•A combined experimental and theoretical study was conducted for acetylacetone (AA).•AA worked better than quinones in photochemical conversion of arsenite.•Excited AA could act as an electron shuttle/redox mediator.•Concerted redox conversion of arsenite and nitrate occurred in the UV/AA process.•The UV/AA process was more efficient than the UV/H2O2 in arsenite oxidation.The redox conversion of arsenite and nitrate has direct effects on their potential environment risks. Due to the similar reduction potentials, there are few technologies that can simultaneously oxidize arsenite and reduce nitrate in one process. Here, we demonstrate that a diketone-mediated photochemical process could efficiently do this. A combined experimental and theoretical investigation was conducted to elucidate the mechanisms behind the redox conversion in the UV/acetylacetone (AA) process. Our key finding is that UV irradiation significantly changed the redox potential of AA. The excited AA, 3(AA)*, acted as a semiquinone radical-like electron shuttle. For arsenite oxidation, the efficiency of 3(AA)* was 1–2 orders of magnitude higher than those of quinone-type electron shuttles, whereas the consumption of AA was 2–4 orders of magnitude less than those of benzonquinones. The oxidation of arsenite and reduction of nitrate could be both accelerated when they existed together in UV/AA process. The results indicate that small diketones are some neglected but potent electron shuttles of great application potential in regulating aquatic redox reactions with the combination of UV irradiation.Download high-res image (242KB)Download full-size image

The nontoxicity of titanium (Ti) and the potential to produce valuable photocatalysts from the final coagulated sludge constitute the main advantages of Ti-based coagulants over conventional ones. However, the low effluent pH and the too-fast hydrolysis limit the wide application of Ti-salt coagulants. Prehydrolysis, to some extent, is helpful to improve the coagulation performance of Ti-salt coagulants. However, the prehydrolyzed polytitanium chloride (PTC) still suffers from narrow applicable dose/pH range. A novel and efficient Ti-based coagulant, denoted as titanium xerogel coagulant (TXC), was successfully prepared by the sol–gel method with TiCl4 as the precursor and acetylacetone as a modifying agent. Compared with TiCl4, a PTC, and a commercial polyferric sulfate, the resulting TXC possessed a larger floc size, better settling property, and wider applicable coagulant dose/pH range. Moreover, the effluent pH after TXC coagulation was not significantly reduced, avoiding the corrosion problem sometimes caused by the low effluent pH. TXC exhibited good coagulation performance for several real wastewaters, especially for the wastewaters of low turbidity. These results demonstrate that gelation was a more effective strategy than prehydrolysis to overcome the inherent weaknesses of Ti salts as a type of promising coagulants.

•Laccase and mediator were co-immobilized through a self-initiated one-pot process.•Acetylacetone could serve as a mediator for both free and immobilized laccase.•The enzymatic transformation of MG was enhanced by the co-immobilization.Laccase is a green biocatalyst. It works with molecular oxygen and produces water as the only by-product. However, its practical application is far less than satisfactory due to the low stability/poor reusability of free laccase and the potential secondary pollution caused by dissolved mediators. To address those bottlenecks in laccase-based catalysis, a novel biocatalyst (Immo-LMS) was fabricated by simultaneously immobilizing both laccase and a mediator (acetylacetone, abbreviated as AA) into a hydrogel through the laccase-AA initiated polymerization. This self-initiated immobilization process avoided the forced conformational change of laccase in the passive embedding to pre-existing carriers. Resulting from the effective cooperation of laccase and AA, the Immo-LMS had the highest substrate conversion quantity to malachite green, followed by the sole immobilized laccase and the immobilized laccase with an external mediator. Besides the improved activity, the Immo-LMS showed enhanced stability. The good performance of the Immo-LMS suggests that the co-immobilization of laccase and mediator through the self-initiated one-pot process was a promising strategy for the immobilization of laccase, which is expected to be helpful to cut down the running cost as well as the potential toxicity that come from mediators in the practical application of laccase.

