Effects of chemical oxidation on sorption and desorption of PAHs in typical Chinese soils

Remediation of crude oil contaminated soil through an integrated biological-chemical-biological strategy

A plausible approach to remediating petroleum contaminated soil is the integration of chemical and biological treatments. Using appropriate chemical oxidation, the integrated remediation can be effectively achieved to stimulate the biodegradation process, consequently bolstering the overall remediation effect. In this study, an integrated biological-chemical-biological strategy was proposed. Both conventional microbial degradation techniques and a modified Fenton method were employed, and the efficacy of this strategy on crude oil contaminated soil, as well as its impact on pollutant composition, soil environment, and soil microorganism, was assessed. The results showed that this integrated remediation realized an overall 68.3 % removal rate, a performance 1.7 times superior to bioremediation alone and 2.1 times more effective than chemical oxidation alone, elucidating that the biodegradation which had become sluggish was invigorated by the judicious application of chemical oxidation. By optimizing the positioning of chemical treatment, the oxidization was allowed to act predominantly on refractory substances like resins, thus effectively enhancing pollutant biodegradability. Concurrently, this oxidating maneuver contributed to a significant increase in concentrations of dissolvable nutrients while maintaining appropriate soil pH levels, thereby generating favorable growth conditions for microorganism. Moreover, attributed to the proliferation and accumulation of degrading bacteria during the initial bioremediation phase, the microbial growth subsequent to oxidation showed rapid resurgence and the relative abundance of typical petroleum-degrading bacteria, particularly Proteobacteria, was substantially increased, which played a significant role in enhancing overall remediation effect. Our research validated the feasibility of biological-chemical-biological strategy and elucidated its correlating mechanisms, presenting a salient reference for the further studies concerning the integrated remediation of petroleum contaminated soil.

Transport and retention of n-hexadecane in cadmium-/naphthalene-contaminated calcareous soil sampled in a karst area

Studying the transport of petroleum hydrocarbons in cadmium-/naphthalene-contaminated calcareous soils is crucial to comprehensive assessment of environmental risks and developing appropriate strategies to remediate petroleum hydrocarbons pollution in karst areas. In this study, n-hexadecane was selected as a model petroleum hydrocarbon. Batch experiments were conducted to explore the adsorption behavior of n-hexadecane on cadmium-/naphthalene-contaminated calcareous soils at various pH, and column experiments were performed to investigate the transport and retention of n-hexadecane under various flow velocity. The results showed that Freundlich model better described the adsorption behavior of n-hexadecane in all cases (R2 > 0.9). Under the condition of pH = 5, it was advantageous for soil samples to adsorb more n-hexadecane, and the maximum adsorption content followed the order of: cadmium/naphthalene-contaminated > uncontaminated soils. The transport of n-hexadecane in cadmium/naphthalene-contaminated soils at various flow velocity was well described by two kinetic sites model of Hydrus-1D with R2 > 0.9. Due to the increased electrostatic repulsion between n-hexadecane and soil particles, n-hexadecane was more easily able to breakthrough cadmium/naphthalene-contaminated soils. Compared to low flow velocity (1 mL/min), a higher concentration of n-hexadecane was determined at high flow velocity, with 67, 63, and 45% n-hexadecane in effluent from cadmium-contaminated soils, naphthalene-contaminated soils, and uncontaminated soils, respectively. These findings have important implications for the government of groundwater in calcareous soils from karst areas.

Insights into the effects of Fenton oxidation on PAH removal and indigenous bacteria in aged subsurface soil

Combined chemical oxidation and bioremediation is a promising method of treating polycyclic aromatic hydrocarbon (PAH) contaminated soil, wherein indigenous soil bacteria play a critical role in the subsequent biodegradation of PAHs after the depletion of the oxidant. In this study, different Fenton conditions were applied by varying either the oxidation mode (conventional Fenton (CF), Fenton-like (LF), modified Fenton (MF), and graded modified Fenton (GMF)) or the H₂O₂ dosage (0%, 3%, 6%, and 10% (v/v)) to treat PAH contaminated soil. The results revealed that when equal dosages of H₂O₂ are applied, PAHs are significantly removed following oxidation treatment, and the removal percentages obeyed the following sequence: CF > GMF > MF > LF. In addition, higher dosages of H₂O₂ improved the PAH removal from soil treated with the same oxidation mode. The ranges of total PAHs removal efficiencies in the soil added 3%, 6%, and 10% of H₂O₂ (v/v) were 18.04%∼59.48%, 31.88%∼71.83%, and 47.56%∼78.16%, respectively. The PAH removal efficiency decreased with increasing ring numbers for the same oxidation treatment. However, the negative influences on soil bacterial abundance, community composition, and function were observed after Fenton treatment. After Fenton oxidation, the bacterial abundance in the soil received 3%, 6%, and 10% of H₂O₂ (v/v) decreased 1.96-2.69, 2.44-3.22, and 3.09-3.42 orders of magnitude compared to the untreated soil. The soil bacterial abundance tended to be impacted by the oxidation mode and H₂O₂ dosage simultaneously. While the main factor influencing the soil bacterial community composition was the H₂O₂ dosages. The results of this study showed that different oxidation mode and H₂O₂ dosage exhibited different effects on PAHs removal and soil bacteria (including abundance, community composition, and function), and there was a trade-off between the removal of PAHs and the adverse impact on soil bacteria.

