Contaminant Removal Mechanisms Research Articles

The Effect of Concentrated Electron Donors on the Solubility of Trichloroethene

AbstractThe use of enhanced in situ bioremediation to remove chlorinated ethenes in sources areas is an emerging application of this remediation technology. Contaminant mass removal likely involves a number of different physicochemical processes; however, recent vendor claims suggest that the addition of highly concentrated electron donor solutions to increase the aqueous solubility of trichloroethene (TCE) is a significant mechanism of contaminant mass removal. To assess the validity of these claims, experimental measurements of the solubility of TCE in aqueous solution with eight typical electron donors at concentrations ranging from 0.5% to 25% are presented. The results of these measurements, in accordance with the theoretical understanding of solubility, indicate that the salts of carboxylic acids, including common electron donors (e.g., sodium lactate), depress the solubility of TCE in accordance with the theoretical predictions of the Setschenow equation. Three of the electron donors tested (ethanol, acetic acid, and lactic acid) increased the solubility of TCE. The largest increase in solubility resulted from ethanol addition, with only a fourfold increase at an ethanol concentration of 25%. These results suggest that salting‐out effects are yet another consideration, along with stoichiometric electron donor requirements, adverse geochemical impacts, utilization by dechlorinating organisms, and electron donor longevity, for electron donor selection.

Microwave-assisted stripping of oil contaminated drill cuttings

The application of microwave heating technology to conventional gas stripping processes has been investigated in the remediation of contaminated drill cuttings. The technical feasibility and limitations of nitrogen and steam stripping processes are demonstrated, and it is shown that the combination of microwave heating with the stripping process offers a step change in performance. Order of magnitude improvements in processing time are shown for the microwave-assisted processes, as well as greatly improved levels of remediation. The mechanisms of contaminant removal are discussed, along with the phenomena which occur with microwave heating processes. The energy requirements of each of pure gas and microwave-assisted processes are also discussed, and the potential applications of each technology are highlighted relative to the overall remediation requirements.

Passive Treatment of Acid Mine Drainage in Bioreactors using Sulfate‐Reducing Bacteria

Acid mine drainage (AMD), characterized by low pH and high concentrations of sulfate and heavy metals, is an important and widespread environmental problem related to the mining industry. Sulfate-reducing passive bioreactors have received much attention lately as promising biotechnologies for AMD treatment. They offer advantages such as high metal removal at low pH, stable sludge, very low operation costs, and minimal energy consumption. Sulfide precipitation is the desired mechanism of contaminant removal; however, many mechanisms including adsorption and precipitation of metal carbonates and hydroxides occur in passive bioreactors. The efficiency of sulfate-reducing passive bioreactors is sometimes limited because they rely on the activity of an anaerobic microflora [including sulfate-reducing bacteria (SRB)] which is controlled primarily by the reactive mixture composition. The most important mixture component is the organic carbon source. The performance of field bioreactors can also be limited by AMD load and metal toxicity. Several studies conducted to find the best mixture of natural organic substrates for SRB are reviewed. Moreover, critical parameters for design and long-term operation are discussed. Additional work needs to be done to properly assess the long-term efficiency of reactive mixtures and the metal removal mechanisms. Furthermore, metal speciation and ecotoxicological assessment of treated effluent from on-site passive bioreactors have yet to be performed.

Behaviour of Bulk and Trace Organics in a Full Scale Membrane Bioreactor for Landfill Leachate Treatment

Landfill leachates are generally highly pollutant wastewaters which have percolated through a deposit of solid waste. In many states it is now required to collect and treat landfill leachates to avoid contamination of surface and groundwater bodies. A state of the art technology for landfill leachate treatment uses membrane bioreactor technology prior to potential polishing with dense membrane processes, activated carbon adsorption or chemical oxidation to make the effluent suitable either for discharge. Landfill leachate contains a wide range of potentially hazardous chemical contaminants. This paper investigates contaminant removal mechanisms in the membrane bioreactor, particularly the impact of ultrafiltration on the retention of bulk organics, which are characterized by size exclusion chromatography, and endocrine disrupting trace organics. The xenoestrogenic chemical bisphenol A deals as an indicator substance and is analysed in raw leachate, activated sludge and final effluent. Bisphenol A removal rates of up to 99% were found. Some retention of the low-molecular bisphenol A compound in ultrafiltration of activated sludge biomass has been observed and attributed to an association with macromolecules and colloids, which are retained by the porous membrane.

