Bioremediation of Heavy Metals from Soil and Aquatic Environment: An Overview of Principles and Criteria of Fundamental Processes

Soil-sediment connectivity through Bayesian source tracking in an urban naturalised waterway via microbial and isotopic markers

Riverine sediments are important habitats for microbial activity in naturalised waterways to provide potential ecosystem services that improve stormwater quality. Yet, little is known about the sources of these sediment microbes, and the factors shaping them. This study investigated the dominant source of sediments in a tropical naturalised urban waterway, using two Bayesian methods for microbial and isotopic 13C/15N markers concurrently. Additionally, key factors shaping microbial communities from the surrounding landscape were evaluated. A comprehensive two-year field survey identified source land covers of interest based on topology and soil context. Among these land covers, riverbanks were the dominant source of sediments contribution for both edaphic and microbial components. The physico-chemical environment explains most of the variation in sediment communities compared to inter-location distances and microbial source contribution. As microbes provide ecosystem services important for rewilding waterways, management strategies that establish diverse sediment microbial communities are encouraged. Since riverbanks play a disproportionately important role in material contribution to sediment beds, management practices aimed at controlling soil erosion from riverbanks can improve overall functioning of waterway systems.

A review of amendments for simultaneously reducing Cd and As availability in paddy soils and rice grain based on meta-analysis

Arsenic (As) and cadmium (Cd) accumulation in rice grains is a global food safety issue, and various methods and materials have been used to remove or reduce As and Cd in agricultural soils and rice grains. Despite the availability of synthesized materials capable of simultaneous As and Cd reduction from soil and rice grains, the contributions, efficiency, and main ingredients of the materials for As and Cd immobilization remain unclear. The present study first summarized the biogeochemistry of As and Cd in paddy soils and their transfer in the soil-food-human continuum. We also reviewed a series of reported inorganic and organic materials for simultaneous immobilization of As and Cd in paddy soils, and their reduction efficiency of As and Cd bioavailability were listed and compared. Based on the abovementioned materials, the study conducted a meta-analysis of 38 articles with 2565 observations to quantify the impacts of materials on simultaneous As and Cd reduction from soil and rice grains. Meta-analysis results showed that combining organic and inorganic amendments corresponded to effect sizes of -62.3% and -67.8% on As and Cd accumulation in rice grains, while the effect sizes on As and Cd reduction in paddy soils were -44.2% and -46.2%, respectively. Application of Fe based materials significantly (P < 0.05) reduced As (-54.2%) and Cd (-74.9%), accounting for the highest immobilization efficiency of As and Cd in rice grain among all the reviewed materials, outweighing S, Mn, P, Si, and Ca based materials. Moreover, precipitation, surface complexation, ion exchange, and electrostatic attraction mechanisms were involved in the co-immobilization tactics. The present study underlines the application of combined organic and inorganic amendments in simultaneous As and Cd immobilization. It also highlighted that employing Fe-incorporated biochar material may be a potential strategy for co-mitigating As and Cd pollution in paddy soils and accumulation in rice grains.

Microbial mediated remediation of heavy metals toxicity: mechanisms and future prospects

Heavy metal pollution has become a serious concern across the globe due to their persistent nature, higher toxicity, and recalcitrance. These toxic metals threaten the stability of the environment and the health of all living beings. Heavy metals also enter the human food chain by eating contaminated foods and cause toxic effects on human health. Thus, remediation of HMs polluted soils is mandatory and it needs to be addressed at higher priority. The use of microbes is considered as a promising approach to combat the adverse impacts of HMs. Microbes aided in the restoration of deteriorated environments to their natural condition, with long-term environmental effects. Microbial remediation prevents the leaching and mobilization of HMs and they also make the extraction of HMs simple. Therefore, in this context recent technological advancement allowed to use of bioremediation as an imperative approach to remediate polluted soils. Microbes use different mechanisms including bio-sorption, bioaccumulation, bioleaching, bio-transformation, bio-volatilization and bio-mineralization to mitigate toxic the effects of HMs. Thus, keeping in the view toxic HMs here in this review explores the role of bacteria, fungi and algae in bioremediation of polluted soils. This review also discusses the various approaches that can be used to improve the efficiency of microbes to remediate HMs polluted soils. It also highlights different research gaps that must be solved in future study programs to improve bioremediation efficency.

Hyperconfined bio-inspired Polymers in Integrative Flow-Through Systems for Highly Selective Removal of Heavy Metal Ions

Access to clean water, hygiene, and sanitation is becoming an increasingly pressing global demand, particularly owing to rapid population growth and urbanization. Phytoremediation utilizes a highly conserved phytochelatin in plants, which captures hazardous heavy metal ions from aquatic environments and sequesters them in vacuoles. Herein, we report the design of phytochelatin-inspired copolymers containing carboxylate and thiolate moieties. Titration calorimetry results indicate that the coexistence of both moieties is essential for the excellent Cd2+ ion-capturing capacity of the copolymers. The obtained dissociation constant, KD ~ 1 nM for Cd2+ ion, is four-to-five orders of magnitude higher than that for peptides mimicking the sequence of endogenous phytochelatin. Furthermore, infrared and nuclear magnetic resonance spectroscopy results unravel the mechanism underlying complex formation at the molecular level. The grafting of 0.1 g bio-inspired copolymers onto silica microparticles and cellulose membranes helps concentrate the copolymer-coated microparticles in ≈3 mL volume to remove Cd2+ ions from 0.3 L of water within 1 h to the drinking water level (<0.03 µM). The obtained results suggest that hyperconfinement of bio-inspired polymers in flow-through systems can be applied for the highly selective removal of harmful contaminants from the environmental water.

A synthetic bacterial consortium improved the phytoremediation efficiency of ryegrass on polymetallic contaminated soil

Polymetallic contamination of soils caused by mining activities seriously threatens soil fertility, biodiversity and human health. Bioremediation is thought to be of low cost and has minimal environmental risk but its effectiveness needs to be improved. This study aimed to identify the combined effect of plant growth and microbial strains with different functions on the enhancement of bioremediation of polymetallic contaminated soil. The microbiological mechanism of bioremediation was explored by amplicon sequencing and gene prediction. Soil was collected from polymetallic mine wastelands and a non-contaminated site for use in a pot experiment. Remediation efficiency of this method was evaluated by planting ryegrass and applying a mixed bacterial consortium comprising P-solubilizing, N-fixing and SO4-reducing bacteria. The plant-microbe joint remediation method significantly enhanced the above-ground biomass of ryegrass and soil nutrient contents, and at the same time reduced the content of heavy metals in the plant shoots and soil. The application of the composite bacterial inoculum significantly affected the structure of soil bacterial communities and increased the bacterial diversity and complexity, and the stability of co-occurrence networks. The relative abundance of the multifunctional genera to which the strains belonged showed a significant positive correlation with the soil nutrient content. Genera related to carbon (C), nitrogen (N), phosphorus (P), and sulphur (S) cycling and heavy metal resistance showed an up-regulation trend in heavy metal-contaminated soils after the application of the mixed bacterial consortium. Also, bacterial strains with specific functions in the mixed consortium regulated the expression of genes involved in soil nutrient cycling, and thus assisted in making the soil self-sustainable after remediation. These results suggested that the remediation of heavy metal-contaminated soil needs to give priority to the use of multifunctional bacterial agents.

Long-term and combined heavy-metal contamination forms a unique microbiome and resistome: A case study in a Yellow River tributary sediments

Microorganisms have developed mechanisms to adapt to environmental stress, but how microbial communities adapt to long-term and combined heavy-metal contamination under natural environmental conditions remains unclear. Specifically, this study analyzed the characteristics of heavy metal composition, microbial community, and heavy metal resistance genes (MRGs) in sediments along Mang River, a tributary of the Yellow River, which has been heavily polluted by industrial production for more than 40 years. The results showed that the concentrations of Cr, Zn, Pb, Cu and As in most sediments were higher than the ambient background values. Bringing the heavy metals speciation and concentration into the risk evaluation method, two-thirds of the sediment samples were at or above the moderate risk level, and the ecological risk of combined heavy metals in the sediments decreased along the river stream. The high ecological risk of heavy metals affected the microbial community structure, metabolic pathways and MRG distribution. The formation of a HM-resistant microbiome possibly occurred through the spread of insertion sequences (ISs) carrying multiple MRGs, the types of ISs carrying MRGs outnumber those of plasmids, and the quantity of MRGs on ISs is also higher than that on plasmids. These findings could improve our understanding of the adaptation mechanism of microbial communities to long-term combined heavy metal contamination.

