Progress on the photocatalytic reduction of hexavalent Cr (VI) using engineered graphitic carbon nitride

Abstract

The existence of chromium in hexavalent oxidation state is highly toxic to aquatic environment. Photocatalytic reduction of hexavalent Cr (VI) into Cr (III) has emerged as a desirable technology due to their prospect in solar energy utilization, high efficiency and low cost. Graphitic carbon nitride (g-C3N4)-based photocatalysts are ideal for Cr (VI) reduction due to their inherent features including; visible-light responsive narrow bandgap, suitable conduction band potential, high physicochemical stability, unique optical and electronic properties. Herein, various surface-interface strategies to modify g-C3N4 including heterojunction formation, doping, structural regulation, co-catalyst loading and construction of nitrogen vacancies are elaborated for improving the Cr(VI) photoreduction efficiency. The review also highlights the effect of operational reaction conditions such solution pH, g-C3N4 dosage, Cr (VI) concentration, temperature, light source, organic acid additives and co-existing ions influencing Cr (VI) reduction efficiency. Finally, we attempt to propose the existing issues based on the current research and future aspects of engineered g-C3N4 for Cr (VI) photoreduction.

Full text

Process
Safety
and
Environmental
Protection
152
(2021)
663–678
Contents
lists
available
at
ScienceDirect
Process
Safety
and
Environmental
Protection
jou
rn
al
hom
ep
age:
www.elsevier.com/locate/psep
Progress
on
the
photocatalytic
reduction
of
hexavalent
Cr
(VI)
using
engineered
graphitic
carbon
nitride
Vasudha
Hasijaa,
Pankaj
Raizadaa,∗,
Pardeep
Singha,
Narinder
Vermab,
Aftab
Aslam
Parwaz
Khanc,d,
Arachana
Singhe,
Rangabhashiyam
Selvasembianf,
Soo
Young
Kimg,
Chaudhery
Mustansar
Hussainh,
Van-Huy
Nguyeni,
Quyet
Van
Leg,∗
aSchool
of
Advanced
Chemical
Sciences,
Faculty
of
Basic
Sciences,
Shoolini
University,
Solan,
HP,
173229,
India
bFaculty
of
Management
Sciences
&
Liberal
Art,
Shoolini
University,
Solan,
HP,
173229,
India
cCenter
of
Excellence
for
Advanced
Materials
Research,
King
Abdulaziz
University,
P.O.
Box
80203,
Jeddah
21589,
Saudi
Arabia
dChemistry
Department,
Faculty
of
Science,
King
Abdulaziz
University,
P.O.
Box
80203,
Jeddah
21589,
Saudi
Arabia
eAdvanced
Materials
and
Processes
Research
Institute,
Hoshangabad
Road,
Bhopal,
MP,
462026,
India
fDepartment
of
Biotechnology,
School
of
Chemical
and
Biotechnology,
SASTRA
Deemed
University,
Thanjavur,
613401,
Tamil
Nadu,
India
gDepartment
of
Materials
Science
and
Engineering,
Institute
of
Green
Manufacturing
Technology,
Korea
University,
145,
Anam-ro
Seongbuk-gu,
Seoul
02841,
South
Korea
hDepartment
of
Chemistry
and
Environmental
Science,
New
Jersey
Institute
of
Technology,
Newark,
NJ
07102,
USA
iFaculty
of
Biotechnology,
Binh
Duong
University,
Thu
Dau
Mot,
Vietnam
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
30
March
2021
Received
in
revised
form
18
June
2021
Accepted
27
June
2021
Available
online
30
June
2021
Keywords:
Graphitic
carbon
nitride
Photocatalytic
activity
Enhancement
strategies
Cr
(VI)
reduction
Reaction
parameters
a
b
s
t
r
a
c
t
The
existence
of
chromium
in
hexavalent
oxidation
state
is
highly
toxic
to
aquatic
environment.
Pho-
tocatalytic
reduction
of
hexavalent
Cr
(VI)
into
Cr
(III)
has
emerged
as
a
desirable
technology
due
to
their
prospect
in
solar
energy
utilization,
high
efficiency
and
low
cost.
Graphitic
carbon
nitride
(g-C3N4)-
based
photocatalysts
are
ideal
for
Cr
(VI)
reduction
due
to
their
inherent
features
including;
visible-light
responsive
narrow
bandgap,
suitable
conduction
band
potential,
high
physicochemical
stability,
unique
optical
and
electronic
properties.
Herein,
various
surface-interface
strategies
to
modify
g-C3N4including
heterojunction
formation,
doping,
structural
regulation,
co-catalyst
loading
and
construction
of
nitrogen
vacancies
are
elaborated
for
improving
the
Cr(VI)
photoreduction
efficiency.
The
review
also
highlights
the
effect
of
operational
reaction
conditions
such
solution
pH,
g-C3N4dosage,
Cr
(VI)
concentration,
tem-
perature,
light
source,
organic
acid
additives
and
co-existing
ions
influencing
Cr
(VI)
reduction
efficiency.
Finally,
we
attempt
to
propose
the
existing
issues
based
on
the
current
research
and
future
aspects
of
engineered
g-C3N4for
Cr
(VI)
photoreduction.
©
2021
Institution
of
Chemical
Engineers.
Published
by
Elsevier
B.V.
All
rights
reserved.
Contents
1.
Introduction
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664
2.
Research
methodology
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665
2.1.
In-depth
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2.2.
Searching
and
reporting
of
literature
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665
3.
Surface
and
interface
engineering
of
g-C3N4.
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.665
3.1.
Heterojunction
formation
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665
3.2.
Doping.
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.668
3.3.
Structural
regulation
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668
3.4.
Construction
of
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vacancies
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669
3.5.
Co-catalyst
loading.
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.669
∗Corresponding
authors.
E-mail
addresses:
pankajchem1@gmail.com
(P.
Raizada),
quyetbk88@korea.ac.kr
(Q.V.
Le).
https://doi.org/10.1016/j.psep.2021.06.042
0957-5820/©
2021
Institution
of
Chemical
Engineers.
Published
by
Elsevier
B.V.
All
rights
reserved.

V.
Hasija
et
al.
Process
Safety
and
Environmental
Protection
152
(2021)
663–678
4.
Effect
of
operational
reaction
conditions
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670
4.1.
Solution
pH.
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.670
4.2.
Photocatalyst
dosage.
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.671
4.3.
Cr
(VI)
concentration
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672
4.4.
Effect
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additives
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672
4.5.
Other
factors
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675
5.
Conclusion
and
perspectives
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675
Acknowledgements
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675
References
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675
1.
Introduction
The
anthropogenic
activities
and
biochemical
processes
con-
tribute
to
heavy
metals
(HMs)
ions
occurrence
in
water
systems.
Among
the
toxic
and
intrinsically
pervasive
HMs,
hexavalent
chromium
Cr
(VI)
is
a
common
contaminant
in
surface
and
ground
water
because
of
its
wide
employment
in
stainless
steel
production,
leather
tanneries,
chromate
pigments,
printing
inks,
metallurgy
and
textile
industries,
etc
(Rangabhashiyam
and
Balasubramanian,
2018;
Singh
et
al.,
2017a).
In
aqueous
media,
chromium
occurs
mainly
in
less
noxious
trivalent
state
Cr
(III)
and
highly
toxic
hex-
avalent
Cr
(VI)
(Hariharan
et
al.,
2020;
Pare
et
al.,
2009).
The
safe
permissible
limit
of
Cr
(VI)
is
5
␮gm−3in
air
and
100
␮gL-1 in
water
as
recommended
by
Occupational
Safety
and
Health
Admin-
istration
(OSHA)
and
Environmental
Protection
Agency
(EPA),
respectively
(Jiang
et
al.,
2019).
The
toxicity
of
Cr
(VI)
has
been
reported
to
be
∼3000
times
more
carcinogenic
than
Cr
(III)
pos-
ing
severe
threat
to
the
human
health
due
to
its
water-soluble,
mobile,
non-biodegradable,
and
penetrating
nature
as
it
forms
free
radicals
(Miretzky
and
Cirelli,
2010).
In
disparity,
Cr
(III)
is
a
vital
micronutrient,
and
is
precipitated
out
of
the
water
in
the
form
of
Cr
(OH)3due
to
its
low
mobility
or
is
readily
separated
by
natural
clays
(Farooqi
et
al.,
2021).
Thus,
to
significantly
minimize
the
fatal
hazards
of
Cr
(VI)
it
is
imperative
to
develop
effective
strategies
for
the
elimination
Cr
(VI)
pollutant.
The
conventional
Cr
(VI)
remediation
techniques
have
been
practiced
but
have
shortcomings.
These
are:
(i)
Adsorption
tech-
nology
limited
by
multistep
preparation
and
poor
recyclability
of
adsorbent
and
partial
removal
of
Cr
(VI)
from
aqueous
media
(Rangabhashiyam
et
al.,
2019).
(ii)
Ultrafiltration
is
accompanied
by
membrane
fouling,
recontamination
of
water
and
expensive
oper-
ational
cost.
(iii)
Ion-exchange
method
involves
replacement
of
Cr
(VI)
ions
by
another
metal
ions
in
water
reducing
the
membrane
stability.
(iv)
Eco-friendly
bioremediation
is
a
time-consuming
pro-
cess
dependent
upon
phytochemicals
and
microbial
action
for
Cr
(VI)
reduction
(Sani
et
al.,
2020).
(v)
Chemical
reduction
is
an
eco-
nomical
approach
involving
kinetically
sluggish
reduction
of
Cr
(VI)
to
Cr
(III)
but
causes
toxic
chemical
sludge
formation
(Azeez
et
al.,
2021).
(vi)
Electro-kinetic
treatment
demands
pH
maintenance
and
low-voltage
direct
current.
In-spite
of
the
utility
of
these
tech-
niques,
they
evince
major
drawbacks
that
make
them
undesirable
for
Cr
(VI)
remediation.
Fortunately,
the
contemporary
solution
to
surmount
the
chal-
lenges
of
conventional
methods
is
photocatalytic
reduction
of
Cr
(VI)
to
Cr
(III).
In
view
of
the
inherent
benefits
of
heterogenous
pho-
tocatalysis
such
as
inexhaustible
solar
energy
driven,
cost-effective
reactions
with
minimal
production
of
hazardous
products.
The
knowledge
on
the
mechanism
of
photocatalytic
Cr
(VI)
reduction
is
undoubtedly
vital
for
the
development
of
efficient
photocata-
lysts
which
can
regulate
the
valence
states
of
chromium
metal
ions.
The
three
complementary
and
indispensable
photocatalysis
steps
(Kumar
et
al.,
2020;
Hasija
et
al.,
2021a,b)
are:
(i)
solar
illumina-
tion
of
photocatalyst
to
mediate
charge
excitation
and
separation.
(ii)
bulk
diffusion
of
electrons
onto
the
reactive
surface
followed
by
(iii)
efficient
redox
reactions
i.e.;
reduction
on
the
conduction
band
(CB)
and
oxidation
on
valence
band
(VB).
Of
note,
the
photo-
catalytic
Cr
(VI)
reduction
to
Cr
(III)
proceeds
via
the
photoinduced
electrons
when
the
CB
position
is
more
negative
than
the
reduction
potential
of
Eo(Cr2O7−2/Cr+3)
=
1.23
V,
Eo(HCrO4-/Cr+3)
=
1.35
V,
Eo
(CrO4−2/Cr+3)
=-0.13
V
vs.
NHE
(Liang
et
al.,
2015).
Also,
the
reduc-
tive
reactive
oxidative
species
i.e.;
O2
•-
radicals
produced
on
the
basis
of
standard
reduction
potential
of
-0.33
V
vs.
NHE
is
involved
in
reducing
Cr
(VI).
This
indeed
provides
a
ground-breaking
route
of
remediation-by-reduction
over
a
suitable
photocatalyst.
The
first
time
reported
photocatalytic
Cr
(VI)
reduction
was
over
TiO2pho-
tocatalyst
by
Fu
et
al.
in
1998
(Fu
et
al.,
1998).
However,
application
of
TiO2photocatalyst
is
restricted
by
only
5
%
ultraviolet
light
absorption
resulting
from
the
narrow
bandgap
of
3.2
eV,
reassem-
bly
of
photogenerated
electron-hole
pairs
and
low
recovery
rate
(Fujishima
and
Honda,
1972).
In
most
of
the
present
studies
metal-
based
photocatalysts
including
CdS,
WO3,
ZnO,
Ag2S,
Fe2O3,
CuS,
CuO,
SnS2,
BiVO4,
and,
polyoxometalates,
etc
have
been
widely
employed
in
Cr
(VI)
reduction
(Cherifi
et
al.,
2021).
Nevertheless,
the
quantum
yield
of
Cr
(VI)
photoreduction
is
less
due
to
the
rapid
electron-hole
pairs
recombination,
low
recyclability
and
toxicity
caused
by
leaching
of
metals.
Consequently,
this
demands
for
the
exploration
on
metal
free
photocatalysts
which
is
economical
and
bio-compatible
for
achiev-
ing
high
Cr
(VI)
photoreduction
efficiency.
Hence,
metal-free
graphitic
carbon
nitride
(g-C3N4)
has
received
in-depth
research
due
to
its
earth-abundant
composition
(carbon
and
nitrogen)
with
strong
covalent
bonds
contributing
to
high
thermal
and
chemical
stability
(Raizada
et
al.,
2021).
Taking
into
account
the
relatively
narrow
bandgap
of
2.7
eV,
with
suitable
-1.09
eV
CB
and
+1.56
eV
VB
potentials
readily
promotes
Cr
(VI)
photoreduction
reactions
on
absorption
of
light
over
optical
wavelength
upto
460
nm
in
visible
region
(Hasija
et
al.,
2020).
More
importantly,
g-C3N4pos-
sesses
2-dimensional
sp2hybridized
-
conjugated
framework
with
six
nitrogen
lone-pair
electrons
that
increases
the
availabil-
ity
of
photogenerated
electrons
for
reduction
reaction
(Sudhaik
et
al.,
2018).
The
pathway
of
Cr
(VI)
reduction
by
pristine
g-C3N4
follows
sequential
three-electron-transfer
reaction
[Eq.
1]
or
O2
•−
mediated
reduction
process
[Eq.
2]
with
formation
of
Cr
(V)
as
the
intermediate
(Su
et
al.,
2020).
Cr (VI)+
e−→
Cr (V)+
e−→
Cr (IV)+
e−→
Cr (III)(1)
O.−
2+
Cr (VI)→
Cr (V)+
O2(2)
Notwithstanding
the
desirable
Cr
(VI)
photoreduction
efficiency
is
only
achieved
by
g-C3N4after
conquering
the
limitations
of
small
specific
surface
area,
rapid
electron-hole
pairs
recombination
which
reduces
solar
light
absorption
and
quantum
efficiency.
To
overcome
these
limitations
various
fundamental
approaches
have
been
explored
targeting
the
surface
and
interface
designing
of
g-
C3N4to
elevate
the
Cr
(VI)
photoreduction.
In
2012,
the
visible
light
Cr
(VI)
photoreduction
was
first
time
reported
in
ZnO/g-C3N4com-
posite
by
transference
of
CB
electrons
from
the
CB
of
g-C3N4to
that
of
ZnO
(Liu
et
al.,
2012).
They
demonstrated
that
g-C3N4based
het-
664

