Unmodified (UN),
acid-treated (AT) and microwave-acid-treated (MAT)
activated carbons were optimized for their solute removal efficacies
by adjusting feed mixture compositions and process conditions. Acetaminophen,
benzotriazole, and caffeine were used either individually or as binary/ternary
mixtures in this study. The process conditions considered were the
pH, adsorbent dosage, and type of adsorbent. Experimental responses
such as total adsorbent loading (
q
total
) and total percentage removal (PR
total
) were fitted with
empirical models that had high adjusted
R
2
(>0.95), insignificant lack of fit (
p
-value
> 0.22),
and high model predictive
R
2
(>0.93).
Mixture compositions of the feed were found to interact significantly
not only among themselves but with process variables as well. Hence,
adsorption optimization must simultaneously consider mixture as well
as process variables. The conventional response surface methodology
for mixtures, termed as ridge analysis, optimizes mixture compositions
at specified values of process variables. An improved steepest ascent
method which considers mixture and process variables simultaneously
was developed in this work. This could track the path of steepest
ascent toward globally optimal settings, from any arbitrary starting
point within the design space. For the chosen adsorbent, optimal settings
for feed mixture compositions and pH were found to change along this
steepest ascent path. The feed compositions, pH, and adsorbent dosage
identified for maximum adsorbent utilization were usually quite different
from those identified for maximum total percentage removal. When both
these objectives were optimized together, the most favorable compromise
solutions for
q
total
and PR
total
were, respectively, 264.1 mg/g and 43.4% for UN, 294.9 mg/g and
52.5% for AT, and 336.6 mg/g and 55.9% for MAT.
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