During the reduction process, the oxidation–reduction potential
(ORP) was detected as dropping from −102.6 to
−225.5 mV quickly, and then remaining stable in the first
15 min, later increasing sharply up to 28.9 mV after 90 min.
This indicates reductive conditions for effective dye removal
within the first 15 min. Moreover, pH variation was also detected
during reduction, first rising sharply from 7.1 to 8.3
during 4 min, and then decreasing gradually to 6.9 with time,
as soon as the NZVI particles contacted dye solution with initial
concentration of 100 mg L−1. Meanwhile, declining TOC
concentration and ADMI color index were observed from 40
to 22.2 mg L−1 as well as ADMI color index dramatically decreasing
from 17,376 to less than 3211, respectively