Arsenic and cadmium availability and its removal in paddy farming areas

Arsenic (As) and cadmium (Cd) accumulation in rice grain is a global concern threatening food security and safety to the growing population. As and Cd are toxic non-essential elements poisonous to animal and human at higher levels. Its accumulation in agro-ecosystems pose a public health risk to consumers of agro-ecosystem products. Due to their hazards, As and Cd sources should be cleared, avoiding entering plants and the human body. As and Cd removal in soils and grains in agro-ecosystems has been conducted by various materials (natural and synthesized), however, there are little documentation on their contribution on As and Cd removal or reduction in rice grains. This identified knowledge gap necessitate a systematically review to understand efficiency and mechanisms of As and Cd availability reduction and removal in paddy farming areas through utilization of various synthetic and modified materials. To achieve this, published peer reviewed articles between 2010 and 2024 were collected from various database i.e., Science Direct, Web of Science, Google Scholar, and Research Gate and analyzed its content in respect to As and Cd reduction and removal. Furthermore, collected data were re-analyzed to determine standardized mean differences (SMD) with 95% confidence intervals (CI). Based on 96 studies with 228 observations involving Fe, Ca, Si, and Se-based materials were identified, it was found that application of Fe, Ca, Si, and Se-based materials potentially reduced As and Cd in rice grains among various study sites and across studies. Among the studied materials, Fe-based materials observed to be more efficient compared to other utilized materials. However, there little or no information on performance of materials when used in combination and how they can improve crop productivity and soil health, thus requiring further studies. Thus, this study confirm Fe, Ca, Si, and Se modified materials have significant potential to reduce As and Cd availability in paddy farming areas and rice grains, thus necessary effort must be made to ensure materials access and availability for farmers utilization in paddy fields to reduce As and Cd accumulation.

Iron Plaque: A Shield against Soil Contamination and Key to Sustainable Agriculture

Soils play a dominant role in supporting the survival and growth of crops and they are also extremely important for human health and food safety. At present, the contamination of soil by heavy metals remains a globally concerning environmental issue that needs to be resolved. In the environment, iron plaque, naturally occurring on the root surface of wetland plants, is found to be equipped with an excellent ability at blocking the migration of heavy metals from soils to plants, which can be further developed as an environmentally friendly strategy for soil remediation to ensure food security. Because of its large surface-to-volume porous structure, iron plaque exhibits high binding affinity to heavy metals. Moreover, iron plaque can be seen as a reservoir to store nutrients to support the growth of plants. In this review, the formation process of iron plaque, the ecological role that iron plaque plays in the environment and the interaction between iron plaque, plants and microbes, are summarized.

Assessment of microbial communities from cold mine environments and subsequent enrichment, isolation and characterization of putative antimony- or copper-metabolizing microorganisms

Mining activities, even in arctic regions, create waste materials releasing metals and metalloids, which have an impact on the microorganisms inhabiting their surroundings. Some species can persist in these areas through tolerance to meta(loid)s via, e.g., metabolic transformations. Due to the interaction between microorganisms and meta(loid)s, interest in the investigation of microbial communities and their possible applications (like bioremediation or biomining) has increased. The main goal of the present study was to identify, isolate, and characterize microorganisms, from subarctic mine sites, tolerant to the metalloid antimony (Sb) and the metal copper (Cu). During both summer and winter, samples were collected from Finnish mine sites (site A and B, tailings, and site C, a water-treatment peatland) and environmental parameters were assessed. Microorganisms tolerant to Sb and Cu were successfully enriched under low temperatures (4°C), creating conditions that promoted the growth of aerobic and fermenting metal(loid) tolerating or anaerobic metal(loid) respiring organism. Microbial communities from the environment and Sb/Cu-enriched microorganisms were studied via 16S rRNA amplicon sequencing. Site C had the highest number of taxa and for all sites, an expected loss of biodiversity occurred when enriching the samples, with genera like Prauserella, Pseudomonas or Clostridium increasing their relative abundances and others like Corynebacterium or Kocuria reducing in relative abundance. From enrichments, 65 putative Sb- and Cu-metabolizing microorganisms were isolated, showing growth at 0.1 mM to 10 mM concentrations and 0°C to 40°C temperatures. 16S rRNA gene sequencing of the isolates indicated that most of the putative anaerobically Sb-respiring tolerators were related to the genus Clostridium. This study represents the first isolation, to our knowledge, of putative Sb-metabolizing cold-tolerant microorganisms and contributes to the understanding of metal (loid)-tolerant microbial communities in Arctic mine sites.

Integrated application of multiple indicators and geographic information system-based approaches for comprehensive assessment of environmental impacts of toxic metals-contaminated agricultural soils and vegetables

Conventional monitoring and mapping approaches are laborious, expensive, and time-consuming because they need a large number of data and consequently extensive sampling and experimental operations. Therefore, due to the growing concern about the potential of contamination of soils and agricultural products with heavy metals (HMs), a field experiment was conducted on 77 farm lands in an area of 2300 ha in the southeast of Shiraz (Iran) to investigate the source of metal contamination in the soils and vegetables and to model spatial distribution of HMs (iron, Fe; manganese, Mn; copper, Cu; zinc, Zn; cadmium, Cd; nickel, Ni, and lead, Pb) over the region using geographic information system (GIS) and geostatistical (Ordinary Kriging, OK) approaches and compare the results with deterministic approaches (Inverse Distance Weighting, IDW with different weighting power). Furthermore, some ecological and health risks indices including Pollution index (PI), Nemerow integrated pollution index (NIPI), pollution load index (PLI), degree of contamination (Cdeg), modified contamination degree (mCd), PIaverage and PIvector for soil quality, multi-element contamination (MEC), the probability of toxicity (MERMQ), the potential ecological index (RI), total hazard index (THI) and total carcinogenic risk index (TCR) based on ingestion, inhalation, and dermal exposure pathways for adults and children respectively for analyzing the noncarcinogenic and carcinogenic risks were calculated. Experimental semivariogram of the mentioned HMs were calculated and theoretical models (i.e., exponential, spherical, Gaussian, and linear models) were fitted in order to model their spatial structures and to investigate the most representative models. Moreover, principal component analysis (PCA) and cluster analysis (CA) were used to identify sources of HMs in the soils. Results showed that IDW method was more efficient than the OK approach to estimate the properties and HMs contents in the soils and plants. The estimated daily intake of metals (DIM) values of Pb and Ni exceeded their safe limits. In addition, Cd was the main element responsible for ecological risk. The PIave and PIvector indices showed that soil quality in the study area is not suitable. According to mCd values, the soils classified as ultra-high contaminated for Cu and Cd, extremely high for Zn and Pb, very high, high, and very low degree of contamination for Ni, Mn, and Fe, respectively. 36, 60, and 4 % of the sampling sites had high, medium, and low risk levels with 49, 21, and 9 % probability of toxicity, respectively. The maximum health risk index (HRI) value of 20.42 with extremely high risk for children was obtained for Ni and the HI for adults and children were 0.22 and 1.55, respectively. The THI values of Pb and Cd were the highest compared to the other HMs studied, revealing a possible non-cancer risk in children associated with exposure to these metals. The routes of exposure with the greatest influence on the THI and TCR indices were in the order of ingestion > inhalation > dermal. Therefore, ingestion, as the main route of exposure, is the route of greatest contribution to health risks. PCA analysis revealed that Fe, Mn, Cu, and Ni may originate from natural sources, while Fe was appeared to be controlled by fertilizer, and Cu primarily coming from pesticide, while Cd and Pb were mainly associated with the anthropogenic contamination, atmospheric depositions, and terrific in the urban soils. While, Zn mainly originated from fertilization. Findings are vital for developing remediation approaches for controlling the contaminants distribution as well as for monitoring and mapping the quality and health of soil resources.

In vitro and in vivo screening of bacterial species from contaminated soil for heavy metal biotransformation activity

Heavy metals (HMs) are widely used in various industries. High concentrations of HMs can be severely toxic to plants, animals and humans. Microorganism-based bioremediation has shown significant potential in degrading and detoxifying specific HM contaminants. In this study, we cultivated a range of bacterial strains in liquid and solid nutrient medium containing different concentrations of different HMs to select and analyze bacteria capable of transforming HMs. The bacterial strains most resistant to selected HMs and exhibiting the ability to remove HMs from contaminated soils were identified. Then, the bacterial species capable of utilizing HMs in soil model experiments were selected, and their ability to transform HMs was evaluated. This study has also generated preliminary findings on the use of plants for further removal of HMs from soil after microbial bioremediation. Alcaligenes faecalis, Delftia tsuruhatensis and Stenotrophomonas sp. were selected for their ability to grow in and utilize HM ions at the maximum permissible concentration (MPC) and two times the MPC. Lysinibacillus fusiformis (local microflora) can be used as a universal biotransformation tool for many HM ions. Brevibacillus parabrevis has potential for the removal of lead ions, and Brevibacillus reuszeri and Bacillus safensis have potential for the removal of arsenic ions from the environment. The bacterial species have been selected for bioremediation to remove heavy metal ions from the environment.

