Reduction of Cr(VI) by Escherichia coli BL21 in the presence of redox mediators

Recent advances in enhanced technology of Cr(VI) bioreduction in aqueous condition: A review

Due to the extensive application of chromate in industry, chromium-contaminated water has emerged as a significant hidden danger that threatens human health and the safety of the ecological environment. The reduction of Cr(VI) to Cr(III) through microbial processes has become one of the most notable methods for remediating water polluted by chromium due to its economic efficiency and environmentally friendly nature. However, several issues persist in its practical application, such as low reduction rates, the need for additional nutrients, and challenges in solid-liquid separation. Therefore, there is a growing focus on seeking enhanced methods for Cr(VI) microbial reduction, which has become a key area of research. This review represents the initial effort to systematically classify and summarize the means of enhancing Cr(VI) microbial reduction. It categorizes the enhancement methods into two main approaches: microbial-based and multi-method combined enhancement, offering detailed explanations for their mechanisms. This research provides both inspiration and theoretical support for the practical implementation of the Cr(VI) microbial reduction method.

Metabolic mechanism of Cr(VI) pollution remediation by Alicycliphilus denitrificans Ylb10

Cr(VI) is a well-known toxic pollutant and its remediation has attracted great attention. It is important to continuously discover and explore new high-efficiency Cr(VI) reducing bacteria to further improve the efficiency of Cr(VI) pollution remediation. In this paper, metabolic mechanism of Cr(VI) reduction in a new highly efficient Cr(VI) reducing bacterium, Alicycliphilus denitrificans Ylb10, was investigated. The results showed that Ylb10 could tolerate and completely reduce 450 mg/L Cr(VI). Cr(VI) can be reduced in the intracellular compartment, membrane and the extracellular compartment, with the plasma membrane being the main active site for Cr(VI) reduction. With the addition of NADH, the reduction efficiency of cell membrane components for Cr(VI) increased 2.3-fold. The omics data analysis showed that sulfite reductase CysJ, thiosulfate dehydrogenase TsdA, nitrite reductase NrfA, nitric oxide reductase NorB, and quinone oxidoreductase ChrR play important roles in the reduction of Cr(VI), in the intracellular, and the extracellular compartment, and the membrane of Ylb10, and therefore Cr(VI) was reduced by the combined action of several reductases at these three locations.

Thermodynamic considerations on the combined effect of electron shuttles and iron(III)-bearing clay mineral on Cr(VI) reduction by Shewanella oneidensis MR-1

Electron shuttles (ESs) and Fe-bearing clay minerals are commonly found in subsurface environments and have shown potential in enhancing the bioreduction of Cr(VI). However, the synergistic effect of ESs at different redox potentials and Fe-bearing clay minerals on Cr(VI) bioreduction, as well as the fundamental principles governing this process, remain unclear. In our study, we investigated the role of ESs and Fe(III) in Cr(VI) bioreduction. We found that the acceleration of ESs and Fe(III) are crucial factors in this process. Interestingly, the promotion of ESs on Cr(VI) and Fe(III) showed opposite trends. Electrochemical methods confirmed the limited steps are the extent of reduced ESs and the redox potential difference between ESs and Fe(III), separately. Furthermore, we investigated the combined effect of ESs and NAu-2 on Cr(VI) bioreduction. Our results revealed two segments: in the first segment, the ES (5-HNQ) and NAu-2 did not synergistically enhance Cr(VI) reduction. However, in the second segment, ESs and NAu-2 demonstrated a synergistic effect, significantly increasing Cr(VI) reduction by MR-1. These bioreduction processes all follow linear free energy relationships (LFERs). Overall, our study highlights the fundamental principles governing multivariate systems and presents a promising approach for the remediation of Cr(VI)-contaminated sites.

