Importance of organic amendment characteristics on bioremediation of PAH-contaminated soil

Bioaccumulation and Bioremediation of Heavy Metals in Fishes-A Review

Heavy metals, the most potent contaminants of the environment, are discharged into the aquatic ecosystems through the effluents of several industries, resulting in serious aquatic pollution. This type of severe heavy metal contamination in aquaculture systems has attracted great attention throughout the world. These toxic heavy metals are transmitted into the food chain through their bioaccumulation in different tissues of aquatic species and have aroused serious public health concerns. Heavy metal toxicity negatively affects the growth, reproduction, and physiology of fish, which is threatening the sustainable development of the aquaculture sector. Recently, several techniques, such as adsorption, physio-biochemical, molecular, and phytoremediation mechanisms have been successfully applied to reduce the toxicants in the environment. Microorganisms, especially several bacterial species, play a key role in this bioremediation process. In this context, the present review summarizes the bioaccumulation of different heavy metals into fishes, their toxic effects, and possible bioremediation techniques to protect the fishes from heavy metal contamination. Additionally, this paper discusses existing strategies to bioremediate heavy metals from aquatic ecosystems and the scope of genetic and molecular approaches for the effective bioremediation of heavy metals.

Seasonal variation of temperature affects HMW-PAH accumulation in fishery species by bacterially mediated LMW-PAH degradation

Currently, the specific mechanism generating seasonal variation in polycyclic aromatic hydrocarbons (PAHs) via bacterial biodegradation remains unclear, and whether this alteration affects PAH bioaccumulation is unknown. Therefore, we performed a study between 2015 and 2020 to investigate the effects of seasonal variation on bacterial communities and PAH bioaccumulation in the Pearl River Estuary. Significantly high PAH concentrations in both aquatic and fishery species were determined in dry seasons (the mean ∑₁₆PAH concentration: water, 37.24 ng/L (2015), 30.83 ng/L (2020); fish, 51.01 ng/L (2015) and 72.60 ng/L (2020)) compared to wet seasons (the mean ∑₁₆PAH concentration: water, 22.38 ng/L (2015), 19.40 ng/L(2020); fish, 25.28 ng/L (2015) and 32.59 ng/L (2020)). Distinct differences in taxonomic and functional composition of bacterial communities related to biodegradation of low molecular weight PAHs (LMW-PAHs) were observed between seasons, and the concentrations of PAHs were negatively correlated with seasonal variation in temperature. Temperature-related specific bacterial taxa (e.g., Stenotrophomonas) directly or indirectly participated in LMW-PAH degradation via encoding PAH degradation enzymes (e.g., protocatechuate 4,5-dioxygenase) that subsequently led to bioaccumulation of high molecular weight PAHs (HMW-PAHs) in wild and fishery species due to LMW-PAHs in the water. Based on this alteration, the ecological risk posed by PAHs decreased in wet seasons, and an unbalanced spatio-temporal distribution of PAHs was observed in this estuary. These results suggest that seasonal variation of temperature affects HMW-PAH accumulation in fishery species via bacterially mediated LMW-PAH biodegradation.

Phytoremediation of a pyrene-contaminated soil by Cannabis sativa L. at different initial pyrene concentrations

This study proposes the phytoremediation of a pyrene (PYR)-contaminated soil by Cannabis sativa L. The experimental campaign was conducted along a 60 days period using three different initial PYR concentrations (i.e., 50, 100 and 150 mg kg TS^-1 of soil) in 300 mL volume pots under greenhouse conditions (18-25 °C and 45-55% humidity). After 60 days of hemp growth and flourishing, the highest PYR removal reached approximately 95% in planted soil, 35% higher than in the unplanted control. PYR accumulation was observed in both roots and aerial parts of the plant, with a higher PYR uptake at increasing initial PYR concentrations in soil. The initial PYR concentration affected the growth and, thus, the phytoremediation potential of C. sativa L., which was the result of different removal mechanisms. Overall, the lowest initial PYR concentration was the one that resulted in the highest PYR removal. The interaction between the plant roots and microorganisms in rhizosphere was likely associated with PYR removal in this study. The highest DHO activity of 66.26 μg INTF g^-1 TS^-1 was observed in the soil spiked with 50 mg PYR·kg TS^-1.

