Reverse osmosis is used as a tertiary treatment for wastewater reclamation. However, sustainable management of the concentrate (ROC) is challenging, due to the need for treatment and/or disposal. The objective of this research was to investigate the efficiency of homogeneous and heterogeneous Fenton-like oxidation processes in removing propoxur (PR), a micro-pollutant compound, from synthetic ROC solution in a submerged ceramic membrane reactor operated in a continuous mode. A freshly prepared amorphous heterogeneous catalyst was synthesized and characterized, revealing a layered porous structure of 5–16 nm nanoparticles that formed aggregates (33–49 μm) known as ferrihydrite (Fh). The membrane exhibited a rejection of >99.6% for Fh. The homogeneous catalysis (Fe3+) exhibited better catalytic activity than the Fh in terms of PR removal efficiencies. However, by increasing the H2O2 and Fh concentrations at a constant molar ratio, the PR oxidation efficiencies were equal to those catalyzed by the Fe3+. The ionic composition of the ROC solution had an inhibitory effect on the PR oxidation, whereas increased residence time improved it up to 87% at a residence time of 88 min. Overall, the study highlights the potential of heterogeneous Fenton-like processes catalyzed by Fh in a continuous mode of operation.

  • Propoxur removal from ROC solution in a submerged ceramic membrane reactor was studied.

  • Homogeneous (Fe3+) and heterogeneous (ferrihydrite) Fenton-like oxidation were compared.

  • While Fe3+ exhibited better catalytic activity than Fh, increased H2O2 and Fh concentrations led to similar PR removal as Fe3+.

  • Maximal PR removal of 87% was obtained at a residence time of 88 min.

  • The composition of the ROC inhibited the PR oxidation.

The ever-increasing application of reverse osmosis (RO) in municipal and industrial wastewater treatment has provided an affordable alternative for the sustainable reclamation of water resources. Nonetheless, this process produces a concentrated waste stream (ROC) that needs to be treated and/or disposed of. Municipal wastewater ROC contains rejected substances at 4–7 times higher concentrations than the feed water (Yaqub et al. 2022) and has a reduced volume of 15–20% (Xiang et al. 2019). The composition of the ROC depends on the properties of the raw wastewater and the specific wastewater treatment process applied. It consists mainly of nutrients, dissolved inorganic salts, and organic substances such as micro-pollutants (MPs). The range of the physicochemical parameters of ROC can be found in Deng (2020) and Valdés et al. (2021).

MPs consist of natural and anthropogenic substances, including pharmaceuticals, personal care products, steroid hormones, industrial chemicals, pesticides, and surfactants. The concentrations of micro-pollutant in ROC range from a few ng/L to hundreds of mg/L. The occurrence of 55 emerging MPs in nanofiltration and RO municipal wastewater concentrates was reviewed by Deng (2020).

Several studies have comprehensively reviewed the physicochemical and biological techniques explored for the removal of MPs, including adsorption, membrane processes, advanced oxidation processes (AOPs), and their integration (Rathi et al. 2021; Sivaranjanee & Kumar 2021; Shahid et al. 2021; Kumar et al. 2022; Morin-Crini et al. 2022).

Management of ROC often poses challenges, as removal of organic pollutants from ROC by conventional methods is difficult due to high salinity and the organic pollutants recalcitrant nature. High salinity inhibits microbial activity mainly by causing water loss from the cell membranes and reducing enzyme activities. This can result in plasmolysis or cell death (Shi et al. 2015). Additionally, high salinity can impact oxidation processes by scavenging bulk and/or surface oxidants, decreasing dissolution, inactivating active sites, and altering the nature of the organics (Yuan et al. 2022).

AOPs are utilized for the partial or complete mineralization of contaminants through in-situ generation of highly reactive and low selectivity radicals, including hydroxyl, hydroperoxyl, and superoxide. Sulfate and chloride radicals are also included in a wider definition of AOPs. AOPs are based on ozone, hydrogen peroxide electrochemical reactions, photolysis, sonolysis, and their combinations. Several recent comprehensive reviews have been published on AOPs in general and specifically for treating emerging trace organic contaminants (Giwa et al. 2021; Krishnan et al. 2021; Tufail et al. 2021; Priyadarshini et al. 2022; Saravanan et al. 2022; Gonzaga et al. 2023; Wang et al. 2023). Moreover, Xiang et al. (2019), Deng (2020), and Arola et al. (2019) have reviewed various AOPs that are applicable to ROC treatment for municipal wastewater reclamation.

The Fenton (H2O2/Fe2+) and Fenton-like (H2O2/Fe3+) reaction can be either homogeneous or heterogeneous, with the latter involving the use of solid catalyst such as iron oxyhydroxide, iron oxides (ferrihydrite, goethite, hematite, and magnetite), and zero-valent iron. Iron-based catalysts have several advantages, including their abundance, low cost, high catalytic activity, low toxicity, efficient recovery, chemically stability over a wide pH environmental friendliness, and sustainability (Hussain et al. 2021; Thomas et al. 2021). In both Fenton and Fenton-like systems, the primary reactions generate hydroxyl radicals (HO•), hydroperoxy (HO2•), and peroxide radicals O2, as shown in Equations (1)–(3). The rate constant of reaction 1 (40–80 1/M s) is much higher than that of reaction 2 (0.1–1 × 10−2 1/M s) (Thomas et al. 2021), indicating the importance of the Fe3+/Fe2+ redox cycle for highly efficient oxidation.

