Effect of oxygen content on the thermal desorption of polychlorinated biphenyl-contaminated soil

Best management practices for minimizing undesired effects of thermal remediation and soil washing on soil properties. A review

The use of remediated soils as end-of-life materials raises some challenges including policy and regulation, permits and specifications, technological limitations, knowledge and information, costs, as well as quality and performance associated with using them. Therefore, a set of procedures must be followed to preserve the quality and fundamental properties of soil during a remediation process. This study presented a comprehensive review regarding the fundamental impacts of thermal desorption (TD) and soil washing (SW) on soil characteristics. The effects of main operating parameters of TD and SW on the physical, chemical, and biological properties of soil were systematically reviewed. In TD, temperature has a more remarkable effect on physic-chemical and biological characteristics of soil than heating time. Therefore, decrease in temperature within a suitable range prevents unreversible changes on soil properties. In SW, more attention should be paid to extraction process of contaminants from soil particles. Using the right dosage and type of chelating agents, surfactants, solvents, and other additives can help to avoid problems with recovery or treatment using conventional methods. In addition, this review introduced a framework for implementing sustainable remediation approaches based on a holistic approach to best management practices (BMPs), which, besides reducing the risks associated with different pollutants, might provide new horizons for decreasing the unfavourable impacts of TD and SW on soil.

Recent advances in PCB removal from historically contaminated environmental matrices

Despite being drastically restricted in the 1970s, polychlorinated biphenyls (PCBs) still belong among the most hazardous contaminants. The chemical stability and dielectric properties of PCBs made them suitable for a number of applications, which then lead to their ubiquitous presence in the environment. PCBs are highly bioaccumulative and persistent, and their teratogenic, carcinogenic, and endocrine-disrupting features have been widely reported in the literature. This review discusses recent advances in different techniques and approaches to remediate historically contaminated matrices, which are one of the most problematic in regard to decontamination feasibility and efficiency. The current knowledge published in the literature shows that PCBs are not sufficiently removed from the environment by natural processes, and thus, the suitability of some approaches (e.g., natural attenuation) is limited. Physicochemical processes are still the most effective; however, their extensive use is constrained by their high cost and often their destructiveness toward the matrices. Despite their limited reliability, biological methods and their application in combinations with other techniques could be promising. The literature reviewed in this paper documents that a combination of techniques differing in their principles should be a future research direction. Other aspects discussed in this work include the incompleteness of some studies. More attention should be given to the evaluation of toxicity during these processes, particularly in terms of monitoring different modes of toxic action. In addition, decomposition mechanisms and products need to be sufficiently clarified before combined, tailor-made approaches can be employed.

Insight into degradation mechanism of PCBs from thermal desorption off-gas over iron-based catalysts

Iron-based catalysts were developed to achieve the hydrodechlorination (HDC)/oxidation of polychlorinated biphenyls (PCBs) from thermal desorption off-gas, and Fe₃O₄/γ-Al₂O₃ showed higher dechlorination efficiency than Fe₂O₃/γ-Al₂O₃. The optimal Fe loading resulted in 95.5% degradation efficiency and 76.9% toxicity reduction of gaseous PCBs, and the optimal Fe₃O₄/γ-Al₂O₃ exhibited excellent stability during a 60-h test. The gas chromatography-mass spectrometry analysis of intermediate products indicated the presence of two competitive degradation pathways, namely, hydrodechlorination and oxidation with Fe₃O₄/γ-Al₂O₃ as catalyst. During the first stage (reductive dechlorination), the reductive activity of iron-based catalysts was effectively enhanced in the presence of water, which was confirmed by density functional theory (DFT) calculations. The removal of chlorine atoms was found in the order of meta > para > ortho. During the second stage (oxidation), hydroxyl and superoxide anion radicals were found to attack PCBs on the surface of Fe₃O₄/γ-Al₂O₃. This study provides an insight into the HDC and oxidation mechanism of gaseous PCBs over iron-based catalysts.

Thermal remediation of cyanide-contaminated soils：process optimization and mechanistic study

Site soils with persistent cyanide compounds (primarily iron-cyanide complex) pose potential hazards to the environment and require remediation before redevelopment. This study evaluated the possibility of thermal treatment on remediation of cyanide-contaminated soils via batch heating experiments spanning a wide temperature range (200-500 °C). The change with operation variables of total cyanide and some reaction intermediates (e.g. CN^-) was analyzed in order to elucidate the optimal variables that guarantee cyanide removal while generating no hazardous byproducts. Temperature, heating time and cyanide species have been found to be important parameters influencing removal/destruction of cyanide in soils. For soils bearing K₃[Fe(CN)₆] and K₄[Fe(CN)₆], a removal efficiency of >99.9% can be obtained with temperatures over 350 °C at 1 h, while for samples bearing Fe₄[Fe(CN)₆]₃, a higher temperature (>450 °C) is needed to obtain an equivalent efficiency. During heating, the iron-cyanide complexes decomposed, releasing highly toxic free cyanides, which will subsequently be oxidized. However, a small percentage of free cyanide can always be detected as a result of incomplete oxidation, thus caution should be taken to minimize the accumulation of free cyanide during thermal treatment.

