In this special issue, articles based on selected platform and poster presentations at the 5th European Bioremediation Conference held in Chania, Greece, July 4–7, 2011, are collected. This quite successful Conference series has established a long-standing relationship with JCTB since this is the third special issue associated with the Conference. Moreover, the journal, through our publishers Wiley & Sons, regularly sponsors prizes for the best student and poster presentations.
These papers represent a variety of biological, abiotic and biotechnological approaches to aid in the transformation, mineralization and removal of toxic compounds and biogenic residues to innocuous or added-value products. The conference's two parallel tracks also represent the two main topics of this issue: (i) novel approaches for the degradation, detoxification, analysis and decontamination of hazardous chemicals using microbes and plants and (ii) the optimization of energy recovery from biomass, investigated from molecular to process engineering scales. In more detail, the articles embrace recent advances in microbial methane and hydrogen recovery from energy crops, agricultural and (food-)industrial wastes, detoxification and extraction of organic and inorganic environmental pollutants using microbes and plants, plus a variety of novel abiotic approaches aimed to enhance biological transformation and translocation processes. In the following, seventeen original research papers are presented within a bracket of each one Perspective and Technical Note.
Microbial remediation of compounds of environmental concern and priority pollutants
Polycyclic aromatic hydrocarbons (PAH)
In their Perspective paper, Yan and Reible1 present a novel electrochemical remediation approach allowing for the integration of active PAH detoxification mechanisms, i.e. biodegradation, into passive leaching prevention measures, i.e. capped sediments. Horizontal low-voltage carbon electrodes integrated into the cap allow for redox-control and the provision of electron acceptors into these by design anaerobic environments. These reactive caps enable for the biodegradation of vertically migrating PAH in an environment that is confined from replenishment of redox acceptors from meteoric waters.
Beside the presence of electron acceptors, hydrocarbon biodegradation in solid contaminated matrices is often limited by the strong sequestration of aromatic compounds into structurally similar natural and anthropogenic matter, rendering pollutants inaccessible to microbes. An integrated approach combining bioremediation and secondary utilization of industrial and household wastes is explored by Scherr et al.2 in demonstrating the applicability of lipid contaminant extraction with thermally abused and spent cooking oils to increase microbial PAH accessibility in historically contaminated soils.
In the past years, increasing attention has been directed into the investigation of the environmental behaviour of alkylated and substituted tar oil contaminants aside the 16 priority PAH listed by the US Environment Protection Agency. In their paper, Vasilieva et al.3 are evaluating data obtained from comprehensive, 2-D gas chromatography using a principal component approach, enabling for the identification of aromatic contaminants residing in soils after bioremediation. These include several both yet unrecognized and EPA-listed compounds.
Polychlorinated biphenyls (PCB) and phenols
Two contributions focus on microbial aspects of PCB dechlorination and provide evidence of the involvement of yet unidentified PCB dechlorinating microbes in PCB dechlorination. The application of zero-valent iron (ZVI) particles as a source of cathodic hydrogen is currently investigated for a variety of dechlorination processes. Zanaroli et al.4 successfully demonstrated the dechlorination-enhancing effect of nano-scale ZVI to PCB-contaminated marine sediments. The subsequently induced changes in biogeochemical conditions favoured the enrichment of a yet uncharacterized bacterial phylotype closely related to Dehalobium chlorocoercia DF-1. Dudková et al.5 developed a stable, sediment-free anaerobic enrichment from river sediment that was able to dechlorinate PCBs in the absence of haloprimers. Their results point towards the occurrence of highly unusual PCB dechlorination patters and possibly also involve yet unrecognized dechlorinating organisms.
Abiotic catalytic hydrogenation aimed at improving the treatability of nitrophenols in wastewater was investigated by Diaz et al.6 Catalytic nitrophenol hydrogenation using Alumina-supported rhodium and palladium catalysts produced amino- and nitro-amino phenols. Monoaminocompounds were found to be better degradable in an anaerobic sludge than their mother compounds. Thus, catalytic hydrogenation may be an efficient strategy to increase the efficacy of a biological post-treatment step in a conventional anaerobic sludge system. For the treatment of chlorophenol-contaminated groundwater, a biological approach using a SBR was investigated by the same research group. Here, Monsalvo et al.7 conclude that both strategies of co-substrate addition (phenol) and bioaugmentation with Pseudomonas putida have a more beneficial effect on 4-CP degradation in activated sludge than time-intensive biomass acclimation.
Textile dyes and caprolactam
As opposed to bacterial degradation processes, the ability of aquatic fungi, a generally scarcely exploited resource in engineered biodegradation, to participate in pollutant detoxification has been less well recognized. In this context, the study by Junghanns et al.8 is amongst the pilot studies. There, the ability of immobilized Phoma sp., an aquatic ascomycete, to decolorize acid, reactive and direct textile dyes from wastewater streams was successfully demonstrated.