Electrokinetic combined peroxymonosulfate (PMS) remediation of PAH contaminated soil under different enhance methods

Because of the high hydrophobicity, low volatility, and high sorption capacity of PAHs, their remediation in contaminated soil is challenging. Electrokinetic (EK) enhanced chemical remediation is an emerging dual technology employed in this study, using a new oxidant peroxymonosulfate (PMS) to remediate PAHs contaminated soil. Here, PMS migration under electric field and the remediation efficiency for the PAHs polluted soil were assessed. We observed that the PMS removal efficiencies (59.7%-82.8%) were higher than those with persulfate (PS) (53.9%-78.5%), indicating PMS's superior oxidation capacity for PAHs. Although oxidant PMS can decontaminate PAHs in polluted soils, its removal of PAHs was only 11.0% without the enhanced methods. The enhancements increased the removal efficiency for PAHs from 0.33 to 2.10 times. At fixed catholyte pH of 4, the highest removal efficiency (34.1%) was achieved because it enhanced PMS migration from cathode to anode. These findings suggested that PMS was a potential oxidant for EK remediation, and some enhancements must be applied in EK combined PMS remediation PAHs polluted soil.

Enhancing remediation of PAH-contaminated soil through coupling electrical resistance heating using Na2S2O8

Soil polycyclic aromatic hydrocarbons (PAHs) contamination caused by factory relocations is a serious environmental issue across the world. Electrical resistance heating (ERH) and chemical oxidation are two promising in-situ methods for treating volatile and semi-volatile organic pollutants in contaminated soil. Coupling of ERH and chemical oxidation technologies to improve the remediation efficiency for PAH-contaminated soil was estimated in this study. PAH removal ratio in contaminated soils using ERH treatment were significantly negatively correlated with the boiling point of the pollutants (P = 0.002), and 21.63% (DBA high boiling point) to 71.53% (Nap low boiling point) of PAHs in the contaminated soil were removed in 120 min. With oxidant Na₂S₂O₈ coupling, the removal ratio were increased as more oxidant was added. For one Phe, 35.90% was removed by ERH treatment and increased to 52.90% and 79.42% when 0.05 or 2.5 mmol/g oxidant was added, respectively. PAHs with higher boiling points had more obvious removal ratio, such as Bap, which increased from 23.50% to 85.47% when coupling ERH with Na₂S₂O₈, and Phe which increased from 35.90% to 79.42%. Relationships between boiling points and PAH removal ratio changed with coupled oxidants, indicating a change of mechanism from volatilization to coupling effects of volatilization and oxidation with the introduction of Na₂S₂O₈. A dynamic experiment showed that Na₂S₂O₈ can accelerate 45.50% of the treatment process. The results of this research demonstrated a novel, cost-effective coupling approach for remediating soil contaminated by organic pollutants.

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.

Accumulated Gibbs free energy as a quantitative measure of desorption hysteresis associated with the formation of metastable states

The persistence of metastable states was proposed in the literature as one explanation for sorption-desorption hysteresis (SDH) of organic compounds on soils and sediments. When such metastable states freely exchange sorbate molecules with the surroundings and there is no spontaneous exit of a whole system from that state, it is possible to determine the extra Gibbs free energy (ΔG^ext) accumulated in a system due to the persistence of metastable states. A novel contribution of this paper is the characterization of SDH, in which the sorption isotherm (SI) and desorption isotherm (DI) do not close a loop, in terms of free energy needed to create "frozen", metastable states. To that end, liquid phase sorption of non-ionized sorbates is considered and by integrating over the sorption-desorption sequence, ΔG^ext and an integral hysteresis index (IHI) were obtained. Experimental data collected from the literature on aqueous sorption and desorption of polyaromatic hydrocarbons, triazines and ureas were examined on soils, sediments, organic matter-rich sorbents, montmorillonites and fullerene. Positive ΔG^ext values were obtained to quantify the thermodynamic potential for spontaneous exit from a metastable state that is not implemented due to the kinetic barriers. Relating the ΔG^ext values to sorbate molecular structure and sorbent properties may allow the prediction of SDH for various chemicals on sorbents in which the sorbate-induced perturbation of a sorbent matrix is believed to be a cause for the formation of persistent metastable states and the appearance of a non-closed sorption-desorption sequence.

