Impact of composting strategies on the treatment of soils contaminated with organic pollutants

Chemical pollution of the environment has become a major source of concern. Studies on degradation of organic compounds have shown that some microorganisms are extremely versatile at catabolizing recalcitrant molecules. By harnessing this catabolic potential, it is possible to bioremediate some chemically contaminated environmental systems. Composting matrices and composts are rich sources of xenobiotic-degrading microorganisms including bacteria, actinomycetes and lignolytic fungi, which can degrade pollutants to innocuous compounds such as carbon dioxide and water. These microorganisms can also biotransform pollutants into less toxic substances and/or lock up pollutants within the organic matrix, thereby reducing pollutant bioavailability. The success or failure of a composting/compost remediation strategy depends however on a number of factors, the most important of which are pollutant bioavailability and biodegradability. This review discusses the interactions of pollutants with soils; look critically at the clean up of soils contaminated with a variety of pollutants using various composting strategies and assess the feasibility of using composting technologies to bioremediate contaminated soil.     

Phytoremediation of contaminated soils and groundwater: lessons from the field

Background, aim, and scope

The use of plants and associated microorganisms to remove, contain, inactivate, or degrade harmful environmental contaminants (generally termed phytoremediation) and to revitalize contaminated sites is gaining more and more attention. In this review, prerequisites for a successful remediation will be discussed. The performance of phytoremediation as an environmental remediation technology indeed depends on several factors including the extent of soil contamination, the availability and accessibility of contaminants for rhizosphere microorganisms and uptake into roots (bioavailability), and the ability of the plant and its associated microorganisms to intercept, absorb, accumulate, and/or degrade the contaminants. The main aim is to provide an overview of existing field experience in Europe concerning the use of plants and their associated microorganisms whether or not combined with amendments for the revitalization or remediation of contaminated soils and undeep groundwater. Contaminations with trace elements (except radionuclides) and organics will be considered. Because remediation with transgenic organisms is largely untested in the field, this topic is not covered in this review. Brief attention will be paid to the economical aspects, use, and processing of the biomass.

Conclusions and perspectives

It is clear that in spite of a growing public and commercial interest and the success of several pilot studies and field scale applications more fundamental research still is needed to better exploit the metabolic diversity of the plants themselves, but also to better understand the complex interactions between contaminants, soil, plant roots, and microorganisms (bacteria and mycorrhiza) in the rhizosphere. Further, more data are still needed to quantify the underlying economics, as a support for public acceptance and last but not least to convince policy makers and stakeholders (who are not very familiar with such techniques).     

Using nanoscale zero-valent iron for the remediation of polycyclic aromatic hydrocarbons contaminated soil

The sites contaminated with recalcitrant organic compounds, such as polycyclic aromatic hydrocarbons (PAHs) with multiple benzene rings, are colossal and ubiquitous environmental problems. They are relatively nonbiodegradable and mutagenic, and 16 of them are listed in the U.S. Environment Protection Agency priority pollutants. Thus, the efficient and emerging remediation technologies for removal of PAHs in contaminated sites have to be uncovered urgently. In this decade, the zero-valent iron (ZVI) particles have been used successfully in the laboratory, pilot, and field, such as degradation of chlorinated hydrocarbons and remediation of the other pollutants. Nevertheless, as far as we know, little research has investigated for soil remediation; this study used nanoscale ZVI particles to remove pyrene in the soil. The experimental variables were determined, including reaction time, iron particle size, and dosage. From the results, both the micro- and nanoscales of ZVI were capable of removing the target compound in soil, but the higher removal efficiencies were by nanoscale ZVI because of the massive specific surface area. The optimal operating conditions to attain the best removal efficiency of pyrene were obtained while adding nanoscale ZVI 0.1 g/g soil within 60 min and 150 rpm of mixing. Thus, nanoscale ZVI has proved to be a promising remedy for PAH-contaminated soil in this study, as well as an optimistically predictable application for additional pilot and field studies.     

