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This study presents a critical review on the application of magnetite-based catalysts to industrial wastewater decontamination by heterogeneous Fenton oxidation. The use of magnetic materials in this field started only around 2008 and continues growing increasingly year by year. The potential of these materials derives from their higher ability for degradation of recalcitrant pollutants compared to the conventional iron-supported catalysts due to the presence of both Fe(II) and Fe(III) species. In addition, their magnetic properties allow their easy, fast and inexpensive separation from the reaction medium. The magnetic materials applied up to now can be classified in three general groups: magnetic natural minerals, in-situ -produced magnetic materials and ferromagnetic nanoparticles. A survey of the catalysts investigated so far is presented paying attention to their nature and competitive features in terms of activity and durability. [ABSTRACT FROM AUTHOR]

Highlights: [•] CWPO of chlorophenols with Fe x O y /γ-Al2O3 catalysts is more efficient than Fenton oxidation. [•] Increasing the temperature improves both the oxidation rate and mineralization degree. [•] The Fe x O y /γ-Al2O3 catalysts are highly active and stable upon CWPO of chlorophenols. [•] CWPO (Fe3O4/γ-Al2O3) at high temperature is the preferable strategy for chlorophenols oxidation. [•] Fe3O4/γ-Al2O3 catalyst can be easily recovered from the liquid phase by a magnet. [Copyright &y& Elsevier]

Abstract: Degradation of chlorophenols by a sequential combination of hydrodechlorination (HDC) and catalytic wet peroxide oxidation (CWPO) using a new magnetic Pd–Fe/γ-Al2O3 catalyst has been studied. This catalyst is active in both hydrodechlorination of chlorophenols and decomposition of H2O2 for the oxidation of organic compounds. The sequential combination of HDC and CWPO allows overcoming some of the drawbacks of both treatments applied independently. The HDC step achieves the complete dechlorination of chlorophenols, so that the subsequent CWPO does not lead to the formation of highly toxic chlorinated by-products and reduces significantly the organic load of the effluent. The results showed that the presence of iron in the Pd catalyst improved significantly its hydrodechlorination rate, achieving the complete dechlorination of chlorophenols in a short reaction time (≈15 min), giving rise to phenol and cyclohexanone. The CWPO of synthetic mixtures of phenol and cyclohexanone showed that a high phenol concentration promotes the oxidation of all the organic species, but the presence of cyclohexanone seems to hinder the formation of aromatic radicals limiting the effectiveness of the CWPO step. Therefore, the effective combination of HDC and CWPO requires that the HDC step achieves the complete dechlorination of chlorophenols but no further hydrogenation is needed. The Pd–Fe/γ-Al2O3 catalyst showed a high activity in both HDC and subsequent CWPO of chlorophenols being easily separated and recovered from the reaction medium due to its ferromagnetic properties. In spite of a moderate loss of activity, the complete dechlorination of chlorophenol and a negligible ecotoxicity of the final effluents were maintained upon successive applications of HDC + CWPO in a four-cycles test. [Copyright &y& Elsevier]

CWPO has proved to be effective for the treatment of representative pharmaceuticals (sulfamethoxazole, atenolol, metronidazole, diltiazem, trimethoprim and ranitidine) in different water matrices (ultrapure water, surface water, WWTP effluent and hospital wastewater). Complete removal of the pollutants and the aromatic intermediates was achieved using the stoichiometric dose of H 2 O 2 , a catalyst (Fe 3 O 4 /γ-Al 2 O 3 ) load of 2 g L −1 , pH 3 and temperature of 50–75 °C. Accordingly, the ecotoxicity was reduced to negligible values. The degradation was faster when the pharmaceuticals were together, being the reaction time for the elimination of the most refractory species (metronidazole) shortened from 4 h to 1 h. The mineralization of the drugs was fairly different, being the most reactive species those containing several aromatic rings ( X TOC ∼ 80%) and the most refractory that bearing an imidazolium ring ( X TOC ∼ 35%). The water matrix affected the kinetics of the process but in all cases complete conversion of the drugs was reached within 1 h. The presence of dissolved organic matter (surface water) seemed to promote drugs degradation while the occurrence of inorganic ions (real WTTP and hospital effluents) partially inhibited it due to scavenging effects. Remarkably, the process was successfully operated at the typical concentrations of main micropollutant sources (μg L −1 ). [ABSTRACT FROM AUTHOR]

