7.6 Combined MW/US in the Preparation of Adsorbents
107
7.6 Combined MW/US in the Preparation of Adsorbents
The US/MW synergistic method has been used to reduce Graphene Oxide (GO) to
rGO (Song et al 2016). A suspension of GO and ethylenediamine was heated to
110 °C under MW irradiation (2450 MHz, 300 W) and discontinuous ultrasonic
irradiation (25 kHz, 750 W, 2 s US and 1 s interruption) for 30 min. Raman spectra
were compared with those of rGO from the liquid phase exfoliation of graphite and
the results confirmed that reduction had occurred. After the treatment of rGO with
melamine foam, under the previously described conditions (MW/US combined irradiation), the rGO-modified melamine foam showed high porosity, superhydrophobicity and superoleophilicity. After a test, it was shown to possess high selectivity
and an improved ability to collect a variety of oils and organic solvents from water.
While subsequent MW and US irradiation has been used for the preparation of a
number of adsorbents, a recent study makes use of simultaneous irradiation to prepare
amine-functionalized MIL-53(Al) (Ge et al. 2016). As depicted in Table 7.1, combined MW/US irradiation gave significant rate enhancements and improved yields,
as compared to the hydrothermal method, and contributed to the decreased external surface areas of the monocrystals that may be due to increased crystal size. In
adsorbency tests, the MW/US irradiation-prepared MIL-53(Al) showed a substantial increase in CO 2 adsorption capacity over those prepared under conventional
conditions.
The effect of US and MW irradiation pretreatment on the biogas production,
solids removal and dewaterability of anaerobically digested sludge has been studied
by Yeneneh et al. (2013). Combined MW/US pretreatment significantly improved
biogas production and specific methane yield after 17 days of the anaerobic degradation of a synthetic sludge sample that had been inoculated with digested sewage
sludge. US pretreatment released extracellular polymeric substances which consist
of short-chain organic matter and facilitated the release of exo-enzymes from cells
that assist in the breakdown of organic materials into readily biodegradable fractions.
MW irradiation enhanced the degradation of organic compounds and the formation
of volatile acid. Furthermore, MW/US-treated sludge showed significantly higher
dewaterability and shorter capillary suction time than the other pretreatment protocol. The optimum operating conditions were found to be 2 min of MW irradiation
followed by 6 min of US.
The use of US and MW technologies has also been considered an appropriate,
rapid, safe and sustainable method for the depolymerization of starch into sugars.
The delignification of lignocellulosic biomass is a matter of great interest (Singh
et al. 2014). With the advent of nanoscience, researchers and industries have paid
significant attention to the production of nanocellulose (NC). This high value-added
product has applications that range from composites, paper, packaging, paints and
oil to personal and medical care (Rajinipriya et al. 2018). The MW-promoted liquefaction of wood residues has provided a new approach to the extraction of celluloseenriched residues from lignocellulosic biomass. In addition, US nanofibrillation can
be used to isolate cellulose nanofibers from chemically purified cellulose fibres (Xie
107
7.6 Combined MW/US in the Preparation of Adsorbents
The US/MW synergistic method has been used to reduce Graphene Oxide (GO) to
rGO (Song et al 2016). A suspension of GO and ethylenediamine was heated to
110 °C under MW irradiation (2450 MHz, 300 W) and discontinuous ultrasonic
irradiation (25 kHz, 750 W, 2 s US and 1 s interruption) for 30 min. Raman spectra
were compared with those of rGO from the liquid phase exfoliation of graphite and
the results confirmed that reduction had occurred. After the treatment of rGO with
melamine foam, under the previously described conditions (MW/US combined irradiation), the rGO-modified melamine foam showed high porosity, superhydrophobicity and superoleophilicity. After a test, it was shown to possess high selectivity
and an improved ability to collect a variety of oils and organic solvents from water.
While subsequent MW and US irradiation has been used for the preparation of a
number of adsorbents, a recent study makes use of simultaneous irradiation to prepare
amine-functionalized MIL-53(Al) (Ge et al. 2016). As depicted in Table 7.1, combined MW/US irradiation gave significant rate enhancements and improved yields,
as compared to the hydrothermal method, and contributed to the decreased external surface areas of the monocrystals that may be due to increased crystal size. In
adsorbency tests, the MW/US irradiation-prepared MIL-53(Al) showed a substantial increase in CO 2 adsorption capacity over those prepared under conventional
conditions.
The effect of US and MW irradiation pretreatment on the biogas production,
solids removal and dewaterability of anaerobically digested sludge has been studied
by Yeneneh et al. (2013). Combined MW/US pretreatment significantly improved
biogas production and specific methane yield after 17 days of the anaerobic degradation of a synthetic sludge sample that had been inoculated with digested sewage
sludge. US pretreatment released extracellular polymeric substances which consist
of short-chain organic matter and facilitated the release of exo-enzymes from cells
that assist in the breakdown of organic materials into readily biodegradable fractions.
MW irradiation enhanced the degradation of organic compounds and the formation
of volatile acid. Furthermore, MW/US-treated sludge showed significantly higher
dewaterability and shorter capillary suction time than the other pretreatment protocol. The optimum operating conditions were found to be 2 min of MW irradiation
followed by 6 min of US.
The use of US and MW technologies has also been considered an appropriate,
rapid, safe and sustainable method for the depolymerization of starch into sugars.
The delignification of lignocellulosic biomass is a matter of great interest (Singh
et al. 2014). With the advent of nanoscience, researchers and industries have paid
significant attention to the production of nanocellulose (NC). This high value-added
product has applications that range from composites, paper, packaging, paints and
oil to personal and medical care (Rajinipriya et al. 2018). The MW-promoted liquefaction of wood residues has provided a new approach to the extraction of celluloseenriched residues from lignocellulosic biomass. In addition, US nanofibrillation can
be used to isolate cellulose nanofibers from chemically purified cellulose fibres (Xie