Remediation of PAHs contaminated soil by extraction using subcritical water

Environmental remediation of emerging contaminants using subcritical water: A review

The continuous rise of emerging contaminants (ECs) in the environment has been a growing concern due to their potentially harmful effects on humans, animals, plants, and aquatic life, even at low concentrations. ECs include human and veterinary pharmaceuticals, hormones, personal care products, pesticides, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), organic dyes, heavy metals (HMs), and others. The world's growing population contributes to the release of many kinds of chemicals into the environment, which is estimated to be more than 200 billion metric tons annually and results in over 9 million deaths. The removal of these contaminants using conventional physical, chemical, and biological treatments has proven to be ineffective, highlighting the need for simple, effective, inexpesive, practical, and eco-friendly alternatives. Thus, this article discusses the utilization of subcritical water oxidation (SBWO) and subcritical water extraction (SBWE) techniques to remove ECS from the environment. Subcritical water (water below the critical temperature of 374.15 °C and critical pressure of 22.1 Mpa) has emerged as one of the most promising methods for remediation of ECs from the environment due to its non-toxic properties, simplicity and efficiency of application. Furthermore, the impact of temperature, pressure, treatment time, and utilization of chelating agents, organic modifiers, and oxidizing agents in the static and dynamic modes was investigated to establish the best conditions for high ECs removal efficiencies.

A Review: Subcritical Water Extraction of Organic Pollutants from Environmental Matrices

Most organic pollutants are serious environmental concerns globally due to their resistance to biological, chemical, and photolytic degradation. The vast array of uses of organic compounds in daily life causes a massive annual release of these substances into the air, water, and soil. Typical examples of these substances include pesticides, polychlorinated biphenyls (PCBs), and polycyclic aromatic hydrocarbons (PAHs). Since they are persistent and hazardous in the environment, as well as bio-accumulative, sensitive and efficient extraction and detection techniques are required to estimate the level of pollution and assess the ecological consequences. A wide variety of extraction methods, including pressurized liquid extraction, microwave-assisted extraction, supercritical fluid extraction, and subcritical water extraction, have been recently used for the extraction of organic pollutants from the environment. However, subcritical water has proven to be the most effective approach for the extraction of a wide range of organic pollutants from the environment. In this review article, we provide a brief overview of the subcritical water extraction technique and its application to the extraction of PAHs, PCBs, pesticides, pharmaceuticals, and others form environmental matrices. Furthermore, we briefly discuss the influence of key extraction parameters, such as extraction time, pressure, and temperature, on extraction efficiency and recovery.

Microbe combined with Fe2+-heat activated persulfate to decompose phenanthrene in red soil: comparison of acid-resistant degrading microflora and indigenous bacteria

This work is designed to counteract the deficiency of targeted research on the PAHs polluted specific soil, especially when the chemicals extremely denatured it. Phenanthrene-contaminated red soil was treated through two-stage process: persulfate oxidation (on dosages of 3.48%, 5.21%, and 6.94%, combined with Fe2+ and β-cyclodextrin, then heated) followed by biodegradation (indigenous bacteria vs. acid-resistant PAHs-degrading microflora (named ADM)) for 90 days. The dosage of oxidant greatly affected the removal efficiencies, which ranged from 46.78 to 85.34% under different treatment. After undergoing oxidation, the soil pH dropped below 3.0 synchronously and retained relatively strong oxidation state. The indigenous bacteria in red soil showed considerable degradation potential that will not vanish upon the sudden change of soil properties, whose average combined removal reached 95.43%, even higher than subgroups of bioaugmentation, but the population structure showed extremely simplex (Proteobacteria as superior occupied proportion of 91.77% after 90-day rehabilitation). The ADM screened from the coking wastewater was dominated by Klebsiella (75.4%) and Pseudomonas (23.6%), whose cooperation with 6.94% persulfate made the residual PHE reduced to less than 50 mg·kg-1 in about 28 days. High-throughput sequencing analysis showed that the microbial community composition of the ADM applied-group was more abundant in the later stage of remediation. ADM inoculation has the advantages of shortening the restoration period and having a positive impact on the soil micro-ecology.

