Application of calcium peroxide in water and soil treatment: A review

Integrated electrokinetic processes for the remediation of phthalate esters in river sediments: A mini-review

Concerning the contamination of phthalate esters (PAEs) in river sediments, this mini-review introduces four recently reported novel "integrated electrokinetic (EK) processes" for the remediation purpose, namely two combined technologies of the EK process and advanced oxidation process (EK-AOP Processes) and two combined technologies of the EK process and biological process (EK-BIO Processes). The following is a comprehensive summary for these remediation processes: (1) the EK process coupled with nano-Fe₃O₄/S₂O₈^2- oxidation process - Test results have shown that nanoscale Fe₃O₄ played a significant role in activating persulfate oxidation. Even a recalcitrant compound like di(2‑ethylhexyl)phthalate (DEHP), its concentration in test sediment was reduced to 1.97 mg kg^-1, far below the regulatory levels set by Taiwan EPA; (2) the EK process integrated with a novel Fenton-like process catalyzed by nanoscale schwertmannite (nano-SHM) - Test results have revealed that simultaneous injection of nano-SHM slurry and H₂O₂ into the anode reservoir and sediment compartment is a good practice. 70-99% in removal efficiency was obtained for various target PAEs; (3) enhanced in situ bioremediation coupled with the EK process for promoting the growth of intrinsic microorganisms by adding H₂O₂ as an oxygen release compound (ORC) - Test results have demonstrated that an intermittent mode of injecting lab-prepared ORC directly into the contaminant zone would be beneficial to the growth of intrinsic microorganisms in test sediment for in situ bioremediation of target PAEs; and (4) coupling of a second-generation ORC (designated 2G-ORC) with the EK-biological process - Test results have proved that 2G-ORC is long-lasting and can be directly utilized as the carbon source and oxygen source for microbial growth resulting in an enhanced biodegradation of PAEs. Except DEHP having a residual concentration of 4 μg kg^-1, all other target PAEs in test sediment were totally removed by this novel combined remediation process.

MP-UV/CaO2 as a pretreatment method for the removal of carbamazepine and primidone in waste activated sludge and improving the solubilization of sludge

Medium-pressure ultraviolet light (MP-UV) combined with calcium peroxide (CaO₂) as a pretreatment technology for removing carbamazepine (CBZ) and primidone (PMD) in waste active sludge (WAS) and improving the solubilization of sludge were investigated. CBZ and PMD were effectively removed and the removal fitted pseudo-first kinetics under MP-UV/CaO₂ treatment with R² > 0.97. The higher CaO₂ dosage and lower initial volatile suspended solids (VSS) concentration were conductive to the removal of CBZ and PMD. Of the CaO₂ hydrolysates, Ca(OH)₂ played a more important role than H₂O₂ during MP-UV/CaO₂ treatment. The removal of the target compounds was attributed to direct photolysis and indirect photolysis caused by •OH, ³DOM^*, and ¹O₂, in which •OH played a vital role with > 62.2% contribution to the overall degradation rate. A model predicting the steady concentration of •OH in WAS ([VSS] ≈ 8.6 g L^-1) under MP-UV/CaO₂ treatment with CaO₂ dosage ranging from 0 to 0.5 g g^-1-VSS was proposed and validated. Moreover, major intermediates of CBZ and PMD were detected and the probable transformation pathways during MP-UV/CaO₂ treatment were proposed. In addition, MP-UV/CaO₂ promoted the sludge solubilization effectively. Considering both the pharmaceutical degradation and sludge solubilization, the optimum operation condition with 0.2 g-CaO₂ g^-1-VSS combined with 7 h MP-UV irradiation is recommended. Under this condition, more than 92.3% of CBZ and 90.3% of PMD were removed, and soluble chemical oxygen demand (SCOD) increased by 657% and 13.6% compared with sole 10 h CaO₂ (0.2 g g^-1-VSS) treatment and 7 h MP-UV treatment, respectively.

