Hexavalent chromium (chromium VI) refers to chemical compounds that contain the element chromium in the +6 oxidation state. Virtually all chromium ore is processed via hexavalent chromium. Hexavalent chromium is used for the production of stainless steel, textile dyes, wood preservation, leather tanning, and as anti-corrosion and conversion coatings as well as a variety of niche uses.
In 2010, the Environmental Working Group studied the drinking water in 35 American cities. The study was the first nationwide analysis measuring the presence of the chemical in U.S. water systems. The study found measurable hexavalent chromium, Cr (VI), in the tap water of 31 of the cities sampled, with Norman, Oklahoma, at the top of list; 25 cities had levels that exceeded California's proposed limit. Cr (IV) is a confirmed carcinogen and causes irritation and ulcers in the stomach, causes kidney and liver damage, and may even lead to death when taken at more than the accepted limit.
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Many algae have immense capability to sorb (adsorb and absorb) metals, and there is considerable potential for using them to treat wastewaters. Metal sorption involves binding on the cell surface and to intracellular ligands. The adsorbed metal is several times greater than intracellular metal. The surface carboxyl group is most important for metal binding. Metal sorption is affected by the concentration of metal and biomass in solution, pH, temperature, cations, anions and metabolic stage of the organism. Algae can effectively remove metals from multi-metal solutions, in many cases better than any other organism. (Mehta and Gaur, 2005)
A number of physicochemical methods, such as chemical precipitation (caustic, sulfide), adsorption, solvent extraction, ion exchange, membrane separation and charcoal filtration have been commonly used for removing toxic metals from wastewaters (Eccles, 1999). However, these methods have several disadvantages, such as incomplete metal removal, expensive equipment and monitoring system requirements, high reagent or energy requirements and generation of toxic sludge or other waste products that require separate and costly disposal. Further, they may be ineffective or extremely expensive when metal concentration in wastewater is in the range 10ââ‚¬"100 mg l. Algae, however can detect and adsorb metals down to levels in the parts per billion. This not only means that it can effectively remove low levels of metal contamination but can also be used as a sensitive environmental monitoring agent.
Bacterial/algal symbiotic bioremediation of Cr has distinct advantages over abiotic remediation. Aerobic bacteria and algae work together to adsorb and absorb Cr. The algae create O2 for the bacteria and the bacteria create CO2 for the algae. In addition, when hydrocarbon contaminants are also present (which is frequently the case), both algae and bacteria are capable of transforming the hydrocarbon contaminants and the bacteria can utilize the organic carbon as an electron donor, removing the need to maintain anaerobic conditions.
Comparing bioremoval with conventional heavy metal removal methods indicate that several potential advantages are possible with bioremoval processes including:
1. use of naturally abundant renewable biomaterials that can be cheaply produced;
2. ability to treat large volumes of wastewater due to rapid kinetics;
3. high selectivity in terms of removal and recovery of specific heavy metals;
4. ability to handle multiple heavy metals and mixed waste;
5. high affinity, reducing residual metals to below ppb in many cases;
6. less need for additional expensive process reagents which typically cause
disposal and space problems;
7. operation over a wide range of physicochemlcal conditions including
temperature, pH, and presence of other ions (including Ca 2+ and Mg2+);
8. relatively low capital investment and low operational costs;
9. greatly improved recovery of bound heavy metals from the biomass; and
10. greatly reduced volume of hazardous waste produced.
(Wilde and Benemann, 1993)
The accumulation of heavy metals in algae involves two processes: an initial rapid (passive) uptake followed by a much slower (active) uptake (Bates et al., 1982). During the passive uptake, metal ions adsorb onto the cell surface within a relatively short span of time (few seconds or minutes), and the process is metabolism independent. Active uptake is metabolism-dependent, causing the transport of metal ions across the cell membrane into the cytoplasm. In some instances, the transport of metal ions may also occur through passive diffusion owing to metal-induced increase in permeability of the cell membrane (Gadd, 1988). Generally, adsorption contributes much more to metals remediation, even >80% (Mehta, Singh, and Gaur, 2002; Mehta, Tripathi, and Gaur, 2000), than does absorption to total metal accumulation by algal cells. After the first minute of exposure to Cu, >90% of total metal content was found adsorbed on the surface of Scenedesmus subspicatus (Knauer, Behra, and Sigg, 1997).
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Algal cells can also adsorb Cr (VI) with considerable ease at low pH values (<2) (Kratchovil and Volesky, 1998; DÅ'nmez and Aksu, 2002). Many species take up high levels of CR at very high temperatures; up into the 400 to 600C range. Removal of Cr (VI) by algae is an anionic process as well as through its reduction to the cationic Cr (III) under strongly acidic conditions. In nature Cr occurs in these two forms only, with Cr (VI) being much more toxic to biota than Cr (III). At low pH, the algal biomass provides protons for the reduction of Cr (VI) to Cr (III). The soluble form, Cr (VI) is reduced to the less toxic Cr (III) and becomes a precipitated insoluble hydroxide which can then be transformed in the anaerobic zone to a biogenic ore. This allows for its safe removal from the process stream.
In work by Doshi, (Doshi, et al., 2007) the biosorption of metal ions were tested on two naturally occurring algal blooms, HD-103 and HD-104. HD-103 was primarily Cladophora sp. And HD-104 was primarily Spirulina. In HD-103, the uptake of trivalent Cr was very high 347mg/g of biomass and in HD-104 was 306mg/g biomass.
Effective photoreduction has been reported of Cr (VI) in Chlorella vulgaris (Deng, et al., 2006). Likewise Chlorella was successfully used for remediation of Cr (VI) from tannery wastes in Lahore, India (Rehman and Shakoori, 2001). The alga Distigma proteus was able to reduce Cr (VI) by 97% after 8 days in culture. (Rehman, et al., 2006)
Clearly, microalgae is capable of effectively remediating Cr in industrial wastewater.