Deep investigation on different effects of Cl− in transformation of reactive species in Fe(II)/NH2OH/PDS and Fe(II)/NH2OH/H2O2 systems

• Fe(II)/NH 2 OH/PDS and Fe(II)/NH 2 OH/H 2 O 2 systems followed different kinetic rules. • Cl − had opposite effects on BA degradation in Fe(II)/NH 2 OH/PDS and Fe(II)/NH 2 OH/H 2 O 2 systems. • Cl − changed relative contents of ·OH, SO 4 ·− and Fe(IV) in Fe(II)/NH 2 OH/PDS system. • Cl − competitively consumed ·OH with target pollutants in Fe(II)/NH 2 OH/H 2 O 2 system. Hydroxylamine (NH 2 OH) has been verified to efficiently strengthen pollutants oxidation in Fe(II)/peroxydisulfate (PDS) and Fe(II)/H 2 O 2 systems. However, the different effects of hydroxylamine salts types were rarely recognized. Herein, the effects of two commonly used hydroxylamine salts (i.e. NH 2 OH·HCl and (NH 2 OH) 2 ·H 2 SO 4 ) on oxidation kinetics and reactive species composition were compared in Fe(II)/PDS and Fe(II)/H 2 O 2 systems for the first time. Pseudo first order kinetics could only describe benzoic acid (BA) oxidation well in Fe(II)/NH 2 OH/H 2 O 2 system, which was related to the different concentration changes of Fe(III) determined by [ Oxidant ] [ N H 3 O H + ] 2 . Hydroxylamine salts types influenced not kinetic rules, but reaction rates of target compounds. The empirical reaction rate constant of BA in Fe(II)/NH 2 OH·HCl/PDS system was 141.5% of that in Fe(II)/(NH 2 OH) 2 ·H 2 SO 4 /PDS system under the same concentration of NH 2 OH (1.4 mM), while the apparent reaction rate constant in Fe(II)/NH 2 OH·HCl/H 2 O 2 system was 68% of that in Fe(II)/(NH 2 OH) 2 ·H 2 SO 4 /H 2 O 2 system. This opposite effect resulted from the differences in primary reactive species compositions and their interactions with Cl − . Reactive species identification indicated that Cl − would decrease the contribution of ferryl ion (Fe(IV)) and transform sulfate radical (SO 4 ·− ) to hydroxyl radical (·OH) in Fe(II)/NH 2 OH/PDS system, while it competitively consumed the only reactive species ·OH in Fe(II)/NH 2 OH/H 2 O 2 system. This study highlights the importance of reductants types on strengthening Fenton oxidation and offers a reference for reasonable construction of the relevant systems.