A magnetic γ-Fe2O3@PANI@TiO2 core–shell nanocomposite for arsenic removal via a coupled visible-light-induced photocatalytic oxidation–adsorption process

Arsenic polluted groundwater impairs human health and poses severe threats to drinking water supplies and ecosystems. Hence, an efficient method of simultaneous oxidation of As(iii) to As(v), and removal of As(v) from water has triggered increasing attention. In this study, a magnetic γ-Fe2O3 core–shell heterojunction nanocomposite was synthesized by means of hydrothermal crystallization of TiO2 on the surface of the magnetic core–shell loaded with polyaniline (γ-Fe2O3@PANI@TiO2). As an efficient photocatalyst coupled with adsorption, γ-Fe2O3@PANI@TiO2 has a high light utilization and good adsorption capacity. Notably, the nanocomposite has excellent stability at various initial pH values with good reusability. Among the co-existing ions investigated, PO43− has the greatest competitive reaction. The photocatalytic oxidation of As(iii) on γ-Fe2O3@PANI@TiO2 is dominated by the synergy of several active substances, with superoxide free radicals and photogenerated holes being the major players.


S1.1 Characterization of the as-synthesized nanocomposites
The Fourier transformed infrared spectroscopy (FTIR Perkin-Elmer L1600300) was carried out to analyze functional groups related information. Raman spectroscopic measurements were performed on a DXR Raman Microscope System (ThermoFisher, USA) with an excitation wavelength of 780 nm. The XRD patterns were obtained on a DX-2007 X X-ray diffractometer (Cu Kα radiation, λ =0.15418 nm, 40 kV, 40 mA). The SEM images of all samples was measured on a JSM-7610F emission scanning electron microscopy (JEOL, Japan). The TEM images were obtained on a Tecnai G2 transmission electron microscopy (JEOL, Japan) at an accelerating voltage of 200 kV. N 2 adsorption-desorption isotherms were recorded by micrometrics instrument (ASAP 2020). X-ray photoelectron spectra (XPS) of the samples were determined by an AXIS ULTRA HAS spectrometer (Kratos Co, UK).
Magnetic properties were measured on a vibrating sample magnetometer (East Changing ET9007) with 1 T applied field at an ambient temperature.
Thermogravimetric analyses (TGA) was analyzied by a NETZSCHSTA449F3 thermogravimetric Analyzer (NETZSCH, Germany) from zero to 850 °C under nitrogen atmosphere at heating rate of 5 °C·min -1 . Photoluminescence spectroscopy (FLS920, EDINBURGH INSTRUMENTS) was conducted to analyze the combination of photogenerated holes and electrons with a Xe900 lamp excitation. The optical properties of γ-Fe 2 O 3 @PANI@TiO 2 nanocomposites were seen in UV-vis DRS analysis (Lambda 750S). The Brunauer-Emmett-Teller (BET) surface area was measured by nitrogen adsorption/desorption isotherms at 77 K using an AutosorbiQ analyzer (Quantachrome, USA). The electron spin resonance (ESR) was conducted with a 300-10/12 electron spin resonance spectrometer (Bruker, Germany) to test the photogenerated holes and superoxide free radical of the composite materials. Solid powder was directly used in the photogenerated hole experiment. The response signal was the difference between light and dark conditions. The g factor was corrected by standard sample diphenylpyrazine hydrazine (DPPH). The samples were dispersed in 10 mg/L dimethylpyridine n-oxide (DMPO) methanol solution during the test of superoxide free radicals, and the test was conducted after five minutes of visible light irradiation (Xe lamp). The measured magnetic field intensity was 3,400−3,700 G; the microwave frequency was 9.1 GHz, and the microwave intensity was 10 mW.

S1.2 Arsenic removal by photocatalytic oxidation and adsorption experiments
Batch experiments were conducted to examine the performance of magnetic γ-Fe 2 O 3 @ PANI@TiO 2 heterojunction composites for As(III) photocatalytic oxidation/ adsorption (a coupling process) under the conditions of visible light irradiation, variable initial pH, co-existing ions, etc. The effect of reaction kinetics and associated mechanism were evaluated in different tests (i.e., dark absorption, light reaction, catalyst concentration, initial concentration). Other than specifically identified, all the following tests were conducted in 50-mL test tubes with triplicates (n = 3) in open atmosphere, and the average of the results are reported here.

S1.2.1 Effect of solution pH
The pH value of the solution was adjusted with HCl and NaOH of 0.1 M and 0.5 M, respectively, within a pH range of 2.0−10.0. An aliquot 0.5 g/L of γ-Fe 2 O 3 @PANI@TiO 2 products were used for the reaction. The initial concentration of As(III) was 20 mg/L.

S1.2.2 Comparison experiments
The pH value of the solution was 5.0. The catalyst dosage was 0.5 g/L, and the initial concentration of As(III) was 20 mg/L.

S1.2.4 Effect of initial As(III) concentration
The initial As(III) concentrations were at 0.067-0.801 mmol/L with initial pH of 5.0 and the catalyst dosage of 0.5 g/L.

S1.2.5 Kinetic experiments
In dark and light irradiation experiments, in the dark adsorption part and the photocatalytic reaction after one, two, three, four, and five hours of irradiation, a sample aliquot was transferred to a colorimetric tube, and making sure the dark sample was not exposed to any light. The pH value of the solution was 5.0, the catalyst dosage was 0.5 g/L, and the initial concentration of As(III) was 10 mg/L. In the experiment of catalyst concentration influence, catalyst amounts of 0.2, 0.6, 1.0, and 1.4 g/L were used. The initial As(III) concentration was set to 5, 10, 15, and 20 mg/L and other reaction conditions were the same.

S1.2.6 Catalyst stability/performance test
Catalyst stability/performance in repeated experiments was also investigated.
The initial concentration of As(III) of 20 mg/L and a dose of γ-Fe 2 O 3 @PANI@TiO 2 of 0.5 g/L were used for 5-h oxidation/adsorption tests; then, the test solution was suspended in 100 ml of 0.5 M NaOH solution, under the conditions of stirring at 100 RPM (to induce desorption) for five hours. After the desorption, the magnets were used for the catalyst separation from water, and the recovered γ-Fe 2 O 3 @PANI@TiO 2 nanocomposites were washed with a large amount of deionized water, air-dried and then utilized for the next experimental cycle. The above cycle was repeated five times.

S1.2.7 Experiments with a free radical trapping reagent
Free radical trapping agent experiments can determine the role of different typical reactive species in the As(III) photocatalytic oxidation. In this study, four reagents were employed: silver nitrate (6 mmol/L), ammonium oxalate (AO), benzoquinone (BQ) and isopropyl alcohol (IPA) (all at 1 mmol/L) for electrons, photogenerated holes, superoxideradicals and hydroxylradicals, respectively. The pH of the solution was 5.0, the catalyst dosage was 0.5 g/L, and the initial As(III) concentration was 10 mg/L. Controls included test systems with all above ingredients but without 1) adding the trapping reagent (blank), 2) adding the catalyst (no photocatalyst).