Heterogeneous Fenton-like reactions, catalyzed by iron oxide, are initiated through surface complexation (Equation (4)) followed by a ligand-to-metal electron transfer within the surface complex (Equation (5)). This generates HO2• and O2. Hydroxyl radicals are then formed by the reaction with surface ferrous ions (Equation (6)) (Chen et al. 2021). The primary difference in the oxidation mechanism between homogeneous and heterogeneous Fenton processes is that the former occurs in the bulk solution, whereas the latter occurs in the solid–liquid boundary layer and/or on the surface of the heterogeneous catalyst. Leaching of iron from the surface of the heterogeneous catalysts may induce a homogeneous Fenton reaction.
formula
(1)
formula
(2)
formula
(3)
formula
(4)
formula
(5)
formula
(6)

The Fenton process is a preferred AOP due to its versatile applications, capacity to withstand interference, simple operation, rapid degradation rate, and improved biodegradability of organic contaminants. However, it exhibits certain limitations such as a restricted operational pH range of 2–4 and the need for neutralization. The formation of iron sludge and consequent discharge of iron into the environment further contribute to its drawbacks (Zhang et al. 2019). The heterogeneous Fenton process offers several advantages over the homogeneous Fenton process. These include reduced iron sludge production, a wider working pH range, catalyst reusability, reduced iron leaching, long-term catalyst stability, and effective Fe3+/Fe2+ interconversion (Xavier et al. 2013; Sreeja & Sosamony 2016; Zhang et al. 2019). However, it is limited by the diffusion rate of the target compounds onto the surface of the catalyst and/or its boundary layer, resulting in a slower oxidation rate than the homogeneous process at the same concentrations (Litter & Slodowicz 2017). The catalyst's synthetic conditions and costs, as well as the reactor, pose challangers (Zhang et al. 2019).

Propoxur (PR), 2-isopropoxy phenyl N-methyl carbamate, commercially known as Baygon® is a broad-spectrum insecticide that can persist in the environment and pose a risk to human health and the ecosystem. It has been detected in surface water, groundwater, and soil, indicating widespread environmental contamination. Exposure to PR has been linked to various adverse health effects, such as nervous system stimulation, enzyme activation, stunted growth, and cancer (Vandana et al. 2001). AOPs can be effective for the removal of PR from contaminated water hence contribute to reducing its potential adverse effects on human health and the environment. It has been reported that PR was oxidized via direct photolysis (UV) (Benitez et al. 1994; Sanjuán et al. 2000), photo-catalysis (UV/TiO2) (Sanjuán et al. 2000), UV/TiO2/GAC (Lu 1999), O3/UV (Benitez et al. 1994), and electrochemically (Guimarães Selva & Longo Cesar Paixão 2016).

In this study, the removal of PR from synthetic ROC solution by homogeneous and heterogeneous Fenton-like oxidation in a ceramic membrane reactor was compared. PR was chosen as a model MP compound due to its high water solubility and widespread use as a broad-spectrum insecticide.

Materials

The chemicals were of analytical grade and were used as-received without further purification. PR was supplied by Supelco (China), NaCl, KCl, NaHCO3 (Frutarom Industries Ltd, Israel), Na2SO4 (Bio-Lab Ltd, Israel), MgCl2·6H2O (Merck, Germany), CaCl2·2H2O (Spectrum, USA), Na2HPO4 (Riedel-de Haën, Germany), and FeCl3·6H2O (Alfa Aesar, USA).

Synthetic reverse osmosis concentrate solution

Synthetic ROC solution was prepared by dissolving NaCl, NaHCO3, Na2SO4, CaCl2·2H2O, KCl, Na2HPO4·2H2O, and MgCl2·6H2O in deionized water (DI). The composition of the solution is listed in Table 1. PR oxidation experiments were conducted in both DI and ROC solutions. It is important to note that the addition of both the homogeneous and heterogeneous catalysts introduces counter ions to the aqueous solution, namely sodium and chloride.

Table 1

Feed synthetic ROC composition (Woo et al. 2019)

IonClHCO3SO42−Na+Ca2+K+PO43−Mg2+pHTDS
(mg/L) 1,000 488 250 819 100 70 31 20 7.8 2,778 
IonClHCO3SO42−Na+Ca2+K+PO43−Mg2+pHTDS
(mg/L) 1,000 488 250 819 100 70 31 20 7.8 2,778 

Synthesis and characterization of the catalyst

A freshly prepared amorphous ferric chloride-based catalyst was synthesized using the precipitation method. The catalyst was prepared by diluting a stock solution of FeCl3·6H2O (1.3 M) in DI water to the desired concentration, followed by the addition of a predetermined amount of NaOH solution (1.0 M) to achieve the desired pH.

