Emerging trends and future outlook on chromium removal in the lab, pilot scale, and industrial wastewater system: an updated review exploring 10 years of research

Chromium (Cr) is widely recognized as a carcinogenic metal, and numerous technologies have been studied on a lab scale to manage the pollution caused by Cr contamination in wastewater. However, the removal of Cr presents several challenges and limitations in industrial wastewater management. These issues highlight the ongoing need for research to discover more efficient methods for remediating Cr from wastewater. The proposed review summarizes the current limitations, gaps, and state-of-the-art technologies on Cr removal in industrial wastewater systems over the past 10 years. It aims to lay the groundwork for future research and innovation in Cr remediation for industrial applications. The review emphasizes that conventional physicochemical techniques are often insufficient and highlights the necessity of implementing advanced integrated systems. The limitations related to industrial scaling up are also deeply investigated. Special attention is given to differentiating research conducted at laboratory, pilot, and industrial levels. The findings reveal that limited research has been conducted on an industrial scale, with most investigations focusing on treating tannery and electroplating wastewater. A few studies have also been reported on wastewater from textile, mining, steel mills, pigments, and wood processing. Despite the existence of high-performance systems demonstrated in lab-scale studies, only a handful of treatment techniques have effectively removed Cr at an industrial scale. Nevertheless, innovative breakthroughs in advanced integrated systems show promise for improved performance in the future.

Hydrochar as a bio-based adsorbent for heavy metals removal: A review of production processes, adsorption mechanisms, kinetic models, regeneration and reusability

The spread of heavy metals throughout the ecosystem has extremely endangered human health, animals, plants, and natural resources. Hydrochar has emerged as a promising adsorbent for removal of heavy metals from water and wastewater. Hydrochar, obtained from hydrothermal carbonization of biomass, owns unique physical and chemical properties that are highly potent in capturing heavy metals via surface complexation, electrostatic interactions, and ion exchange mechanisms. This review focuses on removing heavy metals by hydrochar adsorbents from water bodies. The article discusses factors affecting the adsorption capacity of hydrochars, such as contact time, pH, initial metal concentration, temperature, and competing ions. Literature on optimization approaches such as surface modification, composite development, and hybrid systems are reviewed to enlighten mechanisms undertaking the efficiency of hydrochars in heavy metals removal from wastewater. The review also addresses challenges such as hydrochar regeneration and reusability, alongside potential issues related to its disposal and metal leaching. Integration with current water purification methods and the significance of ongoing research and initiatives promoting hydrochar-based technologies were also outlined. The article concludes that combining hydrochar with modern technologies such as nanotechnology and advanced oxidation techniques holds promise for improving heavy metal remediation. Overall, this comprehensive analysis provides valuable insights to guide future studies and foster the development of effective, affordable, and environmentally friendly heavy metal removal technologies to ensure the attainment of safer drinking water for communities worldwide.

Chromium in plant-soil nexus: Speciation, uptake, transport and sustainable remediation techniques

Heavy metal (HM) pollution has become a serious global problem due to the non-biodegradable nature of the HMs and their persistence in the environment. Agricultural soil is a non-renewable resource that requires careful management so that it can fulfill the increasing demand for agricultural food production. However, different anthropogenic activities have resulted in a large-scale accumulation of HMs in soil which is detrimental to soil and plant health. Due to their ubiquity, increased bioavailability, toxicity, and non-biodegradable nature, HM contamination has formed a roadblock in the way of achieving food security, safety, and sustainability in the future. Chromium (Cr), specifically Cr(VI) is a highly bioavailable HM with no proven role in the physiology of plants. Chromium has been found to be highly toxic to plants, with its toxicity also influenced by chemical speciation, which is in turn controlled by different factors, such as soil pH, redox potential, organic matter, and microbial population. In this review, the different factors that influence Cr speciation were analyzed and the relationship between biogeochemical transformations of Cr and its bioavailability which may be beneficial for devising different Cr remediation strategies has been discussed. Also, the uptake and transport mechanism of Cr in plants, with particular reference to sulfate and phosphate transporters has been presented. The biological solutions for the remediation of Cr contaminated sites which offer safe and viable alternatives to old-style physical and chemical remediation strategies have been discussed in detail. This review provides theoretical guidance in developing suitable approaches for the better management of these remediation strategies.

