Proceedings Tailings and Mine Waste 2011
Vancouver, BC, November 6 to 9, 2011
Methods for Removing Selenium from Aqueous Systems
Lucas Moore, Ph.D., Amir Mahmoudkhani, Ph.D.
Kemira Oil and Mining, Atlanta, USA
Abstract
Selenium is among the list of oxyanions that lead to contamination in mining aqueous waste streams. Though
the elemental forms are toxic, the aqueous oxyanions are more so. The most common forms of selenium
released during mining processes are the aqueous forms, selenite and selenates. The common treatment
technologies to date can be summed up in these major categories: media filtration, chemical treatment, and
biomediated removal. These methods can often involve many expensive processing steps that may also be
limited by variables such as total dissolved solids, presence of other ions, and ability to maintain microbial
growth. We have developed an innovative chemical technology that can successfully reduce selenates to a level
below the EPA recommendations. This new technology offers a unique and viable solution which is transparent
to the above mentioned limitations.
Introduction
Selenium is a naturally occurring metalloid that belongs to the chalcogen group. Selenium is widely
applied in global industries such as electronics, fertilizers, fungicides, antidandruff shampoo, and many
more. Commercial quantities of selenium are generated during copper electrolytic refining (USGS
2007; Lenz 2009). Selenium, in small quantities (0.1-0.5 ppm dry weight) is a micronutrient that is
part of everyday life. Having said that, selenium becomes toxic at concentrations > 3 ppm dry weight.
The National Primary Drinking Water Standard is 50 ppb for total selenium and the National Fresh
Water Quality Standard is 5 ppb for total selenium (EPA 2001; EPA 2011). British Columbia has
placed a standard of 2 ppb for total selenium. In nature, selenium is most commonly observed as
selenate, selenite, or selenide. (Figure I) Though complexed selenium is of low toxicity, selenate (SeVI)
and selenites (SeIV) are very toxic. These two forms of selenium are generally found in water, and
display bioaccumulation and bioavailability. Under acidic conditions, the extremely toxic and
corrosive hydrogen selenide gas can be generated from selenium containing species. The presence of
selenates and selenites in waste water is an immediate problem. If left untreated, selenium will
bioaccumulate and pose a threat to all aquatic life downstream. The EPA listed selenium in at least 508
of the 1,636 sites listed on the National Priorities List (ATSDR 2011). The National Priorities List is a
list of sites containing high levels of hazardous waste in the USA and these sites are financially eligible
for the Federal Superfund program.
Proceedings Tailings and Mine Waste 2011
Vancouver, BC, November 6 to 9, 2011
Figure I: Most common oxidation states of selenium
Chemically, selenium behaves very similarly to sulfur. Thus, selenium is most often associated with
sulfur containing ores such as pyrite, sphalerite, and chalcopyrite (Adams 2005). These sulfate/sulfite
containing ores are prevalent in the mining industry of metals such as copper, nickel, silver, lead, and
uranium. Consequently, selenium contamination has become an emerging issue in such mining
processes. (Figure II) Selenium contamination typically occurs in the aqueous stream, and this stream
must be treated prior to discharge. However, exposure to environmental conditions that the
contaminants were not exposed to prior to unearthing could result in the contaminants becoming
mobile while still in a tailing pond/pit or even the initial mine site.
Figure II: Example of a mine site and possible routes for contamination.
Selenium is also a major impurity in the production of sulfuric acid and mining/utilization of fossil
fuels. Coal, for example, has been reported to contain 0.4-24 ppm selenium prior to processing or usage
(Lemly 2004). (Table I) During coal processing/usage, selenium may become further concentrated as
seen in the fly ash (0.2-500 ppm). As with coal, oil is also a source of selenium contamination. Crude
oil can contain levels of selenium between 500-2000 ppb, while oil shale has been reported to contain
levels between 1.3-5.2 ppm. Refinery waste water has been reported with levels of selenium
contamination as high as 75 ppb. Such mining and industrial activities have led to an increase in
selenium contamination that can be found from urban to rural areas, mountains to plains, deserts to
rainforests, and arctic to tropics. Hence, treating the waste streams from these processes have become
of high interest. Selenium is not only a localized threat, but also a global threat that is increasing as
various industrial activities increase. (Figure III)
Table I: An example of the selenium concentrations found during the mining, processing and
usage of coal.