The particle sizes were measured using a laser diffraction particle size analyzer (Mastersizer 2000, Malvern, UK). The zeta potential was measured using a ZetaSizer Nano-ZS analyzer (Malvern, UK) in a disposable folded capillary cell (DTS1060). The morphology of the synthesized catalyst was studied using Philips CM120 Transmission Electron Microscopy (TEM) and an Olympus BH2 Light Microscope connected to an Optronics CCD camera. Raman spectra were obtained with a Horiba Jobin Yvon LABRAM HR micro-Raman spectrometer equipped with a Nd-YAG laser (100 mW, 532.2 nm) and diffraction gratings of 1,800 grooves 1/mm. The Raman shifts were detected using a Peltier-cooled, slow-scan, CCD matrix detector, and ranged from 180 to 450 1/cm.

Flow-through ceramic membrane reactor

A submerged single silicon carbide ceramic flat sheet membrane (Cerafiltec, Germany) with a pore size of 0.1 μm and filter area of 0.01 m2 was used in an acrylic glass reactor with dimensions of 14.3 × 3.7 × 2.0 cm (l × w × d). The schematics of the experimental system is shown in Figure 1. A ceramic membrane was chosen since it exhibits a high oxidation resistance, narrow pore size distribution, and high mechanical stability. Mixing was guaranteed by air bubbles generated by an OxyMax 200 air pump (Oase, Germany) at a flow rate of 1.5 L/min. Air-scouring also prevents membrane fouling by the heterogeneous catalyst.
Figure 1

The submerged ceramic membrane reactor experimental system.

Figure 1

The submerged ceramic membrane reactor experimental system.

Close modal

A Masterflex peristaltic pump (model 77800-50, Cole-Parmer, USA) was used to feed a solution containing PR and hydrogen peroxide into the reactor. The flow rates were set at 0.13–0.59 L/h, resulting in residence times (RTs) of 20–90 min. A second pump with an identical flow rate was used to pumped the product water (i.e., permeate) at fluxes of 13–59 L/m2 h and a pressure of <25 mbar. The submerged membrane operated in a dead-end mode.

At the beginning of the experiment, the heterogeneous catalyst was added to the reactor at concentrations ranging from 252 to 1,495 mg/L. The experiments were carried out either in ROC or DI water solutions with a volume of 192 mL, a PR concentration of approximately 1 mg/L, and hydrogen peroxide concentrations ranging from 140 to 1,096 mg/L at molar ratios of 0.7–1.1 iron to hydrogen peroxide. A third pump was used to continuously dose the homogeneous catalyst. These experiments were carried out in ROC solution, at pH 3.0, 150 mg/L H2O2, and 250 mg/L Fe3+.

Experiments were conducted to test the effects of sulfate, chloride, and bicarbonate on the PR oxidation in ROC solution, both in the presence and absence of these ions. The experiments were performed at pH 4.0, 150 mg/L H2O2, and 250 mg/L catalyst. Notably, these experiments were conducted using a full ROC solution composition, with the omission of one of the aforementioned anions. This is in contrast to the batch to the batch experiments described in the next section, which were carried out in a single salt solution with PR and the tested ion dissolved in DI water.

pH values of 3.0 and 4.0 were maintained constant throughout the experiments using the homogeneous and heterogeneous catalysts, respectively. Experiments were carried out for at least five RTs after the system had reached a steady state. The reported values of C/C0 represent the normalized concentration at steady-state conditions.

Batch experiments

The efficiency of batch PR oxidation in ROC solution was investigated using both homogeneous and heterogeneous catalysts. The homogeneous catalyst was tested at pH values of 2.5, 3.0, and 3.5, while the heterogeneous catalyst was tested at pH values of 4.0, 5.0, 6.0, and 7.8. The lowest pH tested for the heterogeneous catalyst was 4, as this is the threshold for ferric chloride coagulation (Wei et al. 2017). The pH was maintained constant throughout the experiments. The experiments were conducted in 100 mL glass beakers containing 50 mL of ROC solution with 1 mg/L of PR for up to 4 h. The molar ratio of Fe to H2O2 was kept constant at 1, with 84 mg/L of the homogeneous catalyst (50 mg/L H2O2) and 250 mg/L of the heterogeneous catalyst (150 mg/L H2O2). The concentrations of the reactants were adjusted to achieve comparable PR removal rates with both catalysts. The solution was stirred using a magnetic stir bar at room temperature (22 ± 2 °C) and sampled periodically to measure the concentrations of PR, hydrogen peroxide, and iron. Prior to analysis, samples were filtered using a 0.45 μm filter to separate the heterogeneous catalysts from the solution.

Batch Fenton-like oxidation experiments designed to study the effect of the catalyst concentration were carried out in a glass beaker containing 25 mL ROC solution with 1 mg/L PR for up to 3.5 h. These experiments were conducted at pH 3.0 and 50 mg/L H2O2 for the homogeneous catalyst, and at pH 4.0 and 150 mg/L H2O2 for the heterogeneous catalyst. The different reactant concentrations were used so as to obtain similar PR removal efficiencies using both of the catalysts. The catalyst dosages were varied from 8 to 93 mg/L for the homogeneous catalyst and 71 to 256 mg/L for the heterogeneous catalyst, while the oxidant dosage was kept constant.

