Phytoremediation Crops and Biofuels

Past, present and future trends in the remediation of heavy-metal contaminated soil - Remediation techniques applied in real soil-contamination events

Most worldwide policy frameworks, including the United Nations Sustainable Development Goals, highlight soil as a key non-renewable natural resource which should be rigorously preserved to achieve long-term global sustainability. Although some soil is naturally enriched with heavy metals (HMs), a series of anthropogenic activities are known to contribute to their redistribution, which may entail potentially harmful environmental and/or human health effects if certain concentrations are exceeded. If this occurs, the implementation of rehabilitation strategies is highly recommended. Although there are many publications dealing with the elimination of HMs using different methodologies, most of those works have been done in laboratories and there are not many comprehensive reviews about the results obtained under field conditions. Throughout this review, we examine the different methodologies that have been used in real scenarios and, based on representative case studies, we present the evolution and outcomes of the remediation strategies applied in real soil-contamination events where legacies of past metal mining activities or mine spills have posed a serious threat for soil conservation. So far, the best efficiencies at field-scale have been reported when using combined strategies such as physical containment and assisted-phytoremediation. We have also introduced the emerging problem of the heavy metal contamination of agricultural soils and the different strategies implemented to tackle this problem. Although remediation techniques used in real scenarios have not changed much in the last decades, there are also encouraging facts for the advances in this field. Thus, a growing number of mining companies publicise in their webpages their soil remediation strategies and efforts; moreover, the number of scientific publications about innovative highly-efficient and environmental-friendly methods is also increasing. In any case, better cooperation between scientists and other soil-related stakeholders is still required to improve remediation performance.

Coupling Plant Biomass Derived from Phytoremediation of Potential Toxic-Metal-Polluted Soils to Bioenergy Production and High-Value by-Products—A Review

Phytoremediation is an attractive strategy for cleaning soils polluted with a wide spectrum of organic and inorganic toxic compounds. Among these pollutants, heavy metals have attracted global attention due to their negative effects on human health and terrestrial ecosystems. As a result of this, numerous studies have been carried out to elucidate the mechanisms involved in removal processes. These studies have employed many plant species that might be used for phytoremediation and the obtention of end bioproducts such as biofuels and biogas useful in combustion and heating. Phytotechnologies represent an attractive segment that is increasingly gaining attention worldwide due to their versatility, economic profitability, and environmental co-benefits such as erosion control and soil quality and functionality improvement. In this review, the process of valorizing biomass from phytoremediation is described; in addition, relevant experiments where polluted biomass is used as feedstock or bioenergy is produced via thermo- and biochemical conversion are analyzed. Besides, pretreatments of biomass to increase yields and treatments to control the transfer of metals to the environment are also mentioned. Finally, aspects related to the feasibility, benefits, risks, and gaps of converting toxic-metal-polluted biomass are discussed.

Characterizing an engineered carotenoid-producing yeast as an anti-stress chassis for building cell factories

Background

A microorganism engineered for non-native tasks may suffer stresses it never met before. Therefore, we examined whether a Kluyveromyces marxianus strain engineered with a carotenoid biosynthesis pathway can serve as an anti-stress chassis for building cell factories.

Results

Carotenoids, a family of antioxidants, are valuable natural products with high commercial potential. We showed that the free radical removal ability of carotenoids can confer the engineered host with a higher tolerance to ethanol, so that it can produce more bio-ethanol than the wild type. Moreover, we found that this engineered strain has improved tolerance to other toxic effects including furfurals, heavy metals such as arsenate (biomass contaminant) and isobutanol (end product). Furthermore, the enhanced ethanol tolerance of the host can be applied to bioconversion of a natural medicine that needs to use ethanol as the delivery solvent of hydrophobic precursors. The result suggested that the engineered yeast showed enhanced tolerance to ethanol-dissolved hydrophobic 10-deacetylbaccatin III, which is considered a sustainable precursor for paclitaxel (taxol) bioconversion.

Conclusions

The stress tolerances of the engineered yeast strain showed tolerance to several toxins, so it may serve as a chassis for cell factories to produce target products, and the co-production of carotenoids may make the biorefinary more cost-effective.