Influence of Organic Amendments and Moisture Regime on Soil CO2-C Efflux and Polycyclic Aromatic Hydrocarbons (PAHs) Degradation

In this study, a 30-day incubation experiment was performed to investigate the interactive effects of soil moisture content and two types of organic manure (animal manure: M and wheat straw: WS) on organic C mineralization and the degradation of PAH compounds. Specifically, washed sandy soil sample free from PAHs was treated with combined standard solution containing six different PAHs; pyrene (Pyr), fluoranthene (Flt), benzo[a]pyrene (BaP), benzo[g,h,i]perylene (BghiP), benzo[k]fluoranthene (BkF), and indeno[123-cd]pyrene (IP). The soil samples treated with PAHs were amended with M or WS and then, the soil samples were incubated and subjected to two levels of moisture content (50% and 100% field capacity, FC). The results indicate that CO2–C rates were the highest at day 1, but they tended to be decreased sharply when incubation time increased. The results showed that the higher rate of CO2-C efflux rate and cumulative were observed in M and WS treatments at 100% FC. Applying organic amendments at 50% FC increased the total cumulative CO2-C from 21.6 mg kg−1 to 228 mg kg−1 for M and to 216 mg kg−1 for WS. Meanwhile, applying organic amendments at 50% FC increased the total cumulative CO2-C from 30 mg kg−1 to 381 mg kg−1 for M and to 492 mg kg−1 for WS. The highest increases at 100% FC could be explained by the optimum water content at field capacity. PAHs concentrations decreased significantly in the presence of organic amendments in relation to enhance CO2-C efflux (soil respiration) and to decrease soil pH. It could be concluded that applying organic amendments might be a useful technique to remediate soil PAHs through mineralization.

Mitigation of petroleum-hydrocarbon-contaminated hazardous soils using organic amendments: A review

The term "Total petroleum hydrocarbons" (TPH) is used to describe a complex mixture of petroleum-based hydrocarbons primarily derived from crude oil. Those compounds are considered as persistent organic pollutants in the terrestrial environment. A wide array of organic amendments is increasingly used for the remediation of TPH-contaminated soils. Organic amendments not only supply a source of carbon and nutrients but also add exogenous beneficial microorganisms to enhance the TPH degradation rate, thereby improving the soil health. Two fundamental approaches can be contemplated within the context of remediation of TPH-contaminated soils using organic amendments: (i) enhanced TPH sorption to the exogenous organic matter (immobilization) as it reduces the bioavailability of the contaminants, and (ii) increasing the solubility of the contaminants by supplying desorbing agents (mobilization) for enhancing the subsequent biodegradation. Net immobilization and mobilization of TPH have both been observed following the application of organic amendments to contaminated soils. This review examines the mechanisms for the enhanced remediation of TPH-contaminated soils by organic amendments and discusses the influencing factors in relation to sequestration, bioavailability, and subsequent biodegradation of TPH in soils. The uncertainty of mechanisms for various organic amendments in TPH remediation processes remains a critical area of future research.

Bioaugmentation strategy to enhance polycyclic aromatic hydrocarbons anaerobic biodegradation in contaminated soils