Thermal desorption of PCBs contaminated soil with calcium hydroxide in a rotary kiln

In this study, thermal desorption was combined with the addition of calcium hydroxide to remediate polychlorinated biphenyls (PCBs) contaminated soil, collected from a storage point for PCB-contaminated capacitors and transformers. The thermal desorption test conditions were varied from 300 to 600 °C, both with blank soil and with 1% Ca(OH)₂ added. The results showed that the synergistic thermal desorption was effective to removal most of PCBs. At 600 °C, the removal efficiency (RE) reached 94.0% in presence of Ca(OH)₂, higher than that of 90.9% in blank soil. The dechlorination was significant when compared with blank soil. Ca(OH)₂ effectively decreased both the sum and the toxic equivalence quantity (TEQ) value of the 12 dioxin-like PCBs, with the RE based on TEQ of 90.0%. Ca(OH)₂ strengthened the removal, dechlorination and detoxication of PCBs. The synergistic effect factor proved the promotion did exist in the presence of Ca(OH)₂.

Thermal desorption for remediation of contaminated soil: A review

Soil pollution has become a global environmental concern. Thermal desorption is one of the methods commonly used to remediate contaminated soil. This method has received increasing attention for remediating contaminated sites due to its advantages, such as suitability to different types of contaminants, short treatment period, high efficiency, high safety, and capability to recycle soil and contaminants. This paper provides a comprehensive review of studies on thermal desorption. Introduction of the mechanism, classification, and cost of thermal desorption is presented. Factors affecting the performance of thermal desorption (heating temperature, heating time, heating rate, carrier gas, soil particle size, moisture content, initial concentration of contaminants, and additives) are reviewed. Thermal desorption produces off-gases, which are mostly organic compounds and may result in secondary pollution. Thus far, treatment methods for off-gas have not been systematically investigated. This paper also summarizes current methods for treatment of off-gas in the soil field and of volatile organic compounds in atmospheric and water pollution fields. Several feasible off-gas treatment methods are also comparatively analyzed, and the corresponding principles, advantages, disadvantages, and application ranges are discussed.

Removal of PCDD/Fs, PCP and mercury from sediments: Thermal oxidation versus pyrolysis

A continuous pilot-scale system (CPS) equipped with effective air pollution control devices (APCDs) is used for remediating the sediments contaminated with PCDD/Fs, PCP and Hg simultaneously. The removal efficiencies of these three pollutants in sediments collected from seawater pond and river, respectively, are evaluated via thermal treatment processes. PAHs and CBz formed during thermal oxidation and pyrolysis are also analyzed for better understanding the behaviors of chlorinated organic compounds. Experimental results indicate that low-molecular-weight PAHs are closely related to the formation of CBz, PCDD/Fs, and CPs, while low chlorinated PCDD/Fs and CBz are predominant in flue gas with thermal oxidation. However, the PM concentration is higher in thermal oxidation than pyrolysis due to the higher air flow rate of thermal oxidation. It may bring more particles out of the furnace and have a greater potential to form PCDD/Fs within APCDs. Besides, the high air flow also dilutes the Hg vapor in flue gas and would require more energy to condense and collect Hg with the quench tower. Furthermore, for removal of total amount of PCDD/Fs, pyrolysis is better than thermal oxidation. Thus, pyrolysis is more suitable for remediating the contaminated sediment. The removal efficiencies of PCDD/Fs, PCP and Hg in sediments achieved with pyrolysis increase with increasing operating temperature and retention time in CPS. Overall, the residual concentrations of PCDD/Fs and PCP in river sediment are higher than that in seawater-pond sediment since significant formation of tar is observed due to higher organic matter content in river sediment.

PCDD/F formation during thermal desorption of chlorobenzene contaminated soil

Unintentional formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) is observed and investigated during the thermal desorption in an airflow of a sandy soil, doped artificially with either 1,2-dichlorobenzene (1,2-DiCBz) or hexachlorobenzene (HCBz) using a lab-scale experimental set-up. At all temperatures investigated (200, 250, 300, 350 and 400 °C), this thermal treatment creates significant amounts of PCDD, PCDF and polychlorinated biphenyls (PCB), starting from 1,2-DiCBz. The highest yield of PCDD/F formed from 1,2-DiCBz occurs at 250 °C, with a total (gas + residual soil) output of 117 and 166 pg/g PCDD and PCDF, respectively. Most output reports to the gas phase and the PCDD/F signature is significantly different for residue and gas phase. Also PCB are formed, at a scale of 224 ng/g (300 °C). Compared with 1,2-DiCBz, HCBz converts into PCDD/F even more actively at 350 and 400 °C: the total PCDD/F output created attains 967 pg/g PCDD and 465 pg/g PCDF at 350 °C. As a precursor, 1,2-DiCBz favours formation of PCDF, while PCDD predominates, when the HCBz contaminated soil is treated.