V.
Hasija
et
al.
Process
Safety
and
Environmental
Protection
152
(2021)
663–678
erojunction
compensates
the
drawbacks
of
pristine
g-C3N4through
improved
charge
separation.
2.
Research
methodology
This
review
begins
with
the
overview
of
Cr
(VI)
sources
con-
taminating
water
resources
and
conventional
methods
undertaken
in
the
sequestration
of
Cr
(VI).
We
tend
to
highpoint
the
benefi-
cial
prospects
and
mechanism
of
Cr
(VI)
photocatalytic
reduction
process.
Additionally,
a
brief
explanation
on
fundamental
prop-
erties
and
drawbacks
of
g-C3N4semiconductor
comprises
of
the
introduction
Section
1.
The
subsequent
Section
3
of
the
review
elaborates
surface-interface
strategies
for
engineering
g-C3N4
including
heterojunction
formation,
incorporating
dopants,
struc-
tural
regulation,
vacancy
construction,
and
co-catalyst
loading
for
enhanced
Cr
(VI)
reduction
efficiency.
The
influential
operating
parameters
such
as
solution
pH,
g-C3N4dosage,
Cr
(VI)
con-
centration,
temperature,
light
source,
organic
acid
additives
and
co-existing
ions
effecting
Cr
(VI)
reduction
efficiency
are
discussed
in
Section
4.
Lastly,
the
article
summarizes
emerging
challenges
and
future
perspectives
of
engineered
g-C3N4-based
photocatalysts
for
large-scale
Cr
(VI)
reduction.
2.1.
In-depth
analysis
The
scope
of
the
review
is
to
delineate
the
updated
research
on
Cr
(VI)
removal
from
contaminated
wastewater
in
a
cleaner
viewpoint.
To
manage
the
large
set
of
reported
literature,
we
have
followed
a
systematic
research
methodology.
Initially,
research
questions
were
formulated
to
identify
the
need
of
this
review.
The
following
research
questions
are
the
pillars
to
design
an
informative
in-depth
review
targeted
to
overcome
the
societal
con-
finements.
•What
is
the
current
situation
of
Cr
(VI)
removal
techniques
and
their
limitations?
•What
are
the
strategies
to
engineer
g-C3N4photocatalyst
for
enhanced
Cr
(VI)
reduction?
•What
influencing
reaction
parameters
determine
the
Cr
(VI)
reduction
efficiency?
2.2.
Searching
and
reporting
of
literature
Several
previously
published
review
articles
have
provided
an
account
of
research
advancements
of
g-C3N4in
variant
aspects,
for
instance
photocatalytic
water
splitting
(Zhang
et
al.,
2017),
CO2
reduction
(Liu
et
al.,
2016),
and
pollutant
degradation
(Teixeira
et
al.,
2018).
However,
no
researchers
have
focused
to
gather
the
research
progress
on
photocatalytic
Cr
(VI)
reduction
utiliz-
ing
g-C3N4.
As
above-mentioned,
a
bibliometric
exploration
was
carried
out
in
the
Scopus
database
with
keywords
“graphitic
car-
bon
nitride”
and
“photocatalytic
Cr
(VI)
reduction”.
It
was
reviewed
that
nearly
about
290
research
articles
were
published
from
2012
to
till
date
as
illustrated
in
[Fig.
1a].
The
most
assessed
literature
was
published
in
Journal
of
Colloids
and
Interface
Science,
Journal
of
Photochemistry
and
Photobiology
A,
Chemosphere,
Nano
today,
Journal
of
Cleaner
Production
and
Chemical
Engineering
Journal.
Out
of
the
available
data,
the
articles
containing
mechanistic
insight
about
the
photocatalytic
Cr
(VI)
reduction
via
g-C3N4were
included
in
the
discussion
of
this
review.
Also,
the
reports
involving
dual
applicability
of
g-C3N4were
not
ignored
but
the
entire
review
has
revolved
specifically
around
Cr
(VI)
reduction.
After
screening
the
reference
content,
we
noticed
substantial
ground
breaking
surge
up
research
on
modification
strategies
for
engineering
g-C3N4as
presented
in
[Fig.
1b].
On
the
whole,
the
proposed
review
reflects
Fig.
1.
(a)
Graphical
illustration
of
publications
from
Scopus
data
base
on
photocat-
alytic
Cr
(VI)
reduction
via
graphitic
carbon
nitride
in
the
years
2012
to
March
2021,
and
(b)
Pie
chart
representing
literature
review
on
engineered
graphitic
carbon
nitride
using
the
mentioned
modification
strategies.
g-C3N4-mediated
photocatalysis
as
non-pollutant,
economic,
and
efficient
Cr
(VI)
reduction
technique.
3.
Surface
and
interface
engineering
of
g-C3N4
The
surface
and
interface
modification
of
g-C3N4is
currently
considered
to
be
the
most
effective
route
for
enhanced
Cr
(VI)
pho-
toreduction
efficiency.
For
example,
formation
of
heterojunctions
lowers
electron
hole
pair
recombination
rate
and
doping
of
g-C3N4
results
in
bandgap
tuning,
enlarged
surface
area
and
extended
solar
spectrum
absorption.
Generally,
the
vacancy
construction
alters
the
electronic
structure,
and
co-catalyst
loading
accelerates
charge
car-
rier
migration.
The
other
method
to
improve
the
photoactivity
of
g-C3N4is
structural
regulation
like
morphologies,
crystallinity,
size
and
specific
surface
area.
These
modification
strategies
ultimately
engineer
g-C3N4based
photocatalysts
to
achieve
efficient
Cr
(VI)
reduction.
3.1.
Heterojunction
formation
One
of
the
promising
engineering
strategies
to
mitigate
the
limitations
of
pristine
g-C3N4semiconductor
is
via
heterojunc-
tion
formation.
The
core
of
heterostructures
designing
involves
association
of
two
semiconductors
of
well-matched
band
align-
ment
positions.
Based
on
the
band
alignment,
between
CB
and
VBs
the
type-II
conventional
heterostructures
possesses
suitable
redox
potentials
and
work
function
difference
for
regulation
of
effective
charge
transfer.
In
type-II
heterojunctions
the
migration
of
pho-
toinduced
electrons
with
a
higher
CB
potential
of
semiconductor
665

