Evaluation of persulfate oxidative wet scrubber for removing BTEX gases

Generation and engineering applications of sulfate radicals in environmental remediation

Sulfate radical (SO4•-)-based advanced oxidation processes (AOPs) have become promising alternatives in environmental remediation due to the higher redox potential (2.6-3.1 V) and longer half-life period (30-40 μs) of sulfate radicals compared with many other radicals such as hydroxyl radicals (•OH). The generation and mechanisms of SO4•- and the applications of SO4•--AOPs have been examined extensively, while those using sulfite as activation precursor and their comparisons among various activation precursors have rarely reviewed comprehensively. In this article, the latest progresses in SO4•--AOPs were comprehensively reviewed and commented on. First of all, the generation of SO4•- was summarized via the two activation methods using various oxidant precursors, and the generation mechanisms were also presented, which provides a reference for guiding researchers to better select two precursors. Secondly, the reaction mechanisms of SO4•- were reviewed for organic pollutant degradation, and the reactivity was systematically compared between SO4•- and •OH. Thirdly, methods for SO4•- detection were reviewed which include quantitative and qualitative ones, over which current controversies were discussed. Fourthly, the applications of SO4•--AOPs in various environmental remediation were summarized, and the advantages, challenges, and prospects were also commented. At last, future research needs for SO4•--AOPs were also proposed consequently. This review could lead to better understanding and applications of SO4•--AOPs in environmental remediations.

Modeling of NO mass transfer characteristics absorbed in sodium persulfate solution with a bubble reactor

Sodium persulfate solution is considered as an effective wet denitrification medium, however, it is unclear that the influence of the operating conditions on mass transfer characteristics parameters during the absorption of NO with sodium persulfate solution. To determine the key mass transfer characteristics parameters, the specific interfacial area a and the mass transfer coefficients kL,  kG, were determined based on the Danckwerts method during CO₂ absorption in a bubble column. kL, kG  and a were calculated by correlations between the mass transfer coefficients of NO and CO₂. Results showed that the specific interfacial area increased 77.64 m^-1, the liquid phase mass transfer coefficient increased 2.49 × 10^-4 m·s^-1, and the gas phase mass transfer coefficient increased 0.71 × 10^-5mol·Pa^-1·s^-1·m^-2 with superficial gas velocity increasing from 0.6 to 1.4 L·min^-1. With the temperature of sodium persulfate solution increasing from 293 to 333 K, the specific interfacial area decreased 42.66 m^-1, while the liquid phase mass transfer coefficient and the gas phase mass transfer coefficient increased 3.89 × 10^-4 m·s^-1 and 1.18 × 10^-5mol·Pa^-1·s^-1·m^-2, respectively. The experiments results determined the correlations of a, kL, and kG  with the temperature of the absorption phase and the superficial velocity of the gas. It can serve as a guide to the enhancement of the sodium persulfate wet denitrification process.

Removal of Gaseous Volatile Organic Compounds by a Multiwalled Carbon Nanotubes/Peroxymonosulfate Wet Scrubber

In this study, a wet scrubber coupled with a persulfate-based advanced oxidation process [carbocatalysts/peroxymonosulfate (PMS)] was demonstrated to efficiently remove gaseous volatile organic compounds (VOCs). The removal efficiency of a representative VOC, styrene, was stable at above 98%, and an average mineralization rate was achieved at 76% during 2 h. The removal efficiency of the carbocatalysts/PMS wet scrubber for styrene was much higher than that of pure water, carbocatalysts/water, or PMS/water systems. Quenching experiments, electron spin resonance spectroscopy, in-situ Raman spectroscopy and density functional theory (DFT) calculations indicated that singlet oxygen (¹O₂) and oxidative complexes are the main reactive oxygen species and that both contributed to styrene removal. In particular, carbonyl groups (C═O) in the carbocatalyst were found to be the active sites for activating PMS during styrene oxidation. The role of ¹O₂ was discovered to be benzene ring breaking and a possible non-radical oxidation pathway of styrene was proposed based on time-of-flight mass spectroscopy which was further verified by DFT calculations. In particular, the electron transfer process of multi world carbon nanotubes-PMS* in styrene oxidation was further studied in-depth by experiments and DFT calculations. The unstable vinyl on styrene was simultaneously degraded by the oxidative complexes and ¹O₂ into benzene, and finally oxidized by ¹O₂ into H₂O and CO₂. This study provides an effective method for VOC removal and clearly illustrates the complete degradation mechanism of styrene in a nonradical PMS-based process by a wet scrubber.