Sources, effects and present perspectives of heavy metals contamination: Soil, plants and human food chain

Heavy metal (HM) poisoning of agricultural soils poses a serious risk to plant life, human health, and global food supply. When HM levels in agricultural soils get to dangerous levels, it harms crop health and yield. Chromium (Cr), arsenic (As), nickel (Ni), cadmium (Cd), lead (Pb), mercury (Hg), zinc (Zn), and copper (Cu) are the main heavy metals. The environment contains these metals in varying degrees, such as in soil, food, water, and even the air. These substances damage plants and alter soil characteristics, which lowers crop yield. Crop types, growing circumstances, elemental toxicity, developmental stage, soil physical and chemical properties, and the presence and bioavailability of heavy metals (HMs) in the soil solution are some of the factors affecting the amount of HM toxicity in crops. By interfering with the normal structure and function of cellular components, HMs can impede various metabolic and developmental processes. Humans are exposed to numerous serious diseases by consuming these affected plant products. Exposure to certain metals can harm the kidneys, brain, intestines, lungs, liver, and other organs of the human body. This review assesses (1) contamination of heavy metals in soils through different sources, like anthropogenic and natural; (2) the effect on microorganisms and the chemical and physical properties of soil; (3) the effect on plants as well as crop production; and (4) entering the food chain and associated hazards to human health. Lastly, we identified certain research gaps and suggested further study. If people want to feel safe in their surroundings, there needs to be stringent regulation of the release of heavy metals into the environment.

Risk Assessment and Sources Apportionment of Toxic Metals in Two Commonly Consumed Fishes from a Subtropical Estuarine Wetland System

The widespread occurrence of heavy metals in aquatic environments, resulting in their bioaccumulation within aquatic organisms like fish, presents potential hazards to human health. This study investigates the concentrations of five toxic heavy metals (Pb, Hg, Zn, Cu, and Cr) and their potential health implications in two economically important fish species (Otolithoides pama and Labeo bata) from a subtropical estuarine wetland system (Feni estuary, Bangladesh). Muscle and gill samples from 36 individual fish were analyzed using energy dispersive X-ray fluorescence (EDXRF). The results revealed that the average quantities of heavy metals in both fishes' muscle followed the declining order of Zn (109.41-119.93 mg/kg) > Cu (45.52-65.43 mg/kg) > Hg (1.25-1.39 mg/kg) > Pb (0.68-1.12 mg/kg) > Cr (0.31-5.82 mg/kg). Furthermore, Zn was found to be present in the highest concentration within the gills of both species. While the levels of Cu, Zn, and Cr in the fish muscle were deemed acceptable for human consumption, the concentrations of Pb and Hg exceeded the permissible limits (>0.5 mg/kg) for human consumption. Different risk indices, including estimated daily intake (EDI), target hazard quotient (THQ), hazard index (HI), and carcinogenic or target risk (TR), revealed mixed and varying degrees of potential threat to human health. According to the EDI values, individuals consuming these fish may face health risks as the levels of Zn, Cu, and Cr in the muscle are either very close to or exceed the maximum tolerable daily intake (MTDI) threshold. Nevertheless, the THQ and HI values suggested that both species remained suitable for human consumption, as indicated by THQ (<1) and HI (<1) values. Carcinogenic risk values for Pb, Cr, and Zn all remained within permissible limits, with TR values falling below the range of (10-6 to 10-4), except for Zn, which exceeded it (>10-4). The correlation matrix and multivariate principal component analysis (PCA) findings revealed that Pb and Cr primarily stemmed from natural geological backgrounds, whereas Zn, Cu, and Hg were attributed to human-induced sources such as agricultural chemicals, silver nanoparticles, antimicrobial substances, and metallic plating. Given the significance of fish as a crucial and nutritious element of a balanced diet, it is essential to maintain consistent monitoring and regulation of the levels and origins of heavy metals found within it.

Microbial strategies for copper pollution remediation: Mechanistic insights and recent advances

Environmental contamination is aninsistent concern affecting human health and the ecosystem. Wastewater, containing heavy metals from industrial activities, significantly contributes to escalating water pollution. These metals can bioaccumulate in food chains, posing health risks even at low concentrations. Copper (Cu), an essential micronutrient, becomes toxic at high levels. Activities like mining and fungicide use have led to Copper contamination in soil, water, and sediment beyond safe levels. Copper widely used in industries, demands restraint of heavy metal ion release into wastewater for ecosystem ultrafiltration, membrane filtration, nanofiltration, and reverse osmosis, combat heavy metal pollution, with emphasis on copper.Physical and chemical approaches are efficient, large-scale feasibility may have drawbackssuch as they are costly, result in the production of sludge. In contrast, bioremediation, microbial intervention offers eco-friendly solutions for copper-contaminated soil. Bacteria and fungi facilitate these bioremediation avenues as cost-effective alternatives. This review article emphasizes on physical, chemical, and biological methods for removal of copper from the wastewater as well asdetailing microorganism's mechanisms to mobilize or immobilize copper in wastewater and soil.

Role of Plant-Growth-Promoting Rhizobacteria in Plant Machinery for Soil Heavy Metal Detoxification

Heavy metals migrate easily and are difficult to degrade in the soil environment, which causes serious harm to the ecological environment and human health. Thus, soil heavy metal pollution has become one of the main environmental issues of global concern. Plant-growth-promoting rhizobacteria (PGPR) is a kind of microorganism that grows around the rhizosphere and can promote plant growth and increase crop yield. PGPR can change the bioavailability of heavy metals in the rhizosphere microenvironment, increase heavy metal uptake by phytoremediation plants, and enhance the phytoremediation efficiency of heavy-metal-contaminated soils. In recent years, the number of studies on the phytoremediation efficiency of heavy-metal-contaminated soil enhanced by PGPR has increased rapidly. This paper systematically reviews the mechanisms of PGPR that promote plant growth (including nitrogen fixation, phosphorus solubilization, potassium solubilization, iron solubilization, and plant hormone secretion) and the mechanisms of PGPR that enhance plant-heavy metal interactions (including chelation, the induction of systemic resistance, and the improvement of bioavailability). Future research on PGPR should address the challenges in heavy metal removal by PGPR-assisted phytoremediation.

Integrated morphological, physiological and transcriptomic analyses reveal response mechanisms of rice under different cadmium exposure routes

Rice (Oryza sativa) is one of the major cereal crops and takes up cadmium (Cd) more readily than other crops. Understanding the mechanism of Cd uptake and defense in rice can help us avoid Cd in the food chain. However, studies comparing Cd uptake, toxicity, and detoxification mechanisms of leaf and root Cd exposure at the morphological, physiological, and transcriptional levels are still lacking. Therefore, experiments were conducted in this study and found that root Cd exposure resulted in more severe oxidative and photosynthetic damage, lower plant biomass, higher Cd accumulation, and transcriptional changes in rice than leaf Cd exposure. The activation of phenylpropanoids biosynthesis in both root and leaf tissues under different Cd exposure routes suggests that increased lignin is the response mechanism of rice under Cd stress. Moreover, the roots of rice are more sensitive to Cd stress and their adaptation responses are more pronounced than those of leaves. Quantitative PCR revealed that OsPOX, OsCAD, OsPAL and OsCCR play important roles in the response to Cd stress, which further emphasize the importance of lignin. Therefore, this study provides theoretical evidence for future chemical and genetic regulation of lignin biosynthesis in crop plants to reduce Cd accumulation.

An Overview of Bacteria-Mediated Heavy Metal Bioremediation Strategies

Contamination-free groundwater is considered a good source of potable water. Even in the twenty-first century, over 90 percent of the population is reliant on groundwater resources for their lives. Groundwater influences the economical state, industrial development, ecological system, and agricultural and global health conditions worldwide. However, different natural and artificial processes are gradually polluting groundwater and drinking water systems throughout the world. Toxic metalloids are one of the major sources that pollute the water system. In this review work, we have collected and analyzed information on metal-resistant bacteria along with their genetic information and remediation mechanisms of twenty different metal ions [arsenic (As), mercury (Hg), lead (Pb), chromium (Cr), iron (Fe), copper (Cu), cadmium (Cd), palladium (Pd), zinc (Zn), cobalt (Co), antimony (Sb), gold (Au), silver (Ag), platinum (Pt), selenium (Se), manganese (Mn), molybdenum (Mo), nickel (Ni), tungsten (W), and uranium (U)]. We have surveyed the scientific information available on bacteria-mediated bioremediation of various metals and presented the data with responsible genes and proteins that contribute to bioremediation, bioaccumulation, and biosorption mechanisms. Knowledge of the genes responsible and self-defense mechanisms of diverse metal-resistance bacteria would help us to engineer processes involving multi-metal-resistant bacteria that may reduce metal toxicity in the environment.