Influencing factors and mechanism of Cr(VI) reduction by facultative anaerobic Exiguobacterium sp. PY14

Microbial reduction is an effective way to deal with hexavalent chromium [Cr(VI)] contamination in the environment, which can significantly mitigate the biotoxicity and migration of this pollutant. The present study investigated the influence of environmental factors on aqueous Cr(VI) removal by a newly isolated facultative anaerobic bacterium, Exiguobacterium sp. PY14, and revealed the reduction mechanism. This strain with a minimum inhibitory concentration of 400 mg/L showed the strongest Cr(VI) removal capacity at pH 8.0 because of its basophilic nature, which was obviously depressed by increasing the Cr(VI) initial concentration under both aerobic and anaerobic conditions. In contrast, the removal rate constant for 50 mg/L of Cr(VI) under anaerobic conditions (1.82 × 10-2 h-1) was 3.3 times that under aerobic conditions. The co-existence of Fe(III) and Cu(II) significantly promoted the removal of Cr(VI), while Ag(I), Pb(II), Zn(II), and Cd(II) inhibited it. Electron-shuttling organics such as riboflavin, humic acid, and anthraquinone-2,6-disulfonate promoted the Cr(VI) removal to varying degrees, and the enhancement was more significant under anaerobic conditions. The removal of aqueous Cr(VI) by strain PY14 was demonstrated to be due to cytoplasmic rather than extracellular reduction by analyzing the contributions of different cell components, and the end products existed in the aqueous solution in the form of organo-Cr(III) complexes. Several possible genes involved in Cr(VI) metabolism, including chrR and chrA that encode well-known Chr family proteins responsible for chromate reduction and transport, respectively, were identified in the genome of PY14, which further clarified the Cr(VI) reduction pathway of this strain. The research progress in the influence of crucial environmental factors and biological reduction mechanisms will help promote the potential application of Exiguobacterium sp. PY14 with high adaptability to environmental stress in Cr(VI) removal in the actual environment.

Enhanced microbial reduction of Cr(VI) in soil with biochar acting as an electron shuttle: Crucial role of redox-active moieties

Biochar has been proven to participate in the biotic reduction of hexavalent chromium (Cr(VI)) in environment since its involvement may accelerate the extracellular electron transfer (EET). However, roles of the redox-active moieties and the conjugated carbon structure of biochar in this EET process remain unclear. In this study, 350 °C and 700 °C were selected to produce biochar with more O-containing moieties (BC350) or more developed conjugated structures (BC700), and their performances in the microbial reduction of soil Cr(VI) were investigated. Our results showed that BC350 presented a 241% increase of Cr(VI) microbial reduction after 7-day incubation, much higher than that of BC700 (39%), suggesting that O-containing moieties might play more important roles in accelerating the EET process. Biochar, especially BC350 could serve as an electron donor for microbial anaerobic respiration, but its contribution (73.2%) as an electron shuttle for EET was dominant to the enhanced Cr(VI) reduction. The positive correlation between electron exchange capacities (EECs) of pristine and modified biochars and the corresponding maximum reduction rates of Cr(VI) evidenced the crucial role of redox-active moieties in electron shuttling. Moreover, EPR analysis suggested the nonnegligible contribution of semiquinone radicals in biochars to the accelerated EET process. This study demonstrates the crucial role of redox-active moieties, i.e., O-containing moieties in mediating the EET process during the microbial reduction of Cr(VI) in soil. Findings obtained will advance the current understanding of biochar as an electron shuttle participating in the biogeochemical processes of Cr(VI).

Column study of enhanced Cr(VI) removal by bio-permeable reactive barrier constructed from novel iron-based material and Sporosarcina saromensis W5

In this study, the feasibility of Cr(VI) removal from synthetic groundwater by bio-permeable reactive barrier constructed from novel iron-based material (SiO₂/nano-FeC₂O₄ composite, SNFC) and Sporosarcina saromensis W5 was investigated. According to breakthrough study, an enhanced Cr(VI) removal was found in Bio-SNFC column. The Cr(VI) removal performances of biotic column with 0.2 g biomass and 0.4 g biomass were 16.2 mg/g and 17.9 mg/g, respectively, which were 19.6% and 32.1% higher than that of abiotic column (13.5 mg/g). However, excessive biomass (0.9 g) would cause pore clogging and have a negative impact on the Cr(VI) removal performance of the biotic column, whose removal capability (29.1%) was lower than that of abiotic column. The introduction of proper microorganisms enhanced the utilization of iron and enabled a higher proportion of Fe(II) in biotic column, which provided more reactive sites for Cr(VI) removal. The solid phase characterization indicated the generation of Fe(III) oxide/hydroxide on SNFC surface. The removal of Cr(VI) in Bio-SNFC column was depended on reduction-precipitation, and the final products related to chromium were mainly Cr(OH)₃ and Cr₂O₃. The present work provides a new and sustainable remediation technology for in situ bioremediation of Cr(VI)-contaminated groundwater.

