The ability of heavy metals bioaccumulation to cause toxicity in biological systems-human, animals, microorganisms and plants-is an important issue for environmental health and safety. Recent biotechnological approaches for bioremediation include biomineralization (mineral synthesis by living organisms or biomaterials), biosorption (dead microbial and renewable agricultural biomass), phytostabilization (immobilization in plant roots), hyperaccumulation (exceptional metal concentration in plant shoots), dendroremediation (growing trees in polluted soils), biostimulation (stimulating living microbial population), rhizoremediation (plant and microbe), mycoremediation (stimulating living fungi/mycelial ultrafiltration), cyanoremediation (stimulating algal mass for remediation) and genoremediation (stimulating gene for remediation process). The adequate restoration of the environment requires cooperation, integration and assimilation of such biotechnological advances along with traditional and ethical wisdom to unravel the mystery of nature in the emerging field of bioremediation. This review highlights better understanding of the problems associated with the toxicity of heavy metals to the contaminated ecosystems and their viable, sustainable and eco-friendly bioremediation technologies, especially the mechanisms of phytoremediation of heavy metals along with some case studies in India and abroad. However, the challenges (biosafety assessment and genetic pollution) involved in adopting the new initiatives for cleaning-up the heavy metals-contaminated ecosystems from both ecological and greener point of view must not be ignored.

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Municipal sewage triggers a stress prone environment to accumulate polyhydroxyalkanoates (PHAs) in the cytosol of bacteria. In view of that, different Bacillus species were isolated from municipal sewage and screened for evaluating their efficacy of PHA production. Growth parameters such as temperature, pH, glucose concentration and carbon nitrogen combinations were optimized with respect to higher biomass production as it is analogous to PHA accumulation. Under optimized conditions, the Bacillus species produced 3.09 g/L of PHAs which was estimated as a higher yield in comparison to other similar strains. Fourier transform infrared spectroscopic analysis of the extracted polyhydroxybutyrate confirmed the distinct peak corresponding to C=O group, whereas proton nuclear magnetic resonance ( 1 H NMR) and differential scanning colorimetric analysis exhibited detailed insight of its chemical structure and properties by reflecting monomeric unit. The high yielding bacterial isolate was identified by 16S rDNA sequencing and the sequence was confirmed as Bacillus subtilis with an accession no. KP172548 after submission to NCBI data base. The potential bacterium may be further exploited for cost effective and mass scale production of biopolymer.

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An investigation was conducted to study the interaction between Cd and Ca, Zn and organic matter for Cd-phytoremediation in sunflower on the alluvium soil of the Sheila Dhar Institute (SDI) experimental farm, Allahabad (India). Application of 40 ppm Zn produced 11.18% extra dry matter (DM) content and 5.8% extra seed yield over the control. We recommended 1.0% Ca, 40 ppm Zn and 20 tons/ha of compost to enhance dry matter yield and diminish the Cd accumulation in 15 ppm Cd- ethylenediaminetetraacetic (EDTA)-treated plots up to 1/12 folds in sunflower (<0.21 ppm), which indicated phytoremediation of Cd-contaminated soil through soil-plant-rhizospheric processes.

The study of phytoremediation potential of Helianthus annuus L was conducted in the sewage-irrigated Indo-Gangetic alluvial soils, India. Calcium @ 1.0% and Zn @ 40 ppm enhanced the yield of H. annuus L and minimized the toxicity of Cr in the investigated soils. The study indicated that H. annuus L is highly sensitive to Cr and Zn in terms of metallic pollution; and may be used as indicator plant. For Cr-phytoremediation, humic acid treatment @ 500 mL/acre induced the Cr-accumulation in roots (p < 0.007) and in shoots (p < 0.015), which was recorded 3.21 and 3.16 mg/kg in root and shoot of H. annuus L, respectively. We suggest that H. annuus L fulfils the necessary condition for efficiently increasing species bioaccumulation after soil treatment with humic acid in Cr-polluted sewage-irrigated soils through soil- plant rhizospheric processes.

The ability of hyperaccumulator oilcake manure as compared to chelates was investigated by growing Calendula officinalis L for phytoremediation of cadmium and lead contaminated alluvial soil. The combinatorial treatment T6 [2.5 g kg(-1) oilcake manure+5 mmol kg(-1) EDDS] caused maximum cadmium accumulation in root, shoot and flower up to 5.46, 4.74 and 1.37 mg kg(-1) and lead accumulation up to 16.11, 13.44 and 3.17 mg kg(-1), respectively at Naini dump site, Allahabad (S3). The treatment showed maximum remediation efficiency for Cd (RR=0.676%) and Pb (RR=0.202%) at Mumfordganj contaminated site (S2). However, the above parameters were also observed at par with the treatment T5 [2.5 g kg(-1) oilcake manure +2 g kg(-1) humic acid]. Applied EDDS altered chlorophyll-a, chlorophyll-b, and carotene contents of plants while application of oilcake manure enhanced their contents in plant by 3.73-8.65%, 5.81-17.65%, and 7.04-17.19%, respectively. The authors conclude that Calendula officinalis L has potential to be safely grown in moderately Cd and Pb-contaminated soils and application of hyperaccumulator oilcake manure boosts the photosynthetic pigments of the plant, leading to enhanced clean-up of the cadmium and lead-contaminated soils. Hence, the hyperaccumulator oilcake manure should be preferred over chelates for sustainable phytoremediation through soil-plant rhizospheric process.