Proceedings Tailings and Mine Waste 2011
Vancouver, BC, November 6 to 9, 2011
Material/Waste
Earth's Crust
Surface Water
Coal
Coal Storage Pile (Leachate)
Coal Cleaning (Process Water)
Coal Cleaning (Solid Waste)
Coal Cleaning (Solid Waste Leachate)
Coal Burner Ash (Bottom Ash)
Precipitator Ash (Fly Ash)
Scrubber Ash (Fly Ash)
Fly Ash (Leachate)
Flue Gas Desulfurization (Process Water)
Flue Gas Desulfurization (Sludge)
Boiler Cleaning Water
Coal Ash Slurry
Ash Settling Ponds
Ash Pond Effluents
Ash Disposal Pit (Leachate)
Coal Gasification (Solid Waste)
Coal Gasification (Solid Waste Leachate)
Gasification (Solid Waste Leachate)
Coal Liquifaction (Process Water)
Coal Liquification (Solids Waste)
Selenium
Concentration
0.2 ppm
0.2 ppb
0.4-24 ppm
1-30 ppb
15-63 ppb
2.3-31 ppm
2-570 ppb
7.7 ppm
0.2-500 ppm
70-440 ppm
40-610 ppb
1-2700 ppb
0.2-19 ppm
5-151 ppb
50-1500 ppb
87-2700 ppb
2-260 ppb
40-950 ppb
0.7-17.5 ppm
0.8-100 ppb
0.8-100 ppb
100-900 ppb
2.1-22 ppm
Proceedings Tailings and Mine Waste 2011
Vancouver, BC, November 6 to 9, 2011
Figure 3: A map providing various sites that have reported high levels of selenium
contamination as of 2004. (Coal mining/combustion, oil refining, phosphate mining, gold mining,
silver mining, nickel mining, metal smelting, and landfill leachate)
Existing Treatment Technologies
Elemental selenium is relatively insoluble in aqueous systems and not biologically active, which makes
removal much simpler and prevents bioaccumulation. Having said that, the most common form of
selenium released during the previously mentioned mining processes are the aqueous forms, selenite
and selenates, which are water soluble oxyanions. The most common technologies to date can be
summed up in these major categories: media filtration, chemical treatment, and biomediated removal.
However, it is important to mention that to date, there is not an ultimate solution for the challenging
environmental contamination with selenium (Golder 2009).
Physical Treatment
Media filtration is a physical treatment method. These can be as simple as filtering through sand (Kuan
1998), clay (Goh 2004), titanium dioxide (Zhang 2009), or can be as exotic as filtering through ion
exchange resins or a membrane (reverse osmosis and nanofiltration) (Stripeikis 2001). Many of these
media are commonly used in the water treatment industry. Two common problems associated with
filtration media are the increased amount of waste, and the potential for fouling or scaling of the
membrane. Many of these types of methods also have sensitivities to other ions such as nitrates,
sulfates, and chlorides, which lead to the inability to remove selenates.
Membrane filtration is a method that is commonly used, but can be quite costly. In a closed gold mine
in California, a reverse osmosis system was applied (Golder 2009). In this case, the water was already
contaminated and contained in a waste pond where aqueous waste was no longer being produced. In an
effort to prevent such contamination from reaching the drinking water reservoir, Golder was contracted
to implement the reverse osmosis system. In this case, 100 US gallons/min of water was treated. Due
to a high level of total dissolved solids, only 40% of the selenium was removed.
