The immobilization of heavy metals in soil by bioaugmentation of a UV-mutant Bacillus subtilis 38 assisted by NovoGro biostimulation and changes of soil microbial community

Sustainable bioremediation and reuse of heavy metal-contaminated dredged sediments using Bacillus subtilis

Urbanization has led to heavy metal contamination of dredged sediments, posing severe environmental and health risks. This study investigated the efficacy of Bacillus subtilis-bioremediation and reuse Heavy Metal contaminated sediment as construction material. Recognizing the limitations of conventional calcium chloride, alternative calcium sources for enhanced remediation were explored. The results demonstrate that utilizing calcium hydroxide (0.625 M) as a cementing reagent resulted in optimal compressive strength while minimizing heavy metal leaching. A substantial reduction in leachability: 97.8% for cadmium, 92% for nickel, and 98% for zinc, was observed as determined by USEPA Method 1311. Sequential extraction procedure analysis revealed the effective immobilization of heavy metals within the sediment matrix, primarily through their conversion to metal carbonates and their association with organic matter. This eco-friendly bioremediation approach, combining bacterial activity with sustainable cement stabilization, presents a promising remediation strategy for contaminated dredged sediments, enabling the safe reuse in engineering applications.

Bacillus Subtilis as an Excellent Microbial Treatment Agent for Environmental Pollution: A Review

The use of microorganisms in environmental biotreatment is gaining attention, particularly Bacillus subtilis (B. subtilis), recognized for its effectiveness in wastewater treatment and soil remediation. Its success stems from its diverse biological activities and adaptability, which improve environmental quality and ecological balance. This paper reviews the remediation capabilities and mechanisms of B. subtilis, focusing on its applications in water purification and soil pollution management. B. subtilis facilitates pollutant degradation and adsorption through enzyme production, organic acids, unique cell wall properties, and interactions with other microorganisms. The article addresses current challenges and future directions, emphasizing the need for enhanced cultivation, screening, and genetic engineering of functional strains. Understanding the interactions of these strains with other microorganisms and studying their ecological and toxicological impacts are essential for optimizing microbial remediation, providing both theoretical and practical foundations for bioremediation efforts.

Bacterial biomineralization of heavy metals and its influencing factors for metal bioremediation

Increasing industrial pollution and certain hazardous agricultural practices have led to the discharge of heavy toxic metals into the environment. Among different bioremediation techniques, biomineralization is the synthesis of biomineral crystals extracellularly or intracellularly. Several bacteria, such as Bacillus cereus, Pseudomonas stutzeri, Bacillus subtilis, and Lactobacillus sphaericus have been found to induce heavy metal precipitation and mineralization for bioremediation. This article summarizes the different biomineralization mechanisms of bacterial-induced heavy metal biomineralization, mainly microbial-induced carbonate precipitation (MICP), microbial-induced phosphate precipitation (MIPP), and microbial-induced sulphide precipitation (MISP). Moreover, bacterial structures such as cell wall, biofilm, and extracellular polymeric substances (EPS) influence mineralization and control bacterial compartmentalization of heavy metal precipitation. Several genes control the efficiency of biomineralization in bacteria, such as ureA, ureB, ureC, phoA, dsrA, dsrB, dsrC, dsrD, dsrE, luxS, and ompR. This biomineralization mechanism provides new and broad prospects for its application in soil improvement, industrial applications, and wastewater treatments. In addition, bacterial genetic modification holds immense potential for advancing the biomineralization process to meet diverse environmental and industrial needs.

The duality of sulfate-reducing bacteria: Reducing methylmercury production in rhizosphere but enhancing accumulation in rice plants

Sulfate-reducing bacteria (SRB) are known to alter methylmercury (MeHg) production in paddy soil, but the effect of SRB on MeHg dynamics in rhizosphere and rice plants remains to be fully elucidated. The present study investigated the impact of SRB on MeHg levels in unsterilized and γ-sterilized mercury-polluted paddy soils, with the aim to close this knowledge gap. Results showed that the presence of SRB reduced MeHg production by ∼22 % and ∼17 % in the two soils, but elevated MeHg contents by approximately 55 % and 99 % in rice grains, respectively. Similar trend at smaller scales were seen in roots and shoots. SRB inoculation exerted the most profound impact on amino acid metabolism in roots, with the relative response of L-arginine positively linking to MeHg concentrations in rhizosphere. The SRB-induced enrichment of MeHg in rice plants may be interpreted by the stronger presence of endophytic nitrogen-related microbes (e.g. Methylocaldum, Hyphomicrobium and Methylocystis) and TGA transcription factors interacting with glutathione metabolism and calmodulin. Our study provides valuable insights into the complex effects of SRB inoculation on MeHg dynamics in rice ecosystems, and may help to develop strategies to effectively control MeHg accumulation in rice grains.

Zero-valent iron (ZVI) facilitated in-situ selenium (Se) immobilization and its recovery by magnetic separation: Mechanisms and implications for microbial ecology

Selenium (Se(VI)) is environmentally toxic. One of the most popular reducing agents for Se(VI) remediation is zero-valent iron (ZVI). However, most ZVI studies were carried out in water matrices, and the recovery of reduced Se has not been investigated. A water-sediment system constructed using natural sediment was employed here to study in-situ Se remediation and recovery. A combined effect of ZVI and unacclimated microorganisms from natural sediment was found in Se(VI) removal in the water phase with a removal efficiency of 92.7 ± 1.1% within 7 d when 10 mg L-1 Se(VI) was present. Soluble Se(VI) was removed from the water and precipitated to the sediment phase (74.8 ± 0.1%), which was enhanced by the addition of ZVI (83.3 ± 0.3%). The recovery proportion of the immobilized Se was 34.2 ± 0.1% and 92.5 ± 0.2% through wet and dry magnetic separation with 1 g L-1 ZVI added, respectively. The 16 s rRNA sequencing revealed the variations in the microbial communities in response to ZVI and Se, which the magnetic separation could potentially mitigate in the long term. This study provides a novel technique to achieve in-situ Se remediation and recovery by combining ZVI reduction and magnetic separation.

The role and mechanism of Bacillus megaterium strain A14 in inhibiting the cadmium uptake by peanut plants in acidic red soil

AIMS

This study explores the potential of cadmium (Cd)-resistant bacteria, specifically Bacillus megaterium A14, to decrease Cd accumulation in peanuts, a crop susceptible to metal uptake from contaminated soils, by understanding the bacterium's impact on plant Cd absorption mechanisms.

METHODS AND RESULTS

Through pot experiments, we observed that A14 inoculation significantly increased peanut biomass under Cd stress conditions, primarily by immobilizing the metal and reducing its bioavailability. The bacterium effectively changed the distribution of Cd, with a notable 46.53% reduction in the exchangeable fraction, which in turn limited the expression of genes related to Cd transport in peanuts. Additionally, A14 enhanced the plant's antioxidant response, improving its tolerance to stress. Microbial analysis through 16S sequencing demonstrated that A14 inoculation altered the peanut rhizosphere, particularly by increasing populations of Firmicutes and Proteobacteria, which play crucial roles in soil remediation from heavy metals.

CONCLUSION

The A14 strain effectively counters Cd toxicity in peanuts, promoting growth through soil Cd sequestration, root barrier biofilm formation, antioxidant system enhancement, suppression of Cd transport genes, and facilitation of Cd-remediating microorganisms.

Enhancing soil health to minimize cadmium accumulation in agro-products: the role of microorganisms, organic matter, and nutrients

Agro-products accumulate Cd from the soil and are the main source of Cd in humans. Their use must therefore be minimized using effective strategies. Large soil beds containing low-to-moderate Cd-contamination are used to produce agro-products in many developing countries to keep up with the demand of their large populations. Improving the health of Cd-contaminated soils could be a cost-effective method for minimizing Cd accumulation in crops. In this review, the latest knowledge on the physiological and molecular mechanisms of Cd uptake and translocation in crops is presented, providing a basis for developing advanced technologies for producing Cd-safe agro-products. Inoculation of plant growth-promoting rhizobacteria and arbuscular mycorrhizal fungi, application of organic matter, essential nutrients, beneficial elements, regulation of soil pH, and water management are efficient techniques used to decrease soil Cd bioavailability and inhibiting the uptake and accumulation of Cd in crops. In combination, these strategies for improving soil health are environmentally friendly and practical for reducing Cd accumulation in crops grown in lightly to moderately Cd-contaminated soil.