This work explores the potential application of naturally-occurring minerals as inexpensive catalysts in heterogeneous Fenton, namely catalytic wet peroxide oxidation (CWPO). The availability, low cost and environmentally friendly character of those materials make them interesting candidates for such application. The performance of magnetite, hematite and ilmenite as CWPO catalysts has been tested under different working conditions, which include temperature (25–90 °C), H 2 O 2 dose (250–1000 mg L −1 ) and catalyst concentration (1–4 g L −1 ). The operating temperature plays a key role on the rate of H 2 O 2 decomposition so that with magnetite H 2 O 2 conversion after 4 h increased from 8 to 99% by increasing the temperature from 25 to 90 °C. Based on the reaction mechanism proposed, a kinetic model was developed which successfully described the experimental results on H 2 O 2 decomposition. The catalytic performance of the minerals tested at temperatures above the ambient was demonstrated using phenol (100 mg L −1 ) as target pollutant. Unprecedented efficiencies of H 2 O 2 consumption, higher than 80% were achieved, allowing high oxidation and mineralization, i.e. complete phenol conversion and almost 80% TOC reduction at 75 °C with a catalyst loading of 2 g L −1 and the theoretical stoichiometric amount of H 2 O 2 for complete mineralization of phenol (500 mg L −1 ). Magnetite is particularly attractive, since it showed the highest activity and can be easily separated from the liquid phase given its magnetic properties. All the minerals tested suffered low iron leaching and magnetite and hematite showed a good reusability upon three consecutive runs. However, in this case long-term durability is not a crucial issue, given the availability and low cost of these minerals. [ABSTRACT FROM AUTHOR]

• CWPO of the neonicotinoid pesticides listed in the EU Watch List was investigated. • Fast elimination (20 min) of all pesticides and aromatic by-products was achieved. • Mineralization yields above 50% were reached, being the final products short-chain acids. • The application of CWPO warranted the complete removal of ecotoxicity. • The versatility of the process was demonstrated in different water matrices. Neonicotinoid pesticides family is nowadays identified as the most important type of insecticides in the world. Their consequent widespread occurrence in the environment represents not only a well-known risk for bees but also a significant negative impact in aquatic ecosystems. In this work, the capability of catalytic wet peroxide oxidation (CWPO) (Fe 3 O 4 -R400/H 2 O 2) as a low-cost and environmentally-friendly system for the treatment of the neonicotinoid pesticides listed in the EU Watch List (Decision 2018/840) (acetamiprid (ACT), clothianidin (CLT), imidacloprid (IMD), thiacloprid (THC) and thiamethoxam (THM)) has been investigated. Remarkably, complete elimination of the pollutants (1000 μg L−1) and the aromatic by-products was reached in 20 min reaction time operating at 25 °C, 1 atm, and pH 0 = 5, with the stoichiometric H 2 O 2 amount (~4–5 mg L−1) and 1 g L−1 catalyst load. The reactivity order of the insecticides decreased as follows: THC > IMD > THM > CLT > ACT, being the pseudo-first order rate constant values within the range of 0.26–0.61 min−1. Notably, high mineralization yields were obtained (>50%), being the final effluents non-toxic. As example, the oxidation pathway of ACT was proposed. Finally, the catalytic system was tested in real surface water. [ABSTRACT FROM AUTHOR]

• The removal of all the pharmaceuticals included in the EU Watch List was investigated. • The inexpensive magnetite catalyst showed a high activity and a reasonable stability in CWPO. • Total conversion of all drugs was reached under ambient temperature and circumneutral pH. • The pollutants were highly mineralized (>50%), being the final products short-chain acids. • The process was successfully tested in three real WWTP effluents. The European Union (EU) approved in 2015 a Watch List of pollutants of emerging concern especially harmful for the aquatic environment (Decision 2015/495/EU). In this work, we explore the application of an inexpensive and environmentally-friendly catalytic system based on modified natural magnetite (Fe 3 O 4 -R400) and H 2 O 2 for the removal of all the pharmaceuticals included in the EU Watch List (hormones: estrone, 17-beta-estradiol and 17-alpha-ethinylestradiol; macrolide antibiotics: azithromycin, clarithromycin and erythromycin; and the non-steroid anti-inflammatory diclofenac). The experiments were carried out with a representative initial concentration of the drugs (1000 μg L−1) at a circumneutral pH value (pH 0 = 5) within the temperature range of 17–50 °C. A low catalyst concentration (0.2 g L−1) was used while the applied amount of H 2 O 2 corresponded to the stoichiometric one for the complete mineralization of the drugs (4–6 mg L−1). Operating under ambient-like conditions, total conversion of the target pollutants as well as the aromatic intermediates was achieved. Notably, total organic carbon was strongly reduced (>50%), being the residual carbonaceous products short-chain organic acids. The applicability of the catalytic system was demonstrated using different real wastewater treatment plant (WWTP) effluents spiked with the mixture of pharmaceuticals. Apart from the complete removal of pharmaceuticals in relatively short reaction times (60–90 min), the disappearance of total coliforms was warranted. Remarkably, the catalyst showed a reasonable stability, with low iron leaching (<0.7 mg L−1) and maintaining its magnetic properties almost unchanged. [ABSTRACT FROM AUTHOR]

