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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: [•] The two-step reaction (HDC–CWPO) represents an efficient solution for chlorophenols breakdown. [•] A short-time HDC step before CWPO improves the rate of mineralization. [•] Complete dechlorination of chlorophenols and a high mineralization (80%) was reached. [•] The effluents obtained upon HDC–CWPO process presented negligible ecotoxicity values (<0.5TU). [•] Both HDC (Pd/γ-Al2O3) and CWPO (Fe/γ-Al2O3) catalysts showed a highly stable performance. [Copyright &y& Elsevier]

Abstract: A ferromagnetic γ-Al2O3-supported iron catalyst has been prepared and its activity and stability in catalytic wet peroxide oxidation (CWPO) have been compared with those of a previous iron-based conventional catalyst using the same support. Both catalysts were characterized by nitrogen adsorption–desorption isotherms, ICP, TXRF, XRD, XPS, elemental analysis and Mössbauer spectroscopy. The behavior of these catalysts in CWPO of chlorophenols has been related with the nature of the dominant iron species (magnetite or hematite). The results showed that the magnetic catalyst improved significantly the decomposition of H2O2 leading to an increased degradation and mineralization of chlorophenols. Complete conversion of those target pollutants and more than 75% TOC reduction were achieved after 4h under stoichiometric H2O2 dose, 100mgL−1 initial chlorophenol concentration, 1gL−1 catalyst, pH 3 and 50°C temperature. Moreover, complete dechlorination of all the chlorophenols tested was achieved, being the residual organic by-products short-chain acids without significance in terms of ecotoxicity. The catalyst showed a remarkable stability in long-term continuous experiments with limited Fe leaching, below 5% of the initial loading after 100h on stream. An additional clear advantage of the new catalyst is its easy separation and recovery from the reaction medium by applying an external magnetic field. [Copyright &y& Elsevier]

Abstract: Hydrodechlorination of 4-chlorophenol using Pd/A2O3 as catalyst at mild conditions (30°C, 1atm) was carried out in plain water, buffered (CH3COOH/NaCH3COO, NaH2PO4/Na2HPO4 and NaHCO3/Na2CO3) and alkaline NaOH media at different pH. The highest reaction rate was achieved when the process was carried out at alkaline NaOH solution of pH=8.5. Continuous long term experiments (τ: 5.4kgcat h/mol) were performed in plain water and alkaline medium using Pd/Al2O3 and Pd/AC as catalysts. The former showed an appreciable loss of activity upon time on stream, which was associated with the formation of palladium chloride complexes. However, a residual activity remained practically stable, reaching very similar 4-chlorophenol conversion (36%) in both reaction media. The Pd/AC catalyst maintained a constant activity at a 70% 4-chlorophenol conversion once the steady state was reached. This suggests that the AC supported catalyst is less susceptible to the chloride poisoning produced during the reaction. [Copyright &y& Elsevier]

Abstract: The kinetics of gas-phase hydrodechlorination (HDC) of dichloromethane (DCM) with a Pd on activated carbon commercial catalyst has been studied in a fixed bed reactor within the 200–350°C temperature range. Different kinetic equations have been checked for describing the evolution of DCM. Statistical and physical reliable parameters were found for a Langmuir–Hinshelwood type expression with DCM adsorption as the controlling step. The reaction pathway and the kinetic model have been elucidated from the evolution of dichloromethane and the identified reaction products (methane, monochloromethane, ethane and ethylene). Adsorbed CH2Cl* radical, CH2 ** and C2H4 ** carbene species are proposed as the three intermediates evolving to the final reaction products. An apparent activation energy of 23.9±2.3kJmol−1 has been obtained for dichloromethane disappearance. [Copyright &y& Elsevier]

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]

In this work, polymer-based spherical activated carbon (PBSAC) (dp = 500 μm) has been used as support to synthesize several Pd/PBSAC (1% wt.) catalysts. These catalysts have been tested in the hydrodechlorination (HDC) of 4‐chlorophenol (4-CP) in aqueous phase under ambient-like conditions (30 °C, 1 atm) at [4-CP] 0 = 3.9 mmol L −1 ; [catalyst] 0 = 1 g L −1 and 50 N mL H 2 min −1 . A sequential calcination (200 °C, air atmosphere) − reduction (80 °C, H 2 atmosphere) treatment was required to obtain active HDC catalysts with a convenient proportion of zero-valent and electrodefficient Pd species. This catalyst allowed to achieve complete conversion of 4-CP in 90 min reaction time, giving rise to phenol as main reaction product. An expanded kinetic model accounting for both consecutive reaction and sorption processes was used to successfully fit the experimental data. The values obtained for the effective rate constant of 4-CP disappearance (3.26–10.88 × 10 −4 s −1 ) and apparent activation energy (31.6 kJ mol −1 ) were close to the previously reported for powdered supported Pd catalysts. Remarkably, the catalyst showed a high stability in long-term continuous application (100 h on stream). [ABSTRACT FROM AUTHOR]

[Display omitted] • A Pd-based catalytic membrane reactor with controlled feeding of H 2 was developed. • The Pd/CMR was tested in continuous flow HDC under ambient conditions. • The Pd/CMR showed an outstanding activity and stability for up to 200 h. • The activity of the catalytic system was not affected by the initial DCF concentration. • The versatility of the system was successfully demonstrated in a tap water matrix. This work is focused on the development of a Pd-based catalytic membrane reactor (Pd/CMR) for its application in continuous flow hydrodechlorination (HDC) of chlorinated micropollutants. A cylindrical inert porous alumina membrane was decorated on the outer with well-dispersed Pd nanoparticles (0.2 % wt.). The Pd/CMR was fully characterized by different techniques. The catalytic system was applied for the degradation of the persistent anti-inflammatory drug diclofenac (DCF) under ambient conditions. A remarkable stability was observed upon long-term HDC reaction, with a reasonable DCF conversion (60 %). Notably, as the process could be described by pseudo-first order kinetic, the initial concentration did not affect the final conversion achieved. Finally, the versatility of the system was successfully demonstrated using a real aqueous matrix (tap water). All in all, Pd/CMR exhibited a remarkable feasibility for the implementation of the HDC technology on a larger scale, showing an extraordinary catalytic performance for up to 200 h. [ABSTRACT FROM AUTHOR]