Cr(VI) Removal by Recombinant Escherichia coli Harboring the Main Functional Genes of Sporosarcina saromensis M52

Hexavalent chromium [Cr(VI)], a recognized heavy metal pollutant, has attracted much attention because of its negative impact on the ecological environment and human health. A chromium-resistant strain, Sporosarcina saromensis M52, was discovered, and the functional genes orf2987, orf3015, orf0415, and orf3237 were identified in the strain by genomics. With the advancement of DNA recombination and gene-splicing technology, genetic engineering technology was used to produce recombinant strains 2987, 3015, 0415, and 3237. The study revealed Cr(VI) tolerance in the order of M52 ≈ 2987 > 3015 ≈ 0415 > 3237 and reduction abilities in the order of M52 ≈ 2987 > 3015 > 0415 ≈ 3237. SEM-EDS, XRD, FT-IR and XPS were utilized to examine the surface structure of the recombinant strains and analyze the surface components and main functional groups. A comprehensive review of the recombinant strains’ capacity to tolerate and reduce Cr(VI) revealed that orf2987 and orf0415 were the main functional genes in Sporosarcina saromensis M52, which may play a key role in removing Cr(VI) and protecting the strain, respectively. The optimum pH for recombinant strains 2987 and 0415 was 7.5–8.5, and the optimum temperature was 37°C. Cu²⁺ had the greatest promotional effect when Cr(VI) was removed by them, while SDS had an inhibitory effect. This research provided the foundation for further study into the mechanism of Cr(VI) reduction in Sporosarcina saromensis M52, as well as a theoretical basis for the development of effective engineered strains to repair Cr(VI) contamination.

Biotic and Abiotic Biostimulation for the Reduction of Hexavalent Chromium in Contaminated Aquifers

Hexavalent chromium is a carcinogenic heavy metal that needs to be removed effectively from polluted aquifers in order to protect public health and the environment. This work aims to evaluate the reduction of Cr(VI) to Cr(III) in a contaminated aquifer through the stimulation of indigenous microbial communities with the addition of reductive agents. Soil-column experiments were conducted in the absence of oxygen and at hexavalent chromium (Cr(VI)) groundwater concentrations in the 1000–2000 μg/L range. Two carbon sources (molasses and EVO) and one iron electron donor (FeSO4·7H2O) were used as ways to stimulate the metabolism and proliferation of Cr(VI) reducing bacteria in-situ. The obtained results indicate that microbial anaerobic respiration and electron transfer can be fundamental to alleviate polluted groundwater from hazardous Cr(VI). The addition of organic electron donors increased significantly Cr(VI) reduction rates in comparison to natural soil attenuation rates. Furthermore, a combination of organic carbon and iron electron donors led to a longer life span of the remediation process and thus increased total Cr(VI) removal. This is the first study to investigate biotic and abiotic Cr(VI) removal by conducting experiments with natural soil and by applying biostimulation to modify the natural existing microbial communities.

Application of different redox mediators induced bio-promoters to accelerate the recovery of denitrification and denitrifying functional microorganisms inhibited by transient Cr(VI) shock

The transient hexavalent chromium (Cr(VI)) shock may directly inhibit the denitrification process of municipal wastewater treatment plants (WWTPs), which is difficult to recover in a short time. This study developed four nontoxic bio-promoters (combination of L-cysteine, flavin adenine dinucleotide (FAD), biotin, cytokinin and different redox mediators) to quickly restore the denitrification performance after high-loading Cr(VI) suppressing. After feeding with 100 mg/L of Cr(VI) for 42 cycles (T, 4 h), the removal efficiency of nitrate was reduced by 85.00%, and nitrite was accumulated simultaneously. The denitrification performance was recovered quickly with the addition of bio-promoters, introducing redox mediators showed noticeable superiority on the bio-inhibition release. Compared with sodium humate and riboflavin, the AQDS induced bio-promoter achieved the best nitrate removal recovery performance within only 28 T, and the recovery rate was 2.16 times faster than the natural recovery. Microbial analysis showed that Cr(VI) specially inhibited napA-type denitrifiers, and the OTU numbers sharply dropped by 48.74%. Redox mediators induced bio-promoters could effectively recover the abundance of napA-type and nirS-type denitrifying microorganisms, which was consistent with the change of nitrate removal efficiency. This study offers a cost-effective approach to deal with Cr(VI) shock problem, which may promote the development of bio-promoters for WWTPs.

