Combination of bioleaching by gross bacterial biosurfactants and flocculation: A potential remediation for the heavy metal contaminated soils

Alleviation of cadmium uptake in rice (Oryza sativa L.) by iron plaque on the root surface generated by Providencia manganoxydans via Fe(II) oxidation

Iron plaque is believed to be effective in reducing the accumulation of heavy metals in rice. In this work, a known soil-derived Mn(II)-oxidizing bacterium, LLDRA6, which represents the type strain of Providencia manganoxydans, was employed to investigate the feasibility of decreasing cadmium (Cd) accumulation in rice by promoting the formation of iron plaque on the root surface. Firstly, the Fe(II) oxidation ability of LLDRA6 was evaluated using various techniques including Fourier Transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, phenanthroline photometry, and FeS gel-stabilized gradient assays. Subsequently, the formation of iron plaque on the root surface by LLDRA6 was investigated under hydroponic and pot conditions. Finally, Cd concentrations were examined in rice with and without iron plaque through pot and paddy-field tests. The results showed that LLDRA6 played an efficient role in the formation of iron plaque on seedling roots under hydroponic conditions, generating 44.87 and 36.72 g kg- 1 of iron plaque on the roots of Huazhan and TP309, respectively. In pot experiments, LLDRA6 produced iron plaque exclusively in the presence of Fe(II). Otherwise, it solely generated biofilm on the root surface. Together with Fe(II), LLDRA6 effectively reduced the concentrations of Cd in Huazhan roots, straws and grains by 25%, 46% and 44%, respectively. This combination also demonstrated a significant decrease in the Cd concentrations of TP309 roots, straws and grains by 20%, 52% and 44%, respectively. The data from the Cd translocation factor indicate that obstruction of Cd translocation by iron plaque predominantly occurred during the root-to-straw stage. In paddy-field tests, the Cd concentrations of grains harvested from the combination treatment of LLDRA6 and Fe(II) exhibited a decline ranging from 40 to 53%, which fell below the maximum acceptable value for Cd in rice grains (0.2 mg kg- 1) as per the China national standard for food security (GB2762-2017). Meanwhile, the relevant phenotypic traits regarding the yield were not adversely affected. These findings have demonstrated that LLDRA6 can impede the uptake of Cd by rice in Cd-contaminated soils through the formation of iron plaque on roots, thus providing a promising safe Cd-barrier for rice production.

Synthesis of surface-modified porous polysulfides from soybean oil by inverse vulcanization and its sorption behavior for Pb(II), Cu(II), and Cr(III)

Guided by efficient utilization of natural plant oil and sulfur as low-cost sorbents, it is desired to tailor the porosity and composition of polysulfides to achieve their optimal applications in the management of aquatic heavy metal pollution. In this study, polysulfides derived from soybean oil and sulfur (PSSs) with improved porosity (10.2-22.9 m2/g) and surface oxygen content (3.1-7.0 wt.%) were prepared with respect to reaction time of 60 min, reaction temperature of 170 °C, and mass ratios of sulfur/soybean oil/NaCl/sodium citrate of 1:1:3:2. The sorption behaviors of PSSs under various hydrochemical conditions such as contact time, pH, ionic strength, coexisting cations and anions, temperature were systematically investigated. PSSs presented a fast sorption kinetic (5.0 h) and obviously improved maximum sorption capacities for Pb(II) (180.5 mg/g), Cu(II) (49.4 mg/g), and Cr(III) (37.0 mg/g) at pH 5.0 and T 298 K, in comparison with polymers made without NaCl/sodium citrate. This study provided a valuable reference for the facile preparation of functional polysulfides as well as a meaningful option for the removal of aquatic heavy metals.

DNA-SIP delineates unique microbial communities in the rhizosphere of the hyperaccumulator Sedum alfredii which are beneficial to Cd phytoextraction

Rhizo-microbe recruited by hyperaccumulating plants are crucial for the extraction of metals from contaminated soils. It is important, but difficult, to identify the specific rhizosphere microbes of hyperaccumulators shaped by root exudation. Continuous 13CO2 labeling, microbial DNA-based stable isotope probing (DNA-SIP), and high throughput sequencing were applied to identify those rhizosphere microorganisms using exudates from the Cd hyperaccumulator Sedum alfredii. In contrast to its non-hyperaccumulating ecotype (NAE), the hyperaccumulating ecotype (HAE) of S. alfredii strongly changed the rhizosphere environment and extracted a 5-fold higher concentration of Cd from contaminated soil. Although both HAE and NAE harbored Streptomyces, Massilia, Bacillus, and WPS-2 Uncultured Bacteria with relative abundance of more than 1% in the rhizosphere associated with plant growth and immunity, the HAE rhizosphere specifically recruited Rhodanobacter (2.66%), Nocardioides (1.16%), and Burkholderia (1.01%) through exudates to benefit the extraction of Cd from soil. Different from the bacterial network with weak cooperation in the NAE rhizosphere, a closed-loop bacterial network shaped by exudates was established in the HAE rhizosphere to synergistically resist Cd. This research reveals a specific rhizosphere bacterial community induced by exudates assisted in the extraction of Cd by S. alfredii and provides a new perspective for plant regulation of the rhizo-microbe community beneficial for optimizing phytoremediation.

TEMPO oxidized cellulose nanocrystal (TOCNC) scaffolded nanoscale zero-valent iron (nZVI) for enhanced chromium removal

