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(a) Graphical illustration of publications from Scopus data base on photocatalytic 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.
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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 (V...
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Context 1
... progress on photocatalytic Cr (VI) reduction utilizing g-C 3 N 4 . As above-mentioned, a bibliometric exploration was carried out in the Scopus database with keywords "graphitic carbon 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) ...
Context 2
... the discussion of this review. Also, the reports involving dual applicability of g-C 3 N 4 were 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-C 3 N 4 as presented in [Fig. 1b]. On the whole, the proposed review reflects g-C 3 N 4 -mediated photocatalysis as non-pollutant, economic, and efficient Cr (VI) reduction ...
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Gaseous CO2 reduction driven by solar energy is a promising solution to the current energy crisis and environmental problems. Although thermocatalysts, electrocatalysts, and photocatalysts are developed as classical strategies for CO2 reduction, it remains a challenge for high efficiency and CO2 net reduction during this process. Here, a multi-fiel...
Citations
... Faster electron-hole recombination delays the reaction kinetics and this situation can be overcome by either forming heterojunctions or by the addition of hole scavengers. Formic acid is generally used as a hole scavenger which prevents electron-hole recombination [8,16,42,43]. By trapping holes, formic acid inhibits the instant recombination of electrons and holes generated during photon interaction with the photocatalyst, increasing the concentration of available free electrons. ...
... A similar trend is obtained in previous batch studies of Cr(VI) adsorption (Rafiaee et al. 2020;Shang et al. 2014;Georgieva et al. 2015). Cr(VI) can exist in several ionic states HCrO 4 , CrO 4 2− , and Cr 2 O 7 2− (Valentín et al. 2019; Hasija et al. 2021). High adsorption at lower pH takes place due to electrostatic attraction between HCrO 4 − and cationic adsorbent surface. ...
The era of post-industrialization has remarkably increased the concentration of Cr(VI), negatively affecting environmental sustainability. Therefore, Cr(VI) removal from wastewater before discharging into the water bodies is of utmost importance from the ecosystem point of view. This investigation has been undertaken to determine the adsorption efficiencies and capacities of two locally available aquatic weedy plants, namely, Brachiaria mutica (Forssk.) (B. mutica) and Cyperus rotundus L. (C. rotundus), for the remediation of Cr(VI) from aqueous solution. The characterization of the adsorbents has been performed with the help of Fourier transform infrared spectroscopy (FTIR) and Scanning Electron Microscopy (SEM). The batch study was taken into consideration to determine the impact of different parameters. The results of FTIR showed the presence of various functional groups such as amine, alkane, alkyne, and aromatic rings on the adsorbent’s surface. The adsorption efficiencies for B. mutica and C. rotundus were 80% and 87% at optimized conditions, i.e., 1 pH, 1 g adsorption dose, 20 mg/l initial concentration of metal, 90 min contact time with the 150 rpm shaking speed, and 30 °C temperature. The highest adsorption capacities noticed were 5.07 mg/g and 5.64 mg/g for B. mutica and C. rotundus, respectively. Freundlich isotherm was more appropriate for both adsorbents B. mutica and C. rotundus with the R² of 0.87 and 0.91, respectively. Pseudo-first-order kinetics was more favored in case of B. mutica, while pseudo-second-order kinetic favored by C. rotundus. According to thermodynamic study, the adsorption process for C. rotundus was exothermic and non-spontaneous at high temperature, while at low temperature, B. mutica shows endothermic as well as non-spontaneous nature. Both plants are unwanted and found abundantly in the aquatic environment. Therefore, using these aquatic plants as efficient bioadsorbents for the removal of Cr(VI) has double benefits.
... Cr(VI) is a highly soluble metal ion that can affect humans through the food chain, causing bronchitis, liver problems, kidney problems, brain injury, and potentially lung cancer [17]. Owing to having high solubility and favorable physical and biological characteristics over a wide pH range, Cr(VI) is more toxic than Cr(III) [18]. Considering all of this, the concentration of chromium should be kept to a minimum level. ...
... Cr(VI) is highly harmful to human health and can cause respiratory problems and even the formation of cancer cells [8][9][10]. Humans can come into contact with the metal through the respiratory tract, water, or even food [11]. Due to its high occurrence, added to the damage generated to living organisms, several technologies have been developed and improved aiming at its removal [12][13][14][15][16][17][18][19][20][21][22][23][24]. ...
