Haiqian Zhao

Influence of a reagents addition strategy on the Fenton oxidation of rhodamine B: control of the competitive reaction of ·OH

The Fenton system (Fe2+/H2O2) generates ·OH with a high oxidation potential. However, as reactants themselves, H2O2 and Fe2+ can act as ·OH initiators as well as ·OH scavengers, leading to the need for a high dosage of reactants and increased costs. As a mixing-sensitive reaction, the ·OH-related reaction kinetics (·OH with Fe2+, H2O2, and RhB) was determined from the reaction rates (which were a constant in this work) and stoichiometry, in which the latter could be regulated by an addition strategy of Fenton reagents. This suggests that ·OH competitive reactions could be controlled by applying a macrolevel addition strategy. Herein, the effects of different addition approaches of Fe2+ and H2O2 on ·OH competitive reactions were quantitatively and systematically studied by analyzing the removal of the model pollutant RhB. We found that without stirring, and compared with a one-time addition, once H2O2 or Fe2+ was added in a step-wise pattern (e.g., one drop by one drop, 2 times, or 4 times), a high concentration of H2O2 or Fe2+ existed in a localized place for a longer period, resulting in a lower proportion of ·OH reacting with RhB, which we ascribed to an enhanced reaction between Fe2+, H2O2, and ·OH. However, when H2O2 and Fe2+ were added from two close points without stirring, a larger proportion of ·OH was scavenged by H2O2 and Fe2+; while under stirring, even a one-time addition of H2O2 or Fe2+ could cause severe scavenging of ·OH. The results also revealed a linear relationship between the RhB removal percentage and wavelength blue-shifts. This study showed that microlevel ·OH competitive reactions could be controlled by applying a macrolevel addition strategy of Fenton reagents without the addition of external chemicals. The results suggest this methodology can also offer an approach to lower ·OH invalid consumption by regulating the addition strategy in bigger reactors.

The role of quinone cycle in Fe2+–H2O2 system in the regeneration of Fe2+

ABSTRACT The reaction between Fe2+ and H2O2 generates highly reactive ·OH. However, the weak conversion from Fe3+ to Fe2+ limits its continuous reaction. Here, the difference between the Fenton system and modified Fenton system for the regeneration of Fe2+ was analyzed. A UV-vis spectrometer and redox potential measurements were used to detect Fe2+ concentration. Results indicated that Fe2+ could be better regenerated in the modified Fenton system. The regeneration of Fe2+ was facilitated by the consumption of NH2OH, while in hydroquinone (HQ)- and 1,4-bezoquinone (1,4-BQ)-modified Fenton systems, the quinone cycle could be built up and Fe3+ could be converted to Fe2+ continuously. However, results showed that HQ and 1,4-BQ reacted with ·OH, which caused a gradual decline in the enhancement effect. In order to keep Fe2+ concentration stable for a longer time, the influence of [HQ/1,4-BQ]0/[Fe2+]0 on Fe2+ concentration was carefully studied. When the mole ratio was 5:1, Fe2+ concentration remained nearly 90% of total iron at 40 min. But when the mole ratios were 0.5:1 and 0.1:1, Fe2+ concentration decreased to a very low level at 20 min. Oxidation–reduction potential (ORP) results further confirmed the role of quinone cycle. GRAPHICAL ABSTRACT

Optimization of NO oxidation by H2O2 thermal decomposition at moderate temperatures

H2O2 was adopted to oxidize NO in simulated flue gas at 100–500°C. The effects of the H2O2 evaporation conditions, gas temperature, initial NO concentration, H2O2 concentration, and H2O2:NO molar ratio on the oxidation efficiency of NO were investigated. The reason for the narrow NO oxidation temperature range near 500°C was determined. The NO oxidation products were analyzed. The removal of NOx using NaOH solution at a moderate oxidation ratio was studied. It was proven that rapid evaporation of the H2O2 solution was critical to increase the NO oxidation efficiency and broaden the oxidation temperature range. the NO oxidation efficiency was above 50% at 300–500°C by contacting the outlet of the syringe needle and the stainless-steel gas pipe together to spread H2O2 solution into a thin film on the surface of the stainless-steel gas pipe, which greatly accelerated the evaporation of H2O2. The NO oxidation efficiency and the NO oxidation rate increased with increasing initial NO concentration. This method was more effective for the oxidation of NO at high concentrations. H2O2 solution with a concentration higher than 15% was more efficient in oxidizing NO. High temperatures decreased the influence of the H2O2 concentration on the NO oxidation efficiency. The oxidation efficiency of NO increased with an increase in the H2O2:NO molar ratio, but the ratio of H2O2 to oxidized NO decreased. Over 80% of the NO oxidation product was NO2, which indicated that the oxidation ratio of NO did not need to be very high. An 86.7% NO removal efficiency was obtained at an oxidation ratio of only 53.8% when combined with alkali absorption.

Roles of free radicals in NO oxidation by Fenton system and the enhancement on NO oxidation and H2O2 utilization efficiency

ABSTRACT Low H2O2 utilization efficiency is the main problem when Fenton system was used to oxidize NO in flue gas. To understand the behaviour of the free radicals during NO oxidation process in Fenton system is crucial to solving this problem. The oxidation capacity of and on NO in Fenton system was compared and the useless consumption path of and that caused the low utilization efficiency of H2O2 were studied. A method to enhance the oxidation ability and H2O2 utilization efficiency by adding reducing additives in Fenton system was proposed. The results showed that both of and were active substances that oxidize NO. However, the oxidation ability of radicals was stronger. The vast majority of and was consumed by rapid reaction , which was the primary reason for the low utilization efficiency of H2O2 in Fenton system. Hydroxylamine hydrochloride and ascorbic acid could accelerate the conversion of Fe3+ to Fe2+, thereby increase the generation rate of ·OH and decrease the generation rate of . As a result, the oxidation ability and H2O2 utilization efficiency were enhanced. GRAPHICAL ABSTRACT

Free radical behaviours during methylene blue degradation in the Fe2+/H2O2 system

ABSTRACT Behaviours of the free radicals during the methylene blue (MB) oxidation process in the Fe2+/H2O2 system were studied to reveal the reason for the low utilization efficiency of H2O2. The roles of , and radicals were proven to be different in the MB oxidation process. The results showed that radicals had a strong ability to oxidize MB; however, they were not the main active substances for MB degradation due to the low concentration in the traditional Fe2+/H2O2 system. radicals could not oxidize MB. radicals were the main active substances for MB oxidation. In the short initial stage, the utilization efficiency of H2O2 was high, because the generation rate of was much higher than that of . More radicals were involved in the MB oxidation reaction. In the long deceleration stage (after the short initial stage), a large amount of H2O2 was consumed, but the amount of oxidized MB was very small. Most of the radicals were consumed via the rapid useless reaction between and in this stage, resulting in the serious useless consumption of H2O2. It is a feasible method to improve the utilization efficiency of H2O2 by adding suitable additives into the Fe2+/H2O2 system to weaken the useless reaction between and .