Recent advances in the photocatalytic reduction of carbon dioxide

Fossil fuels are currently the major energy source and are rapidly consumed to supply the increasing energy demands of mankind. CO₂, a product of fossil fuel combustion, leads to climate change and will have a serious impact on our environment. There is an increasing need to mitigate CO₂ emissions using carbon–neutral energy sources. Therefore, research activities are devoted to CO₂ capture, storage and utilization. For instance, photocatalytic reduction of CO₂ into hydrocarbon fuels is a promising avenue to recycle carbon dioxide. Here we review the present status of the emission and utilization of CO₂. Then we review the photocatalytic conversion of CO₂ by TiO₂, modified TiO₂ and non-titanium metal oxides. Finally, the challenges and prospects for further development of CO₂ photocatalytic reduction are presented.     

Biosorbents for solid-phase extraction of toxic elements in waters

Some trace metals are highly toxic for the environment. There is therefore a need for reliable methods for the determination of metals at trace levels. To this end, new sample pretreatment methods such as separation, preconcentration and speciation prior to the determination of metal ions have developed rapidly. Biosorption has become a major tool for solid-phase extraction methods. This review covers selected biosorbents such as algae, bacteria, filamentous fungi and yeasts, as new sorbents used in the solid-phase extraction of metal ions from various water sample matrices. A survey of the literature over 2004-2014 shows possible applications of selected new sorbents available for use in trace metal analysis in waters using solidphase extraction. We highlight the preconcentration of the toxic elements prior to their determination by atomic spectrometry.     

Redox processes in water remediation

Groundwater is the main source of drinking water and water for agricultural and industrial usage. Therefore, groundwater contamination is prevented and contaminated groundwater is remediated to protect public health and the environment. Methods to remediate groundwater contamination have been recently developed. The use of redox processes in water remediation technologies has not been properly reviewed. Numerous water remediation technologies, such as ultrasonication, bioremediation, electrokinetics and nanotechnology, are closely related to redox processes. Redox processes control the chemical speciation, bioavailability, toxicity, mobility and adsorption of water pollutants in environment. Here, we review (1) general introduction of redox processes, (2) applicability of redox processes in water remediation, and (3) catalytic enhancement of redox potentials to explore its wide applicability in environmental remediation.     

High boron removal by functionalized magnesium ferrite nanopowders

Hydroxyl-enriched materials are promising boron adsorbents. However, the use of these materials is hampered by issues of separation, recovery, and selectivity, notably due to the presence of interfering ions. Therefore, we synthesized here a cheap magnetic nanopowder, which was further functionalized with polyvinyl alcohol and glycidol to produce boron-selective adsorbents. We studied their selectivity and removal efficiency using batch and fixed-bed systems. Sorption was studied at both concentrated and trace amounts of boron. Results show that nanopowders have 5.3–6.5 nm pore sizes and 145–203 m²/g surface areas, using Brunauer–Emmett–Teller analysis. Polyvinyl alcohol-functionalized particles removed 93 % of boron at 5 mg/L at pH 7 in 30 min, whereas only 68 % of boron was removed by glycidol-functionalized particles. However, at higher boron concentration, of 50 mg/L, glycidol-functionalized particles showed higher adsorption affinity of 68.9 mg/g. We conclude that internal hydroxyl groups of polyvinyl alcohol-functionalized particles are less accessible at higher boron concentration. This is the first report on magnesium ferrites for boron recovery. The spent adsorbents were separated easily from the aqueous media by an external magnet and repeatedly used. Overall, our findings demonstrated that the hydroxyl-enriched magnetic nanopowders are a better alternative to the existing boron adsorbents regarding magnetic separation, reusability, and selectivity.     

High amounts of free aromatic amino acids in the protein-like fluorescence of water-dissolved organic matter

Fluorescence spectroscopy is widely used to study water pollution. The fluorescence of water natural organic matter can be classified into two groups: the protein-like fluorescence originating from aromatic amino acids and the humic fluorescence originating from humic substances. Actually, the precise molecular origin of the protein-like fluorescence is unknown because this fluorescence may be caused by either free amino acids, peptides or proteins. Therefore, we studied the molecular origin of the protein-like fluorescence of Suwannee River natural organic matter and fractions A, B and C + D obtained by size exclusion chromatography/polyacrylamide gel electrophoresis. Fractions were analyzed by reversed-phase high-performance liquid chromatography. The electrophoretic mobilities of fractions varied in the order C + D > B > A and the molecular size in the opposite order. Our results show that the protein-like fluorescence is almost exclusively located in high molecular size fraction A and medium molecular size fraction B. Retention times and fluorescence emission spectra of authentic free aromatic amino acids tyrosine and tryptophan were identical with the retention times and emission spectra of several chromatographic peaks of fractions A and B. More than 50 % of the protein-like fluorescence is due to free aromatic amino acids incorporated in water natural organic matter.     