Effects of heavy metals on bacterial growth parameters in degradation of phenol by an Antarctic bacterial consortium

Antarctica has often been perceived as a pristine continent until the recent few decades as pollutants have been observed accruing in the Antarctic environment. Irresponsible human activities such as accidental oil spills, waste incineration and sewage disposal are among the primary anthropogenic sources of heavy metal contaminants in Antarctica. Natural sources including animal excrement, volcanism and geological weathering also contribute to the increase of heavy metals in the ecosystem. A microbial growth model is presented for the growth of a bacterial cell consortium used in the biodegradation of phenol in media containing different metal ions, namely arsenic (As), cadmium (Cd), aluminium (Al), nickel (Ni), silver (Ag), lead (Pb) and cobalt (Co). Bacterial growth was inhibited by these ions in the rank order of Al < As < Co < Pb < Ni < Cd < Ag. Greatest bacterial growth occurred in 1 ppm Al achieving an OD600 of 0.985 and lowest in 1 ppm Ag with an OD600 of 0.090. At a concentration of 1.0 ppm, Ag had a considerable effect on the bacterial consortium, inhibiting the degradation of phenol, whereas this concentration of the other metal ions tested had no effect on degradation. The biokinetic growth model developed supports the suitability of the bacterial consortium for use in phenol degradation.

The metabolic mechanisms of Cd-induced hormesis in photosynthetic microalgae, Chromochloris zofingiensis

Cadmium, an environmental pollutant, is highly toxic and resistant to degradation. It exhibits toxicity at elevated doses but triggers excitatory effects at low doses, a phenomenon referred to as hormesis. Microalgae, as primary producers in aquatic ecosystems, demonstrate hormesis induced by cadmium, though the specific mechanisms are not yet fully understood. Consequently, we examined the hormesis of cadmium in Chromochloris zofingiensis. A minimal Cd2+ concentration (0.05 mg L-1) prompted cell proliferation, whereas higher concentrations (2.50 mg L-1) inhibited growth. The group exposed to higher doses exhibited increased levels of reactive oxygen species (ROS), malondialdehyde (MDA), glutathione (GSH), superoxide dismutase (SOD), and catalase (CAT). Contrastingly, the group exposed to low doses exhibited a moderate antioxidant response without significantly increasing ROS. This implies that increased levels of antioxidative components counteract excessive ROS, maintaining cellular redox balance and promoting growth under conditions of low Cd2+. Validation experiments have established that NADPH oxidase-derived ROS primarily coordinates the hormesis effect in microalgae. Comparative transcriptome analysis has proved the involvement of antioxidant systems and photosynthesis in regulating hormesis. Notably, Aurora A kinases consistently displayed varying expression levels across all Cd2+ treatments, and their role in microalgal hormesis was confirmed through validation with SNS-314 mesylate. This study unveils the intricate regulatory mechanisms of Cd-induced hormesis in C. zofingiensis, with implications for environmental remediation and industrial microalgae applications.

Characterization of heavy metal-associated bacteria from petroleum-contaminated soil and their resistogram and antibiogram analysis

The aim of the current study was to screen and identify heavy metal (chromium, cadmium, and lead) associated bacteria from petroleum-contaminated soil of district Muzaffarabad, Azad Jammu and Kashmir, Pakistan to develop ecofriendly technology for contaminated soil remediation. The petroleum-contaminated soil was collected from 99 different localities of district Muzaffarabad and the detection of heavy metals via an atomic absorption spectrometer. The isolation and identification of heavy metals-associated bacteria were done via traditional and molecular methods. Resistogram and antibiogram analysis were also performed using agar well diffusion and agar disc diffusion methods. The isolated bacteria were classified into species, i.e., B. paramycoides, B. albus, B. thuringiensis, B. velezensis, B. anthracis, B. pacificus Burkholderia arboris, Burkholderia reimsis, Burkholderia aenigmatica, and Streptococcus agalactiae. All heavy metals-associated bacteria showed resistance against both high and low concentrations of chromium while sensitive towards high and low concentrations of lead in the range of 3.0 ± 0.0 mm to 13.0 ± 0.0 mm and maximum inhibition was recorded when cadmium was used. Results revealed that some bacteria showed sensitivity towards Sulphonamides, Norfloxacin, Erythromycin, and Tobramycin. It was concluded that chromium-resistant bacteria could be used as a favorable source for chromium remediation from contaminated areas and could be used as a potential microbial filter.

Effects of seasonal temperature regimes: Does Cyprinus carpio act as a health hazard during the construction of Suki Kinari hydropower project on Kunhar River in Pakistan?

The main purpose of the current study was to assess the levels of trace elements (iron, lead, zinc, copper, and manganese) in both water and fish muscles of common carp (Cyprinus carpio) in the Kunhar River during the development of the Suki Kinari hydropower project (SKHPP). Additionally, the aim was to shed light on the potential health hazards associated with the consumption of fish by residents. Surface water and fish (muscle and liver) from ten specific sampling locations in the Mansehra district (affected by an SKHPP) along the river were examined to determine the levels of trace elements. The findings divulged that the water at all locations exhibited concentration levels of iron (Fe), lead (Pb), and manganese (Mn) that surpassed the benchmarks established by the World Health Organization in 2011. Conversely, the concentration levels of copper (Cu) and zinc (Zn) fell beneath the stipulated standards. Moreover, the concentrations of Mn, Zn, and Pb were found to be excessively high. The findings presented in the present study offer a comprehensive comprehension of the spatial and distribution characteristics of trace elements in both water and fish species along the Kunhar River, taking into consideration the impact of the SKHPP. Additionally, our data emphasize the potential health hazards that may arise from the prolonged consumption of fish by the local population.

Physio-anatomical modifications and element allocation pattern in Alternanthera tenella Colla. associated with phytoextraction of chromium

Intensive industrial activities have elevated chromium (Cr) concentrations in the environment, particularly in soil and water, posing a significant threat due to its cytotoxic and carcinogenic properties. Phytoremediation has emerged as a sustainable and economical alternative for detoxifying pollutants. In this context, an attempt has been made to assess the efficacy of Cr remediation by the invasive plant Alternanthera tenella Colla. The study investigated morphological, anatomical, and physiological adaptations in plant tissues in response to 240 µM of K2Cr2O7, considering elemental distribution patterns and bioaccumulation potential. Growth parameter assessments revealed a notable 50% reduction in root elongation and biomass content; however, the plant exhibited a comparatively higher tolerance index (47%) under Cr stress. Chromium significantly influenced macro and micro-elemental distribution in plant tissues, particularly in roots and leaves. Structural modifications, including changes in the thickness and diameter of xylem walls in the root, stem, and leaf tissues of Cr-treated A. tenella, were observed. Distinct cell structural distortions and Cr deposit inclusions in the xylem wall and inner parenchyma cells were distinct. Under Cr stress, there was a reduction in pigment content and metabolites such as proteins and soluble sugars, while proline, phenol, and malondialdehyde showed a twofold increase. The concentration of Cr was higher in the shoots of A. tenella (185.7 mg/kg DW) than in the roots (179.625 mg/kg DW). With a high BCFroot value (16.23) and TF > 1, coupled with effective mechanisms to cope with metal stress, A. tenella emerges as an ideal candidate for chromium phytoextraction.

Unraveling the multifaceted mechanisms and untapped potential of activated carbon in remediation of emerging pollutants: A comprehensive review and critical appraisal of advanced techniques

The rapid global expansion of industrialization has resulted in the discharge of a diverse range of hazardous contaminants into the ecosystem, leading to extensive environmental contamination and posing a pressing ecological concern. In this context, activated carbon (AC) has emerged as a highly promising adsorbent, offering significant advantages over conventional forms. For instance, AC has demonstrated remarkable adsorption capabilities, as evidenced by the successful removal of atrazine and ibuprofen using KOH and KOH-CO2-activated char, achieving impressive adsorption rates of 90% and 95%, respectively, at an initial dosage of 10 mg L-1. Moreover, AC can effectively adsorb aromatic compounds through π-π stacking interactions. The aromatic rings in organic molecules can align and interact with the carbon atoms in AC's structure, leading to effective adsorption. In this review, by employing a systematic analysis of recent research findings (majorly from 2015 to 2023), an in-depth exploration of AC's evolution and its wide-ranging applications in adsorbing and remediating emerging pollutants, including dyes, organic contaminants, and hazardous gases and mitigating the adverse impacts of such emerging pollutants on ecosystems have been discussed. It serves as a valuable resource for researchers, professionals, and policymakers involved in environmental remediation and pollution control, facilitating the development of sustainable and effective strategies for mitigating the global impact of emerging pollutants.