To examine the effect of accompanying ions on the removal of PR, batch experiments were conducted in the presence of each of the ROC components dissolved in DI water at the concentrations listed in Table 1. The experiments were conducted in glass beakers containing 50 mL of solution for up to 25 min. The homogeneous catalyst was used at pH 3.0 with 25 mg/L H2O2, while the heterogeneous catalyst was used at pH 4.0 with 150 mg/L H2O2. The molar ratio of Fe to H2O2 was kept constant at 1.

The counter ion for the cations (calcium, potassium, and magnesium) was chloride, while the counter ion for the anions (chloride, bicarbonate, phosphate, and sulfate) was sodium. Initially, the effect of the counter ions was tested by dissolving NaCl in DI water at a concentration of 1,628 mg/L. The results served as a benchmark for determining the effect of the other tested ions.

Analytic methods

The concentration of PR was analyzed using an Agilent 1260 Infinity LC (Agilent, US) equipped with a Zorbax Eclipse XDB-C18 column. The eluent consisted of a 70:30 (v/v) mixture of HPLC-grade acetonitrile and HPLC-grade water, at a flow rate of 0.4 mL/min. PR concentration was measured at a wavelength of 271 nm, using an injection volume of 10 μL, and a column oven temperature of 30 °C.

Hydrogen peroxide concentration was determined by the Ghormley method (Klassen et al. 1994). Iron concentration was determined using Hach method 8008 FerroVer® Iron Reagent Powder Pillows (DR2800 spectrophotometer, Hach, Germany). All experiments carried out in triplicate revealed a standard deviation average of up to 8% in PR analysis and 5% in hydrogen peroxide and iron analysis.

Characterization of the heterogeneous catalysts

Amorphous ferric oxi/hydroxide agglomerates were freshly prepared before each experiment through the precipitation method, by the addition of 1 M NaOH to FeCl3·6H2O solution in DI water at room temperature. TEM images revealed a mesh-like structure of porous aggregated nanoparticles, as shown in Figure 2(a) and 2(b). The aggregates were composed of spherical particles that ranged in size from 5 to 16 nm, as measured by Image-Pro (v10) analysis software. The micrometer mean size of the aggregates ranged from 33 to 44 μm in ROC solution and from 35 to 49 μm in DI water (Figure 2(c)). The structure corresponds to an amorphous Fe(III) hydroxide hydrate known as ferrihydrite (Fh), which is a hydrous iron oxide mineral with a poorly ordered structure composed of small crystallites with a wide range of sizes and shapes, a water content of 15–25% (Cornell et al. 1989), and a relatively high specific surface area up to 700 m2/g (Eusterhues et al. 2008).
Figure 2

Aggregates characterization (a and b) Cryo-TEM images, (c) mean aggregate sizes, and (d) zeta potential (4.5 mM Fe).

Figure 2

Aggregates characterization (a and b) Cryo-TEM images, (c) mean aggregate sizes, and (d) zeta potential (4.5 mM Fe).

Close modal

Smaller aggregates were obtained in the ROC solution as compared to DI water, as seen in Figure 2(c). This is because the electrolyte ions hinder condensation and polymerization processes, resulting in smaller particle sizes (Wei & Semiat 2017). Measurement of the zeta potential (Figure 2(d)) revealed a higher point of zero charge (8.0) in DI water than in ROC solution (6.1). The electrolytes decrease the zeta potential, indicating reduced electrostatic repulsion, initially promoting the aggregation of iron nanoparticles before the ion hindering effect becomes predominant (Wei & Semiat 2017). Also seen in Figure 2(c) is that larger aggregates were obtained in both DI and ROC solutions as the pH was increased. It has been reported that the threshold of ferric chloride coagulation is around pH 4. At pH < 4, repletion between positively charged monomers limits the bridging process. As the pH increases, the generation of polycationic structures is promoted, resulting in precipitated particles in the aqueous solution (Wei et al. 2017).

Air drying of the Fh aggregates (at room temperature) transformed it into hematite, as indicated by the bands of the Raman spectrum in Figure 3. Fe atom vibrations are responsible for the low-frequency modes in the range of 200–300 1/cm, whereas vibrations of O atoms are responsible for bands in the 400–650 1/cm range. The 227 1/cm band is attributed to the movement of iron cations along the c-axis. The bands at 410 1/cm are assigned to the symmetric mode of O atoms in relation to other cations in a plane that is perpendicular to the crystallographic c-axis, and also to Fe–O stretching vibrations (Jain et al. 2019). It is reported that Fh is thermodynamically unstable and gradually transforms to either α-FeO(OH) (goethite) by a mechanism of dissolution–precipitation or α-Fe2O3 (hematite) by dehydration/internal rearrangement pathway (Hanesch 2009). These crystalline products form by competing mechanisms, and the proportion of each in the final product depends on the relative rates of formation (Cornell et al. 1989).
Figure 3

Raman spectrum of dehydrated Ferrihydrite in room temperature.

Figure 3

Raman spectrum of dehydrated Ferrihydrite in room temperature.