Degradation of pharmaceuticals by ultrasound-based advanced oxidation process

Water pollution by pharmaceutically active compounds is an emerging issue. Toxicological studies reveal that pharmaceuticals are indeed toxic for living organisms. The lack of suitable treatment technology for the complete removal of pharmaceuticals is therefore a major challenge. Advanced oxidation processes are emerging removal techniques that have many advantages versus conventional technologies. Many studies indicate that advanced oxidation processes, either in single or in combination with other degradation techniques, can enhance the degradation of pharmaceuticals in aqueous solutions. Here, we review the degradation of pharmaceuticals by sonolysis, an oxidation processes using ultrasound. In this technique, hydroxyl radicals are generated by pyrolytic cleavage of water molecules. We review the influence of operational parameters, additives and hybrid techniques on the degradation of pharmaceuticals. The maximum degradation of organic compounds was observed in the frequency range of 100–1000 kHz, which is in the high-frequency medium-power ultrasound. Even though almost all the experiments presented more than 90 % removal and good biodegradability of the target compound, good mineralization and the toxicity removal were hardly achieved. The efficiency of the degradation varies with water matrixes and varying pH. Major pathways of degradation are hydroxylation, dehalogenation, demethylation, decarboxylation, deamination, etc. More hybrid techniques have to be developed to scale up the application of ultrasound.     

Silver nanoparticle antimicrobial activity explained by membrane rupture and reactive oxygen generation

Silver nanoparticles are widely used as antimicrobial compounds based on empirical observations. However, there is few knowledge on the mechanism ruling the antimicrobial activity and toxicity of Ag nanoparticles. Here, we investigated this mechanism. Nano-Ag was synthesised by thermal co-reduction. Mutagenicity analysis was performed using Salmonella typhimurium histidine auxotrophic strains TA 98 and TA 100 at nano-silver concentrations of 100 to 500 µg per plate. Dose-dependent analysis for reactive oxygen species generation has been performed using 2,7-dichlorofluorescein diacetate dye. Membrane integrity has been analyzed at 260 nm, before and after treatment. We also used scanning electron microscopy, membrane permeabilization test, and superoxide formation determinations. Results show that the average particle size of Ag nanoparticle is 60.4 ± 3.8 nm. The minimum inhibitory concentration of Ag nanoparticles for E.coli is 30 µg/mL; the minimum bactericidal concentration is 40 µg/mL. Ames mutagenicity tests showed negative results, which may be explained by the antimicrobial activity of nano-silver. Bacterial inner wall were indeed ruptured, and cytoplasmic content was released after 5 min of treatment in a dose-dependent manner. We thus propose that reactive oxygen generation and alteration of membrane integrity and permeability are the major mechanism of antimicrobial activity of nano-silver.     

Methyl jasmonate improves radical generation in macrophyte phytoremediation

Phytoremediation is the use of plants to decontaminate and improve waters and soils. Pleustonic macrophytes are plant models for research in waters. In a phytoremediation study, the elicitation of Pistia stratiotes with methyl jasmonate or salicylic acid suggests that oxytetracycline modification rate coefficients could be increased more than threefold. Here we present the elicitation of Pistia stratiotes apical primary root tips. We detected reactive oxygen species generation by X-band electron paramagnetic resonance spectroscopy, using the spin trap α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN). Elicitation using 0.8 mM methyl jasmonate during 1 h increased the relative spin-trapped radical concentration by +12 %. Further, results indicate acute plant toxicity above 0.24 mM salicylic acid.     

Is there an inverted U-shaped curve? Empirical analysis of the Environmental Kuznets Curve in agrochemicals

As the largest contributor to water impairment, agriculture-related pollution has attracted the attention of scientists as well as policy makers, and quantitative information is being sought to focus and advance the policy debate. This study applies the panel unit root, heterogeneous panel cointegration, and panel-based dynamic ordinary least squares to investigate the Environmental Kuznets Curve on environmental issues resulting from use of agricultural synthetic fertilizer, pesticide, and film for 31 provincial economies in mainland China from 1989 to 2009. The empirical results indicate a positive long-run co-integrated relationship between the environmental index and real GDP per capita. This relationship takes on the inverted U-shaped Environmental Kuznets Curve, and the value of the turning point is approximately 10,000–13,000, 85,000–89,000 and over 160,000 CNY, for synthetic fertilizer nitrogen indicator, fertilizer phosphorus indicator and pesticide indicator, respectively. At present, China is subject to tremendous environmental pressure and should assign more importance to special agriculture-related environmental issues.     

Simple model of sludge thickening process in secondary settlers

In wastewater treatment plants (WWTPs), a secondary settler acts as a clarifier, sludge thickener, and sludge storage tank during peak flows and therefore plays an important role in the performance of the activated sludge process. Sludge thickening occurs in the lower portions of secondary clarifiers during their operation. In this study, by detecting the hindered zone from the complete thickening process of activated sludge, a simple model for the sludge thickening velocity, u _s = aX ^b (a = 0.9925SSVI _3.5, b = 3.541ln(SSVI _3.5)+12.973), describing the potential and performance of activated sludge thickening in the hindered zone was developed. However, sludge thickening in the compression zone was not studied because sludge in the compression zone showed limited thickening. This empirical model was developed using batch settling data obtained from four WWTPs and validated using measured data from a fifth WWTP to better study sludge thickening. To explore different sludge settling and thickening mechanisms, the curves of sludge thickening and sludge settling were compared. Finally, it was found that several factors including temperature, stirring, initial depth, and polymer conditioning can lead to highly concentrated return sludge and biomass in a biologic reactor.