Recent advances in the chemical oxidation of gaseous volatile organic compounds (VOCs) in liquid phase

The chemical oxidation of gaseous volatile organic compounds (VOCs) in liquid phase may possess great advantages in its high removal efficiency, mild conditions, good reliability, wide applicability, and little potential secondary pollution, which has aroused extensive research interests in the past decade. This Overview Article summarizes the latest achievements to eliminate VOCs by chemical oxidation in liquid phase including gas-liquid mass transfer, homogeneous/heterogeneous oxidation, electrochemical oxidation, and coupling technologies. Important research contributions are highlighted in terms of mass transfer, catalytic materials, removal/mineralization efficiency, and reaction mechanism to evaluate their potential industrial applications. The current challenges and future strategies are discussed from the viewpoint of the deep degradation of refractory VOC substrates and their intermediates. It is anticipated that this review will attract more attention toward the development and application of chemical oxidation methods to clear complex industrial organic exhaust gas.

Iron-activated persulfate oxidation degrades aqueous Perfluorooctanoic acid (PFOA) at ambient temperature

Perfluorooctanoic acid (PFOA, C₈HF₁₅O₂) is an industrial surfactant that is highly resistant to natural breakdown processes such as those mediated by heat, hydrolysis, photolysis, and biodegradation. Many efforts have been developed to breakdown PFOA to less harmful species due to its widespread human exposure and potential toxicity. However, these methods require high temperature or specialized equipment with serious disadvantages of high energy cost for long-term use. We investigated the effectiveness of PFOA degradation by ferrous iron-activated persulfate oxidation (IAPO) under various aqueous geochemical conditions. Approximately 64% of PFOA (initial concentration = 1.64 μmol L^-1) was degraded after 4 h under illuminated anoxic conditions at ambient temperature. This degradation rate and magnitude support the potential use of IAPO as a novel inexpensive and environmentally friendly method to remediate PFOA in soil and groundwater.

Insights into the mechanism of hydrogen peroxide activation with biochar produced from anaerobically digested residues at different pyrolysis temperatures for the degradation of BTEXS

The disposal of large amounts of biogas residue from anaerobically digested waste is a burden on environment protection. Porous biochars (BCs) were synthesized from biogas residue at three pyrolysis temperatures (300 °C, 550 °C, and 800 °C) and used to catalyze H₂O₂ for the degradation of benzene, toluene, ethylbenzene, xylene isomers (ortho, para, and meta), and styrene (BTEXS) to develop a new use for biogas residues. The prepared BCs were characterized through scanning electron microscopy, Brunauer-Emmett-Teller method, Fourier transform infrared spectrometry, and X-ray photoelectron spectroscopy. Results showed that BC800/H₂O₂ had the highest BTEXS degradation performance over 6 h. The degradation kinetic data were most consistent with the pseudo-second-order model. The different catalytic effect of the three BCs pyrolyzed at different temperatures were attributed to the dominant active sites (C-O/C-OH/C=C/C=O groups, pyridinic N, and graphitic N) that induced the production of reactive oxygen species (ROS). ROS-quenching experiments indicated that the degradation of BTEXS by BC300/H₂O₂, BC550/H₂O₂, and BC800/H₂O₂ involved ∙OH, ∙O₂^-, and ¹O₂. ∙OH was the dominant ROS in BC300/H₂O₂ and BC550/H₂O₂, and ¹O₂ was the dominant ROS in BC800/H₂O₂. Our findings provided new insight into the different catalytic mechanisms for BC production at different pyrolysis temperatures and demonstrated that a porous BC catalyst with high utilization value could be prepared from biogas residue and could hold considerable potential for application in BTEXS treatment in the future.

