Ferrous chloride

Methanol (purity ≥ 99.9%), ethanol (purity ≥ 99.8%), tin (II) chloride dehydrate, and sodium hydroxide (purity ≥ 97%) were purchased from Fisher Chemical (Pittsburgh, PA, USA). Sodium carbonate (purity ≥ 99%), ferric chloride (purity ≥ 99%), ferrous chloride (purity ≥ 99%) and crystal violet (CV) were obtained from Acros Organics (Geel, Belgium). Reactive blue 4 (RB4) was supplied by Sigma-Aldrich (Natick, MA, USA). All chemicals were analytical grade and used as received. Rice bran was harvested from a local Taichung grinding mill in Taiwan. After cleaning, the rice bran was dried for 6 h in an oven at 110 °C. The dried rice bran was ground in a mill and sieved (20 and 100 mesh) and fractions of 0.15–0.85 mm were selected for the adsorbent preparation.

Chemicals including ferrous chloride (FeCl₂·4H₂O, 99%), ferric chloride hexahydrate (FeCl₃·6H₂O, 99%), zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O, 98%), and sodium carbonate (Na₂CO₃, 99.95%) were purchased from ACROS Organics™ (USA). HCl (36.5–38.0%), ammonium hydroxide solution (NH₄OH, 28–30%), diethylene glycol (C₄H₁₀O₃, 99%), ethylene glycol – EG (C₂H₆O₂, 99%), citric acid – CA (C₆H₈O₇, 99%) were supplied by Sigma Aldrich.

2D iron oxides were synthesized on mica substrates via hydrate-assisted CVD method. The reaction was conducted in a one-inch quartz tube heated by a three-zone furnace (Lindberg/Blue M). ~800 mg CaSO₄·2H₂O powder (99.5%, Alfa Aesar) was put at the upstream as the water source, the temperature of which is set at ~130 °C. ~15 mg Ferrous chloride (99.5%, Alfa Aesar) was placed in an alumina boat downstream. Freshly cleaved fluorophlogopite mica (Taiyuan Fluorphlogopite Mica Company Ltd, 10 × 10 × 0.2 mm) is used as the substrate and put 1–3 cm away from Ferrous chloride. Prior to the growth, the furnace was purged by 400 sccm high-purity Ar gas for 15 min to remove the residual air. Then, 100 sccm Ar were inlet as the carrier gas during the whole growth process. O₂ was introduced into the CVD system at the beginning of heating for different times to synthesize different phases of iron oxides. The growth zone of Ferrous chloride was heated to 550–700 °C for 20 min, and kept for another 10 min for growth. As for other oxides, corresponding metal chlorides are employed, and the growth conditions are similar with iron oxides. The detailed synthesis recipes of transition-metal-based oxides are provided in Supplementary Table 9.

Iron salts—ferric chloride and ferrous chloride—were procured from Alfa Aesar. Ammonia and absolute ethanol were purchased from Rankem and Merck, respectively. MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) was procured from Sigma Aldrich. Tannic acid (TA) was purchased from Thomas Baker. All the reagents were used as received without any further purification or modification. Milli Q water was used to prepared aqueous solutions. Nitrogen gas was obtained from Sigma Gases. All procedures were performed at room temperature unless otherwise stated.