Microbial weathering of montmorillonite and its implication for Cd(II) immobilization

Interactions between silicate bacteria and silicates are very common in nature and hold great potential in altering their mutual physicochemical properties. But their interactions in regulating contaminants remediation involving performance and mechanisms are often overlooked. Here, we focused on the interactions between silicate bacteria (Paenibacillus polymyxa, PP; Bacillus circulans, BC) and a soil silicate montmorillonite (Mt), and their impact on Cd(II) immobilization. The obtained results showed that Mt greatly promoted the growth of the bacteria, resulting in a maximum 10.31 times increase in biomass production. In return, the bacteria strongly enhanced the Cd(II) adsorption on Mt, with adsorption capacities increased by 80.61%-104.45% in comparison to the raw Mt. Additionally, the bacteria-Mt interaction changed Cd(II) to a more stabilized state with a maximum reduction of 38.90%/g Mt in bioavailability. The enhancement of Cd(II) adsorption and immobilization on the bacterial modified Mt was caused by the following aspects: (1) the bacteria activities altered the aggregation state of Mt and made it better dispersed, thus more active sites were exposed; (2) the microbial activities brought about more rough and crumpled surface, as well as smaller Mt fragments; (3) a variety of microbial-derived functional groups were introduced onto the Mt surface, increasing its affinity for heavy metals; (4) the main Cd(II) immobilization mechanism was changed from ion exchange to the combination of ion exchange and functional groups induced adsorption. This work elucidates the potential ecological and evolutionary processes of silicate bacteria-soil clay mineral interactions, and bears direct implications for the clay-mediated bioremediation of heavy metals in natural environments.

The negative effects of the excessive nitrite accumulation raised by anaerobic bioaugmentation on bioremediation of PAH-contaminated soil

Nitrite accumulation in anaerobic bioaugmentation and its side effects on remediation efficiency of polycyclic aromatic hydrocarbon (PAH)-contaminated soil were investigated in this study. Four gradient doses of PAH-degrading inoculum (10^4, 10^5, 10^6 and 10^7 cells/g soil) were separately supplied to the actual PAH-contaminated soil combining with nitrate as the biostimulant. Although bioaugmented with higher dose of inoculum could effectively improve the biodegradation efficiencies in the initial stage than sole nitrate addition but also accelerated the accumulation of nitrite in soil. The inhibition effects of nitrite swiftly occurred following the rapid accumulation of nitrite in each experiment group, restraining the PAH-degrading functionality by inhibiting the growth of total biomass and denitrifying functions in soil. This study revealed the side effects of nitrite accumulation raised by bioaugmentation on soil microorganisms, contributing to further improving the biodegrading efficiencies in the actual site restoration.

Application potential of biofertilizer-assisted Pennisetum giganteum in safe utilization of mercury-contaminated paddy fields

High mercury (Hg) bioaccumulation in crops such as rice in Hg-contaminated areas presents a potential health hazard to humans and wildlife. To develop a safe alternative technique, bacillus-inoculated biofertilizer, citric acid, earthworms, and selenium-modified activated clay were compared for their ability to regulate Hg bioaccumulation in Pennisetum giganteum (P. giganteum). This biofertilizer significantly increased Bacillus sp. abundance in the soil by 157.12%, resulting in the removal of 27.52% of water-soluble Hg fractions through volatilization and adsorption mechanisms. The variation in bioavailable Hg in the soil significantly reduced the total Hg concentration in P. giganteum young leaves, old leaves, stems, and roots of P. giganteum by 74.14%, 48.08%, 93.72%, and 50.91%, respectively (p < 0.05), which is lower than the Chinese feed safety standard (100 ng g-1). The biofertilizer inhibitory potential was highly consistent with that of the selenium-modified activated clay. Biofertilizers significantly reduced the methylmercury concentration in various P. giganteum tissues (p < 0.05), whereas selenium-modified activated clay failed to achieve a comparable effect. This biofertilizer-assisted planting pattern can achieve an economic income quadruple that of the rice planting pattern in the Hg-contaminated paddy fields. Because of its significant environmental and financial applications, the biofertilizer-assisted planting pattern is expected to replace Hg-contaminated paddy fields.

Zinc and cadmium change the metabolic activities and vegetable cellulose degradation of Bacillus cellulasensis in vegetable soils

Bacillus cellulasensis Zn-B isolated from vegetable soil was highly adaptable to Zinc (Zn) and Cadmium (Cd). Cd, but not Zn, adversely affected the total protein spectrum and functional groups of Bacillus cellulasensis Zn-B. Up to 31 metabolic pathways and 216 metabolites of Bacillus cellulasensis Zn-B were significantly changed by Zn and Cd (Zn&Cd). Some metabolic pathways and metabolites related to functional groups of sulfhydryl (-SH) and amine (-NH-) metabolism were enhanced by Zn&Cd addition. The cellulase activity of Bacillus cellulasensis Zn-B was up to 8.58 U mL^-1, increased to 10.77 U mL^-1 in Bacillus cellulasensis Zn-B + 300 mg L^-1 Zn, and maintained at 6.13 U mL^-1 in Bacillus cellulasensis Zn-B + 50 mg L^-1 Cd. The vegetables' cellulose content was decreased by 25.05-52.37% and 40.28-70.70% under the action of Bacillus cellulasensis Zn-B and Bacillus cellulasensis Zn-B + 300 mg L^-1 Zn. Those results demonstrated that Zn could significantly enhance cellulase activity and biodegradability of Bacillus cellulasensis Zn-B to vegetable cellulose. Bacillus cellulasensis Zn-B can survive in vegetable soil accumulated with Zn&Cd. The tolerance concentration and adsorption capacity of Bacillus cellulasensis Zn-B to Zn were up to 300 mg L^-1 and 56.85%, indicating that Bacillus cellulasensis Zn-B acting as a thermostability biological agent had an essential advantage in accelerating the degradation of discarded vegetables by Zn and were beneficial to maintain organic matter content of vegetable soil.

Remediation of Mercury-Polluted Farmland Soils: A Review

Mercury (Hg) bioaccumulation in Hg-polluted farmlands poses high health risk for humans and wildlife, and remediation work is urgently needed. Here, we first summarize some specific findings related to the environmental process of Hg in Hg-polluted farmlands, and distinguish the main achievements and deficiencies of available remediation strategies in recent studies. Results demonstrate that farmland is a sensitive area with vibrant Hg biogeochemistry. Current remediation methods are relatively hysteretic whether in mechanism understanding or field application, and deficient for large-scale Hg-polluted farmlands in view of safety, efficiency, sustainability, and cost-effectiveness. New perspectives including environment-friendly functional materials, assisted phytoremediation and agronomic regulations are worthy of further study as their key roles in reducing Hg exposure risk and protecting agricultural sustainability.

Application of biochar-immobilized Bacillus sp. KSB7 to enhance the phytoremediation of PAHs and heavy metals in a coking plant

The co-existence of heavy metals and polycyclic aromatic hydrocarbons (PAHs) challenges the remediation of polluted soil. This study aimed to investigate whether a combined amendment of biochar-immobilized bacterium (BM) could enhance the phytoremediation of heavy metals and PAHs in co-contaminated soil. The Bacillus sp. KSB7 with the capabilities of plant-growth promotion, metal tolerance, and PAH degradation was immobilized on the peanut shell biochar prepared at 400 °C and 600 °C (PBM4 and PBM6, respectively). After 90 days, PBM4 treatment increased the removal of PAHs by 94.17% and decreased the amounts of diethylenetriamine pentaacetic acid-extractable Zn, Pb, Cr, and Cu by 58.46%, 53.42%, 84.94%, and 83.15%, respectively, compared with Kochia scoparia-alone treatment. Meanwhile, PBM4 was more effective in promoting K. scoparia growth and reducing the uptake of co-contaminants. The abundance of Gram-negative PAH-degrader and 1-aminocyclopropane-1-carboxylic deaminase-producing bacteria within rhizosphere soil was significantly improved after PBM4 treatment. Moreover, the relative abundance of the Bacillus genus increased by 0.66 and 2.05 times under PBM4 treatment compared with biochar alone and KSB7, indicating that KSB7 could colonize in the rhizosphere soil of K. scoparia. However, the removal of PAHs and heavy metals after PBM6 and 600 °C biochar-alone treatments caused no obvious difference. This study suggested that low-temperature BM-amended plant cultivation would be an effective approach to remove PAHs and heavy metals in co-contaminated soil.