The conventional carboxymethyl cellulose (CMC) stabilization hampered available active sites of adsorption and reduction, due to irregular shape of nanoscale zero-valent iron (nZVI) particles with augmented average size and passivated surface, leading to insufficient removal and poor resistance against complex environmental conditions. Herein, we presented (2,2,6,6-Tetramethylpiperidine-1-oxyl)-mediated (TEMPO-mediated) oxidation of cellulose nanocrystal (TOCNC) scaffolded nZVI (nZVI@TOCNC) with enhanced efficiency for chromium removal in comparison with CMC stabilized nZVI (nZVI@CMC). The anchoring of nZVI at the functional sites of TOCNC was initiated by liquid-phase chemical reduction method. The nZVI@TOCNC showed improved nZVI distribution with uniform particle size and thinner shell (∼1 nm). Characterizations using FT-IR, XPS and XRD demonstrated that bindings between TOCNC and nZVI were through hydrogen bonds, electrostatic attractions, coordination-covalent bonds and bidentate chelation. TOCNC with shorter branch-chain (-COC-) surrounding the nZVI could potentially form a porous and compact "mesh" to rigidly encapsulate nZVI, while CMC wrapped around nZVI in the way of traditional polymeric stabilizers. Thus, 0.5 g/L nZVI@TOCNC achieved 99.96% Cr (Ⅵ) removal efficiency (20 mg/L) at pH = 7 and the removal capacity were up to 55.86 mg/g. The nZVI@TOCNC consistently presented higher removal efficiency than nZVI@CMC under wide pH range (3-7). Cr (Ⅵ) was reduced to Cr (Ⅲ) by nZVI@TOCNC with deposition of CrxFe1-x (OH)3 and Cr2O3. The predominant mechanisms of removal probably consisted of electrostatic attractions, reduction, co-precipitation and surface complexation. The pseudo-second-order kinetic model well-fitted the sorption kinetic, indicating TOCNC scaffold stabilized nZVI for efficient reduction of Cr (Ⅵ) through multi-layer adsorption. As a template and delivery carrier, TOCNC shows promising potential to further improve the capability and practice of nZVI for in situ treatment of industrial waste water with heavy metal pollution.

Rhizosphere interface microbiome reassembly by arbuscular mycorrhizal fungi weakens cadmium migration dynamics

The prevalence of cadmium (Cd)-polluted agricultural soils is increasing globally, and arbuscular mycorrhizal fungi (AMF) can reduce the absorption of heavy metals by plants and improve mineral nutrition. However, the immobilization of the rhizosphere on cadmium is often overlooked. In this study, Glomus mosseae and Medicago sativa were established as symbiotes, and Cd migration and environmental properties in the rhizosphere were analyzed. AMF reduced Cd migration, and Cd2+ changed to an organic-bound state. AMF symbiosis treatment and Cd exposure resulted in microbial community variation, exhibiting a distinct deterministic process (|βNTI| > 2), which ultimately resulted in a core microbiome function of heavy metal resistance and nutrient cycling. AMF increased available N and P, extracellular enzyme activity (LaC, LiP, and CAT), organic matter content (TOC, EOC, and GRSP), and Eh of the rhizosphere soil, significantly correlating with decreased Cd migration (p < 0.05). Furthermore, AMF significantly affected root metabolism by upregulating 739 metabolites, with flavonoids being the main factor causing microbiome variation. The structural equation model and variance partial analysis revealed that the superposition of the root metabolites, microbial, and soil exhibited the maximum explanation rate for Cd migration reduction (42.4%), and the microbial model had the highest single explanation rate (15.5%). Thus, the AMF in the rhizosphere microenvironment can regulate metabolite-soil-microbial interactions, reducing Cd migration. In summary, the study provides a new scientific explanation for how AMF improves plant Cd tolerance and offers a sustainable solution that could benefit both the environment and human health.

Fabrication of electrospun cellulose/chitosan/ball-milled bone char membranes for efficient and selective sorption of Pb(II) from aqueous solutions

Separation materials have received increasing attention given their broad applications in the management of environmental pollution. It is desired to balance the contradiction between high separation efficiency and selectivity of separation materials. The integration of ball-milled bone chars with electrospun membranes might achieve this balance. In this study, electrospun cellulose/chitosan/ball-milled bone char (CL/CS/MB) membranes were by well-dispersing ball-milled bone chars with nanoscale size (98.9-167.5 nm) and developed porosity (40.2-373.1 m2/g) in the electrospinning solvent. The synergistic integration of distributed MBs (5.4-31.5 wt.% of loading hydroxyapatite on the membrane matrix) allowed the efficient sorption of Pb(II) with fast kinetics (20.0 min), excellent capacity (219.9 mg/g at pH 5.0, T 298 K), and favorable selectivity coefficients (2.76-6.79). The formation of minerals was dominant for the selective sorption of Pb(II) by combining the spectral analysis and quantitative determination. The surface complexation with O-/reductive N-species, the cation exchange with inorganic Ca2+, the electrostatic attraction with deprotonated O-, and the cation-π coordination with the aromatic carbon via the π-electrons should be not ignored for the capture of Pb(II). This work demonstrated the feasibility of electrospun CL/CS/MB membranes as a promising candidate for the remediation of aquatic pollutants.

Rhizospheric bacteria: the key to sustainable heavy metal detoxification strategies

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

Biological and green remediation of heavy metal contaminated water and soils: A state-of-the-art review

Contamination of the natural ecosystem by heavy metals, organic pollutants, and hazardous waste severely impacts on health and survival of humans, animals, plants, and microorganisms. Diverse chemical and physical treatments are employed in many countries, however, the acceptance of these treatments are usually poor because of taking longer time, high cost, and ineffectiveness in contaminated areas with a very high level of metal contents. Bioremediation is an eco-friendly and efficient method of reclaiming contaminated soils and waters with heavy metals through biological mechanisms using potential microorganisms and plant species. Considering the high efficacy, low cost, and abundant availability of biological materials, particularly bacteria, algae, yeasts, and fungi, either in natural or genetically engineered (GE) form, bioremediation is receiving high attention for heavy metal removal. This report comprehensively reviews and critically discusses the biological and green remediation tactics, contemporary technological advances, and their principal applications either in-situ or ex-situ for the remediation of heavy metal contamination in soil and water. A modified PRISMA review protocol is adapted to critically assess the existing research gaps in heavy metals remediation using green and biological drivers. This study pioneers a schematic illustration of the underlying mechanisms of heavy metal bioremediation. Precisely, it pinpoints the research bottleneck during its real-world application as a low-cost and sustainable technology.

