Ultrasound-assisted soil washing processes using organic solvents for the remediation of PCBs-contaminated soils

Ultrasonic Activation of Persulfate for the Removal of BPA in 20, 28, and 300 kHz Systems

The effect of ultrasound (US) on persulfate (PS) activation was investigated to determine whether acoustic cavitation can effectively induce PS activation for bisphenol A (BPA) degradation at 20, 28, and 300 kHz under various temperature conditions. The optimal liquid volume in the vessel was geometrically determined to be 400, 900, and 420 mL at 20, 28, and 300 kHz, respectively, using KI dosimetry and sonochemiluminescence image analysis. The pseudo-1st-order reaction kinetic constants in the only PS, only US, and US/PS processes at 20, 28, and 300 kHz were obtained under 5-10 ℃, 15-20 ℃, 25-30 ℃, 45-50 ℃, 55-60 ℃, and no temperature control conditions. No notable BPA degradation occurred at 5-10 ℃, 15-20 ℃, and 25-30 ℃ in the only PS processes for all frequencies. The highest sonochemical BPA degradation was obtained at 300 kHz, and much lower BPA degradation was observed at 45-50 ℃ and 55-60 ℃ for all frequencies in the only US processes. No notable enhancement of BPA degradation was observed at 5-10 ℃, 15-20 ℃, and 25-30 ℃ in the US/PS processes compared to the only US processes for all frequencies. At 20 kHz and temperatures between 55 and 60 ℃, the highest BPA degradation was obtained, with a synergistic effect of 171 %. However, the enhancement might be due to the instant or local temperature increase, and not due to acoustic cavitation. No notable PS activation by US irradiation was observed in the US/PS processes in this study. The profiles of the generated sulfate ion concentrations in the US/PS processes confirmed this. Some previous studies found high synergistic effects, whereas others have found low or no synergistic effects in US/PS processes.

Investigation of fullerene and non-fullerene materials in organic photocatalysts on the efficiency of photocatalytic degradation of polychlorinated biphenyls

The photocatalytic degradation of polychlorinated biphenyls (PCBs) is advancing, yet the efficiency of degradation within the visible spectral range continues to encounter significant challenges. In this study, two biochar-based organic semiconductor photocatalysts, Active Carbon@PTQ10 (5,8-Dibromo-6,7-difluoro-2-(2-hexyldecoxy)quinoxaline; trimethyl-(5-trimethylstannylthiophen-2-yl)stannane): ITIC-Th (Propanedinitrile,2,2'-[[6,6,12,12-tetrakis(5-hexyl-2-thienyl)-6,12-dihydrodithieno[2,3-d: 2',3'-d'] -s-indaceno[1,2-b:5,6-b'] dithiophene-2,8-diyl] bis[methylidyne(3-oxo-1H-indene-2,1(3H)-diylidene)]] bis-) (AC@PI) and Active Carbon@PTQ10: PC71BM (6,6)-phenyl C71 butyric acid methyl ester), were synthesized using a wide bandgap material, PTQ10, as the electron donor, along with a non-fullerene material, ITIC-Th, and a fullerene material, PC71BM, as the acceptors, respectively. Under optimized conditions, AC@PI degraded 93.4 % of 2,2 ',4,4 '-tetrachlorobiphenyl (PCB 47) within 60 min. By incorporating a non-fullerene acceptor (ITIC-Th), AC@PI exhibits a larger surface photopressure, a lower hole-electron transfer ratio, a broader absorption spectrum (400 - 1000 nm), and enhanced structural stability. AC@PI can generate photogenerated electrons and holes, as well as superoxide anions (O2-) and hydroxyl radicals (OH), through type II heterojunctions, which contributes to its exceptional properties. This study synthesized novel organic semiconductor catalysts that offer a green, efficient, and non-toxic method for the degradation of aromatic pollutants, such as polychlorinated biphenyls.