Mechanochemical remediation of fluoranthene contaminated soil and biotoxicity evaluation

A mechanochemical (MC) method was employed for the remediation of soil contaminated with fluoranthene (C₁₆H₁₀, FL) a four-ringed polycyclic aromatic hydrocarbon (PAH) containing three benzene rings and a central five-membered heterocyclic ring, with the effects of soil inorganic components, milling conditions, and the degradation mechanism investigated. Results showed that the addition of SiO₂ and kaolin to soil resulted in a greater increase in the effectiveness of FL removal than other inorganic additives. After 3 hours of milling at 500 rpm, the FL removal rate from SiO₂ containing soil, reached 99.26%, with the removal efficiency increasing in accordance with an increase in milling duration and speed. The milled samples were characterized by FT-IR, Raman spectroscopy, and GC-MS analysis, revealing the mechanism of FL degradation, including destruction of the aromatic skeleton structure and the formation of amorphous carbon and graphite. The MC remediation method was applied to FL contaminated soil, showing that FL was efficiently degraded in soil without any soil additives, resulting in a significant reduction in the biotoxicity of the remediated soil. The organic matter, moisture content and pH of the actual soil changed slightly after mechanical ball milling. Thus, the MC method has high potential in the remediation of PAH-contaminated soils.HIGHLIGHTSA mechanochemical (MC) method for the degradation of fluoranthene was assessed.The use of silica and kaolin as soil additives enhances fluoranthene remediation.Fluoranthene can be efficiently removed from contaminated soil by milling alone.The degradation mechanism was skeleton structure destruction and carbonization.The biotoxicity of soil was significantly reduced by milling.

Evaluation of Heavy Metals in Soil Wastewater Stream

Environmental pollution is one of the main factors that significantly affect biological systems and human health. Soil pollution with heavy metals is an increasingly pressing problem worldwide. In general, heavy metals are stable and do not decompose, unlike other organic pollutants. The quantity of them is natural components of soil crust, the remaining come from human activities, which may result from the extensive use of sewage. In the present study, a methodology aimed at simultaneous quantification of 16 heavy elements in soil of 3 different regions was developed. The concentration of 16 soil heavy metals (Se, Cd, V, Be, As, Mn, Co, Zn, Fe, Cr, Pb, Ni, Cu, Mo, Hg, and Ti) was measured in 11 sampling along Riyadh, Qassim, and Medina, Kingdom of Saudi Arabia from 3 sites soil treated with sewage water. These chemical minerals were identified in the samples using an ICPE-9000 spectrometer. The assessment of heavy metal contamination was derived using enrichment factors (EF), the pollution load index (PLI), and geoaccumulation index (I_geo). This study revealed that the soil is predominantly polluted by Cd, As, and Mo of Riyadh and Medina and As, Mo, and Cd of Qassim region at site B and site C, respectively. As recorded, the highest concentration value of 5000 mg/kg for Fe at site (B) followed by Cu. The I_geo value of Cd is 1.1520 in Medina region. The I_geo value of Se is 3.2395 in Medina region, while its cumulative geographical index decreased in the regions of Riyadh and Qassim, which amounted to 2.6114 and 2.1699, respectively. The I_geo values of the rest of the minerals in the three regions studied indicated that the soil is unpolluted, while it was slightly to moderately polluted for both Mo and Hg in most regions studied. The minerals in the soil at all sites studied were less than the general average concentration. With the exception of mercury, molybdenum, arsenic, cadmium, and selenium, whose concentration exceeded the permissible and recommended limits. The increasing order of concentration of minerals was Be < V < Cd < Hg < Mo < Co < Zn < Pb < Cr < Se < As < Ni < Ti < Mn < Cu < Fe at all sites, respectively.