This paper proposes an innovative bioaugmentation approach for the remediation of polycyclic aromatic hydrocarbon (PAH) contaminated soils, based on a novel habitat-based strategy. This approach was tested using two inocula (i-24 and i-96) previously enriched through an anaerobic digestion process on wheat straw. It relies on the application of allochthonous microorganisms characterized by specific functional roles obtained by mimicking a natural hydrolytic environment such as the rumen. The inocula efficiency was tested in presence of naphthalene alone, benzo[a]pyrene alone, and a mix of both of them. In single-contamination tests, i-24 inoculum showed the highest biodegradation rates (84.7% for naphthalene and 51.7% for benzo[a]pyrene). These values were almost 1.2 times higher than those obtained for both contaminants with i-96 inoculum and in the control test in presence of naphthalene alone, while they were 3 times higher compared to the control test in presence of benzo[a]pyrene alone. In mixed-contamination tests, i-96 inoculum showed final biodegradation efficiencies for naphthalene and benzo[a]pyrene between 1.1 and 1.5 higher than i-24 inoculum or autochthonous biomass. Total microbial abundances increased in the bioaugmented tests in line with the PAH degradation. The microbial community structure showed the highest diversity at the end of the experiment in almost all cases. Values of the Firmicutes active fraction up to 7 times lower were observed in the i-24 bioaugmented tests compared to i-96 and control tests. This study highlights a successful bioaugmentation strategy with biological components that can be reused in multiple processes supporting an integrated and environmentally sustainable bioremediation system.

A review on the valorisation of food waste as a nutrient source and soil amendment

Valorisation of food waste offers an economical and environmental opportunity, which can reduce the problems of its conventional disposal. Food waste is commonly disposed of in landfills or incinerated, causing many environmental, social, and economic issues. Large amounts of food waste are produced in the food supply chain of agriculture: production, post-harvest, distribution (transport), processing, and consumption. Food waste can be valorised into a range of products, including biofertilisers, bioplastics, biofuels, chemicals, and nutraceuticals. Conversion of food waste into these products can reduce the demand of fossil-derived products, which have historically contributed to large amounts of pollution. The variety of food chain suppliers offers a wide range of feedstocks that can be physically, chemically, or biologically altered to form an array of biofertilisers and soil amendments. Composting and anaerobic digestion are the main large-scale conversion methods used today to valorise food waste products to biofertilisers and soil amendments. However, emerging conversion methods such as dehydration, biochar production, and chemical hydrolysis have promising characteristics, which can be utilised in agriculture as well as for soil remediation. Valorising food waste into biofertilisers and soil amendments has great potential to combat land degradation in agricultural areas. Biofertilisers are rich in nutrients that can reduce the dependability of using conventional mineral fertilisers. Food waste products, unlike mineral fertilisers, can also be used as soil amendments to improve productivity. These characteristics of food wastes assist in the remediation of contaminated soils. This paper reviews the volume of food waste within the food chain and types of food waste feedstocks that can be valorised into various products, including the conversion methods. Unintended consequences of the utilisation of food waste as biofertilisers and soil-amendment products resulting from their relatively low concentrations of trace element nutrients and presence of potentially toxic elements are also evaluated.

Assessment of European and hybrid aspen clones efficiency based on height growth and removal percentage of petroleum hydrocarbons-a field trial

Soils polluted by organic or inorganic pollutants are an emerging global environmental issue due to their toxic effects. A phytoremediation experiment was conducted to evaluate the extraction potential of three European aspen clones (R2, R3, and R4) and seven hybrid aspen clones (14, 27, 34, 134, 172, 191, and 291) grown in soils polluted with hydrocarbons (includes polycyclic aromatic hydrocarbons (PAH) and total petroleum hydrocarbons (TPH)). Height growth, plant survival rates, and .hydrocarbon removal efficiencies were investigated over a 4-year period at a site in Somerharju, Luumaki Finland, to assess the remediation potential of the clones. Hydrocarbon content in the soil was determined by gas chromatography and mass spectrometry. The results revealed that hybrid aspen clones 14 and 34 and European aspen clone R3 achieved greater height growth (171, 171, and 114 cm, respectively) than the other clones in the study. Further, the greatest removals of PAH (90% at depth 10-50 cm) and (86% at depth 5-10 cm) were observed in plot G15 planted with clone R2. Furthermore, the greatest TPH removal rate at 5-10 cm depth (C₂₂-C₄₀, 97%; C₁₀-C₄₀, 96%; and C₁₀-C₂₁, 90%) was observed in plot 117 with clone 134. However, other clones demonstrated an ability to grow in soils with elevated levels of TPH and PAH, which indicates their tolerance to hydrocarbons and their potential capacity for phytoremediation of hydrocarbon-polluted soils. Our study suggests that European aspen and hybrid aspen clones could be used for the remediation of soils polluted with PAH and TPH.