Remediation of per- and polyfluoroalkyl substances (PFAS) contaminated soil via soil washing with various water-organic solvents

The feasibility of soil washing for remediating PFAS-contaminated clay soil using various water-organic solvents was systematically investigated based on the combination of batch and column tests. Batch tests using 22 types of solvents highlighted that 0 % (water) and 5 % solvents could effectively extract PFCAs (≤ C9), while long-chain PFCAs (≥ C10) and PFSAs required 80 % solvents for optimal extraction, with efficiency in the order of EtOH ≤ MeOH < Acetonitrile (ACN), suggesting a strong correlation with carbon chain lengths and functional head groups. Column tests with six selected washing solutions indicated rapid desorption of PFOA and PFOS initially, peaking at liquid-to-solid (L/S) ratios of 3-4 for 0 % and 5 % solutions, and at an L/S ratio of 1 for 80 % solutions. To remediate 1 kg-dry soil to meet the legislatively permissible levels for groundwater in Japan (PFOA + PFOS < 50 ng/L), 11 L of 0 % solution (water) or 5 L of 80 % ACN are required for washing out PFOA, while 62 L of 0 % solution (water) or 53 L of 80 % ACN for PFOS. Future research should address the treatment of PFAS-rich wastewater generated from washing PFAS-contaminated soils and the impacts of washing solutions on soil.

Simultaneous removal of PAHs and heavy metals from soil by combining electrokinetic-assisted delivery of persulfate and ultrasound activation

Herein, we proposed and demonstrated a novel approach of combining electrokinetics (EK)-assisted delivery of persulfate (PS) and ultrasound-induced thermal activation of PS and release of heavy metals, to synchronously remove organic pollutants and heavy metals from soil. Results showed that in tested soil, the mass transfer efficiency of PS induced by electromigration was approximately 3.36 times that by electroosmotic flow and 1416 times that by diffusion. Compared to the PS injection alone, EK-assisted delivery of PS significantly increased the degradation rate of phenanthrene, improving from 21.60 % to 76.23 %. Due to the applied ultrasound, the temperature of soil was observed to reach target temperatures∼50℃ for effective PS activation, and hydroxyl radicals and alkyl-like radicals were detected in the soil. Moreover, the decomposition of PS on the cathode electrode inhibited the electrolytic water reaction, avoiding the focusing effect and potential flattening. Concurrently, ultrasound thermally activated PS promoted the release of heavy metals from soil. As a result, the removal rates of Pb, Zn and Cd near the anode region increased from 0.00 %, 5.49 % and 11.72 % to 11.89 %, 82.64 % and 86.30 %, respectvcely. These results indicated the novel approach holds the potential for simultaneous removal of organic pollutants and heavy metals.

Ultrasound effects on restricted silica gelation during silica extraction from Pyro-Metallurgical copper slag under acidifying conditions

Pyro-metallurgical copper slag (CS) waste was used as the source material for ultrasound (US) silica extraction under acidification processes with 26 kHz with HCl, HNO₃, and H₂SO₄ at different concentrations at 100, 300, and 600 W. During acidifying extraction processes, US irradiation inhibited silica gel formation under acidic conditions, especially at lower acid concentrations of less than 6 M, whereas a lack of US irradiation led to enhanced gelation. When US stopped, gelation occurred to a considerable degree, suggesting that the gel particle size distribution was aggregated in the 3-400 µm size range. However, with US, the size was mainly in the 1-10 µm range. Results of elemental analysis indicated that US treatment decreased the co-precipitation of other metal ions such as Fe, Cu, and Al sourced from CS for lower acidic medium, whereas the higher concentration medium accelerated silica gelation and the co-precipitation of other metals. With acids of HCl and HNO₃, and H₂SO₄, the gelations were less likely to occur at 6 M and 3 M during US irradiation, but acidic extraction without US was efficient for silica gelation and co-precipitation of other metals in the purified silica. The silica extraction yield with H₂SO₄ concentration of 3 M was 80% with 0.04% of Fe, whereas the silica product from HCl 6 M had a 90% extraction yield with only 0.08% of Fe impurity. In contrast, even though the non-US system of HCl 6 M had a higher yield at 96%, the final product had 0.5% Fe impurity, which was much higher than the US system. Consequently, the US extraction process was quite noticeable for silica recovery from CS waste.