Effects and mechanisms of calcium peroxide on purification of severely eutrophic water

In consideration of severe eutrophication of scenic water caused by pollutants released from sediments in summer, calcium peroxide (CaO₂) was adopted as the slow release peroxide to improve the water quality in a field experiment of 600 m². The effect of CaO₂ on the overlying water, interstitial water, sediment, and sediment microorganisms of scenic water was studied. Results for two months indicated that the dissolved oxygen (DO) concentration of the overlying water in the test zone was 3.78 times that in the control zone; the oxidation-reduction potential (ORP) in the overlying water and sediment increased significantly (p = 0.002 and p = 0). Meanwhile, CaO₂ could effectively inhibit the release of nitrogen (N) and phosphorus (P) from the sediment and could obviously reduce the concentrations of N and P in the overlying water by enhancing the microbiological action. Moreover, the average concentrations of total nitrogen and total phosphorus in the overlying water of the test zone were 46.27% and 50.51% of those in control zone, respectively, and the concentrations of N and P in the interstitial water decreased during the entire experiment. In addition, CaO₂ decreased the relative abundance of anaerobic bacteria in the sediment, whereas it increased that of aerobic bacteria and promoted the appearance of the functional bacteria, such as Nitrospirae and Thermodesulfoba. In conclusion, CaO₂ can improve the DO and ORP in the eutrophic water effectively and change the microbial community in the sediment to a certain extent, thereby controlling the pollutants released from the sediment and reducing the N and P concentrations in the overlying water. Thus, CaO₂ can effectively realize the purification and restoration of the severely eutrophic scenic water.

Insight on the generation of reactive oxygen species in the CaO2/Fe(II) Fenton system and the hydroxyl radical advancing strategy

Calcium peroxide (CaO₂) is a stable hydrogen peroxide (H₂O₂) carrier, and the CaO₂/Fe(II) system has been applied for treatment of various pollutants. It is commonly reported in the literature that hydroxyl radical (HO^●) and superoxide radical anions (O₂ ^●-) are the two main reactive oxygen species (ROSs) generated in the CaO₂/Fe(II) system. However, many of the reported results were deduced from degradation performance rather than specific testing of radical generation. Thus, the specific generation of ROSs and the influence of system conditions on ROSs yield is still unclear. To our knowledge, this is the first study specifically focusing on the generation of HO^● and O₂ ^●- in the CaO₂/Fe(II) system. Experimental conditions were optimized to investigate the production of HO^● and O₂ ^●-. The results showed the influences of CaO₂, Fe(II), and solution pH on HO^● and O₂ ^●- generation, and the HO^● generation efficiency was reported for the first time. In addition, the ROSs generation pathways in the CaO₂/Fe(II) system were elucidated. A strategy for enhancing HO^● yield is developed, based on the continuously dosing Fe(II). This proposed strategy has implications for the effective application of in situ chemical oxidation employing CaO₂/Fe(II) for groundwater remediation.

Application of magnesium peroxide (MgO2) nanoparticles for toluene remediation from groundwater: batch and column studies

In the present study, magnesium peroxide (MgO₂) nanoparticles were synthesized by electro-deposition process and characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The batch experiments were conducted to evaluate the MgO₂ half-life (600 mg/L) in groundwater under various temperatures (4, 15, and 30 °C) and initial pH (3, 7, and 12). The effect of Fe²⁺ ions (enhanced oxidation) on the toluene remediation by MgO₂ was also investigated. Nanoparticles were injected to sand-packed continuous-flow columns, and toluene removal (50 ppm) was studied within 50 days at 15 °C. The results indicated that the half-life of MgO₂ at pH 3 and 12 were 5 and 15 days, respectively, in comparison to 10 days at the initial pH 7 and 15 °C. The nanoparticles showed 20 and 7.5 days half-life at 4 and 30 °C temperatures, respectively. Injection of Fe²⁺ ions indicated an impressive effect on toluene removal by MgO₂, and the contaminant was completely removed after 5 and 10 days, in the batch and column experiments, respectively. Confocal laser scanning microscope (CLSM) analysis indicated that the attached biofilm had a significant role in the decontamination of groundwater. Comparison of bioremediation and enhanced oxidation resulted in a considerable insight into the application of magnesium peroxide in groundwater remediation. Graphical abstract ᅟ.

Electrolytic control of hydrogen peroxide release from calcium peroxide in aqueous solution

The in situ generation of hydrogen peroxide (H₂O₂) for water treatment is more practical than the use of liquid H₂O₂, which is costly to store and transport. Calcium peroxide (CaO₂), a solid carrier of H₂O₂, can release H₂O₂ on dissolution in water. However, the constant H₂O₂ release rate of CaO₂ has been a bottleneck constraining its wider application. In this study, a practical electrochemical method using a divided cell is developed to control the rate of release of H₂O₂ from CaO₂. The results show that the rate of H₂O₂ release from CaO₂ is enhanced in the anolyte. The increase in H₂O₂ release is positively correlated with the current. Under a current of 100 mA, the H₂O₂ concentration was 2.5 times higher after 30 min of electrolysis than in the control experiment in which no current was applied. Water electrolysis in the anodic compartment generates protons that not only: (i) en-hance dissolution of CaO₂ and release of H₂O₂, but also (ii) neutralize the alkaline pH resulting from CaO₂ dissolution, thus providing new advantages for the use of CaO₂. This effective technique may be suitable for the sophisticated control of H₂O₂ release in environmental applications.

