Environmental Science & Engineering - www.esemag.com - January 2005
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Arsenic in the environment

By Derek K. French

Close-up SEM photos of ferric adsorptive media.

Arsenic is a naturally occurring metalloid element, the 20th most-abundant element in the earth's crust and is the 12th most abundant element in the human body. Industrial sources include wood preservatives, metallurgical (alloying and smelting) processes and mining runoff. Combustion of fossil fuels is also a source of arsenic in the environment through atmospheric deposition. Tasteless and odourless, arsenic has been used as a poison for killing weeds, rats and insects and is toxic to humans in quite low concentrations. Most treatment will be for naturally occurring sources of arsenic; of these, about 97% will be for groundwater.

Arsenic is introduced into water through the dissolution of minerals and ores; concentrations in groundwater in some areas are elevated as a result of erosion from local rocks. Runoff and industrial dumping have contaminated river sediments, groundwater wells and surface waters. As a dissolved substance, it will be transported by water in motion as well as by diffusion from a gradient of high concentration to areas of lower concentration. When it is pumped for drinking and irrigation, arsenic laden water is again re-distributed.

Health Effects
The effect on the human body to lowlevel exposure of arsenic is a matter of ongoing study. The US EPA has set the threshold of acceptable mortality at a maximum of one death per 1,000 people for carcinogens. A 1992 study estimated that the lifetime risk at a level of 50 µg/L would lead to as much as an additional 13 cancer-related deaths per year per thousand people. 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 fetus. Sub-lethal effects include diabetes, keratosis, heart disease and high blood pressure. Toxicity is dependent on diet and health, but is cumulative. The body, through deposition in the hair and nails, very slowly excretes arsenic. Acute exposure can sometimes be addressed by chelation therapy, although this is not an option for long-term ingestion.

In 1999, the US National Academy of Sciences (NAS) completed a review of updated scientific data on arsenic and recommended that the US EPA lower the standard as soon as possible down from the previous 50 µg/L. Using the most up-to-date information and methodology, the NAS estimated that the rate of cancer deaths from arsenic at 3 µg/L is between four and 10 per ten thousand people. In 2001, the EPA announced that the new limit in drinking water would be lowered to 10 µg/L, and that water supplies would have to be compliant by 2006. The Canadian Federal Government has recently proposed a revised arsenic limit for the Federal Drinking Water Guidelines. The proposed limit of 5 µg/L has recently been posted for comment.

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. Depending on the reducing or oxidizing condition in the groundwater and on the pH, one species or another will dominate. As V (arsenate) is the form usually found in surface water and occasionally in groundwater. As III (arsenite) is found in groundwater sources. Arsenite is more toxic than arsenate. In water at a pH of less than 6, arsenite is fully protonated and therefore exists as a neutral molecule. Within the Drinking Water Guidelines mandated 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. Arsenate in solution is either a monovalent or a divalent anion in the pH range of drinking water.

Schematic of a small adsorptive system for a small municipal drinking water supply.

Arsenic Treatment Technology
The selection of the most appropriate technology for water treatment obviously comes down to performance and beyond this must examine capital cost, long-term operational costs and complexity of operation. The primary operational costs are related to waste residual management. All arsenic removal processes will generate arsenic-laden wastes. Disposal methods will depend upon the type of waste and Federal and Provincial regulations. Reviewing the type of waste generated by removal method is imperative as removal and disposal must comply with the Toxicity Characteristic Leaching Procedure (TCLP) to determine if solid wastes can be approved for landfill. Resins and spent adsorbent materials are dealt with in the same manner as sludge. A certified laboratory can run TCLP test on wastes generated by trial processes. Check with the manufacturer to determine what analyses have already been performed; also check web sites of the accrediting organizations for listings of approved materials, e.g., www.nsf.org.

Removing Arsenic
Proven arsenic removal technologies rely on adsorption or ionic rejection. Having a complete raw water analysis is critical to the treatment evaluation process. The most common interferences with treatment are pH, anions such as silica, fluoride, phosphate and sulphate, and cations like iron, manganese, etc. Iron present in the water can be beneficial. Precipitated iron can adsorb arsenic V for removal by filtration.

Arsenic can be removed from water using a variety of processes including iron adsorption, activated alumina adsorption, ion exchange, reverse osmosis, nanofiltration, lime softening, iron and alum coagulation and coagulation- assisted microfiltration. Because most of these methods rely on ionic charge, As V will be easier to remove than As III in the normal working pH range of drinking waters, and, therefore, if arsenite is present, pre-oxidation should be the first step. Bench testing, e.g., jar or column, will take out much of the guesswork as to the applicability of the methodology and provide the practical, decision-making data to treat a specific water chemistry.

A typical oxidation and filtration schematic used in all types of processes.