Another method is ion exchange resins, which work by reversibly exchanging a more desirable ion
with a contaminated one. These ion exchange resins can be altered to fit either a specific ion, or left
broad enough to remove a series of ions. Having said that, there are cases where the resin cannot
Proceedings Tailings and Mine Waste 2011
Vancouver, BC, November 6 to 9, 2011
distinguish between ions as well. Due to the similar chemical nature and reactivity of sulfates and
selenates, it is quite difficult to separate the two using an ion exchange resin; thus, a significant
performance decrease is observed in sulfate rich environments. This performance decrease can be
overcome by forcing the formation of a barium scale via the addition of BaCl2. A combination of
precipitation/ion exchange can reduce selenium contamination levels from 1000 ppm to 0.1 ppm.
Resins can also be cleaned and reused, leaving a concentrated selenium waste to be disposed of. As
mentioned previously, the large quantities of waste generated is a significant concern when considering
such a treatment method.
Chemical Treatment
Chemical treatment can be categorized into three classes: precipitation (Zhang 2008, Rovira 2008,
Hayashi 2009, Geoffroy 2010), cementation, and coagulation (Golder 2009). The treatment works by
adjusting the physical or chemical properties of the dissolved contaminant or suspended matter in a
way that will enhance the ability to agglomerate. The particles can then be removed by flotation,
filtration, or gravity settling. Coagulants (Ferrous, Ferric, Aluminate) work by altering the surface
charge of the contaminants, thus allowing for the agglomeration of the particles into a flocculated
precipitate. The floc size can be increased by the addition of a polymeric flocculant, such as
polyacrylamides. Selenites are quite easily removed using any of these methods; however, selenates are
not as reactive. In order to remove the bulk of the selenate contamination with a chemical treatment
method, a reduction step must be incorporated. Another major disadvantage of most chemical
treatment possibilities is in the high quantity of chemicals being consumed, consequently leading to the
need to treat the resulting solid waste. The literature suggests there is often an inconsistency with
reducing selenium to the regulated limit.
One way to remove selenates was referenced in the previous section. Barium can be used to form a
scale/precipitate of barium selenates/selenites that can be filtered. Zero-valent iron can also be used to
remove selenates, as well as selenites (Zhang 2008, Rovira 2008, Hayashi 2009). The iron can first
reduce selenate to selenite, forming ferric and ferrous hydroxides. In turn, the ferric and ferrous
hydroxides can complex with selenites, resulting in a precipitate. At Barrick’s Richmond Hill Mine,
ferric sulfate was used to precipitate “selenium” at a pH of 4.5 and this reduced selenium
concentrations to 12-22 ppb.
Electrocoagulation (Mavrov 2006), photoreduction, and adsorption to acid-treated peanut shells (ElShafey 2007) are other methods that have been discussed in the literature. However, these methods are
very much still in the lab stages and have not progressed into actual trials.
Biotreatment
It has been claimed that an effective method for removing selenite and selenate from aqueous systems
is via the microbial reduction of selenates to elemental selenium (Cohen 2006, Harrison 2010). As with
the previously mentioned methods, there are problems associated with the biomediated reduction.
Often, the presence of other ions such as nitrates can decrease the effectiveness of the biological
systems in reducing the selenates and selenites. If such a problem occurs, a pretreatment step will be
necessary prior to introducing mine-produced water into the biological treatment area. Consequently,
the capital expense will be further increased.
In active microbial reduction, process water is added to the bottom of a reactor where the water flows
upward into a microbial “sludge”. It is in this microbial “sludge” where the selenates and selenites are
reduced to selenium, which is then removed from the top of the reactor. The literature lists molasses,
wood chips, and distiller’s grains as possible media for such microbial activity (Golder 2009).
Proceedings Tailings and Mine Waste 2011
Vancouver, BC, November 6 to 9, 2011
However, one of disadvantages of the microbial reduction process is the elevated concentrations of
total suspended solids; a successful microbial reduction would require pretreatment. Such a process has
been applied at the USEPA Kennecott site for 6 months, and data suggests a decrease in selenium from
1950 ppb to less than 2 ppb. This process was also applied at Duke Energy in North Carolina in the
presence of high total dissolved solids, resulting in a 99.3% reduction in selenium after 9 months. In
addition to the high capital costs associated with this technique, another disadvantage is the microbial
maintenance of parameters such as nutrients, energy, and temperature that is required to sustain
adequate reduction.
