The effect of interaction between Bacillus subtilis DBM and soil minerals on Cu(II) and Pb(II) adsorption

Plant Growth-Promoting Rhizobacteria (PGPR) Assisted Bioremediation of Heavy Metal Toxicity

Due to a variety of natural and anthropogenic processes, heavy metal toxicity of soil constitutes a substantial hazard to all living beings in the environment. The heavy metals alter the soil properties, which directly or indirectly influence the agriculture systems. Thus, plant growth-promoting rhizobacteria (PGPR)-assisted bioremediation is a promising, eco-friendly, and sustainable method for eradicating heavy metals. PGPR cleans up the heavy metal-contaminated environment using various approaches including efflux systems, siderophores and chelation, biotransformation, biosorption, bioaccumulation, precipitation, ACC deaminase activity, biodegradation, and biomineralization methods. These PGPRs have been found effective to bioremediate the heavy metal-contaminated soil through increased plant tolerance to metal stress, improved nutrient availability in soil, alteration of heavy metal pathways, and by producing some chemical compounds like siderophores and chelating ions. Many heavy metals are non-degradable; hence, another remediation approach with a broader scope of contamination removal is needed. This article also briefly emphasized the role of genetically modified PGPR strains which improve the soil's degradation rate of heavy metals. In this regard, genetic engineering, a molecular approach, could improve bioremediation efficiency and be helpful. Thus, the ability of PGPRs can aid in heavy metal bioremediation and promote a sustainable agricultural soil system.

Adsorption of Pb (II) and Cu (II) by magnetic beads loaded with xanthan gum

Green and environmentally friendly and efficient separation adsorbents have attracted much attention in the treatment of heavy metal ions wastewater. In this study, xanthan gum (XG) was supported by fly ash magnetic beads (FAMB) to prepare adsorbent XG@FAMB. The effects of XG@FAMB dosage, pH value of the solution, adsorption time, and initial Pb (II) and Cu (II) concentration on its adsorption performance for Pb (II) and Cu (II) were investigated. The results show that under the conditions of pH 6, dosage of XG@FAMB 4.0 g/L, adsorption time 120 min, and initial concentration 60 mg/L, the maximum adsorption capacity of XG@FAMB for Pb (II) and Cu (II) was 14.93 mg/g and 14.88 mg/g, respectively. The adsorption process of Pb (II) and Cu (II) by XG@FAMB could be better described by the quasi-second-order kinetic model and Langmuir isothermal adsorption model, that is, the adsorption process is monolayer adsorption controlled by chemical action. The adsorption mechanism is that Pb (II) and Cu (II) coordinate with oxygen-containing functional groups hydroxyl and carboxyl on XG@FAMB surface, accompanied by electrostatic adsorption. XG@FAMB has the advantages of environmental protection of XG and easy solid-liquid separation of FAMB, and has a good removal effect on Pb (II) and Cu (II).

Electrochemical regeneration of granular activated carbon using an AQS (9,10- anthraquinone-2-sulfonic acid)/PPy modified graphite plate cathode

In the present study, we investigate the regeneration efficiency of Rhodamine B (RhB)-saturated granular activated carbon (GAC) in an electrochemical regeneration system by using a 9,10-anthraquinone-2-sulfonic acid/polypyrrole modified graphite plate (AQS/PPy-GP) cathode. The response surface methodology based on the Box-Behnken design (RSM-BBD) approach was used to optimize regeneration parameters, whereby the optimum condition of the independent variables was as follows: applied current = 155 mA, concentration of supporting electrolyte = 0.13 M, and regeneration time = 7 h. The electrochemical regeneration system with the AQS/PPy-GP electrode achieved high regeneration efficiency and significantly reduced energy consumption. H₂O₂ concentration generated in the electrolysis system was notably increased, and the time of complete degradation of organics was shortened by 25% compared to the electrode without modification. The mechanism for RhB degradation was proposed as AQS acting as a catalyst to promote the formation of H₂O₂. The regeneration study showed that AQS/PPy-GP cathode had appreciable reusability for GAC regeneration with a regeneration efficiency of 76.6% after 8 regeneration cycles. In summary, the electrochemical regeneration based on AQS/PPy-GP cathode would have practical industrial applications in treating spent activated carbons.