Effect of Biowastes on Soil Remediation, Plant Productivity and Soil Organic Carbon Sequestration: A Review

High anthropogenic activities are constantly causing increased soil degradation and thus soil health and safety are becoming an important issue. The soil quality is deteriorating at an alarming rate in the neighborhood of smelters as a result of heavy metal deposition. Organic biowastes, also produced through anthropogenic activities, provide some solutions for remediation and management of degraded soils through their use as a substrate. Biowastes, due to their high content of organic compounds, have the potential to improve soil quality, plant productivity, and microbial activity contributing to higher humus production. Biowaste use also leads to the immobilization and stabilization of heavy metals, carbon sequestration, and release of macro and micronutrients. Increased carbon sequestration through biowaste use helps us in mitigating climate change and global warming. Soil amendment by biowaste increases soil activity and plant productivity caused by stimulation in shoot and root length, biomass production, grain yield, chlorophyll content, and decrease in oxidative stress. However, biowaste application to soils is a debatable issue due to their possible negative effect of high heavy metal concentration and risks of their accumulation in soils. Therefore, regulations for the use of biowastes as fertilizer or soil amendment must be improved and strictly employed to avoid environmental risks and the entry of potentially toxic elements into the food chain. In this review, we summarize the current knowledge on the effects of biowastes on soil remediation, plant productivity, and soil organic carbon sequestration.

Comparing performances, costs and energy balance of ex situ remediation processes for PAH-contaminated marine sediments

This study proposes a comparison of different ex situ technologies aimed at the removal of polycyclic aromatic hydrocarbons from marine sediments in terms of performances, costs and energy balance. In accordance with the principles of water-energy nexus, anaerobic bioremediation, soil washing and thermal desorption were investigated under low liquid phase and temperature conditions using phenanthrene (PHE) as model compound. After 42 days of anaerobic bioremediation, the highest PHE biodegradation of 68 and 64% was observed under denitrifying and methanogenic conditions, respectively, accompanied by N₂ and CH₄ production and volatile fatty acid accumulation. During soil washing, more than 97% of PHE was removed after 60 min using a solid-to-liquid ratio of 1:3. Along the same treatment time, low-temperature thermal desorption (LTTD) allowed a PHE removal of 88% at 200 °C. The economic analysis indicated that LTTD resulted in a higher cost (i.e. 1782 € m^-3) than bioremediation and soil washing (228 and 371 € m^-3, respectively). The energy balance also suggested that bioremediation and soil washing are more sustainable technologies as a lower required energy (i.e. 16 and 14 kWh m^-3, respectively) than LTTD (i.e. 417 kWh m^-3) is needed.

Impact of digestate and its fractions on mineralization of 14C-phenanthrene in aged soil

The impact of whole digestate (WD) and its fractions (solid [SD] and liquid [LD]) on ¹⁴C-phenanthrene mineralization in soil over 90 d contact time was investigated. The ¹⁴C-phenanthrene spiked soil was aged for 1, 30, 60 and 90 d. Analysis of water-soluble nitrogen, phosphorus, total (organic and inorganic) carbon, and quantitative bacterial count were conducted at each time point to assess their impact on mineralization of ¹⁴C-phenanthrene in soils. Indigenous catabolic activity (total extents, maximum rates and lag phases) of ¹⁴C-phenanthrene mineralization were measured using respirometric soil slurry assay. The soil amended with WD outperformed the SD and LD fractions as well as showed a shorter lag phase, higher rate and extent of mineralization throughout the study. The digestates improved (P < 0.05) the microbial population and nutritive content of the soil. However, findings showed that spiking soil with phenanthrene generally reduced the growth of microbial populations from 1 to 90 d and gave a lower nutritive content in comparison with the non-spiked soil. Also, soil fertility and bacteria count were major factors driving ¹⁴C-phenanthrene mineralization. Particularly, the non-phenanthrene degraders positively influenced the cumulative mineralization of ¹⁴C-phenanthrene after 60 d incubation. Therefore, the digestates (residue from anaerobic digestion) especially WD, which enhanced ¹⁴C-phenanthrene mineralization of the soil without minimal basal salts medium nor additional degraders should be further exploited for sustainable bioremediation of PAHs contaminated soil.