PCDD/F formation during thermal desorption of p,p'-DDT contaminated soil

Thermal treatment of polychlorinated biphenyls (PCB) contaminated soil was shown in earlier work to generate polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF). In this study, the PCDD/F were studied arising during the remediation of p,p'-DDT contaminated soil by thermal desorption. Three kinds of soil (sandy, clayey and lateritic soil) were tested to investigate the effect of soil texture on PCDD/F formation. Those soils were artificially polluted with p,p'-DDT, obtaining a concentration level of 100 mg/kg. Thermal desorption experiments were conducted for 10 min at 300 °C in an air atmosphere. The total concentration of PCDD/F generated for three soils were 331, 803 and 865 ng/kg, respectively, and TeCDD and TeCDF were dominant among all PCDD/F congeners. After thermal desorption, the total amount of PCDD/F generated both in soil and in off-gas correlated positively with the amount of DDT added to soil. In addition, a possible pathway of the formation of PCDD/F was presented.

Remediation of PCB-contaminated soil using a combination of mechanochemical method and thermal desorption

The combination of mechanochemical method and thermal desorption for remediating polychlorinated biphenyls (PCBs) in contaminated soil was tested in this study. The effects of grinding time and heating time on PCB removal efficiency were investigated. The contaminated soil, mixed with CaO powder at a weight ratio of 1:1, was first ground using a planetary ball mill. After 4 h of grinding, the total PCB concentration and its toxic equivalence quantity (TEQ) decreased by 74.6 and 75.8%, respectively. Then, after being heated at 500 °C for 60 min, the residual PCBs in mechanochemical + thermal treated soil decreased to 247 ng/g, resulting in a removal efficiency of 99.95%. The removal effect can be promoted by longer grinding time and heating time; however, increased energy consumption was inevitable. The combination of grinding time and heating time should be optimized in a practical remediation process.

Combined mechanochemical and thermal treatment of PCBs contaminated soil

This study combines a preliminary mechanochemical treatment and a subsequent thermal desorption for remediating soil, contaminated with polychlorinated biphenyls (PCBs).

Suppression of PCDD/Fs during thermal desorption of PCBs-contaminated soil

Thermal treatment of polychlorinated biphenyls (PCBs) contaminated soil was shown in earlier work to generate new PCBs, as well as polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). In this study, this thermal desorption was conducted with addition of three distinct inhibitors, including ammonium sulphate, urea and calcium oxide, to inhibit the formation of PCDDs and PCDFs when remediating PCBs-contaminated soil. Experiments were conducted for 40 min at 400 °C after adding 1 wt.% of inhibitor. Both the total PCDD/Fs and international toxic equivalent quantity (I-TEQ) reduced when inhibitors were introduced. Of the three compounds tested, CaO shows the highest inhibition efficiency, 92.2 % for total PCDD/Fs and 95.6 % for I-TEQ. The amount of CaO added also influences the suppression efficiency of PCDD/Fs. These results suggest that promoting desorption and destruction of precursors is probably the mechanism of suppression.

Pyrolytic Treatment and Fertility Enhancement of Soils Contaminated with Heavy Hydrocarbons

Pyrolysis of contaminated soils at 420 °C converted recalcitrant heavy hydrocarbons into "char" (a carbonaceous material similar to petroleum coke) and enhanced soil fertility. Pyrolytic treatment reduced total petroleum hydrocarbons (TPH) to below regulatory standards (typically <1% by weight) within 3 h using only 40-60% of the energy required for incineration at 600-1200 °C. Formation of polycyclic aromatic hydrocarbons (PAHs) was not observed, with post-pyrolysis levels well below applicable standards. Plant growth studies showed a higher biomass production of Arabidopsis thaliana and Lactuca sativa (Simpson black-seeded lettuce) (80-900% heavier) in pyrolyzed soils than in contaminated or incinerated soils. Elemental analysis showed that pyrolyzed soils contained more carbon than incinerated soils (1.4-3.2% versus 0.3-0.4%). The stark color differences between pyrolyzed and incinerated soils suggest that the carbonaceous material produced via pyrolysis was dispersed in the form of a layer coating the soil particles. Overall, these results suggest that soil pyrolysis could be a viable thermal treatment to quickly remediate soils impacted by weathered oil while improving soil fertility, potentially enhancing revegetation.