DIAMONDS: FROM JEWELRY TO ENVIRONMENTAL APPLICATIONS
I remember that striking and elegant ring on display in the store, dazzling passersby with its diamond housed in a gold arch, unaffordable for many.
Diamonds, like graphite and amorphous carbon are allotropes (property by which an element has different molecular structures) of carbon. Due to their make-up and molecular structure, they are the most stable and hardest element in nature.
Veronica Carrera holds an undergraduate degree in chemistry from the Universidad Central del Ecuador as well as a master's in molecular chemistry from the Universidad Alcala de Henares and is currently pursuing a doctorate in engineering sciences at the Universidad Iberoamericana de Mexico. She also has 6 years of work experience in the oil industry supervising drilling fluids and environment and 8 years of experience as regards environmental inspections, and possesses a vast knowledge of the chemical industry as relates to research processes.
Most natural diamonds are formed under conditions of extreme pressure and temperature, found up to 190 km deep in the earth’s mantle. They are brought close to the Earth’s surface by magma from deep volcanic eruptions, which cools into igneous rocks known as kimberlites and lamproites (Hermann Berg,2015).
In jewelry, the price of a diamond known as a ‘gemstone’ is based on the cut (proportions - polish - symmetry), color, clarity, weight (carat) and fluorescence. The fewer number of inclusions (impurities) a diamond has, the greater its value. More colorlessness will also increase its value and the greater its weight, in spite of subpar cutting, also increases a diamonds value (Hershey, 1940).
Diamonds and graphite
Diamonds are the second most stable form of carbon after graphite. Its notoriety as a material is owed to its physical characteristics resulting from its crystalline structure. Specifically, of the materials known to mankind, diamonds possess a high hardness and thermal conductivity (the quantity of heat transmitted through a material). These properties have resulted in the main industrial application of diamonds in cutting and polishing tools in addition to other applications such as the manufacturing of electrodes for treatment of wastewater.
However, diamonds valued mainly for their hardness and thermal conductivity in the industrial sector explains why 80% of extracted diamonds, unfit to be used as gemstones and known as ‘bort’, have industrial applications.
Some of the more common uses of this type of “bort” diamond include heavy-duty bearings, specialized windows and drill bits, as well as being used as semiconductors in the manufacture of chips.
Environmental applications of diamonds
Some of the applications of diamonds include those related to the environment, where technology has transcended, and this allotrope, because of its internal make-up, is doped in another element, such as boron (BDD) in a silicon substrate and forms electrodes for electrochemical applications, where doping is the intentional process of adding impurities to an extremely pure semiconductor, such as a diamond, in order to change its electrical properties.
Important technological characteristics of this thin boron-doped diamond layer include: an inert surface with a low absorptive capacity; considerable stability against corrosion, even in acid mediums; and an extreme window for potential action in aqueous and non-aqueous electrolytes. In addition to the above mentioned properties, diamond electrodes are proving to be a promising solution for tertiary electrochemical treatments of wastewater containing persistent organic pollutants (POPs), such as pesticides, steroid hormones, etc.
On the other hand, boron, an electricity-conducting metalloid whose intermolecular spaces allow bonding with those of the bort diamond because of its thermal conduction, makes for a good electrode for electrochemical purposes.
An increase in the production of wastewater with a high content of toxic organic pollutants, has resulted in an increase in research and businesses associated with the treatment of this wastewater. Nowadays, there are many viable technologies including biological, physical and chemical processes. Specifically, electrochemical oxidation technologies that provide more versatility, energy efficiency, ease of automation and greater environmental compatibility. This has made technological development possible and its use in the destruction of highly persistent toxic and biorefractory pollutants (products that inhibit biodegradation processes, such as benzene, chloroform, methylene chloride and styrene, among others (Chen, 2004).
Many wastewater treatment plants were inadequately designed to handle the removal of emerging contaminants, such as pharmaceutical products, steroid hormones, beauty and health products supposed to degrade in natural conditions. Likewise, POPs contain organochlorinated compounds, such as pesticides and industrial grade chemicals, common to fatty tissue (El-Shahawi, 2010).The primary and secondary remediation processes present inefficiencies within the above mentioned types of contaminants. On the other hand, advanced oxidation processes make use of the formation of hydroxyl radicals to degrade even the most recalcitrant (difficult to degrade) pollutants in wastewater flows.
In general, the performance of electrochemical processes is decided by a complex interaction between parameters that can be optimized effectively and inexpensively. This performance aims to achieve the mineralization (destruction) of the contaminants or their decomposition.
The role of the electrochemical cell within the electrochemical and oxidation-reduction processes is worth noting in the context of the elements and their oxidation states that many a student has devoted entire days to attempting to understand. It is within this experimental device that electricity is generated by means of a chemical reaction or, failing that, energy is received from an external source thus enabling the oxide reduction reactions to occur it its system.
Cells are characterized by having the following specific components:
- an anode where the oxidation reaction or electron loss takes place;
- a cathode where the reduction reaction takes place resulting in the gaining of electrons;
- a voltmeter that measures the electric potential difference between the anode and cathode by permitting the flow of electrons closing the circuit.