A review on the clean-up technologies for heavy metal ions contaminated soil samples

The soil contamination with heavy metal ions is one of the grave intricacies faced worldwide over the last few decades by the virtue of rapid industrialization, human negligence and greed. Heavy metal ions are quite toxic even at low concentration a swell as non-biodegradable in nature. Their bioaccumulation in the human body leads to several chronic and persistent diseases such as lung cancer, nervous system break down, respiratory problems and renal damage etc. In addition to this, the increased concentration of these metal ions in soil, beyond the permissible limits, makes the soil unfit for further agricultural use. Hence it is our necessity, to monitor the concentration of these metal ions in the soil and water bodies and adopt some better technologies to eradicate them fully. From the literature survey, it was observed that three main types of techniques viz. physical, chemical, and biological were employed to harness the heavy metal ions from metal-polluted soil samples. The main goal of these techniques was the complete removal of the metal ions or the transformation of them into less hazardous and toxic forms. Further the selection of the remediation technology depends upon different factors such as process feasibility/mechanism of the process applied, nature and type of contaminants, type and content of the soil, etc. In this review article, we have studied in detail all the three technologies viz. physical, chemical and biological with their sub-parts, mechanism, pictures, advantages and disadvantages.

Bioleaching and immobilizing of copper and zinc using endophytes coupled with biochar-hydroxyapatite: Bipolar remediation for heavy metals contaminated mining soils

Copper and zinc are toxic heavy metals in soils that require development of feasible strategies for remediation of contaminated soils around the mine areas. In this study, the processing conditions and mechanisms of immobilization and bioleaching for remediation of highly contaminated soils with heavy metals are investigated. Soil remediation is carried out using a bioleaching-immobilization bipolar method. The results show that LSE03 bacteria provide efficient leaching result and immobilization on Cu²⁺ and Zn²⁺. Among the bacterial metabolites, cis, cis-muconic acid and isovaleric acid play major roles in the bioleaching process. The bacterial extracellular polymeric substances are rich in a variety of organic acids that show a significant decrease in content after the adsorption process, indicating that all of these substances are involved in the binding of heavy metals. Characterization of the endophytes and immobilizing agents with FTIR, TEM-mapping, and XPS techniques reveal the ability of both bacteria and composites to adsorb Cu-Zn as well as the main functional groups of -OH, -COOH, -PO₄^3-, and -NH. According to the heavy metals species analyses, competitive adsorption experiments, and bioleaching desorption experiments, it is planned to carry out the bipolar remediation of contaminated soil through immobilization followed by bioleaching process. After bipolar remediation processing, 97.923% and 96.387% of available Cu and Zn are respectively removed. Soils fertility significantly increases in all cases. Our study provides a green, practical, and environmentally friendly treatment method for soils contaminated with high concentrations of heavy metals.

Tea saponin enhanced bioleaching of Fusarium solani to remove hexavalent chromium from soil

Hexavalent chromium pollution is one of the most serious types of site pollution. In this study, a microorganism was screened to remove most hexavalent chromium from soil by leaching in 24 h. After ITS sequencing, the microorganism was identified as belonging to the genus Fusarium solani. The optimization experiment of leaching conditions determined that the removal rate reached the maximum 80% when the rotation speed was 200 rpm, the liquid-soil ratio was 15:1, the temperature was 35℃, and the pH was 7. The study has also shown that tea saponin can effectively strengthen the leaching of Fusarium solani to remove hexavalent chromium from the soil. Compared with tea saponin, the strengthening effect of glucose and rhamnolipid was relatively small. The removal rate of hexavalent chromium reached 85% when the added amount of tea saponin was 0.02 g/mL. The leaching solution destroyed part of the iron-manganese nodule structure of the soil, and its hydroxyl, carboxyl, and other groups complexed metal ions into the solution to achieve the purpose of removing hexavalent chromium. However, since the main crystal of the soil was SiO₂, there was no obvious change in the XRD of the soil. Toxicity test showed that after leaching, the content of hexavalent chromium leached was 0.28 mg/L (< 1.5 mg/L), which meet the entry standard of the landfill site.

A review on the bioleaching of toxic metal(loid)s from contaminated soil: Insight into the mechanism of action and the role of influencing factors

The anthropogenic activities in agriculture, industrialization, mining, and metallurgy combined with the natural weathering of rocks, have led to severe contamination of soils by toxic metal(loid)s. In an attempt to remediate these polluted sites, a plethora of conventional approaches such as Solidification/Stabilization (S/S), soil washing, electrokinetic remediation, and chemical oxidation/reduction have been used for the immobilization and removal of toxic metal(loid)s in the soil. However, these conventional methods are associated with certain limitations. These limitations include high operational costs, high energy demands, post-waste disposal difficulties, and secondary pollution. Bioleaching has proven to be a promising alternative to these conventional approaches in removing toxic metal(loid)s from contaminated soil as it is cost-effective, environmentally friendly, and esthetically pleasing. The bioleaching process is influenced by factors including pH, temperature, oxygen, and carbon dioxide supply, as well as nutrients in the medium. It is crucial to monitor these parameters before and throughout the reaction since a change in any, for instance, pH during the reaction, can alter the microbial activity and, therefore, the rate of metal leaching. However, research on these influencing factors and recent innovations has brought significant progress in bioleaching over the years. This critical review, therefore, presents the current approaches to bioleaching and the mechanisms involved in removing toxic metal(loid)s from contaminated soil. We further examined and discussed the fundamental principles of various influencing factors that necessitate optimization in the bioleaching process. Additionally, the future perspectives on adding omics for bioleaching as an emerging technology are discussed.

Removal of hazardous ions from aqueous solutions: Current methods, with a focus on green ion flotation

Hazardous ions, like those of heavy metals, cause significant health and environmental problems when they are discharged into water resources naturally or through various industrial processes. Removing these ions from water is of significant importance in the provision of high-quality water for drinking and agricultural usage. This work discusses current techniques that are frequently used for the removal of heavy-metal ions from aqueous solutions by absorption, particularly the use of biodegradable surfactants in ion flotation. Certain new surfactants promise high efficiency in their use in the ion-flotation process and in their application in industrial-water treatment to remove heavy metals. As an example, this work demonstrates the high efficiency of surfactants based on an amino-acid (L-cysteine) in removing a range of heavy-metal ions in a simple, single-stage ion-flotation process. High foaming ability, the ability to operate in various temperatures and pHs, decomposing into natural products and high binding affinity for heavy-metal ions make the cysteine-based surfactants a highly suitable compound to replace current commercial surfactants in ion- and froth-flotation processes. Removal of particular ions can also be achieved in ion flotation; a suitable choice of parameters, such as pH and surfactant concentration, favours the surfactant binding to those ions. Further intensive work is required to develop an optimal process to recover valuable elements from waste solutions.

