No abstract

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@biochar-amended soils.

The discovery of materials as soil conditioners for the improvement of soil nitrogen-use efficiency provides a promising strategy for simultaneously addressing production and pollution problems in agriculture. Herein, adsorption–desorption performance and mechanisms of ammonium (NH4 +–N) by a poly­(acrylic acid)-grafted chitosan and biochar composite (PAA/CTS/BC) were studied to evaluate its potential in soil amendment. Furthermore, mathematical models were further applied to explore the effects of PAA/CTS/BC on controlling soil NH4 +–N loss. Results suggest that PAA/CTS/BC showed a high capacity of NH4 +–N adsorption, with a maximum value of 149.25 mg·g–1 at 25 °C, considerably higher than most of the reported BC-based adsorbents. Fast adsorption of NH4 +–N was completed with an efficiency of >90% within 20 min, primarily attributed to the dominant electrostatic interactions with carboxyl groups. Adsorbed NH4 +–N was capable of being easily re-released with the variation of the liquid-phase concentration because of the weak binding of NH4 +–N with the material. These results demonstrate that the PAA/CTS/BC can be considered as a nitrogen slow-release medium to dynamically maintain a plant-available NH4 +–N concentration in the soil. Finally, the mathematical simulation of NH4 +–N flow in the PAA/CTS/BC-amended soil revealed that soil NH4 +–N loss was effectively reduced by 24.18–31.77% at a 1.57–2.02% material dosage during a 10-week rice cultivation.