Ink removal is one of the major technical obstacles, and over the years, several
chemical and enzymatic processes have been tried in alleviating problems related to
deinking process. Compared to the traditional chemical-based deinking, the use of
the enzymatic alternative has some advantages including better brightness, greater
freeness, and superior strength of fibers [108]. Moreover, the enzymatic process is
more efficient and can reduce the use of deinking chemicals, and hence it is
considered as environmentally benign [108]. Enzymatic deinking, in addition to
higher pulp cleanliness, allows to improve strength properties, reduce alkali usage,
render fiber brightness, increase pulp freeness, and minimize the presence of fine
particles in the pulp [109].
Cellulases, lipases, amylases, laccases, pectinases, and xylanases have been used
in recycled paper deinking and bleaching processes [110–113]. The mechanism of
these enzymes in achieving the desired deinking is quite different. For instance, the
lipase hydrolyzes the oil-based binders/resins in the inks, whereas cellulases and
xylanases detach the ink from the fiber surface through hydrolysis of the fine fibers
from toner surfaces [114]. Amylases degrade the starch used in the papermaking and
thereby facilitate the ink release and its removal by the subsequent flotation step.
However, it is essential to note that the enzymes, specially cellulases, should not be
used at higher doses as it can affect the quality of the pulp. Significant loss of fines
may result in undesirable poor fiber bonding [115].
Paper recycling involves swelling of the cellulose fibers, a phenomenon which
facilitates the removal of ink and other additives from the cellulose fibers
[116]. Since high pH renders better swelling of fibers, paper recycling is often
accomplished at higher pH. Moreover, the alkali can directly interact with the dye,
weaken its association to the fibers, and facilitate its removal [117]. Thus, enzymes
considered for paper recycling applications should be operationally stable at alkaline
conditions, and this offers economic, technical, as well as environmental benefits.
The other challenge in paper recycling is the complete removal of adhesive
contaminants which are often referred to as stickies [118–121]. The presence of
stickies can pose serious problems in recycling process and affect the quality of the
recycled paper properties. It can adhere to the recycling and papermaking machines
and lead to fouling and equipment plugging. Macro-stickies which are in the range of
100 to 1,000 μm in size are the most difficult adhesive contaminants to remove
during the recycling process [122]. Efficient removal of these stickies has been one
of the major issues in paper recycling. An enzymatic method that uses alkaline active
esterases and lipases which act on adhesives resulted in an encouraging removal
efficiency of stickies and concomitantly improve the pulp quality [122, 123].
4.1.5 Synthesis
With the emergence of green chemistry, the synthesis of substances using enzymes
has become increasingly important. These synthetic reactions often use enzymes
which are cell-free or cell-bound in processes respectively known as biocatalysis and
biotransformation. These processes are widely considered as safe, mild, efficient,
18
G. Mamo and B. Mattiasson
chemical and enzymatic processes have been tried in alleviating problems related to
deinking process. Compared to the traditional chemical-based deinking, the use of
the enzymatic alternative has some advantages including better brightness, greater
freeness, and superior strength of fibers [108]. Moreover, the enzymatic process is
more efficient and can reduce the use of deinking chemicals, and hence it is
considered as environmentally benign [108]. Enzymatic deinking, in addition to
higher pulp cleanliness, allows to improve strength properties, reduce alkali usage,
render fiber brightness, increase pulp freeness, and minimize the presence of fine
particles in the pulp [109].
Cellulases, lipases, amylases, laccases, pectinases, and xylanases have been used
in recycled paper deinking and bleaching processes [110–113]. The mechanism of
these enzymes in achieving the desired deinking is quite different. For instance, the
lipase hydrolyzes the oil-based binders/resins in the inks, whereas cellulases and
xylanases detach the ink from the fiber surface through hydrolysis of the fine fibers
from toner surfaces [114]. Amylases degrade the starch used in the papermaking and
thereby facilitate the ink release and its removal by the subsequent flotation step.
However, it is essential to note that the enzymes, specially cellulases, should not be
used at higher doses as it can affect the quality of the pulp. Significant loss of fines
may result in undesirable poor fiber bonding [115].
Paper recycling involves swelling of the cellulose fibers, a phenomenon which
facilitates the removal of ink and other additives from the cellulose fibers
[116]. Since high pH renders better swelling of fibers, paper recycling is often
accomplished at higher pH. Moreover, the alkali can directly interact with the dye,
weaken its association to the fibers, and facilitate its removal [117]. Thus, enzymes
considered for paper recycling applications should be operationally stable at alkaline
conditions, and this offers economic, technical, as well as environmental benefits.
The other challenge in paper recycling is the complete removal of adhesive
contaminants which are often referred to as stickies [118–121]. The presence of
stickies can pose serious problems in recycling process and affect the quality of the
recycled paper properties. It can adhere to the recycling and papermaking machines
and lead to fouling and equipment plugging. Macro-stickies which are in the range of
100 to 1,000 μm in size are the most difficult adhesive contaminants to remove
during the recycling process [122]. Efficient removal of these stickies has been one
of the major issues in paper recycling. An enzymatic method that uses alkaline active
esterases and lipases which act on adhesives resulted in an encouraging removal
efficiency of stickies and concomitantly improve the pulp quality [122, 123].
4.1.5 Synthesis
With the emergence of green chemistry, the synthesis of substances using enzymes
has become increasingly important. These synthetic reactions often use enzymes
which are cell-free or cell-bound in processes respectively known as biocatalysis and
biotransformation. These processes are widely considered as safe, mild, efficient,
18
G. Mamo and B. Mattiasson