A novel sensitive fluorescent probe of S2O82− and Fe3+ based on covalent post-functionalization of a zirconium(iv) metal–organic framework

A novel sensitive and selective PL probe has been designed and prepared to detect S₂O₈²⁻ and Fe³⁺ in an aqueous environment by modification of amino groups in the ligand of the MOF UiO-66-NH₂ based on a covalent PSM method.

Use of different kinds of persulfate activation with iron for the remediation of a PAH-contaminated soil

Contamination of soils by persistent pollutants is considered an important matter of increasing concern. In this work, activated persulfate (PS) was applied for the remediation of a soil contaminated with polycyclic aromatic hydrocarbons (PAHs), such as anthracene (ANT), phenanthrene (PHE), pyrene (PYR) and benzo[a]pyrene (BaP). PS activation was performed by different ways; where ferric, ferrous sulfate salts (1-5mmol·L(-1)) and nanoparticles of zerovalent iron (nZVI) were used as activators. Moreover, in order to improve the oxidation rate of contaminants in the aqueous phase, the addition of sodium dodecyl sulfate (SDS), as anionic surfactant, was tested. On the other hand, it was also studied the role of humic acids (HA), as reducing agent or surfactant, on PAHs conversion. Removal efficiencies near 100% were achieved for ANT and BaP in all the runs carried out. Nevertheless, remarkable differences on removal efficiencies were observed for the different techniques applied in case of PHE and PYR. In this sense, the highest conversions of PHE (80%) and PYR (near 100%) were achieved when nZVI was used as activator. Similar results were obtained when activation was carried out either with Fe(2+) or Fe(3+). This can be explained by the presence of quinone type compounds, as 9,10-anthraquinone (ATQ), that can promote the reduction of Fe(3+) into Fe(2+), permitting PS radicals to be generated. On the other hand, the addition of HA did not produce an improvement of the process while surfactant addition slightly increases the PAHs removal. Furthermore, a kinetic model was developed, describing the behavior of persulfate consumption, and contaminants removal under first order kinetics.

Selection of oxidant doses for in situ chemical oxidation of soils contaminated by polycyclic aromatic hydrocarbons (PAHs): A review

In situ chemical oxidation (ISCO) is a promising alternative to thermal desorption for the remediation of soils contaminated with organic compounds such as polycyclic aromatic hydrocarbons (PAHs). For field application, one major issue is the selection of the optimal doses of the oxidizing solution, i.e. the oxidant and appropriate catalysts and/or additives. Despite an extensive scientific literature on ISCO, this choice is very difficult because many parameters differ from one study to another. The present review identifies the critical factors that must be taken into account to enable comparison of these various contributions. For example, spiked soils and aged, polluted soils cannot be compared; PAHs freshly spiked into a soil are fully available for degradation unlike a complex mixture of pollutants trapped in a soil for many years. Another notable example is the high diversity of oxidation conditions employed during batch experiments, although these affect the representativeness of the system. Finally, in this review a methodology is also proposed based on a combination of the stoichiometric oxidant demand of the organic pollutants and the design of experiments (DOE) in order to allow a better comparison of the various studies so far reported.

Synergistic role of different soil components in slow sorption kinetics of polar organic contaminants

We observed that the sorption kinetics of nitrobenzene and 2,4-dinitrotoluene (two model polar compounds) was significantly slower than that of 1,4-dichlorobenzene and phenanthrene (two model apolar compounds). The difference was attributable to the strong non-hydrophobic interactions between the polar molecules and soil. Interestingly, sorption kinetics of the polar sorbates to the soil organic matter-free soil, humic/fulvic acid-free soil, and extracted humic acids was very fast, indicating that different soil components played a synergetic role in the observed slow kinetics. We propose that slow sorption kinetics of highly polar sorbates stems mainly from the strong specific interactions (H-bonding, electron donor-acceptor interactions, etc.) with humic/fulvic acids; such specific interactions occur when sorbate molecules diffuse through humic/fulvic acids coiled, in relatively compressed confirmations, within the complex, tortuous, and porous soil matrices formed by mineral grains/particles and soil organic matter.

