Phytoremediation removal rates of benzene, toluene, and chlorobenzene

A comprehensive review and perspective research in technology integration for the treatment of gaseous volatile organic compounds

Volatile organic compounds (VOCs) are harmful pollutants emitted from industrial processes. They pose a risk to human health and ecosystems, even at low concentrations. Controlling VOCs is crucial for good air quality. This review aims to provide a comprehensive understanding of the various methods used for controlling VOC abatement. The advancement of mono-functional treatment techniques, including recovery such as absorption, adsorption, condensation, and membrane separation, and destruction-based methods such as natural degradation methods, advanced oxidation processes, and reduction methods were discussed. Among these methods, advanced oxidation processes are considered the most effective for removing toxic VOCs, despite some drawbacks such as costly chemicals, rigorous reaction conditions, and the formation of secondary chemicals. Standalone technologies are generally not sufficient and do not perform satisfactorily for the removal of hazardous air pollutants due to the generation of innocuous end products. However, every integration technique complements superiority and overcomes the challenges of standalone technologies. For instance, by using catalytic oxidation, catalytic ozonation, non-thermal plasma, and photocatalysis pretreatments, the amount of bioaerosols released from the bioreactor can be significantly reduced, leading to effective conversion rates for non-polar compounds, and opening new perspectives towards promising techniques with countless benefits. Interestingly, the three-stage processes have shown efficient decomposition performance for polar VOCs, excellent recoverability for nonpolar VOCs, and promising potential applications in atmospheric purification. Furthermore, the review also reports on the evolution of mathematical and artificial neural network modeling for VOC removal performance. The article critically analyzes the synergistic effects and advantages of integration. The authors hope that this article will be helpful in deciding on the appropriate strategy for controlling interested VOCs.

Biotechnology and nanotechnology for remediation of chlorinated volatile organic compounds: current perspectives

Chlorinated volatile organic compounds (CVOCs) are persistent organic pollutants which are harmful to public health and the environment. Many CVOCs occur in substantial quantities in groundwater and soil, even though their use has been more carefully managed and restricted in recent years. This review summarizes recent data on several innovative treatment solutions for CVOC-affected media including bioremediation, phytoremediation, nanoscale zero-valent iron (nZVI)-based reductive dehalogenation, and photooxidation. There is no optimally developed single technology; therefore, the possibility of using combined technologies for CVOC remediation, for example bioremediation integrated with reduction by nZVI, is presented. Some methods are still in the development stage. Advantages and disadvantages of each treatment strategy are provided. It is hoped that this paper can provide a basic framework for selection of successful CVOC remediation strategies.

Investigating plant uptake of organic contaminants through transpiration stream concentration factor and neural network models

Uptake of seven organic contaminants including bisphenol A, estriol, 2,4-dinitrotoluene, N,N-diethyl-meta-toluamide (DEET), carbamazepine, acetaminophen, and lincomycin by tomato (Solanum lycopersicum L.), corn (Zea mays L.), and wheat (Triticum aestivum L.) was measured. The plants were grown in a growth chamber under recommended conditions and dosed by these chemicals for 19 days. The plant samples (stem transpiration stream) and solution in the exposure media were taken to measure transpiration stream concentration factor (TSCF). The plant samples were analyzed by a freeze-thaw centrifugation technique followed by high performance liquid chromatography-tandem mass spectrometry detection. Measured average TSCF values were used to test a neural network (NN) model previously developed for predicting plant uptake based on physicochemical properties. The results indicated that moderately hydrophobic compounds including carbamazepine and lincomycin have average TSCF values of 0.43 and 0.79, respectively. The average uptake of DEET, estriol, acetaminophen, and bisphenol A was also measured as 0.34, 0.29, 0.22, and 0.1, respectively. The 2,4-dinitrotoluene was not detected in the stem transpiration stream and it was shown to degrade in the root zone. Based on these results together with plant physiology measurements, we concluded that physicochemical properties of the chemicals did predict uptake, however, the role of other factors should be considered in the prediction of TSCF. While NN model could predict TSCF based on physicochemical properties with acceptable accuracies (mean squared error less than 0.25), the results for 2,4-dinitrotoluene and other compounds confirm the needs for considering other parameters related to both chemicals (stability) and plant species (role of lipids, lignin, and cellulose).

Quantification of toluene phytoextraction rates and microbial biodegradation functional profiles at a fractured bedrock phytoremediation site

This field study evaluated the efficacy of a mature hybrid poplar phytoremediation system for the remediation of toluene in a fractured bedrock aquifer site. Phytoextraction activity of the trees and the ecology and biodegradation potential of root-colonizing bacteria that ultimately influence how much toluene is transported from the roots and phytoextracted to the aboveground point of measurement were explored. Peak-season toluene mass removal rates ranging from 313 to 743 μg/day were quantified using passive in planta contaminant sampling techniques and continuous heat dissipation transpiration measurements in tree stems. Root bacterial microbiome structure and biodegradation potential were evaluated via high-throughput sequencing and predictive metagenomic functional modelling of bacterial 16S rRNA genes in roots. Poplar roots were colonized mostly by Proteobacteria, Actinobacteria, and Bacteroidetes. Distinct, more uniform communities were observed in roots associated with trees planted in the toluene source area compared to other areas, with differences apparent at lower taxonomic levels. Significant enrichment of Streptomyces in roots was observed in the source area, implicating that genus as a potentially important poplar endophyte at toluene-impacted sites. Moreover, significantly greater aerobic toluene biodegradation capacity was predicted in these roots compared to other areas using taxonomic functional modelling. Together with passive sampling, the molecular results provided supporting evidence of biodegradation activity in the source area and contextualized the detected phytoextraction patterns. These results support the application of phytoremediation systems for aromatic hydrocarbons in environments with complex geology and demonstrate field-validated monitoring techniques to assess phytoextraction and biodegradation in these systems.