Sustainable and efficient technologies for removal and recovery of toxic and valuable metals from wastewater: Recent progress, challenges, and future perspectives

Due to their numerous effects on human health and the natural environment, water contamination with heavy metals and metalloids, caused by their extensive use in various technologies and industrial applications, continues to be a huge ecological issue that needs to be urgently tackled. Additionally, within the circular economy management framework, the recovery and recycling of metals-based waste as high value-added products (VAPs) is of great interest, owing to their high cost and the continuous depletion of their reserves and natural sources. This paper reviews the state-of-the-art technologies developed for the removal and recovery of metal pollutants from wastewater by providing an in-depth understanding of their remediation mechanisms, while analyzing and critically discussing the recent key advances regarding these treatment methods, their practical implementation and integration, as well as evaluating their advantages and remaining limitations. Herein, various treatment techniques are covered, including adsorption, reduction/oxidation, ion exchange, membrane separation technologies, solvents extraction, chemical precipitation/co-precipitation, coagulation-flocculation, flotation, and bioremediation. A particular emphasis is placed on full recovery of the captured metal pollutants in various reusable forms as metal-based VAPs, mainly as solid precipitates, which is a powerful tool that offers substantial enhancement of the remediation processes' sustainability and cost-effectiveness. At the end, we have identified some prospective research directions for future work on this topic, while presenting some recommendations that can promote sustainability and economic feasibility of the existing treatment technologies.

The combination of aerobic digestion and bioleaching for heavy metal removal from excess sludge

In this study, bioleaching is employed for removing heavy metals from excess sludge generated during municipal wastewater treatment. To avoid organic matter impact on bioleaching, aerobic digestion was performed as pretreatment of the bioleaching or accompanied with the bioleaching. The results showed that the leaching amounts of heavy metals from the process of aerobic digestion accompanied with bioleaching was 2.3 times more than that of the process of aerobic digestion followed by bioleaching. The stable-state proportions of Zn, Cu, Ni and Mn increased by 83%, 94%, 96% and 91%, respectively, in the process of aerobic digestion accompanied with bioleaching, and moreover, the reduction rate of MLSS increased by 22.7%. Although the content of ammonia nitrogen and total phosphorus in sludge decreased after bioleaching treatment, they were still much higher than the soil background value. It indicates that the treated sludge still has agricultural value. High throughput sequencing analysis showed that the relative abundance of acid-producing bacteria (Romboutsia, Clostridium, Tricibacter, and Intestinibacter) significantly increased from 0% to 28.6%, 6.9%, 3.9%, and 2.4%. The enrichment of these acidogenic bacteria was the main reason for the pH decrease, which was conducive to the removal of heavy metals from sludge.

Cadmium-tolerant bacteria: current trends and applications in agriculture

Cadmium (Cd) is considered a toxic heavy metal; nevertheless, its toxicity fluctuates for different organisms. Cadmium-tolerant bacteria (CdtB) are diverse and non-phylogenetically related. Because of their ecological importance these bacteria become particularly relevant when pollution occurs and where human health is impacted. The aim of this review is to show the significance, culturable diversity, metabolic detoxification mechanisms of CdtB and their current uses in several bioremediation processes applied to agricultural soils. Further discussion addressed the technological devices and the possible advantages of genetically modified CdtB for diagnostic purposes in the future.

A biogenic microbial biosurfactin that degrades difenoconazole fungicide with potential antimicrobial and oil displacement properties

Surfactin is a bacterial lipopeptide and an influential biosurfactant mainly known for excellent surfactant ability. The amphiphilic nature of surfactin helps it to sustain under hydrophobic and hydrophilic conditions. In this investigation, a bacterium strain (BTKU3) that produces biosurfactant were isolated from oil-contaminated soil. Based on the blue agar plate (Bap) assay, the BTKU3 strain was found to be promising for biosurfactant production. This strain was later identified as a Lysinibacillus sp. by 16S rRNA sequencing. The characteristics of extracted bacterial surfactin were evidenced by FTIR with the presence of amine, C-H, CO, CC, esters, thiocarbonyl and asymmetric aliphatic C-H stretch molecular structural groups. Further, the extracted bacterial biosurfactant material was subjected to Liquid Chromatography-Mass Spectroscopy (LCMS), and it was identified and confirmed as surfactin with an elution time of 3.1 min and m/z value of 1034. The emulsification and oil displacement tests further proved the surfactin ability with 83% of coconut oil emulsion index and 80 % oil displacement ability with diesel, respectively. Lysinibacillus sp. BTKU3 strain also proved its efficacy in the degradation of difenoconazole by utilizing a capacity of 9.1 μg ml^-1. Thus, it is inferred that the Lysinibacillus sp. BTKU3 strain plays a significant role in the production of surfactin, which positively acts as an antimicrobial agent and reduces contaminants in polluted sites.

Bioleaching for environmental remediation of toxic metals and metalloids: A review on soils, sediments, and mine tailings

Owing to industrial evolution, a huge mass of toxic metals, including Co, Cu, Cr, Mn, Ni, Pb, and Zn, and metalloids, such as As and Sb, has inevitably been released into the natural environment and accumulated in soils or sediments. Along with modern industrialization, many mineral mines have been explored and exploited to provide materials for industries. Mining industries also generate a vast amount of waste, such as mine tailings, which contain a high concentration of toxic metals and metalloids. Due to the low economic status, a majority of mine tailings are simply disposed into the surrounding environments, without any treatment. The mobilization and migration of toxic metals and metalloids from soils, sediments, and mining wastes to water systems via natural weathering processes put both the ecological system and human health at high risk. Considering both economic and environmental aspects, bioleaching is a preferable option for removing the toxic metals and metalloids because of its low cost and environmental safety. This chapter reviews the recent approaches of bioleaching for removing toxic metals and metalloids from soils, sediments, and mining wastes. The comparison between bioleaching and chemical leaching of various waste sources is also discussed in terms of efficiency and environmental safety. Additionally, the advanced perspectives of bioleaching for environmental remediation with consideration of other influencing factors are reviewed for future studies and applications.