Characterization of soil microbial community activity and structure for reducing available Cd by rice straw biochar and Bacillus cereus RC-1

The combination of biochar and specific bacteria has been widely applied to remediate Cadmium-contaminated soil. But little is known about how such composites affect the dynamic distribution of metal fractions. This process is accompanied by the alternations of soil properties and microbial community structures. Composite of rice straw biochar and Bacillus cereus RC-1 were applied to investigate its impacts on Cd alleviation and soil microbial diversity and structure. The bacterial/biochar composite treatment decreased the fraction of HOAc-extractable Cd by 38.82%, and increased residual Cd by 23.95% compared to the untreated control. Moreover, compared with the untreated control, the composite treatment significantly increased the soil pH by about 1.5 units, and the activities of catalase, urease and invertase enzymes were increased by 42.39%, 30.50% and 31.20%, respectively. Composite treatment increased soil bacterial and fungal alpha diversity, the relative abundance of Bacillus, Streptomyces, Arthrobacter, and Aspergillus species were also increased. Mantel test and correlation analysis indicated that the effects associated with fungal communities in influencing soil properties were lower than that those of bacterial communities by different treatment. Aggregated boosted tree (ABT) models analysis showed that soil chemical proprieties (as determined by SOM, CEC, AN, etc.,) contributed over 50% of the changes in bacterial and fungal communities by the composite treatment. The co-occurrence network results showed that all treatments enhanced the correlation between OUT groups and improved the possible relationships in the bacterial and fungal communities, especially the interrelationships between bacteria and fungi after the Cd fractions stabilized. These findings provide a new insight to optimal strategies for the remediation of Cd-contaminated soil.

Microbial-assisted soil chromium immobilization through zinc and iron-enriched rice husk biochar

Soil chromium toxicity usually caused by the tannery effluent compromises the environment and causes serious health hazards. The microbial role in strengthening biochar for its soil chromium immobilization remains largely unknown. Hence, this study evaluated the effectiveness of zinc and iron-enriched rice husk biochar (ZnBC and FeBC) with microbial combinations to facilitate the chromium immobilization in sandy loam soil. We performed morphological and molecular characterization of fungal [Trichoderma harzianum (F1), Trichoderma viride (F2)] and bacterial [Pseudomonas fluorescence (B1), Bacillus subtilis (B2)] species before their application as soil ameliorants. There were twenty-five treatments having ZnBC and FeBC @ 1.5 and 3% inoculated with bacterial and fungal isolates parallel to wastewater in triplicates. The soil analyses were conducted in three intervals each after 20, 30, and 40 days. The combination of FeBC 3%+F2 reduced the soil DTPA-extractable chromium by 96.8% after 40 days of incubation (DAI) relative to wastewater. Similarly, 92.81% reduction in chromium concentration was achieved through ZnBC 3%+B1 after 40 DAI compared to wastewater. Under the respective treatments, soil Cr(VI) retention trend increased with time such as 40 > 30 > 20 DAI. Langmuir adsorption isotherm verified the highest chromium adsorption capacity (41.6 mg g⁻¹) with FeBC 3% at 40 DAI. Likewise, principal component analysis (PCA) and heat map disclosed electrical conductivity-chromium positive, while cation exchange capacity-chromium and pH-organic matter negative correlations. PCA suggested the ZnBC-bacterial while FeBC-fungal combinations as effective Cr(VI) immobilizers with >70% data variance at 40 DAI. Overall, the study showed that microbes + ZnBC/FeBC resulted in low pH, high OM, and CEC, which ultimately played a role in maximum Cr(VI) adsorption from wastewater applied to the soil. The study also revealed the interrelation and alternations in soil dynamics with pollution control treatments. Based on primitive soil characteristics such as soil metal concentration, its acidity, and alkalinity, the selection criteria can be set for treatments application to regulate the soil properties. Additionally, FeBC with Trichoderma viride should be tested on the field scale to remediate the Cr(VI) toxicity.

Bacillus subtilis Impact on Plant Growth, Soil Health and Environment: Dr. Jekyll and Mr. Hyde

The increased dependence of farmers on chemical fertilizers poses a risk to soil fertility and ecosystem stability. Plant growth-promoting rhizobacteria (PGPR) are at the forefront of sustainable agriculture, providing multiple benefits for the enhancement of crop production and soil health. Bacillus subtilis is a common PGPR in soil that plays a key role in conferring biotic and abiotic stress tolerance to plants by induced systemic resistance (ISR), biofilm formation, and lipopeptide production. As a part of bioremediating technologies, Bacillus spp. can purify metal contaminated soil. It acts as a potent denitrifying agent in agroecosystems while improving the carbon sequestration process when applied in a regulated concentration. Although it harbors several antibiotic resistance genes (ARGs), it can reduce the horizontal transfer of ARGs during manure composting by modifying the genetic makeup of existing microbiota. In some instances, it affects the beneficial microbes of the rhizosphere. External inoculation of B. subtilis has both positive and negative impacts on the endophytic and semi-synthetic microbial community. Soil texture, type, pH, and bacterial concentration play a crucial role in the regulation of all these processes. Soil amendments and microbial consortia of Bacillus produced by microbial engineering could be used to lessen the negative effect on soil microbial diversity. The complex plant-microbe interactions could be decoded using transcriptomics, proteomics, metabolomics, and epigenomics strategies which would be beneficial for both crop productivity and the well-being of soil microbiota. Bacillus subtilis has more positive attributes similar to the character of Dr. Jekyll and some negative attributes on plant growth, soil health, and the environment akin to the character of Mr. Hyde.

Bioaugmentation and bioaugmentation-assisted phytoremediation of heavy metal contaminated soil by a synergistic effect of cyanobacteria inoculation, biochar, and purslane (Portulaca oleracea L.)

In recent decades, soil contamination with heavy metals has become an environmental crisis due to their long-term stability and adverse biological effects. Therefore, bioremediation is an eco-friendly technology to remediate contaminated soil, which the efficiency requires further research. This study was designed to comparatively investigate two strategies: bioaugmentation by using a cyanobacterial species (Oscillatoria sp.) and bioaugmentation-assisted phytoremediation by using Oscillatoria sp. and purslane (Portulaca oleracea L.) for the bioremediation of soil contaminated by heavy metals (Cr (III), Cr (VI), Fe, Al, and Zn). Various quantities of biochar (0.5, 2, and 5% (w/w)) were used as an amendment in the experiments to facilitate the remediation process. The results of the bioaugmentation test showed that applying biochar and cyanobacteria into contaminated soil significantly increased the chlorophyll a, nitrogen, and organic carbon contents. In contrast, the extractable fractions of Cr (III), Cr (VI), Zn, Al, and Fe declined compared with those of the control treatment. The highest reduction content (up to 87 %) in the extractable portion was obtained for Cr (VI). The development of longer root and hypocotyl lengths and vigour index from lettuces and radish seeds grown in the remediated soil confirmed the success of remediation treatments. Moreover, the findings of the bioaugmentation-assisted phytoremediation test displayed a reduction in the bioavailable fraction of Cr (III), Cr (VI), Zn, Al, and Fe. Cr (III) presented the highest reduction (up to 90 %) in metal bioavailability. With cyanobacteria inoculation and biochar addition, the shoot and root lengths of purslane grew 4.6 and 3-fold while the heavy metal accumulation decreased significantly. Besides, these treatments enhanced the tolerance index (TI) quantities of purslane whereas diminished its bioaccumulation coefficient (BAC) and bioconcentration factor (BCF) values. For all heavy metals (except Zn), translocation factor (TF) and BAC values were found to be less than 1.0 at all treatments, indicating the successful phytoextraction by the purslane. These results suggest that the purslane can be considered an excellent phytoextracting agent for soils contaminated with heavy metals.

Ecological responses of bacterial assembly and functions to steep Cd gradient in a typical Cd-contaminated farmland ecosystem

The response of soil bacterial communities from farmland ecosystems to cadmium (Cd) pollution, in which a steep concentration gradient of more than 100 mg/kg has naturally formed, has not previously been fully reported. In this study, a field investigation was conducted in a typical severe Cd-polluted farmland ecosystem, and the bacterial community response to the steep Cd gradient was analyzed. The results showed that Cd concentration sharply decreased from 159.2 mg/kg to 4.18 mg/kg among four sampling sites alongside an irrigation canal over a distance of 150 m. Bacterial diversity and richness were significantly lower in highly polluted sites, and random forest analysis indicated that Cd gradient played a decisive role in reducing alpha diversity. Redundancy analysis (RDA) and co-occurrence network indicated that the synergistic effects of pH, Cd, and phosphorus were the main drivers shaping community structure. The functional results predicted by BugBase suggested that the bacterial community may adapt to the harsh environment by recruiting Cd-resistant microbes and improving oxidative stress tolerance of the whole community. Cd-resistant microorganisms such as Burkholderia, Bradyrhizobium, and Sulfurifustis, which directly or indirectly participate in diminishing oxidative damage of Cd, may play essential roles in maintaining community stability and might be potential bacterial resources for the bioremediation of Cd pollution.