V.
Hasija
et
al.
Process
Safety
and
Environmental
Protection
152
(2021)
663–678
I
to
lower
CB
potential
semiconductor
II.
Also,
there
is
simulta-
neous
migration
of
holes
from
a
lower
VB
of
semiconductor
II
to
semiconductor
I
with
a
higher
VB
potential
(Hasija
et
al.,
2021c).
The
traditional
type-II
staggered
alignment
was
firstly
inves-
tigated
in
CdS/TiO2heterostructure
by
Serpone
et
al.,
in
1984
demonstrative
of
improved
interparticle
electron
transfer
(Serpone
et
al.,
1984).
The
terminology
“type-II”
originates
from
the
presence
of
two
other
types
of
junctions
i.e.,
straddling
type-I
and
broken
type-III
heterostructure.
Both
the
type-I
and
type-III
heterojunc-
tions
are
discarded
from
discussion
due
to
their
inconsiderate
band
alignments,
leading
to
rapid
electron
hole
pairs
recombination.
In
stark
contrast,
a
strong
internal
electric
field
(IEF)
at
the
interface
of
type-II
heterojunction
is
responsible
for
charge
separation
and
the
potential
difference
determines
the
charge
migration
to
the
respective
reactive
photocatalytic
surface
(Hasija
et
al.,
2021b).
•g-C3N4based
type-II
heterojunction
systems
The
intimate
contact
between
two
semiconductor
photocat-
alysts
in
type-II
staggered
configuration
follows
the
electron
transference
route
from
higher
to
lower
CB
and
reverse
pathway
for
photogenerated
holes
(Soni
et
al.,
2021;).
For
the
pristine
g-C3N4,
the
photogenerated
electrons
in
the
CB
has
a
tendency
to
re-bounce
back
to
VB
due
to
coulombic
effect,
causing
undesirable
quench.
Since,
the
electrons
in
the
CB
have
potential
of
-1.1
eV,
which
is
much
lower
than
most
of
the
other
semiconductors
leads
to
favourable
and
rapid
migration
of
photoexcited
electrons
to
the
CB
of
adjacent
semiconductor
with
a
higher
potential
(Yao
et
al.,
2019).
Simultaneously,
the
photogenerated
holes
follows
reverse
transfer-
ence
route.
As
explained
in
[Fig.
2a]
the
charge
transfer
mechanism
in
p-n
heterostructure
Ag3PO4/g-C3N4composite
followed
migra-
tion
of
photogenerated
electrons
from
the
CB
of
g-C3N4(-1.12
eV)
to
CB
of
Ag3PO4(0.45
eV)
facilitated
by
IEF.
This
resulted
in
dual
pho-
toreduction
of
94.1
%
Cr
(VI)
by
photogenerated
electrons
on
CB
of
Ag3PO4and
O2
•−released
by
g-C3N4(Shun
et
al.,
2020).
The
high-
est
electron
hole
pair
separation
in
g-C3N4/SnS2heterojunction
was
indicative
by
electron
impedance
spectra
(EIS)
results
[Fig.
2b]
with
smallest
Nyquist
semicircle
for
30
wt
%
g-C3N4/SnS2.
This
was
in
accordance
to
the
charge
transfer
mechanism
route
[Fig.
2c]
where
generated
IEF
in
space
charge
region
directs
the
migration
of
pho-
togenerated
electrons
from
g-C3N4to
CB
of
SnS2,
resulting
in
99
%
surface
adsorbed
Cr
(VI)
reduction
efficiency
(Sun
et
al.,
2014a).
An
increased
specific
surface
area
determined
by
Brunauer-Emmett-
Teller
(BET)
theory,
is
crucial
for
reduction
of
adsorbed
Cr
(VI).
This
was
ensured
in
a
study
where
the
BET
analysis
results
showed
an
increase
in
surface
area
from
35.2,
45.7
to
97.2
m2g-1 for
pristine
g-C3N4,
CeO2and
g-C3N4/CeO2as
shown
in
Fig.
2d.
An
increase
in
surface
area
prevented
the
stacking
of
g-C3N4sheets
with
an
improved
96
%
Cr
(VI)
removal
efficiency,
whereas
only
27
%
and
45
%
was
obtained
over
pristine
g-C3N4and
CeO2,
respectively
(Barathi
et
al.,
2021).
Similarly,
specific
surface
area
of
pristine
g-C3N4was
improved
to
101.61
m2g-1 from
15
m2g-1 on
coupling
with
Co-
Fe
layered
double
hydroxide
(LDH)
with
100
%
Cr
(VI)
removal
efficiency
within
90
min
of
visible
light
irradiations
(Ou
et
al.,
2020).
•g-C3N4based
Z-scheme
photocatalysts
Certainly,
g-C3N4based
type-II
photocatalytic
system
sup-
presses
electron
hole
pair
recombination
but
at
the
expense
of
redox
potential,
lowering
the
reductive
ability.
Consequently,
development
of
Z-scheme
photocatalytic
system
has
emerged
from
biomimicking
artificial
photosynthesis
process
(Hasija
et
al.,
2019b;
Fu
et
al.,
2018).
In
an
artificial
Z-scheme
photocatalytic
system
there
is
involvement
of
minimum
two
semiconductors
in
staggered
band
structure
configuration.
The
first
traditional
Z-scheme
pho-
tocatalyst
was
investigated
by
Bard
in
1979,
explaining
the
fermi
level
alignments
in
n-type
semiconductors
with
a
shuttle
redox
mediator
in
liquid
phase
(Bard,
1979).
The
commonly
employed
redox
mediator
includes
electron
acceptor
with
corresponding
donor
pairs
i.e.;
IO3−/I−,
NO3−/NO2−,
Fe+3/Fe+2 ,
[Co(bpy)3]3+/2+,
and
[Co(phen)3]3+/2+ which
serves
as
a
wheel
between
two
semi-
conductors
without
any
immediate
contact.
The
process
proceeds
via
reduction
of
electron
acceptor
with
an
appropriate
reduction
potential
by
the
CB
photogenerated
electrons
into
electron
donor.
Simultaneously,
the
electron
donor
possessing
a
suitable
oxidation
potential
is
oxidized
by
the
VB
photogenerated
holes
into
electron
acceptor
(Li
et
al.,
2016;
Ong
et
al.,
2016)
However,
the
aqueous
shuttle
redox
mediated
Z-scheme
system
has
several
limitations
mainly
restricted
functioning
of
process
in
only
liquid
phase
(Natarajan
et
al.,
2018).
It
is
difficult
to
main-
tain
the
long-term
chemical
stability
of
redox
mediators
in
a
wide
pH
range.
In
most
case
each
electron/donor
redox
pairs
have
its
own
suitable
operating
pH
(Li
et
al.,
2021a,
2021b).
For
instance,
the
Z-scheme
system
involving
IO3−/I−redox
mediator
proceeds
through
six-electron
process
(IO−
3+
3H2O
+
6e−↔
I−+
6OH−)
in
basic
or
neutral
conditions
which
necessitates
the
use
of
effectual
co-catalysts
to
facilitate
multielectron
reaction
(Abe
et
al.,
2001).
Besides,
unwanted
side
reactions
in
acidic
conditions
generate
I3−which
is
difficult
to
reduce
and
get
accumulated
in
the
solu-
tion
during
the
process.
In
addition,
the
electron
acceptor/donor
redox-involved
backward
reactions
inevitably
reduce
the
number
of
available
charge
carriers
for
targeted
photocatalytic
reactions
(Liao
et
al.,
2021).
Precisely,
the
moderately
low
potential
values
of
liquid-phase
redox
mediators
expedite
the
electron
donor/acceptor
ability
of
the
system
(Kumar
et
al.,
2021).
The
substantial
drawbacks
and
unsatisfactory
photocatalytic
efficacy
of
traditional
Z-scheme
led
to
the
discovery
of
second
generation
of
Z-scheme
which
involves
solid-state
mediator
as
electron
transference
bridge
(Zhou
et
al.,
2014).
In
g-C3N4based
all-solid-state
(ASS)
Z-scheme,
noble
metals
like
Au,
Ag,
and
car-
bon
family;
graphene,
graphene
oxide,
reduced
graphene
oxide,
fullerene,
and
carbon
nano
tubes
are
employed
as
solid
electron
mediators
for
rapid
photocarriers
separation
(Xia
et
al.,
2019).
How-
ever,
the
limited
exploration
of
ASS
photocatalytic
systems
is
due
to
random
adherence
of
solid
conductor
onto
the
surface
of
pho-
tocatalyst
which
changes
the
functionality
from
electron
mediator
to
co-catalysts
(Sharma
et
al.,
2020).
In
addition,
noble
metals
and
carbonaceous
conductors
often
suffers
from
light
shielding
effect
owing
to
their
substantial
light
absorption
in
the
solar
spectrum
(Singh
et
al.,
2020b).
Consequently,
the
ASS
Z-scheme
photocat-
alytic
system
is
unsatisfying
and
has
paved
the
way
to
mediator
free
direct
Z-scheme
system.
The
constant
efforts
to
conquer
the
problems
associated
with
carriers
passing
through
a
channel
pro-
vided
by
electron
mediator,
a
mediator-free
Z-scheme
attained
widespread
attention
since
Yu
et
al.
proposed
the
concept
of
direct
Z-scheme
system
involving
TiO2and
g-C3N4photocatalysts
(Yu
et
al.,
2013).
The
obscurities
and
inadequacies
in
type-II
hetero-
junction
system
were
evidenced
in
the
comparative
study
of
type-II
and
Z-scheme
mechanism
in
g-C3N4-Bi12GeO20 heterostructure.
As
shown
in
type-II
mechanism
[Fig.
3a]
the
electrons
in
the
CB
of
g-C3N4(-1.12
eV)
migrate
to
CB
of
Bi12GeO20 (-0.25
eV)
which
is
incompetent
to
reduce
O2to
O2
•−.
On
the
contrary,
in
direct
Z-
scheme
photocatalytic
system
[Fig.
3b]
the
built-in-IEF
between
the
negatively
charged
Bi12GeO20 surface
and
positively
charged
sur-
face
of
g-C3N4resulted
in
effective
charge
carrier
separation
and
eventual
release
of
O2.- for
100
%
Cr
(VI)
reduction
within
3
h
visi-
ble
light
irradiations
(Wan
et
al.,
2017).
Although
the
configuration
of
Z-scheme
photocatalytic
system
is
similar
to
staggered
type-II,
yet
the
charge
transfer
route
is
different
and
thermodynamically
more
favourable.
The
density
function
theory
(DFT)
calculations
and
Bader
charge
analysis
were
studied
by
Hong
et
al.,
(Hong
et
al.,
2019)
to
support
the
direct
Z-scheme
inter
cross-sectional
electron
666

V.
Hasija
et
al.
Process
Safety
and
Environmental
Protection
152
(2021)
663–678
Fig.
2.
(a)
Diagrammatic
representation
of
plausible
mechanism
for
Cr
(VI)
photoreduction
over
the
Ag3PO4/g-C3N4composite
under
visible
light
irradiation
(␭
>
420
nm)
(Reproduced
with
permission
from
reference.
(Shun
et
al.,
2020).
Copyright
with
order
license
no.
5011410013931),
(b)
Electrochemical
impedance
spectroscopy
(EIS)
profiles
of
pristine
g-C3N4,
SnS2,
and
g-C3N4/SnS2composites,
(c)
Photocatalytic
reduction
mechanism
for
aqueous
Cr
(VI)
over
g-C3N4/SnS2type-II
heterojunction
under
visible
light
irradiation
(␭
>
420
nm)
(Reproduced
with
permission
from
reference.
(Sun
et
al.,
2014a).
Copyright
with
1104159-1.),
(d)
Brunauer-Emmett-Teller
(BET)
N2adsorption
and
desorption
isotherm
graph
of
g-C3N4,
CeO2and
15
%-CeO2/g-C3N4photocatalysts.
(Reproduced
with
permission
from
reference
(Barathi
et
al.,
2021).
Copyright
with
order
license
no.
5012411089267).
Fig.
3.
Schematic
illustration
of
charge
migration
in
Bi12GeO20 /g-C3N4heterojunction
(a)
conventional
type-II
(b)
Direct
Z-scheme
mechanism
(Reproduced
with
permission
from
reference.
(Wan
et
al.,
2017).
Copyright
with
order
license
no.
5012420225880),
(c)
Band
structure
representation
and
direct
Z-scheme
photocatalytic
Cr
(VI)
reduction
mechanism
over
g-C3N4/UiO-66
MOF
under
visible-light
irradiation
(Reproduced
with
permission
from
reference.
(Hong
et
al.,
2019).
Copyright
with
order
license
no.
5012420608590),
(d)
Photoluminescence
(PL)
spectra
of
MIL-101(Fe),
g-C3N4and
MIL-101(Fe)/g-C3N4photocatalyst
(Reproduced
with
permission
from
reference.
(Zhao
et
al.,
2020).
Copyright
with
order
license
no.
5012420902371).
667