Hexavalent chromium (Cr(VI)) is a highly toxic form of chromium, which can be found in industrial effluents from various sectors, such as the metallurgical, tanning, and pigment industries. The presence of Cr(VI) in the environment is a concern due to its negative effects on human health and the ecosystem since it is carcinogenic, mutagenic, and can cause damage to the respiratory, renal, hepatic, and dermatological systems. Adsorption is a sustainable alternative for the removal of Cr(VI) from the environment since it is an efficient, low-cost technique and can be adjusted according to specific environmental conditions. The use of algae biomass activated with chemical agents can be a promising solution to improve the adsorption capacity of the material and contribute to the mitigation of contamination by Cr(VI) and the protection of human health and the environment. The results of the study indicate that the activation of the red macroalgae Gelidium sp. with zinc chloride (ZnCl2) resulted in improvements in the adsorbent properties of the material for the removal of Cr(VI) in aqueous solutions. The physical characteristics of the material were analyzed, and it was observed that the surface area increased from 2.90 to 3.12 m² g⁻¹ after activation with ZnCl2. Furthermore, changes in the surface structure of the material were observed, with the presence of irregularities, mainly after the adsorption of Cr(VI). The analysis of functional groups indicated that the main groups present in the native biomass remained after activation with zinc, and new diffraction peaks also appeared, indicating the chemical modification of the material. The adsorption experiments were carried out under different conditions, such as pH, dosage, temperature, Cr(VI) concentration, and contact time. It was observed that the adsorption was favored under acidic conditions, with a dose of 0.05 g L⁻¹ of activated biomass. Equilibrium was reached quickly in the first few minutes, and the general kinetic model best fitted the experimental data. The kinetic adsorption capacity was higher for the activated material (226 mg g⁻¹) compared to the native material (114 mg g⁻¹). Increasing the concentration of Cr(VI) in the solution resulted in an increase in the adsorption capacity, indicating that the driving force gradient was greater at higher concentrations of the contaminating ion. The isothermal data were well fitted by the Koble-Corrigan heterogeneous surface model, and the maximum adsorption capacities were estimated at 126 mg g⁻¹ and 240 mg g⁻¹ for native and activated biomass, respectively, at the highest Cr(VI) concentration studied (150 mg L⁻¹). The results indicate that the activation of the red macroalgae Gelidium sp. with zinc chloride improved its adsorbent properties for the removal of Cr(VI) ions, with high adsorption capacity and efficiency, demonstrating a high potential for application in the removal of Cr(VI) metal ions in aqueous solutions.
Graphical Abstract
... According to Fig. 7a, 99% reduction of Cr(VI) at pH 2 was achieved with UV light irradiation in 60 minutes, whereas at pH 10, 180 minutes were needed to reach Cr(VI) reduction of 46.2% because the acidic medium promoted reduction in Eq. (5). Typically, Cr(VI) species exist as tetrahedral oxocompounds in the form of CrO 2− 4 at pH > 6 and HCrO − 4 at pH 2-6 (Hasija et al., 2021). So, the increase of pH leads to the formation of CrO 2− 4 to reduce the photocatalytic reduction of Cr(VI). ...
... With pH values < pH pzc , anionic species of Cr(VI) adsorbed onto the composite surface and Cr(VI) reduction was more effective at lower pH. This finding is similar to other studies on the influence of decreasing solution pH on Cr(VI) photocatalytic reduction (Bao et al., 2019;Hasija et al., 2021;Hsu et al., 2013). ...
... Photocatalytic technology can reduce highly toxic Cr(VI) to Cr(III), one of the essential trace metal elements for humans. Cr(III) is 1000 times less toxic than Cr(VI) [12]. In addition, Cr(III) tends to be separated by natural minerals which can be easily released from water as Cr(OH) 3 [13]. ...
... In addition, Cr(III) tends to be separated by natural minerals which can be easily released from water as Cr(OH) 3 [13]. Many semiconductor photocatalysts such as bismuth-based materials [14], graphite carbon nitride [12], metal-organic frameworks [15], layered double hydroxides [16], and other materials have been widely applied to the photoreduction of Cr(VI). Biochar derived from the biomass pyrolysis is a low-cost carbonaceous material [17]. ...
... Heterogeneous photocatalysis has a significant potential to degrade xenobiotics present in trace amounts (a few micrograms to hundreds of nanograms per liter) in water and air. Recently, heterogeneous photocatalysis has also been considered as a promising approach for the reduction of highly toxic Cr (VI) [1][2][3][4]. ...