• Carbon-encapsulated iron nanoparticles (CE-nFe) prepared by hydrothermal carbonization. • CE-nFe showed a core-shell structure composed by zero-valent iron and graphitized carbon. • The adsorbent led to a fast removal of DCF, MNZ and SMX (less than 30 min). • The saturated adsorbents were effectively regenerated by heterogeneous Fenton oxidation. • The micropollutant's nature affected the regeneration yield in the order: SMX > DCF > MNZ. Adsorption represents the most plausible technology for micropollutants removal from water nowadays. Nevertheless, the regeneration of the saturated carbon materials is still an important challenge, being these solids in practice commonly disposed. This work aims at overcoming this issue by using innovative carbon-encapsulated iron nanoparticles (CE-nFe). This material was synthesized by a low-cost and green method viz. hydrothermal carbonization (HTC), using olive mill wastewater as carbonaceous source. The solid was fully characterized by different techniques (magnetic properties, elemental analyses, N 2 -sorption isotherms, pH PZC , ICP, XRD and TEM). It showed a clear core-shell structure of around 40 nm in diameter. The core was mainly formed by zero-valent iron and the shell by graphitized carbon. Accordingly, it showed an essentially mesoporous structure, with a specific surface area of 169 m2 g−1, and a clear hydrophobic character (pH PZC = 10). Its adsorption performance was investigated using three relevant micropollutants (diclofenac (DCF), sulfamethoxazole (SMX) and metronidazole (MNZ)). A very fast removal of the micropollutants was achieved (30 min at the most, with rate constants in the range of 0.11–0.41 g mg−1 min−1). The adsorption isotherms revealed the vertical packing of the adsorbate molecules onto the adsorbent active centers, being the data successfully described by the GAB model. The saturated adsorbents were effectively regenerated by heterogeneous Fenton oxidation, taking advantage of the iron core of CE-nFe and the opened mesoporous carbon shell. The regeneration efficiency increased with increasing the operating temperature (25–75 °C) and contact time (1–4 h), as well as the H 2 O 2 dose up to 6 g L-1. The micropollutant nature affected the adsorbent regeneration yield in the order: SMX > DCF > MNZ, consistent with their reactivity towards Fenton oxidation. [ABSTRACT FROM AUTHOR]

The occurrence of harmful cyanobacterial blooms has unabated increased over the last few decades, posing a significant risk for public health. In this work, we investigate the feasibility of catalytic wet peroxide oxidation (CWPO) promoted by modified natural magnetite (Fe 3 O 4 -R400/H 2 O 2), as an inexpensive, simple-operation and environmentally-friendly process for the removal of the cyanotoxins that show the major impact on drinking water: microcystins (MC-LR and MC-RR), cylindrospermopsin (CYN), anatoxin- a (ATX) and saxitoxin (STX). The performance of the system was evaluated under ambient conditions and circumneutral pH (pH 0 = 5) using relevant cyanotoxin concentrations (100–500 μg L−1). The nature of the cyanotoxins determined their reactivity towards CWPO, which decreased in the following order: MC-RR > CYN > MC-LR ≫ ATX > STX. In this sense, microcystins and CYN were completely removed in short reaction times (1–1.5 h) with a low catalyst concentration (0.2 g L−1) and the stoichiometric amount of H 2 O 2 (2–2.6 mg L−1), while only 60–80% conversion was achieved with ATX and STX in 5 h. In these cases, an intensification of the operating conditions (1 g L−1 catalyst and up to 30 mg H 2 O 2 L−1) was required to remove both toxins in 1 h. The impact of the main components of freshwaters i.e. natural organic matter (NOM) and several inorganic ions (HCO 3 −, HPO 4 2-, SO 4 2-) on the performance of the process was also investigated. Although the former led to a partial inhibition of the reaction due to HO· scavenging and catalyst coating, the latter did not show any remarkably effect, and the versatility of the process was finally confirmed in a real surface water. To further demonstrate the effectiveness of the catalytic system, the toxicity of both the initial cyanotoxins and the resulting CWPO effluents was measured with the brine shrimp Artemia salina. Remarkably, all CWPO effluents were non-toxic at the end of the treatment. Image 1 • The catalytic system Fe 3 O 4 -R400/H 2 O 2 is effective for the removal of cyanotoxins. • The reactivity of cyanotoxins decreased in the order: MC-RR > CYN > MC-LR ≫ ATX > STX. • The versatility of the process was demonstrated in different water matrices. • Oxidation effluents were non-toxic to the brine shrimp Artemia salina. [ABSTRACT FROM AUTHOR]