## 1 Introduction  
In the environment, the two most stable oxidation states of chromium are its hexavalent (Cr(VI)) and trivalent (Cr(III)) forms [1]. Cr(III) is considered less toxic than Cr(VI), because it is conventionally considered an essential micronutrient in animal and human diets. By contrast, Cr(VI) is highly toxic, mutagenic, carcinogenic, and teratogenic to biological systems and is readily taken up by organisms via the sulfate transport system [2]; hence, most regulatory agencies worldwide consider Cr(VI) a priority pollutant [3;4]. Chromium compounds are used in chromium electroplating, metal processing, leather tanning, metal corrosion inhibition, papermaking, pigment manufacturing, and other industrial applications [5]. Cr(VI)-containing industrial effluents without proper treatment are released, intentionally or accidentally, into environments in developing and underdeveloped countries [6]; thus, the contamination of surface waters and groundwaters with Cr(VI) is a potential concern. Concentrations of Cr(VI) in these effluents must be reduced to permissible limits by using the appropriate technology before the effluents are discharged into the environment. The Environmental Protection Agency in Taiwan has set the maximum contaminant levels for Cr(VI) in most industrial effluents at 0.5 mg L\({}^{-1}\) and for fisheries water, drinking water, groundwater, and surface water at 0.05 mg L\({}^{-1}\). Many physical and chemical methods have been developed for Cr(VI) removal, such as precipitation, adsorption, ion exchange, electrochemical treatment, evaporation, and reverse osmosis [5]. Nevertheless, existing methods are inefficient and not economically feasible, especially when Cr(VI) contamination in wastewater must be reduced to a low level to meet strict environmental standards, such as 0.5 mg L\({}^{-1}\)[7]. Several researchers have reported that the biological detoxification of Cr(VI) by bacteria and their enzymes is considered one of the most practical removal methods, especially at low-to-medium concentrations of Cr(VI), due to its ecological compatibility and low cost [4;8]. Bacteria endowed with the capacity to reduce Cr(VI) levels are termed chromium-reducing bacteria (CRB) [9]. Numerous varieties of CRB reduce Cr(VI) to Cr(III), including _Bacillus cereus_[10], _Ochrobarrum_ sp. [11], _Paenibacillus ferrarius_[12], _Exigubacterium aestuarii_[13], _Shewanella_ sp., _Desulfovibrio_ sp., _Enterobacter_ sp., _Microoccus_ sp., _Pseudomonas_ sp. [8], and several other species [5]. Although most of these CRB have been isolated from various Cr(VI)-contaminated sites, the availability of CRB is an essential prerequisite to meeting strict environmental standards, regardless of their removal efficiency or capacity. Thus, the feasibility of chromate reductase in removing Cr(VI) is considered, because it functions without the requirement of high organics concentration and complicated biochemistry processes like bacterial cells but possesses a high substrate specificity for Cr(VI) reduction. Cr(VI) reduction or detoxification by CRB is regarded as an enzyme catalysis reaction attributed to soluble chromate reductase or cell membrane-bound chromate reductase [14]. Cell immobilization techniques are typically employed in wastewater treatments, because they result in a solid-liquid separation that is more stable, and thus, the products are easier to reuse [15]. Similarly, the use of immobilized enzymes instead of living bacterial cells may overcome the limit of the toxicity of Cr(VI) to the chromate reductase and produce dilute Cr(VI) residue, in addition to achieving the aforementioned advantages [16]. Chromate reductase is divided into four categories: (1) cytochrome C, (2) flavin protein, (3) old yellow enzymes, and (4) hydrogenases [7].