Biosorption potential and molecular characterization of metal-resistant autochthonous microbes from tannery solid waste

This study encompasses isolation and screening of heavy metal-resistant fungal and bacterial strains from tannery solid waste (TSW). Twelve fungal strains and 25 bacterial strains were isolated from TSW. The growth of fungal strains was observed against different heavy metals ranging from 10 to 1050 mg L^-1 and the growth of bacteria was observed in metal concentrations ranging from 10 to 1200 mg L^-1. Five multi-metal-resistant fungal isolates belonging to the genus Trichoderma and ten bacterial isolates belonging to the genus Bacillus showed good metal resistance and biosorption potential. They were identified through molecular techniques, fungi based on ITS region ribotyping, and bacteria based on 16S rRNA ribotyping. The fungal strains were characterized as T. hamatum (TSWF-06), T. harzianum (TSWF-11), T. lixii (TSWF-02), and T. pseudokoningii (TSWF-03, TSWF-10). The bacterial strains were characterized as Bacillus xiamenensis (TSW-02), B. velezensis (TSW-05), B. piscis (TSW-06), B. safensis (TSW-10), B. subtilis (TSW-14, TSW-15, TSW-17) B. licheniformis (TSW-19), B. cereus (TSW-20), and B. thuringiensis (TSW-22). The fungal strains, namely, T. pseudokoningii (TSWF-03) and T. harzianum, proved to be two multi-metal-resistant strains with good biosorption efficiency. Unlike fungi, bacterial strains showed metal-specific resistance. The strains Bacillus xiamenensis, B. subtilis (TSW-14), and B. subtilis (TSW-15) showed good biosorption efficiency against Cr, B. safensis against Cu, B. piscis, and B. subtilis (TSW-17) against Pb and B. licheniformis and B. thuringiensis against Zn. The autochthonous fungal and bacterial strains can therefore be employed to clean metal-contaminated environments.

Insight Into Microbes and Plants Ability for Bioremediation of Heavy Metals

Contamination of ground and surface water, soil, and air by harmful and carcinogenic chemicals is one of the most prevalent problems in the modern industrialized world. Heavy metal toxicity has demonstrated to be paramount hazardous and there are various risks associated with it. In addition, these heavy metals have adverse effects on human health and plant physiology. The field of bioremediation has undergone an impactful revolution in recent years due to an exponential increase in various issues related to soil and water pollution. Bioremediation is an advanced and efficient technology, which involves the use of biological means such as microorganisms and plants to degrade heavy metal contaminants. Among the millions of microbes present in the ecosystem, the highest metal adsorption ability is possessed by species belonging to genus Penicillium, Streptomyces, Bacillus, Rhizopus, Chlorella, Ascophyllum, Sargassum, and Aspergillus. Among different plant species, Allium, Eucalyptus, Helianthus, and Hibiscus are the main heavy metal absorbers. The present review concentrates on the research in the bioremediation of important heavy metals through the use of plants and microbes.

Biotreatment potential of co-contaminants hexavalent chromium and polychlorinated biphenyls in industrial wastewater: Individual and simultaneous prospects

Co-existence of polychlorinated biphenyls (PCBs) and hexavalent chromium (Cr(VI)) in the environment due to effluent from industries has aggravated the pollution problem. Both contaminants can alter chemical interactions, processes and impair enzymatic activities in the ecosystem that results in negative impacts on aquatic and terrestrial life. Previously, research has been performed for the fate and transfer of these contaminants individually, but simultaneous removal approaches have not received much attention. Cr(VI) exists in a highly toxic form in the environment once released, whereas location of chlorine atoms in the ring determines PCBs toxicity. Lower chlorinated compounds are easily degradable whereas as high chlorinated compounds require sequential strategy for transformation. Microorganisms can develop different mechanism to detoxify both pollutants. However, occurrence of multiple contaminants in single system can alter the bioremediation efficiency of bacteria. Use of metal resistance bacterial for the degradation of organic compounds has been widely used bioaugmentation strategy. Along with that use of sorbents/bio sorbents, biosurfactants and phytoremediation approaches have already been well reported. Bioremediation strategy with dual potential to detoxify the Cr(VI) and PCBs would be a probable option for simultaneous biotreatment. Application of bioreactors and biofilms covered organic particles can be utilized as efficient bioaugmentation approach. In this review, biotreatment systems and bacterial oxidative and reductive enzymes/processes are explained and possible biotransformation pathway has been purposed for bioremediation of co-contaminated waters.