Survival, Height Growth, and Phytoextraction Potential of Hybrid Poplar and Russian Olive (Elaeagnus Angustifolia L.) Established on Soils Varying in Salinity in North Dakota, USA

Salt-affected soils in the Northern Great Plains, USA, can impact the long-term survival and growth of trees recommended for agroforestry systems, with Russian olive (Elaeagnus angustifolia L.) being one of few options that survives on these sites. Similarly, hybrid poplars have been used for phytotechnologies on high-salinity soils throughout the world. The objective of this study was to test the survival, height growth, and phytoextraction potential of eight hybrid poplar clones (Populus deltoides Bartr. ex Marsh. × P. nigra L. ‘Robusta’, ‘DN17’, ‘DN182’, ‘DN5’; P. deltoides × P. maximowiczii A. Henry ‘NC14104’, ‘NC14106’; P. nigra × P. maximowiczii ‘NM2’, ‘NM6’) versus Russian olive grown on soils categorized according to initial salinity levels: low (0.1 to 3.9 dS m−1), medium (4.0 to 5.9 dS m−1), and high (6.0 to 10.0 dS m−1). Seven trees per genotype were grown in each salinity treatment at a spacing of 3 × 3 m for four years in Burleigh County, North Dakota. Survival and height were determined following the first four growing seasons, and leaf phytoextraction potential of Al, Ca, Cd, Fe, K, Mg, Mn, Na, and Zn was measured for one-year-old trees. Soil salinity decreased over time, reflecting the phytoextraction potential of the trees. Russian olive did not survive as well as expected, having lower overall survival than three of the hybrid poplar clones (‘DN17’, ‘DN5’, ‘NM6’). At the end of three years when trees were removed per a landowner maintenance agreement, 86%, 71%, and 43% of the Russian olive trees were alive in the low-, medium-, and high-salinity soils, respectively. At this time, ‘NM2’ was the only hybrid poplar clone with similar survival to Russian olive in the high-salinity soils. Russian olive had greater Na, Cd, and Fe leaf concentrations than the hybrid poplar clones, but it also had the worst uptake of Ca and Mg of all genotypes. For hybrid poplar, the P. deltoides × P. nigra genomic group had the broadest clonal variability among all traits, with ‘Robusta’ and ‘DN182’ exhibiting great potential for establishment on high-salinity soils. ‘Robusta’ and ‘DN17’ are the same genotype but they came from different nursery sources (i.e., hence their different nomenclature), and they did not differ for height nor leaf phytoextraction. Populus deltoides × P. maximowiczii clones were not suitable for the soil conditions and silvicultural applications (e.g., tree shelters) of the current study, while P. nigra × P. maximowiczii clones exhibited the most stable performance across all years and salinity treatments. Both ‘NM2’ and ‘NM6’ had superior fourth-year survival and height, as well as average or above average phytoextraction of all elements tested.

The enhanced removal and phytodegradation of sodium dodecyl sulfate (SDS) in wastewater using controllable water hyacinth

Surfactant is an emerging and popular pollutant in both rural and urban areas and its treatment efficacy by phytoremediation is rarely reported. Water hyacinth was utilized to clean anionic surfactant sodium dodecyl sulfate (SDS) in water and its growth and physiological activities were regulated with Chromolaena odorata L. extract. SDS was effectively removed from the water and then transferred to both root and aerial part of water hyacinth. Part of SDS was converted into low-molecular weight degradation intermediates by the hydrogen abstraction reactions in water hyacinth. The removal efficiency and the degradation of SDS were evidently strengthened accompanying with enhanced root activity and ascorbate peroxidase (APX) activity in the presence of Chromolaena odorata L. extract. Meanwhile, the growth of water hyacinth was effectively controlled, exhibiting low-growth rate (≤0.036 g.day^-1). Furthermore, the root was considered as the major organ to degrade SDS, which was correlated to the remarkable increase in APX activity and a slight increase in root activity under both SDS and extract stress. In conclusion, water hyacinth managed with Chromolaena odorata L. extract should be proposed as an eco-friendly biotechnical treatment for the surfactant.

Toluene biodegradation in the vadose zone of a poplar phytoremediation system identified using metagenomics and toluene-specific stable carbon isotope analysis

Biodegradation is an important mechanism of action of phytoremediation systems, but performance evaluation is challenging. We applied metagenomic molecular approaches and compound-specific stable carbon isotope analysis to assess biodegradation of toluene in the vadose zone at an urban pilot field system where hybrid poplars were planted to remediate legacy impacts to an underlying shallow fractured bedrock aquifer. Carbon isotope ratios were compared spatio-temporally between toluene dissolved in groundwater and in the vapor phase. Enrichment of ¹³C from toluene in the vapor phase compared to groundwater provided evidence for biodegradation in the vadose zone. Total bacterial abundance (16S rRNA) and abundance and expression of degradation genes were determined in rhizosphere soil (DNA and RNA) and roots (DNA) using quantitative PCR. Relative abundances of degraders in the rhizosphere were on average higher at greater depths, except for enrichment of PHE-encoding communities that more strongly followed patterns of toluene concentrations detected. Quantification of RMO and PHE gene transcripts supported observations of active aerobic toluene degradation. Finally, spatially-variable numbers of toluene degraders were detected in poplar roots. We present multiple lines of evidence for biodegradation in the vadose zone at this site, contributing to our understanding of mechanisms of action of the phytoremediation system.