Theoretical exploration of VOCs removal mechanism by carbon nanotubes through persulfate-based advanced oxidation processes: Adsorption and catalytic oxidation

Carbon-catalyzed persulfate activation for the removal of gaseous volatile organic compounds (VOCs) has not been reported yet, and the corresponding fundamental mechanisms of VOCs adsorption and the subsequent VOCs degradation remain controversial. In this work, theoretical chemistry calculations were carried out to explore the VOCs removal mechanism by the persulfate-based advanced oxidation processes (P-AOPs) for VOCs removal over single walled carbon nanotubes (SWCNT). This study provided detailed theoretical insights into the SWCNT/P-AOPs for VOCs treatment in terms of adsorption, activation, mineralization, and diffusion of VOCs or peroxymonosulfate (PMS). Various VOCs were found to be preferentially adsorbed onto SWCNT, and the adsorption strength of VOCs was found to be significantly dependent on their polarizability. On the other side, PMS adsorbed on SWCNT could be efficiently activated through accepting π electron in the sp² carbon matrix of SWCNT rather than the electrons at dangling bonds to generate •OH radicals attributed to the strong interaction between PMS and SWCNT. Formaldehyde was then taken as an example to evaluate the catalytic degradation pathways via SWCNT/P-AOPs. Under the attack of •OH radicals, the ultrafast degradation pathway of formaldehyde with no byproduct CO was identified with ultralow reaction energy barrier and large energy release. In addition, factors affecting the adsorption of organic compounds were identified and the detailed PMS activation pathway was present directly in this work. Above all, this work extended the carbons/P-AOPs system to VOCs abatement and presented systematic evidences for the essential mechanisms associated with VOCs adsorption and PMS activation by SWCNT, and the corresponding removal pathway and mechanism were also understood.

A novel combined system using Na2S2O8/urea to simultaneously remove SO2 and NO in marine diesel engine exhaust

The novel combined system using Na₂S₂O₈/urea was used to simultaneously absorb nitric oxide and sulfur dioxide emissions from marine diesel engines as well as inhibit the formation of nitrate in cleaning wastewater to meet the increasingly stringent requirements of regulations. The influences of reaction temperature, Na₂S₂O₈ concentration, urea concentration, SO₂ concentration, NO concentration and pH value on SO₂ removal efficiency, NO removal efficiency and nitrate concentration were investigated. The experimental results showed that different reaction temperatures had different influences on SO₂ removal efficiency, NO removal efficiency and nitrate concentration. An increase in Na₂S₂O₈ could improve the absorption of NO. The addition of urea could effectively improve the removal efficiency of NO and reduce the nitrate concentration. The removal efficiencies of 1000 ppm NO and 1000 ppm SO₂ achieved 100 % with 0.2 mol/L Na₂S₂O₈ and 2 mol/L urea at 70℃, and the nitrate content was 8.56 mg/L which was far lower than the regulatory requirement of 60 mg/L. The acidic condition (pH ≤ 5.5) not only facilitated the absorption of NO but also reduced the generation of nitrate. According to the experimental results, the novel combined system was promising to be applied to the control technology of marine diesel engine exhaust.