Identification of metal binding motifs in protein frameworks to develop novel remediation strategies for Hg2+ and Cr(VI)

Amino acid sequences in metal-binding proteins with chelating properties offer exciting applications in biotechnology and medical research. To enhance their application in bioremediation studies, we explicitly aimed to identify specific metal-binding chelating motifs in protein structures for two significant pollutants, such as mercury (Hg²⁺) and chromium Cr(V1). For this purpose, we have performed an extensive coordination chemistry approach by retrieving Hg²⁺ and Cr(V1) binding protein structures from the protein database and validated using the B-factor, a term defining uncertainty of the atoms and with occupancy to obtain the best binding motifs. Our analysis revealed that acidic amino acids like aspartic acid, glutamic acid, and basic amino acids such as cysteine and histidine are predominant in coordinating with these metals. The order of preference in Hg²⁺-bound structures is predicted to be Cys > His > Asp > Glu, and for Cr(V1) is His > Asp > Glu. Examination of the atomic coordinates and their distance from each metal revealed that the sulfur atoms of cysteine showing more preference towards Hg²⁺coordination with an atomic distance ranging from 1.5 to 2.9 Å. Likewise, oxygen atoms of aspartic acid, glutamic acid and nitrogen atoms of histidine are within 2 Å of Cr(V1) coordination. Based on these observations, we obtained C-C-C, C-X(2)-C-C-(X)2-C, H-C-H motifs for Hg²⁺, and D-X(1)-D, H-X(3)-E motif for Cr(V1) to be shared within the coordination space of 3 Å. As a future scope, we propose that the identified metal-binding chelating motifs are oligopeptides and can display on the surface of microorganisms such as Escherichia coli and Saccharomyces cerevisiae for effective removal of natural Hg²⁺ and Cr(V1) through biosorption. Hence, our results will provide the basis for futuristic bioremediation.

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.

Biosorption of heavy metals by dry biomass of metal tolerant bacterial biosorbents: an efficient metal clean-up strategy

Heavy metals discharge at an unrestrained rate from various industries into the environment pose serious human health problems. Considering this, the present study aimed at exploring the metal biosorbing potentials of bacterial strains recovered from polluted soils. The bacterial strains (CPSB1, BM2 and CAZ3) belonging to genera Pseudomonas, Bacillus and Azotobacter expressing multi-metal tolerance ability were identified to species level as P. aeruginosa, B. subtilis and A. chroococcum, respectively, by 16S rRNA partial gene sequence analysis. The biosorption of cadmium, chromium, copper, nickel, lead and zinc by three dead bacterial genera were studied as a function of metal concentration, variable pH of the medium and reaction (contact) time. The three bacterial strains exhibited a tremendous metal removal ability which continued even at the highest tested concentration of some metals. Later, a decline in the percentage of biosorbed metals was recorded as the metal concentration was increased with the simultaneous generation of a driving force to overcome mass transfer resistance for movement of metal ions between the solution and the surface of adsorbent. Among test bacteria, B. subtilis biosorbed a maximum of 96% chromium at 25 μg mL^-1 while the maximum percentage (91%) of biosorbed metals recorded at 400 μg Cd mL^-1 was observed for P. aeruginosa. The sorption of metal ions by dead biomass of three bacterial genera at optimum conditions followed the order-(i) B. subtilis BM2: Pb > Cu > Ni > Cd > Cr, (ii) A. chroococcum CAZ3: Cr > Cd > Cu > Ni > Pb and (iii) P. aeruginosa CPSB1: Cd > Cr > Ni > Cu > Pb > Zn. It was found that the optimum pH for metal adsorption ranged between pH 8 and 9 which, however, declined substantially at pH 5.0 for all three bacterial strains. In general, the biosorption of Cd, Cr, Cu, Ni and Pb by B. subtilis and A. chroococcum and such metals along with Zn by P. aeruginosa occurred maximally up to 60 min of bacterial growth. The adsorption data with regard to five metals provide an outstanding fit to the Langmuir and Freundlich isotherms. The biosorptive ability of three bacterial genera correlated strongly (r² > 0.9) with each metal. The bacteria belonging to two Gram-negative genera Pseudomonas (P. aeruginosa) and Azotobacter (A. chroococcum) and one Gram-positive genus Bacillus (B. subtilis) demonstrated exceptional metal removal efficiency and, hence, provides a comprehensive understanding of metal-bacteria sorption process which in effect paves the way for detoxifying/removing metals from contaminated environment.