Tween 80 assisted washing ciprofloxacin-contaminated soil, and recycled it using active chlorines

Active chlorines (ACs) can selectively oxidize contaminants with benzene rings to recycle surfactants, which greatly facilitates the resource cycle. This paper firstly utilized Tween 80 to assist in ex-situ washing the ciprofloxacin (CI) contaminated soil, including the solubilization experiment, shake washing and soil column washing, all of which showed that 2 g/L of Tween 80 (TW 80) was the most effective in removing CI. Then electrochemically treated the collected soil washing effluent (SWE) at 10 V with an electrolyte of 20 mM NaCl + 10 mM Na₂SO₄; Pre-experiments screened the range of electrode spacing, pH and temperature, based on which an orthogonal design Table L₉ (3⁴) was designed. Visual analysis and ANOVA were performed on the ciprofloxacin removal efficiency and Tween 80 retention efficiency during the orthogonal experiments in 9 groups, and the results showed that CI was usually degraded within 30 min, and 50% of TW 80 was still present at the end of the experiment, and there was no significant effect of all three factors. LC-MS demonstrated that CI was mainly degraded synergistically by ·OH and ACs, and ·OH effectively reduced the biotoxicity of the SWE, so the mixed electrolyte may be more suitable for the electrochemical recycling system of ACs. This paper conducted the washing remediation study of CI-contaminated soil for the first time, and applied the theory of selective oxidation by ACs on benzene ring to treat the SWE, which provides a new treatment idea for antibiotic-contaminated soil.

Effects of gas saturation and sparging on sonochemical oxidation activity in open and closed systems, Part I: H2O2 generation

Cavitational/sonochemical activity can be significantly enhanced or reduced depending on the gases dissolved in the liquid. Although many researchers have suggested the order of importance of dissolved gas conditions that affect the degree of sonoluminescence (SL), sonochemiluminescence (SCL), and compound degradation, the most suitable gas condition for sonochemical oxidation reactions is currently unknown. In this study (Part I), the effects of gas saturation and sparging on the generation of H₂O₂ were investigated in a 28-kHz sonoreactor system. Four gas modes, saturation/closed, saturation/open, sparging/closed, and sparging/open, were applied to Ar, O₂, N₂, and binary gas mixtures. The change in dissolved oxygen (DO) concentration during ultrasonic irradiation was measured and was used as an indicator of whether the gaseous exchange between liquid and air altered the gas content of the liquid. Considerable difference in the DO concentration was observed for the gas saturation/open mode, ranging from -11.5 mg/L (O₂ 100 %) to +4.3 mg/L (N₂ 100 %), while no significant difference was observed in the other gas modes. The change in the gas content significantly reduced the linearity for H₂O₂ generation, which followed pseudo-zero-order kinetics, and either positively or negatively affected H₂O₂ generation. Ar:O₂ (75:25) and Ar:O₂ (50:50) resulted in the highest and second-highest H₂O₂ generation for both gas saturation and sparging, respectively. In addition, gas sparging resulted in much higher H₂O₂ generation for all gas conditions compared to gas saturation; this was because of the significant change in the cavitational active zone and concentrated ultrasonic energy, which formed a bulb-shaped active zone, especially for the Ar/O₂ mixtures adjacent to the transducer at the bottom. The sparging flow rate and position also significantly affected H₂O₂ generation; the highest H₂O₂ generation was obtained when the sparger was placed at the bottom adjacent to the transducer, with a flow rate of 3 L/min. In Part II, the generation of nitrogen oxides, including nitrite (NO₂^-) and nitrate (NO₃^-), was investigated using the same ultrasonic system with three gas modes: saturation/open, saturation/closed, and sparging/closed.