Impact of physiochemical properties, microbes and biochar on bioavailability of toxic elements in the soil: a review

The pollution of toxic elements (TEs) in the ecosystem exhibits detrimental effects on the human health. In this paper, we debated remediation approaches for TEs polluted soils via immobilization methods employing numerous amendments with reverence to type of soil and metals, and amendment, immobilization competence, fundamental processes and field applicability. We argued the influence of pH, soil organic matter, textural properties, microbes, speciation and biochar on the bioavailability of TEs. All these properties of soil, microbes and biochar are imperative for effective and safe application of these methods in remediation of TEs contamination in the ecosystem. Further, the application of physiochemical properties, microbes and biochar as amendments has significant synergistic impacts not only on absorption of elements but also on diminution of toxic elements.

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.

Field study on the soil bacterial associations to combined contamination with heavy metals and organic contaminants

The understanding of soil microbial associations to combined contamination would substantially benefit the restoration of damaged ecosystems, which is currently limited at the field scale. In this study, we investigated the soil bacterial associations to combined contamination with metals (Cd, Cu, Hg, Pb, and Zn), polyaromatic hydrocarbons (PAHs), and polybrominated diphenyl ethers (PBDEs). Samples were collected from field sites under five land-use patterns with electronic waste recycling. Results showed that the contents of Cd (0.22-12.86 mg/kg), Cu (17-14,136 mg/kg), Pb (4.6-77,014 mg/kg), Hg (0.28-22 mg/kg), Zn (26-42,495 mg/kg), PAHs (4.6-1753 μg/kg), and PBDEs (1.9-1079 μg/kg) varied significantly across sites. We observed positive correlations between catalase activity and heavy metals, indicative of a resistance response to the oxidative stress induced by metals. Furthermore, the bacterial community diversity was found to be determined primarily by PBDEs, whereas acenaphthylene, available phosphorus, and 2,2',3,3',4,5,6-heptabrominated diphenyl ether (BDE-183) were the three major drivers affecting community composition. The co-occurrence network constructed for bacterial communities exposed to combined contamination was non-random with scale-free, small-world and modularity features. We further proposed functional roles of the modules including stress resistance, hydrocarbon degradation, and nutrient cycling. Overall, the findings of redundancy analysis, variation partition analysis and the co-occurrence network indicated that soil bacterial community under combined contamination cooperated to survive. Members including Rhodoplanes and Nitrospira were capable of degrading PAHs and PBDEs in various pathways, while others, including Acinetobacter, Citrobacter, and Pseudomonas, reduced the metal toxicity to the community. Our findings provide new insights into the responses of soil bacteria, particularly in terms of inter-specific relationships, under combined contamination at the field scale.

Copper bioavailability, uptake, toxicity and tolerance in plants: A comprehensive review

Copper (Cu) is an essential element for humans and plants when present in lesser amount, while in excessive amounts it exerts detrimental effects. There subsists a narrow difference amid the indispensable, positive and detrimental concentration of Cu in living system, which substantially alters with Cu speciation, and form of living organisms. Consequently, it is vital to monitor its bioavailability, speciation, exposure levels and routes in the living organisms. The ingestion of Cu-laced food crops is the key source of this heavy metal toxicity in humans. Hence, it is necessary to appraise the biogeochemical behaviour of Cu in soil-plant system with esteem to their quantity and speciation. On the basis of existing research, this appraisal traces a probable connexion midst: Cu levels, sources, chemistry, speciation and bioavailability in the soil. Besides, the functions of protein transporters in soil-plant Cu transport, and the detrimental effect of Cu on morphological, physiological and nutrient uptake in plants has also been discussed in the current manuscript. Mechanisms related to detoxification strategies like antioxidative response and generation of glutathione and phytochelatins to combat Cu-induced toxicity in plants is discussed as well. We also delimits the Cu accretion in food crops and allied health perils from soils encompassing less or high Cu quantity. Finally, an overview of various techniques involved in the reclamation and restoration of Cu-contaminated soils has been provided.

