Electric Field‐Assisted Uptake of Hexavalent Chromium Ions with In Situ Regeneration of Carbon Monolith Adsorbents

Abstract The uptake of hexavalent chromium (Cr(VI)) ions from wastewater is of great significance for environmental remediation and resource utilization. In this study, a self‐designed instrument equipped with an oxidized mesoporous carbon monolith (o‐MCM) as an electro‐adsorbent is developed. o‐MCM with a super hydrophilic surface displayed a high specific surface area (up to 686.5 m2 g−1). With the assistance of an electric field (0.5 V), the removal capacity of Cr(VI) ions is as high as 126.6 mg g−1, much higher than that without an electric field (49.5 mg g−1). During this process, no reduction reaction of Cr(VI) to Cr(III) ions is observed. After adsorption, the reverse electrode with 10 V is used to efficiently desorb the ions on the carbon surface. Meanwhile, the in situ regeneration of carbon adsorbents can be obtained even after ten recycles. On this basis, the enrichment of Cr(VI) ions in a special solution is achieved with the assistance of an electric field. This work lays a foundation for the uptake of heavy metal ions from wastewater with the assistance of the electric field.


Materials and chemical reagents
Resorcinol, formaldehyde, anhydrous sodium sulfate, and anhydrous ethanol were purchased from Tianjin Damao Chemical Reagent Factory. Block polyether F-127 was purchased from Shanghai Yuanye Biotechnology Co., Ltd. Potassium dichromate (K 2 Cr 2 O 7 ) was purchased from Sahn Chemical Technology (Shanghai) Co., Ltd. All reagents were directly used without a further treatment.

Synthesis of o-MCM
Using resorcinol and formaldehyde as carbon material precursors, a three-dimensional carbon monolith was synthesized by a self-assembly high-temperature carbonization method. [1] Typically, 45 g of resorcinol and 18.75 g of block polyether F127 were dissolved in a solvent mixture of 152 mL of absolute ethanol and 120 mL of deionized water under magnetic stirring at 25 °C, followed by the addition of 1.56 g of 1,6 hexanediol. The amine was added to the above solution and stirred for about 10 min, then 61.5 mL of formaldehyde solution was quickly added to the mixed solution and stirred for 20 min. The obtained white homogeneous emulsion was then transferred to an oven at 50 °C for 24 h, and then continued to be cured at 70 °C for 24 h and 100 °C for 12 h to obtain the solid reactant phenolic resin.
The monolithic mesoporous carbon material (MCM) was obtained by pyrolyzing the phenolic resin at 700, 800, 900 and 1000 °C for 2 h at a heating rate of 2 °C/min in an Ar atmosphere, respectively. Finally, the MCM material was immersed in 100 mL of 3%wt H 2 O 2 solution for a certain time by ultrasonic immersion to improve its hydrophilicity. The obtained MCM materials could be o-MCMT@X, T and X are noted as the pyrolysis temperature and S3 ultrasonic time, respectively.

Characterization
The surface morphology and microstructure of the prepared samples were characterized by field emission scanning electron microscope (SEM, Zeiss Merlin) at 5 kV. Transmission electron microscopy (TEM) was taken in a JEOL JEM-2100F microscope operating at 200 kV.
The specific surface area (BET) of the samples was measured on an ASAP 2010 analyzer by N 2 adsorption. Raman spectra were obtained from a LabRAM Arram Micro Raman spectrometer with an excitation wavelength of 633 nm. X-ray photoelectron spectroscopy (XPS) was performed on a quito-axis ultrasonography (DLD) spectrometer equipped with an aluminum X-ray source, and the binding energy was referenced to the C1s peak at 284.6 eV.
Fourier transform infrared spectroscopy (FTIR) was used to analyze the surface functional groups at wavelengths of 1500-4000 cm -1 by Nicolet IS50 infrared microscope.
Thermogravimetric analysis (TG) was performed by heating the sample to 1000°C at a heating rate of 10°C/min in an air atmosphere by STA449C. The resistance of the material is measured by VICTOR VC890C+ multimeter.

Electrochemical testing
Cyclic voltammetry (CV) and electrochemical impedance (EIS) measurements were performed using an electrochemical analyzer (CHI660E, CH Instruments, USA). In the CV test, platinum and silver/silver chloride electrodes were used as counter and reference electrodes, respectively. Conductive carbon paper was used as the working electrode by coating MCM, naphthol solution and ethanol solution. All CV measurements were performed with 0.1 M KHCO 3 solution. The electrolyte was degassed with purified argon before CV S4 measurements, and argon was passed through the solution during all measurements, The scan rate was set to 5 mV/s. In the EIS test and IR compensation experiment, platinum and silver/silver chloride electrodes were used as counter and reference electrodes, respectively.
The MCM was ground and used as the working electrode. All measurements were performed with 0.1 M Na 2 SO 4 solution. The electrolyte was degassed with purified argon gas prior to measurement, and argon gas was continuously bubbled into the solution during the measurement.

Electro-sorption/desorption performance test
Electro-adsorption: In this experiment, potassium dichromate was used as the model pollutant, MCM was used as the adsorbent, and a self-designed equipment was used as the electrosorption device with a two-electrode system (Figure 1). We set the H 2 O 2 -modified MCM adsorbent as the working electrode, and 400 mL potassium dichromate solution was adsorbed by MCM (~0.7 g) through a peristaltic pump under the continuous stirring. In this work, the carbonization temperature (700~1000 ℃), potassium dichromate solution concentration (25~200 mg/L), applied voltage value (0~1.5 V), pH (1~13), electrical conductivity of MCM were studied respectively. Effects of medium concentration (0~20 g/L) and H 2 O 2 modification time on removal of chromium ions were also tested. The concentration changes of hexavalent chromium and total chromium ions were determined by the ultraviolet spectrophotometer and flame atomic absorption photometer (AAS).

Isotherms and kinetic study
In order to describe the electroadsorption kinetics of Cr(VI) on MCM adsorbent, pseudofirst-order and pseudo-second-order kinetic models were used to fit the experimental data. [2] The linear forms of these two models can be expressed as follows, Pseudo-first-order: Pseudo-second-order: where q e and q t are the electrosorption capacities at equilibrium conditions and at time t, respectively, t is the total electrosorption time, and k 1 and k 2 are the rate constants of the pseudo-first-and pseudo-second-order equations, respectively.
The adsorption isotherms of the adsorbents were fitted by Langmuir and Freundlich models with 0 V and 0.5 V applied, respectively [3] .
Langmuir model： Freundlich model: where C e (mg/L) is the initial Cr 6+ concentration, q e (mg/g) is the amount of Cr 6+ adsorbed on the adsorbent under equilibrium conditions, q max (mg/g) is the maximum adsorption amount of the adsorbent, b ( L/mg), k f and n are constants in the models.