Assessment of phytoremediation potential of native plant species naturally growing in a heavy metal-polluted industrial soils

Abstract The present study was carried out in Hayat Abad Industrial Estate located in Peshawar to assess the levels of cadmium (Cd) that were present in the soil as well as the plant parts (Roots and shoots). To evaluate the phytoremediation potential of the plants different factors i.e. Bioconcentration Factor (BCF), Translocation Factor (TF), and Bioaccumulation Coefficient were determined. These plants were grown in their native habitats (BAC). We have analysed, cadmium concentration from soil which are collected from 50 different locations ranged from 11.54 mg/Kg (the lowest) to 89.80 mg/Kg (highest). The maximum concentration (89.80 mg/Kg) of cadmium was found in HIE-ST-16L Marble City and HIE-ST-7 Bryon Pharma (88.51 mg/Kg) while its minimum concentration (12.47 mg/Kg) were detected in the soil of Site (HIE-ST-14L Royal PVC Pipe) and (11.54 mg/Kg) at the site (HIE-ST-11 Aries Pharma). Most plant species showed huge potential for plant based approaches like phyto-extraction and phytoremediation. They also showed the potential for phyto-stabilization as well. Based on the concentration of cadmium the most efficient plants for phytoextraction were Cnicus benedictus, Parthenium hysterophorus, Verbesina encelioides, Conyza canadensis, Xanthium strumarium, Chenopodium album, Amaranthus viridis, Chenopodiastrum murale, Prosopis juliflora, Convolvulus arvensis, Stellaria media, Arenaria serpyllifolia, Cerastium dichotomum, Chrozophora tinctoria, Mirabilis jalapa, Medicago polymorpha, Lathyrus aphaca, Dalbergia sissoo, Melilotus indicus and Anagallis arvensis. The cadmium heavy metals in the examined soil were effectively removed by these plant species. Cerastium dichotomum, and Chenopodium murale were reported to be effective in phyto-stabilizing Cd based on concentrations of selected metals in roots and BCFs, TFs, and BACs values.

Arsenic pollution cycle, toxicity and sustainable remediation technologies: A comprehensive review and bibliometric analysis

Arsenic pollution and its allied impacts on health are widely reported and have gained global attention in the last few decades. Although the natural distribution of arsenic is limited, anthropogenic activities have increased its mobility to distant locations, thereby increasing the number of people affected by arsenic pollution. Arsenic has a complex biogeochemical cycle which has a significant role in pollution. Therefore, this review paper has comprehensively analysed the biogeochemical cycle of arsenic which can dictate the occurrence of arsenic pollution. Considering the toxicity and nature of arsenic, the present work has also analysed the current status of arsenic pollution around the world. It is noted that the south of Asia, West-central Africa, west of Europe and Latin America are major hot spots of arsenic pollution. Bibliometric analysis was performed by using scopus database with specific search for keywords such as arsenic pollution, health hazards to obtain the relevant data. Scopus database was searched for the period of 20 years from year 2003-2023 and total of 1839 articles were finally selected for further analysis using VOS viewer. Bibliometric analysis of arsenic pollution and its health hazards has revealed that arsenic pollution is primarily caused by anthropogenic sources and the key sources of arsenic exposure are drinking water, sea food and agricultural produces. Arsenic pollution was found to be associated with severe health hazards such as cancer and other health issues. Thus considering the severity of the issue, few sustainable remediation technologies such as adsorption using microbes, biological waste material, nanomaterial, constructed wetland, phytoremediation and microorganism bioremediation are proposed for treating arsenic pollution. These approaches are environmentally friendly and highly sustainable, thus making them suitable for the current scenario of environmental crisis.

Research progress and hotspots on microbial remediation of heavy metal-contaminated soil: a systematic review and future perspectives

Microbial remediation technology has received much attention as a green, ecological, and inexpensive technology, and there is great potential for the application of microbial remediation technology for heavy metals (HMs) contaminated soil alone and in conjunction with other technologies in environmental remediation. To gain an in-depth understanding of the latest research progress, research hotspots, and development trends on microbial remediation of HMs-contaminated soil, and to objectively reflect the scientific contributions and impacts of relevant countries/regions, institutions, and individuals of this field, in this manuscript, ISI Web of Knowledge's Web of Science™ core collection database, data visualization, and analysis software Bibliometrix, VOSviewer, and HistCite Pro were used to collect and analyze the relevant literature from 2000 to 2022, and 1409 publications were subjected to scientometric analyses. It involved 327 journals, 5150 authors, 75 countries/regions, and 2740 keywords. The current progress and hotspots on microbial remediation of HMs-contaminated soil since the twenty-first century were analyzed in terms of the top 10 most productive countries (regions), high-yielding authors, source journals, important research institutions, and hotspots of research directions. Over the past 22 years, China, India, and the USA have been the countries with the most articles. The institution and author with the most publications are the Chinese Acad Sci and Zhu YG, respectively. Journal of Hazardous Materials is the most productive journal. The keywords showed 6 co-occurrence clusters. These findings revealed the research hotspots, knowledge gaps, and future exploration trends related to microbial remediation of HMs-contaminated soil.

Techno-economic analysis of phytoremediation: A strategic rethinking

Phytoremediation is a cost-effective and environmentally sound approach, which uses plants to immobilize/stabilize, extract, decay, or lessen toxicity and contaminants. Despite successful evidence of field application, such as natural attenuations, and self-purification, the main barriers remain from a "promising" to a "commercial" approach. Therefore, the ultimate goal of this paper is to examine factors that contribute to phytoremediation's underutilization and discuss the real costs of phytoremediation when the time and land values are considered. We revisit mechanisms and processes of phytoremediation. We synthesize existing information and understanding based on previous works done on phytoremediation and its applications to provide the technical assessment and perspective views in the commercial acceptance of phytoremediation. The results show that phytoremediation is the most suitable for remote regions with low land values. Since these regions allow a longer period to be restored, land vegetation covers can be established in more or less time like natural attenuation. Since the length of phytoremediation is an inherent limitation, this inherent disadvantage limits its adoption in developed business regions, such as growing urban areas. Because high land values could not be recovered in the short term, phytoremediation is not cost-effective in those regions. We examine the potential measures that can enhance the performance of phytoremediation, such as soil amendments, and agricultural practices. The results obtained through review can clarify where/what conditions phytoremediation can provide the most suitable solutions at a large scale. Finally, we identify the main barriers and knowledge gaps to establishing a vegetation cover in large-scale applications and highlight the research priorities for increased acceptance of phytoremediation.

Bio-mediated detoxification of heavy metal contaminated soil and phytotoxicity reduction using novel strain of Brevundimonas vancanneytii SMA3

Heavy metals pose a serious environmental threat on a global scale due to their toxicity towards livings. Therefore, removing harmful metals from the environment has become more challenging in recent years. The objective of this study is to isolate, examine, and characterize naturally existing bacteria that possess the ability to mitigate and detoxify heavy metals such as cadmium, mercury, and lead. The selected bacteria SMA3 actively demonstrated metal tolerance during screening and was then employed for biosorption study using a lab-scale technique. The bacterium belonged to Brevundimonas sp., according to 16 S rRNA analysis. To enhance the removal efficiency of SMA3, response surface methodology (RSM) was employed, resulting in the identification of optimized conditions (pH 7, temperature 30 °C and shaking speed 120 rpm) for achieving maximum removal percentage (69.5 % of Cd, 58.6 % of Hg, and 85.1 % of Pb) within 72 h. The structural changes induced by microbial treatment were demonstrated by comparing the findings of FESEM images and FTIR spectra confirming the disappearance of C ^ C, C]O peaks along with C]O, C-O-C, C-H, and O-H bond destabilization following bioaccumulation. Moreover, in terms of phytotoxicity evaluation, it was observed that the treated soil, containing both heavy metals and the selected potent bacterial strain, exhibited reduced toxicity, resulting in improved germination and growth parameters for the seeds of Solanum lycopersicum (tomato plant). Overall, the selected bacterial strain demonstrated its potential for effectively removing multiple metals from the metal contaminated environment.

Mitigation of heavy metal stress in the soil through optimized interaction between plants and microbes

Agricultural as well as industrial processes, such as mining and textile activities, are just a few examples of anthropogenic activities that have a long-term negative impact on the environment. Each of the aforementioned factors increases the concentration of heavy metals in soil. Heavy metal contamination in soil causes a wide range of environmental issues and is harmful to microbes, plants, and animals. Because of their non-biodegradability and toxic effects, preventing additional metal contamination and remediating the vast majority of contaminated sites around the world is critical. Hence, this review focuses on the effects of metal contamination on soil microbes, as well as plant-microbe interactions. Plant-associated probiotics reduce metal accumulation; the introduction of beneficial microbes is regarded as one of the most promising approaches to improving metal stress tolerance; thus, the study focuses on plant-microbe interactions as well as their actual implications via phytoremediation. Plant-microbe interaction can play an important role in acclimating vegetation (plants) to metalliferous conditions and should thus be studied to improve microbe-aided metal tolerance in plants. Plant-interacted microbes reduce metal accumulation in plant cells and metal bioaccumulation in the soil through a variety of processes. A novel phytobacterial approach, such as genetically modified microbes, is now being used to improve heavy metal cleanup as well as stress tolerance among plants. This review examines our current understanding of such negative consequences of heavy metal stresses, signaling responses, and the role of plant-associated microbiota in heavy metal stress tolerance and interaction.