V.
Hasija
et
al.
Process
Safety
and
Environmental
Protection
152
(2021)
663–678
transfer
route
between
UIO-66
metal
organic
framework
(MOF)
and
g-C3N4.
The
results
revealed
combination
of
C
atom
in
UIO-66
with
N
and
C
atoms
of
g-C3N4with
a
decrease
of
positive
charge
on
C
atom
of
g-C3N4from
1.53
to
0.92.
Whereas,
an
increase
in
posi-
tive
charge
from
0.11
to
0.48
on
C
atom
of
UIO-66
which
is
clearly
indicative
of
photogenerated
electrons
migration
from
CB
of
UIO-
66
to
VB
of
g-C3N4(-0.91
eV)
[Fig.
3c],
mainly
responsible
for
the
99
%
Cr
(VI)
removal
efficiency.
Similarly,
coupling
of
g-C3N4with
MIL-
101
(Fe)
MOF
led
to
92.6
%
Cr
(VI)
reduction,
whereas
the
pristine
g-C3N4and
MIL-101
(Fe)
resulted
in
only
48.1
and
53.8
%
within
60
min
of
540
nm
of
visible
irradiations.
These
results
were
well-
in
agreement
with
the
photoluminescence
(PL)
spectra
[Fig.
3d]
showing
the
highest
PL
emission
peak
intensity
for
pristine
g-C3N4
indicative
of
maximum
electron-hole
pairs
recombination.
How-
ever,
after
the
heterojunction
formation
with
MIL-101
(Fe),
a
lower
PL
peak
is
observed
for
MIL-101
(Fe)/
g-C3N4composite
signifi-
cant
of
enhanced
charge
separation
(Zhao
et
al.,
2020).
The
direct
Z-scheme
photocatalysts
functions
as
a
sieve
for
consumed
pho-
toexcited
electron
hole
pairs
with
low
redox
potential,
improving
redox
ability
and
reducing
the
light-shielding
effect.
3.2.
Doping
Doping
has
substantial
influence
on
electronic
structure,
surface
morphology
and
porosity
of
the
semiconductor.
The
incorporation
of
an
elemental
dopant
changes
the
electronic
structure
which
low-
ers
the
bandgap
energy
and
expands
light
absorption.
The
primary
purpose
of
doping
is
to
improve
the
electron
hole
pair
separa-
tion
through
altering
electronic
localization,
increasing
in-planar
electron
density,
and
extending
the
-conjugated
system
(Hasija
et
al.,
2019a).
Bandgap
tuning
is
achieved
through
incorporating
dopant
into
the
highly
conjugated
g-C3N4-carbon
matrix
induc-
ing
charge
polarization
and
unbalanced
electron
distribution
due
to
the
difference
in
electronegativity
between
dopant
and
host
C,
N
atoms
(Jiang
et
al.,
2017).
The
tuned
g-C3N4bandgap
is
due
to
orbital
hybridization
between
impurity
level
of
dopant
and
molecular
orbital
of
g-C3N4which
improves
charge
separation
and
broadens
solar
absorption
range.
The
DFT
computations
revealed
formation
of
new
energy
bandgap
below
the
CB
of
g-C3N4by
Br
dopant
inferring
n-type
doping
by
substitution
of
N
atom
by
Br
in
g-C3N4.
The
density
of
states
graph
[Fig.
4a,
b]
was
significant
of
increased
energy
density
of
Br-doped
g-C3N4due
to
electron
delocalization,
contributing
to
bandgap
lowering
from
0.81
eV
to
0.46
eV.
The
improved
Cr
(VI)
photoreduction
from
25.
1
%
to
61.6
%
was
attributed
to
the
release
of
electrons
and
OH•radicals
(Wang
et
al.,
2020a).
The
strong
influ-
ence
of
dopant
incorporation
on
electronic
structure
of
g-C3N4was
found
in
S-doped
g-C3N4.
The
VB-XPS
spectra
[Fig.
4c]
depicted
up-shifting
of
VB
potential
by
0.57
eV
from
1.26
eV
of
1
%
S-doped
g-C3N4.
Besides
incredible
perk
of
narrowed
bandgap,
an
extended
near
infrared
absorption
upto
700
nm
was
obtained
in
S-doped
g-C3N4exhibiting
100
%
Cr
(VI)
removal
efficiency
after
3
h
light
illumination
(Cui
et
al.,
2018b).
An
increased
specific
surface
area
of
the
photocatalysts
improves
the
Cr
(VI)
reduction
photocatalytic
efficacy
by
providing
a
greater
number
of
active
sites
for
photogenerated
electrons.
The
synergistic
influence
of
different
morphologies,
enlarged
surface
area
and
porosity
was
observed
in
a
comparative
study
of
porous
nanosheets
(PNs)
and
bulk
S-doped
g-C3N4deposited
on
reduced
graphene
oxide.
The
Cr
(VI)
photoreduction
results
indicated,
85.2
%
of
Cr
(VI)
removal
over
S-doped
g-C3N4/5
%
reduced
graphene
oxide
PNs,
whereas
73.9,
51.5,
36.9
and
14.2
%
Cr
(VI)
were
reduced
by
S-doped
g-C3N4PNs,
g-C3N4/reduced
graphene
oxide
PNs,
g-
C3N4PNs,
and
bulk
g-C3N4,
respectively.
The
essence
of
high
Cr
(VI)
degradation
rate
was
due
to
increment
in
specific
surface
area
[Fig.
4d]
upto
188.5
m2/g
in
S-doped
g-C3N4deposited
on
reduced
graphene
oxide
PNs
higher
than
that
of
nanosheets
(92.3
m2/g),
and
bulk
g-C3N4(4.885
m2/g).
Fig.
4d
inset
represented
less
than
4
nm
pores
size
distribution
type-IV
curve
obtained
by
Barrett-Joyner-
Halenda
(BJH)
indicative
of
mesoporous
region
with
hysteresis
loop
at
high
relative
pressure
for
S-doped
g-C3N4deposited
on
reduced
graphene
oxide
PNs
(Zheng
et
al.,
2020).
Zhang
and
co-workers
reported
90
%
and
20.36
%
Cr
(VI)
reduc-
tion
over
O-doped
g-C3N4,
and
pristine
g-C3N4within
150
min
of
visible-light
illumination.
The
excellent
Cr
(VI)
reduction
abil-
ity
was
attributed
to
the
structural
defects
and
irregularity
in
the
uniformity
of
g-C3N4nanoplates
on
introduction
of
O
dopant
as
confirmed
by
XRD
diffraction
peak
and
TEM
images.
The
non-
uniformity
in
the
stacked
structure
of
g-C3N4was
beneficial
for
lowering
the
band
gap
from
2.72
eV
to
1.61
eV
with
shift
in
absorption
wavelength
upto
480
nm.
In
addition,
O
dopant
largely
increased
the
BET
surface
area
of
pristine
g-C3N4from
1.6
m2/g
to
11.6
m2/g
and
improvement
in
porosity
from
30.6
nm
to
37.6
nm
(Zhang
et
al.,
2021).
In
another
study,
the
diverse
morphology
of
g-C3N4was
explored
wherein,
hollow
microspheres
of
O-doped
g-
C3N4were
obtained
via
solvothermal
method
which
resulted
in
75
%
Cr
(VI)
within
3
h
of
illumination.
It
was
inferred
that
incorpora-
tion
of
O
dopant
into
the
g-C3N4heptazine
lattice
caused
structural
alterations.
For
example,
optimized
delocalization
of
-electrons
in
O-doped
g-C3N4framework
due
to
defects
induced
by
O
dopant
increased
the
light
absorption
ability
to
680
nm.
Meanwhile,
the
reduced
C
N
bond
length
by
O
doping
led
to
decrease
in
bandgap
from
2.29
to
1.87
eV
(Wang
et
al.,
2017a,
2017b).
3.3.
Structural
regulation
The
impact
of
different
atmosphere
on
the
morphology
and
structure
of
g-C3N4photocatalysts
has
been
a
vital
factor
affect-
ing
the
photocatalytic
activity.
For
instance,
H2impulsed
porous
g-C3N4nanosheets
exhibited
91.3
%
Cr
(VI)
removal
ability,
while
61.3
%
Cr
(VI)
was
removed
by
pristine
g-C3N4.
This
was
ascribed
to
the
rich
porosity
(pore
volume
0.52
cm3/g)
and
high
surface
area
(75
m2/g),
in
comparison
to
0.10
cm3/g
and
12
m2/g
of
pristine
g-C3N4offering
more
accessible
active
sites
for
electron
delocaliza-
tion
thus
reducing
electron
hole
pair
recombination.
Noteworthily,
the
increased
life
time
of
porous
g-C3N4charge
carriers
was
found
by
time-resolved
fluorescence
decay
spectra
[Fig.
5a]
performed
at
460
nm.
It
was
inferred
that
both
porous
and
pristine
g-C3N4
decayed
in
exponential
form
with
average
lifetime
decay
of
4.76
ns
and
3.89
ns,
respectively
(Chen
et
al.,
2021).
Like-wise
anneal-
ing
g-C3N4in
N2atmosphere
raised
the
CB
potential
to
-1.34
eV
from
-1.15
eV
making
it
more
suitable
for
release
of
O2.−radi-
cals
as
interpreted
by
the
X-ray
photoelectron
spectroscopy
(XPS)
and
electron
spin
resonance
(ESR)
spectra.
Also,
the
peaks
at
576.8
and
586.4
eV
were
attributed
to
Cr
2p3/2 and
Cr
2p1/2 of
Cr
(III),
respectively
confirmative
of
91
%
Cr
(VI)
reduction
into
less
toxic
Cr
(III)
after
120
min
of
visible
light
irradiations
(Wang
et
al.,
2019a).
He
et
al.,
prevented
agglomeration
of
g-C3N4by
homogenous
dis-
persion
of
diatomite
micro-disks
followed
by
loading
of
plasmonic
Ag/AgCl
nanoparticles,
which
resulted
in
46.3
times
higher
Cr
(VI)
reduction
due
to
plasmonic
effect
(He
et
al.,
2019).
Notably,
crys-
tallinity
optimization
of
bulk
g-C3N4is
a
beneficial
approach
to
reduce
the
recombination
centers
for
charge
carriers.
The
con-
trolled
etching
with
7
%
NaNO2solution
produced
highly
condensed
g-C3N4nanosheets
which
displayed
78
%
Cr
(VI)
photocatalytic
reduction,
whereas
ability
of
bulk
g-C3N4was
merely
25
%
after
120
min
of
visible
light
irradiations
(Cui
et
al.,
2019).
Accord-
ingly,
structural
regulation
of
g-C3N4is
explored
to
overcome
the
shortcomings
of
nano-sized
powders
and
obtain
large
surface
area
morphologies.
668