Bioremediation of PAH-Contaminated Soils: Process Enhancement through Composting/Compost

Bioremediation of contaminated soils has gained increasing interest in recent years as a low-cost and environmentally friendly technology to clean soils polluted with anthropogenic contaminants. However, some organic pollutants in soil have a low biodegradability or are not bioavailable, which hampers the use of bioremediation for their removal. This is the case of polycyclic aromatic hydrocarbons (PAHs), which normally are stable and hydrophobic chemical structures. In this review, several approaches for the decontamination of PAH-polluted soil are presented and discussed in detail. The use of compost as biostimulation- and bioaugmentation-coupled technologies are described in detail, and some parameters, such as the stability of compost, deserve special attention to obtain better results. Composting as an ex situ technology, with the use of some specific products like surfactants, is also discussed. In summary, the use of compost and composting are promising technologies (in all the approaches presented) for the bioremediation of PAH-contaminated soils.

Removal of polycyclic aromatic hydrocarbons during anaerobic biostimulation of marine sediments

This study proposes the supplementation of digestate, fresh organic fraction of municipal solid waste (OFMSW) and a nutrient solution during the anaerobic biostimulation of marine sediments contaminated by polycyclic aromatic hydrocarbons (PAHs). The experimental activity was conducted with four PAHs (i.e. phenanthrene, anthracene, fluoranthene and pyrene) under controlled mesophilic conditions (37 ± 1 °C) in 100 mL serum bottles maintained at 130 rpm. After 120 days of incubation, the highest total PAH degradation of 53 and 55% was observed in the experiments with digestate + nutrients and OFMSW + nutrients, respectively. Phenanthrene was the most degraded PAH and the highest removal of 69% was achieved with OFMSW + nutrients. The anaerobic PAH degradation proceeded through the accumulation of volatile fatty acids and the production of hydrogen and methane as biogas constituents. The highest cumulative biohydrogen production of 80 mL H₂·g VS^-1 was obtained when OFMSW was used as the sole amendment, whereas the highest biomethane yield of 140 mL CH₄·g VS^-1 was obtained with OFMSW + nutrients. The evolution of PAH removal during anaerobic digestion revealed a higher impact of the methanogenic phase rather than acidogenic phase on PAH degradation.

Effect of immature and mature compost addition on petroleum contaminated soils composting: kinetics

PURPOSE

The kinetic studies and effect of amendment addition on the performance of the composting process in reduction of total petroleum hydrocarbons (TPH) from petroleum contaminated soils (PCS) were investigated in the present research.

METHODS

Seven composting experiments containing various mixing ratios of PCS to unfinished compost (UC) and finished compost (FC) were set up and operated for 14 weeks. The mixing rations consisted of 1:0 (as control experiment), 1:0.3, 1:0.6, and 1:1. The initial C/N/P and moisture contents of the composting piles were adjusted to 100/5/1 and 50-55%, respectively.

RESULTS

Results showed that 50.09-79.49% of TPH was removed in the composting experiments after 14 weeks. The highest and lowest removal rates were achieved in the ratios of 1:1 and 1:0.3, respectively. Moreover, application of UC as amendments and bulking agent is more efficient than FC. The biodecomposition of TPH was fitted to the first-order kinetic with the half lives and rate constants of 5.63-11.55 days and 0.060-0.123 d^-1, respectively. The bacteria detected from the composting treatments were Staphylococcus sp., Bacillus sp., and Pseudomonas sp.

CONCLUSIONS

The study confirmed the suitability of composting process for PCS bioremediation and superiority of UC than FC as bulking agent.