ACC deaminase containing endophytic bacteria ameliorate salt stress in Pisum sativum through reduced oxidative damage and induction of antioxidative defense systems

Approximately 6% of the world's total land area and 20% of the irrigated land are affected by salt stress. Egypt is one such country affected by salt-stress problems. This paper focuses on the role of isolated bacteria, such as Bacillus subtilis and Pseudomonas fluorescens, in alleviating the harmful effects of salt stress. The results show that the irrigation of plants with different concentrations of saline water (0, 75, and 150 mM NaCl) leads to significantly decreased growth criteria, photosynthetic pigments (i.e., chl a, chl b, and carotenoids), and membrane stability index (MSI) values. Moreover, malondialdehyde (MDA), glutathione content, endogenous proline, the antioxidant defense system, 1-aminocyclopropane-1-carboxylic acid (ACC) content, ACC synthase (ACS), ACC oxidase (ACO), and Na+ content were significantly increased under NaCl-stress exposure. On the contrary, treatment with endophytic bacteria significantly increased the resistance of pea plants to salt stress by increasing the enzymatic antioxidant defenses (i.e., superoxide dismutase, catalase, peroxidase, and glutathione reductase), non-enzymatic antioxidant defenses (i.e., glutathione), osmolyte substances such as proline, and antioxidant enzyme gene expression. As a result, endophytic bacteria's use was significantly higher compared to control values for indole-3-acetic acid (IAA), gibberellic acid GA3, MSI, and photosynthetic pigments. The use of endophytic bacteria significantly decreased Na+ accumulation while, at the same time, promoting K+ uptake. In conclusion, the induction of endophytic bacterium-induced salt tolerance in pea plants depends primarily on the effect of endophytic bacteria on osmoregulation, the antioxidant capacity, and ion uptake adjustment by limiting the uptake of Na+ and, alternatively, increasing the accumulation of K+ in plant tissue.

Effects of magnetic biochar-microbe composite on Cd remediation and microbial responses in paddy soil

There is growing global interest in the bioremediation of cadmium (Cd) using combinations of biochar and microorganisms. However, the interactions among biochar, introduced and indigenous microorganisms remain unclear. Accordingly, a 90 day microcosm experiment was conducted to investigate this by adding Bacillus sp. K1 strain inoculated rice straw biochar (SBB) and magnetic straw biochar (MBB) into a Cd contaminated paddy soil from Hunan, China. All treatments were incubated aerobically (60% water holding capacity) or anaerobically for 90 d. During both soil incubations, Bacillus sp. K1 successfully colonized in soil with composites applications. Soil pH was significantly increased from acid to neutral, and available Cd decreased with the addition of both composites. The better remediation efficiency of MBB than SBB under anerobic conditions was attributed to the transformation of acetic acid-extractable Cd into the residual fraction, caused by Cd²⁺ bonding with crystal Fe₃O₄. The application of the two kinds of composites caused similar changes to both microbial communities. There was a slight decrease in indigenous microbial alpha diversity with the MBB aerobic application, while the total population number of bacteria was increased by 700%. Both the redundancy analysis and Mantel analyses indicated that pH and microbial biomass C contributed to the colonization of Bacillus sp. K1 with SBB under aerobic conditions, and with MBB under anerobic conditions, respectively. The research provides a new insight into interactive effects and investigates immobilization mechanisms involved of bacterial/biochar composites in anaerobic and aerobic soils.

Effects of natural organic matter on cadmium mobility in paddy soil: A review

Cadmium (Cd) contamination in paddy soil has caused public concern. The uptake of Cd by rice plants depends on soil Cd mobility, which is in turn substantially influenced by organic matter (OM). In this review, we first summarize the fate of Cd in soil and the role of OM. We then focus on the effects of OM on Cd mobility in paddy soil and the factors influencing the remedial effectiveness of OM amendments. We further discuss the performance of straw incorporation in the remediation of Cd-contaminated paddy soils reported in laboratory and field studies. Considering the huge production of organic materials (such as straw) in agriculture, the use of natural OM for soil remediation has obvious appeal due to the environmental benefits and low cost. Although there have been successful application cases, the properties of OM amendments and soil can significantly affect the remedial performance of the OM amendments. Importantly, straw incorporation alone does not often decrease the mobility of Cd in soil or the Cd content in rice grains. Careful evaluation is required when considering natural OM amendments, and the factors and mechanisms that influence their remedial effectiveness need further investigation in paddy soil with realistic Cd concentrations.

Application of mixed bacteria-loaded biochar to enhance uranium and cadmium immobilization in a co-contaminated soil

This research explored the effect of biochar pyrolyzed from five different materials on U and Cd immobilization in soil. The results showed that all biochars improved the soil properties and microbial metabolic activities, and effectively immobilized U and Cd, especially corn stalk biochar. Subsequently, three strains Bacillus subtilis, Bacillus cereus, and Citrobacter sp. were mixed in a 3:3:2 proportion as a kind of mixed bacteria (MB9) that could adsorb U and Cd effectively. Two types of MB9-loaded biochar were synthesized by physical adsorption and sodium alginate embed method and referred to as AIB and EIB, respectively. MB9-loaded biochar showed superior U and Cd immobilization performance. At 75 d, the highest reduction in the DTPA- extractable U and Cd (69 % and 56 %) was achieved with the 3% AIB amendment. Additionally, compared to the addition of biochar or MB9 alone, AIB was more effective in promoting celery growth and reducing U and Cd accumulation. Finally, the microbial community structure analysis suggested that the relative abundance of Citrobacter genus and Bacillus genus was significantly increased, suggesting that the mixed bacteria MB9 was successfully colonized. These findings may provide a feasible technology for green and cost-effective remediation of heavy metal contamination in farmland soil.

Effects of compaction on lead availability in contaminated soils with contrasting texture

The effects of soil compaction on porosity (α), bulk density (ρs), and saturated hydraulic conductivity (Ksat) can create a physical barrier in the soil, reducing the vertical movement of toxic elements in the soil profile. However, the indirect effects of compaction in altering the forms and availability of heavy metals in soil have not been well-studied. This study examined the influence of compaction on forms of lead (Pb) in soils with contrasting texture. Four levels of compaction were imposed on a sandy loam and a clayey soil, which were artificially contaminated based on their maximum Pb adsorption capacity. Compaction had different effects on Pb forms depending on soil texture. In the sandy loam soil, compaction had a dual beneficial effect in mitigating the impact of Pb contamination, since it decreased Ksat, reducing metal transport to deeper soil layers, and also prevented transformation to more available Pb forms (soluble and exchangeable). Instead, there was an increase in the most environmentally stable forms of Pb (inner sphere adsorption on iron and manganese oxides). In the clayey soil, compaction caused a significant increase in soluble and exchangeable Pb, accompanied by a significant reduction in environmentally stable Pb (inner sphere adsorption on gibbsite and kaolinite). In addition, studies about Pb contents under compacted soil layers should be investigated, mainly in clayey soils with edible crops, and environmental remediation practices that involve the machines traffic (for example, phytoremediation-successive cultivation of Pb-hyperaccumulating plants) should be used with care to minimise the compaction of clayey soils.

Reduction mechanism of Cd accumulation in rice grain by Chinese milk vetch residue: Insight into microbial community

Chinese milk vetch is an efficient approach to reduce Cd accumulation in rice, nevertheless, its reduction mechanism is not well understood. In this study, we investigated the rice grain Cd, soil properties and microbial community in a Cd-polluted paddy field amended with milk vetch residue (MV) or without (CK) during rice growth period. We found that milk vetch residue averagely decreased the Cd content in rice grain by 45%. Decrease of Cd in rice mainly attributed to the inhibition of Cd activation by milk vetch residue at heading stage probably by the formation of HA-Cd (Humic Acid) and CdS. Increased pH and organic matter (OM) promoted the reduction of available Cd. In addition, nonmetric multidimensional scaling (NMDS) analysis revealed that microbial community structure was significantly different between MV and CK treatment (r = 0.187, p = 0.002), and the core functions of differentially abundant genera were mainly associated with N-cycling, organic matter degradation and sulfate-reducing. The application of milk vetch residue increased the abundance of sulfate-reducing bacteria (SRB) by 8-112% during the rice growth period, which may involve in promoting the transformation of Cd to a more stably residual Cd (CdS). Canonical correspondence analysis (CCA) and mantel test analysis indicated that available K (p = 0.004) and available N (p = 0.005) were the key environmental factors of shaping the SRB. Altogether, changes in soil properties affected microbial structure and functional characteristics, especially the response of SRB in MV treatment would provide valuable insights into reducing the bioavailability of Cd in soil.

