Anthracite-Bentonite And Bentonite Clays For Oil Spill Cleanup And Landfill Linings
Oil spills, industrial waste incidents and leaching from waste storage are ongoing problems in both the developed and developing worlds. Should such pollutants reach water courses, processes such as eutrophication and death of aquatic life could result(1). Some of the leading, and robust, treatments of oily and heavy metal contamination in water and waste storage are based around bentonite and anthracite.
What Are Bentonite And Anthracite?
Bentonites are aluminium phyllosilicate clays comprised primarily of montmorillonite. Montmorillonite is a dioctahedral smectite and has a crystal structure of mixed geometries; an octahedral geometry sandwiched between two layers of tetrahedral geometry. Na-bentonite is one of the most common, and is valued for its swelling ability(2). It originates from volcanic ash that was deposited in marine environments. Anthracite is one of the superior forms of coal, widely available and with multitude uses, it is an inexpensive and reliable material.
Many of bentonite clay’s applications stem from its unique porosity profile, i.e. it is highly porous. Porosity levels are closely aligned with surface area, and surface area is related to how adsorbent a material can be. Adsorption is the physical phenomenon where a solid ‘holds’ onto molecules from a gas or liquid solute. As a general rule of thumb, specific properties of an adsorbent notwithstanding, the greater the surface area, the more adsorbent a material can be. Coal in general, and anthracite in particular, are classified as porous materials. Lower grade coals are described as macroporous, whereas higher-grade coals such as anthracite are characterised as predominantly microporous(3).
Bentonite For Spills
Bentonite has found use for the treatment of oil (and other organic) wastes/spills - it acts alongside other components as a highly porous material, physically adsorbing the oil and thus allowing for facile removal via filtration. Oil spills are often some of the most dangerous types of chemical contamination at sea and in watercourses, being responsible for the loss of marine and plant life. As such, rapidly deployable and highly reliable methods to remove oil and other organic contaminants from water must be attained. Bentonite clay plays the starring role in this effort as primary adsorbent. Bentonite may be used alone, modified (see below) or alongside anthracite. Other sorbents for oil from water removal include peat, fibreglass and the ion exchange resin Amberlite(4)
Bentonite-Anthracite For Oil Spills
Bentonite-anthracite mixtures are useful for the separation and removal of oil, oily substances and poorly water soluble organics from liquids.
Oil in oil-in-water emulsions can be isolated using powdered bentonite clays, with the substantially increased macro-scale surface area acting in a complimentary method to the micro/nano-scale surface area provided by the bentonite clay itself(5). Activated carbon, bentonite and deposited anthracite have been used together to treat oil-water emulsions in the petroleum sector. Research has found that adsorption rates (and thus oil removal rates) increase with greater exposure time to the sorbent(6).
A mixture of 30% bentonite and 70% coal dust (by weight) was shown to remove oil and heavy organics from oil/water emulsions at conversions of up to 98%. Efficiencies of this magnitude are explained by the fact that the bentonite-anthracite is highly organophilic(7). Particles sizes ranged from 0.85 to 2.36 mm, thereby once swelled with pollutant can easily be filtered away. Part of the mechanism by which bentonite is an excellent sorbent in that paper relies on the replacement of the sodium or calcium counterion with the nitrogen end of a quaternary amine - the enhanced organophilic nature will result in a pronounced swelling in organic materials(8), and thus it can be said to be selective for organics such as crude oil, petroleum and benzene. Anthracite has a similar density to bentonite in the bulk and has the effect of slowing any advanced absorption into the swelling clay. The ability to absorb liquids of all types diminishes rapidly at high temperatures(9). For such applications, bentonite-anthracite can be added to a pool of contaminated water, or the contaminated water can be passed through a bentonite-anthracite filter.
A filtration system for the removal of oil and hydrocarbons from bilge water in the shipping sector ensures discharge of oil-free water by using a combined peat-anthracite-bentonite filter, which is overall both hydrophobic and oleophilic in nature(10). Patent authors claim that it is advantageous in the maritime environment as the filter cakes (contaminated peat-anthracite-bentonite) can be burned for disposal.
Bentonite-Anthracite For Industrial/Mining Water Contamination
Problems like acid mine drainage are relatively common and persistent problems facing the mining sector. The groundwater near one coal mine in Nigeria had tested positive for iron concentrations in the ca. 1,300 mg/L range of iron, at which levels toxicity is a problem. Using a combined coal fly ash, anthracite and bentonite clay regime, these levels were brought down significantly(11). Other contaminants in acid mine drainage can include heavy metal ions and sulfates. The set-up in this work was to allow mine drainage water to be contained by a bentonite-coal adsorbent barrier, removing in excess of 80% of iron contaminants. An 8x30 mesh, 0.97 m2/g bentonite-anthracite filter column has been used in industrial wastewater streams to remove heavy metals, and although the filter did not perform as well as the expensive zeolite ‘clinoptilolite’ at removing all traces, it did outperform in terms of organic removal from solution in addition to the metals. The authors suggested that using a dual filter system availing of bentonite-anthracite’s robustness, reliability and inexpensive nature, alongside a zeolite filter could provide maximum benefits(12).
