Egeria najas is a submerged aquatic plant, and the literatures on resourcification of submerged aquatic plant biochar remain limited. Until now, there has been no study on submerged aquatic plant biochar supported nZVI that is widely applied for removal of diversified contaminants in solution. In this study, an efficient approach to the preparation of Egeria najas- derived biochar supported nZVI composite is first developed for Cr(VI) removal in wastewater. The adsorption behavior and mechanism of Cr(VI) removal on the as-prepared Egeria najas- derived biochar/nZVI (EN@nZVI) composite were investigated. The results of FTIR and XPS indicate that the EN@nZVI surface had many functional groups such as R-COOH, R-OH, R-NH 2 and R-C-O-C, etc, which could provide active sites during the adsorption process. The BET results showed that the EN@nZVI had large specific surface area and average pore, which were 142.49 m 2 /g and 9.85 nm, respectively. EN@nZVI demonstrated high reactivity for Cr(VI) removal. Compared with nZVI, Cr(VI) removal efficiency by EN@nZVI is 50% higher than that of nZVI within 0.5 h. Furthermore, the maximum adsorption capacity of Cr (VI) was 56.79 mg/g and the energy of activation (E a ) was 31.30 kJ/mol. The adsorption process was well described by the pseudo-second order model and Sips adsorption isotherm model. The reaction mechanism of Cr(VI) removal was a multi-step removal mechanism, involving adsorption, surface complex formation, reduction and ion exchange reaction.
The aquatic plant Egeria najas based biochar was innovatively constructed by modifying it with Fe 2 (SO 4 ) 3 (Fe 2 (SO 4 ) 3 @EN-BC) for Cr(VI) removal. The characterizations of Fe 2 (SO 4 ) 3 @EN-BC confirmed that Fe 2 (SO 4 ) 3 was attached to biochar in form of Fe 2 O 3 after hydrolysis and calcination under high temperature. The results showed that Fe 2 (SO 4 ) 3 @EN-BC had a high removal efficiency of Cr(VI) with approaching 99.88%, which was about 7 times higher compared to unmodified biochar (EN-BC). Batch experiments of Cr(VI) removal included initial concentration, reaction temperature, solution pH and kinetics were conducted. The adsorption kinetics could be more accurately explained using pseudo-second order kinetics model , indicating the chemical adsorption controlled the reaction process. According to the Langmuir model, the maximum adsorption capacities for Cr(VI) were 138.79, 159.36 and 190.91 mg/g at 298, 313 and 333 K, respectively. Moreover, based on FTIR and XPS results, the Fe 2 (SO 4 ) 3 @EN-BC surface was loaded with many functional groups conducive to Cr(VI) adsorption. The main removal mechanisms of Cr(VI) by Fe 2 (SO 4 ) 3 @EN-BC included adsorption and reduction, and the percentage of Cr(VI) removed by adsorption and reduction were 19.1% and 80.9%, respectively.
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