Hexavalent Chromium: Sources, Toxicity, and Remediation
Bidyut Saha (Associate Editor),
Aziz Amine (Associate Editor)
Francis Verpoort (co Editor-in-Chief)
Chromium is a relatively new entry as a major toxic metal after lead, cadmium, and mercury. Natural and anthropogenic sources are responsible for the entry of chromium into the ecosystem. Manganese from birnessite ore converts trivalent chromium to hexavalent chromium of chromite ore under suitable conditions in the soil. The reaction is supposed to occur in three steps:
1. Adsorption of trivalent chromium on the surface sites of manganese dioxide.
2. Oxidation of trivalent chromium to hexavalent chromium by tetravalent manganese.
3. At the end, hexavalent chromium is desorbed from the surface.
Chromium is used extensively in various small- and large-scale industries. Chromium and its compounds are extensively used in industry, with the most common and important sources coming from the electroplating, catalyst sector, tanning, water cooling, pulp production, dyes and pigments, film and photography, wood preservation, and alloy manufacture industries. Petroleum refining processes and chromite ore processing have introduced chromium into the soil, air, and water. Chrome plating is a special type of electroplating Chrome plating baths are of two types: hexavalent baths and trivalent baths. Hexavalent chromium baths are of frequent use. Typical hexavalent chromium bath composition is as follows: (a) electrolytic solution: chromic acid (b) anode: lead with tin up to 7%; (c) operating temperature: 45–60 °C (d) plating current: 1.5–3.0 kA/m2. About 35% of used chromium is discharged in the effluent as trivalent and hexavalent chromium. Chromates of barium, lead, and zinc provide the pigments of lemon chromium, chromium yellow, chromium red, chromium orange, zinc yellow, and zinc green glass.
Chromium compounds have been used in the formulation of wood preservatives for more than a century. These are ‘‘Wolman compounds’’ (based on sodium fluoride and nitrophenol with sodium dichromate or potassium dichromate), copper chromate (CC), copper–chromium–arsenic (CCA), copper– chromium–boron (CCB), copper–chromium–fluoride (CCF), and copper–chromium–phosphate (CCP). A considerable quantity of tanning powder basic chrome sulfate is used in chrome tanning to convert polypeptide collagen strands in the hide to a cross-linked helix. Indications of chromium toxicity in plants include reduction of seed germination, retardation of growth, reduction of yield, inhibition of enzymatic activities, weakening of photosynthesis, nutrient, oxidative disparities, and genetic mutation. Breathing hexavalent chromium-containing material can cause perforation of the nasal septum, asthma, bronchitis, pneumonitis, inflammation of the larynx and liver, and increased incidence of bronchogenic carcinoma. Skin contact of hexavalent chromium compounds can produce skin allergies, dermatitis, dermal necrosis, and dermal corrosion. Stomach cancer is the result of the ingestion of hexavalent chromium. The hexavalent form of chromium is more toxic than the trivalent form. This is due to (a) due to structural similarity of chromate with sulphate, hexavalent chromium passes the cell membrane via sulphate uptake pathway, but trivalent chromium does not
(b) hexavalent chromium (t2g0 eg 0) is a labile center, but trivalent chromium (t2g3 eg 0) is a inert center (c) hexavalent chromium is a strong oxidizing agent and (d) trivalent chromium undergoes precipitation under biological condition. Several methods are utilized to remove chromium from the contaminated site. These include mainly adsorption, reduction followed by chemical precipitation, ion exchange, reduction, electrochemical precipitation, solvent extraction, membrane separation, and bioremediation.
The Topical Issue welcomes papers on (but not limited to):
1. Natural sources of hexavalent chromium/Presence of chromium in ground water
2. Anthropogenic sources of hexavalent chromium
3. Airborne hexavalent chromium
4. Chromium toxicity in plants
5. Chromium toxicity and its health effect
6. Chromium in food crops and chicken meat
7. Hexavalent chromium detection by photoelectrochemical sensor
8. Chemical speciation of chromium in soil
9. Chemical speciation of chromium in water
10. Chromium remediation from soil using adsorbent
11. Hexavalent chromium remediation from water using adsorbent
12. Hexavalent chromium substitution
Deadline for submission: 30 September 2021
Prof. Bidyut Saha, Department of Chemistry, The University of Burdwan, Burdwan, East Bardhaman, WB, India
Prof. Saha is involved in higher studies and research for more than twenty years. His interests are micellar catalysis, remediation of the toxic metal, chemical kinetics and inorganic reaction mechanism. He has contributed more than 120 research articles.