Modified Bentonite Clays For Oil Adsorbents
Several examples of modified bentonite have been shown to be useful in this area. One study used the radical polymerisation of acrylic acid onto Na-bentonite granules at low loading. The authors found that sorption of oil-water mixtures at ambient temperature (ranges from 10 to 21 °C) was noticeably increased at the higher range(13). Looking at a more complex system, Na-bentonite, acrylic acid, sodium-type montmorillonite and various acrylamide linkers were used to create a “superabsorbent” nanocomposite(14) for water and some oils; with the combined clay and montmorillonite content at around 80%. Whilst such a nanocomposite differs from the previous modified Na-bentonite example, the absorbance value was found to be 1,201 g/g of nanocomposite.
In a related study on kaolin-type clays, also aluminosilicates, acid modification of such clay was found to cause the development of a highly defined porous structure with low mechanical strength. Surface areas of ca. 29 m2/g were achieved with pore sizes of 2-5 nm. Further treating this acid-modified clay with a strong base afforded a porous clay with 20-40 nm pores(15). These data suggest that similar treatment of chemically compatible bentonite clay in the same manner would afford analogous results. Bentonite clays modified with dimethyl-di(hydrogenated) tallow (i.e. animal fat) have been shown in oily liquids to effectively remove aromatic residues(16).
Due to the obvious parallels between crude oil and water insoluble organic solvents (such as benzene, cyclohexane, dichloromethane etc.), the porosity afforded by bentonite has been applied as an adsorbent to these cases(17).
Bentonite-Anthracite For Landfill Linings
One of the major scourges in the modern world is the quantity of waste sent to landfill - some of it toxic. Naturally, some waste streams are unavoidable, and in some countries methods of waste separation are not routinely practiced. In such cases, it is crucial to minimise the risk of potentially toxic landfill waste leaching out into the surroundings, into water courses causing risk to life. Bentonite clay, owing to its unique porosity profile, is able to prevent runoff and leaching from landfill sites. Bentonite is rarely used alone, rather, usually in concert with other materials such as anthracite.
Mixtures of bentonite and coal can be used as landfill linings and toppers, to prevent unnecessary and potentially harmful runoff to the local environment. Based on its excellent absorbance properties, bentonite-anthracite has been shown to be useful in the absorbance - and thus trapping of - heavy metals including cadmium, lead and nickel(18) when bentonite was used in a 2:1 ratio to anthracite, in addition to an amount of sand also present.
A rare but nonetheless potentially hazardous contaminant that can be present in landfill, particularly if such landfill is heavily soiled with wastes from food processing or agriculture is enzymes. Chloridazon and metribuzin are two such contaminants that are known to be able to leach from soil and waste piles(19) - an alginate-supported bentonite-anthracite controlled release formulation has been deployed to slow the leaching of chlorizadon and metribuzin from soils. The study employed granulated anthracite-bentonite. This is an example of anthracite-bentonite essentially being utilised as a herbicide.
Essential to landfill/waste applications is the ability to withstand pressure, and it has been found that bentonite-anthracite and sand-bentonite-coal form dense mixtures with good compressive strength properties(20).
Operating Considerations And Comparison To Other Methods
As it is found widely distributed throughout the world, bentonite clay and anthracite are inexpensive, and thus they are good candidates together for the use cases mentioned above. High materials cost for traditional clean up methods can be a concern(21). Some of the major concerns with traditional methods for oil and organic spill clean ups include a low oil-water separation efficiency, high material cost and low organic/oil adsorption capacity(22). Naturally, these problems can be partly negated by using more of the traditional method - such as vegetable- and mineral-based sorbents - but this defeats the principle of using the minimum quantity of materials possible and generates more waste. One often overlooked property of using bentonite clays as sorbents is its ‘environmentally friendly’ and non-toxic nature(23).
One report suggests that powdered bentonite clay is a significantly more effective adsorbent and remover of oils from water(24) than activated charcoal, up to seven times more so. Citing the fact that bentonite does not suffer the same ‘blinding’ of pores that charcoal does, it is stated that this is a more cost effective and scalable model. In cases of both bentonite-charcoal and bentonite-anthracite, the bentonite is the primary sorbent.
- Anthracite and bentonite clay are both naturally occurring, porous materials, that are inexpensive to acquire, work with and dispose of.
- Their properties of porosity and general long term stability lead to their use as sorbents for the treatments of spills and waste streams.
- Oil spills and the removal of organic/oily waste from water; heavy metal removal from water waste and aiding in containing contamination resulting from landfill sites are the primary applications.
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