Abstract Contamination of agricultural soils with heavy metals present lethal consequences in terms of diverse ecological and environmental problems that entail entry of metal in food chain, soil deterioration, plant growth suppression, yield reduction and alteration in microbial community. Metal polluted soils have become a major concern for scientists around the globe. In more recent times, armed with new knowledge and understanding, removal of heavy metals using different applications has emerged as a solution for waste treatment and contaminant remediation in water and soil. However, the description of metal toxicity to the plants and its removal and degradation from the soil is limited. There are a number of reports in the literature where PGP bacterial inoculation and various chelating agents improves metal accumulation and it’s detoxification in different plant parts without influencing plant growth. Therefore, there is a need to select some useful chemicals which possess the potential to improve plant growth as well as expedite the phytoremediation of metals. In this review, we have discussed the mechanisms possessed by different chelating agents to promote plant growth and phytoremediation of metals. We anticipate that this analysis of interconnected systems will lead to the discovery of new research fields. 

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Principles and Applicability of Integrated Remediation Strategies for Heavy Metal Removal/Recovery from Contaminated Environments

Integrative effects of microbial inoculation and amendments on improved crop safety in industrial soils co-contaminated with organic and inorganic pollutants.

Natural illite–smectite and stevensite Moroccan clays were used for the simultaneous removal of lead and cadmium from aqueous medium. The clays were employed in raw state and extruded as honeycomb monoliths form without any additives, which confirms the novelty of this approach in water treatment. The experiments were done in batch conditions with continuous stirring and using a recirculated flow, respectively. In addition to a characterization of the clays by XRF, XRD, TGA, laser granulometry, N2 physisorption, FTIR spectroscopy, SEM-EDS and evaluation of the cation exchange capacity, special attention was paid to the influence on the co-adsorption of variables such as adsorbent dosage, contact time and initial concentration of Cd2+ and Pb2+. Pseudo-second order kinetics and good fitting to Redlich–Peterson model for both heavy metals were found. Our results also suggest that Pb2+ and Cd2+ uptake is controlled by chemisorption with predominance of Langmuir characteristics. No significant depletion of the metals retention attributable to competition was observed, particularly for the stevensite (maximum retention capacity of 1.2 mg Pb2+/g and 4.6 mg Cd2+/g) that showed higher specific surface area. For both clays, cadmium ions adsorption was relatively favoured in the bimetallic solution, and the honeycombs kept the powders performance. Honeycomb monoliths as a compact adsorbent offer a promising way of water treatment thanks to their stability and easy incorporation into dynamic processes avoiding the issues of pressure drop under wastewater circulation. 

Clay honeycomb monoliths for the simultaneous retention of lead and cadmium in water

Two sorbents are successfully synthesized based on the chemical modification of gellan gum (GG). First, thiosemicarbazide is grafted onto GG to produce GEG-C. In a second step, GEG-C is functionalized with a derivative of tributyl phosphate (dTBP, giving GEG-P sorbent). The sorption properties are compared for recovery of Cd(II) from aqueous solutions. GEG-C is expected to bind Cd(II) through carboxylic, amine, and thiocarbonyl groups, while phosphate groups in GEG-P contribute to increase metal sorption. The chemical modification improves not only the uptake kinetics (equilibrium time ≈30 min, vs. 45 min) but also the sorption capacities (≈2.79 vs. 1.60 mmol Cd g−1), at optimum pH (i.e., pH 5). The functionalization also improves (a) the sorbent stability (weaker reduction of the loss in sorption efficiency after five cycles of reuse compared with GEG-C) and (b) the sorption selectivity in the presence of competitor ions (from equimolar multicomponent solutions), especially at pH 4. Different sorption mechanisms may be involved depending on the pH (deprotonation of reactive groups) including chelation (onto deprotonated amine and phosphonate groups) and ion exchange (protonated groups with metal cations). Cadmium sorption is reversed by contact with 0.3 M HCl solution. The functionalization (GEG-P) strongly increases the selectivity coefficient for Cd(II) against major elements in mining effluent, especially at pH higher than 5. These results confirm the interest of multifunctionality in the development of new sorbents based on renewable resources (such as gellan gum). 

Functionalization of thiosemicarbazide/gellan gum for the enhancement of cadmium removal from aqueous solutions – Grafting of tributyl phosphate derivative

Hexavalent chromium (Cr(VI)) contaminated in soil or water system poses a serious threat to human health. Developing an efficient remediation strategy and treatment method towards Cr(VI) in situ from wastewater is truly under an urgent demand in many countries or regions suffered from this heavy metal threaten. Converting Cr(VI) to Cr(III) with less toxicity by a biological or photocatalytic reduction processing is commonly considered as one of the effective strategies. However, how to effectively enhance the performance of Cr(VI) reduction processing remains a technical challenge. In line with this issue, a novel approach has been proposed to combine a biological and photocatalytic reduction processing together to improve the Cr(VI) remediation. In this system, a novel Cr(VI)-reducing anaerobic bacterium Paraclostridium bifermentans G3 isolated from termite’s gut would detoxify Cd(II) to cadmium sulfide (CdS) nanoparticles that could be precipitated in situ on the cell surface to create a CdS-Paraclostridium bifermentans complex. More importantly, with this complex system, the Cr(VI) reduction processing was significantly improved, where Cr(VI) was completely reduced by the complexes in the case of Cr(VI) concentration at 260 mg/L after 90 min under light culture. As a comparison, if the biological agent, P. bifermentans, was applied alone, it would come out a different reduction efficiency at 81.9% of Cr(VI) after 90 min reaction. These results suggested that the proposed complex system of Cr(VI)-reducing bacteria combined with CdS nanoparticles may potentially provide a novel and in-situ approach for a sustainable enhancement of Cr(VI) remediation to wastewater. 

Light-driven sustainable enhancement of Cr(VI) reduction via the combination of Cr(VI)-reducing bacteria, Paraclostridium bifermentans with CdS nanoparticles

Industrial and developmental activities have raised heavy metal loads in wastewater and their inappropriate treatment before disposal causes grave dilemma to the environment. Chromium (Cr), cadmium (Cd) and lead (Pb) are highly toxic heavy metals and extensively used hence, excessively generated with disposing effluent from the industries like paint, battery, electroplating, tannery, paper and pulp, textile, smelting and mining has raised the public attention owing to health concerns. Microbial-remediation processes are considered as environmentally benign options and their integration with existing treatment techniques can construct advanced types of systems in terms of their cost and efficiency due to their synergistic action. These microbial-integrated techniques are gaining popularity these days. There are varieties of microbial-integrated strategy which has been investigated carefully for efficient removal of Cr, Cd, and Pb and has been published in leading journals. In this review, we have critically discussed the microbial–integrated strategies and recent advances in microbial-integrated techniques for proficient removal of Cr, Pb, and Cd from industrial wastewater in a cheaper and cleaner way. In addition, the advantages and disadvantages of the integrated system also have been discussed in this review. The microbial integrated systems such as bioelectrochemical, photo-bioelectrochemical, nano-microbial, and constructed wetland showed excellent performance and have tremendous potential for large scale exploitation. To conclude, microbial-integrated techniques proved as excellent remedial options for Cr, Pb and Cd contaminated wastewater that has been discussed well in this review. These integrated processes may be cost-effective, viable, more efficient and less time taking than the existing techniques and have intense opportunity. 

Carole Sigman

Papers

A review on microbial-integrated techniques as promising cleaner option for removal of chromium, cadmium and lead from industrial wastewater