Biosorption of Cr(VI) by Halomonas sp. DK4, a halotolerant bacterium isolated from chrome electroplating sludge

This study evaluated Cr(VI) biosorption by a halotolerant gram-negative bacterium Halomonas sp. DK4 isolated from chrome electroplating sludge. The bacterium could withstand high concentrations of Cr(VI) exhibiting a minimal inhibitory concentration (MIC) of 250 mg/L. Plackett-Burman design confirmed glucose, KH₂PO₄, NaCl, inoculum size, and initial Cr(VI) concentration as significant variables influencing the Cr(VI) removal ability of the bacterium. The suspended culture of Halomonas sp. DK4 was able to remove 81% (100 mg/L) of Cr(VI) in optimized MSM medium from aqueous solutions within 48 h. The bacterium also removed 59% Cr(VI) in the presence of 15% NaCl concentration within 72 h. The main mechanism involved in Cr(VI) removal by Halomonas sp. DK4 was determined to be biosorption which was best explained using the Langmuir isotherm model, wherein the maximum adsorption of 150.7 mg/g was observed under equilibrium conditions. Kinetic studies reveal that chemisorption of Cr(VI) by Halomonas sp. DK4 was a rate-limiting process which followed pseudo-second-order kinetics (R² = 0.99). Bacterial biomass exhibited maximum adsorption of 70.3% Cr(VI) at an initial concentration of 100 mg/L under optimal conditions. Fourier transform infrared spectroscopy (FTIR) analysis confirmed the presence of hydroxyl, carboxyl, amide, and phosphate groups on the bacterial surface which may be involved in Cr(VI) adsorption. Scanning electron microscopy coupled energy dispersive X-ray (SEM-EDX) analysis revealed morphological changes in the bacterial cell and accumulation of Cr(VI) on the cell surface. These results suggest the potential application of Halomonas sp. DK4 in the removal of Cr(VI) from saline chromium-containing industrial wastewaters.

Application of enhanced bioreduction for hexavalent chromium-polluted groundwater cleanup: Microcosm and microbial diversity studies

Hexavalent chromium (Cr⁶⁺) is a commonly found heavy metal at polluted groundwater sites. In this study, the effectiveness of Cr⁶⁺ bioreduction by the chromium-reducing bacteria was evaluated to remediate Cr⁶⁺-contaminated groundwater. Microcosms were constructed using indigenous microbial consortia from a Cr⁶⁺-contaminated aquifer as the inocula, and slow-releasing emulsified polycolloid-substrate (ES), cane molasses (CM), and nutrient broth (NB) as the primary substrates. The genes responsible for the bioreduction of Cr⁶⁺ and variations in bacterial diversity were evaluated using metagenomics assay. Complete Cr⁶⁺ reduction via the biological mechanism was observed within 80 days using CM as the carbon source under anaerobic processes with the increased trivalent chromium (Cr³⁺) concentrations. Cr⁶⁺ removal efficiencies were 83% and 59% in microcosms using ES and NB as the substrates, respectively. Increased bacterial communities associated with Cr⁶⁺ bioreduction was observed in microcosms treated with CM and ES. Decreased bacterial communities were observed in NB microcosms. Compared to ES, CM was more applicable by indigenous Cr⁶⁺ reduction bacteria and resulted in effective Cr⁶⁺ bioreduction, which was possibly due to the growth of Cr⁶⁺-reduction related bacteria including Sporolactobacillus, Clostridium, and Ensifer. While NB was applied for specific bacterial selection, it might not be appropriate for electron donor application. These results revealed that substrate addition had significant impact on microbial diversities, which affected Cr⁶⁺ bioreduction processes. Results are useful for designing a green and sustainable bioreduction system for Cr⁶⁺-polluted groundwater remediation.

Biotransformation of chromium by root nodule bacteria Sinorhizobium sp. SAR1

The present study aims to address the problem of chromium (Cr) toxicity by providing important insights into the mechanisms involved in its bioremediation. Among the 22 Rhizobium and Sinorhizobium isolates obtained from Sesbania sesban root nodules, Sinorhizobium sp. SAR1 (JX174035.1) tolerated the maximum Cr concentration (1mM) and hence was used for further studies. The excess secretion of extra polymeric substances, as seen from scanning electron micrographs, could be a probable mechanism of adaptation to the Cr stress. The Energy dispersive X-ray spectroscopy data did not show any peaks of Cr. The biosorption studies done on the isolate gave maximum adsorption capacity as 285.71mg/g. The isotherm studies showed a better fit to Langmuir isotherm. The Weber and Morris plot established that the phenomenon of adsorption was governed by film diffusion mechanism. The FTIR analysis suggested the role of cell wall components and extracellular polymeric substances in Cr adsorption to the biomass of Sinorhizobium. On the basis of these results a compiled mechanism of Cr (VI) adsorption and its biotransformation into Cr (III) by Sinorhizobium sp. SAR1 is explained. This work outlines a comprehensive detail for the exact phenomenon of Cr biotransformation by Sinorhizobium sp. SAR1. These results may further help in developing and enhancing effective bioremediation approaches.