•The photochemistry of CNTs in aqueous suspensions was investigated.•CNTs could be photo-ionized and trap hydrated electrons.•CNTs could both generate and scavenge reactive oxygen species.•Inner filter effect dominates in the impact of CNTs on dye photo-degradation.It is reported that carbon nanotubes (CNTs) could either generate reactive oxygen species (ROS) under light irradiation or serve as high-efficient scavenger for ROS. However, it is unclear which role predominates as CNTs enter into aquatic environment. To answer this question, a systematic study of the photochemistry of a pristine and a surface-functionalized CNTs in aqueous suspensions was investigated with both time-resolved and steady state analytical approaches. The transient absorption spectra demonstrate that CNTs could be photo-ionized and trap hydrated electrons upon high energy irradiation. In steady state UV irradiation, CNTs could promote the generation of ROS, such as 1O2 and OH. However, in the presence of H2O2, the OH scavenging effect predominated in the aqueous suspensions of CNTs. The presence of CNTs suppressed the photo-degradation of dye pollutants, as an integrated result of inner filter effect, adsorption effect, and ROS generation and scavenging effect. The results provide useful information for the understanding of the environmental implications of CNTs.

Non-metal doping is an effective approach to improve the visible light responsive photocatalytic activity of TiO2-based materials. To investigate the effect of doping sequence on the photocatalytic activity of non-metal doped TiO2, four TiO2-based nanocrystallites were fabricated with both doping-in-sol and doping-in-gel (adsorption–calcination) methods. Experimental results demonstrate that the doping-in-gel method could effectively suppress the growth of crystallite size and the resultant photocatalyst had higher dopant content. The TiO2 nanoparticle obtained from the doping-in-gel approach had a much better photocatalytic performance than the three doping-in-sol counterparts, although the band gap of the former was larger than those of the other three. The enhanced photocatalytic activity could be attributed to the synergetic effects of the higher dopant content, the smaller crystallite size, the larger specific surface area, and the better dispersion in dye solutions. The results indicate that adsorption exhausted TiO2 gel might be a good precursor for TiO2-based photocatalysts. The consequent utilization of the TiO2-based materials as adsorbents and photocatalysts may endow them with renascent vitality. Besides, the results here shed light on the morphology and activity control of non-metal doped TiO2 with the sol–gel process.

Lifetime is a key index in the evaluation of environmentally functional materials. Although it is well known that adsorption is the first step in photo-catalysis, very little work has been done on the sequential use of materials as both adsorbents and photocatalysts. In this work, two titania-based materials, TiO2 xerogel and TiO2 photocatalyst nanoparticles, were fabricated and evaluated as adsorbent and photocatalyst for the remediation of contaminated water with an azo dye, Acid Orange 7 (AO7), as the modeling pollutant. The TiO2 xerogel showed a high adsorption capacity to AO7 (769 mg/g) and could be regenerated easily with diluted NaOH solution (0.01 mol/L) for several cycles. The exhausted xerogel was calcined at 400 °C for 3 h and used as a photocatalyst for the degradation of AO7. Compared to the nanoparticles directly prepared from fresh TiO2 xerogel, the TiO2 nanoparticles from adsorption exhausted xerogel showed a much higher photocatalytic activity upon both UV and visible light irradiation. Thus the titania-based materials were endowed with improved performance as well as prolonged lifetime.

Photo-induced reversible sorption of azo dyes on titania xerogels is observed, which is based on the photo-switchable change of the acetylacetonato anchors in the xerogels. The photoisomerization of azo compounds that regulates many gate-keeper-like host–guest interaction systems is excluded as the determinant.

Free radical-mediated advanced photochemical oxidation processes are extensively studied for the degradation of variant pollutants. However, their practical application in dye-rich wastewater treatment is limited because of the poor efficiency and the slow degradtion rate caused by the poor reaction selectivity. Here we report that two small molecular diketones could act as effective photoactivators for dye degradation through a non-free radical pathway. Under ultraviolet (UV) irradiation, diketones form charge-transfer complexes with dyes, which might turn the target dyes from shields (inner filters in some photochemical processes, such as UV with H2O2 and UV with TiO2) into spears. As a result, the diketone-mediated process is much more efficient and target-selective. Moreover, diketones and their degradation products have good biocompatibility, making this novel process potentially suitable as a pretreatment step in sequential chemical–biological wastewater treatment.

