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Co-existing of arsenate (As(V)) and arsenite (As(III)) in aquatic environment have posed a great threat to humans due to the high mobility and toxicity. Developing practical methods for the removal of arsenic contamination in water source is vital for improving human wellbeing. Here, a novel nanostructured Fe-Ti-Mn composite oxide (FTMO) for the efficient removal of arsenic was synthesized using a simultaneous oxidation and co-precipitation method. The adsorbent was amorphous or poorly crystallized and was aggregated from heterogeneous nano-sized particles, with a high specific surface area of 434.3 m2/g. Batch experimental results showed that the FTMO effectively removed both As(V) and As(III), and the maximal adsorption capacities of arsenic species at 25 °C were 74.4 mg/g (As(V)) and 122.3 mg/g (As(III)), respectively. The arsenic removal process was endothermic and spontaneous, and the uptake of arsenic was highly pH-dependent. The inhibitive influence of competitive substances on the adsorption behavior followed by the order of: SO42−

Hydroxylamine showed an outstanding performance on enhancing the oxidation of pollutants in Fe(II) involved advanced oxidation processes, while the detailed reaction schemes have not been fully revealed. Specific functions of hydroxylamine in the oxidation of benzoic acid with Fe(II)/persulfate (PDS) system were explored. With the addition of hydroxylamine, degradation kinetics of benzoic acid deviated from both two-stage kinetics and pseudo first order kinetics, but could be interpreted well with binomial regression analysis. Degradation rate constant (kobs) of benzoic acid was calculated and showed the same variation trend with [hydroxylamine][Fe(III)]2/([Fe(II)][H+])2, the value of which was changed during reaction processes. A detailed kinetic model for simulating the degradation profile of benzoic acid with hydroxylamine acceleration was proposed for the first time and indicated that interactions of hydroxylamine and Fe(III) were fast equilibrium reactions, which was a dominant factor influencing the oxidation kinetics of benzoic acid in Fe(II)/hydroxylamine/PDS system. Comparative study showed that when 1.4 mM of ascorbic acid was added into Fe(II)/PDS system, degradation kinetics of benzoic acid was similar to that enhanced by hydroxylamine. However, when 0.6 mM or 1.0 mM of ascorbic acid was added, oxidation kinetics still presented as the two-stage profile. Kinetic simulations indicated that Fe(II) was produced slower from Fe(III)-ascorbic acid complexes than that with hydroxylamine, which caused the difference in oxidation kinetics. This study could improve our understanding about the effect of hydroxylamine and other reductants in promoting pollutants elimination in Fe(II)/PDS system.

Hydroxylamine is commonly known to enhance the performance of Fe(II)-based advanced oxidation processes (AOPs) by promoting Fe(II) regeneration from Fe(III). Recently, ferryl ion (Fe(IV)) was found as an important reactive specie in Fe(II)/peroxydisulfate (PDS) system, while the interaction between hydroxylamine and Fe(IV) in Fe(II)/hydroxylamine/PDS system was still ambiguous. Therefore, the role of hydroxylamine, especially in interaction with reactive species, was systematically studied therein. With methyl phenyl sulfoxide (PMSO) as a probe compound, an original function of hydroxylamine to transform the major reactive species from Fe(IV) to radicals (SO4 - and OH) for potential pollutants oxidation was clarified. Experimental results combined with Density Functional Theory (DFT) calculations showed that SO4 -, OH and Fe(IV) all contributed to PMSO oxidation. Hydroxylamine showed the greatest promotional effect on the production of SO4 -, while the production of Fe(IV) was inhibited with concentrations of hydroxylamine increasing from 0.2 to 1.4 mM. Compared with PMSO, hydroxylamine showed stronger scavenging ability towards Fe(IV) than SO4 - and OH. Thus, more than 50% of radicals was consumed by PMSO and Fe(IV) mainly reacted with hydroxylamine to regenerate Fe(II). In a word, hydroxylamine played a dual role during potential pollutants oxidation in Fe(II)/hydroxylamine/PDS system, namely (1) transforming the major reactive species from Fe(IV) to radials (SO4 - and OH) and (2) regenerating Fe(II) from both Fe(IV) and Fe(III). This study provides an insight into the role of hydroxylamine and its interactions with Fe(IV), which advances our understanding on similar Fe(II)/reductants/PDS systems.

Though hydroxylamine (NH2OH) is effective for accelerating pollutants degradation in Fenton and Fenton-like systems, the effect of anions simultaneously introduced by the hydroxylamine salts have always been ignored. Herein, effect of two commonly used hydroxylamine salts, hydroxylamine hydrochloride (NH2OH·HCl) and hydroxylamine sulfate [(NH2OH)2·H2SO4], for the degradation of dimethyl phthalate (DMP) in peroxymonosulfate (PMS)/Fe(II) system was comparatively investigated. Degradation efficiency of DMP with NH2OH·HCl was 1.6 times of that with same dosages of (NH2OH)2·H2SO4. SO4·−, Fe(IV) and ·OH formed in the PMS/Fe(II)/NH2OH system, but ·OH was the major species for DMP degradation. Addition of Cl− significantly improved the production of ·OH and Cl·, and the exposure dose of ·OH (CT·OH) was more than 10 times that of CTCl· as the concentration of Cl− increased to 1 mM. Calculations based on branching ratios of Cl· and ·OH indicated that the reactions of Cl− with SO4·− and Cl· with H2O were not the only production sources of ·OH in the system. Further experiments with methyl phenyl sulfoxide (PMSO) as the probe indicated that Cl− would facilitate the shift of reactive species from Fe(IV) to radicals (SO4·− or ·OH) in the system. Both hydroxylation and nitration intermediate products were detected in the oxidation of DMP. Cl− promoted the formation of hydroxylation intermediates and reduced the formation of nitration intermediates. This study revealed for the first time that Cl− could shift reactive species from Fe(IV) to radicals in PMS/Fe(II) system, raising attention to the influence of the coexisting anions (especially Cl−) for pollutants oxidation in iron-related oxidation processes.