Cyanide can be found in industrial wastewater streams generated by the production of a broad range of goods and materials, including refined metals and alloys, leather, paints, pigments, wood preservatives, electronics, and chemicals, among others. Given that cyanide is both highly toxic and highly regulated, however, you may be wondering what to do if you have cyanide in your industrial wastewater.
The short answer is that cyanide in wastewater streams will usually demand some form of wastewater treatment. In this article, we’ll offer an overview of the technologies that are commonly used for removing cyanide from industrial wastewater, and take a look at the benefits and drawbacks of each as they relate to various industries and processes.
Does your facility need to remove cyanide from its wastewater?
At its most basic, cyanide is any substance that contains a specific type of carbon-nitrogen anion group. This can include free cyanide anions (CN–), as well as a number of different compounds, such as potassium cyanide (KCN), cyanogen chloride (CNC), and many others. While these various cyanide species have different chemical characteristics, all forms of cyanide are considered toxic and are therefore subject to strict regulatory guidelines in many nations across the globe. In short, if your industrial facility deals with any type of cyanide, you’ll likely need to take steps to monitor and treat wastewater to ensure that cyanide levels remain below legal limits.
Specific regulations around the handling and discharge of cyanides may vary based on a number of factors, including your industry, facility location, and your chosen wastewater discharge strategy. Industrial facilities that discharge wastewater to public waterways in the United States, for example, are subject to stringent limits on cyanide, typically with monthly average effluent limits set around 0.005 ppm or less. Facilities are also explicitly prohibited from simply diluting waste streams to comply with limits on cyanide in effluent streams. As such, facilities with cyanide content in their effluent streams will likely need to implement a wastewater treatment solution to comply with any applicable regulatory requirements.
What are the best technologies for removing cyanide from wastewater?
There are many wastewater treatment technologies that are appropriate for cyanide removal, and as with any separation or treatment solution, choosing the best cyanide removal technology comes down to matching your facility’s specific needs. One of the most important factors to consider is which cyanide species are present in your waste stream. For wastewater treatment purposes, cyanide species are typically classed into two groups:
- Strong acid dissociable (SADs): SADs are cyanide compounds where the cyano group (CN) shares a strong bond with a metal such as gold, iron, silver, or cobalt. SAD cyanides usually cannot be oxidized or broken down, and must instead be removed through some means of physical separation, such as precipitation or membrane filtration.
- Weak acid dissociable (WADs): WADs are cyanides that are easily dissociable at or below neutral pH, including free cyanide anions (CN–), hydrogen cyanide (HCN), and cyanide species where the cyano group shares weak bonds with metals such as cadmium, copper, nickel, and zinc. Compared to SADs, WADs are more toxic and less chemically stable. WAD cyanides can typically be broken down by biological or chemical means to yield less toxic products in a process known as oxidation or cyanide destruction.
In addition to evaluating which cyanides are present in wastewater, your facility should also consider factors such as stream pH, temperature, flow rate, volume, and the current and target concentration of cyanide in the stream. Consideration of each of these factors is critical in understanding which types of cyanide removal and wastewater treatment strategies are best for your facility. Below, we’ll summarize the most common approaches for the removal or reduction of cyanide content in wastewater, and offer an explanation of the pros and cons of each.
Chemical precipitation
Chemical precipitation is among the more popular separation technologies for streams with SAD cyanide content. The process consists of adding metal or metal cations to a wastewater stream, which react with the cyanides present to form new metal-cyanide complexes or precipitates. Precipitated particles can then be removed from the liquid stream through a physical separation process, such as sedimentation or media filtration.
One of the ways in which chemical precipitation is commonly used for cyanide removal is the treatment of SAD gold cyanide solution through the addition of zinc. In this example, the zinc displaces the gold, resulting in a gold precipitate, and a new WAD zinc cyanide complex that can be broken down through a separate oxidation reaction. Another common example is a related process known as flotation. In this example, a cationic surface-active reagent, or surfactant, is added to a SAD cyanide solution. The cationic surfactant reacts with the anionic cyanide complex to form an organic salt precipitate.
The bottom line is that chemical precipitation is a relatively simple and economical solution that is effective for the treatment of streams with SAD cyanide complexes, particularly for facilities needing a continuous treatment solution. Despite these benefits, facilities need to consider that chemical precipitation alone may not be adequate for meeting cyanide removal goals, and may therefore need to be used in conjunction with other treatment technologies, such as following a precipitation step with physical separation and/or an oxidation step. Other potential drawbacks of chemical precipitation include changes to pH, as well as the need to manage stream pH to facilitate the desired reaction.
Adsorption
Adsorption is among the most commonly used methods of cyanide removal for process and wastewater streams with relatively low concentrations of cyanide. Adsorption works by leveraging forces of molecular attraction to remove cyanide (and other contaminants) from a liquid stream. The process consists of passing process or wastewater through some type of adsorbent media that serves to attract and retain contaminants while allowing the liquid effluent to flow through.
Adsorption offers many benefits, including relatively low costs of operation, materials, and waste discharge. Additionally, adsorbed cyanide can in some cases be desorbed and concentrated for recycling and reuse. It is worth noting, however, that adsorption has some limitations that need to be accounted for in order to realize these benefits. Among these is the fact that adsorption is most effective at neutral or near-neutral pH levels. Additionally, many types of adsorption media non-selectively remove a variety of stream constituents, which may be a problem for facilities that have complex wastewater streams, or those who require selective removal to support recycling or reclamation efforts.