Close modal

Fenton-like oxidation of PR

Control experiments were conducted in DI water with PR, PR + H2O2 (130 mg/L), and PR + Fh (250 mg/L) at a residence time of 44 min and pH of 4.0. In these experiments, the feed PR concentration was 1 mg/L. The results revealed negligible changes in the PR concentrations, indicating that the ceramic membrane and/or the Fh did not adsorb the PR and the hydrogen peroxide did not oxidize it without a catalyst. An Fh rejection of >99.6% by the ceramic membrane was obtained at pH ≥ 4.0.

Batch experiments were conducted to determine the optimal pH for the PR oxidation using Fe3+ and Fh at pH 2.5–3.5 and 4.0–7.8, respectively. Figure 4 displays the PR removal and H2O2 decomposition at the tested pH values. The results indicate that pH 3.0 exhibited the highest PR removal catalyzed by the Fe3+ (88% within 3 h), while PR was removed at 50 and 57% at pH 2.5 and 3.5, respectively. Corresponding to the PR oxidation, the highest decomposition of hydrogen peroxide was obtained at pH 3.0. Catalysis by Fh exhibited maximal PR removal (95%) at pH 4.0 after 4 h. The removal of PR decreased as the pH increased, with only 16% removed at pH 7.8. Lower hydrogen peroxide decomposition was obtained at an acidic pH.
Figure 4

PR removal (filled columns) and H2O2 decomposition (empty columns) as a function of ROC solution pH (Fe3+: 50 mg/L H2O2, 84 mg/L Fe3+, t = 3 h; Fh: 150 mg/L H2O2, 250 mg/L Fh, t = 4 h).

Figure 4

PR removal (filled columns) and H2O2 decomposition (empty columns) as a function of ROC solution pH (Fe3+: 50 mg/L H2O2, 84 mg/L Fe3+, t = 3 h; Fh: 150 mg/L H2O2, 250 mg/L Fh, t = 4 h).

Close modal

The pH can have an impact on heterogeneous catalysts by altering the solubility of iron on the catalyst surface and/or activity of its active sites. The fact that the amorphous Fh catalysis was not efficient at elevated pH levels suggests that the pH did not affect its active sites but rather the solubility of iron. An increase in pH slows the iron leaching from the heterogeneous catalyst, leading to inactivation in the bulk solution via hydrolysis and ferric hydroxide sludge precipitation (Wang et al. 2016). Homogeneous Fenton reactions are feasible at pH < 4 because the interconversion of Fe2+ and Fe3+ maximizes the process efficiency (Hussain et al. 2021). Acidity aids in protonating hydrogen peroxide, which enhances its reactivity and increases its propensity to produce hydroxyl radicals rather than decomposing into water (Jung et al. 2009).

Typical results

Figure 5 displays typical results for the PR and hydrogen peroxide decay during the oxidation reaction by H2O2/Fh in the submerged ceramic membrane reactor. The results are presented as a plot of the normalized concentration (C/C0) as a function of normalized time, i.e., the reaction time divided by the residence time (t/τ). As seen, there are two unique regions in Figure 5. The first region comprises the first 2.3 RTs and is referred to as the start-up of the reactor, where a decay in the PR and hydrogen peroxide concentrations is observed. The second region is from 2.3 RTs onward, at which steady-state conditions prevail. In this region, the experimental points are dispersed around an average value shown in Figure 5 as a solid line.
Figure 5

Propoxur removal and hydrogen peroxide decomposition as a function of the normalized time (DI water, PR = 1 mg/L, H2O2 = 150 mg/L, Fh = 260 mg/L, RT of 44 min, pH 4).

Figure 5

Propoxur removal and hydrogen peroxide decomposition as a function of the normalized time (DI water, PR = 1 mg/L, H2O2 = 150 mg/L, Fh = 260 mg/L, RT of 44 min, pH 4).

Close modal

Heterogeneous and homogeneous Fenton-like PR oxidation

The removal of PR in ROC solution and hydrogen peroxide decomposition, catalyzed by Fe3+ and Fh, were compared. As seen in Figure 6, Fe3+ exhibited better catalytic performance than the Fh. In the presence of Fe3+, a total of 72% PR was removed, and 39% of hydrogen peroxide was decomposed, while only 13 and 2%, respectively, were removed in the presence of Fh. A possible explanation for these outcomes could be attributed to the limited mass transfer of the substrates to the heterogeneous catalyst surface (Zhang et al. 2013). In addition, the surface area and number of active sites of the catalyst can restrict heterogeneous oxidation (Ioffe et al. 2022). Heterogeneous catalysts have low activity due to the slow reduction of Fe3+ in the Fe3+/Fe2+ redox cycle (1–3 × 10−3 1/M s) (Gao et al. 2020).
Figure 6

PR removal and H2O2 decomposition catalyzed by Fe3+ (pH 3) and Ferrihydrite (pH 4) (ROC solution, PR = 1 mg/L, H2O2 = 150 mg/L, Fe = 250 mg/L, RT of 44 min).

Figure 6

PR removal and H2O2 decomposition catalyzed by Fe3+ (pH 3) and Ferrihydrite (pH 4) (ROC solution, PR = 1 mg/L, H2O2 = 150 mg/L, Fe = 250 mg/L, RT of 44 min).