Insights into the removal efficiencies of aged polycyclic aromatic hydrocarbons in humic acids of different soil aggregate fractions by various oxidants

Chemically oxidative removal of polycyclic aromatic hydrocarbons (PAHs) in soil is related to their occurrence state. Whether the heterogeneity of natural organic matter has an effect on the occurrence of PAHs in soil and, if there is an effect, on the oxidative removal efficiency of PAHs remains unknown. In this study, the removal efficiencies of 16 priority PAHs aged in humic acids (HAs) of different soil aggregate fractions by various oxidants were investigated by combining soil fractionation and microreaction experiments. Results showed that the accumulations of PAHs in particulate HA (P-HA) and microaggregate occluded HA (MO-HA) mainly occurred in the early period of the aging time frame. In contrast, PAH accumulation in non-aggregated silt and clay associated HA (NASCA-HA) was relatively slow and tended to saturate in the late period of the aging time frame. The cumulative contents of PAHs throughout the entire aging period in MO-HA and NASCA-HA were significantly greater than that in P-HA. The aged PAHs in P-HA and NASCA-HA exhibited the highest and lowest removal efficiencies, respectively. This ranking was mainly governed by the molecular size and polarity of HAs. Sodium persulfate and potassium permanganate had the highest removal efficiencies in total PAHs in HAs, with average efficiencies of 85.8% and 79.1%, respectively, in P-HA. Hydrogen peroxide had the lowest degradation efficiency in PAHs. In particular, the degradation efficiency of total PAHs in NASCA-HA was lowered to 31.0%. PAH congeners in HAs showed a large difference in oxidative removal efficiency. Low-ring PAH was more easily degraded than medium- and high-ring PAHs, and in most treatments, fluoranthene and pyrene in the medium ring and benzo[a]pyrene in the high ring demonstrated higher efficiencies than other PAHs with the same number of rings. Our findings are useful in promoting the accurate and green remediation of PAH-contaminated soils.

Concentrations, distributions, sources, and risk assessment of polycyclic aromatic hydrocarbons in topsoils around a petrochemical industrial area in Algiers (Algeria)

Fifty-five samples were collected from topsoils around a petrochemical industrial area at the east of Algiers (Algeria) and analyzed for 16EPA priority PAHs in the aim to determine the concentrations, the distributions, and the possible sources of polycyclic aromatic hydrocarbons (PAHs). The results of the quantification are then devoted to the assessment of the potential risks as the toxicity, the risk for the ecosystem, and the risk for the human health. The sampling sites were classified into four categories: rural, suburban, urban, and industrial-urban. A new extraction method based on the insertion of a preliminary step, using hot water, was proposed to improve the extraction efficiency. Principal component analysis (PCA) and selected diagnostic ratio of PAHs were used to investigate the source apportionment of these PAHs. The potential toxicity, the ecological, and human health risk of PAHs in soil were estimated using the toxic equivalent quotient, the risk quotient, and the total lifetime cancer risk (TLCR) methods, respectively. The proposed new protocol gave improved recovery rates for the sixteen EPA PAHs particularly for low molecular weight PAHs, with satisfactory repeatability (RSD < 10%). The Σ16PAHs concentrations were varied from 143.73 to 4575.65 μg kg^-1 with a mean value of 1209.56 μg kg^-1. Σ16PAH concentrations found for the industrial areas would be 2 times higher than for urban soils and 3 times higher than for the rural soils. The biplots of PCA and the five diagnostic ratios suggested that the most sources of PAHs in the rural, the suburban, and the urban areas are traffic emissions, biomass burning, and coal combustion sources. Some points of the urban-industrial area are from the petroleum source. The found Σ16PAH concentrations and theirs calculated TEQs showed the following trend: industrial-urban > urban > suburban >rural. The potential cancer of human health risks calculated through TLCR results indicated that the exposure to the 7EPA PAH-contaminated soils produces negligible cancer risk to human health.

