# The Double Face of Metals: The Intriguing Case of Chromium
## 8 Chromium Remediation  
Since heavy metals are not degradable, their accumulation in the environment (soil, water and air) may occur, thus contaminating the food chain, engendering a danger to human health. Hexavalent chromium and its oxyanions (e.g., Cr\({}_{2}\)O\({}_{7}^{2-}\), CrO\({}_{4}^{2-}\), HCrO\({}_{4}^{-}\)) are priority targets for the Environmental Protection Agency (EPA) due to their high degree of toxicity. Heavy metal removal is generally carried out by conventional methods involving absorption by ion exchange or chemical resins, that present the limits that they often require co-reagents or exaggerated regeneration chemicals [131] and can undergo slow kinetics with long processing times of the order of hours [132]. Bioremediation may decrease chromium toxicity: some microorganisms are able to secrete chromate reductase that turns highly toxic hexavalent chromium into nontoxic trivalent chromium [133]. _Pseudomonas_ sp. Cr13 can tolerate high concentrations of Cr(VI) and thus it partially removes Cr(VI) [134; 135]. Recently, _Acinetobacter_ sp. Cr1 strain was found to be effective for removal  
Figure 3: **Chromium epigenetic effects on (A) DNA, (B) histones and (C) miRNAs.** (A) The methylation of DNA in the presence of DNA methyltransferase involves the bond of a methyl group to the cytosine forming 5-methylcytosine. (**B**) The H3 and H4 histone tails undergo covalent reactions that include methylation and acetylation. The acetylation of the \(N\)-terminal lysine residue is a process regulated by histone lysine acetyltransferases. Histones are also methylated on lysine thanks to histone lysine methyltransferases. (**C**) MicroRNAs are subjected to a covalent methylation modification coordinated by methyltransferases.  
of Cr(VI) [136]. Plants can also be used for phytoremediation by absorbing Cr from soil into plant organs [137]. Poly(vinyl)ferrocene-carbon nanotube (PVF-CNT) electrodes were used for the removal of anionic chromium from water, with a high adsorption capacity (>100 mg/g at saturation) and singular properties of regeneration. Both the release and the regeneration of the electrodes leads to the conversion of the hexavalent pollutant into the less harmful trivalent form [138]. The use of plants to remove nondegradable contaminants from the soil is called phytoextraction and is a green technology used as an awesome alternative to traditional physical and chemical methods that does not impact ecosystems and demands modest economic investment. It has been applied to some heavy metals, such as lead, cadmium and nickel [139; 140] and recently mercury and chromium [141; 142]. Recently, in a study in the Thi Vai river catchment (Vietnam), mangrove afrorestation areas in appropriate locations were suggested to be used for their phytoremediation potential [143]. Novel chitosan-based thin sheet nanofiltration membranes, prepared with chitosan/polyvinyl alcohol/montmorillonite clay, have been recently also proposed for the rejection of chromium [144]. Recently, the use of probiotics was reported to be useful to weaken or hamper the toxic effects of Cr(VI). In fact, some species of _Lactobacillus_, _Streptococcus_, _Bacillus_ and _Bifidobacterium_ present both in the mouth and gut of humans and in fermented foods are able to bind and detoxify some toxic substances, such as Cr(VI) [145].