Exothermic Reaction in the Cleaning of Wastewater by a Fe2O3/Coconut Shell Activated Carbon/H2O2 Heterogeneous Fenton-like System

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Exothermic Reaction in the Cleaning of Wastewater by a Fe2O3/Coconut Shell Activated Carbon/H2O2 Heterogeneous Fenton-like System


Human activities and industrial processes produce a variety of wastes or emissions, and energy recovery is an important strategy for minimizing waste and the adverse impact on the environment. Various technologies, such as waste incineration for power generation, biogas fermentation, and flue gas recovery, are commonly used; however, the energy converted during wastewater treatment has rarely been studied and utilized. Wastewater source heat pump technology is a green energy technology that utilizes a small amount of high-grade electrical energy to enhance the low-grade heat energy in wastewater that cannot be directly utilized for enterprise production or winter heating [1,2]. The temperature drop of wastewater after heat extraction is generally controlled within 5 °C due to the wastewater treatment process. Therefore, analyses of the wastewater treatment process focused on wastewater temperature increases to enhance the external heat supply of wastewater source heat pumps and improve energy utilization are of great significance.
o-Phenylenediamine (OPD) is a raw material for the preparation of pesticides, fungicides, medical drugs, and various chemical products, and is easily discharged into the environment with industrial wastewater, agricultural runoff water, and domestic wastewater; it is potentially hazardous to aquatic organisms and human health [3,4]. Advanced oxidation technology is currently used to treat difficult-to-degrade wastewater. This technology mainly involves Fenton- [5], ozone- [6], electrochemical- [7] and microwave-assisted [8], as well as photocatalytic, oxidations [9]. Of these, the Fenton process is the most commonly utilized wastewater oxidation technology owing to its convenient dosing and high oxidizing capacity [10,11]. The Fenton reaction is a Fenton reagent composed of ferrous ions and oxidant H2O2, which produces a short but extremely active substance, namely hydroxyl radical (·OH), which has a significant oxidizing ability to convert pollutants into non-toxic products, including CO2 and H2O. However, a homogeneous Fenton reaction is only feasible when the pH value is lower than 4. When the pH value exceeds 4, iron ions are converted into ferric hydroxide sludge, and some catalysts are lost, thus decreasing the efficiency of the Fenton reaction; furthermore, the treatment and disposal of solid sludge also produces additional costs, which limits its application to a certain extent [12,13,14]. Multiphase Fenton technology is an advanced oxidation technology that replaces the iron ions in traditional Fenton technology with multiphase Fenton catalysts to catalyze the production of free radicals in hydrogen peroxide; it reduces some process limitations, such as sludge production and chemical inputs, thereby reducing costs [12]. For example, Pelalak et al. investigated the reaction mechanism of multimetallic catalysts in the Fenton process, describing effective methods to enhance the activity, electron transfer rate, and generation of hydroxyl radicals [15]. Dong et al. investigated Fe/Al2O3-loaded catalysts for the degradation of wastewater and proposed an efficient and low-cost solution for Fenton treatment with loaded catalysts [16]. Liu et al. investigated the performance of elemental iron-loaded catalysts for the degradation and treatment of sulfamethoxazole wastewater and measured the contributions of adsorption and Fenton oxidation to the degradation rate [17]. Zhang et al. prepared a non-homogeneous Fenton catalyst containing iron resin, which showed high degradation rates for organic wastewater with multiple reuses [18]. The selection of catalyst materials that overcome the disadvantages of existing materials, such as high costs and complicated preparation processes, has become a major area of research.
The application of biomass feedstock as a sustainable resource in wastewater treatment processes has increased worldwide. Coconut shell activated carbon (CSAC) is a biomass activated carbon prepared from coconut shell, an agricultural waste and sustainable resource. CSAC is a commonly used catalyst due to its large surface area and porosity [19]. For example, Li et al. synthesized CuFe2O4@CSAC for the degradation of tetrabromobisphenol A (TBBPA) and demonstrated that the surface active sites are the key factors affecting the degradation performance [20]. Pang et al. prepared coconut shell composites with high photocatalytic properties for methyl orange [21]; the metal oxide particles were uniformly loaded onto the CSAC, which improved the thermal stability and specific surface area of the material. Sun et al. developed a CSAC catalyst for the synthesis of H2/CO, obtaining a conversion rate greater than 90%, and showed that CH4/CO2 content has a great influence on the synthesis of H2/CO [22].
Previous studies have mainly been concerned with the degradation of wastewater using catalysts and conversion of organic matter, and studies on energy utilization in wastewater degradation processes are lacking. In this study, a CSAC-loaded Fe2O3 multiphase Fenton-like catalyst was prepared using 2–4 mm industrial-grade granular CSAC as the carrier, and the degradation performance of the Fe2O3@CSAC/H2O2 multiphase Fenton-like system and exothermic pattern of the degradation of OPD simulated wastewater were investigated. The newly developed catalyst had good degradation performance for OPD wastewater, and the degradation of wastewater increased the temperature of the system so that it contained more recoverable thermal energy, providing a theoretical basis for the energy utilization of wastewater. The study schematic is shown in Figure 1.

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