An overview on heavy metal resistant microorganisms for simultaneous treatment of multiple chemical pollutants at co-contaminated sites, and their multipurpose application

Anthropogenic imbalance of chemical pollutants in environment raises serious threat to all life forms. Contaminated sites often possess multiple heavy metals and other types of pollutants. Elimination of chemical pollutants at co-contaminated sites is imperative for the safe ecosystem functions, and simultaneous removal approach is an attractive scheme for their remediation. Different conventional techniques have been applied as concomitant treatment solution but fall short at various parameters. In parallel, use of microorganisms offers an innovative, cost effective and ecofriendly approach for simultaneous treatment of various chemical pollutants. However, microbiostasis due to harmful effects of heavy metals or other contaminants is a serious bottleneck facing remediation practices in co-contaminated sites. But certain microorganisms have unique mechanisms to resist heavy metals, and can act on different noxious wastes. Considering this significant, my review provides information on different heavy metal resistant microorganisms for bioremediation of different chemical pollutants, and other assistance. In this favour, the integrated approach of simultaneous treatment of multiple heavy metals and other environmental contaminants using different heavy metal resistant microorganisms is summarized. Further, the discussion also intends toward the use of heavy metal resistant microorganisms associated with industrial and environmental applications, and healthcare. PREFACE: Simultaneous treatment of multiple chemical pollutants using microorganisms is relatively a new approach. Therefore, this subject was not well received for review before. Also, multipurpose application of heavy metal microorganisms has certainly not considered for review. In this regard, this review attempts to gather information on recent progress on studies on different heavy metal resistant microorganisms for their potential of treatment of co-contaminated sites, and multipurpose application.

Bioremediation of toxic heavy metals (THMs) contaminated sites: concepts, applications and challenges

Heavy metal contamination is a global issue, where the prevalent contaminants are arsenic (As), cadmium (Cd), chromium (Cr)(VI), mercury (Hg), and lead (Pb). More often, they are collectively known as "most problematic heavy metals" and "toxic heavy metals" (THMs). Their treatment through a variety of biological processes is one of the prime interests in remediation studies, where heavy metal-microbe interaction approaches receive high interest for their cost effective and ecofriendly solutions. In this review, we provide an up to date information on different microbial processes (bioremediation) for the removal of THMs. For the same, emphasis is put on oxidation-reduction, biomineralization, bioprecipitation, bioleaching, biosurfactant technology, biovolatilization, biosorption, bioaccumulation, and microbe-assisted phytoremediation with their selective advantages and disadvantages. Further, the literature briefly discusses about the various setups of cleaning processes of THMs in environment under ex situ and in situ applications. Lately, the study sheds light on the manipulation of microorganisms through genetic engineering and nanotechnology for their advanced treatment approaches.

Bioremediation of cadmium polluted soil using a novel cadmium immobilizing plant growth promotion strain Bacillus sp. TZ5 loaded on biochar

Bioremediation of cadmium polluted soil using biochar (BC) and plant growth promotion bacteria (PGPB) have been widely concerned. In our study, a novel Cd immobilizing PGPB strain TZ5 was isolated based on the Cd immobilizing potential and plant growth promotion (PGP) traits. Further, changes of surface morphology and functional groups of TZ5 cells were observed after exposed to Cd²⁺ by SEM-EDS and FTIR analyses. Then, the strain TZ5 was successfully loaded on BC as biochemical composites material (BCM). Pot experiment indicated that the percentage of acetic acid-extractable Cd in BCM treatments significantly decreased by 11.34 % than control. Meanwhile, BCM significantly increased the dry weight of ryegrass by 77.78 %, and decreased the Cd concentration of ryegrass by 48.49 %, compared to control. Microbial counts and soil enzyme activities in rhizosphere were both significantly improved by BCM. Furthermore, the proportion of relative abundance of Bacillus genus was enhanced after treated by BCM, which indicated that the strain TZ5 was successfully colonized in the rhizosphere. This study provided a practical strategy for bioremediation of Cd contaminated soil.

Lead Toxicity: Health Hazards, Influence on Food Chain, and Sustainable Remediation Approaches

Lead (Pb) toxicity has been a subject of interest for environmental scientists due to its toxic effect on plants, animals, and humans. An increase in several Pb related industrial activities and use of Pb containing products such as agrochemicals, oil and paint, mining, etc. can lead to Pb contamination in the environment and thereby, can enter the food chain. Being one of the most toxic heavy metals, Pb ingestion via the food chain has proven to be a potential health hazard for plants and humans. The current review aims to summarize the research updates on Pb toxicity and its effects on plants, soil, and human health. Relevant literature from the past 20 years encompassing comprehensive details on Pb toxicity has been considered with key issues such as i) Pb bioavailability in soil, ii) Pb biomagnification, and iii) Pb- remediation, which has been addressed in detail through physical, chemical, and biological lenses. In the review, among different Pb-remediation approaches, we have highlighted certain advanced approaches such as microbial assisted phytoremediation which could possibly minimize the Pb load from the resources in a sustainable manner and would be a viable option to ensure a safe food production system.

Ecological influences of the migration of micro resin particles from crushed waste printed circuit boards on the dumping soil

About 0.8 million tons of resin particles, which were generated from the recovery of waste printed circuit boards, were dumped on soil at Qingyuan city of China. Resin particles not only belong to micro plastic but also contain brominated flame retardants and heavy metals. There is little information about soil pollution caused by the dumped resin particles. This study found resin particles would transfer from soil surface into soil at least 10 mm downward for six months. Average content of bromine in soil within 10 cm exceeded 2500 mg/kg. The highest content of Pb, Zn, and Cu was 3450, 1143 and 1450 mg/kg, which were approximately 6.9, 2.3 and 3.6 times as much as Grade Ⅲ soil standard of China. Micro plastic, brominated flame retardants, and heavy metals made significant effects on soil bacterial community. Bacterial diversity was destroyed and the number of resistant bacteria increased obviously such as Acinetobacter, Pseudomonas and Paracoccus. This paper presented the ecological destroy of soil when the resin particles were deposited on soil surface. It also suggested the government to urgently manage the resin particles produced in the recovery of waste printed circuit boards.

Isolation and Characterization of Biosurfactant-Producing Bacteria From Oil Well Batteries With Antimicrobial Activities Against Food-Borne and Plant Pathogens

Microbial biosurfactants, produced by fungi, yeast, and bacteria, are surface-active compounds with emulsifying properties that have a number of known activities, including the solubilization of microbial biofilms. In an on-going survey to uncover new or enhanced antimicrobial metabolite-producing microbes from harsh environments, such as oil-rich niches, 123 bacterial strains were isolated from three oil batteries in the region of Chauvin, Alberta, and characterized by 16S rRNA gene sequencing. Based on their nucleotide sequences, the strains are associated with 3 phyla (Actinobacteria, Proteobacteria and Firmicutes), as well as 17 other discrete genera that shared high homology with known sequences, with the majority of these strains identified to the species level. The most prevalent strains associated with the three oil wells belonged to the Bacillus genus. Thirty-four of the 123 strains were identified as biosurfactant-producers, among which Bacillus methylotrophicus strain OB9 exhibited the highest biosurfactant activity based on multiple screening methods and a comparative analysis with the commercially available biosurfactant, Tween 20. B. methylotrophicus OB9 was selected for further antimicrobial analysis and addition of live cultures of B. methylotrophicus OB9 (or partially purified biosurfactant fractions thereof) were highly effective on biofilm disruption in agar diffusion assays against several Gram-negative food-borne bacteria and plant pathogens. Upon co-culturing with B. methylotrophicus OB9, the number of either Salmonella enterica subsp. enterica Newport SL1 or Xanthomonas campestris B07.007 cells significantly decreased after 6 h and were not retrieved from co-cultures following 12 h exposure. These results also translated to studies on plants, where bacterized tomato seedlings with OB9 significantly protected the tomato leaves from Salmonella enterica Newport SL1 contamination, as evidenced by a 40% reduction of log10 CFU of Salmonella/mg leaf tissue compared to non-bacterized tomato leaves. When B. methylotrophicus 0B9 was used for bacterized lettuce, the growth of X. campestris B07.007, the causal agent of bacterial leaf spot of lettuce, was completely inhibited. While limited, these studies are noteworthy as they demonstrate the inhibition spectrum of B. methylotrophicus 0B9 against both human and plant pathogens; thereby making this bacterium attractive for agricultural and food safety applications in a climate where microbial-biofilm persistence is an increasing problem.