Using Nanoscale Zero-Valent Iron for the Remediation of Polycyclic Aromatic Hydrocarbons Contaminated Soil

Abstract

The sites contaminated with recalcitrant organic compounds, such as polycyclic aromatic hydrocarbons (PAHs) with multiple benzene rings, are colossal and ubiquitous environmental problems. They are relatively nonbiodegradable and mutagenic, and 16 of them are listed in the U.S. Environment Protection Agency priority pollutants. Thus, the efficient and emerging remediation technologies for removal of PAHs in contaminated sites have to be uncovered urgently. In this decade, the zero-valent iron (ZVI) particles have been used successfully in the laboratory, pilot, and field, such as degradation of chlorinated hydrocarbons and remediation of the other pollutants. Nevertheless, as far as we know, little research has investigated for soil remediation; this study used nanoscale ZVI particles to remove pyrene in the soil. The experimental variables were determined, including reaction time, iron particle size, and dosage. From the results, both the micro- and nanoscales of ZVI were capable of removing the target compound in soil, but the higher removal efficiencies were by nanoscale ZVI because of the massive specific surface area. The optimal operating conditions to attain the best removal efficiency of pyrene were obtained while adding nanoscale ZVI 0.1 g/g soil within 60 min and 150 rpm of mixing. Thus, nanoscale ZVI has proved to be a promising remedy for PAH-contaminated soil in this study, as well as an optimistically predictable application for additional pilot and field studies.     

Biological remediation of explosives and related nitroaromatic compounds

Nitroaromatics form an important group of recalcitrant xenobiotics. Only few aromatic compounds, bearing one nitro group as a substituent of the aromatic ring, are produced as secondary metabolites by microorganisms. The majority of nitroaromatic compounds in the biosphere are industrial chemicals such as explosives, dyes, polyurethane foams, herbicides, insecticides and solvents. These compounds are generally recalcitrant to biological treatment and remain in the biosphere, where they constitute a source of pollution due to both toxic and mutagenic effects on humans, fish, algae and microorganisms. However, relatively few microorganisms have been described as being able to use nitroaromatic compounds as nitrogen and/or carbon and energy source. The best-known nitroaromatic compound is the explosive TNT (2,4,6-trinitrotoluene). This article reviews the bioremediation strategies for TNT-contaminated soil and water. It comes to the following conclusion: The optimal remediation strategy for nitroaromatic compounds depends on many site-specific factors. Composting and the use of reactor systems lend themselves to treating soils contaminated with high levels of explosives (e.g. at former ammunition production facilities, where areas with a high contamination level are common). Compared to composting systems, bioreactors have the major advantage of a short treatment time, but the disadvantage of being more labour intensive and more expensive. Studies indicate that biological treatment systems, which are based on the activity of the fungus Phanerochaete chrysosporium or on Pseudomonas sp. ST53, might be used as effective methods for the remediation of highly contaminated soil and water. Phytoremediation, although not widely used now, has the potential to become an important strategy for the remediation of soil and water contaminated with explosives. It is best suited where contaminant levels are low (e.g. at military sites where pollution is rather diffuse) and where larger contaminated surfaces or volumes have to be treated. In addition, phytoremediation can be used as a polishing method after other remediation treatments, such as composting or bioslurry, have taken place. This in-situ treatment method has the advantage of lower treatment costs, but has the disadvantage of a considerable longer treatment time. In order to improve the cost-efficiency, phytoremediation of nitroaromatics (and other organic xenobiotics) could be combined with bio-energy production. This requires, however, detailed knowledge on the fate of the contaminants in the plants as well as the development of efficient treatment methods for the contaminated biomass that minimise the spreading of the contaminants into the environment during post harvest treatment.     

