Arsenic Removal in Groundwater
Arsenic is naturally occurring in groundwater in both the arsenite and arsenate forms. Found in rocks, soil, water, air and biota, arsenic’s natural concentrations in soil typically range from 0.1 to 40 mg/kg.
Arsenic’s carcinogenic role was noted more than 100 years ago, and it has also been linked to diabetes and neurological disorders. Because of its negative effects, the U.S. EPA revised the maximum contaminant level (MCL) for arsenic from 50 to 10 ppb, which became effective Jan. 23, 2006.
Prior to the implementation of the new arsenic MCL, the U.S. Geological Survey estimated that at least 6% of all community water supplies would be non-compliant with the new standard, with an anticipated cost of almost $200 million per year needed to comply.
Several proven treatment technologies are available that remove arsenic to concentrations below 10 ppb, including coprecipitation, adsorption and ion exchange. Selecting an appropriate treatment technology for the reduction of total arsenic in drinking water requires careful and thorough consideration of site-specific conditions. Some of these factors include: analysis of raw water quality, degree of treatment required, space available for treatment, simplicity of the process, and the treatment/disposal of residual waste generated from the primary treatment process.
Iron has a very strong affinity for arsenic. When arsenite is exposed to iron in the presence of an oxidant, insoluble arsenate is formed in a process known as coprecipitation. This process has the advantage of low cost, reusable backwashed media and benign waste.
Also, arsenic removal through coprecipitation often can be implemented in existing systems designed for iron and manganese removal. HMO filters utilize hydrous manganese oxide media to remove arsenic through coprecipitation in the presence of iron. When iron is not a natural constituent in the water, it can be added in the form of ferric chloride. The process further utilizes a pre-oxidant, such as 12.5% sodium hypochlorite, to oxidize the iron to ferric hydroxide. Simultaneously, any arsenite present is oxidized to arsenate, which is adsorbed on the ferric hydroxide carrier floc as ferric arsenate and then further adsorbed on the media. Catalytic activity further accelerates the conversion of iron and arsenite, thereby enabling the removal of total arsenic at high surface loading rates.
HMO filters must be periodically backwashed, and the wastewater may be discharged to a publicly owned treatment works or to a backwash water recovery system. The waste from HMO filtration of arsenic will contain ferric arsenate, a benign salt, which can be dewatered and disposed of as a non-hazardous waste subject to passing criteria for the EPA Toxic Characteristic Leaching Procedure, and the California Waste Extraction Test as appropriate when direct wastewater discharge is not possible.
Adsorption occurs when molecules of one substance attach to the surface of another. When iron-based adsorption systems are used for arsenic removal, arsenic molecules attach to the surface of an iron-based adsorption media.
As with the coprecipitation method, adsorption of arsenic in drinking water typically relies on a strong attraction between arsenic and iron. Both species of arsenic can be removed from drinking water through adsorption using a granular ferric oxy-hydroxide media. In this process, the media is typically utilized on a single-use basis to treat pre-chlorinated groundwater containing arsenate in the range of 11 to 40 ppb with neutral pH conditions. The arsenic adsorption capacity of the media is measurably enhanced at lower pH conditions (pH 6 to 6.5).
Ion exchange (IX) is a reversible exchange of ions adsorbed on a surface with ions in solution that come in contact with the surface. In water treatment IX systems, ions are released from the surface of the resin in exchange for other ions in solution. The direction of the exchange depends on the affinities of the resin for the available ions and the concentrations of the ions in solution.
In groundwater, arsenic is present as an anion. Arsenic can be removed using ion exchange systems with anion exchange resins and a salt brine. Ion exchange may be considered when groundwater contains interfering ions such as silica, sulfate and phosphate in sufficiently high concentrations as to preclude the use of adsorption based on the high frequency of media replacement.
Using IX for the removal of arsenic can result in high concentrations of arsenic that are discharged during regeneration. Proper disposal of discharged waste must, therefore, be taken into consideration.