Chromate Resistance Mechanisms in Leucobacter chromiiresistens

Chromate is one of the major anthropogenic contaminants on Earth. Leucobacter chromiiresistens is a highly chromate-resistant strain, tolerating chromate concentrations in LB medium of up to 400 mM. In response to chromate stress, L. chromiiresistens forms biofilms, which are held together via extracellular DNA. Inhibition of biofilm formation leads to drastically decreased chromate tolerance. Moreover, chromate is reduced intracellularly to the less-toxic Cr(III). The oxidation status and localization of chromium in cell aggregates were analyzed by energy-dispersive X-ray spectroscopy coupled to scanning transmission electron microscopy and X-ray absorption spectroscopy measurements. Most of the heavy metal is localized as Cr(III) at the cytoplasmic membrane. As a new cellular response to chromate stress, we observed an increased production of the carotenoid lutein. Carotenoid production could increase membrane stability and reduce the concentration of reactive oxygen species. Bioinformatic analysis of the L. chromiiresistens genome revealed several gene clusters that could enable heavy-metal resistance. The extreme chromate tolerance and the unique set of resistance factors suggest the use of L. chromiiresistens as a new model organism to study microbial chromate resistance.IMPORTANCE Chromate is a highly toxic oxyanion. Extensive industrial use and inadequate waste management has caused the toxic pollution of several field sites. Understanding the chromate resistance mechanisms that enable organisms to thrive under these conditions is fundamental to develop (micro)biological strategies and applications aiming at bioremediation of contaminated soils or waters. Potential detoxifying microorganisms are often not sufficient in their resistance characteristics to effectively perform, e.g., chromate reduction or biosorption. In this study, we describe the manifold strategies of L. chromiiresistens to establish an extremely high level of chromate resistance. The multitude of mechanisms conferring it make this organism suitable for consideration as a new model organism to study chromate resistance.

Biosorption and biotransformation of hexavalent chromium [Cr(VI)]: A comprehensive review

Chromium (VI) is one of the most common environmental contaminant due to its tremendous industrial applications. It is non-biodegradable as it is a heavy metal, and hence, of major concern. Therefore, it is pertinent that the remediation method should be such that brings chromium within permissible limits before the effluent is discharged. Several different strategies are adopted by microorganisms for Cr (VI) removal mostly involving biosorption and biotransformation or both. These mechanisms are based on the surface nature of the biosorbent and the availability of reductants. This review article focuses on chromium pollution problem, its chemistry, sources, effects, remediation strategies by biological agents and detailed chromium detoxification mechanism in microbial cell. A summary of applied in situ and ex situ chromium bioremediation technologies is also listed. This can be helpful for developing technologies to be more efficient for Cr (VI) removal thereby bridging the gap between laboratory findings and industrial application for chromium remediation.

Insight into Cr6+ reduction efficiency of Rhodococcus erythropolis isolated from coalmine waste water

A microbial treatment of Cr⁶⁺ contaminated wastewater with a chromium reducing bacteria isolated from coal mine area was investigated. In a series of batch study metal removal was executed under different parametric conditions which include pH (2-7), temperature (20-50 °C), initial Cr⁶⁺ concentration (1-100 mg/L), substrate utilization and its overall effect on biomass generation. Impact of oxygen availability was checked at different agitation speed and its role on the remedial process. Liquid phase reduction of Cr⁶⁺ was measured in terms of substrate reduction and total biomass yield. The bacterium species isolated was able to tolerate Cr⁶⁺ over a wide range from 1 to 100 mg/L before it reached minimum inhibition concentration. Apart from Cr⁶⁺, the bacterial isolate showed tolerance towards Fe, As, Cu, Ag, Zn, Mn, Mg and Pb. Removal mechanism adopted by the bacterium recommended that it employed accumulation of Cr⁶⁺ as Cr³⁺ both within and outside the cell. Classical Monod equation was used to determine the biokinetics of the bacterial isolate along with the interference of metal ion concentration and substrate utilization. Cr⁶⁺ removal was found prominent even in bimetallic solutions. The bacterial isolate was confirmed to be Rhodococcus erythopolis by 16s rRNA molecular characterization. Thus the bacterial isolate obtained from the coal mine area proved to be a potential agent for microbial remediation of Cr⁶⁺ laden waste water.