... Currently, surface and interfacial modification of g-C 3 N 4 is considered the most effective way to improve the photoreduction efficiency of Cr(VI). The formation of heterojunctions, for example, reduces the recombination rate of electron-hole pairs, and the doping of g-C 3 N 4 leads to band gap tuning, expanded surface area and broadened solar spectral absorption [16]. Broadly speaking, the construction of vacancies alters the electronic structure, while the loading of the cocatalyst accelerates the migration of charge carriers. ...
Cr(VI) has great solubility and mobility in water, and is a class of highly toxic heavy metals. Illite modified g-C3N4 (IL-CN) materials were successfully synthesized and their photocatalytic activities for Cr(VI) reduction under visible light irradiation were tested in this study. Due to carbon and oxygen vacancies and defects by IL-CN, the modified IL-CN materials exhibited larger specific surface area and pore volume, and stronger electron-hole pair separation capacities. As a result, the IL-CN catalysts exhibited excellent catalytic activity for Cr(VI) reduction. Approximately 91.8 % of Cr(VI) was reduced to Cr(III) after 60 min of treatment under photocatalytic reaction of 2/IL-CN, which was much higher than those of the single g-C3N4 and single illite materials. There existed an optimum ratio of illite/g-C3N4 to realize the greatest Cr(VI) reduction efficiency. Acidic conditions and relatively higher dosage of IL-CN were conducive to Cr(VI) reduction. Cr(VI) reduction was mainly initiated by free electron in the photocatalytic system of modified IL-CN materials, while the role of O2⁻ was quiet weak. Furthermore, the stability and recycling of the IL-CN catalysts were evaluated.
... The rising levels of TMP from ng/L to µg/L in water are due to its high clinical use in the treatment of bacterial infections [1][2][3]. Cr (VI) species are potentially carcinogenic and mutagenic due to their biotransformation ability and high oxidizability into free radicals [4]. Both TMP and Cr (VI) once discharged in water cannot be effectively eliminated via traditional wastewater technique sowing to the formation of secondary pollutants. ...
The strategic designing of graphitic carbon nitride (CN)by elemental doping is likely to generate nitrogen vacancies which benefit by serving as electron trapping sites or by introducing additional energy levels (mid-gap states). In this work, uracil-assisted oxygen doped graphitic carbon nitride (OCNv-Ux) was synthesized by the hydrothermal method. The OCNv-U40 exhibited vigorous photocatalytic efficiency for 98% trimethoprim (TMP) antibiotic degradation and 90.8% Cr (VI) reduction under visible light irradiations. A combined inference from experimental results and simulation studies indicates band gap reduction from 2.7 to 2.34 eV for extended visible light absorption up to 620 nm. In particular, the density of states comprehends band structure regulation by the electron redistribution resulting in valence band up shifting and downshift of the conduction band, as validated by Mott-Schottky spectra. The synergistic outcomes of nitrogen vacancies and oxygen doping sites restrain the recombination of electron-hole pairs by mid-gap state formation and also promote high oxygen molecule adsorption on the nitrogen vacancies. The pioneering advantage of highly adsorbed oxygen molecule leads to the generation of plentiful superoxide anion radicals along with indirect production of hydroxyl radicals via in-situ produced H2O2, evident by radical scavenging and electron paramagnetic resonance experiments. The effect of chloride and nitrate anions showed inhibitory effect on the TMP degradation efficiency. The identification of formed intermediates and TMP degradation mechanism involving oxidation, hydroxylation, and cleavage was proposed, as investigated by TMP DFT studies. This research accomplishes significant enhancement in pristine CN photocatalytic activity by the virtue of increased specific surface area, fully exposed N-vacancies active sites, optimized electronic, structure and visible light active band gap.
... CrO 4 2− + H + ↔ 1/2Cr 2 O 7 2− + 1/2H 2 O, pK = − 7.31, both the protonic chromates are strong acid). Cr(VI) is the hazardous substance that causes harm to humans, animals, bacteria, and plants because of the labile, non-biodegradable, and penetrating properties when it forms free radicals (Hasija et al., 2021). The United States Environmental Protection Agency (USEPA) has categorized Cr(VI) as a Group A contaminant with a maximum permissible level of 100 μg/L in drinking water, while Cr(VI) of below 0.05 mg/L is listed by World Health Organization (WHO) and is also globally regulated in surface waters (Dhal et al., 2013;Ukhurebor et al., 2021). ...