A review for recent advances on soil washing remediation technologies

Contaminated soils have caused serious harm to human health and the ecological environment due to the high toxicity of organic and inorganic pollutants, which has attracted extensive attention in recent years. Because of its low cost, simple operation and high efficiency, soil washing technology is widely used to permanently remove various pollutants in contaminated soils and is considered to be the most promising remediation technology. This review summarized the recent developments in the field of soil washing technology and discusses the application of conventional washing agents, advanced emerging washing agents, the recycling of washing effluents and the combination of soil washing and other remediation technologies. Overall, the findings provide a comprehensive understanding of soil washing technology and suggest some potential improvements from a scientific and practical point of view.

Combination of Soxhlet extraction and catalytic hydrodebromination for remediation of tetrabromobisphenol A contaminated soil

As a widely used brominated flame retardants (BFRs), tetrabromobisphenol A (TBBPA) has been detected in various environmental matrices and is known to cause negative effects on both the environment and human health. In this study, a combined method was developed for the abatement of TBBPA contaminated soil based on successive steps of solvent extraction (SE) and catalytic hydrodebromination (HDB) over Pd/C. The results showed that TBBPA could be efficiently extracted from the TBBPA contaminated soil with polar solvents. Subsequently, TBBPA could be completely hydrodebrominated over Pd/C in ethanol, via multistep ultimately yielding bisphenol A. Moreover, NaOH, NH₃H₂O, and Et₃N were more favorable to promote the HDB of 4-TBBPA over Pd/C, and 100% bromide atom removal ratio of TBBPA was achieved within 40 min when [NaOH]₀/[organic-Br]₀ was more than 1.10 in ethanol. However, the catalytic activity of Pd/C decreased with the repeated use in ethanol. To study the mechanism for this phenomenon, fresh and used catalysts were analyzed by characterization techniques including scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), and energy dispersive X-ray spectrometer (EDS). It was found that the deactivation of Pd/C catalyst caused by the gradual accumulation of NaBr could be recovered by washing with water. On the basis of these studies, an effective and practical system for the combined method of SE and catalytic HDB over Pd/C was developed to dispose BFRs contaminated soils.

Quantification of sonochemical and sonophysical effects in a 20 kHz probe-type sonoreactor: Enhancing sonophysical effects in heterogeneous systems with milli-sized particles

Even though acoustic cavitation has been widely investigated, only few researchers focused on the relationship between sonochemical and sonophysical activities and on the enhancement of sonophysical activity. In this study, sonochemical and sonophysical activities were investigated in a heterogeneous system to understand the relationship between these two activities and to suggest optimal conditions for ultrasonic desorption/extraction processes comprising milli-sized glass beads. The sonochemical activity was quantitatively analyzed using potassium iodide dosimetry in homogeneous and heterogeneous systems. Sonophysical activity was quantitatively and qualitatively analyzed using paint-coated bead desorption tests and aluminum foil erosion tests under three probe positions of "T" (1 cm below the liquid surface), "B" (1 cm above the vessel bottom), and "M" (midpoint between "T" and "B"). Three different sizes of glass beads (diameter: 0.2, 1.0, and 4.0 mm) were used in this study. The highest sonochemical activity was obtained at "B" in both homogeneous and heterogeneous systems. However, three times lower sonochemical activity was observed in the heterogeneous system than in the homogeneous system because significant attenuation and unstable reflection of ultrasound occurred in the bead layer and suspension. Higher sonophysical activity was observed, when the bead size decreased and the probe approached the bottom. However, no significant sonophysical activity was detected when the beads were attached to the bottom. Therefore, the sonophysically active region was the zone around the probe body, opposite to the ultrasound irradiation tip, and only suspended beads could undergo severe cavitational actions. This was confirmed via aluminum foil tests. Several erosion marks on the foil were observed in the area around the probe body, whereas no severe damage was observed at the bottom. Moreover, the degree of sonophysical activity did not change for various saturating gases. This might be due to the different thresholds of sonochemical and sonophysical activities.