動電学的手法による汚染土壌の浄化修復

Electrokinetic remediation is an emerging technology that can be used to remove contaminants from heterogeneous fine-grained soils in situ. Contaminants in the subsurface are removed by the application of a low level direct current electric field across the contaminated soil. The primary contaminant transport and removal mechanisms are electroosmotic advection and ionic migration. However, there are many complex physicochemical reactions occurring simultaneously during the process that may enhance or retard the cleanup process. Nevertheless, the feasibility of the process has been demonstrated by results obtained from many bench scale and large scale laboratory and pilot scale field experiments performed on various soils.This paper reviews the fundamental concepts and the practical aspects of the technology. Laboratory studies of the technique are outlined and major factors affecting the removal process are examined. Enhancement techniques and hybrid process are considered and examples of field trials are described.

Modeling turbulent diffusion and advection of indoor air contaminants by Markov chains.

Turbulent eddy diffusion models are used to describe a continuous concentration gradient with distance from an in-room contaminant emission source. A refined diffusion model termed the Drivas model also accounts for contaminant reflection by wall surfaces and partially accounts for removal by exhaust air. This article develops two models based on Markov chains to describe indoor air contaminant dispersion by turbulent diffusion and advection, and removal by the exhaust airflow. Markov model I is equivalent to the Drivas model and is computationally simple. Markov model II can provide more realism by accounting for the locations of air inlets and outlets, advective flow patterns, in-room reflective surfaces, and contaminant removal mechanisms at specific room positions. The price paid for this greater realism is greater computational complexity. Both Markov models are explicitly probabilistic and estimate the expected concentration values at given room positions.

Practical Aspects of In-Situ Electrokinetic Extraction

Electrokinetic extraction in an emerging technology that can be used to remove contaminants from heterogeneous fine-grained soils in situ. Contaminants in the subsurface are removed by the application of a direct-current electric field across the contaminated soil. The primary contaminant transport and removal mechanisms are electroosmotic advection and ionic migration. However, there are many complex physicochemical reactions occurring simultaneously during the process that may enhance or retard the cleanup process. Nonetheless, the viability of the technology has been established by results obtained from many bench-scale and large-scale laboratory and pilot-scale field experiments performed on various soils. This paper will review the fundamental concepts of the technology and discuss some important practical aspects and design criteria of the technology for field implementations. An example on cost analysis of the technology is also presented to demonstrate the use of various equations presented in the paper and to illustrate the cost-effectiveness of the technology.

Predictive Relationships for Uptake of Organic Contaminants by Hybrid Poplar Trees

Twelve organic compounds commonly found at hazardous waste sites were studied for uptake by hybrid poplar trees. The vegetative uptake of many of these compounds has not previously been demonstrated for plant species being utilized for phytoremediation, such as hybrid poplar trees. Experiments were conducted hydroponically utilizing 14C-labeled compounds to ascertain translocation and fate. Predictive relationships for the translocation and partitioning to plant tissues were developed from the experimental data. Translocation and partitioning relationships based on compounds' octanol−water partitioning coefficients (log Kow) produced the best results, but the relationships did not allow for fully accurate prediction of each contaminant's fate. Translocation and subsequent transpiration of volatile organic compounds (VOCs) from the leaves to the atmosphere was shown to be a significant pathway. As full-scale phytoremediation systems are deliberated, the pathways investigated here should be considered in terms of a contaminant removal mechanism and potential contamination of the vegetation.

