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.

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

Heavy metals influence the population size, diversity, and metabolic activity of bacteria. In turn, bacteria can develop heavy metal resistance mechanisms, and this can be used in bioremediation of contaminated areas. The purpose of the present study was to understand how heavy metals concentration influence on diversity and distribution of heavy metal-resistant bacteria in Araca Bay, Sao Sebastiao, on the Sao Paulo coast of Brazil. The hypothesis is that activities that contribute for heavy metal disposal and the increase of metals concentrations in environment can influence in density, diversity, and distribution of heavy metal-resistant bacteria. Only 12 % of the isolated bacteria were sensitive to all of the metals tested. We observed that the highest percentage of resistant strains were in areas closest to the Sao Sebastiao channel, where port activity occurs and have bigger heavy metals concentrations. Bacterial isolated were most resistant to Cr, followed by Zn, Cd, and Cu. Few strains resisted to Cd levels greater than 200 mg L−1. In respect to Cr, 36 % of the strains were able to grow in the presence of as much as 3200 mg L−1. Few strains were able to grow at concentrations of Zn and Cu as high as 1600 mg L−1, and none grew at the highest concentration of 3200 mg L−1. Bacillus sp. was most frequently isolated and may be the dominant genus in heavy metal-polluted areas. Staphylococcus sp., Planococcus maritimus, and Vibrio aginolyticus were also isolated, suggesting their potential in bioremediation of contaminated sites.

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Diversity and Distribution of Heavy Metal-Resistant Bacteria in Polluted Sediments of the Araça Bay, São Sebastião (SP), and the Relationship Between Heavy Metals and Organic Matter Concentrations

In this study, a bacterial strain was recovered from the chickpea root surface and its heavy metal tolerance ability, bioaccumulation, biosorption, chromium reduction potential, and phosphate solubilization activity were determined. Bacterial strain (OS3) was molecularly characterized through 16S rRNA partial gene sequencing and confirmed strain sequences belonged to the Ensifer adhaerens , inveterate by the BLASTn and phylogenetic analysis and achived accession number from Genbank # HE681418.1. Strain OS3 was able to tolerate up to 250 µg/ml Cd, 500 µg/ml Cr, 800 µg/ml each of Cu and Zn and 1000 µg/ml Ni. Moreover, strain OS3 was able to bioaccumulate and biosorb varying concentrations of heavy metals and showed maximum accumulation of Ni (95%) and least accumulation of Pb (74%) at a temperature of 34 °C and pH 7. The Langmuir and Freundlich adsorption constants were evaluated from the isotherms with a correlation coefficient ( r 2  > 0.98). The strain reduced 200 µg/ml of Cr (VI) completely after 96 h growth at 35 °C. Apart from bioremedaition ability, bacterial strain OS3 expressed a significant inorganic phosphate (P) solubilizing activity and solubilized inorganic P upto 303 µg/ml with a 2.2 solubilization index. Also, strain OS3 demonstrated plant growth promoting traits even under metal stress and secreted IAA, siderophore, HCN and ammonia under in vitro conditions. This study demonstrated that E. adhaerens survived well, even when exposed to higher concentrations of heavy metals and retained plant growth promoting activities. The biosorption ability, chromium reducing potential and ability to secrete plant growth regulators makes this organisms an interesting choice for metal removal (bioremediation) and enhancing crop production even in metal polluted soils.

Ensifer adhaerens for heavy metal bioaccumulation, biosorption, and phosphate solubilization under metal stress condition

Eleven bacterial strains were isolated from soil samples collected from mine tailings. Bacterial strains were checked for tolerance against heavy metals (Cr, Cd, Ni), using the agar dilution method. All the strains showed multiple tolerances against heavy metals, but the most promising results appeared in strains BCr3, BCd33, and BNi11: they were tolerant to 15 mM of Cr6+, 7.5 mM of Cd2+, and 10 mM of Ni2+, respectively. The effect of heavy metals on bacterial growth was tested together with their ability to grow in different pH, NaCl, and temperature values. Bacterial isolates grew well between pH 7.5 and 8.5. The optimum temperature for maximum growth was between 35 and 37°C, and no significant change in bacterial growth was observed in the presence of 2% NaCl. In addition, the bioaccumulation potential of bacterial strains was investigated. Bacterial strains BCr3, BCd33, and BNi11 showed high bioaccumulation ability of Cr (68.7%), Cd (72.4%), and Ni (69.8%), respectively. All bacterial isolates...

Identification and characterization of Cr-, Cd-, and Ni-tolerant bacteria isolated from mine tailings

Abstract The aim of the study was to assess the differences in the bacterial community physiological profiles in soils contaminated with heavy metals versus soils without metal contaminations. The study’s contaminated soil originated from the surrounding area of the Szopienice non-ferrous metal smelter (Silesia Region, Poland). The control was soil unexposed to heavy metals. Metal concentration was appraised by flame atomic absorption spectrometry, whereas the the community-level physiological profile was determined with the Biolog EcoPlatesTM system. The soil microbiological activity in both sites was also assessed via dehydrogenase activity. The mean concentrations of metals (Cd and Zn) in contaminated soil samples were in a range from 147.27 to 12265.42 mg kg−1, and the heavy metal contamination brought about a situation where dehydrogenase activity inhibition was observed mostly in the soil surface layers. Our results demonstrated that there is diversity in the physiological profiles of microorganisms inhabiting contaminated and colntrol soils; therefore, for assessment purposes, these were treated as two clusters. Cluster I included colntrol soil samples in which microbial communities utilised most of the available substrates. Cluster II incorporated contaminated soil samples in which a smaller number of the tested substrates was utilised by the contained microorganisms. The physiological profiles of micro-organisms inhabiting the contaminated and the colntrol soils are distinctly different.

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Community-level physiological profiles of microorganisms inhabiting soil contaminated with heavy metals

Several mechanisms are developed by the microorganisms to tolerate few high concentrations of heavy metals. One of these mechanisms dependent upon anabolic and catabolic energy of microorganisms is the bioaccumulation of heavy metals. In present work, approximately four varieties of bacteria have been isolated from the ash dyke sample of four thermal power plants of Chhattisgarh, i.e., Bharat Aluminium Company (BALCO), Chhattisgarh State Electricity Board (CSEB), Korba, Thermal Power Cooperation (NTPC), Bilaspur and KSK Akaltara, Chattisgarh. Out of one hundred fifty isolates, three were capable to grow in varying concentration of heavy metals. The strains were tested for their tolerance against six different types of heavy metals dominant in the ash samples viz. Pb, Hg, Ni, Co, Cu, Mn. Their maximum resistance existed up to 0.6mM/ml of the above mentioned different metals under lab standard conditions. Three isolates are found suitable for the multiple metal resistance ability viz SM2, SM3, and SM12. These are categorized as Bacillus cereus (SM2, SM3), and Bacillus subtilis (SM12) after performing 16S rDNA sequencing.

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Sonali Banerjee

Papers

Microbial Observation in Bioaccumulation of Heavy Metals from the Ash Dyke of Thermal Power Plants of Chhattisgarh, India