Bentonite addition to a PCB-contaminated sandy soil improved the growth and phytoremediation efficiency of Zea mays L. and Alternanthera sessilis L

In this study, the removal of 17 selected PCB_i congeners was assessed in a transformer oil-contaminated soil amended with bentonite clay powder applied at the three levels of 0, 2, and 4% and cultivated by Zea mays L. or Alternanthera sessilis L. in a pot experiment. Results showed that Z. mays and A. sessilis were able to reduce the residual concentrations of the PCB_i congeners in the contaminated soil significantly (p < 0.05). The average reductions in the ƩPCB_i due to Z. mays or A. sessilis cultivations were 34.3 and 21.4%, respectively, depending on initial soil ƩPCB_i loading and plant growth period. Moreover, addition of bentonite led to significant (p < 0.05) enhancements in plant growth and dissipation of residual soil PCB_i congeners under Z. mays and A. sessilis cultivations. Addition of 4% bentonite to the soil was found to have the greatest positive impact on PCB_i removal so that average PCB_i dissipations in the soil were 56.1 and 51.8% after growing Z. mays and A. sessilis, respectively. It might be concluded that the combined phytoremediation and bentonite addition is an effective technique for removing PCB_i and remediating transformer oil-contaminated coarse-textured soils.

The Difference of Photosynthetic Responses to the Cadmium Stress Between a Wild Soybean (Glycine soja Sieb. et Zucc.) and a Cultivated Soybean

The effects of Cd on a halophyte of Doingying wild soybean (DY-03262) were studied by comparing it with a cultivated soybean (SN-11) via investigating photosynthetic activity, chlorophyll fluorescence and Cd concentrations in leaves after treatment with different concentrations of Cd (0, 50, and 100 μM) for 10 days. There were no significant differences in photosynthetic rate (PN), actual photochemical efficiency of PSII (ΦPSII), stomatal conductance (gs), and photochemical quenching (qP) between the both soybean species without Cd treatment, but they decreased greater in DY-03262 than that in SN-11 leaves after Cd treatment. The study demonstrated that the decrease in PN were significantly correlated with Cd concentrations in the leaves of both soybeans species. The greater decreases in photosynthetic activity of the DY-03262 were due to the higher Cd concentrations in its leaves.

Ricinus communis L. (Castor bean), a potential multi-purpose environmental crop for improved and integrated phytoremediation

Phytoremediation is a plant based environmental cleanup technology to contain (rendering less toxic), sequester and degrade contaminated susbtrates. As can be seen from data metrics, it is gaining cosiderable importance globally. Phytoremediation approach is being applied for cleanup of inorganic (potentially toxic metals), organic (persistent, emergent, poly-acromatic hydrocarbons and crude oil etc.) and co-contaminated (mixture of inorganic and organic) and/or polluted sites globally. Recently new approaches of utilizing abundantly available natural organic amendments have yielded significant results. Ricinus communis L. (Castor bean) is an important multipurpose crop viz., Agricultural, Energy, Environmental and Industrial crop. The current status of knowledge is abundant but scattered which need to be exploited for sustainable development. This review collates and evaluates all the scattered information and provides a critical view on the possible options for exploiting its potential as follows: 1. Origin and distribution, 2. Lead toxicity bioassays, 3. Progress in arbuscular mycorrhizal fungi-assisted phytoremediation, 4. Promising bioenergy crop that can be linked to pytoremediation, 5. A renewable source for many bioproducts with rich chemical diversity, 6. It is a good biomonitor and bioindicator of atmospheric pollution in urban areas, 7. Enhanced chelate aided remediation, 8. Its rhizospheric processes accelerate natural attenuation, 9. It is suitable for remediation of crude oil contaminated soil, 10. It is an ideal candidate for aided phytostabilization, 11. Castor bean is a wizard for phytoremediation and 12. Its use in combined phytoextraction and ecocatalysis. Further, the knowledge gaps and scope for future research on sustainable co-generation of value chain and value addition biobased products for sustainable circular economy and environmental security are described in this paper.