Different caprolactam oligomers are hazardous by-products of caprolactam production, itself a precursor of different synthetic polyamides, including Nylon-6. A possible pathway for the microbial transformation of linear caprolactam oligomers was revealed by Esikova et al.9 to be the oxidative transamination to dicarboxylic acids; the key enzymes' synthesis appears to be determined by CAP plasmids. The elucidation of degradative pathways and their molecular stakeholders will contribute to support biological treatment of waste from polyamide production as opposed to the chemical treatment methods currently in effect.
Energetic valorisation of biogenic agro-industrial residues
Fossil fuels are the main drivers of climate change and play key roles in global environmental pollution and political instability. In contrast, the exploitation of specially grown and waste biomass in microbial energy production can be considered to contribute prominently to potential exit strategies. These include the microbial production of methane and hydrogen from energy crops and waste biomass from households, agriculture, food- and non-food industry.
Cappelletti et al.10 investigated biological hydrogen production by a variety of hyperthermophilic Thermotoga strains in suspended and attached cells. Carbohydrate-rich food industry by-products, molasses—pioneering data are presented here for thermophilic conditions—and cheese whey were processed efficiently to hydrogen. Process cost-efficiency can be significantly increased with the cultivation of Thermotoga on a highly simplified medium which is not connected to a significant loss in hydrogen production efficacy.
Three contributions are concerned with the optimization of methane yield from biomass. The effect of biomass properties in terms of biochemical methane production and community structure were investigated on thirteen different agro-industrial substrates by Merlino et al.11 Here, Methanosarcinales were found to dominate the archaeal community structures largely irrespective of the biomass used. Bertin et al.12 optimized a methane-producing consortium for the treatment of the poorly digestible wet fraction of municipal solid waste. An increase in methane production was connected to the enrichment of different hydrolytic and acidogenic Firmicutes and Bacteroidetes strains, while the archaeal population, composed of two Methanosarcina sp. strains, was constant over the entire experimental duration. Garcia-Mancha et al.13 demonstrated the energetic valorisation of wastewater from used industrial oils in methanogenic granular sludge bed reactors. Industrial oil treatment in anaerobic digestion requires mesophilic conditions to efficiently treat water with organic loading rates exceeding 10 g COD/L.
The use of plants to alleviate environmental pollution, be it of metals, organic pollutants or wastewater, is a sustainable strategy for the cost-efficient treatment of contaminated environmental matrices. Two contributions investigate the use of plants native to India for petroleum hydrocarbon bioremediation with a variety of indigenous sedge14 and carpetgrass15 species. Fertilization significantly increased the extent of pollution reduction in all cases.
Soil amendments can be used to increase the phytoextraction efficiency for heavy metals. Differently charged natural zeolites were investigated as soil amendments to increase phytoaccumulation of lead and zinc by Lai et al.16. Uncharged zeolites were found to be of preferential use in phytostabilisation measures, while ammonia- or CO2-charged zeolites were recommended for use in phytoextraction.
Possible inhibitory effects of landfill leachates from different reactor phases of municipal landfills to plants were explored by Kalčíková et al.17. Leachates from the acidic phase were more toxic to plants than from the stable, methanogenic phase, and phytoremediation is recommended for the latter liquids.
The exposure of a variety of hydrophytes to coherent light, in an approach designated as laser biotechnology, to stimulate plant-related decontamination processes, was demonstrated by Dobrowolski et al.18.
This issue is closed by a Technical Note19. Here, the benefits of using chelators introduced into soil to enhance plant tolerance to heavy metals, specifically for mercury, were weighed against possible adverse effects of contaminant leaching to groundwater. In the article by Smolińska and Król19, results indicate that citric acid as a chelator results in lower potential leaching of mercury into the groundwater than the use of EDTA and KI for the similar purpose.
Novel approaches in enviro-industrial biotechnology
The articles collected in this special issue reflect recent developments and novel strategies in the areas of environmental pollution mitigation and waste valorisation. They span from the characterization of archaeal and bacterial community responses to different environments5, 7, 10–13, the elucidation of metabolic pathways9, novel analytical technologies3 and environmental nano-technology4 to the exploitation of organisms for environmental biotechnology from yet less well characterized environments8 or geographical regions14, 15. In addition, a number of abiotic approaches to increase pollution attenuation efficacy are introduced, including electrochemistry1 and catalytic hydrogenation6, soil amendments to modify processes governing interactions between pollutants and plants16, 19 or microbes, offering the possibility of waste re-use2 and, eventually, laser-based plant biotechnology18.