1. Introduction
Urbanization, growing population and industrialization have led to a
considerable release of manmade pollutants that due to their synthetic
origin are recalcitrant to biodegradation, causing their persistence and
accumulation in the environment. In addition, most of them are highly
toxic posing wildlife and human health at risk. Therefore, the removal
of the aforementioned pollutants from wastewater before being discharged
in the environment is an urgent need. However, conventional wastewater
treatments are inefficient in the removal of such type of pollutants and
emerging technologies are either costly, non-environmentally friendly or
non-feasible on a large scale. This has driven forward the search for
new efficient, cost-effective and ecological wastewater treatment
technologies. In this sense, the use of ligninolytic enzymes produced by
certain microorganisms are particularly interesting due their broad
substrate specificity and natural origin. Among the different existing
ligninolytic enzymes, laccases (E.C. 1.10.3.2; p-bezenediol: dioxygen
oxidoreductases) have attracted increasing interest during the last
decades on account of they only need environmental molecular oxygen to
exert their catalytic action producing water as the only by-product.
Their catalytic site consists of four copper atoms that are classified,
according to their electron paramagnetic resonance and UV-vis
spectroscopy characteristics, in three types and located at different
sites.[1] The type 1 copper (T1) is involved in
the oxidation of the substrate. The type 2 copper (T2) and the two type
3 coppers (T3) form a trinuclear cluster where the molecular oxygen is
reduced to water. The connection between the two sites (i.e., the T1
site and the T2-T3 cluster site) is guaranteed by a tripeptide coupling
of amino acids (His-Cys-His) (Figure 1).[1]
Laccases are widely distributed in Nature and, thus, they have been
found in bacteria, fungi, higher plants and
insects.[2] Among them, laccases from white-rot
fungi are particularly interesting since they have the highest redox
potential. In addition, fungal laccases are often glycosylated what
gives them conformational stability and protection from inactivation by
radicals and proteolysis.[3,4] Also, recently the
mechanism of molecular oxygen reduction for high-redox potential
laccases has been described and has been found that contrary to the
low-redox potential laccases, the T1 copper reduction is very
fast[5] and, hence, their high appeal for
biotechnological processes. Consequently, laccase production and
application as a biocatalyst have been increasingly reported in the
literature.[4,6,7] However, despite laccase
production cost is an important issue for its industrial
exploitation[8], techno-economic analyses have
been scarcely reported
On the other hand, regarding laccase applications, the use of free
enzymes for wastewater treatment holds different drawbacks such as
non-reusability, high cost, lack of long-term stability and sensibility
to different denaturing agents (e.g., pH, temperature, mechanical stress
and inhibiting compounds that may occur in wastewater). In fact, there
is no recorded industrial application of free
laccase.[9] The above-mentioned drawbacks can be
solved by immobilising laccase enzymes in/on solid carriers. The
selection of a suitable solid carrier for laccase immobilisation is
essential for the efficiency of the developed bioprocess. An ideal
carrier should protect both laccase structure and activity under
different operational and environmental conditions while keeping its own
physical integrity. In addition, it should have high affinity for
laccase enzyme, be inert, easily available, low-cost and environmentally
friendly. Recently, Kyomuhimbo and Brink (2023) have reviewed the
different supports used for laccase
immobilisation.[10]
Different immobilisation techniques than can be categorised in physical
(adsorption, encapsulation, entrapment) and chemical (covalent binding,
crosslinking) and different solid carriers have been tested to make
laccase enzymes reusable and increase their stability as recently
reviewed by Alvarado-RamÃrez et al. (2021).[11]Also, Zhou et al. (2021) reviewed the different immobilisation methods
and carriers used to immobilise laccase as well as the application of
the immobilised laccases in water
purification.[12] Each immobilisation protocol
presents its advantages and disadvantages (Table 1). So, there is not a
preferred method for laccase immobilisation and will depend on the
laccase source, the carrier, the intended application and the operating
conditions.[13] Therefore, the design of new
laccase immobilisation protocols is still worth of investigation.
Current trends are oriented towards the use of a low quantity of laccase
enzyme, minimum chemical reagents and biodegradable and low-cost
materials, such as agro-industrial wastes, as solid
carriers.[9] To scale up the wastewater treatment
process, the immobilised laccases are placed in different reactor
configurations. This review explores the reported use of
immobilised-laccase reactors for pollutant removal and wastewater
treatment from 2020 up to date.