Simultaneous removal of p-nitrophenol and Cr(VI) using biochar supported green synthetic nano zero valent iron-copper: Mechanistic insights and toxicity evaluation

Nano zero-valent iron/copper bimetallic particles supported by peanut shell biochar were synthesized with green tea extracts (GT-BC@nZVI/Cu) and used for simultaneous removal of p-nitrophenol (PNP) and Cr (VI) from groundwater. GT-BC@nZVI/Cu was characterized by SEM, TEM, FTIR, XRD and XPS. The effect of pH, temperature, coexisting ions and humic acid on the removal of PNP and Cr (VI) were studied. Results showed that when pH value was 5 and the temperature was 313 K, the removal rates of PNP and Cr (VI) by GT-BC@nZVI/Cu reached 93% and 100 % within 30 min, respectively. Pseudo-first-order, pseudo-second-order and W-M models were used to analyze the experimental results. The adsorption isotherms of Langmuir and Freundlich were also evaluated. The analysis of these models shows that the pseudo-second-order kinetics and Langmuir adsorption isotherm can better describe the removal reaction of PNP and Cr (VI). The presence of coexisting anions (Cl-, SO42-, NO3- and CO32-) showed various effects on the removal of PNP and Cr(VI), while humic acid evidently inhibited the removal of PNP and Cr (VI). Moreover, the reusability of GT-BC@nZVI/Cu demonstrated that the removal of Cr(VI) and PNP are 62.45% and 47.32 % after three cycles, respectively. Mung bean seeds germinated 100% in both the GT-BC@nZVI/Cu suspension and the treated solution after 5 days of cultivation. The mechanism of Cr(VI) removal involves adsorption, reduction and co-precipitation. GC-MS analysis identified p-aminophenol as the reduced product and p-benzoquinone and hydroquinone as the oxidized product. These results demonstrated that the degradation of PNP is a combination of reduction and oxidation.