Phytoremediation of polyaromatic hydrocarbons, anilines and phenols

Phytoremediation technologies based on the combined action of plants and the microbial communities that they support within the rhizosphere hold promise in the remediation of land and waterways contaminated with hydrocarbons but they have not yet been adopted in large-scale remediation strategies. In this review plant and microbial degradative capacities, viewed as a continuum, have been dissected in order to identify where bottle-necks and limitations exist. Phenols, anilines and polyaromatic hydrocarbons (PAHs) were selected as the target classes of molecule for consideration, in part because of their common patterns of distribution, but also because of the urgent need to develop techniques to overcome their toxicity to human health. Depending on the chemical and physical properties of the pollutant, the emerging picture suggests that plants will draw pollutants including PAHs into the plant rhizosphere to varying extents via the transpiration stream. Mycorrhizosphere-bacteria and -fungi may play a crucial role in establishing plants in degraded ecosystems. Within the rhizosphere, microbial degradative activities prevail in order to extract energy and carbon skeletons from the pollutants for microbial cell growth. There has been little systematic analysis of the changing dynamics of pollutant degradation within the rhizosphere; however, the importance of plants in supplying oxygen and nutrients to the rhizosphere via fine roots, and of the beneficial effect of microorganisms on plant root growth is stressed. In addition to their role in supporting rhizospheric degradative activities, plants may possess a limited capacity to transport some of the more mobile pollutants into roots and shoots via fine roots. In those situations where uptake does occur (i.e. only limited microbial activity in the rhizosphere) there is good evidence that the pollutant may be metabolised. However, plant uptake is frequently associated with the inhibition of plant growth and an increasing tendency to oxidant stress. Pollutant tolerance seems to correlate with the ability to deposit large quantities of pollutant metabolites in the 'bound' residue fraction of plant cell walls compared to the vacuole. In this regard, particular attention is paid to the activities of peroxidases, laccases, cytochromes P450, glucosyltransferases and ABC transporters. However, despite the seemingly large diversity of these proteins, direct proof of their participation in the metabolism of industrial aromatic pollutants is surprisingly scarce and little is known about their control in the overall metabolic scheme. Little is known about the bioavailability of bound metabolites; however, there may be a need to prevent their movement into wildlife food chains. In this regard, the application to harvested plants of composting techniques based on the degradative capacity of white-rot fungi merits attention.     

Chlorophenols and other related derivatives of environmental concern: properties, distribution and microbial degradation processes

Chlorophenols are chlorinated aromatic compound structures and are commonly found in pesticide preparations as well as industrial wastes. They are recalcitrant to biodegradation and consequently persistent in the environment. A variety of chlorophenols derivatives compounds are highly toxic, mutagenic and carcinogenic for living organisms. Biological transformation by microorganisms is one of the key remediation options that can be exploited to solve environmental pollution problems caused by these notorious compounds. The key enzymes in the microbial degradation of chlorophenols are the oxygenases and dioxygenases. These enzymes can be engineered for enhanced degradation of highly chlorinated aromatic compounds through directed evolution methods. This review underscores the mechanisms of chlorophenols biodegradation with the view to understanding how bioremediation processes can be optimized for cleaning up chloroaromatic contaminated environments.     

Dissipation of polycyclic aromatic hydrocarbons (PAHs) in the rhizosphere: Synthesis through meta-analysis

Polycyclic aromatic hydrocarbons (PAHs) are widespread and persistent organic pollutants with high carcinogenic effect and toxicity; their behavior and fate in the soil-plant system have been widely investigated. In the present paper, meta-analysis was used to explore the interaction between plant growth and dissipation of PAHs in soil based on the large body of published literature. Plants have a promoting effect on PAH dissipation in soils. There was no difference in PAH dissipation between soils contaminated with single and mixed PAHs. However, plants had a more obvious effect on PAH dissipation in freshly-spiked soils than in long-term field-polluted soils. Additionally, a positive effect of the number of microbial populations capable of degrading PAHs was observed in the rhizosphere compared with the bulk soil. Our meta-analysis established the importance of the rhizosphere effect on PAH dissipation in variety of the soil-plant systems.     

Impact of electrokinetic remediation on microbial communities within PCP contaminated soil

Electrokinetic techniques have been used to stimulate the removal of organic pollutants within soil, by directing contaminant migration to where remediation may be more easily achieved. The effect of this and other physical remediation techniques on the health of soil microbial communities has been poorly studied and indeed, largely ignored. This study reports the impact on soil microbial communities during the application of an electric field within ex situ laboratory soil microcosms contaminated with pentachlorophenol (PCP; 100mg kg(-1) oven dry soil). Electrokinetics reduced counts of culturable bacteria and fungi, soil microbial respiration and carbon substrate utilisation, especially close to the acidic anode where PCP accumulated (36d), perhaps exacerbated by the greater toxicity of PCP at lower soil pH. There is little doubt that a better awareness of the interactions between soil electrokinetic processes and microbial communities is key to improving the efficacy and sustainability of this remediation strategy.     

