Application of δ‐MnO2 and biochar materials in an arsenic‐contaminated groundwater

Two activated biochar materials, peanut char (δ‐MnO2/A‐PC) and corn char (δ‐MnO2/A‐CC), were used to treat an arsenic solution containing 97.5% As(III) and 2.5% As(V). After reacting with δ‐MnO2/A‐PC for 24 h, 18.8% of As(III) and 35.4% of As(V) remained in the solution, revealing that some As(III) was oxidized to As(V) and the other was removed by adsorption. However, δ‐MnO2/A‐CC caused the solution to retain 15.6% of As(III) and 41.7% of As(V) under the same conditions, indicating that δ‐MnO2/A‐CC had higher oxidation for arsenic species than δ‐MnO2/A‐CC. Adsorption capacities for δ‐MnO2/A‐PC and δ‐MnO2/A‐CC to arsenic were 1.50 and 1.53 mg/g in a solution with 0.5 ppm As(III), respectively. After coating with δ‐MnO2, the proportion of mesopore surface areas of δ‐MnO2/A‐CC increased from 33.3% to 79.0%, but their mesopore volumes increased from 67.6% to 89.4%. Fourier‐transform infrared spectroscopy and X‐ray diffraction analyses demonstrated that δ‐MnO2 was coated onto the surfaces of the biochars. The 600°C‐ACC had a higher specific surface area, 221 m2/g, than the δ‐600°C‐APC, 81.5 m2/g; δ‐MnO2/A‐CC could attach more Mn (38.2%) than δ‐MnO2/A‐PC (27.8%). The elemental analysis revealed that δ‐MnO2/A‐PC and δ‐MnO2/A‐CC had similar carbon contents of 26.2%. Practitioner Points The δ‐MnO2/biochar adsorbent can oxidize As(III) into As(V) in the groundwater. δ‐MnO2/biochar adsorbed large amounts of As(III) and As(V). Adsorbent that contains more δ‐MnO2 has a higher oxidation capacity. The δ‐MnO2/biochar made from corn stalks could combine with more δ‐MnO2. A novel adsorbent made from wastes peanut vine or corn stalk is able to remove the toxicity of arsenic from a contaminated groundwater.