Oxidation
Arsenic speciation must be done to determine whether all or part of the arsenic present is trivalent. If arsenite is present, it should be oxidized to facilitate removal. Water that contains only As V does not require further oxidation for treatment. Several methods have been shown to effectively oxidize As III to As V. Chemical feeds, including free chlorine, hypochlorite, ozone and potassium permanganate, have all been shown to oxidize arsenic. Solid oxidizing media, such as those used for iron and manganese removal and sulphide removal, have also performed well, as long as the contact time is adequate. Some methods that were expected to be successful, such as aeration, were not able to oxidize As III to As V. It is also important to note that monochloramine and chlorine dioxide are not effective oxidizers of arsenite.

An EPA demonstration in Climax, Minnesota, using adsorptive media.
Adsorptive Processes
Adsorbent media are engineered materials used in fixed beds for the removal of arsenic. A fixed-bed adsorbent system usually consists of a tank (or tanks) filled with media. It is typically operated in downflow service with the ability to backwash the vessel. A number of media are available; most are based on iron and aluminium. The removal mechanism is a reaction between the arsenic and the surface of the media. The tank size is determined by the media's flow requirements. Fine mesh media and deep bed depths increase the contact efficiency, but also increase the pressure drop and pumping energy requirements. Low velocity and long contact time will improve media capacity and reduce leakage, but require larger tanks and greater media inventories. Any system will be a compromise of system cost, floor space required, arsenic capacity and leakage and the interval for media regeneration or replacement.

a) Activated alumina
Activated alumina (AA) is formed by thermal dehydration of an aluminium hydroxide. Its principal characteristic is a high surface area and high porosity. It is able to remove a wide range of anions using an ion exchange mechanism with the hydroxylated surface. Two screen size ranges of granules are commonly used for arsenic removal, 28x48 and 14x28. The finer material has more surface area per unit volume, which gives it a higher arsenic removal capacity. The finer material also has a higher pressure drop, and that must be considered in a process design. The major water chemistry factors affecting adsorbency are pH, competing ions, EBCT and arsenic oxidation state. Activated alumina is most effective at lower pH values and depending on the alkalinity of the water, it may be cost-effective to reduce the pH of the feed to an activated alumina system and raise the pH before distribution.

Activated alumina has been used as a regenerable media for removal of fluoride and phosphate in wastewater treatment applications but is both complex and costly to regenerate using a two step process. Typically for water applications AA is used as a disposable adsorbent.

An activated alumina design will encompass pH adjustment, empty bed contact calculations (EBCT) and calculations of loading rates per unit area of media. All these factors influence bed volume capacity and subsequently usable time between media change outs. Recent studies by the American Water Works Association Research Foundation (AWWARF) indicate that AA's low cost and disposability makes it an attractive choice for smaller municipal systems as well as point-ofentry for individual dwellings. Overall, activated alumina adsorption is probably the best-understood and characterized method for arsenic removal.

b) Iron adsorbents
Iron-based adsorbents operate on the adsorptive affinity of iron oxides for arsenic. A number of manufacturers currently have media that appear to work to varying degrees.

Iron adsorbents are less susceptible to pH changes than aluminium-based adsorbents, but they still show reduced capacity as the pH increases. The manufacturers of the media typically give pH 5.5 to 9 as an operating range. There is data that indicates carbonate and bicarbonate content interfere with hydrous ferric oxide's (HFO) adsorption of As V. Studies indicate that sorption of arsenate onto HFO drops from 90% to 10% over a rise of approximately two pH units.

c) Ion exchange resin
Anion exchange resin is another means to remove arsenic. Great care must be taken during the design phase because the resin is susceptible to chromatographic peaking - the process where all the sites on a bead can become filled with both target and non-target ions. At that point, non-target ions that are preferred by the resin will "bump" the arsenic off, and if left running long enough, the effluent arsenic concentration can exceed the influent. It is, therefore, very important to regenerate the resin on a conservative schedule. Resins generally will not remove As III, therefore pre-oxidation (and if chlorine is used, dechlorination) is a required step.

One manufacturer recommends at least 36" and a specific flow rate of about 2.0 gpm/ft3, based on a capacity of 1,400 gallons of 100 µg As V. A significant feature of resin is the ability to regenerate. A brine-regenerated resin allows the user to maintain the unit, and there are methods to reduce brine use by recycling it a number of times. Function is apparently not a factor of pH, but competing ions, sulphate, in particular, must be taken into account to determine regeneration timing. Waste generated from the regeneration phase will produce a liquid waste containing high levels of arsenic, therefore waste disposal must be considered.

Close-up of Macrolite media used in large arsenic filtration applications.
Precipitation and Filtration Processes
Precipitation relies on adsorption as the chemical means of drawing arsenic out of solution. In this process, a metal hydroxide adsorbent is created in the water, allowed to react with the arsenic and then removed by filtration. Arsenite still needs to be pre-oxidized to arsenate for this process to be effective, particularly where aluminum or calcium salts are used. Arsenic is sorbed to the ironbased floc and removed concomitantly by filtration. The key consideration around iron precipitation and filtration is the management of backwash residuals. Typically the most economical choice for large volume water treatment is to utilize an iron addition, oxidation and filtration process. The key to success is to have a filter that effectively filters iron, as arsenic removal effectiveness will have a direct correlation to iron removal efficiency.

