Simultaneous and Efficient Capture of Inorganic Nitrogen and Heavy Metals by Polyporous Layered Double Hydroxide and Biochar Composite for Agricultural Nonpoint Pollution Control

Agricultural nonpoint pollution has been recognized as the main source of aquatic contaminants worldwide, such as inorganic nitrogen (ION) and heavy metals (HMs). It is an important challenge to simultaneously and efficiently immobilize soil ION and HMs in farmland. Herein, we present a polyporous Mg/Fe-layered double hydroxide and biochar composite (Mg/Fe-LDH@biochar) with the efficient coadsorption capacity of ION and HMs for the mitigation of agricultural nonpoint pollution toward aquatic systems. The Mg/Fe-LDH@biochar showed strong adsorption toward ION (i.e., NH4+-N and NO3--N) and HMs (i.e., Cu, Zn, Ni, Pb, and Cd), with maximum capacity of 98.53 mg of NH4+-N/g, 27.09 mg of NO3--N/g, 295.80 of mg Cu/g, 141.70 mg of Zn/g, 75.59 mg of Ni/g, 1264.10 mg of Pb/g, and 126.30 mg of Cd/g, respectively. More attractively, by deionized water extraction, the adsorbed ION on the composite was more easily rereleased, with a desorption percentage of about 42.33 +/- 6.87% NO3--N and 1.42 +/- 0.78% NH4+-N, than that of HMs (<1.0%). This difference is primarily related with the strength of bonding forces of ION and HMs when adsorbed on Mg/Fe-LDH@biochar, in the sequence of NO3--N (van der Waals force and electrostatic attraction) < NH4+-N (hydrogen bonding) < HMs (ionic/coordinate bonding). Finally, to examine the performance of Mg/Fe-LDH@biochar for practical applications in farmland, column leaching experiments were successfully conducted by stimulated rainfall events. The addition of Mg/Fe-LDH@biochar into soils could greatly reduce the leaching of ION and HMs simultaneously, with reduction ratios of >60, >40, and >90% for NH4+-N, NO3--N, and HMs, respectively, at 3.0% addition. Moreover, there was no leaching risk of Fe ions into the water body from Mg/Fe-LDH@biocharamended soils.