Close modal

Effect of H2O2/iron concentration

In order to find the best conditions for the flow-through experiments, the effect of the catalyst concentration on the PR removal was studied in batch experiments. In these experiments, the dosage of catalyst was changed while that of the oxidant was constant. The increase in the catalyst concentration led to an increase in the molar ratio of iron to hydrogen peroxide. As seen in Figure 7, an increase in the molar ratio resulted in augmented removal of the PR, up to a molar ratio of 0.75 with a removal of 93%. A further increase in the molar ratio did not improve the PR removal. It is established that higher catalyst dosages facilitate the Fenton process, particularly when using heterogeneous iron-based catalysts. However, at a certain concentration, the scavenging of hydroxyl radicals by the catalyst itself may inhibit the reaction (Hussain et al. 2021). Based on these results, flow-through experiments were conducted using both catalysts, at a molar ratio of 0.75 Fe to H2O2.
Figure 7

PR removal as a function of the molar ratio Fe to H2O2 using Fe3+ (pH 3, H2O2 = 50 mg/L, t = 3 h) and Fh (pH 4, H2O2 = 150 mg/L, t = 4 h) as catalysts (Batch experiments, PR = 1 mg/L).

Figure 7

PR removal as a function of the molar ratio Fe to H2O2 using Fe3+ (pH 3, H2O2 = 50 mg/L, t = 3 h) and Fh (pH 4, H2O2 = 150 mg/L, t = 4 h) as catalysts (Batch experiments, PR = 1 mg/L).

Close modal

Both catalysts exhibited similar removal efficiencies at the same molar ratio (Figure 7). However, to achieve equivalent removal percentages as Fe3+, the heterogeneous oxidation required three times higher concentrations of H2O2 and Fh, as well as prolonged reaction time. As mentioned earlier, heterogeneous catalysis can be limited by factors such as mass transfer, catalyst surface area, number of active sites, and the reduction rate of Fe3+.

In order to equilibrate the efficiency of PR removal catalyzed by Fh with that of Fe3+ in the ceramic membrane reactor, the concentrations of hydrogen peroxide and Fh were increased while keeping the molar ratio of Fe to H2O2 constant at 0.75. The reaction was carried out at residence time for 44 min. The results, displayed in Figure 8(a), show a linear relationship between the H2O2 and Fh concentrations and PR removal up to 1,100 and 1,500 mg/L H2O2 and Fh, respectively. At these concentrations, PR removal (3.6 ± 0.105 μM) was equal to that catalyzed by the Fe3+ (3.5 ± 0.157 μM), with higher H2O2 decomposition during the heterogeneously catalyzed PR oxidation (Figure 8(b)).
Figure 8

(a) PR removal at a molar ratio of 0.75 Fe to H2O2 as a function of H2O2 concentration; data labels are for Fh concentrations; (b) PR removed and H2O2 decomposed, catalyzed by Fe3+ (pH 3, 150 and 250 mg/L H2O2 and Fe3+, respectively) and Fh (pH 4, 1,100 and 1,500 mg/L H2O2 and Fh, respectively). ROC solution, PR = 1 mg/L, RT of 44 min.

Figure 8

(a) PR removal at a molar ratio of 0.75 Fe to H2O2 as a function of H2O2 concentration; data labels are for Fh concentrations; (b) PR removed and H2O2 decomposed, catalyzed by Fe3+ (pH 3, 150 and 250 mg/L H2O2 and Fe3+, respectively) and Fh (pH 4, 1,100 and 1,500 mg/L H2O2 and Fh, respectively). ROC solution, PR = 1 mg/L, RT of 44 min.

Close modal

During the homogeneous Fenton-like oxidation process, Fe3+ was continuously dosed into the reactor. Initially, about five times as much Fh was added as compared to Fe3+ (1,500 and 250 mg/L, respectively). Within about six RTs, the total iron consumed (253 g/m3 product water) was equalized for both catalysts, after which the Fe3+ concentration exceeded that of the Fh. This suggests a preference for heterogeneous catalysis during prolonged operations, as less Fh would be required. Additionally, using Fh as a catalyst has the added advantage that the product water does not contain any iron.

Effect of residence time

Figure 9 displays the PR removal, catalyzed by Fh, at steady state as a function of residence time (20–90 min) in both DI and ROC solutions. In the ROC solution, the concentrations of the catalyst and oxidants were about 7 times higher (1,500 and 1,100 mg/L, respectively) compared to the DI water solution (250 and 150 mg/L, respectively). This was necessary to achieve higher PR removal efficiencies, as the lower concentrations resulted in only 12 and 30% removal at RTs of 44 and 88 min, respectively. The effect of the ROC ionic composition on the PR oxidation is discussed in detail in the next section.
Figure 9

PR removal in DI water and ROC solutions as a function of residence times at pH 4 (PR = 1 mg/L, DI water solution: H2O2 = 150 mg/L, Fh = 260 mg/L; ROC solution: H2O2 = 1,000 mg/L, Fh = 1,470 mg/L).