Biosurfactant is a powerful tool for the bioremediation of heavy metals from contaminated soils

Heavy metal toxicity has become a pressing ecological problem that affects the ecosystems through bioaccumulation, representing a serious public health hazard. Many conventional strategies have been developed and applied to decontaminate and restore metal-contaminated areas. However, these conventional approaches are not very suitable and environmentally safe for heavy metal remediation because of their high operational costs, high energy requirements, post-waste disposal problems, and secondary pollutant generation. Thus, biosurfactant-based bioremediation of heavy metals is a sustainable and promising approach because of its biodegradation capability, economic effectiveness, and ecofriendly nature. Pseudomonas sp., Bacillus sp., Citrobacter freundii, and Candida tropicalis have been isolated as potential sources of biosurfactants and produce compounds such as surfactin, rhamnolipids, and sophorolipids. Owing to the severity of heavy metal pollution in certain parts of the environment, biosurfactants have garnered great interest and attention as an emerging multi-functional technology of the new century for successful removal of heavy metal pollutants. The present study describes the role of biosurfactants in the bioremediation of heavy metals from contaminated environments. Moreover, the interaction mechanism underlying biosurfactant-metal complexation and metal remediation are discussed. Based on the review of the literature, further research is warranted to elucidate the mechanistic roles and explore the structural characterization and gene regulation of biosurfactants to improve their productivity and expand their applicability in bioremediation.

Remediation of Metal/Metalloid-Polluted Soils: A Short Review

The contamination of soil by heavy metals and metalloids is a worldwide problem due to the accumulation of these compounds in the environment, endangering human health, plants, and animals. Heavy metals and metalloids are normally present in nature, but the rise of industrialization has led to concentrations higher than the admissible ones. They are non-biodegradable and toxic, even at very low concentrations. Residues accumulate in living beings and become dangerous every time they are assimilated and stored faster than they are metabolized. Thus, the potentially harmful effects are due to persistence in the environment, bioaccumulation in the organisms, and toxicity. The severity of the effect depends on the type of heavy metal or metalloid. Indeed, some heavy metals (e.g., Mn, Fe, Co, Ni) at very low concentrations are essential for living organisms, while others (e.g., Cd, Pb, and Hg) are nonessential and are toxic even in trace amounts. It is important to monitor the concentration of heavy metals and metalloids in the environment and adopt methods to remove them. For this purpose, various techniques have been developed over the years: physical remediation (e.g., washing, thermal desorption, solidification), chemical remediation (e.g., adsorption, catalysis, precipitation/solubilization, electrokinetic methods), biological remediation (e.g., biodegradation, phytoremediation, bioventing), and combined remediation (e.g., electrokinetic–microbial remediation; washing–microbial degradation). Some of these are well known and used on a large scale, while others are still at the research level. The main evaluation factors for the choice are contaminated site geology, contamination characteristics, cost, feasibility, and sustainability of the applied process, as well as the technology readiness level. This review aims to give a picture of the main techniques of heavy metal removal, also giving elements to assess their potential hazardousness due to their concentrations.

Optimization of biosurfactant production from Pseudomonas sp. CQ2 and its application for remediation of heavy metal contaminated soil

The present study was conducted to enhance the biosurfactant production yield of Pseudomonas sp. CQ2 isolated from the Chongqing oilfield (China). Besides, the capability of biosurfactant and underlying mechanism for remediation of heavy metal contaminated soil was also investigated. Our results suggested that maximum biosurfactant production (40.7 g/L) was attained at 35 °C by using soybean oil and ammonium nitrate as carbon and nitrogen sources with pH 7, rotational speed of 175 rpm and inoculation ratio of 3%). The removal efficiencies of 78.7, 65.7 and 56.9% for Cd, Cu and Pb respectively were achieved at optimized bioleaching conditions (pH: 11, soil/solution ratio: 30:1 and non-sterilized soil), comparative tests between common chemical surfactants (SDS, Tween-80) and biosurfactants demonstrated the larger removal capacity of biosurfactants. Through SEM-EDX, it was found that the granular material disappeared, the content of Cd, Cu and Pb decreased significantly, and the soil surface became smooth with hole formation after soil washing following bioleaching. ATR-FTIR results showed that the carboxyl functional groups in biosurfactants could chelate heavy metals. These results indicated that biosurfactants from Pseudomonas sp. CQ2 could effectively eliminate Cd, Cu, and Pb from soil.

Ethylenediaminetetraacetic Acid (EDTA) Mitigates the Toxic Effect of Excessive Copper Concentrations on Growth, Gaseous Exchange and Chloroplast Ultrastructure of Corchorus capsularis L. and Improves Copper Accumulation Capabilities

Copper (Cu) is an important micronutrient for a plant’s normal growth and development. However, excess amount of Cu in the soil causes many severe problems in plants—which ultimately affect crop productivity and yield. Moreover, excess of Cu contents causes oxidative damage in the plant tissues by generating excess of reactive oxygen species (ROS). The present experiment was designed to investigate the phytoextraction potential of Cu, morpho-physiological features and biochemical reaction of jute (Corchorus capsularis L.) seedlings using ethylenediaminetetraacetic acid (EDTA) of 3 mM under different Cu levels (0 (control), 50 and 100 μM) in a hydroponic nutrient solution (Hoagland). Our results showed that elevated Cu rates (50 and 100 μM) in the nutrient solution significantly reduced plant height, fresh and dry biomass, total chlorophyll content and gaseous exchange attributes in C. capsularis seedlings. As the concentration of Cu in the medium increased (50 and 100 μM), the level of malondialdehyde (MDA) and oxidative stress in C. capsularis seedlings also increased, which could have been controlled by antioxidant activity in particular plant cells. In addition, rising Cu concentration in the nutrient solution also increased Cu uptake and accumulation in roots and leaves as well as affected the ultrastructure of chloroplast of C. capsularis seedlings. The addition of EDTA to the nutrient solution significantly alleviated Cu toxicity in C. capsularis seedlings, showing a significantly increase in plant growth and biomass. MDA contents was not significantly increased in EDTA-induced plants, suggesting that this treatment was helpful in capturing ROS and thereby reducing ROS in in C. capsularis seedlings. EDTA modification with Cu, although the bioaccumulation factor in roots and leaves and translocation factor for the leaves of C. capsularis seedlings has significantly increased. These results indicate that C. capsularis has considerable potential to cope with Cu stress and is capable of removing a large quantity of Cu from the Cu-contaminated soil while using EDTA is a useful strategy to increase plant growth and biomass with Cu absorption capabilities.