The effects of perennial ryegrass and alfalfa on microbial abundance and diversity in petroleum contaminated soil

Enhanced rhizosphere degradation uses plants to stimulate the rhizosphere microbial community to degrade organic contaminants. We measured changes in microbial communities caused by the addition of two species of plants in a soil contaminated with 31,000 ppm of total petroleum hydrocarbons. Perennial ryegrass and/or alfalfa increased the number of rhizosphere bacteria in the hydrocarbon-contaminated soil. These plants also increased the number of bacteria capable of petroleum degradation as estimated by the most probable number (MPN) method. Eco-Biolog plates did not detect changes in metabolic diversity between bulk and rhizosphere samples but denaturing gradient gel electrophoresis (DGGE) analysis of PCR-amplified partial 16S rDNA sequences indicated a shift in the bacterial community in the rhizosphere samples. Dice coefficient matrices derived from DGGE profiles showed similarities between the rhizospheres of alfalfa and perennial ryegrass/alfalfa mixture in the contaminated soil at week seven. Perennial ryegrass and perennial ryegrass/alfalfa mixture caused the greatest change in the rhizosphere bacterial community as determined by DGGE analysis. We concluded that plants altered the microbial population; these changes were plant-specific and could contribute to degradation of petroleum hydrocarbons in contaminated soil.     

Effects of 2,4-dichlorophenol, pentachlorophenol and vegetation on microbial characteristics in a heavy metal polluted soil

The aim of this study was to evaluate the soil microbial characteristics in historically heavy-metal polluted soil, which was also affected by organic co-contaminants, 2,4-dichlorophenol or pentachlorophenol, which often occur due to the conventional use of pesticides. It was observed that the normalized microbial biomass (microbial biomass per unit soil organic C) of the contaminated soil was very low, less than 1% in both non-planted and ryegrass planted soil, and showed a decreasing trend with the treatment of organic co-contaminants. The microbial biomass and substrate-induced respiration (SIR) in the ryegrass planted soil were much larger, as compared with the non-planted soil with or without organic pollutants. The different resistant bacterial community and its physiological diversity in the rhizosphere further suggested that the effect of vegetation on microbial activity was not just a general increase in the mass or activity of pre-existing microorganisms, but rather acted selectively on microbial growth so that the relative abundance of different microbial groups in soil was changed. In sum, high concentrations of organic co-contaminants, especially pentachlorophenol (PCP), could strengthen the deterioration of microbial ecology. The adverse effect of heavy metal-organic pollutants on the soil microbial biomass and activity might be the reason for the slow degradation of PCP that has high chlorinated and high toxicity. Vegetation might be the efficient way to assist in improving and restoring the utilization of agricultural ecosystems. The beneficial microbial effect of vegetation could cause the rapid dissipation of 2,4-dichlorophenol (2,4-DCP) that has less chlorinated and less toxicity in the planted soils.     

Effects of alfalfa and organic fertilizer on benzo[a]pyrene dissipation in an aged contaminated soil

BACKGROUND: A climate-controlled pot experiment was conducted to investigate the effects of planting alfalfa and applying organic fertilizer on the dissipation of benzo[a]pyrene from an aged contaminated agricultural soil. RESULTS: Short-term planting of alfalfa inhibited the dissipation of benzo[a]pyrene from the soil by 8.9%, and organic fertilizer enhanced benzo[a]pyrene removal from the soil by 11.6% compared with the unplanted and unfertilized treatments, respectively. No significant interaction was observed between alfalfa and organic fertilizer on benzo[a]pyrene dissipation. Sterilization completely inhibited the removal of benzo[a]pyrene from the soil indicating that its degradation by indigenous microorganisms may have been the main mechanism of dissipation. Furthermore, significant positive relationships were observed between benzo[a]pyrene removal and the contents of soil ammonium nitrogen, nitrate nitrogen, and total mineral nitrogen at the end of the experiment, suggesting that competition between plants and microorganisms for nitrogen may have inhibited benzo[a]pyrene dissipation in the rhizosphere of alfalfa and the addition of organic fertilizer may facilitate microbial degradation of benzo[a]pyrene in the soil.     

Effects of 2,4-dichlorophenol,pentachlorophenol and vegetation on microbial characteristics in a heavy metal polluted soil

The aim of this study was to evaluate the soil microbial characteristics in historically heavy-metal polluted soil, which was also affected by organic co-contaminants, 2,4-dichlorophenol or pentachlorophenol, which often occur due to the conventional use of pesticides. It was observed that the normalized microbial biomass (microbial biomass per unit soil organic C) of the contaminated soil was very low, less than 1% in both non-planted and ryegrass planted soil, and showed a decreasing trend with the treatment of organic co-contaminants. The microbial biomass and substrate-induced respiration (SIR) in the ryegrass planted soil were much larger, as compared with the non-planted soil with or without organic pollutants. The different resistant bacterial community and its physiological diversity in the rhizosphere further suggested that the effect of vegetation on microbial activity was not just a general increase in the mass or activity of pre-existing microorganisms, but rather acted selectively on microbial growth so that the relative abundance of different microbial groups in soil was changed. In sum, high concentrations of organic co-contaminants, especially pentachlorophenol (PCP), could strengthen the deterioration of microbial ecology. The adverse effect of heavy metal-organic pollutants on the soil microbial biomass and activity might be the reason for the slow degradation of PCP that has high chlorinated and high toxicity. Vegetation might be the efficient way to assist in improving and restoring the utilization of agricultural ecosystems. The beneficial microbial effect of vegetation could cause the rapid dissipation of 2,4-dichlorophenol (2,4-DCP) that has less chlorinated and less toxicity in the planted soils.     