Enhanced adsorption of Pb(II) by nitrogen and phosphorus co-doped biochar derived from Camellia oleifera shells

We describe the synthesis of a series of novel nitrogen- and phosphorus-enriched biochar (activated carbon, AC) nanocomposites via the co-pyrolysis of Camellia oleifera shells (COSs) with different weight ratios of ammonium polyphosphate (APP) (w_APP: w_COSs = 1-3:1). The physicochemical characteristics of these nanocomposites (APP@ACs) were investigated via X-ray diffraction (XRD), Raman spectroscopy, N₂ adsorption/desorption analysis, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR). The results revealed that the APP@ACs exhibited richer N- and P-containing functional groups than unmodified AC. In addition, the removal performance of APP@AC-3 with respect to Pb(II) (723.6 mg g^-1) was greatly improved relative to unmodified AC (264.2 mg g^-1). Kinetic and equilibrium data followed the pseudo-second-order kinetic model and Langmuir model, respectively. The removal mechanism could be attributed to partial physisorption and predominant chemisorption. The N₂ adsorption/desorption isotherms demonstrated that pore-volume properties could be an effective physical trap for Pb(II). Furthermore, the XPS and FTIR analysis revealed that the chemical removal mechanism of the APP@ACs is surface complexation via N-containing and P-containing functional groups. These findings indicate that the co-pyrolysis of COSs and APP leads to the formation of nitrogen- and phosphorus-containing functional groups that facilitate excellent activated carbon-based (biochar) adsorption performance.

Effects of EDTA on adsorption of Cd(II) and Pb(II) by soil minerals in low-permeability layers: batch experiments and microscopic characterization

Ethylenediaminetetraacetic acid (EDTA) can serve as a washing agent in the remediation of low-permeability layers contaminated by heavy metals (HMs). Therefore, batch adsorption experiments, where pure quartz (SM1) and mineral mixtures (SM2) were used as typical soil minerals (SMs) in low-permeability layers, were implemented to explore the effects of different EDTA concentrations, pH, and exogenous chemicals on the HM-SM-EDTA adsorption system. As the EDTA concentration increased, it gradually cut down the maximum Cd adsorption capacities of SM1 and SM2 from approximately 135 to 55 mg/kg and 2660 to 1453 mg/kg; and the maximum Pb adsorption capacities of SM1 and SM2 were reduced from 660 to 306 mg/kg and 19,677 to 19,262 mg/kg, respectively. When the initial mole ratio (MR = moles of HM ions/sum of moles of HM ions and EDTA) was closer to 0.5, the effect of EDTA was more effective. Additionally, EDTA worked well at pH below 7.0 and 4.0 for Cd and Pb, respectively. Low-molecular-weight organic acids (LMWOAs) affected the system mainly by bridging, complexation, adsorption site competition, and reductive dissolution. Cu²⁺, Fe²⁺ ions could significantly increase the Cd and Pb adsorption onto SM2. Notably, there were characteristic changes in mineral particles, including attachment of EDTA and microparticles, agglomeration, connection, and smoother surfaces, making the specific surface area (SSA) decrease from 16.73 to 12.59 m²/g. All findings indicated that EDTA could effectively and economically reduce the HM adsorption capacity of SMs at the reasonable MR value, contact time, and pH; EDTA reduced the HM adsorption capacity of SMs not only by complexation with HM ions but also by decreasing SSA and blocking active sites. Hence, the acquired insight from the presented study can help to promote the remediation of contaminated low-permeability layers in groundwater.

Changes in soil microbial communities at Jinsha earthen site are associated with earthen site deterioration

BACKGROUND

Earthen sites are immobile cultural relics and an important part of cultural heritage with historical, artistic and scientific values. The deterioration of features in earthen sites result in permanent loss of cultural information, causing immeasurable damage to the study of history and culture. Most research on the deterioration of earthen sites has concentrated on physicochemical factors, and information on microbial communities in earthen sites and their relationship with the earthen site deterioration is scarce. We used high-throughput sequencing to analyze bacterial and fungal communities in soils from earthen walls with different degree of deterioration at Jinsha earthen site to characterize the microbial communities and their correlation with environmental factors, and to compare microbial community structures and the relative abundances of individual taxa associated with different degree of deterioration for identifying possible marker taxa.