V.
Hasija
et
al.
Process
Safety
and
Environmental
Protection
152
(2021)
663–678
Fig.
4.
Density
of
states
(DOS)
calculated
through
density
functional
theory
(DFT)
(a)
pristine
g-C3N4,
(b)
Br-doped
g-C3N4(Reproduced
with
permission
from
reference.
(Wang
et
al.,
2020a).
Copyright
with
order
license
no.
5012421171445),
(c)
X-ray
photoelectron
spectroscopy
(XPS)-valence
band
spectra
of
pristine
g-C3N4and
1.0
%
S-doped
g-C3N4(Reproduced
with
permission
from
reference.
(Cui
et
al.,
2018b).
Copyright
with
order
license
no.
5012421415507),
(d)
N2adsorption-desorption
isotherms
and
the
corresponding
pore
size
distribution
(the
inset)
calculated
by
the
Barrett-Joyner-Halenda
(BJH)
method
for
bulk
g-C3N4,
g-C3N
porous
nanosheets
and
S-doped
g-C3N4/5
%
reduced
graphene
oxide
porous
nanosheets
(Reproduced
with
permission
from
reference.
(Zheng
et
al.,
2020).
Copyright
with
order
license
no.
5012430176848).
3.4.
Construction
of
nitrogen
vacancies
The
presence
of
N-vacancies
in
g-C3N4lattice
have
the
advan-
tages
of
narrowing
the
band
gap
by
modulating
electronic
structure,
expanding
visible
light
response,
and
reducing
electron
hole
pair
recombination
(Wang
et
al.,
2020d).
For
the
first
time
reported,
N-vacancy
induced
g-C3N4was
prepared
by
formalde-
hyde
mediated
chemical
reduction.
N-vacancy
occurred
at
the
N(C)3lattice
sites,
confirmed
by
the
N1
s
XPS
spectrum
[Fig.
5b]
with
binding
energies;
398.58
eV,
399.37
eV,
and
401.10
eV
correspond-
ing
to
sp2hybridized
atom
(C
N
=
C),
tertiary
N
atom
N(C)3and
uncondensed
terminal
N
H
species,
respectively.
Furthermore,
the
atomic
percentage
of
C/N
increased
from
0.947
to
0.978
indica-
tive
of
the
existence
of
nitrogen
defects,
resulting
in
58
%
higher
Cr
(VI)
photoreduction
than
bulk
g-C3N4and
P-25
TiO2(Ma
et
al.,
2020).
The
synergistic
effect
of
doping
and
vacancy
generation
has
been
well
explored.
For
example,
P,
S
co-doped
g-C3N4with
feeble
N-vacancy
were
created
by
supramolecular
self-assembly
method.
The
band
structure
of
pristine
g-C3N4was
modulated
on
genera-
tion
of
N-vacancies
as
the
VB
potential
raised
to
1.41
V
from
1.61
V.
Whereas,
CB
potential
lowered
to
-0.97
V
from
-1.18
V
promoting
the
generation
of
O2.−radicals
responsible
for
73.87
%
Cr
(VI)
reduc-
tion
within
2
h
of
420
nm
irradiations
(Yu
et
al.,
2021).
A
remarkable
Cr
(VI)
photoreduction
efficiency
(95
%)
was
obtained
using
P,
Mo-
co-doped
g-C3N4with
N-vacancy,
while
pristine
g-C3N4exhibited
only
22
%
in
2
h
of
540
nm
irradiations.
This
was
ascribed
to
the
uplifting
of
VB
maximum
from
2.12
eV
to
1.50
eV
was
observed
due
to
shallow
N-vacancy
states
above
and
partly
overlapping
with
VB
of
P,
Mo-co-doped
g-C3N4thus
lowering
the
bandgap
from
2.73
to
2.10
eV.
In
evidence,
the
PL
spectra
emission
peak
at
445
nm
is
indicative
of
minimum
charge
carrier
recombination
due
to
the
electron
re-localization
at
the
terminal
N-sites.
This
was
well-in
agreement
with
transient
photocurrent
response
graph
as
depicted
in
[Fig.
5c]
with
maximum
value
for
P,
Mo-co-doped
g-C3N4sugges-
tive
of
high
charge
mobility
and
separation
(Chen
et
al.,
2019).
The
generated
N-vacancies
offers
efficient
separation
of
charge
carriers,
reduced
bandgap,
improved
electron
mobility,
and
subsequently
contribute
to
an
enhanced
photoreduction
performance.
3.5.
Co-catalyst
loading
The
strong
interfacial
electronic
interactions
between
co-
catalyst-semiconductor
tends
to
extract
the
charges
out
of
the
photocatalytic
surface
and
serve
as
active
sites
for
molecule
oxi-
dation
and
reduction
(Maleh
et
al.,
2020).
Remarkably,
a
reductive
co-catalyst
integration
is
advantageous
for
g-C3N4to
extend
the
charge
carrier
lifetime
to
facilitate
Cr
(VI)
photoreduction
pro-
cess.
A
clear
insight
into
the
role
of
co-catalysts
presence
was
revealed
in
Pd
co-catalysed
g-C3N4,
where
Pd
captured
the
pho-
togenerated
electrons
from
CB
of
g-C3N4,
with
99.6
%
Cr
(VI)
reduction.
A
comparative
study
of
the
variant
shapes
of
Pd
i.e.;
nano-cones,
nanocubes
and
Pd-black
(powder)
was
also
conducted
and
found
22.6,
21.6,
and
7.4
m2/g
BET
surface
area.
This
was
consistent
with
the
maximum
electron
hole
pair
separation
in
Pd
nano-cones/g-C3N4<
Pd
nanocubes/g-C3N4<
Pd-black
(powder)/
g-C3N4<
pristine
g-C3N4confirmed
by
the
PL
and
EIS
spectra
(Wu
et
al.,
2019).
The
fine
distribution
of
sulfur
nanoparticles
(SNPs)
of
particle
size
(2−7
nm)
on
the
surface
of
g-C3N4prevented
the
aggregation
of
g-C3N4nanosheets
and
resulted
in
99
%
Cr
(VI)
reduction
efficiency
via
the
following
eqs.
[3,
4]
SNPs/g
−
C3N4+
h
→
h+
VB +
e−
CB (3)
HCrO−
4+
e−
CB +
7H+→
7H2O
+
Cr+3(4)
As
we
know,
g-C3N4lacks
the
potential
to
generate
OH•radical,
as
the
required
potential
of
+2.38
V
for
OH•/OH
is
more
positive
669

V.
Hasija
et
al.
Process
Safety
and
Environmental
Protection
152
(2021)
663–678
Fig.
5.
(a)
Time-resolved
fluorescence
decay
spectra
monitored
at
460
nm
by
time-correlated
single-photon
counting
of
g-C3N4and
porous
g-C3N
nanosheets
(Reproduced
with
permission
from
reference.
(Chen
et
al.,
2021).
Copyright
with
order
license
no.
501243681334),
(b)
X-ray
photoelectron
N
(1
s)
spectra
of
pristine
g-C3N4and
nitrogen
deficient
g-C3N4(Reproduced
with
permission
from
reference.
(Ma
et
al.,
2020).
Copyright
with
order
license
no.
5012430875326),
(c)
Transient
photocurrent
responses
of
pristine
g-C3N4,
P-doped
g-C3N4,
Mo-doped
g-C3N4,
and
P,
Mo
co-doped
g-C3N4(Reproduced
with
permission
from
reference.
(Chen
et
al.,
2019).
Copyright
with
order
license
no.
5012431041401),
(d)
Mechanistic
viewpoint
of
the
charge
migration
route
followed
by
g-C3N4co-catalysed
sulphur
nanoparticles
for
photocatalytic
reduction
of
Cr
(VI)
to
Cr
(III)
(Reproduced
with
permission
from
reference
(Bankole
et
al.,
2021).
Copyright
with
order
license
no.
5012431230134).
than
VB
of
SNPs
(1.45
eV).
However, •CO2−(1.9
V)
are
generated
through
consumption
of
holes
by
formic
acid
(HCOOH)
to
oxidize
HCrO4−as
explained
in
Eqs.
(5,
6)
2HCOO−+
h+
VB →
CO2+
H2+
CO.−
2(5)
HCrO−
4+
CO.−
2+
5H+→
Cr+3+
CO2+
H2O
(6)
The
above
dual
redox
mechanism
of
Cr
(VI)
to
Cr
(III)
[Fig.
5d]
was
a
thermodynamically
feasible
reaction
due
to
appropriate
inter-
facial
contact
between
g-C3N4and
SNPs
co-catalyst,
facilitating
electron
migration
from
CB
of
g-C3N4to
SNPs.
The
SNPs
co-catalyst
also
increased
the
surface
area
as
confirmed
by
BET
isotherm
results
where
improved
surface
area
of
25.53
m2/g
was
obtained
for
3
%
SNPs/g-C3N4much
larger
than
4.5
m2/g
of
pristine
g-C3N4(Bankole
et
al.,
2021).
Therefore,
the
rate
of
Cr
(VI)
reduction
is
significantly
enhanced
upon
co-catalyst
deposition
over
g-C3N4due
to
its
ability
of
facile
charge
carrier
migration.
Conclusively,
the
above-explained
surface-interface
engi-
neering
strategies
for
pristine
g-C3N4fulfils
the
fundamental
requirements
of
photocatalytic
reactions,
including
the
sufficient
photon
absorption,
minimized
electron-hole
pairs
reassembly,
and
catalytic
redox
reactions.
Upon,
reviewing
the
literature
it
was
observed
that
most
effective
method
to
enhance
the
Cr
(VI)
pho-
toreduction
is
via
heterojunction
formation.
Broadly,
both
type-II
and
Z-scheme
heterojunction
systems
requires
positioning
of
two
semiconductors
in
staggered
alignment
for
rapid
electron
migra-
tion,
longer
lifetime
of
charge
carriers
and
broader
light
absorption.
Howbeit,
to
obtain
a
lowered
band
gap
for
extended
visible
light
absorption
and
enlarged
surface
area
tailoring
of
g-C3N4by
incor-
poration
of
dopants
is
opted.
The
criterion
for
doping
is
highly
selective
dependent
upon
the
nature,
size
and
electronegativity
of
dopant.
A
well-modulated
doped
g-C3N4framework
enhances
electronic
and
optical
properties.
Importantly,
g-C3N4structural
regulation
of
the
morphology
and
porosity
have
influence
on
the
photocatalytic
efficiency.
Previ-
ous
research
has
inferred
that
mesoporous
structure
prevents
the
agglomeration
of
lattice,
facilitates
mass
transfer
of
charge
carriers,
enlarges
the
specific
surface
area
and
also
shortens
the
migration
distance
of
carriers.
A
breakthrough
in
modification
strategies
was
procured
on
introduction
of
vacancies
in
pristine
g-C3N4which
contributes
in
retarding
the
electron-hole
pairs
recombination
rate
and
lowering
the
band
gap.
The
co-catalyst
loading
serves
as
elec-
tron
sink
on
the
reductive
site
of
g-C3N4for
high
efficiency
Cr
(VI)
reduction.
The
engineered
g-C3N4tends
to
resolve
the
predica-
ments
of
pristine
g-C3N4while,
simultaneously
preserving
the
stability
of
structure.
4.
Effect
of
operational
reaction
conditions
Aside
the
wide
investigation
on
the
strategies
to
improve
the
redox
abilities
of
g-C3N4photocatalyst,
many
operational
parame-
ters
considerably
influence
the
efficiency
of
Cr
(VI)
photoreduction.
A
huge
number
of
sound
literature
bring
forth
the
understanding
of
variation
in
reaction
conditions
for
faster
and
more
appreciable
reduction
of
Cr
(VI)
contaminated
waste
water.
4.1.
Solution
pH
The
indispensable
environmental
factor
for
photocatalytic
Cr
(VI)
reduction
is
the
pH
as
it
regulates
the
valence
state,
physio-
chemical
properties,
and
solubility
of
Cr
compound
in
the
solution
670