Remediation of soil contaminated with pyrene using ground nanoscale zero-valent iron

The sites contaminated with recalcitrant polycyclic aromatic hydrocarbons (PAHs) are serious environmental problems ubiquitously. Some PAHs have proven to be carcinogenic and hazardous. Therefore, the innovative PAH in situ remediation technologies have to be developed instantaneously. Recently, the nanoscale zero-valent iron (ZVI) particles have been successfully applied for dechlorination of organic pollutants in water, yet little research has investigated for the soil remediation so far. The objective in this work was to take advantage of nanoscale ZVI particles to remove PAHs in soil. The experimental factors such as reaction time, particle diameter and iron dosage and surface area were considered and optimized. From the results, both microscale and nanoscale ZVI were capable to remove the target compound. The higher removal efficiencies of nanoscale ZVI particles were obtained because the specific surface areas were about several dozens larger than that of commercially microscale ZVI particles. The optimal parameters were observed as 0.2 g iron/2 mL water in 60 min and 150 rpm by nanoscale ZVI. Additionally, the results proved that nanoscale ZVI particles are a promising technology for soil remediation and are encouraged in the near future environmental applications. Additionally, the empirical equation developed for pyrene removal efficiency provided the good explanation of reaction behavior. Ultimately, the calculated values by this equation were in a good agreement with the experimental data.     

Study on the possibility of hydrogen peroxide pretreatment and plant system to remediate soil pollution

Hydrogen peroxide was widely selected as the chemical oxidant in chemical remediation or as the donor of oxygen in in situ aerobic bioremediation of organic pollutants. In this paper, hydrogen peroxide pretreatment and plant system was done to examine its possibility to remediate the heavy metal contaminated soil or heavy metal-organic combined contaminated soil. Heavy metal contaminated soil was collected from the heavily industrialized area, in Fuyang county, Zhejiang province, China. And heavy metal-organic combined contaminated soil was prepared from the same contaminated soil by spiking 100 microg g(-1) 2,4-dichlorophenol (2,4-DCP). Results showed that H2O2 could improve the dissipation of 2,4-DCP and enhance the availability of Cu and Zn in soil. The greatly increased DOC (dissolved organic carbon) in the oxidation process was probably the main reason for the greatly increased water soluble Cu in higher pH condition. Water soluble Zn, however, easily rebound to soil components with the time being and had no positive relation with dissolved organic carbon. Planting with ryegrass influenced the behavior of pollutants in soil. It was observed that the dissipation of 2,4-DCP could be enhanced by the presence of plant roots and the availability of Cu and Zn in the planted soil was changed due to the mobilization and rebound mechanisms in the rhizosphere. Co-contamination of 2,4-DCP caused the greater availability of Cu and Zn in H2O2 pretreatment. But with the ryegrass planting, it was easier to rebound to the less available phase in the rhizosphere. Both Cu and Zn concentration in shoots increased with the H2O2 treatment. Therefore our results suggested that H2O2 pretreatment was probably a promising way for promoting the dissipation of persistent organic pollutants and enhancing the solubility of Cu and Zn in soil. A combination of H2O2 pretreatment and suitable plant might be an efficient alternative for remedying heavy metal or heavy metal-organic contaminated soil.     

