Aqueous arsenic(V) remediation and redox transformation of arsenic(III) to arsenic(V) using Fe3O4/Douglas fir biochar

Magnetite nanoparticles were deposited on Douglas fir biochar (Fe3O4/DFBC) using aqueous, NaOH-induced chemical co-precipitation from Fe2+/Fe3+ salt solutions. Fe3O4/DFBC was used to remediate As(V)-contaminated water. Kinetics and isotherms were studied. pH 5 was selected as the optimized pH due to low iron leaching and closeness to groundwater pH. Adsorption equilibrium was reached after 3 h, 2 h, and 1 h for 0.5, 5, and 50 mg/L initial As(V) concentrations, respectively. Adsorption was exothermic, and the Langmuir capacity was 6.33 mg/g at 25 oC. Ionic strength, impacts of Fe3O4/DFBC particle size, and competitive ion/ molecule effects during As(V) adsorption were studied. Continuous-flow fixed-bed column breakthrough studies performed at 0.5, 5, and 50 mg/L of As(V) at pH 5 exhibited maximum capacities of 3.47, 3.99, and 3.72 mg/g, respectively. Aqueous potassium phosphate was used successfully for column regeneration. Fe3O4/DFBC was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Mössbauer spectroscopy before and after As(III) and As(V) adsorption. Mössbauer found the "Fe3O4" was composed of several phases. A key target was the study of simultaneous toxic As(III) adsorption and its transformation to As(V) from pH 1-13. The highest removal of As(III) and oxidized As(V) was obtained at pH 3. The relationship between iron leaching and pH was investigated and the pH-dependent surface adsorption was monitored using X-ray photoelectron spectroscopy (XPS) from pH 1 to 13. Readsorption of leached iron occurred via the formation of insoluble iron-arsenate and arsenites at concentrations exceeding their Ksp values at the pHs. One goal of this study was to enhance the understanding of the adsorption characteristics needed for initial scaling of a treatment facility that can efficiently remediate arsenic-contaminated wastewater. Experiments were conducted using batch and fixed bed continuous flow columns to optimize adsorption process parameters under various circumstances and solution matrices. Another goal was to further establish the surface structures of the chemisorbed arsenates versus pH.