Ultrasound-assisted heterogeneous activation of persulfate and peroxymonosulfate by asphaltenes for the degradation of BTEX in water

This study investigated - for the first time - the simultaneous degradation of benzene, toluene, ethylbenzene and o-xylene (BTEX) by persulfate (PS) and peroxymonosulfate (PMS) activated by asphaltenes (Asph) under ultrasound (US) irradiation. Advantageous properties such as high thermal stability, low production cost and extensive availability make asphaltenes as an appealing carbonaceous material for heterogeneous catalysis. The application of asphaltenes in PS/US increased the degradation of BTEXs from 31%, 34%, 35%, 32%-78%, 94%, 98% and 98%, while the removal of these compounds in PMS/US system was improved from 26%, 27%, 24%, 20%-76%, 91%, 97%, 97%, respectively. PS and PMS activation followed a typical sulfate-radical based advanced oxidation processes. In terms of activation of PS and PMS, the particles of asphaltenes intensified formation of reactive radicals by creating additional centers of cavitational events. Moreover, owing to π-π stacking interaction between asphaltenes and sp²-hybridized systems of BTEX, the contaminants undergo adsorption on the surface of asphaltenes and subsequent oxidation by formed radicals. The radical route of BTEX degradation in both PS/US/Asph and PMS/US/Asph systems was mainly contributed by sulfate (SO₄^•-) and hydroxyl radicals (HO^•) and coexisting superoxide radical anions (O₂^•-) played a minor role.

Removal of styrene in air stream by absorption combined with electrochemical oxidation

Aqueous solution absorptions are widely used as an effective way for the treatment of noxious gases discharged from various industrial processes. However, this technology may encounter problems in removing gaseous pollutants with low Henry constants, such as styrene from contaminated air. In this study, a novel electrochemical absorption reactor was devised to remove these air pollutants. The reactor consists of five pairs of stacked mesh electrodes. Each pair of mesh electrodes consists of a Ti/RuO₂ anode and a Ti cathode. The dimension of mesh electrode is 100 mm × 100 mm with 3 mm × 5 mm rhombic holes evenly distributed. The distance between two neighbouring electrodes is 25 mm. The simulated gas was introduced into the reactor from the bottom of the reactor by a gas distributor. The experimental result shows that styrene in the air was effectively removed by the electrochemical absorption reactor, and the removal increased with the increase of current density applied to the reactor. It was found that almost 100% styrene removal was achieved in 1% NaCl solution with 1 pH value and a current density of 0.04 A/cm² applied to the reactor. The major liquid phase products from styrene oxidation were confirmed to be 1-Phenyl-1, 2-ethanediol and benzaldehyde.

Simultaneous NO and SO2 removal by aqueous persulfate activated by combined heat and Fe2+: experimental and kinetic mass transfer model studies

This study evaluates the chemistry, kinetics, and mass transfer aspects of the removal of NO and SO₂ simultaneously from flue gas induced by the combined heat and Fe²⁺ activation of aqueous persulfate. The work involves experimental studies and the development of a mathematical model utilizing a comprehensive reaction scheme for detailed process evaluation, and to validate the results of an experimental study at 30-70 °C, which demonstrated that both SO₂ and Fe²⁺ improved NO removal, while the SO₂ is almost completely removed. The model was used to correlate experimental data, predict reaction species and nitrogen-sulfur (N-S) product concentrations, to obtain new kinetic data, and to estimate mass transfer coefficient (K_La) for NO and SO₂ at different temperatures. The model percent conversion results appear to fit the data remarkably well for both NO and SO₂ in the temperature range of 30-70 °C. The conversions ranged from 43.2 to 76.5% and 98.9 to 98.1% for NO and SO₂, respectively, in the 30-70 °C range. The model predictions at the higher temperature of 90 °C were 90.0 and 97.4% for NO and SO₂, respectively. The model also predicted decrease in K_La for SO₂ of 1.097 × 10^-4 to 8.88 × 10^-5 s^-1 (30-90 °C) and decrease in K_La for NO of 4.79 × 10^-2 to 3.67 × 10^-2 s^-1 (30-50 °C) but increase of 4.36 × 10^-2 to 4.90 × 10^-2 s^-1 at higher temperatures (70-90 °C). This emerging sulfate-radical-based process could be applied to the treatment of flue gases from combustion sources. Graphical abstract.