Figure 9

PR removal in DI water and ROC solutions as a function of residence times at pH 4 (PR = 1 mg/L, DI water solution: H2O2 = 150 mg/L, Fh = 260 mg/L; ROC solution: H2O2 = 1,000 mg/L, Fh = 1,470 mg/L).

Close modal

As seen in Figure 9, a linear relationship was observed in DI water solutions, between residence time and PR removal, with higher removal rates as residence time increased. The highest PR removal of 89.7% was obtained at a RT of 87.5 min. In contrast, an asymptotic relationship between residence time and PR removal was observed in the ROC solution. Only a small change in the PR removal performance was found following the RT of 64 min. Longer RTs typically result in higher removal rates due to increased reaction time. Insufficient contact between the target compounds and the heterogeneous catalyst, as well as the limiting mass transfer rates, may lead to a lower oxidation efficiency (An et al. 2022). Nonetheless, PR oxidation catalyzed by Fe3+ (150 and 250 mg/L H2O2 and Fe3+, respectively) was not affected by the RTs of 20–60 min (data not shown). These results suggest that, for the tested concentrations of the hydrogen peroxide and Fe3+, RT of 20 min was sufficient to effectively remove the PR to a maximum level of 68 ± 3.4%, and that the reaction rate was shorter than 20 min.

Solution composition

To better understand the effect of the ROC solution on the PR Fenton-like oxidation (both homogenous and heterogeneous), batch experiments were performed in DI water solution with the addition of each of the ROC solution components at the concentrations listed in Table 1. It should be noted that the concentration of the ions may also contribute to their effect on the PR oxidation. The results were compared to those of PR oxidation in NaCl solution (1.6 g/L) in order to eliminate the effect of the counter ions, chloride and sodium, that accompany the tested ions. These experiments were conducted at a molar ratio of 1 Fe to H2O2 with six times higher concentrations of H2O2 catalyzed by Fh. The higher concentrations were chosen to achieve PR removal within the time frame of Fe3+.

As seen in Figure 10, the cations (i.e., K+, Ca2+, and Mg2+) exhibited a negligible effect on the PR oxidation. This may be attributed to their inability to scavenge radicals because they are in their maximum oxidation state (Burns et al. 1999). The anions, on the other hand, inhibited the PR oxidation in the following decreasing order: HPO4 > HCO3 > Cl. While the catalysis by Fe3+ was not affected by the sulfate, it had the most inhibitory effect on the PR oxidation catalyzed by Fh. The difference in the inhibitory effect of the sulfate in the presence of Fe3+ and Fh may be attributed to the different pH values at which the oxidation took place (3 and 4, respectively). Sulfate forms a complex with ferric ions at pH >3.5, resulting in the precipitation of Fe2(SO4)3 (Barik & Mohapatra 2011); thus, it only affected the Fh catalysis.
Figure 10

Propoxur decay in the presence of each of the ROC components during oxidation at a molar ratio of 1 Fe to H2O2 (a) Fh catalysis (H2O2 = 150 mg/L, pH 4.0) and (b) Fe3+ catalysis (H2O2 = 25 mg/L, pH 3.0).

Figure 10

Propoxur decay in the presence of each of the ROC components during oxidation at a molar ratio of 1 Fe to H2O2 (a) Fh catalysis (H2O2 = 150 mg/L, pH 4.0) and (b) Fe3+ catalysis (H2O2 = 25 mg/L, pH 3.0).

Close modal

Inorganic ions may inhibit AOPs through two main mechanisms: hydroxyl radical scavenging and iron complexation. Table 2 lists the reactions of the inorganic anions present in the ROC solution with hydroxyl radicals. These inorganic radicals exhibit lower reactivity, higher selectivity, and longer half-lives than hydroxyl radicals (De Laat et al. 2004; Patra et al. 2020). Inorganic anions may also interfere with the formation of O2·/HO2· radicals hindering the regeneration of Fe2+ from Fe3+ reduction (Equation (2)), resulting in lower hydroxyl radicals generation (Equations (1); De Laat et al. 2004).

Table 2

Primary reactions of hydroxyl radical with inorganic anions

ReactionRate constant (1/M s)Ref.
 4.3 × 109 Buxton et al. (1998)  
 1.4 × 107 de Oliveira et al. (2015)  
  Kim et al. (2019)  
 8.5 × 106 Buxton et al. (1998)  
 3.9 × 108 Buxton et al. (1998)  
 2.2 × 106 Martire & Gonzalez (2001)  
 8.0 × 105 Martire & Gonzalez (2001)  
 2.6 × 106 Martire & Gonzalez (2001)  
ReactionRate constant (1/M s)Ref.
 4.3 × 109 Buxton et al. (1998)  
 1.4 × 107 de Oliveira et al. (2015)  
  Kim et al. (2019)  
 8.5 × 106 Buxton et al. (1998)  
 3.9 × 108 Buxton et al. (1998)  
 2.2 × 106 Martire & Gonzalez (2001)  
 8.0 × 105 Martire & Gonzalez (2001)  
 2.6 × 106 Martire & Gonzalez (2001)  