Selective recovery of lead and zinc through controlling cathodic potential in a bioelectrochemically-assisted electrodeposition system

Proper treatment of mining wastewaters is critically important to minimize contamination by heavy metals contained in those wastewaters. Herein, a bioelectrochemically-assisted electrodeposition (BES-EDP) system was developed and investigated for selective removal and recovery of Pb and Zn from a mimicked smelting wastewater. It was observed that those two metals were reduced at different cathodic potentials and electrodeposition time. At a cathodic potential of -0.75 V vs. Ag/AgCl, 98.5 ± 1.4 % of Pb was recovered after 10 h of reaction while there was little Zn deposition. Increasing the cathodic potential to -1.2 V could achieve 98.7 ± 0.7 % of Zn with the electrodeposition time of 6 h. The composition of the deposits confirmed the results from solution analysis and metal oxides were also formed during metal reduction. The diffusion impedance was much higher than the charge transfer resistance, suggesting that the diffusion process was a rate limiting step for electrodeposition. The diffusion process was verified by chronoamperometry with a good fit in Cottrell equation. The electrodeposition equilibrium constant k₀ was determined as 3.76 cm s^-1. Those results have demonstrated the feasibility of using bioelectricity to assist with selective metal recovery and warrant further investigation of technologies for sustainable management of mining wastewaters.

Discovery of a novel native bacterium of Providencia sp. with high biosorption and oxidation ability of manganese for bioleaching of heavy metal contaminated soils

Heavy metal removal from contaminated soils is a long-term challenging problem important for global economics, environment, and human health. Marine and freshwater-originated Mn(II)-oxidizing bacteria are considered as the promising bioremediation agents for environmental applications. However, practical application of soil-originated Mn(II)-oxidizing bacteria remains to be developed for contaminated soil remediation. In this work, the Mn(II) biosorption/oxidation mechanism of a new soil-originated bacterium and its bioleaching efficiency of heavy metals from soils was studied in detail. First, we found, isolated and identified a new highly Mn(II)-tolerant bacterial strain Providencia sp. LLDRA6 from heavy metal-contaminated soils. Next, strain LLDRA6 demonstrated its high Mn(II) biosorption capacity in aqueous solution. Then, Mn(II) adsorption by LLDRA6 was largely proven to be a synergistic effect of (i) Mn(II) precipitation on the cell surface, (ii) oxidation of Mn(II) into BioMnO_x on the cell surface, and (iii) intracellular accumulation of insoluble MnCO₃. Finally, combination bioleaching by the bacterium of Providencia sp. LLDRA6 and its formed BioMnO_x was proposed to develop a potential environment-friendly and cost-effective technique to remediate severely heavy metal-contaminated soils. The bioleaching tests demonstrated that the combination of Providencia sp. LLDRA6 and BioMnO_x exhibited an excellent removal efficiency for heavy metals of Pb (81.72%), Cr (88.29%), Cd (90.34%), Cu (91.25%), Mn (56.13%), and Zn (59.83%) from contaminated soils, resulting in an increase of removal efficiency in the range of 1.68-26.4% compared to Providencia sp. LLDRA6 alone. Moreover, the bacterial leachate facilitated the residual fraction of metals to transform into the easily migratory fractions in soils. These findings have demonstrated that strain LLDRA6 has high adsorption ability to remove heavy metals from contaminated soils, thus providing a promising bio-adsorbent for environmental bioremediation.

Copper-induced oxidative stress, initiation of antioxidants and phytoremediation potential of flax (Linum usitatissimum L.) seedlings grown under the mixing of two different soils of China

Flax (Linum usitatissimum L.), one of the oldest cultivated crops, continues to be widely grown for oil, fiber and food. Furthermore, the plants show a metal tolerance dependent on species so is ideal for research. Present study was conducted to find out the influence of copper (Cu) toxicity on plant biomass, growth, chlorophyll content, malondialdehyde (MDA) contents, proline production, antioxidative enzymes and metal up taken by L. usitatissimum from the soil grown under mixing of Cu-contaminated soil with natural soil by 0:1 (control), 1:0, 1:1, 1:2 and 1:4. Results revealed that, high concentration of Cu in the soil affected plant growth and development by reducing plant height, plant diameter and plant fresh and dry biomass and chlorophyll contents in the leaves compared with the control. Furthermore, Cu in excess causes generation of reactive oxygen species (ROS) such as superoxide radical (O^-) and hydroxyl radicals (OH), which is manifested by high malondialdehyde (MDA) and proline contents also. The increasing activities of superoxidase dismutase (SOD) and peroxidase (POD) in the roots and leaves of L. usitatissimum are involved in the scavenging of ROS. Results also showed that L. usitatissimum also has capability to revoke large amount of Cu from the contaminated soil. As Cu concentration in the soil increases, the final uptake of Cu concentration by L. usitatissimum increases. Furthermore, the soil chemical parameters (pH, electrical conductivity and cation exchange capacity) were increasing to highest levels as the ratio of Cu concentration to the natural soil increases. Thus, Cu-contaminated soil is amended with the addition of natural soil significantly reduced plant growth and biomass, while L. usitatissimum is able to revoke large amount of Cu from the soil and could be grown as flaxseed and a potential candidate for phytoremediation of Cu.