Current progress on fluoride occurrence in the soil environment: Sources, transformation, regulations and remediation

Fluorine is a halogen element widely distributed in nature, but due to excessive emissions from industrial manufacturing and agricultural production, etc., the soil is over-enriched with fluoride and the normal growth of plants is under stress, and it also poses a great threat to human health. In this review, we summarized the sources of fluoride in soil, and then analyzed the potential mechanisms of fluoride uptake in soil-plant systems. In addition, the main influences of soil ecosystems on plant fluoride uptake were discussed, soil management options to mitigate fluoride accumulation in plants were also summarized. The bioremediation techniques were found to be a developmental direction to improve fluoride pollution. Finally, we proposed other research directions, including fluoride uptake mechanisms in soil-plant systems at the molecular expression levels, development of visualization techniques for fluoride transport in plants, interactions mechanisms between soil microhabitats and plant metabolism affecting fluoride uptake, as well as combining abiotic additives, nanotechnology and biotechnology to remediate fluoride contamination problems.

Bioremediation of industrial wastewater heavy metals using solo and consortium Enterobacter spp

Heavy metals are considered the most common pollutants in industrial wastewater areas. Out of thirty bacterial isolates, only 3 isolates sighted the highest metal resistance activity for Zn+2, Fe+2, Pb+2, Co+2, Mn+2, Ni+2, and Cd+2. The biochemical and DNA homology identification with similarities 99.58%, 99.79%, and 99.86% of those isolates was identified and deposited in WDCM, respectively, as Enterobacter kobei OM144907 SCUF0000311, Enterobacter cloacae OM180597 SCUF0000312, and Enterobacter hormaechei OM181067 SCUF0000313. The minimum tolerance activity (MIC) of heavy metal concentrations against E. kobei and E. cloacae was 25, 15, and 15 mmol/l for Ni+2, Fe+2, and Mn+2, respectively, and 10 mmol/l for Zn+2, Pb+2, Co+2, and Cd+2, while against E. hormaechei, it is 15 mmol/l for Ni+2, Fe+2, and Mn+2 and 10 mmol/l for Zn+2, Pb+2, Co+2, and Cd+2. The consortium and solitary application of bacterial isolates towards heavy metal removal at 100%, 200%, and 300% industrial wastewater concentrations were conducted and showed that more than 90% removal of Zn+2, Fe+2, Pb+2, Mn+2, Ni+2, and Cd+2 from a non-concentrated polluted sample (100%) was reported by the three strains. With doubling the polluted sample concentration (200%), the highest removal efficiency for Zn+2, Pb+2, Mn+2, Ni+2, and Cd+2 was reported by E. cloacae as 70. 75, 66, 65, and 57%, respectively. Removal efficiency after increasing the polluted sample concentration to 300% showed that E. cloacae removed above 45% of all tested heavy metals except Pb+2. Ultimately, E. cloacae exposed the highest efficiency with recommendations for heavy metals removal under higher concentrations.

Ecological and health risk assessment associated with translocation of heavy metals in Lycopersicum esculentum from farmland soil treated with different composts

To meet the United Nations' Sustainable Development Goals, agricultural soil which is a non-renewable natural resource must be carefully managed. Heavy metals present in agricultural soil may imperil food security and instigate extreme risks to human health. Organic wastes have been long known for valuable amendments to soil thereby, improving overall soil health. In the present study, Echhornia crassipes, Hydrilla verticillata, and vegetable waste, was utilized to prepare compost amendments. Lycopersicum esculentum was used to metal uptake from compost amended soils. 5%, 10%, 15%, 25%, and 35% compost: soil (w/w) were studied to understand metal translocation in plants. Potential Ecological risk indices showed that while the degree of risk was medium for the natural soil, it reduced to slight for the soil amended with WHC and VWC for all compositions. The non-carcinogenic risks associated with the human health reduced on application of the composts, however, they still remained substantial for Fe, As, and Pb for WHC, HVC, and VWC composts at higher application ratios, especially among children. On the other hand, the carcinogenic health index values which were calculated to estimate the risk associated with ingestion of L. esculentum, showed a decrease in risk for all the metals studied, upon soil amendment. Soil amended with HVC compost showed an increase in carcinogenic risk for As, Pb, and Cr. Finally, we conclude that biological soil remediation is economical and a sustainable land management strategy that may lead to green and clean remediation solutions for metal contaminated soil.

Removal of toxic metal Cd (II) by Serratia bozhouensis CdIW2 using in moving bed biofilm reactor (MBBR)

The use of bioreactor technology to treat industrial wastewater containing heavy metals has created new perspectives. Cadmium metal is one of the toxic heavy metals that have harmful effects on human health and the environment. This research work presents a comprehensive approach for aqueous cadmium removal through biosorption in a moving bed biofilm reactor (MBBR). The bacterium resistant to Cd(II) (350 mg/L) CdIW2 was selected among 8 cadmium tolerant bacteria isolated from the industrial wastewater of the metal industry. 16S rRNA gene and phenotypic analysis showed that the bacterium CdIW2 is similar to Serratia bozhouensis. The highest biosorption capacity of 65.79 mg/g was acquired in optimal conditions (30 min, pH = 6, 0.5 g/L, and 35 °C). The biosorption of the CdIW2 strain was consistent with the Langmuir isotherm and the pseudo-second order kinetic and showed the process's spontaneous thermodynamic and endothermic results. The removal rate 91.74% of MBBR in batch mode was obtained in 72 h and 10 mg/L of Cd(II). Furthermore, continuous mode bioreactor analysis has shown high efficiency at intel loading rates of 6-36 mg/L. day for cadmium removal. The second order kinetic (Grau) was chosen as the suitable model for modeling the MBBR process. Although several studies have evaluated the removal of various types of heavy metals, none of the studies involved the use of a metal-resistant strain in an MBBR bioreactor.

Bio/phyremediation potential of Leptospirillum ferrooxidans and Ricinus communis on metal contaminated mine sludge

The heavy metal pollution is a serious environmental pollution around the globe and threatens the ecosystem. The physicochemical traits (pH, Electrical conductivity, hardness, NPK, Al, Fe, Cd, Cr, Pb, Mg, and Mn) of soil sample collected from the polluted site were analyzed and found that the most of the metal contents were beyond the acceptable limits of national standards. The metals such as Mn (1859.37 ± 11.25 mg kg-1), Cd (24.86 ± 1.85 mg kg-1), Zn (795.64 ± 9.24 mg kg-1), Pb (318.62 ± 5.85 mg kg-1), Cr (186.84 ± 6.84 mg kg-1), and Al (105.84 ± 5.42 mg kg-1) were crossing the permissible limits. The pre-isolated L. ferrooxidans showed considerable metal tolerance to metals such as Al, Cd, Cr, Pb, Mg, and Mn at up to the concentration of 750 μg mL-1 and also have remediation potential on polluted soil in a short duration of treatment. The greenhouse study demonstrated that the bio/phytoremediation potential of metal tolerant L. ferrooxidans and R. communis under various remediation (A, B, and C) groups. Surprisingly, remediation group C demonstrated greater phytoextraction potential than the other remediation groups (A and B). These results strongly suggest that coexistence of L. ferrooxidans and R. communis had a significant positive effect on phytoextraction on metal-contaminated soil.

Effect of Dietary Selenium on the Growth and Immune Systems of Fish

Dietary selenium (Se) is an essential component that supports fish growth and the immune system. This review attempts to provide insight into the biological impacts of dietary Se, including immunological responses, infection defense, and fish species growth, and it also identifies the routes via which it enters the aquatic environment. Dietary Se is important in fish feed due to its additive, antioxidant, and enzyme properties, which aid in various biological processes. However, excessive intake of it may harm aquatic ecosystems and potentially disrupt the food chain. This review explores the diverse natures of dietary Se, their impact on fish species, and the biological methods for eliminating excesses in aquatic environments. Soil has a potential role in the distribution of Se through erosion from agricultural, industrial, and mine sites. The research on dietary Se's effects on fish immune system and growth can provide knowledge regarding fish health, fish farming strategies, and the health of aquatic ecosystems, promoting the feed industry and sustainable aquaculture. This review provides data and references from various research studies on managing Se levels in aquatic ecosystems, promoting fish conservation, and utilizing Se in farmed fish diets.