Biodegradation of mixed polycyclic aromatic hydrocarbons by pure and mixed cultures of biosurfactant producing thermophilic and thermo-tolerant bacteria

Applicability of thermophilic and thermo-tolerant microorganisms for biodegradation of polycyclic aromatic hydrocarbons (PAHs) with low water solubility is an interesting strategy for improving the biodegradation efficiency. In this study, we evaluated utility of thermophilic and thermo-tolerant bacteria isolated from Unkeshwar hot spring (India) for biodegradation of four different PAHs. Water samples were enriched in mineral salt medium (MSM) containing a mixture of four PAHs compounds (anthracene: ANT, fluorene: FLU, phenanthrene: PHE and pyrene: PYR) at 37 °C and 50 °C. After growth based screening, four potent strains obtained which were identified as Aeribacillus pallidus (UCPS2), Bacillus axarquiensis (UCPD1), Bacillus siamensis (GHP76) and Bacillus subtilis subsp. inaquosorum (U277) based on the 16S rRNA gene sequence analysis. Degradation of mixed PAH compounds was evaluated by pure as well as mixed cultures under shake flask conditions using MSM supplemented with 200 mg/L concentration of PAHs (50 mg/L of each compound) for 15 days at 37 °C and 50 °C. A relatively higher degradation of ANT (92%- 96%), FLU (83% - 86%), PHE (16% - 54%) and PYR (51% - 71%) was achieved at 50 °C by Aeribacillus sp. (UCPS2) and mixed culture. Furthermore, crude oil was used as a substrate to study the degradation of same PAHs using these organisms which also revealed with similar results with the higher degradation at 50 °C. Interestingly, PAH-degrading strains were also positive for biosurfactant production. Biosurfactants were identified as the variants of surfactins (lipopeptide biosurfactants) based on analytical tools and phylogenetic analysis of the surfactin genes. Overall, this study has shown that hot spring microbes may have a potential for PAHs degradation and also biosurfactant production at a higher temperature, which could provide a novel perspective for removal of PAHs residues from oil contaminated sites.

Assembling a genome for novel nitrogen-fixing bacteria with capabilities for utilization of aromatic hydrocarbons

Metagenome from refinery wastewater treatment plant running under nitrogen stress was analyzed for mining of novel aromatic hydrocarbon-degrading bacteria. The sequence data were assembled using metaspade followed by binning using the Metabat tool to assemble genome; where coverage and depth were calculated using bowtie and samtools. The analysis picked a novel genome belonging to family Bradyrhizobiaceae, identified based on 16S rDNA gene which was supported by CheckM and Kraken analysis. Using RAST, the assembled genome showed the capabilities for nitrogen fixation with the utilization of multiple hydrocarbon substrates with 14 different types of oxygenases as mapped by Minpath. An additional genetic feature like genes for stress and resistance towards heavy metals and antibiotic suggested that the genome has gone through the rigorous process of adaptation. If such bacteria could be cultivated then it will open the broad window of bioremediation strategies under nitrogen stress environment.

Toxicity and Bioremediation of Heavy Metals Contaminated Ecosystem from Tannery Wastewater: A Review

The discharge of untreated tannery wastewater containing biotoxic substances of heavy metals in the ecosystem is one of the most important environmental and health challenges in our society. Hence, there is a growing need for the development of novel, efficient, eco-friendly, and cost-effective approach for the remediation of inorganic metals (Cr, Hg, Cd, and Pb) released into the environment and to safeguard the ecosystem. In this regard, recent advances in microbes-base heavy metal have propelled bioremediation as a prospective alternative to conventional techniques. Heavy metals are nonbiodegradable and could be toxic to microbes. Several microorganisms have evolved to develop detoxification mechanisms to counter the toxic effects of these inorganic metals. This present review offers a critical evaluation of bioremediation capacity of microorganisms, especially in the context of environmental protection. Furthermore, this article discussed the biosorption capacity with respect to the use of bacteria, fungi, biofilm, algae, genetically engineered microbes, and immobilized microbial cell for the removal of heavy metals. The use of biofilm has showed synergetic effects with many fold increase in the removal of heavy metals as sustainable environmental technology in the near future.