Another biomediated route to removing selenium is via biofilms. The literature suggests that this is a
route that can also be applied to selenium removal. The use of Desulfomicrobium sp. was proven to
decrease the selenate concentration to sub-micromolar concentrations when lactate and sulfate were
used as the growth media (Hockin 2006). In limited levels of sulfate concentrations, the dominant
species of selenium measured is selenide; however, at an excess of sulfates, the selenate is
enzymatically reduced to selenium. It is important to mention that a disadvantage to this method is the
decrease of activity observed in the presence of elevated nitrate levels.
Unlike the majority of the other biomediated pathways, passive microbial reduction has a relatively low
capital expense. It is successful at reducing both selenite and selenate with minimal supervision
required, but will leave a large amount of waste. This method is also temperature sensitive and
requires a significant increase in process time.
There are other biotreatment methods, but many of them require anaerobic conditions that may be
problematic on the industrial scale with the amount of mining water being released (Oremland 1989,
Lee 2007).
Results and Discussion
In-Situ Solidification – Chemisorption Treatment Method
The focus of this work is to introduce an innovative approach using in-situ solidification –
chemisorption method for treatment of contaminated process waters. Such a method involves the
collaboration of the chemical and physical treatment methods mentioned above. An insoluble
amorphous sorbent possessing active sites for the chemisorption of oxyanionic species, such as
selenate, was prepared by the chemical modification of an inorganic silica based polymeric material.
(Figure IV)
Proceedings Tailings and Mine Waste 2011
Vancouver, BC, November 6 to 9, 2011
Figure IV: Schematic representation of in-situ solidification – chemisorption method.
The addition of a promoter (Pr-1) will induce the cross-linking of the inorganic polymer in solution,
thus allowing for maximization of the amount of active sites available, hence increasing the potential
contact and interaction with the selenates. Encapsulation of the chemisorbed contaminant is believed
to occur due to the irregular nature of the cross-linking process, thus producing an immobilized
amorphous solid mass. The resulting solid mass can be removed from solution with relative ease by
gravitational settling, filtration, or other conventional solid removal methods.
Adsorption is a process where the substance, contaminant in this case, is transferred from the liquid
phase (solution) to the solid surface. Adsorption involves the inter-phase accumulation/concentration of
substances at the surface or interface, which can be between any two phases such as liquid-solid.
Chemical adsorption or chemisorption takes place as a result of chemical bond being formed between
the solute (dissolved species) and the adsorbent, comparable with those leading to the formation of
chemical compounds. (Figure V) There are many factors affecting adsorption such as nature of the
adsorbent, nature of adsorbate, nature of the solvent, and others. Adsorption processes are capable of
removing contaminants if the adsorbent (solid surface) is selected carefully and the solution chemistry
is controlled.
Figure V: Chemisorption of selenium oxyanionic species on inorganic polymeric sorbent
Proceedings Tailings and Mine Waste 2011
Vancouver, BC, November 6 to 9, 2011
The new water treatment technology developed here is based on in-situ solidification of a sorbent and
chemisorption of contaminant species onto the resulting sorbent. An amorphous solid is formed by insitu cross-linking of a modified inorganic polymer based on silicates. The process, as schematically
shown in Figure VI, involves the formation of the sorbent and the chemisorption of contaminated
species onto sorbent active sites. This single stage treatment takes place in a continuously stirred (400 –
500 rpm) mixing tank. Inorganic polymeric system is dosed at 20 – 1000 ppm to the contaminated
water based on the level of contaminant. After 1 – 3 hours mixing, the aliquot is transferred into a
gravity settling tank to allow for precipitation of the suspended solids. The clear supernatant is
decontaminated process water and may be discharged, or undergo further treatment if necessary. Faster
solid separation may be achieved by filtration and/or the use of common organic flocculants such as
anionic polyacrylamides. The separated solids from this process may be safely landfilled.
