To improve the adsorption efficiency of pollutants by biochar, preparing graphene-like biochar (GBC) or nitrogen-doped biochar are two commonly used methods. However, the difference in the nitrogen doping (N-doping) effects upon the adsorption of pollutants by pristine biochar (PBC) and GBC, as well as the underlying mechanisms, are still unclear. Take the tetracycline (TC) as an example, the present study analyzed the characteristics of the adsorption of TCs on biochars (PBC, GBC, N-PBC, N-GBC), and significant differences in the effects of N-doping on the adsorption of TCs by PBC and GBC were consistently observed at different solution properties. Specifically, N-doping had varied effects on the adsorption performance of PBC, whereas it uniformly improved the adsorption performance of GBC. To interpret the phenomenon, the N-doping upon the adsorption was revealed by the QSAR model, which indicated that the pore filling (V_M) and the interactions between TCs with biochars (E_ad-v) were found to be the most important two factors. Furthermore, the density functional theory (DFT) results demonstrated that N-doping slightly affects biochar’s chemical reactivity. The van der Waals (vdWs) and electrostatic interactions are the main forces for TCs-biochars interactions. Moreover, N-doping mostly strengthened the electrostatic interactions of TCs-biochars, but the vdWs interactions of most samples remained largely unaffected. Overall, the revealed mechanism of N-doping on TCs adsorption by biochars will enhance our knowledge of antibiotic pollution remediation.

Background: Zinc oxide nanoparticle ( n ZnO) and chemicals with different mode of action (MOA, i.e., narcotic and reactive) were frequently detected in the Yangtze River. Organisms are typically exposed to mixtures of n ZnO and other chemicals rather than individual n ZnO. Toxicity of n ZnO is caused by the dissolution of Zn 2+ , which has been proved in the field of single toxicity. However, it is still unclear whether the released Zn 2+ plays a critical role in the n ZnO toxicity of n ZnO-chemicals mixtures. In the present study, the binary mixture toxicity of n ZnO/Zn 2+ and chemicals with different MOA was investigated in acute (15 min) and chronic (12 h) toxicity test upon Vibrio fischeri ( V. fischeri ). The joint effects of n ZnO and tested chemicals were explored. Moreover, two classic models, concentration addition (CA) and independent action (IA) were applied to predict the toxicity of mixtures. Results: The difference of toxicity unit (TU) values between the mixtures of Zn 2+ -chemicals with those of n ZnO-chemicals was not significant ( P > 0.05), not only in acute toxicity test but also in chronic toxicity test. The antagonistic or additive effects for n ZnO-chemicals can be observed in most mixtures, with the TU values ranging from 0.75-1.77 and 0.47-2.45 in acute toxicity test and chronic test, respectively. We also observed that the prediction accuracy of CA and IA models was not very well in the mixtures where the difference between the toxicity ratios of the components was small (less than about 10), with the mean absolute percentage error (MAPE) values ranging from 0.14-0.67 for CA model and 0.17-0.51 for IA model, respectively. Conclusion: We found that the dissolved Zn 2+ mainly accounted for the n ZnO toxicity in the mixtures of n ZnO-chemicals, and the joint effects of these mixtures were mostly antagonism and additivity. CA and IA models were unsuitable for predicting the mixture toxicity of n ZnO-chemicals at their equitoxic ratios.