Page 128 - Materials Chemistry, Second Edition

Textile Wastewater Treatment by Advanced Oxidation Processes 109

potential toxicity of the transformation products obtained from the enzymatic
treatment.
Laccase immobilization on supports such as nanoporous silver and gold particles
(Mazur et al., 2007), mesostructured foams (Rekuć et al., 2009), or epoxy-activated
resins (Kunamneni et al., 2008; Lloret et al., 2012) has been successfully performed
during recent years. However, few biocatalysts have been used for practical applications
due to their poor robustness (DiCosimo et al., 2013). Hence, different strategies for
laccase immobilization were evaluated on magnetic supports to choose a robust nano-
biocatalyst for the decolorization of dyes. The covalent bonding between the support
and carboxylic groups (oleic acid) did not result in satisfactory immobilization, which
may be attributed to excessive linking of EDC (1-ethyl-3-(3-dimethylaminopropyl)
carbodiimide hydrochloride) with the enzyme, leading to crosslinking among
enzymes (due to the presence of both -NH and –COOH groups in the enzyme mol-
2
ecule). Activity loss due to excessive crosslinking of enzyme was also observed in
other studies, with immobilization yields below 14% (Majumder et al., 2008; Kumar
et al., 2014). However, other immobilization processes were successful, and the best
results were obtained for silica-coated mNPs, which were selected for the decoloriza-
tion of dyes.
The capacity of the immobilized enzymes to remove 20 mg L of the triphenyl-
−1
methane dye Methyl Green and 100 mg L of the anthraquinone dye Reactive Blue
−1
19 was assessed in repeated batch operation for 24 h. A control with functionalized
nanoparticles lacking laccase was performed. Immobilized laccase reached a decol-
orization percentage higher than 95% in the first cycle, which was maintained for 5
th
cycles, diminishing slightly to 87% after the 10 cycle (Figure 6.5).

100 250

80 200
MG decolorization (%) 60 150 Enzyme activity (U L –1 )

100
40
20 50

0 0
1 2 3 4 5 6 7 8 9 10
Oxidative cycles
FIGURE 6.5 Methyl Green decolorization yield (%, gray bars) and residual laccase activity
(○) in subsequent cycles of enzymatic treatment with laccase immobilized onto mNP.