Composting is a process known from ancient times which is widely used nowadays for the stabilization of biodegradable municipal/agro-industrial wastes and the preparation of organic fertilizers. A number of different organic materials can be utilized as compost substrates. The microbial consortia that develop in the composting pile during the process are responsible for the breakdown of organic matter as well as for the degradation of more amenable environmental contaminants (petroleum-derived products, monoaromatics, organic solvents etc.). In recent years the degradation of more persistent organic pollutants (PAHs, PCBs and chlorinated pesticides) during composting treatments has been proved.
In bioremediation practices, composting consists of mixing of polluted soils with typical compost substrates in order to achieve the decontamination/detoxification of such contaminated matrices. Although co-composting approaches have been already used for remediation at full scale (e.g. by the US Army for the treatment of TNT contaminated soils) in case of pollution with POPs there are still some limitations that needs to be overcome to make this an established technology.
This presentation will focus mainly on the tests performed to optimize the composting of PAHs contaminated soils so that it could be used in practice.
A first set of experiments was aimed at evaluating the suitability of various waste materials and mixtures for the co-composting of PAHs contaminated soils. Two different soils (1st ΣPAHs 370 mg kg-1, the highest concentration of pyrene and fluoranthene; 2nd ΣPAHs 6000 mg kg-1, the highest concentration of phenanthrene and anthracene) and 5 different organic waste mixtures were tested. The volume of each compost pile was approx. 0.75 m3 and the ratio of soil to organic waste was 1:1 (w/w dry basis). Compost piles were aerated by re-digging the whole content of composters 4 times a year.
Since the beginning of the experiment, temperature in the piles’ core, air quality and respiratory gases, were monitored continuously, while concentration of PAHs and microbial parameters (PLFA) were assessed during the thermophilic, cooling and maturation phase throughout a period of 2 years. When the concentrations of PAHs dropped significantly and composts were mature a battery of ecotoxicological and leachate tests was performed. These analyses proved that PAHs were degraded effectively during the composting process as their degradation in both soils varied from 95 to 98 % after 2 years (with more than 90 % of the initial PAHs content was degraded within the first year). Ecotoxicological tests suggested that no toxic metabolites were produced as there was no toxicity associated to the mature composts. According to our results, the composition of the organic waste does not play a substantial role on the extent of PAHs degradation if suitable humidity and carbon to nitrogen ratio are provided, although some differences in the rate of contaminants degradation were observed.
A second set of experiments was aimed at assessing the optimal organic waste to soil ratio. According to the literature, optimal volumetric ratio of soil to compost mixture in order to reach thermophilic conditions is around 30 %, but this is known to highly depend on the quality of both organic waste and soil. In addition, it is not clear yet whether it is necessary to reach the termophilic conditions in order to trigger PAHs degradation. Therefore, experiments were carried out in smaller volumes (150 l), in thermally insulated rotary composter. The organic waste that proved to be the most conducive to PAHs degradation in the previous test was selected along with a soil similar to 1st soil described above (with a slightly higher PAHs concentration, i.e. ΣPAHs 540 mg kg-1). The soil to compost substrate ratios tested were in the range of 10 to 50 vol.%. After 113 days of incubation, the residual concentration of PAHs ranged from 4 to 6% of the original concentration regardless of compost composition, while in the control microcosm (pure contaminated soil) it accounted to 80%. These results show that an efficient PAHs removal is achieved during composting even with a high soil to compost volumetric ratio (up to 50 vol. %). However, these tests were conducted in thermally-insulated composters, under ideal conditions and larger scale tests (0.75 m3 as described previously) are currently in progress. Preliminary results from these latest set up show that the degradation of PAHs is speeded up significantly, even in the presence of 65 vol.% of soil in the compost mixture compared to the control microcosm.
The project was supported by the Technology agency of the Czech Republic (project No. TE01020218).
The touristic magnet of the Valdemarsvik fjord lies on Sweden’s southeast coast in a geotechnically unstable area with high risk of landslides. For many years Valdemarsvik was the largest single source of chromium for the Baltic Sea area, producing about 250kg/year. That, and the now-closed Lundsberg Läder chromium tannery, created contamination that is being addressed today at this area of high importance for marine tourism, boating, fishing and bathing.
The Municipality of Valdemarsvik and the Swedish Environmental Protection Agency are financing the remediation of the fjord and awarded a €25M ($31.7M) contract to DEME Environmental Contractors (DEC) in January 2012.The project requires dredging the contaminated seabed in the inner part of the fjord, then re-using the dredged sediment at Grännäsviken, an onshore fill area.
The actual dredging went on in the course of 2013. From an area of about 350,000m2 with water depths ranging from 1-14m, DEC has removed all sediment with a chromium concentration above 500 mg/kg. About 200,000m3 of sediment has been removed. This will reduce future chromium discharge into the Baltic Sea by as much as 90%.
Given the unstable nature of the site DEC first installed 400,000 m of lime-cement pillars to a maximum depth of 22m near the shores in order to minimize the risk of settlement and landslides during dredging and landfill construction.
