Functional biochar fabricated from red mud and walnut shell for phosphorus wastewater treatment: Role of minerals.

A novel functional biochar (BC) was prepared from industrial waste red mud (RM) and low-cost walnut shell by one facile-step pyrolysis method to adsorb phosphorus (P) in wastewater. The preparation conditions for RM-BC were optimized using Response Surface Methodology. The adsorption characteristics of P were investigated in batch mode experiments, while a variety of techniques were used to characterize RM-BC composites. The impact of key minerals (hematite, quartz, and calcite) in RM on the P removal efficiency of the RM-BC composite was studied. The results showed that RM-BC composite produced at 320 °C for 58 min, with a 1:1 mass ratio of walnut shell and RM, had a maximum P sorption capacity of 15.48 mg g-1, which was more than double that of the raw BC. The removal of P from water was found to be facilitated significantly by hematite, which forms Fe-O-P bonds, undergoes surface precipitation, and exchanges ligands. This research provides evidence for the effectiveness of RM-BC in treating P in water, laying the foundation for future scaling-up trials.

Theoretical study on the degradation mechanism, kinetics and toxicity for aqueous ozonation reaction of furan derivatives.

The compounds including furan-2,5-dicarboxylic acid (FDCA), 2-methyl-3-furoic acid (MFA), and 2-furoic acid (FA), containing Furan ring are considered to be possessing high ozone reactivity, although in depth studies of their ozonation processes have not been carried out yet. Hence, mechanism, kinetics and toxicity by quantum chemical, and their structure activity relationship are being investigated in this study. Studies of reaction mechanisms revealed that during the ozonolysis of three furan derivatives containing C=C double bond, furan ring opening occurs. At temperature (298 K) and pressure of 1 atm, the degradations rates of 2.22 × 103 M-1 s-1 (FDCA), 5.81 × 106 M-1 s-1 (MFA) and 1.22 × 105 M-1 s-1 (FA) suggested that the reactivity order is: MFA > FA > FDCA. In the presence of water, oxygen and ozone, the Criegee intermediates (CIs) as the primary products of ozonation would produce lower molecule weight of aldehydes and carboxylic acids by undergoing degradation pathways. The aquatic toxicity reveals that three furan derivatives play green chemicals roles. Significantly, most of the degradation products are least harmful to organisms residing in the hydrosphere. The mutagenicity and developmental toxicity of FDCA is minimum as compared to FA and MFA, which shows the applicability of FDCA in a wider and broader field. Results of this study reveal its importance in the industrial sector and degradation experiments.

Theoretical study on the formation of Criegee intermediates from ozonolysis of pentenal: An example of trans-2-pentenal.

In this study, we investigated the reaction mechanism and kinetics of ozone with trans-2-pentenal using density functional theory (DFT) and conventional transition state theory (CTST). At 298 K and 1 atm, the gas-phase reaction mechanisms and kinetic parameters were calculated at the level of CCSD(T)/6-311+G(d,p)//M06-2X/6-311+G(d,p). Both CC and CO bond cycloaddition as well as hydrogen abstraction were found. The calculations indicated that the main reaction path is 1,3-dipole cycloaddition reactions of ozone with CC bond with the relatively lower syn-energy-barrier of 3.35 kcal mol-1 to form primary ozonide which decomposed to produce a carbonyl oxide called a Criegee intermediate (CI) and an aldehyde. The subsequent reactions of CIs were analysed in detail. It is found that the reaction pathways of the novelty CIs containing an aldehyde group are extremely similar with general CIs when they react with NO, NO2, SO2, H2O, CH2O and O2. The condensed Fukui function were calculated to identify the active site of the chosen molecules. At 298 K and 1 atm, the reaction rate coefficient was 9.13 × 10-18 cm3 molecule-1 s-1 with atmospheric lifetime of 1.3 days. The calculated rate constant is in general agreement with the available experimental data. The branching ratios indicated that syn-addition pathways are prior to anti-addition. The atmospheric ratios for CIs formation and the bimolecular reaction rate constants for the Criegee intermediates with the variety of partners were calculated. Our theoretical results are of importance in atmospheric chemistry of unsaturated aldehyde oxidation by ozone.

Theoretical Calculation on the Reaction Mechanisms, Kinetics and Toxicity of Acetaminophen Degradation Initiated by Hydroxyl and Sulfate Radicals in the Aqueous Phase.

The •OH and SO4•- play a vital role on degrading pharmaceutical contaminants in water. In this paper, theoretical calculations have been used to discuss the degradation mechanisms, kinetics and ecotoxicity of acetaminophen (AAP) initiated by •OH and SO4•-. Two significant reaction mechanisms of radical adduct formation (RAF) and formal hydrogen atom transfer (FHAT) were investigated deeply. The results showed that the RAF takes precedence over FHAT in both •OH and SO4•- with AAP reactions. The whole and branched rate constants were calculated in a suitable temperature range of 198-338 K and 1 atm by using the KiSThelP program. At 298 K and 1 atm, the total rate constants of •OH and SO4•- with AAP were 3.23 × 109 M-1 s-1 and 4.60 × 1010 M-1 s-1, respectively, considering the diffusion-limited effect. The chronic toxicity showed that the main degradation intermediates were harmless to three aquatic organism, namely, fish, daphnia, and green algae. From point of view of the acute toxicity, some degradation intermediates were still at harmful or toxic level. These results provide theoretical guidance on the practical degradation of AAP in the water.

