2.5. Experimental design
The effects of various experimental parameters on NL-DCPE
process were optimized using Response Surface Methodology
(RSM), which was a mathematical and statistical methods for
designing experiments, building models, evaluating the influence
of variables, and searching optimum conditions of variables to
predict targeted responses (Montgomery, 1996). Based on the
data of single-factor experiment, a three-level BBD with three factors
was investigated to obtain the optimum extraction conditions.
Three independent variables, namely the concentration of
L44, sample pH and stripping agent pH were studied at three
levels, and the factor levels were coded as 1 (low), 0 (central
point) and 1 (high). The response of the experiment was the
extraction efficiency (E) of Pb(II). The range and levels of independent
variables are presented in Table (S) 1. All the experimental
design matrix and data analyses were performed using the software
Design-Expert 8.0.