipitated as hydroxides, adsorbing arsenic. A second adsorptive reaction occurs at the water/media interface where localized zones of high pH assure not only the continued formation, but the maintenance of an active, adsorbent, hydroxide floc.
The mechanism is not unlike the reactions occurring in an activated alumina system, with the exception that the adsorptive media process requires no regeneration and can be backwashed like a conventional sand filter. The backwash water can be decanted and reclaimed allowing approximately 99 percent recovery and water recycle. Since the backwash water is non-hazardous, it can also be drained into a sanitary sewer.
Electrodialysis (ED) and electrodialysis reversal (EDR)
ED is an electrochemical membrane process initially developed for the treatment of saline or brackish waters.3 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 labor intensive. It also creates a concentrate which must be disposed of and is quite 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 pretreatment to minimize fouling of the membrane. ED/EDR systems are not considered to be economically viable for any but very small installations.
This process, also known as membrane softening, uses an ultra-low-pressure membrane designed to allow only passage of particles less than one nanometer (10 Angstroms) in size. It is, thus, very efficient (more so than reverse osmosis) in the removal of dissolved matter, but is, of course, not selective for arsenic only. Like all other membrane processes, extensive pretreatment is necessary to prevent fouling of the delicate and expensive membranes caused by particulate matter, scaling or biofouling.
Coagulation assisted membrane process (CAMP)
Coagulation assisted membrane process (CAMP) is considered to be a promising technology for arsenic removal because it can be applied over a wide range of water quality that contains high turbidity, iron, manganese, sulfate and nitrate. Low-pressure membranes (e.g. microfiltration and ultrafiltration) are very effective for removing particulate arsenic; but without a pre-coagulation step, low-pressure membranes are ineffective for removing soluble arsenic.
Metal-based coagulants, such as ferric chloride, can be used to bind the arsenic which is removed with the ferric floc on the membrane. The use of low-pressure membranes eliminates the breakthrough of arsenic-laden coagulant flocs (a typical occurrence with conventional granular media filters) by taking advantage of the membranes’ particle barrier. Factors affecting the CAM process include ferric chloride dosage, pH, mixing and floc formation (contact time). As with all membrane processes, provision for adequate pretreatment to control feed water quality should be taken to protect the membrane from fouling caused by particulate matter, scaling and biofouling to optimize membrane performance and life. Disposal of the reject coagulant (which is not considered to be a hazardous waste) can be to a sanitary sewer.
Due to the fact that varying ground water quality can significantly affect arsenic removal processes, pilot testing at each well site is recommended. Raw water quality analyses should be made prior to pilot testing to determine all of the constituents in the water that can affect arsenic removal processes. The pilot filter system should be designed to treat all of the constituents in the raw water that will affect the efficiency of the treatment process. On-site testing should be verified by raw and treated water samples that are tested by an independent certified laboratory. The pilot process should include pretreatment equipment as dictated by the raw water quality analysis to assure continued treatment to below U.S. EPA standards and to maximize process runs and optimize media/membrane life. The pilot system must verify removal of arsenic throughout the process run cycle as well as determine pretreatment chemical requirements and the following costs: operations, labor, media disposal and replacement, membrane disposal and replacement and/or regenerant brine disposal.
Arsenic, long known as the poison of choice because of its legendary toxicity at high doses, has been implicated in skin and internal carcinogenesis, thus the requirement to be regulated at the current EPA level of 0.010 mg/L or 10.0 µg/L.
The new maximum contaminant level (MCL) will largely drive and dictate the installation of arsenic removal processes. Of the removal processes available, iron and arsenic oxidation, followed by coagulation/filtration, appear to be the most promising. Meanwhile, many researchers are exploring modifications to conventional treatment, with a particular emphasis on enhancing existing coagulation/flocculation operations.
Pilot testing should be conducted at each well site to verify arsenic removal rates, process design and operational costs.
- National Academy of Sciences, Drinking Water and Health, Vol. III, National Academy Press, Washington DC, 1980.
- McKee and Wolf, Water Quality Criteria, Water Quality Control Board, State of California, 1963.
- Montgomery, J.M., Consulting Engineers, Water Treatment Principles and Design, Wiley Interscience Publications, 1985.
- Rubel, Jr., Frederick, P.E., Design Manual: Removal of Arsenic from Drinking Water by Adsorptive Media, March 2003.
- Dutta, A. and Chauduri, M., Removal of Arsenic From Groudnwater by Lime Softening with Powdered Coal Additive, JWater, SRT-Aqua 40.1, 1991.
- Edwards, M., Chemistry of Arsenic Removal During Coagulation and Iron and Manganese Oxidation, JAWWA 86.9, 1994.
- AWWA Final Report, National Drinking Water Advisory Council, 2001.
- AWWA Water Quality and Treatment (Fourth Edition.
- Environmental Protection Agency (EPA), Final Arsenic Rule, January 20, 2001.
- Feasibility Studies of Using Coagulation Assisted Membrane Processes For Arsenic Removal, HDR Engineering, Bellevue, Wash., 2001
- Pontius, F.W., Safe Drinking Water Act Advisor, American Water Works Assoc., 1994
- Pontius, F.W., et al., Health Implications of Arsenic in Drinking Water, JAWWA 86.9, 1994.
- Schroeder and Balassas, Journal of Chronic Diseases, 19:85-106, 1966.
- Stewart, H.T. and Kessler, K.J., Evalutaion of Arsenic Removal by Activated Alumina Filtration at a Small Community Public Water System, Journal NEWWA, Sept. 1991.
This article was prepared for Pureflow© by Frank Baumann, PE, formerly Chief of the Sanitation and Radiation Laboratories Branch of the Department of Health Services, State of California. For further information, contact Pureflow Filtration Systems Division, California Environmental Controls, Inc., P.O. Box 469, 6739 South Washington Avenue, Whittier, Calif. 90608; telephone 800/926-3426; fax 562/693-5257 or email [email protected]. Or visit the company’s website, www.pfdiv.com. Opinions contained herein are those of the author and do not necessarily reflect policy of the State of California Department of Health Services.