Removal of naturally occurring iron by oxidation and filtration can remove arsenic as a side benefit. In cases where As V is present, aeration to oxidize Fe II to Fe III may be sufficient to precipitate iron hydroxide and with it the arsenic. Where As III is present with Fe II, stronger oxidation is required. In a pilot study performed by Kinetico, oxidation and filtration were sufficient to remove 20 µg of As III in the presence of 0.7-0.9 mg/l of Fe II. A The current arsenic limit listed in the Canadian Drinking Water Guidelines is going to be lowered but the new limit is not yet known. free chlorine residual of greater than 0.5 mg/l was sufficient for iron and arsenic oxidation. Arsenic in the effluent was typically less than 2 µg.

Arsenic will also adsorb to calcium hydroxide. Plants using lime softening for hardness reduction are likely to also see a reduction in arsenic where the influent is less than 50 µg . Consistent reduction may require additional treatment beyond the typical settling clarification of a lime softening system. It is unlikely that a lime softening system would be installed specifically for the purpose of arsenic removal, but it may already be occurring in existing lime softening systems.

Filters for the floc include Macrolite ceramic media, microfiltration membranes, sand and variations of the sand filtration process. Surface filters, such as bag or cartridge filters, would also be effective, although the replacement frequency of the filter elements will limit their use in this application.

Other Membrane Processes
Microfiltration (MF) and ultrafiltration (UF) for arsenic removal are not really considered membrane separation processes as much as coagulation and filtration processes. MF and UF do not reject arsenic in solution as their pore sizes are too large to exclude low molecular weight dissolved solids. MF and UF processes typically include ferric addition so the arsenic removal is accomplished by filtration of the resulting iron hydroxide floc.

a) Reverse Osmosis
Early test data in RO membrane tests indicated that As V was rejected more than As III. This was consistent with typical reverse osmosis performance where weakly ionized or nonionic species were poorly rejected. A 1988 EPA Project Summary indicated 95% rejection of As V and 60-70% rejection of As III.

More recent tests on newer membranes indicate removal of As III is similar to removal of As V across the pH range of 4-8. These results are somewhat unexpected due to the lack of charge of the As III species in the pH range. The rejection may be due to molecular weight cut off, rather than the charge of the arsenic species. Recent data from testing of Kinetico household TFC RO membranes showed a minimum of 98.5% rejection of As V.

This level of performance allows RO membranes to be used as point-ofuse, residential drinking water treatment devices. Polishing can be done via an adsorbent cartridge to add a level of safety.

b) Nanofiltration
Experiments with a nanofilter membrane having a measured molecular weight cut off of 180-340 g/g-mole were predictably consistent with rejection due to the ionization of arsenic. Percent rejection of As V increases with increasing pH, from 30% at pH 4.5 to 80% at pH 8.2. Percent rejection of As III was less than 10% across the same pH range. These results would be expected with the low rejection of nonionic species, partial rejection of monovalent ions and high rejection of divalent ions in nanofiltration.

Summary of Treatment Technologies
If we treat oxidation as a separate subject, all of the arsenic removal technologies can be put in two categories, membrane separations and adsorbents. Membrane separations include reverse osmosis and nanofiltration membranes, both of which are crossflow processes that rely on size exclusion to remove arsenic, not chemical reactions. Adsorbents include any technology that “binds” arsenic to a material that can retain the arsenic in a complex form that allows for removal and disposal.

Adsorbent systems include fixed bed disposable adsorbent media, regenerable adsorbent media, metal hydroxides precipitated from solution and ion exchange resins.

Precipitated iron and aluminium hydroxides remove arsenic by reactions similar to the fixed bed media. It was originally thought that adding ferric chloride or aluminium sulphate formed metal arsenate compounds. At high µg/l levels of arsenic that may be the case, but at part per billion levels the arsenic is sorbed to the metal hydroxide floc, similar to fixed bed adsorbents.

Naturally occurring iron in groundwater can be used for a precipitated adsorbent system. Oxidizing ferrous iron to ferric iron will form a floc that when filtered out, can remove arsenic as well. Use of a chemical capable of oxidizing arsenic III along with the iron eliminates concerns over arsenic speciation.

The current arsenic limit listed in the Canadian Drinking Water Guidelines is going to be lowered but the new limit is not yet known. Due to the previous lowering of US EPA arsenic limits, removal technologies have advanced and much is understood about how to effectively remove arsenic from drinking water. A key to successful removal is to work with suppliers that offer multiple removal technologies. A single supplier should be able to help you evaluate many removal technologies in side by side comparisons and assist in picking the one that makes the most sense.

This article is a segment taken from Kinetico’s Arsenic Treatment Guidebook.
Derek K. French is with Municipal Water Systems.
Contact email dfrench@kinetico.com.

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