V.
Hasija
et
al.
Process
Safety
and
Environmental
Protection
152
(2021)
663–678
Fig.
6.
(a)
Distribution
of
Cr
(VI)
species
at
different
pH
values
ranging
from
acidic
to
basic
medium,
(b)
Effect
of
varying
pH
values
on
the
photoreduction
of
Cr
(VI)
in
g-C3N4/BiFeO3heterojunction,
(Reproduced
with
permission
from
reference.
(Hu
et
al.,
2019).
Copyright
with
order
license
no.
5012431395687),
(c)
Effect
of
g-C3N4
dosages
(0.1
to
0.2
gL−1)
on
Cr
(VI)
removal
rate
under
460
nm
irradiations
(Reproduced
with
permission
from
reference.
(Song
et
al.,
2019).
Copyright
with
order
license
no.
5012440136404),
(d)
Effect
of
Cr
(VI)
concentration
ranging
from
50
mgL-1 to
100
mgL-1 over
pristine
g-C3N4and
g-C3N4/Na-bentonite
composites
(Reproduced
with
permission
from
reference.
(Guo
et
al.,
2019).
Copyright
with
order
license
no.
5012440276143).
(Zhao
et
al.,
2019).
Typically,
Cr
(VI)
species
exists
as
tetrahedral
oxo
compounds
[Fig.
6a]
in
the
form
of
CrO4−2at
pH
>
6.0
and
as
HCrO4-
at
neutral
or
alkaline
(pH
2.0–6.0).
In
reducing
conditions,
Cr
(III)
is
thermodynamically
stable
and
is
dominant
at
more
acidic
(pH
<
3.0)
and
pH
>
3.5.
In
g-C3N4/BiFeO3heterojunction
the
Cr
(VI)
ion
removal
efficiency
[Fig.
6b]
was
100
%
at
pH
2,
whereas
at
pH
8
only
35
%
of
Cr
(VI)
ions
were
reduced
after
120
min
of
visible
light
irradiations.
It
was
inferred
that
abundant
H+(acidic
medium)
promotes
reduction
by
photogenerated
electrons
according
to
eq.
[7]
HCrO−
4+
7H++
3e−
CB →
Cr+3+
4H2O
(7)
Whereas,
under
alkaline
conditions,
the
existence
of
electrostatic
repulsions
between
g-C3N4/BiFeO3heterojunction
and
Cr
(VI)
inhibited
the
interaction
with
photogenerated
electrons,
thereby
reducing
the
Cr
(VI)
removal
rate
(Hu
et
al.,
2019).
In
single
com-
ponent
porous
g-C3N4system
(Wei
et
al.,
2017),
the
facile
action
of
photogenerated
electrons
over
Cr2O7−2occurred
in
acidic
con-
ditions.
The
decreased
electron
cloud
density
of
oxygen
atoms
in
Cr2O7-2 led
to
the
reduction
of
Cr
(VI)
into
Cr
(III)
by
H+when
pH
was
adjusted
to
3
using
dil.
H2SO4
2Cr2O−2
7+
12e−+
16H+→
4Cr+3+
8H2O
+
3O2(8)
However,
without
adjusting
pH
(neutral
solution),
aqueous
Cr
(VI)
could
not
be
reduced
by
photogenerated
electrons
due
to
the
high
electron
density
of
the
oxygen
atoms
eq.
[9].
2Cr2O−2
7+
12e−→
4CrO−
2+
3O2(9)
The
similar
pH-dependent
Cr
(VI)
reduction
was
observed
in
a
study
involving
g-C3N4-palygorskite
nano-network
structure
of
Cr
(VI)
reduction
was
performed
at
different
pH
values
of
2,
4,
6,
8
and
10
over.
It
was
found
that
Cr
(III)
precipitated
as
Cr
(OH)3on
the
photocatalytic
surface
at
higher
pH
values.
The
maximum
Cr
(VI)
reduction
efficiency
of
85.1
%
was
obtained
at
pH
=
2
(Zhang
et
al.,
2019).
In
summary,
the
effect
of
solution
pH
can
be
concluded
as
follows:
•The
predominant
reduction
of
Cr
(VI)
is
in
acidic
medium
and
a
decrease
in
pH
tends
to
improve
the
reducibility
of
Cr
(VI).
•From
the
simulation
studies
it
was
inferred
that
HCrO4−is
the
major
species
below
pH
5,
whereas
CrO4-2 is
above
pH
7
(Yang
et
al.,
2016).
•The
thermodynamic
driving
force
for
Cr
(VI)
reduction
decreases
by
79
mV
with
an
increase
of
pH
by
one
unit
(Wang
et
al.,
2004).
•The
higher
reduction
rate
with
an
increase
in
acidity
is
attributed
to
the
higher
susceptibility
of
HCrO4−than
CrO4-2 to
undergo
reduction
(Mytych
et
al.,
2003).
•In
neutral
or
alkaline
conditions,
the
suppressed
photocatalytic
activity
is
due
to
the
formation
of
Cr
(III)
precipitates
in
the
form
of
Cr
(OH)3.
4.2.
Photocatalyst
dosage
In
view
of
the
photocatalytic
mechanism-concentration
depen-
dent
aspect,
it
is
significant
to
review
the
dependence
of
Cr
(VI)
photoreduction
on
varying
the
photocatalyst
dosage.
Fig.
6c
showed
the
effect
of
varying
g-C3N4dosages
(0.1
to
0.2
gL−1)
which
led
to
increased
Cr
(VI)
removal
rate
from
51
%
to
99
%
within
10
min
of
460
nm
irradiations.
The
high
efficiency
rate
was
attributed
to
more
surface-active
sites
obtained
on
increasing
the
g-C3N4pho-
tocatalyst
(Song
et
al.,
2019).
Li
et
al.,
confirmed
that
on
varying
the
concentration
of
DyVO4from
2.8
to
8.3
%
in
DyVO4/B-doped
g-
C3N4composite
the
removal
rate
of
Cr
(VI)
first
increased
and
then
decreased.
At
an
optimum
concentration
of
5.7
%
DyVO4,
maximum
of
82.3
%
of
Cr
(VI)
was
reduced
within
120
min
of
visible
light
irradi-
ations.
Whereas,
at
8.3
%
and
2.8
%
DyVO4dosage,
75
%
and
60
%
of
Cr
671

V.
Hasija
et
al.
Process
Safety
and
Environmental
Protection
152
(2021)
663–678
Fig.
7.
(a)
Efficiency
for
total
Cr
removal
in
Cr
(VI),
Cr
(III)
and
Cr
(III)
in
Cr
(VI)-RhB
coexistence
system
over
TiO2/g-C3N4photocatalyst
(Reproduced
with
permission
from
reference.
(Lu
et
al.,
2017).
Copyright
with
order
1102765-1),
(b)
Simultaneous
photoreduction
and
photooxidation
reaction
rate
of
Cr
(VI)
and
phenol
using
polyaniline
sensitized
g-C3N4-ZnFe2O4heterojunction
(Reproduced
with
permission
from
reference.
(Patnaik
et
al.,
2018).
Copyright
with
order
license
no.
5012440469469),
(c)
Effect
of
different
temperature
on
Cr
(VI)
reduction
over
g-C3N4/SnS2/SnO2ternary
composite
(Reproduced
with
permission
from
reference.
(Yang
et
al.,
2018).
Copyright
with
order
license
no.
5012440639791),
(d)
Photoreduction
of
Cr
(VI)
under
various
light
sources
over
BUC-21
MOF/g-C3N4composite
(Reproduced
with
permission
from
reference.
(Yi
et
al.,
2019).
Copyright
with
order
1102641-1.).
(VI)
was
reduced,
respectively.
It
was
also
found
that
in
the
absence
of
photocatalyst,
little
change
of
Cr
(VI)
was
found,
inferring
that
self-photoreduction
of
Cr
(VI)
is
limited
(Li
et
al.,
2019b).
Similar
trend
was
observed
for
g-C3N4/TiO2composite
in
which
optimum
6
wt%-g-C3N4displayed
100
%
Cr
(VI)
reduction
rate.
However,
on
increasing
the
weight
ratio
of
g-C3N4to
10,
20,
50
%
there
was
a
decrement
in
photocatalytic
efficiency.
This
was
mainly
because,
excess
of
g-C3N4serve
as
recombination
centres
for
photogener-
ated
electron-hole
pairs
and
suppress
the
effective
charge
transfer
route
(Lu
et
al.,
2015).
Gu
et
al.,
claimed
analogous
trend
of
Cr
(VI)
reduction
which
was
a
steady
increase
and
then
decreased
when
concentration
of
Co9S8in
Co9S8/g-C3N4was
exceeded
beyond
15
wt
%.
This
was
evident
from
the
Cr
(VI)
reduction
rate
constants
over
5
%,
10
%,
15
%,
20
%
and
40
%
Co9S8which
were
0.50,
0.58,
0.63,
0.51,
0.34
min−1,
respectively.
The
maximum
of
89
%
Cr
(VI)
reduc-
tion
was
obtained
at
ideal
dosage
of
15
%
Co9S8.
A
higher
amount
of
Co9S8could
lead
to
aggregation
of
Co9S8thereby,
blocking
the
surface-active
sites
(Gu
et
al.,
2019).
4.3.
Cr
(VI)
concentration
Only
a
few
reports
have
revealed
decrease
in
Cr
(VI)
reduc-
tion
efficiency
at
higher
initial
Cr
(VI)
concentration
due
to
the
requirement
of
more
photocatalytic
reductive
sites.
Also,
the
Cr
(VI)
concentration
affects
the
kinetics
behaviour
of
Cr
(VI)
reduction
(Wang
et
al.,
2016a).
This
was
clearly
evident
in
the
study
involving
g-C3N4/Na-bentonite
composites
where
reduction
efficiency
of
Cr
(VI)
[Fig.
6d]
increased
to
53.2
%
and
then
declined
to
40.2
%
with
an
increase
in
initial
concentration
from
50
mgL−1to
100
mgL−1.
The
results
were
well
assessed
by
first-order
kinetics
equation,
showing
highest
reduction
rate
of
0.0053
min−1at
50
mgL−1,
while
low-
est
rate
of
0.0042
min−1was
obtained
at
100
mgL−1(Guo
et
al.,
2019).
The
typical
effect
of
Cr
(VI)
concertation
was
obtained
in
the
experiment
involving
biochar-coupled
g-C3N4nanosheets.
The
photoreduction
efficiency
gradually
enhanced
when
the
initial
con-
centration
of
Cr
(VI)
increased
from
2.5
mg/L
to
10
mg/L.
However,
an
increment
in
Cr
(VI)
concentration
to
40,
60,
80,
160
and
320
mg/L
resulted
in
decreased
photo-reductive
potential
of
biochar-
coupled
g-C3N4.
The
results
indicated
that
high
concentration
of
Cr
(VI)
caused
accumulation
of
excessive
Cr+3 in
the
pores
of
g-
C3N4surface,
consequently
lowering
the
separation
of
Cr
(III)
from
electron
hole
pairs
(Li
et
al.,
2019c).
4.4.
Effect
of
additives
The
influence
of
additives
on
photocatalytic
reduction
perfor-
mance
of
g-C3N4is
relatively
complex
due
to
the
co-existence
of
organic
species
and
additives.
For
instance,
the
synergistic
Cr
(VI)
reduction
and
rhodamine-B
(RhB)
degradation
was
studied
over
coupled
TiO2nanorods/g-C3N4nanosheets
under
visible
light
irra-
diations.
The
reduction
efficiency
of
Cr
(VI)
was
found
to
increase
3.42
times
higher
in
Cr
(VI)-RhB
composite
system
than
in
the
absence
of
RhB
[Fig.
7a].
In
addition,
RhB
degradation
increased
from
79.62
%
to
98.94
%
within
70
min
of
irradiations
in
the
pres-
ence
of
Cr
(VI).
This
was
because
Cr
(VI)
served
as
electron
scavenger
for
TiO2photogenerated
electrons
and
RhB
consumed
the
holes
in
672