Bioremediation of cadmium-contaminated paddy soil using an autotrophic and heterotrophic mixture

Cadmium (Cd) pollution poses a serious risk to human health and ecological security. Bioremediation can be a promising and effective remediation technology for treating Cd contaminated soils. In this study, seven heterotrophic strains were isolated from Cd contaminated soil and 7 autotrophic strains were isolated from acid mine drainage. Cd removal efficiencies were compared after leaching with autotrophic bacteria (Att-sys), heterotrophic isolates (Htt-sys) and cooperative leaching systems (Co-sys) in laboratory agitating reactors. The results indicated that Cd removal efficiency of Co-sys (32.09%) was significantly higher than that of Att-sys (23.24%) and Htt-sys (0.74%). By analyzing the soil microbial community in different bioleaching systems, we found that the addition of heterotrophic isolates significantly promoted the growth of some heavy metal resistant inhabitants (Massilia, Alicyclobacillus, Micromonospora, etc.), and Co-sys had a minor effect on the growth of soil indigenous microbes. In Co-sys, the content of the four Cd fractions all decreased compared with other leaching systems. The analysis of soil physicochemical parameters during the leaching process showed that pH and ORP (oxidation reduction potential) were not the only determinants for Cd removal efficiency in Co-sys, synergistic metabolic activities of autotrophic and heterotrophic strains may be other determinants. This study demonstrated that cooperative bioremediation may prove to be a safe and efficient technique for field application in heavy metal soil pollution.

Bioremediation can be a promising and effective remediation technology for treating Cd contaminated soils. Cooperative bioremediation using heterotrophic and autotrophic mixtures proved to be an efficient, short-term bioremediation strategy for heavy metal contaminated soil.

Biochemical traits of Bacillus subtilis MF497446: Its implications on the development of cowpea under cadmium stress and ensuring food safety

The present study aimed at assessment of different application methods of Bacillus subtilis MF497446 to induce development of cowpea ensuring food safety under cadmium (Cd) stress. Also, isolation, plant growth promoting (PGP) traits and 16 S rRNA-based identification of Bacillus subtilis MF497446 is documented. Out of 24 Bacillus isolates (AS1-AS24), only four isolates (AS4, AS12, AS14 and AS22) showed greater Cd tolerance up to 18 mg L^-1. The greatest PGP traits under Cd stress were displayed by Bacillus isolate (AS12); which, also, enhanced seedling elongation and vigor index of cowpea under Cd stress. Phylogenetic analysis, based on 16 S rRNA, confirmed that this promising Bacillus isolate (AS12) belongs to Bacillus subtilis and is referred to as B. subtilis MF497446. Treatment of inoculation+soaking for 90 min of cowpea seeds by B. subtilis MF497446 resulted in the best development of cowpea plants under Cd stress (up to 9 mg kg^-1); as fresh and dry masses of cowpea increased from 6.80 to 1.54 to 12.35 and 2.59 g plant^-1, respectively. Moreover, shoot and root lengths were 19.66 and 28.33 cm when cowpea seeds were treated by B. subtilis MF497446 (inoculation+soaking for 90 min) compared to 11.33 and 10.66 cm, respectively, for control (Cd stress only). Application of B. subtilis MF497446 (as inoculation+soaking for 90 min) reduced Cd accumulation and bioconcentration factor in cowpea plants by 29.2 and 28.9%, respectively, compared to control (Cd stress only). These results clearly reveal that applying of B. subtilis MF497446 to crops grown on Cd-contaminated soil enhances plant growth and eliminates (or at least diminishes) the risks to human health ensuring food safety.

Effects of Bacillus thuringiensis HC-2 Combined with Biochar on the Growth and Cd and Pb Accumulation of Radish in a Heavy Metal-Contaminated Farmland under Field Conditions

The objective of this study was to explore the effect of heavy metal-resistant bacteria and biochar (BC) on reducing heavy metal accumulation in vegetables and the underlying mechanism. We tested Bacillus thuringiensis HC-2, BC, and BC+HC-2 for their ability to immobilize Cd and Pb in culture solution. We also studied the effects of these treatments on the dry weight and Cd and Pb uptake of radish in metal-contaminated soils under field conditions and the underlying mechanism. Treatment with HC-2, BC, and BC+HC-2 significantly reduced the water-soluble Cd (34-56%) and Pb (31-54%) concentrations and increased the pH and NH4+ concentration in solution compared with their vales in a control. These treatments significantly increased the dry weight of radish roots (18.4-22.8%) and leaves (37.8-39.9%) and decreased Cd (28-94%) and Pb (22-63%) content in the radish roots compared with the control. Treatment with HC-2, BC, and BC+HC-2 also significantly increased the pH, organic matter content, NH4+ content, and NH4+/NO3- ratio of rhizosphere soils, and decreased the DTPA-extractable Cd (37-58%) and Pb (26-42%) contents in rhizosphere soils of radish. Furthermore, BC+HC-2 had higher ability than the other two treatments to protect radish against Cd and Pb toxicity and increased radish biomass. Therefore, Bacillus thuringiensis HC-2 combined with biochar can ensure vegetable safety in situ for the bioremediation of heavy metal-polluted farmland.

An overview of field-scale studies on remediation of soil contaminated with heavy metals and metalloids: Technical progress over the last decade

Soil contamination by heavy metals and metalloids has been a major concern to human health and environmental quality. While many remediation technologies have been tested at the bench scale, there have been only limited reports at the field scale. This paper aimed to provide a comprehensive overview on the field applications of various soil remediation technologies performed over the last decade or so. Under the general categories of physical, chemical, and biological approaches, ten remediation techniques were critically reviewed. The technical feasibility and economic effectiveness were evaluated, and the pros and cons were appraised. In addition, attention was placed to the environmental impacts of the remediation practices and long-term stability of the contaminants, which should be taken into account in the establishment of remediation goals and environmental criteria. Moreover, key knowledge gaps and practical challenges are identified.

Effects of Bacillus subtilis and nanohydroxyapatite on the metal accumulation and microbial diversity of rapeseed (Brassica campestris L.) for the remediation of cadmium-contaminated soil

This study investigated the effects of the co-application of Bacillus subtilis and nanohydroxyapatite (NHAP) on plant growth, soil cadmium (Cd) dynamics, and the microbiological characteristics (such as enzyme activity and bacterial species richness) of the rhizosphere soil. Rapeseed was used as a model plant in pot experiments. Different concentrations of B. subtilis and 0.5% NHAP were applied alone and in combination to Cd-contaminated soil. The Cd contents in soils and plants as well as the rhizospheric microorganism diversity were assessed. The addition of B. subtilis or NHAP alone increased the soil Cd content and decreased the plant Cd content, while their co-application more effectively increased the soil and plant Cd contents than either treatment alone. B. subtilis and NHAP reduced the plant Cd content by 43.15-57.04% compared with that in the control. Rhizosphere community richness and bacterial diversity were significantly increased after co-application of B. subtilis and NHAP. Co-application of B. subtilis and NHAP effectively promoted rapeseed growth and improved Cd-contaminated soil remediation.

Evaluation of the potential of pelletized biomass from different municipal solid wastes for use as solid fuel

Four different municipal solid wastes (dog manure, horse manure, apple pomace waste and tea waste) and an industrial by-product (NovoGro) were used to produce solid fuel pellets. The mixtures followed a raw material to NovoGro ratio of 50:1. The pellets diameters varied between 4 and 5 mm, and the average length was 20 mm. The dog manure, horse manure, apple pomace waste and tea waste pellets were denoted as DN, HN, AN and TN, respectively. The combustion characteristics of the pelletized fuels were investigated, such as total moisture, ash content, calorific value and ash fusion point, etc. The physicochemical properties were analyzed by using a number of analytical techniques including X-ray fluorescence spectrometry (XRF), X-ray diffraction spectrometry (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The results of the mechanical, thermal and morphological properties show that the raw materials were effectively combined with the NovoGro binder; furthermore, the DN, HN and TN pellets exhibited excellent mechanical and thermal properties, including high calorific values (>16.30 MJ/kg), high resistance to mechanical shock (>99%), high volatile matter contents, optimal softening temperatures and optimal ash contents. However, the high K, Ca, and Si contents of the AN can form low-melting-point eutectics, which can cause slagging. Moreover, the AN materials had large particle sizes, and high cellulose and hemicellulose contents led to high total moistures, low softening temperatures and low calorific values. The AN was not suitable for use as a fuel. The results suggested that NG is an effective binder for pelletization of biomass and showed the feasibility of using municipal solid wastes for energy production.