•Both dispersed and resin-supported ZVIs can oxidize As(III) in O2 and H2O2 systems.•As(III) oxidation by dispersed ZVI was mainly through the Fenton-like reactions.•The function of the supported ZVI differs from that of the dispersed ZVI.•The difference suggests that the support matrix interfered in As(III) removal.The goal of this study is to assess the differences in As(III) removal kinetics and mechanisms between dispersed zero-valent iron (d-ZVI) and resin-supported zero-valent iron (D201-ZVI) in the presence of dissolved oxygen and hydrogen peroxide. Experimental results show that As(III) could be removed by all the studied systems (d-ZVI/O2, d-ZVI/H2O2, D201-ZVI/O2, D201-ZVI/H2O2). The d-ZVI/H2O2 system was more efficient than D201-ZVI/H2O2 for the oxidation of As(III). Similar trends were observed in O2 system for both solids. The kinetic behaviors as well as the influence of a hydroxyl radical scavenger (2-propanol) on the oxidation of As(III) at different pH suggest that the oxidation of As(III) in the d-ZVI/O2 and d-ZVI/H2O2 systems occurred mainly through Fenton-like reactions. The oxidation of As(III) in the D201-ZVI/O2 and D201-ZVI/H2O2 systems might be expected as follows: As(III) was firstly adsorbed onto the surface of the D201-ZVI, and then oxidation may proceed mainly through a non-Fenton mechanism that directly converts H2O2 into O2 and H2O. In addition, certain iron oxides in the D201-ZVI could also serve as oxidants for As(III) oxidation. The significant differences between the dispersed and supported ZVIs suggest that the supporting matrix interfered in the removal process, which deserves a further investigation.Graphical abstract

Using solar energy for the decontamination of wastewater is a promising solution to the water-energy nexus. Current advanced oxidation processes have an unsatisfactory efficiency in the treatment of dye wastewater due to the non-selectivity of hydroxyl radicals. More efficient photochemical approaches for dye degradation are highly needed. Three diketones, biacetyl, acetylacetone, and acetonylacetone, were proven to be potent activators for the photodecoloration of azo, triarylmethane and anthraquinone dyes. The photodegradation kinetics of Acid Orange 7 in the UV/diketone processes was much faster than that in the UV/H2O2 system. Photo-induced energy and electron transfer were possible mechanisms for dye degradation in the diketone systems. Adducts of dye and acetylacetone were identified, indicating a unique dye degradation route through adduct formation and decomposition. Unlike acting only as the target substrate of ˙OH in advanced oxidation processes, the dyes played vital roles in the UV/diketone processes. The findings here provide new insights for designing more efficient technologies for environmental remediation, based on diketone photochemistry.

•Adsorption of six aromatic compounds on four activated carbons was conducted.•Freundlich affinity coefficients (K) were obtained from the adsorption isotherms.•The K index was used as a descriptor in the linear solvation energy relationship.•The LSER was applicable to predict the adsorption of the aromatic adsorbates.•The LSER could also reflect the natures of the adsorbents.A correlation capable of predicting adsorption capacities from the commonly available physicochemical properties of an adsorbate is of great significance to the engineering design of adsorption process. Apart from van der Waals force, dipole–dipole, induced dipole–dipole, and hydrogen-bonding donor–acceptor interactions exist between aromatic compounds (AOCs) and activated carbons. Correlations between the solubility normalized Freundlich affinity coefficients (KFS) of six AOCs on four activated carbons and the linear solvation energy relationship (LSER) variables of the AOCs were established. The modeling results demonstrate that the LSER model could be applied to predict the adsorption of AOCs on the activated carbons with KFS as a measure of the relative interaction strengths between the adsorbents and the adsorbates. Although derived from the properties of the adsorbates, the LSER modeling parameters were correlated with the surface chemistry of the studied activated carbons, suggesting that the LSER models could also reflect the natures of the adsorbents.

Adsorption is controlled by an array of attractive forces between adsorbent, adsorbate, and solvent molecules. Such forces work interactively, making the interpretation and prediction of sorption processes difficult. By carefully designing the experimental matrix, the effects of adsorbent surface chemistry and pore structure on the adsorption of aromatic compounds were isolated from the complicated web of interactions. Two parameters, γ and δ, were created to describe the relative adsorption affinity index of activated carbons to adsorbates and the occupancy rate of activated carbons by active sites that can lead to formation of water clusters. Taking the space availability and the relative adsorption affinity index into account, a correlation between the Freundlich adsorption affinity coefficient and the characteristics of adsorbent was established. With this correlation, if the Freundlich adsorption affinity coefficient of a compound on one carbon is known, its adsorption affinity coefficient on another carbon might be predictable if the surface chemistries and pore structures of both carbons are available.