Microbe and plant assisted-remediation of organic xenobiotics and its enhancement by genetically modified organisms and recombinant technology: A review

Environmental problems such as the deterioration of groundwater quality, soil degradation and various threats to human, animal and ecosystem health are closely related to the presence of high concentrations of organic xenobiotics in the environment. Employing appropriate technologies to remediate contaminated soils is crucial due to the site-specificity of most remediation methods. The limitations of conventional remediation technologies include poor environmental compatibility, high cost of implementation and poor public acceptability. This raises the call to employ biological methods for remediation. Bioremediation and microbe-assisted bioremediation (phytoremediation) offer many ecological and cost-associated benefits. The overall efficiency and performance of bio- and phytoremediation approaches can be enhanced by genetically modified microbes and plants. Moreover, phytoremediation can also be stimulated by suitable plant-microbe partnerships, i.e. plant-endophytic or plant-rhizospheric associations. Synergistic interactions between recombinant bacteria and genetically modified plants can further enhance the restoration of environments impacted by organic pollutants. Nevertheless, releasing genetically modified microbes and plants into the environment does pose potential risks. These can be minimized by adopting environmental biotechnological techniques and guidelines provided by environmental protection agencies and other regulatory frameworks. The current contribution provides a comprehensive overview on enhanced bioremediation and phytoremediation approaches using transgenic plants and microbes. It also sheds light on the mitigation of associated environmental risks.

Assessing bioavailability of complex chemical mixtures in contaminated soils: Progress made and research needs

Understanding the distribution, behaviour and interactions of complex chemical mixtures is key for providing the evidence necessary to make informed decisions and implement robust remediation strategies. Much of the current risk assessment frameworks applied to manage land contamination are based on total contaminant concentrations and the exposure assessments embedded within them do not explicitly address the partitioning and bioavailability of chemical mixtures. These oversights may contribute to an overestimation of both the eco-toxicological effects of the fractions and the mobility of contaminants. In turn, this may limit the efficacy of risk frameworks to inform targeted and proportionate remediation strategies. In this review we analyse the science surrounding bioavailability, its regulatory inclusion and the challenges of incorporating bioavailability in decision making process. While a number of physical and chemical techniques have proven to be valuable tools for estimating bioavailability of organic and inorganic contaminants in soils, doubts have been cast on its implementation into risk management soil frameworks mainly due to a general disagreement on the interchangeable use of bioavailability and bioaccessibility, and the associated methods which are still not standardised. This review focuses on the role of biotic and abiotic factors affecting bioavailability along with soil physicochemical properties and contaminant composition. We also included advantages and disadvantages of different extraction techniques and their implications for bioavailability quantitative estimation. In order to move forward the integration of bioavailability into site-specific risk assessments we should (1) account for soil and contaminant physicochemical characteristics and their effect on bioavailability; (2) evaluate receptor's potential exposure and uptake based on mild-extraction; (3) adopt a combined approach where chemical-techniques are used along with biological methods; (4) consider a simplified and cost-effective methodology to apply at regulatory and industry setting; (5) use single-contaminant exposure assessments to inform and predict complex chemical mixture behaviour and bioavailability.

The potential impact on the biodegradation of organic pollutants from composting technology for soil remediation

Large numbers of organic pollutants (OPs), such as polycyclic aromatic hydrocarbons, pesticides and petroleum, are discharged into soil, posing a huge threat to natural environment. Traditional chemical and physical remediation technologies are either incompetent or expensive, and may cause secondary pollution. The technology of soil composting or use of compost as soil amendment can utilize quantities of active microbes to degrade OPs with the help of available nutrients in the compost matrix. It is highly cost-effective for soil remediation. On the one hand, compost incorporated into contaminated soil is capable of increasing the organic matter content, which improves the soil environment and stimulates the metabolically activity of microbial community. On the other hand, the organic matter in composts would increase the adsorption of OPs and affect their bioavailability, leading to decreased fraction available for microorganism-mediated degradation. Some advanced instrumental analytical approaches developed in recent years may be adopted to expound this process. Therefore, the study on bioavailability of OPs in soil is extremely important for the application of composting technology. This work will discuss the changes of physical and chemical properties of contaminated soils and the bioavailability of OPs by the adsorption of composting matrix. The characteristics of OPs, types and compositions of compost amendments, soil/compost ratio and compost distribution influence the bioavailability of OPs. In addition, the impact of composting factors (composting temperature, co-substrates and exogenous microorganisms) on the removal and bioavailability of OPs is also studied.

