How to remove chromium from Industrial Waste Water?
Chromium is a heavy metal with several distinguishing characteristics, including hardness, high polish, a high melting point, and resistance to tarnish and corrosion. Native chromium metal is not commonly found in nature, but rather in the form of ore deposits primarily composed of a compound known as chromite (iron chromium oxide). Chromite ore is mined from deposits all over the world, with South Africa, Kazakhstan, India, and Turkey producing the most.
Metallurgy has the highest demand for chromium in industry, as the metal is used extensively in the production of stainless steel and other metal alloys, as well as in chrome plating applications. The chemical industry consumes a significant amount of chromium due to the use of compounds such as chrome oxide, chromic acid, and potassium dichromate as catalysts and reagents in a variety of manufacturing processes.
Excess chromium levels in wastewater from these and other industrial activities must be carefully monitored and managed.
Let's look at which technologies are the most effective at removing chromium from industrial water and wastewater, as well as the key benefits and drawbacks of each as they apply to different industries and processes.
1-Using chemical precipitation to remove chromium
Chemical precipitation is a separation method in which chemical precipitants are added to a liquid stream in order to drive contaminant ions out of the solution so that they can be removed via physical separation. Chemical precipitants used for chromium separation include relatively inexpensive chemicals such as calcium hydroxide, sodium hydroxide, magnesium oxide, or calcium magnesium carbonate, making chemical precipitation a generally economical, simple, and popular chromium removal treatment option.
While chemical precipitation is very effective for removing trivalent chromium (Cr(III)), the process becomes a little more complicated for streams containing hexavalent chromium (Cr(VI)), which necessitates an additional reduction step. This usually entails adding a reduction agent (typically sodium bisulfate, sodium meta-bisulfite, iron sulphate, or gaseous Sulphur dioxide) that acts on Cr(VI) to reduce it to Cr (III).
2-Ion exchange for chromium removal
Ion exchange (IX) is a physical-chemical process that removes contaminants from a solution by swapping out ions with similar electrical charges. IX has several advantages, including cost-effectiveness, convenience, and reversibility, and it is an excellent choice for facilities looking to recover and reuse chromium from their waste streams.
An IX resin substrate performs the work of an IX unit, which must be carefully selected based on process conditions and stream characteristics. Strong acid cation (SAC) resins, weak acid cation (WAC) resins, or chelating resins can be used to remove trivalent chromium. Strong base anion (SBA) resins, on the other hand, can remove hexavalent chromium. To ensure consistent and adequate chromium removal, facilities must plan for regular maintenance and resin regeneration cycles.
3-Flocculation and coagulation
Coagulation and flocculation are wastewater treatment technologies used to remove contaminants from colloidal or fine-particle streams. The process involves adding chemical coagulants to the stream and gently mixing it to encourage the agglomeration (or "flocculation") of fine particles into large particles that settle out of the liquid stream. Coagulation and flocculation can be used to remove organic materials, color, odor, suspended particles, and heavy metals such as chromium. Coagulation and flocculation are commonly used as a primary wastewater treatment step to reduce the overall load of chromium or other heavy metals, which can impair the performance of biological treatment, which is commonly used in municipal wastewater treatment plants.
Adsorption is a treatment method that uses molecular forces of attraction to extract contaminants from a liquid stream. The adsorption process involves passing a liquid stream through porous adsorbent media. Because soluble contaminants are more attracted to the adsorbent media than to the water in the stream, the contaminants bind to the media's surface while the liquid effluent flows through. Adsorption is an effective method for separating and removing chromium and other heavy metals from streams with low concentrations of heavy metal contaminants.
5-Electrochemical reduction for chromium removal
Electrochemical reduction is becoming a more popular treatment method for Cr-containing streams (VI). An electrical current is passed through metal electrodes, causing them to dissolve and release ions into the solution. The ions then oxidize, reducing Cr(VI) to a more easily dissolvable Cr (III). Depending on the system design, electrochemical reduction may be followed by a separate precipitation reaction, or precipitation may occur concurrently.
There are numerous advantages to electrochemical reduction, including the ability for the reaction to proceed at a neutral pH and the use of few chemicals, which can be advantageous from both a cost and an environmental standpoint.
Membrane filtration is a physical separation process that involves passing a liquid stream through a semi-permeable filtration membrane that separates materials based on the size exclusion principle. The membrane has specially sized pores that allow the liquid stream and smaller particles to pass through while retaining targeted contaminants.
Ultrafiltration (UF), nano-filtration (NF), reverse osmosis (RO), and electro-dialysis are the most common types of membrane filtration used for chromium separation. In some cases, multiple types of membrane filtration are used in sequence to maximize membrane performance and longevity while also removing enough chromium. Membrane filtration is also an excellent choice for facilities looking to reclaim chromium, as both NF and RO can generate chromium-rich retentate that can be reused in process streams.
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