Risk assessment of soil heavy metals associated with land use variations in the riparian zones of a typical urban river gradient

Urbanization-induced land use changes in riparian area alter soil and water regimes in complex ways, which may also affect the migration and transformation of soil heavy metals and increase the risk of release. In this study, soil samples from the riparian zone of Beiyun River, which located in the rapidly urbanized Beijing metropolis, were collected and analyzed for heavy metals (As, Cd, Cr, Cu, Mn, Ni, Pb, and Zn). Then their zoning distribution pattern along river (section 1 to section 4 from upper to low reaches) and the correlation of heavy metals between riparian soils and riverine sediments were investigated. Results showed that the average soil heavy metal concentrations of Cd, Cr, Cu and Zn in riparian zone were approximately 2.2, 1.7, 1.9 and 2.0 times higher than the background values. Sectionally, the concentrations of Cd, Ni, Pb and Zn displayed a decreasing order with section 2 > section 3 > section 4 > section 1, while the highest values of Cr and Cu were found in section 3. The concentrations of all heavy metals except Cr in artificial garden land were higher than those in other land use types, and the concentrations of Cr among five land use types were in the order of grass land > farmland > artificial garden land > forest land > forest-grass land. Generally, most of the heavy metals in the riverine sediments had higher contents than those in riparian zones, especially Cu and Zn. There was a decreasing order for the average geo-accumulation index (I_geo) of measured heavy metals in the soils of riparian zone: Zn (0.15) > Cr (0.08) > Cu (0.07) > Cd (-0.08) > As (-0.57) > Pb (-0.67) > Mn (-0.75) > Ni (-0.86), whereas they had different "high-low" orders in different land use types. The I_geo index indicated most regions of riparian zone were moderately polluted with Cd, Cr, Cu and Zn, especially in grass land and forest land. Also, Cd, Cr and Zn in riparian zone have positive relationships with the concentrations in riverine sediments. Health risk assessment showed that the contribution of ingestion HQ to HI was the highest among the three exposure pathways (ingestion, inhalation and dermal contact), and children had higher non-carcinogenic risk and carcinogenic risk index than adult. Our findings suggest that land use and soil in riparian zone should be protected and managed scientifically to control the riverine pollution and ensure human health.

Pollution, Source, and Relationship of Trace Metal(loid)s in Soil-Wheat System in Hebei Plain, Northern China

To study the complex migration and transformation of trace metal(loid)s in a soil–wheat system, 225 pairs of surface soil and wheat samples were collected from the Taihang Mountains front plain, Hebei Province, northern China. The concentrations and pools (F1, water-soluble; F2, exchangeable; F3, carbonate-bound; F4, humic acid-bound; F5, Fe–Mn oxide-bound; F6, organic matter-bound; and F7, residual) of Cu, Pb, Zn, Cr, Ni, Cd, and Hg, and the soil properties of the samples were analyzed. The sum of the F1, F2, F3, and F4 proportions of Cd was higher than that of the other trace metal(loid)s, implying that Cd has greater mobility. We found a significant correlation (p < 0.01) between pools of trace metal(loid)s and the corresponding elements in wheat and a significant correlation (p < 0.01) between pools of trace metal(loid)s and pH, cation exchange capacity, clay, and total organic carbon. The results of principle component analysis (PCA)indicated that Cr, Ni and As mainly come from natural sources and Cu, Pb, Zn, and Cd from mixed groups related to farming and industry, Hg come from the coal burning. In addition, the total target hazard quotients showed the presence of harmful levels of trace metal(loid)s in wheat.

Phytoremediation: Environmentally sustainable way for reclamation of heavy metal polluted soils

Soil contamination with toxic metals is a widespread environmental issue resulting from global industrialization within the past few years. Therefore, decontamination of heavy metal contaminated soils is very important to reduce the associated risks and for maintenance of environmental health and ecological restoration. Conventional techniques for reclamation of such soils are expensive and environmental non-friendly. Phytoremediation is an emerging technology implementing green plants to clean up the environment from contaminants and has been considered as a cost-effective and non-invasive alternative to the conventional remediation approaches. There are different types of phytoremediation including, phytostabilization, phytostimulation, phytotransformation, phytofiltration and phytoextraction, the latter being most extensively acknowledged for remediation of soils contaminated with toxic heavy metals. Recent literature is gathered to critically review the sources, hazardous effects of toxic heavy metals and environmentally sustainable phytoremediation technique for heavy metal polluted soils to offer widespread applicability of this green technology. Different strategies to enhance the bioavailability of heavy metals in the soil are also discussed shortly. It can be concluded that phytoremediation of heavy metal contaminated soils is a reliable tool and necessary for making the land resource accessible for crop production.

Seasonal and spatial contamination statuses and ecological risk of sediment cores highly contaminated by heavy metals and metalloids in the Xiangjiang River

To reveal seasonal and spatial variations of heavy metals and metalloids (HMMs) in sediment of the Xiangjiang River, a total of 24 water and 649 sediment samples were collected from six sampling stations in the Songbai section of the river which had been polluted by HMMs for 100 years. Their contamination statuses and ecological risk were determined by enrichment factor (EF), geo-accumulation index (I_geo), pollution load index (PLI), and mean probable effect concentration quotients (mPECQs) analyses. The results revealed a unique seasonal distribution of metals in the sampling stations: The highest concentrations were revealed in the dry seasons (autumn and winter) and the lowest during the wet seasons (spring and summer). It exhibited a greater seasonal variation in the estuary sediment cores (sites ME and MW) than in the cores of other sites. Moreover, the highest concentrations of the tested metals were also found in the estuary sediment cores in the dry seasons (autumn and winter). The highest vertical concentrations of Pb, Zn, Cu, Ni, As, Fe, and Mn were observed at the depths of 16-36 cm in all of the sampled sediment cores. The EF, I_geo, PLI, and mPECQs values of all samples in autumn were higher than in summer. Cd posed the highest ecological risk in all seasons, although its concentrations were lower compared to other studied elements. Our results will benefit to develop feasible sediment quality guidelines for government monitor and remediate the local sediments in the Xiangjiang River.