Evaluation of robustness of activated sludge using calcium-induced enhancement of respiration

Robustness of an activated sludge system, describing uncertainty and operational risk, was evaluated using the absence or presence of calcium-induced enhancement of respiration (CaER) effect. Generally, the fast-growing system was susceptible to external environmental variations, of which the sludge exhibited significant CaER effect under normal operational conditions, while the slow growing system showed less significant CaER effect. However, sludge in both systems exhibited CaER effect under stressed conditions of decreasing temperature or ammonia shocking. Therefore, the absence of CaER effect indicates a more robust system, while the presence of CaER effect indicates a susceptible system. Additionally, a method to identify safe and dangerous shocking was established by a hybrid usage of absence or presence of CaER effect and recovery index (RI) curve type. The evaluation of robustness could help determining when adjustment should be really taken to cope with the uncertainty, and thus holds a high promise for field application.

Mineralization of sulfolane in aqueous solutions by Ozone/CaO2 and Ozone/CaO with potential for field application

Mineralization of sulfolane in aqueous systems by CaO₂/O₃ and CaO/O₃ was investigated in this study. If 1.6 g/L of oxidants (CaO₂ and CaO) were used along with 5 L/min of O₃ in a batch reactor, degradation of sulfolane followed a pseudo-first order kinetics model. Both sulfolane and TOC were totally removed in less than 40 and 150 min respectively. For these treatments, the pH of the aqueous solutions was above 11, which made O₃ more effective in removing sulfolane. However, the high pH of the solution didn't improve TOC removal. For TOC removal the presence of CaO₂ and CaO was necessary. Once these conditions were optimised in the lab, field experiments were designed and evaluated to treat contaminated ground water samples. The field tests were successful in mineralization of sulfolane within a reasonable time (4 h). Sulfolane degradation took 150 min in these experiments. The pH of the water samples was brought to near neutral (pH = 6.5) by bubbling CO₂.

Strong vibration-catalysis of ZnO nanorods for dye wastewater decolorization via piezo-electro-chemical coupling

A novel vibration-catalytic performance based on piezo-electro-chemical coupling of zinc oxide (ZnO) nanorods for wastewater decolorization was characterized through the product of piezoelectric performance and electrochemical process. The vibration-catalytic decolorization ratio for acid orange 7 (AO7) solution (5 μM) was up to ∼ 80%. The oxidizing hydroxyl radical (OH) of the intermediates of the vibration-catalytic reactions is observed, indicating the production of piezoelectrically-induced electric charges. The dependence of ZnO addition mass, initial dye concentration and the recycling utilization times of ZnO on dye decolorization ratio were systematically studied. The vibration-catalysis mediated by ZnO, with the advantages of high efficiency and recycling utilization, is potential for dye wastewater decolorization treatment.

Benzene-contaminated groundwater remediation using calcium peroxide nanoparticles: synthesis and process optimization

Nano-size calcium peroxide (nCaO₂) is an appropriate oxygen source which can meet the needs of in situ chemical oxidation (ISCO) for contaminant remediation from groundwater. In the present study, an easy to handle procedure for synthesis of CaO₂ nanoparticles has been investigated. Modeling and optimization of synthesis process was performed by application of response surface methodology (RSM) and central composite rotatable design (CCRD) method. Synthesized nanoparticles were characterized by XRD and FESEM techniques. The optimal synthesis conditions were found to be 5:1, 570 rpm and 10 °C for H₂O₂:CaSO₂ ratio, mixing rate and reaction temperature, respectively. Predicted values showed to be in good agreement with experimental results (R ² values were 0.915 and 0.965 for CaO₂ weight and nanoparticle size, respectively). To study the efficiency of synthesized nanoparticles for benzene removal from groundwater, batch experiments were applied in biotic and abiotic (chemical removal) conditions by 100, 200, 400, and 800 mg/L of nanoparticles within 70 days. Results indicated that application of 400 mg/L of CaO₂ in biotic condition was able to remediate benzene completely from groundwater after 60 days. Furthermore, comparison of biotic and abiotic experiments showed a great potential of microbial stimulation using CaO₂ nanoparticles in benzene remediation from groundwater.