The respective electrodes (the anode and cathode), as per their design, must be considered in their manufacturing process. In water, the medium in which the salt bridge is present, the process of oxidation or electron loss —2H2O → O2 + 4H++4e— favors the production of oxygen and the reduction or gaining of electrons —2H2O + 2e → H2 + 2OH-— and the production of hydrogen through an electrolytic process wherein water is broken down by the electrical current the forms the above compounds. Because of this, the advantages posed by anodes and cathodes as adequate catalysts of subsequent reactions should be taken into account in the manufacture thereof.
The materials ideal for the manufacture of electrodes for organic contaminant degradation must be stable in a medium electrolysis, affordable and exhibit high organic oxidation activity for degradation and low activity to secondary reactions (e.g. oxygen evolution). As a result, many anodic materials have been evaluated to determine their suitability for this. As per the model put forward in previous research (Comninellis, 1994), anodic materials fall into two categories:
active anodes with a low potential for oxygen production that thus make adequate electro-catalysts and favor partial and selective oxidation (e.g. conversion). This includes carbon and graphite, platinum-based anodes, and iridium and rutenium-based oxides; inactive anodes with a high potential for oxygen evolution and, therefore, are poor electro-catalysts allowing full and non-selective oxidation of organic contaminants to CO2 for an electro-generation of hydroxyl radicals. Inactive anodes are sourced from materials such as antimony-doped tin dioxide, lead dioxide and boron-doped diamond (BDD).
In terms of inactive anodes, BDDs possess several technological properties that set them apart from conventional electrodes, as well as a wide potential window, meaning they can act within a wide voltage interval in contrast with other materials used for this in aqueous and non-aqueous electrolytic solutions. As a result of the diamonds’ high quality, the action window is greater than 3V, thus permitting the greatest amount of organic components —which up until recently could only be removed— to be mineralized.
However, the foundations and applications of wastewater treatment with a powerful electro-oxidation methodology using undivided cells with BDD electrons, allows for the mineralization of organic pollutants (destruction of organic matter in carbon dioxide and water) present in wastewater due to the high production of hydroxyl radicals on the surface of the BDD as a result of water oxidation and where reactions, such as that of Fenton, Fe2+ in acid solution and H2O2, electro-generated in the cathode, are used.
The model assumes that the initial reaction by both active and inactive anodes relates to the oxidation of water that fosters the formation of physisorbed radicals (radicals absorbed into the surface that do not interact with it). The surface of active anodes interacts strongly with the OH and forms highly oxidating superoxides. This occurs when high oxidation is possible in a metal oxide anode, above the standard potential evolution of O2.
On inactive anodes, where oxide formation is excluded, hydroxyl radicals, also known as active physisorbed oxygen, permit a non-selective oxidation of organic compounds, which result in their complete combustion and generates carbon dioxide and water.
Therefore, anodes with a low overpotential for O2 evolution (e.g. anodes that are adequate catalysts for electrochemical oxidation reactions (OER)) bring about a partial oxidation of the organic compounds, while anodes with a high overpotential for O2 evolution (insufficiently catalytic anodes) favor the complete oxidation of organic material to CO2, making them ideal electrodes for wastewater treatment such as PbO2 and BDD, owing to the weak M-OH link (Martínez-Huitle, 2015).
However, the cost of these electrodes is typically thought to surpass expected wastewater treatment costs. This view is mistaken as the constitution of this electrode occurs over a silicate substrate covered with a fine, micron-sized crystalline boron-doped diamond layer thus enabling the degradation of matter that previously could not be removed through repeated treatment and persists in the environment, untreatable using conventional means. Treatment times of persistent and emergent pollutants become feasible with advanced oxidation treatment using BDD electrodes.
To this end the Pontificia Universidad Catolica del Ecuador in collaboration with the Universidad Iberoamericana Ciudad de Mexico is degrading water polluted with pesticide and PCBs, which have persisted for years in the environment in an attempt to leave only nitrogen and phosphate residue behind —harmless substances as regards pesticides— and the generation of a minimum amount of carbon dioxide. The group of researchers from both universities is optimizing available resources and opening various comprehensive research projects as regards the degradation of these compounds while minimizing the consumption of electrical power and opening a potential window for future investment companies specialized in wastewater treatment.
Chen, G., Electrochemical technologies in wastewater treatment. Separation and Purification Technology, 2004. 38(1): p. 11-41.
El-Shahawi, M.S., et al., An overview on the accumulation, distribution, transformations, toxicity and analytical methods for the monitoring of persistent organic pollutants. Talanta, 2010. 80(5): p. 1587-97.
Comninellis, C., Electrocatalysis in the electrochemical conversion/combustion of organic pollutants for waste water treatment. Electrochimica Acta, 1994. 39(11-12): p. 1857-1862.
Martínez-Huitle, C.A., et al., Single and Coupled Electrochemical Processes and Reactors for the Abatement of Organic Water Pollutants: A Critical Review. Chemical Reviews, 2015. 115(24): p. 13362-13407.
Hermann Berg (2008), "Johann Wilhelm Ritter – The Founder of Scientific Electrochemistry, “Review of Polarography, Vol. 54, No. 2, pp. 99-103.
Hershey, W. (1940). The Book of Diamonds (en inglés). Nueva York: Hearthside Press.