Mechanism of removal and degradation characteristics of dicamba by biochar prepared from Fe-modified sludge.

The pyrolysis of excess sludge derived from wastewater treatment plants to prepare biochar can achieve the mass-reduction and harmlessness of solid waste, but it is also necessary to further explore the application prospect of these biochars as a resource for wastewater treatment. In this study, Fe-modified biochar (BC-Fe) was prepared by pyrolysis of excess sludge modified by FeCl3 solution. The molecular structure, elemental valence state, and composition of biochars were comprehensively investigated. The results showed that, compared with the biochar prepared from sludge without modification (BC-blank), the O/C ratio of BC-Fe increased from 0.07 to 0.12, and the (N + O)/C ratio increased from 0.21 to 0.27, indicating increased polarity and weakened aromaticity. The ratio of integrated intensity of the D band and G band in the Raman spectrum increased from 1.34 to 2.40, showing the increased defect structure of the biochar obtained by Fe modification. In the reaction between BC-Fe and dicamba, the removal rate of dicamba reached 92.1% within 180 min, which was far higher than the 17.8% of BC-blank. It was confirmed the adsorption removal dominated and accounted for 70.6% of the dicamba removal by BC-Fe, and the adsorption capacity of biochar could be significantly enhanced by Fe-modification by 5.3 times. Moreover, the persistent free radicals (PFRs) on the surface of biochar was detected by an electron paramagnetic resonance analyzer, and the decline of PFRs signals after the reaction revealed that PFRs participated in the degradation process of dicamba. Through Q-TOF analysis, it could be concluded that dicamba was first converted to 3,6-dichlorosalicylic acid (DCSA) by PFRs reduction and then further transformed to 3,6-dichlorogentisic acid (DCGA). This study provided a reference for the understanding of the removal mechanism of dicamba by Fe-modified biochar and offered an application potential of biochar derived from Fe-containing sludge for the pollution control of dicamba pesticide pollutants.

Understanding the role of cations and hydrogen bonds on the stability of aerobic granules from the perspective of the aggregation and adhesion behavior of extracellular polymeric substances.

Extracellular polymeric substances (EPS) were essential for the granulation and stability of aerobic granular sludge (AGS). In this study, the effects of electrostatic interactions, bridging effect of divalent cations, and hydrogen bonds on the EPS-EPS and EPS-surface interaction were verified by enhancing or reducing the specific interaction with the addition of cations or urea. The size and the surface properties of EPS aggregates were investigated, the adhesion behavior and viscoelasticity of EPS were analyzed by quartz crystal microbalance with dissipation monitoring. The changes of EPS in response to the various condition were analyzed by infrared spectroscopy and fluorescence spectrum. The electrostatic repulsion between EPS could be significantly reduced by Ca2+ addition. With the bridging effect, 10 µM of Ca2+ could reduce the negative charge of EPS more effectively than 200 µM of Na+. As Ca2+ could form the complex with the protein and Ca2+ was more inclined to bind with COO-, the Ca2+ took advantage of boosting the EPS-EPS and EPS-surface interaction than Mg2+ at the same ionic strength, which resulted in the denser structure of calcium-treated EPS. The destruction of hydrogen bonds by urea addition reduced the EPS-EPS and EPS-surface interaction, which confirmed the potential existence of hydrogen bonds in the interaction of EPS-EPS and EPS-surface. The removal of hydrogen bonds of EPS destroyed the protein's secondary structure and caused the unfolded state of the protein, which led to the looser structure of the EPS layer.