Enhanced sorption of naphthalene and p-nitrophenol by nano-SiO2 modified with a cationic surfactant

In this study, we observed that modification of nano-oxides (e.g., nano-SiO2) with cationic surfactants (e.g., cetyl pyridinium chloride, CPC) could be a potential way to make nano-oxides be superior sorbents with a partition mechanism for the sorptive removal of organic contaminants from wastewater where the coated CPC was an effective organic phase for partitioning. The partitioning of nonpolar naphthalene into coated CPC was induced by hydrophobic effect alone and presenting linear isotherms, while that of polar p-nitrophenol was induced by not only the hydrophobic effect but also the hydrogen-bonding interaction and presenting isotherm nonlinearity. The sorption affinity for naphthalene and p-nitrophenol partitioning into the coated CPC and the configuration of coated CPC remained unchanged although the amounts of coated CPC were increased. Linear relationships were established between the coated CPC amounts and the sorption capacities of naphthalene or p-nitrophenol, which could be used to predict the sorption of organic contaminants on surfactant-modified nano-oxides. In addition, these observed results would be also valuable for estimating the environmental behaviors and risks of nano-SiO2 and organic contaminants because nano-SiO2 would be inevitably coated with ubiquitous surfactants in the environment due to the discharging from the wide domestic and industry applications.

Study of dynamic sorption and desorption of polycyclic aromatic hydrocarbons in silty-clay soil

This study reported a well controlled laboratory experiment of high concentration PAHs solute, containing fluorene, phenanthrene, fluoranthene and pyrene, through a nearly homogeneous soil column to reveal sorption and desorption behavior of these four PAHs in soil. The duration of the experiment was 64 days and the flow rate through the soil column was a constant which equals to 2000 mL d(-1). The result showed that the mechanism of isothermal sorption and desorption of fluorene can be perfectly described by the Langmuir model, and the correlation coefficients were greater than 0.997. The first-order Lagergren and the Bangham equation can precisely describe the rate of sorption of PAHs, while the rate of desorption can be represented by the second-order kinetics model. The results of the desorption experiment indicated that the desorption hysteresis of fluorene was evident. Few phenanthrene, fluoranthene and pyrene were desorpted to the aqueous phase for the chemical bond with the clay minerals. The most important process determining the behavior of PAHs in soils and their availability to further transformations was the sorption to soil solids with further sequestration and desorption to the aqueous phase.

Determination of potassium permanganate demand variation with depth for oxidation-remediation of soils from a PAHs-contaminated coking plant

Bench-scale experiments were conducted to investigate the potassium permanganate demand, a key parameter for in situ chemical oxidation (ISCO) system design, and its variation with depth in PAHs-contaminated site of a coking plant. The concentrations of permanganate decreased rapidly during the first 8 d of the reaction process. The reaction follows first order kinetics, with rate constant ranging from 0.01 to 0.3/h. The total oxidant demand (TOD) is significantly higher for clayey silt fill than for soils of other lithology. The typical TOD is about 50 g MnO(4)(-)/kg soil for clayey silt fill, 20-40 g MnO(4)(-)/kg soil for silt, silty clay and 1-7 g MnO(4)(-)/kg soil for fine sand. Statistical analysis revealed that TOD was positively correlated with total organic carbon (TOC) content, clay content and PAHs concentrations, besides sand content, meanwhile TOC was the parameter with the strongest influence on oxidant demand. After 32 d duration of oxidation, PAHs in all tested soils were effectively removed, with total removal percent ranging from 78% to 99%, and small molecular weight PAHs were removed to a greater extent than high molecular weight PAHs. Parameters obtained in this study, combined with soil bulk density, soil porosity and soil moisture, can be used for full-scale ISCO system design and application in coking contaminated site.

A contaminant trap as a tool for isolating and measuring the desorption resistant fraction of soil pollutants

Bioremediation of contaminated soils often leaves a desorption-resistant pollutant fraction behind in the soil, which in the present study was isolated with a combination of diffusive carrier and infinite diffusive sink. Such a diffusive sink was made by casting a composite of silicone and activated carbon into the bottom of a large glass. Field-contaminated soil samples were then suspended in a cyclodextrin solution and incubated in such glasses for the continuous trapping of PAH molecules during their release from the soil matrix. The PAH concentrations remaining in the soil were determined by exhaustive extraction and compared with a biodegradation experiment. The concentration decline in the first soil was faster in the contaminant trap than in the biodegradation experiment, but the halting of the biodegradation process before reaching the legal threshold level was well indicated by the contaminant trap. The PAH concentrations in the second soil hardly decreased in the traps at all, in good agreement with the biodegradation experiment. The PAHs in this soil appeared to be "stuck" by strong sorption. The contaminant trap proved to be a practical approach to the isolation and quantification of the desorption-resistant PAH fraction.