V.
Hasija
et
al.
Process
Safety
and
Environmental
Protection
152
(2021)
663–678
Table
1
Previous
researches
in
the
application
of
engineered
g-C3N4-based
photocatalysts
for
photocatalytic
reduction
of
Cr
(VI).
Photocatalytic
system
Reaction
conditions
Mechanistic
route
Bandgap
(Pristine
g-
C3N4/Engineered
g-C3N4)
BET
(Pristine
g-C3N4
/Engineered
g-C3N4)
Reduction
effi-
ciency/Reaction
time
References
Pristine
g-C3N4
L.S;
300
W
Xe
lamp,
[P.C]
=
30
mg,
Cr
(VI)
=
2×10-4 M
-2.7
eV -100
%/
3
h(Hu
et
al.,
2014)
pH
=
5,
␭;
420
nm
Pristine
g-C3N4
L.S;
300
W
Xe
lamp,
[P.C]
=50
mg,
Cr
(VI)=10
ppm
-2.65
eV -92
%/60
min
(Wang
et
al.,
2019b)
pH
=
3.5,
␭;
420
nm
O-doped
g-C3N4
L.S;
300
W
Xe
lamp,
[P.C]
=50
mg,
Cr
(VI)
=
25
mg/L,
-2.72
eV/1.87
eV -75
%/
3
h(Wang
et
al.,
2017a,
2017b)
pH
=
2,
␭;
540
nm
S-doped
g-C3N4
L.S;
300
W
LED
irradiations,
[P.C]
=0.2
g,
Cr
(VI)=10
mg/L
-2.6
eV/1.98
eV 10.9
m2/g,
18.5
m2/g 85
%/
120
min
(Gong
et
al.,
2020)
pH
=
3,
␭;
420
nm
Fe+3 doped
g-C3N4
L.S;
500
W
Xe
lamp,
[P.C]
=
30
mg,
Cr
(VI)
=
20
ppm
-2.81
eV/2.66
eV -92.9
%/
120
min
(Liang
et
al.,
2016)
pH=
-,
␭;
445
nm
C
doped
g-C3N4/CeO2
L.S;
300
W
Xe
lamp,
[P.C]
=0.2
g/L,
Cr
(VI)=10
mg/L
-2.76
eV/2.58
eV 12.0
m2/g,
35.9
m2/g 99.5
%/40
min (Xu
et
al.,
2021)
pH
=
5,
␭;
420
nm
P
doped
g-C3N4/SnS L.S;
Solar
simulator,
[P.C]
=0.2
g/L,
Cr
(VI)=10
mg/L
Type-II -485.99
m2/g 100
%/
60
min
(Sun
and
Park,
2020)
pH
=
5,
␭;
440
nm
g-C3N4intercalated
NaClO
L.S;
300
W
Xe
lamp,
[P.C]
=50
mg,
Cr
(VI)=40
ppm
2.72
eV/2.59
eV 11.3
m2/g,
85.7
m2/g 85
%/
100
min
(Cui
et
al.,
2018a)
pH=
-,
␭;
540
nm
g-C3N4intercalated
formate
ion
L.S;
300
W
Xe
lamp,
[P.C]
=0.1
g/100
mL,
Cr
(VI)
=
20
mg/L
-2.73
eV/2.78
eV -55
%/4
h
(Dong
and
Zhang,
2013)
pH=
-,
␭;
420
nm
g-C3N4treated
with
H2SO4
L.S;
300
W
Xe
lamp,
[P.C]
=10
mg,
Cr
(VI)=5
mg/
L
-2.77
eV/2.98
eV -95
%/100
min
(Wang
et
al.,
2020b)
pH
=
2,
␭;
540
nm
g-C3N4/MoS2
L.S;
simulated
sunlight
with
AM
1.5,
[P.C]
=
20
mg,
Cr
(VI)=10
mg/L,
Type-II -6.70
m2/g,
21.69
m2/g 81
%/
50
min
(Wu
et
al.,
2020)
pH=
-,
␭;
540
nm
g-C3N4/Bi2S3
L.S;
300
W
Xe
lamp,
[P.C]
=
20
mg,
Cr
(VI)
=
20
mg/L,
Type-II 2.70
eV/2.35
eV -92.5
%/150
min (Chen
et
al.,
2017)
pH=
-,
␭;
540
nm
Chitosan/g-C3N4/TiO2
L.S;
800
W
Xe
lamp,
[P.C]
=0.33
mg/L,
Cr
(VI)
=
30
mg/L,
Type-II -
-90
%/
240
min (Li
et
al.,
2021a,
2021b)
pH
=
2,
␭;
540
nm
CoS2/g-C3N4/reduced
graphene
oxide
L.S;
350
W
Xe
lamp,
[P.C]
=
20
mg,
Cr
(VI)
=
20
ppm,
Type-II -
-99.8
%/
120
min
(Wang
et
al.,
2020c)
pH
=
2,
␭;
550
nm
TiO2/g-C3N4/reduced
graphene
oxide
L.S;
300
W
Xe
lamp,
[P.C]
=50
mg,
Cr
(VI)=100
mg/L,
Type-II -8.9
m2/g/
178.6
m2/g 97
%/4
h
(Li
et
al.,
2019a)
pH
=
3,
␭;
540
nm
g-C3N4/Ti-doped
mesoporous
silica
(SBA-15)
L.S;
300
W
Xe
lamp,
[P.C]
=0.5
g,
Cr
(VI)=10
mg/L
Type
-II --100
%/
120
min (Liu
et
al.,
2017)
pH
=
2.3,
␭;
540
nm
g-C3N4/Fe3O4/CoMoO4
L.S;
50
W
LED,
[P.C]
=0.1
g
Cr
(VI)=100
mg/L Type-II -14.6
m2/g,
41.7
m2/g 23.2
times
greater
than
pristine
g-C3N4
(Yangjeh
et
al.,
2019)
pH
=
2,
␭;
540
nm
g-C3N4/CoAl
hydrotalcites
L.S;
300
W
Xe
lamp,
[P.C]
=1.0
mg/mL Type-II -
-10.6
times
higher
than
pristine
g-C3N4
(Xiong
et
al.,
2020)
Cr
(VI)=50
mg/L
pH
=
3,
␭;
540
nm
673

V.
Hasija
et
al.
Process
Safety
and
Environmental
Protection
152
(2021)
663–678
Table
1
(Continued)
Photocatalytic
system
Reaction
conditions
Mechanistic
route
Bandgap
(Pristine
g-
C3N4/Engineered
g-C3N4)
BET
(Pristine
g-C3N4
/Engineered
g-C3N4)
Reduction
effi-
ciency/Reaction
time
References
g-C3N4/Zn0.25Cd0.75 S
L.S;
500
W
halogen
lamp,
[P.C]
=40
mg/mL Type-II -
-99
%/
25
min
(Sun
et
al.,
2014b)
Cr
(VI)=50
mg/L
pH
=
2.5,
␭;
510
nm
g-C3N4/ZnO
L.S;
300
W
Xe
lamp,
[P.C]
=100
mg/mL Type-II -5.8
m2/g,
28.5
m2/g 98.0
%/
90
min
(Zhong
et
al.,
2020)
Cr
(VI)=10
mg/L
pH
=
2.5,
␭;
525
nm
Fe+3 doped
g-C3N4/MoS2
L.S;
500
W
Xe
lamp,
[P.C]
=
30
mg/mL Type-II -28.7
m2/g,
28.5
m2/g 91.4
%/
120
min (Wang
et
al.,
2016b)
Cr
(VI)
=
20
ppm
pH=-,
␭;
557
nm
g-C3N4/ZnS
L.S;
500
W
Xe
lamp,
[P.C]
=
30
mg/mL Type-II 28.6
m2/g,
48.3
m2/g 91.3
%/120
min
(Wang
et
al.,
2019c)
Cr
(VI)=10
mg/L
pH
=
6.9,
␭;
557
nm
Sr0.25H1.5 Ta2O6·H2O/Ag/
g-C3N4
L.S;
300
W
Hg
lamp,
[P.C]
=
25
mg/mL
All-solid-
state
Z-scheme
-- 98
%/
50
min
(Xin
et
al.,
2016)
Cr
(VI)=10
mg/L
pH=-,
␭;
540
nm
SrTa2O6/Ag/
g-C3N4
L.S;
300
W
Hg
lamp,
[P.C]
=
25
mg/mL
All-solid-
state
Z-scheme
-0.47
cm2/g,
10.50
cm2/g 73.2
%/
60
min
(Su
et
al.,
2015)
Cr
(VI)=10
mg/L
pH=-,
␭;
540
nm
g-C3N4/
Fe0/MoS2
L.S;
300
W
Xe
lamp,
[P.C]
=
20
mg
All-solid-
state
Z-scheme
-28.6
m2/g,
39.38
m2/g 96.2
%/
45
min (Wang
et
al.,
2017a,
2017b)
Cr
(VI)=15
ppm
pH=-,
␭;
440
nm
g-C3N4/Ag/Bi4O7
L.S;
300
W
Xe
lamp,
[P.C]
=15
mg,
Cr
(VI)=50
mg/L,
All-solid-
state
Z-scheme
-8
m2/g,
162
m2/g 98
%/
60
min (Ye
et
al.,
2019)
pH
=
3,
␭;
550
nm
g-C3N4/Ag/TiO2
L.S;
100
W
Xe
arc
lamp,
[P.C]
=
20
mg
All-solid-
state
Z-scheme
-
-100
%/150
min
(Ghafoor
et
al.,
2019)
Cr
(VI)
=
20
ppm
pH
=
3,
␭;
540
nm
g-C3N4/BiOBr/carbon-
dots
L.S;
50
W
LED
lamp,
[P.C]
=0.1
gDirect
Z-scheme
-14.6
m2/g,
47.1
m2/g 21.7
times
higher
than
pristine
g-C3N4/
30
min
(Khaneghah
et
al.,
2018)
Cr
(VI)
=
250
mL
pH=-,
␭;
540
nm
g-C3N4/BiOI
L.S;
500
W
Xe
lamp,
[P.C]
=50
mg/mL Direct
Z-scheme
-11.8
m2/g,
17.2
m2/g 99
%/1.5
h
(Zhang
et
al.,
2020)
Cr
(VI)=10
ppm
pH
=
2,
␭;
680
nm
g-C3N4/BiOI
L.S;
500
W
Xe
arc
lamp,
[P.C]
=
20
mg Direct
Z-scheme
-12.7
m2/g,
18.6
m2/g 79.2
%/
15
min
(Jiang
et
al.,
2020)
Cr
(VI)
=
20
mL
pH=-,
␭;
365
nm
VB
of
g-C3N4.
This
led
to
an
efficient
suppression
in
recombination
of
electron-hole
pairs
with
a
dramatic
enhancement
in
photocat-
alytic
activity
of
co-exiting
Cr
(VI)-RhB
system
in
comparison
to
single
system
(Lu
et
al.,
2017).
Similarly,
synergistic
Cr
(VI)
pho-
toreduction
and
phenol
photooxidation
results
were
obtained
in
polyaniline
sensitized
g-C3N4-ZnFe2O4heterojunction.
The
results
depicted
in
[Fig.
7b]
validates
the
enrichment
in
photoactivity
at
molar
concentration
[2
×10−5M]
of
phenol
and
Cr
(VI).
The
single
Cr
(VI)
species
reduction
was
74.0
%,
while
for
phenol
was
75.1
%
under
2
h
of
visible
light
irradiations.
However,
in
the
mutual
presence
of
each-other,
the
photoreduction
and
oxidation
activity
improved
97.8
%
and
85.1
%,
respectively.
The
explanation
to
the
synergis-
tic
effects
was
ascribed
to
the
electron
donating
ability
of
phenol
which
reacts
irreversibly
with
photogenerated
holes,
thereby
gen-
erating
a
greater
number
of
electrons
for
Cr
(VI)
reduction
(Patnaik
et
al.,
2018).
Fortunately,
addition
of
organic
acids
as
hole
scavengers
has
been
reported
to
improve
the
Cr
(VI)
removal
efficiency
in
g-
C3N4mediated
photocatalytic
reduction
reactions.
The
presence
of
hole
scavengers
is
accounted
beneficial
due
to
the
following
reason.
During
the
Cr
(VI)
photoreduction
process
in
water,
the
CB
photogenerated
electrons
are
responsible
for
Cr
(VI)
reduction,
while
photogenerated
holes
in
the
VB
are
generally
consumed
in
oxidation
of
water
to
oxygen.
Since,
water
oxidation
is
a
more
com-
plicated
and
rate-determining
step
involving
four-electron
transfer
reaction
(Litter,
2017).
Therefore,
in
the
presence
of
hole
scav-
enger
effective
electron-hole
pairs
separation
and
maximum
Cr
(VI)
reduction
is
achieved
as
claimed
by
researchers.
Saha
et
al.,
revealed
the
dual
role
of
HCOOH;
firstly,
in
hole
scavenging
of
g-
C3N4to
make
photogenerated
electrons
in
CB
more
available
for
Cr
(VI)
reduction
as
presented
in
Eq.
(10)
HCOOH
+
h+
VB →
H2+
CO2(10)
In
this
way
HCOOH
tends
to
slow
the
recombination
rate
of
pho-
togenerated
electrons
-holes
and
also
increase
the
availability
of
protons.
Secondly,
the
CO2−/CO2generated
has
reduction
potential
of
-1.9
V
vs
NHE
capable
of
Cr
(VI)
reduction
according
to
pathway
as
depicted
in
Eqs.
(11,
12)
COOH−+
h+
VB →
H++
CO−
2(11)
674