Kinetics and mechanism of the advanced oxidation process of Cr(III) to Cr(VI) by SO4 −˙ free radicals in slightly acidic simulated atmospheric water

In a previous work, we showed that the oxidation of Cr(III) to Cr(VI) by OH˙ present in the atmospheric water droplets has the potential to threaten the people’s health since non-toxic species is transformed into environmental carcinogens. The same oxidation might be initiated by the SO4 −˙ free radicals. Here, we shed some light on the detailed mechanisms of this oxidation reaction occurring in ambient atmosphere. Steady state irradiation and pulse radiolysis technique were used to generate SO4 −˙. The advanced oxidation process mechanism was investigated at pH 4 and 6 selected as typical values of cloud water acidity. Our findings showed that the oxidation is pseudo-first order with respect to Cr(III) and is pH dependent. In the suggested reaction mechanism, the electron transfer proceeds via an inner sphere mechanism, with formation of the [Cr(III)–SO4 −˙] precursor adduct, followed by an electron transfer inside the adduct, from Cr(III) to SO4 −˙, to form Cr(IV): Cr(III) + SO 4 − · ⇌ [ C r ( III ) – SO 4 − · ] → Cr(IV) + SO 4 2 − . $${\rm{Cr(III)}} + {\rm{S}}{{\rm{O}}_4}^{ - \cdot}[Cr({\rm{III}})-{\rm{S}}{{\rm{O}}_4}^{ -\cdot }] \to {\rm{Cr(IV)}} + {\rm{S}}{{\rm{O}}_4}^{2 - }.$$ At pH 4, the equilibrium constant and the rate constant are 7.52 × 104 M−1 and 2.47 × 104 s−1, respectively. At pH 6 these values become 1.90 × 105 M−1 and 1.41 × 104 s−1, respectively.

Impacts of inorganic anions and natural organic matter on thermally activated persulfate oxidation of BTEX in water

The present study investigated the impacts of water matrix constituents (CO₃^2-, HCO₃^-, Cl^-, Br^-, PO₄^3-, HPO₄^2-, H₂PO₄^-, NO₃^-, SO₄^2- and natural organic matters (NOM) on the oxidation of a mixture of benzene, toluene, ethylbenzene, and xylenes (BTEX) by thermally activated persulfate (PS). In the absence of matrix constituents, the BTEX oxidation rates decreased in the following order: xylenes > toluene ≈ ethylbenzene > benzene. HCO₃^-/CO₃^2- and NOM inhibited the BTEX oxidation and the inhibiting effects became more pronounced as the HCO₃^-/CO₃^2-/NOM concentration increased. SO₄^2-, NO₃^-, PO₄^3- and H₂PO₄^- did not affect the BTEX oxidation while HPO₄^2- slightly inhibited the reaction. The impacts of Cl^- and Br^- were complex. Cl^- inhibited the benzene oxidation while 100 mM and 500 mM of Cl^- promoted the oxidation of m-xylene and p-xylene. Br^- completely suppressed the benzene oxidation while 500 mM of Br^- strongly promoted the oxidation of xylenes. Detailed explanations on the influence of each matrix constituent were discussed. In addition, various halogenated degradation byproducts were detected in the treatments containing Cl^- and Br^-. Overall, this study indicates that some matrix constituents such as NOM, HCO₃^-, CO₃^2-, H₂PO₄^-, Cl^- and Br^- may reduce the BTEX removal efficiency of sulfate radical-based advanced oxidation process (SR-AOP) and the presence of Cl^- and Br^- may even lead to the formation of toxic halogenated byproducts.

