Chromium exists in oxidation states ranging from −IV to +VI, inclusively. The compounds exhibit a wide range of geometries including square planar, tetrahedral, octahedral, and various distorted geometries. Ore refining, chemical and refractory processing, cement-producing plants, automobile brake lining, catalytic converters for automobiles, leather tanneries, and chrome pigments contribute to the atmospheric burden of chromium. Hexavalent chromium is known to have 100-fold more toxicity than trivalent chromium, for both acute and chronic exposures because of its high water solubility and mobility, as well as easy reduction. The respiratory tract is the major target organ for hexavalent chromium following the inhalation exposure in humans. Chronic inhalation exposure to hexavalent chromium results in effects on the respiratory tract, with perforations and ulcerations of the septum, bronchitis, decreased pulmonary function, pneumonia, and nasal itching and soreness as reported. Chronic human exposure to high levels of hexavalent chromium by inhalation or oral exposure may produce effects on the liver, kidney, gastrointestinal, and immune systems, and possibly the blood. Dermal exposure to hexavalent chromium may cause contact dermatitis, sensitivity, and ulceration of the skin.

Natural surfactants or biosurfactants are amphiphilic biological compounds, usually extracellular, produced by a variety of microorganisms from various substances including waste materials. There is increasing interest on this topic because of their unique properties such as low toxicity, functionality under extreme conditions, based on renewable substances and biologically degradable nature. The diversity of these molecules supports their potential application in the field of petroleum, medicine, agriculture, food, cosmetics etc. They are also effective in curtailing the green-house effect by reducing the emission of CO₂. They can be termed as ‘green’ because of their low toxicity, biodegradability and relative stability under a wide range of physicochemical environments. In spite of possessing diverse structures and better physicochemical properties than chemical surfactants, biosurfactants are not able to compete with their synthetic counterparts because of their high production & downstream costs. The commercial realization of these eco-friendly biomolecules is restricted by low productivity, expensive downstream processing and lack of appropriate understanding of the bioreactor systems for their production. But we expect that in future better reactor design and product recovery technology would be developed and overproducer microbial strain would be screened. Then production cost would be decreased and yield would be increased i.e. the production would be both ecologically & economically favored. The present review deals with an overall view on biosurfactants, their properties, advantages & disadvantages, production, characterization, application along with a recommendation for future research.

This review contains up-to-date knowledge and recent advancements on the essentiality, sources, and toxicological profile of nickel and its different compounds.