V.
Hasija
et
al.
Process
Safety
and
Environmental
Protection
152
(2021)
663–678
Cr2O−2
7+
6CO−
2+
14H+→
2Cr+3+
7H2O
+
6CO2(12)
The
rate
of
photoreduction
results
confirmed
the
benefits
of
HCOOH
as
only
23
%
of
Cr
(VI)
was
reduced
by
g-C3N4alone
whereas,
85
%
was
reduced
over
HCOOH/g-C3N4mixture
in
2
h
of
illumination
(Saha
et
al.,
2020).
Another
advantage
of
organic
acid
addition
is
the
lowering
of
g-C3N4bandgap
and
increase
in
BET
surface
area.
On
addition
of
HCl
and
HNO3in
g-C3N4the
esti-
mated
bandgap
of
pristine
g-C3N4,
g-C3N4/HNO3and
g-C3N4/HCl
was
2.52,
2.49
and
2.47
eV
and
specific
surface
area
was
8.5,
9.9
and
10.15
m2/g,
respectively
(Zhang
et
al.,
2015).
These
studies
indicate
synergistic
effects
of
organic
acid
additives
to
ameliorate
the
Cr
(VI)
photoreduction
ability.
4.5.
Other
factors
The
operating
temperature
is
a
significant
parameter
for
the
lab-
oratory
and
large-scale
experiments
due
to
its
beneficial
impact
on
the
photocatalytic
Cr
(VI)
reduction
process
(Singh
et
al.,
2013)
For
example,
a
slight
temperature
dependent
Cr
(VI)
reduction
perfor-
mance
was
observed
by
Yang
et
al.,
in
g-C3N4/SnS2/SnO2ternary
composite.
Fig.
7c
demonstrated
100
%
Cr
(VI)
reduction
efficiency
at
140 ◦C
in
comparison
to
38.1
and
67
%
obtained
at
100 ◦C
and
180 ◦C,
respectively.
With
respect
to
the
results
obtained
it
was
inferred
that
adverse
effect
of
higher
temperature
is
aggregation
of
g-C3N4based
photocatalysts
thereby
reducing
the
catalytic
activ-
ity
(Yang
et
al.,
2018).
The
increased
Cr
(VI)
reduction
efficacy
was
also
obtained
in
g-C3N4treated
at
hydrothermal
temperature
of
80,
100
and
120 ◦C
which
were
40.3
%,70.4
%
and
77.2
%,
respectively.
The
enhanced
effect
of
temperature
on
Cr
(VI)
reduction
can
be
elu-
cidated
as
higher
temperature
is
favourable
for
overcoming
large
activation
barrier
(Wei
et
al.,
2016).
The
type
of
light
source
also
has
impact
on
the
Cr
(VI)
photore-
duction
as
revealed
in
the
study
involving
BUC-21
MOF/g-C3N4
composite.
The
effect
of
simulated
white
light
and
real
sunlight
was
evaluated
as
shown
in
Fig.
7d,
which
depicted
100
%
Cr
(VI)
reduction
under
real
sunlight
within
60
min
irradiation
time.
While,
simulated
sunlight
irradiation
time
was
prolonged
to
100
min
to
achieve
100
%
reduction
of
Cr
(VI)
(Yi
et
al.,
2019).
In
addition,
the
light
intensity
during
the
photocatalytic
reaction
is
also
a
vital
factor.
For
example,
Huang
et
al.,
proposed
that
with
increased
light
intensity
a
large
number
of
photons
were
available
for
elec-
tronic
transitions
in
ternary
g-C3N4/TiO2/SnO2composite.
It
was
observed
that
Cr
(VI)
reduction
efficiency
reached
100
%
with
800
W
xenon
lamp.
However,
92
%,
94
%,
and
99
%
reduction
potential
was
achieved
using
only
100
W,
300
W,
and
500
W
of
light
power,
respectively
(Huang
et
al.,
2021).
The
discrepancies
in
Cr
(VI)
pho-
toreduction
efficiency
are
suggested
due
to
varying
experimental
conditions
as
summarized
in
Table
1.
5.
Conclusion
and
perspectives
The
review
represents
graphitic
carbon
nitride
(g-C3N4)
as
a
promising
visible-light
photocatalyst
for
Cr
(VI)
reduction
into
Cr
(III)
due
to
its
high
photostability,
unique
optical
and
electronic
properties,
narrow
bandgap
(2.7
eV),
with
appropriate
conduction
band
and
valence
band
potentials.
The
Cr
(VI)
remediation
by
reduc-
tion
is
possible
when
the
photogenerated
electrons
have
more
negative
potential
than
the
reduction
potential;
Eo(Cr2O7−2/Cr+3)
=
1.23
V,
Eo(HCrO4-/Cr+3)
=
1.35
V,
Eo(CrO4−2/Cr+3)
=
-0.13
V
vs.
NHE.
Providentially,
the
conduction
band
potential
(-1.09
eV)
of
g-C3N4
is
suitable
for
Cr
(VI)
reduction.
Moreover,
the
ability
of
photogener-
ated
electrons
to
reduce
molecular
oxygen
into
O2
•-
radicals
(-0.33
V)
prevents
the
interference
of
O2,
which
is
a
substantial
advantage
for
Cr
(VI)
photoreduction
process.
However,
the
rapid
recombina-
tion
of
photogenerated
charge
carriers
and
small
specific
surface
area
of
g-C3N4tends
to
reduce
the
visible
light
harnessing
ability
and
quantum
efficiency.
The
present
review
concludes
rational
construction
of
compos-
ites
mostly
in
the
form
of
type-II
heterojunctions
as
highly
explored
providing
advantages
of
improved
electron
hole
pair
separation
and
retained
reduction
potential
of
g-C3N4.
The
simple
chemical
composition
of
g-C3N4from
carbon
and
nitrogen
allows
its
ele-
mental
doping
to
narrow
the
bandgap
resulting
in
extended
solar
energy
absorption.
The
other
methods
to
improve
to
reductive
abil-
ity
of
g-C3N4is
via
vacancy
generation
and
co-catalyst
loading
which
alters
the
electronic
structure
and
accelerates
charge
carrier
migration,
respectively.
The
fine
tuning
of
g-C3N4surface
enables
expansion
of
pore
volume,
specific
surface
area
to
create
abundant
active
sites
and
prevent
aggregation
of
g-C3N4nanosheets.
In
view
of
the
reported
literature,
we
anticipate
that
optimizing
pH
to
acidic
medium
and
addition
of
hole
scavenging
organic
acids
as
beneficial
aspect
for
Cr
(VI)
reduction.
To
date,
in
spite
of
the
notable
research
undertaken
in
the
Cr
(VI)
reduction
using
engineered
g-C3N4there
are
still
some
challenges
which
needs
to
be
addressed.
(i)
The
uncertainty
of
mechanism:
It
is
necessary
to
further
investigate
the
actual
mechanism
of
Cr
(VI)
photocatalytic
reduction.
Since,
some
studies
claim
the
simultaneous
par-
ticipation
photogenerated
electrons
and
O2
•−radicals
in
Cr
(VI)
reduction
process
which
requires
experimental
validation
for
the
extent
of
contribution
of
every
reactive
species.
Also,
the
dual
functionality
i.e.,
co-existence
of
photooxidation
and
photoreduction
process
of
g-C3N4photocatalysts
leads
to
com-
plexity
of
mechanism.
This
demands
for
implementation
of
advanced
characterization
techniques
to
explore
the
dominant
conclusive
pathway.
(ii)
pH
sensitivity:
The
activity
of
Cr
(IV)
photoreduction
is
suppressed
under
neutral
and
alkaline
medium,
which
is
detrimental
for
the
desirable
reduced
Cr
(IV)
quantum
yield.
Consequently,
research
on
the
photoreduction
of
Cr
(IV)
under
complex
environment
needs
to
be
reinforced.
(iii)
Toxicity
of
intermediates:
The
photoreduction
pathway
of
toxic
Cr
(VI)
ions
into
Cr
(III)
is
accompanied
with
the
release
of
Cr
(V)
and
Cr
(IV)
as
intermediates.
The
future
studies
should
target
upon
the
toxicity
assessment
of
intermediates
in-addition
to
the
final
Cr
(III)
ions.
(iv)
Photostability
of
g-C3N4:
Due
to
the
multiple
factors
gov-
erning
Cr
(VI)
reduction
efficiency,
it
is
of
great
concern
to
ensure
the
stability
of
g-C3N4during
the
photocatalysis
pro-
cess.
For
instance,
the
stability
of
g-C3N4lattice
can
be
distorted
under
high-intensity
light
irradiations
due
to
the
thermal
effect
of
photons.
The
optimized
addition
of
organic
acids
as
hole
scavengers
is
suggested
for
the
continuation
of
photocatalysis
process.
From
this
viewpoint,
substantial
efforts
would
pave
route
for
cost-effective
development
of
g-C3N4photocatalyst
for
large
scale
Cr
(VI)
photoreduction
application.
Declaration
of
Competing
Interest
The
authors
report
no
declarations
of
interest.
Acknowledgements
This
research
was
supported
by
Brain
Pool
Program
through
the
National
Research
Foundation
of
Korea
(NRF)
funded
by
the
Min-
istry
of
Science
and
ICT
(grant
number
2020H1D3A1A04081409).
675

V.
Hasija
et
al.
Process
Safety
and
Environmental
Protection
152
(2021)
663–678
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