Recent advances in conventional and contemporary methods for remediation of heavy metal-contaminated soils

Remediation of heavy metal-contaminated soils has been drawing our attention toward it for quite some time now and a need for developing new methods toward reclamation has come up as the need of the hour. Conventional methods of heavy metal-contaminated soil remediation have been in use for decades and have shown great results, but they have their own setbacks. The chemical and physical techniques when used singularly generally generate by-products (toxic sludge or pollutants) and are not cost-effective, while the biological process is very slow and time-consuming. Hence to overcome them, an amalgamation of two or more techniques is being used. In view of the facts, new methods of biosorption, nanoremediation as well as microbial fuel cell techniques have been developed, which utilize the metabolic activities of microorganisms for bioremediation purpose. These are cost-effective and efficient methods of remediation, which are now becoming an integral part of all environmental and bioresource technology. In this contribution, we have highlighted various augmentations in physical, chemical, and biological methods for the remediation of heavy metal-contaminated soils, weighing up their pros and cons. Further, we have discussed the amalgamation of the above techniques such as physiochemical and physiobiological methods with recent literature for the removal of heavy metals from the contaminated soils. These combinations have showed synergetic effects with a many fold increase in removal efficiency of heavy metals along with economic feasibility.

Optimization of copper, lead and cadmium biosorption onto newly isolated bacterium using a Box-Behnken design

Due to the progressive development of industrial and technological activities, heavy metal contamination is increasing each year and it poses a serious health and environmental risk. Microorganisms are capable of removing heavy metals from a contaminated environment. In this work, 51 microbial strains were isolated from heavy metal contaminated water and soil. The JAW1 strain, identified as Pseudomonas azotoformans, was selected and applied in bioremediation of the specific mixture of metals (Cd, Cu, and Pb) in an aqueous medium. The Box-Behnken design was used to optimize the biosorption process, with three factors: pH, initial metal concentration, concentration of the biosorbent. For the strain P. azotoformans JAW1, the optimal conditions were pH = 6.0, 25mg/L of each metal and 2g/L, following removal levels were achieved: Cd 44,67%; Cu 63,32%; Pb 78,23%. The possible interactions of cell-metal ions were evaluated using FT-IR analysis. The study indicated the presence of groups, which may be responsible for bonding of metal ions. The studies conducted on bioremediation mechanisms indicated that metal accumulation could occur on the cell surface (biosorption) where the amount of adsorbed metals reached: Cd 98,57%, Cu 69,76%, Pb 88,58%. P. azotoformans JAW1 exhibited a potential for application in the bioremediation of mining wastewater with complex metal contaminations.

Adsorption of heavy metals from aqueous solution by UV-mutant Bacillus subtilis loaded on biochars derived from different stock materials

Two kinds of biochars, one derived from corn straw (CBC) and one from pig manure (PBC), were used as the carriers of a bacterium (B38) to adsorb heavy metals in solution. CBC exhibited high affinity to Hg(II), while PBC showed large adsorption capacity of Pb(II). After loading with B38, the sorption capacity of the co-sorbents were enhanced for Pb(II), but weakened for Hg(II). In a binary system, the overall adsorption capacity to Hg-Pb (CBC+B38, 136.7mg/g; PBC+B38, 181.3mg/g) on co-sorbents was equal to the sum of the single-component values for Hg(II) and Pb(II). Electrostatic interactions and precipitation are the major mechanisms in the adsorption of Hg(II). In contrast, cation-π interactions and precipitation were involved in the sorption process of Pb(II). Moreover, the sorption sites of Hg(II) and Pb(II) partially overlapped on the biochar surface, but were different on co-sorbents. Hence, the co-sorbents have an advantage over the biochar alone in the removal of heavy metal mixtures.

Enhanced cadmium phytoremediation of Glycine max L. through bioaugmentation of cadmium-resistant bacteria assisted by biostimulation

This study examined the potential of three strains of cadmium-resistant bacteria, including Micrococcus sp., Pseudomonas sp. and Arthrobacter sp., to promote root elongation of Glycine max L. seedlings, soil cadmium solubility and cadmium phytoremediation in G. max L. planted in soil highly polluted with cadmium with and without nutrient biostimulation. Micrococcus sp. promoted root length in G. max L. seedlings under toxic cadmium conditions. Soil inoculation with Arthrobacter sp. increased the bioavailable fraction of soil cadmium, particularly in soil amended with a C:N ratio of 20:1. Pot culture experiments observed that the highest plant growth was in Micrococcus sp.-inoculated plants with nutrient biostimulation. Cadmium accumulation in the roots, stems and leaves of G. max L. was significantly enhanced by Arthrobacter sp. with nutrient biostimulation. A combined use of G. max L. and Arthrobacter sp. with nutrient biostimulation accelerated cadmium phytoremediation. In addition, cadmium was retained in roots more than in stems and leaves and G. max L. had the lowest translocation factor at all growth stages, suggesting that G. max L. is a phytostabilizing plant. We concluded that biostimulation-assisted bioaugmentation is an important strategy for improving cadmium phytoremediation efficiency.

Evaluation of biochars from different stock materials as carriers of bacterial strain for remediation of heavy metal-contaminated soil

Two kinds of biochars, one derived from corn straw and one from pig manure, were studied as carriers of a mutant genotype from Bacillus subtilis (B38) for heavy metal contaminated soil remediation. After amendment with biochar, the heavy metal bioavailability decreased. Moreover, the heavy metal immobilization ability of the biochar was enhanced by combining it with B38. The simultaneous application of B38 and pig manure-derived biochar exhibited a superior effect on the promotion of plant growth and the immobilization of heavy metals in soil. The plant biomass increased by 37.9% and heavy metal concentrations in the edible part of lettuce decreased by 69.9–96.1%. The polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) profiles revealed that pig manure-derived biochar could enhance the proliferation of both exotic B38 and native microbes. These results suggest that B38 carried by pig manure-derived biochar may be a promising candidate for the remediation of soils contaminated by multiple heavy metals.

Encapsulated Pseudomonas putida for phenol biodegradation: Use of a structural membrane for construction of a well-organized confined particle

Phenols are toxic byproducts from a wide range of industry sectors. If not treated, they form effluents that are very hazardous to the environment. This study presents the use of a Pseudomonas putida F1 culture encapsulated within a confined environment particle as an efficient technique for phenol biodegradation. The innovative encapsulation technique method, named the "Small Bioreactor Platform" (SBP) technology, enables the use of a microfiltration membrane constructed as a physical barrier for creating a confined environment for the encapsulated culture. The phenol biodegradation rate of the encapsulated culture was compared to its suspended state in order to evaluate the effectiveness of the encapsulation technique for phenol biodegradation. A maximal phenol biodegradation rate (q) of 2.12/d was exhibited by encapsulated P. putida at an initial phenol concentration of 100 mg/L. The biodegradation rate decreased significantly at lower and higher initial phenol concentrations of 50 and up to 3000 mg/L, reaching a rate of 0.1018/d. The results also indicate similar and up to double the degradation rate between the two bacterial states (encapsulated vs. suspended). High resolution scanning electron microscopy images of the SBP capsule's membrane morphology demonstrated a highly porous microfiltration membrane. These results, together with the long-term activity of the SBP capsules and verification that the culture remains pure after 60 days using 16S rRNA gene phylogenetic affiliation tests, provide evidence for a successful application of this new encapsulation technique for bioaugmentation of selected microbial cultures in water treatment processes.

Application of phosphate solubilizing bacteria in immobilization of Pb and Cd in soil

In the present study, heavy metal (HM)-tolerant phosphate solubilizing bacteria (PSB) were isolated and their performance during the remediation of Pb and Cd in contaminated soil was studied. A total of 16 bacterial strains and one consortium were isolated, and the consortium had the highest phosphate solubilizing ability and HM tolerance. Great variations between the Fourier transform infrared (FTIR) spectra of consortium cells before and after adsorption of Pb²⁺ and Cd²⁺ revealed that amide I/amide II bonds and carboxyl on the cell surface were involved in binding of metal ions. High-throughput sequencing technique revealed that the consortium was composed of Enterobacter spp., Bacillus spp., and Lactococcus spp. The consortium was added into contaminated soil, and its potential ability in dissolution of phosphate from Ca₃(PO₄)₂ and subsequent immobilization of HMs was tested. Results showed that when Ca₃(PO₄)₂ was applied at 10.60 mg/g soil, PSB addition significantly increased soil available phosphate content from 12.28 to 17.30 mg/kg, thereby enhancing the immobilization rate of Pb and Cd from 69.95 to 80.76% and from 28.38 to 30.81%, respectively. Microcalorimetric analysis revealed that PSB addition significantly improved soil microbial activity, which was possibly related with the decreased HMs availability and the nutrient effect of the solubilized phosphate. The present study can provide a cost-effective and environmental-friendly strategy to remediate multiple HM-contaminated soils.