The complexation of anions with aqueous and/or surface-bound Fe2+ and Fe3+ affects the distribution and reactivity of the iron species (De Laat et al. 2004) for both homogeneous and heterogeneous catalysts. For heterogeneous catalysis, surface complexations may block active sites on the Fh surface, decreasing the oxidation efficiency (Sheng et al. 2020). The four anions' surface complexation on Fh is considered inner-sphere for phosphate and carbonate, outer-sphere for chloride, and both inner- and outer-sphere for sulfate, depending on the pH, ionic strength, surface coverage, and hydration (Kumar et al. 2014). The molecular structure of sulfate inner-sphere complexes was determined as bidentate − binuclear (Gu et al. 2016) while that of carbonate and phosphate double-corner bidentate complexes (Kumar et al. 2014). In the solution, sulfate, chloride, and phosphate anions form complexes such as FeSO4+, FeSO4, FeCl2+, FeCl+ (de Oliveira et al. 2015), FeH2PO42+, FeHPO4+, and Fe(H2PO4)2+ (Al-Sogair et al. 2002).

To further explore the impact of SO4−2, HCO3, and Cl on PR oxidation, flow-through experiments were conducted in the absence of these anions. It is important to note that unlike the batch experiments, which were conducted in a single salt solution, the experiments described here were carried out in the full ROC composition, with one of the above-mentioned anions excluded. These anions were selected due to their significant effect on Fh-catalyzed PR oxidation, with Cl having the highest concentration in the ROC solution. As expected, excluding the anions from the ROC solution did not significantly affect the oxidation of the PR under steady-state conditions (Figure 11(a)). The number of active sites available for continuous catalytic cycles is one order of magnitude lower than the total anions fed to the reactor: 0.57 mmol sites and 5.88–7.88 mmol of total anions, based on a two-line Fh site density of 0.112 mol sites/mol Fe (Chen et al. 2021). This indicates that there is an excess of anions competing for the available surface sites for complexation, even when one of the anions is excluded. The binding of the anions to the Fh surface sites hinders surface reactions by decreasing the number of surface sites available for reaction with H2O2 and PR. The absence of sulfate and chloride affected the number of RTs required to reach steady state, as illustrated in Figure 11(b). These results may suggest that outer-sphere complexation is the predominant mechanism of the anion adsorption onto the Fh in the ROC solution, as bicarbonate, which forms only inner-sphere complexation, has a negligible effect on the number of RTs required to reach steady state.
Figure 11

(a) Box-whisker plot of the PR steady-state concentrations and (b) number of residence times required to reach steady state in full ROC composition and ROC excluding HCO3, SO42−, and Cl (PR = 1 mg/L, H2O2 = 150 mg/L, Fh = 250 mg/L, RT of 44 min, pH 4).

Figure 11

(a) Box-whisker plot of the PR steady-state concentrations and (b) number of residence times required to reach steady state in full ROC composition and ROC excluding HCO3, SO42−, and Cl (PR = 1 mg/L, H2O2 = 150 mg/L, Fh = 250 mg/L, RT of 44 min, pH 4).

Close modal

The removal of PR from synthetic ROC solution was studied using homogeneous (Fe3+) and heterogeneous (Fh) Fenton-like oxidation in a submerged ceramic membrane reactor was studied. The freshly synthesized heterogeneous Fh catalyst had an amorphous, layered, porous structure consisting of nanoparticles (5–16 nm) in aggregates with a pHPZC of 6.1. The size of the aggregates ranged from 33 to 49 μm, depending on the pH of the solution.

Batch experiments revealed that the most favorable conditions for PR oxidation were achieved at a molar ratio of 0.75 iron to hydrogen peroxide and pH values of 3.0 and 4.0 for homogeneous and heterogeneous catalysis, respectively. The flow-through experiments showed that Fe3+ exhibited higher catalytic activity than Fh, with 72 and 13% of PR removed and 39 and 2% of hydrogen peroxide consumed, respectively. By increasing the concentrations of hydrogen peroxide and Fh, the PR removal could be increased to comparable levels as those obtained by Fe3+ catalysis. Although Fh required about five times higher reactant concentrations, the total iron consumed was equal to that of the Fe3+ within six RTs. After which, the Fe3+ concentration exceeded that of the Fh, as ≥99.6% rejection of the Fh was obtained by the ceramic membrane. Hence, for prolonged operations, less Fh would be required. A maximal PR removal of 87% was obtained at a residence time of 88 min.

The ionic composition of the ROC solution was found to inhibit the PR oxidation, requiring higher reactant concentrations to achieve the same removal efficiencies as in DI water solution. PR oxidation in a single salt solution of the ROC composition revealed a negligible effect of the cations, while, the anions hindered PR removal by scavenging hydroxyl radicals and forming iron complexations. Under steady-state conditions, excluding bicarbonate, sulfate, or chloride from the ROC solution did not significantly affect PR oxidation. However, in the absence of sulfate and chloride, the number of RTs required to reach steady state increased, suggesting that outer-sphere complexation is the predominant mechanism of the anion adsorption onto the Fh in the ROC solution.

The authors would like to express their gratitude for the funding provided by the German Ministry of Education and Research (BMBF) and Israeli Ministry of Science & Technology (MOST) through the German-Israeli water technology cooperation (project WT1902).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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