Ball milled Mg/Al hydroxides modified nitrogen-rich biochar for arsenic removal: performance and governing mechanism

Abstract

Layered double hydroxides (LDHs) are widely used as effective adsorbents for wastewater treatment due to their simple synthesis, controllable structure, strong stability, large surface area, and large interfacial spacing. In this study, modified-biochar (BMBC) and Mg/Al modified-biochar composite (Mg/Al-BC) were directly prepared using ball milling technology to effectively adsorb As(V), and nitrogen-rich biochar was obtained through pyrolysis using shrimp shells as precursors. Compared to pristine biochar, the oxygen-containing functional groups of Mg/Al-BC increased by 71.9%, and the particle average diameter decreased from 14.26 nm to 12.56 nm. The kinetics and isothermal models of arsenic adsorption were examined in batch experiments to investigate the impacts of pH, temperature, and co-existing anions. The adsorption capacities for As(V) followed the order: Mg/Al-BC > BMBC > BC, with their respective maximum adsorption capacities measured at 22.65, 6.73, and 0.48 mg/g. The arsenic adsorbed onto Mg/Al-BC was dependent on pH and coexisting anions. Precipitation, ion exchange, surface complexation, and electrostatic interaction were the possible governing adsorption mechanisms. Protonation of pyridinic-N/quinone groups in biochar contributed to the electrostatic attraction between arsenic anion and quaternary ammonium cation. Stable reusability indicates that the ball milled Mg/Al-BC composite could be a promising adsorbent for arsenate removal from polluted water. Graphical Abstract: [Figure not available: see fulltext.]