Selection of Zygosaccharomyces rouxii strains resistant to cadmium with improved removal abilities through ultraviolet-diethyl sulfate cooperative mutagenesis

Cd2+ resistance and bioaccumulation capacity were selected from parental Zygosaccharomyces rouxii (CRZ-0) while maintaining NaCl tolerance using protoplast mutagenesis technology. Ultraviolet-diethyl sulfate (UV-DES) cooperative mutagenesis, followed by preliminary screening and rescreening, was used to select the mutant strain CRZ-9. CRZ-9 grew better than CRZ-0 in YPD medium with 20 or 50 mg L-1 of Cd2+. Scanning electron microscopy observations and flow cytometry tests indicated that CRZ-9 was more effective at eliminating reactive oxygen species (ROS) generated by Cd2+, which led to less cellular structural damage and lower lethality. Furthermore, compared with CRZ-0, CRZ-9 exhibited increased potential for application with higher Cd2+ removal ratio, wider working pH range, and lower biomass dosage in Cd2+ bioaccumulation. The mutant strain CRZ-9 possessed improved Cd2+ resistance and bioaccumulation capacity and therefore is a promising strain to remove Cd2+ from wastewater.

Cd immobilization and reduced tissue Cd accumulation of rice (Oryza sativa wuyun-23) in the presence of heavy metal-resistant bacteria

Two metal-resistant Bacillus megaterium H3 and Neorhizobium huautlense T1-17 were investigated for their immobilization of Cd in solution and tissue Cd accumulation of rice (Oryza sativa wuyun-23) in the Cd-contaminated soil. Strains H3 and T1-17 decreased 79-96% of water-soluble Cd in solution and increased grain biomass in the high Cd-contaminated soil. Inoculation with H3 and T1-17 significantly decreased the root (ranging from 25% to 58%), above-ground tissue (ranging from 13% to 34%), and polished rice (ranging from 45% to 72%) Cd contents as well as Cd bioconcentration factor of the rice compared to the controls. Furthermore, H3 and T1-17 significantly reduced the exchangeable Cd content of the rhizosphere soils compared with the controls. Notably, strain T1-17 had significantly higher ability to reduce Cd bioconcentration factor and polished rice Cd uptake than strain H3. The results demonstrated that H3 and T1-17 decreased the tissue (especially polished rice) Cd uptake by decreasing Cd availability in soil and Cd bioconcentration factor and the effect on the reduced polished rice Cd uptake was dependent on the strains. The results may provide an effective synergistic bioremediation of Cd-contaminated soils in the bacteria and rice plants and bacterial-assisted safe production of rice in Cd-contaminated soils.

Enhanced bioremediation of lead-contaminated soil by Solanum nigrum L. with Mucor circinelloides

Strain selected from mine tailings in Anshan for Pb bioremediation was characterized at the genetic level by internal transcribed spacer (ITS) sequencing. Results revealed that the strain belongs to Mucor circinelloides. Bioremediation of lead-contaminated soil was conducted using Solanum nigrum L. combined with M. circinelloides. The removal efficacy was in the order microbial/phytoremediation > phytoremediation > microbial remediation > control. The bioremediation rates were 58.6, 47.2, and 40.2% in microbial/phytoremediation, microbial remediation, and phytoremediation groups, respectively. Inoculating soil with M. circinelloides enhanced Pb removal and S. nigrum L. growth. The bioaccumulation factor (BF, 1.43), enrichment factor (EF, 1.56), and translocation factor (TF, 1.35) were higher than unit, suggesting an efficient ability of S. nigrum L. in Pb bioremediation. Soil fertility was increased after bioremediation according to change in enzyme activities. The results indicated that inoculating S. nigrum L. with M. circinelloides enhanced its efficiency for phytoremediation of soil contaminated with Pb.

Isolation of As-tolerant bacteria and their potentials of reducing As and Cd accumulation of edible tissues of vegetables in metal(loid)-contaminated soils

In this study, three As-tolerant bacteria Ralstonia eutropha Q2-8, Rhizobium tropici Q2-13, and Exiguobacterium aurantiacum Q3-11 were isolated from the rhizosphere and bulk soils of Chinese cabbage. The strains were characterized for their production of indole-3-acetic acid (IAA) and siderophores, their effects on soil metal(loid) bioavailability and organic matter content, and their effects on the edible tissue growth and metal(loid) accumulation of Chinese cabbage and radish in the metal(loid)-contaminated soil. The strains produced IAA and siderophores and increased the edible tissue biomass (ranging from 74% to 124%) of the vegetables compared to the controls. Furthermore, strain Q2-8 reduced As contents (ranging from 22% to 50%), while strains Q2-13 and Q3-11 decreased Cd contents (ranging from 21% to 53%) of the edible tissues of the vegetables compared to the controls. Strains Q2-8, Q2-13, and Q3-11 decreased the DTPA-extractable Cd contents (ranging from 16% to 41%) and increased the organic matter contents of the rhizosphere soils compared to the controls. The results showed the effects of the strains on the increased edible tissue growth and reduced As and Cd uptake of the edible tissues and highlighted the possibility to develop a new bacterial-assisted technique for reduced metal(loid) uptake of vegetables in the metal(loid)-contaminated soils.

Synergistic effects of plant growth-promoting Neorhizobium huautlense T1-17 and immobilizers on the growth and heavy metal accumulation of edible tissues of hot pepper

A plant growth-promoting Neorhizobium huautlense T1-17 was evaluated for its immobilization of Cd and Pb in solution. Meanwhile, the impacts of T1-17, immobilizers (vermiculite and peat) and their combination on the fruit biomass and heavy metal accumulation of hot pepper were characterized. T1-17 could significantly reduced water-soluble Cd and Pb in solution. T1-17, vermiculite+T1-17, peat, and peat+T1-17 significantly increased the fruit biomass (ranging from 45% to 269%) and decreased the fruit Cd (ranging from 66% to 87%) and Pb (ranging from 30% to 56%) contents and water-soluble Cd and Pb (ranging from 23% to 59%) contents of the rhizosphere soils compared to the controls. T1-17+vermiculite or peat had higher ability to increase the fruit biomass than T1-17 or vermiculite or peat. Furthermore, T1-17+peat had higher ability to reduce the water-soluble Cd and Pb contents of the rhizosphere soil and the fruit Pb uptake of hot pepper. The results showed that T1-17 and the immobilizers alleviated the heavy metal toxicity and decreased the fruit heavy metal uptake of hot pepper. The results also showed the synergistic effects of T1-17 and the immobilizers on the growth and Cd and Pb accumulation of hot pepper.

Bio-remediation of acephate-Pb(II) compound contaminants by Bacillus subtilis FZUL-33

Removal of Pb(2+) and biodegradation of organophosphorus have been both widely investigated respectively. However, bio-remediation of both Pb(2+) and organophosphorus still remains largely unexplored. Bacillus subtilis FZUL-33, which was isolated from the sediment of a lake, possesses the capability for both biomineralization of Pb(2+) and biodegradation of acephate. In the present study, both Pb(2+) and acephate were simultaneously removed via biodegradation and biomineralization in aqueous solutions. Batch experiments were conducted to study the influence of pH, interaction time and Pb(2+) concentration on the process of removal of Pb(2+). At the temperature of 25°C, the maximum removal of Pb(2+) by B.subtilis FZUL-33 was 381.31±11.46mg/g under the conditions of pH5.5, initial Pb(2+) concentration of 1300mg/L, and contact time of 10min. Batch experiments were conducted to study the influence of acephate on removal of Pb(2+) and the influence of Pb(2+) on biodegradation of acephate by B.subtilis FZUL-33. In the mixed system of acephate-Pb(2+), the results show that biodegradation of acephate by B.subtilis FZUL-33 released PO4(3+), which promotes mineralization of Pb(2+). The process of biodegradation of acephate was affected slightly when the concentration of Pb(2+) was below 100mg/L. Based on the results, it can be inferred that the B.subtilis FZUL-33 plays a significant role in bio-remediation of organophosphorus-heavy metal compound contamination.