Fe-modified
biochars have been widely used in removal of Cr(VI)
from water due to the resulting modified surface functional groups
and magnetization property. However, few studies have synthetically
investigated modification methods and synthesis parameters on the
improvement of the removal efficiency of Cr(VI) by Fe-modified biochars.
Herein, 10 types of corn straw-based magnetic biochars were produced
using pre-modification and post-modification methods with various
modifier ratios, and the highest heating temperature (HHT). Cr(VI)
removal results suggest that the removal efficiency of pre-modified
biochars ranged from 50.7 to 98.6%, which was much higher than that
of post-modified (6.6–21.6%) and unmodified biochars (0.4–7.6%).
The effect of synthesis methods on Cr(VI) adsorption was in the following
order: Fe-modification method > modifier ratio > HHT. The adsorption
kinetics and isotherm results of three types of pre-modified biochars
were well fitted with the pseudo-second-order model (R
2 > 0.99) and the Langmuir adsorption model (R
2 > 0.99), respectively, indicating the surface
homogeneity
of the pre-modified biochars and unilayer chemisorptions of Cr(VI).
Characterization results show that iron oxides or zerovalent iron
particles were successfully deposited onto the surface of biochars
and magnetism was introduced. A good Pearson correlation (r = −0.9694) between the removal efficiency and pH
value in modified biochar suggests that the lower pH value may offer
more positive charges and promote electrostatic attraction. Therefore,
the dominant mechanism for enhanced Cr(VI) adsorption on pre-modified
biochar was electrostatic attraction, resulting from its distinguished
acidity nature. Our findings provide new insights into the high-efficiency
removal of Cr(VI) onto Fe-modified magnetic biochars and will benefit
future design of more efficient magnetic biochars.