The most significant factor to consider is that adsorbent media have somewhat limited capacity, and will need to be replaced periodically to ensure adequate contaminant removal performance. In general, the higher the concentration of cyanide and/or other contaminants in a stream, the more frequently adsorbent media will need to be replaced. To optimize the performance of adsorption technologies for cyanide removal, facilities sometimes choose to implement a pre-treatment step consisting of chemical addition of silver or copper salts. Facilities can also sometimes achieve cost savings by sourcing certain industrial byproducts as absorbent media. Indeed, activated carbon, which is by far the most common adsorbent media used for cyanide removal, can be derived from byproducts such as nutshells, coffee grounds, and olive pits, while other byproducts, such as calcinated eggshells and coffee husks have also shown promise as effective materials for cyanide adsorption. In general, though, the capacity limitations of adsorbent media usually mean that adsorption technologies are best suited for streams that have relatively low concentrations of cyanide; for facilities that experience only occasional or seasonal elevations in cyanide levels; or for facilities that are willing to implement some form of pre-treatment.
Ion exchange
Ion exchange (IX) is a physical-chemical treatment process where a liquid stream is passed through a resin substrate that facilitates the exchange of charged ions, such as cyanide anions. The resin used in an IX system is carefully selected based on the ionic charge of the targeted contaminant(s). For streams where cyanide removal is desired, facilities generally use either a chelating resin or a strong base anion (SBA) resin. When a stream enters the IX column, the resin selectively captures cyanide anions from the solution, retaining them until the resin is regenerated.
Generally speaking, IX is a good fit for facilities looking to reduce cyanide content below stringent regulatory limits; for those looking to treat large volumes of water with relatively low concentrations of cyanide; or for facilities where selective removal of cyanide and/or valuable metals recovery is desired. While IX can be a good solution for these situations and priorities, there are also some notable drawbacks. Most significantly, an IX system requires moderate effort and cost for maintenance and operational support. The higher the concentration of cyanide, the more frequently a facility will need to regenerate its resins, resulting in higher costs due to greater consumption of regenerant chemicals and shorter resin life. Additionally, IX can be sensitive to pH, so facilities need a plan to maintain consistent stream pH and/or consider whether pH adjustment is needed to optimize IX system performance. Lastly, facilities need to plan for treatment and disposal of wastes resulting from IX system use, which can include cyanide-contaminated resins, regeneration chemicals, and rinse water. Generally speaking, the higher the concentration of cyanide in a stream, the less cost-effective IX becomes due to higher maintenance and waste discharge costs.
Membrane separation
Membrane filtration is a type of physical separation technology that uses a semi-permeable barrier to selectively remove certain contaminants from a liquid stream. In the past few decades, advances in system design and membrane materials have contributed to a growth in the adoption of membrane separation technologies over traditional physical-chemical separation methods.
In membrane separation, a facility will pass a liquid stream through a porous membrane, usually by applying pressure to the stream. The pores in the membrane are precisely sized in order to retain targeted ions, molecules, or particles while allowing water and other stream constituents to pass through. Facilities typically leverage either reverse osmosis (RO) or electrodialysis for the removal of cyanide anions. In some cases, a facility may also implement a microfiltration or ultrafiltration system to boost efficiency by pretreating streams ahead of downstream RO units.
Membrane separation is highly effective for producing effluents with low levels of cyanide, making it a good solution for direct dischargers who need to comply with stringent wastewater regulations. Additionally, membrane separation results in a highly concentrated reject stream that takes up relatively little volume and therefore entails minimal disposal costs. Despite these benefits, however, facilities looking into membrane separation for cyanide removal purposes should be aware of a few disadvantages, which can include costs for system maintenance and membrane replacement, along with relatively high energy consumption, and flow rate limitations.
Destruction/Oxidation
Also known as oxidation, cyanide destruction is a process where a chemical oxidant (or another agent) is used to break down the carbon-nitrogen triple bond in WAD cyanides. This oxidation reaction forms cyanate (OCN–), which is far less toxic than cyanide, and is less persistent in the environment.
There are a few different types of cyanide destruction, with chemical addition being the most popular method. Facilities can choose from a number of commonly used chemical oxidants, including hydrogen peroxide, chlorine, oxygen, hypochlorite, and sulfur dioxide, among others. All chemical oxidants share in having a high electron affinity, meaning that they are able to attract electrons away from the cyanide anion to form cyanate. Most chemical oxidants require a strongly alkaline pH of 12 or more, although some, like chlorine dioxide, are effective at a moderately alkaline pH as low as 9. Its effectiveness and relative technical simplicity make chemical oxidation a good fit for higher volume streams with WAD cyanides, especially those with levels of cyanide that exceed the limits supported by biological treatment options. Despite its effectiveness for treating WAD cyanides, however, chemical oxidation can be costly due to the consumption of chemicals used for oxidation and pH control, as well as effluent discharge costs. Additionally, oxidation is largely ineffective for the remediation of SAD cyanide species.
While somewhat less common, there are alternative forms of cyanide destruction that can be highly effective for certain contexts. This includes bio-oxidation, a method that uses biomass consisting of bacteria, fungi, algae, yeasts, and/or plants to naturally oxidize cyanide. This form of biological treatment can be a good choice for wastewater with relatively low levels of cyanide. Other methods of cyanide oxidation include electrolysis and photolysis, which utilize an electrical current or UV radiation, respectively, to catalyze the transfer of electrons to break cyanides down into less toxic constituents.
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