To meet stringent environmental demands dredging took place within silt screens to prevent turbidity outside the dredging area. Furthermore environmental dredging tools were applied to minimize the spill and the spill’s chromium content to below 20g/m².
Dredged sediment was transported by barges to the site Grännäsviken, where it was screened and stabilized with cement to obtain a shear strength of minimal 25kPa. The stabilized material was then used as backfill on the site.
Any water released from the sediments was treated for suspended solids and chromium VI by means of a mobile water treatment plant.
Excavation followed by landfilling is the most common method for treating soils contaminated by metals. Except from that landfilling is not sustainable in a longer perspective, potential valuable metals present in the landfilled masses are not utilized and returned into the societal cycle but left in the landfill with risks of future leaching. Thus, alternative treatment methods are needed. One interesting method is soil washing with metal recovery. By removing the metals not only valuable substances can be recovered but in addition the soil residues become cleaner. Earlier studies by our group on similar soil samples show that leaching using acidic waste process water efficiently release Cu from the samples and that the Cu can be recovered. In this project the method is further developed and evaluated. In addition a special focus has been on the soil residues in order to investigate how the proposed remediation method influences on the soil properties.
Soil samples strongly polluted with copper (Cu) were collected from two sites (A and B) in Sweden. The soil samples were washed with a strongly acidic process water. The leaching efficiency for Cu was optimized for the parameters liquid-to-solid-ratio (L/S) and dilution of the acidic process water. It was also indicated that in one of the sites sieving of the soil before leaching could reduce the amounts of soil needed to be treated, while in the other soil sample the Cu was more or less evenly distributed between different soil particles sizes. The final leaching parameters that were used in a scaled up batch experiment were; 30 minutes leaching time for the soil washing, L/S of 8, acidic process water diluted with milliQ water to 75/25. In order to release weakly sorbed metal ions the residues were thereafter washed with milliQ water.
The original soil from site A is classified as clayish fine sand/coarse silt, while the soil from site B is classified as slightly clayish sand and coarse silt. After leaching, the distribution between different particle types was slightly changed, but in general the soil residues were similar to the original ones. This was also confirmed by the scanning electron microscope SEM images. The pH values in the final soils were acidic and around 4. However, already in the original soils the pH values were <5. Based on these results the soil function “soil as filter and buffer for heavy metals” were evaluated using the TUSEC (technique for soil evaluation and categorization for natural and anthropogenic soils) manual. Originally the soil from site A was of class 5 i.e. “very low capacity of binding and buffering heavy metals”, while site B was of class 4 i.e. “low capacity”. After the soil washing remediation process both soils belongs to class 5. Consequently, the acidic leaching did not influence on the soil properties to a large extent.
After acidic leaching and water washing the Cu contents in the soil samples are reduced five times or more. However, the Cu contents in both soils still exceed the Swedish regulation for “less sensitive land use” (MKM) and cannot directly be put back to the former contaminated site. Instead they might be landfilled. The original soils cannot even be deposited in landfills for hazardous waste, while after treating the soils according to the proposed method the Cu leaching is decreased 6 times and the solid residues can be treated in landfills for non-hazardous waste.
To summarize, the proposed method clearly shows a potential not only to remediate Cu polluted soils but also to recover and reuse the Cu from the generated leachates. Even though the previously highly polluted soils cannot be directly put back at site the solid residues can be deposited in landfills for non-hazardous waste, which is an improvement compared to the original soils that cannot even be deposited in a landfill for hazardous waste.
Teresa Castelo-Grande1, Paulo A. Augusto2 and Domingos Barbosa1
1Laboratório de Engenharia de Processos, Ambiente e Energia (LEPAE), Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465, Porto, Portugal.
2Departamento de Ingenieria Química y Textil, Facultad de Ciencias Químicas, Universidad de Salamanca, Plaza de los Caídos 1-5, 37008 Salamanca, Spain.
Summary
An innovative environmental friendly technique, which combines supercritical extraction with carbon dioxide (CO2) and ultrasounds, is studied by applying it to the removal of atrazine from soil matrices. The obtained recovery for atrazine is higher than the corresponding values for supercritical extraction with CO2, and similar to the recoveries obtained when organic cosolvents are used. Besides high values of recovery, this new technique does not significantly affect the structure of the soil and does not leave any type of residue.
Introduction
Supercritical extraction (SCE) with carbon dioxide (CO2) has been suggested for the removal of hazardous substances from solid matrices and liquids [1-3], however, to increase the selectivity and the recovery of some types of contaminants, mainly polar substances, this technique is used with organic cosolvent (e.g., methanol and acetone). As SCE with CO2 is considered a environmentally friendly remediation technique, the use of organic cosolvents is a major drawback.
Ultrasounds have been used, mainly in analytical techniques, to enhance the extraction from natural products and other solid matrices [4-6]. These studies have been mainly focused in convencional solid-liquid extraction.The increase in the recovery by using ultrasounds is mainly due to the phenomena of cavitation, which consists in the formation, growth and collapse of gas/vapour bubbles in a liquid medium. This generates micro-turbulence and very high temperatures and pressures (close to 1000 atm and 5000 K) in the vicinity of these bubbles. Because supercritical fluids have densities close to that of liquids, we would expect a similar phenomena to occur in supercritical extraction.