Bioreduction performances and mechanisms of Cr(VI) by Sporosarcina saromensis W5, a novel Cr(VI)-reducing facultative anaerobic bacteria

This study reported a novel facultative anaerobic Cr(VI)-reducing bacteria (Sporosarcina saromensis W5) and investigated its Cr(VI) removal performances and removal mechanisms. The strain W5 was able to grow and reduce Cr(VI) under aerobic and anaerobic environment, and exhibited considerable Cr(VI) reduction capabilities under a wide range of pH (8.0-13.0), temperature (20-40 °C) and initial Cr(VI) concentration (50-800 mg/L). The addition of Cd²⁺ severely inhibited its growth and Cr(VI) removal, while Cu²⁺ and Fe³⁺ significantly enhanced the removal efficiencies. The strain W5 could utilize various electron donors and mediators to accelerate Cr(VI) reduction. Aerobic Cr(VI) reduction mainly occurred in cytoplasm and the final products were soluble organo-Cr(III) complexes. Anaerobic Cr(VI) reduction was located in both cytoplasm and membrane, and the reduction products were soluble organo-Cr(III) complexes and Cr(III) precipitates. The functional groups of hydroxyl, carboxyl and phosphoryl on cell surface participated in the combination with Cr(III). Due to its facultative anaerobic property, S. saromensis W5 offers itself as a promising engineering strain for the bioremediation of Cr(VI)-contaminated areas, especially in hypoxia environments.

Chemical-Assisted Microbially Mediated Chromium (Cr) (VI) Reduction Under the Influence of Various Electron Donors, Redox Mediators, and Other Additives: An Outlook on Enhanced Cr(VI) Removal

Chromium (Cr) (VI) is a well-known toxin to all types of biological organisms. Over the past few decades, many investigators have employed numerous bioprocesses to neutralize the toxic effects of Cr(VI). One of the main process for its treatment is bioreduction into Cr(III). Key to this process is the ability of microbial enzymes, which facilitate the transfer of electrons into the high valence state of the metal that acts as an electron acceptor. Many underlying previous efforts have stressed on the use of different external organic and inorganic substances as electron donors to promote Cr(VI) reduction process by different microorganisms. The use of various redox mediators enabled electron transport facility for extracellular Cr(VI) reduction and accelerated the reaction. Also, many chemicals have employed diverse roles to improve the Cr(VI) reduction process in different microorganisms. The application of aforementioned materials at the contaminated systems has offered a variety of influence on Cr(VI) bioremediation by altering microbial community structures and functions and redox environment. The collective insights suggest that the knowledge of appropriate implementation of suitable nutrients can strongly inspire the Cr(VI) reduction rate and efficiency. However, a comprehensive information on such substances and their roles and biochemical pathways in different microorganisms remains elusive. In this regard, our review sheds light on the contributions of various chemicals as electron donors, redox mediators, cofactors, etc., on microbial Cr(VI) reduction for enhanced treatment practices.

Mechanistic insights into adsorption and reduction of hexavalent chromium from water using magnetic biochar composite: Key roles of Fe3O4 and persistent free radicals

Magnetic biochar (MBC) has been used to remove hexavalent chromium (Cr(VI)) from water, but the roles of Fe₃O₄ and persistent free radicals (PFRs) in MBC in Cr(VI) removal are still less investigated. In this work, the MBC synthesized by microwave co-pyrolysis of solid-state FeSO₄ and rice husk was employed to remove Cr(VI) from water. In comparison to the rice husk biochar (BC), the MBC exhibits the 3.2- and 11.7-fold higher adsorption and reduction efficiency of Cr(VI), resulting in the higher Cr(VI) removal efficiency (84.3%) and equilibrium adsorption capacity of MBC (8.35 mg g^-1) than that (26.5% and 2.63 mg g^-1) of BC. Multiple characterization results revealed that the high Cr(VI) removal performance of MBC was mainly attributed to the presence of active Fe₃O₄ and carbon-centered PFRs in the porous and graphitic MBC. The Fe₃O₄ not only provided active chemisorption/reduction sites for Cr(VI) via its Fe(II)_oct and Fe(III)_oct coordination, but also facilitated the generation of more active electron donating carbon-centered PFRs than carbon-centered PFRs with an oxygen atom in the graphitic structure to reduce Cr(VI). The presence of Fe₃O₄ also elevated 36.7 m² g^-1 of BET-surface area and 0.043 cm² g^-1 of pore volume of MBC, promoting the Cr(VI) removal. The Fe₃O₄ and carbon-centered PFRs contributed to ∼81.8% and ∼18.2% of total Cr(III) generation, respectively. In addition, the initial solution pH was responsible for determining the relative significance of Cr(VI) adsorption and reduction. This study provides new insights into the mechanisms of Cr(VI) removal from water by the MBC.