Anoxic treatment of trifluralin-contaminated soil

Amending anoxic soils with stoichiometric amounts of sodium acetate led to the complete transformation of trifluralin within the 45 day treatment period. Under these conditions, a maximum trifluralin transformation rate of 4.9 mg kg −1 of soil per day was estimated, which corresponded to a chemical half life of 11.9 days. Regression analyses indicated that the zero order rate model provided the best fit to the experimental data, suggesting that the trifluralin transformation rate is independent of concentration during acetate addition. Using radiolabeled trifluralin, it was determined that the principal contaminant transformation mechanisms were degradation and bound residue formation (i.e., irreversible adsorption). Volatilization and mineralization of trifluralin were found to be negligible over the 45 day treatment period. Using poisoned controls, it was determined that trifluralin transformation under acetate-amended conditions was biologically mediated. Amending trifluralin contaminated soils with stoichiometric amounts of iron sulfide resulted in complete trifluralin transformation within 24 hours of treatment. A maximum trifluralin transformation rate of 380 mg kg −1 of soil per day was estimated for this system, which corresponded to a chemical half life of 4.4 h. The rates of trifluralin transformation followed the first-order kinetic model during iron sulfide addition. Using radiolabeled trifluralin, it was found that chemical degradation was the principal removal mechanism. Neither volatilization nor mineralization was found to be a significant contaminant removal mechanism during iron sulfide treatment. Poisoned controls indicated that trifluralin transformation was mediated primarily by an abiotic chemical reaction mechanism. Additional study is required to clarify the rate limiting steps so that full scale soil treatment systems may be properly designed.

Oxidation behavior of aqueous contaminants in the presence of hydrogen peroxide and filter media

Hydrogen peroxide has been used as an oxidant to degrade contaminants in solutions and soils. A poor understanding of the numerous variables that are involved makes it difficult to determine dominant contaminant removal mechanisms. Our primary objective was to examine the relationship between contaminant (quinoline and nitrobenzene) degradation rate and the rate of hydrogen peroxide decomposition on filter media. Both batch and continuous flow column experiments were conducted. In general, the rate of contaminant degradation was proportional to the rate of hydrogen peroxide decomposition, but the mass of contaminant removed depended on the amount of hydrogen peroxide decomposed, filter medium concentration, and filter medium characteristics. For increasing filter medium concentration and equivalent loss of hydrogen peroxide, the mass of contaminant degraded was found to decrease. In addition, acid-hydroxylamine treatment of the selected filter medium, to examine the role of reducible metal oxide coatings, resulted in greater contaminant removals than the parent material despite a slower hydrogen peroxide decomposition rate. The observed hydrogen peroxide decomposition and contaminant oxidation results are consistent with a reaction scheme whose central elements include: (1) a rate limiting filter medium surface catalyzed reaction initiating hydrogen peroxide decomposition with the formation of a reactive intermediate, (2) a competing reaction of the intermediate with the filter medium surface, and (3) reaction of the same intermediate with the aqueous organic contaminant. Loss of quinoline and nitrobenzene is most likely a solution phase reaction because sorption of these compounds was small over the pH range 7–8 and oxidation efficiency did not increase with increasing filter medium concentration, which would be expected if the reactions were occurring on the surface. Finally, enhanced oxidation of quinoline and nitrobenzene on the treated material is explained by more efficient use of the reactive intermediates for contaminant oxidation due to a reduction in the number of scavenging sites associated with reducible metal oxide coatings.

Cryogenic Blasting Using Carbon Dioxide Media for the Removal of Organic Coatings

This paper describes the operation of the «Cold Jet» cryogenic blasting process and the mechanism of coatings and contaminant removal by the effect of thermal shock rather than mechanical abrasion. The measurement of cleaning effectiveness is presented, as performed by optically stimulated electron emission (OSEE) equipment. Case studies are described which illustrate applications for the technique on both metallic and polymeric substrates