Crude Oil Contaminated Soil: Its Neutralization and Use

The paper represents the research results for the process of crude oil-contaminated soil neutralization with the use of a neutralizer obtained on the basis ofhumic substances. Using physical methods (gas and liquid chromatography, fluorimetry, atomic absorption spectrometry, IR spectrometry), the element and group compositionswere determined for the crude oil-contaminated soil, neutralizer, and neutralized soil. Optimal parameters were determined for the process of the crude oil-contaminated soil neutralization under laboratory conditions:weight ratios of the crude oil-contaminated soil, neutralizer and water, and the temperature and neutralization process duration. The technological scheme was developed for the neutralization of the crude oil-contaminated soil in field conditions. It was found that low-boiling point hydrocarbon fractions (C12–C17) disappear completely at neutralization, the content of high-boiling point hydrocarbon fractions (C20–C23) is essentially increased, and the content of oil components and metals, including the toxic ones, is decreased. The engineering characteristics for nine mixtures of the stabilized soil, containing the neutralized soil, were evaluated under laboratory conditions and the conditions were determined for their use in road construction (with regard to road category, characteristic pavement layer, minimal air temperature). An experimental road section was constructed with the use of the stabilized soil with neutralized soil (40%).

Temperature-driven variation in the removal of heavy metals from contaminated tailings leaching in northern Norway

High amounts of tailings with a low recycling rate are generated during mining and smelting processes, and a lot of environmental problems were caused by heavy metal leaching from tailings. Temperature is a key point in heavy metals leaching, and knowing the effects of temperature on tailings leaching is useful for tailings management. A small-scale batch leaching experiment was conducted at different temperatures to test temperature-driven heavy metal leaching from tailings in the arctic area. The variation in the leaching of heavy metals from tailings was investigated by a small-scale batch leaching experiment. Results showed that 10 °C is a threshold temperature for the leaching activity of the tested elements. Fe, Cr, and Cu are significantly correlated with temperature in the leaching. Leaching rates of Cr, Cu, and Ni increase as temperature rises. Leaching rates of Cr, Cu, Ni, V, and Zn change by a polynomial model with temperatures, whereas that of Fe changes with a linear model. V shows an antagonistic relationship with Cu, Fe, and Ni in the leaching. However, Cu, Cr, Ni, and Fe show a synergistic relationship. Discovering the threshold temperature of leaching tailings in the arctic area and concluding the influence factors and the relationship between heavy metals leaching and temperature are useful for tailings management.

Remediation approach for organic compounds and arsenic co-contaminated soil using the pressurized hot water extraction process

Successful remediation of soil with co-existing organics contaminants and arsenic (As) is a challenge as the chemical and remediation technologies are different for each group of pollutants. In this study, the treatment effectiveness of the pressurized hot water (PHW) extraction process was investigated for remediation of soil co-contaminated with phenol, crude oil, polycyclic aromatic hydrocarbons (PAHs), and As. An elimination percentage of about 99% was achieved for phenol, and in the range of 63-100% was observed for the PAHs at 260°C for 90 min operation. The performance of PHW extraction in the removal of total petroleum hydrocarbons was found to be 86%. Of the 87 mg/kg of As in untreated soil, 67% of which was eliminated after treatment. The removal of organic contaminants was mainly via desorption, dissolution and degradation in subcritical water, while As was eliminated probably by oxidation and dissolution of arsenic-bearing minerals. According to the experimental results, the PHW extraction process can be suggested as an alternative cleaning technology, instead of using any organic solvents for remediation of such co-contaminated soil.

Assessment of sediment mutagenicity in areas under the influence of a contaminated site undergoing a remediation process

