Contaminated waters, selenium removal

Gleason KJ, Yu J, Bunge AL, and Wright JD. Removal of selenium from contaminated waters using emulsion liquid membranes. In Bartsch RA, Way JD, eds. Chemical Separations with Liquid Membranes, Washington, DC American Chemical Society, ACS symposium series 642, 1996 342-360. 

Removal of Selenium from Contaminated Waters Using Emulsion Liquid Membranes... 

To develop an effective, optimized ELM process for selenium removal, it is necessary to understand the forms of the selenium ions present in contaminated aqueous streams. Selenium can exist in several oxidation states -II, 0, IV, and VI (13). In naturally occurring ground or surface waters, inorganic selenium is found in the IV (selenite) and VI (selenate) states (14). In petroleum refinery wastewater effluents, the form of selenium depends on the process stream. In stripped sour water the selenium exists primarily in the Se(-II) oxidation state as either organoselenium compounds or selenocyanate. In the final biotreated effluent, selenium exists primarily as selenite [Se(IV)] and selenate [Se(VI)] oxyanions (1). 

Unfortunately, in contaminated waters and wastewaters, innocuous anions like sulfate are inevitably present as well, usually at concentrations several orders of magnitude greater than that of the selenium anions. For example, sulfate concentrations in refinery wastewaters approach 800 mg/L (1), while sulfate concentrations in agricultural drainage water are in the 3000-4000 mg/L range (16). Consequently, it is difficult to remove the selenium without also having to remove far larger quantities of other, non-hazardous materials as well. 

GLEASON ET AL. Removal of Selenium fiom Contaminated Waters... 

Sodium selenite has also been incorporated into styrene—butadiene mbber and used in a pellet form which results in the slow release of selenium into water. These pellets have been placed in lakes in Sweden which have fish contaminated with mercury owing to high levels of that element in the water. The selenium released by the pellets reacts with mercury to form insoluble, heavy mercury selenide which setties to the lake bottom and removes mercury from the ecosystem (126). 

Phytovolatilization involves the use of plants and plant-associated soil microbes to take up contaminants from the soil, transform them into volatile forms, and release them into the atmosphere (Lin, 2008). Phytovolatilization occurs as growing trees and other plants take up water and the organic and inorganic contaminants. Metalloids, such as selenium. As, and tin, can be methylated to volatile compounds or mercury that can be biologically transformed to elemental Hg. Phytovolatilization has been primarily used for the removal of mercury and selenium. 

Acetone, 1-butanol, ethanol, methanol, and water are useful solvents for selenourea the solutions in acetone or 1-butanol seem to be the least air-sensitive. Aqueous solutions can be stabilized by the addition of acid. Commercial samples of selenourea are sometimes contaminated with elemental selenium, which can be removed by filtration of the ligand solution. Solid selenourea may be exposed to the atmosphere for short periods without appreciable decomposition, but as a general rule it is advisable to handle selenourea and selenourea complexes in a nitrogen atmosphere and to avoid prolonged exposure to light. 

If the retort water is considered for domestic or irrigation uses, it would have to be treated to remove a number of contaminants. Of the major constituents in retort water, the NH4, HCOa", and organic compounds in the water clearly make it unsuitable for other uses (18). Of the trace constituents, the arsenic and selenium concentrations listed in Table IV are above the maximum permissible concentrations for drinking water (36). The boron concentration may make water unsuitable for irrigation (22). Other studies have found silver and lead concentrations in retort waters have exceeded the maximum permissible concentrations for drinking water (1,2,36). Numerous studies for the treatment of the retort water have been initiated (37). The objectives of these studies are usually to find a method to recover the ammonia and organic material from the water so that treatment costs will be lowered through by-product recovery. 

Kharaka Y. K., Ambates G., and Presser T. S. (1996) Removal of selenium from contaminated agricultural drainage water by nanofiltration membranes. Appl. Geochem. 11, 797-802. 

Selenium enters the refinery as contaminates of processed crude oils. As the refiner processes the cmde oil, the selenium contaminate migrates fi om the crude oil into the refinery s striped sour water (SSW) streams. Once in the SSW stream, the selenium contaminate, which is an environmental concern, is now part of the wastewater stream. The problem for the refiner is how to efficiently remove the selenium prior to discharging the water to an appropriate catch, such as a bay or municipal water treatment plant. 

Packed beds of activated alumina can be used to remove arsenic, fluoride, selenium, silica, and natural organic material (NOM) anions from water. The mechanism, which is one of exchange of contaminant anions for surface hydroxides on the alumina, is generally called adsorption, although ligand exchange is a more appropriate term for the highly specific surface reactions involved (2). 

Activated alumina s role in a water filter is similar to that of carbon. When water comes into contact with it, it attracts and removes certain contaminants, notably arsenic, fluoride, selenium, and LEAD. When activated carbon and alumina are used together, the filter can be highly effective in removing lead, as well as a number of other contaminants. Success of lead removal varies from brand to brand. 

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