Electrochemical Detection of Deuterium Kinetic Isotope Effect on Extracellular Electron Transport in Shewanella oneidensis MR-1

Direct electrochemical detection of c-type cytochrome complexes embedded in the bacterial outer membrane (outer membrane c-type cytochrome complexes; OM c-Cyts) has recently emerged as a novel whole-cell analytical method to characterize the bacterial electron transport from the respiratory chain to the cell exterior, referred to as the extracellular electron transport (EET). While the pathway and kinetics of the electron flow during the EET reaction have been investigated, a whole-cell electrochemical method to examine the impact of cation transport associated with EET has not yet been established. In the present study, an example of a biochemical technique to examine the deuterium kinetic isotope effect (KIE) on EET through OM c-Cyts using a model microbe, Shewanella oneidensis MR-1, is described. The KIE on the EET process can be obtained if the EET through OM c-Cyts acts as the rate-limiting step in the microbial current production. To that end, before the addition of D2O, the supernatant solution was replaced with fresh media containing a sufficient amount of the electron donor to support the rate of upstream metabolic reactions, and to remove the planktonic cells from a uniform monolayer biofilm on the working electrode. Alternative methods to confirm the rate-limiting step in microbial current production as EET through OM c-Cyts are also described. Our technique of a whole-cell electrochemical assay for investigating proton transport kinetics can be applied to other electroactive microbial strains.

Proton Transport in the Outer-Membrane Flavocytochrome Complex Limits the Rate of Extracellular Electron Transport

The microbial transfer of electrons to extracellularly located solid compounds, termed extracellular electron transport (EET), is critical for microbial electrode catalysis. Although the components of the EET pathway in the outer membrane (OM) have been identified, the role of electron/cation coupling in EET kinetics is poorly understood. We studied the dynamics of proton transport associated with EET in an OM flavocytochrome complex in Shewanella oneidensis MR‐1. Using a whole‐cell electrochemical assay, a significant kinetic isotope effect (KIE) was observed following the addition of deuterated water (D₂O). The removal of a flavin cofactor or key components of the OM flavocytochrome complex significantly increased the KIE in the presence of D₂O to values that were significantly larger than those reported for proton channels and ATP synthase, thus indicating that proton transport by OM flavocytochrome complexes limits the rate of EET.

Dissolved organic matter as a terminal electron acceptor in the microbial oxidation of steroid estrogen

Steroid estrogen in natural waters may be biodegraded by quinone-reducing bacteria, dissolved organic matter (DOM) may serve as a terminal electron acceptor in this process. The influence of temperature, pH, dissolved oxygen and light illumination on the reduction efficiency of anthraquinone-2-disulfonate (AQS) was investigated using 17β-estradiol (E2) as the target species. The optimum reduction conditions were found to be in the dark under anaerobic conditions at pH 8.0 and 30 °C. Quinone-reducing bacteria can use the quinone structure of DOM components as a terminal electron acceptor coupling with microbial growth to promote biodegradation. Compared with other DOM models, AQS best stimulated E2 biodegradation and the mediating effect was improved as the AQS concentration increased from 0 to 0.5 mM. However, further increase had an inhibiting effect. Natural DOM containing lake humic acid (LHA) and lake fulvic acid (LFA) had a very important accelerating effect on the degradation of E2, the action mechanism of which was consistent with that defined using DOM models. The natural DOM contained more aromatic compounds, demonstrating their greater electron-accepting capacity and generally more effective support for microorganism growth and E2 oxidation than Aldrich humic acid (HA). These results provide a more comprehensive understanding of microbial degradation of steroid estrogens in anaerobic environments and confirm DOM as an important terminal electron acceptor in pollutant transformation.