RESULTS

The relative abundances of Proteobacteria and Firmicutes were higher and that of Actinobacteria lower with higher degree of deterioration. At the genus level, the relative abundances of Rubrobacter were highest in all sample groups except in the most deteriorated samples where that of Bacteroides was highest. The relative abundance of the yeast genus Candida was highest in the severely deteriorated sample group. The bacterial phylum Bacteroidetes and genus Bacteroides, and fungal class Saccharomycetes that includes Candida sp. were specific for the most deteriorated samples. For both bacteria and fungi, the differences in community composition were associated with differences in EC, moisture, pH, and the concentrations of NH₄⁺, K⁺, Mg²⁺, Ca²⁺ and SO₄^2-.

CONCLUSION

The microbial communities in soil with different degree of deterioration were distinctly different, and deterioration was accompanied with bigger changes in the bacterial than in the fungal community. In addition, the deteriorated soil contained higher concentrations of soluble salts. Potentially, the accumulation of Bacteroides and Candida plays an important role in the deterioration of earthen features. Further work is needed to conclude whether controlling the growth of the bacteria and fungi with high relative abundances in the deteriorated samples can be applied to alleviate deterioration.

The effects of carbon disulfide driven functionalization on graphene oxide for enhanced Pb(II) adsorption: Investigation of adsorption mechanism

The surface properties of graphene oxide (GO) have been identified as the key effects on the adsorption of Pb(II) from aqueous solutions in this study. This study reveals the effect of the surface reactivity of GO via Carbon Disulfide (CS₂) functionalization for Pb(II) adsorption. After successfully preparing CS₂ functionalized GO (GOCS), the specific techniques were applied to investigate Pb(II) adsorption onto GOCS. Results indicated that the new sulfur-containing functional groups incorporated onto GOCS significantly enhanced Pb(II) adsorption capacity on GOCS than that of GO, achieving an improvement of 31% in maximum adsorption capacity increasing from 292.8 to 383.4 mg g^-1. The equilibrium adsorption capacity for GOCS was 280.2 mg g^-1 having an improvement of 83.2% over that of 152.97 mg g^-1 for GO at the same initial concentration of 150 mg L^-1 under the optimal pH of 5.7. Moreover, the results of adsorption experiments showed an excellent fit to the Langmuir and Pseudo-Second-Order models indicating the monolayer and chemical adsorption, respectively. The mechanism for Pb(II) adsorption on GOCS was proposed as the coordination, electrostatic interactions, cation-pi interactions, and Lewis acid-base interactions. The regeneration study showed that GOCS had an appreciable reusability for Pb(II) adsorption with the adsorption capacity of 208.92 mg g^-1 after five regeneration cycles. In summary, GOCS has been proved to be a novel, useful, and potentially economic adsorbent for the high-efficiency removal of Pb(II) from aqueous solutions.

Acceleration and centralization of a back-diffusion process: Effects of EDTA-2Na on cadmium migration in high- and low-permeability systems

Pollutant accumulation in the low-permeability zones (LPZs) in groundwater systems is regarded as a secondary source, and its consequent back-diffusion can extend the timeframe of pump-and-treat remediation. However, the bioavailability and mobility of heavy metals and the medium characteristics can be changed during the process. This study investigated the accumulation and back-diffusion law of toxic metals and the effects of ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) on them by implementing a series of tank experiments. In these experiments, a cadmium solution was injected first, and deionized water or EDTA-2Na constantly washed the system consisting of different medium layers. The experimental results showed that the cadmium breakthrough curves had some concentration gradient reverse points where the curves fluctuated with elution by deionized water, which did not exist when EDTA-2Na was the eluent. In these scenarios, the mass of accumulated cadmium in the media before elution was large, with a value of 931 mg (153 mg/kg), when the low-permeability medium was clay. However, when EDTA-2Na was injected together with cadmium, the value dropped to 319 mg (52.3 mg/kg), greatly reducing the cadmium accumulation. Additionally, the use of EDTA-2Na as an eluent resulted in the appearance of a secondary peak in the breakthrough curve, showing that EDTA-2Na accelerated and centralized the back-diffusion. Notably, the reduced cadmium accumulation in LPZs with the elution by EDTA-2Na was partly due to a reduced adsorption capacity of the clay minerals. The above results can advance the technology related to pump-and-treat remediation.