•AcAc could affect the photoconversion of PPCPs through three pathways.•UV/AcAc reduced the toxicities of PPCPs to similar extents as UV/H2O2 did.•Natural water matrices enhanced the photodecomposition of tetracyclines.•Dissolved oxygen played crucial roles in the photoconversion of PPCPs.Acetylacetone (AcAc) has proven to be a potent photo-activator in the degradation of color compounds. The effects of AcAc on the photochemical conversion of five colorless pharmaceuticals were for the first time investigated in both pure and natural waters with the UV/H2O2 process as a reference. In most cases, AcAc played a similar role to H2O2. For example, AcAc accelerated the photodecomposition of carbamazepine, oxytetracycline, and tetracycline in pure water. Meanwhile, the toxicity of tetracyclines and carbamazepine were reduced to a similar extent to that in the UV/H2O2 process. However, AcAc worked in a way different from that of H2O2. Based on the degradation kinetics, solvent kinetic isotope effect, and the inhibiting effect of O2, the underlying mechanisms for the degradation of pharmaceuticals in the UV/AcAc process were believed mainly to be direct energy transfer from excited AcAc to pharmaceuticals rather than reactive oxygen species-mediated reactions. In natural waters, dissolved organic matter (DOM) played a crucial role in the photoconversion of pharmaceuticals. The role of H2O2 became negligible due to the scavenging effects of DOM and inorganic ions. Interestingly, in natural waters, AcAc first accelerated the photodecomposition of pharmaceuticals and then led to a dramatic reduction with the depletion of dissolved oxygen. Considering the natural occurrence of diketones, the results here point out a possible pathway in the fate and transport of pharmaceuticals in aquatic ecosystems.Download high-res image (207KB)Download full-size image

•Acetylacetone (AA) enhanced the reduction of nitrate by zero valent iron (ZVI).•The ligand effects caused by AA were compared with five other ligands.•Dissociable protons and reduction potential change were important factors.•The potential of AA as an accelerant in nitrate reduction by ZVI was evaluated.Surface passivation is a key limiting factor in the application of zero-valent iron (ZVI) for water remediation. Addition of ligands is a useful approach to overcome this issue. In this work, a small amount of acetylacetone (AA) (0.5 mM) was found highly efficient to enhance the reduction of nitrate by ZVI at near neutral conditions (pH 6.0) with the formation of considerable black coating on ZVI. At an initial nitrate concentration of 20 mg N/L, the pseudo first-order reduction rate constant of nitrate in the ZVI-AA-NO3− system was 0.0991 h−1, which was 52 times higher than that in the ZVI-NO3− system. Under otherwise identical conditions, the other five ligands, including EDTA, formate, acetate, oxalate, and phosphate, had negligible effects. Based on the pKa values of these ligands and the final species of iron, the ligand effects on nitrate reduction by ZVI were summarized from three aspects: (1) the ability to offer potentially dissociable protons from the ligands; (2) the complexation ability to eliminate iron (hydr)oxide precipitates from the surface of ZVI; and (3) the ability to lower down the redox potentials of iron species. The good performance of AA in these three aspects makes it advantage over the other ligands. A cycle test up to six runs demonstrates that AA could continuously take effect in the ZVI system. The results here point out the potential of AA as an effective ligand in ZVI system for enhanced contaminant transformation.Download high-res image (147KB)Download full-size image

Using solar energy for the decontamination of wastewater is a promising solution to the water-energy nexus. Current advanced oxidation processes have an unsatisfactory efficiency in the treatment of dye wastewater due to the non-selectivity of hydroxyl radicals. More efficient photochemical approaches for dye degradation are highly needed. Three diketones, biacetyl, acetylacetone, and acetonylacetone, were proven to be potent activators for the photodecoloration of azo, triarylmethane and anthraquinone dyes. The photodegradation kinetics of Acid Orange 7 in the UV/diketone processes was much faster than that in the UV/H2O2 system. Photo-induced energy and electron transfer were possible mechanisms for dye degradation in the diketone systems. Adducts of dye and acetylacetone were identified, indicating a unique dye degradation route through adduct formation and decomposition. Unlike acting only as the target substrate of ˙OH in advanced oxidation processes, the dyes played vital roles in the UV/diketone processes. The findings here provide new insights for designing more efficient technologies for environmental remediation, based on diketone photochemistry.

Photo-induced reversible sorption of azo dyes on titania xerogels is observed, which is based on the photo-switchable change of the acetylacetonato anchors in the xerogels. The photoisomerization of azo compounds that regulates many gate-keeper-like host–guest interaction systems is excluded as the determinant.