Figure VI: Schematic representation of water treatment in this work.
Treatment of Selenium Containing Water Samples
Optimization of this new technology for the treatment of selenate containing water led to the
development of a series of inorganic polymeric materials that ranged in their chemical compositions
and affinities toward cross-linking in aqueous media. (Figure VII) When using the inorganic polymer
type 1 (IP-1*) to treat water samples containing ppm levels of selenate, the efficiency of selenate
removal was more than 99% under our process conditions. As with most treatment technologies, a
reduction in efficiency was observed as the initial selenium level is decreased to the lower ppb range;
however, some treatments successfully reduced selenate levels to below 1 ppb.
Proceedings Tailings and Mine Waste 2011
Vancouver, BC, November 6 to 9, 2011
Figure VII: Efficiency of the inorganic polymer systems for selenate removal from water samples.
To better understand the technology and to customize it for individual aqueous systems, the treatment
method was further developed by optimizing the ratios of the best performing sorbent (IP-1) and the
sorption promoting chemical (promoter, Pr-1). IP-1* consists of IP-1 and Pr-1 at an optimized ratio that
maximized selenate removal; it resulted in > 99% selenate removal. The efficiency of the treatment
process was monitored via ICP analyses of decontaminated water samples, as well as XRF analyses to
evaluate the quantity of absorbed contaminant.
The IP-1* technology was applied towards more realistic mine conditions. A dosage demand curve
was generated by applying IP-1* towards water samples that contained 1100 ppm sulfates, 1087 ppm
total dissolved solids (TDS), and a conductivity of 1625 uS/cm. Concentrations of IP-1* ranging from
300 ppm to 6000 ppm were applied, and these studies each yielded selenium concentrations reduced
from 1000 ppb to below 5 ppb. (Figure VII)
IP-1* Selenium Removal
Selenium Remaining (ppb)
6
5
4
3
2
1
0
0
1000
2000
3000
4000
5000
6000
7000
Treatment Concentration (ppm)
Figure VIII: IP-1* dosage demand curve in the presence of other ions.
Conclusions
A new chemisorption technology that incorporates both physical and chemical treatment methods was
developed. This technology was evaluated for removing selenium from aqueous systems. The sorbent
was formed in-situ by use of silicate-based inorganic polymeric materials. Selenium oxyanions were
then chemically adsorbed onto the active sites within the cavities of the sorbent material, thus removing
Proceedings Tailings and Mine Waste 2011
Vancouver, BC, November 6 to 9, 2011
> 99% of the total selenium from water. The chemical nature of this treatment system is diverse,
robust, and it was customized via the addition of a promoter to further increase efficiency of selenium
removal. Silicate-based inorganic polymers provide an economical and versatile solution for treatment
of contaminated mining process waters within an operational and environmental-friendly process. The
sensitivities that are commonly associated with the existing treatment technologies, such as elevated
concentrations of common cations and anions (e.g. Ca2+, Fe2+, Cl- and SO42-) were not observed in the
initial screening of this inorganic polymer system. Ongoing work evaluating this technology on actual
contaminated mine process waters shows promising initial data.
Experimental
Materials
Sodium selenate (Na2SeO4) were purchased from Aldrich and used as received with no further
purification. Lab-made aqueous solutions containing selenium were prepared by dissolution of the
above chemicals in the city of Atlanta tap water. Caution! Sodium selenate is extremely toxic and
should be handled and disposed according to regulations for toxic substances.
Instruments
In this study, a Thermo Scientific ICP-AES system model iCAP 6500 equipped with a charge injection
device (CID) detector and a CETAC ASX-520 autosampler was used for determination of the selenium
species in water samples. Low detection limits (1 ppb for selenium) were achieved by preconcentration of 100 mL aqueous samples. Quantitative elemental analyses of trace elements were
conducted on a Bruker S4 Explorer wavelength-dispersive X-ray fluorescence spectrometer. Element
distributions of selenium before and after treatments were used for qualitative and quantitative analyses
of chemisorption of the contaminant species on inorganic polymeric solid sorbent.
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