Biological groundwater treatment for chromium removal at low hexavalent chromium concentrations

The objective of this work is to develop and evaluate biological groundwater treatment systems that will achieve hexavalent chromium reduction and total chromium removal from groundwater at hexavalent chromium (Cr(VI)) groundwater concentrations in the 0-200 μg/L range. Three lab-scale units operated, as sequencing batch reactors (SBR) under aerobic, anaerobic and anaerobic-aerobic conditions. All systems received groundwater with a Cr(VI) content of 200 μg/L. In order to support biological growth, groundwater was supplemented with milk, liquid cheese whey or a mixture of sugar and milk to achieve a COD concentration of 200 mg/L. The results demonstrate that a fully anaerobic system or an anaerobic-aerobic system dosed with simple or complex external organic carbon sources can lead to practically complete Cr(VI) reduction to Cr(III). The temperature dependency of maximum Cr(VI) removal rates can be described by the Arrhenius relationship. Total chromium removal in the biological treatment systems was not complete because a significant portion of Cr(III) remained in solution. An integrated system comprising of an anaerobic SBR followed by a sand filter achieved more than 95% total chromium removal thus resulting in average effluent total and dissolved chromium concentrations of 7 μg/L and 3 μg/L, respectively.

Enhanced azo dye removal in a continuously operated up-flow anaerobic filter packed with henna plant biomass

Effects of henna plant biomass (stem) packed in an up-flow anaerobic bio-filter (UAF) on an azo dye (AO7) removal were investigated. AO7 removal, sulfanilic acid (SA) formation, and pseudo first-order kinetic constants for these reactions (kAO7 and kSA) were higher in the henna-added UAF (R2) than in the control UAF without henna (R1). The maximum kAO7 in R1 and R2 were 0.0345 and 0.2024 cm(-1), respectively, on day 18; the corresponding molar ratios of SA formation to AO7 removal were 0.582 and 0.990. Adsorption and endogenous bio-reduction were the main AO7 removal pathways in R1, while in R2 bio-reduction was the dominant. Organics in henna could be released and fermented to volatile fatty acids, acting as effective electron donors for AO7 reduction, which was accelerated by soluble and/or fixed lawsone. Afterwards, the removal process weakened over time, indicating the demand of electron donation and lawsone-releasing during the long-term operation of UAF.

Molasses as an efficient low-cost carbon source for biological Cr(VI) removal

In the present study, indigenous microorganisms from industrial sludge were used to reduce the activity of Cr(VI). Molasses, a by-product of sugar processing, was selected as the carbon source (instead of sugar used in a previous work) as it is a low-cost energy source for bioprocesses. Initially, experiments were carried out in suspended growth batch reactors for Cr(VI) concentrations of 1.5-110 mg/L. The time required for complete Cr(VI) reduction increased with initial Cr(VI) concentration. Initial molasses concentration was also found to influence the Cr(VI) reduction rate. The optimal concentration for all initial Cr(VI) concentrations tested was 0.8 gC/L. Experiments were also carried out in packed-bed reactors. Three different operating modes were used to investigate the optimal performance and efficiency of the filter, i.e. batch, continuous and SBR with recirculation. The latter mode with a recirculation rate of 0.5L/min lead to significantly high Cr(VI) reduction rates (up to 135 g/m(2)d). The results of this work were compared with those of a similar work using sugar as the carbon source and indicate that molasses could prove a feasible technological solution to a serious environmental problem.

Reduction of Cr(VI) into Cr(III) by Spirulina dead biomass in aqueous solution: kinetic studies

Microalga species are potential scavengers of heavy metals. The active functional groups of a number of biomaterials are capable to detoxify Cr(VI) to Cr(III). A 2nd order kinetic model was developed in terms of concentration of protonated acidic groups of Spirulina sp. biomass for the reduction of Cr(VI) into Cr(III). Cr(VI) reduction reaction grounded on the concentration of functional groups was validated over a broad range of pH, temperature and anionic strength. Lower pH favoured Cr(VI) reduction reaction and the experimental results well fitted to the kinetic model. The overall rate constant, kt, decreased logarithmically from 22.7 to 2.8 mM⁻¹ s⁻¹ with rise of pH from 0.5 to 6.0. Whereas, k(t) increased nearly by 23% with elevation temperature from 25 to 45 °C. Higher concentration (>0.235 mM) of background anions (Cl⁻, SO₄²⁻ and NO₃⁻) was resulted in decreases in k(t) values. The rate constant expression developed can be employed to quantify Cr(VI) reduction into Cr(III) using Spirulina biomass.