Vaping, Environmental Toxicants Exposure, and Lung Cancer Risk

Lung cancer (LC) is the second-most prevalent tumor worldwide. According to the most recent GLOBOCAN data, over 2.2 million LC cases were reported in 2020, with an estimated new death incident of 1,796,144 lung cancer cases. Genetic, lifestyle, and environmental exposure play an important role as risk factors for LC. E-cigarette, or vaping, products (EVPs) use has been dramatically increasing world-wide. There is growing concern that EVPs consumption may increase the risk of LC because EVPs contain several proven carcinogenic compounds. However, the relationship between EVPs and LC is not well established. E-cigarette contains nicotine derivatives (e.g., nitrosnornicotine, nitrosamine ketone), heavy metals (including organometal compounds), polycyclic aromatic hydrocarbons, and flavorings (aldehydes and complex organics). Several environmental toxicants have been proven to contribute to LC. Proven and plausible environmental carcinogens could be physical (ionizing and non-ionizing radiation), chemicals (such as asbestos, formaldehyde, and dioxins), and heavy metals (such as cobalt, arsenic, cadmium, chromium, and nickel). Air pollution, especially particulate matter (PM) emitted from vehicles and industrial exhausts, is linked with LC. Although extensive environmental exposure prevention policies and smoking reduction strategies have been adopted globally, the dangers remain. Combined, both EVPs and toxic environmental exposures may demonstrate significant synergistic oncogenicity. This review aims to analyze the current publications on the importance of the relationship between EVPs consumption and environmental toxicants in the pathogenesis of LC.

Antarctic heavy metal pollution and remediation efforts: state of the art of research and scientific publications

In Antarctica, human activities have been reported to be the major cause of the accumulation of heavy metal contaminants. A comprehensive bibliometric analysis of publications on heavy metal contamination in Antarctica from year 2000 to 2020 was performed to obtain an overview of the current landscape in this line of research. A total of 106 documents were obtained from Scopus, the largest citation database. Extracted data were analysed, and VOSviewer software was used to visualise trends. The result showed an increase in publications and citations in the past 20 years indicating the rising interest on heavy metal contamination in the Antarctic region. Based on the analysis of keywords, the publications largely discuss various types of heavy metals found in the Antarctic water and sediment. The analysis on subject areas detects multiple disciplines involved, wherein the environmental science was well-represented. The top countries and authors producing the most publication in this field were from Australia, China, Brazil and Chile. Numerous efforts have been exercised to investigate heavy metal pollution and its mitigation approaches in the region in the past decades. This paper not only is relevant for scholars to understand the development status and trends in this field but also offers clear insights on the future direction of Antarctic heavy metal contamination and remediation research.

Microbial Biofilms for Environmental Bioremediation of Heavy Metals: a Review

Heavy metal pollution caused due to various industrial and mining activities poses a serious threat to all forms of life in the environment because of the persistence and toxicity of metal ions. Microbial-mediated bioremediation including microbial biofilms has received significant attention as a sustainable tool for heavy metal removal as it is considered safe, effective, and feasible. The biofilm matrix is dynamic, having microbial cells as major components with constantly changing and evolving microenvironments. This review summarizes the bioremediation potential of bacterial biofilms for different metal ions. The composition and mechanism of biofilm formation along with interspecies communication among biofilm-forming bacteria have been discussed. The interaction of biofilm-associated microbes with heavy metals takes place through a variety of mechanisms. These include biosorption and bioaccumulation in which the microbes interact with the metal ions leading to their conversion from a highly toxic form to a less toxic form. Such interactions are facilitated via the negative charge of the extracellular polymeric substances on the surface of the biofilm with the positive charge of the metal ions and the high cell densities and high concentrations of cell-cell signaling molecules within the biofilm matrix. Furthermore, the impact of the anodic and cathodic redox potentials in a bioelectrochemical system (BES) for the reduction, removal, and recovery of numerous heavy metal species provides an interesting insight into the bacterial biofilm-mediated bioelectroremediation process. The review concludes that biofilm-linked bioremediation is a viable option for the mitigation of heavy metal pollution in water and ecosystem recovery.

Recent Advances in Microbial-Assisted Remediation of Cadmium-Contaminated Soil

Soil contamination with cadmium (Cd) is a severe concern for the developing world due to its non-biodegradability and significant potential to damage the ecosystem and associated services. Industries such as mining, manufacturing, building, etc., rapidly produce a substantial amount of Cd, posing environmental risks. Cd toxicity in crop plants decreases nutrient and water uptake and translocation, increases oxidative damage, interferes with plant metabolism and inhibits plant morphology and physiology. However, various conventional physicochemical approaches are available to remove Cd from the soil, including chemical reduction, immobilization, stabilization and electro-remediation. Nevertheless, these processes are costly and unfriendly to the environment because they require much energy, skilled labor and hazardous chemicals. In contrasting, contaminated soils can be restored by using bioremediation techniques, which use plants alone and in association with different beneficial microbes as cutting-edge approaches. This review covers the bioremediation of soils contaminated with Cd in various new ways. The bioremediation capability of bacteria and fungi alone and in combination with plants are studied and analyzed. Microbes, including bacteria, fungi and algae, are reported to have a high tolerance for metals, having a 98% bioremediation capability. The internal structure of microorganisms, their cell surface characteristics and the surrounding environmental circumstances are all discussed concerning how microbes detoxify metals. Moreover, issues affecting the effectiveness of bioremediation are explored, along with potential difficulties, solutions and prospects.

Nanoremediation strategies to address environmental problems

A rapid rise in population, extensive anthropogenic activities including agricultural practices, up-scaled industrialization, massive deforestation, etc. are the leading causes of environmental degradation. Such uncontrolled and unabated practices have affected the quality of environment (water, soil, and air) synergistically by accumulating huge quantities of organic and inorganic pollutants in it. Environmental contamination is posing a threat to the existing life on the Earth, therefore, demands the development of sustainable environmental remediation approaches. The conventional physiochemical remediation approaches are laborious, expensive, and time-consuming. In this regard, nanoremediation has emerged as an innovative, rapid, economical, sustainable, and reliable approach to remediate various environmental pollutants and minimize or attenuate the risks associated with them. Owing to their unique properties such as high surface area to volume ratio, enhanced reactivity, tunable physical parameters, versatility, etc. nanoscale objects have gained attention in environmental clean-up practices. The current review highlights the role of nanoscale objects in the remediation of environmental contaminants to minimize their impact on human, plant, and animal health; and air, water, and soil quality. The aim of the review is to provide information about the applications of nanoscale objects in dye degradation, wastewater management, heavy metal and crude oil remediation, and mitigation of gaseous pollutants including greenhouse gases.

Rhizospheric bacteria: the key to sustainable heavy metal detoxification strategies

The increasing rate of industrialization, anthropogenic, and geological activities have expedited the release of heavy metals (HMs) at higher concentration in environment. HM contamination resulting due to its persistent nature, injudicious use poses a potential threat by causing metal toxicities in humans and animals as well as severe damage to aquatic organisms. Bioremediation is an emerging and reliable solution for mitigation of these contaminants using rhizospheric microorganisms in an environmentally safe manner. The strategies are based on exploiting microbial metabolism and various approaches developed by plant growth promoting bacteria (PGPB) to minimize the toxicity concentration of HM at optimum levels for the environmental clean-up. Rhizospheric bacteria are employed for significant growth of plants in soil contaminated with HM. Exploitation of bacteria possessing plant-beneficial traits as well as metal detoxifying property is an economical and promising approach for bioremediation of HM. Microbial cells exhibit different mechanisms of HM resistance such as active transport, extra cellular barrier, extracellular and intracellular sequestration, and reduction of HM. Tolerance of HM in microorganisms may be chromosomal or plasmid originated. Proteins such as MerT and MerA of mer operon and czcCBA, ArsR, ArsA, ArsD, ArsB, and ArsC genes are responsible for metal detoxification in bacterial cell. This review gives insights about the potential of rhizospheric bacteria in HM removal from various polluted areas. In addition, it also gives deep insights about different mechanism of action expressed by microorganisms for HM detoxification. The dual-purpose use of biological agent as plant growth enhancement and remediation of HM contaminated site is the most significant future prospect of this article.

Synthesis and characterization of keratinase laden green synthesized silver nanoparticles for valorization of feather keratin

This study focuses on the efficient and cost-effective synthesis of silver nanoparticles (AgNPs) using plant extracts, which have versatile and non-toxic applications. The research objectives include synthesizing AgNPs from readily available plant extracts, optimizing their production and multi scale characterization, along with exploring their use for enzyme immobilization and mitigation of poultry feather waste. Among the plant extracts tested, the flower extract of Hibiscus rosa-sinensis (HF) showed the most potential for AgNP synthesis. The synthesis of HF-mediated AgNPs was optimized using response surface methodology (RSM) for efficient and environment friendly production. Additionally, the keratinase enzyme obtained from Bacillus sp. NCIM 5802 was covalently linked to AgNPs, forming a keratinase nanocomplex (KNC) whose biochemical properties were evaluated. The KNC demonstrated optimal activity at pH 10.0 and 60 °C and it displayed remarkable stability in the presence of various inhibitors, metal ions, surfactants, and detergents. Spectroscopic techniques such as FTIR, UV-visible, and X-ray diffraction (XRD) analysis were employed to investigate the formation of biogenic HF-AgNPs and KNC, confirming the presence of capping and stabilizing agents. The morphological characteristics of the synthesized AgNPs and KNC were determined using transmission electron microscopy (TEM) and particle size analysis. The study highlighted the antimicrobial, dye scavenging, and antioxidant properties of biogenic AgNPs and KNC, demonstrating their potential for various applications. Overall, this research showcases the effectiveness of plant extract-driven green synthesis of AgNPs and the successful development of keratinase-laden nanocomplexes, opening possibilities for their use in immobilizing industrial and commercial enzymes.