To analyse this possibility, the supercritical extraction of atrazine from soil samples, with and without ultrasounds, was studied and the results compared.
Materials and Methods
This study was carried out in a semi-continuous supercritical extraction unit consisting of an extractor (with a capacity of 80 cm3), to which supercritical carbon dioxide was continuously fed at the specified pressure. The extractor is inside a thermostated air bath, to mantain the temperature constant, and has an ultrasonic transducer connected to its walls (this transducer is connected to an ultrasound generator). Each extraction essay lasts for 7-8 hours, and the range of temperatures and pressures studied were 303 – 333 K and 10 – 25 MPa.
The experiments were done with soil samples (30 – 35 g) impregnated with known amounts of atrazine, and the recovery of atrazine was quantified by HPLC.
Results and Discussion
The results obtained for the extraction of atrazine, with and without ultrasounds, are summarized in Figure 1 for an operating pressure of 24.5 MPa. This figure clearly shows that the use of ultrasounds enhances the extraction of atrazine leading to an increase of 60 – 80% in its recovery
Conclusion and References
A new environmentally friendly remediation technique for soils, which joins ultrasounds and supercritical extraction with carbon dioxide, is studied for the extraction of atrazine. This preliminary results show that this is a promising remediation technique for soils contaminated with pesticides and other hazardous substances, which does not affect the soil structure and does not leave any type of residues.
[1] J. Sunarso and S. Ismadji, Journal of Hazardous Materials 161, 1 (2009).
[2] G. Anitescu and L.L. Tavlarides, Journal of Supercritical Fluids 38, 167 (2006).
[3] M.N. Baig, G.A. Leeke, P.J. Hammond and R.C.D. Santos, Environmental Pollution 159, 1802 (2011).
[4] S.R. Shirsath, S.H. Sonawane and P.R. Gogate, Chemical Engineering and Processing 53, 10 (2012).
[5] E. Riera, Y. Golás, A. Blanco, J.A. Gallego, M. Blasco and A. Mulet, Ultrasonics Sonochemistry 11, 241 (2004).
[6] H. Bagherian, F.Z. Ashtiani, A. Fouladijatar and M. Mohtashamy, Chemical Engineering and Processing 50, 1237 (2011).
Acknowledgments
The authors would like to acknowledge the Centro de Biotecnologia e Química Fina (CBQF), of the Escola Superior de Biotecnologia, Universidade Católica Portuguesa, Portugal, for the use of the supercritical extraction equipment.
The success of in-situ remediation strongly depends on the level of knowledge and a quality description of the geological structure of the contaminated site. Most failed applications of in-situ remediation technologies were based on successful laboratory and field pilot tests. The failure of the remediation methods is usually not a failure of the technology itself, but it is a failure of the insufficient description of the geological and hydrogeological conditions at the site. The basic conditions for successful remediation include a detailed description of contaminant distribution, proper understanding of the groundwater flow, a detailed geological description, a description of stratification and fracturation, a description of aquifer inhomogeneities, and an understanding of the directions and velocities of groundwater flow. In recent years, high resolution methods have been used for a very detailed description of the contamination as well as the geological and hydrogeological conditions. A complex of methods related to well-logging technology is a very effective system for high resolution diagnosis.
Some data obtained from well-logging cannot be obtained by other methods. Well-logging is irreplaceable from this perspective. One such group of data is used to clarify the groundwater flow in the borehole and clarify its relation with the geological and tectonic structure and the construction of the well. Using an appropriate set of well-logging methods we can determine depths of permeable layers and of open fractures in which there is a flow, it is also possible to measure the intensity of flow. The measurement can determine whether there is a flow across the borehole or whether there is water “short circuit” between two permeable layers. Well-logging can also be used to determine the groundwater flow direction. The advantage of well-logging is its ability to detect fast but also very slow flow: centimeters per day, and slower. If there is a group of wells at a location, it is possible to measure this group of wells to draw conclusions about groundwater flow not only in the wells themselves but also in the whole rock body. This method is widely applied to sites with contaminated groundwater. Based on the results it is possible not only to describe the current state of migration of the contaminated water (in favorable cases it is possible to also record the form of pollution i.e. water insoluble) but a fairly accurate estimate can be made of the further spread of the contamination plume. Well-logging thus provides important information that can be used in planning the optimal remedial method as well as during the actual remediation: in the risk analysis stage, during the remedial work and also in the course of monitoring after completion of the remediation.
This paper describes examples of measured data from sites and demonstrates the significance of well-logging measurements for a detailed understanding, enhancement and optimization of in-situ remedial systems. Detailed measurement and interpretation of natural conditions together with a detailed description of the contaminant distribution provide essential information for the design and dimensioning of the in-situ application of reactive agents and monitoring of in-situ remedial technologies. The presented data are from the interstitial soil environment as well as from a fractured bedrock site.
Acknowledgements: We would like to thank the TA ČR for their financial support (project No. TE01020218, research centre “NANOBIOWAT” and project FP7 No. 309517 “NANOREM”)