Soil contamination enters aquatic ecosystems affecting sediment quality. The region studied is the Taquari River, Brazil, close to a site contaminated by wood preservatives, with a runoff route into the river. The first stage of the remediation process (In this article, the terms intervention and remediation have been used with slightly different meanings. We consider intervention to be the first phase of the remediation process, which aims to remove active sources) was an intervention to remove the main active sources. The Salmonella/microsome assay and polycyclic aromatic hydrocarbons (PAHs) were used to assess sediment quality in organic extracts during different intervention phases. The strains used were TA98, TA97a, and TA100 with and without S9mix (±S9). The results indicated the presence of pro-mutagens at site Ta010 (closest to the contaminated site) in all samplings, and the highest result occurred before intervention for TA100 + S9 (1,672 ± 215.9 rev/g). These values decreased during (83 ± 23.6 rev/g) and after this process (403 ± 105.9 rev/g), although the PAHs concentrations increased. Samples from this site presented PAHs with a carcinogenic potential during the assessed periods. After intervention, Ta006 (4 km downstream from Ta010) showed the most significant mutagenesis for TA100 + S9 (764 ± 230.2 rev/g) and, although the total PAHs values were lower, the species considered carcinogenic had higher concentrations. Mutagenesis predicted values of PAHs confirmed that carcinogenic species were predominantly detected by TA100, and the other PAHs by TA97a strains. Marked contaminant release to the river was observed, mainly in Ta010 at different periods. Mutagenicity and PAHs values in an internal stream, upstream from Ta010, showed a dispersion route of these agents. Thus, contamination in Ta010 and possible contribution to Ta006, after intervention, provides a warning regarding environmental quality in the region. Environ. Mol. Mutagen. 59:625-638, 2018. © 2018 Wiley Periodicals, Inc.

The use of ultrasound-assisted anaerobic compost tea washing to remove poly-chlorinated dibenzo-p-dioxins (PCDDs), dibenzo-furans (PCDFs) from highly contaminated field soils

The remediation of dioxin-contaminated soil of a specific coastal area previously employed for the manufacture of pentachlorophenol (PCP) in southern Taiwan's Tainan City has attracted much attention of researchers there. This work addresses the possibility of providing an effective and environmentally friendly option for removing PCDD/Fs from soil in that field. Soil screening/sieving was first conducted to assess particle distribution. Fine sand was observed to be the major component of the soil, accounting for more than 60% of the total mass. A combination of ultrasonification and mechanical double-blade agitation was used to facilitate the washing of the soil using the biosurfactant anaerobic compost tea. More than 85 and 95% of total removal efficiencies were achieved for moderately and highly contaminated soils after 6 and 10 washing cycles, respectively, under ambient temperature, a soil/liquid ratio 1:2.5, 700 rpm, and over a relatively short duration. These results were achieved through the collision and penetration effects of this combined treatment as well as PCDD/F partitioning between the particles and anaerobic compost tea. This study represents the first to report the use of anaerobic compost tea solvent to wash soil highly contaminated by dioxin. It was concluded that anaerobic compost tea, rich in non-toxic bio-surfactants (e.g., alcohols, humic acids), can be used to improve bioavailability and bioactivity of the soil making bio-attenuation and full remediation more efficient.

Immobilization and reduction of bioavailability of lead in shooting range soil through hydrothermal treatment

Immobilization of Pb in contaminated soil by hydrothermal treatment (HT) under subcritical conditions was investigated using a lab-scale apparatus. The Pb immobilization was evaluated thorough investigating the transformation of Pb fractions and by single chemical extraction. The results showed that HT and treatment temperature significantly affected the immobilization through redistribution of Pb fractions. The results of bioavailability and eco-toxicity assessment demonstrated a drastic decrease in the bioavailable fraction from 41.33 to 14.66%, and an increase in the non-bioavailable fraction from 2.90 to 15.76%. Moreover, the leaching potential of Pb was significantly reduced after treatment. Based on the risk assessment code (RAC), the treated soil residues exhibited medium risk (21.7-14.6 of RAC value for treated soil), which represented a decrease from high risk (41.3 for untreated soil). Immobilization was associated with Pb fractionation from weakly bound to stable fractions. A variety of mechanisms including adsorption and precipitation were suspected to be responsible for Pb immobilization. The findings of this study suggest that the HT may facilitate quantitative reduction of the bioavailability and eco-toxicity of Pb-contaminated soil. Considering the effectiveness of HT in remediating contaminated soil with organic pollutants, this approach might enable remediation of soil co-contaminated with organics and heavy metals.