A systematic review on the bioremediation of metal contaminated soils using biochar and slag: current status and future outlook

Heavy metals contaminated soils are posing severe threats to food safety worldwide. Heavy metals absorbed by plant roots from contaminated soils lead to severe plant development issues and a reduction in crop yield and growth. The global population is growing, and the demand for food is increasing. Therefore, it is critical to identify soil remediation strategies that are efficient, economical, and environment friendly. The use of biochar and slag as passivators represents a promising approach among various physicochemical and biological strategies due to their efficiency, cost-effectiveness, and low environmental impact. These passivators employ diverse mechanisms to reduce the bioavailability of metals in contaminated soils, thereby improving crop growth and productivity. Although studies have shown the effectiveness of different passivators, further research is needed globally as this field is still in its early stages. This review sheds light on the innovative utilization of biochar and slag as sustainable strategies for heavy metal remediation, emphasizing their novelty and potential for practical applications. Based on the findings, research gaps have been identified and future research directions proposed to enable the full potential of passivators to be utilized effectively and efficiently under controlled and field conditions.

Biosorption of aqueous Pb(II) by H3PO4-activated biochar prepared from palm kernel shells (PKS)

The conversion of palm kernel shells (PKS), a major agricultural waste from the palm oil sector, into a potentially high-value biosorbent for heavy metals-contaminated wastewater treatments was explored in this work. Following carbonization, the activated PKS was chemically activated by soaking the biochar in a phosphoric acid (H3PO4) solution at 25 °C. The low-temperature approach benefits from less dangerous acid fume production and operational challenges when compared to the high-temperature procedure. The properties of the biochar were characterized by BET, FTIR, and SEM. The effects of H3PO4 dosage, initial Pb(II) concentration, and adsorbent dosage on removing Pb(II) from synthetic wastewater were investigated in the adsorption study. The activation of PKS biochar with high H3PO4 concentrations led to enhanced removal efficiency. The pseudo-second-order (PSO) kinetic model fitted the experimental data well (R2 0.99), indicating that chemisorption was likely involved in the adsorption of Pb(II) onto activated PKS. Pb(II) sorption was possibly promoted by the presence of phosphate moieties on the adsorbent surface. The Langmuir isotherm best described the sorption of Pb(II) onto the activated PKS (R2 0.97), giving the calculated maximum adsorption capacity (qm) of 171.1 μg/g. In addition to physical sorption, possible adsorption mechanisms included functional group complexation and surface precipitation. Overall, activating PKS biochar with H3PO4 at room temperature could be a promising technique to improve the adsorbent's adsorption efficiency for Pb(II) removal from wastewater.

Omics approaches in effective selection and generation of potential plants for phytoremediation of heavy metal from contaminated resources

Soil and water pollution, rapid industrialization, contaminated irrigation-water, increased waste-production and surge in agricultural land leads to the accumulation of Heavy Metals (HM) with time. HM contamination has raised concern over the past years and new remediation strategies are required to deal with it. HM-contaminated soil is often used for the production of food, which makes a gateway for toxic metals into the food-chain, thereby affecting food security and human health. To avoid HM-toxicity, decontamination of important resources is essential. Therefore, exploring phytoremediation for the removal, decomposition and detoxification of hazardous metals from HM-contaminated sites is of great significance. Hyper-accumulator plants can efficiently remove HMs. However, despite many hyper-accumulator plant species, there is a research gap in the studies of phytotechnology. Hence biotechnological efforts advocating omics studies i.e. genomics, transcriptomics, proteomics, metabolomics and phenomics are in order, the purpose being to select and enhance a plant's potential for the process of phytoremediation to be more effective. There is a need to study newly developed high-efficiency hyper-accumulator plants as HM-decontaminator candidates for phytoremediation and phytomining. Therefore, this review focuses on various strategies and bio-technological methods for the removal of HM contaminants from sites, with emphasis on the advancement of phytoremediation, along with applications in cleaning up various toxic pollutants.

Narrative of hazardous chemicals in water: Its potential removal approach and health effects

H₂O is essential for life to exist on earth; it is important to guarantee both the quality and supply of water to satisfy world demand. However, it became contaminated by a number of hazardous, inorganic industrial pollutants, which caused a number of issues like irrigation activities and unsafe human ingestion. Long-term exposure to harmful substances can result in respiratory, immunological, and neurological illnesses, cancer, and problems during pregnancy. Therefore, removing hazardous substances from wastewater and natural water sources is crucial. It is necessary to develop an alternate method that can effectively remove these toxins from water bodies, as conventional methods have several drawbacks. This review primarily aims to achieve the following goals: 1) to discuss the distribution of harmful chemicals: 2) to give specifics on numerous possible strategies for getting rid of hazardous chemicals, and 3) its effects on the environment and consequences for human health have been examined.

Bioaccumulation and Bioremediation of Heavy Metals in Fishes-A Review

Heavy metals, the most potent contaminants of the environment, are discharged into the aquatic ecosystems through the effluents of several industries, resulting in serious aquatic pollution. This type of severe heavy metal contamination in aquaculture systems has attracted great attention throughout the world. These toxic heavy metals are transmitted into the food chain through their bioaccumulation in different tissues of aquatic species and have aroused serious public health concerns. Heavy metal toxicity negatively affects the growth, reproduction, and physiology of fish, which is threatening the sustainable development of the aquaculture sector. Recently, several techniques, such as adsorption, physio-biochemical, molecular, and phytoremediation mechanisms have been successfully applied to reduce the toxicants in the environment. Microorganisms, especially several bacterial species, play a key role in this bioremediation process. In this context, the present review summarizes the bioaccumulation of different heavy metals into fishes, their toxic effects, and possible bioremediation techniques to protect the fishes from heavy metal contamination. Additionally, this paper discusses existing strategies to bioremediate heavy metals from aquatic ecosystems and the scope of genetic and molecular approaches for the effective bioremediation of heavy metals.

Characterization of a cadmium-resistant functional bacteria (Burkholderia sp. SRB-1) and mechanism analysis at physiochemical and genetic level

In this study, the capacity of cadmium (Cd)-resistant plant growth-promoting bacteria (PGPB) Burkholderia sp. SRB-1 (SRB-1) and its mechanisms were explored through morphological characterizations, biochemical response, plant growth-promoting traits, and functional gene expression patterns. The results showed that SRB-1 was an excellent Cd-resistant bacteria (MIC was 420 mg L^-1), and its maximum Cd removal rate reached 72.25%. Biosorption was the main removal method of Cd for SRB-1, preventing intracellular Cd accumulation and maintaining cellular metabolism. Various functional groups on the cell wall were involved in Cd binding, which deposited as CdS and CdCO₃ on the cell surface according to XPS analysis and might be critical for reducing Cd physiochemical toxicity. Furthermore, metals exporting (zntA, czcA, czcB, czcC), detoxification (dsbA, cysM), and antioxidation (katE, katG, SOD1) related genes were annotated in the SRB-1 genome. The results of Cd distribution and antioxidative enzyme activity in SRB-1 also illustrated that Cd²⁺ efflux and antioxidative response were the main intracellular Cd-resistant mechanisms. These conclusions were further verified by qRT-PCR analysis. Overall, the strategies of extracellular biosorption, cation efflux, and intracellular detoxification jointly build the Cd-resistant system, which invested Burkholderia sp. SRB-1 with potential for bioremediation in heavily Cd-contaminated environmental sites.

Heavy metals in vegetables: a review of status, human health concerns, and management options

For sustainable global growth, food security is a prime concern issue, both quantitatively and qualitatively. Adverse effects on crop quality from contaminants like heavy metals have affected food security and human health. Vegetables comprise the essential and nutritious part of the human diet as they contain a lot of health-promoting minerals and vitamins. However, the inadvertent excess accumulation of heavy metals (As, Cd, Hg, and Pb) in vegetables and their subsequent intake by humans may affect their physiology and metabolomics and has been associated with diseases like cancer, mental retardation, and immunosuppression. Many known sources of hazardous metals are volcano eruptions, soil erosion, use of chemical fertilizers in agriculture, the use of pesticides and herbicides, and irrigation with wastewater, industrial effluents, etc. that contaminate the vegetables through the soil, air and water. In this review, the problem of heavy metal contamination in vegetables is discussed along with the prospective management strategies like soil amendments, application of bioadsorbents, membrane filtration, bioremediation, and nanoremediation.