Abstract
With increasing awareness of reduction of energy and CO2 footprint, more waste is considered recyclable for generating value-added products. Here we reported the negatively-valued iron mud, a waste from groundwater treatment plant, was successfully converted into magnetic adsorbent. Comparing with the conventional calcination method under the high temperature and pressure, the synthesis of the magnetic particles (MPs) by Fe2+/Fe3+ coprecipitation was conducted at environment-friendly condition using ascorbic acid (H2A) as reduction reagent and nitric acid (or acid wastewater) as leaching solution. The MPs with major component of Fe3O4 were synthesized at the molar ratio (called ratio subsequently) of H2A to Fe3+ of iron mud ≥ 0.1; while amorphous ferrihydrite phase was formed at the ratio ≤ 0.05, which were confirmed by vibrating sample magnetometer (VSM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). With the ratio increased, the crystalline size and the crystallization degree of MPs increased, and thus the Brunauer-Emmett-Teller (BET) surface and the cation-exchange capacity (CEC) decreased. MPs-3 prepared with H2A to Fe3+ ratio of 0.1 demonstrated the highest methylene blue (MB) adsorption of 87.3 mg/g and good magnetic response. The adsorption of MB onto MPs agreed well with the non-linear Langmuir isotherm model and the pseudo-second-order model. Pilot-scale experiment showed that 99% of MB was removed by adding 10 g/L of MPs-3. After five adsorption-desorption cycles, MPs-3 still showed 62% removal efficiency for MB adsorption. When nitric acid was replaced by acid wastewater from a propylene plant, the synthesized MPs-3w showed 3.7 emu/g of saturation magnetization (Ms) and 56.7 mg/g of MB adsorption capacity, 2.8 times of the widely used commercial adsorbent of granular active carbon (GAC). The major mechanism of MPs adsorption for MB was electrostatic attraction and cation exchange. This study synthesized a magnetic adsorbent from the negatively-valued iron mud waste by using an environment-friendly coprecipitation method, which had a potential for treatment of dye wastewater.
Highlights
With the trend of urbanization, more and more waste is generated in big cities and must be treated in a renewable and sustainable way
Fe3+ was partially reduced by ascorbic acid (H2A) after iron mud was dissolved with nitric acid and the magnetic adsorbent was synthesized via Fe2+/Fe3+ coprecipitation at room temperature
After separation from the solid fraction, Si was totally removed from, while Fe in the liquid fraction was re-precipitated to Fe3O4 under alkaline conditions and its content gradually increased when more H2A was added from magnetic particles (MPs)-1 to MPs-5 (Fig 1), indicating the crystallization of Fe3O4 increased with more available Fe2+
Summary
With the trend of urbanization, more and more waste is generated in big cities and must be treated in a renewable and sustainable way. Despite the Fe3O4 particles were successfully synthesized in both studies, the production cost may be significantly increased by adding pure ferrous ions in coprecipitation. H2A was successfully used by Gupta et al in Fe3+/Fe2+ coprecipitation [17] It was used by Nene et al [18] to reduce Fe(ACAC) in diphenyl-ether solution but the reactions had to be conducted by refluxing the reactants at a high temperature of 190 ̊C for 1 hour. Fe3+ was partially reduced by H2A after iron mud was dissolved with nitric acid and the magnetic adsorbent was synthesized via Fe2+/Fe3+ coprecipitation at room temperature. It was for the first time to report to synthesize Fe3O4 particles at the environment-friendly room temperature by H2A reduction of Fe3+ ions in low solid content of iron mud for Fe2+/Fe3+ coprecipitation
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