Remediation approaches for polycyclic aromatic hydrocarbons (PAHs) contaminated soils: Technological constraints, emerging trends and future directions

For more than a decade, the primary focus of environmental experts has been to adopt risk-based management approaches to cleanup PAH polluted sites that pose potentially destructive ecological consequences. This focus had led to the development of several physical, chemical, thermal and biological technologies that are widely implementable. Established remedial options available for treating PAH contaminated soils are incineration, thermal conduction, solvent extraction/soil washing, chemical oxidation, bioaugmentation, biostimulation, phytoremediation, composting/biopiles and bioreactors. Integrating physico-chemical and biological technologies is also widely practiced for better cleanup of PAH contaminated soils. Electrokinetic remediation, vermiremediation and biocatalyst assisted remediation are still at the development stage. Though several treatment methods to remediate PAH polluted soils currently exist, a comprehensive overview of all the available remediation technologies to date is necessary so that the right technology for field-level success is chosen. The objective of this review is to provide a critical overview in this respect, focusing only on the treatment options available for field soils and ignoring the spiked ones. The authors also propose the development of novel multifunctional green and sustainable systems like mixed cell culture system, biosurfactant flushing, transgenic approaches and nanoremediation in order to overcome the existing soil- contaminant- and microbial-associated technological limitations in tackling high molecular weight PAHs. The ultimate objective is to ensure the successful remediation of long-term PAH contaminated soils.

TNT and RDX degradation and extraction from contaminated soil using subcritical water

The use of explosives either for industrial or military operations have resulted in the environmental pollution, poses ecological and health hazard. In this work, a subcritical water extraction (SCWE) process at laboratory scale was used at varying water temperature (100-175 °C) and flow rate (0.5-1.5 mL min(-1)), to treat 2,4,6-trinitrotoluene (TNT) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) contaminated soil, to reveal information with respect to the explosives removal (based on the analyses of soil residue after extraction), and degradation performance (based on the analyses of water extracts) of this process. Continuous flow subcritical water has been considered on removal of explosives to avoid the repartitioning of non-degraded compounds to the soil upon cooling which usually occurs in the batch system. In the SCWE experiments, near complete degradation of both TNT and RDX was observed at 175 °C based on analysis of water extracts and soil. Test results also indicated that TNT removal of >99% and a complete RDX removal were achieved by this process, when the operating conditions were 1 mL min(-1), and treatment time of 20 min, after the temperature reached 175 °C. HPLC-UV and ion chromatography analysis confirmed that the explosives underwent for degradation. The low concentration of explosives found in the process wastewater indicates that water recycling may be viable, to treat additional soil. Our results have shown in the remediation of explosives contaminated soil, the effectiveness of the continuous flow SCWE process.

Decontamination of PCBs-containing soil using subcritical water extraction process

Polychlorinated biphenyls (PCBs) are one of the excision compounds listed at the Stockholm convention in 2001. Although their use has been heavily restricted, PCBs can be found in some specific site-contaminated soils. Either removal or destruction is required prior to disposal. The subcritical water extraction (SCWE) of organic hazardous compounds from contaminated soils is a promising technique for hazardous waste contaminated-site cleanup. In this study, the removal of PCBs by the SCWE process was investigated. The effects of temperature and treatment time on removal efficiency have been determined. In the SCWE experiments, a removal percentage of 99.7% was obtained after 1h of treatment at 250°C. The mass removal efficiency of low-chlorinated species was higher than high-chlorinated congeners at lower temperatures, but it was oppositely observed at higher temperatures because the lower chlorinated congeners are formed by dechlorination of higher chlorinated congeners. Gas chromatography/mass spectrometry analysis confirmed that the PCBs underwent partial degradation. Several degradation products including mono- and di-chlorinated biphenyls, oxygen-containing aromatic compounds, and small-size hydrocarbons were identified in the effluent water, which were not initially present in the contaminated soil.