Bingdang Wu

Iron in non-hydroxyl radical mediated photochemical processes for dye degradation: Catalyst or inhibitor?

The acetylacetone (AA) mediated photochemical process has been proven as an efficient approach for decoloration. For azo dyes, the UV/AA process was several to more than ten times more efficient than the UV/H2O2 process. Iron is one of the most common elements on the earth. It is well known that iron can improve the UV/H2O2 process through thermal Fenton and photo-Fenton reactions. What will be the role of iron in the UV/AA process? Could iron-AA complexes act as photocatalysts in environmental remediation? To answer these questions, the photo-degradation of an azo dye, Acid Orange 7 (AO7), was conducted under the variant combinations of AA with iron species in both ionic (Fe2+, Fe3+) and complex (Fe(AA)3) forms. The pseudo-first-order decoloration rate constants of AO7 in these photochemical processes followed such an order: UV/Fe(II)/AA<UV/Fe(III)/AA<UV/Fe(AA)3<UV/AA. The results demonstrate that iron species, in either ionic or complex form, acts as an inhibitor rather than a catalyst in the UV/AA process. Based on spectroscopic analysis, the inner filter effect of iron and the competition between Fe(III) and AA for the complexation with AO7 were attributed to the inhibition effect of iron on the UV/AA process. The understanding of the role of iron provides insight into the practical application of the UV/AA process.

The photochemistry of carbon nanotubes and its impact on the photo-degradation of dye pollutants in aqueous solutions

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 (1)O2 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.

Preparation and Evaluation of Titanium-Based Xerogel as a Promising Coagulant for Water/Wastewater Treatment

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.

Feasibility of the UV/AA process as a pretreatment approach for bioremediation of dye-laden wastewater

Biodegradability and toxicity are two important indexes in considering the feasibility of a chemical process for environmental remediation. The acetylacetone (AA) mediated photochemical process has been proven as an efficient approach for dye decolorization. Both AA and its photochemical degradation products had a high bioavailability. However, the biocompatibility and ecotoxicology of the UV/AA treated solutions are unclear yet. In the present work, we evaluated the biocompatibility and toxicity of the UV/AA treated solutions at both biochemical and organismal levels. The biodegradability of the treated solution was evaluated with the ratio of 5-d biological oxygen demand (BOD5) to chemical oxygen demand (COD) and a 28-d activated sludge assay (Zahn-Wellens tests). The UV/AA process significantly improved the biodegradability of the tested dye solutions. Toxicity was assessed with responses of microorganisms (microbes in activated sludge and Daphnia magna) and plants (bok choy, rice seed, and Arabidopsis thaliana) to the treated solutions, which showed that the toxicity of the UV/AA treated solutions was lower or comparable to that of the UV/H2O2 counterparts. The results are helpful for us to determine whether the UV/AA process is applicable to certain wastewaters and how the UV/AA process could be effectively combined into a sequential chemical-biological water treatment.

Acetylacetone as an efficient electron shuttle for concerted redox conversion of arsenite and nitrate in the opposite direction

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.

Nonnegligible Generation of Hydroxyl Radicals from UVC Photolysis of Aqueous Nitrous Oxide

Nitrous oxide (N2O) is widely used in radiation-chemistry and photochemistry as a scavenger to convert a hydrated electron ( eaq-) into a hydroxyl radical (·OH). However, few investigations pay attention to the photochemistry of dissolved N2O itself. The effects of purged N2O on photochemical processes are unclear and neglected. In the present work, the effects of N2O on the hydroxylation of terephthalic acid (TPA) were investigated with both medium-pressure and low-pressure mercury lamps as the light sources. Under short-wavelength UV (200-300 nm) irradiation, N2O accelerated the decay of TPA and the formation of 2-hydroxylterephthalic acid (hTPA). The effective quantum yield of ·OH from the photolysis of dissolved N2O at 254 nm was determined as 1.15-1.63, which was far larger than those of NO3- (0.09) and NO2- (0.046). On the basis of the kinetic analysis in N2 and N2O purged solutions, isotope fractionation with heavy oxygen water, and ·OH scavenging experiments with tert-butyl alcohol, the contribution of the ·OH radicals generated from the photolysis of N2O to the formation of hTPA (61.7%) was determined to be 1 order of magnitude higher than that from the converted eaq- (6.5%). These results demonstrate that using N2O and ·OH probes to quantify photogenerated eaq- in UVC irradiation might lead to false results. The work here is helpful for the proper design of scavenging and probing experiments by the combination of N2O and ·OH probes.