Nitrogen vacancy induced in situ g-C3N4 p-n homojunction for boosting visible light-driven hydrogen evolution.

Graphitic carbon nitride (g-C3N4) as a novel photocatalyst with great potentials has been extensively employed in solar-driven energy conversion. Herein, the novel in situ g-C3N4 p-n homojunction photocatalyst with nitrogen vacancies (NV-g-C3N4) is successfully fabricated via hydrothermal synthesis followed by two-step calcination. The in situ NV-g-C3N4 homojunction can be employed as an effective photocatalyst for hydrogen generation through water splitting under visible light, and the optimum rate constant of 3259.1 µmol.g-1.h-1 is achieved, which is 8.7 times as high as that of pristine g-C3N4. Moreover, the markedly increased photocatalytic performance is ascribed to the enhanced light utilization, large specific surface area and unique nitrogen-vacated p-n homojunction structure, which provides more active sites and improves the separation of photo-excited electron-hole pairs. Besides, the underlying mechanism for efficient charge transportation and separation is also proposed. This work demonstrates that the remodeling of g-C3N4 p-n homojunction with nitrogen vacancies is a feasible way as highly efficient photocatalysts and might inspire some new strategies for energy and environmental applications.

Persulfate activation by Fe(III) with bioelectricity at acidic and near-neutral pH regimes: Homogeneous versus heterogeneous mechanism.

The combination of persulfate (PS) activation by iron ions with electrochemical process (electro/Fe3+/PS) is a promising advanced oxidation process. However, almost all these systems were performed in an unbuffered solution and actually under acidic pH condition, with the electricity being frequently supplied by external power. Considering the high buffering capacity of wastewater and energy saving, peroxydisulfate (PDS) activation by Fe(III) species with bioelectricity provided by microbial fuel cell (MFC) for bisphenol A (BPA) oxidation was investigated at fixed near-neutral pH as well as acidic pH. The results indicate that 90.8% of BPA could be removed at pH 2.5. Though the iron existed in the form of precipitate, BPA could still be efficiently removed at pH 6.0. The precipitate formed in the system at pH 6.0 was identified as the amorphous iron oxyhydroxides. Sulfate radicals in the bulk solution and that adsorbed on the precipitate were the dominant reactive species responsible for the oxidation of BPA in the homogeneous and heterogeneous MFC/Fe(III)/PDS processes, respectively. The mechanisms of BPA degradation at both pH values were proposed via EPR and quenching tests as well as XPS analysis. The effects of operating parameters, the mineralization, the mineralization current efficiency and energy consumption were also explored.

Oxidation of organic contaminant in a self-driven electro/natural maghemite/peroxydisulfate system: Efficiency and mechanism.

Electro-assisted iron-mediated persulfate (PS) activation process has been successfully employed to oxidize organic contaminant. However, a majority of iron-based catalysts used for PS activation was synthesized through complicated or demanding procedures and may have potential risks on environment during the preparation process. Herein, natural maghemite (NM) which is abundant on the earth was employed to activate peroxydisulfate (PDS) in an electrolytic cell. The voltage was provided by microbial fuel cell (MFC) instead of external power as reported in the previous studies, so as to establish a self-driven electro/natural maghemite/PDS system (MFC/NM/PDS) for the oxidation of acid orange 7 (AO7). The results showed that above 90% removal efficiency of AO7 was achieved in a wide range of pH (3.0-9.0) after 100min reaction. Singlet oxygen was identified for the first time during PDS activation and surface bound sulfate radicals served as the dominant active species responsible for AO7 oxidation. The underlying mechanism of AO7 elimination in the MFC/NM/PDS system was elucidated through quenching tests, electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) techniques. The variation of TOC and cytotoxicity to Escherichia coli was explored. The intermediate products formed were identified using LC-TOF-MS technique and a possible pathway of AO7 degradation was proposed.

Hydronium jarosite activation of peroxymonosulfate for the oxidation of organic contaminant in an electrochemical reactor driven by microbial fuel cell.

Electro-assisted Fenton-like (EAFL) system based on sulfate radicals (SO4-) has been extensively explored for the degradation of recalcitrant organic contaminant. Nevertheless, external power supply should be provided uninterruptedly in the EAFL process and thus the high energy consumption is ineluctable. Recently, microbial fuel cell (MFC), a bio-electrochemical system where exoelectricigens are used to catalyze fuels into electricity energy has gained popularity mainly due to its renewability. Herein, a novel heterogeneous EAFL system, hydronium jarosite (HJ) activation of peroxymonosulfate (PMS) in an electrochemical reactor driven by an uncoated single-chamber MFC (MFC/HJ/PMS), was employed to decolorize acid orange 7 (AO7). The results suggest that the MFC/HJ/PMS system can remove AO7 efficiently in a wide pH range (3-9). The concentration of total iron leached could meet European Union discharge standards and hydronium jarosite could be used at least three circles. The results of electron paramagnetic resonance analysis and radical scavenging experiments indicate SO4- is the major active species responsible for the AO7 elimination. The work provides an efficient, energy-saving and cost-effective approach to treat organic wastewater and develops the conceivable utilization of hydronium jarosite, precipitates produced in hydrometallurgical process.