Influence of activated sewage sludge amendment on PAH removal efficiency from a naturally contaminated soil: application of the landfarming treatment

The removal of polycyclic aromatic hydrocarbons (PAHs) from a naturally co-contaminated soil by PAHs and heavy metals with an initial concentration of 620 mg of total PAHs kg^-1 dry soil was investigated. The efficiency of landfarming in removing phenanthrene, pyrene, benzo(a)pyrene and the group of total 16 US EPA PAHs was evaluated. The process was biostimulated by adding centrifuged activated sewage sludge (SS) as an organic amendment. The tested ratios of contaminated soil to SS were 1:2, 1:1, 1:0.5 and 1:0 as wet weight basis. The process performance was monitored through chemical, microbiological and ecotoxicological analyses during 105 days of incubation. The results of analyses demonstrated that the treatment without centrifuged SS achieved a significantly higher total 16 US EPA PAH removal efficiency (i.e. 32%) compared to treatments with amendment. In the same treatment, the removal efficiency of the PAH bioavailable fraction was 100% for phenanthrene, benzo(a)pyrene and the group of total 16 US EPA PAHs, whereas 76% for pyrene. Ecotoxicity test performed with bioluminescent bacteria Vibrio fischeri confirmed the effectiveness of landfarming. Finally, the results showed that indigenous microorganisms under certain and controlled operating conditions have greater potential for PAH biodegradation compared to allochthonous microorganisms.

Current State of Knowledge in Microbial Degradation of Polycyclic Aromatic Hydrocarbons (PAHs): A Review

Polycyclic aromatic hydrocarbons (PAHs) include a group of organic priority pollutants of critical environmental and public health concern due to their toxic, genotoxic, mutagenic and/or carcinogenic properties and their ubiquitous occurrence as well as recalcitrance. The increased awareness of their various adverse effects on ecosystem and human health has led to a dramatic increase in research aimed toward removing PAHs from the environment. PAHs may undergo adsorption, volatilization, photolysis, and chemical oxidation, although transformation by microorganisms is the major neutralization process of PAH-contaminated sites in an ecologically accepted manner. Microbial degradation of PAHs depends on various environmental conditions, such as nutrients, number and kind of the microorganisms, nature as well as chemical property of the PAH being degraded. A wide variety of bacterial, fungal and algal species have the potential to degrade/transform PAHs, among which bacteria and fungi mediated degradation has been studied most extensively. In last few decades microbial community analysis, biochemical pathway for PAHs degradation, gene organization, enzyme system, genetic regulation for PAH degradation have been explored in great detail. Although, xenobiotic-degrading microorganisms have incredible potential to restore contaminated environments inexpensively yet effectively, but new advancements are required to make such microbes effective and more powerful in removing those compounds, which were once thought to be recalcitrant. Recent analytical chemistry and genetic engineering tools might help to improve the efficiency of degradation of PAHs by microorganisms, and minimize uncertainties of successful bioremediation. However, appropriate implementation of the potential of naturally occurring microorganisms for field bioremediation could be considerably enhanced by optimizing certain factors such as bioavailability, adsorption and mass transfer of PAHs. The main purpose of this review is to provide an overview of current knowledge of bacteria, halophilic archaea, fungi and algae mediated degradation/transformation of PAHs. In addition, factors affecting PAHs degradation in the environment, recent advancement in genetic, genomic, proteomic and metabolomic techniques are also highlighted with an aim to facilitate the development of a new insight into the bioremediation of PAH in the environment.