Selective oxidative degradation of toluene for the recovery of surfactant by an electro/Fe²⁺/persulfate process

An electro/Fe(2+)/persulfate process has been conducted for toluene removal from surfactant (SDS) flushing solution, and the pseudo-second-order reaction rate constant (k2 value) of toluene removal has been optimized by a response surface methodology (RSM). The results indicated that in this process, the reaction between persulfate and externally added Fe(2+) generates sulfate-free radicals, and at the same time, Fe(2+) is electro-regenerated at the cathode by the reduction of Fe(3+). RSM based on Box-Behnken design (BBD) has been applied to analyze the experimental variables, of which the concentrations of persulfate and Fe(2+) showed a positive effect on the rate constant of toluene removal, whereas the concentration of SDS showed a negative effect. The interactions between pairs of variables proved to be significant, such as between SDS, persulfate, and Fe(2+) concentrations. ANOVA results confirmed that the proposed models were accurate and reliable for analysis of the variables of the electro/Fe(2+)/persulfate process. The shapes of the 3D response surfaces and contour plots showed that the SDS, persulfate, and Fe(2+) concentrations substantially affected the k2 value of toluene removal. The results indicated that increasing persulfate or Fe(2+) concentration increased the k2 value, whereas increasing SDS concentration decreased the k2 value. The reaction intermediates have been identified by GC-MS, and a plausible degradation pathway for toluene degradation is proposed.

Application of persulfate to remediate petroleum hydrocarbon-contaminated soil: feasibility and comparison with common oxidants

In this study, batch experiments were conducted to evaluate the feasibility of petroleum-hydrocarbon contaminated soil remediation using persulfate oxidation. Various controlling factors including different persulfate and ferrous ion concentrations, different oxidants (persulfate, hydrogen peroxide, and permanganate), and different contaminants (diesel and fuel oil) were considered. Results show that persulfate oxidation is capable of treating diesel and fuel oil contaminated soil. Higher persulfate and ferrous ion concentrations resulted in higher diesel degrading rates within the applied persulfate/ferrous ion molar ratios. A two-stage diesel degradation was observed in the batch experiments. In addition, treatment of diesel-contaminated soil using in situ metal mineral activation under ambient temperature (e.g., 25°C) may be a feasible option for site remediation. Results also reveal that persulfate anions could persist in the system for more than five months. Thus, sequential injections of ferrous ion to generate sulfate free radicals might be a feasible way to enhance contaminant oxidation. Diesel oxidation efficiency and rates by the three oxidants followed the sequence of hydrogen peroxide>permanganate>persulfate in the limited timeframes. Results of this study indicate that the application of persulfate oxidation is a feasible method to treat soil contaminated by diesel and fuel oil.

Evaluation of activated carbon for remediating benzene contamination: adsorption and oxidative regeneration

This study investigated the potential usage of activated carbon (AC) as a permeable reactive barrier material for the adsorption of benzene contaminant. Sodium persulfate (SPS) or pyrite (FeS(2)) activated SPS oxidation was used for the regeneration of spent AC. Results indicate that persulfate oxidation of AC caused a loss of electrons and a reduction in adsorptive capacity due to the formation of acidic functional groups on the AC. Concerning the reactants that can be used for oxidation of the benzene contaminants, SPS/FeS(2)/AC, as oppose to SPS/AC, can achieve benzene destruction in both the aqueous and the sorbed phases. Furthermore, regeneration of benzene spent AC by SPS or SPS/FeS(2) revealed that SPS oxidation resulted primarily in desorption of benzene over direct oxidation of AC sorbed benzene. In contrast, the SPS/FeS(2) system achieved complete oxidation of desorbed benzene in the aqueous phase while also oxidizing sorbed benzene. Results of re-adsorption show that oxidative regeneration recovered around 70% of the AC adsorption sites and the remaining capacity was mostly occupied by the residual benzene on the AC. This study demonstrates that SPS or FeS(2) activated SPS oxidation is an effective alternative method for the regeneration of spent AC.