PAH dissipation in spiked soil: impacts of bioavailability, microbial activity, and trees

While trees have demonstrated potential in phytoremediation of several organic contaminants, little is known regarding their ability to impact the common soil contaminant PAHs. Several species of native North American trees were planted in soil artificially contaminated with three PAHs. Plant biomass, PAH dissipation, and microbial mineralization were monitored over the course of one year and environmental conditions were allowed to follow typical seasonal patterns. PAH dissipation and mineralization were not affected by planting. Extensive and rapid loss of PAHs was observed and attributed to high bioavailability and microbial activity in all treatments. The rate of this loss may have masked any significant planting effects. Anthracene was found to be more recalcitrant than pyrene or phenanthrene. Parallel soil aging studies indicated that sequestration to soil components was minimal. Contrary to common inferences in literature, amendment with decaying fine roots inhibited PAH degradation by the soil microbial community. Seasonal variation in environmental factors and rhizosphere dynamics may have also reduced or negated the effect of planting and should be taken into account in future phytoremediation trials. The unique root traits of trees may pose a challenge to traditional thought regarding PAH dissipation in the rhizosphere of plants.     

Using biochar for remediation of soils contaminated with heavy metals and organic pollutants

Soil contamination with heavy metals and organic pollutants has increasingly become a serious global environmental issue in recent years. Considerable efforts have been made to remediate contaminated soils. Biochar has a large surface area, and high capacity to adsorb heavy metals and organic pollutants. Biochar can potentially be used to reduce the bioavailability and leachability of heavy metals and organic pollutants in soils through adsorption and other physicochemical reactions. Biochar is typically an alkaline material which can increase soil pH and contribute to stabilization of heavy metals. Application of biochar for remediation of contaminated soils may provide a new solution to the soil pollution problem. This paper provides an overview on the impact of biochar on the environmental fate and mobility of heavy metals and organic pollutants in contaminated soils and its implication for remediation of contaminated soils. Further research directions are identified to ensure a safe and sustainable use of biochar as a soil amendment for remediation of contaminated soils.     

Remediation of Soil Contaminated with Pyrene Using Ground Nanoscale Zero-Valent Iron

Abstract

The sites contaminated with recalcitrant polycyclic aromatic hydrocarbons (PAHs) are serious environmental problems ubiquitously. Some PAHs have proven to be carcinogenic and hazardous. Therefore, the innovative PAH in situ remediation technologies have to be developed instantaneously. Recently, the nanoscale zero-va-lent iron (ZVI) particles have been successfully applied for dechlorination of organic pollutants in water, yet little research has investigated for the soil remediation so far. The objective in this work was to take advantage of nanoscale ZVI particles to remove PAHs in soil. The experimental factors such as reaction time, particle diameter and iron dosage and surface area were considered and optimized. From the results, both microscale and nanoscale ZVI were capable to remove the target compound. The higher removal efficiencies of nanoscale ZVI particles were obtained because the specific surface areas were about several dozens larger than that of commercially microscale ZVI particles. The optimal parameters were observed as 0.2 g iron/2 mL water in 60 min and 150 rpm by nanoscale ZVI. Additionally, the results proved that nanoscale ZVI particles are a promising technology for soil remediation and are encouraged in the near future environmental applications. Additionally, the empirical equation developed for pyrene removal efficiency provided the good explanation of reaction behavior. Ultimately, the calculated values by this equation were in a good agreement with the experimental data.     

Layer of organic pine forest soil on top of chlorophenol-contaminated mineral soil enhances contaminant degradation

Chlorophenols, like many other synthetic compounds, are persistent problem in industrial areas. These compounds are easily degraded in certain natural environments where the top soil is organic. Some studies suggest that mineral soil contaminated with organic compounds is rapidly remediated if it is mixed with organic soil. We hypothesized that organic soil with a high degradation capacity even on top of the contaminated mineral soil enhances degradation of recalcitrant chlorophenols in the mineral soil below. We first compared chlorophenol degradation in different soils by spiking pristine and pentachlorophenol-contaminated soils with 2,4,6-trichlorophenol in 10-L buckets. In other experiments, we covered contaminated mineral soil with organic pine forest soil. We also monitored in situ degradation on an old sawmill site where mineral soil was either left intact or covered with organic pine forest soil. 2,4,6-Trichlorophenol was rapidly degraded in organic pine forest soil, but the degradation was slower in other soils. If a thin layer of the pine forest humus was added on top of mineral sawmill soil, the original chlorophenol concentrations (high, ca. 70 μg g^?1, or moderate, ca. 20 μg g^?1) in sawmill soil decreased by >40 % in 24 days. No degradation was noticed if the mineral soil was kept bare or if the covering humus soil layer was sterilized beforehand. Our results suggest that covering mineral soil with an organic soil layer is an efficient way to remediate recalcitrant chlorophenol contamination in mineral soils. The results of the field experiment are promising.