Synthesis of MgFe layered double hydroxide/biochar composite with satisfactory NH4+ adsorption via HPO42− intercalation

An effective adsorbent for ammonium (NH4+) is necessary to remove this pollutant from wastewater and prevent eutrophication and algal blooms. Layered double hydroxides (LDHs) are effective adsorbents that are often combined with biochar (BC) for improved dispersion and stability. However, further modification is needed to improve their NH4+ selectivity and adsorption capacity. Herein, an MgFe LDH/BC composite was intercalated with hydrogen phosphate using coprecipitation followed by copyrolysis to increase the active sites numbers and thus NH4+ adsorption was increased by chemical precipitation. The effects of the associated Mg/Fe molar ratio and copyrolysis temperature on the NH4+ adsorption performance was experimentally investigated. The best NH4+ adsorption capacity was achieved at a Mg:Fe molar ratio of 3:1 and copyrolysis temperature of 400°C. The behavior of MgFe LDH/BC composite was in accordance with the Langmuir isotherm and pseudo-second-order kinetic models, implying that an occurrence of chemisorption in NH4+ adsorption primarily. Notably, the theoretical maximum adsorption capacity reached 55.94 mg g−1. Further, spectral and microstructural analyses indicated that the composite adsorbed NH4+ through complexation, electrostatic attraction, and hydrogen bonding. Overall, this study is expected to provide a reference for preparing selective NH4+ adsorbents with a high adsorption capacity.