Arsenate removal from aqueous solutions by Mg/Fe-LDH-modified biochar derived from apple tree residues

ABSTRACT

The development of non-toxic and inexpensive materials for arsenic removal is required due to water sources being polluted by arsenic in many countries around the world. The main aim of this study was to characterise the capacity and behaviour of Mg/Fe layered double hydroxides/biochar [Magnesium/Iron-Layered Double Hydroxide (Mg/Fe-LDH)] composite for arsenate adsorption from solution. Apple tree pruning residues were used to produce biochar at 500 °C under oxygen-limited atmosphere. Mg/Fe-LDH-biochar was synthesised using a spontaneous in situ co-precipitation method. Batch experiments were used for the assessment of the kinetics, isotherms, and the effects of initial solution pH (4, 6, 8, and 10), ionic strength (0.01, 0.1, and 0.2 mol L⁻¹), and co-occurring anions (carbonate and phosphate) on the arsenate removal. Scanning electron microscope images showed Mg/Fe-LDH were loaded on the biochar porous structure, and X-ray diffraction analysis affirmed the presence of crystalline LDH minerals in Mg/Fe-LDH-biochar. Surface modification of biochar by Mg/Fe-LDH increased the maximum arsenate adsorption capacity (3.6 mg g⁻¹) ten times compared to unmodified biochar (0.35 mg g⁻¹). Arsenate removal capacity increased from 4.2 % to 54.2 % with modification of biochar by Mg/Fe-based LDH. Kinetic studies indicated that >90 % of Mg/Fe-LDH-biochar arsenate adsorption from a starting concentration of 10 mg L⁻¹ occurred in the first 120 min. Pseudo-second order and Langmuir models described well the kinetics and isotherm of arsenate adsorption by biochar and Mg/Fe-LDH-biochar. Mg/Fe-LDH-biochar showed maximum arsenate removal capacity at pH 6. Increasing solution ionic strength and the presence of phosphate and carbonate anions suppressed arsenate removal by Mg/Fe-LDH-biochar. In summary, surface modification of biochar using Mg/Fe-LDH produced a potentially more cost-effective, locally available, reusable, and non-toxic arsenic adsorbent for decontamination of surface- and groundwater.