The agricultural use potential of the detoxified textile dyeing sludge by integrated Ultrasound/Fenton-like process: A comparative study

Enhancing industrial sludge detoxification is of scientific and practical significance in confronting urban development and stringent environmental regulations. A strategy combining ultrasound (US) with the zero-valent iron/EDTA/Air (ZEA) process was proven to be eco-friendly, being efficient in the removal of toxic compounds from textile dyeing sludge in our previous studies. In this paper, therefore, the detoxification effects of three advanced oxidation processes (US, ZEA, US/ZEA) on textile dyeing sludge were comparatively evaluated for the first time through alteration of the sludge's physico-chemical parameters (e.g., macronutrients, heavy metals, and persistent organic pollutants) and toxicity (plants and aquatic biota), by which the appropriateness of industrial sludge's agricultural use was assessed. The results showed that US led to the least alteration of the physico-chemical properties, and the treated sludge became less biodegradable, as demonstrated by XPS. With ZEA treatment, persistent organic pollutants (POPs) were degraded by oxidation, and heavy metals were more leachable, leading to effective detoxification with a relatively low sludge dose, but an excessive amount of EDTA would negatively change the fertilizing properties of the sludge. However, the integration of US and ZEA could avoid this situation, as US promoted the degradation of EDTA and POPs, thus causing the least inhibition or even a noticeable stimulation of plant growth when the sludge dosage was 7.5 t_dw/ha (recommended dosage by the latest legislation in China). Aquatic organism toxicity tests further confirmed that US/ZEA treatment realized the most significant toxicity reduction, leading to the slightest environmental disruption. This study could be instructive in providing guidance for industrial sludge management considering agricultural use.

Simultaneous adsorption of As(III), Cd(II) and Pb(II) by hybrid bio-nanocomposites of nano hydroxy ferric phosphate and hydroxy ferric sulfate particles coating on Aspergillus niger

To develop an efficient, convenient and cost-effective method to simultaneously remove pollution of As(III), Cd(II) and Pb(II) in wastewater, a strategy to fabricate hybrid bio-nanocomposites ((n-HFP + n-HFS)@An) of nano hydroxy ferric phosphate (n-HFP) and hydroxy ferric sulfate (n-HFS) particles coating on Aspergillus niger was applied. The scanning electron microscope and energy dispersive spectrum analyses showed that (n-HFP + n-HFS)@An composites had been successfully developed which well solved the self-agglomeration problem of the nano particles. Comparing to the bulk nanoparticles, the adsorption rates of the (n-HFP + n-HFS)@An composites for the three metals were promoted 145.34, 28.98 and 25.18% and reached 76.84, 73.62 and 94.31%, respectively. Similarly, the adsorption capacities for As(III), Cd(II), and Pb(II) were 162.00, 205.83 and 730.79 mg/g, respectively. Moreover, the pseudo-second-order kinetic model was more relevant to the adsorption on the three metals by (n-HFP + n-HFS)@An, and adsorbing As(III) was fitted to the Freundlich isotherm model, while the adsorption on Cd(II) or Pb(II) was related to the Langmuir isotherm model. In addition, the adsorption of Cd(II) and Pb(II) was associated with transformation of hydroxyl groups and precipitation with phosphate. As(III) was adsorbed through exchange between AsO₂^- and SO₄^2- in the (n-HFP + n-HFS)@An composites.

Discerning three novel chromate reduce and transport genes of highly efficient Pannonibacter phragmitetus BB: From genome to gene and protein

Here, Pannonibacter phragmitetus BB was investigated at genomic, genetic and protein levels to explore molecular mechanisms of chromium biotransformation, respectively. The results of Miseq sequencing uncovered that a high-qualified bacterial genome draft was achieved with 5.07 Mb in length. Three novel genes involved in chromate reduce and transport, named nitR, chrA1 and chrA2, were identified by alignment, annotation and phylogenetic tree analyses, which encode a chromate reductase (NitR) and two chromate transporters (ChrA1 and ChrA2). Reverse transcription real-time polymerase chain reaction (RT-qPCR) analyses showed that the relative quantitative transcription of the three genes as the maximum reduction rate of Cr(VI) were significantly up-regulated with the increasing initial Cr(VI) concentrations. However, at the maximum cell growth points nitR was in a low transcription level, while the transcription of chrA1 and chrA2 were hold at a relatively high level and decreased with the increasing initial Cr(VI) concentrations. The ex-situ chromate reducing activity of NitR was revealed a V_max of 34.46 µmol/min/mg enzyme and K_m of 14.55 µmol/L, suggesting feasibility of the reaction with Cr(VI) as substrate. The multiple alignment demonstrates that NitR is potentially a nicotinamide adenine dinucleotide phosphate (NADPH) dependent flavin mononucleotide (FMN) reductase of Class I chromate reductases. Our results will prompt a large-scaled bioremediation on the contaminated soils and water by Pannonibacter phragmitetus BB, taking advantage of uncovering its molecular mechanisms of chromium biotransformation.

Fabrication of Bifunctional Chitosan-Based Flocculants: Characterization, Assessment of Flocculation, and Sterilization Performance

In this study, a series of chitosan-based quaternary ammonium graft flocculants, namely chitosan-graft-poly(acrylamide and methacryloyl ethyl trimethyl ammonium chloride) [CTS-g-P(AM-DMC)], was successfully synthesized by plasma initiation, and the as-prepared [CTS-g-P(AM-DMC)] had both flocculation and sterilization functions. Various characterization techniques were used to study the structure and physicochemical properties of the chitosan-based flocculants. ¹H nuclear magnetic resonance spectroscopy (¹H NMR), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction spectroscopy (XRD), and thermogravimetric analysis/differential scanning calorimetry (TG/DSC) confirmed the successful synthesis of CTS-g-P(AM-DMC). Scanning electron microscopy (SEM) analysis exhibited that CTS-g-P(AM-DMC) contained a smooth convex and porous structure with an enormous surface area. CTS-g-P(AM-DMC) was then used to flocculate the simulated wastewater of the kaolin suspension and the Salmonella suspension. Besides external factors, such as the dosage of flocculant and pH, the effect of the internal factor graft ratio was also evaluated. The experimental results showed that CTS-g-P(AM-DMC) also revealed a strong sterilization effect, aside from the excellent flocculation effect. Moreover, the sterilization mechanism was investigated through a series of conductivity measurements and the analysis of fluorescence-based cell live/dead tests. The results indicated that CTS-g-P(AM-DMC) destroyed the cell membrane of Salmonella through its grafted quaternary ammonium salt, thereby exhibiting sterilization property.